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Pd/MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery  

2011-09-23 13:26:11|  分类: 默认分类 |  标签: |举报 |字号 订阅

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Electrochemical and Solid-State Letters, 2010, Vol. 13, No. 11, pp. A165–A167
?2010 The Electrochemical Society. All rights reserved.


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Arjun Kumar Thapa, Kazuki Saimen, and Tatsumi Ishihara*,z
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan
(Submitted: 21 June 2010; revised: 26 July 2010; published online: 3 September 2010)

The oxidation of a carbon binder that occurred during a charge in a carbon–MnO2 air electrode for a Li-air battery resulted in an excessively high charge potential of 4.2 V. The air electrode activity for the Li-air battery was studied on various metals or metal oxides and the mixture of Pd and MnO2 shows the high activity to oxidation and reduction of Li to form Li2O2 or Li2O, respectively. Although the discharge capacity decreased, the application of Pd/MnO2 without a carbon binder for the air electrode is effective to decrease the charge potential and to improve the energy efficiency from 60 to 89%. ?2010 The Electrochemical Society



Contents
BODY OF ARTICLE
Experimental
Results
Conclusion
Acknowledgment
REFERENCES
FIGURES
TABLES
FOOTNOTES

Recently, there has been an increasing demand for high capacity electrical storage devices for use in electric and/or hybrid vehicles. Lithium-air batteries have attracted considerable attention due to their extremely large specific capacity. Such large specific capacity is because the cell consists of lithium metal as an anode and an air electrode for the activation of oxygen in air and, hence, these metal–air batteries are a simple structure. Among various metal–air battery systems, the lithium-air battery is the most attractive because it has the highest energy density per unit weight. The cell discharge reaction occurs between Li and oxygen to yield Li2O (4Li+O2-->2Li2O) or Li2O2, with a theoretical discharge voltage of ca. 3.0 V and a theoretical specific energy density of up to 5200  Wh  kg?1. In practice, the storage of oxygen in the battery is unnecessary because air can be used directly. Therefore, the theoretical specific energy (excluding oxygen) is 11.140  kWh  kg?1, which is much higher than that of other advanced batteries and methanol direct fuel cells. Abraham and Jiang reported a Li-air battery using a nonaqueous electrolyte.1 They suggested that lithium peroxide is a discharge product, based on 2Li+O2-->Li2O2, which resulted in a theoretical voltage of 3.10 V. However, due to low oxygen solubility in a nonaqueous electrolyte, the power density of a Li-air battery using a nonaqueous electrolyte is low.2,3 In lithium-air batteries, nonaqueous electrolytes are used on the anode to eliminate the potentially dangerous reaction between metallic lithium and water. Dobley et al.4 and Kuboki et al.5 employed liquid organic solvents for the electrolyte. When employing an organic3 or ionic liquid-based electrolyte solution,5 the products of the cell reaction produce insoluble Li2O and/or Li2O2, which precipitates in the pores of the porous carbon-based air electrode to block further intake of oxygen, abruptly terminating the discharge reaction. Recently, a lithium-air rechargeable battery employing MnO2 for an air electrode has been reported.6,7 However, a decrease in charge potential is requested because of low energy efficiency (60% of the reported cell) and for this purpose, further improvement in catalytic activity for the air electrode is required. In this study, the air electrode activity of Li-air batteries based on various metals and metal oxides was studied and the mixture of Pd and MnO2 showed the high activity to oxidation and reduction of Li to form Li2O2 or Li2O, respectively.

 


Experimental

All chemicals used in this study were analytical grade. The electrochemical characterizations were carried out using a Swagelok-type cell. The cathode was formed by casting a mixture of electrolytic manganese oxide (EMD), palladium (Pd), and poly(tetrafluoroethylene) (PTFE) (mol ratio of 70:20:10) and then pressing the mixture onto a stainless steel mesh. A lithium foil was used as an anode and was separated by a porous polypropylene film (Mitsubishi Chemical, Celgard 3401). The cell was gastight, except for the stainless steel mesh windows exposing the porous cathode to the O2 atmosphere. The electrolyte used was lithium bis(trifluoro-methanesulfonyl)imide–ethylene carbonate (EC):diethyl carbonate (DEC) [1 M (3:7 volume ratio)] donated by Ube Chemical Co., Ltd., Japan. The charge–discharge performance was carried out in the voltage range of 4.0–2.0 V at a constant current of 0.1  mA  cm?2 and the cell was maintained in an O2 atmosphere to avoid any negative effects of humidity and CO2. We normalized the observed capacity by the total weight of air electrode but not with the weight of carbon for capacity comparison in this study.

Cyclic voltammetry was measured by using the same Swagelok cell as for the analysis of the electrode reaction. A small amount of Li2O, Li2O2, and Li2CO3 powder as a standard material was mixed with teflonized acetylene black and EMD as the working electrode and Li metal was used for both the reference and counter electrodes. The electrolyte used was 1 M LiPF6-EC:dimethyl carbonate (1:2 by volume) supplied by Ube Chemical Co., Ltd., Japan. The cyclic voltammetry experiment was carried out at a scan rate of 10  mV  s?1 in the voltage range of 2.0–4.0 or 2.0–4.5 V using a potential/galvanostat (Hokuto Denko HVS-100).

The examination of the discharge electrodes involved the disassembling the cell in a glove box, rinsing the cathode electrode thrice with DEC, removing the solvent under vacuum, and then sealing the electrodes in a vinyl bag in an argon-filled glove box to prevent any reaction with moisture in the air. Raman spectroscopy was carried out using a Horiba Jobin Yvon HR800 with 744 nm initial excitation laser.

 


Results

Figure 1 shows the cyclic voltammetry of Li2O, Li2O2, and Li2CO3 mixed with a standard MnO2 compound in the first cycle. The potential was swept to positive from open-circuit voltage up to 4.5 V and then decreased to 2.0 V. Peaks representing the decomposition of the standard compound were observed. As shown in Fig. 1, the oxidation of Li2O and Li2O2 to Li metal, which occurred ~3.3 and 3.4 V, respectively, agrees well with the theoretically expected values. However, the oxidation of Li2CO3 occurred at a potential higher than 4.2 V,8 which was slightly higher than the theoretical value (4.0 V). Therefore, if the discharge products were only Li2O or Li2O2, then a potential of ~3.4  V would be sufficient to charge a battery using a MnO2/C air catalyst. However, if Li2CO3 was formed, then a potential higher than 4.2 V would be required to decompose Li2CO3 into Li+, CO2, and O2. Considering that a potential above 4.2 V was required for charge, it is likely that Li2CO3 was formed in the present Li-air battery. In fact, X-ray diffraction and Raman spectroscopy measurements show the Li2CO3 formation but no Li2O2 or Li2O was observed on the air electrode after discharge. Therefore, the charge potential could be decreased by preventing the formation of Li2CO3.

Pd/MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery - 伯虎 - 锂空气电池文献 Figure 1.

Because the charge and discharge performance of the Li-air battery was measured in a pure oxygen atmosphere, two origins were considered for the formation of Li2CO3. One was the electrochemical oxidation of a carbon binder and the other was the decomposition of an organic electrolyte. Because the electrochemical windows of the employed organic electrolyte were higher than 4.0 V, the formation of CO2 might be assigned to the oxidation of the carbon binder. To prevent the oxidation of carbon, an alternative electrode catalyst, which is active to oxygen reduction and oxidation, is required. In this study, several oxides and metals were examined as the air electrode using cyclic voltammetry. In these experiments, we used Li metal as both counter and reference electrodes. Table I summarizes the coulomb amount for the oxidation and reduction of Li/air battery at first and fifth cycles. As shown in Table I, many of the catalysts studied had an increased capacity after five cycles, indicating good reversibility. In our study, Pd and MnO2 had fairly good capacities after five cycles and were suitable for air electrode catalysts. Therefore, the modification of MnO2 with Pd was investigated for use in reversible Li-air batteries.

We investigated the charge–discharge measurement of Li-air batteries using electrolytic manganese dioxide (Brunauer, Emmett, and Teller surface area: 6.0  m2  g?1)/palladium/PTFE air electrodes without carbon binder, as shown in Fig. 2(a). The charge–discharge measurements were carried out in the voltage range of 4.0–2.0 V vs Li/Li+ at a current density of 0.025  mA  cm?2. An initial discharge capacity of 158  mAh  g?1 with a highly reversible capacity was observed. The difference between the charge and discharge voltage was as small as DeltaV=0.4  V, demonstrating that the battery had become more reversible (energy efficiency 89%). In fact, the capacity remained stable over 10 cycles, whereas the capacity becomes much smaller. With increasing addition of palladium to manganese oxide air electrode, the initial discharge capacity increased to 178  mAh  g?1, as shown in Fig. 2(b). However, excess Pd decreased the capacity because of the low surface area of Pd. The charge and discharge voltage difference (DeltaV) of the MnO2/Pd/PTFE (60/30/10) air electrode was ~0.3  V. Therefore, the combination of palladium with manganese oxide appeared to be highly effective in decreasing the charge potential to nearly its theoretical value. Therefore, Pd/MnO2 was highly active to the air electrode of a Li-air battery.

Pd/MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery - 伯虎 - 锂空气电池文献 Figure 2.

Raman spectroscopy is one of the most useful techniques to analyze the Li2O2 or Li2O present in the electrode. So, we have measured the Raman spectra of lithium/air battery using various amounts of EMD/Pd/PTFE air electrodes before charge and after discharge to 2.0 V, as shown in Fig. 3. The Raman spectra of the EMD/Pd/PTFE (75/15/10) air electrode consist of a peak at 652  cm?1, which is MnO2 before charge. After discharge to 2.0 V, the new peaks appeared at 261, 730, and 540  cm?1, which are assigned to Li2O2 and Li2O, as shown in Fig. 3. Raman analysis suggested that the formation of Li2O2 and Li2O was confirmed; however, no Li2CO3 was observed after discharge. Therefore, the present EMD/Pd/PTFE air electrode prevents the formation of Li2CO3, resulting in the decreased charge potential. In addition, the observed discharge capacity came from the reduction of O2 but not from the Li+ ion intercalation.

Pd/MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery - 伯虎 - 锂空气电池文献 Figure 3.

Although the superior cyclability and decreased charge potential were achieved by Pd/MnO2 for the air electrode, the discharge capacity was decreased without using a carbon binder. This may be a result of the small surface area of MnO2. Therefore, the expansion of the surface area is required. In this study, we used a small amount of PTFE-coated acetylene black for the binder in an air electrode of a Li-air battery. The discharge potential is hardly changed by mixing with acetylene black; however, the charge potential was slightly increased from 3.6 to 3.7 V, as shown in Fig. 4(a). However, the discharge capacity was much improved, i.e., a capacity of 257  mAh  g?1 is exhibited. This improvement in capacity could be explained by the increased reaction area. Cycle stability is an important issue for Li-air batteries. Figure 4(b) shows the cycle stability of the battery using Pd/MnO2–PTFE-coated acetylene black for the air electrode. Evidently, a stable capacity for charge and discharge was observed over 20 cycles and a small degradation during cycling was observed. However, the energy density of the present Li-air battery was increased up to 28% using the Pd/MnO2 electrode. Débart et al.9 reported that alpha-MnO2 nanorod is useful for a stable cycle stability up to 10 cycles. In this study, the observed cycle stability is evidently much improved. Consequently, this study reveals that Pd/MnO2 mixed with a small amount of acetylene black is highly effective for preventing Li2CO3 formation, resulting in the high reversibility for charge and discharge.

Pd/MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery - 伯虎 - 锂空气电池文献 Figure 4.


Conclusion

This study reveals that Pd mixed with MnO2 is active for the air electrode of a Li-air battery. By using Pd/MnO2–PTFE coated with acetylene black, the charge potential can decrease to 3.7 V and a stable capacity of ca. 225 mAh/g was sustained for 20 cycles of charge and discharge. Therefore, Li-air batteries could be used as a secondary battery. The improvement of capacity is also expected by improving the surface area of the MnO2 catalyst and this is now under study.

 


Acknowledgment

This work was financially supported by Li-EAD project of the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Kyushu University assisted in meeting the publication costs of this article.

 


REFERENCES



K. M. Abraham and Z. Jiang, J. Electrochem. Soc., 143, 1 (1996). [ISI] first citation in article
J. Read, J. Electrochem. Soc., 149, A1190 (2002). [ISI] first citation in article
J. Read, K. Mutolo, M. Ervin, W. Behl, J. Wolfenstine, A. Driedger, and D. Foster, J. Electrochem. Soc., 150, A1351 (2003). [ISI] first citation in article
A. Dobley, J. DiCarlo, and K. M. Abraham, in Proceeding of the 41st Power Sources Conferences, Philadelphia, PA, p. 61 (2004). first citation in article
T. Kuboki, T. Okuyama, T. Ohsaki, and N. Takami, J. Power Sources, 146, 766 (2005). first citation in article
T. Ogasawara, A. Debart, M. Holfazel, P. Novak, and P. G. Bruce, J. Am. Chem. Soc., 128, 1390 (2006). [MEDLINE] first citation in article
A. Dobley, C. Morein, and K. M. Abraham, Abstract 823, The Electrochemical Society Meeting Abstracts, Los Angeles, CA, Oct 16–21, 2005. first citation in article
R. Imhof and P. Novak, J. Electrochem. Soc., 146, 1702 (1999). first citation in article
A. Débart, A. J. Peterson, J. L. Bao, and P. G. Bruce, Angew. Chem., Int. Ed., 47, 4521 (2008). [MEDLINE] first citation in article


CITING ARTICLES


This list contains links to other online articles that cite the article currently being viewed.
Nanostructured Diamond Like Carbon Thin Film Electrodes for Lithium Air Batteries
Yin Yang et al., J. Electrochem. Soc. 158, B1211 (2011)


FIGURES


Pd/MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery - 伯虎 - 锂空气电池文献 Full figure (26 kB)

Fig. 1. (Color online) Cyclic voltammetry of (a) Li2O-, (b) Li2O2-, and (c) Li2CO3-supported MnO2 catalyst electrode for a lithium-air battery in an O2 atmosphere at 25°C. Scan rate was 10  mV  s?1. First citation in article


Pd/MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery - 伯虎 - 锂空气电池文献 Full figure (15 kB)

Fig. 2. Charge–discharge curves for a lithium-air battery with manganese oxide-supported palladium electrode in an O2 atmosphere between 2.0 and 4.0 V at a current density of 0.02  mA  cm?2. (a) EMD/Pd/PTFE (70/20/10) electrode and (b) EMD/Pd/PTFE (60/30/10) electrode. First citation in article


Pd/MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery - 伯虎 - 锂空气电池文献 Full figure (19 kB)

Fig. 3. (Color online) Raman spectra of different weight ratios of EMD/Pd/PTFE electrode for lithium-air battery before charge and after discharge to 2.0 V; (a) EMD/Pd/PTFE (75/15/10) electrode before charge, (b) EMD/Pd/PTFE (70/20/10) electrode after discharge, and (c) EMD/Pd/PTFE (60/30/10) electrode after discharge. First citation in article


Pd/MnO2 Air Electrode Catalyst for Rechargeable Lithium/Air Battery - 伯虎 - 锂空气电池文献 Full figure (27 kB)

Fig. 4. Charge–discharge curve and cycle stability for a lithium-air battery with a manganese oxide-supported palladium electrode in an O2 atmosphere between 2.0 and 4.0 V at a current density of 0.25  mA  cm?2. (a) Charge–discharge curve and (b) cyclability. First citation in article


TABLES

Table I. Discharge capacity of cycles 1 and 5.
Catalyst Capacity of first cycle
(mAh  g?1)
Capacity of cycle
(mAh  g?1)
Capacity retention
per cycle
(%)
MnO2 262 653 248
Co3O4 199 304 152
NiO 298 362 121
Fe2O3 264 285 108
Pt 1a 1a 100
Pd 277 859 310
RuO2 317 330 104
CuO 292 658 225
V2O5 216 829 383
MoO3 152 152 100
Y2O3 238 213 89
Ir2O3 345 354 102
aElectrolyte decomposition.
First citation in article

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