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Galvanostatic intermittent titration technique (GITT, conducted on VMP3 Bio-Logic®) is a measurement where a series of constant current cycling steps are interspersed with a relaxation time, during which no current passes through the cell and the potential quickly increases/decreases to reach equilibrium potential. Thanks to this technique, information about thermodynamic behavior of the active material present in the positive electrode can be obtained. Constant current (i = 98 μA cm-2; C/20 rate) was applied for 20 min, followed by a relaxation step of 30 min.§ It is important to notice that sensitivity of Li2S powder to air moisture strongly limited the number of techniques that could be applied for evaluation, or required some modifications of standard procedures. Scanning Electron Microscopy (SEM; LEO 1530, Gemini) was used for powder/electrode morphology observation (at accelerating voltage of 5 kV. X-Ray Diffraction (XRD) measurements were performed on D8 Bruker Advanced with CuKα (λ = 1.5406 Å). In case of both techniques, special precautions were taken into consideration while preparing air-sensitive samples. The detailed procedure was described previously (refer to 2.2.3). 4.2.2. Morphology of the electrodes and structural characterization Figure 4-2 shows the SEM photos of as-received Li2S powder (Alfa Aesar) grinded with SuperP®. As-received Li2S purchased from Sigma Aldrich and grinded with carbon gave very similar global view. The appearance of final Li2S electrode resemble to the Li2S/SuperP® SEM photo, and did not differ depending on the powder batch either. The size of Li2S particles is ranging from 5 μm to 20 μm. It can clearly be noticed that the Li2S particles are not uniformly covered by nano-carbon spheres, as it was seen previously for sulfur particles (see Figure 3-4b). Such difference may result from different hardness of both materials. Elemental sulfur is very soft (1.5 – 2 in the Mohs scale238) and basic grinding in the mortar provided complete enrobing with SuperP®. On the contrary, Li2S particles are much harder and probably mechanical grinding is not sufficient to provide intimate contact between Li2S and carbon. A visual indication of that difference could also be seen from the colors of the mixed powders: S/SuperP® totally turned black after grinding, while Li2S/SuperP® was rather grayish after similar grinding. § Such short relaxation time was selected in order not to provoke eventual shuttle phenomenon. However, it was sufficient to be near the stable potential. Chapter 4: Li2S electrode 113PDF Image | Accumulateur Lithium Soufre
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