Synthesis of Hierarchical Hollow-Structured Single-Crystalline

Article pubs.acs.org/JPCC

Synthesis of Hierarchical Hollow-Structured Single-Crystalline Magnetite (Fe3O4) Microspheres: The Highly Powerful Storage versus Lithium as an Anode for Lithium Ion Batteries Q. Q. Xiong, J. P. Tu,* Y. Lu, J. Chen, Y. X. Yu, Y. Q. Qiao, X. L. Wang, and C. D. Gu State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Hierarchical hollow Fe3O4 (H−Fe3O4) microspheres are prepared by the controlled thermal decomposition of an iron alkoxide precursor, which is obtained via an ethylene glycol (EG) mediated solvothermal reaction of ferric chloride hexahydrate (FeCl3·6H2O), sodium acetate (NaAc), and polyvinylpyrrolidone (PVP). The microspheres are characterized by the assembly of highly oriented primary nanoparticles and have a single-crystal feature. As the anode materials for the lithium-ion batteries, the resultant H−Fe3O4 microspheres show high specific capacity and good cycle stability (851.9 mAh g−1 at 1 C and 750.1 mAh g−1 at 3 C up to 50 cycles), as well as enhanced rate capability. The excellent electrochemical performance can be attributed to the high interfacial contact area between the microspheres and electrolyte, and good accommodation of volume change arising from the synergetic effect of the hierarchical hollow structure. It is suggested that the H−Fe3O4 microsphere synthesized by this method is a promising anode material for high energy-density lithium-ion batteries.

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INTRODUCTION With the increasing demand of lithium-ion batteries (LIBs) for promising power sources in electric vehicles (EV) and hybrid electric vehicles (HEV), much attention has been paid to alternative anode materials with high capacity, long cycling lifetime, and high safety.1−8 Recently, extensive efforts have been focused on magnetite (Fe3O4) because of its high capacity (928 mAh g−1), nontoxicity, natural abundance, low cost, and high electronic conductivity (2 × 104 S m−1).9,10 The electrochemical conversion reaction of Fe3O4 with Li+ is shown in reaction 1:11 Fe3O4 + 8Li+ + 8e− ↔ 3Fe + 4Li2O

contact of active material/electrolyte, and the short diffusion length of Li+. In well-designed nanostructures, not only the Li+ diffusion is much easier, but also the strain associated with Li+ intercalation and the volume expansion of active materials are often better accommodated, resulting in significantly improved electrochemical performance.15 Recently, the capacity retention of iron oxides can be enhanced by fabricating active materials into hollow/nanostructures or porous/nanostructures.16−22 To our knowledge, hollow microspherical structure has long been a focus of research.23−27 Therefore, we consider introducing this special structure into the LIB applications. Herein, we present a facile one-pot synthetic route for preparation of unique hierarchical hollow Fe3O4 (H−Fe3O4) microspheres assembling with nanoparticles as primary building blocks by an EG mediated solvothermal reaction (EG as a reductant)28,29 of FeCl3·6H2O, NaAc, and PVP. In our system, PVP served as a surface stabilizer, which is crucial to the formation and transformation of the hollow interiors. The hierarchical hollow structure has a great relationship with the reversible capacity, increasing electric contact areas and greatly promoting Li+ diffusion. The voids in the H−Fe3O4 materials can buffer against the local volume change during the insertion/ desertion process of Li+.

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Each formula unit of Fe3O4 can react with eight Li to form a composite containing Fe nanoclusters embedded in amorphous Li2O matrix, which reversibly converts back to Fe3O4 on charging process.12 The quite reversible reaction ensures excellent columbic efficiency and high reversible capacity of Fe3O4. However, the use of Fe3O4 as anodes in LIBs is still hampered. The capacity retention of Fe3O4 is somewhat disappointing because of the aggregation of Fe3O4 particles during cycling, large volume variation (>200%) that inherently accompanies the conversion reaction process and severe destruction of the electrode.13,14 These holdbacks can partially be solved by fabricating nanostructured materials. Construction of delicate nanostructures of Fe3O4 is an effective strategy to improve the cyclability for large surface area, the sufficient © 2012 American Chemical Society

Received: January 8, 2012 Revised: February 13, 2012 Published: February 21, 2012 6495

dx.doi.org/10.1021/jp3002178 | J. Phys. Chem. C 2012, 116, 6495−6502

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION

Synthesis. Hierarchical H−Fe3O4 microspheres were synthesized by forced hydrolysis of ferric chloride solution with polyvinylpyrrolidone (PVP) at elevated temperatures. In a typical hydrothermal method, PVP (1.0 g) was dissolved in 40 mL of ethylene glycol (EG), followed by the addition of 5 mmol of ferric chloride hexahydrate (FeCl3·6H2O) and 30 mmol of sodium acetate (NaAc). The mixture was stirred vigorously for 15 min, and then was sealed in a Teflon-lined stainless-steel autoclave. The autoclave was heated to 200 °C and maintained at this temperature for 10 h, and then was cooled to room temperature. The resulting black product was washed with ethanol and deionized water several times, and was finally dried at 80 °C in an oven overnight. To prepare solid Fe3O4 nanoparticles for comparison, the same amounts of FeCl3·6H2O and NaAc were used alone without the addition of PVP, then followed by the same conditions and procedures applied in the synthesis of H−Fe3O4 microspheres. Characterization. The morphology and microstructure of the products were characterized by X-ray diffractometer (XRD, Rigaku D/max 2550 PC, Cu Kα), scanning electron microscopy (SEM, Hitachi S-4700 and FESEM, FEI Sirion100), and transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV). Electrochemical Investigation. The electrochemical tests were performed using a coin-type half cell (CR 2025). The slurry consisted of 70 wt % Fe3O4, 15 wt % acetylene black, and 15 wt % polyvinylidene fluoride (PVDF) dissolved in N-methyl pyrrolidinone (NMP) and was pasted on copper foil with 12 mm diameter. Then, the slurry was dried at 90 °C for 24 h in a vacuum. Test cells were assembled in an argon-filled glovebox with the metallic lithium foil as the counter electrode, 1 M LiPF6 in ethylene carbonate (EC)−dimethyl carbonate (DME) (1:1 in volume) as the electrolyte, and a polypropylene (PP) microporous film (Cellgard 2300) as the separator. The galvanostatic charge−discharge tests were conducted on a LAND battery program-control test system at different rates between 0.01 and 3.0 V at room temperature (25 ± 1 °C). Cyclic voltammetry (CV) was performed on the CHI660C electrochemical workstation in the potential range 0−3.0 V (vs Li+/Li) at a scan rate of 0.1 mV s−1. For the electrochemical impedance spectroscopy (EIS) measurements, the excitation voltage applied to the cells was 5 mV and the frequency range was from 100 kHz to 10 mHz.

Figure 1. XRD patterns of H−Fe3O4 and solid Fe3O4 microspheres.

the morphology of solid Fe3O4 particles with the welldistributed size of 200 nm, which are assembled by several small particles. The detailed morphological and structural features of the powders are also examined by TEM. Figure 3a further confirms the hierarchical microsphere structure of H− Fe3O4. With a careful examination, the secondary structure of hierarchical H−Fe3O4 microspheres can be easily observed (Figure 3b). The selected area electron diffraction (SAED) pattern shown in the inset of Figure 3b, which is recorded from a whole microsphere, reveals that a H−Fe3O4 microsphere is single-crystalline and all diffraction rings can be indexed to Fe3O4 with cubic symmetry, which is similar to the results reported by Deng et al.30 and Zhu et al.31 The high-resolution TEM (HRTEM) analysis in Figure 3c corresponds to four representative areas taken from the parts of Figure 3b in rectangular frames, which shows that the whole microsphere preferentially grows along the same direction, indicating again the single-crystalline nature of a microsphere. It is suggested that the primary nanoparticles are highly oriented, resulting in the single-crystal feature of hierarchical H−Fe3O4 microspheres. The distance between adjacent lattice planes is measured at about 0.21 nm, which is in good agreement with the (400) plane of Fe3O4. It can be seen that the Fe3O4 products obtained without the addition of PVP are solid sphere-like structures (Figure 3d). The SAED pattern in the inset also reveals that the solid Fe3O4 microspheres are singlecrystalline. To further confirm the hierarchical hollow microsphere structure, nitrogen adsorption and desorption measurements were performed to estimate the texture properties. The nitrogen adsorption and desorption isotherm and pore size distribution curve (inset) of the H−Fe3O4 microspheres are shown in Figure 4. The isotherm of the H−Fe3O4 microspheres exhibits a hysteresis loop at the P/P0 ranges of 0.55−0.95, indicating the presence of mesopores. The Brunauer− Emmett−Teller (BET) surface area of the H−Fe3O4 microspheres is calculated to be 12.27 m2 g−1, which is much higher than that of the solid sphere-like Fe3O4 (5.43 m2 g−1). The plot of pore size distribution determined by the Barrett−Joyner− Halenda (BJH) method (Figure 4, inset) shows that there are two peaks, a dominant peak around 16.4 nm and a broad peak at 310 nm. The mesopores on the hollow sphere can be attributed to the interspaces of the constituent particles, while the broad peak indicates the void in the hollow sphere. The relatively large specific surface area and high porosity offer large

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RESULTS AND DISCUSSION The XRD pattern in Figure 1 reveals that all the diffraction peaks of the two powders can be unambiguously assigned to the pure face-centered cubic structural (Fd3m space group) magnetite Fe3O4 (JCPDS card no. 19-0629). Compared with the solid Fe3O4 particles, the peaks of hierarchical H−Fe3O4 broaden, indicating the formation of small size particles. Figure 2a shows the SEM image of the monodispersed hierarchical H−Fe3O4 microspheres with diameters of 400−500 nm. The relatively high-magnification SEM image is shown in Figure 2b. The surface of the hollow spheres is composed of closely packed nanoparticles with diameters ranging from 30 to 40 nm as primary building blocks. At the same time, the hollow interior of these spheres is confirmed though the observation of a broken one, which shows a large void space and a welldefined shell. Such a microstructure provides a high surface area that is beneficial to an electrode material. Figure 2c and d shows 6496



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Figure 2. SEM micrographs of (a) H−Fe3O4 microspheres, (b) a magnified image of H−Fe3O4 microspheres, (c) solid Fe3O4 microspheres, and (d) a magnified image of solid Fe3O4 microspheres.

Figure 3. TEM micrographs of (a) H−Fe3O4 microspheres and (b) a magnified image of a single H−Fe3O4 microsphere, with a corresponding SAED pattern (inset). (c) HRTEM images of H−Fe3O4 microspheres. (d) TEM micrograph of solid Fe3O4 microspheres, with a corresponding SAED pattern (inset).

materials/electrolyte contact area and promote Li+ diffusion.13,16 In order to investigate the growth mechanism of such hollow microstructures, we carried out time-dependent experiments.

The related transformation in the morphology of the asprepared samples with different reaction times is shown in Figure 5. Submicrometer particles first form as aggregates with a size of about 50 nm after 0.5 h (Figure 5a). Then, solid 6497



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obvious peaks observed through the XRD pattern of the product. However, after reaction for 2 h, the cubic phase of Fe3O4 was obtained (see the Supporting Information, Figure S1). To generally illustrate the formation mechanism of H−Fe3O4 microspheres, the synthetic procedure is drawn as shown in Figure 6, which can be understood as a result of a nucleationoriented aggregation-recrystallization mechanism from primary nanocrystals under high-temperature solvothermal conditions.32 In simple terms, primary magnetite nanocrystals first nucleate in a supersaturated solution, resulting from the solvent-mediated hydrolysis of Fe3+. Afterward, the newly formed nanocrystals aggregate into round spheres, driven by minimization of total surface energy. During the aggregation process, the adjacent primary nanocrystals align in an orderly way by an oriented-attachment mechanism so that the particles share a planar interface in a common crystallographic orientation.33,34 Therefore, the formed aggregates present a single-crystal characteristic. Compared with the procedure of fabricating solid Fe3O4, the H−Fe3O4 microspheres were grown simply by employing PVP which served as a surface stabilizer,35 to form an organic layer on the surface of the nanocrystals and induce the assembly and growth of Fe3O4 crystals. Then, the inner crystallites of the aggregate go through the mass transfer to the outer shell by a dissolution-recrystallization process at the cost of the small crystals, which have higher surface energies and solubility than the larger ones.31,36 Through a lengthened ripening process, outward migration of crystals would result in continued expansion of interior space within the original aggregates, and the inner space of the spheres is further increased. Therefore, the hollow microspheres would be

Figure 4. Nitrogen adsorption−desorption isotherms with corresponding BJH desorption pore size distributions (inset) of H−Fe3O4 microspheres.

sphere-like structures with an average size of 400 nm are formed with a synthesis time up to 2 h (Figure 5b). When the reaction time is increased to 7 h, the solid Fe3O4 microspheres turn into mainly quasi-hollow Fe3O4 composed of nanoparticles, as shown in Figure 5c, which can be inferred from a strong contrast between the dark edges and the pale center. However, when the reaction time is increased to 15 h, the partial hollow Fe3O4 microspheres break (Figure 5d). Meanwhile, the hollow interior became larger. In addition, we acquired orange precipitation when the reaction time was 0.5 h, which showed the amorphous feature, since there were no

Figure 5. TEM images of the products obtained at different reaction times: (a) 0.5 h, (b) 2 h, (c) 7 h, (d) 15 h. 6498



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Figure 6. Schematic illustration of the formation of H−Fe3O4 and solid Fe3O4 microspheres.

broken, since the decomposition and recrystallization processes last too long. On the other hand, although the hollowing process can be well explained by Ostwald ripening, it is noted that the time needed to form hollow structures in our process using PVP as the structure stabilizer is much shorter (generally, the time needed for structure evolution from solid to hollow structure by the Ostwald ripening process is longer than 12 h).37−39 In view of the potential application as an anode in LIBs, we investigated its ability to reversibly react with Li+ of H−Fe3O4 microspheres in comparison with that of the solid ones. Figure 7 shows the CV curves of H−Fe3 O 4 and solid Fe 3 O4 microspheres for the first three cycles in the potential range 0.0−3.0 V (vs Li/Li+) at ambient temperature at a scan rate of 0.1 mV s−1. The CV curves of the two Fe3O4 electrodes are similar. As shown in Figure 7a, the sharp reduction peak at about 0.6 V is observed in the first cathodic scan for the H− Fe3O4 microspheres, which can be attributed to the reduction of Fe3+ or Fe2+ to Fe0 and the irreversible reaction with the electrolyte. In this step, the conversion of Fe3O4 to Fe and the formation of amorphous Li2O are the main reason of the irreversible capacity during the discharge process. Meanwhile, two anodic peaks at about 1.63 and 1.85 V correspond to the reversible oxidation of Fe0 to Fe2+/Fe3+, which agrees well with early studies.15,40 From the second cycle, both cathodic and anodic peaks are positively shifted in the subsequent cycles due to the structural modification after the first cycle. However, the current density and the integrated area of the H−Fe3O4 microspheres are larger than those of solid Fe3O4 owing to the sufficient reaction with Li+ of hierarchical hollow structure. It also should be noted that the peak at 1.85 V for the second or third cycle of the H−Fe3O4 microsphere electrode, which can be indexed to the reversible reaction to Fe3O4, are more obvious and intensive than that of the solid one, indicating that the hierarchical hollow structure leads to sufficient oxidation reaction process during the anodic cycling. Figure 8 shows the first two galvanostatic charge−discharge curves for the H−Fe3O4 and solid Fe3O4 microsphere electrodes between 0.01 and 3.0 V at 0.1 C. The first curves of the two materials are similar, indicating that the different morphologies do not change the electrochemical nature of Fe3O4. A voltage plateau can be observed at about 0.8 V, followed by a sloping curve down to the cut voltage of 0.01 V, which are typical characteristics of voltage trends for the Fe3O4

Figure 7. CV curves of (a) H−Fe 3 O4 and (b) solid Fe 3 O 4 microspheres from the first cycle to the third cycle at a scan rate of 0.1 mV s−1 in the potential range 0−3.0 V (versus Li/Li+).

electrode.40−42 The second discharge curves of the H−Fe3O4 and solid Fe3O4 microspheres are both different from the first, suggesting drastic, Li+-driven, structural or textural modifications.13 Furthermore, the charge voltage plateaus of both the electrodes are higher than the discharge one. The polarization (i.e., voltage hysteresis between charge and discharge) could be 6499



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Figure 8. Charge−discharge profiles of (a) H−Fe3O4 and (b) solid Fe3O4 microsphere electrodes between 0.01 and 3.0 V at 0.1 C.

due to the limited lithium diffusion kinetics during the intercalation/deintercalation process.43 However, the H− Fe3O4 microspheres show smaller voltage differences of the plateaus, indicating that the hierarchical hollow structure possesses lower electrochemical polarization and induces better reversibility in the discharge/charge processes. The first specific discharge capacity of H−Fe3O4 and solid Fe3O4 microspheres are 1260.8 and 1214.3 mAh g−1, respectively. The extra capacity of the electrodes compared with the theoretic capacity resulted from the formation of solid electrolyte interface (SEI) film and possibly interfacial Li+ storage during the first discharge process.44,45 Figure 9a and b show the cyclability of the H−Fe3O4 and solid Fe3O4 microsphere electrodes at 1 and 3 C after the first five cycles at 0.1 C, respectively. The specific reversible capacity of H−Fe3O4 microspheres after 50 cycles is 851.9 mAh g−1 at 1 C and 750.1 mAh g−1 at 3 C, much higher than that of solid Fe3O4 (513.3 mAh g−1 at 1 C and 211.7 mAh g−1 at 3 C). In addition, the H−Fe3O4 microspheres exhibit better capacity retention. After 50 cycles, the H−Fe3O4 microspheres could sustain 92.4 and 86.0% capacity of the 6th cycle at 1 and 3 C, respectively, compared with 60.8 and 29.0% for the solid Fe3O4. The capacity fade of both the electrodes at 3 C is a little faster than cycling at 1 C. The indirect contact of active materials/ current collector and supplementary inactive interfaces via the traditional slurry coating procedure will limit the electron transfer between the Fe3O4 and the current collector, especially at high rates. Even though, the capacity fade of the H−Fe3O4 microspheres is much less influenced by the discharge rate. The good capacity retention of H−Fe3O4 microsphere electrode is attributed to its hierarchical hollow nanostructure, which can provide large specific surface area to facilitate the Li+ and electron transportation. In addition, the structure of H−Fe3O4 microspheres could keep stable by buffering the volume expansion. Therefore, the H−Fe3O4 microspheres can exhibit much better cycling performance than the solid Fe3O4, especially at high discharge rates. Figure 9c presents the Coulombic efficiencies of H−Fe3O4 and solid Fe3O4 microsphere electrodes at 3 C. It obviously demonstrates that the H− Fe3O4 microspheres exhibit higher initial Coulombic efficiency (78.8−75.8%). In the subsequent cycles, the Coulombic efficiency of H−Fe3O4 microspheres is steady and almost

Figure 9. (a) Cycling performance of H−Fe3O4 and solid Fe3O4 microspheres at 1 C (a) and 5 C (b) from the 6th cycle to the 50th cycle after the first 5 cycles at 0.1 C. (c) Coulombic efficiency of H− Fe3O4 and solid Fe3O4 microspheres.

near 100%, while the Coulombic efficiency of solid Fe3O4 is unstable and much lower than that of H−Fe3O4. It means that the hierarchical hollow nanostructure is much more stable than the solid sphere-like structure during cycling. To further provide insight behind these numerical data, the SEM images of H−Fe3O4 and solid Fe3O4 microsphere electrodes after 50 6500



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cycles at 1 C are shown in Figure S2 the Supporting Information. In addition to the much improved cycling performance, the H−Fe3O4 microspheres also show significantly enhanced high rate performance compared with the solid Fe3O4 ones, as shown in Figure 10. With increasing the rate from 0.1 to 5 C,

Figure 10. Rate performance of H−Fe 3 O 4 and solid Fe 3 O 4 microspheres.

the discharge capacities of the two materials decrease gradually, indicating the diffusion-controlled kinetics process for the electrode reaction.46 When the rate reaches 5 C, the capacity of the H−Fe3O4 microspheres still retains 654.5 mAh g−1. However, the discharge capacity of the solid Fe3O4 microspheres drops dramatically and only 407.9 mAh g−1 can be maintained. When the rate is lowered to 0.1 C again, the discharge capacity of 946.1 mAh g−1 is regained for the H− Fe3O4 microspheres, demonstrating the excellent reversibility. To the best of our knowledge, Fe3O4 synthesized in this experiment shows better rate retention and cycling capability than those in the previous works.14,40,42,47,48 The enhanced cycling performance and rate capability are due to the high electrochemical activity and stability of hierarchical hollow nanostructure. Also the high capacity achieved at a high rate implies that this type of electrode can be a promising candidate for high power applications. AC impedance measurements as shown in Figure 11 are conducted to verify that the hierarchical hollow nanostructure is responsible for the good performance of the cells with the Fe3O4 electrode. The Nyquist plots obtained for the H−Fe3O4 and solid Fe3O4 microsphere electrodes after 3 and 50 cycles (at 0.1 C) in the fully charged state were collected for comparison. The Nyquist plots of both the electrodes consist of a depressed semicircle where a high-frequency semicircle and a medium-frequency semicircle overlap each other and a long low-frequency line. The intercept on the Z real axis in the highfrequency region corresponds to the resistance of electrolyte (Rs). The semicircle in the middle frequency range indicates the charge-transfer resistance (Rct), relating to charge transfer through the electrode/electrolyte interface. The inclined line in the low-frequency region represents the Warburg impedance (Zw), which is related to solid-state diffusion of Li+ in the electrode materials.49 We mainly focus on the comparison of Rct. As one can observe, after 3 cycles, there are no obvious changes of Rct between the two electrodes. However, after

Figure 11. Nyquist plots of H−Fe3O4 and solid Fe3O4 microsphere electrodes after 3 cycles (in red circles) and 50 cycles (in black squares) in the frequency range from 100 kHz to 10 mHz.

cycled for 50 times, the value of Rct of H−Fe3O4 increases slightly, whereas the one of the Fe3O4 microsphere electrode increases strikingly. Therefore, it is suggested that the interparticle resistance of the electrode is suppressed by the hierarchical hollow nanostructure, and consequently, the value of Rct for the H−Fe3O4 microsphere electrode is smaller than that of the Fe3O4 one, which indicates the electrons and Li+ can transfer more effectively in the interface of active materials and electrolyte, thus resulting in the enhanced electrode reaction kinetics and better cycling performance of the cells during the charge/discharge process.

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CONCLUSIONS

In summary, we report a simple one-pot strategy of hydrothermal synthesis to fabricate single-crystalline H− Fe3O4 microspheres in large scale. The H−Fe3O4 microspheres exhibit high discharge capacity, good cycle stability (851.9 mAh g−1 at 1 C and 750.1 mAh g−1 at 3 C up to 50 cycles), and enhanced rate capability, due to the high interfacial contact area of the active materials with the electrolyte and good accommodation of volume change. The utilization of hierarchical hollow structure is suggested as a favorable strategy toward the development of transition metal oxides as highperformance electrode materials for LIBs. 6501



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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns of H−Fe3O4 microspheres obtained at different reaction times and SEM images of the two samples after 50 cycles at 1 C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 571 87952856. Fax: +86 571 8795 2573. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The assistance of Dr. Jun Zhang for SEM analysis and Mr. Xinting Cong and Dr. Wei Huang for TEM analysis are grateful acknowledged.

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REFERENCES

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