Cage and Microheterogeneity Dynamics Control Ionic Transport in Ionic Liquid-Based K-Ion Battery Electrolytes.

We report the dynamics and microheterogeneity of an ionic liquid-based electrolyte consisting of potassium bis(fluorosulfonyl)imide [KFSI] salt and N-methyl N-propyl pyrrolidinium bis(fluorosulfonyl)imide [PYR13FSI] ionic liquid at different concentrations and temperatures for a potassium-ion battery using computer simulation. We utilized classical molecular dynamics simulations on the nanosecond scale to study concentration- and temperature-dependent structural and dynamic changes. The K+ coordinates to ∼5.8 FSI- and PYR13+ coordinates to ∼7.9 FSI- in their first solvation shell at 303 K for 1 m concentration. At lower concentration, the most probable solvation structure of K+ is penta-coordinated FSI-; however, at higher concentration, it is hexa-coordinated. The potential of mean force calculations shows the energy levels along the reaction coordinates of K+-FSI- and PYR13+-FSI-. K+-FSI- forms a stable configuration when K+ and FSI- are 0.51 nm apart, whereas PYR13+-FSI- forms a stable configuration when PYR13+ and FSI- are 0.63 nm apart. A higher energy barrier in the PMF of K+-FSI- than that of PYR13+-FSI- indicates stronger interactions between K+ and FSI-. At higher temperatures, higher values of conductivity and diffusivity were observed. The activation energy of ionic transport of 10.3 kJ mol-1 was obtained from the Arrhenius formulation. The K+-FSI- ion-cage showed a higher lifetime than the PYR13+-FSI- ion-cage for both concentration and temperature variations. The relation between the transport property and ion-cage lifetime is established. The van Hove function and non-Gaussian parameter quantify the heterogeneity in the dynamics of these electrolytes.