Synthesis and characterization of aluminate polymer electrolytes and their blends with poly(ether)s

Solid State Ionics 133 (2000) 295–301 www.elsevier.com / locate / ssi

Synthesis and characterization of aluminate polymer electrolytes and their blends with poly(ether)s K. Matsushita, Y. Shimazaki, M.A. Mehta*, T. Fujinami Department of Materials Science, Faculty of Engineering, Shizuoka University, Hamamatsu 432 -8561, Japan Received 24 March 2000; received in revised form 14 July 2000; accepted 21 July 2000

Abstract A series of single ion conducting aluminate polymer electrolytes were synthesized and their blends with poly(ether)s characterized. A great improvement of mechanical properties and processability was obtained upon blending with poly(ethylene oxide) or an ethylene oxide–propylene oxide copolymer. Enhancement of the ionic conductivity of blended polymer electrolytes was observed by adding LiCF 3 SO 3 and cationic transference numbers were determined to be about 0.56.  2000 Elsevier Science B.V. All rights reserved. Keywords: Polymer electrolyte; Aluminate polymer; Lithium ion conductor; Single ion conductor; Transference number

1. Introduction Ion conducting polymer electrolytes have been attracting interest as safer alternatives to liquid electrolytes and are attractive for use in a variety of all solid state electrochemical devices [1–5]. The majority of polymer electrolytes are bi-ion conductors in which conduction is a result of both cation and anion motion. In more recent years, single Li 1 ion conductors have become the focus of attention because of their potential for use in high performance lithium secondary batteries [6,7]. In particular, inorganic–organic hybrid polymer electrolytes containing ‘ate’ complex structures, (e.g. aluminate, borate, etc.), have been investigated by us and other groups [8–13]. We have demonstrated that incorpora*Corresponding author. Tel. / fax: 181-53-478-1176. E-mail address: [email protected] (M.A. Mehta).

tion of Lewis acid sites into the inorganic backbone facilitated delocalization of anionic charge, reduced cation–anion pairing and resulted in a significant conductivity enhancement. In this paper we report the synthesis and characterization of some new aluminate polymer electrolytes (APE) and their blends with poly(ether)s. In addition, we discuss the effect of added low molecular weight salt (LiCF 3 SO 3 ) on the conductivity and transport properties of the blends.

2. Experimental Triethyleneglycol monomethylether (TEGMME, Tokyo Kasei) was dried by dry nitrogen bubbling followed by distillation. Poly(ethyleneglycol) monomethylether (MW 350), [PEGMME(350)], was dried by dry nitrogen bubbling under partial vacuum. Both

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materials were stored over molecular sieves prior to use. The ether polymers poly(ethylene oxide) (PEO, Aldrich, MW 5310 6 ) and ethylene oxide–propylene oxide copolymer (EO–PO),([(CH 2 CH 2 O) 0.9 (CH 2 6 CH(CH 3 )O) 0.1 ] n , Sumitomo Seika, MW 1310 ) were dried at 508C for 24 h under vacuum. Lithium aluminium hydride (LiAlH 4 , Aldrich, 1 M solution in tetrahydrofuran) and phenyl boronic acid (PhB(OH) 2 , Aldrich) were used as supplied. The remaining reagents, LiCF 3 SO 3 (Aldrich), 4,49-sulfonyl diphenol (Kanto), hydroquinone (Aldrich), and all solvents were dried appropriately before use. In order to rigorously exclude moisture, all manipulations were carried out on a dry nitrogen / vacuum line. Intermediates and final products were transferred to an argon dry-box prior to use. The APEs were synthesized as shown in Scheme 1. Lithium aluminum hydride (typically 2 mmol) was added to 30 ml of tetrahydrofuran (THF) and was cooled to 2788C in a dry ice–acetone bath. TEGMME (4 mmol) diluted with 5 ml of THF was slowly added dropwise while stirring. The reaction mixture was allowed to return to room temperature and was stirred for a further 3 h (Step 1). Hydroquinone (2 mmol) was diluted in 15 ml of THF and cooled to 2788C. The previous solution was added

dropwise, returned to room temperature and the reaction mixture stirred for a further 12 h (step 2). THF was removed by heating to 708C at atmospheric pressure, followed by heating for 36 h under vacuum. A black rigid polymer APE1-1 was obtained (99.8% yield). By using PEGMME(350) in step 1, longer ether chains, containing an average of 7.2 ether oxide units, were incorporated into the polymer. A black rigid polymer APE1-2 was obtained (99.5% yield). Lithium ion conducting polymer electrolytes incorporating electron withdrawing –SO 2 2 groups in the polymer backbone were synthesized using 4,49sulfonyl diphenol as the diol in step 2. The polymer with short ether chains, (n53), APE2-1 was obtained as a strong but flexible white solid (99.7% yield), while the polymer with longer ether chains (n57.2), APE2-2, was obtained as a white semisolid (99.5% yield). The aluminate polymer incorporating Lewis acidic boron in the polymer backbone was synthesized as previously reported [13] using phenyl boronic acid as the diol in step 2. The polymer with short ether chains (n53), APE3-1 was obtained as a soft solid and that with long ether chains (n57.2) was obtained as a semi-solid. For all APE, stoichiometric

Scheme 1.

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production of hydrogen at each step and the absence of residual –OH in the IR spectra of the final products was confirmed. Blends of aluminate polymers with PEO and EO– PO were obtained by mixing predissolved solutions in acetonitrile (for PEO) or THF (for EO–PO) followed by subsequent removal of the solvent. For systems with added salt, the appropriate amount of LiCF 3 SO 3 was added at the same stage. The composition of the blended and composite polymer electrolytes prepared is summarized in Table 1. Polymer electrolyte films for electrical measurement were obtained by hot pressing between PTFE disks using a PTFE spacer (0.39 mm) to control film thickness. Ionic conductivities were determined by ac impedance measurement in the frequency range 1 MHz–1 mHz (signal amplitude 10 mV) using a Solartron 1260 frequency response analyzer and 1287 electrochemical interface. Cells consisted of polymer films sandwiched between polished stainless steel blocking electrodes. The bulk electrolyte resistance was determined from the width of the high

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frequency arc of the Cole–Cole plot. Cationic transference numbers of samples sandwiched between non-blocking lithium electrodes, were determined by the ac impedance / dc polarization method of Evans [14] modified by Choe [15]. Thermal characteristics of the polymer electrolytes were determined by differential scanning calorimetry on a Perkin Elmer Pyris 1 DSC using heat–cool– reheat cycles at 108C / min.

3. Results and discussion The ionic conductivities of the pure aluminate polymer electrolytes are illustrated in Fig. 1. For all three systems, an enhancement in ionic conductivity was obtained by increasing the ether side chain length from 3 to 7.2 repeat units. This is commonly observed in this type of polymer electrolyte [12,13] and can be ascribed to the increased organic component of the polymer since Li 1 ion motion is promoted by the segmental motion of the oligoether

Table 1 Compositions of polymer electrolytes prepared No.

APE

Poly(ether)

Wt. (APE): Wt. (Poly(ether))

x (Added LiCF 3 SO 3 )a

C–O–C: Li 1

1 2 3 4 5 6 7 8 9 10 12 13 14 15 16 17 18 19 20 21 22 23 24

APE2-2 APE2-2 APE2-2 APE2-2 APE2-2 APE2-2 APE2-2 APE2-2 APE2-2 APE2-2 APE2-2 APE2-2 APE2-2 APE3-1 APE3-1 APE3-1 APE3-1 APE3-2 APE3-2 APE3-2 APE3-2 APE3-2 APE3-2

EO–PO EO–PO EO–PO EO–PO PEO PEO PEO EO–PO EO–PO EO–PO PEO PEO PEO EO–PO EO–PO EO–PO PEO EO–PO EO–PO PEO PEO PEO PEO

9:1 8:2 7:3 6:4 9:1 8:2 7:3 8:2 8:2 8:2 8:2 8:2 8:2 3:7 5:5 7:3 5:5 5:5 5:5 5:5 5:5 5:5 5:5

– – – – – – – 0.5 1.0 1.5 0.5 1.0 1.5 – – – – – 0.5 – 0.5 1.0 1.5

17:1 20:1 24:1 36:1 17:1 20:1 24:1 14:1 10:1 8:1 13:1 10:1 8:1 30:1 16:1 10:1 17:1 33:1 22:1 34:1 22:1 17:1 13:1

a

x5(no. mol Li 1 from APE) /(no. mol Li 1 from added salt).

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Fig. 1. Ionic conductivity of aluminate polymers.

side chains. Comparison of APE2 electrolytes with APE1 electrolytes indicated that higher conductivities were obtained for the former. The presence of electron withdrawing –SO 2 2 groups in the polymer backbone resulted in a reduction in the charge density on the central aluminate ion by delocalisation. Thus, reduced ion pairing in the system resulted in an increase in the number of charge carriers and accounted for the enhancement of ionic conductivity. Activation energies (Ea ) for conduction were determined by simple Arrhenius fitting of the APE1 and APE2 data. Ea (APE2-1, 87 kJ / mol).Ea (APE2-2, 66 kJ / mol).Ea (APE1-1, 56 kJ / mol).Ea (APE1-2, 37 kJ / mol). It was of interest to note that for the same side chain length activation energies were greater for APE2 than APE1. Thus one can predict that at extremely low temperatures, (in the region 0 to 2108C) a cross-over in the conductivity order would occur. At temperatures approaching the glass transition temperature it can be visualized that segmental motion of the ether chains becomes restricted and any residual conduction is due to activated hopping. When hopping between potential wells, the greater the distance between the minima, the greater the activation energy for hopping. In the APE2 system, the distance between anion sites is

increased and this was thought to be a possible cause of the observed increased activation energy. For application in all solid state electrochemical devices, it is important for polymer electrolyte films to exhibit good mechanical properties, ease of handling and processability. APE1-1 and APE2-1 were extremely rigid materials due to the presence of aromatic rings in the polymer backbone and low ethylene oxide content. APE1-2, with longer ether side chains, was easily processible and exhibited good room temperature mechanical properties however those of APE2-2, APE3-1 and APE3-2 were insufficient. A common method of improving the performance of polymeric materials is to form a blend with polymers possessing the required characteristics. For this reason, blends of APE2-2, APE3-1 and APE3-2 with the poly(ethers) PEO (semicrystalline) and EO–PO (almost completely amorphous), known to exhibit good mechanical properties as well as providing an ion conducting pathway, were investigated. The variation of ionic conductivity at 308C with weight percent of poly(ether) in the polymeric component of the material (i.e. exclusive of salt), is illustrated in Fig. 2. for APE2-2. Both systems exhibit a shallow conductivity maximum in the region 10–20 wt.% poly(ether) content. DSC

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Fig. 2. Variation of ionic conductivity at 308C with weight percent of poly(ether) in APE2-2 / EO–PO (d), APE2-2 / PEO (s) systems.

curves demonstrated a clear suppression of PEO crystallinity by incorporation of APE2-2, however, small but significant melting endotherms in the region of about 658C were observed. These were ascribed to melting of uncomplexed crystalline PEO. Conductivity maxima are often observed in the region of C–O–C: Li 1 516–20:1 for poly(ether) based polymer electrolytes [16,17]. Increasing APE2-2 content from this composition can be expected to result in a slight increase in ion pairing, however, the shallow nature of the curve in the high APE2-2 concentration region might suggest that the variation in the number of charge carriers is small. Reduction in APE2-2 content results in a reduced Li 1 ion concentration and therefore a reduction in ionic conductivity. Mechanical properties of the APE1 and APE2 systems were greatly improved by blending. Indeed, easily processable, strong films could be obtained by incorporation of 20% poly(ether). Polymer electrolyte films with good room temperature mechanical properties were obtained by blending the APE3-1 system with 30% or more poly(ether) however, in order to prevent flow at higher temperatures (.708C) 50% or more poly(ether) was necessary for the APE3-2 system. The ionic conductivities of APE3 blends at several temperatures are listed in Table 2. Blending with

Table 2 Ionic conductivity of APE3 / poly(ether) blended electrolytes Temp. (8C)

Conductivity (S / cm) 50:50 blend PEO

50:50 blend EO–PO

Pure APE

30 60 80

2.0310 28 1.8310 27 2.1310 26

2.6310 27 1.8310 26 4.1310 26

4.3310 28 3.1310 27 1.2310 26

APE3-1 APE3-1 APE3-1

30 60 80

1.8310 28 1.5310 27 2.7310 26

2.3310 27 1.2310 26 2.9310 26

1.1310 26 5.0310 26 1.1310 25

APE3-2 APE3-2 APE3-2

PEO resulted in suppression of low temperature conductivity for both APE3 electrolytes. Despite an overall suppression of crystallinity compared to PEO, DSC curves again exhibited melting endotherms due to the presence of some uncomplexed PEO. Indeed, at temperatures approaching and greater than 658C, the conductivity of both APE3–PEO blends increased in a manner typical of PEOn LiX systems [17,18], reflecting the increasingly amorphous nature of the electrolyte. It is often observed to be difficult to form completely compatible polymer blends. The fact that melting endotherms ascribed to uncomplexed PEO were very small and greatly reduced in the composite PEO–APE systems studied compared to pure PEO

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would suggest a high degree of compatability between the APE and PEO. Peaks ascribed to uncomplexed PEO are commonly observed in PEO– lithium salt systems and thus the presence of such peaks in the present composite systems may not necessarily indicate the occurrence of (micro)phase separation although this possibility cannot be ruled out. The conductivity of APE3-1 (n53) increased greatly over the entire temperature range upon blending with EO–PO, however, that for APE3-2 (n57.2) decreased. It was thought that the optimum C–O–C: Li 1 composition for this system lay close to 14:1 and thus blending with EO–PO brought the system closer to this composition for APE3-1 and further away for APE3-2. In addition, the pure APE3-2 was a softer polymer than APE3-1 and the effect of blending was to restrict the segmental motion of the ether chains. We reported previously [11–13] that polymer electrolytes based on ate complex structures (in this case aluminate) exhibit single ion conductivity. The presence of what is essentially a polymeric salt in blends with poly(ether)s can be expected to enhance the dielectric constant of the material over that of the pure poly(ether)s, thus influencing the dissolution and dynamics of ions from any codissolved low molecular weight salt. We therefore decided to investigate the effect of added LiCF 3 SO 3 on the conductivity and transport properties of APE2-2 and APE3-2 blends. Ionic conductivities of the 80%APE2-2–20% PEO electrolyte containing different quantities of added salt are shown in Fig. 3a. The x axis shows the molar ratio of Li 1 from added salt (i.e. from LiCF 3 SO 3 ) to that from APE2-2. Ionic conductivity was observed to pass through a maximum when the ratio of added salt to APE2-2 was 1:1. A conductivity enhancement of one order of magnitude was obtained. The glass transition temperatures (T g ), determined by DSC, are illustrated in Fig. 3b. In the composition range investigated, T g increased almost linearly with added LiCF 3 SO 3 . This was ascribed to the increase in ether oxygen– Li 1 crosslinking concomitant with increased salt concentration resulting in reduced ether chain segmental motion. The conductivity trend could therefore be explained by the compromise between increasing carrier concentration and reduced ion

Fig. 3. (a) Ionic conductivity (b) glass transition temperature (T g ) of 80%APE2–2 / 20% PEO electrolyte containing added LiCF 3 SO 3 .

mobility. In addition, at the highest salt concentrations, ion pair formation will be expected to become significant causing a reduction in carrier concentration despite a net increase in salt concentration. Conductivity maxima were observed for identical compositions in both the 80%APE2-2–20% EO–PO system and 50%APE3-2–50% PEO system. Of the blended systems containing LiCF 3 SO 3 , optimum room temperature conductivity of 5.4310 26 S / cm was obtained for the 50%APE3-2–50% EO– PO, x50.5, (sample 20 in Table 1.) In order to investigate the effect of added LiCF 3 SO 3 on the transport properties of APE–poly(ether) blends, cationic transference numbers at 708C were measured for samples 12 and 22. These electrolytes will hereafter be referred to as composite electrolyte 12 and 22, respectively. The transference numbers (T 1 ) were determined as 0.55 and 0.56, respectively. The cationic transference numbers of simple bi-ion conductors such as PEO containing

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dissolved lithium salts are known to vary greatly with the measurement method used and there has been a great deal of discussion as to the most appropriate and accurate technique [18,19]. Comparison of values obtained by similar techniques however gives an indication of the relative magnitude and would suggest that T 1 values for the PEO–LiCF 3 SO 3 system most generally fall in the range 0.2–0.45, i.e. ,0.5, although occasionally values of up to 0.5 have been reported [14,16,20,21]. The values of T 1 obtained for composite electrolytes 12 and 22 of 0.55 and 0.56 are therefore slightly higher than for simple polyether–LiCF 3 SO 3 systems and may be ascribed to the use of the polymeric salt in the system.

4. Conclusion Aluminate polymer electrolytes exhibiting significant lithium ion conductivity were easily prepared. Improved mechanical properties and processability were obtained by blending with polyethers. Ionic conductivity of the blended systems was enhanced by addition of LiCF 3 SO 3 and cationic transference numbers in the region of 0.56 were obtained.

Acknowledgements The authors are grateful to the Lithium Battery Energy Storage Technology Research Association (LIBES) and the Ministry of Education, Science, Sports and Culture, Japan for financial support.

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