Reduced Intercalation Energy Barrier by Rich Structural Water in Spinel ZnMnO for High-Rate Zinc-Ion Batteries.

Aqueous zinc-ion batteries are considered promising next-generation systems for large-scale energy storage due to low cost, environmental friendliness, and high reversibility of the Zn anode. However, the interfacial charge-transfer resistance for the insertion of divalent Zn into cathode materials is normally high, which limits the kinetics of Zn transfer at the cathode/electrolyte interface. This study reveals the presence of rich structural water in spinel ZnMnO (ZnMnO·0.94HO, denoted as ZMO), synthesized by a scalable and low-temperature process, significantly overcoming the great interfacial charge-transfer resistance. ZMO exhibits excellent electrochemical performance toward Zn storage, that is, high capacity (230 and 101 mA h g at 0.5 and 8 A g), high specific energy/specific power (329 W h kg/706 W kg and 134 W h kg/11,160 W kg), and stable cycle retention (75% after 2000 cycles at 4 A g) can be achieved. On the contrary, the controlled sample ZMO-450 with deficient structural water, prepared by post-heat treatment of ZMO at 450 °C, demonstrates limited discharge capacity (45 and 15 mA h g at 0.5 and 8 A g). As examined by electrochemical impedance spectroscopy, rich structural water in ZMO effectively reduces the activation energy barrier upon Zn insertion, rendering fast interfacial kinetics for Zn storage. Benefiting from rich structural water in ZMO, the involvement of Zn during the charge/discharge process exhibits good reversibility, as characterized by X-ray diffraction and X-ray photoelectron spectroscopy.