Electrochemical Properties and Crystal and Electronic Structures of Spinel αMgCo2-xMnxO4-(1 - α)Mg(Mg0.33V1.67-yNiy)O4 for Magnesium Secondary Batteries

In the present study, αMgCo_1.5Mn_0.5O₄-(1 - α)Mg_1.33V_1.57Ni_0.1O₄was synthesized by a reverse co-precipitation method. The products were assigned to a spinel structure with a space group Fd_3m based on powder X-ray diffraction analysis. Inductively coupled plasma-atomic emission spectroscopy revealed that the composition was not close to the target composition; the Mg content was higher and the V content was lower than expected. Charge/discharge cyclic tests showed that the discharge capacity exceeded 180 mAh g-1 and that the cyclability was reversible up to 60 cycles at 90°C with a cut-off voltage of 0.945 to -1.055 V vs. Ag/Ag+ (3.545-1.545 V vs. Mg/Mg2+) for α = 0.3. The electronic structure was analyzed using the maximum-entropy method based on a Rietveld analysis, and it was found that Mg insertion was easier for α = 0.3, 0.5, 0.9 than for MgCo_1.5Mn_0.5O₄and Mg_1.33V_1.57Ni_0.1O₄. The strain in MO6 octahedra for α = 0.1, 0.3, 0.5, 0.7 was smaller than that for MgCo_1.5Mn_0.5O₄and Mg_1.33V_1.57Ni_0.1O₄, which suggests that the host structure was stabilized by forming a solid solution. A Rietveld analysis after the first discharge and second charge confirmed a partly reversible phase transition from the spinel phase to a rock-salt phase. The valence of the transition metals was examined by X-ray absorption fine structure (XAFS) measurements, and the Co and Mn K-edge spectra revealed that Co and Mn were present as Co2.67+ to Co2+ and Mn4+ to Mn3+ species for α = 0.3. Both Co and Mn redox processes are considered to contribute to Mg intercalation. Extended XAFS (EXAFS) Co K-edge and Mn K-edge spectra for the powder specimens and after the first discharge and second charge showed that bonds between Co, V atoms and nearest-neighbor O atoms were distorted after the first discharge, and that this distortion was relaxed after the second charge.