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without sacrificing the practical discharge capacities. However, this architecture is not yet optimized for power rate applications. The benefit of NwC over Al collector is mainly visible when comparing highly loaded electrodes (> 4 mgsulfur cm-2). It is then obvious that much higher capacity values (of even 300 mAh g-1) can be obtained with NwC-based electrodes, and with lower polarization. We also proved that the reason for such improved capacity values is not directly related with the fact that NwC collector offers an additional conductive surface area for solid products precipitation (Li2S/Li2S2?). More important is its high porosity (~ 80 %) and 3D conductive network, which efficiently improve both ionic and electronic conductive pathways, because of easily accessible volume for the electrolyte/polysulfides penetration and fast electron propagation through the carbon fibers. It also provides a rigid and stable carbon matrix for the electrode, which is beneficial from the point of view of dissolution/precipitation cycles. Last but not least, one of the main obstacles we systematically observed was the appearance of micro short circuits during charge, most likely due to dendritic growth on the negative lithium electrode, since we were working with relatively highly loaded positive electrodes, thus meaning high depths of lithium stripping/plating. The problem related to metallic lithium electrode gave us the motivation to go into the direction of safer lithium-ion/sulfur (Li-ion/S) cells, by eliminating metallic lithium and replacing it by silicon negative electrode. In the first step, investigation of the Li2S positive electrode vs. lithium was performed and compared to the one of Li/S cell. If taking apart the first charge process of the Li/Li2S cell, further cycles exhibit the same behavior and limitations as s classical Li/S battery. The initial charge, however, is very particular and characteristic, mostly because of the poor ionic and electronic conductivities of Li2S particles, their micrometric size and poor solubility in the organic solvents. In particular, the electrochemical process occurring upon initial charge is different from further charge cycles. Indeed, Li2S material seems to be present during all the 1st charge, which limits the overall oxidation reaction and induces large polarization in the main part of the charge profile. Moreover, the equilibrium potential is fixed by the equilibria existing in solutions and involving medium-to-long polysulfides (S42-, S62-, S82-), up to the appearance of S8 at the end of charge. After having thoroughly studied the performances of Li/Li2S cell, in the next step a complete metallic lithium-free cell – Si/Li2S – was also analyzed and promising results were obtained. The cycling properties were very close to the ones obtained when using metallic Li, and further improvements of Si/Li2S cell may bring additional benefits. The fundamental investigations of this work were carried out thanks to the application of two pertinent techniques: Electrochemical Impedance Spectroscopy (EIS) and in situ and operando X-Ray diffraction (XRD). With the use of XRD technique, we successfully analysed the structural changes of active material inside a Li/S battery during few initial cycles. Our in situ synchrotron-based results clearly indicate the formation of crystalline Li2S on the positive electrode, starting from the very beginning of lower discharge plateau. We also proposed a sequential discharge 230 Conclusions & PerspectivesPDF Image | Accumulateur Lithium Soufre
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