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Lu et al. Advanced Rechargeable Lithium Batteries was calcined at 700°C under flowing N2 atmosphere for 3 h to obtain NC@SWCNTs host. Preparation of NC@SWCNTs@Se1-xSx NC@SWCNTs@Se1-xSx composites were prepared with the help of supercritical CO2 (SC-CO2) fluid, which is reported in our previous works (Fang et al., 2018a; Fang et al., 2018b; Fang et al., 2020). Firstly, S and Se powders with different molar ratios (S:Se 7:3, 8:2, 9:1) were put into stainless-steel milling jars, respectively. The pre-mixed Se and S mixture was obtained after ball milling (500 rpm) for 12 h. Subsequently, 0.6 g pre-mixed Se and S mixture and a piece of NC@ SWCNTs (3 cm × 3 cm × 3 cm, ∼0.4 g) were put into a stainless-steel jar. Then, CO2 was pumped into the jar until the gaseous pressure reached 8.5 MPa. After the jar was kept at 32°C for 24 h, NC@ SWCNTs@Se1-xSx precursor was obtained by rapidly releasing CO2. Then, NC@SWCNTs@Se1-xSx precursor was sealed in a quartz glass tube under vacuum. Finally, the sealed quartz glass tube was heated to 400°C for 3 h to obtain NC@SWCNTs@Se1-xSx. The samples with different Se/S molar ratios were labeled as NC@SWCNTs@Se0.3S0.7, NC@SWCNTs@Se0.2S0.8 and NC@SWCNTs@Se0.1S0.9, respectively. For comparison, NC@SWCNTs hosts impregnated with Se or S were prepared by using the same SC-CO2 method and named NC@ SWCNTs@Se and NC@SWCNTs@S, respectively. Materials Characterizations The morphologies and microstructures of samples were observed on field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F30) equipped with an energy-dispersive spectroscopy (EDS) detector. X-ray diffraction (XRD) patterns were recorded on Rigaku Ultima IV powder X-ray diffractometer by using Cu Kα radiation (λ 0.15418 nm). Raman spectra were performed by Renishaw InVia Raman spectrometer (λ 532 nm). Thermogravimetric analysis (TGA) was conducted on SDT Q600 analyzer (TA Instruments) under a flowing Ar atmosphere. Electrochemical Measurements NC@SWCNTs@Se1-xSx cathodes were cut into disks of 15 mm in diameter and 2mm in height. CR2025 coin-type cells were assembled in an Ar-filed glove box (MIKROUNA, moisture <1.0 ppm, oxygen <1.0 ppm) with NC@SWCNTs@Se1-xSx composites as cathodes, commercial microporous polypropylene membrane (Celgard 2400) as separator, and lithium metal as anode. A solution of 1.0 M LiPF6 in a co-solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, volume ratio) was used as electrolyte. The dosage of electrolyte in coin- type cells is 15 μl mg−1 (based on the mass of Se1-xSx). Li-Se1-xSx cells were cycled in the voltage range of 1.0–3.0 V on a battery testing system (Shenzhen Neware Technology Co. Ltd.). Cyclic voltammetry (CV) was performed on a CHI650B electrochemical workstation (Chenhua, Shanghai, China). RESULTS AND DISCUSSION The morphology and microstructure of NC@SWCNTs host are characterized by SEM and TEM as illustrated in Figure 2. As vividly depicted in Figure 2A and Supplementary Figure S1, NC@SWCNTs host exhibits a 3D honeycombed network structure, fully inheriting the 3D interconnected framework of melamine foam. Local magnification SEM images (Figures 2B,C) demonstrate that numerous interlaced SWCNTs are covered the surface of melamine foam derived carbon skeletons, as well as SWCNTs are formed into small sheets between carbon skeletons. This unique interconnecting structure not only endows NC@ SWCNTs a highly conductive 3D network to accelerate the electron/ion transport, but also effectively enhances the mechanical strength and flexibility of NC@SWCNTs host. Moreover, TEM results (Figure 2D) further indicate that SWCNTs are crisscrossed in carbon skeletons, forming an intertwined 3D network structure. On the basis of EDS results (Figure 2E), the main elements in NC@SWCNTs are C, O and N, which are uniformly distributed in NC@SWCNTs. Notably, N signal is derived from melamine foam since melamine has high content of N. According to the above analysis, NC@SWCNTs host has a typical 3D network structure that is composed of SWCNTs-coated N-doped carbon skeleton derived from melamine foam and wafery sheets interwoven by SWCNTs. The pores and layer gaps in NC@SWCNTs host are conducive to loading more Se1-xSx active materials. Meanwhile, the 3D interconnected conductive network framework can not only effectively promote redox kinetics, but also endow NC@ SWCNTs host with strong mechanical properties to buffer the volume expansion during cycling. Additionally, the doped N is also beneficial to the adsorption of intermediates. After Se1-xSx impregnation, compared to NC@SWCNTs host, NC@SWCNTs@Se1-xSx composites well maintain the original morphology of NC@SWCNTs (Supplementary Figure S2). Moreover, no discernible Se1-xSx particles can be found at the surface of NC@SWCNTs. Additionally, according to EDS mapping results, the C, N, Se and Se signals are overlapped well, suggesting Se1-xSx composites are uniformly permeated into the pores and layer gaps of NC@SWCNTs host with the assistance of SC-CO2 due to the good permeability, excellent diffusivity and high solubility of SC-CO2. Furthermore, elemental analyses (Supplementary Table S1) of NC@SWCNTs@Se1-xSx show that molar ratios of Se to S in NC@SWCNTs@Se1-xSx conform to the design values. NC@SWCNTs@Se1-xSx composites are further revealed by XRD and Raman analysis. As illustrated in Figure 3A, all the samples have a wide peak in 2θ ranging from 15 to 40o, corresponding to the existence of NC@SWCNTs. Meanwhile, the characteristic diffraction peaks of Se and S are clearly observed in NC@SWCNTs@Se and NC@SWCNTs@S samples, respectively. With the introduction of Se, no characteristic diffraction peak of Se is detected in NC@SWCNTs@Se1-xSx composites. However, some characteristic diffraction peaks of S with low intensity can be still observed, indicating a small amount of Se may occupy S position and further form Se1-xSx in NC@SWCNTs host (Yao et al., 2017). To further investigate the bond between Se and S, Raman spectra were depicted in Figure 3B. Apparently, all the samples have three characteristic peaks located at 260, 375 and 470cm−1, respectively, which are assigned to Se-Se, Se-S and S-S Frontiers in Chemistry | www.frontiersin.org 3 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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