Unveiling the power of sulfide solid electrolytes for next-generation all-solid-state lithium batteries

The demand for power sources for electric vehicles and portable devices has increased rapidly. All-solid-state lithium-ion batteries are promising attention due to their safety and energy density. Especially, the development of solid electrolytes is necessary to realize all-solid-state batteries. Solid electrolytes with high ionic conductivity like sulfides, oxides, and chlorides are used for long-distance conduction in the entire all-solid-state batteries. In addition, those with low ionic conductivity like lithium salts are also used as coating materials and sintering aids. Our group reported active materials and electrolytes using Li2SO4 [1, 2].One of the features required for solid electrolytes is wide electrochemical windows. Although various sulfide solid electrolytes have been studied due to their high ionic conductivity and ductility, they have narrow electrochemical windows [3], leading to oxidative and reductive decomposition during cell cycling. Thus, it is important to consider the electrochemical windows of solid electrolytes.Conventionally, the electrochemical windows of solid electrolytes have been investigated using electrochemical techniques like cycling voltammetry and linear sweep voltammetry (LSV). The working electrode is composed of the objective solid electrolytes and conductive carbon. However, for some materials with low ionic conductivity, it is challenging to ensure ionic conduction at long distances in the composite working electrode. Therefore, few reports have shown that the redox potential of a low ionic-conductive material including Li2SO4 is experimentally determined by electrochemical tests, although the electrochemical stability of lithium ion conductors was investigated using first-principles calculation methods [3, 4].In this study, we used LSV measurements using all-solid-state cells to investigate the redox potential of Li2SO4. The composite working electrode comprised Li2SO4, vapor-grown carbon fiber (VGCF), and oxide solid electrolyte. Li2SO4 and VGCF were ball-milled and the obtained Li2SO4-VGCF and oxide electrolyte were mixed in an agate mortar and pestle to prepare composite working electrodes. As oxide electrolytes, 90Li3BO3·10Li2SO4 glass-ceramic (LBSO) [2] was used in cells for an anodic scan, and 67Li2O·33LiI glass (LOI) [5] was used in cells for a cathodic scan because they have better oxidation or reduction tolerance than sulfide electrolytes. In addition, it was found that the decomposition of sulfide electrolytes affected the LSV curves when sulfide electrolytes were used in a composite working electrode or as a separator layer that directly contacted with the composite working electrode. Therefore, the cell configuration was Li-In/Li3PS4/LBSO (or LOI)/Li2SO4-VGCF-LBSO (or LOI). To confirm the decomposition of Li2SO4 after the LSV measurements, X-ray photoelectron spectroscopy (XPS) measurements were performed on the cells, and the redox mechanism of Li2SO4 was discussed.In the anodic scan, a current peak derived from the oxidation of Li2SO4 was observed at approximately 4.3 V vs. Li-In (4.9 V vs. Li+/Li), which was close to the standard electrode potential of oxidation from SO4 2– to S2O8 2– [6]. In addition, in the cathodic scan, a current peak derived from the reduction of Li2SO4 was observed at approximately 1.5 V vs. Li-In (2.1 V vs. Li+/Li), which almost corresponds to the potential of reduction from SO4 2– to SO3 2– [6]. Furthermore, the XPS analyses confirmed that Li2SO4 was finally reduced to Li2S and Li2O.In summary, the LSV measurements were performed with the all-solid-state cell using oxide electrolytes instead of sulfide electrolytes, which eliminated the overlap of a current caused by the decomposition of sulfide electrolytes. The oxidative and reductive decomposition voltages of Li2SO4 were close to each standard electrode potential. We also found a way to experimentally determine the electrochemical potential of any materials by modifying the cell design. This study will help spread the importance of potential windows and the realization of all-solid-state lithium-ion batteries. Reference s : [1] K. Nagao et al., Sci. Adv. 6 (2020) eaax7236.[2] M. Tatsumisago et al., JCS-Japan, 125 (2017) 433.[3] Y. Zhu et al., ACS Appl. Mater. Interfaces, 7 (2015) 23685.z[4] W. D. Richards, et al., Chem. Mater., 28 (2016) 266.[5] Y. Fujita et al., Electrochemistry, 89 (2021) 334.[6] A. J. Bard et al., Standard Potentials in Aqueous Solution, Routledge (2017).

All-solid-state batteries (ASSBs) using sulfide solid electrolytes with high room-temperature ionic conductivity are expected as promising next-generation batteries, which might solve the safety issues and enable the utilization of lithium metal as the anode to further increase the energy density of cells. Most researchers in the academic community currently focus on developing novel sulfide solid electrolytes, clarifying the interface issues between sulfide electrolytes and solid electrodes and mechanism of lithium dendrite growth in ASSB. However, there is a lacking in the technical route analysis about the commercialization of ASSBs based on sulfide solid electrolytes. This review mainly introduces the specific preparation methods of various parts in sulfide-based ASSBs, including the preparation methods of sulfide solid electrolyte particles, sulfide-based composite electrolyte membranes, composite cathodes and anodes, and analyzes the advantages and disadvantages of these methods. In addition, several schemes of ASSB assembly are also introduced. Finally, a perspective of large-scale production of sulfide-based ASSBs is provided, which is expected to accelerate the commercialization of sulfide-based ASSBs.

All-solid-state secondary batteries that employ inorganic solid electrolytes are desirable because they are potentially safer than conventional batteries. The ionic conductivities of solid electrolytes are currently attracting great attention. In addition to the conductivity, the mechanical properties of solid electrolytes are important for improving the energy density and cycle performance. However, the mechanical properties of sulfide electrolytes have not been clarified in detail. Here, we demonstrate the unique mechanical properties of sulfide electrolytes. Sulfide electrolytes show room temperature pressure sintering. Ionic materials with low bond energies and a highly covalent character, which is promising for achieving a high ionic conductivity, tend to be suitable for room-temperature processing. The Young's moduli of sulfide electrolytes were measured to be about 20 GPa, which is an intermediate value between those of typical oxides and organic polymers.

Enhancement of the rate capabilities for all-solid-state batteries through the surface oxidation of sulfide solid electrolytes

Indium doped sulfide solid electrolyte with tamed lithium dendrite and improved ionic conductivity for all-solid-state battery applications

Enhancing the ionic conductivity of sulfide solid electrolytes (SEs) through dual‐doping is a well‐established approach, yet the atomic‐level mechanisms driving these improvements remain elusive. By dual‐doping Ge and Cl into the Li10GeP2S12 (LGPS) framework, we synthesized Ge/Cl‐doped LGPS (Li10+xGe1+2xP2−2xS12−xClx, x = 0.3) with an ionic conductivity of 12.4 mS/cm at 25°C, a value that stands among the highest for LGPS‐type SEs. This achievement emphasizes the pivotal role of dopant selection in modulating Li‐ion transport mechanisms, thereby enhancing SE performance. Our research elucidates the intricate atomic mechanisms responsible for this enhanced ionic conductivity, with a particular focus on the synergistic effects of Ge and Cl dual‐doping. Integrating advanced multianalytical techniques, including experiments and atomistic modeling (machine‐learning‐assisted molecular dynamics simulations and density functional theory calculations), we provide comprehensive insights into the structure–property relationship in Ge/Cl‐doped LGPS SEs. Our findings reveal that Cl doping significantly enhances the paddle‐wheel dynamics, while Ge doping promotes cooperative Li diffusion through the formation of Li interstitials. This dual‐doping approach not only elucidates the structural and functional dynamics of SEs but also paves the way for designing dopants to enhance ionic conductivity. The insights gained from this study offer a strategic direction for developing higher‐performance SEs, highlighting the importance of tailored dopant selection in advancing energy storage technologies.

Lithium-ion batteries (LIBs) have played a pivotal role in advancing energy storage technologies, enabling high-demand applications such as electric vehicles and renewable energy storage systems. Despite their widespread use, LIBs face significant challenges in terms of energy density and intrinsic safety issues. These challenges have spurred interest in all-solid-state batteries (ASSBs), which utilize solid electrolytes (SEs) to offer safe alternatives to conventional liquid electrolyte-based LIBs. Among the various types of SEs explored to date, halide SEs exhibit improved compatibility with high-voltage oxide cathodes, as compared with sulfide-based SEs. Thus, halide SEs may be utilized as cathode coating materials for ASSBs using sulfide SEs with high ionic conductivity. However, the specific material properties that determine the chemical/electrochemical characteristics of halide coatings have not been clearly studied yet.In this study, we synthesize lithium yttrium chloride (Li3YCl6, LYC) SEs using different methods (solid-state synthesis and ammonium-assisted liquid-phase synthesis) at various heat-treatment temperatures to control phase compositions and crystal structures. These allow us to systematically explore the phase-structure-property relationship of LYC SEs and their roles as coating materials for LiNi0.90Co0.05Mn0.05O2 (NCM). Our experimental findings reveal that the phase, crystal structure and crystallinity of LYC significantly influence the microstructure, ionic conductivity, interfacial compatibility, and thermal stability of LYC-coated NCM (NCM@LYC). The crystal structure and crystallinity of synthesized LYC SEs are explored by Rietveld refinement of X-ray diffraction patterns. Notably, LYC SEs prepared via wet synthesis under optimal heat-treatment conditions exhibit superior Li-ion transport kinetics due to the enhanced crystallinity of phase-pure LYC. This leads to the improved interfacial resistances of NCM@LYC, as evidenced by impedance analyses of NCM composite electrodes. Therefore, the NCM@LYC composite electrodes with the optimized LYC SE show significant improvement in rate capability and cycle lifetimes. This research elucidates the critical role of the synthesis conditions in controlling the properties of halide coating materials and thus in improving the interfacial kinetics and stability of composite cathodes of ASSBs. The insights gained from the phase-structure-property relationships provide valuable guidelines for the development of highly conductive and stable SEs for next-generation batteries.

All-solid-state lithium metal batteries (ASSLMBs) with argyrodite-type Li6PS5Cl solid electrolyte (SE) show potential to replace current commercially available lithium-ion batteries by virtue of their potentially high energy density and safety. However, the interface incompatibility between sulfide solid electrolyte and lithium metal leads to an increase in interfacial resistance and the rapid growth of lithium dendrites, which greatly impede its commercial application. To alleviate the above problems, here a cell construction model has been put forward, in which composite polymer electrolytes (CPEs) with high ionic conductivity and good interface compatibility against lithium metal are employed as the interphase between Li6PS5Cl solid electrolyte and lithium metal. CPEs can prevent direct contact between lithium anode and Li6PS5Cl, and a soft CPE layer can solve the problem of high interfacial resistance caused by bad contact between the Li6PS5Cl solid electrolyte and the lithium metal anode. As a result, the symmetric cells Li/CPEs/Li6PS5Cl/CPEs/Li show stable polarization voltage over 140 h at a current density of 0.2 mA at 60 °C. Compared with the rapidly decaying capacity of the LiFePO4/Li6PS5Cl/Li cell, the initial discharge capacity of LiFePO4/Li6PS5Cl/CPEs/Li is 93.74 mAh/g, and after 15 cycles at 0.5 C, the capacity retention rate is still 75.1%. Therefore, the successful design of this cell structure based on the interface modification of Li6PS5Cl solid electrolyte provides good prospects for the commercial application of ASSLMBs.

Sulfide-based all-solid-state batteries (ASSBs) have ushered in a new era of energy storage technology, offering the tantalizing prospect of unprecedented energy density and safety. However, the poor electrode-electrolyte interface between Li anodes and sulfide solid electrolytes has hindered its practical application. In this review, the primary focus lies in the current fundamental understanding, challenges, and optimization strategies regarding the interface chemistries between sulfide solid electrolytes and Li anode. First, an in-depth discussion is conducted and provides a detailed summary of the interfacial challenges that exist between the Li anode and sulfide solid electrolytes. Among these challenges, poor interfacial compatibility and stability stand out as the two crucial issues. Subsequently, effective approaches are systematically explored to surmount these issues. These encompass the component optimization and structural design of the bulk anode, doping and coating strategies of the sulfide solid electrolytes, and interface design between the Li anode and sulfide solid electrolytes. Finally, the insights are present into the limitations of current studies, perspectives, and recommendations for the further development of sulfide-based solid-state batteries, aiming to offer a comprehensive and enlightening overview for interface engineering, which is of great significance for the integration of applicable all-solid-state Li metal batteries (ASSLMBs).

Ceramic sulfide solid-electrolytes are amongst the most promising materials for enabling solid-state lithium ion batteries. With ionic conductivities that can meet or exceed liquid-electrolytes, such ceramic sulfides are limited in practice chiefly by a narrow electrochemical stability window of approximately 1.7-2.1 V vs Lithium metal. In addition, ceramic sulfides are frequently plagued by interfacial reactions when combined with common electrode active materials. In this talk, methods for the stabilization of both the bulk electrochemical decompositions and the interfacial reactions will be discussed. Solid-state cells stable up to 5V, including layered Li-Co-O and spinel Li-Mn-Ni-O, will be presented and the stabilization mechanisms will be discussed from a detailed theoretical perspective. Ceramic sulfides are known substantially swell during electrochemical decay. Such swelling has been shown to provide a viable means by which to stabilize electrochemical decomposition in lithium ion batteries[1]. Evidence and theoretical understanding of stability window expansion as the result of mechanical constriction will be discussed for both Li10GeP2S12 and Li10SiP2S12 systems. Further discussion on the impact of interfaces will focus on our novel high-throughput computational schema[2], which has enabled the evaluation of nearly 70,000 material DFT calculation for interfacial stability with Li10SiP2S12. These new computational methods found over 2,000 high-voltage coating materials that are electrochemically inert against Li10SiP2S12 at standard cathode voltages. The culmination of these approaches, mechanical bulk stabilization combined with interfacial coating layers, is the design of solid-state full cells with high voltage stability and good cyclability. The principal focus of this talk will be a discussion on the theoretical underpinnings of these methods and a comparison with experimental data. [1] Fan Wu*, William Fitzhugh*, Luhan Ye, Jiaxin Ning, Xin Li. Advanced Sulfide Solid Electrolyte by Core-Shell Structural Design. Nature Communications, (9), 4037 (2018). [2] William Fitzhugh*, Fan Wu*, Wenye Deng, Pengfei Qi, Luhan Ye, Xin Li. A High-Throughput Search for Functionally Stable Interfaces in Sulfide Solid-State Lithium Ion Conductors. Advanced Energy Materials. Under review.

All-solid-state batteries with inorganic solid electrolytes are recognized as an ultimate goal of rechargeable batteries because of their high safety, versatile geometry and good cycle life. Compared to thin-film batteries, increasing the reversible capacity of bulk-type all-solid-state batteries using electrode active material particles is difficult because contact areas at solid–solid interfaces between the electrode and electrolyte particles are limited. Sulfide solid electrolytes have several advantages of high conductivity, wide electrochemical window, and appropriate mechanical properties such as formability, processability, and elastic modulus. Sulfide electrolyte with Li7P3S11 crystal has the highest Li+ ion conductivity of 1.7 × 10-2 S cm-1 at 25 °C. It is far beyond the Li+ ion conductivity of conventional organic liquid electrolytes. The Na+ ion conductivity of 7.4 × 10-4 S cm-1 is achieved for Na3.06P0.94Si0.06S4 with cubic structure. Moreover, formation of favorable solid–solid interfaces between electrode and electrolyte is important for realizing solid-state batteries. Sulfide electrolytes have better formability than oxide electrolytes. Consequently, a dense electrolyte separator and closely attached interfaces with active material particles are achieved via room-temperature sintering of sulfides merely by cold pressing without heat treatment. Elastic moduli for sulfide electrolytes are smaller than that of oxide electrolytes, and Na2S-P2S5 glass electrolytes have smaller Young’s modulus than Li2S-P2S5 electrolytes. Cross-sectional SEM observations for a positive electrode layer reveal that sulfide electrolyte coating on active material particles increases interface areas even with a minimum volume of electrolyte, indicating that the energy density of bulk-type solid-state batteries is enhanced. Both surface coating of electrode particles and preparation of nanocomposite are effective for increasing the reversible capacity of the batteries. Our approaches to form solid–solid interfaces are demonstrated.

As lithium rechargeable batteries are considered a potential candidate for large‐scale energy storage applications in devices such as electric vehicles (EVs) and smart grids, their safety has become of prime concern. This calls for the need to replace the flammable organic liquid electrolyte (LE) with an inorganic solid electrolyte (SE), and thus, develop bulk‐type all‐solid‐state lithium batteries (ASLBs), fabricated using a scalable process. Sulfide SEs are considered the most competitive candidate owing to their high conductivity at room temperature (10−3–10−2 S cm−1), which is comparable to that of LEs, and their ductility, which enables the fabrication of ASLBs simply using cold pressing. In the present review, issues and challenges to be faced for the fabrication of bulk‐type ASLBs using sulfide SEs are presented and discussed, with a special focus on the development of SEs, compatibility of the electrode materials with SEs, and structure of the composite electrodes. Recent progress made with the aim of addressing the aforementioned issues and challenges is also presented, to provide an outlook on the future of SEs and ASLBs.

All-solid-state batteries (ASSBs) working at room and mild temperature have demonstrated inspiring performances over recent years. However, the kinetic attributes of the interface applicable to the subzero temperatures are still unidentified, restricting the low-temperature interface design and operation. Herein, a host of cathode interfaces are constructed and investigated to unlock the critical interface features required for cryogenic temperatures. The unstable interface between LiNi0.90Co0.05Mn0.05O2 (Ni90) and Li6PS5Cl (LPSC) sulfide solid electrolyte (SE) results in unfavorable cathode-electrolyte interphase (CEI) and sluggish lithium-ion transport across the CEI. After inserting a Li2ZrO3 (LZO) coating layer, the activation energy of the Ni90@LZO/sulfide SE interface can be reduced from 60.19 kJ mol-1 to 41.39 kJ mol-1 owing to the suppressed interfacial reactions. Through replacing the LPSC SE and LZO coating layer by the Li3InCl6 (LIC) halide SE, both a highly stable interface and low activation energy (25.79 kJ mol-1) can be achieved, thus realizing an improved capacity retention (26.9%) at -30 °C for the Ni90/LIC/LPSC/Li-In ASSB. Moreover, theoretical evaluation clarifies that cathode/SE interfaces with high ionic conductivity and low energy barrier are favorable to the Li+ conduction through the interphase and the Li+ transfer across the cathode/interphase interface. These critical understandings may provide guidance for low-temperature interface design in ASSBs.

Surface engineering of nickel-rich single-crystal layered oxide cathode enables high-capacity and long cycle-life sulfide all-solid-state batteries

Interface compatibility between sulfide solid electrolytes and Ni-rich oxide cathode materials: factors, modification, perspectives