Hydroxymethyl functionalized poly(ether ether ketone) (OHPEEK) was exploited as the separator of lithium ion battery (LIB). The stable OHPEEK backbone endows the membrane with high anti-shrinkage property (0% until 160°C and only 18.3% at 240°C) and high thermal stability (degradation temperature up to 282°C). Due to the abundant polar groups of the OHPEEK, the wettability of OHPEEK membrane is improved, reflected by that the contact angle reduces to 24.6°C, and the electrolyte uptake rises to 204% at 30°C. The OHPEEK membrane shows a higher conductivity in the electrolyte (1M LiPF6/EC/DME) compared with the Celgard 2400 PP separator. As a result, the LIB with the OHPEEK membrane possesses 0.3%–36.8% higher discharge capacity at the C-rate from 0.1 to 4C and satisfactory cycling performance.