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Ionic conductivity and electrochemical characterization of novel interpenetrating polymer network electrolytes
Solid State Ionics 147 (2002) 391 – 395 www.elsevier.com/locate/ssi
Ionic conductivity and electrochemical characterization of novel interpenetrating polymer network electrolytes Xinping Hou, Kok Siong Siow* Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore
Abstract We report here our investigation into the ionic conductivity and other electrochemical properties of this micro-phase separation type solid-state electrolyte (SPE). The novel polymer electrolyte has been obtained by swelling an interpenetrating polymer network (IPN) with liquid electrolyte solutions of inorganic lithium salts dissolved in a plasticiser or mixture of plasticizers such as ethylene carbonate (EC), propylene carbonate (PC), g-butyrolactone (g-BL) and dimethyl carbonate (DMC). The interpenetrating networks are prepared by sequential interpenetration of cross-linked methoxyoligo(oxyethylene)methacrylate (Cr-MOEnM, where n represents number of unit – CH2CH2O – ) and cross-linked poly(methylmethacrylate) (PMMA). The IPN electrolytes exhibit conductivities in the range of 4.510 4 to 1.410 3 S cm 1 at ambient temperature. Cyclic voltammetry of the IPN electrolytes on stainless steel electrode shows electrochemical stability windows of 5.0, 4.2 and 4.0 V vs. Li+/Li for IPN electrolytes with 1 M LiClO4/EC-DMC (1:1 by volume), 1 M LiBF4/g-BL and 1 M LiSO3CF3/EC-PC (1:1 by volume), respectively. The impedance of the Li/electrolyte interface for the IPN electrolyte with 1 M LiClO4/EC-DMC under open circuit conditions is found to increase rapidly over the first 30 h and then level off, in contrast to the case for the CrMOEnM network electrolyte (i.e., a network without PMMA) where the impedance increases continuously with time. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Polymer electrolytes; Interpenetrating polymer network (IPN); Phase separation; Ionic conductivity; Electrochemical stability
1. Introduction The goal of polymer electrolyte research has been to develop polymer electrolytes with high ionic conductivity, good mechanical properties and minimum or no deleterious electrode reactions. Inorganic fillers have been used to form composite polymer electrolytes in order to improve the electrochemical and mehanical properties of polymer electrolytes [1– 3]. This
*
Corresponding author. Tel.: +65-874-2923; fax: +65-7791691. E-mail address: [email protected] (K.S. Siow).
approach, though effective, is limited by the number of inorganic fillers that can be used. Polymer electrolytes based on polymer blends is another approach that has witted a great potential in overcoming the drawbacks of polymer electrolytes mentioned above [4,5]. Interpenetrating polymer network (IPN) is a special kind of polymer blend that swells but does not dissolve in solvents, and its creep and flow are suppressed [6]. An IPN may be made from (i) one polymer that can complex or hold lithium salts effectively and thus exhibits high ionic conductivity, and (ii) another polymer that is mechanically and electrochemically stable to lithium electrodes. Such an IPN system would, in principle, make an ideal polymer
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 0 3 4 - 6
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X. Hou, K.S. Siow / Solid State Ionics 147 (2002) 391–395
electrolyte that is furthermore able to accommodate the thermal and volumetric changes occasioned by cycling of the lithium anode. We have synthesized a micro-phase separation type solid-state electrolyte (SPE) based on interpenetrating polymer networks (IPNs) of cross-linked methoxyoligo(oxyethylene)methacrylate (abbreviated as MOEnM) and poly(methylmethacrylate) (abbreviated as PMMA) [7]. This is a special kind of polymer electrolyte, which lies between the chemically cross-linking gelled SPE and the porous SPE [8]. They can swell with more liquid electrolyte solution than the ordinary porous SPE because of the strong interaction of electrolyte solution and one of the interpenetrating components. The ionic conductivity and the electrochemical stability of the electrolytes as well as the interfacial stability of lithium electrode/IPN electrolytes interface have been investigated. The synthesis procedure is shown in Scheme 1. The dispersion of PMMA in Cr-MOEnM and cross-linking both inhibit the crystallization of oligo(oxyethylene) side chain, making the IPN a homogeneous macrostructure (like the gel SPE), physically preventing separation of the matrix polymer and the electrolyte solution. At the same time, it maintains sufficient osmosis pressure because of interaction of oligo(oxyethylene) side chain of Cr-MOEnM with
liquid electrolyte solution and the strong swelling capability of IPN (like the porous SPE) [8].
2. Experimental The preparation of IPN networks has been described before [7]. The IPN networks obtained were immersed in liquid electrolyte solutions and form the micro-phase separation electrolytes. The preparation of the electrolytes and the installation of cells were carried out in dry box. An EG&G Potentiostat/Galvanostat Model 273A coupled with a lock-in Amplifier 5210 was employed for all the electrochemical measurements.
3. Results and discussion 3.1. Ionic conductivity In this work, three kinds of IPN electrolytes were prepared by swelling the IPN of MOE12M/25 wt.% PMMA (abbreviated as IPN25) with lithium salt solutions of 1 M LiClO4/EC-DMC, 1 M LiBF4/g-BL and 1 M LiCF3SO3/EC-PC, respectively. The plots of ionic conductivity vs. reciprocal of temperature for
Scheme 1.
X. Hou, K.S. Siow / Solid State Ionics 147 (2002) 391–395
393
Fig. 1. Arrhenius plots of the IPN electrolytes with different electrolyte solutions.
the three IPN electrolytes are shown in Fig. 1. All the three plots in Fig. 1 show linear relationship, suggesting that the ionic conductive behaviour of the IPN electrolytes follows the Arrhenius equation. 3.2. Electrochemical stability The electrochemical stability of the IPN25 electrolytes was studied using cyclic voltammetry (CV). Fig. 2 shows the cyclic voltammograms obtained between 1.4 and 5.4 V or 2 and 6 V on stainless steel electrode vs. Li+/Li at a scan rate of 1 mV s 1. From the magnitude of the current responses, the decomposition voltages of the electrolytes on stainless steel electrodes are found to be 5.0, 4.2 and 4.0 V vs. Li+/Li for the IPN25 electrolytes with 1 M LiClO4/EC-DMC, 1 M LiBF4/g-BL and 1 M LiSO3CF3/EC-PC, respectively. The cathodic peaks located between 1.4 and 2.0 V for IPN25 electrolytes with 1 M LiBF4/g-BL and 1 M LiSO3CF3/EC-PC are probably due to the reduction of some kinds of decomposed products of the IPN25 electrolytes during the anodic scan past the decomposition voltages. The CV curves of the second scan for the IPN25 electrolyte with 1 M LiBF4/g-BL also shows two cathodic and two anodic peaks in the potential range of 1.4 to 5.4 V. No cathodic current
Fig. 2. Cyclic voltammograms for the IPN electrolytes on stainless steel working electrodes vs. Li+/Li. Potential scan rate: 1 mV s 1; stainless steel electrode surface: ca. 1.3 cm2. (a) IPN25 with 1 M LiClO4/EC-DMC; (b) IPN25 with 1 M LiBF4/g-BL; (c) IPN25 with 1 M LiSO3CF3/EC-PC.
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X. Hou, K.S. Siow / Solid State Ionics 147 (2002) 391–395
peak is observed when the anodic potential limit is maintained less positive than the decomposition potential, as shown in Fig. 3. The above results illustrate that all the three IPN25 electrolytes have good electrochemical stability windows enough for lithium bat-
Fig. 4. Time evolution of the resistance of Rb and Ri of symmetric lithium cells at room temperature: (a) Li/IPN25 with 1 M LiClO4/ EC-DMC/Li; (b) Li/IPN0 with 1 M LiClO4/EC-DMC/Li.
teries, and among them, the IPN25 electrolyte with 1 M LiClO4/EC-DMC is the best. 3.3. Interfacial stability of lithium/IPN electrolytes
Fig. 3. Cyclic voltammograms for the IPN25 electrolytes on the stainless steel working electrodes vs. Li+/Li between 1.5 and 4.0 V. Potential scan rate: 1 mV s 1; stainless steel electrode surface: ca. 1.3 cm2.
The stability of the lithium interface with the IPN25 electrolyte was studied by monitoring the impedance responses of a symmetrical Li/IPN25 electrolyte/Li cell for a period of 1 month at room temperature under open circuit conditions. On the basis of the solid polymer layer model developed by Thevenin and Muller [9], we got the bulk resistance (Rb) and the interface resistance (Ri) of the IPN electrolytes. The Ri values in Fig. 4(a) increase sharply from 20 to 1760 V during the first 32 h and then vary in a narrow range with storage time and, finally, level around 2.1 kV after storing for more than 100 h. Comparing with the Ri values in Fig. 4( b), which increase continuously with storage time, we can conclude that this is attributed to the stability
X. Hou, K.S. Siow / Solid State Ionics 147 (2002) 391–395
role of the PMMA component in the IPN25. PMMA can impart good adhesiveness to solid electrolytes while making them stable to atmosphere moisture [10,11]. In the case of our IPN25 electrolyte, PMMA helps to reduce the effects of moisture on the electrolyte and to retard the growth of passivation film between the electrolyte and the lithium electrode. Acknowledgements The authors are grateful to the National University of Singapore for a research grant for this work. References [1] F. Croce, B. Scrosati, S. Mariotto, Chem. Mater. 4 (1992) 1134.
395
[2] G.B. Appetecchi, G. Dautzenberg, B. Scrosati, J. Electrochem. Soc. 143 (1996) 6. [3] J. Hou, G.L. Baker, Chem. Mater. 10 (1998) 3311. [4] H.-J. Rhoo, H.-T. Kim, J.-K. Park, T.-S. Huang, Electrochim. Acta 42 (1997) 1571. [5] P. Chu, H. Jen, F. Lo, C. Lang, Macromolecules 32 (1999) 4738. [6] S.C. Kim, L.H. Sperling (Eds.), IPNs Around the World: Science and Engineering, Wiley, Chichester, 1997. [7] X. Hou, K. Siow, Polymer, in press. [8] K. Murata, S. Izuchi, Y. Yoshihisa, Electrochim. Acta 45 (2000) 1501. [9] J.G. Thevenin, R.H. Muller, J. Electrochem. Soc. 134 (1987) 273. [10] R. Mani, T. Mani, J.R. Stevens, J. Polym. Sci., Part A: Polym. Chem. 30 (1992) 2025 – 2031. [11] T. Mani, R. Mani, J.R. Stevens, Solid State Ionics 60 (1993) 113 – 117.
Ionic conductivity and electrochemical characterization of novel interpenetrating polymer network electrolytes Xinping Hou, Kok Siong Siow* Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore
Abstract We report here our investigation into the ionic conductivity and other electrochemical properties of this micro-phase separation type solid-state electrolyte (SPE). The novel polymer electrolyte has been obtained by swelling an interpenetrating polymer network (IPN) with liquid electrolyte solutions of inorganic lithium salts dissolved in a plasticiser or mixture of plasticizers such as ethylene carbonate (EC), propylene carbonate (PC), g-butyrolactone (g-BL) and dimethyl carbonate (DMC). The interpenetrating networks are prepared by sequential interpenetration of cross-linked methoxyoligo(oxyethylene)methacrylate (Cr-MOEnM, where n represents number of unit – CH2CH2O – ) and cross-linked poly(methylmethacrylate) (PMMA). The IPN electrolytes exhibit conductivities in the range of 4.510 4 to 1.410 3 S cm 1 at ambient temperature. Cyclic voltammetry of the IPN electrolytes on stainless steel electrode shows electrochemical stability windows of 5.0, 4.2 and 4.0 V vs. Li+/Li for IPN electrolytes with 1 M LiClO4/EC-DMC (1:1 by volume), 1 M LiBF4/g-BL and 1 M LiSO3CF3/EC-PC (1:1 by volume), respectively. The impedance of the Li/electrolyte interface for the IPN electrolyte with 1 M LiClO4/EC-DMC under open circuit conditions is found to increase rapidly over the first 30 h and then level off, in contrast to the case for the CrMOEnM network electrolyte (i.e., a network without PMMA) where the impedance increases continuously with time. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Polymer electrolytes; Interpenetrating polymer network (IPN); Phase separation; Ionic conductivity; Electrochemical stability
1. Introduction The goal of polymer electrolyte research has been to develop polymer electrolytes with high ionic conductivity, good mechanical properties and minimum or no deleterious electrode reactions. Inorganic fillers have been used to form composite polymer electrolytes in order to improve the electrochemical and mehanical properties of polymer electrolytes [1– 3]. This
*
Corresponding author. Tel.: +65-874-2923; fax: +65-7791691. E-mail address: [email protected] (K.S. Siow).
approach, though effective, is limited by the number of inorganic fillers that can be used. Polymer electrolytes based on polymer blends is another approach that has witted a great potential in overcoming the drawbacks of polymer electrolytes mentioned above [4,5]. Interpenetrating polymer network (IPN) is a special kind of polymer blend that swells but does not dissolve in solvents, and its creep and flow are suppressed [6]. An IPN may be made from (i) one polymer that can complex or hold lithium salts effectively and thus exhibits high ionic conductivity, and (ii) another polymer that is mechanically and electrochemically stable to lithium electrodes. Such an IPN system would, in principle, make an ideal polymer
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 0 3 4 - 6
392
X. Hou, K.S. Siow / Solid State Ionics 147 (2002) 391–395
electrolyte that is furthermore able to accommodate the thermal and volumetric changes occasioned by cycling of the lithium anode. We have synthesized a micro-phase separation type solid-state electrolyte (SPE) based on interpenetrating polymer networks (IPNs) of cross-linked methoxyoligo(oxyethylene)methacrylate (abbreviated as MOEnM) and poly(methylmethacrylate) (abbreviated as PMMA) [7]. This is a special kind of polymer electrolyte, which lies between the chemically cross-linking gelled SPE and the porous SPE [8]. They can swell with more liquid electrolyte solution than the ordinary porous SPE because of the strong interaction of electrolyte solution and one of the interpenetrating components. The ionic conductivity and the electrochemical stability of the electrolytes as well as the interfacial stability of lithium electrode/IPN electrolytes interface have been investigated. The synthesis procedure is shown in Scheme 1. The dispersion of PMMA in Cr-MOEnM and cross-linking both inhibit the crystallization of oligo(oxyethylene) side chain, making the IPN a homogeneous macrostructure (like the gel SPE), physically preventing separation of the matrix polymer and the electrolyte solution. At the same time, it maintains sufficient osmosis pressure because of interaction of oligo(oxyethylene) side chain of Cr-MOEnM with
liquid electrolyte solution and the strong swelling capability of IPN (like the porous SPE) [8].
2. Experimental The preparation of IPN networks has been described before [7]. The IPN networks obtained were immersed in liquid electrolyte solutions and form the micro-phase separation electrolytes. The preparation of the electrolytes and the installation of cells were carried out in dry box. An EG&G Potentiostat/Galvanostat Model 273A coupled with a lock-in Amplifier 5210 was employed for all the electrochemical measurements.
3. Results and discussion 3.1. Ionic conductivity In this work, three kinds of IPN electrolytes were prepared by swelling the IPN of MOE12M/25 wt.% PMMA (abbreviated as IPN25) with lithium salt solutions of 1 M LiClO4/EC-DMC, 1 M LiBF4/g-BL and 1 M LiCF3SO3/EC-PC, respectively. The plots of ionic conductivity vs. reciprocal of temperature for
Scheme 1.
X. Hou, K.S. Siow / Solid State Ionics 147 (2002) 391–395
393
Fig. 1. Arrhenius plots of the IPN electrolytes with different electrolyte solutions.
the three IPN electrolytes are shown in Fig. 1. All the three plots in Fig. 1 show linear relationship, suggesting that the ionic conductive behaviour of the IPN electrolytes follows the Arrhenius equation. 3.2. Electrochemical stability The electrochemical stability of the IPN25 electrolytes was studied using cyclic voltammetry (CV). Fig. 2 shows the cyclic voltammograms obtained between 1.4 and 5.4 V or 2 and 6 V on stainless steel electrode vs. Li+/Li at a scan rate of 1 mV s 1. From the magnitude of the current responses, the decomposition voltages of the electrolytes on stainless steel electrodes are found to be 5.0, 4.2 and 4.0 V vs. Li+/Li for the IPN25 electrolytes with 1 M LiClO4/EC-DMC, 1 M LiBF4/g-BL and 1 M LiSO3CF3/EC-PC, respectively. The cathodic peaks located between 1.4 and 2.0 V for IPN25 electrolytes with 1 M LiBF4/g-BL and 1 M LiSO3CF3/EC-PC are probably due to the reduction of some kinds of decomposed products of the IPN25 electrolytes during the anodic scan past the decomposition voltages. The CV curves of the second scan for the IPN25 electrolyte with 1 M LiBF4/g-BL also shows two cathodic and two anodic peaks in the potential range of 1.4 to 5.4 V. No cathodic current
Fig. 2. Cyclic voltammograms for the IPN electrolytes on stainless steel working electrodes vs. Li+/Li. Potential scan rate: 1 mV s 1; stainless steel electrode surface: ca. 1.3 cm2. (a) IPN25 with 1 M LiClO4/EC-DMC; (b) IPN25 with 1 M LiBF4/g-BL; (c) IPN25 with 1 M LiSO3CF3/EC-PC.
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X. Hou, K.S. Siow / Solid State Ionics 147 (2002) 391–395
peak is observed when the anodic potential limit is maintained less positive than the decomposition potential, as shown in Fig. 3. The above results illustrate that all the three IPN25 electrolytes have good electrochemical stability windows enough for lithium bat-
Fig. 4. Time evolution of the resistance of Rb and Ri of symmetric lithium cells at room temperature: (a) Li/IPN25 with 1 M LiClO4/ EC-DMC/Li; (b) Li/IPN0 with 1 M LiClO4/EC-DMC/Li.
teries, and among them, the IPN25 electrolyte with 1 M LiClO4/EC-DMC is the best. 3.3. Interfacial stability of lithium/IPN electrolytes
Fig. 3. Cyclic voltammograms for the IPN25 electrolytes on the stainless steel working electrodes vs. Li+/Li between 1.5 and 4.0 V. Potential scan rate: 1 mV s 1; stainless steel electrode surface: ca. 1.3 cm2.
The stability of the lithium interface with the IPN25 electrolyte was studied by monitoring the impedance responses of a symmetrical Li/IPN25 electrolyte/Li cell for a period of 1 month at room temperature under open circuit conditions. On the basis of the solid polymer layer model developed by Thevenin and Muller [9], we got the bulk resistance (Rb) and the interface resistance (Ri) of the IPN electrolytes. The Ri values in Fig. 4(a) increase sharply from 20 to 1760 V during the first 32 h and then vary in a narrow range with storage time and, finally, level around 2.1 kV after storing for more than 100 h. Comparing with the Ri values in Fig. 4( b), which increase continuously with storage time, we can conclude that this is attributed to the stability
X. Hou, K.S. Siow / Solid State Ionics 147 (2002) 391–395
role of the PMMA component in the IPN25. PMMA can impart good adhesiveness to solid electrolytes while making them stable to atmosphere moisture [10,11]. In the case of our IPN25 electrolyte, PMMA helps to reduce the effects of moisture on the electrolyte and to retard the growth of passivation film between the electrolyte and the lithium electrode. Acknowledgements The authors are grateful to the National University of Singapore for a research grant for this work. References [1] F. Croce, B. Scrosati, S. Mariotto, Chem. Mater. 4 (1992) 1134.
395
[2] G.B. Appetecchi, G. Dautzenberg, B. Scrosati, J. Electrochem. Soc. 143 (1996) 6. [3] J. Hou, G.L. Baker, Chem. Mater. 10 (1998) 3311. [4] H.-J. Rhoo, H.-T. Kim, J.-K. Park, T.-S. Huang, Electrochim. Acta 42 (1997) 1571. [5] P. Chu, H. Jen, F. Lo, C. Lang, Macromolecules 32 (1999) 4738. [6] S.C. Kim, L.H. Sperling (Eds.), IPNs Around the World: Science and Engineering, Wiley, Chichester, 1997. [7] X. Hou, K. Siow, Polymer, in press. [8] K. Murata, S. Izuchi, Y. Yoshihisa, Electrochim. Acta 45 (2000) 1501. [9] J.G. Thevenin, R.H. Muller, J. Electrochem. Soc. 134 (1987) 273. [10] R. Mani, T. Mani, J.R. Stevens, J. Polym. Sci., Part A: Polym. Chem. 30 (1992) 2025 – 2031. [11] T. Mani, R. Mani, J.R. Stevens, Solid State Ionics 60 (1993) 113 – 117.