Facilitated Lithium Storage in MoS2 Overlayers Supported on

redox process is about −247 kcal/mol at 240 °C, which is given by HSC chemistry). ...... Konda Shiva , H.S.S. Ramakrishna Matte , H.B. Rajendra...
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J. Phys. Chem. C 2007, 111, 1675-1682

1675

Facilitated Lithium Storage in MoS2 Overlayers Supported on Coaxial Carbon Nanotubes Qiang Wang†,‡ and Jinghong Li*,† Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Tsinghua UniVersity, Beijing 100084, China, and Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, China ReceiVed: October 10, 2006; In Final Form: NoVember 14, 2006

The discoveries of carbon and inorganic fullerene-like nanotubes with a wide spectra of possible applications have stimulated multi- and interdisciplinary research activities. In this paper, we prepared MoS2 overlayers supported on coaxial carbon nanotubes and investigated lithium storage/release properties in relation to their structural properties. The coaxial nanoarchitecture was successfully synthesized by a designed solution-phase route in the low temperature range, which was characterized by X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The reversible lithium-storage behaviors involved in the nanoarchitecture were elucidated by means of various techniques including galvanostatic methods, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). A thorough investigation of the composition-structure-property relationships of the coaxial nanoarchitecture highlighted the importance of the underlying carbon nanotubes in improving the lithium storage/release properties of the MoS2 sheath through a unique synergy at the nanoscale. This work should be notably significant for the design of new multifunctional nanoarchitectures by the wet-chemistry process, applicable for energy conversion and storage of the future.

Introduction In response to the increasing demands for energy conversion and storage worldwide, considerable efforts in this aspect have to be directly connected with a challenging search for new material concepts as well as multidisciplinary approaches.1-7 In particular, various unique properties of carbon nanotubes (CNTs) associated with their small dimensions, high anisotropy, and intriguing tube-like structures have prompted many researchers to design and fabricate semiconductor/carbon nanotube nanocomposites for energy conversion and storage devices. For example, the high surface area along the carbon nanotube surface facilitates impregnation of metal nanoparticles quite effectively. By making use of this feature, carbon nanotube-supported metal catalysts with very small particle sizes have exhibited very high activities for electrocatalytic applications in fuel cells devices.8,9 Another interesting property of carbon nanotubes is their optical response and their utilization in light energy conversion devices. The modification of carbon nanotubes with semiconductor nanoparticles might provide a new nanostructure for use in photocurrent generation with a high photoconversion efficiency.10-12 Recently, to utilize both advantages of double-layer capacitance and pseudo-capacitance, metal oxide/carbon nanotube composite electrodes for supercapacitor applications have been introduced due to their enhanced stability and high conductivity.13,14 Alternatively, efforts have been devoted to encapsulating high capacity lithium-storage compounds within the hollow cavity of carbon nanotubes by using an infiltration technique, which could significantly alter the properties of carbon nanotubes and the filler material.15,16 * Corresponding author. Tel.: +86 10 6279 5290; fax: +86 10 62771149; e-mail: [email protected]. † Tsinghua University. ‡ University of Science and Technology of China.

Similarly to graphite, inorganic fullerene-like (IF) materials such as WS2 and MoS2 also feature a layered structure, in which the atoms are covalently bonded to form two-dimensional (2D) layers that are stacked together through weak van der Waals interactions.17 It is not surprising, therefore, that the twodimensional sheets can analogously be rolled up along specific directions to form tubular structures.18 The discoveries of inorganic fullerene-like nanotubes have opened a challenging new field and have further offered an extensive range of potential applications.17-26 Recently, Chen et al. have demonstrated that the characteristic structures of IF nanotubes such as TiS2 and MoS2 make them promising materials for hydrogenstorage systems, although it is difficult at the present stage to claim special insight into the mechanism of hydrogen adsorption.22-24 In addition, Dominko et al. have found that MoS2 nanotube bundles can controllably store and release relatively large amounts of lithium ions in their one-dimensional (1D) channels, and can form the basis for a suitable and safe electrode material for lithium-storage systems.25,26 However, such excitement needs to be tempered because ensuring fast and reversible lithium storage is not an easy task. Concerning the characteristics of such reactions, the fundamental issues are rooted in both the relief of the stress induced by cycling and in the establishment of a percolating electronic conducting pathway through the electrode.1-4 By taking advantage of carbon nanotubes, it is our fundamental design consideration if we are able to prepare coaxial nanoarchitectures, in which carbon nanotubes are sheathed within IF nanotubes. It is believed that the nanometersized multifunctional heterostructures, created through carefully assembling multiple nanotubes, may work synergistically to deliver both high capacity and good cyclability. In principle, exploiting on the nanoarchitectures with enhanced functionality could lay the foundation for a novel class of highly performing batteries and would signify an innovative concept to energy

10.1021/jp066655p CCC: $37.00 © 2007 American Chemical Society Published on Web 01/04/2007

1676 J. Phys. Chem. C, Vol. 111, No. 4, 2007 conversion and storage fields as a result of their shape-specific and quantum size effects. Despite this significant potential, a major challenge in the synthesis of the coaxial nanoarchitectures is encountered when components are assembled at the nanoscale. Recently, some developments in the generation of single- and multilayered MS2coated carbon nanotubes (M ) Nb, Re, and W) by a templateassisted growth technique have been reported.27-30 However, it is noteworthy that a characteristic feature of the previous method is that high reaction temperatures (>900 °C) or large activation energies are needed to overcome the activation barrier associated with the bending of the otherwise flat twodimensional layers. In addition, these studies have mainly focused on establishing synthetic strategies and characterization of the composite systems. To our knowledge, only one study has been reported on the assembly of CNT/MoS2 duplex nanotubes by the solution-phase route, but no function or property of the system was addressed.31 In the present study, we have provided an alternative solutionphase route to synthesize MoS2 overlayers supported on coaxial carbon nanotubes at low temperatures. As an example of the potential applications, the reversible lithium-storage behaviors of the nanoarchitecture have been investigated. These favorable results suggest that coaxial tubular nanoarchitectures offer a promising direction for developing hybrid nanoarchitectures with enhanced properties through the cooperative contribution of each component. Experimental Procedures Synthesis. High-purity multiwalled carbon nanotubes were produced by a catalytic chemical vapor deposition method and subsequently annealed at vacuum pressures between 10-3 and 10 Pa and temperatures between 1500 and 2150 °C.32 In a typical coating synthesis, 2 mmol of Na2MoO4 and 6 mmol of CH3CSNH2 were added into 30 mL of distilled water. Then, 10 mol/L HCl was dropped into the solution while stirring to adjust the pH value to less than 1, and then 50 mg of purified carbon nanotubes was loaded into the solution. After ultrasonic dispersion for about 30 min, the solution was transferred into a Teflon-lined stainless steel autoclave and heated at 240 °C for 12-36 h. After cooling to room temperature, the resulting precipitates were collected by filtration, rinsed with copious amounts of distilled water, and further heat-treated at 400 °C for 2 h in a flowing argon gas atmosphere (99.99% purity). Control experimental results indicated that the reaction temperature plays a crucial role in the formation of MoS2-coated carbon nanotubes: the products prepared at temperatures ranging from 180 to 220 °C were mainly composed of MoS2 nanoparticles and bare carbon nanotubes; when the reaction temperature increased to 240 °C, a large quantity of MoS2-coated carbon nanotubes was obtained. The initial product obtained by the solution-phase reactions exhibited low and broad diffraction peaks. Annealing had some effect on the crystal growth; thus, the crystallinity of the products could be greatly increased by heating treatment. Thus, we took the product after annealing as an example for characterization. Characterization. Power X-ray diffraction (XRD) was performed on a Bruker D8-Advance X-ray powder diffractometer with monochromatized Cu KR radiation (λ ) 1.5418 Å). The 2θ range used in the measurements was from 10 to 70°. High resolution transmission electron microscopy (HRTEM) images were taken with a JEOL-2010F transmission electron microscope using an accelerating voltage of 200 kV. Raman spectra were taken under ambient conditions by using a RM

Wang and Li 2000 microscope confocal Raman spectrometer (Renishaw PLC.). The spectrometer used the 632.8 nm line of a He-Ne laser at 17 mW of power, which was focused over the specimen on the order of a 20 µm size with on-axis illumination and 90° backscattered Raman light collection. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab 220iXL electron spectrometer from VG Scientific using 300W Al KR radiation. The base pressure was about 3 × 10-9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. Electrochemical Measurements. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out using three-electrode cells with lithium metal as the counter and reference electrodes. Galvanostatic measurements were carried out using two-electrode cells with lithium metal as the counter electrode. The working electrodes were fabricated by compressing the mixture of 90 wt % active materials and 10 wt % ploytetrafluoroethylene (PTFE) onto an aluminum foil. The pellets of the same mass were dried in vacuum at 100 °C for at least 8 h and then assembled as cells in an Ar-filled Labconco glovebox. The electrolyte solution was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) with a volume ratio of EC/DMC/DEC ) 1:1:1. Cyclic voltammograms (CVs) were recorded from 3.0 and 0.0 V at various scan rates, using a CHI 802B electrochemical workstation (CHI Inc.). Voltage-capacity plots were recorded from 3.0 and 0.1 V at a current density of 0.6 mA/cm-2 using a Roofer Battery Tester. Electrochemical impedance measurements were carried out on a PARSTAT 2273 Potentiostat/galvanostat (Advanced Measurement Technology Inc.). The Nyquist plots were recorded potentiostatically by applying an AC voltage of 5 mV amplitude in the 100 kHz- to 10 mHz frequency range. All measurements were carried out at room temperature. Results and Discussion HRTEM observations revealed that the whole outer surfaces of our carbon nanotubes have been continuously and unevenly sheathed with darker fringe contrasts than the carbon layers due to stronger scattering by Mo and S atoms. The coaxially grown coating layers typically consist of -one to five walls parallel to the basal planes of the carbon nanotube matrix. The variation of the wall number was frequently observed and presumably determined by the local concentration of H2S (mentioned later). Figure 1a-c shows the HRTEM images of the intensely dark overlayers grown around the outer surface of the carbon nanotubes. The adjacent coating layers are separated by ca. 6.2 Å, and the value matches the interlayer distance in ordinary layered hexagonal MoS2 where each Mo atom layer is sandwiched between two S atom layers.17 The inner carbon layer separation is ca. 3.4 Å, identical to the separation in bare carbon nanotubes. The selected area electron diffraction (SAED) pattern, characterized by the partly overlapped reflections derived from the tubular sheath and core, is shown in Figure 1d. A set of characteristic reflections was clearly observed, which matches the diffraction pattern for the hexagonal MoS2 structure.33 The characteristic reflections, which belong to carbon nanotubes, are relatively weak due to the well-structured morphology of the tubular sheath and have a larger spacing than those of the sheath due to smaller layer separations. According to the energy-dispersive X-ray spectrum (EDX) analysis in Figure 1e, it is clear that except for Mo and S (as well as C, which presumably arises from the central carbon nanotubes), no other atoms such as O and Na exist. At this time, we are

Facilitated Lithium Storage in MoS2 Overlayers

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Figure 2. XRD patterns of (a) tubular MoS2-sheath/carbon-core nanoarchitecture and (b) bare carbon nanotubes.

Figure 1. (a-c) Typical HRTEM images of tubular MoS2-sheath/ carbon-core nanoarchitecture. (d) Typical SAED pattern. (e) EDX spectrum.

Figure 3. Raman spectra of (a) tubular MoS2-sheath/carbon-core nanoarchitecture and (b) bare carbon nanotubes.

unable to evaluate the Mo/S ratio accurately due to the overlap of Mo (L level) and S (K level) at ca. 2.3 keV. However, this is supported by the following XPS results, which provide direct evidence to conclude that carbon nanotubes are actually sheathed within MoS2 layers. Although we do not have more details to elucidate the growth mechanism of the coaxial nanoarchitecture unambiguously at this stage, the previously mentioned results allow us to suggest a possible process responsible for this formation. As we know, at temperatures above 150 °C, CH3CSNH2, as a sulfurization reagent, easily decomposes and thus generates H2S based on the reaction: CH3CSNH2 + 2H2O f CH3COONH4 + H2S, while Mo (VI) can be easily reduced in solution by H2S.34 Therefore, one can reasonably conclude that it was an oxidation-reduction process that should be responsible for the formation of MoS2: 4Na2MoO4 + 9H2S + 6HCl f 4MoS2 + Na2SO4 + 12H2O + 6NaCl (the ∆rGm for the redox process is about -247 kcal/mol at 240 °C, which is given by HSC chemistry). The resulting MoS2 may deposit easily on the outer surfaces of the carbon nanotube matrix owing to the nearly identical crystal structures of the graphite and layered MoS2, resulting in the formation of a tubular core/shell nanoarchitecture (synthesis of MoS2 in the absence of carbon nanotubes can be found in Figures S1 and S2 of the Supporting Information). The number of MoS2 walls formed on a particular carbon nanotube segment is presumably determined by the local concentration of H2S. As the H2S concentration along the direction of the carbon nanotube template presumably decreased, the amount of H2S was insufficient for a multiwalled structure, which has to do with an assembly of piled-up carbon nanotubes cutting off each other’s access to the solution. Consequently, the MoS2 tube fades away through intermediate double- and

single-walled stages. It is reasonable to expect that this strategy can be extended to the preparation of other coaxial layered metal dichalcogenide/carbon nanotubes by the introduction of appropriate precursors into the reaction. In addition to HRTEM, further information regarding the composition and structure was obtained from XRD and Raman spectra analysis. For comparison, we have also plotted the patterns from bare carbon nanotubes. Figure 2 shows the XRD pattern of the coaxial nanoarchitecture. All MoS2-related reflections in Figure 2a are consistent with the previous HRTEM observations and indexed to hexagonal MoS2 structure with lattice constants a ) 3.161 Å and c ) 12.299 Å (JCPDS card no. 37-1492). The strong [002] peak can be clearly seen, which is indicative of a well-stacked layered structure and high crystallinity of the MoS2 sheath.21 As expected, the CNTs-related reflections mostly overlap with those of the MoS2 sheath, and their intensity becomes significantly weak. The only reflection centered at ca. 26.4° corresponds to the [002] peak, similar to that of bare carbon nanotubes, which indicates that the MoS2 coating does not lead to much distortion of the carbon nanotubes. Raman experiments were carried out using 632.8 nm excitation. As shown in Figure 3, some bands ranging from 200 to 700 cm-1 for the coaxial nanoarchitecture are nearly identical with those for the carbon-free MoS2 structure in Figure S2, corresponding well to the symmetry modes reported previously.35 It is well-known that the Raman spectra of carbon nanotubes consist of a G band (in-plane stretching, E2g mode) and a D band (the disordering or an amorphous C residue). Analogous to that found in the WS2-coated carbon nanotubes,28 the coaxial nanoarchitecture also shows significantly reduced G and D bands as compared to bare carbon nanotubes. Reasonably, this fact is

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Figure 4. (a) Wide survey XPS spectrum of tubular MoS2-sheath/carbon-core nanoarchitecture. (b) Mo 3d spectrum. (c) S 2p spectrum. (d) C 1s spectrum.

supposed to highlight that the lattice vibrations of carbon nanotubes, prominently related to the variations of the electronic structure through electron-phonon coupling, are largely constrained by the particular surrounding of the MoS2 sheath. On the basis of the previous suggestions, it is possible to state that the depressed intensity of the D and G peaks assigned to the inner tubes can be ascribed to the loss of resonance conditions possibly caused by a screening effect of the outer shell. The previously mentioned XRD results may be another reflection of this phenomenon. Further evidence for the oxidation states and composition was provided from XPS analysis. The wideangle XPS (Figure 4a) of the coaxial nanoarchitecture shows the predominant presence of C (74.9 at %), Mo (6.8 at %), and S (18.2 at %). No obvious peaks for other elements or impurities were observed in the survey spectrum. Figure 4b,c shows welldefined spin-coupled Mo and S doublets at the almost same binding energies as those for hexagonal MoS2 structure.34,35 In Figure 4b, there are two strong peaks at ca. 229.4 and 232.5 eV, which can be attributed to Mo3d5/2 and Mo3d3/2 binding energies, respectively. In Figure 4c, the peaks at ca. 162.2 and 163.0 eV can be indexed to S2p3/2 and S2p1/2 binding energies, respectively. Furthermore, the quantification of the peaks gives an S/Mo atomic ratio of 1.97, which is very close to the stoichiometrical MoS2. Figure 4d shows the C1s spectra of coated carbon nanotubes (upper spectrum) and bare carbon nanotubes (lower spectrum). There is almost no difference in shape between the two spectra, implying the absence of surface sulfur groups on the coated carbon nanotubes. The peak at ca. 284.8 eV arises from the graphite-like carbon atoms of the carbon nanotube walls.36 Tarascon et al. recently demonstrated the surprising reversible reactivity of transition metal oxides versus lithium based on a conversion process reaction. Such conversion reactions turn out not to be specific to oxides but can be extended to sulfides,

nitrides, or fluorides and offer numerous opportunities to lead to impressive capacity gains. Another attractive aspect is that the potential of these conversion reactions depends on the ionocovalence of the M-X (X ) O, S, N, or P) bonding and therefore can be tuned into a continuous line from 0 to 3.5 V by changing the nature of the anion X.1-4 In light of such remarks, the electrochemical behavior of transition metal sulfides toward lithium is of prime fundamental interest. In spite of their attractive capacity, none of these sulfides shows good conversion reaction kinetics and capacity retention over the studied voltage range (0-3.5 V).37-39 We believe that sulfide-coated carbon nanotubes will pave the way for their kinetic concerns and, hence, provide a solution to the practical application of such conversion reactions. Cyclic voltammetry has been conducted to confirm the reversibility of lithium storage/release involved in the coaxial nanoarchitecture. As presented in Figure 5a, lithium storage in the first cycle proceeds though successive steps deduced from two well-defined cathodic peaks centered at ca. 0.8 and 0.4 V. However, subsequent lithium release shows only one anodic peak located at ca. 2.4 V. As a consequence, the cathodic peak at ca. 0.4 V almost disappears in the second cycle, while, instead, one new cathodic peak at ca. 1.8 V appears. The formation of a gel-like polymeric layer resulting from electrochemically driven electrolyte degradation (mentioned later), which is a phenomenon also observed in other systems operating through conversion reactions, should be mainly responsible for the change in the first two cyclic voltammograms. From the second cycle onward, it exhibits an excellent reversibility and cyclability evidenced from the almost overlapped lithium storage/release curves. Another excellent property of the coaxial nanoarchitecture is the high rate capability. Results are shown in Figure 5b, in which the rate of 0.5 mV/s was first employed, and after 5 cycles, the rate was increased in stages to 6 mV/s. At higher rates, the shape of the

Facilitated Lithium Storage in MoS2 Overlayers

Figure 5. (a) Cyclic voltammograms of tubular MoS2-sheath/carboncore nanoarchitecture at a sweep rate of 0.5 mV/s. (b) Steady-state cyclic voltammograms of tubular MoS2-sheath/carbon-core nanoarchitecture at various sweep rates. Arrows indicating direction of increasing sweep rate, 0.5, 1, 2, 4, and 6 mV/s. Anodic peaks: negative currents and cathodic peaks: positive currents.

curve was still satisfactory, which indicates quick dynamics of lithium storage/release in a reversible manner. As far as MoS2 is concerned (cyclic voltammograms of MoS2 synthesized in the absence of carbon nanotubes can be found in Figure S3 of the Supporting Information), such an impressive high rate capability has never been observed, which is attributed partly to the high compositional and structural stability after the first two cycles and partly to the transport advantages for both electrons and lithium ions within this special nanoarchitecture. The voltage-capacity plots for the coaxial nanoarchitecture in the initial two cycles at a relatively high current density of 0.6 mA/cm-2 are presented in Figure 6a. By comparing these results with those of MoS2 synthesized without carbon nanotubes (see Figure S4 of the Supporting Information), it is evident that the nanoarchitecture undergoes lithium storage/release evolution totally similar to that of MoS2 but apparently different from that observed with bare carbon nanotubes (see Figure S5 of the Supporting Information). According to the previous XRD and Raman results, it can be reasonably concluded that such behavior is attributable to the masking effect from the MoS2 sheath, which greatly suppresses the lithium storage/release characteristics exhibited by the underlying carbon nanotubes owing to the strict thermodynamic and kinetic constraints especially in the case of a high rate (the carbon nanotube core only amounts to ca. 13% of the composite weight). For illustrative convenience, it may be assumed that the MoS2 sheath makes the dominant contribution to the lithium storage/release, while the underlying carbon nanotubes play a prominent role in the electronics, thermodynamics, and kinetics. In agreement with the previous

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1679 cyclic voltammetry results, two well-pronounced lithium-storage plateaus can be observed in the first cycle: the short plateau at ca. 0.9 V can be attributed to a structure modification of MoS2 from trigonal prismatic to octahedral coordination, which is driven by a lowering of the electronic energy for the octahedral structure when electrons are donated from lithium to MoS2,40 while the long plateau at ca. 0.6 V can be attributed to a conversion reaction process, which first entails the in situ decomposition of MoS2 into Mo nanoparticles embedded into a Li2S matrix and then the formation of a gel-like polymeric layer resulting from electrochemically driven electrolyte degradation. The reversible formation of Li2S at the nanoscale based on the reaction: MoS2 + 4Li S Mo + 2Li2S, accompanying the redox of Mo nanoparticles and the reversible growth of the gel-like layer, should be the plausible reason for the reversible capacity and the sloping voltage regions in the subsequent cycles. Similar conversion processes attributed to a reversible formation of a lithium-bearing solid-electrolyte interface involving oxides, nitrides, fluorides, phosphides, and sulfites have already been established.1-4 The polymeric layer, resulting from kinetically activated electrolyte degradation, was shown to form from the Li electrochemical reduction of numerous binary phases, differentiating either by the nature of the 3d metal (Ni, Fe, and Co) or that of the anion (S, F, N, and O). Figure 6b shows the variation of lithium-storage capacity as a function of cycle number for the coaxial nanoarchitecture at 0.6 mA cm-2, where substantial capacity fading is often observed for MoS2 (see Figure S4 of the Supporting Information) and other metal sulfides.37-39 The larger initial capacity exceeding 700 mAh/g as compared to that in the subsequent cycles is characteristic of such conversion processes. After the first irreversible capacity loss, it presents a favorable retention of capacity during the subsequent cycles with a Coulombic efficiency above 98%. From the fifth to the 50th cycle, the average capacity loss is less than 0.8 mAh/g per cycle and comparable to that observed with bare carbon nanotubes, and the reversible capacity approaching 400 mAh/g is approximately twice that of bare carbon nanotubes under the same condition. With regard to capacity and cyclability, this is a noticeable improvement over previous works on MoS2 nanotubes, wherein only approximately 1/3 of lithium is released reversibly.25,26 To understand the interfacial electronic/ionic properties and to examine the sensitivity with respect to cycling, we performed electrochemical impedance measurements on the coaxial nanoarchitecture. The Nyquist plots at an open circuit voltage are shown in Figure 6c. Comparing the spectrum recorded after the fifth cycle with the spectrum recorded after the 50th cycle, there are no pronounced differences. As for the two spectra, a welldefined semicircle at intermediate frequencies and a straight line inclined at a constant angle to the real axis at low frequencies were observed, corresponding, respectively, to the chargetransfer resistance at the electrolyte/electrode interface and the solid-state diffusion resistance of lithium ions within the electrode.41 Also, it should be highlighted that no noticeable semicircle at high frequencies can be specifically attributed to the resistance of the solid electrolyte interface (SEI) film caused by a moderate decomposition of electrolytes. Further evidence for this phenomenon was inferred from the previous cyclic voltammetry and galvanostatic measurements, where neither an obvious cathodic peak nor a distinctive voltage plateau is responsible for the formation of the SEI film. As cycling continues, the intermediate frequency semicircle becomes slightly large, and no substantial modification of the charge-transfer resistance

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Figure 6. (a) Voltage-capacity plots for tubular MoS2-sheath/carbon-core nanoarchitecture at a current density of 0.6 mA/cm-2. Filled symbols: the first cycle and open symbols: the second cycle. (b) Variation of lithium-storage capacity as a function of cycle number for tubular MoS2sheath/carbon-core nanoarchitecture (filled symbols) and bare carbon nanotubes (open symbols) under the same conditions. (c) Nyquist plots of tubular MoS2-sheath/carbon-core nanoarchitecture at the fifth (open symbols) and 50th (filled symbols) cycle. Inset is the enlargement of the Nyquist plots. (d) Raman spectrum. (e and f) HRTEM images of tubular MoS2-sheath/carbon-core nanoarchitecture after 50 cycles in the discharged state.

value is detectable up to the 50th cycle. For the MoS2 electrode (see Figure S4 of the Supporting Information), the Nyquist plots are characteristic of two partially overlapped semicircles in the high and medium-frequency ranges and a sloping straight line in the low frequency range: the size of the semicircle drastically increases with increasing cycle number, and the separation of the two overlapped semicircles seems to be more obvious after long-term cycling. One plausible reason for the enhanced chargetransfer properties with the MoS2 on carbon nanotubes might be that the coaxial nanoarchitecture provides effective and fast interconnecting pathways for both electron conduction and ion transport, and thus, the charge-transfer reaction can occur easily across the electrolyte/electrode interface.42 In the case of the diffusion impedance, the slope in the low frequency region deviates from the ideal Warburg impedance with θ ) 45°, which

is characteristic of the blocking electrode-type behavior,43 furthermore indicating that a high rate performance can be expected for the coaxial nanoarchitecture. One way of increasing our understanding of lithium storage/release behavior within the duplex nanotubes, especially in view of the previous conversion reaction, is by using Raman and TEM studies during the charge/ discharge processes. After charging and discharging for 50 cycles, the electrode in the discharged state was opened in an Ar-filled glovebox, and the material was recovered, washed with DEC, and protected by an airtight container prior to microstructure analysis. As shown in Figure 6d, several more important peaks were observed at 225.6, 377.4, 405.1, 450.9, and 632.4 cm-1, which correspond well to the hexagonal MoS2 LA(M), E2g1(Γ), A1g(Γ), 2×LA(M), and A1g(M) + LA(M) symmetry modes, respectively.34 The low magnification TEM

Facilitated Lithium Storage in MoS2 Overlayers (Figure 6e) examination of the core/shell nanotubes after 50 charging and discharging cycles confirms that the tubular geometry is maintained and that the templated MoS2 nanotubes are still detectable around the whole outer surface of the carbon nanotubes. This is consistent with the Raman spectroscopy results, indicating that the decomposition and reformation of MoS2 is good reversibly. It can be seen that the crystalline phase of the MoS2 sheath is partly preserved as confirmed from the high magnification TEM image (Figure 6f). Nevertheless, some amorphous regions remain, which have also been observed in other systems, and these amorphous regions should be certainly kinetically favorable for successive lithium storage/release reactions. In light of the previous papers,37-39 it appears that the reduction of metal sulfides by lithium consists, during the first discharge, of two separated and distinct processes (a conversion and a polymer-like process) that become highly intermixed upon cycling, with a predominance of the polymerlike process over the conversion process for long cycling. It is well-documented that such a conversion reaction was demonstrated to be thermodynamically feasible and kinetically favored by the presence of the polymeric gel-like layer.4 The growth of the amorphous polymeric layer was found to be beneficial to the capacity retention, and this can be explained by the protecting role of the polymer film toward Mo nanoparticle agglomeration. Grounded on the present available experimental results, it could be concluded that there are three possible effects resulting from the underlying carbon nanotubes within the coaxial nanoarchitecture. First, the large internal void in carbon nanotubes is believed to easily buffer or absorb the mechanical stress attributed to the local volume variation of the MoS2 sheath during repetitive lithium storage/release, which has been identified as a main cause for the loss of cyclability. Second, the good electrical conductivity of the carbon nanotubes is more effective in maintaining the MoS2 sheath electrically connected during all cycling. The situation is obviously different from the physical mixture, where detachment could easily lead to a permanent loss of electrical contact. Finally, given the unique lithium storage/release mechanism of MoS2, wherein Mo nanoparticles promote the reversible reaction of a polymeric layer responsible for reversible lithium-storage capacity, the phase segregation to Mo nanoparticles, which can inevitably cause continuous capacity fading, would be further prevented due to dispersion effects of the underlying carbon nanotubes. Further work is, however, clearly warranted to understand the structural changes occurring during cycling. In fact, small cell polarization has been observed in Figure 5, which could originate from either of these effects. Conclusion The rational selection and assembly of materials are central issues in the development of energy conversion and storage applications. As an illustrative example, we have demonstrated a simple and effective solution-phase route for MoS2 overlayers supported on coaxial carbon nanotubes in this paper, which can be potentially extended to the preparation of other coaxial metal dichalcogenide/carbon nanotubes. On the basis of our results in the present study, the unique nanoarchitecture has been confirmed to demonstrate highly reversible capacity (approaching 400 mAh/g) and excellent cyclability, which is one of the best cyclabilities so far reported for metal chalcogenides. The outstanding performance is most likely attributable to a unique synergy at the nanoscale between the CNTs core and the MoS2 sheath and highlights the key role of the former in in-

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