Better understanding the role of a water-in-salt electrolyte for designing a stable, high-performance Zn//organic battery

In the context of eco-sustainability, low cost, and safety, rechargeable aqueous Zn//organic batteries emerge as promising candidates for large-scale energy storage. However, their practical application is limited by challenges such as the poor cycling stability of organic anodes when redox-active molecules are not covalently anchored to the electrode, and interfacial side reactions at the zinc cathode (e.g., hydrogen evolution, dendrite growth, corrosion). Among the strategies proposed to simultaneously address these issues at both electrodes, “water-in-salt” electrolytes (WiSEs) stand out as particularly attractive. Yet, their impact on the long-term cycling stability and suppression of parasitic reactions in Zn//organic batteries remain poorly understood. Here, we investigate the performance of a Zn//chloranil battery in ZnCl₂ electrolytes ranging from dilute (0.5 mol/kg) to WiSE concentrations (30 mol/kg). We show that highly concentrated ZnCl₂ electrolytes effectively suppress the dissolution of reduced chloranil, especially when combined with a hydrophobic binder such as polytetrafluoroethylene. An optimal ZnCl₂ concentration is identified, yielding excellent cycling stability, high coulombic efficiency, low self-discharge, and good rate capability. Mechanistic studies reveal that charge storage at the chloranil electrode proceeds via reversible proton-coupled electron transfer, facilitated by the strong acidity of concentrated ZnCl₂ solutions. At the Zn anode, we provide a quantitative explanation for the huge potential shift observed at increasing ZnCl₂ concentrations, attributed to reduced water activity and decreased hydration of Zn²⁺ ions. These mechanistic insights allow to rationalize the nearly constant cell voltage across a wide concentration range and to better understand the marked suppression of zinc corrosion at high salt concentrations, primarily due to kinetic effects. Leveraging these insights, we design a high-performance Zn//chloranil battery delivering 1.1 V and achieving one of the highest areal capacities (3.0 mA·h/cm2) reported for a Zn//organic battery, along with outstanding cycling stability, retaining 90% capacity after 600 cycles at 0.1 A/g.