A strategy for suitable mass production of a hollow Si@C nanostructured anode for lithium ion batteries

Abstract

Si with high theoretical capacity has long suffered from its large volume variation and low electrical transport linked to poor cycling stability and rate performance. Here, a facile approach is reported to mass produce nanostructured Si@carbon with a tunable size of silicon nanoparticles. We performed carbon coating of Si nanoparticles by polyacrylonitrile (PAN) emulsifying and then carbonization. The hollow Si@C nanostructure was obtained via direct etching of Si nanoparticles with HF solution which is more advanced and has better controllability. When evaluated as an anode material for lithium-ion batteries, the C–Si nanocomposites exhibit excellent reversibility and cycling performance. A high capacity of 700 mA h g⁻¹ can be retained after 100 cycles at current densities of 250 mA g⁻¹. The rate capability of the C–Si microfibers is also improved. The special structure is believed to offer better structural stability upon prolonged cycling and to improve the conductivity of the material. This simple strategy could also be applied to prepare other carbon coatings of hollow energy materials.

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Introduction

Lithium ion batteries are attractive energy storage devices with high gravimetric and volumetric capacity and the ability to deliver high rates of power.^1–3 Si is a promising anode material for next-generation lithium ion batteries because of its exceptionally high theoretical capacity (∼4200 mA h g⁻¹) and low discharge potential (∼0.2 V).^4–6 Furthermore, silicon is environmentally friendly and abundant in the Earth's crust.^7,8 However, the associated huge volume expansion during lithiation (∼400%) could cause pulverization, contact losses of Si and unstable solid electrolyte interphase (SEI) formation, thus impeding the development of Si anode.^9–11

In order to improve both the capacity and cycle life of the Si anodes, different ways have been tested to limit the side effects on the electrode integrity. Among them, nanocomposite concept represents one attractive route² and exciting progress has been made, typically by engaging well-defined Si nanostructures including nanowires,¹² core–shell nanofibers,¹³ nanotubes,¹⁴ nanospheres,¹⁵ and their composites with carbon materials.^16–19 Nevertheless, high energy-consuming apparatuses and costly synthesis processes of the aforementioned Si based nanostructured materials can hardly be avoid, resulting in their limited potential for scalable manufacturing.²⁰ More recently, electrodes made from Si nanoparticle slurries have been investigated by several research groups as a potentially manufacturing-compatible route.^{7,10,15,20–23} Although these has been successful in extending the cycle life of silicon, nanoparticles electrodes have introduced new fundamental challenges, including higher surface area which could lower the coulombic efficiency, low tap density which could leads to low volumetric capacity and generally poor electrical properties due to the higher interparticle resistance.¹⁶ Inspired by the structure of a pomegranate fruit, Liu et al. proposed a hierarchical structured silicon anode that tackles most of the problems and resulting in superior electrochemical performance.¹⁶

In an effort to develop Si LIB anodes that could be produced in a quantity comparable to the industrial standard and also support good electrochemical performance, herein, we developed an encapsulation strategy and method to manage voids of Si@C anodes with pomegranate-shaped silicon–carbon composite structure building on the previous work. We demonstrate that two unique features of our approach for realizing this excellent structure are the carbon coating by polyacrylonitrile (PAN) emulsification and size control of Si nanoparticles by directly HF etching. The resultant hollow Si@C nanostructured microparticles showed excellent electrochemical performance for a scalable, electrochemically stable, and highly efficient anode.

Experimental

Synthesis of hollow Si@C clusters

For the carbon coating of the Si nanoparticles (50–100 nm, Alfa Aesar), a polyacrylonitrile (PAN, Alfa Aesar) solution was first prepared by dissolving 100 mg of PAN in 4 ml of N,N-dimethylformamide (DMF). The resulting slurry was left swollen and dissolved at 80 °C for 2 h. Then, 60 mg of Si nanoparticles were dispersed in the solution by ultrasonication for 2 h, and then stirred overnight to make the nanoparticles dispersed adequately in the solution (Fig. 1b). Deionized water was added dropwise from pipet into the mixture while stirring to make the PAN solution emulsify together with Si nanoparticles (Fig. 1c). The mud-like sample was then placed in a tube furnace and carbonized at 1000 °C for 2 h with a heating rate of 10 °C min⁻¹ under vacuum condition to decompose PAN, and obtained Si@C samples (Fig. 1d). The massive sample was ground to obtain a fine powder of Si@C microparticles with agate mortar. The obtained powder was then soaked in 4 wt% HF aqueous solution. HF can react with Si nanoparticles directly and control the particle size by controlling the etching time. A void space was hence created between Si core and carbon shell. The sample was centrifugated and washed with ethanol for three times. The final hollow Si@C nanostructures powders were obtained after drying in a vacuum oven at 80 °C for 2 h. Detailed synthesis process for the samples is schematically illustrated in Fig. 1a.

Fig. 1 Hollow Si@C microsphere electrode design and fabrication. Schematic diagram (a) and digital images (b–f) of the synthesis process of the hollow Si@C microsphere electrode.

Characterization

Morphologies of the as-synthesized products were examined using a JSM-7001F scanning electron microscope (SEM). Transmission electron microscopy (TEM) measurements were taken with a FEI Tecnai G2 F30 microscope operated at 300 kV. Raman spectra were obtained using a Laser Micro-Raman spectrometer (Renishaw inVia). The crystal structure of the samples was characterized by X-ray diffraction (XRD) with Cu Kα radiation. Thermogravimetric analysis (TGA) was performed using a Perkin Elmer Pyris 6 TG analyzer (heating rate of 5 °C min⁻¹) in static-air conditions.

Electrochemical characterization

Electrochemical measurements were performed using two-electrode electrochemical cells employing the prepared anode materials with different etching time as the working electrode and lithium foil as the counter electrode. The working electrodes were prepared by mixing hollow Si@C microparticles (80%), acetylene carbon black (10%) and sodium alginate binder (10%) in deionized water to form slurry that was then coated onto a copper foil and dried overnight at 100 °C in a vacuum oven. The loadage of active materials was accurately determined using a microbalance (Mettle Toledo XP2U, 0.1 μg resolution). Standard cells (CR2032) with lithium foils as counter electrode, polypropylene micromembrane as the separator, 1 M LiPF₆ in ethylene carbonate (EC) and diethyl carbonate (DEC) with a weight ratio of 1 : 1 as the electrolyte, were assembled in an Ar-filled universal glove box with an oxygen and water vapor pressure less than 0.3 ppm. Cyclic voltammogram of the composite electrodes were scanned at 0.05 mV s⁻¹ in a voltage window of 0–3 V on an Ivium electrochemical workstation. Electrochemical impedance spectra (EIS) were recorded from 10⁵ to 0.01 Hz. For cycling and rate performance, the electrodes were galvanostatically charged and discharged in a voltage cutoff of 0.01–1.0 V at various rates on a multichannel Neware battery testing system.

Result and discussion

Fig. 1 shows the preparation process and the optical images of the intermediated substances, from which we can seen that the sample is suitable for large-scale preparation. The schematic fabrication of hollow Si@C microparticles is illustrated in Fig. 1a. The Si nanoparticles were coated with PAN solution and the PAN solution would be emulsified into mud-like samples together with Si nanoparticles when deionized water was added into the solution which simplified the carbon coating process. After high temperature treatment, the carbonized samples with plum pudding structure were shown in Fig. 1d. Then, the sample was milled into powders of Si@C microparticles with the particle size of 1–50 μm. We find that the Si nanoparticles can react with HF directly. We control the particle size of Si in the Si@C clusters to form void space for accommodating the large volume change of Si during the charge/discharge process. The size of the void space can be adjusted by simply control the etching time. In our previous research,^24–26 we find that a certain thickness of oxide will formed on the surface of Si nanoparticles. The silicon oxide would react with HF and fresh surface of Si nanoparticles was exposed to the HF solution, then silicon oxide layer formed again on the freshly exposed surface of Si nanoparticles which would be continuous removed by HF etching. In this way, the particle size of Si nanoparticles can be decreased in such a continuous and iterative process. As seen in TEM images of Fig. 5, we fabricated hollow Si@C microparticles with pomegranate structure.

Fig. 2a shows a representative SEM image of the individual hollow Si@C microparticle with a diameter of ∼10 μm. The microparticle with pomegranate structure is consists of hollow Si@C nanoparticles and the carbon framework. Fig. 2b presents the high magnification SEM image of Si@C microparticle before HF etching. It can be seen that the Si nanoparticles with a diameter of dozens of nanometers are aggregated but well encapsulated within the carbon matrix and carbon shell is connected to adjacent carbon shell. There are many pores in the microparticles and carbon shell from where the HF solution can permeate into the interior of the microparticles. The pores were formed from the volume shrinkage of PAN during the carbonization process. Fig. 2c shows the hollow Si@C samples after HF etching for 10 h. The magnified SEM image shows the local structure of silicon nanoparticles and the conductive carbon framework. Silicon nanoparticles become smaller and are individually encapsulated by the carbon framework, with well-defined and uniform void spaces between the silicon and carbon that accommodate the volume expansion of the silicon.^7,27 Fig. 2d show the image of the sample after HF etching for more than 2 days. Combined with the TEM image shown in Fig. 5f, we can see that Si nanoparticles were removed completely by HF etching and carbon was present at the interior of the microparticle acting as a mechanically robust conductive framework.

Fig. 2 (a) SEM image of an individual hollow Si@C microsphere. (b) High magnification SEM image of Si@C microparticle before HF etching. (c) Typical SEM image of hollow Si@C samples after HF etching for 10 h. (d) SEM image of the carbon framework after etching away silicon.

Fig. 3 shows the high resolution TEM (HRTEM) images around the edge of the particles before and after carbon coating. Fig. 3a shows the Si nanoparticle without carbon coating. There is a thin amorphous SiO₂ layer on the surface of the nanoparticle which will be etched in subsequent steps. Fig. 3b shows the Si@C composite after carbon coating which is the intermediate composite of hollow Si@C. The image shows a clear boundary between Si nanoparticle and amorphous shell. The thickness of carbon shell is in the range of 5–10 nm. The image clearly shows lattice fringes in the crystalline Si nanoparticles. The measured lattice spacings observed in HRTEM are 0.31 nm and 0.19 nm, corresponding well to the (111) and (220) lattice spacings of the cubic Si structure, respectively. The SAED patterns and fast Fourier transform (FFT) image (insets of Fig. 3a and b) recorded from the nanoparticles reveal the single-crystalline structure of cubic Si.

Fig. 3 (a) TEM image of Si nanoparticle without carbon coating. There is a thin amorphous SiO₂ layer on the surface of the nanoparticle which will be etched in subsequent steps. (b) High resolution TEM image with SAED of carbon coated Si nanoparticle (before etching). The Si@C composite is the intermediate material of hollow Si@C.

The chemical structure of the samples is further confirmed using X-ray diffraction (XRD) and Raman spectra. As shown in the XRD pattern of hollow Si@C sample (Fig. 4a), all major diffraction peaks are indexed to the characteristic reflections of cubic Si structure, which reveal the crystalline structure of Si and amorphous nature of carbon shell and framework. The result is in good agreement with Raman spectral analysis results. Fig. 4b shows the Raman spectra of the Si@C microparticles before and after HF etching. Three characteristic peaks below 1000 cm⁻¹ are ascribed to the crystalline Si core. The peak at 511 cm⁻¹ is assigned to the typical Raman mode of Si. Two peaks at 1345 and 1587 cm⁻¹ correspond to the disordered D and G bands of carbon, typical of amorphous carbon and graphitized carbon, respectively.^28,29 The peak intensity at 511 cm⁻¹ of hollow Si@C sample is reduced compared to the sample before HF etching. It indicates that the Si content decreased after HF treatment and reflects the formation of the void in the Si@C nanostructure. The relatively high value of 0.99 for I_D/I_G ratio implies a low graphitization degree in the composite.³⁰ It indicates that the samples are mainly disordered carbons and coupled with partial graphite layers, which can provide better electronic conductivity as an anode material.³¹

Fig. 4 (a) XRD pattern of hollow Si@C samples after HF etching for 10 h (hollow Si@C-10 h). (b) Raman spectrum of Si@C microparticles before and after HF etching.

In order to find out the process of Si etching and the relationship between void volume in the hollow Si@C and etching time, we investigated a group of samples, including Si@C microparticles before HF etching (Si@C-0) and hollow Si@C microparticles with HF etching time of 2 h (hollow Si@C-2 h), 5 h, 10 h, 16 h and 48 h, by using TEM and thermogravimetric (TG) analysis. Fig. 5 shows the representative TEM images of the group of samples and reveals the development of increased void volume in the Si@C nanocomposites. The interspace between the Si core and the hollow carbon shell observed in the TEM images increases with HF etching time, indicating the feasibility of synthesis with this method. The Si core is completely removed after HF etching for about 48 h (Fig. 5f). As seen in Fig. 5d, the carbon sphere of Si@C-10 h has a diameter of ∼1.57 times of the diameter of the Si core, which is theoretically suitable to accommodate the volume change without breaking/degrading after repeated cycling considering the expansion coefficient of Si.³² Fig. 6 presents the thermogravimetric (TG) analysis curves of Si@C microparticles without HF etching (Si@C-0) and etching for 10 h (hollow Si@C-10 h) from 30–700 °C. The TG profiles show three decomposition steps in the temperature ranges of 30–450, 450–650, and 650–700 °C, which confirm complete carbon decomposition near 650 °C. The Si contents in Si@C-0 h and hollow Si@C-10 h are about 78% and 65%, respectively. In the inset of Fig. 6, we summarized the Si contents of the group of samples and plotted the Si mass% vs. HF etching time, from which a linear decrease of the Si contents upon the etching time is observed. The proposed method has good controllability for the synthesis of hollow Si@C nanocomposites.

Fig. 5 Representative TEM images of Si@C microparticles before HF etching (a) and hollow Si@C microparticles with HF etching time of 2 h (b), 5 h (c), 10 h (d), 16 h (e) and 48 h (f).

Fig. 6 Thermogravimetric (TG) analysis curves of Si@C microparticles without HF etching (Si@C-0) and etching for 10 h (hollow Si@C-10 h). The inset shows the relationship between HF etching time and Si content in Si@C microparticles. The Si content was determined by TG analysis.

The electrochemical performance of the electrodes with different etching time was investigated in two electrodes coin cells with Li metal as the counter electrodes. The anodes were evaluated using deep galvanostatic charge/discharge cycles from 1 to 0.01 V. All the specific capacity values in this letter are reported using the total mass of the composites electrodes. Fig. 7a shows a comparison of cycle performance of hollow Si@C-10 h, hollow Si@C-16 h and Si@C-0 h. The electrodes were tested at current density of 250 mA g⁻¹. The volume change of silicon during cycling is known to be about 400%, corresponding to a carbon shell/Si core diameter ratio of ∼1.5 for hollow Si@C nanocomposites.^31,32 Therefore, hollow Si@C-10 h has buffer voids large enough to allow the Si to expand freely without mechanical destruction of the carbon shells in theory. Obviously, Si@C-0 h possesses higher capacity in the first 20 cycles because of its higher Si content. However, upon increasing the charge and discharge cycles, the capacity shows a sustained downward trend. This is because there was no buffer voids between carbon shell and Si core resulting in destruction of the carbon shell. In contrast, hollow Si@C-10 h shows better electrochemical performance. For the first cycle of Si@C-10 h electrodes, the charge and discharge capacities reach 1727 and 1346 mA h g⁻¹. Hollow Si@C-16 h shows lower reversible capacity about 600 mA h g⁻¹ but stable cycling performance due to its low Si contents but large enough void. However, cycle stability is crucial for commercial application and the capacity of 600 mA h g⁻¹ is already very high compared with commercial anode materials. The result suggests that the capacity retention of the hollow Si@C electrodes is greatly improved by the reasonable design of the material structure. Hollow Si@C-10 h also showed excellent cycle stability under high current density. At high charge/discharge current densities ranging from 250 mA g⁻¹ to 2 A g⁻¹, high and stable capacities in the electrodes were demonstrated. Fig. 7b shows the influence of the charge/discharge currents on the capacity retention of the electrodes. With the enhanced constant current, the capacity decreases regularly with continued cycling. The reversible capacity decreases from 1329 mA h g⁻¹ at the current density of 250 mA g⁻¹ to 600 mA h g⁻¹ at the current density of 2 A g⁻¹. When the current density returns to 250 mA g⁻¹, the capacity can be largely recovered, indicating that the sample has a good electrochemical reversibility and structural integrity. Fig. 7c shows the charge/discharge voltage profile of the electrodes under different current densities corresponds to Fig. 7b. The plateaus at ∼0.2 V during discharge and at 0.45 V during charge are similar to the typical charge/discharge behavior of silicon.

Fig. 7 Electrochemical characteristics of the synthesized hollow Si@C microspheres electrodes. (a) Cycle performance comparison of Si@C-0, hollow Si@C-10 h and hollow Si@C-16 h at current densities of 250 mA g⁻¹. Capacity retention (b) and galvanostatic charge/discharge profiles (c) of hollow Si@C-10 h electrodes cycled at various current densities raging from 500 mA g⁻¹ to 2A g⁻¹.

To further characterize the electrochemical performance and elucidate lithiation/delithiation stability, cells made with hollow Si@C-10 h were subjected to both cyclic voltammetry (Fig. 8a) and electrochemical impedance spectroscopy (EIS, Fig. 8b). A broad irreversible peak in the voltage of about 0.7 V at the cathodic branch of the first cycle CV can be attributed to the formation of a solid electrolyte interphase (SEI) on the surface of active materials.³³ The peak starting at ∼0.1 V corresponds to the conversion of the Si to the Li_xSi phase. The two peaks at ∼0.38 V and 0.52 V in the anodic branch corresponds to delithiation of the Li_xSi phase to Si.^30,34 After further cycles, an additional broad peak at ∼0.19 V appears during the cathodic scan, and the anodic peaks at ∼0.38 V and 0.52 V became broader and stronger, which is a common characteristic for the transition from crystalline silicon to amorphous silicon due to lithiation/delithiation.^28,34,35 The increasing CV curve area is due to initial activation of the material, enabling more Li to react with Si.³⁴ The stability of the electrode microparticles is supported by results of the EIS spectrum taken from cells before discharge, after discharge 50 and 100 cycles, respectively. As shown in Fig. 8b, a decrease in the total resistance was observed after 50 cycles, which could be attributed to the increasing conductivity of the electrode during lithiation.^20,36,37 No evident impedance increase was detected between the 50th and 100th cycle, indicating the formation of a stable SEI layer owing to the unique electrode design and the advantage of the conductive carbon 3D framework.^7,21,27 The change of structure during the discharge/charge process is demonstrated in Fig. 9. The well-defined void space around each primary particle slows it to expand without deforming the overall morphology. The structure has several advantages for LIB alloy anodes. First, the hollow structure between carbon shell and Si core allows for the Si nanoparticle to expend without breaking the carbon coating or disrupting the SEI layer outside the secondary particle which prevent the continual rupturing and reformation of the SEI;^7,16 second, the carbon 3D framework functions as an electrical highway and a mechanical backbone so that all nanoparticles are electrochemically active;^10,15,16,20 third, most SEI formed on the outer surface of the microspheres instead of on individual nanoparticles, which not only limits the amount of SEI, but also reduce the consumption of active material.¹⁶ Furthermore, the micro-sized material may improving the energy density and coulombic efficiency which limited by low tap density and high surface area of nano-sized material.^10,15,16 An addition advantage of hollow Si@C microparticles is that the preparation method is sample and low cost.

Fig. 8 (a) Cyclic voltammograms profiles for hollow Si@C-10 h. The first five cycles are shown. (b) EIS results for hollow Si@C-10 h anode before discharge and after 50th and 100th cycles.

Fig. 9 Schematic illustration for the structure change of hollow Si@C microsphere during the discharge/charge process.

Conclusions

In conclusion, we demonstrated an encapsulation strategy and method to manage voids of Si anodes with pomegranate-shaped silicon–carbon composite structure. PAN emulsification and then carbonization is an effective way to coat the carbon layer onto the Si and form conductive and mechanical 3D framework. The hollow structure was obtained via direct etching of Si nanoparticles with HF solution. This method is sample and the production process and production cycle are greatly reduced which is more advanced and has better controllability. And the silicon particle size can be regulated at will. Meanwhile, we are free to adjust the C/Si ratio of the products. The hollow Si@C nanostructured microparticles showed excellent electrochemical performance for a scalable, electrochemically stable, and highly efficient anode. In addition to Si, the preparation method can also be applied to other high capacity alloy-type anode materials to improve cycle life and coulombic efficiency.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51125008, 11274392).

References

1. J. R. Szczech and S. Jin, Energy Environ. Sci., 2011, 4, 56–72.
2. A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon and W. Van Schalkwijk, Nat. Mater., 2005, 4, 366–377.
3. J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587–603.
4. X. Su, Q. L. Wu, J. C. Li, X. C. Xiao, A. Lott, W. Q. Lu, B. W. Sheldon and J. Wu, Adv. Energy Mater., 2014, 4, 1300882.
5. B. A. Boukamp, G. C. Lesh and R. A. Huggins, J. Electrochem. Soc., 1981, 128, 725–729.
6. M. Winter, J. O. Besenhard, M. E. Spahr and P. Novák, Adv. Mater., 1998, 10, 725–763.
7. N. Liu, H. Wu, M. T. McDowell, Y. Yao, C. M. Wang and Y. Cui, Nano Lett., 2012, 12, 3315–3321.
8. S. Y. Reece, J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers and D. G. Nocera, Science, 2011, 334, 645–648.
9. J. G. Tu, W. Wang, L. W. Hu, H. M. Zhu and S. Q. Jiao, J. Mater. Chem. A, 2014, 2, 2467–2472.
10. D. S. Jung, T. H. Hwang, S. B. Park and J. W. Choi, Nano Lett., 2013, 2092–2097.
11. Z. H. Chen, L. Christensen and J. R. Dahn, J. Electrochem. Soc., 2003, 150, A1073–A1078.
12. H. Yoo, J. I. Lee, H. Kim, J. P. Lee, J. Cho and S. Park, Nano Lett., 2011, 11, 4324–4328.
13. H. Wu, G. Y. Zheng, N. A. Liu, T. J. Carney, Y. Yang and Y. Cui, Nano Lett., 2012, 12, 904–909.
14. Z. Z. Lu, T. L. Wong, T. W. Ng and C. D. Wang, RSC Adv., 2014, 4, 2440–2446.
15. A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala and G. Yushin, Nat. Mater., 2010, 9, 353–358.
16. N. Liu, Z. Lu, J. Zhao, M. T. McDowell, H.-W. Lee, W. Zhao and Y. Cui, Nat. Nanotechnol., 2014, 3776.
17. B. Wang, X. L. Li, X. F. Zhang, B. Luo, M. H. Jin, M. H. Liang, S. A. Dayeh, S. T. Picraux and L. J. Zhi, ACS Nano, 2013, 7, 1437–1445.
18. Y. Park, N. S. Choi, S. Park, S. H. Woo, S. Sim, B. Y. Jang, S. M. Oh, S. Park, J. Cho and K. T. Lee, Adv. Energy Mater., 2013, 3, 206–212.
19. L. Su, Y. Jing and Z. Zhou, Nanoscale, 2011, 3, 3967–3983.
20. B. R. Liu, P. Soares, C. Checkles, Y. Zhao and G. H. Yu, Nano Lett., 2013, 13, 3414–3419.
21. H. Wu, G. H. Yu, L. J. Pan, N. A. Liu, M. T. McDowell, Z. A. Bao and Y. Cui, Nat. Commun., 2013, 4, 1943.
22. L. Su, Z. Zhou and M. Ren, Chem. Commun., 2010, 46, 2590–2592.
23. P. Zhang, L. Wang, J. Xie, L. Su and C.-a. Ma, J. Mater. Chem. A, 2014, 2, 3776.
24. H. Cui, Y. Sun, G. Z. Yang, G. W. Yang and C. X. Wang, Appl. Phys. Lett., 2009, 94, 083108.
25. H. Cui, C. X. Wang, G. W. Yang and D. Jiang, Appl. Phys. Lett., 2008, 93, 203113.
26. H. Cui, C. X. Wang and G. W. Yang, Nano Lett., 2008, 8, 2731–2737.
27. X. M. Ma, M. X. Liu, L. H. Gan, P. K. Tripathi, Y. H. Zhao, D. Z. Zhu, Z. J. Xu and L. W. Chen, Phys. Chem. Chem. Phys., 2014, 16, 4135–4142.
28. D. Shao, D. P. Tang, Y. J. Mai and L. Z. Zhang, J. Mater. Chem. A, 2013, 1, 15068–15075.
29. T. Doi, M. Tagashira, Y. Iriyama, T. Abe and Z. Ogumi, J. Appl. Electrochem., 2012, 42, 69–74.
30. X. Y. Zhou, J. J. Tang, J. Yang, J. Xie and L. L. Ma, Electrochim. Acta, 2013, 87, 663–668.
31. S. Chen, M. L. Gordin, R. Yi, G. Howlett, H. Sohn and D. Wang, Phys. Chem. Chem. Phys., 2012, 14, 12741–12745.
32. X. L. Li, P. Meduri, X. L. Chen, W. Qi, M. H. Engelhard, W. Xu, F. Ding, J. Xiao, W. Wang, C. M. Wang, J. G. Zhang and J. Liu, J. Mater. Chem., 2012, 22, 11014–11017.
33. J. H. Kong, W. A. Yee, Y. F. Wei, L. P. Yang, J. M. Ang, S. L. Phua, S. Y. Wong, R. Zhou, Y. L. Dong, X. Li and X. H. Lu, Nanoscale, 2013, 5, 2967–2973.
34. F. Hassan, V. Chabot, A. R. Elsayed, X. Xiao and Z. Chen, Nano Lett., 2014, 14, 277–283.
35. C. K. Chan, H. L. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui, nature nanotechnology, 2008, 3, 31–35.
36. E. Pollak, G. Salitra, V. Baranchugov and D. Aurbach, J. Phys. Chem. C, 2007, 111, 11437–11444.
37. L. Su, Y. Zhong, J. Wei and Z. Zhou, RSC Adv., 2013, 3, 9035.

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