Rational design and preparation of few-layered MoSe₂ nanosheet@C/TiO₂ nanobelt heterostructures with superior lithium storage performance

Abstract

A heterostructure of MoSe₂ nanosheets decorated on C/TiO₂ nanobelts was successfully fabricated through a facile hydrothermal process with the assistance of glucose. Ultrathin MoSe₂ nanosheets fully covered on TiO₂ nanobelts were coupled with carbon components as strong skeletons, which possess high contact surface area, efficient electronic transport frameworks and sufficient void spaces. As expected, the MoSe₂@C/TiO₂ heterostructures exhibit remarkable cycling stability (discharge capacity of 987.4 mA h g⁻¹ after 100 cycles at a current density of 500 mA g⁻¹) and rate capability (retained capacity of 860 mA h g⁻¹ even at a current rate of 3000 mA g⁻¹) as anode materials in lithium-ion batteries. The excellent electrochemical performance can be attributed to the structural advantages of MoSe₂@C/TiO₂ heterostructures, enabling great potential as anode materials in lithium-ion batteries.

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Introduction

Considerable attention has been paid to the development of novel anode materials for high-performance lithium-ion batteries (LIBs) in order to fulfill the ever-growing demand for various electronic devices. Similar to graphite, transition metal dichalcogenide (TMD) layered materials have attracted great interest as anode materials in the field of energy storage.¹ With their particular sandwiched structure of metal atoms layered between two chalcogen layers, TMDs exhibit distinguished performance. Thin layered nanosheets can offer fast diffusion paths for intercalation/deintercalation of Li⁺ and high specific surface area for better contact with the electrolyte. Besides that, environmental friendliness and the ability to assemble with diverse substrates have greatly promoted investigation of TMDs layered materials as ideal anode materials for lithium-ion or sodium-ion storage.

As one of the representative TMDs materials, MoSe₂ reveal relatively high theoretical specific capacity and have been appreciated as good candidate anode materials for energy storage devices.^2–4 However, MoSe₂ nanostructure anodes still expose some inherent problems impeding its practical applications. Accompanying the process of repeated discharge and charge, large volume changes will cause severe structure collapse. Meanwhile particle aggregation can form unstable solid electrolyte interface (SEI) film, which results in fast cycle capacity fading, low coulombic efficiency and limited rate capacity.^5,6 In order to avoid and resolve these obstacles, an effective strategy is to develop synthesis of MoSe₂ hybrid nanostructures with various substrate matrices. Most of the previous researches focused on MoSe₂/carbon nanocomposites, including carbon spheres,⁷ graphene,⁸ carbon cloth⁹ and organic carbon.¹⁰ In these nanocomposites, the improved electrochemical performance demonstrates the synergistic effect of MoSe₂ and carbon matrices. However, the excessive interface between carbon matrices and electrolytes leads to considerable side reactions and forms a thick SEI film on the carbon matrices.¹¹ In this respect, optimized framework materials for MoSe₂ based heterostructures with effective synergy are highly desirable and imperative.

Alternatively, TiO₂ has been has been widely used as a backbone material for hybrid systems owing to its structural integrity during the charge/discharge process (low volume change of less than 4%) and chemical stability.^12–14 In addition, TiO₂ anode exhibits a relatively high lithium insertion/extraction voltage at about 1.7 V, which can efficiently avoid the formation of SEI layers and thus improve safety compared with carbon matrices.

In addition, recently some hybrid systems based on 1D TiO₂ nanostructures have been adopted to overcome the demerits of the individual materials, thus improving the anode performance in LIBs.^15–18

Based on the strategies mentioned above, we rationally designed and successfully synthesized new MoSe₂ nanosheets@C/TiO₂ nanobelts hybrid nanostructures by a facile glucose-assisted hydrothermal process, and used them as electrode materials for lithium-ion batteries. Here, the stable skeletons of TiO₂ nanobelts showed good structural stability as expected and further buffered severely volume change of MoSe₂ nanosheets by providing sufficient void spaces. The low cycle life of MoSe₂ nanosheets and the low specific capacity of TiO₂ nanobelts could be simultaneously improved by the synergistic effect between MoSe₂ and TiO₂. Secondly, what is particularly worth mentioning is that the addition of glucose skillfully achieved killing two birds with one stone: glucose could not only play a critical role during synthesis process to promote uniform cover of few-layered MoSe₂ nanosheets on TiO₂ skeletons, but also couple with TiO₂ nanobelts to provide fast and shortened pathways for electron transportation. Moreover, comparing with agglomerated MoSe₂ nanoflowers, MoSe₂ nanosheets could uniformly stretch with the robust mechanical support of the stable skeletons, resulting high specific surface area with electrolyte. The advisable combination of these two components takes advantages of each other and reveals favorable synergistic effect. As a result, such composites synergistically enhance the effective utilization of MoSe₂, buffer the volume variation and achieve good cycling stability.

Experimental section

Preparation of TiO₂ nanobelts

TiO₂ nanobelts were synthesized through a facile hydrothermal process followed by annealing in air.¹⁹ In a typical synthesis, TiO₂ (P25) powder was dispersed in 10 M NaOH aqueous solution and heated at 180 °C for 24 h in Teflon-lined stainless steel autoclaves. Then the obtained Na₂Ti₃O₇ powder was washed thoroughly and immersed in 0.1 mol L⁻¹ HCl aqueous solution to obtain the H-titanate (H₂Ti₃O₇) nanobelts. Finally the H-titanate nanobelts were annealed in muffle furnace and rough surfaces were obtained which act as nucleation centers for the nucleation and growth of MoSe₂ nanosheets.

Preparation of MoSe₂ nanosheets@C/TiO₂ nanobelts heterostructures

The formation process of MoSe₂@C/TiO₂ heterostructures was described as follows. Typically, 50 mg Se powder was dissolved in 5 mL N₂H₄·H₂O under constant stirring for 4 h to form N₂H₄·H₂O–Se dark red-brown solution in ambient air. In parallel, 50 mg as-synthesized TiO₂ nanobelts were dispersed in 25 mL 0.05 M glucose aqueous solution through ultrasonication for 30 min, followed by mixing with 160 mg Na₂MoO₄·2H₂O based on a nominal Mo : Se molar ratio of 1 : 1 (with a molar ratio of Ti : Mo ≈ 1). The mixed glucose solution was then added to the N₂H₄·H₂O–Se solution slowly and transferred into a 50 mL Teflon-lined stainless steel autoclaves, which were heated in an circulation oven at 200 °C for 24 h. After cooling to room temperature naturally, the black precipitate was collected by centrifugation, washed thoroughly with ethanol, and dried at 60 °C for 12 h. In order to get rid of the redundant Se powder and obtain the highly crystalline MoSe₂ nanosheets, the as-synthesized MoSe₂@C/TiO₂ heterostructures were further treated in a tube furnace at 600 °C for 2 h in a high purity argon atmosphere flowing at 200 sccm. For comparison, one control sample MoSe₂/TiO₂ was synthesized by a homogeneous prepared process except for the addition of glucose, meanwhile pure MoSe₂ nanosheets were also prepared in identical conditions without the addition of TiO₂ nanobelts and glucose.

Material characterization

Powder X-ray diffraction (XRD) patterns were acquired by X-ray diffraction (D/max 2550, Rigaku Corporation, Cu-K_α radiation λ = 0.15406 nm). The morphologies of the as-prepared samples were characterized using a field emission scanning electron microscope (FSEM, Nova Nano SEM 230, FEI), a field emission transmission electron microscope (FTEM, JEM-2100F, JEOL) and a spherical aberration correction lens field emission transmission electron microscope (FTEM, Titan G2 60-300, FEI) coupled with an energy dispersive X-ray spectrometer (Super-X EDX). X-ray photoelectron spectroscopy (XPS) was performed using ESCALAB 250Xi, ThermoFisher-VG Scientific. The thermogravimetric analysis (TGA, NETZSCH STA 449C) was carried out in air heating to 800 °C at a rate of 10 °C min⁻¹.

Electrochemical measurements

To investigate the electrochemical properties of the as-synthesized samples, coin cell (CR2016) configuration was assembled in an argon-filled glove box (MB-OX-SE1, MERAUN, German, O₂ and H₂O levels less than 1 ppm). The MoSe₂@C/TiO₂ heterostructures, MoSe₂/TiO₂ nanocomposites, pure MoSe₂ nanosheets and TiO₂ nanobelts were used as working cathodes while lithium foil served as the reference electrodes. The electrolyte was 1 M LiPF₆ dissolved in EC/DMC/DEC (1 : 1 : 1 by volume) and a membrane film (Celgard 2400) was used as the separator. Typically, for preparing the working electrodes, 80 wt% of the active material, 10 wt% carbon black and 10 wt% carboxyl methyl cellulose (CMC) were mixed with distilled water and alcohol mixture solution, followed by stirring at a constant speed for 12 h to form homogeneous slurry and pasting uniformly onto a copper foil. Then the coated copper foil was dried in vacuum oven at 70 °C overnight and punched into disk-shaped electrodes with a diameter of 12 mm. The cells were aged for 12 h before the measurements to ensure percolation of the electrolyte to the electrodes. Galvanostatic discharge–charge cycling tests were conducted using a battery tester (LAND CT2001A) at different current rates with the potential ranging from 1 mV to 3 V. Cyclic voltammetry (CV) was performed using an electrochemical workstation (PARSTAT 4000, Ametek) from 0.01 V to 3 V at a scanning rate of 0.2 mV s⁻¹. Electrochemical impedance spectrometry (EIS) was also carried out with the same electrochemical workstation in the frequency range of 100 kHz to 10 mHz at an open circuit potential at room temperature.

Results and discussion

The phase composition and crystal structure of as-synthesized TiO₂ nanobelts, MoSe₂ nanosheets and MoSe₂@C/TiO₂ heterostructures were studied by XRD. As shown in Fig. 1, the TiO₂ in the samples shows standard peaks of anatase (JCPDS: 21-1272) and monoclinic (JCPDS: 46-1237) phase. The XRD pattern of MoSe₂ shows the characteristic peaks which can be indexed to the hexagonal MoSe₂ phase with P6₃/mmc space group (JCPDS: 29-0914, drysdallite-2H) and rhombohedral crystal phase with R3m space group of MoSe₂ (JCPDS: 20-0757). As for MoSe₂@C/TiO₂ heterostructures, the detected peaks at 2θ = 13.7°, 31.4°, 53.3°, 55.9° in the pattern can be assigned to the (002), (100), (106), (110) of the hexagonal phase MoSe₂, respectively, and peak at 2θ = 36.5° can be assigned to the rhombohedral phase. The results confirm the co-existence of TiO₂ and MoSe₂ crystal phases in the heterostructures.

Fig. 1 XRD patterns of MoSe₂@C/TiO₂ heterostructures, pure MoSe₂ nanosheets and bare TiO₂ nanobelts.

In order to study the morphologies and microstructures of all the samples, field emission scanning electron microscopy (FSEM) and field emission transmission electron microscopy (FTEM) were performed. Fig. 2a presents that monodisperse TiO₂ nanobelts with diameters of 100–300 nm are obtained. The low magnification TEM image shown in the inset of Fig. 2a reveals rough surface of pure TiO₂ nanobelts. As can be seen in Fig. 2b, the morphology of MoSe₂ can be described as a flower-like sphere with an average diameter of 100 nm. Fig. 2c and d show the SEM and TEM images of the as-synthesized MoSe₂@C/TiO₂ hybrid nanostructures after hydrothermal and annealed process. It reveals TiO₂ nanobelts are uniformly covered by MoSe₂ nanosheets with the assistant of glucose. The enlarged image is shown in Fig. 2e. It can be observed that the MoSe₂ nanosheets are flexible, and curly, indicative of their ultrathin 2D nature. Furthermore, the rings in SAED (Fig. 2f) arise from the stacking of MoSe₂ nanosheets with different crystallographic orientations, consistent with XRD results. As illustrated in Fig. 2g, the scattering diffraction spots in inner circle can be ascribed to interplanar spacing of anatase TiO₂ (101), which is also regarded as the most thermodynamically stable crystal facet of anatase TiO₂.¹⁹ The concentric rings can be assigned as diffraction from (100), (103) and (110) planes of hexagonal phase MoSe₂.

Fig. 2 SEM and TEM images: (a) the bare TiO₂ nanobelts and low magnification TEM image shown in the inset, (b) pure MoSe₂ nanoflowers, (c and d) MoSe₂@C/TiO₂ heterostructures, (e) high magnification TEM image of rectangle region in (d), (f) shows the SEAD patterns with illustration in (g).

From HRTEM image of composites in Fig. 3a, it can be seen that the interplanar distance of 0.65 nm is apparently larger than the d-spacing of (002) planes of hexagonal phase MoSe₂. It is expected that MoSe₂ nanosheets with an enlarged interlayer distance can accommodate more Li⁺. Meanwhile, the interlayered spacing of approximate 0.23 nm and 0.19 nm are observed, which can be perfectly indexed as (103) and (105) plane of the hexagonal phase MoSe₂, respectively. The top view high-resolution TEM image of heterostructures in Fig. 3b indicates that most of the shells in the hybrid nanostructures consist of 3–7 layers of MoSe₂ nanosheets. Furthermore, to investigate the effect of glucose on the morphology of heterostructures, one control sample was synthesized according to the same conditions except for the addition of glucose. Fig. S1† shows that TiO₂ nanobelts are covered by MoSe₂ thick flakes, simultaneously a large amount of them tend to aggregate into dispersive nanoparticles. Based on these results, it can be inferred that glucose is beneficial to suppress clustering of MoSe₂ nanosheets, maintain ultrathin structure superiority and act as a binder to promote uniform cover of few-layered MoSe₂ nanosheets on TiO₂ nanobelts.^11,20

Fig. 3 HRTEM images of MoSe₂@C/TiO₂ heterostructures.

The detailed local elemental mapping (Fig. 4) demonstrates that Mo and Se elements are homogeneously dispersed in the entire sample region. Remarkably, Ti elements orderly assemble to belts acting as a backbone, and the majority of O elements distribute at center belts in consistent with Ti. Meanwhile, it is deserved to be mentioned that C elements exhibit uniformly distribution in accordance with Mo and Se. In combination with synthesis procedure, it can be inferred that this is likely to come from residual glucose carbon groups after hydrothermal process.¹¹ The energy-dispersive spectrometry (EDS) analysis of the prepared MoSe₂@C/TiO₂ is presented in Fig. S2.† It can be seen that the composite mainly contains Mo, Se, Ti, O and C. The atomic ratios of Se : Mo and O : Ti is 1.92 and 1.99, respectively, which is very close to the theoretical value of MoSe₂ and TiO₂. Furthermore, the molar ratio of Ti : Mo is about 2.18.

Fig. 4 Elemental mapping images of MoSe₂@C/TiO₂ heterostructures.

In addition, TGA measurement was employed under air flow to further determine the loading amount of single component in the composite. As shown in Fig. 5, the TiO₂ nanobelts show a slight weight change about 2.49%, while both pure MoSe₂ and MoSe₂@C/TiO₂ have notable weight loss. The weight increase in the range of 300–400 °C can be attributed to the formation of SeO₂ during the oxidation of MoSe₂. As temperature increase, significantly weight loss is associated with the gasification of SeO₂, the further oxidation of MoSe₂ and redundant carbon groups, following the final products MoO₃. So, approximate amount of MoSe₂ can be calculated from TGA analysis by assuming that the whole weight loss is due to the oxidation of TiO₂ and MoSe₂ to MoO₃. X represents the mass fraction of MoSe₂ content in the heterostructures in the following equation.

X × 28.68% + (1 − X) × 2.49% = 20.53% (1)

X = 68.9%

Fig. 5 TG curves of MoSe₂@C/TiO₂ heterostructures, pure MoSe₂ nanosheets and bare TiO₂ nanobelts.

We also carried out X-ray photoelectron spectroscopy (XPS) to investigate the chemical states of Mo, Se, Ti, O and C in the MoSe₂@C/TiO₂ heterostructures and element composition on the surface (Fig. 6). Fig. 6a shows the C 1s spectrum, which can be fitted into three peaks: the carbon in C–C and C–OH appeared at 284.8 and 286.7 eV respectively, and the peak at 282.5 eV is assigned to carbon in C–Mo.¹⁰ In the high resolution Mo 3d spectrum of the composites (Fig. 6b), four separate peaks are presented indicating the Mo atoms are in two different chemical states. Two peaks at 228.8 and 232.0 eV can be ascribed to the expected Mo (+4) 3d_5/2 and 3d_3/2 respectively,²¹ while the other two peaks at 233.0 and 236.0 eV can be attributed to Mo (+6) 3d_5/2 and 3d_3/2 respectively, which are likely due to the existence of small amount of MoO₄²⁻ that is not completely reduced.¹⁹ Meanwhile, in Fig. 6c the binding energies of Se 3d_5/2 at 54.3 eV and Se 3d_3/2 at 55.0 eV indicated the (−2) oxidation chemical state of Se. The 2p peak of Ti element exhibits two peaks in Fig. 6d at 459.0 and 464.7 eV, corresponding to the Ti 2p_3/2 and Ti 2p_1/2 respectively, confirming the existence of Ti (+4) state. The XPS results further prove the co-existence of MoSe₂ and TiO₂ in the heterostructures.

Fig. 6 XPS spectra comparison of MoSe₂@C/TiO₂ heterostructures, pure MoSe₂ nanosheets and bare TiO₂ nanobelts: (a) C 1s peaks, (b) Mo 3d peaks, (c) Se 3d peaks and (d) Ti 2p peaks.

By comparison, in pure MoSe₂ nanosheets, Mo (+4) 3d_5/2, 3d_3/2, Se (−2) 3d_5/2 and Se 3d_3/2 peaks are located at 229.04, 232.24, 54.64 and 55.34 eV, respectively. After TiO₂ nanobelts were decorated on the MoSe₂ nanosheets, the binding energy of Mo 3d and Se 3d of MoSe₂@C/TiO₂ shifted about 0.24 and 0.34 eV to the lower energy direction. Moreover in pure TiO₂ nanobelts, Ti (+4) 2p_3/2 and 2p_1/2 are located at 458.2 and 463.9 eV, respectively, while after loaded MoSe₂ nanosheets, the binding energy shifted about 0.80 eV to higher energy direction. Therefore, the collective shifts of these peaks can be interpreted as mutual effect of varied electron environment, which can further confirmed the successful synthesis of heterostructures. In addition, the detailed compositional analysis reveals that surface Mo and Se atomic ratio is 5.91% and 11.61%, respectively, corresponding to the formula of MoSe₂. Moreover, the molar ratio of Ti : Mo is 2.26, which is also close to results of EDS.

The annealed MoSe₂@C/TiO₂ heterostructure was assembled into coin cells and evaluated for lithium-ion electrochemical performance. The MoSe₂/TiO₂ nanocomposites, pure TiO₂ nanobelts and MoSe₂ nanosheets were also employed to the same tests for comparison in assistant to understand discharge–charge process in detailed. Fig. 7a shows the first three cyclic voltammetry (CV) profiles of MoSe₂@C/TiO₂ heterostructures in the voltage range 0.01–3 V vs. Li⁺/Li at scan rate of 0.2 mV s⁻¹. As can be seen, the CV behavior is generally in agreement with MoSe₂ and TiO₂ nanostructures reported previously.^11,20,22 During the initial discharge process, three main cathodic peaks appear at around 1.45 V, 0.93 V and 0.37 V. The first cathodic peak at 1.45 V corresponds to the lithiation of TiO₂ nanobelts according to reaction (2). The pronounced peak at 0.93 V is due to the intercalation of Li⁺ into the layered MoSe₂ lattice accompanied with the formation of Li_xMoSe₂ (reaction (3)). The other cathodic located at 0.37 V is associated to decomposition reaction from Li_xMoSe₂ to Mo metal and Li₂Se (reaction (4)).

TiO₂ + xLi⁺ + xe⁻ ↔ Li_xTiO₂ (2)

MoSe₂ + xLi⁺ + xe⁻ → Li_xMoSe₂ (3)

Li_xMoSe₂ + Li⁺ + e⁻ → Mo + Li₂Se (4)

Fig. 7 Electrochemical performance of MoSe₂@C/TiO₂ heterostructures, MoSe₂/TiO₂ nanocomposites, pure MoSe₂ nanosheets and bare TiO₂ nanobelts in lithium-ion batteries: (a) CV profiles of MoSe₂@C/TiO₂ heterostructures; (b) discharge–charge voltage profiles of MoSe₂@C/TiO₂ heterostructures; (c and d) cycling and rate performance of MoSe₂@C/TiO₂ heterostructures, MoSe₂/TiO₂ nanocomposites, pure MoSe₂ nanosheets and bare TiO₂ nanobelts between 0.01 and 3 V.

In the reversible charge process, the first two anodic peaks observed at 1.56 V and 1.65 V can be assigned to pseudo-capacitive lithium-ion storage behavior of TiO₂(B), which are in pairs of S-peaks with subsequent discharges peaks at 1.48 V and 1.53 V.^23–25 The sharp anodic peak at 2.13 V indicates the lithium extraction process of Li₂Se to Se. Moreover in the successive cathodic sweeps, the broad peak at around 1.9 V can be interpreted as the lithiation process of Se to form Li₂Se, which is similar with the well-known peak of formation Li₂S in lithium-sulfur battery systems.²⁶ This apparent change between the first and subsequent discharges is consistent with the previous reports, illustrating that MoSe₂ experiences an irreversible phase transition in the initial discharge. Even more, a small pair of successive cathodic peak at 1.25 V and anodic peak at 1.4 V is attributed from transformation of Mo and MoSe₂ which is in accordance with pure MoSe₂ nanosheets. Notably, the areas of CV curves of MoSe₂@C/TiO₂ heterostructures in Fig. 7a are obviously larger than pure MoSe₂ nanosheets and bare TiO₂ nanobelts (Fig. S3†), further indicating hybrid composites exhibit higher specific capacity.

Fig. 7b shows the representative galvanostatic discharge–charge (GDC) voltage profiles of the MoSe₂@C/TiO₂ heterostructures for the first, second and fiftieth cycles at a constant current density of 500 mA g⁻¹. The plateaus in the voltage profiles are in accordance with the distinct peaks in the CV curves. For comparison, the pure MoSe₂ nanosheets and bare TiO₂ nanobelts are also tested (Fig. S4†). As can be seen, two apparent voltage plateaus at 1.4 V and 0.9 V, and small plateau at 0.5 V can be observed in the initial discharge, and the subsequent GDC cycles generally move up with plateaus at 2.0 V and 1.5 V. The MoSe₂@C/TiO₂ heterostructures exhibit the initial discharge and charge specific capacities of 1153.3 mA h g⁻¹ and 861.8 mA h g⁻¹ respectively, giving a coulombic efficiency (CE) of 74.4%, which can be mainly ascribed to the gel-like polymeric layer formation on the MoSe₂@C/TiO₂ and side reactions of electrolyte reduction in the initial cycle.^20,27 The discharge and charge specific capacities in the second cycle are 866.8 mA h g⁻¹ and 831.7 mA h g⁻¹, respectively, with a higher coulombic efficiency of 96%. This value can be retained as 98% even after 50 charge–discharge cycles. It should be noted that the total mass of heterostructures was used in calculating specific capacities. Comparatively, the pure MoSe₂ nanosheets exhibited initial discharge and charge specific capacities of 991.3 mA h g⁻¹ and 726.1 mA h g⁻¹, respectively, and the corresponding initial CE is 73.2%. The initial discharge and charge specific capacities of TiO₂ are 282 mA h g⁻¹ and 140.8 mA h g⁻¹, respectively, with corresponding initial CE of 49.5%. It can be seen that the MoSe₂@C/TiO₂ heterostructures possess superior Li⁺ insertion and extraction capabilities to the single component, further demonstrating the favorable synergistic effect of the MoSe₂@C/TiO₂ heterostructures.

The MoSe₂@C/TiO₂ heterostructures also exhibit excellent cycling stability and rate performance. Fig. 7c displays the comparative rate capability of MoSe₂@C/TiO₂ heterostructures with MoSe₂/TiO₂ nanocomposites, pure MoSe₂ nanosheets and bare TiO₂ nanobelts. MoSe₂@C/TiO₂ heterostructures exhibit a reversible discharge specific capacity of around 920 mA h g⁻¹ in the first ten cycles at current density of 500 mA g⁻¹, and retain a capacity of around 860 mA h g⁻¹ even at the maximum current rate of 3000 mA g⁻¹. After cycling under high current densities, the current density decrease to 500 mA g⁻¹ and the MoSe₂@C/TiO₂ electrodes reveal an increased discharge specific capacity of 1070 mA h g⁻¹. While the MoSe₂/TiO₂ nanocomposites electrodes deliver a combined rate performance of pure MoSe₂ and bare TiO₂. The behavior of capacity change is very similar to that of bare TiO₂ during the early cycle, which is 309 mA h g⁻¹ of 500 mA g⁻¹ to 263 mA h g⁻¹ of 3000 mA g⁻¹. After the current density return to 500 mA g⁻¹, an increased capacity is obtained, and this is the significant characteristic of MoSe₂. The results confirmed that both the ability of Li⁺ store and exposed contact area of MoSe₂ thick flakes and nanoparticles are utterly inadequate to few-layered MoSe₂ nanosheets.

Apart from the excellent rate capability and high reversible capacity, MoSe₂@C/TiO₂ heterostructures also exhibit good cycling behavior. Fig. 7d displays the cycling performance of MoSe₂@C/TiO₂ heterostructures, MoSe₂/TiO₂ nanocomposites, pure MoSe₂ nanosheets and bare TiO₂ nanobelts at a current density of 500 mA g⁻¹ after 100 cycles. The MoSe₂@C/TiO₂ heterostructures and MoSe₂ nanosheets electrodes deliver increased capacity in accordance with expectation. In the case of MoSe₂@C/TiO₂ heterostructures, the discharge specific capacity increased from 866 mA h g⁻¹ to 1050 mA h g⁻¹ with coulombic efficiency over 97%. After transitory float of capacity, the discharge capacity of the MoSe₂@C/TiO₂ heterostructures electrodes remains at 987.4 mA h g⁻¹ in 100th cycle with coulombic efficiency of 98.9%. While pure MoSe₂ nanosheets exhibit severe increase of specific capacities up to 1007 mA h g⁻¹ in first sixty cycles. However there is a sharp decline of nearly 50% specific capacities during following cycling process, and this represents destructive structural collapse. In contrast, the specific capacity of bare TiO₂ nanobelts retains at about 130 mA h g⁻¹ over 100 cycles, which shows the remarkable cycling stability. Note that MoSe₂/TiO₂ nanocomposites exhibit synthetic results of individual components as expected. Due to the compact structure, structural collapse of thick flakes and particles is limited, following relatively stability instead of sharp decline. Therefore, the heterostructures capacities significantly exceed those of either individual component even over 100 cycles, thus the superior performance of MoSe₂@C/TiO₂ heterostructures is confirmed. In addition, the SEM images (Fig. S5†) of MoSe₂@C/TiO₂ heterostructures electrodes after cycling for 100 cycles show that original 1D morphological characteristic still retained, demonstrating that robust skeletons could accommodate stress from cycling.

The increased specific capacity could be explained in detail as following reasons: (i) the layered structures of few-layered MoSe₂ nanosheets provide more contact of electroactive components and electrolyte, which may exist activation of Li⁺ during cycling process; (ii) as described in CV analysis, similar with MoS₂ electrodes,²⁸ Li–Se charge–discharge process makes the major contribution to the reversible capacity during later cycling. These gradual float of specific capacity is owing to the presence of these changes in electrochemical process. Thus, with more clustered few-layered nanosheets, it is accessible that MoSe₂ electrodes exhibit further increase capacity and larger extent structural collapse than unfold few-layered nanosheets in heterostructures.

In order to better understand the kinetics of electrode reaction between electrodes and electrolyte, EIS measurements were carried out for all electrodes. Fig. 8 shows the Nyquist plots of MoSe₂@C/TiO₂ heterostructures and pure MoSe₂ nanosheets, in which the inset represents the equivalent circuit model fitting well with experimental results. R_e represents the internal resistance of the test battery, R_f and Q₁ are related to the resistance of the SEI film corresponding to the high frequency semicircle. The medium frequency semicircle could be attributed to the charge transfer resistance R_ct and double-layer capacitance Q₂ of the electrode–electrolyte interface, and a sloping line at low frequency region is associated with lithium ion diffusion processes. Obviously, compared to pure MoSe₂ nanosheet (R_ct = 554.8 Ω), the MoSe₂@C/TiO₂ heterostructures (R_ct = 44.8 Ω) show a smaller semicircle diameter, indicating that MoSe₂@C/TiO₂ remarkably enhanced the rapid electron transport owing to the close contact between few-layered MoSe₂ nanosheets and conductive skeletons with the addition of glucose.

Fig. 8 EIS of MoSe₂@C/TiO₂ heterostructures and pure MoSe₂ nanosheets.

It is deserved to be mentioned that the average specific capacity of our MoSe₂@C/TiO₂ heterostructures is superior to previously reported MoSe₂ based products (Table S1†). The impressive electrochemical performance of MoSe₂@C/TiO₂ can be assigned to the rationally design of heterostructures. Firstly, the addition of glucose plays a critical role in synthesize process to maintain few-layered MoSe₂ nanosheets uniformly covering on TiO₂ skeletons. Ultrathin feature of the MoSe₂ nanosheets can store more Li⁺ and facilitate fast Li⁺ diffusion; on the other hand, unfolded ultrathin MoSe₂ nanosheets expose more contact surface, which avoid the aggregation of MoSe₂ nanosheets and accelerate penetration of electrolyte. Moreover, TiO₂ nanobelts skeletons could accommodate stress from cycling, maintain structural integrity and buffer severely volume change of MoSe₂ nanosheets. Further coupled with carbon compounds, robust skeleton act as beneficial electronic co-conductor and provide fast pathways for electron transportation.

Conclusions

In summary, we rationally designed and successfully synthesized MoSe₂ nanosheets@C/TiO₂ nanobelts heterostructures by a facile hydrothermal method with assistance of glucose. A systematic study of structure, morphology and electrochemical performance of the as-prepared MoSe₂@C/TiO₂ heterostructures was carried out. With rationally designed structural advantages, the MoSe₂@C/TiO₂ heterostructures serve as good anode materials which exhibit favorable reversibility, excellent capacity and remarkable stability. Even after deep cycling at 3000 mA g⁻¹, the specific capacity recovers to the same levels of 1070 mA h g⁻¹ when the current density is returned back to 500 mA g⁻¹. Furthermore, a very high discharge capacity of 987.4 mA h g⁻¹ after 100 cycles is obtained even at a current density of 500 mA g⁻¹. Therefore, this work opens a potential avenue for skillfully combination of diverse nanostructures by fostering strengths and circumventing weaknesses to develop other novel high-performance anode materials.

Acknowledgements

The authors are grateful for the financial support from the National Science Foundation of China (51302325), Science Fund for Distinguished Young Scholars of Hunan Province (2015JJ1016), Program for New Century Excellent Talents in University (NECT-12-0553), the Scientific Research Foundation for the Returned Oversea China Scholars, the Hunan Youth Innovation Platform and Program for Shenghua Overseas Talent (90600-903030005; 90600-996010162) from Central South University (CSU).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01579d

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