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be directly related with the evolution of Li negative electrode, and growth/modification of its passivation layer. The growth of the semicircle size (i.e. increase of the resistance) shows rather logarithmic behavior, with rapid increase during initial few hours, reaching at some point relatively stable value (in this particular example: ~ 220 Ω), as shown on Figure 6-7. Figure 6-7. Resistance evolution of the MF semicircle during 250 h of storage of a Li/S cell. * As previously mentioned, it is very likely that, due to rapid sulfur dissolution and formation of polysulfides already during initial 10 h of storage, the capacitive behavior is not observed any more. The vertical straight line disappears progressively, giving rise to a small semicircle in the LF range, at the characteristic frequency very close to 1 Hz. Once the global ‘shape’ of the Nyquist plot is getting stabilized (after ~ 50 h), further aging results in an increase of the MF semicircle resistance only (as demonstrated on Figure 6-7), while the LF semicircle stays practically unchanged, with characteristic frequency values always at ~ 1 Hz. It is also important to note that our positive electrode almost does not contribute to the MF semicircle, since coated on NwC collector cloth, which provides enhanced electronic conductivity. If using a classical sulfur electrode, i.e. coated on Al foil, and of a larger bulk internal resistance resulting from inhomogeneity of the coating for example (of even 20 Ω, as extensively discussed in section 2.3.3), the contribution of the positive electrode to the MF loop would be more pronounced, however, still a dominating response would come from the lithium electrode. * The error coming from the MF fitting was very small, i.e. ~ 1 %, therefore not included on the graph as an error bars. Chapter 6: EIS and low temperature studies 199PDF Image | Accumulateur Lithium Soufre
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