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Uniform hematite nanocapsules based on an anode material for lithium ion batteries
Electrochemistry Communications 12 (2010) 382–385
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Uniform hematite nanocapsules based on an anode material for lithium ion batteries Hyun Sik Kim a, Yuanzhe Piao b,c, Soon Hyung Kang a, Taeghwan Hyeon b,*, Yung-Eun Sung a,* a
School of Chemical and Biological Engineering and Research Center for Energy Conversion and Storage, Seoul National University, Seoul 151-744, Republic of Korea National Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea c Department of Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Suwon 443-270, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 24 November 2009 Received in revised form 29 December 2009 Accepted 29 December 2009 Available online 4 January 2010 Keywords: Lithium ion battery Anode material Hematite Nanocapsules
a b s t r a c t Uniform a-Fe2O3 nanocapsules with a high surface area were synthesized by a novel wrap–bake–peel approach consisting of silica coating, heat treatment and finally the removal of the silica coating layer. The length, diameter and shell thickness of the hematite nanocapsules were about 65, 15 and 5 nm, respectively. The electrochemical properties of the a-Fe2O3 nanocapsules were investigated by cyclic voltammetry and charge/discharge measurements. The a-Fe2O3 nanocapsules showed a high reversible capacity of 888 mAh/g in the initial cycle and 740 mAh/g after 30 cycles as well as good capacity retention. This excellent electrochemical performance was attributed to the high surface area, thin shell and volume space of the hollow structure. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, nanostructured 3d-metal oxides MOx (M = Cu, Mn, Fe, Co, Ni, etc.) have been widely studied as anode materials for lithium-ion battery (LIB) owing to their high energy capacity [1,2]. The properties of nanomaterials strongly depend on their size and morphology. By controlling the particle size, the quantization effect was widely examined and the surface area and robustness was influenced by modifying the particle shape. Hence, research attention has focused on controlling the nanoparticle size, shape and structure in order to attain unique properties for LIB [3]. Above all, interest on the 3-dimensional hollow sphere has focused on the prominent position on account of the high surface area and porous structure [4]. In terms of materials, hematite (a-Fe2O3) is considered a promising active lithium intercalation host due to its high theoretical capacity (1007 mAh/g), environmental friendliness, and low cost. As the particle size and morphology of hematite nanostructures exert a key influence on their electrochemical performance for lithium storage, hematite nanostructures showing different morphologies and particle sizes have been synthesized in order to enhance the electrochemical performance [5,6]. In a similar context, the hollow nanostructured hematite possesses several advantages for LIB application such as the extended contact area between the active material and the electrolyte caused by their high surface area, the short lithium diffusion length * Corresponding authors. Tel.: +82 2 880 1889; fax: +82 2 888 1604. E-mail addresses: [email protected] (T. Hyeon), [email protected] (Y.-E. Sung). 1388-2481/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.12.040
resulting from the thin shell and the hollow space in the central part that buffers the volume expansion during cycling. We developed a new route for synthesizing uniform-sized hematite nanocapsules with a high surface area and thin shell in order to achieve excellent electrochemical reactivity toward lithium. Stable, enhanced electrochemical performance due to the peculiar hematite nanocapsules was achieved.
2. Experimental In a typical synthetic procedure of spindle-shaped b-FeOOH nanoparticles, 0.02 M FeCl36H2O was dissolved into 2 L of deionized water at 80 °C under magnetic stirring and the particles were washed with water. To perform the silica coating of the b-FeOOH nanoparticles, 300 ml of ammonium hydroxide (30 wt.%) was added to a solution containing 5 L of ethanol and 500 ml of deionized water. After being pre-coated with polyvinylpyrrolidone, the as-prepared b-FeOOH nanoparticles were dispersed in the solution. To obtain the uniform silica shell/b-FeOOH core nanocomposite, 7 ml of tetraethoxysilane was then added to the mixture solution at room temperature for 10 h with vigorous stirring. The composite was heated to 500 °C for 5 h under an air atmosphere to produce the silica shell/hollow hematite nanostructures. The iron oxide/silica nanostructures were immersed in 0.1 M of NaOH solution with sonication for 5 h to remove the silica shell. The related synthesis procedure is briefly summarized in schematic form in Fig. 1. X-ray diffraction (XRD) patterns were obtained using a Rigaku Dmax 2500 diffractometer. The morphology of the a-Fe2O3 nano-
H.S. Kim et al. / Electrochemistry Communications 12 (2010) 382–385
Fig. 1. Synthetic procedure of uniform hematite nanocapsules by wrap–bake–peel process.
capsules was examined by high-resolution transmission electron microscopy (HR-TEM, JEM-2010, JEOL). The Brunauer–Emmett– Teller (BET) method was used to confirm the surface area via a Micromeritics ASAP-2000 nitrogen absorption analyzer. Cyclic voltammetry (CV) was carried out in the voltage range of 0.0–3.0 V with a scan rate of 0.1 mV/s using a Solartron 1480 multistat instrument. The working electrode was constituted by mixing Fe2O3 (active material), Super P (conducting material), and polyvinylidene fluoride (binder) at a weight ratio of 70:10:20 and the mixture slurry was spread onto Cu foil and then pressed for use as the working electrode. The cells were galvanostatically charged and discharged at a constant current density of 100.7 mA/g in the voltage range from 0.01 to 3.0 V on a TOSCA3100 battery cycler (Toyo Co., Japan).
3. Results and discussion The typical XRD patterns of the b-FeOOH nanorod and hematite nanocapsules are shown in Fig. 2a and b. All the diffraction peaks of Fig. 2a can be readily indexed to a pure tetragonal phase of b-FeOOH, which agrees well with the standard values (JCPDS 75-1594). After wrap–bake–peel procedure, the b-FeOOH nanoparticle was entirely converted into rhombohedral hematite (a-Fe2O3) structure (JCPDS 33-0664), as shown in Fig. 2b. The diffraction pattern shown in Fig. 2b indicates that the (1 1 0) orientation is strongly preferred for the iron oxide nanotubes [7]. No impurities were de-
Fig. 2. XRD patterns of (a) b-FeOOH nanorods and (b) hematite nanocapsules prepared by calcination in air.
383
tected in the patterns and the broad peaks of the XRD pattern indicated that the products had a very thin walls. The size and morphology of the as-prepared b-FeOOH were characterized by HR-TEM, as shown in Fig. 3a. The b-FeOOH nanorod with a uniform diameter of 15 nm and length of 65 nm were given a uniform surface coating of silica material to produce b-FeOOH/silica nanostructures. Fig. 3b certified the presence of a thin silica layer with a thickness of about 10 nm along the entire surface of the b-FeOOH particles. Afterwards, the b-FeOOH/silica nanostructures were calcined at 500 °C for 5 h under air atmosphere. In this process, the thermal dehydroxylation reaction occurred, where the b-FeOOH nanoparticle was completely converted into hollow hematite structure due to the interaction between the outer surface of iron oxide and the inner part of the silica shell, as shown in Fig. 3c. Finally, the low and high TEM images of the hematite after removal of the silica shell by 0.1 M NaOH are exhibited in Fig. 3d and e. The length, diameter and shell thickness of the uniform hematite nanocapsules were determined to be about 65, 15 and 5 nm, respectively. Without silica coating, the hematite particle in Fig. 3f was composed of irregular nanoparticles, with an approximate diameter of 100 nm, that were aggregated into micrometer-sized, poorly shaped particles after heat treatment. Without the silica shell, the b-FeOOH nanoparticles were not converted to hollow structures after calcination at 500 °C, but were rather fused together to form large clusters. Obviously, the coated silica shell plays an important role in preventing aggregation of the hematite nanoparticles and in generating uniform hollow nanocapsule structures during calcination. In this work, the volume of the formed iron oxide was reduced with increasing processing temperature due to the thermal decomposition of b-FeOOH. Therefore, the formation of hollow nanostructures was attributed to the reduction of the volume during thermal treatment, and the growth and spread of the crystalline nuclei to form an iron oxide film covering the silica inner walls to the strong interaction of the iron oxide species with the silica inner wall [8,9]. Furthermore, for detailed morphology comparison, the BET nitrogen adsorption–desorption measurements revealed that the specific area of the nanocapsules was 165 m2/g, compared to only 14 m2/g for the hematite aggregates. The large surface area of the hematite nanocapsules resulted from the increase of the total contact area, including the inner and outer walls of the hollow structure that are easily accessible from electrolyte, and this was expected to enhance the specific capacity. Fig. 4a and b shows the initial five cycles of the hematite aggregates and hematite nanocapsules at a current density of 0.1C. In the first discharge curve, there was a weak potential plateau at 1.70 V, corresponding to lithium insertion into the crystal structure of a-Fe2O3, and an extended potential plateau at 0.77 V, corresponding to the reduction of Fe3+ to Fe0, while there was only one plateau for hematite aggregates [10]. Meanwhile, at the first potential plateau correlated with lithium insertion at 1.7 V, close to 1.3 mol of lithium ion in the hematite nanocapsules were consumed, corresponding to a capacity of 218 mAh/g, whereas the hematite aggregate consumed only 0.3 mol of lithium ion. These results revealed that the morphological discrepancies such as the particle size and the real surface area affected the lithium intercalation performance in the crystal structure of a-Fe2O3 before the structural transformation. The penetration of electrolyte toward the entire surface area was especially difficult in the case of the aggregated hematite electrode. The electrochemical reaction occurred in the partially exposed surface area. The first and second discharge capacities of hematite nanocapsules were 1223 and 981 mAh/g respectively, which were higher than those of the hematite aggregates (1107 and 821 mAh/g). The large irreversible capacity in the first cycle was mainly attributed to the well known decomposition of electrolyte during the discharge process [11]. The
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H.S. Kim et al. / Electrochemistry Communications 12 (2010) 382–385
Fig. 3. TEM images of (a) b-FeOOH nanorods, (b) silica-coated b-FeOOH, (c) silica/iron oxide nanocomposites after calcinations (d), (e) the final hematite nanocapsules and (f) hematite aggregates without silica shell.
Fig. 4. Voltage profiles of (a) hematite aggregates, (b) hematite nanocapsules, (c) cyclic voltammograms and (d) cycle performance of hematite nanocapsules.
electrochemical activity of the hematite nanocapsules showed an improved discharge capacity that was attributed to the increasing ratio of atoms left on the surface area compared to that of the hematite aggregates. Fig. 4c shows the cyclic voltammograms of the hematite nanocapsules for three cycles at the scan rate of 0.1 mV/s. In the cathodic process of the first cycle, two obvious peaks, one weak and one spiky, were observed at 1.66 and 0.64 V, respectively. These were
attributed to the lithium insertion in the crystal structure of aFe2O3, the reduction of Fe3+ to Fe0, and the irreversible reduction reaction of electrolyte. Meanwhile, in the anodic process, only a main peak with a broad curve was recorded at about 1.78 V, which was attributed to the reversible oxidation of Fe0 to Fe3+. In the subsequent cycles, however, the cathodic peaks were shifted to a higher potential, while their peak intensity and integrated area become similar. These results revealed the good reversibility of the reduc-
H.S. Kim et al. / Electrochemistry Communications 12 (2010) 382–385
tion process of Fe3+ to Fe0, due to the high surface area and thin shell layer, which shortened the diffusion length of the lithium ions and thereby ensured the complete reversibility of the lithium intercalation process. Fig. 4d shows the discharge capacity of the hematite aggregates and hematite nanocapsules. The capacity of the hematite nanocapsules was retained at 740 mAh/g after 30 cycles, which was equivalent to 84% of the second discharge capacity, while the hematite aggregates exhibited a capacity retention of 32%. The good capacity retention of the hematite was mainly attributed to its hollow structure, which alleviated the volume expansion during cycling, because the limited volume space provided along the inner wall hindered the volume expansion during cycles. The structural collapse from the volume expansion was principally regarded as an issue of recovery in terms of cycle retention. In this situation, the introduction of hollow nanocapsules into the LIB anode material will facilitate the development of new architecture to produce devices with stable retention and further novel synthesis tools for applications to various fields. 4. Conclusion Hematite nanocapsules with a uniform diameter of 14 nm, length of 65 nm and shell thickness of 5 nm were prepared by a wrap–bake–peel process. The a-Fe2O3 nanocapsules exhibited excellent electrochemical performance, with an initial capacity of 888 mAh/g and a capacity retention of 84% after 30 cycles, due to the high surface area induced by the hollow nanostructure, the restriction of volume expansion along the limited inner volume and the short lithium diffusion length resulting from the thin shell.
385
This wrap–bake–peel synthesis tool can be used for various kinds of material to produce hollow nanostructures as a high-performance anode material for LIB. Acknowledgments This work was supported by the WCU program through the National Research Foundation of Korea funded by the Ministry of Education, Science & Technology (R31-10013), the Division of Advanced Batteries in the NGE (Next generation engine) Program (Project No. 10028960-2007-11) and the Korean Ministry of Education, Science and Technology through the National Creative Research Initiative Program of the Korea Science and Engineering Foundation (KOSEF). References [1] P. Poizot, S. Larueelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496. [2] W.Y. Li, L.N. Xu, J. Chen, Adv. Funct. Mater. 15 (2005) 851. [3] C. Wu, P. Yin, X. Zhu, C.O. Yang, Y. Xie, J. Phys. Chem. B 110 (2006) 17806. [4] Y. Feng, L. Yang, S. Fang, Z. Qiu, Electrochem. Acta 54 (2009) 6244. [5] Y. NuLi, P. Zhang, Z. Guo, H. Liu, J. Electrochem. Soc. 155 (2008) A196. [6] S. Zeng, K. Tang, T. Li, Z. Liang, D. Wang, Y. Wang, Y. Qi, W. Zhou, J. Phys. Chem. C 112 (2008) 4836. [7] T. Yu, J. Joo, Y.I. Park, T. Hyeon, J. Am. Chem. Soc. 128 (2006) 1786. [8] F. Bondioli, A.M. Ferrari, C. Leonelli, T. Manfredini, Mater. Res. Bull. 33 (1998) 723. [9] S.W. da Silva, R.C. Pedroza, P.P.C. Sartoratto, D.R. Rezende, A.V. da Silva Neto. M.A.G. Soler, P.C. Morais, J. Non-Cryst. Solids 352 (2006) 1602. [10] J. Chen, L. Xu, W. Li, X. Gou, Adv. Mater. 17 (2005) 582. [11] H. Liu, G. Wang, J. Park, J. Wang, H. Liu, C. Zhang, Electrochem. Acta 54 (2009) 1733.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Uniform hematite nanocapsules based on an anode material for lithium ion batteries Hyun Sik Kim a, Yuanzhe Piao b,c, Soon Hyung Kang a, Taeghwan Hyeon b,*, Yung-Eun Sung a,* a
School of Chemical and Biological Engineering and Research Center for Energy Conversion and Storage, Seoul National University, Seoul 151-744, Republic of Korea National Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea c Department of Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Suwon 443-270, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 24 November 2009 Received in revised form 29 December 2009 Accepted 29 December 2009 Available online 4 January 2010 Keywords: Lithium ion battery Anode material Hematite Nanocapsules
a b s t r a c t Uniform a-Fe2O3 nanocapsules with a high surface area were synthesized by a novel wrap–bake–peel approach consisting of silica coating, heat treatment and finally the removal of the silica coating layer. The length, diameter and shell thickness of the hematite nanocapsules were about 65, 15 and 5 nm, respectively. The electrochemical properties of the a-Fe2O3 nanocapsules were investigated by cyclic voltammetry and charge/discharge measurements. The a-Fe2O3 nanocapsules showed a high reversible capacity of 888 mAh/g in the initial cycle and 740 mAh/g after 30 cycles as well as good capacity retention. This excellent electrochemical performance was attributed to the high surface area, thin shell and volume space of the hollow structure. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, nanostructured 3d-metal oxides MOx (M = Cu, Mn, Fe, Co, Ni, etc.) have been widely studied as anode materials for lithium-ion battery (LIB) owing to their high energy capacity [1,2]. The properties of nanomaterials strongly depend on their size and morphology. By controlling the particle size, the quantization effect was widely examined and the surface area and robustness was influenced by modifying the particle shape. Hence, research attention has focused on controlling the nanoparticle size, shape and structure in order to attain unique properties for LIB [3]. Above all, interest on the 3-dimensional hollow sphere has focused on the prominent position on account of the high surface area and porous structure [4]. In terms of materials, hematite (a-Fe2O3) is considered a promising active lithium intercalation host due to its high theoretical capacity (1007 mAh/g), environmental friendliness, and low cost. As the particle size and morphology of hematite nanostructures exert a key influence on their electrochemical performance for lithium storage, hematite nanostructures showing different morphologies and particle sizes have been synthesized in order to enhance the electrochemical performance [5,6]. In a similar context, the hollow nanostructured hematite possesses several advantages for LIB application such as the extended contact area between the active material and the electrolyte caused by their high surface area, the short lithium diffusion length * Corresponding authors. Tel.: +82 2 880 1889; fax: +82 2 888 1604. E-mail addresses: [email protected] (T. Hyeon), [email protected] (Y.-E. Sung). 1388-2481/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.12.040
resulting from the thin shell and the hollow space in the central part that buffers the volume expansion during cycling. We developed a new route for synthesizing uniform-sized hematite nanocapsules with a high surface area and thin shell in order to achieve excellent electrochemical reactivity toward lithium. Stable, enhanced electrochemical performance due to the peculiar hematite nanocapsules was achieved.
2. Experimental In a typical synthetic procedure of spindle-shaped b-FeOOH nanoparticles, 0.02 M FeCl36H2O was dissolved into 2 L of deionized water at 80 °C under magnetic stirring and the particles were washed with water. To perform the silica coating of the b-FeOOH nanoparticles, 300 ml of ammonium hydroxide (30 wt.%) was added to a solution containing 5 L of ethanol and 500 ml of deionized water. After being pre-coated with polyvinylpyrrolidone, the as-prepared b-FeOOH nanoparticles were dispersed in the solution. To obtain the uniform silica shell/b-FeOOH core nanocomposite, 7 ml of tetraethoxysilane was then added to the mixture solution at room temperature for 10 h with vigorous stirring. The composite was heated to 500 °C for 5 h under an air atmosphere to produce the silica shell/hollow hematite nanostructures. The iron oxide/silica nanostructures were immersed in 0.1 M of NaOH solution with sonication for 5 h to remove the silica shell. The related synthesis procedure is briefly summarized in schematic form in Fig. 1. X-ray diffraction (XRD) patterns were obtained using a Rigaku Dmax 2500 diffractometer. The morphology of the a-Fe2O3 nano-
H.S. Kim et al. / Electrochemistry Communications 12 (2010) 382–385
Fig. 1. Synthetic procedure of uniform hematite nanocapsules by wrap–bake–peel process.
capsules was examined by high-resolution transmission electron microscopy (HR-TEM, JEM-2010, JEOL). The Brunauer–Emmett– Teller (BET) method was used to confirm the surface area via a Micromeritics ASAP-2000 nitrogen absorption analyzer. Cyclic voltammetry (CV) was carried out in the voltage range of 0.0–3.0 V with a scan rate of 0.1 mV/s using a Solartron 1480 multistat instrument. The working electrode was constituted by mixing Fe2O3 (active material), Super P (conducting material), and polyvinylidene fluoride (binder) at a weight ratio of 70:10:20 and the mixture slurry was spread onto Cu foil and then pressed for use as the working electrode. The cells were galvanostatically charged and discharged at a constant current density of 100.7 mA/g in the voltage range from 0.01 to 3.0 V on a TOSCA3100 battery cycler (Toyo Co., Japan).
3. Results and discussion The typical XRD patterns of the b-FeOOH nanorod and hematite nanocapsules are shown in Fig. 2a and b. All the diffraction peaks of Fig. 2a can be readily indexed to a pure tetragonal phase of b-FeOOH, which agrees well with the standard values (JCPDS 75-1594). After wrap–bake–peel procedure, the b-FeOOH nanoparticle was entirely converted into rhombohedral hematite (a-Fe2O3) structure (JCPDS 33-0664), as shown in Fig. 2b. The diffraction pattern shown in Fig. 2b indicates that the (1 1 0) orientation is strongly preferred for the iron oxide nanotubes [7]. No impurities were de-
Fig. 2. XRD patterns of (a) b-FeOOH nanorods and (b) hematite nanocapsules prepared by calcination in air.
383
tected in the patterns and the broad peaks of the XRD pattern indicated that the products had a very thin walls. The size and morphology of the as-prepared b-FeOOH were characterized by HR-TEM, as shown in Fig. 3a. The b-FeOOH nanorod with a uniform diameter of 15 nm and length of 65 nm were given a uniform surface coating of silica material to produce b-FeOOH/silica nanostructures. Fig. 3b certified the presence of a thin silica layer with a thickness of about 10 nm along the entire surface of the b-FeOOH particles. Afterwards, the b-FeOOH/silica nanostructures were calcined at 500 °C for 5 h under air atmosphere. In this process, the thermal dehydroxylation reaction occurred, where the b-FeOOH nanoparticle was completely converted into hollow hematite structure due to the interaction between the outer surface of iron oxide and the inner part of the silica shell, as shown in Fig. 3c. Finally, the low and high TEM images of the hematite after removal of the silica shell by 0.1 M NaOH are exhibited in Fig. 3d and e. The length, diameter and shell thickness of the uniform hematite nanocapsules were determined to be about 65, 15 and 5 nm, respectively. Without silica coating, the hematite particle in Fig. 3f was composed of irregular nanoparticles, with an approximate diameter of 100 nm, that were aggregated into micrometer-sized, poorly shaped particles after heat treatment. Without the silica shell, the b-FeOOH nanoparticles were not converted to hollow structures after calcination at 500 °C, but were rather fused together to form large clusters. Obviously, the coated silica shell plays an important role in preventing aggregation of the hematite nanoparticles and in generating uniform hollow nanocapsule structures during calcination. In this work, the volume of the formed iron oxide was reduced with increasing processing temperature due to the thermal decomposition of b-FeOOH. Therefore, the formation of hollow nanostructures was attributed to the reduction of the volume during thermal treatment, and the growth and spread of the crystalline nuclei to form an iron oxide film covering the silica inner walls to the strong interaction of the iron oxide species with the silica inner wall [8,9]. Furthermore, for detailed morphology comparison, the BET nitrogen adsorption–desorption measurements revealed that the specific area of the nanocapsules was 165 m2/g, compared to only 14 m2/g for the hematite aggregates. The large surface area of the hematite nanocapsules resulted from the increase of the total contact area, including the inner and outer walls of the hollow structure that are easily accessible from electrolyte, and this was expected to enhance the specific capacity. Fig. 4a and b shows the initial five cycles of the hematite aggregates and hematite nanocapsules at a current density of 0.1C. In the first discharge curve, there was a weak potential plateau at 1.70 V, corresponding to lithium insertion into the crystal structure of a-Fe2O3, and an extended potential plateau at 0.77 V, corresponding to the reduction of Fe3+ to Fe0, while there was only one plateau for hematite aggregates [10]. Meanwhile, at the first potential plateau correlated with lithium insertion at 1.7 V, close to 1.3 mol of lithium ion in the hematite nanocapsules were consumed, corresponding to a capacity of 218 mAh/g, whereas the hematite aggregate consumed only 0.3 mol of lithium ion. These results revealed that the morphological discrepancies such as the particle size and the real surface area affected the lithium intercalation performance in the crystal structure of a-Fe2O3 before the structural transformation. The penetration of electrolyte toward the entire surface area was especially difficult in the case of the aggregated hematite electrode. The electrochemical reaction occurred in the partially exposed surface area. The first and second discharge capacities of hematite nanocapsules were 1223 and 981 mAh/g respectively, which were higher than those of the hematite aggregates (1107 and 821 mAh/g). The large irreversible capacity in the first cycle was mainly attributed to the well known decomposition of electrolyte during the discharge process [11]. The
384
H.S. Kim et al. / Electrochemistry Communications 12 (2010) 382–385
Fig. 3. TEM images of (a) b-FeOOH nanorods, (b) silica-coated b-FeOOH, (c) silica/iron oxide nanocomposites after calcinations (d), (e) the final hematite nanocapsules and (f) hematite aggregates without silica shell.
Fig. 4. Voltage profiles of (a) hematite aggregates, (b) hematite nanocapsules, (c) cyclic voltammograms and (d) cycle performance of hematite nanocapsules.
electrochemical activity of the hematite nanocapsules showed an improved discharge capacity that was attributed to the increasing ratio of atoms left on the surface area compared to that of the hematite aggregates. Fig. 4c shows the cyclic voltammograms of the hematite nanocapsules for three cycles at the scan rate of 0.1 mV/s. In the cathodic process of the first cycle, two obvious peaks, one weak and one spiky, were observed at 1.66 and 0.64 V, respectively. These were
attributed to the lithium insertion in the crystal structure of aFe2O3, the reduction of Fe3+ to Fe0, and the irreversible reduction reaction of electrolyte. Meanwhile, in the anodic process, only a main peak with a broad curve was recorded at about 1.78 V, which was attributed to the reversible oxidation of Fe0 to Fe3+. In the subsequent cycles, however, the cathodic peaks were shifted to a higher potential, while their peak intensity and integrated area become similar. These results revealed the good reversibility of the reduc-
H.S. Kim et al. / Electrochemistry Communications 12 (2010) 382–385
tion process of Fe3+ to Fe0, due to the high surface area and thin shell layer, which shortened the diffusion length of the lithium ions and thereby ensured the complete reversibility of the lithium intercalation process. Fig. 4d shows the discharge capacity of the hematite aggregates and hematite nanocapsules. The capacity of the hematite nanocapsules was retained at 740 mAh/g after 30 cycles, which was equivalent to 84% of the second discharge capacity, while the hematite aggregates exhibited a capacity retention of 32%. The good capacity retention of the hematite was mainly attributed to its hollow structure, which alleviated the volume expansion during cycling, because the limited volume space provided along the inner wall hindered the volume expansion during cycles. The structural collapse from the volume expansion was principally regarded as an issue of recovery in terms of cycle retention. In this situation, the introduction of hollow nanocapsules into the LIB anode material will facilitate the development of new architecture to produce devices with stable retention and further novel synthesis tools for applications to various fields. 4. Conclusion Hematite nanocapsules with a uniform diameter of 14 nm, length of 65 nm and shell thickness of 5 nm were prepared by a wrap–bake–peel process. The a-Fe2O3 nanocapsules exhibited excellent electrochemical performance, with an initial capacity of 888 mAh/g and a capacity retention of 84% after 30 cycles, due to the high surface area induced by the hollow nanostructure, the restriction of volume expansion along the limited inner volume and the short lithium diffusion length resulting from the thin shell.
385
This wrap–bake–peel synthesis tool can be used for various kinds of material to produce hollow nanostructures as a high-performance anode material for LIB. Acknowledgments This work was supported by the WCU program through the National Research Foundation of Korea funded by the Ministry of Education, Science & Technology (R31-10013), the Division of Advanced Batteries in the NGE (Next generation engine) Program (Project No. 10028960-2007-11) and the Korean Ministry of Education, Science and Technology through the National Creative Research Initiative Program of the Korea Science and Engineering Foundation (KOSEF). References [1] P. Poizot, S. Larueelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496. [2] W.Y. Li, L.N. Xu, J. Chen, Adv. Funct. Mater. 15 (2005) 851. [3] C. Wu, P. Yin, X. Zhu, C.O. Yang, Y. Xie, J. Phys. Chem. B 110 (2006) 17806. [4] Y. Feng, L. Yang, S. Fang, Z. Qiu, Electrochem. Acta 54 (2009) 6244. [5] Y. NuLi, P. Zhang, Z. Guo, H. Liu, J. Electrochem. Soc. 155 (2008) A196. [6] S. Zeng, K. Tang, T. Li, Z. Liang, D. Wang, Y. Wang, Y. Qi, W. Zhou, J. Phys. Chem. C 112 (2008) 4836. [7] T. Yu, J. Joo, Y.I. Park, T. Hyeon, J. Am. Chem. Soc. 128 (2006) 1786. [8] F. Bondioli, A.M. Ferrari, C. Leonelli, T. Manfredini, Mater. Res. Bull. 33 (1998) 723. [9] S.W. da Silva, R.C. Pedroza, P.P.C. Sartoratto, D.R. Rezende, A.V. da Silva Neto. M.A.G. Soler, P.C. Morais, J. Non-Cryst. Solids 352 (2006) 1602. [10] J. Chen, L. Xu, W. Li, X. Gou, Adv. Mater. 17 (2005) 582. [11] H. Liu, G. Wang, J. Park, J. Wang, H. Liu, C. Zhang, Electrochem. Acta 54 (2009) 1733.