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.

Polymerized ionic liquids or poly(ionic liquids) (polyILs) have been considered as promising hosts for fabrication of gel polymer electrolytes (GPEs) containing ionic liquids. In this work, a novel GPE based on a polyIL, poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PDADMA TFSI), and a high lithium-concentration phosphonium ionic liquid, trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide (P_111i4FSI), is prepared. The composition-dependent behaviour of the GPEs is investigated by differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS) and solid-state nuclear magnetic resonance (solid-state NMR). The effects of Al₂O₃ nano-particles on the polymer electrolyte properties are also discussed. It is shown that the introduction of high lithium-concentration ionic liquids into the polyIL can effectively decrease the glass transition temperature (T_g) of the resulting GPE, leading to improved ion dynamics and higher ionic conductivity. The Al₂O₃ nano-particles effectively enhanced the mechanical stability of the GPEs. Most importantly, although adding PDADMA TFSI to the ionic liquids decreases the diffusion coefficient of both Li⁺ and anions, a greater decrease in the anion diffusion is observed, resulting in a higher Li⁺ transport number (as evaluated by NMR) than that seen in the original ILs. Finally, a highly conductive free-standing GPE membrane is fabricated, and extremely stable lithium symmetrical cell performance is demonstrated.