A comparative study of nanostructured α and δ MnO₂ for lithium oxygen battery application

Abstract

α- and δ-MnO₂ nanomaterials with different morphology like urchins and flowers are successfully synthesized by a low temperature hydrothermal synthesis. The prepared nanostructures were applied as electrocatalysts for air cathodes in Li air batteries. The synthesized materials possess high electrocatalytic activity and the MnO₂ catalysed electrodes doubled the initial cycling capacity of the Li air cells without any catalysts. We also observed reduced over potential and upon cycling with limited capacity, a very stable performance was obtained.

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Introduction

The rapidly increased development of portable electronics and electric vehicles demands a high energy and high power secondary battery technology.¹ At this point the remarkably high energy density of metal air batteries has attracted the focus of all battery researchers. The development of metal air batteries became attractive since they use metal anode and the oxygen in the air as the cathode. Theoretically, with an environmentally abundant oxygen cathode, the metal air battery could be lighter, cheaper and long lasting (regenerative) with high energy density.^2,3 The metal air batteries have many possible candidates as the aqueous (Ca, Al, Fe, Cd, and Zn–air) and non-aqueous (Li–air) cells.⁴ Recently, various other batteries, like Na-ion/Na–air, hybrid cells, sulphur batteries, etc. have also been studied for the applications due to their high energy density and light weight.^5,6 Owing to an outstanding specific capacity of 3842 mA h g⁻¹ comparable to that of gasoline, however, it is considered that Li-air battery far exceeds all other batteries including Li-ion batteries which are presently used in EV.^3,4 Moreover, the Lithium air batteries have many advantages over fuel cells and other energy storage systems in terms of their availability, portability, cost and wide range of operating conditions. As an emerging technology these batteries still have some challenges to overcome mainly the sluggish electrochemistry on the oxygen reduction reaction (ORR) in air cathode. In order to improve the ORR and overall performance of this cell, specific activities of the catalysts used in air cathode should be improved.^7–9

MnO₂ are one of the promising and most suitable catalyst materials for high performance lithium air batteries.^10–12 Manganese oxides have their advantages of abundance, low cost, high activity in alkaline media and non-toxicity and can be applied as promising catalysts in air electrode for both alkaline fuel cells and metal–air batteries. Cao et al.¹³ have studied the mechanism of the ORR in MnO₂ catalysed air electrodes in alkaline solution. They found that the catalytic activities of MnO₂ with different crystalline structures are in the sequence β-MnO₂ < λ-MnO₂ < γ-MnO₂ < α-MnO₂ ≈ δ-MnO₂. Bruce et al.¹⁴ and few others^13–16 have also compared all types of MnO_x, such as α-MnO₂, β-MnO₂, bulk α-, β-, γ-, λ-MnO₂ and commercial Mn₂O₃ and Mn₃O₄ and concluded that α-MnO₂ nanowires are the most effective catalysts for lithium–air battery with their crystal structure and high surface area. So it is clear that the morphology and crystalline structure of MnO₂ nanomaterials plays a main role in the catalytic property of the material. Even though there have been lots of previous studies in MnO₂ nanomaterials, it is still a great challenge to synthesize pure and uniform MnO₂ nanostructures with different crystalline phases for better electrocatalytic activity. Also, to promote the wide application of MnO_x in ORR and air cathodes, a systematic study on its influential parameters is necessary.

In this work, we have chosen the best crystalline structures of MnO₂ for ORR from previous literatures and comparatively investigated them as electrocatalysts of Li–air cathode. As far as we know there has not been any comparison between α-MnO₂ and δ-MnO₂ nanomaterials or their application in Li–O₂ battery. The intention of this study is to comparatively investigate the catalytic properties of MnO₂ nanomaterials with different phases and morphologies. Sea urchin shaped α-MnO₂ and layered birnessite (flower) structured δ-MnO₂ were prepared successfully by hydrothermal synthesis. The formation of different morphologies like urchin and layered birnessite structures were achieved by controlling the hydrothermal reaction parameters. This synthetic route requires no templates or catalysts. The synthesized nanostructures exhibit high performance in lithium air battery with steady life cycle.

Experimental

The sea urchin shaped α-MnO₂ nanoparticles or nanourchins were prepared by the addition of 0.34 g MnSO₄·H₂O, 0.54 g of K₂S₂O₈ and 2 mL of H₂SO₄ into 40 mL of deionized water under stirring condition for 15 minutes. Then the solution was transferred to teflon-lined stainless steel autoclave and kept in a preheated electric oven at 110 °C for 6 hours. The autoclave was then cooled down to room temperature and the brown precipitate was obtained. The product was centrifuged and washed with deionized water several times and then dried at 60 °C for 8 hours in air. In a typical synthesis of flower like δ-MnO₂ layered birnessite, a 15 mL aqueous solution containing 0.2 g of MnSO₄·3H₂O and 0.5 g of KMnO₄ was transferred into teflon-lined stainless steel autoclave and kept in a preheated electric oven at 140 °C for 2 hours. The final product was cooled to room temperature, filtered and washed with distilled water and then dried at 80 °C for 6 hours in air. All chemicals were of analytical grade and used as received without further purification.

Results and discussion

Fig. 1 gives the XRD spectra for both α- and δ-MnO₂ nanomaterials. The spectra show the growth of pure α- and δ-phases of MnO₂ with tetragonal crystalline (I4/m space group) α-MnO₂ (nanourchins) and monoclinic amorphous (C2/m space group) δ-MnO₂ (nanoflowers) structures (JCPDS 44-0141). The (001) and (002) peaks in δ-MnO₂ corresponds to its layered structure. The lattice parameters of both α- and δ-MnO₂ nanomaterials are calculated by Rietvald analysis; the measured lattice parameters of α-MnO₂ are a = b = 9.8 Å and c = 2.8 Å comparable to the standard values a = b = 9.78 Å and c = 2.8 Å.¹⁹ The lattice parameters for δ-MnO₂ phases were also measured to be a = 5.1 Å, b = 2.8 Å and c = 7.1 Å comparable to the standard values of a = 5.15 Å, b = 2.84 Å and c = 7.17 Å.²⁰ In addition, the surface area of α-MnO₂ (nanourchins) is 50 m² g⁻¹ and δ-MnO₂ is 67.82 m² g⁻¹ measured by Brunauer, Emmett and Teller analysis.

Fig. 1 XRD spectra of α- and δ-MnO₂ nanomaterials.

The formation of different nanostructures of synthesized α-MnO₂ and δ-MnO₂ are confirmed by FESEM and TEM measurements, as shown in Fig. 2(a–h). The FESEM images are shown in the inset of (b and f) in Fig 2. The FESEM image 2b clearly shows the formation of uniform α-MnO₂ nanourchins with 2–2.5 μm diameter which consists of several straight and radially grown nanorods with uniform diameter of 50 nm. The FESEM image 2f shows the core corona architecture of birnessite-type flowers of δ-MnO₂ in which the corona is composed of nanosheets which are grown perpendicularly arranged from the core. The nanosheets possess uniform thickness of approximately 10 nm. The TEM images in Fig. 2(a–d) of α-MnO₂ also show the clear formation of nanourchins by the radially aligned nanorods. The HRTEM image 2c shows the lattice fringes along nanourchins' axes indicating the alignment of the primary nanorods in urchin. The SAED pattern 2d and HRTEM analysis suggest that the nanorods in nanourchins grow along the (001) direction (c axis). The lattice distance was calculated to be 0.69 nm which corresponds to the (110) plane which can be seen in the SAED pattern and consistent with XRD. The TEM of flowers 2(e–h) shows the formation of thick flowers with overlapping nanosheets. The HRTEM image in 2g shows anisotropic growth of the flowers with amorphous structure in the SAED (2h).

Fig. 2 TEM images of α-MnO₂ (a–d) and δ-MnO₂ (e–h) nanomaterials; inset (b) and (f) shows FESEM images of α- and δ-MnO₂ respectively.

MnO₂ can form many polymorphs by interlinking the basic MnO₆ octahedron in different configurations. Fig 3(a and b) clearly shows the schematic crystal structure of the two types of MnO₂. Birnessite-type (δ-MnO₂) has a 2D lamellar structure (Fig 3b) with an interlayer distance of 0.73 nm. Structurally, α-MnO₂ is constructed from the double chains of edge-sharing MnO₆ octahedron (Fig 3a) which are linked at the corners to form (2 × 2) + (1 × 1) tunnel structures (the sizes of the (2 × 2) and (1 × 1) tunnels are 0.46 and 0.18 nm, respectively). These tunnels extend in a direction parallel to the c axis of the tetragonal unit cell.¹⁷

Fig. 3 Crystal structures of α- and δ-MnO₂ nanomaterials.

On the bases of our observation and results the formation mechanism of sea urchin and layered birnessite structures can be explained by Ostwald ripening process.²¹ The formation mechanism of sea urchin can be explained as follows. Initially the high concentration of the precursor leads to a rapid formation of large number of nuclei followed by the slow crystal growth. Nanorods epitaxially grow along the surface of initial microspheres and form the solid urchins. Starting from the nanorods located on the outside there is subsequent crystallization process of cores resulting in interior cavity forms the α-MnO₂ urchins. Layered birnessite (δ-MnO₂) can also be rationalized as follows. Initially large numbers of nuclei are formed in a short time and they aggregate to form amorphous spheres by the reaction between MnSO₄ and KMnO₄. Then there is slow heterogeneous growth of 2D nanosheets on the dense core resulting in small flowers determined by the intrinsic layered crystal structure.²¹

For lithium battery studies, a pellet type electrode pressed on Ni mesh current collector was made by mixing (α-MnO₂/δ-MnO₂) catalyst with conductive KB carbon (1 : 2) in isopropyl alcohol. The pellet was pressed on Ni mesh and dried over night at 100 °C. Thus made cathode was assembled with Li anode in 1M LiTFSI (TEGDME) (1 : 1) electrolyte in Swagelok™ cells. The batteries were tested in a BTS 2004 (JAPAN) battery tester with at 0.1 mA cm⁻² within a potential range of 2–4.3 V in room temperature and 1 atm O₂. The charge–discharge profiles of α-MnO₂ and δ-MnO₂ catalysed Li air cells are given in Fig 4. The addition of catalysts has significantly increased the discharge capacity and overall performance of the cells. The first discharge curve of α-MnO₂ nanourchins reached a maximum capacity of 6125.5 mA h g⁻¹ at 2.0 V with a current density of 0.1 mA cm⁻². A flat plateau is observed at 2.7 V. On charging, a maximum capacity of 6125.5 V at 4.3 V is obtained with 100% reversibility upon Li⁺ intercalation with no loss of capacity. The over potential of this charge–discharge curves is 1.2 V and open circuit potential is 3.35 V. This capacity obtained is thrice than that of the original capacity of the cells without any MnO₂ catalysts. Similarly, the first discharge and charge curve of δ-MnO₂ layered birnessite flower catalysts exhibit a maximum capacity of 3674 mA h g⁻¹. There is no irreversible capacity on charging. This capacity is almost half of that of air cell with α-MnO₂ nanourchins' catalyst but almost twice than that of the original capacity without any catalysts. The results we obtained are significantly higher than α-MnO₂ and GO/α-MnO₂ obtained by Yu et al.²² Yu et al. also observed rapid degradation of capacity during charge–discharge cycle and a very high charging over potential. While both our synthesized MnO₂ nanomaterials make a suitable catalyst for the Li air cathode, α-MnO₂ is better than δ-MnO₂ in terms of stability and capacity. This difference in specific capacity can be explained in terms of their phase and crystal structures.^17,18 The increased electrocatalytic activity of the α-MnO₂ must be because of the large tunnel structures which favour the intercalation of ions in the lattice framework. In addition it has been reported that α-MnO₂ contains more defects and OH⁻ groups which are beneficial to surface adsorption of O₂ and dissociation of O–O bonds.¹⁶ The average Mn–O bond lengths of layered birnessite δ-MnO₂ and α-MnO₂ are about 1.94 and 1.98 Å, respectively.²³ Therefore with a stronger bond birnessite δ-MnO₂ shows poor activity resulting in a lower electrocatalytic activity in the cell performance. The electrocatalytic activity is determined by the reversible adsorption/desorption of reactants/products into and out of the structure. The layered birnessite δ-MnO₂ enables adsorption of more O₂ molecules because of its large interlayer spacing which is 0.7 nm, but the MnO₆ octahedral sheet of layered birnessite δ-MnO₂ is much denser than that of α-MnO₂. So the overall reducibility of birnessite δ-MnO₂ is lower than that of α-MnO₂.¹⁶ Very recently Jung et al.²⁴ has reported the electrocatalytic property of metal-decorated MnO₂ catalysts. Their electrochemical studies using the RDE and hybrid Li–O₂ batteries also suggest that the metal-decorated α-MnO₂ can be used as an efficient bi-functional catalyst for rechargeable hybrid Li–O₂ batteries. They have also observed the reduced over potential with metal-decorated α-MnO₂ electrocatalysts but with time dependent limited cycling.

Fig. 4 Charge–discharge profiles of α- and δ-MnO₂ nanomaterials.

To improve the cycle performance we limited the depth of discharging–charging.^25,26 Fig. 5(a and b) shows the cycle life versus specific capacity of the battery with urchin shaped α-MnO₂ and layered birnessite δ-MnO₂ catalysts. A uniform discharge–charge profile with 0.8 mA h was obtained for more than 35 cycles with 100% efficiency with urchin shaped α-MnO₂. There is a small increase in voltage around after every 10 cycles. A uniform discharge–charge profile with 0.8 mA h was obtained for 20 cycles with 100% efficiency for δ-MnO₂ and significant increase in the over potential of δ-MnO₂ catalysts. After 20 cycles the instability of δ-MnO₂ causes the increase in over potential which results in electrolyte decomposition which can be seen from the deterioration of charge curve. The charge–discharge efficiency of the air cells with urchin shaped α-MnO₂ and layered birnessite δ-MnO₂ catalysts are also given in Fig 5(c and d). The results obtained suggest that, the synthesis method used assures good catalytic performance of the catalysts and α-MnO₂ can potentially applied as electrocatalyst for air cathode. For a practical approach, further studies can be focussed on graphene based MnO₂ composites cathodes to improve the capacity retention and stable performance of the cell.

Fig. 5 Cycle life efficiency of α- and δ-MnO₂ nanomaterials.

Conclusions

To summarize, a systematic study has been carried out on the catalytic properties of MnO₂ based nanostructures as electrocatalysts for Li air batteries. Urchin shaped α-MnO₂ and layered birnessite δ-MnO₂ catalysts were synthesized by hydrothermal synthesis and applied in Li–O₂ batteries. The Li–O₂ batteries with MnO₂ catalysts perform explicitly better than without any catalysts proving α-MnO₂ to be potential electrocatalysts for air cathodes. Among the two phases and structures of MnO₂ used, urchin shaped α-MnO₂ exhibits very good catalytic activity with very high capacity and stable cycling than layered birnessite δ-MnO₂ catalysts. The catalytic performance of α-MnO₂ is associated with the morphology and size of the particles, and also with their crystal structures with (2 × 2) tunnels.

Acknowledgements

This work was supported by the Human Resources Development program (no. 20114030200060) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2013R1A1A2012656). One of our authors Mr Awan Zahoor is thankful to the NED University of Engineering & Technology, Pakistan, for their financial support rendered for his PhD.

Notes and references

1. S. Yoda and K. Ishihara, J. Power Sources, 1997, 68, 3–7.
2. M. Armand and J. M. Tarascon, Nature, 2008, 451, 652–657.
3. R. Black, B. Adams and L. F. Nazar, Adv. Energy Mater., 2012, 2, 801–815.
4. J. S. Lee, S. T. Kim, R. Cao, N. S. Choi, M. Liu, K. T. Lee and J. Cho, Adv. Energy Mater., 2011, 1, 34–50.
5. Q. Sun, Y. Yang and Z. Fu, Electrochem. Commun., 2012, 16(1), 22–25.
6. Z. Lin, Z. Liu, W. Fu, N. J. Dudney and C. Liang, Angew. Chem., Int. Ed., 2013, 52, 7460–7463.
7. Z. L. Wang, D. Xu, J. J. Xu and X. B. Zhang, Chem. Soc. Rev., 2014 10.1039/c3cs60248f.
8. G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson and W. Wilcke, J. Phys. Chem. Lett., 2010, 1, 2193.
9. A. Zahoor, M. Christy, Y. J. Hwang and K. S. Nahm, J. Electrochem. Sci. Technol., 2012, 3(1), 14–23.
10. Y. Shao, S. Park, J. Xiao, J. Zhang, Y. Wang and J. Liu, ACS Catal., 2012, 2, 844–857.
11. D. Capsoni, M. Bini, S. Ferrari, E. Quartarone and P. Mustarelli, J. Power Sources, 2012, 220, 253–263.
12. J. Wang, Y. Li and X. Sun, Nano Energy, 2013, 2, 443–467.
13. Y. L. Cao, H. X. Yang, X. P. Ai and L. F. Xiao, J. Electroanal. Chem., 2003, 557, 127–134.
14. A. Débart, A. J. Paterson, J. Bao and P. G. Bruce, Angew. Chem., Int. Ed. Engl., 2008, 47(24), 4521.
15. F. Cheng, J. Zhao, W. Song, C. Li, H. Ma, J. Chen and P. Shen, Inorg. Chem., 2006, 45, 2038.
16. F. Cheng, Y. Su, J. Liang, Z. Tao and J. Chen, Chem. Mater., 2010, 22, 898–905.
17. W. Xiao, D. Wang and X. W. Lou, J. Phys. Chem. C, 2010, 114, 1694.
18. Y. Wang, H. Liu, M. Bao, B. Li, H. Su, Y. Wen and F. Wang, J. Alloys Compd., 2011, 509, 8306.
19. X. Wang and Y. Li, J. Am. Chem. Soc., 2002, 124(12), 2880–2881.
20. X. Zhang, B. Li, C. Liu, Q. Chu, F. Liu, X. Wang, H. Chen and X. Liu, Mater. Res. Bull., 2013, 48, 2696–2701.
21. H. C. Zeng, Curr. Nanosci., 2007, 3, 177.
22. Y. Yu, B. Zhang, Y. He, Z. Huang, S. Oh and J. Kim, J. Mater. Chem. A, 2013, 1, 1163.
23. S. H. Liang, F. Teng, G. Bulgan, R. L. Zong and Y. F. Zhu, J. Phys. Chem. C, 2008, 112, 5307.
24. K. N. Jung, A. Riaz, S. B. Lee, T. H. Lim, S. J. Park, R. H. Song, S. Yoon, K. H. Shin and J. W. Lee, J. Power Sources, 2013, 244, 328.
25. R. Padbury and X. Zhang, J. Power Sources, 2011, 196(10), 4436–4444.
26. D. Xu, Z. l. Wang, J. J. Xu, L. L. Zhang and X. B. Zhang, Chem. Commun., 2012, 48, 6948.

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