N-doped carbon/MoS₂ composites as an excellent battery anode

Abstract

In this work, we have synthesized N-doped carbon/MoS₂ (C/MoS₂) composites by a simple hydrothermal method. It was found that the C/MoS₂ composite manifested a high specific capacity of 611 mA h g⁻¹ and excellent cycling performance compared to bare MoS₂ and N-doped carbon.

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Lithium-ion batteries (LIBs) have become one of the main power sources for portable electronic devices and electric vehicles (EVs) due to their no memory effect, high energy densities, and good cycling stability.^1–3 At present, graphite is one of the most common anode materials, however, it has a low theoretical specific capacity (372 mA h g⁻¹).⁴ The further development of LIBs has been limited to the capacity of anode materials in a large scale. Thus, scientists have turned to study the transition metal materials as potential anodes, such as metals, metal oxides, and metal sulfides.^5–12 In this case, layered molybdenum disulfide (MoS₂) has become one of the most promising alternatives. The reaction type of Li⁺ inserted into molybdenum disulfide layered is: MoS₂ + 4Li⁺ + 4e⁻ = Mo + 2Li₂S.^13,14 So it has been regarded as a potential anode material in LIBs.¹⁵ However, the reversibility of MoS₂ was found to be poor.¹⁶ To date, some synthetic methods have been proposed to improve the cycling performance of MoS₂, such as integrating MoS₂ with different carbonaceous materials, like amorphous carbon,^17,18 graphene^19–22 and carbon nanotubes (CNTs)^23–25 and so on. N-Doped carbon derived from zeolitic imidazolate framework-8 (ZIF-8) exhibits porous structure, and has high specific surface area.²⁶

In the communication, we have successfully synthesized the C/MoS₂ composites by a simple hydrothermal method using N-doped carbon polyhedron as the nucleus of MoS₂. The as-synthesized samples were characterized by a series of technologies, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis and X-ray photoelectron spectroscopy (XPS). The formation mechanism of C/MoS₂ composites was also proposed. When tested as anode material, the as-synthesized C/MoS₂ composites manifested high specific capacity and excellent cycling performance than bare MoS₂ and N-doped carbon.

XRD characterization was performed to confirm structures of the as-synthesized three samples. As shown in Fig. 1a, all the diffraction peaks of MoS₂ and C/MoS₂ could be indexed to the rhombohedral structure of MoS₂. The thermal gravimetric analysis (TGA in Fig. 1b) was used to determine the amount of MoS₂ and N-doped carbon in the C/MoS₂ composite. In Fig. 1b, it can be seen that the weight loss totally was measured about 40%. Among these, nearly 10% of the mass loss occurred at about 100 °C, which is due to the existence of the adsorbed water in the sample. So the weight loss of the composite material due to react at is about 30%. To the reaction phase, the first weight loss at about 360 °C was attributed to the oxidation of MoS₂ to MoO₃. The second time weight loss occurs at about 420 °C caused by the combustion of the N-doped carbon. Through the fact that molybdenum content remains constant, the content of N-doped carbon was calculated to be about 25% in the C/MoS₂ composite.

Fig. 1 (a) XRD patterns of C, MoS₂ and C/MoS₂; (b) TGA curve of C/MoS₂.

Fig. 2 shows the morphology of the as-synthesized of C, MoS₂ and C/MoS₂, respectively. In SEM image (Fig. 2a and b) and TEM image (Fig. 2c), we could find out that the C sample is composed of regular N-doped carbon polyhedron, which were prepared via removing ZnO by acid treatment, followed by annealing ZIF-8. The C sample has a size of about 50 nm. As shown in SEM image (Fig. 2d and e) and TEM image (Fig. 2f), the MoS₂ sample is composed of nanoflowers assembled by nanosheets with a thickness of less than 10 nm. Fig. 2g–i present SEM and TEM images of C/MoS₂ sample, which has a similar morphology with MoS₂ nanoflowers. In addition, some N-doped carbon polyhedron can observed, as shown in Fig. 2i. Here, the formation of MoS₂ nanoflowers can be mainly attributed to its crystal growth habit.¹⁰

Fig. 2 (a–c) SEM and TEM images of C; (d–f) SEM and TEM images of MoS₂; (g–i) SEM and TEM images of C/MoS₂.

The elemental compositions of C/MoS₂ and C samples were analysed by (XPS). Fig. 3 presents the survey spectrum of C/MoS₂ and C samples, respectively. The content of N element is 5.84% and 24.23% for C/MoS₂ and C samples, respectively. Fig. S1a and b† present the S_2p and Mo_3d spectrum of C/MoS₂, respectively. The S_2p spectrum in Fig. S1a† was deconvoluted to two individual peaks, while the Mo_3d spectrum in Fig. S1b† was deconvoluted to three individual peaks. Fig. S1c and d† present the N_1s XPS spectrum of C/MoS₂ and C samples, respectively. In this work, nitrogen types can be divided into pyridinic N (398.7 eV), pyrrolic N (400.3 eV), graphitic N (401.2 eV), and oxidized N (402.8 eV), respectively.²⁷ The content of pyridinic N, pyrrolic N, graphitic N and oxidized N in N-doped C polyhedron is 10.8%, 6.7%, 5.9%, 0.9%, respectively, while the content of pyridinic N, pyrrolic N, graphitic N and oxidized N in C/MoS₂ is 0.9%, 0.6%, 0.6%, 0.1%, respectively. In addition, Fig. S2† shows the N₂ adsorption–desorption isotherm and corresponding pore size distributions of three samples. It also indicates that the specific surface area of C, MoS₂ and C/MoS₂ samples is 610, 13.6 and 23 m² g⁻¹, respectively. The data shows that the specific surface area of the composite is bigger than pure MoS₂. And this should be caused by the combination of C materials.

Fig. 3 The XPS survey spectrum of C/MoS₂ and C samples.

Fig. 4a–c shows the charge/discharge voltage profiles of the C, MoS₂ and C/MoS₂ electrodes at a current density of 50 mA g⁻¹. As shown in Fig. 4a–c, there is the large irreversible capacity loss, which is mainly because of the formation of the SEI film in the initial cycle. To investigate the electrochemical performance of the C, MoS₂ and C/MoS₂ samples as the LIB anodes, the cyclic voltammetry (CV) measurements were carried out in the potential window of 0.01–3.0 V at a scan rate of 0.2 mV s⁻¹. Fig. 4d–f shows the CV curves for the first 3 cycles of C, MoS₂ and C/MoS₂ samples at a scan rate of 0.2 mV s⁻¹. In the first cathodic process in Fig. 4f, two reduction peaks at about 1.9 V and 1.5 V are showing Li intercalation into MoS₂ and conversion into Mo and Li₂S by the following two reactions:

MoS₂ + xLi⁺ + xe⁻ = Li_xMoS₂ (1)

Li_xMoS₂ + 4Li⁺ + 4e⁻ = 2Li₂S + Mo/Li_x (2)

Fig. 4 (a–c) The charge–discharge curves of C/MoS₂, MoS₂ and C samples; (d–f) the cyclic voltammograms (CVs) for the first 3 cycles of C, MoS₂ and C/MoS₂ samples at a scan rate of 0.2 mV s⁻¹.

While the third reduction peak at about 0.55 V could be due to the decomposition of the irreversible electrolyte. In the anodic scanning, two oxidation peaks appear at approximately 2.25 V and 1.78 V, represents the oxidation reaction from Mo to Mo⁴⁺ and Mo⁶⁺, Li₂S to sulfur. In the later cycles, the reduction peaks at 1.82 V and 1.33 V are due to the reductions/oxidations between Mo and Mo⁶⁺, and the oxidation peaks at 2.3 V and 1.9 V is the sulfide redox reaction.

Fig. 5a shows the cycling performance of the C, MoS₂ and C/MoS₂ electrodes. Among the three electrodes, the C/MoS₂ electrode shows the highest storage capacity in comparison with the other electrode materials. The discharge/charge capacities in the 1st cycle are 1284/783 mA h g⁻¹ for the C/MoS₂, while 1712/740 mA h g⁻¹ for the C sample and 1143/514 mA h g⁻¹ for the MoS₂ sample, respectively. After 50 cycles, the discharge/charge capacities of the C/MoS₂ electrode still remain at 611/605 mA h g⁻¹, respectively, indicating better electrochemical performance than the C and MoS₂ samples. In Fig. S3,† the coulombic efficiencies of C, MoS₂ and C/MoS₂ electrodes are above 99%, except for the first cycle. Fig. 5b shows the discharge capabilities of the C, MoS₂ and C/MoS₂ electrodes at different current densities between 50 mA g⁻¹ and 1000 mA g⁻¹, respectively. The C/MoS₂ electrode clearly shows a higher specific capacity than the other electrodes after 10 cycles. The discharge capacity drops gradually as the current density increases for the three samples. For the C/MoS₂ electrode, the discharge specific capacities are 708, 588, 458, 302, 201 and 165 mA h g⁻¹ at 50, 100, 200, 500, 800 and 1000 mA g⁻¹, respectively. While the discharge capacity of the C and MoS₂ electrodes are only 886/546, 409/165, 237/70, 140/32, 46/15 and 33/15 mA h g⁻¹ at the same rate. The reversible capacity restores to 684 mA h g⁻¹ when the discharge rate returns back to 50 mA g⁻¹, which is higher than the C and MoS₂ electrodes. These results indicate that those of the C/MoS₂ electrode shows better cycling performance and rate performance than the C and MoS₂ electrodes. The enhanced electrochemical performance of the C/MoS₂ electrode can be attributed to the hybrid structure, which can improve the electric conductivity of electrode and keep high capacity. In Fig. S4† shows the Nyquist plots of the C, MoS₂ and C/MoS₂ electrodes at open potential before cycling test. In similar with the impedance spectra of other MoS₂-based electrodes, the impedance spectroscopy is composed of low frequency semicircle and high frequency of straight line. From the impedance curves of the materials, we know that the electrical conductivity of C/MoS₂ is better than pure MoS₂, which is used to explain the excellent performance of C/MoS₂.

Fig. 5 (a) Cycling performance of the three samples; (b) rate capability of three electrodes at various current densities between 0.01 and 3 V.

In summary, we have fabricated the C/MoS₂ composites by solvothermal method. The as-synthesized C/MoS₂ shows significantly improved cycling performance with a high capacity of 611 mA h g⁻¹ after 50 cycles at a current density of 50 mA g⁻¹, much better than the bare C and MoS₂ samples as anode materials for LIBs. This method might be extended to the synthesis of other composite materials to enhance the energy storage capacity and rate ability.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51302079).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra24674a

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