Properties on Novel PVDF-HFP-Based Composite Polymer Electrolyte With Vinyltrimethoxylsilane-Modified ZSM-5Wei Xiao,Xinhai Li,Huajun Guo,Zhixing WangSchool of Metallurgical Science and Engineering,Central South University,Changsha410083,ChinaA kind of novel poly(vinylidenefluoride-co-hexafluoro-propylene)(PVDF-HFP)-based composite polymer elec-trolyte doped with vinyltrimethoxylsilane(DB171sil-ane)-modified ZSM-5is prepared by phase inversion method(denoted as M-ZSM-5membrane).Physical and chemical properties of M-ZSM-5membrane are studied by SEM,FTIR,TG-DSC,EIS,and LSV.The results show that thermal and electrochemical stability can reach4008C and5V,respectively;temperature de-pendence of ionic conductivity follows Vogel–Tamman–Fulcher relation and ionic conductivity at room temper-ature is up to 4.2mS/cm;the interfacial resistance reaches a stable value about325O after5days stor-age at room temperature,which suggests that it can be potentially suitable as electrolyte in polymer lithium ion POS.,33:629–635,2012.ª2012 Society of Plastics EngineersINTRODUCTIONMore and more attention has been paid to gel polymer electrolytes(GPEs)in lithium ion battery due to their excel-lent performances,such as no-leakage of electrolyte,high energy density,flexible geometry,and improved safety haz-ards[1–6].Among the GPEs,the poly(vinylidenefluoride–hexafluoropropylene)(PVDF–HFP)-based GPEs have been fabricated and studied extensively at present because they have some appealing properties,in which PVDF–HFP ma-trix has a high dielectric constant(e¼8.4)that facilitates a higher concentration of charge carriers,and also comprises both an amorphous and a crystalline phase[7–12].The amorphous phase of the polymer assists higher ionic con-ductivity while the crystalline phase acts as a mechanical support for the polymer electrolyte[5,7,13].To further improve the ionic conductivity and mechanical properties of the GPEs simultaneously,adding inorganic nanopar-ticles,such as silica(SiO2),alumina(Al2O3),zirconia (ZrO2),and titania(TiO2),into polymer matrix has been proved to be a feasible approach[6,14–20].In addition to those advantages,they can act as solid plasticizers hindering the reorganization of polymer chains and can interact with polar groups by Lewis acid–base reaction[21,22].So the properties such as ionic conductivity,lithium ion transfer-ence number,and activation energy for ions transport can gain much improvement.Moreover,surface modification technology provides a new way to prepare polymer electro-lyte with excellent properties[23].For example,the com-posite polymer electrolytes doped with nano-SiO2,which is modified with different groups,including vinylene,glyci-doxy,mercapto,chloropropyl,octyl,methacryl,and amino groups,show significant improvement in terms of ionic conductivity,and the ionic conductivity of the modified composite polymer electrolyte is even close to the value of traditional liquid electrolyte at room temperature[23–26]. More attention has been turned to pay to the molecular sieves such as MCM-41,SBA-15,and ZSM-5due to their high surface area,peculiar molecular structures and strong Lewis acidity[27–30],but very few reports exist in the liter-ature addressing composite polymer electrolytes doped with silane-modified molecular sieves[31].In the present work, the novel PVDF-HFP-based composite polymer electrolyte doped with vinyltrimethoxylsilane-modified ZSM-5is pre-pared by phase inversion method.Physicochemical and electrochemical properties are studied by SEM,Fourier transform infrared spectrum(FTIR),thermogravimetry and differential scanning calorimeter(TG-DSC),electrochemi-cal impedance spectroscopy(EIS),and linear sweep voltammetry(LSV)and show dramatic improvement. EXPERIMENTALMaterialsMolecular sieve ZSM-5(10–50nm,purchased from Tianjin Chemist Scientific Ltd.in China)was dried for12Correspondence to:Xinhai Li;e-mail:xwylyq2009@Contract grant sponsor:Major Provincial Science and Technology Pro-grams of Hunan;contract grant number:2009FJ1002.Contract grant sponsor:Central College on the2010Operational Costs of Basic Research Project;contract grant number:2010QZZD0101.DOI10.1002/pc.22156Published online in Wiley Online Library().V C2012Society of Plastics EngineersPOLYMER COMPOSITES—-2012h at1208C prior to use.Poly(vinylidenefluoride-co-hexa-fluoropropylene)(PVDF-HFP,Atofina,Kynar Flex,12 wt%HFP)was used as polymer matrix in the experiment and dried under vacuum for12h at808C prior to use. Analytical grade N,N-dimethylformamide(DMF)and poly(ethylene glycol)with low molecular weight of200 (PEG-200)were directly used as solvent and pore-forming agent,respectively,without further purification. Preparation of Silane-Modified Molecular SieveA certain amount of molecular sieve ZSM-5was added into5%DB171silane–ethanol mixed solution(5:95,v/v) with continuous vigorous stirring for48h at408C,then the above homogeneous solution was centrifuged at 15,000rpm and dried in vacuum at1008C for24h to fur-ther remove residual organic solvents.For the sake of convenience,the modified ZSM-5is denoted as M-ZSM-5in the following exposition.Preparation of Polymer ElectrolyteA certain content of as-prepared M-ZSM-5was dis-persed in DMF solution,and a certain mass of PVDF-HFP and PEG-200were added to the well-dispersed solu-tion with continuous stirring for3–4h at408C to form homogeneous and viscous slurry,in which the mass ratio between the M-ZSM-5and the PVDF-HFP is about1:10. The casting solution was cast onto a glass plate with a doctor blade to form the wet membrane after leaving still for2h at408C,followed by being immersed into deion-ized water at room temperature for12h to obtain the polymer electrolyte membrane.The resulting membranes were further dried under vacuum at608C for24h to remove the residual solvent.The membranes prepared were about120–150l m in thickness.Then the desirable polymer electrolytes were prepared by immersing the as-prepared membranes into the1.0M LiPF6-ethylene car-bonate(EC)/dimethyl carbonate(DMC)/ethylmethyl car-bonate(EMC)(1:1:1,w/w/w)liquid electrolyte solution (provided by Dongguan Shanshan Battery Materials Co. Ltd.in China)at room temperature for1h,which was carried out in a dry-box under argon gas atmosphere to avoid moisture.The PVDF-HFP-based composite polymer electrolytes modified with and without ZSM-5were also prepared in our work as control samples according to the above-mentioned experimental steps.Properties CharacterizationA scanning electron microscope(SEM,JSM6301F) with an accelerating voltage of20kV was used to exam-ine the membrane surface sputter coated with gold under vacuum atmosphere.The PEPARAGN1000instrument with a wave number resolution of2cm21in the fre-quency of4000to400cm21was used to record the FTIR of M-ZSM-5.TG-DSC measurements were carried out on a Perkin-Elmer Pyris-1analyzer.The measurements were performed at a heating rate of108C/min from20to 4008C.Aflow of nitrogen gas was maintained over the perforated pan to avoid any contact with atmospheric moisture.The weights of samples were maintained in the range of12–15mg and an empty aluminum pan was used as a reference.The liquid electrolyte uptake(A)was cal-culated using the following relation Eq.1,where,w1and w0are the weights of the wet and dry membranes,respec-tively.A%¼w1Àw0w03100%(1)The ionic conductivity of the composite polymer elec-trolyte was determined by EIS.The as-prepared electro-lyte membranes were sandwiched between two stainless steel(SS)blocking electrodes to form the SS/composite polymer electrolyte/SS model cells.The EIS tests were measured over an AC oscillation10mV frequency range of1to105Hz at various temperatures(293–363K,the cells were thermostated during measurements)using a CHI660b frequency response analyzer(Shanghai Chen-hua,China).Electrochemical stability window of the polymer electrolyte was determined by running LSV in three-electrode cell using stainless steel as the blocking working electrode,lithium as both the counter,and the reference electrode and polymer electrolyte as the electro-lyte.The LSV tests were carried out using the same sys-tem as those in EIS at a scan rate of5mV/s.In addition, the interfacial stability was studied by investigating the resistance change with different storage times of the blocking model cell Li/composite polymer electrolyte/Li.RESULTS AND DISCUSSIONSurface Morphology AnalysisFigure1presents SEM images of three kinds of differ-ent polymer electrolyte membranes with different adulter-ants.It can be obviously seen that adding molecular sieve into polymer matrix plays a major role in the surface morphology of fabricating polymer electrolyte mem-branes,no matter molecular sieve is modified with DB171silane or pared with Fig.1A,the size and the amount of micro-pores on the surface and inner decreases gradually and increases distinctly in Fig.1B and C,respectively,which may be partly attributed to the Lewis acid–base interaction between polymer matrix PVDF-HFP and molecular sieve,partly to molecular sieve as a cross-linking center during recrystallization of PVDF-HFP.On one hand,with molecular sieves added into polymer matrix,the inner-layer has more micropores and demonstrates better connectivity,which can further promote more electrolyte entrapment ratio;on the other hand,the more uniform surface morphology demonstrated in Fig.1C implies it has best compatibility between the630POLYMER COMPOSITES—-2012DOI10.1002/pcpolymer matrix and electrode,which can be well con-firmed in the following experiments.It can be seen that M-ZSM-5modified PVDF-HFP membrane not only shows most uniform surface morphology,but also presents most plentiful interconnected micropores.FTIR Analysis ResultsThe direct evidence for the reaction mechanism is pro-vided by FTIR recorded for the DB171and M-ZSM-5showed in Fig.2.The CH 3asymmetric stretching vibra-tion occurs at 2,975–2,950cm 21while the CH 2absorp-tion occurs at about 2,930cm 21.The symmetric CH 3vibration occurs at 2,885–2,865cm 21while the CH 2absorption occurs at about 2,870–2,840cm 21.The absorption bands appearing at 1,193,1,090,1,010,and 968cm 21are ascribed to IR vibrating absorption of ÀÀSi ÀÀO ÀÀCH 3bonds,and the 1,600–1,650cm 21is to C ¼¼C stretching absorption in the IR spectrum of DB171[32].A problem worthy to be pointed out is that the cor-responding band showing red shift observed at 945–1,100cm 21is ascribed to IR vibrating absorption of ÀÀSi ÀÀO ÀÀSi bonds,and the 1,600–1,650cm 21represent-ing C ¼¼C stretching absorption becomes weak in the of the spectrum of DB171-modified ZSM-5,which can be explained that silanol (Si ÀÀOH)from hydrolysis of DB171silane interacts with free hydroxyl (ÀÀOH)on the surface of the molecular sieve ZSM-5.The distinct differ-ences between the mentioned spectra indicate that molec-ular sieve is successfully modified by DB171silane.The two IR absorption peaks near 3,428and 1,639cm 21in Fig.2may be assigned to the presence of the oxygen–hydrogen single bond (O ÀÀH)from the samples contain-ing absorbable impurity water [24,25].Thermal and Electrochemical StabilityThermal and electrochemical stability are considered as the most important two factors in practicalapplicationsFIG.1.SEM images of different polymer electrolyte membranes [(A)PVDF-HFP membrane,(B)ZSM-5modified PVDF-HFP membrane,and (C)M-ZSM-5modified PVDF-HFPmembrane].FIG.2.FTIR spectra of DB171and DB171modified ZSM-5.[Color figure can be viewed in the online issue,which is available at .]DOI 10.1002/pcPOLYMER COMPOSITES—-2012631of polymer lithium ion battery.Figure 3presents the TG-DSC plots of polymer electrolyte membranes prepared by phase inversion method.Obviously,Fig.3A about the TG plots reveals that the pure PVDF-HFP electrolyte mem-brane is inferior to the ZSM and M-ZSM modified mem-brane in term of thermal stability,and the thermal stabil-ity of the membrane with M-ZSM-5can even reach 4008C with little decomposition,which can well meet the practical demands about the thermal property of lithium ion battery.Fig.3B shows the DSC plots of the three kinds of membranes.The crystallinity of the membranes can be calculated according to the following relation x %¼D H f /D H H mf from the DSC curves,where,D H Hmf is the standard enthalpy of fusion of pure PVDF,104.7J/g,and D H f is enthalpy of fusion of the PVDF-HFP-based poly-mer electrolyte membrane [10,33,34].The crystallinity of the membranes with the uptake ratio and ionic conduc-tivity at room temperature is listed in the Table 1.It is obviously seen that the values of uptake ratio and ionic conductivity increase with decreasing of the crystal-linity,indicating that molecular sieve plays a critical role in properties of the electrolyte membrane,which is ac-cordance with the above experimental results.The effects of the M-ZSM-5to enhance uptake ratio and ionic con-ductivity at room temperature may be due to abundant interconnected micro-pores in the polymer electrolyte membranes and interfacial reaction between the polymer matrix and molecular sieves.The plots of electrochemical stability window about the three kinds of electrolyte membranes at room temper-ature are demonstrated in pared with the three curves,the M-ZSM-5membrane displays the highest electrochemical stability window about 5V,which can meet practical applications requirements and implies that the M-ZSM-5plays a positive role in term of electro-chemical stability.Based on both excellent thermal and electrochemical stability,the M-ZSM-5membrane would be used as the most promising polymer electrolyte mem-brane candidate in the rechargeable polymer lithium ion battery in the future.Ionic ConductivityThe effect of M-ZSM-5on the ionic conductivity of polymer electrolyte membrane is further investigated by temperature-dependent ionic conductivity.As demon-strated in Fig.5,the ionic conductivity increases with temperature increase and the ionic conductivity of the membranes doped with molecular sieve is higher thantheFIG.3.Plots of TG (A)and DSC (B)about three kinds of polymer electrolyte membranes by phase inversion method.[Color figure can be viewed in the online issue,which is available at .]TABLE 1.Results of crystallinity,ionic conductivity,and uptake ratio about the three kinds of polymer electrolyte membranes.Crystallinity(/%)Ionic conductivity/(mS/cm)Uptake ratio (%)PVDF-HFP membrane 100.0 2.01276.52ZSM-5doped PVDF-HFP membrane50.02 3.64792.99M-ZSM-5doped PVDF-HFP membrane31.264.23598.21FIG.4.Results of LSV about the three kinds of polymer electrolyte membranes.[Color figure can be viewed in the online issue,which is available at .]632POLYMER COMPOSITES—-2012DOI 10.1002/pcpure PVDF-HFP membrane,whether it is modified by sil-ane or not,which suggests that increase temperature low-ering the activation energy for the ions transfer between the micropores in the polymer matrix can improve ionic conductivity and molecular sieves with their unique mole-cule space structure not only can provide more passage-ways for the ionic migration,but also as Lewis acid inter-act with polymer matrix to decrease the crystallinity degree for forming more amorphous areas.Furthermore,the ionic conductivity is not related linearly to the recip-rocal temperature,which is different from the reported composite polymer electrolyte doped by some other inor-ganic fillers and maybe obeys the Vogel–Tamman–Fulcher (VTF)relation [35,36].As the temperature increases,the polymer can expand easily and produce free volume.In other words,as temperature increases,the free volume increases.The resulting conductivity,represented by the overall mobility of ions and the polymer,is deter-mined by the free volume around the polymer chains,which leads to an increase in ion mobility andsegmentalFIG.5.Temperature dependence of ionic conductivity of the three kinds of different polymer electrolyte membranes.[Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]FIG.6.Nyquist plots of three kinds of Li/as-prepared electrolyte/Li cells with various times [(A)Pure PVDF-HFP membrane;(B)ZSM-5modified PVDF-HFP membrane;and (C)M-ZSM-5modified PVDF-HFP membrane].[Color figure can be viewed in the online issue,which is available at .]DOI 10.1002/pc POLYMER COMPOSITES—-2012633mobility that will assist ion transport and virtually com-pensate for the retarding effect of the ion clouds[36]. Interfacial PropertiesInterfacial compatibility between the polymer electro-lyte and the electrode is another important factor to ensure an acceptable performance in practical applica-tions.In the polymer electrolyte system,the passive layer between the electrode and the electrolytes grows with the time,but the uncontrolled layer plays a vital role in prac-tical applications[8].To understand the interfacial stabil-ity between electrode and electrolyte membranes,Nyquist plots of three kinds of Li/as-prepared electrolyte/Li sym-metric cells at open circle are monitored with various times.It is quite obvious from Fig.6that the resistances R i of electrolyte membranes modified by molecular sieves reach a stable value of730O(ZSM-5modified mem-brane)and325O(M-ZSM-5modified membrane)respec-tively after5days storage,however,the resistance R i of the pure PVDF-HFP membrane presents the tendency to keep growing with time under the same conditions.The results indicate that,on one hand,the passivefilm forms between the electrolyte and electrode at the beginning and reaches a stable value when adding the molecular sieve ZSM-5into the polymer matrix,thus the membranes modified by ZSM-5present lower resistances and exhibit better compatibility with the electrode[22];on the other hand,the addition of the ZSM-5can trap any impurities such as water and trace organic solvent,inhibiting the de-structive reaction on the electrode,which can improve the compatibility between the electrode and electrolyte mem-brane[10,37,38].Moreover,the M-ZSM-5modified membrane shows the best compatibility among these membranes,in the view of the authors,which can be attributed to the special surface reaction activity groups formed during the reaction of ZSM-5modified by DB171 silane.And the research on the reaction and ionic conduc-tion mechanisms about the novel polymer electrolyte are in progress in our group.CONCLUSIONSThis work introduces preparation and performances of the novel PVDF-HFP-based polymer electrolyte with DB171-modified molecular sieve ZSM-5.The practical applications such as high ionic conductivity,excellent thermal and electrochemical stability and good interfacial properties with electrode of the novel M-ZSM-5electro-lyte ensure it as the most promising polymer electrolyte for the new rechargeable lithium ion battery. 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