Anion‐Dominated Calendar Aging in Aqueous Zinc Metal Batteries

Zinc metal batteries (ZMBs) are promising candidates for large‐scale energy storage. While the majority of current research focuses on cyclic aging, the impact of calendar aging on zinc anodes has received far less attention. Here, we reveal that zinc deposition morphology, governed by anion chemistry, plays a decisive role in calendar degradation by amplifying interfacial corrosion and accelerating “dead” zinc accumulation. Notably, intrinsic corrosion rates alone do not predict degradation severity; for instance, triflate anion shows low corrosion but induces highly porous deposits and rapid aging. To address this, we propose a comprehensive electrochemical descriptor to quantify deposition morphology beyond traditional morphological characterization. Moreover, we introduce an acetatization strategy that tailors Zn2+ solvation and promotes dense plating, effectively suppressing side reactions and “dead” zinc formation. This approach effectively curbs calendar degradation, yielding nearly a 49‐fold improvement in capacity retention. Under demanding conditions (6‐hour aging intervals per cycle), practical anode‐free Cu||ZnI2 cells (~3 mAh cm–2) exhibit high cycling stability, maintaining 96.2% of their initial capacity over 1000 hours. These findings provide a comprehensive understanding of zinc anode degradation and offer a viable route to address calendar aging in ZMBs.