Symposium EN09—Advances in Conversion Electrodes for Reliable Electrochemical Energy Storage

In 2000, during his noble prize lecture Prof. Herbert Kroemer stated ``The interface is the device''[1]; he was referring to the phenomenal success in design and application of semiconductor heterojunction devices in microelectronics. Arguably, Kroemer's statement has an ever-increasing relevance across a range of technologies far beyond transistors, where heterojunctions found their initial success. Researchers are increasingly finding that interfaces between materials represent a rich space for the exploration of exotic properties that are not present in bulk materials, such as two-dimensional electron gases (or liquids) and quantum topological states. It is clear that the importance of the interface can only grow with the evolution of modern technological applications. Simultaneously, progress in computational materials science in describing complex interfaces is critical for improving the understanding and performance of energy materials, including rechargeable batteries and catalysts [2].
In this talk, I will elucidate how first-principles calculations, based on density functional theory, can be applied to large pools of materials to rationalize their behaviors when forming functional interfaces [3–7]. Focus will be given to topical materials in rechargeable batteries, where controlling interfaces is paramount to curb degradation phenomena [3–7].
[1] H. Kroemer, “Nobel Lecture: Quasielectric fields 1065 and band offsets: teaching electrons new tricks,” Rev. Mod. Phys. 73, 783–793 (2001).
[2] K. T. Butler, G. Sai Gautam and P. Canepa, Designing Interfaces In Energy Materials Applications With First-Principles, npj Computational Materials (2019).
[3] P. Canepa, G. Sai Gautam, D. C. Hannah, R. Malik, M. Liu, K. G Gallagher, K. A Persson and G. Ceder, Chem. Rev., 117, 4287–4341(2017)
[4] P. Canepa et al, Particle Morphology and Lithium Segregation to Surfaces of the Li7La3Zr2O12 Solid Electrolyte, Chem. Mater., 30 (9) 3019–3027 (2018).
[5] T. Chen, G. Ceder, G. Sai Gautam and P. Canepa, Evaluation of Mg compounds as coating materials in Mg batteries, Front. Chem. (2019) 10.3389/fchem.2019.00024
[6] T. Chen, G. Sai Gautam and P. Canepa, Ionic transport in Potential Coating Materials, Chem. Mater., 31 (19) 8087-8099 (2019).
[7] Z. Deng, G. Sai Gautam, S. K. Kolli, J.-N. Chotard, A. K. Cheetham, C. Masquelier and P. Canepa, Phase Behavior in Rhombohedral NaSiCON Electrolytes and Electrodes, 32 (18) 7908-7920 (2020).

To significantly boost the energy of the state-of-art lithium ion (Li-ion) batteries, one of the most effective approaches is to replace graphite anode with Li metal which is ultralight but energy concentrated. However, its thermodynamically instable nature in liquid electrolytes cuases many well-known problems such as dendrite formation which plagues the implementation of the proposed technology. Although many approaches have been proposed to rescue Li metal anodes, most of the work are performed in small-scale coin cells and tested in the conditions drastically different from the reality. A full knowledge of Li metal activities at the cell level is lacking but extremely critical for the success of developing next-generation rechargeable Li metal batteries. This talk will discuss the fundamental challenges of utilizing Li metal anode in at cell-level. A Li metal prototype pouch cell with 350 Wh/kg energy with more than 350 cycles will be demonstrated. The key fundamentals that enable the long-term cycling of Li metal anodes in pouch cells are discussed and the root causes of the poor cycling of realistic Li metal pouch cells have been revisited. A series of fundamentally new insights have been provided to inspire scientific innovations to tackle the real challenges of developing next-generation battery technologies.

Conversion cathode materials have traditionally been paired with lithium metal for use in non-aqueous primary batteries to enable high energy density. Due to the demand for high energy density rechargeable batteries, there is great interest in replacing conventional lithium insertion materials currently used in lithium-ion batteries with conversion cathodes and lithium metal anodes. Essentially, these batteries are similar to lithium primary batteries but with the added complication of recharge. Rechargeability is extremely challenging due to the need to chemically convert species with large volume expansion and multiple discharge products back to their original forms during charge. In conjunction, these already challenging battery systems also rely on repeatable and efficient electroplating and stripping of lithium metal. Despite decades of research, efficient and reliable lithium-metal anode cycling remains a major challenge in commercializing rechargeable batteries based on conversion chemistries.

Interfacial stack pressure in cells consisting of conversion cathodes and lithium-metal anodes is a critical parameter to tune and control. The large volume changes associated with cycling high capacity conversion cathodes and lithium-metal anodes will likely cause significant pressure changes in cells during cycling, unlike the small volume changes typically observed for intercalation electrodes. Furthermore, our previous studies suggest that the absence of interfacial compression greatly impacts lithium-metal anode morphology and Coulombic efficiency during cycling.¹ In an effort to understand these impacts more systematically, we fabricated lithium versus copper pouch cells and subjected them to varied applied interfacial compression.

We cycled pouch cells at high and low current densities in 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane (4 M LiFSI in DME) for 50 cycles at a variety of pressures ranging from 0-10 MPa. This very promising electrolyte was used because it has been shown to enable lithium-metal anode cycling with high Coulombic efficiency, a dense morphology, and a compact solid electrolyte interphase.² We disassembled pouch cells after the first and 51^st plating cycles to understand morphology of the lithium deposits using cryo focused ion beam and cryo electron microscopy techniques. We confirm through these experiments that interfacial compression plays an important role in lithium-metal anode plating and stripping behavior. Lithium morphology and Coulombic efficiency can be improved through tuning the applied interfacial compression.

Supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission 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. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.

1. Harrison, Katharine L., Kevin R. Zavadil, Nathan T. Hahn, Xiangbo Meng, Jeffrey W. Elam, Andrew Leenheer, Ji-Guang Zhang, and Katherine L. Jungjohann. "Lithium self-discharge and its prevention: direct visualization through in situ electrochemical scanning transmission electron microscopy." ACS nano 11, no. 11 (2017): 11194-11205.
2. Qian, Jiangfeng, Wesley A. Henderson, Wu Xu, Priyanka Bhattacharya, Mark Engelhard, Oleg Borodin, and Ji-Guang Zhang. "High rate and stable cycling of lithium metal anode." Nature communications 6, no. 1 (2015): 1-9.

High-power electrochemical energy storage devices are important components for various industrial applications, ranging from individual electronics to grid storage. The demand for power and energy supply is equally imperative in actual use and is keen to expand in the future. Thus, it is highly desirable to design new electrochemical batteries and supercapacitors to mitigate the trade-off between power density and energy density. On the other hand, as growing requirements for intelligent electronic devices and internet-of-things, extensive attentions have been attracted to functional (particularly, smart and stimuli-responsive) energy storage devices, which are rapidly responsive to the variations of devices or the external environment, e.g., configuration, voltage, deformation, light, and temperature, etc. Meanwhile, the portable, implantable, and wearable electronics are advancing toward miniaturization as well as ultralight, and safe, long-term, and high-speed operation, thus stimulating the urgent pursuit for miniaturized energy storage devices.
In this lecture, we will present our recent efforts in developing functional graphene and 2D materials for high-power energy storage devices, especially for the flexible/micro-supercapacitors with smart functions. Electrochromism, thermo-response, and photo-response can be integrated into such devices which provide the possible means to monitor the electrochemical process using external stimulus, thus opening up windows for realizing the power systems for intelligent electronic devices. Towards realizing high-power electrochemical batteries, we will discuss our recent progress in the development of dual-ion energy storage devices, which involve different charge storage chemistry in contrast to the conventional “rocking-chair” mechanism.

Metallic zinc as a rechargeable anode material for aqueous batteries has gained tremendous attention with merits of intrinsic safety, low cost, and high theoretical volumetric capacity (5,854 mAh/cm3). Among zinc-based batteries, Zn-air batteries are promising with the highest theoretical volumetric energy density (4,931 Wh/L). Rechargeable zinc anode has recently achieved progress in neutral electrolytes, yet developed slowly in alkaline electrolytes, which are kinetically favorable for air cathodes. Passivation, dissolution, and hydrogen evolution are three main reasons for irreversibility of zinc anodes in alkaline electrolytes, which limits the rechargeability and usable energy density. In this talk, I will present our recent works on using nanoscale material design to overcome passivation, dissolution, and hydrogen evolution issues of zinc anode, towards a deeply rechargeable zinc-based battery. I will also introduce the battery-gas chromatography quantitative analysis, as well as in situ microscopy methodologies we have developed, to quantify gas evolution side reaction, as well as visualize the reaction on electrodes during operation.

Recently, aqueous Zinc ion batteries have caught research attention due to intrinsic safety, low cost, and high theoretical volumetric capacity. However, insufficient utilization of Zn ions from anode can result in an inferior battery performance and limit practical application. Using 3D printed structures, the active surface area of Zn anode can be increased efficiently utilizing the Zn ions from anodes for battery reactions. Here, we develop a 3D printable zinc-carbon composite anode using direct ink writing (DIW) method to obtain porous anode structure with an increased surface area for high performance Zn ion batteries. Porous anode structure exhibits an increased electrochemical performance as compared to commercial Zn plate anodes resulting in higher specific capacity. By varying filament distancing and path, we can 3D print the zinc anode with different active surface areas, area to volume ratio, and porosity. Carbon in the composite improved conductivity, and mechanical stability of 3D printed zinc anode. Our 3D printed composite anodes allows flexible designing of batteries surpassing conventional battery designs such as coin cells or pouch cells and can be used to design printed energy storage systems.

The rise of flexible electronics calls for cost-effective and scalable batteries with good mechanical and electrochemical performance. In this presentation, we report on our development of printable, polymer-based AgO-Zn batteries that feature flexibility, rechargeability, high areal capacity, and low impedance. Using elastomeric substrate and binders, the current collectors, electrodes, and separators can be easily screen-printed layer-by-layer and vacuum-sealed in a stacked configuration. The batteries are customizable in sizes and capacities, with the highest obtained areal capacity of 54 mAh/cm2 for primary applications. Advanced micro-CT and EIS were used to characterize the battery, whose mechanical stability was tested with repeated twisting and bending. The batteries were used to power a flexible E-ink display system that requires a high-current drain and exhibited superior performance than commercial coin-cell batteries. The developed battery presents a practical solution for powering a wide range of electronics and holds major implications for the future development of practical and high-performance flexible batteries.

For energy storage to become ubiquitous in the grid, safe, reliable low-cost electrochemical storage technologies that can be manufactured at high volumes with low capital expenditures are needed. Rechargeable alkaline batteries based on the use of a Zinc conversion anode are well suited due to Zn’s high capacity (820 mAh g^-1), elemental abundance and established materials supply chain resulting in low production costs. Alkaline-based cells are also inherently safe and do not have the temperature limitations of Li-ion or Pb-acid batteries, thereby removing the need for complicated thermal management control strategies, and providing for simpler systems with lower integration costs. To realize the highest energy density batteries Zn needs to be coupled with a similarly low cost, abundant and high capacity conversion electrode. Historically Zn/MnO₂, Zn/CuO and Zn/S are primary battery chemistries; however, MnO₂ (616 mAh g^-1), CuO (674 mAh g^-1) or S (1675 mAh g^-1) conversion cathodes are enticing candidates and if a reversible battery can be proven.

This talk will cover recent endeavors to develop cost effective and reliable energy dense alkaline batteries for grid storage applications in a Department of Energy, Office of Electricity (OE) funded program, whose success depends on the ability to effectively pair Zn anodes with a high capacity conversion cathode. This presentation will include an introduction into the OE funded program and the recent developments of rechargeable Zn/MnO_2. Progress towards rechargeable Zn/Cu₂S and Zn/CuO batteries, which have now been demonstrated at commercially relevant areal capacities (~ 10-40 mAh cm-²) and energy densities (~ 100-250 Wh/L), and aspects of their battery chemistries and cycling properties will also be provided.

This work was supported by the U.S. Department of Energy, Office of Electricity, Energy Storage Program, Dr. Imre Gyuk, Program Manager, and the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program 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. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

The inherent safety issues of Li-ion batteries and the high-risk supply chain of necessary elements in their active materials provides ample reason to revisit safer aqueous-based batteries. In terms of the specific energy of the system (not the single cell) and the system cost per kW h^–1, zinc-based batteries can outshine Li-ion and are undergoing a 21^st-century renaissance. We have demonstrated that a monolithic three-dimensional (3D) aperiodic form factor zinc “sponge” developed at the U.S. Naval Research Laboratory (NRL) suppresses formation of dendrites, even when cycling the cell versus nickel, silver, or air cathodes at high rate and to deep utilization of all the zinc in the cell. This performance breakthrough arises because the entire electrode volume is directly wired to the current collector through an inner core of always-present metallic zinc thereby improving uniformity of electroreaction. The co-designed feature of through-connected porosity in 3D plays an important role in ensuring within the confined volume that the ultimate end-product of discharge in alkaline electrolyte, zinc oxide, precipitates onto that internal electrified interface. The historic limitations of zinc anodes in alkaline electrolyte are now history paving the road to next-generation, safer electrochemical energy storage.

Rechargeable alkaline Zn-MnO₂ batteries have become the subject of a large research effort because they provide a pathway to high energy density rechargeable batteries that are potentially low cost (<$50/kWh) and have a non-flammable electrolyte. Since aqueous batteries are more limited in voltage than non-aqueous systems, achieving high reversible capacity is key to increasing energy density and lowering cost. A full two-electron reaction of MnO₂ from its layered birnessite form to the layered hydroxide pyrochroite results in a theoretical capacity of 617 mAh/g.

(1) δ-MnO₂ + 2H₂O + 2e^- ↔ Mn(OH)₂ + 2OH^-

However, discharge of reaction (1) proceeds through a mixed mechanism: initially the layered structure is maintained and charge is compensated by insertion of ions; later the layered structure breaks down through dissolution and a new layered structure is formed. The charging reaction is similar in nature, but not necessarily a simple reversal of the same path. Furthermore, side reactions can permanently reduce the capacity, as material is instead converted to a stable and resistive spinel form Mn₃O₄ that results in electrode failure. Layered birnessite, which has the ideal structure δ-MnO₂, can accommodate foreign cations in the interlayer as well as interlayer water. For example K-birnessite incorporates some K⁺ from the electrolyte as well as some neutral H₂O as δ-(K_x)MnO₂ ● wH₂O. However, the K⁺ charge is compensated by some reduction of Mn^IV to Mn^III, and because K⁺ is not electrochemically active in the stability window of water, this results in a theoretical capacity much lower than 617 mAh/g. If the inserted cations are instead electrochemically active (as with some transition metals) they can themselves participate in the electrochemical reaction and provide capacity. It has long been known that doping δ-MnO₂ with Bi³⁺ reduces or eliminates the detrimental side reactions resulting in Mn₃O₄, producing a rechargeable electrode. This doping is most effective in thin electrodes of low areal capacity. However, a dual-doping strategy involving both Bi³⁺ and Cu²⁺ extends the effect to high areal capacity electrodes that are thick, have high mass loading, or both. In such a case there are many electrochemically active species in the material, and the sequential electrochemistry of Mn(IV/III), Cu(II/I), Mn(III/II), Cu(I/0), and Bi(III/0) have all been observed on the surface of an MnO₂ particle using operando μ-XANES during electrode discharge.

Here we present findings concerning the mechanistic role of intercalated cations in the conversion reactions of doped δ-MnO₂, with a particular focus on the part played by Bi³⁺. Bi-doped MnO₂ can be prepared in a number of ways, and is often treated in the literature as a single interchangeable material. However, there is evidence that different synthesis methods result in materials that differ in morphology, crystallinity, and in amount and site of Bi-doping. In this study we prepare a series of crystalline δ-(K_x, Bi_y)MnO₂ ● wH₂O materials in which y is systematically varied, and these model compounds allow the structural effect of Bi³⁺ to be observed. Operando characterization of these compounds during cycling reveal the dynamic effect of Bi dopants and their interaction with Mn during reaction (1). A multimodal study of synchrotron X-ray diffraction (XRD) and quick X-ray absorption (QAS) shows that during charging of Bi-doped MnO₂ there is an extended period of overlapping electrochemical activity of Bi(0/III) and Mn(II/III), and that this period corresponds to the disappearance of the discharge product, crystalline Mn(OH)₂, but no corresponding charge product is observed by diffraction. Operando Raman spectroscopy reveals that during this period the vibrational signature of δ-MnO₂ is present, which indicates a disordered δ-MnO₂ is the initial charge product. Without Bi³⁺ this disordered material is not observed.

This presentation will deal with the development of novel materials and operando methods for the study and characterization of battery materials. The presentation will begin with a brief overview of the methods employed. Particular emphasis will be placed on the use of X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) X-ray microscopy and tomography and confocal Raman under active potential control. The utility of these methods will be illustrated by selected examples including Li/S, Li/Se batteries and Li metal deposition and dendritic growth, as well as the use of organic based materials. The presentation will conclude with an assessment of future directions.

Lithium sulfur (Li–S) battery as one of the most promising next generation energy storage technologies has been extensively studied because of the high theoretical energy, low cost, and environmental friendliness of sulfur.¹ Significant progresses have been made on new materials and advanced characterization techniques for mechanism understandings.² However, further commercialization is currently hindered by the limited cell energy and cycle life of realistic batteries.³ To close the gap between the material-level discoveries to the practical cell-level demonstration, we designed and fabricated realistic Li–S pouch cells, compared the key parameters required for high energy batteries, investigated the reaction processes and their correlations to the cycle behaviors and failure mechanisms. It is found in addition to the electrolyte depletion, heterogeneous electrolyte diffusion and redistribution are dominant factors of the high energy cell decay at electrolyte extremely lean conditions.⁴ Utilization of spatially resolved characterization and theoretical simulation on the practical pouch cells provides a full spectrum view of the dynamic interactions of key cell components in Li–S batteries. New finding of the research will be presented and discussed at the meeting.

References
1. A. Manthiram, Y. Fu, S.-H. Chung, C. Zu and Y.-S. Su, Chemical reviews, 2014, 114, 11751-11787.
2. X. Ji, K. T. Lee and L. F. Nazar, Nature materials, 2009, 8, 500-506.
3. D. Lv, J. Zheng, Q. Li, X. Xie, S. Ferrara, Z. Nie, L. B. Mehdi, N. D. Browning, J. G. Zhang and G. L. Graff, Advanced Energy Materials, 2015, 5, 1402290.
4. L. Shi, S.-M. Bak, Z. Shadike, C. Wang, C. Niu, P. Northrup, H. Lee, A. Y. Baranovskiy, C. S. Anderson and J. Qin, Energy & Environmental Science, 2020.

Solid electrolytes (SEs) have become a practical option for lithium ion and lithium metal batteries due to their improved safety over commercially available ionic liquids. The most promising of the SEs are the thiophosphates whose excellent ionic conductivities at room temperature are comparable to those of the commercially-utilized ionic liquids. Hybrid solid-liquid electrolytes exhibit higher ionic conductivities than their bare solid electrolyte counterparts due to decreased grain boundary resistance, enhanced interfacial contact with electrodes, and decreased degradation at the interface.
In this study, we evaluate a series of hybrid electrolytes made from a ‘solvate’ electrolyte in both Li-Li symmetric cells and in a full cell with a Li₂S cathode relative to their bare Li/SE/Li counterparts. Interestingly, hybrid electrolytes made by combining HFE-modified solvates and SE exhibit all the benefits of the interlayer modified SEs, without the necessity of pellet manufacture. The solvate-integrated cathode layer is additionally beneficial for achieving high active material utilization, maintaining intimate interfacial contact, and providing buffer space for volume contraction and expansion. The hybrid Li₂S cell exhibited superior cycling performance compared to the solid-state cells in terms of Li2S loading, utilzation, and cycling stability. Finally, we discuss the use of hybrid electrolytes to enable other cathode chemistries, particularly those involving conversion.

Electrochemical energy storage has always been an essential part of the application of electricity, as electricity itself cannot be stored. Aqueous electrochemical energy storage is one of the most important and long-lasting research fields due to its cost-effective and environmentally benign nature. The utilization of transition metal oxides/hydroxides in rechargeable batteries has attracted increasing research interests over the years for their multiple valence states, making it possible to have one or more redox couples and the relatively high abundance of transition metal on earth. Among them, the iron as the fourth most abundant element on the earth crust saw its application in the energy storage as early as when Thomas Edison developed the famous Nickel-Iron Battery in 1901.
Our study was devoted to bringing further understanding of the electrochemical behavior of lepidocrocite (γ-FeOOH) in the alkaline system and its application as the anode in full cells. Unlike commonly used alkaline solution with high concentration, we focus on a mild environment (pH=12, 13). By bringing in additional anion group into the system, we observed enhanced capacity with a different redox behavior, which was attributed to the formation of the green rust phase by in-situ X-ray Diffraction measurements. Full cell data showed that the capacity retention was also improved significantly by enabling the anion intercalation . Our study opens a way to improve the performance of iron-based electrode materials in an alkaline system.

Lithium-Sulfur batteries are among the most promising for desired high energy density applications such as electric vehicles. The batteries are usually configured with a Li metal anode and a sulfur-carbon (S-C) cathode electrode, in addition to an electrolyte typically composed by an ether solvent, a salt, and some additives. However, the formation of soluble polysulfide species during discharge reactions, and their migration to the anode side create severe problems that result in drastic reduction of the battery capacity during cycling. One possible solution is to modify the structure and chemical composition of the S-C cathode, by creating chemical S-C bonds. One of the best materials that contain such S-C bonds results from special thermal treatments (under an excess of sulfur) of polyacrylonitrile (PAN), that results in a new material called SPAN. During discharge, this material does not generate long-chain polysulfides, thus avoiding one of the main problems of this type of batteries. However, the underlying microscopic mechanisms occurring in the cathode during lithiation are not yet fully understood. In this work, we use ab initio molecular dynamics simulations to model a cathode material that reproduces experimental data of SPAN's graphitization and conjugated ordering, sulfur-carbon covalent bonding, sulfur loading, and elemental composition, including nitrogen doping. Our study reveals atomistic details regarding voltage profiles including the roles of S, N, and C atoms during lithiation, and provides new insights into the origins of irreversible capacity loss between the first and second cycles.

A crucial prerequisite for a high energy lithium-sulfur (Li-S) battery is the integration of a high-loading sulfur cathode, a lean amount of electrolyte, and a limited Li anode 1-4. However, simultaneous application of these parameters often leads to the rapid deterioration of the cell performance. Fundamental mechanisms of the cell failure are still not very clear; materials that can fulfill both high energy density and long cycle life of the Li-S batteries are still facing significant challenges. Highly porous and nanosized carbon materials are widely explored as effective sulfur hosts to improve Li-S battery’s performance. However, these nano materials tend to form extremely porous and thick sulfur cathodes which require a large amount of electrolyte for pore filling, and thus a very high electrolyte to sulfur ratio (E/S ratio > 10 µL/mgs) is usually used for cell test. To address those issues, rational designs for both materials and electrode structures are urgently needed to enable the operation of low porosity sulfur cathodes (<50%) under lean electrolyte condition (E/S ratio < 4 µL/mgs), conserving more proportion of electrolyte for cell cycling. In this research, sulfur/carbon composite with controllable secondary particle sizes were synthesized and used as example materials to understand the impacts of materials and electrodes on sulfur reactions at realistic conditions. It is found that the larger sulfur/carbon particles (> 90 µm) demonstrate significant superiorities over the smaller ones (< 20 µm) in terms of electrolyte permeability, sulfur utilization rate, electrochemical polarization, and shuttling effect. As a result, at an extremely low electrode porosity of 45%, the high loading sulfur cathode (>4 mg/cm2) can deliver a high initial discharge capacity of 1001 mAh/g under lean electrolyte conditions (E/S ratio = 4 µL/mgs) with a much improved cycling stability. New findings of the research will be presented and discussed at the symposium.

References
1. Lv, D.; Zheng, J.; Li, Q.; Xie, X.; Ferrara, S.; Nie, Z.; Mehdi, L. B.; Browning, N. D.; Zhang, J.-G.; Graff, G. L.; Liu, J.; Xiao, J., High Energy Density Lithium-Sulfur Batteries: Challenges of Thick Sulfur Cathodes. Advanced Energy Materials 2015, 5 (16).
2. Shi, L.; Bak, S.-M.; Shadike, Z.; Wang, C.; Niu, C.; Northrup, P.; Lee, H.; Baranovskiy, A. Y.; Anderson, C. S.; Qin, J.; Feng, S.; Ren, X.; Liu, D.; Yang, X.-Q.; Gao, F.; Lu, D.; Xiao, J.; Liu, J., Reaction heterogeneity in practical high-energy lithium–sulfur pouch cells. Energy & Environmental Science 2020, 13 (10), 3620-3632.
3. Lu, D.; Li, Q.; Liu, J.; Zheng, J.; Wang, Y.; Ferrara, S.; Xiao, J.; Zhang, J. G.; Liu, J., Enabling High-Energy-Density Cathode for Lithium-Sulfur Batteries. ACS Appl Mater Interfaces 2018, 10 (27), 23094-23102.
4. Liu, J.; Bao, Z.; Cui, Y.; Dufek, E. J.; Goodenough, J. B.; Khalifah, P.; Li, Q.; Liaw, B. Y.; Liu, P.; Manthiram, A.; Meng, Y. S.; Subramanian, V. R.; Toney, M. F.; Viswanathan, V. V.; Whittingham, M. S.; Xiao, J.; Xu, W.; Yang, J.; Yang, X.-Q.; Zhang, J.-G., Pathways for practical high-energy long-cycling lithium metal batteries. Nature Energy 2019, 4 (3), 180-186.

Li-S batteries have experienced remarkable progress in the past decade, mainly in terms of cycle life. The shuttling process, the main capacity fading mechanism, was addressed by a variety of strategies targeting all of sulfur electrode, electrolyte, and separator. Nonetheless, the main progress was based on ether-based electrolytes that have a certain level of compatibility with Li metal counter electrode. To achieve more competitive energy density, electrolyte amount should be controlled to be low thus satisfying the necessity of lean electrolyte conditions. In this talk, I will introduce recent progress in identifying high donor electrolytes. High solubility of polysulfides contributes directly to boosting the volumetric energy density. I will also discuss how to secure the compatibility with Li metal anode so that high donor electrolytes can be adopted in commercially viable conditions.

Batteries with energy density higher than that of state-of-the-art Li-ion batteries are considered critical for mass adoption of electric automobiles. Alkali metal-X batteries such as Li-O₂, Na-O₂, K-O₂ and Li-S batteries have been extensively researched over the past few years as high energy density alternatives to Li-ion batteries. Amongst these battery types, metal-oxygen batteries afford the highest energy density for a given alkali metal. Elementary steps that are common to all alkali metal-oxygen batteries are, i) the electrolyte crystal growth during discharge and ii) dissolution of alkali metal peroxides such as Li₂O₂ (in Li-O₂ batteries) or superoxides such as NaO₂ or KO₂ (in Na- or K-O₂ batteries, respectively). In general, at least one of these steps was found to be slow or inefficient in aprotic metal-oxygen batteries. However, the parameters that limit the rate of crystal growth and dissolution, and therefore the ultimate practical energy densities and rechargeability attainable in these batteries, are poorly understood. For example, Li₂O₂ and NaO₂, the discharge products in Li- and Na-O₂ batteries respectively, are both electronic insulators. Therefore, electrochemical deposition of Li₂O₂ and NaO₂ might lead to battery electrode passivation and to low practical specific energy. However, electrode-passivation limited capacities were observed in Li-O₂ batteries, but not in Na-O₂ cells. In this talk, we will present experimental results backed by theoretical calculations that suggest the capacity limitations in alkali metal-oxygen batteries can be overcome by suitable electrolyte design that enhances solution-mediated electrochemical deposition of Li₂O₂ and NaO₂. This mechanism leads to a higher specific energy than that limited by electrode passivation. Further, we will discuss the role of ion-pairing on the growth of crystals in metal-oxygen batteries and propose suitable additives to enhance crystal growth in metal-oxygen batteries. Finally, we will also discuss design rules for selecting electrolyte solvents that favor the solution deposition of discharge products while also being electrochemically stable.

The iron oxyfluoride (FeOF) cathode has been attracting great research interests since it inherits both the high output voltage of fluorides (FeF_x) and the good kinetics of oxides (FeO_x). The lithiation of the rutile FeOF starts with intercalation up to a certain concentration (Li_0.6FeOF) followed by conversion reaction to form new phases (e.g., rock salt LiFeO_x, LiF, and Fe metal). It is not surprising that the thermodynamically favorable conversion reaction does not happen in the very beginning due to kinetic barriers for solid-state phase transformation. The atomistic mechanism of the competition between the intercalation vs. conversion for FeOF is yet to be understood. In this work, a combined study with atomic simulations and experiments is performed to resolve this issue.

We propose by density functional theory (DFT) calculations a new intermediate rock salt structure, which has lower interface energy with the dominating converted phases (LiFeO₂ and FeO). Therefore, the intermediate rock salt structure will have a smaller barrier for conversion reactions. It is identified from the transition state calculations via NEB method that phase transition from the intercalated rutile structure to the intermediate rock salt structure is accompanied by a volume expansion. Therefore, compression stress imposed by a coating layer can prevent the transformation to the intermediate rock salt structure and delay the onset of the conversing reaction. This explained the previous experimental observation that a thin solid electrolyte (LiPON) deposition on the FeOF cathode extended the Li insertion process to higher concentrations (Li_1.0FeOF). Since the intercalation process is more reversible than the conversion reaction, the LiPON coated FeOF cathode retained 89% of its capacity for more than 100 cycles. This suggests that mechanical constraints can be used to control the competition of intercalation and conversion reactions to design durable high capacity conversion-type cathode materials for Li-ion batteries.

Graphite Fluoride (CF_x) exhibits one of the highest theoretical energy capacities among primary batteries, and has recently been reported to be rechargeable in Na/CF_x cells[1]. Although CF_x is widely used, key aspects of CF_x cell operations remain poorly understood. For example, the practical operational voltage is far below the theoretical (thermodynamic) average of 4.66 V for Li/CF_x [2] (Fig. 1a). The discharge rate is slow, and rechargeability for Li/CF_x, which can lead to high energy density secondary batteries, has yet to be demonstrated. A deeper understanding of the CF_x discharge mechanism, which is still disputed, will improve battery capabilities. In this presentation, we will report ab initio molecular dynamics (AIMD) simulations of CF_x defluorination by a Li-droplet on CF_x edges (i.e., under short-circuit conditionS). We also perform static, zero temperature Density Functional Theory (DFT) calculations of equilibrium voltages motivated by insights obtained in these AIMD simulations. From these predictions, we propose a discharge mechanism with a voltage range which is in broad agreement with our Galvanostatic Intermittent Titration Technique (GITT) measurements. We also calculate defluorination energy barriers associated with C-F bond breaking and relate these predictions to kinetic values estimated from experiments. Finally, solvent effects and constant-voltage electrochemical (including “overpotential”) conditions will be considered in DFT calculations.

References
[1] Y. Shao et al., “Synthesis and Reaction Mechanism of Novel Fluorinated Carbon Fiber as a High Voltage Cathode Material for Rechargeable Na Batteries.” Chem. Mater. 28, 1026 (2016).
[2] A.J. Valegra et al., “Thermodynamic and Kinetic Data of Carbon Fluorine Compounds.” NTIS Report AD-776 990 (1972).

Transition-metal fluorides (MFs) such as FeF₃ and CuF₂ are promising cathode materials and offer hope for high energy-density Li batteries (>500 Wh/kg). They operate based on multi-electron conversion reactions and thus store more Li ions per structural unit compared to the intercalation-based cathodes (e.g. no more than 1 for LiCoO₂ and LiFePO₄). Such unique Li-storage mechanism is accompanied by significant chemical and structural transformations at the nanoscale, which pose serious challenges for the reversibility and energy efficiency of the conversion-reconversion processes. In this presentation, I will first discuss the progress in microstructural design and preparation of MF-based conversion cathodes and in situ/operando mechanistic studies into the conversion mechanism, using selected results from our own research and others reported in literature. Then I will show some “unconventional” but rational applications of the conversion chemistry in Li-ion batteries. In the end, I will discuss some emerging opportunies for the conversion cathodes, such as in thin-film solid-state batteries and high-entropy structural design.

Iron fluoride (FeF3) is considered as a potential cathode for sodium-ion batteries due to its high capacity and low cost. However, the dissolution of active materials during cycling limits its further development. Herein, we design thin protective films at the surface of cathode materials by in-situ and ex-situ method. In in-situ method, an electrolyte of sodium-difluoro(oxalate)borate (NaDFOB) dissolved in a ternary solvent was utilized to develop the in-situ protective films. Ex-situ formed coatings are developed by atomic layer deposition technique. Both coatings minimize the active material dissolution and significantly improved the overall electrochemical performance of FeF₃ cathodes in SIBs.

Biological energy transduction occurs through stepwise redox reactions of various organic molecules, such as quinone derivatives, and flavin cofactors. The redox reactions in the biological system have inspired the design of biomimetic materials for renewable energy and environmental science. Recently, the exploitation of biological redox reactions has provided a new approach to the design of organic electrode materials for rechargeable batteries. Redox-active bioorganic molecules led to the development of high-performance battery systems owing to their reversible redox activities. Unfortunately, battery performance of organic-based electrodes is still lacking of energy and power density to be implemented into the practical energy storage system. To tackle this issue, the organic molecules with more redox moieties and higher conductivity have been explored. In addition, to improve the stability of organic molecules as well as inter-charge transfer, nano-dimensional networking between carbon conductor and organic molecules is of great interest so far. However, the organic-battery performance can be influenced by molecular geometry and molecular orbital structure. Therefore, we envisage the cooperative conformational change of organic molecules during battery operation. Herein, we unveiled that the proton-coupled redox reaction of single organic molecule in an aqueous solution can be translated to the Li⁺-coupled redox reaction of single organic molecule in a lithium-based organic electrolyte by using phenoxazin-3-one (i.e., resorufin) as a new bio-inspired redox-active molecule. The resorufin exhibits high reversibility during the repeated proton-coupled redox process owing to its stable intermediates, protons on the imine and carbonyl group in the molecular structure. Because of the high redox reversibility of resorufin, it has been employed in various biochemical applications such as electrochemical biosensor for biomolecules.
In this report, we elaborate the cooperative conformational change of the single molecule during the redox reaction, which ultimately lead to the outstanding battery performance for the first time. Phenoxazin-3-one is selected as the model single organic molecule. Our analyses using operando Raman spectroscopy and DFT calculations confirmed the conformational flexibility in the molecular shape, enabling formation of strong π-π interactions between redox-active organic molecule and carbon. The strong π-π interaction of the single organic molecule with carbon resulted in the excellent battery performance. Our work provides in-depth understanding about cooperative conformational change in the single molecule and its effect on the battery performance. Furthermore, we suggest a phenoxazin-3-one as a new redox-active molecules derived from biochemical redox reaction for energy storage materials in Li-ion rechargeable batteries.