Are Ionic Liquid-Based Electrolytes So Safe

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 terms of energy density. But carbonate mixtures used as electrolyte solvents could lead to safety issues, due to their high flammability and low flash points. Formulating electrolytes with negligible volatility and reduced flammability such as ionic liquids (ILs) could be safer. Ionic liquids have low vapour pressure, high flash point,^1,2 wide electrochemical window, good ionic conductivity and high decomposition temperature (Td).³But little information is available on their behaviour under abuse conditions, such as car crashes or any abnormal use (shortcut, overheat).

From the literature, imidazolium and pyrrolidinium cations associated to bis(trifluoromethanesulfonyl)imide anion [NTf₂] display the highest thermal stability⁴ (Figure 1, left) and the best performances as electrolyte solvents for lithium-ion batteries^5,6,7. We investigated through several techniques (dynamic and long term TGA, …) the thermal decomposition of 1‑butyl‑3‑methylimidazolium bis(trifluoromethanesulfonyl)imide [C₁C₄Im][NTf₂], 1‑butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [PYR₁₄][NTf₂] and their corresponding lithium ion conductive electrolyte solutions, resulting from their mixture with 1 mol.L^-1 of LiNTf₂ ([C₁C₄Im][Li][NTf₂] and [PYR₁₄][Li][NTf₂] respectively).

Long term experiments (2 to 15 h at 350°C) revealed the highest thermal stability for imidazolium-based solutions. For both families, the major volatile decomposition products were identified as butene isomers. Recombined alkyl cations (e.g. C₁Im, C₄Im, C₄C₄Im and C₁C₁Im) were detected in the residual liquid phases, implying no mass loss during degradation.

Flammability and heat of combustion were studied with a fire propagation apparatus (ISO12136). The solutions were found very weakly combustible (Figure 1, right), and LiNTf₂ showed a flame retardant effect in the case of [PYR₁₄][Li][NTf₂]. Flammable gases were emitted due to cation decomposition, while toxic effluents result from the anion decomposition.⁸

 [C₁C₄Im][Li][NTf₂], [PYR₁₄][Li][NTf₂] and commercial carbonates were used as electrolytes in the batteries (coin cells and pouch cells), with Li₄Ti₅O₁₂ and LiNi_1/3Co_1/3Mn_1/3O₂as negative and positive electrodes respectively. Batteries were submitted to cycling and overcharge. After any experiment, the gaseous phases of the batteries were analysed with a specific setup coupled with a mass spectrometer.

References

¹ A. Lewandowski et al., J Power Sources, 2009, 194, 601.

² A. O. Diallo et al., Sep Purif Technol, 2012, 97, 228.

³ P. Wasserscheid and T. Welton, Ionic liquids in synthesis, Wiley-VCH, 2003.

⁴ L. Chancelier et al., in preparation.

⁵ D. R. MacFarlane et al., Energy & Environmental Science, 2013, 7, 232.

⁶ S. Seki et al., ECS Electrochemistry Letters,2012, 1, A77.

⁷ A. Balducci et al., J. Power Sources, 2011, 196, 9719.

⁸ L. Chancelier et al., Phys. Chem. Chem. Phys., 2014, 16, 1967.