Surface phase transformation and CaF2 coating for enhanced electrochemical performance of Li-rich Mn-based cathodes

•Li1.2Mn0.54Ni0.13Co0.13O2 is pre-activated by different amounts of Na2S2O8.•40 wt% Na2S2O8-treated sample shows the best electrochemical properties.•Appropriate Na2S2O8-treatment alleviates the structure conversion upon cycling.•Subsequent CaF2 coating further stabilizes the interface structure.

To overcome the voltage decay upon cycling and increase the initial coulombic efficiency of the layered Li-rich Mn-based oxides, the double modification combining Na2S2O8 treatment with CaF2 coating has been first proposed in this study. The precondition Na2S2O8 treatment activates the Li2MnO3 phase gently and generates a stabilized three-dimensional spinel structure on the surface of particles, leading to a suppression of surface reaction and structure conversion during the subsequent electrochemical process. The mitigation of phase transformation for Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 alleviates the voltage decay and energy density degradation upon long-term charge-discharge cycling. In order to further restrain the capacity loss derived from the HF attack and manganese dissolution, 40 wt% Na2S2O8 treated-sample has been modified by an amorphous CaF2 layer with nano-scale thickness. The first-reported CaF2-coated/40 wt% Na2S2O8 treated-Li1.2Mn0.54Ni0.13Co0.13O2 presents excellent electrochemical properties with a high initial coulombic efficiency of 99.2%, a capacity retention rate of 89.2% after 200 cycles and a high-rate capability of 152.1 mAh g−1 at 3 C. The double surface modification offers a smart design concept for Li-rich Mn-based oxides to meet the practical requirements for advanced lithium ion batteries in electric vehicles.