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Lu et al. Advanced Rechargeable Lithium Batteries FIGURE 1 | Schematic illustration of the synthetic process of NC@SWCNTs@Se1-xSx. hybrid electric vehicles with strict restrictions on battery volume (Lin et al., 2021; Sun et al., 2021). Meanwhile, selenium is a semiconductor with much higher electronic conductivity (1 × 10−3 S m−1) than sulfur, which is conducive to excellent kinetic behavior (Zhang et al., 2020). Nevertheless, in the current research stage of Li-Se batteries, there are still many problems in Se cathode materials, such as relatively unfavorable higher cost and lower gravimetric capacity (675 mA h g−1) when compared to Li-S batteries (1,675 mA h g−1) (Fang et al., 2018b). In order to offset the drawbacks of Se and S and complement each other’s advantages, a solid solution of Se and S (Se1-xSx, 0 < x < 1) has been proposed as high-performance cathode materials for lithium storage. Se1-xSx is a class of chemical compounds with different Se-S ratios, which not only owns a higher theoretical capacity than pure Se, but also has increased electronic conductivity and accelerated reaction kinetics than pristine S (Abouimrane et al., 2012; Wei et al., 2016; Xu et al., 2019). However, similar to S, Se1-xSx cathode materials also suffer from poor cycle lifespan and low Coulombic efficiency due to the dissolution and shuttling of intermediates (Chen et al., 2019; Du et al., 2020). Since Se1-xSx cathode materials exhibit similar electrochemical behaviors to S, the strategies of immobilizing S should also be effective for Se1-xSx (Luo et al., 2014). At present, the main host materials of Se1-xSx are carbonaceous materials (Sun et al., 2021), such as hollow carbon spheres (Xu et al., 2015; Hu et al., 2020), mesoporous carbon (Han et al., 2019), carbon nanotubes (Fan et al., 2018; Guan et al., 2019; Shen et al., 2020), carbon fiber (Chen et al., 2014; Zhang et al., 2017), graphene (Tang et al., 2016; Chen et al., 2019) and carbonized polyacrylonitrile (Li et al., 2018). Generally, expect serving as hosts for Se1-xSx, these carbonaceous materials play another dual role of establishing conductive frameworks to facilitate ions/electrons transport and inhibiting the shuttle effect (Hu et al., 2020). Nevertheless, the physical adsorption ability of nonpolar pristine carbon materials to polar intermediates is too weak to effectively prevent the dissolution and diffusion of intermediate (Sun et al., 2016; Nazarian-Samani et al., 2021). Research shows heteroatom- doped (B, N, O, etc.) carbon hosts can effectively improve the electrochemical performance of Li-Se1-xSx batteries due to the strong chemical affinity of polarized carbon surface, which can significantly trap the soluble intermediates to inhibit the shuttle effect and side reactions in the electrolyte (Guo et al., 2016; Zhang et al., 2017; Fan et al., 2018; He et al., 2018; Sun et al., 2021). Herein, a series of Se1-xSx cathode materials with optimized Se/ S ratio are incorporated into N-doped three-dimensional (3D) porous carbon matrix to form novel freestanding Se1-xSx foamy cathodes (NC@SWCNTs@Se1-xSx) with the assistance of supercritical CO2 fluid (Figure 1). In carbonate-based electrolyte, NC@SWCNTs@Se1-xSx cathodes exhibit single- phase transformation during charge/discharge. Benefiting from the rationally designed structure and chemical composition, NC@SWCNTs@Se1-xSx cathodes with high conductivity and strong adsorption present superior electrochemical performance. EXPERIMENTAL SECTION Preparation of NC@SWCNTs Melamine foam (3 cm × 3 cm × 3 cm) was washed with anhydrous ethanol and dried in an oven at 80°C for 12 h, then immersed in SWCNTs/NMP suspension (0.4%). After 6 h, the melamine foam impregnated with SWCNTs/NMP suspension was taken out and dried in a vacuum oven at 80°C for 24 h to obtain the precursor of melamine/SWCNTs. The above precursor Frontiers in Chemistry | www.frontiersin.org 2 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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