Electrochemical and physicochemical properties of small phosphonium cation ionic liquid electrolytes with high lithium salt contentElectronic supplementary information (ESI) available. See DOI: 10.1039/c5cp00205b

Electrolytes of a room temperature ionic liquid (RTIL), trimethyl(isobutyl)phosphonium (P 111i4 ) bis(fluorosulfonyl)imide (FSI) with a wide range of lithium bis(fluorosulfonyl)imide (LiFSI) salt concentrations (up to 3.8 mol kg −1 of salt in the RTIL) were characterised using a combination of techniques including viscosity, conductivity, differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), nuclear magnetic resonance (NMR) and cyclic voltammetry (CV). We show that the FSI-based electrolyte containing a high salt concentration ( e.g. 1 : 1 salt to IL molar ratio, equivalent to 3.2 mol kg −1 of LiFSI) displays unusual transport behavior with respect to lithium ion mobility and promising electrochemical behavior, despite an increase in viscosity. These electrolytes could compete with the more traditionally studied nitrogen-based ionic liquids (ILs) in lithium battery applications. A novel phosphonium ionic liquid as potential candidate for lithium battery electrolytes.