Thermal-Switching and Repeatable Self-Protective Hydrogel Polyelectrolytes for Energy Storage Applications of Flexible Electronics

Abstract

Currently, electrochemical energy storage devices are one of the main energy sources of portable flexible electronics. However, the continuous heat accumulation inside the energy storage device possesses the risk of inducing thermal runaway. Preventing thermal hazards and dynamically adjusting thermal behavior are of paramount importance to solve the safety issues caused by flexible energy storage devices, especially for various wearable consumer electronic products that are in close contact with the human skin. Here, we report a self-protective flexible supercapacitor (SP-FSC) with reversible thermal-switching behavior based on temperature-responsive poly(N-isopropylacrylamide-co-N-methylolacrylamide) (PNIPAM/NMAM) hydrogel polyelectrolytes. As the temperature increases, the specific capacitance is gradually reduced because the thermal association between the copolymer molecular chains of the electrolyte can suppress the migration of conductive ions and switch off the ion-transport channels, as well as hydrophilic–hydrophobic transition upon the surface of the hydrogel. Benefiting from thermal reversibility of the PNIPAM copolymer network, the capacity loss can be restored after cooling down and the self-protection function of the SP-FSC is repeatable even during five heating/cooling cycles, instead of the existing one-time strategy. Furthermore, the SP-FSC can exhibit different electrochemical performances with a wide temperature window (25–70 °C). Therefore, this work offers an appealing and nondisposable avenue to address thermal safety issues for flexible energy storage sources.