Resting but not idle: unveiling the mechanistic origin of resting losses for zinc anodes

Abstract

Resting losses in aqueous zinc metal batteries (AZMBs) can exceed over 15% of anode capacity within hours; severe enough to cripple grid-scale deployment. These losses are reported to occur regardless of electrolyte (ZnSO₄, Zn(OTf)₂, ZnCl₂), current collector (Cu, Ti, Ni, C, stainless steel, Pt), temperature, or other conditions. This is widely attributed to the thermodynamic advantage of the hydrogen evolution reaction (HER, 0 V vs. SHE) over Zn/Zn²⁺ (–0.76 V vs. SHE), driving rapid dissolution of plated Zn on the current collector with spontaneous H₂ release. By contrast, analogous non-aqueous Li and Na systems show only a fraction of such losses, highlighting corrosion mechanisms unique to Zn. Understanding the mechanistic origins of capacity fading under resting conditions remains a major barrier and must be addressed to establish the design principles needed to mitigate high resting losses in AZMBs. In this work, we use fluorescence microscopy to reveal lateral propagation of cathodic current across current collectors, showing that spontaneous electron exchange occurs not only on deposited Zn but also on the current collector itself, substantially accelerating corrosion. Applying mixed potential theory, we show that electron transfer kinetics, thermodynamic reduction potentials, and concentration polarization collectively govern corrosion rates. We apply in situ operando electrochemical mass spectrometry to precisely quantify resting corrosion rates and faradaic efficiencies, and our results clearly show that these rates vary by several orders of magnitude depending on the HER kinetics of the current collector. Mechanistically, our results show that galvanic corrosion extends beyond just the anode active materials, to the current collector and other coin cell components, including casings, spacers, and springs; whose catalytic activity toward HER exacerbates irreversible consumption of active Zn. We show that these hidden corrosion pathways operate under both resting and cycling conditions, explaining paradoxical HER observations during Zn stripping. By systematically linking deposited Zn, current collectors, and non-electrode components, this study unifies prior observations of resting corrosion, provides a mechanistic understanding of capacity fading origins, and establishes design principles to mitigate losses.