Sulfide solid electrolyte synthesized by liquid phase approach and application in all-solid-state lithium batteries

Current commercialized lithium-ion batteries generally suffer from safety issues due to using flammable organic liquid electrolytes. All-solid-state lithium batteries employing solid electrolytes instead of organic liquid electrolytes and separators possess the advantages of both good safety and high energy density , which are expected to be the most promising energy storage devices for the next generation electric vehicles and smart grid. Sulfide solid electrolytes are regarded as crucial components for all-solid-state rechargeable batteries for the merits of their high room temperature ionic conductivities that approach or exceed liquid organic electrolytes and excellent mechanical ductility. The preparation methods of sulfide solid electrolytes are mainly divided into three categories, i.e. solid-state sintering, ball milling and liquid-phase method. However, solid-state sintering and ball milling are time-consuming accompanied by high energy consumption. At the same time, the synthesized electrolyte particles are large in size, which seriously limits the practical application of sulfide solid electrolytes. In contrast, the liquid-phase method, using organic solvents as the medium, can synthesize sulfide solid electrolytes with controlled particle sizes, which is a simple and time-saving process and more suitable for large-scale production. In this review, we begin by introducing the crystal structures and ion transport mechanisms of major sulfide solid electrolytes including Li 2 S–P 2 S 5 binary sulfide solid electrolytes, Li 10 GeP 2 S 12 and Li 6 PS 5 X (X ​= ​Cl, Br, I) ternary systems, and summarize the progress of sulfide solid electrolytes prepared by liquid phase method in recent years. Based on the solubility state of the reagents in the solvent, the liquid-phase synthesis of sulfide solid electrolytes can be categorized into suspension type, solution type and mixed type, and their reaction mechanisms are discussed separately. Subsequently, we summarize the effect of solvents on the properties of liquid-phase synthesized sulfide solid electrolytes, such as purity, morphology, crystallinity and ionic conductivity. In addition, the application of liquid-phase synthesized sulfide solid electrolytes for all-solid-state lithium batteries is presented from six aspects: sulfide solid electrolytes coated on active materials, electrolyte-active material composites, electrolyte injection into porous electrodes, interfacial modification at solid-solid contact triple-interfaces within electrode layers, electrolyte elemental doping and electrolyte film preparation, which demonstrates the superior scalability of the liquid-phase method and the diverse application prospects. Finally, according to the current research status of the sulfide solid electrolytes synthesized by liquid phase method, the advantages and limitations of the liquid phase synthesis of sulfide solid electrolytes are also analyzed, providing the development direction for the liquid phase synthesized sulfide solid electrolyte for all-solid-state lithium batteries in future.