Ion Association and Electrolyte Structure at Surface Films in Lithium-Ion Batteries

Abstract

During lithium-ion battery charging and discharging, carbonate electrolytes degrade from redox side reactions to produce electrode surface films. The composition of these films depends on the composition of the electrolyte layer at the electrode surface and the structure of the ion solvation shells found therein. However, both the composition and structure of an electrolyte at an interface can vary significantly from their counterparts in the bulk, as reported previously at air and mineral surfaces. Hence, a circular relationship holds in which the surface films formed depend on the electrolyte structure, which in turn is impacted by the presence of the surface film. In this work, three impacts from solid interfaces on carbonate electrolytes are considered: ion accumulation, ion pairing, and solvent exchange dynamics. By considering these effects at four different surfaces of varying solvent affinities (LiF, Li2CO3, Li2EDC, and graphite), we explore the impact of solvent–surface interactions and ion–surface interactions on these interfacial behaviors. Classical molecular dynamics provides a route to explore the molecular structure at the electrolyte boundary, and two different electrolytes are considered to investigate the role of ion association on the accumulation, pairing, and dynamics. By considering the changes as a result of switching between one electrolyte with mostly solvent-separated ions to another with contact ion pairs, we provide evidence that both bulk ion association and the solvent–surface interaction are key descriptors of ion aggregation at the electrode surface. The insights from these simulations inform not only about the impacts of battery interfaces on the surrounding electrolyte but also on the origins of differences reported between classical molecular dynamics simulations at these interfaces.