Possible Explanation for the Efficiency of Al-Based Coatings on LiCoO2: Surface Properties of LiCo1-xAlxO2 Solid Solution

Aluminum-based coatings are commonly used in lithium-ion batteries to modify the surface of LiCoO2 particles, to limit cobalt dissolution in the electrolyte at high voltage. It was shown that the formation of a LiCo1-xAlxO2 solid solution occurs at the interface between the coating and the core material. In this paper, we investigated the surface properties of LiCo1-xAlxO2 materials by X-ray photoelectron spectroscopy. We explored the surface acid-base properties of these materials by adsorption of gaseous probe molecules (NH3 and SO2) followed by XPS analyses. We showed that the basic character of the LiCo1-xAlxO2 surface strongly decreases when x increases, which makes these materials less reactive than LiCoO2 toward acidic species (such as HF) that are present in LiPF6-based electrolytes. This is a possible explanation for the efficiency of Al-based coatings to protect LiCoO2 against cobalt dissolution in the electrolyte.