Recent Progress on Lithium Argyrodite Solid-State Electrolytes

Abstract: All-solid-state batteries have attracted significant attention as next-generation energy-storage devices for electric vehicles and smart grids because of their excellent safety and high energy density. Research on solid electrolytes with high ionic conductivity at room temperature, good chemical/electrochemical stability, and superior electrode compatibility is important for promoting the development of all-solid-state batteries. Sulfide electrolytes have become a hot topic among different inorganic solid electrolytes because of their relatively high Li-ion conductivity (~10-3 Sžcm-1) and low solid-solid interfacial resistance between the solid electrolytes and electrode particles. Among these sulfide electrolytes, lithium argyrodite solid electrolytes have attracted much attention owing to their high Li-ion conductivity at room temperature and relatively low cost. However, many problems still need to be solved before their practical application, such as difficulties in batch preparation, poor air stability, narrow chemical/electrochemical stability window, and poor interface stability towards high-voltage active materials. Extensive research has been conducted by many research groups to solve these problems and significant progress has been achieved. This review summarizes the current research on the structural information, ion conduction behaviors, synthesis routes, modification methods for improving the chemical/electrochemical stability properties, and applications of lithium argyrodite electrolytes combined with various cathode and anode materials in all-solid-state batteries based on our own research and published works of others. Two types of synthesis routes, the solid-state reaction route and the liquid solution route, are used to prepare lithium argyrodite electrolytes. Typically, electrolytes obtained by the former method deliver higher conductivities than those obtained by the latter. Multiple characterization methods, including alternating current (AC) impedance, molecular dynamics (MD) simulations, spin lattice relaxation in 7Li nuclear magnetic resonance (NMR), and 1D/2D Li exchange NMR, have been applied to probe Li-ion diffusion in the bulk of a signal particle across the interface section between two electrolyte particles, across the cathode, and across electrolyte particles. Increasing the number of Li vacancies via halogen substitution and element doping has been widely applied to increase the Li-ion conductivity of argyrodite electrolytes. Improvements in air stability for these argyrodite electrolytes have been achieved using element doping (such as O, Sb, and Sn) based on the hard-soft-acid-base theory and surface coating strategies. Interface contact and stability between the active materials and solid electrolytes play a key role in battery performance. Owing to the poor chemical/electrochemical stability of cathode materials, homogenous surface coatings and lithium halide electrolyte additives have been introduced into the configuration to isolate the direct contact between sulfides and active materials in the cathode mixture. Poor lithium metal compatibility inhibits the application of lithium argyrodite electrolytes in solid-state lithium metal batteries with high energy densities. Elemental doping in lithium argyrodites can form lithium alloys that impede the growth of lithium dendrites, and the surface modification of lithium metal anodes is helpful in constructing solid-state batteries with lithium metal anodes. Furthermore, research on lithium argyrodite electrolyte film preparation has also been conducted to develop a new solid-state battery construction route. In addition, the challenges and problems are analyzed, and possible research directions and development trends of lithium argyrodite solid electrolytes are proposed.