First-principles study of the magnesiation of olivines: redox reaction mechanism, electrochemical and thermodynamic properties

Development of Mg batteries relies heavily on the thermodynamics and kinetics of the magnesiation of cathode materials that favor the high mobility of Mg ions in the host lattice and energy densities at least comparable to Li ion batteries. In this paper, we performed Density Functional Theory studies to understand the thermodynamics of the magnesiation of olivine compounds. The redox reaction mechanism was revealed in the magnesiation process of MnSiO4. Mn4+ in the host structure was first reduced to Mn3+ resulting in the formation of Mg0.5MnSiO4. Further reduction of Mn3+ to Mn2+ took place to form MgMnSiO4. We also investigated the electrochemical and thermodynamical properties of the magnesiation of olivine compounds. Except for Fe2+/Fe3+, the redox potential of magnesiation-demagnesiation for olivine compounds showed TM2+/TM3+ redox couples at 3-4 V vs. Mg/Mg2+ and TM3+/TM4+ redox couples about 4 V with TM = Mn, Fe, Co and Ni. Full magnesiation of MnSiO4 to MgMnSiO4 showed the largest volume expansion of approx. 20%. We concluded that magnesiation process of olivine compounds showed high thermodynamic similarities with lithiation and Mg0.5FePO4 could be the preferred compound for reversible Mg battery cathodes.