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(a) (b) Figure 4-5. Voltage profiles of Li/Li2S (in blue) and Li/S (in black) cells cycled at C/20. Comparison of discharge (a) and charge (b) voltage profiles. In both cases, capacities are presented with respect to the sulfur mass. The active material loadings for both electrodes (Li2S and S8-based) are as follow: 1.23 mgLi2S cm-2 and 2.46 mgsulfur cm-2. These data confirm the fact that, once the cell starts to run, solid active material present in the electrode is changing its form into liquid state, and further cell operation is exactly the same for both Li2S and S8-based systems, no matter the initial active material used during the electrode fabrication. The presence of sharp overpotential peak at the beginning of charge (blue circle on Figure 4-5b) may bring another explanation for the initial and characteristic activation barrier of Li2S electrode (blue circle on Figure 4-4a). It has been shown that at the end of discharge, nano- sized particles of Li2S are formed200 (refer to chapter 5). On the contrary, initial particles of Li2S used for electrode preparation are much bigger (of about 10 – 20 μm). As a matter of fact, less energy is needed to break a passivation layer made of nano-Li2S, while higher activation energy is required to activate a fresh electrode of Li2S after preparation, resulting in more remarkable voltage jump. Figure 4-6 shows an example of the capacity retention over 100 cycles obtained at C/20, with initial cycle performed with the 3.8 V – 1.7 V potential window, followed by 2.8 V – 1.7 V for the next cycles. Such capacity retention is very close to the one already observed when using sulfur as a starting electrode, i.e. drastic capacity fade during few initial cycles, followed by rather stable capacity. The reason for such behavior would be similar as for sulfur electrode: active material loss, pulverization of the electrode, continuous electrode passivation. Discharge capacity values, when calculated with respect to the sulfur mass, are very similar as compared to the cells with ‘S-on-Al’ electrodes (stable capacity at about 350 – 400 mAh g-1). Chapter 4: Li2S electrode 117PDF Image | Accumulateur Lithium Soufre
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