Introduction
Ternary metal molybdates (M2Mo3O8) are an emerging class of important inorganic functional materials which are widely used in various fields such as electromagnetic response, corrosion inhibitors, photoelectric catalysis [1‒3]. In recent years, with the rapid development of high energy density lithium ion batteries, transition metal molybdates and tungstates (M2Mo3O8, M2W3O8 and so on) have attracted much attention as ideal candidates to substitute for the traditional graphite anode materials, due to their high theoretical capacity, low cost and structural diversity [4]. Although various M2Mo3O8 (M: Co, Mn, Zn, Fe) have been synthesized and tested as Lithium ion batteries anodes, their unfavorable microstructures such as high density, aggregated particles or platelets inevitably resulted in inferior capability and cyclability [5‒11]. Synthesis of hollow sphere structures of carbon and inorganic materials have been an effective strategy to tackle the above challenges [12‒15]. The hollow structures with large specific surface area and high porosity, can not only provide more active sites, but also shorten the Li+ diffusion lengths. Furthermore, the porous hollow structure is also benefited to facilitate electrolyte penetration, thus providing more interface area between electrode material and electrolyte [16‒19]. However, the M2Mo3O8 with hollow sphere structures are rarely reported [20], due to the complexity and aggregation of their microstructures.
The bubble-template-assisted hydrothermal synthesis method is a unique and interesting synthesis method for the construction of hollow metal oxide/sulfide spheres. Since Eschenauer’ group proposed the concept of bubble iterative positioning in 1994 [21], many hollow metal oxide/sulfide structures such as NiO, CoO, MoS2 [22‒25] have been synthesized and studied as lithium ion battery electrodes. Herein, we applied the bubble-template-assisted hydrothermal synthesis method and combined it with calcination to produce the Fe2Mo3O8 mesoporous hollow spheres and improved their lithium storage properties. The obtained Fe2Mo3O8 material have the unique hollow sphere architecture with a wall thickness of about 100 nm. When evaluated as a lithium ion battery anode, the Fe2Mo3O8 hollow spheres deliver a high reversible capacity and improved cycling capability. The well-constructed morphology and superior electrochemical performance of Fe2Mo3O8 hollow spheres, indicated that the method is a new approach towards the synthesis of other molybdate and tungstate based hollow spheres with low cost and high performance.
Experimental
Materials synthesis
Iron(III) nitrate 9-hydrate (Fe(NO3)3·9H2O, 1.077 g) was dissolved in the mixed solution of 36 mL ethylene glycol and 4 mL nitric acid. Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, 0.706 g) was dissolved in 40 mL ethylene glycol. Then the iron nitrate solution was added dropwise into ammonium molybdate solution under magnetic stirring in 30 min. The mixture was then transferred into a 100 mL Teflon lined stainless steel autoclave and heated at 160 °C for 12 h. After the reaction and cooling, the product was collected by centrifugation and dried at 60 °C for 12 h. Then the powder was annealed in Ar atmosphere at 500 °C for 2 h with a heating rate of 5 °C∙min-1.
Material characterizations
The morphologies of materials were characterized by field emission scanning electron microscope (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F20). The X-ray diffraction (XRD, Rigaku, D/max-2200 PC) patterns were acquired on a Rigaku Ultima IV X-ray diffractometer with Cu Ka (l = 1.54178 Å) radiation. The X-ray photoelectron spectroscopy (XPS) was performed with a ULVAC-PHI5000 spectrometer with Al Ka radiation. The Brunauer-Emmett-Teller (BET) surface area was determined by the nitrogen adsorption isotherm at 77 K using a Micromeritics ASAP 2460 surface area and porosity analyzer.
Electrochemical measurements
The electrochemical properties were evaluated using coin cells (CR2032) assembled in an Ar filled glove box with lithium foil as a counter electrode. The working electrodes were prepared by mixing the active material (Fe2Mo3O8) with acetylene black and polyvinylidene fluoride in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone. The slurry was then coated on the copper foil and dried at 110 °C for 12 h under vacuum. The Celgard 2500 polypropylene film and 1 mol∙L-1 lithium hexafluorophosphate (LiPF6) in ethylene carbonate and dimethyl carbonate (1:1 in volume)) were used as a separator and the electrolyte, respectively. Cyclic voltammetry (CV) was performed on a CHI 660E electrochemical workstation. The galvanostatic charge-discharge tests were performed using a Newaresles battery test system (Shenzhen, China) with a cutoff voltage of 3.00-0.01 V vs. Li/Li+ at room temperature. The electrochemical impedance spectral measurements were carried out in the frequency range from 100 kHz to 0.01 Hz.
Results and discussion
Figure 1 illustrates the schematic procedures for the general synthesis of Fe2Mo3O8 hollow spheres. Under the solvothermal condition, ethylene glycol reduces Fe3+ to Fe2+ and synchronously released bubbles of low boiling oxidation products such as acetaldehyde and glycolaldehyde etc. [26,27]. In order to reduce surface free energy, the Fe2+ and MoO42- ions concentrated on the surface of bubble templates and nucleated to form iron(II) molybdate FeMoO4 [28,29]. Then, the FeMoO4 nuclei were aggregated together forming the initial shell, whose morphology and phase structure were shown in Fig. S1 (cf. Electronic Supplementary Material, ESM). After calcination under Ar at 500 °C for 2 h, the spherical shell was preserved, while the FeMoO4 phase was converted to Fe2Mo3O8 due to the phase transition of the crystals. Consequently, three dimensions (3D) Fe2Mo3O8 hollow spheres self-assembled by the small nanoparticles were formed.
Figure 2(a) shows a typical FESEM image of the as-synthesized Fe2Mo3O8 hollow spheres. Spherical morphology with a rough surface and diameter in the range of 0.7‒1.2 mm can be clearly seen. Some aggregated nanoparticles appear near the hollow spheres as well as the precursors (Fig. 2(a), Fig. S1), which may be due to the immature growth of crystals during the formation of hollow spheres. Figure 2(b) shows the magnified SEM image of the crack edge of hollow spheres. Particles ranging in size from 10 to 20 nm were observed on the surface of the spheres. Figures 2(c) and 2(d) are the TEM and high resolution TEM (HRTEM) images of as-synthesized Fe2Mo3O8, respectively. The hollow sphere structure is distinctly identified, the TEM images also confirmed the average shell thickness is 120±10 nm (Fig. 2(c)). The HRTEM image reveals the interplanar distance of the lattice fringes of 0.25 nm, corresponding to the (112) crystal planes of Fe2Mo3O8 (Fig. 2(d)).
The XRD characterizations further determined the crystallographic structures of the as-synthesized Fe2Mo3O8. As shown in Fig. 2(e), the observed diffraction peaks are well indexed to the hexagonal phase of Fe2Mo3O8 with the cell parameters of a = 5.773 Å, b = 5.773 Å, c = 10.054 Å, a = 90.0°, b = 90.0° and g = 120.0° (space group P63mc(186), JCPDS PDF no 36-0526). The corresponding structure of the unit cells is schematically illustrated in Fig. 2(f). In the typical crystal structure of Fe2Mo3O8, hybrid octahedral and tetrahedral Fe-O coordinations are connected by octahedral Mo3-cluster into a three-dimensional network.
The chemical compositions of the as-synthesized Fe2Mo3O8 were also identified via XPS analysis. The XPS survey spectrum in Fig. 3(a) reveals the signals of Fe, Mo, and O. The peaks at 711.4 and 724.7 eV in the high-resolution XPS spectrum of Fe 2p are attributed to Fe2+ 2p3/2 and Fe2+ 2p1/2, respectively (Fig. 3(b)). Mo 3d core level (Fig. 3(c)) splits into two characteristic doublets. Peaks centered at 234.1 and 230.1 eV correspond to the Mo(IV) 3d5/2 and 3d3/2, respectively. Another pair of peaks at 235.7 and 232.4 eV indicate the Mo(VI) state because of the partial surface oxidation during the sample preparation for XPS characterization in air or change in oxygen co-ordination due to laser degradation process, which is similar to many other M2Mo3O8 or molybdenum-based compounds [5,9,30,31]. Figure 3(d) displays the high-resolution XPS spectrum of O 1s. The peak at 530.7 eV can be ascribed to the bonding state of Mo-O or Fe-O. Another peak at 532.1 eV can be attributed to the surface adsorbed moisture [5,10].
To study the porosity and pore structures of Fe2Mo3O8 hollow spheres, N2 adsorption and desorption isotherms were measured on the samples. As shown in Fig. S2 (cf. ESM), the isotherms present the characteristics of type IV isotherm with a certain hysteresis loop appeared at P/P0>0.4, indicating that amount of mesopores present in the sample [6]. The resulting sample possesses a BET surface area of 66.5 m2∙g-1 with a pore volume of 0.147 cm3∙g-1 and an average pore size of ~11 nm. This surface area value is much higher than those of 3‒52 m2∙g-1 for many other molybdates [6,8,10], which is plausibly attributed to the unique hollow sphere structures as well as the self-assembly of nanoparticles that may introduce nanoscale slits or pores on the shells. Such porous structures can accelerate the formation of the electrolyte-electrode interfaces and shorten the transport length for both lithium ions and electrons, which are beneficial for the superior lithium storage [15].
The Li-ion storage properties of the as-synthesized Fe2Mo3O8 hollow spheres were further explored as lithium ion battery anodes. Figure 4(a) displays the CV curves of the initial three cycles in the voltage range of 0.01-3.0 V (vs. Li+/Li) at a scan rate of 0.1 mV∙s-1. The CV shape is similar with those reported in literature [8,10,32,33]. There is a general sentiment that the initial cathodic peak at ~0.64 V and another large peak at ~0.1 V are attributed to the reduction of Fe2+ and Mo4+ to metallic Fe and Mo, respectively, as well as the formation of solid electrolyte interphase film [8,10,30]. The formation of Mo-metal was confirmed from the ex situ TEM characterization when cycled to 0.01 V during 1st cycle (Fig. S3, cf. ESM). Accordingly, the speculated lithium-reaction mechanism is following: Fe2Mo3O8 + 16Li+ + 16e-→ 2Fe0 + 3Mo0 + 8Li2O. Subsequently, the broad anodic peak located in the voltage range of 1.2–1.7 V is attributed to the mixed oxidation processes of metallic Fe and Mo [5,8,30]. The speculated equations are 2Fe+ 2Li2O ↔ 2FeO+ 4Li+ + 4e‒ and 3Mo+6Li2O↔3MoO2 +12Li++12e‒, respectively, which are consistent with the results in Fe 2p and Mo 3d XPS spectra (Fig. 3). In the following cycles, the decrease in the intensity of the cathodic and anodic peaks is an indication of the capacity fading, which is similar to many previous reports of M2Mo3O8 [5,33,34]. However, the following second and third CV curves are almost identical, indicating that the reaction is stable and reversible.
Figure 4(b) shows the representative discharge-charge curves for the Fe2Mo3O8 hollow spheres at a current density of 100 mA∙g-1 with a cutoff potential window of 0.01‒3 V vs. Li/Li+. The initial discharge and charge capacities are 1189 and 997 mA·h∙g-1, respectively, which are higher than the theoretical capacity of 813 mA·h∙g‒1 based on lithium storage mechanism of redox conversion. The excess capacity of as-prepared Fe2Mo3O8 is probably due to the interfacial charge storage and metallic cluster-like Li storage [35]. The initial irreversible capacity loss of ~16% is generally originated from the formation of solid electrolyte interphase films, as well as the irreversible trapping of lithium ions by the Li2O lattice [8,9,33]. However, the capacity losses of the second and third cycles are decreasing obviously to ~2.7%, suggesting the good reversibility of the Fe2Mo3O8 electrode.
To reveal its rate capability, the Fe2Mo3O8 electrode was then cycled under various current densities ranging from 0.1 to 1 A∙g-1, as shown in Fig. 4(c). The Fe2Mo3O8 electrode can deliver the average discharge capacities of 974, 806, 636, 369 mA·h∙g-1 at current densities of 0.1, 0.2, 0.5 and 1 A∙g-1, respectively. Notably, when the current rate decreases back to 0.1 A∙g-1, a high discharge capacity of 812 mA·h∙g-1 can still be regained, suggesting the desirable tolerance of the Fe2Mo3O8 electrode.
Figure 4(d) presents the EIS of the Fe2Mo3O8 electrode before cycle and after the first cycle. The shrinkage of the semi-circles at the high-medium frequencies region is corresponding to the significant decrease of the charge transfer resistance (Rct) after cycling, which is identified by the impedance parameters derived from the equivalent circuit model (Table S1, cf. ESM). Additionally, the Li+ diffusion coefficient (DLi+) of Fe2Mo3O8 electrode is 1.51 × 10‒11 cm2∙s‒1, which was calculated corresponding to the slope of the linear plot of Z' versus w‒1/2 (Fig. S4, cf. ESM), indicating a relatively sluggish diffusion kinetics that associated with Li+ diffusion in the bulk electrode [36‒38].
Figure 4(e) shows the repeated discharge–charge cycling performance of the Fe2Mo3O8 electrode at the current densities of 0.1 A∙g-1. It is clearly seen that the Fe2Mo3O8 electrode displays an obvious capacity loss in the first cycle, and then remains stable in following cycles. Consequently, a robust capacity retention of 866 mA·h∙g-1 is demonstrated over 70 cycles, which retains 87% of the initial reversible capacity. Furthermore, the corresponding Coulombic efficiency values for the whole cycling process stay constant at ~98% after initial several cycles, which further indicates the good stability of the Fe2Mo3O8 electrode.
By comparing the evaluations of our synthesized Fe2Mo3O8 hollow spheres with other M2Mo3O8 molybdates, it is clearly seen that the Fe2Mo3O8 hollow spheres delivery higher or comparable electrochemical properties as Lithium ion batteries anodes (Table S2, cf. ESM). Furthermore, the recently reported Fe2Mo3O8 blocks via solid-state synthesis at 1000 °C and 8 MPa pressure [9] also exhibit much lower reversible capacities of 420 mA∙h∙g-1 at 50 mA∙g-1 than our sample. The enhanced lithium storage properties of the as-synthesized Fe2Mo3O8 in this work can be attributed to the special nanostructure of hollow spheres constructed by fine nanoparticles. These mesoporous hollow spheres can facilitate the electrolyte penetration and simultaneously contact the electrolyte on the inner and outer surfaces of hollow spheres, which not only shortens the diffusion path of lithium ions, but also increases the exposure of effective active sites for Li+ insertion/extraction [15].
Fig.4 The electrochemical performances of Fe2Mo3O8 hollow spheres: (a) CV curves at 0.1 mV∙s-1; (b) discharge-charge voltage profiles; (c) rate performance; (d) electrochemical impedance spectroscopy (EIS) profiles; (e) cycling performance and Coulombic efficiency of the Fe2Mo3O8 electrode. |
Conclusions
In summary, we utilized the bubble-template-assisted hydrothermal synthetic method combined with a simple calcination to prepare the Fe2Mo3O8 mesoporous hollow spheres with improved lithium ion storage properties. The shells of the hollow spheres have a thickness of about 120 nm on average. They were consisted of small self-assembled Fe2Mo3O8 nanoparticles with the sizes ranging from 10 to 20 nm. Furthermore, the prepared sample possesses a surface area of 66.5 m2∙g-1 and an average pore size of ~11 nm. These unique mesoporous hollow spheres can facilitate the electrolyte penetration and provide more active sites for the Li+ ions insertion/extraction. When evaluated as a lithium ion battery anode, the proposed Fe2Mo3O8 hollow spheres exhibit highly reversible capacity and good long-term cycling stability. We anticipate that this feasible strategy will lead to new opportunities for the rational design and synthesis in other ternary metal molybdates and tungstates.