Towards atomistic simulations of the SEI in Li metal batteries

Abstract With their high capacity and energy density, Li metal anodes have been intensively researched over the last few decades. Dendrite growth and electrolyte degradation, however, remain obstacles for their commercial implementation. Designing a stable solid-electrolyte interphase (SEI) is an important step in passivating the degradation reactions and mitigating dendrite growth. The advent of machine learning (ML) force fields has accelerated simulations with accuracy similar to quantum mechanical methods. Here we explore the predictions from a fine-tuned foundation model for simulating the interface between Li metal anodes and carbonate-based liquid electrolytes. We find the fine-tuned foundation model quantitatively agrees with DFT calculations, that the force field is remarkably stable, and that the decomposition products at the SEI fall within what is expected to be possible. We find that bare Li reacts with the liquid electrolyte giving typical electrolyte degradation products. We then study Li metal anodes passivated with model inorganic SEI components and find that the presence of these coatings significantly reduces electrolyte decomposition. While certain coatings are more stable and passivate further surface reactions, other coatings allow electrolyte reduction to proceed. In general, (100) coatings appeared more stable than (111) coatings. LiF (100) is found to be the most structurally and chemically stable coating even at high temperatures, fully suppressing electrolyte reduction reactions. The influence of the coatings on interfacial stability is further studied with the program for large scale electronic structure calculations. Results show that electrolyte reduction occurs via electron transfer from the surface of coatings. We also quantify the Li self-diffusion coefficient in the metal, coating and electrolyte from the fine-tuned foundation model for the interface simulations. It is found that the more stable coatings also suppress Li diffusion. Designing a stable SEI which is electronically insulating and ionically conducting requires optimizing the trade-off between these various opposing factors.