Novel gel polymer electrolyte membranes with excellent thermal stability are fabricated via a combination of physical blending and chemical cross-linking procedures. Precursor porous membranes made of poly(vinylidene fluoride) (PVDF) and polystyrene-poly(ethylene oxide)-polystyrene (PS-PEO-PS) triblock copolymer composites are prepared by a phase-inversion technique, and the gel polymer electrolyte membranes are finished by in situ hypercrosslinking of the PS segments in precursor membranes. The latter cross-linking procedure could consolidate pore configuration and thus greatly enhance the thermal stability of the obtained cross-linked composite membranes. The membranes with optimal PS/PEO ratios can retain reasonable porosity with little dimensional shrinkage at high temperatures up to 260 °C. Gel polymer electrolytes with these cross-linked membranes as matrices exhibit much higher ionic conductivities (up to 1.38 × 10 S cm at room temperature) than those based on pure PVDF membranes. Li/LiFePO half cells assembled with these gel polymer electrolytes exhibit good cycling performance and rate capability. These results indicate that the Friedel-Crafts reaction based hypercrosslinking is an efficient method to construct highly heat-resistant polymer electrolytes for lithium ion batteries, particularly advantageous in applications that require high-temperature usage.