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In this case, the Rexp (2nd step) parameter is much larger than the theoretical one (R’ = 0.849), indicating an efficiency of Li2S formation much lower than 1. As the efficiency of Li2S formation is near 1 during the first step, we can determine the amount of Li2S formed during the first step, and in turn we can deduce the capacity (in mAh) associated to the formation of Li2S during the second step. The positive electrode is composed of 27 mg of S8, leading to a theoretical capacity of 45.2 mAh. As 16 electrons are exchanged per mole of sulfur, an exchange of one electron corresponds to 2.82 mAh. Assuming the complete reaction of S42- to form Li2S (through the following reaction: 2 S42- + 12 e- → 8 S2-), the production of Li2S should correspond to 33.9 mAh (i.e. 122.82 mAh = 33.9 mAh). During the first step of Li2S production, 4.6 mAh (refer to Figure 5-12) are exchanged experimentally, instead of 33.9 mAh theoretically calculated based on the amount of sulfur present in the electrode. Thus, only ~ 14 % of awaited/expected Li2S quantity is practically formed at this moment of discharge (at 550 mAh g-1). Using the Li2S peak area recorded at the end of discharge, i.e. at 980 mAh g-1, and the ex situ XRD data, the amount of crystalline Li2S, which is practically deposited on the electrode and detected by in situ XRD, is about 24 % (equation (19)) of the Li2S quantity expected at the end of discharge, which corresponds to 8.1 mAh in total, and only 3.5 mAh for the second step (as 4.6 mAh are exchanged during the first step of Li2S formation): /&#*#$&#-A2 30#BBh&& -A !h#$%&∗%-A2 30#B5505Kh%LM /&#* #$&# -A 2 30 (#B 550 5Kh %LM) = 864 ∗ 0.14 = 0.24 = 24 % 509 (18) Chapter 5: In situ and operando XRD During the second step of Li2S formation, the charge exchanged is equal to 11.6 mAh (refer to Figure 5-12), meaning that about 8.1 mAh are consumed for another reduction process, not related to crystalline Li2S production. If we determine the amount of charge used for the S42- /S22- reaction: 2S42- +4e- →4S22- then 11.3 mAh (42.82 mAh = 11.3 mAh) would be needed for this electrochemical process. Experimentally, 8.1 mAh are consumed by another reduction process, which represents 72 % of the theoretical charge associated with S42-/S22- reaction. The collected data show that near 24 % of S42- is involved in the formation of S2-, with only 10 % during the second step (14 % for the first step), while 72 % may be involved in the formation of S22- afterwards. If we then add the amounts of S42- consumed during the two proposed reactions, almost complete reduction of S42- is obtained (72 % + 24 % = 96 %). Therefore, when using the amount of Li2S formed and the ‘extra’ exchanged capacity, the composition (in mol%) of the Li2S/Li2S2 177PDF Image | Accumulateur Lithium Soufre
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