“Room Temperature Molten Salt”-Based Polymer Electrolyte Enabling a High-Rate and High-Thermal Stability Hybrid Li/Na-Ion Battery

“Water-in-salt” electrolytes have enlarged the electrochemical window of aqueous electrolytes to 3.0 V. However, the practical application of this electrolyte faces the challenge of high cost. Recently, we have proposed a low-cost inorganic room temperature molten salt (RTMS) electrolyte with a widened electrochemical window of 3.1 V. Herein, the RTMS electrolyte has been integrated with a hydrophilic polymer by ultrafast polymerization through electron beam irradiation to further enlarge the anode limit, increase the ionic conductivity, and improve the thermal stability. The double-redox-active Prussian blue analogues of the cobalt hexacyanoferrate cathode (NaCoHCF) in the RTMS-based polymer electrolyte prepared by electron beam (EB) irradiation (e-RPE) show electrochemical performance with a high capacity of 137 mAh·g^–1 at 1C and 100 mAh·g^–1 at 5C. More significantly, at a high temperature of 60 °C, the NaCoHCF electrode in e-RPE exhibits a high capacity of 120 mAh·g^–1 at 5C and a high capacity retention of 92% over 100 cycles at 1C. Compared to RTMS, the RTMS-based polymer electrolyte not only expands the hydrogen evolution limit but also shows high thermal stability, which is favorable for the electrochemical performance of NaCoHCF at high temperature. Furthermore, the battery with e-RPE is intrinsically safe and can be widely used in large-scale energy storage and wearable device applications.