Aging Investigations of Various Electrolytes By Means of IC/ESI-MS and CE/ESI-MS

Lithium-ion batteries (LIBs) are already state of the art energy sources for portable electronic devices like mobile phones and laptops. The most important disadvantage of LIBs is the aging of the cell constituents, which minimizes the storage and the cycle lifetime. Reduction, oxidation and chemical degradation of solvents, conductive salts and additives, have a major negative influence on capacity and lifetime. ^[1-3]

In this work, the thermal decomposition products of electrolytes containing the widely used lithium hexafluorophosphate (LiPF₆) as a conducting salt are investigated by two different developed separation methods: Ion Chromatography (IC) (see Figure 1) and Capillary Electrophorese (CE). Both separation methods are coupled by Electrospray Ionization Mass Spectrometer (ESI-MS) identifying developed decomposition products. In Figure 1, some decomposition products of LiPF₆ are presented: hydrofluoric acid (HF) and difluorophosphate (PO₂F₂^-) ^[4]. We introduced the measurement method CE/ESI-MS for electrolytes to confirm the decomposition products of the already developed IC/ESI-MS method. Beyond that, CE has the advantage that only small amounts (nL range) of electrolytes for injection are required and that it is possible to measure cations and anions in the same measurement.   

Most commercial available electrolytes in LIBs are based on organic solvents, there they are easily flammable and a safety risk for people and the environment in case of accident. Ionic liquids (ILs) are discussed as future electrolytes in LIBs because of theirs advantageous physical properties like non-flammability and non-volatility.^[5]For the application in future LIBs, it is important to gain more information about their thermal stability and the influence of lithium salts to the aging process of ILs.

Therefore, the effect of electrolyte salts like LiPF₆ on the thermal decomposition of ILs is measured as well by our developed CE/ESI-MS and the new IC/ESI-MS methods. In Figure 2, the decomposition products of the ILs PYR₁₃FSI and PYR₁₃TFSI mixed with LiPF₆ are illustrated. It is obvious that PYR₁₃TFSI did not show any degradation products mixed with LiPF₆, while PYR₁₃FSI showed the following decomposition products: SO₃F^- and H₂NO₃S^-.

References

[1] S.S. Zhang, J. Power Sources 2006 162, 1379-1394.

[2] H.F. Xiang et al., J. Power Sources 2009 191, 575-581.

[3] S.E. Sloop et al., J. Power Sources 2003 119-121, 330-337.

[4] L. Terborg et al., Anal. Chim. Acta, 2011, 714, 121-126.

[5] G.B. Appetecchi et al., J. Power Sources, 2006, 153, 1685-1691.