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When looking at Fig. 3.4, it also becomes obvious that treating the material at el- evated temperatures significantly increases agglomeration. The samples shown in Figs. 3.4b and 3.4c are prepared from the same batch of ball-milled Li2S, coated with roughly the same amount of carbon. Because of different carbon precursors (Fig. 3.4b: ethylene, Fig. 3.4c: acetylene), the CVD process was run at different temperatures. At 700 °C, the secondary particle size is increased almost tenfold compared to 400–450 °C. When using methane, which decomposes at around 900 °C, the agglomeration is even worse (data not shown). EDX confirms that neither the oxygen nor the carbon content is increased during the ball-milling step, compare Figs. 3.2 and 3.3. As expected, an increased carbon content is detected after the CVD coating. In principle this increase could be used to calculate the amount of carbon deposited and thereby the thickness of the coating according to Fig. 2.4. However, measurements of various coating runs report 3–16 % increase in carbon content, yielding a carbon coating with an average thickness of 200–900 nm, depending on the particle size distribution. This result is way beyond the intended amount of coating and does not agree with the visual impression of the SEM images. The reason for this discrepancy is the limited penetration depth of the incident electron beam [129, chap. 6.3]. Electrons are preferentially scattered in the top layers of the particles, which of course inflates the share of the carbon detected, since most of the bulk Li2S does not contribute to the EDX signal. 3.1.2 Scanning electron microscopy of electrodes Besides the handling of the active material, the optimization of the electrode prepa- ration was the processing step which needed the most tweaking and tuning. In the following section, SEM micrographs of sulfur/carbon electrodes are presented, illus- trating problems encountered and solutions found. The newly developed active material requires optimized handling, processing, and examination. In particular, the mixing and doctor blading of the electrode slurry needed adjustments in order to achieve good results. Because of the susceptibility of the material and the small batch size, no automatic coating machine is used, but all electrodes are doctor bladed by hand. In this context, SEM images are used for quality control and process optimization. First, electrodes prepared from differently mixed slurries are compared in Fig. 3.5. An electrode prepared according to the recipe published in Ref. [77] is depicted in Fig. 3.5a. This electrode is prepared from a very thin, low-viscosity slurry. Even though it looks nicely solid and uniform, the load- ing of 0.1–0.5 mg/cm2 is considerably lower than the target range of 1.5–2.5 mg/cm2. While alternating steps of coating and drying could build up a thicker electrode, this procedure is associated with various problems ranging from poor mechanical stability 39PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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