Design and synthesis of 3D hierarchical NiCo₂S₄@MnO₂ core–shell nanosheet arrays for high-performance pseudocapacitors

Abstract

We report on the development of 3D hierarchical NiCo₂S₄@MnO₂ core–shell nanosheet arrays on Ni foam for supercapacitors. In our design, the highly conductive NiCo₂S₄ nanosheets can serve not only as a good pseudocapacitive material, which can be contributed to the capacitance of the whole electrode, but also as a 3D conductive scaffold for loading MnO₂ materials, which can overcome the limited conductivity of MnO₂ itself. Furthermore, the 3D NiCo₂S₄@MnO₂ hybrid electrode can provide efficient and rapid pathways for ion and electron transport. These merits together with the elegant synergy between NiCo₂S₄ and MnO₂ lead to a high areal capacitance of 2.6 F cm⁻² at 3 mA cm⁻² and good cyclic stability.

---

1. Introduction

Supercapacitors, also known as ultracapacitors or electrochemical capacitors, hold a great promise in the future portable electronics, hybrid electronic vehicles and a number of microdevices due to their high specific power, long cycling life and fast charge and discharge rates.^1–4 Supercapacitors can be often distinguished into two major types according to the charge-storage mechanism involved in energy storage as well as the active materials used.^2,5 One is electrical double-layer capacitors (EDLCs), where electrical energy is stored by electrostatic accumulation of charges in the electric double-layer near electrode–electrolyte interfaces, commonly use carbon-active materials with high surface area as electrodes.^6–8 Another type of supercapacitor is the so-called pseudocapacitor, in which electrical energy is mainly stored by fast and reversible faradaic reactions, thus offering much higher specific capacitance than EDLCs.^9–11 Typical active pseudocapacitive materials include transition metal oxides and conducting polymers. Among these available electrode materials, MnO₂ has received tremendous interest as the most promising electrode material due to its many advantages such as low-cost, natural abundance, environmental friendliness and most importantly, high theoretical specific capacitance (1370 F g⁻¹).^6,12–14 However, due to its poor electric conductivity (10⁻⁵–10⁻⁶ S cm⁻¹),^12,15 the observed specific capacitance values for MnO₂ are much lower than its theoretical value.

Recently, much attention has been focused on designing and constructing three-dimensional (3D) hybrid nanostructures with large surface area and short diffusion path for electrons and ions. In particular, the additive/binder-free electrode architectures, which can avoid the “dead surface” and tedious process in traditional slurry-coating electrode and significantly improve the utilization rate of electrode materials even at high rates.^3,5,9 For example, Cheng et al.¹⁶ synthesized 3D Co₃O₄@MnO₂ hierarchical porous nanoneedle array with a specific capacitance of 1693.2 F g⁻¹. Lou et al.¹⁷ developed hierarchical NiCo₂O₄@MnO₂ core–shell heterostructured nanowire arrays with a specific capacitance of 3.31 F cm⁻². However, the conductivity of these backbone materials should be further improved to support fast electron transport. More recently, transition metal sulfides have been extensively studied as novel electrode materials with improved electrochemical performance.^18–24 NiCo₂S₄ material has higher electrochemical activity than its corresponding single component sulphides.^18,21,25 Most importantly, NiCo₂S₄ exhibited an electrical conductivity ∼100 times as high as that of NiCo₂O₄.^26,27 Of notable examples include the work by Wang et al.,²⁶ who developed NiCo₂S₄ single crystalline nanotube arrays for loading additional electroactive materials, the as-formed Co_xNi_1−x(OH)₂/NiCo₂S₄ electrode showed the highest areal capacitance of 2.86 F cm⁻², good rate capability, and cycling stability. Dong et al.²⁸ reported NiCo₂S₄@MnO₂ core–shell heterostructures electrode materials showed a specific capacitance of 1337.8 F g⁻¹ and excellent cycling stability, however, the fabrication processes for electrode are complicated. Therefore, NiCo₂S₄ materials can serve as an ideal scaffold for loading additional electroactive pseudocapacitive materials to improve its electrical conductivity and capacitive performance.

Herein, we present the design and fabrication of 3D hierarchical NiCo₂S₄@MnO₂ core–shell nanosheet arrays on Ni foam and their improved capacitive performance. This unique design has the following merits: firstly, highly conductive NiCo₂S₄ nanosheets will provide electron “superhighways” for charge storage and delivery, which could overcome the limited conductivity of MnO₂ itself and improve the utilization rate of electrode materials. Secondly, both of the NiCo₂S₄ and MnO₂ are good pseudocapacitive electrode materials, of which a material-combination will provide synergistic and multifunctional effects. Thirdly, the 3D hierarchical NiCo₂S₄@MnO₂ materials directly grown on Ni foam could avoid “dead” volume caused by the tedious process of mixing active materials with polymer binder/conductive additives. Benefiting from the compositional features and the unique electrode structure, the NiCo₂S₄@MnO₂ hybrid electrode exhibits greatly improved electrochemical performance with high areal capacitance (2.6 F cm⁻² at 3 mA cm⁻²) and good cyclic stability.

2. Results and discussion

Our strategy for fabricating 3D hierarchical NiCo₂S₄@MnO₂ core–shell nanosheet arrays as pseudocapacitive electrode materials involves three-step, as shown in Fig. 1. Firstly, mesoporous NiCo₂O₄ nanosheet arrays were grown vertically on Ni foam via a hydrothermal and annealing process. Secondly, a facile anion exchange reaction has been performed to achieve NiCo₂S₄ nanosheet arrays, which involves treating NiCo₂O₄ nanosheet arrays in a Na₂S aqueous solution under hydrothermal conditions. During this process, S²⁻, HS⁻, and H₂S in Na₂S solution can serve as the sulfur sources to convert NiCo₂O₄ to NiCo₂S₄ homogeneously and keep the original morphology.⁴ Thirdly, the MnO₂ nanosheets were unique coated onto NiCo₂S₄ nanosheet arrays through an efficient and controllable electrodeposition process. The NiCo₂S₄@MnO₂ core–shell nanosheet arrays directly grown on Ni foam could avoid “dead” volume caused by the tedious process of mixing active materials with polymer binder/conductive additives. What is more, the highly conductive NiCo₂S₄ nanosheet arrays grown on Ni foam can not only contribute to the capacitance of the whole electrode but also overcome the limited conductivity of MnO₂ itself.

Fig. 1 Schematic illustration for the fabrication process of the 3D hierarchical NiCo₂S₄@MnO₂ core–shell nanosheet arrays on Ni foam.

The X-ray diffraction (XRD) patterns of the NiCo₂O₄ and NiCo₂S₄ materials are shown in Fig. 2, demonstrating the overall crystal structure and phase purity of the products. The as-prepared materials are scratched down from Ni foam substrate before measurement in order to reduce the impact of the Ni foam. Prior to ion exchange, the diffraction peaks for the NiCo₂O₄ (black curve) can be well ascribed to the cubic phase NiCo₂O₄ (JCPDS no. 20-0781). After the conversion, all the diffraction peaks for the NiCo₂S₄ (blue curve) can be indexed to the cubic phase NiCo₂S₄ (JCPDS no. 20-0782), suggesting successful conversion of NiCo₂O₄ into NiCo₂S₄.

Fig. 2 XRD patterns of the as-synthesized NiCo₂O₄ and NiCo₂S₄ samples and the corresponding standard XRD patterns.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are employed to investigate the morphologies and microstructures of the NiCo₂O₄ and NiCo₂S₄ nanosheet arrays on Ni foam. It is revealed that high-density NiCo₂O₄ nanosheets were uniformly grown on the Ni foam with a smooth surface, and they interconnect with each other to form a wall-like structure (Fig. 3a and b). From Fig. S1,† it can be seen that the NiCo₂O₄ nanosheets possess a highly porous structure, which is favorable for subsequent ion exchange reaction.^22,29 The mesoporous NiCo₂O₄ nanosheets allow sulfur sources to diffuse easily into the core of each NiCo₂O₄ nanosheet. After ion exchange reaction with Na₂S, the surface of the obtained NiCo₂S₄ nanosheets becomes rough, while maintaining the original array architecture, alignment, and physical contact with the substrates (Fig. 3c and d). A high-magnification SEM image, Fig. 3d, shows that numerous pores can be discerned on the surface of the NiCo₂S₄ nanosheets. Fig. 3e presents the typical TEM images of an individual NiCo₂S₄ nanosheet, from which the mesoporous structure with the sizes in the range of a few nanometers can be observed clearly. The high-resolution TEM image (HRTEM) shown in Fig. 3f reveals that the NiCo₂S₄ has a lattice fringe with interplane spacing of 0.191 nm, corresponding to the {422} plane of NiCo₂S₄. These results demonstrate that the NiCo₂O₄ nanosheets were successfully transformed into NiCo₂S₄ nanosheets.

Fig. 3 Low- and high-magnification SEM images of (a and b) NiCo₂O₄ nanosheet arrays and (c and d) NiCo₂S₄ nanosheet arrays. (e and f) TEM and HRTEM images of NiCo₂S₄ nanosheet.

Due to the highly conductive of NiCo₂S₄ nanosheet, it can serve as a 3D conductive scaffold for loading MnO₂ electrode materials so as to enhance the electrochemical capacitor performance. Fig. 4a–c show SEM images of the 3D hierarchical NiCo₂S₄@MnO₂ core–shell nanosheet arrays, demonstrating that the MnO₂ were successfully coated on the NiCo₂S₄ nanosheet arrays. The integration of MnO₂ into the array does not deteriorate the ordered structure, and the thickness of the composites nanosheets increased to about ∼100–150 nm. The MnO₂ nanosheets are interconnected with each other to form a highly porous surface morphology. Thus, the electrolyte can transportation not only at the active materials surface but also throughout the bulk due to the presence of convenient diffusion channels. TEM image in Fig. 4d clearly shows that the surface of NiCo₂S₄ nanosheet is enclosed by a thin and continuous MnO₂ layer. Energy dispersive X-ray (EDX) spectrum (Fig. 4e) demonstrates that the Ni, Co, Mn, O and S can be detected in the hybrid composites. The Cu and C signals come from the carbon-supported Cu grid. Moreover, X-ray photoelectron spectroscopy (XPS) studies also determine the chemical composition of the composites as shown in Fig. 4f. The Mn 2p_3/2 peak is centered at 642.3 eV and Mn 2p_1/2 peak at 654.2 eV, with a binding energy separation of 11.9 eV, which are in good agreement with previous reported peak binding energy separation observed in MnO₂.³⁰

Fig. 4 (a–c) Low- and high-magnification SEM images, (d) TEM image and (e) EDX spectra of the 3D hierarchical NiCo₂S₄@MnO₂ core–shell nanosheet arrays, (f) XPS spectra of Mn 2p for the NiCo₂S₄@MnO₂ composites.

We then investigate the electrochemical properties of the NiCo₂S₄@MnO₂ electrode in a three-electrode configuration using 1 M NaOH aqueous solution as electrolyte, with a Pt counter electrode and a saturated calomel electrode (SCE) reference electrode. Fig. 5a shows the cyclic voltammogram (CV) curves of the MnO₂, NiCo₂O₄, NiCo₂S₄ and NiCo₂S₄@MnO₂ electrodes at a scan rate of 10 mV s⁻¹ in a potential window of −0.1 to 0.5 V. the pristine NiCo₂O₄ electrode materials showed a pair of redox peaks, which can be attributed to the following faradaic redox reactions:^31,32 NiCo₂O₄ + OH⁻ + 2H₂O ⇄ NiOOH + 2CoOOH + H₂O + e⁻, CoOOH + OH⁻ ⇄ CoO₂ + H₂O + e⁻. Similarly, a pair of redox peaks can also be observed in the CV curve for NiCo₂S₄ electrode, which is mainly attributed to the faradaic redox reactions related to MS/MSOH and MSOH/MSO, where M refers to Ni or Co.^26,33 It is also demonstrated that the NiCo₂S₄ electrode materials present pseudocapacitive properties. Moreover, the integrated CV area for the NiCo₂S₄ electrode is significantly larger than the primal NiCo₂O₄ electrode, indicating a substantial improvement of electrochemical capacitance due to higher electric conductivity of NiCo₂S₄ electrode materials for facilitating electron and electrolyte ion transport. Moreover, the NiCo₂S₄@MnO₂ electrode exhibits much higher current densities than that of MnO₂ and NiCo₂S₄ electrode due to the synergistic effects between MnO₂ and NiCo₂S₄ electrode materials and highly conductive of NiCo₂S₄ electrode materials: (1) mesoporous NiCo₂S₄ nanosheets with good electrical conductivity directly grown on Ni foam will provide electron “superhighways” for charge storage and delivery, which could overcome the limited conductivity of MnO₂ itself nanosheets. (2) Both of the NiCo₂S₄ and MnO₂ are good pseudocapacitive electrode materials, of which a material-combination will provide synergistic and multifunctional effects. It is worth noting that the CV integrated areas (Fig. S2†) of the pure and treated (underwent hydrothermal in Na₂S solution) Ni foam are both negligible as compared with that of the NiCo₂S₄@MnO₂ electrode, revealing the almost no capacitance contribution of the current collector. The galvanostatic charge–discharge (CD) measurements are performed in the voltage range of −0.1 to 0.45 V at a current density of 3 mA cm⁻², as shown in Fig. 5b. It clearly shows that the NiCo₂S₄@MnO₂ electrode exhibits much longer discharging time than that of the other electrodes, demonstrating enhanced capacitances. The areal capacitance of the electrode materials was calculated based on the discharge curves (Fig. S3†) measured at different current densities using the following equation:¹⁰ C = It/SΔV, where I (A) is the current used for the charge–discharge, t (s) is the discharge time, ΔV (V) is the voltage interval of the discharge, and S is the geometrical area of the electrode, as shown in Fig. 5c. Significantly, the areal capacitance of NiCo₂S₄@MnO₂ electrode was much higher than that of the other electrodes in the whole current density range. The highest areal capacitance obtained for the NiCo₂S₄@MnO₂ electrode was 2.6 F cm⁻² at a current density of 3 mA cm⁻², which is much higher than the values obtained for the NiCo₂S₄ electrode (1.33 F cm⁻²), NiCo₂O₄ electrode (0.44 F cm⁻²) and MnO₂ electrode (0.057 F cm⁻²). Even at a current density of 20 mA cm⁻², the NiCo₂S₄@MnO₂ electrode still yielded a high areal capacitance of 1.17 F cm⁻². To the best of our knowledge, the areal capacitance performance of the NiCo₂S₄@MnO₂ electrode reported here is also much higher than most of previously reported electrode materials, such as Co₃O₄@MnO₂ (0.56 F cm⁻² at 11.25 mA cm⁻²),⁵ NiCo₂S₄/CFC (1.33 F cm⁻² at 1 mA cm⁻²),³⁴ (Co_xNi_1−x)₉S₈/HCNA (1.32 F cm⁻² at 1 mA cm⁻²),³⁵ Co_0.85Se (929.5 mF cm⁻² at 1 mA cm⁻²).³⁶ Electrochemical impedance spectroscopy (EIS) is also applied to investigate electrochemical behaviors of the electrodes. Fig. S4† shows the EIS spectrum of the NiCo₂S₄ and NiCo₂S₄@MnO₂ electrode materials. The equivalent series resistance (ESR) of the NiCo₂S₄@MnO₂ electrode materials (0.11 Ω) is much smaller than that of the bare NiCo₂S₄ (0.31 Ω), indicating a lower diffusion resistance. The long-term cycling performance of the NiCo₂S₄ and NiCo₂S₄@MnO₂ electrodes was evaluated at a scan rate of 50 mV s⁻¹. As shown in Fig. 5d, the overall capacitance retention for the NiCo₂S₄ electrode is ∼52% after 3000 cycles. In contrast, the NiCo₂S₄@MnO₂ electrode displayed a remarkable long-term cycling stability with ∼103.9% capacitance retention after 5000 cycles. The enhanced pseudocapacitive performances of the NiCo₂S₄@MnO₂ electrode material are mainly attributed to their unique mesoporous core–shell structure and highly conductive of NiCo₂S₄ electrode materials which provide more active sites for efficient electrolyte ion transportation not only at the active materials surface but also throughout the bulk. After long-term cycling test, the structural integrity and basic morphology of the NiCo₂S₄@MnO₂ and NiCo₂S₄ electrode are overall well preserved with little deterioration, which are shown by the SEM image (Fig. S5†). The poor cycling stability of the NiCo₂S₄ electrode could be attributed to the following reasons: during the fast and reversible faradaic reactions, the capacitance decreases maybe relate to the degradation of the active material in the process of OH⁻.^23,37 However, the MnO₂ can act as a buffer for the volume change during a cycling test, providing an assurance for better cycling performance of the NiCo₂S₄@MnO₂ electrode.

Fig. 5 (a) CV curves, (b) CD curves and (c) specific capacitances of the MnO₂, NiCo₂O₄, NiCo₂S₄ and NiCo₂S₄@MnO₂ electrode materials. (d) Cycling stability of NiCo₂S₄ and NiCo₂S₄@MnO₂ electrode materials.

3. Conclusions

In summary, we have designed and successfully prepared 3D hierarchical NiCo₂S₄@MnO₂ core–shell nanosheet arrays on Ni foam for high-performance pseudocapacitors. The highly conductive NiCo₂S₄ materials can serve as an ideal scaffold for loading MnO₂ materials, which can overcome the limited conductivity of MnO₂ itself. Moreover, The NiCo₂S₄ materials may also function as active materials for charge storage and make contribution to the capacitance. The 3D hierarchical NiCo₂S₄@MnO₂ electrode materials show outstanding electrochemical performance in supercapacitors such as high areal capacitance (2.6 F cm⁻² at 3 mA cm⁻²) and good cyclic stability (∼103.9% capacitance retention after 5000 cycles).

4. Experimental section

Synthesis of NiCo₂S₄ nanosheet arrays

All the reagents here were analytical grade (purchased from Sinopharm) and used without further purification. The NiCo₂O₄ nanosheet arrays were synthesized according to our previous work.³⁸ In a typical synthetic procedure, a piece of Ni foam was carefully cleaned with 3 M HCl solution in an ultrasound bath for 30 min to remove the NiO layer on the surface, and then washed by deionized water and absolute ethanol several times. 0.3 g Ni(NO₃)₂·6H₂O, 0.6 g Co(NO₃)₂·6H₂O and 0.8 g hexamethylenetetramine (HMT) were dissolved into a mixed solution of 30 mL deionized water and 20 mL ethanol under vigorous magnetic stirring. After stirring for 30 min, as-obtained solution was transferred into a 60 mL polytetrafluoroethylene (PTFE) (Teflon)-lined autoclave with a piece of clean Ni foam immersed into the reaction solution. The autoclave was sealed and maintained at 95 °C for 8 h in an electric oven, and then cooled down to room temperature. The products on the Ni foam were carefully washed with deionized water and absolute ethanol, and then dried at 60 °C overnight. Afterward, the samples were annealed at 300 °C for 2 h at a ramping rate of 1 °C min⁻¹. For synthesis of NiCo₂S₄ nanosheet arrays, the NiCo₂O₄ nanosheet arrays grown on Ni foam was immersed into a 60 mL aqueous solution containing 1 g of Na₂S and kept at 95 °C for 24 h. After cooling down naturally to the room temperature, the NiCo₂S₄ nanosheet arrays were taken out and washed with deionized water and ethanol, then dried at 60 °C for 8 h under vacuum. The mass loading of the NiCo₂S₄ on Ni foam was calculated to be around 1.5 mg cm⁻².

Electrochemical deposition of MnO₂ nanosheets on the NiCo₂S₄ nanosheet arrays

The Ni foam with NiCo₂S₄ nanosheet arrays was used as a working electrode for electrodeposition of MnO₂ nanosheets, which was deposited at 0.9 V (vs. SCE) in a solution containing 0.346 g manganese ammonium (MnAc₂), 0.0308 g ammonium acetate (NH₄Ac) and 2 mL dimethyl sulfoxide (DMSO) for 10 min at room temperature. After depositing, the as-prepared NiCo₂S₄@MnO₂ electrode materials were washed with deionized water and absolute ethanol, and then dried at 60 °C for 2 h under vacuum.

Materials characterization

As-synthesized products were characterized with a D/max-2550 PC X-ray diffractometer (XRD; Rigaku, Cu-Kα radiation), a scanning electron microscopy (SEM; S-4800), a transmission electron microscopy (TEM; JEM-2100F) equipped with an energy dispersive X-ray spectrometer (EDX) and X-ray photoelectron spectroscopy (XPS, PHI5000VersaProbe).

Electrochemical measurement

Electrochemical performances were performed on an Autolab (PGSTAT302N potentiostat) using a three-electrode mode in a 1 M NaOH solution. The reference electrode and counter electrode were SCE and platinum, respectively. The Ni foam supported electrode materials acted directly as the working electrode.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant nos 21171035, 51472049 and 51302035), the Key Grant Project of Chinese Ministry of Education (Grant no. 313015), the PhD Programs Foundation of the Ministry of Education of China (Grant nos 20110075110008 and 20130075120001), the National 863 Program of China (Grant no. 2013AA031903), the Science and Technology Commission of Shanghai Municipality (Grant no. 13ZR1451200), the Fundamental Research Funds for the Central Universities, the Program Innovative Research Team in University (IRT1221), the Shanghai Leading Academic Discipline Project (Grant no. B603) and the Program of Introducing Talents of Discipline to Universities (no. 111-2-04).

References

1. C. Liu, F. Li, L. P. Ma and H. M. Cheng, Adv. Mater., 2010, 22, 28.
2. G. P. Wang, L. Zhang and J. J. Zhang, Chem. Soc. Rev., 2012, 41, 797.
3. X. H. Lu, Y. X. Zeng, M. H. Yu, T. Zhai, C. L. Liang, S. L. Xie, B. Muhammad-Sadeeq and Y. X. Tong, Adv. Mater., 2014, 26, 3148.
4. J. Xu, Q. F. Wang, X. W. Wang, Q. Y. Xiang, B. Liang, D. Chen and G. Z. Shen, ACS Nano, 2013, 7, 5453.
5. J. P. Liu, J. Jiang, C. W. Cheng, H. X. Li, J. X. Zhang, H. Gong and H. J. Fan, Adv. Mater., 2011, 23, 2076.
6. G. H. Yu, L. B. Hu, M. Vosgueritchian, H. L. Wang, X. Xie, J. R. McDonough, X. Cui, Y. Cui and Z. N. Bao, Nano Lett., 2011, 11, 2905.
7. H. P. Cong, X. C. Ren, P. Wang and S. H. Yu, Energy Environ. Sci., 2013, 6, 1185.
8. H. Jiang, P. S. Lee and C. Z. Li, Energy Environ. Sci., 2013, 6, 41.
9. C. Zhou, Y. W. Zhang, Y. Y. Li and J. P. Liu, Nano Lett., 2013, 13, 2078.
10. P. H. Yang, Y. Ding, Z. Y. Lin, Z. W. Chen, Y. Z. Li, P. F. Qiang, M. Ebrahimi, W. J. Mai, C. P. Wong and Z. L. Wang, Nano Lett., 2014, 14, 731.
11. G. Zhang and X. W. Lou, Adv. Mater., 2013, 25, 976.
12. W. F. Wei, X. W. Cui, W. X. Chen and D. G. Ivey, Chem. Soc. Rev., 2011, 40, 1697.
13. X. H. Lu, T. Zhai, X. H. Zhang, Y. Q. Shen, L. Y. Yuan, B. Hu, L. Gong, J. Chen, Y. H. Gao, J. Zhou, Y. X. Tong and Z. L. Wang, Adv. Mater., 2012, 24, 938.
14. Y. Huang, Y. Y. Li, Z. Q. Hu, G. M. Wei, J. L. Guo and J. P. Liu, J. Mater. Chem. A, 2013, 1, 9809.
15. J. Desilvestro and O. Haas, J. Electrochem. Soc., 1990, 137, 5C.
16. D. Z. Kong, J. S. Luo, Y. L. Wang, W. N. Ren, T. Yu, Y. S. Luo, Y. P. Yang and C. W. Cheng, Adv. Funct. Mater., 2014, 24, 3815.
17. L. Yu, G. Q. Zhang, C. Z. Yuan and X. W. Lou, Chem. Commun., 2013, 49, 137.
18. W. Chen, C. Xia and H. N. Alshareef, ACS Nano, 2014, 8, 9531.
19. W. Hu, R. Q. Chen, W. Xie, L. L. Zou, N. Qin and D. H. Bao, ACS Appl. Mater. Interfaces, 2014, 6, 19318.
20. H. C. Chen, J. J. Jiang, L. Zhang, D. D. Xia, Y. D. Zhao, D. Q. Guo, T. Qi and H. Z. Wan, J. Power Sources, 2014, 254, 249.
21. S. J. Peng, L. L. Li, C. C. Li, H. T. Tan, R. Cai, H. Yu, S. Mhaisalkar, M. Srinivasan, S. Ramakrishna and Q. Y. Yan, Chem. Commun., 2013, 49, 10178.
22. X. H. Xia, C. R. Zhu, J. S. Luo, Z. Y. Zeng, C. Guan, C. F. Ng, H. Zhang and H. J. Fan, Small, 2014, 10, 766.
23. D. P. Cai, D. D. Wang, C. X. Wang, B. Liu, L. L. Wang, Y. Liu, Q. H. Li and T. H. Wang, Electrochim. Acta, 2015, 151, 35.
24. T. Peng, Z. Y. Qian, J. Wang, D. L. Song, J. Y. Liu, Q. Liu and P. Wang, J. Mater. Chem. A, 2014, 2, 19376.
25. L. Yu, L. Zhang, H. B. Wu and X. W. Lou, Angew. Chem., Int. Ed., 2014, 53, 3711.
26. J. W. Xiao, L. Wan, S. H. Yang, F. Xiao and S. Wang, Nano Lett., 2014, 14, 831.
27. Y. F. Zhang, M. Z. Ma, J. Yang, C. C. Sun, H. Q. Su, W. Huang and X. C. Dong, Nanoscale, 2014, 6, 9824.
28. J. Yang, M. Z. Ma, C. C. Sun, Y. F. Zhang, W. Huang and X. C. Dong, J. Mater. Chem. A, 2015, 3, 1258.
29. C. W. Kung, H. W. Chen, C. Y. Lin, K. C. Huang, R. Vittal and K. C. Ho, ACS Nano, 2012, 6, 7016.
30. L. H. Bao, J. F. Zang and X. D. Li, Nano Lett., 2011, 11, 1215.
31. H. Chen, L. F. Hu, M. Chen, Y. Yan and L. M. Wu, Adv. Funct. Mater., 2014, 24, 934.
32. G. Q. Zhang, H. B. Wu, H. E. Hoster, M. B. Chan-Park and X. W. Lou, Energy Environ. Sci., 2012, 5, 9453.
33. L. F. Shen, J. Wang, G. Y. Xu, H. S. Li, H. Dou and X. G. Zhang, Adv. Energy Mater., 2014, 5, 1400977.
34. M. Sun, J. J. Tie, G. Cheng, T. Lin, S. M. Peng, F. Z. Deng, F. Ye and L. Yu, J. Mater. Chem. A, 2015, 3, 1730.
35. L. Wan, J. W. Xiao, F. Xiao and S. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 7735.
36. A. Banerjee, S. Bhatnagar, K. K. Upadhyay, P. Yadav and S. Ogale, ACS Appl. Mater. Interfaces, 2014, 6, 18844.
37. X. Zhao, B. M. Sanchez, P. J. Dobson and P. S. Grant, Nanoscale, 2011, 3, 839.
38. K. B. Xu, X. J. Huang, Q. Liu, R. J. Zou, W. Y. Li, X. J. Liu, S. J. Li, J. M. Yang and J. Q. Hu, J. Mater. Chem. A, 2014, 2, 16731.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05554g

‡ These authors contributed equally to this work.

---