Symposium F.EN07—Innovative Materials and Cell Design, Processing and Manufacturing Strategies for Solid-State Batteries

Introduction
Recently, rechargeable batteries are strongly desired as energy storage systems. In particular, Li-ion batteries have been realized for practical use from consumer use (e.g., smartphone, laptop computer) to large-scaled energy storage devices such as for in/output control of renewable energy and electric vehicle owing to their high-energy-density and cycle performances. However, safety will be the most important demands with the scale of batteries, and several ignition accidents have been reported. ‘Solvent-free’ all-solid-state batteries exhibit quite high safety and have high-energy-density due to realization of thin-film electrolyte layer without separators. Generally, solid electrolytes are mainly categorized to polymer and inorganic electrolytes. Polymer electrolyte have a sufficient self-standing property, formability of stable interface with electrode and mechanical properties. However, it exhibits relatively low ionic conductivity and Li cation transport number. On the other hand, inorganic electrolyte shows relatively high ionic conductivity, even though it have grain boundary (GB) and be easily broken by external force (e.g. dropping and expansion / contraction with charge-discharge processes). Therefore, in this study, we propose polymer / inorganic hybrid electrolyte for usage of both advantages. Hybrid electrolytes of P(EO/PO) network polymer and two-types of Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) presence/absence GB resistance were investigated for developing high-performance all-solid-state Li batteries.
Experimental
All samples were prepared in a glove box under Ar atmosphere. LiN(SO₂CF₃)₂, rhombohedral (r, having GB resistances) and amorphous (a, not having GB resistances) LAGP (Toshima mfg.), DMPA (photo initiator) and acetonitrile (solvent) were dissolved into polyether-based macromonomer (P(EO/PO) (TA-210, Dai-ichi Kogyo Seiyaku)). Amount of hybridized LAGP((a) or (r)) were 0~200 wt% with polymer electrolytes, respectively. Mixed samples were dried in vacuum for 12h to evaporate acetonitrile. Then the slurry was casted between two glass plates separated by spacers (0.5 mm) and radical polymerized by UV irradiation at 5 min. Then, ionic conductivity were measured by non-blocking electrode cells (Li metal / electrolyte / Li metal). In addition, molecular-level structures of prepared samples were analyzed by high-energy X-ray diffraction (HEXRD) at SPring-8 (BL04B2). LAGP-free polyether electrolyte, polyether/ LAGP(a)150wt% hybrid electrolyte and LAGP(a) powder were enclosed in a sample tube and HEXRD measurements were examined.
Results and discussion
From the result of impedance measurement by blocking electrode systems, possibility of influence of GB may not appear when using an amorphous material was suggested. To compare the effects of GB, non-blocking electrode systems, clear differences between amorphous and rhombohedral were observed by the difference of frequency properties [1]. Impedance spectrum by non-blocking electrode systems of LAGP-free electrolyte and all amorphous LAGP systems showed two semicircular arcs. In both systems, high and low frequency components were assigned as bulk resistance and interfacial resistance with Li metal. On the other hand, rhombohedral LAGP systems showed ellipse behavior because GB resistance appears between the two resistance components. This result shows that amorphous LAGP obtain no GB resistance component into amorphous inorganic / polyether hybrid electrolyte. Also, the radial distribution function of three samples was calculated by HEXRD results. A periodic structure appeared in all electrolytes. Hybrid material of amorphous polymer and inorganic systems also could be measured. The results of this measurement will provide spectroscopic evidence supporting the elimination of GBs. Therefore, influence of GB by using amorphous inorganic electrolyte into polyether electrolyte were clarified.

[1] M. Kato, K. Hiraoka, S. Seki, J. Electrochem. Soc., 167, 070559 (2020).

In contrast to Lithium, Sodium is abundant worldwide. Therefore, it is reasonable to investigate its potential as a substitute for Lithium in battery cells. In addition, solid electrolytes for Sodium-ions show promising characteristics for all-solid-state batteries. If such an electrolyte is made of a Sodium-ion conducting ceramic cells are not inflammable, do not tend to decompose and offer high mechanical strengths to stop dendrite growth. The cells are much safer and are able to operate at higher temperatures, if the solid electrolytes replace the liquid electrolyte and the separator.

For the design of all-solid-state batteries, the thermal properties of the used electrolytes need to be known accurately. As a promising solid electrolyte for sodium-ion cells, Na₃Zr₂(SiO₄)₂PO₄-powder with NASICON structure was synthesized by Sol-Gel-method using Sodium nitrate, Nitric acid, Ammonium dihydrogen phosphate, Tetraethyl orthosilicate and Zirconium (IV) oxynitrate hydrate as precursor materials.
Subsequently, this powder material was sintered by Field Assisted Sintering Technology (FAST) and conventional sintering.

The sintered ceramic pellets were investigated with a focus on thermal properties such as heat capacity and phase transitions using differential scanning calorimetry and thermal conductivity using laser flash analysis. The ionic conductivity was measured by impedance spectroscopy. Stoichiometry of the samples was studied by ICP-OES.

All-solid-state batteries (ASSBs) have attracted enormous attention as one of the critical future technologies for safe and high energy batteries. With the emergence of many highly conductive solid electrolytes in recent years, the bottleneck is no longer Li-ion diffusion within the electrolyte. Instead, many ASSBs are limited by their low Coulombic efficiency, poor power performance, and short cycling life due to the high resistance at the interfaces within ASSBs. In this talk, I will highlight the complex characteristics of interphases, namely the composition, distribution, and electronic and ionic properties of the cathode−electrolyte and electrolyte−anode interfaces. We emphasize the significant role of chemo-mechanical behavior of the interfaces, especially when the ASSBs under constant stack pressure. Multiscale and multimodal characterization methods to explore, diagnose, and understand the dynamic and buried interfaces and interphases are essential to pave the pathway for practical high-energy ASSB.

All solid-state batteries with lithium anodes have the potential of outperforming lithium-ion batteries with respect to energy density, safety, and reliability. However, considerable challenges exist, particularly for ceramic-based electrolytes, with regards to their design for vehicle applications. The thin film configurations that currently exist have low practical energy densities, are difficult to scale, and require expensive vacuum technologies. To overcome these difficulties, we have been using a scalable freeze tape casting method to produce porous garnet scaffolds that can be co-sintered with thin, dense components to form bilayers. The scaffold is then infiltrated with active material and other components, and lithium is attached to the other side of the bilayer to form a cell. The use of a secondary soft electrolyte obviates the need for a co-sintering step, which carries with it the risk of adverse reactions between the active material and the garnet. Batteries prepared this way may be able to achieve specific energies in excess of 400 Wh/kg. We will discuss what still needs to be done to make this a reality.

It is well established that the miniaturization of batteries has not kept pace with the miniaturization of electronics. Three-dimensional (3D) batteries, which were developed with the intent of improving microbattery performance, have had limited success because of fabrication challenges and material constraints. Solid-state, 3D batteries have been particularly susceptible to these shortcomings, as conformal electrolytes for 3D configurations suffer from either having low conductivity or having interfacial reactions that lead to high electrolyte-electrode ionic resistances. In the work reported here, we overcome these solid electrolyte limitations by using an ionogel electrolyte, a subclass of solid electrolytes comprised of a room temperature ionic liquid electrolyte confined within an inorganic matrix. The high conductivity of the ionogel electrolyte (10^-3 S cm^-1) ensures sufficient ion transport for high power densities.
Rather than using a 3D geometry, we selected a different platform that not only demonstrates the benefit of a non-planar electrode configuration but also enables easier fabrication using established materials and processes. Specifically, we constructed a solid-state 2.5D battery consisting of a 3D LiFePO₄ (LFP) post array and a planar, Li anode separated by the ionogel electrolyte, which forms around the individual LFP posts and, while in the sol state, penetrates the 3D electrode array. The ability to process the ionogel as a liquid, which then gels to a macroscopic solid phase while maintaining the liquid-like behavior at the nanoscale, satisfies the conformality and wetting issues often required for complex electrode architectures. This 2.5D battery offers high areal energy densities from the post array while the high conductivity solid electrolyte enables high power densities (4 mWh cm^-2 at 2.8 mW cm^-2). These values for the 2.5D solid-state microbattery exceed the energy and power densities of any 3D solid-state system. In addition, the battery exhibited minimal capacity loss when cycled at 0.5 mA cm^-2 over 50 cycles and complementary interfacial studies show that the 2.5D battery avoids the interface impedances which have limited other solid-state 3D battery systems. A multiphysics model derived for the 2.5D geometry provides guidance for achieving significantly higher energy and power densities.

All-solid-state batteries combining an alkali metal anode and a high-voltage cathode have the potential to achieve high energy density. However, interface stability remains a major challenge. On the anode side, dendrite formation limits the maximum plating current density and prevents fast charging, while on the cathode side the limited oxidative stability of most solid electrolytes is a major challenge, especially when the battery is charged beyond 4 V.
Here we demonstrate that the critical current density for dendrite formation in the archetypical ceramic solid electrolyte Na-β”-alumina is ten times higher than that measured on a garnet-type Li₇La₃Zr₂O₁₂ electrolyte under equivalent conditions and reaches up to 12 mA/cm² at room temperature [1]. Further, we show that above the melting temperature of sodium, a cumulative capacity of >10 Ah of sodium can be plated and stripped at an enormous current density of 1000 mA/cm² without dendrite formation [2].
On the cathode side, we have been developing hydroborate solid electrolytes reaching ionic conductivities >1 mS/cm at room temperature [3]. We already demonstrated the integration of these electrolytes with a 3 V class cathode [4, 5], but recently managed to obtain stable cycling with a 4 V class cathode via a self-forming passivating interface [6]. Combined with their low gravimetric density <1.2 g/cm³, low toxicity, high thermal and chemical stability, stability vs lithium and sodium metal anodes, soft mechanical properties enabling cold pressing [4], compatibility with solution infiltration [7], and potential for low cost [8, 9], hydroborate electrolytes represent a promising option for the cathode side of a competitive future all-solid-state battery.

References:
[1] M.-C. Bay, M. Wang, R. Grissa, M. V. F. Heinz, J. Sakamoto, C. Battaglia, Adv. Energy Mater. 2019, 201902889
[2] D. Landmann, G. Graeber, M. V. F. Heinz, S. Haussener, C. Battaglia, submitted
[3] L. Duchêne, A. Remhof, H. Hagemann, C. Battaglia, Energy Storage Mater. 2020, 25, 782
[4] L. Duchêne, R.-S. Kühnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, Energy Environ. Science 2017, 10, 2609
[5] R. Asakura, L. Duchêne, R.-S. Kühnel, A. Remhof, C. Battaglia, ACS Appl. Energy Mater. 2019, 2, 6924
[6] R. Asakura, D. Reber, L. Duchêne, A. Remhof, H. Hagemann, C. Battaglia, submitted
[7] 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
[8] A. Gigante, L. Duchêne, R. Moury, M. Pupier, A. Remhof, H. Hagemann, ChemSusChem 2019, 12, 4832
[9] S. Payandeh, R. Asakura, P. Avramidou, D. Rentsch, Z. Lodziana, R. Cerny, A. Remhof, C. Battaglia, Chem. Mater. 2020, 32, 1101

Solid-state batteries have emerged as the most promising contender to displace conventional Li-ion batteries in applications such as electric vehicles with their potential for greater energy density and improved safety. In order to transition from hype to true disruptor however, significant performance, manufacturing, and cost advancements must still be demonstrated. The electrolyte is the most important component of a solid-state cell, and it must combine high conductivity with anode and cathode compatibility and allow for high-throughput cell production. Oftentimes, the highest conductivity sulfide electrolytes are compatible only with graphite or higher voltage LTO anodes which limits the energy density advantages of the solid-state cell. Likewise, most inorganic solid-state cells today require production methods such as high-temperature sintering or warm isostatic pressing that are not compatible with the Li-ion production infrastructure that has been critical in driving down costs in recent years.

Solid Power is addressing each of these challenges by combining its novel sulfide electrolytes with manufacturing that closely mirrors that of Li-ion mass production today. Solid Power has a truly all-solid material set based on electrolytes that deliver 2-9 mS/cm at room temperature with stability against both Li metal anodes and Ni-rich NMC cathodes. This presentation will cover key aspects of material design for solid-state cells with an emphasis on manufacturability. Solid Power’s pilot roll-to-roll production line will be introduced along with performance of cells produced on the line. Solid Power is currently producing multi-layer pouch cells up to 20 Ah in capacity with an eye toward early electric vehicle qualification. The presentation will also include a discussion of low temperature performance, quick charge capability, and abuse testing.

Stringent chemical and mechanical constraints apply to the interfaces in solid state batteries. They have to be (electro)chemically stable against the chosen electrode materials and applied potentials. In addition, they have to be tolerant to the various mechanical issues that occur in solid state batteries such as electrode swelling/shrinking, stress concentrations at current focusing areas etc. I will present the requirements for chemically stable interfaces between various cathode and electrolyte materials and discuss to what extent coatings can address chemical incompatibility. A repeated stripping and plating of a lithium metal anode presents significant challenges. I will present results from modeling to understand the interplay between current density, stress, and lithium growth through the electrolyte. Our results indicate that in good conductors the build-up of stress to the point that it fractures the electrolyte is unlikely, indicating that other mechanisms are responsible for lithium penetration through the solid electrolyte. We also find that conductivity of the solid electrolyte plays a central role in the mechanical reliability of a solid-state battery.

The advent of solid-state batteries has spawned a recent increase in interest in lithium conducting solid electrolytes, especially in the lithium thiophosphates. However, many open questions remain when trying to optimize electrolytes and understand solid state battery chemistries.

In this presentation, we explore the influence of inductive effects in superionic conductors showing that in Li₁₀GeP₂S₁₂, these can strongly affect the ionic transport.
We further show a better understanding of the structure-transport properties of the novel lithium and sodium rare-earth halides that show promising ionic conductivities for their use in solid state batteries.
Lastly, we show how solvent treatment of the lithium argyrodite Li₆PS₅Cl affects battery performance despite having no influence on the structural stability, suggesting stronger influences of slurry processing than previously believed.

We will discuss the development of Na-ion and K-ion conducting films using atomic layer deposition (ALD) and their progress towards nanoscale solid-state batteries (SSBs) based on more abundant alkali metals such as sodium and potassium. Building upon our achievement with 3D Li-ion SSBs¹, the enabling feature of which was the development of a lithium phosphorus oxynitride (LiPON) solid-state electrolyte (SSE)², we present two new solid electrolytes for Na-ion and K-ion batteries. The ALD process for these films can be generalized to involve the thermal reaction of the respective alkali tert-butoxide (XO^tBu, X = Li, Na, K) with diethylphosphoramidate (DEPA). The NaO^tBu-DEPA process produced a sodium phosphorus oxynitride (NaPON) film, analogous to its cousin LiPON. Surface analysis by X-ray photoelectron spectroscopy revealed that the NaPON (Na₄PO₃N) film closely resembled that of LiPON (Li₂PO₂N), containing similar atomic ratios (N/P = 1) and two nitrogen species (=N– and >N–), deeming their classification as phosphazenes.³ However, the KO^tBu-DEPA process resulted in a potassium phosphate (KPO) film containing ~1% nitrogen and a considerable amount of carbon incorporation. Electrochemical analysis through electrochemical impedance spectroscopy (EIS) was employed to probe the materials ionic transport behavior. NaPON displayed an ionic conductivity of 1.0 × 10^-7 S/cm at 25 °C and up to 2.5 × 10^-6 S/cm at 80 °C, with an activation energy of 0.53 eV. The ionic conductivity and activation energy of NaPON are in close agreement with that of ALD LiPON and other ALD-fabricated Li-ion conductors. EIS measurements of KPO displayed a lower ionic conductivity of 7.4 × 10^-9 S/cm at 25 °C, which is attributed to loss of nitrogen in the obtained structure. The NaPON and KPO ALD processes represent the first ALD synthesized Na-ion and K-ion conducting films. A holistic review of LiPON, NaPON, and KPO will be presented, ranging from their reaction mechanisms to their structure and ionic transport properties. Lastly, we will focus on the aim of obtaining a 3D Na-ion SSB enabled by the NaPON SSE. This includes consideration of the conformality of NaPON in nanoporous structures, electrochemical stability, and demonstration of planar Na-ion SSBs with thin-film electrodes.

1. Pearse, A.; Schmitt, T.; Sahadeo, E.; Stewart, D. M.; Kozen, A.; Gerasopoulos, K.; Talin, A. A.; Lee, S. B.; Rubloff, G. W.; Gregorczyk, K. E., Three-Dimensional Solid-State Lithium-Ion Batteries Fabricated by Conformal Vapor-Phase Chemistry. ACS Nano 2018, 12 (5), 4286-4294.
2. Pearse, A. J.; Schmitt, T. E.; Fuller, E. J.; El-Gabaly, F.; Lin, C.-F.; Gerasopoulos, K.; Kozen, A. C.; Talin, A. A.; Rubloff, G.; Gregorczyk, K. E., Nanoscale Solid State Batteries Enabled by Thermal Atomic Layer Deposition of a Lithium Polyphosphazene Solid State Electrolyte. Chemistry of Materials 2017, 29 (8), 3740-3753.
3. Nuwayhid, R. B.; Jarry, A.; Rubloff, G. W.; Gregorczyk, K. E., Atomic Layer Deposition of Sodium Phosphorus Oxynitride: A Conformal Solid-State Sodium-Ion Conductor. ACS Applied Materials & Interfaces 2020, 12 (19), 21641-21650.

Lithium ion batteries (LIBs) typically utilize organic liquid electrolytes, but their flammability often places limits on their use. Solid electrolytes may alleviate concerns regarding flammability, but the interfacial resistance between a solid electrolyte and an electrode is one major hindrance for their application. The introduction of a buffer layer at the electrode-electrolyte interface may reduce this resistance and improve battery performance. In this work, an acetonitrile containing buffer layer was introduced between a graphite anode and a PEO-LiTFSI electrolyte. This electrode was examined in a half-cell configuration. Impedance spectra for cells with and without the buffer layer were obtained at assembly and after three charge/discharge cycles. At assembly, the interfacial impedance of the cells with a buffer layer was reduced by 25% when compared to unbuffered cells, and it continued to decrease as the cells were cycled. Improved contact between the active material and electrolyte is a potential reason for the lowered interfacial impedance. The addition of a buffer layer shows promise towards the goal of making an all-solid LIB with acceptable electrochemical performance.

Solid-state batteries have a potential to enhance energy density and offer greater safety when compared with the conventional Li-ion batteries. However, a bottleneck with using a solid-state electrolyte lies in mechanically integrating the solid-state electrolyte with the active materials (electrodes). For example, inserting and extracting Li into electrode lattices expands electrode materials upto 10% in volume. These chemo-mechanical strains in the electrode induces misfit stresses at the electrode/electrolyte interfaces, which weaken the structural integrity and electrical conductivity of the battery material composite. In today's talk, we will show how epitaxial strains in thin-film electrodes can be engineered to deter delamination. We develop a continuum model to explore the interplay between phase-transformation microstructures and substrate strains in thin-film electrodes. We apply this model to the vanadium-oxide system, and show how tensile straining of electrodes deters delamination and enhances reversible cycling of electrode materials. We plot a “phase-diagram” for thin-film electrodes as a function of battery voltage and substrate strain—this plot identifies a narrow-range of electro-mechanical boundary conditions under which a thin-film electrode can be repeatedly cycled with minimum mechanical degradation. Broadly, our work provides a battery design framework to mechanically engineer electrodes, and to enhance battery safety and lifespan.

Conventional Li-ion batteries using liquid electrolytes suffer from inherent safety issues due to poor thermal stability of liquid electrolytes. Development of All-solid Li-ion Batteries by substituting liquid electrolytes to non-flammable solid electrolytes can solve this issue. Li₇La₃Zr₂O₁₂(LLZO) has been considered as a major candidate for making safe and high capacity cells due to its high ionic conductivity and chemical compatibility with Li anode.
However, cathode side instability still needs to be overcome. Co-sintering is needed to gain good contact between cathode and LLZO, and to reduce contact resistances. However, this process leads to formation of detrimental Li-insulating interphases. Despite the computational studies on layered oxide cathode|LLZO predicting low chemical reactivity, many experimental studies including works from our group, show formation of detrimental secondary phases at the interface during sintering. Moreover, predicted decomposed phases do not match with experimental findings.
The discrepancy comes from the effect of gas environment, which was not addressed in former computational or experimental works. Recent works from our group addressed the detrimental role of the CO₂ in the gas environment, on the interface stability of on LiCoO₂|LLZO, LiNi_0.6Mn_0.2Co_0.2O₂|LLZO interfacial stability. Li₂CO₃ formed in both systems because of reaction with CO₂(g) in ambient air. This is precursor to the formation of Li-deficient detrimental secondary phases such as La₂Zr₂O₇ and La containing perovskite phases, LaCoO₃, La(Ni,Co)O₃.
In this work, we systematically investigated effect of gas environment by annealing samples in pure O₂, humidified O₂ (O₂ with 2wt% H₂O), N₂, and CO₂ gas environments. Humidified O₂ and CO₂ environment were chosen to unravel effect of H₂O(g) and CO₂(g). We chose N₂ to investigate possibility of using it as a low-cost alternative for pure O₂ environment. We deposited thin film(~100nm) LiNi_0.6Mn_0.2Co_0.2O₂ on top of solid electrolyte by RF sputtering, and post-annealed samples to simulate the sintering process. The sample design placed interfacial region within detection depth of X-ray Absorption Spectroscopy. This thin film approach enabled us to detect the onset of interfacial degradation, even when formed secondary phases lack crystallinity. We correlated results from chemical characterization techniques (X-ray Absorption Spectroscopy, X-Ray Diffraction) with Electrochemical Impedance Spectroscopy to quantify the effect of interfacial degradation on cell performances.
The result showed that O₂ environment enables the best stability, and actually improves the interface. Interfacial resistance decreased as annealing temperature increased in O₂, and interface remained chemically stable up to 700°C. Annealing in N₂ environment also showed chemical stability up to 700°C as no secondary phases were observed. However, we could see slight reduction from the sample, showing that N₂ environment is less favorable compared to pure O₂. We found that annealing at 500°C in humidified O₂ led to hydration of the sample, causing interfacial resistance increase. However, we also observed interfacial resistance decrease along with dehydration at 700°C, showing that the detrimental effect of H₂O(g) can be avoided with sufficiently high sintering temperature. CO₂ environment was shown as most detrimental, as we observed total loss of cathode|electrolyte capacitance, together with major degradation products. This comparison concludes that avoiding CO₂ is important for maintaining a well adhered cathode-LLZO interface that transfers Li ions fast.
We acknowledge funding from U.S. Army Research Office through Institute for Solider Nanotechnologies. (Cooperative Agreement Number W911NF-18-2-0048)

Seeking better performing solid-state electrolytes for post-Li batteries, complex hydride-based compounds have recently drawn attention thanks to several remarkable properties. They show chemical robustness and good thermal stability, but also superior oxidative stability and lower density with respect to oxides and sulphides.[1][2] Interestingly, such class of materials displays order-disorder phase transitions that dramatically increases the ionic conductivity, achieving superionic regime (σ > 1 mS cm^-1).[3] However, such rearrangement generally occurs far from room temperature (rt), thus limiting practical applications. Therefore, increasing the disorder in the structure to lower the phase transition towards ambient temperature is the key for further development. To do so, the combination of two different hydroborate salts can result in a new phase that decreases/suppresses the phase transition, achieving fast cation motion even at rt.[4]
Recently, electrochemical studies on a wide series of binary anion mixtures based on closo- and carbacloso-hydroborates pointed out that their oxidative stability is limited to the decomposition voltage of the less stable anion.[5] As an example, in the Na₄(CB₁₁H₁₂)₂(B₁₂H₁₂), which shows the highest oxidative stability of the pool, the practical voltage window corresponds to the oxidation of the [B₁₂H₁₂]^2- cage.[6] It was already demonstrated that complete halogenation of a [B₁₂H₁₂]^2- leads to a framework less prone to the oxidation, thus potentially enhancing the overall electrochemical stability while employed in a solid-state conductor. On this basis, the novel phase Na₄(CB₁₁H₁₂)₂B₁₂Br₁₂ obtained by complete bromination of Na₂[B₁₂H₁₂] and subsequent solid-state high-energy ball-milling with Na[CB₁₁H₁₂] will be presented. DC conductivity at rt is found to be 0.8 mS cm^-1, while reaching 2.2 ms cm^-1 at 40°C. Temperature-dependent electrochemical impedance spectroscopy reveals a non-Arrhenius behavior of the DC conductivity between – 50°C and 150 °C, although without any conductivity jump due to order/disorder phase transition. Crystal structure analysis using temperature-dependent synchrotron powder diffraction will be also presented. The electrochemical characterization of the oxidative stability by means of slow-rate scan cyclic voltammetry evidenced a wide voltage window up to 4.35 V vs. Na⁺/Na. Such improvement opens the gate to the use of high-voltage positive electrodes, such as Na₃V₂(PO₄)₂F₃ (NVPF), thus achieving post-Li solid-state batteries with more competitive electrochemical performance.

[1] S. Kim, H. Oguchi, N. Toyama, T. Sato, S. Takagi, T. Otomo, D. Arunkumar, N. Kuwata, J. Kawamura, S. Orimo, Nat. Commun. 10 (2019) 1081.
[2] R. Asakura, L. Duchêne, R.S. Kühnel, A. Remhof, H. Hagemann, C. Battaglia, ACS Appl. Energy Mater. 2 (2019) 6924.
[3] W.S. Tang, M. Matsuo, H. Wu, V. Stavila, W. Zhou, A.A. Talin, A. V. Soloninin, R. V. Skoryunov, O.A. Babanova, A. V. Skripov, A. Unemoto, S.-I. Orimo, T.J. Udovic, Adv. Energy Mater. 6 (2016) 1502237.
[4] W.S. Tang, K. Yoshida, A. V. Soloninin, R. V. Skoryunov, O.A. Babanova, A. V. Skripov, M. Dimitrievska, V. Stavila, S. Orimo, T.J. Udovic, ACS Energy Lett. 1 (2016) 659.
[5] M. Brighi, F. Murgia, R. Cerny, Cell Reports Phys. Sci. 1 (2020) 100217.
[6] R.T. Boeré, J. Derendorf, C. Jenne, S. Kacprzak, M. Keßler, R. Riebau, S. Riedel, T.L. Roemmele, M. Rühle, H. Scherer, T. Vent-Schmidt, J. Warneke, S. Weber, Chem. - A Eur. J. 20 (2014) 4447.

Despite being considered promising as a potential solid electrolyte for solid-state Li-ion batteries, Al-doped Li-La-Zirconate (LLZO) suffers from compositional/structural instability in the presence of moisture. In this regard, our prior work has indicated that the formation of La₂Zr₂O₇ in the bulk (in addition to Li₂CO₃ formation at surface) of Al-doped LLZO upon exposure to air leads to structural/mechanical instability and spontaneous cracking/disintegration due to the development of internal stresses [Kobi et al.; J. Eur. Ceram. Soc. 38 (2018) 4707]. This causes reduction in the Li-ion conductivity by at least ~3 orders of magnitude within 3 weeks of exposure to air. In order to address this major problem, earth abundant Mg has been attempted as a dopant, fully or partially replacing the Al-dopant [Kobi et al.; J. Electrochem. Soc. 167 (2020) 120519, Amardeep et al.; Scripta Mater. 162 (2019) 214]. As has been realized, Mg-doping also allows stabilization of the desired cubic garnet phase at room temperature, leading to almost similar Li-ion conductivity as for Al-doped LLZO. More importantly, Mg-doping supresses, in significant terms, the formation of deleterious phases and associated cracking upon long term exposure to air; thus causing no notable reduction in the Li-ion conductivity even after more than 3 weeks of air exposure. The outstanding air stability, concomitant absence of impurity phases at the surface and good Li-ion conductivity cast their influence also on the interfacial resistance at electrode/electrolyte interface. Thus, the optimized Al/Mg co-doped LLZO exhibits a very low area-specific resistance of ∼18 Ω cm²; which is also among the lowest reported to-date, sans any additional surface/interfacial engineering.

Solid-state batteries are generally considered to be safer than their liquid-state counterparts due to their decreased potential for fire or short circuiting. The fabrication of solid-state batteries relies on the application of stack crimping pressure that increases the interfacial surface contacts between electrolytes and the electrodes. However, excessive compressive crimping stresses (that occur in cell assembly) can give rise to cracking phenomena that can degrade battery performance and lead to thermal runaway or failure. It is, therefore, important to develop an understanding of failure mechanisms in solid-state Li-ion electrolytes. In this paper, we use a combination of in-situ optical microscopy and Digital Imaging Correlation (strain mapping) techniques to study compressive deformation and cracking phenomena in a novel solid state Li-ion electrolyte. The stress states associated with the different stages of compressive deformation are also presented along with those due to charge-discharge cycles. The implications of the results are discussed for the material design of robust solid-state Li ion batteries.

Next generation of energy storage devices may largely benefit from fast and solid Li⁺ ceramic electrolyte conductors to allow for safe and efficient batteries and fast data calculation. For those applications, the ability of Li-oxides to be processed as thin film structures and with high control over Lithiation and phases at low temperature is of essence to control conductivity. Through this presentation we review the field from a new angle, not only focused on the classics such as Li-ionic transport and electrochemical stability window for Li-solid state battery electrolytes, but focusing on opportunities and challenges routes in thermal and ceramic processing of the components and their assemblies with electrodes. Also, we will carefully review and give perspectives on the role of solid state battery ceramic strategies for the electrolyte on the electrode interfaces and towards charge transfer and vs. current densities. In other words, it will be a little ceramicist (own) love story on the good and the evil we can design by smart ceramic design at the interfaces originating by the very first choices made in the electrolyte ceramic structure and material design. In the second part of the talk we will discuss new opportunities on low temperature processing of solid state electrolyte ceramics that do not technically require “classic sintering” and avoid prior particle calcination; instead demonstrating opportunities to use liquid based direct densification routes and vacuum techniques to design solid electrolytes and grafting interfaces to new hybrid and solid state battery prototypes targeted at processing below 700C for all parts. Collectively, the insights on solid state energy storage provide evidence for the functionalities that those Li-solid state material designs can have for cost and mass manufacturable solid state and hybrid battery prototypes.
References:
1. A Low Ride on Processing Temperature for a Fast Li Conduction in Garnet Solid State Battery Films R Pfenninger, M. Struzik, I Garbayo, E Stilp, JLM Rupp, Nature Energy, 4, 475–4832019, 2019
2. Lithium-Containing Thin Films JLM Rupp, R Pfenninger, M. Struzik, A Nenning, I Garbayo, US 62/718,838 (2018)
3. Methods of Fabricating Thin Films Comprising Lithium-Containing Materials JLM Rupp, R Pfenninger, M. Struzik, A Nenning, I Garbayo, US 62/718,842 (2018)
4. Glass-Type Polyamorphism in Li-Garnet Thin Film Solid State Battery Conductors
I Garbayo, M Struzik, WJ Bowman, R Pfenninger, JLM Rupp, Advanced Energy Materials, 1702265 (2018)
5. Solid-state Electrolyte and Method of Manufacture Thereof. ZD Hood, Y Zhu, L Miara, JLM Rupp. US 62/713,366 (2018)
6. Solution-Processed Solid-state Electrolyte and Method of Manufacture Thereof. Y Zhu, ZD Hood, L Miara, JLM Rupp US 62/713,428 (2018)

We recently reported an ultrafast high temperature sintering (UHS) for solid state electrolytes (Science, 2020, May Cover). In this talk, I will discuss our progress in applying the UHS in the following various aspects for solid state batteries.

(1) High-performance solid-state electrolyte. The long sintering time of conventional syntheses can lead to Li loss in garnet SSEs caused by the evaporation of Li and the formation of secondary phases that lead to lower ionic conductivity. In contrast, the UHS technique enables us to tune the sintering time in units of seconds, which provides excellent control in terms of the Li content and grain growth. I will discuss our progress on applying UHS method to sintering various solid-state electrolyte, from compositions to microstructures. By programming the temperature and time (T-t), we can establish process diagrams to sinter porous vs. dense structures.

(2) Printed thin film solid-state electrolyte. I will talk about a solution-based printing process followed by rapid (~3 seconds) high-temperature (~1500 °C) reactive sintering for the fabrication of high-performance ceramic SSE films. The SSEs exhibit a dense, uniform structure and a superior ionic conductivity of up to 1 mS/cm, comparable to the bulk conductivity.

(3) Multilayer solid-state electrolyte and electrolyte-electrode co-sintering for solid state batteries. The rapid sintering also allows us to sintering multilayer solid-state electrolytes with significantly reduced interlayer diffusion.

A UMD spinoff, HighT-Tech LLC, is commercializing the above inventions (www.highT-tech.com).

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. To overcome these limitations in ceramic synthesis and processing, we report 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. The UHS method can directly sinter oxide precursors into solid, dense ceramics in seconds. Compared with previous 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 provides 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 solid-state batteries.

The low fracture toughness of ceramic solid electrolytes can significantly limit battery performance and reliability. While small dimensions are generally needed for faster ion transport, these length scales also restrict the approaches that can be used to improve fracture resistance. Nanoscale reinforcements are thus a logical option for improving the fracture resistance of ceramic electrolytes. Reduced graphene oxide (rGO) and hexagonal boron nitride (hBN) have been successfully used to reinforce a variety of polymer and engineering ceramics, where significant changes in the elastic modulus and toughness have been obtained with relatively low volume fractions. In this talk, the reinforcement capability of small amounts of rGO and hBN added to oxide-based lithium-ion conductors will be discussed. Furthermore, a framework to assess the relevant toughening length scales and to provide guidelines for designing toughened solid electrolytes using nanomaterials will be presented.

Research from our Nanostructures for Electrical Energy Storage (NEES) Energy Frontier Research Center has shown that 3D architectures are key to enhanced performance in thin film batteries. As thin film processing and patterning have expanded to include battery materials, microfabrication is increasingly capable of creating high performance 3D solid state batteries (3D-SSBs). We have recently fabricated and studied two archetypal 3D-SSB designs: one employing multiple, coplanar battery layers and one using vertical channels of concentric batteries. Our experimental results validate the benefits of both 3D architectures for simultaneous high power and high energy.

Here we first summarize these results and highlight the distinctly different process sequences necessary for fabrication of the different 3D-SSBs. We then present new results from multiphysics modeling and simulation (COMSOL) that allow us to scale the energy and power metrics to different dimensions and configurations of the two designs. The electro-chemo-mechanical models also provide the basis for further examining the consequences of the 3D design. Calculations of the local stress distribution, Joule heating at high power, and transport properties at nano- and mesoscale will be shown. We find that although the physics is the same, the geometric differences play a large role in the ultimate performance of the 3D-SSBs.

These results indicate that 3D-SSBs may exceed the performance of conventional liquid electrolyte cells and also thin film solid-state batteries that use thick composite electrodes and sintered electrolytes, because thin film fabrication (1) enables designs optimized for simultaneous high power and energy and (2) minimizes excess and passive materials that do not contribute to performance. Results show that multilayer 3D thin film batteries can produce 2-3X higher energy density and 10X higher power density than conventional batteries. From these we share an emerging picture of cell design rules for further development of 3D-SSBs.

Polymer gel electrolytes have been widely used in lithium-ion batteries since 1999, where they compete with liquid electrolytes. However, they do not fully qualify as solid-state electrolytes, since they are used in combination with separators and, despite common claims concerning safety, they are mostly used to improve the cohesion between battery elements and help the wetting of the separator onto which the polymers (usually PVDF-HFP) are coated. On the other hand, commercial lithium metal batteries use solvent-free (PEO-based) solid polymer electrolytes (SPEs) without any separator, but the operation temperature for automotive application of SPEs is limited to > 70°C since the electrolyte is not sufficiently conductive at room temperature. Besides, solvent-free SPEs suffer from low anodic stability, which restricts the energy density of Li metal polymer batteries.
Research on SPEs has long been targeting single-ion conductors to prevent the establishment of concentration gradients and the resulting increase in electrolyte resistance and ultimately, the lithium depletion at metallic anodes, responsible for the fast growth of dendrites. However, until recently, these single-ion SPEs suffered from the same drawbacks as regular salt-in-polymer SPEs, with high operation temperatures and a limited anodic stability that prevents the use of high energy cathodes. On the other hand, studies on gel electrolytes are less clear regarding their aims, as they are often use unsubstantiated safety claims that are only fully valid for ‘dry’ SPEs.[1]
Thus, an overview of recent results on gel and polymer electrolytes for various types of batteries will be given. In particular, the use of non-fluorinated polymer matrixes such as crosslinked polymethacrylic acid (PMMA) to form gel electrolytes in situ for state-of-the-art lithium-ion batteries [2,3](or its fluorinated ionomeric version for forming crosslinked single-ion conductor gels) will be discussed. Besides, non-fluorinated and cheap ionomers such as lithium polyacrylate (LiPAA) were also proposed as water-in-ionomer electrolytes for high voltage aqueous lithium-ion batteries [4]. Finally, for next generation lithium metal batteries, nanophase separated single-ion conductors membranes were easily processed from aromatic polyethersulfone block copolymers to allow high lithium plating current densities (i.e. 2 mA cm^-2 at 20°C) and to enable the operation of high voltage lithium metal battery down to 0°C. Finally, since PEO presents considerable advantages in terms of cathodic stability, especially critical for divalent cation batteries (i.e. Ca-metal) the strategies to allow for high voltage cathode operation in combination with PEO will be discussed as well.
Acknowledgments
The research has been partially done within the VIDICAT project, funded by the European Commission under GA n°829145 within the Horizon 2020 programme.
References
[1] C. Iojoiu, E. Paillard, Polymer Electrolytes for Solid-State Lithium Batteries, in: S. Passerini, D. Bresser, A. Moretti, A. Varzi (Eds.), Encyclodedia Electrochem. Sect. Batter., 2nd ed., Wiley-VCH Verlag, Weinheim, Germany, 2020: pp. XX–XX.
[2] A. Hosseinioun, E. Paillard, In situ crosslinked PMMA gel electrolyte from a low viscosity precursor solution for cost-e ff ective , long lasting and sustainable lithium-ion batteries, J. Memb. Sci. 594 (2020) 117456. doi:10.1016/j.memsci.2019.117456.
[3] A. Hosseinioun, P. Nürnberg, M. Schönhoff, D. Diddens, E. Paillard, Improved lithium ion dynamics in crosslinked PMMA gel polymer electrolyte, RSC Adv. 9 (2019) 27574–27582. doi:10.1039/C9RA05917B.
[4] X. He, B. Yan, X. Zhang, Z. Liu, D. Bresser, J. Wang, R. Wang, X. Cao, Y. Su, H. Jia, C.P. Grey, H. Frielinghaus, D.G. Truhlar, M. Winter, J. Li, E. Paillard, Fluorine-free water-in-ionomer electrolytes for sustainable lithium-ion batteries, Nat. Commun. 9 (2018) 5320. doi:10.1038/s41467-018-07331-6.

Solid-state lithium (Li) metal batteries based on solid polymer electrolytes are attracting a lot of interest due to good safety and high energy density. Among various polymer electrolytes, poly(vinylidene fluoride) (PVDF)-based electrolytes present higher ionic conductivity, large mechanical strength, and good electrochemical and thermal stability. In this talk, we will discuss the influence of (inorganic or organic) additives and residual solvent on the performance of the electrolytes as well as solid-state lithium metal batteries. A small amount of solvent [e.g., N,N-dimethylformamide (DMF)] is left in the electrolytes to bond with Li ions as Li-DMF complexes, not as free solvent, which could promote the Li-ion transport with the assistance of PVDF chains. Thus, by tuning the solvation effect and/or the synergistic effect with additives, the overall performance of the PVDF-based electrolytes including ionic conductivity, electrochemical stability window and stability against a Li anode can be significantly enhanced. With the fine-tuned PVDF-based electrolytes, the solid-state lithium metal batteries with a LiFeO₄ or LiCoO₂ cathode can possess a long cycle life of 1000 cycles and a high capacity retention at room temperature.

Solid polymer electrolytes (sPE) offer a pathway for safer, less flammable lithium batteries. However, developing a polymer that provides high Li⁺ mobility as well electrochemical stability remains a challenge, because ion conductive functional units in the polymer main-chain (e.g., polycarbonate and polyether) usually suffer from poor electrochemical stability at high and low potentials. In the first part of this presentation, I will show our design and synthesis of an sPE with pendent carbonate on a hydrocarbon backbone, that has excellent conductivity and electrochemical stability simultaneously. This pendant polycarbonate is different from conventional polycarbonate electrolytes because the carbonate moiety is in the sidechain, which mitigates the stability problem of polycarbonate backbone while still providing high ionic conductivity when used with a plasticizer. Ionic conductivity as high as 1.1 mS/cm was obtained at 22^oC. Stable lithium metal plating using and stripping using the sPE was observed for 1,200 hours and electrochemical stability up to 4.6 V vs Li⁺/Li has been demonstrated. The low interfacial resistance (<160 ohm cm² at 22^oC) and reasonable ionic conductivity have enabled acceptable cycling performance in a Li-LiFePO₄ battery at 0.98 mA/cm² for 2,300 cycles. In the second part of this presentation, I will show our second-generation pendent polymer electrolyte with a block co-polymer structure, to enable facile tuning of the properties.
This presentation is based on our recent publications:
1. Yubin He, Nian Liu, Paul Kohl. Journal of the Electrochemical Society, 2020, DOI: 10.1149/1945-7111/ab9759.
2. Yubin He, Nian Liu, Paul Kohl. Journal of Power Sources, under review

Solid state electrolyte is the key component to enable the successful use of solid state lithium metal batteries. Poly(ethylene oxide) (PEO)-based solid polymer electrolyte is a promising candidate with advantages including flexibility, chemical stability, and combability with large-scale manufacturing¹. However, PEO-based electrolytes have a relatively low room temperature conductivity, between 10^-9-10^-5 S/cm, which is attributed to PEO crystallization¹. The randomly oriented, non-conductive PEO crystalline lamellae increase the tortuosity of the conduction pathway and hinder ion transport in the amorphous phase².
In this work, our objective is to improve the room-temperature ionic conductivity of PEO-based electrolytes by decreasing the tortuosity of the conduction pathway through alignment of the PEO crystalline phase. We use PEO/LiCF₃SO₃ (EO to Li molar ratio is 16:1) as our model polymer electrolyte. In order to achieve alignment of the PEO crystalline phase, a hot-draw method is implemented for the first time to achieve the desired alignment of PEO lamellae. The polymer electrolyte film is heated to above PEO’s melting temperature in N₂ environment followed by hot drawing till a desired strain is reached. The sample is then cooled down to room temperature to allow for crystallization with the strain maintained during the crystallization process. The hot-draw method enables a shape-stable membrane even after the tensile stress is released. Wide angle X-ray scattering (WAXS) experiments show that as the hot draw strain increases to 200%, the orientation of PEO’s (120) and (032) planes are observed. As a result, the through-plane conductivity increases by over a factor of 2. The effect of hot draw strain, temperature, and crystallization rate will be discussed.

References
(1) Zhao, Q.; Stalin, S.; Zhao, C.-Z.; Archer, L. A. Designing Solid-State Electrolytes for Safe, Energy-Dense Batteries. Nat. Rev. Mater. 2020, 5 (3), 229–252. https://doi.org/10.1038/s41578-019-0165-5.
(2) Cheng, S.; Smith, D. M.; Li, C. Y. Anisotropic Ion Transport in a Poly(Ethylene Oxide)–LiClO4 Solid State Electrolyte Templated by Graphene Oxide. Macromolecules 2015, 48 (13), 4503–4510. https://doi.org/10.1021/acs.macromol.5b00972.

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. Recently we discovered that the network structure of a gel-cast material can be further conditioned by influencing the structural evolution during drying, such as changing sample aspect ratio. This means of processing control appears to introduce a beneficial directionality in the structure that ensues. Here we will elucidate the structural developments that take place and how they lead to improved charge carrier mobility. Acknowledgment: NSF DMR-1610742

A critical challenge for solid-state batteries (SSBs) is the development of solid electrolytes (SEs) with high Li⁺ conductivity and good chemical/mechanical compatibility with Li metal anodes and high voltage cathodes. Crystalline ß-Li₃PS₄ (LPS) represents a promising SE candidate for next-generation SSBs due to its high ionic conductivity (e.g., 1.6 × 10^-4 S/cm at room temperature). However, a major bottleneck for this material is the lack of scalable processing methods to produce ß-LPS films, which are typically fabricated using top-down approaches such as cold pressing. Furthermore, the nano-/polycrystalline structure of many LPS-based SEs leads to nonuniform current densities across the LPS/Li interface and Li dendrite propagation during battery operation.
This presentation will discuss the synthesis and characterization of a new class of composite LPS/poly(ethylene oxide) (LPS/PEO) solid electrolytes which are completely amorphous and exhibit Li⁺ ionic conductivities up to 1 × 10^-6 S/cm at room temperature. The presence of the PEO component facilities the material’s processability, and synthesis conditions can be tuned to suppress crystallization of the ß-LPS phase at elevated temperature (140 – 200 °C). This presentation will describe how the structure (studied using cryo-TEM, vibrational spectroscopy, and neutron scattering) and composition of LPS/PEO composites impact their Li⁺ conductivity and viability for SSBs.

Acknowledgements
This research is sponsored by the United States Department of Energy through the Office of Energy Efficiency and Renewable Energy (EERE) and Vehicle Technologies Office (VTO). A portion of this research was conducted at the Center for Nanophase Materials Sciences and Spallation Neutron Source, which are DOE Office of Science User Facilities.

All-solid-state Li-ion battery based on solid electrolytes is a promising next-generation battery technology with high energy density, intrinsic safety, long cycle life, and wide operational temperatures. However, the lack of solid electrolyte materials that satisfy multiple requirements, such as high ionic conductivity, good stability, and interfacial compatibility with the electrode, are impeding the development of this new battery technology. To resolve these materials challenges, we develop and leverage an array of data-driven computation techniques including machine learning to discover and design novel solid-state Li-ion conductors as solid electrolytes for all-solid-state batteries. The data-driven approach enables rapid searching over a large materials space of tens of thousands of materials with highly diverse structures and chemistries. Dozens of novel solid-state conductors are discovered through our data-driven materials search. Our data-driven analyses provide unique insights into the fundamental understanding about solid-state Li-ion conductors beyond traditional physical mechanistic studies. Our study demonstrates a new paradigm of using machine learning techniques for materials discovery that overcome the data-scarcity challenges.

All-solid-state batteries are increasingly seen as essential for next-generation energy applications in consumer electronics and electric vehicles. Intense research effort has been devoted to designing safe, high energy density, long-life all-solid-state batteries. A major challenge has been to find solid electrolyte materials that can both support superionic conduction [1] and great interfacial stability against battery electrodes [2]. Sulfide-based solid-state electrolyte materials have demonstrated high ionic conductivity that exceeds the conductivity of liquid electrolytes. However, the stability of sulfides is rather limited and their instability and toxicity upon air or water exposure is a potential safety concern from battery industry’s point of view. Oxide materials, on the other hand, have shown excellent air and electrochemical stability, but the ionic conductivity is generally lower than sulfide materials.

The ability of computational modeling based on density-functional-theory methods to accurately predict intrinsic properties of solid electrolyte materials has made modeling a critical tool for understanding the behavior of solid electrolytes and their integration in solid-state batteries [3]. Computational approaches have enabled the efficient design and discovery of new functional materials with desired properties, such as low cost, high alkali ionic conductivity, great phase and electrochemical stability, accelerating the development of all-solid-state batteries.

In this talk I will present recent findings in the physical and chemical design principles for solid-state electrolyte materials with high ionic conductivity and stability with solid-state battery electrodes. More specifically, I will discuss crystallographic features in solid-state conductors which would enable fast ionic transport. I will also show progress of applying high-throughput screening in the computational design and discovery of lithium, sodium solid-state electrolytes and coating materials.

[1] Yan Wang et al., “Design principles for solid-state lithium superionic conductors,” Nature Matererials 14, 1026-1031, (2015).
[2] Y. Xiao, Y. Wang, S.-H. Bo, J. C. Kim, L. Miara, G. Ceder, “Understanding interface stability in solid-state batteries”, Nature Review Materials 5, 105-126 (2020).
[3] G. Ceder, S. Ong, Y. Wang, “Predictive modeling and design rules for solid electrolytes.” MRS Bulletin, 43(10), 746-751 (2018).

The continued improvement in operating time and lifetime of electric vehicles and portable electronic devices require higher energy density in 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 systematically evaluate the thermodynamic stability of a broad range of solid-state chemistries with common cathodes [1]. A materials trade-off is found 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. These computational findings reaffirm previously demonstrated coating and solid electrolyte chemistries and suggest that several new chemistries, including lithium phosphates and lithium ternary fluorides, are promising solid-state chemistries stable with high-voltage cathodes. Our study provides guiding principles for designing coating and solid electrolyte materials with long-term stability with high-voltage cathodes for lithium-ion batteries.

[1] Nolan et al., ACS Energy Lett. 2019, 4, 2444–2451

Cubic garnet structure Li7La3Zr2O12 (LLZO) and doped LLZO show great promise as solid electrolytes for the next-generation solid-state Li-ion batteries due to their relatively high stabilities and bulk Li conductivities. The Li transport mechanism in the crystalline bulk structure of LLZO is well understood. However, the cell performance also depends critically on the Li transport in defected and non-crystalline LLZO phases that occur at grain boundaries or in the interphases near the electrodes. Characterizing Li transport in the narrow interphase regions is experimentally challenging, and the complex atomic structures of these phases also typically prevent first-principles modeling.
Here we develop a machine-learning (ML) model that enables the large-scale atomistic simulation of complex LLZO structures with near first-principles accuracy at a fraction of the computational cost. The model is based on an artificial neural network (ANN) potential trained on density-functional theory (DFT) reference data with an approach that we have previously developed [1-3], and it can be used to study the Li migration in crystalline and non-crystalline LLZO phases. The ANN potential has been constructed to describe defected LLZO and Al/Ga-doped LLZO compositions, since doping is needed to stabilize the cubic LLZO phase that exhibits good Li conductivity. The model is thoroughly validated by comparison with DFT calculations, confirming that our ANN potential is reliable for molecular dynamics simulations of complex LLZO structures with different Al/Ga dopant concentrations.
Our accurate and efficient LLZO ANN potential will form the basis of an extended ML approach for the modeling of grain boundaries, solid-electrolyte interphases, and cathode-electrolyte interphases in LLZO-based solid-state batteries.

[1] N. Artrith and A. Urban, Comput. Mater. Sci. 114 (2016) 135-150.
[2] N. Artrith, A. Urban, and G. Ceder, Phys. Rev. B 96 (2017) 014112.
[3] A. Cooper, J. Kästner, A. Urban, N. Artrith, npj Comput. Mater. 6 (2020) 54.

All-solid-state Li-ion batteries are receiving great attention because of various advantages such as enhanced safety, high energy density, and high durability. Up to date, several solid-state electrolytes (SSEs) including Li₁₀GeP₂S₁₂, Li₇La₃Zr₂O₁₂, and Li_1.3Al_0.3Ti_1.7(PO₄)₃ have been proposed from previous experiments or computational screening from publicly available materials databases. However, none of those SSEs are fully satisfactory due to insufficient ionic conductivity, chemical or mechanical instabilities.
In this work, using DFT calculations, we take one step further. Instead of simply exploring a pre-existing materials database, we carry out a large scale high-throughput screening of potential Li-ion conductors that can display the high ionic conductivity upon aliovalent substitution. Aliovalent substitution of the non-Li cation in Li compounds is known to enhance the ionic conductivity greatly, by enabling the diffusion of Li-ions to occur in a concerted manner.[1] Starting with 15,654 ternary compounds, we identify 436 materials to meet the various preliminary chemical, topological, and bandgap criteria. For these materials, we seek potential high-energy sites for extra Li ions that are introduced to compounds when an aliovalent substitution occurs, considering structural or local potential information. Then, we explicitly add Li-ions on the confirmed high-energy sites and evaluate their vibrational frequency to assess the smoothness of the potential energy surface (PES) on which the extra Li-ions will move. Based on several materials giving rise to low vibrational frequencies, i.e., smooth PES, we create the new crystal structures by substituting non-Li elements with lower valencies and adding extra Li-ions. Finally, we examine their Li-ion conductivities using ab-initio molecular dynamics simulations. As a result of the computational design and high-throughput screening, we suggest candidate SSEs, one of them are predicted to have high Li ionic conductivity of 19 mS/cm, the large bandgap of 6.8 eV, and the energy above hull as low as 9 meV/atom, which exceeds the performance of the state-of-the-art Li₁₀GeP₂S₁₂.

[1] Nat. Comm., 8, 1 (2017)

Models of the electrical double-layer (EDL) at electrode/liquid-electrolyte interfaces no longer hold for all-solid-state electrochemistry. In this work, a new model for the EDL at a solid-state electrochemical interface based on the "Poisson-Fermi-Dirac equation'' is formulated. By combining this model with density functional theory predictions, the interconnected electronic and ionic degrees of freedom in all-solid-state batteries, including the electronic band bending and defect concentration variation in the space-charge layer, are captured self-consistently. Along with a general mathematical solution, the EDL structure is presented in various materials that are thermodynamically stable in contact with a lithium metal anode: the solid electrolyte LLZO and the solid interlayer materials LiF, Li₂O, and Li₂CO₃. The model further predicts the optimum interlayer thicknesses to minimize the electrostatic barrier for lithium transport at relevant solid-state battery interfaces.

Electrochemo-mechanical strains are inherently present at electrode-electrolyte interfaces in solid-state Li-ion batteries. This originated from electrode expansion/contraction during battery charge/discharge, as well as volumetric changes due to interfacial reactions and electrolyte decomposition [1]. These strains reach ∼3% (∼10 GPa stresses) [1]. It is important to quantify how these strains affect Li-ion conduction, detrimental or beneficial, to understand their implication on battery performance. Here we investigate biaxial strain effect on Li-ion conductivity of bulk β-Li₃PS₄, sulfide solid-electrolyte, as a model system. We use Ab Initio Molecular Dynamics to quantify strain impact within elastic regime (± 2%). In response to biaxial strain, depending on the crystallographic-plane of strain (xy, yz, zx) and temperature (550–650 K), Li-ion diffusivity can decrease or increase up to a factor of 2. Extrapolations to room temperature indicate that up to ∼5-fold conductivity increase can be anticipated. This is comparable to impact of chemical doping and Li-off-stoichiometry in this material [2]. Atomistic mechanism behind the response of Li-ion diffusivity to strain is also being assessed, based on disorder and phonon modes [3].

[1] H.-K. Tian, A. Chakraborty, A.A. Talin, P. Eisenlohr and Y. Qi, Evaluation of The Electrochemo-Mechanically Induced Stress in All-Solid-State Li-Ion Batteries, J. Electrochem. Soc., 167, 090541, 2020
[2] N.J.J. de Klerk, E. van der Maas, and M. Wagemaker, Analysis of Diffusion in Solid-State Electrolytes through MD Simulations, Improvement of the Li-Ion Conductivity in β-Li3PS4 as an Example, ACS Appl. Energy Mater., 1, 7, 3230–3242, 2018
[3] S. Muy, J.C. Bachman, L. Giordano, H.-H. Chang, D.L. Abernathy, D. Bansal, O. Delaire, S. Hori, R. Kanno, F. Maglia, S. Lupart, P. Lamph and Y. Shao-Horn, Tuning mobility and stability of lithium ion conductors based on lattice dynamics, Energy Environ. Sci., 11, 850-859, 2018

The correlation between structure and dynamics in different Li and Na solid electrolyte materials was investigated by a combination of different nuclear magnetic resonance (NMR) techniques, including magic-angle spinning NMR, temperature-dependent NMR lineshape analysis, NMR relaxometry, and field-gradient NMR. In that way, we are able to investigate the dynamics of Li or Na ions on different time scales from some nanoseconds to a few seconds, and thus we can observe the local hopping of these charge carriers as well as the long-range transport over several micrometers.
Some examples will be given for Li₃PS₄, Li₆PS₅Cl, Li_0.29La_0.57TiO₃, and NASICON structures for Li and Na electrolytes [1-7]. The results of these NMR experiments are compared to those obtained by impedance spectroscopy and different diffraction techniques in order to obtain information about the diffusion pathways in these crystal structures and the energy landscapes probed by the mobile ions.
[1] H. Stöffler, T. Zinkevich, M. Yavuz, A.-L. Hansen, M. Knapp, J. Bednarčík, S. Randau, F. H. Richter, J. Janek, H. Ehrenberg, S. Indris, J. Phys. Chem. C 123, 10280-10290 (2019).
[2] H. Stöffler, T. Zinkevich, M. Yavuz, A. Senyshyn, J. Kulisch, P. Hartmann, T. Adermann, S. Randau, F. Richter, J. Janek, S. Indris, H. Ehrenberg, J. Phys. Chem. C 122, 15954-15965 (2018).
[3] M. Kaus, H. Stöffler, M. Yavuz, T. Zinkevich, M. Knapp, H. Ehrenberg, S. Indris, J. Phys. Chem. C 121, 23370-23376 (2017).
[4] C. Dietrich, D. A. Weber, S. J. Sedlmaier, S. Indris, S. Culver, D. Walter, J. Janek, W. G. Zeier, J. Mater. Chem. A 5, 18111-18119 (2017).
[5] T. Zinkevich, A. Fiedler, M. Guin, F. Tietz, O. Guillon, H. Ehrenberg, S. Indris, Solid State Ionics 348, 115277 (2020).
[6] M. Kaus, M. Guin, M. Yavuz, M. Knapp, F. Tietz, O. Guillon, H. Ehrenberg, S. Indris, J. Phys. Chem. C 121, 1449-1454 (2017).
[7] M. Guin, S. Indris, M. Kaus, H. Ehrenberg, F. Tietz, O. Guillon, Solid State Ionics 302, 102-106 (2017).

All-solid-state is an ideal configuration of batteries. The solid electrolyte is a key technology for the development of all-solid-state batteries. Among the electrolytes proposed, the sulfide system is a candidate because of its high ionic conductivity. The LGPS (Li₁₀GeP₂S₁₂) electrolyte exhibits a high bulk conductivity of over 10^-2 S cm^-1 at room temperature and is promising for applications requiring batteries with high power and energy densities [1]. Moreover, material variations of the LGPS electrolytes provided suitable combinations of the electrodes and the electrolyte. For example, the LGPS-group materials, such as Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 and Li_9.6P₃S₁₂, provided high power and high energy density to the all-solid-state cells [2]. These results derive from the intrinsic nature of the solid electrolytes, indicating the advantages of the all-solid-state devices over conventional electrochemical devices.
Systematic substitutions of the constituent elements were examined after the first report of Li₁₀GeP₂S₁₂. The simplest substitution system was Si and Sn analogs. Solid solutions of the Ge-Si, Ge-Sn, and Si-Sn systems were examined. Among the substitution systems, the lithium tin-silicon system with Li_10.35[Sn_0.27Si_1.08]P_1.65S₁₂ showed high ionic conductivity which is comparable to the original LGPS material. On the other hand, the LGPS-type structure also appeared in the L-P-S ternary system. Li_9.6P₃S₁₂ with the LGPS-type showed high electrochemical stability which might be used for low potential ranges such as Li and carbon anode.
Partial substitution of the anion-site was studied for the halogen and oxygen systems. The oxygen and halogen substitution generally improved the electrochemical stabilities. The oxygen doped Li-P-S-O system showed higher electrochemical stability. Based on a wide composition range of the materials search, the LGPS materials were also found in the Li-A-P-S-X (A: Si, Ge, Sn, X: halogen) and Li-P-S-X (X: halogen) systems. For example, Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 showed the highest conductivity of 25 mScm^-1 at room temperature. The halogen substitution systems of the L-P-S-X systems developed electrochemical stability.
The present study summarizes the materials varieties and formation ranges of the LGPS materials in these ternary and quaternary systems. In addition to the variety of the materials, the electrolyte-electrode combination which is another important parameter to achieve high battery characteristics is summarized. We also indicate the properties of the all-solid-state cells using these LGPS-type solid electrolytes with various anode and cathode systems.

References
[1] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, A. Mitsui, Nature Materials, 10, 682 (2011).
[2] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, Nature Energy, 1, 201630, (2016).
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Crystallographic defects exist in many redox active energy materials, e.g., battery and catalyst materials, which significantly alter their chemical properties for energy storage and conversion. In this presentation, we will address how these defects, e.g., grain boundaries and geometrically necessary dislocations, govern the charging distribution in solid layered oxide cathodes. The presentation will be divided into two sections, each addressing a specific defect type.
Architecting grain crystallographic orientation can modulate charge distribution and chemomechanical properties for enhancing the performance of polycrystalline battery materials. However, probing the interplay between charge distribution, grain crystallographic orientation, and performance remains a daunting challenge. Herein, we elucidate the spatially resolved charge distribution in lithium layered oxides with different grain crystallographic arrangements and establish a model to quantify their charge distributions. While the holistic “surface-to-bulk” charge distribution prevails in polycrystalline particles, the crystallographic orientation-guided redox reaction governs the charge distribution in the local charged nanodomains. Compared to the randomly oriented grains, the radially aligned grains exhibit a lower cell polarization and higher capacity retention upon battery cycling. The radially aligned grains create less tortuous lithium ion pathways, thus improving the charge homogeneity as statistically quantified from over 20 million nanodomains in polycrystalline particles. This study provides an improved understanding of the charge distribution and chemomechanical properties of polycrystalline battery materials.
Crystallographic defects, such as geometrically necessary dislocations, are reported to influence the redox reactions in battery particles through single-particle, multimodal, in situ synchrotron measurements. Through Laue X-ray microdiffraction, many crystallographic defects are spatially identified and statistically quantified from a large quantity of diffraction patterns in many layered oxide particles, including geometrically necessary dislocations, tilt boundaries, and mixed defects. The in situ and ex situ measurements, combining microdiffraction and X-ray spectroscopic imaging, reveal that LiCoO₂ particles with a higher concentration of geometrically necessary dislocations provide deeper charging reactions, indicating that dislocations may facilitate redox reactions in layered oxides during initial charging. The present study illustrates that a precise control of crystallographic defects and their distribution can potentially promote and homogenize redox reactions in battery materials.

We investigated the influence of doping Al, Ta, Nb, W on structure and dynamics in LLZO. Using the dopants Nb or Ta yields cubic structured LLZO (Li₇La₃Zr₂O₁₂) with highest ionic conductivities amongst this class of solid state electrolyte. In this study, we demonstrate that these dopants can cause a symmetry reduction in LLZO leading to its crystallization in a different space group than usually considered: I-43d instead of Ia-3d. Neutron powder diffraction and ⁶Li-NMR are utilized to localize the Li atoms in these structures and the findings are compared to the structure of Al- and W-doped LLZO. The changes in the local structure arising from different occupations of the Li- and Zr-sites in each Al-, Nb-, Ta- or W-doped LLZO samples is evaluated with total scattering PDF analysis. The decrease of the crystal symmetry caused by Nb and Ta is supposed to have only minor influence on the bulk ionic conductivity. A 24-fold Li position splits into two 12-fold positions, so number and occupation of Li positions stays the same. To this end, impedance spectroscopic and temperature dependent ⁶Li NMR measurements are used to determine the Li ion conductivity. We provide evidence, that the structural differences indeed can be very well distinguished by diffraction and ⁶Li NMR. This enables us to deepen our understanding of the effect that doping / substitution has on the crystal lattice and thus the ionic conductivity.

The replacement of the presently used liquid electrolytes by a non-flammable solid electrolyte is an important avenue to create safer batteries. The Natrium Superionic CONductor (NaSiCON) Na_1+xZr₂Si_xP_3-xO₁₂ (0 < x < 3) that displays high bulk ionic conductivity and good stability towards other NaSiCON-based electrodes is a good solid electrolyte in NaSiCON-based batteries. Despite the sizeable share of research on Na_1+xZr₂Si_xP_3-xO₁₂, the structural and thermodynamic properties of NaSiCON require better understanding for more efficient synthesis and optimization as a solid electrolyte, which often follows chemical intuition. Here, we analyze the thermodynamic properties of the NaSiCON electrolyte by constructing the Na_1+xZr₂Si_xP_3-xO₁₂ phase diagram, based on density functional theory calculations, a cluster expansion framework, and Monte Carlo simulations. Specifically, we build the phase diagram as a function of temperature and composition (0 < x < 3) for the high-temperature rhombohedral structure, which has been also observed in several positive electrode materials, such as Na₃Ti₂(PO₄)₃, Na₃V₂(PO₄)₃ and Na₃Cr₂(PO₄)₃. Through the phase diagram, we identify the concentration domains providing the highest Na⁺-ion conductivity and previously unreported phase-separation behavior across three different single-phase regions. Further, we note the similarities in the phase behavior between Na_1+xZr₂Si_xP_3-xO₁₂ and other NaSiCON-based mono-transition metal electrodes and discuss the potential competition between thermodynamics and kinetics in experimentally observed phase separation. Our work is an important addition in understanding the thermodynamics of NaSiCON-based materials and in the development of inexpensive Na-ion batteries. From our results we propose that the addition of SiO₄^4– moieties to single-transition metal NaSiCON-phosphate-based electrodes will slow significantly the kinetics toward phase separation.

Reference
(1) Deng, Z.; Sai Gautam, G.; Kolli, S. K.; Chotard, J.-N.; Cheetham, A. K.; Masquelier, C.; Canepa, P.. Phase Behavior in Nasicon Electrolytes and Electrodes, 2020. https://doi.org/10.26434/chemrxiv.12370334.v1.

The composition and structure of solid electrolytes are critical to their ability to enable the Li metal anode for high energy batteries. Perhaps the most successful solid electrolyte in terms of enabling the Li anode is the amorphous electrolyte lithium phosphorus oxynitride or LiPON. One of the most significant drawbacks of LiPON is the ionic conductivity is limited to ~3 x 10^-6 S/cm. Developing a solid electrolyte with the advantages of LiPON, but with higher ionic conductivity, could help improve solid-state Li metal batteries. There have been a few recent reports that Si-doped LiPON, or LiSiPON, can have an ionic conductivity of 10^-5 S/cm. In this work, we present a detailed modeling and experimental approach to characterize the structure of LiSiPON in comparison to that of LiPON. The first part of this work involves a careful look at ab-initio MD simulations of the materials. The addition of the Si increases the amount of O and N bridges between phosphate/silicate tetrahedra. These bridges have been linked to higher ionic conductivity in LiPON and could be the origin of the improved conductivity with Si doping. The second part of this work presents experimental neutron pair distribution functions. The first key result of the neutron scattering is the agreement between the experimental and simulated pair distribution functions. The neutron scattering also shows that both LiPON and LiSiPON have a pseudocrystalline repeating structure with a periodicity of ~2.3 Å.

The rapid growth of electric mobility is characterized by a strong interest of academic and industrial research laboratories on the so-called “All-Solid-State-Batteries”. Indeed, conventional commercialized batteries are based on liquid electrolyte composed by flammable compounds. The first target of solid-state technology is thus to solve concerns regarding safety issue for the more and more numerous users of electric vehicle. In addition, promising energy density (Wh.kg^-1 and Wh.l^-1) superior to conventional lithium-ion batteries are supposed thanks to the use of lithium metal as anode material (3860 mAh.g^-1). Solid electrolytes and particularly sulfide-based solid electrolyte have attracted attention because they present low temperature operation and a high ionic conductivity allowing fast charge application if lithium dendrites ⁽¹⁾ can be solved. Among these, the Li₆PS₅Cl Argyrodite (space group ) discovered in 2008 ⁽²⁾ is the subject of many recent studies due to its simplicity to obtain by mecano-synthesis ⁽³⁾ as well as a high ionic conductivity up to 3 mS.cm^-1 at room temperature ⁽⁴⁾. This material seems promising for its use in composite electrodes, but also in separator for all-solid-state-batteries made by dry or by wet process ⁽⁵⁾. Nevertheless, intrinsic chemical instabilities ⁽⁶⁾ and decomposition reactions by contact with active material lead to electrochemical performances below expectations. Moreover, an industrial lock to solid-state technology development is the difficulty to produce those batteries on current Li-ion production lines based on slurry coating processes ⁽⁷⁾.

In this work, we were first interested in the deep characterization of solid electrolyte only, to assess structure and ionic conductivity of the material. In a second time, the electrochemical stability of the phase was tested by galvanostatic cycle in order to verify the electrochemical window recently proposed by Wagemaker’s team. Then, all solid-state cells were assembled by dry mix to prove solid electrolyte viability in batteries. Finally, a wet process approach will be discussed in order to match with industrial prospects. First, a protocol of active material tuned with solid electrolyte will be introduced, showing intimate contacts bewteen particles and enhanced Li⁺ percolation network. Secondly, preliminary results with slurry composed of tuned active material, carbon additive and binder coated on current collector using doctor-blade method will be presented. The goal of resulting all-solid-state cells is to give a concrete and practical example about solid-state technology in order to assess its industrial development possibility.

References
(1) J. Kasemchainan, S. Zekoll, D. Spencer Jolly, Z. Ning, G. O. Hartley, J. Marrow, and P. G. Bruce, Nature Materials, 18 (2019) 1105.
(2) H-J. Deiseroth, S-T Kong, H. Eckert, J. Vannahme, C. Reiner, T. Zaiß, and M. Schlosser, Angewandte Chimie, 120 (2008) 767.
(3) S. Boulineau, M. Courty, J-M. Tarascon, and V. Viallet, Solid State Ionics, 221 (2012) 1.
(4) S. Wang, Y. Zhang, X. Zhang, T. Liu, Y-H. Lin, Y. Shen, L. Li, and C-W. Nan, ACS Applied Materials Interfaces, 10 (2018) 42279.
(5) Y-J. Nam, D-Y. Oh, S-H. Jung, and Y-S. Jung, Journal of Power Sources, 375 (2018) 93.
(6) T. Schwietert, V. Arszelewska, C. Yu, C. Wang, A. Vasileiadis, N. de Klerk, J. Hageman, T. Hupfer, I. Kerkamm, Y. Xu, E. van der Maas, E. M. Kelder, S. Ganapathy, and M. Wagemaker, Nature Materials, (2020). (7) J. Schnella, T. Günthera, T. Knochea, C. Vieidera, L. Köhlera, A. Justa, M. Kellerb, S. Passerinib, G. Reinhart, Journal of Power Sources, 382 (2018) 160.

Closo-hydroborates and carba-closo-hydroborates recently emerged as a promising, yet costly class of solid electrolytes for all-solid-state batteries.[1-4] Here, we introduce nido-hydroborates, MB₁₁H₁₄ (M = Li, Na), with open cage-like structures as a new building block for solid electrolytes. We synthesized and purified MB₁₁H₁₄ (M = Li, Na) starting from NaBH₄ as a cost-effective chemical feedstock.[5] Two-dimensional (2D) nuclear magnetic resonance combined with mass spectrometry techniques were used to analyze the structure of the new building blocks. High Li/Na ionic conductivity of up to > 1 mS/cm at 25 °C is achieved by combining nido- with closo/ carba-closo-hydroborates into cubic unit cells. Crystal structures are solved and refined using X-ray diffraction and density functional theory. Cyclic voltammetry reveals electrochemical stability of ~ 2.6 V vs Li⁺/Li or Na⁺/Na, respectively. Galvanostatic cycling in symmetrical cells with lithium or sodium metal electrodes shows only a small overpotential increase after > 500 cycles at 50 μA/cm². These results demonstrate that despite the thermodynamic instability of nido-hydroborates versus metallic anodes, stable interfaces are formed and the electrolytes were successfully employed in half cells using TiS₂ as cathode active material. Finally, we discuss routes to synthesize carba-closo-hydroborates from nido-hydroborate precursors by closing the cage with a carbon atom, opening a way for the cost-efficient synthesis of hydroborate-based solid-state electrolytes for Li/Na batteries.

[1] L. Duchêne, A. Remhof, H. Hagemann and C. Battaglia, Energy Storage Mater., 2020, 25, 782–794.
[2] L. Duchêne, D. H. Kim, Y. B. Song, S. Jun, R. Moury, A. Remhof, H. Hagemann, Y. S. Jung and C. Battaglia, Energy Storage Mater., 2020, 26, 543–549.
[3] F. Murgia, M. Brighi and R. Černý, Electrochem. commun., 2019, 106, 106534.
[4] L. Duchêne, R.-S. Kühnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, Energy Environ. Sci., 2017, 10, 2609-2615
[5] S. Payandeh, R. Asakura, P. Avramidou, D. Rentsch, Z. Lodziana, R. Cerny, A. Remhof and C. Battaglia, Chem. Mater., 2020, 32, 1101–1110.

A new class of materials has been discovered in 2015 [1], the high entropy oxides, which expand to ionic compounds the concept of High Entropy Alloys. The particularity of these materials is their stabilisation at high temperature by their entropy of configuration, that predominant in their total Gibbs Energy, given by Eq. (1):

(1) △_rG = △_rH - T△_rS

To obtain an entropy-stabilized oxide, by maximizing their configurational entropy, we need to mix, at least, 5 binary oxides in equimolar proportions and heat them above a critical temperature (corresponding to △_rG =0), followed by quenching to froze the structure at room temperature. In these conditions, they form a solid metastable simple crystal structure where the cations are randomly dispersed on the cationic sublattice.

First studies, performed in our lab, have shown the large possibilities of composition by substituting cations or modulating their ratio from the initial formula (MgCoNiCuZn)O when keeping the charge balance between cations and anions (M²⁺ and O^2-) [2] and their very promising properties such as their colossal dielectric constant [2] and their alkali conductivity (Li⁺ and Na⁺) [3]. These specificities make them promising candidates for solid-state electrolytes.

With the ambition to improve the ionic conductivity of Li⁺ in these compounds, currently 1 mS.cm^-1 at room temperature in (MgCoNiCuZn)_0.7Li_0.3O [3], we have studied the (MgCoNiCuZn)_1-xLi_xO series to understand the charge compensation mechanisms that occur when Li⁺ substitutes a 2+ element. Thanks to X-Ray Diffraction, coupled to thermogravimetric analysis and X-ray photoelectron spectroscopy, we found out that, for low fraction of Li⁺, compensation involves a partial oxidation of Co²⁺ into Co³⁺ and then, for large fractions of Li⁺, a combination of cobalt oxidation and a formation of oxygen vacancies [4].


[1] C.M. Rost et al., Nature Commununications. 6, 8485, 2015
[2] D. Bérardan et al., Physical Status Solidi RRL 10, 328, 2016
[3] D. Bérardan et al., Journal of Materials Chemistry A 4, 9536, 2016
[4] N. Osenciat, et al.. Journal of the American Ceramic Society. 2019; 102: 6156– 6162

The antiperovskite superionic conductors Li₃OCl and Li₃OBr show great differences in ionic conductivity. The more polarizable Li₃OBr shows a lower ionic conductivity than Li₃OCl, contradicting the idea that more polarizable framework is beneficial to achieving high ionic conductivity. In this work, we study the influence of substituting Cl with Br on the local structure, lattice dynamics and Li ionic conductivity of Li₃OCl_1-xBr_x, based on first-principles calculations. We find that the incorporation of Br does soften the overall lattice stiffness of Li₃OCl_1-xBr_x, but the accompanied local structural distortion plays a dominant role on changing the activation energy for Li diffusion, which could increase the site energy as well as the Li ion vibration frequency. We suggest that the more distorted initial octahedral site and less distorted saddle trigonal-plane site lead to better ionic transport. In addition, the correlation between the activation energy and the pre-exponential factor of diffusivity extracted from Arrhenius plot of Li₃OCl_1-xBr_x is found to be nonlinear, which is due to the decreasing migration entropy with increasing activation energy.

Li- and Na-based hydroborates are known for their high ion dynamic, leading to liquid-like cation mobility after an order-disorder phase transition, suggesting their use as solid electrolytes for all-solid batteries.^[1,2] However this transition takes place at elevate temperature, outside the typical operation range of a conventional battery .^[3–5]
A systematic study on Na-based closoborates and carba-closoborates has been conducted by means of anion mixing, in order to stabilize room temperature Na-conducting phases, relying on increased structural disorder. The effect of such mixing is the suppression of any phase transition, leading to six novel Na-conductors with a Na-conductivity close to 1 mS cm^-1 at room temperature.
Due to the three-dimensional aromatic nature of the boron cage, closoborates are among the more robust available polyanions;^[6,7] the six presented phases are indeed thermodynamically stable in the investigated temperature range 100 < T < 700 K, while depending from the mixture the electrochemical stability window varies from 3 to 4.1 V versus Na⁺/Na, allowing their implementation in high -voltage next generation solid-state batteries.^[8]
The physical/electrochemical properties of this family will be presented, with particular remark to the non-Arrhenius conductivity behavior, arising from the ion-ion interaction. Their flexibility, in terms of operating voltage window, allowed also to check their compatibility towards different class of cathode materials such as NaCrO₂^[9,10], Na₂Fe₂(SO₄)₃^[11] and Na₃V₂(PO₄)₂F₃.

References:
[1] J. B. Varley, K. Kweon, P. Mehta, P. Shea, T. W. Heo, T. J. Udovic, V. Stavila, B. C. Wood, ACS Energy Lett. 2017, 2, 250.
[2] K. E. Kweon, J. B. Varley, P. Shea, N. Adelstein, P. Mehta, T. W. Heo, T. J. Udovic, V. Stavila, B. C. Wood, Chem. Mater. 2017, 29, 9142.
[3] T. J. Udovic, M. Matsuo, A. Unemoto, N. Verdal, V. Stavila, A. V. Skripov, J. J. Rush, H. Takamura, S. Orimo, Chem. Commun. 2014, 50, 3750.
[4] T. J. Udovic, M. Matsuo, W. S. Tang, H. Wu, V. Stavila, A. V. Soloninin, R. V. Skoryunov, O. A. Babanova, A. V. Skripov, J. J. Rush, A. Unemoto, H. Takamura, S. I. Orimo, Adv. Mater. 2014, 26, 7622.
[5] W. S. Tang, M. Matsuo, H. Wu, V. Stavila, W. Zhou, A. A. Talin, A. V Soloninin, R. V Skoryunov, O. A. Babanova, A. V Skripov, Adv. Energy Mater. 2016, 6, 1502237.
[6] J. ichi Aihara, J. Am. Chem. Soc. 1978, 100, 3339.
[7] R. J. Wiersema, M. F. Hawthorne, Inorg. Chem. 1973, 12, 785.
[8] M. Brighi, Oral Contribution at Swiss & Surrounding Battery days 2019, Dübendorf (Switzerland)
[9] L. Duchêne, R. S. Kühnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, Energy Environ. Sci. 2017, 10, 2609.
[10] F. Murgia, M. Brighi, R. Černý, Electrochem. commun. 2019, 106, 106534.
[11] M. Brighi, F. Murgia, R. Černý, Cell Report Physical Science 2020, 1, 100217

Stringent chemical and mechanical constraints apply to the interfaces in solid state batteries. They have to be (electro)chemically stable against the chosen electrode materials and applied potentials. In addition, they have to be tolerant to the various mechanical issues that occur in solid state batteries such as electrode swelling/shrinking, stress concentrations at current focusing areas etc. I will present the requirements for chemically stable interfaces between various cathode and electrolyte materials and discuss to what extent coatings can address chemical incompatibility. A repeated stripping and plating of a lithium metal anode presents significant challenges. I will present results from modeling to understand the interplay between current density, stress, and lithium growth through the electrolyte. Our results indicate that in good conductors the build-up of stress to the point that it fractures the electrolyte is unlikely, indicating that other mechanisms are responsible for lithium penetration through the solid electrolyte. We also find that conductivity of the solid electrolyte plays a central role in the mechanical reliability of a solid-state battery.

All-solid-state batteries (ASSBs) have attracted enormous attention as one of the critical future technologies for safe and high energy batteries. With the emergence of many highly conductive solid electrolytes in recent years, the bottleneck is no longer Li-ion diffusion within the electrolyte. Instead, many ASSBs are limited by their low Coulombic efficiency, poor power performance, and short cycling life due to the high resistance at the interfaces within ASSBs. In this talk, I will highlight the complex characteristics of interphases, namely the composition, distribution, and electronic and ionic properties of the cathode−electrolyte and electrolyte−anode interfaces. We emphasize the significant role of chemo-mechanical behavior of the interfaces, especially when the ASSBs under constant stack pressure. Multiscale and multimodal characterization methods to explore, diagnose, and understand the dynamic and buried interfaces and interphases are essential to pave the pathway for practical high-energy ASSB.

The advent of solid-state batteries has spawned a recent increase in interest in lithium conducting solid electrolytes, especially in the lithium thiophosphates. However, many open questions remain when trying to optimize electrolytes and understand solid state battery chemistries.

In this presentation, we explore the influence of inductive effects in superionic conductors showing that in Li₁₀GeP₂S₁₂, these can strongly affect the ionic transport.
We further show a better understanding of the structure-transport properties of the novel lithium and sodium rare-earth halides that show promising ionic conductivities for their use in solid state batteries.
Lastly, we show how solvent treatment of the lithium argyrodite Li₆PS₅Cl affects battery performance despite having no influence on the structural stability, suggesting stronger influences of slurry processing than previously believed.

All-solid-state batteries combining an alkali metal anode and a high-voltage cathode have the potential to achieve high energy density. However, interface stability remains a major challenge. On the anode side, dendrite formation limits the maximum plating current density and prevents fast charging, while on the cathode side the limited oxidative stability of most solid electrolytes is a major challenge, especially when the battery is charged beyond 4 V.
Here we demonstrate that the critical current density for dendrite formation in the archetypical ceramic solid electrolyte Na-β”-alumina is ten times higher than that measured on a garnet-type Li₇La₃Zr₂O₁₂ electrolyte under equivalent conditions and reaches up to 12 mA/cm² at room temperature [1]. Further, we show that above the melting temperature of sodium, a cumulative capacity of >10 Ah of sodium can be plated and stripped at an enormous current density of 1000 mA/cm² without dendrite formation [2].
On the cathode side, we have been developing hydroborate solid electrolytes reaching ionic conductivities >1 mS/cm at room temperature [3]. We already demonstrated the integration of these electrolytes with a 3 V class cathode [4, 5], but recently managed to obtain stable cycling with a 4 V class cathode via a self-forming passivating interface [6]. Combined with their low gravimetric density <1.2 g/cm³, low toxicity, high thermal and chemical stability, stability vs lithium and sodium metal anodes, soft mechanical properties enabling cold pressing [4], compatibility with solution infiltration [7], and potential for low cost [8, 9], hydroborate electrolytes represent a promising option for the cathode side of a competitive future all-solid-state battery.

References:
[1] M.-C. Bay, M. Wang, R. Grissa, M. V. F. Heinz, J. Sakamoto, C. Battaglia, Adv. Energy Mater. 2019, 201902889
[2] D. Landmann, G. Graeber, M. V. F. Heinz, S. Haussener, C. Battaglia, submitted
[3] L. Duchêne, A. Remhof, H. Hagemann, C. Battaglia, Energy Storage Mater. 2020, 25, 782
[4] L. Duchêne, R.-S. Kühnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, Energy Environ. Science 2017, 10, 2609
[5] R. Asakura, L. Duchêne, R.-S. Kühnel, A. Remhof, C. Battaglia, ACS Appl. Energy Mater. 2019, 2, 6924
[6] R. Asakura, D. Reber, L. Duchêne, A. Remhof, H. Hagemann, C. Battaglia, submitted
[7] 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
[8] A. Gigante, L. Duchêne, R. Moury, M. Pupier, A. Remhof, H. Hagemann, ChemSusChem 2019, 12, 4832
[9] S. Payandeh, R. Asakura, P. Avramidou, D. Rentsch, Z. Lodziana, R. Cerny, A. Remhof, C. Battaglia, Chem. Mater. 2020, 32, 1101

All solid-state batteries with lithium anodes have the potential of outperforming lithium-ion batteries with respect to energy density, safety, and reliability. However, considerable challenges exist, particularly for ceramic-based electrolytes, with regards to their design for vehicle applications. The thin film configurations that currently exist have low practical energy densities, are difficult to scale, and require expensive vacuum technologies. To overcome these difficulties, we have been using a scalable freeze tape casting method to produce porous garnet scaffolds that can be co-sintered with thin, dense components to form bilayers. The scaffold is then infiltrated with active material and other components, and lithium is attached to the other side of the bilayer to form a cell. The use of a secondary soft electrolyte obviates the need for a co-sintering step, which carries with it the risk of adverse reactions between the active material and the garnet. Batteries prepared this way may be able to achieve specific energies in excess of 400 Wh/kg. We will discuss what still needs to be done to make this a reality.