A New Coating Method for Alleviating Surface Degradation of LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Material: Nanoscale Surface Treatment of Primary Particles

Structural degradation of Ni-rich cathode materials (LiNi_<i>x</i>M_1–<i>x</i>O₂; M = Mn, Co, and Al; <i>x</i> > 0.5) during cycling at both high voltage (>4.3 V) and high temperature (>50 °C) led to the continuous generation of microcracks in a secondary particle that consisted of aggregated micrometer-sized primary particles. These microcracks caused deterioration of the electrochemical properties by disconnecting the electrical pathway between the primary particles and creating thermal instability owing to oxygen evolution during phase transformation. Here, we report a new concept to overcome those problems of the Ni-rich cathode material via nanoscale surface treatment of the primary particles. The resultant primary particles’ surfaces had a higher cobalt content and a cation-mixing phase (<i>Fm</i>3̅<i>m</i>) with nanoscale thickness in the LiNi_0.6Co_0.2Mn_0.2O₂ cathode, leading to mitigation of the microcracks by suppressing the structural change from a layered to rock-salt phase. Furthermore, the higher oxidation state of Mn⁴⁺ at the surface minimized the oxygen evolution at high temperatures. This approach resulted in improved structural and thermal stability in the severe cycling-test environment at 60 °C between 3.0 and 4.45 V and at elevated temperatures, showing a rate capability that was comparable to that of the pristine sample