Thermal stability and fire safety of ionic liquid-based electrolytes for lithium-ion batteries

International audience ; Due to environmental and energy concerns, batteries for electric vehicles are highly studied and developed. They are mainly based on lithium-ion technology, considered more efficient in energy density. Carbonate mixtures used as electrolyte solvents could lead to safety issues due to their high flammability and low flash points. Formulating electrolyte solutions with nonvolatile and nonflammable ionic liquids (ILs) instead of carbonate mixtures could be safer as they have low vapour pressure and high flash point. Moreover, they are good ionic conductors, present wide electrochemical window and high decomposition temperature (Td). But little information is available on their behaviour under abuse conditions like car crashes or any abnormal use (shortcut, overheat). In order to understand the possible decomposition mechanisms of ILs and their mixture with lithium salt during abuse tests, a thermodynamic study of these compounds was carried out. A critical literature study of the Td determined by thermogravimetric analysis (TGA) and consequent experiments highlighted the stability of the NTf2 anion combined with two cationic families: imidazolium and pyrrolidinium. We chose 1-butyl-3-methylimidazolium [C1C4Im] and 1-butyl-1-methylpyrrolidinium [PYR14] cations, bis(trifluoromethanesulfonyl)imide [NTf2] anion. The electrolytes were composed of their mixture with 1 mol.L-1 of LiNTf2 andreferred to as [C1C4Im][Li][NTf2]and [PYR14][Li][NTf2]. Nevertheless, TGA does not give sufficient information on the nature of volatile emissions, so the thermal decomposition of [C1C4Im][NTf2] and [PYR14][NTf2] and their corresponding electrolytes was studied. Long term experiments with different techniques (2 to 15 h at 350°C) showed the highest stability of imidazolium-based solutions (Figure 1). For both families, the volatile decomposition products were mainly identified as butene isomers. Recombined alkyl cations (e.g. C1Im, C4Im, C4C4Im and C1C1Im) were detected in the residual liquid phases, implying ...