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3 Experimental work on lithium sulfide cathodes – results and discussion In this chapter, the results of the experiments and analyses outlined in chapter 2 are presented. First, the characterization of materials by means of microscopy, spec- troscopy, and chemical methods is reported and discussed in section 3.1 with a focus on the processing of the Li2S particles. Next, the impact of the core-shell structure on the cells’ performance is analyzed. Electrochemical tests of full cells are presented and discussed in section 3.2, including cycling, impedance spectroscopy, and cyclic voltammetry. A critical assessment of the results is conducted in section 3.3 and fi- nally, conclusions are drawn. 3.1 Analysis of materials and electrodes Various techniques have been employed to study the materials in the Li/S cell in order to understand and optimize the carbon coating as well as the electrode preparation steps. In the following, these results will be presented and discussed. 3.1.1 Scanning electron microscopy of materials Microscopy is one of the main analysis methods used in this work. SEM images and EDX spectra of the raw materials in the positive electrode are shown in Figs. 3.1 and 3.2, respectively. One ingredient of the electrode, the binder SBR, is missing because it is rapidly decomposed by the microscope’s electron beam, making imaging and spectroscopy impractical. Besides giving an estimate of the materials’ purity, particle size, and agglomeration state, these images are recorded for comparison with the material after treatment. Thereby, the effect of each step of the preparation may be investigated, which is of particular interest for the processing of the Li2S powder. For the pristine, commercial Li2S, the particle size is determined to be 4.0 ± 2.0 μm, with some outliers as big as 50 μm. This finding confirms that the commercial material is not suitable for use in Li/S batteries unless the particle size is significantly reduced. 34PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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