Graphene Hybrid Nanoflowers with Enhanced Electrochemical

Subscriber access provided by EPFL | Scientific Information and Libraries

Article

MoS2/graphene hybrid nanoflowers with enhanced electrochemical performances as anode for Lithium-ion Batteries Honglin Li, Ke Yu, Hao Fu, Bangjun Guo, Xiang Lei, and Ziqiang Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00890 • Publication Date (Web): 20 Mar 2015 Downloaded from http://pubs.acs.org on March 24, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

MoS2/Graphene Hybrid Nanoflowers with Enhanced Electrochemical Performances as Anode for Lithium-ion Batteries Honglin Li, Ke Yu*, Hao Fu, Bangjun Guo, Xiang Lei, and Ziqiang Zhu. Key Laboratory of Polar Materials and Devices (Ministry of Education of China), Department of Electronic Engineering, East China Normal University, Shanghai 200241, China.

ABSTRACT. In this work, we studied the synthesis and electrochemical performance of MoS2 and reduced graphene oxide (rGO) hybrid nanoflowers for use as anode material in lithium ion batteries (LIBs). The morphology and microstructure of the samples were characterized by field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), Xray diffraction (XRD) and X-ray photoelectron spectrometry (XPS). Herein, the composite nanoflowers delivered a significant enhanced reversible specific capacity and charge/discharge cycle stabilities as anode in comparison with pristine MoS2. Electrochemical impedance spectroscopy (EIS) measurements indicated that the incorporation of rGO significantly reduced the contact resistance and the improved electrochemical performances could be attributed to the synergy effect between the functions of MoS2 and rGO. A high reversible capacity of 1150 mAh/g at a current of 0.1 A/g could retain without fading after 60 cycles. The rate performance of the composite was also improved, and the specific capacity remained a relative high value of

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

∼890 mAh/g even at a current of 1000 mA/g. In order to further systematically study the mechanism of the improved LIBs performances for composite, we constructed the corresponding models based on experiment data and conducted first-principles calculation. Nudged elastic band (NEB) method was employed to study the diffusion of Li in different structures. The calculated results proved that the diffusion barrier for Li in MoS2/graphene was significantly lower than that of in pristine MoS2 and presented a theoretical explanation for a better diffusivity property. The high specific capacity and excellent cycling stability of these hybrid nanoflowers are competent as a promising anode material for high-performance LIBs.

KEYWORDS: MoS2, Battery, Graphene, Mechanism 1. Introduction

With the continuously increasing environmental concern, the mass production of green energy sources and the efficient energy storage methods are becoming more and more urgent for the construction and utilization of sustainable energy in response to the worldwide pollution and energy dilemma.1 As one of the most promising candidates, LIBs have became one of the predominant power sources of electric vehicles or portable electronics, etc., for their high energy density, low gravimetric density and long cycle life.2 Currently, graphite is the most widely used commercialized anode material in LIBs. However, the thermodynamic equilibrium saturation composition of LiC6 delivers a low theoretical capacity of 372 mAh/g for graphite, thus it is important to find alternatives in high performance LIBs and explore new anode materials with higher reversible specific capacity as well as superior cyclic stability for LIBs.3 Recently, 2dimensional nanostructures have attracted considerable attention because they are promising alternatives for LIBs anode materials. MoS2 and analogous materials, such as MoS2, WS2 and SnS2, have recently arouse great interest due to their high theoretical capacities and become of a


2

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


high interest in several other energy storage and conversion applications due to its price efficiency and performance competitiveness.4 Utilization in fuel cells and biomass conversion/hydro-deoxygenation are two such examples.5 MoS2 based materials have high theoretical capacities, this is because these compounds can store massive Li ions through a series of reactions and Li ions also can easily intercalate/de-intercalate to/from weak van der Waals bounded layers structure. Since the layers spacing of MoS2 (0.615 nm) is remarkably greater than that of graphite (0.335 nm), the insertion of lithium ions should be easier in comparison with graphite and the geometric construction of the active materials can remain stable without apparent volume expansion in lithiation/delithiation processes. The high theoretical capacities and fast Li ions movement in MoS2 based materials are mainly originated from two aspects. On one hand, MoS2 can be converted into Mo-Li2S and provide a substantial redox capacity. 4 moles lithium can incorporate into 1 mole MoS2 in the corresponding conversion processes and deliver a capacity of about 670 mAh/g, which is significantly higher than that of commercialized graphite.6 Besides, the weak interaction between neighbouring layers of MoS2 is also in favour of the insertion and extraction of Li ions, preventing the pulverization phenomena.7 Hence, a number of MoS2 nanostructures with various morphologies have been synthesized for LIBs. However, the practical cyclic stability is still unsatisfactory when MoS2 is used as anode material in LIBs and it impedes the extensive use for LIBs. Firstly, the conversion lithiation intermediate product Li2S is liable to react with the electrolyte and forms a thick gel-like polymeric layer,8 which inhibits a further reactions and leads to a degraded stability and rate capability. This disadvantage is inherent and cannot be easily changed by nanostructure modifications.9 For another, the extremely low electronic conductivity also results in the structural destruction during cyclic processes and then a rapid


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

capacity fading. These are the major problems faced by practical applications for MoS2 based LIBs.10 In consideration of the above disadvantages, integrating MoS2 with conducting materials like carbon nanotube, amorphous carbon or graphene ought to be a valid method to improve the corresponding electrochemical performances.11 Graphene, with the advantages of excellent chemical, electronic properties and mechanical strength, should be an optimal compounding medium with pristine MoS2.12 The structural and morphological compatibility between graphene and MoS2 sheets make it an excellent integration to achieve the ideal properties.13 During charge and discharge processes, graphene can cushion the changes in volume for the storage of Li ions due to its outstanding mechanical properties. Besides, the high conductivity and charge carrier mobility can also improve the electrical conductivity in electrode reaction.14 Therefore, the composite of MoS2 and graphene can exhibit excellent electrochemical performance by fully utilizing the advantages of the two structures. In experiments, Yu et al. prepared MoS2/graphene nanoflake arrays with 3D architecture which exhibited superior high rate performance for the stable structure and large interfacial area between electrode-electrolyte.15 Zhang et. successfully synthesized morphology-controlled MoS2 nanosheets in the presence of graphene and acid through a hydrothermal method. The obtained MoS2 ultrathin nanosheets present excellent rate performance as well as superior charge/discharge cycle abilities for Li storage.16 Liu et al. developed a facile method to fabricate an independent MoS2/rGO hybrid paper cross-linked by PEO. The synthesized structure also delivered a high specific capacity, cyclic stability and high rate performance.17 Herein, we report a simple hydrothermal method for the fabrication of MoS2/rGO (MG) nanoflowers. The MG composite exhibited significant enhanced electrochemical properties including excellent cycling stability and high rate performance. With regarding to the mechanism


4

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


study of the improved performance for the incorporation of MoS2 with rGO, there is hardly any report related to this aspect. In this paper, we constructed the Li inserted models based on experiment data and detailedly analysed the charge density difference, Bader charges, binding energies and barrier energies of the two structures by first-principles calculation. Li diffusion behavior is a key factor for the rate capability of LIBs. Fast Li diffusion characteristic will bring high rate capability of LIBs. In many applications such as grid storage, high rate capability is more important than energy capacity. So it is necessary to study the diffusion character of Li in MoS2/graphene for use as anode material of LIBs and this work is the first time to report diffusion characteristic of Li in MoS2@graphene composite. In brief, the obtained results adequately explained the reason why composite exhibits better performances and the related analytical methods also offer a certain theoretical foundation for a further study of high performance LIBs. 2. Experimental and calculation details 2.1 Synthesis of MG

Firstly, graphene oxide (GO) was synthesized by a modified Hummers method.18 100 mg the fabricated GO were dispersed into 60 mL distilled water and then ultra-sonicated for 30 min at 400 r/min. The MG composite was fabricated by a one-step hydrothermal method through a reaction with Na2MoO4·2H2O and NH2CSNH2 containing GO. 1 g Na2MoO4·2H2O and 1.2 g of NH2CSNH2 were dissolved into the above GO aqueous solution and kept stirring for 30 min, meanwhile. Afterwards, the mixture solution was transferred into a Teflon-lined autoclave and heated at 180 ℃ for 24h. In this process, the released H2S from NH2CSNH2 under hydrothermal condition could reduce the as-prepared GO sheets to rGO.19 After cooling to room temperature naturally, the black precipitates were obtained and washed carefully with distilled water and


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

anhydrous ethanol in sequence, then dried at 60 ℃ for 12 h. Pristine MoS2 was prepared by the same procedures as described above in the absence of GO. Commercial MoS2 (C-MoS2) was purchased from Lingfeng. 2.2 Characterization of MG nanostructures.

The lattice structures of the different samples were characterized by X-ray diffraction (D8 Advance/ BRUKER AXS GMBH) with Cu-Kα radiation (λ=0.1541 nm, over the 2θ range of 5– 70 ° with a step size of 0.02 °). The morphologies of the samples were obtained by FESEM (JEOL-JSM-6700F). TEM and high-resolution transmission electron microscopy (HRTEM) observations were performed on a JEOL JEM-2100. The samples used for TEM characterization were prepared by dropping the colloidal solution to a holey carbon-coated copper grid, which then were dried in air. XPS was performed with an ESCALAB 250Xi instrument. Raman spectra analyses were carried out by a Jobin-Yvon Lab RAM HR 800 micro-Raman spectrometer. 2.3 Electrochemical studies

The electrochemical performances were evaluated under room temperature through a twoelectrode coin cell (CR 2025). The working electrodes were prepared by casting a slurry of 80 wt% active material (MG/pristine MoS2/C-MoS2), 10 wt% carbon black and 10 wt% PVDF on a copper foil. The coated copper foils were then dried at 80 ℃ in vacuum for 24 h, and then compressed into pieces before use. Li sheets were served as the counter electrode and reference electrode, a polypropylene film (Celgard 2400) was used as a separator between the working electrode and Li foil. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate/dimethyl carbonate/ diethyl carbonate. Galvanostatic discharge and charge cycles of the cells were performed under different voltage range between 0.01 V and 3.0 V at room temperature. Cyclic voltammetry (CV) measurements were performed on CH Instrument 660C with a range of 0.01-


6

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.00 V at a scan rate of 0.5 mV s-1. EIS were obtained over the frequency range from 100 KHz to 0.01KHz. All cells related procedures were operated in an Ar-filled glovebox. 2.4 Computational details

Our calculations were carried out under density functional theory (DFT), with the projectoraugmented wave (PAW) method implemented in the Vienna ab initio simulation package (VASP).20 The valence electron considered were: Mo (p4s5d4), S (s2p4) C (s2p2) and Li (s1p0), respectively. The generalized gradient approximation (GGA) functional of Perdew, Burke and Ernzerhof (PBE) was employed to deal with the exchange and correlation potentials.21 All calculations were performed with a 3×3×1 supercell and a 450 eV cutoff energy was used for the calculations. The method of conjugate gradient energy minimization was used for geometry relaxation. The convergence criterion for energy between two consecutive steps was chosen to be 1E-5 eV, and for each atom upon ionic relaxation the maximum Hellmann-Feynman force was less than 0.01 eV/Å. In this work, the NEB method was employed to study the diffusion barriers of Li migration in pristine and graphene composited MoS2 for comparison.22 Between the initial and final configurations of the diffusion paths, each image maintained equal distance to nearby images and searched for its potential lowest energy along the reaction paths.23 It was performed with linear interpolating 9 images, which we considered were enough to get the variation trends.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

3. Results and discussion 3.1 Characterization of the synthesized samples

Figure 1. SEM and TEM images of the pristine MoS2 and MG composite. (a) and (b) are the SEM images of the pristine MoS2 in low- and high-magnifications. (c) and (d) are TEM and HRTEM images of the pristine MoS2. (e) and (f) are the SEM images of MG composite in lowand high-magnifications. (g) and (h) are TEM and HRTEM images of MG composite. Insert figure in (g) is the FFT pattern of red square obtained from the HRTEM image.

Figure 1a and b are SEM images of the pristine MoS2, it show a sphere morphology mainly consisted by 2D flowerlike nanopetals and the synthesized nanopetals are grown in high density. Figure 1c and d show TEM and HRTEM images of the pristine MoS2, which further prove the 2D nanopetals and the aggregated nature of pristine MoS2. Figure 1e and f show SEM images of MG composite. It presents a clear change in comparison with pristine MoS2. SEM images of the two structures form Figure 1b and e have the same scale, while the spherical structures in Figure 1e are dramatically increased and obvious more than that of pristine MoS2. This means that much more MoS2 can be utilized in the corresponding electrochemical processes and make more efficient use of active substances. The nanopetals displayed in Figure 1f indicate that MoS2 in


8

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MG composite are grown in a more free and loose way. The MoS2 in these hybrid agglomerates are mainly consisted by limited-layer MoS2 structures and tightly coupled with hybrid rGO sheets. The HRTEM image of Figure 1d shows that pristine MoS2 tend to stack with an interlayer distance of 0.62 nm, corresponding to the (002) plane of MoS2. In contrast, the TEM image of the MG nanostructure shown in Figure 1g presents scattered and mild appearance of MoS2 grown on rGO sheets characterized by varied brightness. The HRTEM image and the corresponding fast Fourier transform (FFT) pattern prove the crystalline structure of MoS2. Further, the HRTEM image of the MG composite shown in Figure 1h reveals that the lattice fringe spacing is about 1.02 nm, which is consistent with the following XRD result. This lattice fringe spacing should be ascribed to the interlayer distance of the adjacent MoS2 layers, in the middle of which are the inserted rGO sheets and this structure can be considered as the sandwich structure. Generally, the formation of pristine MoS2 nanoflowers may be related to the amorphous primary nanoparticles and nucleation in certain reaction conditions. During initial reaction period, numerous amorphous primary MoS2 nanoparticles take shape in the solution. One thing to note is that there are two functions NH2CSNH2 acted in hydrothermal process: it acts as both S resource and reductant that the as-prepared GO sheets can be easily reduced to graphene by H2S released from NH2CSNH2. When the temperature raises to a certain value, these primary nanoparticles are freely and spontaneously aggregated into solid spheres and then curl to petals gradually in the surface for the layered nature of MoS2. As for the mechanism of MG nucleation, we consider the formation of MoS2 nanoflowers on rGO is greatly affected by the oxygenated functional groups in GO. In the process of the hydrothermal reduction to rGO, MoS2 may grow on these highly active sites to form “seeds” on the GO surface, also, graphene


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

itself is 2D structured and MoS2 has the nature of growing into flower in this environment, it will result in the confined growth of MG nanoflowers finally.

Figure 2. (a) XRD patterns of the C-MoS2, pristine MoS2 spheres and MG composite. (b) Raman spectra of the E21 g and A1g vibrational modes for pristine MoS2 and MG composite. Insert figures show the atomic displacements of the E21 g and A1g vibrational modes in MoS2. (c) and (d) are C 1s spectra of GO and MG composite, respectively. (e) and (f) are XPS spectra of Mo 3d and S 2p, respectively.

Figure 2a displays XRD patterns of the C-MoS2, pristine MoS2 and MG composite. The diffraction peaks for C-MoS2 correspond to the planes of MoS2 (JCPDS 37-1492). The strong (002) peak around 14° with 0.62 nm spacing signifies the natural layered MoS2 along the c axis.


10

Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In contrast, pristine MoS2 shows a much broadened and subdued (002) peak, which implies that the stacked MoS2 layers along the c axis are much less than C-MoS2. Additionally, the peaks of MG composite become more weaker, indicating the incorporation of rGO can further suppress the stacked natural of MoS2 layers. The diffraction peak 1 of MG around 9° should be indexed to the interlayer distance of the adjacent MoS2 layers in the composite. The broad peak 2 around 16° corresponds to the diffraction between MoS2 layers and rGO surfaces.4 The peak 3 around 24° is attributed to rGO sheets.24 Based on the above HRTEM images and XRD patterns, we in turn confirmed the MoS2 layers and the formation of sandwich structure. The rGO sheets hybrid structure can not only improve the conductivity of the composite but also result in an expansion of the adjacent MoS2 layers, which is in favour of the migration of Li ions. Raman spectra shown in Figure 2b are further used to analyse the corresponding structures. The two peaks at 382 and 407 cm-1 attribute to the E12g and A1g vibrational modes of pristine MoS2, respectively. As is known that the E12g associates in-layer displacements of Mo and S atoms while A1g associates out-of-layer symmetric displacements of sulfur atoms along the caxis and the two vibrational modes are orthogonal to each other.25 It has been reported that the frequency of A1g mode will present a red-shift as the layer number of MoS2 decreases.26 Figure 2b shows that a slightly red-shift of A1g mode is observed for MG composite in comparison with pristine MoS2, implying that the layer number decreases after combination with rGO, which is in accord with the above XRD analysis. The chemical states of Mo, S and C of the pristine and MG composite are investigated by XPS. Figure 2c and d show the C 1s XPS peak-fitting results of GO and MG composite, respectively. In Figure 2c, four resolved peaks of oxygenated functional groups (C(O)-O, C=O, C-O-C) and sp2-hydridized C-C exhibit a considerable degree of oxidation. While the C 1s peak-fitting


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

results of MG composite shown in Figure 2d only presents C-C peak and much smaller peaks of C=O and C-O-C relative to that of GO. One thing to note here is that the disappearance of the C(O)-O peak indicates that the GO sheets have been reduced to graphene. The Mo 3d and S 2s regions of pristine MoS2 are shown in Figure 2e and three peaks can be observed. The two main peaks of Mo 3d5/2 (229.1 eV) and Mo 3d3/2 (232.4 eV) are characteristic of MoS2 and 226.1 eV corresponds to S 2s of MoS2. Sulfur species of pristine MoS2 are shown in Figure 2f. The main doublet located at binding energies of 161.9 and 163.1 eV correspond to the S 2p3/2 and S 2p1/2 of pristine MoS2, respectively.27 As for MG composite, the above binding energy are all slightly larger than that of pristine MoS2. These results signify that there appears an interaction between MoS2 and rGO in the composite, which is derived from the electron transfer between closely contacted MoS2 and rGO sheets and it also illustrates that the MG composite is not a simple physical mixing of MoS2 and rGO, so the interaction of electrons between MoS2 and rGO can effectively increase the conductivity of the composite. In order to further illustrate the interaction between Li and MoS2/graphene in a visual manner, we conduct charge density difference analysis based on first-principles calculation in section 3.3. 3.2 Electrochemical performance


12

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) First three galvanostatic charge/discharge profiles of MG composite; (b) Cyclic voltammograms of MG electrode at a scanning rate of 0.5 mV s-1 during the first three cycles; (c) and (d) are cycling behaviours and rate capabilities of the three structures: C-MoS2, pristine MoS2 and MG composite, respectively. Figure 3a presents the first three charge/discharge voltage profiles of MG at 100 mA/g. It is shown that two plateaus around 1.1 and 0.6 V are observed in the first discharge (lithiation) process. The plateau at 1.1 V represents the formation of LixMoS2. The plateau at 0.6 V can be ascribed to a conversion reaction process, in which MoS2 decomposes into Mo particles that embedded in Li2S matrixes. The following electrolyte degradation can lead to the formation of gel-like polymeric layer. In the next discharge curves, the MG composite displays potential plateaus around 1.9 V along with the disappearance of plateau at 0.6 V in the first discharge. During the charge (delithiation) process, MG composite shows an obvious potential plateau around 2.1 V, which accords with the following CV curves. As for the first charge/discharge process, the lithiation capacity is 1521 mAh/g, and the delithiation capacity reaches 1110 mAh/g.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

In the corresponding discharge/charge processes, the irreversible reactions, such as lithium trapping inside the lattice and the formation of solid electrolyte interphase (SEI), can result in the irreversible capacity loss.28 One thing to note is that the discharge and charge curves are well overlapped from the second cycle and thus an excellent cycling performance achieved. Figure 3b exhibits the CV of the MG electrode. This CV behaviour is generally consistent with the reported results.29 For the first cathodic sweep, the peak around 0.75 V is indicative of the insertion process of Li ions into MoS2 and meanwhile transforms into an octahedral structure to form the triangular prism. The other peak at 0.3 V is assigned to the reduction process of MoS2 into Mo particles inserted into Li2S matrixs: MoS2+4Li→Mo+2Li2S, meanwhile, the electrochemical process impel the degradation of electrolyte and the SEI film take shape. In the following cathodic sweeps, two new peaks around 1.0 and 1.8 V appear, implying the appearance of a multi-step Li ions insertion mechanism. In the anodic sweeps, there exist the broad peak around 1.7 V and a sharp peak at 2.4 V. The peak around 1.7 V is indicative of two-step Li ions removal from Mo while 2.4 V is ascribed to the oxidation process of Li2S into sulfur.30 Thus, the elementary substances of electrode mainly consist of Mo and S instead of MoS2 after the first cycle. Figure 3c shows the cycling behaviour of the three electrodes at 100 mA/g. It is shown that the C-MoS2 electrode delivers a significantly reduced discharge capacity over 300 mAh/g after 60 cycles. The synthesized pristine MoS2 sphere structure shows a much elevated capacity while the cyclic stability is still not good enough and a capacity of 550 mAh/g sustained after 60 cycles. In comparison with C-MoS2, this improved behaviour can be attributed to the MoS2 with spherical hierarchical structure can provide more reactive sites at interface. The nanopetals rooted in the pristine MoS2 structure can effectively shorten the diffusion paths of Li ions and favour the active materials to obtain a higher specific capacity. Besides, the voids between the


14

Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanopetals can dramatically buffer stress to protect the active materials from degeneration during the lithiation/ delithiation processes. Hence, both the cycling and rate performance of the pristine MoS2 sphere structure with dense petals have been enhanced. After hybridized with rGO sheets, the capacity and cyclic stability of MG composite get a further improvement. It is remarkable that the MG composite delivers a superior capacity of about 1150 mAh/g in the initial few cycles and the capacity can still sustain after 60 th cycle or even after more than 60 cycles. This is suffice to elucidate the high stability of the fabricated composite. Excellent rate performance is an important aspect to obtain high power density in LIBs. Figure 3d shows the rate cycling behaviour of the three samples, in which MG composite demonstrates the optimal rate performance. Even though at 1000 mA/g, the specific capacity can still remain at a relative high value of ∼890 mAh/g, which is higher than that of C-MoS2 and pristine MoS2 at a low current density of 100 mA/g. We consider that the composition of MoS2 with the highly conductive rGO sheets can effectively increase the contact area with electrolyte and provide much more reaction sites and thus benefit the migration of Li ions, leading to an excellent rate behaviours of the MG composite. Compared with the previous related studies, Shyamal et al. prepared composite of MoS2 and amorphous carbon, which showed a discharge capacity of 755 mAh/g with Coulombic efficiencies of 98% after 100th cycle at constant current density of 100 mA/g.9 Li et al. found that hierarchical hollow MoS2 nanotubes prepared by solvothermal reaction exhibited a reversible capacity of 727 mAh/g at 100 mA/g after 100 cycles.12 Liu et. al synthesized flexible and robust MoS2–graphene hybrid paper, which had the highest initial discharge capacity of 1240 mAh/g, with only 29% irreversible loss to 890 mAh/g after 100 cycles at a current of 100 mA/g.17 Wang et al. reported that the hybrid urchin-like


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

nanostructures synthesized by solvothermal method, which displayed the a specific capacity of 853 mAh/g at a current density of 50 mA/g after 60 cycles.19

Figure 4. (a) EIS of pristine MoS2 and MG composite electrodes. (b) Atomic model of MG composite. In order to further study the superior electrochemical performance of MG composite in comparison with pristine MoS2, the EIS of pristine MoS2 and MG composite are compared in Figure 4a. All impedance measurements were carried out at 60% depth of discharge (DOD) after 10 cycles. The atomic structure diagram, which is based on the XRD and TEM analyses, is shown in Figure 4b. The two curves both consist of a semicircle and an inclined line. The semicircle is related with the charge transfer resistance (Rct) of the electrode/electrolyte interface, while the inclined line is correlated to the Warburg impedance (Zw) that reflects the Li ions diffusion character in the electrochemical processes.31 It can be observed that the semicircle of the MG is clearly smaller than that of the pristine MoS2 electrode, which contributes to a better electrochemical performance of the MG electrode. This result denotes that the combination of MoS2 with rGO can effectively increase the conductivity of the MG composite structure and enhance the electron transport during the lithiation/delithiation processes, resulting in significant


16

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


improved electrochemical performances. The combination of the rGO can offer a tight coupling between MoS2 nanopetals and rGO matrix, forming a hybrid nanostructure with higher conductivity and the volume expansion also can be effectively buffered simultaneously. In the previous report, the electron transfer is 10-fold faster in graphene than in graphite for the presence of corrugation on the graphene surface.32 In this case, the highly delocalized p conjugation in rGO surface can interact with the outer electrons of S atom in MoS2. It is conclude that the incorporation MoS2 with rGO sheets can increase their contact areas and form synergistic effects with each other. In addition, the flexibility of rGO can stabilize the structure of the MG composite and restrain the aggregation of nature-stacked MoS2 during the lithiation/delithiation processes.33 Finally, the specific morphology of flower-like MG composite with the enlarged interlayer spacing is readily accessible to the electrolyte and is beneficial to the lithiation/delithiation processes of Li ions, thus enhance the utilization of active materials. It is worth mentioning that at the low frequency region, a more vertical straight line is observed for MG than that of pristine MoS2. For this result, we preliminarily infer that the combination MoS2 with rGO can enhance the diffusivity of Li ions, which is responsible for a more vertical line of MG than pristine MoS2. In order to verify this hypothesis, we further conduct the diffusion barriers analyses based on pristine MoS2 and MoS2/graphene composite using NEB method by first-principles calculation. 3.3 Computational analysis


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

Figure 5. Charge density difference distributions for (a) 1 Li inserted MoS2, (b) 1 Li inserted MoS2/graphene, (c) 2 Li inserted MoS2 and (d) 2 Li inserted MoS2/graphene. The red spheres represent Li atoms. Depletion and accumulation space are revealed in wathet blue and yellow, respectively. (e), (f), (g) and (h) are the corresponding densities of four structures plotted in planes passing through Li and paralleled with c axes. The dashed lines and the red circles denote the regains of charge accumulation/depletion and positions of Li, respectively.

In this section, we calculated the charge density differences for Li inserted pristine and graphene composited MoS2 in Figure. 5, which give us information regarding to the charge redistribution. The charge density difference is defined as: ∆n(r ) = nsub/Li (r ) - nsub (r ) - n Li (r ) , where nsub/Li (r )

is the electron density of the substrate–Li composite system, whereas nsub (r ) and n Li (r )

are the separate electron densities of the substrate and isolated Li, respectively. In computing of nsub (r )

and n Li (r ) , all atoms are kept at the same positions as they were in the insertion structures.

Before further calculations, all related structures have been fully relaxed. From these figures, we can elucidate the charge transfer between the inserted Li and substrate. Figure 5a and b show the charge density difference of 1Li inserted pristine MoS2 and MoS2/graphene composite. The


18

Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


yellow regions represent charge accumulation while the blue represent charge depletion. From the charge density difference pattern of 1Li inserted pristine MoS2, it is obvious that the major charge transfer happens between the inserted Li atom and its subjacent S atoms. While for the 1Li inserted MoS2/graphene composite, the charge transfer condition becomes complex. An obvious accumulation space appears above the graphene layer, implying a certain amount of the charge transfer from Li to C atoms. A co-electron takes shape between the graphene layer and its adjacent S atom layer in the composite. It should be noted that after the first discharge/charge cycling, the reversible reaction MoS2+4Li ↔ Mo+2Li2S will play a leading role in the subsequent processes.1, 6 So then we also construct the Li2S modes based on Stephenson et al. report.6 The corresponding charge density difference are shown in Figure 5c and d. It is clear to see that obvious depletion spaces appear between two Li atoms and the co-electrons also take shape between Li atoms and its adjacent S or C atoms. In order to further analyze the bonding states of the different structures, we draw the corresponding contour plots of four structures as shown in Figure 5e to h. All plots shown here were computed in the planes passing through the Li and paralleled to c axes. For 1Li inserted pristine MoS2 structure, we clearly see charge accumulation along the Li atom and it adjacent S atom, implying that the bond has a covalent bonding character. However, the charges are major close to S atoms, this is due to a larger electronegativity of S compared to Li. As for 1Li inserted MoS2/graphene, the main difference between the two structures is that the density around Li in MoS2/graphene is less that of pristine MoS2 as shown in red circles. We speculate that the interaction between the inserted 1Li and pristine MoS2 is stronger than that of MoS2/graphene composite. A further analysis based on Bader charge to quantitatively study the charge transfer conditions should be conducted. When 2Li atoms are inserted into pristine and graphene composited MoS2 for the formation of Li2S,


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

obvious depletion regions appear between the two Li atoms. Besides, the density around 2Li in MoS2/graphene composite is also less than that of pristine MoS2.

Table 1. The calculated changes in Li atoms charges (△Q) and Li binding energies (Eb) for the 1/2-Li inserted structures of pristine and graphene composited MoS2. Here a negative value of △Q indicates charge loss. △Q(e) Eb(eV) MoS2-1Li

-0.873 -2.106

MoS2-2Li

-1.729 -2.001

MoS2/graphene-1Li -0.864 -2.058 MoS2/graphene-2Li -1.714 -1.747

Generally, Bader charge is applied to describe the charge transfer between different portions of the system. It is developed by Bader using the atoms-in-molecules approach.34 For each atom j, a region of space Ωj is distributed by trajectories of steepest density descent（ −∇n ）and starting r r

close to the nucleus. The Bader charge Qj of atomic is expressed as Q j = Z j − ∫ n(r )dr , where Zj is Ωj

the nuclear charge of atom j (all electrons considered in the corresponding pseudopotential). As calculated above that the significant charge density difference between Li and S/C atoms, distinct differences are also reflected in the changes of Bader charges. Our calculated results are listed in Table 1. The changes of Bader charges for 1Li atom inserted in pristine MoS2 and MoS2/graphene are -0.873 and -0.864 e, respectively, implying more electrons from Li atom would transfer to substrate for pristine MoS2. This difference is also applied to 2Li inserted


20

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


structures. 1.729 and 1.714 e charges would transfer form 2Li atoms to substrate for pristine MoS2 and MoS2/graphene composite, respectively. Binding energy Eb is an another important physical factor to reflect and determine the binding properties of inserted or adsorbed systems. In this work, we employ binding energy to quantitative describe the insertion characteristic of Li for the two structures. The binding energy of Li E nLib (Ins) is defined as:35 b E nLi ( Ins ) = Entot ( Ins ) − E( n −1) tot ( Ins ) − E Li , n = 1,2

(1)

where Entot (Ins) is the total energy of n Li inserted supercell, E( n−1) tot ( Ins) and E Li are the total energy of the (n-1) Li inserted structure and Li, respectively. The calculated results are summarized in Table 1. The calculated binding energy of 1Li inserted pristine MoS2 is -2.106 eV, which is slightly lower than that of graphene composited MoS2 of -2.058 eV. This value is 2.001 eV for 2Li inserted pristine MoS2 and still lower than that of the composite. These calculated results clearly show that Li atom and the substrate of pristine MoS2 has a stronger interaction compared to MoS2/Graphene composite in both 1Li and 2Li inserted situations. Generally, it is safe and reasonable to attribute the significantly increased capacity of MG composite to a higher effective interaction area between MoS2 and electrolyte, shortened diffusion paths of Li ions, restrained formation of thick gel-like polymeric layer, or the enhanced charge carrier transport for the high conductivity of graphene. Herein, a new view point is proposed that the increased capacity is also closely related to the difference in binding energy between pristine and graphene composited MoS2. According to the definition of Eq. 1, it is accessible to us that a smaller value denotes the corresponding inserted structure is more energetically favorable and stable. This may be helpful for the intentional insertions or some kinds of performance improvements. While for LIBs system, a lower binding energy of lithiation


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

will in turn leads to a suppressed delithiation capacity. As analyzed by the above charge density difference, Bader charges and binding energies, it is undoubted that the interaction between the inserted Li and matrix for pristine MoS2 is stronger than that of graphene composited MoS2. This will cause a portion of Li fixed between the interlamination of MoS2 in the form of Li2S and finally contribute to a degenerated capacity.

Figure 6. (a) and (b) are the schematics of Li diffusion in pristine and MoS2/graphene composite based on NEB method, respectively. (c) Calculated diffusion barriers of Li for pristine MoS2 (black) and MoS2/graphene composite (red). (d) Schematic diagram of Li insertion mode for the composite structure. According to the transition state theory,36, 37 the diffusion constant is mainly determined by the diffusion barrier, and a lower diffusion energy barrier will result in a faster diffusion rate. The Li migration based on Li2S modes in pristine and graphene composited MoS2 were calculated under the same computing conditions and the diffusion modes are shown in Figure 6a and b,


22

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


respectively. The red arrow denotes the diffusion path of a Li atom from the most stable site to an another equivalent site. Figure 6c shows the calculated Li migration barriers for layered pristine MoS2 and MoS2/graphene composite based on experiment data. It can be seen that in pristine MoS2 host, the calculated diffusion barrier for Li migration is about 0.91 eV, whereas for hybrid MoS2/graphene structure, the diffusion barrier decreases to 0.73 eV. Generally, diffusion coefficient can be expressed as: D ∝ exp(− E barrier / k B T ) , where kB is the Boltzmann constant and T is the temperature. The energy barrier decrease of 0.18 eV would result in the increase of diffusivity by a factor of 103 at room temperature. Therefore, it can draw a conclusion that the Li migration rate in the hybrid MoS2/graphene structure is significantly enhanced.38 The enhanced diffusivity of Li in the layered MoS2/graphene structure makes it attractive as a promising anode material with high rate capability. In brief, the significantly enhanced electrochemical performances are attributed to its unique construction and morphology. The increased nanoflowers of MG composite can efficiently shorten the diffusion distance of Li ions, the expanded interlayer distance further promotes intercalation process and alleviates the strain induced by the intercalation of Li ions. First-principles calculation implies that the combination with graphene can effectively lower the diffusion barriers of Li and enhance the diffusivity. Also, the synergistic effect between MoS2 and graphene can greatly improve the conductivity of the composite structure and is beneficial to the corresponding electrochemical processes. In this work, the enhanced conductivity can greatly contribute to a higher LIBs performance of the MG composite in comparison with pristine MoS2 as discussed above. We attribute the optimized electrical conductivity of MG structure to the synergetic effect between rGO and MoS2. Figure 6d displays Li inserted mode for MoS2/graphene composite structure. The selective growth of highly dispersed MoS2 on rGO proved by SEM and TEM are free of aggregation and


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

the electrical coupling to the graphene can make rapid electron transport from MG to the electrode. The increased diffusivity of Li in composite structure also contributes to a better rate performance of LIBs. 4. Conclusion

In summary, 3D hierarchical MG composite with abundant nanopetals was successfully synthesized by a simple hydrothermal route. The structure and morphology of the corresponding samples were characterized by SEM, TEM, HRTEM, XRD, Raman spectra, and XPS. Structural characterizations showed that the MG composite was composed of a few-layered MoS2 nanopetals with an expanded interlayer spacing. Electrochemical measurements demonstrated that the MG composite could deliver a reversible capacity of about 1150 mA/g at a current density of 100 mA/g over 60 cycles. Even at a high current density of 1000 mA/g, the specific capacity is retained ~890 mAh/g for 10 cycles. As a comparison, the C-MoS2 and pristine MoS2 displayed lower specific capacities, poorer cyclic stabilities and worse rate performances. In this work, the MG composite exhibited excellent electrochemical performance with advantages of 2dimensional conductive rGO and effective suppression of the MoS2 degeneration during lithiation/delithiation processes. Besides, the increased layer distance between neighbouring MoS2 layers provide sufficient space for Li ions intercalation, a lower diffusion energy barrier of Li also facilitate a faster diffusion processes in the MoS2 host, leading to an enhanced chargedischarge reversibility.

Although the synthesized MG composite exhibits excellent

electrochemical performance for use in LIBs and detailed theoretical studies have been made, there is still research space for the study of MoS2 based materials. In order to analyse the corresponding diffusion behaviour of more Li ions, molecular dynamics and/or Monte Carlo methodology should be used for a further study.


24

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Corresponding Author

Corresponding author. Tel.: +86 21 54345198; E-mail address: [email protected] (Ke Yu). ACKNOWLEDGMENT

The authors acknowledge financial support from the NSF of China (Grant No. 61274014, 61474043, 61425004) and Innovation Research Project of Shanghai Education Commission (Grant No. 13zz033). REFERENCES 1

Chang, K.; Chen, W. X. L-Cysteine Assisted Synthesis of Layered MoS2/graphene Composites with Excellent Electrochemical Performances for Lithium Ion Batteries. ACS Nano 2011, 6, 4720-4728.

2

Hu, L. R.; Ren, Y. M.; Yang, H. X.; Xu, Q. Fabrication of 3D Hierarchical MoS2/Polyaniline and MoS2/C Architectures for Lithium-ion Battery Applications. ACS Appl. Mater. Interfaces 2014, 6, 14644-14652.

3

Liu, Y. C.; Zhao, Y. P.; Jiao, L. F.; Chen, J. A Graphene-like MoS2/graphene Nanocomposite as a High performance Anode for Lithium Ion Batteries. J. Mater. Chem. A, 2014, 2, 13109–13115.

4

Grilc, M.; Likozar, B.; Levec. J. Hydrodeoxygenation and Hydrocracking of Solvolysed Lignocellulosic Biomass by Oxide, Reduced and Sulphide Form of NiMo, Ni, Mo and Pd Catalysts. Applied Catalysis B: Environmental 2014, 150–151, 275–287.

5

Veryasov, G.; Grilc, M.; Likozar, B.; Jesih. A. Hydrodeoxygenation of Liquefied Biomass on Urchin-like MoS2. Catalysis Communications, 2014, 46, 183-186.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6

Page 26 of 31

Tyler, S.; Li, Z.; Brian, O.; David, M. Lithium Ion Battery Applications of Molybdenum Disulfide (MoS2) Nanocomposites. Energy Environ. Sci. 2014, 7, 209-231.

7

Su, L.; Jing, Y.; Zhou, Z. Li Ion Battery Materials with Core–shell Nanostructures. Nanoscale 2011, 3, 3967-3983.

8

Wang, Q.; Li, J. Facilitated Lithium Storage in MoS2 Overlayers Supported on Coaxial Carbon Nanotubes. J. Phys. Chem. C 2007, 111, 1675-1682.

9

Shyamal, K. D.; Rajesh, M.; Navaneedhakrishnan, J.; Lynden, A. A. Self-assembled MoS2–Carbon Nanostructures: Influence of Nanostructuring and Carbon on Lithium Battery Performance. J. Mater. Chem. 2012, 22, 12988-12992.

10

Qin, W.; Chen, T. Q.; Pan, L. K.; Niu, L. Y.; Hu, B. W.; Li, D. S.; Li, J. L.; Sun, Z. MoS2reduced Graphene Oxide Composites via Microwave Assisted Synthesis for Sodium Ion Battery Anode with Improved Capacity and Cycling Performance. Electrochimica Acta, 2015, 153, 55-61.

11

Gao, P. Y.; Yang, Z. X.; Liu, G. M.; Qiao, Q. D. A Facile Process was Developed to Synthesize MoS2/MWNT Composites. Ceram. Int. 2015, 41, 1921-1925.

12

Li, G. D.; Zeng, X. Y.; Zhang, T. D.; Ma, W. Y.; Lia, W. P.; Wang, M. Facile Synthesis of Hierarchical Hollow MoS2 Nanotubes as Anode Materials for High-performance Lithiumion Batteries. CrystEngComm, 2014, 16, 10754–10759.

13

Li, Y. M.; Lv, X. J.; Lu, J.; Li, J. H. Preparation of SnO2-Nanocrystal/GrapheneNanosheets Composites and Their Lithium Storage Ability. J. Phys. Chem. C 2010, 114, 21770–21774.


26

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14

Dip, K. N.; Uttam, K. S.; Devika, C.; Sagar, M.; Shaibal, K. S. Atomic Layer Deposited MoS2 as a Carbon and Binder Free Anode in Li-ion Battery. Electrochimica Acta 2014, 146, 706-713.

15

Yu, H.; Ma, C.; Ge, B.; Chen, Y.; Xu, Z.; Zhu, C.; Li, C.; Yang, Q. O.; Gao, P.; Li, J. Three-Dimensional Hierarchical Architectures Constructed by Graphene/MoS2 Nanoflake Arrays and Their Rapid Charging/Discharging Properties as Lithium-Ion Battery Anodes. Chem. Eur. J. 2013, 19, 5818-5823.

16

Zhang, K.; Kim, H. J.; Shi, X. J.; Lee, J. T.; Choi, J. M.; Song, M. S.; Jong, H. P. Graphene/Acid Coassisted Synthesis of Ultrathin MoS2 Nanosheets with Outstanding Rate Capability for a Lithium Battery Anode. Inorg. Chem. 2013, 52, 9807-9812.

17

Liu, Y. T.; Zhu, X. D.; Duan, Z. Q.; Xie, X. M. Flexible and Robust MoS2–Graphene Hybrid Paper Cross-linked by a Polymer Ligand: A High-performance Anode Material for Thin Film Lithium-ion Batteries. Chem. Commun. 2013, 49, 10305-10307.

18

Li, Y. G.; Wang, H. L.; Xie, L. M.; Liang, Y. Y.; Hong, G. S.; Dai, H. J. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296–7299.

19

Wang, Y.; Xing, G. Z.; Han, Z. J.; Shi, Y. M.; Wong, J. I.; Huang, Z. X.; Kostya, O.; Yang, H. Y. Pre-lithiation of Onion-like Carbon/MoS2 Nano-urchin Anodes for Highperformance Rechargeable Lithium Ion Batteries. Nanoscale 2014, 6, 8884–8890.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20

Page 28 of 31

Liang, T.; Gregory, S. W.; Scott, S. P.; Susan, B. S.; Simon, R. P. Energetics of Oxidation in MoS2 Nanoparticles by Density Functional Theory. J. Phys. Chem. C 2011, 115, 1060610616.

21

Kan, M.; Wang, J. Y.; Li, X. W.; Zhagn, S. H.; Li, Y. W.; Kawazoe, Y.; Sun, Q.; Jena, P. Structures and Phase Transition of a MoS2 Monolayer. J. Phys. Chem. C 2014, 118, 15151522.

22

Henkelman, G.; Uberuaga, B. P.; Jonsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901-9904.

23

Olsen, R. A.; Kroes, G. J.; Henkelman. Comparison of Methods for Finding Saddle Points without Knowledge of The Final States. J. Chem. Phys. 2004, 121, 9776-9792.

24

Chang, K.; Chen, W. X. In Situ Synthesis of MoS2/graphene Nanosheet Composites with Extraordinarily High Electrochemical Performance for Lithium Ion Batteries. Chem. Commun. 2011, 47, 4252-4254.

25

Li, Q.; Walter, E. C.; van der Veer, W. E.; Murray, B. J.; Newberg, J. T.; Bohannan, E. W.; Switzer, J. A.; Hemminger, J. C.; Penner, R. M. Molybdenum Disulfide Nanowires and Nanoribbons by Electrochemical/Chemical Synthesis. J. Phys. Chem. B 2005, 109, 3169-3182.

26

Plechinger, G.; Heydrich, S.; Eroms, J.; Weiss, D.; Schuller, C.; Korn, T. Raman Spectroscopy of the Interlayer Shear Mode in Few-layer MoS2. Appl. Phys. Lett. 2012, 101, 101906.


28

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27

Laursen, A. B.; Kegnaes, S.; Dahl, S.; Chorkendorff, I. Molybdenum Sulfides-efficient and Viable Materials for Electro-and Photoelectrocatalytic Hydrogen Evolution. Energy Environ. Sci. 2012, 5, 5577-5591.

28

Xiao, J.; Wang, X. J.; Yang, X. Q.; Xun, S. D.; Liu, G.; Koech, P. K.; Liu, J.; Lemmon, J. P.

Electrochemically

Induced

High

Capacity

Displacement

Reaction

of

PEO/MoS2/graphene Nanocomposites with Lithium. Adv. Funct. Mater. 2011, 21, 28402846. 29

Zhao, C. Y.; Kong, J. H.; Yang, L. P.; Yao, X. Y.; Phua, S. L.; Lu, X. H. The Dopamine– Movi

Complexation-assisted

Large-scale

Aqueous

Synthesis

of

a

Single-layer

MoS2/Carbon Sandwich Structure for Ultrafast, Long-life Lithium-ion Batteries. Chem. Commun. 2014, 50, 9672-9675. 30

Pham, V. H.; Kim, K. H.; Jung, D. W.; Singh, K.; Oh, E. S.; Chung, J. S. Liquid Phase Coexfoliated MoS2–Graphene Composites as Anode Materials for Lithium Ion Batteries. J. Power Sources 2013, 224, 280-286.

31

Hu, S.; Chen, W.; Zhou, J.; Yin, F.; Uchaker, E.; Zhang, Q. F.; Cao, G. Z. Preparation of Carbon Coated MoS2 Flower-like Nanostructure with Self-assembled Nanosheets as Highperformance Lithium-ion Battery Anodes. J. Mater. Chem. A 2014, 2, 7862–7872.

32

Li, W.; Tan, C.; Lowe, M. A.; Abruna, H. D.; Ralph, D. C. Electrochemistry of Individual Monolayer Graphene Sheets. ACS Nano 2011, 5, 2264-2270.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

33

Page 30 of 31

Wang, Z.; Chen, T.; Chen, W. X.; Chang, K.; Ma, L.; Huang, G. C.; Chen, D. Y..; Lee. J. Y. CTAB-assisted Synthesis of Single-layer MoS2–Graphene Composites as Anode Materials of Li-ion Batteries. J. Mater. Chem. A 2013, 1, 2202-2210.

34

Tang, W.; Sanville, E.; Henkelman, G. A Grid-based Bader Analysis Algorithm without Lattice Bias. J. Phys.: Condens. Matter 2009, 21, 084204-7.

35

Van de Wallea, C. G.; Neugebauer, J. First-principles calculation for Defects and Impurities: Applications to III-nitrides. J. Appl. Phys. 2004, 95, 3865-3870.

36

Kang, B.; Ceder, G. Battery Materials for Ultrafast Charging and Discharging. Nature 2009, 458, 190-193.

37

Vineyard, G. H. Frequency Factors and Isotope Effects in Solid State Rate Processes. J. Phys. Chem. Solids 1957, 3, 121-127.

38

Su, J. C.; Pei, Y.; Yang, Z. H.; Wang, X. Y. First-principles Investigation on Crystal, Electronic Structures and Diffusion Barriers of NaNi1/3Co1/3Mn1/3O2 for Advanced Rechargeable Na-ion Batteries. Comp. Mater. Sci. 2015, 98, 304-310.


30

Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


182x151mm (72 x 72 DPI)


Graphene Hybrid Nanoflowers with Enhanced Electrochemical

Recommend Documents