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Electrochem 2020, 1 236 4. Cathode and Separator Materials 4.1. General Work Employing Graphene and Carbon Cathode Materials Much of the work on Li-S batteries has focused on cathode materials or separators to provide a cathode interface [1,2,4–7], and most of those studies have employed carbon-based materials as a conductive material, physical support, or both. When lithium polysulfide (LPS) and a carbon nanofiber (CNF) composite layer are combined as a cathode and the surface is terminated with a Mo nanotube/carbon nanotube (CNT) thin film, for example, effective regulation of the electrochemical redox reactions can be attained (Figure 8) [46]. This is mediated by the Mo centers, which can immobilize the lithium polysulfide species and attenuating self-discharge activity, which is only 3% after 72 h of rest. The cell so constituted displays a high active sulfur utilization of 1401 mAh g−1 (0.1 C) and has only 0.06% decay/cycle over the first 500 cycles operating at 1 C. When the cathode loading is rather high 7.64 mg cm−2 and the cell is operated for 100 cycles, a high reversible areal capacity (4.75 mAh cm−2) is retained upon operation at 0.2 C. The nanoscale design of this material emphasizes what can be accomplished when adsorption sites are positioned in close proximity to both cathode material and conductive layers to effectively regulate electrochemical reaction and diffusion processes. The very small amount of Mo needed is also a plus for cost, disposal and sustainability. Electrochem 2020, 2, FOR PEER REVIEW 12 Figure 8. Schematic showing electrochemical behavior or electrodes comprising carbon Figure 8. Schematic showing electrochemical behavior or electrodes comprising carbon nanofiber/lithium polysulfide (CNF/LPS) (a), CNF/LPS/carbon nanotube (CNT) (b) and nanofiber/lithium polysulfide (CNF/LPS) (a), CNF/LPS/carbon nanotube (CNT) (b) and CNF/LPS/Mo/CNT (c), the latter of which prevents free diffusion of LPS and catalyzes their CNF/LPS/Mo/CNT (c), the latter of which prevents free diffusion of LPS and catalyzes their conversion conversion to lithium sulfide. Reprinted with permission from reference [46]. Copyright 2020 to lithium sulfide. Reprinted with permission from reference [46]. Copyright 2020 American American Chemical Society. Chemical Society. Sheets comprising a few layers of graphene can also be conveniently fabricated onto carbon nanofibers to give materials appropriate to serve the dual roles of cathode scaffold and interlayer Sheets comprising a few layers of graphene can also be conveniently fabricated onto carbon between the cathode and separator [89]. This configuration of materials led to a Li-S battery with nanofibers to give materials appropriate to serve the dual roles of cathode scaffold and interlayer improved charge-discharge capacity and stability. After 100 cycles (0.5 C), a capacity of 820 mAhg−1 between the cathode and separator [89]. This configuration of materials led to a Li-S battery with was observed, while a capacity of 640 mA h g−1 was still observed even after 500 cycles. A Li-S battery has also been achieved in which a Li2S6 catholyte is hosted by a carboxyl-modified graphene oxide sponge having the highest specific pore volume reported to date (6.4 cm3g−1) [47]. The highly porous structure of this material successfully sequesters lithium polysulfides while displaying high electron and electrolyte transportation with commensurately improved charge storage capacity. A discharge capacity of 1607 mAh g−1 (0.1 C) and areal capacity of 3.53 mAh cm−2 were observed. Importantly, a high sulfur loading (6.6 mg cm−2) can be used and the devices still achieve nearly 80%PDF Image | Lithium-Sulfur Batteries: Advances and Trends
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