Lithium-rich material 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2 is successfully prepared by a co-precipitation method. Electrochemical impedance spectroscopy (EIS) is measured under various charge/discharge voltages in the first and second cycles. A reasonable equivalent circuit model is designed for EIS analysis and the fitting curves are aligned well with the experimental curves, suggesting that the proposed equivalent circuit model is an ideal model for lithium battery. According to the analysis on the shape changes of the semicircle in high-frequency and mid-frequency regions during the charging and discharging process, it can be considered that the lithium-rich material is a composite structure of Li2MnO3 phase and LiNi1/3Co1/3Mn1/3O2 phase after its preparation, but it becomes a solid solution structure after the first charging process. The changes of Rct also support above conclusions. According to the changes of the straight line in the low-frequency region and the changes of RSEI during the charging and discharging process, it can be confirmed that the solid electrolyte interphase (SEI) film on the surface of the cathode is gradually formed during the first charging process, and it has been very stable and has stable Li+ migration ability after two cycles. The calculation results of Li+ solid-phase diffusion coefficient also supports above conclusions.