Article pubs.acs.org/IC
Synthesis of Hierarchical Sb2MoO6 Architectures and Their Electrochemical Behaviors as Anode Materials for Li-Ion Batteries Xuan Lu,† Zhenyu Wang,† Lu Lu,‡ Guang Yang,*,‡ Chunming Niu,† and Hongkang Wang*,† †
Center of Nanomaterials for Renewable Energy (CNRE), State Key Lab of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, China 710049 ‡ Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, China 710049 S Supporting Information *
ABSTRACT: We report a facile microwave-hydrothermal synthesis of hierarchical Sb2MoO6 architectures assembled from single-crystalline nanobelts, which are first demonstrated as anode materials for lithium-ion batteries (LIBs) with superior electrochemical properties. Sb2MoO6 delivers a high initial reversible capacity of ∼1140 mA h/g at 200 mA/g with large initial Coulombic efficiency of ∼89%, and a reversible capacity of ∼878 mA h/g after 100 cycles at 200 mA/g. As a new anode, the electrochemical behaviors are investigated through ex situ TEM and XPS measurements, revealing that the superior electrochemical performance is attributed to the novel hierarchical structures and the synergistic interaction between both the active Sb- and Mo-species, in which the in situ generated Li2O−MoOx serves as matrix and efficiently buffers the volume changes of the Li−Sb alloying−dealloying upon cycling.
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INTRODUCTION Hierarchical architectures assembled from low-dimensional nano-building-blocks have attracted considerable attention because of their structure-dependent physicochemical properties and widespread applications in many areas including lithium-ion batteries (LIBs), solar cells, gas sensors, catalysis, and so on.1−6 Therefore, fabrication of hierarchical architectures with delicate compositional and morphological control has become fascinating and vital for both fundamental studies and practical applications. Among the family of metal oxides, mixed-valence metal oxides with different metal cations have demonstrated good electrochemical behaviors owing to the interfacial effects and synergistic effects between multiple metal species.7−11 As a typical mixed metal oxide, antimony− molybdenum oxide (Sb2MoO6) exhibits interesting physical properties such as ferroelectricity, piezoelectricity, and pyroelectricity, and has been widely used as a selective catalyst for oxidative esterification and alkene oxidation,12−15 but has not yet been investigated as anode for LIBs, most likely because of the lack of suitable synthetic approaches. As a potential anode material for LIBs, Sb2MoO6 crystallizes with a layered structure built up by alternate stacking of αSb2O3 related {Sb2O2}n2n+ and SnF4 type {MoO4}n2n− layers,14 while both Sb2O316−18 and MoO319−21 are promising anode materials for LIBs with lithium storage based on the alloying− dealloying reaction and the conversion reaction, respectively. Sb2O3 as anode has been suggested to be based on a two-step © XXXX American Chemical Society
reaction mechansim with a theoretical capacity of 552 mA h/g, which first undergoes structure destruction during the electrochemcial reactions, resulting in the formation of metallic Sb and the irreversible Li2O in the first discharge process according to the reaction of Sb2O3 + 6Li+ + 6e− → 2Sb + 3Li2O. Then, Sb can reversibly react with lithium ions via alloying−dealloying according to the reaction of Sb + 3Li+ + 3e− ↔ Li3Sb.22−24 As a conversion type anode material, MoO3 possesses a higher theoretical capacity of 1117 mA h/g based on the reaction of MoO3 + 6Li+ + 6e− ↔ 2Mo + 3Li2O,19,25 in which the Li2O is reversible, different from the irreversible Li2O formed from the Sb2O3 anode. Therefore, it is fundamentally interesting and practically important to prepare and investigate the electrochemical behaviors of Sb2MoO6. Currently, synthetic Sb2MoO6 has been realized as dominant by the solid-state chemical reaction between equimolar Sb2O3 and MoO3 at higher temperatures,12−14,26,27 which usually results in poor size and phase regulation. Also, to the best of our knowledge, there are few reports on facile synthesis of Sb2MoO6 by wet-chemical methods, which allows for fine structural control on the aspect of size, phase, and composition on the nanoscale. Therefore, synthesis of Sb2MoO6 with confirmed compound structure and delicate nano/microstructures is still a big challenge. Received: April 7, 2016
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DOI: 10.1021/acs.inorgchem.6b00856 Inorg. Chem. XXXX, XXX, XXX−XXX
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electrolyte. Specific capacity was reported on the basis of the mass of Sb2MoO6 active materials, and the loading amount of active material in the electrodes was in the range 1.0−1.2 mg/cm2.
Herein, we report the successful wet-chemical synthesis of hierarchical Sb2MoO6 nanostructures assembled from nanobelts via a facile microwave hydrothermal route using SbF3 and Na2MoO4 as Sb and Mo sources, without the use of any additives or surfactants. Moreover, the electrochemical activity of the hierarchical Sb 2 MoO 6 nanostructures was first investigated as a LIB anode material, and the electrochemical reaction mechanism was explored via the ex situ TEM and XPS analyses.
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RESULTS AND DISCUSSION Hierarchical Sb2MoO6 architectures composed of nanobelts were synthesized via a facile microwave hydrothermal route using SbF3 and Na2MoO4 as starting materials with Sb/Mo molar ratio of 2. Figure 1a shows the X-ray diffraction (XRD)
EXPERIMENTAL SECTION
All chemicals including antimony(III) fluoride (SbF3, Alfa Aesar), sodium molybdate(VI) dihydrate (Na2MoO4·2H2O, J&K), Sb2O3 (Alfa Aesar), and MoO3 (Alfa Aesar) were used as received without any further purification. Materials Synthesis. Microwave-assisted hydrothermal method was applied to synthesize Sb2MoO6 nanostructures, using SbF3 as Sb source and Na2MoO4·2H2O as Mo source. In a typical synthesis, 0.1 g of Na2MoO4·2H2O (0.4 mmol) and 0.15 g of SbF3 (0.8 mmol) were added into 30 mL of deionized water, and stirred for 5 min to ensure that all reagents were dispersed homogeneously and dissolved. The precursor solution with light green color was then transferred into a 50 mL microwave digestion tank. The digestion tank was sealed and heated at 160 °C for 90 min under microwave irradiation. After the reaction tank cooled to room temperature, the green precipitate was collected by centrifugation, thoroughly washed with water and ethanol, and then dried in a vacuum oven at 60 °C overnight. Materials Characterization. Powder X-ray diffraction (XRD) patterns of the products were recorded on a Bruker D2 PHASER Xray diffractometer using Cu Kα radiation (λ = 1.5418 Å) with an operating voltage of 30 kV and a current of 10 mA in the range 10− 80°. The morphologies of the products were examined by a scanning electron microscope (SEM, FEI Quanta 250F). Transmission electron microscope (TEM) images were recorded by a JEOL-2100 TEM operated at 200 kV. The high-resolution transmission electron microscope (HRTEM) images were carried out on a FEI-Titan TEM at an acceleration voltage of 200 kV. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) was performed by using an aberration-corrected JEOL ARM200F instrument with energy-dispersive X-ray spectroscopy (EDS) for elemental analysis. X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi) was applied to determine the compositions and chemical states. X-ray fluorescence (XRF) microanalysis was used to determine the relative Sb/Mo contents of the product, and was performed using a wavelength dispersive XRF-1800 instrument (SHIMADZU, Kyoto, Japan) equipped with a rhodium target tube and flow proportional counter detector (FPC) under vacuum conditions at 40 kV and 70 mA. Electrochemical Measurements. Electrochemical measurements were performed using CR2025 coin type cells assembled in an Ar-filled glovebox with H2O and O2 content less than 1.0 ppm. The working electrodes were fabricated by casting a paste composed of the active material (Sb2MoO6), conductive agent (acetylene black, AB), and binder (poly(acrylic acid), PAA, Mw = 100 000, Sigma-Aldrich) in a weight ratio of active material/AB/PAA = 8:1:1 on copper foil, using water as solvent. Then, the electrodes were dried in a vacuum oven at 120 °C overnight. Lithium foil was used as the counter electrode, and a Celgard 2400 microporous membrane was used as separator. An electrolyte containing 1.0 M LiPF6 dissolved in ethylene carbonate/ dimethyl carbonate (1:1 in volume) was used. Galvanostatic discharge/charge tests of the batteries were carried out on a battery test system (NEWARE BTS, Neware Technology Co., Ltd., China) in the range 0.01−3.0 V (vs Li/Li+) at room temperature (25 °C). Cyclic voltammetry (CV) was carried out using an electrochemical station (Autolab PGSTAT 302N) in the range 0.01−3.0 V at a scan rate of 0.2 mV/s. For ex situ TEM and XPS analyses, the batteries at different discharge and charge states were disassembled, and the electrodes were washed thoroughly with dimethyl carbonate and ethanol to remove the
Figure 1. (a) XRD pattern of the as-prepared Sb2MoO6, and stimulated and standard XRD patterns of Sb2MoO6 with only main diffraction peaks are shown for comparison. (b, c) Schematic illustration of (b) unit cell of monoclinic Sb2MoO6 and (c) its layered structure.
pattern of the as-prepared product, which matches well with simulated XRD pattern and the standard pattern of Sb2MoO6 (JCPDS 33-1491). No extra peaks are detected, indicating the high purity of the product. Both energy dispersive spectroscopy (EDS) and X-ray fluorescence (XRF) analyses reveal that the atomic ratio of Sb/Mo in the product is ≈2 (Figure S1, Table S1), which is consistent with the stoichiometric Sb2MoO6, further implying the product is a phase-pure compound. Moreover, X-ray photoelectron spectroscopy (XPS) reveals the chemical states of Sb(III)28 and Mo(VI)29 (Figure S2). Figure 1b shows a computational simulated unit cell of Sb2MoO6 with a triclinic structure and the space group of P1̅, which is in agreement with the XRD result. As illustrated in Figure 1c, Sb2MoO6 displays a distorted three-dimensional layered structure consisting of alternate octahedral MoO6 monolayer and tetrahedral SbO4 bilayers linked by corner sharing of O atoms. The morphology of the as-prepared Sb 2 MoO 6 was characterized by SEM and TEM. SEM images in Figure 2a,b reveal the hierarchical architecture of Sb2MoO6 with overall size of ∼5 μm, which are self-assembled from one-dimensional (1D) nanobelts. The 1D nanobelt is very thin (10−15 nm) with a length of 1−3 μm and width of 100−200 nm. The HRTEM images taken from the cross-section and plane view of single nanobelts are shown in Figure 2c,d, respectively. The lattice spacing of 0.33 nm in Figure 2c can be indexed to the (003) plane of Sb2MoO6, while three sets of lattice fringes with spacings of 0.33, 0.25, and 0.33 nm correspond to (11̅2), (22̅0), and (11̅2̅) planes of Sb2MoO6 (Figure 3d), respectively. Moreover, the angle between (11̅2) and (22̅0) planes is ∼48°, which is consistent with the theoretical value in the triclinic phase of Sb2MoO6. Figure 2e shows the HAADF image of a single Sb2MoO6 nanobelt, and the corresponding EDS elemental maps of Sb, Mo, and O are shown in Figure 2f−h, B
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Figure 2. (a) SEM and (b) TEM images of the as-prepared Sb2MoO6 hierarchical architectures with inset of part b showing the typical nanobelt building blocks. (c) HRTEM image taken from the cross-section of a single Sb2MoO6 nanobelt. (d) HRTEM image taken from the inset nanobelt in plane view. (e) HAADF image of a single hierarchical Sb2MoO6 architecture and the corresponding EDS maps of (f) Sb, (g) Mo, and (h) O elements.
by the ex situ TEM analysis (Figure 4a). This peak disappears in the next scan, indicating its irreversibility. Another reversible reduction peak at ∼0.75 V typically corresponds to the Li−Sb alloying process (Sb → LixSb), while the oxidation peak at ∼1.2 V relates to the dealloying of LixSb (LixSb → Sb).30,31 It is also observed that the reduction/oxidation peaks (0.75/1.2 V) are both slightly shifted in the subsequent cycles, implying the structural or compositional changes after electrochemical reactions, which match well those of previous reports30,31 and commercial Sb2O3 (Figure S7). Interestingly, another oxidation peak located at ∼1.4 V becomes more prominent and shows relatively higher current intensity, as compared with that of Sb2O3 film electrode,10 MSb2O6 (M = Ni, Co),15 and commercial Sb2O3 (Figure S7), which can be attributed to the formation of Sb-based oxides.15 Correspondingly, a weak reduction peak at ∼1.3 V emerges in the subsequent CV cycles, which can be ascribed to the re-reduction of Sb-based oxides. Simultaneously, a new minor reduction peak located at ∼1.75 V emerges, which together with the one at ∼1.3 V may be evolved from the irreversible prominent reduction peak at ∼1.55 V, arising from the phase separation of the Sb2MoO6 electrode in the first discharge process. Meanwhile, a broad oxidation peak at around 2.5 V seems to emerge in the second CV cycle, which may relate to the partial minor contribution of Mo-related species on lithium storage, as revealed by comparing that with CV curves of the MoO3 anode (Figure S7). Most interestingly, the ratio reverse between the two oxidation peaks at ∼1.2 and ∼1.4 V in Sb2MoO6 as compared with that of Sb2O3 may be
respectively, confirming no observable phase or elemental separation in Sb2MoO6. Experiments were also carried out to investigate the formation process of the stoichiometric Sb2MoO6 hierarchical structures by varying the reaction parameters such as temperature, Sb/Mo ratio, solvent, and so on. As known from the Sb−Mo−O phase diagram, two other stable phases, Sb2Mo10O31 and Sb4Mo10O31, possibly coexist with Sb2MoO6. However, our results show that only Sb2MoO6 was produced under current wet-chemical synthesis, even varying the Sb/Mo molar ratio from 2/1 to 2/10 and 4/10, while the morphology was almostly kept in the form of hierarchical architecture assembled from nanobelts with few changes (Figures S3−4), indicating Sb2MoO6 is the only stable phase in such aqueous reaction systems. In addtion, H2O plays the key role in the formation of the Sb2MoO6 phase, and directly participates in the chemical reaction, which may be described as 2SbF3 + Na2MoO4 + 2H2O → Sb2MoO6 + 4HF + 2NaF. This was verfied by the fact that mainly Sb2O3 was produced when H2O was replaced by organic solvents such as dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP) (Figures S5−6). The electrochemical properties of Sb2MoO6 were first investigated as anode material for LIBs using lithium foil as the counter electrode. Figure 3a shows the CV curves of the Sb2MoO6 electrode in the initial three cycles. In the first cathodic scan, a prominent reduction peak located at ∼1.55 V is observed and can be attributed to the reduction of Sb2MoO6 and formation of Sb metal nanoparticles, which was confirmed C
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Figure 3. (a) Cyclic voltammogram (CV) curves of Sb2MoO6 at a scan rate of 0.2 mV/s in 0.01−3.0 V. (b) Galvanostatic discharge−charge profiles and (c) cycle performance of Sb2MoO6 with corresponding Coulombic efficiency at a current density of 200 mA/g in 0.01−3.0 V. (d) Rate performance of Sb2MoO6 at current densities in the range 100−800 mA/g in 0.01−3.0 V.
capacities are highly dependent on the current densities, implying kinetic control of the lithiation/delithiation process.32 The superior rate performance of the Sb2MoO6 electrode can be attributed to the better Li+ diffusion kinetics, which is verified by electrochemical impedance spectroscopy (EIS). As shown in Figure S8, Nyquist plots of the Sb2MoO6 electrode at different states exhibit a much smaller semicircle diameter (990 mA h/g) at each current density. The discharge capacities are 1180, 1120, 1049, and 990 mA h/g at current densities of 100, 200, 400, and 800 mA/g each after 10 cycles, respectively. When recovering the current density to 100 mA/g, a discharge capacity of ∼1000 mA h/g is maintained after successive 20 cycles, indicating the high rate capability. In addition, it is observed that the charge D
DOI: 10.1021/acs.inorgchem.6b00856 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. (a) TEM image with inset of HRTEM image of the discharged Sb2MoO6 at 1.55 V. (b) TEM image of the discharged Sb2MoO6 at 0.01 V. (c) TEM and (d) HRTEM images of the charged Sb2MoO6 at 3.0 V, and (e) the corresponding HADDF image and its elemental mapping of the (f) Mo and (g) Sb elements.
between Sb 3d5/2 and O 1s, the broad peak centered at around 531.5 eV can be split into Sb 3d5/2 peaks coming from Sb metal and Sb-oxides, and O 1s peaks coming from SEI in states of C− O groups and carbonates (Figure 5a−c). When first discharged to 1.55 V, peaks belonging to Sb metal emerge, which is consistent with the HRTEM result (Figures 4b and 5a). When further discharged to 0.01 V, peaks belonging to Sb-alloy are observed, indicating the formation of Li−Sb alloys, which disappear when charged to 3.0 V. These results clearly confirm the reduction of Sb2MoO6 into Sb metal, which contibutes to the lithium storage via the reversible lithiation−deliathiation (Sb + xLi+ + xe− ↔ LixSb). Interestingly, Sb-oxide presents in the electrode at different discharge and charge states, which on one hand is due to the presence of unactivitated Sb2MoO6 in the first discharge−charge cycle and on the other hand due to the oxidation when exposed in air during the TEM sample preparation. Notably, no Li−Sb alloy and trace Sb metal peaks are present in the charged electrode (Figure 5c), which may indicate the partial reoxidization of Sb into Sb2O3, consistent with the CV results (Figure 3a). Moreover, it has been reported that Sb2O3 is possible to perform as a reversible anode according to the reaction Sb2O3 + 12Li+ + 12e− ↔ 2Li3Sb + 3Li2O,23,24,31 and the theoretical capacity associated with this reaction is as high as 1103 mA h/g, but it is difficult to completely decompose Li2O and reoxidize Li3Sb to Sb2O3. In contrast, lithium ions might be stored as Li2O at the interface of metallic Mo nanoparticles, and nanosized Mo could recombine with O atoms and release Li ions; hence, Li2O becomes reversible. More interestingly, Mo usually functions as a charge
expansion because of the Li−Sb alloying. When further charged to 3.0 V, the amorphous nanobelt undergoes shrinkage (diameter: ∼70 nm) owing to the dealloying of LixSb. Notably, the large nanoparticles with various shapes and tens of nanometers are produced within the amorphous nanobelts (Figure 4c), which are embedded and isolated by the amorphous matrix, instead of separating from the 1D matix. Figure 4d shows the HRTEM image of the embedded nanopaticles, and the spacing of the lattice fringes from the large particle is clearly observed with a value of 0.38 nm, which is solely ascribed to the (003) plane of the rhombohedral Sb (JCPDS 35-0732). Besides, small nanocrystallites are also observed, which shows the lattice spacing of 0.32 nm belonging to the Sb2O3 (Figure 4d). Figure 4e shows the HADDF image of the charged Sb2MoO6 electrode, within which the Sb-species are apparently accumulated due to the formation of Sb nanoparticles, while the Mo-species are uniformly distributed without aggregation, indicating the MoOx together with Li2O serving as a matrix to confine the Li−Sb alloying−dealloying reactions. Also, the enhanced confinement may originate from the layered structure of Sb2MoO6 with a single {MoO4}n2n− layer sandwiched between {Sb2O2}n2n+ layers, which is consistent with the previous report.32 This is greatly helpful for maintaining the structral integrity of the electrode and finally contributes to the superior electrochemical performance of the Sb2MoO6 electrode. Ex situ XPS measurements were performed to determine the chemical states of the Sb2MoO6 electrodes at different discharge and charge states (Figure 5). Due to the overlap E
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Figure 5. (a−c) Sb 3d and O 1s and (d−f) Mo 3d XPS spectra of Sb2MoO6 electrodes at different discharge−charge states at 50 mA/g in 0.01−3.0 V: (a, d) discharged to 1.55 V, (b, e) discharged to 0.01 V, (c, f) charged to 3.0 V.
tranfer center owing to its multiple valence states,36 which is proposed to enhance the reversible formation−decomposition of Li2O with the assistance of Mo-related species. As shown in Figure 5d−f, the chemical state evolution of Mo element at different discharge and charge states is demonstrated, revealing that both Mo(VI) and Mo(IV) dominate in either the discharged or charged electrodes, but no metallic Mo was detected, indicating that the conversion reaction of Mo6+ ↔ Mo0 may not take place in the Sb2MoO6 anode in the first cycle. As shown in Tables S2−4, the relative content of Mo6+ and Mo4+ was roughly calculated to be 1:1 in all three states, which may be related to the partially irreversible formation of Li2MoO3 when Mo(VI) reacted with lithium ions. In order to explain this abnormal phenomenon, it should be noted that the crystalline Sb2MoO6 became amorphous in the first discharge process, in which Mo-species may present as amorphous MoO3
and MoO2 (or Li2MoO3) confined within the Li2O matrix (as supported by Figure 4e−g). As a result of this confinement effect, the formation of Mo metal nanoparticles is impossible.32 Even though Mo nanodots may be produced in the Li2O matrix, the metallic Mo would be unstable and prone to be oxidized. Moreover, MoO2 has a stronger bond dissociation energy than that of MoO3, which makes it difficult to lithiate MoO2 through a conversion reaction but may with an intercalation−deintercalation mechanism.37 In addition, owing to the atomic-level mixing of Sb−Mo atoms, the charge transfer from Mo to Sb is facilitated because of the larger Pauling electronegativity of Sb.38 This may make MoOx more likely to serve as a medium to promote charge transfer from Mo to Sb, but which needs more detailed in situ studies. Nevertheless, the MoOx serving as matrix can buffer the volume changes of the F
DOI: 10.1021/acs.inorgchem.6b00856 Inorg. Chem. XXXX, XXX, XXX−XXX
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CONCLUSIONS In summary, we have reported the wet-chemical synthesis of Sb2MoO6 with hierarchical structures assembled from nanobelt building blocks via a facile microwave hydrothermal method without use of any surfactants, and first demonstrated their electrochemical properties as anode material for LIBs. Structural investigation reveals that the as-prepared Sb2MoO6 is a phase-pure compound with stoichiometric composition. As a new anode material, Sb2MoO6 showed superior electrochemical properties, including high initial reversible capacity, large initial Coulombic efficiency, good cycle, and rate performance. Moreover, ex situ TEM and XPS analyses revealed that the superior electrochemical properties were attributed to the synergistic interaction between both the active Sb- and Mo-species, based on the combination mechanism of alloying−dealloying reaction and conversion reaction. In addition, the MoOx and Li2O matrix in situ generated during the electrochemical reactions efficiently buffered the volume changes of the Li−Sb alloying−dealloying upon cycling, indicating the synergistic enhancement effect on lithium storage performance.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00856. Listings of EDS, XRF, XPS results of the as-prepared samples; XRD patterns and SEM images of samples obtained with different Sb/Mo molar ratio and different solvents; CV curves of Sb2MoO6, MoO3, and Sb2O3; and Nyquist plots of the Sb2MoO6 electrode at different states. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grants 51402232 and 51521065), the Fundamental Research Funds for the Central Universities in China, and the Natural Science Basis Research Plan in Shaanxi Province of China (No. 2015JQ5131). The authors also thank Dr. Lu Lu and Mr. Chuansheng Ma for TEM measurements, Ms. Yanzhu Dai for SEM measurement, and Dr. Qingqin Ge for XPS analyses.
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