Gel electrolytes are being intensively explored for aqueous rechargeable zinc-ion batteries, especially towards high performance and multi-functionalities. Water plays a central role on the fundamental properties, interface reaction/interaction, and performance of the gel-type zinc electrolyte. In this review, the influence of water on the physiochemical properties of gel electrolytes is focused on. The correlation between water activity and the fundamental properties of zinc electrolytes is presented. Current approaches and challenges in manipulating water activity and the consequent influence on the electrochemical stability, transport, and interface kinetics of gel electrolytes are summarized. An outlook on approaches to tuning and investigating water activity is provided to shed light on the design of advanced gel electrolytes.

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