The use of highly concentrated, binary ionic liquid/Li salt electrolytes with the novel asymmetric anion FTFSI results in improved rate capability and capacity retention at 20°C, as compared to Li⁺-diluted systems, in Li-metal and Li-ion cells [1]. This work explores the connection between the bulk electrolyte properties and the molecular organization to give 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%, 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 via the exchange of anions in the first Li⁺ solvation shell [2]. The good rate performance is likely directly influenced by the presence of the larger Li⁺ complexes, which promote Li⁺-ion transport (as opposed to Li⁺-complex transport) and increase the Li⁺ availability at the electrode. In fact, electrophoretic NMR shows that Li is predominantly migrating in negatively charged Li-anion clusters towards the anode. However, this vehicular transport mechanism has decreasing relevance at elevated Li⁺ concentrations.

References

G. A. Giffin, A. Moretti, S. Jeong and S. Passerini, J. Power Sources, 2017, 342, 335-341.

G. A. Giffin, A. Moretti, S. Jeong, K. Pilar, M. Brinkkötter, S. G. Greenbaum, M. Schönhoff and S. Passerini, ChemSusChem, 2018, doi:10.1002/cssc.201702288.

M. Brinkkötter, G. A. Giffin, A. Moretti, S. Jeong, S. Passerini and M Schönhoff, Chem Comm, 2018, submitted for publication