Ionic liquids for energy storage applications, imidazolium or pyrrolidinium ?

Due to environmental and energy concerns, batteries for electric vehicles are highly studied and developed. Previously they were based on Lead, Nickel-Cadmium and Nickel metal hydride technologies, but at the present Li-ion are considered more efficient in energy density. Unfortunately, these batteries use carbonated solvents as electrolytes which lead to safety issues because of their high tlammability and low flash points. The replacement of these electrolytes by ionie liquids (ILs) is now widely considered as they have low vapour pressure and high flash point. They are also good ionie conductors, and present wide electrochemical window and high decomposition temperature (Td). But little information is available on their behaviour under abusive conditions like car crashes or any abnormal use (shortcut, overheat). In order to begin the understanding of the possible degradation of Ils du ring abusive tests a thermodynamic study of these compounds was carried out. A first approach is a criticalliterature study of the Td determined by thermogravimetric analysis (TGA). This study lead to the definition of adapted experimental parameters, and consequent experiments highlighted the stability of the NTf2 anion combined with two cation families: imidazolium and pyrrolidinium. Several trends were subsequently obtained, depending on the anion, the alkyl chain length, the functionalities and the c2-H substitution. But TGA does not give information on the nature of volatile emissions, so the comparison between c1c4ImNTf2 and PYR14 NTf2 with in situ and ex situ techniques has been carried out. Diffuse reflectance infra red spectroscopy associated with gas chromatography and mass spectroscopy was used to determine the decomposition products after two hours heating at 250 and 350°C. The flammability and heat of combustion will also be reported. The second part of the work is dedicated to the electrochemical stability of c1 c4 ImNTf2 and PYR14 NTf2, in the presence of lithium salt LiNTf2 and electrodes (Li4 Ti5o12 and LiNixMnxCox02, x=0.33). Cycling tests are performed on specifie designed cells to allow the in operando analysis of the emitted gases by mass spectrometry.