Journal of Power Sources 324 (2016) 33e40
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Synthesis of nano-sized silicon from natural halloysite clay and its high performance as anode for lithium-ion batteries Xiangyang Zhou a, 1, Lili Wu a, 1, Juan Yang a, *, Jingjing Tang b, Lihua Xi a, Biao Wang a a b
School of Metallurgy and Environment, Central South University, Changsha, 410083, China Department of Mechanical and Engineering, The Hong Kong Polytechnic University, Hong Kong, China
h i g h l i g h t s Halloysite clay is converted into ultrafine Si nanoparticles. The as-prepared HeSi is composed of many interconnected Si nanoparticles. The interconnected network enhances the electrochemical performance of electrodes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2016 Received in revised form 11 May 2016 Accepted 16 May 2016 Available online 21 May 2016
Recently, nanostructured Si has been intensively studied as a promising anode candidate for lithium ion batteries due to its ultrahigh capacity. However, the downsizing of Si to nanoscale dimension is often impeded by complicated and expensive methods. In this work, natural halloysite clay was utilized for the production of Si nanoparticles through selective acid etching and modified magnesiothermic reduction processes. The physical and chemical changes of these samples during the various processes have been analyzed. The as-prepared HeSi from halloysite clay is composed of many interconnected Si nanoparticles with an average diameter of 20e50 nm. Owing to the small size and porous nature, the HeSi nanoparticles exhibit a satisfactory performance as an anode for lithium ion batteries. Without further modification, a stable capacity over 2200 mAh g1 at a rate of 0.2 C after 100 cycles and a reversible capacity above 800 mAh g1 at a rate of 1 C after 1000 cycles can be obtained. As a result, this synthetic route is cost-effective and can be scaled up for mass production of Si nanoparticles, which may facilitate valuable utilization of halloysite clay and further commercial application of Si-based anode materials. © 2016 Elsevier B.V. All rights reserved.
Keywords: Silicon Halloysite clay Magnesiothermic reduction Lithium-ion battery anodes
1. Introduction High energy lithium-ion batteries (LIBs) are becoming the favorable selection for use in portable electronic devices, electric vehicles and large-scale stationary energy storage systems [1e3]. Silicon (Si) has lately attracted tremendous attention due to its large theoretical capacity (Li15Si4, 3579 mAh g1 at room temperature) in comparison with the relatively low specific capacity (372 mAh g1) of the graphite anode [4e6]. In spite of this promising feature, Si based anodes always suffer from a large volume change during Liþ insertion and extraction (>300%), leading to the pulverization of active material and capacity degradation of
* Corresponding author. E-mail address:
[email protected] (J. Yang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2016.05.058 0378-7753/© 2016 Elsevier B.V. All rights reserved.
electrode [7,8]. Recently, pioneering researches have shown that decreasing the size of Si into nanoscale dimension promises an effective strategy to mitigate the physical strain and alleviate the pulverization problem [9e11]. Si with different nano-sized morphology, such as Si nanoparticles [12,13], nanotubes [14,15], nanowires [16,17] and porous nanostructures [18,19], have been wildly exploited and recognized. However, as the second most abundant element in the Earth’s crust, Si is rarely found in nature in its uncombined form, which is widely distributed in clays, sands, rocks and planets as various forms of silica or silicates. Although significant progress on high-performance Si anodes has been explored, the existing methods for producing nanostructured Si anodes are still limited to high temperature pyrolysis of toxic and expensive silane/polysilane/halosilane precursors, which limits the successful implementation of Si based anodes [16,19e21]. It is highly desirable to develop a cost-effective and environmentally
34
X. Zhou et al. / Journal of Power Sources 324 (2016) 33e40
friendly method to obtain nanostructured Si. As a result, the magnesiothermic reduction of silica has gained much attention because of its low operating temperatures. This method has resulted in the synthesis of various types of nanostructured Si with different costeffective silica precursors, such as diatomaceous earth, rice husks, beach sand and bamboo leaves [22e27]. Halloysite is a natural nanomaterial that is abundantly available as a raw material and can be mined from deposits. It has a tubular morphology with a chemical formula of Al2Si2O5(OH)4$2H2O. Compared to kaolinite, halloysite has an additional monolayer of water molecules between the alumina and silica layers, resulting in the presence of multiple rolled layers [28,29]. In general, depending on the deposits, the dimensions of the halloysite tubules vary between 0.02 and 30 mm in length, 30 and 190 nm in external diameter, and 10e100 nm in inner diameter [30]. Historically, halloysite has been mainly used in the manufacturing of porcelain products. With the rapid development of nanoscience and nanotechnology in recent years, natural halloysite is expected to play important roles in several promising applications, especially in waste water treatments and nanoscale templates and containers [31,32]. Large deposits of natural halloysite have also been found worldwide, such as in Australia, the United States, China, New Zealand, Mexico and Brazil. Although purification is always necessary, halloysite nanoclay is very inexpensive (~$4/kg) and the annual global supply of halloysite exceeds approximately 50,000 tons [28,32,33]. Therefore, halloysite clay is a natural reservoir for nanostructured silica. Considering the unique structure and natural abundance of
halloysite clay, scalable synthesis of Si with desired nanostructure is expected. Herein, we report a method to convert halloysite clay into ultrafine Si nanoparticles by selective acid etching of halloysite clay and followed by magnesiothermic reduction with the assistance of NaCl. Without further modification, the as-prepared Si nanoparticles can deliver a high reversible capacity over 2200 mAh g1 at a rate of 0.2 C (1C ¼ 3500 mA g1) after 100 cycles and a capacity above 800 mAh g1 at a rate of 1 C after 1000 cycles. 2. Experimental 2.1. Synthesis of Si nanoparticles from halloysite clay Halloysite clay was directly exploited from Hunan, China. Before experiment, halloysite clay was first dissolved in deionized (DI) water to separate the deposition of undissolved clay from emulsion dispersion. After that halloysite clay was collected by filtration, washed with DI water and dried under vacuum at 60 C. In a typical acid etching process of halloysite, 5 g of the pretreated halloysite clay was dispersed in 500 ml of 2 M sulfuric acid and stirred at 100 C for 10 h to selective etching of aluminum oxide. After washing in water and drying, a white HeSiO2 remnant was obtained. The Si nanoparticles were prepared by a modified magnesiothermic reduction. In a typical synthesis, 0.8 g of HeSiO2 and 8 g of NaCl were dissolved in DI water under vigorous stirring, after stirred for 1 h, the mixture was dried at 120 C to remove all water. Next, this mixture powder was mixed with 0.72 g of Mg powder
Fig. 1. (a) Synthetic route of HeSi nanoparticles from halloysite clay through selective acid etching and magnesiothermic reduction. (b) XRD patterns of bare halloysite clay, HeSiO2 and HeSi nanoparticles.
X. Zhou et al. / Journal of Power Sources 324 (2016) 33e40
and then sealed in an autoclave reactor in an argon-filled glove box. The reactor was placed into a tube furnace and heated to 700 C for 6 h. After cooling down to room temperature, the resulting product was immersed in 1 M HCl for 6 h to remove NaCl, MgO and Mg2Si and washed for several times. The powder was then immersed in 5% hydrofluoric acid for 30 min to remove unreacted SiO2 with subsequent washing with DI water. The collected HeSi nanoparticles were dried under vacuum at 60 C.
2.2. Characterization
35
were characterized by field emission scanning electron microscopy (SEM, Nova NanoSEM230) and field emission transmission electron microscopy (TEM, JEOL JEM-2100F) respectively. In order to improve the conductivity for SEM observation, samples were sputter-coated with gold by low-vacuum sputter coating. X-ray diffraction (XRD) patterns were collected with Rigaku-TTRIII with Cu Ka radiation at a scanning rate of 10 min1. The surface element analyses of samples were recorded by X-ray photoelectron spectroscopy (XPS, ThermoFisher-VG Scientific ESCALAB 250Xi with monochromatic Al KR radiation of 30 eV pass energy in 0.05 eV step over an area of 650 mm 650 mm). All binding energies were calibrated using the C 1 s peak at 284.6 eV as the reference. The ICP
The surface morphologies and internal structures of the samples
Fig. 2. (a) SEM and (b) TEM images of natural halloysite clay. (c) SEM and (d) corresponding EDS spectrum of HeSiO2. (e and f) TEM images of HeSiO2.
36
X. Zhou et al. / Journal of Power Sources 324 (2016) 33e40
analyses were measured on a SPECTRO BLUE SOP spectrometer. The Raman spectra were obtained using a LabRAM HR800 from HORIBA JOBIN YVON. Specific surface areas and pore size distributions (PSD) were measured with the Quantachrome Nova Win 2 equipment.
2.3. Electrochemical characterization The working electrodes were prepared by mixing the HeSi active material, Super P and sodium alginate binder at a weight ratio of 60:20:20 in water solvent. Then, the slurry was pasted on a cooper foil and dried at 60 C for 12 h. The electrodes were punched in the form of disks, and the active material mass loading is typically 0.44e0.56 mg cm2. Prior to cell fabrication, the electrodes were kept in vacuum oven at 60 C for 4 h. Coin cells were assembled in an argon-filled glove box (Super 1220/750, Shanghai Mikrouna Co. Ltd) with lithium foil as a counter electrode, Celgard 2400 as a separator and 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 by volume) with 10 wt% fluoroethylene carbonate additives as the electrolyte. The assembled unit cells were measured on a LAND-CT2001A battery tester in the potential range of 0.01e1.2 V versus Li/Liþ at a certain current density value. The specific capacity was calculated based on the mass loading of Si in the electrode. Cyclic voltammetry (CV) measurement was conducted on an electrochemical workstation (PARSTAT MC) over the potential range 0.01e1.2 V versus Li/Liþ at a scan rate of 0.1 mV s1.
3. Results and discussion The optical images and crystal structure developments after selective acid etching and magnesiothermic reduction are presented in Fig. 1. Sulfuric acid was selected to etch alumina sheets from halloysite, which is similar to the works done for kaolinite dealumination [34e37]. As we can see, the collected HeSiO2 has a white color and its XRD pattern exhibits a broad peak at approximately 23 , indicating the presence of an amorphous phase of SiO2, which is completely different from the characteristic data of halloysite. To convert HeSiO2 into HeSi, traditional carbothermic reduction of silica requires a high processing temperature, which is higher than the melting point of Si, while magnesiothermic reduction carried out at around 650e700 C looks like a more viable choice (SiO2(s) þ 2 Mg(g) / Si(s) þ 2MgO(s)) [38]. Yet, this process is an exothermic reaction, the local reaction temperature will out of control, leading to the aggregation of Si particles. To solve this defect, NaCl (melting point: 801 C) was used as a heat scavenger and kept the reaction temperature below 801 C during fusion [39]. Moreover, the addition of NaCl can be easily removed or recycled. Based on this method, the as-prepared HeSi nanoparticles exhibit a brown color and its XRD pattern reveals five well-resolved diffraction peaks, which can be well assigned to the (111), (220), (311), (400) and (331) planes of Si structure (JCPDS, no. 75-0589). Given the possible residual Al element in the HeSiO2, a minor feature at the low angle side of the diffraction peak at around 27 may be attributed to the existence of Al2(SiO4)O (JCPDS, no. 89-
Fig. 3. (a) SEM, (b and c) TEM and (d) HR-TEM images of HeSi. The insert in (b) shows the corresponding SAED pattern of HeSi.
X. Zhou et al. / Journal of Power Sources 324 (2016) 33e40
1483). The morphology changes and internal structures of the samples before and after selective acid etching were characterized by SEM and TEM. Natural halloysite clay, which has a length of 0.2e1.5 mm, an average external diameter of 50e75 nm and an average lumen diameter of 5e15 nm, is consist of many halloysite nanotubes as shown in Fig. 2a and b. After complete removal of the alumina sheets from halloysite, the obtained HeSiO2 retained the rodlike structure but with a rough surface, and it is mainly composed of Si and O with a molar ratio of nearly 1:2, as indicated by the EDS analysis (Fig. 2c and d). It should be noted that the Au peak arises from the electrically-conducting layer of gold and the C peak arises from the conductive adhesive. Detail surface structure of HeSiO2 can be observed from Fig. 2e and f. Upon complete removal of alumina, HeSiO2 loses its original tubular morphology and transforms into a nanorod decorated with many nanoparticles of ca. 10 nm diameter. In addition, some holes appeared on the surface of the wall, which hints the structural damages during the dealumination process and the porous structure of HeSiO2. In addition, a study on acid etching of halloysite reported similar results [40,41]. Following magnesiothermic reduction of HeSiO2 was carried out, magnesium atoms bonded with oxygen atoms from silica to form MgO by-products, leading to the formation of SieSi bond with neighboring atoms randomly. After removing the by-products, the as-prepared HeSi was further characterized in terms of its surface morphology and internal structure. Fig. 3a shows the SEM image of HeSi composed of large amounts of nanoparticles less than
37
100 nm, while the original structure is not maintained. Corresponding EDS analysis (Fig. S1) reveals that Si is the predominant element in the HeSi nanoparticles. TEM images (Fig. 3b and c) show that HeSi is composed of many interconnected Si nanoparticles, and these nanoparticles are nearly spherical like. In addition, the highly nanoporous interconnected network can be obviously observed. Considering structural collapse caused by removal of oxygen from silica, unsaturated bonds on Si atoms will bond with neighboring Si atoms randomly. This SieSi bonding chemistry is a process to optimize the surface energy of each nanoparticle, which may result in surface diffusion and surface reconstruction, in particular to a silica precursor with nanoscale dimension. From the lattice fringe of high-resolution TEM (HRTEM) image (Fig. 3d), an interplanar spacing around 0.31 nm can be observed, corresponding to the (111) plane of the crystalline Si. In addition, the diffraction rings in the selected area diffraction (SAED) pattern can be indexed to be Si. The mesoporous structures of HeSi were further verified by nitrogen adsorption and desorption isotherm curves. It can be found that the HeSi has a specific surface area of 125 m2 g1 with pore size distribution peaked around 1e8 nm analyzed by BrunauerEmmettTeller (BET) and BarrettJoynerHalenda (BJH) measurements respectively (Fig. 4a and b). Numerous researches and studies confirm that materials with high specific surface area and porous structure may be favorable to achieving high capacity and long cycling life [42,43]. Moreover, the surface chemical composition and crystallinity of HeSi were characterized. The XPS
Fig. 4. (a) Nitrogen adsorption-desorption isotherm and (b) PSD of HeSi. (c) XPS spectrum and (d) Raman spectrum of HeSi.
38
X. Zhou et al. / Journal of Power Sources 324 (2016) 33e40
spectrum (Fig. 4c) of HeSi shows a strong peak at about 99 eV and a little peak appearing at about 103.5 eV, which can be attributed to the Si and amorphous SiO2 respectively. This result indicates a SiOx layer outside the Si nanoparticles, which have been confirmed in previous literature [20,44]. In addition, the Mg 1 s, Cu 2p, Fe 2p and Al 2p XPS spectra of HeSi were provided in Fig. S2, which may be introduced by the autoclave reactor and reactants. As a result, only Al element can be detected, which may be derived from the aluminum sheets of halloysite clay. The weight percentages of Mg, Cu, Fe and Al in HeSi nanoparticles were measured by ICP techniques (Table S1) to be 0.018%, 0.01%, 0.0081% and 0.073%. These fractional residues may enhance the electronic conductivity of
HeSi. The Raman spectrum (Fig. 4d) of HeSi shows a clear peak corresponding to crystalline Si at 503 cm1, which is lower than that of bulk Si at 520 cm1. This result indicates the size of HeSi is nanoscale, which is governed by the particle size dependent manner [45,46]. The nano-sized HeSi samples were investigated as anode materials for LIBs. Due to the small size and porous nature of HeSi, excellent electrochemical performances are anticipated. To characterize the electrochemical properties of HeSi as an anode for LIBs, CV measurements were performed over the potential range 0.01e1.2 V versus Li/Liþ at a scan rate of 0.1 mV s1. The CV curves (Fig. 5a) shows a broad peak at around 1.15 V in the first cathodic
Fig. 5. (a) CV curves of HeSi in the selected cycles at a scan rate of 0.1 mV s1 in the potential window of 0.01 to 1.2 V. (b) The charge/discharge profiles at typical cycles of H-Si electrode. (c) Cycling performance of HeSi and M-Si electrodes at a rate of 0.2 C. (d) Rate performance of HeSi electrode at various C rates. (e) Long-term cycling performance of HeSi electrode at a rate of 1 C.
X. Zhou et al. / Journal of Power Sources 324 (2016) 33e40
branch, suggesting the formation of (solid-electrolyte interphase) SEI films on the surface of the electrode [47]. Disappearance of this peak in the following scans indicates the existence of a stable SEI film that prevents the further decomposition of electrolyte. The distinct peaks below 0.1 V correspond to the Li alloying process from a crystalline Si into an amorphous LixSi phase [4,48]. During the following cycles, a new cathodic peak around 0.2 V appeared, which can be consistent with the lithiation process with amorphous Si. Meanwhile, two anodic peaks at around 0.39 and 0.51 V can be observed during the delithiation process, related to the phase transformation from LixSi phase to amorphous Si [49]. In addition, the increased peak intensity indicates a kinetic enhancement of the electrode occurred over the first few cycles. After ten cycles, the profiles become almost overlapped, suggesting the completion of activation process and electrochemical stability of HeSi electrode. Fig. 5b shows the discharge/charge profiles at typical cycles of HeSi electrode tested at a rate of 0.2 C in the potential window from 0.01 to 1.2 V. The initial discharge and charge capacities are approximately 3440.4 and 2723.1 mAh g1 respectively, corresponding to the initial columbic efficiency of 79.2%. The irreversible capacity loss of the electrode during the lithiation process in the first cycle is mainly attributed to the decomposition of electrolyte occurs at around 1.0 V, leading to the formation of SEI films. The distinct plateau profile below 0.1 V in the initial discharging process is relating to Li alloying process of crystalline Si. After that, crystalline Si converts into an amorphous phase. As the cycle number increased, the capacity of HeSi decreased continuously due to the stress-strain caused by volume change, but with a much slow rate. Fig. 5c shows cycling performance of HeSi electrode tested at a rate of 0.2 C. For comparison, Metallurgical Si (M-Si) produced by carbothermic reduction was used as a reference. The average mass loading of M-Si electrode is typically 0.66e0.78 mg cm2, wherein the mass ratio of active material: carbon black: binder ¼ 6:2:2. From SEM images (Fig. S3) of the M-Si, although some small particles can be observed, the average size of the M-Si was micro-sized. The HeSi electrode exhibits a superior charge capacity of 2224.3 mAh g1 after 100 cycles, which is 6 times larger than the theoretical capacity of graphite. The capacity retention of HeSi electrode after 100 cycles is 81.7%, while the M-Si electrode undergoes drastic volume changes during the lithiation/delithiation process, thus leading to a rapid capacity fading. This excellent cycling performance of the prepared HeSi could be ascribed to the small size and well-organized structure, which could provide void space for Si expansion and open channels for the impregnation of electrolyte. For comparison, some typical Si-based anodes synthesized through magnesiothermic reduction in recent years have been summarized, and the cycling performances of the as-prepared HeSi nanoparticles are comparable or even higher than these reports (Table S2). Fig. 5d shows rate performance of HeSi electrode with the rate ranging from 0.1 C to 5 C. The HeSi electrode delivers the charge capacity around 2800, 2600, 2000, 1600, 1200 and 500 mAh g1 at the rate of 0.1, 0.2, 0.5,1, 2 and 5 C, respectively. After the C-rate returned to 0.1 C, a capacity around 2700 mAh g1 could still be restored. The long-term cycling performance of HeSi electrode at high current density was also evaluated as shown in Fig. 5e. To allow the HeSi electrode exhibits its excellent cycling performance, a rate of 0.2 C was carried out for the first 20 cycles to enhance the interparticle conductivity and kinetics of the HeSi electrode. After that, the electrode was cycled at a rate of 1 C. The HeSi electrode delivers a reversible capacity of 838.9 mAh g1 after 1000 cycles, which is still much higher than the theoretical capacity of graphite. Moreover, after the activation of a few cycles, the coulombic efficiency could maintain stability over 99.5% for the rest of cycles. The high reversible capacity and good cycling
39
performance can be attributed to the small size and high internal porosity of HeSi, which would provide open channels for the impregnation of electrolyte into pores and enable the volume expansion of Si without rupturing the SEI films of electrodes. 4. Conclusions In summary, we have developed a simple and low-cost method to convert natural halloysite clay into ultrafine Si nanoparticles. Selective acid etching of alumina sheets and modified magnesiothermic reduction of silica precursor were investigated. The growth of interconnected HeSi is interpreted to optimize the surface energy of each nanoparticle, which may result in surface diffusion and surface reconstruction. Moreover, the nanoscale dimensions of the obtained HeSi give it excellent electrochemical performance as lithium ion battery anodes. We anticipate this method will boost the development of both scalable application of Si anode and valuable utilization of halloysite clay. Acknowledgements Funding for this work was provided by the National Nature Science Foundation of China (Grant no. 51204209 and 51274240) and Grants from the Project of Innovation-driven Plan in Central South University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.05.058. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
M. Armand, J.-M. Tarascon, Nature 451 (2008) 652e657. J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2009) 587e603. H. Wu, Y. Cui, Nano Today 7 (2012) 414e429. M. Obrovac, L. Christensen, Electrochem. Solid State Lett. 7 (2004) A93eA96. J. Maranchi, A. Hepp, P. Kumta, Electrochem. Solid State Lett. 6 (2003) A198eA201. J. Li, J. Dahn, J. Electrochem. Soc. 154 (2007) A156eA161. J.R. Szczech, S. Jin, Energy Environ. Sci. 4 (2011) 56e72. X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B.W. Sheldon, J. Wu, Adv. Energy Mater. 4 (2014). B. Gao, S. Sinha, L. Fleming, O. Zhou, Adv. Mater. 13 (2001) 816e819. Y. Yao, M.T. McDowell, I. Ryu, H. Wu, N. Liu, L. Hu, W.D. Nix, Y. Cui, Nano Lett. 11 (2011) 2949e2954. X.H. Liu, L. Zhong, S. Huang, S.X. Mao, T. Zhu, J.Y. Huang, ACS Nano 6 (2012) 1522e1531. langer, D.M. Schleich, T. Brousse, Adv. C. Martin, O. Crosnier, R. Retoux, D. Be Funct. Mater. 21 (2011) 3524e3530. X. Zhao, C.M. Hayner, M.C. Kung, H.H. Kung, Adv. Energy Mater. 1 (2011) 1079e1084. H. Wu, G. Chan, J.W. Choi, Y. Yao, M.T. McDowell, S.W. Lee, A. Jackson, Y. Yang, L. Hu, Y. Cui, Nat. Nanotechnol. 7 (2012) 310e315. J.K. Yoo, J. Kim, Y.S. Jung, K. Kang, Adv. Mater. 24 (2012) 5452e5456. C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31e35. C.K. Chan, R.N. Patel, M.J. O’connell, B.A. Korgel, Y. Cui, ACS Nano 4 (2010) 1443e1450. C. Du, C. Gao, G. Yin, M. Chen, L. Wang, Energy Environ. Sci. 4 (2011) 1037e1042. M. Ge, J. Rong, X. Fang, A. Zhang, Y. Lu, C. Zhou, Nano Res. 6 (2013) 174e181. H. Kim, M. Seo, M.H. Park, J. Cho, Angew. Chem. Int. Ed. 49 (2010) 2146e2149. A.M. Morales, C.M. Lieber, Science 279 (1998) 208e211. L. Shen, X. Guo, X. Fang, Z. Wang, L. Chen, J. Power Sources 213 (2012) 229e232. N. Liu, K. Huo, M.T. McDowell, J. Zhao, Y. Cui, Sci. Rep. 3 (2013). L. Wu, J. Yang, J. Tang, Y. Ren, Y. Nie, X. Zhou, Electrochimica Acta 190 (2016) 628e635. Z. Favors, W. Wang, H.H. Bay, Z. Mutlu, K. Ahmed, C. Liu, M. Ozkan, C.S. Ozkan, Sci. Rep. 4 (2014). W.-S. Kim, Y. Hwa, J.-H. Shin, M. Yang, H.-J. Sohn, S.-H. Hong, Nanoscale 6 (2014) 4297e4302. L. Wang, B. Gao, C. Peng, X. Peng, J. Fu, P.K. Chu, K. Huo, Nanoscale 7 (2015) 13840e13847.
40
X. Zhou et al. / Journal of Power Sources 324 (2016) 33e40
[28] E. Joussein, S. Petit, J. Churchman, B. Theng, D. Righi, B. Delvaux, Clay Miner. 40 (2005) 383e426. [29] L. Guimaraes, A.N. Enyashin, G. Seifert, H.A. Duarte, J. Phys. Chem. C 114 (2010) 11358e11363. [30] P. Yuan, P.D. Southon, Z. Liu, M.E. Green, J.M. Hook, S.J. Antill, C.J. Kepert, J. Phys. Chem. C 112 (2008) 15742e15751. [31] P. Yuan, D. Tan, F. Annabi-Bergaya, Appl. Clay Sci. 112 (2015) 75e93. [32] Y.M. Lvov, D.G. Shchukin, H. Mohwald, R.R. Price, ACS Nano 2 (2008) 814e820. [33] Y. Zhang, A. Tang, H. Yang, J. Ouyang, Appl. Clay Sci. 119 (2016) 8e17. [34] D.R. Vollet, J.D. Macedo, Y.P. Mascarenhas, Appl. Clay Sci. 8 (1994) 397e404. [35] M. Rıza Altıokka, H. Akalın, N. Melek, S. Akyalçın, Industrial Eng. Chem. Res. 49 (2010) 12379e12382. [36] J.J. Fitzgerald, A.I. Hamza, C.E. Bronnimann, S.F. Dec, J. Am. Chem. Soc. 119 (1997) 7105e7113. [37] E. Aglietti, J.P. Lopez, E. Pereira, Appl. Clay Sci. 3 (1988) 155e163. [38] Z. Bao, M.R. Weatherspoon, S. Shian, Y. Cai, P.D. Graham, S.M. Allan, G. Ahmad, M.B. Dickerson, B.C. Church, Z. Kang, Nature 446 (2007) 172e175. [39] W. Luo, X. Wang, C. Meyers, N. Wannenmacher, W. Sirisaksoontorn, M.M. Lerner, X. Ji, Sci. Rep. 3 (2013).
[40] E. Abdullayev, A. Joshi, W. Wei, Y. Zhao, Y. Lvov, ACS Nano 6 (2012) 7216e7226. [41] Y. Zhang, L. Fu, H. Yang, Colloids Surfaces A Physicochem. Eng. Aspects 414 (2012) 115e119. [42] H. Xiang, K. Zhang, G. Ji, J.Y. Lee, C. Zou, X. Chen, J. Wu, Carbon 49 (2011) 1787e1796. [43] M. Ge, J. Rong, X. Fang, C. Zhou, Nano Lett. 12 (2012) 2318e2323. [44] M. Morita, T. Ohmi, E. Hasegawa, M. Kawakami, M. Ohwada, J. Appl. Phys. 68 (1990) 1272e1281. [45] Z. Iqbal, S. Veprek, A. Webb, P. Capezzuto, Solid State Commun. 37 (1981) 993e996. [46] R.-p. Wang, G.-w. Zhou, Y.-l. Liu, S.-h. Pan, H.-z. Zhang, D.-p. Yu, Z. Zhang, Phys. Rev. B 61 (2000) 16827. [47] W.-R. Liu, Z.-Z. Guo, W.-S. Young, D.-T. Shieh, H.-C. Wu, M.-H. Yang, N.-L. Wu, J. Power Sources 140 (2005) 139e144. [48] M. Obrovac, L. Krause, J. Electrochem. Soc. 154 (2007) A103eA108. [49] L. Zhang, W. Hao, H. Wang, L. Zhang, X. Feng, Y. Zhang, W. Chen, H. Pang, H. Zheng, J. Mater. Chem. A 1 (2013) 7601e7611.
