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A Simple Method for the Complete Performance Recovery of Degraded Ni-rich LiNi0.70Co0.15Mn0.15O2 Cathode via Surface Reconstruction.

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 LiNi0.70Co0.15Mn0.15O2 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 LiNi0.70Co0.15Mn0.15O2 shows equivalent electrochemical performance compared to the pristine one. This is because the lithium ions in Li2CO3 layer return to the surface lattice of LiNi0.70Co0.15Mn0.15O2, 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.