Ultrathin Li–Si–O Coating Layer to Stabilize the Surface Structure and Prolong the Cycling Life of Single-Crystal LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Materials at 4.5 V

Single-crystal LiNi1–x–yCoxMnyO2 cathode materials can effectively suppress intergranular cracks that usually is seen in commercial polycrystal LiNi1–x–yCoxMnyO2 cathode materials. However, the surface structure degradation for single-crystal LiNi1–x–yCoxMnyO2 cathode materials is still aggravated at a higher cutoff voltage (over 4.5 V). In this work, we prepare single-crystal LiNi0.6Co0.2Mn0.2O2 cathode materials via a solid-state method and then coat an ultrathin Li–Si–O layer on their surface by a wet coating method. The results show that the single-crystal LiNi0.6Co0.2Mn0.2O2 cathode materials with a Li–Si–O coating layer deliver excellent cycling performance even at a higher cutoff voltage of 4.5 V. The optimized Li–Si–O-modified sample displays a capacity retention of 90.6% after 100 cycles, whereas only 68.0% for unmodified single-crystal LiNi0.6Co0.2Mn0.2O2. Further analysis of the cycled electrodes reveals that the surface structure degradation is the main reason for the decrease of electrochemical performance of single-crystal LiNi0.6Co0.2Mn0.2O2 at a high voltage (4.5 V). In contrast, with Li–Si–O coating, this phenomenon can be suppressed effectively to maintain interfacial stability and prolong the cycling life.