Changes in the Local Structure of LiMgyNi0.5-yMn1.5O4 Electrode Materials during Lithium Extraction

The effects of composition and preparation temperature on the structure of LiMgyNi0.5-yMn1.5O4 (y = 0, 0.25, 0.5) compounds are studied by EPR, FTIR, X-ray, and neutron diffraction. For y 0.25, cation ordering in a P4332 superstructure takes place on increasing the annealing temperature from 450 to 750 C. In contrast, a loss of octahedral cation ordering and partial reduction of transition metals are found for LiNi0.5Mn1.5O4 when preparation temperature increases from 700 to 800 C. The EPR behavior of LiMg0.5Mn1.5O4 is determined from localized Mn4+ ions, whereas residual antiferromagnetic correlations between Ni2+ and Mn4+ ions give rise to strong resonance absorption for LiNi0.5Mn1.5O4. The magnetic dilution of the Ni2+ sublattice by Mg2+ or Mg2+\Ni2+ causes strong changes in an apparent g-factor, whereas the line width undergoes little changes. When LiMgyNi0.5-yMn1.5O4 oxides are used as positive electrode materials in test lithium anode cells, the capacity in the 5-V region decreases with decreasing Ni content. Nevertheless, cycling in the 3-V region showed a net improvement on increasing Mg content. Lithium extraction from LiNi0.5Mn1.5O4 (up to 70%) leads to a loss of intensity in the EPR signal as a consequence of the oxidation of paramagnetic Ni2+ to diamagnetic Ni4+ without significant changes in local environment of Mn4+. For fully delithiated Li1xNi0.5Mn1.5O4 oxide, the EPR spectrum from localized Mn4+ ions is observed, indicating an exhaustion of paramagnetic Ni2+ ions in the vicinity of Mn4+ ions.