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 degrees C and only 18.3% at 240 degrees C) and high thermal stability (degradation temperature up to 282 degrees 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 degrees C, and the electrolyte uptake rises to 204% at 30 degrees C. The OHPEEK membrane shows a higher conductivity in the electrolyte (1 M 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 4 C and satisfactory cycling performance.