Polypyrrole-assisted synthesis of roselike MoS₂/nitrogen-containing carbon/graphene hybrids and their robust lithium storage performances

Abstract

Roselike MoS₂/nitrogen-containing carbon/graphene (MoS₂/NC/G) hybrids were successfully synthesized via a facile polypyrrole (PPy)-assisted hydrothermal approach in combination with high-temperature calcination. The obtained MoS₂/NC/G hybrids manifest roselike MoS₂ composed of nanosheets coupled uniformly on NC/G nanosheets due to the strong interactions between MoS₂ and the abundant nitrogen-containing functional groups of NC. When used as anode materials for lithium ion batteries, the MoS₂/NC/G hybrids exhibit enhanced electrochemical energy storage performances compared to the bare MoS₂ nanosheets, including high specific capacity (1570.6 mA h g⁻¹ at 0.1 A g⁻¹), excellent rate capability (704.8 mA h g⁻¹ at 5 A g⁻¹) and good cycling stability (96.4% capacity retention after 100 cycles at 0.2 A g⁻¹). The enhanced lithium storage properties of the MoS₂/NC/G hybrids can be ascribed to the boosted electronic conductivity arising from the novel hybrid nanostructures of MoS₂/NC/G.

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Introduction

Rechargeable lithium-ion batteries (LIBs) have been regarded as one of the most promising power technologies in the applications of energy storage stations, electric vehicles, and so on.^1,2 High energy and power density, excellent security, and long cycling performance are key features for the development of LIBs.³ Nowadays, commercially used carbon-based anode materials are limited by their intrinsically low theoretical capacity value of 372 mA h g⁻¹.⁴ Molybdenum disulfide (MoS₂), a layered transition metal dichalcogenide, in which S–Mo–S layers are held together by weak van der Waals forces, is regarded as a promising anode material in LIBs because of its high theoretical specific capacity (ca. 670 mA h g⁻¹).⁵ However, the rate capability and cycling stability of MoS₂ become poor in subsequent cycles owing to the low conductivity and poor cyclability of Li₂S.⁶ Moreover, the pulverization and aggregation of MoS₂ during the discharge–charge cycle resulting from a significant volume change can lead to fast capacity fading.⁷

A variety of methods have been employed to address the above issues. Previous reports mostly focus on the size and morphology control of MoS₂,⁸ or construction of composite materials of MoS₂ and conductive carbonaceous materials, such as carbon nanotubes,⁹ carbon nanofibers,⁶ or graphene.^7,10–12 For example, Wang et al.⁸ have reported a simple method for the synthesis of 3D tubular architectures constructed using single-layered MoS₂ via a mixed solution reaction, and exhibited greatly improved Li⁺ storage properties. Lou et al.⁹ have reported a simple glucose-assisted hydrothermal method to directly grow MoS₂ nanosheets on the CNT backbone, which have shown greatly enhanced lithium storage properties compared with the pure MoS₂. Graphene, a single layer of sp² carbon atoms, has attracted tremendous interest owning to its novel geometrical structure and excellent electrical conductivity.¹³ Meanwhile, it is an ideal substrate for the growth and anchoring of nanomaterials, such as metal, metal oxide, and metal sulfide nanoparticles, which can function as catalysts for the hydrogen evolution reaction,¹⁴ lithium storage,¹⁵ capacitance,¹⁶ Raman enhancement,¹⁷ and so on. MoS₂/graphene nanocomposites have already been successfully applied as anode materials for LIBs.^7,10–12 Nevertheless, owing to the general incompatibility between graphene and inorganic nanoparticles, the growth of inorganic nanoparticles with uniform morphologies and controllable sizes on a graphene substrate is technically difficult.

Recently, nitrogen-containing carbon (NC) has received considerable interest because nitrogen-containing functional groups can not only provide more active sites for lithium storage, but also be used for the conjugation of metallic ions to help the growth of MoS₂ on the surface of graphene.^18–20 Polypyrrole (PPy) is one of the most promising conducting polymers, which facilitates the incorporation of nitrogen-containing functional groups into a carbon matrix. In addition, the pyrrole monomers can be easily polymerized on both surfaces of graphene oxide (GO) due to the π–π stacking between the PPy and GO layers,^21,22 which inspires us to find a new route to fabricate hybrid nanostructures in which MoS₂ could grow uniformly on the surfaces of PPy/GO. Herein, we report an efficient strategy for the synthesis of MoS₂/NC/G hybrids using a facile PPy-assisted hydrothermal approach in combination with high-temperature calcination, which exhibit excellent lithium storage performances including high specific capacity, excellent rate capability and good cycling stability.

Experimental

Synthesis of GO

Graphene oxide (GO) was synthesized from natural graphite flakes using a modified Hummers’ method.²³

Synthesis of the PPy/GO composite

Pyrrole (4 mL) and GO (200 mg) at the weight ratio of 10 : 1 were dispersed in 900 mL of 0.1 mol L⁻¹ HCl aqueous solution using ultrasonication for 20 min. Then 100 mL of HCl (0.1 mol L⁻¹) aqueous solution containing ammonium peroxydisulfate (13.6 g) was rapidly added to the above mixed suspension at 0–5 °C. The polymerization reaction was carried out for 12 h without any disturbance. Then the black precipitate was filtered off, washed with ammonium hydroxide, deionized water, and ethanol several times and then dried at 80 °C for 24 h.

Synthesis of the MoS₂/NC/G hybrids

The obtained PPy/GO composites (ca. 120 mg) were redispered into 60 mL of H₂O with the assistance of ultrasonication for about 30 min. Then, 0.8 g sodium molybdate (Na₂MoO₄·2H₂O) and 1.0 g thiourea were added to the PPy/GO suspension using ultrasonication for another 30 min. The mixture was transferred to a Teflon-lined autoclave and kept at 200 °C for 20 h. After cooling down to room temperature, the black precipitate was collected using centrifugation, washed with ethanol and water several times, and dried at 80 °C for 24 h. Afterwards, the MoS₂/PPy/GO products were further annealed at 800 °C for 2 h under an atmosphere of 5% H₂, balanced by Ar for 2 h with a heating rate of 3 °C min⁻¹. The preparation of the bare MoS₂ nanosheets is similar to that for the MoS₂/NC/G hybrids except for the addition of PPy/GO.

Characterization

The crystalline structures of the samples were characterized using X-ray powder diffraction (XRD Rigaku D-max-γA XRD with Cu Kα radiation, λ = 1.54178 Å) from 5° to 90°. SEM images were collected using field-emission scanning electron microscopy (FE-SEM, JSM-6700F from JEOL) and TEM images were collected using transmission electron microscopy (TEM, FEI Tecnai G20). Thermogravimetric (TG) analyses were performed on a TG instrument (NETZSCH STA 449C) using a heating rate of 10 °C min⁻¹ in air from 30 °C to 700 °C. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Perkin-Elmer PHI 550 spectrometer with Al Kα (1486.6 eV) as the X-ray source. The XPS spectra were taken after all binding energies were referenced to the C 1s neutral carbon peak at 284.8 eV, and the elemental compositions were determined from the peak area ratios after correcting for the sensitivity factor for each element.

Electrochemical test

The electrochemical measurements were carried out using a CR2032-type coin cell at room temperature. The working electrodes were fabricated by mixing the active materials, carbon black (Super-P), and poly(vinyl difluoride) (PVDF) at a weight ratio of 70 : 20 : 10 and the mixture was mixed with N-methyl pyrrolidone (NMP) to form a slurry before pasting onto pure Cu foil. The electrode area was 1.54 cm². Pure lithium foil was used as the counter electrode and separated using 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 testing. The galvanostatic discharge–charge experiments were performed over the voltage range of 0.01–3.0 V (vs. Li⁺/Li) at different current densities using a LAND CT2001A battery tester. Electrochemical impedance spectroscopy (EIS) measurements were carried out using 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. Cyclic voltammetry (CV) curves were performed using the same workstation as for the EIS measurements at a scanning rate of 0.1 mV s⁻¹.

Results and discussion

The synthetic procedure of the MoS₂/PPy/GO hybrids is illustrated in Fig. 1. To improve the compatibility between GO (Fig. S1a†) and MoS₂, PPy was first coated uniformly on both surfaces of GO via in situ polymerization of pyrrole. As shown in Fig. 1b, the thickness and lateral dimensions of the PPy/GO composite nanosheets (Fig. S1b†) are 20–25 nm and 1–2 μm, respectively. After hydrothermal growth of MoS₂, it is found that roselike MoS₂ composed of nanosheets was coupled on the PPy/GO composite nanosheets due to the strong interaction between MoS₂ and PPy (Fig. 1c). The MoS₂/PPy/GO hybrids can be converted into MoS₂/NC/G hybrids using high temperature calcination.

Fig. 1 The overall synthetic procedure of the MoS₂/PPy/GO hybrids. The SEM images correspond to GO (a), PPy/GO (b), and MoS₂/PPy/G (c).

Fig. 2a and b show typical SEM images of the MoS₂/NC/G hybrids. The low-magnification SEM image (Fig. 2a) reveals that the product has a two-dimensional structure and the surface is covered with roselike MoS₂ subunits. As shown in the high-magnification SEM image (Fig. 2b), it is clear that the PPy/GO composite nanosheets are converted into NC/G composite nanosheets, as indicated by the arrow, due to the carbonization and thermal reduction processes. Moreover, the roselike MoS₂ subunits composed of thin nanosheets are coupled strongly on the surfaces of the NC/G composite nanosheets, which is similar to that of the MoS₂/PPy/GO hybrids, indicating that the MoS₂/NC/G hybrids possess high thermal stability. Compared with the MoS₂/NC/G hybrids, bare MoS₂ tends to form microspheres composed of nanosheets (Fig. S2a and b†). The roselike MoS₂ subunits closely attached on the surfaces of the NC/G nanosheets can be further confirmed via TEM observation (Fig. 2c). In addition, the NC/G nanosheets can also be found in the TEM image as described by the arrow. A typical HRTEM image of the MoS₂/NC/G hybrids is represented in Fig. 2d. The observed fringes correspond to the interplanar distances of 0.66 nm, which are in good agreement with the lattice spacing of the (002) planes of the bare MoS₂ nanosheets (Fig. S2c†).²⁴ Meanwhile, the fringes with an interplanar spacing of 2.09 Å corresponding to the (200) facet of the Mo₂N based on a face-centered cubic crystal structure can be clearly observed in the HRTEM image (Fig. S2d†).²⁵ Intriguing transition metal nitrides have superior properties in some respects such as high conductivity,²⁶ which are desired for LIBs.

Fig. 2 SEM images of the MoS₂/NC/G hybrids (a and b), TEM image of a single MoS₂/NC/G hybrid (c), and high resolution TEM image of the MoS₂ nanosheets (d).

The crystallographic structure and phase purity of the annealed MoS₂/NC/G hybrids are measured using XRD analysis (Fig. S3†). The XRD pattern of the products in the first step can be well indexed to hexagonal MoS₂, in good agreement with the standard data (JCPDS card no. 37-1492; space group P63/mmc; a = 3.161 Ǻ, c = 12.299 Ǻ). Besides the XRD peaks of the hexagonal MoS₂, the XRD patterns of the products in the second step show another peak, which can be well matched with the strong peaks of the (111) and (200) planes of cubic Mo₂N (JCPDS card no. 25-1366), which may be caused by slow nitridation under the condition of PPy pyrolysis.

XPS measurements were applied to analyze the chemical states of the elements in the MoS₂/NC/G hybrids and the bare MoS₂ nanosheets. It can be clearly seen that a peak corresponding to N 1s appears at around 399 eV, suggesting the presence of N in the hybrids (Fig. S4†). There is no peak related to N and C species detected from the bare MoS₂ nanosheets. The surface elemental compositions from XPS analysis show that 3.7 at% of N is contained in MoS₂/NC/G hybrids. The N 1s XPS spectrum of the MoS₂/NC/G hybrids is displayed in Fig. 3a, and fitted using the three components of the binding energies of about 401.0, 398.3, and 397.7 eV. The peaks at 398.3 and 401.0 eV are assigned to the pyridinic N and quaternary N, respectively.²⁷ The peak at around 397.7 eV can be assigned to N 1s which is characteristic for a metal nitride material²⁸ and is in agreement with the XRD and HRTEM results. The presence of nitrogen at the carbon surface enhances the reactivity and electric conductivity according to previous reports.^29,30 Peaks located at 229.6 and 232.6 eV can be attributed to Mo 3d_5/2 and 3d_3/2, respectively. The adsorption–desorption isotherm of the MoS₂/NC/G hybrids exhibits a type IV adsorption branch with an H3 hysteresis loop, which is characteristic of the mesoporous structure (Fig. 4a). The corresponding Brunauer–Emmett–Teller (BET) surface area is calculated to be 30.43 m² g⁻¹, which is much higher than that of the bare MoS₂ nanosheets at about 2.35 m² g⁻¹. The pore size distributions of the MoS₂/NC/G hybrids are shown in Fig. 4b. The pore size distributions tend to concentrate on 0.5–1.5 nm and 2.5–10 nm, revealing the presence of meso- and micro-pores in the NC/G nanosheets. However, the bare MoS₂ nanosheets exhibit much fewer pores than the MoS₂/NC/G hybrids. The porous structures of the MoS₂/NC/G hybrids would offer a more favorable route for lithium ion diffusion, which will benefit their electrochemical lithium storage performances. TG analysis was employed to determine the content of MoS₂ present in the MoS₂/NC/G hybrids. The MoS₂/NC/G hybrids underwent significant weight loss mainly below 500 °C (Fig. S5b†), which can perhaps be attributed to the combustion of NC and graphene, and oxidation of MoS₂ to MoO₃.⁹ We confirmed that the remaining product after the TG measurement is MoO₃ (shown in the XRD pattern in Fig. S6†), which has a weight fraction of about 79%, estimating that the MoS₂ content in the MoS₂/NC/G hybrids is approximately 87%.

Fig. 3 Typical high-resolution XPS spectra of N 1s and Mo 2p (a), and Mo 3d and S 2s (b) for the MoS₂/NC/G hybrids.

Fig. 4 N₂ adsorption–desorption isotherms (a) and the pore size distribution (b) of the MoS₂/NC/G hybrids and the bare MoS₂ nanosheets.

CV measurements were performed to investigate the phase transformations and ionic diffusion processes of the MoS₂/NC/G hybrids and the bare MoS₂ nanosheets during the electrode reaction processes. In the first cathodic scan (Fig. 5a), the 0.96 V peak is assigned to the Li⁺ intercalation into the MoS₂ layers to form Li_xMoS₂, and the 0.36 V peak is attributed to the complete reduction of MoS₂ to Mo and Li₂S. These peaks disappear completely in the subsequent processes. Instead, new peaks at 1.90, 1.06, and 0.25 V emerge, which perhaps are attributed to a multi-step Li⁺ insertion mechanism. In the anodic scans, the peaks at around 2.42 V can be associated with the conversions of Li₂S to S.³¹ It is crucial that the oxidation peak of the MoS₂/NC/G hybrid electrode at 2.42 V does not change in subsequent anodic sweeps (corresponding to the reversible decomposition of Li₂S), indicating the reversibility of the lithiation/delithiation process and the lowest internal diffusion resistance of the MoS₂/NC/G hybrids. As for the bare MoS₂ nanosheets, the CV profile (Fig. 5b) in the first cycle is similar to that of the MoS₂/NC/G hybrids. However, higher oxidation peak potentials and lower corresponding reduction peak potentials compared to the MoS₂/NC/G hybrids are observed, indicating the large polarization of the bare MoS₂ nanosheets during the charge and discharge processes. The polarization is associated with the transfer delay of the electrons on the active material/electrolyte interface.^32,33 In contrast, the MoS₂/NC/G hybrid electrode facilitates the electron and ion transfer during the electrochemical reaction processes.

Fig. 5 The CV curves of the MoS₂/NC/G hybrids (a), and bare MoS₂ nanosheets (b) measured in the voltage range of 0.01–3.0 V with a scan rate of 0.1 mV s⁻¹.

The lithium storage performances of the MoS₂/NC/G hybrids and the bare MoS₂ nanosheets were further evaluated via galvanostatic charge–discharge cycling. As shown in Fig. 6a, the plateaus on the charge/discharge curves of the MoS₂/NC/G hybrids and the bare MoS₂ nanosheets are consistent with the conversion peaks on their CV curves (Fig. 5a and b). The initial discharge and charge capacities of the MoS₂/NC/G hybrids were found to be 1570.6 and 1143.7 mA h g⁻¹, respectively. Such a high initial lithium storage capacity might be associated with the hybrid nanostructure of MoS₂/NC/G. The irreversible capacity loss of approximately 27.2% can be mainly attributed to the formation of the solid–electrolyte interface (SEI). The rate performances of the MoS₂/NC/G hybrids and the bare MoS₂ nanosheets were examined at various current densities in the range of 0.1–5 A g⁻¹ (Fig. 6c). Reversible capacities of around 1570.6, 1182.9, 1153.7, 1056.4, 947.0, and 704.8 mA h g⁻¹ are achieved for the MoS₂/NC/G hybrids at 0.1, 0.2, 0.5, 1, 2, and 5 A g⁻¹, respectively, while the bare MoS₂ nanosheets deliver inferior capacities of 1226.5, 721.3, 548.3, 443.9, 375.5, and 248.6 mA h g⁻¹ at 0.1, 0.2, 0.5, 1, 2, and 5 A g⁻¹, respectively. When the current rate reverses back to 0.1 A g⁻¹, the discharge capacities can recover to the original value immediately, indicating that the MoS₂/NC/G hybrids possess good cycling stability due to the high structure stability.

Fig. 6 Galvanostatic discharge–charge profiles of the bare MoS₂ nanosheet electrode (a) and the MoS₂/NC/G hybrid electrode (b) at various rates. A comparison of the rate performance of the MoS₂/NC/G hybrid electrode and the bare MoS₂ nanosheet electrode (c). EIS spectra of the of the MoS₂/NC/G hybrid and the bare MoS₂ nanosheet cells after the CV test (d). Equivalent circuits for the MoS₂/NC/G hybrid electrode (the inset in d).

The kinetic differences of the MoS₂/NC/G hybrid and MoS₂ nanosheet electrodes were investigated using EIS after the CV test. As shown in Fig. 6d, the high-frequency semicircle corresponds to the solid SEI film resistance (R_f) and the constant phase element (CPE1), the semicircle in the medium-frequency region is assigned to the charge-transfer impedance (R_ct) and the constant phase element of the electrode–electrolyte interface (CPE2), and W is associated with the Warburg impedance. Obviously, the SEI film resistance (R_f) and charge-transfer resistance (R_ct) of the MoS₂/NC/G hybrids based on the modified equivalent circuit (the inset in Fig. 6d) are fitted to be 52.3 and 56.8 Ω, respectively, which are significantly lower than those of the bare MoS₂ nanosheets (at 119 and 614 Ω, respectively). The results confirm that the NC/G nanosheets have distinctively boosted the electronic conductivity of the MoS₂/NC/G hybrids and facilitate the efficient and fast transfer of electrons and Li⁺ ions, leading to the high rate capability of the MoS₂/NC/G hybrids.

Fig. 7 gives the long-term cycling performance of the MoS₂/NC/G hybrid and the bare MoS₂ nanosheet electrodes at a current rate of 0.2 A g⁻¹. During the first 40 cycles, the discharge capacity for the MoS₂/NC/G hybrid stabilizes at around 1021 mA h g⁻¹, and then gradually increases to 1093 mA h g⁻¹ at the 100th cycle (96.4% of the second cycle capacity). The increasing specific capacity with cycling is common for various nanostructured metal oxide/sulfide electrodes and could be attributed to the growth of the gel-like polymeric layer and possibly the electrochemical activation of the hybrid composites.^34,35 In contrast, the bare MoS₂ nanosheets deliver fast capacity fading, and a capacity of only around 285 mA h g⁻¹ was retained after 100 cycles (42.3% of the second cycle capacity). These results reveal the excellent electrochemical stability and cycling performance of the MoS₂/NC/G hybrids.

Fig. 7 The cycling performances of the MoS₂/NC/G hybrid and MoS₂ nanosheet electrodes at 0.2 A g⁻¹.

Conclusions

In conclusion, we have developed a facile PPy-assisted approach to successfully synthesize MoS₂/NC/G hybrids in which roselike MoS₂ subunits composed of nanosheets are closely attached on the surfaces of the NC/G nanosheets. The presence of PPy can not only couple MoS₂ with graphene oxide, but also be converted into NC to increase the electronic conductivity of the hybrids. As a result, the MoS₂/NC/G hybrids exhibit improved lithium ion storage performances including higher specific capacity, superior rate performance, and good cycling stability in comparison with the bare MoS₂ nanosheets. This facile strategy can be extended to fabricate other hybrid electrode materials to couple two incompatible components which may serve as ideal candidates in catalysts, sensors, supercapacitors, as well as LIBs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51272113, no. 51272115), A Project of Shandong Province Higher Educational Science and Technology Program (J13LA10), A Project of Shandong Province Higher Educational Science and Technology Program (J14LA15), A Project of Shandong Province Higher Educational Science and Technology Program (J15LA12) and Natural Science Foundation of Shandong Province (no. ZR2012EMM001).

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Footnote

† Electronic supplementary information (ESI) available: SEM and TEM images, XRD patterns, XPS survey spectra, TG curve. See DOI: 10.1039/c5ra09092j

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