Flame retardant separators are important to prevent or limit lithium ion battery fires and thermal runaway. The separator in lithium ion battery cells prevents direct contact between anode and cathode, leading to internal short circuit, resulting in overheating, and then decomposition of anode material to release oxygen which can react with the electrolyte and the separator. Conventional separators are based on polyethylene and polypropylene, which can shrink under heat (leaving holes) or burn. Strategies to address this battery safety problem include use of higher performance polymers, incorporation of minerals into the polymer (titanium, aluminium, zirconium, silica), ceramic-polymer multilayer separators and use of flame retardant additives in the separators (e.g. ATH, MDH, organic and inorganic phosphates, melamine compounds …). The problem would be reduced with solid state electrolytes but to date these show low electrical performance and often high sensitivity to moisture or oxygen.
“Recent progress in flame-retardant separators for safe lithium-ion batteries”, X. Zhang et al., Energy Storage Materials 37 (2021) 628–647 https://doi.org/10.1016/j.ensm.2021.02.042