Tunnel-structure MnO2 nanospheres as high-capacity and reversible cathode materials for rechargeable aqueous zinc-ion batteries

Manganese-based materials are widely employed as electrode materials for batteries, with particular attention being paid to MnO₂ owing to its notable merits of high theoretical capacity, low toxicity, and elevated output voltage. Nevertheless, the practical application of MnO₂ is impeded by its inadequate rate capability and swift capacity decline. The MnO₂ nanospheres with a tunnel structure (α-MnO₂ NSs) were synthesized through a simple one-step co-precipitation method. Reaping the benefits of its tunnel-like structure and nanoscale dimensions, the α-MnO₂ NSs exhibited a remarkable specific capacity of 462 mAh g⁻¹ at 0.1 A g⁻¹, excellent rate capability of 160 mAh g⁻¹ at 1.0 A g⁻¹, impressive reversible capacity retention (0.1 A g⁻¹(re)/0.1 A g⁻¹= 112.8%), and good cycle performance with a capacity retention rate of 75.7% after 500 cycles at 1.0 A g⁻¹. The energy storage mechanism was investigated through electrochemical kinetic analysis and ex-situ testing. Initial inserting involved H⁺ incorporation into α-MnO₂ NSs electrode, leading to increased OH^- near the surface and Zn₄SO₄(OH)₆·5H₂O formation. Decreasing H⁺ concentration led to Zn²⁺ insertion and a second discharge platform. The electrochemical kinetics of the α-MnO₂ NSs are influenced by both diffusion and capacitance mechanisms. The superior diffusion coefficient average guarantees improved rate performance and cyclic stability even under high current densities., highlighting the importance of nanostructure in promoting ion diffusion. Therefore, α-MnO₂ NSs exhibit promising potential as both safe and efficient cathode materials for energy storage batteries.