Ion Solvation and Dynamics at Solid Electrolyte Interphases: A Long Way from Bulk?

Rechargeable lithium-ion battery technology has revolutionized energy storage for small electronic devices. However, a deeper understanding is required of the interfaces present in such devices for this technology to continue to advance. While many insights into the solid electrolyte interphase (SEI) for lithium-ion systems have been collected from decades of study, many questions also remain. In particular, this work is interested in exploring SEI composition and its impact on electrolyte structure and dynamics. By using a previously tested classical molecular dynamics approach, the impact of the crystallinity of the SEI interface as well as its content of organic and inorganic species is assessed. It is found that the presence of an amorphous SEI results in the accumulation of ions at the interface, ordering of solvent molecules, and a slowing down of solvation dynamics. These behaviors are intensified when crystal surfaces from LiF and Li2CO3 are considered. In addition to these general observations, the changes in lithium and PF6- solvation structure are also considered as they approach the SEI interface. In both cases, the drive to aggregate arising from greater ion accumulation is shielded in part from interaction with the charged groups in the SEI interface. This competition is tilted toward salt aggregation in the case of crystalline SEI, suggesting a significantly different solvation environment than typically seen in bulk for adsorbed species.