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The commercial electrolyte of lithium-ion batteries (LIBs) is usually liquid state supplemented by a porous polyolefin separator to prevent short circuit. Recent years, polymer electrolyte shows great potential for its flexibility and enhanced safety. And novel single-ion conductor gel polymer electrolyte (SIGPE) could effectively suppress the anion polarization by immobilizing anions on the polymer bone. Further enhancing the thermal stability of SIGPE plays a significant role for the safety of the LIBs. Herein, a novel bifunctional core–shell structured composite was prepared, consisting of thermal stable polyimide as core and a composite single-ion conductor polymer. The relations of fiber morphology with spinning parameters and heat-treatment temperature were studied carefully. As a result, the composite core–shell structured membrane shows excellent high dimensional thermal stability up to 190 °C. Moreover, when the charged half battery was stored at 150 °C, the core–shell structured SIGPE cell shows excellent potential stability while ordinary cell quickly short out, greatly enhancing the safety of the LIBs. The ionic conductivity of the plasticized gel electrolyte could reach 1.7*10−5 S cm−1 with a Li-ion transference number of 0.76. The assembled LiFePO4 battery shows stable cycle performance and remained 85% capacity after 100 cycles. [ABSTRACT FROM AUTHOR]