Highly Porous

Feb 20, 2009 - Sulfur/highly porous carbon (HPC) composites were synthesized by ... The microstructure of the HPC and the composite was characterized ...
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J. Phys. Chem. C 2009, 113, 4712–4716

Synthesis and Electrochemical Performance of Sulfur/Highly Porous Carbon Composites C. Lai, X. P. Gao,* B. Zhang, T. Y. Yan, and Z. Zhou Institute of New Energy Material Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed: October 26, 2008; ReVised Manuscript ReceiVed: December 21, 2008

Sulfur/highly porous carbon (HPC) composites were synthesized by thermally treating a mixture of sublimed sulfur and HPC. The microstructure of the HPC and the composite was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) surface area. The specific surface area of HPC reaches up to 1472.9 m2/g, which is sharply reduced to 24.4 m2/g in the sulfur/HPC composite with 57 wt % sulfur. The electrochemical performance of the composites as cathode materials in organic electrolytes was studied by the galvanostatic method and cyclic voltammetry. The sulfur/HPC composite with 57 wt % sulfur delivers the initial high specific capacity up to 1155 mAh/g and a stable capacity of 745 mAh/g after 84 cycles at the current density of 40 mA/g. In addition, it is demonstrated that the excellent cycling stability of the sulfur/HPC composite can be obtained at different current densities. On the basis of the analysis of the microstructure and electrochemical performance, it is confirmed that HPC can effectively prevent the shuttle behavior of the lithium/sulfur battery. 1. Introduction Lithium-sulfur (Li-S) batteries have attracted increasing attention due to their higher theoretical capacity (1675 mAh/ g), low cost, and environmental friendliness.1-3 However, previous studies of Li-S batteries with organic electrolytes revealed some serious problems, including low utilization of active material and poor cycle life, because of the insulating nature of sulfur and the solubility of polysulfides generated during the electrochemical reaction process.3-9 In a typical discharge process, two potential plateaus can be clearly observed.4,6,7,9-11 First, sulfur accepts electrons at a high potential plateau of about 2.3-2.4 V, forming a chain of lithium polysulfides. Next, lithium polysulfides are further reduced at a low potential plateau. These electrochemical reaction processes were proposed by Mikhaylik10 as follows:

S08 + 4e- ) 2S24 2S2+ S24 + 4e ) 2S 2

As for the generation and dissolution of lithium polysulfides, a shuttle mechanism is suggested. When polysulfides dissolve into organic electrolytes, they can diffuse to the lithium anode to react with the lithium metal in a parasitic reaction, leading to the serious lithium corrosion. Therefore, such shuttle decreases the charge-discharge efficiency and becomes an obstacle to get higher specific capacity.8-12 To overcome the problems, many attempts were made, including the optimization of the organic electrolyte and the room temperature ionic liquid to prevent the dissolution of polysulfides,11-13 and the preparation of the sulfur-conductive polymer composites14,15 or the sulfur-carbon composites.13,16-18 In particular, it is necessary to introduce conductive additives and strong adsorbent agents with large surface area to the system. Porous carbon materials, such as mesoporous carbon,13 active carbon,16,17 and carbon nanotubes,18 have proved to be good candidates to improve the capacity and cycling stability of the sulfur electrodes due to * Corresponding author. [email protected].

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large surface area, porous structure, and excellent conductivity. Therefore, the synthesis of the sulfur/porous carbon composite with highly developed porous structure and better electronic conductivity is a key issue for rechargeable Li-S batteries. In this work, highly porous carbon (HPC) material with good conductivity was synthesized by thermally treating polyacrylonitrile (PAN) with a salt.19 The obtained HPC material showed a high specific surface area up to 1500 m2/g and became semiconductive when thermally treated above 500 °C.19 The sulfur/HPC composites were then prepared by thermally treating the sublimed sulfur and HPC. The effect of HPC was also investigated on the electrochemical performance of the resulting composites. 2. Experimental Section 2.1. Sample Preparation and Characterization. HPC was prepared via a process as described in ref 19. First, PAN (1 g) and sodium carbonates (3 g) were soaked in 100 mL of N,Ndimethylformamide. After the solvent was evaporated, the mixture was heated to 150 °C in Ar with a rate of 3 °C /min, and then the mixture was further heated to 750 °C with a rate of 8 °C/min, and finally held at this temperature for 2 h. The reaction product was washed with distilled water and ethanol, and dried at 60 °C for 24 h under vacuum. The as-prepared porous carbon was mixed with the elemental sulfur in weight ratios of 1:5 and 1:10, respectively. The sulfur/carbon mixtures were heated to 150 °C for 6 h to make the melted sublimed sulfur diffuse into the pores of HPC in a sealed vessel. The temperature was increased to and kept at 300 °C for 3 h under Ar atmosphere to vaporize the sulfur covered on the outer surface of HPC. Finally, the composites with 57 and 75 wt % sulfur were obtained, respectively. The characterization of the HPC and sulfur/HPC composites was carried out by X-ray diffraction(XRD,RIGAKUD/max-2500),Brunauer-Emmett-Teller (BET, ASAP 2020) analysis, scanning electron microscopy (SEM, HITACHI S-3500N), and transmission electron microscopy (TEM, FEI Tecnai 20). 2.2. Electrochemical Test. The working electrode was prepared by compressing a mixture of the sulfur/HPC composite,

10.1021/jp809473e CCC: $40.75  2009 American Chemical Society Published on Web 02/20/2009

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Figure 1. SEM images of HPC (a), sulfur/HPC composite with 57 wt % sulfur (b), and sulfur/HPC composite with 75 wt % sulfur (c).

acetylene black, and poly(tetrafluoroethylene (PTFE) in a weight ratio of 70:20:10. Lithium metal was used as the counter and reference electrodes. The electrolyte, purchased from the Performance Materials Co. (Ferro, Suzhou), was LiPF6 (1 M) in a mixture of propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) in a volume ratio of 1:4:5. The galvanostatic charge/discharge tests at the current densities of 40, 80, 160, and 400 mA/g were performed to evaluate the electrochemical capacity and cycle life of the electrodes at room temperature under a LAND-CT2001A instrument. The cutoff potentials for charge and discharge were set at 3.0 and 1.0 V vs Li+/Li, respectively. The cyclic voltammetry (CV) measurement was conducted with a CHI 600A electrochemical workstation at a scan rate of 0.1 mV/s. 3. Results and Discussion The microstructure evolution can be clearly observed for thermally treated PAN. A layer-like highly conjugated structure formed when PAN was thermally treated above 600 °C.19,20 Typical SEM and TEM images of the HPC sample are illustrated in Figures 1a and 2. The as-prepared sample shows flake-like morphology with highly developed porous structure. Most of the pores possess a diameter of less than 3 nm.21 The developed porous structure is favorable for the diffusion of melt sulfur and the access connection of the electrolyte throughout the porous carbon. Figure 1b is the SEM image of the sulfur/HPC composite with 57 wt % sulfur. There is no apparent difference between the composites and the original porous carbon, suggesting that the sublimed sulfur could diffuse into and be restricted in the mesopores or micropores of the porous

carbon.13,16-18 This can be further confirmed by the BET analysis that the specific surface area of the sulfur/HPC composite is reduced to 24.4 m2/g from initial 1473.2 m2/g of HPC. When the sulfur content is increased to 75 wt %, the macroporous structure is destroyed seriously and large amount of fragments are observed, as shown in Figure 1c. The high sulfur vapor pressure may be responsible for the structure being destroyed during the process of thermally treating the mixture of sulfur and HPC in a weight ratio of 1:10 in a sealed vessel. Figure 3 presents XRD patterns for the elemental sulfur, PAN, HPC, and sulfur/HPC composite with different amounts of sulfur. After thermally treating at 750 °C, the characteristic peaks of PAN disappear. The broad peak at about 24 °C appears, indicating the amorphous characteristic of the as-prepared HPC. As shown in Figure 3b, there is no characteristic peaks of sulfur for the sulfur/HPC composite with 57 wt % sulfur. It is believed that embedded sulfur exists in fine particles and highly dispersed state, resulting in the amorphous composite.17,22 When the mixture of sulfur and HPC was thermally treated at 150 °C, sulfur would melt and diffuse into the pores of HPC due to the strong adsorbability of porous carbon. At higher temperature, sulfur existing at the external surface of HPC was sublimed. In the meantime, sulfur gas could also diffuse into the micropores of HPC. However, when the sulfur content was rather high, a portion of sulfur crystallized out of the pores of HPC, as shown in Figure 3c. The composite with 75 wt % sulfur demonstrates a mixed state of individual crystalline and amorphous structures. Accordingly, it is concluded that sublimed sulfur can be embedded in the developed pores of HPC during the thermal treatment process at a temperature higher than the melting point

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Figure 2. TEM images of HPC.

Figure 3. XRD patterns of HPC (a), sulfur/HPC composite with 57 wt % sulfur (b), and sulfur/HPC composite with 75 wt % sulfur (c). Insets are the XRD patterns of PAN and sulfur.

of sulfur. In particular, for the composite with 57% sulfur, sulfur can exist in a highly dispersed amorphous state in the pores of HPC. Cyclic voltammograms (CVs) of the sulfur/HPC composites are presented in Figure 4. As shown in Figure 4a, there are two main peaks near 2.3 and 1.6 V in the cathodic process in the first cycle for the composite with 57 wt % sulfur content, corresponding to the change from sulfur to lithium polysulfides and the further reduction of higher-order lithium polysulfides to lithium sulfides.5,9,10,23,24 Similar CV behaviors were also observed for sulfur/active carbon composites.17 The potential peak near 2.3 V disappears in the following cycles, because irreversible reaction happens and lithium polysulfides are hardly converted back to sulfur after the first discharge.7 The lower potential peak near 1.6 V is shifted to a higher potential of 1.72 V. The anodic peak retains the same current and potential position, indicating the good cycle stability of the sulfur/HPC composite. When sulfur is embedded into the pores of carbon materials, it is considered that the sulfur may form some special complexes with carbon due to the strong adsorbability of HPC.17 In the sulfur/HPC composite, additional energy is necessary to complete the electrochemical reaction in the cathodic process, leading to a lower discharge potential in comparison with pure sulfur. After extraction of lithium ions, the interactions of such

Figure 4. Cyclic voltammograms of the sulfur/HPC composite with 57 wt % sulfur (a) and 75 wt % sulfur (b) at a scan rate of 0.1 mV/s.

complexes become weak and the electrodes present a higher potential after the first cathodic process. For the composite with 75 wt % sulfur, there are two major peaks in the cathodic process, but their peaks gradually disappear in the subsequent cycles. In the anodic process, no potential peaks are observed. The poor electrochemical reversibility of the composite electrode may be related to the sulfur existing out of the pores of porous

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Figure 6. Initial discharge curves of the sulfur/HPC composite with 57 wt % sulfur at different current densities.

Figure 5. Charge/discharge curves of the sulfur/HPC composite with 57 wt % sulfur (a) and 75 wt % sulfur (b) at the current density of 40 mA/g.

carbon. During the cathodic process of the composite with 75 wt % sulfur, lithium polysulfides, generated from the sulfur out of the pores, dissolve into the organic electrolyte with a high viscousness, resulting in the low diffusion of the lithium ions and polysulfides.25-27 In the subsequent anodic process, lithium ions can hardly diffuse to the surface of electrodes to complete the electrochemical reaction. In addition, part of the generated Li2S with a poor conductivity and solubility can cover the surface of active materials and prevent the further electrochemical reaction.4,25-27 The galvanostatic charge/discharge curves of the sulfur/HPC composites are shown in Figure 5. Discharge potential values of upper and lower plateau coincide with the peak potential change in CV curves. For the composite with 57 wt % sulfur, the initial specific capacity of the composite is 1155 mAh/g. After the first cycle, the composite shows a good electrochemical reversibility. The sloping charge and discharge potential plateaus are 2.2 and 1.9 V, respectively. The solubility of polysulfides depends on the solvent used in the organic electrolyte, and hence the charge/discharge curves of the sulfur electrode depend on the solvents used.4,6,9,11,25 In the electrolytes with high polysulfide solubility, the Li-S battery can be operated as a liquid cathode system, which demonstrates a flat potential plateau.10 However, the generated polysulfides may be restricted in the pores of the porous carbon for the sulfur/HPC composite in the organic electrolyte with mixed solvents of PC, EC, and DEC, leading

Figure 7. Cycling curves of the sulfur/HPC composite with 57 wt % sulfur from the second cycle at different current densities. More cycles at the current density of 40 mA/g are given in the inset.

to the sloping potential plateau. As shown in Figure 5b, the composite electrode with 75 wt % sulfur just shows a specific capacity of about 445 mAh/g at a current density of 40 mA/g. Moreover, after the first discharge process, the electrode with 75 wt % sulfur cannot be charged again. As mentioned above, we believe that the dissolution of polysulfide and the generation of Li2S with a poor conductivity and solubility are responsible for this phenomenon. It is difficult to complete the reversible electrochemical reaction after the first discharge process. Therefore, the composite with 75 wt % sulfur shows low charge/ discharge efficiency. Figure 6 displays initial discharge curves of the sulfur/HPC composite with 57 wt % sulfur at different current densities. At the current density of 80, 160, and 400 mA/g, the discharge capacities are 1031, 917, and 903 mAh/g, respectively. Furthermore, the discharge potential plateau at 400 mA/g is only changed slightly as compared to that at 40 mA/g, indicating a good electrochemical kinetics of the sulfur/HPC composite cathode. The cycling curves of the sulfur/HPC composite with 57 wt % sulfur at different current densities are shown in Figure 7. Excellent cycling stability is observed at different current densities, although the reversible specific capacities are different. The composite presents a stable capacity of about 745 mAh/g after 84 cycles at the current density of 40 mA/g. Interestingly, after resting for 3 days, the discharge capacity recovers to 824

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Lai et al. 40 mA/g. In particular, after 3 days of resting, the composite still presented a high capacity of 770 mAh/g after 110 cycles at the current density of 40 mA/g. With increasing sulfur content, poor charge/discharge efficiency was observed, mainly caused by the sulfur existing out of the pores of HPC. The high specific surface area, developed porous structure, and good electrical conductivity of HPC play a key role in improving the capacity and cycling stability of sulfur-containing composite cathodes. Acknowledgment. This work was supported by the 863 Program (2007AA03Z225), 973 Program (2009CB220100), and TSFC (07JCZDJC00400) in China. References and Notes

Figure 8. Proposed scheme for the electrochemical reaction process of the sulfur/HPC composite cathode.

mAh/g (inset in Figure 7), and the composite still maintains a stable capacity of about 770 mAh/g after 110 cycles, meaning that lithium polysulfides can exist stably in the nanopores of HPC without any loss during resting. It is further demonstrated that the pores with stable lithium polysulfides have a dominant effect on the cycle performance of the sulfur/HPC composite. By considering all of the above results, a scheme for the electrochemical reaction process of the composite is proposed in Figure 8. The initial irreversible reaction probably occurs at the interface between sulfur and porous carbon due to the favorable electron transport from conductive porous carbon to sulfur. The increase of the contact area between sulfur and porous carbon would enhance the capacity for the work electrode, especially in the lower potential plateau.9,11 In the following cycles, the charge/discharge process corresponds to the transform between high-order polysulfides and low-order polysulfide and lithium sulfide. Therefore, it is obvious that the improvement of the electrochemical performance of the sulfur/ HPC composite is mainly attributed to the unique microstructure of the HPC. The larger specific surface area can ensure sulfur existing in a highly dispersed state in the composite. The high electrical conductivity of the HPC, which is derived from the highly conjugated structure by thermally treating PAN at 750 °C, is another important factor. In addition, mesopores or micropores can effectively prohibit the dissolution of polysulfides in organic electrolytes, due to their physical adsorption as discussed above. The developed porous structure can also facilitate the transport of organic electrolyte with lithium ions and thus improve the electrochemical performance of the composite cathode.21,28-32 4. Conclusion To improve the electrochemical performance of Li-S rechargeable batteries, a highly porous carbon material with the specific surface area of 1472.9 m2/g was employed as conductive matrix and adsorbed agent for the sulfur cathode materials. Sulfur can be embedded into the pores of HPC in a highly dispersed state by thermally treating the mixture of sulfur and HPC. The sulfur/HPC composite with 57 wt % sulfur delivered the initial specific capacity up to 1155 mAh/g and the stable capacity of 745 mAh/g after 84 cycles at the current density of

(1) Zhu, M. Y. U.S. Patent Application no. 5814420, 1998. (2) Jin, B.; Kim, J. U.; Gu, H. B. J. Power Sources 2003, 117, 148. (3) Peled, E.; Gorenshtein, A.; Segal, M.; Sternberg, Y. J. Power Sources 1989, 26, 269. (4) Rauh, R. D.; Abraham, K. M.; Pearson, G. F.; Surprenant, J. K.; Brummer, S. B. J. Electrochem. Soc. 1979, 126, 523. (5) Yamin, H.; Gorenshtein, A.; Penciner, J.; Sternberg, Y.; Peled, E. J. Electrochem. Soc. 1988, 135, 1045. (6) Peled, E.; Sternberg, T.; Gorenshtein, A.; Lavi, Y. J. Electrochem. Soc. 1989, 136, 1621. (7) Ryu, H. S.; Ahn, H. J.; Kim, K. W.; Ahn, J. H.; Lee, J. Y. J. Power Sources 2006, 153, 360. (8) Jung, Y. J.; Kim, S. Electrochem. Commun. 2007, 9, 249. (9) Cheon, S. E.; Ko, K. S.; Cho, J. H.; Kim, S. W.; Chin, E. Y.; Kim, H. T. J. Electrochem. Soc. 2003, 150, A796. (10) Mikhaylik, Y. V.; Akridge, J. R. J. Electrochem. Soc. 2004, 151, A1969. (11) Shim, J.; Striebel, K. A.; Cairns, E. J. J. Electrochem. Soc. 2002, 149, A1321. (12) Yuan, L. X.; Feng, J. K.; Ai, X. P.; Cao, Y. L.; Chen, S. L.; Yang, H. X. Electrochem. Commun. 2006, 8, 610. (13) Wang, J.; Chew, S. Y.; Zhao, Z. W.; Ashraf, S.; Wexler, D.; Chen, J.; Ng, S. H.; Chou, S. L.; Liu, H. K. Carbon 2008, 46, 229. (14) Wang, J. L.; Yang, J.; Xie, J. Y.; Xu, N. X. AdV. Mater. 2002, 14, 963. (15) Wang, J.; Chen, J.; Konstantinov, K.; Zhao, L.; Ng, S. H.; Wang, G. X.; Guo, Z. P.; Liu, H. K. Electrochim. Acta 2006, 51, 4634. (16) Wang, J. L.; Liu, L.; Ling, Z. J.; Yang, J.; Wan, C. R.; Jiang, C. Y. Electrochim. Acta 2003, 48, 1861. (17) Wang, J. L.; Yang, J.; Xie, J. Y.; Xu, N. X.; Li, Y. Electrochem. Commun. 2002, 4, 499. (18) Zheng, W.; Liu, Y. W.; Hu, X. G.; Zhang, C. F. Electrochim. Acta 2006, 51, 1330. (19) Jobst, K.; Sawtschenko, L.; Schwarzenberg, M.; Wuckel, L. Synth. Met. 1992, 47, 279. (20) Breads, J. L.; Salaneck, W. R. J. Chem. Phys. 1986, 85, 2219. (21) Jobst, K.; Sawtschenko, L.; Schwarzenberg, M.; Wuckel, L. Synth. Met. 1992, 47, 287. (22) Wang, J. L.; Yang, J.; Wan, C. R.; Du, K.; Xie, J. Y.; Xu, N. X. AdV. Funct. Mater. 2003, 13, 487. (23) Choi, Y. J.; Kim, K. W.; Ahn, H. J.; Ahn, J. H. J. Alloys Compd. 2008, 449, 313. (24) Kumaresan, K.; Mikhaylik, Y.; White, R. E. J. Electrochem. Soc. 2008, 155, A576. (25) Chang, D. R.; Lee, S. H.; Kim, S. W.; Kim, H. T. J. Power Sources 2002, 112, 452. (26) Marmorstein, D.; Yu, T. H.; Striebel, K. A.; McLarnon, F. R.; Hou, J.; Cairns, E. J. J. Power Sources 2000, 89, 219. (27) Cheon, S. E.; Ko, K. S.; Cho, J. H.; Kim, S. W.; Chin, E. Y.; Kim, H. T. J. Electrochem. Soc. 2003, 150, A800. (28) Choi, Y. S.; Kim, S.; Choi, S. S.; Han, J. S.; Kim, J. D.; Jeon, S. E.; Jung, B. H. Electrochim. Acta 2004, 50, 833. (29) Long, J. W.; Dunn, B.; Rolison, D. R.; White, H. S. Chem. ReV. 2004, 104, 4463. (30) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Angew Chem., Int. Ed. 2008, 47, 2930. (31) Owens, B. B.; Passerini, S.; Smyrl, W. H. Electrochim. Acta 1999, 45, 215. (32) He, X. M.; Pu, W. H.; Ren, J. G.; Wang, L.; Wang, J. L.; Jiang, C. Y.; Wan, C. R. Electrochim. Acta 2007, 52, 7372.

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