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[Display omitted] • Carrageenan-based hydrogels with high concentration LiCl (up to 13 M) are synthesized via one-pot method above 90 °C. • CG/PAAm-7Li/K hydrogel shows a high conductivity of 1.9 S/m at − 40 °C. • CG/PAAm-7Li/K based supercapacitor exhibits excellent low-temperature electrochemical properties. Stretchable carrageenan (CG) based double-network (DN) hydrogel has great potential in flexible energy storage devices. To achieve high conductivity at sub-zero temperatures, one-pot synthesis method is adopted for the fabrication of CG/polyacrylamide DN hydrogel with high LiCl concentration (termed CG/PAAm-xLi). In this method, a high temperature above 90 °C is required to fully melt CG network in the presence of high-concentration LiCl and to add the monomers of the second polymer network. Furthermore, by incorporation of small amounts of KCl, the obtained CG/PAAm-7Li/K DN hydrogel shows strong tensile properties with tensile elongation of 567.7%, and fracture energy of 967.3 kJ/m2. In addition, the conductivity of the CG/PAAm-7Li/K DN hydrogel achieves 1.9 S/m at − 40 °C. The flexible supercapacitor prepared by using CG/PAAm-7Li/K hydrogel electrolyte exhibits excellent low-temperature electrochemical properties (specific capacitance of 73.4 F/g, and capacitance retention of 95.6% after 20,000 cycles at − 40 °C). This high-temperature strategy may be extended to construct other ionic polysaccharide-based hydrogel electrolytes with high mechanical property and excellent low-temperature electrochemical performance, showing a broad prospect in the field of flexible electronics. [ABSTRACT FROM AUTHOR]