Li Coordination of a Novel Asymmetric Anion in Ionic Liquid-in-Li Salt Electrolytes

We analyze the influence of the asymmetry of the anion on coordination and transport processes in a Li salt/ionic liquid system. The relatively new asymmetric 2,2,2-trifluoromethylsulfonyl-<i>N</i>-cyanamide (TFSAM) anion was investigated in Pyr₁₄TFSAM_{(1–<i>x</i>)}LiTFSAM_<i>x</i> over a broad concentration range (up to <i>x</i> = 0.7 Li salt) and was compared to the well-known bis­(trifluoromethanesulfonyl)­amide (TFSA) anion. In contrast to the TFSA-based system, the system with TFSAM has no phase transition over the whole concentration range. Raman spectroscopy and NMR chemical shifts elucidate the Li coordination in detail. Up to <i>x</i> = 0.3, the asymmetric anion coordinates to Li⁺ only via the cyano group. With increasing Li salt fraction, the contribution of Li–oxygen coordination increases. This coordination effects influence the transport properties of the system, as examined via pulsed-field-gradient NMR (PFG-NMR). Although the overall diffusivity of both systems is decreasing because of viscosity effects, the relative diffusivity of the Li cation is increasing with <i>x</i>. This suggests a change in the transport mechanism depending on the Li salt fraction. Interestingly, the contribution of <i>structural diffusion</i> at high Li salt concentrations (<i>x</i> ≥ 0.6) seems to be higher in the TFSAM system, influenced by the nonsymmetric coordination, while in the TFSA system, the <i>vehicular transport</i> seems to be still predominant at <i>x</i> ≥ 0.6.