Theoretical Prediction of the Strong Solvent Effect on Reduced Ethylene Carbonate Ring-Opening and Its Impact on Solid Electrolyte Interphase Evolution

With continuous growth in demand for lithium-ion batteries (LIBs) with higher capacity retention and longer lifetime, solid electrolyte interphase (SEI) formation allowing for tunable structure is critical to improve efficiency in current LIBs and implement next-generation technologies. To enable engineering of the SEI layer, a deeper understanding of the mechanisms and kinetics underlying its formation processes is required. We first demonstrate the importance of the rate of ring opening of reduced ethylene carbonate (c-EC^-) to its chain-like conformation (o-EC^-). According to our classical molecular dynamics simulations, c-EC^- is far more likely to diffuse away from the anode than o-EC^-, implying that longer c-EC^- lifetimes would correlate with thicker SEI layers and reduced capacity retention. Our work then illustrates that the activation barrier for the c-EC^- → o-EC^- reaction can be strongly influenced by the entropic effects associated with the reorganization of the surrounding molecules. Ab initio molecular dynamics simulations combined with metadynamics sampling clearly demonstrate a considerable decrease (by a factor of 2.5) in the ring-opening barrier when considering a 50/50 mixture of cyclic EC and linear dimethyl carbonate as the solvent rather than pure liquid EC. This is not predicted by implicit solvent methods owing to their failure to capture the effect of solvent geometry. Our findings also suggest that other perturbations of the local environment not considered, such as those due to interfaces, anions, or other reduction products, may also significantly affect the diffusion/reaction kinetics of EC^- radical anion, and, thus, SEI formation and evolution.