Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is a promising polymer material widely used for the production of ion-exchange membranes. In this study, quaternization was performed by reacting brominated PPO with 2-(dimethylamino)ethyl methacrylate, resulting in chemical modification to allow cross-linking. The PPO-based ionomer was then filled into a polyethylene porous substrate to prepare a pore-filled anion-exchange membrane (PFAEM) having excellent electrochemical and mechanical properties. In addition, the chemical stability of the PFAEMs could be greatly improved by cross-linking using an appropriate cross-linker. The prepared membranes were also applied to an all-vanadium redox flow battery to evaluate their performances, and as a result, it was confirmed that the best energy efficiency (87.3%) was superior to that of the commercial membrane (84.8%).