Flame-retarding quasi-solid polymer electrolytes for high-safety lithium metal batteries

Lithium metal batteries (LMBs) have gained significant attention due to their potential for high energy density. However, the commonly used liquid carbonate electrolytes in LMBs are highly flammable and prone to leakage, which can lead to safety concerns such as gas production, cell swelling, fire, and even explosions during thermal runaway. To address these safety issues, all-solid electrolytes, including solid polymeric and inorganic electrolytes, have been proposed as the ultimate solution. However, all-solid polymeric electrolytes suffer from poor conductivity in the bulk electrolyte, while all-solid inorganic electrolytes face challenges with poor contact at the electrode/electrolyte interfaces, making it difficult to commercialize all solid-state LMBs. As a compromise solution, flame-retarding semi-solid polymeric electrolytes have emerged as a promising alternative in recent years, offering improved safety, excellent electrochemical performance, and great potential for commercialization. In this review, we introduce the concept that semi-solid polymeric electrolytes should possess four key dimensions of performance: physical characteristics, electrochemical characteristics, interfacial characteristics, and cost characteristics. Besides, we discuss the recent research advancements in various types of flame-retarding semi-solid polymer electrolytes, including phosphate ester-based quasi-solid polymer electrolytes, ionic liquid-based quasi-solid polymer electrolytes, nitrile-based quasi-solid polymer electrolytes, fluorine-based quasi-solid polymer electrolytes, and other non-flammable quasi-solid polymer electrolytes. We also examine their flame-retarding properties, mechanisms, and electrochemical performance. Furthermore, we discuss the challenges and potential directions for future development of flame-retarding quasi-solid polymer electrolytes.