Ultralong Lifespan for High-Voltage LiCoO 2 Enabled by In Situ Reconstruction of an Atomic Layer Deposition Coating Layer.

Although the rapid development of electrical energy storage devices has slowed down environmental pollution, their large-scale application has posed huge challenges to battery-related mineral resources; thus, extending the lifespan of high-voltage lithium cobalt oxide (LCO) is of great importance. Surface oxide coating is considered as the most common low-cost modification method for addressing unstable cycling performance. However, studies have shown that the oxide layer would further react with an electrolyte, while the investigation on the corresponding component evolution is lacking. Herein, a typical example utilizing the above reaction to realize surface reconstruction is presented. Applying atomic layer deposition (ALD), originally, an ultrathin Al 2 O 3 layer is coated on the LCO surface; however, this coating layer has undergone reconstruction after reacting with electrolyte decomposition products during the cycling. Compared with simple coating, the in situ formed Li 3 AlF 6 layer has a tighter binding to the LCO surface while possessing good Li + conductivity and electrochemical stability. In addition, the unique properties of the ALD technology allow us to achieve ultrathin (1 nm) and conformal coating, which is beneficial for electronic conductivity and cycling stability. Furthermore, the surface phase transition layer stripping failure mechanism has first been revealed to explain the loss of Co and O, while the reconstructed Li 3 AlF 6 effectively suppresses the surface stripping. Thus, excellent high-voltage performance has been realized (an 89% capacity retention after 1000 cycles at 4.5 V and an 88% capacity retention after 200 cycles at 4.6 V). This work casts a new understanding on the surface reconstruction of the oxide coating layer, which is also significant for other electrode materials' modification.