A binder-free sulfur/carbon composite electrode prepared by a sulfur sublimation method for Li–S batteries

Abstract

A binder-free sulfur/carbon composite electrode was prepared by a sulfur sublimation method. Sulfur nanoparticles fill large pores in a carbon paper substrate and primarily have a monoclinic crystal structure. The composite electrode shows a long cycle life of over 200 cycles with good rate performance in Li–S batteries.

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Rechargeable lithium–sulfur (Li–S) batteries, which were first proposed in the 1960s, have become a promising candidate for energy storage devices over the last decades. Among many battery candidates, Li–S batteries attract much attention because of the high specific capacity of sulfur (i.e., 1672 mA h g⁻¹) and high specific energy (i.e., 2600 W h kg⁻¹).^1–3 In addition, sulfur has several other appealing advantages such as low cost, abundance, and environmental benignity. However, despite these particular characteristics, several issues limit the practical use of sulfur cathodes. The major challenges include the insulating character of sulfur (i.e., 5 × 10⁻³⁰ S cm⁻¹ at 25 °C), the low utilization of large sulfur particles in electrodes, the shuttle of dissolved polysulfides in liquid electrolyte, low coulombic efficiency, and fast capacity decay.^2,4,5 The conventional method of making sulfur electrodes by mixing sulfur powder, carbon, and polymer binder is inappropriate for making uniform and high performance electrodes. Poor contact between large sulfur particles and carbon results in low utilization and inhomogeneous distribution of current upon cycling.^6,7

To overcome these issues, many approaches including the synthesis of sulfur–carbon nanocomposites,^7–12 fabrication of novel cathode and cell configurations,^13–21 and making polysulfide-blocking separators have been developed.^22,23 The primary methods for making sulfur–carbon nanocomposites are (i) heat treatment and (ii) solution-based synthesis.^9,24,25 The heat treatment method is to impregnate a micro or mesoporous carbon matrix with melted sulfur, which can result in nanoscaled sulfur in the carbon matrix. The solution-based synthesis is to precipitate sulfur particles in solution through a heterogeneous nucleation reaction, which tends to form larger sulfur particles.

Sulfur can sublime at elevated temperature. Hagen et al. sublimed sulfur into a vertical-aligned carbon nanotube (CNT) substrate.²⁶ Fu et al. sublimed sulfur into a CNT paper current collector using argon as a carrier gas.²⁷ Herein, we present a study on a binder-free sulfur/carbon composite electrode prepared by the sulfur sublimation method in air. At certain temperature and air flow rate, sulfur can be melted and vaporized. Solid sulfur nanoparticles can be deposited into commercial binder-free carbon paper which also acts as a current collector in batteries. Compared with the other methods for making sulfur/carbon composite electrodes, this method has several advantages. Firstly, it is a green, solvent-free method and the sulfur powder undergoes a physical deposition process maintaining its intrinsic composition. Secondly, the sulfur vapor can infiltrate large pores in the carbon paper forming intimate contact with carbon. Finally, it is scalable and provides another synthesis route for making high performance sulfur–carbon composite electrodes.

Fig. 1a shows the experimental setup in a fume hood with constant air flow. To prepare the composite electrode, sulfur powder was uniformly loaded in a beaker. A disc of carbon paper was put on top of the beaker. The beaker was heated at about 200 °C, at which sulfur vapor can be formed modestly and continuously. When the sulfur is vaporized, sulfur rings or sulfur clusters are small enough to penetrate small pores and get deposited in the carbon matrix. The air flow enhances sulfur infiltration in the carbon paper. This process helps break down large sulfur particles into small ones which are beneficial for achieving high utilization of sulfur in batteries. Under these conditions, desirable sulfur contents can be obtained within 15 min. Four deposition times (i.e., 2, 4, 8, and 15 min) resulted in sulfur loading of approximately 0.4, 0.9, 1.7, and 2.8 mg cm⁻², and these electrodes are designated as SE-2, SE-4, SE-8 and SE-15, respectively. The prepared electrode was cut into ∼1 cm² discs, each contains 1.9 mg carbon.

Fig. 1 (a) Schematic illustration of the experimental process for making a binder-free sulfur/carbon composite electrode by the sulfur sublimation method, (b) XRD patterns of the sulfur/carbon composite electrodes SE-2, SE-4, SE-8, and SE-15, and the control sample of commercial sulfur on carbon paper.

X-Ray Diffraction (XRD) was used to qualitatively analyze the sulfur crystal structure formed in the electrodes. For comparison, commercial sulfur on a piece of carbon paper was also examined by XRD. Fig. 1b shows the XRD patterns in the 2θ range between 13° and 40° wherein the characteristic peaks of sulfur and carbon can be seen. The main peak at 26.0° is a characteristic peak of (002) crystal plane of carbon nanotubes.²⁸ The commercial sulfur powder on the carbon paper shows peaks that are matched with the XRD pattern of the orthorhombic sulfur.^29,30 The SE-2 doesn't show any peaks of sulfur besides the carbon peak, indicating a small amount of sulfur was deposited in the sample. The SE-4 shows a few peaks, but they cannot be assigned to either orthorhombic crystal or monoclinic crystal structure. When the deposition time increases to 8 and 15 min, several major peaks of monoclinic sulfur crystal can be seen along with few small unknown peaks. It is reported that sulfur undergoes a phase transition from the orthorhombic to monoclinic structure when the temperature is 200 °C.²⁹ Vaporized sulfur rings re-stack into the favorite monoclinic crystal structure that is stable at elevated temperature.^29,31 The monoclinic sulfur may transition to other structures, e.g., orthorhombic structure, over a long period of time, but it is not interest of this work.

The morphology of the sulfur/carbon composite electrode was examined by SEM, as shown in Fig. 2a. Such carbon paper has been used in Li/polysulfide batteries as it can hold polysulfide solution and cycled products.¹⁸ It can be seen that a lot of irregular sulfur particles were deposited in the voids of the carbon paper. Some sulfur particles are large, which are in the range of a few microns. Overall, it is a uniform composite electrode. The magnified SEM image in the inset picture shows that the large sulfur particles are in the form of many nanoparticles filling all space between carbon nanofibers and nanotubes. The SEM results show that this sulfur sublimation method can facilitate breaking down large sulfur particles and dispersing sulfur nanoparticles into a carbon matrix. These sulfur nanoparticles would have good contact with carbon in the electrode, which can improve the ion and electron transport within the composite electrode in batteries.

Fig. 2 (a) SEM image of a prepared electrode, the inset figure is a magnified SEM image showing sulfur nanoparticles formed within voids of the carbon paper, (b) pore size distribution of the blank carbon paper (CP), SE-2, SE-4, and SE-8.

Fig. 2b shows the pore size distribution within the blank carbon paper and composite electrodes. In the blank carbon paper, a broad pore size in the range of 8–33 nm is observed. The carbon paper consists of thin carbon nanotubes and thick carbon nanofibers as seen in the SEM image in Fig. S1† and 2a forming a variety of pores among them. As sulfur nanoparticles are formed in the carbon paper, the volume of large pores starts to decrease and smaller pores start to appear. For example, the SE-4 has a significant volume of pores with a diameter centered at about 7 nm which is smaller than all pores in the blank carbon paper and SE-2. In contrast, the SE-8 only shows a volume of pores with a diameter centered at about 3 nm and almost all pores between 8 and 33 nm are gone. The small pores in the SE-4 and 8 can only be formed between or within the newly formed sulfur nanoparticles, which filled all large pores in the carbon paper as seen in Fig. 2a.

The SE electrodes exhibited different cycling behaviour, as shown in Fig. 3a. The SE-2 and SE-4 show higher discharge capacities and better cycling stability than the SE-8 and SE-15 due to the low sulfur contents. The initial discharge capacity of the SE-2 and SE-4 can be as high as ∼1400 mA h g⁻¹, in contrast to ∼1100 mA h g⁻¹ of the SE-8 and SE-15. The SE-2, SE-4, and SE-8 maintain a high reversible capacity of 850, 1000, and 820 mA h g⁻¹ after 200 cycles, respectively; however, the SE-15 shows continuous capacity decay to 300 mA h g⁻¹ after 120 cycles. The long sublimation time (15 min) results in high sulfur loading (2.8 mg cm⁻²) in the SE-15. The formed sulfur particles are very large in the electrode, which result in low utilization of sulfur and fast capacity fade. All cells show a high coulombic efficiency of >95% during cycling. The carbon paper in the SE electrodes contributes a negligible discharge capacity (<0.004 mA h), which is determined in a control cell, as shown in Fig. S2.†

Fig. 3 (a) Cyclability and coulombic efficiency of the cells with SE-2, SE-4, SE-8, and SE-15 electrodes at C/5 rate, (b) the 1^st cyclic voltammograms of the cells without LiNO₃ additive in the electrolyte at a potential sweep rate of 0.05 mV s⁻¹ between 1.5 and 3.0 V, (c) cyclability and coulombic efficiency of the electrode with sulfur loading of 1.1 mg cm⁻² at C/2 rate, (d) rate capability of the cell used in (c).

Fig. 3b shows the 1^st cyclic voltammograms (CV) of cells with these electrodes. All cells show two typical cathodic peaks representing the reduction reactions of sulfur to low-order polysulfides and low-order polysulfides to Li₂S. As the sulfur loading increases, the peak shifts to lower potential and the peak area increases. Similarly, all cells show distinguishable anodic peaks which represent the reverse oxidation of Li₂S to high-order polysulfides/sulfur.³² Due to the high sulfur contents in the SE-8 and SE-15, the anodic peaks are much broader and higher than those in the SE-2 and SE-4. In addition, the 2^nd anodic peak at high potential increases more significantly than the 1^st anodic peak at low potential as the sulfur content increases. This indicates slower electrode kinetics and more incomplete conversion of active material to elemental sulfur in the anodic sweep as the sulfur content is higher.

To further evaluate long cycle life, an electrode with sulfur loading of 1.1 mg cm⁻² was cycled for 300 cycles at C/2 rate, as shown in Fig. 3c. A high initial discharge capacity of over 1300 mA h g⁻¹ and a reversible capacity of 1000 mA h g⁻¹ in the first 50 cycles were obtained. Afterwards, the cell maintains a reversible capacity of about 700 mA h g⁻¹ after 300 cycles. The capacity fade is only 0.17% per cycle and the voltage plateaus are relatively stable (not shown). The coulombic efficiency is over 90%. With LiNO₃ additive in the electrolyte, lithium metal anode was passivated which stops aggressive reduction of polysulfides, therefore improves capacity retention and coulombic efficiency.^33,34 The rate capability of the cell is shown in Fig. 3d. A specific capacity of about 1300 mA h g⁻¹ is achieved in the 1^st cycle at C/10 rate and is stable at 1150 mA h g⁻¹ after 10 cycles. A reversible capacity of 1100 mA h g⁻¹ is achieved at C/5 rate. The specific capacity is around 1000, 950, and 850 mA h g⁻¹ at C/2, 1C, and 2C rate, respectively. When the rate was switched back to C/10 rate after 2C rate, the capacity went back to 1100 mA h g⁻¹. At C/5 and C/2 rates, the specific capacities are quite similar as those obtained before the rate testing cycle. These results show the good reversibility and rate capability of the cells enabled by the sulfur nanoparticles deposited in the carbon paper electrodes.

Fig. 4a shows the SEM image of a cycled electrode. After one cycle at the charged state, the electrode was washed by pure DME solvent for 1 hour to remove all soluble species and then scanned under SEM for comparison with the as-prepared electrode shown in Fig. 2a. The carbon paper is uniformly filled and covered with insoluble charged products, probably elemental sulfur which is amorphous as shown in Fig. S3.† The sulfur morphology changed completely from nanoparticle to continuous film, as compared with the SEM image in Fig. 2a. The carbon paper provides a robust matrix holding all cycled products, therefore maintaining stable cycle life as shown in Fig. 3. To further improve cycle life, polymer like polyvinylpyrrolidone (PVP) can be used to modify carbon paper, therefore improving cycling stability.¹⁸ Fig. 4b shows the Nyquist plot of the cell after different cycling status. The intercepts of Nyquist plots in the high-frequency are attributed to the bulk resistance of the liquid electrolyte and the semicircles in the high-medium frequency regions are charge transfer resistance of the electrode/electrolyte interfaces.³⁵ The linear segment in the low-frequency region corresponds to the diffusion limitation within the electrodes. It is shown the cell has a low bulk resistance (20 ohms) and a low charge transfer resistance (22 ohms) before cycling. The monoclinic sulfur nanoparticles in the electrode have a good contact with carbon and the electrolyte, which helps to maintain a low charge transfer resistance. After the 1^st cycle at the charged state, the charge transfer resistance increases to 57 ohms which is due to the change in sulfur morphology as shown in Fig. 4a. After the 20^th and 50^th cycle, the bulk resistance slightly increases to 38 ohms and 31 ohms, respectively, and the charge transfer resistance increases to 82 ohms and 118 ohms, respectively. The bulk resistance increase is due to the depletion of electrolyte and dissolved polysulfide in it, and the charge transfer resistance increases is due to the insulating property at the electrode/electrolyte interface making the charge transfer more difficult.

Fig. 4 (a) SEM image of the SE electrode after one cycle, (b) Nyquist plots of the cell after different cycles.

In summary, we have successfully prepared binder-free sulfur/carbon composite electrodes by a sulfur sublimation method in air. The sulfur deposited in the electrode mainly has a monoclinic crystal structure and it is in the form of nanoparticles filling all large pores in the carbon paper. Several composite electrodes with a variety of sulfur loading were prepared and evaluated in batteries. The electrodes show high utilization of sulfur, good cycling stability, and rate capability when the sulfur loading is <2.0 mg. The cell with sulfur loading of 1.1 mg cm⁻² was cycled over 300 cycles at C/2 rate, remaining a reversible capacity of over 700 mA h g⁻¹ and a coulombic efficiency of over 90%. This study demonstrates that the sulfur sublimation method is a clean, scalable, and viable route for making high performance binder-free sulfur/carbon composite electrodes for Li–S batteries.

Acknowledgements

This work was partially supported by the Research Experiences for Teacher Advancement In Nanotechnology (RETAIN) program funded by NSF grant RET-1406995 and the Enhanced Mentoring Program with Opportunities for Ways to Excel in Research (EMPOWER) grant from the Office of the Vice Chancellor for Research at IUPUI. We would like to acknowledge the Integrated Nanosystems Development Institute (INDI) at IUPUI for use of their JEOL7800F Field Emission Scanning Electron Microscope, which was awarded through NSF grant MRI-1229514. We would like to acknowledge the INDI for use of their Bruker D8 Discover X-Ray Diffraction Instrument, which was awarded through the NSF grant MRI-1429241.

Notes and references

1. X. Ji and L. F. Nazar, J. Mater. Chem., 2010, 20, 9821.
2. A. Manthiram, Y. Fu, S.-H. Chung, C. Zu and Y.-S. Su, Chem. Rev., 2014, 114, 11751.
3. P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat. Mater., 2012, 11, 19.
4. A. Manthiram, Y. Fu and Y.-S. Su, Acc. Chem. Res., 2012, 46, 1125.
5. Y. Cui and Y. Fu, J. Power Sources, 2015, 286, 557.
6. D. Bresser, S. Passerini and B. Scrosati, Chem. Commun., 2013, 49, 10545.
7. S. Xin, L. Gu, N.-H. Zhao, Y.-X. Yin, L.-J. Zhou, Y.-G. Guo and L.-J. Wan, J. Am. Chem. Soc., 2012, 134, 18510.
8. Y. X. Yin, S. Xin, Y. G. Guo and L. J. Wan, Angew. Chem., Int. Ed., 2013, 52, 13186.
9. Y. Fu, Y.-S. Su and A. Manthiram, ACS Appl. Mater. Interfaces, 2012, 4, 6046.
10. X. Ji, K. T. Lee and L. F. Nazar, Nat. Mater., 2009, 8, 500.
11. N. Jayaprakash, J. Shen, S. S. Moganty, A. Corona and L. A. Archer, Angew. Chem., Int. Ed., 2011, 50, 5904.
12. C. Liang, N. J. Dudney and J. Y. Howe, Chem. Mater., 2009, 21, 4724.
13. H. Wang, Y. Yang, Y. Liang, J. T. Robinson, Y. Li, A. Jackson, Y. Cui and H. Dai, Nano Lett., 2011, 11, 2644.
14. R. Elazari, G. Salitra, A. Garsuch, A. Panchenko and D. Aurbach, Adv. Mater., 2011, 23, 5641.
15. J. Guo, Z. Yang, Y. Yu, H. c. D. Abruña and L. A. Archer, J. Am. Chem. Soc., 2012, 135, 763.
16. Y. Fu and A. Manthiram, J. Phys. Chem. C, 2012, 116, 8910.
17. J. Song, M. L. Gordin, T. Xu, S. Chen, Z. Yu, H. Sohn, J. Lu, Y. Ren, Y. Duan and D. Wang, Angew. Chem., Int. Ed., 2015, 54, 4325.
18. Y. Cui and Y. Fu, ACS Appl. Mater. Interfaces, 2015, 7, 20369.
19. C. Huang, J. Xiao, Y. Shao, J. Zheng, W. D. Bennett, D. Lu, L. V. Saraf, M. Engelhard, L. Ji and J. Zhang, Nat. Commun., 2014, 5, 3015.
20. M. Wu, Y. Cui and Y. Fu, ACS Appl. Mater. Interfaces, 2015, 7, 21479.
21. G. Zhou, L. Li, D.-W. Wang, X.-y. Shan, S. Pei, F. Li and H.-M. Cheng, Adv. Mater., 2015, 27, 641.
22. S.-H. Chung and A. Manthiram, J. Phys. Chem. Lett., 2014, 5, 1978.
23. H. Yao, K. Yan, W. Li, G. Zheng, D. Kong, Z. W. Seh, V. K. Narasimhan, Z. Liang and Y. Cui, Energy Environ. Sci., 2014, 7, 3381.
24. D.-W. Wang, Q. Zeng, G. Zhou, L. Yin, F. Li, H.-M. Cheng, I. R. Gentle and G. Q. M. Lu, J. Mater. Chem. A, 2013, 1, 9382.
25. K. Jin, X. Zhou, L. Zhang, X. Xin, G. Wang and Z. Liu, J. Phys. Chem. C, 2013, 117, 21112.
26. M. Hagen, S. Dörfler, H. Althues, J. Tübke, M. Hoffmann, S. Kaskel and K. Pinkwart, J. Power Sources, 2012, 213, 239.
27. K. Fu, Y. Li, M. Dirican, C. Chen, Y. Lu, J. Zhu, Y. Li, L. Cao, P. D. Bradford and X. Zhang, Chem. Commun., 2014, 50, 10277.
28. A. Fallah-Shojaei, K. Tabatabaeian, F. Shirini and S. Z. Hejazi, RSC Adv., 2014, 4, 9509.
29. S. Moon, Y. H. Jung, W. K. Jung, D. S. Jung, J. W. Choi and D. K. Kim, Adv. Mater., 2013, 25, 6547.
30. S. Zhao, C. Li, W. Wang, H. Zhang, M. Gao, X. Xiong, A. Wang, K. Yuan, Y. Huang and F. Wang, J. Mater. Chem. A, 2013, 1, 3334.
31. D.-W. Wang, G. Zhou, F. Li, K.-H. Wu, G. Q. M. Lu, H.-M. Cheng and I. R. Gentle, Phys. Chem. Chem. Phys., 2012, 14, 8703.
32. Y. Fu, Y. S. Su and A. Manthiram, Angew. Chem., Int. Ed., 2013, 52, 6930.
33. Y. V. Mikhaylik and J. R. Akridge, J. Electrochem. Soc., 2004, 151, A1969.
34. S. S. Zhang, Electrochim. Acta, 2012, 70, 344.
35. Y. Fu and A. Manthiram, Chem. Mater., 2012, 24, 3081.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07120a

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