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mechanism related to the low voltage plateau. First, the reduction of S42- into Li2S occurs with efficiency close to 1, whereas for higher DOD%, a competitive reaction appears, which is very likely to be the formation of Li2S2 phase. During charge, disappearance of Li2S seems to also occur in a two-step mechanism, with higher efficiency at the beginning of the charge process, while the complete disappearance of Li2S was obtained at 75% of DOD. A large hysteresis between discharge (Li2S formation) and charge (Li2S consumption) was also noticed, proving the different reactions pathways taking place. At the end of charge, soluble polysulfides are oxidized into solid S8. It was found that sulfur after recrystallization does not come back to its pristine structure, but it appears as another allotrope: monoclinic β-sulfur. During oxidation of soluble polysulfides, crystallization of solid sulfur may be more favourable and easier in a less ordered form of sulfur (β-S8) as compared with the thermodynamically stable and ordered α- S8 phase, initially present in the electrode. Very similar behaviour was observed during further cycles. These evolutions were also confirmed at higher C-rate (C/8). In addition to XRD investigation, Electrochemical Impedance Spectroscopy was applied to the Li/S system for further understanding of discharge/charge mechanisms. EIS studies gave a large overview on the reactions occurring upon cycling in a Li/S cell. To our best knowledge, it was the first time such completed analyses were performed, together with deep interpretation of obtained results at large range of characteristic times. In order to correctly attribute the response of Nyquist plot obtained through EIS measurements on two-electrode cells, the use of symmetric coin cells approach was a helpful and indispensable. EIS results were found to be in a good agreement with the observations obtained through in situ XRD measurements. We have shown that during the initial cycle, metallic lithium anode significantly contributes to the initial OCV impedance response of Li/S cell, mainly through its passivation layer. In addition, in a complete Li/S cell, only the LF response is related to the sulfur electrode, and is visible as a vertical line describing blocking character at OCV. This capacitive character is disappearing quite quickly during the cell storage, accompanied by a potential decrease, mainly lying on the easy partial sulfur dissolution in the electrolyte, and further polysulfides formations (thus inducing self-discharge process). Several tests performed (in situ EIS upon cycling recorded on classical two-electrodes cells, supported by symmetric coin cells at different DOD% during discharge) permitted to propose a simple equivalent circuit preserved all along the cycling. The circuit includes the electrolyte resistance response (at HF region), followed by the three R/CPE elements connected in series between HF → MF → LF regions, associated to (i) the positive electrode bulk contribution, (ii) the passivating film formed at lithium/electrolyte interphase and (iii) the charge transfer reaction of polysulfides at the positive electrode surface, respectively. The LF response was related to the diffusion processes in the overall cell. It was found that the resistance of the electrolyte is strongly dependent on the polysulfides composition present in the electrolyte. The maximum resistance values were found for the moments, where the highest concentration of soluble species is obtained, i.e. beginning of solid 231 Conclusions & PerspectivesPDF Image | Accumulateur Lithium Soufre
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