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5.7 Conclusions 5.7.1 Achievements First of all, it is a major methodological achievement of this work to enable simulations of all relevant electrochemical techniques with just one consistent set of physically- based governing equations and one authoritative set of parameters. This is an accom- plishment completely novel to the domain of Li/S batteries; even many established chemistries lack such a capable model. Regarding the simulations, transport phenomena and charging modes of the cell were studied in section 5.1, on the basis of charge/discharge profiles and cycling plots obtained from the simplified global two-step model. Results of the more detailed multi-step model were introduced in section 5.2, with a parameter set obtained pri- marily from Ref. [196], and other pre-existing literature. The results include charge/ discharge plots, also at different rates, as well as concentration profiles of lithium ions and polysulfide species during cycling. In addition, impedance spectra of the cell at various SoC were presented. Based on the the evolution of solids in the positive electrode, the two voltage plateaus during discharge could be explained conclusively. In section 5.3, we could show that a rigorous calibration of the model in its present form is possible and meaningful, using data available from literature as well as data recorded experimentally. A highly selective determination of individual parameters could be achieved, since various key features of the charge/discharge profiles and impedance spectra are very sensitive to certain physical parameters of the model, e.g. the hysteresis of the plateau voltages, the onset and slope of the transition region be- tween the voltage plateaus, or the high-frequency intercept of the impedance spectra, among others. The simulation results obtained from the calibrated multi-step model are considered plausible and could be partly validated in section 5.4. Even though dis- crepancies remain (see below), the model enables a thorough discussion of the charge and discharge behavior, cyclic voltammetry profiles, and electrochemical impedance spectra at different stages during the discharge. We could show what the performance would look like for different formation efficiencies, i.e. different fractions of activated Li2S. In addition to reproducing experiments, many properties of the battery can be studied, which are not easily accessible to the experiment: for example, the volume changes in the positive electrode were analyzed and put in relation to the concentra- tions of dissolved polysulfides in section 5.5. The concentration of Li+ across the cell was studied at different SoC. Finally, the model can assist with the interpretation of reversible and irreversible cell degradation. New reactions or effects can be accom- modated easily in order to quickly study phenomena like the polysulfide shuttle or various degradation modes. Even though the accuracy of such studies cannot be ex- 132PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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