Research Article www.acsami.org
Immobilizing Polysulfides with MXene-Functionalized Separators for Stable Lithium−Sulfur Batteries Jianjun Song,†,‡ Dawei Su,† Xiuqiang Xie,† Xin Guo,† Weizhai Bao,† Guangjie Shao,*,‡ and Guoxiu Wang*,†,§ †
Centre for Clean Energy Technology, Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia ‡ State Key Laboratory of Metastable Materials Science and Technology, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China § College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, China S Supporting Information *
ABSTRACT: Lithium−sulfur batteries have attracted increasing attention as one of the most promising candidates for next-generation energy storage systems. However, the poor cycling performance and the low utilization of sulfur greatly hinder its practical applications. Here we report the improved performance of lithium−sulfur batteries by coating Ti3C2Tx MXene nanosheets (where T stands for the surface termination, such as -O, -OH, and/or -F) on commercial “Celgard” membrane. In favor of the ultrathin two-dimensional structure, the Ti3C2Tx MXene can form a uniform coating layer with a minimum mass loading of 0.1 mg cm−2 and a thickness of only 522 nm. Owing to the improved electric conductivity and the effective trapping of polysulfides, the lithium−sulfur batteries with MXene-functionalized separators exhibit superior performance including high specific capacities and cycling stability. KEYWORDS: MXene, Ti3C2Tx, separator, polysulfides, lithium−sulfur batteries
1. INTRODUCTION Recently, great progress has been achieved in developing lithium ion batteries (LIBs) that power portable electronic devices such as mobile phones, laptops, and personal digital gadgets. However, the current LIB technology is not capable to satisfy the specific energy requirements for large-scale grid applications and electric vehicles (EVs).1,2 Lithium−sulfur (Li− S) batteries are regarded as one of the most promising potential candidates for next-generation energy storage systems due to the natural abundance of sulfur, high theoretical capacity, and high specific energy density (1675 mAh g−1 and 2600 Wh kg−1, respectively).3,4 However, the development of Li−S batteries have been seriously hindered by the capacity decay and poor cycling stability, which originates from the low electrical conductivity of sulfur and its discharge products Li2S2/Li2S,5,6 and the migration of the soluble polysulfides (Li2S4−8) across the separator during charge/discharge process (the “shuttle effect”).7−11 So far, many approaches have been developed to address these problems. The most popular strategy is to design nanostructured cathodes by confining sulfur within conductive frameworks, including porous carbon materials,4,12−16 carbon nanofibers,17,18 carbon nanotubes (CNTs),19 conductive polymers,20−22 graphene/graphene oxide,23−29 and metal oxides.30,31 This strategy can not only enhance the intimate © 2016 American Chemical Society
conductive contact among the insulating sulfur particles but can also restrict the dissolution and shuttling of polysulfide intermediates via physisorption or chemisorption, thus enhancing the electrochemical performance. Recently, extensive efforts have been made to restrict the migration of dissolved polysulfides across separators.15,32,33 The methods involve introducing a functional interlayer between separator and sulfur cathode or a coating layer on the cathode side of the separator.34−42 Arumugam Manthiram et al. reported that inserting a functional porous carbon interlayer between the separator and sulfur cathode suppressed the shuttle effect of polysulfide intermediates,43 which greatly improved the utilization efficiency of the sulfur and the cycling performance of the batteries. Balach and his co-workers prepared a series of functional separators to enhance cycling stability and rate capability of Li−S batteries, including coating mesoporous carbon,7 RuO2 nanoparticle-decorated mesoporous carbon,44 and N and S dual-doped mesoporous carbon.45 They demonstrated capabilities to suppress the migration of polysulfides. Zeng et al. reported the sulfonated acetylene Received: July 22, 2016 Accepted: October 10, 2016 Published: October 10, 2016 29427
DOI: 10.1021/acsami.6b09027 ACS Appl. Mater. Interfaces 2016, 8, 29427−29433
Research Article
ACS Applied Materials & Interfaces black-coated separator, which can restrain polysulfide anions.46 Moreover, the ion conductivity was not influenced. Thus, the rate and cycling performances of the Li−S batteries using this functional separator show great improvement. Fan et al. prepared an effective functionalized separator by using a graphitic carbon nitride coating,47 which contained enriched polysulfide adsorption sites of pyridinic-N. The cells exhibited a high discharge capacity of 840 mA h g−1 after 400 cycles at 0.5 C. Kim et al. synthesized a poly(acrylic acid)-coated singlewalled carbon nanotube interlayer to restrict the migration of polysulfides.48 In favor of the synergistic effect of PAA and SWNT, the Li−S cells show a high cycling performance and good rate retention over 200 cycles. Two-dimensional graphene oxide/graphene also exhibited a good effect for modification of separator membranes.49,50 Huang et al. reported that an ultrathin graphene oxide (GO) membrane could provide both a physical and a chemical barrier to suppress the migration of polysulfides51 and remarkably increase the Coulombic efficiency. Zhou et al. prepared a flexible sulfur electrode by coating graphene and sulfur on polypropylene membrane.52 The graphene layer could act as an internal current collector and a reservoir for soluble polysulfides and thus extended the cycling stability. Similar to graphene, MXenes are a large family of two-dimensional early transition-metal carbides or carbonitrides.53−56 They are prepared through selectively extracting the “A” layers from the MAX phases,57 where M is an early transition metal, A is a group IIIA or IVA element, and X is carbon and/or nitrogen. MXenes have attracted worldwide attention since first reported by Gogotsi et al. in 2011,54 owing to the high conductivity and highly active two-dimensional structure. MXenes have demonstrated versatile applications in many platforms.56,58−61It is also proven that the highly active two-dimensional surfaces of MXenes can effectively immobilize the soluble polysulfides by metal−sulfur interactions.62,63 In addition to exhibiting high electronic conductivity, MXenes show unique advantages in improving the electrochemical performance of Li−S batteries. Herein we report a functional separator for trapping soluble polysulfides by coating Ti3C2Tx MXene (T stands for the surface termination, such as -O, -OH, and/or -F) nanosheets on commercial polypropylene membranes to improve the cycling stability and rate capability of Li−S cells. Benefiting from their ultrathin two-dimensional structure, the Ti3C2Tx nanosheets can form a conformal coating layer without obvious pores or gaps on the surface of the separator with a mass loading of only 0.1 mg cm−2. Due to the improved electric conductivity and the effective trapping of polysulfides, the cell with a MXenefunctionalized separator delivered high discharge capacity of 550 mAh g−1 after 500 cycles with a capacity decay of only 0.062% per cycle at 0.5 C.
of 1:250. The suspension was ultrasonicated for 30 min and then centrifuged at 3500 rpm for 20 min. After that, the supernatant dispersion was collected. The yield of delaminated Ti3C2Tx nanosheets is about 20%. Preparation of MXene-Modified Polypropylene (MPP) Separator. Five milliliters of Ti3C2Tx dispersion was mixed with 10 mL of ethanol and was then ultrasonicated for 20 min. Finally, the MPP separators were obtained by vacuum filtering the above suspension through a commercial polypropylene (PP) membrane (Celgard 2400), followed by drying in a vacuum oven at 50 °C overnight. The coating weight and thickness of Ti3C2Tx nanosheets is about 0.1 mg cm−2 and 522 nm, respectively. Preparation of S/Carbon Black (S/CB) Composites. The S/CB composites were prepared by a conventional melt-diffusion method. Typically, the commercial sulfur powder (Sigma-Aldrich) and carbon black (super P) were mixed and ground in a mortar for 1 h. The mixture was then heated at 155 °C for 12 h. According to thermogravimetric analysis, the obtained S/CB composites showed 68% sulfur loading. 2.2. Material Characterizations. X-ray diffraction (XRD) patterns of the as-prepared samples were performed with a Bruker D8 diffractometer within 2θ range from 5° to 65°. The sulfur loading of S/CB composites was analyzed based on themogravimetric analysis (SDT 2960) from room temperature to 600 °C under N2 atmosphere with a heating rate of 10 °C min−1. The morphology of samples was investigated by field-emission scanning electron microscopy (Zeiss Supra 55VP) and transmission electronic microscopy (TEM, JEOL JEM-ARM200F, at an accelerating voltage of 220 kV). Elemental mapping were conducted with a Zeiss Evo LS15. 2.3. Electrochemical Measurements. Coin-type (CR2032) cells were assembled in an argon-filled glovebox, in which water and oxygen contents were less than 1 ppm. The test cells consisted of a lithium anode and a sulfur-containing cathode separated by a PP separator (Celgard 2400) or MPP separator. The sulfur-containing electrode was prepared by mixing the S/CB composites, poly(vinylidene fluoride) (PVDF, Sigma-Aldrich), and carbon black (super P) at a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone (Sigma-Aldrich) solvent. The mixture was placed onto aluminum foil and dried in a vacuum oven at 60 °C overnight. The sulfur loading in the electrode is about 1.2 mg cm−2. A nonaqueous solution of 1 M lithium bis(trifluoromethane sulfonel)imide (LiTFSI) in solvent mixture of 1,2-dimethoxyethane and 1,3-dioxolane (1:1, v/v) was used as the electrolyte with 0.1 M LiNO3 as an additive. The amount of electrolyte was 30 μL in each cell. The Li−S cells were galvanostatically discharged and charged over a potential range of 1.7−2.8 V (versus Li/Li+) on a LAND CT2001A testing system. Electrochemical impedance spectroscopy (EIS) results were collected on a CHI 660C electrochemical workstation from 105 Hz to 10−2 Hz with an applied amplitude of 5 mV.
3. RESULTS AND DISCUSSION Ti3C2Tx nanosheets were prepared through the delamination of Ti3C2Tx powder, which was synthesized after the LiF/HCl treatment of Ti3AlC2. The morphology and structure of exfoliated Ti3C2Tx nanosheets were investigated by SEM and TEM. After exfoliation, the Ti3C2Tx MXene was delaminated into micrometer sized ultrathin 2D nanosheets (Figure 1a). The TEM image (Figure 1b) further confirms the ultrathin 2D structure of the Ti3C2Tx nanosheets because it is almost transparent under the electron beam. Figure S1 shows the XRD of Ti3C2Tx nanosheets, which exhibits an amorphous broad peak and is in agreement with the previous report,23 demonstrating that the Al layers of Ti3AlC2 were replaced by oxygen (i.e., OH) and/or F. Figure 1c shows the surface morphology of the MPP separator. Owing to the ultrathin 2D structure, the delaminated Ti3C2Tx nanosheets can easily form a uniform coating layer on the surface of the separator with a minimum loading amount of only 0.1 mg cm−2, and no obvious
2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. Synthesis of Delaminated Ti3AlC2 Nanosheets. Ti3AlC2 powder was synthesized as reported previously.64 Multilayered Ti3C2Tx MXene was prepared by LiF/HCl treatment of Ti3AlC2. Typically, 0.5 g of lithium fluoride (LiF) was introduced into 5 mL of HCl solution (7.5 M). After stirring for 5 min to dissolve LiF, 0.5 g of Ti3AlC2 powder was slowly introduced into the above solution. The suspension was kept at 40 °C for 24 h under stirring. Then the mixture was centrifuged (3500 rpm) and washed with deionized water six times. The final suspension was then filtered and dried in air. The yield of multilayered Ti3C2Tx MXene is almost 100%. For the delamination of Ti3C2Tx, the obtained Ti3C2Tx powder was dispersed in deionized water with a weight ratio of Ti3C2Tx:water 29428
DOI: 10.1021/acsami.6b09027 ACS Appl. Mater. Interfaces 2016, 8, 29427−29433
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ACS Applied Materials & Interfaces
Figure 2. Typical SEM images of pristine lithium (a) and the cycled metallic lithium of the Li−S cells using PP (b) and MPP (c) separators. (d) Elemental mapping of the MPP separator after cycling.
Figure 1. SEM image (a) and TEM image (b) of Ti3C2Tx MXene nanosheets. (c) Low-magnification surface SEM image and the digital image (inset) of the MPP separator. (d) EIS curves of fresh Li−S batteries using PP and MPP separators, respectively.
demonstrating that the soluble polysulfides can be effectively confined and reutilized by the coated Ti3C2Tx nanosheets. Figure 3 shows the schematic cell configuration of lithium− sulfur cells using a PP separator or a MPP separator. In
pores or gaps can be found in the surface. Therefore, the coated Mexene layer can effectively immobilize the soluble polysulfide on the cathode region of the cell. In addition, the thickness of the Ti3C2Tx nanosheet coating layer is only 522 nm (Figure S2). To the best of our knowledge, it is the thinnest among the previously reported modified separators, which can facilitate fast lithium ion transport. EIS of the Li−S cells with PP separators and MPP separators before cycling was used to study the effect of the modified separator on the electrode reaction impedance (Figure 1d). The Nyquist profiles of both cells present a quasi-semicircle at the middle frequency region and a sloping line at the low frequency region, which is associated with the charge transfer process and the Warburg diffusion process, respectively. However, the charge transfer resistance (Rct) in the cell with the MPP separator shows a much lower value (45.06 Ω) than that of the cell with the PP separator (101.2 Ω). The results clearly prove that the Ti3C2Tx nanosheet coating can remarkably decrease the resistance of the electrode owing to its high electronic conductivity. To investigate the influence of the MPP separator on trapping dissolved polysulfides, the cells were discharged and charged 100 cycles at 1 C and then disassembled in an inert glovebox. The separators were thoroughly rinsed with DME/ DOL solvent. The morphology of the metallic lithium and MPP separators after cycling are shown in Figure 2. The SEM image (Figure 2a) of fresh metallic lithium presents a very smooth surface. However, as shown in Figure 2b, a rough surface of the lithium anode with severe corrosion is found in the cell with the PP separator after electrochemical testing, which can be ascribed to deposition of the side reaction product (Li2S2/Li2S) between the lithium anodes and shuttled polysulfides during the charge/discharge processes.41 In contrast, the cycled lithium anode using a MPP separator exhibits a relatively uniform and smooth surface (Figure 2c). The elemental mapping of cycled separator in Figure 2d displays a uniform distribution of strong sulfur signals, which is consistent with the morphological analysis of the lithium anode,
Figure 3. Schematic configuration of the Li−S cells using PP and MPP separators.
conventional cells, the porous PP separator is beneficial for fast ion transport in the cell.7 However, it also undesirably allows the soluble polysulfides to shuttle throughout the pores, leading to the occurrence of side reactions and the loss of active materials. In contrast, the Ti3C2Tx nanosheet barriers on the MPP separator show a uniform distribution, so they can effectively restrain the free migration of dissolved polysulfides through the porous PP membranes by physisorption and chemisorption and thus facilitate the utilization of sulfur and the extended cycle life of cells. To confirm the impact of MPP separators on electrochemical performance, the Li−S cells with PP and MPP separators were discharged and charged from 0.2 to 1 C stepwise and then switched back to 0.2 C. The simple S/CB composites (Figures S3 and S4) were used as cathode materials. Figure 4a and 4b shows the first charge/discharge curves of the cells with PP and MPP separators at 0.2, 0.5, and 1 C, respectively. Each discharge curve shows two typical discharge potential plateaus. The upper discharge voltage plateau around 2.3 V stands for the conversion of sulfur to high-order polysulfides (Li2S4−8). The lower discharge plateau around 2.1 V is related to their further reduction to Li2S2/Li2S.18 The two continuous plateaus in charge curves at 2.2 and 2.4 V indicate the oxidation 29429
DOI: 10.1021/acsami.6b09027 ACS Appl. Mater. Interfaces 2016, 8, 29427−29433
Research Article
ACS Applied Materials & Interfaces
Figure 4. First charge/discharge curves of the Li−S cells with a PP separator (a) and MPP separator (b) at 0.2, 0.5, and 1 C, respectively. (c) Comparative analysis of the potential difference of Li−S cells with PP and MPP separators between the charge and discharge plateaus at 0.2, 0.5, and 1 C, respectively. (d) The corresponding rate performance of the Li−S cells with PP and MPP separators.
densities were switched to 0.5 and 0.2 C, the cells with PP separators show lower reversible capacities of only 513 and 640 mAh g−1, corresponding to 81.1% and 66.8% of the original discharge capacities. This result clearly demonstrates that Ti3C2Tx nanosheet coating on separators can effectively enhance the rate capability of Li−S batteries because of the strong trapping effect for polysulfides. Figure 5 shows the long-term cycling performance of the cells using MPP separators. The cells deliver excellent
reactions of Li2S2/Li2S to Li2S8/S8 during the charging processes. The charge/discharge curves of the cells using MPP separators show more stable and flat charge/discharge plateaus at various rates compared to the cells with PP separators. In addition, the cells using MPP separators show very low voltage plateau gaps in the different charge/discharge curves, with only 140, 175, and 230 mV of plateau potential difference at 0.2, 0.5, and 1 C (as shown in Figure 4c). On the contrary, the cells using PP separators show high values of 259, 456, and 733 mV at the above current rates, respectively. The results prove that the better redox reaction kinetics and higher reversibility can be achieved by the modified separator having a uniform Ti3C2Tx nanosheet coating. Figure 4d shows the rate performance of the Li−S cells with a PP separator and MPP separator. The cells with MPP separators deliver much higher reversible capacities than the cells with PP separators. With the increase of current densities, the advantages of the cells with MPP separators become more obvious. At a low rate of 0.2 C, the cells with MPP separators achieved a high discharge capacity of 1046.9 mAh g−1. With the increase of current density, the cells with MPP separators still delivered discharge capacities of 848.7 and 743.7 mAh g−1 at 0.5 and 1 C, respectively. In particular, when the current densities were switched back to 0.5 and 0.2 C, high reversible capacities of 770 and 822.1 mAh g−1 (90.7% and 78.5% of the original discharge capacities) were obtained at 0.5 and 0.2 C, respectively, reflecting its stable and reversible characteristics by utilizing the Ti3C2Tx nanosheet-modified separators. The capacities of the cells using PP separators exhibit a sharp decrease from 957 mAh g−1 to 632.5 and 431.7 mAh g−1 with the increase of rates from 0.2 C to 0.5 and 1 C, respectively. When the current
Figure 5. Long-term cycling performance of Li−S batteries with MPP separators at rates of 0.5 C and 1 C, respectively.
performance with discharge capacities of 550 mAh g−1 at 0.5 C, and 495 mAh g−1 at 1 C, after 500 cycles, which are much higher than that of the cells with PP separators (Figure S5). Most importantly, the cells deliver a high Columbic efficiency of almost 100% for most cycles and only present a very small capacity decay of 0.062% per cycle at 0.5 C, which shows better 29430
DOI: 10.1021/acsami.6b09027 ACS Appl. Mater. Interfaces 2016, 8, 29427−29433
Research Article
ACS Applied Materials & Interfaces
4. CONCLUSIONS Ti3C2Tx MXene nanosheets were employed to modify the commercial Celgard separators in lithium−sulfur batteries. With this functional separator, the simple sulfur/carbon black composite cathodes exhibited superior cyclability and reversibility. The highly conductive Ti3C2Tx nanosheet layer can greatly decrease the internal resistance of cells. The ultrathin two-dimensional structure of Ti3C2Tx nanosheets with highly active polar sites can form a uniform coating layer to efficiently immobilize the soluble polysulfides. Therefore, two-dimensional MXenes materials are ideal candidates to modify separators and enhance the electrochemical performance of lithium−sulfur batteries.
performance than most of the previously reported modified separators,7,35,37,39,40,42,44,46,47 as summarized in Table S1. Figure 6 presents the cycling performance of the cells using PP separators and the cells using MPP separators with different
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09027. Thickness analysis of separators, XRD, TGA, additional SEM images, and electrochemical testing data (PDF)
Figure 6. Cycling performance of the cells using PP separators and the cells using MPP separators with different sulfur loadings.
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AUTHOR INFORMATION
Corresponding Authors
sulfur loading to investigate the influence of MPP separators on electrochemical performances of higher sulfur loadings at 0.2 C. For a sulfur loading of 1.2 mg cm−2, the cells with PP separators and MPP separators exhibit initial discharge capacities of 1095 and 1246.3 mAh g−1, respectively. After cycling for 30 cycles, the cells with the MPP separators still maintain an excellent discharge capacity of 860.7 mAh g−1 while the cells with PP separator show a low capacity retention of 50.7% and a discharge capacity of only 556 mAh g−1 after 30 cycles. The corresponding charge/discharge curves of the 1st and 30th cycles for the cells with PP and MPP separators are also shown in Figure S6. The cells with PP separators show severe polarization between the two charge/discharge curves. For comparison, no obvious change of the plateau potential can be found in the charge/discharge curves for the cells using MPP separators, which further demonstrates the high reversibility. When the sulfur loading reaches 2.1 and 2.8 mg cm−2, the cells with MPP separators still show good cycling performances with high capacity retentions of 88% and 89.66% with respect to the highest capacity achieved in the first few cycles, and high discharge capacities of 894.7 and 850.9 mAh g−1 after 30 cycles, respectively, which are much better than that of cells with PP separators. The results clearly prove that a MPP separator can also effectively improve the electrochemical performance of high sulfur loading. The improved electrochemical performances can be ascribed to the uniform coating of highly electrically conductive Ti3C2Tx nanosheets. This coating layer on a separator can act as a second current collector to reduce the internal resistance of a cell,43 which facilitates faster redox kinetics. Furthermore, the ultrathin 2D structure of Ti3C2Tx nanosheets can form a uniform coating layer to suppress polysulfide shuttling. In conjunction with the highly polar active surfaces, it can serve as an effective reservoir for immobilizing soluble polysulfides from the cathode region of the cell via both physisorption and chemisorption. This avoids the side reactions with the lithium anode due to the shuttling effect of polysulfides through the separator, thus enhancing the reutilization of sulfur and extending the cycling life of Li−S batteries.
*E-mail:
[email protected]; tel: +86-335-8061569; fax: +86-335-8059878. *E-mail:
[email protected]; tel: +61 2 95141741; fax: +61 2 95141460. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Australian Renewable Energy/Agency (ARENA) project (ARENA 2014/ RND106) and partially supported by the China Scholarship Council (grant 201508130079).
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REFERENCES
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