Advanced Electrolytes for Rechargeable Lithium Metal Batteries with High Safety and Cycling Stability

With the rapid development of advanced energy storage equipment, particularly lithium-ion batteries (LIBs), there is a growing demand for enhanced battery energy density across various fields. Consequently, an increasing number of high-specific-capacity cathode and anode materials are being rapidly developed. Concurrently, challenges pertaining to insufficient battery safety and stability arising from liquid electrolytes (LEs) with flammability persistently emerge. LEs possess the advantages of exceptional ionic conductivity and can operate within a broader temperature range. After two decades of continuous development in commercial applications, it currently stands as the most widely employed electrolyte material in lithium-ion batteries. However, the existing LE primarily consists of a carbonate electrolyte with a low flash point, low boiling point, and flammable and volatile nature, thereby rendering fire and explosion risks inevitable. Compared with LEs, solid-state electrolytes (SSEs) exhibit relatively good flame retardancy and possess the potential to inhibit lithium dendrite formation, and they are regarded as promising electrolyte materials. Nevertheless, numerous challenges of SSEs still need to be addressed at this stage. The inadequate solid-solid contact between the solid electrolyte and the electrode material, as well as the insufficient contact stability, significantly impact the cycling stability of solid-state batteries. Furthermore, unlike liquid electrolytes, the solid electrolyte lacks fluidity and cannot effectively penetrate the pores of porous electrodes, necessitating additional cathode design considerations. The incompatibility with existing liquid battery production processes and high cost further impede the advancement of solid-state batteries. In response to the challenges associated with solid-state batteries, recent research has introduced in situ solidification solutions. By transformation of the liquid into a solid electrolyte within the battery, this method facilitates excellent interfacial contact between the electrolyte and electrode material while ensuring compatibility with existing production equipment. Consequently, these advantages have propelled in situ solidification to become a prominent research methodology for solid-state batteries. Currently, electrolyte research is undergoing a transitional period from liquid to solid-state, accompanying the emergence of numerous hybrid solid-liquid electrolytes (HSLEs). HSLEs not only exhibit the high ionic conductivity characteristic of liquid electrolytes but also enhance battery safety and stability to a certain extent. HSLEs are found in various forms, including hybrid systems comprising inorganic solid electrolytes and LEs, as well as gel systems consisting of polymer electrolytes and LEs. Additionally, there are in situ solidification technologies that enable the gel electrolyte to be formed internally within the battery. This concept introduces the development status of electrolytes with improved safety and stability from the perspectives of LEs, SSEs, and HSLEs.