LiTFSI structure and transport in ethylene carbonate from molecular dynamics simulations

Molecular dynamics (MD) simulations using a many-body polarizable force field were performed on ethylene carbonate (EC) doped with lithium bistrifluoromethanesulfonamide (LiTFSI) salt as a function of temperature and salt concentration. At 313 K Li+ was coordinated by 2.7-3.2 EC carbonyl oxygen atoms and 0.67-1.05 TFSI- oxygen atoms at EC:Li = 10 and 20 salt concentrations. In completely dissociated electrolytes, however, Li+ was solvated by approximately 3.8 carbonyl oxygen atoms from EC on average. The probability of ions to participate ion aggregates decreased exponentially with an increase in the size of the aggregate. Ion and solvent self-diffusion coefficients and conductivity predicted by MD simulations were in good agreement with experiments. Approximately half of the charge was transported by charged ion aggregates with the other half carried by free (uncomplexed by counterion) ions. Investigation of the Li+ transport mechanism revealed that contribution from the Li+ diffusion together with its coordination shell to the total Li+ transport is similar to the contribution arising from Li+ exchanging solvent molecules in its first coordination shell with solvents from the outer shells.