Symposium EN04—Towards High Safety and High Energy Density—Solid-State Batteries

Lithium conducting garnets in the family of doped lithium lanthanum zirconates (nominally Li₇La₃Zr₂O₁₂, LLZO) are of great interest as oxide solid electrolytes for solid-state lithium-metal batteries. Despite the promising properties of LLZO in terms of electrochemical stability and ionic conductivity, processing of garnet solid electrolytes is still a challenge. Many synthetic approaches have been investigated to-date, including sol-gel, co-precipitation, spray pyrolysis, combustion, and molten salt synthesis, not to mention the standard solid-state reaction method. Most of the aforementioned methods can generate LLZO with the expected properties, but nearly all are either costly or not amenable to producing thin films of LLZO, which are necessary for practical solid-state batteries based on lithium conducting garnets. Interestingly, in nearly all of these approaches the lanthanum zirconate pyrochlore phase (La₂Zr₂O₇, LZO) is seen as an intermediate phase before conversion of precursors to LLZO, nearly irrespective of precursor type. In this work, nanocrystalline, off-stoichiometric, doped pyrochlores based on the LZO structure are synthesized as a quasi-single-source precursor for formation of LLZO, maintaining a 3:2 ratio of La:Zr instead of the conventional 1:1 ratio in stoichiometric pyrochlores, while also incorporating necessary dopants to stabilize the cubic phase of LLZO after densification. Using these novel, nanosized pyrochlore precursors, LLZO is formed by the simple addition of a Li₂O source. Conversion of doped LZO to LLZO in molten salts is demonstrated as a novel synthetic approach, requiring only low synthesis temperatures (as low as 400 °C). Additionally, doped pyrochlores may be directly converted to garnets in a single reactive sintering step, resulting in both high density (> 90% relative density) and ionic conductivity (> 0.5 mS cm^-1), presenting a streamlined approach to formation of garnet solid-electrolytes amenable to more facile processing methods than conventional ceramic sintering.

All-solid-state batteries could deliver a step change in energy density and improved safety through the use of an alkali metal anode and a ceramic electrolyte. However, dendrites form at the anode that penetrate the ceramic electrolyte is one of the greatest challenge facing solid-state batteries. While dendrite formation on charging directly leads to short-circuit and cell failure, the formation of voids on discharging can also lead to interfacial contact loss and trigger dendrite initiation. To enable the use of an energy dense alkali metal anode, voids and dendrites both need to be fundamentally understood.

We have investigated the process of stripping as a function of current density and stack pressure in both Li/Li₆PS₅Cl/Li and Na/Na-β''-alumina/Na cells, revealing the dominant role of metal creep in preventing void accumulation and cell failure. By using a combination of 3-electrode cells, scanning electron microscopy and X-ray tomography, we show that there is a critical stripping current density, above which voids will not only form at the interface but increase in number and size on cycling, eventually leading to dendrite formation on charge. The combined effect of stack pressure and temperature have been studied to better understand and potentially eliminate void formation on stripping. We show that by applying high stack pressure, or moderate pressure at higher temperature, higher current densities can be achieved with stable cycling.

For a better mechanistic understanding of Li ingress into solid electrolytes, we've utilised in-situ phase-contrast tomography combined with spatially mapped X-ray diffraction to follow penetration of Li into ceramic electrolyte and the associated propagation of cracks. Based on our observation, the new understanding of how dendrites grow in solid-state batteries will be discussed.

Designing solid electrolytes for all-solid-state-batteries that can withstand the extreme elec-trochemical conditions in contact with an alkali metal anode and a high-voltage cathode is challenging, especially when the battery is cycled beyond 4 V. Here we demonstrate that a hydroborate solid electrolyte Na₄(CB₁₁H₁₂)₂(B₁₂H₁₂), built from two types of cage-like anions with different oxidative stability, can effectively passivate the interface to a 4 V-class cathode and prevent impedance growth during cycling. We show that [B₁₂H₁₂]²⁻ anions decompose below 4.2 V vs Na⁺/Na to form a passivating interphase layer, while [CB₁₁H₁₂]⁻ anions remain intact, providing sufficient ionic conductivity across the layer. Our interface engineering strategy enables the first demonstration of a 4 V-class hydroborate-based all-solid-state battery combining a sodium metal anode and a cobalt-free Na₃(VOPO₄)₂F cathode without any artificial protective coating. When cycled to 4.15 V vs Na⁺/Na, the cells feature a discharge capacity of 104 mAh g⁻¹ at C/10 and 99 mAh g⁻¹ at C/5, and an excellent capacity and energy retention of 78% and 76%, respectively, after 800 cycles at C/5 at <0.2 MPa at room temperature. Increasing the pressure to 3.2 MPa enables a discharge capacity of 117 mAh g⁻¹ at C/10 with a mass loading of 8.0 mg cm⁻², corresponding to an areal capacity close to 1.0 mAh cm⁻². The cell holds the highest average discharge cell voltage of 3.8 V and specific energy per cathode active material weight among all-solid-state sodium batteries reported so far. Combined with their low gravimetric density <1.2 g/cm³, low toxicity, high thermal and chemical stability [2], stability vs lithium and sodium metal anodes [3], soft mechanical properties enabling cold pressing [4], compatibility with solution impregnation [5] and infiltration [6], and potential for low cost [7, 8], hydroborate electrolytes represent a promising option for a competitive future all-solid-state battery technology.

References:
[1] R. Asakura, D. Reber, L. Duchêne, S. Payandeh, A. Remhof, H. Hagemann, C. Battaglia, Energy Environ. Science, DOI: 10.1039/d0ee01569e
[2] R. Asakura, L. Duchêne, R.-S. Kühnel, A. Remhof, C. Battaglia, ACS Appl. Energy Mater. 2019, 2, 6924
[3] L. Duchêne, R.-S. Kühnel, D. Rentsch, A. Remhof, H. Hagemann, C. Battaglia, Chem. Comm. 2017, 53, 4195
[4] L. Duchêne, A. Remhof, H. Hagemann, C. Battaglia, Energy Storage Mater. 2020, 25, 782
[5] L. Duchêne, R.-S. Kühnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, Energy Environ. Science 2017, 10, 2609
[6] L. Duchêne, D. H. Kim, Y. B. Song, S. Jun, R. Moury, A. Remhof, H. Hagemann, Y. S. Jung, C. Battaglia, Energy Storage Mater., 2020, 26, 543
[7] A. Gigante, L. Duchêne, R. Moury, M. Pupier, A. Remhof, H. Hagemann, ChemSusChem 2019, 12, 4832
[8] S. Payandeh, R. Asakura, P. Avramidou, D. Rentsch, Z. Lodziana, R. Cerny, A. Remhof, C. Battaglia, Chem. Mater. 2020, 32, 1101

Li metal anode, after being in the doghouse for several decades, revived in recent years due to the merits of lowest potential (-3.04 V vs. SHE), low density and high specific capacity (3860 mAh/g) under the circumstance that the intercalation chemistry is reaching the ceiling of energy density. Nonetheless, it is not until the chronic diseases related with Li metal and organic electrolyte, such as dendrite growth, excessive N/P ratio, being knocked off can its commercialization be realized. Plagued by the incompatibility with organic electrolytes, one of the research directions branches out towards the combination of Li and inorganic solid-state electrolytes, where the poor interface brings a new challenge. Focusing on resolving the issue, here, a Li metal ink was prepared by introducing biomass-derived carbon particles into molten Li.^[1] Due to the significantly decreased surface tension, the ink is able to directly write on copper foils or other substrates that customized thickness and ultrathin Li foils with a remarkably small thickness (<10 μm) can be achieved. Moreover, when the ink directly writes on garnet-type LLZTO pellets, a desirable interface is obtained and therefore an extremely low interfacial resistance of 6 Ω cm² is delivered, in sharp contrast to 939 Ω cm² of pure Li and the garnet. Thanks to the distinguished interfacial compatibility, a critical current density of 2.5 mA/cm² and stable plating/stripping more than 800 h in symmetric cells are achieved. Full cells consisting of the ink, garnets and NCM523 also enable to deliver stable cycling performance. Due to the successful partnership with non-flammable solid-state electrolytes, the Li metal ink may have a chance to bring us very close to the use of solid-state lithium metal batteries with high safety and high energy density.
Reference
[1] Wu W., et al. A writable lithium metal ink. Sci. China Chem., 2020, 63, 1483-1489.

Interfaces play crucial, but still poorly understood roles in the performance of secondary solid-state batteries (SSBs). Using crystallographically oriented and highly faceted thick cathodes, we directly assess the impact of cathode crystallography and morphology on long-term performance of SSBs. The controlled interface crystallography, area, and microstructure of these cathodes enables understanding interface instabilities unknown (hidden) in conventional thin film and composite solid-state electrodes. A generic and direct correlation between cell performance and interface stability is revealed for a variety of both lithium and sodium-based cathodes and solid electrolytes. Our findings highlight that minimizing interfacial area, rather than its expansion as is the case in conventional composite cathode, is key to both understanding the nature of interface instabilities and improving cell performance. Our findings also point to the use of dense and thick cathodes as a new path for increasing the energy density and stability of SSBs.

The continued improvement in operating time and lifetime of electric vehicles and portable electronic devices requires higher energy density lithium ion batteries. The energy density of lithium-ion batteries can be increased by implementing high-voltage cathodes, but these cathodes are reactive and unstable during cycling with the electrolyte. To design coatings or solid electrolytes that can stabilize these cathodes, an understanding of how different chemistries interact with high-voltage cathodes is critically needed. We employ novel thermodynamic analyses based on a large-scale computation database to systematically evaluate the thermodynamic stability of a broad range of solid-state chemistries with common cathodes. By analyzing a large number of materials in high-throughput computation, we find a trade-off in that materials stable with lithiated cathodes are often unstable with delithiated cathodes, which limits the possible choice of materials stable throughout the cycling voltage. In addition to reaffirming previously demonstrated coating and solid electrolyte chemistries, our computation predicts several new chemistries, including lithium phosphates and lithium ternary fluorides, are promising solid-state chemistries stable with high-voltage cathodes. Additionally, we systematically study the chemical and electrochemical stability of coatings at the interface between promising solid electrolytes and high-capacity cathodes for all-solid-state lithium-ion batteries, which are a promising next-generation battery technology. Based on our new computation approach and our high-throughput materials analyses, we offer suggestions to improve the stability of the interface for application in all-solid-state batteries. Our computational study provides guiding principles for designing coating materials with long-term stability with high-voltage cathodes for lithium-ion batteries.

An all solid-state approach is fast becoming the most promising direction in realizing the goal of rechargeable Li-ion batteries with improved safety and performance for widespread use in portable electronics and electric vehicle applications. Such an approach employing solid electrolytes enables the use of Li metal as the anode, thereby extending access to much higher energy densities than previously envisioned in cells with liquid electrolytes. While a variety of material systems such as oxides, anti-perovskites, garnets, sulfides, and polymers, to name a few, have been extensively explored for their suitability as solid electrolytes (SE), most SEs are known to be unstable in contact with Li metal, leading to formation of decomposition products that can affect the performance of the electrochemical cell. Among these SE material systems, a class of sulfides called argyrodites (Li₆PS₅X, X = Cl, Br, I) have been identified as most promising candidates for practical solid-state Li-ion batteries owing not only to their relatively high ionic conductivities but also easy manufacturability and scalability^[1].

Our study probes the chemistry at the interface of Li metal and argyrodite SE surface and the evolution of decomposition products thus formed through X-ray photoelectron spectroscopy (XPS) under ex-situ and in-situ conditions. Previous studies by Janek and coworkers, Schlenker et al. and Wood et al.^[2-4] have reported interfacial characteristics in related cell materials, considering some of these aspects, but contrasting chemistries and a lack of consensus on electrode-electrolyte interphase products underscores the complexity of such a system. In my talk, I will discuss the effect of using different methods of depositing/plating Li metal on the SE surface while comparing and contrasting the interphasial products therein. I will also present a characterization of the evolution of the interface when subjected to electrochemical cycling under “in-operando” conditions, closely simulating a practical system. Additionally, investigations of the effect of surface roughness of the argyrodite SE as a means to understanding Li metal to SE contact will also be reported.

References:
[1] Pasta, M., Armstrong, D., Brown, Z.L., Bu, J., Castell, M.R., Chen, P., Cocks, A., Corr, S.A., Cussen, E.J., Darnbrough, E., et al. “2020 roadmap on solid-state batteries”. Journal of Physics: Energy 2 (2020), p 032008
[2] Wenzel, S., Sedlmaier, S.J., Dietrich, C., Zeier, W.G., and Janek, J., “Interfacial reactivity and interphase growth of argyrodite solid electrolytes at lithium metal electrodes”. Solid State Ionics 318 (2018) pp 102–111
[3] Schlenker, R., Stepien, D., Koch, P., Hupfer, T., Indris, S., Roling, B., Miß, V., Fuchs, A., Wilhelmi, M., and Ehrenberg, H. Understanding the Lifetime of Battery Cells Based on Solid-State Li₆PS₅Cl Electrolyte Paired with Lithium Metal Electrode. ACS Applied Materials and Interfaces 12 (2020), pp 20012–20025
[4] Wood, K.N., Steirer, K.X., Hafner, S.E., Ban, C., Santhanagopalan, S., Lee, S.H., and Teeter, G. Operando X-ray photoelectron spectroscopy of solid electrolyte interphase formation and evolution in Li2S-P2S5 solid-state electrolytes (2018) Nature Communications 9, pp 1-10.

The all-solid-state battery based on the Li metal anode is a promising next-generation energy storage system, but is currently limited by the low current density and short cycle life of the anode. Further research to improve the Li metal anode is impeded by the lack of understanding in its failure mechanisms at the lithium-solid interfaces, in particular the fundamental atomistic processes responsible for interface failure. Here, we perform the large-scale atomistic modeling study of lithium stripping and plating on solid-electrolyte interfaces by explicitly considering key fundamental atomistic processes and interface atomistic structures. Our simulations found the interface failure mechanisms and the effects of interface structures, lithium diffusion, adhesion energy, and applied pressure on such failure. By systematically varying the independent parameters of our simulations, we provide a guiding map of selecting solid-state lithium cells, in which indicates the required high interfacial adhesion energy and high applied pressure for inhibiting interface failure during cycling. Optimal solid interfaces and new research strategies are also predicted for the research and development of solid-state Li-metal batteries.

Solid-state batteries have seen a dramatic increase in research in recent years because of their ability to address safety challenges associated with flammable liquid electrolytes, and the potential to enable Li metal anodes. However, the formation of solid-solid interfaces poses unique challenges compared to solid-liquid interfaces. This requires new methods to study the fundamental behavior of solid-solid interfaces, and understand their dynamic evolution during cycling¹.

In this talk, I will present a suite of multi-modal in situ/operando characterization approaches that we have used to study Li metal-solid electrolyte interfaces during cycling. First, to gain an improved understanding of the electrochemical stability, I will discuss operando X-ray photoelectron spectroscopy (XPS) analysis of lithium metal-solid electrolyte interfaces². This approach allows us to directly observe interphase formation and evolution as the electrochemical potential of the solid-electrolyte surface is biased to potentials below the thermodynamic potential for Li plating. A range of sulfide and oxide ceramic electrolytes were explored, since they exhibit a range of (in)stability levels during Li metal plating.

To compliment these spectroscopic measurements, a range of in situ/operando microscopy techniques will also be presented. First, operando optical microscopy results will be presented, which allow for direct observation of the nucleation and growth of Li filaments both into, and out of, solid electrolyte surfaces^2-3. By time synchronizing the optical video analysis with the electrochemical signatures of plating and stripping, new insights into the dynamic evolution of morphology and associated electrochemical analysis can be obtained. This micro-scale imaging will also be complimented by in situ analysis at the nanoscale. By integrating the observations across this multi-modal characterization approach, the implications for SSB performance and stability will be discussed, and critical needs for future research will be described.

1) K. B. Hatzell, X. C. Chen, C. L. Cobb, N. P. Dasgupta, M. B. Dixit, L. E. Marbella, M. T. McDowell, P. P. Mukherjee, A. Verma, V. Viswanathan, A. S. Westover, W. G. Zeier “Challenges in Lithium Metal Anodes for Solid State Batteries” ACS Energy Lett. 5, 922 (2020).
2) A. L. Davis, R. Garcia-Mendez, K. N. Wood, E. Kazyak, K.-H. Chen, G. Teeter, J. Sakamoto, N. P. Dasgupta “Electro-Chemo-Mechanical Evolution of Sulfide Solid Electrolyte/Li Metal Interfaces: Operando Analysis and ALD Interlayer Effects” J. Mater. Chem. A 8, 6291 (2020).
3) E. Kazyak, R. Garcia-Mendez, W. S LePage, A. Sharafi, A. L. Davis, A. J. Sanchez, K.-H. Chen, C. Haslam, J. Sakamoto, N. P. Dasgupta “Li Penetration in Ceramic Solid Electrolytes: Operando Microscopy Analysis of Morphology, Propagation, and Reversibility” Matter 2, 1 (2020).

Thin film batteries can be valuable model systems to investigate interface charge dynamics and to serve as playgrounds for interface engineering, without the complexity of bulk systems. Here we investigate the charge transfer properties between the lithium garnet Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte and the LiCoO₂ cathode using a thin-film model system, with the aim of reducing the interface resistance and allowing high charge-discharge rates.
We developed a method for fabricating crystalline LLZO thin films using magnetron co-sputtering followed by an annealing step at 700°C (significantly below the standard processing temperatures). The resulting 500 nm-thick Ga-doped LLZO thin films show densities and ionic conductivities (2×10^-4 S/cm) comparable to the values observed in bulk ceramic pellets.[1]
Based on this thin film fabrication process, we fabricated an all-thin-film model system to investigate the LLZO / LCO cathode interface. This architecture provides an easy access to the interface for characterization, allowing one to identify the degradation processes taking place at the interface under high-temperature co-sintering. Introducing an in situ-lithiated Nb₂O₅ diffusion barrier at the interface, it was possible to lower the LLZO / LCO charge transfer resistance to about 50 Ω cm². The low interfacial resistance combined with the high conductance through the LLZO thin-film electrolyte allows charge transfer at high charge-discharge rates up to 40 C (0.9 mA/cm²).[2]

[1] Sastre, Jordi, et al. "Lithium Garnet Li₇La₃Zr₂O₁₂ Electrolyte for All-Solid-State Batteries: Closing the Gap between Bulk and Thin Film Li-Ion Conductivities." Advanced Materials Interfaces (2020): 2000425.
[2] Sastre, Jordi, et al. "Fast Charge Transfer across the Li₇La₃Zr₂O₁₂ Solid Electrolyte/LiCoO₂ Cathode Interface Enabled by an Interphase-Engineered All-Thin-Film Architecture." ACS Applied Materials & Interfaces 12.32 (2020): 36196-36207.

Li₇La₃Zr₂O₁₂ (LLZO) garnet-type lithium-ion conductors are being investigated as a promising solid electrolyte for solid-state lithium batteries. To enable a functional all-solid-state configuration intensive investigations have focused on reducing the cathode/electrolyte interfacial resistance which contributes the most to cell performance loss [1]. Among the commercial cathode materials investigated so far, LiCoO₂ (LCO) is one of the most stable with garnet electrolytes as only a superficial reaction has been detected between the two materials. However, even this minor reaction would block the Li-ion transport through the interface, resulting in deteriorated cell performance.
In this work, we demonstrate that a thin aluminum oxide layer (5 nm) can be an effective interlayer to impede the formation of harmful interphase and enable facile Li-ion transfer between LCO and LLZO garnet. Room-temperature-sputtered LCO thin films were employed to form an interface with the garnet electrolyte and annealed at various temperatures to reveal the effect of the extent of the interfacial reaction on the Li-ion transfer across the interface. The aluminum oxide layer was then introduced between LCO and the garnet electrolyte by sputtering a metallic aluminum layer which is then annealed together with the upper LCO layer in oxygen. Compared to the cells without an aluminum oxide interlayer, those with the interlayer exhibited much-improved performance, i.e., a stable discharge capacity of up to 110 mAh/(g LCO) at a C/10 rate and a rate capability up to the 3C rate. Atomic layer deposition was also employed to fabricate the aluminum oxide interlayer and a similarly improved cell performance was observed. The specific conditions for implementing the two methods for depositing the aluminum oxide layer and their effectiveness are compared to distinguish their capabilities in practical cell application.

[1] T. Liu, Y.Y. Ren, Y. Shen, S. X. Zhao, Y. H. Lin, C. W. Nan, Journal of Power Sources, 324 (2016) 349-357.

To keep pace with the ever-increasing demands for high energy density, low cost, and long cycle life of rechargeable batteries, advanced battery designs are needed. The greatest improvement over conventional batteries is expected to come from the use of metallic lithium as an anode. However, non-uniform electroplating of lithium metal results in the formation of dendrites, which greatly shortens battery life.
Researchers from Samsung have recently shown that introducing an amorphous interlayer of Ag-C composite leads to a long term stability without dendrite formation.[1] However, the reason why the interlayer shows this advantageous behaviour is not understood. We deposit amorphous carbon interlayers with different properties between anode and solid-state electrolyte by direct current and high power impulse magnetron sputtering. We show the influence of the microstructure and conductivity of the carbon interlayer on lithium plating through lithium phosphorus oxynitride and garnet-type solid electrolytes. Our results shed light on the key factors that enable homogeneous lithium plating and thus the use of lithium metal in solid-state batteries.

[1] Y.-G. Lee et al. High-energy long-cycling all-solid-state lithium metal batteries enabled by silver–carbon composite anodes. Nat. Energy 5, 299–308 (2020)

All-solid-state batteries offer great promise for safer and higher-energy-density storage compared to conventional liquid-based Li-ion batteries. Yet they suffer greatly from high interfacial resistance caused by poor physical contact, structural and chemical heterogeneity and formation of undesired secondary phases that are detrimental to Li-ion transport. To overcome these challenges, atomic-scale visualization of the interfaces and a detailed understanding of how the structure and chemistry of the interface evolve during processing and operation, which ultimately dictates device performance is critical. In this talk, I will address how we combine high-temperature ab initio molecular dynamics simulations with machine-learning and global optimization algorithms to study the structural and chemical evolutions of solid-electrolyte/cathode interfaces in all solid-state lithium batteries. I will also demonstrate how we integrate first-principles based simulations with high-resolution microscopy and spectroscopy to unravel the atomic details of these interfaces under different processing conditions and discuss the implications of structural and chemical inhomogeneity at the interfaces towards Li-ion transport. Examples will be discussed in this talk includes the garnet and perovskite solid-electrolytes interfacing with layered lithium cobalt oxide cathode.

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

The introduction of cryogenic cooling of specimens in the (scanning) transmission electron microscopy (S/TEM) has recently allowed in understanding various quantum interfaces and phase co-existence in strongly correlated systems [1]. Understanding those quantum properties at the fundamental level had been difficult due to limited characterization techniques with inadequate spatial and temporal resolution. Quantum materials must be studied at cryogenic temperatures because many of the relevant properties in these quantum materials only manifest themselves at these low temperatures. Cryogenic S/TEM has also led to the observation of critical structural information related to battery interfaces at atomic resolution, which are traditionally difficult to achieve because they are air-sensitive and are prone to electron beam damage in the S/TEM [2-5]. Thus, most materials require cryo-transfer and-cryo preservation of the sample using cryogenic coolant (liquid nitrogen), limiting the studying of the sample to post-mortem analysis. The cryo-electrical holder previously tested was limited in applying electrical stimulus to the sample at cryogenic temperature, and the primary focus was on imaging[5]. Here, we present the development cryo-electrical biasing S/TEM holder that simultaneously allows electrical stimulus and high-resolution imaging of a sample in-situ at various cryogenic temperatures.

The newly developed cryo-electrical holder will enable the understanding of critical interfaces in battery systems and surface-electronic behavior of quantum topological insulators such in two-dimensional (2D) materials, among others, for artifact-free evaluation of the studied devices in high-resolution imaging and spectroscopy modes. Here, we present an example of studying battery processes using a single nanowire system from room temperature down to liquid nitrogen temperature as close as around -170°C. Electrical biasing is performed in a nanowire sample that sits across the electrodes on the biasing chip. A constant current experiment at cold temperatures on the nanowire shows a voltage drop as the reaction proceeds with a growth of the dendrite layer plated on the nanowire's surface. The cryo-electrical TEM holder will be vital in enabling scientists to expand the knowledge of structure-property relationships in materials, specifically the relation between temperature and electronic properties, and will allow for the accelerated development of the next generation of electronic, quantum, and energy storage materials devices [6]

References
[1] K. A. Moler, Nat. Mater. 2017, 16, 1049.
[2] Y. Li, et al., Science (80-. ). 2017, 358, 506 LP.
[3] X. Wang, et al., Nano Lett. 2017, 17, 7606.
[4] M. J. Zachman, et al., Nature 2018, 560, 345.
[5] K.A. Spoth, et al., Microsc. Microanal (Suppl 2) 2019, 1660-1661
[6] KK, DHA, and NS acknowledge funding from the Department of Energy, Office of Basic Energy Sciences, SBIR Grant # DE-SC0019627.

High Li mobility is required in cathode materials as well as in solid-state electrolytes. While computationally driven searches have had considerable success in identifying new solid-state electrolytes in sulfides, considerably less high mobility oxide conductors have been discovered. In this presentation, I will show the four mechanisms that we have identified to produce high Li mobility in solids. These insights apply universally to solid ion conductors as well as to high-rate cathode materials. For each of the four mechanisms we have performed a high-throughput computational search through the relevant part of known compound space in order to develop new solids with the potential to have very high mobility. Our results indicate that there many oxides that are likely to have considerably higher Li-ion conductivity than LLZO. Time permitting, I will also show how the same principles apply to creating high rate cathode materials.

Ceramic-based solid-state electrolytes (SSEs) are attractive materials for improving battery safety. To develop improved ceramic SSEs, computational predictions based on first principles methods can be a valuable tool in accelerating materials discovery. Experimental confirmation is essential after such predictions; however, materials screening rates are limited by the long processing time of conventional ceramic synthesis and sintering techniques, which are also prone to poor compositional control due to volatile element loss. These problems also cause great challenges for the manufacturing of solid state batteries.
To overcome these limitations in SSE synthesis and manufacturing, we develop an ultrafast high-temperature sintering (UHS) process for the fabrication of ceramic materials by radiative heating that features a record-high temperature of up to 3,000 ^oC and an ultrafast heating rate of up to 100,000 ^oC/minute (Science 368.6490 (2020): 521-526). The UHS method can directly sinter oxide precursors into solid, dense ceramics in seconds. Compared with previous furnace-based fabrication techniques, the UHS process is >100–1000-times faster (e.g., reducing the sintering time from hours to ~10 s). As a result, we are able to achieve excellent compositional control of ceramics containing volatile components (e.g., Li in solid-state electrolytes), as well as prevent uncontrolled grain growth for outstanding material performance. Additionally, the UHS process can also be applied to manufacture SSE membranes with high conductivities and provide excellent sintering at material interfaces with limited interdiffusion, which are essential for devices such as thin film batteries. Furthermore, this technique is compatible with 3D printing to produce novel ceramic structures and devices that are otherwise impossible to achieve by other rapid sintering methods. Finally, the UHS process is universal, allowing us to synthesize a wide range of new ceramic materials with novel composition and structure. This technique has the potential to transform and expand the discovery of ceramic compounds, with significant impacts for rapid materials screening and manufacturing of solid-state batteries.
We also extended the method for sintering printed thin film batteries (Science advances 6.47 (2020): eabc8641) and multielement metals (Advanced Science, Rapid Synthesis and Sintering of Metals from Powders, accepted), which will be discussed briefly as well.

Owing to its potential to improve battery performance compared to state-of-the-art Li-ion, there has recently been a resurgence in the development of Li metal anode technology. However, to date, efforts to cycle Li metal using liquid electrolytes with sufficient coulombic efficiency and safety have been largely insufficient to demonstrate commercial viability for electric vehicles. However, Li-ion conducting solid electrolytes are attracting considerable attention to overcome the issues associated with liquid electrolytes. One such solid electrolyte that meets many of these requirements is cubic Li-garnet of the nominal composition Li₇La₃Zr₂O₁₂ (LLZO). One of the major issues with cubic LLZO is increasing its critical current (charging current at which dendrites form). It was been shown that the critical current of LLZO is a function of its electrical and mechanical properties. It is the purpose of this talk to discuss the effect of three different aliovalent dopants (Al,Ta,Ga) on the electrical (total, grain, grain boundary conductivity), mechanical (hardness, fracture strength, fracture toughness) and critical current of hot-pressed near theoretical dense cubic Li₇La₃Zr₂O₁₂. These results will be correlated with the resulting microstructure. This information is required if LLZO is to be used as an electrolyte in solid-state batteries. In addition, implications of these results on the performance of all-solid-state batteries with a Li metal anode will be discussed.

With the aim of making lithium batteries safer, the scientific community is looking in recent years to replace the liquid solvents used as electrolyte with a solid ionic conductor compound. Several families of materials have been developed, leading to major improvements in this technology (NASICON, perovskites, Garnets [1]). In addition, the thio-phosphate family is widely explored and several compounds have been discovered in the pseudo-binary Li2S-P2S5 diagram such as Li3PS4, Li₇P₃S₁₁ or Li₇PS₆ [2]. In 2011, R. Kanno and al. [3] had discovered a new phase: Li₁₀GeP₂S₁₂ showing ionic conduction of 12 mS/cm. Unfortunately, this structure is unstable with respect to lithium metal [4] and germanium remains a very expensive element. In order to improve the stability of this structure, a partial substitution of sulfur by oxygen has been successfully proposed [5]. Very recently, the germanium-free phase Li_9.6P₃S₁₂ has been obtained and exhibits better stability towards lithium despite a lower conductivity [6]. Recently we published about the effect of adding oxygen in this compound [7]. Here, we present the effect of added oxygen on the structure, on the ionic conductivity and during cycling in all solid-state battery.
[1] F. Zheng, M. Kotobuki, S. Song, M. O. Lai, and L. Lu, Review on Solid Electrolytes for All-Solid-State Lithium-Ion Batteries, Journal of Power Sources 389, 198 (2018).
[2] Ö. U. Kudu, T. Famprikis, B. Fleutot, M.-D. Braida, T. Le Mercier, M. S. Islam, and C. Masquelier, A Review of Structural Properties and Synthesis Methods of Solid Electrolyte Materials in the Li2S − P2S5 Binary System, Journal of Power Sources 407, 31 (2018).
[3] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, and A. Mitsui, A Lithium Superionic Conductor, Nature Materials 10, 682 (2011).
[4] S. Wenzel, S. Randau, T. Leichtweiß, D. A. Weber, J. Sann, W. G. Zeier, and J. Janek, Direct Observation of the Interfacial Instability of the Fast Ionic Conductor Li ₁₀ GeP ₂ S ₁₂ at the Lithium Metal Anode, Chemistry of Materials 28, 2400 (2016).
[5] Y. Sun, K. Suzuki, K. Hara, S. Hori, T. Yano, M. Hara, M. Hirayama, and R. Kanno, Oxygen Substitution Effects in Li10GeP2S12 Solid Electrolyte, Journal of Power Sources 324, 798 (2016).
[6] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, and R. Kanno, High-Power All-Solid-State Batteries Using Sulfide Superionic Conductors, Nature Energy 1, 16030 (2016).
[7] A. Neveu, V. Pelé, C. Jordy, and V. Pralong, Exploration of Li–P–S–O Composition for Solid-State Electrolyte Materials Discovery, Journal of Power Sources 467, 228250 (2020).

Argyrodites, with fast Li⁺ ion conduction, are promising for applications in rechargeable solid-state batteries. Here, we investigate a new compositional space of argyrodite superionic conductors, Li_6-xPS_5-xXY_x [, X and Y are halides], with a remarkably high ionic conductivity of 24 mS/cm at 25 ^oC. In addition, the extremely low Li-migration barrier of 0.155 eV makes it distinct and promising for low-temperature operation of SSBs. Average structure analysis via neutron Bragg diffraction reveals the retention of parent argyrodite structure with cubic space group (F-43m) with significant anion site disorder among the Cl^-, Br^-, and S^2- anions at Wyckoff 4a and 4d sites. Neutron pair distribution functions reveal the short-range order of anions in a monoclinic space group. Further characterization of the local structures by solid-state NMR confirms anion reordering. High-resolution ⁶Li NMR reveals lithium disorder induced by diversifying the anion sublattice. ⁷Li NMR relaxometry shows gradual increase in Li ion dynamics, eventually yielding a “melted” Li⁺ sublattice with a flattened energy landscape when increasing x. In addition, the diversity of anion species and Li-deficiency induce hyper coordination and coordination entropy for the Li-sublattice, resulting in higher jump rates of lithium ions. Electrochemical impedance spectroscopy unveils enhanced Li⁺-ion transport with increasing x. This study demonstrates that mixed-anion frameworks can help stabilize highly conductive structures in a compositional space otherwise unstable with lower anion diversity.

Liquid crystals (LC) are known for forming hexagonal, smectic, and gyroid phase structures. These traits are favorable for LC electrolytes as they may contain ordered ion transport pathways that allow for higher ionic conductivity than isotropic materials. Furthermore, LC system is helpful for studying ion-transport mechanism in ion-aggregates which is a novel concept introduced by Winy and Frischknecht. Current research on LC electrolytes is mostly focused on ion-transport properties of organic ion pairs or metal-ions with solvation sites. Despite the promising potential, thus far there have been limited research performed on non-solvating LC electrolytes that lack polar matrices for ion-solvation; limited works of non-solvating LC have been done and focused on sulfonate/metal ion pairs with addition of solvent to promote charge dissociation. The ion transport property with non-solvating and higher charge delocalization anions have rarely been reported. Previously, we reported on the synthesis and characterization of side-chain, non-solvating single-ion LC polymer electrolytes with various anions and found that delocalization effect of the anion has a strong influence on the Li+ conductivity. It was shown using broadband dielectric spectroscopy analysis that the ion transport is correlated with dielectric relaxation. In this contribution, we report on the ion transport properties of an ionic liquid crystalline electrolyte with –TFSI anions for lithium-based batteries. This class of materials exhibits a wide smectic phase temperature window and a conductivity above 10E-5 at 60 ^oC. The influence of addition of high dielectric constant moiety on phase transition, ion conductivity, and transport mechanism will also be discussed.

Solid-state rechargeable batteries, which afford new chemistry with enhanced energy density and safety, are promising energy storage media. Solid electrolytes with fast ion conduction, good compatibility with electrodes, and superior electrochemical stability are key to the success of solid-state batteries. The development of such solid electrolytes rely on our fundamental understanding of important processes, such ion conduction and dendrite formation, which requires advanced characterization tools that can non-invasively examine various aspects of these processes.

Nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) are known as powerful and versatile tools to study structure and ion dynamics, particularly suitable for investigating the aforementioned topics. In this discussion, we will present our work on: i) ion transport mechanisms and their dependence on structure and compositions in superionic conductors studied with NMR spectroscopy and relaxometry. The insights from this study have enabled the discoveries of new materials with unprecedented ionic conductivities with the lowest activation barriers for Li⁺ion transport. ii) The origin of dendrite formation in solid-state batteries and its propagation process investigated with 3D in situMRI combined with tracer-exchange. This unravels dendrite formation process in the solids, very different from liquid-electrolyte based systems.

All-solid-state lithium batteries (ASLBs) employing solid-state electrolytes (SEs) are considered as promising next-generation energy storage systems with high safety due to the elimination of the flammable liquid electrolyte used in convention lithium ion batteries. Among various SE candidates, sulfide SEs are one of the most studied species because of their ultrahigh ionic conductivity. However, sulfide SEs suffer from poor compatibility with the conventional transition metal oxide cathode, which is a challenging issue for the further application in ASLBs. In this work, an ASLB pairing a novel thioantimonate conductor, Li_6.6Ge_0.6Sb_0.4S₅I, with high capacity sulfide cathode FeS₂ is characterized by operando techniques, and a superior performance is achieved. Generally, FeS₂ is highlighted with high specific capacity, earth abundance and environmentally benignity, but constrained by fast-decaying performance in batteries using liquid electrolyte. Herein, sulfide SEs can successfully address the severe mass loss caused by the aggregation of Fe⁰ nanoparticles and shuttle effect of polysulfides during cycling. Li_6.6Ge_0.6Sb_0.4S₅I exhibits high stability with FeS₂ in an optimized voltage range. The operando energy dispersive X-ray diffraction (EDXRD) was firstly used to in situ study the interface stability. As a result of excellent compatibility between FeS₂ and Li_6.6Ge_0.6Sb_0.4S₅I, outstanding capacity and cycling stability is achieved at the same time.

Solid-state lithium metal batteries have garnered significant interest in recent years due to the potential for solid-state electrolytes (SSEs) to enable a lithium metal anode by suppressing dendrite growth and eliminating hazardous liquid electrolytes. However, the development of solid-state lithium metal batteries has been limited by numerous challenges that exist at the interface between lithium and SSEs, such as lithium metal penetration through the electrolyte, void formation at the interface, and electrochemical decomposition to form an interphase. Here, we characterize the interface between lithium metal and the sulfide SSE Li₁₀SnP₂S₁₂ using operando synchrotron X-ray tomography. Owing to the high ionic conductivity of Li₁₀SnP₂S₁₂ ( > 10^-3 S cm^-1), we were able to electrochemically test symmetric Li/Li₁₀SnP₂S₁₂/Li cells at the relatively high current density of 1 mA cm^-2 while simultaneously collecting 3D images of the cell. Interphase growth and the formation of interfacial voids were observed throughout the electrochemical experiments. Segmentation and detailed image analysis enabled quantitative analysis of these phases, which were coupled to electrochemical measurements to establish links between interphase growth and void formation to cell failure. We ultimately found that the loss of interfacial contact area caused by void formation at the lithium interface is primarily responsible for failure, with current constriction effects exacerbating the voltage polarization in the cell due to the formation of smaller contact spots with highly localized current flow. Our results highlight the power of operando X-ray imaging to probe buried interfaces in solid-state batteries during relevant electrochemical processes.

Li-ion batteries (LIBs) using solid-state electrolytes (SSEs), known as all-solid-state batteries (ASSBs), have gained much attention due to their improved safety as well as energy density compared with the current generation of LIBs. However, Li microstructure/dendrite growth still persists as the major issue that largely compromises stability and causes short circuits of batteries especially at high cycling rates, which hinders the success of ASSBs for practical applications. While much effort has been invested to understand where, when, and how Li microstructures grow in SSEs, direct evidence has yet to emerge to illustrate where the growth of Li dendrite initiates in SSEs, either from SSEs−Li interfaces or the bulk of SSEs and how it propagates. Herein, for the first time, we report 3D images of Li microstructures to reflect the variations of lithium density distribution in Li₇La₃Zr₂O₁₂ (LLZO) upon electrochemical polarization of a Li/LLZO/Li cell via non-invasive ⁷Li Magnetic Resonance Imaging (MRI). By combining ex situ high-resolution 3D MRI of cycled LLZO pellets and in situ 2D MRI of the Li/LLZO/Li cell upon electrochemical cycling, the process of Li microstructure formation is revealed, which proves to be significantly different from the liquid-electrolyte-based LIBs. The techniques developed and demonstrated in this work will also benefit studies of other material systems.

The need for improved battery performance and safety has created the impetus to replace carbon-based anodes used in state-of-the-art batteries with lithium metal. However, it is generally known that lithium metal anodes cannot cycle with state-of-the-art liquid electrolytes due to dendrite formation. Owing to its stability and stiffness, the solid-state electrolyte, lithium lanthanum zirconium oxide (LLZO), is known to physically stabilize lithium during cycling. However, LLZO is typically polycrystalline and there is evidence that grain boundaries potentially serve to initiate lithium filament growth. Thus, there is a need to better understand the behavior and properties of the individual grain boundaries and how grain boundary networks may or may not affect lithium filament penetration in LLZO.

Single crystal and bi-crystal LLZO doped with aluminum were fabricated using rapid induction hot pressing. Grain size and crystallographic orientation were confirmed by x-ray diffraction and electron backscatter diffraction. The ability to grow grains larger than a millimeter enabled electrochemical characterization of single grains and grain boundaries based on orientation and position. Characterization techniques included electrochemical impedance spectroscopy and chronoamperometry. This work presents electrochemical characterization of single grain boundaries and single crystals, increasing the understanding of grain boundary and bulk effects on solid-state electrolyte performance. With this understanding, optimal design and fabrication of solid-state electrolytes can be determined to allow for the implementation of lithium metal anodes into advanced battery technology.

References
1. Krauskopf, T; Richter, FH; Zeier, WG; Janek, J. “Physicochemical Concepts of the Lithium Metal Anode in Solid-State Batteries.” Chemical Reviews 2020 120 (15), 7745-7794.
2. Sharafi, A.; Haslam, CG; Kerns, RD; Wolfenstein, J; Sakamoto, J. “Controlling and Correlating the Effect of Grain Size with the Mechanical and Electrochemical Properties of LLZO Solid-State Electrolyte.” Journal of Materials Chemistry A 2017 5 (40), 21491-21504.

The solid-state lithium garnet structured electrolyte Li_6.25Al_0.25La₃Zr₂O₁₂ (LLZO) has shown strong viability in the fabrication of solid-state lithium batteries given its stability with lithium metal anodes than can drive nearly double the specific energy density of conventional lithium ion batteries. However, the integration into low cost battery systems with high capacity for electric vehicles requires a means of minimizing lithium transport distance from the electrolyte into cathode of a bulk 3D electrolyte to overcome the increased ohmic resistance of solid-state lithium garnets. To reduce lithium transport length in bulk solid batteries, our approach is to utilize scalable and aqueous based freeze tape casting to create low tortuosity porous scaffolds of LLZO co-sintered with a dense LLZO film to form a bi-layer battery architecture that can be incorporated with high nickel NMC cathodes. LLZO bi-layer micro-structure development for Li ion transport and NMC incorporation as well as methods to ameliorate interfacial resistance in the solid-state composite cathode without added liquid/polymer electrolytes will be presented.

Rechargeable lithium-ion batteries (LIBs) have become the main power sources for portable electronic devices, and their applications have rapidly expanded to electric vehicles and large-scale energy storage systems due to their high energy density and excellent cycle life. Although current commercialized LIBs employing liquid electrolytes exhibit superior cycle performance compared to other rechargeable battery systems, there are still concerns related to the use of liquid electrolytes, such as solvent leakage, high volatility and flammability, which make full utilization for large capacity applications very challenging due to the safety issues. As a strategy for enhancing battery safety, all-solid-state lithium battery assembled with a solid-state electrolyte has gained great attention. Among the various types of solid electrolytes, solid polymer electrolytes have attractive properties such as absence of leakage problem, non-flammability, easy processing for producing a thin film, low cost, design flexibility, cuttable shapes and good interfacial contacts with electrodes. However, solid polymer electrolytes exhibited low ionic conductivities at ambient temperature, and their mechanical properties were often poor and thin free-standing films could not be obtained without a thermal curing process. In our work, different types of polymer-based hybrid solid electrolytes with high ionic conductivity and good mechanical property are prepared and applied to rechargeable lithium- batteries. First, the solid polymer electrolytes supported by porous polymer membrane are prepared and their electrochemical properties are characterized. Second, the solid-state hybrid electrolytes composed of ion-conductive polymer and oxide-based inorganic conductive materials are investigated. Third, the hybrid solid electrolytes based on highly conductive Li₆PS₅Cl was prepared in the form of thin film and their electrochemical characteristics are investigated. All types of the polymer-based solid-state electrolytes are applied to Li/LiNi_xCo_yMn_1-x-yO₂ cells, and their electrochemical performance will be reported.

All-solid-state lithium-ion batteries (ASLBs) have been proposed due to the expectation that they can overcome two major problems on the present lithium-ion batteries based on organic liquid electrolyte : (1) thermal instability of the liquid electrolyte with flammable organic solvent, (2) limitted energy density of battery. Sulfide based solid electrolytes are attracting attention, owing to their high formability and high ionic conductivity which enable favorable interfaces between solid electrolyte and active material particles. The well-connected interfaces can lead to increasing in lithium ion channels and energy density of active materials, however, it also causes a side reactions which form a resistive layer and decreases long cycling life of ASLBs. In this regard, many researchers focused on solving interface problems of ASLBs. To solve these problems, it is important to analyze the stability issues concerning sulfide solid electrolytes and their interfacial reactions accurately with various analysis techniques.
In this talk, we will present the results of comparative analysis on chemical and electrochemical stability with two sulfide-based solid electrolytes, Li₇P₃S₁₁ and Li₆P₅SCl which have been widely studied. We will also report the results of electrochemical characterization for composite cathodes by applying each of solid electrolytes. Furthermore, our recent research results of post-mortem analysis for composite cathodes using X-ray based analytical techniques will also be presented.

Application of lithium-ion batteries (LIBs) has been expanded to large-scale energy storages, such as electric vehicles and energy storage systems. However, safety issues of conventional LIBs have emerged as the most critical concern in recent years. In this respect, all-solid-state batteries (ASBs) have been regarded as one of the most promising alternatives. Especially, ASBs employing sulfide solid electrolytes have shown outstanding performances compared with those fabricated with others such as oxides and polymer electrolytes. Specifically, Li argyrodites are unique due to their solution processability that offers the practical fabrication process of ASB electrodes and intimate ionic contacts between active materials and SEs. However, solution-processable Li argyrodites have been studied only for a composition of Li₆PS₅X (X = Cl, Br, I) with insufficiently high Li⁺ conductivities (~10^-4 S cm^-1). Recently, notable progress on new-compositional Li argyrodites has been achieved.

In this presentation, compositional design of solution-processable Li argyrodites and microstructural analyses are presented. Also, promising electrochemical performances of ASBs prepared by an infiltration of solution-processable new Li argyrodites are shown.


References
[1] Adv. Energy Mater. 2018, 8, 1800035.
[2] Adv. Mater. 2016, 28, 1874–1883.
[3] Nano Lett. 2017, 17, 3013−3020.
[4] Nano Lett. 2020, 20(6), 4337–4345.

The solidification of electrolytes using inorganic materials has great potential of enhancing safety and energy density. While sulfide solid electrolytes (SEs) have been considered as a promising candidate due to their high conductivity and mechanical deformability, the chemical instability for organic polar solvents complicates the wet-slurry fabrication of sheet-type electrodes and SE films for all-solid-state Li batteries (ASLBs). Thus far, only a limited number of organic solvents has been investigated with lacking systematic approach. In addition, the disruption of interfacial Li⁺ conduction by binders has been regarded as the origin of the poor electrochemical performances. This could be relieved by hybridizing with liquid electrolytes but at the expense of the ASLBs' thermal stability.
In this presentation, we will show our recent development of dry polymer electrolyte (DPE) binders via a tactical approach considering reactivities between organic solvents and SEs. New liquid-free DPE-based Li⁺-conductive binders that could cope with practically adaptable processing solvents are thus tested. Significant improvements in utilizing electrode active materials and their outstanding thermal stability, enabled by slurry-fabricable DPE-based binders, are demonstrated.

NASICON-type oxide Li_1+xAl_xTi_2–x(PO₄)₃ (LATP) is expected to be a promising solid electrolyte (SE) for all-solid-state batteries (ASSBs) owing to its high ion conductivity and chemical stability. However, its interface properties with electrodes on the atomic scale remain unclear, but it is crucial for rational control of the ASSBs performance. Herein, we focused on the LATP SE with x = 0.17 and investigated the electron and ion transfer behaviors at the interfaces with the Li metal negative electrode and the LiCoO₂ (LCO) positive electrode via explicit interface models and density functional theory calculations. Ti reduction was found at the LATP/Li interface. For the LATP/LCO interface, the results indicated the Li-ion transfer from LCO to LATP upon contact until a certain electric double layer is formed under equilibrium, in which LCO is partially reduced. The calculation results agree well with experimental observations. Co–Ti exchange was also found to be favorable where the Li ion moves with Co³⁺ to LATP. We also explored the possible interfacial processes during annealing by simulating the oxygen removal effect and found that oxygen vacancy can be more easily formed in the LCO at the interface. It implies that partial Li ions move back to LCO for the local charge neutrality. The calculation results are well in line with the experiments. We also demonstrated higher Li chemical potential around the LATP/LCO interfaces, leading to the dynamical Li-ion depletion upon charging. The calculation results and the deduced mechanisms well explain the experimental results so far [1-2] and provide insights into the interfacial electron and ion transfer upon contact, during annealing, and charging [3].

References:
[1] Yamamoto, Y. and Muto, S. et al. J. Am. Ceram. Soc. 2020, 103, 1454–1462.
[2] Tsuchiya, B., Iriyama, Y., and Morita, K. et al. Adv. Mater. Interfaces 2019, 6, 1900100.
[3] Tian, H.-K. and Tateyama, Y. et al. ACS Appl. Mater. Interfaces 2020, 12, 54752–54762.

For the development of all-solid-state Li batteries with improved safety and energy density, solid electrolytes (SEs) showing a high Li⁺ conductivity (≥10^-3 S cm^-1) are the key. This is met by several types of inorganic compounds of sulfides, oxides, borohydrides, and halides. Among them, sulfide materials have been considered to be highly promising due to their mechanical deformability which enables scalable cold-pressing-based fabrication. However, uncoated conventional layered LiMO₂ (M = Ni, Co, Mn, Al) cathodes showed poor performances in combination with sulfide SEs, which is due to poor oxidation stability of sulfide SEs and (electro)chemical reactions between LiMO₂ and sulfide SEs. Since new halide SEs of Li₃YX₆ (X = Cl, Br) showing high Li⁺ conductivities (0.51 and 1.7 mS cm^-1 for Li₃YCl₆ and Li₃YBr₆, respectively) and excellent (electro)chemical oxidation stabilities as well as mechanical deformability were reported, much efforts have been focused on the developments of halide superionic conductors. Despite advances in developments of halide SEs, high Li⁺ conductivities of ~10^-3 S cm^-1 have only been achieved using scarce and expensive elements such as Y, Er, Sc, and In.
In this presentation, our recent results on the development of new cost-effective Li⁺ superionic conductor, Fe³⁺-substituted Li₂ZrCl₆, and their application to all-solid-state batteries will be presented.

The application area of lithium ion batteries (LIB) has been expanded from mobile electronics to electric vehicles and energy storage systems. Safety issue of LIBs employing organic liquid electrolytes has mounted as a serious problem. In this regard, all-solid-state lithium batteries using inorganic solid electrolytes (SEs) are considered as one of the most promising candidates. While sulfide SEs have been extensively investigated for practical , ASLBs owing to their high ionic conductivities and mechanical sinterability, they suffer from poor electrochemical stability, especially when combined with conventional layered LiMO₂ cathodes. Recent reinvestigation of halide SEs derived via mechanochemical method (Li₃YCl₆) has demonstrated high ionic conductivities of >10^-3 S cm^-1, which was contrasted by low conductivities of 10^-4~10^-6 S cm^-1 for samples prepared by conventional heat-treatment in the past. Importantly, halide SEs showed excellent (electro)chemical oxidation stabilities, which enables stable cycling of conventional LiMO₂ cathodes. Thus far, it has been considered that multiple factors such as bulk/local structures and contents of Li and central metals could affect ionic conductivities of halide SEs.
In this presentation, effects of crystal structures of Li₃YbCl₆ substituted with tetravalent metals on Li⁺ conductivities, which are modulated by heat-treatment temperature, are presented.

Li-ion batteries have revolutionized the portable electronics industry and empowered the electric vehicle (EV) revolution. Unfortunately, the traditional Li-ion chemistry is approaching its physicochemical limit. The demand for higher density (longer range), high power (fast charging) and safer EVs has recently revamped the interest in solid state batteries (SSB). Historically, research has focused on improving the ionic conductivity of solid electrolytes, yet ceramic solids now deliver sufficient ionic conductivity. The barriers lie within the interfaces between the electrolyte and the two electrodes, in the mechanical properties throughout the device, and in processing scalability.
In 2018 the Faraday Institution, the UK’s independent institute for electrochemical energy storage research, launched the SOLBAT (solid-state lithium metal anode battery) project aimed at understanding the fundamental science underpinning the problems of SSBs, recognising that the paucity of such understanding is the major barrier to progress.
In my talk I will present an overview of the fundamental challenges that are impeding the development of SSBs, the advances in science and technology necessary to understand the underlying science, and the multidisciplinary approach that the SOLBAT researchers are taking to face these challenges.

References
1. Pasta M, Armstrong D, Brown ZL, Bu J, Castell MR, Chen P, et al. 2020 roadmap on solid-state batteries. J Phys Energy. 2020;2(3):32008.

Solid-state lithium (Li)-ion batteries are attracting a lot of interest due to good safety and high energy density. Sulfide solid electrolytes have many advantages such as high ionic conductivity and easy processability. Recently, monolithic solid-state Li-ion batteries that utilize sulfide electrolytes as both the active material and ionic conductor have been developed. With a monolithic structure, the active material loading and active sites for electrochemical reaction can be increased and the unwanted interfacial reaction between the cathode and the electrolyte is avoided. In this talk, we will discuss monolithic solid-state batteries with argyrodite Li₆PS₅Cl and Li₂S-P₂S₅ glass-ceramic solid electrolytes. First, the synthetic methods of these sulfide electrolytes are introduced and the property optimization of the electrolytes is given. Second, monolithic all-solid-state batteries based on Li₆PS₅Cl and Li₂S-P₂S₅ sulfide electrolytes are fabricated and tested. The monolithic batteries show long cycle life and high discharge capacity at room temperature. The electrochemical activity of these sulfides is systematically analyzed and the main factors that affect the long-term cycling performance of the monolithic batteries are discussed.

Lithium ion batteries (LIBs) deliver a high energy density, a long cycle stability and a high energy efficiency. While the intrinsic safety problem of LIBs impedes the application of current LIBs in large scale energy storage. Aqeuous rechargeable lithium ion batteries resolve several challenges of conventional LIBs. However, the electrochemical stability window of aqueous electrolytes is less than 2.0 V, beyond which H₂ or O₂ gas generated owing to the electrolysis of H₂O. Recently, the ground-breaking of “water-in-salt” electrolyte (WiSE) successfully expanded the ESW of water to 3.0 V. To enhance energy density of WiSE batteries, the catholic limitation of 1.9 V needs to further reduced, and a high Coulombic efficiency (CE) of > 99.9% at a matched areal capacities of cathode/anode at a low charge/discharge rates is required for a long-term cycling. Here, by incorporating WiSE with UV-curable methacrylic polymer, we designed a solid-state aqueous polymer electrolyte (SAPE), in which the abundant hydrophilic groups stabilized water molecules due to the sluggish water mobility in SAPE and formed a water-less thin passivation interphase between anode and electrolyte. Moreover, the anode was pre-coated with a strongly basic water-free solid polymer electrolyte to further promote the formation of passivation interphase. All of these contributed the formation of a more robust SEI to reduce the water reduction reactions on anode surface. An extended electrochemical stability window of 3.86 V is achieved at 12 mol kg^-1 aqueous polymer electrolyte enabled 3 V full cells with an unprecedented high initial CE of 90.5% and average CE of 99.97%. The SAPE exhibits intrinsic safety and tolerance of drastic mechanical abuse, the flexible full cell with supper robustness can be widely used for low-cost and high-safety flexible electronic devices. The UV-cured polymerization process possesses the merit of facile and high efficiency, and the whole process have the potential to be practical used in a large scale. Also, the high-voltage bipolar cell is fabricated to demonstrate aqueous solid state battery. Overall, this newly developed reduced salt concentration WiSE not only reduces the cost of aqueous electrolyte, but also opens up another perspective on future directions and guidance for the design of aqueous electrolyte for high-energy-density Li-ion batteries in practical applications.

Alloy anode materials are promising for Li-ion batteries, as they have greater specific capacity than conventional graphite electrodes. Metals that form alloys with Li can electrochemically react to form Li-rich alloys at room temperature, such as Li₂₂Sn₅, Li₁₃In₃, and Li₁₅Si₄, with specific capacities of 990 mAh g^-1 for Sn and 1012 mAh g^-1 for In, as compared to 372 mAh g^-1 for graphite. However, the severe volume changes associated with charging and discharging alloy anodes remain a problem. In the case of all-solid-state batteries, these electrode volume changes can substantially exacerbate chemomechanical degradation because of the all-solid nature of the battery system and the propensity for fracture of the solid electrolyte. Here, we investigate electrochemical transformations in alloy materials in solid-state batteries and correlate these transformations to stress evolution within the solid-state stack. Metal foils with bicontinuous porosity are shown to exhibit improved capacity and cycling behavior in solid-state batteries made from sulfide-based electrolytes. In particular, porous indium showed much better specific capacity, capacity retention, and cycle life than dense indium foil with similar mass loading in solid-state batteries. The porous metal foils are synthesized by chemically dealloying Li-rich alloys in dry methanol and directly used them as the anode for the solid-state cells. This performance enhancement is likely due to the capability of the porous material to accommodate volume changes during charging/discharging while minimizing mechanical damage to the solid electrolyte. An integrated force sensor within the solid-state cells allows for real-time measurement of subtle changes of stack pressure within the cell, and the porous vs. dense films show different evolution of stack pressure. We have also investigated the impact of anode geometry (foil vs. particulate anode structure) on stress evolution and accumulated mechanical damage. Our results demonstrate the importance of controlling volume changes and stress evolution at the anode of solid-state batteries.

All-solid-state batteries (ASSBs) using fast-ion conductors as electrolytes are promising energy storage technology for enhanced energy density and safety. Among them, Li-argyrodites have attracted increasing attention because of their high ionic conductivity above 10 mS/cm and structural flexibility that can tolerate a variety of modifications in the anion sublattices. Here, we synthesized Li_6-xPS_5-xBr_1+x with different amount of Br⁻ at Wyckoff 4d sites which are predominantly occupied by S^2- in the pristine structure. The highest ionic conductivity of 11 mS/cm at 25 °C is achieved with low activation energy of 0.18 eV for Li_5.3PS_4.3Br_1.7. The influence of Br⁻/S²⁻ mixing on ion conduction is systematically investigated with multinuclear solid-state NMR coupled with X-ray diffraction and impedance spectroscopy. Distinctive atomic arrangements (4S, 3S1Br, 2S2Br, 1S3Br, and 4Br) at 4d sites in the sublattices and a statistically random distribution of Br⁻and S²⁻ at 4d sites are observed with ³¹P NMR. The resulting local structures regulate the jump rates of their neighboring Li ions and Li redistribution. As a result, the increased Li⁺ occupancy at Wyckoff 24g sites promotes fast ion conduction. Meanwhile, ⁶Li→⁷Li tracer-exchange NMR unveils that Li (24g) is more frequently visited than Li (48h). Experimental evidence combined with density functional theory calculations has revealed that the particular arrangement of 1S3Br at 4d sites maximizes overall Li⁺ conduction. This insight applies to other argyrodites and will be useful to the design of new fast-ion conductors.

Identifying new materials that combine high ionic conductivity with structural and electrochemical stability so far remains a slow trial and error search process. To rationally accelerate materials design and exploit the opportunities in the materials genome a dependable rapid screening of materials is required so that promising structures can be shortlisted for higher level computational as well as experimental characterization. Here we report on the progress of our softBV bond-valence site energy-based automated pathway analysis that provides rapid and simplified visualization of pathways, in order to bridge the gap between experimentalists and simulation specialists [1,2]. Thereby meaningful approximate predictions of ion transport pathways can be achieved from crystal structure models within seconds or minutes providing a speedup factor of 3 to 5 orders of magnitude compared to DFT-based NEB methods. Combined with a graphical user interface our software suite (that is available free for academic use from [2]) this aims to enable experimentalists to quickly identify candidate solid electrolyte materials. We also aim to integrate the pre-screening into an automated workflow for subsequent DFT characterization [3].Results are benchmarked against both experimental and DFT NEB migration barriers. Besides the migration barriers the approach now also comprises an AI-based dopant predictor focusing on bond-valence-based crystal chemical descriptors to assist experimentalists in exploring favorable substitutional doping strategies.
We will also compare the predictability of absolute room temperature conductivities from static energy landscape analysis, bond-valence based empirical MD simulations and ab initio molecular dynamics (AIMD) simulations. While for small fast-ion conductor structures at sufficiently high temperatures AIMD appears to be the gold standard, the less reliable but computationally empirical approaches have an advantage in modelling complex disordered interfaces at low temperatures over longer periods. This eliminates the hazards involved in extrapolations down to room temperature properties for the frequent cases of order-disorder phase transitions at intermediate temperatures. As an example we will discuss lithium and sodium compounds containing multiple (poly)anions, in particular the combination of thiophosphate and halide anions or various MS₄ polyanions. Based on computational screening using our bond valence site approach and DFT studies several thiophosphate halides along the A₃PS₄-LiX (Cl, Br, I; A = Li, Na) tie line [4] and the A_x(MS₄)_y(M’S₄)_z phase space [5] have been explored and their properties discussed based on BVSE pathway models and molecular dynamics simulations in combination with experimental (X-ray and neutron) diffraction, solid state NMR and electrochemical characterisation. The simplicity of the approach also facilitates the study of homogeneity ranges as exemplified for the solid solution systems Li_4-xPS4I_x (0<x<0.67) and Na_9+x(MS₄)_3-x(SnS₄)_x or compound series such as LiTX₄. Thereby we also explore under which circumstances the predictions based on static BV calculations have to be complemented by dynamic simulations to capture the role of polyanion librations in promoting ion transport.
References:
[1] L.L. Wong, K.C. Phuah, R. Dai, H. Chen, W.S. Chew, S Adams; submitted.
[2] http://www.dmse.nus.edu.sg/asn/software.html
[3] B. He, S. Chi, A. Ye et al.; npj Scientific Data 7 (2020) 151;
L. Zhang, B. He; Q. Zhao et al.; Advanced Functional Materials 30 (2020) 2003087.
[4] R. Prasada Rao, H. Chen, S. Adams; Chemistry of Materials 31 (2019) 8649-8662.
[5] A. Sorkin, S. Adams, Materials Advances 1 (2020) 184-196.

High temperature and prolonged thermal annealing for the crystallization of cathode films in thin-film solid-state batteries (TF-SSBs) restricts the choice of current collector and substrates and causes lithium loss in the cathode. This work explores photonic-based alternatives for cathode crystallization, specifically xenon flash-lamp annealing (FLA), ultra-violet excimer laser irradiation (UV-laser) and infrared laser (IR) annealing. The effect of these methods is systematically compared to that of thermal annealing in terms of processing time, crystal structure and electrochemical performance of two model thin-film cathodes LiMn₂O₄ (LMO) and LiCoO₂ (LCO).

FLA can crystallize LMO cathode films in less than 6 minutes compared to the reference thermal processing time of 60 minutes at 600 °C. The performance of the FLA-processed LMO cathodes (crystallinity, capacity, diffusion coefficient) is comparable to that of the thermal reference. LCO cathodes with thicknesses up to 5 µm could be crystalized by FLA.

To demonstrate the fast crystallization of thin-film cathodes on temperature-sensitive substrates, we applied FLA to LCO cathode films deposited on flexible aluminum foil. Flexible TF-SSBs composed of LCO as cathode, lithium phosphorus oxynitride (LiPON) as solid electrolyte and Li metal as anode were fabricated and exhibited an energy density and power density up to 800 µWh cm^-2 and 7000 µW cm^-2, respectively. Such performance is comparable to state-of-the-art thermally-annealed TF-SSBs on rigid substrates.

Solid electrolytes are a key component in enabling new technological advances for rechargeable batteries by mitigating many of the challenges associated with the use of liquid organic electrolytes. One of the key properties of solid electrolytes is their ability to transport ions between the anode and the cathode. This ion migration is usually characterized by measuring the ionic conductivity by means of impedance spectroscopic measurements. However, the ionic conductivity is proportional to both the density and the mobility of mobile ions. Only the mobility represents the actual velocity of the mobile ions.

Using lithium phosphorus oxynitride (LiPON), we show how measuring the temperature-dependent current transients can be used to independently quantify both the mobility and the density of mobile ions in solid electrolytes. By changing the mobile ion density in Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte by co-sputtering LLZO and Li₂O, we show how samples that seem to have the same ionic conductivity can still differ in ionic mobility. Finally, we employ this method to quantify the effects of Al and Ga doping on the ion migration in LLZO solid electrolytes. The proposed approach in quantification of mobile ions can be extended to other solid electrolytes for a better understanding of ion migration and the influence on battery performance.

Solid Li-ion conducting electrolytes are one pathway towards future, energy-dense, intrinsically-safe solid-state batteries. Thus, it is of interest to study novel fast Li-ion solid electrolyte materials. Here we report the synthesis and characterization of a new family of oxide solid electrolytes. We detail the compositions that were explored and give the results of the synthesis, densification and the structural, physical and electrochemical characterization and its interface with electrode materials. The properties of the new materials will be compared and contrasted to well-known solid Li electrolytes such as garnet and NASICON structured materials.

Conventional lithium-ion batteries that use liquid electrolytes are not very desirable due to lack of chemical stability, safety issues and cost of production. Solid state electrolytes (SSEs) not only have the potential to correct these drawbacks, but improve performance characteristics such as energy density and power density can also be improved. To simultaneously achieve high ionic conductivity and mechanical stiffness, a composite materials design approach for creating the novel SEEs is indicated. Here we report on our development of hybrid organic-inorganic electrolytes, in which a silica backbone is formed by sol-gel synthesis routes to provide a mechanically rigid backbone. The fluid in this nano-porous structure is subsequently replaced with polymer solutions. The polymer is grafted onto the backbone through reactive groups, and thus anchored into structure to establish the ion conducting phase. This unique approach allows us to decouple the influence of mechanical properties on ionic transport properties of materials and achieve both high elastic stiffness and ionic conductivity. We discovered that the network structure of a gel-cast material can be further conditioned by influencing the structural evolution during drying. Changing sample aspect ratio various degrees of anisotropy and spatial gradients can be achieved in the network topology. This is revealed through nano-mechanical characterization of the materials, using Brillouin light scattering. Since there is a strong correlation between the adiabatic elastic modulus and the activation energy of ion hopping, this anisotropy also affects ionic conductivity. Here we elaborate on strategies to harness this structural conditioning to create better performing SSE (Acknowledgement: NSF-DMR 1610742.)

The introduction of cathode materials that have both intrinsically higher capacities as well as higher operating potentials can help to further move towards the usage of electric vehicles and renewable energies. In this regard, high energy density cathodes which are cobalt free are of high interest in the field of batteries, mainly due to their lower cost, as well as for environmental reasons. The high cost of cobalt [1], as well as the environmental conditions under which it is extracted are the main determining facts which rise major concerns in the continuing use of cobalt containing cathodes.
In this regard, cobalt free cathodes such as LiNi_0.5Mn_1.5O₄ (LNMO) are of high interest due to its high operating potential (4.7 V vs Li⁺/Li) as well as its high specific capacity (147mAh/g). However, regardless of the intensive research done over the years, this material has not yet really made it into the market. One of the reasons being the contradictory results reported in literature trying to correlate the composition, material properties and electrochemical performance with the crystal structure as well as differentiating its bulk and interface properties when in contact with electrolytes [2].
For this reason, in this work, we would like to present the potential of using a thin film LiNi_0.5Mn_1.5O₄ model system to address this issue. So, it is possible to relate more accurately the composition, crystallinity and cation ordering of this material with its electrochemical performance.
Thin film LNMO has been previously reported as model system for study of cation ordering in LNMO. Yet only the effect of the pressure during RF sputter deposition was investigated so far, which, however, can affect both the cation ordering and the composition of the thin film [3],[4]. As such the correlation between the electrochemical performance with the cation ordering was challenging, as it is also dependent on composition. Herein, we present an approach to control the cation ordering of a thin film LNMO while maintaining the composition constant based on controlling the cooling rate during the annealing step after sputtering at the same deposition pressure.

References:
[1] Materials Today, Volume 18, Issue 5, 2015, Pages 252-264
[2] Journal of Power Sources 467 (2020) 228318
[3] Chem. Mater. 2017, 29, 14, 6044-6057
[4] Energy Storage Materials 15 (2018) 396-406

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.

Lithium ion batteries (LIBs) have been in prevalence for over two decades. They find use in portable electronics, electric vehicles and also grid storage. The conventional Li-ion battery technology employs electrolytes that have flammable organic solvents as key constituents. In addition, these electrolytes are thermodynamically unstable at voltages beyond 4 V; stability at voltages beyond 4 V is essential for the development of high voltage and therefore, high energy density Li-ion batteries. Moreover, state-of-the-art lithium ion batteries use a graphitic anode, which has a low theoretical specific capacity of 372 mAh/g. Due to this low specific capacity, conventional LIBs are limited in scope with respect to their energy and power density. Lithium metal anodes with a theoretical specific capacity of 3860 mAh/g are ideal for the development of high energy density Li-ion batteries. However, Lithium metal when used with liquid electrolytes tends to form filament like structures called dendrites due to uneven deposition of Li metal leading to short battery lifetime and also possible battery fires.
Over the years, solutions to this challenge included the development of solid-state electrolytes that could mechanically suppress dendrite growth. Solid-state electrolytes are non-flammable, mechanically robust and (electro)chemically stable. Several solid (inorganic and organic materials) have been proposed as candidate materials. However, dendrite growth is experimentally observed in most of these solid electrolytes. For example, garnet based Li-ion conductor Li₇La₃Zr₂O₁₂ (LLZO) and its doped variants have been extensively studied as potential candidates for utility in solid-state batteries. Garnet solid-state electrolytes have several favourable properties that are desired for solid-state Li-ion battery applications. However, dendrite growth at current densities as low as 100 µA/cm² has been observed. Improvements to the interface between lithium anode and the solid electrolyte via the use of metal interlayers and surface modification techniques, have been shown to mitigate dendrite growth at current densities exceeding 100 μA/cm². When such interfacial improvements have been coupled with an external mechanical pressure of several hundred kilopascal, cells were shown to sustain higher current densities without any dendrite growth. Although such high stack pressures are impractical for batteries development.
In this work we have synthesized high ion conductivity pure cubic phase LLZTO and studied dendrite growth mechanism. We observe that voids at Li/LLZTO interface act as field concentrators and have prime role to play in nucleation of dendrites. We suggest that these voids might be inevitable owing to nature of this battery system and propose the use of electron conducting interlayers which have no solubility with lithium metal as way of suppressing voids and mitigating dendrite growth.

Solid electrolytes (SEs) face significant technical challenges, in large part because
lithium/sodium dendrites readily penetrate through SEs, leading to short circuits, even
though early-on the high mechanical strength of ceramic separators was expected to
suppress lithium growth. The ability of such a soft material (Li or Na metal) to penetrate
through a ceramic is surprising from the point of view of models widely used in the Li
battery field. We introduce a concept, new to the battery field, for suppressing
penetration of lithium dendrites through SEs by putting the SE surfaces into states of
residual compressive stress. For a sufficiently high compressive stress, cracks have
difficulty forming, and cracks that do form are forced to close, blocking dendrite
penetration. This approach is widely used to solve commercially important stress
corrosion cracking problems in metals and static fatigue problems in ceramics and
glasses (e.g., Gorilla Glass). However, the technique will not be useful for SEs if the Li
ion transport rate through a SE is substantially reduced when the SE is in compression.
Our molecular dynamics calculations for Li ion transport through a common SE
demonstrate that the introduction of even very high residual compressive stresses (10
GPa) has only a modest effect on Li ion transport kinetics, suggesting that the
approach is viable and capable of providing a new paradigm for developing highperformance
and mechanically strong SEs.

High Li-ion conductivity solid electrolytes can allow for higher energy density and safer solid-state Li-ion batteries. One promising solid electrolyte is Li₇La₃Zr₂O₁₂ (LLZO) as it has both a high conductivity and stability. However, processing of ceramic electrolytes that are thin, dense, and have limited grain boundaries is a challenge. Additionally, understanding the effect of these properties on battery performance is not well understood. To analyze the effect of LLZO grain structure, a resistor mesh model was created. By solving for voltage using successive over-relaxation of Kirchhoff’s Law, we can calculate the conductivity and diffusion coefficient of a material with varying grain sizes, grain boundary thicknesses, and voids. This model was coupled with a continuum battery performance model, which uses the Nernst-Planck equation to track the concentration of Li throughout a battery with an LLZO electrolyte, a Li metal negative electrode, and a LiCoO₂ positive electrode. Then, using the concentration data, the overpotential from diffusion, charge transfer, and resistance were found using the Nernst equation, Butler-Volmer equation, and the integral of the electric field in the electrolyte, respectively. By adding the overpotentials to the equilibrium voltage based on state of charge, we obtained charge/discharge curves, energy, and power for the battery under varying conditions. Combining these models, we can determine the effect of grain structure on overall battery performance. This modeling method can be used to evaluate the suitability of solid-state electrolyte materials and manufacturing methods.

As expected, the model predicted increasing energy output as grain size increased and grain boundary thickness decreased, due to the low conductivity of LLZO grain boundary. Similarly, it was able to predict increasing energy at lower C-rates. It was found that below ratios of grain size to grain boundary thickness of 10000, the diffusion coefficient begins to appreciably decrease. The low electrolyte diffusion coefficient has the largest detrimental effect at high C-rates, where the electrolyte becomes the limiting factor and is unable to support the Li diffusion.

Electrolytes control battery recharge time and efficiency, anode/cathode stability, and ultimately safety, consequently electrolyte optimization is crucial for the design of modern energy storage devices. However, conductivity and general transport properties of the cation/anion pair(s) dissolved in the electrolyte often pose a technological limitation to the viability of the battery, and progress in fundamental insights into the origin of transport limitations are challenging yet extremely valuable. We adopt theoretical and molecular modeling techniques to shine light on transport properties and correlation effects in solid polymer electrolyte systems.

Poly(ethylene oxide) (PEO) -based solid polymer electrolytes have a long history of research due to the easy processability and good transport properties, yet new observations that challenge our conventional understanding are still reported, especially at high salt concentrations relevant for technological applications. Our study of such regimes for PEO-Li-bis(trifluoromethane)sulfonimide (TFSI) reveals the central role of the anion in coordinating and hindering Li ion movements. In particular, we observe significant competition between the anion and the polymer backbone to coordinate lithium atoms and surprising formation of asymmetric cation-anion clusters. The latter observation resonates well with recent experimental findings, where negatively-charged Li-anion clusters were speculated to exist to justify the negative lithium transference number measured in this system. We then leverage the atomistic understanding developed for PEO to propose and study a new class of poly(anhydride)-based solid polymers electrolytes. Remarkably, we find that the electrolyte consisting of Li-TFSI dissolved in poly(cis-hept-4-enoicanhydride) has a 50% higher conductivity than PEO and a lithium transference number as high as 0.79. We then rationalize these results by investigating the complex anion-polymer chain competition in coordinating lithium.

Solid-state electrolytes (SSEs) are promising candidates for resolving the intrinsic limitations of the organic liquid electrolyte currently employed in Li-ion batteries. Nevertheless, an SSE must fulfil several requirements to be employed in an all-solid state battery (SSB), such a high ionic conductivity. Complex hydrides (e.g. LiBH₄) are suggested as solid-state electrolytes.¹ Among the different polymorphs of LiBH₄, only the hexagonal phase, which is stable at temperatures above 110°C, has a remarkable high ionic conductivity (~10^-3 S cm^-1 at 120 °C). To practically access a room temperature (RT) SSB, a promising approach to enhance the Li-ion conductivity of LiBH₄ at RT is the development of new high conductive interface by mixing it with oxide nanoparticles (such as SiO₂, Al₂O₃ and MgO).²
In this work the Li-ion conductivity of LiBH₄ has been enhanced by means of MgO-mixing, optimizing the composition of LiBH₄-based composites in order to obtain a RT operating SSE. The optimum composition of the mixture results 53 v/v % of MgO, showing a Li-ion conductivity of 2.86 10^-4 S cm^-1 at 20 °C, four order or magnitude higher than pure LiBH₄ and comparable to the Li-ion conductivity of a liquid electrolyte. The improved Li-ion conductivity relies on the formation of a conductive interface that can be described by a core-shell model where the fraction of LiBH₄ (the core) is in direct contact with the oxide (the shell).
The formation of the composite does not affect the electrochemical stability window, which is similar to that of pure LiBH₄ (about 2.2 V vs. Li⁺/Li). The mixture has been incorporated as solid-electrolyte in a TiS₂/Li all-solid-state Lithium metal battery. A freshly prepared battery failed at RT only after 5 cycles. On the other hand, a stable solid electrolyte interphase can be obtained by a pre-conditioning cycling at 60 °C. Afterward, a capacity retention of about 80 % at the 30^th cycle was obtained operating at RT. We illustrate that the addition of oxide nanoparticles to LiBH₄ offers a promising strategy to obtain novel SSE candidates for Li-based SSB.³
(1) Matsuo, M.; Orimo, S. Lithium Fast-Ionic Conduction in Complex Hydrides: Review and Prospects. Adv. Energy Mater. 2011, 1 (2), 161–172. https://doi.org/10.1002/aenm.201000012.
(2) Gulino, V.; Barberis, L.; Ngene, P.; Baricco, M.; de Jongh, P. E. Enhancing Li-Ion Conductivity in LiBH₄-Based Solid Electrolytes by Adding Various Nanosized Oxides. ACS Appl. Energy Mater. 2020, 3 (5), 4941–4948. https://doi.org/10.1021/acsaem.9b02268.
(3) Gulino, V.; Brighi, M.; Murgia, F.; Ngene, P.; de Jongh, P. E.; Černý, R.; Baricco, M. Room Temperature Solid-State Lithium-Ion Battery Using LiBH₄-MgO Composite Electrolyte. ACS Appl. Energy Mater. 2021, accepted. https://doi.org/10.1021/acsaem.0c02525.

The race to safer and higher-energy batteries is prompting a transition from the liquid electrolyte-based lithium-ion batteries to all-solid-state lithium batteries. Recent studies have identified unique properties of organic battery electrode materials such as moderate redox potentials and mechanical softness which are uniquely beneficial for all-solid-state batteries based on ceramic electrolytes. Here we further explore the promise of organic materials and demonstrate a sulfide electrolyte-based organic-lithium battery with a specific energy of 828 Wh kg^–1, rivaling the state-of-the-art of all-solid-state batteries. Two innovation steps are responsible for the accomplishment. First, the combination of lithium anode and the high-capacity cathode material pyrene-4,5,9,10-tetraone ensures a high theoretical specific energy. Second, the microstructure of the organic cathode is optimized with the introduction of cryomilling, a technique common to processing soft materials but not familiar to electrode fabrication. The cathode material utilization increases to 99.5% as a result, up from the 55-89% previously reported for ceramic electrolytes-based solid-state organic batteries. The improvement highlights the special requirements of solid-state organic electrodes for microstructural engineering while preserving the chemical integrity of components.

Considering the high energy consumption during processing, and the low compliance and adhesion of ceramic electrolytes, the integration of polymer into ceramic electrolytes provides a way to mitigate the interfacial9issues. However, the severe ion concentration gradient, low ionic conductivity,and instability toward Li metal and high-voltage cathodes become the major concerns in applying hybrid electrolytes. In this work, we report a single-ion-conducting hybrid electrolyte with a new electrolytechemical stable polymer and LLZO. The composite electrolyte exhibited a high Li transference15number of 0.94 and electrochemical stability up to 5.6 V vs Li/Li+. Promising averaged Coulombic efficiencies of 99.97% and 99.91% were achieved in cells with LiNi0.8Co0.15Al0.05O2 (NCA) and LiNi0.6Mn0.2Co0.2O2 (NMC622) cathodes for 400 and 200 cycles, respectively.

Promoting fast ion conduction in solid electrolytes is of technological importance to the development of all-solid-state rechargeable batteries. Among solid electrolytes, it has reported that Li-Argyrodite has the potential to achieve high Li-ion conductivity on par with conventional organic liquid electrolytes. The anion-site disorder has been proposed as the root cause of flattening the energy landscape for 3D Li⁺ migration. However, the structural origin in terms of ion migration pathways responsible for the high Li⁺ conductivity remains unclear. It, therefore, prompts us to investigate the Li-ion conduction in Li-Argyrodite (Li₆PS₅X, X = Cl, Br, and I) using variable-temperature neutron diffraction in combination with maximum entropy method (MEM) analysis, bond valence site energy (BVSE), and impedance spectroscopy. Several novel insights are obtained in this work: (1) Li⁺ is found suitable to occupy an interstitial site 16e, which then facilitates the 3D ion conduction by 48h–16e–48h jump with lower activation energy. This finding indicates that the 48h–48h jump alone is insufficient to describe the commonly accepted picture of long-range cage-to-cage ion conduction, (2) we find that the Li⁺ conductivity is, despite rare discussion, influenced by the Li⁺ distribution that can be altered by effective anion charge locating in the center of Li-cage, and (3) a rational approach, i.e., equal negative charge (ENC), is employed to improve the Li⁺ conductivity (from 6 mS/cm at r.t. in Li₆PS₅Cl) through manipulating the distribution of anion charge over 4c and 4a sites in Li_5.7PS_4.7ClBr_0.3 (9 mS/cm at r.t.).

Solid-state lithium metal (Li°) batteries (SSLMBs) are considered as the most promising alternatives to improve the energy density and safety of state-of-the-art liquid-based lithium-ion batteries.¹ Solid polymer electrolytes (SPEs) with excellent processability and flexibility have attracted great attention in the development of practical SSLMBs.

The chemistry of lithium salts plays a pivotal role in dictating the physicochemical and electrochemical performance of SPEs, thus influencing the cyclability of SSLMBs.² In addition to the most popular salt, lithium bis(trifluoromethanesulfonyl)imide anion (LiTFSI), several new salts have been proposed to improve further the ionic conductive of SPEs, such as lithium (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (LiDFTFSI),² lithium (trifluoromethanesulfonyl)(N-bis(methoxyethyl)sulfonyl)imide (LiEFA).³ The implementation of these salts in SPEs resulted in significant improvement of lithium-ion conductivities. However, to date, little attention has been paid to chiral salts and their possible impacts on the physicochemical and electrochemical properties of SPEs.

In this work, we report a new type of chiral salts built from commercially available camphorsulfonic acid and their use as electrolyte salts for poly(ethylene oxide) (PEO)-based SPEs. The fundamental properties of the neat salts and PEO-based electrolytes are comprehensively characterized, in terms of surface morphology, thermal stability, phase transition, ionic conductivity, and electrochemical stability... The role of chirality on the properties of the PEO-based electrolytes is intensively revealed via a combination of experimental and computational methods.⁴

References
[1] X. Judez, G. G. Eshetu, C. Li, L. M. Rodriguez-Martinez, H. Zhang and M. Armand, Joule, 2 (2018), 2208–2224.
[2] H. Zhang, U. Oteo, H. Zhu, X. Judez, M. Martinez-Ibañez, I. Aldalur, E. Sanchez-Diez, C. Li, J. Carrasco and M. Forsyth, Angew. Chem. Int. Ed., 131 (2019), 7911–7916.
[3] H. Zhang, F. Chen, O. Lakuntza, U. Oteo, L. Qiao, M. Martinez-Ibañez, H. Zhu, J. Carrasco, M. Forsyth and M. Armand, Angew. Chem. Int. Ed., 131 (2019), 12198–12203.
[4] Q. Lixin, S. Alexander, Z. Yan, M.-I. Maria, S.-D. Eduardo, L. Elias, T. Marcel, J. Patrik, Z. Heng and A. Michel, J. Electrochem. Soc. 167 (2020), 120541.

The interest for solid-state electrolyte has increased over the last decade with the emergence of high-performance materials which avoid safety issues associated to inflammable solvents. Anti-perovskite compounds, which present an inverse perovskite-type structure where the anionic and cationic sites have been exchanged, are promising materials as solid electrolyte for all-solid-state batteries as they show a high ionic conductivity [1]. The stability of the cubic structure depends on the size of the ions and can result in a distorted system with a lower symmetry such as an orthorhombic or tetragonal structure.

This work investigates the Li_3-xH_xOCl compounds to understand the relationship between the structure and the ionic mobility of the lithium ions in anti-perovskite compounds. For these compounds, the cubic structure leads to a greater ionic mobility due to the anisotropic migration pathway. Li₂OHCl shows a phase transition from orthorhombic to cubic structure around 37 °C which leads to an increase of conductivity of one order of magnitude. To stabilize the cubic phase at room temperature, the Li/H ratio has been modified and the Li_2.1H_0.9OCl has been synthesized. Li₂OHCl and Li_2.1H_0.9OCl are characterized by variable temperature ⁷Li and ¹H static and Magic-Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) and by Pulsed-Field Gradient NMR (PFG-NMR). The two compounds are also studied by variable temperature synchrotron X-Ray Diffraction (XRD), Electrochemical Impedance Spectroscopy (EIS) and other characterization techniques. Other similar materials with an anti-perovskite structure where the halide has been partially or totally substituted by a superhalogen (BH₄^-, BF₄^- and AlH₄^-) have been investigated by density functional theory calculations and tend to show a higher ionic conductivity [2] as shown experimentally by Sun et al [3]. First syntheses were achieved with BH₄^- and characterized by XRD, EIS and NMR.

References:
[1] Y. Zhao and L. L. Daemen J. Am. Chem. Soc. 2012, 134, 15042-15047.
[2] H. Fang et al. J. Mater. Chem. A 2017, 5, 13373-13381.
[3] Y. Sun et al. J. Am. Chem. Soc. 2019, 141, 5640-5644.

Recently, there was been renewed interest in the use of pure alkali metals (e.g., Li, Na) as anodes for rechargeable batteries. Using a pure alkali metal anode with a liquid electrolyte can lead to dendrite shorting and flammability issues. To overcome the issues associated with liquid electrolytes solid-state batteries consisting of Li-ion and Na-solid electrolytes combined with Li and Na anodes and solid cathodes are attracting considerable attention. In the case of a solid-state battery it is anticipated that the alkali metal anode will be under load to maintain contact between it and the solid-state electrolyte. For example, one of the major issues with these batteries is that at high discharging currents voids form near the electrolyte/metal interface which results in an increase in cell resistance which eventually leads to cell failure. One way to achieve high discharge currents is to increase stack (cell) pressure which increases the creep rate of the metal and consequently, prevents void formation and hence, avoiding cell death. Thus, it is important to important to determine the creep mechanism of metal anodes to be able to predict and understand the effect of stack pressure on creep rate and hence, cell life. In addition, to creep behavior of the anode other mechanical properties such as; yield stress and elastic constants must be determined in order to optimize the performance of solid-state batteries with Na or Li anodes. It is the purpose of this talk to present and compare the elastic properties, yield stress in tension/compression and the creep behavior in tension/compression for Na to Li and limited existing values in the literature and show how these properties effect the charging/discharging behavior and cycle life of solid-state batteries with Li and Na anodes. In addition, provide new insights in the design, modeling, and understanding of solid-state batteries utilizing alkali metal anodes

Na-ion conducting solid electrolytes are a key to develop all-solid-state sodium rechargeable batteries. Sulfide materials are suitable electrolytes because of their high conductivity and good deformability. In 2012, we reported cubic-Na₃PS₄ metastable phase, which was precipitated by crystallization of the mother glass, exhibited the conductivity of 10^-4 S cm^-1 [1]. Na₃SbS₄ with a higher Na⁺ conductivities of 10^-3 S cm^-1 is developed [2]. In this study, we have focused on cation-substituted Na₃SbS₄ electrolytes to increase their conductivity. The electrolytes were prepared via mechanochemistry, followed by heat-treatment to enhance their crystallinity. A part of Sb was replaced by W to form solid-solutions Na_3-xSb_1-xW_xS₄ and increased their conductivity. The sulfide superionic conductor with the composition of Na_2.88Sb_0.88W_0.12S₄ exhibited a room temperature conductivity of 3.2 × 10⁻² S cm⁻¹ in a sintered body [3], which is higher than the best Li⁺ conductivity of 2.5 × 10⁻² S cm⁻¹ in LGPS-type Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 [4]. Partial substitution of Sb with W induced the Na vacancy doping and the tetragonal to cubic phase transition. A similar conductivity increase was also observed by a partial substitution of Mo. Conductivity enhancement in the W- and Mo-substituted Na₃SbS₄ was discussed by the first-principles calculations [5]. In addition, the Na_2.88Sb_0.88W_0.12S₄ electrolyte was successfully prepared from an aqueous solution [6] and its precursor solution is useful for close contact with active material particles. All-solid-state Na/S cells with the Na₃SbS₄ electrolyte showed an almost full reversible capacity of 1560 mAh per gram of S and good cyclability at 25^oC [7].

Acknowledgements: This work was supported by Element Strategy Initiative of MEXT, Grant Number JPMXP0112101003 and JSPS KAKENHI Grant Number 18H01713 and 19H05816.

References
[1] A. Hayashi et al., Nat.Commun. 3 (2012) 856.
[2] A. Banerjee et al., Angew. Chem. Int. Ed. 55 (2016) 9634.
[3] A. Hayashi et al, Nat.Commun. 10 (2019) 5266.
[4] Y. Kato et al., Nat. Energy, 1 (2016) 16030.
[5] R. Jalem et al., Chem. Mater., 32 (2020) 8373.
[6] S. Yubuchi et al., J. Mater. Chem. A, 8 (2020) 1947.
[7] T. Ando et al., Electrochem. Commun., 116 (2020) 106741.

Anhydrous sodium sulfide (Na₂S) is a key component in sodium-sulfur batteries as well as an important chemical reagent. However, anhydrous Na₂S is currently prohibitively expensive for applications outside of research labs (>$10 g^-1) and purity is a concern. Herein, we compare the properties of three forms of anhydrous Na₂S: (i) commercially supplied, (ii) Na₂S produced through dehydration and purification of commercial hydrate flakes (Na₂S-xH₂O), and (iii) Na₂S formed by the reaction of hydrogen sulfide with dissolved sodium alkoxide and recovered through solvent evaporation. Crystallinity, purity, thermal stability, and morphology of the various forms of Na₂S were characterized by XRD, FTIR/Raman, TGA, and SEM respectively. Vacuum annealing of low-cost Na₂S hydrate at 150 °C produced anhydrous Na₂S. This dehydrated material retains impurity signatures attributed to polysulfide (Na₂S_x) and sulfite groups (S=O) that were also observed in commercially supplied Na₂S. These impurities could be removed by heating to 400 °C under flowing H₂, the kinetics of which are well-described by a shrinking core model. The solution-based approach resulted in the direct synthesis of crystalline Na₂S anhydride at low temperatures (100 °C) without need for further purification. Both approaches presented herein are inherently scalable with materials costs that are one to two orders of magnitude lower than the current price of anhydrous Na₂S.

The garnet-type Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte is of particular interest because of its high Li-ion conductivity, wide electrochemical window, and good chemical stability under atmospheric conditions, suitable for practical all-solid-state batteries (ASSBs). However, some stability issues, including the reduction of Zr [1]and the contact loss [2], at the LLZO/Li interface has been observed in a number of studies, leading to poor cycle stability and high interfacial resistance. Herein, we have revealed the origin of these instabilities by performing a comprehensive first-principles investigation with a high-throughput interface structure search scheme, based on the density functional theory framework. [3,4]Based on the constructed phase diagrams of low-index surfaces, we found that the coordinatively unsaturated (i.e. coordination number < 6) Zr sites exist widely on the low-energy LLZO surfaces. These undercoordinated Zr sites are reduced once the LLZO surface is in contact with the Li metal, leading to the chemical instability of the LLZO/Li interface. The employments of the approaches such as controlling the synthesis atmosphere are needed for preventing the reduction of LLZO against the Li metal. Besides, the calculated formation and adhesion energies of interfaces suggest that the Li wettability on the LLZO surface is dependent on the termination structure, which may result in the inhomogeneous Li depletion and contact loss. The present analysis with comprehensive first-principles calculations provides a novel perspective for the rational optimization of the interface between LLZO electrolyte and Li metal anode in the ASSB. [5]
References:
[1] Y. Zhu, J. G. Connell, S. Tepavcevic, P. Zapol, R. Garcia-Mendez, N. J. Taylor, J. Sakamoto, B. J. Ingram, L. A. Curtiss, J. W. Freeland, D. D. Fong, and N. M. Markovic, Adv. Energy Mater. 9, 1803440 (2019).
[2] T. Krauskopf, H. Hartmann, W. G. Zeier, and J. Janek, ACS Appl. Mater. Interfaces 11, 14463 (2019).
[3] B. Gao, P. Gao, S. Lu, J. Lv, Y. Wang, and Y. Ma, Sci. Bull. 64, 301 (2019).
[4] B. Gao, R. Jalem, Y. Ma, and Y. Tateyama, Chem. Mater. 32, 85 (2020).
[5] B. Gao, R. Jalem, and Y. Tateyama, ACS Appl. Mater. Interfaces 12, 16350 (2020).

Due to their potential of improved safety, energy density, and operating temperature ranges, all-solid-state batteries (ASBs) have been emerging as a promising power source for electric vehicles. Sulfide solid electrolytes (SEs), such as Li₁₀GeP₂S₁₂ (12 mS cm^-1) and Li_5.5PS_4.5Cl (12 mS cm^-1), are attractive candidates for practical ASBs because of their high ionic conductivities being comparable to those of conventional liquid electrolytes (~10 mS cm^-1) and mechanical sinterability. In ASBs that lacks soft components, electrochemo-mechanical evolutions originating from breathing volumetric strains of electrode active materials upon charge-discharge are imperative in the viewpoints of not only improving performances but also comprehensive understanding complex interfacial phenomena.
In this presentation, we report newly developed operando differential electrochemical pressiometry (DEP) for ASBs. The pressure change signals reflecting volume changes of electrode active materials feature phase transitions of electrode active materials in LiMO₂/Gr all-solid-state cells. Thus, it is demonstrated that DEP analysis enables offering nondestructive determination of state-of-charges of ASBs. Complementary analysis results via ex situ X-ray diffraction measurements is also presented.

An all-solid-state battery with a lithium metal anode is a strong candidate for surpassing conventional lithium-ion battery capabilities. However, undesirable Li dendrite growth and low Coulombic efficiency impede their practical application. We have found that a silver-carbon nanocomposite anode layer, when employed in combination with solid electrolyte, prohibits the dendrite formation and allows the homogeneous Li metal plating and stripping on the surface of current collector. The high Coulombic efficiency thus achieved also eliminates the need of excessive Li metal, further increasing the cell energy density and decreasing the production cost. The silver particles are dissolved in the Li metal during the charging process and appear to assist the homogeneous Li plating. On the other hand, the carbon layer keeps the solid electrolyte physically separated from Li metal, preventing the chemical degradation of solid electrolyte and the Li metal penetration through it. Consequently, the nanocomposite anode formed on a current collector can offer high energy density (>900 Wh/L) and long cycle life (>1,000 times) in the solid-state batteries (0.6 Ah prototype pouch cell).