Temperature effects on a nano-porous ZnCo₂O₄ anode with excellent capability for Li-ion batteries

Abstract

In this study, nano-porous ZnCo₂O₄ anodes are synthesized via a hydrothermal method with subsequent annealing at different temperatures. X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are carried out to study the crystal structure, pore size distribution, surface morphology and characteristics, respectively. With increasing sintering temperature, the morphology of ZnCo₂O₄ turns to sphere-like with a porous structure. The as-prepared porous ZnCo₂O₄ nanospheres synthesized at the optimal condition of 600 °C (ZCO-600) demonstrate a high capacity of 1800 mA h g⁻¹ and a good retention performance of 1242 mA h g⁻¹ after 30 cycles. The AC data shows that the ZCO-600 anode gives a lower impedance compared to ZCO at other temperatures. The porous nanostructure and large surface area are responsible for the superior performance. Moreover, nano-porous ZCO synthesized at 500 °C gave a better cycle performance and much higher rate capability compared to the samples synthesized at temperatures of 600 °C and 700 °C, because of its superior structural properties.

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1. Introduction

In recent years, lithium-ion batteries (LIBs) have been extensively applied in many electronic applications like portable electronic devices, electric vehicles (EVs), hybrid electric vehicles (HEVs), energy storage systems (ESSs), and so forth because of their high energy density, perfect lifespan and environmental friendliness.^1–4 The development of high energy capacity, long cycle life and low environmental impact materials for LIBs used in the above devices has become very important.⁵

Considering anode materials, graphite is the primary one used in the market. However, the theoretical capacity of a commercial graphite anode is only 372 mA h g⁻¹, which cannot meet our demand for high energy capacity in the future.⁶ Therefore, it is necessary to look for better materials to replace graphite. In recent research, 3D transition metal oxides with nanostructures and porous characteristics have been found to be good at improving the cycle performance and coulombic efficiency with high capacity.^7–10

In previous reports, transition metal oxides such as NiO,¹¹ Fe₂O₃ (ref. 12) and Co₃O₄ (ref. 13) have shown excellent electrochemical performance. Among all of them, Co₃O₄ exhibits the best anodic performance with high theoretical capacity as high as ∼900 mA h g⁻¹.^1,14–21 However, cobalt is toxic and expensive thus it is unfriendly to our environment and the commercial costs are too high to bear. In order to solve the above disadvantages, cobalt is being partially replaced in Co₃O₄ with eco-friendly and cheaper alternative metals such as Ni,¹⁰ Zn,²² Mg,²³ Fe²³ and Mn,^24,25 which are also electrochemically active for Li insertion and extraction. Spinel anode materials have been reported with NiCo₂O₄,^27,28 ZnCo₂O₄,^29–34 MgCo₂O₄,²³ FeCo₂O₄,²³ MnCo₂O₄/CoMn₂O₄,^24,35 and CuCo₂O₄.^26,36 Among all of them, ZnCo₂O₄ has strong potential as an electrode material for lithium storage and could provide the highest capacity because both Zn and Co are more electrochemically active with respect lithium than other alternative metals.³⁷ Therefore, ZnCo₂O₄ is considered as the most attractive material to substitute for a graphite anode in a Li-ion battery due to its perfect electrochemical properties such as high reversible capacity, excellent cycle performance and good environmental friendliness.³⁴

So far, ZnCo₂O₄ nanomaterials with different shapes have been widely studied, such as nanoparticles,²⁹ nanoflakes,³⁸ nanotubes,³⁷ nanowires³⁹ and nanorods,⁴⁰ all prepared using different methods and all exhibited excellent electrochemical performances. However, little literature focuses on the effect of the synthetic temperature on the structural change and electrochemical performance.

Thus, in this study, we attempt to systematically synthesize ZCO with different annealing temperatures from 500 °C to 700 °C. Characterization techniques, including different electrochemical analyses and BET measurements were used to establish the relationship between the structure and electrochemical properties. The results could provide important information for material design of ZCO as a potential anode for Li-ion batteries in the future.

2. Experimental section

2.1. Preparation of the electrode materials

The sphere-like ZnCo₂O₄ with a porous nanostructure was synthesized using a hydrothermal method. Typically, 0.5425 g of zinc oxide (ZnO, 99.5%, SIGMA-ALDRICH) was dissolved in ∼5 mL of dilute nitric acid (HNO₃, 65%, SIGMA-ALDRICH) and 60 mL of DI water to get the solution. Then, 3.8805 g of cobalt nitrate (Co(NO₃)₂·6H₂O, 98%, SHOWA) and 0.8406 g of citric acid (C₆H₈O₇·1H₂O, 99.5%, SIGMA-ALDRICH) were added to dissolve in the above solution under continuous stirring for 10 min and the pH was adjusted to 7 by adding ammonium hydroxide (NH₄OH, 25%, FISHER). The mixed solution was subsequently transferred to a 100 mL Teflon-lined stainless steel autoclave. The autoclave was heated to 180 °C for 24 h. After heating, the autoclave was air-cooled to room temperature and the resulting pink precipitate was washed with DI water several times and then dried at 80 °C. The precipitates were separately annealed at 500 °C, 600 °C and 700 °C for 3 h to obtain the final products.

2.2. Material characterization

The products were characterized using powder X-ray diffraction (XRD, Bruker D8 Advance Eco) with Cu Kα radiation (λ = 1.5418 Å). The thermogravimetric analysis (TGA/DTG, TA Q50) measurements of the ZnCo₂O₄ precursors were carried out at a heating rate of 5 °C min⁻¹ from room temperature to 800 °C. The morphology and structure of the products were analysed using scanning electron microscopy (SEM, Hitachi S-4100) and transmission electron microscopy (TEM), then element mapping was done using energy-dispersive X-ray spectroscopy (EDS). The Brunauer–Emmett–Teller (BET) surface area and pore size were tested using Micromeritics Tristar 3000.

2.3. Electrochemical measurements

The electrochemical performances of the products were measured using CR2032 coin cells. The working electrode was composed of 60 wt% active materials, 25 wt% KS-6, 5 wt% super-P, 6 wt% CMC and 4 wt% SBR, coated on 10 μm copper foil, which was dried at 120 °C for 8 h in a vacuum system to remove residual water. The electrolyte consisted of 1 M LiPF₆ in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1 : 1 in volume ratio). The discharge/charge test was performed using an AcuTech System in the voltage range of 0.01–3.5 V at room temperature. The mass loading of these samples was in the range of 2.4–4 mg cm⁻². The cyclic voltammograms (CV) were measured using a CH Instruments Analyzer CHI 6273E at a scan rate of 0.001 mV s⁻¹ between 0.01 V and 3.5 V, then the AC impedance was tested in the frequency range from 1–100 000 Hz.

3. Results and discussion

Fig. 1 shows the XRD patterns of ZnCo₂O₄ synthesized with different annealing temperatures of 500 °C, 600 °C and 700 °C. The diffraction peaks at 2θ values of 18.89°, 31.29°, 36.84°, 38.51°, 44.82°, 55.67°, 59.35°, 65.26° and 72.28° correspond to the diffraction planes (111), (220), (311), (220), (400), (422), (511) and (440), respectively. All these diffraction peaks match well with the cubic spinel structure of JCPDS-23-1390 in the XRD database. The absence of any diffraction peaks of Co₃O₄ and ZnO confirmed the high purity of the products, and the schematic crystal structure of spinel ZnCo₂O₄ was demonstrated. Furthermore, based on the full width at half maximum of the diffraction peak (311), the crystal size of the products was calculated using Scherrer’s equation: D = κλ/β1/2 cos θ, where κ is shape factor, β is the line broadening at half the maximum intensity, and cos θ is the Bragg angle. All the calculated average crystal sizes of ZnCo₂O₄ synthesized at 500 °C and 600 °C were about 24 nm, but when the annealing temperature was increased to 700 °C, the average crystal size became about 37 nm. It might have resulted from the fusion of the nanoparticles with each other at the higher temperature.

Fig. 1 XRD pattern of ZnCo₂O₄ synthesized at 500 °C, 600 °C and 700 °C.

Fig. 2 shows the TGA and DTG results of the thermal properties of the precursor. There are two obvious weight losses during the heating process. The first one that occurred below 200 °C was attributed to the loss of free water or other organic molecules, while the second one was due to the thermal decomposition of the precursors into ZnCo₂O₄ by releasing CO₂, corresponding to sharp exothermic peaks located at 188.3 °C and 242.7 °C in the DTG curve. The large total weight loss of 54.7%, and no weight loss or exothermic peaks found over 300 °C, indicated that the precursor was transformed to ZnCo₂O₄ completely. Therefore, to ensure that the ZnCo₂O₄ obtained after annealing was pure phase, a temperature above 500 °C was chosen as the annealing temperature for the synthesis of ZnCo₂O₄.

Fig. 2 TGA/DTG curves of the ZnCo₂O₄ precursor.

Fig. 3 shows the SEM images of ZnCo₂O₄ synthesized at different annealing temperatures of 500 °C, 600 °C and 700 °C. It can be seen from Fig. 3(a) that the structure consists of ZnCo₂O₄ nanoparticles with diameters of 20–30 nm, but the shape of the structure is hardly confirmed at the lower synthetic temperature of 500 °C. With increasing the annealing temperature to 600 °C, shown in Fig. 3(b), the nanoparticles assemble into sphere-like structures. For the higher temperature of 700 °C, it can be seen that the sphere-like structures become rough and larger because of the fusion between the nanoparticles. In the high-magnification images in Fig. 3(d)–(f), all of the structures have a porous nature, especially at 600 °C. Obviously, for the higher temperature of 700 °C, the phenomenon of sintering occurs and most of these pores disappear. The porosity of ZCO, indeed, could be controlled by tuning the annealing temperature. The electrochemical properties, such as the capacity, cycling stability as well as rate performance will be different. Nano-sized particles with high porosity could be good for enhancing Li-ion intercalation and diffusion into the spinel lattice. It also becomes much easier for the electrolyte to penetrate into the active material, thus enhancing contact areas between the electrolyte and ZnCo₂O₄ and shortening the Li-ion diffusion distance in the structure. Fig. 4(a) shows the ZnCo₂O₄ synthesized at 600 °C, the diameters of the ZnCo₂O₄ particles are about 20–30 nm, which match well with the SEM observation. The HRTEM image of ZCO shown in Fig. 4(b) reveals a lattice fringe with an interplanar spacing of 0.25 nm, corresponding to the (311) plane of the spinel ZnCo₂O₄ phase. From the result of the SAED shown in Fig. 4(c), the d values of 0.286 nm, 0.244 nm and 0.202 nm correspond well to the Miller indices of (220), (311) and (400) for a ZnCo₂O₄ crystal. The EDS information of ZnCo₂O₄ is shown in Fig. 5(a). The elements Zn, Co and O can only be seen in the nanostructure, indicating the formation of pure phase ZnCo₂O₄. Fig. 5(b) also shows the EDS mapping results, the elements Zn, Co and O are evenly distributed all over the structure.

Fig. 3 SEM images of ZnCo₂O₄ synthesized at different annealing temperatures: low magnification of (a) 500 °C, (b) 600 °C and (c) 700 °C; high magnification of (d) 500 °C, (e) 600 °C and (f) 700 °C.

Fig. 4 (a) TEM image, (b) HRTEM image, and (c) SAED pattern of ZnCo₂O₄ synthesized at 600 °C.

Fig. 5 (a) EDS analysis and (b) EDS mapping of the as-prepared ZnCo₂O₄.

The specific surface areas and pore size distribution of ZnCo₂O₄ nanospheres synthesized at different annealing temperatures were characterized using BET analysis using nitrogen adsorption–desorption, as shown in Fig. 6. It can be seen that the specific surface areas of ZnCo₂O₄ synthesized at 500 °C and ZnCo₂O₄ synthesized at 600 °C are about 11.67 m² g⁻¹ and 12.48 m² g⁻¹, respectively. The specific surface area of ZCO-600 was similar to that of ZCO-500. As for ZCO-700 shown in Fig. 6(c), its specific surface area is only 7.81 m² g⁻¹. The decrease of surface area from 500–600 °C to 700 °C might be due to the fusion between the ZnCo₂O₄ nanoparticles at the higher annealing temperature. All these ZCO samples showed a narrow pore size distribution below 10 nm because nano-sized ZnCo₂O₄ particles were formed in the pores. The enhanced surface area of ZCO-500 and ZCO-600 could be attributed to the smaller diameters and larger quantities of nanoparticles and nanopores.

Fig. 6 N₂ adsorption–desorption isotherms and BJH pore size distributions of ZnCo₂O₄ synthesized at (a) 500 °C, (b) 600 °C and (c) 700 °C.

According to the previous study, the entire electrochemical process can be explained as follows:

ZnCo₂O₄ + 8Li⁺ + 8e⁻ → Zn + 2Co + 4Li₂O (1)

Zn + Li⁺ + e⁻ ↔ LiZn (2)

Zn + Li₂O ↔ ZnO + 2Li⁺ + 2e⁻ (3)

2Co + 2Li₂O ↔ 2CoO + 4Li⁺ + 4e⁻ (4)

2CoO + 2/3Li₂O ↔ 2/3Co₃O₄ + 4/3Li⁺ + 4/3e⁻ (5)

With the above electrochemical processes, the cyclic voltammetry test (CV) and charge/discharge test can be discussed clearly below.

Fig. 7(a) shows the cyclic voltammograms of ZCO-600 in the first three cycles. The reduction peak in the first cycle shows an intense irreversible reaction at ∼0.15 V, which might be due to the decomposition of ZnCo₂O₄ to Zn⁰ and Co⁰ according to eqn (1) with the formation of a solid electrolyte interphase (SEI). In comparison, the discharge of the second and third cycle show peaks at ∼0.4 V and ∼0.7 V, which are indicative of different electrochemical reactions occurring during the two processes. However, in the first three cycles, two oxidation peaks at ∼1.9 V and 2.3 V in the anodic polarization can be observed in all cycles, and the peaks can be attributed to the oxidation of Zn⁰ to Zn²⁺ and Co⁰ to Co³⁺ (eqn (3)–(5)), respectively. To compare the charge/discharge degree of the Li-ion in ZnCo₂O₄ at different annealing temperatures, the CV curves of these samples in the third cycle were tested and shown in Fig. 7(b). It can be observed that the positions of the redox peaks for ZCO-500 and ZCO-600 were quite close, which demonstrate a similar phenomenon for Li-ion diffusion in these porous structures and a similar specific surface area shown in the BET data. The potential of the reduction reactions of ZCO-600 and ZCO-500 occurs at ∼1.2 V vs. Li/Li⁺. However, for the ZCO-700 sample, an obvious polarization phenomenon was observed in the CV tests. First, the currents of reduction and oxidation for ZCO-700 were lower than those of ZCO-500 and ZCO-600. Second, the onset potential for reduction decreased to 0.9 V vs. Li/Li⁺. The difference of electrochemical properties might be due to a diffusion issue. From the viewpoint of diffusion, the nano-size and porous nature of the ZCO-500 and ZCO-600 samples provide a short diffusion path for Li ions into the materials. For ZCO-700, however, the lower specific surface area and pore volume might result from a problem with the charge and discharge processes.

Fig. 7 (a) The first three cycles of the CVs for ZnCo₂O₄ synthesized at 600 °C and (b) CV curves of ZnCo₂O₄ synthesized at 500 °C, 600 °C and 700 °C in the 3^rd cycle.

Fig. 8 shows charge/discharge curves of ZnCo₂O₄ synthesized at 600 °C at a current density of 0.1C in the voltage range of 0.01–3.5 V in the first three cycles. It can clearly be seen that there is a plateau at ∼0.9 V in the first charging process. For the second and third cycle, the plateau shifts to ∼1.2 V and becomes steeper, which is consistent with the CV results shown in Fig. 7(a). The initial charge and discharge capacities are 1800 and 1452 mA h g⁻¹, respectively. The coulombic efficiency in the first cycle is as high as 80.6%. The irreversible capacity loss in the first cycle is ∼19.4%, which is associated with the formation of an SEI film. In addition, the coulombic efficiency of the second cycle and third cycle is enhanced to as high as 98.9% and 98.8%, and the charge/discharge curves almost overlap, indicating the high reversibility of the electrochemical properties of ZCO-600 and that the result is well-matched with the CV results shown before.

Fig. 8 First three cycles of the discharge/charge curves of ZnCo₂O₄ synthesized at 600 °C.

Fig. 9(a) shows the cycle performance of ZnCo₂O₄ synthesized at 500 °C, 600 °C and 700 °C at a current density of 0.1C. It can be observed that the initial charge/discharge capacity of ZCO-600 is the highest one compared to ZCO-700 and ZCO-500. The change in capacity indicated that the capacities of ZnCo₂O₄ synthesized at 600 °C and 700 °C started to decline after 12 cycles, but the capacity of ZnCo₂O₄ synthesized at 500 °C not only did not decline but also increased, exhibiting the more perfect cycle performance. After 30 cycles, the reversible capacities of ZnCo₂O₄ synthesized at 500 °C, 600 °C and 700 °C are 1075, 1242 and 713 mA h g⁻¹, respectively. Among these samples, both ZCO-500 and ZCO-600 still retained the higher capacity and good retention, which might be attributed to the porous nanosphere structure. From the BET results shown in Fig. 6, the more complete structure and higher surface area of ZCO-500 and ZCO-600 provide Li ions with a shorter diffusion distance and enhance the contact areas between the electrolyte and ZnCo₂O₄, exhibiting better capacity and retention ability than at other annealing temperatures. The coulombic efficiency of ZCO-500, ZCO-600 and ZCO-700 in the first 30 cycles, shown in Fig. 9(a), shows close to 100% efficiency for these samples, which indicates their good cyclability. For larger current tests, Fig. 9(b) demonstrates 100 cycle tests at 1C for these three samples. Obviously, ZCO-500 gave a superior cycling performance compared to those of the other samples. The reversible capacity of ZCO-500 could be maintained to ∼500 mA h g⁻¹ which is ∼1.34 times than that of a graphite anode.

Fig. 9 (a) Cycle performance of ZnCo₂O₄ synthesized at 500 °C, 600 °C and 700 °C for 30 cycles at 0.1C, (b) cycle performance of ZnCo₂O₄ synthesized at 500 °C, 600 °C and 700 °C for 100 cycles at 1C, and (c) rate performance of ZnCo₂O₄ synthesized at 500 °C, 600 °C and 700 °C.

The rate performance of ZCO at different annealing temperatures was also investigated. As shown in Fig. 9(c), the average discharge capacity of ZCO-600 decreases from ∼1250, ∼1100, ∼950 and ∼1000 mA h g⁻¹ with increasing current from 1C, 2C, 5C, and 10C. In the subsequent cycle, the capacity rebounded to ∼1100 mA h g⁻¹ with the current back to 1C. ZCO-500, however, demonstrates a much higher capacity than ZCO-600 and ZCO-700. Under 10C, the capacity of ZCO-500 was as high as ∼900 mA h g⁻¹. The results demonstrate again that the nanostructure plays a very important role in both the cycle stability and rate capability.

The AC impedance results of ZCO synthesized at 500 °C, 600 °C and 700 °C after charging in the third cycle is shown in Fig. 10. The diameters of the semicircles for ZCO-500, ZCO-600 and ZCO-700 are 320 Ω, 160 Ω and 725 Ω, respectively. The ZCO-600 sample exhibited the lowest impedance. In order to understand the major contribution of impedance in these three samples, we used a typical equivalent circuit (shown in the inset in Fig. 10). Herein, R₁, R_SEI and R_CT are the impedances resulting from the electrolyte, SEI film and charge transfer of ZCO, respectively. Some of the corresponding fitting data are displayed in Table 1. First of all, the R₁ values for these three samples are almost the same, which is due to the similar impedance of the interface between the electrode and electrolyte. For the contribution of the SEI film, as shown in Table 1, the R_SEI values were also very close because the surface chemistry of ZCO synthesized at different annealing temperatures was similar. Significantly, the dramatic difference in impedance was the charge transfer resistance. The R_CT values of ZCO-500, ZCO-600 and ZCO-700 were 243, 84.5 and 602, respectively, thanks to the nano-porous nature, which facilities both the reaction kinetics and the diffusion of Li ions.

Fig. 10 AC impedance of ZnCo₂O₄ synthesized at 500 °C, 600 °C and 700 °C in the third cycle.

Table 1 Impedance parameters calculated from the equivalent circuit model

ZnCo2O4	R1	RSEI	RCT
500 °C	2.801	66	243
600 °C	2.236	62	84.5
700 °C	2.738	64	602

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Based on the above results, ZnCo₂O₄ synthesized at 600 °C could provide the best electrochemical properties. To exhibit the superiority of the ZnCo₂O₄ anode materials in this study, Table 2 shows the synthesis methods, electrochemical properties and annealing temperatures from previous research. Compared with this study, it can clearly observed that the 1^st cycle discharge capacity and the 30^th cycle charge capacity are all superior to related references, demonstrating the excellent electrochemical properties.

Table 2 Comparison of ZnCo₂O₄ anode materials from different groups

Synthesis method	Morphology	1^st cycle discharge capacity (mA h g^−1)	30^th cycle charge capacity (mA h g^−1)	Annealing temperature	Reference
Hydrothermal method	Nanospheres	1800	1242	600 °C	This study
Hydrothermal method	Parallel columns	1332	∼900	400 °C	30
Hydrothermal method	Microspheres	1087	∼810	700 °C	33
Co-precipitation	Nanowires	1331	∼1100	500 °C	39
Hydrothermal method	Nanorods	1509	∼850	N/A	40
Hydrothermal method	Flower-like	1430	∼895	400 °C	41
Co-precipitation	Nanoflakes	1458	∼1200	500 °C	42

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4. Conclusions

In this study, nano-porous ZnCo₂O₄ anodes with large surface areas are successfully synthesized via a hydrothermal method. The SEM and TEM images show that ZnCo₂O₄ nanospheres with diameters of about 20–30 nm could be obtained. The BET results show that ZCO-500 and ZCO-600 can provide a larger surface area. The initial charge capacity of ZnCo₂O₄ synthesized at 600 °C can reach as high as 1800 mA h g⁻¹, which can be attributed to the short diffusion distances and large surface area provided by the nanostructure. After 30 cycles, the reversible capacity still remained at 1242 mA h g⁻¹. The rate performance test indicates that ZCO-500 delivers the highest capacity of as high as ∼900 mA h g⁻¹ at 10C. The AC impedance results further prove that ZCO-600 gave the lowest surface layer resistance. The major enhancement of impedance resulted from the charge transfer resistance. All the results reveal that via tuning the synthesis temperature, the nano-porous ZnCo₂O₄ anodes are expected to be a potential anode material for Li-ion batteries in the future.

Acknowledgements

The authors are thankful for financial support from the National Science Council under contract no. NSC-101-3113-P-002-026.

Notes and references

1. P. Poizot, S. Grugeon, L. Dupont and J. M. Tarascon, Nature, 2000, 407, 496–499.
2. Y. G. Li, B. Tan and Y. Y. Wu, Nano Lett., 2008, 8, 265–270.
3. H. L. Wang, L. F. Cui, Y. Yang, H. S. Casalongue, J. T. Robinson, Y. Y. Liang, Y. Cui and H. J. Dai, J. Am. Chem. Soc., 2010, 132, 13978–13980.
4. L. W. Ji, Z. Lin, M. Alcoutlabi and X. W. Zhang, Energy Environ. Sci., 2011, 4, 2682–2699.
5. M. C. Qiu, L. W. Yang, X. Qi, J. Li and J. X. Zhong, ACS Appl. Mater. Interfaces, 2010, 2(12), 3614–3618.
6. J. M. Tarascon and M. Armand, Nature, 2001, 414, 359–379.
7. G. X. Wang, H. Liu, J. Liu, S. Z. Qiao, G. Q. Max Lu, P. Munroe and H. Ahn, Adv. Mater., 2010, 22, 4994.
8. J. Chen, X. H. Xia, J. P. Tu, Q. Q. Xiong, Y. H. Yu, X. L. Wang and C. D. Gu, J. Mater. Chem., 2012, 22, 15056–15061.
9. Y. F. Deng, Q. M. Zhang, Z. C. Shi, L. J. Han, F. Peng and G. H. Chen, Electrochim. Acta, 2010, 8, 2828–2832.
10. L. Jingfa, X. Shenglin, L. Yurong, J. Zhicheng and Q. Yitai, ACS Appl. Mater. Interfaces, 2013, 5, 981–988.
11. A. K. Mondal, D. W. Su, Y. Wang, S. Q. Chen and G. X. Wang, Chem.–Asian J., 2013, 8, 2828–2832.
12. L. Zhang, H. B. Wu, R. Xu and X. W. (David) Lou, CrystEngComm, 2013, 15, 9332–9335.
13. N. Jayaprakash, W. D. Jones, S. S. Moganty and L. A. Archer, J. Power Sources, 2012, 200, 53–58.
14. W.-Y. Li, L.-N. Xu and J. Chen, Adv. Funct. Mater., 2005, 15, 851.
15. X. W. Lou, D. Deng, J. Y. Lee, J. Feng and L. A. Archer, Adv. Mater., 2008, 20, 258.
16. S. L. Xiong, C. Z. Yuan, X. G. Zhang and Y. T. Qian, Chem.–Eur. J., 2009, 15, 5320.
17. C. C. Li, X. M. Yin, T. H. Wang and H. C. Zeng, Chem. Mater., 2009, 21, 4984.
18. X. Wang, X. L. Wu, Y. G. Guo, Y. T. Zhong, X. Q. Cao, Y. Ma and J. N. Yao, Adv. Funct. Mater., 2010, 20, 1680.
19. Y. Wang, H. J. Zhang, L. Lu, L. P. Stubbs, C. C. Wong and J. Y. Lin, ACS Nano, 2010, 4, 4753.
20. S. L. Xiong, J. S. Chen, X. W. Lou and X. C. Zeng, Adv. Funct. Mater., 2012, 22, 861.
21. X. L. Xiao, X. F. Liu, H. Zhao, D. F. Chen, Z. Liu, J. H. Xiang, Z. B. Hu and Y. D. Li, Adv. Mater., 2012, 24, 5762.
22. C. C. Ai, M. C. Yin, C. W. Wang and J. Sun, J. Mater. Sci., 2004, 39, 1077–1079.
23. Y. Sharma, N. Sharma, G. V. S. Rao and B. V. R. Chowdari, Solid State Ionics, 2008, 179, 587–597.
24. J. F. Li, S. L. Xiong, X. W. Li and Y. T. Qian, Nanoscale, 2013, 5, 2045–2054.
25. H. Liu and J. Wang, J. Electron. Mater., 2012, 41(11), 3107–3110.
26. Y. Sharma, N. Sharma, G. V. S. Rao and B. V. R. Chowdari, J. Power Sources, 2007, 173, 495–501.
27. R. Alcantara, M. Jaraba, P. Lavela and J. L. Tirado, Chem. Mater., 2002, 14, 2847.
28. J. F. Li, S. L. Xiong, Y. R. Liu, Z. C. Ju and Y. T. Qian, ACS Appl. Mater. Interfaces, 2013, 5, 981.
29. Y. Sharma, N. Sharma, G. V. Subba Rao and B. V. R. Chowdari, Adv. Funct. Mater., 2007, 17, 2855.
30. D. Deng and J. Y. Lee, Nanotechnology, 2011, 22, 355401.
31. N. Du, Y. F. Xu, H. Zhang, J. X. Yu, C. X. Zhai and D. R. Yang, Inorg. Chem., 2011, 50, 3320.
32. Y. C. Qiu, S. H. Yang, H. Deng, L. M. Jin and W. S. Li, J. Mater. Chem., 2010, 20, 4439.
33. L. L. Hu, B. H. Qu, C. C. Li, Y. J. Chen, L. Mein, D. N. Lei, L. B. Chen, Q. H. Li and T. H. Wang, J. Mater. Chem. A, 2013, 1, 5596.
34. B. Liu, J. Zhang, X. F. Wang, G. Chen, D. Chen, C. W. Zhou and G. Z. Shen, Nano Lett., 2012, 12, 3005.
35. L. Hu, P. Zhang, H. Zhong, X. R. Zheng, N. Yan and Q. W. Chen, Chem.–Eur. J., 2012, 18, 15049.
36. W. Kang, Y. Tang, W. Li, Z. Li, X. Yang, J. Xu and C. S. Lee, Nanoscale, 2014, 6, 6551–6556.
37. L. Wei, H. Xianluo, S. Yongming and H. Yunhui, J. Mater. Chem., 2012, 22, 8916–8921.
38. Q. Yongcai, Y. Shihe, D. Hong, J. Limin and L. Weishan, J. Mater. Chem., 2010, 20, 4439–4444.
39. D. Ning, X. Yanfang, Z. Hui, Y. Jingxue, Z. Chuanxin and Y. Deren, Inorg. Chem., 2011, 50, 3320–3324.
40. H. Liu and J. Wang, Electrochim. Acta, 2013, 92, 371–375.
41. S. G. Mohamed, T. F. Hung, C. J. Chen, C. K. Chen, S. F. Hu, R. S. Liu, K. C. Wang, X. K. Xin, H. M. Liu, A. S. Liu, M. H. Hsieh and B. J. Lee, RSC Adv., 2013, 3, 20143–20149.
42. X. Song, Q. Ru, B. Zhang, S. Hu and B. An, J. Alloys Compd., 2014, 585, 518–522.

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