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Various Structured Molybdenum based Nanomaterials as Advanced Anode Materials for Lithium ion Batteries Zexing Wu, Wen Lei, Jie Wang, Rong Liu, Kedong Xia, Cuijuan Xuan, and Deli Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16251 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 24, 2017

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ACS Applied Materials & Interfaces

Various Structured Molybdenum based Nanomaterials as Advanced Anode Materials for Lithium ion Batteries Zexing Wu, Wen Lei, Jie Wang, Rong Liu, Kedong Xia, Cuijuan Xuan, Deli Wang*

Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P.R. China.

E-mail: [email protected].

KEYWORDS

various molybdenum compounds; few layered ultra-small structure; carbon sphere; anode materials; lithium ion battery;

ABSTRACT

A facile and scalable solvothermal high-temperature treatment strategy was developed to construct few-layered ultra-small MoS2 with less than three layers. These are embedded in carbon spheres (MoS2-C) and can be used as advanced anode material for lithium ion batteries (LIBs). In the resulting architecture, the intimate contact between MoS2 surface and carbon spheres can effectively avert aggregation and 1 ACS Paragon Plus Environment


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volume expansion of MoS2 during the lithiation–delithiation process. Moreover, it improves the structural integrity of the electrode remarkably, while the conductive carbon spheres provide quick transport of both electrons and ions within the electrode. Benefiting from this unique structure, the resulting hybrid manifests outstanding electrochemical performance, including an excellent rate capability (1085, 885, and 510 mAh g-1 at 0.5, 2, and 5 A g-1), and a superior cycling stability at high rates (maintaining 100% of the initial capacity following 500 cycles at 0.5 A g-1). Using identical methods, molybdenum carbide and phosphide supported on carbon spheres (Mo2C-C, and MoP-C) were prepared for LIBs. As a result, MoS2-C exhibits outstanding lithium storage capacities due to its specific layered structure. This study investigates large-scale production capabilities of few-layered structure ultra-small MoS2 for energy storage, and compared lithium storage performance of molybdenum compounds thoroughly.

1. Introduction

Due to an ever-increasing requirement for long-cycle life and high-performance lithium-ion batteries (LIBs), various anode materials are being extensively investigated. Current examples are: carbon1, metal oxides2-3, chalcogenides4, carbides, and phosphides5. Among these numerous anode materials, layered transition metal sulfides (e.g. Mo6, W7, and Sn8) have attracted widespread attention due to their remarkable physical and chemical properties. Layered structure of these materials is held together by van der Waals interactions between each layer (similar to graphite).

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Small Li ions possess a low charge and can easily be intercalated into and extracted from the interlayer structures without significant volume expansion9. Thus, the two dimensional (2D) layer structure of metal sulfides is considered to be a potential anode material and has received tremendous attention for next generation lithium ion batteries10-11.

As a typical 2D layer transition metal chalcogenides, molybdenum sulfide (MoS2) compounds have recently attracted extensive scientific interest due to their four-electron transfer reaction during lithiation–delithiation process. This enable the observed high theoretical capacity (669 mAh g-1) that is far superior to a graphite anode (372 mAh g-1)12. However, practical application of bulk MoS2 as an anode material is still unsatisfactory due to its low intrinsic electric conductivity and structural destruction during the lithiation–delithiation process. This has been shown to lead to dramatic electrode pulverization, subsequently resulting in unsatisfactory cycling stability, and slow MoS2 dynamics13-14. To settle these issues, tremendous strategies have been put forward to improve electronic conductivity and structural integrity of MoS2 by introducing diverse carbon matrixes forming hybrids or synthesizing a MoS2 nanostructure with various specific morphologies15-17. Numerous MoS2-carbon hybrids have been investigated as LiBs anodes, e.g. MoS2 nanosheets embedded in interconnected carbon matrixes 18, carbon nanotubes19, and graphene6. The conductive carbon metrics can not only ameliorate the conductivity of active materials but also buffer the volume expansion during the



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lithiation–delithiation process. Therefore, hybrid composites can be expected to possess excellent cycling and rate activities. Apart from combination with carbon materials, another method to buffer the strain and barrier for lithium intercalation is the preparation of few-layered or even single-layered MoS2 with a range of morphologies15. Few-layered MoS2 has been reported to perform superior to bulk MoS220, due to an increased number of channels and shortened paths for fast diffusion and insertion of Li storage21. Furthermore, reducing the size of the incorporated few layered MoS2 nanoplates is also an important avenue to enhance the lithium storage capacity22-23.

With the exception of MoS2, metal carbides and phosphides have been extensively investigated for LIBs, due to their predominant electric and thermal conductivities, excellent mechanical and chemical stability, and high theoretical capacity for lithium storage10,

24-26

. However, a performance comparison of metal

chalcogenides, carbides, and phosphides that were all considered as advanced anode materials LIBs, lithium storage has not yet been investigated. In this work, molybdenum compounds (MoS2, Mo2C and MoP) supporting on carbon spheres were prepared using an identical route, due to the fabrication way impact the properties and activities of the catalysts evidently27. Moreover, during the process of synthesis, no toxic gas was used and the target phase was easily controlled. Unexpectedly, few-layered MoS2 (below three layers) was easily produced without the assistance of surfactants or chemical vapor deposition20, 28. On contrast, the layers of MoS2 stacked seriously compared with MoS2-C (Scheme 1), demonstrating that the carbon spheres 4 ACS Paragon Plus Environment

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plays an important role on avoiding the layers aggregation. Furthermore, the synthetic production of MoP in this study was simpler compared to traditional avenues29 and Mo2C possesse nanoparticles of 3 nm size. The close surface-to-surface contact between molybdenum compounds and the carbon matrix not only effectively restrains the aggregation of molybdenum compounds and buffers the volume expansion, but also enables fast electron and ion transportation across the interface of carbon spheres and molybdenum compounds.

Scheme 1 Schematic illustration for the synthesis of MoS2-C and MoS2. 2. Experimental Methods

Sample preparation The carbon spheres supported molybdenum compounds (MoS2, Mo2C and MoP) were synthesized by solvothermal-high temperature treatment process. In a typical synthesis, 180 mg of (NH4)6Mo7O24.6H2O and 1000 mg of glucose were dissolved in the mixture of 20 mL ethylene glycol and 10 mL distilled water and stirred for 30 minutes. Afterwards, the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated in an oven at 200 oC for 10 h. Then, the products were washed by using ethanol and water several times, followed by freeze drying. Carbon 5 ACS Paragon Plus Environment


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spheres supported MoS2 (MoS2-C) and MoP (MoP-C) were obtained by mixing the solvothermal products with (NH4)2HPO4 and NH2CSNH2 with molar ratios of 1:1 (Mo:P) and 1:2 (Mo:S) sufficiently and then heated at 900 oC for 2 h under flowing H2 (8%)/Ar. Mo2C-C was obtained by carbonizing the solvothermal products directly with the same procedure. For control experiment, MoS2 was prepared using the same process in the absence of glucose.

Physical Characterization XRD was performed by X'Pert PRO diffractometer with a rate of 4o min-1. TGA was performed on TA Q-500 instrument under flowing air with a rate of 10 °C min-1. XPS were performed by an AXIS-ULTRA DLD-600W Instrument. The morphologies and microstructures were collected by SEM, Sirion200 and TEM (JEOL, Japan). STEM images were performed using an aberration-corrected Hitachi HD2700C at 200 keV. Raman spectra were collected by a LabRam HR800 spectrometer with a 532 nm laser excitation.

Electrochemical measurements Electrochemical measurements were evaluated in CR 2032 coin cells assembled in argon-filled gloved box with lithium metal as the counter electrode and Celgard 2320 membrane as the separator. The working electrode is composed of prepared material, carbon black (super-p) and polymer binder (polyvinylidenefluoride; PVDF) with a ratio of 80:10:10 (weight ratio) and select Cu-foil as the current collector. The active materials loading of all electrodes were about 1.5-2 mg cm-2. The commercial 6 ACS Paragon Plus Environment

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electrolyte containing 1 M LiPF6 in a 1:1 ratio of EC (ethylene carbonate): DEC (diethylene carbonate) was used. Galvanostatic charging and discharging between 0.05 and 3 V vs. Li+/Li of the cells were performed on a NEWARE battery tester at room temperature. Cyclic voltammetry (CV) was performed on a CHI 760E electrochemical working station at a scan rate 0.1 mV s-1.

3. Results and Discussion

XRD patterns of the prepared molybdenum compounds supported on carbon spheres (MoP-C, Mo2C-C, and MoS2-C) and pure MoS2, are illustrated in Figure 1 and Figure S1. These are in accordance with the reference patterns of MoP (JCPDS NO. 24-0771), Mo2C (JCPDS NO. 77-0720), and MoS2 (JCPDS NO. 37-1492). Corresponding diffraction peaks are labeled accordingly and no peaks of by-products were observed, indicating phase purity of the prepared materials. Figure 1a shows a strong single (002) diffraction peak at 13.8o of pure MoS2, corresponding to stacked layers of MoS2 along the c axis. However, the intensity of MoS2 decreased due to the introduction of carbon spheres. Especially, the disappearance of the (002) plane peak, indicating almost no stacking of MoS2 in the c direction even subsequent to annealing. Raman measurement is an important way to examine the layer number of MoS2. The peaks at 407 cm-1 derived from the out-of-plane Mo-S phonon mode A1g which is preferentially excited for the edge-terminated film owing to polarization dependence. However, the peak at 383 cm-1 corresponds to the in-plane Mo–S phonon mode (E12g ) and is preferentially excited by the terrace-terminated surface (Figure 1b).



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Furthermore, the frequency differences of the two typical Raman modes (A1g and E12g) are widely utilized to confirm layer numbers30 of the synthesized MoS2-C and bulk MoS2, are 23 cm-1 and 25.1 cm-1, respectively (Figure 1b). This demonstrates that the composite of MoS2-C consists of less than three layers which is in accordance with the XRD pattern 31.

Figure 1 (a) XRD patterns of bulk MoS2 and MoS2-C as well as corresponding Raman spectra (b). High resolution spectra of Mo 3d (c) and S 2p (d) in MoS2-C.

X-ray photoelectron spectroscopy (XPS) was conducted to investigate chemical composition and valence of the MoS2-C nanocomposite. The high resolution of Mo 3d in Figure 1c was divided into three categories according to the peaks, in which 226.4 eV corresponds to S 2s. The peaks located at 229.5 eV and 232.4 eV were assigned to Mo 3d5/2 and Mo 3d 3/232, respectively. The two remaining peaks at 232.9 eV and 235.7 eV (Mo6+ 3d3/2/3d5/2) were attributed to the oxidation of Mo33. For S 2p (Figure 1d), peaks at 161.8 eV and 163.1 eV indicate the presence of divalent 8 ACS Paragon Plus Environment

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sulfide ions (S2-)

34

, while peaks located at 162.2 eV and 163.5 eV reveal sulfur

oxidation. These binding energies are all in accordance with the values of MoS2 crystal.

Microstructures of the synthesized catalysts were characterized in detail via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It can be clearly seen that carbon spheres and amorphous molybdenum compounds formed subsequent to solvothermal treatment (Figure S2a, b), which is in accordance with the XRD pattern (Figure S3). No difference of the carbon spheres was observed, even after high temperature treatment (Figure S2c). Pure MoS2 is apparently composed of small nanosheets with 30-50 nm and has a well layered structure with d(002) = 0.64 nm, which agrees well with the XRD pattern. There are at least six to twelve layers bonded together via van der Waals interactions. The MoS2-C composite in Figure 2c consists of many ultra-small MoS2 (yellow dotted line) nanosheets with single to triples layers that are highly dispersed in the composite. This is in accordance with the results of XRD and Raman measurements. The interlayer spacing of the (002) planes was 0.69 nm (Figure 2c), which was larger than bulk MoS2 (0.64 nm). The existence of carbon support avoided catalysts aggregation and is suspected to enlarge the interlayer distance. However, both Mo2C-C and MoP-C are composed of nanoparticles and Mo2C possesses a relatively small diameter compared to MoP, demonstrating that the precursors exhibit significant influence on the morphology during high-temperature treatment. Mo2C nanoparticles, which are uniformly well-anchored on the carbon spheres, were about 3 nm in diameter and had a 9 ACS Paragon Plus Environment


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d-spacing of 0.23 nm, corresponding to the (012) plane of Mo2C (Figure S2 d, and e). Compared to Mo2C-C, MoP-C consisted of a larger diameter with 20-30 nm, and a corresponding lattice distance of 0.32 nm to the (001) plane (Figure S2 h, and i) was observed. The elemental composition analyses depicted in Figure 2d and Figure S2 f, 2g and 2j-l, clearly show the molybdenum compounds to be distributed homogeneously throughout the carbon spheres matrix, also verifying the existence of a carbon matrix not previously discovered in the XRD pattern. The mass loading of carbon

spheres

supported

molybdenum

compounds

was

investigated

via

thermogravimetric analysis (TGA) (see Figure S4).

Figure 2 TEM images of MoS2 (a, b), and MoS2-C (c). STEM image of MoS2-C and elemental mappings of C, Mo and S (d).

Electrochemical properties of the prepared molybdenum compounds and pure MoS2 were first investigated via cyclic voltammograms (CVs) within a potential range of 0.05-3.0 V (vs. Li+/Li) at a sweep rate of 0.1 mV s-1 (see Figure 3). There are 10 ACS Paragon Plus Environment

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two reduction peaks in the first cathodic scan of the MoS2 electrode (Figure 3a). One peak at around 1.16 V can be attributed to the intercalation of Li+ in the interlayer spacing MoS235-36 (reaction 1), inducing the phase transformation change of MoS2 from trigonal prismatic to octahedral structure37. The two irregular shoulders located at 0.8 V and 1.4 V maybe contribute to Li+ inserted the lattice of MoS238-39. The second peak at 0.58 V is characteristic for LixMoS2 to Mo and Li2S40-41 (equation 2). The complete discharge process can be ascribed to the following reactions: MoS2 + Li+ + xe- → LixMoS2 (1) LixMoS2 + (4-x) Li+ + (4-x) e- → Mo + 2Li2S (2) During the anodic scan, the oxidation peaks at 1.45 and 2.28 V related to partial oxidation of Mo to MoS2 and complete formation of MoS26. After first cycle, the dominant reduction peak at around 1.8 V was characteristic for the formation of a gel-like polymeric gel42. For MoS2-C, the peak intensity and integrated area kept well, implying an improved cycling stability relative to MoS2. Compared to MoS2, slight shifts in the reduction peak potentials or changes in the shape of the reduction peaks can be caused by the overlap of the electrochemical lithium storage in both MoS2 and carbon spheres41,

43

. A peak at 1.45 V was observed in MoS2-C not MoS2 and

corresponds to the oxidation of Mo to MoS2. This indicates that Mo was not oxidized to MoS2 in carbon free MoS2. The XPS measurement after cycling demonstrated that the electrode is composed of MoS2 and Li, which proved the transformation from Mo to MoS2. Previous reports point towards the activation energies for solid-state double decomposition reactions to increase with increasing particle size44-46. As a result, only 11 ACS Paragon Plus Environment


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one anodic peak was observed in MoS2 due to the substantial particle size leading to an irreversible reaction. The peak located at 0.36 V in MoS2 was not appeared in MoS2-C which is maybe concealed by the overlap of electrochemical lithium storage in both MoS2 and carbon spheres41. A broad and irreversible peak located at 0.68 V and 0.5 V in the first cycle (Figure 3c and d) for Mo2C-C and MoP-C10,

47

,

respectively. This may correspond to the formation of irreversible solid electrolyte

Figure 3 CV curves for MoS2-C (a), MoS2 (b), Mo2C-C (c) and MoP-C (d) electrodes at a scan rate of 0.1 mV s-1. interphase (SEI) films. However, this peak disappears during the following cycles, while an apparent redox pair located at 1.2/1.35 V can be observed for Mo2C-C, corresponding to the conversion reaction between Mo2C and Li+ (Mo2C + xLi+ + xe↔

2Mo + LixC)24,

48

. For MoP-C, the Li storage mechanism is intercalation

reactions49. The peaks located at 1.15 V/1.5 V could attribute to the reversible phase transitions of LixMoP49. Peak intensity and position remain unchanged throughout the


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following cycles, indicating a highly reversible electrochemical reaction of the electrode materials. Figure 4 present the discharge and charge profiles of the prepared molybdenum based electrode materials, conducted at a current density of 100 mA g-1 between 0.05-3.0 V (vs. Li+/Li). For MoS2 and MoS2-C, two prominent potential plateaus at 1.1 V and 0.4 V for MoS2, and 1.5 V and 0.7 V for MoS2-C, were observed in the discharge curve, suggesting a two-step lithiation process of MoS2. However, only one plateau at 2.1 V for MoS2, and two peaks at around 1.4 V and 2.2 V during the charge step correspond to the partial oxidations of Mo to MoS2 and Li2S to S, indicating that Mo in pure MoS2 cannot partially be oxidized into MoS2. The discharge and charge curves of MoS2 and MoS2-C were in accordance with the CV profiles. Initial discharge capacities of pure MoS2, MoS2-C, Mo2C-C, and MoP-C electrodes were 1010, 1630, 1570 and 1385 mAh g-1, respectively. The initial charge capacities were 835, 1200, 1110 and 748 mAh g-1, respectively. The corresponding initial Coulombic efficiencies were 82.6%, 73.6%, 70.7%, and 54%, respectively. The irreversible capacity loss may be caused by electrolyte decomposition as well as formation of a solid electrolyte interlayer (SEI) layer on the surface of the electrode materials during the discharge process2, 50. The low initial Coulombic efficiency could also be ascribed to a low lithium storage capacity and a large initial irreversible capacity loss of carbon spheres in the composites 2, which accounts for only about 46% at 0.1 A g-1 (Figure S7). Evidently, MoS2-C exhibits the highest lithiation and delithiation capacities. The volumetric energy density for MoS2-C electrode is 2389 Wh m-3 at a current density 13 ACS Paragon Plus Environment


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of 500 mA g-1 according to the equation S1. The outstanding lithium ion battery performance of MoS2-C maybe derived from its specific structure, such as few layer and ultra-small size structure, which can provide more active sites relative to nanoparticle morphologies (MoP-C and Mo2C). Mo2C-C nanomaterial possess smaller nanoparticles compared with MoP-C which have more reactive sites for lithium ions and electrode-electrolyte interface, thus Mo2C-C exhibit much more li-ion capacity relative to MoP-C.

Figure 4 Discharge and charge profiles of MoS2-C (a), MoS2 (b), Mo2C-C (c) and MoP-C (d) at a current density of 0.1 A g-1. Long term cycling performance of MoS2 and carbon sphere supported molybdenum compound electrodes at a current density of 100 mA g-1 are shown in Figure 5a. The capacity of MoS2 significantly decayed and retained only 100 mAh g-1 after 100 cycles. In a stark contrast, MoS2-C, Mo2C-C, and MoP-C displayed highly improved capacities of 1067 mAh g-1, 762 mAh g-1, and 575 mAh g-1 after 100 cycles, retaining 90.8%, 69.7%, and 67.6% of their initial capacities, respectively. As 14 ACS Paragon Plus Environment

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reported in previous studies51, the combination of a carbon matrix and molybdenum compounds resulted in significant effects on conductivity, dispersibility of both components, and theoretical capacity of the electrode, which further synergistically affects final lithium storage. It is worth noting, that the cycling performance of MoS2-C is superior to that of Mo2C-C and MoP-C, due to its specific structure. Furthermore, Coulombic efficiency for all samples rapidly increases from relatively low efficiency for the first cycle to above 90% after two cycles, stabilizing at nearly 100% thereafter, indicating fast ion and electron transfer in the prepared electrodes.

Figure 5 Cycling performance of MoS2-C, Mo2C-C, MoP-C, and MoS2 electrodes and corresponding Coulombic efficiency at a current density of 0.1 A g-1 (a). Rate performance of MoS2-C and MoS2 electrodes at various current densities ranging from 0.1 to 5 A g-1 (b). Cycling performances of MoS2-C, Mo2C-C, and MoP-C as well as corresponding Columbic efficiency at a current density of 0.5 A g-1 (c).



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Figure 5b and Figure S 6a, and b show the rate performance of MoS2 and carbon sphere supported molybdenum compound electrodes at various current densities ranging from 0.1 to 5 A g-1. The MoS2-C electrode exhibits an exceptionally high rate capability and the discharge capacities of 880 and 508 mAh g-1 at rather high current densities of 2 and 5 A g-1, respectively are especially striking. Compared to pure MoS2, Mo2C-C, and MoP-C (Figure S 6a, and b), MoS2-C possess the highest capacity and rate performance. Notably, when the current density returned to 0.1 A g-1, after cycling at various rates, the capacity recovered up to 1170 mAh g-1, further implying a stable structure and excellent reversibility of the MoS2-C electrode. The few-layered ultra-small MoS2-C electrode herein possess long term cycling performance even at a high current density of 500 mA g-1 (Figure 5c), which is superior to both Mo2C-C and MoP-C. The flexible feature of MoS2 layers enhanced the robustness of the electrode structure endows the prepared MoS2-C exhibits excellent durability21. TEM measurements were conducted to investigate the morphologies changes of molybdenum compounds after cycling. As shown in Figure S8a, the layered structure of MoS2-C kept well after 500 cycles, which can illustrate the satisfactory stability of the prepared nanomaterials. For MoP-C (Figure S8b), the nanoparticles size of MoP become smaller after cycling, inducing the lower Coulombic efficiency of the initial several cycles52. And then the morphology can keep unchanged in the subsequent cycles which makes MoP-C possess excellent durability in the following cycles. The size of Mo2C-C nanoparticles changed not evidently before and after 500 cycles which endows the outstanding stability (Figure 16 ACS Paragon Plus Environment

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S8c). Obviously, the layers of MoS2 aggregated seriously even after 100 cycles at a low current density, demonstrating the important effects of carbon spheres on enhancing the li-ion capacity and stability (Figure S8d). XPS was conducted on MoS2-C after 100 cycles (Figure S5), the binding energy at 54.8 eV in high resolution of Li 1s is corresponding to metal Li53. For Mo 3d, the binding energy at 229.1 eV and 232.1 eV are corresponding to Mo 3d3/2 and Mo 3d5/2 of Mo4+ 54. Meanwhile, the binding energy at 226.4 eV is corresponding to S2s. As discussed above, the electrode was mainly composed of metal Li and MoS2 after cycling which demonstrating the reversible reactions. Capacity slightly decayed during initial cycles, but increased gradually thereafter, especially that of MoS2-C. This is a common phenomenon for anode materials55-56 and all compounds exhibited a Coulombic efficiency of nearly 100%.

To elucidate the relationship between electrochemical performance and electrode kinetics of various cells, electrochemical impedance spectroscopy (EIS) was performed. Figure S9 depicts Nyquist plots of bulk MoS2 and carbon spheres supported molybdenum compounds as electrode materials. It is very clear that all electrodes share a common character with a depressed high frequency semicircle (Rct) and a corresponding linear tail in the low frequency region. For an equivalent circuit model and the corresponding parameters, see Figure S9. The semicircle diameter for the MoS2-C electrode (Rct=158 Ω) was much smaller than MoS2 (Rct=218 Ω), indicating improved charge transfer performance for the MoS2-C electrode as well as excellent conductivity. The electrical conductivity of the composites was notably 17 ACS Paragon Plus Environment


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improved by adding a matrix of carbon spheres, which is likely the reason for the outstanding cycling performance of the MoS2-C electrode. Evidently, the diameter of the semicircle for the MoS2-C electrode (Rct=123 Ω) decreased after cycling and compared to the fresh cell, indicating a decreasing charge transfer resistance after five cycles. For different molybdenum compounds, both Mo2C-C (Rct=98 Ω) and MoP-C (Rct=83 Ω) possess much smaller resistance values compared to the MoS2-C electrode (Rct=158 Ω) (Figure 6), indicating Mo2C-C and MoP-C electrodes to exhibit better charge transfer performance as well as excellent conductivity. However, lithium storage performance of MoS2-C was superior to Mo2C-C and MoP-C, indicating that resistance was not the key parameter for carbon sphere supported molybdenum compounds. As already discussed, molybdenum compounds exhibited various morphologies and MoS2-C possessed a few-layered structure, beneficial for lithium storage. However, Mo2C-C and MoP-C were composed of nanoparticles and Mo2C had a much smaller diameter compared to MoP. Therefore, the variable morphology might mainly affect the lithium storage difference of the different molybdenum compounds. The layered structure of MoS2 not only functions as an attractive host for lithium intercalation, but also converts into Mo and Li2S with an additional and substantial gain in capacity.


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Figure 6 Nyquist plots of Mo2C-C, MoS2-C and MoP-C. 4. Conclusions

In summary, a simple and scalable strategy was developed to prepare carbon spheres supported molybdenum compounds for lithium ion storage applications. Among the tested compounds, MoS2-C possess the most predominant electrochemical performance. A capacity of 1067 mAh g-1 can be reserved after 100 cycles at a current of 100 mA g-1. When cycled at 2 and 5 A g-1, favorable capacities of 880 and 508 mAh g-1 could be achieved for MoS2-C, exhibiting superior rate capacity. Furthermore, a reversible capacity of 1070 mAh g-1 can be retained during 500 cycles at a current of 500 mA g-1 for MoS2-C, indicating excellent cycling stability. This outstanding electrochemical performance can be explained twofold: (1) the carbon spheres could avoid aggregation of MoS2 during high temperature treatment, maintaining close contact between carbon spheres and MoS2 nanoplates, enhancing the conductivity of the electrode. (2) The prepared MoS2-C exhibits a few-layered structure and ultra-small morphology that can avert the structural destruction during the lithiation–delithiation process, shorten the lithium ion diffusion route, and reduce electroconductive resistance. This simple synthetic strategy provides insights for 19 ACS Paragon Plus Environment


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future mass-production of few-layered ultra-small metal sulfides for large scale battery applications and for the selection of viable metal compounds for lithium storage applications. The lithium storage capacity comparison of various molybdenum based nanomaterials will guide the investigation of molybdenum compounds on energy storage application.

ASSOCIATED CONTENT

Supporting Information.

Detailed additional SEM and TEM images, electrochemical performance. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation (21306060, 21573083), the Program for New Century Excellent Talents in Universities of China (NCET-13-0237), the Doctoral Fund of Ministry of Education of China (20130142120039), 1000 Young Talent (to Deli Wang), and initiatory financial 20 ACS Paragon Plus Environment

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support from Huazhong University of Science and Technology (HUST). The authors thank the Analytical and Testing Center of HUST for allowing use its facilities.

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