Hollow Fe3O4 microspheres as anode materials for lithium-ion batteries

Hollow Fe3O4 microspheres as anode materials for lithium-ion batteries

Electrochimica Acta 75 (2012) 123–130 Contents lists available at SciVerse ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/loca...

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Electrochimica Acta 75 (2012) 123–130

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Hollow Fe3 O4 microspheres as anode materials for lithium-ion batteries Hyung-Seok Lim a , Byoung-Young Jung a , Yang-Kook Sun b,∗∗ , Kyung-Do Suh a,∗ a b

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

a r t i c l e

i n f o

Article history: Received 4 January 2012 Received in revised form 27 March 2012 Accepted 22 April 2012 Available online 27 April 2012 Keywords: Li-ion battery Anode materials Iron oxide Hollow structure Transition metal oxide

a b s t r a c t In this study, we proposed a unique method for hollow Fe3 O4 microspheres and confirmed their electrochemical properties as anode materials for lithium-ion batteries. Poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres were prepared by simple ionic attraction between hydrogel microspheres with negative charge and magnetic Fe3 O4 nanoparticles under alkaline conditions. The poly(MAA/EGDMA) core spheres were removed by heat treatment in order to form the hollow structure of Fe3 O4 microspheres. Their hollow structure prevents cracking of the electrode during the volume change of repetitive Li-ion insertion and extraction reactions and improves the Li-ion transfer during cycling. The morphologies and structure of the hollow Fe3 O4 microspheres were confirmed by scanning electron microscopy, focused ion beam-scanning electron microscopy, transmission electron microscopy, optical microscopy and X-ray diffraction. The electrochemical performance of the composite electrode was evaluated by constant current charging and discharging, cyclic voltammetry and cycling performance at various cycling rates. The results showed excellent cycle stability compared with a composite electrode containing bare Fe3 O4 nanoparticles. These results indicate that the unique structures of Fe3 O4 microspheres contribute to the excellent life and high reversible capacity of the battery when they are used as an anode of a lithium-ion battery. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Rechargeable lithium-ion batteries (LIBs) have been widely utilized as power sources for portable electric devices and hybrid electric vehicles because they have high energy density [1–8]. LIBs are also potential power sources for electric vehicles (EV) and energy storage systems (ESS). Their energy density and power density depend on both physical and chemical properties of the cathode and anode materials. Graphite has been used as an anode material for lithium-ion batteries due to its high columbic efficiency and excellent cycle performance. However, the low specific capacity of graphite (∼372 mAh/g) cannot satisfy the needs for EV and ESS [9]. Thus, the exploration for new anode materials has become critical for high performance LIBs, including materials such as Li–Si, Li–Sn alloy and transition metal oxides (MO, M = Cu, Fe, Co) [10]. Among these materials, iron oxides have been considered one of the most promising candidates for anode materials in LIBs due to their high specific capacities (926 mAh/g). In addition iron oxide has safety benefits compared with graphite because the potential

∗ Corresponding author. Tel.: +82 2 2220 0526; fax: +82 2 2220 4680. ∗∗ Co-Corresponding author. Tel.: +82 2 2220 1749; fax: +82 2 2298 5416. E-mail addresses: [email protected] (Y.-K. Sun), [email protected] (K.-D. Suh). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.04.082

of the iron oxide anode is higher than that of the graphite, which reduces metallic deposition on the anode during charge [11,12]. However, an anode containing iron oxide has poor cyclic performance during continuous cycling because of large volume changes during lithium insertion/extraction [13–15]. In order to reduce this problem, research on carbon coatings [16,17], nanocomposites and nanostructures [18,19] has been reported. In addition to the above problems, high reversible specific capacity, high columbic efficiency, long life performance and good rate capability are also important factors for LIB systems. Recently, it was reported that the hollow structure and porous shell of microspheres can provide void spaces for active materials from the electrode during volume expansion [20–22], resulting in good mobility of lithium ions in LIB systems, better cycling performance and high reversible capacity. In this study, novel hollow Fe3 O4 microspheres were synthesized by ionic adsorption of Fe3 O4 nanospheres onto pH-responsive hydrogel particles followed by heat treatment. The fabrication process of designed particles is shown in Scheme 1. After the removal of the template by heat treatment at 500 ◦ C, the hollow structure was maintained with a porous Fe3 O4 shell because the Fe3 O4 nanoparticles were connected to each other during heat treatment. The as-prepared samples exhibited stable cycle performance when used as an anode for lithium ion batteries.

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Scheme 1. Schematic illustration of the fabrication process of the hollow Fe3 O4 microspheres.

2. Experimental details

2.2. Synthesis of Fe3 O4 nanoparticles

2.1. Materials

Fe3 O4 nanoparticles were prepared by a precipitation oxidation method as follows: PEG (Mw = 4000, 70 g) and FeSO4 ·7H2 O (3 g) were dissolved in distilled water in a 250 ml four-necked flask equipped with a reflux condenser and a mechanical stirrer operated at a speed of 400 rpm for 30 min. Then an aqueous solution containing 40% H2 O2 was added. The pH of the mixture was adjusted with 3 M NaOH solution. The reaction was allowed to proceed for 6 h at 50 ◦ C at pH 13. The obtained magnetic fluid was repeatedly purified by magnetic field separation and decantation. The Fe3 O4 nanoparticles were washed several times with distilled water and freeze dried.

Iron(II) sulfate heptahydrate (FeSO4 ·7H2 O, Aldrich), hydrogen peroxide (H2 O2 , 30%, Dae-Jung Chemicals & Metals Co., Ltd.), sodium hydroxide (NaOH, Sigma Aldrich Chemical Co., Ltd.), methacrylic acid (MAA, Junsei Chemical Co., Ltd.), ethylene glycol dimetharylate (Egdma, Tokyo Kasei Kogyo Co., Ltd.), ␣,␣ -azobis(isobutyronitrile) (AIBN, Junsei Chemical Co., Ltd.), acetonitrile (AN, Oriental Chemical Industries), buffer solution (Samchun Pure Chemical Co., Ltd.), ethyl alcohol anhydrous (ethanol, eJung Chemicals & Metals Co., Ltd.) and polyethylene glycol (PEG, Mw = 4000, Yakuri Pure Chemical Co., Ltd.) were used as received.

Fig. 1. (a) SEM and (b) XRD pattern of Fe3 O4 nanoparticles.

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Fig. 2. Images of (a) OM, (b) SEM and (c) FT-IR spectra of poly(MAA/EGDMA) hydrogel microspheres.

2.3. Synthesis of poly(methacrylic acid/ethylene glycol dimetharylate) hydrogel microspheres The pH-sensitive crosslinked poly(methacrylic acid-coethylene glycol dimethacrylate) (poly(MAA/EGDMA)) hydrogel microspheres were synthesized by distillation–precipitation polymerization. MAA (3.76 g), EGDMA (0.24 g) and AIBN (0.08 g) were dissolved in a mixture containing acetonitrile (68 g) and distilled water (12 g). The mixture was polymerized at 80 ◦ C for 40 min. The resulting gel particles were washed several times with ethanol and distilled water and dried under vacuum. 2.4. Preparation of hollow Fe3 O4 microspheres To prepare the hollow Fe3 O4 microspheres composed of Fe3 O4 nanoparticles, poly(MAA/EGDMA)/Fe3 O4 core–shell particles were prepared by ionic adsorption. The prepared poly(MAA/EGDMA) particles were swollen in pH 11 buffer solution at a mechanical stirring speed of 200 rpm for 2 h. To prepare the 0.5 wt% CTAB solution, CTAB powder (1 g) was dissolved in 200 ml of pH 11 buffer solution, and Fe3 O4 nanoparticles were dispersed in CTAB solution with mechanical stirring at 200 rpm and sonicated for 2 h. The CTAB solution containing Fe3 O4 nanoparticles was slowly added drop-wise into the pH 11 buffer solution contained swollen poly(MAA/EGDMA) microspheres and stirred for 4 h. The pH of the mixture containing poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres was decreased drastically with ethanol. The contracted poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres were washed

repeatedly with ethanol and dried under vacuum. The as-prepared microspheres were heated to 500 ◦ C at 5 ◦ C/min and maintained for 5 h at 500 ◦ C to remove the PMAA core. 2.5. Characterization of the hollow Fe3 O4 microspheres All particles were characterized with X-ray diffraction (XRD, Rigaku, C/MAX 2500), thermo gravimetric analysis (TGA, TG 209F3, NETZSCH), Fourier-transform infrared spectra (FT-IR, Nicolet, Magna IR-550), scanning electron microscopy (SEM, JSM-6300, JEOL), focused-ion beam scanning electron microscopy (FIB-SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL, JEM-2000EX) and optical microscopy (OM, Olympus, BX51). 2.6. Electrochemical performance testing of the hollow Fe3 O4 microspheres To evaluate the electrochemical properties of each sample, coin-type cells were used. The working electrode was prepared by mixing active material (70%, bare Fe3 O4 nanoparticles or the hollow Fe3 O4 microspheres) and Super-P (10 wt.%, as conducting additive) with and carboxymethylcellulose (CMC, 10%, Daicel Fine Chem. Co.) and styrene butadiene rubber (SBR, 10%, Zeon Co.). The electrodes were dried for 20 min at 110 ◦ C. Coin-type cells (2032) of Li/1 M LiPF6 in ethylene carbonate and diethylene carbonate (EC:DEC, 1:1 by volume, provided by Techno Semichem Co., Ltd., Korea) with a Celgard 2400 microporous polypropylene membrane as the separator were assembled in an argon-filled glove box. The

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Fig. 3. OM images of (a) non-swollen poly(MAA/EGDMA), (b) swollen poly(MAA/EGDMA) at pH 11, (c) poly(MAA/EGDMA)/Fe3 O4 and (d) contracted poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres at pH 6.

cells were galvanostatically cycled in the voltage range between 0.02 and 3 V at currents of 0.15 mA and 0.3 mA. To evaluate the rate performance, the cycling current was elevated to 14 mA at 30 ◦ C.

3. Results and discussion 3.1. Characteristics of the hollow Fe3 O4 microspheres Fig. 1a shows the SEM image of Fe3 O4 nanoparticles, which were aggregated with each other and formed nanoclusters because they have a large surface-to-volume ratio, high surface energy and experience magneto-dipole interparticle interactions. Fig. 1b shows six characteristic peaks in a XRD pattern for Fe3 O4 (2 = 30.3◦ , 35.7◦ , 43.3◦ , 54.1◦ , 57.4◦ and 62.9◦ ), marked by their indices ((2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0)). Fig. 2 shows (a) OM, (b) SEM images and (c) FT-IR spectra of the cross-linked poly(MAA/EGDMA) microspheres after polymerization. The monodispersed spherical shapes of the particles were observed after distillation–precipitation polymerization, as shown in Fig. 2a. The SEM image shows oval shaped particles from the cross-linked poly(MAA/EGDMA) without moisture after drying. In the FT-IR spectrum, the cross-linked poly(MAA/EGDMA) particles had a strong and broad peak near 3500 cm−1 due to the vibration of hydroxyl stretching of the carboxylic acid group. The spectrum shows a C O peak at a wavenumber of 1730 cm−1 corresponding to the stretching vibration of the carbonyl group of the carboxylic acid. Poly(MAA/EGDMA) microspheres were successfully synthesized by distillation–precipitation polymerization. The particle size and the surface charge of the poly(MAA/ EGDMA) microspheres and the Fe3 O4 nanoparticles in cetyl trimethyl ammonium bromide (CTAB) solution are shown in Table 1. When the pH value of the solution was 11, the average diameter and surface charge of the swollen poly(MAA/EGDMA)

particles were 4.18 ␮m and −18.2 mV, respectively. The dispersed Fe3 O4 nanospheres with cationic surfactant had an average diameter of 53 nm and positive charge of +16.7 mV. Fig. 3 shows OM images of (a) non-swollen poly(MAA/EGDMA), (b) swollen poly(MAA/EGDMA), (c) poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres in a strong alkali condition (pH 11) and (d) contracted poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres in an acid condition (pH 5). In pH 11 solution, the volume of the crosslinked poly(MAA/EGDMA) particles was expanded as shown in Fig. 3b. Due to the ionic interaction between the poly(MAA/EGDMA) and Fe3 O4 nanoparticles, Fe3 O4 nanoparticles were adsorbed onto the surfaces of the swollen poly(MAA/EGDMA) microspheres. After adjusting the pH value to a neutral condition, a magnetic shell composed of Fe3 O4 nanospheres was formed on the surfaces of the poly(MAA/EGDMA) microspheres, as shown in Fig. 3d. Fig. 4 shows SEM images of the prepared poly(MAA/EGDMA) /Fe3 O4 core–shell microspheres at (a) low and (b) high magnification. The spherical shaped poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres had average diameters of approximately 2.4 ␮m and have uneven surfaces. Under higher magnification, almost all the Fe3 O4 nanospheres with diameters ranging from 10 nm to 100 nm were randomly adsorbed onto the surfaces of poly(MAA/EGDMA) microspheres. In addition, the assembled Fe3 O4 clusters were packed because of the contraction process. In the case of

Table 1 Average diameters and zeta potential values of swollen poly(MAA/EGDMA) particles and dispersed Fe3 O4 nanoparticles in CTAB solution at pH 11.

Poly(MAA/EGDMA) microsphere Fe3 O4 nanoparticle in CTAB solution

Particle size

Zeta () potential

4.18 ␮m 53 nm

−18.2 mV +16.7 mV

Average diameters and zeta potential values of swollen poly(MAA/EGDMA) particles and dispersed Fe3 O4 nanoparticles in CTAB solution at pH 11.

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Fig. 5. TGA plots of the poly(MAA/EGDMA)Fe3 O4 core–shell microspheres and Poly(MAA/EGDMA) bare particles under gaseous mixture-mainly N2 at a heating rate of 5 ◦ C/min. (For interpretation of the references to color in the text citation, the reader is referred to the web version of the article.)

Fig. 4. SEM images of the poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres at (a) low and (b) high magnification.

Fe3 O4 nanoparticles formed a porous-structured shell with a high volume of void spaces following heat treatment at 500 ◦ C for 5 h. The cross-sectional image shows that the hollow structure of the Fe3 O4 microsphere was maintained, and Fe3 O4 nanoparticles were connected to each other after heating, as shown in Fig. 6d. The thickness of the Fe3 O4 shell was approximately 320 nm. Fig. 7 shows the XRD patterns of the poly(MAA/EGDMA)/Fe3 O4 core–shell and the hollow structured Fe3 O4 microspheres. The main peaks are at 2 = 30.09◦ , 35.42◦ , 43.00◦ , 54.05◦ , 57◦ and 62.51◦ , which are consistent with the XRD peaks of Fe3 O4 observed in the poly(MAA/EGDMA)/Fe3 O4 core–shell both before and after heat treatment at 500 ◦ C for 5 h. These results indicate that the poly(MAA/EGDMA)/Fe3 O4 core–shell and the hollow microspheres contained Fe3 O4 nanoparticles, and their crystalline phases were not transformed by heating at 500 ◦ C for 5 h in gaseous mixturemainly N2 . 3.2. Electrochemical properties

the poly(MAA/EGDMA) particles, their spherical shapes were transformed after drying, as shown Fig. 2b. However, the hollow poly(MAA/EGDMA)/Fe3 O4 core–shell particles were maintained even after drying due to their Fe3 O4 shell, as shown in Fig. 4. TGA thermograms of the poly(MAA/EGDMA) and poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres are shown in Fig. 5. TGA curve (red line) of the poly(MAA/EGDMA) bare particles showed a gradual weight loss below 650 ◦ C. The first weight loss took place at below 300 ◦ C, which can be attributed to either the evaporation of moistures or the decomposition of labile oxygen functional groups. The second weight loss arose between 350 ◦ C and 450 ◦ C, which might be corresponding to the degradation of carbon skeleton. TGA curve of the poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres appeared to have similar weight loss trend, but the weight which can be attributed to the PEG decomposition on the surface of Fe3 O4 nanoparticles almost lost around at 500 ◦ C, the poly(MAA/EGDMA)/Fe3 O4 core–shell microsphere loss amounted to 76.8 wt.%. This indicates that a mass of 23.2 wt.% corresponds to the Fe3 O4 portion after heat treatment. TEM and FIB-SEM images of poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres are shown in Figs. 6a and c. The Fe3 O4 nanoparticles were aggregated and adsorbed on the surface of the poly(MAA/EGDMA) microsphere, which lead to formation of uneven surfaces on the hollow Fe3 O4 microspheres. Fig. 6b shows that poly(MAA/EGDMA) particles decomposed, and the adsorbed

To identify all of the electrochemical reactions, the cyclic voltammograms (CVs) of the as-prepared hollow Fe3 O4 microspheres are presented in Fig. 8. In the cathodic process of the first cycle, the peak is observed at 1.52 V, which can be attributed to the reduction of the irreversible reaction with the electrolyte. Two obvious peaks were observed at 0.95 V and 0.75 V, which should be attributed to the reaction Fe3 O4 + 8Li+ + 8e− ↔ 3Fe0 + 4Li2 O. During the discharge step, the irreversibility is due to the formation of amorphous Li2 O and the conversion of Fe3 O4 to Fe [23]. In the anodic process, two peaks are observed at about 1.6 V and 1.9 V, corresponding to the oxidation of Fe0 to Fe2+ and Fe3+ , respectively. Since polarization of the active materials occurs in the first cycle, cathodic and anodic peaks are shifted during the subsequent cycles. The electrochemical properties of the hollow Fe3 O4 microspheres and bare Fe3 O4 nanoparticles were measured using coin-type cells. The first and 35th charge–discharge curves of the hollow Fe3 O4 microspheres and bare Fe3 O4 nanoparticle electrodes are displayed in Fig. 9a. The discharge–charge curves were tested at different C-rates of 1/5C (0.3 mA) and 1/10C (0.15 mA). The two electrodes show initial discharge capacities of 972 mAh/g and 945 mAh/g, respectively. While the coulombic efficiency of the bare Fe3 O4 nanoparticles in the electrode was measured at 78.94%, the electrode of the hollow Fe3 O4 microspheres showed a higher effi-

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Fig. 6. (a) TEM and (c) cross-sectional images of the poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres before heat treatment and (b) TEM and (d) cross-sectional images of the poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres after heat treatment at 500 ◦ C under gaseous mixture mainly N2 .

ciency of 89.71% in the first cycle. In addition, a capacity decay of approximately 67% occurred at the 35th cycle with the bare Fe3 O4 nanoparticle electrode. On the other hand, a low capacity decay of

12% was observed at the 35th cycle with a hollow Fe3 O4 microsphere electrode. Fig. 9b shows the cycling performances of the hollow Fe3 O4 microsphere electrode at a current of 0.3 mA and a bare Fe3 O4 nanoparticle electrode at a current of 0.15 mA, respectively. The discharge capacity of the hollow Fe3 O4 microsphere electrode was

Fig. 7. XRD patterns of the poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres: (a) before and (b) after heat treatment.

Fig. 8. The cyclic voltammograms of the hollow Fe3 O4 microspheres with a scan rate of 0.2 mV/s. The cycle numbers are indicated in the graph.

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to their unique structure and nanoparticles, which combines the advantages of reducing volume change during Li-ion insertion and extraction and short diffusion length. The hollow Fe3 O4 composite electrodes show good rate capability with an average discharge capacity of 810 mAh/g at a current of 0.15 mA (1/10C), 791 mAh/g at a current of 0.3 mA (1/5C), 760 mAh/g at a current of 0.7 mA (1/2C). At high current densities, poor cycling performance appeared because their low electrical conductivity. Even if the capacity decay appeared starting at a current of 1.4 mA (1C) and continued up to 14 mA (10C), the capacity could go back to more than 800 mAh/g after a high discharge/charge current, as shown in Fig. 10. The surface of electrode was still maintained without pulverization, through with slight aggregation of the surface during continuous 60 cycles at different current densities as shown in Supporting Fig. 1. 4. Conclusions The ionic contraction between poly(MAA/EGDMA) microspheres and magnetic Fe3 O4 nanoparticles can be used to synthesize poly(MAA/EGDMA)/Fe3 O4 core–shell microspheres. After heat treatment, the hollow Fe3 O4 microspheres were prepared simply, and their crystalline phase did not change. The electrode made of the hollow Fe3 O4 powder showed excellent electrochemical performance with a stable and reversible capacity. The nanoparticles-assembled hollow Fe3 O4 microspheres can improve cyclic stability of transition metal oxides when they are used as anode materials for lithium-ion batteries because their unique structure better accommodates volume changes and reduces Li-ion diffusion length. Acknowledgements

Fig. 9. Electrochemical performance of Fe3 O4 cells: (a) voltage profiles of the hollow Fe3 O4 nanoparticles at 0.1C and (b) cycling performance of the hollow and bare Fe3 O4 electrodes in the 0.02–3 V voltage window at 0.2C.

gradually increased from 856 mAh/g at the third cycle to 912 mAh/g at the 42nd cycle. On the other hand, the discharge capacity of the bare Fe3 O4 nanoparticle electrode rapidly began to decrease at the 7th cycle and was 300 mAh/g at the 42nd cycle. The enhanced cycling performance with the hollow Fe3 O4 microspheres was due

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0004476). This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20114010203150). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.electacta.2012.04.082. References

Fig. 10. The specific capacity of the hollow Fe3 O4 electrode as a function of the cycling rate (0.1–10C).

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