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MnO2 nanoflakes coated on multi-walled carbon nanotubes for rechargeable lithium-air batteries  

2011-09-23 16:11:28|  分类: 默认分类 |  标签: |举报 |字号 订阅

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Electrochemistry Communications
Volume 13, Issue 7, July 2011, Pages 698-700


Jiaxin Lia, Ning Wanga, Yi Zhaoa, Yunhai Dinga, Lunhui GuanCorresponding Author Contact InformationaE-mail The Corresponding Author
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, YangQiao West Road 155#, Fuzhou, Fujian 350002, PR China

Received 6 April 2011; revised 15 April 2011; Accepted 15 April 2011. Available online 28 April 2011.


 
Abstract

Manganese dioxide (MnO2) nanoflakes were uniformly coated on multi-walled carbon nanotubes (MWNTs) by immersing MWNTs into an aqueous KMnO4 solution. Directly using the MnO2/MWNT composites (containing 40 wt.% MWNTs) as lithium-air battery electrodes enhances kinetics of the oxygen reduction and evolution reactions, thereby effectively improving energy efficiency and reversible capacity.


Research highlights

? MnO2 nanoflakes uniformly coated on MWNTs were studied as cathode materials in lithium-air batteries. ? MnO2/MWNTs deliver discharge capacity of 796 mAh/g(electrode). ? MnO2/MWNTs composites effectively improved energy efficiency and reversible capacity in lithium-air batteries.



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Keywords: Lithium-air batteries; MnO2/MWNT cathodes; Electrochemical performance; Air electrodes




Article Outline
1. Introduction
2. Experimental
3. Results and discussion
4. Conclusion
Acknowledgment
References
1. Introduction

Interest in the development of alternative energy storage/conversion devices with high power and energy density has considerably increased because of environmental problems and fossil fuel depletion. The lithium-air battery is an attractive type of metal-air battery because its theoretical energy density excluding O2 is 11140 Wh/kg. A lithium-air battery that uses non-aqueous electrolytes was first reported in 1996 [1]. Its cell had an open-circuit potential of ~ 3 V and an energy density of 250–350 Wh/kg. There have been extensive studies on the effects of many key factors on the electrochemical properties of lithium-air batteries; these factors include electrolyte composition, cathode formulation, moisture barrier, the oxygen reduction catalyst, and the physical properties of the cathode [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] and [13]. The foremost challenge that needs to be addressed in rechargeable lithium-air batteries is how to enable the reactions (i.e., 2Li+ + 2e– + O2 ← → Li2O2 and 4Li+ + 4e– + O2 ← → 2Li2O) to remain reversible in Li+-containing aprotic electrolytes. Therefore, developing effective air cathode materials, including carbon materials and O2 catalysts, is vital to ensuring reversible reactions [9].


Manganese oxide (MnO2) is a catalyst commonly used in the cathode of lithium-air batteries because of its low cost, low toxicity, high average voltage, and energy compatibility [2] and [4]. Normally, MnO2 nanomaterials, such as nanocrystals, nanotubes, and dendritic clusters, are mixed with different carbon-based materials, including carbon black, carbon foam, and graphite, that act as air cathodes [1], [2], [3], [4], [5], [6], [7], [8] and [9]. To optimize the cathodes in the current study, MnO2 was directly coated on multi-walled carbon nanotubes (MWNTs) [14], which typically have large surface areas and fine conductance, by immersing MWNTs in an aqueous KMnO4 solution. The MWNTs act as reducing agents for the preparation of the MnO2/MWNT composites, and as carbon-based materials for the cathodes. The MnO2/MWNTs were directly used as air cathodes in a lithium-air battery. The cathodes exhibit a relatively low charge potential of 3.8 V and a considerable capacity of 1768 mAh/g(carbon) (796 mAh/g(electrode)) at 70 mA/g in the absence of oxygen.


2. Experimental

All chemicals were of analytical grade and used as received. The MWNTs were purchased from Shenzhen Nanotech Port (Shenzhen, China) and used as received. MnO2 nanoflakes were synthesized by immersing MWNTs in an aqueous KMnO4 solution, as shown in Scheme 1. KMnO4 (200 mg) was dispersed in 50 mL de-ionized water and heated to 80 °C. Then, 100 mg of MWNTs was added into the solution. The mixtures were maintained at 80 °C for 24 h. The pH of the solution was adjusted to 2.5 by adding 2 M HCl. The suspension was filtered and washed with de-ionized water, and then dried at 100 °C overnight. The sample was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermo-gravimetric analysis (TGA).





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Scheme 1. 

Schematic of the synthesis of MnO2 nanoflakes coated on MWNTs.





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The electrochemical behaviors were measured in a Swagelok cell [7] with a 0.5 cm2 hole placed on the cathode side to enable oxygen flow in. All the cells were assembled in a dry argon-filled glove box. The air cathodes were prepared by casting the slurry mixtures of 90 wt.% MnO2/MWNTs and 10 wt.% polyvinylidene difluoride (PVDF) onto a nickel foam current collector. The air electrode disks had an area of 0.785 cm2. The typical loading of the MnO2/MWNTs was 1.3 ± 0.2 mg/cm2. A commercially available electrolyte solution of 1 M LiPF6 in PC:EC:DME (1:1:1 in volume) was impregnated into a Celgard 2300 membrane and sandwiched between a lithium metal anode and air cathode. The cells were cycled by LAND 2001A at room temperature with a lower voltage limit of 2.0 V and an upper limit of 4.15 V versus Li+/Li at different current densities.


3. Results and discussion

On the basis of our previous results, we determined that the MWNTs have a porous structure with mean diameters of 20–40 nm [14]. Coating the MnO2 nanoflakes on the surface of the MWNTs causes the MWNTs to thicken. The diameter of the products increases to approximately 40–60 nm [Fig. 1(a) and (b)], indicating that the MWNTs act not only as reducing agents, but also as carriers of MnO2 nanoflakes. The MnO2 layer is about 10 nm thick. The XRD pattern of the MnO2/MWNTs is shown in Fig. 1(c). Aside from the peaks of the MWNTs, three broad peaks at 2θ around 12°, 37°, and 66° are observed. These peaks can be indexed to α-MnO2, including its amorphous phase, a result consistent with the reports of Thapa [3] and Ma [15]. Fig. 1(d) shows the TGA curves of the as-received MnO2/MWNTs. The MnO2/MWNTs exhibit a weight loss of 25 to 1000 °C. The 10% weight loss between 25 and 150 °C is attributed to the liberation of adsorbed water in the composites. The additional weight loss of 40% between 300 and 450 °C corresponds to the loss of MWNTs. Thus, the amount of MWNTs in the MnO2/MWNT composites is 40%. Furthermore, the final weight loss of ~ 2% between 860 and 910 °C is attributed to the conversion of MnO2 to Mn3O4[16].





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Fig. 1. 

SEM image (a), TEM and HR-TEM images (b), XRD pattern (c), and TGA curve (d) of the MnO2/MWNT nanocomposites.





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Fig. 2(a) shows the initial discharge curves of the MnO2/MWNT cathodes in Ar (as background) and O2 at 30, 70, 120, 180, and 300 mA/g(carbon) between 2.0 and 4.15 V. The electrodes in Ar present small specific capacities above 2.0 V. The oxygen component of the OH- and adsorbed H2O in the MnO2/MWNT composites is tentatively assumed to contribute to the small specific capacities. The battery delivers a large capacity of 2247 mAh/g(carbon) at a current density of 30 mA/g(carbon) in O2. The specific capacity and open voltage decrease when the current density increases. The discharge capacities decrease to 1480 mAh/g(carbon) at a current density of 120 mA/g, and 570 mAh/g(carbon) at a current density of 300 mA/g(carbon). The limited solubility and diffusivity of oxygen significantly affect the rate capability of the air battery. Thus, the oxygen was insufficient for enabling discharge reaction at the high current density. Fig. 2(b) displays the cycling performance of the MnO2/MWNT cathodes at 180 mA/g(carbon) (i.e., 0.15 mA/cm2(electrode)) between 2.0 and 4.15 V in Ar and O2. The capacities in Ar are lower than those in O2 during the cycling process. The battery exhibits an initial charge capacity of 1310 mAh/g(carbon) in O2, and then the capacity fades to 557 mAh/g(carbon) after six cycles.





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Fig. 2. 

(a) Discharge curves of the MnO2/MWNT cathodes at different current densities; (b) Cycling performance of the MnO2/MWNT cathodes at a current of 180 mA/g.





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For comparison, we also directly mixed α-MnO2 (50 wt.%), MWNTs (40 wt.%), and PVDF (10 wt.%) together to fabricate air cathodes. Fig. 3 shows that the MWNT cathodes deliver a low initial capacity, whereas the mixed MnO2/MWNT cathodes deliver a lower initial capacity of 1256 mAh/g(carbon) (at 70 mA/g(carbon)) compared with the 1768 mAh/g(carbon) capacity of the MnO2/MWNT cathodes. The cyclic ability of the mixed MnO2/MWNT cathodes is also low.





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Fig. 3. 

Cycling performance of MnO2/MWNT, mixed MnO2/MWNT, and all-MWNT cathodes at a current of 70 mA/g.





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The charge potential is flat at ~ 3.8 V, as shown in Fig. 2(b). Thus, the MnO2/MWNT cathodes exhibit a considerably lower charge potential (~ 3.8 V) than do the mixed MnO2 cathodes reported by Bruce (~ 4.0 V) [6] and Yu (~ 4.2 V) [7] at a comparable current density of 0.15 mA/cm2(electrode) (i.e., 180 mA/g(carbon)). The electrochemical properties of the MnO2/MWNT cathodes are also comparable with those indicated in the latest reports. The capacity of 796 mAh/g(electrode) [i.e., 1768 mAh/g(carbon), Fig. 2(a)], excluding O2 at 70 mA/g, is comparable with the previously reported capacity of 730 mAh/g(electrode)[6]. Ishihara reported that the mesoporous α-MnO2 mixed cathode exhibited a much lower capacity of 365 mAh/g(electrode), excluding O2 at 0.025 mA/cm2(electrode)[3]. In the present study, the capacity is 1013 mAh/g(electrode) (i.e., 2250 mAh/g(carbon)) at 0.025 mA/cm2(electrode). Moreover, the electrochemical properties of the MnO2/MWNT air cathodes are comparable with those of a PtAu/C catalyst investigated by Lu, especially in terms of high rate performance [13]. The high performance obtained in the current work may be due to the unique structure of MnO2/MWNTs. The electrochemical impedance spectra (EIS) indicate that the MnO2 catalyst directly supported on MWNTs can exhibit better conductance, resulting in the high activity of the air cathodes. However, these electrochemical mechanisms still require further elucidation.


4. Conclusion

MnO2/MWNT composites were synthesized, measured, and directly used as air cathodes in lithium-air batteries. These air cathodes enhance oxygen reduction and evolution reactions, and effectively improve the energy efficiency and cyclic ability. The MnO2/MWNT cathodes exhibit a low charge potential of 3.8 V and a considerable capacity of 1768 mAh/g(carbon) (796 mAh/g(electrode)), excluding O2 at 70 mA/g. We conclude that this approach affords provides an easy and effective route for the fabrication of other composite cathodes that exhibit better electrochemical properties.


Acknowledgment

L. H. Guan is thankful for the financial support provided by the National Key Project on Basic Research (grant nos. 2009CB939801 and 2011CB935904), Natural Science Foundation of Fujian Province (grant no. 2010J05041), and Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (grant no. SZD09003).



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