Jae-Min
Jeong
a,
Kyoung G.
Lee
a,
Sung-Jin
Chang
b,
Jung Won
Kim
c,
Young-Kyu
Han
d,
Seok Jae
Lee
a and
Bong Gill
Choi
*c
aCenter for Nanobio Integration & Convergence Engineering (NICE), National Nanofab Center, 291 Daehak-ro, Yuseong-gu, Daejeon 305-806, Republic of Korea
bDepartment of Chemistry, Chung-Ang University, Seoul 156-756, Republic of Korea
cDepartment of Chemical Engineering, Kangwon National University, Samcheok 245-711, Republic of Korea. E-mail: bgchoi@kangwon.ac.kr
dDepartment of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715, Republic of Korea
First published on 19th November 2014
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.
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.
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+ 3d5/2 and Mo4+ 3d3/2, 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 CO (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 E12g and A1g modes of the hexagonal MoS2 crystal, respectively.30,31 The E12g mode involves the in-layer displacement of Mo and S atoms, while the A1g 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 A1g and E12g 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 ID/IG for MoS2@C compared to MoS2@PD is attributed to the increase in microcrystalline graphitic carbon through pyrolysis in the inert atmosphere.34
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− → LixMoS2 | (1) |
LixMoS2 + 4Li+ + 4e− → 2Li2S + Mo | (2) |
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 2nd and 3rd 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
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 (RCT) 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.
Footnote |
† Electronic supplementary information (ESI) available: TEM images, XPS, TGA and Raman spectra. See DOI: 10.1039/c4nr06215a |
This journal is © The Royal Society of Chemistry 2015 |