- Email: [email protected]
Solid electrolytes based on poly(amide 6-b-ethylene oxide)
a __ @
SOLID STATE
d
__
ELSEVIER
Solid State Ionics 91 (1996)
Solid electrolytes
IONICS
123-130
based on poly(amide
M-ethylene
oxide)
R.A. Zoppi”, C.M.N.P. Fonseca, Marco-A. De Paoli, S.P. Nunes Institute de Quimica, Universidade Estadual de Campinas, C. Postal 6154, CEP 13081-970, Campinas, SP, Brazil Received
4 January
1996; accepted
28 February
1996
Abstract An amide 6-b-ethylene oxide copolymer (PebaxB) was investigated as an ionic conducting matrix. Transparent and flexible films containing LiClO, were prepared with ionic conductivities in the range of 10m6 S cm-’ at room temperature. The conductivity values gradually increased when the films were heated ( 10m4 S cm-’ at 100°C). The conductivity dependence on temperature could be described according to the classical Arrhenius plots over the temperature range of 22 to 100°C. The glass transition temperature was determined by differential scanning calorimetry. The T, of the Pebax/LiClO, system increases for samples containing high salt concentration, as expected. The polymer electrolyte morphology was analyzed by transmission electron microscopy. Salt aggregation was observed in all samples. This was confirmed by electron spectroscopic images selective for chlorine. The electrochemical stability window of the Pebax/LiClO, electrolytes as investigated by cyclic voltammetry was in the range of 4.0 V. Keywords: Polymer
electrolyte;
Poly(amide
6-b-ethylene
oxide); Ionic conductivity
1. Introduction Polymeric electrolytes with lithium salts dissolved in a polyether matrix have been widely studied ever since the pioneering works of Wright and co-workers [l-3] and Armand et al. [4,5]. These systems are of great interest in view of their potential applications in solid state batteries [6,7] and electrochromic devices [8-lo]. The preparation and characterization of polymer electrolytes with different polymer matrices has been recently reviewed by Takeoka et al. [ 1 I]. Most of the systems described nowadays are based *Corresponding +55-192-393-805.
author.
E-mail:
0167-2738/96/$15.00 Copyright PII SO167-2738(96)00383-9
[email protected];
01996
fax:
on polymers carrying ethylene oxide groups in the main or side chain. The chemical structure of the polymer matrix is often modified in order to reduce the cristallinity and to maintain a low glass transition temperature. HydrinB elastomers which contain ethylene oxide and epichlorohydrin repeating units have been investigated as polymer electrolytes [12]. It was verified that the ionic conductivity is about 10m5 S cm-’ at room temperature when poly(ethylene glycol) is used as a plasticizer. The presence of low concentrations of zeolite particles also improves the ionic conductivity [ 121. Poly(N-vinylacetamide) has been evaluated for a polymer electrolyte, using LiCF,SO, and poly(ethylene glycol) as electrolyte and plasticizer, respectively. The samples had ionic
Elsevier Science B.V. All rights reserved
124
R.A. Zoppi et al. I Solid State tonics 91 (1996) 123-130
conductivities in the range of lop4 S cm- ’ at 80°C [ 131. The influence of ion-pairing in poly(ethylene oxide) containing salts of Zn+‘, Pb+’ and Ni+* in their anhydrous form was investigated by different techniques of [141. Copolymers PolY(methylmetacrylate-g-propylene oxide) have been complexed with K+, Na+ and Ca+2 picrates and the efficiency of this complexation was evaluated by further extraction [ 151. Polymer electrolytes based on polyesters with oxide) segments poly(ethylene and containing lithium salts showed conductivities in the range of 10m6 - 10m5 S cm-’ at 25°C and were flexible and totally amorphous [16]. For polyesters of thiodipropionic acid the conductivity is ca. 10m5 S cm-’ at room temperature. In this case, it was found that both the oxygen and sulfur atoms contribute to solvating lithium ions [17]. Another interesting variation was described by Angel1 et al., who mixed lithium salts with small quantities of poly(propylene oxide) and poly(ethylene oxide). The new ‘polymer-in-salt’ materials showed glass transitions low enough to remain rubbery at room temperature while preserving good lithium-ion conductivities and high electrochemical stability [IS]. Glycidyl methacrylate homopolymers and copolymers are convenient precursors for the synthesis of polymer gel electrolytes. The solutions of these polymers and inorganic salts in appropriate solvents can be converted into gels as a result of crosslinking via oxirane groups in the presence of cationic initiators. The gel conductivity is lop3 S cm-’ at room temperature [ 191. In this work, a new ionic conductor was obtained by dissolving LiClO, in a copolymer matrix with polyamide 6 and poly(ethylene oxide) blocks. Poly(ethylene oxide)-salt systems have been extensively described by others [20-22,241. Using the amide-6ethylene oxide copolymer, the poly(ethylene oxide) block should care for an ionic conductivity as high as that poly(ethylene oxide)-salt electrolytes and the polyamide block should improve the flexibility, allowing the preparation of transparent, flexible and better handling films. We were able to prepare these films by casting of a copolymer solution containing LiClO,. The solid electrolytes were characterized by complex impedance spectroscopy, cyclic voltammetry, differential scanning calorimetry and transmission electron microscopy.
2. Experimental 2.1. Sample preparation A copolymer with polyamide 6 and poly(ethylene oxide) blocks, with the trade name PebaxB 4011, was supplied by Elf Aquitaine. Polymer electrolytes were prepared by mixing LiClO, in a copolymer solution (3 wt% in n-butanol) which was stirred and heated at 60°C. The mixture was placed in a Petri dish and the solvent was evaporated at room temperature. Transparent films with 1, 5, 10, 13, 15, 17, 20 or 25 wt% LiClO, were obtained. The film thickness was ca. 0.01 mm as measured with a Mytotuo micrometer. 2.2. Electrochemical
characterization
The electrochemical characterization of these films was carried out in a dry box under argon. The electrochemical stability was analyzed by cyclic voltammetry using a 273 PAR potentiostat in the range from - 0.5 to 5 V (versus Li”), at 0.010 V s-’ and 80°C. The electrolytes were pressed between a stainless steel electrode and a Li” sheet, and packed in a button cell (0.785 cm2 area). For impedance measurements, two stainless steel electrodes were used. The measurements were performed using a 1255 HF Schlumberger Solar&on frequency response analyzer connected to a 273 PAR potentiostat which was interfaced to a 286 IBM/PC AT microcomputer. The range of analyzed frequencies was lo-’ to 105Hz. Each sample was allowed to equilibrate for 1 h at any temperature before measurement. The temperature range was 22 to 100°C. 2.3. Transmission
electron microscopy
Samples were cut at - 80°C with a diamond knife in an Ultracut-S/FC4E Reichert-Jung microtome. Ultrathin films (70 nm thickness) were observed in a Zeiss CEM-902 transmission electron microscope with a spectrometer which uses inelastic electrons to form element specific images. 2.4. Thermal analysis Thermogravimetric analysis was carried out in a 990 Du Pont thermal analyzer using Ar flow, with
125
R.A. Zoppi et al. I Solid State Ionics 91 (1996) 123-130
10°C min-’ as the rate of temperature increase. Differential scanning calorimetry was performed with a 2910 MDSC TA Instruments thermal analyzer using the following heating program: heating from 25 to 100°C; cooling from 100 to - 100°C; heating from - 100 to lOO”C, at rate of 10°C min-‘. Only the second heating curve is shown here.
3. Results and discussion 3.1. Copolymer
characterization
The copolymer chemical composition was determined in a 2400 Perkin Elmer CHN elemental analyzer. It contains 40 wt% polyamide 6 blocks and 60 wt% poly(ethylene oxide) blocks. Thermal analysis showed that pure Pebax starts losing mass only at 390°C. A 2% residue remains (Fig. 1). In the presence of 0, polyamides generally degrade at temperatures near 300°C. In the absence of air, the polyamide degradation is caused by the C-N
TEMPERATURE
-150
I
I
I
50
150
Fig. 1. Thermogravimetric
3.2. Polymer electrolytes
characterization
3.2. I. Electrochemical stability The essential requirements for a polymer electrolyte to be used in electrochemical devices are: electrochemical stability window higher than 4.0 V and chemical inertia during the redox process which takes place on the working and counter electrodes. Preliminary results on cyclic voltammetry of poly(ethylene oxide) containing lithium or sodium salts were reported by Armand and co-workers [20,21] and Rigaud [22]. They found an electrochemical stability of 2.8 V for poly(ethylene oxide)/LiI electrolytes, and a value of almost 4.0 V for poly(ethylene oxide)/NaCF,SO, systems. Fig. 2 shows the cyclic voltammetry of the stainless steel/polymer electrolyte/Li” system, which was carried out at 80°C under Ar. The electrochemical stability region was ca. 4.0 V (from 1 to 5 V). On the cathodic side, the electrochemical window is limited by the pseudoreversible lithium deposition process. On the anodic side, the stability range is limited by the anion oxidation process. The Cloy anions have been mostly investigated because of their good stability during the reduction process [24]. However, the nature of their redox behavior is not completely understood [25]. It has been proposed that ClO, is oxidized to form a radical [24]:
(“Cl
-50
TEMPERATURE
homolytic bond cleavage and subsequent formation of double bonds, amides and nitrile groups [23].
I
250
(“C)
curve of pure PebaxB.
E/V
vs
Li
Fig. 2. Cyclic voltammetry of PebaxILiCIO, (20 wt% LiCIO,) at 80°C. using stainless steel and Li” electrodes and a scan rate of 0.010 v s-‘.
ClO,
. ;,J,/
R.A. Zappi et al. I Solid State Ionics 91 (1996) 123-130
126
-e-
+ ClO,
-6.2
followed
by either disproportionation,
c10;
ClO, + 0,
+
b -6.6
or attack on the methylenic (ethylene oxide) chains ClO; + - CH, -
+
-6.4
hydrogens
of the poly-
-7.0 -J -6.6 -7.2
HClO, + .CH -
followed by further degradation
0”
of the polymer chain
R
-
0
5
IO
15
20
25
wt % LiCIO,
r221. 3.2.2. Ionic conductivity of PebaxlLiClO, electrolytes The impedance plots for stainless steel/PebaxLiClO, /stainless steel system at different temperatures are shown in Fig. 3. Similar plots were recorded for different salt contents. A fairly welldefined semicircle suggests that the lithium salt added to the Pebax matrix complexes with the oxygen atom of the ethylene oxide blocks and the behavior of the complex under potential field is similar to that of poly(ethylene oxide)-Li complexes [241. The ionic conductivity as a function of salt concentration was calculated by measuring the resistance corresponding to the diameter of the semicircle in the impedance plots. As shown in Fig. 4, a maximum conductivity was obtained for electrolytes containing 13 wt% LiClO,. Similar behavior has
0.20,
000
1
0.05
0. IO
0.15
z / 1iPn
0.20
0.25
cm-~
Fig. 3. The complex plane impedance plots of stainless steel/ Pebax-LiClO,/stainless steel system at (a) 22, (0) 30 and (A) 40°C. Films with 20 wt% LiClO,.
Fig. 4. Dependence of ionic conductivity LiCIO, weight fraction at 22°C.
of Pebax electrolytes
on
been reported for different polymer electrolytes [11,16,17,19,26], and it has been assigned to two opposite effects: the increasing conductivity in the low salt concentration region might be attributed to the increase in the number of charge carriers. On the other hand, the conductivity decrease at higher salt concentrations may be due to a decrease in charge carrier mobility caused by the stiffening of the matrix chains with the ion complexation acting as crosslinking nodes. This stiffening effect was verified using differential scanning calorimetry, Fig. 5. It is interesting to note that Pebax has two glass transition temperatures and two endothermic peaks. A T, was detected near -53°C and T,,, near 15”C, and were assigned to the glass transition and fusion process of poly(ethylene oxide) blocks [27]. The T, near 40°C and the T, near 205”C, were assigned to the glass transition and fusion process of the polyamide 6 blocks [27]. These results suggest that both copolymer blocks are immiscible. Furthermore, adding LiClO,, an increase of Tg assigned to poly(ethylene oxide) was observed and its fusion process gradually disappeared. By considering that Li+ complexation acts as physical crosslinks, this behavior can be understood since it has been reported that an increase in the degree of crosslinking inhibits the crystalization [27]. The temperature dependence of the ionic conductivity of Pebax-LiClO, systems is shown in the Arrhenius plots, Fig. 6. Kobayashi et al. [28] have reported that in polymer-salt solid state ionic conductors such plots are not completely linear since near T, the first term in brackets in E!.q. (1) is not small
127
R.A. Zoppi et al. I Solid State Ionics 91 (1996) 123-130
w
::
h
20
I
I
30 I
I
I
0
-50
I
50
TEMPERATURE
I
100
(“Cl
Fig. 5. Differential scanning calorimetry of Pebax containing different LiClO, weight fractions. Insert: glass transition temperature of Pebax-LiCIO, as a function of salt weight fraction.
-4.5 7
-
-5.0
-
-5.5
‘” D -6.0
-
T-65
-
5
ml (+ = r,,
exp{ - [E + WI~E]IKT}
(2)
25
i 1
number of total ions, y is the correction factor for the overlapping of free volume, u* is the smallest volume required for ion hopping, u is the relative volume at temperature T, f,is the fraction of free volume at temperature T,, Aa is the difference between the heat expansion coefficient at T > T, and T
so that linear Arrhenius plots are obtained. The activation energies were calculated from the Arrhenius plots for different salt concentrations, Fig. 7. It is noted that the activation energy for ionic conductivity increases with increase of LiClO, content. This behavior can be understood since the mobility of the ethylene oxide blocks decreases (T, increases) when the salt content increases. A similar explanation was used by Kobayashi et al. [28] for polymethacryloyl-oligo-(oxyethylene) containing LiClO, or LiPF,. When compared to polymethacryloyl-oligo-(oxyethylene) with the same
60
-70
-
-7.5
-
-6.0
-
-6.5
-
1000/T
(K)
Fig. 6. Arrhenius plot, log (T versus T-‘, for Pebax-LiCIO, (A) 5, (w) 13, (a) 17 and (+) 20 wt% salt.
u=
~Oexp{-[yu*lv(
f,+h(T-
a
T,))
+ (E+ W/2E)/KT]}. u, is the pre-exponential
with
0.00
’ 0.05
2
’ 0.10 wt %
8
’ 0.15
’ 0.20
1
J 0.25
LiCI04
(1)
constant proportional
to the
Fig. 7. Activation LiClO, electrolytes
energy of the ionic conductivity as a function of the salt content.
in Pebax-
128
R.A. Zoppi et al. I Solid State Ionics 91 (1996) 123-130
LiCIO, content, Pebax-LiClO, electrolytes have a lower activation energy. This effect reflects the fact that the decrease of mobility adding LiClO, is less drastic in the Pebax-LiClO, systems and that Pebax is a better solvent for LiClO, than polymethacryloyloligo-(oxyethylene). 3.2.3. Morphological characterization Fig. 8a shows the conventional elastic image for a pure Pebax film with dark and spherical regions. The composition of these regions was investigated subtracting images formed by inelastic electrons with an energy loss AE =420 eV, right above the ionization edge of nitrogen (401 eV) and AE = 380 eV, below it. The resulting element specific image is shown in Fig. 9a. A local concentration of nitrogen was detected. These images confirm that n-butanol is a nonsolvent for the polyamide 6 blocks or any residual homopolyamide 6, which form micelles with an average diameter of 200 nm. Block segregation was also confirmed by the two T, observed in DSC experiments.
Fig. 8. Transmission electrons (AE=O).
electron microscopy
of Pebax films containing
For the polymer electrolytes investigated here independently of the LiCIO, weight fraction in the electrolytes dark spherical regions were also observed in Fig. 8b-d. By selecting inelastic electrons (energy loss AE = 220 eV right above the ionization edge of chlorine and at AE = 180 eV, below it), electron spectroscopic images for chlorine, Fig. 9bd, were obtained. There is a local concentration of chlorine showing that in polymer electrolytes a salt aggregation occurs. The size of these salt aggregates is also of the order of 200 nm. Despite these aggregations, the polymer electrolytes are transparent.
4. Conclusions Solid state polymer electrolytes were obtained using Pebax and LiClO,. The ionic conductivity increased with the salt concentration. At room temperature Pebax films with 13 wt% LiClO, had an ionic conductivity of ca. lop5 S cm-’ and at lOO”C,
(a) 0, (b) 5, (c) 15 and (d) 25 wt% LiCIO,. Images obtained with elastic
129
R.A. Zoppi et al. I Solid State tonics 91 (1996) 123-130
Fig. 9. Transmission electron microscopy of Pebax films containing (a) 0, (b) 5, (c) 15 and (d) 25 wt% LiCIO,. images selective for (a) nitrogen (AE=401 eV) and (b-d) chlorine (AE= 200 eV).
electrolytes preca. 10m4 S cm-‘. Pebax-LiClO, sented an electrochemical stability window of 4 V. For all LiClO, weight fraction investigated, transmission electron microscopy showed a salt aggregation in the polymer matrix. This aggregation, however, did not affect the film transparency which was quite good. The films were flexible and had the basic requirements for application in solid state electrochemical devices.
Acknowledgments The authors thank FAPESP
for financial
support.
References [l] D.E. Fenton, J.M. Parker and PIV.Wright, Polymer 589.
14 (1973)
Electron
spectroscopic
VI P.V Wright, Br. Polymer J. 7 (1975) 319. [31 P.V Wright, J. Polym. Sci. Polym. Phys. Edn. 14 (1976) 955. [41 M.B. Armand, J.M. Chabagno and M.J. Duclot, in: Fast Ion Transport in Solids, eds. P Vashishta, J.N. Mundy and G.K. Shenoy (North-Holland, Amsterdam, 1979) pp. 131-136. PI M.B. Armand, J.M. Chabagno and M.J. Duclot, in: Extended Abstracts, Second International Conference on Solid Electrolytes (Scotland, 1978). A.M. Marinangeli, M. Mastragostino, L. 161 C. Arbizzani, Meneghello, T. Hamaide and A. Guyot, J. Power Sources 43-44 (1993) 453. [71 M. Armand, J.Y. Sanchez, M. Gauthier and Y. Choquette in: The Electrochemistry of Novel Materials, eds. J. Lipkowski and P.N. Ross (VCH, New York, 1994) pp. 65-110. L. Meneghello, X. Andrieu WI C. Arbizzani, M. Mastragostino, and T. Vicedo, Proc. Mat. Res. Sot. Symp., 293 (1993) 169. 191 T. Mani and J.R. Stevens, Polymer 33 (1992) 834. A. Zanelli, G. Casalbore-Miceli and A. [lOI M. Mastragostino, Geri, Synth. Met. 68 (1995) 157. 1111 S. Takeoka, H. Ohno and E. Tsuchida, Polym. Adv. Tech. 4 (1993) 53. L.G. Scanlon, R.A. Marsch, B. Kumar u21 N. Munichandraiah, and A.K. Sircar, J. Appl. Electrochem. 24 (1994) 1066. u31 S. Iwatsuki, M. Kubo and M. Ohtake, Chem. Lett. (1992) 519.
130
R.A. Zoppi et al. I Solid State lonics 91 (1996) 123-130
[14] A. Wendsjo, J.O. Thomas and J. Lindgren, Polymer 34 (1993) 2243. [15] K.M. Novack and C.M.F. Oliveira, Polym. Bull. 31 (1993) 449. [16] B. Fang, C.P. Hu, H.B. Xu and S.K. Ying, Polym. Commun. 32 (1991) 382. [17] P. Manaresi, M.C. Bignozzi, F. Pilati, A. Munari, M. Mastragostino, L. Meneghello and A. Chiolle, Polymer 34 (1993) 2422. [ 181 CA. Angell, C. Liu and E. Sanchez, Nature 362 ( 1993) 137. [19] E. Zygadko-Monikowska, Z. Florjanczyk and W. Wieczorek, J.M.S. Pure Appl. Chem. A31 (1994) 1121. [20] M.B. Armand in: Proc. Electrochemical Society, Workshop on Lithium Non-aqueous Batteries (Cleveland, OH, 1980). [21] M.B. Armand, M.J. Duclot and P. Rigaud, Solid State Ionics 3/4 (1981) 429. [22] P Rigaud, PhD Thesis, Universite Scientifique et Medicale, Institut Nationale Polytechnique, Grenoble, 1980.
[23] J. Zimmerman in: Encyclopedia of Polymer Science and Engineering, eds. H.F. Mark, N.M. Bikales, C.G. Overberger, G. Menges, J.l. Kroschwitz, 201 ed., Vol. 11 (John Wiley, New York, 1988) p. 315. [24] CA. Vincent, Prog. Solid State Chem. 17 (1987) 145. [25] B. Scrosati in: Polymer Electrolyte Reviews 1, eds. J.R. MacCallum and C.A. Vincent (Elsevier Applied Science Publishers, London, 1987). [26] H.L. Mei, Y. Okamoto and T. Skotheim, Polym. Adv. Tech. 1 (1990) 239. [27] H.E. Bair, P.K. Gallagher, M. Jaffe, Y.P. Kbanna, J.J. Maurer, E.M. Pearce, R.B. Prime, D. Raucher, S.W. Shalaby, W.W. Wendlandt and B. Wunderlich, in: Thermal Characterization of Polymeric Materials, ed. E.A. Turi (Academic Press, Florida, 1981) pp. 150, 247, 385 and 617. [28] N. Kobayashi, M. Uchiyama, K. Shigehara and E. Tsuchida, J. Phys. Chem. 89 (1985) 987.
SOLID STATE
d
__
ELSEVIER
Solid State Ionics 91 (1996)
Solid electrolytes
IONICS
123-130
based on poly(amide
M-ethylene
oxide)
R.A. Zoppi”, C.M.N.P. Fonseca, Marco-A. De Paoli, S.P. Nunes Institute de Quimica, Universidade Estadual de Campinas, C. Postal 6154, CEP 13081-970, Campinas, SP, Brazil Received
4 January
1996; accepted
28 February
1996
Abstract An amide 6-b-ethylene oxide copolymer (PebaxB) was investigated as an ionic conducting matrix. Transparent and flexible films containing LiClO, were prepared with ionic conductivities in the range of 10m6 S cm-’ at room temperature. The conductivity values gradually increased when the films were heated ( 10m4 S cm-’ at 100°C). The conductivity dependence on temperature could be described according to the classical Arrhenius plots over the temperature range of 22 to 100°C. The glass transition temperature was determined by differential scanning calorimetry. The T, of the Pebax/LiClO, system increases for samples containing high salt concentration, as expected. The polymer electrolyte morphology was analyzed by transmission electron microscopy. Salt aggregation was observed in all samples. This was confirmed by electron spectroscopic images selective for chlorine. The electrochemical stability window of the Pebax/LiClO, electrolytes as investigated by cyclic voltammetry was in the range of 4.0 V. Keywords: Polymer
electrolyte;
Poly(amide
6-b-ethylene
oxide); Ionic conductivity
1. Introduction Polymeric electrolytes with lithium salts dissolved in a polyether matrix have been widely studied ever since the pioneering works of Wright and co-workers [l-3] and Armand et al. [4,5]. These systems are of great interest in view of their potential applications in solid state batteries [6,7] and electrochromic devices [8-lo]. The preparation and characterization of polymer electrolytes with different polymer matrices has been recently reviewed by Takeoka et al. [ 1 I]. Most of the systems described nowadays are based *Corresponding +55-192-393-805.
author.
E-mail:
0167-2738/96/$15.00 Copyright PII SO167-2738(96)00383-9
[email protected];
01996
fax:
on polymers carrying ethylene oxide groups in the main or side chain. The chemical structure of the polymer matrix is often modified in order to reduce the cristallinity and to maintain a low glass transition temperature. HydrinB elastomers which contain ethylene oxide and epichlorohydrin repeating units have been investigated as polymer electrolytes [12]. It was verified that the ionic conductivity is about 10m5 S cm-’ at room temperature when poly(ethylene glycol) is used as a plasticizer. The presence of low concentrations of zeolite particles also improves the ionic conductivity [ 121. Poly(N-vinylacetamide) has been evaluated for a polymer electrolyte, using LiCF,SO, and poly(ethylene glycol) as electrolyte and plasticizer, respectively. The samples had ionic
Elsevier Science B.V. All rights reserved
124
R.A. Zoppi et al. I Solid State tonics 91 (1996) 123-130
conductivities in the range of lop4 S cm- ’ at 80°C [ 131. The influence of ion-pairing in poly(ethylene oxide) containing salts of Zn+‘, Pb+’ and Ni+* in their anhydrous form was investigated by different techniques of [141. Copolymers PolY(methylmetacrylate-g-propylene oxide) have been complexed with K+, Na+ and Ca+2 picrates and the efficiency of this complexation was evaluated by further extraction [ 151. Polymer electrolytes based on polyesters with oxide) segments poly(ethylene and containing lithium salts showed conductivities in the range of 10m6 - 10m5 S cm-’ at 25°C and were flexible and totally amorphous [16]. For polyesters of thiodipropionic acid the conductivity is ca. 10m5 S cm-’ at room temperature. In this case, it was found that both the oxygen and sulfur atoms contribute to solvating lithium ions [17]. Another interesting variation was described by Angel1 et al., who mixed lithium salts with small quantities of poly(propylene oxide) and poly(ethylene oxide). The new ‘polymer-in-salt’ materials showed glass transitions low enough to remain rubbery at room temperature while preserving good lithium-ion conductivities and high electrochemical stability [IS]. Glycidyl methacrylate homopolymers and copolymers are convenient precursors for the synthesis of polymer gel electrolytes. The solutions of these polymers and inorganic salts in appropriate solvents can be converted into gels as a result of crosslinking via oxirane groups in the presence of cationic initiators. The gel conductivity is lop3 S cm-’ at room temperature [ 191. In this work, a new ionic conductor was obtained by dissolving LiClO, in a copolymer matrix with polyamide 6 and poly(ethylene oxide) blocks. Poly(ethylene oxide)-salt systems have been extensively described by others [20-22,241. Using the amide-6ethylene oxide copolymer, the poly(ethylene oxide) block should care for an ionic conductivity as high as that poly(ethylene oxide)-salt electrolytes and the polyamide block should improve the flexibility, allowing the preparation of transparent, flexible and better handling films. We were able to prepare these films by casting of a copolymer solution containing LiClO,. The solid electrolytes were characterized by complex impedance spectroscopy, cyclic voltammetry, differential scanning calorimetry and transmission electron microscopy.
2. Experimental 2.1. Sample preparation A copolymer with polyamide 6 and poly(ethylene oxide) blocks, with the trade name PebaxB 4011, was supplied by Elf Aquitaine. Polymer electrolytes were prepared by mixing LiClO, in a copolymer solution (3 wt% in n-butanol) which was stirred and heated at 60°C. The mixture was placed in a Petri dish and the solvent was evaporated at room temperature. Transparent films with 1, 5, 10, 13, 15, 17, 20 or 25 wt% LiClO, were obtained. The film thickness was ca. 0.01 mm as measured with a Mytotuo micrometer. 2.2. Electrochemical
characterization
The electrochemical characterization of these films was carried out in a dry box under argon. The electrochemical stability was analyzed by cyclic voltammetry using a 273 PAR potentiostat in the range from - 0.5 to 5 V (versus Li”), at 0.010 V s-’ and 80°C. The electrolytes were pressed between a stainless steel electrode and a Li” sheet, and packed in a button cell (0.785 cm2 area). For impedance measurements, two stainless steel electrodes were used. The measurements were performed using a 1255 HF Schlumberger Solar&on frequency response analyzer connected to a 273 PAR potentiostat which was interfaced to a 286 IBM/PC AT microcomputer. The range of analyzed frequencies was lo-’ to 105Hz. Each sample was allowed to equilibrate for 1 h at any temperature before measurement. The temperature range was 22 to 100°C. 2.3. Transmission
electron microscopy
Samples were cut at - 80°C with a diamond knife in an Ultracut-S/FC4E Reichert-Jung microtome. Ultrathin films (70 nm thickness) were observed in a Zeiss CEM-902 transmission electron microscope with a spectrometer which uses inelastic electrons to form element specific images. 2.4. Thermal analysis Thermogravimetric analysis was carried out in a 990 Du Pont thermal analyzer using Ar flow, with
125
R.A. Zoppi et al. I Solid State Ionics 91 (1996) 123-130
10°C min-’ as the rate of temperature increase. Differential scanning calorimetry was performed with a 2910 MDSC TA Instruments thermal analyzer using the following heating program: heating from 25 to 100°C; cooling from 100 to - 100°C; heating from - 100 to lOO”C, at rate of 10°C min-‘. Only the second heating curve is shown here.
3. Results and discussion 3.1. Copolymer
characterization
The copolymer chemical composition was determined in a 2400 Perkin Elmer CHN elemental analyzer. It contains 40 wt% polyamide 6 blocks and 60 wt% poly(ethylene oxide) blocks. Thermal analysis showed that pure Pebax starts losing mass only at 390°C. A 2% residue remains (Fig. 1). In the presence of 0, polyamides generally degrade at temperatures near 300°C. In the absence of air, the polyamide degradation is caused by the C-N
TEMPERATURE
-150
I
I
I
50
150
Fig. 1. Thermogravimetric
3.2. Polymer electrolytes
characterization
3.2. I. Electrochemical stability The essential requirements for a polymer electrolyte to be used in electrochemical devices are: electrochemical stability window higher than 4.0 V and chemical inertia during the redox process which takes place on the working and counter electrodes. Preliminary results on cyclic voltammetry of poly(ethylene oxide) containing lithium or sodium salts were reported by Armand and co-workers [20,21] and Rigaud [22]. They found an electrochemical stability of 2.8 V for poly(ethylene oxide)/LiI electrolytes, and a value of almost 4.0 V for poly(ethylene oxide)/NaCF,SO, systems. Fig. 2 shows the cyclic voltammetry of the stainless steel/polymer electrolyte/Li” system, which was carried out at 80°C under Ar. The electrochemical stability region was ca. 4.0 V (from 1 to 5 V). On the cathodic side, the electrochemical window is limited by the pseudoreversible lithium deposition process. On the anodic side, the stability range is limited by the anion oxidation process. The Cloy anions have been mostly investigated because of their good stability during the reduction process [24]. However, the nature of their redox behavior is not completely understood [25]. It has been proposed that ClO, is oxidized to form a radical [24]:
(“Cl
-50
TEMPERATURE
homolytic bond cleavage and subsequent formation of double bonds, amides and nitrile groups [23].
I
250
(“C)
curve of pure PebaxB.
E/V
vs
Li
Fig. 2. Cyclic voltammetry of PebaxILiCIO, (20 wt% LiCIO,) at 80°C. using stainless steel and Li” electrodes and a scan rate of 0.010 v s-‘.
ClO,
. ;,J,/
R.A. Zappi et al. I Solid State Ionics 91 (1996) 123-130
126
-e-
+ ClO,
-6.2
followed
by either disproportionation,
c10;
ClO, + 0,
+
b -6.6
or attack on the methylenic (ethylene oxide) chains ClO; + - CH, -
+
-6.4
hydrogens
of the poly-
-7.0 -J -6.6 -7.2
HClO, + .CH -
followed by further degradation
0”
of the polymer chain
R
-
0
5
IO
15
20
25
wt % LiCIO,
r221. 3.2.2. Ionic conductivity of PebaxlLiClO, electrolytes The impedance plots for stainless steel/PebaxLiClO, /stainless steel system at different temperatures are shown in Fig. 3. Similar plots were recorded for different salt contents. A fairly welldefined semicircle suggests that the lithium salt added to the Pebax matrix complexes with the oxygen atom of the ethylene oxide blocks and the behavior of the complex under potential field is similar to that of poly(ethylene oxide)-Li complexes [241. The ionic conductivity as a function of salt concentration was calculated by measuring the resistance corresponding to the diameter of the semicircle in the impedance plots. As shown in Fig. 4, a maximum conductivity was obtained for electrolytes containing 13 wt% LiClO,. Similar behavior has
0.20,
000
1
0.05
0. IO
0.15
z / 1iPn
0.20
0.25
cm-~
Fig. 3. The complex plane impedance plots of stainless steel/ Pebax-LiClO,/stainless steel system at (a) 22, (0) 30 and (A) 40°C. Films with 20 wt% LiClO,.
Fig. 4. Dependence of ionic conductivity LiCIO, weight fraction at 22°C.
of Pebax electrolytes
on
been reported for different polymer electrolytes [11,16,17,19,26], and it has been assigned to two opposite effects: the increasing conductivity in the low salt concentration region might be attributed to the increase in the number of charge carriers. On the other hand, the conductivity decrease at higher salt concentrations may be due to a decrease in charge carrier mobility caused by the stiffening of the matrix chains with the ion complexation acting as crosslinking nodes. This stiffening effect was verified using differential scanning calorimetry, Fig. 5. It is interesting to note that Pebax has two glass transition temperatures and two endothermic peaks. A T, was detected near -53°C and T,,, near 15”C, and were assigned to the glass transition and fusion process of poly(ethylene oxide) blocks [27]. The T, near 40°C and the T, near 205”C, were assigned to the glass transition and fusion process of the polyamide 6 blocks [27]. These results suggest that both copolymer blocks are immiscible. Furthermore, adding LiClO,, an increase of Tg assigned to poly(ethylene oxide) was observed and its fusion process gradually disappeared. By considering that Li+ complexation acts as physical crosslinks, this behavior can be understood since it has been reported that an increase in the degree of crosslinking inhibits the crystalization [27]. The temperature dependence of the ionic conductivity of Pebax-LiClO, systems is shown in the Arrhenius plots, Fig. 6. Kobayashi et al. [28] have reported that in polymer-salt solid state ionic conductors such plots are not completely linear since near T, the first term in brackets in E!.q. (1) is not small
127
R.A. Zoppi et al. I Solid State Ionics 91 (1996) 123-130
w
::
h
20
I
I
30 I
I
I
0
-50
I
50
TEMPERATURE
I
100
(“Cl
Fig. 5. Differential scanning calorimetry of Pebax containing different LiClO, weight fractions. Insert: glass transition temperature of Pebax-LiCIO, as a function of salt weight fraction.
-4.5 7
-
-5.0
-
-5.5
‘” D -6.0
-
T-65
-
5
ml (+ = r,,
exp{ - [E + WI~E]IKT}
(2)
25
i 1
number of total ions, y is the correction factor for the overlapping of free volume, u* is the smallest volume required for ion hopping, u is the relative volume at temperature T, f,is the fraction of free volume at temperature T,, Aa is the difference between the heat expansion coefficient at T > T, and T
so that linear Arrhenius plots are obtained. The activation energies were calculated from the Arrhenius plots for different salt concentrations, Fig. 7. It is noted that the activation energy for ionic conductivity increases with increase of LiClO, content. This behavior can be understood since the mobility of the ethylene oxide blocks decreases (T, increases) when the salt content increases. A similar explanation was used by Kobayashi et al. [28] for polymethacryloyl-oligo-(oxyethylene) containing LiClO, or LiPF,. When compared to polymethacryloyl-oligo-(oxyethylene) with the same
60
-70
-
-7.5
-
-6.0
-
-6.5
-
1000/T
(K)
Fig. 6. Arrhenius plot, log (T versus T-‘, for Pebax-LiCIO, (A) 5, (w) 13, (a) 17 and (+) 20 wt% salt.
u=
~Oexp{-[yu*lv(
f,+h(T-
a
T,))
+ (E+ W/2E)/KT]}. u, is the pre-exponential
with
0.00
’ 0.05
2
’ 0.10 wt %
8
’ 0.15
’ 0.20
1
J 0.25
LiCI04
(1)
constant proportional
to the
Fig. 7. Activation LiClO, electrolytes
energy of the ionic conductivity as a function of the salt content.
in Pebax-
128
R.A. Zoppi et al. I Solid State Ionics 91 (1996) 123-130
LiCIO, content, Pebax-LiClO, electrolytes have a lower activation energy. This effect reflects the fact that the decrease of mobility adding LiClO, is less drastic in the Pebax-LiClO, systems and that Pebax is a better solvent for LiClO, than polymethacryloyloligo-(oxyethylene). 3.2.3. Morphological characterization Fig. 8a shows the conventional elastic image for a pure Pebax film with dark and spherical regions. The composition of these regions was investigated subtracting images formed by inelastic electrons with an energy loss AE =420 eV, right above the ionization edge of nitrogen (401 eV) and AE = 380 eV, below it. The resulting element specific image is shown in Fig. 9a. A local concentration of nitrogen was detected. These images confirm that n-butanol is a nonsolvent for the polyamide 6 blocks or any residual homopolyamide 6, which form micelles with an average diameter of 200 nm. Block segregation was also confirmed by the two T, observed in DSC experiments.
Fig. 8. Transmission electrons (AE=O).
electron microscopy
of Pebax films containing
For the polymer electrolytes investigated here independently of the LiCIO, weight fraction in the electrolytes dark spherical regions were also observed in Fig. 8b-d. By selecting inelastic electrons (energy loss AE = 220 eV right above the ionization edge of chlorine and at AE = 180 eV, below it), electron spectroscopic images for chlorine, Fig. 9bd, were obtained. There is a local concentration of chlorine showing that in polymer electrolytes a salt aggregation occurs. The size of these salt aggregates is also of the order of 200 nm. Despite these aggregations, the polymer electrolytes are transparent.
4. Conclusions Solid state polymer electrolytes were obtained using Pebax and LiClO,. The ionic conductivity increased with the salt concentration. At room temperature Pebax films with 13 wt% LiClO, had an ionic conductivity of ca. lop5 S cm-’ and at lOO”C,
(a) 0, (b) 5, (c) 15 and (d) 25 wt% LiCIO,. Images obtained with elastic
129
R.A. Zoppi et al. I Solid State tonics 91 (1996) 123-130
Fig. 9. Transmission electron microscopy of Pebax films containing (a) 0, (b) 5, (c) 15 and (d) 25 wt% LiCIO,. images selective for (a) nitrogen (AE=401 eV) and (b-d) chlorine (AE= 200 eV).
electrolytes preca. 10m4 S cm-‘. Pebax-LiClO, sented an electrochemical stability window of 4 V. For all LiClO, weight fraction investigated, transmission electron microscopy showed a salt aggregation in the polymer matrix. This aggregation, however, did not affect the film transparency which was quite good. The films were flexible and had the basic requirements for application in solid state electrochemical devices.
Acknowledgments The authors thank FAPESP
for financial
support.
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