A Flexible Dual‐Ion Battery Based on Sodium‐Ion Quasi‐Solid‐State Electrolyte with Long Cycling Life

Abstract Sodium‐based dual‐ion batteries (SDIBs) have attracted much attention for their advantages of high operating voltage, environmental friendliness, and especially low cost. However, the electrochemical performances of the reported SDIBs are still unsatisfied due to the decomposition problem of traditional liquid electrolyte under high working voltage. Development of quasi‐solid‐state electrolytes (QSSEs) with excellent electrochemical stability at high voltage is a possible means to improve their properties. In this work, a flexible SDIB based on a QSSE, consisting of poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) three‐dimensionally cross‐linked with Al 2 O 3 nanoparticles, which exhibits a porous 3D structure with dramatically enhanced ionic conductivity up to ≈1.3 × 10 −3 S cm −1 , facilitating fast ionic migration of both anions and cations, is reported. This quasi‐state SDIB exhibits a high specific capacity of 96.8 mAh g −1 at a current rate of 5 C and excellent cycling stability with a capacity retention of 97.5% after 600 cycles at 5 C, which is the best performance of the SDIBs. Moreover, excellent flexibility and a wide working temperature range (−20 to 70 °C) have been realized for this battery, suggesting its potential for high‐performance flexible energy storage applications.