In recent times, various ionic liquids have been considered as potential electrolytes for lithium batteries. The cyclic performance of batteries, in particular those with high open circuit voltage, is dependent on the electrochemical stability window of the electrolyte. This research involves theoretical calculations, based on density functional theory, of the electrochemical stability of ionic liquids with various imidazolium cations and a range of anions.
S. Kazemiabnavi et al., J. Phys. Chem. B (2016) 120 (25) 5691-5702
The oxidation and reduction potentials were examined using the thermodynamic cycle approach as well as based on calculations of the energy of the frontier molecular orbitals. Our results indicate that [CnMIM]+[PF6]- are the most stable ionic liquids due to the high oxidation potential of [PF6]-, while [CnMIM]+[TFSI]- are the least stable among the ionic liquids considered in the study.
By comparing the results to experimental values, we demonstrated that the thermodynamic cycle method provides accurate quantitative predictions of the oxidation and reduction potentials of ionic liquid cations and anions, while the values obtained from HOMO/LUMO method overestimated the potentials by up to 50 %. The results further indicate that the thermodynamic cycle approach can be utilized to populate the electrochemical stability window data for a range of ionic liquids, which in turn can be used to guide the selection of the ionic liquid electrolytes. This information is especially essential for the design of high-voltage rechargeable batteries, where the electrochemical stability window is critical.
Lithium-air batteries show excellent promise in meeting current demands in large scale energy storage applications such as electric vehicles. However, the performance and safety of these batteries are highly dependent on the electrochemical stability and physicochemical properties of the electrolyte.