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slow “refresh” cycle, like the 11th cycle in Figs. 5.4 and 5.5, the material utilization can be increased to a level almost equal to a fresh cell. This refresh cycle also restores the uniformity of the sulfur distribution (data not shown). Another issue directly associated with the composition of the cell still persists, though: When fully charged, there is only very little pore space left in the electrode (blue area in Fig. 5.3). As discussed in chapter 4, the microstructure is not simulated explicitly in this work. Instead, each phase is assumed to occupy one continuous volume. This assumption is not justified anymore for very low porosities. At some point, no continuous path may exist in the electrolyte phase, long before its volume fraction is exactly zero. Because of the above assumption, this is not an issue for the simulations, but the very low porosity is likely to cause problems in a real cell. 1.0 0.8 0.6 0.4 0.2 0.0 Figure 5.3: Volume fractions vs. time. The cell is cycled at a nominal rate of C/10, but the 11th cycle was run at a lower rate of C/100. The reversible degradation caused by regular cycling can be undone by the slow “refresh” cycle. Discharge capacities for the simulation discussed above are plotted in Fig. 5.4, con- firming that the reversible capacity fade can be all but undone. Nevertheless, it is desirable to reduce this effect as much as possible, which leads to the question of how to increase the highest tolerable rate, i.e. the rate at which no significant reversible degradation occurs due to material redistribution. It turns out that there are two op- tions: The first and most obvious possibility to improve the situation is to choose a more suitable electrolyte. In Fig. 5.5, the cell simulated above is compared to another cell with a different electrolyte. In this electrolyte, the transport of dissolved sulfur (and polysulfides) is repressed. This is achieved by dividing diffusion coefficients by electrolyte lithium sulfide sulfur carbon black & binder 94 0 50 100 150 200 Time / h Volume fractionPDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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