Deep eutectic solvent-based eutectogels consisting of ZnCl2 and lignin for quasi-solid-state supercapacitors†

Eutectogels have attracted attention for use in flexible electronics and energy storage because of their high conductivity and good mechanical properties. In this work, using ZnCl₂ as a hydrogen bond acceptor and lactic acid (LA) as a hydrogen bond donor, an inorganic salt-based deep eutectic solvent (DES) was designed. The DES was used to extract lignin from poplar sawdust, and then poly(vinyl alcohol) was added to the mixed DES/lignin solution to obtain the eutectogels. Due to extensive hydrogen bonding and Zn²⁺-induced coordination, the eutectogels present high ionic conductivity, good stretchability, and wide temperature tolerance. In particular, the presence of lignin endows the eutectogels with self-adhesive properties and enhanced mechanical strength. By regulating the lignin content, the optimal eutectogel sample exhibited an ionic conductivity of 23.8 mS cm⁻¹, tensile strength of 331 kPa, and elongation at break of 1000%, and it could be stable for a long time at −20 °C. Furthermore, the eutectogel quasi-solid-state electrolyte in a zinc-ion hybrid supercapacitor has a high capacity of 251 mA h g⁻¹ at 0.5 A g⁻¹, and it provides stable adhesion and mechanical strength at low temperatures. After 10 000 charge/discharge cycles, these capacitors demonstrated over 86% capacitance retention and 100% coulombic efficiency.