Coordination polymer-derived mesoporous Co₃O₄ hollow nanospheres for high-performance lithium-ions batteries

Abstract

A green and convenient approach has been developed to fabricate mesoporous Co₃O₄ hollow nanospheres composed of nanosized building subunits, which involves a morphology-inherited and thermolysis-induced transformation of Co-based coordination polymers. When evaluated as anode materials for lithium-ion batteries, the as-fabricated hollow nanospheres exhibited excellent electrochemical performance with high reversible specific capacity, excellent cycling stability (851 mA h g⁻¹ after 100 cycles) and exceptional rate capability (1053, 976, 887, and 695 mA h g⁻¹ at the current densities of 0.3, 0.5, 1 and 5 A g⁻¹, respectively).

---

Introduction

In the past decades, rechargeable lithium ion batteries (LIBs) have been widely applied in various portable electronic devices and are regarded as the most promising power sources for hybrid electric vehicles (HEVs) and plug-in HEVs (PHEVs).^1–3 Graphite materials are commonly used as anode materials for LIBs. However, their relatively low capacity (∼372 mA h g⁻¹) makes it difficult for them to meet the high power and energy density demands of next-generation LIBs. Consequently, considerable efforts have been devoted to exploring high-performance electrode materials.^4–14

Cobalt oxide (Co₃O₄), an important member of the transition metal oxide family, is considered to be a promising anode material owing to its high theoretical capacity of 890 mA h g⁻¹.¹⁵ Unfortunately, it suffers from a large volume change upon insertion and extraction of lithium-ions and subsequently particle pulverization, which may lead to poor cycling performance and low rate capability. To address this problem, various well-defined micro/nanostructures of Co₃O₄ including nanowires, nanotubes, nanosheets, nanorings, nanocapsules and nanoflowers have been fabricated, with the aim to reduce their volume changes and improve their structural stability.^16–25 Among them, porous structures with a hollow feature are of particular interest since their porosity can ensure efficient electrolyte penetration and their interior space can allow the volume variation, which can lead to improved anode performance.^26–29 To date, the templating approach and the solvothermal method have been employed to achieve the porous hollow Co₃O₄ structures. On the other hand, coordination polymers (also known as metal–organic frameworks or MOFs), composed of metal ions/clusters and organic bridging ligands, have been demonstrated as promising precursors to construct the morphology-inherited porous metal oxides.^30–33 For example, Co-based MOFs have been employed as sacrificial template to prepare Co₃O₄ nanocages.³² MOF-71 serving as a removable precursor in the synthesis of mesoporous nanostructured Co₃O₄ has also been reported.³³ Nevertheless, even with MOFs-templating strategies, they are usually tedious and involve the use of surfactants or toxic organic solvents. Therefore, in view of environmental protection and resource conservation, it is still highly desirable to develop a green and convenient method for the rational design and synthesis of porous hollow Co₃O₄ nanostructures.

Herein, we report a green and convenient strategy to fabricate mesoporous Co₃O₄ hollow nanospheres via the formation of Co-based coordination polymer (CP) nanospheres followed by calcination at appropriate temperature. When evaluated as anodes for LIBs, the as-fabricated Co₃O₄ hollow nanospheres manifested a superior storage capability and excellent rate capability.

Results and discussion

The schematic illustration of the as-synthesized porous Co₃O₄ hollow nanospheres is shown in Fig. 1. First, Co²⁺ from dissolving cobalt nitrite in distilled water was coordinated with 2-methylimidazole during hydrothermal process to form nanosphere structured Co-based coordination polymer (Co-CP) at room temperature. A subsequent thermal conversion process was then introduced to transform Co-CP into Co₃O₄ hollow nanospheres.

Fig. 1 Schematic illustration of fabrication of Co-CP-derived Co₃O₄ hollow nanospheres.

The size and morphology of Co-CP was investigated by FESEM, as shown in Fig. 2a. It is clearly observed that the obtained Co-CP particles were monodisperse with sizes ranging from 100 to 200 nm and had sphere-like morphology. The magnified FESEM image (the inset in Fig. 2a) reveals that these nanospheres had relatively smooth surface. After being annealed in a furnace with a flowing argon gas and then air, the Co₃O₄ hollow nanospheres could be obtained by thermal-induced decomposition. A typical FESEM image of the products indicates that they preserved well the size and sphere shape of the Co-CP precursor (Fig. 2b). The magnified FESEM image further reveals that they had a rough surface in texture consisting of tiny nanoparticles (Fig. 2c). Meanwhile, the energy-dispersive X-ray spectroscopy (EDS) analysis confirmed that the nanospheres were only composed of Co and O elements (Fig. S1, ESI†). X-ray diffraction (XRD) analysis in Fig. 2d confirms that the annealed products can be assigned to a pure cubic spinel Co₃O₄ (JCPDS card no. 42-1467, space group Fd3m).

Fig. 2 (a) FESEM image of Co-PC nanospheres, (b) low- and high-magnified (c) FESEM images of Co₃O₄ nanospheres, and (d) XRD patterns of Co₃O₄ nanospheres.

The detailed structure of Co₃O₄ nanospheres was further characterized by transmission electron microscope (TEM). A low-magnification TEM image in Fig. 3a shows non-uniform contrast between the center and edges of the nanosphere shell, indicating a hollow interior feature of Co₃O₄. The magnified TEM image in Fig. 3b further confirms that this sphere-like structure was porous and composed of non-regular subunits with sizes ranging from 10 to 20 nm. Fig. 3c shows an HRTEM image of Co₃O₄ nanosphere, in which the lattice fringes with the d-spacings of 0.466 and 0.285 nm corresponded well to the (111) and (220) lattice plans of Co₃O₄, respectively (top-right inset). The corresponding fast Fourier transform pattern indicates the dominant crystallization orientation (bottom-left inset). The highly exposed reactive (111) crystal panes may be beneficial to improved lithium storage in Co₃O₄. The texture and porosity of Co₃O₄ hollow nanospheres were quantified by measuring the N₂ adsorption–desorption isotherm in Fig. 3d. The hysteresis loops imply that the Co₃O₄ hollow nanospheres possessed a typical mesoporous structure and had a large BET specific surface area of 123.6 m² g⁻¹. The pore-size distribution calculated using the Barrett–Joyner–Halenda (BJH) reveals that the majority of the mesoporous were around 10 nm in diameter.

Fig. 3 (a) Low- and (b) high-magnified TEM images of mesoporous Co₃O₄ hollow nanospheres; (c) HRTEM image of Co₃O₄ hollow nanospheres with the top-right and the bottom-left inset being an enlarged HRTEM image and the corresponding FFT pattern, respectively; (d) N₂ sorption isotherms of Co₃O₄ hollow nanospheres with the inset showing the pore-size distribution calculated using the BJH method.

Considering the advantages of mesoporous Co₃O₄ hollow nanospheres as anodes for LIBs, electrochemical measurements were carried out based on the standard half-cell configuration. Cyclic voltammetry (CV) curves of the as-prepared Co₃O₄ hollow nanospheres electrode are shown in Fig. S2 (ESI†). During the first scan, a main cathodic peak located at a potential of 0.86 V can be assigned to the reduction process of Co₃O₄ to metallic Co, the electrochemical formation of Li₂O, and the reversible formation of a solid electrolyte (SEI) layer.^34–36 Meanwhile, an anodic peak at 2.13 V is observed, corresponding to the reverse reaction including the re-oxidation of Co to Co₃O₄ and the decomposition of Li₂O. From the second cycle, the main cathodic peak shifts to a higher potential of 1.08 V, while the anodic peak remains almost unchanged. Furthermore, the subsequent CV curves exhibit good reproducibility and almost overlap, indicating a good reversible reactions in the electrode materials. Fig. 4a shows representative discharge–charge voltage profiles of the mesoporous Co₃O₄ nanospheres for the 1st, 2nd, 50th and 100th cycles at a current density of 0.1 A g⁻¹ in the voltage range of 0.01–3 V versus Li⁺/Li. It can be seen that the first discharge curve exhibits a long flat voltage plateau at about 1 V, and this voltage plateau is upshifted in the following cycles, which is consistent with typical charge–discharge voltage profiles of Co₃O₄.²³ The first discharge and charge capacities are 1275 and 859 mA h g⁻¹, respectively, leading to an irreversible capacity loss of 33%. This phenomenon is mainly attributed to the irreversible processes such as the inevitable formation of a solid electrolyte interface (SEI) layer and the decomposition of the electrolyte, which is common for transition metal oxides.²⁸ The curve of capacity versus cycle number at a current density of 0.1 A g⁻¹ is shown in Fig. 4b. From the second cycle onwards, the mesoporous Co₃O₄ hollow nanospheres electrode exhibited remarkable capacity retention upon prolonged cycling, and a reversible capacity as high as 851 mA h g⁻¹ could still be retained at the end of 100 charge–discharge cycles. In addition, after three cycles, its coulombic efficiency gradually climbs to 97.5% and levels off afterwards, suggesting a facile lithium insertion/extraction associated with efficient transport of ions and electrons in the electrode. Notably, even at a higher current density of 1 A g⁻¹, the capacity of the Co₃O₄ hollow nanospheres also maintain the value of 696 mA h g⁻¹ after 500 cycles, demonstrating its suitability for high-rate applications (Fig. S3, ESI†). The morphology of the electrode after 100 cycles was shown in Fig. S4 (ESI†). Although the Co₃O₄ nanospheres in the electrode are not clear due to the formed SEI layers, the characteristic of hollow structures is basically retained.

Fig. 4 Electrochemical properties of the mesoporous Co₃O₄ hollow nanospheres as electrodes in LIBs: (a) galvanostatic charge–discharge voltage profiles at the current density of 0.1 A g⁻¹, (b) cycling performance at the current density of 0.1 A g⁻¹, (c) rate capability at various current densities between 0.1 and 5 A g⁻¹; (d) comparison of the rate capabilities of the Co₃O₄ hollow nanospheres in this work and the previously reported Co₃O₄ nanostructures-based electrodes.

Besides the good specific capacity and excellent cyclability, the rate capability is also crucial for high-performance LIBs. From the rate capability tests shown in Fig. 4c, the electrode delivered the average capacities of 1077, 1053, 976, and 887 mA h g⁻¹ at 0.1, 0.3, 0.5 and 1 A g⁻¹, respectively. Even at a high rate of 5 A g⁻¹, the specific capacity was still as high as 695 mA h g⁻¹, which is much higher than the theoretic capacity of graphite electrodes (372 mA h g⁻¹). Moreover, after the high-rate charge–discharge cycling, a capacity of 1126 mA h g⁻¹ was still regained upon the reduction of the rate to 0.1 A g⁻¹, indicating the good reversibility of the electrode materials. Fig. 4d compares the rate performance between the mesoporous Co₃O₄ nanospheres and the previously reported Co₃O₄ nanostructures-based electrodes.^{20,22–24,29,32–35} Our Co₃O₄ nanospheres were tolerant to varied current rates and exhibited a much more stable high-rate performance with the minimum decrease in capacity as the current density increased.

The excellent electrochemical performance might be related to the unique structural features in several aspects: (1) the building subunits with an ultra-small size allow short distance for the diffusion of Li-ions and facilitate their fast transport, which is crucial to the rate capability; (2) the void space within the hollow interior can effectively endure the volume expansion/contraction during the Li-ions insertion/extraction processes, and hence alleviate the pulverization of the electrode and improve the cycling capability; (3) the porosity in the shells and resultant high specific surface area can provide abundant channels and sites for efficient electrolyte penetration between the Co₃O₄ particles and a large electrode-electrolyte contact area for rapid electrochemical reaction.

Conclusions

In summary, we have successfully designed and synthesized mesoporous Co₃O₄ hollow nanospheres by using Co-based coordination polymers as both the precursor and the self-sacrificing template. The as-synthesized Co₃O₄ combines the benefits of a mesoporous shell, a hollow interior structure and ultrafine building blocks. In view of their unique structural advantages, these Co₃O₄ hollow nanospheres electrodes have exhibited high lithium storage capacities, superior cycling performance and excellent rate capability.

Experimental

Materials synthesis

All chemicals were purchased from commercial sources and used without purification. In a typical synthesis, 0.291 g Co(NO₃)₂·6H₂O and 2.624 g 2-methylimidazole were dissolved in 5 ml and 200 ml of deionized (DI) water, respectively. These two solutions were then mixed vigorously for 5 min, and the resulting solution was incubated at room temperature for 24 h. The as-obtained precipitate was isolated by centrifugation, thoroughly washed with de-ionized water and ethanol, and finally vacuum-dried at 60 °C. To prepare Co₃O₄ nanospheres, the powder of Co-CP nanospheres was placed in a tube furnace and then heated to 400 °C for 30 min with a ramp of 2 °C min⁻¹ under a nitrogen gas flow. Afterwards, the nitrogen gas flow was switched off, and the furnace was still kept at this temperature in air for another 60 min. Lastly, the product was taken out, showing that its colour had changed from purple to black.

Materials characterization

The morphologies and structures were characterized by a field emission scanning electron microscope (FESEM, JEOL JSM7600F) and high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F), respectively. The phase of the products was identified by X-ray powder diffractometer (Bruker D8 Advance) with Cu Kα radiation (λ = 1.5406 Å). The Brunauer–Emmett–Teller (BET) specific surface areas of the products were calculated from the N₂ physisorption at 77 K using a Quantachrome Instruments Autosorb AS-6B equipment.

Electrochemical measurements

The electrochemical measurements were carried out with CR2032-type coin cells. The working electrode was prepared by mixing 80 wt% active materials (Co₃O₄ hollow nanospheres), 10 wt% Super-P carbon black and 10 wt% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidinone (NMP) solution. The mixed slurry was then coated onto copper foil and dried at 80 °C for 12 h. The mass loading of active materials is about 1.5 mg cm⁻². The coin cells were assembled in an argon-filled glove box using lithium metal as the counter electrode, Celgard 2400 membranes as the separator, and 1 M LiPF₆ in a mixture of ethylene carbonate and dimethyl carbonate (1 : 1 in volume) as the electrolyte. Cyclic voltammetry (CV) measurements were recorded using a CHI 760D electrochemical workstation. The galvanostatic charge and discharge tests were performed on a NEWARE battery instrument in the potential range of 0.01–3 V.

Acknowledgements

This work was supported by Ministry of Education, Singapore (Academic Research Fund TIER 1 – RG128/14), Nanyang Environment and Water Research Institute (Core Fund), Nanyang Technological University, Singapore, National Natural Science Foundation of China (51102278, 11547028) and China Postdoctoral Science Foundation (2015M571865).

Notes and references

1. M. Armand and J. M. Tarascon, Nature, 2008, 451, 652.
2. J. Cabana, L. Monconduit, D. Larcher and M. R. Palacin, Adv. Mater., 2010, 22, E170.
3. L. G. Lu, X. B. Han, J. Q. Li, J. F. Huang and M. G. Ouyang, J. Power Sources, 2013, 226, 272.
4. V. Aravindan, J. Gnanaraj, Y. S. Lee and S. Madhavi, J. Mater. Chem. A, 2013, 1, 3518.
5. J. Liu and D. F. Xue, Nanoscale Res. Lett., 2010, 5, 1525.
6. J. Liu, Y. C. Zhou, F. Liu, C. P. Liu, J. B. Wang, Y. pan and D. F. Xue, RSC Adv., 2012, 2, 2262.
7. J. Liu, C. P. liu, Y. L. Wan, W. Liu, Z. S. Ma, S. M. Jin, J. B. Wang, Y. C. Zou, P. Hodgson and Y. C. Li, CrystEngComm, 2013, 15, 1578.
8. R. B. Wu, X. K. Qian, Y. Yu, H. Liu, K. Zhou, J. Wei and Y. Z. Huang, J. Mater. Chem. A, 2013, 1, 11126.
9. S. H. Lee, V. Sridhar, J. H. Jung, K. Karthikeyan, Y. S. Lee, R. Mukherjee, N. Koratkar and I. K. Oh, ACS Nano, 2013, 7, 4242.
10. Z. H. Wen, G. H. Lu, S. Mao, H. Kim, S. M. Cui, K. H. Yu, X. K. Huang, P. T. Hurley, O. Mao and J. H. Chen, Electrochem. Commun., 2013, 29, 67.
11. R. B. Wu, X. K. Qian, K. Zhou, J. Wei and P. M. Ajayan, ACS Nano, 2014, 8, 6297.
12. R. W. Mo, Z. Y. Lei, K. N. Sun and D. Rooney, Adv. Mater., 2014, 26, 2084.
13. R. B. Wu, D. P. Wang, X. H. Rui, B. Liu, K. Zhou, A. W. K. Law, Q. Y. Yan, J. Wei and Z. Chen, Adv. Mater., 2015, 27, 3038.
14. J. Liu, C. Wu, D. D. Xiao, P. Kopold, L. Gu, P. A. v. Aken, J. Maier and Y. Yu, Small, 2016, 12, 2354.
15. M. V. Reddy, G. V. Suba Rao and B. V. Chowdari, Chem. Rev., 2013, 113, 5364.
16. C. C. Li, X. M. Yin, L. B. Chen, Q. H. Li and T. H. Wang, Chem.–Eur. J., 2010, 16, 5215.
17. K. Feng, H. W. Park, X. L. Wang, D. U. Lee and Z. W. Chen, Electrochim. Acta, 2014, 139, 145.
18. M. H. Chen, X. H. Xia, J. H. Yin and Q. G. Chen, Electrochim. Acta, 2015, 160, 15.
19. J. Liu, H. Xia, L. Lu and D. F. Xue, J. Mater. Chem., 2010, 20, 1506.
20. H. Huang, W. J. Zhu, X. Y. Tao, Y. Xia, Z. Y. Yu, J. W. Fang, Y. P. Gan and W. K. Zhang, ACS Appl. Mater. Interfaces, 2012, 4, 5974.
21. S. Q. Chen, Y. F. Zhao, B. Sun, Z. M. Ao, X. Q. Xie, Y. Y. Wei and G. X. Wang, ACS Appl. Mater. Interfaces, 2015, 7, 3306.
22. P. P. Su, S. C. Liao, F. Rong, F. Q. Wang, J. Chen, C. Li and Q. H. Yang, J. Mater. Chem. A, 2014, 2, 17408.
23. G. Y. Huang, S. M. Xu, S. S. Lu, L. Y. Li and H. Y. Sun, ACS Appl. Mater. Interfaces, 2014, 6, 7236.
24. D. Fang, L. C. Li, W. L. Xu, G. Z. Li, G. Li, N. F. Wang, Z. P. Luo, J. Xu, L. Liu, C. L. Huang, C. W. Liang and Y. S. Jia, J. Mater. Chem. A, 2013, 1, 13203.
25. W. J. Hao, S. M. Chen, Y. J. Cai, L. Zhang, Z. X. Li and S. J. Zhang, J. Mater. Chem. A, 2014, 2, 13801.
26. B. Zhang, Y. B. Zhang, Z. Z. Miao, T. X. Wu, Z. D. Zhang and X. G. Yang, J. Power Sources, 2014, 248, 289.
27. J. Y. Wang, N. L. Yang, H. J. Tang, Z. H. Dong, Q. Jin, M. Yang, D. Kisailus, H. J. Zhao, Z. Y. Tang and D. Wang, Angew. Chem., Int. Ed., 2013, 52, 6417.
28. R. B. Wu, X. K. Qian, X. H. Rui, H. Liu, B. Yadian, K. Zhou, J. Wei, Y. Long, Q. Y. Yan and Y. Z. Huang, Small, 2014, 10, 1932.
29. D. L. Wang, Y. C. Yu, H. He, J. Wang, W. D. Zhou and H. D. Abruña, ACS Nano, 2015, 9, 1775.
30. R. B. Wu, D. P. Wang, J. Y. Han, H. Liu, K. Zhou, Y. Z. Huang, R. Xu, J. Wei, X. D. Chen and Z. Chen, Nanoscale, 2015, 7, 965.
31. H. W. Song, L. S. Chen and C. X. Wang, J. Mater. Chem. A, 2014, 2, 20597.
32. C. Li, T. Q. Chen, W. J. Xu, X. B. Lou, L. K. Pan, Q. Chen and B. W. Hu, J. Mater. Chem. A, 2015, 3, 5585.
33. Y. Wang, B. F. Wang, F. Xiao, Z. G. Huang, Y. J. Wang, C. Richardson, Z. X. Chen, L. F. Jiao and H. T. Yuan, J. Power Sources, 2015, 298, 203.
34. J. X. Guo, B. Jiang, X. Zhang, L. Tang and Y. H. Wen, J. Mater. Chem. A, 2015, 3, 2251.
35. F. C. Zhang, K. Shi, X. H. Xu, X. Y. Liang, Y. C. Chen and Y. G. Zhang, RSC Adv., 2016, 6, 9640.
36. Z. P. Li, X. Y. Yu and U. Paik, J. Power Sources, 2016, 310, 41.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09608e

---