Facile Surface Modification of MgMn2O4 Positive-Electrode Material for Improving Cycle Performance of Magnesium Rechargeable Batteries

MgMn₂O₄ with a tetragonal spinel structure shows promise as a positive-electrode material in magnesium rechargeable batteries (MRBs), which have drawn considerable attention as post lithium-ion batteries. However, the material currently suffers from poor cycle performance. In this study, we attempt to improve the cycle performance of MgMn₂O₄ via the Zr modification of its particle surface. X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy demonstrate that the surface modification is successfully performed by immersing MgMn₂O₄ powder into a Zr-containing aqueous solution, followed by heat treatment. However, Zr segregation is observed at high Zr concentration. Furthermore, structural analyses using synchrotron X-rays indicate that the Zr modification has an influence on the bulk structure of the MgMn₂O₄ powder. The positive-electrode properties of the powders are investigated using discharge/charge cycle tests, which show that Zr modification can drastically improve the cycle performance and coulombic efficiency. These improvements are supposed to be due to suppression of an unexpected reaction by the Zr-surface modification and lower structural distortion after the modification. These findings clearly demonstrate the significant potential of surface modification as a method for obtaining high-performance MRBs.

Positive-electrode properties and crystal structures of Mg-rich transition metal oxides for magnesium rechargeable batteries

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.