Liquid Phase Exfoliated MoS2 Nanosheets Percolated with Carbon

Aug 19, 2016 - (31) As a result, there is an urgent call to develop electrode materials with high volumetric/areal capacities and able to properly fun...
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Liquid Phase Exfoliated MoS2 Nanosheets Percolated with Carbon Nanotubes for High Volumetric/Areal Capacity Sodium-Ion Batteries Yuping Liu,†,# Xiaoyun He,‡,# Damien Hanlon,‡ Andrew Harvey,‡ Jonathan N. Coleman,*,‡ and Yanguang Li*,† †

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China ‡ School of Physics, CRANN and AMBER Centers, Trinity College Dublin, Dublin 2, Ireland S Supporting Information *

ABSTRACT: The search for high-capacity, low-cost electrode materials for sodium-ion batteries is a significant challenge in energy research. Among the many potential candidates, layered compounds such as MoS2 have attracted increasing attention. However, such materials have not yet fulfilled their true potential. Here, we show that networks of liquid phase exfoliated MoS2 nanosheets, reinforced with 20 wt % single-wall carbon nanotubes (SWNTs), can be formed into sodium-ion battery electrodes with large gravimetric, volumetric, and areal capacity. The MoS2/ SWNT composite films are highly porous, electrically conductive, and mechanically robust due to its percolating carbon nanotube network. When directly employed as the working electrode, they exhibit a specific capacity of >400 mAh/g and volumetric capacity of ∼650 mAh/cm3. Their mechanical stability allows them to be processed into free-standing films with tunable thickness up to ∼100 μm, corresponding to an areal loading of 15 mg/cm2. Their high electrical conductivity allows the high volumetric capacity to be retained, even at high thickness, resulting in state-of-the-art areal capacities of >4.0 mAh/cm2. Such values are competitive with their lithium-ion counterparts. KEYWORDS: liquid phase exfoliation, MoS2 nanosheets, carbon nantube percolation, sodium-ion batteries, free-standing membranes, volumetric and areal capacities

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Recently, interest in transition metal dichalcogenides (TMDs) for efficient energy storage and conversion has been growing rapidly.4,15−18 These materials have a two-dimensional layered structure analogous to graphite and consisting of covalently bonded monolayers brought together via weak van der Waals interactions. The most well studied example among them is MoS2. Up to now, a variety of MoS2-based materials have been exploited for LIBs with some encouraging results.19−23 Because of the chemical similarity between Li+ and Na+, MoS2-based materials have also been actively investigated for SIBs.24−28 Nevertheless, their electrochemical performances as the anode materials of SIBs still fall short of

ithium-ion batteries (LIBs) have been used as the main power source for portable electronics over the past two decades and are considered one of the most promising candidates for hybrid electric vehicles (HEVs) and electric vehicles (EVs).1−4 However, concerns over the cost, availability, and distribution of lithium in the earth’s crust5 have pushed researchers to develop new battery technologies based on more abundant and low-cost electrode materials.6,7 Among them, sodium-ion batteries (SIBs) have recently attracted tremendous interest owing to their similar insertion electrochemistry with their lithium-ion counterparts.8−13 Nevertheless, before SIBs can become commercially competitive with LIBs, there are still a number of unsolved challenges.11−14 For instance, Na ions are about 55% larger in radius than Li ions. This makes it difficult to find a suitable host material to accommodate Na ions and to allow reversible and rapid ion insertion and extraction. © 2016 American Chemical Society

Received: July 10, 2016 Accepted: August 19, 2016 Published: August 19, 2016 8821

DOI: 10.1021/acsnano.6b04577 ACS Nano 2016, 10, 8821−8828

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Figure 1. Liquid phase exfoliated MoS2 nanosheets. (a) Photograph of an as-prepared MoS2 nanosheet dispersion in NMP; (b, c) TEM images of exfoliated MoS2 nanosheets; inset in (c) is the corresponding SAED pattern; (d) histogram of the lateral size distribution of exfoliated MoS2 nanosheets by TEM statistics; (e) SEM and (f) XRD pattern of aggregated MoS2 nanosheets.

can be directly employed as the working electrode of SIBs with an extremely high loading of active materials (up to 15 mg/ cm2) free of any binder, conducting additive, or current collector. It achieves striking specific capacity (>400 mAh/g), volumetric capacity (∼650 mAh/cm3), and areal capacity (>6.0 mAh/cm2).

expectations. Very few studies have been able to simultaneously achieve large capacity (>400 mAh/g), high rate capability (appreciable capacity at >10 A/g), and long cycling stability (>500 cycles) due to the larger size of Na+ ions.29,30 On the other hand, most current battery studies emphasize gravimetric capacities (C/M, mAh/g) but overlook other important metrics such as volumetric (C/V) and areal (C/A) capacities (mAh/cm3 or mAh/cm2). In many practical device applications, the volumetric capacities can be more important than gravimetric capacities, in particular for the portable devices where volume is more important than weight.31 For example, applications in HEVs and EVs require batteries to be not only light in weight but also compact in size. Furthermore, many battery architectures also require a high areal capacity, which is related to electrode thickness (t) and volumetric capacity by C/ A = t × C/V. To achieve this requires an electrode material that can retain high volumetric capacity even at high electrode thickness. Unfortunately, the reported gravimetric capacities of most electrode materials in the literature are usually achieved with very low mass loadings of the active material, typically in the range of 0.5−2 mg/cm2. This leads to optimistic capacity values that do not necessarily translate to high performance (high C/A) when it comes to real batteries.31 As a result, there is an urgent call to develop electrode materials with high volumetric/areal capacities and able to properly function at high mass loadings (>5 mg/cm2).32−34 Herein, we report the production of high-quality MoS2 nanosheet dispersion with high yields using a facile liquid phase exfoliation (LPE) technique.35,36 This method is insensitive to air and water and can be conveniently scaled up to give large quantities of exfoliated material. The resultant dispersion is mixed with single-wall nanotubes (SWNTs) to construct a MoS2/SWNT composite, which is highly porous, extremely conductive, and mechanically robust.23 Furthermore, as motivated by its high mechanical strength, we also prepare a flexible free-standing MoS2/SWNT membrane using vacuum filtration with tunable thickness up to 90 μm. This membrane

RESULTS AND DISCUSSION Liquid phase exfoliation is a scalable and versatile process for producing mono- to few-layer nanosheets from a range of layered materials including graphite, TMDs, and black phosphorus.35 In solvents with a surface tension closely matching the surface energy of these layered materials, prolonged mechanical agitation by ultrasonic waves can result in their effective exfoliation at high yields. MoS2 nanosheets were prepared by LPE of commercially sourced layered powder in N-methylpyrrolidone (NMP), which has previously been established as a good solvent for the LPE of MoS2.36 Figure 1a shows a photograph of a 250 mL bottle of a MoS2−NMP dispersion with a concentration of ∼0.4 mg/mL. It is highly stable and can stand for several weeks without any obvious precipitates. The structure and phase of the exfoliated nanosheets were verified using multiple characterization techniques. Samples for transmission electron microscopy (TEM) were directly prepared from the dispersion. They are observed to be two-dimensional nanosheets with mostly fewlayer thickness even though monolayers are also found (Figure 1b). From the high-resolution TEM, it is evident that these nanosheets are highly crystalline and exhibit in-plane lattice fringes corresponding to the (100) plane of MoS2 (Figure 1c). A selected area electron diffraction (SAED) pattern collected over a single nanosheet is featured with a set of 6-fold diffraction spots, indicating each nanosheet is a high-quality single crystal with (001) termination. Statistical analysis based on TEM images suggests that the lateral size of the nanosheets is distributed over a broad range from ∼100 nm to over 1 μm with a mean length around 300 nm (Figure 1d). In addition, 8822

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cells filled with 1 M NaClO4 in 1:1 (v/v) ethylene carbonate (EC):diethylene carbonate (DEC) electrolyte. The areal loading of the active material was intentionally kept relatively low (∼1 mg/cm2) for this part of the study to unveil its full potential and allow reliable comparison with previous works reported in the literature. Figure 3a depicts the cyclic voltammetry (CV) curves of the MoS2/SWNT composite for the first three cycles. During the initial negative sweep, the CV curve exhibits two pronounced cathodic peaks centered at 0.85 and 0.65 V (versus Na+/Na, the same hereafter), followed by a broad cathodic wave down to the cutoff voltage. On the basis of the sodiation pathway of MoS2, the two peaks are attributed to the stepwise intercalation of layered MoS2 with Na+ ions, whereas the broad wave in the deep cathodic process is contributed by the conversion of MoS2 to form metallic Mo nanograins embedded in the Na2S matrix, together with the irreversible formation of the solid electrolyte interface (SEI).26 During the subsequent discharge, the original cathodic peaks totally vanish, and instead, two new peaks at 2.3 and 1.1 V emerge. This observation is in good agreement with previous reports and indicative of permanent structural modification during the first cycle.29,43 All the positive-going CV curves exhibit four anodic peaks corresponding to a multi-step desodiation process.26,44 Consistent results are also garnered from the galvanostatic charge/discharge experiments at the specific current of 100 mA/g (Figure 3b). The discharge curve of the first cycle is featured with two distinct plateaus at 0.88 and 0.73 V. However, its shape is dramatically altered in the following cycles as a result of the conversion reaction mechanism. The initial discharge capacity of MoS2/SWNT composite reaches ∼770 mAh/g (normalized to the overall composite mass), and ∼440 mAh/g can be recovered and reversibly sustained afterward. Figure 3c compares the cycling stability of the MoS2-alone electrode and MoS2/SWNT composite electrode at the specific current of 200 mA/g. The specific capacity of the former quickly drops from 540 mAh/g at the second cycle to ∼100 mAh/g at the end of 100 cycles. By stark contrast, the incorporation of a percolating SWNT network in the latter significantly promotes its cycling performance. It starts at 410 mAh/g and retains ∼390 mAh/g after 100 cycles, corresponding to an excellent capacity retention of ∼95%. It is worth noting that the initial capacity value of the composite electrode is slightly lower than that of the MoS2-alone electrode due to the incorporation of less electrochemically active SWNTs. A control experiment shows that pure SWNTs have a reversible specific capacity of only ∼90 mAh/g at 100 mA/g (Figure S1). Furthermore, the MoS2/SWNT composite also demonstrates a very impressive performance under large current rates. When the specific current is gradually ramped up, specific capacities of 437, 404, 370, 341, 302, 268, and 192 mAh/g are reversibly measured at 0.05, 0.2, 0.5, 1, 5, 10, and 20 A/g, respectively (Figure 3d). Among a handful of previous studies on MoS2based materials for SIBs, very few can attain >200 mAh/g at 10 A/g. The rate capability of our composite electrode clearly surpasses most of its competitors owing to its advantageous microstructure (Figure S2).25,28,29,45−48 Importantly, this does not come at the cost of inferior cycling stability under large current rates. In fact, the composite electrode can retain ∼70% of its original capacity even after 1000 cycles at 500 mA/g (Figure 3e). This corresponds to a small average capacity loss of 0.03% per cycle and attests to its outstanding long-term cycling performance.

the exfoliated product can be transferred to water and lyophilized to give powders of reaggregated nanosheets, as depicted in the scanning electron microscopy (SEM) image in Figure 1e. Their X-ray diffraction (XRD) pattern matches well with that of 2H-MoS2 without any noticeable impurity (Figure 1f). Pristine MoS2 nanosheet networks have poor intrinsic electrical conductivity and mechanical strength.23,35,37 In order to address these problems for battery applications, they are usually reinforced with proper conductive additives such as graphene19,38 or carbon nanotubes.39−41 A recent study suggests that adding a small amount of SWNTs to MoS2 nanosheets would result in up to 1010 and 40 times increases in electrical conductivity and mechanical toughness, respectively, which are well described by the percolation theory.23 In addition, SWNTs can also effectively prevent the restacking of MoS2 nanosheets, thus increasing the accessibility of their high surface areas. Motivated by these results, here we prepared a MoS2/SWNT composite by physically mixing as-exfoliated MoS2 nanosheets with 20 wt % nanotubes in isopropyl alcohol (IPA), transferring the composite to water, and finally lyophilizing it (see Methods). SEM images of the final composite at low and high magnification are displayed in Figure 2a,b. It can be clearly observed that MoS2 nanosheets are

Figure 2. Morphology and composition analysis of the MoS2/ SWNT composite. (a, b) SEM images, (c) TEM image, and (d) Raman spectrum of the MoS2/SWNT composite.

uniformly dispersed within the percolating SWNT network (Figure 2b). Under TEM, individual MoS2 nanosheets are found to be electrically wired with several SWNTs at the same time (Figure 2c). There is no evidence of macroscopic phase separation of these two components in the composite. Moreover, Raman spectroscopy unambiguously corroborates the presence of MoS2 and SWNTs in the composite as shown in Figure 2d. The radial breathing mode (RBM) at ∼164 cm−1 and the D, G, and G′ bands at 1345, 1592, and 2679 cm−1, respectively, are associated with SWNTs; the two distinct peaks located at 379 and 402 cm−1 are characteristic of the E12g and A1g vibration modes of 2H-MoS2.42 We next assessed the electrochemical performance of the MoS2/SWNT composite as the SIB anode material. It was blended with poly(acrylic acid) binder, blade coated onto Cu foil, and then paired with a metallic Na disk in standard coin 8823

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Figure 3. Electrochemical measurements of the MoS2/SWNT composite as the SIB anode material. (a) CV curves for the first three cycles at a scan rate of 0.2 mV/s; (b) galvanostatic charge/discharge curves of the MoS2/SWNT electrode for the first three cycles at 100 mA/g; (c) cycling stability of MoS2/SWNT at 200 mA/g in comparison with a MoS2-alone electrode; (d) rate capability of MoS2/SWNT at different specific currents as indicated; (e) long-term cycling stability of the MoS2/SWNT composite at 500 mA/g.

Despite its appealing specific capacity at the relatively low areal loading of ∼1 mg/cm2, we note that it does not necessarily translate to superior battery performance for real applications.31,33 This is because the specific capacity usually decreases with increasing electrode thickness. There are a number of reasons for this. The most well-known is that the electrolyte may not penetrate deep into a very thick electrode, and even if it does, the high rate performance may become diffusion-limited. In addition, for low-conductivity electrode materials such as MoS2, the thicker the electrode, the higher the resistance felt by an electron traveling from the current collector to a lithium storage site near the electrode surface. Again, this can limit performance at high rates. Less often considered is that, for mechanically poor materials such as nanosheet networks, the mechanical robustness of the electrode can decrease with thickness.50 This renders thick electrodes mechanically unstable and limits performance. These are significant problems for experimental materials, as most commercial battery anodes require an areal loading of 5−10 mg/cm2 and a thickness around 50−60 μm in order to have an appreciable energy density.51 To resolve the problems described above, an anode material with certain properties is demanded. Maximizing the cell energy density requires maximizing the capacity of the electrode of a given area, i.e., maximizing the areal capacity. Because C/A = t

From the above studies, it has been established that our MoS2/SWNT composite has the combination of large capacity, high rate capability, and excellent cycling stability. We believe that its superior electrochemical performance is the collective effect of several structural factors as summarized below. Compared to chemical synthesis, MoS2 nanosheets exfoliated by the LPE method have high quality49 and are free of impurities.36 These nanosheets are readily dispersible in IPA without the assistance of any surfactant and can be uniformly mixed with SWNTs. The addition of 20 wt % SWNTs is beyond the electrical and mechanical percolation thresholds.23 They form a three-dimensional percolating network, which not only provides a highly conductive pathway for efficient charge transfer but also mechanically reinforces the composite and accommodates the volume change upon extended charge and discharge cycling. This effect is clearly manifested through the control experiment by replacing SWNTs in the composite with the same amount of Super P carbon black (MoS2/SuperP). MoS2/SuperP has inferior cycling stability compared to that of MoS2/SWNT, which is believed to be due to its insufficient mechanical strength to maintain the electrode integrity during cycling (Figure S3a). As a result, the advantageous architecture of TMD nanosheets percolated with one-dimensional CNTs offers a great opportunity for high-performance Li+ or Na+ ion storage. 8824

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porosity as ∼50%. This should easily be sufficient to allow access of the electrolyte throughout the network. Figure 4d selectively shows the cross-section SEM image of a membrane with an areal loading of 5 mg/cm2. Its thickness is largely uniform and determined to be around 30 μm. Close examination reveals that the membrane consists of MoS2 nanosheets densely stacked together and infiltrated with interconnected SWNTs, in a brick-and-mortar-like arrangement (Figure 4e). By contrast, we note that MoS2/SuperP is too fragile to form free-standing membranes under similar conditions due to its lack of sufficient mechanical strength (Figure S3b). These free-standing MoS2/SWNT membranes were directly employed as the electrode film for SIBs without any additional binder or current collector. This is possible only because of the high conductivity and mechanical performance of these composites. They were galvanostatically charged and discharged at an areal current of 0.2 mA/cm2. Capacities were normalized to the membrane mass, volume, or geometric area. As shown in Figure 5a, all thicknesses deliver very similar initial gravimetric and volumetric capacities, with values between 393 and 420 mAh/g and 655−700 mAh/cm3, respectively. Their areal capacities were found to linearly scale with thickness (and so areal loading), exceeding 6 mAh/cm2 for the 93 μm membrane, consistent with a volumetric capacity of C/V = 650 mAh/cm3 (Figure 5b,c). The 15, 30, and 52 μm thick electrodes display excellent stability with virtually no capacity loss after 100 cycles. However, further increasing the membrane thickness (Figure 5a,b,c) results in inferior cycling stability with an areal capacity of only ∼3.7 mAh/cm2 at the end of 100 cycles for the 72 μm thick electrode. This leads us to conclude that the practical upper limit of the membrane thickness probably lies between 50 and 70 μm, beyond which the electrochemical performance of MoS2/SWNT membranes starts to be compromised. It can be seen in Figure 5d, where we examine the electrode stability by plotting the capacity retention from N = 2 to N = 100 cycles versus electrode thickness. The data clearly show very good capacity retention for electrodes with t ≤ 52 μm but a dramatic fall-off in stability for thicker electrodes. As far as we know, there are no previous reports on the areal or volumetric capacities of any SIB electrode materials. However, the significance of our results can be grasped from the comparison with LIB electrode materials. For example, a commercial graphite electrode for LIBs has a volumetric or areal capacity of ∼550 mAh/cm 3 or ∼4 mAh/cm 2 , respectively;32 the best Si-based materials have areal capacities 6.0 mAh/ cm2. Our study here is one of very few experimental reports on the volumetric or areal capacities of SIB electrode materials. These values are outstanding and even superior to those of most LIB electrode materials, as enabled by the hierarchical architecture percolated with the SWNT network. We believe our strategy of combined LPE and vacuum filtration could be extended to the preparation of other TMD composite membranes for high volumetric/areal capacity sodium-ion storage. METHODS Preparation of MoS2/SWNT Composite and Free-Standing Membranes. A MoS2 nanosheet dispersion was obtained by the LPE of MoS2 bulk powders in NMP for 4 h using a horn-probe tip sonicator (Sonics Vibra-cell VCX-750W ultrasonic processor) operating at 60% amplitude and 6 s on and 2 s off pulse while icecooling the dispersion. The resultant raw dispersion was centrifuged for 60 min at 1000 rpm to remove unexfoliated MoS2. Subsequently, the supernatant was decanted and centrifuged for 90 min at 4500 rpm. Sediments were collected and redispersed in IPA to form a nanosheet 8826

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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/acsnano.6b04577. Additional information (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail (J. N. Coleman): [email protected]. *E-mail (Y. Li): [email protected]. Author Contributions #

Y. Liu and X. He contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (51472173 and 51522208), the Natural Science Foundation of Jiangsu Province (BK20140302 and SBK2015010320), Science Foundation Ireland (11/PI/1087), the European Research Council (SEMANTICS), and the European Union Seventh Framework Program under grant agreement no. 604391 (Graphene Flagship) for financial support. We acknowledge support from the Collaborative Innovation Centre of Suzhou Nano Science, the Priority Academic Program Development of Jiangsu Higher Education Institutions and Technology, and the SFI-funded AMBER research center (SFI/12/RC/2278). REFERENCES (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652−657. (2) Kang, K. S.; Meng, Y. S.; Breger, J.; Grey, C. P.; Ceder, G. Electrodes with High Power and High Capacity for Rechargeable Lithium Batteries. Science 2006, 311, 977−980. (3) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: a Review. Energy Environ. Sci. 2011, 4, 3243−3262. (4) Goodenough, J. B. Evolution of Strategies for Modern Rechargeable Batteries. Acc. Chem. Res. 2013, 46, 1053−1061. (5) Clarke, G. M. Lithium-ion Batteries Raw Material Considerations. Chem. Eng. Prog. 2013, 109, 44−52. (6) Li, Y. G.; Dai, H. J. Recent Advances in Zinc-Air Batteries. Chem. Soc. Rev. 2014, 43, 5257−5275. (7) Lin, M. C.; Gong, M.; Lu, B. G.; Wu, Y. P.; Wang, D. Y.; Guan, M. Y.; Angell, M.; Chen, C. X.; Yang, J.; Hwang, B. J.; Dai, H. J. An Ultrafast Rechargeable Aluminium-Ion Battery. Nature 2015, 520, 324−328. (8) Pan, H. L.; Hu, Y. S.; Chen, L. Q. Room-Temperature Stationary Sodium-Ion Batteries for Large-Scale Electric Energy Storage. Energy Environ. Sci. 2013, 6, 2338−2360. (9) Luo, W.; Shen, F.; Bommier, C.; Zhu, H. L.; Ji, X. L.; Hu, L. B. Na-Ion Battery Anodes: Materials and Electrochemistry. Acc. Chem. Res. 2016, 49, 231−240. (10) Xiang, X. D.; Zhang, K.; Chen, J. Recent Advances and Prospects of Cathode Materials for Sodium-Ion Batteries. Adv. Mater. 2015, 27, 5343−5364. (11) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636− 11682. (12) Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F. The Emerging Chemistry of Sodium Ion Batteries for Electrochemical Energy Storage. Angew. Chem., Int. Ed. 2015, 54, 3431−3448. (13) Hong, S. Y.; Kim, Y.; Park, Y.; Choi, A.; Choi, N.-S.; Lee, K. T. Charge Carriers in Rechargeable Batteries: Na Ions vs. Li Ions. Energy Environ. Sci. 2013, 6, 2067−2081. 8827

DOI: 10.1021/acsnano.6b04577 ACS Nano 2016, 10, 8821−8828

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DOI: 10.1021/acsnano.6b04577 ACS Nano 2016, 10, 8821−8828