Cellulose-Based Hydrogel with Fast Ion Transport Kinetics Inducing Flat Grain-Stacking Plating for Aqueous Zinc Metal Batteries

Hydrogel electrolytes have distinguished themselves in aqueous zinc metal batteries due to their strong designability and diverse functions, especially in inducing even zinc deposition and inhibiting side reactions. While the high water content of hydrogel electrolytes can enhance Zn2+ ionic conductivity, it also tends to exacerbate undesirable water-related side reactions; thus, how to realize fast zinc ion transport kinetics under low water content remains a challenge. This work introduces carboxylated cellulose nanofibers with ample and diversified polar functional groups into cross-linking polyacrylamide networks to form hydrogel electrolytes. The strong hydrophilic groups of cellulose nanofibers, including carboxyl, hydroxyl, and inter/intramolecular ether bonds, not only grasp water molecules to facilitate desolvation of [Zn(H2O)6]2+ but also disperse the single-site binding energy to Zn2+ through multiple adjacent O atoms, effectively reconciling the conflicting demands of high Zn2+ ionic conductivity and low water content. Beyond their favorable mechanical properties, the as-prepared hydrogel electrolytes can induce flat grain-stacking zinc deposition, mitigate side reactions, and suppress polyiodide shuttling. Consequently, symmetric/asymmetric batteries and aqueous zinc–iodine batteries demonstrate much improved electrochemical performance. This work provides a reference for intricately designing hydrogel electrolytes for aqueous zinc metal batteries with specific functions by screening molecular structures of the components.