Fabrication of Nano/Micro Hierarchical Fe₂O₃/Ni Micrometer-Wire Structure and Characteristics for High Rate Li Rechargeable Battery

The nano/micro hierarchical Fe₂O₃/Ni micrometer wire, which is a low-cost material, as a negative electrode for Li-ion cells with the high specific capacity at high charge/discharge current rate, was fabricated. Nanocrystalline and mesoporous Fe₂O₃ film was formed on nickel mesh, knitted of nickel micrometer wires, via pyrolytic transformation of FeO(OH)_0.29(NO₃)_0.27(CO₃)_0.22·0.6H₂O film, which was directly deposited on the entire surface of the nickel mesh by chemical bath deposition. The nestlike morphology with nanoflake of iron oxyhydroxide was maintained after the pyrolysis reaction into Fe₂O₃. Moreover, each nanoflake was constructed by several nanometers particles. This specific hierarchical morphology not only provides ideal electrolyte, lithium ion paths, and electronic paths, but it also reduces both the required diffusion length in the active materials and the effective specific current density. The resultant Fe₂O₃ negative electrode, which gives a high specific charge/discharge capacity 780  mAh/g with good cycle performance even in a high charge/discharge current rate of 13  A/g, indicates the possibility for an energy storage device with high energy density at high power density. ©2006 The Electrochemical Society

Contents

• BODY OF ARTICLE
• Experimental
• Results and Discussion
• § 4
• REFERENCES
• FIGURES
• TABLES
• FOOTNOTES

Lithium-ion storage devices have been widely studied for practice application in electric devices, especially for mobile or portable electric devices and electric vehicle (EV) or hybrid EV (HEV). For these industrials needs, the development of electrodes with high specific capacities (=high energy density) and high current densities (=high power density) is necessary. Therefore, the fabrication of materials with high specific capacity at high charge/discharge current rate has been expected.

The advantage of the lithium-ion secondary battery as an energy storage device is its high specific capacity. Poizot et al. reported materials such as nanocrystalline transition metal oxides such as NiO, CoO, and FeO for negative electrodes, which indicate the high specific capacity of around 700  mAh/g at the low rate of 0.07  A/g.¹ Many researchers have studied the transition metal oxide cells.^{2,3,4,5,6,7,8,9,10,11,12,13,14} However, the weak points of the lithium-ion battery, namely, the low power density, has not been overcome.

Electrodes using transition metal oxide with high specific capacity at high charge/discharge current rate have not been reported. There are four problems due to the nature of a lithium-ion battery that should be solved in order to apply a high-rate lithium storage device^15,16,17: (i) Increasing the electronic conductivity of the electrode materials, (ii) decreasing the particle size to reduce the required diffusion length in the active materials, (iii) reducing the effective specific current density in the rapid charge-discharge process, and (iv) realizing high cycle performance even at rapid charge-discharge process.

In order to solve these problems, general processes such as pressing of the powder, physical vapor deposition, and chemical vapor deposition are not suitable because nanostructure control is difficult in these processes. Synthetic processes for nanostructured materials have been developed through the bottom-up process by a chemical bath deposition (CBD),¹⁸ which can control the real nanosize in several nanometers.¹⁹ The resultant nanoparticulate porous materials apply for electrochemical devices making good use of improved and accelerated charge transport phenomena due to high surface areas and high porosity.²⁰

The CBD, in which water is typically used as a solvent, has been utilized for preparing various kinds of metal oxides and sulfides because thin-film materials can be fabricated at low temperatures without expensive and special apparatus required for vapor-phase techniques.^18,21 It is suitable as a low-cost and low-energy consumption process for the environmental problem. Moreover, CBD can deposit the materials on the substrate with complicated morphology due to the reaction in solution. However, the fabrication of metal oxide films that are both nanocrystalline and nanoporous by general CBD was difficult because most metal oxides grow as single crystals.

We have reported the self-template method by CBD.^20,22,23 The deposited metal hydroxide nanosheets instead of metal oxide are easily converted into nanocrystalline and nanoporous metal oxides by pyrolysis as a self-template without nanostructural deformation.^20,22,23

Here, we report the nanostructured Fe₂O₃ on nickel mesh, which are low-cost materials, as negative electrodes for Li-ion cells with the high specific capacity at high charge/discharge current rate to solve the above-mentioned problems. Nanocrystalline and mesoporous Fe₂O₃ film with specific nanostructure was formed on the nickel mesh, knitted nickel micrometer wires, via pyrolytic transformation of iron oxyhydroxide film, which was directly deposited on the entire surface of the nickel mesh by the CBD. The frameworks of the nickel mesh and mesopores of Fe₂O₃ film filled with electrolyte provide ideal electrolyte and lithium ion paths. The nickel wires with low resistivity play a role of electronic path. Moreover, the high surface area of the Fe₂O₃ film reduces the required diffusion length in the active materials and the effective specific current density. The resultant Fe₂O₃ negative electrode shows the high specific capacity at high charge/discharge current rate, which indicates the possibility for an energy storage device with high energy density at high power density.

A solution for the preparation of iron oxyhydroxide was prepared by dissolving Fe(NO₃)₃·4H₂O and urea [(NH₂)₂CO] in methanol/water mixed solution. The 5 g of urea was dissolved into the methanol (20  mL)/water (5  mL) mixed solution. A concentration of metal ions was adjusted to 0.15  mol/dm³. The nickel meshes (200  mesh, diameter: 50  µm) of around 50×30  mm were folded to a size of around 10×10  mm. A part of the mesh was left as a supporting electrode of metal oxides without folding. The upper side of the meshes was covered with aluminum foil to protect against theprecipitation of the powder formed by homogeneous nucleation. The meshes were put into bottles filled with the solutions and sealed up, and were kept at 60°C in a drying oven. After the deposition, the films were rinsed with ethanol and dried at room temperature, then transformed into metal oxides by heating at 300°C for 10  min in air.

Electrochemical measurements were carried out in a three-electrode setup in the twin beaker cell connected with microcapillary working as a separator. The fabricated metal oxide film on the mesh was used as a working electrode. The reference and counter electrodes were prepared by spreading and pressing of lithium metals on nickel mesh (100  mesh). A 1  mol  dm^–3 LiClO₄ in propylene carbonate (PC) was used as an electrolyte. Cell assembly was carried out in a glove box under an argon atmosphere. The cyclic voltammetry and the charge-discharge performance of the materials were investigated in such a three-electrode cell using lithium metal as counter and reference electrodes. The weight in specific capacity (mAh/g) and current rate (A/g) is calculated by only active materials Fe₂O₃.

At first, we indicate the synthesis of Fe₂O₃ nanoflake via the self-template method. Figure 1(a) shows the X-ray diffraction (XRD) pattern of the powder, which was obtained by scratching of the film from the glass substrate, after immersion in the precursor solution at 60°C for 24  h. The FeOOH has a lot of phase such as -, -, and -type. However, the XRD cards of FeOOH do not indicate the 8° of the lower peak. Figure 2 shows the Fourier transform infrared (FTIR) spectrum of the deposited material within a wavelength spectrum ranging from 4000  to  400  cm^–1. The broad peak around 3400  cm^–1 is assigned to the stretching vibration of the O–H bond, (OH), which indicates the presence of hydroxyl ions and/or water in the crystal. The (HOH) mode of water is also observed at around 1635  cm^–1. The peak, at around 1460  cm^–1, results from a 3 vibrational mode of CO. The peak, at 1385  cm^–1, belongs to the 3 vibrational mode of NO. Thus the deposited materials include a structure containing OH, CO₃, and NO groups. The FeOOH crystals show the layer structure.²⁴ Hence, we considered that the OH, CO, and NO groups are intercalated into the FeOOH layers and the resultant FeOOH shows the lower XRD peak of 8° due to the increasing of the layer distance; similar results are also observed in other metal hydroxide materials.^25,26 Based on the elemental analysis by the CHN analyzer and TG-DTA as shown in Table I, the deposited films are indicated as FeO(OH)_0.29(NO₃)_0.27(CO₃)_0.22·0.6H₂O. The ratio of the calculated weight loss from intercalated FeOOH to Fe₂O₃ is similar to that of the experimental weight loss. Therefore, elemental analysis is considered to indicate the correct composition.

Figure 1.

Figure 2.

When the nickel mesh was used for substrate, the entire surface of the nickel mesh covered by the FeOOH film, as shown in Fig. 3(a). The high magnification view (Fig. 3(b)) indicates the specific morphology of FeOOH. This specific morphology is different from the reported morphology including nanowire, rod, and sphere,^{27,28,29,30,31} and resultant Fe₂O₃ morphology is suitable for electrochemical devices as mentioned later. The nestlike morphology of FeOOH was maintained after the pyrolysis reaction at 300°C for 10  min shown in Fig. 3(c). The film thickness on the Ni wire was around 300–600  nm. The transmission electron microscopy (TEM) images of the heat-treated materials are shown in Fig. 4. The nestlike morphology is constructed by assembly of the nanoflakes with a thickness of several nanometers in Fig. 4(a). Moreover, the nanoflake is composed of several nanometer grains as shown in Fig. 4(b). In Fig. 4(b), we can see the lattice images and electron diffraction image corresponding to the interplanar distance of -Fe₂O₃ (104) with d₍₁₀₄₎=2.70  Å. However, it is considered that some parts are low crystallinity or amorphous because we can see amorphous-like images in TEM and a broad XRD pattern. We checked the FTIR spectrum as shown in Fig. 5. The peaks due to -Fe₂O₃ (540  cm^–1, 470  cm^–1),³² and a small amount of residual compounds derived from CO and NO, are observed. The peaks of H₂O are contaminated in the pellet fabrication of the KBr method. Judging from the SEM and TEM images with various magnification, this Fe₂O₃ film shows the hierarchical structure from several nanometers to submicrometers. This hierarchical structure is one of the features of the self-template method and the resultant morphology causes the high porous structure. The pore size distribution from the adsorption isotherm from the N₂ adsorption/desorption analysis (Fig. 6) of the Fe₂O₃ nanoflake films indicates the existence of mesopores (pore size 3.3 and 45  nm) in the films. In this case, the high Brunauer-Emmett-Teller (BET) surface area around 100  m²/g is indicated. This covered structure with high surface area is obtained by the self-template method based on the CBD process, which can coat the entire surface contacting the precursor solution.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

The properties of the lithium-ion battery of Fe₂O₃ as anodes in Li ions cells were tested via cyclic voltammetry (CV). The CV and cycling testing were performed over the voltage range of 0.01–3.5  V versus a Li/Li⁺ reference electrode. The CV curves at the first and second cycle for Fe₂O₃ film by a scan rate of 0.1  mV/s are presented in Fig. 7. The cathodic peak intensity of the second cycle is smaller than that of the first cycle because the first cycle indicates the irreversible capacity based on electrochemical formation of the solid electrolyte interface (SEI) layer in the case of this transition metal oxide as a negative electrode for a lithium-ion battery.^1,33,34 In this first cycle CV, we can see only a broad shoulder at 1.6  V. In reported work,⁵ the first discharge profile of nanosize Fe₂O₃ is different as compared with that of micrometric Fe₂O₃, and a plateau-like step at 1.6  V was observed from nanosize Fe₂O₃. It is considered that the difference of crystallinity between our sample and the reported sample caused the difference of the reaction at 1.6  V. The nanoparticles in the reported work have a facet that indicates high crystallinity. However, nanoparticles in this work do not have a facet. The second cycle indicates the cathodic peaks and the anodic peaks derived from the reversible electrochemical oxidation and reduction of Fe₂O₃ with lithium.^1,5,6 The reaction mechanism of transition metal oxide cells was reported.^1,5,6 According to the reported works, the reaction mechanism of nanoparticulate Fe₂O₃ with lithium is as Eq. (1)⁶

The redox peaks at around 1.5  V are due to the formation of -LiFe₂O₃ and cubic-Li₂Fe₂O₃. The reduction from Fe³⁺ in Fe₂O₃, LiFe₂O₃, and Fe²⁺ in Li₂Fe₂O₃ to Fe⁰ is caused at around 0.75  V. For example, the reduction Fe₂O₃ is as follows³⁵

In theory, the Fe₂O₃ can totally react with 6 Li per formula unit, corresponding to a reversible specific capacity of 1007  mAh/g. However, in the reported works, the Fe₂O₃ reacted with 8 Li per formula unit (1343  mAh/g).^5,6 Our results in Fig. 8 show the similar capacities of over 1007  mAh/g, too. It is considered that the side reactions such as redox layer on the negative electrode using transition metal oxide^6,36 are contained in the reaction with 8 Li. Moreover, it is considered that the native oxide on the Ni mesh or a little NiO foamed by heating at 300°C will indicate the capacity due to the similar mechanism capacity of NiO because the Ni wire with micrometer size is about 450  mg and Fe₂O₃ shell with nanometer size is about 1  mg in this experiment. At first, we considered the effect of native oxide of Ni. Generally, native oxide of Ni wire is about 2.5  nm (Ref. 37) on the surface of Ni wire. Judging from the 300–600  nm Fe₂O₃ with 2.5  nm NiO on the Ni wire, the mass of NiO with 2.5  nm thickness is much less than 1% of the mass of Fe₂O₃ shell with 300–600  nm thickness; the capacitance resulting from 2.5  nm NiO should be negligible. Second, the NiO foamed by heating at 300°C is important. In fact, we also checked the capacitance from the NiO layer by heating the bare Ni mesh without Fe₂O₃ coating layer at 300°C by cycle voltammograms. It seems that about 20% of the Fe₂O₃ coating layer's capacitance is caused by such NiO layer resulting from the bare Ni wire's oxidation at high temperature. However, the amount of oxidized Ni in bare Ni mesh will be much less than that of the Fe₂O₃/Ni mesh because the Ni surface is covered by the 300–600  nm layer of FeO(OH)_0.29(NO₃)_0.27(CO₃)_0.22·0.6H₂O. The amount of oxidized Ni of Fe₂O₃/Ni should be less than that of only the bare Ni mesh. Here, we just want to point out a scientific concept based on hierarchical nano-micro (Fe₂O₃/Ni) structure electrode, although the Ni wire was much heavier than the active material Fe₂O₃ in this system.

Figure 7.

Figure 8.

The lithium storage characteristics of Fe₂O₃ electrode at high charge/discharge current rate were investigated. The lithium storage device with the high specific capacity at high charge/discharge current rate is shown through overcoming the four problems by our Fe₂O₃/Ni covered structure. Figure 8 shows the second charge/discharge cycling curves at constant current rates of 0.78, 7.3, and 13  A/g. In all discharge curves, the potential drops rapidly to reach the Li insertion potential from 2.0  to  1.0  V, and then the potential gradually decreases to the potential based on the reduction reaction from Fe₂O₃ to Li₂O (around 0.75  V). Finally, the curves gradually decrease to 0.01  V. These behaviors of curves are similar to the CV curves as shown in Fig. 7. Both the discharge and charge processes show high specific capacities of around 1215, 1120, and 780  mAh/g at each current rate of 0.78, 7.3, and 13  A/g, respectively. From these charge/discharge curves, high specific capacity of 1120  mAh/g occurs at a very high current rate such as 7.3  A/g. The capacity of 780  mA/g in the case of 13  A/g is around twice the theoretical specific capacity of usual graphitic carbon (372  mAh/g).³⁸ These good high rate properties are caused by the covered structure of nano/micro hierarchical structured Fe₂O₃/Ni mesh. Ni mesh, which is knitted Ni wire of the diameter of micrometer size, with low resistivity solves problem (i). The mesopores in Fe₂O₃ nanoflake and large pores between the Fe₂O₃ nanoflakes result in easy access by the electrolyte, including large organic molecules such as propylene carbonate (PC). Li⁺ ions and counteranions such as ClO can be transported inside the mesopores and react with the Fe₂O₃. The nanocrystalline Fe₂O₃ of around 10  nm reduces diffusion length for the transported lithium ions to half of the grain size around 5  nm. This indicates the solution of problem (ii). The high BET surface area of around 100  m²/g reduces specific current density. The applied current density per surface area to the Fe₂O₃/Ni covered structure with much larger surface area should be much less than that of nickel mesh. The resultant effective current density in Fe₂O₃ is very small. This causes the solution of problem (iii). Therefore, these nanocrystalline and porous Fe₂O₃ films on nickel mesh make it possible for a lithium rechargeable battery with the high specific capacity at high charge/discharge current rate because all Fe₂O₃ nanoparticles including the inner parts can react with lithium ions in a very short time at high current density.

We confirmed the solution of the last problem (iv) via the diagram for discharge capacity and Coulomb efficiency vs cycle number for different current rates from 0.78  to  13  A/g shown in Fig. 9,10, respectively. The discharge capacities of 20th cycle indicate the capacity of around 80 and 70% of second cycle at the high current rates of 0.78 and 7.3  A/g, respectively. The 55% of the second cycle capacity is maintained even in the very high current rate such as 13  A/g. Moreover, the Coulomb efficiency of various current rates presents a high value over 95% even in the high current rate such as 13  A/g as shown in Fig. 10. Generally, lithium insertion/extraction causes a volume change due to the change of crystal structure and the destruction of the film. Hence, this volume change decreases the cycle performance after a number of cycles with charge and discharge process.³⁹ In this electrode, Fe₂O₃ are constructed by nanoparticulate and mesoporous hierarchical structure. Therefore, this specific structure relaxes the tension from volume change by lithium insertion/extraction. In fact, Fe₂O₃ powder indicates the low capacity and the bad cycle performance in spite of very low current rate of 0.1  A/g as shown in Fig. 9. The powder was fabricated by scratching the film, which is synthesized by a similar method to the fabrication of Fe₂O₃/Ni covered structure, from the glass substrate. The powder was mixed and ground with 5  wt  % Teflon [poly(tetrafluoroethylene)] powder as a binder. The mixture was spread and pressed on a nickel mesh (100  mesh) as the working electrode (WE). The active materials on this WE were constructed by nanoparticles and mesopores. However, the WE does not have Fe₂O₃/Ni covered structure for the solution of the four problems. It is considered that in order to fabricate the lithium rechargeable battery with the high specific capacity at high charge/discharge current rate, we have to solve completely the four problems such as our nano/micro hierarchical structured Fe₂O₃/Ni.

Figure 9.

Figure 10.

Figure 11 indicates the relationship between capacity and current rates from 0.78  to  13  A/g. Up to the high current rate of 13  A/g, the high specific capacity, which is near to theoretical capacity, is maintained. The powder electrodes almost do not show the capacities in the case of high current rate. According to our knowledge, they are the largest level-specific capacities for anodic electrode of lithium rechargeable battery at the high current rates such as 7.3 and 13  A/g. If we construct a lithium rechargeable battery using this negative electrode and positive electrodes with high specific capacity at high charge/discharge current rate such as V₂O₅ (plateau 2.5  V versus Li/Li⁺, 280  mAh/g at 13  A/g),⁷ we may obtain a high-energy density of around 300  Wh/kg at the large output density of about 14  kW/kg. (In this case, the weight is calculated by only active materials as negative and positive electrodes.) These values indicate that in the battery both large output density, which is similar to that of EDLC, and large energy density are fabricated and applied to EV and hybrid-EV.

Figure 11.

In summary, we reported the fabrication of a lithium storage device with a high specific capacity at high charge/discharge current rate. The concept of this electrode for a lithium storage device is shown in Scheme 1. Fe₂O₃ nanoparticles around 10  nm reduce the required diffusion length in the active materials and the effective specific current density at high current rate and improve the cycle performance. The mesoporous structure improves the diffusion of electrolyte. The formation of Fe₂O₃/Ni covered structure, which was constructed throughout the negative electrode, increases the electronic conductivity. This concept of using nanocrystalline and mesoporous covered material by CBD for lithium rechargeable battery also can be extended for another rechargeable battery's negative and positive electrode.

Scheme 1. The concept of the electrode for lithium storage device with both the high power density and high energy density.

National Institute of Advanced Industrial Science and Technology assisted in meeting the publication costs of this article.

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Fig. 1. (a) XRD pattern of the deposited material in the solution at 60°C for 24  h. (b)–(e) JCPDS patterns of various FeOOH. (b) JCPDS no. 08-0098, (c) JCPDS no. 18-0639, (d) JCPDS no. 34-1266, and (e) JCPDS no. 44-1415. First citation in article

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Fig. 2. FTIR spectrum for the deposited material. The peaks due to (OH), (HOH), 3 (CO), and 3 (NO) are labeled (), (), (), and (), respectively. First citation in article

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Fig. 3. SEM [low (a) and high (b) magnification] images of the intercalated FeOOH film on the nickel mesh. The film was fabricated by a direct deposition on the mesh in the precursor solution at 60°C. The image of (c) indicates the Fe₂O₃ film after heat treatment at 300°C 10  min. First citation in article

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Fig. 4. (a) TEM images of the Fe₂O₃ film after heat treatment. (b) The nanocrystalline Fe₂O₃ particles of several nanometers and mesopores are observed. The lattice image and electron diffraction corresponds to the interplanar distance of =Fe₂O₃ (104) with d₍₁₀₄₎=2.70  Å. First citation in article

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Fig. 5. FTIR spectrum for the Fe₂O₃. The peaks due to (OH), (HOH), 3 (CO), and 3 (NO) are labeled (), (), (), and (), respectively. First citation in article

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Fig. 6. The pore size distribution of the Fe₂O₃ from N₂ adsorption/desorption isotherms. First citation in article

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Fig. 7. Cyclic voltammograms (CV) of the nano/micro hierarchical structured Fe₂O₃/Ni mesh. The CV were recorded in 1  mol  dm^–3 LiClO₄ in PC electrolyte in the potential range from 0.01  to  3.5  V (vs Li⁺/Li) at a scan rate of 0.1  mV  s^–1. First citation in article

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Fig. 8. Constant current discharge/charge curves vs Li at various current rates. The second cycling curves are shown. Those were recorded in 1  mol  dm^–3 LiClO₄ in PC electrolyte in the potential range from 0.01  to  3.5  V (vs Li⁺/Li). First citation in article

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Fig. 9. Diagram for discharge capacity vs cycle number for various current rates from 0.78  to  13  A/g ( 0.78  A/g, 7.3  A/g, 13  A/g). The discharge capacity using the electrode with pressed Fe₂O₃ nanoparticles at 0.1  A/g is indicated as the mark (). First citation in article

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Fig. 10. Coulomb efficiency vs cycle number for various current rates from 0.78  to  13  A/g ( 0.78  A/g, 7.3  A/g, 13  A/g). The efficiency indicates the high value over 95%. First citation in article

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Fig. 11. Capacity vs various current rates from 0.1  to  13  A/g. Up to high current rate of 13  A/g, the capacity maintains high energy densities (: Fe₂O₃/Ni covered electode, : pressed Fe₂O₃ nanoparticles electrode, : reported capacity with 8 Li per formula unit, : theoretical capacity in the case of 6 Li per formula unit). First citation in article

Table I. Composition analysis. Elemental wt % of N and C were determined by CHN analysis. Fe wt % was determined by the TG-DTA on the basis of final product Fe2O3 at 600°C. H2O wt % was measured by TG-DTA using weight loss before 100°C. Theoretical weight loss is the value of the loss from the intercalculated FeOOH to Fe2O3. Experimental weight loss was determined by the TG-DTA on the basis of final product Fe2O3 at 600°C.
N (wt %)	C (wt %)	Fe (wt %)	H2O (wt %)
3.1145	2.12325	45.6945	8.9008
Chemical formula		Calculated weight loss (wt %)	Expt. weight loss (wt %)
FeO(OH)0.29(NO3)0.27(CO3)0.22·0.6H2O		32.1	34.7

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^zE-mail: hs.zhou@aist.go.jp

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