Ionic conductivity of polymer electrolytes based on phosphate and polyether copolymers

Solid State Ionics 116 (1999) 63–71

Ionic conductivity of polymer electrolytes based on phosphate and polyether copolymers a, a a b Seong Hun Kim *, Jun Young Kim , Han Sang Kim , Hyun Nam Cho a

Department of Textile Engineering, College of Engineering, Hanyang University, Seoul, 133 -791, Korea b Polymer Materials Laboratory, Korea Institute of Science and Technology, Seoul, 130 -650, Korea Received 15 May 1998; accepted 30 July 1998

Abstract Linear polyphosphate random copolymers (LPC) composed of phosphate as a linking agent with poly(ethylene glycol) (PEG) and / or poly(tetramethylene glycol) (PTMG) were synthesized to increase local segmental motion for improved ion transport. Ionic conductivity and thermal behavior of LPC series–LiCF 3 SO 3 complexes were investigated with various compositions, salt concentrations and temperatures. The PEG(70) / PTMG(30) / LiCF 3 SO 3 electrolyte exhibited ionic conductivity of 8.04 3 10 25 S / cm at 258C. Salt concentration with the highest ionic conductivity was considerably dependent on EO / TMO compositions in LPC series–salt systems. Relationship between solvating ability and chain flexibility with various compositions and salt concentrations was investigated through theoretical aspects of the Adam–Gibbs configurational entropy model. Temperature dependence on the ionic conductivity in LPC6 series–salt systems suggested the ion conduction follows the Williams–Landel–Ferry (WLF) mechanism, which is confirmed by Vogel–Tamman–Fulcher (VTF) plots. The ionic conductivity was affected by segmental motion of the polymer matrix. VTF parameters and apparent activation energy were evaluated by a non-linear least square minimization method. These results suggested that the solvating ability of the host polymer might be a dominant factor to improve the ionic conductivity rather than chain mobility.  1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Poly(ethylene glycol); Poly(tetramethylene glycol); Polyphosphate; Ionic conductivity; Polymer electrolyte

1. Introduction In recent years, ion conducting polymers have been extensively investigated because of their potential application as an electrolyte in solid state batteries. Since the conductivity of poly(ethylene oxide) (PEO)–salt complex was reported by Wright [1] in 1975, the polymer-based solid electrolytes have been of growing importance in application to *Corresponding author. E-mail: [email protected]

high energy density rechargeable batteries [2–6]. The solid polymer electrolytes (SPEs) have many advantages such as high energy density, leak proof, volumetric stability, solvent free condition, easy handling and wide electrochemical stability windows. Especially, PEO-based complexes among the systems based on polyether [7] acting as polymer electrolytes were most widely used, because PEO can easily solvate cations by interaction with polar ether groups in the main chain and their optimal spatial disposition. PEO may be a favored solvating

0167-2738 / 99 / $ – see front matter  1999 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00265-3

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medium for the study of ionic conductivity in SPEs comprising polymer–alkali metal salt complexes [1,8], due to its ability to co-ordinate effectively with salt cations and form homogeneous solutions. However, there are several disadvantages to use the systems, the major drawback being that PEO tends to crystallize or form crystalline complexes resulting in dramatic decrease of ionic conductivity with increasing salt concentration and acceptable levels of ionic conductivity can only be obtained above melting temperature. So far as the conductance in polymer electrolytes is concerned, in contrast with inorganic crystals where quenched defects can lead to high ionic mobility, ion transport [9] is shown obviously to take place in the amorphous elastomeric phase [2,10], irrespective of the coexistence of the stoichiometric crystals. The host polymer serves two essential roles: (1) it provides a source of electrons capable of facilitating ion separation by solvating the high charge density cations; (2) it serves as a dynamically mobile background matrix [11]. Therefore it is advantageous to reduce the crystalline regularity and create the amorphous systems that remain in this state throughout the temperature range of interest, as the conducting pathway is well known to be through the disordered part of the polymer matrix and is assisted by large amplitude segmental motions [7]. Thus, macromolecular motion is represented by flexibility of the host polymer to which the mobile ions reversibly attach. The ion conducting mechanism of polymer electrolytes as well as the solvating ability of host polymer to a salt and type of salt used are significant. In the last few years, many approaches have been adopted to reduce the crystallinity of PEO-based electrolytes and increase segmental mobility of the host polymer through copolymerization, grafting, network formation, modification of macromolecules by pendant PEO segments [12], and plasticization of matrix polymers. The systems are polyphosphazene [13,14], poly(itaconate) [2] with side-chain-grafted PEO, and poly(vinylidene fluoride) (PVdF) [15], poly(acrylonitrile) (PAN) [16,17], poly(methyl methacrylate) (PMMA) [18] plasticized with ethylene carbonate (EC), propylene carbonate (PC) or EC–PC mixture. However, most of the present research concerns have been concentrated on the investigation of maximum conductive and determination of eutectic composition.

In this research, linear polyphosphate random copolymers (LPC) were synthesized to increase local segmental motion to aid the transport of ions and ionic conductivity at ambient temperatures. LPC are composed of phosphate as a linking agent, two glycols with different EO unit length and poly(tetramethylene glycol) (PTMG) as a soft segment increasing chain flexibility. Ionic conductivity and thermal behavior of LPC–salt complexes with various salt concentrations and different EO unit length were investigated. The effect of chain flexibility and solvating ability of host polymer on the ionic conductivity of LPC–salt complexes is investigated through the theoretical aspects of the Adam–Gibbs configurational entropy model.

2. Experimental

2.1. Materials Poly(ethylene glycol) (PEG) as an agent involving a solvating unit with different molecular weights (MW) of 600 and 1000 was purchased from Aldrich, and dehydrated in vacuo at 608C for 24 h before use. Poly(tetramethylene glycol) (PTMG, MW 5 1000) was obtained from Mitsubishi Chemical Co. and used without further purification. Methylphosphorodichloridate (MPC, TCI Co.) was used as a linking agent, and lithium triflate (LiCF 3 SO 3 , Aldrich) as a salt was dried in vacuo at 808C for 24 h before use.

2.2. Synthesis Linear polyphosphate random copolymers were obtained by polycondensation of MPC with various compositions of PEG / PTMG (100 / 0, 70 / 30, 50 / 50, 30 / 70), using a solvent mixture composed of acetonitrile and ether, and triethylamine (TEA) as the scavenger of HCl side product. All the solvents were distilled for purification before use, and the resulting solution was refluxed with stirring under nitrogen atmosphere for 48 h at ambient temperature. After the reaction was completed, HCl ? TEA side products were filtered 4–5 times, and the residual solvents were removed by evaporation. The codes for all samples examined in this research are listed in Table 1. LPC denotes the synthesized host polymers (linear polyphosphate random copolymers) containing the

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Table 1 Codes of linear polyphosphate random copolymers MW of PEG

Sample code

PEG content (mol.%)

PTMG content (mol.%)

600

LPC6 LPC673 LPC637

100 70 30

0 30 70

1000

LPC10 LPC1073 LPC1055 LPC1037

100 70 50 30

0 30 50 70

salts LiCF 3 SO 3 and PEG or PEG / PTMG. The symbols ‘6’ and ‘10’ stand for the molecular weight (MW) of PEG600 and PEG1000 used, respectively. The symbols ‘73’, ‘55’, and ‘37’ in the codes represent the composition ratios of PEG / PTMG in LPC.

2.3. Characterization Both monomers and the synthesized polymers were characterized using IR and NMR analysis. IR measurements were carried out using a Magna-IR 550 spectrometer (Nicolet Co.) in the range 400– 4000 cm 21 with a spectral resolution of 2 cm 21 . NMR measurements were carried out using a Varian Unity Inova instrument operating at a resonance frequency of 300 MHz for proton. 1 H-NMR spectra were obtained in chloroform-d solution (1.0 wt.%) at ambient temperature, and tetramethylsilane was used as an internal standard. A 5-mm diameter glass tube was used for measurements and temperature was maintained at 20–308C during the measurements. 1 H-NMR spectra were obtained with spectral width of 4 kHz, rf pulse width of 9 ms and pulse delay time of 2 s. For typical 1 H-NMR spectra, 32 to 64 scans were accumulated and chemical shifts were presented in ppm downfield from tetramethylsilane for 1 H-NMR spectra.

2.5. Thermal transition temperature measurement The thermal behavior of the synthesized polymer complexes was investigated using a Perkin-Elmer DSC7 equipped with a cold head which was liquid nitrogen cooled in the temperature range of 2 100– 1008C with a scanning rate of 108C / min. Each sample was dried in vacuo at 608C for 24 h before measurement, then sealed in an aluminum pan. The calorimeter was calibrated by using indium and water, and T g was taken as the midpoint of the baseline shift that occurs during the glass-to-rubbery transition.

2.6. Conductivity measurement The ionic conductivity of the polymer electrolytes was evaluated by the complex impedance method using a frequency response analyzer (Solatron Co.) equipped with a Mettler hot stage. The bulk resistance of the samples was measured in the temperature range of 298–333 K and with a constant applied signal amplitude of 0.1 V, using ITO glass as a blocking electrode. The data were collected over a frequency range of 1.0 to 1.0 3 10 5 Hz. The ionic conductivity was calculated from the bulk electrolyte resistance value (R b ) found in the complex impedance diagram according to Eq. (1).

2.4. Complex formation Lithium salts were dissolved in the copolymers with various [Li 1 ] / [EO 1 TMO] mole ratios of 0.01, 0.02, 0.05, 0.10, 0.15, 0.20, using methanol as a solvent. The electrolyte solutions were cast on the Teflon plate and dried in vacuo at 708C for 24 h to form polymer electrolyte complex films.

l s 5 ]] Rb ? A

(1)

where l is the thickness of the polymer electrolyte film, and A is the surface area of the polymer electrolyte film.

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3. Results and discussion

3.1. Characterization Specific peaks of IR and NMR spectra of copolymerized LPC are shown in Fig. 1 and listed in Table 2. Absorptions arising from aliphatic C–H in PEG and P=O stretching of MPC generally occur in

the region of 2800 cm 21 and 1250 cm 21 , respectively. The characteristic peaks from P–O–C were shown at 1190–1150 cm 21 and 1100–870 cm 21 . However, characteristic vibrational modes of –OH in PEG monomer at 3300–3200 cm 21 and P–Cl in MPC at 580–440 cm 21 , had practically disappeared in the IR spectrum of LPC. The structure of LPC was further examined by 1 H-NMR spectrum. Chemical shifts of the –OCH 3 group of MPC, –CH 2 –O– CH 2 –, –CH 2 –CH 2 –, and –OH group which arises from the end group of LPC were observed at the positions of 3.7, 3.5–3.6, 1.5–1.6 and 2.0 ppm. On the basis of the above results, the presumed structure of LPC shown in Scheme 1 was well confirmed.

3.2. Ionic conductivity of LPC–LiCF3 SO3 complex systems Salt concentration dependence on the ionic conductivity was described by examining the plots of the log s vs. [Li 1 ] / [EO1TMO] unit ratios for LPC673 in Fig. 2. As a general trend, at low salt concentration, there is a build-up of charge carriers resulting in an increase in ionic conductivity of the electrolyte. However, as more of the salt LiCF 3 SO 3 is added up to 0.20, the progressive increase in T g leads to a drop in chain flexibility and eventually to a drop in ionic conductivity due to the restricted mobility of charge carriers in the more rigid matrix. The highest ionic conductivity for LPC673–salt complexes was shown at salt concentration of 0.05 in the overall range of temperatures investigated. At high salt concentrations, build-up of charge carriers is offset by the retarding effect of ion clouds, thus

Fig. 1. Structure characterization of LPC; (a) IR spectrum; (b) 1 H-NMR spectrum. Table 2 Peak assignment of linear polyphosphate random copolymer 1

IR Wavelength (cm 21 ) Aliphatic –CH 2 – P–O–C P=O

H-NMR Chemical shift (d/ ppm)

2800 1190–1150 1100–870 1250

–OCH 3 –OH (end)

3.7 2.0

–CH 2 –O–CH 2 – –CH 2 –CH 2 –

3.5–3.6 1.5–1.6

Scheme 1. Synthesis of linear polyphosphate random copolymers.

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Fig. 2. Salt concentration dependence on the ionic conductivity for LPC673–LiCF 3 SO 3 complexes at different temperatures.

Fig. 3. Salt concentration dependence on the ionic conductivity for LPC637–LiCF 3 SO 3 complexes at different temperatures.

ionic conductivity decreased as this effect begins to dominate. However, the dramatic drop of ionic conductivity at high salt concentration (.0.05) was considerably reduced with increasing temperature. This ionic conductivity behavior could be rationalized by recognizing the free volume model [19]. As the temperature increases, the polymer can expand easily and produce free volume. Thus ions, solvated molecules or polymer segments can move into the free volume [20]. The resulting conductivity represented by overall mobility of ion and polymer is determined by the free volume around polymer chains. Therefore, as the temperature increases, the free volume increases. This leads to the increase of ion mobility and segmental mobility that will assist ion transport and practically compensate for the retarding effect of the ion clouds. In the case of LPC637–salt complexes, the highest ionic conductivity was shown at a salt concentration of 0.02. The value was about 1.3310 25 S / cm at room temperature, and it was lower than those of LPC673–salt complexes (salt concentration of 0.05, conductivity of 8.04310 25 S / cm) as shown in Fig. 3. This could be explained by EO / TMO compositions contained in LPC6 series–salt systems. The EO unit is more effective for solvating the salts and shows ionic conductivity approximately two orders of magnitude higher than the TMO unit. Thus LPC673–salt complexes with a greater amount of EO unit can solvate more salt and show higher ionic conductivity than LPC637–salt system.

Variation of glass transition temperature and ionic conductivity with salt concentration for LPC637–salt complexes at ambient temperature is shown in Fig. 4. As suggested earlier, in some systems the higher DT g provides higher conductivity, but this is not always the case as reflected in the present system. The radical increase of T g in LPC637–salt complexes occurred at the change of salt concentration from 0.02 to 0.05 accompanying a decrement of ionic conductivity. Obviously, the incorporated salt dissociates to produce mobile ions via solvation of ether group in the polymer backbone, and it remains coordinated to macromolecules, which could be acting as transient crosslinks to increase T g . However, the excessive dissolution of salt in host poly-

Fig. 4. Influence of the concentration of LiCF 3 SO 3 LPC637 on the ionic conductivity at room temperature and T g .

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mer increases the viscosity very markedly, and the salts exist in the state of ion pairs or aggregated ions that would impede ion transport resulting in a significant decrease of conductivity. In addition, at high salt concentrations, the relative number of effective charge carriers diminishes at equivalent levels of segmental mobility. It suggested that solvating ability and free ions decreased as salt levels were increased. Therefore, the relative degree of ionization is reduced at high salt concentration. Salt concentration dependence on the ionic conductivity for LPC10 series–salt complexes with varying EO / TMO compositions is shown in Fig. 5. The highest ionic conductivity (6.1310 25 S / cm) slightly lower than that of LPC673–salt complex occurred at the salt concentration of 0.10 with EO / TMO composition of 70 / 30. PEO / PTMO composition dependence on the ionic conductivity could be interpreted by the same reason in LPC6 series–salt complex systems. However, the conductivity maximum value and salt dependence could be suggested as follows: when polymer backbone is derived from poly(methacrylic acid) the onset of crystallinity occurs when n.8, and a sample with n57 was amorphous [21]. Similarly Tonge et al. [22] found that polyphosphates with pendant oligo(ethylene oxide) units were amorphous for n,7, but that a sample with n57 had a semi-crystalline nature. The length of the EO unit with n522 used in LPC10 series–salt complexes is sufficiently long to solvate more salt than LPC6 series–salt systems. However,

Fig. 5. Salt concentration dependence on the ionic conductivity for LPC10 series–LiCF 3 SO 3 complexes at room temperatures.

LPC10 series–salt complexes could form a local segmental crystallization and crystalline complex with salt, which could interrupt an ionic mobility. This supposition was well illustrated by an Arrhenius-type temperature dependence on the ionic conductivity that would conform appropriately to the electrolytes with a high degree of crystallinity, as shown in Fig. 6.

3.3. VTF parameters and apparent activation energy With the exception of some data for LPC6 series– salt complex systems, temperature dependence on the ionic conductivity was not linear, which suggested that ion conduction followed the Williams– Landel–Ferry (WLF) mechanism [23]. It indicated that ion transport in polymer electrolytes was correlated with polymer segmental motion [24]. Thus, the results were more effectively represented by the empirical Vogel–Tamman–Fulcher (VTF) equation (Eq. (2)), as shown in Fig. 7 and Fig. 8.

F

1 2B ] s 5 AT 2 2 exp ]] T 2 To

G

(2)

where, A and B are constants, and To is a reference temperature. Constant A in the VTF equation is related to the number of charge carriers in the electrolyte system, and constant B in the VTF

Fig. 6. Temperature dependence on the ionic conductivity for LPC10 series–LiCF 3 SO 3 complexes with [Li 1 ] / [EO1TMO] unit ratio of 0.10.

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free volume disappears or the configurational entropy in the electrolyte system approaches zero. The Adam–Gibbs configurational entropy model predicts To to be about 50 K lower than T g of the electrolyte. Application of the Adam–Gibbs configurational entropy model has been used in a reasonably satisfying manner to describe ionic conductivity in poly(itaconate)s with poly(propylene glycol) side chains [25], and polymers prepared from oligo(ethylene oxide) macromonomers [26]. The VTF equation can be derived from the Adam–Gibbs configurational entropy model [27], and then constant B is a complex accumulation of terms; Fig. 7. VTF behavior of ionic conduction for LPC673–LiCF 3 SO 3 complexes.

Fig. 8. VTF behavior of ionic conduction for LPC637–LiCF 3 SO 3 complexes.

equation is related to the activation energy of ion transport associated with the configurational entropy of the polymer chains. To is taken as the temperature at which relaxation times become infinite and the

To S c* Dm B 5 ]]] k B T DCP

(3)

where, k B is the Boltzmann constant, S c* is the minimum configurational entropy required for a cooperative rearrangement of a polymer chain segment involved in ion transport in the matrix (which can be approximated by k B ln2). DCp is the heat capacity changes at temperature T as the system moves from the glassy to the rubbery state, and Dm is the potential energy barrier hindering the cooperative movement of a chain segment whose size is the minimum capable of undergoing a spatial rearrangement independent of its environment, therefore, Dm can be estimated from a knowledge of DCp and To . The values of DCp calculated at the glass transition temperature from DSC measurement were found to lie in the 24–56 J K 21 mol 21 range for the LPC6 series after correction for salt concentration. A non-linear least square analysis was applied to evaluate the relevant parameters, and the values are gathered in Table 3 and Table 4 for LPC673- and LPC637–LiCF 3 SO 3 complexes, respectively. For

Table 3 VTF parameters and activation energies from non-linear least square analysis of ionic conductivity data for LPC673–LiCF 3 SO 3 complexes [Li 1 ] / [O]

B (K)

To (K)

DCp (J K 21 mol 21 )

Dm (kJ mol 21 )

0.01 0.02 0.05 0.10 0.15 0.20

1243.1 1186.8 1097.3 1156.5 1376.2 1530.4

161.2 163.9 169.1 170.4 172.3 176.6

29.1 27.0 24.3 26.8 44.2 56.3

68.4 60.3 49.8 59.8 113.2 159.5

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Table 4 VTF parameters and activation energies from non-linear least square analysis of ionic conductivity data for LPC637–LiCF 3 SO 3 complexes [Li 1 ] / [O]

B (K)

To (K)

DCp (J K 21 mol 21 )

Dm (kJ mol 21 )

0.02 0.10 0.15 0.20

978.3 1051.0 1256.5 1396.5

159.7 168.3 171.8 175.1

25.2 29.1 37.9 48.7

46.7 57.2 88.7 126.1

LPC637–salt complexes, the values of parameter B and the potential energy barrier hindering the cooperative movement of chain segments, Dm, are relatively lower than those of LPC673–salt systems over the entire range of salt concentrations. This means that the segmental movement of polymer backbone represented by chain flexibility occurs more readily in LPC637–salt than LPC673–salt systems as illustrated well in Fig. 9. Deviation of T g between LPC637 and LPC673 systems is shown in Fig. 9. The increase of chain flexibility can assist ion transport to raise the ionic conductivity of the polymer–salt complex. However, comparing with LPC673–salt systems, more flexible LPC637–salt complexes showed slightly lower ionic conductivity irrespective of salt concentrations as shown in Fig. 2 and Fig. 3 previously. These results suggested that the solvating ability of host polymer acts a more crucial role in improving ionic conductivity than

chain flexibility in this study of LPC6 series–salt systems.

4. Conclusion Linear polyphosphate random copolymers (LPC) composed of phosphate as a linking agent with poly(ethylene glycol) and / or poly(tetramethylene glycol) (PTMG) were synthesized. The highest ionic conductivity was shown at salt concentration of 0.05 for LPC673, 0.02 for LPC637 and 0.10 for LPC1073–LiCF 3 SO 3 complexes in the overall range of temperatures studied. These results suggested that the salt concentration with the highest ionic conductivity was considerably dependent on EO / TMO compositions in LPC6 series–salt systems. LPC10 series–LiCF 3 SO 3 complexes showed a slightly lower ionic conductivity than LPC6 series–LiCF 3 SO 3 complexes due to its tendency to crystallize or form a crystalline complex with salt. It would be caused by sufficiently long EO unit length. For LPC6 series–LiCF 3 SO 3 complex systems, temperature dependence on the ionic conductivity was fitted well to VTF behavior. VTF parameters and apparent activation energy could be evaluated by a non-linear least square minimization method. These results suggested that the solvating ability of host polymer might be a dominant factor to improve the ionic conductivity rather than chain mobility of that in LPC6 series– LiCF 3 SO 3 complex system.

Acknowledgements Fig. 9. Variation of glass transition temperature with salt concentration for LPC6 series–LiCF 3 SO 3 complexes.

This research was supported by the Korea Research Foundation (Project No. 9606-060), and J.Y.

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Kim and H.S. Kim are grateful to the Graduate School of Advanced Material and Chemical Engineering for the scholarship support.

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