α-Fe2O3 Submicron Spheres with Hollow and Macroporous Structures

Jan 21, 2014 - Department of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea. ‡ Department of WCU...
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α‑Fe2O3 Submicron Spheres with Hollow and Macroporous Structures as High-Performance Anode Materials for Lithium Ion Batteries Kyung-An Kwon,† Hyung-Seok Lim,† Yang-Kook Sun,‡,§ and Kyung-Do Suh*,† †

Department of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea Department of WCU Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea § Chemistry Department, Faculty of Science, King University, Jeddah, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: α-Fe2O3 submicron spheres with different internal structures were prepared as anode materials for lithium ion batteries (LIBs). Using sulfonated polystyrene (SPS) microspheres as a template, we designed a hollow and macroporous α-Fe2O3 particle structure. The sulfonation degree of polystyrene (SPS) microspheres was controlled by sulfonation reaction time in the range of 24−36 h. After introducing Fe metal precursors by adsorption of ferrous ions into the SPS particles and adding a reduction agent, α-Fe2O3 submicron spheres with hollow and macroporous structures were obtained by heat treatment in an air atmosphere. The internal structure of particles was characterized by scanning electron microscopy, transmission electron microscopy, focused ion beam-scanning electron microscopy, and X-ray diffraction. The electrochemical properties of the hollow and macroporous α-Fe2O3 composite electrodes were investigated by galvanostatic cycling at both constant and variable current rates. The α-Fe2O3 submicron spheres with hollow and macroporous structures exhibited excellent cyclability and rate capability. Electrical impedance spectroscopy was employed to prove the structural effects on the cell performances.

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INTRODUCTION Graphite has been used as an anode material for commercial LIBs because of its high Coulombic efficiency and high safety.1 However, because of the low theoretical capacity (372 mAh g−1) of graphite, major challenges of the next-generation LIBs for high volume applications, such as hybrid vehicles and energy sources, have demanded development of alternative anode materials with high energy density, long cycle life, and high rate capability. Several transition metal oxides (MOs), which have shown higher theoretical Li-ion storage capacities (>600 mAh g−1) than graphite anodes (372 mAh g−1), have been recommended as highly reversible lithium storage materials.2−5 Moreover, MO (where M is Fe, Co, Ni, Cu, etc.) electrodes have excellent abilities as anode materials due to their low toxicity, low cost, and widespread availability. Among these materials, some iron oxides (Fe3O4 and α-Fe2O3) have received considerable attention owing to their high theoretical capacity (926−1007 mAh g−1), huge abundance on earth, environmental benignity, and high resistance to corrosion.6,7 However, iron oxides often suffer from poor kinetics caused by their low electrical conductivity, serious hysteresis between charging and discharging voltage, and rapid capacity fading due to a large volume expansion during cycling.8−10 Many research groups have reported improved © 2014 American Chemical Society

performance of iron oxide-based anodes through carbon coatings to increase the electronic conductivity of electrode materials and strategic structural modifications to alleviate volume expansion during the lithiation and delithiation processes. Recently, we synthesized hollow Fe3O4 submicron spheres using hydrogel microspheres as an ionizable template.11 Although the hollow Fe3O4 submicron spheres exhibited the improvement of cyclability and rate performance, they showed rapid capacity fading at high current rates (5 and 10 C) because of the low electrical conductivity of Fe3O4. To improve this drawback, the rattle type α-Fe2O3 submicron spheres with a thin carbon layer were prepared through the diffusion of iron ions into the hydrogel microspheres and exhibited good cyclability at high current rates.12 However, we could not confirm the difference in electrochemical properties corresponding to the structural differences because of the difficulty of ionization control of hydrogel template. Since the structure and morphology of active materials greatly influence energy density, specific surface area, mass transfer, and charge transfer, it is important to understand the structural effect of active Received: July 4, 2013 Revised: January 15, 2014 Published: January 21, 2014 2897

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Scheme 1. Schematic Illustration of the Synthetic Process of the α-Fe2O3 submicron Spheres with Different Internal Structures

g mol−1, Sigma-Aldrich Chemical Co.), ethanol (Daejung Chemicals), dioctyl sodium sulfosuccinate (aerosol-OT, Wako Pure Chemicals), sodium dodecyl sulfate (SDS, Wako Pure Chemicals), sulfuric acid (Junsei Chemical Co.), sodium hydroxide (NaOH, Sigma-Aldrich Chemical Co.), and hydrogen peroxide (Daejung Chemicals, assay 30%). All aqueous solutions were prepared using distilled water. 2.2. Synthesis of Semi-IPN Polystyrene Particles. Monodisperse semi-interpenetrating polymer network (semiIPN) polystyrene (PS) microspheres were synthesized through the seeded polymerization method. All reactions were carried out in a 250 mL four-neck glass reactor equipped with a mechanical stirrer, a refluxing condenser, and a nitrogen inlet system. Initially, linear PS seed particles were dispersed in 0.25 wt % SLS solution by ultrasonication for 30 min. The monomer mixture consisting of DVB (1 g) and AIBN (0.04 g) was poured into the reactor and stirred vigorously at 35 °C. After 90 min, the swollen particles were stabilized with 1 wt % PVP aqueous solution. Then the reactor was submerged into a preheated thermostatic oil bath, and the polymerization was carried out with a stirring speed of 120 rpm at 70 °C for 12 h. The resulting semi-IPN PS microspheres were centrifuged at 3600 rpm for 10 min. The supernatant solution was decanted, and remaining semi-IPN PS microspheres were repeatedly washed with several centrifugation/redispersion cycles in ethanol and distilled water. Finally, the semi-IPN PS microspheres were dried under a vacuum at room temperature. 2.3. Sulfonation of Semi-IPN PS Particles. The chemical modification of semi-IPN PS microspheres was carried out using sulfuric acid. Three samples were prepared with different sulfonation reaction times (24, 30, and 36 h). The sulfonated semi-IPN PS microspheres (SPS) were obtained using the following approaches. The semi-IPN PS microspheres (0.5 g) were added into the media bottle filled with H2SO4 (50 mL) and dispersed with ultrasonication for 30 min. Subsequently, the bottle containing the sample was placed in a jacketed flatbottom beaker. At constant temperature (50 °C), the mixture was stirred using magnetic stirrer and kept at a fixed reaction time of 24, 30, and 36 h. Subsequently, the sediment was repeatedly filtered and washed with ethanol until the precipitate was neutral. After completing the drying process under a vacuum at room temperature, pink powders were obtained.

materials on battery performances. Herein, therefore, we report a facile approach to control the morphology and compare the electrochemical properties of iron oxide submicron spheres with different structures. In this synthetic system, sulfonated polystyrene (SPS) microspheres were used as sacrificial templates to prepare α-Fe2O3 submicron spheres with hollow and macroporous structures. The sulfonation degrees of SPS microspheres have a great effect on the internal structure of final products after heat treatment. The synthetic process of designed α-Fe2O3 submicron spheres is shown in Scheme 1. In order to understand the structural changes, we measured the sulfonation degrees of SPS microspheres by a confocal laser scanning microscopy (CLSM) and confirmed that sulfonation starts from the outer region of PS microspheres during a sulfonation process. As a result, the SPS microspheres prepared by different sulfonation times have different degrees of hydrophilicity. For these reasons, the diffusion depth of ferrous ions (Fe2+) depends on the sulfonation degree of SPS microspheres and, consequently, two types of α-Fe 2 O 3 submicron spheres with hollow and macroporous structures could be obtained after heat treatment in air. The specific surface area and pore distribution of two samples consisting of many α-Fe2O3 nanograins were quite different because of a significant structure difference. The structural and morphological characteristics and electrochemical properties of two types of α-Fe2O3 submicron spheres were characterized and analyzed by FIB-SEM, TEM, and various electrochemical measurements. Both samples exhibited significantly improved Li-ion storage capabilities with a very high reversible capacity over a theoretical capacity as well as exhibiting high rate capabilities. Finally, we investigated the structural effects on the total resistance, mass resistance, and charge transfer resistance by using electrochemical impedance spectroscopy.

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EXPERIMENTAL SECTION

2.1. Materials. Divinylbenzene (DVB, 80% mixture of isomers, Aldrich Chemical Co.) was used after purification via an inhibitor removal column (Aldrich). 2,2′-Azoisobutyronitrile (AIBN, Junsei Chemical Co.) and benzoyl peroxide (BPO, Daejung Chemicals) were recrystallized with methanol to remove an inhibitor before use. Other materials were used without further purification, including styrene (St, Junsei Chemical Co.), polyvinylpyrrolidone (PVP, Mw ∼ 4.0 × 104 2898

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2.4. Preparation of Hollow and Macroporous α-Fe2O3 Submicron Spheres. The prepared SPS microspheres were dispersed in distilled water (150 mL) by ultrasonication for 30 min. Then, NaOH solution (3 M, 82 mL) was slowly added into above mixture until the pH reached approximately pH 6.5. The pale-green aqueous solution containing FeSO4·7H2O (0.25 g) was added, and mechanical stirring was allowed to proceed for 12 h. After this, to filter the residue, the sample was centrifuged once at 3600 rpm. Thereafter, precipitate was redispersed with distilled water (200 mL). H2O2 (1.872 g) solution was added into the mixture and stirred constantly for 2 h. After repetitive washing with ethanol/distilled water mixture, the Fe metal precursors/SPS composite microspheres were dried under vacuum, then heated to 600 °C at 2 °C/min, and maintained for 4 h at 600 °C to remove the SPS templates. We summarized the sample names of SPS microspheres prepared by sulfonation reaction at different times, as shown in Table 1.

2.6. Characterization of Electrochemical Properties. Electrochemical characterization was carried out using a 2032 coin type cell. Two types of α-Fe2O3 submicron spheres (70 wt %) and super-P (conductive agent, 15 wt %) were mixed with carboxymethylcellulose (CMC, 5 wt % in DI water) and poly(acrylic acid) (PAA, 1 wt % in DI water) as binders. The mixture slurries were coated on a Cu foil by doctor-blade casting. The working electrodes were dried for 20 min at 110 °C and roll-pressed. The coin type cells were assembled under an argon atmosphere. Li metal foil (1 mm thick) was used as a counter electrode, and the commercial electrolyte was composed of a 1 M LiPF6 salt and an ethylene carbonate/ diethyl carbonate solution (EC/DEC, 1:1 by volume, provided by Techno Semichem Co. Ltd., Korea). Electrochemical performance was tested in the voltage range 0.02−3 V (vs Li+/Li) at various current rates (0.05, 0.1, 0.2, 0.5, 1, 2, 3, 5, and 10 C). Electrochemical impedance spectroscopy (EIS) measurements were taken using a Zahner Elektrik IM6 at frequencies ranging from 5 mHz to 100 kHz with an amplitude of 50 mV. Also, the cyclic voltammetry (CV) measurement was tested at a scan rate of 0.1 mV s−1 in the range of 0.02−3 V (vs Li/Li+).

Table 1. Sample Names and Characteristics of SPS Microspheres Prepared by Sulfonation with Different Reaction Times sample name

sulfonation time (h)

mean particle size (μm)

reaction temp (°C)

SPS-24 SPS-30 SPS-36

24 30 36

1.45 1.45 1.45

50 50 50

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RESULTS AND DISCUSSION 3.1. Characterization of the Hollow and Macroporous α-Fe2O3 Microspheres. Figures S1a,b in the Supporting Information show the SEM images of the linear and semi-IPN PS microspheres with a narrow size distribution. The particle size of semi-IPN PS microspheres is a bit larger than that of the linear PS microspheres, and the surface of the particles is smooth. Figure 1 shows the FT-IR spectra of sulfonated semiIPN PS (SPS) microspheres prepared by the sulfonation process with different reaction times. The absorption bands at 1223, 1063, and 1007 cm−1 are associated with the sulfonic acid group (−SO3H), and the peak at 1125 cm−1 was assigned as the symmetric stretch of the sulfone group (−SO2−).13−15 We can identify that all samples were successfully modified with sulfonic acid and sulfone groups. The obvious increase in peak intensity related to the sulfonate symmetric stretch with increasing degree of sulfonation provides proof of the successful incorporation of the sulfonated regions in the polymer networks. It was demonstrated that the absorption intensity of the sulfonic

2.5. Characterization. FT-IR spectroscopy (Magna-IR 760 Nicolet) and a confocal laser scanning microscope (CLSM, IX70 OLYMPUS) were utilized to analyze the chemistry of the sulfonated semi-IPN PS particles. The particle shapes were observed using a scanning electron microscope (SEM, JEOL JSM-6300), focused ion beam-scanning electron microscope (FIB-SEM, Hitachi S-4800), and transmission electron microscope (TEM, JEOL JEM-2000EXII). The surface area and pore volume of the particles were measured using nitrogen adsorption via the Brunauer−Emmett−Teller (BET, AS1A4Quantachrome) and Barrett−Joyner−Halenda (BJH) methods. The amount of metal precursors in the composite particles was calculated via thermogravimetric analysis (TGA, TG 209F3 NETZSCH). All samples were characterized with powder X-ray diffraction (XRD, Rigaku C/MAX 2500).

Figure 1. FT-IR spectra of sulfonated semi-IPN PS (SPS) microspheres prepared by sulfonation process with different reaction times. 2899

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acid and sulfone groups increased with increasing degree of sulfonation. In order to confirm the hydrophilic (sulfonated) regions of the three types of SPS microspheres, CLSM analysis was executed. Figure 2 shows the CLSM images of three samples after labeling sulfonic acid groups with a fluorescent dye (Rhodamine 123) dispersed in water.16 Through this, it is ascertainable that the internal polymer chains were well-modified with sulfonic acid and sulfone groups

as well as the surface of particles. In addition, the CLSM image of SPS-36 (Figure 2c) shows a large and brighter fluorescent region than those of SPS-24 (Figure 2a) and SPS-30 (Figure 2b). The diffusion penetration depth of fluorescent dye proves that the functionalization of the polymer chains starts from the outside and proceeds toward the inside region of polymer particles depending on the sulfonation reaction time. Figure S3 in the Supporting Information shows SEM images and their cross-sectional images of SPS/Fe precursor composite submicron spheres, which were prepared using SPS-24 (Figure S3a), SPS-30 (Figure S3b), and SPS-36 (Figure S3c). Despite Fe metal precursors being introduced into the SPS particles, the average diameters and surface morphologies of all composite particles were similar, with SPS particles with mean diameters of about 1.5 μm and a smooth surface without Fe metal precursors (Figure S2), as can be seen in Figures S3a, c, e. In addition, all samples have undistinguished internal phases without heterogeneous phases resulting from Fe nanocrystals between SPS polymer phase, as shown in Figures S3b, d, f. To investigate the amount of introduced Fe precursors in particles, TGA analysis was conducted (Figure 3).

Figure 3. TGA curves of Fe metal precursors/SPS composite microspheres using SPS microspheres prepared by sulfonation with different reaction times: 24, 30, and 36 h.

Compared to Figure S4, TGA data of the SPS particles before the introduction of Fe metal precursors, all samples contained more than 20% solid residues at 800 °C. Of all samples, the composite particles prepared using SPS-36 as a sacrificing template exhibited the largest amount of residue (Figure 3, blue line, 33%). Ultimately, it is possible to think that the SPS particles having relatively more functional groups such as sulfonic acid and sulfone groups can accept more Fe ions by electrostatic attraction between sulfonic acid groups and Fe ions. After calcination, the average diameters (Figures 4a−c) of the three samples were significantly reduced to submicron range from 400 to 550 nm. Interestingly, there is no morphological and particle size difference between the three samples before calcination as can be seen in the cross-sectional observation (Figure S3). After heating under a gaseous mixture containing air, however, all samples had porous surfaces resulting from the α-Fe2O3 nanograins formed by Ostwald ripening with template removal during heat treatment, as shown in Figure 4. In addition, the sulfonation degree of SPS microspheres as a sacrificing template had an effect on the size distribution and morphology of three resulting α-Fe2O3 submicron spheres. The size distribution of

Figure 2. CLSM images of sulfonated semi-IPN PS microspheres prepared by sulfonation with different reaction times: (a) 24, (b) 30, and (c) 36 h. All sulfonation temperature was fixed at 50 °C. 2900

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Figure 4. SEM images of heat-treated α-Fe2O3 submicron spheres using SPS microspheres prepared by sulfonation with different reaction times: (a, b) 24 h; (c, d) 30 h; (e, f) 36 h.

two types of α-Fe2O3 submicron spheres (hollow and macroporous) is provided as Figure S6. The size distribution data show that the (black line) hollow, (red line) hollow with thick shell, and (blue line) macroporous submicron spheres have size distribution ranges of 400−1150, 230−1208, and 400−985 nm, respectively. All submicron spheres have a broad size distribution. When we have investigated the size distribution through SEM images, however, the hollow, hollow with thick shell, and macroporous α-Fe2O3 submicron spheres exhibited relatively narrow size distribution of 540−575, 482−

508, and 429−448 nm, respectively. These wide size distribution ranges obtained by Zetasizer might be calculated from lots of clusters made of α-Fe2O3 submicron spheres as well as well-distributed submicron spheres. We have found that the diameter and morphology of α-Fe2O3 submicron spheres depend on the sulfonation time of PS template particles. These results are attributed to the different penetrating depths of ferrous ions. CLSM images and TGA curves shown in Figure 2 and Figure S4 can also support this assertion. To ascertain the composition of the heat-treated samples, EDX (energy 2901

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the internal structures of metal oxide particles by adjusting sulfonation degree of SPS microspheres as sacrificing templates. 3. 2. Electrochemical Characteristics. We conducted in a variety of measurements to compare electrochemical properties between hollow (thin shell) and macroporous α-Fe 2 O3 submicron spheres. Figure 7 shows the typical cyclic voltammetry (CV) profiles of the cells structured with α-Fe2O3. In the first cycle (Figures 7a, b), the cathodic current peaks positioned at 0.68 and 0.8 V can be ascribed to a reversible conversion reaction of α-Fe2O3 with the metallic lithium to form Li2O as well as the electrolyte decomposition to form SEI films.17,18 Meanwhile, in the anodic processes, two peaks are presented at about 1.65 and 1.96 V, corresponding to the reversible oxidation of Fe0 to Fe2+ and Fe3+, respectively.19 Afterward, the peak intensities for the two samples decrease, which can be attributed to the extraction of Li from Li2O and the formation of an irreversible SEI layer. Because of polarization of the active materials in the first cycle, the cathodic and anodic peaks are shifted during the subsequent cycles. The rate capabilities of the electrodes containing the hollow and macroporous α-Fe2O3 submicron spheres were evaluated at various charge−discharge current densities up to 10 C with a cutoff voltage between 0.02 and 3.0 V, as shown in Figure 8. Both electrodes show high charge−discharge capacities even at high current rates (∼5 C). Two types of α-Fe2O3 based cells exhibited the gradual increased capacity, which is attributed to the gradual activation by the formation of a pseudocapacitive polymeric gel-like film within the hollow and porous nanostructures.20−23 Specifically, the macroporous α-Fe2O3 composite electrode showed higher capacities than those of the hollow type α-Fe2O3 composite electrode until 5 C and maintained their capacities above 1200 mAh g−1 until 3 C, as shown in Figure 8a. It is noted that the charge and discharge capacities of the electrode with macroporous particles are higher than the theoretical capacity of α-Fe2O3 (1007 mAh g−1). Since the macroporous α-Fe2O3 submicron spheres have larger BET surface area (501 m2 g−1) than that (468 m2 g−1) of the hollow type submicron spheres as shown in Figure S5, macroporous α-Fe2O3 submicron spheres have the stronger activation resulting in rapid increase of capacity. This phenomenon has been widely reported and is attributed to the decomposition of electrolyte to form the SEI layer at the initial state and a gel-like organic layer at a sloped voltage range.24−26 Although the charge−discharge capacities of the hollow α-Fe2O3 composite electrode were lower than those of the macroporous type α-Fe2O3 composite electrode, better stable cycling performance was exhibited over all current rates. These results are assumed to be due to the structural difference between the two types of α-Fe2O3 used as active materials. The pore size distribution and N2 sorption isotherms for the two samples were obtained through the Barrett−Joyner−Halenda (BJH) and Brunauer−Emmett−Teller (BET) methods, as shown in Figure S5. They display that the hollow and macroporous α-Fe2O3 submicron spheres have dominant mesopores with pore size range of 2−20 nm. The insets in Figure S5 show N2 sorption isotherms of (a) hollow and (b) macroporous type α-Fe2O3 submicron spheres. The hollow type sample shows type II isotherm that means the hollow type sample contains diameters exceeding pores. On the other hand, the macroporous type sample shows type IV isotherm that materials have pores in the range of 2−100 nm. These results are corresponded with their hollow and macroporous

dispersive X-ray) spectroscopy was used. Figure S7 shows EDX profiles of heat-treated samples, which were found that there are no other elements except Fe, O, C, and Pt as a coating material. X-ray diffraction (XRD) analysis was used to examine the crystal structure of the heat-treated samples. Figure 5 shows

Figure 5. XRD patterns of heat-treated α-Fe2O3 submicron spheres using SPS microspheres prepared by sulfonation with different reaction times: (a) 24, (b) 30, and (c) 36 h.

the XRD patterns of the two types of hollow (black and blue lines) and (red line) macroporous structured α-Fe 2 O 3 submicron spheres. The positions of the characteristic peaks of particles correlate well with the standard values for the hexagonal α-Fe2O3 phase (JCPDS number 33-0664), indicating that highly crystalline products have been obtained after calcination. The patterns are normalized with respect to the 113 peak, which corresponds to a plane with oxygen atoms. The XRD patterns of all samples show peaks corresponding to the α-Fe2O3 structure. In Figure 6, the cross-sectional images of the heat-treated α-Fe2O3 submicron spheres prepared by sulfonation of SPS microspheres with different reaction times exhibit quite different interiors. The hollow structure of α-Fe2O3 submicron spheres was observed from the heat-treated samples using SPS microspheres sulfonated for relatively shorter reaction times (24 and 30 h). The shell thickness of hollow structured α-Fe2O3 submicron spheres increased with increasing sulfonation degree of the SPS microspheres. On the other hand, a relatively long sulfonation reaction time for SPS microspheres caused the macroporous structure, as can be seen in Figure 6e. TEM images (Figures 6b, d, f) exhibit more clearly the different internal structures. Considering the two types of hollow α-Fe2O3 submicron spheres, there is an obvious difference in shell thickness between the two samples. The thin and thick shells comprised of α-Fe2O3 nanograins were formed by using SPS-24 and SPS30, respectively. In all of TEM images, the α-Fe2O3 nanograins show up as dark spots within the submicron spheres. The shortest time (24 h) for sulfonation process causes the formation of a thin porous shell having a thickness of 50 nm. In the case of a sample using SPS-30, a thicker porous shell ranging from 100 to 110 nm and smaller cavity were observed, as shown in Figure 6d. The macroporous structure was observed from the cross-sectional and TEM images of the heattreated sample using SPS-36, as shown in Figures 6e, f. These results indicate that the various internal structures of α-Fe2O3 submicron spheres depend on the penetrating depth of ferrous ions during metal ion diffusion. It is easily possible to control 2902

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Figure 6. Cross-sectional (FIB-SEM) and TEM images of heat-treated α-Fe2O3 submicron spheres using SPS microspheres prepared by sulfonation with different reaction times: (a, b) 24 h; (c, d) 30 h; (e, f) 36 h.

α-Fe2O3 composite electrode are higher than those of the electrode-containing hollow type α-Fe2O3 submicron spheres due to a higher surface area (Figure S5, inset). At the highest current density (10 C), the capacities of both electrodes rapidly decrease because of the higher impedance came from relatively long diffusion length of Li ions. The better rate capability of the hollow type α-Fe2O3 composite electrode is attributed to the

structures. The hollow structure having an internal cavity and mesopores allow that electrolyte is easily diffused into the submicron spheres and increase the interfacial area between electrolyte and particle and thereby reducing the Li+ ion diffusion length in the submicron spheres. For these reasons, the two types of electrodes may achieve a high rate capability. Besides, the discharge and charge capacities of the macroporous 2903

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macroporous α-Fe2O3 submicron spheres were evaluated for the galvanostatic cycling test as shown in Figure 9a. Two electrodes were cycled between 0.02 and 3 V at a 1 C rate up to 100th cycle. Both electrodes exhibited excellent cycling performance. The charge−discharge capacities of the macroporous composite electrode increased up to about 1600 mAh g−1 at the 80th cycle from 1150 mAh g−1 at the fourth cycle. After 80 cycles, the charge−discharge capacities deliver 1300 mAh g−1 until the 100th cycle. The phenomenon related to rising capacity from the 4th to 80th cycle is not fully understood but might be attributed to the pseudocapacitive polymeric gel-like layer caused by the low voltage decomposition of organic electrolyte.24−26 The origin of capacity fading from the 80th cycle with a low efficiency is likely due to the increase of cell impedance caused by agglomeration of the nanosized grains by the welding effect resulting from the pressure generated by the large volume expansion during the lithiation and delithiation processes.27 The agglomeration of adjacent α-Fe2O3 nanograins causes the increase of Li ion diffusion length and the trapping of SEI films in the particles, leading to the low capacity retention. Meanwhile, the charge− discharge capacities of the hollow α-Fe2O3 composite electrode reached approximately 1080 mAh g−1 at the 100th cycle from 990 mAh g−1 at the 4th cycle. In case of the hollow structured α-Fe2O3 based cell, the charge−discharge capacities were maintained during the cycling without capacity fading, as shown Figure 9a. Through Figures 9c, d, we can identify voltage ranges resulting from the electrochemical reaction of macroporous and hollow particles electrode. Figures 9c, d show the charge−discharge curves of the macroporous (c) and hollow (d) α-Fe2O3 composite electrodes. When the potential decreases to about 0.9 V in the two electrode curves, a definite potential plateau related to the reduction of iron from Fe3+ to Fe can be identified. In Figures 9c, d, a discharge potential plateau with 0.9 V at the 50th cycle become shorter than the potential plateau at the first cycle. This means that the electrochemical reaction related to the faradaic process was decreased during the continuous cycling. However, the nonfaradaic reaction in low voltage ranges (0−0.5 V), such as the freshly formed and growth of a gel-like layer on the surface of α-Fe2O3 submicron spheres, has an effect on the high capacities.28 At the 100th cycle in Figure 9c, the macroporous α-Fe2O3 composite electrode shows a shorter discharge potential plateau at 0.9 V than that of the hollow α-Fe2O3 composite electrode. This indicates that the porous type αFe2O3 submicron spheres cannot maintain an electrochemical reaction similar to the hollow structured α-Fe2O3 submicron spheres during continuous cycling. Figure 9b shows Nyquist plots for each cell before and after cycling. Semicircle, which shows two overlapping semicircles at high frequency, in a Nyquist plot represents both Rct and RSEI. The normalized total resistances were 110 and 15 ohms for the hollow type α-Fe2O3 composite electrode before and after 100 cycles, whereas the macroporous type α-Fe2O3 composite electrode exhibited 190 and 45 ohms, respectively. The diameters of the semicircles for the cycled electrodes are smaller than the electrode before cycling. It can be seen that the reaction kinetics was improved during the several charge/discharge cycles, which resulted in capacity increase, as shown in Figures 8a, b. After 100th cycle, the hollow type α-Fe2O3 submicron based electrode exhibited much smaller charge transfer resistance than that of macroporous type α-Fe2O3 based electrode. These impedance results are well matched with the cyclability data. In Figure 9a, both

Figure 7. Cyclic voltammograms of two types of α-Fe2O3 electrodes containing (a) macroporous α-Fe2O3 submicron spheres and (b) hollow α-Fe2O3 submicron spheres from the first cycle to the second cycle at a scan rate of 0.1 mV s−1 in the voltage range of 0.02−3.0 V.

Figure 8. Rate performance of two types of electrodes containing (a) macroporous α-Fe2O3 submicron spheres and (b) hollow α-Fe2O3 submicron spheres at various rates in the voltage region of 0.02−3.0 V vs Li/Li+.

short diffusion length of Li ions during lithiation−delithiation processes. The two types of electrodes containing hollow and 2904

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Figure 9. (a) Reversible capacities of α-Fe2O3 electrodes containing macroporous and hollow particles at 1 C rate; (b) electrochemical impedance spectroscopy and (b, inset) corresponding equivalent circuit; the charge−discharge performance respecting cycling performance of the electrode containing the as-prepared (c) macroporous α-Fe2O3 and (d) hollow α-Fe2O3 submicron spheres.

electrodes show increased capacities up to about 80th cycle. However, the capacity of macroporous type α-Fe2O3 based electrode slowly decreased up to 100th cycle while the capacity of hollow type α-Fe2O3 based electrode continues to increase up to 100th cycle. Although the macroporous type α-Fe2O3 submicron spheres exhibit higher activation than the hollow type α-Fe2O3 submicron spheres because of higher surface area, the higher impedance came from structural change after long cycling caused capacity decay. In addition, the charge-transfer resistance of the hollow type α-Fe2O3 electrode is much smaller than that of macroporous type α-Fe2O3 composite electrode before and after cycling. This is evidence to support the fact that the hollow structure decreases the cell impedance. Consequently, the structural characteristics of active materials have a great effect on the electrochemical performances. Figure 10 shows the cross-sectional images of two types of αFe2O3 based composite electrodes of (a, b) fresh cells and lithiated cells at (c, d) 10th cycle and (e, f) 100th cycle. From the cross-sectional images of the two electrodes after 100 cycles, we could find that two types of α-Fe2O3 submicron spheres embedded in composite electrodes remained their intrinsic structures with a slight morphological deformation (Figures 10e, f). The hollow type α-Fe2O3 submicron spheres exhibited less volume change of about 17% in comparison with the macroporous type α-Fe2O3 submicron spheres at 10th lithiation process, as shown in Figures 10c,d. After 100 cycles, the cross-sectional image of the electrode containing macroporous submicron spheres (Figure 10f) indicates that α-Fe2O3 submicron spheres were aggregated and welded to each other

Figure 10. Cross-sectional images of two types of α-Fe2O3 submicron spheres in the fresh cells (a and b, before cycling), lithiated cells (c and d, at 10th cycle), and lithiated cells (e and f, at 100th cycle): (a, c, and e) hollow and (b, d, and f) macroporous α-Fe2O3 submicron spheres.

after being cycled. Conversely, Figure 10e shows that the hollow type α-Fe2O3 submicron spheres in the composite electrode maintained their structure even after 100 cycles. Moreover, we have investigated the thickness change of composite electrodes during lithiation and delithiation 2905

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The Journal of Physical Chemistry C processes. Figure S8 shows the cross-sectional images of hollow and macroporous type α-Fe2O3 based fresh, lithiated, and delithiated electrodes. The thickness change of hollow α-Fe2O3 based electrode was about 41%, which was less than the thickness change of about 65% of macroporous based electrode from that of fresh cell. These results indicate that the structure of active materials has influence on the degree of volume variation of electrode. In addition, we have studied the crystalline change of α-Fe2O3 during lithiation and delithiation processes by XRD. At the first, the principal electrochemical process occurring during the charge and discharge processes of α-Fe2O3 proceeds as follows. XRD patterns of lithiated α-Fe2O3 show a body-centered cubic (bcc) lattice structure of Fe resulting from the reaction between Fe2O3 submicron spheres and lithium ions, as shown in Figure S9. The XRD pattern corresponding to lithium oxides was not detected because of their amorphous structure. At the delithiation process, XRD patterns of cubic lithium−iron oxide were not detected. However, the appearance of bcc lattice structure corresponding to Fe was observed.

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2009-0092780). This work was supported by the Human Resources Development program (No. 20124010203290) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

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In summary, we have used three types of sulfonated polystyrene (SPS) particles reacted with sulfuric acid under different sulfonation times as a sacrificing template to introduce Fe precursors. The hollow and macroporous types of α-Fe2O3 submicron spheres consisting of interconnected α-Fe2O3 nanograins were obtained by calcination in air. The different amounts and depth of the introduced Fe metal precursors in the three templates are key factors to control the internal structure and particle size. The electrode made of the hollow and macroporous α-Fe2O3 submicron spheres showed excellent electrochemical performance with high rate capability and reversible capacity above a theoretical capacity. Specifically, the hollow type α-Fe2O3 composite electrode exhibited better cyclability and a high Coulombic efficiency than that of the macroporous α-Fe2O3 composite electrode. These results are attributed to the structural difference of α-Fe2O3 submicron spheres resulting in different cell impedance, which affects the Li ion diffusion.

S Supporting Information *

SEM images of linear, semi-IPN PS (Figure S1), sulfonated PS microspheres with different reaction times (Figure S2), crosssectional images of Fe metal precursors/SPS composite microspheres (Figure S3), TGA data of SPS microspheres prepared by sulfonation with different reaction times, pore size distribution of the macroporous and hollow α-Fe2O3 microspheres (Figure S5), and the cross-sectional images of two types of electrodes after cycling (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +82222200526; e-mail [email protected] (K.-D.S.). Notes

The authors declare no competing financial interest. 2906

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