A composite separator design enabled by phosphorus-nitrogen flame retardants and SiO2 or Al2O3 synergy for safer and more efficient lithium-ion batteries

Conventional polypropylene (PP) separators in lithium-ion batteries (LIBs) suffer from poor thermal dimensional integrity and limited flame retardancy, posing significant safety hazards. In this work, a novel phosphorus-nitrogen-based flame retardant, pentaerythritol diphosphate-1-piperazine ethanol phosphate (PDPE), was synthesized. Its molecular structure features phosphate ester groups and nitrogen-containing piperazine rings, which endow PDPE with a dual flame-retardant function: gas-phase radical quenching and condensed-phase char promotion. PDPE was then integrated with inorganic oxides (SiO 2 or Al 2 O 3 ) to fabricate organic–inorganic hybrid composite separators via a simple and scalable coating method. Meanwhile, the inorganic oxides provide thermal insulation and enhance ionic conductivity, collectively contributing to improved thermal stability and the electrochemical performance of lithium-ion batteries. Among the composites, the PA63 separator (PDPE/Al 2 O 3 ) showed outstanding fire resistance, with a 53 % reduction in peak heat release rate (PHRR) and a 36 % decrease in total heat release (THR), while the char residue increased from 4.38 % (PP) to 25.38 %. It also remained dimensionally stable at 200 ℃ and exhibited a markedly reduced self-extinguishing time. Electrochemical tests showed a discharge capacity of 169.3 mAh/g after 100 cycles at 0.5C and 145.4 mAh/g at 2C, significantly higher than the commercial PP separator (147.7 mAh/g at 0.5C and 91.5 mAh/g at 2C). These results demonstrate the potential of PDPE-based composite separators for next-generation LIBs with superior safety and electrochemical performance.