Ultrathin sandwich-like MoS₂@N-doped carbon nanosheets for anodes of lithium ion batteries

Abstract

In this work, we report on a simple and scalable process to synthesize the core–shell nanostructure of MoS₂@N-doped carbon nanosheets (MoS₂@C), in which polydopamine is coated on the MoS₂ surface and is then carbonized. An intensive investigation using transmission electron microscopy and Raman spectroscopy reveals that the as-synthesized MoS₂@C possesses a nanoscopic and ultrathin layer of MoS₂ sheets with a thin and conformal coating of carbon layers (∼3 nm). The MoS₂@C demonstrates a superior electrochemical performances as an anode material for lithium ion batteries compared to exfoliated MoS₂ and bulk MoS₂ samples. This unique core–shell structure is capable of delivering an excellent Li⁺ ion charging–discharging process as follows: a specific capacity as high as 1239 mA h g⁻¹, a high rate capability even at a high current rate of 10 A g⁻¹ while retaining 597 mA h g⁻¹, and a good cycle stability over 200 cycles at a high current rate of 2 A g⁻¹.

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Introduction

The development of novel electrodes with high capacity, good cyclability and low cost is of pivotal importance for lithium batteries in applications of further portable electronics, electric vehicles and stationary power backups.^1–3 The combination of high capacity, low cost, ease of fabrication and low discharged voltage of the layered metal sulfide (LMS) family (e.g. MoS₂, WS₂ and SnS₂) makes this family an attractive candidate for an anode in lithium ion batteries.^4–6 Moreover, recent investigations have demonstrated that these materials, when they are thinner and close to a monolayer, achieved much better electrochemical performance compared to the pristine bulk materials.^7–9 However, most LMS-based electrodes suffer from limited cyclability and poor rate capability, which are caused by the intrinsically poor electrical conductivity of the above mentioned LMSs as well as the generation of polysulfides Li₂S_x (2 < x < 8) during the Li storage process.¹⁰ There have already been extensive attempts to address these challenges including the construction of carbon–LMS composites and the expansion of the interlayer distance of metal sulfides using polymers.^11–16 Despite these studies, the electrochemical performance of LMSs is still unsatisfactory. Most of the synthesized or mixed LMSs with carbon supports (e.g. graphene, carbon nanotubes and mesoporous carbons) cause undesirable electronic pathways because of their partial carbon coating and non-exfoliating states of LMSs. Although the incorporation of polymers such as polyethylene oxide is an effective way to exfoliate LMS sheets, additional carbon is essential to increase the electrical conductivity of LMSs.¹⁰ Consequently, a fully carbon wrapped and exfoliated LMS with a core–shell structure is capable of delivering a breakthrough to improve electrical conductivity and prevent the dissolution of polysulfides.

Inspired by an adhesive protein in mussels, polydopamine (PD) has recently offered great opportunities for the surface modification of organic and inorganic materials in a variety of applications.^17,18 Recently, Choi et al. discovered the adhesive potential of PD in lithium ion batteries, including the surface modification of separator and adhesive binders for silicon.^19–21 These studies have resulted in the significant enhancement of cell performance. Moreover, Dai et al. reported that PD can serve as an N-doped carbon source with controllable, conformal and ultrathin coatings.²² Although several nanoparticles have been employed to fabricate PD carbon-coated materials,^23,24 N-doped carbon-coated MoS₂ using PD and its electrochemical behavior for lithium ion batteries have yet to be explored.

Taking full account of PD's versatile adhesion and carbon source, here we synthesized an ultrathin MoS₂@C nanostructure by self-polymerizing dopamine followed by a post carbonization process. The outer-shell of N-doped carbon served as rapid and efficient electron transfer pathways while the inner-core of MoS₂ sheets effectively performed the Li⁺ ion charging–discharging process. These unique properties of MoS₂@C as electrodes for lithium ion batteries resulted in better performance than the exfoliated MoS₂ electrode, showing a high level of specific capacitance, high rate capability and good cycling stability.

Experimental

Synthesis of MoS₂@C

Prior to preparing the MoS₂@C sample, lithium-intercalated MoS₂ (Li_xMoS₂) was first prepared following the process in ref. 25. MoS₂ flakes (0.1 g, Sigma-Aldrich) were purged by Ar gas for 2 hours and were then filled with n-butyllithium (1.6 M, 1 mL, Sigma-Aldrich) in a sealed tube. Following an inserting reaction time of 48 hours, the Li_xMoS₂ samples were washed several times with n-hexane and filtered to remove unreacted n-butyllithium. Finally, a solid powder was obtained by drying the sample overnight at room temperature (RT) under vacuum. In order to prepare the PD modified-MoS₂ (MoS₂@PD) sample, Li_xMoS₂ flakes were dispersed in Tris-buffer aqueous solution (75 mL, 10 mM; pH 8.5) and treated with a bath-sonicator for 20 min to exfoliate the MoS₂ sheets. Then, dopamine hydrochloride (150 mg, Sigma-Aldrich) was added and the mixture was stirred at 80 °C for 24 hours. The MoS₂@PD powder was washed using water several times and collected by filtration. The resultant sample was heated in a furnace tube to 400 °C for 2 hours at a heating rate of 1 °C min⁻¹ under N₂ gas, and then further heated up to 800 °C for 3 hours at a heating rate of 5 °C min⁻¹. The obtained sample was denoted as MoS₂@C.

Structural characterization

Transmission electron microscopy (TEM), high-angle annular dark-filed scanning TEM (HAADF-STEM) and elemental mapping were performed using a JEM-2200FS microscope at 200 kV. X-ray photoelectron spectroscopy (XPS) data were collected on a Thermo MultiLab 2000 system with an Al Mgα X-ray source. N₂ adsorption–desorption was determined by Brunauer–Emmett–Teller (BET) measurements using an ASAP-2010 surface area analyzer. The Raman spectra were obtained using a NTEGRA Spectra spectrometer (NT-MDT, Russia). X-ray diffraction (XRD) data were obtained on a Rigaku D/max lllC (3 kW) with a q/q goniometer equipped with a Cu Kα radiation generator. The diffraction angle of the diffractograms was in the range of 2θ = 5°–30°. Thermogravimetric analysis (TGA) was carried out using a Dupon 2200 thermal analysis station.

Electrochemical characterization

Electrochemical measurements were performed using a CR2016 type coin cell with Li metal as the counter/reference electrode, MoS₂@C as the working electrode and 1 M LiPF₆ dissolved in ethylene carbonate–dimethyl carbonate–diethylcarbonate (1 : 2 : 1 v/v) as the electrolyte. Cu foil was used as a current collector. The working electrodes were prepared by mixing 80 wt% active material, 10 wt% conducting carbon black and 10 wt% polyvinylidene fluoride binder in N-methyl-2-pyrrolidone. The active materials were MoS₂@C, exfoliated MoS₂ (e-MoS₂) and bulk MoS₂ powders. The cells were assembled in a glove box filled with argon and galvanostatic charge–discharge cycles were tested using a Wonatech automatic battery cycler at various current densities between 3 and 0.01 V vs. Li⁺/Li at room temperature. The electrochemical impedance spectroscopy measurements were carried out over a frequency range of 10⁵ to 10⁻² Hz at an AC voltage of 10 mV amplitude and RT using a Solartron 1260 impedance/gain-phase analyzer.

Results and discussion

A typical experimental procedure for preparing MoS₂@C nanosheets is described in Fig. 1. First, Li-intercalated MoS₂ (Li_xMoS₂) was synthesized according to a previous report.²⁵ This powder can be easily exfoliated into monolayers in a water solution through forced hydration and sonication, yielding a stable colloidal suspension. After adding dopamine, MoS₂@PD composites were obtained by the oxidative self-polymerization of dopamine onto the surface of MoS₂ sheets dispersed in Tris-buffer solution with a pH of 8.5. This coating method enabled us to obtain a highly conformal coating of PD on the surface of MoS₂ (Fig. S1†). After washing and collecting the MoS₂@PD sample, it can be readily re-dispersed in water with a slightly dark color (Fig. 1). Finally, ultrathin MoS₂@C nanosheets were obtained from the carbonization process using MoS₂@PD powders. This method offers a versatile route towards the scalable fabrication of the conformal carbon-coated MoS₂ sheets.

Fig. 1 Experimental process for the preparation of MoS₂@C using the self-polymerization of dopamine followed by a post carbonization process.

TEM images in Fig. 2a and 2b show a few layers of MoS₂@C nanosheets with a lateral size of several hundred nanometers. The carbon coating and core–shell structures are clearly shown in the high-resolution TEM image in Fig. 2c. The carbon coating layer obtained here is ∼3 nm thick on average (Fig. 2c). This core–shell structure was further investigated by the element mapping images of the energy dispersive X-ray (EDX) analysis. The carbon and nitrogen signals uniformly overlapped with the molybdenum signal of MoS₂. Based on these results, a nano-sized carbon layer conformally covered the individual MoS₂, resulting in ultrathin sandwich-like MoS₂@C nanosheets. In addition, the MoS₂@C sample was investigated using XPS (Fig. S2†). The newly appearing C 1s, N 1s and O 1s peaks of MoS₂@PD indicate the successful polymerization of dopamine on the surface of the MoS₂ sheet. After carbonization, the O 1s peak significantly decreased, while the N 1s peak was relatively unchanged. The chemical state of MoS₂@C was further investigated by high resolution XPS (Fig. S3†). Observation of Mo 3d spectra showed two prominent peaks at 230.8 and 233.2 eV for Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2, respectively.^26,27 The C 1s spectrum of MoS₂@C exhibited a strong and sharp peak of C–C (284.8 eV) with the weak peaks of C–N (286.2 eV) and C=O (287.8 eV). In the N 1S spectrum, two dominant peaks at 397.8 and 400.8 eV are assigned to pyridinic N and graphitic N, respectively.²⁸ The nitrogen content is 2.6% of the elements on the surface of MoS₂@C. In addition, the TGA results show 6.5 wt% of carbon on MoS₂@C (Fig. S4†). This N-doped carbon resulting from the carbonization of polydopamine is beneficial for Li⁺ storage due to the carbon layer's facilitated electrical conductivity and the charge transfer at the interface.²⁹

Fig. 2 (a) and (b) TEM images of MoS₂@C. (c) HR-TEM image of MoS₂@C. (d) HAADF-STEM image of MoS₂@C with EDX mapping of Mo, N and C.

As expected, this thin and conformal coating effectively prevented the aggregation and restacking of MoS₂ sheets. This was confirmed by XRD data (Fig. 3a). The prominent and strong intensity of the (002) peak corresponding to a d-spacing of 0.62 nm for pristine MoS₂ becomes very weak after PD coating and carbonization.²⁵ When observing Raman spectra (Fig. 3b and 3c), two prominent peaks of bulk MoS₂ at 384.6 and 409.9 cm⁻¹ appeared, corresponding to the E¹_2g and A_1g modes of the hexagonal MoS₂ crystal, respectively.^30,31 The E¹_2g mode involves the in-layer displacement of Mo and S atoms, while the A_1g mode involves the out-of-layer symmetric displacements of S atoms along the c axis.^30,31 In particular, the difference between the frequencies (Δ) of the A_1g and E¹_2g modes is known to be an indicator of the number of layers.³² This value was estimated to be ∼24.3 cm⁻¹ for MoS₂@PD and ∼24.2 cm⁻¹ for MoS₂@C by measuring the Raman spectra of several independent MoS₂ nanosheets. This frequency difference indicates that the MoS₂@C is less than 4 layers thick.^30,31 In addition, two other peaks at ∼1372 and ∼1587 cm⁻¹, which are related to the D and G bands of carbon, were observed in both samples (Fig. S5†).³³ The higher intensity ratio of I_D/I_G for MoS₂@C compared to MoS₂@PD is attributed to the increase in microcrystalline graphitic carbon through pyrolysis in the inert atmosphere.³⁴

Fig. 3 (a) XRD pattern of bulk MoS₂, MoS₂@PD and MoS₂@C samples. (b) Raman spectra of bulk MoS₂, MoS₂@PD and MoS₂@C samples. (c) Average difference between the E¹_2g and A_1g peak positions for MoS₂@PD.

The electrochemical performance of MoS₂@C was evaluated by preparing coin-type half cells (Fig. 4). During the first discharge, MoS₂@C measured at 100 mA g⁻¹ exhibited two prominent potential plateaus at approximately 1.1 V and 0.45 V, as shown in Fig. 4a. These are attributed to the phase transformation based on the following reactions (eqn (1) and (2)):¹³

MoS₂ + xLi⁺ + xe⁻ → Li_xMoS₂ (1)

Li_xMoS₂ + 4Li⁺ + 4e⁻ → 2Li₂S + Mo (2)

Fig. 4 (a) Galvanostatic charge–discharge curves of the MoS₂@C electrode cycled at the 1^st, 2^nd and 5^th between 3 and 0.01 V (vs. Li⁺/Li) at a current density of 100 mA g⁻¹. (b) Cyclic performances and CEs of MoS₂@C, e-MoS₂ and bulk MoS₂ electrodes at a current density of 100 mA g⁻¹.

The first potential profiles are consistent with those of other MoS₂ and MoS₂–carbon composite anodes previously studied.^11,14,32,35 The first discharge and charge capacities are 1239 and 1046 mA h g⁻¹, respectively. Based on these values, the first Coulombic efficiency (CE) was calculated to be 84%, which is higher than those of most of the other reported MoS₂-based anodes.^14,36,37 This indicates that the carbon coating induces the stable formation of SEI layers during the first cycle.³⁸ After the first cycle, the potential profiles for the 2^nd and 3^rd processes overlap a great deal and are followed by the reaction:¹³

MoS₂ + 4Li ↔ Mo + 2Li₂S (3)

In the following cycles, a new potential plateau at around 2.1 V appeared, which is indicative of the formation of sulfur-containing substances.^39,40 The unique structure of MoS₂@C enabled it to have a stable cycling performance as an anode for a lithium ion battery. Fig. 4b shows specific capacity as a function of the cycle number over 50 cycles. To demonstrate the superior cycling stability of MoS₂@C, exfoliated MoS₂ (e-MoS₂) alone and bulk MoS₂ were also tested under the identical conditions of MoS₂@C. When measured at a rate of 100 mA g⁻¹, MoS₂@C shows much higher specific capacities than those of e-MoS₂ and MoS₂ during battery cycling. After 50 cycles, the MoS₂@C had a 98% (= 1147 mA h g⁻¹) capacity retention based on a second discharge cycle. In contrast, e-MoS₂ and MoS₂ had the inferior cycling performances of 72% and 44%, respectively.

Furthermore, MoS₂@C had a better rate capability than e-MoS₂ and MoS₂ when increasing the charge–discharge rates from 0.05 to 10 A g⁻¹. All of the obtained capacities for MoS₂@C and MoS₂ were plotted with the applied current densities, as shown in Fig. 5a. MoS₂@C exhibits a decent discharge capacity retention that is continuously higher than that of e-MoS₂. Even when increasing the cycling rate up to 10 A g⁻¹, 48% of the original capacity was maintained. This remaining capacity of 597 mA h g⁻¹ was still higher than the theoretical capacity of commercial graphite (372 mA h g⁻¹). The retention value is also higher than those of other reported MoS₂-based anodes tested at even lower rates, including MoS₂–carbon nanotube composites¹³ and MoS₂–polymer–graphene nanocomposites.¹⁰ Moreover, when the current rate was finally returned to 0.05 A g⁻¹ after a total of 80 cycles, an 85% retention of the initial capacity was still recoverable. The outstanding cycling performance of MoS₂@C was further demonstrated by cycling at a high rate of 2 A g⁻¹. A high retention value (89%) of the original capacity after 200 cycles was still maintained. This value is higher than those of other reported MoS₂-based materials tested at even lower rates, including graphene-like MoS₂ nanoplates⁶ and MoS₂–grapheme.⁴¹

Fig. 5 (a) Rate capabilities of MoS₂@C and e-MoS₂ electrodes at various current densities between 0.05 A g⁻¹ and 10 A g⁻¹. (b) Cyclic performance and CE of the MoS₂@C electrode at a current density of 2 A g⁻¹.

The ultrathin and conformal N-doped carbon coating of MoS₂ is responsible for high capacity, high rate capability and good cycling stability. We found clear evidence of an enhanced charge transfer in MoS₂@C compared to MoS₂ from electrochemical impedance spectroscopy measurements. Fig. 6 shows the Nyquist plots of MoS₂@C and e-MoS₂. On the basis of a semicircle, MoS₂@C had a lower charge transfer resistance (R_CT) than that of e-MoS₂. The N-doped carbon coating enabled MoS₂ to improve the charge transfer at the electrode–electrolyte interface, which is responsible for reversible charge–discharge processes. At the low frequency region, a more vertical straight line was observed for MoS₂@C than e-MoS₂. A few layer N-doped carbon shell enabled us to improve electron conductivity and ion transfer at the interface of the electrode,²³ and thus leading to the enhancement of Li⁺ diffusion which is responsible for the more vertical line of MoS₂@C than e-MoS₂. Moreover, the N-doped carbon coating enabled us to maintain the exfoliated MoS₂ sheets in an ultrathin carbon shell, thus leading to the high surface area. N₂ adsorption–desorption measurements provided a BET surface area of 276.8 m² g⁻¹ for MoS₂@C, which is five-fold higher than that of bulk MoS₂ (56.2 m² g⁻¹) and is also higher than exfoliated MoS₂ (198.5 m² g⁻¹). Therefore, an MoS₂ core embedded in an ultrathin carbon shell with a high surface area efficiently opens a reaction area where electrons can reach in all directions and meet with the inserted Li⁺ ions, thereby enhancing good rate and cycling performances.

Fig. 6 Nyquist plots for the MoS₂@C and e-MoS₂ electrodes.

Conclusions

We developed a versatile and simple method to prepare an effective core–shell structure of MoS₂@C through simultaneous PD coating and carbonization processes. The surface modification of polydopamine enabled the re-stacking of MoS₂ to be effectively prevented, thus leading to a high surface area. In addition, N-doped carbon coated on the surface of MoS₂@C is beneficial for the transport of electrons over electrode materials, which can improve the electrochemical properties. The resultant MoS₂@C was applied to the anodes for lithium ion cells and showed a specific capacitance as high as 1239 mA h g⁻¹, a high rate capability at a high current rate of 10 A g⁻¹ while retaining 597 mA h g⁻¹, and a good cycle stability over 200 cycles at a high current rate of 2 A g⁻¹. This solution-based approach is a simple and scalable method and could thus be readily applicable to other metal sulfide-based energy storage and conversion applications.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning of Korea (MSIP), No. 2010-C1AAA001-0029018 and No. 2014R1A5A2010008. The authors would also like to acknowledge BioNano Health-Guard Research Center funded by MSIP of Korea as Global Frontier Project (grant number H-GUARD_2013M3A6B2078945).

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Footnote

† Electronic supplementary information (ESI) available: TEM images, XPS, TGA and Raman spectra. See DOI: 10.1039/c4nr06215a

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