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3.1.3 Transmission electron microscopy To confirm that a core-shell structure is actually formed when coating the Li2S parti- cles, transmission electron microscopy (TEM) is used. The resulting micrographs were provided by Xiangyun Song1 and can be found in Fig. 3.9. The images suggest that the Li2S core particles are mostly crystalline (pattern on the right hand side of Fig. 3.9) and coated by a thin and rather uniform carbon layer. Based on the TEM images, its thickness is 16–29 nm. The material used for these images has an average particle size of 1.1 ± 0.3 μm (determined by measuring the diameter of numerous particles in a low magnification SEM micrograph) and contains 1.9 ± 0.9 wt.% of carbon coating (deter- mined by weighing during the CVD process). Choosing mcarbon : mtotal = 1.9 % and rcore = 1.1 μm in Eq. (2.1), the coating’s thickness is calculated to be 370 ± 140 nm. This tremendous discrepancy can be explained by two effects: either the average particle size is considerably smaller than determined or carbon is trapped in the agglomerates. To match the results, however, the particles need to be as small as 100 nm or > 80 % of the carbon needs to be unavailable for coating, both of which seems unlikely. Another possible but equally unlikely explanation is that the carbon is preferentially deposited on the CVD boat instead of the Li2S particles, forming carbon black. Since TEM mi- crographs are the most direct way to examine the coating, the figure extracted from these images is considered to be more accurate than the calculations based on weight and geometry. x 100 nm 5 nm Figure 3.9: TEM images of carbon-coated Li2S particles. Left: core-shell structures, 250 000×; right: close-up of the Li2S core, 1 000 000× (at the position marked by “x”). The arrows indicate the carbon coating. 1Senior technician, Environmental Energy Technologies Division, Lawrence Berkeley National Labora- tory, 1 Cyclotron Road, Berkeley, CA 94720, USA 44PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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