Controlling the Aspect Ratio of α-MnO₂ via the Confined-Space Growth Effect to Enhance the Performance in Aqueous Zinc-Ion Storage

α-MnO 2 nanorods with large 2 × 2 tunnels are often selected as storage materials in aqueous zinc-ion batteries due to their high theoretical capacity, environmental friendliness, and low cost. The electrochemical performance is strongly associated with their particle size; however, many techniques for controlling size include undesirable contents, such as carbon-containing components, which reduce the overall energy density of the electrodes. In this paper, Ni(II) ions are introduced to hinder the spatial growth during the preparation of α-MnO 2, hence tuning the aspect ratio and enhancing the zinc-ion storage performance. The confined-space effect is employed by controlling the size of α-MnO 2 particles, in which the initial long rods at the micron scale have become tiny α-MnO 2 nanorods. Smaller particle size electrode materials offer shorter ion diffusion paths, greater electron transport efficiency, and more sufficient solid–liquid interface space due to the small size effect, which also determines the capacity and lifetime of batteries. At the same time, Ni(II) addition selectively hinders the growth direction (001) plane, which effectively shrinks Zn(II) and electron transport paths. As a result, the prepared α-MnO 2 nanorods obtained a high capacity (272 mA h g –1 at 50 mA g –1 ) and stable cycling stability (70% capacity retention after 500 cycles at 500 mA g –1 ). These results demonstrate that the introduction of uncoordinated ions is an effective way to control the metal oxide size and provide a direction in the development of electrode materials for secondary batteries.