In binary ionic liquid/Li salt mixtures with the novel asymmetric anion FTFSI, electrophoretic mobility μi values of all ion species were determined using electrophoretic NMR. Li was determined to migrate in negatively charged Li-anion clusters towards the anode. This vehicular transport mechanism was shown to have decreasing relevance at elevated salt concentrations.

No abstract

The use of highly concentrated ionic liquid-based electrolytes results in improved rate capability and capacity retention at 20 °C compared to Li -dilute systems in Li-metal and Li-ion cells. This work explores the connection between the bulk electrolyte properties and the molecular organization to provide insight into the concentration dependence of the Li transport mechanisms. Below 30 mol %, the Li -containing species are primarily smaller complexes (one Li cation) and the Li ion transport is mostly derived from the vehicular transport. Above 30 mol %, where the viscosity is substantially higher and the conductivity lower, the Li -containing species are a mix of small and large complexes (one and more than one Li cation, respectively). The overall conduction mechanism likely changes to favor structural diffusion through the exchange of anions in the first Li solvation shell. The good rate performance is likely directly influenced by the presence of larger Li complexes, which promote Li -ion transport (as opposed to Li -complex transport) and increase the Li availability at the electrode.

In binary ionic liquid/Li salt electrolytes for lithium‐ion batteries the use of asymmetric anions reduces the tendency for crystallization and enables liquid systems with high Li concentration. The ionic liquid composed of the N‐(methoxyethyl)‐N‐methylpyrrolidinium (Pyr12O1) cation and the (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI) asymmetric anion at molar Li FTFSI fractions up to 0.6 are investigated by 19F and 7Li chemical shifts, 1H, 19F, and 7Li electrophoretic nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations. Thereby, the local coordination environment of Li is elucidated and correlated with the Li+ ion transport properties. At low Li salt fraction, the preferred Li+ coordination is to the trifluoromethanesulfonyl side of the anion, resulting in vehicular Li+ transport in stable, net negatively charged Li‐anion clusters causing negative Li transference numbers. The Li coordination is, however, shifting to the fluorosulfonyl group at salt fractions >0.4, as consistently evidenced by DFT and 19F NMR. Herein, Li+ mobilities give evidence of an increasing relevance of structural Li+ ion transport, which is key toward developing efficient ionic liquid–based batteries. This knowledge will serve further tailored design of cations and anions, which reduces crystallization and promote structural transport in ionic liquids for safe and high‐power batteries.

Two ternary polymer gel electrolyte systems are compared, containing either polyethylene oxide (PEO) or the poly-ionic liquid poly(diallyldimethylammonium) bis(trifluoromethyl sulfonyl)imide (PDADMA-TFSI). Both gel types are based on the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide (PTFSI) and LiTFSI. We study the influence of the polymers on the local lithium ion dynamics at different polymer concentrations using Li spin-lattice relaxation data in dependence on frequency and temperature. In all cases the relaxation rates are well described by the Cole-Davidson motional model with Arrhenius dependence of the correlation time and a temperature dependent quadrupole coupling constant. For both polymers the correlation times are found to increase with polymer concentration. The activation energy of local motions slightly increases with increasing PEO concentration, and slightly decreases with increasing PDADMA-TFSI concentration. Thus the local Li motion is reduced by the presence of either polymer; however, the reduction is less effective in the PDADMA samples. We thus conclude that mechanical stabilization of a liquid electrolyte by a polymer can be achieved at a lower decrease of Li motion when a cationic polymer is used instead of PEO.