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

Abstract

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 Mg_0.5MnSiO₄. 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 Mg_0.5FePO₄ could be the preferred compound for reversible Mg battery cathodes.