Quaternary　ammonium　densely　functionalized　octa-benzylmethyl-containing　poly(arylene　ether　ketone)s　(QA-OMPAEKs)　with　ion　exchange　capacities　(IECs)　ranging　from　1.23　to　2.21　mmol　g−1　were　synthesized　from:　(1)　Ullmann　coupling　extension　of　tetra-benzylmethyl-containing　bisphenol　A;　(2)　condensation　polymerization　with　activated　dihalide　in　the　presence　of　K2CO3;　(3)　selective　bromination　using　N-bromosuccinimide;　and　(4)　quantitative　quaternization　using　trimethylamine.　Both　smallangle　X-ray　scattering　(SAXS)　and　transmission　electron　microscope　(TEM)　characterizations　revealed　distinct　nano-phase　separation　in　QA-OMPAEKs　as　a　result　of　the　dense　quaternization.　The　QA-OMPAEK-20　with　an　IEC　of　1.98　mmol　g−1　exhibited　a　high　SO42−　conductivity　of　11.4　mS　cm−1　and　a　low　VO2+　permeability　of　0.06×10−12　m2　s−1　at　room　temperature,　leading　to　a　dramatically　higher　ion　selectivity　than　Nafion　N212.　Consequently,　the　vanadium　redox　flow　battery　(VRFB)　assembled　with　QA-OMPAEK-20　achieved　a　Coulombic　efficiency　of　96.9%　and　an　energy　efficiency　of　84.8%　at　a　current　density　of　50　mA　cm−2,　which　were　much　higher　than　those　of　the　batteries　assembled　with　Nafion　N212　and　a　home-made　control　membrane　without　distinct　nano-phase　separation.　Therefore,　ion　segregation　is　demonstrated　to　be　a　strategical　route　for　the　design　of　high　performance　anion　exchange　membranes　(AEMs)　for　VRFBs.　©　2019,　Science　China　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.