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

Abstract

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-N-cyanamide (TFSAM) anion was investigated in Pyr14TFSAM(1-x)LiTFSAMx over a broad concentration range (up to x = 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 x = 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 x. This suggests a change in the transport mechanism depending on the Li salt fraction. Interestingly, the contribution of structural diffusion at high Li salt concentrations (x ≥ 0.6) seems to be higher in the TFSAM system, influenced by the nonsymmetric coordination, while in the TFSA system, the vehicular transport seems to be still predominant at x ≥ 0.6.