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Lu et al. Advanced Rechargeable Lithium Batteries the short plateau is probably due to the dissolution of intermediates into the electrolyte (Li et al., 2015). Meanwhile, the long plateau at 1.75–1.88 V is assigned to the conversion of polysulfides/ polyselenides to Li2S/Li2Se (Luo et al., 2014). During the charge process, there is only one sloping plateau at ∼2.12 V, corresponding to the conversion of Li2Se/Li2S to Se0.2S0.8. Figure 4C shows the cyclic performance of NC@SWCNTs@ Se1-xSx cathodes with different Se/S ratios at a current density of 0.2 A g−1. NC@SWCNTs@Se0.2S0.8 cathode delivers the highest initial discharge capacity (2,398.5 mA h g−1) among NC@ SWCNTs@Se0.3S0.7, NC@SWCNTs@Se0.2S0.8 and NC@ SWCNTs@Se0.1S0.9 samples. The initial discharge capacity exceeds the theoretical capacity may be attributed to side reactions and the formation of SEI layer on the surface of electrode (Luo et al., 2014). After 200 cycles, the reversible capacities of NC@SWCNTs@Se0.3S0.7, NC@SWCNTs@Se0.2S0.8 and NC@SWCNTs@Se0.1S0.9 samples are 490, 632 and 360 mA h g−1 with the corresponding capacity retentions of 51.7, 65.3 and 47.9%, respectively. Obviously, NC@SWCNTs@Se0.2S0.8 cathode exhibits the superior cyclic stability. In addition, the rate capabilities of NC@SWCNTs@Se1-xSx cathodes at different current densities are presented in Figure 4D. Compare to other samples, NC@SWCNTs@Se0.2S0.8 cathode demonstrates the best rate performance. The reversible rate capacities of NC@SWCNTs@ Se0.2S0.8 cathode are 998.4, 723.7, 606.8, 506.1, and 415.0 mA h g−1 at the current density of 0.2, 0.5, 0.8, 1.0 and 2.0 A g−1, respectively. When the current density switches back to 0.5 A g−1, the reversible discharge capacity of NC@SWCNTs@Se0.2S0.8 cathode reverts to the initial value. Moreover, as shown in Supplementary Table S2 and Supplementary Figure S4, NC@SWCNTs@Se0.2S0.8 cathode with Se loading of as high as 4.4 mg cm−2 (a relevant areal capacity of as high as 2.78 mA h cm−2) can surpass most reported Se1-xSx cathodes (Luo et al., 2014; Li et al., 2015; Guo et al., 2016; Wei et al., 2016; Li et al., 2017; Yao et al., 2017; Zhang et al., 2017; Hu et al., 2018; Li et al., 2018; Zhu et al., 2018). Such remarkable electrochemical performance of NC@SWCNTs@Se0.2S0.8 cathode mainly is due to the following reasons: 1) Se and S in Se0.2S0.8 solid solution play different roles: Se can significantly improve the electrical conductivity, while S can greatly enhance capacity. 2) N-doped 3D porous carbon matrix and interlaced SWCNTs not only provide storage space for Se1-xSx, but also effectively reinforce the structural stability, and further promote the cycling stability of NC@SWCNTs@Se1-xSx cathodes. CONCLUSION In summary, a series of rationally designed freestanding NC@ SWCNTs@Se1-xSx cathodes with 3D interconnected porous REFERENCES Abouimrane, A., Dambournet, D., Chapman, K. W., Chupas, P. J., Weng, W., and Amine, K. (2012). A New Class of Lithium and Sodium Rechargeable Batteries Based on Selenium and Selenium-Sulfur as a Positive Electrode. J. Am. Chem. Soc. 134, 4505–4508. doi:10.1021/ja211766q structure are developed with the assistance of supercritical CO2 fluid. NC@SWCNTs host with 3D network structure serves as an effective matrix for encapsulating Se1-xSx as well as facilitating ion/electron transport and redox kinetics. Benefiting from the rationally designed structure and optimized chemical composition, NC@SWCNTs@Se0.2S0.8 cathode exhibits excellent cycling stability (632mA h g−1 at 0.2A g−1 at 200 cycle) and remarkable rate performance (415 mA h g−1 at 2 A g−1) in carbonate-based electrolyte. This work offers a feasible approach to develop high-performance Se1- xSx cathodes for advanced Li-Se1-xSx batteries. DATA AVAILABILITY STATEMENT The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author. AUTHOR CONTRIBUTIONS CL: materials preparation, data analysis and manuscript writing; RF: materials preparation and data analysis; KW: manufacture and result analysis of battery; ZX: materials characterization and data analysis; GK: material properties analysis and discussion; YG: electrochemical performance test and results discussion; XH: electrochemical performance test and materials characterization; HH: data analysis and discussion; WZ: data analysis and discussion; YX: experimental design and manuscript revision. All authors contributed to the article and approved the submitted version. FUNDING This research was supported by Zhejiang Provincial Natural Science Foundation of China (LY21E020005), China Postdoctoral Science Foundation (2020M671785 and 2020T130597), National Natural Science Foundation of China (U20A20253) and Zhejiang Provincial Special Support Program for High-level Talents (2020R51004). SUPPLEMENTARY MATERIAL The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2021.738977/ full#supplementary-material Chen, X., Peng, L., Wang, L., Yang, J., Hao, Z., Xiang, J., et al. (2019). Ether- compatible Sulfurized Polyacrylonitrile Cathode with Excellent Performance Enabled by Fast Kinetics via Selenium Doping. Nat. Commun. 10, 1021. doi:10.1038/s41467-019-08818-6 Chen, Y., Li, X., Park, K.-S., Hong, J., Song, J., Zhou, L., et al. (2014). Sulfur Encapsulated in Porous Hollow CNTs@CNFs for High-Performance Lithium- Sulfur Batteries. J. Mater. Chem. A. 2, 10126–10130. doi:10.1039/C4TA01823K Frontiers in Chemistry | www.frontiersin.org 6 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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