Charge Compensation Mechanism during Cycles in Fe-Containing Li2MnO3 Cathode for High Energy Density and Low-Cost Lithium-Ion Batteries

The charge compensation mechanism during cycles was investigated in Fe-containing lithium-rich layered manganese oxides (Li1.26Mn0.52Fe0.22O2), which has a potential to be used as cathode materials for high energy density and low-cost lithium-ion batteries. The capacity of cells with Li1.26Mn0.52Fe0.22O2 cathode in the discharged state was 234 mAhg−1 and the capacity retention was 46% at 30 cycles for the limit voltage from 2.0 to 4.8 V. X-ray diffraction measurements and the X-ray Rietveld analysis indicated that a layered rock-salt structure for the discharge states was transformed into a mixture of the layered rock-salt and spinel structure during cycles and that the ratio of the spinel phase became 32% by weight at 30 cycles. The X-ray absorption spectroscopy (XAS) and soft-XAS measurements revealed that the valence modification from Mn4+ to Mn2+/3+, Fe4+ to Fe3+, and O22− to O2− for the charge-discharge occurred together during cycles. During 30 cycles, the redox reactions of Fe, Mn, and oxide ions gradually became irreversible. Primary cause of above phenomena was that Fe and Mn ions stabilized at Fe3+ and Mn2+ from the surface of the electrode with the structural transformation from the layered rock-salt phase to the spinel phase.