Changes in the Local Structure of LiMgyNi0.5-yMn1.5O4 Electrode Materials during Lithium Extraction
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Abstract
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 Li1-xNi0.5Mn1.5O4 oxide, the EPR spectrum from localized Mn4+ ions is observed, indicating an exhaustion of paramagnetic Ni2+ ions in the vicinity of Mn4+ ions.
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