A nonflammable polymeric gel electrolyte film has been developed for rechargeable lithium battery systems. The gel film consists of poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) swollen with LiPF6 solution of a ternary solvent containing nonflammable trimethyl phosphate (TMP). The addition of TMP to mixed ethylene carbonate plus diethyl carbonate (EC+DEC) solvent prevented the LiPF6 salt from the thermal decomposition. The LiPF6 solutions containing 20vol% or higher content of TMP were fire-retardant. High ionic conductivity was obtained for both the liquid and the gel electrolyte systems containing 55vol% of TMP in the liquid components: 7.1×10−3 Scm−1 for the liquid and 2.9×10−3 Scm−1 for the gel electrolytes at 20°C. A wide potential window (−0.5 to 4.5V versus Li/Li+) and the redox activity of the electrode materials were established for the gel electrolyte containing TMP.