Fluorine-Initiated Carboxyl Group Enhanced Combination Properties of the Polyethylene Separator for Lithium-Ion Batteries

Due to the intrinsic inertness of polyethylene (PE),it is difficultto induce polar oxygen-containing groups onto the PE separator surfaceunder mild conditions on a large scale and further enhance their wettability.Herein, utilizing the ultrastrong oxidation of elemental fluorine(F-2), it was found that F-2 could easily reactwith the PE separator surface via radical-related routes, and thusoxygen would be naturally captured onto the separator surface forits radical affinity. The fluorinated PE separator exhibited significantlyimproved wettability as the water contact angle decreased from 117 & DEG;to 62 & DEG; at the minimum. Therefore, electrolyte uptake of the fluorinatedseparator reached 803.9% (of which the PE electrolyte uptake was 246.2%),and the ionic conductivity increased from 0.29 to 0.52mS/cm. Capacityretention of LiCoCO2/graphite cells assembled by a fluorinatedPE separator increased to 80.4% from 73.2% after 200 cycles of charge-discharge,and the discharge capacity of it also increased 38.83% (from 79.07mAh/g to 109.77 mAh/g) at 1.2 C. Besides, due to the spontaneous couplingbetween direct fluorination induced radicals, micro-cross-linkingspots were generated, and thus, modulus and thermal deformability,which meant service stability of the separator, were also improved.Therefore, direct fluorination could be considered an effective post-treatmentstrategy for high-performance PE separators.