Post-tunable salt gel electrolytes toward flexible supercapacitor switches

Abstract

Convertible hydrogel supercapacitors have emerged as promising energy storage devices in switches, diodes, and transistors. However, inherent weaknesses in ionic conductivity, mechanical properties, and water retention of hydrogel electrolytes seriously hinder their development. Inspired by the hardness conversion of sea cucumber skin, a conductivity and mechanics dual-tunable salt gel electrolyte is successfully designed. The salt gel presents a reversible switching of conductors-insulators and a mechanical regulation between softness and hardness via the dissolution-crystallization transition of sodium acetate trihydrate (SAT). Meanwhile, the salt gels spontaneously grow a layer of “armor” through saturated phase-change salt crystals effectively reducing water evaporation of hydrogel electrolytes. Furthermore, this phase-change soft-rigid conversion strategy will expand the capabilities of gel-based flexible supercapacitors (area capacitance: 258.6 mF cm⁻²), and the capacitance retention rate could still reach 86.9% after 3000 cycles at high temperatures. Moreover, the salt gel supercapacitor is potentially used in over-heat alarm systems. It is anticipated that the strategy of conductivity and mechanics of dual-tunable salt gel would provide a new perspective on the development of energy storage devices, wearable electronics, and flexible robots.