The Lifetime of Shell Anions: How Long Do Ionic Liquid Anions Remain in a Li+ Shell?

In recent years, ionic liquids have been considered strong candidates as electrolytes. Conventional organic electrolytes can lead to lithium dendrite growths that are known to cause fires. Ionic liquids are a favorable replacement due to their large electrochemical windows, low volatility, improved cycling, and inability to catch fire. A limiting factor of ionic liquids is the poor conductivity compared to conventional methods, which is largely influenced by lithium ion transport. Studies have shown that certain mixtures of ionic liquids have more desirable properties. In our study, we have used molecular dynamics simulations to examine various properties in several ionic liquid mixtures with the goal that learning more about these compounds at the molecular level will help us understand their unique properties at the macroscopic level.

Molecular dynamics simulations have been used in the past to measure Li+ diffusion through an electrolyte. For our study, a cubic sample of ionic liquid electrolyte was generated with various amounts of Li+ ions placed in the solution and were allowed to diffuse. The simulation can actually generate videos of such diffusion on very small timescales (i.e. picoseconds, nanoseconds). We wanted to investigate the properties of the group of anions that surround the Li+, or the anion shell, to find the amount of time that an ionic liquid anion stays within the anion shell. It is possible that there is a relationship between the fluidity of the anion shell and Li+ diffusion, as the study examines.

Molecular dynamics solutions also can give us useful information about the spatial orientation of the molecules in our system. The radial distribution function was examined to find peaks that correspond to different spatial configurations between Li+ cation and ionic liquid anion. The findings from the radial distribution function were coupled with a visual aid to illustrate these interactions for different compounds.