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Raman), chemical tests, and the entire spectrum of electrochemical tests, including galvanostatic and potentiostatic cycling, cyclic voltammetry, impedance spectroscopy, and combinations thereof. Finally, other methods or materials that did not work well were briefly discussed in section 2.5. The experimental results were presented in chapter 3, which roughly followed the chronology of this work: First, the characterization of the active material and elec- trodes made thereof was presented in section 3.1. The effectivity of different coating protocols was evaluated by SEM imaging and EDX spectroscopy. The best results were obtained using acetylene as carbon precursor, deposited at 400–450 °C. Both Raman spectra and TEM images confirm that a regular, uniform carbon coating was applied to the particles. Next, SEM images and EDX spectra of several different electrodes were compared to optimize composition, fabrication, and handling. Despite numerous tweaks and improvements to the active material, the electrolyte, and the electrode preparation, Li2S was not contained in the electrode as intended. This could be shown by a chemical test as well as a run in a transparent beaker cell, where the leakage of polysulfides into the electrolyte was evident. Nevertheless, cells prepared from the novel material exhibit a promising behavior as analyzed in section 3.2. To exploit their full potential, the activation (a.k.a. formation) needed to be adjusted, as discussed in section 3.2.3. We could show that almost all the capacity can be activated at voltages as low as 3.4 V. While an estimation of SoC and SoH based on impedance spectra was intended, the reproducibility of the results did not allow for an authoritative analysis. Next, the shape of the charge/discharge profiles was analyzed and found to be generally very similar to that of “regular” Li/S cells with sulfur/carbon electrodes, as analyzed in section 3.2.5. Finally, cycling results were presented for various cells and conditions, demonstrating a slow, but steady capacity decay. While the rate of decay, as well as the initial discharge capacity of slightly above 1000 Ah · kg−1 , leave room for improvement, the remaining capacity sulfur of almost 500 Ah · kg−1 after 200 cycles was higher than any value published for Li2S sulfur based electrodes at that time. The modeling part started with a general recapitulation of the means and purpose of modeling in chapter 4, before the methodology of our physically-based continuum model was presented. The phenomena described by the model and underlying as- sumptions were discussed along with the equations used in the model. These include the transport in the liquid electrolyte, the electrochemical and electrical description of the cell as well as the representation of different phases and their microstructures in the one-dimensional model. Next, two different reaction mechanisms were intro- duced: the simple two-step global model and the more detailed multi-step model. Both comprise a set of electrochemical reactions and corresponding expressions for 138PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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