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Preparation and electrochemical properties of sulfur–acetylene black composites as cathode materials
Electrochimica Acta 54 (2009) 3708–3713
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Preparation and electrochemical properties of sulfur–acetylene black composites as cathode materials B. Zhang, C. Lai, Z. Zhou, X.P. Gao ∗ Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 6 October 2008 Received in revised form 19 January 2009 Accepted 20 January 2009 Available online 29 January 2009 Keywords: Sulfur composite Acetylene black Thermal treatment Cathode material Lithium–sulfur battery
a b s t r a c t A sulfur–acetylene black (AB) composite was synthesized by thermally treating a mixture of sublimed sulfur and AB. The sulfur–AB composites were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) tests. From the results, we confirmed that sulfur was well dispersed on nano-scale and embedded inside nano-pores of the acetylene black with the steric chain structure in the composite. The electrochemical performance of the composite as cathode materials was evaluated by the galvanostatic method, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS). The sulfur–AB composite, which can effectively confine the diffusion of dissolved polysulfides in organic electrolyte and stabilize the structure during the charge and discharge process, showed high capacity and good cycle performance. The discharge capacity of the sulfur–AB composite was maintained at 500 mAh/g after 50 cycles. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid increase in portable electronic devices and electric vehicles (EV), it is necessary to develop secondary batteries with high energy density. Sulfur is very attractive cathode material for lithium–sulfur batteries because of its high theoretical capacity of 1672 mAh/g and theoretical energy density of 2600 Wh/kg, assuming the complete reaction of lithium with sulfur to form Li2 S [1–3]. Furthermore, sulfur has the advantage of low cost, non-toxicity and abundance. Therefore, lithium–sulfur batteries are promising for next generation power sources. However, sulfur is an ionic and electronic insulator (5 × 10−30 S/cm at 25 ◦ C) [4], and lithium sulfides are soluble in organic electrolytes [5–7], which is detrimental to the electrochemical performance of lithium–sulfur batteries. To overcome these problems, sulfur must be incorporated within well-distributed conducting materials as the elementary substance in the cathode. Moreover, the conducting materials should have a highly porous structure to accommodate more active sulfur materials and improve the efficiency of the adsorbing polysulfides. Recently, conductive polymer/sulfur composite materials were prepared by heating a mixture of polyacrylonitrile and sublimed sulfur [8]. Sulfur-polypyrrole composites have also been prepared by an optimized chemical polymerization method [9]. These composites, in which the sulfur particles were embedded in the conductive
∗ Corresponding author. Tel.: +86 22 23500876; fax: +86 22 23500876. E-mail address: [email protected] (X.P. Gao). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.01.056
polymer matrix, displayed improved electrochemical properties. As good conductive materials, carbon nanotubes with large specific surface area and abundant micro-pores have been incorporated with sulfur by thermal methods [10]. The S-carbon nanotube composite showed a high capacity and a good cycle performance due to its effective conductivity and good absorbability. However, carbon nanotubes are very expensive compared with active carbon and are not economical for use in lithium–sulfur batteries. Moreover, active carbon–sulfur composites have shown better performance only when combined with a polymer electrolyte [11]. Although active carbon had large specific surface area, the electrochemical performance of active carbon–sulfur composites was still limited due to the low conductivity caused by the poorly graphitized structure of active carbon. On the other hand, many researchers have optimized the electrolyte based on different organic solvents and polymers to prevent dissolving the polysulfides. Different electrolyte systems, including polymers and gel electrolytes, have been studied [12–19], and room temperature ionic liquids have been added as a solvent or additive in the electrolyte to improve the cycling performance of lithium–sulfur batteries [20–22]. It is well known that acetylene black is an excellent conductive additive used in plastics, rubber, and batteries, due to its good conductivity, liquid absorbing ability, compressibility and elasticity [23]. However, acetylene black is often used as the electrical conductor in the electrode by ball-milling with sulfur [14,15,24–26]. Thermal treatment may be an alternative method to incorporate sulfur with acetylene black and obtain sulfur–acetylene black composites. In this work, we utilize acetylene black as conductive matrix to synthesize carbon–sulfur composites.
B. Zhang et al. / Electrochimica Acta 54 (2009) 3708–3713
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The electrochemical performance of the sulfur–acetylene black (AB) composites as cathode material for rechargeable lithium batteries was investigated. The composites showed good cycle characteristics and high rate capability in liquid electrolyte.
2. Experimental 2.1. Preparation of sulfur–AB composites Sublimed sulfur (Fuchen Chemical, Tianjin) was dried under vacuum (0.1 MPa) at ambient temperature for 24 h before using. Acetylene black (Letai Chemical, Tianjin) was dried at 100 ◦ C for 24 h. The acetylene black (0.1 g) was then mixed with the sublimed sulfur (0.5 g). The mixture was ground for uniformity and was heated at 149 ◦ C for 6 h in a reactor filled with argon gas. At this temperature, the melted sulfur has its lowest viscosity (0.00709 Pa s) [27] and can diffuse into the pores of acetylene black easily. The temperature was increased to 300 ◦ C and was maintained for 2.75 h to vaporize the superfluous sulfur covering the surface of acetylene black, and allow it to diffuse into the acetyleneblack nano-pores. The sulfur content in the composite, calculated by the weight loss of the composite, was 36%. For comparison, the sulfur–AB composite with the same sulfur content was prepared by ball milling sulfur (0.5625 g) and acetylene black (1.0 g). The ball-milling procedure was conducted continuously for 30 min with an interval of 5 min and repeated for 3 h at a speed 200 rps in a ball mill (FRITSCH P6, Germany) with an agate container and agate balls. The diameter of agate balls was 9 mm and 23 mm, respectively, and the weight ratio of ball to powders was 50:1. The characterization of the sulfur–AB composite was measured by X-ray powder diffraction (XRD, RIGAKU D/Max-2500), scanning electron microscopy (SEM, HITACHI S-3500N), transmission electron microscopy (TEM, FEI Tecnai 20), and Brunauer–Emmett–Teller (BET, ASAP2020) tests.
2.2. Preparation and electrochemical testing of the cathodes The sulfur–AB composite, containing 36 wt% sulfur, was mixed with acetylene black and polytetrafluorethylene (PTFE) in a weight ratio of 70:20:10. Ethanol (about 2 ml) was poured into the mixture as a dispersant and then stirred for 30 min to make a homogenous paste. The paste was pressed into a film with a roller. The obtained film, with a thickness of 0.075 mm, was cut into a disk approximately 0.5 cm2 in apparent area and 3 mg in weight. The film disks were dried at 50 ◦ C for 24 h before using. The aluminum collector, composite cathode, polypropylene separator, lithium anode and copper collector were sandwiched in stainless-steel battery holders, which were fitted with polytetrafluorethylene ferrules. A commercial electrolyte, 1 M LiPF6 PC-EC-DEC (1:4:5, v/v/v), was purchased from the Performance Materials Company (Ferro, Suzhou). The batteries were assembled in a glove box filled with high pure Ar. The batteries were sealed with wax (to prevent the electrolyte from being exposed to air) and rested for 4 h before testing. The charge and discharge performance of the batteries was tested with a LAND CT-2001A instrument (Wuhan, China), and the charge and discharge current density was 40 mA/g between 1.0 and 2.8 V at ambient temperature. The specific capacity was calculated on the basis of the active sulfur material. Cyclic voltammetry (CV) experiments were conducted using a CHI 600A potentiostat at a scan rate of 0.1 mV/s. Electrochemical impedance spectra (EIS) of the composites were measured before initial discharge using a Zahner IM6ex electrochemical workstation over a frequency ranges of 10 kHz to 10 mHz.
Fig. 1. TEM images of acetylene black (AB).
3. Results and discussion TEM images of acetylene black, as shown in Fig. 1, show that nano-particles formed a chain-like structure, which should ensure good conductivity. Moreover, the acetylene black had a partially graphitized structure with some nano-pores in the particles, which suggests that its properties are intermediate between those of graphite and amorphous carbon [23]. SEM images of the sulfur, acetylene black and sulfur–AB composite prepared by different methods are presented in Fig. 2. The bare crystalline sulfur had a particle size distribution of 1–20 m. The particle size of the acetylene black is below 100 nm. After incorporating sulfur with the acetylene black by the thermal method described, the morphology of the sulfur–AB composite was almost the same as that of the acetylene black. Therefore, it is proposed that sulfur successfully diffused into the nano-pores of acetylene black and became well dispersed in the composite. On the contrary, for the sulfur–AB composite prepared by ball-milling, it was obvious that most of the sulfur was coated on the outer surface of the acetylene black.
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Fig. 2. SEM images of sulfur (a), acetylene black (AB) (b), sulfur–AB composite prepared by thermal treatment (c), and sulfur–AB composite prepared by ball-milling (d).
XRD patterns of sulfur, acetylene black, and sulfur–AB composites prepared by thermal treatment and ball-milling are shown in Fig. 3. The peaks at about 21.23◦ and 25.38◦ indicate the partially graphitized structure of the acetylene black. The sulfur exists in a crystalline state. The broad diffraction peak of the sulfur–AB composite prepared by thermal treatment clearly indicates that the crystalline sulfur had become amorphous. In contrast, the characteristic peaks of the crystalline sulfur and acetylene black can be clearly observed for the sulfur–AB composite prepared by ballmilling. The pore size distributions of the acetylene black and
Fig. 3. XRD patterns of sulfur (a), acetylene black (AB) (b) sulfur–AB prepared composite by thermal treatment (c), and sulfur–AB composite prepared by ball-milling (d).
sulfur–AB composite prepared by thermal treatment are shown in Fig. 4. The acetylene black possesses a surface area of 65.1 m2 /g with a most probable pore size of about 2.5 nm. After incorporating sulfur with the acetylene black by thermal treatment, the surface area of the sulfur–AB composite was reduced to 33.2 m2 /g, caused by a high filling of the sulfur inside the nano-pores. From the above results, it is confirmed that the sulfur was well dispersed in the nano-pores of the acetylene black in the composite prepared by thermal method, however, the sulfur only covered the surface of acetylene black in the composite prepared by ball-milling due to the sharp reduction of the surface area from 65.1 to 8.9 m2 /g. The sulfur cannot be trapped inside the nano-pores of the acetylene
Fig. 4. The pore size distribution of acetylene black (AB) and sulfur–AB composite prepared by thermal treatment.
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Fig. 5. The first discharge and charge curves of the lithium battery with the sulfur–AB composites prepared by thermal treatment (a) and ball-milling (b) at a charge–discharge current density of 40 mA/g.
black in the composite prepared by ball-milling. Therefore, the thermal treatment is more effective for embedding the sulfur inside the nano-pores of acetylene black and obtaining the uniform sulfur–AB composites. Fig. 5 shows the discharge–charge curves of the sulfur–AB composites prepared by thermal treatment and ball-milling. The sulfur–AB composite prepared by thermal treatment shows two potential plateaus at 2.4 and 1.7 V (vs. Li/Li+ ) in the first cycle, in accordance with the two-step reaction of sulfur with lithium [3,24,28]. As demonstrated above, the sulfur was embedded in the nanopores of the acetylene black in the sulfur–AB composite prepared by thermal treatment. Therefore, the electrochemical reaction between sulfur and lithium during the discharge process would need to overcome the absorbing energy due to the strong adsorbability of acetylene black, leading to a high electrochemical polarization and lower discharge potential plateaus [11]. It is noted that the high reversible charge capacity was obtained for the sulfur–AB composite. The sulfur–AB composite prepared by ballmilling has a flat potential plateau of 2.4 V (vs. Li/Li+ ) with a specific capacity greater than 750 mAh/g in the first cycle. The discharge process does not need to overcome such an absorbing energy due to the high coverage of the sulfur on the surface of the acetylene black, leading to a flat potential plateau of 2.4 V. However, it can only slightly be charged reversibly due to poor electronic conductivity and good dissolvability of the polysulfides on the surface of the acetylene black in the organic electrolyte during the electrochemical process. Cyclic voltammograms of the sulfur–AB composites by thermal treatment and ball-milling are shown in Fig. 6. For the sulfur–AB composite prepared by thermal treatment, there are two cathodic peaks at 1.7 and 2.4 V (vs. Li/Li+ ) in the first cycle, corresponding to the discharge potential plateaus shown in Fig. 5. The small cathodic peak at 2.4 V disappears in the following cycles; however, the cathodic current of the peak at 1.7 V decreases slightly with an obvious potential shift. This suggests that the sulfur content on the surface of the composite is limited, contributing to a low, irreversible capacity after the first cycle. Importantly, there are no apparent current or potential changes of the anodic peak at 2.3 V (vs. Li/Li+ ) in the following cycles, indicating good reversibility of the sulfur–AB composite during the electrochemical process. In the case of the sulfur–AB composite prepared by ball-milling, there are three cathodic peaks in the first cycle, which almost disappear in the following cycles. Moreover, a broad anodic peak with a low current is observed at 2.3 V (vs. Li/Li+ ), which is coincident with the
Fig. 6. Cyclic voltammograms of the sulfur–AB composites by thermal treatment (a) and ball-milling (b) at 0.1 mV/s.
poor charge capacity presented in Fig. 5. The structural change of the composite electrode during cycling was also investigated by XRD, as shown in Fig. 7. The stable structure of the composite prepared by thermal treatment remained after the second cycle. On the contrary, in the composite prepared by the ball-milling method (curve d), the diffraction peaks of the crystalline sulfur disappear, and the diffraction peaks of Li2 S are observed clearly, after the second cycle. The poor cycle performance is primarily related to surface passivation, caused by dissolution of the active sulfur and lithium
Fig. 7. XRD patterns of the sulfur–AB composite prepared by thermal treatment before discharge (a) and after the 2nd cycle (b), the sulfur–AB composite prepared by ball-milling before discharge (c) and after the 2nd cycle (d).
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Fig. 8. Cycle performance of the sulfur–AB composites prepared by thermal treatment (a) and ball-milling (b) at a charge–discharge current density of 40 mA/g and between 1.0 and 2.8 V.
polysulfides in the electrolyte and the surface coverage of insoluble Li2 S in the composite. Thereby, the structural stability of the sulfur–AB composite during cycling is important for controlling the dissolvability of the sulfur and polysulfides and maintaining the reversible electrochemical reaction. The cycle performance of the sulfur–AB composites in the organic electrolyte is shown in Fig. 8. The sulfur–AB composite prepared by thermal treatment had a good cycle performance. The specific capacity of the composite decreased from 934.9 mAh/g in the first cycle to 636.2 mAh/g in the second cycle due to irreversible capacity loss. After the second cycle, the discharge capacity fading was gradual; from the 10th cycle on, the average fading rate per cycle was less than 1%, and the capacity was stabilized to nearly constant at 500 mAh/g after the 50th cycle. The charge and discharge efficiency was close to 100%, indicating excellent electrochemical reversibility of the sulfur–AB composite prepared by thermal treatment. It is clear that the composite structure is beneficial to repeated discharging and charging and can effectively confine polysulfide anion diffusion in the organic electrolyte. On the contrary, the cycle life of the sulfur–AB composite prepared by ball-milling was poor, although the composite had a high initial discharge capacity. It seems that the steric chain-like structure of the acetylene black in the sulfur–AB composite prepared by ball-milling leads to high sulfur utilization in the first cycle. However, it is difficult to maintain the structural stability during cycling, since the sulfur is not embedded in the nano-pores of the composite. The impedance spectra and the equivalent circuits of the sulfur–AB composites are presented in Fig. 9. The impedance spectra are mainly composed of the solution resistance (Re), the constant phase element (CPE), the charge-transfer resistance (Rct) and the Warburg impedance (W1). The fitted charge-transfer resistance and the Warburg impedance of the sulfur–AB composite prepared by thermal treatment are 180.8 and 91 , respectively, much smaller than those of the composite prepared by ball-milling (537.3 and 264.1 ). This may be explained by the supposition that the uniformly dispersed sulfur inside the nano-pores of the sulfur–AB composite is beneficial to the electron and charge transfer on the electrode surface. Moreover, the Warburg impedance of the sulfur–AB composite prepared by thermal treatment can be mainly attributed to the increase in reactive sites in the nano-pores and the well-established electric conductivity of AB. In particular, highly dispersed insulating sulfur on the electrically conductive matrix inside the nano-pores would decrease the diffusion path of lithium ions in the sulfur–AB composite.
Fig. 9. Electrochemical impedance spectra (EIS) of the sulfur–AB composites prepared by thermal treatment and ball-milling before initial discharge.
Generally, sulfur should be dispersed at nano- or even molecular-scale in carbon materials to improve the sulfur utilization in sulfur–carbon composites. Therefore, the carbon materials must have a porous structure with a large surface area. This special structure is beneficial not only to increase the sulfur content in the composites but also to prevent polysulfides from dissolving in the organic electrolyte. Unfortunately, ordinary carbon materials have low electronic conductivity due to poorly graphitized structure; however, they do have highly porous structures with large surface areas. Acetylene black, with its strong absorbing ability, has good conductivity due to its unique partially graphitized steric chain-like structure [23]. In the sulfur–AB composite prepared by thermal treatment, sulfur was absorbed in the nano-pores, which allowed the electrolyte to readily access the amorphous sulfur surface. After the sulfur was reduced, the product of lithium polysulfides dissolved in the organic electrolyte; however, the soluble lithium sulfide was well confined due to the strong absorbing ability of the acetylene black [23]. The large interface area, existing between the acetylene black and the lithium polysulfides, offers many electrochemical reaction sites [29]. Moreover, the steric chain-like structure of the acetylene black can provide an excellent conductive network, which is favorable for the electron and charge transfer. Therefore, the electrochemical performance was improved for the sulfur–AB composite prepared by thermal treatment. 4. Conclusion Acetylene black and sublimed sulfur were used to prepare a sulfur–AB composite by thermal treatment. The sulfur–AB composite, in which the sulfur was embedded inside nano-pores of the acetylene black with the steric chain-like structure, showed a high discharge capacity and good cycle performance in organic electrolyte. The discharge capacity of the sulfur–AB composite decreased slowly after the second cycle and stabilized at 500 mAh/g from the 10th cycle to 50th cycle. Therefore, the sulfur–AB composite can effectively confine the diffusion of dissolved polysulfides in organic electrolyte and maintain the stable structure during the charge and discharge process. The composite prepared by thermal treatment had much better performance than that prepared by ball-milling. Acknowledgements This work is supported by the 863 Program (2007AA03Z225), 973 Program (2009CB220100), and TSFC (07JCZDJC00400) in China.
B. Zhang et al. / Electrochimica Acta 54 (2009) 3708–3713
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Preparation and electrochemical properties of sulfur–acetylene black composites as cathode materials B. Zhang, C. Lai, Z. Zhou, X.P. Gao ∗ Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 6 October 2008 Received in revised form 19 January 2009 Accepted 20 January 2009 Available online 29 January 2009 Keywords: Sulfur composite Acetylene black Thermal treatment Cathode material Lithium–sulfur battery
a b s t r a c t A sulfur–acetylene black (AB) composite was synthesized by thermally treating a mixture of sublimed sulfur and AB. The sulfur–AB composites were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) tests. From the results, we confirmed that sulfur was well dispersed on nano-scale and embedded inside nano-pores of the acetylene black with the steric chain structure in the composite. The electrochemical performance of the composite as cathode materials was evaluated by the galvanostatic method, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS). The sulfur–AB composite, which can effectively confine the diffusion of dissolved polysulfides in organic electrolyte and stabilize the structure during the charge and discharge process, showed high capacity and good cycle performance. The discharge capacity of the sulfur–AB composite was maintained at 500 mAh/g after 50 cycles. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid increase in portable electronic devices and electric vehicles (EV), it is necessary to develop secondary batteries with high energy density. Sulfur is very attractive cathode material for lithium–sulfur batteries because of its high theoretical capacity of 1672 mAh/g and theoretical energy density of 2600 Wh/kg, assuming the complete reaction of lithium with sulfur to form Li2 S [1–3]. Furthermore, sulfur has the advantage of low cost, non-toxicity and abundance. Therefore, lithium–sulfur batteries are promising for next generation power sources. However, sulfur is an ionic and electronic insulator (5 × 10−30 S/cm at 25 ◦ C) [4], and lithium sulfides are soluble in organic electrolytes [5–7], which is detrimental to the electrochemical performance of lithium–sulfur batteries. To overcome these problems, sulfur must be incorporated within well-distributed conducting materials as the elementary substance in the cathode. Moreover, the conducting materials should have a highly porous structure to accommodate more active sulfur materials and improve the efficiency of the adsorbing polysulfides. Recently, conductive polymer/sulfur composite materials were prepared by heating a mixture of polyacrylonitrile and sublimed sulfur [8]. Sulfur-polypyrrole composites have also been prepared by an optimized chemical polymerization method [9]. These composites, in which the sulfur particles were embedded in the conductive
∗ Corresponding author. Tel.: +86 22 23500876; fax: +86 22 23500876. E-mail address: [email protected] (X.P. Gao). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.01.056
polymer matrix, displayed improved electrochemical properties. As good conductive materials, carbon nanotubes with large specific surface area and abundant micro-pores have been incorporated with sulfur by thermal methods [10]. The S-carbon nanotube composite showed a high capacity and a good cycle performance due to its effective conductivity and good absorbability. However, carbon nanotubes are very expensive compared with active carbon and are not economical for use in lithium–sulfur batteries. Moreover, active carbon–sulfur composites have shown better performance only when combined with a polymer electrolyte [11]. Although active carbon had large specific surface area, the electrochemical performance of active carbon–sulfur composites was still limited due to the low conductivity caused by the poorly graphitized structure of active carbon. On the other hand, many researchers have optimized the electrolyte based on different organic solvents and polymers to prevent dissolving the polysulfides. Different electrolyte systems, including polymers and gel electrolytes, have been studied [12–19], and room temperature ionic liquids have been added as a solvent or additive in the electrolyte to improve the cycling performance of lithium–sulfur batteries [20–22]. It is well known that acetylene black is an excellent conductive additive used in plastics, rubber, and batteries, due to its good conductivity, liquid absorbing ability, compressibility and elasticity [23]. However, acetylene black is often used as the electrical conductor in the electrode by ball-milling with sulfur [14,15,24–26]. Thermal treatment may be an alternative method to incorporate sulfur with acetylene black and obtain sulfur–acetylene black composites. In this work, we utilize acetylene black as conductive matrix to synthesize carbon–sulfur composites.
B. Zhang et al. / Electrochimica Acta 54 (2009) 3708–3713
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The electrochemical performance of the sulfur–acetylene black (AB) composites as cathode material for rechargeable lithium batteries was investigated. The composites showed good cycle characteristics and high rate capability in liquid electrolyte.
2. Experimental 2.1. Preparation of sulfur–AB composites Sublimed sulfur (Fuchen Chemical, Tianjin) was dried under vacuum (0.1 MPa) at ambient temperature for 24 h before using. Acetylene black (Letai Chemical, Tianjin) was dried at 100 ◦ C for 24 h. The acetylene black (0.1 g) was then mixed with the sublimed sulfur (0.5 g). The mixture was ground for uniformity and was heated at 149 ◦ C for 6 h in a reactor filled with argon gas. At this temperature, the melted sulfur has its lowest viscosity (0.00709 Pa s) [27] and can diffuse into the pores of acetylene black easily. The temperature was increased to 300 ◦ C and was maintained for 2.75 h to vaporize the superfluous sulfur covering the surface of acetylene black, and allow it to diffuse into the acetyleneblack nano-pores. The sulfur content in the composite, calculated by the weight loss of the composite, was 36%. For comparison, the sulfur–AB composite with the same sulfur content was prepared by ball milling sulfur (0.5625 g) and acetylene black (1.0 g). The ball-milling procedure was conducted continuously for 30 min with an interval of 5 min and repeated for 3 h at a speed 200 rps in a ball mill (FRITSCH P6, Germany) with an agate container and agate balls. The diameter of agate balls was 9 mm and 23 mm, respectively, and the weight ratio of ball to powders was 50:1. The characterization of the sulfur–AB composite was measured by X-ray powder diffraction (XRD, RIGAKU D/Max-2500), scanning electron microscopy (SEM, HITACHI S-3500N), transmission electron microscopy (TEM, FEI Tecnai 20), and Brunauer–Emmett–Teller (BET, ASAP2020) tests.
2.2. Preparation and electrochemical testing of the cathodes The sulfur–AB composite, containing 36 wt% sulfur, was mixed with acetylene black and polytetrafluorethylene (PTFE) in a weight ratio of 70:20:10. Ethanol (about 2 ml) was poured into the mixture as a dispersant and then stirred for 30 min to make a homogenous paste. The paste was pressed into a film with a roller. The obtained film, with a thickness of 0.075 mm, was cut into a disk approximately 0.5 cm2 in apparent area and 3 mg in weight. The film disks were dried at 50 ◦ C for 24 h before using. The aluminum collector, composite cathode, polypropylene separator, lithium anode and copper collector were sandwiched in stainless-steel battery holders, which were fitted with polytetrafluorethylene ferrules. A commercial electrolyte, 1 M LiPF6 PC-EC-DEC (1:4:5, v/v/v), was purchased from the Performance Materials Company (Ferro, Suzhou). The batteries were assembled in a glove box filled with high pure Ar. The batteries were sealed with wax (to prevent the electrolyte from being exposed to air) and rested for 4 h before testing. The charge and discharge performance of the batteries was tested with a LAND CT-2001A instrument (Wuhan, China), and the charge and discharge current density was 40 mA/g between 1.0 and 2.8 V at ambient temperature. The specific capacity was calculated on the basis of the active sulfur material. Cyclic voltammetry (CV) experiments were conducted using a CHI 600A potentiostat at a scan rate of 0.1 mV/s. Electrochemical impedance spectra (EIS) of the composites were measured before initial discharge using a Zahner IM6ex electrochemical workstation over a frequency ranges of 10 kHz to 10 mHz.
Fig. 1. TEM images of acetylene black (AB).
3. Results and discussion TEM images of acetylene black, as shown in Fig. 1, show that nano-particles formed a chain-like structure, which should ensure good conductivity. Moreover, the acetylene black had a partially graphitized structure with some nano-pores in the particles, which suggests that its properties are intermediate between those of graphite and amorphous carbon [23]. SEM images of the sulfur, acetylene black and sulfur–AB composite prepared by different methods are presented in Fig. 2. The bare crystalline sulfur had a particle size distribution of 1–20 m. The particle size of the acetylene black is below 100 nm. After incorporating sulfur with the acetylene black by the thermal method described, the morphology of the sulfur–AB composite was almost the same as that of the acetylene black. Therefore, it is proposed that sulfur successfully diffused into the nano-pores of acetylene black and became well dispersed in the composite. On the contrary, for the sulfur–AB composite prepared by ball-milling, it was obvious that most of the sulfur was coated on the outer surface of the acetylene black.
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Fig. 2. SEM images of sulfur (a), acetylene black (AB) (b), sulfur–AB composite prepared by thermal treatment (c), and sulfur–AB composite prepared by ball-milling (d).
XRD patterns of sulfur, acetylene black, and sulfur–AB composites prepared by thermal treatment and ball-milling are shown in Fig. 3. The peaks at about 21.23◦ and 25.38◦ indicate the partially graphitized structure of the acetylene black. The sulfur exists in a crystalline state. The broad diffraction peak of the sulfur–AB composite prepared by thermal treatment clearly indicates that the crystalline sulfur had become amorphous. In contrast, the characteristic peaks of the crystalline sulfur and acetylene black can be clearly observed for the sulfur–AB composite prepared by ballmilling. The pore size distributions of the acetylene black and
Fig. 3. XRD patterns of sulfur (a), acetylene black (AB) (b) sulfur–AB prepared composite by thermal treatment (c), and sulfur–AB composite prepared by ball-milling (d).
sulfur–AB composite prepared by thermal treatment are shown in Fig. 4. The acetylene black possesses a surface area of 65.1 m2 /g with a most probable pore size of about 2.5 nm. After incorporating sulfur with the acetylene black by thermal treatment, the surface area of the sulfur–AB composite was reduced to 33.2 m2 /g, caused by a high filling of the sulfur inside the nano-pores. From the above results, it is confirmed that the sulfur was well dispersed in the nano-pores of the acetylene black in the composite prepared by thermal method, however, the sulfur only covered the surface of acetylene black in the composite prepared by ball-milling due to the sharp reduction of the surface area from 65.1 to 8.9 m2 /g. The sulfur cannot be trapped inside the nano-pores of the acetylene
Fig. 4. The pore size distribution of acetylene black (AB) and sulfur–AB composite prepared by thermal treatment.
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Fig. 5. The first discharge and charge curves of the lithium battery with the sulfur–AB composites prepared by thermal treatment (a) and ball-milling (b) at a charge–discharge current density of 40 mA/g.
black in the composite prepared by ball-milling. Therefore, the thermal treatment is more effective for embedding the sulfur inside the nano-pores of acetylene black and obtaining the uniform sulfur–AB composites. Fig. 5 shows the discharge–charge curves of the sulfur–AB composites prepared by thermal treatment and ball-milling. The sulfur–AB composite prepared by thermal treatment shows two potential plateaus at 2.4 and 1.7 V (vs. Li/Li+ ) in the first cycle, in accordance with the two-step reaction of sulfur with lithium [3,24,28]. As demonstrated above, the sulfur was embedded in the nanopores of the acetylene black in the sulfur–AB composite prepared by thermal treatment. Therefore, the electrochemical reaction between sulfur and lithium during the discharge process would need to overcome the absorbing energy due to the strong adsorbability of acetylene black, leading to a high electrochemical polarization and lower discharge potential plateaus [11]. It is noted that the high reversible charge capacity was obtained for the sulfur–AB composite. The sulfur–AB composite prepared by ballmilling has a flat potential plateau of 2.4 V (vs. Li/Li+ ) with a specific capacity greater than 750 mAh/g in the first cycle. The discharge process does not need to overcome such an absorbing energy due to the high coverage of the sulfur on the surface of the acetylene black, leading to a flat potential plateau of 2.4 V. However, it can only slightly be charged reversibly due to poor electronic conductivity and good dissolvability of the polysulfides on the surface of the acetylene black in the organic electrolyte during the electrochemical process. Cyclic voltammograms of the sulfur–AB composites by thermal treatment and ball-milling are shown in Fig. 6. For the sulfur–AB composite prepared by thermal treatment, there are two cathodic peaks at 1.7 and 2.4 V (vs. Li/Li+ ) in the first cycle, corresponding to the discharge potential plateaus shown in Fig. 5. The small cathodic peak at 2.4 V disappears in the following cycles; however, the cathodic current of the peak at 1.7 V decreases slightly with an obvious potential shift. This suggests that the sulfur content on the surface of the composite is limited, contributing to a low, irreversible capacity after the first cycle. Importantly, there are no apparent current or potential changes of the anodic peak at 2.3 V (vs. Li/Li+ ) in the following cycles, indicating good reversibility of the sulfur–AB composite during the electrochemical process. In the case of the sulfur–AB composite prepared by ball-milling, there are three cathodic peaks in the first cycle, which almost disappear in the following cycles. Moreover, a broad anodic peak with a low current is observed at 2.3 V (vs. Li/Li+ ), which is coincident with the
Fig. 6. Cyclic voltammograms of the sulfur–AB composites by thermal treatment (a) and ball-milling (b) at 0.1 mV/s.
poor charge capacity presented in Fig. 5. The structural change of the composite electrode during cycling was also investigated by XRD, as shown in Fig. 7. The stable structure of the composite prepared by thermal treatment remained after the second cycle. On the contrary, in the composite prepared by the ball-milling method (curve d), the diffraction peaks of the crystalline sulfur disappear, and the diffraction peaks of Li2 S are observed clearly, after the second cycle. The poor cycle performance is primarily related to surface passivation, caused by dissolution of the active sulfur and lithium
Fig. 7. XRD patterns of the sulfur–AB composite prepared by thermal treatment before discharge (a) and after the 2nd cycle (b), the sulfur–AB composite prepared by ball-milling before discharge (c) and after the 2nd cycle (d).
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Fig. 8. Cycle performance of the sulfur–AB composites prepared by thermal treatment (a) and ball-milling (b) at a charge–discharge current density of 40 mA/g and between 1.0 and 2.8 V.
polysulfides in the electrolyte and the surface coverage of insoluble Li2 S in the composite. Thereby, the structural stability of the sulfur–AB composite during cycling is important for controlling the dissolvability of the sulfur and polysulfides and maintaining the reversible electrochemical reaction. The cycle performance of the sulfur–AB composites in the organic electrolyte is shown in Fig. 8. The sulfur–AB composite prepared by thermal treatment had a good cycle performance. The specific capacity of the composite decreased from 934.9 mAh/g in the first cycle to 636.2 mAh/g in the second cycle due to irreversible capacity loss. After the second cycle, the discharge capacity fading was gradual; from the 10th cycle on, the average fading rate per cycle was less than 1%, and the capacity was stabilized to nearly constant at 500 mAh/g after the 50th cycle. The charge and discharge efficiency was close to 100%, indicating excellent electrochemical reversibility of the sulfur–AB composite prepared by thermal treatment. It is clear that the composite structure is beneficial to repeated discharging and charging and can effectively confine polysulfide anion diffusion in the organic electrolyte. On the contrary, the cycle life of the sulfur–AB composite prepared by ball-milling was poor, although the composite had a high initial discharge capacity. It seems that the steric chain-like structure of the acetylene black in the sulfur–AB composite prepared by ball-milling leads to high sulfur utilization in the first cycle. However, it is difficult to maintain the structural stability during cycling, since the sulfur is not embedded in the nano-pores of the composite. The impedance spectra and the equivalent circuits of the sulfur–AB composites are presented in Fig. 9. The impedance spectra are mainly composed of the solution resistance (Re), the constant phase element (CPE), the charge-transfer resistance (Rct) and the Warburg impedance (W1). The fitted charge-transfer resistance and the Warburg impedance of the sulfur–AB composite prepared by thermal treatment are 180.8 and 91 , respectively, much smaller than those of the composite prepared by ball-milling (537.3 and 264.1 ). This may be explained by the supposition that the uniformly dispersed sulfur inside the nano-pores of the sulfur–AB composite is beneficial to the electron and charge transfer on the electrode surface. Moreover, the Warburg impedance of the sulfur–AB composite prepared by thermal treatment can be mainly attributed to the increase in reactive sites in the nano-pores and the well-established electric conductivity of AB. In particular, highly dispersed insulating sulfur on the electrically conductive matrix inside the nano-pores would decrease the diffusion path of lithium ions in the sulfur–AB composite.
Fig. 9. Electrochemical impedance spectra (EIS) of the sulfur–AB composites prepared by thermal treatment and ball-milling before initial discharge.
Generally, sulfur should be dispersed at nano- or even molecular-scale in carbon materials to improve the sulfur utilization in sulfur–carbon composites. Therefore, the carbon materials must have a porous structure with a large surface area. This special structure is beneficial not only to increase the sulfur content in the composites but also to prevent polysulfides from dissolving in the organic electrolyte. Unfortunately, ordinary carbon materials have low electronic conductivity due to poorly graphitized structure; however, they do have highly porous structures with large surface areas. Acetylene black, with its strong absorbing ability, has good conductivity due to its unique partially graphitized steric chain-like structure [23]. In the sulfur–AB composite prepared by thermal treatment, sulfur was absorbed in the nano-pores, which allowed the electrolyte to readily access the amorphous sulfur surface. After the sulfur was reduced, the product of lithium polysulfides dissolved in the organic electrolyte; however, the soluble lithium sulfide was well confined due to the strong absorbing ability of the acetylene black [23]. The large interface area, existing between the acetylene black and the lithium polysulfides, offers many electrochemical reaction sites [29]. Moreover, the steric chain-like structure of the acetylene black can provide an excellent conductive network, which is favorable for the electron and charge transfer. Therefore, the electrochemical performance was improved for the sulfur–AB composite prepared by thermal treatment. 4. Conclusion Acetylene black and sublimed sulfur were used to prepare a sulfur–AB composite by thermal treatment. The sulfur–AB composite, in which the sulfur was embedded inside nano-pores of the acetylene black with the steric chain-like structure, showed a high discharge capacity and good cycle performance in organic electrolyte. The discharge capacity of the sulfur–AB composite decreased slowly after the second cycle and stabilized at 500 mAh/g from the 10th cycle to 50th cycle. Therefore, the sulfur–AB composite can effectively confine the diffusion of dissolved polysulfides in organic electrolyte and maintain the stable structure during the charge and discharge process. The composite prepared by thermal treatment had much better performance than that prepared by ball-milling. Acknowledgements This work is supported by the 863 Program (2007AA03Z225), 973 Program (2009CB220100), and TSFC (07JCZDJC00400) in China.
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