From Two-Electron to Four-Electron System: Electrolyte Regulation for Advanced Aqueous Zn–I₂ Batteries

Abstract

Aqueous Zn–I₂ batteries (AZIBs) represent an efficient energy storage technology, with the emerging four-electron redox mechanism further enhancing their application value. However, the advancement toward commercial implementation requires addressing key challenges inherent to the electrode–electrolyte interface. Apart from electrode optimization, electrolyte design is a pivotal strategy to tackle the interface issues and an inevitable road to realize the four-electron redox reaction. In recent years, significant research efforts have been directed toward advancing AZIBs through electrolyte engineering. This review systematically summarizes recent progress in electrolyte-regulated AZIBs. First, fundamental principles of AZIBs were presented, including their working mechanisms and inherent challenges related to both the zinc anode and iodine cathode. Furthermore, strategies based on functional additives, highly concentrated electrolytes, cosolvents, Zn salts, and hydrogel electrolytes are analyzed to evaluate their effectiveness in optimizing both traditional two-electron and advanced four-electron redox systems. After thoroughly discussing the zinc utilization and gas evolution of zinc anode, practical AZIBs configurations, that is, soft-pack battery, flexible battery, and microbattery, are reviewed. Finally, prospective directions and development strategies are proposed to advance the practical implementation of AZIBs.

Keywords

aqueous Zn–I₂ battery / electrolyte design / four-electron redox / functional additive / polyiodide shuttle

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