Changes in the Local Structure of LiMg_yNi_0.5-yMn_1.5O₄ Electrode Materials during Lithium Extraction

The effects of composition and preparation temperature on the structure of LiMg y Ni 0.5 - y Mn 1.5 O 4 ( 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 P 4 3 32 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 LiNi 0.5 Mn 1.5 O 4 when preparation temperature increases from 700 to 800 °C. The EPR behavior of LiMg 0.5 Mn 1.5 O 4 is determined from localized Mn 4+ ions, whereas residual antiferromagnetic correlations between Ni 2+ and Mn 4+ ions give rise to strong resonance absorption for LiNi 0.5 Mn 1.5 O 4 . The magnetic dilution of the Ni 2+ sublattice by Mg 2+ or Mg 2+ /Ni 2+ causes strong changes in an apparent g -factor, whereas the line width undergoes little changes. When LiMg y Ni 0.5 - y Mn 1.5 O 4 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 LiNi 0.5 Mn 1.5 O 4 (up to 70%) leads to a loss of intensity in the EPR signal as a consequence of the oxidation of paramagnetic Ni 2+ to diamagnetic Ni 4+ without significant changes in local environment of Mn 4+ . For fully delithiated Li 1 - x Ni 0.5 Mn 1.5 O 4 oxide, the EPR spectrum from localized Mn 4+ ions is observed, indicating an exhaustion of paramagnetic Ni 2+ ions in the vicinity of Mn 4+ ions.