KR20090076277A - Cathode for battery, method for manufacturing thereof, and lithium secondary battery comprising the same - Google Patents

Abstract

A positive electrode is provided to prevent a crystal structure of the positive electrode form being collapsed, by suppressing the reaction of electrolyte and positive electrode, and to ensure cycleability excellent and high temperature storage capacity. A method for manufacturing a positive electrode comprises the steps of: coating a positive active material(20) on a current collector(10); and forming a metal oxide layer on a part or the whole of the positive electrode mixture by using an ALD(atomic layer deposition) method. The ALD(atomic layer deposition) method comprises the steps of: positioning a current collector applied with the positive electrode mixture within a chamber; adding metal atom within a chamber; adding purge gas within the chamber; adding an oxidizing agent at the upper part of the positive electrode mixture to form a metal oxide layer(50); and adding purge gas within the chamber to remove unreacted residual oxidizing agent.

Description

A cathode, a method of manufacturing the same, and a lithium secondary battery comprising the same

The present invention relates to a positive electrode having a positive electrode mixture applied to a positive electrode current collector and a metal oxide film applied to a part or all of the positive electrode mixture, a method of manufacturing the same, and a lithium secondary battery comprising the same. It forms a uniform charge distribution on the surface of the positive electrode active material and induces polarization to promote the movement of lithium ions, thereby suppressing the precipitation of metal ions to show improved performance during high temperature storage.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is increasing rapidly. Among them, many researches have been conducted and commercialized and widely used for lithium secondary batteries with high energy density and discharge voltage. It is used.

The lithium secondary battery has a structure in which a non-aqueous electrolyte containing lithium salt is impregnated in an electrode assembly having a porous separator interposed between a positive electrode and a negative electrode on which an active material is coated on a current collector. The cathode active material is mainly composed of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium composite oxide, and the like, and the cathode oxide is mainly composed of a carbon-based material.

However, such a lithium secondary battery generally has a tendency to significantly reduce its remaining capacity and recovery capacity when stored in a high temperature condition. This is because the metal component of the active material, in particular the positive electrode active material, elutes under the action of the electrolyte under high temperature conditions.

Therefore, various methods for improving performance degradation at high temperature storage have been proposed, but each of these methods has disadvantages.

For example, it is a method of reducing the elution amount of the metal element with time into the electrolyte by reducing the specific surface area of the active material (Y. Xia, et al. J. of Power Source 24,24-28 (1998)). Since the active material particles must be made large, long-term heat treatment is required, and the high-speed discharge characteristics and the low temperature characteristics of the electrodes are deteriorated due to the decrease of the reaction area and the increase in the diffusion distance of lithium ions.

Another method commonly used by other methods is the surface treatment method. In case of surface treatment of heterogeneous elements (Li ₂ CO ₃ , Na ₂ CO ₃ , K ₂ CO _3, etc.) to improve the high temperature preservation characteristics, the capacity decrease is severe and multiple processes must be added. There is this.

In addition, conventionally, a method of manufacturing a cathode by adding a metal oxide to a current collector by mixing a positive electrode active material, a conductive material, and a binder, or a secondary battery using an electrode surface-treated by a deposition method such as CVD is proposed.

Meanwhile, as interest in environmental problems has increased recently, researches on electric vehicles and hybrid electric vehicles, which can replace vehicles using fossil fuels such as gasoline and diesel vehicles, which are one of the main causes of air pollution, are being conducted. have. Research using a lithium secondary battery as a power source of such an electric vehicle, a hybrid electric vehicle, etc. has been actively conducted, and some commercialized and used.

As a cathode active material of a lithium secondary battery used as a power source of such an electric vehicle, a lithium manganese oxide having low cost and high safety has been studied. However, lithium manganese oxides have a problem that the high temperature storage characteristics are worse than lithium cobalt oxides and lithium nickel oxides.

Therefore, there is a high need for a technology capable of solving the high temperature characteristics of the positive electrode active material, particularly lithium manganese oxide, in a lithium secondary battery.

Therefore, in the present invention, the positive electrode active material is peeled off due to the decrease in adhesion between the positive electrode active material and the current collector of the conventional lithium secondary battery as described above. Etc. To solve the conventional problems.

Thus, the present invention was able to solve the above problems by applying a positive electrode mixture including a positive electrode active material, a conductive material, and a binder to the positive electrode current collector, and then coating a metal oxide on the positive electrode mixture using ALD deposition.

Accordingly, an object of the present invention is to provide a positive electrode of a lithium secondary battery having excellent high temperature storage capability and improved cycle characteristics.

In addition, another object of the present invention is to provide a method for producing a positive electrode having the above characteristics.

Still another object of the present invention is to provide a lithium secondary battery including the positive electrode.

By coating the high-k dielectric metal oxide on the current collector using the ALD deposition method as in the present invention, the charge distribution on the surface of the positive electrode active material is uniform and the polarization is induced to promote the movement of lithium ions. It suppresses the precipitation of and shows improved performance at high temperature storage.

In addition, the reaction between the electrolyte and the positive electrode active material prevents the crystal structure of the positive electrode from decaying, and the positive electrode active material is protected by the metal oxide coating film, thereby suppressing the collapse of the layered structure due to the desorption of lithium ions. Improves properties

In addition, by coating ALD (Atomic Layer Deposition) deposition method which is different from the conventional CVD method in coating metal oxide having high dielectric constant, it can be thinly and uniformly coated with desired thickness, and pure metal oxide can be applied without external contamination. Can be.

The positive electrode of the present invention for achieving the above object is characterized in that it comprises a metal oxide layer in part or all of the positive electrode mixture applied on the current collector.

In addition, a method of manufacturing a positive electrode for achieving another object of the present invention comprises the steps of applying a positive electrode mixture on the current collector; Forming a metal oxide layer on the part or all of the positive electrode mixture by ALD deposition, characterized in that it comprises.

In addition, the present invention is characterized by providing a lithium secondary battery comprising a positive electrode having the above characteristics in order to achieve another object.

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

The positive electrode of the present invention is a positive electrode mixture, which is a mixture of a positive electrode active material, a conductive material, and a binder (hereinafter referred to as "positive electrode mixture") on a current collector, and then includes a metal oxide layer thereon.

The positive electrode is prepared by applying the slurry of a positive electrode active material, a conductive material, and a binder on a positive electrode current collector, followed by drying, and optionally, a filler is further added to the mixture. The positive electrode current collector is generally made to a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver and the like can be used.

The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

Specific examples of the positive electrode active material of the present invention include a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + x} Mn _{2 - x} O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO _2, and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , Cu ₂ V ₂ O ₇ and the like; Ni-site type lithium nickel oxide represented by the formula LiNi _{1- x} MxO ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3; Formula LiMn _{2 - x} M _x O ₂ (wherein M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (wherein M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu, or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ and the like, but are not limited to these. Preferably, the positive electrode active material may be lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium manganese-cobalt-nickel oxide, or a combination of two or more thereof.

The binder is a component that assists the bonding of the active material and the conductive material to the current collector, and is generally added in an amount of 1 to 50 wt% based on the total weight of the electrode mixture. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoro Low ethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluorine rubber, various copolymers thereof, and the like.

The conductive material is a component for further improving the conductivity of the electrode active material, and may be added in an amount of 1 to 20 wt% based on the total weight of the electrode mixture. Such a conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The filler is optionally used as a component for inhibiting expansion of the positive electrode, and is not particularly limited as long as it is a fibrous material without causing chemical change in the battery. Examples of the filler include olefinic polymers such as polyethylene and polypropylene; Fibrous materials, such as glass fiber and carbon fiber, are used.

Hereinafter, a method of manufacturing a positive electrode according to the present invention will be described in detail.

First, a positive electrode mixture slurry including the positive electrode active material, the conductive material, the binder, and other additives is prepared on a metal current collector, and applied thereto. Application of the positive electrode mixture slurry is carried out by a method known in the art, and is not particularly limited.

Next, a metal oxide layer is formed on the cathode mixture by ALD deposition.

The ALD deposition method includes placing the current collector coated with a cathode mixture in a chamber, adding a metal atom into the chamber, adding a purge gas into the chamber, adding an oxidant into the chamber, and the chamber. Adding a purge gas into it.

When the current collector is located in the chamber, it is preferable to maintain the temperature inside the chamber at 200 to 500 ° C. If the temperature of the chamber is less than 200 ° C., it is not preferable because the reactivity of the reactants in the subsequent process is not easy, It is not preferable to exceed about 500 DEG C because crystallization of the thin film formed on the current collector is performed.

Further, the pressure inside the chamber is preferably adjusted to be about 0.1 to 3.0 torr. If the pressure is less than 0.1 torr, it is not preferable because the reactivity of the metal atoms added in the subsequent process is not easy, and if it exceeds about 3.0 torr, it is not preferable because the process control is not easy.

Referring to FIG. 1A, after placing the current collector 10 coated with the cathode mixture 20 in a chamber 100 that satisfies the above temperature and pressure conditions, the metal atoms 30 are inserted into the chamber. Is added as reactant. The metal atom has high dielectric properties, and is preferably selected from the group consisting of Zr, Hf, Ti, Ta, Y, Nb, Ba, Sr and their alloys.

Referring next to FIG. 1B, the first portion 30 ′ of the metal atom is formed by providing a reactant that is the metal atom 30 over the positive electrode mixture 20 located in the chamber 100 as described above. It is chemisorbed on the positive electrode mixture 20. In addition, the second portion 30 ″ excluding the first portion of the metal atom is physically adsorbed to the first portion or drifted on the chamber.

The next step is to provide a purge gas into the chamber. Examples of the purge gas include an inert gas such as argon gas or nitrogen gas. In this case, the purge gas is preferably provided for about 0.5 to 20 seconds. As such, by providing a purge gas into the chamber, the second portion drifted in the chamber or physically adsorbed to the first portion is removed as shown in FIG. 1C. As a result, precursor molecules 30a of the chemisorbed first portion remain on the positive electrode mixture.

Instead of providing the purge gas, the inside of the chamber may be maintained in a vacuum state for about 2 to 10 seconds to remove the second portion drifting in the chamber or physically adsorbed to the first portion.

In addition, the introduction of the purge gas and the vacuum purge may be performed together to remove the second part drifting in the chamber or physically adsorbed to the first part.

Next, as shown in FIG. 1D, an oxidant 40 is provided into the chamber after the above steps. As an example of the oxidizing agent, one selected from the group consisting of O ₃ , O ₂ , H ₂ O, plasma O ₂ and remote plasma O ₂ is preferably used alone, and two or more may be mixed and used in some cases.

In particular, in the present invention, it is preferable to provide O ₃ as the oxidant into the chamber for about 1 to 7 seconds. As such, by providing the oxidant, the oxidant chemically reacts with precursor molecules 30a, which is the first portion of the reactant material chemisorbed on the positive electrode mixture, to oxidize the precursor molecules.

Next, a purge gas of the same kind as described above is added to remove the oxidant which has not reacted with the first portion of the metal atom. The type and introduction time of the purge gas are the same as described above. As such, by providing a purge gas into the chamber, oxidants that do not chemically react with the metal atoms are removed from the chamber as shown in FIG. 1E.

Accordingly, the metal oxide layer 50 is formed on the positive electrode mixture 20 of the current collector as shown in FIG. 1F.

If necessary, the above steps may be repeated at least once. In this case, a structure 50 ′ in which a metal oxide layer is stacked is formed on the cathode mixture, and the thickness of the metal oxide layer is It is adjusted according to the number of repetitions of these.

On the other hand, the present invention also provides a secondary battery comprising a positive electrode having a metal oxide layer applied to a part or all of the positive electrode mixture as described above, a negative electrode produced by applying and drying a negative electrode material on a negative electrode current collector, and a separator There is a characteristic.

The negative electrode of the present invention is a negative electrode active material, for example, carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt, Ti which can be alloyed with lithium, and compounds containing these elements; Complexes of metals and compounds thereof with carbon and graphite materials; Lithium-containing nitrides, and the like, but are not limited thereto. Preferably, they are selected from the group consisting of crystalline carbon, amorphous carbon, silicon-based active material, tin-based active material, and silicon-carbon-based active material, alone or two or more. It may be a combination, and in addition to the conventional binder, conductive material, and other additives included in the negative electrode may be included, specific examples or contents thereof are sufficient to be added to the usual level.

The lithium secondary battery of the present invention has a structure in which a lithium salt-containing non-aqueous electrolyte is impregnated in an electrode assembly having a separator interposed between a positive electrode and a negative electrode.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheet or nonwoven fabric made of glass fiber or polyethylene; Kraft paper or the like is used. Typical examples currently on the market include Celgard series (Celgard ^TM 2400, 2300 (manufactured by Hoechest Celanese Corp.), polypropylene separator (manufactured by Ube Industries Ltd. or Pall RAI), and polyethylene series (Tonen or Entek).

In some cases, a gel polymer electrolyte may be coated on the separator to increase the stability of the battery. Representative examples of such gel polymers include polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile and the like. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The lithium salt-containing non-aqueous electrolyte consists of a nonaqueous electrolyte and a lithium salt. As the nonaqueous electrolyte, a nonaqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte, and the like are used.

As said non-aqueous electrolyte, N-methyl- 2-pyrrolidinone, a propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, for example , Gamma-butylo lactone, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, 4-methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolon, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxolon Aprotic organic solvents such as derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyroionate and ethyl propionate can be used. Can be.

Examples of the organic solid electrolytes include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymers containing ionic dissociating groups and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

The lithium salt is a good material to be dissolved in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, LiSCN, LiC (CF 3 SO 2) 3, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, 4-phenyl lithium borate, imide and the like can be used.

In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, etc. Nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride, etc. It may be. In some cases, in order to impart nonflammability, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high temperature storage characteristics.

1A to 1F illustrate a process of forming a metal oxide film of the present invention.

<Short description of drawing symbols>

100 chamber 10 anode current collector

20: positive electrode mixture 30: metal particles

30 ': first portion of chemisorbed metal particles

30 ": second portion of physisorbed or drifting metal particles

30a: first partial precursor of chemisorbed metal particles

40: oxidant 50: metal oxide film

50 ": Stacked metal oxide film

Claims (18)

A positive electrode comprising a metal oxide layer on part or all of the positive electrode mixture applied on the current collector. The anode of claim 1, wherein the metal atoms included in the metal oxide layer are one or more selected from the group consisting of Zr, Hf, Ti, Ta, Y, Nb, Ba, Sr, and alloys thereof. Applying a positive electrode mixture on the current collector; Forming a metal oxide layer on all or part of the positive electrode mixture by ALD deposition method. The method of claim 3, wherein The ALD deposition method Placing a current collector coated with the positive electrode mixture in a chamber; Adding metal atoms into the chamber; Adding a purge gas into the chamber; Adding an oxidant to the upper portion of the positive electrode mixture to form a metal oxide layer; And Adding a purge gas into the chamber to remove unreacted residual oxidant. The method of claim 4, wherein the temperature inside the chamber when the current collector is positioned is maintained at 200 to 500 ° C. 6. The method of claim 4, wherein the pressure inside the chamber when the current collector is positioned is maintained at 0.1 to 3.0 torr. The method of claim 4, wherein the metal atom is at least one selected from the group consisting of Zr, Hf, Ti, Ta, Y, Nb, Ba, Sr, and alloys thereof. The method of claim 4, wherein The first portion of the metal atoms added in the chamber is chemisorbed to the positive electrode mixture, the second portion of the metal atoms except the first portion is physically adsorbed, or drift in the chamber. . The method of claim 4, wherein the purge gas is at least one inert gas selected from the group consisting of argon gas and nitrogen gas. The method of claim 4, wherein And the second portion of the physically adsorbed metal atoms and the atoms drifting in the chamber are removed by the addition of the purge gas. The method of claim 4, wherein the oxidizing agent is at least one selected from the group consisting of O ₃ , O ₂ , H ₂ O, plasma O ₂ , and remote plasma O ₂ . The method of claim 4, wherein the oxidant reacts with the first portion of the metal atom to form a metal oxide film.  The method of claim 3, wherein the electrode is a cathode including a cathode active material, a conductive material, and a binder. The method of claim 13, wherein the positive electrode active material is a layered compound, or a compound substituted with one or more transition metals; Lithium manganese oxide represented by the formula Li _{1 + x} Mn _{2 - x} O ₄ , wherein x is 0 to 0.33; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxide; Ni-site type lithium nickel oxide represented by the formula LiNi _{1- x} MxO ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3; Formula LiMn _{2 -x} M _x O ₂ , wherein M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1, or Li ₂ Mn ₃ MO ₈ , where M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; And Fe ₂ (MoO ₄ ) ₃ . The method of claim 13, wherein the conductive material is at least one selected from the group consisting of graphite, carbon black, conductive fibers, metal powder, conductive whiskey, conductive metal oxide, and polyphenylene derivative. The method of claim 13, wherein the binder is polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone At least one member selected from the group consisting of tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluororubber, and various copolymers thereof. The manufacturing method of the positive electrode made into. A lithium secondary battery comprising the positive electrode according to any one of claims 1 to 2. 18. The lithium secondary battery of claim 17, wherein the battery is used in a medium to large battery system.

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