This work reports the facile surface coating of lithium-rich Li1.2Mn0.54Ni0.13Co0.13O2 cathode material by nano-SnO2 for lithium ion batteries. Thus-obtained nano-SnO2 coated Li1.2Mn0.54Ni0.13Co0.13O2 (denoted as NTO-LMO) material is characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. It is revealed that the SnO2 layer with a thickness of 4–8 nm is uniformly coated on the surface of Li1.2Mn0.54Ni0.13Co0.13O2. This NTO-LMO material exhibits outstanding rate capability and cyclic stability in comparison with pristine material, which should be ascribed to the nano-SnO2 coating layer that limits the side reactions and produces a thin and stable solid electrolyte interface film. More importantly, in contrast to the conventional surface coatings that usually reduce the reversible capacity of active materials, the discharge capacity of our NTO-LMO material increases by 38 mAh g−1 at the current density of 30 mA g−1, which is attributed to the enhanced activation of Li2MnO3 component. The oxygen vacancies in nano-SnO2 coating layer are revealed to facilitate the transfer of high valence state oxygen through the coating layer and be responsible for the promoted activation of Li2MnO3. These insightful findings are very helpful to developing effective strategies for the surface modification of Li-rich oxide materials.