Enhancing the Electrochemical Performances by Wet Ball Milling to Introduce Structural Water into an Electrolytic MnO2/Graphite Nanocomposite Cathode for Zinc-Ion Batteries

The introduction of structural water in cathode materials of zinc-ion batteries can reduce electrostatic interactions to promote zinc-ion diffusion. However, it is difficult to introduce structural water in MnO2 cathodes due to annealing for crystallinity. For the first time, we introduce structural water into MnO2/graphite nanocomposites by simple wet ball milling of a mixture of electrolytic MnO2 and natural graphite. The composites of nanorod MnO2/graphite exhibit a high discharge capacity (312 mA h g–1 at 0.1 A g–1), which is more than twice that of electrolytic MnO2 (130 mA h g–1 at 0.1 A g–1). It also shows an outstanding rate capacity and cyclic stability that retains 80.1% of the incipient capacity after 1000 cycles at 1 A g–1. MnO2/graphite composites with certain structural water and oxygen vacancies exhibit excellent electrochemical properties, mainly because the presence of structural water and oxygen vacancies can promote Zn2+ ion diffusion of the materials. Through the results of density functional theory calculations and experiments, we verify the adsorption between structural water and crystal planes and identify the positions of structural water, mainly on the (102) and (110) planes of ε-MnO2, which make an impact on ion diffusion. This feasible wet ball milling can not only obtain the composite electrode materials with excellent electrochemical performances but also provide an approach for future synthesis of composite materials with structural water and oxygen vacancies.