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explained by more pronounced response of the electrode/collector contact resistance, as the adhesion is getting worse. The resistance increase due to a bad electrode/current collector interface may explain the low capacity obtained at 2C (after more than 20 cycles). Better performance of CMC-based electrode at 2C can be then associated with good interface contact and high efficient conductive pathways inside the electrode. 2.3.5. Conclusions For simply prepared electrodes, we did not observe any significant modifications of the capacity values neither of the capacity retention, when using different carbons of different shapes (fibers vs. nano-spheres) and with different specific surface areas (13 – 60 – 800 m2 g- 1), while keeping the same electrode composition, i.e. S8/carbon/PVdF (80/10/10 wt%). Thus, we did not find any correlation between carbon surface area and practical discharge capacity, which may question the fact of accessible conductive surface area. With EIS technique, we tested the electrical properties of sulfur electrodes (measurements in symmetric coin cells). It was found out that the main contribution in the Nyquist plot is the bulk response of sulfur electrode, which is strongly related to the electrode’s homogeneity and preparation method. The attribution of different components of EIS spectra was performed, and the MF response was correlated with the response of the electrode morphology and efficiency of its electric network, and not with the nature of active material used. Well-dispersed and homogenous inks (made with Dispermat®) resulted in significantly decreased resistance value. It seems that the binder nature (CMC or PVdF) does not matter so much in terms of capacity and fading. However, the differences between two binders are more visible in terms of adhesion properties on post mortem electrodes. CMC-based electrodes may provide better electrode adhesion and lower resistance at OCV potential, which is not necessarily visible in capacity retention upon cycling. In other word, prediction of cycling behavior based on improved resistance of the pristine electrode is not a clue. Nevertheless, these parameters must not be neglected, especially for high rate tests. It was also demonstrated that the preparation method and the homogeneity of the ink priori electrode coating has a crucial effect on the EIS response of the bulk electrode. Last but not least, as previously mentioned, a key parameter relates to the active material loading. In particular, the rate capability as well as the initial discharge capacity is decreasing with increased loading. On the contrary, capacity retention is surprisingly improved with higher loadings, which may arise from the higher polysulfides concentration in the electrolyte upon cycling. This fact needs to be investigated deeper in the future. To go towards higher sulfur electrode loadings, the nature of the current collector was modified, and aluminum was replaced by a 3D porous carbon material. Alternative electrode morphologies were obtained and studied in details. Corresponding results are presented in the next chapter. 70 Chapter 2: S8 electrode on AluminumPDF Image | Accumulateur Lithium Soufre
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Sulfur Deposition on Carbon Nanofibers using Supercritical CO2 Sulfur Deposition on Carbon Nanofibers using Supercritical CO2. Gamma sulfur also known as mother of pearl sulfur and nacreous sulfur... More Info
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