The superior electrochemical performance of a Li-rich layered cathode material with Li-rich spinel Li₄Mn₅O₁₂ and MgF₂ double surface modifications

Although Li-rich layered materials are some of the best potential cathode materials owing to their high capacity (>250 mA h g^-1), low cost and reduced pollution, they still faces some problems, including low initial coulombic efficiency, poor cycling performance, and bad rate capability. In this work, Li-rich spinel Li₄Mn₅O₁₂ and MgF₂ are constructed on the surface of a Li-rich layered material by simple liquid-phase erosion and liquid-phase deposition methods, respectively. The Li-rich spinel Li₄Mn₅O₁₂ layer provides 3D Li-ion channels and it restrains the growth of SEI film and oxygen release. The outermost amorphous MgF₂ layer of coating also favors Li-ion migration and further protects Li₄Mn₅O₁₂ from HF corrosion. It is found that the double surface modifications induce a phase transformation from a layered structure to an Li₄Mn₅O₁₂-type spinel during cycling, which is different from the traditional structural transformation from a layered structure to a LiMn₂O₄ spinel-like structure, and it exhibits a slower structural transformation. Benefiting from these collaborative contributions from Li₄Mn₅O₁₂ and MgF₂, the material shows superior electrochemical properties, including a high initial coulombic efficiency of 96.4%, excellent capacity retention of 80% after 300 cycles, a small voltage decay rate of 1.5 mV per cycle, and a remarkable rate capability.