The Studies Of Sulfur-carbon Composite Cathode For Lithium-Sulfur Battery

Lithium-sulfur batteries?LSBs?,with high theoretical specific capacity of 1675mAh g^-1 and high theoretical energy density of 2600 Wh kg^-1 when used sulfur as cathode,have been regarded as one of the most potential high-energy chemical rechargeable power.In addition,the abundant sulfur and metal lithium resources,low price and environmental friendliness show advantages of LSBs for practical applications.Although LSBs have many advantages,they are facing some inevitable challenges as follows:?1?the highly electrical insulating nature of sulfur(5×10^-30 S cm^-1at 25?)and its discharge products?Li₂S₂,Li₂S?;?2?the shuttle effect of soluble lithium polysulfides?LiPSs?;?3?the discharging product Li₂S cannot generated in electrolytes,while partially deposited on electrode surface and?4?the volume variation?⁸0%?of sulfur cathode during lithiation/delithiation process,which causes an inadequate utilization of active sulfur,leads to inferior cycling durability,a poor rate capability and low coulombic efficiency.Thus,the commercialized application of LSBs has been hindered.More recently,investigators proposed many useful approaches to solve the problem of LiPSs by exploring polar host materials,including transitional-metal oxides,carbides,nitrides and sulfides,which guarantees a good conductive property,possesses greatly intensive chemical adsorption and ultrahigh conversion for LiPSs,and alleviates the volume expansion during discharging process.As mentioned previously,in this work,three novel excellent cathode materials of LSBs with a certain binding capability for LiPSs are prepared.The specific contents are as follows:?1?Sulfur/carbon composites with different sulfur storage approaches and sulfur content have great influence on the electrochemical performance.Sublimated sulfur?S?and low-cost acetylene black?AB?were used as raw materials.The typical methods of sulfur storage include hand-milled,in-site chemical deposition,liquid impregnation and melt-diffusion strategy receiving sulfur/carbon composites with different sulfur content.The electrochemical performance was studied by cyclic voltammetry?CV?measurement,galvanostatic charge/discharge?GCD?tests and electrochemical impedance spectroscopy?EIS?.The results show that sulfur content and sulfur loading methods have obvious influence on the electrochemical properties of sulfur/carbon composites.By melt-diffusion strategy,the sulfur/carbon composite with 50 wt%sulfur content shows impressive electrochemical performance.It's worth noting that the CV results show the highest peak current,the sharpest peak and minimum overpotential of the corresponding electrode.The sulfur/carbon composite has demonstrated an initial discharge capacity of916 mAh g^-11 at 0.2 C,over 50 cycles with the retention rate of 55%.And the excellent conductivity,rapid transmission of Li⁺/e^-and enhanced electrochemical reaction kinetics were further verified by the EIS results.?2?The preparation and electrochemical performance of HsGY@S composite.Our group synthesized a film-like carbon-rich framework which is named as hydrogen substituted graphyne?HsGY?through alkyne metathesis reaction using TPB?1,3,5-tripynylbenzene?as the precursor.XRD,Raman,solid-state NMR,FESEM,TEM and so on were applied to obtain the morphology,composition and structure of the composites.The HsGY film possesses a conductive carbon skeleton with triangle-like pores structure,and also contains inherent?-conjugated carbon skeleton comprised of acetylic linkages and benzene rings,which offers enough space for sulfur,LiPSs intermediates and the large irreversible volume change during the discharging process.In contrast with the commercial rGO@S electrode,HsGY@S electrode shows higher Li⁺/e^-mobility,faster kinetic and thermodynamic process,which is conducive to outstanding prolonged cycling performance and superior rate capability.?3?The preparation and electrochemical performance of Fe-functionalized mesoporous carbonaceous microsphere@sulfur?CMS900@S?composite.Ethanol was used as carbon source and reductant,FeCl₃·6H₂O and I₂ were treated as oxidants,and then undergone solvothermal and pyrolyzed at 900°C in vacuum.And it yielded a conducting Fe-functionalized carbon-based substrate.Sulfur storage is based on the typical in-site chemical deposition method and the melt-diffusion strategy.XRD,Raman,BET,FESEM,TEM and so on were carried out to analyze the morphology,composition and structure of the composites.The carbon microspheres with Fe/Fe₃C components exhibit high conductivity and provide some functional groups on the surface of carbon microspheres effectively immobilizing sulfur via strong chemical entrapment and thus suppressing the shuttle effect of LIPSs during cycling,CMS900@S composite contains80.8 wt%sulfur content.The electrochemical performance was studied by CV measurement,GCD tests and EIS,showing rapid electrochemical reaction kinetics,an excellent cycling property and the superior rate performance.For the long cycling performance,it shows a long-term cycling performance with a high initial discharge capacity of 1269 mAh g^-1,keeping a sulfur utilization of 77%at the 0.1 C rate,and maintaining a specific capacity of 744 mAh g^-1 after 200 cycles with the lowest capacity decay of 0.06%per cycle.During the rate test,CMS900@S cathode possesses a high initial discharge capacity of 1122 mAh g^-1 at 0.2 C and a tremendous rate capacity of511 mAh g^-1 at 2 C,which is an optimal candidate as a cathode for advanced LSBs.?4?The preparation and electrochemical performance of vanadium vioxide@reduced graphene oxide/sulfur?VO₂@rGO/S?composite.Based on GO with abundant oxygen-containing function groups,it was in favor of attachment to V₂O₅surface area.Thus,VO₂ was successfully grown on rGO surface through an easy and extensible solvothermal strategy with isopropanol as a reductant.The morphology,composition and structure of the composites were measured through XRD,Raman,BET,FESEM,TEM and so on.The VO₂@rGO/S composite has several apparent advantages,such as?1?a folded and wrinkled three-dimensionally framework to form half coated heterostructure are capable of sufficiently immersing electrolyte in an interconnected channel and holding the volume expansion during cycling;?2?each VO₂ nanoflake not only has greatly intensive chemical adsorption and ultrahigh conversion for LIPSs to inhibit the“shuttle issue”,but also provides good diffusion channels for Li⁺/e^-on graphene surface;?3?rGO nanosheets act as physical confinement for LIPSs and guarantee good conductive framework of sulfur cathode to enhance the electrochemical reaction kinetics;?4?the designed special heterostructure consists of VO₂ and rGO,performing a synergistic effect,thus leading to outstanding rate capabilities and prominent cycling stability over the long cycles.CV measurement,GCD tests and EIS were employed to confirm great cycling stability and reveal its impressive electrochemical performance for long lifespan LSBs.The VO₂@rGO/S composite with76.1 wt%sulfur content delivers a high initial discharge capacity of 1416 mAh g^-1 at 0.2C and a tremendous rate capacity of 663 mAh g^-1 at 2 C.It also shows an excellent long-term cycling performance that the VO₂@rGO/S,at the 0.2 C rate,offers a high sulfur utilization of 81%after undergoing a high initial discharge capacity of 1358 mAh g^-1,and maintains a specific capacity of 1049 mAh g^-11 with the lowest capacity decay of only 0.06%per cycle after 370 cycles.Moreover,we further studied the electrochemical performance of VO₂@rGO/S cathode with a higher loading of 4 mg cm^-2.A relatively high capacity of about 711 mAh g^-1(2.84 mAh cm^-2)at 0.5 C(3.35 mA cm^-2),and after20 cycles 685 mAh g^-1(2.74 mAh cm^-2)can still be achieved.In summary,this work studies the influence of electrochemical performance about sulfur/carbon composites with different sulfur storage approaches and different sulfur content.And three novel polar host materials were designed and successfully synthesized,then were applied to sulfur cathode in LSBs system.Based on the analysis of their morphology,composition,structure and electrochemical performance,the ameliorative sulfur-based composites possess a stable cycling performance and high ratio charge/discharge properties.Therefore,we are confident that the work provides forceful theoretical and experimental basis for the design of new sulfur-based cathode materials and the development of excellent performance LSBs in the future,and also lays a reliable theoretical foundation to develop LSBs with high specific energy in practical applications.