Ultrahigh conductivity and antifreezing zwitterionic sulfobetaine hydrogel electrolyte for low-temperature resistance flexible supercapacitors

Abstract

In recent years, ionic conductive hydrogels have been developed and applied in the field of energy-storage devices owing to their unique properties, such as high ionic conductivity, mechanical flexibility, liquid-leakage-free operation, and stability. However, latent ice crystals can be generated within the hydrogel at extreme low temperatures, which can greatly limit the activity of ions and thus ionic conductivity, hence limiting their further applications. In this work, a poly(SBMA-co-HEAA) (PSH) hydrogel was prepared by a free radical copolymerization of the zwitterionic sulfobetaine (SBMA) and N-(2-hydroxyethyl)acrylamide (HEAA), which exhibited a highly recoverable stretchable property, excellent adhesive property, and a self-healing capacity. The as-prepared hydrogel with 6 M LiCl solution presented an ultrahigh ionic conductivity (25.8 S m⁻¹) at room temperature. Even at extreme low temperature (−40 °C), its ionic conductivity still remained as high as 2.21 S m⁻¹. Furthermore, the hydrogel electrolyte and an as-prepared MXene/rGO/TA film electrode was assembled into a flexible supercapacitor, which showed a specific capacitance of 218.2 F g⁻¹ (1 A g⁻¹) at room temperature. At −20 °C, the supercapacitor still contributed a specific capacitance of 154.2 F g⁻¹, retaining 70.7% of its room temperature capacitance. Moreover, freezing–thawing looping tests between room temperature and −20 °C or −40 °C showed that there was no irreversible damage to the supercapacitors. It is expected that the as-prepared ultrahigh ionic conductivity antifreezing hydrogel electrolyte would be quite suitable for flexible supercapacitors that need to work at extremely low temperature.