Multi-Scale Mechanical Behavior of the Li3PS4 Solid-Phase Electrolyte

Elucidating the Reductive Decomposition Mechanism in Sulfide Solid Electrolyte Li4SnS4

The sulfide solid electrolyte Li4SnS4 has garnered considerable interest due to its exceptional moisture durability, which is attributed to its stable hydrated state. However, a major limitation of certain sulfide solid electrolytes, including Li4SnS4, is their low reduction durability, which limits their application in the negative electrodes of all-solid-state batteries and impedes qualitative material development assessments. In this study, we introduced a quantitative and straightforward method for evaluating the reductive decomposition of Li4SnS4 to better understand its degradation mechanism. The configuration of the electrochemical evaluation cell was modified from SUS|Li4SnS4|Li to SUS|Li4SnS4|Li3PS4|Li, allowing for stabilization of the reference potential of the counter electrode. The reductive decomposition potential of Li4SnS4 was quantitatively assessed by using cyclic voltammetry in a two-layer electrochemical evaluation cell. We observed a minor irreversible reduction current below +1.2 V and a pronounced decomposition peak at +1.0 V. Notably, reductive decomposition continued below 0 V, which is typically the onset point for Li electrodeposition. Postreduction, the solid electrolyte was comprehensively analyzed through optical microscopy, X-ray diffraction, and X-ray absorption spectroscopy. These analyzes revealed the following: (i) The SnS44- unit in Li4SnS4 initially decomposes into Li2S and β-Sn with the dissociation of the Sn-S bond; (ii) the resulting β-Sn forms LixSn alloys such as Li0.4Sn; and (iii) the ongoing reductive decomposition reaction is facilitated by the electronic conductivity of these LixSn alloys. These findings offer crucial methodological and mechanistic insights into the development of higher-performance solid electrolyte materials.

Anion Engineering for Stabilizing Li Interstitial Sites in Halide Solid Electrolytes for All-Solid-State Li Batteries

Halide solid electrolytes (SEs) have been highlighted for their high-voltage stability. Among the halide SEs, the ionic conductivity has been improved by aliovalent metal substitutions or choosing a ccp-like anion-arranged monoclinic structure (C2/m) over hcp- or bcc-like anion-arranged structures. Here, we present a new approach, hard-base substitution, and its underlying mechanism to increase the ionic conductivity of halide SEs. The oxygen substitution to Li2ZrCl6 (trigonal, hcp) increased the ionic conductivity from 0.33 to 1.3 mS cm-1 at Li3.1ZrCl4.9O1.1 (monoclinic, ccp), while the sulfur and fluorine substitutions were not effective. A systematic comparison study revealed that the energetic stabilization of interstitial sites for Li migration plays a key role in improving the ionic conductivity, and the ccp-like anion sublattice is not sufficient to achieve high ionic conductivity. We further examined the feasibility of the oxyhalide SE for practical and all-solid-state battery applications.

Liquid-Phase Approach to Glass-Microfiber-Reinforced Sulfide Solid Electrolytes for All-Solid-State Batteries

Deformable, fast-ion conducting sulfides enable the construction of bulk-type solid-state batteries that can compete with current Li-ion batteries in terms of energy density and scalability. One approach to optimizing the energy density of these cells is to minimize the size of the electrolyte layer by integrating the solid electrolyte in thin membranes. However, additive-free thin membranes, as well as many membranes based on preprepared scaffolds, are difficult to prepare or integrate in solid cells on a large scale. Here, we propose a scalable solution-based approach to produce bulk-type glass-microfiber-reinforced composites that restore the deformability of sulfide electrolytes and can easily be shaped into thin membranes by cold pressing. This approach supports both the ease of preparation and enhancement of the energy density of sulfide-based solid-state batteries.

Dendrite initiation and propagation in lithium metal solid-state batteries

All-solid-state batteries with a Li anode and ceramic electrolyte have the potential to deliver a step change in performance compared with today's Li-ion batteries^1,2. However, Li dendrites (filaments) form on charging at practical rates and penetrate the ceramic electrolyte, leading to short circuit and cell failure^3,4. Previous models of dendrite penetration have generally focused on a single process for dendrite initiation and propagation, with Li driving the crack at its tip^5-9. Here we show that initiation and propagation are separate processes. Initiation arises from Li deposition into subsurface pores, by means of microcracks that connect the pores to the surface. Once filled, further charging builds pressure in the pores owing to the slow extrusion of Li (viscoplastic flow) back to the surface, leading to cracking. By contrast, dendrite propagation occurs by wedge opening, with Li driving the dry crack from the rear, not the tip. Whereas initiation is determined by the local (microscopic) fracture strength at the grain boundaries, the pore size, pore population density and current density, propagation depends on the (macroscopic) fracture toughness of the ceramic, the length of the Li dendrite (filament) that partially occupies the dry crack, current density, stack pressure and the charge capacity accessed during each cycle. Lower stack pressures suppress propagation, markedly extending the number of cycles before short circuit in cells in which dendrites have initiated.

Codependent failure mechanisms between cathode and anode in solid state lithium metal batteries: mediated by uneven ion flux

An in-depth understanding of the degradation mechanisms is a prerequisite for developing the next-generation all solid-state lithium metal battery (ASSLMB) technology. Herein, synchrotron X-ray computed tomography (SXCT) together with other probing tools and simulation method were employed to rediscover the decaying mechanisms of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM)|Li₆PS₅Cl (LPSCl)|Li ASSLMB. It reveals that the detachment and isolation of NCM particles cause the current focusing on the remaining active regions of cathode. The extent of Li stripping and the likelihood of Li⁺ plating into LPSCl facing the active NCM particles becomes higher. Besides, the homogeneity of Li stripping/plating is improved by homogenizing the electrochemical reactions at the cathode side by LiZr₂(PO₄)₃ (LZP) coating. These results suggest a codependent failure mechanism between cathode and anode that is mediated by uneven Li ion flux. This work contributes to a holistic understanding of the degradation mechanisms in ASSLMBs and opens new opportunities for their further optimization and development.

Antiperovskite Electrolytes for Solid-State Batteries

Solid-state batteries have fascinated the research community over the past decade, largely due to their improved safety properties and potential for high-energy density. Searching for fast ion conductors with sufficient electrochemical and chemical stabilities is at the heart of solid-state battery research and applications. Recently, significant progress has been made in solid-state electrolyte development. Sulfide-, oxide-, and halide-based electrolytes have been able to achieve high ionic conductivities of more than 10^-3 S/cm at room temperature, which are comparable to liquid-based electrolytes. However, their stability toward Li metal anodes poses significant challenges for these electrolytes. The existence of non-Li cations that can be reduced by Li metal in these electrolytes hinders the application of Li anode and therefore poses an obstacle toward achieving high-energy density. The finding of antiperovskites as ionic conductors in recent years has demonstrated a new and exciting solution. These materials, mainly constructed from Li (or Na), O, and Cl (or Br), are lightweight and electrochemically stable toward metallic Li and possess promising ionic conductivity. Because of the structural flexibility and tunability, antiperovskite electrolytes are excellent candidates for solid-state battery applications, and researchers are still exploring the relationship between their structure and ion diffusion behavior. Herein, the recent progress of antiperovskites for solid-state batteries is reviewed, and the strategies to tune the ionic conductivity by structural manipulation are summarized. Major challenges and future directions are discussed to facilitate the development of antiperovskite-based solid-state batteries.

Fluorinated solid electrolyte interphase enables highly reversible solid-state Li metal battery

Solid-state electrolytes (SSEs) are receiving great interest because their high mechanical strength and transference number could potentially suppress Li dendrites and their high electrochemical stability allows the use of high-voltage cathodes, which enhances the energy density and safety of batteries. However, the much lower critical current density and easier Li dendrite propagation in SSEs than in nonaqueous liquid electrolytes hindered their possible applications. Herein, we successfully suppressed Li dendrite growth in SSEs by in situ forming an LiF-rich solid electrolyte interphase (SEI) between the SSEs and the Li metal. The LiF-rich SEI successfully suppresses the penetration of Li dendrites into SSEs, while the low electronic conductivity and the intrinsic electrochemical stability of LiF block side reactions between the SSEs and Li. The LiF-rich SEI enhances the room temperature critical current density of Li₃PS₄ to a record-high value of >2 mA cm^-2. Moreover, the Li plating/stripping Coulombic efficiency was escalated from 88% of pristine Li₃PS₄ to more than 98% for LiF-coated Li₃PS₄. In situ formation of electronic insulating LiF-rich SEI provides an effective way to prevent Li dendrites in the SSEs, constituting a substantial leap toward the practical applications of next-generation high-energy solid-state Li metal batteries.

Quantitative Analysis of Microstructures and Reaction Interfaces on Composite Cathodes in All-Solid-State Batteries Using a Three-Dimensional Reconstruction Technique

The composite cathode of an all-solid-state battery composed of various solid-state components requires a dense microstructure and a highly percolated solid-state interface different from that of a conventional liquid-electrolyte-based Li-ion battery. Indeed, the preparation of such a system is particularly challenging. In this study, quantitative analyses of composite cathodes by three-dimensional reconstruction analysis were performed beyond the existing qualitative analysis, and their microstructures and reaction interfaces were successfully analyzed. Interestingly, various quantitative values of structure properties (such as the volume ratio, connectivity, tortuosity, and pore formation) associated with material optimization and process development were predicted, and they were found to result in limited electrochemical charge/discharge performances. We also verified that the effective two-phase boundaries were significantly suppressed to ∼23% of the total volume because of component dispersion and packing issues.

Influence of Lattice Polarizability on the Ionic Conductivity in the Lithium Superionic Argyrodites Li6PS5X (X = Cl, Br, I)

In the search for novel solid electrolytes for solid-state batteries, thiophosphate ionic conductors have been in recent focus owing to their high ionic conductivities, which are believed to stem from a softer, more polarizable anion framework. Inspired by the oft-cited connection between a soft anion lattice and ionic transport, this work aims to provide evidence on how changing the polarizability of the anion sublattice in one structure affects ionic transport. Here, we systematically alter the anion framework polarizability of the superionic argyrodites Li₆PS₅X by controlling the fractional occupancy of the halide anions (X = Cl, Br, I). Ultrasonic speed of sound measurements are used to quantify the variation in the lattice stiffness and Debye frequencies. In combination with electrochemical impedance spectroscopy and neutron diffraction, these results show that the lattice softness has a striking influence on the ionic transport: the softer bonds lower the activation barrier and simultaneously decrease the prefactor of the moving ion. Due to the contradicting influence of these parameters on ionic conductivity, we find that it is necessary to tailor the lattice stiffness of materials in order to obtain an optimum ionic conductivity.