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Lu et al. Advanced Rechargeable Lithium Batteries FIGURE 4 | (A) CV curves of the NC@SWCNTs@Se0.2S0.8 cathode. (B) Charge/discharge curves of the NC@SWCNTs@Se0.2S0.8 cathode at 0.2 A g−1. (C) Cycle performances and (D) rate performances of NC@SWCNTs@Se1-xSx cathodes. respectively. Meanwhile, the onset decomposition temperatures of NC@SWCNTs@Se0.3S0.7, NC@SWCNTs@Se0.2S0.8, and NC@ SWCNTs@Se0.1S0.9 are between NC@SWCNTs@S and NC@ SWCNTs@Se samples, and gradually increase with increasing Se content in Se1-xSx. It is because the thermal stability of Se is higher than that of S, the higher the content of Se in Se1-xSx, the higher the thermal stability of solid solution. Moreover, the distinct weight losses exist in all the samples, corresponding to the active material in samples. Therefore, the actual contents of S, Se0.3S0.7, Se0.2S0.8, Se0.1S0.9, and Se in NC@SWCNTs@S, NC@ SWCNTs@Se0.3S0.7, NC@SWCNTs@Se0.2S0.8, NC@SWCNTs@ Se0.1S0.9, and NC@SWCNTs@Se are 54.8, 58.7, 52.5, 58.4, and 56.8%, respectively, which are close to the design value of ∼60%. In order to evaluate the electrochemical performance of NC@ SWCNTs@Se1-xSx composites, NC@SWCNTs@Se1-xSx composites are employed as freestanding cathodes in Li-Se1-xSx batteries with carbonate-based electrolyte (LiPF6-EC/DMC). Figure 4A and Supplementary Figure S4 show initial three cyclic voltammetry (CV) curves of NC@SWCNTs@Se1-xSx cathodes at a scanning rate of 0.1 mV s−1 in the potential window from 1.0 to 3.0 V versus Li/Li+. At the initial scan, a sharp reduction peak at ∼1.38 V, a small reduction peak at ∼2.37 V, and a broadened oxidation peak at ∼2.14 V are clearly observed. The small reduction peak at ∼2.37 V disappears after the first scan, while the sharp reduction peak at ∼1.38 V shifts to ∼1.7 V during the subsequent scan. The peak shift indicates the activation process during the first lithification process, and the polarization is effectively reduced thereafter (Luo et al., 2014; Zhu et al., 2018). The subsequent CV curves are well overlapped after the first scan, indicating the good cyclability and reversibility of NC@SWCNTs@ Se0.2S0.8 cathode (Guo et al., 2019). It should be mentioned that the CV curves of NC@SWCNTs@Se1-xSx cathodes are obviously different from S cathode, indicating the introduction of Se changes the electrochemical reaction process of S that is conducive to its stable work in carbonate-based electrolytes. Moreover, galvanostatic charge-discharge curves (Figure 4B and Supplementary Figure S5) of NC@SWCNTs@Se1-xSx cathodes are consistent with CV results. During the first discharge process, there are two plateaus: one is an extremely short plateau at ∼2.38 V, and another is a long plateau at ∼1.75 V. In the subsequent cycles, the short plateau at ∼2.38 V disappears, while the long plateau at ∼1.75 V becomes a little steeper and shifts to ∼1.88 V. The short plateau at ∼2.38 V is attributed to the transformation of Se0.2S0.8 to polysulfides/polyselenides intermediates. And the disappearance of Frontiers in Chemistry | www.frontiersin.org 5 July 2021 | Volume 9 | Article 738977PDF Image | Supercritical CO2 Synthesis of Freestanding Se1-xSx Foamy Cathodes for High-Performance Li-Se1-xSx Battery
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