A Phosphonium Bis(fluorosulfonyl)Imide-Based Ionic Liquid Electrolyte for Lithium Batteries: Tailoring Composition and Performance

The electrolyte is one of the key components of an energy storage device required to achieve high reliability and safety as well as high energy density. Lithium batteries have become one of the most common technologies for portable electronics and are making their way into electric vehicles and stationary storage.

The interaction of a phosphonium-based room temperature ionic liquid (RTIL) with a lithium salt, lithium bis(fluorosulfonyl)imide, as a function of temperature and concentration has been evaluated and correlated with the mechanism of lithium transport. The formulated system could be a potential safe electrolyte for lithium batteries to meet the challenges presented by emerging battery technologies. We have found that the highly conductive and fluid phosphonium ionic liquid based on FSI anion gives excellent performance for lithium batteries, particularly for electrolytes containing higher lithium salt concentrations. The relationship between the physical properties and the mobility of the species present in solution has also been investigated.

A combination of differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), nuclear magnetic resonance (NMR) and X-Ray Photoelectron (XPS) spectroscopy techniques were used to characterise these novel electrolytes. Impedance spectroscopy measurements established that the addition of lithium bis(fluorosulfonyl)imide to the ionic liquid decreased the ionic conductivity over a range of temperatures (from -20 to 120°C) and this was related to an increase of the dynamic viscosity. Interestingly, the mechanism of lithium transport is dependent on the lithium concentration. It is also evident from EIS and NMR that the mobility of the species in solution is dependent on lithium salt concentration, and this impacts the electrolyte transport properties and mechanism. These data give an insight into the fluidity of the species when introducing lithium into the system.

In terms of lithium electrodeposition, cyclic voltammetry (CV) is a powerful tool to better understand the electrochemical stability of lithium metal in the electrolyte to demonstrate the viability of using a lithium based anode in a battery. Cyclic voltammetry measurements showed that reversible electrodeposition of lithium at a nickel electrode could be observed at very high lithium salt concentrations (up to 3.8 mol.kg^-1 of lithium salt) at 25°C. An increase of lithium concentration also revealed an improved efficiency and electrochemical stability at more negative potentials. These data provided an insight into the fundamental mechanism of lithium transport. The results suggest that the binary systems behave as a potential electrolyte including improved safety and stability.