Synthesis, Crystal Structure and Electrode Properties of Spinel-Type MgCo2−xMnxO4

MgCo2−xMnxO4 (x = 0.1, 0.2, 0.4) was prepared using an inverse co-precipitation method. The primary product was determined to have a spinel structure (space group Fd 3 m) based on powder X-ray diffraction data. A Rietveld analysis of synchrotron X-ray diffraction data showed that Mg, Co and Mn in this material were distributed in a disordered manner, meaning that cation mixing had occurred. Charge-discharge testing using MgCo2−xMnxO4/AZ31 cells with Ag reference electrodes demonstrated a discharge capacity of 80 mAhg−1 and a high coulombic efficiency below 60°C, with cut-off voltages in the range of 0.345 to −1.155 V vs. Ag/Ag+ (3.5 to 2.0 V vs. Mg/Mg2+). The improved cycling performance of this material is ascribed to the replacement of a portion of the Co atoms with Mn. The stability of the crystal structure was investigated based on first-principles calculations and the results showed that a model in which Mn occupied only the 16d sites was the most stable. The ordered Mg/Co/Mn structure of this material would be expected to facilitate the diffusion of Mg2+ ions throughout the cathode material in a magnesium secondary battery.