K+ pre-intercalated manganese dioxide with enhanced Zn2+ diffusion for high rate and durable aqueous zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) are attracting extensive research interest because of their safety and low toxicity. However, the development of ZIBs is hindered by sluggish reaction kinetics and serious structural degradation of the cathode materials. Here, tunnel-structured MnO2 hierarchical nanotubes with high pre-intercalated K cation content (alpha-K0.19MnO2) are proposed as a superior cathode for ZIBs. Specifically, the alpha-K0.19MnO2 nanotubes are prepared via a self-sacrificial template method, including a neutral solvent hydrothermal intercalation and a subsequent annealing phase transformation process. When tested as cathodes for ZIBs, a subsequent H+ and Zn2+ intercalation mechanism at different voltage platforms is clarified. The water-solvated H+ first inserts into tunnel cavities and the subsequent insertion of Zn2+ into MnO2 partially changes the MnO2 phase from a tunnel-type structure to a layered-type structure (Zn-buserite). The high content pre-intercalated K cations in the layered-type matrix as pillars stabilize the layered structures and expand Zn2+ migration channels, which can facilitate the diffusion of Zn2+ in the MnO2 cathodes. It is noteworthy that, a K-salt additive is employed to maintain the concentration of K+ in the electrolyte with the aim of inhibiting the extraction of K+ from the alpha-K0.19MnO2 host material during cycling, thereby further boosting the cycling ability.