Reduced Intercalation Energy Barrier by Rich Structural Water in Spinel ZnMn₂O₄ 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 Zn2+ into cathode materials is normally high, which limits the kinetics of Zn2+ transfer at the cathode/electrolyte interface. This study reveals the presence of rich structural water in spinel ZnMn2O4 (ZnMn2O4·0.94H2O, 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–1 at 0.5 and 8 A g–1), high specific energy/specific power (329 W h kg–1/706 W kg–1 and 134 W h kg–1/11,160 W kg–1), and stable cycle retention (75% after 2000 cycles at 4 A g–1) 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–1 at 0.5 and 8 A g–1). As examined by electrochemical impedance spectroscopy, rich structural water in ZMO effectively reduces the activation energy barrier upon Zn2+ insertion, rendering fast interfacial kinetics for Zn storage. Benefiting from rich structural water in ZMO, the involvement of Zn2+ during the charge/discharge process exhibits good reversibility, as characterized by X-ray diffraction and X-ray photoelectron spectroscopy.