The Use of Ultrathin Film Coatings Prepared through Atomic Layer Deposition (ALD) Has Become Widespread for Improving Lithium-Ion Diffusivity of Active Particles, Which Plays a Crucial Role in Determining the Rate Capability of Lithium-Ion Batteries (LIBs). in This Study, the Impact of ALD Coating Thickness on Ionic Diffusivity in CeO2-Coated LiMn2O4 (LMO) Cathode Particles is Comprehensively Investigated through First-Principles Calculations by Focusing on the Trade-Offs between the Physical Properties of the Film and its Impact on the Diffusivity of Ions. Our Findings Indicate that Several Physical Factors Affect the Diffusivity of the Coating, Including the Crystal-Amorphous Structure that Depends on Thickness, as Well as Surface and Bulk Diffusion, and the Influence of Neighboring Atoms. These Factors Contribute to the Observed Trade-Offs in Diffusivity. in Addition, We Observe a Previously Unknown Phenomenon: Variation in Diffusivity Along the Thickness of the Coating, Which Can Be Explained by the Aforementioned Physical Factors. This Study is the First to Provide a Fundamental Atomic-Level Understanding of How the Thickness of the Coating Affects the Diffusivity of Coated Particles. Our Findings Provide Valuable Insights into the Control of ALD Coating Thickness and the Design of Cathodes to Improve the Performance of LIBs.