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5.4.1 Rate capability In Fig. 5.18, simulation results are compared to the experimental data presented in Fig. 3.23, where one cell is discharged at subsequently increasing rates. For technical reasons, the cycling protocol for the simulation uses more restrictive charge and dis- charge cutoff voltages (1.95–2.7V) than the experiment (1.5–2.8V). Especially at the end of discharge, when the remaining active surface area of the cathode approaches zero, the calculation time would otherwise increase disproportionately even though very little charge is actually transferred. Experimentally, the larger voltage window is required to access more strongly bound sulfur that would otherwise stick to e.g. functional groups on the carbon surface or other impurities [67]. Also, note that in the experimental data, some degradation does occur from cycle to cycle, which is not the case for the simulated data. Additionally, an almost quasistatic discharge is simulated at a very low rate of approx. C/1000, representing the thermodynamics of the system. 2.8 2.4 2.0 1.6 0 50 100 150 200 250 300 350 400 Capacity / Ah/kgS Figure 5.18: Simulated discharge profiles at various rates compared to experimental results. While there is no quantitative match, the simulations clearly re- produce the correct trend. TD: Thermodynamic, i.e. quasistatically slow discharge. There is no full agreement between the experimental and simulated data: the in- creasing overpotential toward the end of discharge is reflected well, as is the decreasing discharge voltage. The charge plateau is matched correctly, even though the experi- mental data exhibits a little variation. Also, the shift of the onset of the lower voltage plateau and the disappearance of the voltage overshoot right before the plateau are faithfully reproduced. For three major features, however, the agreement is rather poor: C/50 C/20 C/10 TD C/50 C/20 C/10 117 Cell voltage / V sim. ref.PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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