Direct observation of a non-crystalline state of Li2S-P2S5 solid electrolytes

Direct Visualization of Chemical Transport in Solid-State Chemical Reactions by Time-of-Flight Secondary Ion Mass Spectrometry

Systematic control and design of solid-state chemical reactions are required for modifying materials properties and in novel synthesis. Understanding chemical dynamics at the nanoscale is therefore essential to revealing the key reactive pathways. Herein, we combine focused ion beam-scanning electron microscopy (FIB-SEM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to track the migration of sodium from a borate coating to the oxide scale during in situ hot corrosion testing. We map the changing distribution of chemical elements and compounds from 50 to 850 °C to reveal how sodium diffusion induces corrosion. The results are validated by in situ X-ray diffraction and post-mortem TOF-SIMS. We additionally retrieve the through-solid sodium diffusion rate by fitting measurements to a Fickian diffusion model. This study presents a step change in analyzing microscopic diffusion mechanics with high chemical sensitivity and selectivity, a widespread analytical challenge that underpins the defining rates and mechanisms of solid-state reactions.

Computational Design of Antiperovskite Solid Electrolytes

In the face of the current climate emergency and the performance, safety, and cost limitations current state-of-art Li-ion batteries present, solid-state batteries are widely anticipated to revolutionize energy storage. The heart of this technology lies in the substitution of liquid electrolytes with solid counterparts, resulting in potential critical advantages, such as higher energy density and safety profiles. In recent years, antiperovskites have become one of the most studied solid electrolyte families for solid-state battery applications as a result of their salient advantages, which include high ionic conductivity, structural versatility, low cost, and stability against metal anodes. This Review highlights the latest progress in the computational design of Li- and Na-based antiperovskite solid electrolytes, focusing on critical topics for their development, including high-throughput screening for novel compositions, synthesizability, doping, ion transport mechanisms, grain boundaries, and electrolyte-electrode interfaces. Moreover, we discuss the remaining challenges facing these materials and provide our perspective on their possible future advances and applications.

Disentangling Phase and Morphological Evolution During the Formation of the Lithium Superionic Conductor Li10 GeP2 S12

The structural and morphological changes of the Lithium superionic conductor Li₁₀ GeP₂ S₁₂ , prepared via a widely used ball milling-heating method over a comprehensive heat treatment range (50 - 700 °C), are investigated. Based on the phase composition, the formation process can be distinctly separated into four zones: Educt, Intermediary, Formation, and Decomposition zone. It is found that instead of Li₄ GeS₄ -Li₃ PS₄ binary crystallization process, diversified intermediate phases, including GeS₂ in different space groups, multiphasic lithium phosphosulfides (Li_x P_y S_z ), and cubic Li₇ Ge₃ PS₁₂ phase, are involved additionally during the formation and decomposition of Li₁₀ GeP₂ S₁₂ . Furthermore, the phase composition at temperatures around the transition temperatures of different formation zones shows a significant deviation. At 600 °C, Li₁₀ GeP₂ S₁₂ is fully crystalline, while the sample decomposed to complex phases at 650 °C with 30 wt.% impurities, including 20 wt.% amorphous phases. These findings over such a wide temperature range are first reported and may help provide previously lacking insights into the formation and crystallinity control of Li₁₀ GeP₂ S₁₂ .

Self-organized hetero-nanodomains actuating super Li+ conduction in glass ceramics

Easy-to-manufacture Li₂S-P₂S₅ glass ceramics are the key to large-scale all-solid-state lithium batteries from an industrial point of view, while their commercialization is greatly hampered by the low room temperature Li⁺ conductivity, especially due to the lack of solutions. Herein, we propose a nanocrystallization strategy to fabricate super Li⁺-conductive glass ceramics. Through regulating the nucleation energy, the crystallites within glass ceramics can self-organize into hetero-nanodomains during the solid-state reaction. Cryogenic transmission electron microscope and electron holography directly demonstrate the numerous closely spaced grain boundaries with enriched charge carriers, which actuate superior Li⁺-conduction as confirmed by variable-temperature solid-state nuclear magnetic resonance. Glass ceramics with a record Li⁺ conductivity of 13.2 mS cm^-1 are prepared. The high Li⁺ conductivity ensures stable operation of a 220 μm thick LiNi_0.6Mn_0.2Co_0.2O₂ composite cathode (8 mAh cm^-2), with which the all-solid-state lithium battery reaches a high energy density of 420 Wh kg^-1 by cell mass and 834 Wh L^-1 by cell volume at room temperature. These findings bring about powerful new degrees of freedom for engineering super ionic conductors.

Tackling Structural Complexity in Li2S-P2S5 Solid-State Electrolytes Using Machine Learning Potentials

The lithium thiophosphate (LPS) material class provides promising candidates for solid-state electrolytes (SSEs) in lithium ion batteries due to high lithium ion conductivities, non-critical elements, and low material cost. LPS materials are characterized by complex thiophosphate microchemistry and structural disorder influencing the material performance. To overcome the length and time scale restrictions of ab initio calculations to industrially applicable LPS materials, we develop a near-universal machine-learning interatomic potential for the LPS material class. The trained Gaussian Approximation Potential (GAP) can likewise describe crystal and glassy materials and different P-S connectivities PmSn. We apply the GAP surrogate model to probe lithium ion conductivity and the influence of thiophosphate subunits on the latter. The materials studied are crystals (modifications of Li3PS4 and Li7P3S11), and glasses of the xLi2S-(100 - x)P2S5 type (x = 67, 70 and 75). The obtained material properties are well aligned with experimental findings and we underscore the role of anion dynamics on lithium ion conductivity in glassy LPS. The GAP surrogate approach allows for a variety of extensions and transferability to other SSEs.

Femtosecond X-ray Laser Reveals Intact Sea-Island Structures of Metastable Solid-State Electrolytes for Batteries

Experimental characterization of the nanostructure of metastable functional materials has attracted significant attention with recent advances in computational materials discovery. However, since metastable glass-ceramics are easily damaged by irradiation, damage-free nanoimaging has not been realized thus far. Herein, we propose novel high-contrast coherent diffractive imaging that quantitatively analyzes the intact internal nanostructure of metastable glass-ceramics using femtosecond X-ray pulses. The immersion of sample particles in a solvent helps enhance the reconstructed image contrast and allows us to distinguish an ∼7% electron density difference between an amorphous form and crystals. Furthermore, morphological operations with a band-pass filter quantitatively elucidate the depth information. The evaluated volume ratio of the amorphous to crystalline phases is ∼2.5:1 for the measured metastable (Li₂S)₇₀-(P₂S₅)₃₀ glass-ceramic particle. Sulfide glass-ceramics are used as electrolytes for all-solid-state batteries, which are indispensable for reducing the carbon footprint. Our results will facilitate structural studies on fragile metastable materials with important scientific and industrial implications.

An electrochemically stable homogeneous glassy electrolyte formed at room temperature for all-solid-state sodium batteries

All-solid-state sodium batteries (ASSSBs) are promising candidates for grid-scale energy storage. However, there are no commercialized ASSSBs yet, in part due to the lack of a low-cost, simple-to-fabricate solid electrolyte (SE) with electrochemical stability towards Na metal. In this work, we report a family of oxysulfide glass SEs (Na₃PS_4−xO_x, where 0 < x ≤ 0.60) that not only exhibit the highest critical current density among all Na-ion conducting sulfide-based SEs, but also enable high-performance ambient-temperature sodium-sulfur batteries. By forming bridging oxygen units, the Na₃PS_4−xO_x SEs undergo pressure-induced sintering at room temperature, resulting in a fully homogeneous glass structure with robust mechanical properties. Furthermore, the self-passivating solid electrolyte interphase at the Na|SE interface is critical for interface stabilization and reversible Na plating and stripping. The new structural and compositional design strategies presented here provide a new paradigm in the development of safe, low-cost, energy-dense, and long-lifetime ASSSBs.

Single sodium-ion solid electrolyte that meets the requirements of practical applications is difficult to design. Here, the authors show how kinetic stability via the creation of a self-passivating solid electrolyte interphase allows a homogenous glass solid electrolyte to exhibit remarkable electrochemical stability with sodium metal.

A Nanoscale Design Approach for Enhancing the Li-Ion Conductivity of the Li10GeP2S12 Solid Electrolyte

The discovery of the lithium superionic conductor Li10GeP2S12 (LGPS) has led to significant research activity on solid electrolytes for high-performance solid-state batteries. Despite LGPS exhibiting a remarkably high room-temperature Li-ion conductivity, comparable to that of the liquid electrolytes used in current Li-ion batteries, nanoscale effects in this material have not been fully explored. Here, we predict that nanosizing of LGPS can be used to further enhance its Li-ion conductivity. By utilizing state-of-the-art nanoscale modeling techniques, our results reveal significant nanosizing effects with the Li-ion conductivity of LGPS increasing with decreasing particle volume. These features are due to a fundamental change from a primarily one-dimensional Li-ion conduction mechanism to a three-dimensional mechanism and major changes in the local structure. For the smallest nanometric particle size, the Li-ion conductivity at room temperature is three times higher than that of the bulk system. These findings reveal that nanosizing LGPS and related solid electrolytes could be an effective design approach to enhance their Li-ion conductivity.

Strong Interfacial Adhesion between the Li2S Cathode and a Functional Li7P2.9Ce0.2S10.9Cl0.3 Solid-State Electrolyte Endowed Long-Term Cycle Stability to All-Solid-State Lithium-Sulfur Batteries

The extrinsic cathode interface between the sulfide electrolyte and the Li₂S electrode is always ignored in all-solid-state lithium-sulfur batteries. However, the aggregation of the Li₂S cathode is still observed during cycling. The gradually lost extrinsic contact interface between the cathode and the electrolyte would result in considerable interface resistance and severe capacity decay in the cell due to the lack of efficient electron and ionic conduction at the interface. Herein, a facile dual-doping strategy demonstrates the synthesis of a functional inorganic electrolyte. The obtained Li₇P_2.9Ce_0.2S_10.9Cl_0.3 glass-ceramic electrolyte shows a higher-lithium-ionic conductivity of 3.2 mS cm^-1 at room temperature. Further, UV-vis absorption and ex situ scanning electron microscopy studies confirm robust interfacial adhesion between the functional inorganic electrolyte, Li₇P_2.9Ce_0.2S_10.9Cl_0.3, and the Li₂S cathode. Thus, a stable extrinsic cathode interface is unprecedently built. Finally, the all-solid-state lithium-sulfur battery based on the Li₇P_2.9Ce_0.2S_10.9Cl_0.3 electrolyte delivers a higher reversible initial capacity of 617 mA h g^-1, a lower interface resistance of 25 Ω cm² and much better cycling stability (with a high capacity retention of 89% after 100 cycles) than the pristine Li₇P₃S₁₁ electrolyte.

On the underestimated influence of synthetic conditions in solid ionic conductors

The development of high-performance inorganic solid electrolytes is central to achieving high-energy- density solid-state batteries. Whereas these solid-state materials are often prepared via classic solid-state syntheses, recent efforts in the community have shown that mechanochemical reactions, solution syntheses, microwave syntheses, and various post-synthetic heat treatment routines can drastically affect the structure and microstructure, and with it, the transport properties of the materials. On the one hand, these are important considerations for the upscaling of a materials processing route for industrial applications and industrial production. On the other hand, it shows that the influence of the different syntheses on the materials' properties is neither well understood fundamentally nor broadly internalized well. Here we aim to review the recent efforts on understanding the influence of the synthetic procedure on the synthesis - (micro)structure - transport correlations in superionic conductors. Our aim is to provide the field of solid-state research a direction for future efforts to better understand current materials properties based on synthetic routes, rather than having an overly simplistic idea of any given composition having an intrinsic conductivity. We hope this review will shed light on the underestimated influence of synthesis on the transport properties of solid electrolytes toward the design of syntheses of future solid electrolytes and help guide industrial efforts of known materials.

An Air-Stable and Li-Metal-Compatible Glass-Ceramic Electrolyte enabling High-Performance All-Solid-State Li Metal Batteries

The development of all-solid-state Li metal batteries (ASSLMBs) has attracted significant attention due to their potential to maximize energy density and improved safety compared to the conventional liquid-electrolyte-based Li-ion batteries. However, it is very challenging to fabricate an ideal solid-state electrolyte (SSE) that simultaneously possesses high ionic conductivity, excellent air-stability, and good Li metal compatibility. Herein, a new glass-ceramic Li_3.2 P_0.8 Sn_0.2 S₄ (gc-Li_3.2 P_0.8 Sn_0.2 S₄ ) SSE is synthesized to satisfy the aforementioned requirements, enabling high-performance ASSLMBs at room temperature (RT). Compared with the conventional Li₃ PS₄ glass-ceramics, the present gc-Li_3.2 P_0.8 Sn_0.2 S₄ SSE with 12% amorphous content has an enlarged unit cell and a high Li⁺ ion concentration, which leads to 6.2-times higher ionic conductivity (1.21 × 10^-3 S cm^-1 at RT) after a simple cold sintering process. The (P/Sn)S₄ tetrahedron inside the gc-Li_3.2 P_0.8 Sn_0.2 S₄ SSE is verified to show a strong resistance toward reaction with H₂ O in 5%-humidity air, demonstrating excellent air-stability. Moreover, the gc-Li_3.2 P_0.8 Sn_0.2 S₄ SSE triggers the formation of Li-Sn alloys at the Li/SSE interface, serving as an essential component to stabilize the interface and deliver good electrochemical performance in both symmetric and full cells. The discovery of this gc-Li_3.2 P_0.8 Sn_0.2 S₄ superionic conductor enriches the choice of advanced SSEs and accelerates the commercialization of ASSLMBs.

Under Pressure: Mechanochemical Effects on Structure and Ion Conduction in the Sodium-Ion Solid Electrolyte Na3PS4

Fast-ion conductors are critical to the development of solid-state batteries. The effects of mechanochemical synthesis that lead to increased ionic conductivity in an archetypical sodium-ion conductor Na₃PS₄ are not fully understood. We present here a comprehensive analysis based on diffraction (Bragg and pair distribution function), spectroscopy (impedance, Raman, NMR and INS), and ab initio simulations aimed at elucidating the synthesis-property relationships in Na₃PS₄. We consolidate previously reported interpretations regarding the local structure of ball-milled samples, underlining the sodium disorder and showing that a local tetragonal framework more accurately describes the structure than the originally proposed cubic one. Through variable-pressure impedance spectroscopy measurements, we report for the first time the activation volume for Na⁺ migration in Na₃PS₄, which is ∼30% higher for the ball-milled samples. Moreover, we show that the effect of ball-milling on increasing the ionic conductivity of Na₃PS₄ to ∼10^-4 S/cm can be reproduced by applying external pressure on a sample from conventional high-temperature ceramic synthesis. We conclude that the key effects of mechanochemical synthesis on the properties of solid electrolytes can be analyzed and understood in terms of pressure, strain, and activation volume.

Fundamentals of inorganic solid-state electrolytes for batteries

In the critical area of sustainable energy storage, solid-state batteries have attracted considerable attention due to their potential safety, energy-density and cycle-life benefits. This Review describes recent progress in the fundamental understanding of inorganic solid electrolytes, which lie at the heart of the solid-state battery concept, by addressing key issues in the areas of multiscale ion transport, electrochemical and mechanical properties, and current processing routes. The main electrolyte-related challenges for practical solid-state devices include utilization of metal anodes, stabilization of interfaces and the maintenance of physical contact, the solutions to which hinge on gaining greater knowledge of the underlying properties of solid electrolyte materials.

LiI-Doped Sulfide Solid Electrolyte: Enabling a High-Capacity Slurry-Cast Electrode by Low-Temperature Post-Sintering for Practical All-Solid-State Lithium Batteries

All-solid-state lithium batteries (ASSLBs) based on sulfide solid electrolytes (SEs) have received great attention because of the high ionic conductivity of the SEs, intrinsic thermal safety, and higher energy density achievable with a Li metal anode. However, studies on practical slurry-cast composite electrodes show an extremely limited battery performance than the binder-free pelletized electrodes because of the poor interfacial robustness between the active materials and SEs by the presence of a polymeric binder. Here, we employ a low-temperature post-sintering process for the slurry-cast composite electrodes in order to overcome the binder-induced detrimental effects on the electrochemical performance. The LiI-doped Li₃PS₄ SEs are chosen because the addition of iodine not only improves the Li-ion conductivity and Li metal compatibility but also lowers the glass-transition and crystallization temperatures. Low-temperature post-sintering of composite cathodes consisting of a LiNi_0.6Co_0.2Mn_0.2O₂-active material, LiI-doped Li₃PS₄ SE, polymeric binder, and conducting agent shows a significantly improved electrochemical performance as compared to a conventional slurry-cast electrode containing pre-annealed SEs. Detailed analyses by electrochemical impedance spectroscopy and galvanostatic intermittent titration technique confirm that post-sintering effectively reduces the interfacial resistance and enhances the chemomechanical robustness at solid-solid interfaces, which enables the development of practical slurry-cast ASSLBs with sulfide SEs.

Crystallization behavior of the Li2S-P2S5 glass electrolyte in the LiNi1/3Mn1/3Co1/3O2 positive electrode layer

Sulfide-based all-solid-state lithium batteries are a next-generation power source composed of the inorganic solid electrolytes which are incombustible and have high ionic conductivity. Positive electrode composites comprising LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) and 75Li₂S·25P₂S₅ (LPS) glass electrolytes exhibit excellent charge–discharge cycle performance and are promising candidates for realizing all-solid-state batteries. The thermal stabilities of NMC–LPS composites have been investigated by transmission electron microscopy (TEM), which indicated that an exothermal reaction could be attributed to the crystallization of the LPS glass. To further understand the origin of the exothermic reaction, in this study, the precipitated crystalline phase of LPS glass in the NMC–LPS composite was examined. In situ TEM observations revealed that the β-Li₃PS₄ precipitated at approximately 200 °C, and then Li₄P₂S₆ and Li₂S precipitated at approximately 400 °C. Because the Li₄P₂S₆ and Li₂S crystalline phases do not precipitate in the single LPS glass, the interfacial contact between LPS and NMC has a significant influence on both the LPS crystallization behavior and the exothermal reaction in the NMC–LPS composites.