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ten compared to Tab. A.5. The results clearly indicate that reversible degradation can be alleviated by the improved electrolyte to some degree. Unfortunately, the choice of known electrolytes with a decent performance is rather limited [83]. Also, one is usually not free to choose an arbitrary electrolyte, since ad- ditional requirements need to be met, which are not represented in the model, namely chemical compatibility with all materials, safety, and cost. The second option to reduce the capacity fade does not require any changes to the cell itself, but rather to the cycling protocol. Instead of simply stopping the charge at the predefined cutoff voltage, a constant voltage step is added: The voltage is fixed at the charge cutoff voltage until the current drops below 1 % of the nominal charge cur- rent. This measure, known as CCCV charging, is not only very effective in preventing reversible capacity fade, but also easy to implement and without negative side effects, except for the slightly increased charging time. Conclusions. Using the global two-step model, transport in and cycling of the Li/S cell can be studied. The spatial homogeneity was shown to decrease during cycling, cf. Figs. 5.2 and 5.3. Also, the effect of the electrolyte’s conductivity as well as CCCV vs. simple CC charging was investigated, cf. Fig. 5.5. The main conclusions are that a sig- nificant amount of porosity is needed to avoid pore clogging, and that CCCV charging is absolutely required, even at medium charge/discharge rates. This insight helped to avoid potential issues when designing the electrodes and cells for the experiments as outlined in chapter 2. In addition, the findings presented in Fig. 5.3 influenced the design of the cycling protocol in the experimental section, where a slow “refresh” cycle was added every 25 cycles, cf. pp. 62f. While the global model can illustrate some aspects of the Li/S battery without too much complexity, it cannot reproduce the behavior of the cell precisely. The major limitation of the model is that is does not describe the electrochemical processes in sufficient detail, resulting in inaccurate results for the the cell voltage in general and the discharge and charge profiles in particular. To overcome this issue, a more detailed description of the electrochemical reactions is needed as presented in the following section. 96PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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