Green synthesis of GeO₂/graphene composites as anode material for lithium-ion batteries with high capacity

Abstract

A facile green solution route using only GeO₂ powder, graphene oxide and purified water has been developed to prepare a GeO₂/graphene composite, in which the GeO₂ particles are wrapped in graphene nanosheets. When utilized as an anode material for lithium-ion batteries (LIBs), the composite electrode exhibits a high initial reversible charge capacity of 1637 mA h g⁻¹, while bare GeO₂ particles only show a capacity of 150 mA h g⁻¹. The high capacity of the GeO₂/graphene composites was ascribed to the reversible utilization of Li₂O, which was confirmed by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) techniques. It is noteworthy that the solvent, reagents, as well as instruments we used in the preparation process of the composites are innocuous and inexpensive. Thus, the GeO₂/graphene composite electrode can be used as a promising high capacity anodic material for LIBs.

---

Introduction

Lithium-ion batteries (LIBs) are of great interest because of their high energy density, low self-discharge and widely applications in portable electronic devices, electric vehicles (EVs) and hybrid electric vehicles (HEVs).^1–6 However, the graphite anode in commercially available LIBs has a quite low specific capacity of 372 mA h g⁻¹, which is unable to meet the establishment of energy storage systems and the requirements of large EVs development. Hence, the development of novel anode materials with higher energy capacity is urgently needed.

The IV group elements including Si (4200 mA h g⁻¹) and Ge (1600 mA h g⁻¹) have gathered significant amounts of attention due to their high theoretical capacities. When compared with Si-based materials, Ge exhibits better electric conductivity (10⁴ times higher than that of Si owing to its smaller band gap of 0.6 eV), higher lithium-ion diffusivity (400 times higher than in Si at ambient temperature), isotropic lithiation behavior and better oxidation resistance, albeit its relatively lower capacity and higher cost.^7–15 Furthermore, it shows low charge/discharge potential as well as environmental benignity. These merits enable Ge to be the most promising potential alternative anode material for next-generation LIBs. Unfortunately, similar to Si anode materials, Ge undergoes considerable volume changes upon the lithiation/delithiation cycles, with a volume expansion of about 370%.^16,17 The increasing mechanical stress generated in the electrochemical reaction process may cause the active materials to crack and pulverize as well as undergo exfoliation from the current collector, leading to capacity decay and failure of the battery.^18–24

So far, multiple strategies have been proposed to solve this problem, such as designing Ge nanostructures with different morphologies,^25–28,49 decreasing the particle size,^29–31 developing carbon-based hybrid nanocomposites^32–37 and so forth. Among these methods, one of the most promising ways is to introduce a carbon buffer layer, which not only serves as a conductive substrate for improving the electrical conductivity of the electrode but also accommodates the volume changes of Ge. Graphene, a two-dimensional (2D) carbonic nanomaterial with high electrical conductivity, vast specific surface area (2620 m² g⁻¹), good mechanical flexibility and chemical stability, may be the best choice.^38–41 Most encouragingly, several recent studies on exploring the fabrication of graphene-based Ge anode materials have made fairly good progress. For instance, Fang et al. developed a facile route to prepare a 3D Ge–graphene–carbon nanotube composite, which exhibits a capacity of 863.8 mA h g⁻¹ after 100 cycles and good rate performance.⁴ Jia's group synthesized a three-dimensional GeO₂/graphene nanostructure and the prepared composite electrode shows a higher de-lithiation capacity of 1021 mA h g⁻¹ at 0.2 C and 730 mA h g⁻¹ at 5C.² Lv et al. reported a facile one-step reduction approach to the synthesis of a GeO_x/reduced graphene oxide composite, which shows a high reversible capacity of 1600 mA h g⁻¹ at 100 mA g⁻¹ and 410 mA h g⁻¹ at a high current density of 20 A g⁻¹.⁴² Although having apparently improved the performance of Ge-based anode, these methods generally require sophisticated preparation processes, harmful organic substances (CTAB, PVP, etc.) and expensive germanium precursors (such as GeBr₂, GeCl₄ and GeH₄). Therefore, it is still a challenge to prepare Ge–graphene composites via simple, environmentally friendly procedures.

In this work, a GeO₂/graphene composite was successfully fabricated based on a dissolution re-crystallization mechanism, where graphene nanosheets act as nucleation sites. It is worth noting that the fabrication process was simple, high yielding and eco-friendly and did not require the use of complex instruments. The GeO₂ particles were uniformly embedded in an elastic graphene network, which could effectively alleviate the volume expansion of GeO₂ during the charge and discharge processes. Furthermore, the combination with graphene not only avoids the agglomeration of GeO₂ particles and improves the overall conductivity, but also enables the reversible utilization of Li₂O and accordingly, leads to a high charge capacity.

Experimental section

Preparation of the GeO₂/graphene composite

Graphene oxide (GO) was synthesized according to a literature procedure.⁵¹ Before use, a certain amount of GO powder was dispersed in deionized water via ultrasonication to form a 1 g L⁻¹ GO suspension. The GeO₂/graphene composite was prepared via the in situ growth of GeO₂ (99.99%, Aladdin, Shanghai, China) on the GO surface. Specifically, 0.12 g of GeO₂ was added to 125 mL of the GO suspension (125 mg GO) with stirring for 3 h to form a homogeneous suspension. The homogeneous mixture was dried at 50 °C in air for 2 days. Next, the dried sample was annealed at 300 °C under a N₂ atmosphere for 2 h. A black flake-like material was subsequently obtained. For comparison, pure graphene, pure GeO₂ particles and a GeO₂/graphene composite with 0.12 g GeO₂ and lower graphene content (50 mg GO) were synthesized using the same method.

Characterization

The crystal structures of the as-synthesized samples were characterized by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer with Cu-Kα radiation (λ = 0.154 nm) in the range of 10–90° (2θ) at a scanning speed of 6° min⁻¹. The morphology of the samples was investigated on a Philips XL 30FEG Scanning electron microscope (SEM). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) investigations were carried out on a JEOL JEM-2100F microscope with an accelerate voltage of 20 kV. X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra DLD, Japan) was performed with Al Kα radiation. Raman spectrocopy was conducted on a RM-1000 Renishaw confocal micro-Raman spectrometer with a laser wavelength of 514.5 nm at a laser power of 0.48 mW in the range of 300 to 2000 cm. Thermogravimetric analysis (TGA) was carried out in an air atmosphere at a heating rate of 10 °C min⁻¹ on a Netzsch instrument TG/STA 449F3.

Electrochemical measurements

The GeO₂/graphene composite was used as a LIB anode and metallic lithium was used as the counter electrode. Electrochemical measurements were evaluated using CR2016 coin-type cells. The working electrode was prepared by dispersing the active material, carbon black and polyvinylidene fluoride (PVDF) with a weight ratio of 80 : 10 : 10 in 1-methyl-2-pyrrolidene (NMP) solution to form a homogeneous slurry, which was then uniformly coated on the copper foil current collector. After drying at 60 °C in a vacuum oven for 12 h, copper foil was punched into disks with a diameter of 14 mm. The cell, which was comprised of the copper foil disk, lithium plate, polypropylene film and electrolyte solution of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1 : 1, v/v) was assembled in a glove box under an Ar atmosphere. Galvanostatic charge–discharge cycling was performed on a Land battery measurement system (BT2013A, Wuhan, China) at room temperature. The cyclic voltammetry (CV) tests (0–3 V, scan rate of 0.1 mV s⁻¹) and electrochemical impedance spectroscopy (EIS) (amplitude: 5 mV, frequency: 0.01–100 kHz) were performed on an electrochemical workstation (Zennium, IM6, Germany).

Results and discussion

The GeO₂/graphene composites were fabricated via a green and mild procedure, as illustrated in Fig. 1. When GeO₂ was gradually added to a GO suspension, a homogeneous precursor suspension containing H₂GeO₃ and GO was obtained. During the evaporation process, the concentration of GeO₂ increased and therefore, GeO₂ would recrystallize and precipitate. It is speculated that the GO nanosheets act as nucleation sites for GeO₂.⁴² Thus, GeO₂/graphene composites were synthesized in situ and the yield could reach 100% when the solution was completely evaporated.

Fig. 1 A schematic representation of the procedure used to prepare the GeO₂/graphene composite.

Fig. 2a shows the SEM image of commercial GeO₂ (c-GeO₂), which has an irregular shape with particle sizes in the range of 30–50 μm. After the dissolution and re-crystallization of GeO₂, the particle size of c-GeO₂ was decreased to several tens nanometers to 1 micrometer, as shown in Fig. 2b and the resulting pure GeO₂ (p-GeO₂) still possessed an irregular morphology. Fig. 2c and d display the SEM images of the GeO₂/graphene composite at low and high magnification, respectively. The composite was comprised of nearly spherical GeO₂ sub-microparticles with an average particle size of ∼750 nm and transparent, wrinkled graphene nanosheets, in which the GeO₂ particles were homogeneously enwrapped. It is believed that the agglomeration of graphene between layers can be eased by anchoring the GeO₂ particles.⁴⁰ As a consequence, the highly active surface area of graphene was well retained. The voids between the GeO₂ spheres are beneficial to electrolyte diffusion and lithium ion transport.⁴³ The TEM images, which can be found in ESI Fig. S1,† clearly show the GeO₂ particles with similar sizes are enwrapped in the transparent sheet-like graphene. The SAED patterns of the composites suggest the polycrystalline nature of the GeO₂ component.

Fig. 2 SEM images of (a) commercial GeO₂, (b) the as-prepared pure GeO₂ and (c and d) the GeO₂/graphene composites at (c) low and (d) high magnification.

The crystal structures of p-GeO₂ and the GeO₂/graphene composite samples were characterized by XRD, as shown in Fig. 3a. All the diffraction peaks of p-GeO₂ were perfectly indexed to the hexagonal phase structure of α-GeO₂ (JCPDS card no. 85-1515). The strong and sharp peaks indicate the high crystallinity of GeO₂. The diffraction peaks of the GeO₂/graphene composite are in accordance with p-GeO₂ and no other impurities were detected, implying the high purity of the samples. It is worth noting the peak intensity of the former was lower than that of the latter presumably on account of the presence of the graphene coating layer. However, there are no peaks corresponding to graphene displayed in the pattern, which may be attributed to the inferior crystallization and the lack of tremendous layer-to-layer stacking of graphene as confirmed by SEM (Fig. 2d).^5,33 In addition, the peak overlapping between graphene and GeO₂ at about 25° may also result in the disappearance of the characteristic peaks of graphene.

Fig. 3 (a) The powder X-ray diffraction patterns of p-GeO₂ and the GeO₂/graphene composite, and (b) the Raman spectra of GO and the GeO₂/graphene composite.

To evaluate the weight content of graphene in the GeO₂/graphene composite, TGA was performed in the range of 35–800 °C (ESI Fig. S2†). As shown in Fig. S2,† the weight loss of 10.5% below 200 °C was due to the elimination of absorbed/trapped water molecules. At 200–600 °C, there is a weight loss of 14.2%, which was attributed to the oxidation of graphene.

The structural changes of GO before and after its interaction with GeO₂ were investigated by Raman spectroscopy (Fig. 3b). The Raman spectrum of GO is comprised of two broad peaks. The D-band at 1361 cm⁻¹ is associated with the disorder-induced mode, while the G-band centered at 1600 cm⁻¹ is related to the vibration of sp² hybridized carbon atoms.^44,45 For the GeO₂/graphene composite, there are three peaks in the Raman spectrum. The strong peak located at 443 cm⁻¹ was assigned to the symmetric Ge–O–Ge stretching vibration mode²⁰ and the other two peaks correspond to the D-band (1341 cm⁻¹) and G-band (1611 cm⁻¹), respectively, which are the characteristic peaks of carbonaceous materials.¹⁰ Although the D-band/G-band peak position of the two samples are almost identical, the D/G intensity ratio of the composite (I_D/I_G = 1.12) was significantly increased, indicating a decrease in the average size of the sp² domains due to the reduction of GO.^52,53

The lithium storage mechanisms of the GeO₂/graphene composite and p-GeO₂ were investigated using CV and galvanostatic charge–discharge tests. The CV curves of GeO₂/graphene composites are plotted in Fig. 4a. During the first cathodic sweep, a weak reduction peak is noticed at about 0.9 V, which is most probably due to the formation of the solid electrolyte interface (SEI) layer on the surface of the GeO₂/graphene particles.²⁶ An intense peak appears at 0.02–0.5 V and it can be assigned to the electrochemical conversion of GeO₂ to Ge and the formation of Li_4.4Ge alloys.¹⁶ In the corresponding anodic sweep cycle, the oxidation peak at about 0.6 V can be related to the de-alloying reaction of Li_4.4Ge to Ge and Li.³⁵ Interestingly, at 1.3 V, a broad hump peak can be observed, which is typically attributed to the re-oxidation of Ge.⁴² In the following CV cycles, similar hump peaks still exist in the same voltage position. However, similar peak cannot be observed in the CV curves of p-GeO₂ (Fig. S3†), implies the re-oxidation of Ge does not occur in the p-GeO₂ electrode. The CV analysis suggests that the GeO₂/graphene composites would exhibit higher capacity compared with the p-GeO₂ electrode, and the speculation is confirmed by the following charge–discharge tests (Fig. 4b). In addition, the CV curves obtained for the GeO₂/graphene composite overlap substantially in the following CV scans, indicating the composite has good stability and reversibility in the lithiation/delithiation processes.

Fig. 4 (a) The CV curves obtained for the GeO₂/graphene composite, (b) galvanostatic charge–discharge curves obtained for the p-GeO₂ electrode and GeO₂/graphene composite electrode during the first cycle.

Fig. 4b shows the typical charge–discharge profiles obtained for the p-GeO₂ and GeO₂/graphene composite electrodes in the potential region of 0–3 V versus Li⁺/Li at the current density of 100 mA g⁻¹. The charge capacity of the p-GeO₂ electrode was much lower than the discharge capacity, corresponding to a coulombic efficiency (CE) of only 11.8%. The low CE was related to the decomposition of the electrolyte, incomplete transformation of Li⁺ after insertion into GeO₂, formation of a non-conductive solid electrolyte interphase (SEI) layer and fracture/pulverization of the microstructure of the electrode caused by the huge volume expansion of p-GeO₂.^46–48 In contrast, the first discharge and charge capacities obtained for the GeO₂/graphene composite were 3052 and 1637 mA h g⁻¹ with a CE of 53.6%. It is noteworthy that two apparent plateaus can be observed for the GeO₂/graphene composite electrode in the initial charge curve. The former was located at 0.3–0.6 V and was related to Li_4.4Ge delithiation to Ge and the latter at 1.0–1.5 V was related to the re-oxidation of Ge,^21,42 while the 1.0–1.5 V plateau was not found for p-GeO₂. This was consistent with the results obtained from the cyclic voltammograms (Fig. 4a).

XPS measurements were conducted in order to identify surface composition and chemical states of the GeO₂/graphene composite before (Fig. 5a–c) and after the discharge–charge cycle (Fig. 5d). The XPS survey spectrum indicates that the GeO₂/graphene composite was comprised of the elements Ge, C and O, as shown in Fig. 5a. The high resolution spectrum of C 1s (Fig. 5b) can be separated into three major peaks located at 283.4, 285.0 and 287.3 eV, which can be attributed to sp² hybridized carbon (C=C), epoxy and alkoxy groups (C–O), and carbonyl and carboxylic (C=O) groups, respectively.^7,45,50 The high resolution spectrum of Ge 3d is shown in Fig. 5c and only one peak was observed at 31.9 eV, indicating that before cycling, Ge in the composite has an oxidation state of +4.¹⁰ After the first discharge–charge cycle, to study the valence state of germanium, the high resolution XPS measurement of Ge 3d was performed again as shown in Fig. 5d. There are two peaks located at 31.3 and 33.5 eV, which can be attributed to Ge²⁺ and Ge⁴⁺, respectively.²⁶ The XPS measurements suggest that after the initial charging process, the de-alloyed Ge is re-oxidized, thus, causing the high initial charge capacity (1637 mA h g⁻¹).

Fig. 5 The XPS spectrum of the GeO₂/graphene composite: (a) the survey spectrum and high resolution spectra of (b) C 1s, (c) Ge 3d and (d) Ge 3d after the first discharge–charge cycle.

The cycling performances of the three samples are shown in Fig. 6a. For the p-GeO₂ electrode, the capacity rapidly decays to about 100 mA h g⁻¹ after only 10 cycles. In contrast, the GeO₂/graphene composite (125 mg GO) electrode shows significantly enhanced cycling performance, exhibiting a high charge capacity of 640 mA h g⁻¹ after 80 cycles. The good cycling performance may be ascribed to the small particle size of GeO₂ and the existence of graphene nanosheets, which can not only improve the electrical conductivity, but also accommodate the mechanical strain of the GeO₂ particles during electrochemical reaction processes. While decreasing the graphene content, as shown in Fig. 6a, the GeO₂/graphene composite (50 mg GO) exhibits an initial discharge–charge capacity of 3027.2 and 1886.1 mA h g⁻¹, which are obviously higher than the 125 mg electrode. However, the 50 mg electrode shows worse cycling performance, after 50 cycles the electrode exhibits a charge capacity of 454 mA h g⁻¹. For the pure graphene electrode, although it shows excellent cycling performance (178 mA h g⁻¹ after 80 cycles with an initial charge capacity of 192 mA h g⁻¹, ESI Fig. S4†), it's capacity is quite low.

Fig. 6 (a) The cycling performance of the three samples at a current density of 100 mA g⁻¹. (b) The electrochemical impedance spectra of the p-GeO₂ and GeO₂/graphene composite electrodes after 50 cycles. The inset presents the equivalent circuit model.

With the purpose to further study the influence of adding graphene on the electrical conductivity, we performed electrochemical impedance spectroscopy (EIS) on the p-GeO₂ and GeO₂/graphene composite electrodes before cycling (ESI Fig. S5†) and after 50 cycles (Fig. 6b). As shown in Fig. 6b, the Nyquist plots for both samples were comprised of a depressed semicircle in the high-middle-frequency region and an inclined line in the low-frequency region. Both of them can be analyzed with the same equivalent circuit mode (the inset of Fig. 6b). CPE represents the constant phase-angle element involving the double-layer capacitance and Z_w is the Warburg impedance, which reflects the solid-state diffusion of Li ions into the bulk of the active materials. The intercept at Z′ in the high-frequency region is related to the resistance of the electrolyte (R_s). The semicircle in the high-middle-frequency region is attributed to the charge-transfer resistance (R_ct), showing the charge transfer through the electrode/electrolyte interface. We can see that the R_ct values of the two anodes after 50 cycles are both lower than that of before cycling, which may be associated with the activation of the electrode. It is worth noting that the diameter of the semicircle obtained for the GeO₂/graphene composite electrode after 50 cycling was tremendously decreased in comparison with p-GeO₂. It can be speculated that the graphene in the composite can serve as a highly electrically conductive continuous medium and the electronic transport length in GeO₂ was effectively shortened after adding GO,⁵⁴ suggesting that the GeO₂/graphene electrode exhibits better electrical conductivity. Thus, the electrode reaction kinetics of the composites was significantly improved, resulting in a high reversible capacity when compared with the p-GeO₂ electrode.

Conclusions

In summary, a GeO₂/graphene composite was fabricated using a facile solution approach in the absence of any hazard reagents and solvents, as well as complex instruments. The yield of the GeO₂/graphene composite can reach 100%. When used as an anode material for LIBs, the GeO₂/graphene composite electrode exhibits an increased reversible capacity and improved cyclability when compared with p-GeO₂. The improved Li⁺ storage performance of the composite electrode arises from the reversible utilization of Li₂O and the elastic graphene buffer.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21475085, 21575131) and the key scientific research project of high schools in Henan Province (16A430025).

References

1. C. G. Hu, L. X. Lv, J. L. Xue, M. H. Ye, L. X. Wang and L. T. Qu, Chem. Mater., 2015, 27, 5253.
2. H. P. Jia, R. Kloepsch, X. He, J. P. Badillo, M. Winter and T. Placke, J. Mater. Chem. A, 2014, 2, 17545.
3. X. N. Li, J. W. Liang, Z. G. Hou, Y. C. Zhu, Y. Wang and Y. T. Qian, Chem. Commun., 2015, 51, 3882.
4. S. Fang, L. F. Shen, H. Zheng and X. G. Zhang, J. Mater. Chem. A, 2015, 3, 1498.
5. S. P. Wu, R. Wang, Z. L. Wang and Z. Q. Lin, Nanoscale, 2014, 6, 8350.
6. G. L. Cui, L. Gu, L. J. Zhi, N. Kaskhedikar, P. A. V. Aken, K. Mullen and J. Maier, Adv. Mater., 2008, 20, 3079.
7. S. Fang, L. F. Shen, H. Zheng, Z. K. Tong, G. Pang and X. G. Zhang, RSC Adv, 2015, 5, 85256.
8. Y. W. Lee, D. M. Kim, S. J. Kim, M. C. Kim, H. S. Choe, K. H. Lee, J. I. Sohn, S. N. Cha, J. M. Kim and K. W. Park, ACS Appl. Mater. Interfaces, 2016, 8, 7022.
9. X. W. Li, Z. B. Yang, Y. J. Fu, L. Qiao, D. Li, H. W. Yue and D. Y. He, ACS Nano, 2015, 9, 1858.
10. F. Zou, X. L. Hu, L. Qie, Y. Jiang, X. Q. Xiong, Y. Qiao and Y. H. Huang, Nanoscale, 2014, 6, 924.
11. Y. Son, M. Park, Y. Son, J. S. Lee, J. H. Jang, Y. Kim and J. Cho, Nano Lett., 2014, 14, 1005.
12. M. Pharra, Y. S. Choi, D. Lee, K. H. Oh and J. J. Vlassak, J. Power Sources, 2016, 304, 164.
13. T. T. Qiang, J. X. Fang, Y. X. Song, Q. Y. Ma, M. Ye, Z. Fang and B. Y. Geng, RSC Adv, 2015, 5, 17070.
14. W. Wang, J. W. Qin and M. H. Cao, ACS Appl. Mater. Interfaces, 2016, 8, 1388.
15. A. M. Chockla, K. C. Klavetter, C. B. Mullins and B. A. Korgel, ACS Appl. Mater. Interfaces, 2012, 4, 4658.
16. Z. Chen, Y. Yan, S. Xin, W. Li, J. Qu, Y. G. Guo and W. G. Song, J. Mater. Chem. A, 2013, 1, 11404.
17. M. Li, D. Zhou, W. L. Song, X. G. Li and L. Z. Fan, J. Mater. Chem. A, 2015, 3, 19907.
18. X. L. Wang, W. Q. Han, H. Y. Chen, J. M. Bai, T. A. Tyson, X. Q. Yu, X. J. Wang and X. Q. Yang, J. Am. Chem. Soc., 2011, 133, 20692.
19. L. Mei, M. L. Mao, S. L. Chou, H. K. Liu, S. X. Dou, D. H. L. Ng and J. M. Ma, J. Mater. Chem. A, 2015, 3, 21699.
20. S. Choi, J. Kim, N. S. Choi, M. G. Kim and S. Park, ACS Nano, 2015, 9, 2203.
21. L. X. Zeng, X. X. Huang, X. Chen, C. Zheng, Q. R. Qian, Q. H. Chen and M. D. Wei, ACS Appl. Mater. Interfaces, 2016, 8, 232.
22. D. J. Xue, S. Xin, Y. Yan, K. C. Jiang, Y. X. Yin, Y. G. Guo and L. J. Wan, J. Am. Chem. Soc., 2012, 134, 2512.
23. S. X. Jin, G. Z. Yang, H. W. Song, H. Cui and C. X. Wang, ACS Appl. Mater. Interfaces, 2015, 7, 24932.
24. M. X. Liu, X. M. Ma, L. H. Gan, Z. J. Xu, D. Z. Zhu and L. W. Chen, J. Mater. Chem. A, 2014, 2, 17107.
25. J. W. Liang, X. N. Li, Z. G. Hou, T. W. Zhang, Y. C. Zhu, X. D. Yan and Y. T. Qian, Chem. Mater., 2015, 27, 4156.
26. S. H. Choi, K. Y. Jung and Y. C. Kang, ACS Appl. Mater. Interfaces, 2015, 7, 13952.
27. J. Liu, K. P. Song, C. B. Zhu, C. C. Chen, P. A. V. Aken, J. Maier and Y. Yu, ACS Nano, 2014, 8, 7051.
28. X. S. Liu, J. Hao, X. X. Liu, C. X. Chi, N. Li, F. Endres, Y. Zhang, Y. Li and J. P. Zhao, Chem. Commun., 2015, 51, 2064.
29. X. N. Li, J. W. Liang, Z. G. Hou, Y. C. Zhu, Y. Wang and Y. T. Qian, Chem. Commun., 2014, 50, 13956.
30. T. Kennedy, E. Mullane, H. Geaney, M. Osiak, C. O'Dwyer and K. M. Ryan, Nano Lett., 2014, 14, 716.
31. D. T. Ngo, H. T. T. Le, C. Kim, J. Y. Lee, J. G. Fisher, I.-D. Kim and C. J. Park, Energy Environ. Sci., 2015, 8, 3577.
32. M. H. Seo, M. Park, K. T. Lee, K. Kim, J. Kim and J. Cho, Energy Environ. Sci., 2011, 4, 425.
33. R. Wang, S. P. Wu, Y. C. Lv and Z. Q. Lin, Langmuir, 2014, 30, 8215.
34. J. K. Hwang, C. S. Jo, M. G. Kim, J. Chun, E. Lim, S. Kim, S. Jeong, Y. Kim and J. Lee, ACS Nano, 2015, 9, 5299.
35. K. H. Seng, M. H. Park, Z. P. Guo, H. K. Liu and J. Cho, Nano Lett., 2013, 13, 1230.
36. D. Li, C. Q. Feng, H. K. Liu and Z. P. Guo, J. Mater. Chem. A, 2015, 3, 978.
37. P. Kitschke, M. Walter, T. Ruffer, A. Seifert, F. Speck, T. Seyller, S. Spange, H. Lang, A. A. Auer, M. V. Kovalenko and M. Mehring, J. Mater. Chem. A, 2016, 4, 2705.
38. J. L. Xie, C. X. Guo and C. M. Li, Energy Environ. Sci., 2014, 7, 2559.
39. W. W. Sun and Y. Wang, Nanoscale, 2014, 6, 11528.
40. H. M. Liu and W. S. Yang, Energy Environ. Sci., 2011, 4, 4000.
41. J. G. Ren, Q. H. Wu, H. Tang, G. Hong, W. J. Zhang and S. T. Lee, J. Mater. Chem. A, 2013, 1, 1821.
42. D. P. Lv, M. L. Gordin, R. Yi, T. Xu, J. X. Song, Y. B. Jiang, D. Choi and D. H. Wang, Adv. Funct. Mater., 2014, 24, 1059.
43. C. J. Zhang, Z. Lin, Z. Z. Yang, D. D. Xiao, P. Hu, H. X. Xu, Y. L. Duan, S. P. Pang, L. Gu and G. L. Cui, Chem. Mater., 2015, 27, 2189.
44. D. T. Ngo, R. S. Kalubarme, H. T. T. Le, C. N. Park and C. J. Park, Nanoscale, 2015, 7, 2552.
45. D. Li, K. H. Seng, D. Q. Shi, Z. X. Chen, H. K. Liu and Z. P. Guo, J. Mater. Chem. A, 2013, 1, 14115.
46. I. S. Hwang, J. C. Kim, S. D. Seo, S. Lee, J. H. Lee and D. W. Kim, Chem. Commun., 2012, 48, 7061.
47. D. T. Ngo, H. T. T. Le, R. S. Kalubarme, J. Y. Lee, C. N. Park and C. J. Park, J. Mater. Chem. A, 2015, 3, 21722.
48. X. N. Li, J. W. Liang, Z. G. Hou, W. Q. Zhang, Y. Wang, Y. C. Zhu and Y. T. Qian, J. Power Sources, 2015, 293, 868.
49. M. Javadi, Z. Y. Yang and J. G. C. Veinot, Chem. Commun., 2014, 50, 6101.
50. S. H. Choi, J. H. Lee and Y. C. Kang, ACS Nano, 2015, 9, 10173.
51. N. I. Kovtyukhova, P. J. Ollivier, B. R. Martin, T. E. Mallouk, S. A. Chizhik, E. V. Buzaneva and A. D. Gorchinskiy, Chem. Mater., 1999, 11, 771.
52. Q. Guo and X. Qin, ECS Solid State Lett., 2013, 2, M41.
53. J. F. Liang, W. Wei, D. Zhong, Q. L. Yang, L. D. Li and L. Guo, ACS Appl. Mater. Interfaces, 2012, 4, 454.
54. M. Q. Li, Y. Yu, J. Li, B. L. Chen, A. Konarov and P. Chen, J. Power Sources, 2015, 293, 976.

---

Footnote

† Electronic supplementary information (ESI) available: Additional data for the CV studies, EIS. See DOI: 10.1039/c6ra14819k

---