Underlying Principles and Practical Design Strategies of Hydrogel Electrolytes for Long‐Term Stable Zinc Batteries

Hydrogel electrolytes, featuring tunable polymer networks, strong mechanical robustness, and effective water confinement, have emerged as promising candidates for stabilizing aqueous zinc‐ion batteries (AZIBs). This review provides a comprehensive analysis of the design principles and mechanisms of hydrogel electrolytes for enhancing the electrochemical long‐cycle stability of AZIBs. Hydrogel electrolytes are first compared with traditional aqueous liquid electrolytes, emphasizing their advantages in ion transport regulation, mechanical compliance, and interface compatibility. Key performance parameters—including ionic conductivity, Zn2+ transference number, crystallographic selectivity, and solid electrolyte interphase (SEI) composition—are discussed in relation to hydrogel composition and structure. Based on the essential components of hydrogel systems (hydrophilic polymers, water, and zinc salts), various modification strategies are systematically classified and analyzed, such as polymer backbone engineering, water activity regulation, and Zn2+ solvation environment tailoring. Emerging design concepts are also highlighted, including gradient architectures, dynamic crosslinking, and dual‐network architectures, which contribute to improved mechanical integrity and dendrite suppression during extended cycling. Finally, current challenges are outlined and future directions are proposed in the rational design and functionalization of hydrogel electrolytes to meet the demands of next‐generation energy storage systems, particularly in grid‐scale applications and flexible/wearable electronics.