Fundamental insight into the zinc storage mechanism in amorphous manganese oxide for rechargeable zinc-ion batteries

Abstract

Zinc-Manganese dioxide (MnO₂) batteries have garnered significant interest due to their high energy density, low cost and low toxicity. Crystalline MnO₂ has shown structural transformations during the intercalation and deintercalation of Zn, which influences the performance of the battery. Recent studies have demonstrated that amorphous MnO₂ shows better Zn storage capacity due to an increase in structural defects. However, the underlying mechanism of Zn storage remains poorly understood and, in this study, we investigate the structural dynamics of electrodeposited amorphous MnO₂ during Zn storage. Ex situ Raman spectroscopy and X-ray absorption spectroscopy analysis revealed notable structural alterations in the amorphous MnO₂. Interestingly, no changes were observed in the oxidation state of Mn. However, changes in the oxygen K-edge spectra showed that Zn intercalation and deintercalation primarily occur at the disordered oxygen defect sites. By optimising the electrodeposition conditions for MnO₂, a Zn-MnO₂ battery was fabricated, which initially demonstrated a capacity of 300 mAh g⁻¹. This value subsequently decreased due to structural transformation to a stable capacity of 190 mAh g⁻¹ at a current density of 0.5 A g⁻¹. The findings presented in this work open new avenues for developing alternative amorphous structures that can be tailored for Zn-based batteries.