Monitoring LiNixCoyMn(1-x-y)O2 Degradation in Contact with Li via In Situ Transmission Electron Microscopy.

High-voltage LiNixCoyMn(1-x-y)O2 (NCM) is one of the most promising cathode materials for high-energy-density lithium metal batteries. Significant efforts have been made on inhibiting the surface transition of NCM from the ordered layered phase to the low-ionic-conductivity rock salt phase, which facilitates maintaining a low interfacial impedance for superior cycle performance. However, it is often overlooked that the surface rock salt phase also has a low electronic conductivity, which may alleviate the notorious growth of lithium dendrite-induced short-circuit. In this article, we further demonstrate that the surface rock salt phase of NCM is effective in resisting pulverization from contact with Li via in situ transmission electron microscopy. The ordered layered NCM experiences rapid overlithiation in contact with Li, which triggers lattice expansion and pulverization. The overlithiation-induced degradation is retarded for NCM with the Li-deficient disorder rock salt phase on the surface, which is attributed to the blocked Li+ primary path. Our work revisits the unwanted surface rock salt phase of ordered layer cathodes, which provides a guideline for interface design for long-cycling and high-safety lithium batteries.