Sulfonated hydrogel electrolyte enables dendrite-free zinc-ion batteries

•Sulfonate groups enhance the ionic conductivity and flexibility.•The sulfonate groups induce uniform deposition of Zn.•Solid Zn-ion batteries based on the gel electrolyte exhibit stable cycling. Metallic Zn anode is plagued by severe side reactions and dendrite growth, preventing the commercial development of Zn-ion batteries (ZIBs). Semi-solid state hydrogel electrolyte has received immense attention in aqueous Zn-ion batteries due to their intrinsic advantages (e.g., wider electrochemical windows, good interfacial compatibility, and multi-functional applications). However, their inferior ionic conductivity and mechanical deformation tolerance are the main barriers to realizing the true application in ZIBs. Herein, a hydrogel electrolyte with high ionic conductivity and high flexibility is fabricated through sulfonated chitosan and further cross-linking with polyacrylamide. Accordingly, the ion conductivity of the hydrogel electrolyte increases from 28.2 to 38.1 mS cm−1, whilst tensile strength is elevated from 19.1 to 54.2 kPa. With this hydrogel electrolyte, the Zn||Zn symmetric cells show exceptional cycle stability over 2600 h at the current density of 1 mA cm−2, and the Zn||Cu asymmetric cells demonstrate nearly 100 % coulomb efficiency over 1200 cycles. In addition, NH4V4O10||Zn full cells enable capacity retention of as high as 96.1 % even over 500 cycles, and the as-assembled flexible batteries can stand bending, twisting, and cutting operation without comprising the electrochemical performance.