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Article doc.001pp.com Downloaded via XI'AN JIAOTONG UNIV on September 27, 2019 at 11:54:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Sulfur Vapor-In?ltrated 3D Carbon Nanotube Foam for Binder-Free High Areal Capacity Lithium−Sulfur Battery Composite Cathodes Mengya Li,† Rachel Carter,† Anna Douglas,†,§ Landon Oakes,†,§ and Cary L. Pint*,†,§ †Department of Mechanical Engineering and §Interdisciplinary Materials Science Program, Vanderbilt University, Nashville, Tennessee 37235, United States *S Supporting Information ABSTRACT: Here, we demonstrate a strategy to produce high areal loading and areal capacity sulfur cathodes by using vaporphase in?ltration of low-density carbon nanotube (CNT) foams preformed by solution processing and freeze-drying. Vapor-phase capillary in?ltration of sulfur into preformed and binder-free lowdensity CNT foams leads to a mass loading of ∼79 wt % arising from interior ?lling and coating of CNTs with sulfur while preserving conductive CNT−CNT junctions that sustain electrical accessibility through the thick foam. Sulfur cathodes are then produced by mechanically compressing these foams into dense composites (ρ > 0.2 g/cm3), revealing speci?c capacity of 1039 mAh/gS at 0.1 C, high sulfur areal loading of 19.1 mg/cm2, and high areal capacity of 19.3 mAh/cm2. This work highlights a technique broadly adaptable to a diverse group of nanostructured building blocks where preformed low-density materials can be vapor in?ltrated with sulfur, mechanically compressed, and exhibit simultaneous high areal and gravimetric storage properties. This provides a route for scalable, low-cost, and highenergy density sulfur cathodes based on conventional solid electrode processing routes. KEYWORDS: lithium−sulfur batteries, high areal capacity, high areal loading, cathode, carbon nanotubes, capillary force, high-energy density L ithium−sulfur (Li−S) battery cathodes are a promising replacement for lithium-ion cathodes due to earth abundance and the low cost of sulfur, nontoxic nature of cathode materials, and extraordinary theoretical capacity (∼1675 mAh/g) many times that of traditional cathodes.1−8 The high theoretical energy density (∼2600 Wh/kg) of Li−S batteries is about 3−5 times higher than conventional Li-ion batteries.4,9−11 However, to achieve mass performance approaching this theoretical value, the insulating nature of sulfur, large volume expansion, and polysul?de dissolution remain challenges.2,3,12−14 Speci?cally, the electrochemical reaction between lithium and sulfur ?rst results in high-order polysul?des (HOPSs; Li2S8, Li2S6, and Li2S4) that are soluble in widely used ether-based electrolytes and lead to sulfur mass loss from the cathode. The irreversible loss of HOPSs in the electrolyte leads to poor conversion of low-order polysul?des (cLycOlaPbSilsit;y.L9,i125S−218aPnrdevLioi2uSs)reasnedarcrheshualstsfoicnuspeodoorn capacity and using several strategies to solve these problems, with conductive composite material fabrication,15,19−24 structural con?nement design,25−30 polar binding additives,24,31−34 and additional interlayer con?guration.35−37 Whereas these advances continue to emerge, an engaging area of research has focused on developing routes to maintain the high gravimetric capacity associated with stable sulfur cathodes in electrode architectures where high areal loading and high areal capacity are achieved. This requires scaling-up of fundamental nanoscale design rules to thicken electrode architectures that are at a minimum competitive with the areal capacity of commercial lithium-ion battery cathodes, which is generally between 2 and 4 mAh/cm2.3,6,38−42 Achieving cathodes that maintain gravimetric capacity at substantially higher loading results in less external packaging weight, lower cost associated with expensive current collector materials, and better energy density at the cell level resulting from less inactive material. However, design challenges emerge for high areal capacity electrodes that are not relevant in thinner electrode architectures, and three separate approaches have been primarily used in state-of-the-art e?orts. The most Received: February 28, 2017 Accepted: April 28, 2017 Published: April 28, 2017 © 2017 American Chemical Society 4877 DOI: 10.1021/acsnano.7b01437 ACS Nano 2017, 11, 4877−4884 ACS Nano Article Figure 1. (a) Ultralight CNT sponge with a size of ∼1.5 cm × 0.8 cm × 0.2 cm resting on a dandelion. Schematic illustration (b) bulk 3D CNT foam, (c) carbon microscale morphology formed during solution processing, and (d) interconnected CNTs that make up structure corresponding to SEM imaging. (e) Photograph of a typical 3D CNT foam. SEM images of (f) the 3D CNT foam with carbon ?akes at the microscale and (g) interconnected CNTs with meso- and microporous features at the nanoscale. widely utilized technique for fabricating sulfur cathode composites is melt in?ltration, where bulk carbon and sulfur powder are heated (typically overnight) in a furnace to produce a composite.43−47 These are then mixed with binders and cast in thick slurry coatings to achieve high areal loading. The critical limitation of this technique is the lack of control over the location of insulating sulfur deposits, which inhibit conductive carbon−carbon material interfaces that are critical to achieve full electrical connectivity of the thick electrode and full sulfur conversion. Another prominent route has involved the use of interlayers,34,48,49 where a thick interlayer or membrane is cast over typically elemental sulfur electrodes to overcome polysul?de shuttling, but only utilizing sulfur as active material. The limitation of this approach is that researchers often achieve high areal capacities at the expense of including excess electrochemically inactive mass that lowers the total electrode gravimetric performance and compromises improvements in cell-level energy density over lithium-ion. Currently the most commonly accepted technique for high areal capacity electrodes is the use of a catholyte electrolyte where the carbon electrode is preformed, species known to be soluble are loaded into the electrolyte, and solid sulfur deposits form at the host during operation.50−52 Whereas this technique yields the highest recorded areal capacity,50 the requirement of sulfur-containing electrolytes ushers in new challenges for packaging and manufacturing since cathode development is dependent upon electrolyte properties. In this work we demonstrate a technique where we combine the bene?t of a preformed carbon-based electrode which is a strength of the catholyte approach, with spatial control of sulfur in?ltration at high mass loadings (∼79 wt %) directed by capillary thermodynamics, to produce a sulfur−carbon composite cathode. As a host material we utilize carbon nanotube (CNT) foams, as these are low-density interconnected sponge-like materials that can be produced in thick structures using solution processing methodologies.53−57 The capillary loading of sulfur directs the in?ltration on the interior of CNTs, leaving conductive CNT−CNT electrical pathways unimpeded by insulating sulfur deposits to enable accessible thick electrode geometries. Further, after vapor in?ltration we mechanically compress these low-density materials into electro- des that maintain density above 0.2 g/cm3 and overcome limitations of highly porous and low volumetric density electrode designs. Based on this approach, we observe electrodes that exhibit high areal capacities of 19.3 mAh/cm2 at a mass loading of 19.1 mg/cm2 with a gravimetric capacity of 1039 mAh/gS. Whereas this is among the highest composite cathode performance reported to date, second only to one report on catholyte-in?ltrated cotton,50 our calculations emphasize this areal capacity to be at a level where signi?cantly improved areal capacity is less important to the packaged cell energ 内容过长，仅展示头部和尾部部分文字预览，全文请查看图片预览。 Muralidharan, N.; Pint, C. L. Isothermal Sulfur Condensation into Carbon Scaffolds: Improved Loading, Performance, and Scalability for Lithium Sulfur Battery Cathodes. J. Phys. Chem. C 2017, 121, 7718−7727. (60) Hagen, M.; Hanselmann, D.; Ahlbrecht, K.; Maca, R.; Gerber, D.; Tubke, J. Lithium-Sulfur Cells: The Gap Between the State-of-theArt and the Requirements for High Energy Battery Cells. Adv. Energy Mater. 2015, 5, ***. 4884 DOI: 10.1021/acsnano.7b01437 ACS Nano 2017, 11, 4877−4884 [文章尾部最后500字内容到此结束,中间部分内容请查看底下的图片预览]请点击下方选择您需要的文档下载。

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