Proton-Selective Electrode Skin Enables High-Performance Aqueous Zinc Batteries: in-situ XANS and XAFS Studies

The pressing demand for sustainable energy storage solutions has spurred the burgeoning development of rechargeable aqueous batteries1. Of particular interest are aqueous zinc batteries (AZBs), which directly take cheap zinc metal as the capacity-dense and low-redox-potential anodes, and mild acidic aqueous Zn solutions as the highly ionic conductive, non-flammable, and nontoxic electrolytes2. Until now, various cathode materials capable of accommodating Zn2+, with metal oxides (e.g., V-, Mn-, and Mo-based oxide compounds) as high-voltage and high-capacity materials, have been explored to couple with the Zn metal anode3. However, kinetics-sluggish Zn2+ as the dominant charge carriers in cathodes leads to suboptimal charge-storage capacity and durability of AZBs. The choice of charge carrier ions has a significant impact on the Faradaic reaction kinetics of battery electrodes. In AZBs, H+ or its hydrated form (H3O+) offers more favourable kinetics compared to Zn2+ owing to the smaller size, lighter mass, as well as rapid conduction in aqueous electrolytes governed by the Grotthuss mechanism. Nevertheless, increasing the involvement of H+ in the cathode chemistries is challenging, as the H+ concentration in the mild acidic electrolyte of AZBs is typically several orders of magnitude lower than the Zn2+ concentration. Moreover, a deep understanding of proton-dominated intercalation mechanism helps to maximum the charge storage capacity of electrodes and design high-performance aqueous batteries.