Recent progress in flame-retardant separators for safe lithium-ion batteries

Lithium-ion batteries (LIBs) are considered as one of the most successful energy storage technologies due to the high energy density, long cyclability and no memory effect. With the ever-increasing energy density of LIBs in practical applications, operational safety becomes more critical because unintentional release of energy can lead to devastating accidents related to fire and explosions of LIBs. The flammable polymer separators and liquid electrolytes are the root causes of such safety hazards. For the advancement of safe LIB design, this review summarizes the recent developments towards highly flame-retardant separators and solid-state electrolytes. The composition, structure and the performance of flame-retardant separators for liquid LIBs are firstly discussed, which include inorganic particle blended polymer separators, ceramic material coated separators, inherently nonflammable polymer separators, and separators with flame-retardant additives. Then much attention is paid on the solid-state electrolytes including inorganic solid electrolytes, solid polymer electrolytes, and inorganic/organic composite electrolytes. Future outlooks for the challenges of flame-retardant separators to practical applications are also discussed.