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36 S S: 71% O: 18% C: 11% CO Al 1200 800 400 (a) Commercial Li2S 300 200 100 800 1200 800 400 0 2000 2000 1500 1500 1000 1000 500 500 00 0.0 1.0 2.0 3.0 0.0 1.0 X-ray energy / keV 400 000 0.0 1.0 2.0 3.0 X-ray energy / keV 0.0 1.0 2.0 3.0 X-ray energy / keV (b) Carbon black 0.0 1.0 2.0 3.0 X-ray energy / keV (c) Carbon nanotubes Figure 3.2: EDX spectra of the raw materials of the positive (sulfur) electrode. All spec- tra are recorded at 15 keV incident electron beam energy. Labels indicate the position of the Kα1 line of each detected element and atom percentages for prominent elements. 0.0 1.0 X-ray energy / keV 2.0 3.0 2.0 3.0 (a) after ball-milling X-ray energy / keV (b) carbon-coated using C2H4 at 700 °C (c) carbon-coated using C2H2 at 400–450 °C C C: 100% O Al S S S: 64% C: 22% O: 13% C O Al Figure 3.3: EDX spectra of the active material at various steps during the preparation process. All spectra are recorded at 15 keV incident electron beam energy. Labels indicate the position of the Kα1 line of each detected element and atom percentages for prominent elements. From the recorded EDX spectra, presented in Fig. 3.2, the composition of the sam- ples can be calculated. Note that H and Li are not detected by EDX, cf. section 2.4.1. Judging from the spectra, the carbon components are indeed very pure. Small amounts of oxygen may have been introduced during the installation of the samples in the mi- croscope, e.g. in the form of adsorbed trace water. For the Li2S, however, the situation is different. A pure Li2S sample should ideally show up as 100 % sulfur. The additional carbon content might be caused by stray carbon black particles, which are easily spilled across the sample holder during the evacuation step. Also, the carbon tape used to affix the powder to the aluminum sample holder may be detected by EDX. There is no similar explanation for the high amount of oxygen in the specimen, though. The EDX signature of oxygen is relatively small so that the composition is only accurate to plus/minus several percent. Nevertheless, it has to be assumed that the Li2S particles have been partly converted to LiOH and LiO2 at least at their surfaces. Unfortunately, C C: 97% O: 3% O Al S S S: 61% C: 29% O: 10% C O Al S S: 71% O: 16% C: 13% C O Al Counts Counts Counts Counts Counts CountsPDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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