The most daunting challenge in solid-state polymer electrolyte membranes (PEMs) is to achieve high ionic conductivity close to that of the liquid electrolytes, while maintaining enhanced thermal and mechanical performances. The ionic conductivity in relation to the morphology of PEMs composed of diblock copolymer (polystyrene-block-poly­(ethylene oxide); PS-b-PEO), lithium salt (lithium trifluoromethanesulfonate; LiTf), and ionic liquid (1-ethyl-3-methylimidazolium trifluoromethanesulfonate; EMIMTf) is investigated. The optimized functional nanostructured PEMs are achieved with room-temperature ionic conductivities higher than a 1 mS cm–1 benchmark. The morphology of these microphase-separated electrolytes is composed of a major soft high ionic-conductive PEO/LiTf/IL matrix with minor glassy high-modulus PS nanodomains. The ionic-liquid upload in hybrid electrolytes inhibits the PEO crystallization, reduces the PEO glass transition temperature, promotes an extended PEO chain conformation, and enhances the solubilization of the non-dissociated lithium salt at the PS–PEO domain interfaces. These intrinsic properties caused by the ionic-liquid loading serve to achieve stable and robust nanostructured electrolyte membranes and can explain the achieved benchmark conductivity.