Transference Numbers of Aqueous Polyelectrolyte Solutions

The demand for lithium-ion batteries with improved energy density and charging rates is growing, because of increasing use of consumer electronics and electrification of vehicles. However, lithium-ion batteries have several issues including long charging times. A potential route to circumvent this issue and increase the battery energy density is high transference number electrolytes. Lithium transference numbers using traditional liquid electrolytes are less than 0.5, which indicates the anion carries most of the current. In order to increase this transference number, it has been suggested to use polyelectrolyte solutions in which the polyanion mobility is less than the lithium ion mobility, but molecular-level understanding of counterion dynamics in polyelectrolyte solutions is underdeveloped. Our work uses molecular dynamics simulations of coarse-grained polymer models to elucidate how chain length, counterion size, and solution concentration affect the ion transference number in solution. These simulations show ion transference numbers depend non-monotonically on chain length, and smaller counterions have lower transference numbers. These simulation results can be used to guide experimental design of electrolytes with high lithium ion transference numbers.