Flexible and anti-freezing zinc-ion batteries using a guar-gum/sodium-alginate/ethylene-glycol hydrogel electrolyte

Natural polymer hydrogels are promising candidates for solid-state electrolytes in zinc-ion batteries. They are safe, biocompatible, and mechanically robust. However, the ionic conductivity of most natural polymer-based hydrogels is unsatisfactory, and they are particularly problematic at sub-zero temperatures because freezing severely limits their functionality. In this study, we fabricate composite hydrogels with high ionic conductivity (25.37 mS cm−1). This is done by blending two kinds of natural polymers, guar gum (GG) and sodium alginate (SA). The zinc-ion batteries with GG/SA hydrogel electrolytes show a superior electrochemical performance (354.9 mAh g−1 at 0.15 A g−1, 137.0 mAh g−1 at 6 A g−1 and capacity retention of 91.52% over 1000 cycles) than the zinc-ion batteries with the pure GG hydrogel electrolyte. In addition, to extend the application range of GG/SA hydrogels into the sub-zero temperature region, we introduce ethylene glycol (EG) into GG/SA to form GG/SA/EG anti-freeze hydrogel. Below -20 °C, the GG/SA/EG hydrogel maintains a high ionic-conductivity (6.19 mS cm−1), and the zinc-ion batteries, which were made of GG/SA/EG hydrogel electrolyte, showed an excellent low-temperature discharge performance, for a specific capacity of 181.5 mAh g−1 at 0.1 A g−1. This study describes a new approach for the development of natural polymer-based hydrogel electrolytes with high ionic-conductivity and good anti-freeze capability.