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

Abstract

Single-crystal LiNi_{1-<i>x</i>-<i>y</i>}Co<i>_x</i>Mn<i>_y</i>O₂ cathode materials can effectively suppress intergranular cracks that usually is seen in commercial polycrystal LiNi_{1-<i>x</i>-<i>y</i>}Co<i>_x</i>Mn<i>_y</i>O₂ cathode materials. However, the surface structure degradation for single-crystal LiNi_{1-<i>x</i>-<i>y</i>}Co<i>_x</i>Mn<i>_y</i>O₂ cathode materials is still aggravated at a higher cutoff voltage (over 4.5 V). In this work, we prepare single-crystal LiNi_0.6Co_0.2Mn_0.2O₂ 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 LiNi_0.6Co_0.2Mn_0.2O₂ 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 LiNi_0.6Co_0.2Mn_0.2O₂. 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 LiNi_0.6Co_0.2Mn_0.2O₂ 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.