Abstract
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.
Highlights
IntroductionIn the Li-S battery (LSB) chemistry, cyclo-octasulfur (S8), the most stable sulfur allotrope [1], undergoes a reduction process through multiple long-chain lithium polysulfide intermediates (Li2Sx, 3 < x ≤ 8, or LiPSs) into Li2S2 and Li2S
In the Li-S battery (LSB) chemistry, cyclo-octasulfur (S8), the most stable sulfur allotrope [1], undergoes a reduction process through multiple long-chain lithium polysulfide intermediates (Li2Sx, 3 < x ≤ 8, or LiPSs) into Li2S2 and Li2S.With this two-electron-based reaction per sulfur atom: S8 +16 Li+ + 16 theoretical e−↔ 8 specificLi2S, the capacity sulfur cathode can release a of 1675 mAh·g−1, several folds higher than the transition-metal-oxide-based intercalation cathode materials
It should be noted that NCS nanosheets can be grown onto carbon nanofibers (CNF) via a facile hydrothermal process, their size and thickness are much larger than the NCS obtained here by employing edge-oriented graphene (EOG) as a growth mediator
Summary
In the Li-S battery (LSB) chemistry, cyclo-octasulfur (S8), the most stable sulfur allotrope [1], undergoes a reduction process through multiple long-chain lithium polysulfide intermediates (Li2Sx, 3 < x ≤ 8, or LiPSs) into Li2S2 and Li2S. Energy Material Advances polymers [20], metal oxides [21, 22] and sulfides [23, 24], and single transition metal atoms [25] have all been theoretically and experimentally demonstrated with capability to stabilize LiPSs and catalyze their conversions Among these catalytic candidates, metal sulfides are interesting. Since catalytic function is a surface phenomenon, 2D sheet-like metal sulfides should be selected to minimize their mass loading, which if properly arranged on a conductive carbon scaffold will give an ideal host structure for the sulfur cathode. This conceptual design is straightforward, the studies along this approach are still very limited because of the difficulty in structure controlling. By virtue of the adaptability of the reported synthetic method and the prominent catalytic activity of the resulted product, this hierarchical structure can be further implemented in constructing other materials which are expected to deliver competitive electrochemical performances
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