Porous Co.SUB.3.O.SUB.4 .nanoparticles and method for preparing the same

Abstract

The disclosure relates to porous Co.sub.3O.sub.4 nanoparticles which include flocculated amorphous primary nanoparticles, with air pores formed between the amorphous primary nanoparticles. The porous Co.sub.3O.sub.4 nanoparticles, according to an embodiment of the disclosure, may be in the form of flocculated amorphous primary nanoparticles of 1 nm or less, have a 400 times larger specific surface area than the conventional Co.sub.3O.sub.4 particles, and address the issue with the expansion of Co.sub.3O.sub.4 lattices which may arise when the battery is charged or discharged, thereby providing more reliability when applied to batteries.

Claims

1. Porous Co.sub.3O.sub.4 nanoparticles configured in a form of flocculated amorphous primary nanoparticles represented as chemical formula 1, wherein air pores are formed between the amorphous primary nanoparticles, wherein
Co.sub.3O.sub.4−x  [Chemical formula 1] wherein 0≤x≤0.4.

2. The porous Co.sub.3O.sub.4 nanoparticles of claim 1, wherein the amorphous primary nanoparticles have a size ranging from 0.3 nm to 3 nm.

3. The porous Co.sub.3O.sub.4 nanoparticles of claim 1, wherein the porous Co.sub.3O.sub.4 nanoparticles have a porosity ranging from 20% to 50%.

4. The porous Co.sub.3O.sub.4 nanoparticles of claim 1, wherein the air pores have a size ranging from 0.4 nm to 5 nm.

5. The porous Co.sub.3O.sub.4 nanoparticles of claim 1, wherein the porous Co.sub.3O.sub.4 nanoparticles have a BET specific surface area ranging from 40 m.sup.2/g to 120 m.sup.2/g.

6. Porous Co.sub.3O.sub.4 nanoparticles prepared by forming primary nanoparticles in such a manner as to separate Co clusters by injecting positive ions into lattices of crystalline Co.sub.3O.sub.4 particles while simultaneously allowing the primary nanoparticles to transition to an amorphous phase, wherein the porous Co.sub.3O.sub.4 nanoparticles are configured in a form of flocculated amorphous primary nanoparticles represented as chemical formula 1, wherein
Co.sub.3O.sub.4−x  [Chemical formula 1] wherein 0≤x≤0.4.

7. The porous Co.sub.3O.sub.4 nanoparticles of claim 6, wherein the positive ions are positive ions of an alkaline metal or an alkaline earth metal.

8. The porous Co.sub.3O.sub.4 nanoparticles of claim 6, wherein the positive ions are injected using a constant current or constant voltage method.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete appreciation of the present disclosure and many of the attendant aspects thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

(2) FIG. 1 is a view schematically illustrating a process for preparing porous Co.sub.3O.sub.4 nanoparticles according to an embodiment;

(3) FIG. 2 is a galvanostatic cycling with potential limitation (GCPL) of porous Co.sub.3O.sub.4 nanoparticles preparing device according to an embodiment;

(4) FIG. 3 is a view illustrating the results of TEM measurement on an embodiment and a comparative example;

(5) FIG. 4 is a view illustrating the results of STEM measurement on an embodiment and a comparative example; and

(6) FIG. 5 is a view illustrating the results of tomo-STEM measurement on an embodiment and a comparative example.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(7) Hereinafter, exemplary embodiments of the inventive concept will be described in detail with reference to the accompanying drawings. The inventive concept, however, may be modified in various different ways, and should not be construed as limited to the embodiments set forth herein. Like reference denotations may be used to refer to the same or similar elements throughout the specification and the drawings.

(8) When determined to make the subject matter of the present disclosure unclear, the detailed description of the known art or functions may be skipped.

(9) The terms as used herein are provided merely to describe some embodiments thereof, but not to limit the present disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” and/or “have,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

(10) Hereinafter, embodiments of the present disclosure are described in detail. However, the scope of the disclosure is not limited to the embodiments disclosed herein but is defined by the appended claims.

(11) The disclosure relates to porous Co.sub.3O.sub.4 nanoparticles which include flocculated amorphous primary nanoparticles, with air pores formed between the amorphous primary nanoparticles.

(12) The amorphous primary nanoparticles may be represented as shown in chemical formula 1 below.
Co.sub.3O.sub.4-x  [Chemical formula 1]

(13) where 0≤x≤0.4.

(14) The amorphous primary nanoparticles mean particles that result as crystalline Co.sub.3O.sub.4 particles have air pores built up therebetween and their bonds are broken. The reason why the amorphous primary nanoparticles are so amorphous is that as positive ions are injected into the lattices, the oxygen is removed from Co.sub.3O.sub.4 and, thus, crystalline-amorphous phase transition occurs. Flocculation may be the gathering of primary nanoparticles that are formed as air pores build up inside the secondary particles (Co.sub.3O.sub.4 particles), which are of the parent phase, the and thus, interparticle bonds are broken. The gaps between the primary nanoparticles may be the air pores.

(15) The size of the amorphous primary nanoparticle may range from 0.3 nm to 3 nm. The amorphous primary nanoparticles with a size less than 0.3 nm are nearly impossible to prepare, and although possible, they may be hard to use commercially. If the size of the amorphous primary nanoparticles exceeds 3 nm, no or little confinement effect of a specific size may be achieved. As confinement effects achievable by tiny particles, there may be the catalytic properties, better surface reactions, or surface plasmonic phenomena. Such effects may be significantly lowered if the particle size exceeds 3 nm.

(16) The porosity of the porous Co.sub.3O.sub.4 nanoparticles may range from 20% to 50%. If the porosity is less than 20%, the passage of substrate molecules may be restricted and, thus, reactivity may be lowered. If the porosity exceeds 50%, the passage of the molecules is smooth, but a narrow confinement effect of a specific size may not arise and an increase in reaction efficiency may be limited.

(17) The size of the air pore may range from 0.4 nm to 5 nm. If the size of the air pore is less than 0.4 nm, the passage of substrate molecules may be restricted and, thus, reactivity may be lowered. If the size of the air pore exceeds 5 nm, the passage of the molecules is smooth, but a narrow confinement effect of a specific size may not arise and an increase in reaction efficiency may be limited.

(18) The BET specific surface area of the porous Co.sub.3O.sub.4 nanoparticles may range from 40 m.sup.2/g to 120 m.sup.2/g. This is about 400 times larger than the conventional 20 nm Co.sub.3O.sub.4 particles. If the BET specific surface area is less than 40 m.sup.2/g, the reactivity is lowered and, if the BET specific surface area is more than 120 m.sup.2/g, no narrow confinement effect of a specific size results, restricting an increase in reaction efficiency.

(19) According to an embodiment, the primary nanoparticles are formed in such a manner as to separate Co clusters by injecting positive ions into the lattices of crystalline Co.sub.3O.sub.4 particles, and the primary nanoparticles simultaneously transition to the amorphous phase.

(20) Specifically, if positive ions are injected into the lattices of crystalline Co.sub.3O.sub.4 particles, lattice distortion occurs. Defects in lattice, vacancy defects, or grain boundaries are given depending on the amount of positive ions injected, and the shape and concentration are precisely controlled to form lattices with a new property. The bonds in the lattices may be broken by worsening the lattice distortion and, thus, ultrafine particles may be formed.

(21) According to an embodiment, the intralattice bonds are broken by worsening lattice distortion, so that porous Co.sub.3O.sub.4 nanoparticles in the form of flocculated primary nanoparticles are formed. At this time, the primary nanoparticles are allowed to transition to the amorphous phase by the injected positive ions. The amorphous primary nanoparticles may be represented as chemical formula 1 below.
Co.sub.3O.sub.4-x  [Chemical formula 1]

(22) where 0≤x≤0.4.

(23) FIG. 1 schematically illustrates the above description. FIG. 1 illustrates a process in which as injection of positive ions worsens lattice distortion, Co.sub.3O.sub.4 particles turn into porous Co.sub.3O.sub.4 nanoparticles.

(24) The positive ions may be those of an alkaline metal or alkaline earth metal. For example, the positive ions may be those of lithium (Li), sodium (Na), potassium (K), magnesium (Mg), or calcium (Ca).

(25) The positive ions may be injected by a constant current or constant voltage method.

(26) Now described is a method for preparing porous Co.sub.3O.sub.4 nanoparticles. However, embodiments of the disclosure are not limited thereto.

(27) First, a Co.sub.3O.sub.4 thin film layer and an ion generation layer are placed on both sides of a separation plate (step a).

(28) Specifically, a Co.sub.3O.sub.4 thin film layer is formed on a metal sheet, and then, the metal sheet is placed on a cathode electrode.

(29) The Co.sub.3O.sub.4 thin film layer may be formed by coating the metal sheet with, e.g., mixed slurry of Co.sub.3O.sub.4 powder, a binder, and an organic solvent. The binder may include polyvinylidene fluoride (PVDF), Nafion, poly (acrylic acid)(PAA), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), or poly (ethylene oxide).

(30) The organic solvent may include N-methyl-2-Pyrrolidone (NMP), ethanol, tetrahydrofuran (THF), benzene, KOH/MeOH, MeOH, toluene, CH.sub.2Cl.sub.2, hexane, dimethylformamide (DMF), diisopropyl ether, diethyl ether, dioxane, dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), or chlorobenzene.

(31) The slurry may further include a conducting agent, such as super-p, Ketjenblack, acethylene black, sfg6, or CNT.

(32) The weight ratio of the Co.sub.3O.sub.4 powder to the binder in the slurry may be 90:10 to 99:1.

(33) The weight ratio of the mixture of the Co.sub.3O.sub.4 powder and the binder to the organic solvent may be 10 mg to 0.01 mL through 1 mL.

(34) The weight ratio of the conducting agent to the slurry may be 0.1 mg through 0.5 mg to 10 mg.

(35) The metal sheet may include a copper, aluminum, silver, or stainless steel sheet. As the metal sheet, any metal sheet with good conductivity may be used. A metal sheet with a reduction potential higher than about 0.0V vs. RHE may be electrochemically preferable.

(36) The thickness of the metal sheet may range from 10 μm to 100 μm.

(37) The thickness of the Co.sub.3O.sub.4 thin film layer may range from 10 μm to 400 μm.

(38) The cathode electrode may include stainless steel, aluminum, titanium, copper, nickel, or an alloy thereof. Other various metals or alloys with oxide film passivity may be adopted for the metal sheet.

(39) The thickness of the cathode electrode may range from 10 μm to 1,000 μm.

(40) Next, an ion generation layer is formed on an anode electrode.

(41) The ion generation layer may include an alkaline metal, an alkaline earth metal, an alkaline metal-containing alloy, or an alkaline earth metal-containing alloy, preferably lithium (Li), sodium (Na), potassium (K), magnesium (Mg), or calcium (Ca). The ion generation layer may be in the form of a metal sheet.

(42) The thickness of the ion generation layer may range from 10 μm to 1,000 μm.

(43) The anode electrode may include stainless steel, aluminum, titanium, copper, nickel, or an alloy thereof. Other various metals or alloys with oxide film passivity may be adopted for the metal sheet.

(44) The thickness of the anode electrode may range from 10 μm to 1,000 μm.

(45) The anode electrode and the cathode electrode are connected to terminals to which voltage is applied from the outside.

(46) Then, the Co.sub.3O.sub.4 thin film layer and the ion generation layer are placed on both sides of a separation plate.

(47) The separation plate may be formed by wetting a porous polymer plate with an electrolyte.

(48) The porous polymer plate may include, e.g., polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), alumina-coated polypropylene, cellulose, or polypropylene-polyethylene-polypropylene plate.

(49) The thickness of the porous polymer plate may range from 10 μm to 1,000 μm.

(50) The electrolyte may contain metal ions which are included in the ion generation layer.

(51) Specifically, the electrolyte may include MPF6 (where M is the alkaline and alkaline earth metal ion)-dissolved EC, DEC, DMC, or other organic solvent solution or aqueous solution.

(52) If the thickness of each layer is less than the lower limit, the layer may be easily torn and, if exceeding the upper limit, the layers may be difficult to join together due to an increase in the overall thickness.

(53) Thereafter, a voltage is applied to the Co.sub.3O.sub.4 thin film layer and the ion generation layer (step b).

(54) If a voltage is applied to the Co.sub.3O.sub.4 thin film layer and the ion generation layer, the potential difference causes oxidation, allowing alkaline metal or alkaline earth metal ions to build up on the ion generation layer. The alkaline metal or alkaline earth metal ions are dissolved in the electrolyte and pass through the separation plate to the Co.sub.3O.sub.4 thin film layer. The separation plate, although electrically insulated, may allow ions to pass through. The alkaline metal or alkaline earth metal ion generated from the ion generation layer trigger reduction on the Co.sub.3O.sub.4 thin film layer, allowing the ions to be injected into the lattices. The injected ions may cause lattice distortion and, depending on the degree of distortion, intralattice defects may occur.

(55) The degree of distortion may be adjusted depending on the degree of ion injection (voltage and time). As the voltage initially applied reduces, more ions may be injected into the Co.sub.3O.sub.4 thin film layer, increasing lattice distortion. The defects may include vacancies, oxygen vacancies, or boundaries. If the lattice bonds are broken by worsening the lattice distortion, ultra-fine particles (primary nanoparticles) may be generated and, at this time, the primary nanoparticles may be rendered to transition from the crystalline phase to the amorphous phase by the lattice distortion. If the voltage is further lowered, the ions may be alloyed with the Co metal, resulting in the formation of a cobalt alloy. Thus, a caution needs to be taken. Such a proper amount of ions as to break the lattice bonds needs to be injected. Injection of less ions may cause defects but fail to break the bonds, and injection of more ions may result in the formation of a cobalt alloy.

(56) It is preferable to place the metal sheet on the Co.sub.3O.sub.4 thin film layer. By its conductivity, the metal sheet may transfer electrons to the Co.sub.3O.sub.4 thin film layer. As such, because the metal sheet is stable upon injecting alkaline metal or alkaline earth metal ions into the Co.sub.3O.sub.4 thin film layer by applying voltage, application of the metal sheet is preferable.

(57) To separate the porous Co.sub.3O.sub.4 nanoparticles in the form of flocculated amorphous primary nanoparticles, the Co.sub.3O.sub.4 thin film layer, the ion generation layer, and the separation plate are disassembled to separate the Co.sub.3O.sub.4 thin film layer off, and the Co.sub.3O.sub.4 thin film layer is washed with an organic solvent.

(58) The organic solvent washes the electrolyte off the Co.sub.3O.sub.4 thin film layer. According to reactions, additional washing may be performed with distilled water and/or an acid solution of 0.1 mM to 1 mM.

(59) The organic solvent may include acetone, ethanol, tetrahydrofuran (THF), benzene, KOH/MeOH, MeOH, toluene, CH.sub.2Cl.sub.2, hexane, dimethylformamide (DMF), di-isopropyl ether, diethyl ether, dioxane, dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), or chlorobenzene.

(60) Specifically, the flocculated porous Co.sub.3O.sub.4 nanoparticles taken off while the binder is washed off the Co.sub.3O.sub.4 thin film layer by the organic solvent are put aside, and the remaining alkaline metal or alkaline earth metal ions are washed out with distilled water. In some case, additional washing may be carried out with the acid solution, and water is injected for neutralization. The washed-off porous Co.sub.3O.sub.4 nanoparticles may be dispersed in water or organic solvent or may be dried into a powder.

Embodiment

(61) Hereinafter, embodiments of the present disclosure are described in detail. However, the scope of the disclosure is not limited thereto.

Example of Manufacturing a Device for Preparing Porous Co.SUB.3.O.SUB.4 .Nanoparticles

(62) The Co.sub.3O.sub.4 powder (as commercially available from Avention, Inc., 0.94 g), Super-p (as commercially available from MTI Korea, 0.03 g), and polyvinylidene fluoride (as commercially available from Sigma-Aldrich, 0.03 g) were mixed together, and N-methyl-2-pyrrolidone (1 mL) (NMP, Sigma-Aldrich) of 1 mL was added to the mixture, thus preparing slurry. The slurry was casted on a 25 μm-thick copper sheet and was then dried in a vacuum oven, forming a 200 μm-thick Co.sub.3O.sub.4 thin film layer. The formed Co.sub.3O.sub.4 thin film layer was cut to a size of 8×8 cm.sup.2. A 400 μm-thick stainless steel (as commercially available from Sigma-Aldrich) sheet was cut to a size of 10×10 cm.sup.2 and was used as an electrode. The electrode was placed on a case, with the copper sheet contacting the electrode, thereby forming an ion injector. A 20 μm-thick, 12×12 cm.sup.2-sized polypropylene plate was placed on the Co.sub.3O.sub.4 thin film layer of the ion injector. 1M of LiPF6 EC/DEC electrolyte was dropped on the polypropylene plate and then was wet, forming a separation plate. A 400 μm-thick lithium sheet was cut to a size of 10×10 cm.sup.2 and was placed on the electrode on the case, forming an ion generation layer. The separation plate was positioned between the Co.sub.3O.sub.4 thin film layer and the lithium sheet, and the case was fastened up so that each layer inside comes in tight contact.

(63) The reaction of the Co.sub.3O.sub.4 thin film layer according to application of a voltage to the porous Co.sub.3O.sub.4 nanoparticles preparing device was measured and shown in FIG. 2.

(64) An open circuit voltage in the so-manufactured porous Co.sub.3O.sub.4 nanoparticles preparing device was measured, and a resultant value of about 3V relative to lithium was obtained. Thereafter, as a constant current of −200 μA is applied, the voltage gradually reduces from 3V and reaction thus occurs. If a reaction occurs, the curve turns flat at a specific reaction voltage. This indicates that a specific reaction continues to occur at the specific voltage, and the application of current continues until the reaction ends. The application of current may be regarded as the reaction occurring as, as many lithium ions as the number of electrons move.

(65) Resultantly, between 1.2V and 0.8V, a conversion reaction is caused by the lithium ions and, as oxygen is taken off Co.sub.3O.sub.4, the bonds increase in the early stage but, later, gradually decrease, so that atomization begins. Then, at a lower voltage, an alloying reaction arises in which the lithium ions enter the Co.sub.3O.sub.4 lattices, forming a new phase. At this time, the Co.sub.3O.sub.4 particles transition to the amorphous phase.

Embodiment Preparation of Porous Co.SUB.3.O.SUB.4 .Nanoparticles

(66) −200 uA/cm.sup.2 was applied to the porous Co.sub.3O.sub.4 nanoparticles preparing device, which was manufactured as above, to apply up to 0V (vs. Li+/Li). After the reaction ends, the porous Co.sub.3O.sub.4 nanoparticles preparing device was disassembled, and the Co.sub.3O.sub.4 thin film layer was taken apart. The leftover electrolyte was washed off the Co.sub.3O.sub.4 thin film layer using acetone. Next, the flocculated Co.sub.3O.sub.4 particles, taken off while the binder was washed off with the organic solvent, were put aside. Or, if not taken off by the organic solvent, the flocculated Co.sub.3O.sub.4 particles may be physically scrapped off and then put aside. Then, the Co.sub.3O.sub.4 particles were washed with 1 mM of acetone and then with water, thereby obtaining porous Co.sub.3O.sub.4 nanoparticles.

COMPARISON EXAMPLE

Co.SUB.3.O.SUB.4 .Particles

(67) The Co.sub.3O.sub.4 powder (as commercially available from Avention, Inc., 20 nm particle size) used in the above preparing example was used as a comparison example.

EXPERIMENTAL EXAMPLE

Experimental Example Measure Co.SUB.3.O.SUB.4 .Particle Size

(68) FIGS. 3, 4, and 5 show the results of TEM measurement, STEM measurement, and tomo-STEM measurement, respectively, for the embodiment and the comparative example.

(69) Referring to FIG. 3, the TEM measurement shows that the particles overlap and, thus, the size and inter-particle boundary are difficult to precisely or clearly identify. Thus, STEM measurement was performed. Referring to FIG. 4, it may be identified that the porous Co.sub.3O.sub.4 nanoparticles of the embodiment include about 1 nm primary nanoparticles. Tomo-STEM analysis was performed to figure out the particle size in the three-dimensional (3D) structure phase. Referring to FIG. 5, it may be identified that the porous Co.sub.3O.sub.4 nanoparticles of the embodiment three-dimensionally include about 1 nm primary nanoparticles.