A comparison of the impact of cation chemistry in ionic liquid-based lithium battery electrolytes

There is an increasing interest in ionic liquid electrolytes for battery applications because they are potentially safer alternatives to conventional liquid electrolytes. As the properties of ionic liquid electrolytes strongly depend on the chemistry of the constituent cations and anions, with phosphonium cations often being more favourable, here-in, we compare electrolytes based on methyl-substituted phosphonium and ammonium cations, namely a new tris(amino)-based phosphonium cation with the bis(fluorosulfonyl)imide anion (P1(DMA)3][FSI]) and the previously reported hexamethylguanidinium [HMG][FSI]. Highly concentrated electrolytes containing 50 mol% LiFSI are investigated for their thermal properties and dynamic behaviour as well as in lithium cells. Glass transition temperature, viscosity, ionic conductivity, and the ion diffusion coefficients show dependency on the nature of the cation. The electrochemical performance of the two electrolytes in Li metal symmetrical cells was compared under different cycling conditions, and the interfacial resistance monitored by EIS. Under milder cycling rates (0.5 mA cm-2), the [HMG][FSI] based electrolyte shows lower overpotentials compared to the phosphonium analogue, but this is reversed when Li metal is cycled at high current (1.5 mA cm-2). A higher interfacial resistance during the first 5 cycles was observed in the cell with 50 mol% LiFSI in [HMG][FSI] when cycled at 1.5 mA cm-2 which was consistent with the SEI analysis performed using ex situ X-ray photoelectron spectroscopy that showed the presence of higher amount of carbonate species at the Li metal surface. Finally, both electrolytes supported full cell cycling with LiFePO4 cathodes (2 mA h cm-2) with good rate performance and stability, although P1(DMA)3][FSI]/50 mol% LiFSI gave higher areal capacity values than the HMG analogue when cycled at C/5 (1.6 vs. 1.53 mA h cm-2). Two high-salt-content ionic liquid electrolytes with distinct cationic chemistries were compared. The one with a phosphonium cation showed superior characteristics, particularly in terms of its enhanced capacity when used in lithium metal batteries.