Multi-shelled NiO hollow spheres: Easy hydrothermal synthesis and lithium storage performances

Journal of Alloys and Compounds 685 (2016) 8e14

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Multi-shelled NiO hollow spheres: Easy hydrothermal synthesis and lithium storage performances Hongjing Wu a, *, Yiqun Wang a, Chenhui Zheng a, Jinmeng Zhu b, Guanglei Wu a, c, **, Xuanhua Li b, *** a b c

School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an, PR China School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, PR China Institute of Energy and Environmental Materials, College of Materials Science and Engineering, Qingdao University, Qingdao, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 February 2016 Received in revised form 7 May 2016 Accepted 23 May 2016 Available online 26 May 2016

Complex metal oxide hollow structures are mostly prepared by hard template-based multistep procedures. It is still desirable yet challenging to develop new efficient strategies to fabricate high-quality complex metal oxide hollow structures. Herein, uniform multi-shelled NiO hollow spheres were synthesized; especially quintuple-shelled NiO hollow spheres were synthesized for the first time by a simple shell-by-shell self-assembly capable of controlling the shell numbers, which are achieved by controlling the heat treatment. The research makes a significant contribution to the synthetic methodology of multishelled hollow structures and opens up new opportunities for deeper understanding formation mechanism of the shell-by-shell self-assembly. The obtained quadruple/quintuple-shelled NiO hollow spheres show promising electrochemical performance in anodic lithium storage for Li-ion battery. © 2016 Elsevier B.V. All rights reserved.

Keywords: Nickel oxide Multi-shell Hydrothermal synthesis Anode material

1. Introduction Nanostructured materials have attracted interest in both fundamental and applied research areas due to their unexpected physicochemical properties. Recently, significant effort has been devoted to the fabrication of hollow spheres with multi-shells [1e12]. Different metal oxides with multi-shelled hollow-sphere structures have been synthesized on the basis of the soft/hardtemplate approaches [13]. Multi-shelled metal oxide hollow spheres with attributes such as large surface area and hollow interior as well as properly designed multi-shelled hollow micro-/ nanostructures represent an ideal 3D structure and will be useful as battery materials, supercapacitors, and so forth [14e18]. Despite the progress achieved to date, it is still desirable yet challenging to develop new efficient strategies to fabricate high-quality complex multi-shelled metal oxide hollow structures. Nickel oxide (NiO) is an antiferromagnetic semiconductor with a

* Corresponding author. ** Corresponding author. School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an, PR China. *** Corresponding author. E-mail addresses: [email protected] (H. Wu), wuguanglei@mail. xjtu.edu.cn (G. Wu), [email protected] (X. Li). http://dx.doi.org/10.1016/j.jallcom.2016.05.264 0925-8388/© 2016 Elsevier B.V. All rights reserved.

wide band gap of ~3.6 eV, which has been extensively studied due to its high capacity, environmental friendliness, low cost, and natural abundance [19]. Recently, various metal oxides have been widely studied as anode materials for lithium ion battery (LIB) applications owing to their high theoretical capacities [20e24]. Among them, nanostructures of NiO with high surface areas, short paths for lithium ion transport, and high reactivity for lithium ions intercalation/deintercalation can meet the requirements of new generation lithium ion batteries [22], and various NiO nanomaterials have been investigated as high performance electrode materials for lithium ion batteries, including nanoparticles [21,25], one-dimensional nanostructures (nanorods, nanowires and nanotubes) [26e30], two-dimensional nanostructures (nanobelts and nanosheets) [31e34], and three-dimensional nanostructures (mesoporous materials) [35,36]. However, to the best of our knowledge, there has been no report about multi-shelled NiO hollow spheres for rechargeable lithium ion batteries (LIBs). In this paper, we report a simple yet efficient method to create multi-shelled NiO hollow spheres. The potential use of such fabricated multi-shelled NiO hollow spheres in Li-ion battery was investigated in detail. An attribute of this synthesis approach is the feasibility of straightforward strategy to prepare multi-shelled NiO hollow spheres with a controlled number of shells. Remarkably, complex quadruple/quintuple-shelled NiO hollow spheres showed

H. Wu et al. / Journal of Alloys and Compounds 685 (2016) 8e14

9

better capacity retention and higher rate capability on cycling compared with other NiO samples. After the high-rate chargedischarge cycling, a specific capacity as high as ~548 mA h g
1 can be restored when the current density is reduced to 100 mA g
1.

frequency range from 0.01 Hz to 100 kHz.

2. Experimental

3.1. Characterization of multi-shelled NiO hollow spheres

2.1. Synthesis

XRD analysis was carried out to identify the crystal structure, composition, and orientation of the prepared multi-shelled NiO hollow spheres. As showed in Fig. 1, the prepared NiO precursor is gradually converted into NiO as the calcination temperature increases to 350  C. All the NiO samples display a similar facecentered cubic (FCC with a lattice parameter a ¼ 4.18 Å) NiO phase (ICDD No. 71-1179), except one broad peak centered at 2q around 29.8 owing to the amorphous carbon. TG analysis was used to further monitor the underlying phase transformation from NiO precursor to NiO. Fig 2 shows the TG and DSC curves of the prepared NiO precursor with temperature increments from 25 to 800  C. The weight loss between 25 and 200  C can be attributed primarily to the release of residual and adsorbed water, along with the dissociation of a small amount of intercalated species (e.g., H2O) or adsorbed anions [37]. A sharp weight loss of ~82.1% at the temperature range of 300e450  C is observed, which may be attributed to the removal of the amorphous carbon. As further confirmed by the DSC curve, a strong endothermic peak can be obtained around 400  C. Beyond that temperature, no distinct weight loss presents, suggesting that the resultant NiO crystal shows an excellent thermal stability at high temperatures. Raman spectroscopy is a powerful tool to determine the structure of carbonaceous nanomaterials. Two obvious peaks, located around 1397 and 1592 cm
1 are found in uncalcined and lowtemperature calcined NiO samples (Fig. 3), which can be attributed to the D and G bands of carbon, respectively. A strong band around 1397 cm
1 attributed to the in-plane vibrations of disordered amorphous carbon (D band) is observed, reflecting degree of defects and disorder in the materials. While the other strong band around 1592 cm
1 attributed to the in-plane vibrations of crystalline graphic carbon (G band) reflects the degree of order. The bands observed at 555, 1129 and 1598 cm
1 for calcined NiO samples at 430 and 550  C are ascribed to the longitudinal optical (LO) phonon mode, 2LO combination phonon mode, and two magnon (2 M) excitation in NiO, respectively. Raman results from the NiO samples demonstrate that the NiO precursor remains relatively stable with

Reagents: The reagents including D-glucose and Ni(NO3)2$6H2O were all analytical grade in purity, and bought from Sinopharm Chemical Reagent Co., Ltd. Utrapure water (Millipore Milli-Q grade) with a resistivity of 18.2 MU was used in all the experiments. Synthesis of multi-shelled NiO hollow spheres: In a typical experiment, 0.01 mol D-glucose and 0.02 mol Ni(NO3)2$6H2O were dissolved in 50 ml utrapure water. Then, the solution was homogenized by vigorous stirring. After stirring for 30 min, the resultant mixture was transferred to a 100 ml Teflon-lined autoclave followed by hydrothermal treatment at 180  C for 20 h. The obtained products were washed and filtered off several times using utrapure water and ethanol successively, and finally dried in a vacuum oven at 80  C for 12 h. After synthesis, the products were subjected to annealing at 270, 350, 430 or 550  C for 3 h in air with a heating rate of 2  C min
1 from room temperature to obtain multi-shelled NiO hollow spheres with a controlled number of shells (solid NiO sphere, double-, triple- and quadruple/quintuple-shelled NiO hollow sphere, respectively). 2.2. Characterization X-ray diffraction (XRD) analysis was performed on a Rigaku D/ Max 2500 equipment with Cu Ka radiation operated at 40 kV and 40 mA. Thermogravimetry (TG) and differential scanning calorimetry (DSC) measurements were performed in air with a heating rate of 10  C min
1. Raman spectra were recorded using a Renishaw instrument inVia Reflex with a laser wavelength of 514 nm. The specific surface areas of the as-prepared samples were measured by the Brunauer-Emmett-Teller (BET) method using nitrogen adsorption and desorption isotherms. Pore size distribution was derived from the desorption branch by the BJH method. The morphologies and structures of the as-prepared samples were visualized by using a FEI Quanta 600 FEG scanning electron microscope (SEM) and a FEI Tecnai G2 F20 S-TWIN transmission electron microscope (TEM), and an energy-dispersive X-ray spectroscopy (EDS) attachment was used to obtain the surface element composition.

3. Results and discussion

2.3. Electrochemical analysis For evaluating the electrochemical performance of the materials, a mixture of 70 wt% NiO, 20 wt% acetylene black (AB) and 10 wt% polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP) solvent was taken as the slurry for casting. The slurry was cast on the Cu foil followed by drying at 80  C in a vacuum oven for 12 h. The electrochemical properties of the electrodes were measured by assembling them into coin cells (CR 2023) in an argonfilled glove box. Lithium foil was used as a counter electrode and a polypropylene (PP) film as the separator. The electrolyte was made from 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume). The charging/discharging behavior of all the cells was galvanostatically cycled between 3 and 0.01 V. Capacity retention tests of the assembled cells were carried out for the required rates. Rate capability tests of the cells were performed by changing the rate from 100 to 600 mA g
1 for each 5 cycles. Electrochemical impedance spectroscopy (EIS) measurements were carried out on the sample electrochemical workstation with the

Fig. 1. Wide-angle XRD patterns of the resultant NiO precursor calcined at different temperatures.

10

H. Wu et al. / Journal of Alloys and Compounds 685 (2016) 8e14

Fig. 2. TGA and DSC curves of the uncalcined NiO sample.

Fig. 3. Raman spectra of the resultant NiO precursor calcined at different temperatures.

negligible Raman structural change under the low-temperature calcinations but starts Raman structural transformation after being subject to high-temperature calcination at 430 and 550  C. This conclusion is in good agreement with XRD and TG results. Fig. 4 shows the morphologies of the prepared NiO precursor and NiO hollow spheres with a controlled number of shells upon calcination temperature. The NiO hollow spheres with various shells are harvested separately. Relative to the carbonaceous precursor before heating in Fig. 4a, the TEM image in Fig. 4b shows metal oxide hydrate to ready form outer surface shell (Fig. 4c) when the sample is heated to 270  C as well as the decrease in diameter from 7.3 to 4.7 mm. Interestingly, a heating temperature of 350  C leads to noticeable hollow structures with a distinguishable smaller shell side the outer shell for each particle, a so-called doubleshelled NiO hollow sphere, which has been rare in one-pot synthesis process [38] (Fig. 4d). When the temperature reaches 430  C,

the inner solid core sharply contracts and changes to a core-shell sphere, resulting in a triple-shelled NiO hollow sphere (Fig. 4e). Increasing the temperature further to 550  C leads to a quadrupleshelled NiO hollow sphere (Fig. 4feg). Impressively, we demonstrate that the number of NiO multi-shells can reach a maximum of 5 multi-shells at 550  C (Fig. 4h). By the method described above, we can obtain a small quantity of high specific surface quintupleshelled NiO hollow spheres (Fig. 4h). Elemental analysis by EDX reveals that the elements Ni and O are homogeneously distributed inside the quintuple-shelled NiO hollow spheres, and the individual shells are enriched with Ni and O (Fig. 4h, (1-4)). N2 adsorption-desorption isotherms and the related pore diameter distribution have been measured to further elucidate the multi-shelled hollow structure in NiO. As shown in Fig. 5, all the synthesized NiO samples exhibit type-IV isotherm plots with a sharp capillary condensation steps, indicative of mesoporous structures. Typically, the BET surface areas of double-shelled, tripleshelled, and quadruple/quintuple-shelled NiO hollow spheres are 234.3, 282.6, and 305.6 m2 g
1, respectively, which are much higher than those of the uncalcined NiO (50.8 m2 g
1) and calcined NiO at 270  C (8.6 m2 g
1). The great improvements in specific surface area may arise from the voids among close-packing nano-building blocks [37], and the mesoporous in the multi-shelled hollow spheres. Insert shows the pore size distributions for the NiO samples, where several narrow pore size distributions with an average pore diameter less than 10 nm are generally observed. Because of the large surface area and porous structure, the as-synthesized multi-shelled NiO hollow spheres may have many potential applications such as electrochemical supercapacitor and highperformance lithium ion battery, since they can offer unique advantages for the ion transfer at the electrode/electrolyte interface [39e41]. The formation mechanism of the single-shelled hollow spheres using carbon precursor based templates has been investigated extensively [42]. In our case, the multi-shelled NiO hollow spheres imply that its reaction mechanism could be quite different from the previously reported mechanisms for single shell spheres. We here propose a multi-step process to elucidate the possible formation mechanism of the multi-shelled NiO hollow sphere (Fig. 6).

H. Wu et al. / Journal of Alloys and Compounds 685 (2016) 8e14

11

Fig. 4. TEM images of the resultant NiO precursor calcined at different temperatures: (a) uncalcined, (bec) 270  C, (d) 350  C, (e) 430  C, and (feh) 550  C. (1e4) shows the corresponding EDX elemental analysis in (h).

Fig. 5. N2 adsorption-desorption isotherms and pore size distribution plots (insert) for NiO samples calcined at different temperatures: (a) 350, (b) 430, and (c) 550  C.

Firstly, depending on carbonaceous particles rich with surface functional groups (e.g., eOH and eC]O groups), a dehydration reaction can take place between the surface functional groups and the metal oxide hydrate in solution, leading to formation of a uniform carbonaceous particle@metal oxide hydrate core-shell structure. Importantly, the hydrophilic groups on the surfaces of the newly formed carbonaceous primary spheres may act as the nuclei sites for gradually hydrated Ni species to anchor. Subsequently, metal oxide hydrate on the surface of the carbonaceous particles may further react with the eOH groups formed via the dehydration

and polymerization of the D-glucose, which forms the secondary carbon shell outside the Ni(OH)x layers. Further hydrolysis of Ni salt will provide additional nickel source to form the outer NiO shell upon further condensation of Ni(OH)x species and eventually, leading to the formation of C@NiO@C@NiO sphere precursors under hydrothermal treatment process. In other words, the carbonaceous particle and metal oxide hydrate are assembled shell-byshell via the chemically induced dehydration process to form multi-shelled NiO hollow sphere precursors [43]. Secondly, when the calcination temperature is low (e.g., 270  C),

12

H. Wu et al. / Journal of Alloys and Compounds 685 (2016) 8e14

Fig. 6. Schematic of proposed reaction mechanism of the formation of the multi-shelled NiO hollow spheres.

the contraction of the carbonaceous spheres by degradative oxidation starts slowly before causing any crystallization of the metal oxide hydrates, allowing metal oxide hydrate to ready form outer surface shell. Thirdly, as the annealing temperature increases (e.g., 350e550  C), it accelerates the crystallization of the metal oxide hydrate before causing any combustion of the carbonaceous layer and forms a thermally stable metal oxide shell. Finally, when the temperature reaches the respective critical value (e.g., 350e550  C), the combustion of the carbonaceous layer provides the energy for the phase separation of the cores and shells [44]. Thereby, multishelled NiO hollow spheres can be controllably generated using this simple approach. One of the important features compared with previously reported templated-engaged methods for multi-shelled hollow spheres is the uniform shell-by-shell self-assembly, which can promote the formation of more shells within the carbonaceous sphere templates, making the method universal for various metal oxides. 3.2. Electrochemical properties The obtained multi-shelled NiO hollow spheres possessing large surface area and complex structure are intriguing for lithium ion battery applications. We selected multi-shelled NiO hollow spheres to evaluate their electrochemical properties in lithium half-cells. Fig. 7a shows the first-cycle charge-discharge voltage profiles of four samples of multi-shelled NiO hollow spheres at a current density of 200 mA g
1 in the potential range from 0.01 to 3.0 V. In the first-discharge curves, the potential quickly falls to a 0.6 V plateau and then gradually declines to the cutoff voltage of 0.01 V. The longer the observed plateau is, the higher the achievable power output is [4]. The double-shelled, triple-shelled and quadruple/ quintuple-shelled NiO hollow spheres have ultrahigh initial capacities of 964.3, 1052.9 and 1167 mA h g
1, respectively, far greater than that of the calcined NiO at 270  C (166.8 mA h g
1). All these initial capacities are higher than the theoretical capacity (718 mA h g
1) for bulk NiO [39]. The exceeded capacity is as a result of large surface area, originating from the formed solid electrolyte interface (SEI) layer during the reduction. Fig. S1 shows the charge-discharge curves of the quadruple/ quintuple-shelled NiO hollow spheres in the first 10 cycles at a

scan rate of 200 mA g
1. In the subsequent cycles, the capacity of the quadruple/quintuple-shelled NiO hollow spheres decreases continuously because of the kinetic restrictions associated with the incomplete decomposition of Li2O and continuous formation of SEI. After 10 cycles, the quadruple/quintuple-shelled NiO hollow spheres have a reversible capacity of 425 mA h g
1. With the proceeding of the cyclic discharge-charge, the charge-discharge capacity is gradually faded, while the irreversible capacity is decreased. In contrast, the solid NiO spheres deliver much greater capacity retention (see Fig. S2). The reversible capacity of 83 mA h g
1 after 10 cycles is actually largely stabilized from the second cycle (93 mA h g
1). However, the solid NiO spheres exhibit much lower discharge-charge capacities than the quadruple/ quintuple-shelled NiO hollow spheres. Fig. 7b highlights high initial capacities of all the multi-shelled NiO hollow spheres with respect to that of solid NiO spheres, wherein double-shelled, triple-shelled, and quadruple/quintupleshelled NiO hollow spheres quickly decline via cycling. However, their stabilized discharge-charge capacities after 100 cycles are as high as 139.5 and 139.4 mAh g
1, that are still higher than those of solid NiO spheres after 10 cycles (see Fig. S3). After 30 cycles at a current density of 200 mA g
1, the values of multi-shelled NiO hollow spheres are significantly decreased to 241.4, 210.4, 140 and 80.4 mA h g
1, respectively, which are in good agreement with the results from Pan and Liu [37,39]. The severe capacity degradation and poor cycling performance should be attributed to the large volume expansion/constriction leading to the pulverization and degradation of the electrode. The volume expansion/constriction also leads to the repeated destroy and formation of SEI film and accordingly the continuous consumption of the electrolyte [45]. As the rate capability is also critical for practical applications, we measured discharge curves at different current densities for the multi-shelled NiO hollow spheres (Fig. 7c). As can be seen, the average specific capacities of quadruple/quintuple-shelled NiO hollow spheres are ~934, 664, 492 and 356 mA h g
1 at the current densities of 100, 200, 400 and 600 mA g
1, respectively. After the high-rate charge-discharge cycling, a specific capacity as high as ~548 mA h g
1 can be restored when the current density is reduced to 100 mA g
1. The above results clearly imply that quadruple/ quintuple-shelled NiO hollow spheres are relatively tolerant of various charge and discharge currents, which is preferred for high

H. Wu et al. / Journal of Alloys and Compounds 685 (2016) 8e14

13

Fig. 7. (a) First-cycle charge-discharge curves, (b) cycling performances at a current density of 200 mA g
1, (c) discharge curves at different current densities, and (d) electrochemical impedance spectra (EIS) of the multi-shelled NiO hollow spheres.

power applications. The Nyquist plots for the NiO samples in Fig. 7d show a single semicircle in the high frequency region corresponding to the charge-transfer resistance (Rct) and a sloping straight line in the low frequency range corresponding to solid-state diffusion of lithium (Zw) [46,47]. The semicircle diameter for the quadruple/quintupleshelled NiO hollow spheres is obviously smaller than those in the cases of the other multi-shelled NiO hollow spheres. The chargetransfer resistance (Rct) of the quadruple/quintuple-shelled NiO hollow spheres is smaller than those of the other NiO samples because of larger specific surface area, suggesting that the charge transfer for the former occurs prior to those of the latter, which may facilitate better electrolyte transport and strain release compared to those in the other NiO samples. It is thus demonstrated that the performance of the quadruple/quintuple-shelled NiO hollow spheres as anodes for LIBs should be enhanced. As shown in Fig. 7, the lithium ion battery performance of the quadruple/quintuple-shelled NiO hollow spheres is clearly superior to that of the other types of multi-shelled NiO hollow spheres synthesized. The most likely interpretation of these results is based on a combination of observations. First, the unique multi-shelled structures and increased porosity of the shells lead to an increase in the electrolyte/NiO contact area, a decrease in the effective diffusion distance for both lithium ion and electrons, and thus better rate capabilities [9]. Second, the void space effectively accommodates the dramatic volume change and alleviates the strain during Liþ-insertion/extraction processes [4]. The morphology of the electrode after 30 charge-discharge

cycles at a current density of 200 mA g
1 was presented to illustrate the stability. As shown in Fig. 8, some of the multi-shelled NiO hollow structure still is preserved after such a long time cycling process, indicating high structure stability. However, as a whole, it would cause loss of electrolyte/NiO contact area and corresponding capacity fade with more and more multi-shelled NiO hollow spheres collapse [48]. Therefore, it is still a challenge to further enhance the structure stability of the electrode. 4. Conclusions In summary, uniform multi-shelled NiO hollow spheres were successfully synthesized by an easy hydrothermal method. The number of shells could be rationally designed by adjusting the heating conditions. The method is quite universal which can be expected to prepare other multi-shelled metal oxide hollow structures. When tested as the anode materials for LIBs, these complex quadruple/quintuple-shelled NiO hollow spheres exhibited superior Li storage performances at initial charge/discharge cycles (e.g., a specific capacity of 1167 mA h g
1 at a current density of 200 mA g
1), good rate capacity (e.g., a specific capacity as high as ~ 548 mA h g
1 can be restored when the current density is reduced to 100 mA g
1), and further improvement in cyclic stability is still needed. The better performance of quadruple/quintupleshelled NiO hollow spheres in LIBs originates from the porous hollow multi-shelled microstructure, which guarantees more lithium-storage sites, a shorter lithium-ion diffusion length, and sufficient void space to buffer the volume expansion.

14

H. Wu et al. / Journal of Alloys and Compounds 685 (2016) 8e14

Fig. 8. (a) TEM image and (b) SAED pattern of quadruple/quintuple-shelled NiO hollow spheres after 30 cycles.

Acknowledgements Financial support was provided by National Natural Science Foundation of China (Nos. 50771082, 51571166, 61505167 and 60776822). G.W. thanks the National Natural Science Foundation of China (No. 51407134), China Postdoctoral Science Foundation (No. 2014M562412), and China Postdoctoral Science Special Foundation (No. 2015T81028). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.05.264. References [1] X. Sun, Y. Li, Angew. Chem. Int. Ed. 43 (2004) 3827e3831. [2] Q. He, T. Yuan, S. Wei, N. Haldolaarachchige, Z. Luo, D.P. Young, A. Khasanov, Z. Guo, Angew. Chem. Int. Ed. 51 (2012) 8842e8845. [3] X.Y. Lai, J. Li, B.A. Korgel, Z.H. Dong, Z.M. Li, F.B. Su, J. Du, D. Wang, Angew. Chem. Int. Ed. 50 (2011) 2738e2741. [4] J.Y. Wang, N.L. Yang, H.J. Tang, Z.H. Dong, Q. Jin, M. Yang, D. Kisailus, H.J. Zhao, Z.Y. Tang, D. Wang, Angew. Chem. Int. Ed. 52 (2013) 6417e6420. [5] H. Ren, R.B. Yu, J.Y. Wang, Q. Jin, M. Yang, D. Mao, D. Kisailus, H.J. Zhao, D. Wang, Nano Lett. 14 (2014) 6679e6684. [6] Z.H. Dong, H. Ren, C.M. Hessel, J.Y. Wang, R.B. Yu, Q. Jin, M. Yang, Z.D. Hu, Y.F. Chen, Z.Y. Tang, H.J. Zhao, D. Wang, Adv. Mater. 26 (2014) 905e909. [7] G.Q. Zhang, X.W. Lou, Angew. Chem. Int. Ed. 53 (2014) 9041e9044. [8] L. Zhou, D.Y. Zhao, X.W. Lou, Adv. Mater. 24 (2012) 745e748. [9] A.Q. Pan, H.B. Wu, L. Yu, X.W. Lou, Angew. Chem. Int. Ed. 52 (2013) 2226e2230. [10] L.F. Shen, L. Yu, X.Y. Yu, X.G. Zhang, X.W. Lou, Angew. Chem. Int. Ed. 54 (2015) 1868e1872. [11] F. Wang, L.D. Sun, W. Feng, H.J. Chen, M.H. Yeung, J.F. Wang, C.H. Yan, Small 6 (2010) 2566e2575. [12] Z.C. Wu, M. Zhang, K. Yu, S.D. Zhang, Y. Xie, Chem. Eur. J. 14 (2008) 5346e5352. [13] J. Liu, S.B. Hartono, Y.G. Jin, Z. Li, G.Q. Lu, S.Z. Qiao, J. Mater. Chem. 20 (2010) 4595e4601. [14] M. Chen, C.Y. Ye, S.X. Zhou, L.M. Wu, Adv. Mater. 25 (2013) 5343e5351. [15] C.Z. Yuan, H.B. Wu, Y. Xie, X.W. Lou, Angew. Chem. Int. Ed. 53 (2014) 1488e1504. [16] J. Qi, X.Y. Lai, J.Y. Wang, H.J. Tang, H. Ren, Y. Yang, Q. Jin, L.J. Zhang, R.B. Yu, G.H. Ma, Z.G. Su, H.J. Zhao, D. Wang, Chem. Soc. Rev. 44 (2015) 6749e6773. [17] X.Y. Lai, J.E. Halpert, D. Wang, Energy Environ. Sci. 5 (2012) 5604e5618. [18] Z.H. Dong, X.Y. Lai, J.E. Halpert, N.L. Yang, L.X. Yi, J. Zhai, D. Wang, Z.Y. Tang, L. Jiang, Adv. Mater. 24 (2012) 1046e1049.

[19] X.H. Wang, Z.B. Yang, X.L. Sun, X.W. Li, D.S. Wang, P. Wang, D.Y. He, J. Mater. Chem. 21 (2011) 9988e9990. [20] H. Liu, G. Wang, J. Liu, S. Qiao, H. Ahn, J. Mater. Chem. 21 (2011) 3046e3052. [21] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496e499. [22] P.G. Bruce, B. Scrosati, J.M. Tarascon, Angew. Chem. Int. Ed. 47 (2008) 2930e2946. [23] Y. Li, B. Tan, Y. Wu, Nano Lett. 8 (2008) 265e270. [24] S. Ni, X. Wang, G. Zhou, F. Yang, J. Wang, Q. Wang, D. He, Mater. Lett. 63 (2009) 2701. [25] B. Kang, G. Ceder, Nature 458 (2009) 190e193. [26] C.K. Chan, H.L. Peng, G. Liu, K. Mcllwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31e35. [27] Y.J. Lee, H. Yi, W.J. Kim, K. Kang, D.S. Yun, M.S. Strano, G. Ceder, A.M. Belcher, Science 324 (2009) 1051e1055. [28] M.S. Park, G.X. Wang, Y.M. Kang, D. Wexler, S.X. Dou, H.K. Liu, Angew. Chem. Int. Ed. 46 (2007) 750e753. [29] E. Hosono, T. Kudo, I. Honma, H. Matsuda, H.S. Zhou, Nano Lett. 9 (2009) 1045e1051. [30] X.W. Lou, D. Deng, J.Y. Lee, J. Feng, L.A. Archer, Adv. Mater. 20 (2008) 258e262. [31] L.Q. Mai, B. Hu, W. Chen, Y.Y. Qi, C.S. Lao, R.S. Yang, Y. Dai, Z.L. Wang, Adv. Mater. 19 (2007) 3712e3716. [32] L. Zheng, Y. Xu, D. Jin, Y. Xie, Chem. Mater. 21 (2009) 5681e5690. [33] D.Y. Pan, S. Wang, B. Zhao, M.H. Wu, H.J. Zhang, Y. Wang, Z. Jiao, Chem. Mater. 21 (2009) 3136e3142. [34] G.X. Wang, X.P. Shen, J. Yao, J.S. Park, Carbon 47 (2009) 2049e2053. [35] H. Kim, J. Cho, Nano Lett. 7 (2008) 3688e3691. [36] X.L. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500e506. [37] J.H. Pan, Q.Z. Huang, Z.Y. Koh, D. Neo, X.Z. Wang, Q. Wang, ACS Appl, Mater. Interfaces 5 (2013) 6292e6299. [38] X. Wu, G.Q. Lu, L.Z. Wang, Energy Environ. Sci. 4 (2011) 3565e3572. [39] L. Liu, Y. Li, S.M. Yuan, M. Ge, M.M. Ren, C.S. Sun, Z. Zhou, J. Phys. Chem. C 114 (2010) 251e255. [40] X.H. Wang, L. Qiao, X.L. Sun, X.W. Li, D.K. Hu, Q. Zhang, D.Y. He, J. Mater. Chem. A 1 (2013) 4173e4176. [41] N.N. Wang, L. Chen, X.J. Ma, J. Yue, F.E. Niu, H.Y. Xu, J. Yang, Y.T. Qian, J. Mater. Chem. A 2 (2014) 16847e16850. [42] X.W. Lou, Y. Wang, C. Yuan, J.Y. Lee, L.A. Archer, Adv. Mater. 18 (2006) 2325e2329. [43] H.X. Yang, J.F. Qian, Z.X. Chen, X.P. Ai, Y.L. Cao, J. Phys. Chem. C 111 (2007) 14067e14071. [44] P.F. Xu, R.B. Yu, H. Ren, L.B. Zong, J. Chen, X.R. Xing, Chem. Sci. 5 (2014) 4221e4226. [45] M.V. Reddy, T. Yu, C.H. Sow, Z.X. Shen, C.T. Lim, G.V. Subba Rao, B.V.R. Chowdari, Adv. Funct. Mater. 17 (2007) 2792e2799. [46] H. Han, T. Song, E.K. Lee, A. Devadoss, Y. Jeon, J. Ha, Y.C. Chung, Y.M. Choi, Y.G. Jung, U. Paik, ACS Nano 6 (2012) 8308e8315. [47] R.W. Mo, Z.Y. Lei, K.N. Sun, D. Rooney, Adv. Mater. 26 (2014) 2084e2088. [48] Z.G. Wu, Y.J. Zhong, J.T. Li, X.D. Guo, L. Huang, B.H. Zhong, S.G. Sun, J. Mater. Chem. A 2 (2014) 12361e12367.