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 <i>N</i>-methyl <i>N</i>-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.