Freeze-tolerant and high-stretchable lithium acrylate-based hydrogel polymer electrolyte for superior hybrid supercapacitors*

Hydrogels are an attractive material for all-solid-state supercapacitors due to their flexibility and high-ionic conductivity. However, maintaining the electrochemical performance of supercapacitors in harsh environments, such as mechanical stress or sub-zero temperature operation, is still a prominent issue to be solved. We demonstrate self-healable and freeze-tolerant fiber supercapacitors (FSCs) constructed from heteronetwork polymer electrolytes (NPEs) and metal-organic framework-derived carbon-coated carbon nanotube yarn hybrid electrodes. The NPE, composed of silica nanoparticles as stress buffers and poly(lithium acrylate) containing mobile lithium countercations, are prepared via sol-gel reaction and radical polymerization. The NPE possesses high conductivities of ~10^-1 at 25 °C and 4 x 10^-3 S/cm even at 0 °C. This is due to the Li⁺ binding strongly to water, thereby preventing water evaporation and crystallization. The FSC exhibits a high specific capacitance of 51 F/g, a high power density of 6 kW/kg, a high energy density of 44 Wh/kg, and a wide voltage range of 2.5 V. The electrochemical performance of FSC remains stable under stretching, bending, and cutting/healing cycles, and still retains ~ 92% capacity even at a low temperature (-10 °C). Our study of the combination of heteronetwork polymer electrolytes and hybrid electrodes can provide an effective strategy for the optimal design of supercapacitors, sufficiently stretchable and reliable for use in extreme environments.