Separator with Nitrogen-Phosphorus Flame-Retardant for LiNi_x Co_y Mn_{1- x - y} O₂ Cathode-Based Lithium-Ion Batteries

With the pursuit of high-energy-density for lithium-ion batteries (LIBs), the hidden safety problems of batteries have gradually emerged. LiNi_x Co_y Mn_{1- x - y} O₂ (NCM) is considered as an ideal cathode material to meet the urgent needs of high-energy-density batteries. However, the oxygen precipitation reaction of NCM cathode at high temperature brings serious safety concerns. In order to promote high-safety lithium-ion batteries, herein, a new type of flame-retardant separator is prepared using flame-retardant (melamine pyrophosphate, MPP) and thermal stable Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). MPP takes the advantage of nitrogen-phosphorus synergistic effect upon the increased internal temperature of LIBs, including the dilution effect of noncombustible gas and the rapidly suppression of undesirable thermal runaway. The developed flame-retardant separators show negligible shrinkage over 200 °C and it takes only 0.54 s to extinguish the flame in the ignition test, which are much superior to commercial polyolefin separators. Moreover, pouch cells are assembled to demonstrate the application potential of PVDF-HFP/MPP separators and further verify the safety performance. It is anticipated that the separator with nitrogen-phosphorus flame-retardant can be extensively applied to various high-energy-density devices owing to simplicity and cost-effectiveness.