Facile Synthesis of Antimony Tungstate Nanosheets as Anodes for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) have been widely used in the fields of smart phones, electric vehicles, and smart grids. With its opened Aurivillius structure, tungstate antimony oxide (Sb2WO6, SWO), constituted of {Sb2O2}2n+ and {WO4}2n−, is rarely investigated as an anode for lithium-ion batteries. In this work, Sb2WO6 with nanosheets morphology was successfully synthesized using a simple microwave hydrothermal method and systematically studied as an anode for lithium-ion batteries. The optimal SWO (SWO-60) exhibits a high specific discharge capacity and good rate capability. The good electrochemical performance could be ascribed to mesoporous nanosheets morphology, which is favorable for the penetration of the electrolyte and charge transportation. The results show that this nanostructured SWO is a promising anode material for LIBs.


Introduction
Lithium-ion batteries (LIBs) are widely used in the fields of portable devices, electric cars, and smart grids due to of their advantages, which include a higher working voltage and longer cycle life than many other energy storage systems [1][2][3][4][5][6][7][8][9]. However, the energy density of LIBs is still limited and may not meet demand, especially in the fields of high-performance electric vehicles and unmanned aerial vehicles, effectively hindering further development in these fields [10][11][12][13][14][15][16][17]. The reversible discharge capacity of commercial anodes (graphite), which is only~372 mAh g −1 , cannot meet the increasing demands for high-energy-density applications [18][19][20][21][22][23][24][25][26]. The use of proper anode materials with high capacities is a good strategy to solve the problem of LIBs [17]. Therefore, it is urgent that high-capacity negative materials for rechargeable lithium-ion batteries are developed and optimized.

Fabrication of Sb 2 WO 6
All chemicals were directly used after purchase without further purification. As per the typical method, 0.1045 g Na 2 WO 4 ·2H 2 O and 0.1446 g SbCl 3 were separately dissolved in 20 mL and 10 mL of distilled water, respectively, and then stirred for 15 min. The Na 2 WO 4 ·2H 2 O solution was added to the SbCl 3 solution until a yellow suspension was formed, after which it was stirred for another 30 min. Next, the solution was added to a microwaveable water kettle (50 mL) and fixed in a microwave hydrothermal synthesizer (XH800, Beijing, China). A continuous microwave heating mode was set up to provide a stable heat source at a microwave power of 500 W. The temperature of the mixed solution quickly rose from room temperature to 160 • C and was maintained at 160 • C for different lengths of time (45 min, 60 min, and 75 min, which were denoted as SWO-45, SWO-60, and SWO-75, respectively). Afterwards, the resulting precipitates were filtered and washed several times with absolute ethanol and deionized water, and then the collected precipitation was dried overnight by lyophilization (LGJ-12, Beijing, China).

Material Characterization
Scanning electron micrographs were collected on a field-emission scanning electron microscope (FESEM, JSM-5610LV, JEOL, Akishima, Japan), and transmission electron micrographs and selected area electron diffraction patterns (SAED) were collected on a transmission electron microscope (TEM, JSM-2100F, 200 kV, Hitachinaka, Naka, Japan) to visually observe morphologies, particle sizes, etc. The crystallographic phase of the as-synthesized samples was characterized by X-ray diffraction (XRD, Bruker D8 ADVANCE, Cu kα source), and the 2θ-angle ranged from 10 • -80 • . Nitrogen adsorption-desorption isotherms were acquired by the Brunauer-Emmett-Teller (BET) method at 77 K from a Quadrasorb SI analyzer, and pore size distribution originated from the desorption branch based on the Barrett-Joyner-Halenda (BJH) theory.

Electrochemical Measurements
The electrochemical lithium storage performances were evaluated through a two-electrode system. Specifically, high-purity lithium foil (15.8 mm × 0.5 mm) was employed as the counter and reference electrode. After dissolving carboxymethylcellulose sodium (binder, CMC) in deionized water and stirring gently for 24 h to fabricate the working electrode, the slurry was made by mixing the Sb 2 WO 6 samples, carbon black (Super P), and CMC at a weight ratio of 70:20:10. After fully stirring, the steady slurry was coated onto conductive copper foil and dried at 80 • C for 5 h. The dried electrodes were cut Nanomaterials 2019, 9, 1689 3 of 10 into a 12 mm-diameter disk to serve as a working electrode, and the active material loading in each disk was weighed to be 0.9-1.1 mg, corresponding to 0.8-0.97 mg cm −2 . The batteries were assembled in the glove box with argon atmosphere. During assembly, the Celgard 2400 was used as a separator, while the electrolyte was made by dissolving 1M LiPF 6 into a solvent consisting of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC/DMC, 1:1 in volume ratio). The galvanostatic discharge-charge measurements were performed on a LAND CT2001A in the voltage window from 0.01 to 3.0 V (vs. Li/Li + ). An electrochemical impedance spectroscopy (EIS) was conducted ranging from 100 kHz to 0.01 Hz, and the cyclic voltammetry (CV) was measured in the range of 0.01-3.0 V (vs. Li/Li + ) at 0.5 mV s −1 . The two electrochemical tests discussed above were carried out on an electrochemistry workstation (CHI660E, Shanghai, China).  [29]. Furthermore, no additional phase could be found, indicating that pure Sb 2 WO 6 was successfully synthesized. Nanomaterials 2019, 9, x FOR PEER REVIEW 3 of 11 material loading in each disk was weighed to be 0.9 mg-1.1 mg, corresponding to 0.8-0.97 mg cm −2 .

Structure and Morphology
The batteries were assembled in the glove box with argon atmosphere. During assembly, the Celgard 2400 was used as a separator, while the electrolyte was made by dissolving 1M LiPF6 into a solvent consisting of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC/DMC, 1:1 in volume ratio). The galvanostatic discharge-charge measurements were performed on a LAND CT2001A in the voltage window from 0.01 to 3.0 V (vs. Li/Li + ). An electrochemical impedance spectroscopy (EIS) was conducted ranging from 100 kHz to 0.01 Hz, and the cyclic voltammetry (CV) was measured in the range of 0.01-3.0 V (vs. Li/Li + ) at 0.5 mV s −1 . The two electrochemical tests discussed above were carried out on an electrochemistry workstation (CHI660E, Shanghai, China).  [29]. Furthermore, no additional phase could be found, indicating that pure Sb2WO6 was successfully synthesized. The Sb2WO6 samples synthesized by the different microwave heating times (SWO-45, SWO-60, and SWO-75) were also characterized by a nitrogen adsorption-desorption isotherms measurement. As depicted in Figure 1b-d, typical IV-type isotherms were obtained, indicating that almost no micropores existed in the Sb2WO6 samples [36]. A large number of mesopores (whose diameters are usually smaller than 30 nm) were observed in the pore-size distribution curves (inset in Figure 1b The Sb 2 WO 6 samples synthesized by the different microwave heating times (SWO-45, SWO-60, and SWO-75) were also characterized by a nitrogen adsorption-desorption isotherms measurement. As depicted in Figure 1b-d, typical IV-type isotherms were obtained, indicating that almost no micropores existed in the Sb 2 WO 6 samples [36]. A large number of mesopores (whose diameters are usually smaller than 30 nm) were observed in the pore-size distribution curves (inset in Figure 1b-d).

Structure and Morphology
The specific surface areas, mean pore sizes, and mean crystallite sizes of the three different SWO samples are listed in Table 1. According to the BET equation and the Barrett-Joyner-Halenda plot, the specific surface area of the SWO-60 sample was calculated as~22.07 m 2 g −1 , and the mean pore size appeared at 3.9 nm. According to the Scherrer equation, the mean crystallite size was calculated as 13.82 nm. In contrast, the specific surface area and the mean pore size of the SWO-45 sample was 16.79 m 2 g −1 and 3.7 nm, respectively. This specific surface area was much smaller than that of the SWO-60 sample, and the mean crystallite size of SWO-45 was higher than SWO-60. Although the specific surface area of the SWO-75 sample (~22.98 m 2 g −1 ) was a little higher than the surface area of the SWO-60 sample, the main pore size was smaller than in the SWO-60 sample. It is believed that this mesoporous structure, with appropriate crystallite size, may be favorable for the transport and storage of lithium ions [38]. A scanning electron microscope and a transmission electron microscope were carried out to examine the microstructures and morphology of the SWO samples. As depicted in Figure 2a,b, the as-synthesized SWO-45 and SWO-60 samples exhibit similar nanosheet-like morphology. The nanosheets in SWO-45 ( Figure 2a) are severely agglomerated and unevenly distributed, whereas the SWO-60 nanosheets are uniformly grown, and the thickness of the nanosheets is 12~21 nm, which might favor the charge transport and ion diffusion as well as the penetration of the electrolyte. In contrast, the SWO-75 sample ( Figure 2c) seems to be of nanoparticle assembly, the reason for which may be that the microwave reaction time was too long, resulting in the overgrowth of nanosheets. As depicted in Figure 2d, the transmission electron microscope image further confirms flake morphology of the SWO-60 sample. A high-resolution transmission electron microscope (HRTEM) image is depicted in Figure 2e, with interplanar spacing of 3.3 Å in a single nanosheet, which corresponds to the spacing between the (211) crystal plane of the Sb 2 WO 6 . Figure 2f shows the SAED of SWO-60, indicating that the crystalline structure of SWO-60 is polycrystalline, which agrees well with the X-ray diffraction patterns depicted in Figure 1a.
Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 11 d). The specific surface areas, mean pore sizes, and mean crystallite sizes of the three different SWO samples are listed in Table 1. According to the BET equation and the Barrett-Joyner-Halenda plot, the specific surface area of the SWO-60 sample was calculated as ~22.07 m 2 g −1 , and the mean pore size appeared at 3.9 nm. According to the Scherrer equation, the mean crystallite size was calculated as ~13.82 nm. In contrast, the specific surface area and the mean pore size of the SWO-45 sample was ~16.79 m 2 g −1 and 3.7 nm, respectively. This specific surface area was much smaller than that of the SWO-60 sample, and the mean crystallite size of SWO-45 was higher than SWO-60. Although the specific surface area of the SWO-75 sample (~22.98 m 2 g −1 ) was a little higher than the surface area of the SWO-60 sample, the main pore size was smaller than in the SWO-60 sample. It is believed that this mesoporous structure, with appropriate crystallite size, may be favorable for the transport and storage of lithium ions [38]. A scanning electron microscope and a transmission electron microscope were carried out to examine the microstructures and morphology of the SWO samples. As depicted in Figure 2a,b, the as-synthesized SWO-45 and SWO-60 samples exhibit similar nanosheet-like morphology. The nanosheets in SWO-45 (Figure 2a) are severely agglomerated and unevenly distributed, whereas the SWO-60 nanosheets are uniformly grown, and the thickness of the nanosheets is 12~21 nm, which might favor the charge transport and ion diffusion as well as the penetration of the electrolyte. In contrast, the SWO-75 sample (Figure 2c) seems to be of nanoparticle assembly, the reason for which may be that the microwave reaction time was too long, resulting in the overgrowth of nanosheets. As depicted in Figure 2d, the transmission electron microscope image further confirms flake morphology of the SWO-60 sample. A high-resolution transmission electron microscope (HRTEM) image is depicted in Figure 2e, with interplanar spacing of 3.3 Å in a single nanosheet, which corresponds to the spacing between the (211) crystal plane of the Sb2WO6. Figure 2f shows the SAED of SWO-60, indicating that the crystalline structure of SWO-60 is polycrystalline, which agrees well with the X-ray diffraction patterns depicted in Figure 1a.  To further investigate the chemical composition and valence state of SWO-60, an X-ray photoelectron spectroscopy (XPS) was conducted. In Figure 3b, the binding energies of 530.9 eV and 540.3 eV are in accordance with the Sb 3d 5/2 and Sb 3d 3/2 with 3 + oxidation state, respectively [39]. In addition, the peak at 531.9 eV is the characteristic of O 1s. In Figure 3c, the peak at 767.9 eV could be attributed to the Sb 3p 3/2 binding energy of Sb 2 WO 6 . Additionally, the peaks at 35.9 eV, 38.1 eV, and 34.5 eV are characteristics of W 4f 7/2 , W 4f 5/2, and Sb 4d, respectively, and W 4f 7/2 and W 4f 5/2 are in accordance with the oxidation state of W 6+ [40,41]. To further investigate the chemical composition and valence state of SWO-60, an X-ray photoelectron spectroscopy (XPS) was conducted. In Figure 3b, the binding energies of 530.9 eV and 540.3 eV are in accordance with the Sb 3d5/2 and Sb 3d3/2 with 3 + oxidation state, respectively [39]. In addition, the peak at 531.9 eV is the characteristic of O 1s. In Figure 3c, the peak at 767.9 eV could be attributed to the Sb 3p3/2 binding energy of Sb2WO6. Additionally, the peaks at 35.9 eV, 38.1 eV, and 34.5 eV are characteristics of W 4f7/2, W 4f5/2, and Sb 4d, respectively, and W 4f7/2 and W 4f5/2 are in accordance with the oxidation state of W 6+ [40,41].

Electrochemical Performance
Electrochemical tests were conducted to examine the performances of different SWO samples in lithium-ion batteries (Figure 4). The cyclic voltammetry (CV) of a SWO-60 electrode at a scan rate of 0.5 mV s −1 between 0.01 V and 3 V is depicted in Figure 4a. The reduction peak located at ~1.125 V can be attributed to the formation of solid electrolyte interface (SEI) film, during which Sb2WO6 was decomposed and antimony oxide was converted into metallic antimony [35]. The reduction peak centered at ~0.43V could be attributed to the alloying-reaction that took place between Li and Sb, and the oxidation peak of 1.254V is related to the dealloying of Li3Sb [35,42]. As we can see, from the third cycle on, the cyclic voltammetry curves are well overlapped, indicating good reversibility of the SWO-60 electrode.

Electrochemical Performance
Electrochemical tests were conducted to examine the performances of different SWO samples in lithium-ion batteries (Figure 4). The cyclic voltammetry (CV) of a SWO-60 electrode at a scan rate of 0.5 mV s −1 between 0.01 V and 3 V is depicted in Figure 4a. The reduction peak located at~1.125 V can be attributed to the formation of solid electrolyte interface (SEI) film, during which Sb 2 WO 6 was decomposed and antimony oxide was converted into metallic antimony [35]. The reduction peak centered at~0.43 V could be attributed to the alloying-reaction that took place between Li and Sb, and the oxidation peak of 1.254 V is related to the dealloying of Li 3 Sb [35,42]. As we can see, from the third cycle on, the cyclic voltammetry curves are well overlapped, indicating good reversibility of the SWO-60 electrode.  The discharge-charge profile of the SWO-60 electrode in the 1st, 2nd, 5th, 50th, and 100th cycles at 0.2 A g −1 is shown in Figure 4b. During the first cycle, the initial specific discharge and charge capacities of the Sb2WO6 electrode were ~875.06 and ~678.42 mAh g −1 , respectively, corresponding to 77.52% in initial coulombic efficiency, which could be attributed to the formation of a solid electrolyte interphase during the first discharge process. As shown in Figure 4b, there are two well-defined plateaus at ~1.3 and ~0.7 V in the first discharge curve. The plateau of ~1.3 V disappears after the first cycle, while the ~0.7 V plateau remains in the following cycles, which is consistent with the CV discussed above. The cycling stabilities of different Sb2WO6 electrodes are shown in Figure 4c. The initial specific discharge capacities of the SWO-45, SWO-60, and SWO-75 electrodes are 246.03, 875.06, and 764.5 mAh g −1 at 0.2 A g −1 , respectively. The discharge capacities of the SWO-45 and SWO-75 electrodes decrease rapidly, the specific discharge capacities of which undergo a decrease of ~49% and ~69%, respectively, from the 2nd to the 100th cycle, that is, they decrease at a rate of ~1.9 and ~4.4 mAh g −1 per cycle, respectively. In contrast, the reversible capacity of the SWO-60 electrode still delivers 424.22 mAh g −1 at 0.2 A g −1 after 100 cycles, corresponding to a decrease of ~39%, which is less than the other two electrodes, which may be associated with the uniform distributed nanosheets structure. However, as mentioned in the literature, the cycling stability of SWO-60 is inferior to Sb2WO6 for sodium-ion batteries [29], which may be attributed to the poor conductivity of SWO-60 [27], and its cycling stability could be improved through compositing with amorphous carbon, graphene, and other materials with higher conductivity, such as conducting polymers. Figure 4d illustrates the rate-performance of three electrodes at current densities ranging from 0.1 to 1.1 A g −1 .
The SWO-60 electrode displays reversible discharge capacities of 743.9, 513.7, and 462.2 mAh g −1 at 0.1, 0.7, and 1.1 A g −1 , respectively. The SWO-60 electrode can still deliver 703.6 mA h g −1 when the current density reverts to 0.1 A g −1 , which is much higher than the SWO-45 and SWO-75 electrodes. The discharge-charge profile of the SWO-60 electrode in the 1st, 2nd, 5th, 50th, and 100th cycles at 0.2 A g −1 is shown in Figure 4b. During the first cycle, the initial specific discharge and charge capacities of the Sb 2 WO 6 electrode were~875.06 and~678.42 mAh g −1 , respectively, corresponding to 77.52% in initial coulombic efficiency, which could be attributed to the formation of a solid electrolyte interphase during the first discharge process. As shown in Figure 4b, there are two well-defined plateaus at~1.3 and~0.7 V in the first discharge curve. The plateau of~1.3 V disappears after the first cycle, while the~0.7 V plateau remains in the following cycles, which is consistent with the CV discussed above. The cycling stabilities of different Sb 2 WO 6 electrodes are shown in Figure 4c. The initial specific discharge capacities of the SWO-45, SWO-60, and SWO-75 electrodes are 246.03, 875.06, and 764.5 mAh g −1 at 0.2 A g −1 , respectively. The discharge capacities of the SWO-45 and SWO-75 electrodes decrease rapidly, the specific discharge capacities of which undergo a decrease of~49% and~69%, respectively, from the 2nd to the 100th cycle, that is, they decrease at a rate of 1.9 and~4.4 mAh g −1 per cycle, respectively. In contrast, the reversible capacity of the SWO-60 electrode still delivers 424.22 mAh g −1 at 0.2 A g −1 after 100 cycles, corresponding to a decrease of~39%, which is less than the other two electrodes, which may be associated with the uniform distributed nanosheets structure. However, as mentioned in the literature, the cycling stability of SWO-60 is inferior to Sb 2 WO 6 for sodium-ion batteries [29], which may be attributed to the poor conductivity of SWO-60 [27], and its cycling stability could be improved through compositing with amorphous carbon, graphene, and other materials with higher conductivity, such as conducting polymers. Figure 4d illustrates the rate-performance of three electrodes at current densities ranging from 0.1 to 1.1 A g −1 . The SWO-60 electrode displays reversible discharge capacities of 743.9, 513.7, and 462.2 mAh g −1 at 0.1, 0.7, and 1.1 A g −1 , respectively. The SWO-60 electrode can still deliver 703.6 mA h g −1 when the current density reverts to 0.1 A g −1 , which is much higher than the SWO-45 and SWO-75 electrodes. The excellent rate performance of SWO-60 could be associated with its unique nanosheets morphology, which facilitates rapid lithium-ion conduction and transportation [43].
In order to investigate the electrochemical kinetics of different kinds of electrodes, electrochemical impedance spectroscopy measurements were carried out from 0.1 Hz to 100 kHz in frequency. Figure 5a,b shows the Nyquist plots of three kinds of electrodes in initial their states and after 200 discharge-charge cycling at 0.2 A g −1 , respectively. In the impedance spectra, the semicircles in the high-frequency region correspond to the charge-transfer resistance (R ct ) in the interface of electrodes and electrolyte, and the inclined lines during the low-frequency region are attributed to insertion of lithium ions into the negative electrode [44]. In Figure 5a, the charge transfer resistance of a battery with a SWO-60 electrode (332.8 Ω) is much smaller than those of the SWO-45 (998.4 Ω) and SWO-75 (477.3 Ω) batteries, which is consistent with the good cycling stability of the SWO-60 electrode. Figure 5b shows the EIS of SWO-45, SWO-60, and SWO-75 cells after 200 cycles at 0.2 A g −1 . After cycling, the cell with the SWO-60 electrode also exhibited the lowest charge transfer resistance, which is in good agreement with the fact that the SWO-60 electrode showed the best electrochemical properties. The excellent rate performance of SWO-60 could be associated with its unique nanosheets morphology, which facilitates rapid lithium-ion conduction and transportation [43]. In order to investigate the electrochemical kinetics of different kinds of electrodes, electrochemical impedance spectroscopy measurements were carried out from 0.1 Hz to 100 kHz in frequency. Figure 5a,b shows the Nyquist plots of three kinds of electrodes in initial their states and after 200 discharge-charge cycling at 0.2 A g −1 , respectively. In the impedance spectra, the semicircles in the high-frequency region correspond to the charge-transfer resistance (Rct) in the interface of electrodes and electrolyte, and the inclined lines during the low-frequency region are attributed to insertion of lithium ions into the negative electrode [44]. In Figure 5a, the charge transfer resistance of a battery with a SWO-60 electrode (332.8 Ω) is much smaller than those of the SWO-45 (998.4 Ω) and SWO-75 (477.3 Ω) batteries, which is consistent with the good cycling stability of the SWO-60 electrode. Figure 5b shows the EIS of SWO-45, SWO-60, and SWO-75 cells after 200 cycles at 0.2 A g −1 .
After cycling, the cell with the SWO-60 electrode also exhibited the lowest charge transfer resistance, which is in good agreement with the fact that the SWO-60 electrode showed the best electrochemical properties. To investigate the effect of different microwave hydrothermal times on the structural stability of electrodes, the morphology of electrodes was examined after 200 cycles at 0.2 A g −1 (Figure 6). Figure  6a-c shows the morphology of the SWO-45, SWO-60, and SWO-75 electrodes after cycling, respectively. After 200 cycles, it was found that there were many large cracks and pulverizations in the SWO-45 and SWO-75 electrodes, as shown in Figure 6a,c, respectively. By contrast, the SWO-60 electrode retained its structural integrity even after 200 cycles, as shown in Figure 6b. To investigate the effect of different microwave hydrothermal times on the structural stability of electrodes, the morphology of electrodes was examined after 200 cycles at 0.2 A g −1 (Figure 6). Figure 6a-c shows the morphology of the SWO-45, SWO-60, and SWO-75 electrodes after cycling, respectively. After 200 cycles, it was found that there were many large cracks and pulverizations in the SWO-45 and SWO-75 electrodes, as shown in Figure 6a,c, respectively. By contrast, the SWO-60 electrode retained its structural integrity even after 200 cycles, as shown in Figure 6b.

Conclusions
In summary, mesoporous Sb2WO6 nanosheets were successfully fabricated using a microwave hydrothermal method and were systematically investigated as negative electrodes for LIBs. When the microwave reaction time is 60 min, SWO-60 exhibits the highest reversible capacity and the best rate performance. The good electrochemical performance of the SWO-60 electrode may be ascribed to its uniformly distributed nanosheets structure. Due to the limited inherent electronic conductivity

Conclusions
In summary, mesoporous Sb 2 WO 6 nanosheets were successfully fabricated using a microwave hydrothermal method and were systematically investigated as negative electrodes for LIBs. When the microwave reaction time is 60 min, SWO-60 exhibits the highest reversible capacity and the best rate performance. The good electrochemical performance of the SWO-60 electrode may be ascribed to its uniformly distributed nanosheets structure. Due to the limited inherent electronic conductivity of SWO, our future work will focus on combining SWO with other materials with higher conductivities, such as heteroatom-doped nanocarbon materials and graphene, to further improve their electrochemical performance.
Author Contributions: Y.L., W.Z., and F.R. conceived and designed the experiments; Y.W., F.W., and Z.L. performed the experiments; Y.W., J.L., and K.P. analyzed the data; Y.W. wrote the paper; Y.L., G.W., M.C., and S.W. revised the paper. All the authors have discussed the results and commented on the manuscript.