Dilatometric Study of the Electrochemical Intercalation of Bis(trifluoromethanesulfonyl) imide and Hexafluorophosphate Anions into Carbon-Based Positive Electrodes

“Dual-ion cells” are electrochemical energy storage devices that employ graphitic carbons as positive electrode material. The electrolyte in this cell system does not only behave as a charge carrier but also as active material.[1] The negative electrode in the dual-ion system can be built up by typical anode materials for lithium ion cells such as lithium metal, lithium titanate (Li₄Ti₅O₁₂, LTO) or graphite.[2] During charging, the anions of the electrolyte are intercalated into the layered structure of the graphite positive electrode, while the lithium ions of the electrolyte deposit on the surface of the metallic lithium negative electrode. The gallery height expansion for graphite positive electrodes by bis(trifluoromethanesulfonyl) imide (TFSI^-) intercalation is around 140%[3] and 130% if hexafluorophosphate (PF6^-) is intercalated.[4] On discharge, both ions are stripped/de-intercalated and solvated again in the electrolyte. Furthermore several carbon materials like graphite, carbon black or carbon nanofibers are applied as conductive additives in high voltage cathodes. At potentials above 4.5 V vs. Li/Li⁺, major problems are related to electrolyte degradation and to anion and solvent co-intercalation. Co-intercalation can lead to the destruction of the carbon structure (exfoliation) and therefore to continuous electrolyte degradation.[5,6] The method of electrochemical dilatometry has been used since the 1970s to investigate the height change of layered host compounds when a guest molecule or ion is inserted.[7] The Li⁺ intercalation into different types of graphite or the BF4^-intercalation into carbons are two examples that were already investigated using the electrochemical dilatometer (ECD-1), which was developed by Hahn et al.[8,9] The complete electrode height change or the electrode height change per cycle can be calculated. An illustration of the ECD-1 cell core is presented in Figure 1.

In this work, we report the intercalation of TFSI^- and PF₆^- anions into graphitic and carbon black-based positive electrodes from an ionic liquid-based and organic solvent-based electrolyte using LiTFSI and LiPF₆ as the conductive salts.

Furthermore, we investigated the influence of two electrode binders, namely sodium-carboxymethyl cellulose (CMC) and poly(vinylidene diflouride) (PVdF) on the electrochemical intercalation/de-intercalation behavior of the electrode. Especially, the influence of the electrode binder on the electrode expansion behavior and the electrode material stability are discussed. Additionally, the influence of a constant voltage step at the upper cut-off potential and the influence of a higher operation temperature of 60 °C on the electrode expansion are discussed.

Figure 1: Illustration of the in-situelectrochemical dilatometer cell (ECD-1 (EL-Cell GmbH)). 1: Metal membrane; 2: Working electrode (WE); 3: Glass T-frit; 4: Metallic lithium (CE); 5: CE plunger; 6: Sensor tip; 7: Spacer disc; 8: RE with metallic lithium on top.

References

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[2] S. Rothermel, P. Meister, G. Schmuelling, O. Fromm, H.W. Meyer, S. Nowak, M. Winter, T. Placke, Energy & Environmental Science, 7 (2014) 3412-3423.

[3] G. Schmuelling, T. Placke, R. Kloepsch, O. Fromm, H.-W. Meyer, S. Passerini, M. Winter, J. Power Sources, (2013).

[4] J. A. Read, J. Phys. Chem. C, DOI: 10.1021/jp5115465.

[5] X. Qi , B. Blizanac , A. DuPasquier, T. Placke, P. Meister, M. Oljaca, J. Li, M. Winter, Phys.Chem.Chem.Phys.,16 (2014), 25306.

[6] O. Fromm,P. Meister P, X. Qi, S. Rothermel, J. Huesker, H. Meyer, M. Winter, T. Placke, ECS Transactions, 58 (14) 55-65 (2014).

[7] A. Metrot, P. Willmann, A. Herold, Mater. Sci. Eng., 31 (1977) 83-86.

[8] M. Hahn, O. Barbieri, R. Gallay, R. Kotz, Carbon, 44 (2006) 2523-2533.

[9] M. Hahn, H. Buqa, P.W. Ruch, D. Goers, M.E. Spahr, J. Ufheil, P. Novak, R. Kotz, Electrochem. Solid St., 11 (2008) A151-A154.