METHOD FOR MANUFACTURING IRON-CHROMIUM OXIDE USING ION EXCHANGE RESIN

Abstract

The present invention relates to a method of preparing an iron-chromium oxide using an ion-exchange resin. Moreover, the present invention relates to a method of preparing an iron-chromium oxide that can be used as a cathode material for lithium-ion batteries. According to one aspect of the present invention, it has the effect of providing a cathode material for lithium-ion batteries with a high capacitance, while exhibiting a voltage similar to that of a transition-metal oxide (2-4.5 V vs Li.sup.+/Li).

Claims

1. A method of preparing an iron-chromium oxide, comprising: a step of providing an ion exchange column substituted with Fe.sup.3+ ions; a step of adding a hexavalent chromium (Cr.sup.6+) compound to the ion exchange resin column to obtain an ion-exchanged aqueous solution; and a step of heat-treating the ion-exchanged aqueous solution.

2. The method according to claim 1, wherein, in the step of providing an ion exchange column substituted with Fe.sup.3+ ions, an ion exchange column substituted with Fe.sup.3+ ions is provided by adding an aqueous solution containing one or more selected from the group consisting of FeCl.sub.3, FeCl.sub.3.6H.sub.2O, Fe(NO.sub.3).sub.3, and Fe(NO.sub.3).sub.3.9H.sub.2O to an ion exchange resin.

3. The method according to claim 2, wherein the ion exchange resin is a cation exchange resin.

4. The method according to claim 1, wherein the hexavalent chromium (Cr.sup.6+) compound is CaCrO.sub.4, Na.sub.2CrO.sub.4, or K.sub.2Cr.sub.2O.sub.7.

5. The method according to claim 1, wherein the ion-exchanged aqueous solution comprises Fe.sup.3+ ions and Cr.sup.6+ ions.

6. The method according to claim 1, wherein the heat treatment is performed under a vacuum atmosphere, an oxygen (O.sub.2) atmosphere, a nitrogen (N.sub.2) atmosphere, or an argon (Ar) atmosphere.

7. The method according to claim 1, wherein the heat treatment is performed at 270° C. to 350° C.

8. A cathode for lithium-ion batteries, comprising the iron-chromium compound prepared using the method according to claim 1.

9. The cathode according to claim 8, wherein the iron-chromium compound has a Fe/Cr ratio of 0.2 to 0.5.

10. The cathode according to claim 8, wherein the iron-chromium compound has a monoclinic unit cell.

Description

DESCRIPTION OF DRAWINGS

[0018] FIG. 1 shows the results of thermogravimetric-differential scanning calorimetry analysis for an iron-chromium oxide prepared according to one embodiment of the present invention.

[0019] FIG. 2 shows the results of X-ray diffraction analysis for an iron-chromium oxide prepared according to one embodiment of the present invention.

[0020] FIG. 3 shows the results of X-ray diffraction analysis for an iron-chromium oxide prepared according to one embodiment of the present invention compared to Cr.sub.8O.sub.21 and Cr.sub.2O.sub.5 prepared by a conventional method, respectively.

[0021] FIG. 4 shows the results of Le Bail fitting for the X-ray diffraction pattern of an iron-chromium oxide prepared according to one embodiment of the present invention.

[0022] FIG. 5A includes the scanning electron microscope (SEM) images and transmission electron microscope (TEM) images of an iron-chromium oxide prepared according to one embodiment of the present invention.

[0023] FIG. 5B includes the scanning electron microscope (SEM) images of Cr.sub.8O.sub.21 and Cr.sub.2O.sub.5 prepared by a conventional method, respectively.

[0024] FIGS. 6A to 6C show the results of the battery charge/discharge experiments for the coin cell made of an iron-chromium oxide prepared according to one embodiment of the present invention, and those of Cr.sub.8O.sub.21 and Cr.sub.2O.sub.5 prepared by a conventional method, respectively.

[0025] FIG. 7 shows the results of X-ray photoelectron analysis during the battery cell test performed on an iron-chromium oxide prepared according to one embodiment of the present invention; the tests were performed after stopping the coin cells at different discharge voltages (4.5, 4.0, 3.1, 2.8, 2.5, 2.0 V).

[0026] FIG. 8 shows the results of comparative analysis of X-ray photoelectron spectra of Cr2p for an iron-chromium oxide prepared according to one embodiment of the present invention, and those of the Cr.sub.8O.sub.21 and Cr.sub.2O.sub.5 prepared by a conventional method, respectively.

[0027] FIG. 9 shows the results of X-ray diffraction analysis during the battery cell test performed on an iron-chromium oxide prepared according to one embodiment of the present invention; the tests were performed after stopping the coin cells at different discharge voltages (4.5, 4.0, 3.1, 2.8, 2.5, 2.0 V).

BEST MODE

[0028] The present invention will now be described more fully with reference to the accompanying drawings and contents disclosed in the drawings. However, the present invention should not be construed as limited to the exemplary embodiments described herein.

[0029] The terms used in the present specification are used to explain a specific exemplary embodiment and not to limit the present inventive concept. Thus, the expression of singularity in the present specification includes the expression of plurality unless clearly specified otherwise in context. It will be further understood that the terms “comprise” and/or “comprising”, when used in this specification, specify the presence of stated components, steps, operations, and/or elements, but do not preclude the presence or addition of one or more other components, steps, operations, and/or elements thereof.

[0030] It should not be understood that arbitrary aspects or designs disclosed in “embodiments”, “examples”, “aspects”, etc. used in the specification are more satisfactory or advantageous than other aspects or designs.

[0031] Although terms used in the specification are selected from terms generally used in related technical fields, other terms may be used according to technical development and/or due to change, practices, priorities of technicians, etc. Therefore, it should not be understood that terms used below limit the technical spirit of the present invention, and it should be understood that the terms are exemplified to describe embodiments of the present invention.

[0032] Also, some of the terms used herein may be arbitrarily chosen by the present applicant. In this case, these terms are defined in detail below. Accordingly, the specific terms used herein should be understood based on the unique meanings thereof and the whole context of the present invention.

[0033] Meanwhile, terms such as “first” and “second” are used herein merely to describe a variety of constituent elements, but the constituent elements are not limited by the terms. The terms are used only for the purpose of distinguishing one constituent element from another constituent element.

[0034] In addition, when an element such as a layer, a film, a region, and a constituent is referred to as being “on” another element, the element can be directly on another element or an intervening element can be present.

[0035] Unless defined otherwise, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present invention, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0036] In addition, in the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention unclear. The terms used in the specification are defined in consideration of functions used in the present invention, and can be changed according to the intent or conventionally used methods of clients, operators, and users. Accordingly, definitions of the terms should be understood on the basis of the entire description of the present specification.

[0037] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0038] A method of preparing an iron-chromium oxide according to one embodiment of the present invention is to prepare an iron-chromium oxide using an ion-exchange chromatographic method, and includes a step of providing an ion exchange column substituted with Fe.sup.3+ ions; a step of adding a hexavalent chromium (Cr.sup.6+) compound to the ion exchange resin column to obtain an ion-exchanged aqueous solution; and a step of heat-treating the ion-exchanged aqueous solution.

[0039] In the step of providing an ion exchange column substituted with Fe.sup.3+ ions, an ion exchange column may be provided by adding an aqueous solution containing one or more selected from the group consisting of FeCl.sub.3, FeCl.sub.3.6H.sub.2O, Fe(NO.sub.3).sub.3, and Fe(NO.sub.3).sub.3.9H.sub.2O to an ion exchange resin. More specifically, an ion exchange column substituted with Fe.sup.3+ ions may be provided by passing an aqueous solution containing Fe.sup.3+ ions through a cation exchange resin (cation exchanger) or mixing and stirring a cation exchange resin and an aqueous solution containing Fe.sup.3+ ions. Whether all of the cation exchange resin in the ion exchange column have been substituted with Fe.sup.3+ ions may be confirmed by passing the aqueous solution through the ion exchange column and measuring the pH of the discharged solution. At this time, the pH may be measured using a litmus test paper. More specifically, when H.sup.+ ions are substituted with Fe.sup.3+ ions, an acidic solution having a pH of 7 or less is discharged during the process of substituting H.sup.+ ions with Fe.sup.3+ ions, and a neutral solution is discharged after all H.sup.+ ions are substituted with Fe.sup.3+ ions. Based on this principle, it can be confirmed that all of the cation exchange resin in the ion exchange resin column have been substituted with Fe.sup.3+ ions.

[0040] The ion exchange resin may be a cation exchange resin.

[0041] The hexavalent chromium (Cr.sup.6+) compound may be CaCrO.sub.4, Na.sub.2CrO.sub.4, or K.sub.2Cr.sub.2O.sub.7.

[0042] The ion-exchanged aqueous solution may include Fe.sup.3+ ions and Cr.sup.6+ ions.

[0043] The heat treatment may be performed under a vacuum atmosphere, an oxygen (O.sub.2) atmosphere, a nitrogen (N.sub.2) atmosphere, or an argon (Ar) atmosphere.

[0044] The heat treatment may be performed at 350° C. or lower or at 270° C. to 350° C. More preferably, the heat treatment is performed under an oxygen (O.sub.2) atmosphere, a nitrogen (N.sub.2) atmosphere, an argon (Ar) atmosphere, or vacuum. For example, the heat treatment may be performed under a degree of vacuum of 10.sup.−4 torr or less.

[0045] In addition, the cathode for lithium-ion batteries of the present invention includes the iron-chromium compound prepared using the above method.

[0046] The iron-chromium compound may have a Fe/Cr ratio of 0.2 to 0.5.

[0047] The iron-chromium compound may have a monoclinic unit cell.

[0048] In addition, the present invention may provide the cathode for lithium-ion batteries including the iron-chromium compound prepared using the above method.

Example 1

[0049] A column substituted with Fe.sup.3+ ions was prepared using an aqueous solution containing FeCl.sub.3.6H.sub.2O and cation-exchange resin IR-120(H).

[0050] Subsequently, an aqueous CaCrO.sub.4 solution at 80° C. was passed through the column to exchange the Fe.sup.3+ ions attached to the ion-exchange resin with Ca.sup.2+ ions, and obtain an aqueous solution containing Fe.sup.3+ ions and Cr.sup.6+ ions. The aqueous solution was dried at 80° C. and was subjected to heat treatment at 300-580° C. under a vacuum atmosphere of 1×10.sup.−4 Torr or less to obtain iron-chromium oxide.

Example 2

[0051] The same procedure as in Example 1 was performed except that heat treatment was performed at 270° C. to 350° C. under an oxygen (O.sub.2) atmosphere.

Example 3

[0052] The same procedure as in Example 1 was performed except that heat treatment was performed at 350° C. under an argon (Ar) atmosphere.

Example 4

[0053] The same procedure as in Example 3 was performed except that heat treatment was performed at 350° C. under a nitrogen (N.sub.2) atmosphere.

Measurement Example 1. Thermogravimetric-Differential Scanning Calorimetry (TGA-DSC) Analysis

[0054] Thermogravimetric-differential scanning calorimetry (TGA-DSC) for the iron-chromium oxide prepared in Example 1 was performed at a heating rate of 10° C./min under a nitrogen (N.sub.2) atmosphere of 100 ml/min, and the analysis results are shown in FIG. 1.

Measurement Example 2. X-Ray Diffraction Analysis Using X-Ray Diffractometer (XRD)

[0055] X-ray diffraction analysis for iron-chromium oxides prepared in Example 1 (vacuum atmosphere, heat treatment temperature: 300 to 580° C.), Example 2 (oxygen atmosphere, heat treatment temperature: 270 to 350° C.), Example 3 (argon atmosphere, heat treatment temperature: 350° C.), and Example 4 (nitrogen atmosphere, heat treatment temperature: 350° C.) was performed using an X-ray diffractometer (Rigaku X-ray diffractometer, 40 kV, 100 mA) equipped with a Cu-Kα radiation source, and the analysis results were compared with The International Centre for Diffraction Data (ICDD). XRD diffraction pattern analysis results are shown in FIGS. 2 and 3.

[0056] As shown in FIG. 2, an FeCrO.sub.3 perovskite phase with no battery characteristics was observed in the heat-treated sample prepared at temperatures of 400 and 580° C., which correspond to Example 1. It can be confirmed that heat-treated samples prepared in the temperature range of 270-350° C. under various atmospheres (Examples 2 to 4) exhibited XRD diffraction patterns similar to that of Cr.sub.2O.sub.5 obtained by heat-treatment of CrO.sub.3 at 350° C. under an oxygen atmosphere; each diffraction pattern is shown in FIG. 3. The synthesis of Cr.sub.2O.sub.5 and Cr.sub.8O.sub.21 was sensitive to the heating temperature. In contrast, in the case of the synthesis method performed in the present invention, it was confirmed that the Fe—Cr—O samples synthesized at temperatures between 270° C. and 350° C. under various atmospheres exhibited similar XRD patterns.

[0057] As shown in FIGS. 2 and 3, the XRD diffraction pattern of the Fe—Cr—O compound obtained by heat treatment at 350° C. under vacuum was analyzed using the LeBail fitting method, and the results are shown in FIG. 4. The Cr.sub.2O.sub.5 sample analyzed by Hewston et al. had a monoclinic unit cell of a=12.01 Å, b=8.52 Å, c=9.39 Å; β=92.0°. The analysis results of FIG. 4 shows that the Fe—Cr—O compound had a monoclinic unit cell of a=11.97 Å, b=8.59 Å, c=9.19 Å; β=92.76°.

Measurement Example 3. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Analysis

[0058] FIG. 5A shows a scanning electron micrograph and transmission electron micrograph of iron-chromium oxide prepared in Example 1 (vacuum, heat treatment temperature: 350° C.). Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis for the iron-chromium oxide used for microphotography confirmed that the component ratio of chromium to iron was 3:1. Hexagonal column-shaped crystal clusters were observed at a magnification of 200,000× (FIG. 5A, upper left image), and each hexagonal column-shaped particle consisted of smaller particles with a size of several tens of nanometers observed at a high magnification of 3 millionx (FIG. 5A, upper right image). The shape and size of these small particles were confirmed using transmission electron microscopy (TEM) (FIG. 5A, lower images), and particles with diameters of approximately 30-40 nm were observed. The material observed above is composed of crystal grains with sizes that are at least 100× times smaller (several μm) than the size of Cr.sub.8O.sub.21 and Cr.sub.2O.sub.5 crystals synthesized by heat treatment of CrO.sub.3 at 270 and 350° C., respectively (FIG. 5B), which indicated that the crystallization was achieved via different processes during synthesis. These results are consistent with the change in peak broadening observed in the X-ray diffraction pattern shown in Measurement Example 2.

Measurement Example 4. Charging/Discharging Analysis

[0059] A coin cell was manufactured using iron-chromium oxide (composition ratio of chromium to iron: 3:1) prepared in Example 1 (vacuum, heat treatment: 350° C.) and analyzed in Measurement Example 3, and a battery charge/discharge experiment was performed. The results are shown in FIG. 6A. For a performance comparison analysis with conventional Cr.sub.8O.sub.21 and Cr.sub.2O.sub.5, coin cells were assembled in the same procedure using Cr.sub.8O.sub.21 and Cr.sub.2O.sub.5 powder (specimens analyzed in Measurement Examples 2 and 3), and battery charge/discharge experiments were performed under the same conditions. The results are shown in FIGS. 6B (Cr.sub.8O.sub.21) and 6C (Cr.sub.2O.sub.5).

[0060] The oxide electrodes were fabricated using a mixture of the active materials, acetylene black, and polytetrafluoroethylene (PTFE) taken in a ratio of 70:20:10 weight percentage. One molar LiPF.sub.6 in ethylene carbonate (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC) (1:1:1 by volume) was used as the electrolyte, and a lithium metal foil (14-mm diameter circular disk) was used as the counter and reference electrodes. The electrode, separator, and electrolyte were assembled and sealed in an Ar-filled glove box to obtain the cells.

[0061] As shown in FIGS. 6A-C, when repeated five times at a C/10 rate, a voltage of 2-4.5 V vs Li.sup.+/Li was observed. After five cycles, the charge/discharge capacities of Cr.sub.8O.sub.21, Cr.sub.2O.sub.5, and iron-chromium oxide were 210, 280, and 380 mAh/g, respectively. It was confirmed that Cr.sub.8O.sub.21 and Cr.sub.2O.sub.5 were nearly consistent with previous experimental results. Based on these comparative experimental results and the battery performance analysis using iron-chromium oxide prepared in Example 1 as a cathode material, it was confirmed that a charge/discharge capacity higher than 100 mAh/g was obtained compared with previously developed Cr.sub.8O.sub.21 and Cr.sub.2O.sub.5.

Measurement Example 5. X-Ray Photoelectron Analysis for Electrode Sample after Charging/Discharging

[0062] After the coin cell composed of the iron-chromium oxide used in the charge/discharge experiment performed in Measurement Example 4 was stopped at different discharge voltages (4.5, 4.0, 3.1, 2.8, 2.5, 2.0 V), the coin cell was moved into a glove box, disassembled, and X-ray photoelectron analysis was performed while the cathode electrode was not exposed to air. The results are shown in FIG. 7, indicating a change in the valence states of the chromium ions from Cr.sup.6+ at 4.5 V to Cr.sup.3+ with the insertion of Li ions.

[0063] The comparative analysis between Cr.sub.8O.sub.21, Cr.sub.2O.sub.5, and iron-chromium oxide in Measurement Example 4 revealed the cause of the difference in charge/discharge capacity, which and can be attributed to the difference in the atomic ratios of tetrahedral Cr.sup.6+ to octahedral Cr.sup.3+. Atomic ratio of Cr.sup.6+/Cr.sup.3+ is 2 and 3 for Cr.sub.2O.sub.5 and Cr.sub.8O.sub.21, respectively, as previously reported. The Cr.sub.8O.sub.21 compound, with relatively more Cr.sup.6+ ions which can participate in the oxidation/reduction reaction during battery charging/discharging, showed higher charge/discharge capacity than Cr.sub.2O.sub.5. In the case of iron-chromium oxide prepared in Example 1, the iron-chromium oxide was predicted to have a monoclinic crystal structure similar to that of Cr.sub.2O.sub.5, as shown in FIG. 4, and the ICP-AES analysis indicated that the composition Cr/Fe ratio was 3.0. Based on these results, the crystallographic formula for Fe—Cr—O can be estimated as Fe.sup.3+.sub.xCr.sup.3+.sub.2-x(Cr.sup.6+O.sub.4).sub.2(Cr.sup.6+.sub.2O.sub.7) assuming some Cr.sup.3+ of Cr.sub.2O.sub.5 (═Cr.sub.2.sup.3+(Cr.sup.6+O.sub.4).sub.2(Cr.sub.2.sup.6+O.sub.7)) is substituted with Fe.sup.3+, whereas the ICP-AES (Cr/Fe=3.0) suggested the formula to be Fe.sup.3+.sub.1.5Cr.sup.3+.sub.0.5(Cr.sup.6+.sub.2O.sub.4).sub.2(Cr.sup.6+.sub.2O.sub.7). In this case, the Cr.sup.6+/Cr.sup.3+ ratio was 8, which is 2.5 times greater than that of the Cr.sub.8O.sub.21 (3). The substituted Fe.sup.3+ ions may participate in the oxidation/reduction reaction and contribute to higher capacitive properties. To confirm this experimentally, X-ray photoelectron analysis was performed on Cr.sub.8O.sub.21, Cr.sub.2O.sub.5, and iron-chromium oxide, and the results are shown in FIG. 8. As expected, the relative peak intensities (Cr.sup.6+/Cr.sup.3+) of Cr2p.sub.3/2 were in the order of Fe.sup.3+.sub.1.5Cr.sup.3+.sub.0.5Cr.sup.6+.sub.4O.sub.15>Cr.sub.8O.sub.21>Cr.sub.2O.sub.5, which corresponded with the order of the Cr.sup.6+/Cr.sup.3+ ratio.

Measurement Example 6. X-Ray Diffraction Analysis for Electrode Sample after Charging/Discharging

[0064] After the coin cell used in the charge/discharge experiment performed in Measurement Example 4 was stopped at different discharge voltages (4.5, 4, 3.1, 2.8, 2.5, and 2.0 V), it was moved into a glove box, disassembled, and X-ray diffraction analysis was performed on a sample sealed with a Kapton film. The results are shown in FIG. 9.

[0065] The crystalline peak, which observed in X-ray diffraction patterns of Example 2 (FIGS. 2 and 3) performed on the powder samples, disappeared and new peaks were observed at approximately 45° as lithium was inserted. These results are consistent with the experimental results for Cr.sub.2O.sub.5 previously published in literature (Xu-Yong Feng, Ning Ding, Li Wang, Xiao-Hang Ma, Yong-Ming Li, and Chun-Hua Chen, J. Power Sources, 2013, 222, 184-187.).

[0066] Meanwhile, embodiments of the present invention disclosed in the present specification and drawings are only provided to aid in understanding of the present invention and the present invention is not limited to the embodiments. It will be apparent to those skilled in the art that various modifications can be made to the above-described exemplary embodiments of the present invention without departing from the spirit and scope of the invention.