- Email: [email protected]
Poly[lithium methacrylate-co-oligo(oxyethylene)methacrylate] as a solid electrolyte with high ionic conductivity
Solid State Ionics 17 (1985) 307-311 North-Holland. Amsterdam
POLY [ LITHIUM METHACRYLATE-CO-OLIGO(OXYETHYLENE)METHACRYLATE] AS A SOLID ELECTROLYTE WITH HIGH IONIC CONDUCTIVITY Norlhisa KOBAYASHI,
Masahiro UCHlYAMA and Eishun TSUCHIDA *
Department of Polymer Chemistry, WasedaUniversity, Tokyo 160, Japan
Received 6 May 1985; in revised form received 10 July 1985 Poly[lithium methacrylate-co-oligo(oxyethylene)methacrylate] film was prepared as a polymeric solid electrolyte which showed lithium ionic conductivity of 2 X lo-’ (S/cm). This film contained no organic plasticizer nor low molecular weight Lithium salts and was shown to be a singleion conductor in solid state. Li* ionic conductivity was deeply influenced by the glass transition temperature and lithium methacrylate content of this film. A rechargeable battery composed of metallic lithium/this film/graphite showed better characteristics than any previously reported systems using polymeric solid electrolytes.
1. Introduction Several polymeric solid electrolytes have been presented in this decade, however, there are only a few reports on the single-ion polymeric conductor [l-3]. The intrinsic property of polymeric electrolytes is a bi-ionic conduction and dc conductivity is therefore decreased by long dc polarization, even if alkali metal electrodes are employed. Such a decrease of dc ionic conductivity is quite inconvenient if the polymeric solid electrolytes are to be used in devices driven with dc polarization. It is therefore important to develop single-ion conductors. For several commercial applications, anion movement should be suppressed and the transport number of the cation should be unity. Commonly, polyelectrolyte salts such as poly(methacrylic acid alkali salts) showed low conductivity in the dry state. The important factor for ion transport in such hybrid conductors is believed to be the weak interaction between polymer and cation [4,.5]. Shriver et al. [2] have succeeded in creating an interesting single-ion system. In their system, only chloride ions (Cl-) were revealed as carrier ions. On the other hand, a hybrid system composed of polymer and polyelectrolyte has also been reported as a single-ion conductor [3 1. In the present paper, a lithium methacrylate as carrier
source was copolymerized with oligo(oxyethylene) methacrylate, facilitating salt dissociation and ion transport.
2. Experimental 2. I. Materials Lithium methacrylate #and lithium iso-butyrate * A known amount of reagent-grade lithium hydroxide was reacted with a large excess of distilled methacrylic acid or iso-butyric acid in dry methanol at 25°C for 2 h. The required lithium salt was precipitated by pouring the methanolic solution into dry acetone. The white powder was redissolved and reprecipitated three times. This was washed with dry acetone several times and dried in vacua at room temperature for a day. No contamination by free acid was confirmed by IR and NMR spectroscopies. Oligo(oxyethylene) methacrylate $ Oligo(oxyethylene)mono-methylether with an average number of repeating units of 7 (Aldrich Chem. # CHs
* CH3
A= CH,
c~H-CH~
&KILi
COOLi
I * To whom correspondence
should be addressed.
0 167-2738/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
*CH, L=CH2
308
N. Kobayashi et al./Polyflithium metacrylute-cooligo(oxyothelene)methaqdate]
Inc.) was reacted with a large excess of metallic lithium by refluxing with dry THF for three days under an inert atmosphere. After the removal of unreacted metallic lithium, a dry THF solution of an excess amount of methacryloyl chloride was added dropwise to the solution at 0-5°C. The mixture was reacted for a half day at 0-S”C, then for another day at room temperature. The resulting mixture was carefully concentrated by evaporation at a temperature lower than 30°C, then the solvent was replaced by dry chloroform. Complete evaporation during the replacing of the solvent should be avoided, otherwise spontaneous polymerization could occur. The solution was passed through a basic alumina (Merck, Grade-I) column (typically 9 10 X h30 cm for 40 g product) eluted with dry chloroform in order to remove unreacted methacrylqyl chloride and lithium chloride precipitate. The chloroform solution thus obtained, with a concentration lower than 40 wt%, was refrigerated at 0°C. Yields were typically 60-80%. The structure was confirmed by the NMR spectroscopy in deutero-chloroform: terminal -OCH, 3.3 (3H), -OCH,- 3.6 (26H), -COOC&-I$H20- 4.2 (2H), -C=CH2 5.5,6.1 (2H), methacrylate -CH, 2.0 (3H) in ppm. Other chemicals
Solvents were distilled, stored over molecular seive-4A and redistilled before use. Reagent-grade lithium perchlorate was dried at 180°C in va&o for two days before use.
methacrylate] and of [lithium iso-butyrate/oligo(oxyethylene) methacrylate) hybrid insoluble films, the completion of copolymerization was confirmed. 2.3. Measurements All the measurements were conducted under dry argon atmosphere to avoid moisture at the given temperatures. 2.3.1. Conductivity measurement Metallic lithium electrodes were used for both ac( lV)- or dc(3 V&conductivity measurements. Details of the instrumentation have been described previously [6-lo]. The ac-ionic conductivity was calculated from the complex impedance plots [ 1l] with computer curve fitting. Determination of apparent transport number of lithium ion
Film samples were sandwiched between a metallic lithium anode and a stainless steel cathode. After applying dc(3 Qpolarization for a given period, the cathode was carefully separated from the film. The metallic lithium deposited on the cathode was dissolved in pure water and the concentration of lithium ions was determined by atomic absorption spectroscopy. The apparent transport number of the lithium ions was calculated from the amount of electro-deposited metallic lithium and the charge passed through the cell.
2.2. Film preparation The chloroform solution of oligo(oxyethylene)
methacrylate was evaporated at room temperature with continuous addition of dry methanol. To the resulting methanolic solution, a given amount of lithium methacrylate, lithium iso-butyrate or lithium perchlorate and 1 mol% amount (in respect to the total vinyl monomers) of 2,2’-azobisisobutylonitryle were added. The mixture was developed on a Teflon@ plate and was allowed to evaporate under dry nitrogen flow at 80°C for 1 day. The remaining viscous solution on Teflon@ plate was further evacuated at 80°C for another 2 days over P20, in order to perform both thermal polymerization and complete drying. From NMR spectra of methanolic Soxhlet extracts of Poly[lithium methacrylate-co-oligo(oxyethylene)
J-V curves and discharging process of a rechargeable battery
A rechargeable battery was demonstrated by the cell constitution of metallic lithium (anode)/sample film/graphite (cathode, Union Carbide Co.). This graphite was used after thermal treatment at 250°C for two days in vacua. The time dependence of short circuit current with no external loading was measured with 0 V bias potential. Differential scanning wkwimetry
Thermograms of film samples were obtained with a SEIKO differential scanning calorimeter (Model SSC580, DSC-10) under dry helium atmosphere.
N. Kobayashi et al./Polyflithium
309
metacrylate-co-oIigo(oxyothylene)methacrylatej
X-ray diffraction analysis The X-ray diffraction pattern of film samples was
measured by an X-ray diffractometer (Rigaku Denki Co. Ltd., Model 2026) with Cu-Ka irradiation.
3. Results and discussion Thin films prepared by the thermal polymerization of vinyl monomers were flexible and transparent but were insoluble in any solvent. The thermal polymerization of oligo(oxyethylene) methacrylate (MEO) of more than 4 mol% in THF solution caused gelation. The ac ionic conductivities (~a,_)of film samples of [P(MEO)/lithium iso-butyrate] , [P(MEO)/LiClOJ and [copoly(MEO-lithium methacrylate)] are compared in fig. 1 in respect to the content of lithium salts. Hereafter, we denote these three systems as hybrid (A), hybrid (B) and P(MEO-MALi), respectively. The hybrids (A), and (B) show distinct maxima of ua, at 20 and 60 mol% of the lithium salt content, respectively [lo]. The number of carrier ions was increased by adding the salt (~a,?), however this also caused the increase of Tg(ua,J). A similar profile can be seen for P(MEO--MALi) systems with the maxima of uac at the lower lithium salt content. The Tg value of the P(MEO-MALi) system increases more steeply than that of the hybrid (B) system with increasing lithium ion content (fig. 2). The enhanced interaction of ether oxygens and ILi salt] 40
20
-
-0-
P[MEO-MALi)
-O-
Hybrid(A)
*
Hybrid (6)
I
I
0
10
[ Li
30
20 salt
1
( mot%
1
Fig. 2. Relation between Li salt concentration films determined by DSC.
and Tg of the
lithium cations is the major reason for the increase of the Tg value by adding the lithium salt in the cases of hybrid (A) and (B). The introduction of lithium methacrylate on the chain backbone may cause a stronger effect on Tg, because this salt should act as “hard segment” in the matrix. The u values of hybrid (A) and P(MEO-MALI’) with the same lithium salt concentration are smaller than that of hybrid (B) due to the difference in the dissociation energy of the salt. The intrinsic dissociation energy of lithium iso-butyrate salt or lithium methacrylate salt is still not clear, but it may be larger than that of lithium salts with bulky anions such as
(mol%) 60
-“I +I-
F 2-6
P(MEO-MALI)
-f
Hybrid (A)
-&-
Hybrid
(B)
._ 0 0” -7
0 0
10 ILi
20
30
salt1 (mol%)
Fig. 1. ac ionic conductivity of polymer solid electrolytes.
90
60
30 Time
120
(min)
Fig. 3. Time dependence of dc ionic conductivity in polymer solid electrolyte.
N. Kobayashi et ab/Polyflithium metacrybte-co-oligo(oxyothylene)metha~late]
310
LiCIO, (723 KJ/mol). From the X-ray diffraction pattern, each polymer film is revealed to contain no crystalline salt and dissolve the lithium salt homogeneously even at the higher contents of the salts indicated in fig. 1. Fig. 3 shows the time dependence of dc ionic conductivities (udc) of the three systems with certain salt concentrations which show the highest uac. It should be noted that ode of the P(MEO-MALi) showed time stability, while that of hybrid (A) and (B) decreased markedly with the prolonged supply of dc potential. From the amount of metallic lithium electro-deposited on the cathode measured by the dc polarization (see experimental), the transport number of the lithium cation was estimated as 0.78 + 0.05 and 0.71 f 0.05 for hybrid (A) and (B), respectively. Although the lithium cation is the major carrier ion due to its smaller ionic radius compared with those of perchlorate or carboxylate anions, those anions are still mobile to some extent as illustrated in fig. 4a. Such mobile anions may accumulate near the anode with the dc polarization and may interfere with the diffusion of lithium ion from anode toward the cathode. On the other hand, for the P(MEO-MALi) system, the transport number of the lithium cation is unity, within the error of kO.O.5. As the carboxylate anions of P(MEO-MALi) systems were covalently bonded to polymer chains and showed no mobility, it is structurally reasonable to consider the P(MEO-MALi) system as a single-ion (lithium ion) conductor (see iig. 4b). Fig. 5 shows the temperature dependence of P(MEO-MALi) samples with four different compositions. Log uac showed a non-linear but curved rela-
(W
(a)
/O-0, b . '0
o/o-o
F--o. 6
X-
'0
o’o-o
‘0,
+I 1 0
b Li
‘o-o-
X-O/O
Li*
,o
/”
0
$I
coo-
Li.
/ p,y;,
o-o/O
-0-o
‘0,
lo
Li. X-
7
0-O~O cooLi’
d”
O/O & Li’
b
o-
01
‘0,
X-
/O
P
\
coo-
/j3
Li
0. 0,
‘O-
-0oc
6 -0’
Fig. 4. The difference between (a) biion conductor and 0~) single-ion conductor.
._
z)
-6.5 -
,k
-7.0 -
2.8
u t -a4
5 IO 15 20
3.0 1000/T
3.2
3.4
(l/K)
Fig. 5. Temperature dependence of ionic conductivity in P(MEO-MALi) fiis.
tionship with inverse temperature. This indicates that the ion conduction mechanism of the system obeyed the WLF model rather than the Arrhenius model. uac is therefore a function of Tg of the films. The single-ion properties of P(MEO-MALI) films are suitable to construct solid state rechargeable batteries, because there is no decay in discharge current. This film with a thickness of 180 pm was used to prepare a cell composed of [metallic lithium (negative electrode)/P(MEO-MALi) film/graphite (positive electrode)] . The J-V characteristics of this cell are obtained by monitoring the steady-state current density under the controlled potential as shown in fig. 6, This film containing 15 mol% [MALi] showed an open circuit potential V,, of 3 V and a short circuit current density Jse of 8.0 ,uA/cm2. These values agreed with E’ of Li/Li+ t e- and the ode value of the corresponding polymer film as shown in fig. 3. The time dependence of J,, of this battery with no bias voltage was compared with that of the battery containing the hybrid (A) as solid electrolyte (fig. 7). Although hybrid (A) showed the higher ionic conductivity at the initial stage of closing the circuit, Jsc of this battery rapidly decayed to a lower value reflecting the bi-ionic conduc tive properties. On the other hand, the battery of P(MEO-MALi) film supplied a constant Jsc value of 8.0 PA/cm2 continuously for longer than 3 h. As com-
N. Kobayashi et al./Poly[Iithium metacrylate-codigo(oxyothylene)methacvlateJ
311
4. Conclusion
Graphite
Solid state polyelectrolytes were prepared by the thermal copolymerization of oligo(oxyethylene)methacrylate and lithium methacrylate. The resulting polymer film was flexible and transparent. It showed completely single-ion conduction for lithium ions with a conductivity of lo-’ (S/cm). The battery composed of [metallic lithium/this film/graphite] provided Voc and J,, of 3.0 V and 8.0 pA/cm2, respectively.
Acknowledgement
This work was partially supported by a Grant-inAid for Scientific Researches from the Ministry of Education, Science and Culture of Japan and the Grant for Special Research Projects by the Waseda University. Fig. 6. J-V characteristics of short circuit current of [metallic lithium/P(MEO-MALi)/graphite] cell; [MAW] = 15 mol%.
References
111N. Kobayashi, K. Shigehara and E. Tsuchida, Polym. plete ohmic properties of P(MEO-MALi) films were confirmed in the range of O-3 V, Js, of the battery should be enhanced to around a mA in the film thickness is limited to only a few Mm.These characteristics should satisfy the requirement for commercial solid batteries.
-o-
PNEO-MALI)
-C
Hybrid (A)
0
n
0
I 120
I 80 Time
* 180
(min)
Fig. 7. Time dependence of short circuit current in [metallic lithium/P(MEO-MALi)/graphite] cell.
Prepr. Japan 33 (1984) 485.
[21 L.C. Hardy and D.F. Shriver, Macromolecules 17 (1984) 975. 131 D.J. Bannister, G.R. Davies, I.M. Ward and J.E. McIntyre, Polymer 25 (1984) 1291. I41 B.L. Papke, M.A. Ratner and D.F. Shriver, J. Phys. Chem. Solids 42 (1981) 493. [51 R. Dupon, B.L. Papke, M.A. Ratner, D.H. Whitmore and D.F. Shriver, J. Am. Chem. Sot. 104 (1982) 6247. [61 H. Ohno, H. Mat&a, K. Mizoguchi and E. Tsuchida, Polymer Bull. 7 (1982) 271. [71 E. Tsuchida, H. Ohno and K. Tsunemi, Electrochim. Acta 28 (1983) 591. [S] K. Shigehara, K. Kobayashi and E. Tsuchida, Solid State Ionics 14 (1984) 85. 191 E. Tsuchida and K. Shigehara, Mol. Cryst. Liq. Cryst. 106 (1984) 361. [lo] N. Kobayashi, M. Uchiyama, K. Shigehara and E. Tsuchida, J. Phys. Chem. 89 (1985) 987. [ll] K.S. Cole and R.H. Cole, J. Chem. Phys. 9 (1941) 341.
POLY [ LITHIUM METHACRYLATE-CO-OLIGO(OXYETHYLENE)METHACRYLATE] AS A SOLID ELECTROLYTE WITH HIGH IONIC CONDUCTIVITY Norlhisa KOBAYASHI,
Masahiro UCHlYAMA and Eishun TSUCHIDA *
Department of Polymer Chemistry, WasedaUniversity, Tokyo 160, Japan
Received 6 May 1985; in revised form received 10 July 1985 Poly[lithium methacrylate-co-oligo(oxyethylene)methacrylate] film was prepared as a polymeric solid electrolyte which showed lithium ionic conductivity of 2 X lo-’ (S/cm). This film contained no organic plasticizer nor low molecular weight Lithium salts and was shown to be a singleion conductor in solid state. Li* ionic conductivity was deeply influenced by the glass transition temperature and lithium methacrylate content of this film. A rechargeable battery composed of metallic lithium/this film/graphite showed better characteristics than any previously reported systems using polymeric solid electrolytes.
1. Introduction Several polymeric solid electrolytes have been presented in this decade, however, there are only a few reports on the single-ion polymeric conductor [l-3]. The intrinsic property of polymeric electrolytes is a bi-ionic conduction and dc conductivity is therefore decreased by long dc polarization, even if alkali metal electrodes are employed. Such a decrease of dc ionic conductivity is quite inconvenient if the polymeric solid electrolytes are to be used in devices driven with dc polarization. It is therefore important to develop single-ion conductors. For several commercial applications, anion movement should be suppressed and the transport number of the cation should be unity. Commonly, polyelectrolyte salts such as poly(methacrylic acid alkali salts) showed low conductivity in the dry state. The important factor for ion transport in such hybrid conductors is believed to be the weak interaction between polymer and cation [4,.5]. Shriver et al. [2] have succeeded in creating an interesting single-ion system. In their system, only chloride ions (Cl-) were revealed as carrier ions. On the other hand, a hybrid system composed of polymer and polyelectrolyte has also been reported as a single-ion conductor [3 1. In the present paper, a lithium methacrylate as carrier
source was copolymerized with oligo(oxyethylene) methacrylate, facilitating salt dissociation and ion transport.
2. Experimental 2. I. Materials Lithium methacrylate #and lithium iso-butyrate * A known amount of reagent-grade lithium hydroxide was reacted with a large excess of distilled methacrylic acid or iso-butyric acid in dry methanol at 25°C for 2 h. The required lithium salt was precipitated by pouring the methanolic solution into dry acetone. The white powder was redissolved and reprecipitated three times. This was washed with dry acetone several times and dried in vacua at room temperature for a day. No contamination by free acid was confirmed by IR and NMR spectroscopies. Oligo(oxyethylene) methacrylate $ Oligo(oxyethylene)mono-methylether with an average number of repeating units of 7 (Aldrich Chem. # CHs
* CH3
A= CH,
c~H-CH~
&KILi
COOLi
I * To whom correspondence
should be addressed.
0 167-2738/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
*CH, L=CH2
308
N. Kobayashi et al./Polyflithium metacrylute-cooligo(oxyothelene)methaqdate]
Inc.) was reacted with a large excess of metallic lithium by refluxing with dry THF for three days under an inert atmosphere. After the removal of unreacted metallic lithium, a dry THF solution of an excess amount of methacryloyl chloride was added dropwise to the solution at 0-5°C. The mixture was reacted for a half day at 0-S”C, then for another day at room temperature. The resulting mixture was carefully concentrated by evaporation at a temperature lower than 30°C, then the solvent was replaced by dry chloroform. Complete evaporation during the replacing of the solvent should be avoided, otherwise spontaneous polymerization could occur. The solution was passed through a basic alumina (Merck, Grade-I) column (typically 9 10 X h30 cm for 40 g product) eluted with dry chloroform in order to remove unreacted methacrylqyl chloride and lithium chloride precipitate. The chloroform solution thus obtained, with a concentration lower than 40 wt%, was refrigerated at 0°C. Yields were typically 60-80%. The structure was confirmed by the NMR spectroscopy in deutero-chloroform: terminal -OCH, 3.3 (3H), -OCH,- 3.6 (26H), -COOC&-I$H20- 4.2 (2H), -C=CH2 5.5,6.1 (2H), methacrylate -CH, 2.0 (3H) in ppm. Other chemicals
Solvents were distilled, stored over molecular seive-4A and redistilled before use. Reagent-grade lithium perchlorate was dried at 180°C in va&o for two days before use.
methacrylate] and of [lithium iso-butyrate/oligo(oxyethylene) methacrylate) hybrid insoluble films, the completion of copolymerization was confirmed. 2.3. Measurements All the measurements were conducted under dry argon atmosphere to avoid moisture at the given temperatures. 2.3.1. Conductivity measurement Metallic lithium electrodes were used for both ac( lV)- or dc(3 V&conductivity measurements. Details of the instrumentation have been described previously [6-lo]. The ac-ionic conductivity was calculated from the complex impedance plots [ 1l] with computer curve fitting. Determination of apparent transport number of lithium ion
Film samples were sandwiched between a metallic lithium anode and a stainless steel cathode. After applying dc(3 Qpolarization for a given period, the cathode was carefully separated from the film. The metallic lithium deposited on the cathode was dissolved in pure water and the concentration of lithium ions was determined by atomic absorption spectroscopy. The apparent transport number of the lithium ions was calculated from the amount of electro-deposited metallic lithium and the charge passed through the cell.
2.2. Film preparation The chloroform solution of oligo(oxyethylene)
methacrylate was evaporated at room temperature with continuous addition of dry methanol. To the resulting methanolic solution, a given amount of lithium methacrylate, lithium iso-butyrate or lithium perchlorate and 1 mol% amount (in respect to the total vinyl monomers) of 2,2’-azobisisobutylonitryle were added. The mixture was developed on a Teflon@ plate and was allowed to evaporate under dry nitrogen flow at 80°C for 1 day. The remaining viscous solution on Teflon@ plate was further evacuated at 80°C for another 2 days over P20, in order to perform both thermal polymerization and complete drying. From NMR spectra of methanolic Soxhlet extracts of Poly[lithium methacrylate-co-oligo(oxyethylene)
J-V curves and discharging process of a rechargeable battery
A rechargeable battery was demonstrated by the cell constitution of metallic lithium (anode)/sample film/graphite (cathode, Union Carbide Co.). This graphite was used after thermal treatment at 250°C for two days in vacua. The time dependence of short circuit current with no external loading was measured with 0 V bias potential. Differential scanning wkwimetry
Thermograms of film samples were obtained with a SEIKO differential scanning calorimeter (Model SSC580, DSC-10) under dry helium atmosphere.
N. Kobayashi et al./Polyflithium
309
metacrylate-co-oIigo(oxyothylene)methacrylatej
X-ray diffraction analysis The X-ray diffraction pattern of film samples was
measured by an X-ray diffractometer (Rigaku Denki Co. Ltd., Model 2026) with Cu-Ka irradiation.
3. Results and discussion Thin films prepared by the thermal polymerization of vinyl monomers were flexible and transparent but were insoluble in any solvent. The thermal polymerization of oligo(oxyethylene) methacrylate (MEO) of more than 4 mol% in THF solution caused gelation. The ac ionic conductivities (~a,_)of film samples of [P(MEO)/lithium iso-butyrate] , [P(MEO)/LiClOJ and [copoly(MEO-lithium methacrylate)] are compared in fig. 1 in respect to the content of lithium salts. Hereafter, we denote these three systems as hybrid (A), hybrid (B) and P(MEO-MALi), respectively. The hybrids (A), and (B) show distinct maxima of ua, at 20 and 60 mol% of the lithium salt content, respectively [lo]. The number of carrier ions was increased by adding the salt (~a,?), however this also caused the increase of Tg(ua,J). A similar profile can be seen for P(MEO--MALi) systems with the maxima of uac at the lower lithium salt content. The Tg value of the P(MEO-MALi) system increases more steeply than that of the hybrid (B) system with increasing lithium ion content (fig. 2). The enhanced interaction of ether oxygens and ILi salt] 40
20
-
-0-
P[MEO-MALi)
-O-
Hybrid(A)
*
Hybrid (6)
I
I
0
10
[ Li
30
20 salt
1
( mot%
1
Fig. 2. Relation between Li salt concentration films determined by DSC.
and Tg of the
lithium cations is the major reason for the increase of the Tg value by adding the lithium salt in the cases of hybrid (A) and (B). The introduction of lithium methacrylate on the chain backbone may cause a stronger effect on Tg, because this salt should act as “hard segment” in the matrix. The u values of hybrid (A) and P(MEO-MALI’) with the same lithium salt concentration are smaller than that of hybrid (B) due to the difference in the dissociation energy of the salt. The intrinsic dissociation energy of lithium iso-butyrate salt or lithium methacrylate salt is still not clear, but it may be larger than that of lithium salts with bulky anions such as
(mol%) 60
-“I +I-
F 2-6
P(MEO-MALI)
-f
Hybrid (A)
-&-
Hybrid
(B)
._ 0 0” -7
0 0
10 ILi
20
30
salt1 (mol%)
Fig. 1. ac ionic conductivity of polymer solid electrolytes.
90
60
30 Time
120
(min)
Fig. 3. Time dependence of dc ionic conductivity in polymer solid electrolyte.
N. Kobayashi et ab/Polyflithium metacrybte-co-oligo(oxyothylene)metha~late]
310
LiCIO, (723 KJ/mol). From the X-ray diffraction pattern, each polymer film is revealed to contain no crystalline salt and dissolve the lithium salt homogeneously even at the higher contents of the salts indicated in fig. 1. Fig. 3 shows the time dependence of dc ionic conductivities (udc) of the three systems with certain salt concentrations which show the highest uac. It should be noted that ode of the P(MEO-MALi) showed time stability, while that of hybrid (A) and (B) decreased markedly with the prolonged supply of dc potential. From the amount of metallic lithium electro-deposited on the cathode measured by the dc polarization (see experimental), the transport number of the lithium cation was estimated as 0.78 + 0.05 and 0.71 f 0.05 for hybrid (A) and (B), respectively. Although the lithium cation is the major carrier ion due to its smaller ionic radius compared with those of perchlorate or carboxylate anions, those anions are still mobile to some extent as illustrated in fig. 4a. Such mobile anions may accumulate near the anode with the dc polarization and may interfere with the diffusion of lithium ion from anode toward the cathode. On the other hand, for the P(MEO-MALi) system, the transport number of the lithium cation is unity, within the error of kO.O.5. As the carboxylate anions of P(MEO-MALi) systems were covalently bonded to polymer chains and showed no mobility, it is structurally reasonable to consider the P(MEO-MALi) system as a single-ion (lithium ion) conductor (see iig. 4b). Fig. 5 shows the temperature dependence of P(MEO-MALi) samples with four different compositions. Log uac showed a non-linear but curved rela-
(W
(a)
/O-0, b . '0
o/o-o
F--o. 6
X-
'0
o’o-o
‘0,
+I 1 0
b Li
‘o-o-
X-O/O
Li*
,o
/”
0
$I
coo-
Li.
/ p,y;,
o-o/O
-0-o
‘0,
lo
Li. X-
7
0-O~O cooLi’
d”
O/O & Li’
b
o-
01
‘0,
X-
/O
P
\
coo-
/j3
Li
0. 0,
‘O-
-0oc
6 -0’
Fig. 4. The difference between (a) biion conductor and 0~) single-ion conductor.
._
z)
-6.5 -
,k
-7.0 -
2.8
u t -a4
5 IO 15 20
3.0 1000/T
3.2
3.4
(l/K)
Fig. 5. Temperature dependence of ionic conductivity in P(MEO-MALi) fiis.
tionship with inverse temperature. This indicates that the ion conduction mechanism of the system obeyed the WLF model rather than the Arrhenius model. uac is therefore a function of Tg of the films. The single-ion properties of P(MEO-MALI) films are suitable to construct solid state rechargeable batteries, because there is no decay in discharge current. This film with a thickness of 180 pm was used to prepare a cell composed of [metallic lithium (negative electrode)/P(MEO-MALi) film/graphite (positive electrode)] . The J-V characteristics of this cell are obtained by monitoring the steady-state current density under the controlled potential as shown in fig. 6, This film containing 15 mol% [MALi] showed an open circuit potential V,, of 3 V and a short circuit current density Jse of 8.0 ,uA/cm2. These values agreed with E’ of Li/Li+ t e- and the ode value of the corresponding polymer film as shown in fig. 3. The time dependence of J,, of this battery with no bias voltage was compared with that of the battery containing the hybrid (A) as solid electrolyte (fig. 7). Although hybrid (A) showed the higher ionic conductivity at the initial stage of closing the circuit, Jsc of this battery rapidly decayed to a lower value reflecting the bi-ionic conduc tive properties. On the other hand, the battery of P(MEO-MALi) film supplied a constant Jsc value of 8.0 PA/cm2 continuously for longer than 3 h. As com-
N. Kobayashi et al./Poly[Iithium metacrylate-codigo(oxyothylene)methacvlateJ
311
4. Conclusion
Graphite
Solid state polyelectrolytes were prepared by the thermal copolymerization of oligo(oxyethylene)methacrylate and lithium methacrylate. The resulting polymer film was flexible and transparent. It showed completely single-ion conduction for lithium ions with a conductivity of lo-’ (S/cm). The battery composed of [metallic lithium/this film/graphite] provided Voc and J,, of 3.0 V and 8.0 pA/cm2, respectively.
Acknowledgement
This work was partially supported by a Grant-inAid for Scientific Researches from the Ministry of Education, Science and Culture of Japan and the Grant for Special Research Projects by the Waseda University. Fig. 6. J-V characteristics of short circuit current of [metallic lithium/P(MEO-MALi)/graphite] cell; [MAW] = 15 mol%.
References
111N. Kobayashi, K. Shigehara and E. Tsuchida, Polym. plete ohmic properties of P(MEO-MALi) films were confirmed in the range of O-3 V, Js, of the battery should be enhanced to around a mA in the film thickness is limited to only a few Mm.These characteristics should satisfy the requirement for commercial solid batteries.
-o-
PNEO-MALI)
-C
Hybrid (A)
0
n
0
I 120
I 80 Time
* 180
(min)
Fig. 7. Time dependence of short circuit current in [metallic lithium/P(MEO-MALi)/graphite] cell.
Prepr. Japan 33 (1984) 485.
[21 L.C. Hardy and D.F. Shriver, Macromolecules 17 (1984) 975. 131 D.J. Bannister, G.R. Davies, I.M. Ward and J.E. McIntyre, Polymer 25 (1984) 1291. I41 B.L. Papke, M.A. Ratner and D.F. Shriver, J. Phys. Chem. Solids 42 (1981) 493. [51 R. Dupon, B.L. Papke, M.A. Ratner, D.H. Whitmore and D.F. Shriver, J. Am. Chem. Sot. 104 (1982) 6247. [61 H. Ohno, H. Mat&a, K. Mizoguchi and E. Tsuchida, Polymer Bull. 7 (1982) 271. [71 E. Tsuchida, H. Ohno and K. Tsunemi, Electrochim. Acta 28 (1983) 591. [S] K. Shigehara, K. Kobayashi and E. Tsuchida, Solid State Ionics 14 (1984) 85. 191 E. Tsuchida and K. Shigehara, Mol. Cryst. Liq. Cryst. 106 (1984) 361. [lo] N. Kobayashi, M. Uchiyama, K. Shigehara and E. Tsuchida, J. Phys. Chem. 89 (1985) 987. [ll] K.S. Cole and R.H. Cole, J. Chem. Phys. 9 (1941) 341.