Two phosphonium ionic liquids with high Li+ transport numberElectronic supplementary information (ESI) available: In addition, all data presented in this work are available in the supplementary tables. See DOI: 10.1039/c5cp02799c

This work presents the physicochemical characterization of two ionic liquids (ILs) with small phosphonium cations, triethylpenthylphosphonium bis(trifluoromethanesulfonyl)imide ([P 2225 ][Tf 2 N]) and (2-methoxyethyl)trimethylphosphonium bis(trifluoromethanesulfonyl)imide ([P 222(201) ][Tf 2 N]), and their mixtures with Li + . Properties such as the electrochemical window, density, viscosity and ionic conductivity are presented. The diffusion coefficient was obtained using two different techniques, PGSE-NMR and Li electrodeposition with microelectrodes. In addition, the Li + transport number was calculated using the PGSE-NMR technique and an electrochemical approach. The use of these three techniques showed that the PGSE-NMR technique underestimates the diffusion coefficient for charged species. The Li + transport number was found to be as high as 0.54. Raman spectroscopy and molecular dynamics simulations were used to evaluate the short-range structure of the liquids. These experiments suggested that the interaction between the Li + and the Tf 2 N − anion is similar to that seen with other ILs containing the same anion. However, the MD simulations also showed that the Li + ions interact differently with the cation containing an alkyl ether chain. The results found in this work suggest that these Li + mixtures have promising potential to be applied as electrolytes in batteries. This work presents the physicochemical characterization of two ionic liquids (ILs) with small phosphonium cations, triethylpenthylphosphonium bis(trifluoromethanesulfonyl)imide ([P 2225 ][Tf 2 N]) and (2-methoxyethyl)trimethylphosphonium bis(trifluoromethanesulfonyl)imide ([P 222(201) ][Tf 2 N]), and their mixtures with Li + .