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A three-electrode system, with a reliable reference electrode would be an ideal solution to observe each electrode separately. Nevertheless, this approach cannot be really applied to the Li/S cells, since until now, any stable reference electrode in the presence of soluble lithium polysulfides has been proposed. Kolosnitsyn et al.213 have performed three-electrode cell using metallic lithium as a reference electrode, however, it was rather a supportive approach, while most of the interpretation was done on a two-electrode cell configuration. We did not apply three-electrode configuration in our studies, in order not to question further the reliability of the obtained results. Thus, classical two-electrode coin cell configuration was chosen, with supporting information obtained from the symmetric coin cells (the concept has been already introduced in section 2.3.3), where both electrodes are either metallic Li or ‘S- on-NwC’ cathodes. For some of the symmetric cells, electrolyte was replaced by the ‘catholyte’ solution (0.25 M Li2S6 dissolved in classical electrolyte). It is also important to notice that EIS is a very sensitive technique, and a response of ‘bad’ cables/connections or sample holder may significantly modify the HF resistance value, thus more precautions should be taken in order to improve the measurement precision. Also it is necessary to provide sufficient contact between all the cell components, to minimize the contact resistances-related issues. A good example was evidenced through the careful selection of the stainless steel spacer used during coin cell fabrication (available as 0.5 mm and 1.0 mm thick), depending on the global electrodes thicknesses used in the cell. 6.2.2. EIS measurements Electrochemical Impedance Spectroscopy tests were performed on Bio-Logic® VMP3 multichannel potentiostat, equipped with impedance modules. Each spectrum was recorded in the frequency range of 1 MHz to 10 mHz, with an excitation potential of 10 mV and 13 points per decade. Obtained results were then fitted using ZView software (Scribner Associates Inc.). Before observing the Li/S cell upon cycling, the first step was to determine the impedance response of the system at the starting point (i.e. at OCV), and to assign the origin of the different components on EIS spectra. To this purpose, symmetric coin cell approach appeared to be a very useful tool. In situ EIS was recorded on the Li/S two-electrode coin cells galvanostatically cycled at C/20 rate, in the potential window of 3.0 V – 1.5 V. The EIS spectra were registered every 1 h, after 15 min of relaxation applied prior to the measurement. This period was found as an optimal/compromise value, enough for reaching rather stable voltage (pseudo-equilibrium) and not too long to avoid polysulfide composition changes, self-discharge, disproportionation, etc. 192 Chapter 6: EIS and low temperature studiesPDF Image | Accumulateur Lithium Soufre
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