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Hollow Cobalt Selenide Microspheres: Synthesis and Application as Anode Materials for Na-Ion Batteries You Na Ko, Seung Ho Choi, and Yun Chan Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11963 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on March 1, 2016
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ACS Applied Materials & Interfaces
Hollow Cobalt Selenide Microspheres: Synthesis and Application as Anode Materials for Na-Ion Batteries You Na Ko, Seung Ho Choi, and Yun Chan Kang*
Address: Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713
*Corresponding author. Fax: (+82) 2-928-3584. E-mail address: [email protected]
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ABSTRACT The electrochemical properties of hollow cobalt oxide and cobalt selenide microspheres are studied for the first time as anode materials for Na-ion batteries. Hollow cobalt oxide microspheres prepared by one-pot spray pyrolysis are transformed into hollow cobalt selenide microspheres by a simple selenization process using hydrogen selenide gas. Ultrafine nanocrystals of Co3O4 microspheres are preserved in the cobalt selenide microspheres selenized at 300ºC. The initial discharge capacities for the Co3O4, and cobalt selenide microspheres selenized at 300 and 400ºC are 727, 595, and 586 mA h g−1, respectively, at a current density of 500 mA g−1. The discharge capacities after 40 cycles for the same samples are 348, 467, and 251 mA h g−1, respectively, and their capacity retentions measured from the second cycle onwards are 66, 91, and 50%, respectively. The hollow cobalt selenide microspheres have better rate performances than the hollow cobalt oxide microspheres.
Keywords: energy storage; Na-ion battery; cobalt oxide; cobalt selenide; nanostructure
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INTRODUCTION Recently, there has been a great interest in developing rechargeable batteries as energy storage devices for the replacement of fossil fuels, and the development of alternative energies. Although Li-ion batteries (LIBs) have achieved commercial success, the scarcity of Li and its high cost are serious barriers for their application in large-scale devices and electric vehicles.1-3 Therefore, in order to renew the development of rechargeable batteries, and meet the increasing demand for low-cost energy storage, Na-ion batteries (NIBs) are being considered as the next generation of rechargeable batteries due to their environmental friendliness and low cost.1-3 The development of NIBs is at an early stage, thus the exploration of new anode materials with large reversible capacity is essential. It is well known that NIBs follow a similar working principle to LIBs, because of their analogous storage behavior.4-6 However, a sufficient performance has not yet been achieved with NIBs that would allow them to replace LIBs, due to the kinetic limitation resulting from the larger Na+ ionic radius.5,6 A number of studies have been performed regarding the application of various anode materials and their Na-ion storage performance.7-9 However, the search for efficient anode materials has so far been restricted to a limited set of materials including amorphous carbon, alloys, metal oxides, and metal sulfides.10-23 The challenge to develop NIBs powers the exploration for new anode materials with promising electrochemical capacity. In order to achieve high energy and power density in NIBs, which are essential to replace LIBs, several problems need to be circumvented, such as the poor cyclability and slow rate performance that result from large volume expansions and slow kinetics.6,24,25 Various nanostructured materials have been successfully applied as electrode materials in LIBs to improve their electrochemical performances.26-33 High rate performance and stable cyclability can be achieved using nanostructured electrode materials, because they offer the advantages of a high specific surface area for electrolyte contact, short alkali-ion and electron pathways, and lower strain than bulk materials under volume change. Recently, metal chalcogenides including metal sulfides and selenides have been successfully applied as anode materials for NIBs.34-38 In addition, various cobalt-based compounds such as cobalt oxides and cobalt chalcogenides are well established as anode materials for LIBs, and provide good electrochemical properties.39-46 Therefore, various cobalt-based compounds could be promising candidates as anode material for NIBs. 3
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However, to the best of our knowledge, cobalt oxide and selenide materials have been scarcely investigated as anode materials for NIBs. In this study, the preparation of hollow cobalt oxide and cobalt selenide microspheres and their Na-ion storage properties were investigated. Hollow cobalt oxide microspheres prepared by one-pot spray pyrolysis were transformed into hollow cobalt selenide microspheres through a simple selenization process. The electrochemical properties of the hollow cobalt selenide microspheres were compared in terms of Na-ion storage with the hollow cobalt oxide microspheres.
RESULTS AND DISCUSSION
Scheme 1. Schematic illustration for the preparation procedure of the hollow cobalt selenide microsphere.
The hollow cobalt selenide microspheres were prepared using the two-step process shown in Scheme 1. In the first step, hollow cobalt oxide microspheres are prepared by simple spray pyrolysis using a solution of cobalt nitrate. In the second step, the cobalt oxide microspheres are selenized to prepare hollow cobalt selenide microspheres using selenium metal powders. The cobalt oxide microspheres are selenized in hydrogen selenide (H2Se) vapor, generated by the reaction of selenium powders with hydrogen gas.
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Figure 1. Morphologies of the nanostructured Co3O4 precursor microspheres: (a) SEM image, (b)-(d) TEM images, and (e) SAED pattern.
The morphology of the cobalt oxide microspheres directly prepared by spray pyrolysis is shown in Figure 1. The microspheres are hollow and spherical with a thin shell, as shown in the SEM and TEM images. During spray pyrolysis, hollow microspheres are generally formed due to the fast drying of the droplets and the rapid decomposition of the metal salts at the high temperature. In this study, fast decomposition of cobalt nitrate into Co3O4 occurred at 700ºC in air. The precipitated cobalt nitrate did not melt and form a dense structured powder; thereby, hollow cobalt oxide microspheres were prepared directly by spray pyrolysis. The hollow microspheres are composed of ultrafine aggregates of Co3O4 nanocrystals smaller than 15 nm, as shown in Figure 1c. The shell thickness of the microsphere shown in Figure 1c is 22 nm. The high resolution TEM image shown in Figure 1d reveals clear lattice fringes separated by 0.24 nm, corresponding to the (311) plane of cubic Co3O4. The selected-area electron diffraction (SAED) pattern shown in Figure 1e reveals the polycrystalline cubic Co3O4 phase of the microspheres.
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Figure 2. XRD patterns of the cobalt compounds selenizied at different temperatures.
The hollow Co3O4 microspheres were selenized at 200, 300, and 400ºC for 6 h in order to optimize the structure of the resulting cobalt selenide microspheres. The XRD patterns of the microspheres obtained before and after selenization are shown in Figure 2. The microspheres prepared directly by spray pyrolysis are composed of pure cubic Co3O4. Selenization did not occur at 200ºC because no hydrogen selenide gas was formed. However, small peaks corresponding to the oxygen-deficient CoO phase are observed in the XRD pattern. The nanostructured CoOx microspheres selenized at 200ºC were composed of Co3O4 and CoO with a 74:26 molar ratio, based on quantitative XRD analysis using relative intensity data. The XRD pattern of the cobalt selenide microspheres selenized at 300ºC had main peaks of orthorhombic CoSe2 and minor peaks of cubic CoSe2 and monoclinic Co3Se4. The microspheres selenized at 400ºC had pure cubic CoSe2 crystal structure with no impurity peaks. A complete selenization of the hollow Co3O4 microspheres into CoSe2 microspheres occurred even at the relatively low temperature of 300ºC. The sharp XRD peaks obtained for the cubic-phase CoSe2 microspheres reveal the abrupt crystal growth occurring at 400ºC. 6
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Figure 3. Morphologies of the nanostructured cobalt selenide microspheres selenized at 300 o
C: (a) SEM image, (b)-(d) TEM images, (e) SAED pattern, and (f) elemental mapping
images.
The morphology of the Co3O4 microspheres was completely preserved at a selenization temperature of 200ºC, as shown in Figure S1. The morphologies of the cobalt selenide powders selenized at 300 and 400ºC are shown in Figures 3 and S2, respectively. The cobalt selenide powders selenized at 300ºC contain hollow spherical structures as shown in the SEM and TEM images in Figure 3. The ultrafine Co3O4 nanocrystals shown in Figure 1c are preserved in the cobalt selenide powders selenized at 300ºC (Figure 3c). The high resolution TEM image shown in Figure 3d reveals clear lattice fringes separated by 0.29 nm, corresponding to the (101) plane of orthorhombic CoSe2. The SAED pattern in Figure 3e shows that the microspheres obtained are mainly composed of polycrystalline orthorhombic CoSe2. The elemental mapping images shown in Figure 3f reveal the uniform distribution of 7
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Co and Se throughout the cobalt selenide microspheres. The thin-shelled hollow structure facilitates the selenization of the cobalt oxide microspheres into cobalt selenide even at the relatively low temperature of 300ºC. The cubic-phase CoSe2 powders selenized at 400 °C contain non-spherical shapes, as shown in Figures S2a and S2b. This suggests that crystal growth at a high selenization temperature (400ºC) destroys the spherical morphology of the Co3O4 microspheres. The grain size of the cobalt selenide powder selenized at 400ºC shown in Figure S2c is about 180 nm. The high resolution TEM image shown in Figure S2d reveals clear lattice fringes separated by 0.27 nm, which correspond to the (210) plane of cubic CoSe2. With increasing selenization temperatures, the SAED pattern of the cobalt selenide powders changes from a ring to a spot due to the abrupt crystal growth occurring at high selenization temperature (400ºC). The elemental mapping images of the cubic-phase CoSe2 powder in Figure S2f also show uniform distributions of Co and Se throughout the powder.
Figure 4. Electrochemical properties of the cobalt compounds: (a) first cyclic voltammograms, (b) initial charge/discharge curves, (c) cycling performances at a constant current density at 0.5 A g-1, and (d) rate performances. 8
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The electrochemical properties of the cobalt oxide and cobalt selenide microspheres as anode materials for NIBs were investigated by CV and galvanostatic discharge/charge cycling in the 0.001–3 V vs. Na/Na+ voltage range. Sharp reduction peaks are observed at 0.81 and 0.86 V during the first discharge process in the CV curves of the cobalt selenide materials selenized at 300 and 400ºC (Figure 4a). The electrochemical properties of cobalt selenide with respect to Li- and Na-ions have not been reported previously. Therefore, the electrochemical reaction of cobalt selenide was estimated from that of cobalt sulfide (CoSx). Transition metal selenides react with Li-ions through a conversion reaction similar to that of transition metal sulfides. In the first Li insertion process, the CV curve of the cobalt sulfide also has a reduction peak at around 1.1 V, which can be attributed to the reduction of CoSx to Co according to Equation (1).42-44 CoSx + 2x Li+ + 2x e– → x Li2S + Co
(1)
The Na-ion storage mechanism with CoSe2 in an NIB can be described by Equation (2). CoSe2 + 4 Na+ + 4 e– ↔ Co + 2 Na2Se
(2)
Therefore, the sharp reduction peaks observed in Figure 4a for the CoSe2 materials are attributed to the conversion of CoSe2 to Co metal nanograins and Na2Se. During reversible charging, oxidation peaks are observed around 1.84 and 1.94 V, which are associated with the recovery of CoSe2 material from Co metal and Na2Se. From the third cycle onwards, the peaks in the CV curves overlap substantially as shown in Figure S3, emphasizing the good cycling stability of the hollow CoSe2 microspheres selenized at 300 ºC. The CV curves for the Co3O4 materials with and without CoO impurities show sharp reduction peaks at around 0.05 V for the first discharge process (Figures 4a and S3). In the first discharge process (Na+ insertion), the CV curve of the Co3O4 materials has a sharp reduction peak at around 0.05 V, which can be attributed to the electrochemical conversion of Co3O4 to Co metal nanograins according to Equation (3). Co3O4 + 8 Na+ + 8 e– → 4 Na2O + 3 Co
(3)
In the first charge process (Na+ desertion), oxidation peaks are observed around 0.8 and 1.2 V, which are attributed to the oxidation of Co metal nanograins with Na2O. As an anode material for NIBs, cobalt oxide requires lower potential than cobalt selenide for the electrochemical conversion process involved in Na-ion storage. The equilibrium potential (Eº) for the conversion reaction plateau of metal compounds was theoretically determined using the Gibbs free energy from Nernst law (Equation 4).4,47 9
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Eº = −∆rG/zF
(4)
The use of Na rather than Li ions leads to a lowering of the electrochemical potential for the conversion reaction of metal compounds. Adelhelm et al. reported the typical calculation of the cell potential for the conversion reaction of metal compounds with Li and Na.4 The difference in cell potential between the Li- and Na-based conversion reaction with metal oxide was 0.96 V. In our paper, the electrochemical conversion potentials of the Co3O4 and CoSe2 materials for the NIB are 0.05 and 0.85 V due to the difference of Gibbs energy. The initial discharge and charge curves obtained for the cobalt selenide and cobalt oxide materials at a current density of 500 mA g−1 are shown in Figure 4b. Distinct plateaus are observed for cobalt selenide at around 0.9 and 1.9 V, but these distinct plateaus are only observed in the first discharging process for the cobalt oxide at around 0.1 V. The results of the first discharge and charge curves are in good agreement with those from the CV curves shown in Figure 4a. The initial discharge capacities of the cobalt selenide materials selenized at 300 and 400 °C were 595 and 586 mA h g−1, respectively, and the corresponding initial charge capacities were 498 and 502 mA h g−1. However, the initial discharge capacities of the Co3O4 materials with and without CoO impurities were 466 and 727 mA h g−1, with corresponding initial charge capacities of 336 and 538 mA h g−1. The CoOx microspheres selenized at 200ºC had shorter plateaus at around 0.1 V in the initial discharge curve than that of the Co3O4 microspheres. This result could be due to the fact that CoO has a difficulty attaining sufficient electrochemical decomposition because the cell potential decreases with the decreasing oxidation state of cobalt.48 The initial Coulombic efficiencies of the Co3O4 and hollow cobalt selenide microspheres selenized at 300ºC were 74 and 84%, respectively. The cycling performances of the cobalt selenide and cobalt oxide materials at a current density of 500 mA g−1 are shown in Figure 4c. The discharge capacities after 40 cycles of the cobalt selenide microspheres selenized at 300 and 400ºC were 467 and 251 mA h g−1, respectively, and their capacity retentions measured from the second cycle onwards were 91 and 50%, respectively. The discharge capacities after 40 cycles of the Co3O4 materials with and without CoO impurities were 253 and 348 mA h g−1, respectively. The cobalt selenide microspheres selenized at 300ºC with ultrafine grains showed a better cycling performance than both cobalt selenide material selenized at 400ºC with larger grains and Co3O4. The Coulombic efficiencies of the cobalt selenide microspheres selenized at 300ºC (Figure S4) exhibit a decreasing trend upon cycling, and lower values than those of the cobalt oxide microspheres 10
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after several cycles. This can be attributed to the formation of unstable solid-electrolyteinterface (SEI), which results in capacity fading and low Coulombic efficiencies during cycling.50 These low Coulombic efficiencies of cobalt selenide could be attributed to repeat formation of solid-electrolyte-interface (SEI) layer on cobalt selenide surface due to structural change from large volume expansion during repeatedly cycling.51,52 Generally, electrolyte decomposition in the lithium and sodium ion batteries forms a passivating SEI layer on the electrode surface at the low potential. However, the SEI layer formed on the cobalt selenide surface can be broken as the nanostructure during huge volume expansion. The fresh cobalt selenide surface exposed to the electrolyte forms the SEI layer again via consumption of Na+ ions, resulting in the low Coulombic efficiency of electrode.51,52 The Coulombic efficiencies and cycle stability of the cobalt selenide microspheres are strongly affected by the type of electrolyte. Therefore, the enhancement of the Coulombic efficiencies, and cycle stability of the cobalt selenide microspheres can be expected by optimizing the electrolyte composition.53-56 The rate performances of the cobalt selenide microspheres selenized at 300ºC and phase pure Co3O4 microspheres are shown in Figure 4d, with current densities increasing stepwise from 100 to 900 mA g−1. The hollow cobalt selenide microspheres selenized at 300ºC provided final discharge capacities of 521, 490, 471, and 446 mA h g−1 at current densities of 100, 300, 600, and 900 mA g−1, respectively. On the other hand, the pure cubic Co3O4 microspheres had final discharge capacities of 504, 416, 338, and 278 mA h g−1 at current densities of 100, 300, 600, and 900 mA g−1, respectively. Cobalt selenide material is well known to be a metallic conductor.57,58 Therefore, cobalt selenide microspheres with good electrical conductivity show faster electrochemical reaction with Na at high current densities than those of the cobalt oxide microspheres. Impedance spectra obtained before and after 40 cycles of the cobalt compound microspheres are shown in Figure S5. The Nyquist plots indicate compressed semicircles in the medium frequency range of each spectrum, which describe the charge transfer resistances (Rct) for these electrodes, and straight lines in the low frequency range, which are associated with Na-ion diffusion in the bulk of the active materials.59,60 Before cycling, the radii of the cobalt selenide microspheres with good electrical conductivity are smaller than those of the cobalt oxide microspheres, which demonstrates low charge transfer resistance. The charge transfer resistances of the cobalt compounds microspheres before and after selenization at 200, 300, and 400ºC are 68, 71, 63, and 93 Ω after 40 cycles, respectively. The cobalt 11
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selenide microspheres selenized at 300ºC exhibit the smallest diameter of semicircle and the shortest straight line in the low frequency region, which indicate the low charge transfer resistance and high Na-ion diffusivity. On the other hand, the cobalt selenide microspheres selenized at 400ºC have the largest charge transfer resistance after 40 cycles. The morphology of the cobalt selenide microspheres selenized at 300 and 400ºC obtained after 40 cycles is shown in Figures S6 and S7, respectively. The cobalt selenide microspheres selenized at 300ºC maintain their hollow morphology even after cycling. However, the spherical morphology of the cobalt selenide microspheres with large grain size selenized at 400ºC is compromised after 40 cycles. The structural damage of the cobalt selenide microspheres selenized at 400ºC increases the charge transfer resistance after cycling. The Na-ion diffusion coefficients were calculated by using the relationship between charge transfer resistance and
ω−1/2 in the low-frequency region.61 According to linear fitting, the Na-ion diffusion coefficients at 25oC were calculated to be 2.43 × 10−12 and 5.72 × 10−11 cm2 s−1 for the precursor and cobalt selenide microspheres selenized at 300oC after 40 cycles, respectively. The cobalt selenide microspheres selenized at 300ºC exhibit low charge transfer resistance, high Na-ion diffusivity, and good structural stability during cycling, have good cycling and rate performances for Na-ion storage.
CONCLUSIONS This study confirmed for the first time that cobalt selenide can be successfully applied as an anode material for NIBs. The hollow cobalt selenide microspheres selenized at 300ºC with ultrafine grains provided both high initial discharge capacity and high initial Coulombic efficiency, in addition to good cycling and rate performances for Na-ion storage. Moreover, the electrochemical properties of the hollow cobalt selenide microspheres could be improved by optimizing the electrolyte system used for Na-ion storage. The hollow cobalt oxide microspheres were shown to be advantageous in terms of their high initial capacity, and low voltages for Na-ion storage as anode material for NIBs. The cycling and rate performances of the hollow Co3O4 microspheres could be upgraded by applying the technologies successfully developed for LIBs, notably by using carbon composite anodes. These hollow cobalt oxide and cobalt selenide microspheres are therefore promising anode materials for NIBs.
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EXPERIMENTAL SECTION Synthesis of hollow cobalt oxide and cobalt selenide microspheres The hollow Co3O4 microspheres were prepared by a spray pyrolysis process, comprising a droplet generator, a high-temperature tubular quartz reactor, and a Teflon bag filter (powder collector). A 1.7 MHz ultrasonic spray generator consisting of six vibrators was used to generate a large number of droplets, which were then carried to the quartz reactor via carrier gas. The reactor temperature and flow rate of air used as carrier gas were fixed at 700ºC and 5 L min−1, respectively. For the selenization of the hollow Co3O4 microspheres, commercial selenium metal powders were used as Se source. The hollow Co3O4 microspheres and selenium metal powders were loaded in a covered alumina boat and placed in a quartz tube reactor, as previously described.36 The selenization treatment was performed at temperatures of 200, 300, and 400ºC for 6 h under a flow of 10 vol.% H2/Ar mixture gas. Characterization The morphologies of the cobalt oxide and cobalt selenide microspheres were investigated by field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi) and transmission electron microscopy (FE-TEM; JEM-2100F, JEOL). The crystal structures of the microspheres were investigated by X-ray diffraction (XRD; X’Pert PRO MPD) using Cu Kα radiation (λ = 1.5418 Å) at the Korea Basic Science Institute (Daegu). Electrochemical properties were analyzed in a 2032-type coin cell. The anode was prepared from a mixture of active material, carbon black, and sodium carboxymethyl cellulose in a 7:2:1 weight ratio. Na metal and a microporous polypropylene film were used as counter electrode and separator, respectively. The electrolyte consisted of a solution of 1 M NaClO4 (Aldrich) in a 1:1 volume mixture of ethylene carbonate/dimethyl carbonate with 5 wt.% fluoroethylene carbonate. The discharge and charge characteristics of the samples were investigated by cycling in the 0.001–3 V voltage range at various current densities. Cyclic voltammograms (CVs) were recorded at a scan rate of 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) of the electrode was measured over a frequency range of 100 kHz – 0.01 Hz.
■ ASSOCIATED CONTENT Supporting Information. 13
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SEM and TEM images of the powders selenized at 200 and 400 oC. Cycling voltammograms of the cobalt compounds selenized at different temperatures. Coulombic efficiencies versus cycle number for cobalt compounds microspheres materials. Nyquist plots of the electrochemical impedance spectra of the cobalt compounds. Morphologies and elemental mapping images of the cobalt selenide microspheres obtained after cycling. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; Fax: (+82) 2-928-3584. ACKNOWLEDGMENTS This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (201320200000420). This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2015R1A2A1A15056049).
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Nanospindles as a Superior Anode Material for Lithium-Ion Batteries. Adv. Funct. Mater. 2008, 18, 3941–3946. (31) Guo, Z. P.; Du, G. D.; Nuli, Y.; Hassan, M. F.; Liu, H. K. Ultra-Fine Porous SnO2 Nanopowder Prepared via a Molten Salt Process: a Highly Efficient Anode Material for Lithium-Ion Batteries. J. Mater. Chem. 2009, 19, 3253–3257. (32) Xu, S.; Hessel, C. M.; Ren, H.; Yu, R.; Jin, Q.; Yang, M.; Zhao, H.; Wang, D. α-Fe2O3 Multi-Shelled Hollow Microspheres for Lithium Ion Battery Anodes with Superior Capacity and Charge Retention. Energy Environ. Sci. 2014, 7, 632–637. (33) Cao, X.; Shi, Y.; Shi, W.; Rui, X.; Yan, Q.; Kong, J.; Zhang, H. Preparation of MoS2Coated Three-Dimensional Graphene Networks for High-Performance Anode Material in Lithium-Ion Batteries. Small 2013, 9, 3433–3438. (34) Wang, Y. X.; Chou, S. L.; Wexler, D.; Liu, H. K.; Dou, S. X. High-Performance SodiumIon Batteries and Sodium-Ion Pseudocapacitors Based on MoS2/Graphene Composites.
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Nanoscale 2013, 5, 2045–2054. (47) Oumellal, Y.; Rougier, A.; Nazri, G. A.; Tarascon, J. M.; Aymard, L. Metal Hydrides for Lithium-ion Batteries. Nat. Mater. 2008, 7, 916–921. (48) Klein, F.; Jache, B.; Bhide, A.; Adelhelm, P. Conversion Reactions for Sodium-Ion Batteries. Phys. Chem. Chem. Phys. 2013, 15, 15876–15887. (49) Luo, C.; Xu, Y.; Zhu, Y.; Liu, Y.; Zheng, S.; Liu, Y.; Langrock, A.; Wang, C. Selenium@Mesoporous Carbon Composite with Superior Lithium and Sodium Storage Capacity. ACS Nano 2013, 7, 8003–8010. (50) Darwiche, A.; Marino, C.; Sougrati, M. T.; Fraisse, B.; Stievano, L.; Monconduit, L. Better Cycling Performances of Bulk Sb in Na-Ion Batteries Compared to Li-Ion Systems: An Unexpected Electrochemical Mechanism. J. Am. Chem. Soc. 2012, 134, 20805–20811. (51) Ji, L.; Gu, M.; Shao, Y.; Li, X.; Engelhard, M. H.; Arey, B. W.; Wang, W.; Nie, Z.; Xiao, J.; Wang, C.; Zhang, J.-G.; Liu, J. Controlling SEI Formation on SnSb-Porous Carbon Nanofibers for Improved Na Ion Storage. Adv. Mater. 2014, 26, 2901–2908. (52) Wu, H.; Chan, G.; Choi, J. W.; Ryu, I.; Yao, Y.; McDowell, M. T.; Lee, S. W.; Jackson, A.; Yang, Y.; Hu, L.; Cui, Y. Stable Cycling of Double-Walled Silicon Nanotube Battery Anodes through Solid-Electrolyte Interphase Control, Nat. Nanotechnol. 2012, 7, 310–315. (53) Hu, Z.; Zhu, Z.; Cheng, F.; Zhang, K.; Wang, J.; Chen, C.; Chen, J. Pyrite FeS2 for High18
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Rate and Long-Life Rechargeable Sodium Batteries. Energy Environ. Sci. 2015, 8, 1309– 1316. (54) Pan, H.; Hu, Y.-S.; Chen, L. Room-Temperature Stationary Sodium-Ion Batteries for Large-Scale Electric Energy Storage. Energy Environ. Sci. 2013, 6, 2338–2360. (55) Zhang, K.; Hu, Z.; Liu, X.; Tao, Z.; Chen, J. FeSe2 Microspheres as a High-Performance Anode Material for Na-Ion Batteries Adv. Mater. 2015, 27, 3305–3309. (56) Kim, H.; Hong, J.; Park, Y.-U.; Kim, J.; Hwang, I.; Kang, K. Sodium Storage Behavior in Natural Graphite using Ether-based Electrolyte Systems. Adv. Funct. Mater. 2015, 25, 534– 541. (57) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 Nanoparticles Grown on Carbon Fiber Paper: An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 4897–4900. (58) Maneeprakorn, W.; Malik, M. A.; O’Brien, P. The Preparation of Cobalt Phosphide and Cobalt Chalcogenide (CoX, X = S, Se) Nanoparticles from Single Source Precursors. J.
Mater. Chem. 2010, 20, 2329–2335. (59) Shi, Y.; Wang, J. Z.; Chou, S. L.; Wexler, D.; Li, H. J.; Ozawa, K.; Liu, H. K.; Wu, Y. P. Hollow Structured Li3VO4 Wrapped with Graphene Nanosheets In Situ Prepared by a OnePot Template-Free Method as an Anode for Lithium-Ion Batteries. Nano Lett. 2013, 13, 4715–4720. (60) Yang, S.; Song, H.; Chen, X. Electrochemical Performance of Expanded Mesocarbon Microbeads as Anode Material for Lithium-Ion Batteries. Electrochem. Commun. 2006, 8, 137–142. (61) Ko, Y. N.; Park, S. B.; Jung, K. Y.; Kang, Y. C. One-Pot Facile Synthesis of Ant-CaveStructured Metal Oxide–Carbon Microballs by Continuous Process for Use as Anode Materials in Li-Ion Batteries. Nano Lett. 2013, 13, 5462–5466.
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Article
Hollow Cobalt Selenide Microspheres: Synthesis and Application as Anode Materials for Na-Ion Batteries You Na Ko, Seung Ho Choi, and Yun Chan Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11963 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on March 1, 2016
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Hollow Cobalt Selenide Microspheres: Synthesis and Application as Anode Materials for Na-Ion Batteries You Na Ko, Seung Ho Choi, and Yun Chan Kang*
Address: Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713
*Corresponding author. Fax: (+82) 2-928-3584. E-mail address: [email protected]
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ABSTRACT The electrochemical properties of hollow cobalt oxide and cobalt selenide microspheres are studied for the first time as anode materials for Na-ion batteries. Hollow cobalt oxide microspheres prepared by one-pot spray pyrolysis are transformed into hollow cobalt selenide microspheres by a simple selenization process using hydrogen selenide gas. Ultrafine nanocrystals of Co3O4 microspheres are preserved in the cobalt selenide microspheres selenized at 300ºC. The initial discharge capacities for the Co3O4, and cobalt selenide microspheres selenized at 300 and 400ºC are 727, 595, and 586 mA h g−1, respectively, at a current density of 500 mA g−1. The discharge capacities after 40 cycles for the same samples are 348, 467, and 251 mA h g−1, respectively, and their capacity retentions measured from the second cycle onwards are 66, 91, and 50%, respectively. The hollow cobalt selenide microspheres have better rate performances than the hollow cobalt oxide microspheres.
Keywords: energy storage; Na-ion battery; cobalt oxide; cobalt selenide; nanostructure
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INTRODUCTION Recently, there has been a great interest in developing rechargeable batteries as energy storage devices for the replacement of fossil fuels, and the development of alternative energies. Although Li-ion batteries (LIBs) have achieved commercial success, the scarcity of Li and its high cost are serious barriers for their application in large-scale devices and electric vehicles.1-3 Therefore, in order to renew the development of rechargeable batteries, and meet the increasing demand for low-cost energy storage, Na-ion batteries (NIBs) are being considered as the next generation of rechargeable batteries due to their environmental friendliness and low cost.1-3 The development of NIBs is at an early stage, thus the exploration of new anode materials with large reversible capacity is essential. It is well known that NIBs follow a similar working principle to LIBs, because of their analogous storage behavior.4-6 However, a sufficient performance has not yet been achieved with NIBs that would allow them to replace LIBs, due to the kinetic limitation resulting from the larger Na+ ionic radius.5,6 A number of studies have been performed regarding the application of various anode materials and their Na-ion storage performance.7-9 However, the search for efficient anode materials has so far been restricted to a limited set of materials including amorphous carbon, alloys, metal oxides, and metal sulfides.10-23 The challenge to develop NIBs powers the exploration for new anode materials with promising electrochemical capacity. In order to achieve high energy and power density in NIBs, which are essential to replace LIBs, several problems need to be circumvented, such as the poor cyclability and slow rate performance that result from large volume expansions and slow kinetics.6,24,25 Various nanostructured materials have been successfully applied as electrode materials in LIBs to improve their electrochemical performances.26-33 High rate performance and stable cyclability can be achieved using nanostructured electrode materials, because they offer the advantages of a high specific surface area for electrolyte contact, short alkali-ion and electron pathways, and lower strain than bulk materials under volume change. Recently, metal chalcogenides including metal sulfides and selenides have been successfully applied as anode materials for NIBs.34-38 In addition, various cobalt-based compounds such as cobalt oxides and cobalt chalcogenides are well established as anode materials for LIBs, and provide good electrochemical properties.39-46 Therefore, various cobalt-based compounds could be promising candidates as anode material for NIBs. 3
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However, to the best of our knowledge, cobalt oxide and selenide materials have been scarcely investigated as anode materials for NIBs. In this study, the preparation of hollow cobalt oxide and cobalt selenide microspheres and their Na-ion storage properties were investigated. Hollow cobalt oxide microspheres prepared by one-pot spray pyrolysis were transformed into hollow cobalt selenide microspheres through a simple selenization process. The electrochemical properties of the hollow cobalt selenide microspheres were compared in terms of Na-ion storage with the hollow cobalt oxide microspheres.
RESULTS AND DISCUSSION
Scheme 1. Schematic illustration for the preparation procedure of the hollow cobalt selenide microsphere.
The hollow cobalt selenide microspheres were prepared using the two-step process shown in Scheme 1. In the first step, hollow cobalt oxide microspheres are prepared by simple spray pyrolysis using a solution of cobalt nitrate. In the second step, the cobalt oxide microspheres are selenized to prepare hollow cobalt selenide microspheres using selenium metal powders. The cobalt oxide microspheres are selenized in hydrogen selenide (H2Se) vapor, generated by the reaction of selenium powders with hydrogen gas.
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Figure 1. Morphologies of the nanostructured Co3O4 precursor microspheres: (a) SEM image, (b)-(d) TEM images, and (e) SAED pattern.
The morphology of the cobalt oxide microspheres directly prepared by spray pyrolysis is shown in Figure 1. The microspheres are hollow and spherical with a thin shell, as shown in the SEM and TEM images. During spray pyrolysis, hollow microspheres are generally formed due to the fast drying of the droplets and the rapid decomposition of the metal salts at the high temperature. In this study, fast decomposition of cobalt nitrate into Co3O4 occurred at 700ºC in air. The precipitated cobalt nitrate did not melt and form a dense structured powder; thereby, hollow cobalt oxide microspheres were prepared directly by spray pyrolysis. The hollow microspheres are composed of ultrafine aggregates of Co3O4 nanocrystals smaller than 15 nm, as shown in Figure 1c. The shell thickness of the microsphere shown in Figure 1c is 22 nm. The high resolution TEM image shown in Figure 1d reveals clear lattice fringes separated by 0.24 nm, corresponding to the (311) plane of cubic Co3O4. The selected-area electron diffraction (SAED) pattern shown in Figure 1e reveals the polycrystalline cubic Co3O4 phase of the microspheres.
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Figure 2. XRD patterns of the cobalt compounds selenizied at different temperatures.
The hollow Co3O4 microspheres were selenized at 200, 300, and 400ºC for 6 h in order to optimize the structure of the resulting cobalt selenide microspheres. The XRD patterns of the microspheres obtained before and after selenization are shown in Figure 2. The microspheres prepared directly by spray pyrolysis are composed of pure cubic Co3O4. Selenization did not occur at 200ºC because no hydrogen selenide gas was formed. However, small peaks corresponding to the oxygen-deficient CoO phase are observed in the XRD pattern. The nanostructured CoOx microspheres selenized at 200ºC were composed of Co3O4 and CoO with a 74:26 molar ratio, based on quantitative XRD analysis using relative intensity data. The XRD pattern of the cobalt selenide microspheres selenized at 300ºC had main peaks of orthorhombic CoSe2 and minor peaks of cubic CoSe2 and monoclinic Co3Se4. The microspheres selenized at 400ºC had pure cubic CoSe2 crystal structure with no impurity peaks. A complete selenization of the hollow Co3O4 microspheres into CoSe2 microspheres occurred even at the relatively low temperature of 300ºC. The sharp XRD peaks obtained for the cubic-phase CoSe2 microspheres reveal the abrupt crystal growth occurring at 400ºC. 6
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Figure 3. Morphologies of the nanostructured cobalt selenide microspheres selenized at 300 o
C: (a) SEM image, (b)-(d) TEM images, (e) SAED pattern, and (f) elemental mapping
images.
The morphology of the Co3O4 microspheres was completely preserved at a selenization temperature of 200ºC, as shown in Figure S1. The morphologies of the cobalt selenide powders selenized at 300 and 400ºC are shown in Figures 3 and S2, respectively. The cobalt selenide powders selenized at 300ºC contain hollow spherical structures as shown in the SEM and TEM images in Figure 3. The ultrafine Co3O4 nanocrystals shown in Figure 1c are preserved in the cobalt selenide powders selenized at 300ºC (Figure 3c). The high resolution TEM image shown in Figure 3d reveals clear lattice fringes separated by 0.29 nm, corresponding to the (101) plane of orthorhombic CoSe2. The SAED pattern in Figure 3e shows that the microspheres obtained are mainly composed of polycrystalline orthorhombic CoSe2. The elemental mapping images shown in Figure 3f reveal the uniform distribution of 7
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Co and Se throughout the cobalt selenide microspheres. The thin-shelled hollow structure facilitates the selenization of the cobalt oxide microspheres into cobalt selenide even at the relatively low temperature of 300ºC. The cubic-phase CoSe2 powders selenized at 400 °C contain non-spherical shapes, as shown in Figures S2a and S2b. This suggests that crystal growth at a high selenization temperature (400ºC) destroys the spherical morphology of the Co3O4 microspheres. The grain size of the cobalt selenide powder selenized at 400ºC shown in Figure S2c is about 180 nm. The high resolution TEM image shown in Figure S2d reveals clear lattice fringes separated by 0.27 nm, which correspond to the (210) plane of cubic CoSe2. With increasing selenization temperatures, the SAED pattern of the cobalt selenide powders changes from a ring to a spot due to the abrupt crystal growth occurring at high selenization temperature (400ºC). The elemental mapping images of the cubic-phase CoSe2 powder in Figure S2f also show uniform distributions of Co and Se throughout the powder.
Figure 4. Electrochemical properties of the cobalt compounds: (a) first cyclic voltammograms, (b) initial charge/discharge curves, (c) cycling performances at a constant current density at 0.5 A g-1, and (d) rate performances. 8
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The electrochemical properties of the cobalt oxide and cobalt selenide microspheres as anode materials for NIBs were investigated by CV and galvanostatic discharge/charge cycling in the 0.001–3 V vs. Na/Na+ voltage range. Sharp reduction peaks are observed at 0.81 and 0.86 V during the first discharge process in the CV curves of the cobalt selenide materials selenized at 300 and 400ºC (Figure 4a). The electrochemical properties of cobalt selenide with respect to Li- and Na-ions have not been reported previously. Therefore, the electrochemical reaction of cobalt selenide was estimated from that of cobalt sulfide (CoSx). Transition metal selenides react with Li-ions through a conversion reaction similar to that of transition metal sulfides. In the first Li insertion process, the CV curve of the cobalt sulfide also has a reduction peak at around 1.1 V, which can be attributed to the reduction of CoSx to Co according to Equation (1).42-44 CoSx + 2x Li+ + 2x e– → x Li2S + Co
(1)
The Na-ion storage mechanism with CoSe2 in an NIB can be described by Equation (2). CoSe2 + 4 Na+ + 4 e– ↔ Co + 2 Na2Se
(2)
Therefore, the sharp reduction peaks observed in Figure 4a for the CoSe2 materials are attributed to the conversion of CoSe2 to Co metal nanograins and Na2Se. During reversible charging, oxidation peaks are observed around 1.84 and 1.94 V, which are associated with the recovery of CoSe2 material from Co metal and Na2Se. From the third cycle onwards, the peaks in the CV curves overlap substantially as shown in Figure S3, emphasizing the good cycling stability of the hollow CoSe2 microspheres selenized at 300 ºC. The CV curves for the Co3O4 materials with and without CoO impurities show sharp reduction peaks at around 0.05 V for the first discharge process (Figures 4a and S3). In the first discharge process (Na+ insertion), the CV curve of the Co3O4 materials has a sharp reduction peak at around 0.05 V, which can be attributed to the electrochemical conversion of Co3O4 to Co metal nanograins according to Equation (3). Co3O4 + 8 Na+ + 8 e– → 4 Na2O + 3 Co
(3)
In the first charge process (Na+ desertion), oxidation peaks are observed around 0.8 and 1.2 V, which are attributed to the oxidation of Co metal nanograins with Na2O. As an anode material for NIBs, cobalt oxide requires lower potential than cobalt selenide for the electrochemical conversion process involved in Na-ion storage. The equilibrium potential (Eº) for the conversion reaction plateau of metal compounds was theoretically determined using the Gibbs free energy from Nernst law (Equation 4).4,47 9
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Eº = −∆rG/zF
(4)
The use of Na rather than Li ions leads to a lowering of the electrochemical potential for the conversion reaction of metal compounds. Adelhelm et al. reported the typical calculation of the cell potential for the conversion reaction of metal compounds with Li and Na.4 The difference in cell potential between the Li- and Na-based conversion reaction with metal oxide was 0.96 V. In our paper, the electrochemical conversion potentials of the Co3O4 and CoSe2 materials for the NIB are 0.05 and 0.85 V due to the difference of Gibbs energy. The initial discharge and charge curves obtained for the cobalt selenide and cobalt oxide materials at a current density of 500 mA g−1 are shown in Figure 4b. Distinct plateaus are observed for cobalt selenide at around 0.9 and 1.9 V, but these distinct plateaus are only observed in the first discharging process for the cobalt oxide at around 0.1 V. The results of the first discharge and charge curves are in good agreement with those from the CV curves shown in Figure 4a. The initial discharge capacities of the cobalt selenide materials selenized at 300 and 400 °C were 595 and 586 mA h g−1, respectively, and the corresponding initial charge capacities were 498 and 502 mA h g−1. However, the initial discharge capacities of the Co3O4 materials with and without CoO impurities were 466 and 727 mA h g−1, with corresponding initial charge capacities of 336 and 538 mA h g−1. The CoOx microspheres selenized at 200ºC had shorter plateaus at around 0.1 V in the initial discharge curve than that of the Co3O4 microspheres. This result could be due to the fact that CoO has a difficulty attaining sufficient electrochemical decomposition because the cell potential decreases with the decreasing oxidation state of cobalt.48 The initial Coulombic efficiencies of the Co3O4 and hollow cobalt selenide microspheres selenized at 300ºC were 74 and 84%, respectively. The cycling performances of the cobalt selenide and cobalt oxide materials at a current density of 500 mA g−1 are shown in Figure 4c. The discharge capacities after 40 cycles of the cobalt selenide microspheres selenized at 300 and 400ºC were 467 and 251 mA h g−1, respectively, and their capacity retentions measured from the second cycle onwards were 91 and 50%, respectively. The discharge capacities after 40 cycles of the Co3O4 materials with and without CoO impurities were 253 and 348 mA h g−1, respectively. The cobalt selenide microspheres selenized at 300ºC with ultrafine grains showed a better cycling performance than both cobalt selenide material selenized at 400ºC with larger grains and Co3O4. The Coulombic efficiencies of the cobalt selenide microspheres selenized at 300ºC (Figure S4) exhibit a decreasing trend upon cycling, and lower values than those of the cobalt oxide microspheres 10
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after several cycles. This can be attributed to the formation of unstable solid-electrolyteinterface (SEI), which results in capacity fading and low Coulombic efficiencies during cycling.50 These low Coulombic efficiencies of cobalt selenide could be attributed to repeat formation of solid-electrolyte-interface (SEI) layer on cobalt selenide surface due to structural change from large volume expansion during repeatedly cycling.51,52 Generally, electrolyte decomposition in the lithium and sodium ion batteries forms a passivating SEI layer on the electrode surface at the low potential. However, the SEI layer formed on the cobalt selenide surface can be broken as the nanostructure during huge volume expansion. The fresh cobalt selenide surface exposed to the electrolyte forms the SEI layer again via consumption of Na+ ions, resulting in the low Coulombic efficiency of electrode.51,52 The Coulombic efficiencies and cycle stability of the cobalt selenide microspheres are strongly affected by the type of electrolyte. Therefore, the enhancement of the Coulombic efficiencies, and cycle stability of the cobalt selenide microspheres can be expected by optimizing the electrolyte composition.53-56 The rate performances of the cobalt selenide microspheres selenized at 300ºC and phase pure Co3O4 microspheres are shown in Figure 4d, with current densities increasing stepwise from 100 to 900 mA g−1. The hollow cobalt selenide microspheres selenized at 300ºC provided final discharge capacities of 521, 490, 471, and 446 mA h g−1 at current densities of 100, 300, 600, and 900 mA g−1, respectively. On the other hand, the pure cubic Co3O4 microspheres had final discharge capacities of 504, 416, 338, and 278 mA h g−1 at current densities of 100, 300, 600, and 900 mA g−1, respectively. Cobalt selenide material is well known to be a metallic conductor.57,58 Therefore, cobalt selenide microspheres with good electrical conductivity show faster electrochemical reaction with Na at high current densities than those of the cobalt oxide microspheres. Impedance spectra obtained before and after 40 cycles of the cobalt compound microspheres are shown in Figure S5. The Nyquist plots indicate compressed semicircles in the medium frequency range of each spectrum, which describe the charge transfer resistances (Rct) for these electrodes, and straight lines in the low frequency range, which are associated with Na-ion diffusion in the bulk of the active materials.59,60 Before cycling, the radii of the cobalt selenide microspheres with good electrical conductivity are smaller than those of the cobalt oxide microspheres, which demonstrates low charge transfer resistance. The charge transfer resistances of the cobalt compounds microspheres before and after selenization at 200, 300, and 400ºC are 68, 71, 63, and 93 Ω after 40 cycles, respectively. The cobalt 11
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selenide microspheres selenized at 300ºC exhibit the smallest diameter of semicircle and the shortest straight line in the low frequency region, which indicate the low charge transfer resistance and high Na-ion diffusivity. On the other hand, the cobalt selenide microspheres selenized at 400ºC have the largest charge transfer resistance after 40 cycles. The morphology of the cobalt selenide microspheres selenized at 300 and 400ºC obtained after 40 cycles is shown in Figures S6 and S7, respectively. The cobalt selenide microspheres selenized at 300ºC maintain their hollow morphology even after cycling. However, the spherical morphology of the cobalt selenide microspheres with large grain size selenized at 400ºC is compromised after 40 cycles. The structural damage of the cobalt selenide microspheres selenized at 400ºC increases the charge transfer resistance after cycling. The Na-ion diffusion coefficients were calculated by using the relationship between charge transfer resistance and
ω−1/2 in the low-frequency region.61 According to linear fitting, the Na-ion diffusion coefficients at 25oC were calculated to be 2.43 × 10−12 and 5.72 × 10−11 cm2 s−1 for the precursor and cobalt selenide microspheres selenized at 300oC after 40 cycles, respectively. The cobalt selenide microspheres selenized at 300ºC exhibit low charge transfer resistance, high Na-ion diffusivity, and good structural stability during cycling, have good cycling and rate performances for Na-ion storage.
CONCLUSIONS This study confirmed for the first time that cobalt selenide can be successfully applied as an anode material for NIBs. The hollow cobalt selenide microspheres selenized at 300ºC with ultrafine grains provided both high initial discharge capacity and high initial Coulombic efficiency, in addition to good cycling and rate performances for Na-ion storage. Moreover, the electrochemical properties of the hollow cobalt selenide microspheres could be improved by optimizing the electrolyte system used for Na-ion storage. The hollow cobalt oxide microspheres were shown to be advantageous in terms of their high initial capacity, and low voltages for Na-ion storage as anode material for NIBs. The cycling and rate performances of the hollow Co3O4 microspheres could be upgraded by applying the technologies successfully developed for LIBs, notably by using carbon composite anodes. These hollow cobalt oxide and cobalt selenide microspheres are therefore promising anode materials for NIBs.
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EXPERIMENTAL SECTION Synthesis of hollow cobalt oxide and cobalt selenide microspheres The hollow Co3O4 microspheres were prepared by a spray pyrolysis process, comprising a droplet generator, a high-temperature tubular quartz reactor, and a Teflon bag filter (powder collector). A 1.7 MHz ultrasonic spray generator consisting of six vibrators was used to generate a large number of droplets, which were then carried to the quartz reactor via carrier gas. The reactor temperature and flow rate of air used as carrier gas were fixed at 700ºC and 5 L min−1, respectively. For the selenization of the hollow Co3O4 microspheres, commercial selenium metal powders were used as Se source. The hollow Co3O4 microspheres and selenium metal powders were loaded in a covered alumina boat and placed in a quartz tube reactor, as previously described.36 The selenization treatment was performed at temperatures of 200, 300, and 400ºC for 6 h under a flow of 10 vol.% H2/Ar mixture gas. Characterization The morphologies of the cobalt oxide and cobalt selenide microspheres were investigated by field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi) and transmission electron microscopy (FE-TEM; JEM-2100F, JEOL). The crystal structures of the microspheres were investigated by X-ray diffraction (XRD; X’Pert PRO MPD) using Cu Kα radiation (λ = 1.5418 Å) at the Korea Basic Science Institute (Daegu). Electrochemical properties were analyzed in a 2032-type coin cell. The anode was prepared from a mixture of active material, carbon black, and sodium carboxymethyl cellulose in a 7:2:1 weight ratio. Na metal and a microporous polypropylene film were used as counter electrode and separator, respectively. The electrolyte consisted of a solution of 1 M NaClO4 (Aldrich) in a 1:1 volume mixture of ethylene carbonate/dimethyl carbonate with 5 wt.% fluoroethylene carbonate. The discharge and charge characteristics of the samples were investigated by cycling in the 0.001–3 V voltage range at various current densities. Cyclic voltammograms (CVs) were recorded at a scan rate of 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) of the electrode was measured over a frequency range of 100 kHz – 0.01 Hz.
■ ASSOCIATED CONTENT Supporting Information. 13
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SEM and TEM images of the powders selenized at 200 and 400 oC. Cycling voltammograms of the cobalt compounds selenized at different temperatures. Coulombic efficiencies versus cycle number for cobalt compounds microspheres materials. Nyquist plots of the electrochemical impedance spectra of the cobalt compounds. Morphologies and elemental mapping images of the cobalt selenide microspheres obtained after cycling. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; Fax: (+82) 2-928-3584. ACKNOWLEDGMENTS This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (201320200000420). This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2015R1A2A1A15056049).
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