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may limit the kinetic of the reduction process. The decrease in Li2S efficiency could have several explanations: 1) an increase of the current density associated with a decrease in the electrode surface may change the electrochemical process, with the formation of Li2S2 as intermediate compounds, which is more soluble than Li2S; 2) a direct reaction between Li2S4 and Li2S produced by reduction, which forms more soluble Li2S2 species in accordance to the disproportionation reaction: Li2S4 + 2 Li2S → 3 Li2S2. This reaction may constantly consume Li2S which is produced during the reduction process, thus decreasing the apparent efficiency of Li2S formation. In conclusion, the final discharge product is composed of a Li2S/Li2S2 mixture, which conveniently explains the practical lower discharge capacity (980 mAh g-1), which is a little bit more than half of the theoretical one. Indeed, the missing part of Li2S2 conversion into Li2S (~ 72 %) represents a large part of the theoretical capacity (~ 72 % of 838 mAh g-1), i.e. ~ 600 mAh g-1. This part of the capacity, which seems not to be accessible based to these calculations, allows to explain the main difference between experimentally obtained capacity (980 mAh g- 1) and the theoretical one (1675 mAh g-1). To increase the practical capacity, the conversion of Li2S2 into Li2S should be enhanced, which may be currently limited by the low/non solubility and the poor electronic conductivity of these two species that rapidly passivate the positive electrode during the low voltage discharge plateau. Indeed, as previously demonstrated, post mortem SEM photos (Fig 3-5, chapter 3) of a discharged electrode on NwC collector clearly show that a very thin and dense layer of deposit (estimated to be ~ 150 – 200 nm) was formed at the surface of each fiber. However, if assuming that this layer is composed of pure Li2S, and calculating the amount of Li2S that is formed in this case (knowing the surface area of NwC carbon, density of Li2S and the thickness of this thin deposit), such obtained value is much lower than if calculated based on the electrochemical data (assuming that the entire low voltage plateau corresponds to the formation of Li2S only). Thus, it is really likely that the discharge proceeds through the formation of another intermediate compound, such as Li2S2. It is also clear from SEM photos that, apart from the thin and dense deposit, more ‘volumetric’ solid formation occurs in between the fibers. More investigation should be performed in this direction to conclude on the chemical composition of these different layers. Figure 5-14 shows analogical investigation of the initial charge process. 179 Chapter 5: In situ and operando XRDPDF Image | Accumulateur Lithium Soufre
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