Ionic Liquid Cross-linked Poly(N-isopropylacrylamide) Hydrogel Electrolytes for Self-Protective Flexible Separator-Free Supercapacitors

Thermoresponsive sol–gel transitions of poly(N-isopropylacrylamide) (PNIPAm) aqueous electrolytes have been explored to endow smart overheating self-protection for energy devices, but sol-state electrolytes are not suitable for flexible or wearable energy devices. It has remained challenging to explore PNIPAm hydrogels as smart electrolytes and separators in flexible energy devices. Herein, PNIPAm hydrogels cross-linked by ionic liquids were designed and prepared with gel–gel transitions for flexible supercapacitors, working both as the smart electrolyte and separator. The assembled supercapacitors exhibited desired specific capacitance and long-term stability at room temperature, and the capacitance suddenly dropped to almost zero at 60 °C to switch to self-protection of the supercapacitors. The reversible overheating protections were both obtained in both unbending and bending supercapacitors. The ionic liquid cross-linker played an important role to achieve effective overheating protections by confining more ions in the aggregated PNIPAm phase at high temperatures. Consequently, the ionic liquid cross-linked PNIPAm hydrogel electrolytes here demonstrated the advanced shutdown function of electrochemical performance compared with conventional thermoresponsive electrolytes. This work will inspire a more rational design of thermoresponsive hydrogel electrolytes in the perspective of polymers and aqueous electrolytes to achieve overheating protections.