Sb2O4@rGO Nanocomposite Anode for High Performance Sodium

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Research Article pubs.acs.org/journal/ascecg

Sb2O4@rGO Nanocomposite Anode for High Performance SodiumIon Batteries Kiruthiga Ramakrishnan,† Chandrasekaran Nithya,*,† Bindhya Kundoly Purushothaman,†,§ Nitesh Kumar,†,§ and Sukumaran Gopukumar‡ †

Department of Energy and Environment, National Institute of Technology, Tiruchirappalli, India 620 015 CSIR-Network Institute of Solar Energy, CSIR-Central Electrochemical Research Institute, Karaikudi, India 630 006

‡

S Supporting Information *

ABSTRACT: Research on high performance electrode materials is significant for further development of sodium ion batteries (NIBs). The Sb2O4 anode can be employed as a promising anode material for NIBs owing to its high theoretical capacity of 1227 mAh·g−1. In this paper, we report the Sb2O4@rGO nanocomposite anode for NIBs which exhibit good cyclability and rate capability due to the formation of wrinkled rGO nanosheets during cycling. Well-formed nanowrinkles act as a template for anchoring Sb2O4 particles during cycling and effectively alleviate the strain due to the volume expansion. The improved electrochemical performance is attributed to the shorter Na+ ion diffusion path length from the small nanoparticles and good electrons as well as ion transport from the intimate contact between the active Sb2O4 particles and rGO matrix. At a current density of 0.1 A·g−1, it retains the 94.2% (890 mAh· g−1) of initial reversible capacity after 100 cycles. Over prolonged cycling (after 500 cycles), the Sb2O4@rGO electrode still delivers a reversible capacity of 626 mAh·g−1 at a current density of 0.6 A·g−1. These significant results offer hope for the exploration of making high capacity anodes combined with a reduced graphene oxide matrix to alleviate the strain during cycling. KEYWORDS: Coprecipitation, Reduction, Reduced graphene oxide, Antimony oxides, Volume expansion, Electrochemistry

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The finding of suitable negative anode material for NIBs is a major challenge since the Na+ ion cannot be intercalated to the characteristic graphitic carbons used in LIBs.6 Although the intercalation mechanism of the Na+ ion in electrode materials is similar to that of the Li+ ion, the larger size of the Na+ ion (1.06 Å in radius) as compared to the Li+ ion (0.76 Å in radius) makes it difficult to simply adopt recent strategies anticipated for high-performance LIBs.7 The key challenge facing NIBs is to develop appropriate host materials with the capability for fast and stable sodium-ion insertion/extraction. In particular, only a few Na storage anode materials have demonstrated suitable reversible capacity and adequate cyclability. A momentous breakthrough in the energy density of the Na-ion battery may be achieved by using high capacity anodes. Alloy anodes, metal oxides, sodium titanates such as Na2Ti3O7, metal sulfides, phosphides, organic materials, etc. have been examined so far as anodes for sodium ion batteries.8 Among the reported anodes, Sb-based anode materials, viz., Sb, Sb composites and alloys, Sb based oxides, and Sb2S3 have shown high theoretical capacities for NIBs. Sb2O4 possesses high theoretical capacity (1227 mAh·g−1) as compared to other

INTRODUCTION

The term “technology” represents how (mobile, i-pad, laptop, camera) our day to day life starts and ends. It became the sixth finger of every human being. In return, it asks us for energy storage devices. In the electrochemical energy storage field, lithium-ion batteries have been explored substantially in the past few decades and became a main power source for portable electronic devices, high-power tools, and electric vehicles.1,2 Regrettably, the large-scale commercial manufacturing of lithium ion batteries (LIBs) faces brutal challenges from the increasing cost of lithium and limited reserves. These issue triggered our interest in the development of alternative chemistries for energy storage applications.3 The low cost, high abundance, and easy mining of sodium minerals promote interest in sodium-based electrochemical systems as an alternative for LIBs for energy storage devices. Similar to LIBs, sodium-ion batteries (NIBs) could provide alternative chemistry and might thus become economically more competitive, especially in large-scale energy storage systems. In this aspect, a key role has been recently played by the identification of suitable electrode materials able to ensure enhanced electrochemical performances in terms of cycle life and delivered capacity and their limitations (low energy density than lithium) to forward their execution.4,5 © 2017 American Chemical Society

Received: February 13, 2017 Revised: April 11, 2017 Published: April 19, 2017 5090

DOI: 10.1021/acssuschemeng.7b00469 ACS Sustainable Chem. Eng. 2017, 5, 5090−5098

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ACS Sustainable Chemistry & Engineering Sb based analogues9 however; very few studies have been reported for Sb2O4 anode material for NIBs. According to a recent investigation, Sb2O4 accommodates the sodium ions through conversion followed by alloying type of reactions. The reaction will proceed as follows: Sb2 O4 + 8Na + + 8e− → 2Sb + 4Na 2O

(1)

2Sb + 6Na + + 6e− → 2Na3Sb

(2)

Information, Figure S1). The synthesized GO and Sb2O4@rGO composites were characterized by means of an X-ray diffractometer (Xpert PRO PANalytical PW 3040/60 X’Pert PRO) at a scan rate of 2° min−1 by using Cu Kα radiation (λ = 1.5418 Å), while the voltage and current were held at 40 kV and 20 mA (2θ = 5−60°). The morphology and microstructure of the composites were characterized by means of SEM (VEGA3 SB, TESCAN Instruments) and TEM (FEITecnai-20 G2). Raman spectra were recorded for the synthesized materials in a Renishaw InVia laser Raman microscope with a He−Ne laser (λ = 633 nm). Electrochemical Measurements. The anodes (pristine Sb2O4 and Sb2O4@rGO composite) were prepared by mixing 80 wt % synthesized material, 10 wt % Super-P carbon, and 10 wt % polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP) to form a homogeneous slurry. The slurry was coated on copper foil and dried under ambient conditions. Circular discs with a diameter of 18 mm were punched out and dried in a vacuum at 120 °C for 12 h. The mass loadings of active materials of Sb2O4 and Sb2O4@ rGO are 1.05 mg·cm−2 and 1.12 mg·cm−2, respectively. Finally, coin cells of 2032 type were assembled inside an argon filled glovebox by using the prepared anode as a working electrode, sodium foil as a reference electrode, celgard 2400 as the separator, and NaClO4 in 1:1 ethylene carbonate/propylene carbonate (EC/PC) as the electrolyte. Charge−discharge studies of the coin cells were performed by using a programmable battery tester (Biologic Science Instruments) at suitable current densities in the potential range of 0.01−1.5 V. We took the total mass of the composite in the capacity calculation for showing the capacity of composites. Cyclic voltammograms were recorded by using a Biologic Instruments potentiostat/galvanostat at a scan rate of 0.1 mVs−1 between 0.01 and 1.5 V. Electrochemical impedance spectra were measured by using a Biologic Instruments potentiostat/ galvanostat with an alternating current (AC) voltage signal of 5 mV, and the frequency range was between 100 kHz and 5 mHz.

Sun et al.10 first time attained the discharge capacities of 1120 mAh g−1, 896 mAh g−1, and 724 mAh g−1 in the first, second, and 20th cycle, respectively (at C/70), with Sb2O4/Na. Regardless of their high theoretical capacity, conversely, antimony oxide anode materials are limited in practical consumption because conversion reactions usually suffer from large voltage hysteresis, low reversibility, and high redox potential.11 Thus, it will be extremely significant to overcome the shortcomings and improve its sodium storage capability for NIBs. Graphene based composite anodes are an attractive alternative candidate for NIBs because of their superior electronic conductivity, mechanical strength, and ability to be interfaced with Na active redox components which promote faster Na+ ion diffusion.12,13 Moreover, chemically modified graphenes14 (synthesized from chemical reduction of graphene oxide15), i.e., reduced graphene oxide (rGO) sheets, serve as an excellent matrix to host the active materials providing a necessary electronic path and promote faster Na+ ion diffusion.16 The aforementioned properties of rGO will lead to the long cycle life, high reversible capacity, and superior electrochemical performance.17,18 Herein, we have reported the Sb2O4@rGO nanocomposite as an anode for sodium ion batteries. Most importantly nanosized particles with uniform dispersion on graphene sheets and strong interaction with graphene sheets are significant parameters to improve the NIBs’ performance.

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RESULTS AND DISCUSSION Structure and Morphology Analysis. Figure 1 presents the XRD patterns of GO, Sb2O4, and the Sb2O4@rGO

EXPERIMENTAL SECTION

Synthesis of Sb2O4. Sb2O4 was synthesized using a reductive coprecipitation method, in which two mixtures of solutions (A and B) were prepared separately in double distilled water. Mixture A contained antimony(III) chloride (0.01M) and trisodium citrate dihydrate (0.035 M). Mixture B contained sodium hydroxide (0.035M) and sodium borohydride (0.02 M). Then, mixture B was added drop by drop into mixture A under vigorous stirring, ensuring homogeneity to obtain the required material. The reaction mixture was kept in a water bath at 80 °C for 5 h to improve the crystallinity. After that, the precipitate was filtered and washed with distilled water and ethanol followed by drying at 110 °C for 12 h. Finally, it was calcined at 600 °C for 4 h in an an air atmosphere. Synthesis of Sb2O4@rGO Composite. Graphene oxide was synthesized using the modified Hummers method.14,15 Then, graphene oxide was dispersed in double distilled water and sonicated for about 2 h to obtain a uniform dispersion. To this, as synthesized Sb2O4 powder was added under sonication followed by the addition of sodium borohydride solution and kept under constant stirring for about 2 h. The mixture was filtered, washed repeatedly with water and ethanol, and finally dried at 80 °C for 12 h. Consequently, the prepared Sb2O4@rGO composite was used for further structural and electrochemical characterization. Structural Characterization. TGA (PerkinElemer/TGA4000) of the Sb2O4 and Sb2O4@rGO nanocomposite was analyzed from 0 to 900 °C at a heating rate of 10 °C min−1 in air to understand the thermal decomposition behavior of Sb2O4 and rGO, as well as the percentage of carbon present in the composites (see Supporting

Figure 1. XRD pattern of GO, Sb2O4, and the Sb2O4@rGO composite.

nanocomposite. The diffraction pattern of graphene oxide shows that after oxidation of graphite by the Hummers method, the characteristic peak was obtained around 2θ = 10.09°, indicating that the pristine graphite was oxidized into GO by expanding the d-spacing from 0.34 to 1.01 nm; this is quite consistent with the previous literature.19,20 An increased interlayer distance between consecutive carbon basal planes is attributed to the intercalation of oxygen functional groups into 5091

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ACS Sustainable Chemistry & Engineering the carbon layer structure. The X-ray pattern of Sb2O4 confirms that it exists as a cervantite type structure without any impurity phases. By comparison with the ICDD data (ref. no. 00-0010818), the diffraction peaks at 26.07°, 29.26°, 30.58°, 37.63°, 49.12°, and 65.24° can be assigned to the (111), (112), (004), (020), (024), and (222) planes of Sb2O4 with the Pna21 space group. The XRD pattern of the Sb2O4@rGO nanocomposite exhibits the characteristic peaks of Sb2O4 and rGO. After reduction with NaBH4, the composite does not contain any GO phase and shows only the rGO phase around 2θ = 25.8°, corresponding to the (002) diffraction of the graphitic layered structure with a reduced d-spacing around 0.38 nm.21 It has been reported that the smaller d-spacing in the reduced graphene could be attributed to the effective π−π stacking of tiny graphene sheets with few structural defects.22 These XRD results clearly demonstrate that reduction of GO will partially restore the graphitic crystal structure due to reductive removal of the oxygen containing functional groups. However, the characteristic peak of rGO appears weak in intensity in the composite, which is attributed to the high intensity Sb2O4 peaks weakening the intensity of rGO, and also the content of rGO is little in composites. The size and morphology of the as-synthesized materials are characterized by SEM and TEM analysis. Figure 2 presents the

Figure 3. HRTEM and SAED patterns of (a and b) Sb2O4 and (c and d) Sb2O4@rGO composite.

in folded rGO nanosheets. The size of the Sb2O4 particles on the rGO surface varies from 30 to 100 nm. The SAED pattern (Figure 3d) of the nanocomposite shows the set of intense diffraction rings which can be clearly assigned to the diffractions of Sb2O4 (204, 202, 221) and rGO (002). This demonstrates that the particles present were highly crystalline in nature. The corresponding d-spacings of the planes are calculated as (1.860, 2.241, 1.792 Å) and 3.652 Å. The obtained values for the dspacing of Sb2O4 and rGO are quite consistent with the XRD observations. Laser Raman Spectroscopy. Figure 4 shows the laser Raman spectra of GO, rGO, and Sb2O4@rGO nanocomposites.

Figure 2. SEM images of (a) GO, (b) Sb2O4, and (c) Sb2O4@rGO composite.

SEM images of GO, Sb2O4, and the Sb2O4@rGO nanocomposite. The SEM image of GO shows the micron-sized flakelike morphology. Sb2O4 particles appeared in an irregular morphology with agglomeration. The size of the particles varied from 0.1 to 1 μm. The SEM image of the Sb2O4@rGO nanocomposite clearly shows the Sb2O4 particles resided on rGO nanosheets. Figure 3 depicts the TEM image and SAED patterns of Sb2O4 and the Sb2O4@rGO nanocomposite. The size of the Sb2O4 particle is around 200 nm. Vaguely appearing rings and dots in the SAED pattern indicates that the particles present with agglomeration. In the TEM image of the composite, we can clearly see the Sb2O4 particles embedded

Figure 4. Laser Raman spectra of GO, rGO, and Sb2O4@rGO composite.

The Raman spectrum of graphite is dominated by a strong line (so-called G-line) at 1582 cm−1, which is due to the in-plane bond-stretching phonon mode called the G-mode. In perfect graphene, this phonon is also Raman-active and is predicted at 1585 cm−1. The observed G band at 1593 cm −1 and D band (distortion induced) at 1335 cm−1 in the Raman spectrum of GO23,24 as shown in Figure 4 are indicative of the severe disruption or disorder induced into the sp2 carbon lattice by the 5092

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ACS Sustainable Chemistry & Engineering oxidative synthesis of GO, confirming the successful oxidation of graphite. Raman data further confirm the formation of reduced graphene oxide. The Raman spectra of rGO exhibit two peaks around 1325 and 1597 cm−1, corresponding to the typical D band and G band for carbon material.25 The D and G band positions are very sensitive to the microstructure of carbon materials, which provides the information about defects, disorder, edges, and carbon grain size. The intensity ratio of the D band to the G band (ID/IG) could reflect the degree of disorder of rGO. The ratio of intensities ID/IG is increased from 0.995 (for GO) to 1.02 (for rGO), which is attributed to the fact that reduction creates more sp2 carbon atoms and defects. The increase in ID/IG values, a prominent D band, and a wide G band indicate the loss of long-range ordering between the graphene sheets. The Raman spectrum of the Sb2O4@rGO nanocomposite clearly shows the formation of the composite phase. The four different peaks observed in the spectrum display the complex vibrational modes26,27 of O−Sb−O and Sb−O−Sb bonds. The peaks observed at 200 and 261 cm−1 are from the vibration of O−Sb−O, and the peaks at 401 and 462 cm−1 are from the stretching vibrations of Sb−O−Sb. The composite also exhibits the characteristic D and G bands for rGO. Electrochemical Characterization. The sodium insertion/extraction properties of Sb2O4, rGO, and Sb2O4@rGO as anodes for NIBs are investigated by Galvanostatic charge/ discharge measurements over a voltage range of 0.01 to 1.5 V Vs Na/Na+ at a current density of 0.1 A·g−1 (the first cycle starts from OCV). The initial charge capacities of Sb2O4, rGO, and Sb2O4@rGO are 869, 508, and 944 mAh·g−1 against the discharge capacities of 1227, 1030, and 1077 mAh·g−1, respectively (Figure 5a). The irreversible capacity loss is more for Sb2O4 (358 mAh·g−1) as compared to the Sb2O4@rGO nanocomposite (133 mAh·g−1). The irreversible capacity loss is due to the decomposition of electrolytes on the electrode surface and form the solid electrolyte interface (SEI) in the initial charge/discharge process. Moreover, in the case of metal oxides, formation of Na2O on the anode surface consumes more sodium and forms a metallic phase.28 This is confirmed by HRTEM analysis of the cycled electrode after the first discharge and charge. Figure S2 in the Supporting Information shows the HRTEM images of Sb2O4 in charged and discharged states. This confirms the formation of a Na2O metallic phase (planes of (311) and (220), ICDD Database ref. no. 00-0021285) and Na3Sb ((203) plane, ICDD ref. no. 00-004-0724) in a discharged state. After the first charge, the formed metallic phase is again converted to Sb2O4 (planes of (112), (022), and (111), ICDD ref. no. 00-001-0818) which is confirmed by HRTEM and SAED patterns. In the subsequent cycles, the irreversible capacity loss is gradually decreased and Coulombic efficiency increased. The composite exhibits less irreversible capacity loss as compared to pristine Sb2O4, which is attributed to the rGO matrix assisting to form the thin layer of SEI on the electrode. This is attributed to the surface coverage of Sb2O4 nanoparticles on rGO being able to help to reduce the rGOelectrolyte interaction and reduce the electrolyte side reactions associated with SEI formation, resulting in low irreversible capacity.29 In the present work, the reversible capacities of Sb2O4 and Sb2O4@rGO are 869 and 944 mAh·g−1 at the current density of 0.1 A·g−1 (∼C/10). The achieved capacity is still lower than the theoretical value (1227 mAh·g−1), but it is the highest capacity

Figure 5. (a) Charge/discharge profiles (1st cycle) and (b) cycling performance of rGO, Sb2O4, and Sb2O4@rGO over 100 cycles.

obtained so far as compared to previous work (a reversible capacity of 650 mAh·g−1 was achieved for Sb2O4 at the current rate of C/10).10 As compared to pristine Sb2O4, the composite exhibits high capacity, which is attributed to the rGO sheets serving as a host matrix for Sb2O4 particles, providing a necessary electronic path and shorter sodium ion diffusion path length. However, pristine Sb2O4 exhibits lower capacity due to the presence of large agglomerated particles sluggish the Na+ ion diffusion results low capacity. Cycle stability of electrode material is obviously a most significant parameter for its applicability. The cycling performances (Figure 5b) of rGO, Sb2O4, and Sb2O4@rGO are measured in the potential range of 0.01−1.5 V vs Na/Na+ under a constant current density of 0.1 A·g−1. Remarkably, the Sb2O4@rGO exhibits very stable cycling behavior; after 100 cycles, it still delivers a stable reversible capacity of 890 mAh· g−1, which corresponds to 94.2% of the initial reversible capacity, demonstrating that the interconnected graphene network maintains the structural integrity and electrical conductivity of the electrode.16 For pristine Sb2O4, the capacity is stable up to 25 cycles; afterward the capacity gradually decreases and reaches a capacity of 702 mAh·g−1 with a capacity retention of 81.1% after 100 cycles. The structure was gradually destroyed during repeated sodiation/desodiation processes; because of the large amount of volume expansion, gradual capacity fading results.9 Likewise, rGO sheets retain only 15.4% of the initial capacity after 100 cycles. The poor cycling performance of the rGO sheet is due to during repeated sodiation/desodiation processes, the restacking of rGO sheets 5093

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Figure 6. (a) Rate capability curves. (b) Cycling performance at different current densities. (c) Cycling performance over 500 cycles (current density: 0.6 A·g−1) of Sb2O4@rGO (CE: Coulombic efficiency).

of 0.6 A·g−1 as shown in Figure 6c. The first cycle’s reversible capacity is 787 mAh·g−1, and the capacity is 626 mAh·g−1 even after 500 cycles. After 300 and 500 cycles, the ex situ XRD and HRTEM analyses have been carried out to investigate the stability of the cycled electrode, as shown in Figure 7. The XRD pattern clearly shows the Sb2O4 diffraction pattern with some other peaks which may be due to PVDF and Super-P carbon (used for electrode coating). After 300 cycles, we observed the formation of nanowrinkles, which means that rGO nanosheets turn into thin nanowrinkles. Even after 500 cycles, the XRD pattern clearly shows the Sb2O4 phase, and more nanowrinkles are formed as observed by HRTEM. It is very interesting to note that formed rGO nanowrinkles firmly hold the active Sb2O4 particles on its surface. The well formed nanowrinkles can act as a buffer to accommodate the volume expansion (Figure S3 in the Supporting Information) of the active materials and maintain the structural stability of the electrode on cycling. As a sum of the results, the much better cyclability of the Sb2O4@rGO nanocomposite is attributed to the following several reasons according to Mo et al.:16 (i) During the sodiation/desodiation process, the small size of the Sb2O4 particles reduces the developed strain and prevents the fracture of nanoparticles. (ii) Local cracking is prevented by the developed stress, which is evenly distributed in the whole

taking place, resulting in poor capacity retention over prolonged cycling.30 The rate performance of the Sb2O4@rGO electrode at different current densities is further studied, as shown in Figure 6a and b. When cycled at the current densities of 0.2, 0.6, 1.2, 2.4, 6.1, and 12.2 A g−1, capacities of around 886, 787, 608, 407, 185, and 89 mAh·g−1 resulted, respectively. The cell is cycled back to 0.2 A·g−1; the reversible capacity is 870 mAh·g−1. Notably even at high current densities of 12.2 A·g−1, the cell delivered a reversible capacity of 89 mAh·g−1. Significantly, when the current density returns to lower current density after high rate cycling, the reversible capacity can recover to the original values, indicating that the composite could preserve the integrity of the electrode and thus be tolerant to varied charge and discharge currents which are important for high power applications of rechargeable batteries. The above results demonstrate that the Sb2O4@rGO electrode shows superior rate capability which is attributed to the combination of high electrical conductivity offered by the reduced graphene matrix and shorter diffusion length for both electron and ion transport provided by the small Sb2O4 particles and reduced graphene network. In order to test the cycling stability of the Sb2O4@rGO electrode, the cell is subjected to 500 cycles at a current density 5094

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Figure 7. Ex situ HRTEM and XRD of Sb2O4@rGO nanocomposites (a) after 300 cycles and (b) after 500 cycles.

composite as well as the electrode. (iii) The interconnected graphene network enhances the electrical conductivity as well as decreases the sodium-ion diffusion path length. Figure 8a shows the typical cyclic voltammograms of the half cell at a scan rate of 0.1 mV s−1 between 0.01 and 1.5 V vs Na/ Na+. From the CV curves of Sb2O4 and Sb2O4@rGO, two peaks centered at around 1.3 and 0.7 V are observed in the cathodic process which are attributed to the conversion reaction31 (Sb2O4 + 8Na+ + 8e− → 2Sb + 4Na2O) as well as alloying reaction32 (2Sb + 6Na+ + 6e− → 2Na3Sb). The anodic peaks centered at 1.06 and 1.3 V correspond to the dealloying reaction31 (2Na3Sb → 2Sb + 6Na+ + 6e−) and the formation reaction32 of Sb2O4 (2Sb + 4Na2O → Sb2O4 + 8Na+ + 8e−). The obtained redox peak pairs are consistent with Galvanostatic charge/discharge behavior. The CV curves of the Sb2O4@ rGO composite with an increased number of cycles are shown in Figure 8b. From the CV curves, we confirmed that the conversion reaction of Sb2O4 is reversible, which is identified by the peak at 1.3 V not disappearing in the subsequent cycles. Furthermore, the CV profiles (see Figure S4 in the Supporting Information (CV of rGO)) and charge/discharge measurements showed that in the composites, Sb2O4 particles only take part in the electrochemical reaction, whereas rGO sheets act as a matrix to hold the active Sb2O4 particles very tightly during cycling. Moreover, the rGO sheets serve as a conductive medium for the electron transfer process and provide intimate contact between the active particles.33,34

To further explore the better electrochemical performance of Sb2O4@rGO than that of pristine Sb2O4, we investigated the electrochemical impedance spectroscopy (EIS) before cycling, and the EIS results are shown in Figure 8c. Each of the two Nyquist plots consisted of a depressed semicircle in the highmedium frequency region and a slope in the low frequency region. The semicircle in the high-medium frequency region of the Nyquist plot is assigned to the charge-transfer resistance (Rct) and SEI film resistance (RSEI) between the electrode and electrolyte. The line inclined at a 45° angle is the Warburg region associated with the sodium ion diffusion process in the electrode.35 Obviously, the Sb2O4@rGO composite electrode shows a smaller semicircle diameter as compared to the pristine Sb2O4, which indicates that the composite has a lower interface impedance (Rint = RSEI + Rct), which is quite consistent with cycling behavior. The interface impedance (Rint) of pristine Sb2O4 is 434.92 Ω, whereas the Sb2O4@rGO composite has only 291.84Ω, which is less than the pristine Sb2O4. Z-view software is used to fit the experimental data to simulate (see Figure S5 in the Supporting Information) the obtained experimental data in the present work (an equivalent circuit is proposed to simulate the obtained experimental data and is shown in Figure S6 in the Supporting Information). The circuit elements are Rs (solution resistance in the bulk electrolyte), RSEI and CSEI (the ionic resistance and capacitance present in the SEI layers), and Rct and Cdl (charge transfer resistance and double layer capacitance in the electrode−electrolyte interface), and W is the Warburg impedance (which characterizes the 5095

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Figure 8. Cyclic voltammogram of (a) Sb2O4 and Sb2O4@rGO (1st cycle) and (b) Sb2O4@rGO (1st, 2nd, and 3rd cycles) and (c) Nyquist plots of Sb2O4 and Sb2O4@rGO (before cycling) composites.

diffusion of Na+ ions). Diffusion characteristics of Na+ ions in Sb2O4 and Sb2O4@rGO electrodes are calculated by using the following equation:36,37 D=

Scheme 1. Mechanism of Cycling of Sb2O4@rGO

R2T 2 2A2 n 4F 4C 2σ 2

where R is the gas constant, T is the absolute temperature, n is the number of electrons per molecule oxidized, A is the active surface area, F is the Faraday constant, C is the concentration of Na+ ions, D is the diffusion coefficient, and σ is the coefficient of Warburg impedance obtained from the intersection of the straight line on the real axis.37 The calculated diffusion coefficients of Sb2O4 and Sb2O4@rGO are 5.21 × 10−7 and 3.29 × 10−5cm2 s−1, respectively. The enhanced conductivity is due to the presence of the rGO matrix, which results in the improved electrochemical performance of the Sb2O4@rGO nanocomposite. Table S1 presents the comparison of electrochemical performance of Sb2O4@rGO with the previously reported work. It can be clearly seen that the nanocomposite shows superior electrochemical performance to that shown in the earlier literature. The excellent electrochemical performance of the Sb2O4@ rGO nanocomposite is attributed to the formation of wrinkled rGO nanosheets during galvanostic cycling. Scheme 1 illustrates the proposed mechanism of superior electrochemical performance of the Sb2O4@rGO nanocomposite: (i) Nanosized Sb2O4 particles on the rGO surface can offer a shorter Na+ ion

diffusion pathway as evidenced by EIS observations. (ii) Formation of wrinkled rGO nanosheets during cycling can serve as a buffer to accommodate the volume expansion. (iii) Formed rGO nanowrinkles firmly hold the active Sb2O4 particles on its surface. (iv) In addition, the rGO nanowrinkles can also be served as a conductive medium, which facilitates the charge transport. 5096

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CONCLUSIONS In conclusion, the well-formed Sb2O4@rGO nanocomposite exhibits improved sodium ion storage properties, which could be attributed to the intimate contact between Sb2O4 and rGO nanowrinkles. It is demonstrated that as an anode material for NIBs, the Sb2O4@rGO composite with a combination of small particle size and strong interaction with the rGO network provides long cycling stability and high rate capability. At a current density of 0.6 A·g−1, a capacity of 626 mAh·g−1 is maintained even after 500 cycles. Even at a much higher current density of 12.2 A·g−1, a reversible capacity of 89 mAh·g−1 is still observed. The superior electrochemical performance is attributed to the nanosized Sb2O4 particles as well as the formation of wrinkled rGO nanosheets being able to effectively alleviate the strain caused by the large volume expansion and enhances the electronic conductivity during cycling. All of these results demonstrate that the synthesized Sb2O4@rGO nanocomposite possesses three key advantages: (i) nanosized particles, (ii) the formation of wrinkled rGO nanosheets during cycling, and (iii) strong synergistic coupling effects of rGO and Sb2O4 nanoparticles. More importantly, this work opens the door for the application of high capacity electrodes in next generation NIBs as well as the simple scalable synthesis of graphene-based composites for advanced energy storage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00469. TG curves of Sb2O4 and Sb2O4@rGO composite, HRTEM and SAED patterns of Na2O and Sb2O4 formation after first charge and discharge, HRTEM images of Sb2O4@rGO before cycling and after 400 cycles, CV curve of rGO, fitting curves (Sb2O4 and Sb2O4@rGO composite) of EIS, and equivalent circuit model for describing the EIS behavior (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-9952669945. E-mail: [email protected]. ORCID

Chandrasekaran Nithya: 0000-0002-0761-158X Author Contributions §

Equally contributed.

Funding

This research work was supported by Department of Science and Technology (DST), India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors, Dr. C. Nithya, wishes to thank the Department of Science and Technology (DST), India for an INSPIRE Faculty Award.

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DOI: 10.1021/acssuschemeng.7b00469 ACS Sustainable Chem. Eng. 2017, 5, 5090−5098