10 mol% Fe- and Ni-substituted Li2MnO3 (Li1+x(Fe0.1Ni0.1Mn0.8)1-xO2, 0 < x < 1/3) samples were prepared under different calcination atmospheres (in air or flowing N2). The N2 calcined sample (sample N) exhibited better cycle performance because sample N had a more homogeneous transition metal (TM) distribution. Through X-ray Rietveld analysis, highly TM-deficient spinel phases were detected with layered rock-salt phase after charging. The highly TM-deficient spinel had unusually low TM occupancy on the 16d site (<40%). At the end of discharging, the spinel phase was diminished. Only TM-disordered layered rock-salt phase was observed. At 34th discharged state, the highly TM-deficient spinel phase appeared as a minor phase because of layer to spinel conversion when upper cut-off voltage was set to 4.6 V. However, the highly TM-deficient spinel phase converted to a normal LiMn2O4-like phase when the upper cut-off voltage was set to 4.8 V. The spinel content after cycling can be reduced by synthesizing homogeneous TM distributed samples.