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3.3.2 Possible improvements and outlook This section is not to be confused with the outlook in chapter 6.3. While in the lat- ter a look at the big picture is presented, this section elaborates on more immediate improvements to the existing experimental methods and setup. Processing. After the work for this thesis was completed in late 2013, the issue of non-uniform particle size associated with the ball-milling of Li2S was solved in Elton Cairns’ group by synthesizing particles of well-defined size and shape, see Ref. [133]. Additionally, coating uniformness could be greatly improved while simultaneously reducing the complexity of the coating process by using a rotary furnace. This way, the intermittent mixing steps could be omitted and the continuous stirring is expected to reduce particle agglomeration, too. This would enable an additional annealing step at elevated temperature after the coating is completed without causing excessive agglomeration. These advantages were already known at the time the experiments were conducted. However, no rotary furnace was available for testing. Therefore, a simple hand-operated rotary furnace tube was designed and tested in the regular tube furnace, cf. Fig. 3.27. While the properties of the coating were essentially unchanged, initial results suggest that agglomeration could be prevented to some degree even with this simple setup. Actually, on grounds of these findings, a rotary furnace was set up at LBNL re- cently, which enables a more uniform coating of the Li2S particles. As a result of these measures, the performance of the new active material is expected to improve further. 25 mm Figure 3.27: Photo of the self-made, hand-operated “rotary furnace” (left). The inner tube is fixed inside the outer quartz tube eccentrically (right). When rotat- ing the outer tube, the inner tube is turned upside down, stirring up the Li2S powder inside. Concerning the electrodes, spraying instead of doctor blading could improve both uniformness and loading [166, 167]. Finally, calendering, a standard step in the prepa- ration of electrodes, was omitted because no calendering press was available inside a glovebox, but also because a low porosity might cause problems when operating the cell at higher rates, see section 5.1. Besides reducing the porosity of the positive elec- trode and thereby the amount of electrolyte needed, calendering is known to enhance electronic conductivity and mechanical durability [168]. 66 16 mmPDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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