Tracing the Powerfade: Location and Quantification of the Fluoridic Solid Electrolyte Interphase on Graphite Anodes

Abstract

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. 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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