1. Introduction

Rechargeable lithium-ion batteries (LIBs) have been regarded as one of the most promising power technology in the applications of energy storage stations, electric vehicles, hierarchical electric vehicles and so on especially in the circumstances of energy crisis and global environmental concerns [1, 2]. However, the present commercial lithium batteries can not meet with the requirements of increasing energy density [3, 4]. Molybdenum disulfide (MoS₂), known as a typical two-dimensional (2D) transition-metal sulfide, has a structure analogous to that of graphite, in which S-Mo-S layers are held together by weak van der Waals forces, and has been considered as a promising anode material for LIBs owing to their high-lithium-storage capacity (ca. 670 mA h g^-1) [5, 6]. However, the poor electronic/ionic conductivity between S-Mo-S layers limits its development for anode materials [7, 8]. Moreover, the pulverization and aggregation of MoS₂ during the discharge–charge cycle can lead to fast capacity fading [9, 10].

In recent years, a variety of methods including nanostructure engineering[11, 12], construction of composite materials of MoS₂ and other materials, such as carbon nanotubes, graphene, MoO₂, TiO₂, and so on [13–16], have been employed to address the above issues. The down-sizing strategycanpartly buffer the large volume changes of MoS₂, and shorten the transport paths of electrons and Li⁺ ions [11]. The conductive materials such as amorphous carbon, carbon nanotubes (CNTs), graphene [13, 14], and conducting polymer [7], have been employed to construct MoS₂ composite structures to effectively improve the rate capability of MoS₂ obviously. For examples, Yang et al. [7] have fabricated hierarchical MoS₂/ polyaniline nanowires and exhibited greatly improved Li⁺ storage properties. Ding et al. [13] have prepared MoS₂ nanosheets on CNT backbone with assistance of glucose, which shown greatly enhanced lithium storage properties compared with the pure MoS₂. Among the carbonaceous materials, nitrogen-doped carbon is of unique for improving electrochemical properties by virtue of the increased electronic conductivity and the facilitation of lithium ion transport [17, 18], to date, hierarchical architectures with porous micro/nanostructuresare of greatinterest forlithium storagedue to the dual merits of both the nanoscale primary building blocks and microscale secondary assemblies [19], which can not only improve transport of electrons and Li⁺ ions, but also effectively prevent the agglomeration of nanoscale building blocks [20]. Herein, we report the synthesis of MoS₂ nanosheet arrays supported on hierarchical nitrogen-doped porous carbon (MoS₂@C), which can protect MoS₂ nanostructures from restacking during charge/discharge process. The obtained MoS₂@C exhibit high specific capacity and good rate capability compared with pure MoS₂ nanosheets.

2. Results and discussion

Fig. 1 shows the XRD patterns of the MoS₂ nanosheets and MoS₂@C. All the diffraction peaks can be indexed to hexagonal MoS₂ (JCPDS card No. 37-1492; space group P63/mmc; a = 3.161 Å, c = 12.299 Å). The strongest peak at 14.2° corresponds to the (0 0 2) plane, which is characteristic of the ordered stacking of 2D MoS₂ layers along c-axis. Notably, the 2θ of the (0 0 2) diffraction peak was shifted to a lower-angle compared with 2H-MoS₂, revealing the increased interlayer distance of the (0 0 2) plane. No other characteristic peaks from impurities are detected in the XRD pattern. According to the XRD patterns, the nitrogen-doped porous carbon is amorphous (Fig. S1 in Supporting information) and does not destroy the structure of hexagonal MoS₂.

Fig. 2 depicts typical SEM images of MoS₂@C. As shown in Fig. 2a and b, MoS₂ ultrathin nanosheets can grow in situ on the surfaces of nitrogen-doped porous carbon (Fig. S2 in Supporting information) to form hierarchical MoS₂@C. It is clear that the thickness of MoS₂ nanosheests in MoS₂@C is about 11 nm due to the strong interaction between MoS₂ and nitrogen-doped carbon. Energy-dispersive X-ray spectroscopy (EDX) scanning elemental mapping of Mo, S, C, N and O elements in Fig. S3 in Supporting information also verified the formation of MoS₂ nanosheets are in situ grow on the nitrogen-doped porous carbon. The hierarchical structures of MoS₂@C can be further confirmed by TEM image in Fig. 2c, which shows that aligned MoS₂ nanosheets are closely attached on the surfaces of the nitrogen-doped porous carbon. Typical HRTEM image of MoS₂@C is represented in Fig. 2d. The observed fringes correspond to the interplanar distance of 0.69 nm, which are in good agreement with the lattice spacing of the (0 0 2) planes of pure MoS₂ nanosheets as shown in Fig. 3d [21]. The amorphous nitrogen-doped porous carbon can be also found. When the nitrogen-doped porous carbon was absent in the reaction system, flower-like MoS₂ nanostructures composed of nanosheets with thickness of about 20 nm are clearly observed (Fig. 3a and b), which is much larger than the thickness of MoS₂ nanosheests in MoS₂@C, TEM images of pure MoS₂ nanosheests can also be seen in Fig. 3c, which is in agreement with the abovedescribed SEM observations. The weight content of MoS₂ in MoS₂@C is determined to be 84.49% through thermogravimetric (TG) analysis (Fig. S4 in Supporting information).

Fig. 4a and b shows the representative CV curves of MoS₂@C and pure MoS₂ nanosheets electrode for the first, second, and fifth cycles, respectively. In the first cycle, the cathodic peak at about 0.8 V corresponds to the intercalation of lithium ions into the MoS₂ lattice to form Li_xMoS₂, and another peak at about 0.2 V is attributed to complete reduction of MoS₂. In the following cathodic sweeps, the peaks at 0.8 V and 0.2 V disappear while two new peaks appear at about 1.7 V and 0.7 V, suggesting a multi-step lithium intercalation reaction. There is no significant change in anodic sweeps and it remains at about 2.5 V, corresponding to lithium extraction process, which is in agreement with the reports previously [22–24].

Fig. 5a and b display the galvanostatic discharge–charge profiles of MoS₂@C electrode and pure MoS₂ electrode, respectively. Two voltage plateaus can be observed at about 1.1 V and 0.6 V, which are attributed to the intercalation of lithium ions into the MoS₂ lattice and the complete reduction of MoS₂, respectively [25, 26]. The initial discharge capacity of MoS₂@C electrode is 1305.5 mA h g^-1 at a current density of 100 mA g^-1, which is much higher than that of pure MoS₂ electrode (984 mA h g^-1). As the current density is increased from 200 to 500 and 1000 mA g^-1, the discharge capacities of MoS₂@C electrode are decreased from 713 mA h g^-1 to 535 mA h g^-1 and 438 mA h g^-1, respectively. However, the pure MoS₂ electrode delivers the discharge capacities of 521 mA h g^-1 (200 mA g^-1), 284 mA h g^-1 (500 mA g^-1), and 145 mA h g^-1 (100 mA g^-1). Clearly, the discharge capacity of MoS₂@C electrode is much higher than that of pure MoS₂ electrode, especially at high current density. Fig. 5c shows the comparison of the rate performances of MoS₂ electrode and MoS₂@C electrode. Both pure MoS₂ and MoS₂@C electrodes exhibit good cycling performances at the current density of 200–1000 mA g^-1, because the restacking and condensation of MoS₂ during the charge–discharge process can be avoided. It is clear that the hierarchical MoS₂@C electrode exhibit good rate capability due to the introduction of nitrogen-doped carbon and hierarchical MoS₂@C. When the current density reverses back to 100 mA g^-1, their capacities of pure MoS₂ and MoS₂@C electrodes can recover to the original value immediately, indicating that the electrode materials are highly stable in spite of cycling with high rate. The cycling performance of the hierarchical MoS₂@C and pure MoS₂ electrode at a current density of 0.1 A g^-1 is shown in Fig. 5d. After 50 cycles, the hierarchical MoS₂@C can still be stable at a capacity of 628 mAh g^-1, while pure MoS₂ delivers fast capacity fading, which suggesting that the hierarchical porous carbon can effectively help to keep the overall structural integrity during the discharge/charge process.

In order to understand the electrochemical reaction kinetics and interfacial behavior of the hybrid electrode, the EIS of the electrodes were further examined. Fig. 6 shows the Nyquist plots of MoS₂@C electrode and the pure MoS₂ electrode. Each of the curves consists of a depressed semicircle and a straight line, which are related to the charge transfer process of the electrolyte–electrode interface and the lithium ion diffusion in the bulk of the electrode, respectively. It can be seen that the charge-transfer resistance of MoS₂@C electrode is much lower that of pure MoS₂ electrode, indicating that the introduction of porous nitrogen-doped carbon can indeed improve the electronic conductivity of MoS₂@C electrode. Furthermore, the inclined line of MoS₂@C electrode is also steeper than that of pure MoS₂ electrode owing to the small thickness of MoS₂ nanosheets, which can facilitate Li⁺ ion transfer in the electrode. The entranced electrochemical performances of MoS₂@C electrode can be attributed to the synergistic effects of the increased electronic conductivity and the facilitation of lithium ion transport arising from the hierarchical structures of MoS₂@C.

3. Conclusion

In summary, we have developed a facial hydrothermal route combined with high-temperature calcination to synthesize hierarchical MoS₂@C, in which MoS₂ nanosheet arrays are closely attached on the surfaces of the nitrogen-doped porous carbon. The hierarchical structures are helpful to facilitate the fast and efficient transport of electrons and Li⁺ ions. As expected, the MoS₂@C exhibit enhanced electrochemical performances compared with pure MoS₂ nanosheets. These findings based on the synergistic effect of active materials and porous carbon can be expected to open up new opportunities for designing high performance electrode materials.

4. Experimental

The nitrogen-doped porous carbon was prepared through a similar method using polypyrrole as the precursor [18]. In a typical synthesis for MoS₂@C, nitrogen-doped porous carbon (60 mg) was dispersed into deionized water by ultrasonication for 30 min. Then ammonium molybdate ((NH₄)₆Mo₇O₂₄, 0.584 g) was added to the solution. After being stirred for 5 min, thiourea (1 g) was added. After being stirred for another 5 min, the solution was transferred into a Teflon-lined stainless steel autoclave (100 mL) in an electric oven at 200 ℃ for 24 h and then cooled down to room temperature naturally. The black precipitate was collected by centrifugation, washed with deionized water and ethanol several times, and finally dried overnight at 80 ℃ in an oven. The preparation of pure MoS₂ was similar to that described above, but without the addition of nitrogen-doped porous carbon. The MoS₂ nanosheets and MoS₂@C were further calcined at 800 ℃ in an N₂ atmosphere for 2 h with a heating rate of 3 ℃ min^-1.

The crystalline structure of the MoS₂ nanosheets and MoS₂@C were characterized by X-ray powder diffraction (XRD Rigaku Dmax-γA XRD with CuK_α radiation, λ = 1.54178 Å) from 10° to 90°. SEM images were measured by field-emission scanning electron microscopy (FE-SEM, JSM-6700F from JEOL) and TEM images were measured by a transmission electron microscopy (TEM, JEOL 2100F).

The electrochemical measurements were carried out using CR2032-type coin cell at room temperature. The working electrodes were fabricated by mixing the active materials, carbon black (SuperP), and poly(vinyl difluoride) (PVDF) at aweight ratio of 70:20:10 and the mixture was mixed with n-methyl pyrrolidone (NMP) to form slurry and then pasted onto pure Cu foil. The electrode area was 1.54 cm². Pure lithium foil was used as the counter electrode and separated by a Celgard 2500 membrane separator. A solution of 1 mol L^-1 LiPF₆ in ethylene carbonate/dimethyl carbonate (1:1 by volume) was used as the electrolyte. The cells were assembled in a glove box filled with high purity argon gas and soaked overnight before test. The galvanostatic discharge–charge experiments were performed over a voltage range of 0.01–3.0 V (vs. Li⁺/Li) at different current density using a LAND CT2001A battery tester. Cyclic voltammetry (CV) curves were performed using the same workstation as EIS measurements at a scanning rate of 0.5 mV s-1. Electrochemical impedance spectroscopy (EIS) measurements were carried out on an Autolab PGSTAT302N electrochemical workstation by applying a sine wave with the amplitude of 10.0 mV over the frequency range from 100 kHz to 10 MHz.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 51272113, 51272115, 51672146), A Project of Shandong Province Higher Educational Science and Technology Program (Nos. J13LA10, J14LA15, J15LA12), Development Program in Science and Technology of Qingdao (No. 15-9-1-65-jch).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.10.007.