Mechanism of Mg extraction from MgMn₂O₄ during acid digestion

MgMn₂O₄ having a spinel structure is a very attractive material for the positive electrode in Mg-ion batteries, since its reversible Mg extraction/insertion reaction can lead to a large reversible capacity. While the Mg extraction from MgMn₂O₄ has been reported, the reaction mechanism remains unclear. In this paper, Mg ions were chemically extracted from MgMn₂O₄ by acid digestion at various concentrations to produce Mg_xMn₂O₄ (0 < x < 1). The results showed that Mg extraction from MgMn₂O₄ is a two-step two-phase reaction, via the intermediate Mg_0.5Mn₂O₄ to the fully oxidised Mn₂O₄. The kinetics of Mg extraction was clarified using acid digestion experiments of different durations, and the direct reaction pathway of MgMn₂O₄ oxidation to λ-MnO₂ was the fastest process. This may explain the difficulty in Mg extraction from MgMn₂O₄ using electrochemical methods.