Symposium ET03 : Application of Nanoscale Phenomena and Materials to Practical Electrochemical Energy Storage and Conversion

Fuel produced from the electrochemical splitting of water can be used to power a wide variety of technologies including information communication technologies infrastructure. The slow kinetics of the oxygen evolution reaction (OER) is one of the performance-limiting factors for hydrogen production through electrolysis. OER catalysts are often unstable in alkaline environments exhibiting deactivation and structural transformation causing a significant challenge for use in photoeletrochemical cells and water electrolyzers. Moreover, in nanoscopically thin catalyst layers, OER activity also decreases due to inefficient charge transfer to the electrolyte-catalyst interface. Epitaxial heterostructures are promising to solve these issues, though recent attempts yielded improved stability only at the expense of greatly reduced OER activity. In this presentation, we elucidate the competing factors for deactivation of La_xSr_1-xCoO₃ (LSCO) in nanoscopically thin layers supported on conducting perovskite substrates, and demonstrate heterostructured anodes with simultaneously high activity and stability during electrochemical water splitting in alkaline environments (pH = 13).
Epitaxial thin films of La_1-xSr_xCoO₃, Ba_1-xLa_xSnO₃, and Sr_1-xLa_xTiO₃ were grown by pulsed laser epitaxy. Interface energetics were characterized using in situ X-ray and ultraviolet photoelectron spectroscopies. Scanning transmission electron microscopy and electron energy loss spectroscopy were used to resolve the atomic structures, and scanning nonlinear dielectric microscopy used to probe the nature of the charge carrier character on the heterostructured catalyst surface. Density functional theory calculations were used to assess the impact of the electronic structure of the heterostructured catalyst layers on the overpotential and OER catalytic activity.
While the LSCO undergoes dramatic structural and electronic changes during electrolysis, including leaching of La and Sr from the film to yield a layer of cobalt oxyhydroxide, the thickness dependence of the OER activity will be revealed to be due to inefficiency of charge carrier transport to active sites. We demonstrate engineering of depletion layers widths and chemical stability using heterostructures comprised of nanoscopically thin epitaxial layers of degenerately doped stannate and titanate perovskite structure oxides to yield low overpotentials ~ 300 mV at current densities (~10 mA/cm²) relevant for hydrogen production in electrolyzers and photo-electrochemical cells, at hundreds of hours operations in nanoscopically thin active layers. Implications of the results for applications of nanoscopically thin oxide heterostructures for designs of high activity and stable anodes for carbon neutral energy production via the electrochemical splitting of water will be discussed.

Acknowledgement
C.H. would like to thank the Japan Trust program of the National Institute of Information and Communication Technologies (NICT) for funding.

The development of OER catalysts for PEM water electrolysis requires both activity and stability testing methods. The OER catalyst activity can be estimated by using rotating disk electrode (RDE), flow-channel methods, or in an electrolyzer. The evaluation of catalyst stability is realized using accelerated tests as testing over the whole lifetime (5-10 years) under realistic conditions is not practical.^{1, 2} A protocol for the OER catalyst stability using RDE was proposed by the JCAP group and is now used by other researchers. In this protocol, a constant current is applied in a half-cell configuration and the change in potential is monitored until a sudden increase in potential indicates complete catalyst degradation.³ It was shown recently that the measured catalyst life-time depends on the nature of the electrode substrate onto which the catalyst powder is being supported. Based on this, it was recommended that Au and boron-doped diamond be used as they show better stability of the catalyst under investigation, while glassy carbon and fluorine doped tin oxide electrodes were deemed unsuitable in such stability test (St. T.).²
Here we present a careful examination of the use of RDE for investigating OER catalyst stability. Although the increase in potential in an RDE St. T. is thought to be due to catalyst degradation (dissolution), our findings provide a clear evidence that the change in potential is rather due to an experimental artifact. The source of this artifact are nano- and micro-bubbles formed within the pores of the catalyst layer during the OER that cannot be removed by electrode rotation. These bubbles accumulate and block the OER active sites, resulting in a potential increase, which is mistakenly interpreted as catalyst degradation. Our findings indicate that almost no catalyst degradation takes place at the first phase of the St. T., which can last for several hours. In this phase, the bubbles accumulate and block the active sites, resulting in an artificial increase in the potential. The second phase of the St. T. starts once a threshold potential is realized, where a rapid potential increase is observed, due to catalyst dissolution at high potentials. Gas bubbles accumulation is responsible for the increase in potential, and ultimately resulting in the full degradation of the catalyst layer. A proper St. T. using RDE technique should avoid the accumulation of oxygen bubbles, which is currently under investigation and preliminary results will be also presented.

1. H.-S. Oh, H. N. Nong, T. Reier, A. Bergmann, M. Gliech, J. Ferreira de Araújo, E. Willinger, R. Schlögl, D. Teschner and P. Strasser, J. Am. Chem. Soc., 138(38), 12552–12563 (2016).
2. S. Geiger, O. Kasian, A. M. Mingers, S. S. Nicley, K. Haenen, K. J. J. Mayrhofer and S. Cherevko, ChemSusChem, 41, 15 (2017).
3. C. C. L. McCrory, S. Jung, J. C. Peters and T. F. Jaramillo, Journal of the American Chemical Society, 135(45), 16977–16987 (2013).

Modern societies face the challenge of supplying a continuously increasing energy demand. Intensive research is being conducted all around the globe to ensure that these needs can be satisfied while minimizing carbon emissions and other environmental threats. Electrochemistry holds promise to satisfy this requirement, either by using renewable energy to obtain fuels or store electricity or by converting fuels into electricity. Here catalysis research is pivotal, providing key advances in electrolyzers, fuel cells, and metal-air batteries.

Nature Catalysis, a new journal from the Nature Research group launched in January 2018, provides coverage of top research from the area of electrocatalysis, as well as all other areas of catalysis. Our broad scope, drawing from the work of scientists, engineers and researchers in industry and academia, ensures that published work reaches the widest possible audience. Nature Catalysis brings together researchers from across all chemistry and related fields, publishing work on homogeneous, heterogeneous, and biocatalysis, incorporating both fundamental and applied studies that advance our knowledge and inform the development of sustainable industries and processes.

The need for a sustainable source of hydrogen has been widely recognized, not just as a potential transportation fueling vehicles, but to limit CO₂ production and fossil fuel consumption from existing industrial processes such as ammonia generation. Currently over 95% of hydrogen is made from fossil fuels through natural gas reforming or coal gasification. However, significant growth has occurred in recent years in water electrolysis research, especially in catalyst research for the hydrogen (HER) and oxygen (OER) evolution reactions. Proton exchange membrane (PEM)-based systems are relatively mature in that the technology has been commercialized, but further research and development can achieve significant impact; for example, order of magnitude reductions in catalyst loading. Anion exchange membranes based systems are still under development, with membrane and ionomer stability in the operating environment being a critical issue. In both the AEM and PEM case, there are complex interactions at the electrode level which need to be considered in catalyst and membrane development. First, the liquid electrolyte environment used for catalyst activity screening, where all of the catalyst surface is accessible to the reactant is often not comparable to a complex, 3-dimensional, ionomer-based electrode. Also, similar to automotive fuel cells, the operating environment is highly important and should be considered when claiming improvements over state of the art. For example, catalyst performance at very low current densities may indicate inherent activity but may not represent capability at typical device operating currents. Similarly, a membrane which cannot operate at differential pressure may be highly limited in utility even if more efficient than current commercial solutions. This talk will describe some of the complex interactions that need to be considered, typical operating requirements, and stages of development where relevant conditions should be introduced.

The catalytic oxygen evolution reaction (OER) to extract electrons from water molecules is important for the artificial photosynthesis to generate useful chemicals such as hydrogen and organic compounds [1, 2]. In terms of elements strategy, utilization of abundant element for OER catalysts is remarkably advantageous for future low-costly artificial photosynthetic system. Fe-based OER catalysts, based on the fourth most earth-abundant element, are attractive, but are known to suffer from low OER activity due to limited electrical conductivity and non-ideal electronic structures near the surfaces of these catalysts.
Here, we report a highly crystalline, 10 nm-sized red rust OER catalyst composed of pure β-phase FeOOH(Cl) nanorods (ca. 3 × 13 nm) doped with Ni ions (β-FeOOH(Cl):Ni) [3] and surface-modified with amorphous Ni(OH)₂ (a-Ni(OH)₂, at a Ni to Fe ratio of 22 at.%), which shows the highest level of performance among Fe-rich oxides and oxyhydroxides. This catalyst can be synthesized by a facile one-pot process at room temperature, and colloidal aqueous solutions of the β-FeOOH(Cl)Ni/a-Ni(OH)₂ nanorods are very stable, with no apparent precipitation over a time span of at least one month.
Electrochemical measurements for β-FeOOH:Ni/a-Ni(OH)₂ stacked nanorod anodes deposited on carbon paper (CP) were performed in a 3-electrode configuration using a Ag/AgCl reference electrode and a Pt wire counter electrode. The overpotential during the electrochemical OER over the anodes was 170 mV, and an OER current of 10 mA/cm² was obtained at an overpotential of 430 mV(+1.66 V vs. RHE) in 0.1 M KOH (without subtracting the iR drop). It is suggested that the surface modification with the a-Ni(OH)₂ lowered the OER overpotential of β-FeOOH(Cl):Ni, resulting in the very high current density at low potential compared with Fe-rich oxide and oxyhydroxide electrodes reported previously. Mössbauer spectroscopy also suggested electronic interaction between Fe and Ni species, which may be crucial evidence for the enhanced activity in the Fe-rich OER system [4].
The present cost-effective Fe-based OER catalysts can be widely applied to construct artificial photosynthetic systems for solar fuel generation by combination with CO₂ reduction catalysts.

References
[1] T. R. Cook, et al., Chem. Rev., 110 (2010) 6474. [2] C. C. L. McCrory, et al., J. Am. Chem. Soc., 136 (2013) 16977. [3] T. M. Suzuki, T. Morikawa, et al., Sustainable Energy Fuels, 1 (2017) 636. [4] T. M. Suzuki, T. Morikawa, et al., Bull. Chem. Soc. Jpn., 91 (2018) 778.

Two-dimensional (2D) materials have attracted great interest in catalyzing electrochemical reactions such as water splitting, oxygen reduction, and carbon dioxide reduction. Quantum mechanical simulations have been extensively employed to study the catalytic mechanisms. However, these calculations typically assume that the catalyst has a zero/constant charge for computational simplicity, while in reality, the catalyst usually has a varying charge as the reaction proceeds due to the match between its Fermi level and the applied electrode potential. These contradictions urge an evaluation of the charge effects.
Here using grand canonical density functional theory calculations, we show that the charge on 2D materials can have a much stronger impact on the electrochemical reaction than the charge on 3D metals, which arises from the unique electronic properties of 2D materials. Our work calls for reconsideration of some of the previously suggested electrocatalytic mechanisms of 2D materials by incorporating the charge effects. [1]
[1] Donghoon Kim, Jianjian Shi, Yuanyue Liu, submitted

One of the grand challenges facing humanity today is the development of an alternative energy system that is safe, clean, and sustainable and where combustion of fossil fuels no longer dominates. A distributed renewable electrochemical energy and mobility system (DREEMS) based on cheap renewable electricity could meet this challenge. At the foundation of this new energy system, we have chosen to study a number of electrochemical devices including fuel cells, electrolyzers, and flow batteries. We have been working on the development of hydroxide exchange membrane fuel cells (HEMFCs) and electrolyzers (HEMELs) which can work with nonprecious metal catalysts and inexpensive hydrocarbon polymer membranes. We have developed roadmaps for HEMFCs and HEMELs, the most chemically stable membranes, and the most active nonprecious metal catalysts. We have also studied why hydrogen oxidation and evolution reactions (HOR/HER) are slower in base than in acid for precious metal catalysts. For flow batteries we have developed novel designs, chemistries and cost models e.g., double-membrane aqueous flow batteries with high voltages (i.e., 3 V), single-element-mimic redox pairs, and user friendly physics-based analytical cost models. In this presentation, I will focus on our HEMEL work highlighting a new class of membranes, nonprecious metal catalysts and base/salt-free HEMEL cells for hydrogen production.

Replacing fossil fuels with renewable resources to meet the global need requires a technology that is scalable to the unprecedented level of several terawatts. Natural photosynthesis is the sole existing technology that produces energy dense chemicals on the terawatt scale (> 100 TW). Its key design feature is the closed cycle of H₂O oxidation and formation of the primary reduction products on the shortest possible length scale, the nanometer scale, while separating the incompatible redox environments by an ultrathin membrane. This offers the advantage of minimizing efficiency-degrading mass transport processes and unwanted side reactions.
To incorporate the key feature into artificial photosystems, we assembled ultrathin (2 nm), pinhole-free, molecular wires-embedded SiO₂ membrane on planar and nanotube constructs. This membrane system spatially separates the H₂O oxidation CO₂ reduction, but enables (photo-)electrochemical communication between the incompatible redox reactions by transmitting protons and electrons in a precisely controlled manner, while preventing O₂ transport causing unwanted reverse reactions. This unique mass-transport behavior on planar and nanotube configurations was systematically studied via cyclic voltammetry, electrochemical impedance spectroscopy, and visible light-sensitized short circuit current experiments. The embedded molecular wires’ integrity before and after the mass-transport process was confirmed by time-resolved optical spectroscopy and grazing angle ATR-FT-IR or IRRAS characterization.

Polymer-catalyst interfaces control the energy efficiency of many energy conversion and storage device. The interfacial polymer layers are very thin (typically less than one micron thick). Many interesting structural, mechanical and transport properties in such thin ion containing polymer (ionomer) layers evolve as a result of complex multimodal interfacial interactions, unusual hydration behavior and confinement. Especially ion conductivity at the interface can be drastically different from that in the bulk membranes and the route to this poor ion conduction behavior is not well-understood. It is thus highly needed to systematically study how the ion conduction environment and water uptake change with the change in ionomer structure and film thickness. In this work, three potential fluorocarbon based hydrogen fuel cell ionomers (Nafion, 3M PFIA, 3M PFSA) having single/multiple acids at side chain were studied in sub-micron thick films. All three ionomers have fluorocarbon (PTFE) backbones. The difference between Nafion and 3M PFSA is in the side chain structure, but both has single acid group at the side chains. On the other hand, 3M PFIA has bis(sulfonyl)imide group in addition to perfluorosulfonic acid which makes the polymer more acidic. By tracking the fluorescence response of photoacid dye HPTS incorporated within hydrated ionomer thin films, very interesting trends were obtained regarding the extent of proton transfer. The results, when combined with the information on nanoscale structure and water sorption, clearly indicated that there are many factors controlling the proton conduction behavior in thin ionomer films, in addition to water uptake.

Nanoscale transport and materials are critical to electrochemical energy storage, such as power density, cycling life and safety. In this talk, I will present two examples on advanced tools and fabrication to understand nanoscale transport phenomena and designing of nanoscale materials. The first one is based on an emerging Stimulated Raman Scattering Microscopy, which is three orders of magnitude faster than traditional Raman microscopy. Therefore it can clearly track ion transport in electrolyte together with lithium dendrite, to illustrate their correlations. A positive feedback mechanism has been visualized, which guide methods to suppress lithium dendrite. The second example is through designing nanoscale modification of interfaces between battery electrodes and solid electrolyte. Therefore, the stability between electrodes and electrolyte and the cycling life of corresponding full cells are significantly improved.

The design/discovery of layered materials for applicability in next-generation battery technologies requires a fundamental understanding of the links between the atomic-scale structure, chemistry and the mechanisms and energetics of intercalation and de-intercalation reactions, and a consideration of other solid-state reactions that might compete. The goal of our research is to design/discover layered material microstructures as alternatives to graphite using an innovative combination of atomic-scale modeling, experimental in-situ characterization of the microstructural evolution during (de)intercalation reactions. Density functional theory (DFT) simulations are carried out to investigate the structural accommodation of the layered material during insertion and exertion of the intercalating species (energy barriers, volumetric expansion, and phase transformations). The structural stability of the 2H and 1T phases of MoS₂ during lithiation suggests that a phase transformation of the 2H phase of MoS₂ to the 1T phase may occur when MoS₂ is reacted with Li; the computational study allows different dosages of Lithium ion to be assessed with the aim of testing these the validity of these models using in-situ characterization of the solid-state reactions between Li and MoS₂ in the transmission electron microscope (TEM). The mechanisms of strain relaxation and the energetics of Li intercalation-induced phase transformations in MoS₂ at the atomic scales will be presented. This work is supported by NSF grant No. 1820565.

Understanding the structure and phase changes associated with two-dimensional (2D) layered transition metal dichalcogenides (TMDs) is critical in optimizing performance in lithium-ion batteries. The large interlayer spacing in MoS₂ (∼0.65nm) accommodates species such as alkali metal ions (Li⁺/Na⁺/K⁺) during intercalation. Intercalation is reported to change the electronic structure of the host molecule, resulting in variations in their electrical and optical properties. In this work, we examine the solid-state reactions between Li and MoS₂. Li⁺ ions can be inserted into vdW gap; the reaction is still unclear. Plan-view imaging has been extensively used, however, it is essential to visualize the process with the electron beam being parallel to the basal planes of the layer material to understand the reaction process. Lattice-fringe images have been discussed for several systems but relying on microtoming or simply using curved thin layers, the orientation of the specimen was less than ideally uncontrolled. Here, TEM specimens are made using FIB, and oriented for detailed study of the intercalation process. This study of TMDs uses a Tecnai F30 and a Cs/image-corrected Titan equipped with a direct electron detector camera, K2. This camera has two major advantages: the electron dose can be minimized and quick changes during reactions are recorded; both instruments have EELS and XEDS capabilities. DFT calculations are used to probe the structure and bonding changes during these reactions. Volumetric expansion, energy barriers, phase transformations and the role of doping, defects and interfaces can be modeled. The dynamics of the structural response are modeled using ab initio MD simulations. Electrochemical aspects can be monitored in situ in real-time and at atomic scale to provide understanding of lithium-ion storage mechanisms in these solid-state reactions and thus to test the modeling-based results.
In plan-view specimen, variations normal to the basal plane are not seen. Defects associated with the reactions were monitored real-time. As the reaction between MoS₂ and Li proceeds, white-line defects were observed under high-resolution imaging by TEM. Lower-magnification images show that the defects are not equally spaced and do not correspond to ‘stage’ development. These defects can cross several basal planes in the MoS₂ (either forwards or backwards) but maintain essentially the same width after the step; they are not completely constrained to the vdW gap.
This work is funded by NSF grant No. 1820565. MTJ is at LANL. TEM is at CINT, an Office of Science User Facility operated for the U.S. DOE. Sandia NL is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s NNSA under contract DE-NA-0003525. The views expressed in the abstract do not necessarily represent the views of the U.S. DOE or the U.S. Government.

Electrochemical processes at electrode/electrolyte interfaces (e.g. electric double layer charging, heterogeneous charge transfer and surface binding of reaction species on the electrode) are of vital importance to energy conversion and storage systems including batteries, supercapacitors and electrocatalytic production of fuels. It has been widely acknowledged that the kinetics of the interfacial electrochemical processes are largely determined by the electronic structure (e.g. density of states and electronic occupation) at the electrode/electrolyte interface. We have developed a back-gated electrode structure that utilizes electrostatic charging (induced by a gate bias) to control electrochemical kinetics on ultrathin or 2D materials (5-nm-thick ZnO, monolayer MoS₂ and graphene).^1,2 Such back-gated electrodes are fabricated with nanometer-thick semiconductors on SiO₂/degenerate Si substrates, analogous to the metal–oxide–semiconductor stack in the CMOS technology. Due to the extreme thinness of the electrode materials, the alignment of electronic bands as well as the electronic occupation, at the electrode/electrolyte interface, can be dramatically altered by the gate-induced charge carriers. Thus, real-time, continuous and efficient modulation of reaction kinetics can be achieved on 2D materials by varying the gate bias.
In this presentation, we will use back-gated monolayer MoS₂ as an example to demonstrate how the applied gate bias affects the kinetics of heterogeneous charge transfer and surface binding processes. Specifically, the standard charge transfer rate constant between MoS₂ and ferrocene/ferrocenium redox couple can be tuned by over two orders of magnitude and the catalytic overpotential of hydrogen evolution reaction on 2H-MoS₂ can be reduced by more than 150 mV. Overall, the approach introduced here is generally applicable to investigation and optimization of interfacial electrochemical phenomena in a wide range of electrochemical systems. With the ability to control the band alignment and electronic occupation independent of the electrode potential, the back-gated 2D electrodes will provide new insights to rational design of electrode materials.


(1) Kim, C.-H.; Frisbie, C. D. Field Effect Modulation of Outer-Sphere Electrochemistry at Back-Gated, Ultrathin ZnO Electrodes. J. Am. Chem. Soc. 2016, 138 (23), 7220–7223.
(2) Wang, Y.; Kim, C.-H.; Yoo, Y.; Johns, J. E.; Frisbie, C. D. Field Effect Modulation of Heterogeneous Charge Transfer Kinetics at Back-Gated Two-Dimensional MoS₂ Electrodes. Nano Lett. 2017, 17 (12), 7586–7592.

The performance of electrochemical energy materials depends crucially on the underlying nanoscale processes. The charge-discharge cycles of batteries result in gradual changes in nanoscale structure and chemistry of the different electrode layers with often detrimental consequences for the electrochemical properties [1]. To understand the nanoscale mechanisms causing the degradation of the battery materials and to develop strategies to counteract, high resolution imaging and analysis techniques are indispensable. While high-resolution Transmission Electron Microscopy (TEM) enables imaging of the nanostructures down to atomic resolution, analysis of light elements (Z < 6) and low concentrations (< 0.1 at. %) are difficult using typing analytical tools in a TEM such as Energy Dispersive X-ray Spectroscopy. In comparison, Secondary Ion Mass Spectrometry (SIMS) has an excellent sensitivity (can be as low as ppm range) and all the elements (including isotopes) of the periodic table can be analysed. However, the SIMS image resolution is limited to ~ 50 nm in most commercial SIMS instruments (except some new developments [2] where resolution < 20 nm has been demonstrated). Nevertheless, the resolution is still more than 2 orders of magnitude poorer than TEM imaging. To complement the strengths of TEM and SIMS in the same instrument, we developed an in-situ correlative microscopy technique combining TEM-SIMS [3, 4]. In this presentation, we will demonstrate the application of this new nanoscale characterization technique to elucidate the structural and chemical changes occurring in Li ion battery cathodes containing LiV₃O₈ thin film with different initial microstructures obtained by thermal annealing. Bright-Field TEM and corresponding SIMS images (e.g. Li⁺ and V⁺ maps) from uncycled and cycled samples were obtained to investigate the underlying materials phenomena (such as vanadium dissolution) in the cycled cathodes and to correlate the nanoscale processes with macroscopic electrochemical performance [5].
References:
[1] Q. Zhang et al, Journal of The Electrochemical Society, 164 (7) A1503-A1513 (2017)
[2] D. Dowsett, T. Wirtz, Anal. Chem. 89 (2017) 8957-8965
[3] T. Wirtz, P. Philipp, J.-N. Audinot, D. Dowsett, S. Eswara, Nanotechnology, Vol. 26, 434001, 2015.
[4] L. Yedra, S. Eswara, D. Dowsett, T. Wirtz, Sci. Rep. 6, 28705, 2016
[5] Acknowledgements: LVO cathode samples are synthesized as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under award #DE-SC0012673.

Development of a stable energy-storage device is a fundamental approach to solve energy-related issues. Lithium-ion batteries (LIBs) are one of the most promising candidates because of their high energy density and long cycle life. From electrochemical impedance spectroscopic measurements, the cell resistance of conventional LIB is dominated by charge transfer resistance at electrode/electrolyte interfaces. [1,2] Therefore, we investigate the charge transfer process, i.e. Li insertion/desorption process, at the interface between a graphite anode and 1 M LiPF₆EC electrolyte. The density functional theory (DFT) with effective screening medium (ESM) method [3] combined with the reference interaction site model (RISM), called ESM-RISM, is employed to simulate the Li insertion/desorption process. [4] In this method, the graphite surface (Li_xC₆slab and additional Li⁺) and liquid solution (1 M LiPF₆EC) are represented as quantum mechanical and implicit classical solvation, respectively. The energy landscapes of reaction are revealed under constant electron chemical potential conditions at the interface. Across the transition state where the Li forms a half solvation shell, the reacting Li inside the electrode changes to a full solvation structure in the solution accompanied by electron transfer. The activation energies at the equilibrium potentials of the charge transfer reaction are approximately 0.6 eV, [5] which is consistent with the electrochemical impedance spectroscopy measurements. In the presentation, we explain the details of the ESM-RISM simulation and introduce the energy profiles of the Li insertion/desorption path at the LiC₆/EC LiPF₆interface.

[1] T. Abe, H. Fukuda, Y. Iriyama, and Z. Ogumi, J. Electrochem. Soc. 151, A1120 (2004).
[2] K. Xu, A. von Cresce, and U. Lee, Langmuir 26, 11538 (2010).
[3] M. Otani and O. Sugino, Phys. Rev. B 73, 115407 (2006).
[4] S. Nishihara and M. Otani, Phys. Rev. B 96, 115429 (2017).
[5] J. Haruyama, T. Ikeshoji, and M. Otani, J. Phys. Chem. C 122, 9804 (2018).

Rapidly increasing demand for low-cost, high energy density energy storage motivates researchers to develop advanced and reliable anode materials. Lithium alloying anodes such as Si, Sn, or Ge has three to ten times of charge capacity compared to the traditional graphite electrodes, and thus gathered enormous research interest. One of the major challenges associated with the lithium alloying anodes originates from de/lithiation-induced large volume change (~300%). Such volume change applies excessive cyclic strain on solid electrolyte interphase (SEI) to cause its mechanical failure and continued formation, resulting in poor cycle life. Several electrolyte additives such as fluoroethylene carbonate (FEC) or vinylene carbonate (VC) have been investigated and demonstrated to improve cyclic performance of Si electrodes. However, quantitative evaluation on influence of the additives on mechanical properties of SEI is still challenged.
With this background, we have developed an experimental approach to characterize elastic modulus, yield stress, inelastic deformation behavior, and crack density evolution of SEI formed with carbonate-based electrolytes. An SEI (~100nm) is prepared by lithium thin film - electrolyte (1.2M LiPF6 in ethylene carbonate) reactions on a rectangular free-standing polydimethylsyloxane (PDMS) membrane (~300 - 400nm in thickness). The prepared sample is subjected to bulge testing in an inert environment; various level of controlled pressure is applied to the SEI/PDMS membrane and the corresponding deflection is measured by the atomic force microscopy (AFM). The plane strain elastic modulus and the yield stress of SEI are evaluated from the pressure-deflection relation from the bulge testing. Moreover, a careful observation of SEI surface topography yields the evolution of crack density as a function of applied strain. The experiment is repeated using FEC added electrolytes to investigate the influence of the FEC additive on mechanical stability of SEI.

Multivalent ion batteries (MVIB), or those utilizing Mg, Ca, Zn, and Al, are garnering increasing attention as alternatives to Li-ion batteries in applications where portability is not an issue owing to their high energy density, cost efficiency, and abundance. However, the lack of suitable electrolytes allowing for the reversible plating of metallic anodes has limited the development of MVIBs, especially those involving Ca. This is primarily due to the fact that the solid-electrolyte interphase (SEI), a passivating layer which forms between the electrolyte and anode, often does not allow for the migration of ions in MVIBs. [1,2] In this work, we develop an understanding of the SEI in MVIB systems using a computational approach combining density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. [3] We first identify the principle components of the SEI by studying the decomposition of the solvents and salts comprising various electrolytes on Li, Ca, and Al surfaces using AIMD. Following this, we identify electrolytes which can be used with a Ca metal anode by investigating the diffusion of Ca ions through the likely inorganic compounds produced using DFT. Finally, we investigate the decomposition of these electrolytes in the presence of external electric fields to more fully understand these reactions in electrochemical systems. We anticipate the promising new electrolytes proposed in this work will help guide experimentalists in the development of rechargeable MVIBs.

J.Y. and M.S. were supported by funds from Binghamton University. DFT calculations were performed on the Spiedie cluster at Binghamton University, as well as the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF Grant No. ACI-1053575, under allocations TG-DMR170127 and TG-DMR180009.

[1] Ponrouch et al., Nat. Mater. 15 169 (2016)
[2] Wang et al., Nat. Mater. 17 16 (2017)
[3] J. Young and M. Smeu, J. Phys. Chem. Lett. 9 3295 (2018)

Room temperature ionic liquids (ILs) have recently emerged as highly promising electrolytes for a wide range of emerging energy technologies, including next-generation supercapacitors and ion-batteries, due to their high thermal stability, ionic conductivity and wide electrochemical windows. The chemical and structural diversity of ILs creates a vast design space that could be exploited to optimize the device performance and stability. However, many mechanistic details remain enigmatic, including the fundamental nature of the cation-anion interactions and their relevance in determining structural and electronic properties of the liquids. Having this detailed information for the bulk liquid is a prerequisite for eventually deciphering the complexity the arises at nanostructured electrode interfaces, which are ubiquitous among energy storage devices. In this presentation, we combine high-level first-principles simulations and synchrotron X-ray characterization experiments to unravel the key structural, chemical and electronic properties of several archetypal ILs comprised of imidazolium-based ILs. In particular, we utilize extensive ab initio molecular dynamics simulations to probe the local density distribution and medium-range order of the ILs, which can be directly compared and validated by X-ray scattering measurements. Soft and tender X-ray absorption spectroscopy at the K-edge of fluorine, phosphorus, and sulfur contained on the anion also complements the chemical and electronic picture from the simulations. Our integrated theoretical and experimental approach relates these structural and chemical signatures with the intrinsic cation-anion interactions, by considering ILs with anions having significant differences in the molecular geometry, chemical composition, and charge distribution.

This work was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.