Symposium EN05—Chemomechanical and Interfacial Challenges in Energy Storage and Conversion—Batteries and Fuel Cells

Rechargeable lithium metal batteries can reduce the cost of energy storage for both transportation and grid applications. In order to combat issues of infinite volume change, dendrite growth, and parasitic reactions with electrolytes, we have employed several approaches. We have designed a multifunctional 3D host structure with built-in electrolyte additives which promotes dendrite free deposition by reducing effective current densities while improving current efficiency. In terms of electrolyte chemistry, our guiding principle has been the minimization of reactivity with lithium. In this regard, new multicomponent ether-based electrolytes have shown promise. In addition, we emphasize the importance of the chosen electrolyte to be compatible with leading cathode chemistries such as sulfur and transition metal oxides. Performance of lithium metal cells utilizing different cathodes based on the ether electrolytes will be discussed. Finally, we have pursued the formation of surface coatings that can promote uniform lithium ion flux on the electrode surface to enable dendrite free lithium deposition. Our approach favored the use of in-situ chemical reactions in the electrolyte to form protective coatings, which range from lithium organic carbonates to polymeric lithium ion conductors.
Acknowledgements: This work was supported by the Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research Program (Battery 500 Consortium) under Contract DE-EE0007764. Part of the work used the UCSD-MTI Battery Fabrication Facility and the UCSD-Arbin Battery Testing Facility.

Lithium metal has been considered as an ultimate anode material due to the advantages of the high specific theoretical capacity (3860 mAh g^-1) and the lowest electrochemical potential (-3.04V vs the standard hydrogen electrode) for the next generation lithium ion batteries (LIBs). However, its practical use has been hindered by the Li dendrite growth, large volume changes and the low Coulombic efficiency during cycling, which finally leads the cell failure and safety issues. Various approaches have been suggested; i) Physical suppression of the Li dendrite growth. ii) Regulating Li deposition with dense and planar structures. iii) SEI layer reinforcement iv) Composite electrode engineering. We have focused on the Li protection layer for the control of the Li-ion flux on the Li/electrolyte interphase and the increase of Sand’s time. Herein, we report the novel Li metal protection layer and its application for the stable and high performance of Li metal batteries (LMBs). The mechanical property, electronic conductivity and ionic conductivity have been considered for the design of Li metal protection layer. The designed protection layer could effectively retard the Li dendrite growth with the uniform Li-ion flux and the prevent of the repeated electrolyte decomposition during cycling.

Lithium metal anodes are critical enablers for high energy density next generation batteries, but they suffer from poor morphology control and parasitic reactions. Recent experiments have shown that an external packing force on Li metal batteries with liquid electrolytes extends their lifetimes by inhibiting the growth of dendritic structures during Li deposition. However, the mechanisms by which pressure affects dendrite formation and growth have not been fully elucidated. For example, beneficial pressure effects have been observed even for thin polymer separators whose mechanical properties are not expected to be able to hinder dendrite growth. In this paper we offer an explanation for how dendrite growth can be inhibited when the cell is subjected to an external load, even with a relatively soft separator. We carried out a three-dimensional contact mechanics model based on the semi-analytical method for tracking Li surface and sub-surface stresses for a pouch cell architecture with realistically (micron-scale) rough electrode surfaces subjected to a packing force. Our work shows that the picture normally used to understand dendrite penetration, where micron-scale Li metal protrusions under a fixed local current density press conformally against a separator, is oversimplified. At the larger, sub-mm scales studied here, contact between the Li metal and the separator is highly heterogeneous and far from conformal for surfaces with realistic roughness: the load is carried at a relatively small number of the tallest asperities, while the rest of the Li surface feels no force at all. Yet, dendrite growth is suppressed over the entire Li surface. To explain this observation, we suggest that (1) local contact stresses can be high enough (tens of MPa) at the peaks of Li protrusions (incipient dendrites) so that incremental Li⁺ ions plate instead in regions of low or zero stress; and (2) creep ensures that Li protrusions are gradually flattened. These mechanisms cannot be captured in micron-scale analyses of dendrite growth.

Recent technological advances such as modern electric vehicles and portable personal electronics have enabled by advances in lithium ion batteries. This technology has improved at a relatively steady 8% per year (in terms of specific energy, Wh/kg) since introduction in 1991. To grow the rate of electrified vehicle adoption, larger improvements in battery technology are needed. While there is a range of competing chemistries for next generation batteries, most would benefit from the use of lithium metal anodes as one of the primary means of improving energy density.

Despite the considerable interest and dependence on lithium anodes in future generation batteries, its mechanical properties are poorly understood. Recently our group investigated the elastic, plastic and creep behavior or lithium metal [1]. This presentation will extend the scope from our previous study to battery specific mechanical environments, specifically aspect ratio, temperature and strain speed.

REFERENCES
[1] Masias, A. et al. J. Mat. Sci., 54(3) (2018), 2585-2600.

The lifetime, efficiency and reliability of energy storage devices depends strongly on the complex relationships between chemistry and mechanical properties of the materials within the device. As defined by this symposium, there is a need to characterize the mechanical properties of energy storage materials to provide relevant information for chemo-mechanical simulations and models. Here, we cover the measurement of mechanical properties and deformation using nanoindentation techniques. Specifically, elastic modulus, hardness, and creep deformation will be discussed with reference to the relationship between strength, defects, and length scale. The importance of incorporating as-processed material properties with respect to “bulk” property measurements into simulations will be highlighted. Nanoindentation measurements of elastic modulus, hardness, and creep of lithium, ceramic separators, and thin films will be provided to show current capabilities and the experimental challenges of understanding material deformation at the nanoscale. Challenges include film thickness, surface roughness, and large time dependent deformation at room temperature. Because of these challenges, we have performed nanoindentation experiments that allow for elastic modulus measurements below 10nm of indentation depth, statistically significant hardness measurements on rough surfaces, and creep deformation measurements at fractions of the time required for standard creep testing. To summarize, nanoindentation can be used as a method for examining mechanical behavior of the materials in energy storage devices, but it is important to understand experimental challenges and what can be done today and, in the future, to overcome those challenges.

Interfaces in Li-ion batteries play a crucial role in the reversibility of intercalation or plating behaviors. These interfaces are influenced by the coupling of electrochemical reactions, thermal conditions and mechanical changes. As batteries are exposed to a varying environments (low temperature or aggressive charging rates) performance and safety are altered. These variations can enable undesired side reactions like SEI growth or lithium plating. Herein, plating prone conditions, specifically low temperature and thermal shock) are characterized in experimental and commercial Li-ion batteries. Electrochemical behavior is carefully analyzed with (impedance spectroscopy and incremental capacity) and the physical implications on the system are assessed with non-destructive µ X-ray CT (computed tomography). This combined analysis identifies lithium plating and jellyroll collapse as dominate capacity loss mechanisms in abuse scenarios. An accelerated rate calorimeter (ARC) modified with high temperature borescope is used to assess the implications of these conditions (lithium plating or jellyroll collapse) on thermal runaway behaviors. The careful analysis of failure modes identifies thermal gradients as a high risk condition for lithium plating. Coin cell assemblies were utilized to probe this behavior further. These studies reveal the ability to modulate lithium plating behavior in li-ion and li-metal systems with external thermal gradients. This observation emphasizes the importance of temperature uniformity in commercial systems and provides an avenue for design of high performance lithium metal anodes.

1. Mistry, A. N.; Smith, K.; Mukherjee, P. P., Electrochemistry Coupled Mesoscale Complexations in Electrodes Lead to Thermo-Electrochemical Extremes. ACS Appl Mater Inter 2018, 10 (34), 28644-28655.
2. Carter, R.; Love, C. T., Modulation of Lithium Plating in Li-Ion Batteries with External Thermal Gradient. ACS Appl Mater Inter 2018, 10 (31), 26328-26334.
3. Love, C. T.; Baturina, O. A.; Swider-Lyons, K. E., Observation of Lithium Dendrites at Ambient Temperature and Below. ECS Electrochem Lett 2015, 4 (2), A24-A27.

Lithium (Li) dendrites and mossy Li remain the major obstacle for the application of Li metal electrodes in liquid electrolyte-based batteries because they excessively consume the electrolyte, can penetrate through the separator, and form dead Li. While new electrolytes and artificial solid electrolyte interphases (SEIs) are being developed, mechanical suppression by, for example, applying proper external pressure, functional separators, and surface coatings, is emerging as an effective approach to improve the cycling stability of Li metal electrodes (even with mossy Li) in liquid electrolyte-based batteries. To better design mechanical suppression approaches, it is indispensable to gain a fundamental understanding of the mechanical behavior of bulk Li as well as mossy Li. Although recent progress has been made in measuring the mechanical properties of bulk Li, the mechanical behavior of electroplated mossy Li is unknown. To fill this knowledge gap, we investigated the mechanical behavior of electroplated mossy Li at room temperature using flat punch indentation inside an argon-filled glovebox. It is found that the Young’s modulus of mossy Li depends on the porosity and is smaller than that of bulk Li. Both the mossy and bulk Li show clearly indentation creep behavior. Despite the highly porous microstructure, the impression creep velocity of the mossy Li is far less than that of bulk Li under the same compressive stress. We proposed possible mechanisms for the significantly higher creep resistance of the mossy Li over bulk Li. These findings may be helpful for developing mechanical suppression approaches to improve the cycling stability of Li metal electrodes.

The potential advantages of lithium (Li) metal anodes have been widely touted (lowest reduction potential, etc.). However, the poor stability of Li metal / liquid electrolyte interfaces leads to chronic problems, such as dendrite formation and capacity loss. The possible impact of mechanical effects on interface stability and dendrite formation leads to critical questions that are difficult to probe directly. An important part of this are the stresses that arise during battery cycling. These were measured with in-situ wafer curvature. Using this technique during Li plating and examining film thickness effects, it was possible to separate contributions from the bulk lithium metal and the solid electrolyte interphase (SEI). These investigations show that significant stresses are created in the SEI films. Similar stress evolution measurements were also performed during lithium plating and stripping using both soft (PEO) and hard (LiF) artificial surface layers. The results indicate that stresses in surface films can be tuned to improve performance when artificial SEI layers are employed. A basic chemo-mechanical model indicates that hybridizing the two layers may benefit interface stability and performance in Li metal anodes.

Dendritic growth of alkali metal anode during cycling is one of major issues to be addressed for practical application of alkali metal based rechargeable batteries. In this work, morphological changes of Na and Li metal anode were investigated using SO₂ solvated inorganic liquid electrolyte. We found that Na metal anode showed highly stable cycle performance without dendritic formation during cycling in non-flammable and highly Na⁺-conductive NaAlCl₄-2SO₂ inorganic electrolyte, as a result, showing superior electrochemical performances to those in conventional organic electrolytes, which was possible by inducing polygonal growth of Na deposit using a highly concentrated Na⁺-conducting inorganic electrolyte and also creating highly dense passivation film mainly composed of NaCl on the surface of Na-metal electrode. We also found that surface modification of Li metal with Na-containing SO₂ electrolyte can be an effective way to prevent dendritic Li growth during cell operation. The surface-modified Li metal anode exhibited no dendritic deposits even under a high areal capacity (5 mA h cm^-2) and a high current density (3 mA cm^-2), while the unmodified anode showed typical filamentary Li deposition. The surface-modified Li metal anode also demonstrated significantly enhanced electrochemical performance, which could be attributed to the newly-formed Na-containing inorganic surface layer that exhibits uniform and dense properties.
Keywords: Li metal, dendrite, battery

The stress inevitably imposed during electrochemical reactions is expected to fundamentally affect the electrochemistry, phase behavior and morphology. Using graphene liquid cell electron microscopy, we recently unveiled the strong stress-composition coupling in lithium binary alloys in the nanoscale. During lithiation of core-shell nanoparticles, the generation of non-uniform composition field is directly visualized with in situ graphene liquid cell electron microscopy imaging. Based on this coupling, we demonstrate that we can directionally control the lithium distribution by applying different stresses to lithium alloy materials. We show a few device concepts of stress-composition coupling, including electrochemically driven mechanical energy harvesters and non-volatile electrochemical actuators.

Despite possessing physical properties that justify their use in high voltage, high power aprotic battery chemistries, carbonate-based electrolytes and their resulting solid electrolyte interphases (SEI) do not generally enable morphologically homogeneous plating and stripping to and from metal electrodes. Instead, due to interphase breakdown, freshly plated metal is continuously exposed to the electrolyte. High electrode-electrolyte reactivity causes loss of coulombic efficiency and promotes dendrite growth, ultimately leading to catastrophic cell failure. Efforts to circumvent this issue have often focused on the ex-situ design of an artificial inert SEI or on engineering customized electrolyte systems that are hypothesized to form stable interphases in-situ. Still, due to the difficulty in probing the SEI at the molecular scale, little is known about the dynamics of its formation and breakdown during plating and stripping, and the products thereof. Here we investigate the gas phase products resulting from lithium plating and stripping under operando conditions using a custom electrochemical cell coupled to a gas chromatograph. Under electrochemical activity, we quantify the evolution of several C₁-C₂ species, as well as CO, CO₂ and H₂, that are dynamically formed due to electrolyte decomposition and SEI formation and breakdown. The operando nature of our experiments enables the identification of gases evolved through pathways that have rate-limiting electron-transfer steps, as well as gases that have chemical reactions as rate-limiting steps. Moreover, we show that SEI transformations, such as the chemical conversion of interfacial Li₂CO₃ into Li₂O, release detectable gas signatures, which are then correlated with interfacial composition measured by X-ray photoelectron spectroscopy. Because some species are formed exclusively by pathways involving electron transfer, we correlate gas evolution with plating-stripping coulombic efficiency, and hence capacity loss. By varying galvanostatic conditions, we also observe rate-dependent gas evolution, which we link to surface morphology using ex-situ scanning electron microscopy. We then investigate electrolyte systems that are known to enable morphologically homogeneous plating-stripping, in which gas evolution is expected to be suppressed due to the formation of a likely stable SEI. Finally, we suggest that operando gas analysis can be effective in identifying species and their formation mechanisms that result from parasitic electrode-electrolyte reactions, the evolution of which needs to be suppressed to achieve high cycling stability.

Intercalation and conversion are two fundamental chemical processes for battery materials in response to ion insertion. The interplay between these two chemical processes has never been directly seen and understood at atomic scale. Here, using in situ HRTEM, we captured the atomistic conversion reaction processes during Li, Na, Ca insertion into a WO₃ single crystal model electrode. An intercalation step prior to conversion is explicitly revealed at atomic scale for the first time for Li, Na, Ca. Nanoscale diffraction and ab initio molecular dynamic simulations revealed that after intercalation, the inserted ion-oxygen bond formation destabilizes the transition-metal framework which gradually shrinks, distorts, and finally collapses to an amorphous W and M_xO (M=Li, Na, Ca) composite structure. This study provides a full atomistic picture of the transition from intercalation to conversion, which is of essential importance for both secondary ion batteries and electrochromic devices.

Lithium metal anode has been regarded as the "Holy Grail" of next-generation battery technologies. In order to stabilize the electrodeposited Li metal for safe and high energy batteries, highly concentrated electrolytes (solvate electrolytes) have been proposed to suppress the formation of Li dendritic structure and polysulfide dissolution in lithium-sulfur (Li-S) batteries. (1-5) In this talk, we use in situ transmission X-ray microscopy (TXM) to study the effect of solvate electrolyte on Li plating/stripping process. In situ TXM images show that electrodeposited Li particles with uniform density are formed in solvate electrolyte during initial plating process. The effect of solvate electrolyte on the mechanism of Li growth is studied in detail. Additionally, in situ spectroscopy including Raman and X-ray spectroscopy (X-ray diffraction and X-ray absorption spectroscopy) are used to investigate sulfur reaction mechanism and the interaction between polysulfide and solvate electrolyte. We found that the sulfur species formed in the solvate electrolyte are different from the sulfur species formed in conventional DOL/DME electrolyte. These results suggest that solvate electrolyte changes both polysulfide solubility and sulfur reaction mechanism. We next propose different solvate electrolytes with low polysulfide solubility and high stability toward Li metal to enhance the capacity retention of Li-S batteries.

References:
[1] L. Suo et al., Nat. Commun. 2013, 4, 1481.
[2] K. Dokko et al., J. Electrochem. Soc. 2013, 160, A1304.
[3] M. Cuisinier et al., Energy Environ. Sci., 2014, 7, 2697.
[4] Y. Yamada et al., J. Electrochem. Soc. 2015, 162 (14), A2406.
[5] L. Cheng et al., ACS Energy Lett. 2016, 1, 503.

Presently, there is a high demand for rechargeable batteries which provide high energy and power densities. Generally, the usage of portable electronic devices is limited by the short lifetime and charge capacity of batteries. Therefore, to cope up with the increasing demand of consumers, novel battery electrodes with enhanced volumetric, gravimetric and areal capacities under fast charging rates need to be developed. This demand has prompted intensive research efforts for developing next generation Li-ion battery electrodes, e.g., to replace conventional slurry cast carbon anodes. Electrospinning is considered to be a convenient and promising approach for the fabrication of porous mats with tailored micron/sub-micron diameter fibers containing nanoparticles and polymer binder.^1,2 Electrospun anodes for Li-ion batteries have superior characteristics including: (i) high electrode/electrolyte interfacial area, (ii) flexible interfiber void space of an electrode mat to facilitate electrolyte infiltration, and (iii) micron/sub-micron sized fibers with a high nanoparticle content (short Li⁺ ion transport pathways in the radial fiber direction). Pintauro and co-workers have examined and reported on several different types of electrospun anodes for Li-ion batteries with TiO₂/C³, Si/C⁴ or PVDF/C⁵ nanoparticles with either poly(acrylic acid) (PAA) or poly(vinylidene fluoride) (PVDF) as the polymeric binder. In our current work we are focusing on an electrospun/electrosprayed system where a Si/binder ink was electrospun into fiber with the simultaneous electrospraying of a C/binder ink. Preliminary half-cell charge/discharge data are very promising. For example, an anode containing 61 wt.% Si-PAA fibers (40 wt.% PAA) and 39 wt.% C-PVDF droplets (37 wt.% PVDF) exhibited a terminal discharge gravimetric capacity of 1100 mAh/g (based on the total anode weight) at 0.1C and 600 mAh/g at 1C, with remained constant for 50 charge/discharge cycles. For this anode, the areal capacity was 1.2 mAh/cm² and the coulombic efficiency was ~99%. During this presentation, details will be given on the fabrication method and structure of the fiber/sprayed anodes, along with charge/discharge data at different C rates. The effects of binder type and Si/C weight ratio on anode performance (gravimetric, volumetric, and areal capacities) at different C-rates will be discussed.


References:

(1) Self, E. C.; McRen, E. C.; Wycisk, R.; Pintauro, P. N. LiCoO₂-Based Fiber Cathodes for Electrospun Full Cell Li-Ion Batteries. Electrochim. Acta 2016, 214, 139–146.
(2) Powers, D.; Wycisk, R.; Pintauro, P. N. Electrospun Tri-Layer Membranes for H₂/Air Fuel Cells. J. Memb. Sci. 2018, https://doi.org/10.1016/j.memsci.2018.11.046.
(3) Self, E. C.; Wycisk, R.; Pintauro, P. N. Electrospun Titania-Based Fibers for High Areal Capacity Li-Ion Battery Anodes. J. Power Sources 2015, 282, 187–193.
(4) Self, E. C.; Naguib, M.; Ruther, R. E.; McRen, E. C.; Wycisk, R.; Liu, G.; Nanda, J.; Pintauro, P. N. High Areal Capacity Si/LiCoO₂ Batteries from Electrospun Composite Fiber Mats. ChemSusChem 2017, 10 (8), 1823–1831.
(5) Self, E. C.; McRen, E. C.; Pintauro, P. N. High Performance Particle/Polymer Nanofiber Anodes for Li-Ion Batteries Using Electrospinning. ChemSusChem 2016, 9 (2), 208–215.

Silicon has received much attention as a promising negative electrode material, however, it undergoes extremely large volume changes during lithiation/delithiation. This leads to substantial stresses inside of particle-based electrodes, which are believed to cause poor cycling performance. Composite electrodes that also incorporate oxidized silicon are a cost-effective way to accommodate these stresses and extend cycle life. To obtain fundamental information about chemomechanical phenomena in these composite structures, several different types of materials are being investigated: (1) Si nanoparticles with oxide shells, (2) Si thin films with oxidized surface layers, and (3) composite SiOx particles. The evolution of internal stresses in all of these structures was monitored with precise in-situ curvature in conjunction with parallel electrochemical measurements. Ex situ characterization with electron microscopy, x-ray diffraction, and XPS provide important complementary information about changes in the materials. The different types of materials used for this work make it possible to systematically investigate key length scales, by independently varying oxide layer thicknesses and particle sizes. Analysis of these results requires assessments and models of both the chemical and mechanical effects oxide surface layers and silicon encapsulation. The implications for optimizing these composite electrode structures will also be presented.

Resources used in lithium-ion batteries are becoming more expensive due to demand, and the global cobalt market heavily depends on supplies from countries with high geopolitical risks. Alternative battery technologies including magnesium-ion batteries are therefore desirable. Progress toward practical magnesium-ion batteries have been impeded by an absence of suitable anodes that can operate with conventional electrolyte solvents. Although alloy-type magnesium-ion battery anodes are compatible with common electrolyte solvents, they suffer from severe failure associated with huge volume changes during cycling. Consequently, achieving more than 200 cycles in alloy-type magnesium-ion battery anodes remains a challenge [1]. In this talk I will an unprecedented long-cycle life of 1000 cycles, achieved at a relatively high (dis)charge rate of 3C (current density: 922.5 mA/g) in Mg₂Ga₅ alloy-type anode, taking advantage of near-room-temperatures solid-liquid phase transformations between Ga(liquid) and Mg₂Ga₅(solid)[2]. A combination of Finite-Element Modelling (FEM), electrochemical characterization, and operando wide-angle X-ray scattering (WAXS) is used to investigate this remarkable cycling performance[2]. This concept should open the way to the development of practical anodes for the next generation magnesium-ion batteries.

Keywords: Beyond lithium, Magnesium-ion battery anode, self-healing, solid-liquid phase transformation, operando X-ray scattering.

[1] Y. Cheng, Y. Shao, L.R. Parent, M.L. Sushko, G. Li, N.D. Browning, C. Wang and J. Liu:
Interface promoted reversible Mg insertion in nanostructured tin-antimony alloys Adv. Mater. 27, 6598-6605 (2015).

[2] L. Wang, S.S. Welborn, H. Kumar, Z. Wang, M. Li, V. Shenoy, and E. Detsi:
High-Rate and Long Cycle-Life Alloy-Type Magnesium-Ion Battery Anode Enabled through (De)magnesiation-Induced Near-Room-Temperature Solid-Liquid Phase Transformation
Under Review (2019)

A 0.7 mAh LiFePO₄/graphite Li-ion pouch cell with a 1.0 M LiPF₆ 1:1 Ethylene Carbonate:Ethyl Methyl Carbonate v/v electrolyte was overdischarged by a fixed 35.7 kOhm resistive load applied continuously for 2 weeks. The resistive load was applied after the 5^th conditioning cycle discharge by constant 0.07 mA current to 2.5 V cell voltage. 3-electrode measurements show that the anode increases to 3.4 V vs. Li/Li⁺, which is greater than the copper oxidation potential, after about an hour after application of the fixed resistive load as the cell voltage decreases to <200 mV. At about the 6^th hour of the fixed resistive load overdischarge step, the cell voltage rapidly (<5 minutes) increases from 200 mV to about 1.5 V, plateaus for about 4 hours then decreases during the next ~4 hours to 0.0 V. The 3-electrode results show that the cell voltage increase is driven by the anode potential rapidly decreasing from 3.4 V vs. Li/Li⁺ to about 1.5 V vs. Li/Li⁺ at the 6^th hour for about 4 hours. The cathode potential rapidly decreases from 3.5 to ~3.0 V vs. Li/Li⁺ at the 6^th hour and plateaus for about 4 hours. The cathode and anode potentials then decrease to ~1.0 and ~0.5 V vs. Li/Li⁺, respectively, during hours 10-14 of the fixed resistive load step until the electrode potentials asymptote together and increase to about 2.6 V vs. Li/Li⁺ during the remaining ~322 hours of the fixed resistive load step. When the cell was recharged after the 2 week fixed resistive load overdischarge step the cell charge/discharge capacity increased by about 0.08 mAh. The electrode potential characteristics also changed consistent with an increase in the reversible lithium available in the cell compared to before the fixed resistive load overdischarge.
Visual inspection of the anode shows darkening in some areas of the composite surface. Scanning electron microscopy (SEM)/energy dispersive x-ray (EDX) and x-ray photoelectron spectroscopy (XPS) results show a significant, non-uniform change to the interfacial chemistry of the anode with an increase in the presence of fluorine, phosphorous, copper and lithium on the surface compared to a control cell that was not overdischarged. The increased presence of the phosphorous, lithium and fluorine was more substantial in the visually darkened areas. Hi resolution XPS scans of the Phosphorous 2p, Fluorine 1s, and Oxygen 1s show the 2 week fixed resistive load overdischarge increases the presence of Li_xPF_y, LiF, Li_xPO_yF_z and P₄O₁₀ compounds on the surface of the anode, particularly in the visually darkened areas of the anode. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) of dried electrolyte samples shows an increase in copper content of the electrolyte in an overdischarged cell compared to a cell that was not overdischarged.
These results are consistent with copper dissolution as well as an oxidative breakdown of electrolyte Li⁺ and PF₆^- ions onto the anode surface occuring during a fixed resistive load overdischarge of a conventional LiFePO₄/graphite Li-ion cell. Additionally, a cell modified with added reversible lithium such that the anode does not increase to the copper oxidation potential during fixed resistive load overdischarge did not exhibit a delayed rapid cell voltage increase, indicating that the delayed cell voltage increase in the conventional cell involves a product of copper oxidation.
While a precise reaction mechanism cannot be determined from the present results, the electrochemical potential characteristics of the reactions are measured, the effect on interfacial chemistry of the anode & cell capacity is characterized, and insights into the cell material components involved in the reaction that causes the delayed rapid decrease in anode potential are gained. Overall, a delayed cell voltage increase and the effects of fixed resistive load overdischarge on interfacial chemistry of the anode could have implications for Li-ion cell safety as well as recovery from overdischarge events.

The next generation high-energy batteries comprise light element metals for the cathode, electrolyte, and anode (Li-S, Li-O, and solid electrolytes, respectively) and will be replacing its non-transition metals counterpart. The former materials, however, are known to be highly reactive – the materials are very sensitive to air and electron beam. Sample preparation and experimentation are challenging because it is necessary to transfer samples between milling systems and microscopes while simultaneously maintaining the material’s integrity. A critical requirement in developing these materials for tangible applications is the ability to characterize the materials in a pristine state, without environmental modifications or contamination. Furthermore, transmission electron microscopy (TEM) is a critical analytical technique for battery research and development. Following the advancement of cryo-electron microscopy (cryo-EM) in life sciences, cryo-EM has been adapted for Li battery research, which allows for the preservation of the Li battery materials’ native state during imaging at the atomic scale. This study presents robust controlled environments that protect the material during the sample preparation phase (bulk to focused ion beam [FIB] preparation) and through the multi-length scale electron microscopy characterization phase. At the micrometer scale, scanning electron microscopy (SEM) characterization will show the morphology of the solid electrolyte while the sub-angstrom scale using the TEM providing interfacial chemistry and morphology. High quality, Li ion battery TEM specimens that are free from amorphous and Ga damage will be prepared and imaged under cryo-EM conditions.

The controlled environments workflow for SEM and TEM sample preparation and microscopy characterization involved the preparation of the bulk sample using broad ion beam (BIB) Ar⁺ milling to remove surface oxides. A vacuum/inert gas transfer capsule protected the sample post-BIB milling. A glove box with a positive pressure environment was used to transfer the bulk sample from the vacuum/inert gas transfer capsule to a FIB transfer system, and thereafter, to a FIB system for morphology and elemental characterization and subsequently TEM specimen preparation. A TEM half grid was secured in the cartridge of a vacuum transfer specimen holder. This cartridge was inserted into the FIB by means of the FIB transfer system. A TEM specimen was prepared using standard lift-out methods and polishing steps (30 and 5 kV) in the FIB. The cartridge with the TEM specimen was moved into the FIB transfer system and then to the glove box. Subsequently, the cartridge was mounted on the vacuum transfer specimen holder within the glove box. The TEM specimen holder was then moved to a concentrated Ar⁺ beam milling system. Ar⁺ milling was performed by rastering the beam within a defined area of the TEM specimen at decreasing milling energies. Subsequently, the TEM specimen was loaded on a cryo TEM holder inside the glove box; a glove bag with inert gas protected the TEM specimen during insertion of the cryo holder in the TEM. The cryo holder was then cooled in the TEM.

Cryo-EM imaging and analysis using energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) before and after ion milling to quantify the removal of FIB-induced damage and the workflow’s ability to prevent specimen oxidation and contamination will be presented.

Solid polymer electrolytes (SPEs) have the potential to enhance the safety and energy density of lithium batteries. However, poor interfacial contact between the lithium metal anode and SPE leads to high interfacial resistance and low specific capacity of the battery. In this presentation, I will show a novel strategy to improve this solid–solid interface problem and maintain good interfacial contact during battery cycling by introducing an adaptive buffer layer (ABL) between the Li metal anode and SPE. The ABL consists of low molecular-weight polypropylene carbonate, poly(ethylene oxide) (PEO), and lithium salt. Rheological experiments indicate that ABL is viscoelastic and that it flows with a higher viscosity compared to PEO-only SPE. ABL also has higher ionic conductivity than PEO-only SPE. In the presence of ABL, the interface resistance of the Li/ABL/SPE/LiFePO4 battery only increased 20% after 150 cycles, whereas that of the battery without ABL increased by 117%. In addition, because ABL makes a good solid–solid interface contact between the Li metal anode and SPE, the battery with ABL delivered an initial discharge specific capacity of >110 mAh/g, which is nearly twice that of the battery without ABL, which is 60 mAh/g. Moreover, ABL is able to maintain electrode–electrolyte interfacial contact during battery cycling, which stabilizes the battery Coulombic efficiency.
This presentation is based on our recent publication:
ACS Appl. Mater. Interfaces, 2019, 11 (31), 27906-27912
https://doi.org/10.1021/acsami.9b08285

Battery cathode materials that could host both cationic and anionic redox activities have demonstrated an impressive capacity and, therefore, are the subject of intense research. The interplay among the co-existing redox centers is critical to battery performance. While tremendous efforts have been devoted to investigating the electrochemically-driven redox evolution on the surface and in the bulk of the high-energy-density cathodes at room temperature, their behavior under elevated temperature is an area that lacks in-depth investigation.

In this presentation, I will show our recent systematic investigation of the thermally-driven redox coupling effect in lithium- and manganese-rich (LMR) materials using Li_1.2Ni_0.15Co_0.1Mn_0.55O₂ as an example. Under thermal abuse conditions, we report the structural degradation from the atomic scale up to the particle level. In particular, under mildly elevated temperature (~100 C), we observed long-range charge transfer between the oxygen anions in the bulk and the transition metal cations on the surface. Our results suggest that the thermally-activated ionic mobility in the hosting LMR lattice could lead to the redistribution of lithium-ions, which facilitate the long-range redox coupling effect and maximizes the entropy of the system as it settles to a new equilibrium.

These findings highlight the dynamic nature of redox couples in LMR material at even slightly elevated temperature. Such temperature sensitivity can have profound impacts on both the battery safety and electrochemical performance

We assess the heterogeneous electrochemistry and mechanics in a composite electrode of commercial batteries using synchrotron X-ray tomography analysis and microstructure-resolved computational modeling. We visualize the morphological defects at multi-scales ranging from the macroscopic composite, particle ensembles, to individual single particles. Particle fracture and interfacial debonding are identified in a large set of tomographic data of active particles. Mechanical failure in the regime near the separator is more severe than toward the current collector. The active particles close to the separator experience deeper charge and discharge over cycles and thus are more mechanically loaded. The difference in the Li activity originates from the polarization of the electrolyte potential and the non-uniform distribution of the activation energy for the charge transfer reaction. We model the kinetics of intergranular fracture and interfacial degradation to confirm that the various Li activities are the major cause of the heterogeneous damage. The interfacial failure may reconstruct the conductive network and redistribute the electrochemical activities that render a dynamic nature of electrochemistry and mechanics evolving over time in the composite electrodes. We further quantify the influence of the mechanical damage on the metrics of battery performance. We simulate the electrochemical impedance profile to build a relationship between the interfacial debonding and the impedance of electron transport and surface charge transfer. The mechanical failure disrupts the conduction path of electrons and results in significant polarization and capacity loss in batteries.

We have devised an approach to fabricate dense textured V₂O₅ thin films, which allows us to scrutinize the root cause of capacity fade in V₂O₅ cathodes of Li-ion batteries. Specifically, we performed in-situ measurements of stress of V₂O₅ thin films during 50 electrochemical cycles. Surprisingly, electrochemical cycling appears to induce elastic and rate-independent deformation over a voltage range relevant to battery operation (4 - 2.8 V). However, the compressive stresses gradually increase with cycle number during the first few cycles, likely due to side reactions and/or residual Li left in the V₂O₅, even after delithiation (to 4 V). Further cycling leads to accumulated mechanical (e.g., fracture, delamination) and structural changes which ultimately result in severe capacity fade.

Dissolution of transition metals (TMs) from the cathode and their subsequent accumulation on the anode contributes to the capacity fading of lithium ion batteries (LIBs) regardless of the chemical nature of the cathode. In recent years this stability aspect of LIBs has received growing attention in the community, however, mechanistic understanding of the associated processes is hampered by the complexity of electrochemical interfaces under study. Here, we present the results of our first-principles molecular dynamics based investigation of TM dissolution from the high-voltage spinel-structured LiNi_0.5Mn_1.5O₄ cathode into the liquid organic electrolyte. By employing accelerated free-energy simulations with explicit treatment of the organic electrolyte, we analyze a number of factors affecting the kinetics of TM dissolution including the nature of TM (Ni, Mn) and electrolyte species, the presence of surface protons and oxygen vacancies. The obtained results also suggest a clear correlation between the adsorption strength of electrolyte species and TM dissolution barriers enabling materials screening toward systems less prone to TM dissolution.

The properties of lithium ion batteries (LIB) depend critically on the transport properties in the electrodes. In this work, we focus on the effective transport properties of the granular cathode consisting of active material particles, liquid electrolyte, and additives, i.e. a mixture of binder and carbon black. To study the impact of the granular structure through the effective transport parameters on the cell performance is our objective in this work.
The classical Newman cell model requires effective transport parameters, electronic and ionic, in the solid and liquid phases. Typically, they are obtained by empirical theoretical models such as the well-known Bruggeman relation. In our approach, we compute all needed effective transport properties by resistor network methods, as well as the structural data such as porosity and specific surface area. To this end, either tomographic real data or statistically equivalent virtual structures are imported from project partners. To study the structure property relations of the cathode, we simulate half cells with a lithium foil and a separator of which the effective transport parameters are obtained from Bruggeman relations, for simplicity.
The active material in the cathode is represented as an assembly of spherical particles of a given size and at a given packing factor. A resistor network method will be presented to model the effective transport through this phase. In this approach, each particle corresponds to a node with a certain potential and the edge to a connected neighbor defines a resistor as a function of the bottleneck geometry. This computationally efficient and simplifying approach was validated by comparison to spatially resolved finite element models. Furthermore, a resistor network method will be introduced for the effective transport property in the space surrounding the particles. In this case, the so-called Voronoi tessellation is used where the corners of the resulting cells are the nodes and resistors are assigned to the edges based on the throat geometry in the space between particles. This method was validated by comparison to finite element simulations as well.
In the next step we account for the fact that the space between active material particles is filled by an additive, consisting of a binder-carbon black mixture, on the one side and liquid electrolyte, on the other. A method is presented how to calculate the effective electronic conductivity of the additive phase and verified by comparison to experiments in literature. Depending on the volume fraction of liquid electrolyte in the remaining space between active material particles, the effective ionic conductivity and diffusivity are is determined.
In this way, all effective transport properties needed for cell modeling, are obtained from structural data of cathodes. We present and discuss values of structural and transport properties for some real electrodes. Furthermore, cell simulation shows the C-rate dependence of specific capacity. A characteristic drop at a certain C-rate is found which depends on the electrode thickness. These trends are in good agreement with experimental results by project partners. Finally, we will present a parameter study concerning electrode thickness, particle size, porosity, and specific surface area in view of the effect of these parameters through the related effective transport properties on the cell performance.

Ni-rich layered oxides LiNi_xM_1-xO₂ (x>0.6, M = Co, Mn and/or Al) are technologically important cathode materials for lithium-ion batteries due to their high energy density and good power capability. These materials are usually prepared by a co-precipitation and post-calcination method that produces micron-sized polycrystalline (PC) particles consisting of nanoscale grains. The unique microstructure and anisotropic Vegard coefficient make them vulnerable to grain-boundary fracture (“electrochemical shock”), a major cause for impedance growth and performance degradation. We have recently developed a scalable method to prepare a variety of single-crystal (SC) Ni-rich cathode materials. SC-NMC622 outperforms the commercial PC-NMC622 in cycling stability, rate performance, and thermal stability. Post-cycling structural characterizations reveal that the SC-NMC622 do not crack and its surface undergoes less phase transformation than PC-NMC622. The fracture mechanism of PC and SC layered oxide cathode materials will be discussed and compared. SC Ni-rich cathode materials are critical to fabricating mechanically reliable solid-state batteries and provide a unique platform for studying doping and surface modification methods.

Particle-level reaction heterogeneity in lithium ion batteries can lead to formation of microcracks, local over-charge/discharge, and thus loss of capacity and shortened cycle life. The origin of this phenomenon is still not well understood, with many factors in play including particle morphology, thermodynamic properties, nonuniform electronic connections as well as the applied cycling conditions. In this work, operando measurements using synchrotron-based transmission X-ray microscopy (TXM) are conducted for LiFePO4 (LFP) secondary particles upon slow charging. We show that persistent nonuniform distribution of state-of-charge (SOC) exist during the entire charge process. Unlike the core-shell structure often observed in the cycling of single-crystalline electrode particles, the delithiated phase nucleates at a small number of “hotspots” and grows outwards in a highly anisotropic, finger-like pattern. Our phase-field simulation well captures the experimental phase morphology and explains the observed phase evolution behavior. We propose the incompatibility of lattice expansion among randomly oriented primary particles as the source of the reaction heterogeneity. It induces non-uniform Li-insertion stress among primary particles, which results in spatially varied nucleation and growth energy barriers. As a result, new phase preferentially nucleates at grains where adjacent grains can expand coordinately and follows a growth pathway with the least elastic energy penalty. As many battery compounds exhibit anisotropy in lattice expansion and elasticity, our observation may represent a general phenomenon for secondary electrode particles.

Recently, sodium-ion batteries (SIB) have been considered as a promising alternative to lithium-ion batteries due to the abundance of sodium resources (Na is the Earth's 6^th most abundant element, while Li is 33^rd) and its low extraction costs. Among various SIB candidate cathode materials, the layered oxides NaT_MO₂ (T_M=3d transition metal elements and their mixture) made of earth-abundant elements can reversibly store sodium by intercalation at relatively high voltages (up to ~3.8 V vs Na/Na⁺). However, these cathode materials suffer from a poor rate capability and short cycle life. In this talk, I will present a novel bulk Na_2/3[Ni_1/3Mn_2/3]O₂ structure containing void-type inclusions, which we have developed using ultrafine three-dimensional (3D) bicontinuous nanoporous nickel (NP-Ni) scaffold as template. During the conversion of this 3D NP-Ni scaffold into Na_2/3[Ni_1/3Mn_2/3]O₂ through solid-state reaction, the void space in the NP-Ni gives rise to structural inclusions in the bulk of the Na_2/3[Ni_1/3Mn_2/3]O₂. The as-synthesized cathode materials exhibit outstanding cycling stability with ~94 % capacity retention at 5C after 1000 cycles. When cycled at ultra-fast rates of 50C, the material still shows an unprecedented rate capability with roughly 50 % capacity retention. Kinetic analysis at a scan rate of 1 mV/s reveals that ~86 % of the overall capacity originates from the capacitive contribution, suggesting that sodium storage between the layers is not diffusion-limited. Therefore, High-resolution Transmission Electron Microscopy (HRTEM) analysis was performed to understand the origin of the ultra-fast Na-ion migration in and out of the material. It was found that the Na-intercalation layer spacing increased by ~16 % due to inclusions, compared to pristine material counterparts without inclusions. These structural inclusions were further studied using Small/Wide-angle X-ray Scattering (SAXS/WAXS). Such void inclusion-induced structure can be applied to a wide range of material systems, and will shed new light on the design of high-performance cathode materials.

Vanadium pentoxide (V₂O₅) is one of the most promising cathode materials used in metal-ion batteries due to its high theoretical capacity. However, it still suffers from several limitations such as slow electrochemical kinetics. Although the introduction of Sn impurities in V₂O₅ has been proposed to alleviate the kinetic drawback of the cathode, the origin of the enhancement is still vague. To find fundamental explanations and better understand the role of Sn dopants, we carried out the density functional theory (DFT+U) calculations to identify the most thermodynamically stable defects in the presence of Sn dopants. Specifically, we computed the defect formation energies of various point defects, charge carriers, and defect complexes to determine their relative stabilities in the lattice. We find that Sn could be inserted in the lattice or substituted at a V site. Sn insertion exhibits mixed ionic/covalent interaction with the lattice and donates two electrons which are localized at the two nearby V centers. Alternatively, Sn is found to be more stable to replace at the V lattice site when an oxygen vacancy (V_O) is created at the nearest neighbor position. The Sn-V_O defect complex generates one extra electron localized at the nearby V center. These extra electrons created upon the defect formations are trapped at the V sites which in turn induce distortion of local structure and form small polarons. Small polarons can undergo thermally activated hopping from one V site to the nearby site throughout the lattice where their transport properties could affect the diffusion of Li⁺ ions. In order to study the transport properties of ions and polarons, we employed nudged elastic band (NEB) method to calculate their migration barriers. Overall, computations show that Sn doping increases number of charge carriers in the V₂O₅ cathode. Structural distortion upon doping increases stabilities of Li insertion while does not diminish migration properties of charge carriers. The obtained fundamental understanding of the defect formation in the V₂O₅ based cathode materials could be one way to promote the improvement of the battery technologies.

One-dimensional nanomaterials can offer large surface area, facile strain relaxation upon cycling and efficient electron transport pathway to achieve high electrochemical performance. Hence, nanowires have attracted increasing interest in energy related fields. We designed the single nanowire electrochemical device for in situ probing the direct relationship between electrical transport, structure, and electrochemical properties of the single nanowire electrode to understand intrinsic reason of capacity fading. The results show that during the electrochemical reaction, conductivity of the nanowire electrode decreased, which limits the cycle life of the devices. We have developed a facile and high-yield strategy for the oriented formation of CNTs from metal−organic frameworks (MOFs). The appropriate graphitic N doping and the confined metal nanoparticles in CNTs both increase the densities of states near the Fermi level and reduce the work function, hence efficiently enhancing its oxygen reduction activity. In addition, we demonstrated the critical role of structural H₂O. The results suggest that the H₂O-solvated Zn²⁺ possesses largely reduced effective charge and thus reduced electrostatic interactions with the V₂O₅ framework, effectively promoting its diffusion. We also identified the exciting electrochemical properties (including high electric conductivity, small volume change and self-preserving effect) and superior sodium storage performance of alkaline earth metal vanadates through preparing CaV₄O₉ nanowires. Furthermore, a novel assembled nanoarchitecture was also presented, which consists of V₂O₃ nanoparticles embedded in amorphous carbon nanotubes that are then coassembled within a reduced graphene oxide network. The naturally integrated advantages of each subunit exhibit highly stable and ultrafast sodium-ion storage. Our work presented here can inspire new thought in constructing novel one-dimensional structures and accelerate the development of energy storage applications.

Insight into relationship between Crystal/Interface structure and properties of capacity, stability and rate capability by methods of material-Gene and structure chemistry are important for developing advanced Li-ion batteries. (Ref. 1) Using theoretical calculations combined with experimental in-situ tests, we did extensive studies on the kinetic of Li-ion diffusion for two representative cathode materials: layered Li(Ni_xMn_yCo_z)O₂ (NMC) (x + y + z = 1) and LiFePO₄. We not only focus on the bulk kinetics, but also the kinetics across electrode/electrolyte solid-liquid interface and in the electrolytes.(Ref. 2) Using ab initio calculations combined with experiments, we clarified how the thermal stability of NMC materials can be tuned by the most unstable oxygen. In NMC, insight of Ni/Li disorder has be investigated by theory and experimental e.g. neutron powder diffraction experiments and magnetization measurements. Thus, the variation of Li/Ni exchange ratio vs. TM mole fraction in NMC with different compositions can be well understood and predicted in terms of magnetic frustration and superexchange interactions. (Ref. 3) The formation of the SEI between a graphite anode and a carbonate electrolyte are investigated through combined atomic-scale microscopy and in situ and operando techniques. The insight of “H2O in Salt” and related structure vs. electrochemical window are revealed . (Ref. 4)

Ref.
1. (a)F. Pan* et al., Identify crystal structures by a new paradigm based on graph theory for building materials big data, SCI CHINA Chem, (2019) on-line；（b）A Descriptor of ‘Material Genes’: Effective Atomic Size in Structural Unit of Ionic Crystals, SCI CHINA T&S, (2019)(on-line)；（c）Discovering unusual structures from exception using big data and machine learning techniques, Sci Bull , 64（9）(2019) 612；（f）A new MaterialGo database and its comparison with other high-throughput electronic structure databases for their predicted energy band gap，SCI CHINA T&S, (2019)(on-line)
2. (a)F. Pan* et al., “Kinetics Tuning of Li-ion Diffusion in Layered Li(Ni_xMn_yCo_z)O₂”,JACS., 2015, 137, 8364;.(b) "Janus Solid–Liquid Interface Enabling Ultrahigh Charging and Discharging Rate for Advanced Lithium-Ion Batteries", Nano Lett,2015, 15 6102 (c)“Single-particle performances and properties of LiFePO₄ nanocrystals for Li-ion batteries” Adv. Energy Mater., (Front page ) 2016, 1601894
3. F. Pan* et al., (a) “Tuning of Thermal Stability in Layered Li(Ni_xMn_yCo_z)O₂”,JACS., 2016, 138 , 13326,（b）The Role of Super-Exchange Interaction on Tuning of Ni/Li Disordering in Layered Li(Ni_xMn_yCo_z)O₂, JPCL.,2017,8,5537; (c) Insight into the origin of Lithium/Nickel ions exchange in layered Li(Ni_xMn_yCo_z)O₂ cathode materials, Nano Energy 49 2018
4. F. Pan* et al. (a）In situ quantification of interphasial chemistry in Li-ion battery, Nature Nanotech. 14(1), (2019) 50–56, (c）Understanding Thermodynamic and Kinetic Contributions in Expanding the Stability Window of Aqueous Electrolytes，Chem. 4 (12) (2018), 2872-288

The electro-chemo-mechanical (ECM) effect causes dimensional change in a solid due to change in chemical composition induced by a Faradaic electric current. The ECM effect seriously impairs the functioning of batteries or fuel cells, limiting their lifetime and degrading their performance. However, it is was recently suggested that the electro-chemo-mechanical coupling has the potential for use in actuation^[1]. To explore this idea, we propose the following scheme as a basic design for an ECM actuator: electrode1\WB1\solid-electrolyte(SE)\WB2\electrode2, where WB denotes a working-body comprising a mixed ionic-electronic conductor with a large chemical expansion coefficient.
Theoretical analysis of this actuator design indicates: 1. The speed of actuation is limited by the ionic diffusion coefficient in WBs but not in SE. 2. ECM actuation has several potential advantages: (a) ECM actuators can simultaneously deliver large strain and large stress, which is difficult to achieve with other actuation mechanisms; (b) ECM actuators maintain their state after external voltage is removed; (c) displacement and force generated by an ECM actuator are determined by the amount of charge transferred, which is more readily controlled than electric field-driven devices. 3. The only major shortcoming of ECM actuation is that its energy conversion efficiency cannot exceed a few percent.
To demonstrate the concept, we have constructed a room temperature, nanocrystalline ECM membrane actuator (2mm diameter and »2µm thick) with Gd-doped ceria as SE. We tested two alternative compositions for WB’s: (1) metal/(metal oxide) or (2) nanocrystalline ceria/metal composite. Electrical and electromechanical measurements demonstrated that the actuator response with metal/metal oxide WB’s is limited by the rate of oxygen diffusion from the solid electrolyte to the metal surface. Actuators with ceria/metal composite WB’s provide shorter response time (»20sec) and larger vertical displacement (>3.5µm). These findings suggest that the ECM effect may indeed become a valuable actuation mechanism for MEMS applications.

References
[1] J. G. Swallow, J. J. Kim, J. M. Maloney, D. Chen, J. F. Smith, S. R. Bishop, H. L. Tuller, K. J. Van Vliet, Nat. Mater. 2017, 16, 749.

In the literature, reported 3D-resolved battery models rely on oversimplifications, such as an implicit representation of the carbon-binder domains (CBD) through the use of effective parameters for porosity and tortuosity or by merging CBD with the Active Material (AM) as a single solid phase.

This work’s novelty relies on the explicit representation of CBD, leading to a new level of accuracy in terms of electrochemical modeling. This achievement is made possible thanks to an in-house algorithm, INfinite Number Of phases meshing through Voxelization (INNOV) ^[1]. INNOV can generate a volumetric mesh from data of different types due to its flexible input format. INNOV takes as input binary stack of images to reconstruct the 3D structure. Such an input can arose from tomography imaging, from slicing a 3D object or from Coarse Grained Molecular Dynamics(CGMD) simulations ^[2]. For the latter a function has been developed to convert its output (coordinates of the centers and radii of the particles) into a binary stack of images. This algorithm is designed in the scope of the ARTISTIC Project ^[3] to import a multi-phase volumetric mesh of an electrode (from a CGMD simulation) into COMSOL Multiphysics to simulate the performances of the cell.

The segmentation method of Nielson and Franke ^[4] has been translated and optimized for MATLAB language and modified to suit the COMSOL Multiphysics meshing importation process. To tackle the computational cost of a 3 phases mesh, a “precision” parameter is introduced to downsample the number of nodes, faces and elements. Time-efficient meshing is achieved thanks to the simplicity of the operations and to an optimization through a matrix formalism in MATLAB.

The core of this work is to increase the current level of precision of the modeling of batteries by separating active and inactive materials. In doing so, one must not sacrifice the integrity of the mesostructure geometry. To ensure this, INNOV provides a number of observables, which can be compared to experimental numbers (e.g. arising from tomography characterizations). Among them, two distinct values of porosity are displayed by INNOV: the porosity of the mesh and the porosity of the stack of images. Furthermore, the volume ratio of each phase in the mesh is compared to its value in the stack of images. This can render proof of the overall volume conservation of each phase; however, these are average values and cannot highlight local deformations within the mesh. Another useful insight is the surface coverage between different phases.

Once the integrity of the mesh is ensured, electrochemical simulations can be done to characterize the two structures along different hypotheses for the CBD behavior. The first approach is to consider the CBD as blocking, i.e. there is no diffusion nor intercaltion of Li inside the CBD. The second one is transparent CBD, the Li diffuse inside the CBD with the same diffusion coefficient as in the electrolyte but there is still no intercalation. Such study can highlight the importance of the CBD morphology in a 3D-resolved battery model beyond its role as an electronic conductor.

In conclusion, INNOV offers a time-efficient tool to perform meshing without requiring substantial computational resources. Simulations can later be performed to characterize these meshes with the CBD explicitly considered. It can lead to new approaches to characterize battery microstructures and the impact of each components. .

REFERENCES
[1] M. Chouchane, A. Rucci, A.A. Franco (accepted in ACS Omega, 2019)
[2] A.C. Ngandjong et al. Multiscale Simulation Platform Linking Lithium Ion Battery Electrode Fabrication Process with Performance at the Cell Level. J. Phys. Chem. Lett. (2017). 5966–5972
[3] https://www.u-picardie.fr/erc-artistic/
[4] G.M. Nielson, R. Franke Computing the separating surface for segmented data. IEEE Vis. (1997). 229–233

Ni-rich layered oxides have been attracting the research interests due to their low price and high discharge capacities comparing the commercial Co-based Li-ion battery (LIB). However, one of the critical obstacles to commercialize the Ni-rich cathode is the capacity fade during early cycles which has been widely observed in Ni-rich layered oxide systems (i.e., LiNiO₂, LiNi_1-xM_xO₂, LiNi_1-x-yM_xM_y′O₂, and Li[Ni_1–x–yCo_xMn_y]O₂: M, M′=metal). Indeed, the capacity loss is directly associated with the structural degradation of the cathode, which has been attributed to the cation mixing from the partial reduction of the Ni-ions. In other words, the production of inactive Ni is the main factor of the capacity fade, which is strongly related to oxygen loss. Convincingly, preventing the spontaneous oxygen evolution from the cathode surface could be a critical breakthrough for commercializing the Ni-rich layered oxide.
In this research, we studied the degradation mechanisms of the Ni-rich layered oxide and their protective procedures by introducing the new additives; which has been investigated using the Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂(NCM)/graphite 18650 cylindrical-type cells as well as the first principles Density Functional Theory (DFT) computations. First of all, we comprehended the structural degradations of NCM via the Electron Energy Loss Spectroscopy (EELS), Scanning Transmission Electron Microscope (STEM) High Angular Annular Dark Field (HAADF) and DFT calculations. Here, we confirmed that the reduced Ni-ions on the surface has been penetrated to the bulk by cycling along with the spontaneous oxygen evolutions from the cathode surface. Besides, we proposed the novel functional electrolyte additives to stabilize the surface Ni via ab initio computations. We found the generic trends of CO_x, SO_x, and PO_x compounds in the local atomic environments of Ni-rich cathode surface through computing the intrinsic oxidation/reduction energies, protonation energies, dehydrogenation energies, Li⁺ interactions, and Ni²⁺ interactions. Finally, we studied the additives effect on the cathode surface by close collaboration with the computations and experiments. The cathode surface interacts with the electrolyte and additive molecules which can enhance or prevent the structural degradations. Consequently, incorporating the surface protecting substances on the electrolyte would be an efficient procedure to preserve the cathode surface. Therefore, we investigated the electronic structure, adhesive energy, and surface oxygen protection potentials for the candidate additives, which are phosphorus-based, carbon-based, sulfur-based, and phenyl-based molecules. Our computations and experiments illustrated the spontaneous oxygen evolutions on the surface and sub-surface layers for the charged cathode system, which oxygen evolutions were prevented with the functional additives. In the candidate list, four molecules have been shown strong adhesive tendency on the charged cathode surface, and that molecules successfully prevent the spontaneous oxygen evolutions. Therefore, our proposed functional additives instruct the designing aspect of additives to protect the Ni-rich cathodes. Consequently, we provided an effective procedure to overcome the most significant obstacle for commercializing the Ni-rich LIBs.

The energy generators for human activities, which converts human-induced energies into electricity, is emerging as a next-generation energy harvester for various applications, such as wireless sensors and Internet of Things (IoT) consisting of small-scale elements that are difficult to be integrated with the battery. Previous studies have primarily focused on improving energy generation efficiency of devices through simply merging various energy harvesters with different mechanisms. However, optimizing energy harvesting performance that are diversified depending on the time scale of external stimuli has not been much explored, although it is one of the most important task for practical applications. In this study, the dual energy generator consisting of the tiled arrangement of PDMS and Bismuth telluride is proposed as the efficient energy harvester using thermoelectric–triboelectric mechanisms, induced by the mechanical contact of human skin, and design optimization strategies are suggested according to the input stimuli frequency (0.5–2.5 Hz) and temperature gradient, which is determined by the input variables in operating environment.
To fabricate the dual energy generator, Ag electrode for triboelectricity is deposited on a poly(ethyleneterephthalate) (PET) substrate by a sputtering method, while bismuth telluride tiles involving P-N junctions are attached for thermoelectricity. The empty spaces between tiles are filled with PDMS which acts as a separating and guiding layer for thermoelectric components and transaction of charge carries, respectively.
Depending on the frequency at which human skin contacts the device surface, complex heat transfer of convection and conduction occurs by body temperature, and it leads to various patterns of thermal saturation and temperature distribution. PDMS layer serves both as a triboelectric layer in single electrode mode and an insulation layer that maintains the thermal gradient among bismuth telluride. Interfacing thermoelectric materials with PDMS also increases the amount of generated triboelectricity due to their dielectric properties, leading to feature-size dependent triboelectric energies. The hybrid energy harvester exhibits dominantly triboelectric energy when the touch motion exhibits high frequency. In the opposite case, thermoelectric power dominates the average power due to its continuous power generation.
We establish balanced design guidelines based on this simple rationale, providing detailed analyses of the heat transport properties upon human touch and the optimization strategies for the hybrid energy harvesters. These results outperform the simple merge of hybrid energy harvesters and can be widely applicable to wireless sensors and IoT technologies where human touch occurs environmentally.

Polymeric binder has turned out to be very critical for stable operation of high capacity battery electrodes including silicon (Si) anodes, as the binder could stabilize the electrode films even during the large volume change of active materials. In this talk, I will present novel binder designs focusing on supramolecular chemistries targeting high capacity battery materials represented by silicon. Such binder designs include 1) the use of mussel-inspired catechol functional group, 2) multi-dimensional cross-linkable hydrogen bonding network, 3) self-healing polymer network, 4) host-guest interaction network, and finally 5) elastic binder network incorporating molecular machines. The series of these investigations suggest the usefulness of noncovalent polymer interactions and the future role of supramolecular chemistry in the binder development. The same design principles were also applied to Li metal electrodes, with high elasticity being proved as a unique functionality.

The development of solid-state batteries has encountered a number of problems due to the complex interfacial contact conditions between Li metal and solid electrolytes (SE), where high interface resistance can limit Li ion transport and where dendrite penetration can lead to cell failure. Recent experiments have shown that applying a stack pressure can ameliorate these problems. To understand quantitatively the relationship between pressure, Li microstructure, and the Li-SE interface resistance, we have developed a 3D, time-dependent, and multi-scale contact model for describing the Li-SE interface evolution under a stack pressure. Our simulation considers the surface roughness of the Li and the SE, Li elastoplasticity, Li creep, and the Li metal plating/stripping process. Our results show that the Li-SE projected contact area does not depend only on the stack pressure; instead, it depends primarily on the ratio between stack pressure and Li yield strength, which has been reported to depend on the scale of the Li microstructure and on the presence of Li impurities. Contact elastoplasticity is related to the ratio between the Li yield strength and surface roughness, which is revealed to play a key role in the evolution of Li dendrites and voids. During charge/discharge, there is a competition between Li plating/stripping and creep, where the latter gradually results in a more conformal Li-SE contact and a more homogeneous contact stress field. However, the surfaces do not become conformal as long as there is any charging or discharging, since newly-deposited and newly-stripped Li occurs only at contact points. Simultaneous fitting to very recent experiments from two different research groups requires an effective yield strength of the Li used in those experiments of 16 ± 2 MPa. We suggest that the preferred stack pressure is between 20 and 50 MPa, in order to maintain a relatively small interface resistance and stable stripping without void formation, while limiting material damage and fracture.

Ammonia borane (AB, NH₃BH₃) is a highly promising hydrogen storage material, but, its high dehydrogenation temperature hinders its wider use in practice. The infiltration of AB into the pores of porous materials can lower the dehydrogenation temperature by what is known as the nanoconfinement effect. Nonetheless, it is unclear as to whether this phenomenon stems from a catalytic effect or the nanosize effect. In this work, carbon nanomaterials with an uniform pore size and with inertness to AB were chosen as nanoscaffolds without catalytic sites to control the particle size of AB. It is proved experimentally that the dehydrogenation temperature from AB is inversely proportional to the reciprocal of the particle size, which means that the nanoconfinement effect can be caused soley by the nanosize effect without a catalytic effect.

Ceria and ceria-zirconia solid solutions are important catalytic materials, either serving directly as catalysts themselves or serving as supports for metal nanoparticle catalysts. On the basis of extensive studies of these materials, there is a growing consensus that the catalytic activity correlates with the concentration of reduced Ce3+ species. However, the extent of reduction at the surfaces of these oxides, where catalysis occurs, is largely unknown. To address this gap, we employ angle-resolved X-ray Absorption Near Edge Spectroscopy (XANES) to quantify, under technologically relevant conditions, the Ce3+ concentration in the surface (2-3 nm) and bulk regions of well-defined ceria-zirconia films. Under all measurement conditions, we find an extraordinary level of surface enhancement of the Ce3+ concentration in the surface regions of the films, relative to that in the bulk. Moreover, although the Ce3+ concentration generally increases in the bulk with increasing Zr concentration, the opposite is true in the surface regions, although the surface enhancement remains substantial. Such behavior is entirely unexpected. On the other hand, we measured the relevant surface reaction constant of Zr doped and undoped ceria using electrical conductivity relaxation method (ECR), the result of which shows that adding Zr slows down the reaction by more than one order of magnitude. This suggests that high Zr concentration, which is desirable for increasing thermal stability and bulk oxygen storage capacity, may be detrimental in terms of area-specific catalytic reaction rates.

Thin passive oxides (5-10 nm) such as corundum play crucial roles in several technologies including corrosion inhibition in metals, solid electrolyte interfaces in batteries, gate oxides in microelectronics etc. While these oxides are chemically and electronically inert under normal operational conditions, exposure to high anisotropic electric fields can significantly accelerate ionic transport in such oxide films. One particular example is localized corrosion of metallic Al, Cr surfaces, where passive oxides Al₂O₃, Cr₂O₃ are known to locally disintegrate in the presence of corroding agents such as halide electrolytes. According to the well-known point defect model, the diffusion flux of ionic species play important roles in determining the rates of localized corrosion, but the underlying mechanisms of ionic diffusion under electrochemical conditions are not clearly understood at the atomic level. In this work, we compute the effect of electric fields (0-10 MV/cm) on the migration barrier of Al³⁺ and O^2- ions in hexagonal Al₂O₃. The migration barriers are then integrated with a lattice kinetic Monte Carlo simulation to compute average diffusion fluxes at different temperatures. Calculations of Al³⁺ migration barriers in the absence of electric field reveal the diffusivity of Al³⁺ to be 10^-26 cm²/s at 300 K. This value is too small to contribute to localized corrosion behavior at room temperature. These zero field barriers are computed using the climbing image nudged elastic band method. Further calculations are performed by computing the ground state of the previously generated replicas (for CI-NEB) under homogeneous potential gradients.
An electric field with magnitude up to 10 MV/cm (measured in corrosive environments) is applied perpendicular to the basal plane in Al₂O₃. The energy barriers are found to be asymmetric; favouring Al³⁺ hopping in the direction of the electric field.

In recent years, energy crisis has become one of the most urgent issues that people try to deal with. Microbial fuel cell (MFC) is an environmentally friendly, sustainable technology that converts chemical energy to electricity directly. Biofilms attached on the anode oxidize organic matters during metabolism and produce electrons and protons. Since MFCs can generate electricity and achieve wastewater treatment simultaneously, this novel technology is regarded as a promising solution for increasing energy needs. Although MFCs have high potential for power generation, low power density is the major problem that limits MFCs’ application. Anode materials of MFCs are generally recognized as a key factor for electricity generation, which can directly affect biofilm formation and charge transfer efficiency. Therefore, modifications on anode materials have been widely investigated to improve MFCs’ performances.

In our work, a core-shell structured carbon nanotube (CNT)@graphene oxide nanoribbon (GONR) prepared through a simple unzipping process was proposed as the anode material for MFCs. First of all, the microstructures of CNT@GONR anodes are characterized by scanning electron microscopy, transmission electron microscope, Brunauer–Emmett–Teller method. Secondly, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy are used to analyze elemental composition. Furthermore, the electrochemical properties of the MFCs have been investigated by linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy in our study. CNT@GONR provides large specific surface area, high conductivity, and high electron transfer ability that can improve the power output of MFCs significantly. With comparison to the pure CNT anode, the devices with CNT@GONR anodes performed higher power density (more than 3 times), demonstrating its great potential for enhancing the performances of MFCs.

Fuel cells are promising candidates for clean energy conversion for terrestrial and space applications such as human space travels, which require several technological developments that support the energy-efficient production and preservation of closed systems in microgravity spaceship environments [1]. The overpotential required for the Oxygen Reduction Reaction (ORR) is the main electrochemical factor that diminish practical application of fuel cells [2]. ORR in aqueous solutions occurs mainly by two pathways: the direct four-electron reduction pathway from O2 to H2O, and the two-electron reduction pathway from O2 to hydrogen peroxide (H2O2). In fuel cell processes, the four-electron direct pathway is highly preferred. The two-electron reduction pathway is used in industry for H2O2 production [3]. Carbon nanostructures (Nanocarbons), such as Vulcan and carbon nano-onions (CNOs), have been previously used as catalyst due to high stability and surface area, high electrical conductivity, and mesoporous structure. Studies have revealed that carbon nanostructures and metal-carbon structures show catalytic activity in ORR [5,6]. Rotating disk slurry electrodeposition technique (RoDSE), as electrodeposition process without high temperatures, hazardous compounds, and nor energetic procedures, has been used to deposit metal nanoparticles on carbon to prepare a hybrid catalyst in powder form, [7]. Accordingly, in order to evaluate the ORR essential role in fuel cells in microgravity conditions and space applications, iron nanoparticles supported on Nanocarbons (FeNanocarbons) were synthesized and characterized by RoDSE. The structural properties of the FeNanocarbons were investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, induced coupled plasma, and X-ray photoelectron spectroscopy. FeNanocarbons electrochemical characterization revealed higher performance than Nanocarbons, due to Fe nanoparticle enhances the electronic conductivity and specific capacitance. An analysis of the rotating disk electrode (RDE) technique data was done to evaluate the ORR kinetics, including n-values which are related to the mechanism of oxidation, at the FeNanocarbons, using the Koutechy-Levich (K-L) equation. ORR over FeNanocarbons and Nanocarbons was evaluated in O2 saturated 0.1 M KOH, by a scan rate of 10 mV/s at different rotation rates: 800, 1200, 1600, 2000, and 2400 rpm. Initial fuel cell tests at 6V, utilizing oxygen and RO water, showed that Fe/Nanocarbons and Nanocarbons can generate 0.055 and 0.025 w/w% peroxide concentration, respectively. The system output current was 0.38 amps for Fe/Nanocarbons and 0.25 amps for Nanocarbons. These results suggested that Nanocarbons performs high selectivity toward a two-electron pathway reduction process, whereas Fe/Nanocarbons catalyzes a four-electron route. Therefore, our approach would be promising to control of four- or two-electrons route kinetics of ORR in fuel cells for space technologies, by the nanocarbon source and metal-nanocarbon configurations. References: [1] NASA Strategic Plan, 2018, at: https://www.nasa.gov/sites/default/files/atoms/files/nasa_2018_strategic_plan.pdf. [2] J. K. Nørskov, J. Rossmeisl, A. Logadottir and L. Lindqvist. J. Phys. Chem. B, 108 (46), 17886–17892, 2004. [3] Song, C,; Zhang, J. Electrocatalytic ORR in PEM fuel cell electrocatalysts and catalyst layers. Springer; 2008, 89-134. [4] F. Hennrich, C. Chan, V. Moore, M. Rolandi, and M. O’Connell, “The element carbon,” in Carbon Nanotubes Properties and Applications, M. J. O’Connell, Ed., Taylor & Francis, Boca Raton, Fla, USA, 2006. [5] Xing W, Qiao SZ, Ding RG, Li F, Lu GQ, Yan ZF..Carbon, 44(2):216–24, 2006. [6] Frédéric Haschéa, Mehtap Oezaslan, Peter Strasser, Tim-Patrick Fellinger. Journal of Energy Chemistry 25, 251-257, 2016. [7] D. Santiago, G. G. Rodriguez-Calero, H. Rivera, D. A. Tryk, M. A. Scibioh, and C. R. Cabrera, J. Electrochem. Soc., 157(12), F189 (2010).

Perovskite oxides (ABO3) are considered key players in clean energy conversion applications, including solid oxide fuel/electrolysis cells (SOF/EC) and solar-to-fuel conversion. However, most of the state-of-the-art SOFC materials, such as (La,Sr)MnO₃ (LSM) and (La,Sr)CoO₃ (LSC), suffer from degradation of surface chemistry and oxygen exchange kinetics at elevated temperatures, which limit their long-term application. This degradation is primarily because of Sr segregation and concomitant formation of SrO-like insulating phases at the surface which block the electron transfer and oxygen exchange pathways[1,2]. The formation of inactive surface oxide phase therefore leads to a severe drop in oxygen reduction reaction (ORR) kinetics.
Numerous previous studies observed that applying cathodic polarization induced significant increase in oxygen exchange kinetics at the LSM surface by up to 50-fold. In virtue of in-situ X-ray techniques, it was shown that cathodic polarization actually suppressed Sr segregation and removed the SrO-like secondary phases at the surface[3]. However, several previous studies go against or obscure this observation. For example, a number of previous studies observed that cathodic polarization rather promoted Sr segregation instead of suppressing it, which contradict the observed increase in cathode performance during the activation process[4]. Also, other possible origins of activation process that are also induced under cathodic polarization cannot be simply excluded, such as the formation of oxygen vacancies or extension of the active area by spreading of Mn(II) onto the electrolyte surface. To elucidate if dopant segregation indeed contributes significantly to the activation process, it is important to first understand how polarization affects the dopant segregation behavior of perovskite oxides.
Recent study by our group revealed that the segregation behavior of La_0.8D_0.2MnO₃ (D=dopant) can be explained by the sum of two driving forces; electrostatic (E_elec) and elastic energy (E_ela)[5]. We observed that these two driving forces change with electrical polarization oppositely to each other; E_elec decreased with polarization while E_ela increased with polarization. The convolution of these two driving forces therefore makes a valley-shaped segregation graph. In case of La_0.8Ca_0.2MnO₃ (LCM), however, the contribution from E_ela is negligible due to the negligible size misfit between Ca and La, thus showing monotonically decreasing segregation graph; the higher anodic polarization, the less segregation.
Inspired by the segregation graph of LCM, we studied if applying anodic polarization could remove the dopant oxides at the surface of LCM and if this could consequently activate the surface oxygen exchange kinetics of LCM. By using X-ray photoelectron spectroscopy (XPS), we show that insulating dopant oxide at LCM thin film can actually be removed with anodic polarization and this process is high controllable by following its segregation vs. polarization graph. In-situ electrochemical impedance spectroscopy (EIS) shows that applying anodic polarization for a short time reactivates the initially degraded surface of LCM by removing dopant oxide at the surface. This activation process is distinguished from the previous reports on the activation of LSM in that the process is induced under anodic polarization instead of cathodic one, thus confirming the bipolarity of the activation process. By investigating the kinetics of activation process in combination with X-ray absorption spectroscopy (XAS), we propose a new mechanism behind this exotic activation process under anodic polarization.

1. Wang W et al., Solid State Ionics 2006, 177(15-16):1361.
2. McIntosh S et al., Electrochemical and Solid-State Letters 2004, 7(5):A111.
3. Huber A-K et al., Journal of Catalysis 2012, 294:79.
4. la O’ GJ et al., Journal of The Electrochemical Society 2009, 156(6):B771.
5. Kim D et al., in preparation, 2019.

Propane, which is the main component of Liquified Petroleum Gas (LPG), is commonly used as a fuel for internal combustion engines (ICEs), portable stoves and residential heaters because of its high volumetric energy density and simplicity for storage and transportation in a liquid state. Thus, propane could be a superior candidate fuel for portable fuel cells and remote power generation applications such as an unmanned aerial vehicle (UAV). Direct utilization of this propane is one of the attractive projects in Solid Oxide Fuel Cells (SOFCs) as well application due to its high energy conversion efficiencies and the possibility for simplification of the systems. However, the use of propane directly has been limited due to several challenging issues such as low anode activity and carbon coke. We previously reported Ce(Mn, Fe)O₂ (CMF) and La(Sr)Fe(Mn)O₃ (LSFM), oxidation-tolerant oxide anode for SOFC with reasonably high performance and good tolerance to coking, even when using dry propane as a fuel.
In this study, to enhance catalytic performance and electron conductivity of previous oxide anodes, two ways of surface modification of these oxide anodes were investigated. First, a composite oxide of CMF-LSFM (LSFM@CMF) with a nano-composite particle structure was prepared by a wet process. Nano-sized CMF successfully deposited on LSFM backbones and it was found that surface activity of this composite oxide was much improved compared to simply mixed one. Remarkably high maximum power density of 0.76 Wcm^-2 and 1.24 Wcm^-2 was achieved at 800 ^oC for hydrogen and propane as a fuel. Secondly, we evaluated the surface modification of previous ceramic anode with various transition metal (Ni, Co, Fe, Cu, Ru, Pd), prepared by infiltration method. In particular, Co infiltrated CMF (CMF-Co) and LSFM (LSFM-Co) shows fairly good maximum power density of 0.89 Wcm^-2 and 1.02 Wcm^-2 in propane atmosphere at 800 ^oC. Therefore, it will be discussed that enhancing method for a robust and high performed ceramic electrode with nano-complex could be used as an active oxide electrode in solid oxide electrochemical cells with excellent redox and coking tolerance under high carbon contents.

It has maintained an extensive interest in developing advanced direct electrochemical reduction of CO₂ into value-added fuels and specialty chemicals using renewable energy as an input. However, its application is still constrained partly due to chemomechanical and interfacial challenges related to activity and selectivity toward the particular product of interest. Detailed ab initio molecular dynamics simulations are invaluable in complementing the experimental investigations to unravelling important aspects of the electrochemical reduction reaction mechanisms for various type of catalysts. Here, we present advances in the understanding of reactivity and selectivity in the electrocatalysis of converting CO₂ into methanol using density functional theory and thermodynamics. We will also discuss the role of protons and electrons transfer, the effect of doping and oxidation, and possible processes through different surface-bound reaction intermediates in order to steer catalyst selectivity among the vast number of possible carbon-based products.

Lead (Pb)-embedded polyacrylonitrile-based carbon nanofiber (PAN-CNF) anodes for lithium-ion batteries were prepared via the electrospinning method. PAN is a well-known polymer precursor to prepare CNF that has demonstrated high electrical conductivity and cycling stability over numerous cycles. The use of lead-based anodes with high amounts of carbon has been shown to improve the cycling performance of batteries through increased capacity retention and cycling stability. Despite this, encapsulation of Pb directly into a PAN-based electrospinning solution is not feasible due to the incompatibility of Pb solutions with PAN. In this work, we report the first demonstration of Pb encapsulated in PAN-CNF as anode materials and elucidate the mechanism which allows Pb to be indirectly embedded into PAN-CNF to yield significant enhancement to the specific capacity and cyclability of lithium-ion batteries.

Closo-borates represent a promising but yet under-explored alternative class of electrolytes for all-solid-state batteries. We recently reported ionic conductivities of 1 mS/cm and an electrochemical stability window of 3 V at room temperature for Na₄(B₁₂H₁₂)(B₁₀H₁₀).[1] The mixed-anion configuration stabilizes the ion-conducting high-temperature phase at room temperature.[2] We further demonstrated stable cycling for a 3 V class all-solid-state battery based on this electrolyte consisting of a sodium metal anode and a NaCrO₂ cathode.[3] The cathode composite can be assembled by simple cold pressing, but cycling performance is enhanced through a preliminary solvent-based impregnation step of the cathode particles by a thin electrolyte coating. This coating guarantees intimate contact between cathode particles and electrolyte resulting in reversible and stable cycling with 85% capacity retention after 250 cycles at C/5. We recently extended this approach by demonstrating that closo-borates can also be infiltrated into porous electrodes, where they crystallize into their highly conductive phase.[4]
I will further discuss routes to translate these results to lithium analogues and our efforts to establish low-cost synthesis routes for these non-toxic materials.[5] Our results demonstrate that owing to their physical properties and processability, closo-borate-based electrolytes could play a significant role in the development of a competitive all-solid-state battery technology.

[1] L. Duchêne, R.-S. Kühnel, D. Rentsch, A. Remhof, H. Hagemann, C. Battaglia, Chem. Comm. 2017, 53, 4195
[2] L. Duchêne, S. Lunghammer, T. Burankova, W.-C. Liao, J. P. Embs, C. Copéret, H. M. R. Wilkening, A. Remhof, H. Hagemann, C. Battaglia, Chem. Mater. 2019, 31, 3449
[3] L. Duchêne, R.-S. Kühnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, Energy & Environmental Science 2017, 10, 2609
[4] L. Duchêne, D. H. Kim, R. Moury, A. Remhof, H. Hagemann, Y. S. Jung, C. Battaglia, submitted
[5] S. Payandeh, P. Avramidou, A. Remhof, C. Battaglia, submitted

Advances in solid electrolytes (SEs) with superionic conductivity and stabilized electrode-electrolyte interfaces are key enablers for all solid-state batteries (SSBs) to meet the energy density and cost targets for next generation batteries for electric vehicles. Compared to their oxide counterparts, sulfide- and thiophosphate-based electrolytes offer several key advantages, including (i) exceptionally high ionic conductivities up to 10^-2S/cm (comparable to nonaqueous liquid electrolytes) as recently reported for Li₁₀GeP₂S₁₂and Li_9.54Si_1.74P_1.44S_11.7Cl_0.3(2), (ii) availability of low temperature and inexpensive synthesis routes to produce glass, glass-ceramic, and crystalline structures, and (iii) soft mechanical properties, which facilitates material processing. Among their drawbacks, sulfides have a narrow electrochemical stability window and hence limited stability against lithium metal and cathodes. They also suffer from poor chemical stability and are highly sensitive to moisture. Despite rapid progress in achieving higher bulk ionic conductivities of SEs the rate capability of SSB’s is affected by interfacial side reactions that increase the cell resistance during electrochemical cycling. Computational modeling based on density functional theory (DFT) predicts narrow electrochemical stability windows (typically 1.6-2.5 V vs. Li/Li⁺)for most thiophosphate-based compositions. Due their narrow thermodynamic stability range, most SEs rely on formation of kinetically stabilized interfacial layers with reasonable ionic conductivity and very low electronic conductivity. In addition to the thermodynamical instability, significant mechanical stresses develop in SE due to (i) volume changes of the electrode material during repeated lithiation (delithiation) and (ii) interface roughening from side reactions upon extended cycling. This “chemo-mechanical effect” results in poor interfacial contact between the SE and cathode. The talk will reveiw recent progress on synthesis and characerization of new thiophosphate based cathodes for SSB based on catenation reaction with sulfur to the ionically conductive Li₃PS₄ framework. In addition various approaches to address the high interfacial resistance beween the thiophosphate SE and cathode interfaces will be presented.
Acknowledgement
Research supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program

Attaining stable cathode-solid electrolyte interfaces is a great challenge in sulfide-based all-solid-state sodium batteries (ASSSBs). Currently, these ASSSBs experience low specific energy and poor cycling performance due to the interfacial incompatibility between cathode materials and sulfide electrolytes. A resistive layer forms at the interface when cathode is charged above the anodic stability potential of sulfide electrolyte. We have demonstrated some of the longest-cycling cathode materials and Na anodes through the control of interfacial structure and electrolyte decomposition. For instance, we demonstrated an organic cathode material, pyrene-4,5,9,10-tetraone (PTO), that induces reversible cathode-electrolyte interfacial resistance evolution during cycling as the result of the reversible conversion between the superionic conductor Na₃PS₄ and the resistive oxidation products Na₄P₂S₈/Na₂P₂S₆. Structural and mechanical analyses further showed that a low-modulus cathode material like PTO (Young’s modulus = 4.2 ± 0.2 GPa) can effectively accommodate interfacial stress and maintain intimate interfacial contact with solid electrolytes. The reversible electrolyte decomposition, revealed by time-of-flight secondary ion mass spectrometry (ToF-SIMS), the consistently intimate interfacial contact, visualized by focused ion beam-scanning electron microscopy (FIB-SEM), and the soft nature of cathode material, characterized by nanoindentation, are all first-time reports in the field of solid-state batteries, and they have collectively led to a high specific energy (587 Wh kg^-1) at the active-material level and an 89% capacity retention over 500 cycles, a record cycling stability among all-solid-state Na batteries.

Lithium metal anodes could enable the next generation of high energy density lithium ion batteries required for increased adoption of electric vehicles. The formation of dendrites on charge/discharge cycles, however, is a safety hazard that hinders their commercialization. The use of solid-ion conductors (SICs) as electrolytes may allow for stable, dendrite-free, lithium electrodeposition, provided the properties of the electrode-electrolyte interface obey certain criteria related to the mechanical and chemical properties [1, 2]. Optimal SIC candidates must have properties that are either pressure-driven dendrite-blocking, or density-driven dendrite-suppressing, but not both. This division between two regimes of stable electrodeposition stems from the opposite influences of the SIC shear modulus and those of the partial molar volume of Li⁺ relative to those of the lithium anode. In this work, we use density functional theory (DFT) simulations to investigate a SIC soft polymer composite – LiF@PIM hybrid – in the dendrite-suppressing regime. Lithium ion motion through this material was studied using the nudged elastic band (NEB) method on a LiF surface. The activation energy for lithium movement computed in this manner shows good agreement with experimental results. The molar volume of Li⁺ in the composite, which is experimentally challenging to obtain, was determined using Bader charge analysis. The effects that functional groups representative of the PIM polymer matrix have on our calculated values were also investigated. Our results confirm the electrodeposition stability of this composite, which has been demonstrated to extend cell cycle-life (>300 cycles against ~100 cycles for control cells) [3]. Such nanostructured composites may be integrated into the battery manufacturing and provide a path forward for commercialization of metal anode batteries.

[1] Monroe, C., Newman, J., J. Electrochem. Soc., 152, A396 (2005).
[2] Ahmad, Z. and Viswanathan, V., Phys. Rev. Lett. 119, 056003 (2017).
[3] Fu, C., Venturi, V., Ahmad, Z., Ells, A. W., Viswanathan, V., Helms, B. A., arXiv preprint, arXiv:1901.04910.

Alkali metal ion batteries (AMIBs), which employ alkali metal ions (AMIs) as charge carries, have been intensively investigated for storing the electricity from renewable energy due to the simple working principle and high energy efficiency. Among all available anode candidates, carbonaceous materials show the superior overall performance and low production cost, where the formed solid electrolyte interface (SEI) on the carbonaceous anode is essential to the long-term cycling life of AMIBs. Even though the presence of the anode SEI is critical, its formation and growth together with the influence on the battery performance are less understanding, in particular, when different AMIs are stored. In this work we investigated the interfacial chemistry by choosing different parameters including various AMIs, type of carbonaceous anode, electrolyte composition and concentration as well as electrochemical condition, attempting to understand the formation and functionality of SEI on the initial capacity loss, rate capability and cycling stability of AMIBs. The understanding gained potentially provides reference for the optimization of electrode/electrolyte interface and artificial design of SEI on the carbon anode for AMIBs.


References
Li, Y.Q.; Lu, Y.X.; Adelhelm, P.; Titirici, M.M.; Hu, Y.-S. Chem. Soc. Rev. 2019, in press.
Mogensen, R.; Brandell, D.; Younesi, R. ACS Energy Lett. 2016, 1, 1173-1178.
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Glass-ceramic electrolytes are promising for all-solid-state batteries, yet their enhanced ionic conductivity is not well understood. We investigate the structural difference between ceramic and glass-ceramic Li_1.3Al_0.3Ti_1.7(PO₄)₃ tuned by boron, which is found to enhance conductivities of glass-ceramic by one order of magnitude, yet reduces that of ceramics. Raman and Magic Angle Spinning Nuclear Magnetic Resonance spectra indicate boron are primarily contained in glass phase, while electron energy-loss spectroscopy and high-resolution transmission electron microscopy reveal inhomogenoeus distribution of boron, glassifying and relaxing its grain boundaries. This substantially reduces interfacial resistance at grain boundaries with little effect on bulk resistance, which is further supported by spatial and spectrscopic electrochemical strain microscopy that resolves the local dynamics. The grain bounary resistance of ceramics, on the hand, is increased by boron. We thus establish lower interfacial resistance at glasssified grain boundaries is responsible for higher conductivity in glass-ceramics, while boron can relax their grain boundaries even further.

Solid electrolyte is the key component for solid-state batteries. Among different electrolytes, ceramic sulfide electrolyte shows the highest ionic conductivity. However, narrow intrinsic electrochemical stability window of the electrolyte and interfacial reactions between electrolytes and electrodes remain a problem for the application in solid state batteries. Here we show our unique understanding and strategies to improve electrochemical window and interfacial stability of sulfide electrolyte. A core-shell structured Li_9.54Si_1.74P_1.44S_11.7Cl_0.3(LSPS-Cl) and Li₁₀GeP₂S₁₂ (LGPS) were designed, synthesized and tested. The sulfide electrolytes with core-shell structure show improved stability windows in both electrochemical test and theoretical simulation. The interfacial reaction was alleviated by applying coating materials to separate the sulfide electrolyte from the electrode materials. A high-throughput screening from computational simulation helps find functional coating materials, and the stability between coating materials and electrolyte/electrode materials were demonstrated by experiments. All-solid-state batteries with LiCoO₂ or LiNi_0.5Mn_1.5O₂ high voltage cathode and Li₄Ti₅O₁₂ or lithium metal anode were demonstrated with good cycling performance based on our sulfide electrolytes.

Reference:
Fan Wu, William Fitzhugh, Luhan Ye, Jiaxin Ning, and Xin Li. Advanced sulfide solid electrolyte by core-shell structural design. Nature Communications 9, no. 1 (2018): 4037.
William Fitzhugh*, Fan Wu*, Luhan Ye*, Haoqing Su, Xin Li. Strain stabilized ceramic sulfide electrolytes. Small (2019), in press.
William Fitzhugh, Fan Wu, Luhan Ye, Wenye Deng, Pengfei Qi, and Xin Li. A high throughput search for functionally stable interfaces in sulfide solid-state lithium ion conductors. Advanced Energy Materials (2019): 1900807.

Lithium sulfur (Li-S) cells are promising candidates for high energy density battery systems. However, employing conventional liquid electrolytes leads to continuous degradation in Li-S batteries because of soluble reaction intermediates.[1] Today, ongoing developments in the field of solid electrolytes are drawing attention to all-solid-state Li-S cells in which the so called polysulfide shuttle is physically prevented by a solid separator.[2] Although modern thiophosphate electrolytes provide promising ionic conductivities, degradation reactions at the electrode interfaces due to their limited thermodynamic stability represent a major bottleneck for application. Experimentally, stabilities of thiophosphates up to 5 V vs. Li/Li⁺ are often claimed from cyclic voltammetry (CV) using planar working electrodes. Contradictorily, redox activity of electrolytes was observed at lower potentials in solid-state batteries.[4,5]
In this presentation, we will report how employing a high surface area carbon-electrolyte composite working electrode in a CV setup helps to visualize the practical stability limit of state-of-the-art solid electrolytes.[6] As carbon is a widely used additive in electrodes for solid-state batteries, this approach allows information on the practical stability of the electrolyte of interest. Furthermore, by combining CV and X-ray photoelectron spectroscopy, an insight into the underlying chemistry, i.e. the oxidation of thiophosphate building units, is given. Hereafter, we show the decomposition products to be redox active. Due to the partial reversible cyclability of the oxidized species, the decomposed electrolyte acts as pseudo-active material adding additional cell capacity while degrading the long term cell performance due to its highly resistive nature.
In a solid-state battery, the overall performance is governed by the evolving interphase between carbon and thiophosphate. By restricting the cycling window, the capacity retention can be increased significantly.

References
[1] M. Wild, L. O’Neill, T. Zhang, R. Purkayastha, G. Minton, M. Marinescu, G.J. Offer. Energy Environ. Sci. 8 (2015) 3477-3494.
[2] J. Janek, W.G. Zeier. Nat. Energy 1 (2016).
[3] Y. Zhu, X. He, Y. Mo. ACS Appl. Mater. Interfaces 7 (2015) 23685-23693.
[4] R. Koerver, F. Walther, I. Aygün, J. Sann, C. Dietrich, W.G. Zeier, J. Janek. J. Mater. Chem. A 5 (2017) 22750-22760.
[5] F. Han, T. Gao, Y. Zhu, K.J. Gaskell, C. Wang. Adv. Mater. 27 (2015) 3473-3483.
[6] G.F. Dewald, S. Ohno, M.A. Kraft, R. Koerver, P. Till, N.M. Vargas-Barbosa, J. Janek, W.G. Zeier. Experimental Assessment of the Practical Oxidative Stability of Lithium Thiophosphate Solid Electrolytes. ChemRxiv. (2019) Preprint. https://doi.org/10.26434/chemrxiv.8014715.v1.

Maintaining the physical contact at the electrolyte/electrode interface and preventing mechanical failure are critical to the performance of all-solid-state batteries (ASSB). The ceramics based solid electrolyte and electrode interface tends to have imperfect contact, which can be worsened due to cycling and improved due to cell stack pressure. The solid electrolyte is also subject to electrochemical decomposition at the interface, causing volume change, in addition to the well-known volume change in the electrodes during lithiation-and-delithiation. These chemical strains generate mechanical stress, which leads to fracture in the highly compliant all-solid-state batteries, especially in 3D ASSBs. In this presentation, new continuum models were developed to capture the highly coupled electrochemical-mechanical phenomena. First, we will present a 1D Newman battery model for a film-type Li|LiPON|LiCoO₂ ASSB that incorporated the effect of imperfect contact area with battery performance by assuming the current and Li concentration will be localized at the contacted area. To establish the relationship between the applied pressure and the contact area, we applied Persson’s contact mechanics theory as it uses self-affined surfaces to simplify the multi-length scale contacts in ASSLBs. The model is then used to suggest how much pressures should be applied to recover the capacity drop due to contact area loss. Furthermore, we will introduce a 3D Newman battery model for the experimentally-made 3D Si|LiPON|LiCoO₂ ASSB to incorporate the chemo-mechanical strains due to interfacial decomposition of LiPON and the lithium concentration gradient while cycling. This electrochemo-mechanical model could be used for predicting whether and where the fractures would be produced in order to guide the architecture design of 3D ASSB with balanced energy density and mechanical integrity.

An optimally engineered lithium metal/solid electrolyte interface promises all-solid battery technologies that reap the capacitive benefits of Li metal anodes while mechanically resisting the Li interface evolution and thus prolonging lifetime. Additionally, such systems offer greater safety than chemistries that include flammable liquid electrolytes. However, detrimental interface evolution and short-circuit inducing Li protrusions are observed in solid state batteries despite theoretical mechanical resistance.^1,2 There is debate as to whether these protrusions nucleate at the Li anode or within the ceramic electrolyte as well as the most crucial factors that affect these protrusions. These factors include electrolyte density, pre-existing defects, anode/electrolyte interfacial contact, and imperfect electronic insulation within the electrolyte.³ Understanding the influence of these variables is greatly enhanced by directly imaging the interior of the ceramic material at multiple scales in conjunction with electrochemical experiments and complementary chemical analyses.
In this talk, operando X-ray micro-tomography addresses the contribution of electrolyte density and defects, interfacial contact, and conductivity to structural changes at the interface between Li metal and β-Li₃PS₄ (LPS) ceramic electrolyte with sub-micron resolution. Cells of Li, LPS, and a blocking contact are constructed and studied during cycling at 200 psi and 70°C. Because electrolyte density and initial defects depend on the composition and synthesis of the ceramic conductor, two syntheses of LPS with different particle sizes are compared. Including pressure and temperature as variables addresses the remaining factors of interest as pressure is a key parameter in the quality and stability of interfacial contact while temperature affects both the ionic and electronic conductivity of the ceramic.
In our previous work, image analysis of micro-tomography data has identified sites of Li microstructure growth⁴ while the present work now isolates variable-dependent trends such as pressure-dependent void formation in the Li anode. Additionally, X-ray absorption spectroscopy at the phosphorus and sulfur K-edges tracks chemical changes in the LPS at the anode/electrolyte interface between pristine and cycled cells. Progress toward combining these data with operando transmission x-ray microscopy, which offers spatial resolution in the tens of nanometers, is also presented. Linking structural and chemical changes observed during cycling to the factors that contribute to Li evolution will guide the design of robust ceramic electrolytes with improved performance and safety.
1. L. Porz, T. Swamy, B. W. Sheldon, D. Rettenwander, T. Frömling, H. L. Thaman, S. Berendts, R. Uecker, W. C. Carter, and Y.-M. Chiang, Adv. Energy Mater., 7, 1701003 (2017).
2. E. J. Cheng, A. Sharafi, and J. Sakamoto, Electrochim. Acta, 223, 85–91 (2017).
3. F. Han, A. S. Westover, J. Yue, X. Fan, F. Wang, M. Chi, D. N. Leonard, N. J. Dudney, H. Wang, and C. Wang, Nat. Energy (2019).
4. N. Seitzman, H. Guthrey, D. B. Sulas, H. A. S. Platt, M. Al-Jassim, and S. Pylypenko, J. Electrochem. Soc., 165, 3732–3737 (2018).

X-ray spectroscopic techniques with operando capabilities offer the unique characterization in energy materials and catalysis in regards to the functionality, complexity of material architecture, chemical interactions. A particularly powerful soft x-ray technique is the resonant inelastic x-ray scattering (RIXS), which provides access to elementary excitations, such as d-d (f-f) excitations, vibrational excitations, and charge transfer effects that are critical for energy-related materials and chemical functions. Current in situ x-ray spectroscopy techniques (XAS and RIXS) have provided element-specific access to local chemical states in liquids and at solid/liquid interfaces during electrochemical reactions. The presentation will give a brief introduction on the XAS and RIXS and overview a number of the experimental studies that successfully revealed the electrochemical reactions at solid/liquid interfaces in real time, e.g. electrochemical interface of photocatalysis and batteries. The experimental results demonstrate that the operando soft x-ray characterization provides the unique information for understanding the real reaction mechanism. Also, an extension of this method toward simultaneous spatial- (100 nm) and time-dependent (ns and longer) RIXS probing of electronic and chemical dynamics is envisioned.

Experimental studies aimed to understand the bottlenecks that control the transport and microstructural evolution in ionic ceramics indicate that grain boundary properties impact (positively or negatively) its macroscopic behavior. Scientific arguments to explain the ambiguous behavior in interfaces span a wide range of explanations that include the description of phenomena such as interfacial structural disorder, electrostatic phenomena, segregation of impurities and dopants, and even the appearance of glassy pockets. In this presentation, a thermodynamically consistent variational theory is presented to naturally include the effects of non-diluted segregation, dipolar and self-induced electric field effects, as well as long range, chemically induced (chemomechanical) elastic energy density contributions to the total free energy of the system. The developed model provides a rational basis to understand the stability of charge point defect distributions away from the interface and its effect on the properties and microstructural evolution of ionic polycrystalline ceramics. The theory is demonstrated on chemistries such as GDC, YSZ, and STO and validated against experimentally measured properties.

Solid-liquid interfaces play key roles in the degradation mechanisms of rechargeable batteries, yet they are notoriously difficult to study because they are buried and require in situ probes. Multimodal X-ray characterization spanning many relevant length scales during typical battery operation is vital in understand and overcoming the failure mechanisms of these complex and heterogenous interfaces. We will discuss our multimodal in situ approach combining information from high resolution X-ray microscopy and spectra-microscopy, micro- and nano-tomography, high resolution X-ray diffraction, and X-ray absorption spectroscopy to probe and ultimately understand the dynamics across buried interfaces in energy storage systems. Specifically, we will present our recent results in designing anode architectures to stabilize solid electrolyte interphase. We will also present recent work studying the interfacial and chemomechanical properties in multivalent ion systems.

Solid-state lithium batteries (LBs) are promising next-generation energy storage devices due to high energy density, and long lifetime. However, in solid-state LBs, the high interface resistance (charge-transfer resistance R_i) between electrolyte and positive electrode hinders fast charging and discharging. Although the origin of the interface resistance is still controversial, considerable effects of the structural disorder and the space-charge layer near the interface have been pointed out.[1,2] Notably such structural disorder and space-charge layer are considered to be static factors, therefore the time-dependence of the interface resistance has never been discussed. In this study, we report time-dependent interface resistance between the solid electrolyte Li₃PO₄ (LPO) and 5 V-class positive electrode LiCo_0.5Mn_1.5O₄ (LCMO). To quantitatively evaluate R_i of the electrolyte and electrode interface, model electrodes using (001)-oriented LCMO epitaxial thin films were fabricated, in which the electrode area, crystal structures and orientations are well-defined.
The (001)-oriented LCMO epitaxial thin film on a LaNiO₃ epitaxial film was prepared on a Nb:SrTiO₃(001) substrate by using pulsed laser deposition. Using this model electrode, we fabricated solid-state thin film LBs consisting of Li (negative electrode) on an LPO (solid electrolyte)/LCMO (positive electrode)/LaNiO₃ (current collector) layered structure. All the processes including thin-film fabrication and electrochemical characterization were performed under high vacuum (In-vacuo process).[3]
In cyclic voltammetry, observed redox peaks at around 4.0 V and 5.3 V correspond to the Mn^3+/4+ and Co^3+/4+ redox reactions of spinel-structure LCMO. The interface resistance of the fabricated battery was quantitatively evaluated by electrochemical impedance spectroscopy, and the interface resistance at 4.0 V (13 Ωcm²) was comparable to that of the previously reported solid electrolyte electrode interface.[4] However, the interface resistance at 5.3 V was three orders of magnitude larger than that at 4.0 V, indicating that the interface resistance was rapidly increased with increase in voltage. Although the interface resistance showed hysteresis behavior during charge and discharge process, the interface resistance at 4.0 V was almost recovered from 7.4 × 10² Ωcm² to its original value (12 Ωcm²) about 9.5 hours after discharge process (from 5.5 V to 3.5 V). Thus, the reversibility and time-dependence of the interfacial resistance at 4.0 V was observed after exposure to high voltage above 5 V. These results suggested that the origin of the interfacial resistance at high potential (> 5 V vs Li⁺/Li) was the formation of a Li depletion layer at the Li₃PO₄/LCMO interface and/or a reversible structural change at the interface.

While solid|iquid interfaces are the origin for degradation and transformation in liquid batteries, solid|solid interfaces govern transformations pathways in solid state batteries. The nature of anode|electrolyte and cathode|electrolyte effective interfacial properties (elastic, electrochemical, and morphological) govern transport, durability, and lifetime. Furthermore, the underlying electro-chemo-mechanical coupling that occurs during operation affect stability. During discharge, lithium metal is oxidized to Li⁺ which transports through the electrolyte to the cathode. A vacancy forms at the anode surface and when the discharge rate exceeds the diffusion rate for lithium to fill this vacancy, interface morphology instabilities can occur and pores can form at the surface. Pores can lead ultimately to delamination of the anode, formation of dead lithium, and contribute to poor rate performance. At the cathode, there is a volume expansion upon discharge (i.e. intercalation). The active material in a lithium ion battery electrode undergoes volume changes during electrochemical cycling (i.e. charge and discharge) that can lead to local stress formation. This stress remains localized at the active material because the plastic binder and liquid electrolyte do not transfer stress. Herein, we demonstrate a techique that can probe both interfaces at the same time in-situ. By combining real and reciprocal space imaging and characterization techniques we can track strain at both interfaces and start to describe degadation pathways.

Oxygen reduction reaction (ORR) is a topic of significant interest for the design of efficient and affordable catalysts for fuel cells. However, the sluggish kinetics of ORR has hindered the large-scale application of fuel cells. Platinum (Pt) is regarded as the best metal catalyst for ORR and Pt-based cathodic devices have been developed and researched for years to achieve high efficiency, long durability and stability. In recent years, Pt-free cathode materials such as palladium (Pd) have also attracted much attention and it is worth examining other metals that can promote ORR, In this contribution, we have investigated the mechanism of ORR on Hg doped Au(111) by first principles calculations and experimental work. We have considered the ORR reactions in both acid and alkaline solutions and calculated the adsorption energies of key intermediates including atomic O, O₂, OOH, OH. The possible reaction mechanisms are investigated by plotting the minimum energy pathway and computing the energy barriers for each elementary step. Both the associative mechanism and dissociative mechanism are considered. The associative mechanism includes formation of OOH; and the dissociative mechanism includes the direct dissociation of O₂ into two atomic O. Along the pathway, the reactant and the product differ in acidic media and alkaline solution.
In addition to the bare surface, a H₂O-covered Hg-Au(111) surface is adopted to accurately simulate the mechanism of ORR. With the consideration of water interaction, a new mechanism called H₂O-medaited mechanism has been recently proposed by Liu and co-workers.^1-2 Previous studies of ORR on Pt(111) in alkaline solution has proposed that the H₂O-mediated mechanism is more competitive than the dissociative and associative mechanisms. Here, based on previous experimental observation and DFT studies, one layer of ice-like bilayer structure of water layer is placed on top of Hg-Au(111) surface. This water layer is constructed by a honeycomb (√3×√3 ) R30 pattern with 2/3ML of water coverage, where half of the water molecules are ^*H₂O and the other half have an O-H bond pointing down (H-down). The H₂O-mediated mechanism is investigated by plotting the reaction pathway and calculating the energy barriers in each elementary step. Finally, the potential energy diagrams of the ORR via different mechanisms are plotted and the ORR activities of pure Au and Hg-Au are compared. This work sheds light on the ORR mechanism on gold and Hg modified gold.

References
1.Liu, S.; White, M. G.; Liu, P., Mechanism of oxygen reduction reaction on Pt (111) in alkaline solution: Importance of chemisorbed water on surface. J. Phys. Chem. C 2016, 120 (28), 15288-15298.
2.Liu, S.; White, M. G.; Liu, P., Oxygen Reduction Reaction on Ag (111) in Alkaline Solution: A Combined Density Functional Theory and Kinetic Monte Carlo Study. ChemCatChem 2018, 10 (3), 540-549.

The growing global energy demand continues to derive the search for sustainable energy storage and conversion techniques such as metal-air batteries and fuel cells. In these next-generation energy storage and conversion systems, the electrocatalytic oxygen redox reactions are significantly sluggish in kinetics, limiting their performance. Hence, it is of great significance to develop electrocatalysts that can effectively promote the oxygen redox reactions. Here, we report a simple solution approach for the synthesis of thin WO₃ nanosheets bearing Pd nanoparticles as an electrocatalyst to promote the oxygen reduction reaction (ORR) effectively. Exfoliated WS₂ nanosheets were oxidized to WO₃ in H₂O along with deposition of Pd nanoparticles on the nanosheet surface. The coverage of Pd nanoparticles on the WO₃ surface was adjusted and found to influence the ORR kinetics. As-prepared WO₃/Pd hybrids exhibited excellent catalytic activity and durability in ORR at basic conditions.

Understanding the surface change of electrode materials in contact with an electrolyte, at the atomic-level, is a fundamental step toward achieving better electrochemical performance of rechargeable cells. Various type of water-based aqueous solutions have been proposed as alternative electrolytes to the currently used flammable organic solvents in Li-ion batteries. However, most research on aqueous rechargeable Li-ion cells has mainly focused on the synthetic processing of materials and resulting electrochemical properties rather than in-depth atomic-scale observation at the electrode surface where the initial charge transfer and the (de)intercalation reaction take place. By using LiCoO₂ single crystals with a (001) surface, in this article, we demonstrate the severe Co dissolution from LiCoO₂ surface into an aqueous solution under an O₂-flow environment without any electrochemical cycling. In addition, it is directly identified via atomic-scale scanning transmission electron microscopy that Co can occupy the tetrahedral interstices. Ab initio density functional theory calculations also reveal that this tetrahedral-site occupation is stabilized when cation vacancies are simultaneously present in both Li and Co sites. In addition to demonstrating the importance of controlling of oxygen content in an electrolyte to suppress Co dissolution, the findings in this study emphasize the significant role of vacancies in atomic structure variation and its local stability in Li-intercalation oxides.

Achieving widespread adoption of EVs requires higher energy density and safer, cheaper batteries. The development of solid-state electrolytes to enable Li metal solid-state batteries is one approach to achieve > 350 Wh/kg and > 1200 Wh/L. However, the underpinning mechanisms that control Li metal initiation and propagation in ceramic electrolytes remain unsolved. Most of the research has focused on solid Li-solid LLZO interfaces where researchers have demonstrated > 1 mA/cm² before Li penetration occurs. However, based on our recent study we have observed that the critical current density (CCD) of a molten Li-LLZTO-Li cell is significantly higher (hundreds of mA/cm²). Current densities of this magnitude may have applications for grid storage, but more importantly it elucidates the point that the CCD can increase over a hundred-fold simply by operating at higher temperatures. While the bulk properties of LLZO, such as electronic and ionic conductivity, increase with temperature along with an increasing CCD, there is still a large step increase in CCD across the melting point of Lithium at 180 C. Thus, it is possible to dramatically increase the CCD of a solid electrolyte without changing its intrinsic properties simply by making the interface liquid-solid rather than solid-solid. We coupled our CCD data with Operando cell cycling videos showing lithium dendrite propagation observable macroscopically. We believe the results of this study can help elucidate the vexing phenomenon that allows for a molten metal to penetrate a hard and dense ceramic electrolyte. These findings can establish deeper mechanistic insight into the factors that control the maximum Li plating density in Li metal batteries using ceramic electrolytes.

Mg-ion secondary batteries have been attracting attention as a next-generation rechargeable battery. Since Mg is a divalent ion and is abundant natural resource, Mg-ion secondary batteries can be expected to realize high capacity and low cost. Research on poly(ethylene oxide) (PEO) as a solid polymer electrolyte has been energetically conducted, but it is necessary to further improve their ionic conductivity. Therefore, we focus on zwitterions. Zwitterions are substances that have cation and anion fixed in the parent molecule. Zwitterions have a high polarity. Because of its high polarity, it can be expected to enhance Mg-salt dissociation and the ionic conductivity. In this study, diblock copolymers (PEGMAm-b-SPBn) (m and n denote the unit numbers) were synthesized by RAFT polymerization of an oligoether monomer (PEGMA) and a zwitterionic monomer (SPB). The introduction of SPB block is expected to improve the electrochemical properties of PEGMA-based electrolytes. A given amount of Mg-salt (magnesium (II) bis(trifluoromethylsulfonyl)amide (Mg(TFSA)₂) was added to the copolymers to prepare composites. Thermal and electrochemical measurements were performed. PEGMA₄₄-b-SPB₇₅/Mg(TFSA)₂ exhibited an ionic conductivity value of 2.3 × 10^-5 S cm^-1 at room temperature. On the other hand, the composites of the homopolymer PEGMA₂₂ and Mg(TFSA)₂ exhibited an ion conductivity value of 6.4 × 10⁻⁸ S cm⁻¹ at room temperature. The copolymerization of zwitterion improved the ionic conductivity. In phase AFM measurements, PEGMA₄₄-b-SPB₇₅, which showed the highest ionic conductivity, showed gyroid structure. It is suggested that there is a correlation between the microphase separation structure and the ion conductivity. When the unit ratio of PEGMA : SPB was 1: 1.7, a higher ionic conductivity was obtained as compared with those of other unit ratios. Raman spectroscopy was performed, and the results indicated that the dissociation of the Mg-salt was enhanced in PEGMA-b-SPB/Mg(TFSA)₂ compared with PEGMA/Mg(TFSA)₂.

Proton exchange membrane fuel cell (PEMFC) has been attracting great attention as an alternative energy system to the energy conversion devices based on fossil fuels due to its high efficiency, high energy density, low operating temperature, and fast start-up and shut down. However, there is an obstacle in commercializing PEMFCcialization due tobecause it uses the high price of platinum (Pt) as a catalyst for boosting electrochemical reactions such as hydrogen oxidation and oxygen reduction. Therefore, huge studies have been conducted to develop cheaper catalysts such as non-precious metal catalyst and alloy catalyst. palladium (Pd) is one of them.
In this study, we investigated the Pd nanoparticles encapsulated by carbon shell (Pd@CS) and its electrochemical properties especially durability which is needed to be improved for Pd to be applied in PEMFC instead of Pt. Pd catalyst having a nitrogen doped carbon shell was synthesized by delicately controlling the synthesis order in this study. It is well known that Pd procures are reduced without a reducing agent when it is mixed with aniline monomers. Pd forms Pd nanoparticles while aniline monomers turn into polyaniline (PANI) through oxidative polymerization. The PANI becomes a nitrogen doped carbon shell when it is carbonized with Pd nanoparticles under specific condition. Actually, we optimized the specific condition.
The structure of Pd@CS was characterized through XRD, HR-TEM, and TGA and it was revealed that Pd nanoparticle has well defined carbon shell. To demonstrate the electrochemical properties of Pd@CS, typical half cell and unit cell tests were conducted. Cyclic voltammetry and linear sweep voltammetry were studied for half cell, and current-potential and current-power curves were compared with those of Pt/C in the unit cell test. For durability, we have adopted the test protocol suggested by Department of Energy (DOE) in 2016. From the durability test, it was figured out that the carbon shell supplements the weakness of Pd.

Triboelectric nanogenerators (TENG) are intriguing energy harvesting devices that convert mechanical energy into electricity and could power small portable devices, sensor networks or charge batteries [1;2]. Although it is known that (TENG) based on polarized ferroelectric polymer films show better performance, the origin of this enhancement is not fully clear. To date, it has been accepted that enhancement is observed due to shift of the effective work function of ferroelectric polymer insulator which in turn enhance electron transfer between TENG electrodes. The present study based on measurements in TENG and in piezo-regime reveals that the enhancement is observed due to induction driven by piezoelectric charges. Furthermore, a novel piezoelectric-electrostatic generator has been constructed from inversely polarized polyvinylidene films, which exhibit higher performance than TENG for mechanical energy conversion to electricity uncovering a promising avenue for further development of mechanical energy harvesting nanogenerators.

[1] Z.L. Wang “Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors”, ACS Nano 7 (2013) 9533-9557.
[2] Z.L. Wang, J. Chen, L. Lin “Progress in triboelectric nanogenertors as new energy technology and self-powered sensors”, Energy & Environmental Sci, 8 (2015) 2250-2282.

Optimal operation and design of batteries, particularly lithium-ion batteries (LIBs), in high-power applications such as electric vehicles, requires knowledge of transport properties (e.g., ion transport, thermal transport, etc.) in electrolytes during battery operation. As the electrolytes used in commercial LIBs exhibit large concentration polarization due to comparatively low lithium-ion transference number and salt diffusivity, the polarization is largely related to the transport properties. It is known that internal resistance of the LIBs temporarily rises by repeating high-rate charge/discharge cycles and that it decreases by stopping the battery operation and rest. Because such a reversible resistance rise occurs only during the battery operation, this phenomenon is speculated to be related to the ion concentration distributions in the electrolyte. However, there are few techniques that can be used to quantitatively characterize ion transport behavior in the electrolytes during the battery operation.
Phase-contrast X-ray imaging is a technique that can be used to visualize density differences by detecting the X-ray phase shift caused by an X-ray passing through a sample. The sensitivity of the phase-contrast X-ray imaging is about 1000 times higher than that of conventional X-ray absorption imaging in a hard X-ray region for light elements. Since the electrolytes of batteries are also composed of light elements, the phase-contrast X-ray imaging is expected to be effective in visualizing small density differences in an electrolyte during battery operation. We demonstrated that in situ phase-contrast X-ray imaging technique can quantitatively visualize the salt concentration distributions that arise in electrolytes during battery operation [1].
Another crucial concern about batteries used in high-power applications is the thermal management of batteries. It is known that temperature of the batteries temporarily rises by repeating high-rate charge/discharge cycles and such a temperature distribution causes inhomogeneous reaction distribution of batteries. Therefore, a fundamental understanding of thermal transport properties in battery electrolytes during battery operation is also important. Although there are several conventional temperature measurement methods, such as thermistors, resistive temperature detectors, thermocouples, infrared thermography, etc., these methods detect the temperature directory through physical contact or using infrared and visible light from the object, and therefore, the detectable depth is limited to a few mm from its surface. So, its inner temperature distribution cannot be measured in principle. We have successfully demonstrated that nondestructive observations of inner temperature and thermal flow in heated water by using phase-contrast X-ray imaging [2].
In this presentation, we will show the quantitative evaluation of ion concentration distribution and thermal distribution (inner temperature gradient) in the electrolyte simultaneously during battery operation with high temporal (a few seconds) and spatial (a few microns) resolutions. We envisage that the phase-contrast X-ray imaging will become a versatile tool for evaluating electrolytes, both aqueous and non-aqueous, of many electrochemical systems, which will further our understanding of the dynamic behavior of electrolytes in actual applications.

References: [1] J. Am. Chem. Soc., 140, 1608 (2018). [2] Sci. Rep., 8, 12674 (2018).

Pathway for ions is normally in an aqueous phase. Regardless, the introduction of any aqueous phase in energy conversion devices causes many problems during fabrication, operation, maintenance, and so on. Thus, much efforts have been devoted to develop quasi-solid electrolytes such as gel polymer, ion exchangeable polymers, impregnation of ions in porous matrix and so on. Among the candidates, ion exchangeable polymers are quite often chosen as ion conducting media for energy conversion devices. The technique to introduce ion exchangeable polymers within electrodes for oxidation and reduction reactions is to solidify catalyst inks consisting of electrocatalyst, dispersion of ion exchangeable polymers, controlling solvents and additives by evaporation of all solvents in catalyst inks. Ion exchangeable polymers could be dispersed in various solvents. It causes different shapes of ion exchangeable polymers in solvents, for instance, cylindrical rods, a less-defined large particles, coils and so on. Such different types of ion exchangeable polymers form distinguished structure catalyst layers. In this study, the effect of solvents dispersing ion exchangeable polymers on the performance and durability of catalyst layers was investigated. Electrochemical characterization such as I-V polarization, cyclic voltammetry, impedance and so on and microscopic characterization such as SEM and TEM were carried out to evaluate the performance and durability of catalyst layers.

Acknowledgement
This work was supported by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20173010032100).

The all-solid-state Li-ion battery is attracting attention because it enables the use of a high capacity Li metal anode while easing safety concerns by employing a stable solid electrolyte. It is expected that the use of a solid electrolyte with a high shear modulus would prevent the growth and propagation of destructive lithium dendrites. However, there are several studies showing that dendrites can still penetrate solid electrolytes, causing the cell to short. The exact mechanism of the dendrite penetrations is still under debate. Additionally, a large compressive stress is generally applied to the cell to maintain good contact between the electrode and the solid electrolyte. This can affect the lithium deposition process. Therefore, understanding the relationship between the mechanical stress and lithium dendrite growth is a key factor in decoding the dendrite penetration process.
We developed a new in-situ transmission electron microscopy (TEM) technique to directly observe the lithium dendrite growth process and obtain the force on a Li dendrite to gain insight into the early stages of this destructive phenomenon. The in-situ TEM observation showed a nano-sized lithium pillar growing vertically causing increasing stress. It continued its vertical growth until the force reached a critical value of 13.5 nN, resulting in a maximum stress of 15.8 MPa. The pillar then changed its growth direction and started to expand horizontally. The measured critical stress is much higher than the yield stress of bulk lithium, which indicates that lithium can locally introduce high stresses on solid electrolytes without yielding. The information obtained in this work is useful for constructing a constitutive model of a lithium plating process in an all-solid-state system and understanding the mechanical stability of solid electrolytes particularly at the early stage of lithium deposition.

Researches about the improvement of durability of fuel cell electrocatalysts have addressed for proliferate the fuel cell vehicle [1]. Electrochemical active surface area (ECSA) is one of characteristic parameters to measure durability of platinum based catalysts and is strongly correlated with the size of nanoparticles. Change of nanoparticle size during the potential cycle test is commonly evaluated by TEM observations [2]. The size and distribution of nanoparticles estimated by TEM, however, are originated from limited area of catalyst. To acquire the size and size distribution from whole catalyst, powder X-ray diffraction (XRD) is potential technique but only the crystal size is estimated from Scherrer equation in most cases.
In the present study, we prepared the size-controlled platinum nanoparticles by batch and continuous flow synthesis for catalyst coated glassy carbon GC electrode and MEA fabrication, respectively. After accelerated durability test, the crystal size and size distribution of nanoparticles were evaluated by whole powder peak fitting (WPPF) of powder XRD spectra.
As a model electrocatalyst, size controlled platinum nanoparticles were prepared by batch and continuous flow synthesis. For batch synthesis, platinum nanoparticles were prepared using a previously reported method [3]. For continuous flow synthesis, platinum precursor (H₂PtCl₆) was dissolved in mixed solvent of water and ethylene glycol. Sodium hydroxide was added as additive to the solution. The precursor solution flow into PFA tube placed in an oven at 180 °C. The flow rate was controlled by micro-pump at 3ml min^-1. The synthesized nanoparticle solution was mixed with carbon black (Vulcan XC 72R) and stirred overnight. The platinum loading of catalyst was 30~35 wt% determined by ICP. Preparation of the catalyst coated GC electrodes and MEAs followed procedures reported in the literature [3, 4]. Electrochemical analyses were tested under start-stop potential cycling by the Fuel Cell Commercialization Conference of Japan’s (FCCJ) [4].
The average diameter of platinum nanoparticles (arithmetic mean) estimated by TEM observation was 2.3±0.4 nm. The nanoparticles dispersed well on the carbon support without aggregation irrespective of synthesis method. The distribution of arithmetic diameter was converted to that of volume mean diameter (2.4±0.4 nm) for comparison with XRD results. The WPPF of powder XRD analysis shows similar crystal size and distribution (2.3±0.6 nm) to those estimated by TEM. The result indicated that the WPPF analysis is useful for evaluation of size of nanoparticles from whole catalyst. After 2000 potential cycle test of GC electrode at 60 °C, the size of nanoparticle increased to 4.6±1.1 nm (TEM) and 5.1±1.1 nm (XRD). Evaluation of platinum nanoparticle size in degraded catalysts by TEM has some difficulty since the catalysts contains irregular shape nanoparticles and the contour of nanoparticle is unclear. This difficulty may cause underestimation of average size of nanoparticles. WPPF of powder XRD spectra is more or less structure independent analysis. We believe that WPPF of powder XRD provide more practical crystal size for degraded electrocatalyst.

[1] J. St-Pierre, J. Electrochem. Soc., 165, Y7 (2018).
[2] L. Zhang, W. Shi, B. Zhang, J. Energy Chemstry, 26, 1117 (2017).
[3] P. Joshi, T. Okada, K. Miyabayashi, M. Miyake, Anal. Chem., 90, 6116 (2018).
[4] A. Ohma, K. Shinohara, A. Iiyama, T. Yoshida, A. Daimaru, ECS Trans. 41, 775–784 (2011)

This work has been financially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Li-ion batteries which we use every day suffer from inherent safety issues due to low ignition point of liquid electrolytes. This problem can be solved by using solid electrolytes since they are non-flammable. LLZO (Li7La3Zr2O12) has been considered as a major candidate among the solid electrolytes because of its high conductivity and wide electrochemical window. Therefore, we can potentially develop all solid batteries with high stability and energy density using LLZO.
However, interfacial resistance of solid-state batteries remains too high for commercialization. The major problem comes from the sintering process which is necessary to gain good contact between electrodes and solid electrolyte. During the sintering, detrimental secondary phases form at the interface. These ruins the cell performance by blocking Li transport. There have been many works on the anode|electrolyte interface to improve the stability. However, works on the cathode side are relatively scarce. Considering the importance of cahode|electrolyte interface that has been recently demonstrated by our group (Vardar et al. 2018), we believe that unraveling the degradation mechanism at the cathode|electrolyte interface can be a major milestone in the development of successful solid Li-batteries.
We chose NMC622 (LiNi0.6Mn0.2Co0.2O2) as the cathode and Al-doped LLZO (Al0.24-Li6.28La3Zr2O12) as the solid electrolyte. We deposited thin film cathode (<100nm) on top of pre-prepared solid electrolyte pellets. Afterward, we annealed the samples in the air with different temperature conditions to simulate the sintering process. In the samples with conventional geometry, it is difficult to characterize the cathode|electrolyte interface since they are buried deep inside the samples. In contrast, since we used thin film cathodes, we could characterize interfacial regions by using surface sensitive techniques such as XAS. By combining findings from those techniques with conventional characterization techniques such as XRD and EIS, we could clarify the degradation mechanism at the interface.
We could see major decomposition products at the interface of the sample which was annealed at 700°C. With XRD, we could see the formation of La2Zr2O7, Li2CO3 and a perovskite phase with La on A-site. The exact identification of the perovskite phase could be done by XANES analysis. We could detect drastic shape change of Co L-edge indicating the formation of Co4+(High Spin) (Merz et al. 2010) Based on this finding, we suggest that La(Ni,Co)O3 has formed at the interface. Due to the electronegativity difference between Ni and Co, Co4+(High Spin) could form in La(Ni,Co)O3. (Pérez et al. 1998) Effect of the secondary phase at the interface on the electrochemical property of the cell could be clearly seen in EIS analysis. We could identify additional arc which does not exist in the sample without further annealing. We attribute this feature to the interfacial resistance due to the detrimental secondary phase.
In contrast, we could not see any indication for interfacial degradation in the sample annealed at 300°C. XRD image did not show any secondary phases. We could not see drastic shape change of Co L-edge either. Considering these findings, we suggest that the interface remains stable when the sample is annealed at 300°C. This claim could be further proven by EIS data which did not show any additional arc compared to the as-deposited sample.
In conclusion, we claim that temperature for typical sintering condition(700°C) is too high for solid Li-batteries. This has been proven by multiple characterization techniques clarifying the formation of secondary phases and their detrimental effect on cell performance. As an alternative, we suggest a low-temperature annealing process(300°C) in order to inhibit degradation at the interface.
We acknowledge U.S. Army Research Office for funding the research through the Institute for Soldier Nanotechnologies. (Cooperative Agreement Number W911NF-18-2-0048)

Polymer electrolyte membrane fuel cells (PEMFCs) have gained attention as future power devices due to their high energy density and no pollutants such as carbonaceous materials during operation. However, the commercialization of PEMFCs is interrupted by several issues since their electrocatalysts should satisfy not only activity but also durability. Commonly, carbon-supported Pt nanoparticles are utilized as catalysts for PEMFCs owing to their excellent oxygen reduction reaction (ORR) activity at the cathode. However, the expensive cost of Pt according to low reserves impedes the commercialization of PEMFCs. In order to solve this practical problem, a great deal of effort has focused on the development of cost-effective and highly active electrocatalysts, as demonstrated by the widely investigated alloys of Pt with relatively cheap transition metals such as Ni, Co, and Fe. The above alloying optimizes d-ban structure of Pt, which binding energy between Pt and oxygen species is more favorable than that of the pure Pt as well as decreases the cost of the catalyst thanks to the usage of inexpensive materials. However, it easily is dissolved under electrochemically stressful condition during PEMFC operation even though the alloyed electrocatalysts exhibit high enhanced ORR activity, which requires the development of durable bimetallic catalysts. A number of researches to improve the electrochemical durability have been introduced, for instance, core-shell structures, intermetallic structure, dealloyed surface structure, and additional surface functionalization with organic or inorganic materials. In spite of these efforts, the development of active, durable, and cost-effective ORR electrocatalysts has not been achieved yet. Herein, we propose several types of Pt skin structure PtCo nanocatalysts toward through various post-treatments such as acid treatment, thermal treatment, and additional Pt reduction. The physical and electrochemical properties were investigated and compared to commercial Pt/C. The Pt skin structure nanocatalysts showed enhanced catalytic activity approximately 2-fold compared with commercial Pt/C because of the tuning of d-band structure of Pt by alloying with Co. Furthermore, the Pt skin structure nanocatalysts exhibited the better ORR activity than initial performance of commercial Pt/C despite low loading amount of Pt even after accelerated degradation test (ADT) under harsh condition. Subsequently, the reason for improvement in long-term durability were analyzed via physical characterizations. Based on these results, the supplementary Pt-skin structure formation on the surface of PtCo nanocatalysts diminishes the amount of defect sites of Pt and prevents metal dissolution during electrochemical test. We expect that this study suggests a novel solution for designing active and durable nanocatalysts for ORR and promotes the commercialization of PEMFCs.

Energy crisis has become one of world's most serious challenges, as the rapid growth of the total energy consumption. The renewable energies have been intensively explored and converted into electricity to ensure the sustainable development. The inexhaustible blue energy from the ocean is independent of season, circadian rhythm and weather.^[1] At the same time, triboelectric nanogenerators (TENGs) are considered as one of the most promising approaches for harvesting blue energy.^[2,3] Previously, most of the blue-energy-harvesting TENGs are based on the friction between solids, but the contact / friction area is only a tiny portion of the total surface due to the roughness of the surface and the inefficient contact condition.^[4,5]
In this work, a network of liquid-solid-contact triboelectric nanogenerator (LS TENG) is fabricated to efficiently harvest the huge quantities of blue energy from ocean. Due to the sufficient contact condition of the liquid solid interface, the energy output per cycle of LS TENG is about 48.7 times bigger than that of the solid-solid-contact triboelectric nanogenerator (SS TENG) with same area. The buoy LS TENG can generate several sequential damping signals just by one pulse of triggering, demonstrating the maximum utilization of the vibration energy. This characteristic merit renders it available for the buoy LS TENG to collect the low-frequency energy sufficiently. The output current, transferred charge and the output voltage of the network of 18 LS TENGs are 290 μA, 16725 nC and 300 V.
Additionally, the buoy LS TENG can acquire energy from different types of movements, including the up-down, shaking and rotation movements. The last but not the least, the network of buoy LS TENGs could harvest large quantities of energy, including surface wave energy and submarine current energy, to power portable electric devices or navigation systems. In this work, the electricity energy generated from the LS TENGs is stored in the capacitor to drive a wireless SOS radio frequency (RF) transmitter for ocean emergencies. This work provides a more efficient method of capturing the blue energy and expands its applications.

Reference:
[1] X. Y. Li, J. A. Tao, W. X. Guo, X. J. Zhang, J. J. Luo, M. X. Chen, J. Zhu, C. F. Pan, J. Mater. Chem. A 2015, 3, 22663.
[2] X. Y. Li, J. Tao, J. Zhu, C. F. Pan, APL Mater. 2017, 5, 074104.
[3] X. J. Zhao, G. Zhu, Y. J. Fan, H. Y. Li, Z. L. Wang, ACS Nano 2015, 9, 7671.
[4] C. S. Wu, R. Y. Liu, J. Wang, Y. L. Zi, L. Lin, Z. L. Wang, Nano Energy 2017, 32, 287.
[5] Y. Xi, H. Y. Guo, Y. L. Zi, X. G. Li, J. Wang, J. N. Deng, S. M. Li, C. G. Hu, X. Cao, Z. L. Wang, Adv. Energy Mater. 2017, 7, 1602397.

The development of new multi-functional electrochemical interfaces that can solve challenging problems of clean energy production, storage, and conversion is of paramount importance in the quest to find alternatives to fossil fuel use. Electrocatalysis – the study of electrode processes where the rate of reaction has a strong dependence on the nature of electrochemical interfaces – lies at the heart of the spectrum of electrochemical transformations relevant for resolving these challenges. Ability to tune physicochemical properties of electrochemical interfaces has evolved towards ability to define the nature, arrangement, and transformations of electrode surface atoms along with hydrated ions in the double layer, including the kinetics of electron transfer. Implementation of this approach will be presented for the development of non-precious and platinum-based nanoscale catalysts for the hydrogen evolution reaction, oxygen reduction and evolution reactions. The role of molecular species at the interface such as water, counter cations and anions, for both two and three phase interfaces, will be discussed in terms of ability to identify and understand their influence on reaction kinetics.

Polycrystalline ceramic materials play central roles as electrolytes and electrodes within a wide range of electrochemical technologies, including fuel cells and batteries. Heterovalent doping within ceramic electrolytes (e.g., perovskites) produces mobile oxygen vacancies as well as facilitating the transport of other charged defects, including protons and small polarons. The lattice strain associated with lattice-scale defect concentrations induces mechanical stress. This paper is concerned with modeling thermo- and chemo-mechanical coupling, which can lead to potentially damaging stresses. The primary emphasis is on membrane-electrode assemblies (MEA) fabricated from proton-conducting ceramics. However, the paper will also incorporate recent efforts to extend the models to Li-ion battery electrodes.

All-solid-state lithium batteries are one of the candidates for next-generation energy storage devices due to high energy density and long cyclability. It is well known that the insertion of a thin electrical insulator, such as Al₂O₃, MgO, and ZrO₂, between a solid electrolyte and an electrode is effective on improving battery performances.[1] However, the insulators do not contain lithium ions, and thus, the function of insulators remains unclear. Accordingly, it is crucial to understand the interfacial ionic transport properties across the solid electrolytes and electrodes through thin insulators.
Here, we present quantitative studies of the ionic transport through the thin insulators using thin-film batteries consisting of epitaxial films. This approach provides well-defined crystal orientations and interface structures. To verify the influence of the inserted thin insulator, we fabricated thin film lithium batteries using a very flat model electrode and covered the surface with a thin Al₂O₃ layer.
We prepared anatase Ti_0.996Nb_0.004O₂ (TNO) epitaxial thin films as a very flat model electrode (25 nm thickness, average surface roughness: Ra = 0.28 nm) on a (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT) (100) substrate by employing pulsed laser deposition. Using this model electrode, we fabricated all-solid-state thin-film lithium batteries through in-vacuo process that provides clean interfaces.[2] The thin film batteries consist of Li anode (800 nm), Li₃PO₄ solid electrolyte (450 nm), TNO thin film on LSAT substrate. Al₂O₃ thin layers with a variety of thicknesses ranging from 1 to 50 nm were inserted at the interface of a Li₃PO₄ and a TNO.
Cyclic voltammetry (CV) of the thin film battery without Al₂O₃ insertion showed redox peaks (Ti⁴⁺/Ti³⁺) in good agreement with those reported earlier using organic electrolytes.[3] As the thickness of the inserted Al₂O₃ layer was increased, the difference in the redox peak voltages was enlarged, indicating an increased overpotential. It is surprising to observe the battery redox cycling even though a Al₂O₃ layer completely covered the surface of a TNO electrode. In addition, we found that the overpotential decreased as the cycle number of CV was increased. These results suggest that a lithium ionic conductor has formed electrochemically in the Al₂O₃ layers.

[1] J. H. Woo et al., J. Electrochem. Soc. 159, 1120-1124 (2012).
[2] M. Haruta, T. Hitosugi et al., Solid State Ionics, 285, 118-121 (2016).
[3] J. S. Chen et al., Electrochem. Commun. 11, 2332 (2009).

Cerium oxide (CeO₂) or ceria is a known fast oxide ion conductor with an oxygen vacancy-mediated transport mechanism. The highest ionic conductivities are observed when it is doped with Gd₂O₃ and Sm₂O₃. For this reason, ceria-based materials are widely used electrolyte for solid oxide fuel cells (SOFCs).
Over the past decade, many attemps have been made on the development of fast oxygen-ion conductors in SOFC. Strain engineering has been one of the most effective ways to enhance ionic conductivity. Many researchers have observed various strain-induced enhancements of ion conductivity in nanostructures such as ultrathin, multicoated, and free-standing layers.
In this study, using an atomistic model, we investigate the effects of biaxial extrinsic and local intrinsic lattice strain on oxygen-ion transport in doped ceria via performing molecular dynamics (MD) simulations with well tested interatomic potentials.
We have performed three main analyses to examine the structural integrity of doped ceria: (1) radial distribution function (RDF) analysis for the phase and bond informations, (2) the coordination number (CN) analysis for the vacancies, and (3) mean square displacement (MSD) analysis for the cations and anions.
We have focused on the oxygen ion diffusion of doped ceria by considering five main mechanicsm contributing to accelerate oxygen-ion transport: (1) the influence of type of cations with respect to the ionic size, (2) the influence of cation concentration, (3) the influence of cation distribution, (4) the influence of local lattice intrinsic strain induced by co-doping and, (5) the influence of biaxial extrinsic strain.

References:
ACS Appl. Mater. Interfaces 9, 42415 (2017), Physical Chemistry Chemical Physics 20,10048 (2018), The Journal of Physical Chemistry C 122, 22374 (2018)

The demand for rechargeable Li batteries having more safety and higher energy density has been increased to meet the strong demand of novel applications such as electric vehicles and energy storage system. In this aspect, lithium ion batteries containing typical liquid electrolytes have fundamental limitations because liquid electrolytes can act as fuels in thermal runaway behavior leading to a fire or an explosion of battery and can be decomposed at high potential (> 4.5V) leading to the restricted use of high potential cathodes. Also, liquid electrolytes cause severe safety problems with Li metal, which is the best anode material for high energy density with low potential and high capacity, because Li metal in liquid electrolytes easily leads to the formation of dendrite that can produce a short-circuit between cathode and anode. To address these problems, there are several approaches. One of promising approaches is to apply proper oxide-based solid electrolytes (SEs) instead of liquid electrolyte. Among oxide-based SEs, garnet-type SEs has been a lot of attraction because they can have several advantages over liquid electrolytes in terms of electrochemical window, chemical stability with Li metal, and safety.
In this talk, I will discuss about the newly developed garnet-type SE that has superior electrochemical/chemical properties with respect to the wettability with Li metal, ionic conductivity, and chemical stability with air. Also, I will talk about the efforts in our group to build up all solid-state battery by using the developed garnet-type SE.

All-solid-state batteries promise significant safety and energy density advantages over liquid-electrolyte batteries. The interface between the cathode and the solid electrolyte is an important contributor to charge transfer resistance. Strong bonding of solid oxide electrolytes and cathodes requires sintering at elevated temperatures. Knowledge of the temperature dependence of the composition and charge transfer properties of this interface is important for determining the ideal sintering conditions. To understand the interfacial decomposition processes and their onset temperatures, model cathode systems of LiCoO₂ (LCO) and LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) thin films deposited on cubic Al-doped Li₇La₃Zr₂O₁₂ (LLZO) pellets were studied as a function of temperature, gas composition and electrochemical conditions. The methods combine interface-sensitive techniques, including X-ray photoelectron spectroscopy (XPS), synchrotron X-ray absorption spectroscopy, hard X-ray photoemission (HAXPES), and synchrotron X-ray diffraction. In this talk, we will present the found precipitation products at the interface as a function of synthesis and electrochemical conditions, their role in altering the interface resistance to Li transfer, and compare the LCO and NMC related cathodes in terms of their instability onset conditions. We will also present the role of protective coatings such as LiNbO₃ in interfering with the detrimental interface reactions.

Solid-state batteries (SSB) have been considered as promising next-generation energy storage device with improved safety and potentially increased energy density. Among all the inorganic solid electrolytes (SE), thiophosphates offer competitive advantages in terms of the low cost, high ionic conductivity, low Young’s modulus and yield strength, which makes them a strong candidate for room temperature application in SSB.¹

Despite providing high bulk conductivity and good processability, both theoretical calculations and experimental results suggest that thiophosphates will experience oxidation/decomposition upon charging when being in direct contact with commonly used oxide cathode materials.^2,3 This interfacial instability results in increased interfacial resistance during cycling and highlights the necessity of interfacial protection at the cathode/SE interface. Searching for promising cathode coatings and understand the mechanism of the coating functionality and validity is critical for improving the SSB cycle life. In this work, we predict potential coatings materials for thiophosphate Li₃PS₄ (LPS) system through ab-initio computational calculation, and we experimentally verify the performances of the selected coating materials through direct observation using advanced (scanning) transmission electron microscopy ((S)TEM) analysis with air-free sample preparation. By combining different modes of microscopy techniques (image, diffraction, and spectroscopy), we were able to probe the chemical composition and interface structure in SSB at a resolution of nanoscale. Our work elucidates the correlation between calculated chemical /electrochemical stability and the observed coating integrity during cycling, and provides a powerful method for predicting and probing the interfacial stability issue of the cathode/SE.

[1] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, Nat. Energy. 2016, 1 (4) 16030.
[2] W.D. Richards, L.J. Miara, Y. Wang, J.C. Kim, G. Ceder, Chem. Mater. 2015, 28 (1) 266.
[3] A. Sakuda, A. Hayashi, M. Tatsumisago, Chem. Mater. 2009, 22 (3), 949.

β-Li₃PS₄ is a promising Li-ion conductor (0.2 mS/cm at room temperature). Recently, its air-stability and ionic conductivity were improved by a small amount of ZnO doping (1 mS/cm), making it even more appealing study case. Here, we investigate the influence of interfacial cathode/electrolyte strain on β-Li₃PS₄ ionic conductivity by studying the effect of uniaxial and biaxial strains on Li-ion migrations by means of DFT calculations. Interestingly, migration energies show non-linear response to strain indicating that strain accommodation in lithium-phosphorus-sulfide family is different from that in conventional oxide-ion conductors, and, overall, the effect of strain is smaller. The structural response to strain and atomistic mechanisms behind migration barrier reduction are discussed. Differences between sulfide and oxide solid electrolytes are highlighted.

All-solid-state Li-ion battery based on solid electrolytes is a promising next-generation battery technology with high energy density, intrinsic safety, long cycle life, and wide operational temperatures. However, multiple materials challenges, such as low ionic conductivity of solid electrolytes and poor interfacial compatibility at the solid electrolyte-electrode interfaces, are impeding the development of this new battery technology. To resolve these materials challenges, we develop and leverage an array of ab initio computation techniques to provide unique insights into the fundamental materials limitations and to establish general design principles of materials and solid interfaces. Our first-principles atomistic modeling studies reveal the origin of ultra-fast Li-ion diffusion in lithium super-ionic conductors. Based on the newly gained understanding, we establish design principles for fast ion-conductor materials, and demonstrate these design principles for the computation discovery and design of new lithium super-ionic conductors. In addition, we develop thermodynamic calculations based on the materials-genome database for investigating the compatibility of heterogeneous interfaces between solid electrolytes and electrodes. Key factors affecting the compatibility of the solid electrolyte-electrode interfaces are identified, and interfacial design strategies are proposed from our thermodynamic computation. The demonstrated computation capabilities represent a transferable model in designing new materials and interfaces for emerging technologies.

In the past, advanced characterization methods, no matter in situ or ex situ, are majorly considered as means for understanding the functioning mechanisms of materials. However, they can also be powerful tools for accelerating the materials design loops, especially in the cases where computational designs nowadays can quickly provide dozens and hundreds of promising candidate materials for synthesis trials. High-throughput synthesis is a straightforward approach to accelerate the synthesis trials. However, in the cases where high-throughput synthesis is difficult or too expensive, in situ characterizations on the synthesis process represent a more economical and even more information-rich route for the quick synthesis of target materials.

Here we report multiple case studies where in situ characterizations for a variety of synthesis methods were used to advance our understanding on the thermodynamics and phase diagrams of novel battery materials, including electrodes and solid electrolytes. The discovery of a sodium cathode P2-Na_0.66Li_0.12Fe_0.18Mn_0.70O₂ with very high capacity and outstanding capacity retention will be discussed. In situ XRD and solid state NMR characterizations were used to understand the formation and functioning mechanisms. Novel sulfide based Li+ and Na+ ionic conductors identified with using in situ XRD will also be presented with their structure resolution, ionic conductivity measurement and full-cell battery tests.

Solid-state batteries (SSBs) are considered as the next-generation batteries due to their potential for improved safety, power density, and energy density compared with conventional Li-ion batteries. However, the interfacial reactivity and resulting resistance increase between the cathode and solid-state electrolyte (SSE) can significantly deteriorate the cell performance. Since virtually none of the SSEs are intrinsically stable against both electrodes, coatings are needed that are chemically and electrochemically stable with the SSE and the electrode.

We employ a high-throughput (HT) computational framework to screen Li-containing materials for cathode coating application, focusing on their phase stability, electrochemical and chemical stability, and ionic conductivity. The HT results show that there are clear chemical rules to explain reactivity between electrodes and SSEs and that the optimal coating materials need to be selected for each specific SSE/cathode combination.

We identified polyanionic oxides as promising cathode coating candidates to protect sulfide electrolytes against oxide cathodes, with LiH2PO4, LiTi2(PO4)3, and LiPO3 being particularly appealing choices. Some lithium borates exhibiting excellent (electro)chemical stability at various interfaces are also highlighted. Lastly, factors affecting the stability of coating materials such as Li content and oxygen bonding covalency are also discussed.

The solid-state lithium metal battery is a promising next-generation technology due to the high energy density of the lithium metal anode and the potential for solid-state electrolytes (SSEs) to prevent failure modes seen in liquid electrolyte lithium metal batteries. However, the interfaces between many SSEs and lithium metal are (electro)chemically unstable, and we lack a detailed understanding of how interfacial transformations relate to electrochemical degradation. Using the NASICON-type Li_1.4Al_0.4Ge_1.6(PO₄)₃ (LAGP) as a reactive SSE, we study the reaction processes that occur at the lithium metal/SSE interface and develop a chemo-mechanical understanding of degradation in LAGP. In situ transmission electron microscopy reveals that lithium insertion into LAGP at the interface drives the transformation to form an amorphous phase with expanded volume [1]. In symmetric Li/LAGP/Li cells, the evolution of mechanical stress due to this transformation ultimately causes fracture of the SSE and complete cell failure. Ex situ characterization of LAGP after cycling shows that the morphology of the interphase layer is highly dependent on the applied current density, and the interphase morphology significantly influences mechanical stability. In situ X-ray tomography during electrochemical cycling of symmetric cells reveals the onset of fracture and growth of the crack network throughout cycling [2]. Our results show that the impedance begins to increase when fracture first initiates, indicating that the ionic conductivity of the interphase is comparable to pristine LAGP and is not responsible for the significant increase in impedance. This work highlights that the nature of the reaction at the Li/SSE interface plays a crucial role in determining chemo-mechanical degradation mechanisms, with implications for understanding degradation in a wide range of practical SSEs.

References:
[1] Lewis, J. A.; Cortes, F. J. Q.; Boebinger, M. G.; Tippens, J.; Marchese, T. S.; Kondekar, N.; Liu, X.; Chi, M.; McDowell, M. T. ACS Energy Letters 2019, 4 (2), 591.
[2] Tippens, J.; Miers, J. C.; Afshar, A.; Lewis, J. A.; Cortes, F. J. Q.; Qiao, H.; Marchese, T. S.; Leo, C. V. D.; Saldana, C.; McDowell, M. T. ACS Energy Letters 2019, 1475.

Formation and migration of point defects underlie nearly all materials for energy transformation, such as electrodes and solid electrolytes for lithium-ion batteries and solid oxide fuel cells. The distribution of point defects such as lithium and oxygen vacancies reflect electro-chemo-mechanical equilibria. In this talk, I will present two such examples. First, we show that anisotropic and polycrystalline lithium layered oxide can exhibit extremely non-uniform lithium concentration due to mechanical effect due to orientation distribution of crystallites. Second, we present a general theory framework for understanding how mechanical contribution plays a key role in determining the space-charge effect at interfaces.

Na-ion batteries (SIB) as one of the battery technologies for grid-scale energy storage has attracted great attention recently. SIBs, like Li-ion counterparts, usually have solid electrolyte interphase (SEI) at the anode side and cathode electrolyte interphase (CEI) on cathode surface, which determine the reversibility and rate of the cell reactions for the rest of the cell life. The electrode/electrolyte interphases have not been thoroughly investigated despite the progress in SIB development. Here, we reported our work on the development of advanced electrolyte systems for 1) controlled SEI on hard carbon anodes and hence fast Na-ion transfer; and 2) mitigated cathode surface phase transition and significantly improved cyclability. Hard carbon anodes demonstrated high rate and good low temperature performance in TEGDME based electrolyte. The specific capacity of hard carbon anode is ~180 mAh/g at the current density of ~250 mA/g at room temperature. The specific capacity is ~200 mAh/g at -20oC/ discharge at the current density of ~25 mA/g. The O3-type metal oxide cathode in localized high concentration electrolyte delivers a high specific capacity of ~190 mAh/g between 2-4.2V. The capacity retention is >80% over 1000 cycles between 2-4V with a specific capacity of ~150 mAh/g.

The solid-electrolyte-interphase (SEI) is probably the least understood component in Li-ion batteries. Considerable effort has been put in the understanding of its formation and electrochemistry under realistic battery conditions, but mechanistic insights have been inferred mostly indirectly. Here we show the formation of the SEI between a graphite anode and a carbonate electrolyte using combined atomic scale microscopy and in-situ and operando techniques. In particular, we weigh the graphitic anode during its initial lithiation process with electrochemical quartz crystal microbalance, which unequivocally identifies lithium fluoride (LiF) and lithium alkylcarbonates as the main chemical components at different potentials. In-situ gas analysis confirmed the preferential reduction of cyclic over acyclic carbonate molecules, making its reduction product the major component in SEI. We find that SEI formation starts at graphite edge-sites with dimerization of solvated Li⁺-intercalation between graphite-layers. We also show that this lithium salt, at least in its nascent form, is re-oxidizable, despite the general belief that an SEI is electrochemically inert and its formation irreversible.

Electrochemical devices such as solid oxide fuel/electrolyzer cells (SOFC/SOEC) and all-solid-state batteries employ an ion-conducting and electron-blocking solid electrolyte and can be operated reversibly between charge and discharge. Yet reversing operation modes from SOFC to SOEC, or from battery discharge to charge often lead to more severe degradations, as evidenced by oxygen bubble formation at the grain boundaries in the zirconia electrolyte, Na metal islands in the Na-beta-alumina solid electrolytes of Na-S batteries and similar Li metal islands inside the Li₇La₃Zr₂O₁₂ electrolyte in lithium ion batteries. While conventional wisdoms rationalized such observations by electrode overpotential and thought internal phase formation can be kinetically suppressed by sluggish electron transport through electron-blocking electrolytes, our thermodynamic analysis show (i) possible overpotential at transport bottlenecks inside solid electrolytes, leading to (ii) potential overshoot/undershoot beyond two boundary values at electrode/electrolyte interfaces, thus setting (iii) largest chemical driving forces to precipitate damaging internal phases, and (iv) electrons/holes can be readily transported to form such phases with ionic transference number >0.999 under typical operation conditions of such electrochemical devices. To the end, our work calls for better understanding of materials theory and better characterizations of transport properties for solid electrolytes, in order to achieve higher-energy-density and safer electrochemical devices.

References
[1] Y. Dong, Z. Zhang, A. Alvarez, I.-W. Chen, Overpotentials at Kinetic Bottlenecks Cause Inordinate Internal Phase Formation in Electrochemical Cells, arXiv preprint arXiv:1812.05187 (2018).

Lithium (Li)-metal based anode is of particular interest owing to their high energy density, and specific capacity more than ten times to that of LiC₆ anodes. However, the highly reactive Li-metal inhibit further practical applications due to the growth of undesired Li dendrites and the formation of unstable solid electrolyte interphase (SEI) formation. This Li dendrites growth and continuous formation of SEI consume much electrolyte leading to low CE and capacity fading. In addition, the notorious Li dendrites can pierce the separator causing the short circuit and fire explosion. To improve the cycling performances and safety concerns, herein, we consider a rational approach for designing an artificial Li-metal/electrolyte interface. Facile and cost-effective mechanism of engineering ex situ artificial SEI layer was introduced by using WS₂. The chemical and mechanical stable artificial layer provides both high Li-ion conductivity that facilitates fast Li-ion diffusion and high Young’s modulus enough to suppress the Li dendrite growth. The ability to store Li by the conversion reaction and plating underneath the SEI layer induces a homogenous Li deposition. Such an artificial protected Li symmetrical cell showed significantly long hours plating/stripping cycles and reduced overpotential compared to the bare Li. The full cell configuration with high-loading (11.88 mg.cm^-2) of NMC111 showed astonishing results such as longer stable cycling, higher capacity at higher rates and higher rate capability compared to the bare Li anode. This approach could lead to the development of high energy density and safe Li-metal based batteries.

The development of lithium metal batteries (LMB) is hauled by uncontrollable dendrite growth and short cycling life, which restrained its potential to replace lithium ion batteries (LIB) or bring revolutionary advances to energy storage systems. Engineering solid electrolyte interphase (SEI) by modifying electrolyte recipe is considered the most effective way to improve the performance of LMB. Despite the general acceptance of potential improvements by substituting more Fluorine to organic electrolytes, it is still unclear what differences in decomposition mechanisms or derived SEI components between Fluoroethylene Carbonate (FEC) and Difluoroethylene Carbonate (DFEC) can lead to contradictory results with respect to the general expectation. In this study, using Density Functional Theory (DFT) and the developed "two lithiums" method, we have observed distinctive spontaneous initial bond-breaking mechanisms for FEC and DFEC. FEC breaks by forming CO and LiF, or Li₂CO₃ and CHFCH₂; while DFEC breaks by forming CO, 2LiF and C₂H₂O₂, or Vinylene Carbonate (VC) and 2LiF. DFT confidence measurement C-value indicates absolute energetic favorability of CO forming mechanism for FEC, while approximately equal competence of both mechanisms for DFEC. On the basis of initial bond breaking mechanisms, we provide and discuss the detailed step-by-step energetic pathways and identify the full sets of FEC and DFEC decomposition products. The final SEI components from our analysis are carefully compared to X-ray photoelectron spectroscopy (XPS) results from literature and found good agreements. The results from this study provide the evidence of contrasts in bond breaking mechanisms and final decomposition products between FEC and DFEC, which offer sound explanation of the variations in cell performances and enhanced fundamental understanding of SEI formation. At last, we suggest electrolyte additive with more Fluorine substitution does not necessarily contribute to cell performance upgrade, and a rigorous study using the demonstrated methodology in this work should be performed beforehand to assist electrolyte selection and SEI engineering.

Theoretical investigations are important for understanding the depositional and morphological evolutions at the interface of Li metal and solid electrolyte (SE) in Li-metal solid state batteries (LMSSBs), especially when direct experimental measurements are hard because of the buried nature of the interface between two solids. In this work, we provide theoretical illustrations of the evolution of the Li metal/SE interface under the relevant boundary conditions (BCs) of LMSSBs. We first prove that the effect of mechanical pressure on the interfacial electrochemistry is negligible compared with other factors, such as the charge-transfer and the mass-transfer of Li-ion. We then investigate the Li electrodeposition and Li plastic deformation for a wide range of BCs, including the initial surface irregularity, kinetics of charge transfer and mass transfer of Li-ions, yield strength of Li metal, and applied stack pressure at boundaries. We conclude that the interfacial contact loss and surface overpotential at Li metal/SE can be well controlled by optimizing the electrochemical and mechanical conditions, such as the applied stack pressure and applied current density.

The rapid expansion of electrical vehicle market raises the demand for batteries with improved safety and higher energy density. All-solid-state batteries (SSBs) are widely considered as the next-generation energy storage device that can meet both requirements. They improve safety by removing organic carbonate-based liquid electrolytes and can potentially increase energy density by utilizing a Li-metal anode. However, because of the large solid-electrolyte fraction (30 wt%–50 wt%) typically required in cathode composites to provide sufficient ionic diffusion, the volume fraction of cathode (cathode loading) of current SSBs is low, resulting in low energy density.
Using experiments and modeling, we demonstrate in this work that very high volume fractions of cathode can be fully utilized in a composite cathode as long as the ratio of the solid electrolyte to cathode particle size is controlled in a cold-pressed composite cathode. This conclusion is experimentally verified, and a composite cathode with > 50 vol% cathode loading is demonstrated using Li₂O–ZrO₂ (LZO)-coated LiNi_0.5Mn_0.3Co_0.2O₂ (NMC) as the active material and amorphous 75Li₂S–25P₂S₅ (LPS) as the solid electrolyte.
Our model is applicable to various active materials and solid conductors and provides a generalized guideline for increasing active material loading via particle size optimization.

All-solid-state lithium-ion batteries have attracted significant interest as candidates for the next generation of rechargeable batteries. However commonly observed interfacial reactions between sulfide electrolytes and oxide electrode materials can increase interfacial impedance, thus degrading the battery performance. Interposing an ionic conductor as an interface coating can suppress any unanticipated reactions while maintaining lithium ion diffusion through the battery. However, known lithium-ion-conducting sulfides are usually unstable against cathode materials while the stable oxides are typically poor lithium ion conductors. To increase the space of potential interfacial coatings, we propose that a double-layer coating design could potentially stabilize the electrolyte-electrode interfaces and facilitate the lithium ion transfer. We present the results of a search for stable double-layer coatings with high ionic conductivity. Mobility of lithium ions in the double layer coatings is characterized with a recently developed class of machine learning interatomic potentials, which accesses nanosecond-long molecular dynamics with nearly ab initio accuracy. The identified coating strategies highlight opportunities for creating stable solid-state lithium-ion batteries with fast charge and discharge rates.

The continuing search for methods to engineer safer, smaller, lighter, cheaper, and more stable batteries requires comprehensive understanding of the processes governing their operation at multiple length scales. All-solid-state power sources do not contain flammable organic electrolytes, and, therefore, are safer. However, they suffer from high internal impedances, arising at the interfaces between the solid electrolyte and electrodes. The nanoscopic thickness of these interfaces calls for microscopic tools to study their behavior. Here, using in operando Kelvin Probe Force Microscopy (KPFM), we measure the potential distribution in a solid-state Li-ion battery as a function of its state of charge. The battery was fabricated by sequentially depositing thin layers of Pt (110-130 nm), LiCoO₂ (280-420 nm), LIPON (1100-1200 nm), Si (50-240 nm) Cu or Pt (150-200 nm) onto a Si/SiO₂ wafer (oxide thickness 100 nm). The fabricated battery was cleaved in an Ar atmosphere to expose the stacked layers, mounted on a holder, wired, and safely transferred without exposing to air into a dual-beam instrument that combines a scanning electron microscope (SEM), a Ga-ion focused ion beam (FIB) and an atomic force microscope (AFM) in one vacuum chamber (residual pressure of 10^-4 Pa). The stacked battery was milled to expose a cross-section of the layers, and imaged using SEM and KPFM, while cycling the battery. The acquired potential maps reveal a highly non-uniform interelectrode potential distribution, with most of the potential drop occurring at the electrolyte-Si anode interface in the pristine battery. During the first charge, the potential distribution gradually changes, revealing complex polarization within the LIPON layer due to Li-ion redistribution. Cycling the battery at high rate significantly decreased its capacity, although the capacity loss can be recovered. KPFM imaging allowed the detection of the interface responsible for this capacity loss. The acquired data was compared to first principles calculations shedding light onto the interfacial Li-ion transport in the battery and its reversibility.

Lithium lanthanum zirconium oxide (LLZO), a ceramic oxide is a promising solid electrolyte candidate to enable Li metal anodes due to its high ionic conductivity (1 mS/cm), stability and low interfacial impedance against Li. However, in a full cell configuration, how LLZO is integrated into cathode constructs requires further development. Recently, bilaminar cell configurations consisting of discrete LLZO membranes to protect Li metal anodes, coupled with conventional liquid electrolyte permeated cathodes, has been suggested as a viable approach. In this electrolyte configuration, LLZO would act to stabilize the Li anode and the liquid electrolyte provides facile transport between LLZO and the cathode. However, we have shown that there may be an incompatibility between typical liquid electrolyte (1M LiPF₆ in Ethylene carbonate/Dimethyl carbonate) and LLZO. For example, in hybrid cell configurations, the interfacial impedance increases with time. This work was focused on first developing a mechanistic understanding of the interactions between LLZO and carbonate-based liquid electrolytes. Understanding the reaction pathways between LLZO and liquid electrolytes provided us insight to develop approaches to enable a stable interface between LLZO and the catholyte. In this work, we propose a candidate for the catholyte which is compatible with LLZO and would enable a low interfacial impedance and stable cycling in a full cell configuration.
The results of this work provide an insight into the limitations of using liquid electrolytes with LLZO. We also provide a solution which would help in enabling Li metal anodes using a bilaminar electrolyte configuration.

Atomic layer deposition (ALD) coating technique is proved to be an effective and efficient strategy to improve the capacity and cycle capability of lithium-ion batteries. One of the key roles of the ALD coating layer is to facilitate the species transfer and diffusion in electrode particles. It has been generally believed that this enhancement has resulted from the higher conductivity of the coating layer itself. However, there is a concern about this idea because the proportion of ALD coating layer is very small. Here, we propose a new hypothesis about the role of ALD coating layer in ion transportation. Due to the agglomeration of particles, the surfaces of electrode particles are partially blocked, and, as a result, Li-ion intercalation is not uniform over the surfaces. On the other hand, ALD coating could provide a path to quickly distribute Li-ions over the whole particle surface, leading to enhanced diffusivity of Li-ions through the particles. This hypothesis is validated by both continuum simulation and experiment in this work. In addition, an investigation is conducted to study the ALD coating thickness impact on Li-ion diffusivity via first-principles calculations, which shows that the Li-ion diffusion increases to the maximum value and then decrease as the coating thickness increases. We find that this is related to the higher diffusivity of the ALD coating than that of the active material, and higher diffusivity through the surface than through the bulk. This combined work reveals the influence of ALD coating layer on Li-ion diffusion and provides us a clue why there is an optimal ALD coating thickness for the best battery performance.

Ability to resist penetration of metallic lithium is the major requirement for the electrolyte membrane in an all solid-state lithium cell. In this regard, the lithium phosphorus oxynitride (LiPON) electrolyte has been demonstrated to successfully block Li metal propagation (often termed lithium dendrites) in thin-film cell configuration¹. It is therefore important to understand which properties of LiPON are critical for such stable performance with Li metal so that other electrolytes could be designed following similar metric. Here we concentrate on mechanical properties of LiPON, specifically its resistance to cracking, i.e. fracture toughness. This property appears to be rather important, since metallic lithium penetration has been demonstrated from artificially introduced defects even in the absence of grain boundaries².
By using instrumented nano-indentation we observed unusually high resistance of LiPON to crack initiation and propagation. We compare this behavior to fracture in typical glass materials and other Li-ion conducting glass-ceramic electrolytes. In addition to resistance to cracking LiPON apparently exhibits time-dependent behavior and is capable of partial recovery upon unloading³. We discuss these features in relation to LiPON functionality in all solid state cell with metallic lithium anode.
Acknowledgements: This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE), under contract DE-AC05-00OR22725, was sponsored by the U.S. Department of Energy, Advanced Research Projects Agency for Energy (ARPA-E) through the IONICS program, Award No. DE-AR0000775.

1. Bates, J., Dudney, N.J.,Gruzalski, G., Zuhr, A., Choudhury, A., Luck, C., Robertson, J., Electrical Properties of Amorphous Lithium Electrolyte Thin Films, Solid State Ionics53-56 (1992), 647-654.
2. Porz, L., Swamy, T., Sheldon, B.W., Rettenwander, D., Fromling, T., Thaman, H.L., Berendts, S., Uecker, R., Carter, W.C., Chiang, Y.-M., Mechanism of lithium metal penetration through inorganic solid electrolytes, Advanced Energy Materials7 (2017), 1701003, 12 pp.
3. Herbert, E.G., Tenhaeff, W.E., Dudney, N.J., Pharr, G.M., Mechanical characterization of LiPON films using nanoindentation, Thin Solid Films520 (2011), 413-418.

The Oxygen Reduction Reaction (ORR) is the desired cathode reaction in Proton Exchange Membrane Fuel Cells (PEMFCs). Carbon-supported platinum alloy (PtM/C) nanoparticles are by far the most effective ORR catalysts in PEMFCs. However, the durability of PtM/C is not satisfying. Efforts to improve their durability have been impeded by the poor understandings of their degradation mechanisms. In this study, by conducting accelerated stress tests (ASTs) on a commercial PtCo/C catalyst in both a rotating disk electrode and a PEMFC, we identified that a significant fraction of the electrochemical surface area (ECSA) loss of the PtCo/C upon the AST was recoverable. This recoverable ECSA loss was tentatively ascribed to the poisoning of the Pt surface by the sulfate ions either from the H₂SO₄ electrolyte in a RDE or the Nafion in a PEMFC. We further demonstrated that this poisoning effect can be largely suppressed without compromising the activity by encapsulating PtCo/C into thin functionalized carbon overlayers that were tuned to be permeable to H⁺, O₂, and H₂O, but not to sulfate ions.

Solid-state batteries (SSBs) have been considered as one of the most promising next-generation energy storage systems. However, interfacial issues such as mechanical instability between electrodes and solid-electrolytes are critical factors affecting the cyclic performances of SSBs. Ultrathin Al₂O₃ and polyurea thin film prepared by atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques have been used as a coating layer for Li metal anode and solid-electrolyte materials in SSBs, which helped to boost the cycling performance and mechanical stability by protecting the interface of electrode/electrolyte¹. However, the static and dynamic mechanical response of ALD and MLD films is still unclear. We studied the elastic, fracture, and fatigue behavior of ALD Al₂O₃ and MLD aglucone as well as hybrid MLD/ALD films on a thickness range of 2 nm to 20nm. The static and dynamic mechanical behavior of suspended ALD and MLD films over supporting monolayer graphene was studied using atomic force microscope (AFM) film deflection technique. All ALD films at all thicknesses failed by brittle failure and showed higher stiffness than MLD films. The overall stiffness of ALD and MLD films affected by graphene to a greater extent below the critical thickness of 2 nm and 9 nm, respectively. Remarkable fatigue life was obtained for all Al₂O₃ films from million to several billion cycles under force range of 50% to 80% of static failure forces, while the fatigue cycle numbers increased by decreasing the thickness. Higher stiffness and failure forces of “15nmMLD/5nmALD” hybrid film in compare to “5nmALD/15nmMLD” films were shown to be the reason behind their better electrochemical performances in contact with Li metal anodes².

1.Y. Zhao, K. Zheng, and X. Sun, Addressing Interfacial Issues in Liquid-Based and Solid-State Batteries by Atomic and Molecular Layer Deposition, Joule (2018) 2, 1–22.
2.Y. Zhao*, M. Amirmaleki*, Q. Sun, C. Zhao, A. Codirenzi, L. V. Goncharova, C. Wang, K. Adair, X. Li, X. Yang, F. Zhao, R. Li, T. Filleter, M. Cai, and X. Sun, Natural SEI-Inspired Dual-Protective Layers via Atomic/Molecular Layer Deposition for Long-Life Metallic Lithium Anode, Matter (2019) 1, 1-17

Batteries utilizing multivalent species (e.g., Mg, Ca, Zn, or Al) as the working ion are gaining increasing attention for use in applications where large numbers of inexpensive cells are needed, such as grid storage. Ca in particular offers several advantages, including a reduction potential close to that of Li, high volumetric capacity, and fast solid state diffusion. [1] However, the breakdown of commonly used organic electrolytes causes an ionically insulating passivating layer (the solid electrolyte interphase, or SEI) to form on the electrode surface, preventing reversible plating and stripping of the Ca metal anode. [2] In this work we use density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations to study the interaction of Ca ions with various organic solvents used in typical electrolytes and investigate their breakdown on Ca metal surfaces. [3,4] We find that Ca²⁺ forms a large first solvation shell in ethylene carbonate (EC) and propylene carbonate (PC) and a slightly smaller one in tetrahydrofuran (THF). We then use AIMD to compute the diffusion coefficient of Ca in each solvent using ClO₄^- as a counterion, and find that it diffuses fastest in THF, and slower in EC and PC. Finally, we used AIMD to identify the principle components of the SEI by studying the decomposition of EC on Li, Ca, and Al surfaces. We believe that these results have generated an increased understanding of Ca-based electrolytes and can help in the design of new ones in the future.
.
[1] P. Canepa et al., Chem. Rev. 117 4287 (2017)
[1] D. Monti et al., Frontiers in Chemistry 7 79 (2019)
[2] J. Young and M. Smeu, J. Phys. Chem. Lett. 9 3295 (2018)
[3] J. Young, P. M. Kulick, T. R. Juran, M. Smeu, ACS Appl. Energy. Mater. 2 1676 (2019)

Understanding (electro-)chemical reactions at the electrode-electrolyte interface (EEI) is crucial to promote the cycle life of lithium-ion batteries. In this study, we developed an in situ Fourier-transform infrared spectroscopy (FT-IR) method, which provided unprecedented information on the oxidation of carbonate solvents via dehydrogenation on LiNi_xMn_yCo_1-x-yO₂ (NMC). While ethylene carbonate (EC) was stable against oxidation on Pt up to 4.8 V_Li, unique evidence for dehydrogenation of EC on LiNi_0.8Co_0.1Mn_0.1O₂ (NMC811) at voltages as low as 3.8 V_Li was revealed by in situ FT-IR measurements, which was supported by density functional theory (DFT) results. Unique dehydrogenated species from EC were observed on NMC811 surface, including dehydrogenated EC anchored on oxides, vinylene carbonate (VC) and dehydrogenated oligomers which could diffuse away from the surface. Similar dehydrogenation on NMC811 was noted for EMC-based and LP57 (1 M LiPF₆ in 3:7 EC/EMC) electrolytes. In contrast, no dehydrogenation was found for NMC111 or surface-modified NMC by coatings such as Al₂O₃. In addition, while the dehydrogenation of solvents was observed in 1 M electrolytes with different anions, they were not observed on NMC811 in the concentrated electrolyte (EC/EMC with 3.1 M LiPF₆), indicating lithium coordination could suppress dehydrogenation. Dehydrogenation of carbonates on NMC811 accompanied with rapid growth of interfacial impedance with increasing voltage revealed by electrochemical impedance spectroscopy (EIS), while the electrode-electrolyte combinations without dehydrogenation did not show significant impedance growth. Therefore, minimizing carbonate dehydrogenation on the NMC surface by tuning electrode reactivity and electrolyte reactivity is critical to develop high-energy Li-ion batteries with long cycle life.

It is necessary to reduce operating temperature below 600 °C for solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) toward cost reduction. High temperature proton conducting oxides have gained widespread interest as an electrolyte materials which are alternative to oxygen ion conductors. For typical high temperature proton conducting oxides, aliovalent cation-doped perovskite-type oxides (ABO₃) exhibit proton conductivity originating from the hydration of oxide ion vacancies [1]. The hydration reaction is accompanied by structural deformation, i.e. chemical expansion. The chemical expansion may lead to mechanical failure in electrochemical devices, and thus it is necessary to understand the causes of this process at the atomic scale. In this study, the chemical expansion behaviors of Y-doped strontium cerate (SCY) and Y-doped strontium zirconate (SZY) were comparatively investigated. High temperature X-ray diffraction and thermogravimetric analysis revealed that the cerate exhibits higher chemical expansion. To understand the reason why cerate has experimentally higher chemical expansion than zirconate, density functional theory (DFT) calculations revealed that size of oxide ion vacancies, and covalency between B-site and oxygen played important roles to understand the chemical expansion difference [2]. DFT shows the chemical expansion of SZY is larger than that of SCY when two dopant Y atoms are close each other. However, the reverse is also true for the case in which two dopant Y atoms are distant from each other. The former Y clustering case is energetically more stable in comparison with the latter of Y distant case. When it comes to sintering process of SZY and SCY, it seems that Y atoms are not experimentally diffused on the most stable atomic positions. Moreover, DFT revealed that the size of oxide ion vacancies in SZY is much smaller than that in SCY in the case of Y-clustering; this is the reason why the chemical expansion of SZY is large than that of SCY. However, in case of the Y atoms are distant, oxide ion vacancies’ size is similar between SZY and SCY. Ionic and covalent bonding nature support the trend that SCY experimentally shows larger chemical expansion than SZY.

Acknowledgements
This project was supported by the Japan Society for the Promotion of Science (JSPS) and National Science Foundation (NSF) under the JSPS-NSF Partnerships for International Research and Education (PIRE), the JSPS Core-to-Core Program of Advanced Research Networks (Solid Oxide Interfaces for Faster Ion Transport), the International Institute for Carbon-Neutral Energy Research (I2CNER) of the World Premier International Research (WPI) in MEXT of Japan, and the Advanced Graduate Program in Global Strategy for Green Asia of Kyushu University. Also, this work was funded by JSPS Kaken (Grant-in-Aid for Research Activity start-up) 19K23577.

References
[1] K, Leonard, et al. Int. J. Hydrogen Energy,42,7 (2017) pp.3926-3937
[2] T. Fujisaki et al. Solid State Ionics, 333, (2019) pp.1-8

In order to understand proton transport during a faradaic reaction in an oxide whose structural water content and structure was well-known, we have been investigating the electrochemical de/intercalation of protons in hydrated tungsten oxides, WO_3^●nH₂O (n = 1, 2). Recently, we found that the presence of structural water leads to a transition in the energy storage mechanism from solid-state diffusion limited in WO₃ to surface-limited (or pseudocapacitive) in WO_3^●2H₂O. We also found that WO_3^●2H₂O exhibits much smaller and more reversible mechanical deformation during proton intercalation than WO₃, suggesting that the presence of structural water minimized the electrochemically-driven structural transformations (electrochemomechanics). Surface-limited energy storage in WO_3^●2H₂O would suggest that a facile solid-state proton diffusion mechanism, such as Grotthuss transport, was present in this material. However, the fact that the energy storage process was closely related to the mechanical deformation of the material led us to consider an alternate explanation for the observed kinetic behavior. We investigated the role of structural water on the observed kinetic and electrochemomechanical differences between WO_3^●nH₂O and WO₃ by coupling electrochemical characterization with fundamental studies of the structure and dynamics of both the solid state structure and confined water network. Operando X-ray diffraction (XRD), ex situ quasielastic and inelastic neutron scattering (QENS and INS, respectively), and density functional theory (DFT) calculations as a function of potential and proton content reveal that structural water in WO_3^●nH₂O is extremely stable, which allows for rapid and facile structural transformation during electrochemical proton intercalation. We demonstrate that the facile electrochemomechanics of WO_3^●nH₂O allow for reversible proton intercalation at a scan rate of 2 V s^-1, corresponding to a charge/discharge time of only ~ 500 ms. We hypothesize that it is the structural rigidity of the water network and not the presence of a Grotthuss mechanism, that leads to pseudocapacitive proton storage in the WO_3^●nH₂O. This leads us to propose a new materials design strategy for high power, high energy density storage via the presence of structurally rigid fluids confined in the interlayer of 2D materials that stabilize the solid state structure at high rates.

In the last years, several Platinum-based bimetallic alloys have been studied as possible substituents of pure Platinum catalysts in Proton Exchange Membrane Fuel Cells (PEMFCs) to strongly reduce production cost. In particular, PtNi alloy was found to be a promising alternative to pure Pt due to its very good mass activity and its competitive catalytic activity. The main drawback is related to the acid-based environment in which catalyst has to operate, which causes the dissolution of the less noble metal and the consequent degradation of the whole catalyst layer. In this framework, in situ Grazing Incidence Small Angle X-ray Scattering combined with electrochemistry is used to investigate the real-time kinetics of the surface degradation of PtNi alloy during electrochemical cycling voltammetry. In order to simulate the realistic fuel cell operation conditions, the study has been performed by applying different upper potentials and using different alloy compositions. Obtained results, have been complemented with in situ Electro-Chemical AFM and with Inductively Coupled Plasma Mass Spectrometry. Data revealed in depth the degradation of the aforementioned catalyst, the structural / compositional changes during fuel cell operation, and the asymmetric Ostwald ripening process, here described in electrochemistry the first time.

The authors acknowledge the financial support from project CEROP, which is a CERIC-ERIC internal research project.

Nanocellulose is a renewable, inexpensive and biocompatible nanomaterial with high strength and tunable surface chemistry. Due to high global energy demand and environmental concerns it is vital to find alternative of fossil fuel; in this aspect Polymer electrolyte membrane fuel cells are highly effective and environmental friendly energy source. But higher cost for fabrication of proton exchange membrane in the fuel cell is major factor which makes it expensive. In this regards we have used novel, simple, cost effective and less chemical oriented Nitro-oxidation method to prepare high strength Cellulose nanopore having -COONa and -COOH functionalities with high proton conductivity at (80 ^OC with various RH%), good gas barrier properties and efficient fuel cell performance (37 mS2). Further we have characterized NO-CNF Nanopaper with morphological, spectroscopic and analytical techniques which includes IR, elemental anlysis, SEM, TEM, WXRD, ¹³CPNMR, BET surface area analysis, DMA etc.

A new class of amorphous/nanocrystalline (La,Sr)CoO3-(La,Sr)2CoO4 composite cathodes were developed for IT-SOFCs via co-sputtering of the respective oxides. The cathodes as deposited were amorphous at mid-compositions and could remain so when used at reduced temperatures. They could crystallize into two phase structures at temperatures close to 700 °C, with grains no more than 10 nm in size yielding a structure with extremely high density of hetero-interfaces[1]. EIS responses measured on symmetric cells imply satisfactory ORR performance at temperatures as low as 575^oC especially when the volume fractions are in the range 0.40 < (La,Sr)2CoO4 < 0.60. Moreover it was found that co-sputtered (La,Sr)CoO3-(La,Sr)2CoO4 cathodes, followed in terms of area specific resistance (ASR), had highly stable cathode performance in prolonged testing. Cathodes were resistant to Sr segregation due probably to the high stability of extremely fine two-phase structure.
[1] D Sari, F. Piskin, Z. C. Torunoglu, B Yasar, Y Kalay and Tayfur Ozturk , Solid State Ionics 326 (2018) 124–130

The growing demand for wearable and flexible energy storage devices requires batteries with high capacity and low cost. A rationally designed air electrode with high catalytic activity and robust mechanical properties is the key requirement to realize flexible metal-air batteries for wearable applications. In this work, electrospinning N-doped carbon nanofibers containing iron carbide (Fe₃C@N-CFs) are synthesized and employed as the cathode in the flexible Al-air battery. Benefiting from the excellent catalytic activity of the iron carbide which are uniformly encapsulated in the N-doped carbon matrix, as well as the large specific surface area of the cross-linked network nanostructure, the as-prepared Fe₃C@N-CFs show outstanding catalytic activity and stability during oxygen reduction reaction. The as-fabricated all-solid-state Al-air batteries with Fe₃C@N-CFs catalyst show a stable discharge voltage (»1.7 V) for 8 hours, giving a capacity of 1287.3 mAh g^-1. They are also demonstrated to power a LED watch continuously for over 22 hours, indicating their promising application as energy storage device for flexible electronics.

As high-efficiency energy conversion devices, proton exchange membrane fuel cells (PEMFCs) have been recognized as crucial technologies for electric vehicles. However, PEMFCs still rely on expensive platinum-based electrocatalysts for the sluggish oxygen reduction reaction (ORR). As an emerging alternative, alkaline polymer electrolyte fuel cells (APEFCs) have drawn increasing attention because they can enable the use of non-precious metal electrocatalysts, in particular 3d metals, due to their low cost and high activity. Here, we report on a novel family of Co-Mn spinel oxide nanoparticle catalysts, that exhibit significantly enhanced ORR activity when compared to both Co₃O₄/C and Mn₃O₄/C and which rival Pt/C catalysts in alkaline fuel cells.^1-2 Co-Mn spinels achieved a benchmark peak power density of 1.2 W/cm² at 2.6 A/cm² in membrane electrode assembly (MEA) tests at an optimal loading of 80 wt.% metal oxide on carbon supports.¹
However, the origin of such electrocatalytic activity remains elusive, and necessitates the use of in situ/operando techniques to identify the catalytically active (and relevant) sites under real-time electrochemical conditions. Synchrotron-based in situ X-ray absorption spectroscopy (XAS) is a powerful nondestructive technique to study electrocatalytic mechanisms because it can provide atomic-level information on electrochemical reactions under operating conditions. The high penetration depth of high-energy X-rays, combined with a home-made electrochemical cell, have enabled the operando study of Co-Mn oxide catalysts.³
In situ X-ray absorption near-edge structure (XANES) was employed to track/monitor the oxidation state changes of Co and Mn, not only under steady state (constant applied potential), but also under non-steady state (potentiodynamic cyclic voltammetry) conditions. The periodic conversion between Mn(III,IV), Co(III) and Mn(II,III), Co(II) during the CV scans suggested that Co and Mn redox couples serve as co-active sites (co-catalyst) for the ORR. Rapid X-ray data acquisition, combined with a slow sweep rate in CV, enabled a 3 mV resolution in the applied potential and X-ray measurements, approaching a non-steady (potentiodynamic) state. Changes in the Co and Mn valence states were simultaneous and exhibited periodic patterns that tracked the cyclic potential sweeps. These observations strongly suggest a potential (and very likely) synergistic effect between Co and Mn, which may explain the superior activity of the Co_1.5Mn_1.5O₄/C electrocatalyst over the monometallic oxide counterparts.
Aiming for a practical Pt-free cathode for AEMFCs for automotive applications, non-precious ORR electrocatalysts need to not only fulfill the requirements of high initial ORR activity, but also address durability concerns. We designed a family of Mn-Co-Fe trimetallic oxides to effectively improve the durability of our first-generation Mn-Co catalysts.⁴ The periodic conversion between Mn(III, IV)/Co(III) and Mn(II, III)/Co(II) as well as the essentially constant oxidation state of Fe during the CV suggested collaborative efforts among Mn, Co, and Fe. Mn and Co served as the synergistic coactive sites to catalyze the oxygen reduction, resulting in the observed high activity, while Fe worked to maintain the integrity of the spinel structure, likely contributing to the remarkable durability of the catalyst.
In summary, in situ XAS studies of electrocatalytic systems for the ORR in alkaline media have dramatically advanced our understanding of catalytic systems with various-valence multi-metallic active sites, in general, and for fuel cells, in particular.
References:
1. Y. Yang, et al. ACS Energy Lett. 2019, 4, 1251.
2. Y. Wang, Y. Yang, et al. Nat. Commun. 2019, 10, 1506.
3. Y. Yang, et al. J. Am. Chem. Soc. 2019, 141, 1463.
4. Y. Xiong, Y. Yang, (co-first) et al. J. Am. Chem. Soc. 2019, 141, 4412.

Polymer electrolyte membrane fuel cell (PEMFC) is considered as a promising energy source due to high energy efficiency and environment friendly characteristics. However, its commercialization is hampered by platinum catalysts requiring high cost and having low stability originating from an aggregation and a dissolution. To prevent particle agglomeration and dissolution, some researchers have introduced shells on platinum particles. For example, Chung et al. [“Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction” J. Am. Chem. Soc. 2015, 137, 15478−15485] reported PtFe electro-catalysts coated by carbon shell by adopting polydopamine layer and a carbonization. The carbon shell hinders the aggregation of alloy particles during the pyrolysis step carried out to transform the crystal structure of PtFe from fcc to fct. In addition, this shell protects platinum from dissolution during electrochemical reaction, thus enhances the durability at both half cell and unit cell tests. Chen et al. [“Nanostructured Polyaniline-Decorated Pt/C@PANI Core −Shell Catalyst with Enhanced Durability and Activity” J. Am. Chem. Soc. 2012, 134, 13252−13255] reported that the polymer coated commercial Pt / C had better durability than the uncoated Pt/C. They coated Pt/C with polyaniline and confirmed the carbon shell using a transmission electron microscope. PANI coated Pt/C exhibits improved activity and stability in half-cell test.

In this study, we are going to propose a new method for synthesizing platinum electro-catalyst encapsulated with a carbon shell and prove its improved durability in unit cell performance test. Typically, the carbon shell can be prepared by heating the polymer or carbon source coated on metal particle. However, this method requires extra processes which are generally difficult and tricky. Typical methods for synthesizing platinum core/carbon shell catalyst are comprised of 1) platinum reduction (electrochemical catalyst synthesis), 2) polymer or carbon source coating and 3) pyrolysis (carbon shell formation). In contrast, we have simplified the process to two steps of 1) synthesis of platinum and carbon source complexes and 2) pyrolysis (platinum reduction + carbon shell formation). Platinum-aniline complex made the reduction of synthesis step possible. The structural characteristics were investigated through XRD and HR-TEM and it was revealed that the Pt particles are encapsulated by 2~3 layers of graphitic carbon shells. From the Pt particle size of 5 nm, it could be inferred that aniline prevents the aggregation of Pt while it is decomposing to carbon.
In order to verify that carbon shell is perfectly covering Pt particle and this could contribute to the enhancement of durability of Pt catalyst, as-synthesized catalysts were electrochemically studied following the AST protocol suggested by DOE in 2016. The catalyst showed a similar activity but an enhanced stability in half cell test compared to the result obtained with a commercial Pt/C. Unit cell test had proved the real value of carbon shell. No loss in current density at 0.6 V was observed after 30 K AST cycles carried out following DOE 2016. The analysis of MEA performed after AST revealed that carbon shell still encapsulates the Pt particle and no aggregation and dissolution happen.

Polymer electrolyte membrane (PEM) fuel cells are promising power systems for future automotive applications, due to their low emissions and high efficiency. However, the lifetime of PEM fuel cells can be strongly affected by their various operation modes and transitional states. One such case is start-up/shutdown cycles, during which the air/hydrogen gas boundary will form in the anode gas flow channel and elevate the catalyst-electrolyte interfacial potential difference in the cathode. This increased interfacial potential causes carbon corrosion and Pt dissolution inside the cathode catalyst layer, leading to a loss of active catalyst area and performance degradation. One mitigation strategy is to use a selective catalyst at the anode that inhibits the anode half-cell reaction, which in turn reduces the corresponding corrosion current at the cathode. In this case, the selective anode uses a catalyst with high hydrogen oxidation reaction (HOR) activity and low oxygen reduction reaction (ORR) activity.
In this current work, we investigate the effectiveness of different selective anode catalysts, including iridium (Ir) and platinum (Pt) alloys of alloy composition on the PEM fuel cell’s start-up/shutdown durability. Results show that the anode with Ir on carbon (Ir/C) catalyst has the highest start-up/shutdown durability, with less than 20% of electrochemically active surface area (ECSA) lost after 1100 start-up/shutdown gas purging cycles. In comparison, conventional Pt/C anode causes more than 60% of ECSA loss after the same amount of cycles. Among Pt_xIr_y alloy catalysts, the Ir₃Pt₁/C shows the highest improvement on the start-up/shutdown durability, but not comparable with the Ir/C catalyst. Differences in the HOR and ORR activity of these anode catalysts are compared, and the degradation patterns are analyzed for better understanding of cause of the differences in start-up/shutdown durability.

X-ray total scattering was teamed with Raman and X-ray spectroscopy and related tools to probe the atomic-scale defects and chemomechanical response of MnO₂ nanosheet assemblies. The data reveal a direct link between surface Mn3+ defects and charge storage, where intentionally introducing ~25% Mn3+ defects increases the gravimetric capacitance by a factor of 3. An operando measurement cell was developed for X-ray PDF and XAS studies, and new PDF modeling approaches were developed that yield accurate quantification of the interatomic spacings and defect content vs. charge state. The data demonstrate that the nanosheets breathe in 2-D; in other words, the interlayer spacing between nanosheets remains invariant while the nanosheets expand and contract in the plane of the nanosheets by as much as 1% during charging. X-ray spectroscopy provides the Mn oxidation state which completes the picture of Mn redox. Overall, the study demonstrates that adding ~25% Mn3+ defects increases charge storage by 3X with improvement in charge transfer resistance and dramatically improved cycle stability.

Ongoing development of proton exchange membrane (PEM) fuel cells focuses around the durability and the precious metal content of the membrane electrode assembly (MEA) of the fuel cell stack [1, 2]. The loss of the electrochemical surface area (ECSA) of the platinum (Pt) catalyst in the cathode electrode, particularly induced by voltage cycling, is a very important factor governing the performance degradation of the MEA [3, 4]. Thus, understanding the performance loss mechanism of voltage cycling is crucial for the successful commercialization of the PEM fuel cell for automotive applications

We present a physics-based Pt catalyst degradation model that predicts ECSA loss via Pt particle coarsening and the formation of electronically disconnected Pt particles in the ionomer phase. The continuum electrochemical model includes: dissolution of Pt and subsequent electrochemical deposition on Pt nanoparticles; platinum-oxide coverage that forms a passivating layer, decreasing dissolution of the metal underneath; diffusion of Pt ions in the membrane electrode assembly; and Pt ion chemical reduction in membrane by hydrogen permeating through the membrane from the anode electrode [5-8]. Coarsening of Pt nanoparticles follows the mechanism of source-limited Oswald ripening, and it is investigated by tracking the evolution of the particle size distribution upon cycling. In the model, the Gibbs-Thompson effect drives the redistribution of mass from small to large particles, therefore decreasing the Pt active surface area. Diffusion of dissolved Pt at the micrometer scale leads to permanent mass loss and the formation of Pt bands in the membrane.

Comparing with literature experimental data [3, 4], the analysis provides further understanding of catalyst degradation mechanisms under voltage cycling and accelerated stress test conditions.

References:
[1] B. Groger, H.A. Gasteiger, and J.P. Suchsland, J. Electrochem. Soc. 162(14), A2605 (2015).
[2] Department of Energy; Fuel Cell Technologies Office Multi-Year Research, Development, and Demonstration Plan (2011-2020).
[3] P. Zihrul, I. Hartung, S. Kirsch, G. Huebner, F. Hasche, and H. A. Gasteiger, J. Electrochem. Soc. 163, F492 (2016).
[4] G. S. Harzer, J. N. Schwämmlein, A. M. Damjanović, S. Ghosh, and H. A. Gasteiger, J. Electrochem. Soc. 165(6), F3118 (2018).
[5] R.M. Darling, J.P. Meyers, J. Electrochem. Soc. 150, A1523 (2003).
[6] R.M. Darling, J.P. Meyers, J. Electrochem. Soc. 152, A242 (2005).
[7] W. Bi, T.F. Fuller, Journal of Power Sources 178, 188 (2008).
[8] E. F. Holby, W. Sheng, Y. Shao-Horn and D. Morgan, Energy Environ. Sci. 2, 865 (2009).