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polarization also falls rapidly down to less than 100 mV. Afterward, the polarization slowly decreases during ‘step 2’ of the charge, reaching the minimum value of 50 mV at the end of this step. Such low polarization along the charge plateau (‘step 2’), indicates that the kinetic of the overall reaction occurring at this region is fast. During ‘step 1’ and ‘step 2’, the relaxation potential value, i.e. of about 2.4 V, could be related to the electrochemical reactions involving high-order polysulfides (as this is the potential corresponding to high-order polysulfides oxidation, already reported for Li/S system.). Then, at this state of charge, high-order polysulfides coexist together with Li2S solid phase31. Indeed, the electrode potential, where several electrochemical reactions take place, is fixed by the easiest redox equilibrium, which in our case is the oxidation of high-order polysulfides. Therefore, one of proposed explanation for the atypical first charge profile would be that initially, during the very rapid voltage jump, only a part of Li2S is getting oxidized, and its oxidation is very short. The capacity corresponding to the ‘step 1’ is usually about ~ 50 mAh g-1, which stands only for 5 – 6% of the theoretical value (874 mAh g-1), when assuming complete oxidation of Li2S according to the reaction: 4 Li2S → Li2S4 + 6Li+ + 6e-. In this first oxidation step, the cell voltage is increasing even up to 3.6 V, meaning that the long chain polysulfides may be formed. Furthermore, Li2S2 is known to be non-thermodynamically stable241, thus direct oxidation of Li2S to Li2S4 could be expected. During the pseudo plateau (‘step 2’), it is still not clear but really likely that several electrochemical reactions may occur: slow kinetic oxidation of Li2S and oxidation of mid-to-longer polysulfides i.e. S42-, S62- (this last process fixing the potential) to S82-. Indeed, the equilibrium potential is nearly the same for ‘step 2’ and ‘step 1’, then the electrochemical reactions involved should be very close. As soon as the potential starts to increase again upon charging (‘step 3’), quite high polarization is observed, indicating the slow kinetics of the overall oxidation reaction occurring in that region. However, the relaxation potential is still close to the one obtained in the previous steps (2.4 – 2.45 V; equilibrium potential of ‘step 1’ and ‘step 2’), thus related, once again, to the same electrochemical reactions, i.e. oxidation of middle-to-high-order polysulfides in the presence of Li2S. These data seems to show that, in the third part of the charge curve (‘step 3’), the kinetic of the overall oxidation reaction decreases notably as compared with those observed during ‘step 2’. The kinetic of the overall oxidation reaction could limit the Li2S oxidation into medium-to-high-order polysulfides (S42-, S62-, etc.), which requires a large overpotential. On the contrary, the electrochemical couples dictating the electrode potential are still the same: the oxidation of mid-to-high-order polysulfides. At the end of charge (‘step 4’), the relaxation potential is about 2.8 V, which is consistent with the oxidation of S82- and formation of S8. However, the electrochemical production of sulfur is shifted to higher potential (up to 3.6 V) under the current flow, due to polarization effect. The coexistence of both Li2S and S8 at the end of charge cannot be excluded, as Li2S oxidation limits the overall process, and oxidation of soluble polysulfides is expected to be notably easier. 124 Chapter 4: Li2S electrodePDF Image | Accumulateur Lithium Soufre
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