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(a) (b) Figure 5-10. Evolution of crystallite size (calculated with Scherrer equation, applied to the most intense Li2S reflection (111)), superimposed with peak area evolution and voltage profile. 5.4.3. Ex situ XRD – supporting data for analysis A specific methodology was applied in order to be able to quantify the real amount of Li2S being formed at the end of discharge. The main idea was as follow: since we know the amount of sulfur in the electrode at the initial state (i.e. 27 mg), we could theoretically estimate the amount of Li2S that would be formed at the end of discharge, assuming a complete reaction of S8 to Li2S, where 16 electrons are exchanged. Knowing the amount of Li2S formed experimentally during the second plateau, thanks to the use of the Li2S peak areas at each state of discharge, we are able to compare the practical capacity of this low voltage discharge plateau with the effective amount of Li2S formed. In turn, we were able to determine the efficiency of Li2S formation versus the effective charges exchanged. To perform this quantitative approach, we tried to correlate the Li2S peak area with the quantity of Li2S formed (in moles), and with the quantity/peak area of sulfur material initially present in the electrode. To this purpose, three different powder batches having different mass ratios of S8/Li2S: 2/1, 1/2 and 1/1, were prepared. In order to prepare the S8/Li2S mixtures at desired mass ratios, and to avoid direct mixing of both powders at different oxidation states, in the first step S8 and Li2S powders were grinded with SuperP® (with little amount of cyclohexane), to provide kind of immediate protection of Li2S and S8 from each other. Such obtained mixtures of S8/SuperP® and Li2S/SuperP®, once dried, where then mixed together in a beaker, and the same procedure was applied for three different S8/Li2S ratios. XRD was recorded on three samples obtained for quantitative purposes. All procedure, starting from powders mixing Chapter 5: In situ and operando XRD 173PDF Image | Accumulateur Lithium Soufre
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