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ORIGINAL PAPERSulfur/mesoporous carbon composites combined with γ-MnS as cathode materials for lithium/sulfur batteriesJ.D.Liu &X.S.Zheng &Z.F.Shi &S.Q.ZhangReceived:26June 2013/Accepted:30October 2013/Published online:24November 2013#Springer-Verlag Berlin Heidelberg 2013Abstract With the aim to develop high-performance sulfur electrode,manganese sulfide (MnS)was combined with sulfur/porous carbon composite electrode by a simple precip-itation method.Both X-ray diffraction (XRD)and Fourier transform infrared spectroscopy (FTIR)results show that as-prepared MnS corresponds to Rambergite phase with a hex-agonal structure (γ-MnS).MnS could be uniformly dispersed in the carbon matrix when the content was less than 20wt%.When the content of MnS increased,γ-MnS particles aggre-gated on both outside of the mesoporous carbon channels and the surface of carbon particles.The CV curves of MnS/MC in the first cycle were similar to elemental sulfur,indicating partially decomposed MnS at the surface of mesoporous car-bon.Charge/discharge tests indicated that the initial discharge-specific capacity of S/MnS/MC was 1412.5mAh g −1and remained a reversible capacity of 727.4mAh g −1after 50cycles at a current of 100mA g −1,which is superior to that of sulfur composite electrode without MnS.Keywords Sulfur composite electrode .Manganese Sulfide .Deposition .Lithium/sulfur BatteriesIntroductionLithium –sulfur cell technology represents a dramatic advance over lithium-ion batteries and attracts increasing interest in developing high-density energy storage devices [1,2].Its specific energy is up to 2600Wh/kg assuming the complete reaction of sulfur with lithium to form Li 2S,which is 5–10times that of traditional Li-ion batteries [3].In addition,ele-ment sulfur is abundant,cheap,and environment-friendly.Lithium/sulfur couple is expected to become the next gener-ation of high-capacity energy storage system [4].The key issue with Li/S batteries to become a commercially viable technology is the poor reversibility of sulfur cathodes in presence of the polysulfides dissolved in the electrolyte [5,6].Some nano-structured S/C composites did exhibit a great improvement in the electrochemical utilization and cycling stability of sulfur cathodes [7].It is speculated that such improvement was induced by the sorption properties of po-rous carbon matrix in trapping the polysulfides [8].Porous carbon acted as a micro-container of polysulfides,which was formed in the reduction of S 8molecule,and inhibited its migration to negative electrode.The reduction process and the oxidation process of polysulfide are completed at the surface of porous carbon.The surface of porous carbon only serves as a transport channel of electrons [9].It is shown that Li +did take part in the reduction –oxidation process [10].While high-order polysulfides breaks into low-order polysulfides in the reduction process and low-order polysulfides combines into high-order polysulfides in the oxidation process,lithium-ion should be provided to maintain charge balance.It is our idea that an electrochemically favorable and highly cyclable sulfur cathode may be achieved if lithium intercalation compound such as transitional metal sulfides is combined into cathode.MnS displayed low Li insertion potential during the cycling process [11],so if MnS is combined into cathode,it may take part in the redox process of polysulfides and stabilize it.From this point of view,we prepared MnS/MC (mesoporous carbon)com-posites simply by precipitation and then vaporized sulfur into the mesoporous carbon microspheres by heat treat-ment to obtain S/MnS/MC composites.The morphology of MnS in the matrix of carbon was investigated,and theJ.D.Liu (*):X.S.Zheng :Z.F.Shi :S.Q.ZhangCollege of Chemistry and Chemical Engineering,Fuzhou University,Fuzhou 350108,China e-mail:ljd@Ionics (2014)20:659–664DOI 10.1007/s11581-013-1019-6electrochemical performance of S/MC and S/MnS/MC composite cathode was compared in detail. ExperimentalMaterial synthesisMnS A solution of1.00mmol of Mn(CH3COO)2·4H2O was injected into a solution of1.02mmol of Na2S·9H2O.After stirring for10min,a solution of4ml of N2H4·H2O+NH3 (volume ratio=1:3)was added.The reaction solution was heated to80°C and stirred for24h.A brown-colored precip-itate was collected by centrifugation and washed with distilled water for several times.The final product was dried in a vacuum box at80°C for12h.MnS/MC Mesoporous carbon(MC)was obtained by the established method[12].The synthetic procedure of MnS/ MC was almost the same as that of MnS.The only difference was that prior to reaction,0.1750g MC was added to a solution of1.00mmol of Mn(CH3COO)2·4H2O.The final product was black-colored.The content of MnS was20,33, and50wt%,respectively.S/MnS/MC0.1126g MnS/MC and0.1314g elemental sulfur were mixed and grinded for2h.It was sealed in a tube and then heated to155°C for6h.When cooled down,the tube was broken appropriately.A black-colored product was ob-tained and signed as S/MnS/MC.The content of sulfur was 54wt%.CharacterizationThe X-ray powder diffraction patterns of the solids were recorded on an X'Pert PRO Advance diffractometer(Cu Kαradiationλ=1.51418nm,working voltage35kV,working current35mA,2θmode scope10°–80°,and scan rate6°/ min).The morphology and microstructure of particles were observed using a Nova NanoSEM230Scanning Electron Microscope(America)at an accelerating voltage of10kV. Before observation,Au was deposited onto the surface of the particles(the differentiated ratio is1.5in SEI)to improve the conductivity.Energy dispersive X-ray spectroscopy(EDS) was taken on the SEM.N2adsorption–desorption analysis was measured on an Autosorb-1C-TCD instrument;pore vol-umes were determined using the adsorbed volume.Electrochemical testsThe working cathode was composed of active materials(S/ MnS/MC)(80wt.%),acetylene black(10wt.%)and poly(vinylidene fluoride)(Mw=2×106,10wt.%).N-methyl pyrrolidone(80wt.%)was added and grounded for4h.The resultant slurry was coated onto an aluminum foil with thick-ness of16μm and dried at80°C in a vacuum oven for12h to remove the residual solvent.LiCF3SO3(99%)was dissolved in mixed solvent of ethylene glycol dimethyl ether and diethylene glycol dimethyl ether(volume ratio=1:1)to get a liquid electrolyte.Lithium foils and UBE porous film(Japan) were used as anodes and separators;CR2025-type coin cells were fabricated with the above components in an argon cham-ber(ZKX-2,Nanking China).A Land CT2001A battery test system(Wuhan,China)was used to observe the batteries' charge–discharge capacity and cyclic ability.The cells were charged and discharged at a current value corresponding to 100mA g−1.The voltage range is1.5–3.0V(vs.Li+/Li). Cyclic V oltammetry was performed on a CH Instruments 660D electrochemical workstation(Shanghai China),lithium Fig.1IR patterns of the(a)MC(b)S/MC(50%)(c)MnS and(d) MnS/MC(20%)Fig.2XRD patterns of the(a)MnS(b)MnS/MC(20%)and(c)MnS/ MC(50%)foil was used as counter electrode and reference electrode,and the scan rate was 1mV/s.Results and discussionFigure 1a shows the X-ray diffraction (XRD)patterns of as-prepared MnS and MnS/MC samples.All diffraction peaks of MnS can be ascribed to the characteristic peaks of γ-MnS (JCPDS 40–1289),corresponding to Rambergite phase with a wurzite structure and the lattice parameters of a=b=0.39807,c=0.64514nm.Peaks of 2θ=36.1°and 2θ=49.6°can be seen in the XRD patterns of MnS/MC (50%).It shows that MnS/MC (50%)is the blend of γ-MnS and MC.These peaks are vague in XRD patterns of MnS/MC (20%).When the content of γ-MnS in MnS/MC is less than 20wt%,the XRD patterns of γ-MnS cannot be seen.We conclude that γ-MnS can be uniformly dispersed in the carbon matrix when the content is less than 20wt%.A mathematical analysis of the Bragg peaks was undertaken to calculate the crystallite size of γ-MnS in the MnS/MC (20%)composite using the Scherrer formula,and it was found that the particle size was ~267nm.Fig.3SEM images of the samples a ×5,000MC and b –e MnS/MC (20%);b ×5,000,c ×12,000,d ×30,000,e ×50,000of MC/MnS (20%),as well as f EDS Spetrum obtained from MnS/MC (20%)Figure 2shows the Fourier transform infrared spectroscopy (FTIR)spectra of the four different samples.No obvious absorption peaks are shown above 500cm −1for MC.The absorption peaks around 600cm −1with a shoulder in MnS are attributed to the Mn –S stretching vibrations.The broad peaks around 3,500cm −1in MnS or MnS/MC (20%)are assigned to –OH group that remained at the surface of samples.The peaks around 600cm −1are almost invisible in MnS/MC (20%),indicating certain interaction between MnS and carbon matrix.Figure 3shows the influence of MnS mixing on the mor-phology of MC.As shown,MC exhibited the primary sphere-like particles in the size of 1–2um.After γ-MnS impregna-tion,MC in MnS/MC composite still exhibited a sphere-like morphology,as shown in Fig.3b –e .A large number of γ-MnS particles aggregated together to form an irregular coral-like bulk.It can be observed that γ-MnS particles were distributed both outside the mesoporous carbon channels and the surface of sphere carbon particles.The size of γ-MnS particles in the MnS/MC composite ranged from 100to 300nm,which was in good agreement with the results calculated by the Scherrer formula.The chemical composition of the MnS/MC compos-ites was measured by EDS,as depicted in Fig.3f .It was confirmed that the MnS/MC composites were mainly com-posed of carbon,manganese,and sulfur elements.The content of sulfur was larger than that of manganese,which was due to extensive amount of Na 2S.The N 2adsorption –desorption isotherms of mesoporous carbon MC and MnS/MC (20%)composites are depicted in Fig.4,which exhibited a typical IV-type isotherm with a characteristic of mesoporous materials.The BET surface area and pore volume were 1024m 2/g and 1.1cm 3/g for MC,and 521m 2/g and 0.58cm 3/g for MnS/MC (20%)composites,respectively.Obviously,the BET surface area and pore vol-ume of MnS/MC (20%)composites decreased dramatically,indicating the high loading of MnS in the mesochannels of MC.Q u a n t i t y A b s o r b e d (c m 3/g )Relative Pressure(P/P 0)Relative Pressure(P/P 0)V o l u m e a b s o r b e d (c m 3/g )Fig.4N 2absorption –desorption isotherms of a MC and b MnS/MC (20%).Inset:pore size distributions from the desorption branch through the BJHmethod.0.51.01.52.02.53.0-2246C u r r e n t /m AVoltage/ V vs Li +/LiV o l t a g e (V )Capacity (mAh/g)aFig.5a CV curves of MnS/MC (20%)from 0.5to 3.0V at a scan rate of 1mV/s;b charge –discharge curves of MnS/MC (20%)at a current of 100mA g −1.Specific capacity was calculated by mass of MnS in cathode.Figure 5illustrates the CV curves and charge –dis-charge curves of MnS/MC (20%).The CV curves of MnS/MC (20%)showed two broad cathodic peaks at 2.17and 1.3V,and one anodic peak at 2.57V,respec-tively.Its electrochemical behavior was similar to ele-mental sulfur.It was believed that MnS partially decomposed to elemental sulfur in initial discharge,since MnS is highly active in the surface of the meso-porous carbon.The reaction corresponding to formation of elemental sulfur may be MnS=Mn+S.The initial capacity was high (1380mAh g −1),but dropped drasti-cally at the second cycle (Fig.5b ).The same phenom-enon was also found in literature [11].Since sulfur content in MnS was low,S n 2−(n >6)diffused away from cathode and lost its electrochemical activity;only con-centration of polysulfides reaches certain amount can it shows electrochemical activity.The capacity at subse-quent cycles at 1.5V plateau ranged from 400to 500mAh g −1,indicating that Li-ion inserted into MnS.Figure 6a,b showed the first and the second charge/discharge curves of S/MnS/MC and S/MC cathodes at 100mA g −1.Three plateaus were clearly shown in the dis-charge curve of S/MnS/MC at 2.45,2.15,and 1.80V,corresponding to the elemental sulfur (S 8)reduction to S 62−,S 42−,and S 22−(or S 2−),respectively.In charging process,the voltage showed a small drop in very beginning,as indicated by a circle in Fig.6b .This phenomenon was vague in Fig.6a .This voltage drop can be attributed to the chemical dissolution (oxidization)of Li 2S and Li 2S 2by the high-order polysulfides (PS)formed in the very beginning of charging process,which has been known to contribute to the reversibility of Li/S cells [9].The capacity at low plateau of S/MnS/MC (about 500mAh g −1)was larger than S/MC (about 400mAh g −1),so it was believed that MnS was helpful to enhance revers-ibility of composite cathode.The initial discharge capacities for S/MnS/MC and S/MC were 1412.5and 1215.1mAh g −1,respectively (Fig.4b ).After the first cycle,the discharge capacities of S/MnS/MC and S/MC decreased to 1268.2and 1018.2mAh g −1,respec-tively,which indicated a capacity loss of 144.3and 196.9mAh g −1.The capacity loss reflected the extent of polysulfides dissolved loss.From the results,we can conclude that the dissolved loss of polysulfides in S/MnS/MC was low compared with that of S/MC.After 50cycles,the discharge capacities of S/MnS/MC degraded to 727.4mAh g −1,while that of S/MC wasV o l t a g e (V )Capacity (mAh/g)V o l t a g e (V )Capacity (mAh/g)400600800100012001400d i s c h a r ge c a p a c i t y / m A h .g -1Cycle NumberFig.6a Charge –discharge curves of S/MnS/MC (1:1:1mass ratio),b charge –discharge curves of S/MC (1:1mass ratio),and c cycling performance of S/MnS/MC (1:1:1),S/MC (1:1),and MnS/MC (20%)at a current density of 100mA g −1.Specific capacity was calculated by mass of sulfur in cathode529mAh g−1.The decay rate of S/MnS/MC was relatively low compared with that of S/MC cathode.ConclusionThe MnS loaded S/MC cathodes were prepared by the method of deposition in liquid phase and employed as electrodes for lithium sulfur batteries.γ-MnS can be partially decomposed to elemental sulfur and manganese,due to its highly activity at the surface of porous carbon.By introducing MnS aggrega-tion phase,the reversibility of cathode was enhanced and the electrochemical performances were greatly improved. 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