- Email: [email protected]
Effect of Al2O3 nanoparticles on the electrochemical characteristics of P(VDF-HFP)-based polymer electrolyte
Electrochimica Acta 49 (2004) 4633–4639 www.elsevier.com/locate/electacta
Effect of Al2O3 nanoparticles on the electrochemical characteristics of P(VDF-HFP)-based polymer electrolyte Zhaohui Lia,1,*, Guangyao Sua,1, Deshu Gaoa,1, Xiayu Wanga,1, Xiaoping Lib,1 a
College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, PR China b TCL Hyperpower Batteries Inc., Huizhou, Guangdong 516003, PR China
Received 5 December 2003; received in revised form 9 May 2004; accepted 17 May 2004
Abstract Alumina (Al2O3) nanoparticles have been used as fillers in the preparation of poly(vinylidenefluoride-co-hexafluorpropylene) (P(VDFHFP))-based porous polymer electrolyte. The degree of crystallization of polymer film filled with Al2O3 nanoparticles decreases with increase of the mass fraction of Al2O3 nanoparticles and the amorphous phases of polymer film expand accordingly. The Al2O3 nanoparticles play the role of solid plasticizer for polymer matrix. Nevertheless that excessive Al2O3 nanoparticles existing in polymer matrix leads to micro-phase separation between polymer matrix and fillers. As a result, both ionic conductivity and lithium ions transference number reduces whereas the activation energy for ions transport increases. When the polymer film is filled with 10% of the mass fraction of Al2O3 nanoparticles, polymer electrolyte possesses the ionic conductivity up to 1.95 103 S cm1 and the lithium ions transference number to 0.73 while the activation energy for ions transport of them falls to 5.6 kJ mol1. Effect of Al2O3 on the electrochemical properties of polymer electrolyte has been investigated in this paper. Analysis of FTIR spectra shows that there is the interaction between Al2O3 nanoparticles and polymer chains. ß 2004 Elsevier Ltd. All rights reserved. Keywords: Polymer electrolyte; P(VDF-HFP); Al2O3; FTIR; Electrochemical property
1. Introduction Poly(vinylidenefluoride-co-hexafluorpropylene) (P(VDFHFP)) copolymers have been used as polymer matrix of polymer electrolyte in the most commonly commercialized plastic lithium-ion batteries (PLiONTM) by Telcordia Technologies (formerly Bellcore) since Gozdz et al. found the preparation process of porous film [1–5]. The preparation process can be briefly divided into two stages. First, a film is obtained by evaporating redundant casting solvent with low boiling points from the adhesive polymer solution. Second, the film is immersed into another solvent, which is usually a poor solvent with respect to the polymer material whereas it is a good solvent to the plasticizer. Therefore, this liquid– liquid extraction process obtains a porous film. Prior to use * Corresponding author. Tel.: +86 732 8292206; fax: +86 732 8292477. E-mail address: [email protected] (Z. Li), . 1 ISE member. 0013-4686/$ – see front matter ß 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.05.018
impregnating in nonaqueous electrolyte will activate it. The resulting polymer electrolyte has the ionic conductivity of 103 S cm1. The addition of inorganic fillers such as silica (SiO2), or copper oxide (CuO) or titania (TiO2) nanoparticles into the polymer electrolyte results in the enhancement of physical strength as well as the increase in the absorption level of electrolyte solution [6–8]. In addition to these effects, they act as solid plasticizer hindering the reorganization of polymer chains and can interact with polar groups by Lewis acid–base reaction [9–12]. So the properties such as ionic conductivity, lithium ions transference number and activation energy for ions transport are improved. In this paper, dibutyl phthalate (DBP) and N-methylpyrrolidinone (NMP) are used as organic plasticizer and casting solvent, respectively. A series of porous polymer film filled with various amounts of Al2O3 nanoparticles have been prepared according to the liquid–liquid extraction process. We have studied the effect of Al2O3 nanoparticles on the electrochemical properties of polymer electrolyte such as
4634
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
the ionic conductivity, lithium ions transference number and activation energy for ions transport. The interaction between Al2O3 nanoparticles and polymer matrix is confirmed by FTIR spectra.
2. Experiment 2.1. Materials P(VDF-HFP) copolymer (KnyarFlex12801, containing 12% HFP unit) was obtained as gift from TCL Hyperpower Batteries Inc. Dibutyl phthalate, N-methylpyrrolidinone and ether purchased from Shanghai Chemical Reactants Plant were distilled before used. Nanoscale Al2O3 obtained from Zhejian Mingri Chemicals Co. with average particle size of 30 nm was dried under vacuum at 100 8C for 24 h. The 1 M LiPF6 solution in ethylene carbonate (EC)–diethyl carbonate (DEC) was purchased from Merck Co. without any treatment.
Porous film was prepared by the liquid–liquid extraction process. A certain amount of P(VDF-HFP) was dissolved into NMP at 50 8C. Then the nanoscale Al2O3 particles and DBP were added to the viscous solution and agitated with ultrasonic stirrer. The resulting slurry was cast on a clean glass plate and dried under vacuum at 80 8C for 8 h. After evaporation of NMP, the glass plate was left to cool to room temperature in situ. The polymer film peeled off from glass plate was immersed into ether to extract plasticizer DBP, and dried at 40 8C under vacuum for 2 h. At last, the film was punched into the circular pieces with diameter of 12 mm. 2.3. Electrochemical measurement of polymer electrolyte The circular pieces of polymer film were dried under vacuum at 80 8C for 2 h, then put into dry glove box (H2O content 5 ppm) instantly. It was immerged into 1 mol/L solution of LiPF6/EC–DEC (volume 1:1) for 1 h to make porous polymer electrolyte. After the excrescent solution at the surface of the polymer electrolyte was absorbed with filler paper, the polymer electrolyte membrane was sandwiched between two symmetrical stainless steel blocking electrodes. Using EG&G Potentiostat/Golvannostat M273 conjunction with M5210 Lock-in amplifier electrochemical analysis system we measured the resistance of the polymer electrolyte. The frequency ranged from 1 Hz to 100 kHz, and ac amplitude was 5 mV. The bulk resistance of polymer electrolyte was found from the impedance spectrum. Thus, the ionic conductivity was calculated based on the following equation: d Rb S
tþ ¼
Is ðDVI0 R0 Þ I0 ðDVIs Rs Þ
(2)
2.4. Differential scanning calorimetry (DSC) measurement and FTIR analysis
2.2. Preparation of the porous film
s¼
In the equation, s is the ionic conductivity, Rb the bulk resistance; d and S are the thickness and area of the specimen, respectively. The lithium ions transference number was determined by dc polarization/ac impedance combination method [13,14]. The initial interfacial resistance (R0) of the cell constructed with symmetrical lithium foils was first determined by electrochemical impedance spectroscopy (EIS) measurement. A chronoamperometry measurement was then carried out with a potential difference (DV) of 50 mV, and the initial current (I0) was measured. After 100 s the steady-state current (Is) was obtained. The steady-state resistance (Rs) was determined by EIS measurement again after the chronoamperometry measurement. Therefore, the lithium ions transference number t+ can be calculated from the following equation:
(1)
The thermal property of the porous polymer film filled with different amounts of Al2O3 nanoparticles was investigated with Perkin-Elmer DSC7 instrument. All samples containing no lithium salt were dried under vacuum at 80 8C for 4 h prior to measurement. The measurement was carried out with the heating rate of 10 8C/min at nitrogen atmosphere. The infrared spectra were recorded under nitrogen atmosphere on Perkin-Elmer FTIR 1710 spectrometer, covering a range of 450–4000 cm1 with a resolution of 1 cm1. All samples containing 2 wt.% of LiClO4 and different amounts of Al2O3 nanoparticles were dried as usual.
3. Results and discussions 3.1. Morphology Typical SEM micrograph of the polymer film surface has been shown in Fig. 1. It exhibits that the polymer film possesses porous structure with the diameter ranging from 1 to 10 mm after extraction of plasticizer DBP from polymer matrix. Fig. 2A shows the TEM graph of nanoparticles in the solution of PVDF-HFP/NMP at 5% of the filling mass fraction. The diameter of them is about 200 nm much more than that of primary value (30 nm, provide by manufacturer). From Fig. 2B it can be seen that the nanoparticles grow up to 500 nm or so at 16% of the filling mass fraction. Moreover, it was found that a lot of particles deposit in the polymer solution if the filling mass fraction of nanoparticles is more than 16%.
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
4635
Fig. 1. SEM picture of surface of the porous P(VDF-HFP) film (containing 12% Al2O3).
3.2. Effect of Al2O3 content on the degree of crystallization of polymer film The degree of crystallization of polymer film can be calculated based on the following equation from the DSC curves of polymer film (Fig. 4): Xc ¼
DHm 100% DHm;p
Fig. 3. XRD spectrum of polymer film filled with different amounts of Al2O3 nanoparticles.
3.3. Porosity of polymer film (3) The porosity of polymer film is measured as follows: immerge the film into n-butanol for 1 h, weigh the mass of them before and after the absorption of n-butanol, then calculate the porosity p of polymer film based on the equation p ¼ ðma =ra Þ=ðma =ra þ mP =rp Þ, where ma, mP are the mass of the wet film and the dry film and ra, rp are the density of butanol and polymer, respectively. Fig. 5 shows the porosity dependence of the amount of Al2O3 nanoparticles filling in polymer film. From the figure, it exhibits that the porosity of polymer film is 45% without Al2O3 nanoparticles because the plasticizer DBP, which stays in the polymer film after evaporation of NMP, leaves porous structure after extraction by ether. But the porosity of polymer film rises up to 61% when the polymer film contains 5% of the mass fraction of Al2O3 nanoparticles. Moreover,
16% 14% 12%
Heat Flow / mW
where DHm,p is the heat of fusion for pure a-PVDF, 104.7 J/ g [15], DHm the heat of fusion for P(VDF-HFP) film filled different amounts of nanoparticles. It can be calculated from the integral area of the baseline and each melting curve. The baseline can be determined by connecting two points at which the instant value of its derivative curve becomes zero near melting temperature (Tm). The data of DHm, Tm and the degree of crystallization Xc has both been shown in Table 1. It shows that the degree of crystallization of polymer film decreases with the increase of the amount of Al2O3 nanoparticles filled in. The result can be further confirmed by the XRD spectra for polymer film filled with various amounts of Al2O3 nanoparticles. It is found in Fig. 3 that the intensity of the characteristic peaks for PVDF a-phase crystals at 2u = 17.8, 20.0 and 39.18 decreases with addition of Al2O3 nanoparticles. Maybe the interaction between the Lewis acid groups –OH on the surface of Al2O3 nanoparticles and the basic groups F atoms of polymer chains hinders the motion of polymer segments so the degree of crystallization for polymer film decreases.
10% 8% 5% 0 mass fraction of Al 2O 3 in polymer membrane 80
100
120
140
160
180
0
Tem perature / C
Fig. 2. Transmission electron micrographs of Al2O3 nanoparticles in the solution of P(VDF-HFP)/NMP. (The mass fraction of Al2O3 nanoparticles, A: 5%; B: 16%.)
Fig. 4. DSC curves of PVDF-HFP film filled with different mass fraction of Al2O3 nanoparticles.
4636
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
Table 1 Crystallization degree of PVDF-HFP film filled with varied amounts of Al2O3 nanoparticles Mass fraction of Al2O3 in polymer film (%)
Melting temperature of crystalline, Tm (8C)
Heat of fusion for crystalline, DHm (J g1)
Crystallization degree, Xc (%)
0 5 8 10 12 14 16
162.8 163.1 162.8 162.8 163.1 163.1 162.3
51.59 50.27 49.34 47.02 42.90 35.68 21.48
49.3 48.0 47.2 45.0 41.0 34.1 20.5
when polymer film is filled with 10% of the mass fraction of Al2O3 nanoparticles, they get the largest value of the porosity, 70%. However, it decreases with further addition of Al2O3 nanoparticles into polymer matrix because of the aggregation of nanoparticles. The interfacial layers between Al2O3 nanoparticles and polymer matrix, which has been found by Croce et al. [16], is responsible to the increase of the porosity of polymer film. The micro-spaces occur at the interface between the surface of Al2O3 nanoparticles and polymer matrix where electrolyte solution is stored and become the tunnels for lithium ions migration. The volume of interfacial layers increases with the amount of Al2O3 nanoparticles filling in polymer film at first but decreases with aggregation of Al2O3 nanoparticles. 3.4. Ionic conductivity of polymer electrolyte Polymer film was proved to contain little plasticizer (DBP) by thermogravimetric analysis, and absorb the same mass of electrolyte solution by weighing. The relationship between the mass fraction of Al2O3 nanoparticles and ionic conductivity of polymer electrolyte has been showed in Fig. 6.
It is well known that lithium ions migrate in two ways: (i) move along the molecular chains of polymer, and (ii) move in the amorphous phase of polymer electrolyte [17]. The former is slow transport whereas the latter is fast. Fig. 6 shows that the ionic conductivity of polymer electrolyte increases with the rise of amount of Al2O3 nanoparticles in the polymer film when the mass fraction is not more than 10%. On one hand, the volume of the interfacial layers, which possesses a higher ionic conductivity, increases with the amount of Al2O3 nanoparticles filling in the polymer film, so the number of the tunnels for lithium ions migration enhances. On the other hand, the competition between Al2O3 and F atoms with respect to lithium ions facilitates the migration for lithium ions in polymer electrolyte. In this case, the ionic conductivity of polymer electrolyte increases with the addition of Al2O3 nanoparticles into polymer matrix. On the contrary, excessive Al2O3 nanoparticles filling in polymer matrix lead to the aggregation of nanoparticles, so the volume of the interfacial layers decreases. The effective number of the O atoms that compete for lithium ions with F atoms reduces with the decrease of the surface area of nanoparticles due to the aggregation as well. As a result,
2.0
70
-1
1.8
Porosity / %
-3
Ionic conductivity / x10 S.cm
65
60
55
50
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
45
0.0
-2
0
2
4
6
8
10
12
14
16
18
Mass fraction of Al 2O3 nanoparticles in polymer membrane / %
Fig. 5. Porosity of polymer film filled with different amount of Al2O3 nanoparticles (25 8C).
-2
0
2
4
6
8
10
12
14
16
18
Mass fraction of Al2O3 nanoparticles in porous polymer membrane / %
Fig. 6. The relationship between amounts of Al2O3 nanoparticles and ionic conductivity of P(VDF-HFP) film (25 8C, 5% lithium salt).
Fig. 7 exhibits the ionic conductivities dependence of the temperature ranging from 25 to 55 8C for polymer electrolyte. These curves appears linear so that the activation energy for ions transport Ea can be further obtained by using the simple Arrehnius model [s ¼ s 0 expðEa =RTÞ] instead of the Vogel–Tamman–Fulcher (VTF) model fs ¼ s 0 T 1=2 exp½Ea =ðTT0 Þ ; T0 ¼ Tg 50g, where R is gas content, s the conductivity of polymer electrolyte, s0 the pre-exponential index and T the testing temperature, respectively. According to Arrehnius equation the activation energy for ions transport can be calculated from the slope of the straight line. Fig. 8 shows the relationship between the amount of Al2O3 nanoparticles in polymer film and the activation energy for ions transport. It suggests that the activation energy for ions transport decreases with the amount of Al2O3 nanoparticles when the mass fraction of nanoparticles is below 10% whereas it increases with further addition of Al2O3 nanoparticles into polymer film. The activation energy falls to the bottom of the curve, namely 5.6 kJ mol1, when the mass fraction of Al2O3 nanoparticles equals to 10%. With more tunnels for lithium ions migration and the competition between O atoms of Al2O3 and F atoms of polymer chains with respect to lithium ions, lithium ions transfer easily. As a result, the activation energy for Li+ ions transport decreases. Nevertheless, the aggregation of nanoparticles results in the decrease of the numbers of O atoms that competing with F atoms for Li+ ions and the number of tunnels for lithium ions migration. The migration of lithium ions is hindered in polymer electrolyte, accordingly.
+
3.5. Activation energy for ions transport
Activition energy for Li ion transportion / kJ.mol
the ionic conductivity of polymer electrolyte decreases with further addition of Al2O3 nanoparticles into the polymer film.
-1
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
8.0
7.5
7.0
6.5
6.0
5.5 -2
0
2
4
6
8
10
12
14
16
18
Mass fraction of Al2O3 nanoparticles in porous polymer membrane / %
Fig. 8. The relationship between amounts of Al2O3 nanoparticles in P(VDF-HFP) film and activation energy for ion transport.
3.6. Transference number of lithium ions Figs. 9 and 10 show the chronoamperometry profile and ac impedance spectroscopy for P(VDF-HFP)-based polymer electrolyte containing 10% of the mass fraction of Al2O3 nanoparticles at 25 8C, respectively. From Fig. 9, it is found that the initial current (I0) and the steady-state current (Is) are 280 and 227 mA, respectively. From analysis of Fig. 10, the initial interfacial resistance and steady-state interfacial resistance are 128 and 152 V, respectively. According to Eq. (2) the lithium ions transference number is calculated to equal 0.73. The relationship between amount of Al2O3 nanoparticles and lithium ions transference number has been shown in Fig. 11. It suggests that lithium ions transference number increases with the rising of mass fraction of Al2O3 nanoparticles in polymer film below 10% whereas it decreases with further addition of Al2O3 nanoparticles into polymer film. The result shows that the critical mass fraction of Al2O3
-2.0
280
-2.4
270
-2.8
I0
current / uA
260
-3.2
log
4637
-3.6
250
240
-4.0 Mass fraction of Al2O3 nanoparticles in porous polymer membrane
-4.4
0 12%
Is
230
5% 8% 10% 14% 16%
220
-4.8
-10 3.05
3.10
3.15
3.20
3.25
3.30
3.35
3.40
0
10
20
30
40
50
60
70
80
90
100
110
time / s
1000/T
Fig. 7. log s–1/T curves of P(VDF-HFP) film filled with various amounts of Al2O3 nanoparticles.
Fig. 9. Chronoamperometry profile for P(VDF-HFP) polymer electrolyte (mass fraction of Al2O3, 10%, temperature, 25 8C, potential difference, 50 mV).
4638
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
100 90
before dc polarization after dc polarization
80 70
Zim / ohm
60 50 40 30 s
20
Rb +Rs s
Rb
10
0
0
Rb
Rb +R0
0 20
40
60
80
100
120
140
160
180
200
Zre / ohm
Fig. 10. ac impedance spectroscopy for P(VDF-HFP)-based polymer electrolyte (100 kHz–0.1 Hz, 25 8C).
nanoparticles filling the polymer film is 10%. At this amount the polymer electrolyte possesses the largest transference number of lithium ions. From the relationship between the activation energy for the transport of lithium ions and the mass fraction of nanoparticles in polymer film, it illuminates that the lower the activation energy the more easily lithium ions migrate. Furthermore, the increase of the amount of the interfacial layers facilitates the migration of lithium ions so that the transference number of lithium ions increases firstly but decreases with excessive addition of Al2O3 nanoparticles.
614 cm1 all belong to the vibration of crystal phase for P(VDF-HFP) whereas the frequencies of 879, 841 cm1 are assigned to the vibration of amorphous phase for P(VDFHFP). The absorption peaks appearing at 1278 and 1188 cm1 are assigned to the symmetrical and nonsymmetrical stretching vibration of CF2 group, respectively. The deformed vibration of CH2 groups appears at the frequency of 1403 cm1 [19] that will move to high position with the weakening of interaction between H atoms of CH2 groups and F atoms of CF2 groups. The characteristic absorption vibrations of LiClO4: 1150–1080 cm1, 941 cm1 (symmetrical vibration of ionic pairs between Li+ and ClO4) [20], 627 cm1 (stretching vibration of ClO4), 3434 and 1639 cm1 (stretching and bending vibration of OH bonds for absorbing water, which disappear in polymer electrolyte with full drying under vacuum). The large-scale frequency ranging from 550 to 875 cm1 is assigned to the characteristic vibration of Al2O3. The vibration bonds at 3457 and 1647 cm1 can be assigned to the stretching and bending vibration of OH bonds for absorbing water. Fig. 12b is the FTIR spectra of P(VDF-HFP) film containing various amounts of Al2O3 nanoparticles. The absorption
PVDF-HFP
Al2O3
LiC lO4
3.7. FTIR studies Fig. 12a shows the FTIR spectra of pure P(VDF-HFP) film, LiClO4 and Al2O3. The vibrational bonds at 488 and 510 cm1 are assigned to the wagging and bending vibrations of CF2 group [18]. The frequencies of 1072, 976, 763,
0
500
1000
1500
2000
2500
3000
3500
4000
4500
-1
(a)
Wave / cm
0.75
+
Li ions transferance number / t+
0.70
Al 2 O3 contents 0.65
16 %
0.60
14 %
0.55
12 % 0.50
10 % 8%
0.45
5% 0
0.40 0
5
10
15
20
Al2O3/ %(wt)
0
(b) Fig. 11. The effect of amounts of Al2O3 nanoparticles on the lithium ions transference number for P(VDF-HFP) polymer electrolyte (25 8C).
500
1000
1500
2000
2500
3000
3500
4000
4500
-1
Wave / cm
Fig. 12. FTIR spectroscopy of P(VDF-HFP)-based polymer electrolyte.
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
bonds at 3022 and 2980 cm1, which are assigned to the nonsymmetrical and symmetrical stretching vibration of CH2 groups [21–23], appear after addition of LiClO4 to polymer film whereas they are not distinct peaks in pure P(VDF-HFP) matrix because of the interaction between lithium ions and F atoms. This leads to weakening of the interaction between H atoms of CH2 groups and F atoms of CF2 groups. It is found that the intensity of these two absorptions decreases with the increase of the amount of Al2O3 nanoparticles in polymer matrix. The result suggests that the competition between O atoms of Al2O3 and F atoms of polymer segments with respect to Li+ ions makes F atoms act with H atoms again. They would disappear if the mass of Al2O3 nanoparticles exceeds 16%. In addition the Al2O3 nanoparticles weaken the polarity of CF2 groups so the deformed vibration of CH2 groups moves to higher frequency. From Fig. 12b, it shows that the deformed vibration of CH2 groups moves near to 1407 cm1 after addition of Al2O3 nanoparticles into polymer matrix.
4. Conclusion The porous P(VDF-HFP) film filled with various amount of Al2O3 nanoparticles has been prepared by liquid–liquid extraction process. The degree of crystallization of polymer matrix decreases with the increase of mass fraction of Al2O3 nanoparticles in polymer film due to their solid plasticization effect. The optimal value of mass fraction of Al2O3 nanoparticles in polymer matrix is 10% at which the ionic conductivity is 1.95 103 S cm1, the lithium ions transference number 0.73, the activation energy for ions transport 5.6 kJ mol1. From the analysis of FTIR spectra of P(VDFHFP)-based polymer electrolyte, we have the conclusion that addition of Al2O3 nanoparticles to polymer matrix can weaken the interaction between Li+ ions and F atoms of polymer units.
4639
Acknowledgement The authors gratefully acknowledge the financial support by the important program from the Science Technology Department of Hunan Province and Xiangtan University (02XZX09). References [1] W. Xu, K.S. Siow, Z. Gao, S.Y. Lee, J. Electrochem. Soc. 146 (1999) 4410. [2] X. Qiu, W. Li, S. Zhang, H. Liang, W. Zhu, J. Electrochem. Soc. 150 (2003) A917. [3] H. Huang, S.L. Wunder, J. Power Sources 97–98 (2001) 649. [4] A.S. Gozdz, C.N. Schmutz, J.M. Tarascon, US Patent 5,296,318 (1994). [5] A.S. Gozdz, C.N. Schmutz, J.M. Tarascon, P.C. Warren, US Patent 5,540,741 (1996). [6] K.M. Kim, N. Park, K.S. Ryu, S.H. Chang, Polymer 43 (2002) 3951. [7] M. Caillon-Caravanier, B. Claude-Montigny, D. Lemordant, G. Bosser, J. Power Sources 107 (2002) 125. [8] B. Wang, L. Gu, Mater. Lett. 57 (2002) 361. [9] W. Wieczorek, K. Such, J.R. Steven, Electrochim. Acta 40 (1995) 2251. [10] B. Kumar, S.J. Rodrigues, S. Koka, Electrochim. Acta 47 (2002) 4125. [11] F. Croce, L. Persi, B. Scrosati, F. Serraino-Fiory, E. Plichta, M.A. Hendrickson, Electrochim. Acta 46 (2001) 2457. [12] W. Wieczorek, P. Lipka, G. Zukowska, H. Wycislik, J. Phys. Chem. B 102 (1998) 6968. [13] P.G. Bruce, C.A. Vincent, J. Electroanal. Chem. 225 (1987) 1. [14] J.Y. Song, C.L. Cheng, Y.Y. Wang, J. Electrochem. Soc. 149 (2002) A1230. [15] Y. Rosenberg, A. Sigmann, M. Narkis, S. Shkolnik, J. Appl. Polym. Sci. 43 (1991) 535. [16] F. Croce, G.B. Appetecchi, L. Persi, Nature 394 (1998) 456. [17] B. Huang, Z. Wang, L. Chen, R. Xue, F. Wang, Solid State Ionics 91 (1996) 279. [18] S.Rajendran,O.Mahendran,T. Mahalingam,Eur.Polym.J.38(2002)49. [19] S. Rajendran, O. Mahendran, R.Kanan,Mater. Chem. Phys. 74(2002)52. [20] J. Tang, X. Xuan, J. Wang, Spectrosc. Spectral Anal. 21 (2001) 472. [21] A. Salimi, A.A. Yousefi, Polym. Testing 22 (2003) 699. [22] N. Betz, J. Begue, M. Goncalves, K. Gionnet, G. De´ le´ ris, A. Le Moe¨ l, Nucl. Instrum. Meth. Phys. Res. Sec. B 208 (2003) 434. [23] M.M.E. Jacob, A.K. Arof, Electrochim. Acta 45 (2000) 1701.
Effect of Al2O3 nanoparticles on the electrochemical characteristics of P(VDF-HFP)-based polymer electrolyte Zhaohui Lia,1,*, Guangyao Sua,1, Deshu Gaoa,1, Xiayu Wanga,1, Xiaoping Lib,1 a
College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, PR China b TCL Hyperpower Batteries Inc., Huizhou, Guangdong 516003, PR China
Received 5 December 2003; received in revised form 9 May 2004; accepted 17 May 2004
Abstract Alumina (Al2O3) nanoparticles have been used as fillers in the preparation of poly(vinylidenefluoride-co-hexafluorpropylene) (P(VDFHFP))-based porous polymer electrolyte. The degree of crystallization of polymer film filled with Al2O3 nanoparticles decreases with increase of the mass fraction of Al2O3 nanoparticles and the amorphous phases of polymer film expand accordingly. The Al2O3 nanoparticles play the role of solid plasticizer for polymer matrix. Nevertheless that excessive Al2O3 nanoparticles existing in polymer matrix leads to micro-phase separation between polymer matrix and fillers. As a result, both ionic conductivity and lithium ions transference number reduces whereas the activation energy for ions transport increases. When the polymer film is filled with 10% of the mass fraction of Al2O3 nanoparticles, polymer electrolyte possesses the ionic conductivity up to 1.95 103 S cm1 and the lithium ions transference number to 0.73 while the activation energy for ions transport of them falls to 5.6 kJ mol1. Effect of Al2O3 on the electrochemical properties of polymer electrolyte has been investigated in this paper. Analysis of FTIR spectra shows that there is the interaction between Al2O3 nanoparticles and polymer chains. ß 2004 Elsevier Ltd. All rights reserved. Keywords: Polymer electrolyte; P(VDF-HFP); Al2O3; FTIR; Electrochemical property
1. Introduction Poly(vinylidenefluoride-co-hexafluorpropylene) (P(VDFHFP)) copolymers have been used as polymer matrix of polymer electrolyte in the most commonly commercialized plastic lithium-ion batteries (PLiONTM) by Telcordia Technologies (formerly Bellcore) since Gozdz et al. found the preparation process of porous film [1–5]. The preparation process can be briefly divided into two stages. First, a film is obtained by evaporating redundant casting solvent with low boiling points from the adhesive polymer solution. Second, the film is immersed into another solvent, which is usually a poor solvent with respect to the polymer material whereas it is a good solvent to the plasticizer. Therefore, this liquid– liquid extraction process obtains a porous film. Prior to use * Corresponding author. Tel.: +86 732 8292206; fax: +86 732 8292477. E-mail address: [email protected] (Z. Li), . 1 ISE member. 0013-4686/$ – see front matter ß 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.05.018
impregnating in nonaqueous electrolyte will activate it. The resulting polymer electrolyte has the ionic conductivity of 103 S cm1. The addition of inorganic fillers such as silica (SiO2), or copper oxide (CuO) or titania (TiO2) nanoparticles into the polymer electrolyte results in the enhancement of physical strength as well as the increase in the absorption level of electrolyte solution [6–8]. In addition to these effects, they act as solid plasticizer hindering the reorganization of polymer chains and can interact with polar groups by Lewis acid–base reaction [9–12]. So the properties such as ionic conductivity, lithium ions transference number and activation energy for ions transport are improved. In this paper, dibutyl phthalate (DBP) and N-methylpyrrolidinone (NMP) are used as organic plasticizer and casting solvent, respectively. A series of porous polymer film filled with various amounts of Al2O3 nanoparticles have been prepared according to the liquid–liquid extraction process. We have studied the effect of Al2O3 nanoparticles on the electrochemical properties of polymer electrolyte such as
4634
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
the ionic conductivity, lithium ions transference number and activation energy for ions transport. The interaction between Al2O3 nanoparticles and polymer matrix is confirmed by FTIR spectra.
2. Experiment 2.1. Materials P(VDF-HFP) copolymer (KnyarFlex12801, containing 12% HFP unit) was obtained as gift from TCL Hyperpower Batteries Inc. Dibutyl phthalate, N-methylpyrrolidinone and ether purchased from Shanghai Chemical Reactants Plant were distilled before used. Nanoscale Al2O3 obtained from Zhejian Mingri Chemicals Co. with average particle size of 30 nm was dried under vacuum at 100 8C for 24 h. The 1 M LiPF6 solution in ethylene carbonate (EC)–diethyl carbonate (DEC) was purchased from Merck Co. without any treatment.
Porous film was prepared by the liquid–liquid extraction process. A certain amount of P(VDF-HFP) was dissolved into NMP at 50 8C. Then the nanoscale Al2O3 particles and DBP were added to the viscous solution and agitated with ultrasonic stirrer. The resulting slurry was cast on a clean glass plate and dried under vacuum at 80 8C for 8 h. After evaporation of NMP, the glass plate was left to cool to room temperature in situ. The polymer film peeled off from glass plate was immersed into ether to extract plasticizer DBP, and dried at 40 8C under vacuum for 2 h. At last, the film was punched into the circular pieces with diameter of 12 mm. 2.3. Electrochemical measurement of polymer electrolyte The circular pieces of polymer film were dried under vacuum at 80 8C for 2 h, then put into dry glove box (H2O content 5 ppm) instantly. It was immerged into 1 mol/L solution of LiPF6/EC–DEC (volume 1:1) for 1 h to make porous polymer electrolyte. After the excrescent solution at the surface of the polymer electrolyte was absorbed with filler paper, the polymer electrolyte membrane was sandwiched between two symmetrical stainless steel blocking electrodes. Using EG&G Potentiostat/Golvannostat M273 conjunction with M5210 Lock-in amplifier electrochemical analysis system we measured the resistance of the polymer electrolyte. The frequency ranged from 1 Hz to 100 kHz, and ac amplitude was 5 mV. The bulk resistance of polymer electrolyte was found from the impedance spectrum. Thus, the ionic conductivity was calculated based on the following equation: d Rb S
tþ ¼
Is ðDVI0 R0 Þ I0 ðDVIs Rs Þ
(2)
2.4. Differential scanning calorimetry (DSC) measurement and FTIR analysis
2.2. Preparation of the porous film
s¼
In the equation, s is the ionic conductivity, Rb the bulk resistance; d and S are the thickness and area of the specimen, respectively. The lithium ions transference number was determined by dc polarization/ac impedance combination method [13,14]. The initial interfacial resistance (R0) of the cell constructed with symmetrical lithium foils was first determined by electrochemical impedance spectroscopy (EIS) measurement. A chronoamperometry measurement was then carried out with a potential difference (DV) of 50 mV, and the initial current (I0) was measured. After 100 s the steady-state current (Is) was obtained. The steady-state resistance (Rs) was determined by EIS measurement again after the chronoamperometry measurement. Therefore, the lithium ions transference number t+ can be calculated from the following equation:
(1)
The thermal property of the porous polymer film filled with different amounts of Al2O3 nanoparticles was investigated with Perkin-Elmer DSC7 instrument. All samples containing no lithium salt were dried under vacuum at 80 8C for 4 h prior to measurement. The measurement was carried out with the heating rate of 10 8C/min at nitrogen atmosphere. The infrared spectra were recorded under nitrogen atmosphere on Perkin-Elmer FTIR 1710 spectrometer, covering a range of 450–4000 cm1 with a resolution of 1 cm1. All samples containing 2 wt.% of LiClO4 and different amounts of Al2O3 nanoparticles were dried as usual.
3. Results and discussions 3.1. Morphology Typical SEM micrograph of the polymer film surface has been shown in Fig. 1. It exhibits that the polymer film possesses porous structure with the diameter ranging from 1 to 10 mm after extraction of plasticizer DBP from polymer matrix. Fig. 2A shows the TEM graph of nanoparticles in the solution of PVDF-HFP/NMP at 5% of the filling mass fraction. The diameter of them is about 200 nm much more than that of primary value (30 nm, provide by manufacturer). From Fig. 2B it can be seen that the nanoparticles grow up to 500 nm or so at 16% of the filling mass fraction. Moreover, it was found that a lot of particles deposit in the polymer solution if the filling mass fraction of nanoparticles is more than 16%.
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
4635
Fig. 1. SEM picture of surface of the porous P(VDF-HFP) film (containing 12% Al2O3).
3.2. Effect of Al2O3 content on the degree of crystallization of polymer film The degree of crystallization of polymer film can be calculated based on the following equation from the DSC curves of polymer film (Fig. 4): Xc ¼
DHm 100% DHm;p
Fig. 3. XRD spectrum of polymer film filled with different amounts of Al2O3 nanoparticles.
3.3. Porosity of polymer film (3) The porosity of polymer film is measured as follows: immerge the film into n-butanol for 1 h, weigh the mass of them before and after the absorption of n-butanol, then calculate the porosity p of polymer film based on the equation p ¼ ðma =ra Þ=ðma =ra þ mP =rp Þ, where ma, mP are the mass of the wet film and the dry film and ra, rp are the density of butanol and polymer, respectively. Fig. 5 shows the porosity dependence of the amount of Al2O3 nanoparticles filling in polymer film. From the figure, it exhibits that the porosity of polymer film is 45% without Al2O3 nanoparticles because the plasticizer DBP, which stays in the polymer film after evaporation of NMP, leaves porous structure after extraction by ether. But the porosity of polymer film rises up to 61% when the polymer film contains 5% of the mass fraction of Al2O3 nanoparticles. Moreover,
16% 14% 12%
Heat Flow / mW
where DHm,p is the heat of fusion for pure a-PVDF, 104.7 J/ g [15], DHm the heat of fusion for P(VDF-HFP) film filled different amounts of nanoparticles. It can be calculated from the integral area of the baseline and each melting curve. The baseline can be determined by connecting two points at which the instant value of its derivative curve becomes zero near melting temperature (Tm). The data of DHm, Tm and the degree of crystallization Xc has both been shown in Table 1. It shows that the degree of crystallization of polymer film decreases with the increase of the amount of Al2O3 nanoparticles filled in. The result can be further confirmed by the XRD spectra for polymer film filled with various amounts of Al2O3 nanoparticles. It is found in Fig. 3 that the intensity of the characteristic peaks for PVDF a-phase crystals at 2u = 17.8, 20.0 and 39.18 decreases with addition of Al2O3 nanoparticles. Maybe the interaction between the Lewis acid groups –OH on the surface of Al2O3 nanoparticles and the basic groups F atoms of polymer chains hinders the motion of polymer segments so the degree of crystallization for polymer film decreases.
10% 8% 5% 0 mass fraction of Al 2O 3 in polymer membrane 80
100
120
140
160
180
0
Tem perature / C
Fig. 2. Transmission electron micrographs of Al2O3 nanoparticles in the solution of P(VDF-HFP)/NMP. (The mass fraction of Al2O3 nanoparticles, A: 5%; B: 16%.)
Fig. 4. DSC curves of PVDF-HFP film filled with different mass fraction of Al2O3 nanoparticles.
4636
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
Table 1 Crystallization degree of PVDF-HFP film filled with varied amounts of Al2O3 nanoparticles Mass fraction of Al2O3 in polymer film (%)
Melting temperature of crystalline, Tm (8C)
Heat of fusion for crystalline, DHm (J g1)
Crystallization degree, Xc (%)
0 5 8 10 12 14 16
162.8 163.1 162.8 162.8 163.1 163.1 162.3
51.59 50.27 49.34 47.02 42.90 35.68 21.48
49.3 48.0 47.2 45.0 41.0 34.1 20.5
when polymer film is filled with 10% of the mass fraction of Al2O3 nanoparticles, they get the largest value of the porosity, 70%. However, it decreases with further addition of Al2O3 nanoparticles into polymer matrix because of the aggregation of nanoparticles. The interfacial layers between Al2O3 nanoparticles and polymer matrix, which has been found by Croce et al. [16], is responsible to the increase of the porosity of polymer film. The micro-spaces occur at the interface between the surface of Al2O3 nanoparticles and polymer matrix where electrolyte solution is stored and become the tunnels for lithium ions migration. The volume of interfacial layers increases with the amount of Al2O3 nanoparticles filling in polymer film at first but decreases with aggregation of Al2O3 nanoparticles. 3.4. Ionic conductivity of polymer electrolyte Polymer film was proved to contain little plasticizer (DBP) by thermogravimetric analysis, and absorb the same mass of electrolyte solution by weighing. The relationship between the mass fraction of Al2O3 nanoparticles and ionic conductivity of polymer electrolyte has been showed in Fig. 6.
It is well known that lithium ions migrate in two ways: (i) move along the molecular chains of polymer, and (ii) move in the amorphous phase of polymer electrolyte [17]. The former is slow transport whereas the latter is fast. Fig. 6 shows that the ionic conductivity of polymer electrolyte increases with the rise of amount of Al2O3 nanoparticles in the polymer film when the mass fraction is not more than 10%. On one hand, the volume of the interfacial layers, which possesses a higher ionic conductivity, increases with the amount of Al2O3 nanoparticles filling in the polymer film, so the number of the tunnels for lithium ions migration enhances. On the other hand, the competition between Al2O3 and F atoms with respect to lithium ions facilitates the migration for lithium ions in polymer electrolyte. In this case, the ionic conductivity of polymer electrolyte increases with the addition of Al2O3 nanoparticles into polymer matrix. On the contrary, excessive Al2O3 nanoparticles filling in polymer matrix lead to the aggregation of nanoparticles, so the volume of the interfacial layers decreases. The effective number of the O atoms that compete for lithium ions with F atoms reduces with the decrease of the surface area of nanoparticles due to the aggregation as well. As a result,
2.0
70
-1
1.8
Porosity / %
-3
Ionic conductivity / x10 S.cm
65
60
55
50
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
45
0.0
-2
0
2
4
6
8
10
12
14
16
18
Mass fraction of Al 2O3 nanoparticles in polymer membrane / %
Fig. 5. Porosity of polymer film filled with different amount of Al2O3 nanoparticles (25 8C).
-2
0
2
4
6
8
10
12
14
16
18
Mass fraction of Al2O3 nanoparticles in porous polymer membrane / %
Fig. 6. The relationship between amounts of Al2O3 nanoparticles and ionic conductivity of P(VDF-HFP) film (25 8C, 5% lithium salt).
Fig. 7 exhibits the ionic conductivities dependence of the temperature ranging from 25 to 55 8C for polymer electrolyte. These curves appears linear so that the activation energy for ions transport Ea can be further obtained by using the simple Arrehnius model [s ¼ s 0 expðEa =RTÞ] instead of the Vogel–Tamman–Fulcher (VTF) model fs ¼ s 0 T 1=2 exp½Ea =ðTT0 Þ ; T0 ¼ Tg 50g, where R is gas content, s the conductivity of polymer electrolyte, s0 the pre-exponential index and T the testing temperature, respectively. According to Arrehnius equation the activation energy for ions transport can be calculated from the slope of the straight line. Fig. 8 shows the relationship between the amount of Al2O3 nanoparticles in polymer film and the activation energy for ions transport. It suggests that the activation energy for ions transport decreases with the amount of Al2O3 nanoparticles when the mass fraction of nanoparticles is below 10% whereas it increases with further addition of Al2O3 nanoparticles into polymer film. The activation energy falls to the bottom of the curve, namely 5.6 kJ mol1, when the mass fraction of Al2O3 nanoparticles equals to 10%. With more tunnels for lithium ions migration and the competition between O atoms of Al2O3 and F atoms of polymer chains with respect to lithium ions, lithium ions transfer easily. As a result, the activation energy for Li+ ions transport decreases. Nevertheless, the aggregation of nanoparticles results in the decrease of the numbers of O atoms that competing with F atoms for Li+ ions and the number of tunnels for lithium ions migration. The migration of lithium ions is hindered in polymer electrolyte, accordingly.
+
3.5. Activation energy for ions transport
Activition energy for Li ion transportion / kJ.mol
the ionic conductivity of polymer electrolyte decreases with further addition of Al2O3 nanoparticles into the polymer film.
-1
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
8.0
7.5
7.0
6.5
6.0
5.5 -2
0
2
4
6
8
10
12
14
16
18
Mass fraction of Al2O3 nanoparticles in porous polymer membrane / %
Fig. 8. The relationship between amounts of Al2O3 nanoparticles in P(VDF-HFP) film and activation energy for ion transport.
3.6. Transference number of lithium ions Figs. 9 and 10 show the chronoamperometry profile and ac impedance spectroscopy for P(VDF-HFP)-based polymer electrolyte containing 10% of the mass fraction of Al2O3 nanoparticles at 25 8C, respectively. From Fig. 9, it is found that the initial current (I0) and the steady-state current (Is) are 280 and 227 mA, respectively. From analysis of Fig. 10, the initial interfacial resistance and steady-state interfacial resistance are 128 and 152 V, respectively. According to Eq. (2) the lithium ions transference number is calculated to equal 0.73. The relationship between amount of Al2O3 nanoparticles and lithium ions transference number has been shown in Fig. 11. It suggests that lithium ions transference number increases with the rising of mass fraction of Al2O3 nanoparticles in polymer film below 10% whereas it decreases with further addition of Al2O3 nanoparticles into polymer film. The result shows that the critical mass fraction of Al2O3
-2.0
280
-2.4
270
-2.8
I0
current / uA
260
-3.2
log
4637
-3.6
250
240
-4.0 Mass fraction of Al2O3 nanoparticles in porous polymer membrane
-4.4
0 12%
Is
230
5% 8% 10% 14% 16%
220
-4.8
-10 3.05
3.10
3.15
3.20
3.25
3.30
3.35
3.40
0
10
20
30
40
50
60
70
80
90
100
110
time / s
1000/T
Fig. 7. log s–1/T curves of P(VDF-HFP) film filled with various amounts of Al2O3 nanoparticles.
Fig. 9. Chronoamperometry profile for P(VDF-HFP) polymer electrolyte (mass fraction of Al2O3, 10%, temperature, 25 8C, potential difference, 50 mV).
4638
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
100 90
before dc polarization after dc polarization
80 70
Zim / ohm
60 50 40 30 s
20
Rb +Rs s
Rb
10
0
0
Rb
Rb +R0
0 20
40
60
80
100
120
140
160
180
200
Zre / ohm
Fig. 10. ac impedance spectroscopy for P(VDF-HFP)-based polymer electrolyte (100 kHz–0.1 Hz, 25 8C).
nanoparticles filling the polymer film is 10%. At this amount the polymer electrolyte possesses the largest transference number of lithium ions. From the relationship between the activation energy for the transport of lithium ions and the mass fraction of nanoparticles in polymer film, it illuminates that the lower the activation energy the more easily lithium ions migrate. Furthermore, the increase of the amount of the interfacial layers facilitates the migration of lithium ions so that the transference number of lithium ions increases firstly but decreases with excessive addition of Al2O3 nanoparticles.
614 cm1 all belong to the vibration of crystal phase for P(VDF-HFP) whereas the frequencies of 879, 841 cm1 are assigned to the vibration of amorphous phase for P(VDFHFP). The absorption peaks appearing at 1278 and 1188 cm1 are assigned to the symmetrical and nonsymmetrical stretching vibration of CF2 group, respectively. The deformed vibration of CH2 groups appears at the frequency of 1403 cm1 [19] that will move to high position with the weakening of interaction between H atoms of CH2 groups and F atoms of CF2 groups. The characteristic absorption vibrations of LiClO4: 1150–1080 cm1, 941 cm1 (symmetrical vibration of ionic pairs between Li+ and ClO4) [20], 627 cm1 (stretching vibration of ClO4), 3434 and 1639 cm1 (stretching and bending vibration of OH bonds for absorbing water, which disappear in polymer electrolyte with full drying under vacuum). The large-scale frequency ranging from 550 to 875 cm1 is assigned to the characteristic vibration of Al2O3. The vibration bonds at 3457 and 1647 cm1 can be assigned to the stretching and bending vibration of OH bonds for absorbing water. Fig. 12b is the FTIR spectra of P(VDF-HFP) film containing various amounts of Al2O3 nanoparticles. The absorption
PVDF-HFP
Al2O3
LiC lO4
3.7. FTIR studies Fig. 12a shows the FTIR spectra of pure P(VDF-HFP) film, LiClO4 and Al2O3. The vibrational bonds at 488 and 510 cm1 are assigned to the wagging and bending vibrations of CF2 group [18]. The frequencies of 1072, 976, 763,
0
500
1000
1500
2000
2500
3000
3500
4000
4500
-1
(a)
Wave / cm
0.75
+
Li ions transferance number / t+
0.70
Al 2 O3 contents 0.65
16 %
0.60
14 %
0.55
12 % 0.50
10 % 8%
0.45
5% 0
0.40 0
5
10
15
20
Al2O3/ %(wt)
0
(b) Fig. 11. The effect of amounts of Al2O3 nanoparticles on the lithium ions transference number for P(VDF-HFP) polymer electrolyte (25 8C).
500
1000
1500
2000
2500
3000
3500
4000
4500
-1
Wave / cm
Fig. 12. FTIR spectroscopy of P(VDF-HFP)-based polymer electrolyte.
Z. Li et al. / Electrochimica Acta 49 (2004) 4633–4639
bonds at 3022 and 2980 cm1, which are assigned to the nonsymmetrical and symmetrical stretching vibration of CH2 groups [21–23], appear after addition of LiClO4 to polymer film whereas they are not distinct peaks in pure P(VDF-HFP) matrix because of the interaction between lithium ions and F atoms. This leads to weakening of the interaction between H atoms of CH2 groups and F atoms of CF2 groups. It is found that the intensity of these two absorptions decreases with the increase of the amount of Al2O3 nanoparticles in polymer matrix. The result suggests that the competition between O atoms of Al2O3 and F atoms of polymer segments with respect to Li+ ions makes F atoms act with H atoms again. They would disappear if the mass of Al2O3 nanoparticles exceeds 16%. In addition the Al2O3 nanoparticles weaken the polarity of CF2 groups so the deformed vibration of CH2 groups moves to higher frequency. From Fig. 12b, it shows that the deformed vibration of CH2 groups moves near to 1407 cm1 after addition of Al2O3 nanoparticles into polymer matrix.
4. Conclusion The porous P(VDF-HFP) film filled with various amount of Al2O3 nanoparticles has been prepared by liquid–liquid extraction process. The degree of crystallization of polymer matrix decreases with the increase of mass fraction of Al2O3 nanoparticles in polymer film due to their solid plasticization effect. The optimal value of mass fraction of Al2O3 nanoparticles in polymer matrix is 10% at which the ionic conductivity is 1.95 103 S cm1, the lithium ions transference number 0.73, the activation energy for ions transport 5.6 kJ mol1. From the analysis of FTIR spectra of P(VDFHFP)-based polymer electrolyte, we have the conclusion that addition of Al2O3 nanoparticles to polymer matrix can weaken the interaction between Li+ ions and F atoms of polymer units.
4639
Acknowledgement The authors gratefully acknowledge the financial support by the important program from the Science Technology Department of Hunan Province and Xiangtan University (02XZX09). References [1] W. Xu, K.S. Siow, Z. Gao, S.Y. Lee, J. Electrochem. Soc. 146 (1999) 4410. [2] X. Qiu, W. Li, S. Zhang, H. Liang, W. Zhu, J. Electrochem. Soc. 150 (2003) A917. [3] H. Huang, S.L. Wunder, J. Power Sources 97–98 (2001) 649. [4] A.S. Gozdz, C.N. Schmutz, J.M. Tarascon, US Patent 5,296,318 (1994). [5] A.S. Gozdz, C.N. Schmutz, J.M. Tarascon, P.C. Warren, US Patent 5,540,741 (1996). [6] K.M. Kim, N. Park, K.S. Ryu, S.H. Chang, Polymer 43 (2002) 3951. [7] M. Caillon-Caravanier, B. Claude-Montigny, D. Lemordant, G. Bosser, J. Power Sources 107 (2002) 125. [8] B. Wang, L. Gu, Mater. Lett. 57 (2002) 361. [9] W. Wieczorek, K. Such, J.R. Steven, Electrochim. Acta 40 (1995) 2251. [10] B. Kumar, S.J. Rodrigues, S. Koka, Electrochim. Acta 47 (2002) 4125. [11] F. Croce, L. Persi, B. Scrosati, F. Serraino-Fiory, E. Plichta, M.A. Hendrickson, Electrochim. Acta 46 (2001) 2457. [12] W. Wieczorek, P. Lipka, G. Zukowska, H. Wycislik, J. Phys. Chem. B 102 (1998) 6968. [13] P.G. Bruce, C.A. Vincent, J. Electroanal. Chem. 225 (1987) 1. [14] J.Y. Song, C.L. Cheng, Y.Y. Wang, J. Electrochem. Soc. 149 (2002) A1230. [15] Y. Rosenberg, A. Sigmann, M. Narkis, S. Shkolnik, J. Appl. Polym. Sci. 43 (1991) 535. [16] F. Croce, G.B. Appetecchi, L. Persi, Nature 394 (1998) 456. [17] B. Huang, Z. Wang, L. Chen, R. Xue, F. Wang, Solid State Ionics 91 (1996) 279. [18] S.Rajendran,O.Mahendran,T. Mahalingam,Eur.Polym.J.38(2002)49. [19] S. Rajendran, O. Mahendran, R.Kanan,Mater. Chem. Phys. 74(2002)52. [20] J. Tang, X. Xuan, J. Wang, Spectrosc. Spectral Anal. 21 (2001) 472. [21] A. Salimi, A.A. Yousefi, Polym. Testing 22 (2003) 699. [22] N. Betz, J. Begue, M. Goncalves, K. Gionnet, G. De´ le´ ris, A. Le Moe¨ l, Nucl. Instrum. Meth. Phys. Res. Sec. B 208 (2003) 434. [23] M.M.E. Jacob, A.K. Arof, Electrochim. Acta 45 (2000) 1701.