A highly porous polymer electrolyte with good wettability and excellent thermal dimensional stability is of great importance to enable the high performance of lithium ion batteries (LIBs). This study reports a facile in-situ polymerization process to approach molecular mixing level of poly (vinylidene fluoride-hexa-fluoropropylene) (PVDF-HFP) and the fully aromatic single-ion conducting polymer (fa-SIPE). The molecular level mixing is greatly beneficial for promoting both macro and nanophase separation between the flexible aliphatic PVDF-HFP and the aromatic fa-SIPE. These occur because of the poor physical compatibility of the two polymers and leads to a macro/nano porous single-ion polymer electrolyte (SIPE). Filling the porosity with a Li+-ion coordinating solvent enables high Li+-ion mobility, which is favorable for the development of high power and fast charge LIBs. Remarkably, the highly porous SIPE membrane displays high mechanical stability, high electrolyte wettability and strong thermal dimensional stability, all possibly contributing to excellent stability and reliability of LiBs. This study provides an efficient way of fabricating porous, single-ion polymer electrolyte for LIBs application.