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6.5.2. Symmetric coin cells at different DOD Similar to the approach we performed at OCV, analogical procedure of symmetric coin cells was applied in order to separate the contribution of both electrodes to the complete Li/S cell signal. This time however, it was much more delicate, since the symmetric cells were prepared from previously cycled electrodes and stopped at different moments during discharge. Two DOD% were selected (discharged to 2.1 V and to 1.5 V), based on preliminary observation of Nyquist plot evolution. The choice of ‘2.1 V’ was governed by the fact that, at this level, the Nyquist plot does not evolve that much (spectra #3 or #4 on the Figure 6-9) as compared with the initial state. Moreover, this is the moment where the presence of the soluble polysulfides species is expected. At 1.5 V, the Nyquist plot has changed dramatically at both, MF and LF levels. Therefore, we decided to perform a symmetric coin cell on completely discharged cells as well. The procedure we applied is relatively simple, and is schematically illustrated on Figure 6-10. (a) Figure 6-10. Illustrated concept of symmetric coin cells prepared from pre-cycled half cells to desired DOD (a); details of the experiments (b). Two classical Li/S coin cells (cell A and B) with ‘S-on-NwC’ composite electrodes were discharged to the same potential of 2.1 V (identical discharge profiles were obtained) and relaxed for at least 15 min before EIS was measured. After that, coin cells were opened and sulfur electrode from coin cell A was exchanged with Li electrode from cell B (as shown on schematic illustration on Figure 6-10a). The same separators were kept. Only new coin cells components (casing, spring, spacer, etc.) were used to make these symmetric cells: S8||S8 and Li||Li. EIS was then measured on such prepared symmetric coin cells. All these steps were repeated with another set of two Li/S cells, this time fully discharged to 1.5 V. EIS measured at 2.1 V (refer as ‘experimental’ value, obtained from Li/S cell) was then compared with the ‘calculated’ one, based on the equation: ZLi/S = 1⁄2 ZS/S + 1⁄2 ZLi/Li. We expected some differences between the ‘calculated’ and the ‘experimental’ values, notably in terms of the resistance amplitude, since the symmetric cells were built by recombination of the two positive and negative electrodes. Especially, such divergences in the resistance values are Chapter 6: EIS and low temperature studies (b) 203PDF Image | Accumulateur Lithium Soufre
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