General formation of Mn-based transition metal oxide twin-microspheres with enhanced lithium storage properties

Abstract

In this investigation, we designed a general method to manipulate the controlled preparation of Mn-based transition metal oxide materials with the structure of hierarchical twin-microspheres. Initiated by a precursor solvothermal synthesis of metal carbonate twin-microspheres, the formation of hierarchical twin-microspheres of metal oxides was finalized by thermal annealing in laboratory air. The method allows one to prepare binary and ternary hierarchical twin-microspheres constructed by primary ultrathin nanoparticles. By applying the respective metal salts in the synthesis, six oxide species, including Mn₃O₄, CoMn₂O₄, ZnMn₂O₄, Ni_xMn_3−xO₄, Cu_xMn_3−xO₄, and Fe_xMn_3−xO₄, have been reported herein in order to draw common features via oriented-attachment. Concerning the workability, CoMn₂O₄ twin-microspheres are evaluated as electrode materials for lithium-ion batteries. As expected, the CoMn₂O₄ twin-microspheres exhibit excellent rate performance and impressive cycling stability due to their unique assembled architecture and probably synergetic effects of different metal ions.

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1. Introduction

Recently, transition metals oxides (TMOs), especially mixed valence oxides have attracted research focus due to their high theoretical capacity. Their applications in energy storage devices range from oxygen reduction reaction (ORR) and electrochemical capacitors (ECs) to LIBs.^1–11 Among the 3d transition metal oxide anodes, Mn-based TMOs have been given notable attention owing to their advantages of low cost, abundance and environmental friendliness, high capacity, low operating voltage and so on. Myriad micro/nanostructures of Mn-based binary and ternary oxides, such as MnO,^12–15 Mn₃O₄,^16–19 CoMn₂O₄ (ref. 8–10) and ZnMn₂O₄,^8,20–23 have been successfully synthesized and excellent lithium storage properties have been obtained. In fact, the real performance of electrodes has always encountered issues, such as poor cycling profiles, low rate capabilities, and high operating voltages.²⁴ As established by related research, porous architectures assembled by numerous subclasses of nano-building blocks opened up accessible routes to enhance the reverse capacity and rate capability.^8–11,20,25 In this scenario, a higher surface area offered adequate contact sites between the electrode and electrolyte. The porous configuration served as free transportation channels for Li ions to move into the bulk of the electrode, shortening the whole diffusion duration. Furthermore, the assembled structure could well buffer the volume variation to maintain the good mechanical strength during repeated lithium ions insertion/extraction.

Considering the gigantic potential of porous architectures in LIBs, to rationally engineer the formation of porous micro/nanocomposite materials with novel structures became a challenging task in the nanochemistry community.^26–30 As a kind of special micro/nanostructure, twin-spheres with various architectures have been fabricated through different synthesis routes.^20,25,31,32 For example, Li and coworkers fabricated Co₃O₄ twin-microspheres with an urchin-like structure based on a mechanism of multistep-splitting growth from 1D nanorods and observed enhanced specific capacitances for supercapacitors.³² 3D ZnCo₂O₄ hierarchical twin microspheres were produced via a typical combined process of multistep splitting and in situ dissolution–recrystallization, offering a reversible capacity of 550 mA h g⁻¹ at a rate of 5 A g⁻¹ after 2000 cycles when evaluated as anode materials for lithium-ion batteries (LIBs).²⁵ Not withstanding these advancements, the synthesis of twin-spheres with hierarchical micro/nanostructures is still in a rudimentary state of development in comparison with what has been achieved for their spherical counterparts. Moreover, most synthetic strategies showed accessibility only to the formation of one certain material in their respective processes. Accordingly, it would be highly desirable to develop a feasible, but general strategy to effectively synthesize porous twin-spheres with a relatively general formation mechanism for different functional materials.

In view of the considerations and inspiration mentioned above, herein, a general route had been developed to fabricate hierarchical porous Mn-based oxide (A_xMn_3−xO₄, 0 ≤ x < 3; A = Co, Zn, Ni, Cu, Fe) twin-microspheres via a multiple strategy consisting of modified polyol avenue and subsequent thermal process. To be exact, taking CoMn₂O₄ as an example, monodisperse Co_0.33Mn_0.67CO₃ twin microspheres have been firstly synthesized via an oriented-attachment accompanied by Ostwald ripening. CoMn₂O₄ twin microspheres were then fabricated by annealing the above precursor at 500 °C for 4 h in laboratory air. To the best of our knowledge, this is the first report on the preparation of CoMn₂O₄ twin microspheres via a simple solution-based route. When evaluated as anode materials for LIBs, these new CoMn₂O₄ twin-microspheres exhibit high specific capacity with excellent rate capability and enhanced cycling stability, making them potential electrodes for LIBs. As the current synthetic strategy was extended to prepare Mn₃O₄, ZnMn₂O₄, Ni_xMn_3−xO₄, Cu_xMn_3−xO₄, and Fe_xMn_3−xO₄, similar twin microspheres have been successfully realized.

2. Experimental section

Chemicals

Triethylene glycol (TEG, absolute for analysis) was purchased from Wuxi Zhanwang Chemical Reagent Co. Ltd in China. All the following reagents were supplied by Shanghai Sinopharm Chemical Reagent Co. Ltd: manganese acetate tetrahydrate (Mn(Ac)₂·4H₂O, 99%, AR), cobalt acetate tetrahydrate (Co(Ac)₂·4H₂O, 99%, AR), zinc acetate dihydrate (Zn(Ac)₂·2H₂O, 99%, AR), cobalt acetate tetrahydrate (Ni(Ac)₂·4H₂O, 99%, AR), copper acetate monohydrate (Cu(Ac)₂·H₂O, 99%, AR), iron oxalate dehydrate (FeC₂O₄·2H₂O, 99%, AR) and NH₄HCO₃ (99%, AR). All chemicals are directly used without any treatment.

Synthesis of CoMn₂O₄ twin-microspheres

In a typical synthesis, 0.5 mmol of Co(Ac)₂·4H₂O and 1 mmol of Mn(Ac)₂·4H₂O are firstly dissolved into 40 mL of TEG to form a transparent solution under magnetic stirring for 30 min. Then, 30 mmol of NH₄HCO₃ powder was added to the above mixture. The resultant mixture was continually stirred until getting a homogeneous solution and then transferred into a Teflon lined stainless-steel autoclave (capacity of 50 mL). The autoclave was sealed and maintained at 200 °C for 20 h in an electronic oven. After reaction, the system cooled down to ambient temperature naturally. The obtained product (analyzed as Co_0.33Mn_0.67CO₃ twin-microspheres in the main text) was harvested by centrifugation and washed with water and ethanol for several times. Followed by heat post-treatment at 500 °C under laboratory air for 4 h with a ramp rate of 1 °C min⁻¹, targeted CoMn₂O₄ twin-microspheres could be obtained.

Other Mn-based metal oxides were prepared with the similar procedures as that used for CoMn₂O₄ twin-microspheres except for using different metal acetate/oxalate precursors, which would be specified in the following text.

Materials characterization

The crystallographic phase of as-synthesized samples was characterized by powder X-ray diffraction (XRD, Philips X’Pert Pro Super diffractometer, Cu Kα radiation λ = 1.54178 Å). Morphological and structural investigations were carried out with field-emission scanning electron microscopy (FESEM, JSM-6300F, JEOL). Structural and compositional information was provided by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and affiliated energy-dispersive X-ray spectroscopy (EDX) operated with an accelerating voltage of 200 kV (JEM-2010 and JEM-2100F, JEOL). Surface analysis of the samples was detected by X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical) and the binding energies were calibrated using C 1s peak (284.6 eV) as a reference.

Electrochemical measurements

The active material (CoMn₂O₄ twin-spheres), conductive material (acetylene black), and polymer binder (Carboxy Methylated Cellulose, CMC) were milled for 3 h at a weight ratio of 70 : 20 : 10, and then coated on the surface of a copper foil. The working electrode had a diameter of 12 mm, with a coating thickness of 200 μm and the density of the active material is about 1.0 mg cm⁻². The electrochemical measurements were carried out using 2032 coin cells with lithium foil as both the counter and reference electrodes. 1 M LiPF₆ in a mixture of ethylene carbonate (EC), dimethylcarbonate (DMC) and diethyl carbonate (DEC) (1 : 1 : 1, v/v/v) was used as the electrolyte. The cell was assembled in an argon-filled glovebox with both the moisture and the oxygen content below 1 ppm. The galvanostatic charge–discharge data were collected by CT2001A LAND Cell test system at different current densities. The cyclic voltammetry (CV) was tested within the voltage range from 0.01 to 3.0 V by an electrochemical workstation (CHI760D).

3. Results and discussion

Our strategy for the formation of mixed-metal-oxide complex twined structures is depicted in Fig. 1, taking CoMn₂O₄ as an example. According to the synthetic process outlined, monodisperse Co_0.33Mn_0.67CO₃ twin microspheres, namely the precursor, formed through the following three steps: (i) the formation of randomly dispersive spheres coexisting with discrete small nanoparticles, (ii) the alignment between two spheres with the assistance of TEG ligands (light yellow thin lines between two spheres) and (iii) formation of the twin-spheres based on an oriented-attachment accompanied by Ostwald ripening with expense of small nanoparticles, (also see Fig. S1 and S2, ESI†). The similar crystallographic nature and crystallization habits of CoCO₃ and MnCO₃ made them coprecipitate. The XRD pattern in Fig. S1† confirmed the hexagonal-phase Co_0.33Mn_0.67CO₃, similar to CoCO₃ (78-0278) and MnCO₃ (86-0172). Fig. 3a and b are typical field-emission scanning electron microscopy (FESEM) images of as-formed Co_0.33Mn_0.67CO₃ twin microspheres with average diameter of ∼2.0 μm, clearly showing the panoramic morphological profile and demonstrating a high yield of close to 100%. More materials detection by TEM can be found in the ESI (Fig. S2†).

Fig. 1 Schematic diagram illustrating the formation process of porous CoMn₂O₄ twin microspheres.

After calcination treatment in laboratory air, the XRD pattern (Fig. 2a) of the final product can be readily assigned to body-centered-tetragonal CoMn₂O₄ (JCPDS card no. 77-0471, a = b = 5.784 Å, c = 9.091 Å, α = β = γ = 90°; space group: I4₁/amd) with a distorted spinel structure due to the well-known Jahn–Teller effect of manganese(III). No residues or contaminants have been detected, showing the high purity of the sample. The schematic crystal structure of spinel CoMn₂O₄ is inserted in Fig. 2a. In the spinel structure, the bivalent Co-ions and trivalent Mn-ions occupy the tetrahedral and octahedral void sites, which are isostructural to Co₃O₄. The more detailed elemental composition and the oxidation state of the as-obtained CoMn₂O₄ are further examined by X-ray photoelectron (XPS) measurements and the corresponding results are shown in Fig. 2b–d. All of the binding energies (BEs) in this XPS analysis were corrected for specimen charging by referencing them to the C 1s peak (set at 284.6 eV). By using a Gaussian fitting method, the Co 2p emission spectrum (Fig. 2b) was best fitted with two spin–orbit doublets characteristic of Co²⁺ and Co³⁺, and two shakeup satellites (shown as “Sat.”).³³ The Mn 2p was also fitted with two spin–orbit doublets, characteristic of Mn²⁺ and Mn³⁺, and two little shakeup satellites.³⁴ The high-resolution spectrum for the O 1s region (Fig. 2d) shows three oxygen contributions. Specifically, the peak at 529.8 eV is typical of metal–oxygen bonds.^35,36 The peak sitting at 531.3 eV is usually associated with a higher number of defect sites with low oxygen coordination often observed in materials with small particles.^37,38 The peaks at ∼532.4 eV can be attributed to multiplicity of physi- and chemi-sorbed water at or near the surface.^35,36 These data supplied the surface chemical information of CoMn₂O₄ twin-spheres, composed of different-valence ions involving Co²⁺, Co³⁺, Mn²⁺, and Mn³⁺. Furthermore, the atomic ratio of Co to Mn elements measured from the EDX pattern (Fig. S3†) of the final product is very close to 1 : 2 from different regions detected. According to the above analysis, one can safely conclude that the cobalt manganese oxide samples prepared in this work have the chemical composition of CoMn₂O₄ with a spinel structure.

Fig. 2 (a) XRD patterns of the as-prepared CoMn₂O₄ twin microspheres; inset: the crystal structure of tetragonal spinel CoMn₂O₄. High-resolution XPS spectra of (b) Co 2p, (c) Mn 2p, and (d) O 1s for the CoMn₂O₄ twin microspheres.

Despite the high temperature post-treatment, the CoMn₂O₄ sample still retained the spherical morphology and size of its carbonate precursor, as revealed by the panoramic FESEM image of Fig. S4 in ESI.† High-magnification FESEM examination (Fig. 3c and d) clearly indicated the detailed structural configuration of the as-obtained twin microspheres, composed of numerous uniform nanosized primary nanoparticles. A typical TEM image in Fig. 3e clarified the porous features constructed by stacking nanoparticles, which is consistent with the observation of the FESEM findings. A locally magnified HRTEM image of several nanoparticles of a CoMn₂O₄ twin microsphere recorded for Fig. 3f reveals lattice fringes with interplane spacings of 0.27 nm, corresponding to the (103) planes of spinel CoMn₂O₄. Considering the commensurate lattice fringes, it should be noted that different single-crystalline CoMn₂O₄ grains were also well interconnected, maintaining the same crystallographic orientation, which could enhance the required charge transfer and thus the electron conductivity during LIB operation. In Fig. 3g, an elemental mapping study on a single twin-microsphere further confirms the uniform distributions of elemental Mn, Co, and O in the CoMn₂O₄ hierarchical twin-microspheres.

Fig. 3 FESEM images of Co_0.33Mn_0.67CO₃ twin microspheres (a and b); FESEM (c and d), TEM (e) and high-resolution TEM image (f) of CoMn₂O₄ twin microspheres. (g) STEM image and the corresponding EDX elemental mappings of Mn, Co, and O for a single representative CoMn₂O₄ twin microsphere.

Mixed metal oxides have been considered as a promising class of electrode materials for high-performance energy storage devices. To demonstrate the potential application of such novel complex porous structure, CoMn₂O₄ twin microspheres are evaluated as electrode materials for LIBs. Fig. 4a shows the first five consecutive cyclic voltammograms (CVs) at a scan rate of 0.2 mV s⁻¹ in the potential range of 0.01–3.0 V. The CV curves for the first cycle are obviously different from those for the following cycles, and no significant alteration is seen from the second cycle onwards. As for the 1^st cycle, the broad peak centered at ∼1.25 V and the sharp reduction peak at ∼0.4 V in the cathodic process could be attributed to the reduction of Mn³⁺ to Mn²⁺ as well as Mn²⁺ and Co²⁺ to metallic Mn and Co, respectively. Additionally, the peak at 0.76 V can be ascribed to their irreversible decomposition of solvent in the electrolyte to form the solid–electrolyte interface (SEI). In the conjugated anodic processes, two oxidation peaks at 1.35 V and 1.99 V originated from the oxidation of Mn and Co to Mn²⁺ and Co²⁺, respectively.³⁹ From the second cycle onwards, the repeated reduction–oxidation of MnO and CoO lead to two pairs of distinct redox peaks at 0.49/1.40 V and 1.01/2.03 V, respectively. Furthermore, these CV curves mostly overlapped, demonstrating the good cyclability and stability for the insertion and extraction of lithium ions. On the basis of analyses derived from CVs, the entire electrochemical process for CoMn₂O₄ electrode can be classified as follows:

CoMn₂O₄ + 8Li⁺ + 8e⁻ → Co + 2Mn + 4Li₂O (1)

Co + 2Mn + 3Li₂O → CoO + 2MnO + 6Li⁺ + 6e⁻ (2)

Fig. 4 Electrochemical evaluation of CoMn₂O₄ twin microspheres as an anode of lithium-ion batteries: (a) cyclic voltammetry (CV) curves at a scan rate of 0.1 mV s⁻¹ in the voltage window of 0.01–3.0 V; (b) discharge–charge voltage profiles and (c) cycling performance recorded at a current density of 200 mA g⁻¹; (d) rate capability at different current densities between 0.01 and 3.0 V.

Fig. 4b indicates the pertinent charge–discharge voltage profiles at a current density of 200 mA g⁻¹. The CoMn₂O₄ electrode delivers high first-cycle discharge and charge capacities of 1181 and 831 mA h g⁻¹, respectively, contributing to a moderate irreversible loss of about 30% mainly caused by the formation of SEI during the first charge. In the following cycles, the Coulombic efficiency quickly increased to higher than 98%, showing the good cycling stability.

Fig. 4c indicates the discharge–charge capacity as a function of cycle number explored at a current of 200 mA g⁻¹. Just as analyzed above, a gradual capacity loss happened due to the SEI layer formed on the electrode surface and the compact electric contact established between the current collector and electrode in the initial cycles. Afterwards, the electrode displays a stable cycling performance with high Coulombic efficiency, wherein it still retains a reversible capacity of 890 mA h g⁻¹ and nearly 100% capacity retention after more than 70 cycles. Even at a high current density of 1000 mA g⁻¹, a reversible discharge capacity of 573 mA h g⁻¹ is retained after continuous cycling for more than 50 cycles, corresponding to 77% of the second-cycle discharge capacity (Fig. S5, ESI†), much higher than the previous reports.^{18,19,39–41} The rate capability is further evaluated at different current densities ranging from 200 to 2000 mA g⁻¹. As seen in Fig. 4d, the specific capacities are 790, 740, 664, 580, and 500 mA h g⁻¹ at the current densities of 500, 800, 1000, 1500, and 2000 mA g⁻¹, respectively. Noticeably, when the current rate turns back to 200 mA g⁻¹, the capacity can recover as high as 908 mA h g⁻¹ even after 120 cycles without any loss.

The lithium storage properties achieved in the present study are superior to that of other CoMn₂O₄ structures reported.^{18,19,39–41} For instance, the reversible capacity of the CoMn₂O₄ hierarchical microspheres remains 894 mA h g⁻¹ at a current of 100 mA g⁻¹ after 65 cycles,¹⁸ CoMn₂O₄ hollow microcubes exhibited a capacity of about 624 mA h g⁻¹ with a retention of about 75.5% at a current of 200 mA g⁻¹ after 50 cycles,³³ and CoMn₂O₄ powders only indicated a capacity of 515 mA h g⁻¹ with a retention of about 64% at a current of 69 mA g⁻¹ after 50 cycles. Obviously, the unique structure of the current CoMn₂O₄ complex porous structure is beneficial for enhanced lithium storage properties. Specifically, the integration of smaller primary nanoparticles and pores in the CoMn₂O₄ twin microspheres can provide a short pathway for Li⁺ ion diffusion, leading to high capacity and excellent rate capability. More importantly, the unique porous architecture furnished the CoMn₂O₄ twin microspheres with better structural integrity by partially alleviating the mechanical strain and the large volume change associated with the repeated Li⁺ insertion/extraction processes during cycling, thus lightening the pulverization problem and improving the cycling stability. It should be noted that the twin hierarchical architecture possesses lower surface energy, which can effectively inhibit self-aggregation during the charge–discharge process. Moreover, the enhanced activity is also related to the possible synergetic effects of different metal ions.

More importantly, the strategy used here shows good versatility and can be extended to the exploitation of other Mn-based metal oxides. To be specifically, Mn₃O₄, ZnMn₂O₄, Ni_xMn_3−xO₄, Cu_xMn_3−xO₄, and Fe_xMn_3−xO₄ twin-microspheres are successfully synthesized by simply using their respective metal salts under same conditions. Fig. 5–8 indicates the typical FESEM, TEM, and corresponding mapping images of Mn₃O₄ (Fig. 5a and b), ZnMn₂O₄ (Fig. 5c and d), Ni_xMn_3−xO₄ (Fig. 6), Cu_xMn_3−xO₄ (Fig. 7), and Fe_xMn_3−xO₄ (Fig. 8) porous twin microspheres. As can be seen, similar twin-spherical structures can be prepared on a large scale and high yield for these five different Mn-based metal oxides. The surfaces of the as-obtained products becomes much rougher than those of the corresponding precursors, suggesting the generation of porous structures. The corresponding XRD and EDX characterizations can be found in the ESI (Fig. S6 and S7†). The morphological features are quite analogous in terms of both size and shape to CoMn₂O₄ twin microspheres. These interesting porous twinned structures built from nanometer-sized building blocks may offer a new opportunity for applications involving areas of solar cells, electrocatalysts, supercapacitors and LIBs. Since the present work is focused largely on the development of general routes to synthesize Mn-involved oxide twin spheres and exploit the application in LIBs with CoMn₂O₄ as an example. Therefore, substantial verification of the relevant performances of other Mn-based mixed oxides will be supplied. Although all of the twined structures in this study are exclusively Mn-based transition metal oxides, our preliminary results indicate that it would be possible to impart versatility to other transition metal oxide twin-spheres by precisely adjusting and controlling the experimental parameters, such as the composition of the solvent, the appropriate surfactant, chelating agent, the reaction temperature, and the reaction time. This versatile and controllable synthesis approach could provide a venue to fully optimize the structure and performance of complex porous twined structures.

Fig. 5 (a and b) FESEM images of Mn₃O₄ twin microspheres. (c and d) TEM images of ZnMn₂O₄ twin microspheres, STEM image and the corresponding EDX elemental mappings of Zn, Mn, and O for a single representative ZnMn₂O₄ twin microsphere.

Fig. 6 FESEM (a and b) and TEM (c) of Ni_xMn_3−xO₄ twin microspheres. (d) STEM image and the corresponding EDX elemental mappings of Ni, Mn, and O for a single representative Ni_xMn_3−xO₄ twin microsphere.

Fig. 7 FESEM (a and b) and TEM (c) of Cu_xMn_3−xO₄ twin microspheres. (d) STEM image and the corresponding EDX elemental mappings of Cu, Mn, and O for a single representative Cu_xMn_3−xO₄ twin microsphere.

Fig. 8 FESEM (a and b) and TEM (c) of Fe_xMn_3−xO₄ twin microspheres. (d) STEM image and the corresponding EDX elemental mappings of Fe, Mn, and O for a single representative Fe_xMn_3−xO₄ twin microsphere.

4. Conclusions

In summary, we have successfully developed a general method for the synthesis of various Mn-based transition metal oxide twin-microspheres, including Mn₃O₄, CoMn₂O₄, ZnMn₂O₄, Ni_xMn_3−xO₄, and Cu_xMn_3−xO₄, Fe_xMn_3−xO₄. The preparation strategy first involved synthesizing the twin-microspheres of metal carbonate precursor. Then, by the aid of thermal post-treatment in laboratory air at 500 °C for 4 h, the corresponding carbonate can topotactically convert to respective oxide twin-microspheres without changing the overall morphologies. As an example, CoMn₂O₄ porous twin-microspheres are studied as an anode material for LIBs. Due to the unique mesoporous hierarchical micro/nanostructures and the possible synergetic effects of different metal ions, the as-obtained CoMn₂O₄ twin-microspheres exhibit excellent properties according to their cycling performance and rate capability as well as cycle life for LIBs. Moreover, it should be noted that the method could be extended to synthesize other metal oxides with novel hierarchical architectures.

Acknowledgements

The authors gratefully acknowledge the financial supports provided by the National Basic Research Program of China (the 973 Project of China, no. 2011CB935901), National Natural Science Fund of China (no. 21371108), Shandong Provincial Natural Science Foundation for Distinguished Young Scholar (no. JQ201304), and the National Science Foundation of Shandong Province (no. ZR2012BM018).

References

1. F. Y. Cheng, J. A. Shen, B. Peng, Y. D. Pan, Z. L. Tao and J. Chen, Nat. Chem., 2011, 3, 79.
2. G. Q. Zhang and X. W. Lou, Angew. Chem., Int. Ed., 2014, 53, 9041 (Angew. Chem., 2014, 126, 9187).
3. Y. M. Chiang, Science, 2010, 330, 1485.
4. T. Brezesinski, J. Wang, S. H. Tolbert and B. Dunn, Nat. Mater., 2010, 9, 146.
5. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845.
6. J. R. Miller and P. Simon, Science, 2008, 321, 651.
7. C. G. Morales-Guio, S. D. Tilley, H. Vrubel, M. Gratzel and X. L. Hu, Nat. Commun., 2014, 5, 3059.
8. G. Q. Zhang, L. Yu, H. B. Wu, H. E. Hoster and X. W. Lou, Adv. Mater., 2012, 24, 4609.
9. L. Hu, H. Zhong, X. R. Zheng, Y. M. Huang, P. Zhang and Q. W. Chen, Sci. Rep., 2012, 2, 986.
10. J. F. Li, S. L. Xiong, X. W. Li and Y. T. Qian, Nanoscale, 2013, 5, 2045.
11. C. Z. Yuan, H. B. Wu, Y. Xie and X. W. Lou, Angew. Chem., Int. Ed., 2014, 53, 1488 (Angew. Chem., 2014, 126, 1512).
12. X. W. Li, D. Li, L. Qiao, X. H. Wang, X. L. Sun, P. Wang and D. Y. He, J. Mater. Chem., 2012, 22, 9189.
13. B. Sun, Z. X. Chen, H. S. Kim, H. Ahn and G. X. Wang, J. Power Sources, 2011, 196, 3346.
14. X. Q. Yu, Y. He, J. P. Sun, K. Tang, H. Li, L. Q. Chen and X. J. Huang, Electrochem. Commun., 2009, 11, 791.
15. X. W. Li, S. L. Xiong, J. F. Li, X. Liang, J. Z. Wang, J. Bai and Y. T. Qian, Chem.–Eur. J., 2013, 19, 11310.
16. 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.
17. J. Gao, M. A. Lowe and H. D. Abruna, Chem. Mater., 2011, 23, 3223.
18. Z. C. Bai, N. Fan, Z. C. Ju, C. L. Guo, Y. T. Qian, B. Tang and S. L. Xiong, J. Mater. Chem. A, 2013, 1, 10985.
19. G. Jian, Y. Xu, L. Lai, C. Wang and M. Zachariah, J. Mater. Chem. A, 2014, 2, 4627.
20. Y. R. Liu, J. Bai, X. J. Ma, J. F. Li and S. L. Xiong, J. Mater. Chem. A, 2014, 2, 14236.
21. Y. Y. Yang, Y. Q. Zhao, L. F. Xiao and L. L. Zhang, Electrochem. Commun., 2008, 10, 1117.
22. L. Zhou, H. B. Wu, T. Zhu and X. W. Lou, J. Mater. Chem., 2012, 22, 827.
23. C. Z. Yuan, J. Y. Li, L. R. Hou, L. H. Zhang and X. G. Zhang, Part. Part. Syst. Charact., 2014, 31, 657.
24. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J. M. Tarascon, Nature, 2000, 407, 496.
25. J. Bai, X. G. Li, G. Z. Liu, Y. T. Qian and S. L. Xiong, Adv. Funct. Mater., 2014, 24, 3012.
26. M. H. Oh, T. Yu, S. H. Yu, B. Lim, K. T. Ko, M. G. Willinger, D. H. Seo, B. H. Kim, M. G. Cho, J. H. Park, K. Kang, Y. E. Sung, N. Pinna and T. Hyeon, Science, 2013, 340, 964.
27. X. Y. Lai, J. E. Halpert and D. Wang, Energy Environ. Sci., 2012, 5, 5604.
28. X. W. Lou, L. A. Archer and Z. C. Yang, Adv. Mater., 2008, 20, 3987.
29. J. H. Sun, J. S. Zhang, M. W. Zhang, M. Antonietti, X. Z. Fu and X. C. Wang, Nat. Commun., 2012, 3, 1339.
30. J. B. Joo, Q. Zhang, M. Dahl, I. Lee, J. Goebl, F. Zaera and Y. D. Yin, Energy Environ. Sci., 2012, 5, 6321.
31. F. Li, F. L. Gong, Y. H. Xiao, A. Q. Zhang, J. H. Zhao, S. M. Fang and D. Z. Jia, ACS Nano, 2013, 7, 10482.
32. Y. H. Xiao, S. J. Liu, F. Li, A. Q. Zhang, J. H. Zhao, S. M. Fang and D. Z. Jia, Adv. Funct. Mater., 2012, 22, 4052.
33. S. A. Needham, G. X. Wang, K. Konstantinov, Y. Tournayre, Z. Lao and H. K. Liu, Electrochem. Solid-State Lett., 2006, 9, A315.
34. B. J. Tan, K. J. Klabunde and P. M. A. Sherwood, J. Am. Chem. Soc., 1991, 113, 855.
35. J. F. Marco, J. R. Gancedo, M. Gracia, J. L. Gautier, E. I. Ríos and F. J. Berry, J. Solid State Chem., 2000, 153, 74.
36. T. Choudhury, S. O. Saied, J. L. Sullivan and A. M. Abbot, J. Phys. D: Appl. Phys., 1989, 22, 1185.
37. Y. E. Roginskaya, O. V. Morozova, E. N. Lubnin, Y. E. Ulitina, G. V. Lopukhova and S. Trasatti, Langmuir, 1997, 13, 4621.
38. J. H. Zhong, A. L. Wang, G. R. Li, J. W. Wang, Y. N. Ou and Y. X. Tong, J. Mater. Chem., 2012, 22, 5656.
39. L. Zhou, D. Y. Zhao and X. W. Lou, Adv. Mater., 2012, 24, 745.
40. G. Q. Zhang and X. W. Lou, Angew. Chem., Int. Ed., 2014, 53, 9041.
41. F. M. Courtel, H. Duncan, Y. Abu-Lebdeh and I. J. Davidson, J. Mater. Chem., 2011, 21, 10206.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03645c

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