2014SC.锂电池

Three-Dimensional Sulfur/GrapheneMultifunctional Hybrid Sponges forLithium-Sulfur Batteries with Large ArealMass LoadingSongtao Lu,Yan Chen, Xiaohong Wu, Zhida Wang (b) photograph of three strong GO hydrogel allowingsupporting weight; (c) SEM image of the interior microstructures of theGO spongesand (d) SEM image of interior microstructures of the S-GSwith 80 wt.% sulfur.Figure 2 |X-ray diffraction (XRD) patterns of the GO, GSand S-GSwith80 wt.% sulfur.Figure 3 |IR spectra of the GO, GS and S-GS with 80 wt.% sulfur.www.nature.com/ scientificreportsSCIENTIFIC REPORTS | 4 : 4629 | DOI: 10.1038/srep04629 2a higher current density, thus tending to the lower impedance andthicker electrode also places more of a strain on the transport oflithium ions in the electrolyte, thus possibly decreasethe gravimetricspecific capacity.With continued cycling, asshown in figure 5a, the electrode exhib-ited well-overlapped and flat plateaus, suggesting good stability andreversibility of the electrode. Figure 5b compares the capacity of theelectrode as a function of cycle numbers. Although the initial dis-charge capacity is relatively low (4.93 mAh cm 2 2 of the electrode and513 mAh g2 1 of sulfur), it can continued to increase until the 11thcycle (6.0 mAh cm2 2 of the electrode and 625 mAh g2 1 of sulfur).This behavior can be contributed to the activation step of the S-GSelectrode becausethe surface areaof the as-prepared S-GSis low, andtherefore, it takes some time for the electrolyte to flood the internalsurfaces of the GO sponges. Only under this condition, the deeplyburied sulfur and disulfide bonds can contact with the electrolyte andbecomeelectrochemically active.Subsequently, the capacities almoststabilized and demonstrated little fading upon extended cycling 5.And asa result, the electrode can exhibit a reversible and comparablecapacity of 4.53 mAh cm2 2 after 300 cycles, corresponding to capa-city retention of 75.5%(of its highest capacity of 6.0 mAh cm2 2), andthe decay rate was as low as 0.08% per cycle for 300 cycles. At thesame time, as can be seen from the inserted map in figure 5b, thecoulombic efficiency remained at around 98%. On the basisof suchsuperior cyclic stability, it is reasonableto conclude that the graphenespongesframework could effectively improve the cyclestability of thelithium sulfur batteries, likely through absorption and immobiliza-tion of the polysulfides intermediate and provide better mechanicalsupport to accommodate the volume changes during charge anddischarge.ConclusionIn summary, we have synthesized a3D electrode of sulfur embeddedinto porous graphene sponges for lithium sulfur batteries by a heattreatment, which is simple, highly efficient, and scalable. This 3Darchitecture electrode was able to demonstrate high areal specificcapacity and high retention ratio even at a large areal mass loadingof , 12 mg sulfur/cm 2 ,approximately 6– 12times higher than that ofmost reports. The observed high areal specific capacity of the elec-trode (4.53 mAh cm2 2 after 300 cycles) and slow decay rate at 0.1C(0.08% per cycle after 300 cycles) represents a significant step for-ward for the application of Li2 S batteries.MethodsSynthesisof GO. The grapheneoxide wassynthesizedfrom natural flake graphite byHummers’ method. The concentration of the GO suspensionobtained was, 3.5 mgmL2 1, which wasdetermined by drying 20 mL the suspensionat70uC under vacuumfor 72 h and then weighing the dried GO.Synthesis of GO Sponges (GS). The GO Spongeswasprepared by hydrothermalreduction-assembly of homogeneousGO suspension.Briefly, 10 mL of the GOsuspensionwassealedin a 15-mL Teflon-lined stainlesssteelautoclaveandmaintained at 180uC for 18 h. After beencooled to room temperature, black GShydrogel can be obtained. For GO spongespreparation, the as-preparedgraphenehydrogel wasfreeze-driedto remove absorbedwater.Synthesis of S-GS. The GO spongewascut and shapedinto a circular disc with adiameter of 10 mm (weight of 2.34 mg). Subsequently,appropriate amount of puresulfur wasevenlyput on the GO spongediscand then the sample wereput in quartztubesthat weresealedunder vacuum.Thesulfur impregnation wasfurther carried outby heating the samplein the vacuum-sealedquartz tube under 155uC for 10 h. theweight of the preparedS-GSwas11.68 mg, corresponding the sulfur content in thetotal cathode is 80%and the areal massloading of sulfur is , 11.90 mg cm2 2.Materials characterization . The compositeswere characterizedby X-ray diffraction(XRD) with Cu-Ka irradiation, Fourier transform infrared spectra(FT-IR), and four-probe resistivity measurementsystem.And field emission scanningelectronmicroscope (FESEM) wasapplied to observethe morphology of the synthesizedcompositematerial. Total sulfur loading (, 9.34 mg) in the final electrodesamplewascalculatedby weighing the samplebefore and after sulfur infusion (using a MettlerToledo MS105DU Semi Micro Balance,0.01 mg readability and 6 0.02 mgrepeatability).Figure 4 |The charge – dischargevoltage profiles of the 11th cyclefor the S-GScathode measured during galvanostatic cycled at 0.1C.Figure 5 |(a) Charge-discharge profiles at different cycle numbers aslabeled; (b) Cyclic performance and coulombic efficiency of the S-GScathode forLi-S battery at a current density of 0.1C for 300 cycle.www.nature.com/ scientificreportsSCIENTIFIC REPORTS | 4 : 4629 | DOI: 10.1038/srep04629 3Electrochemical measurement. The electrochemicalexperimental methods usedinthis work were similar to the onesin our previous study. The S-GScircular disc wasusedasthe cathode directly. 2032type coin cellswere assembledin an argon-filledglove box with lithium foil asthe anode.The separatorwaspurchasedfrom Cellgard(model 2400).The electrolyte was0.1 M lithium nitrate and 1.0 M lithium bis-trifluoromethane sulfonylimide in 1, 3-dioxolane and 1, 2-dimethoxyethane (volumeratio 151) (Zhangjiagang Guotai-Huarong New Chemical Materials Co., Ltd).Galvanostaticmeasurementswereconducted using a LAND CT2001A battery testsystembetween1.5 V and 3.0 V (vsLi 1 /Li).1. Shakoor,R. A. et al. Site-SpecificTransition Metal Occupation inMulticomponent Pyrophosphatefor Improved Electrochemicaland ThermalProperties in Lithium Battery Cathodes:A Combined Experimental andTheoretical Study.JAm ChemSoc134, 11740– 11748,doi:Doi 10.1021/Ja3042228(2012).2. Lu, S.T., Cheng, Y. W., Wu, X. H. DOI:10.1038/srep04629 (2014).This work is licensedunder a CreativeCommons Attribution-NonCommercial-NoDerivs 3.0 Unported License.The imagesin this article are included in thearticle ’sCreativeCommonslicense,unlessindicatedotherwisein theimagecredit;if theimageis notincluded undertheCreativeCommonslicense,userswill needtoobtainpermissionfrom thelicenseholderinorder to reproducetheimage.Toviewa copyof this license,visit http://creativecommons.org/licenses/by-nc-nd/3.0/www.nature.com/ scientificreportsSCIENTIFIC REPORTS | 4 : 4629 | DOI: 10.1038/srep04629 4