Reduced Graphene Oxide Composite as a High-Rate Anode

Article pubs.acs.org/JPCC

An SbOx/Reduced Graphene Oxide Composite as a High-Rate Anode Material for Sodium-Ion Batteries Xiaosi Zhou,* Xia Liu, Yan Xu, Yunxia Liu, Zhihui Dai, and Jianchun Bao* Jiangsu Key Laboratory of Biofunctional Materials,School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China S Supporting Information *

ABSTRACT: Antimony has attracted enormous attention as anode materials for sodium-ion batteries owing to its high theoretical gravimetric capacity (∼660 mA h g−1). Despite the outstanding gravimetric capacity advantage, antimony suffers from unsatisfactory electrochemical performance originating from its huge volume changes during repeated sodium insertion/extraction. Herein, we synthesize an SbOx/reduced graphene oxide (SbOx/RGO) composite through a wet-milling approach accompanied by redox reaction between Sb and GO. When used as an anode material for sodium-ion batteries, SbOx/RGO exhibits high rate capability and stable cycling performance. A reversible capacity of 352 mA h g−1 was obtained even at a current density of 5 A g−1. More than 95% capacity retention (409 mA h g−1) was achieved after 100 cycles at a current density of 1 A g−1. The excellent electrochemical performance is due to the Sb−O bonding between nanometer-sized SbOx particles surface and highly conductive RGO, which can not only effectively prevent SbOx nanoparticles from aggregation upon cycling but also promote the electrons and sodium ions transportation.

1. INTRODUCTION There is great demand for high-performance rechargeable batteries for applications in portable electronics, hybrid and electric vehicles, and large-scale stationary energy storage. Until now, lithium-ion batteries (LIBs) have become the most promising candidate due to their long life-span, high power and energy density, safety, and so forth.1−13 However, the limited and unevenly distributed lithium resource could be quickly consumed if LIBs are mass produced in the near future. This issue has stimulated us to investigate sodium-ion batteries (SIBs) because of the natural abundance and low cost as well as environmental benignity of sodium resources.14−19 Previous research in SIBs was mainly focused on the development of stable and high rate cathode materials.20−29 On the anode side, great effort has been devoted to developing carbonaceous materials such as carbon nanowires, carbon nanospheres, and highly disordered carbon.30−41 However, these hard carbon materials exhibit low capacities at high rate due to their limited sodium host sites. Recently, several groups paid considerable attention to antimony (Sb) as SIB anode materials on account of its high theoretical gravimetric capacity (∼660 mA h g−1).42−52 Despite the distinct gravimetric capacity advantage, Sb suffers from unsatisfactory electrochemical performance originating from its huge volume changes during repeated sodium insertion/extraction. After complete sodiation to produce an alloy of Na3Sb, Sb goes through volume expansion of ∼400% relative to its initial state.53 Such massive volume change inevitably triggers pulverization of Sb, loosened contact between Sb and conductive additives, serious Sb particle © XXXX American Chemical Society

aggregation during cycling, and continual solid electrolyte interphase (SEI) formation, thus leading to rapid capacity fading. So far, some rational electrode designs including ball-milled Sb/C nanocomposites and electrospun Sb/C nanofibers have been reported to be effective in enhancing the sodium storage capability.43−50 For example, Yang et al. carried out the pioneering work on the utilization of metallic Sb, which exhibits a reversible capacity of 309 mA h g−1 at a current density of 2 A g−1.44 Wang et al.47 reported Sb/C nanofibers that delivered a high capacity of 104 mA h g−1 under 5 A g−1. Very recently, highly dispersed SbSn nanoparticles encapsulated in porous carbon nanofibers have been fabricated, showing a reversible capacity of 110 mA h g−1 at a super high rate of 10 A g−1.50 Indeed, these composites significantly mitigate the kinetic problems and unstable interface through the shortened sodium ion diffusion length and improved electrical conductivity as well as stabilized SEI film. However, because of the pursuit of high power density, great efforts are still directed to achieve superior rate capability and stable cycling performance. At this stage, a novel Sb-based nanomaterial with favorable nanostructure for electron and sodium ion transport and its facile synthesis are highly desired. In this work, we report a wet-milling approach accompanied by redox reaction between Sb and GO to fabricate an SbOx/ Received: July 17, 2014 Revised: September 23, 2014

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dx.doi.org/10.1021/jp507116t | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Negative ion mode spectra were calibrated on the C−, CH−, CH2−, C2−, and C2H− peaks. Sb L3-edge and C K-edge X-ray absorption near edge structure (XANES) analyses were measured by using beamlines 4B7A and 4B7B at the Beijing Synchrotron Radiation Facility (BSRF), respectively; the resulting data were normalized to the incident photon flux I0 determined by using a fresh gold target. Nitrogen adsorption and desorption isotherms at 77.3 K were recorded on an ASAP 2050 surface area-pore size analyzer. 2.6. Electrochemical Measurements. Electrochemical tests were carried out using CR2032 coin cells. For making working electrodes, SbOx/RGO was mixed with Super-P carbon black and carboxymethyl cellulose sodium wit a weight ratio of 80:10:10 in water using mortar and pestle. The obtained slurry was pasted onto pure Cu foil (99.9%, Goodfellow) and then dried at 80 °C for 10 h under vacuum. The dried electrodes were punched into round discs with a diameter of 1.0 cm and the typical amount of loaded active material was approximately 1.5 mg cm−2. The electrolyte was 1 M NaClO4 in ethylene carbonate/propylene carbonate/fluoroethylene carbonate (1:1:0.1 v/v/v). Glass fibers (GF/D) from Whatman were used as separators and sodium metal was utilized as the counter electrode. The coin cells were assembled in an argon-filled glovebox (H2O, O2 < 0.1 ppm, Mbraun, Germany). The charge and discharge measurements of the batteries were carried out on a Land CT2001A multichannel battery testing system in the fixed voltage window between 0 and 2 V vs Na+/Na at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were recorded on a PARSTAT 4000 electrochemical workstation. CV was measured at a scan rate of 0.1 mV s−1 over the potential window of 0−2 V vs Na+/Na while EIS was tested in the frequency range from 100 kHz to 100 mHz.

reduced graphene oxide composite (SbOx/RGO, x = 0.21) for the first time. Benefiting from the Sb−O bonding between nanometer-sized SbOx particles and highly conductive RGO, not only SbOx nanoparticles aggregation could be well suppressed but electrons and sodium ions transportation could be significantly promoted, thus leading to superior rate capability (352 mA h g−1 under 5 A g−1) and improved cycling performance (409 mA h g−1 after 100 cycles under 1 A g−1).

2. EXPERIMENTAL SECTION 2.1. Synthesis of SbOx/RGO. Graphene oxide (GO) was first synthesized from natural graphite flakes through a modified Hummer’s method.54 200 mg of Sb powders (200 mesh, Alfa Aesar) and 12 mL of GO aqueous suspension (4.0 mg mL−1) were wet milled at 1200 rpm for 12 h under argon atmosphere using a WL-IA particulate miller. The resultant product was collected by centrifugation, washed with ethanol, and dried under vacuum at 70 °C overnight to obtain SbOx/RGO. 2.2. Synthesis of a Physical Mixture of Sb and RGO (Sb + RGO Mixture). RGO was first fabricated by annealing freeze-dried GO under argon flow at 800 °C for 2 h. Afterward, Sb powders and RGO were dry-milled with the same weight ratio (73:25) as that of SbOx/RGO at 1200 rpm for 12 h under argon atmosphere using a WL-IA particulate miller to produce an Sb + RGO mixture. 2.3. Synthesis of Wet-Milled Sb. A 200 mg sample of Sb powder and 12 mL of dionized water were wet milled at 1200 rpm for 12 h under argon atmosphere using a WL-IA particulate miller. The following steps were the same as making SbOx/RGO. 2.4. Synthesis of Wet-Milled GO. A 12 mL sample of graphene oxide aqueous suspension (4.0 mg mL−1) was wet milled at 1200 rpm for 12 h under argon atmosphere using a WL-IA particulate miller. The following steps were the same as making SbOx/RGO. 2.5. Materials Characterization. Scanning electron microscopy (SEM) characterizations were carried out on a JEOL JSM-7600F scanning electron microscope operated at 10 kV. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) characterizations were examined using a JEOL JEM-2100F transmission electron microscope operated at 200 kV. Energy dispersive X-ray spectroscopy (EDS) analysis and scanning transmission electron microscopy (STEM) meaurements as well as elemental mapping analyses were determined on a Tecnai G2 F20 U-TWIN field emission transmission electron microscope attached with an EDAX system. X-ray diffraction (XRD) pattern was characterized by a Rigaku D/max 2500/PC diffractometer using Cu Kα radiation. Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449 F3. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALab250Xi electron spectrometer from VG Scientific using 300W Al Kα radiation. Gas compositions were identified by a gas chromatographer GC-7900 using thermal conductivity detector. Secondary ion mass spectroscopy (SIMS) measurements were obtained by a time-of-flight secondary ion mass spectrometer TOF-SIMS 5 from ION-TOF GmbH (Munster, Germany). A Bi1+ liquid metal ion gun running at a 30 keV beam voltage with a 45° incident angle was performed. All analyses were conducted on analyzed area of 500 × 500 μm2 at 256 × 256 pixels after two prescans under DC mode to remove most of the surface contaminants. Charge compensation with an electron flood gun was utilized during the analysis cycles.

3. RESULTS AND DISCUSSION SbOx/RGO was fabricated from slurry of Sb powder and GO aqueous suspension under argon atmosphere through a wetmilling process. The resultant composite was first characterized using XRD (Figure 1). Obvious peaks indexed to Sb and Sb2O3

Figure 1. XRD pattern of SbOx/RGO and standard XRD pattern of Sb (JCPDS card No. 35−0732). The peaks marked with asterisks correspond to Sb2O3 (JCPDS card No. 43-1071).

can be observed and the characteristic peak of GO at 9.9° is completely disappeared (Figure S1, Supporting Information), suggesting that the redox reaction between Sb and GO probably took place during the wet-milling process. For comparison, both the wet-milled GO and the wet-milled Sb were also investigated by XRD (Figure S1, Supporting Information). The results reveal that GO almost retain its physical structure and only a small amount of Sb is oxidized B



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Figure 2. (a) Sb 3d XPS spectrum of SbOx/RGO. (b) SIMS spectrum of SbOx/RGO. (c) C K-edge XANES spectra of SbOx/RGO and SbOx/RGOHCl. (d) Sb L3-edge XANES spectra of SbOx/RGO and pure Sb2O3.

SbOx/RGO, suggesting that wet-milling with Sb powders caused the reduction of GO (Figure S6, Supporting Information), which is in good agreement with the XRD analysis. Furthermore, SIMS measurements were conducted on SbOx/RGO and Sb + Sb2O3 mixture after most of their surface contaminants were removed (Figures 2b and S7, Supporting Information). The result demonstrates that obvious SbO3COO− peaks can be observed in the SbOx/RGO sample but do not exist in the Sb + Sb2O3 mixture sample, indicating the Sb−O bonding might be formed between SbOx and RGO. In order to disclose the interaction between the two components of SbOx/RGO, we removed Sb2O3 from some SbOx/RGO composite using concentrated hydrochloric acid (SbOx/RGO-HCl), and then performed XANES measurements on SbOx/RGO and SbOx/RGO-HCl (Figure 2c,d). Compared to SbOx/RGO-HCl, the SbOx/RGO composite showed an apparent increase of C K-edge peak intensity at 289.3 eV (Figure 2c), corresponding to carbon atoms in RGO attached to oxygen. This indicated the formation of interfacial Sb−O bonding between SbOx and RGO. Besides, obvious increase in the Sb L3-edge XANES peaks of SbOx/RGO compared to pure Sb2O3 further confirms the generation of Sb−O bonding (Figure 2d), which causes lower electron density at the Sb site, thus resulting in a higher ionic Sb−O bonding in the composite.55 The morphology of the as-synthesized SbOx/RGO composite was characterized using SEM and TEM. As displayed in Figure 3a, the composite presents micrometer-sized with wrinkled RGO coating. The typical thickness of the RGO coating is around 4 nm, which could be clearly observed from the HRTEM image (Figure 3d). Figure 3b shows the highmagnification SEM image of SbOx/RGO. It is easy to see that some SbOx nanoparticles are covered under RGO, as proved by XPS survey scan (Figure S8, Supporting Information), with less than 6.1 at. % Sb could be detected on the surface of SbOx/ RGO. The TEM image of the SbOx/RGO composite demonstrates that the SbOx nanoparticles are uniformly

when they are wet-milled separately, thus confirming that Sb and GO could play the roles of reductant and oxidant for each other when they are wet-milled together, in other words, the electrons can be transferred from Sb to GO during the wetmilling process. To determine the value of x in SbOx/RGO, we made a linear plot between molar ratio (nSb:nSb2O3, y) and intensity ratio of the strongest peaks (I28.7o:I27.7o, k) of crystalline Sb to Sb2O3, which fits the following equation: y = 7.62k + 1.44 (0.31 ≤ k ≤ 2.21) (Figure S2, Supporting Information). As the I28.7o:I27.7o of SbOx/RGO is 1.41, the nSb:nSb2O3 in the composite can be calculated to be 12.2, thus the x in SbOx/RGO is 0.21. Similarly, the oxidation value of the wet-milled Sb can be calculated to be 0.038, i.e., the wet-milled Sb can be expressed as SbO0.038 (Figure S1 and S3, Supporting Information). Additionally, the product atmosphere of SbOx/ RGO was also analyzed using gas chromatography (GC) for safety considerations; however, the result shows that no hydrogen could be detected (Figure S4, Supporting Information), suggesting either the content of hydrogen is below the detection limit of GC or hydrogen takes part in the reduction of GO. The weight percentage of SbOx in the SbOx/RGO were determined by TGA, based on the weight loss of RGO combustion and the weight gain of Sb2O4 generation (Figure S5, Supporting Information). The content of SbOx in the SbOx/RGO composite is calculated to be approximately 75 wt % based on eq S1, Supporting Information, which is lower than the expected value (around 80 wt %). It is probably due to the reaction occurred between Sb and the stainless steel milling tank, leading to the loss of Sb. On the basis of the molar ratio betwen Sb and Sb2O3, the weight percents of Sb and Sb2O3 in the composite can be calculated to be 62.7% and 12.3%, respectively. XPS measurements (Figure 2a) show strong Sb 3d3/2 and Sb 3d5/2 peaks at 539.9 and 530.5 eV, respectively, indicating Sb3+ is the dominant oxidation state of the surface of SbOx/RGO. Compared with the XPS spectrum of GO, a dramatic loss of oxygen-containing functional groups can be observed in that of C



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that the first cathodic scan is different from the following cathodic scans. During the first cathodic scan, the irreversible shoulder peak at around 0.46 V is attributed to the decomposition of the electrolyte to produce SEI film on the surface of the electrode and the irreversible electrochemical reduction of large SbOx particles.7,43 The large reduction peak at 0.35 V corresponds to the Na−Sb alloying reaction to generate Na3Sb alloy compounds, serving as an activation process.50 At the second cathodic scan, the CV curve displays four apparent reduction peaks at 0.95, 0.67, 0.51, and 0.25 V. The first one is assigned to the conversion reaction between SbOx and Na, which is generally irreversible for bulk SbOx, but could become reversible for the nanometer-sized SbOx (reaction 1).7,56 The second and third ones are attributed to two-step alloying reaction of Sb with Na to form NaSb and Na3Sb, respectively (reactions 2 and 3).44,47 The last one is probably due to Na insertion into RGO, which is in good aggreement with the previous reports.49 In the reverse anodic scans, three oxidative peaks positioned at 0.78, 0.85, and 1.54 V are observed. The first two are assigned to the dealloying reactions of Na−Sb compounds,44,47 while the latter one corresponds to the extraction of Na from Na2O, respectively.7,56 In addition, the oxidative peak of Na extraction out of RGO might be overlapped by the Na−Sb dealloying peaks. After the activation process occurred during the first cathodic scan, the peak area and peak positions remained almost steady for the following scans, suggesting stable sodiation/desodiation reactions of the SbOx/RGO electrode.

Figure 3. (a) SEM and (b) high-magnification SEM images of SbOx/ RGO. (c) TEM image of SbOx/RGO. (d) HRTEM image of SbOx/ RGO.

distributed between RGO (Figure 3c). Moreover, nitrogen absorption measurement of the composite determines a Brunauer−Emmett−Teller (BET) surface area of 82.8 m2 g−1 (Figure S9, Supporting Information), which allows for fast sodium ion diffusion. These results imply that the as-formed SbOx/RGO composite possesses three key advantages of Sb−O bonding, nanometer-sized SbOx particles, and highly conductive RGO network, all of which promise a desired performance for sodium storage. Figure 4a shows the cyclic voltammograms (CVs) of the SbOx/RGO electrode in Na+ electrolyte. It can be observed

SbOx + 2x Na + + 2x e− ↔ Sb + x Na 2O

(1)

Sb + Na + + e− ↔ NaSb

(2)

NaSb + 2Na + + 2e− ↔ Na3Sb

(3)

The rate capability of the SbOx/RGO electrode was evaluated under galvanostatic conditions. At a current density of 0.1 A g−1 in the voltage range of 0−2 V vs Na+/Na (Figure 4b), the initial charge and discharge capacities of the SbOx/

Figure 4. (a) CV curves of the first five cycles of the SbOx/RGO electrode. (b) Rate capability of the SbOx/RGO electrode. (c) Cycling performance and Coulombic efficiency of the SbOx/RGO electrode. The first 20 cycles are under 0.1 A g−1, and the remaining 100 cycles are under 1 A g−1. (d) Comparison of rate performance of the SbOx/RGO electrode with other Sb-based sodium-ion battery anodes reported recently. D



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Figure 5. STEM image (a), carbon (b), oxygen (c), and antimony (d) element mapping images of SbOx/RGO. STEM image (e), carbon (f), oxygen (g), and antimony (h) element mapping images of the SbOx/RGO composite after rate test.

RGO electrode are 427 and 703 mA h g−1, respectively, corresponding to a Coulombic efficiency of 61%. Note that the specific capacity values are calculated based on the total mass of the SbOx/RGO composite. By subtracting the capacity contribution from RGO, the initial discharge capacity of 722 mA h g−1 can be reached for SbOx on the basis of eq S2, Supporting Information, suggesting about 99% utilization has been realized for sodium insertion into SbOx with the theoretical capacity of 732 mA h g−1. Likewise, the initial charge capacity of SbOx is calculated to be 527 mA h g−1, indicating 72% utilization has been achieved for sodium extraction from SbOx. Integrated with the pair of redox peaks at 0.95/1.54 V of SbOx nanoparticles, we deduce that the large SbOx particles existed in the SbOx/RGO composite are unfavorable for sodium storage. Thus, the 39% capacity loss of the SbOx/RGO electrode can be attributed to the decomposition of the electrolyte on the surface of the SbOx/ RGO composite to form SEI layer as well as the irreversible electrochemical reduction of large SbOx particles,7,43 which could be improved by presodiation and further crushing the large SbOx particles. Along with the increase of current densities from 0.1 to 10 A g−1, the SbOx/RGO electrode delivers the charge capacities of 448, 459, 447, 430, 408, 352, and 220 mA h g−1 at the current densities of 0.1, 0.2, 0.5, 1, 2, 5, 10 A g−1, respectively (Figure 4b). To the best of our knowledge, this high rate performance has rarely been reported in other Sb-based sodium-ion battery anodes (Figure 4d). An increase of specific capacity of the SbOx/RGO electrode can be observed during the initial cycles. This activation process may be originated from the delayed infiltration of the electrolyte into the composite.57 Impressively, the charge capacity of the SbOx/RGO electrode could immediately recover to 448 mA h g−1 without obvious delay upon returning the current density from 10 to 0.1 A g−1. As current density increased from 0.1 to 10 A g−1, the charge−discharge profiles present similar shapes, with increasing of the voltage difference, which results from polarization loss and mechanical energy dissipation during battery cycling (Figure S10, Supporting Information).47

In control experiments, we prepared Sb + RGO mixture, wetmilled Sb, and wet-milled GO, respectively (see Experimental Section). XRD patterns and TEM images of the wet-milled Sb and the wet-milled GO show that their physical structures are almost maintained in comparison with Sb and GO, respectively (Figure S2 and S11, Supporting Information). Compared to Sb + RGO mixture, the SbOx/RGO composite has smaller particle size distributed between RGO, indicating the oxygencontaining functional groups serve as anchor points for preventing the Sb nanoparticles from reaggregation during the wet-milling process (Figure S12, Supporting Information). When evaluated as anode materials, all of Sb + RGO mixture, wet-milled Sb, and wet-milled GO exhibit lower electrochemical performance than that of SbOx/RGO (Figures 4b and S13−S15, Supporting Information). At a current density of 0.1 A g−1, the Sb + RGO mixture electrode, the wet-milled Sb electrode, and the wet-milled GO electrode manifest initial charge capacities of 495, 13, 127 mA h g−1, but followed by a rapid capacity decay with the increase of current density, respectively, while the SbOx/RGO electrode exhibits much higher sodium-ion storage capacity and much better rate capability. For instance, at a current density of 5 A g−1, the charge capacities of the Sb + RGO mixture electrode, the wetmilled Sb electrode, and the wet-milled GO electrode are 15, 3, and 16 mA h g−1, respectively, while the SbOx/RGO electrode shows a stable capacity of 352 mA h g−1. In addition, the SbOx/ RGO composite demonstrates better cycling stability than that of Sb + RGO mixture, wet-milled Sb, or wet-milled GO. After 100 cycles under 1 A g−1, the charge capacities of Sb + RGO mixture, wet-milled Sb, and wet-milled GO are 33, 2, and 38 mA h g−1, respectively, while SbOx/RGO delivers a high capacity of 409 mA h g−1 (Figure 4c, S13−15, Supporting Information). Moreover, compared with the wet-milled GO electrode and the wet-milled Sb electrode, the SbOx/RGO electrode presents higher initial Coulombic efficiency (Figures S10b, S14b, and S15b, Supporting Information). This is probably due to the initimate RGO coating around SbOx nanoparticles through the Sb−O bonding, which not only E



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4. CONCLUSIONS In summary, we have fabricated an SbOx/reduced graphene oxide (SbOx/RGO) composite as a high-rate anode material for sodium-ion batteries. The fabrication was carried out via a facile wet-milling approach at room temperature. The in situ formed SbOx/RGO composite possesses Sb−O bonding between SbOx nanoparticles surface and highly conductive RGO, which not only suppresses the aggregation of SbOx during cycling but also promotes electrons and Na ions transportation as well as facilitates the generation of stable SEI film. This composite shows high rate capability (352 mA h g−1 even at a current density of 5 A g−1) and excellent cycle stability (100 cycles with 95% capacity retention). Moreover, this rational design of binding SbOx nanoparticles between RGO would be very favorable in improving the electrochemical performance of other high capacity alloy-type anode materials for sodium-ion batteries.

reduces the active surface of SbOx exposed to electrolyte for SEI growth58 but also buffers the substantial volume changes of SbOx during cycling to prevent the pulverization and subsequent inactivation of SbOx. We also compared SbOx/ RGO with other Sb-based sodium-ion battery anodes reported recently, including Sb/C nanocomposites,43,44 Sb/MWCNT nanocomposite,48 Sb/C nanofibers,47,49 and SbSn/C nanofibers,50 and the results are demonstrated in Figure 4d. When the current density is lower than 1.5 A g−1, the rate performance of the SbOx/RGO electrode is not superior to the reported results. However, as the current density is higher than 1.5 A g−1, the SbOx/RGO electrode still shows high capacities and its rate capability outperforms these Sb-based anodes. To reveal the mechanism for the high rate performance of SbOx/RGO, the morphology and structure of the composite before and after rate test were analyzed using STEM and EDS elemental mapping. Combined with the STEM images (Figure 5a, e), the elemental mappings show that the uniform distribution of Sb, O, and C is preserved after cycling (Figure 5b−d,f−h), indicating the Sb−O bonding formed between SbOx and RGO can effectively suppress the agglomeration of Sb particles during repeatedly sodium insertion/extraction. Furthermore, the strong peak of Na in the EDS spectrum and the uniform sodium distribution of the cycled composite confirm that a SEI layer formed on the surface of the SbOx/ RGO electrode (Figure S16, Supporting Information). Electrochemical impedance spectroscopy (EIS) was also performed on the SbOx/RGO electrode before cycling and after rate test (80 cycles) (Figure S17, Supporting Information). Nyquist plots demonstrate that the diameter of the semicircle of the SbOx/ RGO electrode decreases after 80 cycles, indicating an activation step might occur during the rate test.59 The kinetic parameters of the SbOx/RGO electrode before cycling and after 80 cycles were further studied by simulating the EIS spectra based on the modified Randles equivalent circuit (Figure S18, Supporting Information).60 The values of the SEI film resistance Rf of the SbOx/RGO electrode before cycling and after 80 cycles are 137.5 and 55.5 Ω, respectively (Table S1, Supporting Information). In the case of charge-transfer resistance Rct, the values before cycling and after 80 cycles are 22.4 and 14.3 Ω, respectively (Table S1, Supporting Information). These results indicate the unstable SEI film formed before cycling become stabilized and the charge-transfer resistance decreases with the gradual infiltration of the electrolyte into the composite, thus confirming the existence of the activation process and the formation of stable SEI film. The high rate capability of the SbOx/RGO electrode can be attributed to the following aspects: (1) The Sb−O bonding between SbOx and RGO not only facilitate the fast electron transport from highly conductive RGO network to SbOx, but effectively bind SbOx nanoparticles to avoid the aggregation of SbOx during battery testing. (2) The in situ obtained nanometer-sized SbOx particles from wet-milling Sb powder and GO aqueous suspension significantly shortens the diffusion lengths for Na ions. (3) The SbOx/RGO composite possess an initimate RGO coating around SbOx nanoparticles through the Sb−O bonding, which accommodates the large volume change of the SbOx upon sodiation without destroying the electron transport. This in turn allows for the formation of stable SEI film on the RGO coating and protects the SEI from cracking.57

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ASSOCIATED CONTENT

S Supporting Information *

Additional XRD patterns, gas chromatograms, TGA curve, XPS spectra, SIMS spectrum, N2 adsorption−desorption isotherms, TEM images, electrochemical performances, EDS spectrum and elemental mapping, and Nyquist plots. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(X.Z.) E-mail: [email protected]. Telephone/Fax: +8625-85891027. *(J.B.) E-mail: [email protected]. Telephone/Fax: +86-25-85891936. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21175069 and 21171096) and the Scientific Research Foundation for Advanced Talents of Nanjing Normal University (2014103XGQ0073), with financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Program of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials.

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REFERENCES

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