A Simple Method for the Complete Performance Recovery of Degraded Ni-rich LiNi_0.70Co_0.15Mn_0.15O₂ Cathode via Surface Reconstruction

Abstract

The highly active surfaces of Ni-rich cathodes usually result in rapid surface degradation, which is manifested by poor cycle and rate capabilities. In this work, we propose a simple method to restore those degraded surfaces after storage. More importantly, the mechanism of surface degradation and recovery are investigated thoroughly. As storage in moist air, a lithium carbonate (Li₂CO₃) dominated impurity layer formed and tightly coated on the surface of the LiNi_0.70Co_0.15Mn_0.15O₂ particles. Except for the Li₂CO₃ layer, a NiO rock-salt structure was also found at near surface region by high-resolution transmission electron microscopy. These two inert species together impedance the transport of lithium ions and electrons, which result in no capacity at 4.3 V charge cutoff voltage of the stored material. We proposed a simple and effective method, i.e., three h calcination at 800 °C under oxygen flow. The restored LiNi_0.70Co_0.15Mn_0.15O₂ shows equivalent electrochemical performance compared to the pristine one. This is because the lithium ions in Li₂CO₃ layer return to the surface lattice of LiNi_0.70Co_0.15Mn_0.15O₂, and the NiO cubic phase transforms back to the layered structure with the oxidation of Ni²⁺. This method is not only insightful for cathode material design but also beneficial for practical application.