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Anchoring Sb6O13 nanocrystals on graphene sheets for enhanced lithium storage Xiaozhong Zhou, Zhengfeng Zhang, Xiaohu Xu, Jian Yan, Guofu Ma, and Ziqiang Lei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13548 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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ACS Applied Materials & Interfaces

Anchoring Sb6O13 nanocrystals on graphene sheets for enhanced lithium storage Xiaozhong Zhou, a,* Zhengfeng Zhang,a Xiaohu Xu,a Jian Yan,b Guofu Ma,a Ziqiang Leia,* a

Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key

Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, Gansu Province, P.R. China. b

Ningde Amperex Technology Limited, Ningde 352100, Fujian Province, P. R. China.

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ABSTRACT: Sb6O13/reduced graphene oxide (Sb6O13/rGO) nanocomposite was synthesized by solvothermal method using Sb2O3 and graphene oxide as raw material. Based on the physical and electrochemical characterizations, Sb6O13 nanocrystals of 10-20 nm in size were uniformly anchored on rGO sheets, and the nanocomposite displayed a large reversible specific capacity of 1271 mA h g-1 and a excellent cyclability of 1090 mA h g-1 after 140 cycles at 100 mA g-1 when proposed as a potential anode material for lithium ion batteries, emphasizing the advantages of anchoring of Sb6O13 nanocrystals on rGO sheets for the maximum utilization of electrochemical active Sb6O13 and rGO for lithium storage.

KEYWORDS: antimony hexitatridecoxide (Sb6O13), reduced graphene oxide (rGO), reversible conversion-alloying, lithium ion batteries, excess lithium storage


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INTRODUCTION Lithium-ion batteries (LIBs), with well design flexibility and high energy density, have been widely used in most of portable electronic devices such as cell phones and laptop computers.1-2 Unfortunately, the graphite anode, the most wildly used anode material for commercial LIBs, has a relatively low theoretical specific capacity (Li1/6, 372 mA h g-1), thereby restricting its applications in future electric vehicles (EVs). Thus, many alternative anode materials by virtue of conversion reactions or alloying/de-alloying reactions with high theoretical specific capacities have been widely investigated as a potential substitute for graphite.2-8 Among various materials that can storage lithium reversibly, antimony-based oxides/sulfides have considerable attention due to plentiful appealing features including abundance, environmental benignity, low cost, and high theoretical capacity (> 800 mA h g-1), which are very promising for the next-generation LIBs.3,

9-14

It is well known that the conversion reaction with lithium could become

reversible when metal oxide particle size is reduced to nanoscale, due to the in-situ formation of metal nanoparticles during the initial discharge process, which enables the formation and decomposition of Li2O upon succedent cycling.1, 8, 10, 15-18 If the structure and morphology were controlled properly, the antimony-based oxides (SbOx) and sulfides (SbSx) as anode materials would experience not only a conversion reaction of SbOx (or SbSx) with lithium but also an alloying reaction of antimony with lithium, resulting in a remarkably large capacity.10-13 Fu et al.10, 19 observed that the thin films of Sb2O3 and Sb2O4 could react electrochemically with lithium and sodium, which delivered a reversible capacity of 794 mA h g-1 and 896 mA h g-1, respectively, according to the reversible conversion-alloying reactions mechanism.


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Since both antimony element and oxygen element can be combined with lithium to form a corresponding lithiated compound, it is expected that the naturally occurring compound Sb6O13 will exhibit a high theoretical specific capacity when used as a potential electrode for LIBs. Sb6O13 has a defective pyrochlore-tye structure and it can be represented as a structural formula of Sb3+Sb5+2O6O'0.5,20 which can be transformed easily from Sb2O321 or Sb2O5•nH2O (n=0~4)22 through heat-treatment under ambient atmosphere. Similar to Sb2O323 and Sb2O419, theoretically, Sb6O13 can not only undergo the reversible conversion reaction (Equation (1)), but also utilize the alloying/dealloying reaction (Equation (2)) reversibly. Sb6O13 + 26Li+ + 26e- ↔ 6Sb + 13Li2O

(1)

Sb + 3e- + 3Li+ ↔ Li3Sb

(2)

Based on the combination of reversible reaction equation (1) and (2), Sb6O13 will get a large theoretical specific capacity of 1270 mA h g-1, which is 3.4 times larger than that of graphite. Based on the nano-size effect in nanostructured electrode materials, which leads to conversion reaction mechanism with lithium,13, 15, 22, 24 Eq. (1) would also be made reversible electrochemically when the morphology and structure of the Sb6O13 phase were controlled properly, thereby rendering the reversible capacity of Sb6O13 close to 1270 mA h g-1 (7231 mA h cm-3). Graphene with unique properties including good electrical conductivity, low weight, large surface area, and superior mechanical flexibility, is promising material in lithium batteries.25-36 Chemically derived doped graphene exhibits a large reversible specific capacity up to 1549 mA h g-1, attributed to both a faradic capacitance on the edge sites or surface of graphene sheets and lithium insertion into the graphene layers.30-32


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Here, an attempt was done to investigate the electrochemical lithium-storage performance of Sb6O13/rGO composite. The reduced graphene oxide (rGO) with good electroconductivity and mechanical durability was used in composite to enhance the electrochemical performance. The flexible rGO sheets can offer a support to uniformly anchore Sb6O13 nanoparticles and serve as a fine conductive substrate for enabling good electric contact between them, as well as could effectively permit accommodation of large volume variations and minimize electrode destruction from the associated strain during lithiation/de-lithiation process.12, 16, 37-39 Benefiting from the high electrical conductivity and the unique nanostructure, the as-obtained Sb6O13/rGO electrode displays excellent properties including reversible specific capacity, cycling performance and rate capability when proposed as a potential anode material for lithium ion batteries.

EXPERIMENTAL SECTION Materials. Natural flake graphite powder, Sb2O3, ethylene glycol, polyvinylidene fluoride (PVDF), N-methyl pyrrolidinone (NMP) and carbon black, were purchased from Aladdin Biochemical Technology Co. Ltd. (Shanghai, China). Electrolyte (1 M LiPF6 in a mixture of ethylmethyl carbonate (EMC), ethylene carbonate (EC), and dimethyl carbonate (DMC) at a 1:1:1 volume ratio) was provided by Shenzhen Capchem Technology Co., Ltd. (China). Synthesis of Sb6O13/reduced graphene oxide nanocomposite. Firstly, graphene oxide (GO) was prepared using natural flake graphite as raw material through a modified Hummers’ method.40 Using Sb2O3 and GO as raw material, Sb6O13/reduced graphene oxide (Sb6O13/rGO) nanocomposite was synthesized by solvothermal method. Typically, 0.3 g Sb2O3 and 0.3 g GO were ultrasonic dispersed in 40 mL ethylene glycol and 30 mL


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distilled water to form homogeneous suspensions, respectively. Then the above two suspensions were mixed together by severely magnetic stirring for 3.5 hours, and the mixed suspension was transferred to a 100 mL Teflon-lined autoclave with a stainlesssteel shell. The autoclave was scaled and solvothermally treated in a conventional oven at 120 ℃ for 12 hours. After being naturally cooled to ambient temperature, the black precipitate Sb6O13/rGO was collected through suction filtration and washed thoroughly successively with anhydrous ethanol and distilled water before being vacuum dried in at 80 ℃ overnight for characterizations. As a contrast, bare rGO was also prepared through the same process without the addition of Sb2O3 using ethylene glycol as a reducing agent41-42. The bare Sb6O13 was synthesized through heat-treatment of hydrated antimony pentoxide (Sb2O5•nH2O, n=0~4) at 400 ℃ for 3 h under ambient atmosphere. A coground mixture of Sb6O13+rGO was prepared using the above-obtained bare rGO and Sb6O13. Due to the reaction between Sb2O3 and ethylene glycol resulting the formation of ethylene glycol antimony, no any solid product was obtained when treating a mixture of Sb2O3 and ethylene glycol without GO aqueous solution by the same procedure as that for Sb6O13/rGO nanocomposite. Physical characterization. Transmission electron microscopy (TEM, FEI TECNAI TF20, operating at 200 kV) and powder X-ray diffraction (XRD, Rigaku D/max 2400, operating with Cu Kα radiation of λ=0.15416 nm, 40 kV, 150 mA) were used to characterize the morphology, crystal structures and

composition of the as-prepared

samples, respectively. All XRD patterns were analyzed using MDI Jade 5.0 software of Material Data, Inc. (MDI). X-ray photoelectron spectra (XPS) was acquired on a PHI5702 spectrometer with Al Kα X-ray radiation as the X-ray excitation source. The Sb6O13


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content in the as-obtained composite was examined using a thermogravimetric analyzer (TGA) (PrekinElmer, U.S.A.). Characterization of electrochemical performance. The detailed electrochemical performances of the as-obtained samples were investigated by galvanostatic chargedischarge (GSCD) tests and cyclic voltammetry (CV) measurements at ambient temperature. GSCD techniques were performed on a LAND CT2001A battery test system (Wuhan LAND Electronics Co., Ltd., China) in the voltage range of 0.01-3.0 V (vs. Li+/Li). CV techniques were performed on an electrochemical work-station (Autolab PGSTAT128N, Metrohm, Switzerland) in the voltage range of 0.01-3.0 V (vs. Li+/Li) at a scan rate of 0.2 mV/s. The electrochemical properties were measured with 2032-type coin cells assembled in a Ar-filled glove box. In lithium half cells, lithium metal foil was used as the counter electrode, a liquid solution of 1 M LiPF6 in dimethyl carbonate (DMC)/ethylmethyl carbonate (EMC)/ ethylene carbonate (EC) (1:1:1 by volume) as the electrolyte, and a Celgard 2400 polypropylene foil as the separator. The testing electrodes consisting of the active powder material (80 wt.%), polyvinylidene fluoride (PVDF) (10 wt.%) dissolved in N-methyl pyrrolidinone (NMP) as the binder, and carbon black (10 wt.%) as the conductor were used. The ingredients were mixed together by stirring to form a homogeneous slurry, which was coated on a copper foil current collector and then dried at 120 oC for 24 h under a vacuum. For clarification, charging here refers to deintercalation of lithium into Sb6O13-based materials, and discharging refers to intercalation of lithium. The special capacity was accounted for the total weight of the active materials (including Sb6O13 and rGO).

RESULTS AND DISCUSSION ACS Paragon Plus Environment

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Structural Verification. The crystalline phase, crystallinity, particle size, and purity of the obtained pure Sb6O13 phase and Sb6O13/rGO composite were examined by XRD. As shown in Figure 1a, diffraction peaks appearing in both the as-obtained Sb6O13/rGO composite and the bare Sb6O13 can be well-indexed to cubic Sb6O13 (JCPDS card No. 33-0111), which suggested the Sb6O13 was successfully synthesized via solvothermal method using Sb2O3 and GO as raw materials. The crystal structure of Sb6O13 is shown in Figure 1b as a defective pyrochlore-tye structure with structural formula of Sb3+Sb5+2O6O'0.5,20 which contains the rigid (Sb5+O3) framework and the (Sb3+2O’) framework.43 It is interesting that, the diffraction peak at 2θ=10˚ for GO (Figure S1(a), black curve) disappeared, and a wide diffraction peak at 2θ=22˚ for rGO (Figure S1(a), blue curve) appeared in the as-obtained Sb6O13/rGO sample, confirming that the GO was reduced into rGO during the preparation process of Sb6O13/rGO sample. To investigate the formation mechanism for Sb6O13 particle in rGO sheets, the same preparation process of Sb6O13/rGO sample was also performed for the mixture composing of bare rGO and Sb2O3 (rGO+Sb2O3, with a mass ratio of wrGO/wSb2O3=0.3/0.3), and freezing-dried GO and Sb2O3 (FDGO+Sb2O3, with a mass ratio of wrGO/wSb2O3=0.3/0.3) in 70 mL ethylene glycol without water, respectively. And the XRD patterns of the both as-obtained samples, named as rGO+Sb2O3 and FDGO+Sb2O3 respectively, are shown in Figure S1(b), in which a wide diffraction peak at 2θ=22˚ for rGO appeared and no obvious diffraction peak of Sb-based oxides was detected. In other words, little Sb-based oxide can be obtained when solvothermally treating of the mixture composing of Sb2O3 and FDGO or rGO with 70 mL ethylene glycol in the absence of H2O, indicating that H2O in the GO aqueous solution acts as a major role in formation of Sb6O13 particles in rGO sheets. Based on the above investigation, a probable formation process for the Sb6O13/rGO nanocomposite is outlined in Scheme 1. During the solvethermal


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treatment, Sb2O3 was firstly transformed into ethylene glycol antimony due to the reaction between Sb2O3 and ethylene glycol, and then into Sb6O13 nanoparticles anchoring on the thin rGO sheets through hydrolysis and oxidation reactions in-situ on the GO sheets with a large number of oxygen-contained functional groups in presence of GO aqueous solution. The oxygencontaining functional groups such as hydroxyl, epoxy, alkoxy and carboxyl groups on the surfaces of GO nanosheets as preferential nucleation sites are conducive to formation of the Sb6O13 nanoparticles, and do make rGO as a good support for uniformly dispersing and anchoring of the Sb6O13 nanoparticles with a small particle size.7, 44-47

Scheme 1. Scheme of synthesizing Sb6O13/rGO nanocomposite.

Based on the Scherrer’s formula, the average particle size of ca. 15 nm was given for the Sb6O13 particles in both the bare Sb6O13 and as-obtained Sb6O13/rGO nanocomposite. Raman spectra (Figure 1c) are then collected to investigate the structure of Sb6O13/rGO and rGO obtained using the same synthesis process. The tiny peak at about 149 cm-1


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according to A1g band from Sb phase indentifies the presence of Sb6O13 in the as-obtained Sb6O13/rGO. From the high change of D/G intensity ratio in Sb6O13/rGO (ID/IG =1.15 ) compared with rGO (ID/IG =0.91 ), the oxygen-containing functional groups of GO have been removed mostly 37, 48 and rGO with high reduction degree can provide a high electric conductivity and large-current discharge capacity.37 This result indicates that the reduction of GO was strengthen in the presence of Sb2O3, which acts a weak reducing agent together with ethylene glycol. The relatively small 2D peak and distinct D band peak demonstrate that plentiful defects and dangling bonds, and numerous disordered structure, are present,49 which indicates that a relatively larger quantities of defects and disordered edge sites were introduced into the rGO sheets within Sb6O13/rGO nanocomposite due to the incorporated of Sb6O13 nanoparticles during the solvothermal treatment.50 These defects not only could play the role of nucleation sites for Sb6O13 nanocrystals growth and prompt them to anchore on rGO sheets uniformly,47 but also can store extra amount of lithium ions.30-32 To further investigate the structure of as-obtained Sb6O13/rGO nanocomposite, XPS measurements were carried out. As can be seen from the full XPS spectrum of asobtained nanocomposite (Figure S2(a)), only the elements Sb, O and C were observed. The high-resolution spectra were taken on C 1s and Sb 3d regions and could be deconvoluted into various components. The Sb 3d3/2 and 3d5/2 peaks in the Sb 3d corelevel XPS spectra (Figure 1d) clearly show that the Sb6O13/rGO sample have two antimony environments: Sb5+ at 540.8 and 531.0 eV, and Sb3+ at 540.1 and 530.4 eV corresponding to Sb6O13. Additionally, two O 1s signals were obtained at 532.2 and 533.5 eV, which can be assigned to the oxygen element in Sb6O13 and rGO, respectively. For


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the GO sample obtained through modified Hummer’s method, the C 1s spectra shows two intensive peaks for carbon sp2 (Cg, ~284.6 eV) and epoxy/hydroxyl group (C-O, ~286.7 eV), and one relatively weak one for carbonyl groups (C=O, ~288.5 eV). After thermal treating with Sb2O3 at 120℃ for 12 h, the C 1s spectra for the as-obtained Sb6O13/rGO nanocomposite can be resolved into four peaks (Figure 1e), which typically assigned to carbon sp2 (Cg, ~284.6 eV), epoxy/hydroxyl group (C-O, ~286.1 eV), carbonyl groups (C=O, ~288.5 eV), and carboxyl groups (O-C=O, ~290.5 eV), respectively51, and the relative intensities of the peaks for oxygen-containing functional groups was weakened significantly. This could be attributed to the efficient reduction of the GO sheets through the solvothermal treatment and attests the formation of rGO. The relatively lower binding energy (~286.1 eV) for epoxy/hydroxyl group than that (~286.7 eV) in GO indicates a relatively strong interaction between rGO sheets and nanocrystals.


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Figure 1. (a) XRD patterns of bare Sb6O13 and the Sb6O13/rGO nanocomposite. (b) Schematic crystal stucture of Sb6O13. (c) Raman spectra of the Sb6O13/rGO nanocomposite and rGO, Highresolution XPS spectra of Sb6O13/rGO nanocomposite: (d) Sb 3d and O 1s core level spectra, and (e) C 1s core level spectra.

To evaluate the Sb6O13 content in the as-obtained Sb6O13/rGO nanocomposite, thermogravimetric analysis TGA was performed in ambient atmosphere and results are illustrated in Figure 2. The weight loss of Sb2O5•nH2O (n=0~4) starts at about 200 oC and stops at about 400 oC, derived from evaporation of the crystal water and formation of Sb6O13, which is relatively stable between 400~600 oC. The slight weigh loss (ca. 8.1 wt.%) below 200 oC for as-obtained Sb6O13/rGO nanocomposite is mainly associated to the volatilization of absorbed water. The 11.6% weight loss from 200 to 400 oC is ascribed to the oxygen containing functional groups of rGO. Meanwhile, the TGA curve indicates a main weight loss of 49.5% at about 500 oC for Sb6O13/rGO nanocomposite, which can be ascribed to combustion of the rGO. Based on the Equation (3), it can thus be calculated from the TGA curve that the Sb6O13 content is about 33.5 wt.%.

WSb6O13 % =

Wtotal − WH 2O − absorbed − WrGO Wtotal − WH 2O − absorbed

× 100%


(3)

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Figure 2 TG curves of Sb2O5•nH2O (n=0~4) and as-obtained Sb6O13/rGO nanocomposite.

Morphology of the as-obtained Sb6O13/rGO nanocomposite. The morphology and structure of the as-obtained Sb6O13/rGO nanocomposite investigated by TEM are presented in Figure 3. As shown in Figure 3a, the black Sb6O13 nano-particles are uniformly anchored on the gray thin rGO sheets, which confirms the formation of Sb6O13/rGO nanocomposite. Based on the TEM analysis, the particle size of Sb6O13 in the as-obtained nanocomposite is between 10 and 20 nm, in accord with the analysis result based on the XRD pattern using the Scherrer’s equation. Selected area electron diffraction (SAED, inset in Figure 3a) of the Sb6O13/rGO nanocomposite displays two sets of diffraction patterns: (i) diffraction spots consistent well with Sb6O13 spacing (with selected d-spacings of 0.126, 0.189, 0.215, 0.266, and 0.309 nm), and (ii) diffraction rings of crystalline rGO (with d-spacings of 0.21 and 0.12 nm).52 From the high-resolution TEM (HRTEM) image (Figure 3b), a distinct crystal lattice with a measured lattice spacing of ca. 0.295 nm is consistent to the (222) plane of cubic Sb6O13. And the Sb6O13 particle was wrapped with rGO nanosheets with the (200) interplanar distance of ca. 0.39 nm, which is consistent with the analysis result based on the XRD pattern (Figure 1a). As the anode material of LIBs, the rGO nanosheets coating on the active particles play a role of supportor and electric conductor, as well as restrain the aggregation and volume expansion of nanoparticles during lithiation/de-lithiation


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processes, beneficial to prolong the cycling life of LIBs. The aforementioned XRD, Raman, XPS and TEM structural characterizations confirm the successfully obtained the Sb6O13/rGO nanocomposite.

Figure 3. TEM (a), and HRTEM images (b) of Sb6O13/rGO nanocomposite. The corresponding selected area electron diffraction (SAED) pattern is inset in (a).

Electrochemical Performances of the As-obtained Sb6O13/rGO Nanocomposite. The lithiation/de-lithiation behaviour of the as-obtained Sb6O13/rGO nanocomposite for LIBs was evaluated using half-cell cycled between 0.01-3.0 V. The first three cyclic voltammograms (CV) of pure Sb6O13 and the as-obtained Sb6O13/rGO nanocomposite are illustrated in Figure 4a and b, respectively, to indentify all the electrochemical reactions in both electrodes. The reduction peak at 1.24 and 1.16 V in the first cycle for pure Sb6O13 and Sb6O13/rGO electrode, respectively, can be assigned to decomposition of Sb6O13 to form Sb and Li2O, which is similar to those observed in Sb2O3 thin film electrode.10 The redox pairs at ca. 0.72 V during reduction and ca. 1.15 V during oxidation derived from the alloying reaction and de-alloying reaction between Sb and Li.53-54 During the reversed anodic scan, peak occur around 1.48 V originate from the oxidization


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reaction to convert Sb into nano-Sb6O13, which is similar to those observed in Sb2O3 thin film electrode.10 This consequence demonstrates that the nano-sized Sb6O13 phase could be decomposed and re-constructed reversibly during subsequent discharging and charging processes according to the Eq. (1) and (2). Furthermore, the correlative plateau regions can be discovered in the discharge/charge curves of the Sb6O13/rGO electrode (Figure 5a). As the cycling proceeds, for pure Sb6O13, the onset potentials of the reduction peaks at ca. 0.72 V shift negatively while that of the oxidation peak at ca. 1.15 V moves inversely, and the peaks sharply deform and widen (Figure 4a), demonstrating a larger polarization and a sluggish reaction kinetics of electrode, which result in the gradual increased reaction hysteresis. Significantly, at subsequent cycles for the Sb6O13/rGO electrode, the reduction peaks consistently shift right and the oxidation ones shift left relative to those in the first cycle (Figure 4b), illustrating a low polarization change with cycling. Meanwhile, no obvious variations in the curve feature and peak position occur at the subsequent two cycles, demonstrating a superior cyclability of the Sb6O13/rGO electrode. The plot with smaller polarization and narrower voltage separation is acquired for Sb6O13/rGO electrode, which might be explained by the effective compositing with rGO in Sb6O13/rGO nanocomposite, reducing the restraint for the electron and ion transfer. The broad reduction peak between 0.5 and 0.01 V for Sb6O13/rGO electrode shown in Figure 4b overlaps well in the subsequent cycles, which indicates that reversible lithium storage is capable at point defects or/and interfaces on the rGO sheets in form of faradic capacitance on the edge sites or surface of rGO.30-32, 55-61 As the cycling proceeds, the CV curves of Sb6O13/rGO electrode become stable in their shape and intensity, exhibiting the activation and rearrangement process of active materials.


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Figure 4. First three cyclic voltammograms for (a) pure Sb6O13 and (b) the as-obtained Sb6O13/rGO nanocomposite at a scan rate of 0.2 mV s-1. The constant current galvanostatic charge/discharge curves of Sb6O13/rGO electrode between 0.01-3 V vs. Li+/Li under a current density of 100 mA g-1 are represented in Figure 5a. During first discharge profile, three regions were observed. The region between 2.0~1.0 V could be ascribed to the reduction reaction of Sb6O13 to Sb and Li2O, which steepens during the following several cycles, indicating that this step is partly electrochemically irreversible. The plateaus at ca. 0.72 V could be ascribed to alloying reaction between Sb and Li.53-54 The obviously declined region from ~0.5 V in the discharge curve, which contributes about a half of total discharge capacity in ally cycles, could be ascribed to the multiple lithium insertion sites in the Sb6O13/rGO electrode, mainly including interfaces, micro-/mesopores active sites and induced defects,30-32, 5561

which leads to the higher actual capacity than the theoretical capacity calculated based on the

traditional theory. The discharge/charge cycling performance of the as-obtained Sb6O13/rGO electrode, as illustrated in Figure 5b, is investigated between 0.01 and 3.0 V at 100 mA g-1 for 140 cycles. The initial discharge process leads to a very large initial capacity of 2347 mA h g-1. The succedent


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charge process provides a capacity of 1271 mA h g-1. The pronounced initial irreversible capacity (45.8%) loss is due to the incomplete extraction of lithium from the active material and the formation of solid electrolyte interphase (SEI) layer,62 consistent with the CV result. The reversible capacity gradually decreases to a value of ca. 921 mA h g-1 after 30 cycles, and then increases tardily up to 1109 mAh g-1 even in 140 cycles. From the TEM image of the fullylithiated Sb6O13/rGO electrode (as shown in Figure S3), the Sb6O13 nanocrystals were broken into small particles anchored tightly on the rGO nanosheets and the interface between active material and electrolyte is enlarged, resulting in low coulombic efficiencies and capacity fading during the several inital cycles. It should be emphasized that, the specific capacity values given for the electrode are accounted for the total mass of the Sb6O13/rGO nanocomposite, including the rGO nanosheets. This value still gets close to the theoretical value of Sb6O13 (1270 mA h g-1), much larger than that for graphene (744 mA h g-1), previous Sb-based materials.10-12, 14. As shown in the Figure 5c, another way was used to present the capacity contributed by Sb6O13 or rGO during cycling in the Sb6O13/rGO nanocomposite. Either component delivered extremely higher-than-theoretical reversible capacities, hypothesizing the other one displayed the fixed theoretical capacity (744 mA h g-1 for rGO, or 1270 mA h g-1 for Sb6O13) during cycling. Based on the initial reversible specific capacity, the reversible capacity retention of ca. 87.3% can be achieved after 140 cycles. It is notable that the capacity retention of the Sb6O13/rGO nanocomposite is significantly improved compared with the Sb6O13+rGO mixture and the pure Sb6O13, which showed a fast capacity declining (Figure 5b).


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Figure 5 (a) Galvanostatic discharge/charge curves of the Sb6O13/rGO electrode at 100 mA g−1. (b) Coulombic efficiency (right y-axis) and Cycling performance (left x-axis) of the Sb6O13/rGO electrode at 100 mA g−1, compared with Sb6O13+rGO mixture and bare Sb6O13. (c) Capacity contribution from Sb6O13 or rGO during cycling in the Sb6O13/rGO nanocomposite, hypothesizing rGO contributed the fixed theoretical capacity of 744 mA h g-1 or Sb6O13 contributed the fixed theoretical capacity of 1270 mA h g-1 during cycling, respectively. (d) and (e) Rate performance of the Sb6O13/rGO nanocomposite.


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Figure 5d shows the long-term cycling performance of the as-obtained Sb6O13/rGO nanocomposite measured at 500 and 1000 mA g-1, in which the reversible capacity firstly decreases and then stabilizes at ca. 430 and 290 mA h g-1, respectively. To further investigate the electrochemical performance, the rate capability and stability of the Sb6O13/rGO electrode under varied current densities (200, 300, 500, 1000, 2000, 3000 and 5000 mA g-1) for LIBs were demonstrated in Figure 5e. Even at 3000 mA g-1, the electrodes still retain the reversible capacity of 201 mA h g-1. After the higher different current lithiation/de-lithiation process, when current density back to 200 mA g-1, the reversible capacity maintains more than 745 mA h g-1, which is much higher than that of the Sb6O13+rGO mixture as shown in Figure S4, indicating the enhanced rate capability and stability by anchoring Sb6O13 nanocrystals tightly in rGO sheets.

Figure 6 Electrochemical impedance spectroscopy (EIS) plots of pure Sb6O13 and the asobtained Sb6O13/rGO before cycle. Figure 6 shows the comparison of the electrochemical impedance spectra (EIS) between pure Sb6O13 and the as-obtained Sb6O13/rGO before cycle. The data were collected from 100 kHz to 10 mHz. As shown in Figure 6, both EIS curves exhibit a small semicircle in the highmedium frequency region and an inclined line in the low frequency region, respectively,


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representing the charge-transfer resistance and the Li+-diffusion process. Obviously, the asobtained Sb6O13/rGO electrode displays a much smaller charge-transfer resistance than the bare Sb6O13, indicative of the improved electrical conductivity by compositing with rGO. These outstanding electrochemical properties might be owed to the reversible interface lithium storage, and the excellent mutually complementary effect between Sb6O13 and rGO sheets in the nanocomposite. Uniform Sb6O13 nanoparticles of 10-20 nm in size were anchored homogenously on the rGO sheets, which provided a relatively strong interaction between rGO sheets and nanocrystals, just as that in SnO2/rGO nanocomposite16 or Sb/graghene63. And we consider that the flexible structures of rGO sheets and the interaction between rGO sheets and Sb6O13 are both helpful to primely buffer the volume change and suppress the aggregation of the in-situ formed Sb6O13 nanoparticles during lithiation/de-lithiation processes, and ultimately achieve reversible conversion-alloying reaction between antimony oxide and lithium. Moreover, a large amount of defects or micro-pores in the rGO sheets could accommodate additional lithium ions,64 the reversible formation of gel-like film16,

65-68

and the extra lithium

interfacial storage between the primary nanoparticles 6, 56-57, 69-70 could also conduce to the high reversible specific capacity, which have been confirmed in numerous metal oxide electrode materials. Thus, such a composite is able to effectively utilize large surface area,

excellent

mechanical

flexibility,

the

favorable

conductivity,

and

good

electrochemical lithium-storage properties of rGO sheets as well as the short lithium diffusion length, high electrode-electrolyte contact area, and good stability for nanostructured Sb6O13 particles. These above results distinctly reveal that the wellorganized nanostructure, which prevents the aggregation of nanostructured Sb6O13


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particles, facilitates electrolyte immersion and accelerate electrons and lithium ion exchange, is conducive to improving the electrochemical performance.

CONCLUSIONS In summary, through the use of facile solvothermal synthesis to form Sb6O13/rGO nanocomposite, we have enabled Sb6O13 nanocrystals to anchor on rGO sheets and maximally utilize the electrochemical active Sb6O13 and rGO as lithium-storage material for LIBs. The as-obtained Sb6O13/rGO nanocomposite exhibits excellent properties including reversible specific capacity, cycling performance and rate capability for LIBs, benefiting from the nanocrystals Sb6O13 and the conductive rGO sheets. These results would shed light on developing novel electrode materials based on the reversible conversion-alloying reaction mechanism for high performance rechargeable batteries.

ASSOCIATED CONTENT The Supporting Information, including XRD patterns of the rGO, GO, rGO+Sb2O3 and FDGO+Sb2O3, XPS spectra of Sb6O13/rGO nanocomposite and bare GO, TEM image of the Sb6O13/rGO nanocomposite in its fully discharged state, and rate performance of the Sb6O13+rGO mixture, is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION *

Corresponding Authors

Xiaozhong Zhou, E-mail: [email protected], Tel/Fax: +86 931 7972663


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Ziqiang Lei, E-mail: [email protected], Tel/Fax: +86 931 7971261

ACKNOWLEDGEMENTS The authors acknowledge the funding support by the National Natural Science Foundation of China (Grant nos. 51462032), the program for Changjiang Scholars and Innovative Research Team in University (IRT15R56), and the Fundamental Research Funds for universities in Gansu province.

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