A modified synthesis process of three-dimensional

Materials and Design 153 (2018) 9–14

Contents lists available at ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

A modified synthesis process of three-dimensional sulfur/graphene aerogel as binder-free cathode for lithium sulfur batteries Buyin Li ⁎, Qi Xiao, Yuanzheng Luo Key Laboratory of Electronic Information Functional Material of Education Ministry, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• A modified two-step hydrothermal reduction and freeze-drying method of S/ GA was introduced. • The porous structure of S/GA enables a high areal specific capacity of N3mAh/ cm2. • This novel binder-free cathod based on GA simplified the manufacture processes of cathode.

a r t i c l e

i n f o

Article history: Received 30 October 2017 Received in revised form 24 April 2018 Accepted 28 April 2018 Available online 30 April 2018 Keywords: Graphene aerogel Lithium sulfur batteries Cathode materials Binder-free

a b s t r a c t Among the existing rechargeable battery systems, lithium sulfur batteries exhibit ultra-high theoretical specific capacity (1672mAh/g) and specific energy (2600 Wh/kg). However, its practical applications have been limited by several problems, especially the dissolution of lithium polysulfides and shuttle effect, which will cause poor cycling stability. Herein, we synthesized a 3D porous graphene aerogel through a modified two-step hydrothermal reduction method, which has been simultaneously used as the matrix to load sulfur and the shield to constrain the dissolution of polysulfides. The graphene aerogel containing sulfur was mechanically pressed to circular disc and directly used as cathode without any additives, which shows great flexibility and excellent electrical conductivity. Since no extra conductive additives and binder are needed, this binder-free method is also more facile than conventional processes. Meanwhile, the shield was used as lithium polysulfides absorber to suppress the shuttle effect. Compared with conventional lithium sulfur batteries, this novel cathode with unique battery structure enables a higher areal sulfur loading of N2.5mg/cm2 and initial specific capacity of N1100mAh/g. More importantly, the dissolution and shuttle effect of lithium polysulfides were suppressed effectively by the graphene aerogel matrix and shield, greatly improving the cycling stability. © 2018 Elsevier Ltd. All rights reserved.

1. Introduction

⁎ Corresponding author. E-mail address: [email protected] (B. Li).

https://doi.org/10.1016/j.matdes.2018.04.078 0264-1275/© 2018 Elsevier Ltd. All rights reserved.

Environmental protection and energy supply are the most serious challenges which we face today, especially the rapid depletion of fossil energy. The deterioration of environment and the change of energy

10

B. Li et al. / Materials and Design 153 (2018) 9–14

structure make the demand for clean energy more and more urgent in the high developing society. Therefore, the development of more efficient, greener, more economical energy storage devices is particularly critical. During the existing rechargeable battery systems, lithium-ion batteries are widely used in mobile phones, computers and other consumer electronics due to their low cost, high specific energy and small size. On the other hand, with the fast development of electric vehicles (EVs), hybrid electric vehicles (HEVs), flexible devices and robots [1,2], lithium-ion batteries meet great challenges due to their low specific energy of b300 Wh/kg [3]. So, developing batteries with higher specific energy to meet the needs of future social development will affect the survival and development of mankind profoundly. In recent years, lithium sulfur (Li S) batteries have gained a lot of attention due to its ultra-high theoretical specific capacity of 1672mAh/g and specific energy of 2600 Wh/kg [3–5]. Moreover, the raw materials of Li S batteries are low cost and environmentally friendly. However, despite these advantages, the development and commercialization of Li S batteries are still facing many challenges, especially the following: (a) The conductivity of elemental sulfur is very low (at room temperature, its electronic conductivity is about 10~30S/cm), which leads to low utilization of active material of Li-S batteries [6]; (b) The intermediate product lithium polysulfides (Li2Sx, 4 b x b 8) are very soluble in general organic electrolyte, causing the loss of active material. On the other hand, the shuttle effect caused by polysulfides will corrode the lithium anode and further degrade the performance of Li S batteries [7]; (c) In the lithiation of S to Li2S/Li2S2, the volume expansion can reach about 80%, causing the destruction of cathode structure and the fading of capacity. Meanwhile, this volume expansion is also potentially dangerous for the Li S batteries. To overcome these disadvantages, lots of efforts have been devoted. For example, many researchers have tried to develop high performance sulfur based cathode composites to improve the utilization of active materials and cycling performance of Li S batteries, especially the synthesis of matrix materials with excellent electrical conductivity and high sulfur loading. Nowadays, the researches are mainly focused on the suppression of polysulfides dissolution and shuttle effect through novel composite cathode materials, such as carbon-based materials and polymer composites. In the research of high performance sulfur cathode composite and interlayer, carbon materials have become the focus of research. For example, CNTs [8–11], micro/mesoporous carbon [12,13], graphene [14–16] and carbonaceous aerogel [17–20] have all been introduced to synthesize the cathode materials. These carbon-based materials exhibit unique advantages: (a) The good electrical conductivity of carbon materials can overcome the electrical insulation of sulfur and improve the conductivity of cathode; (b) High specific surface area can provide more active sites for electrode reaction, reducing self-discharge and electrode reactive polarization; (c) The adsorption of porous structure can well constrain sulfur and inhibit the dissolution of polysulfides in electrolyte, which is beneficial to improve the utilization of active material. By exploring the carbon materials above, finding appropriate pore size, specific surface area and other characteristics, the energy density and cycling stability of carbon-based composite cathode can be effectively improved. Among numerous carbon materials, graphene has been widely studied and applied due to its unique advantages [20,21]. In order to further improve the electrochemical performance of carbon/sulfur composite cathode and optimize its pore structure and parameters, the research on graphene/sulfur composite should be mainly focused on the following improvements: (a) Increasing the porosity of carbon material for higher sulfur loading of the matrix; (b) Increasing the specific surface area and adjusting pore size, thus enhancing the adsorption of matrix to lithium polysulfides. By adjusting pore structure and parameters of matrix materials through appropriate process designs and production control, the sulfur loading of matrix and the utilization of active material can be increased. Based on above description and discussion, we report a modified two-step hydrothermal reduction and freeze-drying method

to synthesize a three-dimensional sulfur/graphene aerogel (S/GA) composite cathode, as shown in Fig. 1. Compared to general composite, the as prepared S/GA can be pressed into flexible sheet and directly used as cathode without any additives. Meanwhile, the graphene aerogel not only offers an ideal matrix to improve the conductivity of sulfur, but also acts as a shield to constrain the dissolution of polysulfides. The as prepared cathode exhibits a high specific capacity of more than 1100mAh/g and good cycling stability. More importantly, this synthesis method can be integrated with other composites easily, which is promising to further improve the performance of Li S batteries. 2. Experimental section 2.1. Preparation of graphene oxide (GO) The graphene oxide was synthesized by the modified Hummer's method. Typically, 3 g graphite powders were dissolved in 100 mL concentrated sulfuric acid (95%) with magnet stirring of 100 rpm. Then 13.5 g potassium permanganate was added to the solution slowly with stirring of 1 h, and the speed was 800 rpm. Then the solution was heated to 90 °C followed by stirring for 30 min, and 750 mL water was added slowly. After the solution was cooled down, 800 mL of 5% hydrogen peroxide and 300 mL of 10% hydrochloric acid was added. Finally, the solution was washed with a large amount of alcohol and distilled water until the solution is neutral (pH = 7.0). After centrifugation and drying at room temperature for 12 h, the graphene oxide powders can be obtained. 2.2. Preparation of S/GA composite and flexible cathode sheet The preparation of S/GA composite was based on two-step hydrothermal reduction and freeze-drying method of GO. Typically, 158 mg Na2S2O3 and 20 mg as prepared GO powders were dispersed into 13 mL deionized water. 2 mL of 1 M HCl was added to the solution under magnet stirring for 10 min. Then, 30 mg ascorbic acid was added with stirring for another 20 min. For each sample, 2 mL solution was sealed in a cylindrical glass vial, which was then placed into water bath of 95 °C for 30 min. The vial was then placed into dry ice bath for 1 h. When the samples were thawed and recovered to room temperature, they were placed into water bath for another 5 h. The as prepared hydrogel was finally washed with deionized water and then freezedried to obtain S/GA composite. To prepare work electrode, the S/GA composite was directly pressed at 1 MPa into flexible sheet, as shown in Fig. 2. Meanwhile, we have found that the pressure has significant effect on the performance of the S/GA cathode. Specifically, when the pressure is too small, the cathode sheet showed no flexibility, which is unbeneficial to the structural stability of electrode during cycling. On the other hand, if the pressure is too big, it's difficult to offer good electrolyte infiltration into the cathode, and the composite will exhibit poor initial specific capacity. 2.3. Materials characterization We used scanning electron microscopy (SEM, Hitachi SU8010) to characterize the morphology and structure of the S/GA composite and the cathode sheet. Thermogravimetric analysis (TGA, Mettler-Toledo TG/DSC1) was performed to evaluate the sulfur loading of S/GA. 2.4. Electrochemical measurement The as prepared cathode sheet was fabricated into 2032-type coin cells in an Ar-filled Glovebox. Li metal foil was used as counter electrode with Celgard 2400 polyethylene as separator. To prepare electrolyte, the solution of 1.0 M LiTFSI in a mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 ratio, by volume) was prepared with 1 wt% LiNO3 as additives. Galvanostatic charge/discharge, cycling

B. Li et al. / Materials and Design 153 (2018) 9–14

11

Fig. 1. Schematic illustration of the synthesis process of S/GA composite.

Fig. 2. The digital images of the synthesis process of S/GA composite and flexible cathode sheet.

performance and rate capability tests were carried out on the LAND CT2001A instrument. The potential window was controlled from 1.6 V to 2.8 V (vs. Li/Li+). 3. Results and discussions The S/GA composite was synthesized by a modified two-step hydrothermal reduction and freeze-drying method, as mentioned above. In fact, the synthesis process of S/GA based on this method is governed by complex and dynamic liquid-particle and particle-particle interactions. Specifically, directly freezing of GO dispersions can also result in aerogel with porous structure. However, the amount of oxygencontaining groups and freezing condition have significant effects on the morphology of the resulted graphene aerogel. If the first-step reduction time is too short, the negatively charged groups on GO sheets will

generate strong electrostatic repulsion between sheets, which may prevent good connection between flakes during the freezing process. In other words, when carefully controlling the reduction time and temperature, the as prepared graphene aerogel and S/GA will have superior porous structure. The microstructure of S/GA is shown in Fig. 3. It can be observed that the sulfur particles are coated by the porous and folded structure of GA, which can be attributed to the self-assembling of GO sheet during the reduction and the ice-casting during the dry ice bath. With the porous and folded structure of GA, sulfur particles can be dispersed into the matrix uniformly. Meanwhile, the coating of graphene sheet can make the sulfur particles embedded in the matrix more tightly, constraining the dissolution of the polysulfides during the cycling. What's more, this bulk material can be directly pressed into flexible sheet, as shown in Fig. 2, which is promising to simplify the process of cathode. To further

Fig. 3. SEM images of the S/GA under different resolution.

12

B. Li et al. / Materials and Design 153 (2018) 9–14

Fig. 4. (a) The SEM image of the cathode sheet, and corresponding elemental mapping of (b) C and (c) S by EDS.

verify the distribution of sulfur in the GA matrix, the elemental mapping of S and C in the cathode sheet by EDS is shown in Fig. 4. It can be observed that the sulfur has been dispersed in the matrix successfully. To investigate the sulfur loading of the cathode quantitatively, thermogravimetric analyzer under nitrogen atmosphere has been applied, as shown in Fig. 5. Sulfur evaporates at around 200–300 °C. It should be noted that the content of sulfur in the S/GA reaches 62%, which is a relatively high value when calculating by the mass of the whole cathode. Meanwhile, the diameter of the cathode is around 12 mm, and the corresponding areal sulfur loading is N2.5 mg/cm2, which is higher than lots of literatures before.

The S/GA bulk material was pressed into flexible sheet directly without any additives. The cycling performance of S/GA is shown in Fig. 6 to investigate its electrochemical performance. It can be observed that the S/GA exhibits a high initial specific capacity of more than 1100mAh/g, and the corresponding areal capacity is more than 3 mAh/cm2. That's a relatively higher value compared to existing literature. Meanwhile, the S/GA cathode shows good cycling stability after the first few cycles. The reversible capacity is still more than 500mAh/g after 60 cycles at 0.1C. The high specific capacity and good cycling stability can be attributed to the effect of GA, which offers an ideal matrix to improve the electrical conductivity of cathode. On the other hand, the porous and

Fig. 7. The charge/discharge curves of S/GA at 0.1C. Fig. 5. TGA curves of elemental sulfur and the S/GA cathode.

Fig. 6. Cycling performance and corresponding coulombic efficiency of S/GA at 0.1C.

Fig. 8. Long-term cycling performance and corresponding coulombic efficiency of S/GA at 0.5C.

B. Li et al. / Materials and Design 153 (2018) 9–14

13

As the current rate increasing, the specific capacity decreases to about 300mAh/g. However, when the charge/discharge rate restored to 0.1C, the reversible capacity recovered to N500mAh/g, exhibiting good rate capability. In summary, these results confirm that the presence of GA matrix can not only improve the electrical conductivity of cathode and the utilization of active material, but also constrain the dissolution of polysulfides, improving the cycling stability of the S/GA cathode. Meanwhile, the unique structure of the composite offers a simpler process to prepare cathode sheet without any additives. What's more, this modified method can be integrated with other composites, which is promising to further improve the performance of the cathode. 4. Conclusion

Fig. 9. The charge/discharge curves of S/GA at 0.5C.

folded structure acts as a shield to coat the sulfur particles, which constrain the dissolution of the polysulfides. Fig. 7 further shows the corresponding charge/discharge curves of S/GA in the first and second cycles. There are two plateaus during discharge and one plateau during charge can be observed, which represent the reducing and oxidizing reaction of the cathode respectively. It should be noted that the capacity loss of the first cycle is relatively high, which can be ascribed to the structural instability of cathode material at the beginning. So, after the first cycles of stabilization, the cathode exhibits a good cycling stability from 10th to 50th cycles. To further investigate the cycling stability of S/GA, the long-term cycling performance of S/GA at 0.5C was tested, as shown in Fig. 8. The S/ GA cathode exhibits an initial specific capacity of N900mAh/g at 0.5C, and the corresponding charge/discharge curves are shown in Fig. 9. Although the capacity loss of the first cycle is relatively high, the average capacity loss is decreased during the following cycles. After 200 cycles, the reversible capacity is about 300mAh/g. Meanwhile, the coulombic efficiency is still N90% after 200 cycles, illustrating the good electrochemical performance. On the other hand, the rate capability of S/GA has also been tested from 0.1C to 1C, as shown in Fig. 10. It can be observed that the S/GA cathode exhibits an initial specific capacity of more than 1100mAh/g, which is in common with the result of Fig. 6.

Fig. 10. The rate capability of S/GA from 0.1C to 1C.

A modified two-step hydrothermal reduction and freeze-drying method is introduced in this work, which effectively optimized the microstructure of GA, improving the performance of S/GA composite cathode. The more regular and tunable porous structure can make the sulfur dispersed more uniformly into the GA matrix, enabling higher sulfur loading and specific capacity. Compared with pristine composite, the effect of GA matrix can ensure better absorption of lithium polysulfides, which suppresses the loss of active material and enhances the cycling stability of Li S batteries. The as prepared cathode exhibits a high specific capacity of more than 1100mAh/g, and the corresponding areal capacity is more than 3 mAh/cm2. Meanwhile, after the first cycles of stabilization, the cathode shows good cycling stability and rate capability. What's more important, a novel binder-free method based on graphene aerogel to prepare cathode is proposed, which is very convenient to be integrated with other composites and promising to simplify the manufacture processes of cathode, further improving the performance of Li S batteries. Acknowledgements This research was supported by the Fundamental Research Funds for the Central Universities (HUST: No. 2016YXMS205) and the Creative Technology Project of Hubei Province (No. 2016AAA048). References [1] L. Lu, X. Han, J. Li, J. Hua, M. Ouyang, A review on the key issues for lithium-ion battery management in electric vehicles, J. Power Sources 226 (2013) 272–288. [2] R. Marom, S.F. Amalraj, N. Leifer, D. Jacob, D. Aurbach, A review of advanced and practical lithium battery materials, J. Mater. Chem. 21 (27) (2011) 9938–9954. [3] A. Rosenman, E. Markevich, G. Salitra, D. Aurbach, A. Garsuch, F.F. Chesneau, Review on li-sulfur battery systems: an integral perspective, Adv. Energy Mater. 5 (16) (2015) 1500212. [4] M. Wild, L. O'Neill, T. Zhang, R. Purkayastha, G. Minton, M. Marinescu, G. Offer, Lithium sulfur batteries, a mechanistic review, Energy Environ. Sci. 8 (2015) 3477–3494. [5] R. Chen, T. Zhao, F. Wu, From a historic review to horizons beyond: lithium sulphur batteries run on the wheels, Chem. Commun. 51 (1) (2015) 18–33. [6] G. Zhou, S. Pei, L. Li, D.W. Wang, S. Wang, K. Huang, L.C. Yin, F. Li, H.M. Cheng, A graphene-pure-sulfur sandwich structure for ultrafast, long-life lithium sulfur batteries, Adv. Mater. 26 (4) (2014) 625–631. [7] Z.W. Seh, W. Li, J.J. Cha, G. Zheng, Y. Yang, M.T. McDowell, P.C. Hsu, Y. Cui, SulphurTiO2 yolk-shell nanoarchitecture with internal void space for long-cycle lithium sulphur batteries, Nat. Commun. 4 (2013) 1331. [8] M.D. Walle, Z. Zhang, X. You, M. Zhang, J.M. Chabu, Y. Li, Y.N. Liu, Soft approach hydrothermal synthesis of a 3D sulfur/graphene/multiwalled carbon nanotube cathode for lithium sulfur batteries, RSC Adv. 6 (82) (2016) 78994–78998. [9] Y.L. Ding, P. Kopold, K. Hahn, P.A. van Aken, J. Maier, Y. Yu, Facile solid-state growth of 3D well-interconnected nitrogen-rich carbon nanotube-graphene hybrid architectures for lithium sulfur batteries, Adv. Funct. Mater. 26 (7) (2016) 1112–1119. [10] G. Yuan, G. Wang, H. Wang, J. Bai, A novel three-dimensional sulfur/graphene/ carbon nanotube composite prepared by a hydrothermal co-assembling route as binder-free cathode for lithium sulfur batteries, J. Nanopart. Res. 17 (1) (2015) 36. [11] J. Guo, Y. Xu, C. Wang, Sulfur-impregnated disordered carbon nanotubes cathode for lithium sulfur batteries, Nano Lett. 11 (10) (2011) 4288–4294. [12] N. Jayaprakash, J. Shen, S.S. Moganty, A. Corona, L.A. Archer, Porous hollow carbon@ sulfur composites for high-power lithium sulfur batteries, Angew. Chem. 123 (26) (2011) 6026–6030.

14

B. Li et al. / Materials and Design 153 (2018) 9–14

[13] N.W. Li, Y.X. Yin, Y.G. Guo, Three-dimensional sandwich-type graphene@microporous carbon architecture for lithium sulfur batteries, RSC Adv. 6 (1) (2016) 617–622. [14] R. Fang, S. Zhao, S. Pei, X. Qian, P.X. Hou, H.M. Cheng, C. Liu, F. Li, Toward more reliable lithium sulfur batteries: an all-graphene cathode structure, ACS Nano 10 (9) (2016) 8676–8682. [15] J. Cao, C. Chen, Q. Zhao, N. Zhang, Q. Lu, X. Wang, Z. Niu, J. Chen, A flexible nanostructured paper of a reduced graphene oxide-sulfur composite for high-performance lithium sulfur batteries with unconventional configurations, Adv. Mater. 28 (43) (2016) 9629–9636. [16] J. Feng, X. Qin, Z. Ma, J. Yang, W. Yang, G. Shao, A novel acetylene black/sulfur@ graphene composite cathode with unique three-dimensional sandwich structure for lithium sulfur batteries, Electrochim. Acta 190 (2016) 426–433. [17] K. Balakumar, N. Kalaiselvi, High sulfur loaded carbon aerogel cathode for lithium sulfur batteries, RSC Adv. 5 (43) (2015) 34008–34018.

[18] Y. Xie, Z. Meng, T. Cai, W.Q. Han, Effect of boron-doping on the graphene aerogel used as cathode for the lithium sulfur battery, ACS Appl. Mater. Interfaces 7 (45) (2015) 25202–25210. [19] Z. Zhang, Z. Li, F. Hao, X. Wang, Q. Li, Y. Qi, R. Fan, L. Yin, 3D interconnected porous carbon aerogels as sulfur immobilizers for sulfur impregnation for lithium sulfur batteries with high rate capability and cycling stability, Adv. Funct. Mater. 24 (17) (2014) 2500–2509. [20] M. Yu, A. Wang, F. Tian, H. Song, Y. Wang, C. Li, J.D. Hong, G. Shi, Dual-protection of a graphene sulfur composite by a compact graphene skin and an atomic layer deposited oxide coating for a lithium-sulfur battery, Nanoscale 7 (12) (2015) 5292–5298. [21] Y. Shi, L. Wen, G. Zhou, J. Chen, S. Pei, K. Huang, H.M. Cheng, F. Li, Graphene-based integrated electrodes for flexible lithium ion batteries, 2D Materials 2 (2) (2015), 024004.