Study on performances of ZSM-5 doped P(VDF-HFP) based composite polymer electrolyte

ORIGINAL PAPER

Study on performances of ZSM-5doped P(VDF-HFP)based composite polymer electrolyte prepared by steam bath technique

Wei Xiao ?Xinhai Li ?Zhixing Wang ?

Huajun Guo

Received:9February 2012/Accepted:19May 2012óIran Polymer and Petrochemical Institute 2012

Abstract Several novel kinds of poly (vinylidene ?uoride-co-hexa?uoropropylene)(P(VDF-HFP))based composite polymer electrolyte (CPE)membranes doped with ZSM-5(Zeolite sieve of molecular porosity)were fabricated by steam bath technique.The desirable CPE membranes were further prepared by immersing them into the liquid elec-trolyte solution of 1.0M LiPF 6-ethylene carbonate /dime-thyl carbonate/ethylmethyl carbonate (v /v /v:1/1/1)for 1h to be activated at room temperature.Physical and chemical properties of the as-prepared CPEs are studied by scanning electron microscope,Fourier transform infrared,thermo-gravimetry and differential scanning calorimetry and elec-trochemical methods.The results showed that the thermal and electrochemical stabilities of the CPEs can be reached to 350°C and 5V,respectively.Reciprocal temperature dependence of corresponding ionic conductivity follows Vogel–Tamman–Fulcher relation,and ionic conductivity at ambient temperature can be up to 5.1mS cm -1.The fabri-cated Li/CPEs/LiCoO 2coin cells with the CPE membranes prepared by steam bath technique can achieve a high dis-charge capacity about 144.97mAh g -1at 0.1C .At 1.0C ,the capacity can be kept to 89.84%of that at 0.1C for the cell.After 26cycles,the cell prepared with these CPE membranes can keep 97.5%of its initial discharge capacity.These excellent physicochemical and battery properties indicate that these novel CPEs can be potentially used as electrolyte in lithium ion polymer battery.

Keywords ZSM-5áP(VDF-HFP)áComposite polymer electrolyte áSteam bath technique áLithium ion polymer battery

Introduction

In recent years,more and more attention has been focused on lithium ion polymer battery with gel polymer electro-lytes (GPEs)for their outstanding properties,such as no leakage of electrolyte,higher energy density,?exible geometry and improved safety hazards [1–3].The GPEs mainly comprise polymer matrix,entrapped liquid elec-trolyte and proper additives.There are several kinds of polymer matrices widely used in the processes of different GPEs preparation.Among these matrices,poly (ethylene oxide)(PEO)is the earliest and most extensively studied system.Although the ionic conductivity of PEO has been discovered by Fenton et al.[4],its technological impor-tance was only recognized in the early 1980s [5,6].The major drawback of these electrolytes was quite low ionic conductivity at room temperature.The poly (acrylonitrile)(PAN)based electrolytes exhibited exceptional ionic con-ductivity with the order of 1mS cm -1at ambient tem-perature,appreciable transference number about 0.6and wide electrochemical stability window about 4.5V.Unfortunately,these electrolytes underwent severe pas-sivation in contact with lithium metal anode [7–9].Appe-tecchi et al.[10]and Ren et al.[11]have explored the possibility of using poly (methylmethacrylate)(PMMA)as possible host of polymer electrolytes for lithium ion polymer battery.However,the poor mechanical strength of plasticized PMMA eliminated these electrolytes from practical applications.A copolymer of poly (vinylidene ?uoride-hexa?uoropropylene)[P(VDF-HFP)]was used as

W.Xiao áX.Li (&)áZ.Wang áH.Guo

School of Metallurgical Science and Engineering,Central South University,

Changsha 410083,People’s Republic of China e-mail:xwylyq2009@https://www.360docs.net/doc/3317255376.html,

Iran Polym J

DOI 10.1007/s13726-012-0052-z

separator material in the?eld of rechargeable lithium ion https://www.360docs.net/doc/3317255376.html,pared with poly(vinylidene?uoride)(PVDF), P(VDF-HFP)has relatively lower crystallinity due to the copolymerization effect between VDF and HFP[12,13]. Therefore,P(VDF-HFP)with more amorphous areas can be able to entrap larger amounts of liquid electrolytes, while the crystalline regions can still provide suf?cient mechanical integrity for the processing of free-standing membranes[14,15].Furthermore,adding inorganic?llers such as silica(SiO2),copper oxide(CuO)and titania (TiO2)into the polymer electrolyte membranes can result in the enhancement of mechanical strength as well as the improvement of the absorption level of electrolyte solution [16–18].In addition to these effects,they can act as solid plasticizers hindering the reorganization of polymer chains and interact with polar groups by Lewis acid–base reac-tions[1,4,19].Thus,the properties such as ionic con-ductivity,lithium ions transference number and activation energy of ions transport can gain much improvement.

Currently,polymer electrolyte membranes are usually prepared by non-solvent-induced phase separation pro-cesses.Many literatures reported phase inversion method to fabricate the composite polymer electrolyte membranes [12,20–22].For example,the classical Bellcore prepara-tion processes can be brie?y divided into two stages[23]. At?rst,the membrane was obtained by evaporating redundant casting solvents with low boiling point from the adhesive polymer solution.Second,the membrane was immersed into another solvent,which was usually a poor solvent with respect to the polymer matrix whereas it was a good solvent to the plasticizer.Therefore,the micro-porous membrane can be prepared by the liquid–liquid extraction process.The desirable polymer electrolytes can be attained by impregnating the porous membranes,for some time, into the liquid electrolyte to be activated.The resulting polymer electrolyte has the ionic conductivity with the order of1mS cm-1at ambient temperature[12,24,25]. Due to the complicated preparation processes and the considerable noxious solvents used in the traditional phase inversion processes,convenient and environmental pro-cesses to prepare the composite polymer electrolyte(CPE) membranes are under consideration.

In the present work,a novel preparation process,i.e., steam bath technique is proposed to fabricate the polymer electrolyte membrane in the?eld of lithium ion battery. After the free-standing wet membrane fabricated by casting the resultable casting solution onto a glass plate with a doctor blade,the novel steam bath process can form a homogeneous membrane,and then the desirable composite polymer membrane can be attained by immersing the cast wet membrane into80°C steam bath for several hours.The CPEs have been doped with the molecular sieve ZSM-5 (Zeolite sieve of molecular porosity)by the steam bath technique.This aluminosilicate zeolite mineral belongs to the pentasil family of zeolites and has been extensively used in a great deal of catalysis?elds for its high surface area,special channel structures and strong Lewis acidity. Mechanical strength and ionic conductivity of the CPEs can be improved by adding ZSM-5into the polymer matrix [26–28].Physicochemical and electrochemical properties of ZSM-5doped P(VDF-HFP)based composite electrolyte membranes prepared by the steam bath method show sig-ni?cant improvement.In order to investigate the mecha-nism of the ionic conduction in the CPEs in more detail,the distinguished improvements were studied by scanning electron microscope(SEM),Fourier transform infrared (FTIR),thermogravimetry and differential scanning calo-rimetry(TG–DSC),electrochemical impedance spectros-copy(EIS)and linear sweep voltammetry(LSV). Experimental

Materials

P(VDF-HFP)(Ato?na,Kynar Flex,12wt%HFP)was used as polymer matrix in the experiments.Molecular sieve ZSM-5(10–50nm)was purchased from Tianjin Chemist Scienti?c Ltd.Co.,China.P(VDF-HFP)and molecular sieve ZSM-5were dried under vacuum for12h at60and 120°C,respectively,prior to use.Analytical grade solvent N,N-dimethylformamide(DMF)and pore-forming agent poly(ethylene glycol)with low molecular weight of200 (PEG-200)were directly used without further puri?cation. Preparation methods of CPEs

To attain the CPEs,a well-distributed and homogeneous casting solution must be prepared.At?rst,appropriate amount of ZSM-5was added to DMF with ultrasonic vibration for30min,and then a certain mass of P(VDF-HFP)and small amounts of PEG-200were added to the above solution with continuous stirring for3–4h at40°C, in which the weight ratio of ZSM-5to P(VDF-HFP)was kept at1/10.The resulting homogeneous solution was cast onto a glass plate with a doctor blade to form free-standing wet membrane.To attain the desirable dry membrane,the wet free-standing membrane was placed in the steam bath at80°C for3–4h and directly dried under vacuum at 60°C for10–12h,which were labeled as SB and VD membrane,respectively.The PI membrane was prepared by traditional phase inversion method in order to do some comparisons.Then,the desirable CPEs were prepared by immersing the three kinds of membranes,namely,SB,VD, and PI membranes into the1.0M LiPF6-ethylene carbon-ate(EC)/dimethyl carbonate(DMC)/ethylmethyl carbonate

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(EMC)(1/1/1:v/v/v)liquid electrolyte solution(provided by Dongguan Shanshan Battery Materials Co.Ltd.,China) for1h,to be activated at room temperature.The whole activation processes were carried out in a glove box under an atmosphere of argon.The commercialized polyethylene separator,namely PE membrane,was also used as a control in our work.

Physicochemical performances characterization

For observing morphological features of the CPEs,the micro-porous membranes were treated by gold sputtering under vacuum.A SEM(JSM6301F)with an accelerating voltage of20kV was employed to observe the surface of the as-prepared membranes.The FTIR spectra were used to record on a Nicolet6700instrument with a wave-number resolution of2cm-1in the frequency of4,000–400cm-1. For the measurement,the mixed slurry was cast on a KBr wafer and dried via the same step.

TG–DSC measurements were carried out on a Perkin Elmer Pyris-1analyzer.The measurements were performed at a heating rate of10°C min-1from20to800°C.A?ow of argon 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 uptake rate(A)was calculated by the fol-lowing relation:

Ae%T?w1àw0

w0

?100e1T

where w1is the weight of the wet membrane and w0is the weight of dry membrane.

Electrochemical measurements

Ionic conductivity of the CPEs were determined over the frequency range of1–105Hz in the temperature range from 293to363K by EIS using a CHI-660b frequency response analyzer purchased from Shanghai Chenhua Ltd.,China. The electrolyte was sandwiched between two stainless steel (SS)blocking electrodes to fabricate a symmetrical SS/ electrolyte/SS cell.The ionic conductivity was calculated according to the following relation:

r?l=eR?STe2Twhere r is the ionic conductivity,R is the bulk resistance of the symmetrical cell,and l and S are the thickness and the area of the specimen,respectively.

Electrochemical stability window of the CPEs were determined by running LSV tests in three-electrode cell using stainless steel as the blocking working electrode, lithium as both the counter and the reference electrode and the CPEs as the electrolyte.A CHI-660b electrochemical interface was used to run the voltammetry at a scan rate of 5mV s-1.

The charge and discharge tests of the fabricated Li/CPE/ LiCoO22032coin cell were carried out on electrochemical workstation(Xinwei Corp.,Shenzhen,China).In the preparation of LiCoO2electrode,LiCoO2(provided by ATL,Dongguan,China)was used as the active material and carbon black(Super P,MMM carbon,Belgium)was used as a conductive agent,while PVDF was used as a binder with a weight ratio of90/5/5.N-methylpyrrolidone was used as a solvent to prepare the electrode slurry that was then coated on an aluminum foil.The processes of battery assembly were carried out in a full-argon glove box.

Results and discussion

Surface morphology

As separators of lithium ion battery,the surface morphol-ogy of the CPE membranes is very important because the entrapped electrolyte uptake ratio and the compatibility between electrodes and electrolyte are very closely linked with the surface properties of the membranes.Figure1 shows the SEM images of the CPE membranes prepared by different methods.It can be obviously observed that the membranes have many micro-pores with different pore sizes except for the VD membrane displayed in the Fig.1d, which may be mainly attributed to the rapid evaporation of DMF,resulting in forming fewer pores during the pro-cesses of the VD membrane preparation.

As can be seen from Fig.1a and c,we can draw con-clusions in the following three aspects:(a)molecular sieve ZSM-5is homogeneously dispersed on the surface of polymer matrix and none of distinct aggregates was dis-covered;(b)plentiful micro-pores in the inner layer of both SB and PI membranes demonstrat their good inter-con-nectivity,which can further enhance more electrolyte entrapment ratio;(c)due to different driving forces,the pore size of the membrane in Fig.1c is smaller than that of the membrane in Fig.1a,which indicates that the former has higher uptake ratio than the latter.The surface mor-phology of the CPE membranes shows much difference, which can be partly attributed to the addition of ZSM-5and partly to different driving forces on the interface between the polymer matrix and solvent during the process of evaporation.

Ionic conductivity

The EIS plots of the as-prepared different CPEs at room temperature are shown in Fig.2.The bulk resistance values

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of the CPEs show no signi?cant differences in all fre-quency ranges.However,the values in the high frequency display slight differences.It can be obviously observed from Fig.2that the VD membrane shows the maximum resistance value and the other two CPEs exhibit the values with insigni?cant differences.The corresponding ionic conductivity and electrolyte uptake ratio about different kinds of polymer electrolyte membranes are listed in Table 1.The ionic conductivity of both PI and SB mem-branes at room temperature can be up to 3.6and 5.1mS cm -1,respectively,which exceed that of the con-ventional commercial membrane used in lithium ion polymer battery because of the higher uptake ratio of the CPEs.The results are in a good accordance with those of the SEM analysis,which suggests that the good inter-connectivity of the membranes as shown in Fig.1a and c can enhance uptake ratio.However,there must be another factor to affect the ionic conductivity because the uptake ratio and the ionic conductivity do not follow a consistent relation as shown in Table 1.Moreover,the ionic con-ductivity of ZSM-5doped CPEs possesses signi?cant improvement,which could be attributed to the particular structure and surface property of ZSM-5.

The plots about reciprocal temperature dependence of ionic conductivity of the three kinds of different polymer

electrolytes are shown in Fig.3.Obviously,the SB mem-brane possesses the highest ionic conductivity in all tem-perature ranges,which suggests that the CPE membrane prepared by steam bath method can be well improved in terms of ionic conductivity.Furthermore,the ionic con-ductivity is not related linearly to the reciprocal tempera-ment,which is different from the reported composite polymer electrolyte doped with some other inorganic ?llers and maybe obeys the Vogel–Tamman–Fulcher (VTF)relation [29,30].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 determined by the free volume around the polymer chains,which leads to an increase in ion mobility and segmental mobility that will assist ion transport and virtually compensate for the retarding effect of the ion clouds [30].FTIR analysis

In order to understand the formation mechanism of micro-pores in the CPE membranes,FTIR spectra study is carried out in the present work.Figure 4a displays the IR spectrum of the solvent DMF,the absorption band appearing

around

Fig.1SEM images of the composite polymer electrolyte membranes with ZSM-5prepared by a phase inversion method;b direct vacuum drying method;c steam bath method;and without ZSM-5by steam bath method (d )

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1,657–1,696cm -1is ascribed to IR absorption of C=O double bonds.Figure 4b and c represent the IR spectra of the CPE membranes with and without ZSM-5,respectively.The two IR absorption peaks near 3,430and 1,639cm -1in Fig.4b may be assigned to the presence of the hydrogen bond (O–H)from the CPE membranes containing absorb-able impurity water,and peaks near 1,278and 1,188cm -1can be attributed to asymmetric and symmetrical stretching vibrations of the CF 2bond (F–C–F),respectively.The

absorption bands appearing at 1,072,976,763and 614cm -1are ascribed to IR vibrating absorptions of crystalline phase,and the 879and 841cm -1are assigned to the absorption of amorphous phase.Similarly,the corre-sponding absorption bands can be also observed at

3,380,

Fig.2EIS plots of:a SS/electrolyte/SS symmetric cells with different polymer electrolyte at room temperature and b represent the local enlarged plot about

a

Fig.3Reciprocal temperature dependence of ionic conductivity of the three kinds of different polymer electrolyte membranes

Table 1Results of ionic conductivity and uptake ratio of different kinds of polymer electrolyte membranes Sample Ionic conductivity (mS cm -1)Uptake ratio (A %)PI membrane 3.64792.92VD membrane 0.52743.25SB membrane

5.098

95.79

Fig.4FTIR spectra of solvent DMF (a )and the composite polymer electrolytes without ZSM-5(b )and with ZSM-5(c )

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1,628,and1,192cm-1of the spectrum of the CPE mem-branes with ZSM-5in Fig.4c.

It is obviously observed from Fig.4that the two CPEs have the same absorption wave number but different inten-sities in the peaks,which can be attributed to the destroyed micro-structure of the CPE membranes by the addition of molecular sieve ZSM-5.For example,the intensities of peaks at3,380,1,628,and1,192cm-1in the Fig.4c are much weaker than those in the Fig.4b due to the presence of twofold functions of ZSM-5.On one hand,ZSM-5can absorb impurities such as water and decomposed organics, on the other hand,as Lewis base,it can interact with the CF2 bonds of polymer matrix P(VDF-HFP)which can decrease the intensities of CF2absorption bands.

Thermal stability

Thermal stability is always considered as an important parameter to evaluate the practical applications of lithium ion battery.Figure5shows the TG–DSC plots of polymer electrolyte membranes prepared by steam bath method with and without molecular sieve ZSM-5.It is obviously observed from TG plots in Fig.5a that the SB membrane shows thermal stability above350°C whether the mem-brane was modi?ed with ZSM-5or not,and the thermal stability of the membrane with ZSM-5can even reach 385°C,which can well satisfy practical requirements in terms of thermal properties.

There are few differences in the DSC plots of the two kinds of SB membranes as demonstrated in Fig.5b.It suggests that the basic structure of the polymer matrix was not destroyed during the preparation process even if molecular sieve ZSM-5was added to modify polymer matrix.However,the crystallinity of the SB membranes shows obvious differences displayed in the insert section in

Fig.5b.The crystallinity can be measured from DSC curves according to the following relation:

Xe%T?D H f=D H H

mf

?100e3Twhere D H mf H is the standard enthalpy of fusion of pure PVDF,i.e.,104.7J g-1,and D H f is enthalpy of fusion of the P(VDF-HFP)based polymer electrolyte membrane.It can be obviously observed in Fig.5b that value of X decreases from50.02to28.54with the addition of molecular sieve ZSM-5,which implies that the orderly crystallization is destroyed because of the interaction between polymer matrix and inorganic particles.Moreover, the X value of the SB membrane without ZSM-5decreases nearly100%compared with the commercialized PE sep-arator.These results indicate that the membranes,prepared by the steam bath technique,with more amorphous areas can attain much improvement in ionic conductivity and other performances.Electrochemical stability window

Figure6shows electrochemical stability window of the three kinds of CPEs prepared by different methods at room temperature.It is well known that the rapid increasing of current density at high potential results from the decom-position of the CPEs.It can be obviously observed that the PI membrane prepared by the traditional phase inversion method possesses the lowest electrochemical stability window about 4.5V;however,both the SB and VD membranes can be up to5V,which are close to that of the commercialized PE https://www.360docs.net/doc/3317255376.html,bined with the above experimental results,the SB membrane exhibits the excellent practical applications of lithium ion polymer battery,which is not only attributed to its5V electro-chemical stability window but also to high uptake rate and ionic

conductivity.

Fig.5TG(a)and DSC(b)plots of polymer electrolyte membranes prepared by steam bath method with and without molecular sieve ZSM-5(insert section in b shows differences in crystallinity of the SB membranes)

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Rate and cycle performances

Figure 7presents the charge–discharge curves of the Li/CPE/LiCoO 2coin cell at different rates (0.1C ,0.2C ,0.5C and 1.0C ).It can be seen from Fig.7that the discharge capacity of the cell is gradually faded with increasing charge–discharge rate,but keeps relatively high capacity.The cells at 0.1C achieve a high capacity,e.g.,144.97mAh g -1for the cell with CPEs prepared by steam bath technique,and 143.25mAh g -1for the cell with CPEs prepared by phase inversion method.At 1.0C ,the capacity keeps 89.84%of that at 0.1C for the cell with ZSM-5modi?ed CPEs prepared by steam bath technique (Fig.7a),but only 87.18%for the cell with ZSM-5modi?ed CPEs prepared by phase inversion technique (Fig.7b).There-fore,the ZSM-5modi?ed CPEs prepared by steam bath technique are good for use in the lithium ion battery using LiCoO 2as cathode.

Figure 8presents the cycle performance of the Li/CPE/LiCoO 2cell.As can be seen from it,the cell with ZSM-5modi?ed CPEs prepared by steam bath technique has the initial capacity as high as the cell with ZSM-5modi?ed CPEs prepared by phase inversion technique.After 26cycles,the former keeps 97.5%of its initial discharge capacity but the latter keeps only 89.3%.Thus,the cycle performance of the cell assembled with ZSM-5modi?ed CPEs prepared by steam bath method has been also improved.The rate and cycle performances of lithium ion battery gain much improvement when the ZSM-5modi?ed CPEs prepared by phase inversion technique are used as electrolyte.It can be ascribed to the higher ionic conduc-tivity of CPEs and its better compatibility with

electrodes.

Fig.6Results of LSV about four kinds of different polymer electrolyte

membranes

Fig.7Rate performances of the cell Li/CPE/LiCoO 2from 4.25to 2.75V at 0.1C ,0.2C ,0.5C ,and 1.0

C

Fig.8Cycle stability of the cell Li/CPE/LiCoO 2,charged and discharged at 0.5C between 4.25and 2.75V

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Conclusion

Three kinds of polymer electrolyte membranes,namely PI, VD and SB membrane,were prepared by traditional phase inversion,direct vacuum drying and steam bath method, respectively.A combination of SEM,uptake ratio and ionic conductivity tests shows that the SB membrane has the uniform and ordered surface morphology with abundant inter-connected micro-pores in the inner layer of the SB membrane,which can not only improve the interfacial properties but also enhance ionic conductivity up to 5.1mS cm-1.The results of TG–DSC and LSV measure-ments indicate that thermal and electrochemical stability reach350°C and5V,respectively.Moreover,Li/CPE/ LiCoO2cells with the CPE prepared by steam bath tech-nique exhibit good rate and cycle performances.Better practical performances and relatively simpler preparation processes of the SB membrane ensure that it can be used as the most promising polymer electrolyte membrane for the rechargeable lithium ion polymer battery.

Acknowledgments This work has been carried out with the?nan-cial support of the Major Provincial Science and Technology Pro-grams of Hunan(2009FJ1002)and Central College on the2010 Operational Costs of Basic Research Project(2010QZZD0101). References

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