Energy-dense lithium-ion battery (LIB) is in high demand for advanced portable electronics as well as electric vehicle EV applications. However, the flammability of conventional carbonate-based electrolytes makes the safety of the LIB questionable. Here, we demonstrate a nonflammable, thermally stable electrolyte for LIB application in the wide temperature range of −20 °C–50 °C. Specifically, we developed a non-woven fabric-supported quasi-solid flexible polymer electrolyte with a unique liquid electrolyte combination of methyl propionate (MP), trimethyl phosphate (TMP), fluoroethylene carbonate (FEC), and Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer. The resulting composite electrolyte is compatible with Li metal, and in a half-cell (Li||LFP) performs superior to the commercial electrolyte over the wide temperature range. A full-cell with high-loading (cathode loading 13.2 mg/cm2) LFP combined with graphite demonstrated electrochemical performance. An initial discharge capacity of 115 mAh/g at a C/10 rate and capacity retention of 57 % was observed after 100 cycles. The thermal safety of the full cell was evaluated through multimodule calorimetry, where a low heat release of 37 J/g was observed, compared to 1.5 kJ/g with commercial electrolyte. This report aims to demonstrate the great potential of using flame-retardant quasi-solid polymer electrolytes for fabricating safer LIBs.