Electrochemically stable tunnel-type α-MnO₂-based cathode materials for rechargeable aqueous zinc-ion batteries

The purpose of this study is the synthesis of α-MnO₂-based cathode materials for rechargeable aqueous zinc ion batteries by hydrothermal method using KMnO₄ and MnSO₄ as starting materials. The aim is to improve the understanding of Zn²⁺ insertion/de-insertion mechanisms. The as-prepared solid compounds were characterized by spectroscopy and microscopy techniques. X-ray diffraction showed that the hydrothermal reaction forms α-MnO₂ and Ce⁴⁺-inserted MnO₂ structures. Raman spectroscopy confirmed the formation of α-MnO₂ with hexagonal MnO₂ and Ce-MnO₂ structures. Scanning electron microscopy (SEM) confirmed the formation of nanostructured MnO₂ (nanofibers) and Ce-MnO₂ (nanorods). The electrochemical performance of MnO₂ was evaluated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) tests in half-cells. CV results showed the reversible insertion/de-insertion of Zn²⁺ ions in MnO₂ and Ce-MnO₂. GCD cycling tests of MnO₂ and Ce-MnO₂ at 2500 mA/g demonstrated an impressive electrochemical performance, excellent cycling stability throughout 500 cycles, and high rate capability. The excellent electrochemical performance and the good cycling stability of MnO₂ and Ce-MnO₂ nanostructures by simple method makes them promising cathode materials for aqueous rechargeable zinc-ion batteries.