An ultra-thin film coating via Atomic Layer Deposition (ALD) is a viable approach for enhancing battery performance significantly. Despite several demonstrations of this improvement, still the fundamental understanding on the role of thin film is unclear, especially, regarding to interaction between the active material and the coated layer. In this work, an investigation was conducted via a first-principles calculation, focused on interfacial reaction, charge analysis and structural change of coating layer, and lithium ions transport mechanism during lithiation. Comparison between CeO₂ and Al₂O₃ layers could explain the experimental observation, where CeO₂ coating showed better performance. One interesting finding was that lithium ions in the coating layer can intercalate easily to the active materials, which was ignored in the several previous work in literature. This gives us an important clue for the reason of why ALD coating is beneficial to battery.