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MWCNTs and its electrochemical performance
Electrochimica Acta 55 (2010) 8062–8066
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The preparation of nano-sulfur/MWCNTs and its electrochemical performance Jia-jia Chen, Xin Jia, Qiu-jie She, Chong Wang, Qian Zhang, Ming-sen Zheng, Quan-feng Dong ∗ State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China
a r t i c l e
i n f o
Article history: Received 23 November 2009 Received in revised form 15 January 2010 Accepted 15 January 2010 Available online 25 January 2010 Keywords: Nano-sulfur/MWCNTs composite Sulfur-fixed matrix Modified MWCNTs Lithium–sulfur battery Nanomaterial
a b s t r a c t In this work, a novel nano-sulfur/MWCNTs composite with modified multi-wall carbon nano-tubes (MWCNTs) as sulfur-fixed matrix for Li/S battery is reported. Based on different solubility of sulfur in different solvents, nano-sulfur/MWCNTs composite was prepared by solvents exchange method. The composite was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The modified MWCNTs are considered that not only acts as a conducting material, but also a matrix for sulfur. The electrochemical performance of the nano-sulfur/MWCNTs composite was tested. The results indicated that nano-sulfur/MWCNTs composite had the specific capacity of 1380 mAh g−1 , 1326 mAh g−1 and 1210 mAh g−1 in the initial cycle at 100 mA g−1 , 200 mA g−1 and 300 mAh g−1 discharge rates respectively, and remained a reversible capacity of 1020 mAh g−1 , 870 mAh g−1 and 810 mAh g−1 after 30 cycles. The electrochemical performances confirm that the modified MWCNTs as sulfur-fixed matrix show better ability than any other carbon in cathode of Li/S batteries that had been reported. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction In order to meet the challenge of global warming and the finite nature of fossil fuels, it is now essential that new, low-cost and environmentally friendly energy conversion and storage systems are found. The performance of these power supply devices depends intimately on the properties of their materials which hold the key to fundamental advances in energy conversion and storage [1]. Lithium ion battery is now the most promising portable storage device. However, the first and most popular cathode material LiCoO2 has many restrictions on safety and toxicity issues because of its immanence nature. Although the new cathode materials, such as LiFePO4 [2,3] and layer-structured LiMnO2 [4,5] are developed for replacing LiCoO2 in lithium battery, all these transition metal oxide cathode materials show relatively low capacities. A battery based on lithium/sulfur has a theoretical capacity of 1675 mAh g−1 and a theoretical specific energy of 2600 Wh kg−1 , assuming complete reaction to the product Li2 S. Therefore, there is a strong incentive to develop Li/S batteries. Nowadays, there are many researchers focus on finding appropriate sulfur composites to meet the serious problems of low utilization and poor cycle life in sulfur cathodes. However, sulfur is an ionic and electron insulator, it is hard to operate a pure sulfur cathode material at high charge–discharge rates. In order
∗ Corresponding author. E-mail address: [email protected] (D. Quan-feng). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.01.069
to achieve good performance of Li/S battery, dimension of sulfur particles should be reduced. As reported in Ref. [1], the nanomaterial has a better accommodation of the strain of lithium transfer which is considered to have a contribution on improving the cycle life, and a better electrode/electrolyte interface leading to higher charge/discharge rates. The path for electron and Li+ transport was also shorten. Meanwhile, carbon is introduced as a conductive agent which is significant for current collection and, hence, the rate performance and sulfur utilization [6–8]. Choi [8] showed that carbon not only acted as electrical conductor but also supplied electrochemical reaction sites for sulfur. The key to improve Li/S battery is to increase electron conductivity in cathodes. In prior studies, there were two ways to prepare sulfur/carbon composites: thermo-treating and ball milling [9–12]. The first discharge capacities of composites in those published work were low, and they dropped down to 500 mAh g−1 after several cycles even at low rates. Sulfur is difficult to be dispersed with carbon homogeneously resulting in a low sulfur utilization, and the poor cyclic ability of the battery which is caused by sulfur running out from the electrode. In this work, we report a new method to synthesis a high performance composite cathode material for Li/S battery by solvent exchange. The MWCNTs are attractive choice as the electric conductor for sulfur cathode because they could provide more effective electronically conductive network than other conductive additives, such as carbon black (CB), acetylene black (AB) and graphite. Most of all, modified surfaces would improve the interaction of MWCNT with solvents and dispersion, and even enhance MWCNTs adsorption properties to allow grafting of nanoparticles [13,14]. Above all the electrochemical
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performances of the nano-sulfur/MWCNTs composites as cathode material for rechargeable lithium batteries are investigated. 2. Experimental 2.1. Preparation of materials Raw MWCNTs (20–40 nm, ShenZhen, China) were soaked in concentrated nitric acid at 120 ◦ C for 2 h and then washed by distilled water until the filtrate’s pH value reached 7.0. The aim of this procedure was not only to purify the MWCNT, but also to modify the surface of MWCNTs with functional groups. Then, 0.1 g modified MWCNTs were dispersed into 200 ml 1% (w/w) sodium dodecyl sulfate (SDS, AR) aqueous solution, while the sublimation sulfur (AR) dissolved into the purified tetrahydrofuran (THF) to get a saturated solution. After that, 30 ml saturated solution (nearly 0.16 mol l−1 ) was added into the solution containing modified MWCNTs and SDS with continuously strong stirring. The suspension was separated and washed with distillated water three times to remove SDS. The weight ratio of sulfur and modified MWCNTs was 6:4. The reference sample was pure sulfur in nanoscale which prepared by the same method. The morphology of these products was investigated by using scanning electron microscopy (SEM) on a HITACHI S-4800, and their XRD patterns were taking place on a Panalytical X’pert PRO X-ray diffraction (XRD) using Cu K␣ radiation.
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sulfur composites were used to fabricate cathodes. One was nanosulfur/MWCNTs composite mentioned above which was shorted as sample A; another was pure nano-sulfur mixed with the same weight of acetylene black which was named as sample B. All cathodes comprised the same proportion of sulfur (48%) and 10 wt% water soluble polymer n-lauryl acrylate (Chengdu, China). The cathode slurries were prepared by ball milling and then coated onto Al foils. The electrodes were dried at 50 ◦ C under vacuum over night. The testing cells, CR2016-type coin cells, used lithium foils as the counter electrodes, Cellgard2400 microporous membrane as separator and 1.0 mol l−1 LiCF3 SO3 dissolved in dioxolane (DOL) and diethylene glycol dimethyl ether (DEGDME) (1:2, v/v) as the electrolyte. The cells were assembled in an Argon-filled glove box. The discharge–charge tests were carried out with a NEWARE BTS-5 V/5 mA type battery charger (Shenzhen, China) at current densities of 100 mA/g, 200 mA/g and 300 mA/g (base on sulfur) with the voltage range 1.5–3.0 V. The specific capacity was calculated on the basis of the active sulfur material. Cyclic voltammetry (CV) experiments were conducted using a Princeton PAR273A potentiostat at scan rates of 0.1 mV s−1 . Electrochemical impedance spectra (EIS) of the cells were performed using a Zahner IM6 electrochemical workstation, which was carried out in a frequency range between 100 kHz and 100 mHz at potentiostatic signal amplitudes of 5 mV. All experiments were conducted at room temperature. 3. Results and discussion
2.2. Preparation and electrochemical testing of the cathodes In order to investigate the effect of modified MWCNTs on the electrochemical performance of sulfur cathode, two types of
Fig. 1 shows the morphological characterization of raw sulfur, MWCNTs, nano-sulfur and sample A. The raw sulfur has a particle size distribution of 5–100 m (Fig. 1a). The sulfur particles which
Fig. 1. SEM images of (a) raw sulfur; (b) nano-sulfur (sample B); (c) MWCNTs and (d) nano-sulfur/MWCNTs composite (sample A).
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Fig. 2. XRD patterns of (a) MWCNTs, (b) sample B and (c) sample A.
prepared by the solvent exchange method are much smaller than raw sulfur, and the size distribution is unanimous nanoscale which the scale range of sulfur particles is 50–100 nm (Fig. 1b). The diameter of the modified MWCNTs was about 20–40 nm (Fig. 1c). The modified MWCNTs have many functional groups, such as hydroxyl, carboxyl, and carbonyl, can act as the growth points when the sulfur precipitated by the solvent exchange method. The functional groups also provide an intimate contact between sulfur and MWCNTs. While the modified MWCNTs are introduced as sulfurfixed matrix and conducting materials, the sulfur coat uniformly on the surface of MWCNTs. The diameter of sample A was less than 100 nm, which means the sulfur layer is about 40 nm, compared with the MWCNTs without sulfur (Fig. 1d). The structure of products of nano-sulfur, MWCNTs and sample A which are further confirmed by the XRD tests. The results are showed in Fig. 2. The reflections of the nano-sulfur are consistent with Fddd orthorhombic structure. Compared with the pattern of the nano-sulfur, the XRD spectra of sample A exhibit the same trend, and the peak of MWCNTs center at 2 = 26◦ is disappeared. This indicates that no phase transformation of sulfur occurred during the preparation and the sulfur particles are well coated on the surface of the MWCNTs, so there are not any obvious peaks of the MWCNTs. The SEM and XRD results confirm that sulfur particles can distribute uniformly on the surface of MWCNTs while the MWCNTs are modified by the concentrated HNO3 . The two types of sulfur composites, sample A cathode and sample B, were tested to investigate the effect of modified MWCNTs on the electrochemical performance of the sulfur cathode. Fig. 3 depicts initial discharge curves of those two sulfur cathodes at various rates. All discharge curves show two plateaus based on the voltage profiles. The detail mechanisms of electrochemical oxidation and reduction of sulfur, polysulfides and lithium sulfides during discharge/charge were already reported elsewhere [15,16]. The upper plateau (∼2.41 V) is well known as the ring opening of S8 to the linear high order lithium polysulfides (Li2 Sn , n ≥ 4). The lower plateau (∼2.07 V) is caused by the reduction of high order lithium polysulfides to low order lithium polysulfides (Li2 Sn , n < 4), even to Li2 S. Sample B shows a high specific capacity of 1270 mAh g−1 in initial discharge at the current density of 100 mA g−1 which means it has a good utilization of sulfur. However, with the discharge rates increasing, the discharge capacities decrease greatly to 1150 mAh g−1 and 900 mAh g−1 , while at 200 mA g−1 and 300 mA g−1 , respectively. It is caused by low electron conductivity of sulfur baffles electrons entering/exiting sulfur outer orbital. The great electrochemical resistance could not be ignored that increasing current density causes high over
potential decreases discharge voltage, declines sulfur efficiency simultaneity. Meanwhile, the discharge plateau voltages decline greatly when sample B suffers from high discharge current densities. Compared to sample B, we can find that sample A shows better rate performance than sample B. The initial discharge capacities are up to 1380 mAh g−1 , 1330 mAh g−1 and 1210 mAh g−1 , at 100 mA g−1 , 200 mA g−1 and 300 mA g−1 severally. Moreover, the discharge plateau voltages only drop a little bit with increasing discharge currents. These phenomena show that sample A has many advantages than sample B due to obviously influence of MWCNTs. MWCNTs enhance cathode electron conductivity on three ways: (1) MWCNTs themselves are very good conductive framework; (2) Functional groups on the surface of modified MWCNTs can provide an intimate contact between sulfur and MWCNTs arising from the properties of sulfur-fixed as well as polysulfides with nanoscale dimensions, thus affords excellent accessibility of the active material; (3) MWCNTs support reaction sites for sulfur and polysulfides and the entrapment of the sulfur and polysulfides ensures that a more complete redox process takes place, and results in enhanced utilization of the active sulfur material. Besides, hollow tube like sulfur shorts electron transfer path and has larger surface than cubic nano-sulfur. The cyclic voltammograms of the Li/S cells with two composite cathodes were measured at the scan rate of 0.1 mV s−1 as shown in Fig. 4. The 2.4 V anodic peaks are related to the open ring reaction that motioned before, and the peaks between 1.9 V and 2.0 V are caused by the reduction of the high order lithium polysulfides to low order lithium polysulfides (Li2 Sn , n < 4), even to Li2 S [15,16]. These two anodic peaks are corresponding to the two discharge plateau as depicted in Fig. 3. Compare the peaks of two cathodes, the anodic peaks of sample A cathode reveal narrow and high peaks
Fig. 3. First discharge curves of the two cathodes at different discharge rates of (a) 100 mA g−1 (b) 200 mA g−1 (c) 300 mA g−1 .
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Fig. 4. CV curves of the two cathodes at scan range: 3.0–1.5 V, and scan rate: 0.1 mV s−1 .
and the smaller E (potential difference between the peak couple) than those of sample B. The CV results indicate that the electrochemical polarization of sample B is stronger than that of sample A cathode. The cycle performance of the two cathodes in the electrolyte were tested at different rates is shown in Fig. 5. With decreasing sulfur particle sizes dimensions, sulfur utilization would be increased due to the increase of specific surface area. Sample B shows a good utilization of active material in the first discharge at a current density of 100 mA g−1 , the discharge capacity is nearly 1200 mAh g−1 .
Fig. 5. Cyclic and rate performance of Li/S cell with the two cathodes at various discharge rates of (a) 100 mA g−1 (b) 200 mA g−1 (c) 300 mA g−1 .
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However, the discharge capacity drop to 1000 mAh g−1 in the 2nd cycle and gradually fades in the later cycles. After 30 cycles, the discharge capacity only remains at 650 mAh g−1 . Although the capacity after several cycles is lower compared to the first discharge capacity, it is still higher than many other sulfur and carbon composites [9–12]. This also confirms that the sulfur will show a better performance when its dimension was reduced. The cycle performances of sample B are similar to the result obtained at the current density of 100 mA g−1 when it was tested at other current densities of 200 mA g−1 and 300 mA g−1 . The irreversible loss of sample B maybe contributes to the dissolving of sulfur into the electrolyte. Meanwhile, it is difficult to prevent sample B from aggregation during the cycling. So, the modified MWCNTs are introduced as a sulfur-fixed matrix and a conducting material, which can provide good electrochemical reaction sites of sulfur. When sample A is tested as cathode at the same rates of sample B, it shows even better cycle performances than sample B. With a little capacities drop from the 1st to 2nd cycle, the discharge capacity retention is almost unchanged at 1100 mAh g−1 with a current density of 100 mA g−1 . Even with the increasing of discharge rates, the cycle life keeps at a high capacity without any obvious fading. This is may explained by the functional groups of MWCNTs modified by concentrated HNO3 . The functional groups help confine diffusion of polysulfides out of the cathode structure, minimize the loss of the active mass in the cathode and improve the cycling stability. Fig. 6 shows EIS plots of sulfur cathodes in their original statements before discharge. It can be seen from the figures that all the Nyquist plots of sulfur cathodes are composed by a semicircle at high frequencies corresponding to the contact resistance and charge transfer resistance and a short inclined line in low frequency regions due to the ion diffusion within the cathode. Before discharge, sample B shows higher charge-transfer resistances (Rct) than sample A. This is further evidence to show better conductivity of sample A. As all we mentioned above, when sulfur particles are reduced to nanoscale and dispersed uniformly in MWCNTs via a simple solvent exchange method, the electrochemical performance is greatly enhanced. The modified MWCNTs here are not simply acted as conducting materials, but also a well sulfur-fixed matrix. The functional groups, such as hydroxyl, carboxyl, and carbonyl groups, can be acted as the growth point when the sulfur precipitated by the solvent exchange method mentioned above. And those groups maybe further provide a chemical gradient that retards diffusion of polysulfide anions out of the electrode, thus facilitating more complete reaction and improving the cycling stability.
Fig. 6. Electrochemical impedance spectra (EIS) of cells with (a) sample A and (b) sample B before initial discharge.
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4. Conclusions The nano-sulfur/MWCNTs composite was prepared by solvent exchange method. This is a new method to prepare sulfur cathode materials. It can reduce the dimension of the sulfur, thus enhance the electrochemical performance. However, the pure nano-sulfur cannot prevent aggregation during the cycling. So, the MWCNTs modified by concentration HNO3 were introduced with the aim to fix sulfur and prevent the sulfur from aggregation. The functional groups on the carbon surface can play an important role on enhance the cycling of Li/S battery. Other researchers also use MWCNTs to prepare the sulfur composites, but the MWCNTs were without any treat. The results of those cathodes are still poor in cycling at high capacity retention and high charge–discharge rates. The modified MWCNTs can improve MWCNTs interaction with solvents and dispersion, and even can modify MWCNTs adsorption properties to allow the grafting of nanoparticles. The introduced hydroxyl, carboxyl, and carbonyl groups after modification can be acted as the growth point when the sulfur precipitated by the method mentioned above. And the research on the functional groups of MWCNTs will be seen in further reports. Acknowledgments The authors gratefully acknowledge the financial support by the Key Project of NSFC (200933005, 20903077), National 973 Program
(2009CB220102) and the Key Project Founded by Fujian province (2008H0087) and NFFTBS (No. J0630429). References [1] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. Van Schalkwijk, Nature Materials 4 (2005) 366. [2] S.F. Yang, P.Y. Zavalij, M.S. Whittingham, Electrochemistry Communications 3 (2001) 505. [3] L. Persi, F. Croce, B. Scrosati, Electrochemistry Communications 4 (2002) 92. [4] M. Yoshio, H. Nakamura, Y.Y. Xia, Electrochimica Acta 45 (1999) 273. [5] Y.Y. Xia, K. Tatsumi, T. Fujieda, P.P. Prosini, T. Sakai, Journal of the Electrochemical Society 147 (2000) 2050. [6] L.X. Yuan, J.K. Feng, X.P. Ai, Y.L. Cao, S.L. Chen, H.X. Yang, Electrochemistry Communications 8 (2006) 610. [7] H.S. Ryu, H.J. Ahn, K.W. Kim, J.H. Ahn, K.K. Cho, T.H. Nam, J.U. Kim, G.B. Cho, Journal of Power Sources 163 (2006) 201. [8] Y.S. Choi, S. Kim, S.S. Choi, J.S. Han, J.D. Kim, S.E. Jeon, B.H. Jung, Electrochimica Acta 50 (2004) 833. [9] Y.J. Choi, Y.D. Chung, C.Y. Baek, K.W. Kim, H.J. Ahn, J.H. Ahn, Journal of Power Sources 184 (2008) 548. [10] Y.M. Lee, N.S. Choi, J.H. Park, J.K. Park, Journal of Power Sources 119 (2003) 964. [11] J.W. Choi, J.K. Kim, G. Cheruvally, J.H. Ahn, H.J. Ahn, K.W. Kim, Electrochimica Acta 52 (2007) 2075. [12] B. Zhang, C. Lai, Z. Zhou, X.P. Gao, Electrochimica Acta 54 (2009) 3708. [13] D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato, Chemical Reviews 106 (2006) 1105. [14] A. Hirsch, Angewandte Chemie-International Edition 41 (2002) 1853. [15] D. Marmorstein, T.H. Yu, K.A. Striebel, F.R. McLarnon, J. Hou, E.J. Cairns, Journal of Power Sources 89 (2000) 219. [16] Y.V. Mikhaylik, J.R. Akridge, Journal of the Electrochemical Society 151 (2004) A1969.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The preparation of nano-sulfur/MWCNTs and its electrochemical performance Jia-jia Chen, Xin Jia, Qiu-jie She, Chong Wang, Qian Zhang, Ming-sen Zheng, Quan-feng Dong ∗ State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China
a r t i c l e
i n f o
Article history: Received 23 November 2009 Received in revised form 15 January 2010 Accepted 15 January 2010 Available online 25 January 2010 Keywords: Nano-sulfur/MWCNTs composite Sulfur-fixed matrix Modified MWCNTs Lithium–sulfur battery Nanomaterial
a b s t r a c t In this work, a novel nano-sulfur/MWCNTs composite with modified multi-wall carbon nano-tubes (MWCNTs) as sulfur-fixed matrix for Li/S battery is reported. Based on different solubility of sulfur in different solvents, nano-sulfur/MWCNTs composite was prepared by solvents exchange method. The composite was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The modified MWCNTs are considered that not only acts as a conducting material, but also a matrix for sulfur. The electrochemical performance of the nano-sulfur/MWCNTs composite was tested. The results indicated that nano-sulfur/MWCNTs composite had the specific capacity of 1380 mAh g−1 , 1326 mAh g−1 and 1210 mAh g−1 in the initial cycle at 100 mA g−1 , 200 mA g−1 and 300 mAh g−1 discharge rates respectively, and remained a reversible capacity of 1020 mAh g−1 , 870 mAh g−1 and 810 mAh g−1 after 30 cycles. The electrochemical performances confirm that the modified MWCNTs as sulfur-fixed matrix show better ability than any other carbon in cathode of Li/S batteries that had been reported. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction In order to meet the challenge of global warming and the finite nature of fossil fuels, it is now essential that new, low-cost and environmentally friendly energy conversion and storage systems are found. The performance of these power supply devices depends intimately on the properties of their materials which hold the key to fundamental advances in energy conversion and storage [1]. Lithium ion battery is now the most promising portable storage device. However, the first and most popular cathode material LiCoO2 has many restrictions on safety and toxicity issues because of its immanence nature. Although the new cathode materials, such as LiFePO4 [2,3] and layer-structured LiMnO2 [4,5] are developed for replacing LiCoO2 in lithium battery, all these transition metal oxide cathode materials show relatively low capacities. A battery based on lithium/sulfur has a theoretical capacity of 1675 mAh g−1 and a theoretical specific energy of 2600 Wh kg−1 , assuming complete reaction to the product Li2 S. Therefore, there is a strong incentive to develop Li/S batteries. Nowadays, there are many researchers focus on finding appropriate sulfur composites to meet the serious problems of low utilization and poor cycle life in sulfur cathodes. However, sulfur is an ionic and electron insulator, it is hard to operate a pure sulfur cathode material at high charge–discharge rates. In order
∗ Corresponding author. E-mail address: [email protected] (D. Quan-feng). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.01.069
to achieve good performance of Li/S battery, dimension of sulfur particles should be reduced. As reported in Ref. [1], the nanomaterial has a better accommodation of the strain of lithium transfer which is considered to have a contribution on improving the cycle life, and a better electrode/electrolyte interface leading to higher charge/discharge rates. The path for electron and Li+ transport was also shorten. Meanwhile, carbon is introduced as a conductive agent which is significant for current collection and, hence, the rate performance and sulfur utilization [6–8]. Choi [8] showed that carbon not only acted as electrical conductor but also supplied electrochemical reaction sites for sulfur. The key to improve Li/S battery is to increase electron conductivity in cathodes. In prior studies, there were two ways to prepare sulfur/carbon composites: thermo-treating and ball milling [9–12]. The first discharge capacities of composites in those published work were low, and they dropped down to 500 mAh g−1 after several cycles even at low rates. Sulfur is difficult to be dispersed with carbon homogeneously resulting in a low sulfur utilization, and the poor cyclic ability of the battery which is caused by sulfur running out from the electrode. In this work, we report a new method to synthesis a high performance composite cathode material for Li/S battery by solvent exchange. The MWCNTs are attractive choice as the electric conductor for sulfur cathode because they could provide more effective electronically conductive network than other conductive additives, such as carbon black (CB), acetylene black (AB) and graphite. Most of all, modified surfaces would improve the interaction of MWCNT with solvents and dispersion, and even enhance MWCNTs adsorption properties to allow grafting of nanoparticles [13,14]. Above all the electrochemical
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performances of the nano-sulfur/MWCNTs composites as cathode material for rechargeable lithium batteries are investigated. 2. Experimental 2.1. Preparation of materials Raw MWCNTs (20–40 nm, ShenZhen, China) were soaked in concentrated nitric acid at 120 ◦ C for 2 h and then washed by distilled water until the filtrate’s pH value reached 7.0. The aim of this procedure was not only to purify the MWCNT, but also to modify the surface of MWCNTs with functional groups. Then, 0.1 g modified MWCNTs were dispersed into 200 ml 1% (w/w) sodium dodecyl sulfate (SDS, AR) aqueous solution, while the sublimation sulfur (AR) dissolved into the purified tetrahydrofuran (THF) to get a saturated solution. After that, 30 ml saturated solution (nearly 0.16 mol l−1 ) was added into the solution containing modified MWCNTs and SDS with continuously strong stirring. The suspension was separated and washed with distillated water three times to remove SDS. The weight ratio of sulfur and modified MWCNTs was 6:4. The reference sample was pure sulfur in nanoscale which prepared by the same method. The morphology of these products was investigated by using scanning electron microscopy (SEM) on a HITACHI S-4800, and their XRD patterns were taking place on a Panalytical X’pert PRO X-ray diffraction (XRD) using Cu K␣ radiation.
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sulfur composites were used to fabricate cathodes. One was nanosulfur/MWCNTs composite mentioned above which was shorted as sample A; another was pure nano-sulfur mixed with the same weight of acetylene black which was named as sample B. All cathodes comprised the same proportion of sulfur (48%) and 10 wt% water soluble polymer n-lauryl acrylate (Chengdu, China). The cathode slurries were prepared by ball milling and then coated onto Al foils. The electrodes were dried at 50 ◦ C under vacuum over night. The testing cells, CR2016-type coin cells, used lithium foils as the counter electrodes, Cellgard2400 microporous membrane as separator and 1.0 mol l−1 LiCF3 SO3 dissolved in dioxolane (DOL) and diethylene glycol dimethyl ether (DEGDME) (1:2, v/v) as the electrolyte. The cells were assembled in an Argon-filled glove box. The discharge–charge tests were carried out with a NEWARE BTS-5 V/5 mA type battery charger (Shenzhen, China) at current densities of 100 mA/g, 200 mA/g and 300 mA/g (base on sulfur) with the voltage range 1.5–3.0 V. The specific capacity was calculated on the basis of the active sulfur material. Cyclic voltammetry (CV) experiments were conducted using a Princeton PAR273A potentiostat at scan rates of 0.1 mV s−1 . Electrochemical impedance spectra (EIS) of the cells were performed using a Zahner IM6 electrochemical workstation, which was carried out in a frequency range between 100 kHz and 100 mHz at potentiostatic signal amplitudes of 5 mV. All experiments were conducted at room temperature. 3. Results and discussion
2.2. Preparation and electrochemical testing of the cathodes In order to investigate the effect of modified MWCNTs on the electrochemical performance of sulfur cathode, two types of
Fig. 1 shows the morphological characterization of raw sulfur, MWCNTs, nano-sulfur and sample A. The raw sulfur has a particle size distribution of 5–100 m (Fig. 1a). The sulfur particles which
Fig. 1. SEM images of (a) raw sulfur; (b) nano-sulfur (sample B); (c) MWCNTs and (d) nano-sulfur/MWCNTs composite (sample A).
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Fig. 2. XRD patterns of (a) MWCNTs, (b) sample B and (c) sample A.
prepared by the solvent exchange method are much smaller than raw sulfur, and the size distribution is unanimous nanoscale which the scale range of sulfur particles is 50–100 nm (Fig. 1b). The diameter of the modified MWCNTs was about 20–40 nm (Fig. 1c). The modified MWCNTs have many functional groups, such as hydroxyl, carboxyl, and carbonyl, can act as the growth points when the sulfur precipitated by the solvent exchange method. The functional groups also provide an intimate contact between sulfur and MWCNTs. While the modified MWCNTs are introduced as sulfurfixed matrix and conducting materials, the sulfur coat uniformly on the surface of MWCNTs. The diameter of sample A was less than 100 nm, which means the sulfur layer is about 40 nm, compared with the MWCNTs without sulfur (Fig. 1d). The structure of products of nano-sulfur, MWCNTs and sample A which are further confirmed by the XRD tests. The results are showed in Fig. 2. The reflections of the nano-sulfur are consistent with Fddd orthorhombic structure. Compared with the pattern of the nano-sulfur, the XRD spectra of sample A exhibit the same trend, and the peak of MWCNTs center at 2 = 26◦ is disappeared. This indicates that no phase transformation of sulfur occurred during the preparation and the sulfur particles are well coated on the surface of the MWCNTs, so there are not any obvious peaks of the MWCNTs. The SEM and XRD results confirm that sulfur particles can distribute uniformly on the surface of MWCNTs while the MWCNTs are modified by the concentrated HNO3 . The two types of sulfur composites, sample A cathode and sample B, were tested to investigate the effect of modified MWCNTs on the electrochemical performance of the sulfur cathode. Fig. 3 depicts initial discharge curves of those two sulfur cathodes at various rates. All discharge curves show two plateaus based on the voltage profiles. The detail mechanisms of electrochemical oxidation and reduction of sulfur, polysulfides and lithium sulfides during discharge/charge were already reported elsewhere [15,16]. The upper plateau (∼2.41 V) is well known as the ring opening of S8 to the linear high order lithium polysulfides (Li2 Sn , n ≥ 4). The lower plateau (∼2.07 V) is caused by the reduction of high order lithium polysulfides to low order lithium polysulfides (Li2 Sn , n < 4), even to Li2 S. Sample B shows a high specific capacity of 1270 mAh g−1 in initial discharge at the current density of 100 mA g−1 which means it has a good utilization of sulfur. However, with the discharge rates increasing, the discharge capacities decrease greatly to 1150 mAh g−1 and 900 mAh g−1 , while at 200 mA g−1 and 300 mA g−1 , respectively. It is caused by low electron conductivity of sulfur baffles electrons entering/exiting sulfur outer orbital. The great electrochemical resistance could not be ignored that increasing current density causes high over
potential decreases discharge voltage, declines sulfur efficiency simultaneity. Meanwhile, the discharge plateau voltages decline greatly when sample B suffers from high discharge current densities. Compared to sample B, we can find that sample A shows better rate performance than sample B. The initial discharge capacities are up to 1380 mAh g−1 , 1330 mAh g−1 and 1210 mAh g−1 , at 100 mA g−1 , 200 mA g−1 and 300 mA g−1 severally. Moreover, the discharge plateau voltages only drop a little bit with increasing discharge currents. These phenomena show that sample A has many advantages than sample B due to obviously influence of MWCNTs. MWCNTs enhance cathode electron conductivity on three ways: (1) MWCNTs themselves are very good conductive framework; (2) Functional groups on the surface of modified MWCNTs can provide an intimate contact between sulfur and MWCNTs arising from the properties of sulfur-fixed as well as polysulfides with nanoscale dimensions, thus affords excellent accessibility of the active material; (3) MWCNTs support reaction sites for sulfur and polysulfides and the entrapment of the sulfur and polysulfides ensures that a more complete redox process takes place, and results in enhanced utilization of the active sulfur material. Besides, hollow tube like sulfur shorts electron transfer path and has larger surface than cubic nano-sulfur. The cyclic voltammograms of the Li/S cells with two composite cathodes were measured at the scan rate of 0.1 mV s−1 as shown in Fig. 4. The 2.4 V anodic peaks are related to the open ring reaction that motioned before, and the peaks between 1.9 V and 2.0 V are caused by the reduction of the high order lithium polysulfides to low order lithium polysulfides (Li2 Sn , n < 4), even to Li2 S [15,16]. These two anodic peaks are corresponding to the two discharge plateau as depicted in Fig. 3. Compare the peaks of two cathodes, the anodic peaks of sample A cathode reveal narrow and high peaks
Fig. 3. First discharge curves of the two cathodes at different discharge rates of (a) 100 mA g−1 (b) 200 mA g−1 (c) 300 mA g−1 .
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Fig. 4. CV curves of the two cathodes at scan range: 3.0–1.5 V, and scan rate: 0.1 mV s−1 .
and the smaller E (potential difference between the peak couple) than those of sample B. The CV results indicate that the electrochemical polarization of sample B is stronger than that of sample A cathode. The cycle performance of the two cathodes in the electrolyte were tested at different rates is shown in Fig. 5. With decreasing sulfur particle sizes dimensions, sulfur utilization would be increased due to the increase of specific surface area. Sample B shows a good utilization of active material in the first discharge at a current density of 100 mA g−1 , the discharge capacity is nearly 1200 mAh g−1 .
Fig. 5. Cyclic and rate performance of Li/S cell with the two cathodes at various discharge rates of (a) 100 mA g−1 (b) 200 mA g−1 (c) 300 mA g−1 .
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However, the discharge capacity drop to 1000 mAh g−1 in the 2nd cycle and gradually fades in the later cycles. After 30 cycles, the discharge capacity only remains at 650 mAh g−1 . Although the capacity after several cycles is lower compared to the first discharge capacity, it is still higher than many other sulfur and carbon composites [9–12]. This also confirms that the sulfur will show a better performance when its dimension was reduced. The cycle performances of sample B are similar to the result obtained at the current density of 100 mA g−1 when it was tested at other current densities of 200 mA g−1 and 300 mA g−1 . The irreversible loss of sample B maybe contributes to the dissolving of sulfur into the electrolyte. Meanwhile, it is difficult to prevent sample B from aggregation during the cycling. So, the modified MWCNTs are introduced as a sulfur-fixed matrix and a conducting material, which can provide good electrochemical reaction sites of sulfur. When sample A is tested as cathode at the same rates of sample B, it shows even better cycle performances than sample B. With a little capacities drop from the 1st to 2nd cycle, the discharge capacity retention is almost unchanged at 1100 mAh g−1 with a current density of 100 mA g−1 . Even with the increasing of discharge rates, the cycle life keeps at a high capacity without any obvious fading. This is may explained by the functional groups of MWCNTs modified by concentrated HNO3 . The functional groups help confine diffusion of polysulfides out of the cathode structure, minimize the loss of the active mass in the cathode and improve the cycling stability. Fig. 6 shows EIS plots of sulfur cathodes in their original statements before discharge. It can be seen from the figures that all the Nyquist plots of sulfur cathodes are composed by a semicircle at high frequencies corresponding to the contact resistance and charge transfer resistance and a short inclined line in low frequency regions due to the ion diffusion within the cathode. Before discharge, sample B shows higher charge-transfer resistances (Rct) than sample A. This is further evidence to show better conductivity of sample A. As all we mentioned above, when sulfur particles are reduced to nanoscale and dispersed uniformly in MWCNTs via a simple solvent exchange method, the electrochemical performance is greatly enhanced. The modified MWCNTs here are not simply acted as conducting materials, but also a well sulfur-fixed matrix. The functional groups, such as hydroxyl, carboxyl, and carbonyl groups, can be acted as the growth point when the sulfur precipitated by the solvent exchange method mentioned above. And those groups maybe further provide a chemical gradient that retards diffusion of polysulfide anions out of the electrode, thus facilitating more complete reaction and improving the cycling stability.
Fig. 6. Electrochemical impedance spectra (EIS) of cells with (a) sample A and (b) sample B before initial discharge.
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4. Conclusions The nano-sulfur/MWCNTs composite was prepared by solvent exchange method. This is a new method to prepare sulfur cathode materials. It can reduce the dimension of the sulfur, thus enhance the electrochemical performance. However, the pure nano-sulfur cannot prevent aggregation during the cycling. So, the MWCNTs modified by concentration HNO3 were introduced with the aim to fix sulfur and prevent the sulfur from aggregation. The functional groups on the carbon surface can play an important role on enhance the cycling of Li/S battery. Other researchers also use MWCNTs to prepare the sulfur composites, but the MWCNTs were without any treat. The results of those cathodes are still poor in cycling at high capacity retention and high charge–discharge rates. The modified MWCNTs can improve MWCNTs interaction with solvents and dispersion, and even can modify MWCNTs adsorption properties to allow the grafting of nanoparticles. The introduced hydroxyl, carboxyl, and carbonyl groups after modification can be acted as the growth point when the sulfur precipitated by the method mentioned above. And the research on the functional groups of MWCNTs will be seen in further reports. Acknowledgments The authors gratefully acknowledge the financial support by the Key Project of NSFC (200933005, 20903077), National 973 Program
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