Hierarchical NiCo₂S₄ hollow spheres as a high performance anode for lithium ion batteries

Abstract

Hierarchical NiCo₂S₄ hollow spheres are successfully fabricated by a facile hydrothermal method accompanied by a high-temperature annealing and anion exchange process. By using NiCo₂S₄ hollow spheres as an anode material for lithium ion batteries, a high specific capacity of 696 mA h g⁻¹ can be obtained after 100 cycles at a current density of 200 mA g⁻¹. Even at a current density of 2000 mA g⁻¹, the specific capacity still remains at 411 mA h g⁻¹ after 50 cycles. The good electrochemical performance can be attributed to the unique porous features and void spaces within the surface of the hollow spheres, which provide large contact areas between the electrolyte and active materials for electrolyte diffusion and alleviate the volume change during the lithium-ion insertion/extraction process.

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Introduction

Over the past two decades, lithium ion batteries (LIBs) have received extensive attention and been used in portable electronic devices because of their high energy and power densities.^1–3 As for the anode materials in LIBs, commercial graphite with relatively low theoretical specific capacity (372 mA h g⁻¹) and poor rate capability cannot fully meet the energy density requirements for high-performance batteries. Therefore, numerous efforts have been devoted to searching for new anode materials with high specific capacity and good cycling stability.^4–7 Recently, transition metal chalcogenides such as CoS_x,^8–13 SnS_x,^13–16 MoS₂,^17–19 WS₂ (ref. 20–22) and so on were selected as the promising electrode materials in view of their high specific capacity, 2–3 times higher than that of the carbon/graphite-based materials.¹³ However, these materials often suffer from large volume change during lithium ion insertion/extraction and large initial irreversible capacities. To overcome the problem, an effective approach is to construct appropriate nanostructures. Among these alternative candidates, three-dimensional hollow structures have attracted significant attention. Such hierarchical architectures can effectively enhance the surface-to-volume ratio and reduce diffusion distance for lithium ions. Pores and hollow interior can accommodate the large volume change during lithium ion insertion/desertion. For instance, uniform CoS₂ hollow spheres have been fabricated via a facile solvothermal method, which deliver high discharge capacity (1210 mA h g⁻¹) and good cycle stability.²³ Yang's research group reported that mesoporous Co₉S₈ hollow spheres were synthesized through a solvothermal route accompanied by a high-temperature annealing in Ar/H₂.²⁴ Such hollow spheres exhibited a reversible capacity of 1414 mA h g⁻¹ after 100 cycles at 100 mA g⁻¹. Xia and co-authors have been fabricated SnS₂ hollow microspheres with good cycling performance.²⁵ Hierarchical hollow nanoparticles of MoS₂ nanosheets have been synthesized by a solvothermal reaction, which exhibit a high reversible capacity of 902 mA h g⁻¹ at 100 mA g⁻¹ after 80 cycles.²⁶ Therefore, fabricating the materials with hierarchical hollow structures is considered as an effective method to optimize the lithium storage capacity.

Recently, CoS_x (ref. 23 and 27) and NiS (ref. 28 and 29) with different morphologies have been fabricated and their lithium storage capacities have been evaluated. However, the poor cycling performance and the low conductivity will limit their practical application. It has been reported that ternary Ni–Co sulfides have much higher electrical conductivity than nickel sulfides and cobalt sulfides. The higher electronic conductivity is benefit for the rapid transfer of electrons in an electrode, enhancing the electrochemical properties. Up to now, various nanostructured Ni–Co sulfides including nanosheets,³⁰ nanoplates,³¹ nanotubes,^32,33 ball-in-ball hollow spheres³⁴ and urchin like structures^35,36 were prepared and their electrochemical properties for supercapacitors were investigated. To the best of our knowledge, no report has been devoted to investigate the electrochemical properties of Ni–Co sulfides as the anode material for rechargeable lithium ion batteries. Although different morphologies of Ni–Co sulfides have been fabricated, less report has been focused on the fabrication of NiCo₂S₄ hollow structures. In this work, NiCo₂S₄ hollow spheres with high surface area were successfully synthesized by a simple hydrothermal method accompanied by high-temperature calcination and anion exchange. Based on the experimental results, a CO₂ bubble template mechanism is proposed for interpreting the formation of the hollow spheres. Electrochemical measurements indicate that NiCo₂S₄ hollow spheres exhibit high specific capacity, good cycling stability and rate capability.

Experimental section

Synthesis of NiCo₂O₄ hollow spheres

In the typical procedure, 1 mmol of NiCl₂·6H₂O, 2 mmol of CoCl₂·6H₂O and 1.2 g of urea were dissolved into 20 mL of distilled water under the constant stirring. Then the clear solution was transferred into the Teflon-lined stainless steel autoclave with the capacity of 25 mL. The autoclave was sealed and heated at 180 °C for 12 h, and cooled naturally to room temperature. The as-prepared precipitates were centrifugated, washed with distilled water and ethanol, and then dried at 80 °C overnight. Finally, the dried precipitate was annealed at 450 °C for 6 h in air.

Synthesis of NiCo₂S₄ hollow spheres

0.3 g of NiCo₂O₄ was dispersed in 20 mL of distilled water and stirred for 1 h. After that, 0.4 g of Na₂S was introduced into the solution and stirred for another 0.5 h. The mixed slurry was transferred into the 25 mL of Teflon-lined stainless steel autoclave and maintained at 110 °C for 12 h. The powder was collected and washed with distilled water and ethanol. Subsequently, the powder was dried at 80 °C for further characterization.

Characterization

The crystal structure of the samples was recorded by power X-ray diffraction (XRD, Rigaku D/max-2550pc diffractometer, λ = 0.15406 nm). The structure and morphology of samples were detected on FEI Quanta 200F field emission scanning electron microscope (FESEM). Transmission electron microscope (TEM), high resolution TEM and selected area electron diffraction (SAED) were determined on FEI Tecnai G2 S-Twin operated at an accelerating voltage of 200 kV. X-ray photoelectron spectra (XPS) were performed on Axis Ultra Photoelectron Spectrometer with an excitation source of Mg–Al radiation. The surface area and pore size distribution of the samples were characterized by Quantachrome Surface Area Analyzer (QUDRASORB SI, USA). The specific surface area was obtained by the Brunauer–Emmett–Teller (BET) method.

Electrochemical measurements

The electrochemical properties of NiCo₂S₄ as anode materials in lithium ion cells were detected on Neware BTS-5V10mA Battery Test System. The working electrode was fabricated by mixing 70 wt% active material, 15 wt% acetylene black and 15 wt% polyvinylidene difluoride (PVDF) in a N-methylpyrrolidone (NMP) solution. The obtained homogeneous slurry was coated onto a copper foil and dried in the vacuum at 100 °C for 12 h (a total mass is around 1 mg). Cell assembly was carried out in a highly pure argon filled glovebox. A pure lithium foil was used as counter/reference electrode, polypropylene membrane (Celgard 2400) as separator. The electrolyte consists of 1 M LiPF₆ in ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) with a volume ratio of 1 : 1 : 1. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were carried out on CHI660E electrochemical workstation (Shanghai Chenhua Co., Ltd., China).

Results and discussion

The typical synthesis of NiCo₂O₄ contains two crucial steps, i.e., the fabrication of the precursor via a hydrothermal process and the subsequent thermal conversion to NiCo₂O₄. The X-ray diffraction peaks in Fig. S1a† presents the composition of the obtained Ni–Co precursor. All the peaks can be assigned to the CoCO₃ (JCPDS No. 11-0692). The corresponding energy dispersive spectroscope indicates that four elements Ni, Co, C, and O exist in the precursor (Fig. S1b†). For the synthesis of nickel cobalt mix-oxides, the precursor obtained after hydrothermal treatment was annealed at 450 °C. The XRD pattern of the product is shown in Fig. 1a, all the diffraction peaks match well with the spinel NiCo₂O₄ phase (JCPDS No. 20-0781). No other peaks arising from impurities could be observed, indicating the phase purity of NiCo₂O₄ in the product. The morphology of the obtained NiCo₂O₄ was characterized by FESEM. As shown in Fig. 1b, the obtained sample is composed of a large number of microspheres with a diameter of 2–6 μm. The broken spheres in Fig. 1b confirm the hollow structure of the products. The high-magnification FESEM images (Fig. 1c and d) of the NiCo₂O₄ sample provide detailed structural information on the hollow spheres. It is clearly observed that the hollow spheres contain many small nanorods. The average diameter and length are 20 nm and 2 μm, respectively.

Fig. 1 (a) XRD pattern and FESEM images of the NiCo₂O₄: (b) low magnification, (b) middle magnification, (d) high magnification.

To understand the formation mechanism of the hollow sphere, different reaction parameters were performed. When the solvothermal reaction temperature was decreased to 150 °C, urchin-like NiCo₂O₄ precursor can be obtained. Unfortunately, no broken spheres can be observed after 30 min ultrasonic treatment (Fig. S2a and b†). Further decreasing the reaction temperature to 120 °C, the obtained samples were composed of urchin-like structure, microspheres as well as nanoparticles (Fig. S2c†). From the above experimental results, one can conclude that the reaction temperature has much effect on the final morphology of the sample. In this work, urea was used as mineralizer. In the reaction system, the CO₂ bubbles with different size formed by decomposing the urea with increased reaction temperature. Because of the surface tension and the pressure caused by CO₂ atmosphere, CO₂ bubbles are aggregated into big spheres of micrometer size. Then the NiCo₂O₄ precursor nanocrystals start to nucleate homogeneously around the gas–liquid interface between bubble and solvent. As the result, the hollow spheres formed. With the reaction temperature increasing, the pressure inside the hollow spheres increases, leading to the broken hollow spheres, which can be seen in Fig. 1. According to the above discussion, the NiCo₂O₄ precursor hollow spheres can be explained by a bubble template mechanism,^37–40 as illustrated in Scheme S1.† When less amount of urea (0.6 g) was introduced into the reaction system, less CO₂ bubbles generated. As indicated, microspheres, irregular nanoparticles combined with hollow spheres can be obtained (Fig. S2d†). In addition, the reaction time is another important factor that affected the final morphology. After hydrothermal treatment for 2 h, less pink product is obtained. As presented in Fig. S3a,† numerous microspheres constructed by tiny nanorods and some nanoparticles can be obtained. The broken microspheres indicate that the obtained microspheres possess the hollow structure. When the reaction time is prolonged to 4 h, the microspheres become more uniform and the irregular nanoparticles decrease (Fig. S3b†). After hydrothermal reaction time proceeded (8 h), the obtained product is composed of uniform hollow spheres (Fig. S3c and d†). Closer inspection of these hollow spheres demonstrates that the nanorods grow longer and wider, which can be further ascertained in Fig. 1b–d. In the initial stage, the Ni–Co precursor nuclei were generated in the solution and then grew around the CO₂ bubbles. Thus the hollow spheres come into being. With increasing the reaction time, the Ni–Co precursor nuclei continued generating and then nucleated onto the big nanorods forming the regular hollow spheres through the process known as Ostwald ripening. The results further proved that appropriate CO₂ bubbles promoted the formation of NiCo₂O₄ precursor hollow spheres.

Using Na₂S as the sulfur source, the free sulfide anions can react with NiCo₂O₄ to form NiCo₂S₄. Fig. 2a displays the XRD pattern of the sample after 12 h sulfidation. The obtained diffraction peaks are agreed well with the standard patterns of the cubic NiCo₂S₄ (JCPDS No. 20-0782). And no other impurities such as NiCo₂O₄, NiS and CoS can be observed, indicating that this strategy can be successfully applied to fabricate NiCo₂S₄ through anion exchange. In addition, the diffraction peaks are weak and broad, indicating low crystallinity of the sample. The panoramic FESEM image shows that hollow spheres are obtained without apparent deformation in appearance. Such hollow spheres (shell thickness is ∼1 μm) are still constructed by some small nanorods. The composition of the sample is further determined by energy dispersive spectroscopy (EDS), three elements of Ni, Co, S with the atomic ratio of 13.01 : 30.20 : 55.71 are observed in Fig. S4,† which is consistent with the stoichiometric ratio of NiCo₂S₄.

Fig. 2 (a) XRD pattern and FESEM images of the NiCo₂S₄: (b) low magnification, (b) middle magnification, (d) high magnification.

The microstructure of as-prepared NiCo₂S₄ was further determined by transmission electron microscopy (TEM), high resolution TEM (HRTEM) and selected area electron diffraction (SAED) pattern. Fig. 3a further confirms that the hollow sphere is composed of numerous nanorods. As shown in Fig. 3b, the nanorods were porous and constructed by many small nanoparticles. The HRTEM image (Fig. 3c) presents the clear lattice fringes with spacings of 0.286 nm and 0.234 nm, which correspond to the (311) and (400) planes of cubic NiCo₂S₄, respectively. And the ring pattern of the selected area electron diffraction (SAED) in Fig. 3d displays the polycrystalline nature of the NiCo₂S₄.

Fig. 3 (a and b) TEM images, (c) HRTEM image, and (d) corresponding SAED pattern of the NiCo₂S₄ hollow sphere.

X-ray photoelectron spectrum (XPS) was applied to detect the chemical composition of NiCo₂S₄ hollow sphere and the results were presented in Fig. 4. The full XPS spectrum demonstrates three elements Ni, Co and S are detected except the adventitious C and O (Fig. 4a). The XPS spectrum of Ni 2p shows two peaks at 853.1 eV and 870.4 eV, attributed to Ni 2p_3/2 and Ni 2p_1/2, respectively (Fig. 4b).^41,42 The peaks at 778.8 eV and 793.9 eV can be assigned to the Co 2p_3/2 and Co 2p_1/2 of NiCo₂S₄ phase (Fig. 4c).⁴² And the two peaks with binding energy values at 161.8 and 162.8 eV in Fig. 4d, corresponding to S 2p_3/2 and S 2p_1/2, respectively.⁴³

Fig. 4 (a) XPS survey spectra, (b) Ni 2p, (c) Co 2p, and (d) S 2p spectra of the NiCo₂S₄ hollow sphere.

The porous nature of the NiCo₂S₄ hollow sphere was determined by nitrogen gas adsorption–desorption isotherm. It can be seen that the NiCo₂S₄ hollow sphere displays a type IV isotherm with a type H3 hysteresis loop in the relative pressure range of 0.6–1.0 P/P₀, implying the presence of mesopores structure (Fig. 5a). The corresponding pore size distribution in Fig. 5b presents the narrow pore size distribution in the range of 2–15 nm. The pores correspond to the interstitial spaces between the small nanoparticles in the shell, which is in agreement with that observed in SEM and TEM images. The specific surface area of the hollow sphere calculated by the Brunauer–Emmett–Teller (BET) method is 59.37 m² g⁻¹. The porous structure with a high surface area is expected to provide numerous electrochemical reaction sites for intercalation and deintercalation of the lithium ions.

Fig. 5 (a) Nitrogen gas adsorption–desorption isotherm and (b) pore size distribution of the NiCo₂S₄ hollow sphere.

In the previous reports, the NiCo₂S₄ can be used as supercapacitors.^32,34,35 In this work, the NiCo₂S₄ hollow sphere is applied as a potential anode for Li-ion battery. Cyclic voltammetry (CV) is initially used to evaluate the lithium storage properties. In the first cycle of NiCo₂S₄ hollow spheres electrode, as shown in Fig. 6a, the cathodic peak centered at 1.06 V is attributed to the reduction of Co³⁺ and Ni²⁺ to metallic Co and Ni, respectively.^3,44 In the subsequent cycles, the peak shifts to the higher voltage range. The weak peak at 0.56 V originates from the formation of a solid electrolyte interface (SEI) layer. Meanwhile, the anodic peaks at 2.04 V and 2.37 V can be observed, resulting from the sulfuration of metallic Ni and Co to nickel sulfides and cobalt sulfides. On the above analysis and previous reports,^28,45,46 the redox reactions can be described as follows:

NiCo₂S₄ + 8Li⁺ + 8e⁻ → Ni + 2Co + 4Li₂S (1)

Ni + Li₂S ↔ NiS + 2Li (2)

Co + Li₂S ↔CoS + 2Li (3)

Fig. 6 (a) CV curves of the NiCo₂S₄ electrode at a scan rate of 0.5 mV s⁻¹ in the voltage range of 0.01–3.0 V, (b) the charge–discharge curves of the NiCo₂S₄ electrode used as an anode material in lithium-ion battery, (c) corresponding cycle performance and (d) specific capacity as a function of cycle number at various current densities from 100 to 2000 mA g⁻¹ at the voltage ranging from 0.01 to 3.0 V.

To further investigate the Li-ion insertion and extraction properties, CR2025 coin cell was tested by galvanostatic cycling at room temperature at various current rates in the voltage ranging from 0.01 V to 3.0 V versus Li/Li⁺. Fig. 6b demonstrates the voltage–capacity profiles of the as-prepared NiCo₂S₄ hollow sphere electrode for the first, second, and fifth charge/discharge cycles at a current rate of 200 mA g⁻¹, respectively. The NiCo₂S₄ electrode delivers a first discharge capacity of 1251 mA h g⁻¹ and charge capacity of 1016 mA h g⁻¹, respectively, which leads to a coulombic efficiency of 81%. The large irreversible capacity and the low initial coulombic efficiency may be attributed to the formation of a solid–electrolyte interface (SEI) layer on the surface of the electrode.^8,11 For the second and fifth cycles, the discharge capacities are 966 mA h g⁻¹ and 874 mA h g⁻¹. While the charge capacities are 879 mA h g⁻¹ and 811 mA h g⁻¹, respectively. And the coulombic efficiency increased to 91% and 92.8%, respectively. During the cycling process, the porous structure can cushion the volume change of the electrode and effectively alleviate the pulverization problem, resulting in the enhanced cycle stability. As shown Fig. 6c, the coulombic efficiency is closed to 100% after 15 times cycles. The cycling performance of the NiCo₂S₄ electrode is presented in Fig. 6b. The specific discharge capacity decreased gradually to 764 mA h g⁻¹ at the 25^th cycle, then the capacity was stabilized and retained at 696 mA h g⁻¹ after 100 cycles. The capacity retention after the second cycle is ∼72%. In addition, the electrochemical properties of NiCo₂O₄ hollow spheres are also investigated. As can be seen in Fig. S4a,† the NiCo₂O₄ hollow spheres present the initial discharge and charge capacities of 1486 and 1018 mA h g⁻¹, respectively (current density: 200 mA g⁻¹). Fig. S4b† shows the cycling performance of the NiCo₂O₄ hollow spheres at the current rate of 200 mA g⁻¹. The discharge capacity remains at 867 mA h g⁻¹ after 100 cycles, and the corresponding capacity retention after the second cycle is 73.5%, which is comparable to the previous NiCo₂O₄ hollow spheres with a core-in-double shell interior structure,⁴⁷ mesoporous NiCo₂O₄ nanosheets,⁴⁸ porous NiCo₂O₄ nanoflakes and nanobelts.⁴⁴ The results indicate that both NiCo₂O₄ and NiCo₂S₄ demonstrate the good cycle performance. The relatively high discharge capacity and good cycle performance is mainly due to the unique porous structures. The high capacity and stability are ascribed to the synergistic effect of the porous structure and the small nanoparticles that assembled the hollow spheres. The small nanoparticles provide an increased contact area between NiCo₂S₄ or NiCo₂O₄ electrode and electrolyte, which shortens the diffusion length of charge carriers and accommodates the volume change during the Li-ion insertion/extraction process, resulting in the high specific capacity and high rate capability. Meanwhile, the hollow structure of NiCo₂S₄ or NiCo₂O₄ spheres can promote the penetration of electrolyte into the electrode, which mitigates the volume change during charge–discharge process, leading to the good cycling performance. The enhanced electrochemical properties can be observed in other transition metal oxides and chalcogenides mesoporous and hollow structures.^{11,24,44,47,49} From another point of view, the stability of the hollow spheres has much effect on the electrochemical properties. Even after 100 cycles, the overall morphology of hollow spheres is still maintained and no obvious aggregation is observed (Fig. S6a and b†). The result further presents that elastic mesoporous structure can effectively suppress the large volume change during cycling. The good volume stability is helpful to achieve high capacity retention and excellent cycling performance. The rate capability of the NiCo₂S₄ hollow spheres is also evaluated at different current densities and the results are shown in Fig. 6d. The reversible capacity decreases gradually with increasing the current density. But cycle specific capacity still remains at 411 mA h g⁻¹ even at higher current density of 2000 mA g⁻¹. The NiCo₂S₄ hollow spheres display better stability and rate capacity compared to the previously reported Li storage performance for Ni₃S₄ nanoparticles,²⁹ NiS hierarchical hollow microspheres²⁸ and CoS_x with shapes of nanosheets⁴⁶ and hollow spheres.²⁷ For instance, the Ni₃S₄ nanoparticles electrode displays poor cycle stability with the specific capacity decreased from 1402 to 494 mA h g⁻¹ (100^th cycle) at the current density of 140.9 mA g⁻¹. The NiS hierarchical hollow spheres show a capacity retention of 14.2% after only 20 cycles at the current density of 50 mA g⁻¹.²⁸ The ultrathin CoS nanosheets demonstrate a specific capacity of 359 mA h g⁻¹ after 80 cycles at the current density of 58.9 mA g⁻¹.⁴⁶ For the Co₉S₈ hollow spheres, the specific capacity decreased from 1008 to 634 after 100 cycles at the current density of 1103.9 mA h g⁻¹ to 254.9 mA h g⁻¹ at 100 mA g⁻¹.²⁷

To further understand the electrochemical performance of NiCo₂S₄ hollow spheres as anode material, the electrochemical impedance spectra (EIS) are preformed. Fig. 7 shows the Nyquist plots for the fresh cell and the cell after 100 cycles. Both the Nyquist plots depict the similar shape with depressed semicircles in the high and medium-frequency range and a straight line in the low-frequency. The inclined line in the low-frequency range corresponds the Warburg impedance (Z_w), implying solid-state diffusion of Li⁺ in the electrode materials.⁵⁰ While the semicircle in the middle frequency represents the charge-transfer resistance (R_ct), which may be attributed to the charge transfer through the electrode/electrolyte interface. As can be seen in Fig. 7, the charge-transfer resistance increases after 100 times cycling. The lower charge-transfer resistance (R_ct) of the electrode indicates that the lithium ions and electrons can transfer more freely in the electrode/electrolyte interface. The electrochemical impedance result is in agreement with the cycle performance of the electrode first decreasing from the initial discharge capacity of 1251 mA h g⁻¹ to 696 mA h g⁻¹ after 100 cycles.

Fig. 7 Electrochemical impedance spectra (EIS) of (a) the fresh cell and (b) the cell after 100 cycles at a current density of 100 mA g⁻¹.

Conclusions

In summary, hierarchical NiCo₂S₄ hollow spheres were fabricated by hydrothermal method combined with calcination and anion exchange process. The electrochemical measurements results display that the hierarchical NiCo₂S₄ hollow spheres exhibit high discharge capacity and cycle stability. A high specific capacity of 696 mA h g⁻¹ was achieved after 100 cycles at the current density of 200 mA g⁻¹. Even at the rate as high as 2000 mA g⁻¹, the specific capacity higher than 411 mA h g⁻¹ was obtained after 50 cycles. The superior performance is directly brought by the unique porous structure. Such hierarchical structure provides faster ion diffusions and buffers drastic volume changes during the charge–discharge process when used as an anode material for LIBs.

Acknowledgements

The authors are grateful for the financial support from the Natural Science Foundation of China (Project no. 21301086), Natural Science Foundation of Shandong Province (Project no. ZR2013BQ008), and Talent Introduction Fund of Ludong University (Project no. LY2013014).

Notes and references

1. A. S. Aricò, P. Bruce, B. Scrosati, J.-M. Tarascon and W. V. Schalkwijk, Nat. Mater., 2005, 4, 366–377.
2. P. G. Bruce, B. Scrosati and J.-M. Tarascon, Angew. Chem., Int. Ed., 2008, 47, 2930–2946.
3. Y. Chen, M. Zhuo, J. Deng, Z. Xu, Q. Li and T. Wang, J. Mater. Chem. A, 2014, 2, 4449–4456.
4. K. Brezesinski, J. Haetge, J. Wang, S. Mascotto, C. Reitz, A. Rein, S. H. Tolbert, J. Perlich, B. Dunn and T. Brezesinski, Small, 2011, 7, 407–414.
5. A. Pan, H. B. Wu, L. Zhang and X. W. Lou, Energy Environ. Sci., 2013, 6, 1476–1479.
6. J. Zhu, T. Zhu, X. Zhou, Y. Zhang, X. W. Lou, X. Chen, H. Zhang, H. H. Hng and Q. Yan, Nanoscale, 2011, 3, 1084–1089.
7. X. Wang, X. Cao, L. Bourgeois, H. Guan, S. Chen, Y. Zhong, D.-M. Tang, H. Li, T. Zhai, L. Li, Y. Bando and D. Golberg, Adv. Funct. Mater., 2012, 22, 2682–2690.
8. W. Shi, J. Zhu, X. Rui, X. Cao, C. Chen, H. Zhang, H. H. Hng and Q. Yan, ACS Appl. Mater. Interfaces, 2012, 4, 2999–3006.
9. Q. H. Wang, L. F. Jiao, H. M. Du, W. X. Peng, Y. Han, D. W. Song, Y. C. Si, Y. J. Wang and H. T. Yuan, J. Mater. Chem., 2011, 21, 327–329.
10. Y. X. Zhou, H. B. Yao, Y. Wang, H. L. Liu, M. R. Gao, P. K. Shen and S. H. Yu, Chem.–Eur. J., 2010, 16, 12000–12007.
11. Y. N. Ko, S. H. Choi, S. B. Park and Y. C. Kang, Chem.–Asian J., 2014, 9, 572–576.
12. N. Mahmood, C. Zhang, J. Jiang, F. Liu and Y. Hou, Chem.–Eur. J., 2013, 19, 5183–5190.
13. X. Rui, H. Tan and Q. Yan, Nanoscale, 2014, 6, 9889–9924.
14. B. Luo, Y. Fang, B. Wang, J. Zhou, H. Song and L. Zhi, Energy Environ. Sci., 2012, 5, 5226–5230.
15. K. Chang, Z. Wang, G. Huang, H. Li, W. Chen and J. Y. Lee, J. Power Sources, 2012, 201, 259–266.
16. J.-w. Seo, J.-t. Jang, S.-w. Park, C. Kim, B. Park and J. Cheon, Adv. Mater., 2008, 20, 4269–4273.
17. S. Ding, J. S. Chen and X. W. Lou, Chem.–Eur. J., 2011, 17, 13142–13145.
18. H. Hwang, H. Kim and J. Cho, Nano Lett., 2011, 11, 4826–4830.
19. K. Chang and W. Chen, ACS Nano, 2011, 5, 4720–4728.
20. R. Bhandavat, L. David and G. Singh, J. Phys. Chem. Lett., 2012, 3, 1523–1530.
21. C. Feng, L. Huang, Z. Guo and H. Liu, Electrochem. Commun., 2007, 9, 119–122.
22. H. Liu, D. Su, G. Wang and S. Z. Qiao, J. Mater. Chem., 2012, 22, 17437–17440.
23. Q. H. Wang, L. F. Jiao, Y. Han, H. M. Du, W. X. Peng, Q. N. Huan, D. W. Song, Y. C. Si, Y. J. Wang and H. T. Yuan, J. Phys. Chem. C, 2011, 115, 8300–8304.
24. Y. Zhou, D. Yan, H. Xu, J. Feng, X. Jiang, J. Yue, J. Yang and Y. Qian, Nano Energy, 2015, 12, 528–537.
25. J. Xia, G. Li, Y. Mao, Y. Li, P. Shen and L. Chen, CrystEngComm, 2012, 14, 4279–4283.
26. M. Wang, G. Li, H. Xu, Y. Qian and J. Yang, ACS Appl. Mater. Interfaces, 2013, 5, 1003–1008.
27. R. Jin, J. Zhou, Y. Guan, H. Liu and G. Chen, J. Mater. Chem. A, 2014, 2, 13241–13244.
28. Y. Wang, Q. Zhu, L. Tao and X. Su, J. Mater. Chem., 2011, 21, 9248–9254.
29. N. Mahmood, C. Zhang and Y. Hou, Small, 2013, 9, 1321–1328.
30. W. Chen, C. Xia and H. N. Alshareef, ACS Nano, 2014, 8, 9531–9541.
31. J. Pu, F. Cui, S. Chu, T. Wang, E. Sheng and Z. Wang, ACS Sustainable Chem. Eng., 2014, 2, 809–815.
32. H. Wan, J. Jiang, J. Yu, K. Xu, L. Miao, L. Zhang, H. Chen and Y. Ruan, CrystEngComm, 2013, 15, 7649–7651.
33. J. Xiao, L. Wan, S. Yang, F. Xiao and S. Wang, Nano Lett., 2014, 14, 831–838.
34. L. Shen, L. Yu, H. B. Wu, X.-Y. Yu, X. Zhang and X. W. Lou, Nat. Commun., 2015, 6, 6694.
35. Y. Zhang, M. Ma, J. Yang, C. Sun, H. Su, W. Huang and X. Dong, Nanoscale, 2014, 6, 9824–9830.
36. H. Chen, J. Jiang, L. Zhang, H. Wan, T. Qi and D. Xia, Nanoscale, 2013, 5, 8879–8883.
37. D. Song, Q. Wang, Y. Wang, Y. Wang, Y. Han, L. Li, G. Liu, L. Jiao and H. Yuan, J. Power Sources, 2010, 195, 7462–7465.
38. Y. Han, M. Fuji, D. Shchukin, H. Mohwald and M. Takahashi, Cryst. Growth Des., 2009, 9, 3771–3775.
39. J. Yang and T. Sasaki, Chem. Mater., 2008, 20, 2049–2056.
40. T. Tomioka, M. Fuji, M. Takahashi, C. Takai and M. Utsuno, Cryst. Growth Des., 2012, 12, 771–776.
41. J. Li, S. Xiong, Y. Liu, Z. Ju and Y. Qian, ACS Appl. Mater. Interfaces, 2013, 5, 981–988.
42. F. Zheng, D. Zhu and Q. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 9256–9264.
43. S.-L. Yang, H.-B. Yao, M.-R. Gao and S.-H. Yu, CrystEngComm, 2009, 11, 1383–1390.
44. A. K. Mondal, D. Su, S. Chen, X. Xie and G. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 14827–14835.
45. H. Ruan, Y. Li, H. Qiu and M. Wei, J. Alloys Compd., 2014, 588, 357–360.
46. S. Kong, Z. Jin, H. Liu and Y. Wang, J. Phys. Chem. C, 2014, 118, 25355–25364.
47. L. Shen, L. Yu, X.-Y. Yu, X. Zhang and X. W. Lou, Angew. Chem., Int. Ed., 2015, 54, 1868–1872.
48. A. K. Mondal, D. Su, S. Chen, K. Kretschmer, X. Xie, H.-J. Ahn and G. Wang, ChemPhysChem, 2015, 16, 169–175.
49. J. Li, J. Wang, X. Liang, Z. Zhang, H. Liu, Y. Qian and S. Xiong, ACS Appl. Mater. Interfaces, 2014, 6, 24–30.
50. K. M. Shaju, F. Jiao, A. Débart and P. G. Bruce, Phys. Chem. Chem. Phys., 2007, 9, 1837–1842.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14412d

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