Anchoring function for polysulfide ions of ultrasmall SnS2 in hollow carbon nanospheres for high performance lithium–sulfur batteries

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Anchoring function for polysulfide ions of ultrasmall SnS2 in hollow carbon nanospheres for high performance lithium–sulfur batteries

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Xueliang Li a,b,∗ , Linbo Chu a,b , Yiyi Wang a,b , Lisheng Pan a,b a b

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, PR China Anhui Key Laboratory of Controllable Chemical Reaction and Material Chemical Engineering, Hefei 230009, PR China

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Article history: Received 26 August 2015 Received in revised form 27 November 2015 Accepted 1 December 2015 Available online xxx

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Keywords: Li–S batteries Sulfur loading Tin sulfide Hollow carbon nanospheres Polysulfides

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1. Introduction

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Tin sulfide-anchored sulfur-hollow carbon nanospheres (S/AHCNS-SnS2 ) composites are synthesized by uniformly dispersing conductive SnS2 particles into hollow carbon nanospheres activated with potassium hydroxide, followed by impregnating sulfur. Surface morphology and structure of this composite are characterized using a field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), Brunauer–Emmett–Teller (BET) method, X-ray power diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The first discharge capacity of the sample containing 10 wt.% SnS2 exhibits a high special capacity value of about 1237.5 mAh g−1 at 0.2 C, and after 200 cycles retains 924 mAh g−1 . Tin sulfide particles play a favorable role on adsorption towards polysulfides which would influence electrochemical process. This strategy of immobilization of sulfur with small amount of metal sulfide particles anchored in AHCNS provides a considerable approach to elevate the sulfur loading, coulombic efficiency, and cycling stability for Li–S batteries. © 2016 Published by Elsevier B.V.

Recently, the fast-growing market for hybrid electric vehicles and intelligent power grid has put forward the urgent need for batteries with high power and energy density, long lifespan, low cost [1]. Rechargeable lithium–sulfur batteries, due to its low cost, abundance in nature and high theoretical energy density of 2600 Wh kg−1 , have attracted much attention and have met the urgent demand above [2]. However, in spite of these advantages, sulfur cathodes still face several problems urgent to solve which include low coulombic efficiency, poor cycling performance and low intrinsic conductivity as well as high self-discharge. These shortcomings of lithium–sulfur batteries mainly depend on the following three reasons. Firstly, sulfur posses a low intrinsic conductivity of about 5 × 10−30 S cm−1 at 25 ◦ C, which leads to low sulfur loading and low sulfur utilization [3]. Secondly, a terrible “shuttle reaction” is generated during the discharge/charge process [4]. Specifically, dissolved polysulfides ((Li2 Sx , 4 ≤ x ≤ 8)) transfer to the surface of anode and form insoluble precipitates (Li2 S or Li2 S2 ), result in the fading of active materials. Thirdly, a large volume

∗ Corresponding author at: School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, PR China. Tel.: +86 551 62901450; fax: +86 551 62901450. E-mail address: [email protected] (X. Li).

variation of cathode material occurs during the charge and discharge process [5,6]. In order to alleviating the shuttle effect of polysulfides and confining the sulfur, researchers adopted many strategies to address these problems aforementioned, which include constructing mesoporous carbon-sulfur nanocomposites [7–13], operating conductive polymer [14–18] or grapheme [19–24] coated hybrid structures. Although significant progress in increasing the access to the skills of confining sulfur has been made, the extensive utilization of Li–S batteries is still hampered by the inherent defect of sulfur. In order to avoid the formation of soluble polysulfide (Li2 Sx , 4 ≤ x ≤ 8) during the discharge–charge process, S. Xin et al. first prepared a microporous carbon-sulfur nanocomposite by using small molecule S2-4 and infusing them into carbon matrix, which improves the coulombic efficiency and elevates cycling stability of sulfur cathode [25]. However, sulfur loading is restricted due to the nature of Li2 S2 , and Li2 S. Therefore, increasing the sulfur loading and anchoring polysulfides are of great importance in the further research of lithium–sulfur batteries. For the sake of solving this problem, we adopt a strategy of developing a hybrid nanocomposite by incorporating the merits of ultra-small metal sulfide and hollow carbon nanosphere, which can be a feasible way to improve the performance of lithium–sulfur batteries. This strategy is uniformly distributes ultra-small metal sulfide in hollow carbon nanosphere, which considers the intense interaction between metal sulfide and sulfur. For instance, metals (Sn, Co,

http://dx.doi.org/10.1016/j.mseb.2015.12.002 0921-5107/© 2016 Published by Elsevier B.V.

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Teflon-lined autoclave. Then, the yellow solid product was gathered via centrifugation and dried at 100 ◦ C for 4 hours. After the yellow solid product was heated at 750 ◦ C for 1 h with the protection of flowing Ar gas, a silicon–carbon (SiO2 –C) material with core-shell structure was obtained. Last, the silica dioxide core was removed by etching method with 0.1 M HF for 8 h. In order to augment the shell porosity of HCNS, an activated-HCNS (AHCNS) material was treated by KOH activation of the HCNS in the shell. Typically, 800 mg of HCNS powder was first scattered in 7 M KOH solution for 30 min and then mixed for 2 h and impregnated for another 1 d. The mixture was filtered to get the KOH/HCNS composite. After drying in air for 1 d at 65 ◦ C, the composite was heated at 800 ◦ C for 1 h under Ar gas to yield AHCNS. Finally, byproducts were removed via washing the sample with both a 1 M HCl solution and deionized water for three times. Scheme 1. Schematic illustration the S/AHCNS-SnS2 synthetic process.

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Ni, etc.) can react with sulfur to form sulfides under some appropriate conditions. These metal sulfides can also be prepared at a moderate condition and present excellent cycling performance [26–28]. The intense interaction between sulfur and metal make the sulfur in metal sulfides directly produce metal and Li2 S without forming soluble polysulfide, which can load and immobilize more sulfur in carbon nanospheres [29]. Among these metals sulfides, SnS2 is a promising candidate as a highly conductive material. Tin sulfides are grown by sulfurization of Sn precursor at appropriate temperatures and polysulfide clusters are formed in a sulfur rich environment [30]. Also, we select hollow carbon nanospheres as carbon matrix to support sulfur. The unique core-shell structure was used to load higher amounts of sulfur, buffer the volume change during the cycle process and suppress the “shuttle reaction” of polysulfides. Moreover, it is considered that this spherical carbon matrix can readily form close packed arrays [31]. Therefore, hollow carbon nanosphere can be select as proper carbon matrix for lithium–sulfur batteries. For these reasons above, we provide a design of tin sulfide-immobilized sulfur-hollow carbon nanosphere (S/AHCNS-SnS2 -10) nanocomposite with high sulfur loading of 64.2% as a cathode material for lithium–sulfur batteries. A chemical vapor deposition (CVD) method was used to introduce SnS2 nanoparticles and a wet-impregnation approach was applied to load sulfur, as showed in Scheme 1. In the composite of S/AHCNS-SnS2 , hollow structure of AHCNS physical restrains the sulfur and SnS2 nanoparticles serve as “Anchor primer” to combine sulfur/polysulfides, thus availably decreasing the loss of active material. Data from XRD and XPS analysis indicate that this feature is due to the adsorption of polysulfide anions on the framework of carbon nanospheres. The S/AHCNS-SnS2 composite cathodes reveal reversible, stable and high capacities along with cycling performance and good rate property. In addition, the as-prepared S/AHCNS-SnS2 possesses good electrical conductivity and large surface area, which serves it as an advanced material in the domains of energy storage and catalysts, optical devices and electrical.

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2. Experimental

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2.1. Synthesis of AHCNS

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Hollow carbon nanospheres (HCNS) were prepared by selfassembly from tetraethyl orthosilicate (TEOS), formaldehyde (F) and resorcinol (R). Specifically, 6.0 mL of an ammonia aqueous solution (28 wt.%) was added to a mixed solution of 140 mL of ethanol and 20 mL of deionized water. After the solution was stirred for 20 min at 25 ◦ C, TEOS (5.6 mL), R (0.8 g), and F (37 wt.%, 1.12 mL) were supplied into the solution every 15 min. The mixture was mixed for 1 d at 25 ◦ C and stayed at 110 ◦ C for another 1 d in a

2.2. Synthesis of AHCNS-SnS2 The AHCNS was served as the carbon matrix to support active materials. Tin (IV) chloride (SnCl4 ) was added into ethanol alcohol to form a 40% SnCl4 content solution (SnCl4 /EtOH-solution). An multiple wet-impregnation method was used to load tin chloride in AHCNS, that is, the tin (IV) chloride solution was first added to AHCNS and the suspension was subsequently stirred at 80 ◦ C to evaporate the solvent for 10 h. The process mentioned above was repeated until a specific amount of SnCl4 was loaded to AHCNS. The resulting production was then prepared by using atmospheric pressure chemical vapor deposition (APCVD) process at 500 ◦ C for 1 h under argon gas mixed with 5% H2 S environment. The AHCNS-SnS2 with different content of SnS2 (noted as AHCNS-SnS2 -x) can be synthesized by adjusting the amount of SnCl4 . Our study found that the AHCNS deposited with 10% SnS2 has the best anchoring effect for sulfur in lithium–sulfur batteries among 5, 10, and 15% SnS2 . Therefore, the AHCNS-SnS2 material with 10% SnS2 (AHCNS-SnS2 -10) was prepared and employed in electrochemical measurements experiment section. 2.3. Preparation of S/AHCNS-SnS2 nanocomposite The S/AHCNS-SnS2 -5 composite was prepared via APCVD process and wet-impregnation techniques. Sulfur (Sigma–Aldrich) was first dispersed and dissolved in carbon disulfide (CS2 , Aldrich 98%) to form a 40% sulfur content solution (sulfur/CS2 -solution). 3.0 g as-obtained AHCNS-SnS2 powder was added to 18 g of the 40% S-CS2 solution. The resulting production was dried at 70 ◦ C in a vacuum for 1 d, and around 60% of sulfur was loaded into the asobtained composite. Meantime, identical to the S/AHCNS-SnS2 -10 composite preparation method, a sample with 69.4 wt.% of sulfur impregnating into the AHCNS was also obtained (S/AHCNS). In addition, a simple physical mixture of AHCNS and sulfur power (mixed AHCNS-S) corresponding to the 69.4 wt.% of sulfur in AHCNS were also obtained as a reference sample. S/AHCNS-SnS2 10 and S/AHCNS-SnS2 -15 are prepared by a process similar to the preparation of S/AHCNS-SnS2 -5. To obtain the sulfur content in the sulfur-containing samples, thermogravimetry (TGA) was performed on a DSC STA449F3, Germany, with a heating rate of 10 ◦ C min−1 , and highly pure Ar as the purge gas. 2.4. Structural characterization The crystalline phase of the sample prepared was characterized by the XRD (Rigaku D/max-rB) using a Cu-K␣ radiation in the scan range 2 = 10–90◦ . The morphology and structural property of the material were studied by transmission electron microscopy (TEM, JEOL-JEM2010F) and scanning electron microscopy (SEM, JEOL-JSM5610). The pore size distribution of the sample was

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Fig. 1. (a) FESEM image of the AHCNS, (b) enlarged FESEM image of AHCNS, (c) FESEM image of the AHCNS-SnS2 -10, (d) enlarged FESEM image of AHCNS-SnS2 -10, (e) FESEM image of the S/AHCNS-SnS2 -10, (f) enlarged FESEM image of S/AHCNS-SnS2 -10.

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calculated by the Barrett–Joyner–Halenda (BJH) model and nitrogen desorption and adsorption isotherms were collected via employing a Brunauer–Emmett–Teller (BET) measurement. The existence state of element was analyzed by X-ray photoelectron spectroscopy (XPS) measurement.

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2.5. Electrochemical measurements

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The as-prepared S/AHCNS-SnS2 -10 nanocomposites were used as cathode materials for rechargeable lithium–sulfur batteries. The cathode electrodes were obtained by mixing the active materials with Super P, and polyvinylidene fluoride (PVDF) binder in Nmethyl-2-pyrrolidinone (NMP) with weight ratios of 80:10:10 to form a slurry, which was coated on the surface of aluminum foil and then dried under vacuum at 60 ◦ C. Lithium metal piece was served as the anode electrode, and the polypropylene Celgard 2320 film with the microporous structure was used as separator. The electrolyte consisted of 1 M LiTFSI and 0.2 M anhydrous lithium nitrate (analytical grade) in a mixed solvent of dimethyl ether (DME) and 1,3-dioxolane (DOL) at a volume ratio of 1:1, which was bought from the Aladdin Industrial Corporation (FengXian, Shanghai). The cyclic voltammetry (CV) test of the cells was conducted with a CHI 660B (Chenhua, China) at a scan rate of 0.1 mV s−1 . The cells were tested by employing the Land CT2001A charge-discharge system (Wuhan Jinnuo, China) at the rates of 0.2, 0.5, 1, 2 and 5 C between 1.8 and 3.0 V at 25 ◦ C. The specific capacity of the batteries was calculated on the strength of the active sulfur material

acquired from TGA measurement. The electrochemical impedance spectroscopy (EIS) measurement of the cells was conducted with a CHI 660B electrochemical workstation (Chenhua, China), which used AC voltage of 5 mV as power source and was in the frequency range of 100 kHz to 100 mHz.

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3. Results and discussion

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3.1. Structural characterization

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Spherical morphologies are clearly observed by SEM. The surface morphologies of AHCNS, AHCNS-SnS2 -10 composite, and S/AHCNSSnS2 -10 composite are observed in Fig. 1. The FESEM image of the AHCNS presents a spherical structure with a nanoparticle size of about 370 nm in Fig. 1a and b. After depositing with SnS2 or SnS2 and S, the change structure can be acquired from the AHCNS-SnS2 10 or the a S/AHCNS-SnS2 -10 SEM images (Fig. 1c–f), indicating that spherical structured carbon are divided into several successive parts due to the introduction of SnS2 or SnS2 and sulfur particles. The energy dispersive X-ray spectrometry (EDS) elemental maps of the S/AHCNS-SnS2 -10 composite are also observed in Fig. 2f and h. From the EDS maps in the FESEM images, we can readily see that the SnS2 map follows the sketch of the AHCNS matrix (dotted line part of the picture (e), Fig. 2), and the sulfur map accords well with that of SnS2 , suggesting that both SnS2 and sulfur are well scattered in the S/AHCNS-SnS2 -10 composite.

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Fig. 2. FETEM images of (a, b) the AHCNS, (c) the S/AHCNS, (d) line scan analysis of S/AHCNS for image (c), (e, f) the AHCNS-SnS2 -10, (g) the S/AHCNS-SnS2 -10, and (h) the corresponding EDS maps of C, S, Sn and O for image (g).

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The microstructures of AHCNS, AHCNS-SnS2 -10, and S/AHCNSSnS2 -10 were further characterized by selected area electron diffraction (SAED, shown in Fig. 1d) and FETEM (Fig. 2). From Fig. 1a and b, a typical morphology of AHCNS can be watched, but the pore size of AHCNS cannot be distinguished due to the overlapping distribution of AHCNS of pores. When 10% of SnS2 was loaded in AHCNS, SnS2 particles with size around 5–7 nm are well distributed in AHCNS matrix (Fig. 2e). The lattice of SnS2 can be observed in FETEM in Fig. 3a and b and verified by the SAED pattern. Sulfur

impregnation into AHCNS does not change the surface morphology of AHCNS-SnS2 composite (Fig. 2g and h). Moreover, the SAED pattern of S/AHCNS-SnS2 sample in inset of Fig. 1d displays the clear rings composed of sequential spots, which may ascribe to the uniformly distribution of the SnS2 particles on carbon matrix. XRD patterns of the AHCNS, mixed AHCNS-S, S/AHCNS, S/AHCNS-SnS2 -10 are shown in Fig. 4a. The XRD pattern of pristine AHCNS only displays two broad peaks (curve a), indicating that AHCNS is an amorphous carbon structure. After physically

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Fig. 3. (a) FETEM images of the synthesized SnS2 nanoparticls in S/AHCNS-SnS2 -10, (b) HRTEM image of the synthesized SnS2 nanoparticles in S/AHCNS-SnS2 -10.

Fig. 4. (a and b) XRD patterns of AHCNS, mixed AHCNS-S, S/AHCNS, AHCNS-SnS2 -10, S/AHCNS-SnS2 -10 and virgin SnS2 , (c) XPS spectra of S/AHCNS-SnS2 -10 composite, with the insets showing the magnified Sn 3d and S 2p spectrum. (d) N2 adsorption isotherms of AHCNS, AHCNS-SnS2 -10, and S/AHCNS-SnS2 -10 and the corresponding pore size distributions (inset).

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mixing with sulfur, the intense and sharp peaks from the crystalline sulfur (S8 ) can be clearly watched (curve b). The peaks of the crystalline sulfur become weak (curve c) when sulfur was impregnated in the AHCNS matrix using S/CS2 solution. The XRD pattern difference between the impregnated S/AHCNS and the mixed AHCNS-S shown that the sulfur is well dispersed in AHCNS matrix as a very fine crystallites and/or amorphous state when sulfur is impregnated via using the impregnation technique. The XRD pattern of SnS2 nanoparticles show diffraction peaks at 2 = 41.89◦ , 32.12◦ , 30.26◦ , 28.19◦ and 15.02◦ , corresponding to the lattice planes (1 0 2), (1 0 1), (0 0 2), (1 0 0), and (1 1 0), respectively. All the peaks are well indexed to the hexagonal structure of SnS2 nanoparticles (JCPDS No. 23-0677) [32–37]. When SnS2 is deposited into AHCNS

by using APCVD process, XRD pattern of the AHCNS-SnS2 (curve e) nanocomposite can be displayed in Fig. 4a. When compared with the pattern of virgin SnS2 (Fig. 4b), the SnS2 peaks of sample of AHCNS-SnS2 are weak. Undergoing sulfur impregnation into the AHCNS-SnS2 -10 material, SnS2 peaks become weak (curve d), confirming that the sulfur was absorbed by the AHCNS-SnS2 -10. XRD results demonstrate that the sulfur is packaging in the pores of carbon matrix. Detailed XPS analysis on the surface composition of S/AHCNSSnS2 -10 manifests clearly that the sample is composed of element C, Sn and S. As displayed in Fig. 4c, there are strong peaks of Sn 3d and S 2p in the XPS spectra of S/AHCNS-SnS2 -10 nanocomposite. On one hand, in the area of high-resolution Sn 3d spectrum, there are

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Scheme 2. Schematic illustration of the formation and electrochemical reaction process of polysulfides SnSx of the S/AHCNS-SnS2 cathode.

Fig. 5. TGA curves for the samples of S, S/AHCNS, S/AHCNS-SnS2 -5, S/aHCNS-SnS2 10 and S/aHCNS-SnS2 -15.

Table 1 Porosity parameters and SnS2 content of AHCNS and its composites.

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Sample

SBET (m2 /g)

Vmeso (cm3 /g)

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SnS2 (wt.%)

AHCNS S/AHCNS AHCNS-SnS2 -10 S/AHCNS-SnS2 -10

852 68.5 664 72.8

1.6 0.5 0.75 0.55

3.6 3.6 6.3 6.3

– – 10 10

two clear peaks at about 487.0 and 495.4 eV for Sn 3d5/2 and Sn 3d3/2 core levels, revealing that chemical valences of Sn in the S/AHCNSSnS2 -10 composite are in valence states of +4 [38–40]. On the other hand, the high-resolution S 2p core level analysis indicates the existence of S6+ and S2− species in the S/AHCNS-SnS2 -10 nanocomposite [35]. The presence of S2− peaks observed at 162.3 and 163 eV is in good agreement with the literature reported SnS2 nanoparticles. The S6+ peak observed at 167.9 eV shows a trace amount of the −SO4 group observed on the S/AHCNS-SnS2 -10 nanocomposite. In order to approximatively present the formation and electrochemical reaction process of polysulfides SnSx , we can clearly observed in Scheme 2. Pore volume, specific surface area, and pore size distribution of AHCNS, AHCNS-SnS2 -10 and S/AHCNS-SnS2 -10 were measured via using the N2 adsorption-desorption isotherms. The BJH pore size distribution of the AHCNS samples is displayed in Fig. 4d, which demonstrates that the sample reveals a pattern of bimodal pore size distribution. In AHCNS samples, there exist pores with an average size of about 6 nm. However, besides the 6 nm sized pores, the pores with a size of about 14 nm are also significant in the AHCNS sample. The AHCNS possesses a high BET specific surface area of 852 m2 g−1 , as well as a pore volume of 1.6 cm3 g−1 . After loading with SnS2 and S, the N2 sorption isotherms of both AHCNS-SnS2 -10 and S/AHCNSSnS2 -10 maintain type IV isotherm, but the pore size and the total amount of the adsorbed N2 are reduced. After deposited with SnS2 , the 14 nm pores are filled with SnS2 and the BET specific surface area is reduced from 852 m2 g−1 to around 664 m2 g−1 . After sulfur impregnation, the most of pores are filled with sulfur, further reducing the pore volume to around 0.55 cm3 g−1 and BET specific surface area to 72.8 m2 g−1 . The pore remaining can offer free space for volume change of S/polysulfides in the process of charge and discharge in the lithium–sulfur battery. A summary of the physical properties of the samples mentioned above is given in Table 1, which present the porosity parameters and SnS2 content of AHCNS and its composites.

3.2. Thermal analysis The thermal behavior of S, S/AHCNS and S/AHCNS-SnS2 -10 were detailed investigated by thermogravimetry analysis (TGA) in a Ar atmosphere with a temperature ramp rate of 10 ◦ C min−1 and compared in Fig. 5. Different from the evaporation of sulfur in a temperature range of 180–550 ◦ C, the weight loss in S/AHCNS composite shows two-step sulfur-loss curves. The first step sulfur-loss in the temperature range of 180–270 ◦ C releases about 25% of sulfur, which mainly ascribes to the evaporation of sulfur covered on the external surface of S/AHCNS. The second step sulfur-loss takes place in the temperature ranges from 270 ◦ C to 550 ◦ C releases about 44.4% of the rest sulfur, which mainly contribute the evaporation of sulfur covered in the pores of S/AHCNS. Due to the bonding between S and Sn, sulfur starts to release from the S/AHCNS-SnS2 -10 composite at about 210 ◦ C and all of the sulfur (≈64.2 wt.%) can be evaporated when the temperature reaches to around 550 ◦ C. The delay in releasing sulfur from S/AHCNS-SnS2 10 composite indicates that sulfur is indeed stabilized by SnS2 particles in the S/AHCNS-SnS2 -10 composite, which can be ascribe to the intense interaction between SnS2 particles and sulfur. Similarly, TGA curves shown in Fig. 5 present the weight contents of sulfur in the S/AHCNS-SnS2 -5 and S/AHCNS-SnS2 -15 were 67.3% and 61.5%, respectively. 3.3. Electrochemical properties The electrochemical properties of AHCNS/S, S/AHCNS-SnS2 -5, S/AHCNS-SnS2 -10 and S/AHCNS-SnS2 -15 are shown in Fig. 6; the initial charge and discharge curves of the four samples at 0.2 C are shown in Fig. 6a. Specifically, the initial discharge capacities of AHCNS/S, S/AHCNS-SnS2 -5, S/AHCNS-SnS2 -10 and S/AHCNS-SnS2 15 are 1103.97 mAh g−1 , 1216.11 mAh g−1 and 1237.51 mAh g−1 , 1153.22 mAh g−1 , respectively. Among these samples, S/AHCNSSnS2 -10 shows the highest capacity; the result indicates that the capacity firstly increases and then decreases along with the increasing content of tin sulfide. The higher initial discharge capacity can be attributed to higher sulfur utilization, that is, tin sulfide immobilize sulfur in the composite of S/AHCNS-SnS2 -10. Furthermore, we have obtained carbon/sulfur composite containing 10 wt.% SnS2 and the S/AHCNS-SnS2 -10 has the similar pore size with mixed S and AHCNS by surface modification via introduce SnS2

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Fig. 6. Discharge–charge capacity vs. voltage plot (a); cycling performance (pink squares show the coulombic efficiency of S/AHCNS-SnS2 -10 samples) (b) of cells with S/AHCNS-SnS2 -10 (S:64.2%), S/AHCNS-SnS2 -5 (S:67.3%), S/AHCNS-SnS2 -15 (S:61.5%) and S/AHCNS (S:69.4%) at 0.2 C. (c) of the S/AHCNS-SnS2 -10 electrodes in the first five cycles at 0.2 C.

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nanoparticles. It is found that S/AHCNS-SnS2 -10 presents a higher capacity (1237.51 mAh g−1 ) than that of S/AHCNS, which is due to the fact that AHCNS-SnS2 -10 can capture more active materials. Therefore, the stabilization treatment of active materials by anchoring with a suitable SnS2 particle is a significant technique and proves to be an effective method for the loss of active materials.

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The cycling performances of as-prepared materials at 0.2 C are showed in Fig. 6b and c. The S/AHCNS-SnS2 -10 electrode reveals a cathodic peak at about 2.4 and a large peak starts at 2.1 V in the first cycle as displayed in Fig. 6c. The peak at high voltage of 2.4 V (vs Li/Li+ ) is associated to the formation of soluble polysulfides (Li2 Sx , 4 ≤ x ≤ 8) during the charge and discharge process, while the low-voltage peak is related to formation of insoluble polysulfides Li2 S2 and/or Li2 S. The cathodic peak at 2.4 V (vs Li/Li+ ) gets weak in the second and fifth cycle, indicating that high-order polysulfides react with (or dissolve into) electrolyte (0.2 M LiNO3 + 1.0 M LiTFSI + DME/DOL (1:1, v/v)). After 200 cycles, the specific capacities of S/AHCNS, S/AHCNS-SnS2 -5, S/AHCNS-SnS2 -10 and S/AHCNSSnS2 -15 are 635.7, 744.7, 924 and 672.4 mAh g−1 and retain about 57.58%, 61.23%, 74.66% and 58.31% of initial capacities, respectively. It is clear that S/AHCNS-SnS2 -10 presents the best cycling performance among the four composites. It is due to the empty space of S/AHCNS, which can both accommodate high sulfur content and facilitate Li+ migration. The cycling performance of S/AHCNS-SnS2 10 is higher than that of S/AHCNS and the result is attributed to the pore size of S/AHCNS which does not well entrap the polysulfides. Compared with S/AHCNS-SnS2 -5, the discharge capacity of S/AHCNS-SnS2 -10 in Fig. 6b delivers 949.3 mAh g−1 over 100 cycles and retains about 76.7% of initial capacity. The improved cycling performance of S/AHCNS-SnS2 -10 is related to the anchoring function of ultra-small SnS2 particles. This function of SnS2 in stabilizing cycle performance can be demonstrated by XPS result which confirms the interaction between tin and sulfur, for S 2p spectrum (after sulfur treatment, Fig. 4b) has a broad higher binding energy peak centered between 167 and 172 eV. In addition, the peaks positioned at 165.3 and 164.1 eV are assigned to S 2p1/2 and 2p3/2 , but the binding energies are slightly higher than those of the characteristic peaks of elemental sulfur, demonstrating that the possible sulfur signal origins from the polysulfides SnSx . Therefore, XPS results confirm the formation of Sn-polysulfides SnSx . The adsorption behavior of SnS2 nanoparticles reduces the dissolution of polysulfide anions, and thus increases the cycle life. Moreover, the high coulombic efficiency approaching 100% also indicates the shuttle effect has been restrained in pores of S/AHCNS-SnS2 -10. The rate performances of the prepared materials were evaluated by charging to 3.0 V and discharging to 1.8 V at different rates. After 5 cycles at 0.2 C, S/AHCNS-SnS2 -5, S/AHCNS-SnS2 -10, S/AHCNS-SnS2 -15 and S/AHCNS deliver the capacities of 782.4, 1046.8, 897 and 837.5 mAh g−1 , respectively. It can be seen from Fig. 7 that S/AHCNS-SnS2 -10 presents a higher capacity than those of S/AHCNS-SnS2 -5 and S/AHCNS-SnS2 -15 at various rates. Therefore, S/AHCNS-SnS2 -10 has the best rate performance among the prepared materials. Meanwhile, SnS2 -addition material presents a better rate performance than that of S/AHCNS. At rate of 0.2 C, S/AHCNS-SnS2 -10 delivers initial discharge capacities of 1237.51 mAh g−1 followed fast capacity decreasing before third cycle, and 1056.1 mAh g−1 for the fourth cycle and relatively stable capacity for further cycling. It delivers discharge capacities of 929.8, 826, 717.6 and 570.2 mAh g−1 at rates of 0.5 C, 1 C, 2 C and 5 C, respectively. When rate is changed from 5 C to 0.2 C, the discharge capacity is recovered to 1025 mAh g−1 . But for S/AHCNS-SnS2 -5, it is recovered to 799.3 mAh g−1 . S/AHCNS-SnS2 -10 shows the best rate performance among the four electrodes due to the surface modification of SnS2 nanoparticles. Electrochemical impedance spectroscopy (EIS) studies were performed to better understand the electrochemical performances. Fig. 8 shows the Nyquist Plots of S/AHCNS-SnS2 -5, S/AHCNS-SnS2 10, S/AHCNS-SnS2 -15 and S/AHCNS, respectively. The Nyquist plots are composed of a semicircle in the high frequency and an inclined line in the low frequency. An equivalent circuit model inserted in Fig. 8a is used to fitting data points. In this equivalent circuit, it is composed of Rs branched into Rct , Cd and Zw . Rs represents solution

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345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410

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ARTICLE IN PRESS X. Li et al. / Materials Science and Engineering B xxx (2015) xxx–xxx Table 2 Impedance parameters of samples and the apparent diffusion coefficients. Sample

Re ()

Rct ()

D (cm2 s−1 )

S/AHCNS-SnS2 -5 S/AHCNS-SnS2 -10 S/AHCNS-SnS2 -15 S/AHCNS

12.4 11.2 15.7 13.6

75.6 47 120.5 97.5

2.1213 × 10−13 2.3599 × 10−13 1.1924 × 10−13 1.4963 × 10−13

resistance of the electrolyte and Cd is related to surface film. Rct designates charge transfer resistance, and Zw is Warburg impedance and it is related to the diffusion of the ions during charge-discharge process [7,41]. The lithium ion diffusion coefficient is listed in Table 2. For the S/AHCNS, S/AHCNS-SnS2 -5, S/AHCNS-SnS2 -10 and S/AHCNS-SnS2 -15, the charge transfer resistance is 97.5, 75.6, 47 and 120.5  before cycles, respectively. The lowest charge transfer resistance further supports that S/AHCNS-SnS2 -10 has the best cycling stability among the four materials. For the straight line, the lithium ion diffusion coefficient can be acquired by the following formulas (1) and (2) [42–45] Fig. 7. Rate capabilities of S/AHCNS-SnS2 -5, S/AHCNS-SnS2 -10, S/AHCNS-SnS2 -15 and S/AHCNS composites.

D=

R2 T 2 2A2 n4 F 4 C 2  2

Z = Rs + Rct + ω−1/2

413 414 415 416 417 418 419 420 421

422

(2)

423

4. Conclusion In conclusion, the tin sulfide-immobilized sulfur in hollow carbon nanospheres with different contents of tin sulfide are prepared via two facile processes including disperse conductive SnS2 particles into AHCNS and wet-impregnation sulfur. The ultrasmall SnS2 particles loading in AHCNS can effectively stabilize the sulfur loaded in S/AHCNS-SnS2 -10 cathode. The unique structural S/AHCNS-SnS2 -10 composite cathode containing 64.2% sulfur retains capacities of around 924 mAh g−1 at 0.2 C after 200 charge/discharge cycles, which demonstrate the good charge/discharge stability and long life span. The excellent cycling performance of S/AHCNS-SnS2 -10 cathode employed in this study has three distinguishing features: (i) Actived-hollow carbon nanosphere host with free space for volume change of S/polysulfides is considerable. (ii) Ultrasmall SnS2 can improve the stabilization of sulfur and archor the polysulfides; (iii) SnS2 nanoparticles enhance the electronic conductivity of the S/AHCNSSnS2 -10 cathodes. The results demonstrated in this article opens the door to explore high sulfur-loading and good cycling stability cathode material derived from an idea that employing small amount of metal sulfide nanoparticles to immobilize the S/polysulfides for lithium–sulfur batteries. Acknowledgment This work was supported by the Science and Technology Project Q3 of Anhui Province (1301022077).

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412

(1)

where D is the diffusion coefficient, R is the gas constant, T is the absolute temperature, A is the surface area, n is the number of electrons, F is the Faraday’s constant, C is the concentration of ions, ω is the frequency, and  is the Warburg factor which is related to Z . Fig. 8b shows the relationship between Z and ω−1/2 in the low-frequency region, the lithium ion in Li2 S diffusion coefficient of S/AHCNS-SnS2 -10 is 2.3599 × 10−13 cm2 s−1 . The lithium ion diffusion coefficient as shown in Table 2 represents that the S/AHCNS-SnS2 -10 has a fastest ion transfer on the interface of electrode/electrolyte and demonstrates that S/AHCNS-SnS2 -10 presents the best rate performance among the four samples.

Fig. 8. (a) Equivalent circuit and Nyquist plots of electrodes (S/AHCNS, S/AHCNSSnS2 -5, S/AHCNS-SnS2 -10 and S/AHCNS-SnS2 -15) before cycles. (b) Real parts of complex impedance versus ω−1/2 in the low-frequency region for the first discharge.

411

424 425 426 427 428 429 430 431 432 433 434

435

436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456

457

458 459

G Model MSB 13816 1–9

ARTICLE IN PRESS X. Li et al. / Materials Science and Engineering B xxx (2015) xxx–xxx

460

461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494

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