α-MnO2 as a cathode material for rechargeable Mg batteries

Reversible Mg insertion and extraction behaviors of α-MnO2, a promising cathode material for rechargeable Mg batteries, were studied using discharge–charge cycling, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The α-MnO2 with a (2 × 2) tunnel structure as cathode shows a reversible capacity about 240 mAh/g in a cell using Mg metal as anode during the 1st cycle. A reversible reduction/oxidation reaction of Mn ions during the insertion and extraction of Mg ions was clearly observed by both XPS and XAS studies. The extended X-ray absorption fine structure (EXAFS) results show a partial collapse of the tunnel structure in the α-MnO2 during the Mg insertion and extraction which might be responsible for the irreversible capacity loss during the 1st cycle. These studies show that α-MnO2 is a promising candidate as cathode material for rechargeable Mg batteries if the capacity retention can be significantly improved.