Synthesis, electrochemical properties, and changes in crystal and electronic structures during charge/discharge process of spinel-type cathode materials Mg4V5-xNixO12 (x=0, 0.3, 0.6, 1.0) for magnesium secondary batteries

Mg(4)V(5-x)Ni(x)O(12 )was synthesized by a solid-phase method under high vacuum. The product had a spinel structure with a space group Fd-3m based on its powder X-ray diffraction pattern. Elemental mapping by STEM-EDS showed that the particles had a uniform composition. Charge and discharge cycle tests revealed a discharge capacity exceeding 150 mAhg(-1) at 90 degrees C, and cyclability was reversible up to 14 cycles. Mg(Mg0.33V1.57Ni0.1)O-4 exhibited a high discharge capacity of 194 mAhg(-1) at a low rate of 1 mAg(-1). From a Rietveld analysis of the X-ray diffraction pattern for the electrode, the occupancies of the 8a and 16c sites increased after charging. Since the Mg occupancy at the 16c sites changed significantly after charging during the second cycle and after discharging, it is considered that the discharge capacity is largely responsible for Mg occupancy of the 16c sites. A shift towards higher energy was observed in the X-ray absorption near edge structure spectra of V and Ni. Therefore, the charge-discharge mechanism was revealed to involve a redox reaction involving V and Ni.