Possible Explanation for the Efficiency of Al-Based Coatings on LiCoO₂: Surface Properties of LiCo_1−xAl_xO₂ Solid Solution

Aluminum-based coatings are commonly used in lithium-ion batteries to modify the surface of LiCoO 2 particles, to limit cobalt dissolution in the electrolyte at high voltage. It was shown that the formation of a LiCo 1− x Al x O 2 solid solution occurs at the interface between the coating and the core material. In this paper, we investigated the surface properties of LiCo 1− x Al x O 2 materials by X-ray photoelectron spectroscopy. We explored the surface acid−base properties of these materials by adsorption of gaseous probe molecules (NH 3 and SO 2 ) followed by XPS analyses. We showed that the basic character of the LiCo 1− x Al x O 2 surface strongly decreases when x increases, which makes these materials less reactive than LiCoO 2 toward acidic species (such as HF) that are present in LiPF 6 -based electrolytes. This is a possible explanation for the efficiency of Al-based coatings to protect LiCoO 2 against cobalt dissolution in the electrolyte.