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A phase transformation route to Fe2O3–Mn3O4 nanocomposite with improved electrode performance
Materials Letters 107 (2013) 221–224
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A phase transformation route to Fe2O3–Mn3O4 nanocomposite with improved electrode performance$ Seung Mi Oh a, In Young Kim a, Su-Jeong Kim a, Woong Jung b, Seong-Ju Hwang a,n a Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul 120-750, Republic of Korea b Department of Emergency Medicine, Kyung Hee University, Seoul 134-727, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 3 April 2013 Accepted 13 May 2013 Available online 5 June 2013
Intimately mixed nanocomposite of Fe2O3 and Mn3O4 is synthesized by an electrostatically-derived selfassembly between exfoliated MnO2 nanosheets and Fe cations, which is followed by heat-treatment at elevated temperature. The as-prepared Fe–layered MnO2 nanocomposite experiences phase transformations into Fe-substituted Mn3−xFexO4 nanoparticle at 450 1C and Fe2O3–Mn3O4 nanocomposite at 650 1C. The Fe2O3−Mn3O4 nanocomposite shows better performance as anode material for lithium ion batteries than the Fe-substituted Mn3−xFexO4 nanoparticle, indicating the beneficial effect of composite formation on the electrode performance of 3d metal oxide. The present finding underscores that a self-assembly between exfoliated metal oxide nanosheets and metal cations can provide useful precursor for efficient composite electrode materials. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Energy storage and conversion Nanocomposites Particles Nanosize Phase transformation
1. Introduction As promising anode materials for lithium ion batteries, 3d transition metal oxides with low oxidation states attract a great deal of research activity because of their greater discharge capacity than currently commercialized graphitic carbon [1]. Despite their large initial capacity, these materials suffer from severe capacity fading during electrochemical cycling, which is attributable to drastic volume change [2,3]. To circumvent this problem, many attempts are made with nanostructure formation and composite formation with conductive carbon [2,3]. Since the nanoscale mixing of two kinds of electrode materials can improve their cyclability [4], the intimately mixed nanocomposites consisting of two different 3d metal oxide nanocrystals are supposed to show improved electrode performance. Considering the fact that the heat-treatment of MnO2 at elevated temperature yields the lowvalent Mn3O4 material via accompanying oxygen loss [5], the intercalation compounds of exfoliated MnO2 nanosheets with different metal cations can be useful precursors for the Mn3O4based composite electrode materials. To date, we are aware of no
☆ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. n Corresponding author. Tel.: +82 2 3277 4370; fax: +82 2 3277 3419. E-mail address: [email protected] (S.-J. Hwang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.050
reports on the synthesis of mixed metal oxide nanocomposite from the precursor of exfoliated nanosheet. In the present study, the intimately coupled Mn3O4–Fe2O3 nanocomposite is synthesized by the self-assembly between anionic MnO2 nanosheets and Fe cations, and the subsequent heat-treatment at elevated temperatures. The nanocomposites are tested as anode materials for lithium ion batteries to probe the effect of composite formation on the electrode activity of 3d metal oxides. 2. Experimental The exfoliated MnO2 nanosheet was prepared by a protonexchange of layered K0.45MnO2 and the following intercalation of tetrabutylammonium (TBA) ions [6]. The self-assembly between exfoliated MnO2 nanosheets and Fe cations was achieved by mixing the aqueous colloidal suspension of MnO2 nanosheets and the aqueous solution of iron(II) acetate with the Fe/Mn ratio of unity. The reaction proceeded at pH¼9 at 25 1C for 24 h. The resulting precipitate of Fe−layered MnO2 was separated and washed with distilled water. The Mn3−xFexO4 and Fe2O3–Mn3O4 materials were synthesized by the heat-treatment of the Fe−layered MnO2 at 450– 650 1C under N2 flow for 3 h. The crystal structure and morphology of these materials were examined with powder X-ray diffraction (XRD) and electron microscopy, respectively. The chemical composition and thermal behavior of the as-prepared material were studied with energy dispersive spectrometry (EDS) and thermogravimetric
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analysis (TGA), respectively. X-ray absorption near-edge structure (XANES) experiments were carried out at Mn K-edge and Fe K-edge at the beam line 10C at the Pohang Accelerator Laboratory (PAL) in Korea. The electrode functionality of the present materials was examined by galvanostatic charge−discharge cycling with a current density of 90 mA/g. The electrode test was carried out with a cell of Li/1 M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC) (50:50 v/v)/composite cathode. The composite cathode was composed of active material (80%), Super P (10%), and PVDF (10%).
3. Results and discussion The left panel of Fig. 1 represents the powder XRD patterns of the as-prepared Fe−layered MnO2 (denoted as FeMn-RT) and its derivatives calcined at 450 and 650 1C (denoted as FeMn450 and FeMn650), as compared with those of the pristine K0.45MnO2 and its protonated derivative. The protonation process leads to the displacement of the (00l) peaks toward low angle region, indicating a basal expansion from 0.71 to 0.73 nm by the replacement of K+ ions with H3O+ ions. Despite the significant broadening and depression of diffraction peaks, a series of (00l) reflections are still observable for the FeMnRT, indicating the formation of intercalative Fe−layered MnO2 compound with the basal spacing of 0.72 nm. The intercalation structure of the as-prepared compound is transformed into the spinel-structured Mn3−xFexO4 at 450 1C and then to the mixture of α-Fe2O3 and spinel-structured Mn3O4 at 650 1C, as illustrated in the right panel of Fig. 1. According to the EDS analysis, the Fe−layered MnO2 has the Fe/Mn ratio of 0.7. The TGA result indicates that the FeMn-RT shows significant mass loss with several steps (see Supporting information); a mass loss of 15% in the temperature region of 25−250 1C is attributed to the dehydration/dehydroxylation of this material whereas the mass loss at higher than 250 1C corresponds to the reduction of Mn ions with oxygen evaporation. On the basis of Scherrer equation, the particle sizes are estimated to be 19, 24, and 18 nm for Mn3−xFexO4, Fe2O3, and Mn3O4, respectively, confirming the nanocrystallity of these materials. Fig. 2a−c represents the field emission-scanning electron microscopy (FE-SEM) images of FeMn-RT, FeMn450, and FeMn650 correspondingly. The FeMn-RT shows porous morphology created
by the house-of-cards type stacking of MnO2 nanosheets with the lateral size of several hundred nanometers, like other nanosheetbased composites [7,8]. Upon the heat-treatment at 450−650 1C, the sheet-shaped crystals are changed into the porous assembly of polyhedral nanoparticles with the size of 20−30 nm, highlighting the phase transformation of layered MnO2 into non-layered Mn3O4. The phase transition from anisotropic nanosheets to polyhedral nanoparticles upon the heat-treatment is further confirmed by transmission electron microscopy (TEM) (see Supporting information). The EDS analysis reveals the presence of Fe, Mn, and O elements in this material (Fig. 2d), confirming the formation of Fe −layered MnO2 material. The left panel of Fig. 3 represents the Mn K-edge XANES spectra of FeMn-RT, FeMn450, and FeMn650, as compared with those of the pristine K0.45MnO2, Mn2O3, Mn3O4, and MnO. The edge energy of the FeMn-RT is nearly identical to that of the pristine K0.45MnO2, indicating the mixed oxidation state of Mn3+/Mn4+. The heat-treatment at 450−650 1C causes the displacement of the edge position to that of the reference Mn3O4, suggesting the reduction of Mn oxidation state to Mn2+/Mn3+. The as-prepared FeMn-RT exhibits pre-edge peak P/P′ corresponding to the dipoleforbidden 1s-3d transitions [9]. The weak intensity of these peaks indicates the stabilization of manganese ions in centrosymmetric octahedral symmetry. In comparison with FeMn-RT, the calcined derivatives display stronger pre-edge feature, reflecting the formation of spinel-structured Mn3O4 containing noncentrosymmetric MnO4 tetrahedra. All of the present materials display main-edge peaks A, B, and C corresponding to the dipoleallowed 1s-4p transitions [9]. The main-edge feature of the FeMn-RT is quite similar to that of the pristine K0.45MnO2, whereas the FeMn450 and FeMn650 commonly display similar main-edge features to the reference Mn3O4, confirming the local structural change to spinel Mn3O4 structure. The Fe K-edge XANES spectra of FeMn-RT, FeMn450, and FeMn650 are presented in the right panel of Fig. 3, together with the reference spectra of Fe2O3, Fe3O4, and FeO. The as-prepared FeMn-RT and its calcined derivatives commonly exhibit nearly identical edge energy to the reference Fe2O3, indicating the Fe3+ oxidation state in these materials. All the present nanocomposites display a weak pre-edge peak P corresponding to the dipoleforbidden 1s-3d transition [8], highlighting the maintenance of
Fig. 1. (Left) Powder XRD patterns of (a) the pristine K0.45MnO2, (b) protonated manganate, (c) FeMn-RT, (d) FeMn450, and (e) FeMn650. In (e), the miller indices with/ without underlines denote the reflections of Fe2O3/Mn3O4 phases, respectively. (Right) Schematic model for the phase transition of Fe−layered MnO2 upon heat-treatment.
S.M. Oh et al. / Materials Letters 107 (2013) 221–224
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Fig. 2. FE-SEM images of (a) FeMn-RT, (b) FeMn450, and (c) FeMn650. (d) EDS data of FeMn-RT.
Fig. 3. (Left) Mn K-edge and (right) Fe K-edge XANES spectra for (a) FeMn-RT, (b) FeMn450, (c) FeMn650, (d) the pristine K0.45MnO2/Fe2O3, (e) Mn2O3/Fe3O4, (f) Mn3O4/FeO, and (g) MnO.
the octahedral symmetry of iron ion upon self-assembly and heattreatment. In main-edge region, there are several peaks denoted as A, B, and C related to the 1s-4p transitions [8]. The overall shape and position of these main-edge peaks are nearly identical for the present nanocomposites and Fe2O3, confirming the formation of trivalent iron oxide. Summarizing the powder XRD, FE-SEM, and
XANES results, the FeMn450 corresponds to the Fe-substituted 3+ Mn2+/3+ 3−x Fex O4 nanoparticle whereas the FeMn650 is the compo2+/3+ site material of Fe3+ O4 nanoparticles. 2 O3 and Mn3 The calcined nanocomposites are tested as anode materials for lithium ion batteries. Both FeMn450 and FeMn650 materials show similar potential profiles characteristic of 3d metal oxides [1–3]
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Fe2O3−Mn3O4 nanocomposite at 650 1C. The Fe2O3−Mn3O4 nanocomposite shows promising anode performance for lithium ion batteries, which is superior to the Mn3−xFexO4 nanocrystals, highlighting the merit of composite formation. The present findings clearly demonstrate the usefulness of the self-assembled nanocomposite of exfoliated metal oxide nanosheets as precursors for novel efficient nanocomposite electrode materials.
Supporting information TGA, TEM, and potential profiles of the composite materials.
Acknowledgments
Fig. 4. Capacity plots for (a) FeMn450 and (b) FeMn650. The open and close symbols represent the discharge and charge data, respectively.
(see Supporting information). As plotted in Fig. 4, the FeMn650 delivers large discharge capacities of 1050 mAh g−1 for the 1st cycle and 230 mAh g−1 for the 25th cycle, which are significantly larger than those of the FeMn450. The FeMn650 displays a notable capacity decrease in the initial several cycles, which is attributable to the formation of solid electrolyte interphase (SEI) layers and/or to the irreversible reaction with the electrolyte [2,3]. After initial several cycles, only a weak capacity loss occurs for the FeMn650. Conversely, continuous severe fading is observable for the FeMn450, indicating the poorer cyclability of this material than the FeMn650. The present results indicate that the nanocomposite of Fe2O3 and Mn3O4 nanocrystals show much better electrode performance than the Fe-substituted Mn3−xFexO4 nanocrystal, underscoring the beneficial effect of the composite formation. 4. Conclusions Mesoporous nanocomposite of Fe2O3−Mn3O4 is synthesized by the self-assembly of exfoliated MnO2 nanosheet with Fe ions, and the following heat-treatment. The as-prepared Fe-layered MnO2 is transformed into Fe-substituted Mn3−xFexO4 at 450 1C and
This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF2010-C1AAA001-2010-0029065) and by the Core Technology of Materials Research and Development Program of the Korea Ministry of Intelligence and Economy (Grant no. 10041232). The experiments at PAL were supported in part by MOST and POSTECH.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.05. 050.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon J-M. Nature 2000;407:496. Wang Z-H, Yuan L-X, Shao Q-G, Huang F, Huang Y-H. Mater Lett 2012;80:110. Liu Z, Tay SW. Mater Lett 2012;72:74. Bruce PG, Armstrong AR, Gitzendanner R. J Mater Chem 1999;9:193. Seong DY, Kim IY, Kim TW, Song M-S, Hwang S-J. J Phys Chem C 2011;115:13171. Omomo Y, Sasaki T, Wang LZ, Watanabe M. J Am Chem Soc 2003;125:3568. Kim IY, Lee KY, Kim TW, Hwang S-J. Meter Lett 2011;65:894. Kim TW, Ha H-W, Paek M-J, Paek IH, Hyun S-H, Choy J-H, Hwang S-J. J Phys Chem C 2008;112:14853. Kim TW, Yoo H, Kim IY, Ha H-W, Han AR, Chang J-S, Lee JS, Hwang S-J. Adv Funct Mater 2011;21:2301.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A phase transformation route to Fe2O3–Mn3O4 nanocomposite with improved electrode performance$ Seung Mi Oh a, In Young Kim a, Su-Jeong Kim a, Woong Jung b, Seong-Ju Hwang a,n a Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul 120-750, Republic of Korea b Department of Emergency Medicine, Kyung Hee University, Seoul 134-727, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 3 April 2013 Accepted 13 May 2013 Available online 5 June 2013
Intimately mixed nanocomposite of Fe2O3 and Mn3O4 is synthesized by an electrostatically-derived selfassembly between exfoliated MnO2 nanosheets and Fe cations, which is followed by heat-treatment at elevated temperature. The as-prepared Fe–layered MnO2 nanocomposite experiences phase transformations into Fe-substituted Mn3−xFexO4 nanoparticle at 450 1C and Fe2O3–Mn3O4 nanocomposite at 650 1C. The Fe2O3−Mn3O4 nanocomposite shows better performance as anode material for lithium ion batteries than the Fe-substituted Mn3−xFexO4 nanoparticle, indicating the beneficial effect of composite formation on the electrode performance of 3d metal oxide. The present finding underscores that a self-assembly between exfoliated metal oxide nanosheets and metal cations can provide useful precursor for efficient composite electrode materials. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Energy storage and conversion Nanocomposites Particles Nanosize Phase transformation
1. Introduction As promising anode materials for lithium ion batteries, 3d transition metal oxides with low oxidation states attract a great deal of research activity because of their greater discharge capacity than currently commercialized graphitic carbon [1]. Despite their large initial capacity, these materials suffer from severe capacity fading during electrochemical cycling, which is attributable to drastic volume change [2,3]. To circumvent this problem, many attempts are made with nanostructure formation and composite formation with conductive carbon [2,3]. Since the nanoscale mixing of two kinds of electrode materials can improve their cyclability [4], the intimately mixed nanocomposites consisting of two different 3d metal oxide nanocrystals are supposed to show improved electrode performance. Considering the fact that the heat-treatment of MnO2 at elevated temperature yields the lowvalent Mn3O4 material via accompanying oxygen loss [5], the intercalation compounds of exfoliated MnO2 nanosheets with different metal cations can be useful precursors for the Mn3O4based composite electrode materials. To date, we are aware of no
☆ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. n Corresponding author. Tel.: +82 2 3277 4370; fax: +82 2 3277 3419. E-mail address: [email protected] (S.-J. Hwang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.050
reports on the synthesis of mixed metal oxide nanocomposite from the precursor of exfoliated nanosheet. In the present study, the intimately coupled Mn3O4–Fe2O3 nanocomposite is synthesized by the self-assembly between anionic MnO2 nanosheets and Fe cations, and the subsequent heat-treatment at elevated temperatures. The nanocomposites are tested as anode materials for lithium ion batteries to probe the effect of composite formation on the electrode activity of 3d metal oxides. 2. Experimental The exfoliated MnO2 nanosheet was prepared by a protonexchange of layered K0.45MnO2 and the following intercalation of tetrabutylammonium (TBA) ions [6]. The self-assembly between exfoliated MnO2 nanosheets and Fe cations was achieved by mixing the aqueous colloidal suspension of MnO2 nanosheets and the aqueous solution of iron(II) acetate with the Fe/Mn ratio of unity. The reaction proceeded at pH¼9 at 25 1C for 24 h. The resulting precipitate of Fe−layered MnO2 was separated and washed with distilled water. The Mn3−xFexO4 and Fe2O3–Mn3O4 materials were synthesized by the heat-treatment of the Fe−layered MnO2 at 450– 650 1C under N2 flow for 3 h. The crystal structure and morphology of these materials were examined with powder X-ray diffraction (XRD) and electron microscopy, respectively. The chemical composition and thermal behavior of the as-prepared material were studied with energy dispersive spectrometry (EDS) and thermogravimetric
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analysis (TGA), respectively. X-ray absorption near-edge structure (XANES) experiments were carried out at Mn K-edge and Fe K-edge at the beam line 10C at the Pohang Accelerator Laboratory (PAL) in Korea. The electrode functionality of the present materials was examined by galvanostatic charge−discharge cycling with a current density of 90 mA/g. The electrode test was carried out with a cell of Li/1 M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC) (50:50 v/v)/composite cathode. The composite cathode was composed of active material (80%), Super P (10%), and PVDF (10%).
3. Results and discussion The left panel of Fig. 1 represents the powder XRD patterns of the as-prepared Fe−layered MnO2 (denoted as FeMn-RT) and its derivatives calcined at 450 and 650 1C (denoted as FeMn450 and FeMn650), as compared with those of the pristine K0.45MnO2 and its protonated derivative. The protonation process leads to the displacement of the (00l) peaks toward low angle region, indicating a basal expansion from 0.71 to 0.73 nm by the replacement of K+ ions with H3O+ ions. Despite the significant broadening and depression of diffraction peaks, a series of (00l) reflections are still observable for the FeMnRT, indicating the formation of intercalative Fe−layered MnO2 compound with the basal spacing of 0.72 nm. The intercalation structure of the as-prepared compound is transformed into the spinel-structured Mn3−xFexO4 at 450 1C and then to the mixture of α-Fe2O3 and spinel-structured Mn3O4 at 650 1C, as illustrated in the right panel of Fig. 1. According to the EDS analysis, the Fe−layered MnO2 has the Fe/Mn ratio of 0.7. The TGA result indicates that the FeMn-RT shows significant mass loss with several steps (see Supporting information); a mass loss of 15% in the temperature region of 25−250 1C is attributed to the dehydration/dehydroxylation of this material whereas the mass loss at higher than 250 1C corresponds to the reduction of Mn ions with oxygen evaporation. On the basis of Scherrer equation, the particle sizes are estimated to be 19, 24, and 18 nm for Mn3−xFexO4, Fe2O3, and Mn3O4, respectively, confirming the nanocrystallity of these materials. Fig. 2a−c represents the field emission-scanning electron microscopy (FE-SEM) images of FeMn-RT, FeMn450, and FeMn650 correspondingly. The FeMn-RT shows porous morphology created
by the house-of-cards type stacking of MnO2 nanosheets with the lateral size of several hundred nanometers, like other nanosheetbased composites [7,8]. Upon the heat-treatment at 450−650 1C, the sheet-shaped crystals are changed into the porous assembly of polyhedral nanoparticles with the size of 20−30 nm, highlighting the phase transformation of layered MnO2 into non-layered Mn3O4. The phase transition from anisotropic nanosheets to polyhedral nanoparticles upon the heat-treatment is further confirmed by transmission electron microscopy (TEM) (see Supporting information). The EDS analysis reveals the presence of Fe, Mn, and O elements in this material (Fig. 2d), confirming the formation of Fe −layered MnO2 material. The left panel of Fig. 3 represents the Mn K-edge XANES spectra of FeMn-RT, FeMn450, and FeMn650, as compared with those of the pristine K0.45MnO2, Mn2O3, Mn3O4, and MnO. The edge energy of the FeMn-RT is nearly identical to that of the pristine K0.45MnO2, indicating the mixed oxidation state of Mn3+/Mn4+. The heat-treatment at 450−650 1C causes the displacement of the edge position to that of the reference Mn3O4, suggesting the reduction of Mn oxidation state to Mn2+/Mn3+. The as-prepared FeMn-RT exhibits pre-edge peak P/P′ corresponding to the dipoleforbidden 1s-3d transitions [9]. The weak intensity of these peaks indicates the stabilization of manganese ions in centrosymmetric octahedral symmetry. In comparison with FeMn-RT, the calcined derivatives display stronger pre-edge feature, reflecting the formation of spinel-structured Mn3O4 containing noncentrosymmetric MnO4 tetrahedra. All of the present materials display main-edge peaks A, B, and C corresponding to the dipoleallowed 1s-4p transitions [9]. The main-edge feature of the FeMn-RT is quite similar to that of the pristine K0.45MnO2, whereas the FeMn450 and FeMn650 commonly display similar main-edge features to the reference Mn3O4, confirming the local structural change to spinel Mn3O4 structure. The Fe K-edge XANES spectra of FeMn-RT, FeMn450, and FeMn650 are presented in the right panel of Fig. 3, together with the reference spectra of Fe2O3, Fe3O4, and FeO. The as-prepared FeMn-RT and its calcined derivatives commonly exhibit nearly identical edge energy to the reference Fe2O3, indicating the Fe3+ oxidation state in these materials. All the present nanocomposites display a weak pre-edge peak P corresponding to the dipoleforbidden 1s-3d transition [8], highlighting the maintenance of
Fig. 1. (Left) Powder XRD patterns of (a) the pristine K0.45MnO2, (b) protonated manganate, (c) FeMn-RT, (d) FeMn450, and (e) FeMn650. In (e), the miller indices with/ without underlines denote the reflections of Fe2O3/Mn3O4 phases, respectively. (Right) Schematic model for the phase transition of Fe−layered MnO2 upon heat-treatment.
S.M. Oh et al. / Materials Letters 107 (2013) 221–224
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Fig. 2. FE-SEM images of (a) FeMn-RT, (b) FeMn450, and (c) FeMn650. (d) EDS data of FeMn-RT.
Fig. 3. (Left) Mn K-edge and (right) Fe K-edge XANES spectra for (a) FeMn-RT, (b) FeMn450, (c) FeMn650, (d) the pristine K0.45MnO2/Fe2O3, (e) Mn2O3/Fe3O4, (f) Mn3O4/FeO, and (g) MnO.
the octahedral symmetry of iron ion upon self-assembly and heattreatment. In main-edge region, there are several peaks denoted as A, B, and C related to the 1s-4p transitions [8]. The overall shape and position of these main-edge peaks are nearly identical for the present nanocomposites and Fe2O3, confirming the formation of trivalent iron oxide. Summarizing the powder XRD, FE-SEM, and
XANES results, the FeMn450 corresponds to the Fe-substituted 3+ Mn2+/3+ 3−x Fex O4 nanoparticle whereas the FeMn650 is the compo2+/3+ site material of Fe3+ O4 nanoparticles. 2 O3 and Mn3 The calcined nanocomposites are tested as anode materials for lithium ion batteries. Both FeMn450 and FeMn650 materials show similar potential profiles characteristic of 3d metal oxides [1–3]
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Fe2O3−Mn3O4 nanocomposite at 650 1C. The Fe2O3−Mn3O4 nanocomposite shows promising anode performance for lithium ion batteries, which is superior to the Mn3−xFexO4 nanocrystals, highlighting the merit of composite formation. The present findings clearly demonstrate the usefulness of the self-assembled nanocomposite of exfoliated metal oxide nanosheets as precursors for novel efficient nanocomposite electrode materials.
Supporting information TGA, TEM, and potential profiles of the composite materials.
Acknowledgments
Fig. 4. Capacity plots for (a) FeMn450 and (b) FeMn650. The open and close symbols represent the discharge and charge data, respectively.
(see Supporting information). As plotted in Fig. 4, the FeMn650 delivers large discharge capacities of 1050 mAh g−1 for the 1st cycle and 230 mAh g−1 for the 25th cycle, which are significantly larger than those of the FeMn450. The FeMn650 displays a notable capacity decrease in the initial several cycles, which is attributable to the formation of solid electrolyte interphase (SEI) layers and/or to the irreversible reaction with the electrolyte [2,3]. After initial several cycles, only a weak capacity loss occurs for the FeMn650. Conversely, continuous severe fading is observable for the FeMn450, indicating the poorer cyclability of this material than the FeMn650. The present results indicate that the nanocomposite of Fe2O3 and Mn3O4 nanocrystals show much better electrode performance than the Fe-substituted Mn3−xFexO4 nanocrystal, underscoring the beneficial effect of the composite formation. 4. Conclusions Mesoporous nanocomposite of Fe2O3−Mn3O4 is synthesized by the self-assembly of exfoliated MnO2 nanosheet with Fe ions, and the following heat-treatment. The as-prepared Fe-layered MnO2 is transformed into Fe-substituted Mn3−xFexO4 at 450 1C and
This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF2010-C1AAA001-2010-0029065) and by the Core Technology of Materials Research and Development Program of the Korea Ministry of Intelligence and Economy (Grant no. 10041232). The experiments at PAL were supported in part by MOST and POSTECH.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.05. 050.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon J-M. Nature 2000;407:496. Wang Z-H, Yuan L-X, Shao Q-G, Huang F, Huang Y-H. Mater Lett 2012;80:110. Liu Z, Tay SW. Mater Lett 2012;72:74. Bruce PG, Armstrong AR, Gitzendanner R. J Mater Chem 1999;9:193. Seong DY, Kim IY, Kim TW, Song M-S, Hwang S-J. J Phys Chem C 2011;115:13171. Omomo Y, Sasaki T, Wang LZ, Watanabe M. J Am Chem Soc 2003;125:3568. Kim IY, Lee KY, Kim TW, Hwang S-J. Meter Lett 2011;65:894. Kim TW, Ha H-W, Paek M-J, Paek IH, Hyun S-H, Choy J-H, Hwang S-J. J Phys Chem C 2008;112:14853. Kim TW, Yoo H, Kim IY, Ha H-W, Han AR, Chang J-S, Lee JS, Hwang S-J. Adv Funct Mater 2011;21:2301.