Facile synthesis of MnO_x nanoparticles sandwiched between nitrogen-doped carbon plates for lithium ion batteries with stable capacity and high-rate capability

Abstract

In this work, a material consisting of MnO_x nanoparticles sandwiched between nitrogen-doped carbon plates (C/MnO_x/C) has been successfully synthesized via a step-by-step strategy. It is demonstrated that the MnO_x nanoparticles are well sandwiched between the double nitrogen-doped platelike carbon sheets. As an anode material for lithium-ion batteries, the double nitrogen-doped platelike carbon sheets encapsulating MnO_x can not only address the issues related to the aggregation and volumetric changes of manganese oxides during the Li⁺ insertion/extraction, but also effectively shorten the transport path of Li⁺ ions and enhance the conductivity. As a result, the prepared C/MnO_x/C composite exhibits stable cycling performance and superior high rate capability. The reversible capacity of C/MnO_x/C after 100 cycles is as high as 770.9 mA h g⁻¹, which is comparable with the initial capacity at 0.2 A g⁻¹, and even at a high rate at 1 A g⁻¹, it can deliver a high reversible of 443.9 mA h g⁻¹, demonstrating the rational architecture design of the encapsulation of MnO_x with nitrogen-doped platelike carbon layers.

---

Introduction

With the growing market demand for electric vehicles and energy storage systems, lithium-ion batteries (LIBs) have attracted great attention and become a promising power source.^1–3 The electrode materials are one of the most critical influences on electrochemical performances of LIBs.⁴ Therefore, it is important to exploit electrode materials with long cycle life, low cost, high reversible capacity and excellent rate capability.^5,6 Transition metal oxides display higher theoretical reversible capacities than commercial graphite materials due to an unusual conversion reaction mechanism.^7–11 Manganese oxides including MnO, Mn₃O₄, Mn₂O₃ and MnO₂ benefit from high theoretical capacity, low cost, low toxicity, natural abundance and environmental friendliness, have been widely studied as potential anode materials for LIBs.^12–16 However, the application of pure manganese oxides as practical electrodes is still hampered, which is due to their low rate capability arising from the limited electrical conductivity and rapid capacity fading arising from the huge volume change upon cycling.^17–19

To date, many strategies have been made to improve these problems. One of the effective methods is reducing the dimension of the particles to nanometer to shorten the diffusion length of lithium ions and electrons.^20,21 Meanwhile, the synthesis of various nanostructures such as nanotubes, hollow spheres or other porous nanostructures in electrode materials is also an effective way, which can enhance the capacity retention by alleviating the volume changes due to their interior hollow spaces.^14,22–24 In addition, combining manganese oxides with carbon materials is one of the most used methods. Generally, carbonaceous materials such as amorphous carbon, carbon nanotubes, carbon nanofibers and graphene sheets are often used as both a volume buffer to absorb the internal stress and a conductive network to increase ion and electron transport in the electrodes.²⁵ These approaches, such as MnO/reduced graphene oxide composite,²⁶ N-doped MnO/graphene nanosheets,²⁷ MnO/multi-walled carbon nanotubes composite²⁸ and 3D porous MnO/C microspheres,⁶ have been proved to be effective for enhancing the rate performance and cycling stability of manganese oxides-based hybrid electrode. However, most of the manganese oxides and carbonaceous materials were combined in loose state. It is inevitable that manganese oxides will fall off from carbonaceous materials during the repeated charge and discharge process. As a result, those manganese oxides will lose the stress absorption and high conductivity benefited from carbonaceous materials, which will affect the electrochemical performances. Therefore, a rational design of manganese oxides/carbon hybrid is of great significant to solve these problems.

In this work, MnO_x nanoparticles sandwiched between nitrogen-doped carbon plates architecture (C/MnO_x/C) was synthesized. In this structure, MnO_x nanoparticles are evenly dispersed between two thin layers of carbon plates. This structure has novel advantages. On one hand, the double carbonized poly-dopamine sheets can efficiently prevent manganese oxides aggregation and buffer the volume change of MnO_x during the Li-ion insertion/extraction process. On the other hand, the highly conductive N-doped carbon plates could shorten the transport path of Li⁺ ions and enhance the conductivity. In order to obtain the structure, we use basic magnesium carbonate as hard template and dopamine as carbon source to synthesize N-doped platelike carbon, which acts as supporting framework for C/MnO_x/C sandwich plates composite. And then, the precursor of MnO_x nanoparticles were grew on the platelike carbon through the reaction between hydrazine and Mn(Ac)₂ in ethylene glycol. After coating on poly-dopamine, the as-obtained product was annealed in H₂/Ar atmosphere. As expected, the C/MnO_x/C sandwich plates composite exhibits excellent lithium storage performance. It could deliver a reversible capacity of 770.9 mA h g⁻¹ after 100 cycles at a current density of 0.2 A g⁻¹, which is much higher than platelike carbon/MnO_x (543.6 mA h g⁻¹) and MnO_x (81.6 mA h g⁻¹).

Experimental

Synthesis of the platelike carbon

In a typical synthesis, 1 g of basic magnesium carbonate (4MgCO₃Mg(OH)₂·5H₂O) was dispersed in 400 mL of 10 mM Tris solution (pH = 8.5) containing 2 g of polyethylene polypropylene glycol (F127). Then, 1 g of dopamine hydrochloride was added in with continuous stirring in air for about 12 h. Similarly, the precipitate was collected by filtration, washed with deionized water and absolute ethanol then dried at 60 °C. Afterwards, the resulting product was heated at 800 °C for 2 h in N₂ atmosphere with a heating rate of 5 °C min⁻¹. After washing the resulting sample with 1 mol L⁻¹ HCl solutions for 12 h, the carbon was collected by filtration, washed with deionized water and dried at 70 °C for 12 h.

Synthesis of platelike carbon/MnO_x composite

0.1 g of the as prepared platelike carbon (carbonized poly-dopamine) was dispersed in 150 mL ethylene glycol and sonicated for 0.5 h, and then 0.9 g Mn(CH₃COO)₂·4H₂O was added in under stirring, followed by adding 1 mL hydrazine hydrate (80 wt%). The resultant solution was kept stirring for 6 h at 110 °C. After cooling to room temperature, the precipitate was collected by filtration, washed with deionized water. Finally, the precursor material was annealed in Ar/H₂ mixing atmosphere (Ar : H₂ = 95% : 5%,volume) at 550 °C for 2 h at a heating rate of 2 °C min⁻¹. The carbonized poly-dopamine/MnO_x was designated as CM. For comparison, the pure MnO_x was obtained in the absence of porous carbon, in which other conditions were kept the same.

Synthesis of the C/MnO_x/C sandwich plates composite

0.1 g of the as prepared platelike carbon was dispersed in 150 mL ethylene glycol and sonicated for 0.5 h, and then 1.2 g Mn(CH₃COO)₂·4H₂O was added in under stirring, followed by adding 1 mL hydrazine hydrate (80 wt%). The resultant solution was kept stirring for 6 h at 110 °C. Afterwards, the precipitate was dispersed in 400 mL of 10 mM Tris solution containing 0.4 g of F127. After that, 0.2 g of dopamine hydrochloride was added with continuous stirring in air for about 12 h. A precursor material was obtained by filtration, washed with deionized water and dried at 60 °C. Finally, the precursor material was annealed in Ar/H₂ mixing atmosphere (Ar : H₂ = 95% : 5%, volume) at 550 °C for 2 h at a heating rate of 2 °C min⁻¹, and the carbonized poly-dopamine/MnO_x/carbonized poly-dopamine sandwich plates composite was designated as CMCS.

Material characterization

The products were characterized by X-ray diffraction (XRD, Rigaku-TTRIII), scanning electron microscope (SEM, JSM-6360LV), transmission electron microscopy (TEM, JEM-2100F), X-ray photoelectron spectra (XPS, K-Alpha 1063), thermogravimetric analysis (TGA, SDTQ600), Raman spectroscopy (LabRAM Hr800) and elemental analysis (EuroEA3000 analyzer).

Electrochemical measurement

Electrochemical measurements were performed by using coin-type 2025 cells. The working electrodes were prepared as follows. First the as-synthesized active materials, carbon black and polyvinyl-difluoride (PVDF) were mixed in NMP with a weight ratio of 8 : 1 : 1, and then the slurry was pasted onto Cu foil. The electrolyte consists of a solution of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (volume ratio of 1 : 1). Pure lithium foils were used as counter electrodes, and polypropylene membranes from Celgard were used as the separators. The mass loading of the active materials are about 1.0 mg cm⁻². The cells were assembled in a glove-box filled with argon. The charge and discharge performances were carried out on a LAND battery measurement system within a range of 0.01–3 V at different current densities. Cyclic voltammograms experiments were performed in the potential window of 0.01–3.0 V at a scanning rate of 0.2 mV s⁻¹. Electrochemical impedance spectroscopy measurements were performed with electrochemical workstation in the frequency window of 100 000–0.01 Hz.

Results and discussion

The XRD patterns of the as-prepared MnO_x, CM and CMCS are shown in Fig. 1a. The dominant diffractions can be indexed to the cubic MnO (JCPDS no. 07-0230), and the other weak diffractions are ascribed to the hausmannite structure of Mn₃O₄ (JCPDS no. 24-0734) probably owing to the oxidation of MnO in air.²⁹ It can be seen from Fig. 1a that diffraction peaks of CMCS are much lower than those of MnO_x and CM, this is probably due to the surface of CMCS is coated by carbonized poly-dopamine. Fig. 1b shows the XRD patterns of the precursors of CM and CMCS. The dominant diffractions can be indexed to Mn₃O₄ (JCPDS no. 24-0734), and the other weak diffractions are ascribed to Mn(OH)₂ (JCPDS no. 18-0787). The main phase in the precursor of CMCS is Mn₃O₄, which means that most of Mn(OH)₂ in the precursor of CM are oxidized to be Mn₃O₄ in the process of coating poly-dopamine.

Fig. 1 XRD patterns of (a) MnO_x, CM and CMCS and (b) the precursors of CM and CMCS.

The final decomposition products of heating CM and CMCS in air are Mn₂O₃.³⁰ According to the TGA curves shown in Fig. S1,† the final weight of CM and CMCS is 71.7% and 63.0% respectively, which means that the Mn content in CM and CMCS is 49.9 wt% and 43.9 wt%. As shown in Table S1,† the O, C and N in CM evaluated by CHONS element analysis are 16.7 wt%, 30.39 wt% and 3.21 wt%, respectively. Thus, the content of Mn in CM is 49.7 wt% under ignoring other elements, which is consistent with the result of TGA.

Raman spectra of the as-prepared CM and CMCS are shown in Fig. 2a. Two broaden peaks at ca. 1590 cm⁻¹ and 1360 cm⁻¹ assigning to the G band (graphitic carbon) and D band (disordered carbon) of carbon materials³¹ can be observed in the spectra of the composites. Meanwhile, the characteristic peak at 640 cm⁻¹ should be ascribed to the Mn–O vibration mode.²⁷ The signal intensity for the metal–oxygen bond of the sample CM is higher than that of the sample CMCS, which may be due to the fact that the Mn content in CM is higher than that in CMCS and there is carbonized poly-dopamine coated on the surface of CMCS. XPS spectra (Fig. 2b) of the as-prepared of CM and CMCS show the presence of C, N, O and Mn elements. The high-resolution C 1s, N 1s, O 1s and Mn 2p spectra for CMCS are shown in Fig. 2c–f. The C 1s peak could be deconvoluted into three peaks centered at ca. 289.1, 285.5, 284.7 eV corresponding to the C–O, N–sp²C, sp²–sp²C type bonds, respectively. The N 1s spectrum could be deconvoluted into three peaks at 403.9, 400.4 and 398.3 eV (Fig. 2d), which are attributed to graphitic N, pyrrolic N and pyridinic N bonds, respectively. In the O 1s spectrum (Fig. 2e), three deconvoluted peaks at 533, 530.9 and 529.7 eV can be assigned to H–O, C–O and Mn–O, respectively. The H–O bond originates from water absorbed on the sample surface after exposed in air.^27,29,32,33 The high-resolution Mn 2p spectrum (Fig. 2f) exhibits two signals at 641.5 eV for Mn 2p_3/2 and 653.0 eV for Mn 2p_1/2, respectively. However, the binding energies of Mn 2p_3/2 for MnO, Mn₃O₄, Mn₂O₃ and MnO₂ locate in the small range of 642.2–640.4 eV, which are difficult to distinguish due to the measurement resolution in experiment.^34,35 The high-resolution C 1s, N 1s, O 1s and Mn 2p spectra for CM are shown in Fig. S2.†

Fig. 2 (a) Raman spectra of CM and CMCS. (b) XPS spectra of CM and CMCS. XPS spectra of CMCS: (c) high-resolution of C 1s, (d) high-resolution of N 1s, (e) high-resolution of O 1s, (f) high-resolution of Mn 2p.

The morphologies and nanostructures of the synthesized products were investigated by SEM and TEM. As shown in Fig. 3a and 4a, pure MnO_x is composed of small particles with an average size of about 200 nm and these nanoparticles aggregate slightly. In this work, basic magnesium carbonate acted as hard template for carbon, will largely determine the morphology of the as-prepared carbon. Fig. S3† displays the SEM image of basic magnesium carbonate, demonstrating the rough surfaces of interconnected thick sheets. Fig. 3b and 4b show the morphology of the as-prepared carbon. The platelike carbon is composed of large numbers of interconnected thin carbon sheets, indicating that poly-dopamine was synthesized and coated on the surfaces of basic magnesium carbonate. As revealed in Fig. 3c and 4c, the morphology of carbon matrix in CM is maintained. Meanwhile, the MnO_x nanoparticles in CM have two different average sizes, ca. 120 nm and ca. 20 nm, respectively. This is probably due to that the platelike carbon with rough surface acts as supporting material for the MnO_x nanoparticles and restricts these particles size, while the MnO_x nanoparticles grew in the solution without restriction from carbon sheets have a larger size and these particles distribute in the interlayer of the thin carbon sheets. From Fig. 3d, one can see that the surface of CMCS is coated by carbonized poly-dopamine, and there are no MnO_x nanoparticles on the surface of CMCS, demonstrating that MnO_x nanoparticles are well sandwiched between the double nitrogen-doped platelike carbon sheets. Fig. 4d shows that the nanostructure under carbonized poly-dopamine is similar to CM, demonstrating that coating poly-dopamine would not change the nanostructure of CM. Therefore, the difference between CM and CMCS is that MnO_x nanoparticles in CM are distributed on the surface of platelike carbon and the MnO_x nanoparticles in CMCS are sandwiched between the double nitrogen-doped platelike carbon sheets. Fig. S4† is the high-resolution TEM (HRTEM) image of CMCS. The lattice distance of 0.496 nm and 0.258 nm correspond to the (1 0 1) planes of Mn₃O₄ and the (1 1 1) planes of MnO, respectively.^30,36 Mn₃O₄ is outside of MnO probably owing to the oxidation of MnO in air.

Fig. 3 SEM images of (a) MnO_x, (b) platelike carbon, (c) CM, (d) CMCS.

Fig. 4 TEM images of (a) MnO_x, (b) platelike carbon, (c) CM, (d) CMCS.

The electrochemical performances of the composites were investigated as anode materials in lithium ion batteries. Their electrochemical properties were first studied by the cyclic voltammogram curves. Fig. 5a and b show typical CV curves of CM and CMCS electrodes for the initial three cycles at a scan rate of 0.2 mV s⁻¹ in the voltage range of 0.01–3.0 V vs. Li/Li⁺. It is clear to see that CM and CMCS have a reduction peak at ca. 1.3 V, which can be ascribed to the reduction of Mn³⁺ (Mn₃O₄) to Mn²⁺.³⁷ From the second cycle, the reduction peak disappeared, indicating that few or no Mn³⁺ is generated in the oxidation process. In addition, two irreversible reduction peaks at around 0.85 V and 0.6 V correspond to the irreversible reduction of the electrolyte and the formation of a solid electrolyte interphase (SEI) layer, respectively.³⁸ In addition, the sharp reduction peak at ca. 0.13 V refers to the transformation from Mn²⁺ to Mn⁰. The reduction peak shifts to ca. 0.28 V in the subsequent cycles, indicating a microstructure alteration arising from the formation of Mn and Li₂O.³⁹ The main oxidation peak at ca. 1.35 V could be ascribed to the oxidation reaction of Mn⁰ to Mn²⁺. A weak peak appears at ca. 2.1 V corresponding to the decomposition of the polymer/gel layer at high oxidation potential above 2.0 V.⁴⁰ The CV profiles demonstrate that metallic Mn and Li₂O are the end products of discharge to 0.01 V, while MnO is the end product of recharge to 3.0 V for CM and CMCS. Fig. 5c shows the first and second galvanostatic charge–discharge profiles of CMCS at a current density of 0.2 A g⁻¹. The charge–discharge profiles match very well with the above CV. The initial discharge and charge specific capacities capacity are 1190.6 and 746.7 mA h g⁻¹, respectively. The formation of solid electrolyte interphase (SEI) films is the mainly reason for leading the irreversible capacity.³³

Fig. 5 (a) Cyclic voltammogram curves of CM at a scan rate of 0.2 mV s⁻¹. (b) Cyclic voltammogram curves of CMCS at a scan rate of 0.2 mV s⁻¹. (c) Discharge and charge profiles of CMCS at 0.2 A g⁻¹. (d) Cycles performances of CMCS, CM and MnO_x at 0.2 A g⁻¹.

The cycling stability of CMCS electrode was investigated by comparing with that of CM and MnO_x at a current density of 0.2 A g⁻¹. The formation of SEI films is the mainly reason for the capacity loss after the initial cycle, while the diffusion length and volume change for lithium insertion/extraction is normally the influence for the further capacity loss.³⁶ As shown in Fig. 5d, it can be clearly observed that CMCS shows the most excellent cycling performance. After 100 cycles, the capacity of CMCS is still as high as 770.9 mA h g⁻¹. On the contrary, the capacity of CM and MnO_x drops to 543.6 mA h g⁻¹ and 81.6 mA h g⁻¹ after 100 cycles. As shown in Table S2,† the electrochemical performance of the CMCS is superior among those of the reported manganese oxide materials. The theoretical capacity of MnO is 755 mA h g⁻¹ and the capacity of carbon is mostly less than that of MnO. However, the reversible capacities of CMCS and CM are higher than the theoretical value (755 mA h g⁻¹). This can be attributed to the synergistic effect between MnO_x and nitrogen-doped carbon plates, that is polymeric gel-like film formed on the surface of active materials during the repeated charge and discharge processes.⁷

The SEM and TEM were applied to further examine the morphologies of CMCS and CM after 100 cycles. Fig. 6a and b show the morphology of CMCS electrode before discharge/charge and after 100 cycles. It can be seen that the structure of active materials in CMCS electrode surrounded by PVDF can still be maintained in the electrode. In contrast, the morphology of CM electrode after 100 cycles shown in Fig. 5Sb† exhibits a more loose structure and many holes appear on the surface of electrode, which is possible that MnO particles fell from the carbon plate during the process of volume expansion/contraction. It is worth noting that TEM results (Fig. 6d and S5d†) show that both the MnO_x nanoparticles in CM and CMCS turned into a diameter of about 5 nm small particles after 100 cycles. The lattice distance of 0.216 nm correspond to the (2 0 0) planes of MnO. These small particles tended to be larger particles due to the huge surface energy, and then fell from the electrode. This is the primary reason for the capacity loss. As shown in Fig. 6c and S5c,† MnO particles in CMCS still uniformly distributed in double carbonized poly-dopamine sheets, while MnO particles in CM aggregated slightly due to the protection is incomplete came from a single platelike carbon. These large particles fell easily from the electrode, and then formed loose structure and many holes (Fig. S5b†). These results indicate that platelike carbon acts as supporting material, which could alleviate the gradual aggregation of metal grains and the loss of electronic contact of active particles resulting from large volume expansion/contraction during the repeated conversion reaction. However, the MnO_x nanoparticles in CM distributed on the surfaces of platelike carbon will still suffer these issues. Therefore, the capacity of CM electrode drops rapidly after 35 cycles. The MnO_x nanoparticles in CMCS are well encapsulated between the double carbonized poly-dopamine sheets. Even though the volume expansion of MnO_x still occurs, the carbonized poly-dopamine will ensure the structural stability of MnO_x during the Li-ion insertion/extraction process. That's the reason why CMCS could keep a stable reversible capacity after 100 cycles.

Fig. 6 SEM images of CMCS electrode (a) before discharge/charge, (b) after discharge/charge. (c) TEM image of CMCS after discharge/charge. (d) HRTEM image of CMCS after discharge/charge.

The consecutive cycling behavior of CMCS at various charge and discharge current densities, measured after 10 cycles in ascending steps from 0.1 A g⁻¹ to 1.0 A g⁻¹, followed by a repetitive process, is presented. As shown in Fig. 7a, the capacity fades gradually with the increasing of the current density, but retains stable at each current density. Even at 1.0 A g⁻¹, the reversible capacity is 443.9 mA h g⁻¹ in the first procedure and 328.8 mA h g⁻¹ in the second procedure. Importantly, when the two measuring procedures is end and the current density is returned to 0.1 A g⁻¹, the capacity of CMCS is recovered to 713.4 mA h g⁻¹ and remains 732.4 mA h g⁻¹ after 130 cycles. The electrochemical results suggest that the double nitrogen-doped platelike carbon sheets can efficiently buffer the volume change of MnO_x during the Li-ion insertion/extraction process and improve the electrical conductivity.

Fig. 7 (a) Rate performances (discharge) of CMCS at various rates. (b) The electrochemical impedance spectra of CMCS, CM and MnO_x.

EIS is an effective way to evaluate the electron conductivity and Li diffusion of materials. Prior to the EIS tests, the cells were activated for 3 cycles at a current density of 0.2 A g⁻¹ to obtain stable electrodes. The resistance values were evaluated by the equivalent circuit (Fig. S6†). In the equivalent circuit, R_o, R_S, R_ct and Z_W represent the ohmic resistance of the cell, the SEI resistance, the charge-transfer resistance and Warburg diffusion impedance, respectively. According to the pervious papers, R_ct directly reflects the kinetics of electrode reactions and low R_ct generally favors fast charge-transfer response.⁴¹ As shown in Fig. 7b, R_ct of the pure MnO_x, CM and CMCS electrodes are 63.59 Ω, 35.6 Ω and 24.78 Ω, which suggested that CMCS electrode possess a lowest charge transfer resistance. It means that the double carbonized poly-dopamine layer could greatly improve the activation and kinetics of conversion reaction upon Li-ion insertion/extraction. Thus, CMCS shows the best reversible capacity and wonderful rate capability.

Conclusion

In summary, we have demonstrated a facile strategy to fabricate C/MnO_x/C sandwich plates composite using a solvothermal method followed by annealing treatment. The resultant composite exhibits high reversible capacity, excellent cycling stability and exceptional rate performance when used as anode materials in LIBs. It can deliver a reversible capacity of 770.9 mA h g⁻¹ at 0.2 A g⁻¹ after 100 cycles and a stable reversible capacity of 443.9 mA h g⁻¹ at a high current density of 1.0 A g⁻¹. Compared with CM and MnO_x, CMCS has the best electrochemical performances because of the encapsulation of double nitrogen-doped platelike carbon sheets, which can not only address the issues related to manganese oxides aggregation and volumetric changes during the Li⁺ insertion/extraction, but also effectively shorten the transport path of Li⁺ ions and enhance the conductivity.

Acknowledgements

This study were Supported by the National Nature Science Foundation of China (Grant no. 51204209 and 51274240) and Grants from the Project of Innovation-driven Plan in Central South University.

Notes and references

1. K. Su, C. Wang, H. Nie, Y. Guan, F. Liu and J. Chen, J. Mater. Chem. A, 2014, 2, 10000.
2. V. Etacheri, R. Marom, R. Elazari, G. Salitra and D. Aurbach, Energy Environ. Sci., 2011, 4, 3243.
3. W.-M. Chen, L. Qie, Y. Shen, Y.-M. Sun, L.-X. Yuan, X.-L. Hu, W.-X. Zhang and Y.-H. Huang, Nano Energy, 2013, 2, 412–418.
4. Y. Sun, X. Hu, W. Luo and Y. Huang, J. Mater. Chem., 2012, 22, 19190.
5. Y. Xiao, X. Wang, W. Wang, D. Zhao and M. Cao, ACS Appl. Mater. Interfaces, 2014, 6, 2051–2058.
6. H. Hu, H. Cheng, Z. Liu and Y. Yu, Electrochim. Acta, 2015, 152, 44–52.
7. Q. Wang, D.-A. Zhang, Q. Wang, J. Sun, L.-L. Xing and X.-Y. Xue, Electrochim. Acta, 2014, 146, 411–418.
8. L.-L. Xing, B. He, Y.-X. Nie, P. Deng, C.-X. Cui and X.-Y. Xue, Mater. Lett., 2013, 105, 169–172.
9. I. Nam, N. D. Kim, G.-P. Kim, J. Park and J. Yi, J. Power Sources, 2013, 244, 56–62.
10. Y. Xiao, C. Hu and M. Cao, J. Power Sources, 2014, 247, 49–56.
11. J. Qu, Y. X. Yin, Y. Q. Wang, Y. Yan, Y. G. Guo and W. G. Song, ACS Appl. Mater. Interfaces, 2013, 5, 3932–3936.
12. J. Yue, X. Gu, L. Chen, N. Wang, X. Jiang, H. Xu, J. Yang and Y. Qian, J. Mater. Chem. A, 2014, 2, 17421–17426.
13. X. Zhang, Z. Xing, L. Wang, Y. Zhu, Q. Li, J. Liang, Y. Yu, T. Huang, K. Tang, Y. Qian and X. Shen, J. Mater. Chem., 2012, 22, 17864.
14. Z. Bai, N. Fan, Z. Ju, C. Guo, Y. Qian, B. Tang and S. Xiong, J. Mater. Chem. A, 2013, 1, 10985.
15. Z. Bai, Y. Zhang, Y. Zhang, C. Guo, B. Tang and D. Sun, J. Mater. Chem. A, 2015, 3, 5266–5269.
16. L. Xing, C. Cui, C. Ma and X. Xue, Mater. Lett., 2011, 65, 2104–2106.
17. J. Huang, J. Xie, K. Chen, L. Bu, S. Lee, Z. Cheng, X. Li and X. Chen, Chem. Commun., 2010, 46, 6684–6686.
18. X. Sun, Y. Xu, P. Ding, M. Jia and G. Ceder, J. Power Sources, 2013, 244, 690–694.
19. G. L. Xu, Y. F. Xu, J. C. Fang, F. Fu, H. Sun, L. Huang, S. Yang and S. G. Sun, ACS Appl. Mater. Interfaces, 2013, 5, 6316–6323.
20. F. J. Douglas, D. A. MacLaren, F. Tuna, W. M. Holmes, C. C. Berry and M. Murrie, Nanoscale, 2014, 6, 172–176.
21. D. Qiu, L. Ma, M. Zheng, Z. Lin, B. Zhao, Z. Wen, Z. Hu, L. Pu and Y. Shi, Mater. Lett., 2012, 84, 9–12.
22. Z. Bai, X. Zhang, Y. Zhang, C. Guo and B. Tang, J. Mater. Chem. A, 2014, 2, 16755–16760.
23. G. Jian, Y. Xu, L.-C. Lai, C. Wang and M. R. Zachariah, J. Mater. Chem. A, 2014, 2, 4627.
24. W. Luo, X. Hu, Y. Sun and Y. Huang, ACS Appl. Mater. Interfaces, 2013, 5, 1997–2003.
25. T. Wang, Z. Peng, Y. Wang, J. Tang and G. Zheng, Sci. Rep., 2013, 3, 2693.
26. G. Zhao, X. Huang, X. Wang, P. Connor, J. Li, S. Zhang and J. T. S. Irvine, J. Mater. Chem. A, 2015, 3, 297–303.
27. K. Zhang, P. Han, L. Gu, L. Zhang, Z. Liu, Q. Kong, C. Zhang, S. Dong, Z. Zhang, J. Yao, H. Xu, G. Cui and L. Chen, ACS Appl. Mater. Interfaces, 2012, 4, 658–664.
28. W. B. Luo, S. L. Chou, W. Jia-Zhao, Y. C. Zhai and H. K. Liu, Sci. Rep., 2015, 5, 8012.
29. G. Lu, S. Qiu, H. Lv, Y. Fu, J. Liu, X. Li and Y.-J. Bai, Electrochim. Acta, 2014, 146, 249–256.
30. C. Yang, Q. Gao, W. Tian, Y. Tan, T. Zhang, K. Yang and L. Zhu, J. Mater. Chem. A, 2014, 2, 19975–19982.
31. J. Zang, H. Qian, Z. Wei, Y. Cao, M. Zheng and Q. Dong, Electrochim. Acta, 2014, 118, 112–117.
32. C. Zhang, L. Fu, N. Liu, M. Liu, Y. Wang and Z. Liu, Adv. Mater., 2011, 23, 1020–1024.
33. S. Qiu, X. Wang, G. Lu, J. Liu and C. He, Mater. Lett., 2014, 136, 289–291.
34. Z. Cui, X. Guo and H. Li, J. Power Sources, 2013, 244, 731–735.
35. X. Fang, X. Lu, X. Guo, Y. Mao, Y.-S. Hu, J. Wang, Z. Wang, F. Wu, H. Liu and L. Chen, Electrochem. Commun., 2010, 12, 1520–1523.
36. S. Z. Huang, J. Jin, Y. Cai, Y. Li, H. Y. Tan, H. E. Wang, G. Van Tendeloo and B. L. Su, Nanoscale, 2014, 6, 6819–6827.
37. B. Sun, Z. Chen, H.-S. Kim, H. Ahn and G. Wang, J. Power Sources, 2011, 196, 3346–3349.
38. X. Li, D. Li, L. Qiao, X. Wang, X. Sun, P. Wang and D. He, J. Mater. Chem., 2012, 22, 9189.
39. Y. Sun, X. Hu, W. Luo, F. Xia and Y. Huang, Adv. Funct. Mater., 2013, 23, 2436–2444.
40. S. Wang, Y. Xing, H. Xu and S. Zhang, ACS Appl. Mater. Interfaces, 2014, 6, 12713–12718.
41. J. Yang, Q. Liao, X. Zhou, X. Liu and J. Tang, RSC Adv., 2013, 3, 16449.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26411a

---