sulfur batteries

sulfur batteries

G Model ARTICLE IN PRESS APSUSC-27636; No. of Pages 5 Applied Surface Science xxx (2014) xxx–xxx Contents lists available at ScienceDirect Applie...

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ARTICLE IN PRESS

APSUSC-27636; No. of Pages 5

Applied Surface Science xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Multiwalled carbon nanotubes-sulfur composites with enhanced electrochemical performance for lithium/sulfur batteries Xin Zhou Ma, Bo Jin ∗ , Pei Ming Xin, Huan Huan Wang Key Laboratory of Automobile Materials, Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130025, China

a r t i c l e

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Article history: Received 26 February 2014 Received in revised form 3 April 2014 Accepted 4 April 2014 Available online xxx Keywords: Multiwalled carbon nanotubes-sulfur composites Cathode materials Lithium–sulfur batteries

a b s t r a c t Multiwalled carbon nanotubes-sulfur (MWCNTs-S) composites were synthesized by chemical activation of MWCNTs and capillarity between sulfur and MWCNTs. The MWCNTs activated by potassium hydroxide (denoted as K-MWCNTs) were used as conductive additive. The as-prepared K-MWCNTs-S composites can display excellent cycle stability and rate capability with the initial discharge capacity of 741 mAh g−1 and capacity retention of 80% after 50 cycles compared to pure S. The improvement in the electrochemical performance for K-MWCNTs-S composites is attributed to the interstitial structure of the MWCNTs resulted from the strong chemical etching, which can facilitate the insertion and extraction of Li ions and more better percolation of the electrolyte, and also ascribed to enhanced electronic conductivity of K-MWCNTs-S composites. It is indicated that the K-MWCNTs-S composites can be used as the cathode materials for lithium–sulfur batteries. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Rechargeable lithium ion batteries (LIBs) have been used widely in mobile phones, laptop computers, digital cameras, electrical vehicles and hybrid electrical vehicles [1–4]. In rechargeable LIBs, the cathode material is a key component mainly relating to the performance of the batteries. The elemental sulfur is an attractive cathode active material due to low cost, low equivalent weight, no toxicity, high theoretical specific capacity of 1675 mAh g−1 and high energy density of 2600 Wh kg−1 [5], assuming the complete reaction of lithium with sulfur to Li2 S. However, both its electronic and ionic insulting nature leads to poor capacity retention upon cycling and limits the practical applications [6]. In the meantime, the elemental sulfur is reduced into intermediate lithium polysulfides Li2 Sn (n > 2) which are highly soluble in common organic electrolytes. Nevertheless, some of the highly soluble polysulfides are reduced into insoluble Li2 S when contacting with lithium and leads to poor capacity retention. The current method to resolve poor cycling performance is adding conductive additives, restricting the movement of intermediate polysulfides and coating LiFePO4 onto the surface of sulfur [7–10].

∗ Corresponding author. Tel.: +86 431 85095170. E-mail addresses: [email protected], [email protected] (B. Jin).

Carbon nanotubes (CNTs) have been widely used in LIBs due to their many unusually mechanical, electronic, magnetic, physical and electrochemical properties, as introduced as follows. Jin et al. [11] demonstrated that the added MWCNTs not only increased the electronic conductivity and lithium-ion diffusion coefficient but also decreased crystallite size and charge transfer resistance of LiFePO4 -MWCNTs composite. Yuan et al. [12] prepared a novel sulfur-coated MWCNTs composites material with a reversible capacity of 670 mAh g−1 after 60 cycles. Yin et al. [13] synthesized a novel [email protected] core-shell composite via in situ polymerization of acrylonitrile on the surface of MWCNTs, mixing with sulfur and final pyrolysis, and indicated that the homogeneous dispersion and integration of MWCNTs in the composite created an electronically conductive network and reinforced the structural stability, leading to the outstanding electrochemical performance as a cathode material for rechargeable lithium–sulfur batteries. In this study, multiwalled carbon nanotubes-sulfur (MWCNTsS) composites were synthesized by chemical activation of MWCNTs and capillarity between sulfur and MWCNTs [14]. The MWCNTs activated by potassium hydroxide (denoted as K-MWCNTs) were used to open the end, shorten the tube length and create more nanopores on the wall [15]. This provided more channels and cavities for the free migration of Li ions and more better percolation of the electrolyte during the discharge/charge process, leading to the improved electrochemical properties. The structural and morphological performance of K-MWCNTs-S composites was investigated

http://dx.doi.org/10.1016/j.apsusc.2014.04.036 0169-4332/© 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: X.Z. Ma, et al., Multiwalled carbon nanotubes-sulfur composites with enhanced electrochemical performance for lithium/sulfur batteries, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.04.036

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Fig. 1. The synthesis schematic diagram of K-MWCNTs-S. The black lines represent the MWCNTs and the yellow area on the MWCNTs surface is sulfur. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

by X-ray diffraction, high-resolution transmission electron microscope, scanning electron microscopy and field emission scanning electron microscope, and the electrochemical properties were analyzed by cyclic voltammograms and galvanostatic discharge/charge tests. 2. Experimental 2.1. Chemical and synthesis Multi-walled carbon nanotubes (MWCNTs, Nanjing XFNANO Materials Technology Co., Ltd.) with length of 10–20 ␮m and diameter of 30–50 nm were activated with potassium hydroxide (KOH),

Fig. 2. XRD patterns of (a) MWCNTs, (b) K-MWCNTs, (c) K-MWCNTs-S and (d) pure S.

as reported elsewhere [15]. Firstly, 0.5 g MWCNTs and 3.5 g KOH were stirred for 1 h in a 20 ml aqueous solution at room temperature and then dried in a vacuum oven at 100 ◦ C for 24 h. Next, the mixture was heated at 700 ◦ C for 2 h in a tubular furnace under the protection of nitrogen. The heat-treated MWCNTs were washed with deionized water several times to eliminate K2 O, K2 CO3 and residual KOH, and then dried at 60 ◦ C. The achieved black powder was denoted as K-MWCNTs.

Fig. 3. SEM image of (a) pure S and FESEM images of (b) MWCNTs, (c) K-MWCNTs and (d) K-MWCNTs-S.

Please cite this article in press as: X.Z. Ma, et al., Multiwalled carbon nanotubes-sulfur composites with enhanced electrochemical performance for lithium/sulfur batteries, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.04.036

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Fig. 4. TEM images of (a) MWCNTs, (b) K-MWCNTs and (c) K-MWCNTs-S; HRTEM images of (d) MWCNTs, (e) K-MWCNTs and (f) K-MWCNTs-S.

K-MWCNTs and sulfur (at a 1:5 mass ratio) were mixed in 6 ml carbon disulfide (CS2 ) and then sonicated for 1 h. When the CS2 was utterly vaporized at room temperature, the mixture was ground for 30 min and then heated at 155 ◦ C for 24 h under N2 atmosphere in a tubular furnace. After cooling down to room temperature, the K-MWCNTs-S composites were obtained. By calculating the mass change of the mixture before and after heat treating, we knew that the sulfur content in the composites was 75 wt.%. The synthesis route for K-MWCNTs-S composites is illustrated in Fig. 1. 2.2. Material characterizations and performance measurements The crystalline phases were identified with X-ray diffraction (XRD, Dmax/2500PC, Rigaku, Japan) with Cu K␣ radiation ˚ Powder morphologies were observed by scanning ( = 1.5406 A). electron microscope (SEM, ZEISS EVO 18, Germany), field emission scanning electron microscope (FESEM, JSM-6700F, Japan) and highresolution transmission electron microscope (HRTEM, FEI Tecnai G2 F20, operating at an accelerating voltage of 200 kV, USA). The working electrode was prepared by mixing K-MWCNTs-S or pure S with acetylene black and polyvinylidene fluoride in a weight ratio of 80:10:10 in N-methyl-2-pyrrolidinone. The cointype batteries (CR2025) were assembled with lithium metal as the counter/reference electrode and 1 M LiCF3 SO3 in a mixed solvent of dimethoxyethane and dioxolane with a volume ratio of 50:50 as electrolyte. The automatic discharge/charge equipment (LAND CT2001A) was used to perform the galvanostatic discharge/charge tests in a potential range of 1.5–3.0 V at room temperature. The total amount of active material sulfur in the working electrode was used to estimate the specific capacity of battery. Cyclic voltammogram (CV) measurements were carried out on an electrochemical workstation (CHI650D, Shanghai Chen Hua Instruments Ltd.) at a scan

rate of 0.1 mV s−1 from 1.5 to 3.0 V. As a comparison, pure sulfur cathode was also prepared by the same process just in the absence of K-MWCNTs. 3. Results and discussion 3.1. Characterization of the products XRD patterns of pure S, MWCNTs, K-MWCNTs and K-MWCNTsS are shown in Fig. 2. All the patterns of pure S can be indexed to a material having an orthorhombic structure, which is the same as the one listed in the X-ray powder diffraction data file (JCPDS card number 24-0733) by the American Society for Testing Materials as standard. The strong (0 0 2) diffraction peak intensity of the MWCNTs at ca. 26◦ decreases after activation by KOH, demonstrating that the tubular structure of the graphite layer has been destroyed partially and the diffraction peak intensities at ca. 42◦ and 51◦ increases. There is no obvious carbon diffraction peaks in the K-MWCNTs-S composites due to its low content. It is demonstrated that the added MWCNTs do not change the crystal structure of pure S. SEM image of pure S and FESEM images of MWCNTs, K-MWCNTs and K-MWCNTs-S are shown in Fig. 3. As shown in Fig. 3(a), the particle sizes of pure S are about 5–30 ␮m and not uniform. It is obvious that the diameters of the MWCNTs are enlarged after activation by KOH. This extraordinary change of the tubular structure for MWCNTs can be explained by hypothesizing that the carbon atoms at the tips of the nanotubes were eroded away, allowing the tips to open. Because the tips were open, molten KOH was intercalated into the interior cavity and graphitic layers of the MWCNTs, eroding the interior graphitic layer. The graphitic layer was then exfoliated owing to redox reaction [15,16]. As shown in Fig. 3(d), the diameter

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Fig. 6. Discharge and charge curves of (a) pure S and (b) K-MWCNTs-S after different cycles at a current density of 200 mA g−1 . Fig. 5. Cyclic voltammograms of (a) pure S and (b) K-MWCNTs-S at a scan rate of 0.1 mV s−1 at room temperature.

3.2. Performance of pure S and K-MWCNTs-S in lithium-ion batteries of K-MWCNTs-S is slightly larger than that of K-MWCNTs due to the tubular layer of sulfur coating onto the MWCNTs surface and some molten sulfur absorbing into the interior cavity of the K-MWCNTs through capillarity force and surface tension [14]. In order to observe further the change of the MWCNTs after KOH etching and morphological performance of K-MWCNTs-S, TEM and HRTEM were carried out, the results are shown in Fig. 4. Fig. 4(b) demonstrates that the tips of the MWCNTs are opened after KOH etching. As shown in Fig. 4(b) and (e), the change of the MWCNTs after etching is as follows: the tips were open; a large number of nanopores and nanoholes (indicated with white arrows) are obtained, as demonstrated by other literatures [15,16]. This is due to strong chemical etching resulted from potassium stream and carbon dioxide during reaction process of MWCNTs and KOH. KOH is a useful activating reagent for activated carbon materials between C and KOH owing to the redox reaction (C + 4KOH → 4K + CO2 + 2H2 O). It can be noted that KOH etching, which contributes to these extraordinary changes, will help to increase filling yield of the active material S and create more channels for the insertion and extraction of Li ions and more better percolation of the electrolyte resulted from the increased pore volume and specific surface area, Niu et al. [17] also demonstrated that the pore volume and specific surface area of the MWCNTs increased after KOH etching. Fig. 4(c) and (f) indicate that the sulfur is coated onto the MWCNTs surface and the diameter of K-MWCNTs-S is slightly larger compared to K-MWCNTs.

Cyclic voltammograms of pure S and K-MWCNTs-S at a scan rate of 0.1 mV s−1 at room temperature are shown in Fig. 5. It is obvious that two reduction peaks at around 2.3 and 2.0 V and one oxidation peak at about 2.5 V are observed in both pure S and K-MWCNTs-S in the first cycle, as described elsewhere [12,13,18–21]. The peak at around 2.3 V is ascribed to the reduction of sulfur to form the higher order lithium polysulfides (Li2 Sn , n  4). The peak at about 2.0 V corresponds to further reduction of these lithium polysulfides to lower order lithium polysulfides (Li2 Sn , n < 4), even to Li2 S. The oxidation peak at about 2.5 V can be attributed to the oxidation of lithium polysulfides (Li2 Sn , n < 4) and Li2 S to Li2 S8 . As for pure S, both the reduction and the oxidation peak current density greatly decrease during the following cycles compared to the first cycle. Furthermore, in the case of K-MWCNTs-S, both the reduction and the oxidation peak current density have no great change. It is demonstrated that the reversibility and reactivity of K-MWCNTs-S are better than that of pure S. Discharge and charge curves of pure S and K-MWCNTs-S after different cycles at a current density of 200 mA g−1 are shown in Fig. 6. As for pure S and K-MWCNTs-S, all the discharge curves display two typical plateaus ascribed to the two-step reaction of sulfur with lithium during the discharge process, which is consistent with the results of CV measurements (Fig. 5). The upper plateau is assigned to the change of sulfur to lithium polysulfides (Li2 Sn , n  4), and the lower plateau can be attributed to further reduction

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just showed an initial specific capacity of about 445 mAh g−1 at a current density of 40 mA g−1 and could not be charged again after the first discharge process. He et al. [23] reported that the S-OCNT with 72.6 wt.% sulfur and the S-PCNT with 74.4 wt.% sulfur composite cathodes delivered the discharge capacity of about 600 mAh g−1 after 50 cycles at a current density of 100 mA g−1 . Fig. 8 obviously demonstrates that the rate performance of the K-MWCNTs-S is better than that of pure S, and the K-MWCNTs-S can deliver a discharge capacity of 525 mAh g−1 at a current density of 550 mA g−1 , however, only 66 mAh g−1 for pure S at the same current density. 4. Conclusions

Fig. 7. Cycling performance of (a) pure S and (b) K-MWCNTs-S at a current density of 200 mA g−1 at room temperature.

of these lithium polysulfides to lithium polysulfides (Li2 Sn , n < 4), even to Li2 S. Cycling performance of pure S and K-MWCNTs-S at a current density of 200 mA g−1 at room temperature is shown in Fig. 7. Rate performance of pure S and K-MWCNTs-S at different current densities is shown in Fig. 8. As shown in Fig. 7, the K-MWCNTsS delivers the discharge capacities of 741 mAh g−1 in the initial cycle and 592 mAh g−1 after 50 cycles, respectively. However, the discharge capacity of pure S is 296 mAh g−1 in the first cycle, and drops to 111 mAh g−1 after 50 cycles. It is obvious that the cycling stability and the discharge capacity of the K-MWCNTs-S is enhanced compared to pure S. The improvement in the electrochemical performance for K-MWCNTs-S composites is attributed to the interstitial structure of MWCNTs resulted from the strong chemical etching. The interstitial MWCNTs can deposit sulfur onto or into interstitial framework to improve the electronic conductivity of insulate sulfur and consequently provide a fast path for electron transportation. Moreover, the interstitial structure can facilitate the insertion and extraction of Li ions and more better percolation of the electrolyte resulted from the increased pore volume and specific surface area. The electrochemical performance of the K-MWCNTs-S in our study is also excellent compared to other reports [12,22,23]. Yuan et al. [12] demonstrated that the S-coatedMWCNTs cathode with 68 wt.% sulfur gave a reversible capacity of 670 mAh g−1 after 60 cycles at a current density of 100 mA g−1 . Lai et al. [22] reported that the composite electrode with 75 wt.% sulfur

The K-MWCNTs-S composites have been successfully synthesized by chemical activation of MWCNTs and capillarity between sulfur and MWCNTs. The K-MWCNTs-S can deliver the discharge capacity of 592 mAh g−1 after 50 cycles. However, the discharge capacity of pure S is only 111 mAh g−1 after 50 cycles. It is demonstrated that the cycling stability and the discharge capacity of the K-MWCNTs-S is enhanced compared to pure S. The improvement in the electrochemical performance for K-MWCNTs-S composites is attributed to the interstitial structure of MWCNTs resulted from the strong chemical etching, which can facilitate the insertion and extraction of Li ions and more better percolation of the electrolyte, and also ascribed to enhanced electronic conductivity of K-MWCNTs-S composites. These results indicate that the KMWCNTs-S can be used as the cathode material for lithium–sulfur batteries. Acknowledgments The authors acknowledge the financial supports from The Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; International Science and Technology Cooperation Plan, Science and Technology Bureau of Changchun City (Grant No. 11GH05). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

Fig. 8. Rate performance of (a) pure S and (b) K-MWCNTs-S at different current densities.

[22] [23]

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Please cite this article in press as: X.Z. Ma, et al., Multiwalled carbon nanotubes-sulfur composites with enhanced electrochemical performance for lithium/sulfur batteries, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.04.036