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Magnesium Anode for Magnesium Power Sources with Non-Aqueous Electrolyte
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 6 (2019) 101–105
www.materialstoday.com/proceedings
3rd ISE SSRSEU 2018
Magnesium Anode for Magnesium Power Sources with Non-Aqueous Electrolyte D. V. Bondar*, O. V. Kolomoiets, E. M. Shembel
Scientific Research Laboratory of Chemical Power Sources of Ukrainian State University of Chemical Technology, Dnipro, 08540, Ukraine
Abstract The effective use of a magnesium anode in power sources with a non-aqueous electrolyte strongly depends on the properties of the interface between the surface of the magnesium electrode and the non-aqueous electrolyte. In order to optimize electrochemical characteristics of magnesium electrode in the given electrolytes and, as a consequence, to optimize the power source in general, the impedance and galvanic characteristics of the magnesium electrode have been investigated. The electrochemical properties of anode based on magnesium foil and magnesium electrode with a porous structure have been compared. Electrodes with a porous structure have been made using magnesium shavings. The impedance characteristics of the magnesium electrode have been determined in dependence to the composition of the electrolyte, the value of the currents of galvanostatic anodic dissolution, and the storage time after galvanostatic anodic dissolution also determined. The influence of anodic dissolution mode on the impedance spectrum of a magnesium electrode has been investigated. It has been found that absorbing formed films after anodic decomposition exhibit higher conductivity than films formed without actuation. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine. Keywords: magnesium battery, magnesium foil, magnesium porous structure, impedance, galvanostatic cycling, non-aqueous electrolyte.
Introduction Many scientific studies have recently been devoted to the development of electrochemical current sources that use cheap and affordable components. Among them, according to aggregate parameters, such as energy characteristics, safety of operation, price and distribution in the earth's crust, magnesium is the undisputed leader [13]. * Corresponding author. Tel.: +380676141915 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine.
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Despite the fact that the elements of the magnesium anode in non-aqueous electrolytes have high characteristics and a broad potential market, the state of research still does not make it possible to organize their production. This is due to a wide range of problems, including the need to optimize the macrostructure of the magnesium anode and the magnesium/non-aqueous electrolyte interface, and to optimize the composition of the electrolyte both in terms of anode and cathode process in solutions of magnesium salts. This work is devoted to the study and optimization of anodic processes on a magnesium anode. 1. Experimental The study of processes occurring at the interface between phases of a magnesium electrode/electrolyte was carried out in 3-electrode cells of the prismatic structure. As working electrodes, electrodes on the basis of 100 μm thick metal magnesium foil, and composite electrodes with a porous structure based on magnesium shavings with a fraction of less than 70 μm were used. Metallic magnesium was used as auxiliary electrodes and electrodes for comparison. The surface of all electrodes made of metallic magnesium into the elements before cleaning was cleaned with a steel barrel. Magnesium stripping and collecting 3-electrode cells were carried out in glovebox with atmosphere of dry argon. Glymes were used as solvents. 0.5 M solution of Mg (ClO4)2 in diglyme (G2), triglyme (G3) and tetraglyme (G4) were used as electrolytes. Solutions of electrolytes were pre-supported over molecular sieves for two weeks. Impedance and galvanostatic studies were carried out using VoltaLab-40 electrochemical experimental complex with computer programming and registration of received data. All experiments were carried out at +25°С. Impedance studies were conducted in the frequency range from 100 kHz to 100 mHz. The amplitude of the voltage variable was 10 mV. The duration of the discharge of magnesium electrodes in galvanostatic studies was 300–600 s with obligatory relaxation of electrodes at the end of the research. 2. Results Impedances of magnesium electrodes with porous structure and smooth magnesium in electrolytes based on 0.5 M Mg(ClO4)2 in solutions of G2, G3 and G4 of freshly assembled systems are shown in Figures 1.A and 1.B, respectively. The numbers indicate the composition of the electrolytes. It is seen that the value of the active and reactive components of the impedance increases in a number of soluble diglyme - triglyme - tetraglyme. In this case, electrodes with porous structure that are based on magnesium shavings have significantly lower resistance rates.
Fig. 1. Effect of the nature of the solvent on the impedance of electrodes on the basis of powder (A) and metallic (B) magnesium in electrolytes based on 0,5 М Mg(ClO4)2 in solvents : 1 – G2; 2 – G3; 3 – G4
However, the data obtained for a plate metal electrode can be attributed to a unit surface of 1 cm2, whereas the real surface area of Mg-powder-containing electrodes will be much larger. In real batteries the positive effect of the
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developed surface of the magnesium electrode could be manifested. Impedance studies have shown that the nature of the hodographs for the magnesium metal plate and electrodes with porous structure are significantly different. Thus, for electrodes with porous structure in the low frequency region a characteristic vertical motion of the hodograph is observed, whereas metal electrodes are characterized by an explicit semicircle. For all investigated systems in the process of storage there is an increasing of the active and reactive components of the impedance. Especially this is observed in the first day. For electrodes based on metallic magnesium obtained homographs are well described by an equivalent electric circuit with three R-C elements (Fig. 2). The scheme is based on the Wojtz model and does not have a clear physical content. In the scheme, the elements responsible for the capacitive characteristics are intentionally replaced by the elements of the so-called constant phase of the CRE.
Fig. 2. The principle equivalent scheme is based on the model Wojta
For systems with electrodes based on magnesium foil in the electrolyte 0.5 M Mg(ClO4)2 in G2, the elements CRE-T1 and CRE-T3 do not change over time and have values of 8·10–5 and 1·10–4 Ohm, respectively. Their stepby-step indicators CRE-P also do not change over time and have values close to 1. The element of the scheme СРЕТ2 per day increases linearly from 8·10–5 to 2·10–4 Оhm, and its step rate over the entire period of time is 0.7. The behavior of the active elements of the equivalent scheme is shown in Figure 3. It should be noted that the dependencies of Rs and R2 from time to time have an extreme character with a maximum in time close to 3 hours. At the same time, the value of the element R3 almost does not change, and then rapidly increases, goes into stationary position and continues to gradually increase.
Fig. 3. Changing the active elements of the equivalent circuit in time for electrode based on magnesium foil in an electrolyte 0.5 M Mg(ClO4)2 in G2
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It should be noted that the anode dissolution of the electrodes leads to a decrease in impedance. As a result of the surface upgrading, the value of the impedances in the process of surface activation returns to the values of freshly assembled elements. Galvanostatic studies of all systems show that at low currents (10-50 μA/cm2) the polarization of metallic magnesium varies considerably over time. The conditions, under which the equilibrium between processes of anodic dissolution of magnesium and the transfer of its ions through the passivation film is established, are fulfilled. In this case, the value of polarization both for electrodes based on metallic magnesium (Figure 4.A.) and for electrodes of powder magnesium (Figure 4.B.) fluctuates within 0.2-0.5 V. For the electrodes of powder magnesium, the value of polarization in all solvents decreases with an increase in the content of the powder in the electrode.
Fig. 4. Influence of galvanostatic current density on polarization voltage on the metallic-based (A) and powder-based (B) magnesium electrodes in electrolyte 0.5 M Mg(ClO4)2 in G2Current density, μA/cm2: 1 – 10; 2 – 20; 3 – 50; 4– 100; 5 – 200; 6 – 500; 7 – 1000; 8 – 2000.
An increase in the anode current of more than 100 μA/cm2 leads to unstable polarization of electrodes with oscillatory nature. The oscillating nature of the potential change can be explained by a violation of the mechanical integrity of the surface film in the process of anodic dissolution. The appearance of microcracks facilitates the transfer of magnesium ions through the film into a solution and the polarization of the electrode decreases. At the same time, the process of passivating the electrode occurs, which leads to an increase in polarization. The magnitude of the polarization depends on the ratio of the velocities of these two processes. For electrodes based on metallic magnesium, an increase in the amplitude of oscillations is observed with the growth of the value of the current and the decrease of their period with time. In this case, the value of the polarization for currents from 100 to 2000 μA/cm2 is in the range of 0.5-0.8 V. A slightly different nature is observed for electrodes based on powdered magnesium. In them, the oscillatory nature of polarizations occurs at currents of 50-500 μA/cm2, and their values are within 1 V. At currents greater than 500 μA/cm2 oscillatory character is discontinued, but the polarization has a value greater than 2 V. During the storage of both metal and powdered electrodes, an increase in the value of polarization is observed. This indicates an extension of the passivation of the magnesium surface as a result of interaction with the components of the electrolyte. 3. Conclusions 1. 2.
The results of galvanostatic and impedance investigations of magnesium electrodes show that the properties of the passivating film on the surface of the magnesium electrodes, the nature of impedance characteristics, possible mechanical strength depend on the nature of the organic solvent and the pre-history of the electrode. Comparison of the hodographs of different periods of storage of anode based on magnesium indicates an increase in the resistance of the interface between the electrode/electrolyte over time. This indicates an
D.V. Bondar et al./ Materials Today: Proceedings 6 (2019) 101–105
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5. 6.
105
additional passivation of the surface of the magnesium electrode as a result of interaction with the components of the electrolyte. Impedance characteristics of magnesium electrodes with a porous structure are characterized by considerably less resistance than magnesium electrodes based on magnesium foil. In the mode of galvanic discharge of a magnesium anode with a porous structure the limiting factor is the diffusion of magnesium ions in the porous structure of the anode. Thus, there is an effect of salt passivation in the pores of the electrode, the possible complications of removing the products of dissolution from the electrode layer. In this case, an important factor is the dependence of the solubility of the magnesium salt on the nature of the solvent. The authors of the article continue research in the direction of optimization of the macrostructure of the electrode based on magnesium, and optimization of the composition of the electrolyte. The results will be presented in the following publications. Anodes based on magnesium are promising for current sources with non-aqueous electrolytes and cathodes based on manganese oxides MnO2; lithium/magnesium manganese spinel LiyMgxMn2O4, and sulfur S. It should be noted that in a system with a sulfur-based cathode, the use of an anode based on magnesium can significantly reduce self-discharge of the Mg-based battery.
The authors of the article continue research in the direction of optimization of the macrostructure of the electrode based on magnesium, and optimization of the composition of the electrolyte. The results will be presented in the next publications. Acknowledgements This work was supported by the Ministry of Education and Science of Ukraine. Investigations were carried out in the frame of the project No. 42/170790 "Development of the high energy power sources based on Ukrainian magnesium and manganese raw materials for innovative instrument making”. The current article is also connected with NATO SPS 985148 project “Development of New Cathodes for Stable and Safer Lithium-Sulfur Batteries” in accordance with NATO Science for Peace and Security Programme. The scientific head of these projects is Prof. E. M. Shembel, the Chief of the NILHIT. In this research used magnesium foil has been produced by innovative technology, which was developed by Markevich A. V., PhD, the Senior researcher of the NILHIT. The electrodes with porous structure based on magnesium shavings have been made by Yu.V.Polishchuk, PhD, the Associate Professor of the Department of Technical Electrochemistry and Environment Technology. References [1] R.D. Apostolova. Issues of Chemistry and Chemical Technology (Voprosy khimii i khimicheskoi tekhnologii). Vol. 2(111), (2017) 47–62. [2] S. Ha, Y. Lee, S. Woo, B. Koo, J. Kim et al, ACS Appl. Mater. Interfaces, 6 (2014) 4063−4073. [3] S. Tang, RSC Adv.v. 4(22), (2014) 11251–11287.
ScienceDirect Materials Today: Proceedings 6 (2019) 101–105
www.materialstoday.com/proceedings
3rd ISE SSRSEU 2018
Magnesium Anode for Magnesium Power Sources with Non-Aqueous Electrolyte D. V. Bondar*, O. V. Kolomoiets, E. M. Shembel
Scientific Research Laboratory of Chemical Power Sources of Ukrainian State University of Chemical Technology, Dnipro, 08540, Ukraine
Abstract The effective use of a magnesium anode in power sources with a non-aqueous electrolyte strongly depends on the properties of the interface between the surface of the magnesium electrode and the non-aqueous electrolyte. In order to optimize electrochemical characteristics of magnesium electrode in the given electrolytes and, as a consequence, to optimize the power source in general, the impedance and galvanic characteristics of the magnesium electrode have been investigated. The electrochemical properties of anode based on magnesium foil and magnesium electrode with a porous structure have been compared. Electrodes with a porous structure have been made using magnesium shavings. The impedance characteristics of the magnesium electrode have been determined in dependence to the composition of the electrolyte, the value of the currents of galvanostatic anodic dissolution, and the storage time after galvanostatic anodic dissolution also determined. The influence of anodic dissolution mode on the impedance spectrum of a magnesium electrode has been investigated. It has been found that absorbing formed films after anodic decomposition exhibit higher conductivity than films formed without actuation. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine. Keywords: magnesium battery, magnesium foil, magnesium porous structure, impedance, galvanostatic cycling, non-aqueous electrolyte.
Introduction Many scientific studies have recently been devoted to the development of electrochemical current sources that use cheap and affordable components. Among them, according to aggregate parameters, such as energy characteristics, safety of operation, price and distribution in the earth's crust, magnesium is the undisputed leader [13]. * Corresponding author. Tel.: +380676141915 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine.
102
D.V. Bondar et al./ Materials Today: Proceedings 6 (2019) 101–105
Despite the fact that the elements of the magnesium anode in non-aqueous electrolytes have high characteristics and a broad potential market, the state of research still does not make it possible to organize their production. This is due to a wide range of problems, including the need to optimize the macrostructure of the magnesium anode and the magnesium/non-aqueous electrolyte interface, and to optimize the composition of the electrolyte both in terms of anode and cathode process in solutions of magnesium salts. This work is devoted to the study and optimization of anodic processes on a magnesium anode. 1. Experimental The study of processes occurring at the interface between phases of a magnesium electrode/electrolyte was carried out in 3-electrode cells of the prismatic structure. As working electrodes, electrodes on the basis of 100 μm thick metal magnesium foil, and composite electrodes with a porous structure based on magnesium shavings with a fraction of less than 70 μm were used. Metallic magnesium was used as auxiliary electrodes and electrodes for comparison. The surface of all electrodes made of metallic magnesium into the elements before cleaning was cleaned with a steel barrel. Magnesium stripping and collecting 3-electrode cells were carried out in glovebox with atmosphere of dry argon. Glymes were used as solvents. 0.5 M solution of Mg (ClO4)2 in diglyme (G2), triglyme (G3) and tetraglyme (G4) were used as electrolytes. Solutions of electrolytes were pre-supported over molecular sieves for two weeks. Impedance and galvanostatic studies were carried out using VoltaLab-40 electrochemical experimental complex with computer programming and registration of received data. All experiments were carried out at +25°С. Impedance studies were conducted in the frequency range from 100 kHz to 100 mHz. The amplitude of the voltage variable was 10 mV. The duration of the discharge of magnesium electrodes in galvanostatic studies was 300–600 s with obligatory relaxation of electrodes at the end of the research. 2. Results Impedances of magnesium electrodes with porous structure and smooth magnesium in electrolytes based on 0.5 M Mg(ClO4)2 in solutions of G2, G3 and G4 of freshly assembled systems are shown in Figures 1.A and 1.B, respectively. The numbers indicate the composition of the electrolytes. It is seen that the value of the active and reactive components of the impedance increases in a number of soluble diglyme - triglyme - tetraglyme. In this case, electrodes with porous structure that are based on magnesium shavings have significantly lower resistance rates.
Fig. 1. Effect of the nature of the solvent on the impedance of electrodes on the basis of powder (A) and metallic (B) magnesium in electrolytes based on 0,5 М Mg(ClO4)2 in solvents : 1 – G2; 2 – G3; 3 – G4
However, the data obtained for a plate metal electrode can be attributed to a unit surface of 1 cm2, whereas the real surface area of Mg-powder-containing electrodes will be much larger. In real batteries the positive effect of the
D.V. Bondar et al./ Materials Today: Proceedings 6 (2019) 101–105
103
developed surface of the magnesium electrode could be manifested. Impedance studies have shown that the nature of the hodographs for the magnesium metal plate and electrodes with porous structure are significantly different. Thus, for electrodes with porous structure in the low frequency region a characteristic vertical motion of the hodograph is observed, whereas metal electrodes are characterized by an explicit semicircle. For all investigated systems in the process of storage there is an increasing of the active and reactive components of the impedance. Especially this is observed in the first day. For electrodes based on metallic magnesium obtained homographs are well described by an equivalent electric circuit with three R-C elements (Fig. 2). The scheme is based on the Wojtz model and does not have a clear physical content. In the scheme, the elements responsible for the capacitive characteristics are intentionally replaced by the elements of the so-called constant phase of the CRE.
Fig. 2. The principle equivalent scheme is based on the model Wojta
For systems with electrodes based on magnesium foil in the electrolyte 0.5 M Mg(ClO4)2 in G2, the elements CRE-T1 and CRE-T3 do not change over time and have values of 8·10–5 and 1·10–4 Ohm, respectively. Their stepby-step indicators CRE-P also do not change over time and have values close to 1. The element of the scheme СРЕТ2 per day increases linearly from 8·10–5 to 2·10–4 Оhm, and its step rate over the entire period of time is 0.7. The behavior of the active elements of the equivalent scheme is shown in Figure 3. It should be noted that the dependencies of Rs and R2 from time to time have an extreme character with a maximum in time close to 3 hours. At the same time, the value of the element R3 almost does not change, and then rapidly increases, goes into stationary position and continues to gradually increase.
Fig. 3. Changing the active elements of the equivalent circuit in time for electrode based on magnesium foil in an electrolyte 0.5 M Mg(ClO4)2 in G2
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D.V. Bondar et al./ Materials Today: Proceedings 6 (2019) 101–105
It should be noted that the anode dissolution of the electrodes leads to a decrease in impedance. As a result of the surface upgrading, the value of the impedances in the process of surface activation returns to the values of freshly assembled elements. Galvanostatic studies of all systems show that at low currents (10-50 μA/cm2) the polarization of metallic magnesium varies considerably over time. The conditions, under which the equilibrium between processes of anodic dissolution of magnesium and the transfer of its ions through the passivation film is established, are fulfilled. In this case, the value of polarization both for electrodes based on metallic magnesium (Figure 4.A.) and for electrodes of powder magnesium (Figure 4.B.) fluctuates within 0.2-0.5 V. For the electrodes of powder magnesium, the value of polarization in all solvents decreases with an increase in the content of the powder in the electrode.
Fig. 4. Influence of galvanostatic current density on polarization voltage on the metallic-based (A) and powder-based (B) magnesium electrodes in electrolyte 0.5 M Mg(ClO4)2 in G2Current density, μA/cm2: 1 – 10; 2 – 20; 3 – 50; 4– 100; 5 – 200; 6 – 500; 7 – 1000; 8 – 2000.
An increase in the anode current of more than 100 μA/cm2 leads to unstable polarization of electrodes with oscillatory nature. The oscillating nature of the potential change can be explained by a violation of the mechanical integrity of the surface film in the process of anodic dissolution. The appearance of microcracks facilitates the transfer of magnesium ions through the film into a solution and the polarization of the electrode decreases. At the same time, the process of passivating the electrode occurs, which leads to an increase in polarization. The magnitude of the polarization depends on the ratio of the velocities of these two processes. For electrodes based on metallic magnesium, an increase in the amplitude of oscillations is observed with the growth of the value of the current and the decrease of their period with time. In this case, the value of the polarization for currents from 100 to 2000 μA/cm2 is in the range of 0.5-0.8 V. A slightly different nature is observed for electrodes based on powdered magnesium. In them, the oscillatory nature of polarizations occurs at currents of 50-500 μA/cm2, and their values are within 1 V. At currents greater than 500 μA/cm2 oscillatory character is discontinued, but the polarization has a value greater than 2 V. During the storage of both metal and powdered electrodes, an increase in the value of polarization is observed. This indicates an extension of the passivation of the magnesium surface as a result of interaction with the components of the electrolyte. 3. Conclusions 1. 2.
The results of galvanostatic and impedance investigations of magnesium electrodes show that the properties of the passivating film on the surface of the magnesium electrodes, the nature of impedance characteristics, possible mechanical strength depend on the nature of the organic solvent and the pre-history of the electrode. Comparison of the hodographs of different periods of storage of anode based on magnesium indicates an increase in the resistance of the interface between the electrode/electrolyte over time. This indicates an
D.V. Bondar et al./ Materials Today: Proceedings 6 (2019) 101–105
3. 4.
5. 6.
105
additional passivation of the surface of the magnesium electrode as a result of interaction with the components of the electrolyte. Impedance characteristics of magnesium electrodes with a porous structure are characterized by considerably less resistance than magnesium electrodes based on magnesium foil. In the mode of galvanic discharge of a magnesium anode with a porous structure the limiting factor is the diffusion of magnesium ions in the porous structure of the anode. Thus, there is an effect of salt passivation in the pores of the electrode, the possible complications of removing the products of dissolution from the electrode layer. In this case, an important factor is the dependence of the solubility of the magnesium salt on the nature of the solvent. The authors of the article continue research in the direction of optimization of the macrostructure of the electrode based on magnesium, and optimization of the composition of the electrolyte. The results will be presented in the following publications. Anodes based on magnesium are promising for current sources with non-aqueous electrolytes and cathodes based on manganese oxides MnO2; lithium/magnesium manganese spinel LiyMgxMn2O4, and sulfur S. It should be noted that in a system with a sulfur-based cathode, the use of an anode based on magnesium can significantly reduce self-discharge of the Mg-based battery.
The authors of the article continue research in the direction of optimization of the macrostructure of the electrode based on magnesium, and optimization of the composition of the electrolyte. The results will be presented in the next publications. Acknowledgements This work was supported by the Ministry of Education and Science of Ukraine. Investigations were carried out in the frame of the project No. 42/170790 "Development of the high energy power sources based on Ukrainian magnesium and manganese raw materials for innovative instrument making”. The current article is also connected with NATO SPS 985148 project “Development of New Cathodes for Stable and Safer Lithium-Sulfur Batteries” in accordance with NATO Science for Peace and Security Programme. The scientific head of these projects is Prof. E. M. Shembel, the Chief of the NILHIT. In this research used magnesium foil has been produced by innovative technology, which was developed by Markevich A. V., PhD, the Senior researcher of the NILHIT. The electrodes with porous structure based on magnesium shavings have been made by Yu.V.Polishchuk, PhD, the Associate Professor of the Department of Technical Electrochemistry and Environment Technology. References [1] R.D. Apostolova. Issues of Chemistry and Chemical Technology (Voprosy khimii i khimicheskoi tekhnologii). Vol. 2(111), (2017) 47–62. [2] S. Ha, Y. Lee, S. Woo, B. Koo, J. Kim et al, ACS Appl. Mater. Interfaces, 6 (2014) 4063−4073. [3] S. Tang, RSC Adv.v. 4(22), (2014) 11251–11287.