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Anodes
One-Pot Hydrothermal Synthesis of FeMoO4 Nanocubes as an Anode Material for Lithium-Ion Batteries with Excellent Electrochemical Performance Zhicheng Ju,* En Zhang, Yulong Zhao, Zheng Xing, Quanchao Zhuang, Yinghuai Qiang, and Yitai Qian
Metal molybdates nanostructures hold great promise as high-performance electrode materials for next-generation lithium-ion batteries. In this work, the facial design and synthesis of monodisperse FeMoO4 nanocubes with the edge lengths of about 100 nm have been successfully prepared and present as a novel anode material for highly efficient and reversible lithium storage. Well-defined single-crystalline FeMoO4 with high uniformity are first obtained as nanosheets and then self-aggregated into nanocubes. The morphology of the product is largely controlled by the experimental parameters, such as the reaction temperature and time, the ratio of reactant, the solution viscosity, etc. The molybdate nanostructure would effectively promote the insertion of lithium ions and withstand volume variation upon prolonged charge/ discharge cycling. As a result, the FeMoO4 nanocubes exhibit high reversible capacities of 926 mAh g−1 after 80 cycles at a current density of 100 mA g−1 and remarkable rate performance, which indicate that the FeMoO4 nanocubes are promising materials for high-power lithium-ion battery applications.
1. Introduction As a family of inorganic functional material, growing attention has been focused on the metal molybdates (MMoO4) because they hold great promises in the fields of catalytic,[1,2] Dr. Z. Ju, Y. Zhao, Z. Xing, Prof. Q. Zhuang, Y. Qiang School of Materials Science and Engineering China University of Mining and Technology Xuzhou, Jiangsu 221116, P. R. China E-mail: [email protected] E. Zhang, Prof. Y. Qian Key Laboratory of Colloid and Interface Chemistry Ministry of Education, School of Chemistry and Chemical Engineering Shandong University Jinan, Shandong 250100, P. R. China Prof. Y. Qian Hefei National Laboratory for Physical Science at Microscale Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026, P. R. China DOI: 10.1002/smll.201501294 small 2015, 11, No. 36, 4753–4761
optics,[3] magnetism,[4] and electrochemical.[5] Recently, MMoO4 also have been applied as promising electrodes for the improvement of the next-generation lithium-ion batteries (LIBs) due to several advantages, including their high stability, eco-friendliness, and special structure which provides excellent capability to incorporate multiple Li+ ions.[6–8] Consequently, numerous efforts have been devoted to synthesis of novel molybdates like CoMoO4, FeMoO4, MnMoO4, NiMoO4, etc.[9–11] Among metal molybdates, only a limited number of such studies about FeMoO4 have been reported so far because of the complexity of their structure and the complicated synthesis conditions.[12] So far, only several morphologies such as hierarchical hollow spheres[10,13] and nanorods[14] of the FeMoO4 were reported. Therefore, the design and synthesis of FeMoO4 nanomaterials with special morphology still present a great challenge. Furthermore, various physical and chemical properties and reactivities such as magnetism,[15] catalysis,[16,17] gas sensing,[18,19] and energy conversion[20–22] are sensitive to the surface atomic configurations, therefore, tailoring the
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shape and structure of the nanoparticles can remarkably change the activities through the variations in exposed crystal facets.[23,24] It is anticipated that the 3D polyhedral structures would gain various characteristics from different exposed crystal surfaces with different atom densities and crystallographic structures.[25] These features make polyhedral structures nanostructured materials attractive for energy storage and conversion systems.[26] Particularly, there is increasing interest in nanocubes as electrodes for lithium-ion batteries, because they often showed to manifest high capacity and improved cycling stability.[27] Moreover, owing to the special crystal facets they exposed, nanocubes also could be used as an ideal model for researches of surface-related phenomena,[28] which will help us further investigate the relationship between specific surface and properties of crystals. However, up to date only a few materials with nanocube morphology have been synthesized by different strategies, such as noble metals,[29] transitional metal oxides,[16,30] and metal sulfides.[31] The nanocubes of different materials have been assembled via different methods, such as solvothermal synthesis,[32,33] hydrochloric acid–mediated polyol process,[34] nanoscale metal–organic frameworks (NMOFs)-templated approach,[16,35] and seed-mediated method.[34,36] Although diverse cubic nanoparticles have been synthesized, most of the compounds possess high symmetry of the crystal structure; to the best of our knowledge, monoclinic FeMoO4 nanocubes have not been reported before, which attributed to the lower symmetry of the crystal structure and thermodynamic instability increased dramatically in the course of synthesis of complex compounds. In addition, investigations into the formation mechanism as well as their electrochemical properties have been limited. Thus, the development of convenience, economic, and effective approaches to produce FeMoO4 nanocubes functional materials is definitely a challenge. In this report, we developed a simple and effective onepot hydrothermal method to synthesis of single-crystalline FeMoO4 nanocubes. The synthesis involves two common and inexpensive reagents: iron(II) sulfate heptahydrate and sodium molybdate dihydrate as raw materials. It was discovered that FeMoO4 nanocubes were well-defined highly uniformed with uniform edge length in the range of 100 ± 20 nm. During the reaction, FeMoO4 nanosheets were first obtained and then dissolved and self-assembled again into cube-like single-crystalline FeMoO4. This result demonstrates the model of crystal growth is associated with the generation and dissolution of the nanosheets. More importantly, the resultant FeMoO4 nanocubes exhibit high specific capacity and excellent electrochemical cycling stability thus making it as potential anode material in lithium-ion batteries. The wellcontrolled morphology and outstanding electrochemical performance of FeMoO4 nanocubes will offer a good example for the synthesis of other inorganic nanocube structures with low cost and high performance.
2. Results and Discussion Figure 1 shows the X-ray diffraction (XRD) pattern of FeMoO4 prepared through a typical method. The strong and
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Figure 1. XRD patterns: a) α-FeMoO4, b) β-FeMoO4, and c) the as-synthesized sample.
sharp diffraction peaks reflect the as-synthesized products are well crystallized and homogenous orientation of the prepared sample. The diffraction peaks can be well indexed as the β-FeMoO4 (monoclinic phase with the cell parameters of a = 10.29 Å, b = 9.394 Å, c = 7.072 Å, and β = 106.31°, space group C2/m, JCPDS No: 22-0628) mixed with a small amount of α-FeMoO4 (monoclinic phase with cell constants of a = 9.805 Å, b = 8.950 Å, c = 7.660 Å, and β = 114.05°, space group C2/m, JCPDS No: 22-1115). Furthermore, the (-220) crystal plane diffraction peak intensity was significantly enhanced compared to the standard data of JCPDS card, indicating that the sample has a strong preferential orientation growth. It is worth mentioning that previous studies indicate that the structure of β-FeMoO4 and α-FeMoO4 can be converted into each other under certain conditions.[12,37] In order to figure out the morphology and the structure of the obtained samples, scanning electron microscope (SEM) and transmission electron microscopy (TEM) techniques were used to investigate the final products. Figure 2a shows a typical panoramic SEM image of the as-synthesized product, indicating the presence of homogeneous, welldefined nanocube structure with an average edge length of 100 ± 20 nm. There are still a small amount of irregular nanoparticles appear. The corresponding large-area TEM images (Figure 2b,c) also confirmed the phenomenon. This may be attributed to the entire reaction system in unsteady-state reaction diffusion resulting in the insufficiency or inhomogeneous reaction. Further investigation on the morphology and structure of the nanocube was conducted through high-resolution transmission electron microscopy (HRTEM) experiment of randomly selected individual cubic FeMoO4 crystal. The uniform resolution and clear-cut edges all confirm the morphology of these cubic particles. The angles between the edges at the corners of the particles can be divided into two styles: 83.5° and 96.5°. From the corresponding HRTEM image (Figure 2e), it can be found that the clear lattice fringes have a dihedral angle of 83.5° and 96.5°; the interplanar distances are calculated to be 4.17 and 4.24 Å, corresponding to the (-211) and (120) crystal planes of monoclinic β-FeMoO4. High crystallinity of the nanocube is further confirmed by the
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Figure 2. FeMoO4 materials prepared through a typical method: a) SEM image, b,c) TEM image, d) single cube of β-FeMoO4, e,f) HRTEM image of a single cube β-FeMoO4 and its SAED pattern, g) schematic structure of β-FeMoO4, h) single cube of α-FeMoO4, i,j) HRTEM image of a single cube α-FeMoO4 and its SAED pattern, and k) schematic structure of α-FeMoO4.
corresponding selected area electron diffraction (SAED) pattern (Figure 2f), which also clearly exhibits the single-crystal nature. The diffraction spots can also be indexed as (-211) and (120) planes of monoclinic β-FeMoO4, respectively, with the incident electron beam parallel to the [-21-5] zone axes of the crystal. These results indicate that the surfaces of the pseudocubic nanocrystals are bounded by {-211} and {012} planes. The inset of Figure 2f presents a geometrical model of an ideal β-FeMoO4 pseudocube enclosed by {-211} and {012} planes, which are in good agreement with the as-prepared nanocrystals. The scheme in Figure 2g shows a typical crystal structure of monoclinic β-FeMoO4 along the direction of . The monoclinic structure of the β-FeMoO4 crystals has a wolframite-type structure with point group symmetry C2/m and two clusters per unit cell (Z = 2). In these unit cells, the Fe and Mo atoms are coordinated to six oxygen atoms which form the distorted octahedral [FeO6] and [MoO6] clusters (6 vertices, 6 faces, and 12 edges). By further careful observation of the obtained nanocubes, we can find the existence of α-FeMoO4 similarly showing cubic morphology. Figure 2h shows another randomly selected single cubic particle. The lattice fringe with the interfringe distance is about 3.16 Å, which is close to the lattice spacing of (-220) planes of the monoclinic α-FeMoO4 phase indexed from the JPCDS No: 22-1115. Figure 2i,j showed the corresponding SAED pattern and typical crystal structure of monoclinic α-FeMoO4 (along the direction of ), respectively. In the monoclinic α-FeMoO4 structure, the Fe and Mo atoms are also surrounded by six oxygen atoms that form the distorted octahedral [FeO6]/[MoO6] clusters. As can be observed, small differences exhibit in O-Fe-O and O-Mo-O bond angles with β-FeMoO4. It is also worth noting that the small 2015, 11, No. 36, 4753–4761
morphology and size of the α-FeMoO4 nanocubes are similar to β-FeMoO4, and the atomic arrangement and crystal facets of the cubic surface are along different orientations. According to the HRTEM result and crystallographic symmetry, the α-FeMoO4 nanocubes can be indexed to the (220) facets exposed single crystal. A good understanding of the process and parameters controlling the crystallization helps to improve the engineering of the growth of nanoparticles to the desired size and shape. The morphology of the products synthesized at different temperature and time was investigated. It is found that the product was obtained at lower temperature (160 °C) with uneven size distribution (Figure S1a,c, Supporting Information). At the same time, the sample also contains a large bulk substance. When the reaction temperature was increased to 220 °C, the morphology of the product is consistent with the typical experimental conditions (Figure S1b,d, Supporting Information). Reaction time is found to be another important factor that influences the final morphology of FeMoO4. To clarify the construction procedure of the morphology, a series of experiments through various reaction time 5, 10, and 20 h with other conditions kept the same were carried out. Figure 3 illustrates the SEM images of the FeMoO4 obtained at different reaction time. When the reaction time was fixed at 5 h, as shown in Figure 3a, a large number of irregular nanosheets with diameters in the range of 40–50 nm and thickness of about 10 nm were generated and no cubic structure could be observed in the product. Subsequently, as time was extended to 10 h, the nanosheets were all dissolved; at the same time, regular nanocubes with irregular nanoparticles emerged (Figure 3b). When extending the duration of treatment time, the cubic particle would further attract
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Figure 3. Under typical experimental conditions for different reaction periods: a) 5, b) 10, c) 20 h, d) the corresponding XRD patterns, asterisk * represents α-FeMoO4, and e) the schematic illustration of the formation of FeMoO4 cubes.
free-standing FeMoO4 irregular nanoparticles attached on their surface; the attached nanoparticles would fuse and the cubic particle would grow bigger with cubic configuration. Finally, when the reaction time was increased to 20 h, numerous regular and uniform nanocubes were constructed and most of the irregular nanoparticles surrounding the nanocubes disappeared (Figure 3c). It is notable that, even if the reaction time was prolonged to 20 h, the product still has residual nanocrystals, but showed a decreasing trend, which indicating that the whole reaction process is an oriented aggregation and that is still underway. So we can conclude that the increased reaction time is beneficial to the formation of nanocube morphology for this experiment. In this case, the structural transformation from 2D nanosheets to 3D nanocubes is a condition optimization process induced by thermal stimulation, and the formation of the nanocubes is associated with the generation and dissolution of the nanosheets and nanoparticles. It is obvious that these well-shaped cubic center particles were derived from the oriented aggregates of nanosheets and nanoparticles. The XRD patterns of the products shown in Figure 3d demonstrate the conversion process of the crystal phase during the reaction. All of the reflection peaks in the three products can be readily indexed to monoclinic β-FeMoO4 with a small amount of α-FeMoO4 (marked with asterisk *). It is noteworthy that the two phases were simultaneously generated after nucleation stage; when the reaction time
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was prolonged, the reflection peaks of α-FeMoO4 weakened, which might demonstrate that the α-FeMoO4 partially dissolved and β-FeMoO4 is more stable. The whole evolution process from nanosheets to nanocubes of FeMoO4 single crystal can be summarized in the schematic diagram in Figure 3e. In general, nanocube growth in solution typically involves a short nucleation period of primary particles followed by the subsequent growth by two major processes: ripening and aggregation. Nucleation process occurs at the very beginning of the reaction, during which the ions/molecules of reactants will assist as building blocks for the construction of crystallites, then the nanocrystallites will further supply as crystal seeds and evolve into active sites for the product. Continuous growth of the seeds results in the generation of primary two phases of FeMoO4 nanosheets. In the following reaction, the dissolution of nanosheets and the generation of irregular nanoparticles indicate that Ostwald ripening process was carried out. Then, the resulting irregular nanoparticles accumulate and fuse into single crystal enclosed with facets of (-211) and (120), in which oriented aggregation process is expected to take place to form well-defined FeMoO4 cubes with high index crystal facets. The extended cubes would continuously attract free-standing FeMoO4 nanoparticles to orientedly attach on the surfaces, and the fusion process was repeated until nanoparticles were consumed. The morphology and size distribution of the resulting crystal are highly dependent on the reaction environment of
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of most products (Figure 5d) was presented in the form of microrods (≈200 nm in width and 1–2 µm in length). It is obvious that solvents in solution can substantially influence the final morphology of the product; when the viscosity is increased, the size of the product becomes larger and the shape is in favor of being transformed from nanoparticles to microrods. This result implies that the ultimate shape of crystal could be controlled by introducing appropriate solution to alter the growth rates of the crystal facet, and hence the final morphology and structure of the crystal. Based on the above experimental results and the literature reported before,[40] we believe that the Ostwald Figure 4. Varying the ratio of Na2MoO4 and FeSO4 and the other conditions remain unchanged: ripening process and oriented aggregation a) 1:1, b) 1:2, c) 1:3, and d) the corresponding XRD patterns. mechanism should be responsible for the the reagent and solution. A further investigation on the param- explanation of the FeMoO4 nanocube crystal forming proeters controlling the crystallization helps to improve the engi- cess. At the initial stage, FeMoO4 primary nuclei are formed neering of the growth of nanoparticles to the desired size and via the reaction of (MoO4)2− ion and Fe2+ in the mixed liquor. shape. The ratio of reactant Na2MoO4 and FeSO4 has a great Due to the minimization of surface energy driven, two phases influence on the morphology of the final product. As shown of FeMoO4 primary nucleation will be aggregated together in Figure 4, when the ratio of the reactant changed from 1:1 through oriented attachment process; however, a small to 1:3 while keeping the other conditions remain unchanged, amount of nucleus will be affected by the intrinsic anisotropy the morphology of the product gradually changed from nano- of the crystal structure, these nuclei have intrinsic tendency cubes to nanorods and nanoparticles, indicating that FeSO4 to form FeMoO4 nanorods. In fact, at this moment the solupromote the change from nanocubes to very short nanorods. tion has been undergoing a dissolution recrystallization proInterestingly, the edge length of the initial nanocubes is about cess; and studies[41] have shown that in solution PVP is not 100 nm, but when the ratio becomes 1:3, the morphology of only able to stabilize and disperse the FeMoO4 nanoparticles, the product converted into the nanorods with the width of but also helps to select the appropriate positions during the 30 nm and length of 60 ± 20 nm, which may be responsible course of oriented attachment due to the slower aggregation for the relatively high concentration of iron ions that causes rate, thus favoring formation of the final regular quasicubic the system to quickly generate a large number of nuclei and geometry. Therefore, as time goes on, the surfactant is gradually oriented and adsorbed on the plane (β-FeMoO4), resulting in a final product size which is relatively small. According to the previous report,[38,39] because the sur- which restricts the radial growth; in the meanwhile, the process face atomic arrangement and surface affinity for the solvent of dissolution and recrystallization gradually proceeds. When to each orientation are different, solvent effect on the crystal the reaction time is prolonged to 10 h, the rudiment of the morphology is generated via the varied solvent–solute inter- FeMoO4 nanocubes is basically formed and after 20 h, the final actions along different orientations of crystal. In this case, product is obtained. In addition, as the quantity of the FeSO4 different solution environment can affect the growth rates of the crystal facets and the morphology of the product. Figure 5 displays the morphology of the product in different viscosity solvents (ethanol, PEG 200, PEG 400, PEG 600, and water mixed at the ratio of 25:15, respectively; the viscosity and the molecular chain of the solvents: ethanol < PEG 200 < PEG 400 < PEG 600) with other conditions remain unchanged. We can visually notice that particle size of the product synthesized in solution with lower viscosity (shorter molecular chain) is much smaller (about 20 nm) and some short nanorods appear (Figure 5a). When solution viscosity rises, the particle size increases to 70–80 nm and shows a uniform quasicubic-shaped (Figure 5b); if the solution viscosity further increases, the morphology of the product turns to bigger cubes (200–300 nm) with a large number of short rods Figure 5. Glycol is replaced with the same volume of a) ethanol, (40–50 nm in width and ≈300 nm in length) (Figure 5c). When b) polyethylene glycol 200, c) polyethylene glycol 400, d) polyethylene PEG 600 with higher viscosity was used as solvent, the shape glycol 600, and the other conditions remain unchanged. small 2015, 11, No. 36, 4753–4761
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Figure 6. a) Discharge/charge curves of the electrode made from the as-prepared FeMoO4 nanocubes and b) plot of capacity versus cycle number for the FeMoO4 nanocubes electrode in the voltage range of 0.01–3.0 V at a current rate of 100 mA g−1.
is increased, we found the nanocubes gradually decreased and more quasinanorod structures are obtained. This may be related to the competition between PVP and FeSO4. Excess Fe2+ can self-evidently promote the anisotropic growth and the more Fe2+ is added, the more apparent the effect is; further, relatively high concentrations of iron ions cause rapid formation of more nuclei, resulting in a relatively small particle size of the final product; when the viscosity of the solution increases, the Brownian motion gradually weakens, the diffusion rate is also correspondingly slow, which reduces the rate of nucleation of the solution, and further leads to the formation of crystal nuclei grow bigger; and as the viscosity increases, the crystal anisotropy gradually dominates, so that only microrods are present in higher viscosity solution. The Li storage and cycling properties of the synthesized FeMoO4 are investigated based on the coin-type half-cell configuration in the voltage range of 0.01–3.0 V (vs Li+/Li) at a constant current of 100 mA g−1 at room temperature. Voltage versus capacity profiles of the selected cycles are shown in Figure 6. As can be seen from the diagram, the first discharge curve is apparently different from the subsequent cycles. At the initial stage, the voltage curve shows a sharp decrease to 1.67 V where a short plateau sets in. This part delivers a capacity of about 125 mAh g−1 which equates to 1 mol Li (theoretical specific capacity formula: 26.8n × 1000/m (mAh g−1), here n is the number of electrons transferred and M is the molecular weight of FeMoO4; therefore, 1 mole of Li corresponds to capacity of 124 mAh g−1) and this part could be relevant to the formation of LixFeMoO4 compounds. The results of previous studies[42] suggest that a small amount of lithium-ion intercalation does not cause any structural change of FeMoO4. Hereafter, a sloping voltage profile up to 0.84 V followed by a smooth curve to the 0.30 V yielding a capacity of 850 mAh g−1, and then a sloping profile until the cutoff voltage 0.01 V. As for the charge profile, a smooth steep gradient curve can be found till 1.1 V followed via a gentle transition to 2.0 V from which the voltage sharply rised to 3.0 V. The first discharge and charge special capacities are 1356 and 960 mAh g−1, respectively, corresponding to a coulombic efficiency of 70.8%. The measured first discharge capacities are larger than the theoretical data (992.87 mAh g−1), which could be attributed to the insertion of lithium ions into acetylene black[43] and interfacial storage.[44] The irreversible capacity could be ascribed to the formation of stable solid electrolyte interphase (SEI) film on
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the surface of the electrode, and the irreversible destruction of the material structure caused irreversible lithium-ion interfacial storage. Subsequent discharge–charge cycles at 100 mA g−1 present a relatively large capacity fading. After five cycles, the specific capacity declined to 840 mA g−1, then the capacity to stabilization and showing a gradually increasing trend, which has similar phenomenon with MnWO4 nanobar[42] and CoMoO4 nanodisks.[8] The capacity fading in first few cycles could be attributed to the irreversible structural changes and establish an intimate electric contact of the “composite” with the current collector. The electrode maintains a capacity of 926 mAh g−1 after 80 cycles at a current density of 100 mA g−1, and the coulombic efficiency is more than 97% after three cycles which improves excellent electrochemical lithium storage performance and cycling stability of the FeMoO4 nanocubes as expected. Furthermore, the morphology change of MoFeO4 electrode after five cycles at 100 mA g−1 was also researched. The whole morphology of the sample after five charge–discharge cycles is still similar to that of the freshly prepared sample (Figure S2, Supporting Information). This result shows that even charging and discharging do not influence the structure stability of the electrode significantly. Besides the high capacity, the rate capabilities are also a useful and important characterization method for LIB. Therefore, the rate performance of FeMoO4 nanocubes is further investigated at different current densities (Figure 7).
Figure 7. Rate capabilities of the FeMoO4 nanocubes at different current density.
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It was proceeded by increments consisting of ten cycles as the current density increases from 100 to 200, 500, 1000, and 2000 mA g−1, corresponding to the average reversible capacities of FeMoO4 nanocubes of 880, 624, 442, 343, and 215 mAh g−1, respectively. The specific capacity of the nanocubes decreases steadily as the current density increases, but still retains high values. Importantly, after reaching a huge discharge–charge current density (2000 mA g−1), the specific capacity could return to more than 950 mAh g−1 when altering the current density back to the 100 mA g−1. The full recovery of the initial capacity implies that the structure of the electrode is stable even during the high current density. Rate performance indicates that the as-prepared FeMoO4 nanocube is a promising anode candidate to replace commercial graphite as the LIB anode in the field of battery power. The excellent performance with high specific capacity and good cycling stability may be corresponded to the special geometry of the regular cubic, which provides specific surface and promotes the conductive capacity. To further understand the electrochemical discharge/ charge mechanism of FeMoO4 nanocubes anode during cycling, cyclic voltammograms (CVs) experiments were conducted to reveal the continual lithiation/delithiation during the discharge and charge cycles. Figure 8 displays representative CV curve of the FeMoO4 synthesized through the typical experiment for the first, second, third, fourth, and tenth cycles, which is carried out at a scan rate of 0.1 mV s−1 in the voltage range of 0.01–3.0 V versus Li/Li+. As illustrated below, the first lithiation and delithiation process is significantly different from the subsequent cycles. During the first cathodic sweep, four redox current peaks denoted as R11, R12, R13, and R14 which are located at around 1.61, 1.45, 0.70, and 0.43 V can be clearly identified from the CVs. According to the reported literature[45] the irreversible cathodic peak R11 can be assigned to the incorporation of lithium ions into the lattice by insertion, forming an intercalated phase LixFeMoO4 with a small volume expansion. The following two peaks R12 and R13 could be ascribed to the reduction of Mo6+ and Fe2+ to metallic Mo0 and Fe0, respectively.[46,47] The R14 peak might result from the formation of solid electrolyte interfaces.[47,48] Meantime, two
poorly defined anodic peaks O11 and O12 centered at around 1.48 and 2.00 V, respectively, in the first scan mean that the oxidation processes of metallic Fe and Mo had taken place by two steps. As shown in the anode scan, the obvious O11 peak probably arises from the oxidation process from Mo to Mo6+[49] while the broad O12 peak could be caused by the oxidation of Fe0 to Fe2+, stepwise.[46,50] In the subsequent cycles, the initial cathodic peaks at 1.61 and 0.43 V disappear as a result of irreversible phase transition from FeMoO4 to FeO and MoO3 and SEI film formation; cathode scans show three peaks R21, R22, and R23 at 1.37, 0.67, and 0.25 V, while the anode scans show the two peaks O21 and O22 located at 1.48 and 1.9 V gradually became broadened and merged into one broad peak. The two peaks R21 and R23 could be attributed to the lithiation in MoO3 taken place in two stages.[46,51] R21 caused by the Li intercalates with MoO3 and the formation of LixMoO3, while R23 represents Li reacts with LixMoO3 to consequently form metallic Mo0 and Li2O. In addition, the R22 peak could be caused by the reduction of Fe2+. The anodic peaks O21 and O22 represent the oxidation of metallic Mo0 and Fe0 to Mo6+ and Fe2+, respectively. Compared with the first cycle both cathodic and anodic peaks are shifted, which is ascribed to polarization of the electrode material in the first cycle. During the anode polarization process, both the peak current and the integrated area of the anodic peak are reduced, implying that the capacity is decreased during the charging process. However, the two anodic peaks could not be observed in the following cycles, which could be attributed to the irreversible destruction of the structure in the process of the formation of Mo0 and Fe0 from Mo6+ and Fe2+,[28] the formation of the amorphous matrix Li2O, and the formation of stable SEI layer on the surface of the electrode.[8] By CV curve analysis and reference to previous literature, the lithium storage in the FeMoO4 is primarily based on multiple step reactions; the following electrochemical reaction mechanism would be responsible for the formation of FeMoO4 with Li. The first discharge can be explained in the following equation. At the initial stage of lithium intercalation (1), a small amount of lithium is inserted into the FeMoO4 to form LixFeMoO4. Then fine Fe0 and Mo0 grains are generated and dispersed in the Li2O matrix as a result of the complete reduction of LixFeMoO4 (2). For the overall process, the first two reactions are largely irreversible, whereas the last one is highly reversible for lithium storage FeMoO 4 + xLi + + xe − → Li x FeMoO 4
(1)
Li x FeMoO 4 + ( 8 − x ) Li + + ( 8 − x ) e − → Fe 0 + Mo 0 + 4Li 2O
(2)
Meanwhile, the reversible charged and discharge cycle is relevant to the mechanism of equation from (3) to (6)[46,47] Figure 8. Cyclic voltammetry of the FeMoO4 nanocubes at a scan rate of 0.1 mV s−1 in the voltage range of 0.01–3.0 V versus Li+/Li. small 2015, 11, No. 36, 4753–4761
Fe 0 + Li 2 O ↔ FeO + 2e − + 2Li
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Figure 9. Schematic atomistic models of the lithiation/delithiation process of FeMoO4. A) FeMoO4 before lithiation, B) initial stage of Li ion insertion with the formation of LixFeMoO4, C) formation of interconnected Fe0 and Mo0 clusters embedded in the Li2O nanocrystals, and D) formation of MoO3 and FeO nanoparticles by the deconversion reaction.
Mo 0 + 3Li 2 O→ MoO 3 + 6e − + 6Li
(4)
MoO 3 + yLi + + ye − → Li y MoO 3
(5)
Li y MoO 3
+ zLi + ze − → Mo 0 + 3Li
2O
(6)
By considering the discharge/charge information and CV curves, we propose the structural evolution in the conversion reaction of the FeMoO4 as shown in Figure 9. In the first cycle, the lithium ions inserted into FeMoO4 crystalline lattice without changing the crystal structure at the very beginning of lithiation. The further intake of lithium ion leads to a collapse of the FeMoO4 crystalline lattice and then formation of a phase consisting of LixFeMoO4 clusters; and with more lithium ions diffusing to react with FeMoO4, the lattice finally collapses; single-crystalline FeMoO4 were transformed to networks of Mo and Fe nanograins embedded in Li2O matrix. In the following delithiation process, when lithium ions are electrochemically extracted from the Mo, Fe, and Li2O multicrystalline composite, the Li2O is dissociated along with the oxidation of the Mo, Fe clusters, leading to the formation of MoO3 and FeO (instead of the original FeMoO4) nanocrystals. The MoO3/Mo and FeO/Fe clusters would reversibly form during the conversion/deconversion reaction in the subsequent electrochemical cycles. However, to clarify the specific electrochemical reaction process and influencing factors, more sophisticated techniques and works are needed, which is what we need to do next.
3. Conclusion In summary, FeMoO4 nanocubes with an average edge length of 100 ± 20 nm have been successfully prepared by an effective one-pot hydrothermal route. The FeMoO4 nanosheets were first fabricated and then self-assembled to
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the nanocubes. Both phases of FeMoO4 were formed pseudocube morphology; the β-FeMoO4 pseudocube enclosed by {-211} and {012} planes as well as the α-FeMoO4 nanocubes can be indexed to the (220) facets exposed single crystal. The results showed that the reaction time, the ratio of reactant, and the solution viscosity all play significant roles in restricting the size and morphology of the final product. Electrochemical characterization showed the FeMoO4 nanocubes exhibited a high reversible capacity of 926 mAh g−1 after 80 cycles at a current density of 100 mA g−1 and good rate performance, which indicate that the FeMoO4 nanocubes are promising materials for high-power lithiumion batteries applications. The excellent electrochemical performance may be corresponded to the special geometry of the regular cubic, which provides specific surface and promotes the conductive capacity.
4. Experimental Section All reagents were of analytical purity and purchased from Shanghai Chemical Reagents Company without further purification. Na2MoO4 and FeSO4·7H2O are in the molar ratio of 1:2 mixed. Appropriate amounts of PVP and NaAc were dissolved in 40 mL of mixed solution (25 mL H2O and 15 mL glycol) at a constant speed under electromagnetic stirring for 10 min. Then the mixed solution was transferred into an autoclave in an electric oven at 200 °C for a period of 20 h. After heating, the autoclave was cooled to room temperature naturally. The as-synthesized product was centrifuged and washed several times with deionized water and absolute ethanol before vacuum drying at 80 °C. The XRD patterns of the products were carried out by a Bruker D8 advanced X-ray diffractometer equipped with CuKα radiation (λ = 1.5418 Å). TEM images were taken on a JEM-1011 transmission electron microscope, using an accelerating voltage of 100 kV. The HRTEM images and the SAED patterns were taken on a JEM-2100 transmission electron microscope with the accelerating voltage of 200 kV. Doctor-blade technique was used to prepare the electrode for Li cycling. The electrode was composed of active material (70 wt%), conductivity agent (Super P carbon black, 20 wt%), and binder (polyvinylidene fluoride) (10 wt%) with a Cu foil as the current collector. The coin-type cells (size: 2032) were assembled in the argon-filled glove box (Mikrouna, Super 1200/750) with the concentrations of moisture and oxygen below 1 ppm. Li metal was applied as anode. The separator was Celgard 2400 microporous membrane, and the electrolyte was 1 M LiPF6 solution dissolved in ethylene carbonate/dimethyl carbonate (EC/DMC) by 1:1 volume ratio. And galvanostatic charge/discharge cycling was performed using LAND-CT2001A multichannel galvanostat (Wuhan, China) in the voltage range of 0.01–3.0 V (vs Li+/Li) at room temperature. The CV profiles were carried out in the voltage window 0.01–3.0 V at a scan rate of 0.1 mV s−1 by electrochemical workstation (LK2005A, Tianjin, China).
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgement This work was supported by the Fundamental Research Funds for the Central University 2014QNA10.
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Received: May 7, 2015 Revised: June 4, 2015 Published online: July 6, 2015
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Anodes
One-Pot Hydrothermal Synthesis of FeMoO4 Nanocubes as an Anode Material for Lithium-Ion Batteries with Excellent Electrochemical Performance Zhicheng Ju,* En Zhang, Yulong Zhao, Zheng Xing, Quanchao Zhuang, Yinghuai Qiang, and Yitai Qian
Metal molybdates nanostructures hold great promise as high-performance electrode materials for next-generation lithium-ion batteries. In this work, the facial design and synthesis of monodisperse FeMoO4 nanocubes with the edge lengths of about 100 nm have been successfully prepared and present as a novel anode material for highly efficient and reversible lithium storage. Well-defined single-crystalline FeMoO4 with high uniformity are first obtained as nanosheets and then self-aggregated into nanocubes. The morphology of the product is largely controlled by the experimental parameters, such as the reaction temperature and time, the ratio of reactant, the solution viscosity, etc. The molybdate nanostructure would effectively promote the insertion of lithium ions and withstand volume variation upon prolonged charge/ discharge cycling. As a result, the FeMoO4 nanocubes exhibit high reversible capacities of 926 mAh g−1 after 80 cycles at a current density of 100 mA g−1 and remarkable rate performance, which indicate that the FeMoO4 nanocubes are promising materials for high-power lithium-ion battery applications.
1. Introduction As a family of inorganic functional material, growing attention has been focused on the metal molybdates (MMoO4) because they hold great promises in the fields of catalytic,[1,2] Dr. Z. Ju, Y. Zhao, Z. Xing, Prof. Q. Zhuang, Y. Qiang School of Materials Science and Engineering China University of Mining and Technology Xuzhou, Jiangsu 221116, P. R. China E-mail: [email protected] E. Zhang, Prof. Y. Qian Key Laboratory of Colloid and Interface Chemistry Ministry of Education, School of Chemistry and Chemical Engineering Shandong University Jinan, Shandong 250100, P. R. China Prof. Y. Qian Hefei National Laboratory for Physical Science at Microscale Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026, P. R. China DOI: 10.1002/smll.201501294 small 2015, 11, No. 36, 4753–4761
optics,[3] magnetism,[4] and electrochemical.[5] Recently, MMoO4 also have been applied as promising electrodes for the improvement of the next-generation lithium-ion batteries (LIBs) due to several advantages, including their high stability, eco-friendliness, and special structure which provides excellent capability to incorporate multiple Li+ ions.[6–8] Consequently, numerous efforts have been devoted to synthesis of novel molybdates like CoMoO4, FeMoO4, MnMoO4, NiMoO4, etc.[9–11] Among metal molybdates, only a limited number of such studies about FeMoO4 have been reported so far because of the complexity of their structure and the complicated synthesis conditions.[12] So far, only several morphologies such as hierarchical hollow spheres[10,13] and nanorods[14] of the FeMoO4 were reported. Therefore, the design and synthesis of FeMoO4 nanomaterials with special morphology still present a great challenge. Furthermore, various physical and chemical properties and reactivities such as magnetism,[15] catalysis,[16,17] gas sensing,[18,19] and energy conversion[20–22] are sensitive to the surface atomic configurations, therefore, tailoring the
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shape and structure of the nanoparticles can remarkably change the activities through the variations in exposed crystal facets.[23,24] It is anticipated that the 3D polyhedral structures would gain various characteristics from different exposed crystal surfaces with different atom densities and crystallographic structures.[25] These features make polyhedral structures nanostructured materials attractive for energy storage and conversion systems.[26] Particularly, there is increasing interest in nanocubes as electrodes for lithium-ion batteries, because they often showed to manifest high capacity and improved cycling stability.[27] Moreover, owing to the special crystal facets they exposed, nanocubes also could be used as an ideal model for researches of surface-related phenomena,[28] which will help us further investigate the relationship between specific surface and properties of crystals. However, up to date only a few materials with nanocube morphology have been synthesized by different strategies, such as noble metals,[29] transitional metal oxides,[16,30] and metal sulfides.[31] The nanocubes of different materials have been assembled via different methods, such as solvothermal synthesis,[32,33] hydrochloric acid–mediated polyol process,[34] nanoscale metal–organic frameworks (NMOFs)-templated approach,[16,35] and seed-mediated method.[34,36] Although diverse cubic nanoparticles have been synthesized, most of the compounds possess high symmetry of the crystal structure; to the best of our knowledge, monoclinic FeMoO4 nanocubes have not been reported before, which attributed to the lower symmetry of the crystal structure and thermodynamic instability increased dramatically in the course of synthesis of complex compounds. In addition, investigations into the formation mechanism as well as their electrochemical properties have been limited. Thus, the development of convenience, economic, and effective approaches to produce FeMoO4 nanocubes functional materials is definitely a challenge. In this report, we developed a simple and effective onepot hydrothermal method to synthesis of single-crystalline FeMoO4 nanocubes. The synthesis involves two common and inexpensive reagents: iron(II) sulfate heptahydrate and sodium molybdate dihydrate as raw materials. It was discovered that FeMoO4 nanocubes were well-defined highly uniformed with uniform edge length in the range of 100 ± 20 nm. During the reaction, FeMoO4 nanosheets were first obtained and then dissolved and self-assembled again into cube-like single-crystalline FeMoO4. This result demonstrates the model of crystal growth is associated with the generation and dissolution of the nanosheets. More importantly, the resultant FeMoO4 nanocubes exhibit high specific capacity and excellent electrochemical cycling stability thus making it as potential anode material in lithium-ion batteries. The wellcontrolled morphology and outstanding electrochemical performance of FeMoO4 nanocubes will offer a good example for the synthesis of other inorganic nanocube structures with low cost and high performance.
2. Results and Discussion Figure 1 shows the X-ray diffraction (XRD) pattern of FeMoO4 prepared through a typical method. The strong and
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Figure 1. XRD patterns: a) α-FeMoO4, b) β-FeMoO4, and c) the as-synthesized sample.
sharp diffraction peaks reflect the as-synthesized products are well crystallized and homogenous orientation of the prepared sample. The diffraction peaks can be well indexed as the β-FeMoO4 (monoclinic phase with the cell parameters of a = 10.29 Å, b = 9.394 Å, c = 7.072 Å, and β = 106.31°, space group C2/m, JCPDS No: 22-0628) mixed with a small amount of α-FeMoO4 (monoclinic phase with cell constants of a = 9.805 Å, b = 8.950 Å, c = 7.660 Å, and β = 114.05°, space group C2/m, JCPDS No: 22-1115). Furthermore, the (-220) crystal plane diffraction peak intensity was significantly enhanced compared to the standard data of JCPDS card, indicating that the sample has a strong preferential orientation growth. It is worth mentioning that previous studies indicate that the structure of β-FeMoO4 and α-FeMoO4 can be converted into each other under certain conditions.[12,37] In order to figure out the morphology and the structure of the obtained samples, scanning electron microscope (SEM) and transmission electron microscopy (TEM) techniques were used to investigate the final products. Figure 2a shows a typical panoramic SEM image of the as-synthesized product, indicating the presence of homogeneous, welldefined nanocube structure with an average edge length of 100 ± 20 nm. There are still a small amount of irregular nanoparticles appear. The corresponding large-area TEM images (Figure 2b,c) also confirmed the phenomenon. This may be attributed to the entire reaction system in unsteady-state reaction diffusion resulting in the insufficiency or inhomogeneous reaction. Further investigation on the morphology and structure of the nanocube was conducted through high-resolution transmission electron microscopy (HRTEM) experiment of randomly selected individual cubic FeMoO4 crystal. The uniform resolution and clear-cut edges all confirm the morphology of these cubic particles. The angles between the edges at the corners of the particles can be divided into two styles: 83.5° and 96.5°. From the corresponding HRTEM image (Figure 2e), it can be found that the clear lattice fringes have a dihedral angle of 83.5° and 96.5°; the interplanar distances are calculated to be 4.17 and 4.24 Å, corresponding to the (-211) and (120) crystal planes of monoclinic β-FeMoO4. High crystallinity of the nanocube is further confirmed by the
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Figure 2. FeMoO4 materials prepared through a typical method: a) SEM image, b,c) TEM image, d) single cube of β-FeMoO4, e,f) HRTEM image of a single cube β-FeMoO4 and its SAED pattern, g) schematic structure of β-FeMoO4, h) single cube of α-FeMoO4, i,j) HRTEM image of a single cube α-FeMoO4 and its SAED pattern, and k) schematic structure of α-FeMoO4.
corresponding selected area electron diffraction (SAED) pattern (Figure 2f), which also clearly exhibits the single-crystal nature. The diffraction spots can also be indexed as (-211) and (120) planes of monoclinic β-FeMoO4, respectively, with the incident electron beam parallel to the [-21-5] zone axes of the crystal. These results indicate that the surfaces of the pseudocubic nanocrystals are bounded by {-211} and {012} planes. The inset of Figure 2f presents a geometrical model of an ideal β-FeMoO4 pseudocube enclosed by {-211} and {012} planes, which are in good agreement with the as-prepared nanocrystals. The scheme in Figure 2g shows a typical crystal structure of monoclinic β-FeMoO4 along the direction of . The monoclinic structure of the β-FeMoO4 crystals has a wolframite-type structure with point group symmetry C2/m and two clusters per unit cell (Z = 2). In these unit cells, the Fe and Mo atoms are coordinated to six oxygen atoms which form the distorted octahedral [FeO6] and [MoO6] clusters (6 vertices, 6 faces, and 12 edges). By further careful observation of the obtained nanocubes, we can find the existence of α-FeMoO4 similarly showing cubic morphology. Figure 2h shows another randomly selected single cubic particle. The lattice fringe with the interfringe distance is about 3.16 Å, which is close to the lattice spacing of (-220) planes of the monoclinic α-FeMoO4 phase indexed from the JPCDS No: 22-1115. Figure 2i,j showed the corresponding SAED pattern and typical crystal structure of monoclinic α-FeMoO4 (along the direction of ), respectively. In the monoclinic α-FeMoO4 structure, the Fe and Mo atoms are also surrounded by six oxygen atoms that form the distorted octahedral [FeO6]/[MoO6] clusters. As can be observed, small differences exhibit in O-Fe-O and O-Mo-O bond angles with β-FeMoO4. It is also worth noting that the small 2015, 11, No. 36, 4753–4761
morphology and size of the α-FeMoO4 nanocubes are similar to β-FeMoO4, and the atomic arrangement and crystal facets of the cubic surface are along different orientations. According to the HRTEM result and crystallographic symmetry, the α-FeMoO4 nanocubes can be indexed to the (220) facets exposed single crystal. A good understanding of the process and parameters controlling the crystallization helps to improve the engineering of the growth of nanoparticles to the desired size and shape. The morphology of the products synthesized at different temperature and time was investigated. It is found that the product was obtained at lower temperature (160 °C) with uneven size distribution (Figure S1a,c, Supporting Information). At the same time, the sample also contains a large bulk substance. When the reaction temperature was increased to 220 °C, the morphology of the product is consistent with the typical experimental conditions (Figure S1b,d, Supporting Information). Reaction time is found to be another important factor that influences the final morphology of FeMoO4. To clarify the construction procedure of the morphology, a series of experiments through various reaction time 5, 10, and 20 h with other conditions kept the same were carried out. Figure 3 illustrates the SEM images of the FeMoO4 obtained at different reaction time. When the reaction time was fixed at 5 h, as shown in Figure 3a, a large number of irregular nanosheets with diameters in the range of 40–50 nm and thickness of about 10 nm were generated and no cubic structure could be observed in the product. Subsequently, as time was extended to 10 h, the nanosheets were all dissolved; at the same time, regular nanocubes with irregular nanoparticles emerged (Figure 3b). When extending the duration of treatment time, the cubic particle would further attract
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Figure 3. Under typical experimental conditions for different reaction periods: a) 5, b) 10, c) 20 h, d) the corresponding XRD patterns, asterisk * represents α-FeMoO4, and e) the schematic illustration of the formation of FeMoO4 cubes.
free-standing FeMoO4 irregular nanoparticles attached on their surface; the attached nanoparticles would fuse and the cubic particle would grow bigger with cubic configuration. Finally, when the reaction time was increased to 20 h, numerous regular and uniform nanocubes were constructed and most of the irregular nanoparticles surrounding the nanocubes disappeared (Figure 3c). It is notable that, even if the reaction time was prolonged to 20 h, the product still has residual nanocrystals, but showed a decreasing trend, which indicating that the whole reaction process is an oriented aggregation and that is still underway. So we can conclude that the increased reaction time is beneficial to the formation of nanocube morphology for this experiment. In this case, the structural transformation from 2D nanosheets to 3D nanocubes is a condition optimization process induced by thermal stimulation, and the formation of the nanocubes is associated with the generation and dissolution of the nanosheets and nanoparticles. It is obvious that these well-shaped cubic center particles were derived from the oriented aggregates of nanosheets and nanoparticles. The XRD patterns of the products shown in Figure 3d demonstrate the conversion process of the crystal phase during the reaction. All of the reflection peaks in the three products can be readily indexed to monoclinic β-FeMoO4 with a small amount of α-FeMoO4 (marked with asterisk *). It is noteworthy that the two phases were simultaneously generated after nucleation stage; when the reaction time
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was prolonged, the reflection peaks of α-FeMoO4 weakened, which might demonstrate that the α-FeMoO4 partially dissolved and β-FeMoO4 is more stable. The whole evolution process from nanosheets to nanocubes of FeMoO4 single crystal can be summarized in the schematic diagram in Figure 3e. In general, nanocube growth in solution typically involves a short nucleation period of primary particles followed by the subsequent growth by two major processes: ripening and aggregation. Nucleation process occurs at the very beginning of the reaction, during which the ions/molecules of reactants will assist as building blocks for the construction of crystallites, then the nanocrystallites will further supply as crystal seeds and evolve into active sites for the product. Continuous growth of the seeds results in the generation of primary two phases of FeMoO4 nanosheets. In the following reaction, the dissolution of nanosheets and the generation of irregular nanoparticles indicate that Ostwald ripening process was carried out. Then, the resulting irregular nanoparticles accumulate and fuse into single crystal enclosed with facets of (-211) and (120), in which oriented aggregation process is expected to take place to form well-defined FeMoO4 cubes with high index crystal facets. The extended cubes would continuously attract free-standing FeMoO4 nanoparticles to orientedly attach on the surfaces, and the fusion process was repeated until nanoparticles were consumed. The morphology and size distribution of the resulting crystal are highly dependent on the reaction environment of
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of most products (Figure 5d) was presented in the form of microrods (≈200 nm in width and 1–2 µm in length). It is obvious that solvents in solution can substantially influence the final morphology of the product; when the viscosity is increased, the size of the product becomes larger and the shape is in favor of being transformed from nanoparticles to microrods. This result implies that the ultimate shape of crystal could be controlled by introducing appropriate solution to alter the growth rates of the crystal facet, and hence the final morphology and structure of the crystal. Based on the above experimental results and the literature reported before,[40] we believe that the Ostwald Figure 4. Varying the ratio of Na2MoO4 and FeSO4 and the other conditions remain unchanged: ripening process and oriented aggregation a) 1:1, b) 1:2, c) 1:3, and d) the corresponding XRD patterns. mechanism should be responsible for the the reagent and solution. A further investigation on the param- explanation of the FeMoO4 nanocube crystal forming proeters controlling the crystallization helps to improve the engi- cess. At the initial stage, FeMoO4 primary nuclei are formed neering of the growth of nanoparticles to the desired size and via the reaction of (MoO4)2− ion and Fe2+ in the mixed liquor. shape. The ratio of reactant Na2MoO4 and FeSO4 has a great Due to the minimization of surface energy driven, two phases influence on the morphology of the final product. As shown of FeMoO4 primary nucleation will be aggregated together in Figure 4, when the ratio of the reactant changed from 1:1 through oriented attachment process; however, a small to 1:3 while keeping the other conditions remain unchanged, amount of nucleus will be affected by the intrinsic anisotropy the morphology of the product gradually changed from nano- of the crystal structure, these nuclei have intrinsic tendency cubes to nanorods and nanoparticles, indicating that FeSO4 to form FeMoO4 nanorods. In fact, at this moment the solupromote the change from nanocubes to very short nanorods. tion has been undergoing a dissolution recrystallization proInterestingly, the edge length of the initial nanocubes is about cess; and studies[41] have shown that in solution PVP is not 100 nm, but when the ratio becomes 1:3, the morphology of only able to stabilize and disperse the FeMoO4 nanoparticles, the product converted into the nanorods with the width of but also helps to select the appropriate positions during the 30 nm and length of 60 ± 20 nm, which may be responsible course of oriented attachment due to the slower aggregation for the relatively high concentration of iron ions that causes rate, thus favoring formation of the final regular quasicubic the system to quickly generate a large number of nuclei and geometry. Therefore, as time goes on, the surfactant is gradually oriented and adsorbed on the plane (β-FeMoO4), resulting in a final product size which is relatively small. According to the previous report,[38,39] because the sur- which restricts the radial growth; in the meanwhile, the process face atomic arrangement and surface affinity for the solvent of dissolution and recrystallization gradually proceeds. When to each orientation are different, solvent effect on the crystal the reaction time is prolonged to 10 h, the rudiment of the morphology is generated via the varied solvent–solute inter- FeMoO4 nanocubes is basically formed and after 20 h, the final actions along different orientations of crystal. In this case, product is obtained. In addition, as the quantity of the FeSO4 different solution environment can affect the growth rates of the crystal facets and the morphology of the product. Figure 5 displays the morphology of the product in different viscosity solvents (ethanol, PEG 200, PEG 400, PEG 600, and water mixed at the ratio of 25:15, respectively; the viscosity and the molecular chain of the solvents: ethanol < PEG 200 < PEG 400 < PEG 600) with other conditions remain unchanged. We can visually notice that particle size of the product synthesized in solution with lower viscosity (shorter molecular chain) is much smaller (about 20 nm) and some short nanorods appear (Figure 5a). When solution viscosity rises, the particle size increases to 70–80 nm and shows a uniform quasicubic-shaped (Figure 5b); if the solution viscosity further increases, the morphology of the product turns to bigger cubes (200–300 nm) with a large number of short rods Figure 5. Glycol is replaced with the same volume of a) ethanol, (40–50 nm in width and ≈300 nm in length) (Figure 5c). When b) polyethylene glycol 200, c) polyethylene glycol 400, d) polyethylene PEG 600 with higher viscosity was used as solvent, the shape glycol 600, and the other conditions remain unchanged. small 2015, 11, No. 36, 4753–4761
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Figure 6. a) Discharge/charge curves of the electrode made from the as-prepared FeMoO4 nanocubes and b) plot of capacity versus cycle number for the FeMoO4 nanocubes electrode in the voltage range of 0.01–3.0 V at a current rate of 100 mA g−1.
is increased, we found the nanocubes gradually decreased and more quasinanorod structures are obtained. This may be related to the competition between PVP and FeSO4. Excess Fe2+ can self-evidently promote the anisotropic growth and the more Fe2+ is added, the more apparent the effect is; further, relatively high concentrations of iron ions cause rapid formation of more nuclei, resulting in a relatively small particle size of the final product; when the viscosity of the solution increases, the Brownian motion gradually weakens, the diffusion rate is also correspondingly slow, which reduces the rate of nucleation of the solution, and further leads to the formation of crystal nuclei grow bigger; and as the viscosity increases, the crystal anisotropy gradually dominates, so that only microrods are present in higher viscosity solution. The Li storage and cycling properties of the synthesized FeMoO4 are investigated based on the coin-type half-cell configuration in the voltage range of 0.01–3.0 V (vs Li+/Li) at a constant current of 100 mA g−1 at room temperature. Voltage versus capacity profiles of the selected cycles are shown in Figure 6. As can be seen from the diagram, the first discharge curve is apparently different from the subsequent cycles. At the initial stage, the voltage curve shows a sharp decrease to 1.67 V where a short plateau sets in. This part delivers a capacity of about 125 mAh g−1 which equates to 1 mol Li (theoretical specific capacity formula: 26.8n × 1000/m (mAh g−1), here n is the number of electrons transferred and M is the molecular weight of FeMoO4; therefore, 1 mole of Li corresponds to capacity of 124 mAh g−1) and this part could be relevant to the formation of LixFeMoO4 compounds. The results of previous studies[42] suggest that a small amount of lithium-ion intercalation does not cause any structural change of FeMoO4. Hereafter, a sloping voltage profile up to 0.84 V followed by a smooth curve to the 0.30 V yielding a capacity of 850 mAh g−1, and then a sloping profile until the cutoff voltage 0.01 V. As for the charge profile, a smooth steep gradient curve can be found till 1.1 V followed via a gentle transition to 2.0 V from which the voltage sharply rised to 3.0 V. The first discharge and charge special capacities are 1356 and 960 mAh g−1, respectively, corresponding to a coulombic efficiency of 70.8%. The measured first discharge capacities are larger than the theoretical data (992.87 mAh g−1), which could be attributed to the insertion of lithium ions into acetylene black[43] and interfacial storage.[44] The irreversible capacity could be ascribed to the formation of stable solid electrolyte interphase (SEI) film on
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the surface of the electrode, and the irreversible destruction of the material structure caused irreversible lithium-ion interfacial storage. Subsequent discharge–charge cycles at 100 mA g−1 present a relatively large capacity fading. After five cycles, the specific capacity declined to 840 mA g−1, then the capacity to stabilization and showing a gradually increasing trend, which has similar phenomenon with MnWO4 nanobar[42] and CoMoO4 nanodisks.[8] The capacity fading in first few cycles could be attributed to the irreversible structural changes and establish an intimate electric contact of the “composite” with the current collector. The electrode maintains a capacity of 926 mAh g−1 after 80 cycles at a current density of 100 mA g−1, and the coulombic efficiency is more than 97% after three cycles which improves excellent electrochemical lithium storage performance and cycling stability of the FeMoO4 nanocubes as expected. Furthermore, the morphology change of MoFeO4 electrode after five cycles at 100 mA g−1 was also researched. The whole morphology of the sample after five charge–discharge cycles is still similar to that of the freshly prepared sample (Figure S2, Supporting Information). This result shows that even charging and discharging do not influence the structure stability of the electrode significantly. Besides the high capacity, the rate capabilities are also a useful and important characterization method for LIB. Therefore, the rate performance of FeMoO4 nanocubes is further investigated at different current densities (Figure 7).
Figure 7. Rate capabilities of the FeMoO4 nanocubes at different current density.
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It was proceeded by increments consisting of ten cycles as the current density increases from 100 to 200, 500, 1000, and 2000 mA g−1, corresponding to the average reversible capacities of FeMoO4 nanocubes of 880, 624, 442, 343, and 215 mAh g−1, respectively. The specific capacity of the nanocubes decreases steadily as the current density increases, but still retains high values. Importantly, after reaching a huge discharge–charge current density (2000 mA g−1), the specific capacity could return to more than 950 mAh g−1 when altering the current density back to the 100 mA g−1. The full recovery of the initial capacity implies that the structure of the electrode is stable even during the high current density. Rate performance indicates that the as-prepared FeMoO4 nanocube is a promising anode candidate to replace commercial graphite as the LIB anode in the field of battery power. The excellent performance with high specific capacity and good cycling stability may be corresponded to the special geometry of the regular cubic, which provides specific surface and promotes the conductive capacity. To further understand the electrochemical discharge/ charge mechanism of FeMoO4 nanocubes anode during cycling, cyclic voltammograms (CVs) experiments were conducted to reveal the continual lithiation/delithiation during the discharge and charge cycles. Figure 8 displays representative CV curve of the FeMoO4 synthesized through the typical experiment for the first, second, third, fourth, and tenth cycles, which is carried out at a scan rate of 0.1 mV s−1 in the voltage range of 0.01–3.0 V versus Li/Li+. As illustrated below, the first lithiation and delithiation process is significantly different from the subsequent cycles. During the first cathodic sweep, four redox current peaks denoted as R11, R12, R13, and R14 which are located at around 1.61, 1.45, 0.70, and 0.43 V can be clearly identified from the CVs. According to the reported literature[45] the irreversible cathodic peak R11 can be assigned to the incorporation of lithium ions into the lattice by insertion, forming an intercalated phase LixFeMoO4 with a small volume expansion. The following two peaks R12 and R13 could be ascribed to the reduction of Mo6+ and Fe2+ to metallic Mo0 and Fe0, respectively.[46,47] The R14 peak might result from the formation of solid electrolyte interfaces.[47,48] Meantime, two
poorly defined anodic peaks O11 and O12 centered at around 1.48 and 2.00 V, respectively, in the first scan mean that the oxidation processes of metallic Fe and Mo had taken place by two steps. As shown in the anode scan, the obvious O11 peak probably arises from the oxidation process from Mo to Mo6+[49] while the broad O12 peak could be caused by the oxidation of Fe0 to Fe2+, stepwise.[46,50] In the subsequent cycles, the initial cathodic peaks at 1.61 and 0.43 V disappear as a result of irreversible phase transition from FeMoO4 to FeO and MoO3 and SEI film formation; cathode scans show three peaks R21, R22, and R23 at 1.37, 0.67, and 0.25 V, while the anode scans show the two peaks O21 and O22 located at 1.48 and 1.9 V gradually became broadened and merged into one broad peak. The two peaks R21 and R23 could be attributed to the lithiation in MoO3 taken place in two stages.[46,51] R21 caused by the Li intercalates with MoO3 and the formation of LixMoO3, while R23 represents Li reacts with LixMoO3 to consequently form metallic Mo0 and Li2O. In addition, the R22 peak could be caused by the reduction of Fe2+. The anodic peaks O21 and O22 represent the oxidation of metallic Mo0 and Fe0 to Mo6+ and Fe2+, respectively. Compared with the first cycle both cathodic and anodic peaks are shifted, which is ascribed to polarization of the electrode material in the first cycle. During the anode polarization process, both the peak current and the integrated area of the anodic peak are reduced, implying that the capacity is decreased during the charging process. However, the two anodic peaks could not be observed in the following cycles, which could be attributed to the irreversible destruction of the structure in the process of the formation of Mo0 and Fe0 from Mo6+ and Fe2+,[28] the formation of the amorphous matrix Li2O, and the formation of stable SEI layer on the surface of the electrode.[8] By CV curve analysis and reference to previous literature, the lithium storage in the FeMoO4 is primarily based on multiple step reactions; the following electrochemical reaction mechanism would be responsible for the formation of FeMoO4 with Li. The first discharge can be explained in the following equation. At the initial stage of lithium intercalation (1), a small amount of lithium is inserted into the FeMoO4 to form LixFeMoO4. Then fine Fe0 and Mo0 grains are generated and dispersed in the Li2O matrix as a result of the complete reduction of LixFeMoO4 (2). For the overall process, the first two reactions are largely irreversible, whereas the last one is highly reversible for lithium storage FeMoO 4 + xLi + + xe − → Li x FeMoO 4
(1)
Li x FeMoO 4 + ( 8 − x ) Li + + ( 8 − x ) e − → Fe 0 + Mo 0 + 4Li 2O
(2)
Meanwhile, the reversible charged and discharge cycle is relevant to the mechanism of equation from (3) to (6)[46,47] Figure 8. Cyclic voltammetry of the FeMoO4 nanocubes at a scan rate of 0.1 mV s−1 in the voltage range of 0.01–3.0 V versus Li+/Li. small 2015, 11, No. 36, 4753–4761
Fe 0 + Li 2 O ↔ FeO + 2e − + 2Li
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(3)
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Figure 9. Schematic atomistic models of the lithiation/delithiation process of FeMoO4. A) FeMoO4 before lithiation, B) initial stage of Li ion insertion with the formation of LixFeMoO4, C) formation of interconnected Fe0 and Mo0 clusters embedded in the Li2O nanocrystals, and D) formation of MoO3 and FeO nanoparticles by the deconversion reaction.
Mo 0 + 3Li 2 O→ MoO 3 + 6e − + 6Li
(4)
MoO 3 + yLi + + ye − → Li y MoO 3
(5)
Li y MoO 3
+ zLi + ze − → Mo 0 + 3Li
2O
(6)
By considering the discharge/charge information and CV curves, we propose the structural evolution in the conversion reaction of the FeMoO4 as shown in Figure 9. In the first cycle, the lithium ions inserted into FeMoO4 crystalline lattice without changing the crystal structure at the very beginning of lithiation. The further intake of lithium ion leads to a collapse of the FeMoO4 crystalline lattice and then formation of a phase consisting of LixFeMoO4 clusters; and with more lithium ions diffusing to react with FeMoO4, the lattice finally collapses; single-crystalline FeMoO4 were transformed to networks of Mo and Fe nanograins embedded in Li2O matrix. In the following delithiation process, when lithium ions are electrochemically extracted from the Mo, Fe, and Li2O multicrystalline composite, the Li2O is dissociated along with the oxidation of the Mo, Fe clusters, leading to the formation of MoO3 and FeO (instead of the original FeMoO4) nanocrystals. The MoO3/Mo and FeO/Fe clusters would reversibly form during the conversion/deconversion reaction in the subsequent electrochemical cycles. However, to clarify the specific electrochemical reaction process and influencing factors, more sophisticated techniques and works are needed, which is what we need to do next.
3. Conclusion In summary, FeMoO4 nanocubes with an average edge length of 100 ± 20 nm have been successfully prepared by an effective one-pot hydrothermal route. The FeMoO4 nanosheets were first fabricated and then self-assembled to
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the nanocubes. Both phases of FeMoO4 were formed pseudocube morphology; the β-FeMoO4 pseudocube enclosed by {-211} and {012} planes as well as the α-FeMoO4 nanocubes can be indexed to the (220) facets exposed single crystal. The results showed that the reaction time, the ratio of reactant, and the solution viscosity all play significant roles in restricting the size and morphology of the final product. Electrochemical characterization showed the FeMoO4 nanocubes exhibited a high reversible capacity of 926 mAh g−1 after 80 cycles at a current density of 100 mA g−1 and good rate performance, which indicate that the FeMoO4 nanocubes are promising materials for high-power lithiumion batteries applications. The excellent electrochemical performance may be corresponded to the special geometry of the regular cubic, which provides specific surface and promotes the conductive capacity.
4. Experimental Section All reagents were of analytical purity and purchased from Shanghai Chemical Reagents Company without further purification. Na2MoO4 and FeSO4·7H2O are in the molar ratio of 1:2 mixed. Appropriate amounts of PVP and NaAc were dissolved in 40 mL of mixed solution (25 mL H2O and 15 mL glycol) at a constant speed under electromagnetic stirring for 10 min. Then the mixed solution was transferred into an autoclave in an electric oven at 200 °C for a period of 20 h. After heating, the autoclave was cooled to room temperature naturally. The as-synthesized product was centrifuged and washed several times with deionized water and absolute ethanol before vacuum drying at 80 °C. The XRD patterns of the products were carried out by a Bruker D8 advanced X-ray diffractometer equipped with CuKα radiation (λ = 1.5418 Å). TEM images were taken on a JEM-1011 transmission electron microscope, using an accelerating voltage of 100 kV. The HRTEM images and the SAED patterns were taken on a JEM-2100 transmission electron microscope with the accelerating voltage of 200 kV. Doctor-blade technique was used to prepare the electrode for Li cycling. The electrode was composed of active material (70 wt%), conductivity agent (Super P carbon black, 20 wt%), and binder (polyvinylidene fluoride) (10 wt%) with a Cu foil as the current collector. The coin-type cells (size: 2032) were assembled in the argon-filled glove box (Mikrouna, Super 1200/750) with the concentrations of moisture and oxygen below 1 ppm. Li metal was applied as anode. The separator was Celgard 2400 microporous membrane, and the electrolyte was 1 M LiPF6 solution dissolved in ethylene carbonate/dimethyl carbonate (EC/DMC) by 1:1 volume ratio. And galvanostatic charge/discharge cycling was performed using LAND-CT2001A multichannel galvanostat (Wuhan, China) in the voltage range of 0.01–3.0 V (vs Li+/Li) at room temperature. The CV profiles were carried out in the voltage window 0.01–3.0 V at a scan rate of 0.1 mV s−1 by electrochemical workstation (LK2005A, Tianjin, China).
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgement This work was supported by the Fundamental Research Funds for the Central University 2014QNA10.
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Received: May 7, 2015 Revised: June 4, 2015 Published online: July 6, 2015
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