High-performance rechargeable lithium-ion batteries (LIBs) are central for electric vehicles and portable electronics. However, battery thermal runaway and explosion accidents in practical applications have aroused considerable debates on battery safety. The safety issues are directly associated with the flammable polyolefin separators and organic liquid electrolytes. Herein, a poly(vinylidene fluoride-co-hexafluoropropylene)/Li6.75La3Zr1.75Ta0.25O12/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP/LLZTO/PVDF-HFP) trilayer separator with remarkable flammable retardancy and thermal stability is designed. The outer PVDF-HFP layers lead to compatible interfaces with battery electrodes, and the inner LLZTO layer enhances the thermal stability with subtle deformation at 300 °C. LiFePO4 half batteries assembled with the trilayer separator own a discharge capacity up to 153 mAh g−1 at 0.5 C and capacity retention up to 95% over 200 cycles. This work provides a new avenue to safe and high-performance LIBs.