Electrochemical properties and crystal and electronic structure changes during charge/discharge of spinel type cathode-materials Mg_1.33V_1.67-xMn_xO₄ for magnesium secondary batteries

Mg_1.33V_1.67−xMn_xO₄ (x = 0.1 ∼ 0.4) was successfully synthesized using a solid-phase method under a high vacuum condition for the first time. The crystal structures of the products were all assigned to a spinel structure with a space group of Fd3̅ m based on the results of the powder X-ray diffraction profiles and those chemical compositions were determined to be uniform by the STEM observation followed by elemental analysis. Charge and discharge cycle tests indicated that the deliverable discharge capacity depends on the cycle number, Mn composition, and working temperature; Mg_1.33V_1.57Mn_0.1O₄, i.e., x = 0.1, showed the largest discharge capacity of 256 mAh g⁻¹ at 13th cycles while the initial capacity was only 73 mAh g⁻¹ at 90 °C. The local structure was analyzed based on EXAFS spectra at the V K-edge for the pristine and electrode materials, and the results suggested that Mg_1.33V_1.57Mn_0.1O₄ had the smallest lattice distortion due to Mn at 16d sites, and XAFS spectra at the V K-edge indicated the significant change in the oxidation state of V during the charge/discharge cycles. The particularly stable crystal structure and large contribution of charge compensation by V may jointly contribute to the superior charge–discharge property of Mg_1.33V_1.57Mn_0.1O₄.