Free-standing and binder-free sodium-ion electrodes based on carbon-nanotube decorated Li₄Ti₅O₁₂ nanoparticles embedded in carbon nanofibers

Abstract

Free-standing CNT/Li₄Ti₅O₁₂/C composite nanofibers with uniformly dispersed Li₄Ti₅O₁₂ nanoparticles and CNTs in a one-dimensional (1D) carbon nanofiber matrix were developed for high-power electrode materials in Na-ion batteries. The carbon nanofibers act as a matrix to enable better dispersion of Li₄Ti₅O₁₂ and restrict the Li₄Ti₅O₁₂ particle size at the nanoscale. The CNTs were introduced to construct a three-dimensional (3D) network between Li₄Ti₅O₁₂ particles and to enhance the conductivity of the electrode, thereby realizing the full potential of the active materials. Free-standing CNT/Li₄Ti₅O₁₂/C composite nanofibers achieved a discharge capacity of 119 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹, corresponding to 95.2% of the initial charge capacity (125 mA h g⁻¹) and a better rate capability (77 mA h g⁻¹ at 500 mA g⁻¹). This design could also be further extended to other electrode materials, which promises to promote the development of next-generation Li-ion batteries or Na-ion batteries.

---

Introduction

Nowadays, global environmental problems have attracted much attention which are mainly caused by gaseous emissions from the burning of fossil fuels and biomass. So the demand for renewable energy and utility applications are increasing.^1–4 Among them, considerable attention has been paid to Li-ion batteries (LIBs) owing to their high energy density and long service life.^5,6 However, the large scale application of LIBs is still limited by the growing price of Li resource. As an alternative, Na-ion batteries (NIBs) have attracted more attention because Na has similar physical and chemical properties and much more abundant resource than Li. However, it is difficult for many materials to accommodate sodium ions and to allow reversible and rapid ion insertion and extraction since Na ion is ca. 55% larger than Li ion.^7–10

Graphite, the widely used anode material for commercial LIBs, is not suited for a sodium based system, due to its narrow interlayer space that could not allow reversible and rapid ion insertion and extraction.¹¹ Cao et al. reported that hollow carbon nanowires with an ordered layer structure and interplanar distance of ∼0.37 nm could also act as sodium-ion insertion anode.¹² Disorder carbon was found to have both high storage capacity and good cycling performance as anode for NIBs,^13–15 but its relatively low sodium storage voltage (almost 0 V versus Na⁺/Na) easily leads to significant security issues when a slight polarization occurs. Therefore, plenty of efforts have been made to find out other proper electrode materials for NIBs.

Among the non-carbon materials, Na₂Ti₃O₇ has been examined with a discharge plateau at 0.2 V and a charge plateau at 0.4 V. However, the capacity faded fast below 100 mA h g⁻¹ after 100 cycles at 0.2 C rate.¹⁶ Li et al. reported a simple method of mixing commercial red phosphorus and carbon nanotubes could be applied for NIBs anode material, delivering a high reversible capacity of 1675 mA h g⁻¹. But they only reported the results of limited cycle numbers.¹⁷ Sodium-ion insertion in TiO₂,¹⁸ Fe₃O₄,¹⁹ Sn,²⁰ Sb²¹ and their alloys²² based anodes have also been investigated. Spinel Li₄Ti₅O₁₂ is well known as its “zero strain” feature for LIBs, exhibits excellent capacity retention and high thermal stability.²³ Recently, it was reported as an anode material for NIBs, which shows a three-phase separation mechanism during sodium ions insertion and extraction.²⁴ The average storage voltage of spinel Li₄Ti₅O₁₂ anode is ca. 0.9 V, which can avoid of the formation of SEI layers thus ensure its safety compared with hard carbonaceous materials for NIBs.²⁵ However, the poor ion transport properties hamper the practical application of Li₄Ti₅O₁₂.

To reduce the ionic and electronic transportation distance, nanosizing is a popular approach.²⁶ Another method to enhance the electronic conductivity is to dope Li₄Ti₅O₁₂ with metal nonmetal ions or construct the Li₄Ti₅O₁₂ with surface carbon coatings.^27–30 However, the introduced carbon in the Li₄Ti₅O₁₂/C composites is usually amorphous because of the poor graphitization ability of the precursors at the sintering temperature, making it very difficult to increase the rate performance of the Li₄Ti₅O₁₂, especially applied as anode for NIBs.

Carbon nanotubes (CNTs), conventional conductive additives, are particularly attractive in combination with active storage materials owing to their 1D geometry, high aspect ratio, excellent electronic conductivity, and good mechanical and chemical stability. It has been reported that creating Li₄Ti₅O₁₂/CNTs composite thus forming mixed conducting 3D networks could display highly efficient for Li storage.^31,32 In addition, to increase both high volumetric energy and power density of NIBs, it is ideal to prepare ultrathin, lightweight and flexible free-standing electrodes with nanostructures, in which all the materials could participate in charge storage. This design not only can significantly simplify cell packing by eliminating inactive ingredients such as binders and current collectors, but also ensure improved overall performance when the total volume of the device is taken into account.^33–37

In this work, we design and preparation of free-standing CNT-loaded electrospun Li₄Ti₅O₁₂/Carbon composite nanofibers (denoted as CNT/Li₄Ti₅O₁₂/C) by using a combination of electrospinning and sol–gel process. The free-standing CNT/Li₄Ti₅O₁₂/C composite can achieve improved cyclability (119 mA h g⁻¹ after 100 cycles at 100 mA g⁻¹) and rate capability when used as anode for NIBs, which attribute to the special double carbon decorated structure providing special advantages. Firstly, the carbon nanofibers matrix acts as an inhibitor between the Li₄Ti₅O₁₂ particles and prevents the particle growth. This shortens the charge transfer distance and leads to increased sodium diffusion coefficient. Secondly, CNTs are incorporated into the composite nanofibers to further increase the graphitization and electrical properties of the composite nanofibers. Thirdly, the homogeneously dispersed CNTs can offer well conducting three-dimensional (3D) network between Li₄Ti₅O₁₂ particles and enhance the conductivity of the electrode.^38–43

Experimental section

Fabrication of the nanofibers via electrospinning and thermal treatment

Firstly, MWCNTs were carboxyl functionalized by refluxing with concentrated HNO₃ as reported.⁴⁴ In a typical synthesis, 0.075 g functionalized MWCNTs were dispersed in 50 mL PVP–ethanol solution (9.1 wt%). After stirring for 6 h, Li(CH₃COO)₂·H₂O (1.23 g, 12 mmol), titanium(IV) oxyacetylacetonate (3.95 g, 15 mmol) were added. The mixture was kept under stirring for 30 h to form a homogenous and yellow precursor solution. The obtained solution was poured into a syringe with a 20-gauge metal needle. A variable high voltage power supply was used to provide a high voltage (around 16 kV) for electrospinning with 0.6 mL h⁻¹ and the distance between the needle tip and the collector is 15 cm. The ambient humidity was maintained at around 50%. The as-spun fibers were collected and then calcined at 280 °C for 2 h and 700 °C for 12 h under Ar atmosphere with a heating rate of 1 K min⁻¹, thus the CNT-loaded Li₄Ti₅O₁₂ embedded in carbon nanofibers were obtained (denoted as CNT/Li₄Ti₅O₁₂/C). For comparison, electrospun Li₄Ti₅O₁₂/C composite nanofibers were also prepared without adding MWCNTs.

Characterization

The structure characterizations of the samples were studied by X-ray diffraction (XRD, Philips X′pert Pro Super) using Cu Kα radiation. The morphology and crystalline structure of obtained samples were characterized with field emission scanning electron microscopy (FESEM, JEOL JSM-6700, Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM, JEOL 4000EX, Tokyo, Japan). The weight content of carbon in CNT/Li₄Ti₅O₁₂/C nanofibers was measured by elementar analyzer (Vario EL III, Germany). In addition, Raman spectra were recorded on LabRam HR excited by a 514.5 nm laser in order to understand the structural change before and after the addition of MWCNTs.

Electrochemical measurement

The calcined nanofibers were tailored to pieces of sheet and directly used as the working electrodes. The weight of each piece of electrode is around 1 mg. Coin-type cells (CR2032) were assembled in an argon-filled glove box (MBRAUN LABMASTER 130) in which oxygen and moisture concentrations were kept below 0.1 ppm. Na foils were used as counter electrodes. The electrolyte was 1 M NaClO₄ in propylene carbonate (PC) and glass fibers (GF/D) from Whatman were used as separators. Cyclic Voltammetry (CV) measurements were performed between 0.5 ∼ 3.0 V (vs. Na/Na⁺) at a scan rate of 0.1 mV s⁻¹ on a CHI-660D electrochemical work station (Chenhua Instrument Company, Shanghai, China). The galvanostatic charge–discharge tests were conducted at a voltage window of 0.5–3.0 V (vs. Na/Na⁺) with a NEWARE battery test system. The impedance spectra of the cells were carried out with the CHI 660D at charging state of 50% of the capacity after the first 3 cycles. The frequency ranged from 0.01 Hz to 100 kHz and the AC amplitude was 5.0 mV.

Result and discussion

The obtained CNT/Li₄Ti₅O₁₂/C (or Li₄Ti₅O₁₂/C) composite was cut and directly used as anode for NIBs (Fig. 1a). Fig. 1b shows the XRD patterns of CNT/Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C composite nanofibers produced at 700 °C for 12 h, respectively. Both samples displayed identical phase Li₄Ti₅O₁₂. The major identified peaks at 2θ = 18.4, 35.6, 43.2, 47.4, 57.3, 62.8 and 66.3°, correspond to the (111), (311), (400), (331), (333), (440) and (531) planes of a face-centered cubic spinel Li₄Ti₅O₁₂ structure with the Fd3m space group respectively, which can be perfectly indexed to JCPDS card no. 49-0207. The addition of CNTs does not change the phase structure but increase the crystallinity of the composite. There are also small amount impurity phase of TiO₂ (Rutile, JCPDS no. 21-1276) in the composite nanofibers. No peaks of CNTs was detected due to the trace amount of CNTs (mass ratio TiO₂ : CNT = 16 : 1).

Fig. 1 (a) Digital image of free-standing CNT/Li₄Ti₅O₁₂/C electrode. (b) XRD patterns of the Li₄Ti₅O₁₂/C and CNT/Li₄Ti₅O₁₂/C nanocomposite. (c) Raman spectra for the Li₄Ti₅O₁₂/C and CNT/Li₄Ti₅O₁₂/C nanocomposite.

Fig. 1c shows the representative Raman spectrum of the obtained CNT/Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C composite. Both nanofibers display well-known D-band in the range of 1250–1450 cm⁻¹ and G-band between 1550 and 1660 cm⁻¹. The characteristic band at 1350 cm⁻¹ (D-band) is related to the disorder features due to the finite particle size effect or lattice distortion of the graphite crystals while the band at 1595 cm⁻¹ (G-band) is related to the vibration of sp²-bonded carbon atoms in a two-dimensional hexagonal lattice.⁴⁵ The relative intensities (R = I_D/I_G) can be used to analyze the amount of carbon defects in the CNFs. The R value for CNT/Li₄Ti₅O₁₂/C is 1.002, which is lower than that in the Li₄Ti₅O₁₂/C spectrum (R = 1.017), indicating a more graphitic or crystalline carbon structure.⁴⁶ In addition, CNT free sample (Li₄Ti₅O₁₂/C nanofibers) shows higher amount of disordered sections and defects.^47,48 Elemental analysis (Table 1) reveals that the CNT/Li₄Ti₅O₁₂/C composite contains approximately 9.46 wt% of carbon, 0.9 wt% of hydrogen and trace amounts of nitrogen.

Table 1 Elemental analysis result of carbon, hydrogen and nitrogen content in the CNT/Li₄Ti₅O₁₂/C composite

Sample	C [wt%]	H [wt%]	N [wt%]
CNT/Li4Ti5O12/C	9.46	0.93	0.31

---

The FE-SEM image of as-spun CNT free nanofibers is displayed in Fig. 2a. The nanofibers align randomly and show a long and individual fibrous morphology with uniform diameters ranging from 200–300 nm. The nanofibers exhibit a smooth surface and interweave, forming a network structure. The morphology of as-spun nanofibers with CNTs shows similar structure.

Fig. 2 FE-SEM images of the electrospun nanofibers (a), Li₄Ti₅O₁₂/C (b) and CNT/Li₄Ti₅O₁₂/C (c). The inset images are corresponding high magnification pictures, respectively.

As shown in Fig. 2b and c, both of Li₄Ti₅O₁₂/C and CNT/Li₄Ti₅O₁₂/C nanofibers show similar fibrous nanostructure (diameter: 200–400 nm) and keep interconnected after calcination. Compared with as-spun nanofibers, the morphological changes from straight and continuous to break and bunch together after heat treatment, which results from a large weight loss accompanied with gas evolution from the polymer matrix. The high magnification view (inset images of Fig. 2b and c) display the surfaces of these fibers are rough and there are many tiny particles embedded in the fibers. In addition, there is slightly morphology difference in the fiber diameter between Li₄Ti₅O₁₂/C and CNT/Li₄Ti₅O₁₂/C nanofibers, which caused by the viscosity enhancement due to the presence of CNTs in the precursor solution.^49,50

Transmission electron microscopy (TEM) examination was conducted on the CNT/Li₄Ti₅O₁₂/C fibers. As shown in Fig. 3a, the fibers display interconnected structure and the CNTs were well oriented along the fiber axis. The high-resolution Transmission electron microscopy (HRTEM, Fig. 3b) reveals that the Li₄Ti₅O₁₂ nanoparticles (sub 10 nm in diameter) were dispersed in the carbon fiber matrix. The distance between neighboring fringes in the crystalline grains was measured to be 0.48 nm by corresponding with the interplanar spacings between the (111) planes of spinel Li₄Ti₅O₁₂. The carbon matrix effectively prevents the aggregation of the Li₄Ti₅O₁₂ and the smaller particles size shortens the diffusion length of Na⁺/e⁻.

Fig. 3 TEM image (a) and HRTEM image (b) of CNT/Li₄Ti₅O₁₂/C.

The Na-ion insertion–extraction behaviors of CNT/Li₄Ti₅O₁₂/C were investigated by cyclic voltammetry (Fig. 4a). One reduction peak at ∼0.66 V and one oxidation peak at ∼1.08 V can be observed in the first three cycles, which can be ascribed to sodium-ions insertion–extraction in the Li₄Ti₅O₁₂ nanoparticles.²⁵ The difference of CV curves between the first reduction process and the other two reduction processes originates from the irreversible sodium insertion into functional groups on the surface of the amorphous carbon nanofiber derived from the carbonization process of the polymer (PVP). The CV curves of the second and third cycles almost overlap, implying an excellent cyclability of CNT/Li₄Ti₅O₁₂/C.

Fig. 4 (a) CV curves of the free-standing CNT/Li₄Ti₅O₁₂/C electrodes at a scanning rate of 0.1 mV s⁻¹. (b–d) Electrochemical performance of Li₄Ti₅O₁₂/C and CNT/Li₄Ti₅O₁₂/C electrodes cycled between 0.5 V and 3 V vs. Na⁺/Na. (b) Charge–discharge curves of the 1^st cycle at a current rate of 100 mA g⁻¹; (c) Cycling performance at current density of 100 mA g⁻¹. (d) Discharge–charge capacity at different current densities (increased from 50 mA g⁻¹ to 500 mA g⁻¹).

Fig. 4b shows the voltage profiles of free-standing Li₄Ti₅O₁₂/C and CNT/Li₄Ti₅O₁₂/C electrodes in the range of 0.5–3.0 V (vs. Na⁺/Na) at a current density of 100 mA g⁻¹. CNT/Li₄Ti₅O₁₂/C composite nanofibers display voltage plateaus at around 1.0 V (charging) and 0.8 V (discharging), respectively, which are the characteristic behavior of a three-phase separation mechanism during Na⁺ intercalation and deintercalation. By contrast, Li₄Ti₅O₁₂/C composite nanofibers shows higher polarization and its plateaus are at around 1.1 V (charging) and 0.7 V (discharging), respectively. It was found that the first discharge step delivers a specific capacity of 180 mA h g⁻¹ for CNT/Li₄Ti₅O₁₂/C and 127 mA h g⁻¹ for Li₄Ti₅O₁₂/C. While the first charge capacities of the CNT/Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C are 125 and 77 mA h g⁻¹, respectively. One reason for the higher initial capacity of CNT/LTO/C is attributed to the lower current resistance and charge transfer resistance (EIS results in Fig. 5), leading to the higher electronic conductivity that calculated by the formula σ = d/AR (σ is the conductivity, R is the resistance, d is the diameter and A is the specific surface area).³¹ As a result, CNT/LTO/C shows improved kinetic properties. Another reason is believed to be the higher specific surface area of CNT resulting in more surface storage of sodium. The initial Coulombic efficiencies of CNT/Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C were 70% and 61%, respectively. The low Coulombic efficiency and the irreversible capacity in the first cycle are mainly due to the titanium oxide impurities.⁵¹ After several cycles, the Coulombic efficiency could approach ∼100% for both electrodes.

Fig. 5 The AC impedance spectra of Li₄Ti₅O₁₂/C and CNT/Li₄Ti₅O₁₂/C electrodes. Inset is the amplification curves of the impedance in the high-frequency region.

Fig. 4c shows the capacity retention of CNT/Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C electrodes at a current density of 100 mA g⁻¹. Obviously, the CNT/Li₄Ti₅O₁₂/C electrode, with conductive CNTs additive, shows an even more stable cycling performance. It delivered a steady-state reversible capacity of 119 mA h g⁻¹ after 100 cycles. For comparison, the specific capacity of CNT-free nanofibers composite decreased gradually from 84 mA h g⁻¹ after the first 3 cycles to 47 mA h g⁻¹ after 100 cycles under the same condition.

Fig. 4d shows rate capability for the CNT/Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C electrodes upon cycling at different current densities of 50, 100, 200, and 500 mA g⁻¹. At each current, the battery was tested for 10 cycles to ensure the reliability of the results. The specific capacity was stable at a constant current rate, while changes in current density resulted in stepwise dependence of the discharge capacity. When cycling at high current density of 200 mA g⁻¹ and 500 mA g⁻¹, CNT/Li₄Ti₅O₁₂/C also delivered capacities of about 117 mA h g⁻¹ and 77 mA h g⁻¹, respectively. In contrast, the capacity of Li₄Ti₅O₁₂/C decreased promptly at each rate, dropping to 1 mA h g⁻¹ at 500 mA g⁻¹. It can be inferred that CNTs play a significant role in improving the reaction kinetics of Li₄Ti₅O₁₂, especially, at high current rates.

To verify these, half charged CNT free and CNT-decorated Li₄Ti₅O₁₂/C nanofibers electrodes were analyzed using electrochemical impedance spectroscopy (EIS) after cycled for 3 times at a current density of 100 mA g⁻¹ (Fig. 5). In the low-frequency region, straight and oblique lines indicate the Warburg impedance (R_w), corresponding to Na-ion diffusion through the free-standing electrodes. In the high-frequency and medium-frequency region (inset image of Fig. 5), depressed semicircles were observed for both samples, which is attributed to the charge transfer (R_ct) resistance at the active material interface. The increase in the semicircle radius indicates the enhancement in charge transfer resistance. After introduction of CNTs, the R_ct value decreased effectively, indicating that the CNT/Li₄Ti₅O₁₂/C electrode shows better reaction kinetics of Na-ion insertion/extraction during electrochemical cycling.^52–54

Furthermore, all the materials of our flexible freestanding and binder-free electrode could participate in sodium storage, which could improve the volumetric energy and power density of the whole cell. The introduction of carbon black additives and binders to electrode will definitely result in the lower overall energy density of battery.

Thus the relatively good electrochemical performance of CNT/Li₄Ti₅O₁₂/C nanofibers composite can be attributed to the synergistic effects of nanoparticles embedded in 1D nanofibers and conductive CNTs additive: (i) Super tiny Li₄Ti₅O₁₂ nanoparticles shorten the diffusion length of Na⁺/e⁻. (ii) Conductive CNTs in the nanofibers improve the conductivity of the electrode, thus introduce fast electron and ion transport and enhance the electrode reaction kinetics as well as reduce the polarization. (iii) The function of carbon fiber could be favorable to keep good electronic contact for the electrode occurring large volume variation and can act as buffer medium to maintain the electrode stability.^55–57 (iv) The free-standing electrode is thin and lightweight, which could effectively increase volumetric and total gravimetric energy density.

Conclusions

In summary, free-standing CNT/Li₄Ti₅O₁₂/C composite nanofibers with uniformly dispersed Li₄Ti₅O₁₂ nanoparticles and CNTs in 1D carbon nanofibers matrix was developed for high-power electrode materials in Na-ion battery. The use of 1D carbon nanofibers matrix, which forms a conducting 3D nano-network, enables both Na⁺ and electrons to migrate and reach each of the Li₄Ti₅O₁₂ particles, hence realizing the full potential of the active materials. The well-dispersed CNTs in the matrix construct conducting bridges between Li₄Ti₅O₁₂ particles and enhancing the electron transport of the whole electrode. In comparison with the CNT-free electrode, CNT/Li₄Ti₅O₁₂/C nanofibers composite electrode delivered a much higher discharge capacity (119 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 100 cycles) and a better rate capability (77 mA h g⁻¹ at 500 mA g⁻¹). This design could increase volumetric and total gravimetric energy density and also be further extended to other electrode materials, which promises to promote the development of next-generation LIBs or NIBs.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no.21171015, no.21373195, and no.11174334), the “1000 plan” from Chinese Government, program for New Century Excellent Talents in University (NCET), the Fundamental Research Funds for the Central Universities (WK2060140014 & WK2060140016) and sofja kovalevskaja award from Alexander von Humboldt Foundation. L. G. acknowledges the financial support from the “Hundred Talents” program from the Chinese Academy of Sciences.

References

1. H. Ibrahim, A. Ilinca and J. Perron, Renewable Sustainable Energy Rev., 2008, 12, 1221–1250.
2. B. Dunn, H. Kamath and J.-M. Tarascon, Science, 2011, 334, 928–935.
3. M. Contestabile, G. J. Offer, R. Slade, F. Jaeger and M. Thoennes, Energy Environ. Sci., 2011, 4, 3754–3772.
4. Z. Yang, J. Zhang, M. C. W. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon and J. Liu, Chem. Rev., 2011, 111, 3577–3613.
5. J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587–603.
6. J. M. Tarascon and M. Armand, Nature, 2001, 414.
7. S. Komaba, C. Takei, T. Nakayama, A. Ogata and N. Yabuuchi, Electrochem. Commun., 2010, 12, 355–358.
8. Y. Cao, L. Xiao, W. Wang, D. Choi, Z. Nie, J. Yu, L. V. Saraf, Z. Yang and J. Liu, Adv. Mater., 2011, 23, 3155–3160.
9. P. Barpanda, J. Lu, T. Ye, M. Kajiyama, S.-C. Chung, N. Yabuuchi, S. Komaba and A. Yamada, RSC Adv., 2013, 3, 3857–3860.
10. J. Barker, M. Y. Saidi and J. L. Swoyer, Electrochem. Solid-State Lett., 2003, 6, A1–A4.
11. J. Sangster, J. Phase Equilib. Diffus., 2007, 28, 571–579.
12. Y. Cao, L. Xiao, M. L. Sushko, W. Wang, B. Schwenzer, J. Xiao, Z. Nie, L. V. Saraf, Z. Yang and J. Liu, Nano Lett., 2012, 12, 3783–3787.
13. K. Tang, L. Fu, R. J. White, L. Yu, M.-M. Titirici, M. Antonietti and J. Maier, Adv. Energy Mater., 2012, 2, 873–877.
14. L. Fu, K. Tang, K. Song, P. A. van Aken, Y. Yu and J. Maier, Nanoscale, 2014, 6, 1384–1389.
15. W. Li, L. Zeng, Z. Yang, L. Gu, J. Wang, X. Liu, J. Cheng and Y. Yu, Nanoscale, 2014, 6, 693–698.
16. A. Rudola, K. Saravanan, C. W. Mason and P. Balaya, J. Mater. Chem. A, 2013, 1, 2653.
17. W.-J. Li, S.-L. Chou, J.-Z. Wang, H.-K. Liu and S.-X. Dou, Nano Lett., 2013, 13, 5480–5484.
18. Y. Xu, E. Memarzadeh Lotfabad, H. Wang, B. Farbod, Z. Xu, A. Kohandehghan and D. Mitlin, Chem. Commun., 2013, 49, 8973.
19. S. Komaba, T. Mikumo and A. Ogata, Electrochem. Commun., 2008, 10, 1276–1279.
20. Y. Xu, Y. Zhu, Y. Liu and C. Wang, Adv. Energy Mater., 2013, 3, 128–133.
21. J. Qian, Y. Chen, L. Wu, Y. Cao, X. Ai and H. Yang, Chem. Commun., 2012, 48, 7070.
22. L. Xiao, Y. Cao, J. Xiao, W. Wang, L. Kovarik, Z. Nie and J. Liu, Chem. Commun., 2012, 48, 3321.
23. T. Ohzuku, J. Electrochem. Soc., 1995, 142, 1431.
24. Y. Sun, L. Zhao, H. Pan, X. Lu, L. Gu, Y.-S. Hu, H. Li, M. Armand, Y. Ikuhara, L. Chen and X. Huang, Nat. Commun., 2013, 4, 1870.
25. J. Liu, K. Tang, K. Song, P. A. van Aken, Y. Yu and J. Maier, Phys. Chem. Chem. Phys., 2013, 15, 20813–20818.
26. X. Yu, H. Pan, W. Wan, C. Ma, J. Bai, Q. Meng, S. N. Ehrlich, Y.-S. Hu and X.-Q. Yang, Nano Lett., 2013, 13, 4721–4727.
27. L. Shen, H. Li, E. Uchaker, X. Zhang and G. Cao, Nano Lett., 2012, 12, 5673–5678.
28. G. Du, N. Sharma, V. K. Peterson, J. A. Kimpton, D. Jia and Z. Guo, Adv. Funct. Mater., 2011, 21, 3990–3997.
29. K.-S. Park, A. Benayad, D.-J. Kang and S.-G. Doo, J. Am. Chem. Soc., 2008, 130, 14930–14931.
30. Y.-Q. Wang, L. Gu, Y.-G. Guo, H. Li, X.-Q. He, S. Tsukimoto, Y. Ikuhara and L.-J. Wan, J. Am. Chem. Soc., 2012, 134, 7874–7879.
31. J. Shu, L. Hou, R. Ma, M. Shui, L. Shao, D. Wang, Y. Ren and W. Zheng, RSC Adv., 2012, 2, 10306–10309.
32. H.-K. Kim, J.-Y. Kim and K.-B. Kim, Meeting Abstracts, 2010, MA2010–02, 230.
33. Y. Li, B. Tan and Y. Wu, J. Am. Chem. Soc., 2006, 128, 14258–14259.
34. B. Liu, J. Zhang, X. Wang, G. Chen, D. Chen, C. Zhou and G. Shen, Nano Lett., 2012, 12, 3005–3011.
35. J.-Z. Wang, S.-L. Chou, J. Chen, S.-Y. Chew, G.-X. Wang, K. Konstantinov, J. Wu, S.-X. Dou and H. K. Liu, Electrochem. Commun., 2008, 10, 1781–1784.
36. L. Fu, K. Tang, C.-C. Chen, L. Liu, X. Guo, Y. Yu and J. Maier, Nanoscale, 2013, 5, 11568–11571.
37. R. Wang, C. Xu, J. Sun, Y. Liu, L. Gao and C. Lin, Nanoscale, 2013, 5, 6960–6967.
38. Y. Yu, L. Gu, C. Zhu, P. A. van Aken and J. Maier, J. Am. Chem. Soc., 2009, 131, 15984–15985.
39. C. Zhu, Y. Yu, L. Gu, K. Weichert and J. Maier, Angew. Chem., Int. Ed., 2011, 50, 6278–6282.
40. J. Wang, Y. Yu, L. Gu, C. Wang, K. Tang and J. Maier, Nanoscale, 2013, 5, 2647–2650.
41. S. Kedem, J. Schmidt, Y. Paz and Y. Cohen, Langmuir, 2005, 21, 5600–5604.
42. S. Xiao, M. Shen, R. Guo, Q. Huang, S. Wang and X. Shi, J. Mater. Chem., 2010, 20, 5700–5708.
43. L. Zhao, J. Ni, H. Wang and L. Gao, RSC Adv., 2013, 3, 6650–6655.
44. Z. Peining, A. S. Nair, Y. Shengyuan and S. Ramakrishna, Mater. Res. Bull., 2011, 46, 588–595.
45. L. Aldon, P. Kubiak, M. Womes, J. C. Jumas, J. Olivier-Fourcade, J. L. Tirado, J. I. Corredor and C. Pérez Vicente, Chem. Mater., 2004, 16, 5721–5725.
46. E. J. Ra, K. H. An, K. K. Kim, S. Y. Jeong and Y. H. Lee, Chem. Phys. Lett., 2005, 413, 188–193.
47. C. Kim, Y. I. Jeong, B. T. N. Ngoc, K. S. Yang, M. Kojima, Y. A. Kim, M. Endo and J.-W. Lee, Small, 2007, 3, 91–95.
48. C. Kim, K. S. Yang, M. Kojima, K. Yoshida, Y. J. Kim, Y. A. Kim and M. Endo, Adv. Funct. Mater., 2006, 16, 2393–2397.
49. D. Wu, T. Shi, T. Yang, Y. Sun, L. Zhai, W. Zhou, M. Zhang and J. Zhang, Eur. Polym. J., 2011, 47, 284–293.
50. L. Ji, A. J. Medford and X. Zhang, J. Mater. Chem., 2009, 19, 5593–5601.
51. S.-L. Chou, J.-Z. Wang, H.-K. Liu and S.-X. Dou, J. Phys. Chem. C, 2011, 115, 16220–16227.
52. S.-W. Kim, J. Ryu, C. B. Park and K. Kang, Chem. Commun., 2010, 46, 7409–7411.
53. O. Toprakci, H. A. Toprakci, L. Ji, G. Xu, Z. Lin and X. Zhang, ACS Appl. Mater. Interfaces, 2012, 4, 1273–1280.
54. C. Gong, Z. Xue, X. Wang, X.-P. Zhou, X.-L. Xie and Y.-W. Mai, J. Power Sources, 2014, 246, 260–268.
55. L. Ji, K.-H. Jung, A. J. Medford and X. Zhang, J. Mater. Chem., 2009, 19, 4992–4997.
56. R. von Hagen, H. Lorrmann, K.-C. Möller and S. Mathur, Adv. Energy Mater., 2012, 2, 553–559.
57. L. Wu, X. Hu, J. Qian, F. Pei, F. Wu, R. Mao, X. Ai, H. Yang and Y. Cao, Energy Environ. Sci., 2014, 7, 323–328.

---