The manganese metal ions and other transition metal ions in lithium manganite cathode materials will be dissolved into the electrolyte during cycling and storage at charged state, leading to severe capacity fading. Herein, the SnO2-coated Li1.2Mn0.54Co0.13Ni0.13O2 cathode material is prepared successfully by a simple organic liquid-phase method. The data of inductive coupled plasma-atomic emission spectroscopy and ex-XRD suggest that the coating layer can effectively suppress the dissolution of metal ions, which maintains the stability of main structure. The value of the charge transfer impedance is 35.49 Ω cm2 for LLMO-Sn1 after 50 cycles, while the LLMO is 123.30 Ω cm2. The LLMO-Sn1 has the highest discharge capacity of 214.0 mAh·g−1 after 150 cycles in half cell and exhibits the capacity retention of 86.8% after 150 cycles in full-cell. The decomposition reaction peak of LLMO-Sn1 appears at 250.1 °C.