How NaFTA salt affects the structural landscape and transport properties of Pyrr_1,3FTA ionic liquid

Recently, it has been demonstrated that ionic liquids (ILs) with an asymmetric anion render a wider operational temperature range and can be used as a solvent in sodium ion batteries. In the present study, we examine the microscopic structure and dynamics of pure 1-methyl-1-propylpyrrolidinium fluorosulfonyl(trifluoromethylsulfonyl)amide (Pyrr_1,3FTA) IL using atomistic molecular dynamics simulations. How the addition of the sodium salt (NaFTA) having the same anion changes the structural landscape and transport properties of the pure IL has also been explored. The simulated x-ray scattering structure functions reveal that the gradual addition of NaFTA salt (up to 1.2 molal) suppresses the charge alternating feature of the pure IL because of the replacement of the Pyrr⁺ cations with the Na⁺ ions. The Na⁺ ions are majorly found near the oxygen atoms of the anions, but the probability of finding the Na⁺ ions near these atoms slightly decreases with increasing salt concentration. As expected, the Na⁺ ions stay away from the Pyrr⁺ cations. However, the probability of finding the anions around anions increases with increasing salt concentration. The simulated self-diffusion coefficients of the ions in the pure IL reveal slightly faster diffusion of the Pyrr⁺ cations as compared to the FTA^- anions. Interestingly, in the salt solution, despite having smaller size, the diffusion of the Na⁺ ions is found to be lesser than the Pyrr⁺ cations and the FTA^- anions. The analysis of the ionic conductivity and transport numbers reveals that the fractional contribution of the FTA^- anion to the overall conductivity remains nearly constant with increasing salt concentration, but the contribution of Pyrr⁺ cation decreases and Na⁺ ion increases.