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the same result was obtained for all Li2S samples tested (from three different batches, data not shown) and there is no practical way of purifying them or even determining the precise composition of the impurities. Therefore, the material was used for the experiments nevertheless, keeping in mind that the composition of the final electrode may differ from what is intended, cf. section 3.2.2. The material is also imaged during the refinement process. Fig. 3.4a shows Li2S particles after high-energy ball milling. It becomes clear that the particle size is suc- cessfully reduced during that step. More precisely, it averages to 1.2 ± 0.8 μm. Even after ball-milling, the particle size distribution is not very uniform and outliers as large as 4 μm can be found regularly and up to 20 μm rarely. Finally, the coated material is analyzed. An immediate effect of the conductive coating is that the quality of the SEM images is visibly improved because the sample is no longer charged during imaging, cf. [129, chap. 15]. The coating smoothly conforms to the particle shapes, which are still not very regular (compared e.g. to synthesized nanoparticles [133]). Unfortunately, two types of agglomeration can also be observed: First, coated particles stick to each other, creating a cross-linked network. This kind of agglomeration seems to be ubiquitous after coating, but does most likely not pose a problem with regard to the electrochemical performance. On the contrary, it might even improve the long-range electronic conductivity of the electrode and is thus not considered harmful. Second, however, particles which stick to each other before or early during the carbon deposition are coated collectively and are thus trapped under a common “carbon veil”. This may well have a negative impact on the material’s performance because less area is exposed to the electrolyte, causing longer diffusion distances and thereby higher overpotentials in the particles. As a consequence, sulfur utilization is expected to be lower. During the optimization of the coating protocol, it was observed that this kind of agglomeration can be broken up partly during the intermittent grinding steps. Also, it can be decreased by reducing the continuous coating time. Still, even for coating times of only 30 min. per run, agglomeration was detected (data not shown). To improve the situation, a rotary furnace may be needed, as discussed in section 3.3.2. 37PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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