A high-performance acid-base blend ion exchange membrane is designed and prepared for vanadium flow battery (VFB) application. Through transferring H+-SPEEK to Na+-SPEEK, the compatibility of hydrophilic sulfonated poly(ether ether ketone) (SPEEK) and polybenzimidazole (PBI) is improved. A great balance between ion selectivity and proton conductivity for these blend ion exchange membranes based on SPEEK and PBI is realized, due to the internal cross-linked networks induced by the acid-base interaction. A very impressive VFB performance with a coulombic efficiency of 98.5% and an energy efficiency of 89.8% at a current density of 80 mA cm(-2) is thus obtained, which are among the highest values ever reported for ion exchange membranes. Besides, these SPEEK-PBI blend membranes also possess excellent chemical, mechanical and thermal stability, showing very promising prospect for VFB application.