Preparation and characterization of a novel microporous PE membrane supporting composite gel polymer electrolyte

Preparation and characterization of a novel microporous PE membrane supporting composite gel polymer electrolyte

Solid State Ionics 176 (2005) 2829 – 2834 www.elsevier.com/locate/ssi Preparation and characterization of a novel microporous PE membrane supporting ...

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Solid State Ionics 176 (2005) 2829 – 2834 www.elsevier.com/locate/ssi

Preparation and characterization of a novel microporous PE membrane supporting composite gel polymer electrolyte Mujie Yang a,*, Weili Li a, G.G. Wang b, J.Q. Zhang b,c a

Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China b Department of Chemistry, Zhejiang University, Hangzhou 310027, China c China State Key Laboratory for Corrosion and Protection Shenyang 110015, China Received 22 October 2004; received in revised form 19 July 2005; accepted 27 July 2005

Abstract A new type of composite gel polymer electrolyte(CGPE) with a blend of polyethylene glycol 200 maleate, methyl methacrylate(MMA) and poly(methyl methacrylate)(PMMA) coated and polymerized on the microporous polyolefin membrane was prepared by means of ultra-violet crosslinking and then soaked in a lithium salt solution. The chemical construction, morphology, ionic conductivity of composite gel polymer electrolytes, and their interfacial stability between lithium metal electrode were characterized by using infrared spectroscopy, 1H NMR measurement, scanning electron microscopy, alternating current impedance and linear sweep voltammetry, respectively. The ionic conductivity of the CGPE reaches 10 3 S/cm at room temperature and its electrochemical stability window is 4.7 V, which makes it a potential candidate for application as polymer electrolyte in devices. D 2005 Elsevier B.V. All rights reserved. Keywords: Membrane supporting composite gel polymer electrolyte; Polyethylene glycol 200 maleate; UV cross linking; Ionic conductivity

1. Introduction Since the first discovery of ionic conductivity in alkali metal salt complexes of poly(ethylene oxide)(PEO) by P.V. Wright et al. in 1973 [1], number of contributions to the field of Solid Polymer Electrolytes(SPEs) have grown enormously. These polymer electrolytes are solid solutions of alkali metal salts in polymer(PEO) [2]. However, they commonly exhibit conductivities in the range from 10 8 to 10 5 S/cm. The main obstacles to practical applications are, firstly the high degree of crystallinity which is unfavorable for ionic conduction in these complexes, and secondly the low solubility of salt in the amorphous phase [3]. Therefore, considerable attention has been dedicated to the development and characterization of Gel Polymer Electrolytes(GPEs) because of their liquid-like ionic conductivity [4,5]. This system includes a polymer host, an ionic salt and a small molecule organic solvent and can be easily prepared * Corresponding author. Fax: +86 571 87952444. E-mail address: [email protected] (M. Yang). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.07.011

by swelling polar polymer membrane with a lithium salt solution. Unfortunately, this process requires a moisture-free environment because of the higher water sensitivity of the lithium salt, and the swelling process often degrades the mechanical properties, so the application of this electrolyte to devices is far from satisfactory. In 1990s, Bellcore’s group successfully overcame the difficulties and developed a liquid extraction and activation method to prepare the polymeric electrolyte materials for plastic lithium-ion batteries [6,7]. The processes are the plasticization of a copolymer of vinylidene fluoride and hexafluoropropylene (PVdF-HFP), subsequent removal of the plasticizer, and final swelling in an electrolyte solution. This procedure requires critical moisture control during only the last activation step. The ability to absorb and retain the electrolyte solution is critical to applications in lithium batteries. Recently, there have been a number of studies on gel polymer electrolyte which was coated onto a microporous separator [8– 13]. The gelled polymers are accompanied with an inert separator membrane; the support membrane

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endows the final polymer electrolyte matrix with sufficient mechanical properties for practical battery assembly procedures, while the coated polymers are adapted to encapsulate the electrolyte solution in the porous membrane and to further assist in adhering the electrodes to the separator [8]. Poly(ethylene glycol diacrylate)(PEGDA) or poly(ethylene glycol dimethylacrylate)(PEGDMA) used as a crosslink prepolymer were blended with PMMA [8] or PEO [10], and then coated and polymerized on the microporous polyethylene(PE) membrane under heat or UV irradiation. With these polymer-coated membranes, highly conductive polymer electrolytes without solvent exudation were prepared by soaking them in an electrolyte solution. However, these crosslink prepolymers, PEGDA or PEGDMA are very active and hence lead to storage problem. With the aim of developing highly conductive polymer electrolytes with sufficient mechanical strength and high ionic conductivity, we synthesized a new type crosslink prepolymer, polyethylene glycol 200 maleate which was very inert during storage under general condition, while it became active when it was blended with MMA or styrene. We prepared miscible prepolymer solution of polyethylene glycol 200 maleate, MMA and PMMA, PE membrane coated with the prepolymers solution was polymerized in situ under UV irradiation. In microscopic view, polyethylene glycol 200 maleate and MMA were shrunk into a chemically crosslinked solid network on the PE membrane, while PMMA impregnated in the thermally stable polymer blends had high affinity for liquid electrolytes. Then the highly conductive polymer electrolytes without solvent exudation were prepared by soaking the cured membrane in an electrolyte solution. In this paper we investigate the configuration, swelling and electrochemical properties of the composite polymer electrolyte supported by microporous polyolefin membrane.

2. Experimental 2.1. Materials Poly(ethylene glycol) (M n = 200) (Sinopharm Chemical) and p-toluenesulfonic acid(AR) (Sinopharm Chemical) were dried at 80 -C under vacuum, maleic anhydride(AR) (Jiangshu Yonghua Chemical), methyl methacrylate (MMA) (CP) (Sinopharm Chemical) distilled under reduced pressure before use, PMMA(M n, ca. 105), toluene (AR) (Hangzhou ˚ molecular sieve, Chemical) was dehydrated using 4 A acetone (AR) (Hangzhou Chemical). 2.2. Prepolymer synthesis Polyethylene glycol 200 maleate was synthesized in a batch reactor comprising of three-neck, with an argon inlet, an outlet, a water knockout drum, a condensator, and a thermocouple connected to a temperature controller. First, 0.05 mol maleic anhydride and 0.05 mol PEG200 were

dissolved in toluene, p-toluenesulfonic acid (TPSA) as catalyst. The mixture were heated to 150 -C with stirring for 8 h under argon atmosphere. The outgrowth H2O was extracted in the water knockout drum with toluene during the reaction. Then the production was transferred to a rotary evaporator to remove the solvents. The reaction involved is outlined in Scheme 1. 2.3. Membrane supported composite gel polymer electrolyte preparation A typical polymerization procedure was as follows: appropriate amounts of polyethylene glycol 200, MMA and PMMA were dissolved and stirred in anhydrous acetone to become a homogeneous solution, their weight ratio was fixed at 1 : 1 : 1, and the concentration of solution was 0.1 g/ml. Then the photo initiator, benzophenone, and a curing accelerator, triethylamine were added to the solution. A microporous PE separator (ENTEK, thickness: 25 Am, porosity: 60%) was then soaked in the solution for 1 h. The membrane was taken out on a Teflon plate and left to evaporate out the solvent. After the evaporation of acetone, the membrane was cured under UV irradiation (110 W, k = 375 nm) for 1 h to polymerize the mixture. The prepolymer can be coated and polymerized on the PE membrane uniformity under UV irradiation. To prepare the composite polymer electrolyte, the cured membranes were first dried under vacuum at 50 -C for 48 h, and then transferred to the glove box and immersed in the organic liquid electrolyte (DMC : DEC : EC = 1 : 1 : 1 (W / W) LiPF6 1 M, Guotai–Huarong Chemical) for 1 h. After activation, the membranes were removed from the electrolyte solution and excess electrolyte solution on the surface was wiped with filter paper. 2.4. Measurements FTIR measurement was carried out on BRUKER VECTOR-22 spectrometer and 1H NMR measurement was conducted in Advance DMX500, 500 MHz spectrometer (solvent: CDCl3, internal standard: tetramethylsiliane). The cross-sectional morphology of the polymer membrane was observed by scanning electron microscopy (SEM) (XL30-ESEM, PHILIPS) under vacuum after sputtering with gold at 10 mA for 10 min. To study the cured membrane’s swelling capability, percent swelling (S w) was considered. Uptake of an electrolyte solution was determined as follows: S w = 100(W  W 0) / W 0, where W 0 and W were weights of the initial and wet membrane. The ionic conductivity (r) of the membrane supported composite gel polymer electrolytes were determined by AC O O O O n + nHO(CH2CH2O)mH

150°C TPSA

*

C H

C

C

O CO(CH2CH2O)m

*

H n

Scheme 1. Synthesis of the polyethylene glycol 200 maleate prepolymer.

M. Yang et al. / Solid State Ionics 176 (2005) 2829 – 2834

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electrode. LSV measurement was carried out using CHI660A (CH Instruments. Inc). 3438

3. Results and discussion

+ 1641 + 1404

3.1. FTIR and 1H NMR spectra of prepolymer

2877

1128 1729 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber/cm-1 Fig. 1. FTIR spectrum of polyethylene glycol 200 maleate.

impedance spectroscopy (EG&G Model 273A potentiostat). The membrane was sandwiched between two parallel stainless steel (SUS) discs (U : 1 cm). During the measurement, it was mounted in a sealed coin cell to prevent contamination of the sample. The frequency ranged from 100 KHz to 1 Hz at a perturbation voltage of 5 mV. The ionic conductivity (r) was then calculated from the electrolyte resistance (R b) obtained from the intercept of the Nyquist plot with the real axis, the membrane thickness (l), and the electrode area (A) according to the equation r = l / AR b. The electrochemical stability window of the composite gel polymer electrolyte was measured by linear sweep voltammetry (LSV) at a scanning rate of 0.05 V/s. Three electrode-laminated cell was assembled inside a glove box. Stainless steel (SS) was used as working electrode and lithium metal was used both as a counter and as a reference

The IR and 1H NMR spectra of polyethylene glycol 200 maleate obtained are shown in Figs. 1 and 2. The characteristic absorption (cm  1 ), 2877 (m C – H ), 1729 (m C_O ), 1641(m C_C), 1404 (d as, C – H), 1128 (d as, C – O – C) are observed in IR spectrum, and the disappearance of 1820 cm 1 and 1780 cm 1 for anhydride bond means the complete esterification of maleic anhydride. And also, it is found from Fig. 2 that the peaks in chemical shift d (ppm): 6.293 (m, 1 H, –CH_), 4.343– 4.325 (m, 2 H, – CH2COO –), 3.742– 3.467 (m, 4 nH, – (CH2CH2O)n – ), which confirm with the IR spectrum analysis. 3.2. Morphology of polymer-coated PE membrane Typical scanning electron micrographs (SEM) on the surface of the polymer-coated membranes are presented in Fig. 3(a –f). They show pore-like structure on the PE membrane. When the prepolymers are coated on the porous PE membrane, the polyethylene glycol 200 maleate and MMA are viscous liquid. Under UV light, the polymerization of crosslink prepolymers are believed to cause shrinkage of polymer coating on the membrane. At the same concentration, the number of the pores in unit area decreases with the proportion of crosslink prepolymers. At the same

Fig. 2. 1H NMR spectrum of polyethylene glycol 200 maleate.

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Fig. 3. Scanning electron micrographs of the polymer-coated membranes(PCM) with different concentration of the coated prepolymer solution: (a) PCM ((1 : 1 : 0.25) — 0.1 g/ml), (b) PCM ((1 : 1 : 0.5) —0.1 g/ml), (c) PCM ((1 : 1 : 1) — 0.15 g/ml, (d) PCM ((1 : 1 : 2) — 0.1 g/ml) (e) PCM(1 : 1 : 1) — 0.05 g/ml) (f) PCM((1 : 1 : 1) — 0.15 g/ml).

proportion, an increase in the prepolymer concentration causes a decrease in the number of the pores in unit area. Presence of micropore on the surface of the polymer-coated separator leads to efficient uptake of the liquid electrolyte when it is soaked in an electrolyte solution finally leads to gelation of the coated polymer. However, when the concentration of solution reached 0.15 g/ml, fewer micropores could be found, resulting in less absorption of lithiumion electrolyte. 3.3. Swelling and electrical measurements of membrane supporting gel polymer electrolyte The value of DW%, S w and room temperature ionic conductivity varied with weight proportion and solution concentration of prepolymers which are given in Tables 1

and 2, respectively. It can be seen from Table 1 that DW% is not very dependent on the weight proportion of prepolymer while S w and ionic conductivity increase with the content of PMMA at the same concentration (0.10 g/ ml). The increase of S w and ionic conductivity is maybe due to the high affinity of PMMA for liquid electrolytes which can widen the ion-conductive pathway. In Table 2, it Table 1 The effect of weight proportion of prepolymer on the property of polymercoated membrane Weight proportion

DW%

Sw

Ionic conductivity (S/cm)

PCM PCM PCM PCM

91 106 93 85

90 100 135 150

6.89 * 10 4 8.34 * 10 4 1.1 * 10 3 1.35 * 10 3

((1 : 1 : 0.25) —0.10 g/ml) ((1 : 1 : 0.5) —0.10 g/ml) ((1 : 1 : 1) —0.10 g/ml) ((1 : 1 : 2) —0.10 g/ml)

M. Yang et al. / Solid State Ionics 176 (2005) 2829 – 2834 Table 2 The effect of coating solution concentration on the property of polymercoated membrane DW%

Sw

Ionic conductivity (S/cm)

51 93 113.5

117 135 60

9.01 * 10 4 1.1 * 10 3 6.51 * 10 4

2.4x10-3 0.10 g/ml 0.05 g/ml 0.15 g/ml

2.0x10-3

σ, S/cm

Concentration PCM ((1 : 1 : 1) —0.05 g/ml) PCM ((1 : 1 : 1) —0.10 g/ml) PCM ((1 : 1 : 1) —0.15 g/ml)

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1.6x10-3 1.2x10-3

DW: Percent weight increase of coated composite polymer membrane.

8.0x10-4

is seen that DW% increases with prepolymers concentration of the coating solution at the same weight proportion (1 : 1 : 1 by weight). However, the highest S w and ionic conductivity are obtained at concentration of PCM ((1 : 1 : 1) — 0.10 g/ml), both above and below this concentration decreased. S w decreases at high concentration of 0.15 g/ml as the less porosity of polymer-coated membrane and electrolyte can’t be absorbed well in the membrane which reduces the ion-conductive pathway. PCM ((1 : 1 : 1) —0.10 g/ml) and PCM ((1 : 1 : 1) —0.05 g/ml) have similar morphology. However, when S w is considered, the former is higher, which means it absorbs more electrolyte, and lithium-ion can easily transfer through it. Next, we try to investigate the ion conductivity mechanics of PE membrane supporting composite gel polymer electrolyte. Typical AC impedance spectrum for the membrane (CGPE(1 : 1 : 1) —0.10 g /ml, 30 -C) using SS blocking electrodes is shown in Fig. 4. Fig. 4 shows that there is no semicircle observed at high frequency. The result suggests that only the resistive component of polymer electrolyte could be considered at the high amount of plasticizing electrolyte. It is possible to construct a local effective pathway in liquid phase and in gel phase for ionic conduction. The ion mobility in the gel polymer electrolyte is decoupled with the segmental motion of the polymer chain, and it is transferred through gel or liquid electrolyte [2]. Lithium-ion can transport quickly in these phases as the electric potential alternates between positive electrode and negative electrode in an AC field.

4.0x10-4 2.9

3.0

3.1

3.2

3.3

3.4

3.5

3.6

1000/T (1/K) Fig. 5. Dependence of conductivity on the reciprocal of temperature for membrane supporting composite gel polymer electrolytes with various prepolymer concentrations.

The temperature dependence of the ionic conductivity of the polymer electrolytes is generally followed by either a Vogel – Tamman– Fulcher(VTF) (Eq. (1)) or an Arrhenius (Eq. (2)) type equation, depending on whether the ion mobility is coupled with the segmental motion for the polymer chain, r ¼ AT 1=2 expð  B=ðkB ðT  T0 ÞÞÞ

ð1Þ

r ¼ Aexpð  E=kB T Þ

ð2Þ

Where A is a constant which is proportional to the number of carrier ions, B denotes the pseudo-activation energy associated with the motion of the polymer segment, k B represents the Boltzmann constant, and T 0 is a reference temperature (normally associated with the ideal T g at which the free volume is zero, or with the temperature at which the configuration entropy becomes zero). Fig. 5 is a typical plot of ionic conductivity of the electrolyte samples (CGPE((1 : 1 : 1) — 0.05, 0.10, 0.15 g/ ml)) against temperature which reveals a linear relationship. It confirms that the variation in conductivity with temperature follows an Arrhenius relationship. As we all know, increasing temperature results in the expansion of the

200 180 0.008

160

CGPE(1:1:2)-0.10g/ml

140

Current (A)

-Z"(Ohm)

0.006 120 100 80 60

0.004 4.7V 0.002

40 20

0.000

0 0

20

40

60

80

100

120

140

160

180

200

Z'(Ohm) Fig. 4. AC impedance spectra of composite electrolyte membrane (CGPE((1 : 1 : 1) —0.10 g/ml), 30 -C).

3

4

5

6

7

Voltage (V vs Li) Fig. 6. Electrochemical stability window of the CGPE ((1 : 1 : 2) —0.10 g/ ml) by linear sweep voltammogram.

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material, which produces the local empty space, and expends the free volume. It promotes the polymer segments and ionic carriers to move. Hence, as the temperature increase, the R b decreases. This phenomenon also indicates that the charge carriers are decoupled from the segmental motion of the polymer chain and transport occurs via an activated hopping mechanism. The behavior is typically observed in liquid electrolytes and gel-type polymer electrolytes [14,15]. To ascertain the electrochemical stability of the composite gel polymer electrolyte, linear sweep voltammogram (LSV) measurement of the laminated three electrode cells was performed at ambient temperature. The LSV curve of CGPE ((1 : 1 : 2) — 0.10 g/ml) is presented in Fig. 6. The working electrode potential of the cell was varied from 3.0 to 8.0 V (versus Li) at scanning rate of 0.05 V/s. It is evident from Fig. 6 that there is no electrochemical reaction in the potential range from 3.0 to 4.7 V. The onset of current flow at 4.7 V is associated with the decomposition of the electrolyte. From this result, the composite gel polymer electrolyte is thought to be acceptable for high voltage cathode materials, such as LiCoO2 LiNiO2 and LiMn2O4.

4. Conclusion A new type of membrane supporting composite gel polymer electrolytes was prepared via UV crosslinking of polyethylene glycol 200 maleate, MMA and PMMA on microporous polyolefin membranes and soaking them in an electrolyte solution. In these membranes, PE membrane gave chemical and mechanical integrity to the electrolyte system, and the crosslinked polymer encapsulated the electrolyte solution within the porous membrane. The morphology and the ionic conductivity of the composite

polymer electrolytes were found to relate with both the concentration of the coated solution and the relative weight ratio of polymer and crosslink prepolymer. The prepared sample CGPE ((1 : 1 : 2) — 0.10 g/ml) were measured to be 1.35 * 10 3 S/cm and 4.7 V vs. Li / Li+. It displayed a high ionic conductivity and electrochemical stability, exhibiting no solvent leakage. The use of this membrane supporting composite gel polymer electrolyte in practice is promising.

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