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 difficult to induce polar oxygen-containing groups onto the PE separator surface under mild conditions on a large scale and further enhance their wettability. Herein, utilizing the ultrastrong oxidation of elemental fluorine (F2), it was found that F2 could easily react with the PE separator surface via radical-related routes, and thus oxygen would be naturally captured onto the separator surface for its radical affinity. The fluorinated PE separator exhibited significantly improved wettability as the water contact angle decreased from 117° to 62° at the minimum. Therefore, electrolyte uptake of the fluorinated separator reached 803.9% (of which the PE electrolyte uptake was 246.2%), and the ionic conductivity increased from 0.29 to 0.52mS/cm. Capacity retention of LiCoCO2/graphite cells assembled by a fluorinated PE 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 79.07 mAh/g to 109.77 mAh/g) at 1.2 C. Besides, due to the spontaneous coupling between direct fluorination induced radicals, micro-cross-linking spots 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-treatment strategy for high-performance PE separators.