In this work, we focus on Mg-Fe-O and Mg-Ni-O with Mg-rich compositions as positive-electrode materials for magnesium rechargeable batteries, and prepare them by a thermal decomposition of precipitates obtained by a solution method. It is indicated from X-ray diffraction patterns that the Mg-Fe-O and Mg-Ni-O samples have the spinel and rocksalt structures, respectively. X-ray absorption near edge structures indicate that Fe and Ni are trivalent and divalent, respectively, in the Mg-rich oxides. From charge/discharge cycle test, it is demonstrated that the Mg-Fe-O shows higher discharge capacity than the other and then has good cycle performance while keeping a discharge capacity over 100 mA h g-1. To gain deeper understanding on a relationship between the electrode properties and the crystal structure of the Mg-Fe-O, the crystal structure is investigated by a Rietveld refinement using a synchrotron X-ray diffraction profile and an analysis on total correlation functions. It is indicated from these studies that a vacant octahedral site in the spinel structure is partially occupied by the excess Mg in the synthesized sample. This structural feature might result in a stable charge/discharge cycle performance of the Mg-rich Mg-Fe-O.
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