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While the shape of the transition region between the upper and lower voltage plateaus during discharge does change with increasing rate, the effect is significantly overestimated by the model. A possible explanation is that the model does not in- clude any disproportionation reactions, which could greatly speed up effective reac- tion rates. Due to the parallel reaction pathways, “bottlenecks” in the linear chain of electrochemical reduction reactions can be circumvented, lowering the reaction over- potential and thereby mitigating the impact of higher discharge rates. The second discrepancy concerns the voltage profile at the end of charge, which is not even qualitatively reproduced by the simulations. Where the voltage starts to rise steeply in the experiments, hitting the charge cutoff voltage, it levels off or even decreases slightly in the simulations. While there is no plausible explanation for this behavior from a chemical point of view, this effect is technically related to the increas- ing active surface area available for the precipitation of S8, cf. Eq. (4.24). The voltage does reach the charge cutoff voltage eventually (data not shown). However, this only happens if the cell is considerably overcharged, until the electrolyte becomes depleted of polysulfides, as is the case with the multi-step model before reparametrization, compare Figs. 5.7a and 5.8. The third debatable feature is the loss of discharge capacity with increasing rate. The effect is present in the simulations, albeit greatly underestimated. This leads to the question whether the end of discharge is actually exclusively triggered by surface passivation, as suggested by the simulations. One possible explanation for this dis- crepancy would be that the mean pore size is actually smaller than suggested by the electrode’s global porosity. If there are many very small pores, clogging might play a significant role despite the large total pore volume. A pore size distribution anal- ysis could help to clarify this issue; such data is not available for the reference cells, though. 5.4.2 Cyclic voltammetry Furthermore, a cyclic voltammetry run is simulated and analyzed. The experiment presented in Fig. 3.16a is used as reference. The results are plotted in Fig. 5.19. Note that this plot follows the usual convention of assigning a positive current to the anodic sweep (charge) and a negative current to the cathodic sweep (discharge). As for the rate capability plot, there is no quantitative match. Still, the simulation reproduces the main features of the experiment qualitatively very well: The discharge sweep is separated into two peaks, the upper one smaller, the lower one larger, corresponding to the upper and lower voltage plateau during a regular galvanostatic discharge. The charge sweep is composed of only one peak. Still, there are three obvious differences: First, the total area under the curve (equal to the total charge transferred) does not match. This is easily explained, since the 118PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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