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Article
High-performance Lithium-Sulfur Batteries with a Self-assembled MWCNT Interlayer and a Robust Electrode-Electrolyte Interface Hee Min Kim, Jang Yeon Hwang, Arumugam Manthiram, and Yang-Kook Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10812 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 26, 2015
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High-Performance Lithium-Sulfur Batteries with a Self-Assembled MWCNT Interlayer and a Robust Electrode-Electrolyte Interface Hee Min Kim,† Jang Yeon Hwang,† Arumugam Manthiram,‡ ,* and Yang-Kook Sun†,* †
Department of Energy Engineering, Hanyang University, Seoul, 133-791, South Korea, and
‡
Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, Austin, Texas, 78712, United States
ABSTRACT
Elemental sulfur electrode has a huge advantage in terms of charge-storage capacity. However, the lack of electrical conductivity results in poor electrochemical utilization of sulfur and performance. This problem has been overcome to some extent previously by using a bare multiwall carbon nanotube (MWCNT) paper interlayer between the sulfur cathode and the polymeric separator, resulting in good electron transport and adsorption of dissolved polysulfides. To advance the interlayer concept further, we present here a self-assembled MWCNT interlayer fabricated by a facile, low-cost process. The Li-S cells fabricated with the self-assembled MWCNT interlayer and a high loading of 3 mg cm-2 sulfur exhibit a first 1
discharge specific capacity of 1112 mAh g-1 at 0.1 C rate and retain 95.8% of the capacity at 0.5 C rate after 100 cycles as the self-assembled MWCNT interlayer facilitates good interfacial contact between the interlayer and the sulfur cathode and fast electron and lithiumion transport while trapping and reutilizing the migrating polysulfides. The approach presented here has the potential to advance the commercialization feasibility of the Li-S batteries.
Introduction The increase in global economic and population growth is creating an increasing demand for energy. The rapid depletion of fossil fuels and the environmental concerns associated with fossil fuels are prompting the use of renewable energy sources like solar and wind. However, the renewable sources are intermittent, so efficient storage of electricity produced from such renewable sources is critical. Rechargeable batteries are one of the prime approaches for electrical energy storage. The current lithium-ion battery technology based on transitionmetal oxide cathodes and graphite anode have limited energy storage capacity, so there is immense interest to develop alternate high-capacity cathode and anode materials. Lithiumsulfur (Li-S) batteries are appealing in this regard as they offer a high energy density of 2500 Wh kg-1 when coupled with lithium-metal anode due to the large charge-storage capacity of 2
sulfur.1-3 Moreover, sulfur is much more abundant, inexpensive, and non-toxic compared to the transition-metal oxide cathodes. However, there are several challenges with the Li-S batteries, such as low electrical conductivity of sulfur and the discharge product Li2S and the migration of dissolved polysulfides from the cathode to the anode, resulting in low efficiency and inadequate cycle life.4-7 Well-designed carbon-sulfur composites have been extensively explored to overcome these issues.8-10 In these strategies, sulfur was entrapped in carbon structures using porous carbon,11-14 carbon nanotubes (CNT),15-19 and hollow carbon spheres.20-24 Carbon backbone serves as the electron conducting path for sulfur and reduce polysulfide migration by trapping the polysulfides during cycling. However, polysulfide migration still occurs and cycle life is inadequate for practical applications. Manthiram group and other groups introduced the use of carbon-paper interlayers in between the sulfur cathode and the polymer separator or carbon-coated membrane and demonstrated high sulfur utilization with better cycle life due to the suppression of polysulfide diffusion to the anode and the serving of the carbon-paper interlayer as a pseudo-upper current collector. 1, 25-30 We present here the fabrication of an interlayer consisting of MWCNT and the electrolyte in a single fabrication process. Such MWCNT-electrolyte-paper interlayers are much more effective compared to the bare MWCNT-paper interlayer due to the fast ionic and electronic transport and a better interface between the sulfur cathode and the interlayer as well as between the separator and the interlayer.
1 M lithium bis(trifluoromethane sulfonyl)imide in 1,3-dioxolane (DOL) and 1,2dimethoxyethane (DME) (1:1 volume) containing 0.4 M LiNO3 was used as the base electrolyte. In a typical experiment, 0.25 g of MWCNT (Nanolab Inc.) was added to a vial containing 5 mL of the electrolyte inside an Ar-filled glove box and the mixture was vigorously stirred. Then, 60–100 ㎕ of the dispersed MWCNT-electrolyte mixture (3-5 mg MWCNT, electrolyte/sulfur ratio: 13) was applied to the top of the sulfur electrode, which was prepared as described in the section below. For a comparison, the bare MWCNT interlayer was prepared by dispersing the MWCNT in water, filtering, peeling off, and drying.
Electrochemical measurements To prepare the cathode, elemental sulfur powder was mixed with 10 wt. % of carbon (Acetylene Black) and 10 wt. % of PVdF binder in N-methylpyrrolidone (NMP). The homogeneously mixed slurry was then cast onto an aluminum foil and dried at 50℃ for 24 h in a vacuum oven. Cathode disks were then punched out of the cast electrode (electrode size: 14 mm diameter (1.54 cm2) with the following: average sulfur loading amount: 3 - 5 mg cm-2, sulfur content in the total mass of the sulfur cathode: 80 wt.%; sulfur content in the total mass of the sulfur cathode with bare MWCNT interlayer and self-assembled MWCNT interlayer: 52.6 wt.%). Electrochemical testing was performed with R2032 coin-type cells, employing lithium foil as the anode. A microporous polypropylene film (Celgard 2400) was used to separate the cathode and the anode. In the case of MWCNT-electrolyte-paper interlayer, it was placed between the sulfur cathode and the separator without adding additional liquid electrolyte. In the case of bare MWCNT paper (, it was first placed on the sulfur cathode, 4
followed by the addition of liquid electrolyte, and the separator; the liquid electrolyte was 1 M lithium bis(trifluoromethane sulfonyl)imide in 1,3-dioxolane (DOL) and 1,2dimethoxyethane (DME) (1:1=v:v) containing 0.4 M LiNO3. Note that the electrolyte/sulfur ratio (E/S ratio) was approximately 13. The cells were typically cycled in a constant current mode at a 0.5 C rate within the voltage range of 1.9 – 2.8 V versus Li/Li+ at 25℃. The rate specified in this research is based on the mass and theoretical capacity of sulfur (1,675 mAh g-1)
Characterization Morphology of the interlayers was observed with scanning electron microscopy (JSM 6400, JEOL Ltd., Japan) and transmission electron microscopy (JEOL 2010, JEOL Ltd., Japan). For the cross-section, samples of the interlayers were prepared by a focused ion beam system (NOVA 200, FEI Company, USA).
RESULTS AND DISCUSSION The schematic configuration of the bare MWCNT interlayer and the self-assembled MWCNT interlayer cells are shown in Figure 1, and the detailed preparation processes are shown in Figure S1. The electrodes of both cells were composed of elemental sulfur without the impregnation of any other substance. Impregnation usually involves a complicated process, so use of plain elemental sulfur provides energy density advantages. The preparation of the bare MWCNT interlayer involved MWCNT-solvent dispersion, filtration for film formation, drying, peeling off, and shaping (punching for a disk).25 In contrast, the selfassembled MWCNT interlayer preparation involved only a single step of dispersion of the components and then applying onto the top of the sulfur electrode. Thus, the formation 5
process of the self-assembled MWCNT interlayer is more efficient and facile. Both the selfassembled MWCNT interlayer and the bare MWCNT interlayer showed almost identical structure as seen in Figure S2. The morphologies of the bare MWCNT interlayer and the self-assembled MWCNT interlayer after extracting from the cycled cells were investigated with SEM, and are shown in Figure 2. The surfaces of both the interlayers show similar morphology where the MWCNTs are entangled with each other and fully covered the surface of the carbon-sulfur mixture cathode. However, some differences are observed in the entire integrated structure. The surface of the self-assembled MWCNT interlayer shows more tightly intertwined structure forming dense film, while the bare MWCNT interlayer shows several cracks and torn-out part. This incomplete integrated structure of MWCNT film is also observed at the edges of bare MWCNT interlayer. As the bare MWCNT interlayer was separately prepared before the cell assembly, this MWCNT film had its own shape and low flexibility, necessitating reforming their shape to fit to the shape of the sulfur-carbon cathode. The selfassembled MWCNT interlayer, on the other hand, shows a tightly integrated structure of the MWCNT film and the sulfur-carbon cathode. The well-entangled film structure and the tight integration between the self-assembled MWCNT interlayer and the sulfur-carbon cathode may efficiently block polysulfide diffusion and enhance sulfur utilization by providing polysulfide capturing sites and facile electron path. The capturing of dissolved polysulfides during cycling was investigated with TEM by observing the existence of sulfur in the interlayers recovered from the cycled (100 cycles) cells. In Figure 3a, the morphology of the pristine MWCNT shows the tube shape with smooth surface. However, the cycled self-assembled MWCNT interlayer shows a bumpy surface with sediments. To identify the sediments, electron energy loss spectroscopy (EELS) 6
was carried out with TEM. These sediments were confirmed to be sulfur element, which originally existed only in the elemental sulfur cathode of the fresh cell. The TEM and EELS images of cycled bare MWCNT interlayer also showed the same trend. During cycling, sulfur in the cathode has dissolved in the form of polysulfides, diffused out of the cathode, and then deposited onto the bare MWCNT interlayer and the self-assembled MWCNT interlayer as Manthiram’s group reported previously.31 The electrochemical performances of the cells without the interlayer, with the bare MWCNT interlayer (1.95 mg cm-2), and with the self-assembled MWCNT interlayer (1.95 mg cm-2) were investigated with sulfur cathode and lithium-metal anode as shown in Figure 4. All the electrodes were prepared with a sulfur loading density of about 3 mg cm-2, which is relatively high and is an important factor for practical cells. The cell without the interlayer shows poor sulfur utilization with a low initial discharge capacity of 182 mAh g-1 at 0.1 C rate due to the intrinsic insulating nature of sulfur and high polysulfide diffusion to the anode. In contrast, both the cells with the bare MWCNT interlayer and the self-assembled MWCNT interlayer show significantly improved electrochemical performances, with initial discharge capacities of, respectively, 1044 mAh g-1 and 1112 mAh g-1, due to enhanced sulfur utilization and suppressed polysulfide diffusion. While the cell with the bare MWCNT interlayer shows an initial capacity of 783 mAh g-1 at 0.5 C, the cell with the self-assembled MWCNT interlayer delivers a higher initial discharge capacity of 851 mAh g-1 at the same rate. Furthermore, the latter shows ~ 22% more capacity than the former after 100 cycles at 0.5 C rate, as shown in Figure 4c. The rate capabilities of the cells in Figure 4d were further investigated at a current density of 0.1 A g-1 to 3 A g-1. Similar to the above results, at a low current density of 0.1 A g-1 (about 0.06 C rate), the difference in the discharge capacity between the cells having the bare 7
MWCNT interlayer and the self-assembled MWCNT interlayer is not significant. However, at a high current density of 3 A g-1, the cell with the self-assembled MWCNT interlayer shows 2.4 times higher discharge capacity than the that with the bare MWCNT interlayer. The remarkably improved rate capability of the cell with the self-assembled MWCNT interlayer can be attributed to the uniformly integrated dense MWCNT film structure and the more intimate attachment of the MWCNT film to the pristine cathode as confirmed by the SEM images in Figure 2. To clarify the effect of structural benefits of the self-assembled MWCNT interlayer on the improved electrochemical performance, cross-sectional morphology of cycled electrodes was investigated with SEM and FIB. As shown in Figure 5, the bare MWCNT interlayer film section of the electrode possesses plenty of pores and cracks, while the self-assembled MWCNT interlayer section of the electrode shows only tiny pores and densely packed shape. In typical Li-S cells, pore formation in the electrode is beneficial for ion transport by serving as ion channels. However, huge and excessive pores diminish the benefits of MWCNT film on (i) capturing the dissolved polysulfides and (ii) providing electron path. Consequently, the self-assembled MWCNT interlayer provides highly interconnected electron path and sufficient ion path, while concurrently trapping the dissolved polysulfides, during cycling. The self-assembled MWCNT interlayer with a denser packed structure easily to suppresses the migration of polysulfides. Moreover, the self-assembled MWCNT interlayer contains an electrolyte different from the other interlayer. In general, when using the carbon layer with a dense structure, the electrolyte is difficult even to enter uniformly the inner film. Thus, it is difficult to activate elemental sulfur. On the other hand, despite the compact structure, the self-assembled MWCNT interlayer has good lithium-ion path due to the presence of sufficient electrolyte, resulting in high performance in the initial cycles.32,33 It is worth 8
mentioning again that the simple preparation for the self-assembled MWCNT interlayer is advantageous for fabrication of practical cells as there are no additional steps involved. In figure S4, the influence of the amount of MWCNT in the self-assembled MWCNT interlayer on the electrochemical performance is shown. If less than a specified amount of MWCNT is used, the cell performance is relatively low. Sufficient MWCNT is needed for the effective electrochemical utilization of sulfur. However, if the MWCNT amount exceeds a threshold value, then the performance drops as too much MWCNT may act as a blockage for ion diffusion. When the sulfur loading level is 2 mg cm-2, there is no significant difference between the cells having the bare MWCNT and self-assembled MWCNT interlayers, as seen in Figure S5a. However, at higher sulfur loadings of 3 or 4 mg cm-2, the cell with the self-assembled MWCNT interlayer shows better performance with stable cycle life than that with the bare MWCNT interlayer, as seen in Figure 4c and Figure S5b. The bare MWCNT interlayer was found to have cracks when its thickness is large (e.g., > 3 mg of MWCNT in the interlayer), as seen in Figure S4c, so the electrochemical performance is poor when the sulfur loading is high with the bare MWCNT interlayer. When we fabricated the cells with different sulfur loadings of 4 or 5 mg cm-2, but with the same weight (3 mg) of MWCNT in the interlayer, the cell performance deteriorated withy the bare MWCNT interlayer, as seen in Figure S6.
CONCLUSIONS A self-assembled MWCNT interlayer electrode was successfully prepared with a simple process for lithium-sulfur batteries. Through TEM and SEM characterizations, the selfassembled MWCNT interlayer was found to show a more compact structure and tight contact with the pristine carbon-sulfur electrode compared to the bare MWCNT electrode. The self9
assembled MWCNT interlayer electrode cell showed a discharge capacity of 1272 mAh g-1 at a current density of 0.1 A g-1 which is similar to that of the bare MWCNT interlayer electrode cell. However, as the current density increases gradually, the self-assembled MWCNT interlayer electrode cell showed batter rate capability and cycling performance compared to the pristine electrode cell and the bare MWCNT interlayer electrode cell. At a current density of 3.0 A g-1, the self-assembled MWCNT interlayer electrode cell delivered a discharge capacity of 406 mAh g-1, which is 2.4 times higher than that of the bare MWCNT interlayer electrode cell. The aforementioned structural advances of the self-assembled MWCNT interlayer electrode cell are advantageous to improve the electrochemical performances of lithium-sulfur batteries.
ASSOCIATED CONTENT Supporting Information. Specifications and operating conditions for the Li-S cells with the bare MWCNT interlayer and the self-assembled MWCNT interlayer. Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org
AUTHOR INFORMATION Corresponding Author *Tel: +82-2-2220-0524. E-mail: [email protected] (Sun, Y.-K.) *Tel: +1-512-471-1791. E-mail: [email protected] (Manthiram, A.)
Notes All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest
ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2014R1A2A1A13050479) and was also supported by the Global Frontier R&D Program (2013M3A6B1078875) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.
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Figure 2. SEM data and EDS data of the surface (separator side) of (a) bare MWCNT interlayer and (c) self-assembled MWCNT interlayer. SEM data of the cross-section of (b) bare MWCNT interlayer and (d) self-assembled MWCNT interlayer.
Figure 4. Charge/discharge profiles to 100 cycles at 0.5 C rate of (a) bare MWCNT interlayer cell and (b) self-assembled MWCNT interlayer cell. (c) Cyclability and (d) rate capability of the pristine cell (without interlayer), bare MWCNT interlayer cell, and selfassembled MWCNT interlayer cell with a 14 mm diameter elemental sulfur electrode.
