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100 50 0 -50 -100 100 50 0 -50 -100 -150 -200 1000 800 600 400 200 0 3.5 3.0 2.5 2.0 1.5 1.0 3.5 3.0 2.5 2.0 1.5 1.0 1.5 Voltage vs. Li/Li+ / V 2 2.5 3 3.5 0 20 40 60 80 100 Time / h (a) Upsweep to 3.4 V, immediately followed by the downsweep to 1.5 V. 1000 800 600 400 200 0 1.5 Voltage vs. Li/Li+ / V 2 2.5 3 3.5 0 20 40 60 80 100 Time / h (b) Upsweep to 3.4 V, followed by a 20 h rest at constant voltage and a downsweep to 1.5 V. Note that in the left representation, the extra capacity collected during the constant voltage phase is hidden in the vertical line at 3.4 V. Figure 3.16: Two different activation protocols, analyzed by slow cyclic voltamme- try. Left column: Standard representation, i.e. current plotted vs. voltage. Right: Voltage (red) and charge capacity (black) plotted vs. time. 3.2.4 Impedance spectroscopy A typical impedance spectrum of a Li/S cell is shown in Fig. 3.17. There are several distinct features, which can be evaluated qualitatively directly from the graphical rep- resentation, see [132, chap. 18]: First, the high frequency intersect with the real axis directly correlates to charge transport in the liquid electrolyte. Second, the size of the semicircle correlates to the average charge-transfer resistance at the carbon surface. Additional overlapping or separate semicircles may be discernible, representing dif- ferent surfaces, e.g. the lithium electrode. Third, the low-frequency tail is governed by material transport and changing SoC. A straightforward phenomenological approach to interpret EIS results is the creation of equivalent circuit analogs [132, chap. 9]. While this method is suitable for describing even complex electrode morphologies, the val- 56 Current / Ah/kgS Current / Ah/kgS Capacity / Ah/kgS Capacity / Ah/kgS Cell voltage / V Cell voltage / VPDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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