FULL PAPER DOI: 10.1002/asia.201402508
Flame Spray Pyrolysis for Finding Multicomponent Nanomaterials with Superior Electrochemical Properties in the CoOx-FeOx System for Use in Lithium-Ion Batteries Jung Hyun Kim,[a] Jong-Heun Lee,[b] and Yun Chan Kang*[b]
Abstract: High-temperature flame spray pyrolysis is employed for finding highly efficient nanomaterials for use in lithium-ion batteries. CoOx-FeOx nanopowders with various compositions are prepared by one-pot hightemperature flame spray pyrolysis. The Co and Fe components are uniformly distributed over the CoOx-FeOx composite powders, irrespective of the Co/ Fe mole ratio. The Co-rich CoOx-FeOx composite powders with Co/Fe mole ratios of 3:1 and 2:1 have mixed crystal
structures with CoFe2O4 and Co3O4 phases. However, Co-substituted magnetite composite powders prepared from spray solutions with Co and Fe components in mole ratios of 1:3, 1:2, and 1:1 have a single phase. Multicomponent CoOx-FeOx powders with a Co/ Keywords: batteries · energy storage · flame spray pyrolysis · gasphase reaction · nanostructured materials
Introduction
tact area between an electrolyte and anode material have been mainly studied.[3–10, 29, 30] In particular, electrochemical properties of the nanopowders of multicomponent transition-metal oxides prepared by liquid solution methods have been mainly investigated.[12–16] Flame spray pyrolysis, which involves gas-phase reaction processes, has been applied for the large-scale commercial production of single-component metal oxides. Flame spray pyrolysis is also advantageous for the preparation of aggregation-free multicomponent oxide nanopowders.[31–35] Nanopowders with various compositions prepared by flame spray pyrolysis have also been used as anode materials in LIBs.[36–39] However, to the best of our knowledge, the effect of the compositions of multicomponent transition-metal oxides on their electrochemical properties when used as anode materials in LIBs has not been studied. CoFe2O4 powders have been investigated as anode materials for use in LIBs because of their high theoretical capacity and stable electrochemical properties.[10] The electrochemical properties of various CoFe2O4 structures such as hollow nanospheres, mesoporous nanospheres, and nanopowders have been investigated.[9–12] These nanostructured CoFe2O4 materials have been prepared mainly by liquid solution methods such as hydrothermal, co-precipitation, sol– gel, and solvothermal processes.[10–13] However, nanostructured cobalt ferrite materials with various mole ratios of Co and Fe components have been barely studied.[9] In this study, for the first time, high-temperature flame spray pyrolysis was applied for finding highly efficient nano-
Multicomponent transition-metal oxides have been studied as possible anode materials for use in rechargeable lithiumion batteries (LIBs) because their cycling performances are better than those of the individual metal oxides.[1–26] Multicomponent transition-metal oxides are converted into nanocomposites of the respective metal oxides and Li2O immediately after the electrochemical conversion reaction.[2–9] Therefore, the crystal structure of multicomponent transition metal oxides is not the main factor influencing the electrochemical properties of their powders. The ratio of metal components present in multicomponent transition-metal oxides with a homogeneous composition strongly affects their electrochemical properties.[27, 28] In addition, the electrochemical performances of multicomponent transitionmetal oxides at high current densities are also affected by their morphologies, mean particle sizes, and specific surface areas.[1–8] Nanostructured materials supporting the large con[a] J. H. Kim Department of Chemical Engineering, Konkuk University 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701 (Korea) [b] Prof. J.-H. Lee, Prof. Y. C. Kang Department of Materials Science and Engineering Korea University Anam-Dong, Seongbuk-Gu, Seoul 136-713 (Korea) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402508.
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Fe mole ratio of 2:1 and a mixed crystal structure with Co3O4 and CoFe2O4 phases show high initial capacities and good cycling performance. The stable reversible discharge capacities of the composite powders with a Co/Fe mole ratio of 2:1 decrease from 1165 to 820 mA h g 1 as the current density is increased from 500 to 5000 mA g 1; however, the discharge capacity again increases to 1310 mA h g 1 as the current density is restored to 500 mA g 1.
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materials for use in LIB applications. CoOx-FeOx nanopowders with homogeneous compositions were directly prepared by flame spray pyrolysis from aqueous spray solutions with various mole ratios of Co and Fe components in order to find anode nanomaterials with good electrochemical properties. Further, the effect of composition of the CoOx-FeOx nanopowders on their electrochemical properties was investigated.
Results and Discussion The morphologies and elemental mapping images of the composite nanopowders of the CoOx-FeOx system at various Co/Fe mole ratios, prepared directly by flame spray pyrolysis, are shown in Figures 1 and 2. The morphologies and ele-
Figure 2. TEM and elemental mapping images of the single- and multicomponent nanopowders with Co/Fe mole ratios of (a) 0:1, (b) 1:3, (c) 1:2, and (d) 1:1.
well-faceted polymorphs including cubes, hexagonal prisms, tetragonal prisms, and polyhedra, as revealed by high-resolution TEM imaging (Figures 1 and 2). A high preparation temperature of > 2500 8C and the quenching process resulted in various metastable polymorphs.[40] The high-resolution TEM images of pure CoOx and FeOx shown in Figures 1 a and 2 a reveal clear lattice fringes separated by 0.24 and 0.48 nm, respectively, corresponding to the (111) and (111) planes of cubic-symmetry CoO and inverse spinel g-Fe2O3, respectively. The multicomponent CoOx-FeOx nanopowders also showed clear fringes separated by 0.48 nm irrespective of the Co/Fe mole ratios. The mean sizes of the nanopowders with Co/Fe mole ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 as measured from the TEM images were 98, 52, 66, 66, 67, 72, and 80 nm, respectively. The multicomponent CoOxFeOx nanopowders had smaller mean particle sizes than the pure CoOx and FeOx nanopowders. The Brunauer–Emmett– Teller (BET) specific surface areas of the CoOx-FeOx composite nanopowders with Co/Fe mole ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 were determined to be 9, 29, 21, 19, 18, 17, and 16 m2 g 1, respectively. When two solid phases are mixed, each phase prevents the crystal growth in the other phase. Therefore, the CoOx-FeOx composite nanopowders had larger specific surface areas and smaller mean particle sizes than single-component CoOx and FeOx nanopowders. The elemental mapping images shown in Figure 1 and 2 reveal that the Co and Fe components were uniformly distributed over the CoOx-FeOx composite nanopowders, irrespective of the Co/Fe mole ratios. The compositions of the nanosized CoOx-FeOx composite nanopowders were analyzed by inductively coupled plasma–optical emission spectrometry (ICP-OES), and the results are listed in Table S1
Figure 1. TEM and elemental mapping images of the single- and multicomponent nanopowders with Co/Fe mole ratios of (a) 1:0, (b) 3:1, and (c) 2:1.
mental mapping images of pure CoOx and FeOx powders are also shown in Figure 1 a and 2 a, respectively. As shown in the low-resolution TEM images, the single- and multicomponent CoOx-FeOx powders were composed of nanosized particles and displayed non-aggregation characteristics. Submicro- or microsized powders formed directly from droplets several microns in size were not observed. The drying and decomposition of a droplet inside the high-temperature diffusion flame formed microsized composite powder consisting of Co and Fe components. The Co and Fe components completely evaporated in the high-temperature diffusion flame to form vapors of cobalt and iron oxides. Nanocomposite oxide powders of Co and Fe components were formed from these vapors by nucleation and growth processes. The immediate reaction and crystallization of cobalt and iron oxides formed crystalline CoOx-FeOx system composite powders even for a short residence time of the powders in the high-temperature diffusion flame. The single- and multicomponent CoOx-FeOx nanopowders had
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in the Supporting Information. The Co/Fe mole ratios of the spray solutions were well maintained to be nearly equal to those of the nanosized CoOx-FeOx composite nanopowders directly prepared by flame spray pyrolysis. Figure 3 shows the XRD patterns of the single- and multicomponent CoOx-FeOx nanopowders prepared directly by flame spray pyrolysis. The nanopowders prepared from the spray solution with a Co component had a main crystal
Figure 3. XRD patterns of the single- and multicomponent nanopowders with different Co/Fe mole ratios prepared by flame spray pyrolysis.
structure of cubic-symmetry CoO with small impurity peaks corresponding to cubic spinel Co3O4. The high-temperature diffusion flame resulted in the CoO phase. The Co-rich CoOx-FeOx composite nanopowders with Co/Fe mole ratios of 3:1 and 2:1 had mixed crystal structures with CoFe2O4 and Co3O4 phases. Addition of the Fe component changed the crystal structure of cobalt oxide from cubic-symmetry CoO to cubic spinel Co3O4. However, the CoOx-FeOx composite nanopowders with Co/Fe mole ratios of 1:3, 1:2, and 1:1 had crystal structures similar to that of pure maghemite g-Fe2O3. Co-substituted magnetite CoOx-FeOx composite nanopowders with a single phase were prepared from the spray solutions of Co and Fe components with Co/Fe mole ratios of 1:3, 1:2, and 1:1. The electrochemical properties of the single- and multicomponent CoOx-FeOx nanopowders with different Co/Fe mole ratios prepared directly by flame spray pyrolysis are shown in Figure 4. Figure 4 a shows the initial charge and discharge profiles of the nanopowders in the range 0.01–3 V at a constant charge/dicharge rate of 1000 mA g 1. The initial discharge curve of the single-component CoOx nanopowders showed mainly two plateaus at around 0.97 and 0.55 V, respectively, corresponding to lithium storage in CoO and the conversion reaction between CoO and Li.[41, 42] Lithium insertion into the Co3O4 nanostructure by the reduction of Co3O4 to Co metal and the formation of Li2O also contributed to the plateau formation at around 0.97 V.[41, 43] The composite nanopowders with mixed crystal structures of Co3O4 and CoFe2O4 showed small plateaus at around 0.97 V owing to the reduction of Co3O4 to Co metal and Li2O. The initial discharge curve of the single g-Fe2O3 nanopowders showed two plateaus at 0.86 and 0.76 V, which
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Figure 4. Electrochemical properties of the single- and multicomponent nanopowders with different Co/Fe mole ratios prepared directly by flame spray pyrolysis: (a) Initial charge/discharge curves; (b) Cycling properties at a current density of 1000 mA g 1; and (c) Rate performances.
could be attributed to Li insertion into the structure, the reduction of Fe3 + to Fe0, and the formation of amorphous Li2O.[44] However, the initial discharge curves of the Co-substituted magnetite CoOx-FeOx composite nanopowders with a single phase and Co/Fe mole ratios of 1:3, 1:2, and 1:1 showed one main plateau at around 0.69 V, which was attributed to the reduction of CoFe2O4 to nanograins of Co and Fe metals and Li2O.[10] Figure S1 in the Supporting Information shows the cyclic voltammograms (CVs) of the single- and multicomponent nanopowders with different Co/ Fe mole ratios. The CV curves are in good agreement with the initial cycle profiles of the CoOx-FeOx nanopowders with different Co/Fe molar ratios shown in Figure 4 a. The multicomponent CoOx-FeOx nanopowders showed large cathodic peaks located at around 0.6 V, which is attributed to reduction reactions of Fe3 + and Co2 + with Li and the for-
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mation of Li2O.[11] The initial discharge capacities of the CoOx-FeOx nanopowders with Co/Fe molar ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 were 1020, 1261, 1320, 1240, 1198, 1158, and 1279 mA h g 1, respectively, and the corresponding charge capacities were 792, 983, 1026, 973, 950, 923, and 999 mA h g 1, respectively. The initial Coulombic efficiencies of the CoOx-FeOx nanopowders had similar values regardless of their compositions. Figure 4 b shows the cycling performances of the CoOxFeOx nanopowders with different Co/Fe molar ratios at a constant charge/discharge rate of 1000 mA g 1. The discharge capacities of the CoOx-FeOx nanopowders with Co/ Fe molar ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 after 120 cycles were 838, 649, 1101, 485, 227, 130, and 91 mA h g 1, respectively, and the corresponding capacity retentions were 82, 51, 83, 39, 19, 11, and 7 %, respectively. The value of the discharge capacity of pure g-Fe2O3 nanopowders was low (91 mA h g 1 after 120 cycles) because of large volume expansion/contraction during the cycling of gFe2O3.[45] On the other hand, the single-component CoOx nanopowders with a mixed crystal structure of CoO and Co3O4 phases showed better cycling performance than the multicomponent CoOx-FeOx nanopowders except for the sample with a Co/Fe molar ratio of 2:1. The multicomponent CoOx-FeOx nanopowders with a Co/Fe molar ratio of 2:1 and a mixed crystal structure of Co3O4 and CoFe2O4 phases showed the best cycling performance. The structural stability of the transition metal oxides with large volume changes during cycling strongly affected the cycling performances of the powders. Therefore, the structural stabilities of the CoOx, g-Fe2O3, and CoOx-FeOx system with a Co/Fe molar of 2:1 during cycling were evaluated by electrochemical impedance spectroscopy (EIS) measurements, as shown in Figures S2 and S3 in the Supporting information. Impedance measurements were carried out at room temperature on cells after 1 and 100 cycles, in the potential range 0.01– 3.0 V at a current density of 1000 mA g 1. The Nyquist plots of the electrodes obtained after the first cycles (Figure S2, Supporting Information) were composed of a semicircle in the medium-frequency region and an inclined line in the low-frequency region. The medium-frequency semicircle was assigned to the charge-transfer resistance (Rct), and the line inclined at ~ 458 to the real axis corresponded to the lithium diffusion process within the electrodes.[46, 47] Figure S4 in the Supporting Information shows the relationship between the real part of the impedance spectra (Zre) and w 1/2 (where w is the angular frequency in the low-frequency region and is given by w = 2pf) in the low-frequency region and after the first cycles. The charge-transfer resistances obtained after first cycles for the CoOx, g-Fe2O3, and CoOx-FeOx (Co/Fe = 2:1) nanopowders were similar. However, the high slope (s is the Warburg impedance coefficient) of the real part of Zre versus w 1/2 for the CoOx nanopowders indicates a lower lithium ion diffusion rate than those for the g-Fe2O3 and CoOx-FeOx nanopowders. Therefore, the CoOx nanopowders had lower initial discharge and charge capacities at a high charge/discharge rate of 1000 mA g 1 (Figure 4 b).
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The Nyquist plots of the electrodes obtained after 100 cycles are shown in Figure S3 in the Supporting Information. After 100 cycles, the charge-transfer resistances of the CoOx and CoOx-FeOx nanopowders were smaller than that of the bare g-Fe2O3 nanopowders. The high structural stabilities of the CoOx and CoOx-FeOx nanopowders with complex crystal structures resulted in excellent electrochemical properties. On the other hand, the structural degradation of the pure gFe2O3 nanopowders during cycling significantly decreased their discharge capacities from 1279 to 257 mA h g 1 after 60 cycles, as shown in Figure 4 b. Rate performances of the CoOx and CoOx-FeOx (Co/Fe = 2:1) nanopowders are shown in Figure 4 c. The current density was increased from 500 to 5000 mA g 1 in a stepwise manner and was restored to 500 mA g 1. In each step, the measurements were conducted for 10 cycles to evaluate the rate performance. Figure 4 c clearly shows that the capacity gap between the CoOx and CoOx-FeOx (Co/Fe = 2:1) nanopowders widened with increasing current density. The stable reversible discharge capacities of the composite nanopowders with a Co/Fe mole ratio of 2:1 decreased from 1165 to 820 mA h g 1 as the current density was increased from 500 to 5000 mA g 1; furthermore, the discharge capacity increased to 1310 mA h g 1 as the current density was restored to 500 mA g 1. However, the discharge capacities of the CoOx nanopowders decreased from 946 to 242 mA g 1 as the current density was increased from 500 to 5000 mA g 1. The CoOx-FeOx (Co/Fe = 2:1) nanopowders showed better rate performances than the single-component CoOx nanopowders.
Conclusions Single- and multicomponent CoOx-FeOx nanopowders were prepared by flame spray pyrolysis from aqueous spray solutions with various mole ratios of Co and Fe components in order to find anode nanomaterials with good electrochemical properties. The prepared powders had nanoscale sizes and non-aggregation characteristics irrespective of their composition. However, the composition of the nanopowders strongly affected their electrochemical properties. The single-component CoOx nanopowders having mixed crystal structures with CoO and Co3O4 phases and multicomponent CoOx-FeOx nanopowders having a mixed crystal structure with Co3O4 and CoFe2O4 phases showed good cycling performances. The high structural stabilities of the CoOx and CoOx-FeOx nanopowders with complex crystal structures resulted in excellent electrochemical properties. In particular, the CoOx-FeOx (Co/Fe = 2:1) nanopowders showed high initial capacities and good cycling and rate performances. Flame spray pyrolysis was effective in identifying the compositions of transition metal oxide nanopowders that would result in good electrochemical properties for use of the powders as anode materials in LIBs.
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Experimental Section Synthesis of Materials The flame spray pyrolysis system used in this study had a droplet generator, a flame nozzle, a quartz reactor, a powder collector, and a blower. A 1.7 MHz ultrasonic spray generator with six resonators was used to generate droplets that were then carried into a high-temperature diffusion flame by oxygen (carrier gas).[36] Propane (fuel) and oxygen (oxidizer) were used to generate the diffusion flame. The flow rate of the fuel gas was 5 L min 1, and those of the oxidizer and carrier gases were 40 and 10 L min 1, respectively. The mixed solvent comprised EtOH and water in a volume ratio of 1:3. The starting materials used for the synthesis were CoACHTUNGRE(NO3)2·6 H2O and FeACHTUNGRE(NO3)3·9 H2O. The overall concentration of cobalt and iron components was 0.5 m. Characterizations The crystal structures of the powders were investigated by X-ray diffractometry (XRD, Rigaku DMAX-33) using CuKa radiation. The morphological characteristics of the powders were investigated using transmission electron microscopy (TEM, JEOL, JEM-2010). Elemental compositions of the powders were investigated using an inductively coupled plasma-optical emission spectrometer (ICP-OES, PerkinElmer, OPTIMA 4300 DV). Surface areas of the powders were measured by the Brunauer–Emmett–Teller (BET) method with N2 used as the adsorbate gas. Electrochemical Measurements The electrode was fabricated using a mixture of 70 wt % CoOx-FeOx composite powders, 20 wt % carbon black (Super-P) as a conductive material, and 10 wt % sodium carboxymethyl cellulose (CMC) as a binder. Lithium metal and a microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte used was 1 m LiPF6 dissolved in a mixture of fluoroethylene carbonate/dimethyl carbonate (FEC/DMC) in a volume ratio of 1:1 (Techno Semichem Co.). The entire cell was assembled in a glove box in an argon atmosphere. The charge/discharge characteristics of the samples were determined by cycling in the potential range of 0.01–3.0 V at various current densities. Cyclic voltammetry measurements were carried out at a scan rate of 0.1 mV s 1 in the range 0.01–3.0 V. Electrochemical impedance spectroscopy measurements of the electrodes were acquired in the frequency range between 0.01 Hz and 100 kHz at room temperature.
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Flame Spray Pyrolysis for Finding Multicomponent Nanomaterials with Superior Electrochemical Properties in the CoOx-FeOx System for Use in Lithium-Ion Batteries Jung Hyun Kim,[a] Jong-Heun Lee,[b] and Yun Chan Kang*[b]
Abstract: High-temperature flame spray pyrolysis is employed for finding highly efficient nanomaterials for use in lithium-ion batteries. CoOx-FeOx nanopowders with various compositions are prepared by one-pot hightemperature flame spray pyrolysis. The Co and Fe components are uniformly distributed over the CoOx-FeOx composite powders, irrespective of the Co/ Fe mole ratio. The Co-rich CoOx-FeOx composite powders with Co/Fe mole ratios of 3:1 and 2:1 have mixed crystal
structures with CoFe2O4 and Co3O4 phases. However, Co-substituted magnetite composite powders prepared from spray solutions with Co and Fe components in mole ratios of 1:3, 1:2, and 1:1 have a single phase. Multicomponent CoOx-FeOx powders with a Co/ Keywords: batteries · energy storage · flame spray pyrolysis · gasphase reaction · nanostructured materials
Introduction
tact area between an electrolyte and anode material have been mainly studied.[3–10, 29, 30] In particular, electrochemical properties of the nanopowders of multicomponent transition-metal oxides prepared by liquid solution methods have been mainly investigated.[12–16] Flame spray pyrolysis, which involves gas-phase reaction processes, has been applied for the large-scale commercial production of single-component metal oxides. Flame spray pyrolysis is also advantageous for the preparation of aggregation-free multicomponent oxide nanopowders.[31–35] Nanopowders with various compositions prepared by flame spray pyrolysis have also been used as anode materials in LIBs.[36–39] However, to the best of our knowledge, the effect of the compositions of multicomponent transition-metal oxides on their electrochemical properties when used as anode materials in LIBs has not been studied. CoFe2O4 powders have been investigated as anode materials for use in LIBs because of their high theoretical capacity and stable electrochemical properties.[10] The electrochemical properties of various CoFe2O4 structures such as hollow nanospheres, mesoporous nanospheres, and nanopowders have been investigated.[9–12] These nanostructured CoFe2O4 materials have been prepared mainly by liquid solution methods such as hydrothermal, co-precipitation, sol– gel, and solvothermal processes.[10–13] However, nanostructured cobalt ferrite materials with various mole ratios of Co and Fe components have been barely studied.[9] In this study, for the first time, high-temperature flame spray pyrolysis was applied for finding highly efficient nano-
Multicomponent transition-metal oxides have been studied as possible anode materials for use in rechargeable lithiumion batteries (LIBs) because their cycling performances are better than those of the individual metal oxides.[1–26] Multicomponent transition-metal oxides are converted into nanocomposites of the respective metal oxides and Li2O immediately after the electrochemical conversion reaction.[2–9] Therefore, the crystal structure of multicomponent transition metal oxides is not the main factor influencing the electrochemical properties of their powders. The ratio of metal components present in multicomponent transition-metal oxides with a homogeneous composition strongly affects their electrochemical properties.[27, 28] In addition, the electrochemical performances of multicomponent transitionmetal oxides at high current densities are also affected by their morphologies, mean particle sizes, and specific surface areas.[1–8] Nanostructured materials supporting the large con[a] J. H. Kim Department of Chemical Engineering, Konkuk University 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701 (Korea) [b] Prof. J.-H. Lee, Prof. Y. C. Kang Department of Materials Science and Engineering Korea University Anam-Dong, Seongbuk-Gu, Seoul 136-713 (Korea) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402508.
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Fe mole ratio of 2:1 and a mixed crystal structure with Co3O4 and CoFe2O4 phases show high initial capacities and good cycling performance. The stable reversible discharge capacities of the composite powders with a Co/Fe mole ratio of 2:1 decrease from 1165 to 820 mA h g 1 as the current density is increased from 500 to 5000 mA g 1; however, the discharge capacity again increases to 1310 mA h g 1 as the current density is restored to 500 mA g 1.
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materials for use in LIB applications. CoOx-FeOx nanopowders with homogeneous compositions were directly prepared by flame spray pyrolysis from aqueous spray solutions with various mole ratios of Co and Fe components in order to find anode nanomaterials with good electrochemical properties. Further, the effect of composition of the CoOx-FeOx nanopowders on their electrochemical properties was investigated.
Results and Discussion The morphologies and elemental mapping images of the composite nanopowders of the CoOx-FeOx system at various Co/Fe mole ratios, prepared directly by flame spray pyrolysis, are shown in Figures 1 and 2. The morphologies and ele-
Figure 2. TEM and elemental mapping images of the single- and multicomponent nanopowders with Co/Fe mole ratios of (a) 0:1, (b) 1:3, (c) 1:2, and (d) 1:1.
well-faceted polymorphs including cubes, hexagonal prisms, tetragonal prisms, and polyhedra, as revealed by high-resolution TEM imaging (Figures 1 and 2). A high preparation temperature of > 2500 8C and the quenching process resulted in various metastable polymorphs.[40] The high-resolution TEM images of pure CoOx and FeOx shown in Figures 1 a and 2 a reveal clear lattice fringes separated by 0.24 and 0.48 nm, respectively, corresponding to the (111) and (111) planes of cubic-symmetry CoO and inverse spinel g-Fe2O3, respectively. The multicomponent CoOx-FeOx nanopowders also showed clear fringes separated by 0.48 nm irrespective of the Co/Fe mole ratios. The mean sizes of the nanopowders with Co/Fe mole ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 as measured from the TEM images were 98, 52, 66, 66, 67, 72, and 80 nm, respectively. The multicomponent CoOxFeOx nanopowders had smaller mean particle sizes than the pure CoOx and FeOx nanopowders. The Brunauer–Emmett– Teller (BET) specific surface areas of the CoOx-FeOx composite nanopowders with Co/Fe mole ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 were determined to be 9, 29, 21, 19, 18, 17, and 16 m2 g 1, respectively. When two solid phases are mixed, each phase prevents the crystal growth in the other phase. Therefore, the CoOx-FeOx composite nanopowders had larger specific surface areas and smaller mean particle sizes than single-component CoOx and FeOx nanopowders. The elemental mapping images shown in Figure 1 and 2 reveal that the Co and Fe components were uniformly distributed over the CoOx-FeOx composite nanopowders, irrespective of the Co/Fe mole ratios. The compositions of the nanosized CoOx-FeOx composite nanopowders were analyzed by inductively coupled plasma–optical emission spectrometry (ICP-OES), and the results are listed in Table S1
Figure 1. TEM and elemental mapping images of the single- and multicomponent nanopowders with Co/Fe mole ratios of (a) 1:0, (b) 3:1, and (c) 2:1.
mental mapping images of pure CoOx and FeOx powders are also shown in Figure 1 a and 2 a, respectively. As shown in the low-resolution TEM images, the single- and multicomponent CoOx-FeOx powders were composed of nanosized particles and displayed non-aggregation characteristics. Submicro- or microsized powders formed directly from droplets several microns in size were not observed. The drying and decomposition of a droplet inside the high-temperature diffusion flame formed microsized composite powder consisting of Co and Fe components. The Co and Fe components completely evaporated in the high-temperature diffusion flame to form vapors of cobalt and iron oxides. Nanocomposite oxide powders of Co and Fe components were formed from these vapors by nucleation and growth processes. The immediate reaction and crystallization of cobalt and iron oxides formed crystalline CoOx-FeOx system composite powders even for a short residence time of the powders in the high-temperature diffusion flame. The single- and multicomponent CoOx-FeOx nanopowders had
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in the Supporting Information. The Co/Fe mole ratios of the spray solutions were well maintained to be nearly equal to those of the nanosized CoOx-FeOx composite nanopowders directly prepared by flame spray pyrolysis. Figure 3 shows the XRD patterns of the single- and multicomponent CoOx-FeOx nanopowders prepared directly by flame spray pyrolysis. The nanopowders prepared from the spray solution with a Co component had a main crystal
Figure 3. XRD patterns of the single- and multicomponent nanopowders with different Co/Fe mole ratios prepared by flame spray pyrolysis.
structure of cubic-symmetry CoO with small impurity peaks corresponding to cubic spinel Co3O4. The high-temperature diffusion flame resulted in the CoO phase. The Co-rich CoOx-FeOx composite nanopowders with Co/Fe mole ratios of 3:1 and 2:1 had mixed crystal structures with CoFe2O4 and Co3O4 phases. Addition of the Fe component changed the crystal structure of cobalt oxide from cubic-symmetry CoO to cubic spinel Co3O4. However, the CoOx-FeOx composite nanopowders with Co/Fe mole ratios of 1:3, 1:2, and 1:1 had crystal structures similar to that of pure maghemite g-Fe2O3. Co-substituted magnetite CoOx-FeOx composite nanopowders with a single phase were prepared from the spray solutions of Co and Fe components with Co/Fe mole ratios of 1:3, 1:2, and 1:1. The electrochemical properties of the single- and multicomponent CoOx-FeOx nanopowders with different Co/Fe mole ratios prepared directly by flame spray pyrolysis are shown in Figure 4. Figure 4 a shows the initial charge and discharge profiles of the nanopowders in the range 0.01–3 V at a constant charge/dicharge rate of 1000 mA g 1. The initial discharge curve of the single-component CoOx nanopowders showed mainly two plateaus at around 0.97 and 0.55 V, respectively, corresponding to lithium storage in CoO and the conversion reaction between CoO and Li.[41, 42] Lithium insertion into the Co3O4 nanostructure by the reduction of Co3O4 to Co metal and the formation of Li2O also contributed to the plateau formation at around 0.97 V.[41, 43] The composite nanopowders with mixed crystal structures of Co3O4 and CoFe2O4 showed small plateaus at around 0.97 V owing to the reduction of Co3O4 to Co metal and Li2O. The initial discharge curve of the single g-Fe2O3 nanopowders showed two plateaus at 0.86 and 0.76 V, which
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Figure 4. Electrochemical properties of the single- and multicomponent nanopowders with different Co/Fe mole ratios prepared directly by flame spray pyrolysis: (a) Initial charge/discharge curves; (b) Cycling properties at a current density of 1000 mA g 1; and (c) Rate performances.
could be attributed to Li insertion into the structure, the reduction of Fe3 + to Fe0, and the formation of amorphous Li2O.[44] However, the initial discharge curves of the Co-substituted magnetite CoOx-FeOx composite nanopowders with a single phase and Co/Fe mole ratios of 1:3, 1:2, and 1:1 showed one main plateau at around 0.69 V, which was attributed to the reduction of CoFe2O4 to nanograins of Co and Fe metals and Li2O.[10] Figure S1 in the Supporting Information shows the cyclic voltammograms (CVs) of the single- and multicomponent nanopowders with different Co/ Fe mole ratios. The CV curves are in good agreement with the initial cycle profiles of the CoOx-FeOx nanopowders with different Co/Fe molar ratios shown in Figure 4 a. The multicomponent CoOx-FeOx nanopowders showed large cathodic peaks located at around 0.6 V, which is attributed to reduction reactions of Fe3 + and Co2 + with Li and the for-
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mation of Li2O.[11] The initial discharge capacities of the CoOx-FeOx nanopowders with Co/Fe molar ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 were 1020, 1261, 1320, 1240, 1198, 1158, and 1279 mA h g 1, respectively, and the corresponding charge capacities were 792, 983, 1026, 973, 950, 923, and 999 mA h g 1, respectively. The initial Coulombic efficiencies of the CoOx-FeOx nanopowders had similar values regardless of their compositions. Figure 4 b shows the cycling performances of the CoOxFeOx nanopowders with different Co/Fe molar ratios at a constant charge/discharge rate of 1000 mA g 1. The discharge capacities of the CoOx-FeOx nanopowders with Co/ Fe molar ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 after 120 cycles were 838, 649, 1101, 485, 227, 130, and 91 mA h g 1, respectively, and the corresponding capacity retentions were 82, 51, 83, 39, 19, 11, and 7 %, respectively. The value of the discharge capacity of pure g-Fe2O3 nanopowders was low (91 mA h g 1 after 120 cycles) because of large volume expansion/contraction during the cycling of gFe2O3.[45] On the other hand, the single-component CoOx nanopowders with a mixed crystal structure of CoO and Co3O4 phases showed better cycling performance than the multicomponent CoOx-FeOx nanopowders except for the sample with a Co/Fe molar ratio of 2:1. The multicomponent CoOx-FeOx nanopowders with a Co/Fe molar ratio of 2:1 and a mixed crystal structure of Co3O4 and CoFe2O4 phases showed the best cycling performance. The structural stability of the transition metal oxides with large volume changes during cycling strongly affected the cycling performances of the powders. Therefore, the structural stabilities of the CoOx, g-Fe2O3, and CoOx-FeOx system with a Co/Fe molar of 2:1 during cycling were evaluated by electrochemical impedance spectroscopy (EIS) measurements, as shown in Figures S2 and S3 in the Supporting information. Impedance measurements were carried out at room temperature on cells after 1 and 100 cycles, in the potential range 0.01– 3.0 V at a current density of 1000 mA g 1. The Nyquist plots of the electrodes obtained after the first cycles (Figure S2, Supporting Information) were composed of a semicircle in the medium-frequency region and an inclined line in the low-frequency region. The medium-frequency semicircle was assigned to the charge-transfer resistance (Rct), and the line inclined at ~ 458 to the real axis corresponded to the lithium diffusion process within the electrodes.[46, 47] Figure S4 in the Supporting Information shows the relationship between the real part of the impedance spectra (Zre) and w 1/2 (where w is the angular frequency in the low-frequency region and is given by w = 2pf) in the low-frequency region and after the first cycles. The charge-transfer resistances obtained after first cycles for the CoOx, g-Fe2O3, and CoOx-FeOx (Co/Fe = 2:1) nanopowders were similar. However, the high slope (s is the Warburg impedance coefficient) of the real part of Zre versus w 1/2 for the CoOx nanopowders indicates a lower lithium ion diffusion rate than those for the g-Fe2O3 and CoOx-FeOx nanopowders. Therefore, the CoOx nanopowders had lower initial discharge and charge capacities at a high charge/discharge rate of 1000 mA g 1 (Figure 4 b).
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The Nyquist plots of the electrodes obtained after 100 cycles are shown in Figure S3 in the Supporting Information. After 100 cycles, the charge-transfer resistances of the CoOx and CoOx-FeOx nanopowders were smaller than that of the bare g-Fe2O3 nanopowders. The high structural stabilities of the CoOx and CoOx-FeOx nanopowders with complex crystal structures resulted in excellent electrochemical properties. On the other hand, the structural degradation of the pure gFe2O3 nanopowders during cycling significantly decreased their discharge capacities from 1279 to 257 mA h g 1 after 60 cycles, as shown in Figure 4 b. Rate performances of the CoOx and CoOx-FeOx (Co/Fe = 2:1) nanopowders are shown in Figure 4 c. The current density was increased from 500 to 5000 mA g 1 in a stepwise manner and was restored to 500 mA g 1. In each step, the measurements were conducted for 10 cycles to evaluate the rate performance. Figure 4 c clearly shows that the capacity gap between the CoOx and CoOx-FeOx (Co/Fe = 2:1) nanopowders widened with increasing current density. The stable reversible discharge capacities of the composite nanopowders with a Co/Fe mole ratio of 2:1 decreased from 1165 to 820 mA h g 1 as the current density was increased from 500 to 5000 mA g 1; furthermore, the discharge capacity increased to 1310 mA h g 1 as the current density was restored to 500 mA g 1. However, the discharge capacities of the CoOx nanopowders decreased from 946 to 242 mA g 1 as the current density was increased from 500 to 5000 mA g 1. The CoOx-FeOx (Co/Fe = 2:1) nanopowders showed better rate performances than the single-component CoOx nanopowders.
Conclusions Single- and multicomponent CoOx-FeOx nanopowders were prepared by flame spray pyrolysis from aqueous spray solutions with various mole ratios of Co and Fe components in order to find anode nanomaterials with good electrochemical properties. The prepared powders had nanoscale sizes and non-aggregation characteristics irrespective of their composition. However, the composition of the nanopowders strongly affected their electrochemical properties. The single-component CoOx nanopowders having mixed crystal structures with CoO and Co3O4 phases and multicomponent CoOx-FeOx nanopowders having a mixed crystal structure with Co3O4 and CoFe2O4 phases showed good cycling performances. The high structural stabilities of the CoOx and CoOx-FeOx nanopowders with complex crystal structures resulted in excellent electrochemical properties. In particular, the CoOx-FeOx (Co/Fe = 2:1) nanopowders showed high initial capacities and good cycling and rate performances. Flame spray pyrolysis was effective in identifying the compositions of transition metal oxide nanopowders that would result in good electrochemical properties for use of the powders as anode materials in LIBs.
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Experimental Section Synthesis of Materials The flame spray pyrolysis system used in this study had a droplet generator, a flame nozzle, a quartz reactor, a powder collector, and a blower. A 1.7 MHz ultrasonic spray generator with six resonators was used to generate droplets that were then carried into a high-temperature diffusion flame by oxygen (carrier gas).[36] Propane (fuel) and oxygen (oxidizer) were used to generate the diffusion flame. The flow rate of the fuel gas was 5 L min 1, and those of the oxidizer and carrier gases were 40 and 10 L min 1, respectively. The mixed solvent comprised EtOH and water in a volume ratio of 1:3. The starting materials used for the synthesis were CoACHTUNGRE(NO3)2·6 H2O and FeACHTUNGRE(NO3)3·9 H2O. The overall concentration of cobalt and iron components was 0.5 m. Characterizations The crystal structures of the powders were investigated by X-ray diffractometry (XRD, Rigaku DMAX-33) using CuKa radiation. The morphological characteristics of the powders were investigated using transmission electron microscopy (TEM, JEOL, JEM-2010). Elemental compositions of the powders were investigated using an inductively coupled plasma-optical emission spectrometer (ICP-OES, PerkinElmer, OPTIMA 4300 DV). Surface areas of the powders were measured by the Brunauer–Emmett–Teller (BET) method with N2 used as the adsorbate gas. Electrochemical Measurements The electrode was fabricated using a mixture of 70 wt % CoOx-FeOx composite powders, 20 wt % carbon black (Super-P) as a conductive material, and 10 wt % sodium carboxymethyl cellulose (CMC) as a binder. Lithium metal and a microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte used was 1 m LiPF6 dissolved in a mixture of fluoroethylene carbonate/dimethyl carbonate (FEC/DMC) in a volume ratio of 1:1 (Techno Semichem Co.). The entire cell was assembled in a glove box in an argon atmosphere. The charge/discharge characteristics of the samples were determined by cycling in the potential range of 0.01–3.0 V at various current densities. Cyclic voltammetry measurements were carried out at a scan rate of 0.1 mV s 1 in the range 0.01–3.0 V. Electrochemical impedance spectroscopy measurements of the electrodes were acquired in the frequency range between 0.01 Hz and 100 kHz at room temperature.
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