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Hierarchical MoSe2 yolk–shell microspheres with superior Na-ion storage properties† Y. N. Ko,a S. H. Choi,b S. B. Parkc and Y. C. Kang*a
Received 10th May 2014 Accepted 6th July 2014 DOI: 10.1039/c4nr02538e www.rsc.org/nanoscale
Yolk–shell-structured MoSe2 microspheres were prepared via a simple selenization process of MoO3 microspheres. The yolk–shell-structured MoSe2 and MoO3 microspheres delivered initial discharge capacities of 527 and 465 mA h g1 in the voltage range of 0.001–3 V vs. Na/Na+ at a current density of 0.2 A g1, respectively, and their discharge capacities after 50 cycles were 433 and 141 mA h g1, respectively. The yolk–shellstructured MoSe2 microspheres also exhibited outstanding high rate capabilities. The hierarchical yolk–shell structure comprised of wrinkled nanosheets facilitated fast Na-ion and electron kinetics, and buffered the large volume changes encountered during cycling.
Introduction Transition metal dichalcogenides with the formula MX2 (M ¼ Mo, W; X ¼ S, Se) with layered structures have attracted considerable attention because of their unique characteristics.1–14 In layered transition metal dichalcogenides, the metal atom is sandwiched between two chalcogens by strong covalent bonding, while the individual layers are stacked by weak van der Waals interactions, which allow for a variety of atomic intercalations between the layers.15–17 This structural characteristic makes layered transition metal dichalcogenides one of the most promising host materials for atomic intercalations. Therefore, layered transition metal dichalcogenides are of great interest in the eld of energy storage.17–23 In particular, molybdenum dichalcogenides (MoS2, MoSe2) have gained increasing attention in energy storage applications, specically in Na-ion and Li-ion batteries.24–36 Many reports have described the synthesis of molybdenum disuldes;
however, the synthesis of MoSe2 powders has been scarcely reported, especially in the view of application in energy storage.36 Flower-like nanostructures and nanosheets of MoSe2 have been synthesized by the hydrothermal method in which the molybdenum precursor reacts with a selenium precursor under mild conditions.9,10 Another strategy used is chemical vapor deposition, which uses selenium powder to prepare nanostructured MoSe2 materials, such as nanolms, mesoporous rods, and nanosheets.11,12,36,37 However, developing simple methods to obtain nanostructured MoSe2 materials with controlled morphologies, which can be used in energy storage, remains a challenge. Na-ion batteries require porous nanostructured anode materials on account of the large radius and slow reaction kinetics of Na-ions, which causes low reversible capacity and poor cycle life. Yolk–shell-structured materials with core@void@shell conguration have been successfully applied as anode materials for Li-ion batteries.38–44 However, to the best of our knowledge, yolk–shell structured materials are yet to be investigated as anode materials for Na-ion batteries. In this study, we report for the rst time the synthesis of yolk–shell-structured MoSe2 microspheres via a simple selenization process. The as-prepared MoO3 yolk–shell microspheres were selenized to form the MoSe2 yolk–shell microspheres using hydrogen selenide vapors. The yolk–shell structure of the microspheres was maintained even aer the selenization process. The prepared MoSe2 yolk–shell microspheres were aggregates of thin-layered nanosheets. The yolk–shell-structured MoSe2 microspheres exhibited excellent Na-ion storage properties when compared to the yolk–shell structured MoO3 microspheres.
a
Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136–713, Korea. E-mail: [emailprotected]; Fax: +82-2-9283584; Tel: +82-2-3290-3268
b
Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjingu, Seoul 143–701, Korea
c
Department of Chemical and Biomolecular Engineering, Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305–701, Korea † Electronic supplementary 10.1039/c4nr02538e
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Results and discussion The schematic representing the formation of the yolk–shellstructured MoSe2 microspheres is described in Scheme 1. A small alumina boat containing the as-prepared MoO3 yolk–shell microspheres was loaded into a larger alumina boat with a
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Scheme 1 Schematic illustrations for the (a) formation process of a yolk–shell-structured MoSe2 microsphere and (b) selenization process.
cover. The selenium metal powders were loaded outside the small alumina boat and inside the large alumina boat with cover. The MoO3 yolk–shell microspheres were allowed to react with the hydrogen selenide vapor, which was generated by the reaction of the selenium powders with hydrogen gas. The hydrogen selenide vapor was formed steadily during the selenization process. The MoO3 yolk–shell microspheres with clean surfaces transformed into MoSe2 yolk–shell microspheres with rough surfaces at a selenization temperature of 300 C. The prepared MoSe2 yolk–shell microspheres were aggregates of nanosheets with wrinkled ultrathin layers. The MoSe2 nanosheets further loosely aggregated to form the hierarchical yolk– shell structure. Fig. 1 shows the XRD patterns of the microspheres selenized at 300 C for different times. The XRD pattern of the microspheres selenized at 300 C for 3 h showed major peaks belonging to MoO2 and MoO3 phases and small impurity peaks belonging to the MoSe2 phase. The peak intensities of the MoSe2 phase increased with increase in the selenization time,
XRD patterns of the microspheres selenized at 300 C for different times: (a) 0 h, (b) 3 h, (c) 9 h, (d) 12 h, and (e) 24 h. Fig. 1
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while the peak intensities of the tetragonal MoO2 phase decreased. The reduction of the orthorhombic a-MoO3 phase into the tetragonal MoO2 phase occurred before the selenization process. Finally, phase pure MoSe2 microspheres were prepared aer 24 h. The XRD pattern of the MoO3 yolk–shell microspheres is also shown in Fig. 1. The phase pure MoO3 microspheres exhibited sharp XRD peaks. The intensities of the XRD peaks decreased aer the selenization process. The highly crystalline MoO3 microspheres transformed into the layered MoSe2 microspheres by the simple selenization process. The (002) reection at 2q value around 13.7 , which corresponds to the interlayer distance of the MoSe2 layers, revealed a wellstacked structure of the yolk–shell structured MoSe2 microspheres.45 The average stacking height of the MoSe2 layers calculated from the (002) peak using Scherrer's equation was 10 nm. This indicated that the yolk–shell structured MoSe2 microspheres consisted of an average of 17 layers. Fig. 2 shows the overall morphologies of the hierarchical MoSe2 yolk–shell microspheres selenized at 300 C for 24 h. The low resolution FE-SEM image indicates the spherical shape with rough surface of the prepared MoSe2 yolk–shell microspheres. The surfaces of the microspheres are composed of aggregated nanosheets as shown in Fig. 2b. The microspheres were ground by hand using an agate mortar to obtain the SEM image of the fractured microspheres, as shown in Fig. 2c. The SEM image of the fractured microsphere shows the clear yolk–shell structure. The core with a hierarchical structure was located inside the highly porous shell. The spherical morphologies of the MoO3 yolk–shell microspheres were maintained even aer the completion of the selenization process, as shown in Fig. 2 and S1.† The detailed morphologies and elemental mapping images of the hierarchical MoSe2 yolk–shell microspheres are shown in Fig. 3. The low resolution TEM images, as shown in Fig. 3a and b, indicate the yolk–shell structure in which a clear core and a
Fig. 2 Morphologies of the hierarchical MoSe2 yolk–shell microspheres: (a and b) FE-SEM images, (c) FE-SEM image of the fractured microsphere, and (d) schematic illustration of the microsphere.
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TEM and elemental mapping images of the hierarchical MoSe2 yolk–shell microspheres: (a–c) low resolution TEM images, (d and e) high resolution TEM images, (f) SAED pattern, and (g) elemental mapping images. Fig. 3
void space between the core and shell were observed. The MoSe2 yolk–shell powder showed highly rough surfaces, as shown in Fig. 3c. The transparent ultrathin layers were wrinkled and formed the shell with a rough structure. The high resolution TEM images shown in Fig. 3d and e indicate clear lattice fringes separated by 0.62 nm, which correspond to the d-spacing of the (002) plane of the hexagonal MoSe2 phase. The high-resolution TEM image shown in Fig. 3e demonstrates stacked MoSe2 layers with a thickness of about 10 nm. The thickness of the stacked MoSe2 layers agreed well with the results of the XRD observation. The selected area electron diffraction (SAED) pattern shown in Fig. 3f with clear rings is indicative of the polycrystalline structure of the microspheres of the hexagonal MoSe2 phase. The elemental mapping images shown in Fig. 3g exhibit uniform distributions of Mo and Se components throughout the hierarchical MoSe2 yolk–shell microsphere. The clear void space between the core and shell parts was also observed from the elemental mapping images, as shown by the arrow in Fig. 3g. Fig. S2† shows the TEM-energy dispersive X-ray (EDX) spectrum of the hierarchical MoSe2 yolk–shell microspheres. The clear peaks of Mo and Se were observed in the EDX spectrum, while the peak of oxygen was negligible. The molar ratio of Mo and Se was obtained as 34 : 64 by EDX spectroscopy. The results of TEM–EDX also indicated the complete selenization of the layered MoO3 yolk–shell microspheres into layered MoSe2 yolk–shell microspheres. Fig. S3† shows the N2 adsorption and desorption isotherms and pore size distributions of the MoO3 and hierarchical MoSe2 yolk–shell microspheres. The isotherms of two samples can be characterized as type IV with H3 hysteresis loop, which indicates the presence of mesopores. The BET surface areas of the MoO3 and hierarchical MoSe2
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yolk–shell microspheres were 9.0 and 5.4 m2 g1, respectively. The densities of the yolk–shell-structured MoSe2 and MoO3 microspheres measured by mercury porosimetry were 4.5 and 2.0 g ml1, respectively. The electrochemical Na-ion insertion and extraction behaviors of the yolk–shell-structured MoSe2 and MoO3 microspheres were investigated by cyclic voltammetry (CV) and galvanostatic discharge–charge cycling in the voltage range 0.001–3 V vs. Na/ Na+. The CV curves of the yolk–shell structured MoSe2 microspheres for the rst ve cycles at a scan rate of 0.1 mV s1 are shown in Fig. 4a. The electrochemical reaction of MoSe2 can be estimated from that of MoS2 because of their similar crystal structure.36 During the initial discharge process, the yolk–shell structured MoSe2 microspheres displayed three main cathodic peaks at around 1.4, 0.65, and 0.44 V. The rst cathodic peak at 1.4 V corresponds to the intercalation of Na-ions into the MoSe2 lattice and the formation of NaxMoSe2.34,45 The other cathodic peaks observed at around 0.65 and 0.44 V are attributed to the conversion reaction from NaxMoSe2 to Mo metal nanograins and the formation of a gel-like polymeric layer.34,45 In the reversible charge process, the anodic peaks were observed at around 0.69 and 0.79 V, which are associated with the partial oxidation of Mo and the oxidation of Na2Se to Se.45 Starting from the second cycle, the peaks in the CV curves almost overlapped substantially, suggesting the good cycling stability of the yolk– shell-structured MoSe2 microspheres. Fig. 4b shows the discharge–charge proles of the yolk–shellstructured MoSe2 microspheres at a constant current density of 0.2 A g1. The plateaus in the voltage proles are consistent with the distinct peaks in the CV curves. The yolk–shell-structured MoSe2 microspheres exhibited initial discharge and charge capacities of 527 and 448 mA h g1, respectively, and the corresponding initial Coulombic efficiency was 85%. In contrast, the initial discharge and charge capacities of the yolk–shell structured MoO3 microspheres were 465 and 202 mA h g1, respectively, and the corresponding initial Coulombic efficiency was 43% (see Fig. S5†). It can be seen that the yolk–shellstructured MoSe2 microspheres possess superior Na-ion insertion and extraction capabilities to the yolk–shell-structured MoO3 microspheres. The formation enthalpy value of Na2Se (342 kJ mol1) is lower than that of Na2O (418 kJ mol1).46 The low formation enthalpy value as well as special nanostructure of the MoSe2 resulted in their superior electrochemical properties compared with those of the MoO3.47,48 The cycling performances of the yolk–shell structured MoSe2 and MoO3 microspheres were investigated in the voltage range 0.001–3 V vs. Na/Na+ at a current density of 0.2 A g1 (see Fig. 4c). The yolk–shell structured MoSe2 and MoO3 microspheres delivered discharge capacities of 433 and 141 mA h g1 aer 50 cycles, respectively, and the corresponding capacity retentions measured aer the rst cycle were 99 and 66%. The yolk–shell structured MoSe2 microspheres exhibited superior rate performance to the MoO3 microspheres, as demonstrated by the stepwise current density increase in Fig. 4d from 0.1 to 1.5 A g1 and the return to 0.1 A g1. For each step, 10 cycles were carried out to evaluate the rate performance. The yolk– shell-structured MoSe2 microspheres exhibited nal discharge Nanoscale, 2014, 6, 10511–10515 | 10513
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Fig. 4 Electrochemical Na-ion insertion and extraction behaviors of the yolk–shell-structured MoSe2 and MoO3 microspheres: (a) cyclic voltammograms and (b) discharge/charge profiles of the MoSe2 microspheres, (c) cycling performances at a constant current density of 0.2 A g1 and (d) rate performances of the MoSe2 and MoO3 microspheres.
capacities of 442, 399, 382, 369, 364, and 345 mA h g1 at current densities of 0.1, 0.3, 0.5, 0.8, 1.0, and 1.5 A g1, respectively. On the other hand, the yolk–shell-structured MoO3 microspheres exhibited nal discharge cycle capacities of 173, 128, 104, 80, 67, and 49 mA h g1 at current densities of 0.1, 0.3, 0.5, 0.8, 1.0, and 1.5 A g1, respectively. Despite cycling at high current densities, the discharge capacity of the yolk–shellstructured MoSe2 microspheres recovered to 470 mA h g1 when the current density returned to 0.1 A g1. The yolk–shell-structured MoSe2 microspheres showed superior Na-ion storage capabilities to carbon materials reported previously in the literature even at a high current density of 1.5 A g1.49–51 Fig. S6† shows the EIS of the yolk–shell-structured MoSe2 and MoO3 microspheres aer 50 cycles. The Nyquist plots indicate compressed semicircles in the medium frequency range of each spectrum, which describe the charge transfer resistance (Rct) for these electrodes, and straight lines in the low frequency range, which is associated with Na-ion diffusion in the bulk of the active material. The Rct values of the yolk–shellstructured MoSe2 and MoO3 microspheres were 66 and 91 U, respectively. The yolk–shell-structured MoSe2 microspheres exhibited a smaller charge transfer resistance than the yolk– shell-structured MoO3 microspheres. In contrast to the yolk– shell-structured MoO3 microspheres, the MoSe2 microspheres still maintained the straight line at low frequencies aer cycling, which indicates stable Na-ion diffusivity during cycling. The yolk–shell structured MoSe2 microspheres exhibited superior Na-ion storage performances because of their unique structural and morphological properties. First, the synthesized
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MoSe2 microspheres possess layered structure stacked through weak van der Waals interactions, which ensures facile Na-ion intercalation and deintercalation. Second, the hierarchical yolk–shell structure comprised of nanosheets facilitates fast Naion and electron kinetics and buffers the large volume change encountered during cycling.
Conclusions We report for the rst time the synthesis of yolk–shell structured MoSe2 microspheres with superior Na-ion storage properties via a simple selenization process. The MoO3 yolk–shell microspheres with clean surfaces and dense structures transformed into MoSe2 yolk–shell microspheres with porous structures and rough surfaces at a selenization temperature of 300 C. The MoSe2 yolk–shell microspheres were aggregates of wrinkled nanosheets with thin layers. The hierarchical MoSe2 yolk–shell microspheres with well-developed mesopores showed higher initial capacities and better cycling and rate performances than the MoO3 yolk–shell microspheres. The yolk–shell structured MoSe2 microspheres with structural uniqueness exhibited excellent Na-ion storage capabilities, which make this material a promising anode material for Naion batteries.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST)
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(no. 2012R1A2A2A02046367). This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted nancial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (201320200000420).
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