The intrinsically low electrolyte affinity of polyolefin separators limits their electrochemical performance in lithium ion battery. Most of the traditional modification methods which improve the polyolefin separator's electrolyte affinity either rely on the use of thick coating layer that may decrease energy density of the battery or involve multiple laborious steps, making them less competitive for practical applications. Herein, we report a photo-induced functionalization that could significantly improve the electrolyte affinity of the polypropylene separator within 3 minutes. We find that the electrolyte saturated functionalized separator shows improved ionic conductivity, increased lithium ion transfer number, and thus excellent charge-discharge performance. Our results not only demonstrate the high potential of the functionalized separator, but also suggest that the photo-induced functionalization could be used as a general strategy to endow polyolefin separator with task-specific properties required for different battery applications. (C) 2018 The Electrochemical Society.