Highly conductive and anti-freezing Mg/Ca-crosslinked sodium alginate/phytic acid hydrogel electrolytes for flexible supercapacitors.

Conventional hydrogel electrolytes often suffer from reduced ionic conductivity at low temperatures and limited environmental sustainability. Herein, a flexible biopolymer-based hydrogel electrolyte with high ionic conductivity and low-temperature stability is developed through a synergistic Mg/Ca dual-ion crosslinking strategy within a sodium alginate (SA)/phytic acid (PA) framework. Mg2+ serves as a key functional ion, contributing to the formation of a Mg2+-rich hydrated ion-transport environment and the suppression of ice crystallization, whereas a small amount of Ca2+ is retained to reinforce the structural stability of the network. The hydrogel exhibits an ionic conductivity of 135.9 mS cm-1 at room temperature and maintains 56.4 mS cm-1 after continuous storage at -18 °C for 20 days. Notably, the hydrogel retains its flame-retardant and self-extinguishing properties even after 90 days of open-air storage. The assembled flexible solid-state supercapacitors (FSCs) retain 83.96% of their room-temperature capacitance at -18 °C under the same current density, maintain nearly unchanged capacitance during deformation, and retain 88.9% of their initial capacitance after 10,000 cycles. The Mg/Ca dual-ion crosslinking strategy proposed in this work provides a new approach for developing safe, sustainable, and low-temperature-tolerant biopolymer-based hydrogel electrolytes for flexible electrochemical devices.