Ni-enhanced Co₃O₄ nanoarrays grown in situ on a Cu substrate as integrated anode materials for high-performance Li-ion batteries

Abstract

A two-step strategic approach was proposed to synthesize three-dimensional Co₃O₄ nanoarrays fabricated on a Cu substrate surface with a Ni layer as the interface. Firstly, a Ni-nanoseed-layer was prepared on a Cu substrate by electrodepositing Ni. And then Co₃O₄ nanoarrays were in situ grown on the Ni layer via a hydrothermal synthesis. The as-obtained materials were used directly as anodes for Li-ion batteries, and the electrodes maintained a high capacity up to 1150 mA h g⁻¹ at 0.1 C after 30 cycles, and showed an excellent cycling stability and rate capability. The good electrochemical performance is owing to the pre-electrodeposited Ni-nanoseed-layer, because the Ni layer could improve the mechanical adhesion between Co₃O₄ nanoarrays and substrates effectively and increase the conductivity of anodes, without applying binders or conductive additives. This strategic in situ synthesis method will probably open a new avenue for the development of integrated electrode materials for high-performance Li-ion batteries.

---

Introduction

Along with the rapid development of economies and societies, new energy-materials are extensively used in many areas, including portable electronics, electric vehicles, and smart grids.^1–4 Therefore research on battery materials with high capacity, cycling stability and security is necessary. As the Li-ion battery (LIB) has many advantages including high energy density, large output power, and friendly to the environment, it plays an irreplaceable role in the secondary energy supply.^5–8 Transition-metal oxide materials such as cobalt oxide and nickel oxide are promising high-energy-density anode materials. Among them cobalt-based electrode materials are attractive due to their high electrochemical activity and ease of processing,⁹ and Co₃O₄ as anode material has attracted more interest owing to its superior specific capacity (896 mA h g⁻¹ in theory),^10–13 which is much higher than that of commercial graphite anode (∼372 mA h g⁻¹). However, relatively poor capacity retention upon cycling and low rate capability of Co₃O₄ restrict its practical application in LIBs as a high-performance anode. Different approaches have been utilized to improve the electrochemical properties of Co₃O₄, such as the use of Co₃O₄/carbon nanocomposites, nanoscale Co₃O₄ and mesoporous Co₃O₄. In addition to the above approaches, considerable effort has been devoted to develop Co₃O₄ materials with three-dimensional (3D) microstructure,¹⁴ since the unique morphology can significantly enhance electrochemical performance: (1) the 3D microstructure provide better access for Li ions due to a larger electrode/electrolyte interface, and shorter diffusion paths for Li ions and electrons, leading to improved rate capability.^15,16 (2) The free space among nanoarrays can effectively buffer the large volume change during Li insertion/extraction, thus contribute to better cycling stability.^17,18 (3) The 3D nanoarrays show stable structural support and multiple interconnections across nanowires.

Besides the 3D nanoarray microstructure, we inferred that if Co₃O₄ nanoarrays could be in situ grown on the current collector while maintaining a good contact, it would show better cycling performance and durability when used as LIB anode materials directly. Considering the low-cost Cu sheets are common current collecting substrates, we proposed that 3D Co₃O₄ nanoarrays fabricated on Cu substrates are promising anode materials, since this integrated anode can enhance electrical conductivity and enlarge reaction surface.^19–21 However, free-standing Co₃O₄ nanoarrays grown directly on Cu sheets have rarely been realized.²² As the adhesion between nanoarrays and Cu substrates is weak, active materials are easy to fall off during the charging–discharging process, thus greatly reduces the cycling stability and energy density.

In this paper, we successfully prepared Ni-enhanced Co₃O₄ nanoarrays in situ grown on Cu substrate as integrated anode for high-performance LIBs, by a two-step approach. The new synthetic strategy combined two synthesis methods of electrodeposition of Ni-nanoseed-layer and hydrothermal synthesis. We found that pre-electrodepositing Ni-nanoseed-layer onto the Cu substrate could improve the mechanical adhesion between Co₃O₄ nanoarrays and substrates effectively and increase the conductivity of anodes, without applying binders or conductive additives. Those were the principal reasons for improved performance. In addition, we preliminarily speculated that the Ni layer might play some role in influencing the growth of Co₃O₄ nanoarrays, promoting the formation of nanoarrays with dominant (111) crystal planes that show much better performance than (001) planes.²³ Finally, when as-obtained Ni-enhanced Co₃O₄ nanoarrays were investigated as anode materials for LIBs, these integrated electrodes indeed exhibited good specific capacity, cycling stability and rate capability.

Experimental

Electrodeposition of Ni nanoseeds

Firstly, we carefully cleaned the Cu sheet in mixed acid solution for 5 min in order to remove the surface CuO layer, and then used deionized water and ethanol in turn to wash, repeating several times. Subsequently we electrodeposited Ni nanoparticles onto the Cu substrate (1 × 3 cm² in size). The concentration of plating solution was 250 g L⁻¹ NiSO₄·6H₂O, 30 g L⁻¹ NiCl₂·6HO, 35 g L⁻¹ H₃BO₃ and 0.1 g L⁻¹ sodium dodecyl sulfate (SDS), while the pH value was set at 3–5. In the three-electrode electrochemical cell, the acid-treated Cu worked as working electrode, Ni plate worked as counter electrode and reference electrode. Electrodepositing Ni was carried out at 45 °C with a stirring speed of 700 rpm, by imposing a constant current. The Ni depositing time was 60 s.

Synthesis of Co₃O₄ nanoarrays

Co₃O₄ nanoarrays were prepared by a hydrothermal synthesis method. Firstly, 1.46 g of Co(NO₃)₂·6H₂O, 0.37 g of NH₄F and 1.5 g of CO(NH₂)₂ were dissolved in 50 mL of distilled water. After being stirring slightly for 40 min, the solution was transferred into Teflon-lined stainless steel autoclave. The Cu substrate coated by Ni-nanoseed-layer was immersed into the reaction solution with a tilt angle of 45 degree. The stainless steel autoclave was sealed and heated at 120 °C for 5 h, and then the autoclave was cooled down to room temperature. After the reaction, the samples were rinsed with distilled water for several times. Finally, the Ni-enhanced Co₃O₄ nanoarrays in situ grown on Cu substrate was obtained after annealing the samples at 450 °C in air for 3 h, with a color changing of the samples from pink to black.

Characterization

The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus X-ray diffractometer with Ni-filtered Cu-Kα radiation (λ = 0.15406 nm). The scanned 2θ range was between 40° and 95° at room temperature. X-ray photoelectron spectroscopy (XPS, PHI 5700), scanning electron microscopy (SEM, JEOL JSM-7001F) and high-resolution transmission electron microscopy (JEOL JEM-2100).

Electrochemical measurements

Simulation of the battery were directly fabricated from the Co₃O₄–Ni@Cu as the working electrode without any ancillary materials. The electrolyte used was LiPF₆ (1 M)/EC + DEC + DMC (1 : 1 : 1, weight). The galvanostatic charge–discharge tests were performed using LAND battery testing system in the voltage range of 0.01–3.2 V at room temperature.

Results and discussion

Fig. 1 shows the two-step synthesis strategy for the Co₃O₄ nanoarrays by the combination of electrodeposition and hydrothermal synthesis. Firstly, Ni nanoparticles were electrodeposited onto the Cu substrate as seeds.²⁴ this sample was named Ni@Cu for simplicity. Subsequently Co₃O₄ nanoarrays were in situ grown upwards from the Ni-nanoseed-layer on the Cu substrate directly, using hydrothermal synthesis, followed by a calcinations treatment. Impressively, the synthesized integrated anode material showed a high specific capacity, an excellent rate capacity and a good cycling stability.

Fig. 1 Schematic for the growth process of Co₃O₄–Ni@Cu (two-step method).

The as-obtained final products of Ni-enhanced Co₃O₄ nanoarrays were named Co₃O₄–Ni@Cu for simplicity. The Energy-dispersive X-ray spectrometry (EDS) mapping analysis shown in Fig. S1 (ESI†) proves the existence of the Co₃O₄–Ni@Cu. The corresponding XRD patterns of Ni@Cu samples are provided in Fig. 2. Fig. 2a and b show the XRD pattern of the Cu substrates after being electrodeposited with Ni layer for 60 s and 200 s, respectively. It was found that the Ni peak was not obvious in a relatively short electrodepositing time, almost all peaks in Fig. 2a belonging to Cu (Fig. 2a). However, when the Ni electrodepositing time reached 200 s, Ni peaks could be observed clearly in Fig. 2b, indicating that the Ni-seed-layer was successfully deposited on the surface of Cu substrate.^25,26 Moreover, as shown in Fig. S2 (ESI),† the presence of Co₃O₄, Ni and Cu in Co₃O₄–Ni@Cu was proved by XRD spectrum.

Fig. 2 (a) and (b) XRD patterns of the products after being electrodeposited with Ni, while the electrodepositing time was 60 s and 200 s, respectively. (c) XPS peaks of Ni2p of the Cu substrate after being electrodeposited with Ni (Ni@Cu) and (d) XPS peaks of Co2p of the final product (Co₃O₄–Ni@Cu).

In order to clarify the components of the final product of Ni-enhanced Co₃O₄ nanoarrays and find some clues about the growth mechanism, the Ni@Cu and Co₃O₄–Ni@Cu samples were characterized by XPS. As shown in Fig. 2c, the Ni@Cu sample showed Ni2p_3/2 and Ni2p_1/2 peaks which are centered at 852.6 and 873.85 eV, respectively, confirming that the elementary Ni was deposited on the surface of Cu substrates. We suggested that Co₃O₄ nanoarrays were in situ grown from the Ni-nanoseed-layer which was firstly electrodeposited onto Cu substrate. Ni nanoseeds formed a connection layer between the Co₃O₄ nanoarrays and Cu substrate, playing a key role in stabilizing the nanoarrays grown on Cu substrates. In addition, as seen in Fig. 2d, the Co₃O₄–Ni@Cu sample exhibited peaks of Co2p_1/2 and Co2p_3/2 at 795.2 eV and 780.2 eV,⁸ respectively. The XRD and XPS results indicated that the final product was Ni-enhanced Co₃O₄ composite.

The morphology of Ni-nanoseed-layer on Cu substrate (Ni@Cu) was shown in Fig. 3a, we observed that the Ni nanoseeds were successfully growth on the Cu substrate. Similarly, Fig. 3b displayed the SEM photograph of 3D Co₃O₄–Ni@Cu composite nanoarrays, which were electrodeposited with Ni for 60 s and hydrothermal treated at 120 °C for 5 h. The as-obtained Ni-enhanced Co₃O₄ nanowires had an average diameter of 100 nm. In addition, various steps of formation of layers were proved by color changing (as seen in inset), from silver (Ni@Cu) to black (Co₃O₄–Ni@Cu).²⁷ In contrast, Fig. S3 (ESI†) displayed the SEM images of Co₃O₄@Cu nanoarrays without Ni nanoseeds. Moreover, too short or too long electrodepositing time was not favorable for the formation of Co₃O₄–Ni@Cu composite nanoarrays. As shown in Fig. S4 (ESI),† when the electrodepositing time was only 2 s, we observed that the Co₃O₄ nanoarrays were sparser than those with an electrodepositing time of 60 s. By contrast, when the electrodepositing time increased to 6000 s, the anode materials were likely to crack, and Co₃O₄ active materials were inclined to fall from the Ni-nanoseed-layer.

Fig. 3 (a) Low-magnification SEM images of Ni@Cu, the Ni electrodepositing time is 60 s (b)SEM images of the Co₃O₄–Ni@Cu nanoarrays, the hydrothermal synthesis condition: 120 °C for 5 h. Inset: photographs of the products in different reaction steps.

Further structural characterizations of the Ni-enhanced Co₃O₄ nanoarrays and Co₃O₄ nanoarrays without Ni were performed by TEM, as shown in Fig. 4a and c. Obviously, plenty of pores were distributed uniformly on the nanoarray surface due to the gas releasing (CO₂, H₂O) during the calcination process,^28,29 which can provide a quite short diffusion path for Li ions and electrons when used as anode materials for LIBs.^30–32 In Fig. 4b which was taken from Co₃O₄ nanoarrays in situ grown on Cu substrates with Ni-nanoseed-layer, we found that a measured interplanar spacing of 0.467 nm was in good agreement with the spacing of the Co₃O₄ (111) planes.²² In Fig. 4d which was taken from Co₃O₄ nanoarrays grown on Cu substrates directly without Ni-nanoseed-layer, it showed a measured interplanar spacing of 0.403 nm which was in good agreement with the spacing of the Co₃O₄ (001) planes. In general, for HRTEM results we found that Ni-enhanced Co₃O₄ nanoarrays had dominant (111) crystal planes while Co₃O₄ nanoarrays without Ni-nanoseed-layer showed dominant (001) crystal planes. Moreover, in previous work, addition of seed layer in nanostructure synthesis had been found to change the growth orientation and morphologies of nanostructure, such as ZnO, TiO₂ and SnO₂ nanomaterials.^33–36 Based on these work related to seed-layer and HRTEM results, here we preliminarily inferred that the Ni-nanoseed-layer might play some role in influencing the growth of Co₃O₄ nanoarrays and promote the formation of Co₃O₄ nanoarrays with (111) crystal planes. The relationship of Ni-nanoseed-layer and Co₃O₄ nanoarray crystal plane will be studied in future research.

Fig. 4 (a) TEM image of Co₃O₄ nanoarrays in situ grown on Cu substrates with Ni-nanoseed-layer. (b) HRTEM image of the above Co₃O₄ nanoarrays, showing a dominant (111) crystal plane. (c) TEM image of Co₃O₄ nanoarrays grown on Cu substrates directly without Ni-nanoseed-layer. (d) HRTEM image of the above Co₃O₄ nanoarrays, showing a dominant (001) crystal plane.

Recently, nanostructures exposing highly reactive crystal planes begin to exhibit great potentials for electrochemical energy storage. Li et al. systematically summarized the relationship between crystal plane of Co₃O₄ and electrochemical performance, and observed that Co₃O₄ nanoparticles with (111) planes showed better cycling and rate performances than those with (001) planes.²³ To further illustrate, the atomic configurations for various crystal planes of the Co₃O₄ unit cell are shown in Fig. 5. As shown in Fig. 5a and b, the (001) plane contains only 2 Co²⁺ while the (111) plane has 3.75 Co²⁺, showing that the (111) crystal plane has more Co²⁺ than the (001) plane. According to the charge–discharge mechanism of Co₃O₄,^37–39 (111) crystal planes have a faster Co²⁺/Co⁰ redox reaction, which brings better electrochemical performance such as higher rate capability. Therefore, we could predict that Co₃O₄ nanoarrays with dominant (111) crystal planes would exhibit better electrochemical performance than those with dominant (001) crystal planes.

Fig. 5 Theoretical models of various crystal planes of Co₃O₄. The 3D and 2D surface atomic configurations of (a) (001) plane and (b) (111) plane of Co₃O₄.

Formation process of the 3D Co₃O₄–Ni@Cu products was further investigated for different reaction stages, Fig. 6a–d are the SEM images of products obtained at different hydrothermal reaction times. When the hydrothermal treatment time was 20 min, we observed that lots of Co₃O₄ nanorods formed on the Cu substrate surface, as shown in Fig. 6a. However, when the reaction time reached 40 min, non-uniform clusters of nanowires formed initially. After reacting for 1 h, the Cu substrate was completely covered by the fast-growing Co₃O₄ nanoarrays. We obtained the final product after 5 h of hydrothermal treatment (Fig. 6d).

Fig. 6 SEM images of Co₃O₄–Ni@Cu precursors synthesized at 463 K for different time: (a) 20 min, (b) 40 min, (c) 1 h and (d) 5 h.

To further investigate the role of Ni-nanoseed-layer in the Co₃O₄ nanoarrays in situ grown on Cu substrates, we did a controlled experiment, in which we fabricated Co₃O₄ nanoarrays directly grown on the Cu substrates, without pre-depositing Ni-nanoseed-layer on Cu. The anode performance of the Co₃O₄–Ni@Cu nanoarrays as well as the Co₃O₄@Cu for LIBs was both evaluated, with the standard Co₃O₄@Cu/Li and Co₃O₄–Ni@Cu/Li half-cell configurations. The first three charging–discharging voltage profiles of Co₃O₄@Cu and Co₃O₄–Ni@Cu samples were shown in Fig. 7a and b, respectively, with a current rate of 0.1 C and a voltage range of 0.01–3.2 V (versus Li/Li⁺). In the first-discharge curves, both of them exhibit voltage plateaus at 1.25 V, subsequently declines to cutoff voltages of 0.05 V gradually, indicating typical characteristics of voltage profiles trends for Co₃O₄ anode. The Co₃O₄@Cu anode exhibited initial charge and discharge capacities of 1000 mA h g⁻¹ and 1365 mA h g⁻¹, respectively, while the Coulombic efficiency was 73%. Meanwhile, the Co₃O₄–Ni@Cu anode had higher charge and discharge capacities of 1300 mA h g⁻¹ and 1816 mA hg⁻¹, respectively, with a Coulombic efficiency of 71%. The first discharge capacities of the two anodes were higher than the theoretical capacity (890 mA h g⁻¹). The difference can be attributed to the formation of a solid electrolyte interface (SEI) layer and possibly interfacial lithium storage.^40–42 Obviously, the Co₃O₄–Ni@Cu anode show a much higher first discharge capacity than that of Co₃O₄@Cu. Therefore, electrodeposition Ni nanoseeds is suggested to be an effective technique to improve the specific capacity of Co₃O₄ nanoarrays anodes.

Fig. 7 (a) and (b) First three charging–discharging curves for Co₃O₄@Cu and Co₃O₄–Ni@Cu nanoarray anodes, respectively, with a voltage range of 0.01–3.2 V and a current of 0.1 C. (c) Cycling performance of the Co₃O₄@Cu and Co₃O₄–Ni@Cu nanoarray anodes with a current of 1 C. (d) Discharging curves for Co₃O₄@Cu and Co₃O₄–Ni@Cu nanoarray anodes at different rates.

Fig. 7c compares the cycling performance of Co₃O₄@Cu and Co₃O₄–Ni@Cu anode materials with a high current rate of 1 C. The Co₃O₄–Ni@Cu anode exhibited little capacity fading after 100 cycles, maintaining a discharge capacity of 610 mA h g⁻¹ and a Coulomb efficiency of 100%. By contrast, the Co₃O₄@Cu anode showed a discharge capacity of only 200 mA h g⁻¹ after 100 cycles. The discharge capacity of Co₃O₄–Ni@Cu nanoarrays anode was almost triple that of Co₃O₄@Cu anode. Moreover, Fig. S5 (ESI†) highlighted that the Co₃O₄–Ni@Cu nanoarrays anode maintained a quite high specific capacity as high as 1150 mA h g⁻¹ after 30 cycles, with a current rate of 0.1 C. The better cycling performance of Co₃O₄–Ni@Cu anode was suggested to be mainly owed to the pre-electrodepositing Ni-nanoseed-layer which improved the mechanical adhesion between Co₃O₄ nanoarrays and substrates effectively.

As the rate capability is quite critical for practical applications of LIBs, discharge curves at various current densities for the Co₃O₄–Ni@Cu and Co₃O₄@Cu anodes were shown in Fig. 7d. The charge–discharge capacities with different rates (0.1 C, 1 C, 2 C, and 3 C, 1 C = 890 mA g⁻¹) were investigated while each rate was measured 10 times. When compared with Co₃O₄@Cu, the Co₃O₄–Ni@Cu nanoarrays anode showed a much better rate capability. More specifically, Co₃O₄–Ni@Cu anode showed discharge capacities of 1043 mA h g⁻¹, 651 mA h g⁻¹, 595 mA h g⁻¹ and 502 mA h g⁻¹ with rates of 0.1, 1, 2 and 3 C, respectively. It was found that Co₃O₄–Ni@Cu showed only a little capacity fading when the discharge rate increased from 1 C to 3 C. By contrast, without pre-electrodepositing Ni-nanoseed-layer, the Co₃O₄@Cu only maintained discharge capacities of 446 mA h g⁻¹, 227 mA h g⁻¹, 125 mA h g⁻¹ and 26 mA h g⁻¹ with rates of 0.1, 1, 2 and 3 C, respectively. The improved rate capability of Co₃O₄–Ni@Cu nanoarrays anode was suggested to be ascribed to the increased conductivity of anodes, as well as the improved mechanical adhesion between Co₃O₄ nanoarrays and substrates due to the Ni-nanoseed-layer.

Fig. S6 (ESI†) showed the impedance of Co₃O₄–Ni@Cu and Co₃O₄@Cu nanoarray anodes. Both of the impedance spectra had similar characteristics: a depressed semicircle at the high-medium frequency as well as an inclined line at the low frequency which were in good agreement with previously reported impedance spectra of Co₃O₄.⁴³ The inclined lines were ascribed to the lithium diffusion impedance. And the depressed semicircles were attributed to charge impedance.⁴⁴ It indicated that the Co₃O₄–Ni@Cu structure helped improve the conductivity of the anode, when compared with Co₃O₄@Cu anode without Ni-nanoseed-layer. In addition, as shown in Fig. 8, the nanoarrays microstructure of Co₃O₄–Ni@Cu was well preserved after 10 cycles and 30 cycles with a current of 1 C, even after 100 cycles, the skeleton of the Co₃O₄ nanoarrays still maintained (Fig. 8e and f), revealing an excellent structural stability during charge–discharge cycles.

Fig. 8 (a and b) (c and d) (e and f) The SEM images of the Co₃O₄–Ni@Cu nanoarrays tested as LIBs anode after 10, 30 and 100 cycles, respectively.

In this study, the integrated anode composed of Ni-enhanced Co₃O₄ nanoarrays in situ grown on Cu substrate exhibited far better electrochemical performance than those made of Co₃O₄ nanoarrays without Ni-nanoseed-layer. The excellent properties should be owed to several factors: first, Ni-nanoseed-layer improved the mechanical adhesion between Co₃O₄ nanoarrays and Cu substrates effectively, thus enhanced the cycling stability. Second, the Co₃O₄–Ni@Cu system increased the conductivity of anode materials. Third, the 3D Co₃O₄ nanoarrays microstructure provided larger electrode/electrolyte interface, shorter diffusion paths and larger free volume, contributing to better electrochemical performance as anode material for LIBs. Forth, we preliminarily inferred that the Ni layer might play some role in influencing the growth of Co₃O₄ nanoarrays and led to dominant (111) crystal planes, which exhibited better performance than (001) planes. It appears promising to further enhance the electrochemical properties of Co₃O₄ integrated anodes through improving their crystal plane structure and introducing nanoseed layers.

Conclusion

In summary, we demonstrated a facile synthesis method of Co₃O₄ nanoarrays in situ grown on Cu substrates directly, by a combination of pre-electrodepositing Ni-nanoseed-layer and controlled hydrothermal synthesis. The as-prepared Co₃O₄–Ni@Cu nanoarrays electrode exhibited excellent electrochemical performance as anode materials for LIBs: they showed a quite high charge–discharge capacity of ∼1150 mA h g⁻¹ with a current of 0.1 C, while exhibited good cycling stability and rate capability. The high electrochemical performance was owed to the unique feature of the as-obtained Ni-enhanced Co₃O₄ nanoarrays: besides the larger electrode/electrolyte interface, shorter diffusion paths and larger free volume due to the nanoarrays microstructure, the Ni-nanoseed-layer pre-electrodeposited on the Cu substrate could improve the mechanical adhesion between Co₃O₄ nanoarrays and substrates and enhance the conductivity of anodes, without applying binders or conductive additives. Given their facile in situ synthesis and improved performance, this work will open a new avenue for the development of integrated electrode materials for high-performance LIBs.

Acknowledgements

This work was supported by National Natural Science Foundation of China (no. 21276257) and Beijing Natural Science Foundation (2132054) and “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant no. XDA09010103).

References

1. Y. Gogotsi and P. Simon, Science, 2011, 334, 917–918.
2. Y. F. Deng, Z. E. Li, Z. C. Shi, H. Xu, F. Peng and G. H. Chen, RSC Adv., 2012, 2, 4645–4647.
3. H. B. Wu, J. S. Chen, H. H. Hng and X. W. Lou, Nanoscale, 2012, 4, 2526–2542.
4. J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587–603.
5. J. Liu, Y. C. Zhou, F. Liu, C. P. Liu, J. B. Wang, Y. Pan and D. F. Xue, RSC Adv., 2012, 2, 2262–2265.
6. B. Dunn, H. Kamath and J. M. Tarascon, Science, 2011, 334, 928–935.
7. X. M. Liu, Q. Long, C. H. Jiang, B. B. Zhan, C. Li, S. J. Liu, Q. Zhao, W. Huang and X. C. Dong, Nanoscale, 2013, 5, 6525–6529.
8. 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, 6417–6420.
9. Y. Qi, H. Zhang, N. Du, C. X. Zhai and D. Yang, RSC Adv., 2012, 2, 9511–9516.
10. L. Li, K. H. Seng, Z. X. Chen, Z. P. Guo and H. K. Liu, Nanoscale, 2013, 5, 1922–1928.
11. X. H. Xia, J. P. Tu, Y. Q. Zhang, Y. J. Mai, X. L. Wang, C. D. Gu and X. B. Zhao, RSC Adv., 2012, 2, 1835–1841.
12. M. Okubo, E. Hosono, T. Kudo, H. S. Zhou and I. Honma, Solid State Ionics, 2009, 180, 612–615.
13. R. H. Wang, C. H. Xu, J. Sun, Y. Q. Liu, L. Gao and C. C. Lin, Nanoscale, 2013, 5, 6960–6967.
14. H. B. Wu, J. S. Chen, H. H. Hng and X. W. Lou, Nanoscale, 2012, 4, 2526–2542.
15. X. H. Xia, J. P. Tu, Y. Q. Zhang, J. Chen, Xi. L. Wang, C. D. Gu, C. Guan, J. S. Luo and H. J. Fan, Chem. Mater., 2012, 24, 3793–3799.
16. B. G. Choi, S. J. Chang, Y. B. Lee, J. S. Bae, H. J. Kim and Y. S. Huh, Nanoscale, 2012, 4, 5924–5930.
17. W. J. Hao, S. M. Chen, Y. J. Cai, L. Zhang, Z. X. Li and S. J. Zhang, J. Mater. Chem. A, 2014, 2, 13801–13804.
18. W. Wang, M. Tian, A. Abdulagatov, S. M. George, Y. C. Lee and R. G. Yang, Nano Lett., 2012, 12, 655–660.
19. Q. Yang, Z. Y. Lu, Z. Chang, W. Zhu, J. Q. Sun, J. F. Liu, X. M. Sun and X. Duan, RSC Adv., 2012, 2, 1663–1668.
20. Y. Gao, S. Chen, D. Cao, G. Wang and J. Yin, J. Power Sources, 2010, 195, 1757–1760.
21. J. Jiang, J. P. Liu, R. M. Ding, X. X. Ji, Y. Y. Hu, X. Li, A. Z. Hu, F. Wu, Z. H. Zhu and X. T. Huang, J. Phys. Chem. C, 2010, 114, 929–932.
22. X. Y. Xue, S. Yuan, L. L. Xing, Z. H. Chen, B. He and Y. J. Chen, Chem. Commun., 2011, 47, 4718–4720.
23. X. L. Xiao, X. L. Liu, H. Zhao, D. F. Chen, F. Z. Liu, J. H. Xiang, Z. B. Hu and Y. D. Li, Adv. Mater., 2012, 24, 5762–5766.
24. R. P. Silva, S. Eugénio, T. M. Silva, M. J. Carmezim and M. F. Montemor, J. Phys. Chem. C, 2012, 116, 22425–22431.
25. J. Y. Liao, D. Higgins, G. Lui, V. Chabot, X. C. Xiao and Z. W. Chen, Nano Lett., 2013, 13, 5467–5473.
26. B. X. Li, Y. Xie, C. Z. Wu, Z. Q. Li and J. Zhang, Mater. Chem. Phys., 2006, 99, 479–486.
27. R. Xu and H. C. Zeng, J. Phys. Chem. B, 2003, 107, 12643–12649.
28. L. Q. Mai, F. Dong, X. Xu, Y. Z. Luo, Q. Y. An, Y. L. Zhao, J. Pan and J. N. Yang, Nano Lett., 2013, 13, 740–745.
29. Y. J. Feng, R. Q. Zou, D. G. Xia, L. L. Liu and X. D. Wang, J. Mater. Chem. A, 2013, 1, 9654–9658.
30. S. R. Gowda, A. L. M. Reddy, M. M. Shaijumon, X. B. Zhan, L. J. Ci and P. M. Ajayan, Nano Lett., 2011, 11, 101–106.
31. C. K. Chan, H. L. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol., 2008, 3, 31–35.
32. F. Zhan, B. Geng and Y. Guo, Chem.–Eur. J., 2009, 15, 6169–6174.
33. M. Y. Liao, L. Fang, C. L. Xu, F. Wu, Q. L. Huang and M. Saleem, Mater. Sci. Semicond. Process., 2014, 24, 1–8.
34. Z. F. Liu, J. Ya and L. E, J. Solid State Electrochem., 2010, 14, 957–963.
35. Y. F. Wang, B. X. Lei, Y. F. Hou, W. X. Zhao, C. L. Liang, C. Y. Su and D. B. Kuang, Inorg. Chem., 2010, 49, 1679–1686.
36. Z. H. Ibupoto, K. Khun, J. Lu, X. J. Liu, M. S. AlSalhi, M. Atif, A. A. Ansari and M. Willander, J. Cryst. Growth, 2013, 368, 39–46.
37. M. V. Reddy, Z. Beichen, L. J. Nicholette, K. M. Zhang and B. V. R. Chowdari, Electrochem. Solid-State Lett., 2011, 14, A79–A82.
38. Y. M. Kang, M. S. Song, J. H. Kim, H. S. Kim, M. S. Park, J. Y. Lee and S. X. Dou, Electrochim. Acta, 2005, 50, 3667–3673.
39. G. Binotto, D. Larcher, A. S. Prakash, R. Herrera Urbina, M. S. Hegde and J. M. Tarascon, Chem. Mater., 2007, 19, 3032–3040.
40. X. L. Wu, Y. G. Guo, L. J. Wan and C. W. Hu, J. Phys. Chem. C, 2008, 112, 16824–16829.
41. H. Liu, G. X. Wang, J. Liu, S. Z. Qiao and H. Ahn, J. Mater. Chem., 2011, 21, 3046–3052.
42. X. Wang, H. Guan, S. M. Chen, H. Q. Li, T. Y. Zhai, D. M. Tang and Y. Bando, Chem. Commun., 2011, 47, 12280–12282.
43. J. Chen, X. H. Xia, J. P. Tu, Q. Q. Xiong, Y. X. Yu, X. L. Wang and C. D. Gu, J. Mater. Chem., 2012, 22, 15056–15061.
44. X. L. Chen, K. Gerasopoulos, J. C. Guo, A. Brown, C. S. Wang, R. Ghodssi and J. N. Culver, ACS Nano, 2010, 4, 5366–5372.

---

Footnotes

† Electronic supplementary information (ESI) available: Experimental part, EDS mapping, SEM images and cycling performance. See DOI: 10.1039/c4ra13994a

‡ These authors contributed equally to this work.

---