SnLi4.4 nanoparticles encapsulated in carbon matrix as high performance anode material for lithium-ion batteries

SnLi4.4 nanoparticles encapsulated in carbon matrix as high performance anode material for lithium-ion batteries

Nano Energy (2014) 9, 196–203 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/nanoenergy RAPID COMMUNICATION S...

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Nano Energy (2014) 9, 196–203

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

RAPID COMMUNICATION

SnLi4.4 nanoparticles encapsulated in carbon matrix as high performance anode material for lithium-ion batteries Xiulin Fan, Jie Shao, Xuezhang Xiao, Xinhua Wang, Shouquan Li, Hongwei Ge, Lixin Chenn State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Application for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China Received 23 April 2014; received in revised form 27 June 2014; accepted 23 July 2014 Available online 1 August 2014

KEYWORDS

Abstract

Lithium-ion battery; Anode; SnLi4.4; Tin

Induction melting associated with simple ball-milling is utilized to synthesize a [email protected] core– shell hierarchical composite in which nanometer-sized SnLi4.4 particles are uniformly dispersed and encapsulated by carbon matrix. When evaluated as anode materials for lithium ion batteries, the composite exhibits a reversible capacity of 680 mA h g 1 after 200 cycles at 200 mA g 1. A capacity of 310 mA h g 1 is obtained even at a high rate of 5000 mA g 1. The superior electrochemical performance is ascribed to the fact that the prelithiated SnLi4.4 will not exert any expansion stress on the carbon matrix during the subsequent delithiation and lithiation processes, therefore guarantee the sustainable integrity of the composite in the prolonged cycling. The carbon matrix offers continuous transport paths for Li + ions and electrons inside the composite. Meanwhile the carbon can sufficiently prevent the disintegration and aggregation of Sn nanoparticles upon prolonged cycling. The present study effectively circumvents the low initial Coulombic efficiency of the Sn-related nanocomposites and provides a protocol for pairing lithium-free cathodes to make the next-generation high energy lithium ion batteries. & 2014 Elsevier Ltd. All rights reserved.

Introduction n

Corresponding author. E-mail address: [email protected] (L. Chen).

http://dx.doi.org/10.1016/j.nanoen.2014.07.020 2211-2855/& 2014 Elsevier Ltd. All rights reserved.

High capacity alloying anode materials for lithium ion batteries, such as silicon and tin, have stimulated great research interest in the last 10 years [1]. Tin is a promising

SnLi4.4 nanoparticles encapsulated in carbon matrix as high performance anode material for lithium-ion batteries anode active material due to its extremely high volumetric capacity (7313 mA h mL 1) and high gravimetric capacity (993 mA h g 1), which is about 9 and 3 times that of the commercial graphite, respectively [2]. However, one of its primary disadvantages is that large volume variations (260%) will lead to rapid pulverization of Sn particles and loss of capacity during cycling, and thereby hinders its practical applications [3]. Common approaches to improving the cycling performance of tin-based anode are to use nanostructured tin [4], and Sn composites [5]. Quite recently, significantly improved electrochemical performance was achieved for the embedded/confined Sn anodes [6]. These nanocomposites can better accommodate the volume change due to the faster stress relaxations which enhance the cycling performance of the materials. In addition, nanoparticles of anode provide a much shorter diffusion path for the lithium ions, which also improves the rate capability of the electrodes [7]. Unfortunately, nanoscale geometries require advanced processing methods with carefully designed procedures, especially for the low melting point of Sn (232 1C), which results in increased anode powder costs. Although the cycling performance and rate capability were enhanced for the nanostructured tin composites, however, a drawback to this approach is that the high surface area of nanostructured materials significantly increases solid electrolyte interphase (SEI) formation during the first electrochemical cycle, which causes a high irreversible capacity loss and leads to low Coulombic efficiency values such as 25–60% [4c,6i,8], depending on the structure of tin and the composition of the anode composite. The relatively low values of Coulombic efficiency need excess cathode materials to offset the irreversible lithium ions and prevent high specific capacity anodes from being viable when paired with cathodes in a “full cell”. In addition, due to the large surface to volume ratio of nanoparticles, a low tap density is obtained, which reduces the volumetric capacity of the electrode [4d]. Recently, many cathode materials, such as V2O5 [9], MnO2 [10], sulfur [11], and metal fluorides [12] have shown the potential to have much higher Li storage capacity compared with the commercial LiCoO2 and LiFePO4 cathode materials. To further increase the energy of the batteries, it is desirable to pair Sn or Si anodes with high capacity cathodes [13]. Although these cathode materials have many advantages (e.g. low cost, abundance, and high energy); however, neither anode nor any of these cathodes contains lithium. Therefore, for this type of combination, either the cathode or the anode needs to be prelithiated [13]. To circumvent the low initial Coulombic efficiency of Sn-anodes and even provide an anode candidate for the lithium-free cathode materials, we synthesized the hierarchical [email protected] composite. By cost-effective ball milling the pre-lithiated Sn anode materials (SnLi4.4) with carbon black, a facile and scalable method is demonstrated to make Sn anodes viable in practical batteries. Previously, ball-milling has been utilized as an effective way to synthesize the composites used for lithium electrode materials [14]. However, for the Sn-related anode, some rigid abrasives should be added [15] or some special devices are utilized during ball-milling [16] due to the excellent ductility of Sn. After alloying with Li, the material becomes

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brittle and fragile [17], which can be easily smashed into nanoparticles during ball milling. When further ball milled with carbon, the nanometer-sized SnLi4.4 particles become uniformly dispersed in the flexible matrix of carbon, forming a composite with size of microns or sub-microns. Within this hierarchy, SnLi4.4 nanoparticles are well confined by the carbon matrix, and will not exert any expansion stress on the carbon matrix during cycling, therefore resulting in an excellent electrochemical performance in terms of capacity, Coulombic efficiency, and cycling performance. Being in the fully lithiated state, SnLi4.4 overcomes the low initial Coulombic efficiency of as-mentioned Sn nanocomposites and allows the use of these mentioned lithium-free cathodes.

Experimental section Materials Tin foil (99.5%), lithium (99.9%), were purchased from Sinopharm Chemical Reagent Co., Ltd. Materials handling and sample preparation were performed in an argon-filled glovebox, where the oxygen and water concentration were kept below 1 ppm. SnLi4.4 was synthesized by induction melting stoichiometric mixtures of Tin foil and lithium metals in argon atmosphere. The working voltage of induction melting apparatus is 380 V, with a working current of about 10 A. The as-prepared ingots were smashed and then mechanically milled for 40 h under 0.1 MPa of pure argon atmosphere by the Planetary mill (QM-3SP4J, Nanjing) at 400 rpm to prepare SnLi4.4 powder. Then, the SnLi4.4 powder with carbon black (7:3 in weight) was further milled for 40 h to prepare [email protected] composite. Tens grams of composite can be prepared in a vial.

Characterization X-ray diffraction experiments of the samples were performed on an X'Pert Pro X-ray diffractometer (PANalytical, the Netherlands) with Cu Kα radiation at 40 kV and 40 mA. XPS was carried out on a VG ESCALAB MARK II system with Mg Kα radiation (1253.6 eV) at a base pressure of 1  10 8 Torr. All binding energy values were referenced to the C 1s peak of carbon at 284.6 eV with an uncertainty of 70.2 eV. Transmission electron microscopy (TEM, Tecnai G2 F30) and SEM (Hitachi SU-70) equipped with energy dispersive spectrometer (EDS) were performed to examine the morphology and microstructure of the products. During the sample transfer and measurements, a lab-built argon filled container was used to protect the samples from air and moisture.

Electrochemical investigation The electrochemical tests were performed using a coin-type half cell (CR 2025). Metallic lithium was used as the negative electrode. The as-prepared [email protected] composite was uniformly spread onto a nickel foam current collector, and then pressed under a pressure of about 10 MPa. The typical mass loading of the active material was E4 mg/cm2. The electrolyte solution comprised 1 M LiPF6/ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylmethyl carbonate (EMC) (1:1:1 by volume).

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The cells were assembled with a polypropylene (PP) microporous film (Celgard 2300) as the separator. Charge–discharge experiments were performed at different current densities between 0.005 and 3 V using a LAND CT2001A Battery Cycler at room temperature. Cyclic voltammetry (CV) was performed on the CHI660E electrochemical workstation in the potential range 0–3.0 V (vs Li + /Li) at a scan rate of 0.1 mV/s.

Results and discussion Material preparation and characterization Figure 1 shows the XRD patterns of as-cast SnLi4.4, ball milled SnLi4.4 and [email protected] composite. For comparison, the pattern of carbon black is also compiled. All the main peaks of as-cast SnLi4.4 could be indexed to the cubic SnLi4.4 (JCPDS no. 650296, space group: F23, 196), while no peaks of Sn or Li were detected. The diffraction peaks of SnLi4.4 after ball milling appeared much weaker and broader compared to the XRD patterns of the as-cast SnLi4.4 sample, implying a significant decrease in size and crystalline correlation length through ball milling. After ball milled with carbon black, the peaks of SnLi4.4 were further broadened, with no peaks of carbon detected. Therefore, it is reasonable to suggest that the long-range ordered graphite structure is mostly destroyed during ball milling with SnLi4.4. The fragile SnLi4.4 would be nanocrystalline and even disordered after the prolonged ball milling process, resulting in the broadening and weakening of the XRD patterns. Using the Scherrer equation (d=0.9λ/ βcosθ), the average crystalline size of SnLi4.4 was estimated to be 9.5 and 6 nm from 2θ and λ values of the highest peak (822) of SnLi4.4 for the ball milled SnLi4.4 and [email protected] composite, respectively. These results indicate that ball milling is a good approach for reducing the as-cast SnLi4.4 to nanometer-sized particles, due to the evident fragility of the material. The dramatic decrease of the grain size will promote the facile diffusion of the lithium ion during electrochemical cycling. The morphology of the as-prepared composite was studied using transmission electron microscopy (TEM, Figure 2a–d) and scanning electron microscopy (SEM, Figures S1 and S2). Figure 2a shows a typical irregular shaped as-prepared [email protected] particle with dimension at  1.5 mm. In the magnified TEM image (Figure 2b), it can be clearly seen that the

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particles have a hierarchical structure: small primary islandlike SnLi4.4 nanoparticles are uniformly distributed and enclosed in the large secondary carbon particles. Figure 2c displays a set of parallel fringes with the space of 0.34 nm in the shell of the secondary particles, corresponding to the undestroyed (002) planes of carbon. Apart from this set of fringes, few lattices are observed in the surface layer, indicating the amorphization of carbon black during ball milling, in line with the XRD results. In the HADDF-STEM mode, it is more obvious to observe the distribution of different elements, as shown in Figure 2d. The darker color in the composites represents the light element, which is carbon here, and the much brighter color represents the SnLi4.4 alloy particles. Nanoparticles of SnLi4.4 are homogeneously dispersed and wrapped in the carbon matrix. The element distribution is further evidenced by the cross-sectional compositional line profiles of the composite (Figure 2e), which clearly shows that the outer shell of the composite is composed of carbon and the SnLi4.4 nanoparticles are dispersed in the carbon matrix. A typical composite particle generally contains thousands of SnLi4.4 nanoparticles with size from several nanometers to tens nanometers. The SEM elemental mapping image of [email protected] composite confirms that all of the particles are encapsulated by carbon, as shown in Figure S2. In order to further verify if our synthesized [email protected] composite has a high-quality encapsulation structure, a surface-sensitive XPS experiment was carried out to examine the chemical characteristics of the surface of the composites. For comparison, pure SnLi4.4 was also tested. Figure 2f shows the full XPS spectrums of [email protected] composite and pure SnLi4.4, in which the photoelectron lines at a binding energy of about 285 and 532 eV are attributed to C 1s and O 1s, respectively [18]. Intensive peaks of Sn 3d5/2 and 3d3/2 at 486 and 494 eV for the pure SnLi4.4 can be observed, but there is almost an absence of the signals of Sn 3d5/2 and 3d3/2 for [email protected] composite. Because the atomic sensitivity factor of Sn is much higher than that of C [19], absence of its peaks implies that, in our synthesized [email protected] composite, the SnLi4.4 particles appear to be completely sealed inside the carbon layers. The formation of the hierarchical nanostructure for the [email protected] composite is hypothesized to arise from the intrinsic physical properties of the SnLi4.4 and carbon black, and the scheme is shown in Figure 2g. The SnLi4.4 is fragile and brittle while the carbon black is much ductile and flexible. In the initial stages of milling, the ductile carbon black can easily be deformed into lamellae along the (002) planes by ball-powder-ball collisions, while the brittle SnLi4.4 particles can be readily fragmented and comminuted into nanometer size. These fragmented brittle nanoparticles tend to become occluded by the ductile constituents and trapped in the ductile particles [20]. With further milling, the ductile lamellae get convoluted and refined, and meanwhile the brittle nanoparticles get uniformly dispersed among the ductile matrix [20b]. It is believed that the entire core–shell structure will be more favorable to keep the architecture integrity during electrochemical discharge and charge processes compared with the bare and pure SnLi4.4 nanoparticles. In this embedded hierarchical architecture, the carbon matrix offers a robust framework that provides good conductivity and also acts as a mechanical support to prevent the agglomeration and pulverization of the individual Sn nanoparticles during

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Figure 2 (a) and (b) TEM images of [email protected]; (c) HR-TEM image of [email protected]; (d) and (e) HADDF-STEM image of [email protected] and the corresponding cross-sectional compositional line profiles of [email protected] (f) XPS spectra of bare SnLi4.4 without carbon black and [email protected]; (g) schematic illustration of the preparation process of the [email protected] composite, IM and BM mean induction melting and ball milling, respectively.

electrochemical lithiation/delithiation process. As Sn is in the lithiated state (SnLi4.4) initially, it will not exert any expansion stress on the carbon matrix during the subsequent lithiation/delithiation process, therefore the carbon matrix can keep much better stability compared with other confined composites [14a]. The uniform dispersion of nano-SnLi4.4 particles in the composite can effectively reduce the distance that Li-ions and electrons must travel during cycling in the composite.

Electrochemical performance Figure 3a shows the charge–discharge profiles of the [email protected] composite at a current rate of 200 mA g 1 between 0.005 and

3 V. The initial discharge and charge capacities of [email protected] composite are 313 and 743 mA h g 1 with an apparent Coulombic efficiency of 237%. Because the main active species of Sn has been prelithiated, the limited first discharge capacity can be mainly ascribed to the lithiation of carbon, the formation of SEI films and the electrolyte decomposition. The first delithiation capacity is close to the theoretical capacity (775 mA h g 1) of the [email protected] composite (SnLi4.4: C=7:3 by weight), based on the theoretical capacity of carbon (372 mA h g 1) and metallic tin (993 mA h g 1) (the theoretical capacity was calculated by assuming the mixture of SnLi4.4 and carbon according to Ctheoretical =CSn  %mass of Sn +Ccarbon  %mass of carbon=992  0.65+372  0.35=775 mA h g 1. In the equation, the pre-inserted Li has been deducted, therefore the weight of Sn and carbon accounts for 65% and

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35% in the composite, respectively). For the electrochemical cycles, the voltage plateaus occurring near 0.6 V are apparent in both discharge (lithiation) and charge (delithiation) profiles, reflecting the process of Li + insertion and deinsertion. These plateaus are typical characteristics of Sn electrodes [6h,21]. The cycling performance of SnLi4.4 and [email protected] was evaluated under the same conditions, as shown in Figure 3b. The SnLi4.4 electrode shows an initial charge capacity of 906 mA h g 1. Then, the capacity continuously decreases

within the following 40 cycles and exhibits a capacity of about 330 mA h g 1 in the 41th cycle with only 36% capacity retention. For comparison, the [email protected] composite exhibits an excellent cycling performance, although slight decrease of capacity was also observed in the first several cycles. The Coulombic efficiency approaches 99% after the 6th cycle, suggesting its high reversibility. Interestingly, the reversible capacity of the [email protected] composite gradually increases to 700 mA h g 1 from 40th cycle to the 100th cycle. The behavior of capacity increase has been reported in the Sn/C nanocomposite [6g,22], SnO2 nanocomposite [23] and other metal oxide systems [24]. After 200th cycle, a capacity of 680 mA h g 1 can be obtained. These comparisons indicate that the unique architecture with active SnLi4.4 nanoparticles dispersed within carbon matrix and encapsulated by carbon layer can provide high reversible capacity for alloying/dealloying reactions, realizing an almost completely reversible alloying reaction at a wide voltage window. Considering the SnLi4.4 content in the composite, it is clear that the material maintains capacities close to the theoretical values for Sn even after hundreds of charge–discharge cycles, and exhibits minimal irreversible losses and excellent Coulombic efficiency. Such unique nanostructure endows the composite with excellent electrochemical performance as an anode material for lithium ion batteries. To the best of our knowledge, this is the first time to directly utilize the lithiated Sn as anode materials, which exhibits a superior electrochemical performance. Induction melting and ball milling are widely used preparation methods in industry. Therefore, it is believed that asprepared anode material can be easily extended to large scale production. Figure 3c shows the cyclic voltammograms (CVs) of the [email protected] electrode after 200th cycling at a scan rate of 0.1 mV/s between 0 and 3 V. The reduction peaks at 0.2, 0.35 and 0.55 V are overlapped and formed an evident cathodic peak, corresponding to the lithiation of Sn. Oxidation peaks at 0.52, 0.67, and 0.78 V are assigned to delithiation reaction of the LixSn alloy. The unchanged peak position and current intensities imply excellent electrochemical cycling stability of the [email protected] composite. The superior stability is attributed to the favorable [email protected] electrode structure mentioned above. The [email protected] composite also shows favorable high-rate capability, as illustrated in Figure 4. The hierarchical composite can deliver a capacity of  605 mA h g 1 at 500 mA g 1,  545 mA h g 1 at 1000 mA g 1, and 425 mA h g 1 at 2000 mA g 1; even at a current density of as high as 5000 mA g 1, it can still exhibit a charge–discharge capacity of about 310 mA h g 1. More importantly, when the current rate gradually returns to 200 mA g 1, a stable capacity of 650 mA g 1 can revert. Carbon black is networked carbon with excellent electrical connectivity [25]. Under dry ball-milling conditions, the carbon black becomes strongly bonded to the SnLi4.4 nanoparticles, which can provide the [email protected] composite anode with improved current collection capability, thus an excellent rate capability. The significantly improved cycling performance and rate capability could be attributed to the following three main reasons. First, the unique hierarchy with island SnLi4.4 embedded in the carbon matrix can form stable and rapid lithium and ion channels during electrochemical cycling.

SnLi4.4 nanoparticles encapsulated in carbon matrix as high performance anode material for lithium-ion batteries Second, pulverization of the composite incurred by volume expansion would be alleviated, because there is no need to create new volumes in the composite for the incoming Li, as in the case of traditional Sn/C composite. Third, the carbon matrix can effectively prevent the Sn nanoparticles from aggregation and disintegration during electrochemical cycling. To further understand the reaction mechanism and cycling stability of the [email protected] electrode, we performed SEM, EDS, TEM and SAED analysis to characterize the structural and morphological changes of the electrode after 200th dealloying reaction. Figure 5a and b presents SEM images of the [email protected] composite after 200th delithiation reaction. No pulverization and disintegration of the electrode can be observed, which can explain the excellent cycling stability of the [email protected] composite. As previously mentioned, the encapsulated SnLi4.4 nanoparticles will not exert tensile stress on the carbon matrix 1000

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during the cycling, thereby circumvent the inevitable fracture and pulverization of the composite. We also performed the element mapping (C, Sn and F) on the selected area of the composite, as shown in Figure 5c. The C-map shows the carbon matrix. It clearly indicates that the Sn-map exhibits a very homogenous distribution and coincides well with the C-map, demonstrating Sn is highly dispersed in the carbon matrix. This homogeneity of dispersed Sn distribution without any detectable aggregation is favorable for prolonged electrochemical cycling. The signals of F element come from the LiPF6 in the electrolyte. Detailed structure of the [email protected] composite after cycling is also characterized by the TEM, SAED, EDS and HRTEM. The images (Figure 5d and e) clearly show that most of the fine grains are with a diameter of less than 8 nm (although some bigger ones with size of about 20 nm can also be observed) inside the carbon matrix. SAED, EDS and the HRTEM confirmed that these ultra-small particles are nano-Sn particles. The corresponding diffuse ring-like SAED patterns indicate the dramatic reduction of Sn crystallites after cycling. In addition, magnified TEM image of cycled composite shows an outer carbon layer, which is further evidenced by EDS results (inset of Figure 5e). The highresolution TEM image shown in Figure 5f reveals that lattice fringes with a basal distance of 0.29 nm can be observed from the locally magnified image of the Sn nanoparticles (the upper left inset of Figure 5f), which is in good agreement with the (200) lattice spacing of Sn (JCPDS no. 65-0296). No graphite lattice is observed in the carbon matrix, which again confirms the amorphization of carbon black. Based on the above analysis, it is concluded that during repeated electrochemical cycling, the SnLi4.4 will collapse into ultra-small nanoparticles because of the

Figure 5 (a) and (b) SEM images of [email protected] after 200th electrochemical cycles. (c) EDS mapping of Sn, C and F for [email protected] after 200th electrochemical cycles. (d) and (e) TEM images of [email protected] after 200th electrochemical cycles, the insets of (d) and (e) are SAED patterns of the composite and EDS results of A and B areas in (e); (f) HRTEM image of [email protected] after 200th electrochemical cycles.

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tremendous volume variation during delithiation and lithiation, while the composite remains constant. The in situ formed ultra-small Sn nanoparticles are stably embedded in the carbon matrix, which can effectively alleviate the large volume change and curb the pulverization and aggregation of Sn nanoparticles in the subsequent cycling. This can explain why the [email protected] composite could have the amazing cycling performance and rate capability, while the bare SnLi4.4 particles exhibit a rather poor cycling performance.

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[3]

[4]

Conclusions Hierarchical [email protected] composite with SnLi4.4 nanoparticles uniformly dispersed in the carbon matrix was simply and successfully fabricated by induction melting of Sn and Li and subsequent ball milling with carbon black. Using as an anode material for Li ion batteries, the as-prepared [email protected] composite exhibits a charge–discharge capacity of 680 mA h g 1 at 200 mA g 1 after 200th cycle. A capacity of 310 mA h g 1 was obtained even at a high current density of 5000 mA g 1. The excellent electrochemical performance of the [email protected] composite is due to (1) nanoscaled particles of SnLi4.4 distributed in the carbon matrix, which can not only serve as the favorable electron and lithium ion transport channels, but also effectively curb the pulverization and aggregation of Sn nanoparticles in the prolonged cycling; (2) because the active material of SnLi4.4 have been prelithiated, the pulverization incurred by volume expansion would be alleviated and therefore, much more stability can be achieved for the composite. The simplified preparation process and the high electrochemical performance of the [email protected] composite make it promising in the application as the anode materials for lithium-ion batteries, especially for paring with the lithium-free cathode materials for the next generation lithium ion batteries.

[5]

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Acknowledgments The authors gratefully acknowledge the support of the National Natural Science Foundation of China (21303161), Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), the Key Science and Technology Innovation Team of Zhejiang Province (2010R50013), the China Postdoctoral Science Foundation (2012M521167) and the International Postdoctoral Exchange Program (2013).

Appendix A.

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Supporting information

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2014.07.020.

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Jie Shao is currently pursuing his Ph.D.in Department of Materials Science and Engineering at the Zhejiang University under the supervision of Prof. Lixin Chen. He received his B.Sc in Metal Material Engineering from Sichuan University in 2010. His current research interests are the basic researches and applications on renewable energy storage materials, including novel complex hydrides for hydrogen storage, nano/amorphous energy storage materials. Xuezhang Xiao received his Ph.D.in Materials Science from Zhejiang University (ZJU) in 2008. After two-year of postdoctoral research training in Chemical Engineering and Technology of ZJU, he joined the Department of Materials Science and Engineering of ZJU as an Associate Professor in Dec.2010. His main research interests are the basic researches on nano-renewable energy materials. Xinhua Wang received his Ph.D. in Department of Materials Science and Engineering from the University of Tokyo in 2003. He is now a professor in Zhejiang University. His research is focused on energy materials.

Shouquan Li received his bachelor’s degree in Mechatronic Engineering from Zhejiang University. He is currently a senior engineer in Department of Materials Science and Engineering at the Zhejiang University. His interests are basic researches and applications on renewable energy storage materials, including Ni-MH and Lithium-ion batteries.

Hongwei Ge is currently an associate engineer in Department of Materials Science and Engineering at the Zhejiang University. Her research interests are design and characterization of nanostructured materials related to energy storage and conversion devices.

Lixin Chen received his Ph.D. in Department of Materials Science and Engineering from Zhejiang University in 2000. He joined Zhejiang University as a lecturer in 1992 and became a Professor in 2005. His research is focused on energy materials for hydrogen storage, hydrogen energy technologies and secondary batteries.