Understanding the Role of Temperature and Cathode Composition on Interface and Bulk: Optimizing Aluminum Oxide Coatings for Li-Ion Cathodes.

Surface coating of cathode materials with Al2O3 has been shown to be a promising method for cathode stabilization and improved cycling performance at high operating voltages. However, a detailed understanding on how coating process and cathode composition change the chemical composition, morphology, and distribution of coating within the cathode interface and bulk lattice is still missing. In this study, we use a wet-chemical method to synthesize a series of Al2O3-coated LiNi0.5Co0.2Mn0.3O2 and LiCoO2 cathodes treated under various annealing temperatures and a combination of structural characterization techniques to understand the composition, homogeneity, and morphology of the coating layer and the bulk cathode. Nuclear magnetic resonance and electron microscopy results reveal that the nature of the interface is highly dependent on the annealing temperature and cathode composition. For Al2O3-coated LiNi0.5Co0.2Mn0.3O2, higher annealing temperature leads to more homogeneous and more closely attached coating on cathode materials, corresponding to better electrochemical performance. Lower Al2O3 coating content is found to be helpful to further improve the initial capacity and cyclability, which can greatly outperform the pristine cathode material. For Al2O3-coated LiCoO2, the incorporation of Al into the cathode lattice is observed after annealing at high temperatures, implying the transformation from "surface coatings" to "dopants", which is not observed for LiNi0.5Co0.2Mn0.3O2. As a result, Al2O3-coated LiCoO2 annealed at higher temperature shows similar initial capacity but lower retention compared to that annealed at a lower temperature, due to the intercalation of surface alumina into the bulk layered structure forming a solid solution.