Are Ionic Liquid-Based Electrolytes So Safe
Authors: Chancelier Léa , Diallo Alpha Oumar , Santini Catherine C. , Marlair Guy , Gutel Thibaut , Mailley Sophie , Len Christophe 
Published: February 2020

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 batteries5,6,7. 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 LiNi1/3Co1/3Mn1/3O2as 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.