Chemical degradation at electrode/electrolyte interfaces in Li-ion batteries creates the challenge of designing stable cathode materials to avoid the loss of active cathode material at the interface of the cathode and electrolyte.  Surface modification with non-active oxide is regarded as a valid strategy to decrease such degradation based on their role as electronic insulator between cathode and electrolyte. Although different oxides have been reported to enhance stability against chemical degradation, the realization of stability and durability of surface modified cathode materials is not well understood owing to the inhomogeneity and complex morphology of typical cathode powders and the resulting difficulty in properly characterizing the surface structure. Indeed, the specific chemical identity of the non-active oxide on the surface of cathode materials can vary. For instance, Al³⁺ can incorporate as amorphous Al₂O₃ or crystalline LiAl_xCo_1-xO₂, among others.

Herein, we directly synthesized well-defined LiCoO₂ (LCO) nanocrystals with a hydrothermal process and employed as model cathode materials before coating 2nm uniform Al³⁺ oxide thin films on their surface and annealed at different temperatures. Phase analysis have confirmed that Al can be formed as Al₂O₃ at low temperature and as LiAl_xCo_1-xO₂ at high temperature. Microscopy analysis presents direct proof of Al conformally coated on their surface. The electrochemical properties have demonstrated the different effect of coating with different Al-containing phases. Compared to bare LCO nanoplates, amorphous Al₂O₃ film coated on such nanoplates presents higher capacity and average stability whereas LiAl_xCo_1-xO₂ coated film have much higher stability with average capacity. Data from atomic resolution TEM and solid state NMR will be discussed.