Antimony Nanocrystals Encapsulated in Carbon Microspheres

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Sb Nanocrystals Encapsulated in Carbon Microspheres Synthesized by Facile Self-Catalyzing Solvothermal Method for High-Performance Sodium-Ion Battery Anodes Shen Qiu, Xianyong Wu, Lifen Xiao, Xinping Ai, Hanxi Yang, and Yuliang Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10182 • Publication Date (Web): 28 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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ACS Applied Materials & Interfaces

Sb Nanocrystals Encapsulated in Carbon Microspheres Synthesized by Facile Self-Catalyzing Solvothermal Method for High-Performance Sodium-Ion Battery Anodes Shen Qiu,† Xianyong Wu, † Lifen Xiao,*‡ Xinping Ai, † Hanxi Yang† and Yuliang Cao *†

†

College of Chemistry and Molecular Sciences, Hubei Key Lab. of Electrochemical Power

Sources, Wuhan University, Wuhan 430072, China. ‡

College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China

* Corresponding author E-mail: [email protected]; [email protected]

Abstract: Sb@C microspheres are initially synthesized via a facile self-catalyzing solvothermal method and its applicability as an anode material for Na-ion batteries is investigated. The structural and morphological characterizations reveal that Sb@C microspheres are composed of Sb nanoparticles (~20 nm) homogeneously encapsulated in the carbon matrix. The selfcatalyzing solvothermal mechanism is verified through comparative experiments by using different raw materials. The as-prepared Sb@C microspheres exhibit superior sodium storage properties demonstrating a reversible capacity of 640 mAh g-1, excellent rate performance, and

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an extended cycle stability of 92.3% capacity retention over 300 cycles, enabling them as a promising anode material for sodium-ion batteries. Keywords: Sb@C microspheres; Self-catalyzed reaction; Solvothermal synthesis; Anode; Naion batteries

1. Introduction Room-temperature Na-ion batteries (NIBs) have recently received renewed interest as an attractive alternative to lithium-ion batteries in the application of large-scale energy storage due to their natural abundance, low cost and environmental friendliness of sodium resources.1-3 In order to develop practical Na-ion batteries, research efforts have been focused on exploring suitable electrodes with sufficient electrochemical capacity and long cycle life since the larger ionic radius of Na compared to Li results in sluggish diffusion kinetics and structural collapse during volume changes.4-6 Until now, a variety of host cathode materials have been successfully investigated as Na-insertion cathodes with adequate capacity and cycle stability, such as layered oxides,7, 8 phosphates,9, 10 hexacyanoferrate11, 12 and so on. Unfortunately, limited progress has been made in the development of anode materials because major efforts are rather focused on carbonaceous materials.13-16 Although several hard carbons demonstrate sodium storage capabilities with reversible capacities of 200~300 mAh g-1, it has been a nontrivial task to further improve their specific capacity due to limited Na host sites in carbon layers. Besides, a working potential very close to the deposition potential of sodium metal for the carbonaceous materials would cause a significant safety issue. Recently, Na alloy materials have attracted increasing amount of attention for its applicability as a high-performance anode due to their high reversible capacity and suitable charge-discharge


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potential plateau, e.g. Na3Sb (660 mAh g-1), Na15Sn4 (847 mAh g-1) and Na3P (2560 mAh g-1), with a safe potential plateau range of 0.2 ~0.8 V vs. Na/Na+.17-30 Among these alloy anodes, the Sb-based alloy materials have continuously attracted interest because of their high reversible capacity of 600 mAh g-1 with adequate cycling performance.19-25 However, it is noteworthy that like other alloying reactions, the electrochemical Na storage process of Sb is inevitably accompanied by drastic volume expansion of the host materials (volume expansion of 390 %),22 which eventually leads to the pulverization of the electrode materials and fast capacity decay. In order to alleviate this dilemma, many attempts have been devoted to develop Sb-based composites (Sb/C, Sb/inactive material composites)

22,23

and intermetallic compounds (Sb-Sn,

Sb-Cu),24,25 which can buffer volume expansion and enhance the conductive connection during the alloying/dealloying process. For instance, Xiao et al. first reported that a SnSb/C nanocomposite can provide a repeatedly stable and conductive supporter during electrochemical reaction by a self-support mechanism that stabilizes the structure, leading to high capacity (544 mAh g-1) and stable cyclability.26 Subsequently, Sb-based materials (Sb/Ti, SiC/Sb/C) were also reported to demonstrate high Na storage capacity and good cycling performance.27,28 Moreover, Sb-based materials of unique morphology are suggested to be beneficial in the enhancement of the electrochemical Na storage performance, e.g. Sb/C nanofibers22 and Sb hollow nanospheres.30

Therefore, it is concluded that in addition to the uniform distribution of

nanosized-Sb in the rigid matrix to yield a high-performance Sb anode, the unique morphology plays a vital role for the structural stability of the electrode. Henceforth, a feasible strategy to develop the uniform-distribution, nanosized-Sb composite with unique morphology is demanded for high-capacity and stable-cycling of the Sb anode.


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Spherical hard carbon nano-archtechture is generally considered to be a high-capacity and stable template material due to its high conductivity, uniform structure, high tape density and strong tolerance to stress change.31,32 Some nanosized metals or metal oxides encapsulated in the spherical carbon matrix have been successfully synthesized through a hydrothermal process and demonstrate an improved capacity and cycling stability for Li-ion storage.33,34 However, it is arduous to form a uniform distribution of nanosized Sb under hydrothermal conditions because antimony salt (SbCl3) is easily hydrolyzed. Therefore, the facile synthesis of Sb nanocrystals uniformly encapsulated in carbon microspheres still remains a challenge. Herein, a facile selfcatalyzed solvothermal method is introduced by using furfural as a carbon source in the ethanol to synthesize Sb@C microspheres with Sb nanoparticles (~20 nm) uniformly dispersed in the carbon matrix. Due to its unique buffer structure, the Sb@C microspheres demonstrate a high capacity of 640 mAh g-1 at a current density of 50 mA g-1, with an excellent capacity retention of 92.3% after 300 cycles and a remarkable rate performance at 8C rate, demonstrating great promise as high-capacity and long-term cycle anode materials for Na-ion batteries.

2. Experimental Section 2.1．．Material Synthesis and Electrode Preparation. At first, 2 g furfural and 1.14 g SbCl3 were dissolved in 80ml ethanol with vigorous stirring for 2 hour to obtain uniform solution. The mixture was then transferred into a 100 mL Teflonlined stainless steel autoclave and heated up 140 ºC for 10 h. After the reaction was completed, the resulting black precipitate was isolated via filtration and dried in oven at 80 ºC for 12 h. Subsequently, the Sb@C microspheres were obtained by carbonthermal reduction reaction at 600 ºC for 1 h in Ar/ H2 (90:10, v/v). For comparison, the preparation process of the Sb/C particles is


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the same as the Sb@C microspheres just using the super P instead of furfural. In order to realize fair comparison, the Sb@C microspheres and Sb/C particles have similar carbon content. The Sb@C anode was prepared by mixing 80 wt% Sb@C microspheres, 10 wt% super P and 10 wt% Polyacrylic acid (PAA, 25 wt%) to form a slurry, which was then coated onto a copper (Cu) foil and dried at 60 ºC overnight under vacuum.

2.2 Characterization. The Sb@C microspheres were performed by scanning electron microscopy (SEM, ULTRA/PLUS, ZEISS) and by transmission electron microscopy (TEM, JEOL, JEM-2010FEF). The crystalline structures of the as-prepared microspheres were characterized by X-ray diffraction (XRD, Shimadzu XRD-6000). The composition analysis of the Sb@C microspheres and Sb/C particles were performed with TG measurement (Diamond TG/DTA300).

2.3 Electrochemical Measurements The area of the electrode is ~1.13 cm2 while the loading of the whole material is about 2.5 mg/cm2. The charge-discharge performances of the electrode were examined by 2032 coin-type cells using the Sb@C anode as a working electrode and a Na sheet as counter and reference electrode, 1 M NaPF6 dissolved in ethylene carbonate (EC) diethylcarbonate (DEC) (1:1 by volume) with 5% fluoroethylene carbonate (FEC) as the electrolyte, and a Celgard 2400 microporous membrane as separator. The Na sheets were home-made by rolling sodium lumps into plate, and then cut into circulated disks. All the cells were assembled in a glove box with water/oxygen content lower than 1 ppm and tested at room temperature. The galvanostatic charge-discharge test was conducted on a LAND cycler (Wuhan Kingnuo Electronic Co.,


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China). The discharge/charge capacities were calculated based on the pure Sb mass in the composites by deducting the capacity contribution of the pyrolyzed carbon from furfural microsphere and super P (Fig. S4). Cyclic votammetric measurements were carried out with the coin cells at a scan rate of 0.1mV s-1 using a CHI 660c electrochemical work station (ChenHua Instruments Co., China). Electrochemical impedance spectra were recorded by Impedance Measuring Unit (1M 6e, Zahner) with oscillation amplitude of 5mv at the frequency range from 100 m Hz to 100 KHz.

3. Results and Discussion

Figure 1. (a) The XRD patterns of the Sb@C microspheres and the Sb/C particles; (b) The Raman spectroscopy of the Sb@C microspheres and the Sb/C particles. To evaluate the structural and morphological effects of the Sb@C microspheres on their electrochemical performances, an irregular-shaped Sb/C particle was also synthesized by using Super P carbon under the same solvothermal conditions for comparison. As shown in Figure 1a, both the Sb@C microspheres and the Sb/C particles show similar XRD patterns, all of which can be unambiguously indexed to the hexagonal Sb (JCPDS no. 35-0732) sans impurities, implying


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complete reduction of antimony ions to metallic antimony by a carbonthermal reduction reaction. Besides, no characteristic peaks of carbon were detected, indicating the amorphous state of the carbons in these two materials. Based on Scherer formula, the mean crystallite size of the Sb particles in the Sb@C microspheres were calculated to be ~20 nm, while the Sb/C particles exhibit sharp diffraction peaks and very narrow peak width at half-height, indicating a larger Sb particle size. The larger Sb particle size of the Sb/C particles is likely due to non-uniform precipitation and reduction of Sb on the carbon powders (Super P carbon) during the solvothermal and carbonthermal process. The metallic Sb contents in the Sb@C and Sb/C samples were determined to be 41% and 42% from the TG results (Figure S1), respectively, calculated based upon the derivation of the oxidation product of Sb2O4 investigated by XRD (Fig. S1) after heat-treated at 700 oC and the previous literaure.29 The similar carbon content of the Sb@C and Sb/C composites assist in providing an identical baseline to compare their electrochemical performance. Raman spectroscopy was carried out to investigate the surface features of the Sb@C and Sb/C particles. As illustrated in Figure 1b, the Raman spectra of the two samples primarily demonstrate two broad peaks at ~1340 and ~1580 cm-1, corresponding to the disordered-induced D-band and inplane vibrational G-band of the sp2 type carbon, respectively. The relative peak area ratio of IG/ID, which is indicative of the degree of graphitization (or the electric conductivity), was calculated to be 0.37 and 0.62 for Sb@C and Sb/C samples based on Figure 1b, respectively.35 The higher IG/ID value of the Sb/C sample indicates that the Super P carbon powder possess a higher electronic conductivity than the pyrolyzed carbon.


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Figure 2. (a and b) SEM images; (c) TEM image; (d) high resolution TEM image of the Sb@C microspheres The morphological features of the Sb@C microspheres can be clearly seen from SEM and TEM images. As shown in Figure 2a and 2b, the as-prepared Sb@C microspheres demonstrate a uniformly monodispersed spherical shape with diameters ranging from 1 to 2 µm. Figure 2c reveals the homogeneous encapsulation of Sb nanoparticles in the carbon microspheres. Since the carbonization temperature (600 ºC) was below the melting point of Sb (631 ºC), the aggregation of Sb nanoparticles during heat treatment can be effectively prevented and exhibits a final size of 15~20 nm, which is consistent with the result calculated by Scherer formula in


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Figure 1a. The high resolution TEM image in Figure 2d clearly displays the lattice fringes with distance of ~0.308 nm corresponding to the (012) plane of hexagonal Sb (JCPDS no. 35-0732), further confirming the presence of pure Sb nanoparticles in carbon microspheres. The Energy dispersive spectrometer (EDS) analysis (Figure S2) also reveals that the Sb particles are embedded in the carbon matrix, as shown in Figure 2d. The specific surface area and porous structure of the Sb@C microspheres and Sb/C particles are conducted and displayed by nitrogen adsorption-desorption experiments in Figure S3. The N2 adsorption-desorption isotherm of the Sb@C microspheres display sharp knees at relative pressure of 0.1 and a hysteresis loop at high relative pressures, which is typical for a coexistence of microporous and mesorporous structures. The isotherm of the Sb/C particles show no sharp knee in low press region (P/P0 < 0.1) and no hysteresis loop in high pressure region (P/P0 > 0.9), which belongs to the macroporous characteristics. This behavior implies Sb@C microspheres contain a higher content of micro- and mesopores with high surface area (300 m2 g-1), while Sb/C particles possess few macroporous structures with minimal surface area (32 m2 g-1). Therefore, the nanosize of Sb particles, the uniform Sb dispersion in carbon matrix, and the voids of combined pores can effectively buffer the massive volume changes of the Sb phase during Na alloying/dealloying reactions.


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Figure 3. (a) The comparative experiments using different raw materials: (I) single SbCl3; (II) single furfural; (III) furfural-HCl; (IV) SbCl3-furfural. (b) Schematic illustration of the preparation process for the Sb@C microspheres From inspection of XRD, SEM and TEM results, it is discovered that the facile solvothermal process by using SbCl3 and furfural raw materials can successfully prepare monodispersed Sb@C microspheres with Sb nanocrystals homogenously distributed in the carbon matrix. To understand the mechanism of the solvothermal process, comparative experiments were carried out by using different raw materials, i.e., single SbCl3, single furfural, furfural-HCl and SbCl3furfural. As portrayed in Figure 3a, it is distinctly seen that there is a small quantity of white precipitate appearing in the single SbCl3 system (Figure 3a (I)), while no visible changes are seen for the single furfural system (Figure 3a (II)), indicating that SbCl3 alcoholyzed to form


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antimony oxides or hydroxides during the solvothermal process, but the carbonization reaction of the furfural cannot take place. Interestingly, HCl can be added to the furfural system to form a black precipitate (Figure 3a (III)), identical to the SbCl3-furfural system (Figure 3a (IV)), suggesting the occurrence of the carbonization reaction of the furfural. It is assured that the carbonization reaction of the furfural is an acid-catalyzed process. Therefore, the formation of the Sb@C microspheres in the SbCl3-furfural system can be explained that the SbCl3 is first alcoholyzed to produce traces of nanosized antimony hydroxides or oxides and H+ (Figure 3b), subsequently, the produced H+ catalyzes the polycondensation of furfural to generate H2O (Figure 3b). The presence of the generated H2O further accelerates the hydrolysis of SbCl3 to produce additional antimony oxides and H+ catalyst. Thus, the polycondensation of furfural and hydrolysis of SbCl3 repeatedly takes place by an in-situ self-catalyzed reaction. During said reaction, the produced antimony oxide nanocrystals are adsorbed onto the oligomer that was generated from the condensation of furfural. Accompanied by the self-catalyzed reaction, the precursor of antimony compounds embedded in poly-furfural microspheres is gradually formed. Finally, Sb@C microspheres were obtained by a carbon-thermal reduction reaction at 600 ºC. Figure 3b illustrates a possible mechanism of the Sb@C microsphere fabrication. It is clearly shown that the self-catalyzed reaction not only guarantees continuous hydrolysis of SbCl3 generating nanosized antimony oxides, but also successively provides a supporter formed from the condensation of the furfural for antimony oxide nanocrystals to hinder its agglomeration, resulting in the formation of the Sb@C microspheres with Sb nanocrystals homogenously encapsulated in the carbon matrix.


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Figure 4. Electrochemical performance of the Sb@C microsphere electrode: (a) CV curves at a scan rate of 0.1 mV s-1; (b) the charge-discharge profiles at a current rate of 0.01C (1C=600 mA g-1); (c) Long-term cycling performance at a charge-discharge current density of 0.5C; (d) Rate performance of Sb@C microspheres at different rates from 0.2C to 8C The electrochemical Na storage performance of Sb@C microspheres in 1.0 mol L-1 NaPF6 ECDEC electrolyte is given in Figure 4. Figure 4a illustrates the dominant features in the CV plots with a pair of well-defined symmetric oxidation/reduction peaks at 0.8/0.25 V, ascribed to the Na dealloying/alloying reaction of the Sb electrode according to previous studies.19,20 Compared to the second scan, a weak peak appears at ~0.75 V and a larger reduction peak at 0.25 V, which demonstrates that the first negative scan causes decomposition of the electrolyte and formation of a solid-electrolyte interface (SEI) film on the surface of the electrode materials. The charge-


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discharge profiles of the Sb@C electrode during the first two cycles at 0.01C (1C=600 mA g-1) are shown in Figure 4b. In accordance with CV data, the initial discharge curve exhibits a large slope plateau at ~0.5 V compared to the second cycle, representing SEI film formation accompanied by Na insertion into Sb particles. The reversible capacity of the first cycle reaches as high as 640 mAh g-1, demonstrating nearly a full capacity utilization of Sb (Na3Sb: 660 mA g1

). The high capacity might be attributed to the uniform distribution of nanosized Sb and

exceptional electric contact between the Sb nanoparticles and carbon matrix (Figure 3). Figure 4c shows the cycling performance of the Sb@C electrode at a moderate current density of 0.3C. After 300 cycles, the reversible capacity slightly decreases from 494 mAh g-1 to 456 mAh g-1, demonstrating very high capacity retention of 92.3%. For comparison purposes, the Sb/C particles of similar carbon content as Sb@C microspheres were also investigated (Figure S6). Even though the Sb/C electrode delivers an identical initial capacity of 494 mAh g-1, it exhibits poor cycle stability with capacity retention of only 48.6% over 40 cycles; this result further confirms that the drastic improvement of electrochemical properties for the Sb@C microspheres is a result of its smaller particle size and unique morphology that effectively alleviates volume changes and maintains adequate electric contact between the Sb particles and carbon matrix. Even though the coulombic efficiency of the Sb@C electrode was 46% for the first cycle (Figure 4c), it distinctly increased to an optimum value of 95% for the third cycle and then remained steady at 99% for subsequent cycles, demonstrating superior reversibility for the Na storage reaction. The exceptional cycling reversibility of the Sb@C microspheres and Sb/C electrodes is also validated by EIS experiments (Figure S5). Both Sb@C and Sb/C electrodes have similar electrochemical impedance spectra that include two depressed semicircles at the high and medium frequency ranges and a single slope at the low frequency. Two semicircles at the high


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and medium frequencies correspond to the resistance of the SEI film (RSEI and CSEI) and the charge transfer (Rct and Cct) on the electrode/electrolyte interface, respectively, while the inclined line corresponds to sodium diffusion within the bulk of the electrode material. The kinetic differences of the Sb@C and Sb/C electrodes were evaluated by modelling the electrochemical impedance spectra based on an equivalent circuit (upper left inset of Figure S5c). The fitted impedance parameters (inset of Figure S5a, S5b) demonstrate that the Sb@C electrode possesses similar RSEI value and a slightly higher Rct value compared to the Sb/C electrode, originating from the higher electronic conductivity of the Super P carbon powders compared with the pyrolyzed carbon (Figure 1b). After 30 cycles, the values of RSEI and Rct of the Sb/C electrode increase from 16 and 75 Ω during the first cycle to 82 and 125 Ω at the 30th cycle, respectively, indicating structural deterioration during the repeated alloying/dealloying reaction. In contrast, the impedance of the Sb@C microsphere electrode exhibits a decreasing trend as RSEI and Rct are 16 and 86 Ω during the first cycle and 6 and 32 Ω after the 30th cycle. The improved impedance characteristics of the Sb@C electrode are attributed to the maintenance of its unique morphology and the enhanced kinetics through the activation process with cycling. Moreover, to further evidence the structureal stability of the Sb@C microspheres, SEM and TEM images of the Sb@C microspheres presented in Figure S7 are carried out from the electrode after 50 cycles to visualize the morphological change of the composites. Compared to the SEM image of uncycled Sb@C microspheres (Figure 2a), the cycled microspheres can still retain its original size distribution and integrated sphere structure in Figure S7a. The TEM image (Figure S7b) reveals the homogeneous encapsulation of the Sb nanoparticles in the carbon microspheres after charging and discharging. These results demonstrated that the Sb@C


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microspheres have excellent structure stability during charging and discharging so as to contribute good cycling performance. In addition to its superior cycle performance, the Sb@C electrode also demonstrates a strong rate capability at the same charge/discharge current density (Figure 4d). The reversible capacity is 554, 468, 401, 320, 267, 228 mAh g-1 at current densities of 0.2C, 0.5C, 1C, 2C, 5C, 7C (1C= 600 mAh g-1), respectively. Even at a high rate of 8C, the reversible capacity is still capable of reaching 190 mAh g-1, corresponding to ~34% of the capacity at 0.2C; the good rate performance results from the nanosized Sb particles and enhanced electric contact with carbon matrix. In addition, a comparison of the electrochemical performance of the Sb electrodes in the previous works is presented in Table S1, which demonstrates that though the rate capacity of the asprepared Sb microspheres is not enough good, the Sb@C microspheres have better cycling performance than most of the works (seeing the Tab. S1). The enhanced electrochemical performance of the Sb@C microspheres should be attributed to the stable structure during sodiation and desodiation.

4. Conclusions In summary, a facile synthesis of the Sb@C microspheres by a self-catalyzed solvothermal method and subsequent carbon-thermal reduction reaction is successfully achieved. The asprepared Sb@C microspheres display a uniform structure homogenously encapsulated in the carbon matrix, providing a structurally stable host for Na-ion intercalation and deintercalation. The Sb@C microsphere electrode demonstrates a high initial capacity of 640 mAh g-1, impressive cyclability of 93% capacity retention over 300 cycles at a middle rate capability at 200 mA g-1, and excellent rate capability. Enhanced electrochemical performance is primarily attributed to the homogenous distribution of microspheres, superior electric contact of Sb


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nanocrystals in the carbon matrix, and unique spherical morphology that accommodates volume change during Na-ion intercalation and deintercalation. Therefore, the Sb@C microspheres provide a practical anode material capable of high-capacity and cycling-stable for Na-ion batteries. Furthermore, the self-catalyzed solvothermal mechanism is also revealed herein for the first time, in which the hydrolysis of the metal salt and condensation of carbon source promote each other to synthesize unique carbon spheres homogenously encapsulating nanoparticles. The unique structural nano-architecture can be expanded to a various fields such as energy and catalysis.

Associated Content Supporting Information Available. Additional details and figures as mentioned in the text. This material is available free of charge via the Internet at http:// pubs.acs.org.

Author Information Corresponding Authors *E-mail: [email protected]; [email protected]

Acknowledgments We thank financial support by the National Key Basic Research Program of China (No. 2015CB251100), National Science Foundation of China (No. 21333007, 21373155, 21273090), Program for New Century Excellent Talents in University NCET-12-0419) and Hubei National Funds for Distinguished Young Scholars (2014CFA038).


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Table of Contents Graphic

Title: Sb Nanocrystals Encapsulated in Carbon Microspheres Synthesized by Facile SelfCatalyzing Solvothermal Method for High-Performance Sodium-Ion Battery Anodes Shen Qiu,† Xianyong Wu, † Lifen Xiao,*‡ Xinping Ai, † Hanxi Yang† and Yuliang Cao *†

ToC Figure:

Sb@C microsphere is successfully synthesized via a facile self-catalyzing solvothermal method exhibits superior sodium storage properties demonstrating a reversible capacity of 640 mAh g-1, excellent rate performance, and an extended cycle stability of 92.3% capacity retention over 300 cycles, establishing this device as a promising anode material for sodium-ion batteries.


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Sb@C microsphere is successfully synthesized via a facile self-catalyzing solvothermal method exhibits superior sodium storage properties demonstrating a reversible capacity of 640 mAh g-1, excellent rate performance, and an extended cycle stability of 92.3% capacity retention over 300 cycles, establishing this device as a promising anode material for sodium-ion batteries. 236x152mm (113 x 116 DPI)


Antimony Nanocrystals Encapsulated in Carbon Microspheres

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