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Chapter 6: EIS and low temperature studies Figure 6-30. Arrhenius plot obtained with the resistance values of RPS semicircle at the end of charge, at different temperatures. 6.8. Conclusions EIS investigations gave a large overview on the reactions occurring upon cycling in a Li/S cell. To our best knowledge, it was the first time such completed description was performed, together with deep interpretation of obtained results at large range of characteristic frequencies. In order to correctly attribute the response of Nyquist plot obtained through EIS measurements, the use of symmetric coin cells approach was helpful and indispensable. EIS results are in a good agreement with the observations obtained through in situ XRD measurements. First of all, metallic lithium electrode significantly contributes to the impedance response of a complete Li/S cell, especially visible in the initial cycle, mainly through its passivation layer, whereas this contribution is generally omitted in the literature. Our investigation permitted to propose a simple equivalent circuit preserved all along the cycling. The circuit includes the electrolyte resistance response (at HF region), followed by three R//CPE elements connected in series between MF to LF, associated to (i) the positive electrode bulk contribution, (ii) passivating film formed at lithium/electrolyte interphase and (iii) charge transfer reaction of polysulfides at the positive electrode surface. The LF response was attributed to the diffusion processes in the overall cell, however, was not further investigated in this work. The evolution of each parameter was investigated during the first cycle and further ones as well. The main difference between different cycles, is the lower response of the lithium interphase starting with the second cycle. Moreover, we were able to correlate the capacity retention as a perfect match with the electrolyte resistance evolution, which in turn gives an indication of the limiting factors. 227PDF Image | Accumulateur Lithium Soufre
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