LAUSR

To investigate the influence of anion structure on physical properties of electrolytes we used five different anions, namely (FSO2)2N (FSI, FSA), (CN)2N (DCA), (CF3SO2)2N (TFSI, TFSA), CF3SO3 (TfO) and (CF3SO2)N(CN) (TFSAM), and investigated them in a series of polyelectrolytes, ionic liquid (IL)/Li salt mixtures and in ternary ion gels (poly(ionic liquid)/ionic liquid/Li salt), respectively. Both the poly(ionic liquid) (PIL) poly(diallyldimethylammonium) (PDADMA) and the ILs employed in the study have the same pyrrolidinium anion. To isolate solely the effect of the anion a common chloride precursor PIL was used to maintain constant the average degree of polymerization (DPn) and chain dispersity. The systems were analyzed in terms of ionic conductivity, electrochemical stability vs Li+/Li, ion diffusion (multinuclear Pulsed Field Gradient NMR) and local lithium ion dynamics (7Li spin lattice relaxation rates). Concerning optimization of any of these parameters, different orders of the anion series are obtained and discussed in terms of structural aspects and ion coordination. In particular, the coordination strength of the –CN group to the lithium ion is higher than the coordination strength of the oxygens of the –SO2CF3 fragment. As a result the lithium ion in the TFSAM samples is primarily coordinated by the –CN group. Detailed structure/property relationship analysis demonstrated that for those applications of ion gels (capacitors, sensors, etc.) where a high conductivity is a primary demand, while the electrochemical stability is not essential, the FSI anion will be the best choice. However, for energy storage systems such as Li metal batteries preferably TFSI but also TFSAM anions will represent the best compromise of relevant parameters.