Triple-network hydrogel polymer electrolytes: enabling flexible and robust supercapacitors for extreme conditions

Abstract

Hydrogel electrolytes are known for their flexibility and high ionic conductivity, but suffer from mechanical and ion transporting issues at sub-zero temperatures and high-voltage energy storage. To address this, we prepare triple-network hydrogel polymer electrolytes (TNPEs) using single hydrophilic monomer, two hydrophobic monomers, and LiCl salts, via one-step thermal radical polymerization, forming chemically/physically/ionically cross-linked networks. We thoroughly investigate the influence of the distinctive triple networks on the anti-freezing, anti-dehydration, recovery, and adhesion properties of TNPEs using experimental and computational methods. Remarkably, TNPE displays exceptional performance even at sub-zero temperatures, exhibiting outstanding ionic conductivity (5.6 × 10−3 S/cm at −30 ℃), alongside stable mechanical traits and enhanced water retention (80 % after 20 days). The resultant TNPE-based flexible supercapacitors, assembled with activated carbon electrodes, showcase excellent stability under high voltage (2.25 V), boasting superior specific capacitance (24 F/g), energy density (17 Wh/kg), and power density (671 W/kg). The device retains steady electrochemical performance with ∼ 100 % capacitance retention under mechanical deformation (bending and folding) and exhibits stable operation at low temperatures (-30 ℃), thanks to strong electrode–electrolyte adhesion (55 J/m2). Hence, TNPE, capturing both electrochemical and mechanical properties, is expected to provide new insights and opportunities for a new generation of flexible energy storage devices in challenging environments.