Improvement of the ionic conductivity for amorphous

Electrochimica Acta 48 (2003) 1991 /1995 www.elsevier.com/locate/electacta

Improvement of the ionic conductivity for amorphous polyether electrolytes using supercritical CO2 treatment technology Gun-Ho Kwak, Yoichi Tominaga, Shigeo Asai, Masao Sumita * Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-Ku, Tokyo 152-8552, Japan Received 19 May 2002; accepted 28 October 2002

Abstract The influence of the supercritical carbon dioxide (scCO2) on ionic conductivity for polyether electrolytes based on oligo(oxyethylene glycol) methacrylate with lithium triflate, LiCF3SO3, has been investigated. In particular, the present research is a first attempt to improve an ion transport behavior of the polyether electrolytes using scCO2 treatment technique. Consequently, the ionic conductivity of scCO2 treated samples at room temperature was more than ten times elevated by the scCO2 treatment under the condition of 10 MPa and 40 8C. From the Raman spectroscopy, decrease of aggregate ions and increase of free ions for the scCO2 treated samples have been observed. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Poly [oligo(oxyethylene glycol) methacrylate]; Supercritical CO2; Ionic conductivity; Raman spectroscopy; Glass transition temperature

1. Introduction Since Wright et al. first reported the ionic conduction of poly(ethylene oxide) (PEO) and alkali metal salt complexes in 1973 [1], the study of ion transport mechanism and the development of materials have been extensively investigated due to their potential applications such as high-energy density rechargeable batteries, electrochromic display, ion sensors, and so on [2 /5]. In this field, the effective techniques about the improvement of the ionic conductivity for PEO /salt mixture such as addition of the liquid solvent as plasticizer, inorganic particles, and highly dissociable salts have been reported [6,7]. However, the ionic conductivity is on the order of 105 /104 S cm 1 at room temperature because of its high crystallization [8]. With regard to ion transport in solid PEO, the ionic conduction occurs in the amorphous phase above glass transition temperature (Tg), where the transport is induced by local motion of polyether chain segments repeatedly creating new coordination complexes into

* Corresponding author. Tel.: /81-3-5734-2431; fax: /81-3-57342876. E-mail address: [email protected] (M. Sumita).

which the ions may then migrate [9]. Therefore, the modification of the molecular structure to obtain the amorphous polyether electrolytes, for example, forming networks, copolymerization with other functional groups, polyether /salt hybridization and introducing comb-branch units is essential factor for higher ionic conductivity [10 /16]. On the other hand, it is well known that the supercritical fluid (SCF) is the fourth state of pure materials showing above their critical temperature (Tc) and pressure (Pc) conditions, and exhibit interesting behavior by combining the properties of conventional liquids and gases [17]. Specifically, the liquid-like density of SCF shows high solvent ability compared with gas, while gas-like low viscosity lead to high rates of diffusion, which often results in superior mass-transfer characteristics when compared with the liquid state [18]. This also provides considerable control over their thermodynamic and transport characteristics with variations in pressure and temperature. Furthermore, SCF is highly compressible, and the density, viscosity, and dielectric constant can be tuned by only the control of pressure. Carbon dioxide (CO2) is used for the most commonly SCF as reaction solvent. The properties of supercritical CO2 (scCO2) have attracted attention in the preparation of progressively more advanced materials

0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00176-2

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G.-H. Kwak et al. / Electrochimica Acta 48 (2003) 1991 /1995

because of its extraordinary transport properties, and relatively near ambient conditions (Tc /31 8C, Pc /7.4 MPa). Potential applications of scCO2 are known in fields as diverse as the cleaning and dyeing of fibers and textiles, polymerization and polymer processing, purification and crystallization of pharmaceuticals, and as a reaction medium for chemical synthesis. Particularly, scCO2 treatment for polymer processing such as extraction and precipitation are essential techniques. The recent research renaissance in applications of SCF is driven by environmental concerns. During polymer processing, the important role of the CO2 molecule is its plasticizing effect on polymers through the penetration. This effect causes a large decrease in the Tg by increasing the CO2 content inside the polymer phase [19]. Since CO2 molecules can penetrate more easily in amorphous polymers [20] and interact with electron donor species such as carbonyl groups [21,22], the scCO2 treatment can also be expected to plasticize the polyether derivatives and lower the Tg. Moreover, it is expected that the CO2 molecules act as a modifier at the local structure based on ion /ion or ion /polyether interactions [23]. In this study, we tried to improve the ionic conductivity of poly [oligo(oxyethylene glycol) methacrylate] (PMEO) /Li salt mixture through the impregnation with scCO2 fluid. In previous work, the improvement of crystalline PEO /LiCF3SO3 salt mixture using scCO2 treatment technique has been observed [24]. In this paper, the effect of subcritical or supercritical CO2 treatment under the different conditions of temperature and pressure on the ionic conductivity, thermal properties, and the state of ionic association were evaluated.

2. Experiment 2.1. Materials Oligo(oxyethylene glycol) methacrylate (CH2 / C(CH3)/COO /(CH2CH2O)8 /H, MEO, NOF Co.) as monomer of host polymer matrix and lithium trifluoromethane sulfonate (LiCF3SO3, Aldrich) as salt was used for preparation of electrolyte film. The LiCF3SO3 was used after drying at 100 8C under vacuum for 24 h. 2.2. Preparation and scCO2 treatment of the films The starting homogeneous solutions were obtained from the MEO dissolved in dehydrated methanol with LiCF3SO3 and 1.0 mol% of AIBN. The concentration of LiCF3SO3 was expressed as the molar ratio of Li to the repeating unit of oxyethylene (OE), r/[Li]/ [OE] /100, ranging from 2.5 to 20.0 mol%. Upon removal of the solvent, the resulting mixture was dried under a vacuum at 40 8C for 24 h and then stored in a

dry nitrogen gas before preparation of the corresponding films. The freestanding films were obtained from dynamic heating process. The heating rate, 10 8C min 1, was controlled from 30 to 160 8C using the temperature controller unit (CHINO KP1000). The prepared self-standing transparent films (PMEOx LiCF3SO3, x /40, 20, 10, 5) were further dried in the vacuum oven at 40 8C for 12 h. The scCO2 treated samples were prepared using the scCO2 extraction system (JASCO Co., Ltd.) consisting of a delivery pump (SCF-Get), automatic backpressure regulator (SCF-Bpg), and SUS/Ni alloy (Hastelloy) high-pressure reactor (80 ml). The sample films were treated by scCO2 in the reactor at 40, 60, and 80 8C and 5, 10, 15, and 20 MPa for 30 min. After the treatment process, the reactor was cooled in cold water to 20 8C and the extra CO2 gas was released immediately. These films were dried in the vacuum oven at 30 8C for 24 h as a measurement sample. The color of all the original sample films varied from transparent to slightly opaque. This can be explained partly by the change of light refraction in the scCO2 treated samples resulted from the difference of density. 2.3. Measurements The Tg was measured by DSC Thermal Analyzer (TA-50WS, Shimadzu) under nitrogen atmosphere at the heating rates of 10 8C min1 from /80 to 200 8C. The ionic conductivity (ac 1 V) was measured by the complex impedance method using a 4192A LF Impedance Analyzer (Hewlett/Packard) in the frequency range from 100 to 10 MHz. The temperature was varied from 30 to 100 8C at the heating rate of 2.0 8C min 1. The measurement cell was constructed with a pair of parallel SUS plates with 1.0 mm thick Teflon spacer. This entire process was carried out in SUS box filled with dry nitrogen gas. FT-Raman spectroscopy was used to examine the ionic condition of CF3SO3 in the polymeric electrolytes for the original and the scCO2 treated samples. The Raman spectra with a wave number resolution of 2 cm 1 was recorded at room temperature in the region from 1200 to 600 cm 1 using a RFT-800 (JASCO Co., Ltd.) equipped with a Raman module frequencyresponse analyzer. These spectrums were curve-fit using peak analysis program with a straight base line and Gaussian product function for each peak [25].

3. Results and discussion 3.1. Effect of scCO2 fluid on the ionic conductivity The scCO2 treatment pressure dependence of the ionic conductivity for PMEO40 /LiCF3SO3 original and the


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Fig. 1. The scCO2 pressure dependence of PMEO40 /LiCF3SO3 on the ionic conductivity (treatment temperature; 40 8C).

Fig. 2. The scCO2 temperature dependence of PMEO40 /LiCF3SO3 on the ionic conductivity (treatment pressure; 5 MPa).

scCO2 treated samples is shown in Fig. 1. The effect of saturation pressure on the Arrhenius plots at a constant saturation temperature of 40 8C has been evaluated. The original sample showed a relatively low ionic conductivity at 30 8C (3.2 /107 S cm1). Meanwhile, the scCO2 treated sample showed an ionic conductivity higher than that of the original one over the entire measurement temperature range. The maximum value of the ionic conductivity was obtained at treatment pressure of 10 MPa and it showed 5.5 /106 S cm 1 at 30 8C. Above the critical point, the density can be varied by almost an order of magnitude with relatively small changes in pressure. This tunability of the density for scCO2 will result in a variable strength of plasticization [26], thus having a pressure-dependent effect on the ionic conductivity in this system. It was also studied the effect of changing the saturation temperature at constant saturation pressure of 5 MPa on the ionic conductivity. The Arrhenius plots are shown in Fig. 2. It showed a clear trend of increasing the ionic conductivity with increasing the treatment temperature, which can be attributed to decreasing the viscosity of scCO2 fluid and increasing the diffusivity of CO2 molecules in PMEO at slightly below the critical pressure. Hence, under the appropriate conditions, liquid CO2 may be considered as a subcritical fluid. This means that the substance may exhibit some SCFlike properties such as reduced viscosity and density [18]. Consequently, subcritical CO2 fluid displays a limited degree of compressibility and its plasticizing properties may be tunable with pressure. The effects of treatment process under both subcritical and supercritical conditions on the ionic conductivity at 30 8C and Tg were also evaluated as shown in Fig. 3. As an experimental result, it is known that small change in the treatment temperature or pressure could be significant factor for the ion-conductive behavior.

Furthermore, the optimum treatment temperature (40 8C) and pressure (10 MPa) have been determined for the highest ionic conductivity of polyether electrolyte in this system. It is thought that the CO2 molecule lowers the Tg of polymer matrix and affect on the local structure based on the ether oxygen /ion interactions. This effect on the polymer matrix increases the mobility of the polymer chains under supercritical condition, which allows the chains to rearrange into a more ordered configurations, resulting in induced permanent pathways for ion transport [27]. This phenomenon is due to weak interactions between the CO2 molecules and polar groups such as carbonyl or ether groups in amorphous polymers [28]. The variation of the ionic conductivity and Tg for both original and scCO2 treated samples as a function of the Li concentration to OE unit is shown in Fig. 4. Usually, the increase of salt concentration is known to induce the transient crosslinking structure through the

Fig. 3. The effect of treatment process condition on the ionic conductivity at 30 8C and Tg for original PMEO40 /LiCF3SO3 (j, I) and the scCO2 treated samples (treatment temperature, 40 8C; ', ^; 60 8C; ", 2; 80 8C;m, k).


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Fig. 4. Salt concentration dependence of the ionic conductivity at 30 8C and Tg for original (j, I) and the scCO2 treated samples (', ^) (treatment condition; 10 MPa and 40 8C).

formation of intra- and interpolymer coordination with ether oxygens and the dissociated ions [29]. Over the appropriate salt concentration, this effect decreases segmental motion of the polymer chains and thus increases the Tg. In this work, the original sample showed the highest ionic conductivity at around 5 mol% of Li concentration and slowly decreased the ionic conductivity with increasing the salt concentration at more than 10 mol%. On the other hand, the ionic conductivity of all the scCO2 treated samples was higher than that of the original one. Moreover, the maximum ionic conductivity of the treatment samples shifted to the higher Li concentration region at around 10 mol%. This is a remarkable phenomenon because the dissociation of Li salt in PMEO could be promoted by the scCO2 treatment. It is also thought that the scCO2 fluid affected on the PMEO /salt mixtures not only as a plasticizer but also as a dispersing agent for the dissociation of the aggregate ions. Moreover, there was almost no change of the ionic conductivity within 3 months.

Fig. 5. Raman spectrum of the original PMEO10 /LiCF3SO3 (a) and the scCO2 treatment sample (b) (treatment condition; 10 MPa and 40 8C).

ions, ion pairs and highly associated ionic species in ionconducting polymers. The intramolecular modes of the CF3SO3 (triflate) anion, in particular the SO3 symmetric stretch ns(SO3) and the CF3 symmetric deformation ds(CF3) have been used to evaluate the ionic association in polyether electrolytes containing triflate anion [30,31]. They are more sensitive to the salt addition and provide the highest intensity in the electrolyte spectra. In this study, we focused on the behavior of the ns(SO3) mode. The Raman spectra of ns(SO3) mode for original PMEO10 /LiCF3SO3 and the scCO2 treated sample at room temperature is shown in Fig. 5. In this figure, the major component of the ns(SO3) band which has been assigned to free ions is observed at 1032 cm 1. The

3.2. Raman spectroscopy study Raman spectroscopy has been successfully applied to determine quantitatively the relative amount of free

Table 1 The fraction data of each peak due to free ions, ion pairs, and aggregate ions in PMEOx LiCF3SO3 (x/20, 10, 5) original and scCO2 treated samples (10 MPa, 40 8C) Sample (mol%)

P20 (5) P10 (10) P5 (20)

Aggregate ions (1052 cm 1)

Ion pairs (1042 cm 1)

Free ions (1032 cm1)

Tg (8C)

Original (%)

ScCO2 (%)

Original (%)

ScCO2 (%)

Original (%)

ScCO2 (%)

Original (%)

ScCO2 (%)

11 15 25

4 10 21

24 40 38

25 14 35

65 45 37

71 76 44

/47 /42 /37

/55 /58 /38

Relative peak area is obtained by dividing the percent area for a specific peak from the total area of all peaks (the error in percent area is estimated to be 9/5%).


bands in the higher wavenumber range observed at 1042 and 1052 cm 1 were attributed to ion pairs and ion aggregates, respectively [32]. It is clear that the bands of ion pairs and aggregate ions in Fig. 5(b) diminished. It seems that the peak area of free ions increased by the scCO2 treatment. The influence of the scCO2 treatment on each peak fraction and Tg for PMEO /LiCF3SO3 samples is summarized in Table 1. The peak fractions of original PMEOx LiCF3SO3 (x /20, 10, 5) shown in Table 1 presented significant decrease of ion pairs and aggregate ions, and remarkable increase of free ions by the scCO2 treatment. It is well known that the ion aggregation is one of the serious factors limiting the ionic conduction in polyether electrolytes [33]. In this table, it is clear that the increase of free anion concentration caused by the scCO2 treatment is related to the improvement of the ionic conductivity as seen in Fig. 4. Moreover, the Tg of each original sample showed also a large decrease by the scCO2 treatment. From these results, it is suggested that the promotion of ion dissociation as seen in a decrease of fraction for aggregate ions or ion pairs was caused by the penetration of CO2 molecules. In the PMEO /Li salt system, it is thought that the penetrated CO2 molecules act as an electron acceptor (Lewis acid) and interact with an electron donor species (Lewis base) such as triflate anion or ether chains. The effect of CO2 is due to the decrease of Tg, and consequently, caused the improvement of ionic conductivity. In fact, there was almost no difference in the Tg between pure PMEO and the scCO2 treated sample. Therefore, the scCO2 treatment was an effective for dispersion of ions and decrease of Tg.

4. Conclusions The application of scCO2 for polyether electrolyte was effective for high ionic conductivity. As experimental results, the ionic conductivity of PMEO /LiCF3SO3 mixture at room temperature was more than ten times improved by the scCO2 treatment. Especially, the scCO2 treated sample of PMEO10 /LiCF3SO3 showed the largest increase of the ionic conductivity more than 50 times, and the value was 4.0 /105 S cm 1 at 30 8C. In the case of PMEO electrolyte, the optimum treatment condition (10 MPa, 40 8C) was estimated. It was revealed that the scCO2 treatment causes the increase of carrier ions and the decrease of Tg.

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