SbOx as sodium

Journal of Alloys and Compounds 693 (2017) 141e149

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

Porous graphene anchored with Sb/SbOx as sodium-ion battery anode with enhanced reversible capacity and cycle performance Gui-Zhi Wang, Jian-Min Feng*, Lei Dong, Xi-Fei Li, De-Jun Li** Institute of Physics and Materials Science, Tianjin Normal University, Tianjin, 300387, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2016 Received in revised form 13 September 2016 Accepted 15 September 2016 Available online 15 September 2016

Nanocomposites of porous graphene anchored with Sb/SbOx nanoparticles were synthesized through a two-step wet chemical process. SbOx/graphene was initially prepared by refluxing SbCl3 and graphene oxide in benzyl alcohol. Subsequently, the SbOx nanoparticles were partially reduced by NaBH4. The nanocomposites exhibited an enhanced performance for sodium-ion battery with a stable sodium-ion storage capacity of 311.6 mAh g1 at 50 mA g1 after 100 cycles and a rate capability of 205.6 mAh g1 at 800 mA g1. Reduction of SbOx by adding moderate amount of NaBH4 was conducive in improving the nanocomposite's electrochemical performance. The sodium-ion storage capacity of Sb/SbOx/graphene material was 1.4 times higher than that of SbOx/graphene without NaBH4 and 6.2 times higher than that of Sb/graphene with excessive amount of NaBH4. © 2016 Elsevier B.V. All rights reserved.

Keywords: Sodium-ion battery Antimony Graphene Electrochemical performance

1. Introduction Sodium-ion batteries (SIBs) are an attractive energy storage candidate to meet the increasing affordable and safe green energy storage demand [1e3]. SIBs possess abundant raw electrode materials compared with lithium-ion batteries (LIBs) [4,5]. Earth's crust is made up of 2.8% sodium, which is far higher than that of lithium (0.0065%) [6e8]. The energy storage of SIBs depends on the intercalation/deintercalation of Naþ, which is similar to that of LIB [9]. However, the size of Naþ is larger than that of Liþ, which will lead to ion intercalation/deintercalation obstacle, larger volume expansion and shrinkage [5,10]. LIB electrode materials are generally not applicable for SIB. Development of novel electrode materials or modification of the available LIB electrode materials has recently gained attention [11e13]. Different types of SIB electrode, including carbon-based materials of carbon nanofibers [14], hard carbon [15], hollow carbon nanowires [16], carbon spheres [17,18], carbon nanosheets [19], titanium-based materials (TiO2 and sodium titanate [20,21]), alloy materials of metal/metal oxide [11,22,23], and organic compounds [24], have been developed. Antimony is a promising anode material because of its high theoretical capacity of 660 mAh g1 (based on

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.-M. Feng), [email protected] (D.-J. Li). http://dx.doi.org/10.1016/j.jallcom.2016.09.150 0925-8388/© 2016 Elsevier B.V. All rights reserved.

the electrochemical reaction: Sb þ 3Naþ þ 3e / Na3Sb), good kinetic performance and voltage characteristics [3,25,26]. However, antimony anodes are confronted with unavoidable problem because of the large volume changes (near 400%) upon Naþ insertion into Sb [27e29]. These volume changes lead to fracture and loss of electrical contact, as well as continuous growth of solid electrolyte interphase (SEI) film on the antimony surface, resulting in rapid capacity fading [2,30,31]. In order to overcome this obstacle, two effective strategies have been developed. One is Sb-based materials nano-crystallization with special microstructure. Li and his group synthesized Sb2O3 nanowires by a modified solvothermal reactions as anode material for SIBs [32]. Hong and his partners developed one-step electrodeposition process to synthesis morula-like Sb/Sb2O3 as SIBs anode [33]. Nam et al. reported synthesizing a 3D porous Sb-based electrode with the assist of a polypyrrole nanowire network by an electrochemical process and applied as SIBS anode [34]. Liu et al. prepared nanoporousantimony by chemical dealloying methods as SIBs anode [35]. The other is to use carbon-based materials to buffer volume changes in Sb-based electrode via nano-compositing process [36,37]. The introduced carbon component could not only serve as buffer but also could be a conductive matrix to maintain the electrode microstructure and conductivity [16,23,38]. Hou and his group prepared Sb/Acetylene Black composite by a simple chemical reduction method as SIBs anode [26]. Ko and his partners synthesized Sb nanocrystals embedded in carbon microspheres by a spray pyrolysis process as SIBs anode [27]. Li et al. synthesized antimony-

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carbon-graphene fibrous composite by an electrospinning/spray way as SIBs anode [15]. Bitner-Michalska and his cooperator reported a one-step microwave plasma chemical vapor deposition method to synthesize SbxOy/C composite as SIBs anode [29]. Duan and his partners prepared Sb embedded in three-dimensional nitrogen-doped porous carbon matrix by self-wrapping and controlled growth method as SIBs anode [39]. Gao and his group synthesized Sb/cross-linked chitosan by simple chemical reduction as SIBs anode [40]. Lü and her team synthesized Sb/C/Graphene nanocomposite by typical wet-chemical method as SIBs anode [41]. Luo et al. prepared antimony/three-dimension carbon network via the methods of freeze-drying and one-step in-situ carbonization as SIBs anode [31]. Qiu and his group reported Sb@C microspheres by a facile self-catalyzing solvothermal method applied as SIBs anode [42]. Wan and her cooperators prepared Sb nanospheres on graphene by simple chemical and carbonization method as SIBs anode [43]. Wan et al. synthesized Sb/RGO micro/nanocomposite by a simple liquid-phase mix method as SIBs anode [44]. Wang and his team prepared Sb/Amorphous Carbon network by electrostatic spray deposition and heat treatment as SIBs anode [45]. In comparison, graphene owns superior electrical conductivity and chemical stability as well as a super huge specific surface area of 2600 m2g-1 [46,47], which could provide a well accommodation for Sb-based electrode for improving performance [13,36,48]. However, there are few articles focusing the stress on the hybrid structure with sb-based materials uniform anchoring on the surface of graphene with an interfacing band. A firm SbeO band between Sb and graphene could provide a strong combination of the two materials and avoid volume charge of Sb during repeating cycles. Wet chemical process promises full contact of Sb-based materials and graphene by constantly stirring; consequently better forming the SbeO band. Partial reduction step by NaBH4 from SbOx to Sb is due to higher theoretical capacity of antimony, which could gain a desired SIB anode. In this work, we adopted a controllable wet chemical process to prepare porous nanocomposite of Sb/SbOx/graphene. SbOx/graphene was initially prepared by refluxing SbCl3 and graphene oxide (GO) in benzyl alcohol. Partial reduction step was then performed by adding NaBH4 to prepare Sb/SbOx/graphene. The porous morphology and hybrid nano-structure of graphene anchored with Sb/SbOx nanoparticles are conducive to improve the electrochemical performance of the material. Comparison of electrochemical tests and analysis showed that maintaining partial reduction of SbOx was beneficial to improve the nanocomposite electrochemical performance.

2. Experimental 2.1. Sb/SbOx/graphene preparation GO was prepared using a modified Hummer method [49]. Sb/ SbOx/graphene nanocomposites were synthesized using a two-step wet chemical method. First, 1 g of SbCl3 was dissolved in 20 mL of benzyl alcohol. Meanwhile, 0.18 g of GO was dispersed in 60 mL of benzyl alcohol after ultrasonic processing for 0.5 h. Second, SbCl3 and GO solutions were mixed and refluxed at 120 C for 24 h. When the reaction reached 12 h, 0.1 g of NaBH4 was added to reduce SbOx to Sb. For comparison, no NaBH4 and excessive NaBH4 (0.33 g) were added in accordance with the above procedure. In addition, bare graphene obtained without usage of other raw materials. The final product was collected via filtration, washed three times with distilled water and alcohol, and dried at 70 C overnight.

2.2. Morphology and phase characterizations Scanning electron microscopy (SEM; TDCLS-8010, Hitachi Japan) and transmission electron microscopy (TEM; Tecnai G2 F20, FEI, Holland) were conducted to determine the morphology of the product. The phase of the final product was characterized by X-ray diffraction (XRD; D8 Advance, Bruker, German), with Cu/Ka radiation in the range of 10 e80 . Raman spectroscopy (Horiba Jobin Yvon, LabRAM HR800, 17 mW, 514 nm, HeeNe laser) was performed in the range of 100e2300 cm1. The carbon content of the products was evaluated using 15 mg of samples by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (TGA/DSC1/1100, METTLER, Germany) at a heating rate of 10 C/min and an air flow rate of 15 sccm. The energy dispersive electron microscopy (EDS) was recorded under 20 KV. The FT-IR spectra were tested by an IRAffinity-I FT-IR spectrometer (Shimadzu), and the samples for FT-IR measurement were first prepared by grinding the dried powder with KBr and then compressing into thin disks. 2.3. Electrochemical characterizations Electrochemical properties of Sb/SbOx/graphene were evaluated by assembling 2032 coin cells in a glove box filled with argon. Sodium metal foil was used as counter electrode. NaClO4 (1 mol/L) was dissolved in ethylene carbonate, and dimethyl carbonate (3:7 by volume) was added with 10% fluoroethylene carbonate as electrolyte. The working electrode slurry contained 75 wt% active materials, 15 wt% carbon black, and 10 wt% sodium alginate. The electrode was prepared by casting the slurry onto a copper foil and dried at 80 C overnight under vacuum. Afterwards, the electrodes were cut into 12 mm diameter disks and the mass of the active materials was near 1 mg cm2 every disk. We calculated the specific capacity of Sb/graphene, SbOx/graphene and Sb/SbOx/graphene by using the total weight include graphene. The galvanostatic chargeedischarge test was conducted on a LAND cycler (LANHE CT2001A) between 0.01 and 3.0 V vs Naþ/Na. Cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed with Princeton Applied Research VersaSTAT4. CV measurement was carried out at a scan rate of 0.1 mV/s in the range of 0.01e3 V vs Naþ/Na. EIS was tested in the range of 100 kHz to 0.01 Hz. For comparison, SbOx/ graphene and Sb/graphene were also evaluated. 3. Results and discussion Fig. 1a showed the XRD patterns of as-prepared Sb/SbOx/graphene, Sb/graphene, and SbOx/graphene. The phase of the above products could be facilely prepared through wet chemical process by adding a controlled amount of NaBH4. The as-prepared product without adding NaBH4 was mainly SbOx, and the corresponding phase was Sb2O3. When an excessive amount of NaBH4 was added, the corresponding mole ratio of NaBH4 and SbCl3 was 2/1, and pure Sb metal was obtained. Addition of a moderate amount of NaBH4 resulted in 2/5 mol ratio of NaBH4 and SbCl3, and the product with a mixing phase of Sb and SbOx was obtained. The XRD analysis confirmed that Sb2O3 could be indexed to JCPDS card PDF-#652426 and JCPDS card PDF-#42-1466 and Sb could be classified into JCPDS card PDF-#85-1322. However, no evident XRD peaks for graphene were observed on the XRD patterns, which may be submerged by the diffraction peak of antimony-based materials at around 28 . Raman spectroscopy was further adopted to characterize the phase of graphene and graphene-based materials. The Raman


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Fig. 1. XRD (a) Sb/SbOx/graphene, SbOx/graphene and Sb/graphene, Raman spectra (b) of Sb/SbOx/graphene, SbOx/graphene, Sb/graphene, graphene oxide and graphene and FTIR of Sb/SbOx/graphene, graphene oxide and graphene.

spectra of Sb/SbOx/graphene, SbOx/graphene, Sb/graphene, graphene and graphene oxide was shown in Fig. 1b. All spectra demonstrated two typical characteristic bands, namely, D band at 1334 cm1 and G band at 1594 cm1 for graphene, were observed on the Raman spectra. In addition, The D band related to the defective graphitic structure of sp3-boned carbon atoms and the G band corresponded to the in-place stretching motion of sp2boned carbon atoms [50]. Relative intensive ratio (ID/IG) indicated the degree of disorder and defect [51]. The ID/IG value of Sb/SbOx/ graphene, SbOx/graphene, Sb/graphene graphene and graphene oxide were 1.12, 1.11, 1.22, 1.01 and 0.99, respectively. The lowest ID/ IG value of graphene oxide indicated the higher level of oxidation. After reduction, the value became larger. Otherwise, with the increased usage of NaBH4, the ID/IG value gradually added, which confirmed the enhancive reduction of Sb particles from SbOx and disorder degree of graphene. The FTIR spectra of Sb/SbOx/graphene, graphene oxide and graphene were shown in Fig. 1c. For graphene oxide, it was obvious seen that the FTIR spectra demonstrated strong absorption peaks at 580, 1055, 1255, 1394, 1609 and 1729, which were related to OeH stretching, alkoxy CeO bands, epoxy CeO bands, carboxy CeO bands aromatic C]C and C]O stretching. Compared with graphene oxide, the evident decrease of the C]O and CeO peak intensity indicated the adequate reduction from graphene oxide to graphene, which were consistent with the Raman results. Moreover, a new band appeared in Sb/SbOx/graphene at 614 cm1, which could be ascribed to the stretching vibration motions of SbeO. The bands verified a strong interaction between Sb and the oxygencontaining groups in the graphene, following beneficial to the firmly adhesion of Sb/SbOx and graphene [50,51]. Fig. 2a showed the TGA curve of Sb/SbOx/graphene. A slight weight loss of 12 wt% was observed from room temperature to 450 C. This weight loss results in water evaporation and

Fig. 2. TGA (a) and DSC (b) curves of Sb/SbOx/graphene.

dehydration condensation of carbon/carboxyl on the graphene surface. From 450 C to 580 C, a rapid weight loss of 40.5 wt% was observed, which corresponded with the main sharp exothermic peak at 480 C on the DSC curve (Fig. 2b). The weight loss in this range was mainly attributed to graphene oxidation, which could be speculated that the content of graphene was approximately 40.5 wt %. In addition, three small exothermic peaks at 300 C, 508 C, and 550 C were also observed on the DSC curve. Considering the multilevel oxide of Sb, these exothermic peaks should correspond with the different oxidization degrees of Sb and SbOx reaction. After 600 C, the curves became flat, which indicated that the same oxidation of antimony particle (Sb2O4) produced as well as the

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carbon content (graphene) burned out completely. We could use the following equation to calculate the weight ratio of antimony inside the composite: Sb(wt%) ¼ (Sb molecularweight/Sb2O4 molecularweihgt) * (weight of Sb2O4 at 600 C/weight of composite at 20 C). The content of antimony was 18.81% after calculated by the above equation and the corresponding weight ratio of Sb2O3 were 28.69% (Sb/Sb2O3 ¼ 2:3) [15,28]. The composition morphology of the Sb/SbOx/graphene was analyzed by energy dispersive spectroscopy (EDS), as showed in Fig. 3. It was obviously seen many Sb/SbOx particles uniformly dispersed on the sheets of graphene (Fig. 3a). The C, O and Sb elemental mapping images demonstrated the nanocomposite contained these elements and these elements homogeneously distributed on graphene (Fig. 3bed). Fig. 4 presented the morphology of Sb/SbOx/graphene (Fig. 4a, b), SbOx/graphene (Fig. 4c, d) and Sb/graphene (Fig. 4e, f) characterized by SEM. SEM results showed that the as-prepared products consisted of porous-layered structures and some irregular fluffy materials (Fig. 4aed). In Fig. 4a, b, Sb/SbOx particles anchored on the surface of graphene. Moreover, compared with SbOx/graphene, the Sb/SbOx/graphene demonstrated more uniform dispersion. Unfortunately, with the excessive usage of NaBH4, the graphene lost the porous structure to some graphene sheets stacked together and Sb particles fell off from the graphene net (Fig. 4e, f), which will led to large volume change during the repeating cycle process. TEM observations demonstrated that nano particles adhered firmly to the surface of graphene. In addition, TEM analysis further confirmed that Sb/SbOx particles equably attached on the graphene (Fig. 5a, b) than that SbOx particles sparsely dispersed on the graphene (Fig. 5c, d). One lattice-resolved high-resolution TEM image (Fig. 5f) of Sb/SbOx/graphene exhibited a lattice spacing of 0.344 nm, corresponding to the spacing of (012) crystal plane of Sb, which confirmed SbOx has been reduced to Sb. And combining XRD analysis, Sb/SbOx/graphene nano composites have been prepared.

In comparison, SbOx/graphene prepared by milling process by Zhou et al. [48] stacked each other, which was different with the porous structure of Sb/SbOx/graphene. And TEM observations of SbOx/graphene showed that SbOx just loaded on the surface of graphene, not like the tight integration between Sb/SbOx and graphene. In addition, high resolution observations exhibited a lattice spacing of 0.344 nm in the lattice-resolved image, corresponding to the spacing of (012) crystal plane of Sb, which further shown that the presence of metal of Sb was beneficial to obtain high capacity antimony-based SIB anode. The electrochemical performance of the as-prepared Sb/SbOx/ graphene was first analyzed by CV. Fig. 6a showed the three cycles of CV curve. A wide cathodic peak at 0.1e0.7 V was observed in the first reduction process, which corresponded to the formation of SEI film and NaxSb alloys [28]. In the following reduction reactions, the wide peak split into four sharp peaks at 0.28, 0.4, 0.8, and 0.9 V. The first one was related to the Na-ion insertion into graphene [48] and the other three were corresponded to the reduction reaction of SbOx (SbOx þ 2xNaþ þ 2xe / Sb þ xNa2O [28]) and the alloying of Sb with Naþ (2Sb þ 6Naþ þ 6e / 2Na3Sb [36]). Besides, there was a small peak near 0.01 V was resulted from the intercalation of Naions in non-graphitized carbon layers [19]. In the oxidation process, two anodic peaks at 0.75 and 0.8 V were observed, which corresponded to the dealloying reaction process (NaxSb / Sb þ 2xNaþ þ 2xe [48]). In addition, the other wide peak at 1.4 Ve1.8 V was associated with the oxidation of Sb (Sb þ 3Na2O / SbOx þ 2xNaþ þ 2xe [48]). Furthermore, after the first cycle of CV, the following curves were almost overlapped, showing a superb cycle stability of the Sb/SbOx/graphene electrode. The electrochemical conversion process could be described as follows: 2Sb þ 6Naþ þ 6e / 2Na3Sb and Sb2O3 þ 6Naþ þ 6e / 2Sb þ 3Na2O [25,48]. Fig. 6b presented the charge and discharge curves (1st, 2nd, 50th and 100th cycle) of the Sb/SbOx/graphene nanocomposite with a

Fig. 3. SEM (a) of Sb/SbOx/graphene and the C (b), O (c), Sb (d) elemental mapping images of Sb/SbOx/graphene, respectively.


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Fig. 4. SEM of Sb/SbOx/graphene (a, b), SbOx/graphene (c, d) and Sb/graphene (e, f).

stationary current density of 50 mA g1 and a voltage range of 0.01e3 V. In the first discharge curve, there is a long plateau at near 0.6 V, which could be ascribed to the formation of SEI layer and Na3Sb alloy [28]. In the following discharge processes, there were a clear slant plateaus between 0.4 and 1.0 V that due to the formation of the Na3Sb alloy [15]. With repeated cycling process, the position and length of plateaus decreased due to the volume expansion of Sb/SbOx after 100 cycles. In charge process, the electrochemical reaction was demonstrated by two slant plateaus from 0.7 to 1.0 V and 1.5e2.0 V. The first one was related to the desodiation of Na3Sb to Sb and the other one was corresponded to the extraction of Naþ from Na2O [25]. The chargeedischarge plateaus were nearly consistent with the peaks in the CV curve. However, RGO/SbOx [48] and RGO/nano Sb [36] mainly discussed the charge-discharge curves at various current densities and didn't analysis the plateaus corresponding to each electrochemical reactions. Fig. 6c shows the cycle stability of Sb/SbOx/graphene, SbOx/ graphene, Sb/graphene and graphene tested for 100 cycles at a current density of 50 mA g1 in the voltage from 0.01 to 3 V. It was obvious seen that all electrodes exhibited decline in the first 20 cycles, which may result from the adaptive process of electrolyte absorption, sodium-ion diffusion, electrochemical reaction of

sodium-ion and active materials and slight changes of the external conditions (like temperature, humidity and test machine). Graphene electrode demonstrated capacity of 50 mAh g1 after 100 cycles with excellent stability. For Sb/SbOx/graphene, the first irreversible discharge capacity was 682.5 mAh g1 and the second discharge capacity was 428.5 mAh g1. The huge capacity recession was mainly due to the formation of SEI film. The capacity retention of 30th, 50th, 80th and 100th cycle was 93.9%, 88.6%, 87.0% and 85.6%, comparing with 20th cycle. Sb/SbOx/graphene nanocomposite maintained its stability, and the corresponding discharge capacity was 311.6 mAh g1 after 100 cycles. Moreover, we have compared the results in our experiment with that RGO/SbOx [48] and RGO/ nano Sb [36] already published in the literature in Table 1. The comparisons are in the terms of synthesis process (method, raw materials and reacting equipments), Sb content and cycle capability. Although Sb/SbOx/graphene demonstrated lower discharge capacity than that of two electrodes, it needed more simple synthesis process and the reversible capacity delivered by much fewer Sb content than RGO/SbOx and RGO/nano Sb. By contrast, the other two materials exhibited much lower reversible capacity. SbOx/ graphene was only 191.6 mAh g1, and Sb/graphene was less than 60 mAh g1 after 100 cycles. The reason of capacity degradation of

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Fig. 5. TEM of Sb/SbOx/graphene (a, b, e), SbOx/graphene (c, d) and HRTEM (f) of Sb/SbOx/graphene.

Sb/graphene electrode was mainly due to almost no attachment of Sb particles and graphene. This result indicated that the cycle performance of the Sb/SbOx/graphene nanocomposite was effectively improved compared with those of the Sb/graphene and SbOx/ graphene. The enhanced electrochemical performance of Sb/SbOx/graphene was further proved by rate capability test compared with SbOx/graphene. As shown in Fig. 6d, Sb/SbOx/graphene and nanocomposite electrode exhibited discharge capacities of approximately 353.1, 316.9, 275.3, 235 and 205.6 mAh g1 at the current densities of 50, 100, 200, 400 and 800 mA g1, respectively. Nevertheless, the corresponding figures of SbOx/graphene were 253.1, 214.6, 170.2, 120 and 101.3 mAh g1. When the rate is reverted to 50 mA g1 after 50 cycles, the capacity of the Sb/SbOx/ graphene electrode was recovered immediately. This finding indicated that the nanocomposite exhibited good tolerance during repeated Naþ insertion/extraction. The perfect electrical conductivity of graphene dispersed between Sb and SbOx played an effective role to improve the nanocomposite electrochemical performance, which could prevent Sb/SbOx aggregation, facilitate electrolyte immersion and sodium-ion diffusion. The effective effect of graphene on nanocomposite

electrochemical performance can be easily understood. However, the cause of their differences in electrochemical performance needs further investigation. Therefore, EIS test was adopted to evaluate the charge-transfer resistance and impedance of Sb/SbOx/graphene anode and compare SbOx/graphene and Sb/graphene anodes. The EIS spectra plots were recorded at charge states after 10 cycles (Fig. 7a). The features of the three impedance spectra were similar. All the plots are composed of a depressed semicircle in the high-tomedium frequency range and a slopping line in the low frequency region [52]. The semicircle diameter for the Sb/SbOx/graphene in the high-to-medium frequency range was smaller than the other two. The accurate result could be accepted equivalent circuit used to simulate EIS curves (Fig. 7b). These symbols of Rs, Rsf, CPEsf, Rct, CPEct and Zw were represented solution resistance, SEI film resistance, surface film capacitance, charge transfer resistance, double layer capacitance and Warburg impedance [37]. The Rct value of Sb/ SbOx/graphene, SbOx/graphene and Sb/graphene were 90.43, 185.9 and 200.7 U respectively (Fig. 7c), which indicated that the chargetransfer resistance of Sb/SbOx/graphene was lower than those of SbOx/graphene and Sb/graphene. The result also showed that partial reduction of SbOx is conducive to improve the interface between antimony-based nanoparticle and graphene as well as


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Fig. 6. CV profiles at 0.1 mV s1 between 0.01 and 3 V (a), the 1st, 2nd, 50th and 100th cycle of discharge-charge profiles (b), comparative Naþ storage capacity with SbOx/graphene, Sb/graphene and graphene at a 50 mA g1 (c) and comparative rate capability with SbOx/graphene at 50, 100, 200, 400 and 800 mA g1 (d) of Sb/SbOx/graphene.

Table 1 Comparison of our sample with previously published RGO/SbOx and RGO/nano Sb anode materials for sodium ion batteries in terms of synthesis process (method, raw materials and reacting equipments), Sb content and cycle capability. Electrodes

Sb/SbOx/ graphene RGO/SbOx RGO/nano Sb

Synthesis process

Sb content (wt %)

Cycle capability (mAh g1)/Current Density (mA g1)

Method

Raw materials

Reacting equipments

Wet chemical

SbCl3, GO, NaBH4

Refluxing equipments

47.5

315/50

Wet milling Stirring reduction

Sb powder, GO SbCl3, GO, NaBH4, CTAB

Argon, particulate miller Ultrasonic bath and stirring equipments

75 91.5

450/100 641/131

Fig. 7. (a) comparative EIS curves of Sb/SbOx/graphene, SbOx/graphene and Sb/graphene in the 10th cycle (the insert was after fitting), (b) the equivalent circuit used to simulate EIS curves and (c) the Rct value.

enhance the nanocomposite electrochemical performance. The tight integration between Sb/SbOx and graphene was important factor to maintain the stability of Sb/SbOx/graphene anode performance. In order to further investigate the superb

electrochemical performance of Sb/SbOx/graphene, the test coin cell after 100 cycles was disassembled in argon atmosphere, rinsed in ethanol to remove electrolyte, dipped on copper net and observed by EDS and TEM. Fig. 8a showed the SEM image and Fig. 8

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Fig. 8. SEM (a) of Sb/SbOx/graphene and the C (b), O (c), Sb (d) elemental mapping images of Sb/SbOx/graphene after 100 cycles, respectively.

Fig. 9. TEM (a) and HRTEM (b) of Sb/SbOx/graphene after 100 cycles.

bed exhibited the element mappings of Sb/SbOx/graphene after 100 cycles. The element mapping images demonstrated that the equably distribution of C, O and Sb in the composite even after repeating cycles. TEM observations (Fig. 9a) showed that Sb/SbOx particles after Na alloy-dealloy still anchored on the surface of graphene. One lattice-resolved HRTEM image of Sb/SbOx particles exhibited a lattice spacing of 0.344 nm, corresponding to the spacing of (012) crystal plane of Sb, which confirmed the structure well hold the Sb particles. The above analysis confirmed that the hybrid structure remain stable, which still could play its buffer role and conductive matrix role, so that it is beneficial to keep the electrochemical performance of Sb/SbOx/graphene.

4. Conclusions A wet chemical method was developed to synthesize Sb/SbOx/ graphene composites. The detailed characterizations confirm that the composites possess a uniform structure of Sb/SbOx anchored on the graphene surface. Electrochemical tests showed that Sb/SbOx/

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