Monitoring LiNi<sub><i>x</i></sub>Co<sub><i>y</i></sub>Mn<sub>(1–x–y)</sub>O<sub>2</sub> Degradation in Contact with Li via <i>In Situ</i> Transmission\nElectron Micr

High-voltage LiNi<sub><i>x</i></sub>Co<sub><i>y</i></sub>Mn<sub>(1–<i>x</i>–<i>y</i>)</sub>O<sub>2</sub> (NCM) is one of the most promising\ncathode materials\nfor high-energy-density lithium metal batteries. Significant efforts\nhave been made on inhibiting the surface transition of NCM from the\nordered layered phase to the low-ionic-conductivity rock salt phase,\nwhich facilitates maintaining a low interfacial impedance for superior\ncycle performance. However, it is often overlooked that the surface\nrock salt phase also has a low electronic conductivity, which may\nalleviate the notorious growth of lithium dendrite-induced short-circuit.\nIn this article, we further demonstrate that the surface rock salt\nphase of NCM is effective in resisting pulverization from contact\nwith Li via in situ transmission electron microscopy. The ordered\nlayered NCM experiences rapid overlithiation in contact with Li, which\ntriggers lattice expansion and pulverization. The overlithiation-induced\ndegradation is retarded for NCM with the Li-deficient disorder rock\nsalt phase on the surface, which is attributed to the blocked Li<sup>+</sup> primary path. Our work revisits the unwanted surface rock\nsalt phase of ordered layer cathodes, which provides a guideline for\ninterface design for long-cycling and high-safety lithium batteries.