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Polyacrylonitrile electrolytes
Solid State Ionics 150 (2002) 327 – 335 www.elsevier.com/locate/ssi
Polyacrylonitrile electrolytes 1. A novel high-conductivity composite polymer electrolyte based on PAN, LiClO4 and a-Al2O3 Y.W. Chen-Yang *, H.C. Chen, F.J. Lin, C.C. Chen Department of Chemistry and Center for Nanotechnology at CYCU, Chung Yuan Christian University, 32023 Chung-Li, Taiwan, ROC Received 30 July 2001; received in revised form 4 April 2002; accepted 12 May 2002
Abstract In this work, a series of novel solid-type a-Al2O3-containing polyacrylonitrile (PAN)-based composite polymer electrolytes (CPE) with high conductivity and high mechanical property at room temperature has been prepared. The effect of the addition of a-Al2O3 on the properties of the PAN-based composite polymer electrolyte has been analyzed. The best conductivities obtained at room temperature is 5.7 10 4 S cm 1 from the CPE with 7.5 wt.% a-Al2O3 and 0.6 LiClO4 per PAN repeat unit. The stress – strain test result indicates that the membranes prepared possess high yield stress (73 kg cm 2) suitable for serving as separators in the solid-state lithium and lithium ion batteries and high yield elongation (225%) pliable to form good interface with electrodes. Also discussed are the effects of the addition of the ceramics on the interactions in the system and the possible conduction mechanism. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Composite polymer electrolyte; Ionic conductivity; PAN; LiClO4; a-Al2O3
1. Introduction The preparation of ionic conducting solid polymer electrolyte membranes with high ionic conductivity and mechanical property at room temperature has been a goal for many researchers to pursue in the past decade due to the possibility for fabrications of flexible, compact, laminated solid-state structures free from leakage and available in varied geometries [1]. Large research efforts have been devoted to study the PEO-
*
Corresponding author. Tel.: +886-3-4387263; fax: +886-34562107. E-mail address: [email protected] (Y.W. Chen-Yang).
based polymer electrolyte systems which include the solid type, the liquid plasticizer-containing gel type and the filler-containing composite type [2,3]. Among them, the conductivity of the solid-type PEO – LiX electrolytes reaches practically useful values (of about 10 4 S cm 1) only at temperatures of 60 –80 jC. For the gel type, the highest room temperature conductivity of 2 10 3 S cm 1 has been found for a hybrid film of PEO –salt electrolytes gelled with polyacrylonitrile (PAN) [4]. On the other hand, the preparation of composite polymer electrolyte (CPE) has been widely used in improving the mechanical and the interfacial properties of the PEO – LiX electrolytes [5 –9]. It also has been successfully used to enhance the low-temperature conductivity of the PEO – LiX electrolytes by
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addition of the nano-scale TiO2 and Al2O3, in which conductivities of around 10 4 S cm 1 at 50 jC and 10 5 S cm 1 at 30 jC have been obtained [9]. On the other hand, the studies of PAN-based polymer electrolytes have been focused on the liquidplasticizer-containing gel type and widely investigated in many research groups [10 – 12]. As high as 10 3 S cm 1 of ionic conductivity can be easily obtained at room temperature from these electrolytes and the high conductivity of the order 10 2 S cm 1 was obtained in the composite based on PAN – gel/zeolite [13]. However, they all suffered the poor mechanical property and the possible hazard caused by the organic solvent incorporated and a progressive evaporation of excess liquid solvent, which may induce a progressive decrease in the ionic conductivity. PAN has also been used as a polymer matrix to mix with lithium salt to prepare solid-type polymer electrolytes, yet, even though those were considered to have amorphous structure, the best conductivity of the PAN – LiClO4 solid electrolytes obtained at room temperature up to now is only about 6.5 10 7 S cm 1, which is far below the value (10 4 S cm 1) that can be used practically [14]. Recently, detailed investigations of PAN-based ‘‘polymer in salt electrolytes’’ (PISE), in which the salt is the major component (concentrations are greater than 60 wt.%), have been reported by Forsyth et al. [15 – 17]. However, the best conductivity of these PAN – LiTf (75 wt.%) PISEs obtained is only close to 10 6 S cm 1 at 45 jC. In this study, a series of high-conductivity PAN/ LiClO4/a-Al2O3 composite polymer electrolytes with excellent mechanical strength has been prepared. The mechanical property and the temperature dependence of the conductivity of the electrolytes are discussed. Also investigated are the X-ray diffraction (XRD) patterns, DSC thermograms, the FTIR spectra and the cation transport number of the composite electrolytes for explaining the effect of a-Al2O3 on the enhancement of the conductivity.
2. Experimental 2.1. Material Polyacrylonitrile (PAN, Mw: 150,000, Sp2), lithium perchlorate (LiClO4) (reagent grade) and dime-
thylformamide (DMF) were purchased from Aldrich. a-Al2O3 (100 nm) was kindly provided by the Grace Derwey. LiClO4 and a-Al2O3 were dried by vacuum ( < 10 3 Torr) for 24 h at 140 jC before use. 2.2. Preparation of polymer electrolytes The concentration of salt is expressed as the molar ratio of salt fed to a polyacrylonitrile repeat unit, F=[LiClO4]/[CN]. To prepare the electrolyte, first, an appropriate amount of PAN was dissolved with a small amount of DMF. Then, the required quantity ( F value) of the lithium salt was added, and the solution was stirred well. A designed amount of a-Al2O3 powder was then added and the PAN/LiClO4/aAl2O3 solution was stirred continuously by a highintensity ultrasonic finger directly immersed in the solution for 24 h to disperse the particles. After this, the solution was cast on a flat glass and dried in a vacuum oven at a proper temperature to remove the solvent for at least 24 h. The mechanically stable membranes obtained have average thickness of about 100 Am. The DMF residue in the membranes estimated from TGA measurement was less than 10 wt.%. The dried samples were stored in an argon-filled glove box (water is less than 5 ppm) to avoid moisture contamination. Throughout this report, the abbreviation NFxAy is used to identify the polymer electrolyte samples prepared. N represents PAN, F represents LiClO4 concentration (Fx means F = 0.x), A represents a-Al2O3 and y is the wt.% of a-Al2O3. 2.3. Instruments The FTIR spectra were recorded on a Bio-Rad FTS-7 with a wave number resolution of 2 cm 1. The samples were coated on KBr windows. The DSC and TGA measurements were implemented with a Seiko DSC 220C and a Seiko TG/DTA 220, respectively. For DSC measurements, the samples were heated at a rate of 10 K/min from 150 to 180 jC in the first heating, then, cooled back for the second heating with the same heating rate. The ionic conductivities of the polymer electrolytes were measured by the complex impedance method in the temperature range from 30 to 80 jC. The samples were sandwiched between stainless steel blocking electrodes and placed in a temperature-controlled oven at vacuum ( < 10 2 Torr)
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for 2 h before measurement. The experiments were performed in a constant area cylindrical cell of an electrode diameter of 1.76 cm2 and the thickness of the electrolytes was controlled to about 100 Am. The impedance measurements were carried out on a computer-interfaced HP 4192A impedance analyzer over the frequency range of 5 Hz to 13 MHz. The transference numbers were measured using the technique reported [11]. The electrochemical cell comprised a rectangular piece of polymer electrolyte (about 3.14 cm2) sandwiched between two nonblocking electrodes made of lithium foil (about 2 cm2). The impedance of the cell was measured using an Autolab PGSTAT 30 potentiostat/galvanostat analyzer between the frequencies of 5 Hz and 1 MHz. The mechanical properties were measured using a tensile tester ( Q-test) equipped with a 100-N load cell and interfaced to a computer for data collection. For measurements, the samples with the dimension of 1 6 in2 were pulled at a constant rate of 2 in./min. Fig. 1. XRD patterns of: (a) LiClO4, (b) a-Al2O3, (c) PAN, (d) NF6 and (e) NF6A7.5.
3. Results and discussion According to the procedure described above, a series of PAN/LiClO4/a-Al2O3 composite polymers electrolytes with F = 0.2– 0.6 of salt and 0 – 10 wt.% of the filler has been prepared. It is found that the asprepared membranes are transparent or opaque and mechanically strong. But, the membranes lost their dimensional stability when the salt content is higher ( F>0.6) and are not studied here. The properties of the mechanically strong electrolyte membranes are analyzed and discussed as follows. 3.1. X-ray diffraction study Fig. 1 shows the typical XRD patterns of the highsalt electrolytes, NF6 and NF6A7.5, comparing with that of the pure PAN, LiClO4 and a-Al2O3 samples. As can be seen in Fig. 1(a) – (c), the salt, LiClO4, and the ceramic filler, a-Al2O3, are in their crystal form, and the as-prepared pure PAN membrane has obvious crystalline phase as well. However, Fig. 1(d) demonstrates that as the LiClO4 salt is well mixed with the PAN polymer, the individual diffraction patterns of the crystalline LiClO4 and PAN are not revealed in the diffraction pattern of the as-prepared electrolyte NF6
membrane indicating that the membrane is basically amorphous at room temperature. For the NF6A7.5 membrane, a similar featureless pattern is observed in Fig. 1(e) showing that with addition of the a-Al2O3 particles, the corresponding composite polymer electrolyte membrane, NF6A7.5, remains amorphous at room temperature. 3.2. Thermal analysis The thermal properties of the as-prepared polymer electrolytes are studied by DSC measurement. It was found that for the as-prepared ceramic-free electrolytes, as F z 0.4 ( f 44 –55 wt.% of salt content), a glass transition temperature (Tg) well below room temperature was observed in the DSC thermograms indicating that these high-salt electrolytes are in amorphous phase at room temperature. Therefore, the salt in the phase should be in a disordered cluster or aggregate form. This is ascribed to the plasticizing effect caused by the residual solvent. Since no obvious crystal phase is found in the X-ray diffraction pattern as discussed above, the endothermic transition observed in the thermogram of NF6 in Fig. 2(a)
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the presence of the a-Al2O3 particles and the a-Al2O3 filler does not act as a plasticizer. In addition, the TGA measurement showed that the amount of the residual solvent is similar in the aAl2O3-contaning CPE and the corresponding ceramicfree electrolyte implying that the plasticizing effect that resulted from the residual solvent should be similar for the electrolytes with or without the aAl2O3 filler. 3.3. Ionic conductivity
Fig. 2. DSC thermograms of the PAN-based CPEs. (a) First heating, (b) second heating.
peaking at around 53 jC is attributed to melting of the microcrystalline domains that existed. Nevertheless, the microcrystalline domains were destroyed and no recrystallization occurred during cooling from the first heating back to the low temperature as indicated in the second heating thermogram shown in Fig. 2(b). The glass transition temperature of the as-prepared ceramic-free electrolyte decreases with increasing the salt content as listed in Table 1. On the other hand, it is also noticed in Fig. 2 that the thermal behavior of the as-prepared a-Al2O3containing high salt composite polymer electrolytes, NF6A5 and NF6A7.5, is similar to that of the corresponding ceramic-free electrolyte, NF6. However, as indicated in Table 1, the glass transition temperature increases with increasing a-Al2O3 content suggesting that the free volume of the material is reduced due to
The ionic conductivities of the as-prepared polymer electrolytes were obtained from the impedance measurements as described in the previous work [18]. Fig. 3 shows the typical Ac-impedance plots of the NF6 and NF6A7.5 membranes. ZV and ZW represent the real part and the imaginary part of the impedance data, respectively, at the test temperature. The ionic conductivity was then calculated using the value of the bulk resistance, Rb, as determined from the interception of the spur at the ZVaxis, with the thickness and the surface area of the specimen. Fig. 4 displays the ionic conductivities as a function of lithium salt concentration for the as-prepared electrolytes at 30 jC. It is noticed that for the asprepared ceramic-free PAN/LiClO4 polymer electrolyte, the conductivity shows a maximum in the lowsalt range ( F < 0.3) as reported by Yang et al. [14] and most low-salt solid polymer electrolyte systems. However, when F z 0.3, the ionic conductivity increases with increase of the salt content. A similar behavior has been reported for the solvent-free PAN/LiTf PISE system with salt content higher than 60 wt.% by Forsyth et al. [15,17,19] and Ferry et al. [20] and the rubbery PISE system by Angell et al. [21]. In those PISE systems, a connective (percolation) path was postulated for the ion conduction. Accordingly, we believe that for the as-prepared high-salt ceramic-free polymer electrolytes ( F z 0.3), the ion transport asso-
Table 1 Glass transition temperatures for the PAN-based CPEsa Sample Tg (jC) a
NF4 53.2
NF5 70.3
NF6 77.8
NF6A5 74.6
NF6A7.5 67.3
Tg’s are obtained from the first heating thermogram of the DSC measurements.
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Fig. 3. Ac-impedance plots of (a) NF6 (b) NF6A7.5 at 30 jC.
ciates with both the segmental motion of the polymer chain and the long-range path provided by the ion clusters/aggregates in the amorphous phase, which are increased with increasing the salt content. On the other hand, the ionic conductivities of the high-salt a-Al2O3-containing PAN-based CPEs also increase with increasing the salt content in the range measured (0.2 V F V 0.6) and are about one to two orders higher in magnitude than that of the corresponding ceramic-free PAN-based electrolytes indicating that the ionic conductivity is enhanced by the presence of the dispersed a-Al2O3 particles. So, the ceramic filler in these CPEs acts as a conductance enhancer, but, not as plasticizer as mentioned above. As commonly found in composite materials, it is also observed that for the CPEs with F = 0.5, 0.6, the conductivities also increase with increasing the aAl2O3 content, reaches a maximum for those with 7.5 wt.% of a-Al2O3, and then decreases for those
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with higher a-Al2O3 concentration. The enhancement of the conductivity in low filler content is ascribed to the specific interactions of the ceramic surface and the decay at high filler content is attributed to the predominance of the dilution effect as reported by Croce et al. [22]. In addition, it is also noted that the conductivities of several of these PAN-based CPEs reach the practical value (>10 4 S cm 1) at room temperature for rechargeable batteries. For the NF6A7.5 membrane, as high as 5.7 10 4 S cm 1 of the ionic conductivity is obtained, which is the highest ionic conductivity value ever found at ambient temperature for the non-gel-type PAN-based polymer electrolytes reported. Fig. 5 shows the temperature dependence of the ionic conductivity for the CPEs with 7.5 wt.% of aAl2O3 and F = 0.4, 0.5 and 0.6 between 30 and 80 jC. As can be seen, the conductivities of the CPEs increase steadily with increasing T value in the temperature range without any break and the conductivity of NF6A7.5 reaches to 10 3 S cm 1 when T>60 jC. A similar phenomenon was also found for the ceramic filler-containing PEO-based CPEs [22]. To confirm the phenomenon, a measurement of the conductivities in the cooling process was also carried out from 80 to 30 jC and showed that the similar plots without break were obtained for these CPEs implying that the ion transport is irrelevant to the microcrystalline domains detected in the DSC analysis. The linear plots reveal that, in the temperature range, the ionic conductivity
Fig. 4. Changes in conductivities of the PAN-based polymer electrolytes as a function of salt concentration.
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Fig. 5. Temperature dependence of the ionic conductivities of the PAN-based CPEs.
data of all these a-Al2O3-containing CPEs follow Arrhenius equation, r(T) = r0 exp{ Ea/RT}, where r0 is the conductivity pre-exponential factor and Ea is the activation energy for conduction. This is unlike what is usually observed for most traditional amorphous solid polymer electrolytes, for which some curvature with VTF behavior is evident in the data and the conduction mechanism is believed to involve the segmental motion of the polymer. The ionic motion in this a-Al2O3-containing PAN-based CPE system is, therefore, suggested to be decoupled with the segmental movement of the polymer chains. In addition, as listed in Table 2, the Ea values obtained from these linear plots by the least square method are in the range of 1.3 –16 kJ mol 1. These are much lower than the values (55 –140 kJ mol 1) reported for the ceramicfree PAN/LiTf electrolytes, [15] but, as low as that of the gel-type polymer electrolytes [11], indicating that, in the temperature range 30 – 80 jC, the ions in these CPEs can be activated as easily as in the gel-type electrolyte and transport fast due to the presence of the a-Al2O3 particles. 3.4. FTIR spectroscopy To investigate the role of the PAN polymer chain in the ion conduction and the effect of the addition of a-
Al2O3 particles on the ionic conductivity for the asprepared CPE electrolytes, the FTIR spectra of the selected PAN/LiClO 4 , PAN/a-Al 2 O 3 and PAN/ LiClO4/a-Al2O3 electrolytes are studied. Since the structure of PAN involves only a simple C – C backbone and high density of CN groups, the change of the CN vibration absorbance due to the presence of the salt and/or the filler can provide important information concerning the interactions between CN group and the other Lewis acids exist. Fig. 6 shows the FTIR spectra in the region of CN stretching vibration absorption for PAN, NF6, NA7.5 and NF6A7.5 membranes. It is seen that the absorption peak for NA7.5 is almost the same as that for the pure PAN membrane. This implies that with addition of 7.5 wt.% of the a-Al2O3 particles, the CN vibration mode remains unchanged. In other words, no significant interaction between the nitrile groups and the a-Al2O3 particles is revealed in the FTIR spectrum. However, a shoulder at about 2262 cm 1 on the high-frequency side of the symmetric CN stretching mode of PAN peaking at 2242 cm 1 is observed for the PAN/LiClO4 electrolyte, NF6, as reported before [10]. This is attributed to the coordination between the CN groups of PAN and the Li + cations. With the assistance of a BIO-RAD WinIR software package, the absorption between 2200 and 2300 cm 1 was deconvoluted into two contributions with maxima in the 2235 –2250 and 2255– 2270 cm 1 range, respectively, for the spectrum of NF6 electrolyte. It was found that about 50% of the CN groups belonged to the higher frequency vibration mode implying that about half of the CN groups were coordinated with the Li + cations in this ceramic-free electrolyte. However, the intensity of the shoulder absorption found in NF6 is obviously decreased in NF6A7.5. This reveals that the fraction of the coordinated CN groups is significantly reduced in NF6A7.5. In other words, the coordination between the CN groups of PAN and the Li + cations is significantly depressed when a-Al2O3 particles are Table 2 Arrhenius parameters for the PAN-based CPEs with 7.5 wt.% of a-Al2O3 at 30 jC Sample NF4A7.5 NF5A7.5 NF6A7.5
Ea (kJ mol 1.4 16.0 16.2
1
)
r0 (S cm 5.2 10 7.9 10 3.2 10
1 5 2 1
)
r (S cm
1
2.8 10 1.4 10 5.7 10
5
)
4 4
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Fig. 6. FTIR spectra of PAN, NA7.5, NF6 and NF6A7.5 CPEs in the region of CN stretching absorption.
dispersed in the electrolyte. The similar result is also found in all the other CPEs with various F values prepared. On the other hand, no significant difference was found between the FTIR spectrum (not shown) of the high-salt ceramic-free PAN/LiClO4 electrolyte, NF6, and that of the corresponding PAN/LiClO4/a-Al2O3 composite polymer electrolyte, NF6A7.5, in the region of v4 of the ClO4 absorption (620 – 640 cm 1). This indicates that with or without a-Al2O3, the as-prepared PAN-based polymer electrolytes have similar dissolutions of the LiClO4 salt. Accordingly, although the interaction between PAN and Li + ion is depressed due to the presence of the a-Al2O3 particles, the fraction of the dissolved Li + ions remains similar. These dissolved Li + ions are considered to interact with the surface oxygens of the a-Al2O3 particles by the Lewis acid – base interaction in the CPE.
species to the overall conductivity of the polymer electrolyte. As noticed in Table 3, the t + value of the as-prepared NF6 electrolyte is small and difficult to measure, but, it increases with increasing the a-Al2O3 content. This indicates that the cationic species in the a-Al2O3-containing CPE contribute more to the overall conductivity than in the corresponding ceramicfree PAN/LiClO4 electrolyte due to the presence of the a-Al2O3 particles. The t + value of the as-prepared NF6A7.5 CPE sample measured is 0.33, which is close to that of some gel-type electrolytes [11], indicating that the efficiency of the cation transport in the NF6A7.5 CPE is close to that in the gel-type electrolytes and is ascribed to the assistance of the aAl2O3 particles. 3.6. Conduction mechanism Based on the above discussion, the conduction process of the as-prepared a-Al2O3-containing PAN-
3.5. Transference number measurements Since the cationic transference number of an electrolyte, t + , is defined as the net number of faradays of charge carried across the reference plane by the cationic species in the direction of the cathode, during the passage of one faraday of charge across the plane, it is used to evaluate the contribution of the cationic
Table 3 Cationic transference number for the PAN-based CPEs at room temperature Sample
t+
NF6 NF6A3.8 NF6A7.5
– 0.14 0.33
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based CPEs is carried out in the amorphous phase and irrelevant to the microcrystalline domains detected in the DSC analysis. The enhancement of the conductivity due to the presence of the dispersed a-Al2O3 particles in the as-prepared PAN-based CPE is mainly attributed to the increase of the transference number of the Li + ionic species. The depression of the Li + – CN interaction due to the presence of the a-Al2O3 particles supports that the Li + ion transport of the CPE is decoupled with the polymer chain movement and involves the surface groups of the a-Al2O3 particles promoting fast ion transport as proposed for the PEObased CPEs by Croce et al. [19]. The low activation energy and the high cationic transference number obtained are consistent with the ion transport that is assisted by the disordered ionic clusters/aggregates and the dispersed a-Al2O3 particles rather than the segmental motion of the polymer chain. Therefore, a possible conduction mechanism in these as-prepared CPEs is proposed as that the ions transport through a somewhat connective pathway formed by a combination of the disordered ionic clusters/aggregates and the surface groups, such as the Lewis-base oxygens, of the dispersed a-Al2O3 particles. 3.7. Mechanical property The mechanical property of a polymer electrolyte during charge/discharge cycles is vital for a safe and endurable battery. However, for most of the polymer electrolytes, an increase of conductivity usually leads to a decrease of mechanical stability and vice versa. Therefore, very few mechanical property data of the high conductive polymer electrolytes have been reported. Fortunately, the high-conductive PAN-based composite polymer electrolyte membranes prepared in this study are quite strong, hence, their mechanical properties are able to be measured. As listed in Table 4, the pristine PAN is a tough film with high yield stress, high tangent modulus and low yield elongation. With high salt content, the polymer electrolyte NF6 membrane possesses lower tensile strength and higher extentability. Nevertheless, its yield stress is more than two times higher and its yield elongation is much larger than gel-type PAN-based electrolytes. On the other hand, for the a-Al2O3-containing composite polymer electrolyte NF6A7.5 membrane, as high as 73 kg
Table 4 Mechanical properties of PAN and the CPEs Sample
Yield stress (kg cm 2)
Yield elongation (%)
Tangent modulus (kg cm 2)
PAN NF6 NF6A7.5
280 29 73
2 54 225
14,965 162 49
cm 2 of yield stress and 225% of yield elongation are obtained. This implies that the membrane is suitable for use as a separator in batteries and pliable to form a conforming interface with electrodes. The enhancement of the yield stress and the elongation due to the presence of a-Al2O3 particles is ascribed to the structural modification induced via the net Lewis acid – base interactions formed in the CPE system.
4. Conclusion A series of novel solid-type a-Al2O3-containing PAN-based composite polymer electrolytes with high conductivity and high mechanical property at room temperature has been prepared. The best conductivity obtained at 30 jC is as high as 5.7 10 4 S cm 1. It is found that the addition of a-Al2O3 into the PAN/ LiClO4 electrolyte depresses the interactions between PAN and Li + ions and forms the ion –ceramic interactions. The temperature dependence of the conductivity follows Arrhenius equation with low activation energy in the temperature range of 30– 80 jC. The combination of an increase of the cation transference number and the fast ion transport via the ceramic surface leads to a high conductivity for the as-prepared PAN/LiClO4/a-Al2O3 CPEs. The conduction ions in the CPEs are proposed to transport through a somewhat connective pathway formed by a combination of the disordered ionic clusters/aggregates and the surface groups, such as the Lewis-base oxygens, of the dispersed a-Al2O3 particles. Detailed study of the ion transport mechanism by NMR measurements is under investigation.
Acknowledgements The authors wish to thank the National Science Council of ROC for the financial support of this work.
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Polyacrylonitrile electrolytes 1. A novel high-conductivity composite polymer electrolyte based on PAN, LiClO4 and a-Al2O3 Y.W. Chen-Yang *, H.C. Chen, F.J. Lin, C.C. Chen Department of Chemistry and Center for Nanotechnology at CYCU, Chung Yuan Christian University, 32023 Chung-Li, Taiwan, ROC Received 30 July 2001; received in revised form 4 April 2002; accepted 12 May 2002
Abstract In this work, a series of novel solid-type a-Al2O3-containing polyacrylonitrile (PAN)-based composite polymer electrolytes (CPE) with high conductivity and high mechanical property at room temperature has been prepared. The effect of the addition of a-Al2O3 on the properties of the PAN-based composite polymer electrolyte has been analyzed. The best conductivities obtained at room temperature is 5.7 10 4 S cm 1 from the CPE with 7.5 wt.% a-Al2O3 and 0.6 LiClO4 per PAN repeat unit. The stress – strain test result indicates that the membranes prepared possess high yield stress (73 kg cm 2) suitable for serving as separators in the solid-state lithium and lithium ion batteries and high yield elongation (225%) pliable to form good interface with electrodes. Also discussed are the effects of the addition of the ceramics on the interactions in the system and the possible conduction mechanism. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Composite polymer electrolyte; Ionic conductivity; PAN; LiClO4; a-Al2O3
1. Introduction The preparation of ionic conducting solid polymer electrolyte membranes with high ionic conductivity and mechanical property at room temperature has been a goal for many researchers to pursue in the past decade due to the possibility for fabrications of flexible, compact, laminated solid-state structures free from leakage and available in varied geometries [1]. Large research efforts have been devoted to study the PEO-
*
Corresponding author. Tel.: +886-3-4387263; fax: +886-34562107. E-mail address: [email protected] (Y.W. Chen-Yang).
based polymer electrolyte systems which include the solid type, the liquid plasticizer-containing gel type and the filler-containing composite type [2,3]. Among them, the conductivity of the solid-type PEO – LiX electrolytes reaches practically useful values (of about 10 4 S cm 1) only at temperatures of 60 –80 jC. For the gel type, the highest room temperature conductivity of 2 10 3 S cm 1 has been found for a hybrid film of PEO –salt electrolytes gelled with polyacrylonitrile (PAN) [4]. On the other hand, the preparation of composite polymer electrolyte (CPE) has been widely used in improving the mechanical and the interfacial properties of the PEO – LiX electrolytes [5 –9]. It also has been successfully used to enhance the low-temperature conductivity of the PEO – LiX electrolytes by
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addition of the nano-scale TiO2 and Al2O3, in which conductivities of around 10 4 S cm 1 at 50 jC and 10 5 S cm 1 at 30 jC have been obtained [9]. On the other hand, the studies of PAN-based polymer electrolytes have been focused on the liquidplasticizer-containing gel type and widely investigated in many research groups [10 – 12]. As high as 10 3 S cm 1 of ionic conductivity can be easily obtained at room temperature from these electrolytes and the high conductivity of the order 10 2 S cm 1 was obtained in the composite based on PAN – gel/zeolite [13]. However, they all suffered the poor mechanical property and the possible hazard caused by the organic solvent incorporated and a progressive evaporation of excess liquid solvent, which may induce a progressive decrease in the ionic conductivity. PAN has also been used as a polymer matrix to mix with lithium salt to prepare solid-type polymer electrolytes, yet, even though those were considered to have amorphous structure, the best conductivity of the PAN – LiClO4 solid electrolytes obtained at room temperature up to now is only about 6.5 10 7 S cm 1, which is far below the value (10 4 S cm 1) that can be used practically [14]. Recently, detailed investigations of PAN-based ‘‘polymer in salt electrolytes’’ (PISE), in which the salt is the major component (concentrations are greater than 60 wt.%), have been reported by Forsyth et al. [15 – 17]. However, the best conductivity of these PAN – LiTf (75 wt.%) PISEs obtained is only close to 10 6 S cm 1 at 45 jC. In this study, a series of high-conductivity PAN/ LiClO4/a-Al2O3 composite polymer electrolytes with excellent mechanical strength has been prepared. The mechanical property and the temperature dependence of the conductivity of the electrolytes are discussed. Also investigated are the X-ray diffraction (XRD) patterns, DSC thermograms, the FTIR spectra and the cation transport number of the composite electrolytes for explaining the effect of a-Al2O3 on the enhancement of the conductivity.
2. Experimental 2.1. Material Polyacrylonitrile (PAN, Mw: 150,000, Sp2), lithium perchlorate (LiClO4) (reagent grade) and dime-
thylformamide (DMF) were purchased from Aldrich. a-Al2O3 (100 nm) was kindly provided by the Grace Derwey. LiClO4 and a-Al2O3 were dried by vacuum ( < 10 3 Torr) for 24 h at 140 jC before use. 2.2. Preparation of polymer electrolytes The concentration of salt is expressed as the molar ratio of salt fed to a polyacrylonitrile repeat unit, F=[LiClO4]/[CN]. To prepare the electrolyte, first, an appropriate amount of PAN was dissolved with a small amount of DMF. Then, the required quantity ( F value) of the lithium salt was added, and the solution was stirred well. A designed amount of a-Al2O3 powder was then added and the PAN/LiClO4/aAl2O3 solution was stirred continuously by a highintensity ultrasonic finger directly immersed in the solution for 24 h to disperse the particles. After this, the solution was cast on a flat glass and dried in a vacuum oven at a proper temperature to remove the solvent for at least 24 h. The mechanically stable membranes obtained have average thickness of about 100 Am. The DMF residue in the membranes estimated from TGA measurement was less than 10 wt.%. The dried samples were stored in an argon-filled glove box (water is less than 5 ppm) to avoid moisture contamination. Throughout this report, the abbreviation NFxAy is used to identify the polymer electrolyte samples prepared. N represents PAN, F represents LiClO4 concentration (Fx means F = 0.x), A represents a-Al2O3 and y is the wt.% of a-Al2O3. 2.3. Instruments The FTIR spectra were recorded on a Bio-Rad FTS-7 with a wave number resolution of 2 cm 1. The samples were coated on KBr windows. The DSC and TGA measurements were implemented with a Seiko DSC 220C and a Seiko TG/DTA 220, respectively. For DSC measurements, the samples were heated at a rate of 10 K/min from 150 to 180 jC in the first heating, then, cooled back for the second heating with the same heating rate. The ionic conductivities of the polymer electrolytes were measured by the complex impedance method in the temperature range from 30 to 80 jC. The samples were sandwiched between stainless steel blocking electrodes and placed in a temperature-controlled oven at vacuum ( < 10 2 Torr)
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for 2 h before measurement. The experiments were performed in a constant area cylindrical cell of an electrode diameter of 1.76 cm2 and the thickness of the electrolytes was controlled to about 100 Am. The impedance measurements were carried out on a computer-interfaced HP 4192A impedance analyzer over the frequency range of 5 Hz to 13 MHz. The transference numbers were measured using the technique reported [11]. The electrochemical cell comprised a rectangular piece of polymer electrolyte (about 3.14 cm2) sandwiched between two nonblocking electrodes made of lithium foil (about 2 cm2). The impedance of the cell was measured using an Autolab PGSTAT 30 potentiostat/galvanostat analyzer between the frequencies of 5 Hz and 1 MHz. The mechanical properties were measured using a tensile tester ( Q-test) equipped with a 100-N load cell and interfaced to a computer for data collection. For measurements, the samples with the dimension of 1 6 in2 were pulled at a constant rate of 2 in./min. Fig. 1. XRD patterns of: (a) LiClO4, (b) a-Al2O3, (c) PAN, (d) NF6 and (e) NF6A7.5.
3. Results and discussion According to the procedure described above, a series of PAN/LiClO4/a-Al2O3 composite polymers electrolytes with F = 0.2– 0.6 of salt and 0 – 10 wt.% of the filler has been prepared. It is found that the asprepared membranes are transparent or opaque and mechanically strong. But, the membranes lost their dimensional stability when the salt content is higher ( F>0.6) and are not studied here. The properties of the mechanically strong electrolyte membranes are analyzed and discussed as follows. 3.1. X-ray diffraction study Fig. 1 shows the typical XRD patterns of the highsalt electrolytes, NF6 and NF6A7.5, comparing with that of the pure PAN, LiClO4 and a-Al2O3 samples. As can be seen in Fig. 1(a) – (c), the salt, LiClO4, and the ceramic filler, a-Al2O3, are in their crystal form, and the as-prepared pure PAN membrane has obvious crystalline phase as well. However, Fig. 1(d) demonstrates that as the LiClO4 salt is well mixed with the PAN polymer, the individual diffraction patterns of the crystalline LiClO4 and PAN are not revealed in the diffraction pattern of the as-prepared electrolyte NF6
membrane indicating that the membrane is basically amorphous at room temperature. For the NF6A7.5 membrane, a similar featureless pattern is observed in Fig. 1(e) showing that with addition of the a-Al2O3 particles, the corresponding composite polymer electrolyte membrane, NF6A7.5, remains amorphous at room temperature. 3.2. Thermal analysis The thermal properties of the as-prepared polymer electrolytes are studied by DSC measurement. It was found that for the as-prepared ceramic-free electrolytes, as F z 0.4 ( f 44 –55 wt.% of salt content), a glass transition temperature (Tg) well below room temperature was observed in the DSC thermograms indicating that these high-salt electrolytes are in amorphous phase at room temperature. Therefore, the salt in the phase should be in a disordered cluster or aggregate form. This is ascribed to the plasticizing effect caused by the residual solvent. Since no obvious crystal phase is found in the X-ray diffraction pattern as discussed above, the endothermic transition observed in the thermogram of NF6 in Fig. 2(a)
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the presence of the a-Al2O3 particles and the a-Al2O3 filler does not act as a plasticizer. In addition, the TGA measurement showed that the amount of the residual solvent is similar in the aAl2O3-contaning CPE and the corresponding ceramicfree electrolyte implying that the plasticizing effect that resulted from the residual solvent should be similar for the electrolytes with or without the aAl2O3 filler. 3.3. Ionic conductivity
Fig. 2. DSC thermograms of the PAN-based CPEs. (a) First heating, (b) second heating.
peaking at around 53 jC is attributed to melting of the microcrystalline domains that existed. Nevertheless, the microcrystalline domains were destroyed and no recrystallization occurred during cooling from the first heating back to the low temperature as indicated in the second heating thermogram shown in Fig. 2(b). The glass transition temperature of the as-prepared ceramic-free electrolyte decreases with increasing the salt content as listed in Table 1. On the other hand, it is also noticed in Fig. 2 that the thermal behavior of the as-prepared a-Al2O3containing high salt composite polymer electrolytes, NF6A5 and NF6A7.5, is similar to that of the corresponding ceramic-free electrolyte, NF6. However, as indicated in Table 1, the glass transition temperature increases with increasing a-Al2O3 content suggesting that the free volume of the material is reduced due to
The ionic conductivities of the as-prepared polymer electrolytes were obtained from the impedance measurements as described in the previous work [18]. Fig. 3 shows the typical Ac-impedance plots of the NF6 and NF6A7.5 membranes. ZV and ZW represent the real part and the imaginary part of the impedance data, respectively, at the test temperature. The ionic conductivity was then calculated using the value of the bulk resistance, Rb, as determined from the interception of the spur at the ZVaxis, with the thickness and the surface area of the specimen. Fig. 4 displays the ionic conductivities as a function of lithium salt concentration for the as-prepared electrolytes at 30 jC. It is noticed that for the asprepared ceramic-free PAN/LiClO4 polymer electrolyte, the conductivity shows a maximum in the lowsalt range ( F < 0.3) as reported by Yang et al. [14] and most low-salt solid polymer electrolyte systems. However, when F z 0.3, the ionic conductivity increases with increase of the salt content. A similar behavior has been reported for the solvent-free PAN/LiTf PISE system with salt content higher than 60 wt.% by Forsyth et al. [15,17,19] and Ferry et al. [20] and the rubbery PISE system by Angell et al. [21]. In those PISE systems, a connective (percolation) path was postulated for the ion conduction. Accordingly, we believe that for the as-prepared high-salt ceramic-free polymer electrolytes ( F z 0.3), the ion transport asso-
Table 1 Glass transition temperatures for the PAN-based CPEsa Sample Tg (jC) a
NF4 53.2
NF5 70.3
NF6 77.8
NF6A5 74.6
NF6A7.5 67.3
Tg’s are obtained from the first heating thermogram of the DSC measurements.
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Fig. 3. Ac-impedance plots of (a) NF6 (b) NF6A7.5 at 30 jC.
ciates with both the segmental motion of the polymer chain and the long-range path provided by the ion clusters/aggregates in the amorphous phase, which are increased with increasing the salt content. On the other hand, the ionic conductivities of the high-salt a-Al2O3-containing PAN-based CPEs also increase with increasing the salt content in the range measured (0.2 V F V 0.6) and are about one to two orders higher in magnitude than that of the corresponding ceramic-free PAN-based electrolytes indicating that the ionic conductivity is enhanced by the presence of the dispersed a-Al2O3 particles. So, the ceramic filler in these CPEs acts as a conductance enhancer, but, not as plasticizer as mentioned above. As commonly found in composite materials, it is also observed that for the CPEs with F = 0.5, 0.6, the conductivities also increase with increasing the aAl2O3 content, reaches a maximum for those with 7.5 wt.% of a-Al2O3, and then decreases for those
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with higher a-Al2O3 concentration. The enhancement of the conductivity in low filler content is ascribed to the specific interactions of the ceramic surface and the decay at high filler content is attributed to the predominance of the dilution effect as reported by Croce et al. [22]. In addition, it is also noted that the conductivities of several of these PAN-based CPEs reach the practical value (>10 4 S cm 1) at room temperature for rechargeable batteries. For the NF6A7.5 membrane, as high as 5.7 10 4 S cm 1 of the ionic conductivity is obtained, which is the highest ionic conductivity value ever found at ambient temperature for the non-gel-type PAN-based polymer electrolytes reported. Fig. 5 shows the temperature dependence of the ionic conductivity for the CPEs with 7.5 wt.% of aAl2O3 and F = 0.4, 0.5 and 0.6 between 30 and 80 jC. As can be seen, the conductivities of the CPEs increase steadily with increasing T value in the temperature range without any break and the conductivity of NF6A7.5 reaches to 10 3 S cm 1 when T>60 jC. A similar phenomenon was also found for the ceramic filler-containing PEO-based CPEs [22]. To confirm the phenomenon, a measurement of the conductivities in the cooling process was also carried out from 80 to 30 jC and showed that the similar plots without break were obtained for these CPEs implying that the ion transport is irrelevant to the microcrystalline domains detected in the DSC analysis. The linear plots reveal that, in the temperature range, the ionic conductivity
Fig. 4. Changes in conductivities of the PAN-based polymer electrolytes as a function of salt concentration.
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Fig. 5. Temperature dependence of the ionic conductivities of the PAN-based CPEs.
data of all these a-Al2O3-containing CPEs follow Arrhenius equation, r(T) = r0 exp{ Ea/RT}, where r0 is the conductivity pre-exponential factor and Ea is the activation energy for conduction. This is unlike what is usually observed for most traditional amorphous solid polymer electrolytes, for which some curvature with VTF behavior is evident in the data and the conduction mechanism is believed to involve the segmental motion of the polymer. The ionic motion in this a-Al2O3-containing PAN-based CPE system is, therefore, suggested to be decoupled with the segmental movement of the polymer chains. In addition, as listed in Table 2, the Ea values obtained from these linear plots by the least square method are in the range of 1.3 –16 kJ mol 1. These are much lower than the values (55 –140 kJ mol 1) reported for the ceramicfree PAN/LiTf electrolytes, [15] but, as low as that of the gel-type polymer electrolytes [11], indicating that, in the temperature range 30 – 80 jC, the ions in these CPEs can be activated as easily as in the gel-type electrolyte and transport fast due to the presence of the a-Al2O3 particles. 3.4. FTIR spectroscopy To investigate the role of the PAN polymer chain in the ion conduction and the effect of the addition of a-
Al2O3 particles on the ionic conductivity for the asprepared CPE electrolytes, the FTIR spectra of the selected PAN/LiClO 4 , PAN/a-Al 2 O 3 and PAN/ LiClO4/a-Al2O3 electrolytes are studied. Since the structure of PAN involves only a simple C – C backbone and high density of CN groups, the change of the CN vibration absorbance due to the presence of the salt and/or the filler can provide important information concerning the interactions between CN group and the other Lewis acids exist. Fig. 6 shows the FTIR spectra in the region of CN stretching vibration absorption for PAN, NF6, NA7.5 and NF6A7.5 membranes. It is seen that the absorption peak for NA7.5 is almost the same as that for the pure PAN membrane. This implies that with addition of 7.5 wt.% of the a-Al2O3 particles, the CN vibration mode remains unchanged. In other words, no significant interaction between the nitrile groups and the a-Al2O3 particles is revealed in the FTIR spectrum. However, a shoulder at about 2262 cm 1 on the high-frequency side of the symmetric CN stretching mode of PAN peaking at 2242 cm 1 is observed for the PAN/LiClO4 electrolyte, NF6, as reported before [10]. This is attributed to the coordination between the CN groups of PAN and the Li + cations. With the assistance of a BIO-RAD WinIR software package, the absorption between 2200 and 2300 cm 1 was deconvoluted into two contributions with maxima in the 2235 –2250 and 2255– 2270 cm 1 range, respectively, for the spectrum of NF6 electrolyte. It was found that about 50% of the CN groups belonged to the higher frequency vibration mode implying that about half of the CN groups were coordinated with the Li + cations in this ceramic-free electrolyte. However, the intensity of the shoulder absorption found in NF6 is obviously decreased in NF6A7.5. This reveals that the fraction of the coordinated CN groups is significantly reduced in NF6A7.5. In other words, the coordination between the CN groups of PAN and the Li + cations is significantly depressed when a-Al2O3 particles are Table 2 Arrhenius parameters for the PAN-based CPEs with 7.5 wt.% of a-Al2O3 at 30 jC Sample NF4A7.5 NF5A7.5 NF6A7.5
Ea (kJ mol 1.4 16.0 16.2
1
)
r0 (S cm 5.2 10 7.9 10 3.2 10
1 5 2 1
)
r (S cm
1
2.8 10 1.4 10 5.7 10
5
)
4 4
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Fig. 6. FTIR spectra of PAN, NA7.5, NF6 and NF6A7.5 CPEs in the region of CN stretching absorption.
dispersed in the electrolyte. The similar result is also found in all the other CPEs with various F values prepared. On the other hand, no significant difference was found between the FTIR spectrum (not shown) of the high-salt ceramic-free PAN/LiClO4 electrolyte, NF6, and that of the corresponding PAN/LiClO4/a-Al2O3 composite polymer electrolyte, NF6A7.5, in the region of v4 of the ClO4 absorption (620 – 640 cm 1). This indicates that with or without a-Al2O3, the as-prepared PAN-based polymer electrolytes have similar dissolutions of the LiClO4 salt. Accordingly, although the interaction between PAN and Li + ion is depressed due to the presence of the a-Al2O3 particles, the fraction of the dissolved Li + ions remains similar. These dissolved Li + ions are considered to interact with the surface oxygens of the a-Al2O3 particles by the Lewis acid – base interaction in the CPE.
species to the overall conductivity of the polymer electrolyte. As noticed in Table 3, the t + value of the as-prepared NF6 electrolyte is small and difficult to measure, but, it increases with increasing the a-Al2O3 content. This indicates that the cationic species in the a-Al2O3-containing CPE contribute more to the overall conductivity than in the corresponding ceramicfree PAN/LiClO4 electrolyte due to the presence of the a-Al2O3 particles. The t + value of the as-prepared NF6A7.5 CPE sample measured is 0.33, which is close to that of some gel-type electrolytes [11], indicating that the efficiency of the cation transport in the NF6A7.5 CPE is close to that in the gel-type electrolytes and is ascribed to the assistance of the aAl2O3 particles. 3.6. Conduction mechanism Based on the above discussion, the conduction process of the as-prepared a-Al2O3-containing PAN-
3.5. Transference number measurements Since the cationic transference number of an electrolyte, t + , is defined as the net number of faradays of charge carried across the reference plane by the cationic species in the direction of the cathode, during the passage of one faraday of charge across the plane, it is used to evaluate the contribution of the cationic
Table 3 Cationic transference number for the PAN-based CPEs at room temperature Sample
t+
NF6 NF6A3.8 NF6A7.5
– 0.14 0.33
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based CPEs is carried out in the amorphous phase and irrelevant to the microcrystalline domains detected in the DSC analysis. The enhancement of the conductivity due to the presence of the dispersed a-Al2O3 particles in the as-prepared PAN-based CPE is mainly attributed to the increase of the transference number of the Li + ionic species. The depression of the Li + – CN interaction due to the presence of the a-Al2O3 particles supports that the Li + ion transport of the CPE is decoupled with the polymer chain movement and involves the surface groups of the a-Al2O3 particles promoting fast ion transport as proposed for the PEObased CPEs by Croce et al. [19]. The low activation energy and the high cationic transference number obtained are consistent with the ion transport that is assisted by the disordered ionic clusters/aggregates and the dispersed a-Al2O3 particles rather than the segmental motion of the polymer chain. Therefore, a possible conduction mechanism in these as-prepared CPEs is proposed as that the ions transport through a somewhat connective pathway formed by a combination of the disordered ionic clusters/aggregates and the surface groups, such as the Lewis-base oxygens, of the dispersed a-Al2O3 particles. 3.7. Mechanical property The mechanical property of a polymer electrolyte during charge/discharge cycles is vital for a safe and endurable battery. However, for most of the polymer electrolytes, an increase of conductivity usually leads to a decrease of mechanical stability and vice versa. Therefore, very few mechanical property data of the high conductive polymer electrolytes have been reported. Fortunately, the high-conductive PAN-based composite polymer electrolyte membranes prepared in this study are quite strong, hence, their mechanical properties are able to be measured. As listed in Table 4, the pristine PAN is a tough film with high yield stress, high tangent modulus and low yield elongation. With high salt content, the polymer electrolyte NF6 membrane possesses lower tensile strength and higher extentability. Nevertheless, its yield stress is more than two times higher and its yield elongation is much larger than gel-type PAN-based electrolytes. On the other hand, for the a-Al2O3-containing composite polymer electrolyte NF6A7.5 membrane, as high as 73 kg
Table 4 Mechanical properties of PAN and the CPEs Sample
Yield stress (kg cm 2)
Yield elongation (%)
Tangent modulus (kg cm 2)
PAN NF6 NF6A7.5
280 29 73
2 54 225
14,965 162 49
cm 2 of yield stress and 225% of yield elongation are obtained. This implies that the membrane is suitable for use as a separator in batteries and pliable to form a conforming interface with electrodes. The enhancement of the yield stress and the elongation due to the presence of a-Al2O3 particles is ascribed to the structural modification induced via the net Lewis acid – base interactions formed in the CPE system.
4. Conclusion A series of novel solid-type a-Al2O3-containing PAN-based composite polymer electrolytes with high conductivity and high mechanical property at room temperature has been prepared. The best conductivity obtained at 30 jC is as high as 5.7 10 4 S cm 1. It is found that the addition of a-Al2O3 into the PAN/ LiClO4 electrolyte depresses the interactions between PAN and Li + ions and forms the ion –ceramic interactions. The temperature dependence of the conductivity follows Arrhenius equation with low activation energy in the temperature range of 30– 80 jC. The combination of an increase of the cation transference number and the fast ion transport via the ceramic surface leads to a high conductivity for the as-prepared PAN/LiClO4/a-Al2O3 CPEs. The conduction ions in the CPEs are proposed to transport through a somewhat connective pathway formed by a combination of the disordered ionic clusters/aggregates and the surface groups, such as the Lewis-base oxygens, of the dispersed a-Al2O3 particles. Detailed study of the ion transport mechanism by NMR measurements is under investigation.
Acknowledgements The authors wish to thank the National Science Council of ROC for the financial support of this work.
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