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C nanosheet heterostructure arrays for ultra-stable sodium-ion batteries
Accepted Manuscript Interface-engineered MoS2/C nanosheet heterostructure arrays for ultra-stable sodium-ion batteries Haiyan Wang, Hao Jiang, Yanjie Hu, Petr Saha, Qilin Cheng, Chunzhong Li PII: DOI: Reference:
S0009-2509(17)30552-3 http://dx.doi.org/10.1016/j.ces.2017.09.007 CES 13784
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
25 July 2017 26 August 2017 1 September 2017
Please cite this article as: H. Wang, H. Jiang, Y. Hu, P. Saha, Q. Cheng, C. Li, Interface-engineered MoS2/C nanosheet heterostructure arrays for ultra-stable sodium-ion batteries, Chemical Engineering Science (2017), doi: http://dx.doi.org/10.1016/j.ces.2017.09.007
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Interface-engineered MoS2/C nanosheet heterostructure arrays for ultra-stable sodium-ion batteries Haiyan Wang a, Hao Jiang a,*, Yanjie Hu a, Petr Saha b,Qilin Cheng a, Chunzhong Li a,* a
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and
Engineering, East China University of Science and Technology, Shanghai 200237, China b
Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Trida T. Bati 5678, 760 01
Zlin, Czech Republic Email: [email protected] (Prof. H. Jiang) and [email protected] (Prof. C. Li)
Abstract Development of ultra-stable high capacity electrodes is imperative for the widespread commercialization of sodium-ion batteries. Herein, we employed a micro-area etching and surface functionalization strategy to synthesize two-dimensional (2D) MoS2/C nanosheets with a well-defined heterointerface vertically anchored on a carbon cloth. The large MoS2/C nanosheet heterointerface and a high interlayer distance (0.99 nm) not only facilitated Na+ intercalation but also improved the diffusion kinetics of Na+ in the 2D interlayer space. A modulation of the cut-off voltage yielded a high specific capacity of 433 mAh g-1 at 0.2 A g-1 and 232 mAh g-1 at 10 A g-1 within the potential range of 0.4–3.0 V. These values are much higher than that of pure MoS2 nanosheet arrays (162 mAh g-1 at 10 A g-1). More importantly, during the first 1500 cycles, the capacity was maintained at ~320 mAh g-1 at 1 A g-1, while after 10000 cycles, it became approximately ~271 mAh g-1 at 3 A g-1. These are the best values ever reported for MoS2-based anode materials for SIBs. Furthermore, after being assembled into a flexible battery, it withstand repeated bending for over 200 times without any obvious capacity loss. Hence, this material is a promising electrode for future flexible batteries. 1
Keywords: MoS2, heterointerface, micro-area etching, sodium ion batteries, flexible electrode.
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1. Introduction Sodium-ion batteries (SIBs) are regarded as future alternatives to lithium-ion batteries because of the abundance of sodium and its charge storage mechanisms, which are similar to those of lithium (Grey and Tarascon, 2017; Kim et al., 2016; Xu et al., 2015). As the atomic radius of Na+ is higher than that of Li+(1.02Å vs. 0.76 Å), it is difficult to achieve Na+ intercalation using a graphite anode with an interlayer distance of 0.34 nm (Zhou et al., 2016; Stevens and Dahn, 2000; Kim et al., 2015). Hence, the development of anodes with a wider interlayer spacing is pivotal for developing SIBs. In recent years, graphene-like MoS 2 nanosheets have gained attention as potential anode materials for SIBs because of their large interlayer distance (0.64 nm), weak Van der Waals interactions, and a higher degree of safety as compared to traditional hard carbon anodes (Zhang et al., 2016; Xiong et al., 2015). Nevertheless, these anodes usually suffer from inherent low conductivity, easy stacking (like graphene), and visible volume changes (Sun et al., 2017; Xie et al., 2016). Although many MoS2-based materials have been developed to address these issues (Wang et al., 2016; Liu et al., 2016; Zhu et al., 2014; Bang et al., 2014), none of them has been able to deliver both a high specific capacity and long cycle life (>400 mAh g-1 with limited cycles (<400) or more than 1500 cycles with a low capacity (<300 mAh g-1)). It is well-known that MoS2-based anodes mainly store Na+ by intercalation (at 0.4–3 V, (MoS2+ xNa+ + xe- → NaxMoS2)) and conversion (below 0.4 V, (NaxMoS2+ (4-x)Na+ + (4-x)e- → Mo +2Na2S)) reactions (Park et al., 2013). Note that the capacity of electrode materials degrades after their reorganization and pulverization owing to the conversion reaction (Liu et al., 2017; Cabana et al., 2010). Recent studies have shown that ultra-stable capacity can be achieved by avoiding conversion reactions. For example, the MoS2 nanoflowers synthesized by Chen et al. (2014) showed a gradual increase in the specific capacity from 200 to
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296 mAh g-1 in the potential range of 0.4–3 V over 1500 cycles at 1 A g-1. This is the best result reported till date for MoS2-based SIB anode materials. However, the rate performance was only 175 mAh g-1 at 10 A g-1 and the cycling stability of such materials still need to be improved. Moreover, an increase in the cut-off voltage reduces the partial specific capacity of such materials. Therefore, it remains a big challenge to further improve the specific capacity while keeping it unchanged during each charge/discharge over many cycles. On the other hand, the bottleneck for achieving an ideal flexible electrode for the development of flexible batteries is the development of a method to anchor high-performance electrode materials firmly on flexible current collectors (Dong et al., 2016; Su et al., 2017; Zhang et al., 2015). Inspired by previous ab initio calculation-based studies, which have reported that electrode materials rich in heterointerfaces facilitate Li+ intercalation (Miwa1 and Scopel, 2013; Kresse and Furthmüller, 1996; Kresse and Furthmüller, 1996; Fang et al., 2016), herein, we successfully fabricated two-dimensional (2D) and robust MoS2/C nanosheets with well-defined heterointerfaces on the surface of a three-dimensional (3D) carbon cloth by a micro-area etching and functionalization process. The face-to-face contact of the MoS2/C nanosheet arrays (NSAs) maximized the heterointerface with an increase in the interlayer distance (~0.99 nm), thus improving the diffusion kinetics of Na+ in the 2D interlayer space. Consequently, our MoS2/C NSAs exhibited a high specific capacity of 433 mAh g-1 at 0.2 A g-1 and 232 mAh g-1 at 10 A g-1. These values are much higher than that of pure MoS2 nanosheet arrays (162 mAh g-1 at 10 A g-1). More importantly, the specific capacity remained constant at 320 mAh g-1at 1 A g-1 over 1500 cycles and at 271 mAh g-1 at 3 A g-1 over 10000 cycles. These are the best results ever reported for SIBs. Furthermore, the assembled flexible SIBs could withstand repeated bending for over 200 times. Hence, the findings of this study will be beneficial for the development of future flexible energy storage devices.
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2. Experimental section
2.1 Synthesis of MoS2/C NSAs First, a carbon cloth was micro-area etched and functionalized by treating with HNO3/H2SO4 (v/v = 1:3). Then, 0.5 g of thiourea, 0.5 g of Na2MoO4, and 0.8 g of sodium oleate were dissolved in a solvent containing 10 mL of ethanol, 10 mL of distilled water, and 1 mL of oleic acid (OA). The pH value of the resulting solution was adjusted to 1.0 by using a HCl solution. Subsequently, this solution was transferred into a 50-mL Teflon-lined stainless-steel autoclave with two pieces of the treated carbon cloth (12 cm2). The autoclave was then kept in an oven at 180 C for 22 h. The mixture was allowed to cool naturally and then washed with deionized water and absolute ethanol, thus yielding MoS2/OA NSAs on the carbon cloth. The as-obtained MoS2/OA NSAs were then immersed in 150 mL of distilled water containing 120 mg of 2-amino-2-hydroxymethyl-propane-1,3-diol (Tris). To this solution, 120 mg of dopamine (DA) was added to react with OA molecules and then self-polymerize into PDA under dissolved oxygen, forming MoS2/PDA-OA NSAs. Finally, MoS 2/C NSAs were obtained by carbonizing MoS 2/PDA-OA NSAs at 800 C for 2 h under an argon atmosphere. For comparison, pure MoS2NSAs were obtained by directly annealing MoS2/OA NSAs in a tube furnace at 800 C for 2 h under an argon atmosphere.
2.2 Characterization The structure and morphology of the as-prepared products were characterized using a field-emission scanning electron microscope (FESEM, Hitachi S-4800) and a transmission electron microscope (TEM, JEOL-2100F) operating at 200 kV with an X-ray energy dispersive spectrometer (EDS). X-ray diffraction (XRD) studies were carried out using a Rigaku D/Max2550 (Cu Kα radiation) at a scan rate of 1° min -1. X-ray photoelectron spectroscopy (XPS) and Raman spectra were recorded with an AXIS Ultra 5
Delay-Line Detector (DLD) spectrometer (Al Kα X-ray source) and a NEXUS 670 FT-IR Raman spectrometer, respectively. The electrical conductivity of the flexible electrodes was measured using a four-probe (model RTS-8, Guangzhou 4 Probes Tech Industrial Co., Ltd., Guangzhou, China) method.
2.3 Electrochemical Measurements
Electrochemical characterization was carried out using coin-type half cells assembled in an argon-filled glove box. The as-prepared sample (1 cm2 area, mass loading: ~1.1 mg) was directly used as the working electrode. A pure Na foil was employed as both the reference and counter electrodes. A solution of 1 M NaClO4 in propylene carbonate (PC) with 5 wt% fluoroethylene carbonate (FEC) was used as the electrolyte and a glass fibre membrane (Whatmann, Cat. No. 1823-047) was used as the separator. A 2 cm × 4 cm sample of MoS2/C NSAs was used to assemble the flexible half-cells. Cyclic voltammetry (CV) was carried out using Autolab PGSTAT302N at various sweep rates. Electrochemical impedance spectra (EIS) (in the frequency range of 100 kHz to 0.01 Hz in the constant voltage mode) were also obtained using the PGSTAT302N electrochemical workstation. Galvanostatic charge/discharge evaluations were carried out using a LAND-CT2001A test system at various current densities at room temperature.
3. Results and discussion Fig. 1a shows the schematic of the procedure used for fabricating MoS2/CNSAs. The 3D carbon cloth substrates were first activated by micro-area etching. Then, the MoS2/OA nanosheets were vertically grown on the surface of the etched carbon cloth via a hydrothermal process (Figs. S1a and b). The amidation reaction between OA and DA intensively transferred DA into the interlayer spacing of the MoS2 nanosheets. This DA subsequently self-polymerized into polydopamine (PDA), forming MoS2/PDA-OA NSAs (Figs. S1c and d). The MoS2/C NSA flexible electrodes were obtained after carbonization (Fig. 1b). The 6
high-magnification SEM image shown in Fig. 1c shows dense MoS2/C NSAs with a size of ~200 nm and a thickness of ~8.1 nm. The high-resolution TEM image shown in Fig. 1d reveals that the MoS2/C nanosheets possessed a well-defined heterointerface with an interlayer spacing ranging from 0.62 (Fig. S2a) to 0.99 nm. The selected-area electron diffraction (SAED) pattern (Fig. S3) of the MoS2/C nanosheets confirms their hexagonal structure. These results indicate the successful intercalation of a carbon layer (0.34 nm) into the interlayer spacing of the MoS2 nanosheets. The homogeneous elemental distribution of the nanosheets (Fig. 1e) further confirms the formation of 2D MoS2/C nanosheet heterostructures.
Fig. 1. (a) Schematic illustration of the fabrication process of MoS2/C NSAs, (b) low- and (c) high-magnification SEM images, (d) high-resolution TEM image, and (e) TEM-EDS mapping of the MoS2/C NSAs; (f) XRD patterns of MoS2/C NSAs, MoS2/PDA-OA NSAs, MoS2/OA NSAs, and the corresponding carbon cloth substrates; (g) Raman spectra of MoS2/C NSAs and pure MoS2 NSAs.
X-ray diffraction (XRD) and Raman spectroscopy studies were also employed to examine the increase in the interlayer distance of the as-obtained MoS2/C nanosheets. The (002) reflection peak of MoS2/OA NSAs was obtained at 9.0° (Fig. 1f), while that of pure MoS2NSAs was obtained at 14.2° (Fig. S2b). After
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intercalating PDA, the (002) reflection peak further shifted to a smaller angle of 8.2°. This angle increased to 8.9° in the case of the MoS2/C NSA sample having a large interlayer spacing of ~0.99 nm. These results were in good agreement with the TEM observations. Furthermore, the peaks indicated by solid triangles are the diffraction peaks of the etched carbon cloth. The TG curve of the MoS2/C NSA sample peeled off from the carbon cloth substrate is shown in Fig. S4. The figure shows that the mass ratio of the PDA-derived carbon was ~31%. The Raman spectroscopy results (Fig. 1g) reveal that MoS2/C NSAs showed clear amorphous carbon peaks at around 1364 and 1597 cm-1 unlike pure MoS2 NSAs. The intercalated carbon increased the electric conductivity of the nanosheets. In the MoS2-related part (inset of Fig. 1g), the frequency difference (Δk) between
1 2g
and A1g is 22.1 cm-1 for the MoS2/C NSAs (26.0 cm-1 for the MoS2 NSAs), indicative of
an almost single-layer MoS2 nanosheet (Liu et al., 2012; Yu et al., 2013). The aforementioned results show that we successfully synthesized 2D MoS2/C nanosheets with a large heterointerface anchored on a 3D carbon cloth.
Fig.2. O 1s XPS spectra and the corresponding SEM images of the carbon cloth etched for (a, d) 1 day, (b, e) 3 days, and (c, f) 10 days; (g–i) SEM images of the subsequent growth of the MoS 2 nanosheets on the etched carbon cloth.
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The surface micro-area etching of a carbon cloth plays a pivotal role in the synthesis of MoS2/C NSAs. Figs 2a–c show the O 1s XPS spectra of the carbon cloth etched for 1 day, 3 days, and 10 days, respectively. The two peaks at 531.7 and 533.2 eV can be attributed to the presence of C=O and C‒O bonds, respectively (Wang et al., 2015; Ventosa et al., 2012). With an increase in the etching time, more oxygen-containing groups were formed on the surface of the carbon cloth. The corresponding SEM images with different magnifications are also shown in Figs. 2d–f. A gradual increase in the surface roughness was observed. This resulted in an increase in the number of active and nucleation sites for the growth of active materials. Figs 2g–i show the SEM images of the samples after the growth of the MoS2 nanosheets on the carbon cloths etched for different time durations. From Fig. 2i, we can observe dense and vertical MoS2NSAs, which increased the number of active edge sites and open channels for Na + intercalation. Furthermore, the MoS2 nanosheets on the former two carbon cloths appeared seriously damaged because of the subsequent treatments (Fig. S5). On the other hand, the structure of the MoS2 nanosheets on the last carbon cloth (Fig. 1c) remained unchanged even after an ultrasonic treatment for 20 min (Fig. S6). These results indicate that the micro-area etching strategy not only facilitates the direct growth of MoS2NSAs on carbon cloths, but also strengthens the bond between them, thus making it possible to develop an ideal electrode for flexible energy storage devices. Both MoS2/C NSAs and pure MoS2NSAs were directly used as anodes to assemble coin-type half cells for evaluating their respective sodium-ion storage performance. To avoid the structural reorganization and pulverization caused by the conversion reaction, the cut-off voltage was slightly increased to achieve a long cycle life. Fig. S7 in the supporting information shows the three initial CV curves of MoS2/C NSAs over the voltage range of 0.4 and 3.0 V at 0.2 mV s-1. In the first cycle, two reduction peaks at approximately 0.95 and 9
0.50 V, corresponding to the stepwise Na+ intercalation (xNa + MoS2 → NaxMoS2(x ≤ 0.5) and yNa + NaxMoS2 → Nax+yMoS2 (0.5 < x + y< 2), respectively) were observed (Park et al., 2013; Ding et al., 2016). The oxidation peaks at approximately 1.34, 1.57, and 1.97 V correspond to the reverse desodiation process (Li et al., 2015; Cook et al., 2016). The other peaks are attributed to the etched carbon cloths (Fig. S8). In the two subsequent cycles, the CV curves almost overlapped, implying a highly reversible charge/discharge process. Figs 3a and b show the CV curves of MoS2/C NSAs and MoS2NSAs at various scan rates. It can be seen
Fig.3. CV curves at various sweep rates from 0.1 to 1.0 mV s -1 of (a) MoS2/C NSAs and (b) MoS2 NSAs, (c–d) log i vs. log v plots for obtaining b-values based on the above corresponding redox peaks.
that the shape of the MoS2/C NSAs peaks remained almost unchanged even when the scan rate was increased from 0.1 to 1 mV s-1, indicating the fast Na+ insertion and extraction kinetics (Chao et al., 2016) of MoS2/C NSAs. To further support this point, the log i vs. log v plots of MoS2/C NSAs and MoS2NSAs were obtained. The relationship between the sweep rate (v) and peak current (i) obeys a power law:
(Augustyn et al.,
2014; Augustyn et al., 2013;Brezesinski et al., 2010), where a and b are coefficients. In general, sodium 10
storage is diffusion-controlled if
0.5. However, the process is capacitive at
1. As shown in Figs. 3c
and d, MoS2/C NSAs exhibited higher b values for both cathodic (0.86) and anodic (0.99) peaks as compared to MoS2NSAs (0.66 and 0.91), indicating a rapid Na + insertion process with a typical capacitive behaviour. Therefore, the MoS2/C NSAs electrode showed a fast charge/discharge capability and a stable cycling performance.
Fig.4. (a) Rate performances and (b) fast charging capability of MoS2/C NSAs and MoS2 NSAs, (c) the subsequent cycling performance at 1 A g-1 for another 1500 cycles, and (d) the overall cycling performance of MoS2/C NSAs at 3 A g-1 for 10000 cycles.
Fig. S9 shows the first three discharge/charge profiles of MoS2/C NSAs at 0.2 A g-1. The initial discharge capacity reached 760.8 mAh g-1 with a Coulombic efficiency (CE) of 57%. This relatively low CE is mainly attributed to the side reactions caused by the carbon cloth (Fig. S8) and the formation of a solid
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electrolyte interface (SEI) film (Ding et al., 2016; Slater et al., 2013). Therefore, the CE of MoS2/C NSAs increased to 95% in the second cycle. We then evaluated the discharge capability of MoS2/C NSAs at current densities in the range of 0.2–10 A g-1. Fig. 4a, shows that MoS2/C NSAs could deliver an average discharge capacity of 433, 347, 292, 253, 238, and 232 mAh g-1 at 0.2, 1, 3, 6, 8, and 10 A g-1, respectively. The discharge capacity increased to 439 mAh g-1 when the current density was reduced to 0.2 A g-1. Here, the specific capacities were calculated using the total mass of MoS2 and the PDA-derived carbon. Pure MoS2NSAs were also synthesized as control samples. They showed a relatively lower specific capacity of 314 mAh g-1 at 0.2 A g-1and 162 mAh g-1at 10 A g-1 because of their slower electron transfer and sluggish Na+ diffusion kinetics. The EIS results (Fig. S10) reveal that MoS2/C NSAs showed a much lower resistance (317 Ω) than MoS2NSAs (551 Ω), indicating that the carbon layer intercalation greatly accelerated the charge transfer and ion diffusion. We further measured their charge capability, which is also very important for practical applications like powering a cell phone or electric vehicles in a short time. Once the two fully charged batteries completely discharged at 50 mA g-1, the discharge capacities were assigned to their own benchmark values. As shown in Fig. 4b, the capacity of MoS2/C NSAs could be charged to the benchmark value of 89, 86, 83, and 80% at 200, 1000, 3000, and 6000 mA g-1, respectively. On the other hand, MoS2NSAs could only be charged to the benchmark capacity of 80, 73, 70, and 68% at 200, 1000, 3000, and 6000 mA g-1, respectively. The above results show that MoS2/C NSAs exhibited a higher charge and discharge capability than MoS2NSAs. More importantly, MoS2/C NSAs exhibited an almost unchanged capacity retention of 319 mAh g-1 at 1 A g-1 during the additional 1500 cycles (Fig. 4c). It was also observed that MoS2/C NSAs showed a higher cycling stability at a high current density of 3 A g-1 (Fig. 4d). A specific capacity of ~271 mAh g-1 for each cycle could be maintained over the entire duration of 10000 cycles. It is worth mentioning that the nanosheet morphology of the samples could still be maintained with a strong 12
adhesion to the carbon cloth (Fig.S11). To the best of our knowledge, the MoS2/C NSAs developed in this study are the best MoS2-based anode materials ever reported for SIBs in terms of sodium storage capacity (at various current densities) and cycle life. A comprehensive comparison with references has also been provided in Table S1 of the supporting information. The excellent and ultra-stable sodium storage capability of the MoS2/C NSAs developed in this study can be attributed to the following. (a) The 2D MoS 2/C nanosheet with a well-defined heterointerface significantly accelerated the electron transfer. This is clear from the EIS (Fig.S10) and conductivity measurement (Fig. 5a) results. (b) The intercalated carbon layer not only effectively restrained the stacking/restacking of the graphene-like MoS2 nanosheets after a long charge/discharge process, but also increased the interlayer spacing of the MoS2 nanosheets from 0.62 to 0.99 nm, thus guaranteeing rapid intercalation reaction kinetics between the layers even at high rates by strengthening the Na+ mass transfer. (c) The micro-area etching strategy rendered the surface of the carbon cloth with a large number of active sites and functional groups (Fig. 2c), which facilitated the growth of MoS2 nanosheets and strengthened the bond between them (e.g. the MoS2 nanosheets remained undetached even after an ultrasonic treatment for 20 min). The vertical growth of the MoS2 nanosheets on the carbon cloth further enhanced the electron transfer while avoiding the weakness of a low charge transfer efficiency in the c-axis direction for the MoS2 nanosheets. Furthermore, the as-formed NSAs provided an excellent transport channel for the electrolyte. (d) The high cut-off voltage prevented the structural reorganization and pulverization of the active materials caused by the conversion reaction and sodium storage mechanism during the cycles. Therefore, the MoS2/C NSAs anode exhibited an ultra-stable and improved sodium storage capacity.
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Fig. 5. (a) The electronic conductivity values of MoS2/C NSAs and MoS2 NSAs during the bending test for 1000 cycles, (b) the charge/discharge curves of the assembled MoS2/C NSAs flexible battery under the flat state and after 1, 100, and 200 bending cycles, (c–e) a white LED connected to the flexible battery under the flat and bending states and after 200 bending cycles.
To demonstrate the flexibility of the MoS2/C NSA electrodes, their electronic conductivity was measured by a four-probe method under different mechanical deformation conditions. Fig. 5a shows that the conductivity of the MoS2/C NSA electrode was ~8.8 S cm-1 without any obvious change even after 1000 bending cycles. This value is approximately 11 times higher than that obtained for the pure MoS2NSA electrode (~0.8 S cm-1). Such excellent flexibility and high conductivity make MoS2/C NSAs promising anodes for flexible batteries. To further evaluate their performance as SIB anodes we assembled MoS2/C NSAs into a flexible sodium-ion half-cell. Fig. 5b shows the charge/discharge profiles obtained at different bending times. For 1-time bending, the discharge capacity was found to be about 296 mAh g-1. When the bending was done 200 times, a discharge capacity of 279 mAh g-1 (~ 94% capacity retention) was obtained, demonstrating a highly reversible sodium storage capability of the MoS2/C NSA electrode. Furthermore, the assembled flexible half-cell could also power a white light-emitting diode (LED) under both the flat and
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bending states (Figs. 5c and d). The brightness of the LED remained the same even after 200-times bending (Fig. 5e), further unveiling the excellent mechanical flexibility and durability of the electrode.
4. Conclusions In summary, we successfully fabricated vertically grown 2D MoS2/C nanosheets with a well-defined heterointerface on a carbon cloth via a micro-area etching and functionalization strategy. The face-to-face contact of MoS2/C NSAs with the carbon cloth maximized the heterointerface area and increased the interlayer distance (~0.99 nm), which facilitated Na+ intercalation and also improved the diffusion kinetics of Na+ in the interlayer. This led to a high sodium storage capacity and rapid charge/discharge capability. Meanwhile, excellent cycling stability was also achieved by modulating the cut-off voltage to avoid the structural pulverization caused by the conversion reaction. Consequently, the MoS 2/C NSA anode exhibited a high capacity of 433 mAh g-1 at 0.2 A g-1 and 232 mAh g-1 at 10 A g-1 within the potential range of 0.4–3.0 V. These values were much higher than those obtained using pure MoS2 nanosheet arrays (162 mAh g-1 at 10 A g-1). More importantly, the capacity could be maintained at ~320 mAh g-1 at 1 A g-1 for over 1500 cycles and ~271 mAh g-1 at 3 A g-1 after 10000 cycles, outperforming the previously reported MoS2-based SIBs. Furthermore, after being assembled into a flexible battery, the MoS2/C NSA anode could withstand repeated bending for over 200 cycles without an obvious capacity loss, implying significant potential for future flexible batteries.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21522602, 51672082, 91534202), the International Science and Technology Cooperation Program of China (2016YFE0131200),
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the Shanghai Rising-Star Program (15QA1401200), the Basic Research Program of Shanghai (17JC1402300), and the Fundamental Research Funds for the Central Universities (222201718002).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://www.elsevier.com/.
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Graphical abstract
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Highlights 1. The carbon cloths are modified by micro-area etching and functionalization. 2. The maximal 2D heterointerface greatly improves sodium storage capacity. 3. The enlarged interlayer space of 0.99 nm enhances the diffusion kinetics of Na+. 4. An ultrastable capacity retention is achieved for over 10000 cycles.
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S0009-2509(17)30552-3 http://dx.doi.org/10.1016/j.ces.2017.09.007 CES 13784
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
25 July 2017 26 August 2017 1 September 2017
Please cite this article as: H. Wang, H. Jiang, Y. Hu, P. Saha, Q. Cheng, C. Li, Interface-engineered MoS2/C nanosheet heterostructure arrays for ultra-stable sodium-ion batteries, Chemical Engineering Science (2017), doi: http://dx.doi.org/10.1016/j.ces.2017.09.007
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Interface-engineered MoS2/C nanosheet heterostructure arrays for ultra-stable sodium-ion batteries Haiyan Wang a, Hao Jiang a,*, Yanjie Hu a, Petr Saha b,Qilin Cheng a, Chunzhong Li a,* a
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and
Engineering, East China University of Science and Technology, Shanghai 200237, China b
Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Trida T. Bati 5678, 760 01
Zlin, Czech Republic Email: [email protected] (Prof. H. Jiang) and [email protected] (Prof. C. Li)
Abstract Development of ultra-stable high capacity electrodes is imperative for the widespread commercialization of sodium-ion batteries. Herein, we employed a micro-area etching and surface functionalization strategy to synthesize two-dimensional (2D) MoS2/C nanosheets with a well-defined heterointerface vertically anchored on a carbon cloth. The large MoS2/C nanosheet heterointerface and a high interlayer distance (0.99 nm) not only facilitated Na+ intercalation but also improved the diffusion kinetics of Na+ in the 2D interlayer space. A modulation of the cut-off voltage yielded a high specific capacity of 433 mAh g-1 at 0.2 A g-1 and 232 mAh g-1 at 10 A g-1 within the potential range of 0.4–3.0 V. These values are much higher than that of pure MoS2 nanosheet arrays (162 mAh g-1 at 10 A g-1). More importantly, during the first 1500 cycles, the capacity was maintained at ~320 mAh g-1 at 1 A g-1, while after 10000 cycles, it became approximately ~271 mAh g-1 at 3 A g-1. These are the best values ever reported for MoS2-based anode materials for SIBs. Furthermore, after being assembled into a flexible battery, it withstand repeated bending for over 200 times without any obvious capacity loss. Hence, this material is a promising electrode for future flexible batteries. 1
Keywords: MoS2, heterointerface, micro-area etching, sodium ion batteries, flexible electrode.
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1. Introduction Sodium-ion batteries (SIBs) are regarded as future alternatives to lithium-ion batteries because of the abundance of sodium and its charge storage mechanisms, which are similar to those of lithium (Grey and Tarascon, 2017; Kim et al., 2016; Xu et al., 2015). As the atomic radius of Na+ is higher than that of Li+(1.02Å vs. 0.76 Å), it is difficult to achieve Na+ intercalation using a graphite anode with an interlayer distance of 0.34 nm (Zhou et al., 2016; Stevens and Dahn, 2000; Kim et al., 2015). Hence, the development of anodes with a wider interlayer spacing is pivotal for developing SIBs. In recent years, graphene-like MoS 2 nanosheets have gained attention as potential anode materials for SIBs because of their large interlayer distance (0.64 nm), weak Van der Waals interactions, and a higher degree of safety as compared to traditional hard carbon anodes (Zhang et al., 2016; Xiong et al., 2015). Nevertheless, these anodes usually suffer from inherent low conductivity, easy stacking (like graphene), and visible volume changes (Sun et al., 2017; Xie et al., 2016). Although many MoS2-based materials have been developed to address these issues (Wang et al., 2016; Liu et al., 2016; Zhu et al., 2014; Bang et al., 2014), none of them has been able to deliver both a high specific capacity and long cycle life (>400 mAh g-1 with limited cycles (<400) or more than 1500 cycles with a low capacity (<300 mAh g-1)). It is well-known that MoS2-based anodes mainly store Na+ by intercalation (at 0.4–3 V, (MoS2+ xNa+ + xe- → NaxMoS2)) and conversion (below 0.4 V, (NaxMoS2+ (4-x)Na+ + (4-x)e- → Mo +2Na2S)) reactions (Park et al., 2013). Note that the capacity of electrode materials degrades after their reorganization and pulverization owing to the conversion reaction (Liu et al., 2017; Cabana et al., 2010). Recent studies have shown that ultra-stable capacity can be achieved by avoiding conversion reactions. For example, the MoS2 nanoflowers synthesized by Chen et al. (2014) showed a gradual increase in the specific capacity from 200 to
3
296 mAh g-1 in the potential range of 0.4–3 V over 1500 cycles at 1 A g-1. This is the best result reported till date for MoS2-based SIB anode materials. However, the rate performance was only 175 mAh g-1 at 10 A g-1 and the cycling stability of such materials still need to be improved. Moreover, an increase in the cut-off voltage reduces the partial specific capacity of such materials. Therefore, it remains a big challenge to further improve the specific capacity while keeping it unchanged during each charge/discharge over many cycles. On the other hand, the bottleneck for achieving an ideal flexible electrode for the development of flexible batteries is the development of a method to anchor high-performance electrode materials firmly on flexible current collectors (Dong et al., 2016; Su et al., 2017; Zhang et al., 2015). Inspired by previous ab initio calculation-based studies, which have reported that electrode materials rich in heterointerfaces facilitate Li+ intercalation (Miwa1 and Scopel, 2013; Kresse and Furthmüller, 1996; Kresse and Furthmüller, 1996; Fang et al., 2016), herein, we successfully fabricated two-dimensional (2D) and robust MoS2/C nanosheets with well-defined heterointerfaces on the surface of a three-dimensional (3D) carbon cloth by a micro-area etching and functionalization process. The face-to-face contact of the MoS2/C nanosheet arrays (NSAs) maximized the heterointerface with an increase in the interlayer distance (~0.99 nm), thus improving the diffusion kinetics of Na+ in the 2D interlayer space. Consequently, our MoS2/C NSAs exhibited a high specific capacity of 433 mAh g-1 at 0.2 A g-1 and 232 mAh g-1 at 10 A g-1. These values are much higher than that of pure MoS2 nanosheet arrays (162 mAh g-1 at 10 A g-1). More importantly, the specific capacity remained constant at 320 mAh g-1at 1 A g-1 over 1500 cycles and at 271 mAh g-1 at 3 A g-1 over 10000 cycles. These are the best results ever reported for SIBs. Furthermore, the assembled flexible SIBs could withstand repeated bending for over 200 times. Hence, the findings of this study will be beneficial for the development of future flexible energy storage devices.
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2. Experimental section
2.1 Synthesis of MoS2/C NSAs First, a carbon cloth was micro-area etched and functionalized by treating with HNO3/H2SO4 (v/v = 1:3). Then, 0.5 g of thiourea, 0.5 g of Na2MoO4, and 0.8 g of sodium oleate were dissolved in a solvent containing 10 mL of ethanol, 10 mL of distilled water, and 1 mL of oleic acid (OA). The pH value of the resulting solution was adjusted to 1.0 by using a HCl solution. Subsequently, this solution was transferred into a 50-mL Teflon-lined stainless-steel autoclave with two pieces of the treated carbon cloth (12 cm2). The autoclave was then kept in an oven at 180 C for 22 h. The mixture was allowed to cool naturally and then washed with deionized water and absolute ethanol, thus yielding MoS2/OA NSAs on the carbon cloth. The as-obtained MoS2/OA NSAs were then immersed in 150 mL of distilled water containing 120 mg of 2-amino-2-hydroxymethyl-propane-1,3-diol (Tris). To this solution, 120 mg of dopamine (DA) was added to react with OA molecules and then self-polymerize into PDA under dissolved oxygen, forming MoS2/PDA-OA NSAs. Finally, MoS 2/C NSAs were obtained by carbonizing MoS 2/PDA-OA NSAs at 800 C for 2 h under an argon atmosphere. For comparison, pure MoS2NSAs were obtained by directly annealing MoS2/OA NSAs in a tube furnace at 800 C for 2 h under an argon atmosphere.
2.2 Characterization The structure and morphology of the as-prepared products were characterized using a field-emission scanning electron microscope (FESEM, Hitachi S-4800) and a transmission electron microscope (TEM, JEOL-2100F) operating at 200 kV with an X-ray energy dispersive spectrometer (EDS). X-ray diffraction (XRD) studies were carried out using a Rigaku D/Max2550 (Cu Kα radiation) at a scan rate of 1° min -1. X-ray photoelectron spectroscopy (XPS) and Raman spectra were recorded with an AXIS Ultra 5
Delay-Line Detector (DLD) spectrometer (Al Kα X-ray source) and a NEXUS 670 FT-IR Raman spectrometer, respectively. The electrical conductivity of the flexible electrodes was measured using a four-probe (model RTS-8, Guangzhou 4 Probes Tech Industrial Co., Ltd., Guangzhou, China) method.
2.3 Electrochemical Measurements
Electrochemical characterization was carried out using coin-type half cells assembled in an argon-filled glove box. The as-prepared sample (1 cm2 area, mass loading: ~1.1 mg) was directly used as the working electrode. A pure Na foil was employed as both the reference and counter electrodes. A solution of 1 M NaClO4 in propylene carbonate (PC) with 5 wt% fluoroethylene carbonate (FEC) was used as the electrolyte and a glass fibre membrane (Whatmann, Cat. No. 1823-047) was used as the separator. A 2 cm × 4 cm sample of MoS2/C NSAs was used to assemble the flexible half-cells. Cyclic voltammetry (CV) was carried out using Autolab PGSTAT302N at various sweep rates. Electrochemical impedance spectra (EIS) (in the frequency range of 100 kHz to 0.01 Hz in the constant voltage mode) were also obtained using the PGSTAT302N electrochemical workstation. Galvanostatic charge/discharge evaluations were carried out using a LAND-CT2001A test system at various current densities at room temperature.
3. Results and discussion Fig. 1a shows the schematic of the procedure used for fabricating MoS2/CNSAs. The 3D carbon cloth substrates were first activated by micro-area etching. Then, the MoS2/OA nanosheets were vertically grown on the surface of the etched carbon cloth via a hydrothermal process (Figs. S1a and b). The amidation reaction between OA and DA intensively transferred DA into the interlayer spacing of the MoS2 nanosheets. This DA subsequently self-polymerized into polydopamine (PDA), forming MoS2/PDA-OA NSAs (Figs. S1c and d). The MoS2/C NSA flexible electrodes were obtained after carbonization (Fig. 1b). The 6
high-magnification SEM image shown in Fig. 1c shows dense MoS2/C NSAs with a size of ~200 nm and a thickness of ~8.1 nm. The high-resolution TEM image shown in Fig. 1d reveals that the MoS2/C nanosheets possessed a well-defined heterointerface with an interlayer spacing ranging from 0.62 (Fig. S2a) to 0.99 nm. The selected-area electron diffraction (SAED) pattern (Fig. S3) of the MoS2/C nanosheets confirms their hexagonal structure. These results indicate the successful intercalation of a carbon layer (0.34 nm) into the interlayer spacing of the MoS2 nanosheets. The homogeneous elemental distribution of the nanosheets (Fig. 1e) further confirms the formation of 2D MoS2/C nanosheet heterostructures.
Fig. 1. (a) Schematic illustration of the fabrication process of MoS2/C NSAs, (b) low- and (c) high-magnification SEM images, (d) high-resolution TEM image, and (e) TEM-EDS mapping of the MoS2/C NSAs; (f) XRD patterns of MoS2/C NSAs, MoS2/PDA-OA NSAs, MoS2/OA NSAs, and the corresponding carbon cloth substrates; (g) Raman spectra of MoS2/C NSAs and pure MoS2 NSAs.
X-ray diffraction (XRD) and Raman spectroscopy studies were also employed to examine the increase in the interlayer distance of the as-obtained MoS2/C nanosheets. The (002) reflection peak of MoS2/OA NSAs was obtained at 9.0° (Fig. 1f), while that of pure MoS2NSAs was obtained at 14.2° (Fig. S2b). After
7
intercalating PDA, the (002) reflection peak further shifted to a smaller angle of 8.2°. This angle increased to 8.9° in the case of the MoS2/C NSA sample having a large interlayer spacing of ~0.99 nm. These results were in good agreement with the TEM observations. Furthermore, the peaks indicated by solid triangles are the diffraction peaks of the etched carbon cloth. The TG curve of the MoS2/C NSA sample peeled off from the carbon cloth substrate is shown in Fig. S4. The figure shows that the mass ratio of the PDA-derived carbon was ~31%. The Raman spectroscopy results (Fig. 1g) reveal that MoS2/C NSAs showed clear amorphous carbon peaks at around 1364 and 1597 cm-1 unlike pure MoS2 NSAs. The intercalated carbon increased the electric conductivity of the nanosheets. In the MoS2-related part (inset of Fig. 1g), the frequency difference (Δk) between
1 2g
and A1g is 22.1 cm-1 for the MoS2/C NSAs (26.0 cm-1 for the MoS2 NSAs), indicative of
an almost single-layer MoS2 nanosheet (Liu et al., 2012; Yu et al., 2013). The aforementioned results show that we successfully synthesized 2D MoS2/C nanosheets with a large heterointerface anchored on a 3D carbon cloth.
Fig.2. O 1s XPS spectra and the corresponding SEM images of the carbon cloth etched for (a, d) 1 day, (b, e) 3 days, and (c, f) 10 days; (g–i) SEM images of the subsequent growth of the MoS 2 nanosheets on the etched carbon cloth.
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The surface micro-area etching of a carbon cloth plays a pivotal role in the synthesis of MoS2/C NSAs. Figs 2a–c show the O 1s XPS spectra of the carbon cloth etched for 1 day, 3 days, and 10 days, respectively. The two peaks at 531.7 and 533.2 eV can be attributed to the presence of C=O and C‒O bonds, respectively (Wang et al., 2015; Ventosa et al., 2012). With an increase in the etching time, more oxygen-containing groups were formed on the surface of the carbon cloth. The corresponding SEM images with different magnifications are also shown in Figs. 2d–f. A gradual increase in the surface roughness was observed. This resulted in an increase in the number of active and nucleation sites for the growth of active materials. Figs 2g–i show the SEM images of the samples after the growth of the MoS2 nanosheets on the carbon cloths etched for different time durations. From Fig. 2i, we can observe dense and vertical MoS2NSAs, which increased the number of active edge sites and open channels for Na + intercalation. Furthermore, the MoS2 nanosheets on the former two carbon cloths appeared seriously damaged because of the subsequent treatments (Fig. S5). On the other hand, the structure of the MoS2 nanosheets on the last carbon cloth (Fig. 1c) remained unchanged even after an ultrasonic treatment for 20 min (Fig. S6). These results indicate that the micro-area etching strategy not only facilitates the direct growth of MoS2NSAs on carbon cloths, but also strengthens the bond between them, thus making it possible to develop an ideal electrode for flexible energy storage devices. Both MoS2/C NSAs and pure MoS2NSAs were directly used as anodes to assemble coin-type half cells for evaluating their respective sodium-ion storage performance. To avoid the structural reorganization and pulverization caused by the conversion reaction, the cut-off voltage was slightly increased to achieve a long cycle life. Fig. S7 in the supporting information shows the three initial CV curves of MoS2/C NSAs over the voltage range of 0.4 and 3.0 V at 0.2 mV s-1. In the first cycle, two reduction peaks at approximately 0.95 and 9
0.50 V, corresponding to the stepwise Na+ intercalation (xNa + MoS2 → NaxMoS2(x ≤ 0.5) and yNa + NaxMoS2 → Nax+yMoS2 (0.5 < x + y< 2), respectively) were observed (Park et al., 2013; Ding et al., 2016). The oxidation peaks at approximately 1.34, 1.57, and 1.97 V correspond to the reverse desodiation process (Li et al., 2015; Cook et al., 2016). The other peaks are attributed to the etched carbon cloths (Fig. S8). In the two subsequent cycles, the CV curves almost overlapped, implying a highly reversible charge/discharge process. Figs 3a and b show the CV curves of MoS2/C NSAs and MoS2NSAs at various scan rates. It can be seen
Fig.3. CV curves at various sweep rates from 0.1 to 1.0 mV s -1 of (a) MoS2/C NSAs and (b) MoS2 NSAs, (c–d) log i vs. log v plots for obtaining b-values based on the above corresponding redox peaks.
that the shape of the MoS2/C NSAs peaks remained almost unchanged even when the scan rate was increased from 0.1 to 1 mV s-1, indicating the fast Na+ insertion and extraction kinetics (Chao et al., 2016) of MoS2/C NSAs. To further support this point, the log i vs. log v plots of MoS2/C NSAs and MoS2NSAs were obtained. The relationship between the sweep rate (v) and peak current (i) obeys a power law:
(Augustyn et al.,
2014; Augustyn et al., 2013;Brezesinski et al., 2010), where a and b are coefficients. In general, sodium 10
storage is diffusion-controlled if
0.5. However, the process is capacitive at
1. As shown in Figs. 3c
and d, MoS2/C NSAs exhibited higher b values for both cathodic (0.86) and anodic (0.99) peaks as compared to MoS2NSAs (0.66 and 0.91), indicating a rapid Na + insertion process with a typical capacitive behaviour. Therefore, the MoS2/C NSAs electrode showed a fast charge/discharge capability and a stable cycling performance.
Fig.4. (a) Rate performances and (b) fast charging capability of MoS2/C NSAs and MoS2 NSAs, (c) the subsequent cycling performance at 1 A g-1 for another 1500 cycles, and (d) the overall cycling performance of MoS2/C NSAs at 3 A g-1 for 10000 cycles.
Fig. S9 shows the first three discharge/charge profiles of MoS2/C NSAs at 0.2 A g-1. The initial discharge capacity reached 760.8 mAh g-1 with a Coulombic efficiency (CE) of 57%. This relatively low CE is mainly attributed to the side reactions caused by the carbon cloth (Fig. S8) and the formation of a solid
11
electrolyte interface (SEI) film (Ding et al., 2016; Slater et al., 2013). Therefore, the CE of MoS2/C NSAs increased to 95% in the second cycle. We then evaluated the discharge capability of MoS2/C NSAs at current densities in the range of 0.2–10 A g-1. Fig. 4a, shows that MoS2/C NSAs could deliver an average discharge capacity of 433, 347, 292, 253, 238, and 232 mAh g-1 at 0.2, 1, 3, 6, 8, and 10 A g-1, respectively. The discharge capacity increased to 439 mAh g-1 when the current density was reduced to 0.2 A g-1. Here, the specific capacities were calculated using the total mass of MoS2 and the PDA-derived carbon. Pure MoS2NSAs were also synthesized as control samples. They showed a relatively lower specific capacity of 314 mAh g-1 at 0.2 A g-1and 162 mAh g-1at 10 A g-1 because of their slower electron transfer and sluggish Na+ diffusion kinetics. The EIS results (Fig. S10) reveal that MoS2/C NSAs showed a much lower resistance (317 Ω) than MoS2NSAs (551 Ω), indicating that the carbon layer intercalation greatly accelerated the charge transfer and ion diffusion. We further measured their charge capability, which is also very important for practical applications like powering a cell phone or electric vehicles in a short time. Once the two fully charged batteries completely discharged at 50 mA g-1, the discharge capacities were assigned to their own benchmark values. As shown in Fig. 4b, the capacity of MoS2/C NSAs could be charged to the benchmark value of 89, 86, 83, and 80% at 200, 1000, 3000, and 6000 mA g-1, respectively. On the other hand, MoS2NSAs could only be charged to the benchmark capacity of 80, 73, 70, and 68% at 200, 1000, 3000, and 6000 mA g-1, respectively. The above results show that MoS2/C NSAs exhibited a higher charge and discharge capability than MoS2NSAs. More importantly, MoS2/C NSAs exhibited an almost unchanged capacity retention of 319 mAh g-1 at 1 A g-1 during the additional 1500 cycles (Fig. 4c). It was also observed that MoS2/C NSAs showed a higher cycling stability at a high current density of 3 A g-1 (Fig. 4d). A specific capacity of ~271 mAh g-1 for each cycle could be maintained over the entire duration of 10000 cycles. It is worth mentioning that the nanosheet morphology of the samples could still be maintained with a strong 12
adhesion to the carbon cloth (Fig.S11). To the best of our knowledge, the MoS2/C NSAs developed in this study are the best MoS2-based anode materials ever reported for SIBs in terms of sodium storage capacity (at various current densities) and cycle life. A comprehensive comparison with references has also been provided in Table S1 of the supporting information. The excellent and ultra-stable sodium storage capability of the MoS2/C NSAs developed in this study can be attributed to the following. (a) The 2D MoS 2/C nanosheet with a well-defined heterointerface significantly accelerated the electron transfer. This is clear from the EIS (Fig.S10) and conductivity measurement (Fig. 5a) results. (b) The intercalated carbon layer not only effectively restrained the stacking/restacking of the graphene-like MoS2 nanosheets after a long charge/discharge process, but also increased the interlayer spacing of the MoS2 nanosheets from 0.62 to 0.99 nm, thus guaranteeing rapid intercalation reaction kinetics between the layers even at high rates by strengthening the Na+ mass transfer. (c) The micro-area etching strategy rendered the surface of the carbon cloth with a large number of active sites and functional groups (Fig. 2c), which facilitated the growth of MoS2 nanosheets and strengthened the bond between them (e.g. the MoS2 nanosheets remained undetached even after an ultrasonic treatment for 20 min). The vertical growth of the MoS2 nanosheets on the carbon cloth further enhanced the electron transfer while avoiding the weakness of a low charge transfer efficiency in the c-axis direction for the MoS2 nanosheets. Furthermore, the as-formed NSAs provided an excellent transport channel for the electrolyte. (d) The high cut-off voltage prevented the structural reorganization and pulverization of the active materials caused by the conversion reaction and sodium storage mechanism during the cycles. Therefore, the MoS2/C NSAs anode exhibited an ultra-stable and improved sodium storage capacity.
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Fig. 5. (a) The electronic conductivity values of MoS2/C NSAs and MoS2 NSAs during the bending test for 1000 cycles, (b) the charge/discharge curves of the assembled MoS2/C NSAs flexible battery under the flat state and after 1, 100, and 200 bending cycles, (c–e) a white LED connected to the flexible battery under the flat and bending states and after 200 bending cycles.
To demonstrate the flexibility of the MoS2/C NSA electrodes, their electronic conductivity was measured by a four-probe method under different mechanical deformation conditions. Fig. 5a shows that the conductivity of the MoS2/C NSA electrode was ~8.8 S cm-1 without any obvious change even after 1000 bending cycles. This value is approximately 11 times higher than that obtained for the pure MoS2NSA electrode (~0.8 S cm-1). Such excellent flexibility and high conductivity make MoS2/C NSAs promising anodes for flexible batteries. To further evaluate their performance as SIB anodes we assembled MoS2/C NSAs into a flexible sodium-ion half-cell. Fig. 5b shows the charge/discharge profiles obtained at different bending times. For 1-time bending, the discharge capacity was found to be about 296 mAh g-1. When the bending was done 200 times, a discharge capacity of 279 mAh g-1 (~ 94% capacity retention) was obtained, demonstrating a highly reversible sodium storage capability of the MoS2/C NSA electrode. Furthermore, the assembled flexible half-cell could also power a white light-emitting diode (LED) under both the flat and
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bending states (Figs. 5c and d). The brightness of the LED remained the same even after 200-times bending (Fig. 5e), further unveiling the excellent mechanical flexibility and durability of the electrode.
4. Conclusions In summary, we successfully fabricated vertically grown 2D MoS2/C nanosheets with a well-defined heterointerface on a carbon cloth via a micro-area etching and functionalization strategy. The face-to-face contact of MoS2/C NSAs with the carbon cloth maximized the heterointerface area and increased the interlayer distance (~0.99 nm), which facilitated Na+ intercalation and also improved the diffusion kinetics of Na+ in the interlayer. This led to a high sodium storage capacity and rapid charge/discharge capability. Meanwhile, excellent cycling stability was also achieved by modulating the cut-off voltage to avoid the structural pulverization caused by the conversion reaction. Consequently, the MoS 2/C NSA anode exhibited a high capacity of 433 mAh g-1 at 0.2 A g-1 and 232 mAh g-1 at 10 A g-1 within the potential range of 0.4–3.0 V. These values were much higher than those obtained using pure MoS2 nanosheet arrays (162 mAh g-1 at 10 A g-1). More importantly, the capacity could be maintained at ~320 mAh g-1 at 1 A g-1 for over 1500 cycles and ~271 mAh g-1 at 3 A g-1 after 10000 cycles, outperforming the previously reported MoS2-based SIBs. Furthermore, after being assembled into a flexible battery, the MoS2/C NSA anode could withstand repeated bending for over 200 cycles without an obvious capacity loss, implying significant potential for future flexible batteries.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21522602, 51672082, 91534202), the International Science and Technology Cooperation Program of China (2016YFE0131200),
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the Shanghai Rising-Star Program (15QA1401200), the Basic Research Program of Shanghai (17JC1402300), and the Fundamental Research Funds for the Central Universities (222201718002).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://www.elsevier.com/.
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Graphical abstract
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Highlights 1. The carbon cloths are modified by micro-area etching and functionalization. 2. The maximal 2D heterointerface greatly improves sodium storage capacity. 3. The enlarged interlayer space of 0.99 nm enhances the diffusion kinetics of Na+. 4. An ultrastable capacity retention is achieved for over 10000 cycles.
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