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based electrolyte is rather an easy process. Once in liquid solubilized form, sulfur may be more reactive as the solid elemental particles, and may diffuse through the electrolyte to the negative electrode side and get reduced, giving the rise of high-order polysulfides formation. Lithium polysulfides are much more electrochemically active, which may explain the complete disappearance of blocking capacitive behavior of the system upon storage and formation of very small amount of soluble polysulfides, thus giving a rise to the self-discharge process. In order to verify if, indeed, a creation of polysulfides can be detected, three similar Li/S coin cells were prepared and left for storage for 8 h, 70 h and 250 h, during which OCV was controlled and EIS was recorded. The last coin cell, after having its OCV monitored for 250 h, was then left for additional storage of 1.5 months (this time without EIS recording). All three coin cells were opened, and the photos are shown on Figure 6-6. Figure 6-6. Photos of the coin cells dismounted after 8 h, 70 h and 1.5 month of storage (i.e. OCV monitoring with EIS recorded). It can be noticed (even visually) that separator and electrolyte turned into yellowish-reddish color, which clearly proves creation of lithium polysulfides. Longer the storage time was (1.5 month), the more intense color was and the higher dissolved amount seemed to be formed. This also proves that self-discharge occurs relatively easily in Li/S cells, resulting in a drastic potential drop at the beginning (during initial 10 h) of storage time. However, after that time (and during further ~ 250 h of storage), the voltage decrease was relatively slow, and the potential of the cell stabilized at 2.45 V. As concerning the MF semicircle evolution (previously attributed to the passivation layer on the lithium surface), a progressive increase of its amplitude upon storage can be noticed, together with characteristic frequency values slightly shifting into lower range (i.e. 864 Hz at the initial state, 266 Hz after 250 h of storage). Indeed, very similar behavior was found when symmetric Li||Li cells were left upon storage and EIS was recorded every 1 h (Figure 6-4b). Thus, it is very clear that the MF semicircle evolution observed in Li/S cell upon storage can Chapter 6: EIS and low temperature studies 198PDF 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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