English

• INTRODUCTION

  Lithium-ion batteries (LIBs) with high energy density, long lifespan and environmental benign have attracted intensive interest as the major energy storage device.^{[1, 2]} It has been demonstrated that nanostructure electrodes with optimized structure and morpho-logy can achieve high energy and charge density in LIBs compared to bulk materials.^[3] One-dimensional (1D) nanomaterials of silicon/carbon and transition metal oxides such as Co₃O₄, Fe₂O₃ and MnO₂ are often used as anodes for LIBs. For example, Si has low discharge voltage and high theoretical capacity (4200 mA h g^-1), which is 10 times higher than that of commercial graphite anodes.^[4] Low cost transition metal oxides also have a high capacity because of their capability of storing lithium by the conversion mechanism.^[5] Although the design of 1D nanostructures can offer rapid electron transport path in the axial direction and shortened diffusion path for Li⁺, ^{[6, 7]} large volume change during charging and discharging remains a challenge, which leads to pulverization and capacity fading. The introduction of buffer layer, e.g., graphene with good conductivity and flexibility^{[8, 9]} on the surfaces of these electrode nanomaterials^[10-18] has been used to mitigate the pro-blems.

  In comparison with the above mentioned materials, as a typical TMDs compound, MoTe₂ has a sandwich-like layered structure with an interlayer spacing of 0.699 nm, which is larger than that of graphite (0.335 nm), facilitating the process of reversible Li⁺ insertion/extraction.^[19] Hence, the nature of MoTe₂ makes it very suitable as active material for LIBs. Currently, the most common forms of MoTe₂ are nanosheets, thin films^{[20, 21]} and bulk blocks^{[22, 23]} because the native 2D layered structure increases the difficulty to produce 1D nanostructured MoTe₂.^[24] Most 1D MoTe₂ are synthesized by chemical vapor deposition (CVD), ^[25] electron irradiation^[26] and sonochemical technique.^[27] In these methods, the expensive equipment is required and the complex experimental process is involved. The development of a simple way to synthesize 1D MoTe₂ is highly desirable.

  Here, an in-situ reaction of GO encapsulated Mo₃O₁₀(C₂H₁₀N₂) nanowires with Te under Ar/H₂ atmosphere at 500 ^oC has been developed for the production of MoTe₂ nanorods coated with rGO thin layers containing certain Te vacancies. The 1D MoTe₂ nanorods have an average diameter of 100-200 nm and length of 1-3 μm, respectively. When used as anode material, the hybridized MoTe₂/rGO nanorods promote conductivity and cycling stability. As a result, the MoTe₂/rGO nanocomposite delivers a high capacity of 637 mA h g^-1 at 0.2 A g^-1 and long-term lifespan (374 mA h g^-1 after 200 cycles at 0.5 A g^-1), which is much larger than that of bare MoTe₂ nanorods. This work provides a new pathway for the rational design of advanced 1D TMDs electrode materials with enhanced performance.

  RESULTS AND DISCUSSION

  Figure 1 illustrates the schematic preparation of MoTe₂/rGO nanocomposites and bare MoTe₂ nanorods. Typically, the Mo₃O₁₀(C₂H₁₀N₂) nanowires (Figure S1b)^[28] were first mixed with aqueous dispersion of different percentages of GO (the GO contents are 5, 10 and 15 mg, respectively) to obtain GO-coated Mo₃O₁₀(C₂H₁₀N₂)/GO. Then, MoTe₂/rGO-x nanocomposites (x represents the mass of graphene introduced to the precursor) were synthesized by reacting Mo₃O₁₀(C₂H₁₀N₂)/GO with Te powder at a mass ratio of 1:1. In this process, the Mo₃O₁₀(C₂H₁₀N₂)GO nanowires was as 1D template and molybdenum source. Meanwhile, bare MoTe₂ nanorods were also prepared with a similar method without graphene oxide.

  Figure 1

  

  Figure 1.  Schematic illustration for the fabrication of bare MoTe₂ nanorods and MoTe₂/rGO nanocomposite.

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  Crystal Phase and Structure Characterization of MoTe₂. Figure 2a shows XRD patterns of bare MoTe₂ nanorods and MoTe₂/rGO-10 nanocomposite. All the diffraction peaks can be indexed to the monoclinic phase MoTe₂ (JCPDS No. 71-2157) for both bare MoTe₂ nanorods and MoTe₂/rGO-10 nanocomposite. The results indicate that Mo₃O₁₀(C₂H₁₀N₂) has been completely converted to MoTe₂. Furthermore, the position and intensity of the signals are generally similar for bare MoTe₂ nanorods and MoTe₂/rGO-10 nanocomposite, indicating that their crystallinity, size and interlayer spacing maintain consistency. The primary diffraction peak located at 2θ = 12.8° belongs to the (002) crystal plane, which possesses an interlayer spacing of about 0.69 nm.

  Figure 2

  

  Figure 2.  (a) XRD patterns of bare MoTe₂ nanorods and MoTe₂/rGO-10 nanocomposite together with standard pattern of MoTe₂ highlighted with chopsticks, (b) SEM image, (c) TEM image, (d) HRTEM image, (e) SAED pattern, (f) TEM image and corresponding elemental mapping images of (g) Mo, (h) Te, (i) C (scale: 100 nm) in the MoTe₂/rGO-10 nanocomposite.

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  For comparison, the MoTe₂/rGO nanocomposites with various rGO ratios were also synthesized (Figure S2). As the amount of GO increases, the MoTe₂ nanorods are gradually coated by rGO nanosheets. SEM image of MoTe₂/rGO-5 nanocomposite shows obvious incomplete encapsulation of rGO on the surface of nanorods (Figure S3a). In contrast, the surface of MoTe₂/rGO-15 nanocomposite (Figure S3b) is stacked with thick rGO nanosheets.

  As shown in Figure S4, the morphology of bare MoTe₂ nanorods well inherited the Mo₃O₁₀(C₂H₁₀N₂) precursor after tellurization. The nanorods are packed in bundles and have a rough surface. Figure 2b shows the SEM image of MoTe₂/rGO-10 nanocomposite. It can be clearly seen that the product has a uniform distribution and well-defined 1D rod-like morphology. The nanorods have a diameter around 100-200 nm and a length of 1-3 μm. The morphology and structure of the MoTe₂/rGO-10 nanocomposite are further characterized by TEM and HRTEM. Figure 2c shows that several nanorods are randomly arranged and coated with a thin layer of rGO. The rGO nanosheets are well cross-linked with MoTe₂ nanorods, forming an interconnected structure. The HRTEM image (Figure 2d) indicates interplanar spacing is 0.69 and 0.18 nm, corresponding to the (002) plane of MoTe₂ and the (101) plane of rGO, respectively. The SAED pattern (Figure 2e) reveals the polycrystalline structure of MoTe₂/rGO-10 nanocomposite. The elemental distribution of MoTe₂/rGO-10 nanocomposite is further investigated by EDS. Figure 2f-i shows elemental mapping around the nanorods, and the homogenous distribution of elements Mo, Te and C, confirming that MoTe₂ nanorod is uniformly coated by rGO.

  The specific surface area and porosity of the materials were confirmed by BET. As shown in Figure S5, both bare MoTe₂ and MoTe₂/rGO-10 nanocomposite display Type IV isotherms of nitrogen adsorption-desorption profile, indicating their mesporous structure. The specific surface area of MoTe₂/rGO-10 nanocomposite is determined to be 15.4153 m² g^-1, which is slightly larger than that of bare MoTe₂ (9.7428 m² g^-1). The inset of Figure S5 shows their pore-size distributions from BJH (Barrett-JoynerHalenda), showing the pore diameters are in a range of 2-20 nm. Therefore, the MoTe₂/rGO-10 nanocomposite possesses rich interfacial sites for lithium storage. Meanwhile, TGA of the four samples was carried out in order to calculate the weight content of MoTe₂. According to the TG curves (Figure S6), the weight increases on the four samples in the temperature range of 300-450 ℃ can be attributed to the oxidation of MoTe₂ into TeMoO_x in air. Subsequently, the weight loss is related to the oxidation of rGO accompanied by the sublimation of partial MoTeO_x. Finally, the weight losses over bare MoTe₂, MoTe₂/rGO-5, MoTe₂/rGO-10 and MoTe₂/rGO-15 are 29.48%, 34.55%, 36.63% and 39.53%, respectively. The weight percentages of MoTe₂ in the MoTe₂/rGO-5, MoTe₂/rGO-10 and MoTe₂/rGO-15 nanocomposites are determined to be 92.8%, 89.8% and 85.7%, respectively.

  EPR spectroscopy (Figure S7) was used to characterize the internal microstructure of MoTe₂. The characteristic peak at g = 2.00248 indicates the existence of Te vacancies. This may be caused by the easy sublimation of Te under high temperature, and presence of Te vacancy facilitates the formation of distorted 1T phase.^[29] The existence of vacancies may provide more paths to promote the transport of ions.^[30]

  Figure S8a illustrates the XRD patterns of bulk MoTe₂, which matches well with the monoclinic phase MoTe₂ (JCPDS No. 71-2157). Compared to bare MoTe₂ nanorods, the diffraction peaks of bulk MoTe₂ are very sharp, indicating that bulk MoTe₂ has a good crystallinity and large particle size. SEM image (Figure S8b) shows that the block structure of MoTe₂ in nature has irregular shapes and large size of several micrometers.

  XPS was utilized to analyze the surface chemical states of MoTe₂/rGO-10 nanocomposite. As shown in Figure S9a, the survey spectrum affirms the presence of Mo, Te, C and O elements in the hybrid. The Mo 3d spectrum displays prominent peaks at 228.35 and 231.49 eV, corresponding to 3d_5/2 and 3d_3/2 of Mo-Te bonds, ^{[31, 32]} respectively (Figure 3a). The peaks at 232.73 and 235.89 eV are related to Mo-O bonding. In the Te 3d core level spectrum, the peaks positioned at 572.99 and 583.36 eV are displayed for 3d_5/2 and 3d_3/2, respectively, implying the existence of Mo-Te bonds (Figure 3c). In addition, the Te 3d_5/2 and 3d_3/2 peaks located at 576.52 and 586.98 eV are attributable to Te-O bonding. In reality, the Mo-O and Te-O bonding are mainly caused by the inevitable oxidation of MoTe₂/rGO-10 nanocomposite in air.^[31] In comparison with XPS of bare MoTe₂ nanorods, the peak of Mo-Te shifts towards the higher binding energy, indicating that electrons are transferred from MoTe₂ to rGO.^[33] It is possible that Mo and Te atoms may form Mo-O and Te-O bonds with rGO, respectively, which benefits the anchor of rGO to MoTe₂. The C 1S spectrum in Figure S9b demonstrates three peaks at the binding energies of 284.79, 285.89 and 288.52 eV, which are ascribed to C-C, C-OH and C=O, respectively.^[34]

  Figure 3

  

  Figure 3.  High resolution XPS of MoTe₂/GO-10 nanocomposite and bare MoTe₂. (a) Mo 3d, and (b) Te 3d.

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  Electrochemical Measurements of Half-Cell. The electroche-mical performances of diverse electrodes were evaluated as the anodes versus Li foil in the half-cells. As demonstrated in Figure 4a, typical CV curves during the initial six cycles are recorded in the voltage window of 0.01-3.0 V at a scan rate of 0.2 mV s^-1. During the first cycle of discharge progress, the MoTe_­2/rGO-10 nanocomposite demonstrates four main reduction peaks. In detail, the reduction peaks located at 1.23 and 0.92 V are associated with the intercalation reaction, corresponding to the inserting of Li⁺ into MoTe₂ lattice to form Li_xMoTe₂ (MoTe₂ + xLi⁺ + xe⁻ ↔ Li_xMoTe₂) and the sharp peak observed at 0.69 V is mainly associated with the conversion reaction, leading to the formation of metallic Mo and Li₂Te (Li_xMoTe₂ + (4-x)Li⁺ + (4−x)e^- ⇔ Mo + 2Li₂Te).^{[35, 36]} The broad peak around 0.25 V is due to the decomposition of electrolyte and the formation of a solid electrolyte interface (SEI) film.^[35-37] During the following cycles, the four reduction peaks corresponding to the first cycle are all fading away and a distinct peak appears around 1.3 V, which is caused by the decrease in electron polarization with the size reduction of MoTe­₂ nanorods.^[35] In the six cycles of charging, the prominent single anodic peaks are ascribed to the reformulation of MoTe₂ through the oxidation reaction of Mo and Li₂Te. Figure 4b presents the galvanostatic discharge/charge profiles of MoTe₂/rGO-10 nanocomposite at the current density of 0.05 A g⁻¹ (initial three circles) and 0.2 A g⁻¹. All the voltage platforms match well with the distinct peaks in CV curves. In the first cycle, the discharge and charge capacities of MoTe₂/rGO-10 nanocomposite are 651.41 and 519.16 mA h g^-1 at 0.05 A g^-1, respectively, corresponding to an initial Coulombic efficiency of 79.7%. The loss of capacity is mainly from the formation of SEI film and electrolyte decomposition.^[38]

  Figure 4

  

  Figure 4.  (a) CV curves of MoTe₂/rGO-10 nanocomposite for initial six cycles at a scan rate of 0.2 mV s^-1, (b) Initial charge/discharge profiles of the MoTe₂/rGO-10 nanocomposite (1^st-3^rd cycles at 0.05 A g^-1 and 4^th-5^th cycles at 0.2 A g^-1), (c) Cycling performance of the bare MoTe₂ and MoTe₂/rGO-10 nanocomposite at 0.2 A g^-1, (d) Rate capability of bare MoTe₂ nanorods and MoTe₂/rGO-10 nanocomposite, (e) Cycling performance of MoTe₂/rGO-10 nanocomposite at 0.5 A g^-1.

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  Figure 4c demonstrates the cycling performances of MoTe₂/rGO-10 nanocomposite and bare MoTe₂ nanorods at a current density of 0.2 A g^-1 in a voltage range of 0.01-3.0 V. After the initial three cycles of activation at a current density of 0.05 A g^-1, the MoTe₂/rGO-10 nanocomposite exhibits the high reversible capacity of 637 mA h g^-1 over 100 cycles with capacity retention of 97%. While the bare MoTe₂ nanorods only exhibit a discharge capacity of 389 mA h g^-1 in the initial 40 cycles, and the capacity shows a trend of decay in the following cycles. In addition, bare rGO coating can only maintain 227 mA h g^-1 after 50 cycles at 0.2 A g^-1, which is lower than the reversible capacity of MoTe₂ and MoTe₂/rGO-10 (Figure S10). Therefore, the synergistic effects of rGO coating and MoTe₂ nanorods contribute to the enhancement of the electrochemical performance of MoTe₂/rGO-10. This is benefiting from the flexible nature of rGO, which cushions the vo-lume change during the insertion and extraction of Li⁺, resulting in less structural damage. While the good electrical conductivity of rGO facilitates the transfer of electrons, contributing to a significant increase in capacity. In addition, Te vacancies in MoTe₂/rGO-10 nanocomposite can provide rich active sites, which may also contribute to the improvement of electrochemical performance. Therefore, the MoTe₂/rGO-10 nanocomposite exhibits superior electrochemical performance. For the bulk solid MoTe₂, as shown in Figure S11a, it only retains 248 mA h g^-1 after 30 cycles, which is even much lower than that of bare MoTe₂ nanorods (389 mA h g^-1). Meanwhile, bare MoTe₂ nanorods have higher rate capability (Figure S11b). Therefore, the 1D nanostructures can significantly improve the electrochemical performance compared with blocky structures. Figure S12 shows the SEM images of the collected MoTe₂/rGO-10 nanocomposite and bulk MoTe₂ after 100 cycles at 0.2 A g^-1. The 1D structure of MoTe₂/rGO-10 nanocomposite is well preserved, indicating that rGO layers effectively alleviate the volume change of nanorods and maintain the stability of the structure. However, the bulk MoTe₂ loses its original block structure and pulverizes seriously. Thus MoTe₂/rGO-10 nanocomposite delivers an enhanced cycle performance.

  Figure 4d gives the rate performances of the MoTe₂/rGO-10 nanocomposite and bare MoTe₂ nanorods. It is showing that the current density is increased step-wisely from 0.1 to 2 A g^-1. The MoTe₂/rGO-10 electrode shows the high specific capacities of 472, 459, 438, 416, 374 mA h g^-1 at 0.1, 0.2, 0.5, 1, 2 A g^-1, respectively. When the current density is returned back to 0.1A g^-1, the reversible capacity recovers to 583 mA h g^-1, showing a good rate retention and excellent stability. Furthermore, the long-term cycling performance of the MoTe₂/rGO-10 nanocomposite is tested at a current density of 0.5 A g^-1 (Figure 4e). It delivers a discharge capacity of 446 mA h g^-1 after activation with initial three cycles at 0.05 A g^-1 and maintains 360 mA h g^-1 over 200 cycles. The capacity retention is 80.7%, indicating excellent capacity retention and good stability. As can be seen from Table S1, the present MoTe₂/rGO-10 nanocomposite exhibits high capacity and good stability at high current densities in comparison with the related MoTe₂ and typical Mo-based anodes in the literatures.

  Here, we also investigate the effect of GO quantity on the performance. In Figure S13, MoTe₂/rGO-15 nanocomposite shows a lower discharge specific capacity of 523 mA h g^-1 after 100 cycles than MoTe₂/rGO-10 sample. If the graphene was decreased to 5 mg, although MoTe₂/rGO-5 nanocomposite exhibits higher capacity and cycling performance than bare MoTe₂ nanorods, the structural stability is still not good. In reality, the small amount of rGO in the MoTe₂/rGO-5 nanocomposite is not sufficient to completely cover the surface of the nanorods, leading to the decreased conductivity and buffer capacity. With the increase of graphene, MoTe₂/rGO-15 nanocomposite shows good structural stability, but the capacity is lower than that of MoTe₂/rGO-10 nanocomposite. Excessive rGO nanosheets aggregate to form thick folds, which will hinder electron transport and cause poor conductivity, resulting in lower capacity. Therefore, the appropriate amount of rGO in the nanocomposite is the key to improve electro-chemical performance.

  Galvanostatic intermittent titration technique (GITT) and EIS were carried out to understand the Li⁺ diffusion kinetics and intrinsic electrochemical mechanisms of bare MoTe₂ and MoTe₂/rG0-10 nanocomposite anodes. The testing details are given in Figure S14. Figures S15a and S15b show the GITT curves of MoTe₂/rGO-10 in the first discharge/charge process. The potentials at around 1.5 V during the first discharge and at around 2.0 V during the first charge are related to the processes of Li⁺ insertion and extraction, respectively. The corresponding lithium diffusion coefficients at various discharge/charge voltages are calculated (Figures S15c, S15d). It can be seen that the minimum values of the diffusion coefficient appear at 1.2-1.6 V during the discharging progress or around 2.0-2.2 V during the charging progress, which are related to deep diffusion of Li⁺ into/from the active materials. Notably, the MoTe₂/rGO-10 nanocomposite shows a higher diffusion coefficient, which further demonstrates the pre-ferable rate capability and cycling performance. Figure S16 is the Nyquist plots of different electrodes after 100 cycles at 0.2 A g^-1. The EIS curves mainly consist of a semicircle in high frequency region and an oblique line at low frequency. The equivalent circuit diagram shows that R_s is the electrolyte resistance, R_ct represents the charge transfer resistance and Z_w stands for the Warburg impedance associated with the diffusion of Li⁺. According to the fitting results, the charge-transfer resistance (R_ct) of the MoTe₂/rGO-10 nanocomposite is 67 Ω, which is less than that of bare MoTe₂ nanorods (89 Ω) and bulk MoTe₂ (125 Ω). Thus, the MoTe₂/rGO-10 nanocomposite is more favorable for charge transfer.

  In order to gain insights into the relationship between the superior rate performance and pseudocapacitor contribution of the MoTe₂/rGO-10 composite electrode, the CV curves at various scan rates have been done (Figure 5a). The relationship between current and scan rates can be expressed by the following equation: ^{[39, 40]}

  Figure 5

  

  Figure 5.  (a) CV curves of MoTe₂/rGO-10 nanocomposite at different scan rates, (b) Log(i) versus log(v) plots at different oxidation and reduction states of MoTe₂/rGO-10 nanocomposite, (c) Fitted pseudocapacitive contribution (red area) of the MoTe₂/rGO-10 electrode at a scan rate of 0.9 mV s^-1, (d) Ratio of pseudocapacitive (yellow) and diffusion-controlled (green) capacities at various scan rates of MoTe₂/rGO-10 electrode.

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  $ i = av^{b} $

  (1)

  $ \mathrm{log}(i) = b\mathrm{log}(v) + \mathrm{log}(a) $

  (2)

  where i is the peak current, v stands for the scan rate, and a and b are adjustable parameters. The charge storage mechanism is closely related to the b value. Typically, the b value of ~0.5 indicates a diffusion-controlled process, whereas the b value of ~1 implies a capacitive-controlled process. As shown in Figure 5b, the b values corresponding to the anodic and cathodic are 0.8575 and 0.7745, respectively. Thus, the capacitive kinetics is dominant in the MoTe₂/rGO-10 nanocomposite, resulting in a fast Li⁺ insertion/extraction progress. Dunn equation is also used to investigate capacitive contribution under certain scan rate:

  $ i (V) = k_{1}v + k_{2}v^{1/2} $

  (3)

  where i (V) is related to the total current contributions that can be considered as combination of two separate mechanisms, namely capacitive contribution by the surface (k₁v) and diffusion-controlled insertion (k₂v^1/2). Figure 5c illustrates the separation of pseudocapacitive-controlled current (highlighted red area) from the entire CV curve at 0.9 mV s^-1. The calculated red region of 94% represents the contribution of the pseudocapacitive behavior, which determines a rapid lithium-storage behavior at high current density. Meanwhile, a capacitive contribution of 81%, 87%, 90%, 92%, 94% and 95% at the scan rates of 0.1, 0.3, 0.5, 0.7, 0.9, and 1.1 mV s^-1 can be achieved (Figure 5d). It appears that the charge storage process is pseudocapacitance-dominated in the MoTe₂/rGO-10 nanocomposite.

  The excellent electrochemical performance of MoTe₂/rGO-10 nanocomposite anode is mainly attributed to its microscopic structural characteristics. First, 1D nanostructure can not only shorten the diffusion path of Li⁺, but also provide directional paths for fast electron transport. Second, the Te vacancies provide more facile ion diffusion path, which can cause faster mobility for Li⁺ insertion/extraction. Third, the rGO thin layers significantly improve the electrical conductivity of the MoTe₂/rGO-10 nanocomposite, which leads to higher capacity and better rate capability. Fourth, the flexible rGO layers can suppress the volume change of MoTe₂ nanorods and enhance the stability of the structure. As a synergistic result, MoTe₂/rGO-10 nanocomposite anode exhibits excellent electrochemical performance.

  MoTe₂/rGO Versus LCO Full-Cell. A full-cell LIB was assembled by coupling MoTe₂/rGO-10 anode and the commercial LiCoO₂ cathode (LCO). According to the practical capacity, the weight ratio of MoTe₂/rGO-10 and LCO is 1:3. The specific capacities are evaluated based on the mass loading of MoTe₂/rGO nanocomposite anode. Figure S17 shows the CV curve of full cell at a scan rate 0.1 mV s^-1. The CV curve shows a distinct reduction peak at ~1.8 V, which corresponds to the nominal voltage of the full cell. Figure 6a shows the charge/discharge performance curves of MoTe₂/rGO-10//LCO full cell. The charge/discharge capacities in the 1^st cycle are 508 and 180 mA h g^-1, respectively, giving a low coulombic efficiency of 35%, which is mainly due to the polarization and irreversible capacity loss at the MoTe₂/rGO-10 anode. It is increased to 87% for the 3^rd cycle and the charge/discharge capacities are 250 and 219 mA h g^-1, respectively. The MoTe₂/rGO-10//LCO full cell exhibits the reversible capacity of 105 mA h g^-1 over 250 cycles and shows good cycling stability (Figure 6b).

  Figure 6

  

  Figure 6.  (a) Galvanostatic charge/discharge curves at 0.1 C for different cycles of the MoTe₂/rGO-10//LCO full cell in the voltage range of 1.5-3.8 V, (inset shows the 1^st cycle charge/discharge curve), (b) Long-term cycling performance of the MoTe₂/rGO-10//LCO full cell for 250 cycles at 0.1 C.

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  CONCLUSION

  In summary, 1D MoTe₂/rGO nanocomposite anode has been successfully fabricated by a facile in-situ tellurization of GO encapsulated Mo₃O₁₀(C₂H₁₀N₂) nanowires process. It achieves a high reversible capacity of 637 mA h g^-1 at 0.2 A g^-1 and superior rate capability (374 mA h g^-1 at 2 A g^-1), exhibiting significantly improved performance compared to bare MoTe₂ and bulk MoTe₂ electrode. More importantly, the nanocomposite also shows good long cycling stability, revealing a capability decay of 0.09% per cycle for 200 cycles at 0.5 A g^-1. Moreover, the assembled MoTe₂/rGO-10//LCO full cell has good cycling stability with a capacity of 105 mA h g^-1 after 250 cycles at 0.1 C. The excellent electrochemical performance can be attributed to the synergistic effects of 1D nanostructure, rGO thin layers and the presence of Te vacancies. This work can be extended to the synthesis of other 1D TMDs on a large-scale production for energy storage devices.

  EXPERIMENTAL

  Synthesis of Organic-Inorganic Mo₃O₁₀(C₂H₁₀N₂) Nanowires. The organic-inorganic hybridized Mo₃O₁₀(C₂H₁₀N₂) nanowire was synthesized according to the literature.^[41] In a typical procedure, 1.24 g of (NH₄)₆Mo₇O₂₄·4H₂O and 1 mL of ethylenediamine were dissolved in 15 mL of distilled water, and then 1 M HCl aqueous solution was dropped to the mixture with magnetic stirring at room temperature until a white precipitate was formed. After incubation at 50 ℃ for 5 h, the product was washed with deionized water and ethanol several times. The collected sample by centrifugation was dried at 60 ℃ overnight.

  Synthesis of Graphene Oxide Coated Mo₃O₁₀(C₂H₁₀N₂) Nano-wires (Mo₃O₁₀(C₂H₁₀N₂)/GO). Mo₃O₁₀(C₂H₁₀N₂)/GO was synthesized via the following procedure. 0.1 g Mo₃O₁₀ (C₂H₁₀N₂) nano-wires were dispersed into 40 mL H₂O, then mixed with 20 mL, 40 mL and 60 mL of GO dispersion solution (0.25 mg mL^-1). The obtained mixture was stirred for 24 h, resulting in the formation of Mo₃O₁₀(C₂H₁₀N₂)/GO with different GO contents. The obtained suspension was freeze-dried for 12 h.

  Synthesis of MoTe₂/rGO-x Nanorods. The reaction of the as-obtained Mo₃O₁₀(C₂H₁₀N₂)/GO (50 mg) and Te powder (50 mg) were performed in a tubular furnace with a temperature controller. The equipment was heated to 500 ℃ at a rate of 2 ℃ min^-1 and maintained at that temperature for 5 h under flowing Ar/H₂. Meanwhile, GO was thermally reduced to rGO. The mass of coating GO was 5, 10 and 15 mg, respectively, and the resulting products were designated as MoTe₂/rGO-5, MoTe₂/rGO-10 and MoTe₂/rGO-15. For comparison, bare MoTe₂ nanorods without GO was also synthesized with a similar method.

  Synthesis of Bulk MoTe₂. Bulk Mo₃O₁₀(C₂H₁₀N₂) was firstly produced similar to the condition of Mo₃O₁₀(C₂H₁₀N₂) nanowires except that the reaction time was decreased to 1 h. Then the reaction of bulk Mo₃O₁₀(C₂H₁₀N₂) with Te resulted in the formation of bulk MoTe₂.

  Materials Characterization. The crystal structure was studied by X-ray diffraction (XRD) on a Rigaku Corporation UItimaIV diffractometer with Cu Kα radiation (λ = 0.15418 nm). The chemical composition and valence state of the samples were analyzed by X-ray photoelectronic spectroscopy (XPS) on an ESCALAB 250Xi system. The microstructure and morphology of the samples were characterized by scanning electronic microscope (SEM; ZEISS MERLIN Compact, Germany) and high-resolution transmission electron microscopy (HRTEM) with FEG (Talos F200 X) instruments equipped with energy dispersive X-ray spectroscopy (EDS, Oxford SDD X-Max). Electronic paramagnetic resonance spectrometer (EPR, EMXplus-6/1, Bruker, Germany) was used to characterize the existence of defects. BET was used for the surface area determination using MicroActive for TriStar II Plus 2.02. TGA was used to determine the content of MoTe₂ under N₂ atmosphere by NETZSCH TG 209F3.

  Electrochemical Test of Half Cells. The working electrode was prepared by mixing the active materials (bare MoTe₂ nanorods or MoTe₂/rGO nanocomposite), carbon black and polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) at a mass ratio of 8:1:1, respectively. Then slurry was cast onto a piece of clean copper foil and dried under vacuum at 80 ℃ for 12 h. The copper foil was then cut into discs with the diameter of 12 mm. The loading of active materials in the electrode was ≈1-1.3 mg cm^-2. The CR2032 coin-type cells were assembled in an argon-filled glove box with lithium foil as counter and reference electrode (Mikrouna, H₂O ≤ 0.1 ppm, O₂ ≤ 0.1 ppm). Celgard 2400 was used as the separator, 1.0 M LiPF₆ in ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate = 1:1:1 vol% with 1.0% vinylene carbonate was used as the electrolyte. The charge/discharge tests were performed on Lan He battery testing system (Wuhan LAND Electronics Co., China) in the potential range of 0.01-3.0 V versus Li⁺/Li. Cyclic voltammetry (CV) measurement was carried out on a CHI 760E electrochemical workstation between 0.01 and 3 V with different scanning rates. The electrochemical impedance spectro-scopy (EIS) was carried out with the same electrochemical workstation over a frequency range of 100 kHz to 0.01 Hz with an amplitude of 5 mV.

  Fabrication and Electrochemical Measurements of Full Cells.

  The cathode was obtained by mixing the LCO, carbon black and PVDF in NMP with weight ratio of 8:1:1. Then the slurry was coated on aluminum foil and dried overnight at 80 ℃. The loading of LCO in the electrode was ≈3-3.2 mg cm^-2. The CR2032 coin-type cells were assembled in an argon-filled glove box (Mikrouna, H₂O ≤ 0.1 ppm, O₂ ≤ 0.1 ppm). Celgard 2400 was used as the separator, and the mixture of 1.0 M LiPF₆ in ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate = 1:1:1 vol% with 1.0% vinylene carbonate was used as the electrolyte. Galvanostatic measurements were made in the voltage range from 1.5 to 3.8 V at 0.1 C. The specific capacities were evaluated based on the mass loading of MoTe₂/rGO nanocomposite.

  
  ACKNOWLEDGEMENTS: This work was supported by the National Natural Science Foundation of China (21771137). The authors declare no competing interests.
  COMPETING INTERESTS
  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0050
  ADDITIONAL INFORMATION
  For submission: https://mc03.manuscriptcentral.com/cjsc
  
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• 

  Figure 1  Schematic illustration for the fabrication of bare MoTe₂ nanorods and MoTe₂/rGO nanocomposite.

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  Figure 2  (a) XRD patterns of bare MoTe₂ nanorods and MoTe₂/rGO-10 nanocomposite together with standard pattern of MoTe₂ highlighted with chopsticks, (b) SEM image, (c) TEM image, (d) HRTEM image, (e) SAED pattern, (f) TEM image and corresponding elemental mapping images of (g) Mo, (h) Te, (i) C (scale: 100 nm) in the MoTe₂/rGO-10 nanocomposite.

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  Figure 3  High resolution XPS of MoTe₂/GO-10 nanocomposite and bare MoTe₂. (a) Mo 3d, and (b) Te 3d.

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  Figure 4  (a) CV curves of MoTe₂/rGO-10 nanocomposite for initial six cycles at a scan rate of 0.2 mV s^-1, (b) Initial charge/discharge profiles of the MoTe₂/rGO-10 nanocomposite (1^st-3^rd cycles at 0.05 A g^-1 and 4^th-5^th cycles at 0.2 A g^-1), (c) Cycling performance of the bare MoTe₂ and MoTe₂/rGO-10 nanocomposite at 0.2 A g^-1, (d) Rate capability of bare MoTe₂ nanorods and MoTe₂/rGO-10 nanocomposite, (e) Cycling performance of MoTe₂/rGO-10 nanocomposite at 0.5 A g^-1.

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  Figure 5  (a) CV curves of MoTe₂/rGO-10 nanocomposite at different scan rates, (b) Log(i) versus log(v) plots at different oxidation and reduction states of MoTe₂/rGO-10 nanocomposite, (c) Fitted pseudocapacitive contribution (red area) of the MoTe₂/rGO-10 electrode at a scan rate of 0.9 mV s^-1, (d) Ratio of pseudocapacitive (yellow) and diffusion-controlled (green) capacities at various scan rates of MoTe₂/rGO-10 electrode.

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  Figure 6  (a) Galvanostatic charge/discharge curves at 0.1 C for different cycles of the MoTe₂/rGO-10//LCO full cell in the voltage range of 1.5-3.8 V, (inset shows the 1^st cycle charge/discharge curve), (b) Long-term cycling performance of the MoTe₂/rGO-10//LCO full cell for 250 cycles at 0.1 C.

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