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  1. Free, publicly-accessible full text available November 1, 2024
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  4. Many transition-metal-oxide-based catalysts have been investigated to chemically bind soluble lithium polysulfides and accelerate their redox kinetics in lithium-sulfur (Li-S) battery chemistry. However, the intrinsic poor electrical conductivities of these oxides restrict their catalytic performance, consequently limiting the sulfur utilization and the rate performance of Li-S batteries. Herein, we report a freestanding electrocatalytic sulfur host consisting of hydrogen-treated VO2 nanoparticles (H-VO2) anchored on nitrogen-doped carbonized bacterial cellulose aerogels (N-CBC). The hydrogen treatment enables the formation and stabilization of the rutile VO2(R) phase with metallic conductivity at room temperature, significantly enhancing its catalytic capability compared to the as-synthesized insulative VO2(M) phase. Several measurements characterize the electrocatalytic performance of this unique H-VO2@N-CBC structure. In particular, the two kinetic barriers between S8, polysulfides, and Li2S are largely reduced by 28.2 and 43.3 kJ/mol, respectively. Accordingly, the Li-S battery performance, in terms of sulfur utilization and charge/discharge rate, is greatly improved. This work suggests an effective strategy to develop conductive catalysts based on a typical transition metal oxide (VO2) for Li-S batteries. 
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  5. Abstract

    An ultra‐fast electrochemical capacitor (EC) designed for efficient ripple current smoothing was fabricated using vertically oriented MoS2(VOM) nanoflakes deposited on freestanding carbonized cellulose (CC) sheets as electrodes. The daily used cellulose tissue sheets were transformed into electrode scaffolds through a rapid pyrolysis process within a preheated furnace, on which VOM nanoflakes were formed in a conventional hydrothermal process. With these ~10 μm thick VOM‐CC electrodes, ultrafast ECs with tunable frequency response and specific capacitance density were fabricated. The ECs with a cell‐level areal capacitance density of 0.8 mF/cm2at 120 Hz were demonstrated for ripple current filtering from 60 Hz to 60 kHz. At a lower frequency response level, EC cell with a large capacitance density of 4.8 mF/cm2was also demonstrated. With the facile and easily scaled up process to producing the nanostructured electrode, the miniaturized VOM‐CC based ECs have the potential to substitute the bulky aluminum electrolytic capacitors for current smoothing and pulse power applications.

     
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  6. null (Ed.)
    Polysulfide shuttle effect, causing extremely low Coulombic efficiency and cycling stability, is one of the toughest challenges hindering the development of practical lithium sulfur batteries (LSBs). Introducing catalytic nanostructures to stabilize the otherwise soluble polysulfides and promote their conversion to solids has been proved to be an effective strategy in attacking this problem, but the heavy mass of catalysts often results in a low specific energy of the whole electrode. Herein, by designing and synthesizing a free-standing edge-oriented NiCo 2 S 4 /vertical graphene functionalized carbon nanofiber (NCS/EOG/CNF) thin film as a catalytic overlayer incorporated in the sulfur cathode, the polysulfide shuttle effect is largely alleviated, revealed by the enhanced electrochemical performance measurements and the catalytic function demonstration. Different from other reports, the NiCo 2 S 4 nanosheets synthesized here have a 3-D edge-oriented structure with fully exposed edges and easily accessible in-plane surfaces, thus providing a high density of active sites even with a small mass. The EOG/CNF scaffold further renders the high conductivity in the catalytic structure. Combined, this novel structure, with high sulfur loading and high sulfur fraction, leads to high-performance sulfur cathodes toward a practical LSB technology. 
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  7. Core-shell structured sulfur composite nanoparticles (NPs) and their various derivatives have been widely inves- tigated as a promising cathode material for Li-S batteries (LSBs) thanks to their unique features in suppressing the lithium polysulfides shuttle effect, accommodating the sulfur electrode volume change, and providing abundant electrochemically active sites. The commonly used infiltration strategy falls short in producing a near ideal core- shell structure. Accordingly, the strategy of encapsulation, in which the prefabricated sulfur or sulfur precursor nanocore is encapsulated by a subsequently formed host shell has attracted broad interest, and this technique has significantly accelerated the LSB development. To advance the state of the art in producing encapsulated sulfur NPs, it becomes necessary to systematically survey the past relevant works and sum up research gaps. This review first takes an excursion to the infiltration strategy to highlight its limitations, followed by surveys on studies of synthesizing sulfur NPs, encapsulating sulfur NPs, and producing encapsulated sulfur NPs from metal sulfides. The strengths and weaknesses of each method, the resulted NPs, their electrochemical properties and the associated LSB performances are particularly emphasized. The rationales to design and the results of applying structural derivatives of the conventional core-shell configuration are then assessed. The encapsulated sulfur NPs applied in aqueous batteries are also discussed. This comprehensive review on sulfur encapsulation is concluded by a summary on further challenges and opportunities as well as our perspectives on possible future research directions, towards fundamental understanding and practical development of encapsulated sulfur NP-based LSB technology. 
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