Enhanced wetting properties of a polypropylene separator for a lithium-ion battery by hyperthermal hydrogen induced cross-linking of poly(ethylene oxide)

Enhancing the electrolyte wetting of separators by surface modification is very critical to prepare high-performance lithium-ion batteries. Herein, we present a new approach named hyperthermal hydrogen induced cross-linking (HHIC) technology to increase the electrolyte-affinity of polypropylene (PP) separators by covalently cross-linking a thin layer of poly(ethylene oxide) (PEO) on surface-inert PP separators. With the HHIC treatment, the polar functionalities of PEO (e.g. -OH, C-O-C) can be preserved through selective cleavage of C-H bonds and subsequent cross-linking of resulting carbon radicals generated on PEO and PP chains. As proved by solvent rinsing tests, contact angle measurements and Fourier transform infrared spectroscopy, a PEO coating was found firmly fixed on the separator surface, which results in significantly improved wetting with the electrolyte. Electrochemical measurements on subsequent lithium-ion batteries with the modified separator by HHIC treatment exhibit a lower internal resistance but higher capacity retention when compared to the pristine separator. HHIC treatment is concluded to be a highly efficient and environmental-friendly approach for separator surface modification without need for other chemical additives (e.g. chemical cross-linkers, initiators, and catalysts) and can preserve the desired macroscopic material properties of separators such as pore structures and mechanical strength.