Quantifying Absolute Amounts of Electrolyte Components in Lithium-Ion Cells Using HPLC
Authors: Stockhausen Richard , Smith Anna , Ehrenberg Helmut 
Published: July 2022

Capacity loss and solid electrolyte interphase (SEI) buildup are accompanied by continuous electrolyte consumption due to electrochemical decomposition reactions[1],[2],[3]. The quantitative analysis of electrolytes in battery cells usually consists in the determination of concentrations or quantity ratios. As the total electrolyte amount in the cell decreases during cell aging due to electrochemical decompositions, concentrations do not reveal reliable information about the dominant decomposition reactions. Hence, the total electrolyte mass and the absolute consumption of the electrolyte components need to be determined to get reliable insights into cell chemistry. For this purpose, a method for electrolyte extraction from pouch cells and subsequent analysis by high-performance liquid chromatography (HPLC) coupled to an electrospray ionization mass spectrometer (ESI/MS) and an ultraviolet/visible light (UV/Vis) detector is presented that enable a quantification of the absolute amounts of the electrolyte components in battery cells[4].
The investigated pouch cells are initially filled with 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) 1:1 by weight with 3 wt% vinylene carbonate (VC). For the electrolyte extraction, diethyl carbonate (DEC) is injected into the cell, followed by a sealing of the puncture and a storage of the cell for 8 days to enable an intermixing between DEC and the original electrolyte. Afterwards, the electrolyte is extracted through a puncture in the cell. The UV/Vis detector is used for the determination of the concentrations of EC, DMC, VC, and DEC in the extracts, while the quantification of Li+ and PF6
− is performed by the ESI/MS. Based on these concentrations and the weighed mass of the injected DEC, the total mass of the original electrolyte and the absolute amounts of EC, DMC, VC, Li+, and PF6
− in the cell is determined. The investigated electrolytes are extracted from fresh cells, cells after formation, 25 cycles, and about 2000 cycles (with a remaining capacity of 80%). Based on the absolute amounts of EC, DMC, VC, Li+, and PF6
− in the cells at different aging stages, the absolute consumption of the electrolyte components by electrochemical decomposition is eventually calculated.
From the concentrations shown in Figure (a), only a VC decomposition during formation and cycling can be concluded. The consideration of the absolute, consumed amounts of substance during formation and long-term cycling in Figure (b), however, suggests that the EC decomposition is dominant and thus much more pronounced than the VC degradation. Besides, a slight salt and DMC decomposition during long-term cycling is observed. Simultaneously, the quantified electrolyte mass is diminished during formation and cycling, as can be seen in Figure (c).
The pronounced VC consumption during formation and cycling is expected, as VC is an additive for the SEI buildup. As a high fraction of the initially present VC is decomposed, its decomposition can be identified by the concentrations. The decomposition of the other components cannot be detected based on the concentrations, as lower fractions of the initially present amounts are consumed and the total electrolyte amount in the cell decreases during formation and cycling. The strong EC degradation might be related to the tendency of EC to contribute to the Li+ solvation shell, which is associated with a preferential reduction[5]. These results demonstrate that only the determination of the absolute consumed amounts of substance allows for identification of the decompositions in battery electrolytes.

Figure: (a) Overview of the average concentrations of EC, DMC, and VC (quantified by HPLC-UV/Vis), as well as the average concentrations of Li+ and PF6
− (quantified by HPLC-ESI/MS) in cells at different stages of electrochemical aging[4]. (b) The amounts of substance consumed during formation and prolonged cycling altogether determined by HPLC analyses[4]. (c) The electrolyte masses in the fresh cells, the cells after formation, and the cells after prolonged cycling determined by HPLC analyses related to the corresponding initial electrolyte masses[4].

References

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[2] D. Aurbach, Y. Ein-Eli, B. Markovsky, and A. Zaban, J. Electrochem. Soc., 142, 2882 (1995).
[3] H. Ota, Y. Sakata, A. Inoue, and S. Yamaguchi, J. Electrochem. Soc., 151, A1659 (2004).
[4] R. Stockhausen, A. Hofmann, L. Gehrlein, T. Bergfeldt, M. Müller, H. Ehrenberg, and A. Smith, J. Electrochem. Soc., 168, 080504 (2021).
[5] S. A. Delp, O. Borodin, M. Olguin, C. G. Eisner, J. L. Allen, and T. R. Jow, Electrochim. Acta, 209, 498 (2016).

Figure 1