Electrochemical Properties and Crystal and Electronic Structures of Spinel αMgCo_2−xMn_xO₄-(1 − α)Mg(Mg_0.33V_1.67−yNi_y)O₄ for Magnesium Secondary Batteries

Abstract

In the present study, αMgCo_1.5Mn_0.5O₄-(1 − α)Mg_1.33V_1.57Ni_0.1O₄ was synthesized by a reverse co-precipitation method. The products were assigned to a spinel structure with a space group Fd3m 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_0.5O₄ and 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_0.5O₄ and 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 Co^2.67+ 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.