We investigated the effect of coating Fe- and Ni-substituted Li2MnO3 cathodes (Li1.23Mn0.46Fe0.15Ni0.15O2, LMFN) with Sm oxide (Sm2O3). The cells with LMFN and Sm2O3-coated LMFN cathodes respectively showed 85% and 90% capacity retention and generated 4.3 and 1.6 mlg−1 of gas during 50 cycles, which verified that the Sm2O3 coating was effective. Observations with a scanning transmission electron microscope equipped with an energy dispersive X-ray spectroscopy analyzer showed that the Sm2O3 uniformly coated the LMFN. Synchrotron radiation X-ray absorption spectroscopy indicated that the Sm2O3 coating had little effect on the valence and/or electronic state of the Mn, Fe, and Ni during cycles. However, hard X-ray photoemission spectroscopy indicated that the Sm2O3 coating disturbed solid electrolyte interphase growth originating from the chemical reaction with the electrolyte solution and cathode electrode. The irreversible redox reactions of oxygen ions were also considerably suppressed. We believe that the improvements in cell properties had by using the Sm2O3-coated LMFN cathode will contribute to the development of next-generation storage devices.