Mn and Au redox mediators in ZnCl₂ water-in-salt electrolytes: Implications for Zn-ion battery chemistry

• The Mn-electrochemistry in WiSE is free from the issue of “dead MnO₂” • Au rapidly oxidizes in WiSE to AuCl₄⁻, the dissolution rate is ca. twice that of Zn • The electrochemistry of AuCl 4 - closely parallels that of Mn 2+ in WiSE • The areal capacity of ∼1 mAh/cm 2 is achieved in tens of cycles • Oxidation of Mn 2+ yields reversibly cyclable γ-MnO 2 as the main product Redox mediators in water-in-salt electrolytes (WiSE) offer a compelling platform for durable, safe, and efficient Zn-ion battery. Here we investigate two model systems: MnCl 2 and HAuCl 4 dissolved in 15 m ZnCl 2 . Using a carbon positive electrode enables areal capacities of approx. 1 mAh/cm², outperforming traditional electrodes with solid thin-film materials, e.g., phosphate olivines. This capacity is available in a WiSE volume, which fits the standard 2032 coin cell. The WiSE environment substantially alleviates the “dead MnO 2 ” problem, while γ-MnO 2 is generated by anodic oxidation of Mn 2+ over a broad potential region. The charge transfer is diffusion-limited, with ion transport primarily controlled by the viscosity of the WiSE. Remarkably, Au and Mn display strikingly similar electrochemical signatures, each producing broad, asymmetric voltammetric peaks with a formal potential near 1.7 V vs Zn 2+ /Zn, despite Mn redox couples being shifted by ca. 0.7 V below their standard potentials. The observed potential shifts arise from chloromanganate formation as well as from WiSE-specific effects. The potentials are conveniently referenced to the Ru(NH 3 ) 6 3+/2+ couple, which is essentially insensitive to ZnCl 2 concentration. Gold undergoes rapid oxidative dissolution to AuCl 4 - . The Au-Zn alloys are identified by distinct features at anodic stripping, as well as by SEM, EDX and XPS. These findings highlight both the opportunities and mechanistic complexities of liquid-phase redox mediators for high-capacity Zn-ion energy-storage systems with WiSE.