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Figure 2.7: A transparent beaker cell for optically investigating polysulfide formation and transport. important information on the cell, this data can be used to identify faulty cells and to estimate the uniformity of a batch of electrodes. Impedance spectra are recorded using a 10 mV sinusoidal excitation around OCV in the frequency range of 1.0 MHz to 50 mHz with a VSP potentiostat (BioLogic, Claix, France). For some cells, impedance was also recorded during stabilization or intermittently during cycling. For the latter, the discharge was stopped 15–60 s before the EIS recording so that the cell voltage can relax (almost) to OCV. The experimental results are plotted in Nyquist representation [132, chap. 16.1]. For all tests except EIS, cells are stored in controlled temperature chambers (TEC1; TestEquity, Moorpark, CA, USA) at 30 °C in order to minimize the effect of circadian, seasonal, or weather-dependent temperature changes. Cyclic voltammetry. In order to study electrolyte stability and to evaluate differ- ent charging procedures, slow-sweep cyclic voltammetry (CV) was employed, using a VMP3 potentiostat (BioLogic, Claix, France). “Slow” in this context means that the voltage is changed quasistatically, i.e. the rate at which the voltage changes is slow compared to the time it takes for the current to stabilize at the new voltage. Practi- cally, it means that the SoC of the cell is following the voltage so that one CV cycle corresponds to one full charge/discharge cycle. In the cyclic voltammetry (CV) plots, the current (i.e. charge passed per time) is plotted vs. the externally applied voltage. Because of the slow and steady voltage ramp, the current is approximately propor- tional to the total charge which would be passed if the cell was held at that voltage indefinitely. A typical CV experiment, utilizing a voltage ramp in the range of 10– 25 μV/s, runs for several days. 31PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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