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A novel type of magnesium ion conducting polymer electrolyte
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Electrochimica Acta, Vol. 43, Nos 10±11, pp. 1253±1256, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0013±4686/98 $19.00 + 0.00 S0013-4686(97)10026-3
A novel type of magnesium ion conducting polymer electrolyte C. Liebenow Ernst-Moritz-Arndt-UniversitaÈt, Institut fuÈr Anorganische Chemie, Soldtmannstr. 16, D-17487 Greifswald, Germany (Received 16 September 1996; accepted 16 April 1997) AbstractÐPolymer electrolytes consisting of ethyl magnesium bromide solved in poly(ethylene oxide) have been prepared, containing about one molecule of tetrahydrofuran or dibutylether per magnesium ion. Using these gel-like electrolytes, magnesium ion reduction and reoxidation is demonstrated, and the dependence of conductivity on temperature and composition of the electrolyte system is studied. Best results were obtained with a magnesium content of 5±6 wt.% magnesium, corresponding to an ethylene oxide unit to magnesium ratio of 4:1. At 508C, these electrolyte systems show conductivities in the range of 10ÿ5±10ÿ4 S cmÿ1. Enhanced conductivity is found for systems with a higher content of monomer ether per magnesium. # 1998 Elsevier Science Ltd. All rights reserved Key words: magnesium, polymer electrolyte, Grignard reagent.
1. INTRODUCTION Although magnesium is abundant in nature and has a low equivalent weight, its application as anode material is problematic through diculties concerning the passivation layer on its surface, and the lack of appropriate nonaqueous magnesium ion conducting electrolytes. Polymer electrolyte systems based on solvate-free magnesium salts such as MgCl2 or Mg(ClO4)2 dissolved in poly(ethylene oxide) have already been investigated by some authors. But these systems show a very low electrical conductivity at temperatures below 1008C and appear to be purely anion conductors[1±4]. This can be explained by the nonlabile bond formed between magnesium ions and the oxygen of the polymer ether[5]. It is known from liquid systems, that solutions of magnesium salts in organic solvents are not usable for magnesium deposition or dissolution. The electrode reaction in these systems is inhibited due to the strong tendency for the surface of magnesium to become covered by a dense passivation layer. In comparison, organic electrolytes containing Grignard reagents have been shown to be suitable for electrochemical magnesium deposition from aprotic solvents[6]. It has been found that magnesium deposited on suitable substrate materials
like silver or gold could be almost completely reoxidized[7]. The aim of this work was to investigate polymer electrolytes which were prepared by dissolving Grignard reagent in a polymeric ether.
2. EXPERIMENTAL Ethyl magnesium bromide (EtMgBr, 1.2 molar) solutions were prepared by the Grignard reaction in carefully dried tetrahydrofuran (THF) or di-n-butylether (DBE). For the preparation of the polymer electrolytes, poly(ethylene oxide) (PEO, SERVA, M = 200 000) was mixed with dierent volumes of EtMgBr solution. The solvent was evaporated under vacuum at 608C (THF) and 708C (DBE), respectively. Solid or gel-like systems of the composition EtMgBr-P(EO)n-xTHF or EtMgBr-P(EO)n-xDBE were thus obtained. The magnesium and bromide contents were determined by complexometric and argentometric titration, respectively. Impedances were measured in the frequency range from 100 Hz to 100 kHz using a Solartron FRA 1255 connected with a ECI 1286. The samples were sandwiched between gold electrodes. A distance holder was used to prevent further compression of the samples during measurements. The conductivities were calculated
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C. Liebenow
Fig. 1. Cyclic voltammogram of the polymer electrolyte EtMgBr-P(EO)4-2,5THF. Working electrode: Ni, counter electrode: Mg, 258C.
from the electrolyte resistances which were taken from the impedance diagrams. Activation energies were determined from Arrhenius plots of the conductivity in the temperature range of 208C to 608C. Cyclic voltammograms were recorded at sweep rates of 5 mV sÿ1 using a two-electrode cell containing a nickel or silver working electrode and a magnesium counterelectrode. The electrolyte resistance was used for the ir correction. All operations were carried out in a dry argon atmosphere.
Fig. 2. Dependence of the conductivity of the system EtMgBr-P(EO)n-DBE on: (a) the magnesium content, (b) the ethyleme oxide:magnesium ratio.
Fig. 3. Dependence of the conductivity of the system EtMgBr-P(EO)4-DBE on temperature.
Fig. 4. Dependence of the conductivity of the system EtMgBr-P(EO)n-THF on: (a) the magnesium content, (b) the ethylene oxide:magnesium ratio.
Magnesium ion conducting polymer
Fig. 5. Dependence of the conductivity of the systems EtMgBr-P(EO)6.6-4THF and EtMgBr-P(EO)10-0.5THF on temperature.
3. RESULTS AND DISCUSSION The consistency of the polymer electrolytes depends on the ethyl magnesium bromide content. As the solvent could not completely be removed from the polymer electrolyte systems, it in¯uences their mechanical properties considerably. About one solvent molecule per magnesium was found after the evaporation process. Solid materials were obtained at magnesium concentrations below 4 wt.%. Their mechanical properties correspond to those of solid polymeric materials such as LiXPEO electrolytes. Up to 5±8 wt.%, a plastic rubbery material was obtained (gel electrolytes). At higher magnesium concentrations, there is not enough polymer materials to bond all the Grignard compound. The handling of these pastelike materials is more dicult as they react violently with moisture. Figure 1 shows a cyclic voltammogram of the EtMgBr-P(EO)4-2,5THF electrolyte on a nickel electrode. A sample with higher THF content was here chosen to achieve higher conductivity at room temperature. A nearly reversible magnesium deposition and dissolution is demonstrated. This electrochemical activity was found for electrolytes containing more than 3 wt.% magnesium. As this cyclic voltammogram was measured using a two electrode cell, the current is plotted versus the voltage E between the nickel working electrode and the magnesium reference electrode. So it gives no
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exact information about the working electrode potential. As clear evidence of electrochemical magnesium reduction, visible amounts of magnesium were deposited with low current densities (I = 10ÿ5±10ÿ4 A/cm2) over longer periods on silver, nickel or gold electrodes. The cell voltages during these deposition experiments corresponded to the voltage for the reduction process which was observed in the cyclic voltammograms. The dependence of the conductivity on the magnesium content for the DBE-containing systems is shown in Fig. 2(a). The optimal magnesium content of 5.9 wt.% corresponds to a ratio of ethylene oxide to magnesium of 4:1 (Fig. 2(b)). A typical Arrhenius plot measured on these systems is shown in Fig. 3. Straight-line plots were found at temperatures below 608C, corresponding to activation energies in the range 20±35 kJ/mol. Similar results were obtained with polymer electrolytes prepared using THF solution of the Grignard reagent. Here the maximum of conductivity was found at 6 or 7 wt.% (Fig. 4(a)). This corresponds to an ethylene oxide:magnesium ratio of 4:1 (Fig. 4(b)). For this gel-like system an activation energy of 50 kJ molÿ1 was found. The conductivities of electrolytes prepared from THF solutions are a little higher than those prepared from DBE-solutions. The residual monomer ether molecules are supposed to be rather tightly bonded to the magnesium, so it is understandable that the very compact THF molecules hinder the conducting process to a lower extent than the DBE molecules. Higher solvent concentration helps to attain higher conductivities. As seen in Fig. 5 the Arrhenius plot for solvent-rich systems is non-linear. These materials change in the course of experiments as the solvent molecules are partly split o at increasing temperatures. The mechanism of conductivity as well as the nature of the electrochemically active particles are still unknown. The composition of alkyl magnesium halide solutions consists of dierent charged or uncharged MgxRy Brz compounds, as described by the Schlenk equilibrium[8]. The position of the equilibrium depends on concentration, temperature and solvent type. Further investigation on this point will thus be necessary. The presence of Grignard compounds in the polymer electrolytes also plays an important role in the electrode processes as it activates the passivated magnesium surface and allows the electrochemical deposition and oxidation of magnesium.
ACKNOWLEDGEMENTS This work was supported Forschungsgemeinschaft.
by
Deutsche
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C. Liebenow REFERENCES
1. L. L. Yang, A. R. McGhie and G. C. Farrington, J. Electrochem. Soc. 133, 1380 (1986). 2. G. C. Farrington and R. G. Linford, in Polymer Electrolyte Reviews 2. (Edited by J. R MacCallum and C. A. Vincent). Elsevier Applied Science, London p. 271 (1989).
3. A. Patrick, M. Glasse and R. Linford, Solid State Ionics 18/19, 1063 (1986). 4. J. Y. Cheng, M. Z. A. Munshi, B. B. Owens and W. H. Smyrl, Solid State Ionics 28/30, 857 (1987). 5. C. A. Vincent, Electrochim. Acta 40, 2035 (1995). 6. D. M. Overcash and F. C. Mathers, Trans. Electrochem. Soc. 64, 305 (1933). 7. C. Liebenow, J. Appl. Electrochem. 27, 221 (1997). 8. E. C. Ashby, Quart. Rev. 21, 259 (1967).
Electrochimica Acta, Vol. 43, Nos 10±11, pp. 1253±1256, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0013±4686/98 $19.00 + 0.00 S0013-4686(97)10026-3
A novel type of magnesium ion conducting polymer electrolyte C. Liebenow Ernst-Moritz-Arndt-UniversitaÈt, Institut fuÈr Anorganische Chemie, Soldtmannstr. 16, D-17487 Greifswald, Germany (Received 16 September 1996; accepted 16 April 1997) AbstractÐPolymer electrolytes consisting of ethyl magnesium bromide solved in poly(ethylene oxide) have been prepared, containing about one molecule of tetrahydrofuran or dibutylether per magnesium ion. Using these gel-like electrolytes, magnesium ion reduction and reoxidation is demonstrated, and the dependence of conductivity on temperature and composition of the electrolyte system is studied. Best results were obtained with a magnesium content of 5±6 wt.% magnesium, corresponding to an ethylene oxide unit to magnesium ratio of 4:1. At 508C, these electrolyte systems show conductivities in the range of 10ÿ5±10ÿ4 S cmÿ1. Enhanced conductivity is found for systems with a higher content of monomer ether per magnesium. # 1998 Elsevier Science Ltd. All rights reserved Key words: magnesium, polymer electrolyte, Grignard reagent.
1. INTRODUCTION Although magnesium is abundant in nature and has a low equivalent weight, its application as anode material is problematic through diculties concerning the passivation layer on its surface, and the lack of appropriate nonaqueous magnesium ion conducting electrolytes. Polymer electrolyte systems based on solvate-free magnesium salts such as MgCl2 or Mg(ClO4)2 dissolved in poly(ethylene oxide) have already been investigated by some authors. But these systems show a very low electrical conductivity at temperatures below 1008C and appear to be purely anion conductors[1±4]. This can be explained by the nonlabile bond formed between magnesium ions and the oxygen of the polymer ether[5]. It is known from liquid systems, that solutions of magnesium salts in organic solvents are not usable for magnesium deposition or dissolution. The electrode reaction in these systems is inhibited due to the strong tendency for the surface of magnesium to become covered by a dense passivation layer. In comparison, organic electrolytes containing Grignard reagents have been shown to be suitable for electrochemical magnesium deposition from aprotic solvents[6]. It has been found that magnesium deposited on suitable substrate materials
like silver or gold could be almost completely reoxidized[7]. The aim of this work was to investigate polymer electrolytes which were prepared by dissolving Grignard reagent in a polymeric ether.
2. EXPERIMENTAL Ethyl magnesium bromide (EtMgBr, 1.2 molar) solutions were prepared by the Grignard reaction in carefully dried tetrahydrofuran (THF) or di-n-butylether (DBE). For the preparation of the polymer electrolytes, poly(ethylene oxide) (PEO, SERVA, M = 200 000) was mixed with dierent volumes of EtMgBr solution. The solvent was evaporated under vacuum at 608C (THF) and 708C (DBE), respectively. Solid or gel-like systems of the composition EtMgBr-P(EO)n-xTHF or EtMgBr-P(EO)n-xDBE were thus obtained. The magnesium and bromide contents were determined by complexometric and argentometric titration, respectively. Impedances were measured in the frequency range from 100 Hz to 100 kHz using a Solartron FRA 1255 connected with a ECI 1286. The samples were sandwiched between gold electrodes. A distance holder was used to prevent further compression of the samples during measurements. The conductivities were calculated
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C. Liebenow
Fig. 1. Cyclic voltammogram of the polymer electrolyte EtMgBr-P(EO)4-2,5THF. Working electrode: Ni, counter electrode: Mg, 258C.
from the electrolyte resistances which were taken from the impedance diagrams. Activation energies were determined from Arrhenius plots of the conductivity in the temperature range of 208C to 608C. Cyclic voltammograms were recorded at sweep rates of 5 mV sÿ1 using a two-electrode cell containing a nickel or silver working electrode and a magnesium counterelectrode. The electrolyte resistance was used for the ir correction. All operations were carried out in a dry argon atmosphere.
Fig. 2. Dependence of the conductivity of the system EtMgBr-P(EO)n-DBE on: (a) the magnesium content, (b) the ethyleme oxide:magnesium ratio.
Fig. 3. Dependence of the conductivity of the system EtMgBr-P(EO)4-DBE on temperature.
Fig. 4. Dependence of the conductivity of the system EtMgBr-P(EO)n-THF on: (a) the magnesium content, (b) the ethylene oxide:magnesium ratio.
Magnesium ion conducting polymer
Fig. 5. Dependence of the conductivity of the systems EtMgBr-P(EO)6.6-4THF and EtMgBr-P(EO)10-0.5THF on temperature.
3. RESULTS AND DISCUSSION The consistency of the polymer electrolytes depends on the ethyl magnesium bromide content. As the solvent could not completely be removed from the polymer electrolyte systems, it in¯uences their mechanical properties considerably. About one solvent molecule per magnesium was found after the evaporation process. Solid materials were obtained at magnesium concentrations below 4 wt.%. Their mechanical properties correspond to those of solid polymeric materials such as LiXPEO electrolytes. Up to 5±8 wt.%, a plastic rubbery material was obtained (gel electrolytes). At higher magnesium concentrations, there is not enough polymer materials to bond all the Grignard compound. The handling of these pastelike materials is more dicult as they react violently with moisture. Figure 1 shows a cyclic voltammogram of the EtMgBr-P(EO)4-2,5THF electrolyte on a nickel electrode. A sample with higher THF content was here chosen to achieve higher conductivity at room temperature. A nearly reversible magnesium deposition and dissolution is demonstrated. This electrochemical activity was found for electrolytes containing more than 3 wt.% magnesium. As this cyclic voltammogram was measured using a two electrode cell, the current is plotted versus the voltage E between the nickel working electrode and the magnesium reference electrode. So it gives no
1255
exact information about the working electrode potential. As clear evidence of electrochemical magnesium reduction, visible amounts of magnesium were deposited with low current densities (I = 10ÿ5±10ÿ4 A/cm2) over longer periods on silver, nickel or gold electrodes. The cell voltages during these deposition experiments corresponded to the voltage for the reduction process which was observed in the cyclic voltammograms. The dependence of the conductivity on the magnesium content for the DBE-containing systems is shown in Fig. 2(a). The optimal magnesium content of 5.9 wt.% corresponds to a ratio of ethylene oxide to magnesium of 4:1 (Fig. 2(b)). A typical Arrhenius plot measured on these systems is shown in Fig. 3. Straight-line plots were found at temperatures below 608C, corresponding to activation energies in the range 20±35 kJ/mol. Similar results were obtained with polymer electrolytes prepared using THF solution of the Grignard reagent. Here the maximum of conductivity was found at 6 or 7 wt.% (Fig. 4(a)). This corresponds to an ethylene oxide:magnesium ratio of 4:1 (Fig. 4(b)). For this gel-like system an activation energy of 50 kJ molÿ1 was found. The conductivities of electrolytes prepared from THF solutions are a little higher than those prepared from DBE-solutions. The residual monomer ether molecules are supposed to be rather tightly bonded to the magnesium, so it is understandable that the very compact THF molecules hinder the conducting process to a lower extent than the DBE molecules. Higher solvent concentration helps to attain higher conductivities. As seen in Fig. 5 the Arrhenius plot for solvent-rich systems is non-linear. These materials change in the course of experiments as the solvent molecules are partly split o at increasing temperatures. The mechanism of conductivity as well as the nature of the electrochemically active particles are still unknown. The composition of alkyl magnesium halide solutions consists of dierent charged or uncharged MgxRy Brz compounds, as described by the Schlenk equilibrium[8]. The position of the equilibrium depends on concentration, temperature and solvent type. Further investigation on this point will thus be necessary. The presence of Grignard compounds in the polymer electrolytes also plays an important role in the electrode processes as it activates the passivated magnesium surface and allows the electrochemical deposition and oxidation of magnesium.
ACKNOWLEDGEMENTS This work was supported Forschungsgemeinschaft.
by
Deutsche
1256
C. Liebenow REFERENCES
1. L. L. Yang, A. R. McGhie and G. C. Farrington, J. Electrochem. Soc. 133, 1380 (1986). 2. G. C. Farrington and R. G. Linford, in Polymer Electrolyte Reviews 2. (Edited by J. R MacCallum and C. A. Vincent). Elsevier Applied Science, London p. 271 (1989).
3. A. Patrick, M. Glasse and R. Linford, Solid State Ionics 18/19, 1063 (1986). 4. J. Y. Cheng, M. Z. A. Munshi, B. B. Owens and W. H. Smyrl, Solid State Ionics 28/30, 857 (1987). 5. C. A. Vincent, Electrochim. Acta 40, 2035 (1995). 6. D. M. Overcash and F. C. Mathers, Trans. Electrochem. Soc. 64, 305 (1933). 7. C. Liebenow, J. Appl. Electrochem. 27, 221 (1997). 8. E. C. Ashby, Quart. Rev. 21, 259 (1967).