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

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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.

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], [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], [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], [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.

Section snippets

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

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

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

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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