J. Cent. South Univ. Technol. (2007)03-0348-05

DOI: 10.1007/s11771-007-0069-1               

Morphology and conductivit

y of in-situ PEO-LiClO₄-TiO₂

composite polymer electrolyte

PAN Chun-yue(潘春跃), FENG Qing(冯 庆), WANG Li-jun(王丽君),

 ZHANG Qian(张 倩), CHAO Meng(巢 猛)

(School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China)

Abstract: PEO-LiClO₄-TiO₂ composite polymer electrolyte films were prepared. TiO₂ was formed directly in matrix by hydrolysis and condensation reaction of tetrabutyl titanate. The crystallinity, morphology and ionic conductivity of composite polymer electrolyte films were examined by differential scanning calorimetry, scanning electron microscopy, atom force microscopy and alternating current impedance spectroscopy, respectively. The glass transition temperature and the crystallinity of composite polymer electrolytes are decreased compared with those of PEO-LiClO₄ polymer electrolyte film. The results show that TiO₂ particles are uniformly dispersed in PEO-LiClO₄-5%TiO₂ composite polymer electrolyte film. The maximal conductivity of 5.5×10^-5 S/cm at  20 ℃ of PEO-LiClO₄-TiO₂ film is obtained at 5% mass fraction of TiO₂.

Key words: polyethylene oxide (PEO); TiO₂; composite polymer electrolyte; in-situ composite; conductivity

1 Introduction

Ionic conducting solid polymer electrolytes are increasingly used in the design of high energy batteries, electrochemical devices and chemical sensors because of the relative ease in fabricating compact and light mass devices with enhanced safety. An exemplary solid polymer electrolyte is a poly(ethylene oxide) (PEO) system containing lithium salts^[1-4]. However, the polymers in such system tend to crystallize, resulting in low room-temperature ionic conductivity unsatisfied for practical application. Two approaches have been developed to address this problem, that is, incorporation of plasticizers^[5-6] and ceramic fillers^[7-8]. Especially the latter brings about improvement in the ionic conductivity, mechanical properties and interfacial stability with lithium electrode^[9].

An extensive literature survey^[10-12] reveals that almost all polymer-ceramic nanocomposites were prepared through mechanical blending of nanoscale ceramic particles, polymer, and salt in a compatible solvent. However, aggregation of these particles stemming from their high surface energy is a perennial problem that undermines the efficacy of the ceramic fillers. The particle size of the ceramic fillers is increased because of the aggregation of these particles, which leads to worsening of the electrolyte performance. The method of sol-gel process that overcomes such a problem has been widely used for preparing the polymer-ceramic composite materials. The nanosize ceramic fillers are precipitated in situ in the polymer matrix through a series of hydrolysis and condensation reactions of suitable precursors. Thus the ceramic fillers are uniformly distributed in the polymer and exhibit excellent properties. However very few studies have been published using this method for the preparation of composite polymer electrolytes^[13]. In this study, PEO-LiClO₄-TiO₂ composite polymer electrolyte films were prepared. TiO₂ was formed directly in situ in matrix by hydrolysis and condensation reaction of tetrabutyl titanate. The morphology and ionic conductivity of the composite polymer electrolyte films were examined by differential scanning calorimetry(DSC), scanning electron microscopy (SEM), atom force microscopy (AFM) and alternating current (AC) impedance spectroscopy, respectively.

2 Experimental

2.1 Materials

Polyethylene oxide (PEO, relative molecule mass  600 000) was obtained from Aldrich Chemicals and dried at 50 ℃ for 24 h under vacuum before use. Lithium perchlorate (LiClO₄) was analytically pure and dried at 120 ℃ for 24 h under vacuum to remove crystal-water. Acetonitrile (99.5%, mass fraction) was used as the solvent in the film-casting process. Tetrabutyl titanate (Ti(OC₄H₉)₄) was analytically pure.

2.2 Preparation of composite polymer electrolyte

films

PEO and LiClO₄ were dissolved in acetonitrile and the mixture was stirred at room temperature for 3 h to obtain a homogeneous solution. The concentration of PEO in acetonitrile was maintained at 5%(mass fraction) for all solutions. The molar ratio of EO to Li was fixed at 8:1 for all the samples. A calculated amount of tetrabutyl titanate was added into the solution under continuous stirring. The TiO₂ contents of the composite polymer electrolyte films were calculated by assuming complete conversion of tetrabutyl titanate into TiO₂. The residual water in acetonitrile was adequate for hydrolysis, and no water was deliberately added to the mixture. The mixture was vigorously stirred for another 3 h before it was transferred to a Teflon dish to allow the solvents to evaporate. The composite polymer electrolyte films were further dried at 40℃ under vacuum for 2 d to completely remove the residual solvent.

2.3 Measurements of composite polymer electrolyte

films

DSC was performed by use of a Q-10 analyzer (TA, USA). Detailed surface images of the composite polymer electrolyte films were obtained by means of a KYKY-2800 scanning electron microscope (SEM) at 20 kV and Dimension-3000 atom force microscope (AFM) (DI, USA). AC impedance spectroscopy of the composite polymer electrolyte films was measured by use of CHI660 electrochemical workstation (Chenhua, Shanghai, China) between 1 Hz and 1×10⁵ Hz with 30 mV amplitude. The films were sandwiched between two stainless steel electrodes and assembled into a tightly sealed test cell.

3 Results and discussion

3.1 Analysis of DSC for composite polymer electrolyte films

The DSC curves of the PEO-LiClO₄ and the PEO-LiClO₄-5%TiO₂ composite polymer electrolyte films are shown in Fig.1. Firstly about 10 mg of samples were sealed in standard aluminum cans and cooled in a nitrogen atmosphere from room temperature to -60 ℃. The first heating curve was recorded when the samples  were heated from -60 ℃ to 100 ℃ at the rate of     10 ℃/min.

The two DSC curves in Fig.1 show obvious transition at low temperature region assigned to the glass transition temperature (t_g) of composite polymer electrolyte films. All the samples exhibit a relatively sharp endothermic peak near 45 ℃, which could be attributed to the melting of the crystalline region of PEO. The crystallinity (χ_c) of composite polymer electrolyte films can be obtained by comparing the heat of melting (ΔH_m) with that (213.7 J/g)^[14] of 100% crystalline PEO. The important data obtained from the first heating curves are summarized in Table 1.

Fig.1 DSC curves of composite polymer electrolyte films

Table 1 Data of DSC curves for PEO-LiClO₄-TiO₂ composite polymer electrolyte films

It is found that the addition of TiO₂ causes obvious decrease in t_g of composite polymer electrolyte films. This observation can be rationalized in terms of hydrogen bonding between the surface hydroxyl groups of TiO₂ particles and the ether oxygen atoms in the polymer chains. This weakens the complexation between Li⁺ and these ether oxygen atoms, facilitates the migration of Li⁺, perturbs the PEO chain conformation and introduces additional free space among the polymer segments, thus yielding a lower t_g. Furthermore, the crystallinity (χ_c) of composite polymer electrolyte films decreases after the addition of TiO₂ and the value for PEO-LiClO₄-5%TiO₂ composite polymer electrolyte film is the least. They all indicate that the polymer segmental motion can be increased and it is beneficial to ion conduction.

After the first heating, the samples were cooled to -60 ℃ at 20 ℃/ min. The second heating curve was recorded when the samples were heated from -60 ℃ to 100 ℃ at the rate of 10 ℃/ min again. As seen from Fig.2, The value of t_g for the second heating curve is increased from -50.2 ℃ to -33.9 ℃, indicating that there is a marked effect on t_g for the thermal history of the samples. On the other hand, it perhaps should be noted that an exothermic peak is observed at 25.9 ℃ for the second heating curve, which is assigned to the cold crystalline phenomena of PEO. This phenomenon is attributed to the fact that the cooling rate at 20 ℃/ min is too rapid for PEO to form crystal and the crystalline region appears when the samples are secondly heated.

Fig.2 DSC curves for PEO-LiClO₄-5%TiO₂composite polymer electrolyte film

3.2 Morphology of composite polymer electrolyte films

The SEM images of PEO/LiClO₄ polymer electrolyte films (Fig.3(a)) and PEO-LiClO₄-5%TiO₂ composite polymer electrolyte film (Figs.3(b) and (c)) are shown in Fig.3.

The surface is smooth and there are no holes detected in Fig.3(a). It can be seen that TiO₂ particles are uniformly dispersed in the polymer matrix in Fig.3(b). Fig.3(c) is partially magnified image. We can see that the diameter of TiO₂ particles is about 300 nm.

AFM was also used to characterize the film surface morphology of the solid polymer electrolytes (SPEs)^[15]. Fig.4 presents 5 ?m×5 ?m surface plot of PEO-LiClO₄- 5%TiO₂ composite polymer electrolyte film (three- dimensional representation, Fig.4(b)) together with top view of the films (Fig.4(a)).

As seen from Fig.4, many nanoparticles TiO₂ are homogeneously dispersed on the surface. The max perpendicular deviation is about 2 nm in Fig.4(b). The interface of the TiO₂ particles and the polymer matrix is very illegible, that is to say, they are well consistent.

3.3 Conductivity of composite polymer electrolyte films

Fig.5 shows the AC impedance spectra of PEO-LiClO₄ and PEO-LiClO₄-5%TiO₂ composite polymer electrolyte films. The point of intersection between the arc in the high frequency range and the

Fig.3 SEM images of composite polymer electrolyte films (a) PEO-LiClO₄; (b), (c) PEO-LiClO₄-5%TiO₂

straight line in the low frequency range coincides with the bulk resistance of composite polymer electrolyte films. In our experiment, the semicircles in high frequency range are not observed, which may be concluded that the frequency is not high enough. The conductivity of the samples can be calculated by the following equation:

              (1)

where  σ is the conductivity, d and S represent the thickness and the area of composite polymer electrolyte films, and R_b is the bulk resistance. The conductivity of PEO-LiClO₄-TiO₂ composite polymer electrolyte films is shown in Fig.6 and Fig.7.

Fig.4 Top view (a) and three-dimensional surface plot (b) AFM images of PEO-LiClO₄-5%TiO₂ composite polymer electrolyte film

Fig.5 AC impedance spectra of composite polymer electrolyte films

(a) PEO-LiClO₄; (b) PEO-LiClO₄-5%TiO₂

Fig.6 Dependence of conductivity on temperature for PEO-LiClO₄-TiO₂ composite polymer electrolyte

Fig.7 Conductivity of PEO-LiClO₄-TiO₂ composite polymer electrolyte films at different TiO₂ contents

Fig.6 shows the temperature dependency of the conductivity for composite polymer electrolyte films. The plot of lg σ versus 1000/T is approximately linear, suggesting the Arrhenius equation could be a good fit for the experimental data:

σ=σ₀exp[-E_a/(kT)]             (2)

where  σ₀ is the conductivity, k is the conductivity pre-exponential factor and E_a is the activation energy for conduction. The activation energies for PEO-LiClO₄- TiO₂ composite polymer electrolyte are listed in Table 2.

Table 2 Activation energies of PEO-LiClO₄-TiO₂ composite polymer electrolyte films

The conductivity of an electrolyte is determined by the product of the concentration of ion charge carriers and their mobility:

              (3)

where  Z_i is the charge on the ions, n_i is the number of charge carriers, and μ_i is the ionic mobility. Conductivity enhancement could be realized by increasing either the charge carrier number or their mobility. Table 2 shows clearly that the addition of TiO₂ decreases the activation of the composite polymer electrolyte films, indicating the increased mobility of the ions μ_i. It is attributed to the fact that the addition of TiO₂ decreases t_g and χ_c of composite polymer electrolyte films from the DSC results, enhancing the movement of polymer chain and facilitating the migration of Li⁺. It is also generally believed that there are interactions among TiO₂, Li salt and polymer chains^[16-18]. Such interactions could increase the effective charge carrier number n_i. Because both the ion mobility and the number of charge carrier are increased after the addition of TiO₂, the conductivity is higher in PEO-LiClO₄-TiO₂ composite polymer electrolyte films.

As seen from Fig.7, the conductivity increases with increasing the TiO₂ content and attains a maximum value when the mass fraction of TiO₂ is 5%. Subsequently, the conductivity decreases with further increasing the TiO₂ content. This could be understood in terms of badly dispersion of TiO₂ particles, resulting in conduction paths being blocked. The maximum values of conductivity for PEO-LiClO₄-5%TiO₂ composite polymer electrolyte film are 5.5×10^-5 S/cm and 1.1×10^-3 S/cm, respectively, at 20 ℃ and 80 ℃, while the conductivity values of polymer electrolyte without TiO₂ are about 4.7×10^-6 S/cm and 1.5×10^-4 S/cm at 20 ℃ and 80 ℃, respectively.

4 Conclusions

1) Nanocomposite polymer electrolytes based on PEO-LiClO₄ system are synthesized, in which the ceramic filler, TiO₂, is formed in-situ within the polymer matrix by a sol-gel process. Differential scanning calorimetry confirms the suppression of polymer crystallization in the presence of TiO₂. The glass transition temperature (t_g) and the crystallinity (χ_c) of composite polymer electrolyte films all decrease after the addition of TiO₂.

2) SEM and AFM results show that polymer and the ceramic fillers are uniformly mixed at the nanometer level. The average size of TiO₂ particles is about 300 nm.

3) Electrochemical measurements show higher ionic conductivity is obtained when the in-situ formed TiO₂ is used. The conductivity shows a maximum value of   5.5×10^-5 S/cm at 20 ℃ when the mass fraction of TiO₂ is 5%, which is an order of magnitude higher than that in the absence of TiO₂.

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(Edited by YANG Bing)

Received date: 2006-08-24; Accepted date: 2006-09-27

Corresponding author: PAN Chun-yue, Professor; Tel: +86-731-8836961; E-mail: Panchunyue@sina.com