(368c) Lithium Ion Transport Mechanism in PYR14tfsi/PEO Branched Nanopore System: A Polarizable Molecular Dynamics Study

A polarizable molecular dynamics (MD) simulation study was conducted to verify the lithium ion transport mechanism in polymer branched ion channels filled with ionic liquids. Specific compounds of the electrolytes comprised the solution of [lithium][bis(trifluoromethanesulfonyl)imide] (LiTFSI) in [1‐butyl‐1-methylpyrrolidinium][bis(trifluoromethanesulfonyl)imide] (PYR₁₄TFSI) and poly-ethylene oxide (PEO) were branched at the wall of nanopores at regular intervals. MD simulations with polarizable force fields, especially APPLE&P force fields [1], have been performed to precisely analyze Li ion transport mechanism. The effects of the ratio of Li ions to PEO chains and size of nanopores on the structural and dynamic properties of Li ions were examined.

As a result of structural analysis, Li ions prefer to migrate in the longitudinal direction due to the confinement in the radial direction. Interestingly, it was found by cluster analysis that the complex of Li and TFSI ions preferred rather than in the PEO system, although is the most prominent structure in the binary electrolytes (LiTFSI/PYR₁₄TFSI) of similar salt concentration. [2,3] With smaller Li-TFSI complex, the mobility of Li ions was expected to be higher than binary electrolyte systems, and these results were consistent with the result of dynamic properties; Diffusion coefficients and ionic conductivities of Li ions in the longitudinal direction was improved in the PEO system. Particularly, as the ratio of the Li ions to PEO chains increases and the pore size decreases, the mobility of Li ions increases. We also investigated the residual time of Li ions in the complex to analyze transport mechanism of Li ions. In the PEO system, residual time of Li ions for TFSI was decreased compared to the binary electrolytes system. In addition, as a result of single ion trajectory analysis, Li ions encountered more TFSI ions during simulation times in the PEO system.

Ref.

[1] Borodin O., Polarizable force field development and molecular dynamics simulations of ionic liquids. J. Phys. Chem. B 113, 11463–11478 (2009).

[2] Tong J, Wu S, von Solms N, et al. The effect of concentration of lithium salt on the structural and transport properties of ionic liquid‐based electrolytes. Front Chem. 2020;7:1‐10.

[3] Haskins JB, Bennett WR, Wu JJ, et al. Computational and experimental investigation of Li‐doped ionic liquid electrolytes: [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4]. J Phys Chem B. 2014;118:11295‐11309.