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eliminate metallic Li. Therefore, symmetric coin cells composed of two identical§ sulfur electrodes were used, and an example of a Nyquist plot is shown on Figure 2-11. (a) (b) Figure 2-11. Nyquist plot of the symmetric coin cell composed of two identical sulfur positive electrodes (S/SuperP®/PVdF), with loadings of ~ 7.0 mgsulfur cm-2 and electrode coating thickness of ~ 130 μm (a). Corresponding equivalent circuit (b). Three main regions, with different time constants, can clearly be distinguished (and are separated by vertical blue lines for easier visualization): high, medium and low frequency regions. A very narrow fourth region (named as ‘high to medium’ frequency region) is also marked, since a small semicircle can be visible with a characteristic frequency in the range of 150-300 kHz. High frequency (HF) region refers to the electrolyte resistance, coin cell casing, sample holder, and cable connections responses225,226, and can be modeled as a pure resistor (R). The response at the low frequency region (LF) of a porous electrode corresponds to a restricted Warburg Impedance, associated with semi-infinite diffusion, due to the blocking character of the electrode, i.e. impossible intercalation of lithium. Such phenomenon has been previously shown by Levi and Aurbach, and can be directly correlated with the electrode thickness228. However, CPE is also proposed for modelling this part of the spectra212, when the diffusion part and the blocking one are not well-separated in frequency. The interpretation of the middle frequency (MF) region and semicircle(s) present is more controversial, and in the literature generally associated to the charge transfer reaction207,212. However, since the electrode presents a blocking character (visible as a vertical line in the low frequency region, due to the insulating character of sulfur active material), such interpretation can be questionable. Further experiments were performed in order to remove this ambiguity, and are present in the following sections. § Electrodes were carefully selected based on the weight and the thickness of each Ø 14 mm disk, to be as similar as possible. Chapter 2: S8 electrode on Aluminum 59PDF Image | Accumulateur Lithium Soufre
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