Ehrenberg, H.

Pfaffmann, L.

Scheiba, F.

KITopen

KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association Experiment Ambition • Work out a visualization method of the EASA by mans of SEM. • Determination of EASA amount in different carbon anode materials. Approach • Adopting the OsO4 staining procedure commonly used for polymer blends and biologic tissue for EASA analysis in lithium ion batteries. • Lithium shows a strong reaction with OsO4 [4] which is also expected for lithiated graphite. • Li should extracted and diffuses to the particle’s surface forming solid Li-Os-O compounds. • Li-Os-O can be visualized by SEM and therefore allows a visual inspection of the EASA. References [1]   W. Lehnert et al., Chemical Physics, 163; Issue 3, 331-337, (1992). [2]   F. Béguin et al., Journal of Physics and Chemistry of Solids, 65, 211–217,    (2004). [3]   M.E. Spahr et al., Journal of power sources, 153, 300-311, (2006). [4]   M. Zier et al., Journal of Power Sources, 266, 198-207, (2014). New approach to evaluate and visualize the EASA of carbon electrodes using OsO4 Lukas Pfaffmann1, Dr. Frieder Scheiba1, Prof. Dr. Helmut Ehrenberg1 Institute for Applied Materials  1 Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM), 76344 Eggenstein-Leopoldshafen, Germany a) b) Particle surface Lithium-Ion Li-Os-O compound Diffusion direction Graphene layer Fig. 1: Sketch of a graphite particle a) before and b) after the osmium tetroxide staining. EASA is covered with a Li-Os-O compound after staining. Lukas.Pfaffmann@kit.edu Anode EASA Porosity by SEM contrast Porosity by material density Coarse Graphite (CG) 37 % 34 % 36 % Fine Graphite (FG) 35 % 54 % 52 % Amorphous Carbon (AC) 72 % 46 % 38 % Introduction • In graphite Li can diffuse only parallel to graphene layers or through grain boundaries [1] (Fig. 1a). • Li-intercalation only at the edge sites in the graphite where graphene layers end.  → Only a fraction of the surface is active for lithium intercalation. • This area is generally addressed by the concept of the Electrochemical Active Surface Area (EASA) [2]. • The higher the relative amount of EASA in graphite electrodes, the more reactive is the graphite electrode [3].   Difficulty: EASA does not differ optically to the rest of the surface of a graphite particle.  In this work: First time visualization of the EASA by means of Scanning Electron Microscopy (SEM).  Fig. 2:  a):  SEM InLens-image of a 155 x 59 µm² wide anode cross-section of the CG sample with a SOC of 10%. b): corresponding AsB-image. c-e): show the EDX maps of the green marked region in a). Thus the bright regions in b) correspond to the Li-Os-O compound. Table 1:  Results of the three investigated anodes.  Conclusion and Outlook It was demonstrated that the reaction of lithiated graphite with OsO4 is suitable to visualize the EASA by means of electron microscopy. AsB-images could be evaluated to get a quantitative approximation of the EASA.  The reaction of OsO4 with certain Solid Electrolyte Interphase (SEI) components may also help to gain a deeper understanding of the SEI structure. Therefore, XPS analysis of OsO4 reaction products with SEI components are underway to facilitate the interpretation of how SEI layers reacted with OsO4. Experimental • Investigation of three different anodes: Coarse Graphite (CG) with 20 µm particle size, Fine Graphite (FG) with 9 µm particle size and Amorphous Carbon (AC). • Using two State Of Charge (SOC): 5% and 10 %. • After the exposure Ar-Ion-milling cross-section were prepared and analyzed by a SEM. • Angle selective Backscattering (AsB) and energy dispersive X-ray spectroscopy (EDX) confirmed that bright areas in AsB-images correspond to osmium-rich parts on the anode surface (cf. Fig. 2). • Porosity was obtained by both, ratio of grain area in processed AsB-images (grey Fig. 3) to cross-section area and ratio of  material (graphite or amorphous carbon) density in relation to the anode’s mass and volume. Results • AsB-images clearly show that Li-Os-O compound is at some parts of the anode’s surface (cf. Fig 3).    → Li can be extracted of electrodes by OsO4.    → EASA of the particles can be made visible by using the AsB detector. • To count Li-Os-O compound among the EASA, it should be thicker than 100 nm (→ 3 image pixels). Samples with an SOC of 10 % showed better results. • The EASA of a graphite electrode is one third of the grain area, whether big or small particles (cf. Table 1).   • AC shows an EASA twice as large as graphite electrodes.  • The porosity obtained by SEM image contrast coincides with the porosity obtained by density considerations. Fig. 3: a), c) and e):  AsB-image section of cross-sections. The Li-Os-O compound  appears as bright regions.    b), d) and f): Processed AsB-images. Grain-boundaries are markedby black   and EASA by red lines, respectively. The grain area is filled out  with grey.  10 µm 10 µm CG 10 µm 10 µm FG 10 µm 10 µm AC C-Kα O-Kα Os-Kα 5 µm 30 µm 30 µm 

New approach to evaluate and visualize the EASA of carbon electrodes using OsO₄

https://publikationen.bibliothek.kit.edu/220103655/3816994

New approach to evaluate and visualize the EASA of carbon electrodes using OsO₄

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