Electrolytes for Lithium battery applications
Improving lithium-ion battery performance is of technological importance due to increased demand for consumer electronics and the increasing electrification of automobiles. Understanding the effect of chemical detail on the performance is of fundamental interest because the only design parameters are the solvent and anion type. Solvents for battery electrolytes should have a large dielectric constant, to promote salt dissociation, and low viscosity to increase ion diffusion. Mixtures of carbonate molecules have these characteristics. A number of salt species have been considered as potential candidates for battery electrolytes. Lithium bis(trifluoromethane) sulfonimide (TFSI) is a promising ion, because it is electrically stable and has a large conductivity. However, the chemically similar trifluoromethanesulfonate ion (OTf) does not provide the necessary performance as an electrolyte. In our group we are using computer simulation of atomistic models to test various candidates for solvent and anion for battery applications.
A challenging and longstanding problem is the formation of Lithium dendrites in batteries. We are using very coarse models in an effort to elucidate the general principles behind dendrite formation and growth. In a simple model where the Lithium ions are spheres, and the rest of the electrolyte is a continuum we find that the important effects are due to the self-diffusion constant and the local electric field. The dendrites transform from a cauliflower to broccoli shape as the diffusion constant and electric field decrease. We are working towards obtaining a molecular explanation by connecting these studies to the simulation studies using more detailed molecular models.