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) over barm) 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 degrees C, with cut-off voltages in the range of 0.345 to -1.155V vs. Ag/Ag+ (3.5 to 2.0V 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. (C) The Electrochemical Society of Japan, All rights reserved.