Application of Terpolymer Encapsulated Flame-Retardant Separator in Ni-Rich and High-Voltage Lithium-Ion Batteries

Abstract

Increasing the working voltage of a Ni-rich cathode is considered as the most effective way to enhance the energy density of lithium-ion batteries (LIBs). However, high energy density brings safety hazards in LIBs, resulting from the structural instability of the cathode material and the oxidative decomposition of carbonate electrolyte under high-voltage conditions. Directly adding the proper amount of flame retardant to the liquid electrolyte reduces the safety hazards, but causes rapid decay in electrochemical performance. To achieve a balance, flame retardant of triphenyl phosphate (TPP) is encapsulated by poly(butyl methacrylate-acrylonitrile-styrene) (P(BMA-AN-St)) terpolymer during the electrospinning process ascribed from different physical properties of surface energy and polarity. Compared with the self-extinguishing time (SET) of 89.5 s g −1 for commercial polyethylene (PE) separator, the SET value of PT11 separator (P(BMA-AN-St): TPP = 1:1 (w:w)) is sharply reduced to 23.0 s g −1 , although both separators present the similar ionic conductivity of 0.8 × 10 −3 S cm −1 at room temperature. Whether in half-cells of LiNi 0.8 Co 0.15 Al 0.05 O 2 /Separator/Li or full-cells of LiNi 0.8 Co 0.15 Al 0.05 O 2 /Separator/graphite, PE and PT11 separator show parallel cycle stability between 3.0 and 4.5 V. Thus, terpolymer encapsulated flame-retardant separator achieves both improved flame retardancy and competent electrochemical stability, making it become a good candidate for high-energy-density LIBs.