Ultra-Stretchable Anti-Freezing Hydrogel Electrolytes Cross-Linked by Liquid Metal Particle Initiators Toward Soft Energy Storage Devices

Hydrogels are appealing for electrolyte of soft energy storage devices due to their mechanical flexibility and high ionic conductivity. Their polymeric networks containing large amount of water result in mechanical flexibility suitable for soft devices. However, high water content in hydrogels results in insufficient mechanical strength and freezing at sub-zero temperatures. Herein, we developed anti-freezing hydrogels with high mechanical strength by liquid metal (LM)-initiated free-radical polymerization. During the polymerization, we introduced stearyl methacrylate (SMA) to form hydrophobic associations, increasing the physical cross-linking density within the polymer network, resulting in desirable mechanical properties (elongation at break of 907% and tensile strength of 766 kPa). The hydrogel exhibiting ionic conductivity of 4.35 S m⁻¹ at 25 °C after immersing in LiCl solution showed the slightly decreased ionic conductivity (3.39 S m⁻¹) and almost maintained mechanical stretchability (elongation at break of 897%) after being stored at - 20 °C for 12 h. Supercapacitors consisted of the hydrogel electrolyte and activated carbon electrodes achieved a high areal capacitance (93.52 mF cm⁻²) due to rapid ionic mobility through the hydrogel electrolyte and retained 98% of its capacitance after 45,000 charge-discharge cycles due to enhanced polymeric network of electrolyte via LM particles-initiated polymerization and SMA-induced hydrophobic association.