Exceptionally flexible and stable quasi-solid-state supercapacitors via salt-in-polyampholytes electrolyte with non-freezable properties

Hydrogel electrolytes have been intensively studied for a decade, due to their outstanding intrinsic physical properties, such as good flexibility, high stretchability, and high ionic conductivity. However, due to their low mechanical strength (<10³ Pa) and poor ionic conductivity (<1 mS/cm) at low temperatures, many researchers have been focusing on improving these issues. Thus, in our study, we prepared salt-in-polyampholytes (SIPAs) that have two different counterions (Li⁺ and Cl^-) in an aqueous medium via facile one-step UV radical polymerization. Polyampholyte (PA) improves the toughness and elongation by inducing strong ionic interactions between opposite charges in the polymer chains, but also exhibits excellent recovery ability under periodically changing strain and strong adhesion with electrodes. Furthermore, the incorporation of LiCl salt into our PA results in SIPA with high ionic conductivity at 25 °C (134 mS/cm) and even at sub-zero temperatures (15 mS/cm). Consequently, the non-freezable SIPA-based quasi-solid-state supercapacitor (QSS), assembled with activated carbon electrodes, exhibits a wide operating voltage window as high as 2.0 V, a high specific capacitance of 241 F/g, and high energy (34 Wh/kg) and power (598 W/kg) densities, along with excellent cycling stability, retaining 91 % of the initial capacitance even after 20,000 cycles at 1 A/g. Besides, the QSS electrochemical performance remains unaffected under bending and rolling deformations at a low temperature, demonstrating excellent mechanical, thermal, and electrochemical stability of our system. This work provides an effective strategy for designing energy storage devices that are both foldable and reliable, making them suitable for use in extreme environments.