Crystal water for high performance layered manganese oxide cathodes in aqueous rechargeable zinc batteries

Aqueous zinc (Zn)-ion batteries are gaining considerable attention as grid-scale energy storage systems due to their advantages in rate performance, cost, and safety. Here, we report a layered manganese oxide that contains a high content of crystal water (∼10 wt%) as an aqueous zinc battery cathode. The interlayer crystal water can effectively screen the electrostatic interactions between Zn²⁺ ions and the host framework to facilitate Zn²⁺ diffusion while sustaining the host framework for prolonged cycles. By virtue of these 'water' effects, this material exhibits a high reversible capacity of 350 mA h g^-1 at 100 mA g^-1, along with decent cycling and rate performance, in a two-electrode cell configuration. Density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) analyses jointly reveal that upon Zn²⁺ ion intercalation, a stable inner-sphere Zn-complex coordinated with water molecules is formed, followed by the formation of a Zn-Mn dumbbell structure, which gives a clue for the observed electrochemical performance. This work unveils the useful function of crystal water in enhancing the key electrochemical performance of emerging divalent battery electrodes.