Sulfur–carbon nano-composite as cathode for rechargeable
lithium battery bad on gel electrolyte
J.L.Wang *,J.Yang,J.Y.Xie,N.X.Xu,Y.Li
Energy Science and Technology Laboratory,Shanghai Institute of Microsystem and Information Technology,Chine Academy of Sciences,
Shanghai 200050,China
Received 25March 2002;received in revid form 17April 2002;accepted 18April 2002
Abstract
Sulfur–carbon nano-composite with elemental sulfur incorporated in porous carbon was prepared by thermal treatment of a mixture of sulfur and active carbon.The new material was characterized by X-ray diffraction,BET and scanning electron mi-croscopy.The nano-composite,tested at room temperature as cathode in a nonaqueous lithium cell bad on PVDF gel electrolyte,exhibited a reversible capacity of 440mAh g À1at a current density of 0.3mA cm À2.The utilization of electrochemically active sulfur w
as about 90%assuming a complete reaction to the product of Li 2S during cycling.Ó2002Elvier Science B.V.All rights rerved.
Keywords:Elemental sulfur;Nano-composite;Porous carbon;Gel electrolyte;Lithium battery
1.Introduction
On the consideration of the cost and toxicity of LiCoO 2ud extensively in currently commercialized lithium batteries,there is an increasing interest to re-arch other cathodes for rechargeable lithium batteries,such as Li ðAlCo ÞO 2[1,2],vanadium oxide or its nano-composite [3,4],LiFePO 4[5,6]and layer-structured LiMnO 2[7,8].However,all the transition metal oxide cathode materials show relatively low specific capacity.A battery bad on the lithium/elemental sulfur has,in contrast,almost the highest theoretical capacity of 1672mAh g À1of active material and theoretical specific energy of 2600Wh kg À1in all known cathode materials,assuming a complete reaction as the following,2Li þS !Li 2S.Therefore,combining with abundant sources of elemental sulfur in nature,Li/S battery shows great potential for next generation of lithium batteries,such as,micro-batteries for small-size electronic devices em-phasizing the high charge density and power sources for electric vehicles for its low cost.Previous studies of Li/S
cells with liquid electrolytes faced the rious problems of low active material utilization and poor recharge-ability [9].The difficulties led many rearchers to utilize organic sulfides instead of elemental sulfur,which results in a significantly lower theoretical capacity [10–12].
Low active material utilization of elemental sulfur cathode may be due to the dissolution of sulfide into electrolytes,reduced conductivity and insulated reaction products covering the sulfur particles,which prevent a further reaction with the interior active material [13].In this paper,we exploit highly disperd sulfur embedded in porous carbon as cathode material to overcome the above-mentioned problems.
2.Experimental
描写大雪的诗句
2.1.Preparation and characterization of sulfur–carbon nano-composites
Active carbon is an excellent material for superca-pacitor for its very large surface area and abundant nano-and micro-pores [14].Active carbon (Nanjing Forestry Institute,Nanjing,China)with main pore
size
Electrochemistry Communications 4(2002)
499–502
七嫂导航*
Corresponding author.Tel.:+86-21-62511070;Fax:+86-21-32200534.
E-mail address: (J.L.Wang).
1388-2481/02/$-e front matter Ó2002Elvier Science B.V.All rights rerved.PII:S 1388-2481(02)00358-2
of2.5nm and surface area of1080m2gÀ1was mixed with elemental sulfur by a weight ratio of1:5.The mixture was heated to200°C to make elemental sulfur melt and this temperature was kept for6h for melted sulfur to diffu into the pores of active carbon.Then the temperature was enhanced to300°C and kept for3h to vaporize the sulfur covering on the outside surface of active carbon.The thermal treatment was performed under Ar protection.The characterization of the com-posite was carried out by means of X-ray diffraction (XRD,D/max2550V),Brunauer–Emmett–Teller(BET, ASAP2010V5.02)and scanning electron microscopy (SEM,JEOL JSM-6700F).
2.2.Preparation and analysis of PVDF/HFP gel electro-lyte
Gel electrolyte membranes were prepared by pha paration method[15].Three g PVDF–HFP copolymer (Kynar2801,Elf Atochem)and1g nano-SiO2powder were dissolved and disperd in30ml acetone under stirring for3h in a aled container.Then24ml ethanol was slowly added and the gel solution was stirred for3h. The resulting slurry was cast on a glassflat manually and the solvents were evaporated at ambient temperature. With this procedure,approximately60l m-thick, mechanically stable membranes were obtained.The membrane after vacuum drying at50°C can absorb about70wt%liquid electrolyte of PC–EC–DEC(1:4:5, v/v)containing1M LiPF6(from Mitsubishi Chemical). The ionic conductivity of the gel electrolyte after wiping the superfluous liquid was about1.2mS cmÀ1.This value was obtained by running impedance spectroscopy of a cell formed by sandwiching the sample between two stainless steel blocking electrodes.
2.3.Cell asmbling and testing
To fabricate cathodes,70wt%above-mentioned composite containing30wt%sulfur was mixed with20 wt%acetylene black and10wt%polytetrafluroethylene (PTFE)with ethanol rved as dispersant.The mixture was presd onto a nickel mesh[16].It should be men-tioned that carbon mesh is also a good current collector and can replace nickel mesh ud her.After drying un-der vacuum,the cathode and electrolytefilm were soaked in the liquid electrolyte for a while and the su-perfluous liquid was wi
ped.CR2025-type coin cell was asmbled in a glove box with H2O and O2content below1ppm.Anode was lithium sheet.The charge and discharge performances of the cells were investigated with Arbin Cycler(USA)at a current density of0.3mA cmÀ2between1.0and3.0V at room temperature.A Solartron Model1260Frequency Respon Analyzer coupled with1286electrochemical interface was ud for cyclic voltammograms of the cell.3.Results and discussion
Fig.1exhibits the X-ray diffraction patterns for ele-mental sulfur and porous carbon and sulfur–carbon composites.The sulfur in the composite is amorphous. The diffraction peak of the composite at28.7(2h)(C8, d¼3:0295,a¼4:28A)decreas compared with po-rous carbon and even disappears with higher sulfur content as shown in Fig.1(d).The results suggest that there may be some interaction between sulfur and active carbon and the sulfur in the composite is in a highly disperd state.SEM measurements show that the morphology of the composite is very analogous to that of porous active carbon with about100nm size aggre-gated particles.On the other hand,BET tests indicate that the surface area of the composite reduces dramat-ically from initial1008:9m2gÀ1of active carbon to 117:7m2gÀ1.From the test results of view,sulfur of the composite is probably incorporated in the nano-and micro-pores of active carbon.During heating process, molten sulfur,especially sulfur gas in higher tempera-ture period,will diffu into the pores and be trapped in
side becau of the high surface energy and excellent absorbability of active carbon.In fact,the strong trap-ping effect of active carbon makes it possible to prepare the sulfur–carbon composite at a temperature higher than the sublimation point of sulfur.
Fig.2illustrates cyclic voltammograms of composite electrode/gel electrolyte/lithium cell.A cathodic peak near2.3V disappears in the following cycles,
suggesting
an irreversible reaction occurs at this voltage in the first lithium inrtion process.That another cathodic peak near 1.3V shifts toward positive direction (closing to the anodic peak)from cycle to cycle is an indication of the improvement of reversibility of the cell with increasing cycle number.Fig.3prents discharge and charge profiles of cell for the first veral cycles.The composite containing 30wt%sulfur shows good electrochemical reversibility after the first cycle and the average charge and discharge voltages are stabilized at ca.2.3and 1.8V,respectively.
We assume that the sulfur embedded in the fine pores may form some special complexes with carbon or sur-face-bonded oxygen for strong absorbability of active carbon.Therefore,the electrochemical reaction between sulfur and lithium during discharge will need an addi-tional energy to break such complexes,leading to a high electrochemical polarization and a lower discharge voltage than pure sulfur.The charge ,lith-ium extraction,may not involve in such an additional
energy.After lithium extraction,the complexes will re-form but with a lower absorbing energy and the cell exhibits a higher discharge voltage after first cycle.The short plateau near 2.3V in the first discha
rge ems to be related to a slight amount of sulfur outside the pores.With increasing sulfur content in the composite,the discharge voltage about 2.3V become longer.But the cell rechargeability get poor and behave as traditional Li/S batteries rearched by other group [13].
The initial discharge capacity of the composite is about 800mAh g À1,which is larger than 490mAh g À1of theoretical capacity of the composite assuming that sulfur in the composite is completely reduced to Li 2S.The initial high irreversibility may be related little with sulfur becau about 90%of sulfur in the composite reacts reversibly with lithium during the following cy-cles.On the other hand,most of the initial irreversible capacity could not be attributed to electrochemical re-actions of the active carbon matrix,possibly,via irre-versible intercalation,surface filming and the reduction of impurities as the electrodes bad on pure active carbon had only a capacity of 123mAh g À1for the first lithium inrtion (e Fig.3)and a residual capacity of ca.60mAh g À1during cycling,performed just like a supercapacitor [17].A better understanding of the initial high irreversibility remains to be done.
Fig.4shows the cycling performances of the com-posite cathode.The sulfur–carbon composite material demonstrates good cyclability with a stable capacity about 440mAh g À1,which means the utilization of sulfur approaches 90%,following to the reaction,2Li þS !Li 2S.Furthermore,its charge–di
scharge effi-ciency clos to 100%after the first two cycles.The composite with the structure of elemental sulfur em-bedded in pores of active carbon can effectively prevent the dissolution of sulfide and improve the utilization of sulfur.The cycling test is still under way.As a com-parison,a mixture containing the same sulfur
content
prepared by milling ground for30h with300rps,in which sulfur is mostly dissociated on the outside surface of active carbon,cannot be charged,although there is a voltage plateau near2.3V with a specific capacity less than100mAh gÀ1for thefirst discharge.
4.Conclusions
We have demonstrated a simple and effective prepa-ration method of sulfur–carbon nano-composite and gel electrolyte membrane.The composite with a structure of elemental sulfur embedded and trapped in nano-and micro-pores of active carbon shows good cyclability in rechargeable lithium cell bad on gel electrolyte.The reversible capacity of the composite is about440mAh gÀ1and the utilization of sulfur during cycling ap-proaches90%.An apparent drawback of this cell is the relatively low voltage.The low voltage battery with high charge density can eventuallyfind a potential market with the development of electronics,especially SOI (Silicon on Insulator)miconductors for about1.8V or even a lower activation voltage.We will optimize the carbon matrix with suitable pore size and structure for the sulfur–carbon composites to increa the sulfur content,that is,to increa the specific capacity,and further to enhance the electrochemical performances. Acknowledgements
锡纸茄子We thank Dr.Y.X.Yang and G.F.Chen for the uful scientific discussion on the material characterizations.References
[1]G.Ceder,Y.M.Chiang,D.R.Sadoway,M.K.Aydinol,Y.I.Jang,
B.Huang,Nature392(1998)694.
[2]Y.I.Jang,B.Huang,H.F.Wang,D.R.Sadomway,G.Ceder,
Y.M.Chiang,H.Liu,H.Tamura,J.Electrochem.Soc.146(1999) 862.
[3]E.Spahr,P.S.Bitterli,R.Nesper,O.Haas,P.Novak,
乌梅孕妇能吃吗J.Electrochem.Soc.146(1999)2780.
[4]B.C.Gill,D.R.Shackle,T.N.Andern,J.Electrochem.Soc.147
(2000)3575.
[5]S.F.Yang,P.Y.Zavalij,M.S.Whittingham,Electrochem.Com-
mun.3(2001)505.
残疾人工作[6]L.Persi,F.Croce,B.Scrosati,Electrochem.Commun.4(2002)
92.
[7]M.Yoshio,H.Nakamura,Y.Xia,Electrochem.Acta45(1999)公务员个人简历
273.
[8]Y.Xia,T.Fujieda,K.Tatsumi,P.P.Prosini,T.Sakai,
J.Electrochem.Soc.147(2000)2050.
[9]H.Yamin,E.Peled,J.Power Sources9(1983)281.
[10]M.Liu,S.J.Visco,C.D.Jonghe,J.Electrochem.Soc.138(1991)
1891.
[11]N.Oyama,T.Tatsuma,T.Sato,T.Sotomura,Nature373(1995)
598.关于庐山的诗句
[12]F.Matsumoto,M.Ozaki,Y.Inatomi,S.C.Paulson,N.Oyama,
Langmuir15(1999)857.
[13]Marmorstein,T.H.Yu,K.A.Striebel,F.R.Mclarnon,J.Hou,
E.J.Cairns,J.Power Sources89(2000)219.
[14]R.C.Hayward,P.A.Henning, B.F.Chmelka,G.D.Stucky,
Micropor.Mesopor.Mater.44–45(2001)619.
唯美句子简短走心
[15]A.Magistris,P.Mustarelli,E.Quartarone,P.Piaggio,A.Bottino,
Electrochim.Acta46(2001)1635.
[16]T.Sotomura,T.Tatsuma,N.Oyama,J.Electrochem.Soc.143
(1996)3152.
[17]J.Gamby,P.L.Taberna,P.Simon,J.F.Fauvarque,M.Chesneau,
J.Power Sources101(2001)109.
502J.L.Wang et al./Electrochemistry Communications4(2002)499–502
