Quantum and Classical Molecular Dynamics of Ionic Liquids Electrolytes for Na/Li-based Batteries: molecular origins of the conductivity behavior.

Compositional effects on the charge-transport properties of electrolytes for batteries based on room-temperature ionic liquids (RTILs) are well-known. However, further understanding is required about the molecular origins of these effects, in particular regarding the replacement of Li by Na. In this work, we investigate the use of RTILs in batteries, by means of both classical molecular dynamics (MD), which provides information about structure and molecular transport, and abinitio molecular dynamics (AIMD), which provides information about structure. The focus has been placed on the effect of adding either Na+ or Li+ to 1-methyl-1-butyl-pyrrolidinium [C4PYR](+) bis(trifluoromethanesulfonyl)imide [Tf2N](-). Radial distribution functions show excellent agreement between MD and AIMD, which ensures the validity of the force fields used in the MD. This is corroborated by the MD results for the density, the diffusion coefficients, and the total conductivity of the electrolytes, which reproduce remarkably well the experimental observations for all studied Na/Li concentrations. By extracting partial conductivities, it is demonstrated that the main contribution to the conductivity is that of [C4PYR](+) and [Tf2N](-). However, addition of Na+/Li+, although not significant on its own, produces a dramatic decrease in the partial conductivities of the RTIL ions. The origin of this indirect effect can be traced to the modification of the microscopic structure of the liquid as observed from the radial distribution functions, owing to the formation of [Na(Tf2N)(n)]((n-1)-) and [Li(Tf2N)(n)]((n-1)-) clusters at high concentrations. This formation hinders the motion of the large ions, hence reducing the total conductivity. We demonstrate that this clustering effect is common to both Li and Na, showing that both ions behave in a similar manner at a microscopic level in spite of their distinct ionic radii. This is an interesting finding for extending Li-ion and Li-air technologies to their potentially cheaper Na-based counterparts.