Thermal stability and fire safety of ionic liquid-based electrolytes for lithium-ion batteries
Résumé
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 that a degradation with no mass loss occurred. Figure 1: Long term stability test by TGA of IL-based electrolytes, neat ILs and lithium salt at 350 °C for 15 h Flammability and heat of combustion were studied with a fire propagation apparatus. The solutions were found very weakly combustible, and LiNTf2 showed a flame retardant effect. Flammable gases were emitted due to cation decomposition, and the anion decomposition formed toxic effluents. Figure 2: Heat release rate profiles of neat ILs and corresponding electrolytes showing good resistance to ignition ( ca.5 min) Then the electrolytes [C1C4Im][Li][NTf2]and [PYR14][Li][NTf2] were used in batteries constituted of Li4Ti5O12 and LiNi1/3Co1/3Mn1/3O2 electrodes. The cells were submitted to abuse tests such as shortcut or overcharge. A specific setup coupled with a mass spectrometer was developed to analyse in situ the gaseous emissions. All these points will be developed during the communication.