聚合物电池的新材料.pdf

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1、Advanced electrolyte and electrode materials forlithium polymer batteriesF.Croce,A.DEpifanio,J.Hassoun,P.Reale,B.Scrosati*Departimento di Chimica,University di Roma La Sapienza,Rome 00185,ItalyAbstractThe most recent results obtained in our laboratory on the characterization of two classes of polyme

2、r electrolytes and of olivine-type lithiumiron phosphate electrodes are reviewed and discussed,especially in view of their application in advanced lithium batteries.#2003 Elsevier Science B.V.All rights reserved.Keywords:Lithium batteries;Polymer electrolytes;Lithium phosphate electrodes1.Introducti

3、onThe next step in the advancement of the lithium batterytechnology is expectedto be the replacement of the commonliquid electrolyte with a highly conducting polymer mem-brane.Indeed,thin-film,solid polymer electrolyte recharge-able lithium batteries are expected to overcome theperformance of conven

4、tional liquid electrolyte systems.Inaddition,the large-scale production of solid-state batteriescan benefit from the well-established technologies devel-oped in the polymer industry 1.The key component of the lithium polymer battery is theelectrolyte.The proper choice of this component is ruled bya

5、series of requirements which include high ionic conduc-tivity,good mechanical properties and compatibility withthe electrode materials.Two classes of electrolytes can beconsidered.One involves solvent-free membranes formedby blending poly(ethylene oxide)(PEO)with a lithium salt,LiX and the other inc

6、ludes gel-type membranes formed byimmobilization and/or swelling of selected liquid solutionsinpolymermatrices.Inthispaper,wediscusstheapplicationof both classes in advanced,rechargeable lithium polymerbatteries.2.Lithium polymer batteriesPolymer electrolytes formed by blending poly(ethyleneoxide)wi

7、th a lithium salt,LiX,are of interest for applicationas advanced separators in rechargeable lithium cells.Theuseful operating temperatures of these polymer electrolytesare in the 70100 8C range since below 70 8C the poorconductive PEO crystalline phase is stable 2.We haveshown that the conductivity

8、can be improved by the additionto the PEOLiX matrix of active ceramic powders atnano-particle size in order to form nano-composite polymerelectrolytes 3.A typical example is obtained by dispersing10 wt.%of Al2O3(or SiO2)in a PEOLiCF3SO3blend.Thisnano-composite electrolyte is hereafter simply noted a

9、sPEOLiCF3SO3 10w/o SiO2.It has been demonstrated that the ceramic filler enhancesboth the polymer chains flexibility and the polymer chainssolvating power 4,thus finally leading to consistentimprovementsinconductivity.Forinstance,theconductivityof the PEOLiCF3SO3 10w/o Al2O3electrolyte variesfrom10?

10、3to10?5S cm?1passingfrom100to20 8C(versusthe 10?3to 10?7S cm?1values of the ceramic-free counter-part in the same temperature range).In addition to high conductivity,nano-composite electro-lytes have a series of favorable properties which make themof particular value in view of battery application.O

11、ne is theimproved interfacial stability towards the lithium metalelectrode.This is shown in Fig.1 which illustrates the timeevolution of the impedance of a symmetrical Li/nano-com-posite/Li cell kept under open-circuit conditions at 95 8C5.The impedance response evolves as a semicircle whoselow-freq

12、uency intercept is representative of the Li/electro-lyte interface resistance,Ri6.It may be clearly seen thatthe semicircle does not consistently expand upon time,thusfinally demonstrating the invariance of Riand thus,thestability of the interface.Journal of Power Sources 119121(2003)399402*Correspo

13、nding author.Tel.:39-06-446-2866;fax:39-06-491-769.E-mail address:scrosatiuniroma1.it(B.Scrosati).0378-7753/03/$see front matter#2003 Elsevier Science B.V.All rights reserved.doi:10.1016/S0378-7753(03)00260-XThis high stability,which is welcome in terms of theefficiency of the lithium cycling proces

14、s,is supposed to bepromoted by the dispersed ceramic powders,e.g.Al2O3,SiO2,TiO2powders,by specific shielding and scavengingactions.In fact,the ceramic dispersion reinforces themechanical properties of the electrolytes,thus leading tohard interfaces which are expected to inhibit lithium den-drites.I

15、n addition,due to their affinity for liquids,theceramics trap away from the interface traces of liquids(e.g.residual casting solvent),i.e.of those impurities whichare generally very aggressive versus lithium metal.Another beneficial effect of the dispersed ceramics is theenhancement of the lithium t

16、ransference number,tLi.Thiseffect,which reflects positively on the kinetics of thechargedischarge process of the lithium battery,is asso-ciatedtotheceramicssurfacestateswhichcompetewiththepolymer chains in coordinating the lithium salt cations andanions 7.This correlation has been demonstrated by di

17、s-persing ceramics characterized by different extent of sur-faces,and observing a corresponding different increase inone of the tLiof the resulting nano-composites 7.To benoticed that by a proper selection of the ceramic filler,enhancement in tLiup to 30%respect to ceramic-freeelectrolytes can be ac

18、hieved 7.All these favorable properties are somewhat contrasted bythe relatively low electrochemical stability window,sincenano-composite electrolytes suffer of the limited oxidativestability of PEO.Fig.2 shows the currentvoltage curve of astainless-steel electrode in a P(EO)20LiCF3SO3 10w/wAl2O3pol

19、ymer electrolyte cell.The onset of the current,which is representative of the decomposition of the electro-lyte,occurs around 4.0 V versus Li,this indeed suggestingthat the choice of the cathode to be used with this electrolytemay be a critical factor.Among the various possible cathodematerials,iron

20、 phospho-olivines,i.e.LiFePO48 seemparticularly convenient.Indeed,LiFePO4is the perfectcathode for PEO-based lithium batteries due to the flatnessof its two-phase,chargedischarge process which evolves inthe 3.5 V range,i.e.within the stability window of theelectrolyte.However,common iron phosphate e

21、lectrodessuffer from loss of capacity with increasing current density,associated to the diffusion-limited transfer of lithium acrossthe two-phase interface 8.We have shown that an effectiveway to by-pass the aforementioned kinetic limitation is toenhance the iron phosphate inter-particle electronic

22、contactby suitable doping 9.Indeed,optimized metal-doped LiFePO4cathodes behavequite well in lithium,nano-composite electrolyte batteries,as demonstrated by the typical example reported in Fig.3.The battery,which operates on the basis of the followingchargedischarge process:LiFePO4,xLi Li1?xFePO4(1)

23、can be cycled several times with a very limited capacityfading 10.It is also to be noticed from Fig.3 that the cycles evolvewith a chargedischarge efficiency approaching 100%,thisFig.1.Time evolution of the impedance spectra of the Li/P(EO)20-LiCF3SO3 10w/o Al2O3/Li cell stored under open-circuit co

24、nditions at95 8C.Fig.2.Currentvoltagecurveofastainless-steelelectrodeinaP(EO)20LiCF3SO3 10w/w Al2O3polymer electrolyte cell.Counter andreference electrode:Li;t 70 8C;scan rate 0:1 mV s?1.The onset ofthe current occurs at the decomposition voltage of the electrolyte.Fig.3.Cycling response of a Li/P(E

25、O)20LiCF3SO3 10w/w Al2O3/LiFePO4polymer cell at various rates and at 105 8C.400F.Croce et al./Journal of Power Sources 119121(2003)399402confirming the ongoing of a smooth and reversible lithiumstripping-deposition process,as that assured by the abovediscussedlithium/nano-compositeinterfacialstabili

26、ty.Finally,the high rates at which the battery operates confirmthat the metal-doped iron phosphate electrode is indeedcharacterized by fast kinetics.However,although quite satisfactory in terms of cycle lifeand rate capability,the lithiumiron phosphate,nano-com-posite polymer battery can efficiently

27、 operate only above70 8C since the conductivity of the electrolyte is still too lowat lower temperatures.Therefore,this battery can be profit-ably addressed to those applications where temperature isnot a critical parameter,e.g.in the electric vehicle area.For all the other cases,a different class o

28、f polymerelectrolytes is required.A good choice is provided bygel-type membranes formed by the immobilization and/orswelling of selected liquid solutions in a polymer matrix.Atypical example of these membranes is that prepared byswelling a poly(vinylidene fluoride)(PVdF)matrix with aLiPF6ethylene ca

29、rbonatepropylene carbonate solution11.Hereafter,this membrane will be simply noted asLiPF6ECPCPVdF.Also these gel-type membranes can be profitably used aspolymer electrolytes in lithium batteries since,despite theliquid component,they retain a good mechanical integrity(see Fig.4).Furthermore,the con

30、ductivity of these mem-branes is quite high over a wide temperature range,whichinclude ambient and subambient regions.For instance,theconductivity of the LiPF6ECPCPVdF gel membranevaries from 2?10?3to 5?10?4S cm?1passing from 20to?20 8C.Metal-added,modified lithium iron phosphate can be thecathode o

31、f choice also for lithium batteries based on the gelpolymerelectrolytes.Fig.5comparesthecyclingresponseofone example of these batteries with that of a similar batterywhere the gel polymer electrolyte is replaced by the swellingliquid electrolyte.There is no consistent difference betweenthe two cases

32、,with even a slightly improved response for thegel battery.This demonstrates that the selected gel electro-lytes may indeed be competitive with common liquid solu-tions,probablybecauseoftheirhighchemicalandelectrochemical stability.Finally,Fig.5 shows that the bat-teries may operate at very high rat

33、es,this confirming theimproved kinetics of the modified iron phosphate cathode.3.ConclusionThe results here reported show that two classes of elec-trolytes can be profitably used for the development ofadvanced lithium polymer batteries.The first class considersPEOLiX-based,low particle size ceramic-

34、added,nano-composite membranes.These electrolytes can be addressedto batteries operating in the 70100 8C temperature rangewith relevance for the electric vehicle market.The secondclass includes gel-type membranes prepared by swellingsuitable polymer matrices,e.g.PVdF matrices,with liquidlithium salt

35、 solutions.These electrolytes have a high con-ductivity over a wide temperature range and thus,they canbe profitably used for the development of various types ofbatteries,including lithium ion batteries.AcknowledgementsThe financial support of MIUR,COFIN 2002 is acknowl-edged.References1 W.van Schal

36、kwijk,B.Scrosati(Eds.),Progress in Lithium IonBatteries,Kluwer Academic Publishers,New York,2002.Fig.4.Appearance of the LiPF6ECPCPVdF gel membrane.Fig.5.Cycling response of Li/electrolyte/Ag-added LiFePO4batteries atvarious discharge rates and at room temperature.Electrolyte:LiPF6ECPCPVdF gel membr

37、ane and LiPF6ECPC liquid solution,respectively.The cycling capacity is referred to the LiFePO4cathode.Charge rate:first40 cycles,0.2 C;remaining cycles,1 C.F.Croce et al./Journal of Power Sources 119121(2003)3994024012 F.M.Gray,Polymer Electrolytes,Royal Society of ChemistryMonographs,Cambridge,1997

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39、etecchi,F.Croce,L.Persi,F.Ronci,B.Scrosati,Electrochim.Acta 45(2000)1481.8 A.K.Padhi,K.S.Nanjundaswamy,J.B.Goodenough,J.Electrochem.Soc.144(1997)1188.9 F.Croce,A.DEpifanio,J.Hassoun,A.Deptula,T.Olczac,B.Scrosati,Electrochem.Solid State Lett.5(2002)A47.10 F.Croce,F.Serraino Fiory,L.Persi,B.Scrosati,Electrochem.SolidState Lett.4(2001)A121.11 L.Persi,F.Croce,B.Scrosati,Electrochem.Commun.4(2002)929.402F.Croce et al./Journal of Power Sources 119121(2003)399402