EA电子版

This article appeared in a journal published by Elsevier.The attached copy is furnished to the author for internal non-commercial research and education use,including for instruction at the authors institutionand sharing with colleagues.Other uses,including reproduction and distribution,or selling or licensing copies,or posting to personal,institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle(e.g.in Word or Tex form)to their personal website orinstitutional repository.Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:/copyrightElectrochimica Acta 89 (2013) 334–338Contents lists available at SciVerse ScienceDirectElectrochimicaActaj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c taElectrochemical performance of trimethylolpropane trimethylacrylate-based gel polymer electrolyte prepared by in situ thermal polymerizationDong Zhou a ,Li-Zhen Fan a ,∗,Huanhuan Fan a ,Qiao Shi ba Institute of Advanced Materials and Technology,University of Science and Technology Beijing,Beijing 100083,China bShenzhen Capchem Technology Co.,Ltd.,Shenzhen 518118,Chinaa r t i c l ei n f oArticle history:Received 15October 2012Received in revised form 20November 2012Accepted 23November 2012Available online 30 November 2012Keywords:Lithium ion batteries Polymer electrolyteTrimethylolpropane trimethylacrylate In situ thermal polymerizationa b s t r a c tCross-linked trimethylolpropane trimethylacrylate-based gel polymer electrolytes (GPE)were prepared by in situ thermal polymerization.The ionic conductivity of the GPEs are >10−3S cm −1at 25◦C,and contin-uously increased with the increase of liquid electrolyte content.The GPEs have excellent electrochemical stability up to 5.0V versus Li/Li +.The LiCoO 2|TMPTMA-based GPE |graphite cells exhibit an initial dis-charge capacity of 129mAh g −1at the 0.2C ,and good cycling stability with around 83%capacity retention after 100cycles.Both the simple fabricating process of polymer cell and outstanding electrochemical performance of such new GPE make it potentially one of the most promising electrolyte materials for next generation lithium ion batteries.© 2012 Elsevier Ltd. All rights reserved.1.IntroductionLithium-ion batteries are the power sources of choice for future energy storage device,since they are characterized by high energy density,high working voltage,high efficiency,long life and memory-effect-free [1].In recent years,many approaches have been employed to develop lithium-ion polymer batteries with gel polymer electrolytes (GPE),which have little security problems caused by liquid electrolyte leakage as well as flexible process-ability.Many polymer matrixes have been frequently used in gel polymer electrolytes,such as poly(ethylene oxide)(PEO)[2,3],poly(methyl methacrylate)(PMMA)[4,5],poly(acrylonitrile)(PAN)[6,7],and poly(vinylidene fluoride-hexafluoro propylene)(PVDF-HFP)[8,9].These types of electrolytes are generally prepared by dissolving the polymer matrixes in the liquid electrolyte directly,and then solvent-casting the gelled solutions into electrolyte films.Even if the GPEs can achieve high ionic conductivities of around 10−3S cm −1at room temperature [10],their poor dimensional stabilities and cumbersome preparation processes are usually inad-equate for practical application.Recently,in situ synthesis process has been increasingly attracted extensive attention in the preparation of GPEs.In this process,liquid electrolytes,initiators and monomers with low molecular weight,such as acrylic esters [11–14],ethylene oxide∗Corresponding author.Tel.:+861062334311;fax:+861062334311.E-mail address:fanlizhen@ (L.-Z.Fan).[15],are injected into the battery package.Cross-linked GPEs are prepared by free radical polymerization of monomers triggered by methods such as thermal initiation [16–18],UV [19],␥-ray irradia-tion [20]in the presence of liquid electrolyte solvent,and polymer Li-ion batteries are directly formed at the same time.The resulting products are consisted of the polymer network and the liquid elec-trolyte that is the main Li +ion conductor.This synthesis process can provide a simple solution to the preparation and performance optimization of the GPEs.Here,we report a GPE that was prepared by radical polymer-ization using trimethylolpropane trimethylacrylate (TMPTMA)as a new monomer,lauroyl peroxide (LPO)as a thermal initiator in a liquid electrolyte solvent.TMPTMA has three vinyl bonds at the end of the chain and their cross-linked networks are expected to be of high mechanical strength as well as good compatibility with liquid electrolytes.Electrochemical performance of the resulted GPEs prepared by thermal polymerization is investigated with LiCoO 2|GPE |graphite cells.Especially,this study discusses the influ-encing factors of the ionic conductivity and gelation time,which is rarely reported.2.ExperimentalGPE films were prepared by thermal polymerization of a precur-sor solution in a sealed transparent glass container.All the reagents were of battery grade and used without further purification.The precursor was composed of a monomer,an initiator and a liq-uid electrolyte.The TMPTMA (C 18H 26O 6,M w :338)was used as a0013-4686/$–see front matter © 2012 Elsevier Ltd. All rights reserved./10.1016/j.electacta.2012.11.090D.Zhou et al./Electrochimica Acta 89 (2013) 334–338335OOO O OO +O OO OOO R 2CH 3(CH 2)10C OO2+CO 2C 11H 23C 11H 23R 1H 23C 11C H 23C 11C 11H 23Fig.1.The polymerization mechanism of TMPTMA.R 1and R 2represent molecular chains.monomer and the LPO (C 24H 46O 4,M w :398)was used as a ther-mal initiator.The liquid electrolyte was consisted of 1M LiPF 6in a non-aqueous solution of ethylene carbonate (EC)/ethylmethyl carbonate (EMC)/dimethyl carbonate (DMC)with a volume ratio of 1:1:1.In order to characterize the gelation time and electro-chemical performance of the electrolyte material,contents of the initiator and liquid electrolyte was changed 0.1–0.9wt%(the mass of monomer)and 80–97wt%(the total mass of gel),respectively.The precursors were polymerized at a temperature of 50–80◦C to obtain translucent GPE films.Gelation time was recorded when the precursor solution formed translucent gels in the transparent glass container,and the gels did not further transform with extended heating time.All procedures for preparing the GPE films were car-ried out in an Ar-filled glove box.The ionic conductivities of the GPEs were measured by an AC impedance method.The test cell was a small piece of GPE film sandwiched between two stainless steel blocking electrodes,kept at each test temperature for at least half an hour before AC impedance measurements in order to reach thermal equilibrium.The AC impedance measurements were carried out from 100kHz to 1Hz with ac amplitude of 5mV on a CHI660C Electrochemical Workstation (Shanghai Chenhua).The electrochemical stabilities of the GPE films were studied with cyclic voltammetry (CV)in a three-electrode system.Stainless steel blocking electrode and aluminum foil were used as the working electrode respectively while a lithium foil was used as the counter and the reference electrode in this sys-tem.The CV curves were recorded from −0.5to 5V versus Li/Li +at a scanning rate of 1mV s −1using a CHI660C Electrochemical Work-station (Shanghai Chenhua).The cross section morphologies of GPE films rinsed with acetone were observed with a scanning electron microscopy (SEM,EVO 18).The electrochemical properties of the polymer cells were mea-sured by two-electrode coin cells.The cells were assembled in an Ar-filled glove box with the concentrations of moisture and oxy-gen below 1ppm.The TMPTMA-based polymer coin cells were fabricated by the in situ thermal polymerization method directly.The two-electrode coin cells were consisted of positive electrode (90wt%LiCoO 2,5wt%acetylene black and 5wt%PVDF),a Celgard 2400separator,and negative electrode (95wt%graphite and 5wt%PVDF).The precursor solution,with 1wt%initiator and 97wt%1M LiPF 6/EC:EMC:DEC (1:1:1)electrolyte,was filled into the cells.After that,the assembled cells were aged for 2h to ensure the precur-sor solution well wetted into the electrodes.Then the cells were polymerized at 80◦C for half an hour in an oven.Galvanostatic charge–discharge test was carried out at various current densities,x C (x =0.1,0.2,0.5,1),using a LAND-CT2001A cell test instrument with voltage window of 3.0–4.2V at 25◦C.The x value in the x C current density was equivalent to the time for removal of 1M Li +in LiCoO 2in 1/x number of hours,that is,1C =274mAh g −1.3.Results and discussionThe possible mechanism of radical polymerization of TMPMMA,initiated by LPO with a thermal radiation is illustrated in Fig.1.The attack of primary radicals derived by thermal decomposition of LPO initiates a chain free radical and the polymerization proceeds by the chain reaction adding TMPTMA monomer molecules to the free radical ends of growing chain molecules.Finally,a high degree of cross-linking gives rise to three-dimensional network polymers in the liquid electrolyte and forms translucent GPEs,which are tough and flexible.Although a Celgard 2400is used as the separator,the excellent mechanical properties of GPEs still matter because they could give a great improvement to the shock resistance of polymer lithium ion batteries.To visualize the structure of the polymer matrix in GPEs,SEM investigations were carried out for this objective.A typical SEM image shows that an obviously porous network polymer was obtained (Fig.2a),and the pore sizeis about 20␮m (Fig.2b).The polymer matrix acting as internal scaffolding is well cross-linked to establish a three-dimensional framework with high porosity.Liq-uid electrolyte is trapped in such pores and highly dispersed in theFig.2.SEM image of TMPTMA-based gel polymer electrolyte.336D.Zhou et al./Electrochimica Acta 89 (2013) 334–338(a)t / m i nT /o Cσ / m S c m-1σ / m S c m-1σ / m S c m-1(b)Liquid electrolyte content / wt%t / m i n(c)Initiator content / wt%t / m i nFig.3.Room temperature (25◦C)conductivity and gelation time of the GPEs as a function of polymerization temperature (a),liquid electrolyte content (b)and initiator content (c).polymer framework.This structure can be expected to have high mechanical strength and strong ability to absorb liquid electrolyte.The ionic conductivity is the key factor in polymer electrolytes,while the gelation time determines the productivity of polymer lithium ion batteries prepared by in situ thermal polymerization.The gelation time and ionic conductivities of GPEs at 25◦C as a function of polymerization temperatures,electrolyte content,and initiator content are shown in Fig.3.GPEs in Fig.3a are prepared with the same ratio of initiator (0.5wt%)and liquid electrolyte (90wt%)at various polymerization temperatures.The gelation time drops rapidly with an increase in polymerization temperature,from 35min at 50◦C to 4min at 80◦C,corresponding to the improvement of initiator activity and acceleration of the radical polymerization reaction rate.Effects of polymerization temperature on gelation time are unapparent.In Fig.3b,GPEs with the same content of initiator (0.5wt%)and different contents of liquid electrolytes ran-ging from 80to 97wt%are polymerized at 70◦C.As expected,the increased content of liquid electrolyte leads to the decreased contents of reactants (the monomer and initiator),which prolongs the gelation time appreciably.Meanwhile,the ionic conductivity of GPEs continuously increases with the increased liquid electrolyte-2.2-2.1-2.0-1.9-1.8-1.7T /ºCl o g / S c m -11000 T -1/ k -1Fig.4.Ionic conductivities for the TMPTMA-based GPE prepared by 97wt%liquidelectrolyte,1wt%initiator and polymerized at 80◦C and 1M LiFP 6/EC:DMC:EMC liquid electrolyte at various temperatures.content.The ionic conductivity of GPE contained 97wt%liquid elec-trolyte reaches a value of 8.2×10−3S cm −1,approaching the value of corresponding pure liquid electrolyte (8.9×10−3S cm −1).How-ever,lower polymer content leads to a lower mechanical strength of the GPE film [21].The conductivities of the GPEs all reached >10−3S cm −1in the present liquid electrolyte content range,which are sufficient for practical application in Li-ion batteries,indicating that TMPTMA-based GPE has higher ionic conductivity than most of frequently-used GPEs such as PMMA-based [22]and PAN-based [23].The gelation time and ionic conductivities of GPEs polyme-rized at 70◦C with the same content of liquid electrolyte (70wt%)but different initiator contents ranging from 0.1to 0.9wt%are pre-sented in Fig.3c.The results show that the ionic conductivity is not affected by variation in the initiator concentration significantly,while the gelation time is reduced with the increased initiator con-tent,which indicates the slower polymerization rate.To evaluate the dependence of ionic conductivity ( )on tem-perature,ionic conductivities of the GPE films are measured at wide test temperature range from 20to 90◦C.Fig.4shows the changes in the conductivity of a GPE prepared by 97wt%liquid electrolyte,1wt%initiator and polymerized at 80◦C.Apparently,the ionic conductivity increases with increasing test temperature,from 7.4×10−3S cm −1at 20◦C to 1.7×10−2S cm −1at 90◦C.The conductivities for pure liquid electrolyte are also illustrated for comparison.It can be seen that in either GPE or liquid electrolyte,the plots of log versus T −1exhibit a linear relationship in line with the Arrhenius equation [24]:= o exp −E akT(1)where E a the activation energy, o the pre-exponential factor and R the Boltzmann constant.E a can be calculated according to the Eq.(1)from the slopes of the lines.The Arrhenius plot of GPE parallels to those of liquid electrolyte,which means they have similar acti-vation energies.The activation energy is considered to be related to the mobility of ionic carriers [25],indicating that the ionic con-duction in the GPE follows the same mechanism as in pure liquid electrolytes,that is,mainly due to the diffusion of low molecular weight solvents in polymers (low activation energy)instead of the motion of polymer segments (high activation energy).Even though the immobilization of the liquid electrolyte and enhanced viscos-ity ultimately result in a reduction in the conductivity of GPEs,it is required for the settlement of security problems caused by liquid electrolyte leakage.D.Zhou et al./Electrochimica Acta89 (2013) 334–338337Fig.5.Cyclic voltammogram of TMPTMA-based GPE using stainless steel and alu-minum foil as the working electrode respectively,Li as the counter and the reference electrode,at a scan rate of1mV s−1.The electrochemical stability of the GPE was performed by cyclic voltammetry(CV).Fig.5indicates the CV curves of the GPEfilm (following the same preparation route as in Fig.4)tested between −0.5and1.5V vs.Li/Li+.On the stainless steel working electrode, a strong current is observed from about−0.20V in the negative scan,corresponding to the plating of lithium on the stainless steel electrode.In the following reverse scan,the stripping of lithium appears at about0.35V.No peak or noticeable oxidation current was observed up to5.0V except at potential range of−0.5to0.5V, which means the GPE is relatively stable up to5.0V vs.Li/Li+.The excellent electrochemical stability may be attributed to the high oxidation potential of TMPTMA and interactions between the poly-mer chain and lithium salt[26].The phenomenon that the peak currents remain fairly constant with repeated cycle in the CV curve indicates the plating/stripping of lithium are highly reversible.CV curves of the GPEfilm are also tested on the aluminum foil,which is used as the current collector for lithium-ion batteries.As shown in Fig.5,the GPE keeps electrochemically stable up to5.0V,which would be enough to meet the requirements for lithium-ion battery with LiCoO2as a positive material whose charging voltage is about 4.2V.LiCoO2|GPE|graphite cells were fabricated in order to evalu-ate the electrochemical performance of TMPTMA-based polymer lithium ion battery.The component of the GPE prepared by in situ thermal polymerization was identified as1wt%LPO initiator and 97wt%1M LiPF6/EC:EMC:DEC(1:1:1)electrolyte polymerized at 80◦C.The cells were charged and discharged at various current densities after a preprocessing cycle at0.1C between3.0and4.2V. Thefirst charge–discharge curves of LiCoO2|GPE|graphite cells at a rate of0.1C,0.2C,0.5C and1C are plotted in Fig.6.The charge and discharge curves show clear potential plateaus which indicat-ing a reversible cycling process.The cell shows an initial discharge capacity of129mAh g−1at a current density of0.2C and the coulom-bic efficiency approaches94%,almost the same as the lithium ion batteries using1M LiFP6/EC:DMC:EMC liquid electrolyte.The dis-charge capacity of TMPTMA-based cells slightly decreases with increasing current densities.An acceptable discharge capacity of 110mAh g−1was achieved at1.0C,which was81.5%of the capac-ity at0.1C(135mAh g−1)and85.3%of the capacity at0.2C.The reduced high rate capability could be explained primarily by the lower diffusion rate of Li+in the GPEs[27].Cycling stability of TMPTMA-based polymer lithium ion bat-tery concluded from charge-discharge curves is presented in Fig.7. After100cycles at0.2C,the capacity retention ratio for TMPTMA-based polymer lithium ion battery is83%,that is,the discharge capacity is about107mAh g−1.The superior cycling stability of the2004060801001201401603.03.23.43.63.84.04.2Capacity / mAh g-1Voltage/Vvs.Li/Li)(+0.1C0.2C0.5C1 CFig.6.The initial charge–discharge profiles of LiCoO2|GPE|graphite full cell at vari-ous current densities.Capacity/mAhg-1Cycle numberFig.7.The cycling stability of LiCoO2|GPE|graphite full cell at a rate of0.2C.TMPTMA-based polymer lithium ion batteries may be attributed to the interface between electrode and GPE keeping steady during charge–discharge cycles.4.ConclusionsTrimethylolpropane trimethylacrylate-based gel polymer elec-trolytes with a well cross-linked three-dimensional framework and excellent electrochemical properties were successfully prepared by radical polymerization.The ionic conductions in the GPEs were proved to follow the same mechanism as in pure liquid electrolytes. The gel polymer electrolytes exhibited remarkable electrochemical property including high ionic conductivity(>10−3S cm−1at25◦C), wide electrochemical window(>5.0V),highly reversible capacity, superior rate capability and good cycling stability,while the in situ thermal polymerization greatly simplified the assembly process of the polymer lithium ion batteries.These properties potentially made such new gel polymer electrolyte particularly attractive for high-performance lithium ion batteries.AcknowledgementsThis work was supported by the973project(2013CB934001), NSF of China(51172024),Fundamental Research Funds for the Cen-tral Universities of China and State Key Laboratory of New Ceramic and Fine Processing(Tsinghua University).338 D.Zhou et al./Electrochimica Acta89 (2013) 334–338References[1]B.Scrosati,J.Garche,Lithium batteries:status,prospects and future,Journal ofPower Sources195(2010)2419.[2]C.W.Nan,L.Z.Fan,Y.H.Lin,Q.Cai,Enhanced ionic conductivity of polymerelectrolytes containing nanocomposite SiO2particles,Physical Review Letters 91(2003)266104.[3]M.Egashira,H.Todo,N.Yoshimoto,M.Morita,Lithium ion conduction in ionicliquid-based gel polymer electrolyte,Journal of Power Sources178(2008)729.[4]J.H.Park,W.Park,J.H.Kim,D.Ryoo,H.S.Kim,Y.U.Jeong,D.W.Kim,S.Y.Lee,Close-packed poly(methyl methacrylate)nanoparticle arrays-coated poly-ethylene separators for high-power lithium-ion polymer batteries,Journal of Power Sources196(2011)7035.[5]H.R.Jung,W.J.Lee,Electrochemical characteristics of electrospun poly(methylmethacrylate)/polyvinyl chloride as gel polymer electrolytes for lithium ion battery,Electrochimica Acta58(2011)674.[6]P.Raghavan,J.Manuel,X.Zhao,D.S.Kim,J.H.Ahn,C.Nah,Preparation and elec-trochemical characterization of gel polymer electrolyte based on electrospun polyacrylonitrile nonwoven membranes for lithium batteries,Journal of Power Sources196(2011)6742.[7]P.Carol,P.Ramakrishnan,B.John,G.Cheruvally,Preparation and characteriza-tion of electrospun poly(acrylonitrile)fibrous membrane based gel polymer electrolytes for lithium-ion batteries,Journal of Power Sources196(2011) 10156.[8]X.L.Wang,Q.Cai,L.Z.Fan,T.Hua,Y.H.Lin,C.W.Nan,Gel-based compositepolymer electrolytes with novel hierarchical mesoporous silica network for lithium batteries,Electrochimica Acta53(2008)8001.[9]S.Ferrari, E.Quartarone,P.Mustarelli, A.Magistris,M.Fagnoni,S.Protti, C.Gerbaldi, A.Spinella,Lithium ion conducting PVdF-HFP com-posite gel electrolytes based on N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide ionic liquid,Journal of Power Sources195 (2010)559.[10]J.Y.Song,Y.Y.Wang,C.C.Wan,Review of gel-type polymer electrolytes forlithium-ion batteries,Journal of Power Sources77(1999)183.[11]J.A.Choi,Y.Kang,H.Shim,D.W.Kim,H.K.Song,D.W.Kim,Effect of the cross-linking agent on cycling performances of lithium-ion polymer cells assembled by in situ chemical cross-linking with tris(2-(acryloyloxy)ethyl)phosphate, Journal of Power Sources189(2009)809.[12]W.C.Kang,H.G.Park,K.C.Kim,S.W.Ryu,Synthesis and electrochemical prop-erties of lithium methacrylate-based self-doped gel polymer electrolytes, Electrochimica Acta54(2009)4540.[13]H.S.Kim,J.H.Shin,S.I.Moon,M.S.Yun,S.P.Kim,Electrochemical performancesof gel polymer electrolytes using tetra(ethylene glycol)diacrylate,Chemical Engineering Science58(2003)1715.[14]B.Oh,K.Amine,Evaluation of macromonomer-based gel polymer electrolytefor high-power applications,Solid State Ionics175(2004)785.[15]H.Li,X.T.Ma,J.L.Shi,Z.K.Yao,B.K.Zhu,L.P.Zhu,Preparation and properties ofpoly(ethylene oxide)gelfilled polypropylene separators and their correspond-ing gel polymer electrolytes for Li-ion batteries,Electrochimica Acta56(2011) 2641.[16]S.I.Kim,H.S.Kim,S.H.Na,S.I.Moon,Y.J.Kim,N.J.Jo,Electrochemical charac-teristics of TMPTA-and TMPETA-based gel polymer electrolyte,Electrochimica Acta50(2004)317.[17]H.S.Kim,S.I.Moon,Synthesis and electrochemical performances ofdi(trimethylolpropane)tetraacrylate-based gel polymer electrolyte,Journal of Power Sources146(2005)584.[18]K.H.Lee,H.S.Lim,J.H.Wang,Effect of unreacted monomer on perfor-mance of lithium-ion polymer batteries based on polymer electrolytes prepared by free radical polymerization,Journal of Power Sources139(2005) 284.[19]J.R.Nair,C.Gerbaldi,M.Destro,R.Bongiovanni,N.Penazzi,Methacrylic-basedsolid polymer electrolyte membranes for lithium-based batteries by a rapid UV-curing process,Reactive and Functional Polymers71(2011)409.[20]Y.F.Zhou,S.Xie,X.W.Ge,C.H.Chen,K.Amine,Preparation of rechargeablelithium batteries with poly(methyl methacrylate)based gel polymer elec-trolyte by in situ␥-ray irradiation-induced polymerization,Journal of Applied Electrochemistry34(2004)1119.[21]L.X.Yuan,J.D.Piao,Y.L.Cao,H.X.Yang,X.P.Ai,Preparation and performancecharacterization of polymer Li-ion batteries using gel poly(diacrylate)elec-trolyte prepared by in situ thermal polymerization,Journal of Solid State Chemistry9(2005)183.[22]O.Bohnke,G.Frand,M.Rezrazi,C.Rousselot,C.Truche,Fast ion transport in newlithium electrolytes gelled with PMMA.1.Influence of polymer concentration, Solid State Ionics66(1993)97.[23]H.Huang,L.Chen,X.Huang,R.Xue,Studies on PAN-based lithium salt complex,Electrochimica Acta37(1992)1671.[24]M.Nookala,B.Kumar,S.Rodrigues,Ionic conductivity and ambient tem-perature Li electrode reaction in composite polymer electrolytes containing nanosize alumina,Journal of Power Sources111(2002)165.[25]M.Park,X.Zhang,M.Chung,G.B.Less,A.M.Sastry,A review of conduc-tion phenomena in Li-ion batteries,Journal of Power Sources195(2010) 7904.[26]G.B.Appetecchi,F.Croce,B.Scrosati,Kinetics and stability of the lithium elec-trode in poly(methylmethacrylate)-based gel electrolytes,Electrochimica Acta 40(1995)991.[27]D.W.Kim,Electrochemical characterization of poly(ethylene-co-methylacrylate)-based gel polymer electrolytes for lithium-ion polymer batteries, Journal of Power Sources87(2000)78.。