Advancements in flame-retardant strategies for lithium-sulfur batteries: from mechanisms to materials

Due to their extraordinary theoretical energy density, high specific capacity, and environment-friendly nature, lithium-sulfur batteries (LSBs) have been considered the most promising candidates for energy storage. However, in recent years, fire hazards and explosions caused by batteries have seriously endangered the safety of society, and thus, research on LSBs must focus on high safety and electrochemical performance. The combustion and thermal runaway (TR) of LSBs are influenced by their flammable electrolyte, low ignition point of sulfur, and poor thermal stability of polymer separators. In this case, flame-retardant materials such as polyimide and aramid with a high limited oxygen index (LOI) are difficult to burn, making them suitable as separators. Additionally, inorganic flame-retardant materials with three-dimensional structures can be used to load sulfur, whereas nonflammable ionic liquids (ILs) can replace the ether electrolyte to construct high-safety LSBs. Herein, the TR route and flame-retardant mechanism of LSBs in the gas phase and condensed phase are revealed. In addition, we systematically reviewed the research progress on flame-retardant materials used in different components of LSBs, including electrolyte engineering, functional separators, and modified cathodes. Finally, the problems of nonflammable LSBs based on flame-retardant materials are analyzed, and the future perspective trend is highlighted.