Lithium-ion batteries (LIBs) working at high temperature (HT) (80–150°C) (HT-LIBs) should have a number of intrinsic advantages such as high ionic conductivities, fast electrode kinetics, etc. An HT-LIB could at a vehicle systems level reduce the cooler size, as compared to using conventional LIBs. To comply with HT conditions, however, all parts of the battery have to be thermally stable, to not become a security problem or deteriorated. The most often used LIB electrolyte salt (LiPF₆) is unstable at temperatures >60°C [1] and hence it is crucial to explore alternative lithium salts.

The lithium bis(fluorosulfonyl)imide (LiFSI) salt has been suggested as a possible replacement for LiPF₆due to an improved thermal stability, high ionic conductivities and electrochemical stabilities in LIB electrolytes [2,3]. However, some contradictory results exist with respect to its promising thermal stability and non-corrosiveness [4,5].

We have investigated three commercial LiFSI salts and compared their properties with focus on thermal stability and phase transitions – including a vibrational spectroscopy based assessment of salt purity and decomposition products [6]. The salts were found to differ significantly in their thermal stabilities as determined by both dynamic and isothermal TGA. The FT-IR spectra of the salts are close to identical, but several additional bands were identified in the Raman spectra of the least stable salt. The latter allows for a discussion of the origin and role of salt impurities for the observed thermal (in-)stability. Furthermore, by adding each salt to the ionic liquid 1-ethyl-3-methylimidazolium FSI (EMIFSI), Li_0.1EMI_0.9FSI electrolytes were obtained. Galvanostatic cycling and linear sweep voltammetry were both performed to elucidate the influence of the impurities on the electrochemical performance.

[1] S.F. Lux, I.T. Lucas, E. Pollak, S. Passerini, M. Winter, R. Kostecki, Electrochem. Commun. 14 (2012) 47.
[2] H.-B. Han et al.J. Power Sources 196 (2011) 3623.
[3] L. Li, S. Zhou, H. Han, H. Li, J. Nie, M. Armand, Z. Zhou, X. Huang, J. Electrochem. Soc. 158 (2011) A74.
[4] K. Kubota, T. Nohira, T. Goto, R. Hagiwara, Electrochem. Commun. 10 (2008) 1886.
[5] A. Abouimrane, J. Ding, I.J. Davidson, J. Power Sources 189 (2009) 693.
[6] M. Kerner, N. Plylahan, J. Scheers, P. Johansson, submitted

Figure 1: The thermal stabilities of three commercial LiFSI salts.