Self-polymerized hollow Mo-dopamine complex-induced functional MoSe2/N-doped carbon electrodes with enhanced lithium/sodium storage properties

Chaochao Zhao, He Song, Qianyu Zhuang, Quanning Ma, Jun Liang, Hongrui Peng, Changming Mao, Zhonghua Zhang* and Guicun Li*
College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China. E-mail: zhangzh@qust.edu.cn; guicunli@qust.edu.cn; Fax: +86-532-84022900; Tel: +86-532-84022900

Received 3rd February 2018 , Accepted 9th March 2018

First published on 12th March 2018


Self-polymerized hollow Mo-dopamine (PDA-Mo) spherical composites are appealing precursors for constructing robust N-doped carbon-encased Mo-containing electrode materials. Here, a method involving facile chemical precipitation combined with vapor selenization processes has been developed to prepare three-dimensional (3D) hierarchical MoSe2/N-doped carbon microsphere composites. The developed synthetic process eliminates the use of an expensive carbon matrix and toxic reagents, and the preliminary preparation of templates, and is deemed a facile, green, and cost-effective route to fabricate 3D hierarchical architectures. The as-synthesized hierarchical MoSe2/N-doped carbon microsphere composites are constructed from sheet-like MoSe2 layers that are encased by amorphous N-doped carbon. As expected, the as-synthesized 3D hierarchical MoSe2/N-doped carbon microsphere composites have excellent rate capacity (1500 and 600 mA h g−1 at 0.1 A g−1 and 10 A g−1, respectively) and long-life cycling stability (141.7 mA h g−1 remains even after 2000 cycles at 30 A g−1). Besides, the as-synthesized hierarchical MoSe2/N-doped carbon microsphere composites deliver a high capacity of about 570 mA h g−1 at 0.1 A g−1 when used as sodium-ion battery anode materials. Our work provides a successful approach for fabricating 3D hierarchical architectures applying simple synthetic methods, which could shed some light on future research pertaining to the construction of high-performance electrode materials.


1. Introduction

Two-dimensional (2D) MoS2 with a layered structure, characterized by relatively weak van der Waals forces and large spaces between two strongly bonded interlayers,1,2 has received considerable research interest for use in rechargeable lithium-ion and sodium-ion battery systems.3–7 Superseding sulfur atomic layers with larger sized selenium atoms would enable higher reversibility and the faster intercalation/extraction of lithium,8,9 sodium and other charge carriers, owing to weaker electrostatic interactions between the guest ions and the host selenium lattice, making 2D MoSe2 a promising candidate for high power electrode materials.10–12 However, MoSe2 still shows undesirable electrochemical performance, due to its intrinsically low electronic conductivity and issues with how easily it stacks, as well as polyselenide shuttle effects during charge–discharge processes.

Highly electronically conductive carbon materials integrated with various MoSe2 nanostructures have been demonstrated to be one of the most promising avenues for achieving better electrochemical guest ion storage properties.13 Tu and co-workers have reported nitrogen-doped carbon coated MoSe2 microspheres with good rate capability (471 mA h g−1 at 2000 mA g−1).14 Chen and co-workers have demonstrated that a sheet-like MoSe2/C composite delivers a reversible capacity of 576.7 mA h g−1 at 100 mA g−1 after 50 cycles.15 Yang and co-workers have successfully presented long-life and high-rate MoSe2-covered N and P-doped carbon nanosheet structures for sodium-ion batteries, which exhibit a specific capacity of 378 mA h g−1, even after 1000 cycles at 0.5 A g−1.16 Besides, various MoSe2/C composites have been fabricated and investigated as electrode materials for secondary battery technologies, including MoSe2 embedded CNT-reduced graphene oxide microspheres,17 flower-like MoSe2/C composites,18 1T-MoSe2 and carbon nanotube hybridized flexible film,10 MoSe2 nanosheets grown on carbon cloth,16 coaxial-cable MoSe2/hollow carbon nanofiber composites,19 fullerene-like MoSe2 nanoparticle-embedded CNT balls,20 and so on. Thus, combining highly reactive MoSe2 with highly electronically conductive carbon materials is a readily-available and effective solution to improve the electrochemical properties of MoSe2-based electrodes.21–23 However, MoSe2/C composites, particularly those with 3D hierarchical architectures assembled using primary nano-sized structures, often require tedious synthetic processes and expensive carbon matrices.24–27 Some also involve the preliminary preparation of the carbon matrix and its subsequent integration with MoSe2.4,10,16 Furthermore, the weak interactions between MoSe2 and the carbon matrix may lead to poor structural stability in MoSe2/C composites and the reaggregation of MoSe2, which inevitably deteriorates the electrochemical performance of the electrodes.28–30 Fabricating robust 3D hierarchical MoSe2/C composites via facile and effective processes is still a great challenge.

Mo-dopamine complexes represent a family of strongly coupled species based on Mo-containing oxyanions (such as MoO42−) and the catechol groups in dopamine. It has been experimentally demonstrated that Mo-dopamine complexes can serve as an appealing precursor for various carbon-containing Mo-based nanostructures.31–35 On one hand, Mo-dopamine complexes can function as surface coating-layer precursors for fabricating various hollow structures and/or composites. For example, Lou and co-workers have successfully fabricated ultrathin nanosheet-organized hierarchical b-Mo2C nanotubes from Mo-polydopamine coatings on MoO3 nanorod templates.36 Hierarchical C@MoS2@C hollow spheres have also been produced via a modified template method, where MnCO3 spheres and Mo-dopamine complexes serve as sacrificial templates and as Mo and C sources, respectively.37 On the other hand, Mo-dopamine complexes can self-assemble and polymerize to form solid or hollow Mo-polydopamine spherical structures, which can be used as appealing precursors for various Mo-containing nano-sized spherical structures. Wang and co-workers have successfully converted hierarchical Mo-polydopamine hollow spheres into hierarchical hollow MoO2/C and Mo2C/C composites via facile annealing processes.34 Li and co-workers have also developed a facile two-step preparation method for fabricating Mo2C nanoparticles uniformly supported on 3D carbon microflowers using self-polymerized Mo-dopamine spherical precursors.31 With respect to the atomic-scale distribution of the Mo-dopamine precursors, each microflower was mechanically robust enough to maintain its structural integrity. In addition, compared to template methods, facile chemical precipitation combined with annealing processes would be more acceptable and easily commercialized for large-scale production.31,34 Thus, applying self-polymerized hollow or solid Mo-dopamine spherical precursors to the preparation of 3D hierarchical MoSe2/C composites would be an appealing route for constructing robust electrode materials with well-distributed MoSe2.

Here, 3D hierarchical MoSe2/N-doped carbon microsphere composites have been successfully fabricated via facile chemical precipitation combined with annealing processes, using Mo-polydopamine hollow sphere precursors. The developed synthetic process eliminates the use of an expensive carbon matrix and toxic reagents, and the preliminary preparation of templates, and is deemed a facile, green, and cost-effective route to fabricate 3D hierarchical architectures. The as-synthesized hierarchical MoSe2/N-doped carbon microsphere composites are constructed using sheet-like MoSe2 layers that are encased by amorphous N-doped carbon. This multi-scale structured composite not only can shorten the diffusion pathways of charge carriers, but also is conducive to an increase in active sites, and the enhancement of electrode electronic conductivity, as well as electrolyte wettability and penetration.38,39 As expected, the as-synthesized 3D hierarchical MoSe2/N-doped carbon microsphere composites show excellent rate capacity (1500 and 600 mA h g−1 at 0.1 A g−1 and 10 A g−1, respectively) and long-life cycling stability (233.5 and 141.7 mA h g−1 remains even after 2000 cycles at 10 and 30 A g−1, respectively). Besides, the as-synthesized hierarchical MoSe2/N-doped carbon microsphere composites deliver a high capacity of about 570 mA h g−1 at 0.1 A g−1 when used as sodium-ion battery anode materials. Our work provides a successful method for fabricating 3D hierarchical architectures applying simple synthetic methods, which could shed some light on future research pertaining to the construction of high-performance electrode materials.

2. Experimental methods

2.1. Synthesis of the PDA-Mo precursor and MoSe2/C

All chemicals were of reagent grade and used without any further purification. In a typical synthesis, 0.75 g of ammonium molybdate tetrahydrate was dissolved in 210 mL of water under vigorous stirring until the solution became clear. Then 0.9 g of PDA (dopamine hydrochloride) was added into the above solution and the solution became brownish red immediately. After stirring for about 40 min at room temperature, a homogeneous solution was obtained. After that, 450 mL of ethanol was poured into the above solution and it was further reacted for about 20 min, and then 1.2 mL of 25–28 wt% ammonium hydroxide was added dropwise into the solution to adjust the PH to ∼8.5.40 Afterwards the system was reacted for about 4 h under stirring and the brown gray product PDA-Mo was collected using a centrifugal machine, washed with deionized water and alcohol three times, and dried at 60 °C for 24 h.
Synthesis of MoSe2/C. Typically, to obtain MoSe2/C, a combustion boat containing 0.3 g of PDA-Mo precursor was put into a tube furnace and Se powder, in another boat, was put upstream of the tube; the distance between the two boats was about 15 cm and the weight ratio of PDA-Mo/Se powder was about 0.5. The selenization process was performed at 300 °C at a heating rate of 1 °C min−1 for 3 h and then 750 °C for 1 h under an Ar atmosphere. For comparison, MoS2/C and MoS1.8Se0.2/C were prepared in a similar fashion to MoSe2/C. The sulfidation process was performed at 600 °C with a heating rate of 1 °C min−1 for 3 h and then 800 °C for 2 h under an Ar atmosphere.

2.2. Characterization

In order to determine the crystal phases, X-ray diffraction (XRD), using a Rigaku D-max-cA XRD with Cu Kα radiation (k = 1.54178 Å) from 3° to 90°, was used. The morphologies of the as-made products were characterized using field-emission scanning electron microscopy (FE-SEM, JSM 6700F) and transmission electron microscopy (TEM, JEOL 2100F). X-ray photoelectron spectroscopy (XPS) was implemented using a PerkinElmer PHI 550 spectrometer with Al Kα radiation (1486.6 eV) as the X-ray source. To record the specific surface areas of the samples, the Brunauer–Emmett–Teller (BET) method was adopted. Raman spectroscopy measurements were conducted on a laser confocal micro-Raman spectrometer (LabRAMHR800), with an excitation laser beam wavelength of 532 nm.

2.3. Electrochemical measurements

Coin cells of type CR-2032 were used to test electrochemical properties at ambient temperature. The working electrodes were prepared by blending the samples, poly(vinyl difluoride) (PVDF), and carbon black (Super-P) at a weight ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1 and pasting onto pure Cu foil. Pure lithium/sodium foil was used as the counter electrode and a Celgard 2500 membrane separator served as the separator. The electrolyte for the Li-ion batteries was constituted of a solution of 1 mol L−1 LiPF6 in EC (ethylene carbonate)/DC (dimethyl carbonate) (1[thin space (1/6-em)]:[thin space (1/6-em)]1 by volume) and 1 M NaClO4 in propylene carbonate (PC), with the 5 vol% addition of fluoroethylene carbonate (FEC) for the Na-ion batteries. To assemble the Li-ion and Na-ion cells, a glove box filled with high purity argon gas was used. Cyclic voltammetry (CV) curves were obtained using an Autolab PGSTAT302N electrochemical workstation at a scanning rate of 0.1 mV s−1, and electrochemical impedance spectroscopy (EIS) measurements were also performed using this machine, by applying a sine wave with an amplitude of 10.0 mV over a frequency range from 100 kHz to 10 mHz. Meanwhile, the galvanostatic discharge–charge experiments were conducted over a voltage range from 0.01 V to 3.0 V (vs. Li+/Li) at varied rates, using a LAND CT2001A battery tester.

3. Results and discussion

The PDA-Mo precursor was synthesized via a complexation reaction between ammonium molybdate tetrahydrate and dopamine-HCl. And the MoSe2/C was synthesized via the facile selenization of the PDA-Mo precursor with selenium as the Se source. Fig. 1a, presenting an SEM image of the PDA-Mo precursor, reveals that the precursor is in the form of hierarchical spheres with external diameters of about 800 nm and composed of nanoscaled lamellar PDA-Mo complexes. A TEM image (Fig. S5) of the precursor shows that the synthesized sample is a hollow spherical structure assembled from primary nano-sheets. Fig. 1b shows an SEM image of MoSe2/C, and there are no visible differences between the PDA-Mo precursor and MoSe2/C. This shows that the hierarchical structure is maintained after high temperature calcining. In order to further characterize the microstructures of the products, TEM measurements were employed. As shown in Fig. 1c, the nanoscale lamellar assembled structures are confirmed distinctly, and they are embedded in amorphous carbon derived from the carbonization of dopamine. A HRTEM image of MoSe2/C, shown in Fig. 1d, shows clear lattice fringes separated by ∼0.62 nm, corresponding to the d-spacing between the (002) planes of the MoSe2 phase, indicating 6 to 10 layers of stacked MoSe2.
image file: c8qi00101d-f1.tif
Fig. 1 SEM images of the PDA-Mo precursor (a) and MoSe2/C (b); a TEM image of MoSe2/C (c); and a HRTEM image of MoSe2/C (d).

The nitrogen adsorption–desorption isotherms of the three samples in Fig. 2a all show a typical hysteresis loop, which suggests the presence of mesoporous structures. BET calculations show that the MoSe2/C microspheres have a specific surface area of 162.2 m2 g−1, much higher than that of MoS1.8S0.2/C (111.1 m2 g−1) and nearly the same as MoS2/C (160.8 m2 g−1). The porous structure of MoSe2/C with a large surface area is favored in battery materials, since the structure can provide a large area contact area between the electrolyte and electrode. Besides, this structure can also offer lots of space for buffering strain during the discharge–charge process, enabling good rate capability and long-term cycling performance.41,42


image file: c8qi00101d-f2.tif
Fig. 2 N2 adsorption–desorption isotherms (a) and pore size distribution curves (b) for MoSe2/C, MoS1.8Se0.2/C and MoS2/C.

Fig. 3a shows typical XRD patterns of MoS1.8Se0.2/C, MoS2/C, MoSe2/C and the PDA-Mo precursor. The diffraction peaks of the samples can be well indexed to hexagonal MoS2 and MoSe2, and are in good agreement with the standard data (JCPDS card no. 37-1492; and JCPDS no. 29-0914).43,44 The (002) peaks are observed at a 2θ value of around 13.7° for MoSe2/C, which corresponds to the interlayer distance between MoSe2 layers. These results showed the formation of a MoSe2/C structure with good crystallinity after the selenylation process at 300 °C. X-ray photoelectron spectroscopy (XPS) is carried out to analyze the chemical states on the surfaces of the MoSe2/C microspheres (Fig. 3d–f). This shows that Mo, Se, and N elements all exist on the surface of MoSe2/C and the atomic ratio of Mo/Se in MoSe2/C is 0.45 (Se 3d % = 12.7%, Mo 3d % = 5.73%). Fig. 3d shows peaks at 232.2 eV, 229.0 eV, and 236.0 eV, which are assigned as the Mo 3d3/2 peak of MoSe2, Mo 3d5/2 peak of MoSe2 and Mo 3d3/2 peak of Mo–O, respectively.45 The Mo 3d3/2 peak at 232.2 eV and the Mo 3d5/2 peak at 229.0 eV confirm the presence of Mo4+ in MoSe2/C, and the weak peak at 235.8 eV from Mo6+ 3d3/2 can be ascribed to the existence of amorphous MoO3, which may derive from the oxidation of MoSe2/C in air.46 The N 1s spectrum is displayed in Fig. 3e. The N 1s peak at 400.9 eV can be assigned to pyrrolic N. The peak at 398.8 eV is probably related to pyridinic N and the peak at 394.9 eV corresponds to Mo–N bonds.43 The doping of nitrogen in the carbon can greatly improve the reactivity and wettability of the surface, together with tuning the electronic conductivity of the samples. Se 3d5/2 and Se 3d3/2 peaks located at 54.6 and 55.4 eV, respectively, are shown in Fig. 3f, indicating the Se2− chemical state of MoSe2 in the MoSe2/C microspheres.44 The Raman spectra of the as-prepared samples are shown in Fig. 3b and c. There are two broad overlapping bands at around 1350 cm−1 (a disorder-induced D-band) and 1560 cm−1 (a graphitic G-band), revealing the presence of disordered and amorphous carbon. The ID/IG intensity ratio is calculated to be 1.05, indicating the high graphitization degree of dopamine derived carbon. A broad 2D band at 2800 cm−1 is associated with a second-order zone boundary phonon mode for graphitized carbon, which shows the presence of a good conductivity carbon framework, derived from the carbonization of dopamine. It should be noted that a minor band in the lower frequency, at around 250 cm−1, corresponding to the J2 peak of 1T-MoSe2 is observed, which may suggest the presence of 1T-structures in the as-prepared MoSe2/C microspheres.47–51 EDS mapping of MoSe2/C (see ESI Fig. S2) shows the existence of Se, Mo and C elements, suggesting their uniform distribution.


image file: c8qi00101d-f3.tif
Fig. 3 XRD patterns of the PDA-Mo precursor, MoSe2/C, MoS2/C, and MoS1.8Se0.2/C, the JCPDF card of MoS2 No. 37-1492, and the JCPDF card of MoSe2 No. 29-0914 (a); Raman spectra of the samples (b, c); high-resolution Mo 3d XPS spectra of MoSe2/C (d); and the N 1s (e) and Se 3d (f) peaks of MoSe2/C.

Galvanostatic charge–discharge profiles at a current density of 0.1 A g−1 are shown in Fig. 4a. The plateaus in the profiles are consistent with the CV curves. The MoSe2/C microspheres exhibited initial discharge and charge capacities of 1567 and 1154 mA h g−1, respectively. The coulombic efficiency was 73.7% for the first charge–discharge process. The irreversible capacity loss (26.3%) can be mainly due to the formation of a solid-electrolyte interface (SEI) and the decomposition of the electrolyte.52,53 However, in subsequent circulations the corresponding coulombic efficiency was near 100%. Table 1 shows a comparison of the as-prepared MoSe2/C, MoS2/C and MoS1.8Se0.2/C (loading mass, specific capacity and cycling performance). The galvanostatic charge–discharge properties were also investigated at various current densities from 0.1 A g−1 to 10 A g−1 (Fig. S3), and discharge capacities of around 1567.3, 1181.5, 1168.7, 1122.5, 1055.2, 908.8, 716.2 and 597.3 mA h g−1 are achieved for the MoSe2/C microspheres at 0.1, 0.2, 0.5, 1, 2, 5, 8 and 10 A g−1, respectively. The MoS1.8Se0.2/C microspheres deliver capacities of 2193.4, 1492.4, 1285.7, 1117.4, 973.6, 791.2, 602.6 and 539.1 mA h g−1 and the MoS2/C microspheres have capacities of 1949.2, 1373.9, 1182, 1001.7, 837.9, 654.8, 522.6 and 475.2 mA h g−1, at the respective current densities. The rate performances are shown in Fig. 4b. The MoSe2/C microspheres deliver an excellent high-rate capacity of 597.3 mA h g−1 at 10 A g−1 and outstanding stability of 1306.3 mA h g−1 when the current density was put back to 0.1 A g−1 after cycling at various current densities. The experimental results show that the MoSe2/C microspheres we synthesized possess excellent high rate capacities, which is attributed to the highly unique hierarchical structure of MoSe2/C. The hierarchical structure provides abundant voids and additional active sites for lithium ion storage and transport channels for lithium ions. The SEM image shown in Fig. S6 demonstrates that the spherical structure of the MoSe2/C electrode is maintained even after several charge–discharge cycles. Fig. 4c gives the long-term cycling performance of MoSe2/C, MoS1.8Se0.2/C and MoS2/C microsphere electrodes at a current density of 1 A g−1. The capacity still holds at 1313.1, 1134.4 and 1020.6 mA h g−1 for MoSe2/C, MoS1.8Se0.2/C and MoS2/C, respectively, after 100 cycles. The results reveal that MoSe2/C microspheres constructed from MoSe2 nanosheets possess eminent stability and good cycling performance. In order to investigate the long-term cycling performance of the MoSe2/C microspheres at a high current density, the long-term cycling performance of MoSe2/C microspheres at 10 A g−1 and 30 A g−1 is shown in Fig. 4d; the capacities are 233.5 and 141.7 mA h g−1, respectively. The result shows the excellent high capacity and long cycle life of MoSe2/C microspheres when used as lithium ion battery anodes.


image file: c8qi00101d-f4.tif
Fig. 4 Galvanostatic charge–discharge profiles at a current density of 0.1 A g−1 (a); rate behaviors and coulombic efficiencies of MoSe2/C, MoS2/C and MoS1.8Se0.2/C (b); cycling performance and coulombic efficiency of MoSe2/C, MoS2/C and MoS1.8Se0.2/C at 1 A g−1 (c); long-term cycling performance of MoSe2/C microspheres at 10 A g−1 and 30 A g−1 (d); and GITT of MoSe2/C at a current density of 0.1 A g−1 (e, f).
Table 1 A comparison of as-prepared MoSe2/C, MoS2/C and MoS1.8Se0.2/C
Sample Loading mass (mg cm−2) Initial specific capacity (mA h g−1) Cycling performance (mA h g−1)
MoSe2/C 0.404 1567.3 0.5 A g−1: 1168.7
1 A g−1: 1122.5/1313.1 (100 cycles)
5 A g−1: 908.8
10 A g−1: 597.3
MoS2/C 0.404 1949.2 0.5 A g−1: 1182
1 A g−1: 1001.7/1020.6 (100 cycles)
5 A g−1: 654.8
10 A g−1: 475.2
MoS1.8Se0.2/C 0.404 2193.4 0.5 A g−1: 1285.7
1 A g−1: 1117.4/1134.1 (100 cycles)
5 A g−1: 791.2
10 A g−1: 539.1


Galvanostatic intermittent titration techniques (GITT), as shown in Fig. 4e and f, were adopted to investigate the electrochemical behavior of MoSe2/C microspheres. Fig. 4e shows the differences, estimated from the GITT curve, between the cut-off voltage and the open circuit voltage, measured after relaxation, where 0 refers to a fully delithiated state and 100% refers to a fully lithiated state. The difference shown in Fig. 4f reflects the polarization for Li+ insertion and extraction in MoSe2/C microsphere electrodes. For the first discharge, the overpotential continuously decreases with Li+ insertion, indicating that the reactions are kinetically limited. In the following charge process, increasing overpotential is observed, which may be caused by the formation of SEI layers.

To better investigate the electrochemical properties of the MoSe2/C microspheres, we apply the samples as anode materials in Na-ion batteries (see Fig. 5). Fig. 5a and b shows the rate performances of the MoSe2/C microspheres as anode materials in sodium-ion batteries over the voltage range from 0.001–3 V vs. Na/Na +. The MoSe2/C microspheres delivered discharge capacities of 938.6, 447.8, 302.4, 236.8, 178.8, 111.2, 75.6 and 61.1 mA h g−1 at current densities of 0.1, 0.2, 0.5, 1, 2, 5, 8 and 10 A g−1, respectively, indicating that the MoSe2/C microspheres show super Na-ion storage capabilities and good rate properties. Fig. 5c shows the long cycling performance of the MoSe2/C microspheres as anode materials over the voltage range of 0.001–3 V vs. Na/Na+ at a current density of 1 A g−1; it shows that the capacity can be maintained at 138.6 mA h g−1, with the corresponding capacity retention, measured with respect to the initial cycle, being 66%. The results show that MoSe2/C possesses great potential for practical application as an anode material in Na-ion batteries. This glorious electrochemical performance can be attributed to the highly uniform hierarchical structures.


image file: c8qi00101d-f5.tif
Fig. 5 Rate behavior and coulombic efficiency of MoSe2/C (a); galvanostatic charge–discharge profiles at various current densities (b) and cycling performance at 1 A g−1 (c) of MoSe2/C in a Na-ion battery; and galvanostatic charge–discharge profiles at a current density of 0.1 A g−1 of MoSe2/C in a Na-ion battery (d).

4. Conclusions

In summary, we have successfully synthesized N-doped carbon-encased MoSe2 nanosheet microspheres via the combined chemical precipitation and selenization of a Mo-polydopamine spherical precursor. Electrochemical investigations indicate that the carbon-encased MoSe2 microspheres exhibit excellent cycling stabilities and rate capabilities as anode materials in lithium-ion and sodium-ion batteries. The reversible capacity only decreased from 1567.3 to 597.3 mA h g−1 when the charging–discharging current density increased from 0.1 to 10 A g−1 in a lithium-ion battery, and from 938.6 to 61.1 mA h g−1 in a sodium-ion battery. When the current density increases to 10 A g−1 and 30 A g−1 the corresponding capacity can also be maintained at 233.5 and 141.7 mA h g−1, respectively, even after 2000 cycles. The desirable electrochemical properties are ascribed to the multi-scale structured composite, which not only can shorten the diffusion pathways of charge carriers, but also is conducive to an increase in active sites, and enhancement in the electrode electronic conductivity, as well as the electrolyte wettability and penetration. This material, together with the developed synthetic protocols, could provide some highlights for the future development of high performance electrode materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51672146), the Natural Science Foundation of Shandong Province (ZR2016EMM06), a Project of the Shandong Province Higher Educational Science and Technology Program (No. J15LA12), the Development Program in Science and Technology of Qingdao (No. 15-9-1-65-jch) and the Taishan Scholars Program of Shandong Province (No. ts201511034).

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Footnote

Electronic supplementary information (ESI) available: SEM data for MoSe2/C, MoS1.8Se0.2/C and MoS2/C; elemental mapping of MoSe2/C; galvanostatic charge–discharge profiles of MoSe2/C, MoS1.8Se2/C and MoS2/C; EIS spectra of MoSe2/C, MoS1.8Se2/C and MoS2/C microspheres; XPS data for MoS1.8Se2/C, MoS2/C and MoSe2/C; TEM images of Mo-dopamine precursors; and SEM morphology data for MoSe2/C microspheres after cycling tests. See DOI: 10.1039/c8qi00101d

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