Crosslinkable sulfonated poly (diallyl-bisphenol ether ether ketone) (SDPEEK) membranes with different degrees of sulfonation (DS) are prepared and first investigated in vanadium redox flow battery (VFB) application. The SDPEEK membranes with DS of 80% (SD4-6-100) show over an order of magnitude lower permeability of VO2+ ions (2.4 × 10−8 vs. 1.04 × 10−6 cm2 min−1) than currently used Nafion 115. The VFB battery assembled with SD4-6-100 membrane exhibits much higher columbic efficiency (98% vs. 92% at 50 mA cm−2) and longer self-discharge time (180 h vs. 50 h) than that of Nafion 115, however, at similar energy efficiency. The membrane keeps a stable performance after 900 h charge–discharge test. In order to further improve the ions selectivity of SDPEEK membranes with high DS, the SD5-5-50 (DS 100%) membrane is crosslinked under ultraviolet (UV) radiation. The ion selectivity of the membrane is improved obviously after crosslinking, as a result, showing much higher columbic efficiency and energy efficiency.