UC Berkeley

Cathode surface coatings are widely used industrially as a means to suppress degradation and improve electrochemical performance of lithium-ion batteries. However, developing an optimal coating is challenging, as different coating materials may enhance one aspect of performance while hindering another. To elucidate the fundamental thermodynamic and transport properties of amorphous cathode coating materials, here, we present a framework for calculating and analyzing the Li⁺ and O^2- transport and the stability against delithiation in such materials. Our framework includes systematic workflows of ab-initio molecular dynamics calculations to obtain amorphous structures and diffusion trajectories coupled with an analysis of critical changes of the active-ion local environment during diffusion. Based on these data, we provide an estimate of room-temperature diffusivities, including statistical error bars, and the evaluation of the coating suitability in terms of its ability to facilitate Li⁺ transport while blocking O^2- transport. Finally, we add the thermodynamic stability analysis of the coating chemistry within the operating voltage of common Li-ion cathodes. We apply this framework to two commonly used amorphous coating materials, Al₂O₃ and ZnO. We find that (1) in general, a higher Li⁺ content increases both Li⁺ and O^2- diffusivities in both Al₂O₃ and ZnO. Also, Li⁺ and O^2- diffuse much faster in ZnO than in Al₂O₃. (2) However, neither Al₂O₃ nor ZnO is expected to retain a significant concentration of Li⁺ at high charge. (3) ZnO performs much more poorly in terms of O^2- blocking, and hence, Al₂O₃ is preferred for high-voltage cathode applications. These results will help to quantitatively evaluate amorphous materials, such as metal oxides and fluorides, for different performance metrics and facilitate the development of optimal cathode coatings.