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Preparation and electrochemical characterization of the alkaline polymer gel electrolyte polymerized from acrylic acid and KOH solution
Electrochimica Acta 49 (2004) 2533–2539
Preparation and electrochemical characterization of the alkaline polymer gel electrolyte polymerized from acrylic acid and KOH solution Xiaoming Zhu a,b , Hanxi Yang a,∗ , Yuliang Cao a , Xingping Ai a b
a Department of Chemistry, Wuhan University, Wuhan 430072, China Department of Chemistry, College of Xianning, Xianning 437002, China
Received 13 November 2003; received in revised form 19 December 2003; accepted 8 February 2004
Abstract An alkaline polymer gel electrolyte (PGE) film was prepared by solution polymerization of acrylate–KOH–H2 O at room temperature, and the preparation conditions were optimized in view of the mechanical properties and ionic conductivity of the film. The PGE film with the optimized composition of 0.02% K2 S2 O8 , 16.75% acrylic acid and 83.23 wt.% 4 mol l−1 KOH solution is transparent, rubber-like and dimensionally stable with improved mechanical properties as compared with gelled electrolyte. The specific conductivity of the film is 0.288 s cm−1 at room temperature and the conductivity values follow the Arrhenius equation with the activation energy of ∼10 kJ mol−1 . These data suggest that the ionic conduction proceeds in the same mechanism as in aqueous alkaline solution. Experimental results from the laboratory Zn/Air, Zn/MnO2 and Ni/Cd cells using the PGE film as electrolyte demonstrate that the PGE film has almost the same chemical and electrochemical stability as aqueous alkaline solution, and shows good performance characteristics for application of alkaline primary and secondary battery systems. © 2004 Elsevier Ltd. All rights reserved. Keywords: Alkaline batteries; Ionic conductivity; Polymer gel electrolyte; Poly(acrylate–H2 O–KOH); Solution polymerization
1. Introduction Aqueous alkaline batteries, such as Zn/MnO2 , Ni/Cd, and Ni/MH, etc., have widespread applications in today’s electronic world and possess a dominant share in consumer market, especially in portable electronics. These batteries have to be tightly sealed in order to prevent from the leakage of corrosive alkaline solution, and this often brings additional difficulties in geometrical design and minimization of these batteries. During past decades, great efforts have been contributed to develop solid polymer electrolytes instead of aqueous alkaline solutions so as to improve the safety, reliability and processibility of these batteries. Earlier developments of alkaline solid polymer electrolyte were mainly focused on the polymer–salt complexes formed by dissolving inorganic hydroxides in suitable polymer matrixes such as PEO [1,2]. This type of solution-free electrolytes can be built into a thin film with sufficient mechanical strength and display some ionic conductivity, typically in the range of ∗
Corresponding author. Tel.: +86-27-873526; fax: +86-27-87884476. E-mail address: [email protected] (H. Yang).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.02.008
10−5 to 10−8 s cm−1 , at room temperature, but these conductivities are too low, far from the requirements for battery applications. In recent years, an alternative approach to enhance the ionic conductivity is extensively employed by use of so-called polymer gel electrolytes (PGE) in which aqueous alkaline electrolyte is encapsulated in polymeric matrix. This type of electrolytes are usually prepared by directly dissolving the polymer with high concentrations of polar solvating groups in aqueous alkaline solution and then by solvent-casting the gelled solution into an electrolyte film [3–5]. Although the PGE film can achieve very high ionic conductivity of 0.5 to 10−3 s cm−1 at room temperature, their mechanical properties are usually inadequate for practical use. The poor mechanical strength of the alkaline PGE has been well recognized to arise from the incompatibility of polymeric matrix and alkaline solution [6]. Some research studies have attempted to solve this problem by changing the polymer host, alkali salt, and their compositions [3,6,7]. Since the polymers used in previous studies were commercial products with well-defined molecular weights and cross-linking densities, it is difficult to adjust the degrees of
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polymerization and cross-linkage to improve the mechanical strength of the PGE film. In this paper, we report a simple method to prepare the dimensionally stable film of aqueous alkaline PGE by solution polymerization of acrylic acid in concentrated alkaline solution, and discuss the influences of polymerization conditions on the conductivity and mechanical properties of the PGE films. In addition, electrochemical characteristics of the PGE film used for alkaline batteries are also described.
ductance electrode (DJS-1 type, Shanghai, China). The standard conductance electrode consists of a pair of 0.5 cm2 Pt electrodes separated parallel with 0.5 cm space. In order to ensure the perfect contact between the Pt electrodes and alkaline polymer gel electrolyte, the conductance electrode was dipped into the mixed AA–KOH–H2 O solution and then polymerized together into a gelled lump after initiating polymerization. The resistance between the two Pt electrodes was determined by ac impedance technique, at the controlled temperature of 25 ◦ C, using a CHI660A electrochemical analysis systems (Shanghai, China).
2. Experimental 2.3. Electrochemical measurements 2.1. Preparation and characterization Acrylic acid (AA), N,N methylene-bisacrylamide (MBA), KOH, and K2 S2 O8 used in the work were all commercial reagents of analytical grade purchased from Shanghai Pharmaceutical (Group) Corporation. All the reagents were used as received without further treatment. The alkaline PGE film was prepared by casting and initiating polymerization of the mixed solution of acrylic acid and potassium hydroxide with addition of MBA as a cross-linker and K2 S2 O8 as a polymerization initiator. A typical procedure was firstly to dissolve 50 mg MBA into 11.02 g (10.6 ml) acrylic acid in a 100 ml beaker and add dropwise 50 ml 8.4 mol l−1 KOH solution (prior cooled to 0 ◦ C) into the beaker at continuous stirring, and then add 0.4 g 4 wt.% K2 S2 O8 solution into the beaker. Stirring for about 1 min, the solution was poured out onto a Teflon plate to spread out into a liquid layer of about 0.5 mm thickness. The plate was placed for 2 h at room temperature for initiating polymerization. After then, a transparent and elastic PGE film of polyacrylate–KOH–H2 O was formed, which can be easily peeled off for experimental use. The thickness of the finished PGE film was almost the same as the liquid film before polymerization and the composition of the electrolyte film was calculated from the mass balance. The extent of polymerization was characterized by Infrared reflection- absorption spectroscopy on a Nexus 670-FT-IR (Japan) IR spectrometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to determine the “free water” content of the PGE film. To distinguish the evaporation temperatures of the free water and bound water, a slow scan rate of 1.25 ◦ C min−1 was used in the measurements. TGA and DSC analysis were also carried out in N2 atmosphere with flow rate of 100 ml min−1 . X-ray diffractometry was employed to characterize the phase structure of the PGE film, and performed on a Shimadzu lab XRD diffractometer with Cu K␣ and at scan rate of 5 ◦ min−1 . 2.2. Conductance measurements The conductivity of alkaline PGE was determined by measuring the resistance of the PGE using a commercial con-
The feasibility and electrochemical characteristics of the PGE film as electrolyte in alkaline batteries were examined using simulated Zn/MnO2 , Zn/air, and Ni/Cd cells. The laboratory Zn/MnO2 and Zn/air cells were consisted of a piece of the PGE film sandwiched by a Zn anode and a MnO2 or air cathode. The Zn anode was prepared by casting the Zn slurry (Zn powder:CMC binder = 99:1, wt.%) on Cu net, then frozen at –20 ◦ C for 30 min and finally dried under vacuum. The MnO2 cathode was fabricated in the same as the Zn anode except for use of electrolytic MnO2 powder as electroactive material and Ni net as current accumulator. The preparation of air cathode was in the similar way as described previously [8,9]. The experimental Ni/Cd cells were constructed using a sintered nickel cathode sandwiched by two pieces of Cd anodes. Two pieces of the PGE films were placed in-between the cathode and anode operating as both a separator and an ionic conductor. The positive Ni electrodes and negative Cd electrodes were provided by 3J Battery Co. (China) and activated prior to the experiments.
3. Results and discussion 3.1. Polymerization conditions The polymerization reaction of polyacrylate has been well studied and discussed in detail in previous monographs and papers [10–12]. It is now well accepted that the reaction proceeds mainly through a free radical polymerization, as illustrated in Fig. 1. In the solution polymerization of poly(acrylate–KOH–H2 O), the polymerization conditions affects the conductivity and mechanical properties of the PGE film very differently. In general, the mechanical strength of a polymer is proportional to its glass-transition temperature Tg and the Tg value is primarily dependent on the polymer’s molecular ¯ n . A usual way to increase a polymer’s mechaniweightM cal strength is to increase the polymer’s molecular weight. In the case of poly(acrylate–KOH–H2 O), an increase in the molecular weight of polyacrylate can be realized by increasing the concentration of monomer (AA) or by
X. Zhu et al. / Electrochimica Acta 49 (2004) 2533–2539
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Fig. 1. A schematic representation of the polymerization process of polyacrylate.
decreasing the concentrations of initiator K2 S2 O8 and aqueous KOH solvent. However, the polymerization reaction of poly(acrylate–KOH–H2 O) is highly exothermic and the large excessive heat often leads to a violent self-accelerating polymerization if the concentration of monomer (AA) is higher than 25 wt.% and the concentration of aqueous KOH solution is lower than 75 wt.%. In addition, although the lower content of initiator K2 S2 O8 helps to increase the ¯ n value in such a way to increase the glass-transition temM perature Tg so as to improve the mechanical strength of the PGE film, this also lowers the polymerization rate at the initiator content less than 0.01 wt.%. In order to make the GPE film with better mechanical properties, we tried to adjust the concentration ratio of monomer (AA) and KOH solution in the region of 1/10 to 1/3 and found that an appropriate composition of the acrylate–KOH–H2 O solution for the polymerization reaction was 0.02 wt.% K2 S2 O8 , 16.75 wt.% acrylic acid, and 83.23 wt.% aqueous KOH solution. On the other hand, the cross-linking density is also an important parameter in determining the mechanical properties of a polymer gel, which can be adjusted by use of cross-linker. In general, a gelled polymer with higher cross-linking density has better mechanical strength. However, in this work we found that the polymer gel of poly(acrylate–KOH–H2 O) became to a brittle and wax-like solid and lost its flexibility when the cross-linking density
is higher than 1%. In order to get a good compromise between the mechanical strength and flexibility for the PGE film, the content of the cross-linker (MBA) used in this work was about 0.0625 wt.%. In such polymerization conditions, the polymerization reaction of poly(acrylate–KOH–H2 O) can proceed quite mildly at room temperature and finish in 1 h. The PGE film so prepared shows as a rubber-like and dimensionally stable membrane with certain mechanical strength. 3.2. Structural characterization Fig. 2 compares the infrared spectrum of the PGE film with commercial potassium polyacrylate powder. It is clear from the figure that the basic IR features for both materials are almost the same except for observation of an additional IR band at about 2100 cm−1 from the PGE film, due to the IR absorption of contaminant CO2 , which was absorbed in the alkaline PGE film at the experimental exposure in air. This similarity in the IR spectra of the PGE film and polyacrylate suggests that the polymeric network in the PGE film has the same chemical structure as polyacrylate. Besides, the characteristic IR peaks of acrylic acid at 1630 cm−1 (C=C bond), 999 cm−1 (CH2 + CH), 917 cm−1 (O–H), and 1310 cm−1 (C=C=O) as well as other bands all disappeared, the relative intensities and shapes of the main
Fig. 2. A comparison of the IR spectra of the PGE film (a) with commercial potassium polyacrylate powder (b).
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Fig. 3. XRD patterns of the PGE film (a) and commercial potassium polyacrylate powder (b).
IR bands at 1652, 1548, and 1404 cm−1 for the PGE film are all in good agreement with the IR features of polyacrylate, demonstrating a complete polymerization occurring in the acrylate–KOH–H2 O system. Fig. 3 compares the XRD diffractograms of the PGE film and the commercial potassium polyacrylate powder. Both of the samples did not show any characteristic diffraction peak, representing typical amorphous polymeric structure. The failure to observe the XRD signals from the polymeric chains and KOH strongly indicates that the electrolyte is fully solvated in the PGE film, which also lowers the degree of crystalline polymer. For an aqueous polymer gel, water molecules exist in the polymer matrix in three distinctly different states: free water, intermediate water and bound water [13]. Only free water can influence the solvation of KOH and contribute to ionic conduction. Fig. 4 shows the TGA and DSC curves of the PGE film. In the DSC curve, a huge endothermic band is observed below 100 ◦ C, and this band can only be attributed to the evaporation of free water. Correspondingly, a heavy weight loss appears in the TGA curve at the tem-
Fig. 4. TGA (a) and DSC (b) curves of the PGE film.
Fig. 5. Changes in the conductivities of the PGE films and aqueous KOH solutions as a function of KOH concentration at 25 ◦ C.
perature region, which is approximately 31.35 wt.% of the PGE film. Calculated from the weight percent of the starting materials, the free water is about 63 wt.% of total water content in the PGE film. 3.3. Dependences of the conductivity on the polymerization conditions Fig. 5 shows the conductivities of the PGE films polymerized from the alkaline solutions of different KOH concentrations and compared with those of aqueous KOH solution. It can be seen that both of the curves exhibited a parabolic shape with their maximum conductivities at 4–6 mol l−1 . In comparison, the maximum conductivity of the PGE film is 0.288 s cm−1 , about 60% of the maximum value (0.476 s cm−1 ) of KOH solution. Seemingly, the free water of only 30 wt.% content in the PGE film cannot account for this very high conductivity. However, if considering that the total water content is 63 wt.% in the PGE film and all the water molecules including intermediate water and bound water can exchange and participate in ionic conduction, the observed high conductivity of the PGE film is understandable. In a very recent study of alkaline polymer electrolyte, the PGE film based on poly(acrylate–Me4 NOH·5H2 O) was reported to exhibit a very high conductivity of 102 s cm−1 [6]. An interesting phenomenon in the Fig. 5 is the conductivity changes in the region of low KOH concentrations. At the concentrations of KOH lower than 1 mol l−1 , the PGE films exhibit higher conductivities than aqueous KOH solutions. Even polymerized in neutral electrolyte solution (i.e. the solution of acrylic acid was neutralized in the same molar ratio to become a solution of pH 7), the PGE films still shows a considerably high conductivity of 0.1 s cm−1 . The possible explanation for this phenomenon is that the PGE film can provide a larger amount of ions at low KOH concentration due to the dissociation of the coordinated K+ cations on the polyacrylate chains.
X. Zhu et al. / Electrochimica Acta 49 (2004) 2533–2539
Fig. 6. Conductivities of the PGE films polymerized in 4 mol l−1 KOH solutions at 25 ◦ C with different concentrations of monomer (potassium acrylate).
Fig. 6 shows the conductivities of the PGE films prepared from 4 mol l−1 KOH solutions with the monomer concentrations ranging from 1.8 to 3.0 mol l−1 . As mentioned above, higher concentrations of the monomer would cause thermal runaway of the polymerization reaction and lower concentrations lead to a poor mechanical strength of the PGE film. In the range of monomer concentrations, the conductivity of the PGE film decreases with the increase of monomer concentration. This is apparently due to the decrease in the weight fraction of free water in the polymer gel when the monomer concentration is increased, which reduces the dissociation degree of polymer–salt complex correspondingly, and then decreases the ionic conduction. The effects of the concentrations of initiator and cross-linker on the conductivity of the PGE film are given in Fig. 7. In general, the cross-linking density of a polymer gel is directly proportional to the concentration of cross-linker,
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Fig. 8. Plots of log σ vs. T−1 of the PGE film (a) and aqueous KOH solution (b).
while the polymericular weight is inversely proportional to the concentration of initiator. Since the cross-link density and polymericular weight are the determining factors for the segmental motions of a polymer matrix, Fig. 7 actually reflects the influences of the segmental motions of the polymer chains on the conductivity of the PGE film. In Fig. 7, the conductivity of the PGE film remains almost unchanged at the value of 0.29 s cm−1 with increasing concentrations of cross-linker and initiator, suggesting that the ionic conduction is contributed predominately by the liquid phase and the ionic migration brought about by the segmental motion of the polymer chains is negligible in the PGE containing concentrated alkaline electrolyte. In order to understand the ionic conduction mechanism in the PGE film, we measured the conductivity–temperature relationship of the PGE film and compared with aqueous KOH solution. Fig. 8 shows the temperature dependences of the conductivity of the PGE film and aqueous KOH solution. It can be seen from the figure that the logarithmic conductivity (log σ) of these two types of electrolytes decrease linearly with T−1 , showing a very good consistency with the Arrhenius equation (σ = σ0 exp(Ea /RT)). In addition, both of the log σ–T−1 plots in Fig. 8 are in parallel and have the same slops, implying that the ionic conductions in both types of electrolytes have the same activation energy (∼10 kJ mol−1 ), or in other words, proceed in the same mechanism. 3.4. Examples for battery applications
Fig. 7. Effects of the concentrations of initiator and cross-linker on the conductivity of the PGE film.
For battery applications, it requires the electrolyte not only to have sufficient ionic conductivity but also to have chemical and electrochemical stability. Usually the chemical and electrochemical stability of the electrolyte can be evaluated by voltammetry. Fig. 9 compares the cyclic voltammograms of a glassy carbon electrode using both the PGE film and aqueous KOH solution as electrolyte. It is obvious that both types of the electrolytes exhibited almost the same electro-
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Fig. 9. Cyclic voltammograms of a glassy carbon electrode on the PGE film and in aqueous KOH solution.
chemical windows. Except for the anodic oxidation of OH− ions and the cathodic reduction of H2 O at the very positive and negative potential regions, there were not any additional current signals observed for the polymer matrix. This suggests a possible use of the PGE film to replace the aqueous electrolyte in alkaline batteries. The discharge performances of the simulated Zn/MnO2 cells are shown in Fig. 10. The main features of these discharge profiles are all similar to those of aqueous alkaline Zn/MnO2 cells [14]. Calculated from the discharge capacities of MnO2 cathode, the utilization of the cathodic material reached to over 90% at 0.5 C rate and still attained to 75% even at high rate of 2 C. Nevertheless, no negative influences were observed for the Zn/MnO2 cells using the PGE film. These discharge behaviors demonstrate the feasibility of the PGE film for construction of solid alkaline Zn/MnO2 batteries. In order to test the applicability of the PGE film in other alkaline battery systems, we also used the PGE film to prepare Zn/air cells and Ni/Cd cells. The discharge results of the simulated Zn/air cells are shown
Fig. 10. Discharge curves of the simulated Zn/MnO2 cells using the PGE film as electrolyte at the current density of: (a) 0.5 C, (b) 1 C, and (c) 2 C.
Fig. 11. Discharge curves of the simulated Zn/air cells using the PGE film as electrolyte at the current density of: (a) 6.25 mA/cm2 , (b) 12.5 mA/cm2 , and (c) 25 mA/cm2 .
in Fig. 11. It is seen that the discharge curves exhibit the typical characteristics of alkaline Zn/air cells in the discharge plateau and rate capability, and the data are analogous to those reported for aqueous Zn/air cells [15]. The discharge capacity of zinc anode reached to 530 mA h g−1 , corresponding to a utilization of 65%. Fig. 12 compares the charge–discharge curves of the Ni/Cd cells using both of the PGE film and alkaline solution at the same experimental conditions. The two types of the cells exhibited very similar charge–discharge capacity and voltage profile, and the cell using the PGE film even showed slightly higher discharge voltage and lower charging voltage. However, it should be mentioned that if the cells were overcharged, the gaseous evolution at the electrode surfaces can destroy the electrode/PGE film interface and therefore cause a rapid decay in the cell performances. Thus, it needs great attention to prevent from the overcharge in the use of the PGE film for secondary batteries.
Fig. 12. A comparison of the charge–discharge curves of the simulated Ni/Cd cells using the PGE film as electrolyte (a), and using aqueous KOH solution (b).
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4. Conclusions
References
An alkaline polymer gel electrolyte (PGE) film was prepared by solution polymerization of acrylate–KOH–H2 O system, and the preparation conditions affecting the mechanical properties and ionic conductivity of the PGE film were described. It is revealed that the PGE film with the composition of 0.02% K2 S2 O8 , 16.75% acrylic acid, and 83.23 wt.% 4 mol l−1 KOH solution have the ionic conductivity of 0.288 s cm−1 and a good mechanical property adequate for applications in construction of alkaline solid state batteries. Experimental results from the laboratory Zn/Air, Zn/MnO2 and Ni/Cd cells using the PGE film as electrolyte demonstrate that the PGE film has almost the same chemical and electrochemical stability as aqueous alkaline solution, and shows indiscernible impact on the performance characteristics of the alkaline battery systems.
[1] J.F. Fauvarque, S. Guinot, N. Bouziu, E. Salmon, J.F. Penneau, Electrochim. Acta 40 (1995) 2449. [2] S. Guinot, E. Salmon, J.F. Penneau, J.F. Fauvarque, Electrochim. Acta 43 (1998) 1163. [3] A. Lewandowski, K. Skorupska, J. Malinska, Solid State Ionics 113 (2000) 265. [4] C. Yang, J. Power Sources 109 (2002) 22. [5] N. Vassal, E. Salmon, J.-F. Fauvarque, J. Electrochem. Soc. 146 (1999) 20. [6] J. Sun, D.R. MacFarlane, M. Forsyth, Electrochim. Acta 48 (2003) 1971. [7] N. Vassal, E. Salmon, J.-F. Fauvarque, Electrochim. Acta 45 (2000) 1527. [8] S. Muller, F. Holzer, H. Arai, O. Haas, J. New Mater. Electrochem. Syst. 2 (1999) 227. [9] S. Muller, K. Striebel, O. Haas, Electrochim. Acta 39 (1994) 1661. [10] J. Brandrup, E.H. Immergut, Polymer Handbook, John Wiley and Sons, New York, 1989. [11] P. Rempp, W.E. Merrill, Polymer and Synthesis, Huthig and Wepf, Basel, 1991. [12] D.S. Concetta, G. Antonio, P. Daniela, S. Silvio, React. Funct. Polym. 55 (2003) 9. [13] K.D. Yao, W.G. liu, J. Liu, J. Appl. Polym. Sci. 71 (1999) 449. [14] M. Yuji, A. Junichi, T. Hitoshi, J. Power Sources 63 (1996) 305. [15] C. Chakkaravarthy, A.K.A. Waheed, H.V.K. Udupa, J. Power Sources 6 (1981) 203.
Acknowledgements The authors gratefully acknowledge the financial support by the 973 Program, China (Grant No. 2002CB211800) and the Education Department of Hubei Province, China (Grant No. 2002B04002).
Preparation and electrochemical characterization of the alkaline polymer gel electrolyte polymerized from acrylic acid and KOH solution Xiaoming Zhu a,b , Hanxi Yang a,∗ , Yuliang Cao a , Xingping Ai a b
a Department of Chemistry, Wuhan University, Wuhan 430072, China Department of Chemistry, College of Xianning, Xianning 437002, China
Received 13 November 2003; received in revised form 19 December 2003; accepted 8 February 2004
Abstract An alkaline polymer gel electrolyte (PGE) film was prepared by solution polymerization of acrylate–KOH–H2 O at room temperature, and the preparation conditions were optimized in view of the mechanical properties and ionic conductivity of the film. The PGE film with the optimized composition of 0.02% K2 S2 O8 , 16.75% acrylic acid and 83.23 wt.% 4 mol l−1 KOH solution is transparent, rubber-like and dimensionally stable with improved mechanical properties as compared with gelled electrolyte. The specific conductivity of the film is 0.288 s cm−1 at room temperature and the conductivity values follow the Arrhenius equation with the activation energy of ∼10 kJ mol−1 . These data suggest that the ionic conduction proceeds in the same mechanism as in aqueous alkaline solution. Experimental results from the laboratory Zn/Air, Zn/MnO2 and Ni/Cd cells using the PGE film as electrolyte demonstrate that the PGE film has almost the same chemical and electrochemical stability as aqueous alkaline solution, and shows good performance characteristics for application of alkaline primary and secondary battery systems. © 2004 Elsevier Ltd. All rights reserved. Keywords: Alkaline batteries; Ionic conductivity; Polymer gel electrolyte; Poly(acrylate–H2 O–KOH); Solution polymerization
1. Introduction Aqueous alkaline batteries, such as Zn/MnO2 , Ni/Cd, and Ni/MH, etc., have widespread applications in today’s electronic world and possess a dominant share in consumer market, especially in portable electronics. These batteries have to be tightly sealed in order to prevent from the leakage of corrosive alkaline solution, and this often brings additional difficulties in geometrical design and minimization of these batteries. During past decades, great efforts have been contributed to develop solid polymer electrolytes instead of aqueous alkaline solutions so as to improve the safety, reliability and processibility of these batteries. Earlier developments of alkaline solid polymer electrolyte were mainly focused on the polymer–salt complexes formed by dissolving inorganic hydroxides in suitable polymer matrixes such as PEO [1,2]. This type of solution-free electrolytes can be built into a thin film with sufficient mechanical strength and display some ionic conductivity, typically in the range of ∗
Corresponding author. Tel.: +86-27-873526; fax: +86-27-87884476. E-mail address: [email protected] (H. Yang).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.02.008
10−5 to 10−8 s cm−1 , at room temperature, but these conductivities are too low, far from the requirements for battery applications. In recent years, an alternative approach to enhance the ionic conductivity is extensively employed by use of so-called polymer gel electrolytes (PGE) in which aqueous alkaline electrolyte is encapsulated in polymeric matrix. This type of electrolytes are usually prepared by directly dissolving the polymer with high concentrations of polar solvating groups in aqueous alkaline solution and then by solvent-casting the gelled solution into an electrolyte film [3–5]. Although the PGE film can achieve very high ionic conductivity of 0.5 to 10−3 s cm−1 at room temperature, their mechanical properties are usually inadequate for practical use. The poor mechanical strength of the alkaline PGE has been well recognized to arise from the incompatibility of polymeric matrix and alkaline solution [6]. Some research studies have attempted to solve this problem by changing the polymer host, alkali salt, and their compositions [3,6,7]. Since the polymers used in previous studies were commercial products with well-defined molecular weights and cross-linking densities, it is difficult to adjust the degrees of
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polymerization and cross-linkage to improve the mechanical strength of the PGE film. In this paper, we report a simple method to prepare the dimensionally stable film of aqueous alkaline PGE by solution polymerization of acrylic acid in concentrated alkaline solution, and discuss the influences of polymerization conditions on the conductivity and mechanical properties of the PGE films. In addition, electrochemical characteristics of the PGE film used for alkaline batteries are also described.
ductance electrode (DJS-1 type, Shanghai, China). The standard conductance electrode consists of a pair of 0.5 cm2 Pt electrodes separated parallel with 0.5 cm space. In order to ensure the perfect contact between the Pt electrodes and alkaline polymer gel electrolyte, the conductance electrode was dipped into the mixed AA–KOH–H2 O solution and then polymerized together into a gelled lump after initiating polymerization. The resistance between the two Pt electrodes was determined by ac impedance technique, at the controlled temperature of 25 ◦ C, using a CHI660A electrochemical analysis systems (Shanghai, China).
2. Experimental 2.3. Electrochemical measurements 2.1. Preparation and characterization Acrylic acid (AA), N,N methylene-bisacrylamide (MBA), KOH, and K2 S2 O8 used in the work were all commercial reagents of analytical grade purchased from Shanghai Pharmaceutical (Group) Corporation. All the reagents were used as received without further treatment. The alkaline PGE film was prepared by casting and initiating polymerization of the mixed solution of acrylic acid and potassium hydroxide with addition of MBA as a cross-linker and K2 S2 O8 as a polymerization initiator. A typical procedure was firstly to dissolve 50 mg MBA into 11.02 g (10.6 ml) acrylic acid in a 100 ml beaker and add dropwise 50 ml 8.4 mol l−1 KOH solution (prior cooled to 0 ◦ C) into the beaker at continuous stirring, and then add 0.4 g 4 wt.% K2 S2 O8 solution into the beaker. Stirring for about 1 min, the solution was poured out onto a Teflon plate to spread out into a liquid layer of about 0.5 mm thickness. The plate was placed for 2 h at room temperature for initiating polymerization. After then, a transparent and elastic PGE film of polyacrylate–KOH–H2 O was formed, which can be easily peeled off for experimental use. The thickness of the finished PGE film was almost the same as the liquid film before polymerization and the composition of the electrolyte film was calculated from the mass balance. The extent of polymerization was characterized by Infrared reflection- absorption spectroscopy on a Nexus 670-FT-IR (Japan) IR spectrometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to determine the “free water” content of the PGE film. To distinguish the evaporation temperatures of the free water and bound water, a slow scan rate of 1.25 ◦ C min−1 was used in the measurements. TGA and DSC analysis were also carried out in N2 atmosphere with flow rate of 100 ml min−1 . X-ray diffractometry was employed to characterize the phase structure of the PGE film, and performed on a Shimadzu lab XRD diffractometer with Cu K␣ and at scan rate of 5 ◦ min−1 . 2.2. Conductance measurements The conductivity of alkaline PGE was determined by measuring the resistance of the PGE using a commercial con-
The feasibility and electrochemical characteristics of the PGE film as electrolyte in alkaline batteries were examined using simulated Zn/MnO2 , Zn/air, and Ni/Cd cells. The laboratory Zn/MnO2 and Zn/air cells were consisted of a piece of the PGE film sandwiched by a Zn anode and a MnO2 or air cathode. The Zn anode was prepared by casting the Zn slurry (Zn powder:CMC binder = 99:1, wt.%) on Cu net, then frozen at –20 ◦ C for 30 min and finally dried under vacuum. The MnO2 cathode was fabricated in the same as the Zn anode except for use of electrolytic MnO2 powder as electroactive material and Ni net as current accumulator. The preparation of air cathode was in the similar way as described previously [8,9]. The experimental Ni/Cd cells were constructed using a sintered nickel cathode sandwiched by two pieces of Cd anodes. Two pieces of the PGE films were placed in-between the cathode and anode operating as both a separator and an ionic conductor. The positive Ni electrodes and negative Cd electrodes were provided by 3J Battery Co. (China) and activated prior to the experiments.
3. Results and discussion 3.1. Polymerization conditions The polymerization reaction of polyacrylate has been well studied and discussed in detail in previous monographs and papers [10–12]. It is now well accepted that the reaction proceeds mainly through a free radical polymerization, as illustrated in Fig. 1. In the solution polymerization of poly(acrylate–KOH–H2 O), the polymerization conditions affects the conductivity and mechanical properties of the PGE film very differently. In general, the mechanical strength of a polymer is proportional to its glass-transition temperature Tg and the Tg value is primarily dependent on the polymer’s molecular ¯ n . A usual way to increase a polymer’s mechaniweightM cal strength is to increase the polymer’s molecular weight. In the case of poly(acrylate–KOH–H2 O), an increase in the molecular weight of polyacrylate can be realized by increasing the concentration of monomer (AA) or by
X. Zhu et al. / Electrochimica Acta 49 (2004) 2533–2539
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Fig. 1. A schematic representation of the polymerization process of polyacrylate.
decreasing the concentrations of initiator K2 S2 O8 and aqueous KOH solvent. However, the polymerization reaction of poly(acrylate–KOH–H2 O) is highly exothermic and the large excessive heat often leads to a violent self-accelerating polymerization if the concentration of monomer (AA) is higher than 25 wt.% and the concentration of aqueous KOH solution is lower than 75 wt.%. In addition, although the lower content of initiator K2 S2 O8 helps to increase the ¯ n value in such a way to increase the glass-transition temM perature Tg so as to improve the mechanical strength of the PGE film, this also lowers the polymerization rate at the initiator content less than 0.01 wt.%. In order to make the GPE film with better mechanical properties, we tried to adjust the concentration ratio of monomer (AA) and KOH solution in the region of 1/10 to 1/3 and found that an appropriate composition of the acrylate–KOH–H2 O solution for the polymerization reaction was 0.02 wt.% K2 S2 O8 , 16.75 wt.% acrylic acid, and 83.23 wt.% aqueous KOH solution. On the other hand, the cross-linking density is also an important parameter in determining the mechanical properties of a polymer gel, which can be adjusted by use of cross-linker. In general, a gelled polymer with higher cross-linking density has better mechanical strength. However, in this work we found that the polymer gel of poly(acrylate–KOH–H2 O) became to a brittle and wax-like solid and lost its flexibility when the cross-linking density
is higher than 1%. In order to get a good compromise between the mechanical strength and flexibility for the PGE film, the content of the cross-linker (MBA) used in this work was about 0.0625 wt.%. In such polymerization conditions, the polymerization reaction of poly(acrylate–KOH–H2 O) can proceed quite mildly at room temperature and finish in 1 h. The PGE film so prepared shows as a rubber-like and dimensionally stable membrane with certain mechanical strength. 3.2. Structural characterization Fig. 2 compares the infrared spectrum of the PGE film with commercial potassium polyacrylate powder. It is clear from the figure that the basic IR features for both materials are almost the same except for observation of an additional IR band at about 2100 cm−1 from the PGE film, due to the IR absorption of contaminant CO2 , which was absorbed in the alkaline PGE film at the experimental exposure in air. This similarity in the IR spectra of the PGE film and polyacrylate suggests that the polymeric network in the PGE film has the same chemical structure as polyacrylate. Besides, the characteristic IR peaks of acrylic acid at 1630 cm−1 (C=C bond), 999 cm−1 (CH2 + CH), 917 cm−1 (O–H), and 1310 cm−1 (C=C=O) as well as other bands all disappeared, the relative intensities and shapes of the main
Fig. 2. A comparison of the IR spectra of the PGE film (a) with commercial potassium polyacrylate powder (b).
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Fig. 3. XRD patterns of the PGE film (a) and commercial potassium polyacrylate powder (b).
IR bands at 1652, 1548, and 1404 cm−1 for the PGE film are all in good agreement with the IR features of polyacrylate, demonstrating a complete polymerization occurring in the acrylate–KOH–H2 O system. Fig. 3 compares the XRD diffractograms of the PGE film and the commercial potassium polyacrylate powder. Both of the samples did not show any characteristic diffraction peak, representing typical amorphous polymeric structure. The failure to observe the XRD signals from the polymeric chains and KOH strongly indicates that the electrolyte is fully solvated in the PGE film, which also lowers the degree of crystalline polymer. For an aqueous polymer gel, water molecules exist in the polymer matrix in three distinctly different states: free water, intermediate water and bound water [13]. Only free water can influence the solvation of KOH and contribute to ionic conduction. Fig. 4 shows the TGA and DSC curves of the PGE film. In the DSC curve, a huge endothermic band is observed below 100 ◦ C, and this band can only be attributed to the evaporation of free water. Correspondingly, a heavy weight loss appears in the TGA curve at the tem-
Fig. 4. TGA (a) and DSC (b) curves of the PGE film.
Fig. 5. Changes in the conductivities of the PGE films and aqueous KOH solutions as a function of KOH concentration at 25 ◦ C.
perature region, which is approximately 31.35 wt.% of the PGE film. Calculated from the weight percent of the starting materials, the free water is about 63 wt.% of total water content in the PGE film. 3.3. Dependences of the conductivity on the polymerization conditions Fig. 5 shows the conductivities of the PGE films polymerized from the alkaline solutions of different KOH concentrations and compared with those of aqueous KOH solution. It can be seen that both of the curves exhibited a parabolic shape with their maximum conductivities at 4–6 mol l−1 . In comparison, the maximum conductivity of the PGE film is 0.288 s cm−1 , about 60% of the maximum value (0.476 s cm−1 ) of KOH solution. Seemingly, the free water of only 30 wt.% content in the PGE film cannot account for this very high conductivity. However, if considering that the total water content is 63 wt.% in the PGE film and all the water molecules including intermediate water and bound water can exchange and participate in ionic conduction, the observed high conductivity of the PGE film is understandable. In a very recent study of alkaline polymer electrolyte, the PGE film based on poly(acrylate–Me4 NOH·5H2 O) was reported to exhibit a very high conductivity of 102 s cm−1 [6]. An interesting phenomenon in the Fig. 5 is the conductivity changes in the region of low KOH concentrations. At the concentrations of KOH lower than 1 mol l−1 , the PGE films exhibit higher conductivities than aqueous KOH solutions. Even polymerized in neutral electrolyte solution (i.e. the solution of acrylic acid was neutralized in the same molar ratio to become a solution of pH 7), the PGE films still shows a considerably high conductivity of 0.1 s cm−1 . The possible explanation for this phenomenon is that the PGE film can provide a larger amount of ions at low KOH concentration due to the dissociation of the coordinated K+ cations on the polyacrylate chains.
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Fig. 6. Conductivities of the PGE films polymerized in 4 mol l−1 KOH solutions at 25 ◦ C with different concentrations of monomer (potassium acrylate).
Fig. 6 shows the conductivities of the PGE films prepared from 4 mol l−1 KOH solutions with the monomer concentrations ranging from 1.8 to 3.0 mol l−1 . As mentioned above, higher concentrations of the monomer would cause thermal runaway of the polymerization reaction and lower concentrations lead to a poor mechanical strength of the PGE film. In the range of monomer concentrations, the conductivity of the PGE film decreases with the increase of monomer concentration. This is apparently due to the decrease in the weight fraction of free water in the polymer gel when the monomer concentration is increased, which reduces the dissociation degree of polymer–salt complex correspondingly, and then decreases the ionic conduction. The effects of the concentrations of initiator and cross-linker on the conductivity of the PGE film are given in Fig. 7. In general, the cross-linking density of a polymer gel is directly proportional to the concentration of cross-linker,
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Fig. 8. Plots of log σ vs. T−1 of the PGE film (a) and aqueous KOH solution (b).
while the polymericular weight is inversely proportional to the concentration of initiator. Since the cross-link density and polymericular weight are the determining factors for the segmental motions of a polymer matrix, Fig. 7 actually reflects the influences of the segmental motions of the polymer chains on the conductivity of the PGE film. In Fig. 7, the conductivity of the PGE film remains almost unchanged at the value of 0.29 s cm−1 with increasing concentrations of cross-linker and initiator, suggesting that the ionic conduction is contributed predominately by the liquid phase and the ionic migration brought about by the segmental motion of the polymer chains is negligible in the PGE containing concentrated alkaline electrolyte. In order to understand the ionic conduction mechanism in the PGE film, we measured the conductivity–temperature relationship of the PGE film and compared with aqueous KOH solution. Fig. 8 shows the temperature dependences of the conductivity of the PGE film and aqueous KOH solution. It can be seen from the figure that the logarithmic conductivity (log σ) of these two types of electrolytes decrease linearly with T−1 , showing a very good consistency with the Arrhenius equation (σ = σ0 exp(Ea /RT)). In addition, both of the log σ–T−1 plots in Fig. 8 are in parallel and have the same slops, implying that the ionic conductions in both types of electrolytes have the same activation energy (∼10 kJ mol−1 ), or in other words, proceed in the same mechanism. 3.4. Examples for battery applications
Fig. 7. Effects of the concentrations of initiator and cross-linker on the conductivity of the PGE film.
For battery applications, it requires the electrolyte not only to have sufficient ionic conductivity but also to have chemical and electrochemical stability. Usually the chemical and electrochemical stability of the electrolyte can be evaluated by voltammetry. Fig. 9 compares the cyclic voltammograms of a glassy carbon electrode using both the PGE film and aqueous KOH solution as electrolyte. It is obvious that both types of the electrolytes exhibited almost the same electro-
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Fig. 9. Cyclic voltammograms of a glassy carbon electrode on the PGE film and in aqueous KOH solution.
chemical windows. Except for the anodic oxidation of OH− ions and the cathodic reduction of H2 O at the very positive and negative potential regions, there were not any additional current signals observed for the polymer matrix. This suggests a possible use of the PGE film to replace the aqueous electrolyte in alkaline batteries. The discharge performances of the simulated Zn/MnO2 cells are shown in Fig. 10. The main features of these discharge profiles are all similar to those of aqueous alkaline Zn/MnO2 cells [14]. Calculated from the discharge capacities of MnO2 cathode, the utilization of the cathodic material reached to over 90% at 0.5 C rate and still attained to 75% even at high rate of 2 C. Nevertheless, no negative influences were observed for the Zn/MnO2 cells using the PGE film. These discharge behaviors demonstrate the feasibility of the PGE film for construction of solid alkaline Zn/MnO2 batteries. In order to test the applicability of the PGE film in other alkaline battery systems, we also used the PGE film to prepare Zn/air cells and Ni/Cd cells. The discharge results of the simulated Zn/air cells are shown
Fig. 10. Discharge curves of the simulated Zn/MnO2 cells using the PGE film as electrolyte at the current density of: (a) 0.5 C, (b) 1 C, and (c) 2 C.
Fig. 11. Discharge curves of the simulated Zn/air cells using the PGE film as electrolyte at the current density of: (a) 6.25 mA/cm2 , (b) 12.5 mA/cm2 , and (c) 25 mA/cm2 .
in Fig. 11. It is seen that the discharge curves exhibit the typical characteristics of alkaline Zn/air cells in the discharge plateau and rate capability, and the data are analogous to those reported for aqueous Zn/air cells [15]. The discharge capacity of zinc anode reached to 530 mA h g−1 , corresponding to a utilization of 65%. Fig. 12 compares the charge–discharge curves of the Ni/Cd cells using both of the PGE film and alkaline solution at the same experimental conditions. The two types of the cells exhibited very similar charge–discharge capacity and voltage profile, and the cell using the PGE film even showed slightly higher discharge voltage and lower charging voltage. However, it should be mentioned that if the cells were overcharged, the gaseous evolution at the electrode surfaces can destroy the electrode/PGE film interface and therefore cause a rapid decay in the cell performances. Thus, it needs great attention to prevent from the overcharge in the use of the PGE film for secondary batteries.
Fig. 12. A comparison of the charge–discharge curves of the simulated Ni/Cd cells using the PGE film as electrolyte (a), and using aqueous KOH solution (b).
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4. Conclusions
References
An alkaline polymer gel electrolyte (PGE) film was prepared by solution polymerization of acrylate–KOH–H2 O system, and the preparation conditions affecting the mechanical properties and ionic conductivity of the PGE film were described. It is revealed that the PGE film with the composition of 0.02% K2 S2 O8 , 16.75% acrylic acid, and 83.23 wt.% 4 mol l−1 KOH solution have the ionic conductivity of 0.288 s cm−1 and a good mechanical property adequate for applications in construction of alkaline solid state batteries. Experimental results from the laboratory Zn/Air, Zn/MnO2 and Ni/Cd cells using the PGE film as electrolyte demonstrate that the PGE film has almost the same chemical and electrochemical stability as aqueous alkaline solution, and shows indiscernible impact on the performance characteristics of the alkaline battery systems.
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Acknowledgements The authors gratefully acknowledge the financial support by the 973 Program, China (Grant No. 2002CB211800) and the Education Department of Hubei Province, China (Grant No. 2002B04002).