Multi-Scale Modeling of Void Evolution at Li/SEI Interface during Delithiation

Abstract

Lithium metal electrodes promise high energy density batteries due to its high specific capacity (3,860 mAhg–1) and lowest reduction potential, but Li dendrite growth still occurs in both liquid electrolyte and solid electrolytes. In liquid electrolytes, the Li metal surface is always covered by inorganic and organic layers which is called solid electrolyte interphase (SEI) due to the reduction of electrolytes. In solid electrolytes, the Li metal surface is either covered by solid electrolytes itself or by an interlayer formed by the decomposed solid electrolytes. Therefore, the atomistic level of Li stripping and plating processes occur at the Li/SEI interfaces. Maintaining a smooth Li surface during cycling will prevent non-uniform current distribution and suppress Li dendrite growth. The Li surface becomes non-smooth, as collections of Li vacancies (voids) are generated when the stripping process removes Li atoms from the Li/SEI interface faster than vacancies being filled. The accumulation of voids can lead to the interface delamination or pitting as well as to nonuniform current distributions at the interface of Li metal, promoting Li dendrite growth in the following plating process. Ideally, if Li atom diffusion toward the Li/SEI interface is comparable with the rate of Li removal, the surface voids will be digested (filled) by the Li from the bulk, thus a smooth Li surface will be achieved during Li stripping. Therefore, we developed a multi-scale modeling method combining density functional theory (DFT) and Kinetic Monte Carlo (KMC) techniques to study the evolution/formation of voids at different Li/SEI interfaces. The KMC method simulates the void's evolution by capturing the competition between voids generation due to Li removal with the processes to fill the voids, including Li atom diffusion in all directions and the mechanical deformation. DFT is used to determine the stable Li/SEI interfaces, adhesion, Li vacancy formation and diffusion near the Li/SEI interface, and more importantly, providing the Li diffusion barriers toward and away from the Li/SEI interface as the key inputs for the KMC simulation. Based on Li vacancy formation energy landscapes, when voids are generated at the interfaces, Li/LiF interface tends to trap Li vacancies at the interface, while Li/Li2O interface tends to repel the Li vacancies. KMC simulations show consistent results that the voids created at the Li/LiF interface do not migrate to the bulk portion of the Li metal, while the voids at the Li/Li2O interface are filled by diffusion of Li atoms from the bulk, in other words, the voids are digested by the bulk Li. Thus, by varying coating materials, the Li/coating interaction will lead to different morphology of voids evolution at the Li metal surface. This multi-scale modeling approach is anticipated to be applied to a wide range of SEI and SEE materials to maintain a smooth Li surface during the striping process, in order to prevent Li dendrite growth.