Comparative Investigation of MgMnSiO4 and Olivine-Type MgMnSiS4 as Cathode Materials for Mg Batteries

The identification of potential cathode materials is necessary for the development of a new magnesium-based battery technology. Most attempts focus on oxide and sulfide materials, which in general suffer, respectively, of poor Mg mobility and low intercalation voltage. New chemistries should be explored. In this work, we investigate the basic electrode characteristics of olivine-type thiosilicates MgMSiS4 (M = Fe, Mn) with the double challenge of (1) raising the low intercalation voltage of transition metal sulfides and (2) improving the poor Mg diffusion of the oxosilicate counterparts MgMSiO4 (M = Fe, Mn). Density functional theory (DFT) calculations corroborate both expectations. The calculated average Mg deintercalation voltage for MgMnSiS4 (2.31 V) is above that of virtual MgMnS2 compounds (around 1.8 V), accounting for the inductive effect of the Si4+ ion on the transition metal. The calculated energy barriers for Mg diffusion in MgxMnSiO4 are 0.75 eV at x ∼ 1 and higher than 1.1 eV at x ∼ 0 and x = 0.5. The energy barriers decrease to 0.68 and 0.76 eV in MgxMnSiS4 (x ∼ 0, 1), thanks to the more covalent Mn–S bond (compared to the Mn–O bond) that renders less oxidized Mn ions, therefore favoring Mg2+ mobility. Although these results are promising, more work is needed to ensure the potential application of thiosilicates as cathode materials for Mg batteries.