Tracing the Powerfade: Location and Quantification of the Fluoridic Solid Electrolyte Interphase on Graphite Anodes
Authors: Furtmair Michael , Wolters Anika , Simic Sanja , Thannhuber Markus , Ruhl Günther , Sternad Michael 
Published: September 2023

Due to the conversion of the global energy supply from carbon emitting to renewable sources, there is the need for energy storage systems being efficient at high energy- and power densities. Many requirements are fulfilled best by Lithium-Ion Batteries (LIBs), so this technology is successfully part of battery-electric vehicles, cordless power tools and portable electronic devices. The ongoing improvement of anode- and cathode materials, together with adapted electrolytes [1] led to excellent advances in power- and energy densities. However, the increasing amount of total energy per cell underlines the importance of monitoring the cell’s health parameters (e.g. capacity fades, coulombic efficiencies, AC-and DC-impedances) in order to ensure safety and expand the lifetime [2, 3]. The Solid Electrolyte Interphase (SEI) represents a main source for cell impedances [4] and was in the case of LiPF6-based electrolytes found to be best analytically accessible by the quantitative determination of its unsolvable, fluoridic fractions (LiF) using ion-exchange chromatography [5].

During this work, long term cycling tests (T = 25, 40, 60 °C, 2 C charging) of industrial 3 Ah-21700-cells with graphite anode (figure a), NMC 811 cathode and LiPF6-based electrolyte were performed (figure b). Subsequently, after defined cycling steps (10, 250, 500, 750 cycles) the cells were opened, post-mortem analysis (SEM, EDX, “broad ion beam”- (BIB) preparation) together with the drawing of anode/separators samples for the later fluoride determination took place. The samples were cleaned from LiPF6 (washing with DEC) and dried under glovebox conditions (Ar, < 0.5 ppm H2O). With the aim to quantify the SEI layer 1) in the anode bulk and 2) adhering to or inside the separator to trace reversible Li-plating, anode/separators samples were eluted separately with deionized water and the fluoride-concentration of the solutions was determined by ion-exchange chromatography (figure c - f). SEI fractions, like inorganic carbonates, alkyl carbonates, oxides, etc., were investigated using X-ray photoelectron spectroscopy (XPS) along a depth profile (figure g) using Ar-ion sputtering [6, 7].

The results elaborated within this study point out a strong correlation in between the amount of fluoridic SEI and the DC-impedance rise during cycling, especially at elevated temperatures (40, 60 °C). At 25 °C different ageing mechanisms are obvious: In comparison lower fluoride concentrations together with substantial DC impedance gains suggest the temporary and superficial occurrence of reversible Li-plating, blocking important Li-ion paths via the electrolyte (e.g. anode surface pores, separator pores [8, 9]) by residual products of their SEI-films.

Reference
s

[1] Eshetu, G. G.; Zhang, H.; Judez, X.; Adenusi, H.; Armand, M.; Passerini, S.; Figgemeier, E., Production of high-energy Li-ion batteries comprising silicon-containing anodes and insertion-type cathodes. Nat Commun
2021, 12, (1), 5459.

[2] Sternad, M.; Cifrain, M.; Watzenig, D.; Brasseur, G.; Winter, M., Condition monitoring of Lithium-Ion Batteries for electric and hybrid electric vehicles. e & i Elektrotechnik und Informationstechnik
2009, 126, (5), 186-193.

[3] Furtmair, M.; Wolters, A.; Kühnel, F.; Thannhuber, M.; Sötz, V.; Sternad, M., The Impact of Fast-Charging on Cell Ageing of Industrial High-Power Lithium-Ion Batteries. In IMLB 2022, Sydney, Australia, 2022.

[4] Aurbach, D.; Markovsky, B.; Rodkin, A.; Cojocaru, M.; Levi, E.; Kim, H.-J., An analysis of rechargeable lithium-ion batteries after prolonged cycling. Electrochimica Acta
2002, 47, (12), 1899-1911.

[5] Uitz, M.; Sternad, M.; Breuer, S.; Täubert, C.; Traußnig, T.; Hennige, V.; Hanzu, I.; Wilkening, M., Aging of Tesla's 18650 Lithium-Ion Cells: Correlating Solid-Electrolyte-Interphase Evolution with Fading in Capacity and Power. Journal of The Electrochemical Society
2017, 164, (14), A3503-A3510.

[6] Shutthanandan, V.; Nandasiri, M.; Zheng, J.; Engelhard, M. H.; Xu, W.; Thevuthasan, S.; Murugesan, V., Applications of XPS in the characterization of Battery materials. Journal of Electron Spectroscopy and Related Phenomena
2019, 231, 2-10.

[7] Zhu, Y.; Pande, V.; Li, L.; Wen, B.; Pan, M. S.; Wang, D.; Ma, Z. F.; Viswanathan, V.; Chiang, Y. M., Design principles for self-forming interfaces enabling stable lithium-metal anodes. Proc Natl Acad Sci U S A
2020, 117, (44), 27195-27203.

[8] Lagadec, M. F.; Ebner, M.; Zahn, R.; Wood, V., Communication—Technique for Visualization and Quantification of Lithium-Ion Battery Separator Microstructure. Journal of The Electrochemical Society
2016, 163, (6), A992-A994.

[9] Zier, M.; Scheiba, F.; Oswald, S.; Thomas, J.; Goers, D.; Scherer, T.; Klose, M.; Ehrenberg, H.; Eckert, J., Lithium dendrite and solid electrolyte interphase investigation using OsO4. Journal of Power Sources
2014, 266, 198-207.

Acknowledgment

The authors wish to thank H. Schröttner (Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology) for preferential access to preparation and examination equipment.

Figure 1