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cycle, both the total amount and the composition of the ions vary to a great extent. Hence, the electrochemical response of the cell is expected to behave differently at different SoC. This will be further discussed in the following section. 5.2.4 Impedance The results of impedance simulations for different SoC are shown in Fig. 5.9. The upper two panels show a Bode representation, i.e. the real and imaginary part of the cell’s impedance Z versus logarithmic frequency. The lower panel shows the same information in Nyquist representation, i.e. imaginary part versus real part. The cell shows a complicated impedance behavior, which strongly depends on the SoC. At high SoC, a feature at ∼ 0.5 Hz is dominating, with an additional smaller feature at ∼ 10 Hz. At increasing SoC, these features merge at an intermediate frequency. Additionally, impedance strongly increases toward low frequencies < 10 mHz, which is typical of batteries. 5.2.5 Conclusions From the above results, reproduced from Ref. [P2], the following conclusions can be drawn: First of all, the results of the multi-step model are much more realistic and detailed in terms of voltage, current, and dissolved species concentrations compared to the two-step model. Also, the more detailed description enables the simulation of representative impedance spectra – with regard to Li/S batteries, this is a unique feature as of today. Despite the slightly different reaction mechanism, the results are otherwise similar to previous simulations by Kumaresan et al. [196]: For the operat- ing conditions simulated, the “behavior of the Li/S cell is governed by the presence of solid reactant and product phases. The volume fractions of S8 and Li2S in the cathode change considerably during cycling, providing an explanation of the distinct stages during discharge. The model also predicts an asymmetric behavior of phase formation/dissolution when comparing discharge and charge, as well as high charge overpotentials”1. 1cited verbatim from Ref. [P2], p. 187f 102PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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