Unlocking ultra-low temperature performance: an anti-freezing, high-conductivity, biodegradable hydrogel electrolyte for supercapacitors down to −60 °C

Hydrogel electrolytes are highly regarded in supercapacitors for their intrinsic safety and mechanical adaptability, but inevitable freezing at subzero temperatures leads to rapid deterioration of electrochemical performance. To overcome this critical limitation, a freeze-resistant hydrogel electrolyte (SCG-Zn) capable of operating at ultra-low temperatures is developed by integrating biodegradable polysaccharides (sodium hyaluronate and carboxymethyl chitosan), glycerol, and zinc chloride. The synergistic effect of intense chloride ion (Cl^-) hydration and abundant hydrophilic groups (<i>e.g.</i>, -OH, -COOH, and -NH₂) within the hydrogel electrolyte creates a strongly bound water network, significantly suppressing ice crystallization and depressing the freezing point. This unique structure enables SCG-Zn to maintain exceptional ionic conductivity (13.32 mS cm^-1) even at -60 °C (35.75 mS cm^-1 at 25 °C), arising from its high density of zincophilic functional groups and the establishment of continuous Zn²⁺ ion conduction pathways. Supercapacitors assembled with SCG-Zn electrolyte demonstrate exceptional cycling stability, retaining 97.6% capacity after 20 000 cycles at 25 °C. Impressively, even under the harsh condition of -60 °C, facilitated by the sustained ionic conduction of SCG-Zn, a remarkably high-capacity retention of 97.4% is achieved after 20 000 cycles. Furthermore, the assembled flexible devices also exhibit stable performance under repeated mechanical deformations (bending and loading). This work establishes a simple, sustainable but highly effective material for high-performance, reliable energy storage devices capable of operating in extreme cold environments.