ARTECLE

J. Cent. South Univ. (2019) 26: 1493-1502

DOI: https://doi.org/10.1007/s11771-019-4105-8

Facile synthesis of Sb＠Sb₂O₃/reduced graphene oxide composite with superior lithium-storage performance

ZHOU Xiao-zhong(周小中)¹, LU He-jie(陆和杰)¹, TANG Xing-chang(唐兴昌)²,ZENG Ya-ping(曾娅萍)¹, YU Xin(余欣)¹

1. 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, China;

2. State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Sb-based materials have been considered one of the most promising anode electrode materials for lithium-ion batteries, whereas they were commonly synthesized through time-consuming and costly processes. Here, Sb＠Sb₂O₃/reduced graphene oxide (Sb＠Sb₂O₃/rGO) composite was successfully synthesized by a facile one-pot chemical method at ambient temperature. Based on the XRD and TGA analysis, the mass fractions of Sb and Sb₂O₃ in the Sb＠Sb₂O₃/rGO composite are ca. 34.05% and 26.6%, respectively. When used as an alternative electrode for lithium ion batteries, a high reversible capacity of 790.9 mA·h/g could be delivered after 200 cycles with the capacity retention of 93.8% at a current density of 200 mA/g. And a capacity of 260 mA·h/g could be maintained even at 2000 mA/g. These excellent electrochemical properties can be attributed to its well-constructed nanostructure. The Sb and Sb₂O₃ particles with size of 10 nm were tightly anchored on rGO sheets through electronic coupling, which could not only alleviate the stress induced by the volume expansion, suppress the aggregation of Sb and Sb₂O₃ particles, but also improve the electron transfer ability during cycling.

Key words: Sb＠Sb₂O₃/rGO composite; synthesis; electrochemical performance; lithium-ion batteries

Cite this article as: ZHOU Xiao-zhong, LU He-jie, TANG Xing-chang, ZENG Ya-ping, YU Xin. Facile synthesis of Sb＠Sb₂O₃/reduced graphene oxide composite with superior Lithium-storage performance [J]. Journal of Central South University, 2019, 26(6): 1493-1502. DOI: https://doi.org/10.1007/s11771-019-4105-8.

1 Introduction

With the rapid development of society, more and more attention is being paid to the environmental protection and sustainable development, thus cleaner energy resources, such as solar and wind, have been extensively developed. To peak shift operation for these cleaner energy application, energy storage systems (ESSs) are requisite. Due to their high energy density, excellent cycling stability and low-toxicity, lithium ion batteries (LIBs) have been widely applied in portable electronic products, and become the primary candidates for automotive application and ESSs [1]. However, the commercial LIBs with graphite-based anode electrode are unable to meet those new applications considering the lower theoretical specific capacity (372 mA·h/g) of graphite [2]. Therefore, it is imperative to explore substitute electrodes with high specific capacity and long cycle life [3, 4].

Among those alternative anode materials, antimony-based materials have been considered one of the most promising anode electrode materials, attributed to their high theoretical capacity (Sb, 660 mA·h/g; Sb₂O₃, 1103 mA·h/g) [5], as well as appropriate kinetic and voltage characteristics which could effectively prevent the emergence of lithium dendrites [6]. Before practical application in LIBs, several issues such as low practical specific capacity and poor cycling stability should be solved properly. The low practical specific capacities can be due to the low utilization of the active materials and/or the partly irreversible conversion reaction of antimony-based compounds during cycling. Large volume changes during the de-lithiation/lithiation reactions cause the electrode fracture and pulverization [7], as well as the continuous growth of solid electrolyte interphase (SEI) film, resulting in rapid capacity fading [8]. Partial reduction of antimony-based compound to form Sb/Sb₂O₃ composite can relieve the stress induced by volume expansion, which is beneficial to improve its cycling stability [8, 9]. YANG et al [8] synthesized the Sb/Sb₂O₃＠C spheres through a partial reduction method, which delivers a good capacity of 467 mA·h/g after 95 cycles. Formation of nano-sized active materials and carbonaceous modification can greatly enhance their electrochemical performance [10-14]. The nano- sized active materials could enhance the electrochemical reaction kinetics owe to their unique surface microstructure, and promote the reversible reactions [3, 15-17]. With superior electronic conductivity, stable chemical properties and huge specific surface area [18], graphene has been widely used in high-performance LIBs [19, 20]. In our previous reported work, anchoring Sb₂O₃ [10], Sb₂O₄ [11] and Sb₆O₁₃ [12] nanoparticles on reduced graphene oxide (rGO) sheets by a solvothermal method could improve the reversible capacities and electrochemical stabilities to a large extent, which can be attributed to the synergistic effect of the rGO sheets and the nano-sized antimony oxides. The interactions between active antimony oxides and the rGO sheets supporter can effectively inhibit nanoparticles aggregation and enhance the electrical conductivity, resulting in the superior electrochemical performance.

However, these antimony-based materials with excellent electrochemical performance are commonly synthesized through time-consuming and costly processes. Here, a facile one-pot chemical reduction method has been employed to synthesize Sb＠Sb₂O₃/rGO composite at ambient temperature, using Sb₂O₃ as antimony source, graphene oxide (GO) as supporter, ethylene glycol as solvent, and NaBH₄ as reductive agent. As verified, 10 nm Sb and Sb₂O₃ nanoparticles are tightly anchored on rGO sheets. When used for LIBs application, the as-obtained Sb＠Sb₂O₃/rGO composite could deliver a high reversible capacity of 790.9 mA·h/g after 200 cycles at a current density of 200 mA/g, corresponding to the capacity retention of 93.8%.

2 Experimental

2.1 Synthesis of GO, Sb＠Sb₂O₃ and Sb＠Sb₂O₃/ rGO composite

Graphene oxide (GO) was prepared by a modified Hummers’ method [4], using natural flake graphite as raw material. The obtained GO aqueous solution (0.1 mg/mL) was freeze-dried for further use. Sb＠Sb₂O₃/rGO composites were synthesized by a facile one-pot chemical reduction method, as shown in Figure 1. Typically, 1 g of Sb₂O₃ was ultrasonically dispersed in 100 mL ethylene glycol (EG) for 0.5 h. The obtained mixture solution was heated and further stirred at 110 °C for 0.5 h and during which ethylene glycol antimony compound was formed as a result of the reaction between EG and Sb₂O₃ [21]. After being cooled down to ambient temperature, 2.0 g of freeze-dried GO (FDGO) was added and dispersed ultrasonically. Finally, excess NaBH₄ ethanol solution was dropwise added into above mixed suspension and continuously stirred for another 1.5 h. The precipitate was collected by extraction filtration and washed sequentially with ethanol and distilled water, and then dried in an oven at 80 °C to obtain Sb＠Sb₂O₃/rGO composite. As for comparison, Sb＠Sb₂O₃ composite and bare rGO were also synthesized by the same process except for the addition of FDGO and Sb₂O₃, respectively.

Figure 1 Schematic illustration of synthesis procedure for Sb/Sb₂O₃/rGO sample

2.2 Material characterization

Field-emission scanning electron microscope (FE-SEM, Ultra Plus, Carl Zeiss, Germany,) and field-emission transmission electron microscope (TEM, FEI TECNAI TF20) were employed to investigate the microstructures of the samples. The phase compositions were identified by X-ray diffraction (XRD, PANalytical Empyrean) using Jade 5.0 of Material Data, Inc. (MDI). The surface structure was analyzed by X-ray photoelectron spectra (XPS, PHI-5702, Al K_α X-ray radiation). A Pyris Diamond thermogravimetric analyzer (TGA, PrekinElmer, U.S.A.) was used to determin the Sb and Sb₂O₃ contents of Sb＠Sb₂O₃/rGO composite, which was operated between room temperature and 800 °C with a heating rate of 10 °C/min under ambient atmosphere.

2.3 Electrochemical characterizations

Electrochemical lithium-storage properties of the as-obtained samples were evaluated through cyclic voltammetry (CV) and galvanostatic charge- discharge (GSCD) measurements in the voltage range of 0.01-3.0 V vs Li⁺/Li at ambient temperature, using 2025-type coin cells in an argon-filled glovebox. The electrical conductivities were measured by electrochemical impedance spectroscopy (EIS). The GSCD measurements were performed on a battery testing system (CT2001A, Wuhan LAND Electronics Co., Ltd., China). On an electrochemical workstation (Autolab PGSTAT128N, Metrohm, Switzerland), the CV curves were obtained at a scan rate of 0.2 mV/s, as well as the EIS curves obtained over a frequency range of 10^-5-10² kHz at open-circuit potentials. The 2025-type coin cells were assembled with lithium metal wafer as the counter electrode, a Celgard 2400 polypropylene foil as the separator, and 1 mol/L LiPF₆ solution in a dimethyl carbonate/ ethyl methyl carbonate/ethylene carbonate (DMC/EMC/EC, 1:1:1 by volume) mixture as the electrolyte. For the testing electrode, the active powder materials, polyvinylidene fluoride and carbon black were mixed together and magnetically stirred with a mass ratio of 75:15:10 to form a homogeneous slurry, and then blade coated on copper foil substrate. After vacuum-dried at 80 °C overnight, the as-prepared electrodes were cut into discs with a diameter of 10 mm for 2025-type coin cells. The specific capacities were calculated based on the total weight of as-obtained composites.

3 Results and discussion

3.1 Structure, morphology and composition of Sb＠Sb₂O₃/rGO composite

Figure 2(a) shows the XRD patterns of the obtained Sb＠Sb₂O₃ and Sb＠Sb₂O₃/rGO composites. Obviously, the intense diffraction peaks in both samples can be well indexed to the elemental Sb phase with rhombohedral structure (JCPDS card No. 85-1322), as well as several weak peaks for senarmontite cubic structured Sb₂O₃ phase (JCPDS card No. 05-0534), indicating the successful reduction of Sb³⁺ into Sb⁰ and successful synthesis of Sb＠@Sb₂O₃ composite by the facile one-pot chemical reduction method at ambient temperature. According to the semi-quantitative analysis with the as-known reference intensity value (K) of the characteristic diffraction peaks, the mass ratio of Sb to Sb₂O₃ is about 7.47 and 1.28 in Sb＠Sb₂O₃ and Sb＠Sb₂O₃/rGO, respectively. The relative higher Sb₂O₃ content in Sb＠Sb₂O₃/rGO composite can be attributed to the addition of GO, which could play a role of weak oxidant [12]. A wide diffraction peak located at around 2θ=25.2° originated from rGO can be detected, indicating the successful reduction of GO [22]. To further verify the reduction of GO, Raman spectroscopy was employed. As shown in Figure 2(b), compared with bare GO and rGO, the higher integrated intensity ratio of characteristic D band and G band (I_D/I_G= 1.25) for Sb＠Sb₂O₃/rGO composite indicates higher reduction degree for rGO in composite than that for bare rGO [12, 23], which is beneficial for electric conductivity and lithium storage properties [24, 25]. Moreover, a tiny peak located at 146 cm^-1 can be attributed to the A_1g band of the Sb phase, indentifying the presence of Sb in the as-synthesized Sb＠Sb₂O₃/rGO composite.

Figure 2 XRD patterns of Sb＠Sb₂O₃ and Sb＠Sb₂O₃/rGO nanoparticles (a), Raman spectra of GO, rGO and Sb＠Sb₂O₃/rGOcomposite (b), full XPS spectrum (c) and high-resolution C 1s XPS spectra of Sb＠Sb₂O₃/rGOcomposite (d)

The surface structure of the obtained Sb＠Sb₂O₃/rGO composite was analyzed by XPS measurement. Figure 2(c) displays the full XPS spectrum for Sb＠Sb₂O₃/rGO composite, in which only three elements (C, O and Sb) can be detected, confirming that element Sb is present in the Sb＠Sb₂O₃/rGO composite [26, 27]. Figure 2(d) displays the bonding information of C 1s investigated by means of XPS peak-differenating analysis. It can be seen that, the high-resolution XPS spectra of C 1s can be fitted well with four peaks at 288.6, 286.7, 285.2 and 284.6 eV, which are assigned to O—C=O, C=O, C—O, and C—C/C=C, respectively [28]. And the relative weak intensities of the O—C=O and C=O peaks further confirm that GO has been reduced into rGO after the facile one-pot chemical reduction procedure at ambient temperature, which agrees well with the results of XRD and Raman analysis [10, 29]. It is surprising that the binding energy of C—O (285.2 eV) is much lower than that of the standard C—OH, indicating that there is a strong electronic coupling between rGO sheets and Sb/Sb₂O₃ particles in the Sb＠Sb₂O₃/rGO composite [30]. This strong electronic coupling could enhance the electron transfer ability and suppress the aggregation of active particles during de-/lithiation process, leading to excellent electrochemical performance [10, 12, 31].

The mass fraction of Sb＠Sb₂O₃ in the composite Sb＠Sb₂O₃/rGO was estimated by TGA measurement. As shown in Figure 3, a slight mass loss of 7.6 wt% from room temperature to 180 °C is mainly due to evaporation of water that adsorbed on the surface of the sample [32]. The major mass loss of 52.0% from 180 °C to 625 °C is associated with multiple factors, mainly including the volatilization and oxidation of Sb and Sb₂O₃ to form Sb₂O₄ [10, 30], and combustion of the rGO component in the air [33]. According to the XRD and TGA results, the mass fractions of Sb and Sb₂O₃ in the as- synthesized Sb＠Sb₂O₃/rGO composite are 34.0% and 26.6%, respectively.

Figure 3 Thermogravimetric analysis (TGA) curve of as-obtained Sb＠Sb₂O₃/rGO composite

The microstructure characteristics of Sb＠Sb₂O₃ and Sb＠Sb₂O₃/rGO composites were investigated using SEM and TEM technologies. As shown in Figure 4(a), the Sb and/or Sb₂O₃ particles in size of ca. 50 nm (inset in Figure 4(a)) aggregate together to form porous structure. The obtained Sb＠Sb₂O₃/rGO composite displays a bulk with stacked layer structure, and no obvious particles can be observed (Figure 4(b)). According to the element mappings inset in Figure 4(b), three elements C, O and Sb are evenly distributed in the composite, indicating that Sb and Sb₂O₃ particles are uniformly dispersed in obtained Sb＠Sb₂O₃/rGO composite. Figure 4(c) and (d) shows the TEM observations for Sb＠Sb₂O₃/rGO composite, in which particles in size of ca. 10 nm were uniformly dispersed in rGO sheets. The inserted selected area electron diffraction (SAED) in Figure 4(c) displays the diffraction rings characteristic of Sb and Sb₂O₃. Moreover, the high-resolution TEM image of the composite (Figure 4(d)) shows two phases that can be identified as rhombohedral Sb with interplanar spacing of 0.184 nm for (012) plane and cubic Sb₂O₃ with interplanar spacing of 0.279 nm for (400) plane, respectively, further revealing that both Sb and Sb₂O₃ nanoparticles were anchored firmly on rGO sheets. And these results are consistent with the analysis of XRD.

The strong interaction effect between rGO sheets and Sb/Sb₂O₃ particles can be also confirmed through the EIS measurement of the assembled half cells. The EIS curves among bare Sb₂O₃, Sb＠Sb₂O₃ and Sb＠Sb₂O₃/rGO composite are compared in Figure 5. All EIS curves of samples show a semicircle relating to the charge transfer resistance (R_ct) in the medium and high frequency region, as well as a gradient line representing the lithium ion diffusion process in the low frequency region [12, 34]. Distinctly, the obtained Sb＠Sb₂O₃/rGO composite shows smallest R_ct value than those for bare Sb₂O₃ and Sb＠Sb₂O₃ composite, confirming the enhanced electrical contact between rGO sheets and Sb＠Sb₂O₃ particles by strong electric coupling, which is conducive to improving cycling ability.

Figure 4 FE-SEM images of Sb＠Sb₂O₃ (a) and Sb＠Sb₂O₃/rGO composites (b), TEM (c) and high-resolution TEM (d) images of Sb＠Sb₂O₃/rGO composite (Corresponding magnified image, EDS mapping, and selected area electron diffraction (SAED) pattern are insets in panel (a), (b) and (c), respectively)

3.2 Electrochemical properites of obtained samples for LIBs application

For LIBs application, electrochemical properties of the Sb＠Sb₂O₃/rGO composite were evaluated by CV and GSCD tests in the voltage range of 0.01-3.0 V vs Li⁺/Li at ambient temperature, as shown in Figure 5. Figure 5(a) displays the typical CV curves of the Sb＠Sb₂O₃/rGO electrode in the initial three cycles at a scan rate of 0.2 mV/s. During the first cycling, a reduction peak located at ca. 1.15 V can be attributed to the reduction of Sb₂O₃ compound to form Li₂O and nano-sized Sb [10, 35], and another one at ca. 0.55 V is derived from the alloying of Sb to form Li₃Sb and the formation of solid electrolyte interface (SEI) film [10, 36, 37]. In subsequent cycles, the two reduction peaks shift to ca. 1.59 and 0.75 V, respectively, indicating an activation process during the first cycle [10], in which the primal Sb and/or Sb₂O₃ particles have been transferred into ones with smaller size after the first cycle [38], leading to improving the lithiation reaction kinetics. The reduction peak below 0.25 V originates from the insertion of lithium into rGO sheets, in accordance with the alkali metal storage in those carbonaceous electrode [39]. During the charging process, two oxidation peaks located at 1.14 and 1.40 V are respectively owing to the reverse process of the alloying reaction and the reconstitution of Sb₂O₃ phase [10, 40], indicating a reversible conversion reaction between Sb and Sb₂O₃ during cycling ascribed to the strong interaction effect between rGO sheets and Sb＠Sb₂O₃. Moreover, both redox peak pairs at 1.14/0.75 V and 1.40/1.59 V, respectively, corresponding to (de-)alloying and conversion lithium storage mechanism overlapped well during the subsequent cycles, suggesting a good electrochemical stability [41].

Figure 5 CV curves at a scan rate of 0.2 mV/s (a), galvanostatic charge-discharge profiles at a current density of 200 mA/g (b), cycling performance at 200 mA/g (c), and rate capability at various current densities of Sb＠Sb₂O₃/rGO electrode (d)

Moreover, all corresponding voltage plateaus were also detected in the GSCD curves of the Sb＠Sb₂O₃/rGO electrode at a current density of 200 mA/g, as shown in Figure 5(b). There are two distinct voltage plateaus at around 1.3 and 0.7 V in the first discharge process corresponding to the reduction of Sb₂O₃ and the lithiation reaction of Sb and formation of SEI film, respectively, and a slant region below 0.5 V originating from the lithiation of rGO sheets. As well, two plateaus at around 1.0 and 1.4 V during the charge process can be detected for de-alloying reaction of Li₃Sb and reformation of Sb₂O₃. These reversible processes corresponding to (de)alloying and conversion reactions are in accordance with the CV results. The electrochemical cycling performance of the Sb＠Sb₂O₃/rGO electrode has been tested at 200 mA/g for 200 cycles. A high initial reversible specific capacity of 843.5 mA·h/g with an initial coulombic efficiency of 60.6% can be delivered for the Sb＠Sb₂O₃/rGO electrode at 200 mA/g. A rapid capacity fading to 633 mA·h/g in initial 10 cycles is ascribed to those unanchored Sb＠Sb₂O₃ particles, which were pulverized due to large volume changing during the initial several cycles and further became inactive towards lithium. The fading can be relieved by optimizing technological process in the further investigations. It is noteworthy that, the capacity increased gradually since 70 cycles owing to the reversible formation of gel-like film [31]. And the reversible capacity of as high as 790.9 mA·h/g with the capacity retention of 93.8% could be maintained after 200 cycles (Figure 5(c)). The Sb＠Sb₂O₃/rGO electrode also displays excellent rate capacities. As shown in Figure 5(d), the reversible capacities stabilized ca. 740, 620, 522, 425 and 330 mA·h/g at current densities of 50, 100, 200, 500 and 1000 mA/g, respectively. A high reversible capacity of 260 mA·h/g can be obtained even at 2000 mA/g. When the current density was returned to 50 mA/g, the reversible capacity can be fully recovered to above 638 mA·h/g after 10 cycles, indicating high structure stability and superior rate capability of the Sb＠Sb₂O₃/rGO electrode. The above excellent lithium storage properties can be attributed to the well-constructed nanostructure. Sb and Sb₂O₃ particles in size of ca. 10 nm were tightly anchored on rGO sheets through electronic coupling, which could not only suppress the aggregation of active nanoparticles, alleviate the stress caused by the volume expansion, but also enhance the electron transfer ability during de-/lithiation process.

4 Conclusions

1) Sb＠Sb₂O₃/rGO composite has been successfully synthesized by a facile one-pot chemical reduction method at ambient temperature.

2) As an alternative electrode for LIBs application, the as-synthesized Sb＠Sb₂O₃/rGO composite delivers a high reversible capacity of 790.9 mA·h/g after 200 cycles with a capacity retention of 93.8% at 200 mA/g.

3) The excellent electrochemical properties can be attributed to the well-constructed nanostructure. The nano-sized Sb and Sb₂O₃ particles are tightly anchored on rGO sheets through electronic coupling, which could not only alleviate the stress caused by the volume expansion and suppress the aggregation of active nanoparticles, but also enhance the electron transfer ability during cycling.

4) And these findings are useful in synthesizing functional materials through facile and convenient method and developing novel and alternative electrode materials with high electrochemical performance for LIBs.

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(Edited by FANG Jing-hua)

中文导读

具有优越储锂性能的Sb＠Sb₂O₃/还原氧化石墨烯复合材料的简易制备

摘要：锑基材料被认为是制备锂离子电池最具应用前景的负极材料之一，然而复杂而成本昂贵的制备过程严重限制了其应用。本文采用一锅化学还原的简便方法在室温下得到Sb＠Sb₂O₃/还原氧化石墨烯(Sb＠Sb₂O₃/rGO)复合材料。XRD和TGA结果表明，复合材料中Sb和Sb₂O₃的质量分数分别为34.0%和26.6%。将该复合材料用作锂离子电池电极材料使用时，在200 mA/g电流密度下循环200次后可逆比容量仍保持在790.9 mA·h/g，容量保持率高达93.8%；在2000 mA/g电流密度下充放电时仍有260 mA·h/g的可逆比容量。优越的电化学性能得益于所制备复合材料良好的纳米结构，尺寸约为10 nm的Sb和Sb₂O₃颗粒通过电子耦合作用牢牢地锚定在还原氧化石墨烯片层上。该结构既能有效缓冲因储锂过程中产生体积膨胀而引起的应力作用，还可抑制Sb和Sb₂O₃纳米颗粒的团聚，更能提高活性材料的导电性。

关键词：Sb＠Sb₂O₃/rGO复合材料；制备；电化学性能；锂离子电池

Foundation item: Project(51462032) supported by the National Natural Science Foundation of China; Project(17JR5RA066) supported by the Foundation for Distinguished Young Scholars of Gansu Province, China

Received date: 2018-11-19; Accepted date: 2019-02-18

Corresponding authors: ZHOU Xiao-zhong, PhD, Associate Professor; Tel: +86-15117182652; E-mail: zxz20004@163.com; ORCID: 0000-0001-5366-0545; TANG Xing-chang, PhD, Lecturer; Tel: +86-13609359216; E-mail: tangxingchang＠ sohu.com; ORCID: 0000-0001-7934-9376