posted on 2021-03-29, 20:14authored byJustin
R. Pinca, William G. Duborg, Ryan Jorn
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.