The role of a MnO2 functional layer on the surface of Ni-rich cathode materials: Towards enhanced chemical stability on exposure to air

The severe degradation of Ni-rich cathode materials on exposure to air is a crucial restriction for their large-scale application. To overcome this issue, a MnO2 functional layer has been introduced to the surface of LiNi0.8Co0.15Al0.05O2 oxide via a wet-chemical method. Compared with pristine sample, the modified LiNi0.8Co0.15Al0.05O2 shows an enhanced chemical stability because of its low sensitivity to moisture and CO2. The formation of absorbed Li2CO3/LiOH species and spontaneous reduction of Ni3+ to Ni2+ on the surface of MnO2-modified sample have been delayed remarkably, which is confirmed by characterizations of SEM, TEM, XPS and FTIR. Benefit from these merits, the modified LiNi0.8Co0.15Al0.05O2 displays a specific capacity of 183 mAh g(-1) at 0.1 C after air-storage for 40 days, while the pristine sample drops from 195 to 144 mAh g(-1) promptly. Meanwhile, the MnO2 layer inhibits the generation of HF and protects the active material against the erosion of electrolyte in the working cell. Therefore, the MnO2 modified LiNi0.8Co0.15Al0.05O2 after storage shows a capacity retention of 87.1% at 1 C after 100 cycles, which is more stable than that of stored pristine sample (70.3%).