Stable 1T-MoSe2 and Carbon Nanotube Hybridized Flexible Film

May 25, 2017 - Two-dimensional stable metallic 1T-MoSe2 with expanded interlayer spacing of 10.0 Å in situ grown on SWCNTs film is fabricated via a o...
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Stable 1T-MoSe2 and Carbon Nanotube Hybridized Flexible Film: Binder-Free and High-Performance Li-Ion Anode Ting Xiang,†,# Shi Tao,‡,# Weiyu Xu,† Qi Fang,† Chuanqiang Wu,† Daobin Liu,† Yu Zhou,† Adnan Khalil,† Zahir Muhammad,† Wangsheng Chu,† Zhonghui Wang,§ Hongfa Xiang,§ Qin Liu,*,† and Li Song*,† †

National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui 230029, China ‡ Department of Physics and Electronic Engineering, Jiangsu Lab of Advanced Functional Materials, Changshu Institute of Technology, Changshu, Jiangsu 215500, China § School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui 230009, China S Supporting Information *

ABSTRACT: Two-dimensional stable metallic 1T-MoSe2 with expanded interlayer spacing of 10.0 Å in situ grown on SWCNTs film is fabricated via a one-step solvothermal method. Combined with X-ray absorption near-edge structures, our characterization reveals that such 1T-MoSe2 and single-walled carbon nanotubes (abbreviated as 1TMoSe2/SWCNTs) hybridized structure can provide strong electrical and chemical coupling between 1T-MoSe2 nanosheets and SWCNT film in a form of C−O−Mo bonding, which significantly benefits a high-efficiency electron/ion transport pathway and structural stability, thus directly enabling high-performance lithium storage properties. In particular, as a flexible and binder-free Li-ion anode, the 1T-MoSe2/SWCNTs electrode exhibits excellent rate capacity, which delivers a capacity of 630 mAh/g at 3000 mA/g. Meanwhile, the strong C−O−Mo bonding of 1T-MoSe2/SWCNTs accommodates volume alteration during the repeated charge/discharge process, which gives rise to 89% capacity retention and a capacity of 971 mAh/g at 300 mA/g after 100 cycles. This synthetic route of a multifunctional MoSe2/SWCNTs hybrid might be extended to fabricate other 2D layer-based flexible and light electrodes for various applications such as electronics, optics, and catalysts. KEYWORDS: 1T-MoSe2 nanosheets, carbon nanotube, layered hybrid, flexible electrode, lithium-ion battery

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further improvement of electrochemical performance. Hence, the design and fabrication LIBs with light weight and high specific capacity have been considered as the trend of development in energy storage system. In this regard, the evolution of self-dependent soft electrode materials is significant for energy storage system. Recently researchers have designed different kinds of novel battery electrode that supersede or even eradicate the usage of current collectors or binders in traditional battery electrode configuration.9 Among them, single-wall carbon nanotube (SWCNT) films as one of the promising carbon materials have drawn tremendous attention to design lightweight flexible electrodes

owadays, owning to high energy density, long cycle life, and the environmentally friendly character of lithium-ion batteries (LIBs), they have been widely applied to portable electronics devices and large electricitydriven machines such as smartphones, laptops, sports bracelets, and electric automobiles.1−3 Recent progressive techniques in diverse kinds of flexible portable electronic apparatuses need the growth of soft batteries as their energy sources.4−6 In conventional processing of LIB anodes, the active materials are usually attached to copper current collectors using the polymer binder poly(vinylidene fluoride) (PVDF). This sort of anode electrode material is unfit for soft batteries, and the metal current collector of the anode side is a comparatively ponderous part in a LIB, which is equivalent to the heaviness of the electrode material and takes up 10% of the gross mass of the cells.7,8 Besides, the polymer binders, which are usually insulators, hinder ion transfer in the electrolyte and restrict the © 2017 American Chemical Society

Received: May 13, 2017 Accepted: May 25, 2017 Published: May 25, 2017 6483

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Figure 1. (a) XRD patterns of as-obtained 1T-MoSe2/SWCNTs hybrids, in which the (002) peak shifts to lower angle. (b) SEM image and (c) TEM image of the hybrids, revealing that MoSe2 nannosheets are anchored intimately on the surface of an SWCNT bundle. (d) Corresponding HRTEM image showing an enlarged interlayer distance of 10.0 Å.

rate performance in practical applications, which might account for the instability and intrinsic low electronic conductivity of the structure. Recently, there have been some reports of combining 2H-MoSe2 with conductive carbon matrixes (graphene, CNTs, carbon cloth, and so on) to improve the structural integrity and electrical conductivity.17−19 To this end, MoSe2/C composites have shown satisfactory electrochemical performance for LIBs. However, the powder-like hybrid materials cannot meet the robust and flexible requirements of LIBs. In this work, we demonstrate a solvothermal method to prepare a hierarchical nanostructure of 1T-MoSe2/SWCNTs composite. The oxygen groups in SWCNTs induced by air oxidation serve as an anchor for the subsequent growth of MoSe2 nanosheets. By controlling the molar ratio of selenourea and molybdenum pentachloride, 1T-MoSe2 nanosheets with an expanded interlayer spacing of 10.0 Å were selectively grown on an SWCNTs film, forming a designed nanostructure with 1TMoSe2 nanosheets perpendicularly connecting with SWCNTs through a direct coupling at the edge Mo of 1T-MoSe2 nanosheets with oxygen from functional groups present on SWCNTs (Mo−O−C bonds). This structural characteristic endows the 1T-MoSe2/SWCNTs composite anode with many advantages: (1) The flexible and lightweight film realize a bendable and binder-free anode electrode. (2) The expanded interlayer spacing of 10.0 Å and the abundant active edge sites of 1T-MoSe2 nanosheets benefit the diffusion of lithium ions and penetration of electrolyte. (3) Good adhesion of 1T-MoSe2

due to their special physical and chemical characteristics, high specific surface areas, high electron conductivity, mechanical stability, etc.10−12 Recently, MX2 (M = transition metal; X = chalcogen), as typical layered materials, are obtaining more and more attention in LIBs, which possess open 2D channel crystal structures, composed of inexpensive and abundant elements along with high theoretical specific capacity.13,14 Molybdenum diselenide (MoSe2) is a member of the transition metal dichalcogenides (TMDCs) family. Ordinarily, MoSe2 is an intrinsic semiconductor whose band gap is 1.1−1.5 eV.15 MoSe2 consists of lamellar crystal units that possess a sandwich structure of Se−Mo−Se. The crystal units were joined via weak van der Waals forces along the c axis. The specific layer structure is beneficial to the intercalation and extraction of Li ions. On the basis of its crystal structure, MoSe2 has two phases. The 2H-MoSe2 phase, with a trigonal structure, has been known to be semiconducting, while 1T-MoSe2, with an octahedral structure, is metallic. But 1T-MoSe2 prepared by the conventional lithium intercalation−exfoliation method is quite sensitive to the atmosphere and easily transforms back to the 2H phase at room temperature.14 Recently, our group explored stable 1T-MoS2, which can be obtained by NH4+ intercalation, and the interlayer distance of the stable 1T-MoS2 is about 9.8 Å, which is wider than that of 2H-MoS2.16 Therefore, we adopt a similar strategy to fabricate 1T-MoSe2 and employ it as an anode material for LIBs. The 2H-MoSe2 electrode suffered a lot from inferior cycling stability and low 6484

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Figure 2. XPS spectra of Mo 3d (a), Se 3d (b), N 1s (c), C 1s (d), and O 1s (e) core level peak regions of 1T-MoSe2/SWCNTs hybrids. The curves are deconvoluted by Gaussian fitting.

RESULTS AND DISCUSSION The schematic illustration of the preparation of 1T-MoSe2/ SWCNTs using SWCNT films as substrates of a free-standing 1T-MoSe2/SWCNTs film is shown in Figure 1. Initially, the surface of SWCNTs with adsorbed selenourea and molybdenum pentachloride and the oxygen functional groups on the SWCNTs serve as fixing sites and nucleation sites for MoSe2 to grow. As shown in Figure S1, the 1T-MoSe2/SWCNTs film is flexible and reaches a size of tens of centimeters long. Then MoSe2 nanosheets were formed and embedded in the SWCNT matrix during the solvothermal process. Figure 2a shows the X-ray diffraction (XRD) patterns of 1TMoSe2/SWCNTs composites. It is observed that the (002) peak of the 1T-MoSe2/SWCNTs composites shifts to a lower degree of 8.8° compared to that of 2H-MoSe2 . The

nanosheets on SWCNTs via the C−O−Mo bonds ensures a good structural stability and thus is beneficial to the lasting cycling performance. The Mo−O−C bond also provides lithium ions/electrons a transfer highway between metallic 1T-MoSe2 and highly conductive SWCNTs. As a consequence, it is not unexpected that the prepared 1TMoSe2/SWCNTs composite manifests an outstanding invertible capacity of 630 mAh/g and even 3000 mA/g and an excellent cycling performance with a capacity of 971 mAh/g at 300 mA/g after 100 cycles. We expected that the as-prepared hierarchical 1T-MoSe2/SWCNTs composite is a hopeful competitor as anode material for future flexible energy storage devices. 6485

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Figure 3. (a) C K-edge and (b) O K-edge X-ray absorption near-edge structures (XANES) of the SWCNTs film and 1T-MoSe2/SWCNTs hybrids.

accordance with previously reported 1T-MoS2 and 1TWS2.16,20 According to XPS analysis, the content of 1T phase estimated by the deconvolution of the Mo 3d peak is calculated to be 78.6%. The high-resolution N 1s XPS spectra (Figure 2c) was deconvoluted into three different peaks: 297.5 eV for pyridinic N, 299.2 eV for pyrrolic N, and 400.65 eV for NH4+, respectively.21 Nitrogen doping by ammonium released from excessive selenourea during the solvothermal process may result in the presence of pyridinic N and pyrrolic N in the SWCNT film. The C 1s peak is mainly situated at 284.75 eV for CC in Figure 2d. In addition, the other three weak peaks situated at 282.8, 286.2, and 287.5 eV belong to C−H and oxygencontaining functional groups of SWCNTs’ C−O and CO, respectively. As shown in Figure 2e, we deconvoluted the highresolution spectrum of O 1s into three peaks at 531.5, 532.4, and 533.5 eV, corresponding to CO, C−O−Mo, and C− OH, respectively.22−24 From the above Mo, C, and O spectral data, this suggests that the Mo edge of 1T-MoSe2 nanosheets might bond with the oxygen atom in SWCNTs to form a C− O−Mo bond.25 In Figure S8, XRD and XPS tests were performed to characterize the aged sample of 1T-MoSe2/ SWCNTs stored in ethanol for six months. The XRD and XPS data of the fresh and six-month-aged samples are similar, strongly illustrating that the intercalation of ammonium ions and metallic phase in the 1T-MoSe2/SWCNTs film is extremely stable. The thermodynamic stability of 1T-MoSe2NH4 was checked with a Born−Oppenheimer molecular dynamic simulation at the temperature of 500 K for 5 ps. As shown in Figure S9, there are no signs of structure disruption during the simulation time. This means that 1T-MoSe2-NH4 is quite stable. C and O K-edge X-ray absorption near-edge structures (XANES) of the 1T-MoSe2/SWCNTs films were conducted to further study the electronic structure at the interface and reciprocity between 1T-MoSe2 and SWCNTs. XANES is a powerful tool to study the valence state, the coordination environment, and structural information including orbital hybridization, coordination number, and symmetry of the materials.26 From Figure 3a, the XANES spectrum of the C Kedge in the hybrid displays a main peak at 285.0 eV, which belongs to the transitions from C 1s to the vacant orbit of C C π* character. We can acquire the electronic information on the unoccupied densities of state of the π* character from the peak

corresponding interlayer spacing of the (002) plane is about 10.0 Å. This structure is similar to the previously reported 1TMS2 (M = Mo/W) structure, in which ammonium ions intercalated into the interlayer of MS2 and stabilized the metallic 1T-phase.16,20 The excessive selenourea here not only acts as a selenium source but also produces abundant ammonium ions during the reaction. From Figure 2b, MoSe2 nanosheets were uniformly grown on the SWCNT bundle, and these SWCNT bundles cross each other to create a cross-linked network, which effectively enhances the electroconductivity and benefits the electrolyte infiltration (Figure S2). The TEM image in Figure 2c shows that the MoSe2 nanosheets were grown directly onto the external surface of the SWCNT bundle. The HRTEM (Figure 2d) of the growth of MoSe2/SWCNTs reveals that the interlayer spacing of the (002) plane is about 10.0 Å, which agrees with the above XRD calculations. Furthermore, electron energy loss spectroscopy (EELS) mapping analyses (Figure S3) were utilized to illustrate the elemental distribution. Around SWCNTs, it is noted that elements (Mo, Se, N, and O) are also uniformly distributed. Pure MoSe2 as a control sample was synthesized through the same steps except for the addition of the SWCNT film. As shown in Figures S4−6, the morphology of the bare MoSe2 sample is flower-like, composed of nanosheets, which indicates that the addition of an SWCNT film can efficiently prevent the aggregation of MoSe2 nanosheets. We conducted X-ray photoelectron spectroscopy (XPS) tests to study the chemical constitution and valence states of elements in 1T-MoSe2/SWCNTs composites (Figure 2). The XPS spectra verified the existence of Mo, Se, C, O, and N elements. Figure 2a shows that Mo 3d exhibits two main peaks at 231.6 eV for Mo 3d3/2 and 228.5 eV for Mo 3d5/2, respectively. What is noteworthy is that the peaks of Mo 3d shift to lower binding energy compared to 2H-MoSe2, which located at around 229.8 and 232.9 eV. From Figure S7, we can observe an obvious downshift of Mo 3d and Se 3d between 1TMoSe2/SWCNTs and bulk MoSe2. We ascribed the Mo 3d peak at high bonding energy to the existence of a Mo−O bond. The Se 3d spectrum shows two peaks, at 53.9 and 54.8 eV, belonging to 3d5/2 and 3d3/2, respectively. Similarly, we also observed that the Se 3d peaks shift to a lower bonding energy compared with 2H-MoSe2. These indicate the formation of a 1T phase in the 1T-MoSe2/SWCNTs sample, which is in 6486

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Figure 4. Electrochemical performance evaluations of 1T-MoSe2/SWCNTs hybrids. (a) Charge and discharge curves. The inset image is a 1TMoSe2/SWCNTs film electrode. (b) Cycling performance at 300 mA/g. (c) Rate performance. (d) Corresponding Nyquist plots.

curves of the 1T-MoSe2/SWCNTs film electrode for the initial three cycles with a cutoff voltage 0.01−3.0 V at 60 mA/g. In previous studies,18,30 there are two platforms at around 0.6 and 0.9 V, which can be seen in the initial Li-ion extraction process of 2H-MoSe2. The platform at 0.9 V is ascribed to the formation of LixMoSe2 by intercalation of Li ions into the interlayers of MoSe2, accompanied by the conversion of 2H to 1T structure. Another reduction platform at 0.6 V reflects that the LixMoSe2 converts to Mo metal and Li2Se, and a gel-like polymeric layer and the SEI layer are simultaneously formed. Interestingly, there is no obvious platform at 0.9 V in the first reduction process in our study. This can be attributed to the distorted 1T phase of MoSe2 in our 1T-MoSe2/SWCNTs composites, which will not change phase when the Li ions insert into the interlayers. An evident platform at approximately 2.2 V in the first Li-ion extraction process arises, which may correspond to the conversion from Mo to MoSe2. It is worth mentioning that the charge/discharge curves are alike in the following cycles, suggesting that the electrochemical process of the 1T-MoSe2/SWCNTs composites is very stable. The 1TMoSe2/SWCNTs composites possess a discharge capacity of 1495 mAh/g and corresponding charge capacity of 1001 mAh/ g in the initial cycle; therefore the initial Coulombic efficiency is calculated to be 67%. In contrast, the pure MoSe2 control sample yields capacities of only about 600 mAh/g at 0.1 C, and the capacities drop very quickly.18,30 The enhanced performance of our 1T-MoSe2/SWCNTs is ascribed to the synergistic effect between the SWCNTs and the 1T-MoSe2 nanosheets. Cyclic voltammetry (CV) was also conducted. As shown in Figure S12, in the first Li-ion insertion process, we can observe a wide peak at 0.6 V, another peak at 1.85 V, and a peak located

intensity. It is worth noting that the peak intensity is reduced after combining with 1T-MoSe2, manifesting that charge transfer occurs from 1T-MoSe2 to C 2p-derived π* states in SWCNTs at the junction of the 1T-MoSe2/SWCNTs hybrid. The intensity of another peak at 288.2 eV, which belongs to the C K-edge spectrum, is increased. This peak is attributed to carbon atoms of SWCNTs integrated with oxygen or other species. Figure 3b is the O K-edge XANES spectra of 1TMoSe2/SWCNTs hybrid and the O K-edge spectra for pure SWCNTs, and the 1T-MoSe2/SWCNTs show characteristic peaks at 531.5 and 540 eV, which belongs to a π* CO transition and σ* C−O transition.27 It is worth noting that there is an extra peak in the 1T-MoSe2/SWCNTs at 530 eV, which should be ascribed to O 2p-states in the d(t2g) conduction band, which is composed of 4d Mo and 2p O orbitals, which indicates the formation of a Mo−O bond.28 This further suggests that there is interfacial C−O−Mo bonding in the 1T-MoSe2/SWCNTs heterostructures, which agrees with the above results. The thermogravimetric analysis (TGA) was performed in the range of room temperature to 700 °C in air at 10 °C min−1 to investigate the content of active material in the 1T-MoSe2/SWCNTs hybrid. As shown in Figure S10, the accurate loading of 1T-MoSe2 in the 1T-MoSe2/SWCNTs composites is calculated to be 73.10%.18,29 The tensile test was carried out to characterize the mechanical properties. The extension of the 1T-MoSe2/SWCNTs film can reach 22.3%, and the breaking strength is about 34 MPa, calculated from the stress−strain curve in Figure S11. The electrochemical Li ions’ charge and discharge behavior of the 1T-MoSe2/SWCNTs was investigated by galvanostatic charge/discharge cycling. Figure 4a is the charge/discharge 6487

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Figure 5. (a) Cyclic voltammogram (CV) curves of LiFePO4/(1T-MoSe2/SWCNTs) flexible full cell cycled between 0.01 and 3 V at a scan rate of 0.5 mV/s. (b) Charge and discharge curves of the flexible full cell at a current density of 60 mA/g (the mass of the 1T-MoSe2/SWCNTs hybrids is 0.5 mg). Photographs of a flat (c) and bent (d) flexible full cell when carrying out the CV test.

at approximately 2.2 V in the first Li-ion extraction process, which is in agreement with the charge/discharge curves. Notably, after 100 cycles, the reversible capacity of 1TMoSe2/SWCNTs can remain at 971 mAh/g at 300 mA/g (Figure 4b). There is no big decay observed in the galvanostatic charge/discharge curve after 100 cycles (Figure S13). In Figure S14a, we characterized the film electrode after 100 cycles of the charge/discharge process. We can observe that the film was not fractured even after 100 cycles. The TEM image in Figure S14b reveals that MoSe2 nanosheets were tightly anchored on the SWCNT surface, resulting in high stability of the hybrid structure during the charge/discharge process. More importantly, the rate performance of the 1T-MoSe2/SWCNTs sample was researched by cycling at diverse current densities. As shown in Figure 4c, the rate capability is still remarkable even at high current density. Our 1T-MoSe2/SWCNTs electrode possesses a superior reversible capacity of 1118, 981, 949, 812, 725, and 630 mAh/g at 60, 300, 600, 1200, 1800, and 3000 mA/g, respectively. While increasing the current density, the discharge capacity of the bare MoSe2 control sample decreases rapidly.30 We suggest that the long life and remarkable rate performance of 1T-MoSe2/SWCNTs can be attributed to the fast and wide lithium ion/electron transfer channel fabricated by metallic 1T-MoSe2 and highly conductive

SWCNTs through a C−O−Mo bond. Figure 4d is the Nyquist plots of 1T-MoSe2/SWCNTs and the bare MoSe2 control sample. It can be seen that the Nyquist curve is composed of a semicircle and a straight line, relating to charge transfer resistance (Rct) at the interface of the electrolyte/electrode and Li+ diffusion resistance (Zw) in the electrode, respectively. Comparing the curves of 1T-MoSe2/SWCNTs composites and bare MoSe2, we can easily note that the diameter of the semicircle of the 1T-MoSe2/SWCNTs composites is much smaller than that of the bare MoSe2 control sample, indicating that the 1T-MoSe2/SWCNTs composites can possess much lower charge transfer resistance. Furthermore, the slope of the straight line of the 1T-MoSe2/SWCNTs composites is higher than the bare MoSe2, suggesting a much faster Li+ diffusion. To demonstrate the flexibility of 1T-MoSe2/SWCNTs films, we assembled flexible full cells consisting of LiFePO 4 electrodes, a separator, and MoSe2/SWCNTs films, shown in Figure S15. In Figure S16, we can observe that when bending the cell, it still powers a yellow LED without conspicuous dimming, as the flat one had. The CV test of the flexible full cell was also performed. Figure 5 depicts the CV curves of a LiFePO4/(1T-MoSe2/SWCNTs) flexible full cell cycled at 0.5 mV/s scan rate. There are an oxidation peak at about 1.9 V and a reduction peak at 1.1 V in the CV curves, corresponding to 6488

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Scheme 1. Schematic illustration of the solvothermal procedure with an SWCNT film as substrate for the preparation of stable 1T-MoSe2/SWCNTs hybrids.

the process of extraction of Li ions from the cathode and the intercalation of Li ions into the anode.31 It is obvious that the peaks of the flat cell (Figure 5c) are almost the same as those of the bent one (Figure 5d). The overlapping of the CV curves in Figure 5a suggests the excellent reversibility and flexibility of the full cell with the 1T-MoSe2/SWCNTs electrode during the electrochemical cycling process. Moreover, from Figure 5b, it can be seen that the first charge capacity is large. In the first charge process, the Li ions were inserted into the interlayers of MoSe2, accompanied by the decomposition of the electrolyte and the formation the SEI layer. In the subsequent cycles, the charge capacity is similar. The capacity shows a negligible decrease even after 50 bendings compared to the original flat battery. For a better understanding of the structure and mechanism, Scheme 2 shows the electrochemical process scheme for our

1T-MoSe2/SWCNTs hybrid according to the above analyses. The remarkable performance as well as high durability of the 1T-MoSe2/SWCNTs hybrid with multiple synergistic structures can be attributed to three aspects: (1) the vertically grown 1T-MoSe2 nanosheet with enlarged interlayer spacing favors easier intercalation−deintercalation of Li+ and offers abundant active edge sites for lithium reaction. (2) The metallic 1TMoSe2 nanosheets grown on the SWCNTs, in the form of C− O−Mo bonds, endows a high-efficiency electron/ion transport pathway in which the electron and ion can be transferred quickly from the metallic MoSe2 plane to SWCNTs. (3) The strong C−O−Mo bond between 1T-MoSe2 and SWCNTs ensures its structural stability against substantial volume changes during the charge−discharge process.

CONCLUSIONS In conclusion, a one-step solvothermal method was successfully developed to fabricate metallic 1T-MoSe2 nanosheets with an expanded interlayer distance embedded on SWCNTs. The oxygen-containing groups in SWCNTs enabled the grown sites to bond to the edge Mo, leading to the formation of vertical 1T-MoSe2 nanosheets in the form of C−O−Mo bonding. The strong C−O−Mo bond between 1T-MoSe2 and SWCNTs ensured the structural stability, preventing structural decomposition during the charge−discharge process. The 1T-MoSe2/ SWCNTs film electrode delivers a superior reversible capacity of 971 mAh/g at 300 mA/g after 100 cycles. The highly conductive SWCNTs and metallic conductivity of 1T-MoSe2 constructed an electron/ion transfer channel, leading to a remarkable rate performance of 630 mAh/g even at 3000 mA/ g. Therefore, such a 1T-MoSe2/SWCNTs hybrid is expected to be a potential material for flexible energy storage devices.

Scheme 2. Schematic illustration showing the electrochemical process in a 1T-MoSe2/SWCNTs electrode.

MATERIALS AND METHODS Synthesis of the SWCNT Film. We used the CVD method to synthesize carbon nanotubes.32 Argon was chosen to act as a protective atmosphere. The temperature was raised to 1000 °C with a heating rate of 30 °C/min and 200 sccm argon airflow, then raised to 1100 °C with a rate of 10 °C/min, and the catalyst was heated to 90 °C simultaneously. When the temperature reached 1100 °C, the argon flow was changed to 1000 sccm, and at the same time we inlet 3 sccm of methane flow. After 2 h of reaction, the furnace was turned off and gradually cooled to room temperature. The obtained carbon nanotubes were purified by calcining at 350 °C for 12 h in air, then immersed in 37% HCl for 1 week. The purified carbon nanotubes were immersed in ethanol for further use. 6489

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cell battery (LiFePO4/ separator/MoSe2/SWCNTs film) for lighting a yellow LED (PDF)

Synthesis of 1T-MoSe2/SWCNT Hybrids. We purchased the N,N-dimethylformamide (DMF), molybdenum chloride (MoCl5) powder, and selenourea (CH3CSeNH2) from Alfa Aesar. We used all chemicals directly without further treatment. Molybdenum pentachloride (0.1 mmol) and 3 mmol of selenourea were put into 20 mL of DMF. The solution was vigorously stirred for 30 min. Then a piece of carbon nanotube film was immersed into the homogeneous solution for 12 h. Finally the solution was transferred into a 45 mL Teflon-lined stainless steel autoclave, which was put into a 220 °C oven and kept for 2 days. We used water and ethanol to wash the product three times, and the final obtained product was stored in ethanol. Characterizations. We used a D8-Advance power diffractometer equipped with a Cu Kα radiation source (λ = 1.541 78 Å) to record XRD patterns of samples. We used a VG ESCALAB MK II X-ray photoelectron spectrometer with a Mg Kα = 1253.6 eV source to conduct XPS measurements. We used a field emission SEM (15 kV, JEOL, JSM-6700F) and TEM (JEOL JEM2010) to characterize the morphology of the MoSe2/SWCNTs film. We used a DTG-60H to determine the content of active materials in the hybrid; the test was conducted from 30 to 700 °C at 10 °C min−1 in air flux. A JEOL JEMARM200F TEM (200 kV) equipped with a spherical aberration corrector was used to carry out the EELS mapping analyses. The X-ray absorption near-edge structure spectra (XANES) of C and O K-edge were performed at the XMCD beamline in the National Synchrotron Radiation Laboratory (NSRL). Simulation and Calculation. To explore the stability of MoSe2NH4, the quantum molecular dynamics simulation was performed in the canonical ensemble using a Nosé thermostat method. The simulated temperature of the system was up to 500 K. The time step was set at 1 fs, and the total simulation time was 5 ps. Electrochemical Measurements. We used Li metal as a counter electrode to assemble the CR2032-type coin cells, and all the processes were carried out in a glovebox filled with argon. The separator was Celgard 2400. The electrolyte was a solution of 1 M LiPF6 that was dissolved in ethylene carbonate and dimethyl carbonate with a 1:1 volume ratio. We adopted a slurry tape casting procedure to prepare the flexible cell assembled with LiFePO4 electrodes as counter electrodes. We used N-methyl-2-pyrrolidinone to dissolve the mixture consisting of 10 wt % acetylene black, 80 wt % LiFePO4, and 10 wt % PVDF. The obtained slurry was taped on aluminum foil and then transferred to a vacuum oven at 100 °C for 12 h. We used an aluminum-plastic film to encapsulate the sandwich structure consisting of LiFePO4 electrode, a separator, and the MoSe2/SWCNTs film. The electrochemical performance of the cells was assessed on a Land CT 2001 battery tester in the range of 0.01−3 V. The flexible binder-free MoSe2/SWCNTs film was put into a vacuum oven at 100 °C for 12 h, and then the dried film was used directly as an anode electrode. The Nyquist plots with an amplitude of 10 mV from 106 Hz to 0.01 Hz and CV curves of cells were obtained on a CHI660D electrochemical workstation.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Li Song: 0000-0003-0585-8519 Author Contributions #

T.X. and S.T. contributed equally to this work. L.S. and Q.L. supervised the project and designed the experiments. T.X. carried out most of the experiments and analyzed the data. W.Y. and Q.F. provided the SWCNT film. C.W., D.L., Y.Z., A.K., and Z.M. carried out partial experimental characterizations. Z.W. and H.X. conducted flexible full-cell experiments. T.X., Q.L., S.T., W.C., and L.S. cowrote the paper. All authors discussed the results. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support from the 973 (2014CB848900) program, NSF (11375198, U1532112, 11574280), a project funded by China Postdoctoral Science Foundation (2017M612105), CAS Key Research Program of Frontier Sciences (QYZDB-SSW-SLH018), CAS Interdisciplinary Innovation Team, National Postdoctoral Program for Innovative Talents (BX201600141), and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) Nankai University. L.S. thanks the CAS Hundred Talent Program of China and the recruitment program of global experts. We thank Dr. Xiuling Li for her help with the simulation. REFERENCES (1) Wang, H. L.; Cui, L. F.; Yang, Y.; Casalongue, H. S.; Robinson, J. T.; Liang, Y. Y.; Dai, H. Mn3O4− Graphene Hybrid as a High-Capacity Anode Material for Lithium Ion Batteries. J. Am. Chem. Soc. 2010, 132, 13978−13980. (2) Qie, L.; Chen, W. M.; Wang, Z. H.; Shao, Q. G.; L, X.; Hu, X. L.; Zhang, W. X.; Huang, Y. H. Nitrogen-Doped Porous Carbon Nanofiber Webs as Anodes for Lithium Ion Batteries with a Superhigh Capacity and Rate Capability. Adv. Mater. 2012, 24, 2047−2050. (3) Ji, L. W.; Lin, Z.; Alcoutlabi, M.; Zhang, X. W. Recent Developments in Nanostructured Anode Materials for Rechargeable Lithium-ion Batteries. Energy Environ. Sci. 2011, 4, 2682−2699. (4) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652−657. (5) 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. (6) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2009, 22, 587−603. (7) Cui, L. F.; Hu, L. B.; Choi, J. W.; Cui, Y. Light-Weight FreeStanding Carbon Nanotube-Silicon Films for Anodes of Lithium Ion Batteries. ACS Nano 2010, 4, 3671−3678. (8) Jia, X. L.; Yan, C. Z.; Chen, Z.; Wang, R. R.; Zhang, Q.; Guo, L.; Wei, F.; Lu, Y. F. Direct Growth of Flexible LiMn2O4/CNT LithiumIon Cathodes. Chem. Commun. 2011, 47, 9669−9671. (9) Fang, X.; Ge, M. Y.; Rong, J. P.; Zhou, C. W. Free-Standing LiNi0.5Mn1.5O4/Carbon Nanofiber Network Film as Lightweight and High-Power Cathode for Lithium Ion Batteries. ACS Nano 2014, 8, 4876−4882.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03329. Optical image, SEM, EELS mapping of 1T-MoSe2/ SWCNTs film; SEM, TEM, and HRTEM images of bare 1T-MoSe2 sample; comparison of XPS spectra between 1T-MoSe2/SWCNTs composites and bulk MoSe2; XRD and XPS spectra of fresh and aged 1T-MoSe2/SWCNTs sample; TGA of 1T-MoSe2/SWCNTs composites; the initial four CV curves of 1T-MoSe2/SWCNTs film; charge and discharge curves of 1T-MoSe2/SWCNTs hybrids; optical image and TEM of 1T-MoSe2/SWCNTs film after 100 cycles of charge−discharge prccess; photograph of the sandwich structure of the flexible full-cell battery; illustration of flat and bent flexible full6490

DOI: 10.1021/acsnano.7b03329 ACS Nano 2017, 11, 6483−6491

Article

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DOI: 10.1021/acsnano.7b03329 ACS Nano 2017, 11, 6483−6491