Microporous poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) membranes were prepared using the phase-separation method. Then, the membranes were immersed in liquid electrolyte to form polymer electrolytes. The effects of PMMA on the morphology, degree of crystallinity, porosity, and electrolyte uptake of the PVDF membrane were studied. The addition of PMMA increased the pore size, porosity and electrolyte uptake of the PVDF membrane, which in turn increased the ionic conductivity of the polymer electrolyte. The maximum ionic conductivity at room temperature was 1.21 × 10−3 S cm−1 for Sample E70. The polymer electrolyte was investigated, along with lithium iron phosphate (LiFePO4) as cathode for all solid-state lithium-ion rechargeable batteries. The lithium metal/E70/LiFePO4 cell yielded a stable discharge capacity of 133 mAh g−1 after up to 50 cycles at a current density of 8.5 mA g−1.