ISSN 1023-1935, Russian Journal of Electrochemistry, 2009, Vol. 45, No. 1, pp. 101–107. © Pleiades Publishing, Ltd. 2009. Original Russian Text © O.V. Yarmolenko, Yu.V. Baskakova, G.Z. Tulibaeva, L.M. Bogdanova, E.A. Dzhavadyan, B.A. Komarov, N.F. Surkov, B.A. Rozenberg, O.N. Efimov, 2009, published in Elektrokhimiya, 2009, Vol. 45, No. 1, pp. 107–112.

Effect of Solvents on Properties of Polymer Gel-Electrolyte Based on Polyester Diacrylate O. V. Yarmolenkoz, Yu. V. Baskakova, G. Z. Tulibaeva, L. M. Bogdanova, E. A. Dzhavadyan, B. A. Komarov, N. F. Surkov, B. A. Rozenberg, and O. N. Efimov Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akad. Semenova 1, 142432 Chernogolovka, Moscow Region, Russia Received November 15, 2007

Abstract—New polymer gel electrolytes based on polyester diacrylates and LiClO4 salt solutions in organic solvents are developed for lithium ion and lithium polymer batteries with a high ionic conductivity up to 2.7 × 10–3 Ohm–1cm–1 at the room temperature. To choose the optimum liquid electrolyte composition, the dependence is studied of physico–chemical parameters of new gel electrolytes on the composition of the mixture of aprotic organic solvents: ethylene carbonate, propylene carbonate, and γ-butyrolacton. The bulk conductivity of gel electrolytes and exchange currents at the gel electrolyte/Li interface are studied using the electrochemical impedance method in symmetrical cells with two Li electrodes. The glass transition temperature and gel homogeneity are determined using the method of differential scanning calorimetry. It is found that the optimum mixture is that of propylene carbonate and γ-butyrolacton, in which a homogeneous polymer gel is formed in a wide temperature range of –150 to +50°C. Key words: polymer gel electrolyte, Li electrode, polyester diacrylate, Li electrode/gel electrolyte phase boundary DOI: 10.1134/S1023193509010145

INTRODUCTION The interest towards polymer electrolytes is primarily due to the problems of safety and improvement of the manufacturing procedure for lithium power sources [1–3]. Besides, reversibility of the electrode reaction at a lithium electrode remains a fundamental problem. Metallic lithium is easily corroded in the course of cycling in organic solvents, which hinders its practical application in lithium batteries, despite its obvious advantage due to high capacity (3860 A h/kg). The main problem consists in minimizing all side reactions of electrolyte decomposition and treeing at a Li electrode. Solid polymer electrolytes with the polymer/salt composition do not provide high ionic conductivity due to low ion mobility. A compromise solution consists in using polymer gel electrolytes (PGE) containing a liquid organic solvent. Due to this, additions can be introduced into PGE that are adsorbed at the electrode surface and decrease the rate of lithium electrode corrosion. Polymer gel electrolytes can be of two types: homogeneous gel and phase separated gel. In a homogeneous gel, the electrolyte solution is embedded into the polymer network at a molecular level, i.e., PGE is prepared in a single stage by cross-linking of an oligomer or a monomer in the liquid organic electrolyte medium. A z

Corresponding author: [email protected] (O.V. Yarmolenko).

phase separated gel includes a porous polymer frame and a solution of electrolyte that occupies all connected pores, i.e., first a polymer film is poured, the inert solvent is removed and then the porous polymer film is impregnated by liquid aprotic electrolyte. The main factor determining the PGE stability is the degree of affinity between the polymer matrix and organic solvent. This factor also affects the mechanical strength of a PGE film and its conductivity in a wide temperature range, especially in electrochemical cells with a thin–film electrolyte and high electrode surface area. In the case of low degree of affinity between the polymer and electrolyte, a microphase layering to predominantly polymer and solvent–enriched phases can occur in PGE, which, in its turn, can decrease the PGE bulk conductivity and decrease the exchange currents at a Li electrode. To increase the electrolyte electrochemical stability in the lithium power sources, highly polar easily freezing solvents are used (e.g., ethylene carbonate, Tm = 36°C). It can be assumed that the increase in the degree of affinity between the solvent and polymer will allow “mitigating” the problem of PGE resistance increase and conductivity decrease at the decrease of temperature, as the increase in the interaction of solvent molecules with the polymer will hinder the microphase layering of PGE at the temperature decrease. The comparison of the properties of PGE [4] containing polyvinylidene fluoride (PVDF), copolymer of

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polyvinylidene fluoride with hexafluoropropylene (PVDF–HFP), polyacrylonitrile (PAN) (Mw = 150000), and polymethylmethacrylate (PMMA) (Mw = 12000) as a polymer component allowed observing the following regularities. The rate of solvent loss is changed in the series of PMMA ≤ PAN
PVDF–HPP ≤ PVDF according to the decrease in the degree of affinity of the solvent towards the polymer and to the increase of the polymer crystallinity degree. The polymers with lower crystallinity degree feature better swelling and the solvent is uniformly dispersed in the polymer matrix. PGE based on new polyester diacrylates (PEDA) [5] that showed high ionic conductivity can be used for development of lithium–ion and lithium–polymer batteries. PEDA obtained by condensation of a mixture of the 2-hydroxyethylacrylate and diisocyanate anodic polymerization products contain up to 10% of crowntype macrocyclic compounds required, as shown earlier [5–9] to provide high ionic conductivity both within the polymer matrix and on its interface with the lithium electrode. A special feature of the structure of the PEDA main chain is alternation of ether and ester groups in each link. Besides, NHCO are additionally introduced into the main chain, which increases the polarity of the molecule and assists a stronger retention of electrolyte molecules within the pores of the polymer matrix. The presence of double bonds at the ends of the PEDA chain provides the possibility of obtaining a cross-linked polymer on its basis in the liquid electrolyte medium and gel formation. The aim of this work was to study the effect of the nature of solvents (ethylene carbonate (EC), propylene carbonate (PC), and γ-butyrolacton (GBL) on the physico-chemical properties of PGE based on PEDA for choosing the optimum composition of liquid electrolyte providing the formation of a uniform nonlayering gel. In the study, solvents with high dielectric constant and high boiling temperature were chosen, as the polymer gel in the electrolyte is formed by thermal solidification of PEDA and the low-boiling solvent can be evaporated in the course of the synthesis. EXPERIMENTAL The technology of polymer gel electrolyte preparation includes preparation of a PGE solution by mixing calculated amounts of initial components (liquid electrolyte, PEDA, polymerization initiating agent (2.2azo-bis-isobutironitrile (AIBN)), pouring the PGE solution onto a horizontal surface and thermal solidification in a film. Preparation of a Reactor for Polymer Gel Electrolyte Synthesis The reactor consists of two glasses with a Teflon leakproof seal with the thickness of 0.3–0.8 mm connected by special clamps. To prevent the polymer adhesion to the glass, thoroughly washed and degreased

glasses with the size of 100 × 150 mm were thrice treated by a 15% dimethylchlorosilane solution in toluene. To remove toluene, the glasses were washed by an ammonia solution in distilled water and baked in a drying oven at 200°ë for 6 h. Synthesis of Polymer Gel Electrolyte The PGE solution was poured into a glass reactor and thermal solidification was carried out at 60–80°ë at stepwise temperature increase for five hours. Herewith, thin uniform transparent PGE films were obtained with the thickness of 0.2–0.6 mm. The liquid electrolyte used was 1 M LiClO4 solution in organic solvents of different compositions: EC/PC (1 : 1 wt); EC/GBL (1 : 1 wt); PC/GBL (1 : 1 wt); GBL. The latter solvent was chosen for comparison as we had earlier studied it well [9, 10]. PEDA based on oligohydroxyethylacrylate (OHEA) and 4,4'-dicyclohexylmethanediisocyanate (PEDA*) was chosen for the formation of a polymer matrix in the liquid electrolyte medium. The method of PEDA* synthesis was described earlier [11]. PEDA*, basing on the synthesis of initial OHEA, contains up to 10 wt % of crown-type structures: 1,6-dioxo-14-crown-4 [12]: O C

O

O

O

O

C O

The earlier developed liquid chromatography method in exclusion and critical modes was used for characterization of PEDA* [12]. PEDA* can be polymerized on its terminal double bonds forming a crosslinked polymer in the presence of polymerization initiation agents. In the study, the chosen PEDA* had the following characteristics: ån = 1570, Mw = 2780, íg = −32°ë. The polymerization kinetics of the PEDA* solution in a liquid electrolyte with the composition of 20 wt % PEDA, 2 wt % AIBN as a thermal polymerization initiation agent, 78 wt % liquid electrolyte were studied using the isothermal calorimetry method at a DAK-1 calorimeter. The kinetic solidification curve of the PEDA* solution in a liquid electrolyte is presented in Fig. 1, where α is the conversion degree by double bonds of –ë=ë–. Thus, a complete oligomer cross-linking and its conversion to a polymer occurs at α = 1. The following solidification mode is chosen on the basis of kinetic data: 60–80°ë at a stepwise temperature increase for 5 h. A gel is formed in the course of thermal solidification, which is a cross-linked polymer matrix based on PEDA* with its micropores filled by liquid electrolyte.

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The compatibility of components in polymer gel electrolytes was determined for all four compositions in the temperature range of –150 to +50°C on the basis of the data obtained using the differential scanning calorimetry method at a DSC 822e Mettler-Toledo device with an embedded Star software at the temperature scanning rate of 5°C/min.

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α 1.0

5 4

0.8 3 0.6 2

Method of Differential Scanning Calorimetry Differential scanning calorimetry (DSC) is a method based on measuring the difference in heat fluxes from the studied sample and reference sample [13]. In the course of the experiment, a crucible with the sample and an empty crucible (reference) are cooled to –150°ë using liquid nitrogen. Then both crucibles are heated at a similar rate of 5°C/min. The instrument registers the difference between emitted heat amounts. The dependence of the rate of emitted heat on the temperature increase is plotted. The x-axis corresponds to temperature while the y-axis corresponds to the difference in the rates of heat emission by two heaters at a given temperature. The obtained information allows determining the character of the occurring processes and charactering the properties of the studied sample (liquid, polymer, gel). The plot obtained using DSC features certain differences between the first–order and second–order phase transitions. Crystallization and melting (first–order phase transition) are due to heat emission or absorption, so they are registered on a plot in the form of exo- or endothermal peaks. In the case of glass transition (second–order phase transition), the curve features no pit or spike, as the polymer does not emit or absorb hidden heat during glass transition. The only change observed is in the polymer heat capacity. This is due to the fact that the polymers have a higher thermal conductivity at the temperature above the glass transition (Tg), as compared to lower temperatures. The change does not occur immediately but is extended in a certain temperature range. This makes the choice of a given specific Tg value somewhat difficult. It is usually considered that Tg corresponds to the middle of the curving.

0.4

1

0.2

200

400

600 Time (min)

Fig. 1. Kinetic curves of PEDA* polymerization at 80°C in liquid electrolyte at different concentration of the initiation agent (AIBN, wt %): (1) 0.2; (2) 0.5; (3) 1.0; (4) 1.5; (5) 2.0; α is the conversion degree by double bonds.

mer electrolyte/electrode [5, 6] and polymer gel electrolyte/electrode interface [7–9], which was convincingly proved in [9], where convergence of experimental and calculated complex plane plots is shown. RESULTS AND DISCUSSION The method of thermal solidification was used to obtain new PGE based on PEDA* and liquid electrolyte of 1 M LiClO4 solution in organic solvents of the following composition: EC/PC (1 : 1 wt); EC/GBL (1 : 1 wt); PC/GBL (1 : 1 wt); GBL. Physico-chemical properties of new PGE are studied using the DSC and electrochemical impedance methods. Cdl

Electrochemical Studies Electrochemical impedance was measured in the frequency range from 12 to 105 Hz at the measured signal amplitude of 5–10 mV using a LCR819 Goodwill Instruments Ltd immitance meter. Symmetrical cells of two types were used: 1) with blocking electrodes of stainless steel (SS); 2) with reversible lithium electrodes. The measurement results were treated according to the model of electric double layer adsorption relaxation according to the ZView2 software. This model (Fig. 2) was first suggested by B.M. Grafov and E.A. Ukshe [14, 15] for the processes occurring at the electrode/solid electrolyte interface. It was further successfully used for calculation of the processes occurring at the polyRUSSIAN JOURNAL OF ELECTROCHEMISTRY

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Re

RF

CA

RA

ZW

Fig. 2. Equivalent circuit of the electrochemical cell with reversible electrodes, where Re is the bulk electrolyte resistance, RF is the charge transfer resistance, Cdl is the double layer capacitance, RÄ is the adsorption resistance, CÄ is the adsorption capacitance, ZW is the diffusion impedance. No. 1

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1 (‡)

crystallization (‡)

0 –0.5

Tg = –103°C

Tg = –114°C –1

crystallization

–1.0

melting

W, mW

W, mW

–2 –1.5 3

(b)

melting

–3 crystallization 3

(b)

0 0

Tg = –99°C crystallization

–3

–3

melting –6

–100

–50

0

Tg = –114°C

T,°C

melting –6

–100

–50

0

T,°C

Fig. 3. DSC diagrams: (a) for PGE based on 20 wt % PEDA and 1 M LiClO4 in EC/PC; (b) for liquid electrolyte 1 M LiClO4 in EC/PC; the y-axis is the rate of heat emission, the x-axis is the temperature in °C.

Fig. 4. DSC diagrams: (a) for a gel electrolyte based on 20 wt % PEDA* and 1 M LiClO4 in EC/GBL; (b) for 1 M LiClO4 liquid electrolyte in EC/GBL; the designations on the axes are similar to those in Fig. 3.

Naturally, PGE in the working temperature range must be uniform, i.e., no crystallization, melting, and layering processes must occur in it. Studies of the phase state of PGE and liquid electrolytes using the DSC method showed that all diagrams in the studied temperature range (–150 to +50°C) in all diagrams feature a temperature transition characterizing the glass transition temperature of PGE or liquid electrolyte in the range from –99 to –133°C. Besides, exothermal crystallization peaks and endothermal melting peaks of the mixture components in the negative temperature range are observed in the diagrams for PGE with liquid electrolyte based on the EC/PC (1 : 1 wt), EC/GBL (1 : 1 wt), GBL solvents (Fig. 3–5). This means that these liquid electrolytes and also gel electrolytes based on these solvents cannot be used in the low–temperature range.

A single temperature transition is observed only in the case of PGE with the liquid electrolyte with the composition of 1 M LiClO4 solution in a mixture of PC/GBL (1 : 1 wt). It has the form of a step (Fig. 6), which characterizes the glass transition temperature of the whole system (–117°ë) evidencing the compatibility of the polymer matrix based on PEDA with liquid electrolyte. Besides, no crystallization or melting of the mixture components are observed in this system in a wide temperature range (–100 to +50°C). On the basis of the obtained data, one can conclude that PGE with the composition of 20 wt % PEDA, 2 wt % AIBN as a thermal polymerization initiation agent, 78 wt % 1 M LiClO4 solution in the mixture of PC/GBL (1 : 1 wt) is optimal for potential use in the widest temperature range from –100 to +50°C.

Table 1. Bulk conductivity of PGE with the composition of 20 wt % PEDA*, 78 wt % 1 M LiClO4 in a mixture of organic solvents: 2 wt % 2-azo-bis-isobutironitrile at 20°C Solvents (1 : 1 wt)

Bulk conductivity, Ohm–1 cm–1

The impedance is measured for symmetrical electrochemical cells with the composition of NS/PGE/NS at the room temperature. The results of calculation of PGE bulk conductivity are presented in Table 1. As seen from Table 1, the highest bulk conductivity is typical for PGE based on GBL and PC/GBL mixture.

EC/PC EC/GBL PC/GBL GBL

1.6 × 10–3 1.0 × 10–3 2.5 × 10–3 2.7 × 10–3

The studies of the dependence of charge transfer resistance at the PGE/lithium electrode interface on the electrolyte composition and temperature are performed. Exchange currents at this interface and their activation energies are calculated.

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–1

0 crystallization Tg = –117°C

105 (‡)

(‡) Tg = –117°C –0.5

–2

W, mW

W, mW

melting –3 crystallization 5 T = –133°C g

–1.0 1

(b)

(b)

0

0 –1

–5

Tg = –119°C

–2

–10 melting

–15 –100

–50

–3 0

–100

T,°C

–50

0 T,°C

Fig. 5. DSC diagrams: (a) for a gel electrolyte based on 20 wt % PEDA* and 1 M LiClO4 in GBL; (b) for 1 M LiClO4 liquid electrolyte in GBL; the designations on the axes are similar to those in Fig. 3.

Fig. 6. DSC diagrams: (a) for a gel electrolyte based on 20 wt % PEDA* and 1 M LiClO4 in PC/GBL; (b) for 1 M LiClO4 liquid electrolyte in PC/GBL; the designations on the axes are similar to those in Fig. 3.

Impedance complex plane plots of the Li/PGE/Li cells for all four electrolyte compositions are similar. For example, Fig. 7 presents the characteristic dependence of the impedance complex plane plots on the temperature (from –18 to 40°C) for a cell with two reversible Li electrodes and PGE based on 1 M LiClO4 in PC/GBL.

which can be confirmed by the triple decrease of the activation energy of this process (Table 2). The stability constants of the forming Li+/crown-ether complex, same as that of any other complex grows at the temper-

Fig. 8 shows the exchange current dependence at the PGE/Li interface on the electrolyte composition and temperature. The activation energy of a reversible process Li+ Li+ + e is calculated at the PGE/Li interface. The calculation results are presented in Table 2. As seen from Fig. 8, the exchange currents at the PGE/Li interface at the temperatures below the room temperature (from –18 to 10°C) weakly depend both on the solvent composition and on the temperature. The character of these dependences is changed after heating, especially in the case of PGE based on 1 M LiClO4 in GBL. The absence of significant differences in the exchange currents at low temperatures for electrolytes of different compositions can be caused by adsorption of crown-type structures on the Li electrode. Crownethers affect significantly the lithium cation transfer at the lithium/polymer gel electrolyte interface, as we showed earlier [8–9]. They desolvate the lithium ion and thus promote the electrode reaction: Li+ + e Li, RUSSIAN JOURNAL OF ELECTROCHEMISTRY

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–ImZ, Ω

1 2 3 4 5

10000

5000

0

5000

10000

15000 ReZ, Ω

Fig. 7. Impedance complex plane plots of the Li/PGE/Li electrochemical cell for the electrolyte with the composition of 20 wt % PEDA* in 1 M LiClO4 in PC/GBL at the temperatures of, °C: (1) –18, (2) –5, (3) 10, (4) 20, (5) 40. No. 1

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ature decrease. Thus, as seen from Fig. 8, predominant effect on the exchange currents at the Li/gel electrolyte interface in the temperature range from –18 to 10°C is produced by complexation of crown-ether with the Li+ ion. The solvation shell type in this case produces the minimum effect. In the case of higher temperatures (10–40°C), the crown–type structures can be desorbed and the effect of the nature of the solvent on the electrochemical properties of PGE increases. As seen from Table 2, the highest exchange current activation energy at the Li/gel electrolyte interface is typical for γ-butyrolacton. Thus, one can conclude that this solvent forms the strongest solvate shell (Fig. 9) that hinders Li+ transport towards the metallic lithium surface.

i0 × 10 5, Ä/Òm2 15 1 2 3 10 4

5

0

–20

0

20

40 T, °C

Fig. 8. Exchange current dependence at the PGE/Li interface on the electrolyte composition and temperature. (1) EC/PC, (2) EC/GBL, (3) PC/GBL, (4) GBL.

0.122 nm

0.1995 nm Li+

Fig. 9. Structure of the solvate complex of Li+ with four molecules of γ-butyrolacton.

Table 2. Activation energy of the Li Li+ + e process for the PGE/Li interface in different solvents for different temperature ranges Solvents EC/PC EC/GBL PC/GBL GBL

Activation energy, eV –18–10°C

10–40°C

0.31 ± 0.04 0.30 ± 0.09 0.51 ± 0.08 0.59 ± 0.27

1.50 ± 0.34 1.42 ± 0.50 1.39 ± 0.38 1.87 ± 0.58

CONCLUSION In order to choose the optimum solvent composition for formation of a uniform gel electrolyte, the effect of the nature of solvents on the properties of polymer gel electrolytes based on new polyester diacrylates is studied. An advantage of the latter is that crown–type structures providing high conductivity are present within them from the outset, as implied by the synthesis method. New polymer gel electrolytes based on 1 M LiClO4 solution are obtained in the following solvents and their mixtures: ethylene carbonate, propylene carbonate, and γ-butyrolacton. The physico-chemical properties of the obtained thin-film polymer gel electrolytes are studied using the method of differential scanning calorimetry and electrochemical impedance method. It is found on the basis of the performed studies that the optimum composition for practical application in the synthesis of a polymer gel electrolyte is the following one: 20 wt % PEDA*, 2 wt % 2,2-azo-bisisobutironitrile as a PEDA* radical polymerization initiation, and 78 wt % 1 M LiClO4 solution in the mixture of PC/GBL (1 : 1 wt), resulting in the formation of a uniform gel electrolyte with a high bulk conductivity of about (1.0–3.5) × 10–3 Ohm–1 cm–1 in the temperature range from –18 to +40°ë. The authors are grateful to G.E. Estrina, G.S. Zaspinok, and V.V. Komratov for analysis of polyester diacrylate samples. ACKNOWLEDGMENTS The work was financially supported by the Russian Foundation for Basic Research (project no. 05-0850087, project no. 06-03-32520) and OKhNM RAS (program no. 8). REFERENCES 1. Skundin, A.M., Efimov, O.N., and Yarmolenko, O.V., Usp. Khim., 2002, vol. 71, p. 378.

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EFFECT OF SOLVENTS ON PROPERTIES OF POLYMER GEL-ELECTROLYTE 2. Kolosnitsyn, V.S., Dukhanin, G.P., Dumler, S.A., and Novakov, I.A., Zh. Prikl. Khim. (S.-Peterburg), 2005, vol. 78, p. 3 [Russ. J. Appl. Chem. (Engl. Transl.), vol. 78, p. 1]. 3. Song, J.Y., Wang, Y.Y., and Wan, C.C., J. Power Sources, 1999, vol. 77, p. 183. 4. Kim, C.S. and Oh, S.M., Electrochim. Acta, 2001, vol. 1323, p. 46. 5. Yarmolenko, O.V., Ukshe, A.E., Movchan, T.I., Efimov, O.N., and Zueva, A.F., Elektrokhimiya, 1995, vol. 31, p. 388. 6. Yarmolenko, O.V., Ukshe, A.E., Yakushchenko, I.K., Movchan, T.I., and Efimov, O.N., Elektrokhimiya, 1996, vol. 32, p. 508 [Russ. J. Electrochem. (Engl. Transl.), vol. 32, p. 468]. 7. Yarmolenko, O.V., Belov, D.G., and Efimov, O.N., Elektrokhimiya, 2001, vol. 37, p. 321 [Russ. J. Electrochem. (Engl. Transl.), vol. 37, p. 280]. 8. Yarmolenko, O.V. and Efimov, O.N., Elektrokhimiya, 2005, vol. 41, p. 646 [Russ. J. Electrochem. (Engl. Transl.), vol. 41, p. 568].

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9. Baskakova, Yu.V., Yarmolenko, O.V., Shuvalova, N.I., Tulibaeva, G.Z., and Efimov, O.N., Elektrokhimiya, 2006, vol. 42, p. 1055 [Russ. J. Electrochem. (Engl. Transl.), vol. 42, p. 949]. 10. Yarmolenko, O.V., Efimov, O.N., Obolonkova, E.S., Ponomarenko, A.T., Kotova, A.V., Matveeva, I.A., and Zapadinskii, B.I., Vysokomolek. soed., Ser. A., 2004, vol. 46, p. 1292. 11. Rozenberg, B.A., Bogdanova, L.M., Boiko, G.N., Gur’eva, L.L., Dzhavadyan, E.A., Surkov, N.F., Estrina, G.A., and Estrin, Ya.I., Vysokomolek. soed., Ser. A., 2005, vol. 47, p. 952. 12. Estrina, G.A., Komarov, B.A., Estrin, Ya.I., and Rozenberg, B.A., Vysokomolek. soed., Ser. A., 2004, vol. 46, p. 207. 13. Entsiklopediya polimerov (Encyclopaedia of Polymers), Moscow: Sov. entsiklopediya, 1977, p. 928. 14. Grafov, B.M. and Ukshe, E.A., Elektrokhimiya, 1974, vol. 10, p. 1875. 15. Ukshe, E.A. and Bukun, N.G., Tverdye elektrolity (Solid Electrolytes), Moscow: Nauka, 1977.

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