Spherical composite as high-rate and ultra-stable anode material for sodium ion batteries

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1 Nano Research DOI /s Nano Res 1 Spherical nano-sb@c composite as high-rate and ultra-stable anode material for sodium ion batteries Ning Zhang 1, Yongchang Liu 1, Yanying Lu 1, Xiaopeng Han 1, Fangyi Cheng 1, and Jun Chen 1,2 ( ) Nano Res., Just Accepted Manuscript DOI /s on June 12, 2015 Tsinghua University Press 2015 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer -review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 Spherical composite as high-rate and ultra-stable anode material for sodium ion batteries Ning Zhang, Yongchang Liu, Yanying Lu, Xiaopeng Han, Fangyi Cheng and Jun Chen* Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin , China Spherical composite prepared by a one-pot method was used as an anode material for SIBs, showing ultra-high rate capability and long cycling stability.

3 Nano Research DOI (automatically inserted by the publisher) Research Article Spherical composite as high-rate and ultra-stable anode material for sodium ion batteries Ning Zhang 1, Yongchang Liu 1, Yanying Lu 1, Xiaopeng Han 1, Fangyi Cheng 1 and Jun Chen 1,2 ( ) Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014 KEYWORDS Antimony, micro-nanostructured composite, anode material, sodium ion batteries ABSTRACT An aerosol spray pyrolysis technique is used to synthesize spherical nano-sb@c composite. Instrumental analyses reveal that the micro-nanostructured composite with an optimized Sb content of 68.8 wt% is composed of ultrasmall Sb nanoparticles (10 nm) uniformly embedded within a spherical porous carbon matrix (denoted as 10-Sb@C). The content and size of Sb can be controlled by altering the concentration of the precursor. As anode material of sodium ion batteries, 10-Sb@C provides the discharge capacity of 435 mah g -1 in the second cycle and 385 mah g -1 (a capacity retention of 88.5 %) after 500 cycles at 100 mah g -1. In particular, the electrode exhibits excellent rate capability (355, 324 and 270 mah g -1 at 1000, 2000 and 4000 ma g -1, respectively). Such high-rate performance for Sb-C anode has rarely been reported previously. The remarkable electrochemical behavior of 10-Sb@C is attributed to the synergetic effects of ultrasmall Sb nanoparticles with a uniform distribution and a porous carbon framework, which could effectively alleviate the stress associated with large volume change and suppress the agglomeration of the pulverized nanoparticles during prolonged charge-discharge cycling.

4 2 Nano Res. 1. Introduction Driven by the fast development of portable electronics and electric vehicles, the need of lithium ion batteries (LIBs) has been growing [1, 2]. While, one major concern is that the inevitable raising cost of Li source due to the limited abundance and the uneven distribution of Li salts on the earth s crust. As a viable alternative, sodium ion batteries (SIBs) have recently attracted much attention because of the greater abundance, more wide distribution and lower cost of Na source [3-5]. In the research filed of electrode materials, SIBs are following the footsteps of LIBs due to that Na holds similar chemical properties to Li. However, the investigation is still insufficient, mainly because that Na + is larger and heavier than Li +, limiting Na + insertion and extraction from host materials [6-8]. Particularly, graphite is a widely-commercialized anode for LIBs, but possesses low capacity for Na storage due to its interlayer distance mismatching the larger Na + [9]. Even as the most promising anode for LIBs [10], silicon has not been reported the electrochemical sodiation behavior in experiment because Si could not insert Na ion like that of Li ion. Thus, extensive explorations have been launched to find suitable anode materials for SIBs [11-14]. Anodes with alloying reactions (such as Sn, Ge and Sb) have been studied in SIBs, due to their higher gravimetric and volumetric specific capacity than that of carbonaceous material (e.g. graphite) [15-17]. Among them, metallic Sb has attracted much attention as an anode material for SIBs owing to its theoretical sodium storage capacity (660 mah g -1 ) and is considered as a proper anode candidate for SIBs [17-21]. However, alloying anodes usually undergo large volume change during Na + insertion and extraction, which leads to loss of electric contact, and eventually rapid capacity fading [22, 23]. To improve battery performance, nanostructured active materials are frequently proposed because of the facile stress release [24, 25]. In addition, reducing the particles size of alloying materials to nanoscale could facilitate the transfer of alkali metal ion, benefiting high rate performance [26, 27]. However, nanoparticles tend to aggregate during cycling, limiting its long-term cycle life. To effectively settle these problems, homogeneously dispersing nanoparticles in a conductive matrix such as carbon could effectively accommodate volume expansion, inhibit the aggregation of nanoparticles and maintain the integrity of the whole electrode [28, 29]. For example, Sb nanoparticles ( nm) with the decoration of N-rich carbon nanosheets were synthesized by sol-gel method [30], displaying 305 mah g -1 at 50 ma g -1 after 60 cycles and 220 mah g -1 at 2 A g -1. However, the Sb content of 15.6 wt% is low, which would limit the specific capacity of the electrode. Wang s group had synthesized Sb/C fibers (54 wt% Sb) with 30 nm Sb nanoparticles encapsulated in 400 nm carbon fibers using electrospinning method [31]. The as-prepared sample showed a reversible capacity of 350 mah g -1 after 300 cycles under 100 ma g -1. Very recently, Cao and co-workers also reported Sb-C nanofibers through electrospinning technique and subsequent calcination. Sb nanoparticles of 20 nm embedded in carbon fibers with 38 wt% Sb content show a stable cycle performance [32]. However, fabricating uniformly Sb nanoparticles (about 10 nm) dispersed in carbon matrix with high Sb loading by one-step method remains a challenge. In particular, the high-rate performance to meet practical applications is still needed. Three-dimension spherical composites are generally considered as stable and high packing density electrode materials, as well as easily handling during electrode manufacturing [28]. Recent studies disclosed that spray-based method is a robust way to prepare various nanomaterials. Compared with some traditional methods (hydrothermal route, sol-gel method, ball-milling, etc.), the spray-assisted approach shows a simple operation, rapid reaction and scalable production, which has been used a while for wide range of industrial products [33, 34]. Generally, it is facile and inexpensive to prepare micro-sized spherical carbon composites with various nanostructures [34-37]. In this work, we report a one-pot strategy for in-situ fabrication of a spherical porous nano-sb@c composite employing aerosol spray pyrolysis and applied it as anode of SIBs. The micro-nanostructured composite realizes the ideal structure in which ultrasmall Sb nanoparticles (10 nm) were uniformly embedded in spherical conductive carbon matrix. The nano-sb@c electrode

5 Nano Res. 3 exhibits high-rate capability and ultra-stable cyclability, owing to the unique structure with good stress accommodation, rapid Na + diffusion and excellent integrity protection of the electrode. 2. Experimental section 2.1 Materials synthesis: The spherical nano-sb@c composite was synthesizd through an aerosol spray pyrolysis method, as schematically shown in Figure S1 (supporting informantion (SI)). The size of Sb particles and the morphology of the Sb@C composites can be controlled by changing the precursor composition and reaction temperature (Figure S2-3, SI). For the synthesis of ultrasmall Sb nanoparticles (10 nm) uniformly embedded within a spherical porous carbon matrix (denoted as 10-Sb@C), a brief procedure follows. The resorcinol formaldehyde (RF) resin solution was prepared by dissolving 7.7 g resorcinol in 7.5 ml formaldehyde at room temperature and evaporating in water bath with heating at 50 o C for 2h to obtain a colloidal solution. The as-prepared RF solution and 0.1M SbCl3 were dissolved in 250 ml ethanol, stirring to prepare an aqueous solution. The precursor solution was atomized with argon flow by a constant output nebulizer, in which aerosol droplets were passed through a tubular furnace at 800 o C for residence time about 2 s. The carbonization of RF and the decomposition of Sb source simultaneously yield nano-sb@c composite. For comparison, Sb@C composite with Sb size of 20 nm (denoted as 20-Sb@C) and bulk Sb (denoted as bulk-sb@c) were synthesized using the same synthesized procedures, but just changing the concentration of SbCl3 to 0.2 M and 0.5 M, respectively. Since metal ions act as a catalyst to promote the formation of carbon in a short time, the pyrolyzed carbon could not be synthesized directly by aerosol spray pyrolysis method if the precursor solution did not contain Sb 3+. The pure Sb powder was purchased from Sigma-Aldrich. 2.2 Materials Characterization: The structures of Sb@C composites were investigated by X-ray diffraction (XRD, Rigaku MiniFlex600, Cu Karadiation) and transmission electron microscopy (TEM, JEOL 2100F). The morphologies of the composites were characterized by field-emission scanning electron microscopy (SEM, JEOL JSM7500F). The carbon contents and structures of the as-prepared samples were detected using a TG-DSC analyzer (NETZSCH, STA 449 F3) in air from room temperature to 600 o C at a heating rate of 5 C min -1 and confocal Raman microscope (DXR, Thermo-Fisher Scientific, 532 nm excitation). 2.3 Electrochemical measurements: Electrochemical performance was measured using CR2032 coin-type cells. Na metal and glass fiber were used as counter electrode and separator, respectively. The anodes of sodium ion batteries were fabricated by blending the active material (AM), SP carbon and carboxymethyl cellulose (CMC) in deionized water with a ratio of AM:SP:CMC=80:10:10. The obtained slurry was pasted onto a copper foil with an active material loading of ~ 0.8 mg cm -2, and dried at 110 o C for 10 hours in vacuum. The electrolyte was 1.0 M NaClO4 dissolved in propylene carbonate (PC) with 5% fluoroethylene carbonate (FEC). Cyclic voltammograms (CVs) were tested at a scan rate of 0.1 mv S -1 from 0.01 V to 2.0 V. The discharge/charge cycling performance was investigated in the voltage range of 0.01 V to 2.0 V at various current densities, using a LAND-CT2001A battery-testing system. Electrochemical impedance spectroscopy (EIS) was performed on an AC voltage of 5 mv amplitude in the frequency range from 100 khz to 100 mhz. Before EIS tests, the assembled cells were first discharged/charged at 100 ma g -1 for 5 cycles to stabilize the cells. 3. Results and discussion Figure 1a shows the SEM image of nano-sb@c composite synthesized by aerosol spray pyrolysis, exhibiting spherical shape with an average diameter of 350 nm (Figure S4, SI). From the TEM images in Figure 1b-c, it is clearly seen that Sb nanograins (black dots) are finely located in the spherical carbon matrix (grey color). The average size of Sb nanoparticles is about 10 nm (Figure S5, SI), being denoted as 10-Sb@C. The high resolution TEM (HRTEM) image (inset of Figure 1c) displays a set of parallel fringes with the space of nm, corresponding to the (012) plane of Sb crystal structure (JCPDS No ). Figure 1d-e shows the TEM elemental mapping images of 10-Sb@C, indicating that Sb nanoparticles are homogeneously Nano Research

6 4 Nano Res. distributed and confined within the carbon framework. For comparison, composites with 20 nm Sb nanoparticles (denoted as and bulk Sb (denoted as were also prepared, as shown in Figure S6 (SI). Figure 1 (a) SEM image of 10-Sb@C composite. (b) TEM image, and (c) Magnified TEM image of 10-Sb@C. Inset: high-resolution TEM image (HRTEM). TEM elemental mapping images of 10-Sb@C for carbon (d) and Sb (e). Figure 2a shows the X-ray diffraction (XRD) patterns of 10-Sb@C, 20-Sb@C and bulk-sb@c. All of the XRD peaks can be well indexed to rhombohedral Sb (JCPDS No ). No impurities are detected because (1) when ethanol is used as the solvent, no hydrolysis of SbCl3 occurs, and (2) carbon in-situ decomposed by RF resin can effectively protect Sb particles against oxidizing. Figure 2b displays the Raman spectra of Sb@C composites. Compared with pure antimony, the peaks of the as-prepared samples around 110, 150 and 260 cm -1 are typical metallic Sb signals. Moreover, there are two characteristic peaks at 1330 and 1600 cm -1, attributed to disorder-induced feature (D band) and the E2g mode of graphite (G band) of carbon materials, respectively. Moreover, the porosity of 10-Sb@C was performed by Brunauer Emmett Teller (BET) measurement (Figure S7, SI). The nitrogen absorption desorption isotherms is a type-iv isotherm, suggesting that the composite contains numerous mesopores. This was formed by gas volatilization during the decomposition of RF resin along with pyrolysis of SbCl3. From the BET data, the composite holds a high surface of m 2 g -1, and the Barrett Joyner Halenda (BJH) pore size distribution of 10-Sb@C is narrow and centered around a diameter of 3.8 nm. The presence of a porous structure is important to good ionic conductivity due to well-impregnated active materials with electrolyte. This also provides sufficient void space for expansion and shrinking of nanoparticles during sodiation/desodiation cycling. The composition of 10-Sb@C composite was measured by thermogravimetric analysis (TGA) in air, as shown in Figure S8 (SI). From 100 to 230 o C, almost no weight loss was observed, indicating that the composite is stable in air up to 230 o C. In addition, the TGA curve exhibits two step weight increase and one step weight loss after 230 o C. The first weight increase from 230 to 320 o C is associated with the oxidation of metallic Sb (2 Sb + 3 O2 2 Sb2O3). The followed weight loss from 320 to 470 o C is mainly due to the decomposition of carbon component (C + O2 CO2). The second weight increase after 470 o C is attributed to further oxidation of Sb2O3 (Sb2O O2 Sb2O4). The two step oxidation products of Sb is confirmed by XRD analysis after heating the 10-Sb@C at 320 o C and 600 oc in air for 1 hour, respectively (Figure S9, SI). In addition, the TGA curves of 20-Sb@C and bulk-sb@c are shown in Figure S10 (SI). The content of Sb in 10-Sb@C is 68.8% and the volumetric density of 10-Sb@C is ~1.98 g cm -3. Moreover, the content of Sb in 20-Sb@C and bulk-sb@c are 71.6% and 77.8%, respectively. The mass loading of Sb is higher than that in other reported Sb-C composites [30-32].

7 Nano Res. 5 Figure 2 (a) XRD patterns for 10-Sb@C, 20-Sb@C and bulk-sb@c compared with JCPDS No (b) Raman spectra of 10-Sb@C, 20-Sb@C and bulk-sb@c compared with pure Sb. The electrochemical properties of the Sb@C composite electrodes were investigated in coin 2032 cells using sodium as the counter electrode. The capacity in this study was calculated on the basis of total mass of the Sb@C composite. Figure 3a shows the cyclic voltammograms (CV) of 10-Sb@C vs. Na + /Na at a scan rate of 0.1 mv s -1 between 0.01 V V. In the first cathodic scan, a sharp peak below 0.5 V corresponds to Na + insertion into Sb accompanied with the formation of solid electrolyte interface (SEI) [31]. In the second and subsequent scans, the CV curves are well overlapped, suggesting good cycling stability of the electrode. Two reduction peaks at 0.66 V and 0.47 V are assigned to the sodium alloying with Sb to form Na xsb (x 3) and Na3Sb respectively [18]. Moreover, a cathodic peak near 0.0 V is attributed to sodium insertion into carbon material, which is also observed in the CV behavior of the pyrolyzed carbon electrode (Figure S11, SI). In the anodic scans, two peaks at 0.9 V and 1.0 V almost merge into a broad peak, corresponding to the desodiation reaction of Na 3Sb and NaxSb, respectively. Figure 3b shows the charge/discharge profiles of 10-Sb@C anode in the initial three cycles between 0.01 V- 2.0 V at 100 ma g -1. The first sodiation step delivers a capacity of 657 mah g -1 with a coulombic efficiency of 63.4%, providing charge capacity of 417 mah g -1. The capacity loss of 240 mah g -1 in the initial cycle is mainly attributed to the irreversible formation of SEI film, which is also observed for the carbon electrode (Figure S12a, SI). In particular, the followed discharge curves maintain a reversible capacity at around 435 mah g -1, indicating good cycling stability. In addition, the charge/discharge curves of 10-Sb@C with other active material loadings (1.2 mg cm -2 and 1.6 mg cm -2 ) were performed at 100 ma g -1, which show similar reversible capacity of ~ 430 mah g -1 (Figure S13, SI). Since carbon electrode only can deliver a reversible capacity of 55 mah g -1 (Figure S12b, SI) at the same testing conditions, the initial reversible capacity of 435 mah g -1 is very close to the theoretical capacity (471 mah g -1 ) for 10-Sb@C composite (660 mah g % + 55 mah g % = 471 mah g -1 ). Figure 3c depicts the rate performance of the as-prepared Sb@C composites at various current densities from 100 ma g -1 to ma g -1. The 10-Sb@C electrode displays a reversible capacity of 420, 380, 355, 324 and 270 mah g -1 at 200, 500, 1000, 2000, and 4000 ma g -1, respectively. Even at a higher current density of 8000 and ma g -1, it can deliver 178 and 146 mah g -1, respectively. This demonstrates excellent rate capability. Impressively, a high capacity of 433 mah g -1 is recovered when the current density is switched from ma g -1 to 100 ma g -1. In contrast, the 20-Sb@C electrode shows an inferior rate performance with the desodiation capacities of 387, 348, 323, 295, 244 and 134 mah g -1 at 200, 500, 1000, 2000, 4000 and 8000 ma g -1, respectively. It is noted that the bulk-sb@c exhibits the worst performance with 296, 255 and 187 mah g -1 at 200, 1000 and 4000 ma g -1, respectively. The 10-Sb@C electrode shows better performance because that the smaller size of Sb nanoparticles provides the shorter diffusion distance, which is beneficial for the high-rate capability. Compared with the bulk-sb@c electrode, the rate performance of the spherical Sb@C Nano Research

8 6 Nano Res. nanocomposites and is much better, reflecting the effect of the carbon matrix protection during cycling and the nanoscaled Sb particles could be fully utilized to store sodium ions with higher capacity. Moreover, the rate performance of is also better to those for Sb-C composites anodes for SIBs reported previously (Figure 3d) [4, 30-32, 38, 39]. The outstanding rate capability of should benefit from the synergistic effect offered by porous carbon network and ultrasmall Sb particles. Figure S14 (SI) shows the electrochemical impendence spectra (EIS) of the three composites. The charge-transfer resistance of and anodes are 53.0, and Ω on the basis of the equivalent circuit, respectively. The smaller resistance leads to faster electron transfer in electrode and better rate capability. Figure 3e displays the cycling performance of and pure Sb electrodes at 100 ma g -1 between 0.01 V 2.0 V. The 10-Sb@C exhibits much better cycling stability than that of the other electrodes. After 500 cycles, 10-Sb@C anode can still deliver a desodiation capacity of 385 mah g -1 with a high capacity retention of 88%, which is much higher than that of the reported carbonaceous anode materials [9, 12]. The capacity displays a slight decay in the initial cycles, but the coulombic efficiency approaches 100% after several cycles. This demonstrates superior reversibility of the 10-Sb@C composite. Only after 100 cycles, the 20-Sb@C and bulk-sb@c electrodes show low capacities of 335 mah g -1 and 295 mah g -1 respectively. While, the pure Sb anode exhibits a fast capacity decrease and a poor cycling stability. Moreover, the cycling performance of 10-Sb@C, 20-Sb@C, bulk-sb@c and pure Sb at 1000 ma g -1 with the active material loading of 1.4 mg cm -2 is shown in Figure S15 (SI). Obviously, the 10-Sb@C electrode shows the best cycling stability. After 500 cycles, a reversible capacity of 325 mah g -1 is obtained with a high coulombic efficiency around 100%. This means a practical feasibility in SIBs. The outstanding electrochemical performance of 10-Sb@C is ascribed to the following three main merits. First, the ultrasmall size of Sb nanoparticles could effectively reduce the strain generated during the sodiation and desodiation process and then suppress the fracture of Sb particles. Second, the homogeneous dispersion of Sb nanograins located in carbon could generate a balanced stress upon cycling over the whole electrode, and thus improve long-term cycle life. Finally, the porous carbon frame not only could prevent the aggregation of nanoparticles and accommodate the large volume change, but also allow for easy mass transfer between the electrolyte and electrode. Furthermore, this design could be extended to develop other materials suffered from the large volume change during charge/discharge cycling.

9 Nano Res. 7 Figure 3 (a) Cyclic voltammograms of the initial 4 cycles for 10-Sb@C electrode from 2.0 V to 0.01 V vs. Na + /Na at 0.1 mv s -1. (b) Charge/discharge profiles at the initial three cycle of 10-Sb@C electrode. (c) Rate capability 10-Sb@C electrode at various current densities from 100 ma g -1 to ma g -1. (d) This work with the comparison to previously reported Sb-C composites (capacities are all calculated on the basis of the whole mass of Sb/C). (e) Cycling performance of 10-Sb@C, 20-Sb@C, bulk-sb@c and pure Sb at a current density of 100 ma g -1. To further understand the sodiation/desodiation mechanism of Sb@C electrode, ex-situ XRD and HRTEM were performed at selected charged/discharged states between 0.01 V and 2.0 V at 100 ma g -1. Figure 4a shows the ex-situ XRD patterns of 10-Sb@C at selected stages during the first cycle. The XRD pattern of fresh electrode is well indexed to metallic Sb (JCPDS No ). At the half of the 1st discharge, the main peak (around 28 o ) intensity of Sb is decreased and no new peak is observed, because the amorphous phase Na xsb could not be detected by XRD. This result is in good Nano Research

10 8 Nano Res. agreement with previous report [18]. At the end of the 1st discharge, the remarkable peaks around 19 o and 34 o appeared, indicating the phase formation of Na3Sb [5, 18]. The almost vanishing of the diffraction peaks of Sb implies that Sb can be fully utilized to store sodium in the 10-Sb@C composite. During charge, the typical peaks of Na 3Sb gradually disappeared and transformed into an amorphous intermediate. With a fully charge, the characteristic peaks of Sb were observed again. This means that desodiation of sodium-alloyed phase occurs to form Sb. Figure 4b shows the ex-situ HRTEM image of 10-Sb@C electrode after discharging to 0.01 V. Clearly, the discharge product with the interplanar spacing of nm is well in agreement with (110) plane of Na3Sb alloy. The size of Na3Sb nanoparticles is about nm, confirming that the stable carbon framework can effectively accommodate the volume change during cycling. After charged to 2.0 V, the Na3Sb transforms into Sb metal (Figure 4c), displaying the lattice fringes with d-space of nm. The above observations manifest the reversible sodiation/desodiation mechanism, which is described in Equations 1 and 2. Sb + xna + + xe - NaxSb (1) Na xsb + (3-x) Na + + (3-x) e - Na 3Sb (2) Figure 4 (a) Ex-situ XRD patterns at selected stages of discharge and charge of the electrode. Ex-situ HRTEM images of 10-Sb@C discharged at 0.01 V (b) and charged at 2.0 V (c), respectively.

11 Furthermore, the structural stability of electrodes was investigated using TEM. Figure 5a shows the TEM image of after 100 cycles at 100 ma g -1. It can be clearly seen that the composite still remains its original spherical morphology after undergoing intensive cycling. The TEM elemental mapping images (Figure 5b-c) reveal that the inner-dispersion of Sb nanograins is still uniform. These results demonstrate that the unique structure can effectively suppress the volume change and alleviate the aggregation of Sb nanoparticles, thus retaining the integrity of the whole electrode and extending the cycle life. In contrast, the morphological changes of 20-Sb@C and bulk-sb@c after 100 cycles at 100 ma g -1 are shown in Figure S16 (SI). Both of their structures are collapsed and most Sb particles are pulverized as well as aggregated to some extent. hybrid material displays superior performance as the anode for SIBs because its unique structure could offer synergistic effect to alleviate large volume change, suppress the aggregation of Sb nanoparticles and shorten the path length for ion transport during cycling. The nano-sb@c composite shows a reversible capacity of 435 mah g -1 at 100 ma g -1 and the capacity retention of 88 % over 500 cycles. More importantly, a high-rate performance is obtained with 324 mah g -1 at 2000mA g -1. To the best of our knowledge, this result is much better than previously reported Sb-C composite. The facile synthesis method and excellent performance should shed light on the practical development of nano-sb@c composite as high rate capability and long cycle life electrode for rechargeable SIBs. Acknowledgements This work was supported by the Programs of National 973 (2011CB935900), NSFC ( ), MOE (B12015 and IRT13R30), and the Fundamental Research Funds for the Central Universities. Electronic Supplementary Material: Supplementary material (Schematic diagram of the synthesis process, SEM images of other samples, TEM images of 20-Sb@C and bulk-sb@c, TGA curves, XRD patterns Figure 5 (a) TEM image of 10-Sb@C after 100 cycles at 100 ma g -1. (b-c) TEM elemental mapping images for C and Sb, respectively. 4. Conclusions In summary, we have reported a one-pot method for the synthesis of spherical porous nano-sb@c composite by an aerosol spray pyrolysis technique. The micro-nanostructured composite is composed of nano-sb particles (10 nm) hom ogeneously embedded in a spherical porous carbon frame. Moreover, the carbon content and the size of Sb nanograins could be controlled by altering the reaction conditions. The of Sb@C composite after heat treatment, N2 adsorption-desorption isotherms of 10-Sb@C, cycling performance of carbon, EIS of Sb@C composites) is available in the online version of this article at (automatically inserted by the publisher). References [1] Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; van Schalkwijk, W. Nanostructured materials for advanced energy conversion and storage devices. Nat. Mater. 2005, 4, [2] Guo, Y. G.; Hu, J. S.; Wan, L. J. Nanostructured materials for electrochemical energy conversion and storage devices. Adv. Mater. 2008, 20, [3] Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium-ion batteries. Adv. Funct. Mater. 2013, 23,

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