KR101556299B1 - Cathode for lithium secondary battery comprising amorphous lithium transition metal oxide coating layer and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a positive electrode for a lithium secondary battery comprising a positive electrode active material layer on which a slurry containing a positive electrode active material, a binder and a conductive agent is applied, and an amorphous lithium transition metal oxide coating layer formed on the positive electrode active material layer, will be.
According to the cathode for a lithium secondary battery manufactured in the present invention, an amorphous lithium transition metal oxide coating layer is formed on the cathode active material layer by the RF sputtering method to improve the electrical conductivity of the electrode and act as a protective film against unstable electrolytes, , It is possible to provide a lithium secondary battery having excellent charge / discharge cycle characteristics even at high energy and high output.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode for a lithium secondary battery including an amorphous lithium transition metal oxide coating layer,

The present invention relates to a positive electrode for a lithium secondary battery comprising an amorphous lithium transition metal oxide coating layer and a method of manufacturing the same, and more particularly, to a positive electrode for a lithium secondary battery comprising a positive electrode active material layer coated with a slurry containing a positive electrode active material, A positive electrode for a lithium secondary battery including an amorphous lithium transition metal oxide coating layer formed on a positive electrode active material layer, and a method of manufacturing the same.

Recently, with regard to the trend toward miniaturization and light weight of portable electronic devices, there is an increasing need for high performance and large capacity of lithium secondary batteries to be used as power sources for these devices. Also, there is an intensive research on lithium secondary batteries having excellent stability and high economy .

In general, composite metal oxides such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , and LiNi _{1 -x} Co _x O ₂ (0 <x <1) are used as a cathode active material of a lithium secondary battery. Of these cathode active materials, LiCoO ₂ is the first commercially available material because of its excellent electrode life and high-speed discharge efficiency, but it is disadvantageous in that it is expensive. LiNiO ₂ , which is a nickel-based positive electrode active material, exhibits battery characteristics of the highest discharge capacity, but has a disadvantage in that it has a short life span and is vulnerable at high temperatures. On the other hand, LiMn ₂ O ₄ is easier to synthesize, less expensive, and environmentally friendly, but Mn is present on an average of 3.5 sides. Due to the Jnn-Teller twist phenomenon caused by local Mn ^{3 +} ions during charge / discharge, lithium manganese oxide And the manganese ion dissolves in the electrolyte due to the reaction with the organic solvent used as the electrolyte, and the life of the electrode is drastically reduced, which is more severe at high temperatures.

Accordingly, a method of replacing a lithium metal oxide cathode active material component with a metal component and a method of forming a metal oxide coating layer on the entire surface of the cathode active material have been proposed to solve the above problems.

Specifically, the conventional techniques are described as follows. A cathode active material including a glass coating layer, which is an amorphous oxide coated on lithium oxide, having improved high temperature storage characteristics and lifetime characteristics, is prepared by forming a metal oxide coating layer on the entire surface of a cathode active material (Korean Unexamined Patent Publication No. 10-2002-0066548), a method of forming a metal oxide, a metal hydroxide, a metal oxyhydroxide, a metal oxycarbonate, a metal hydroxycarbonate, or an element derived from an acid and a base on the surface of a cathode active material (Korean Patent Laid-Open No. 10-2003-0033913) is known.

LiNi _x M _y Co _z O ₂ (M is at least one element of Mn and Al and x + y + z is an element improving the performance of LiNiO ₂ by replacing metal components in the lithium metal oxide cathode active material component. = 1) has been reported (JP-A-12-149945).

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide an amorphous lithium transition metal oxide coating layer on a positive electrode active material layer to improve the electrical conductivity of the electrode and act as a protective layer against unstable electrolyte, And to provide a positive electrode for a lithium secondary battery having improved charge / discharge cycle characteristics even at high energy and high output, and a method for manufacturing the same.

In order to accomplish the above object, the present invention provides a cathode active material comprising a cathode active material layer on which a slurry containing a cathode active material, a binder and a conductive agent is applied on a current collector; And an amorphous lithium transition metal oxide coating layer formed on the cathode active material layer.

The positive electrode active material is a composite oxide of LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _{1 -x} Co _x O ₂ (0 <X <1), Li-Ni-Mn composite oxide, Oxide, and the like.

The binder is any one selected from the group consisting of polyethylene, polypropylene, polyvinyl alcohol, polyvinyl chloride, styrene-butadiene rubber, cellulose-based polymer, nitrile-based polymer, and fluorine-based polymer.

The conductive agent is characterized by being conductive carbon, conductive metal or conductive polymer.

Wherein the amorphous lithium transition metal oxide is represented by the following general formula (1).

&Lt; General Formula 1 &

Li _x -Mn _{2- y} -M _y -O _z (0.9 = x = 1.2, 0.35 = y = 0.85, 3.8 = z = 4.2)

(Wherein M is one or more selected from the group consisting of Ni, Cu, Cr, Co, Fe, Ti, V, Zn and Mo)

The amorphous lithium transition metal oxide coating layer has a thickness of 150 to 200 nm.

Further, the present invention provides a positive electrode for a lithium secondary battery, A negative electrode containing a negative active material; Separation membrane; And a non-aqueous electrolyte.

The present invention also provides a method for producing a slurry, comprising: i) dispersing a cathode active material, a binder and a conductive agent in an organic solvent to obtain a slurry; ii) applying the slurry onto a current collector and drying to form a cathode active material layer; And iii) depositing an amorphous lithium transition metal oxide coating layer on the positive electrode active material layer by RF sputtering. The present invention also provides a method for manufacturing a positive electrode for a lithium secondary battery.

The positive electrode active material in step i) may be LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _{1 -x} Co _x O ₂ (0 <X <1) -Co-based composite oxides.

The binder in step i) is any one selected from the group consisting of polyethylene, polypropylene, polyvinyl alcohol, polyvinyl chloride, styrene-butadiene rubber, cellulose-based polymer, nitrile-based polymer, and fluorine-based polymer.

The conductive agent in step i) is characterized by being conductive carbon, conductive metal or conductive polymer.

The organic solvent in step i) is any one selected from the group consisting of water, methanol, ethanol, propanol, isopropanol, N-methylpyrrolidone, and mixtures thereof.

The current collector in the step ii) is an aluminum foil.

In the RF sputtering method in the step iii), 50 to 200 W of microwave, argon gas of 80 to 100 sccm, and oxygen gas of 5 to 25 sccm are charged and deposited for 30 to 40 minutes.

The amorphous lithium transition metal oxide of step iii) is represented by the following general formula (1).

&Lt; General Formula 1 &

Li _x -Mn _{2- y} -M _y -O _z (0.9 = x = 1.2, 0.35 = y = 0.85, 3.8 = z = 4.2)

(Wherein M is one or more selected from the group consisting of Ni, Cu, Cr, Co, Fe, Ti, V, Zn and Mo)

The amorphous lithium transition metal oxide coating layer in step iii) has a thickness of 150 to 200 nm.

According to the cathode for a lithium secondary battery manufactured in the present invention, an amorphous lithium transition metal oxide coating layer is formed on the cathode active material layer by the RF sputtering method to improve the electrical conductivity of the electrode and act as a protective film against unstable electrolytes, , It is possible to provide a lithium secondary battery having excellent charge / discharge cycle characteristics even at high energy and high output.

1 is a conceptual diagram illustrating a manufacturing process of a positive electrode for a lithium secondary battery including an RF sputtering process according to an embodiment of the present invention.
FIG. 2 (a) is a cross-sectional view of a cathode in which an amorphous lithium transition metal oxide coating layer is formed on a cathode active material layer according to an embodiment of the present invention, and FIG. 2 (b) A scanning electron microscope (SEM) photograph of the cross section of the anode which was not observed.
3 is an X-ray diffraction (XRD) graph showing the crystal structure of the surface of an amorphous lithium transition metal oxide coating layer formed according to an embodiment of the present invention.
4 is an X-ray diffraction (XRD) graph showing the crystal structure of a cathode active material layer in which an amorphous lithium transition metal oxide coating layer is not formed according to a comparative example of the present invention.
5 is a graph showing changes in discharge capacity according to the number of charging / discharging cycles of a lithium secondary battery manufactured from Examples and Comparative Examples of the present invention.
6 is a differential scanning calorimetry (DSC) graph showing the thermal characteristics of a lithium secondary battery produced from Examples and Comparative Examples of the present invention.

Hereinafter, a cathode for a lithium secondary battery including an amorphous lithium transition metal oxide coating layer according to the present invention and a method for producing the same will be described in detail with reference to the drawings.

The present invention relates to a positive electrode active material layer in which a slurry containing a positive electrode active material, a binder and a conductive agent is applied on a current collector; And an amorphous lithium transition metal oxide coating layer formed on the cathode active material layer.

First, in the present invention, LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ or LiNi _{1 -x} Co _x O ₂ (0 <X <1), which is a cathode active material for a lithium secondary battery, is preferably used as the cathode active material, A Li-Ni-Mn composite oxide or a Li-Ni-Mn-Co composite oxide may be used without limitation.

In addition, the binder may be prepared by uniformly dispersing the cathode active material, the conductive agent, and the like in an organic solvent to form a uniform slurry, keeping the shape of the electrode plate by binding the powdery cathode active material and the conductive agent well, Can be used without limitation as long as it can bond and electrically conduct. Examples thereof include general plastics such as polyethylene, polypropylene, polyvinyl alcohol, and polyvinyl chloride; synthetic rubbers such as styrene-butadiene rubber; and cellulose-based ones such as methyl cellulose, ethyl cellulose, carboxymethyl cellulose, or hydroxypropyl cellulose Polymer or a nitrile-based polymer such as polyacrylonitrile, poly (acrylonitrile-methyl methacrylate) copolymer, or poly (acrylonitrile-methacrylic acid) copolymer, or polyvinylidene fluoride, Fluorine-based polymers such as fluoroethylene, poly (vinylidene fluoride-hexafluoropropylene) copolymer and the like are preferable, and polyvinylidene fluoride is particularly preferably used.

Examples of the conductive carbon include natural graphite, artificial graphite, carbon black, acetylene black, carbon black, carbon nanotubes, or carbon fibers as the conductive material, and conductive metals such as carbon black, Copper, nickel, aluminum, silver, or the like. As the conductive polymer, polyphenylene, polypyrrole, or polythiophene may be preferably used, but the present invention is not limited thereto.

In the present invention, an amorphous lithium transition metal oxide coating layer is formed on the cathode active material layer containing the cathode active material, the binder and the conductive agent. The amorphous lithium transition metal oxide may be represented by the following general formula (1).

&Lt; General Formula 1 &

Li _x -Mn _2-y -M _y -O _z (0.9? X? 1.2, 0.35? _Y? 0.85, 3.8? _Z ? 4.2)

(Wherein M is one or more selected from the group consisting of Ni, Cu, Cr, Co, Fe, Ti, V, Zn and Mo)

The thickness of the amorphous lithium transition metal oxide coating layer is preferably 150 to 200 nm. If the thickness of the coating layer is less than 150 nm, it may be difficult to perform the function of a protective film for inhibiting the elution of manganese from the unstable electrolyte. If it is more than 200 nm, adhesion with the positive electrode active material layer is deteriorated and deposition of the coating layer is not smooth.

The present invention also provides a method for producing a slurry, comprising: i) dispersing a cathode active material, a binder and a conductive agent in an organic solvent to obtain a slurry; ii) applying the slurry onto a current collector and drying to form a cathode active material layer; And iii) depositing an amorphous lithium transition metal oxide coating layer on the positive electrode active material layer by RF sputtering. The present invention also provides a method for manufacturing a positive electrode for a lithium secondary battery.

First, the cathode active material, the binder and the conductive agent in step i) are used as they are. The organic solvent is not particularly limited as long as it can uniformly disperse the cathode active material, the binder and the conductive agent. Specific examples thereof include water, methanol, ethanol, propanol, isopropanol, N-methylpyrrolidone, Is preferably used.

Next, an aluminum foil is used as the current collector in the step ii), and the mixture slurry obtained in the step i) is applied to an aluminum foil and then dried at 70 to 80 for 6 to 8 hours to form a cathode active material layer do.

Subsequently, an amorphous lithium transition metal oxide coating layer is deposited on the cathode active material layer formed in the step ii) by RF sputtering. The amorphous lithium transition metal oxide coating layer is deposited by applying 50 to 200 W of microwave, 80 to 100 sccm of argon gas and 5 to 25 sccm of oxygen gas Deposition for 30 to 40 minutes. When the flow rate of the argon gas is less than 80 sccm, the amorphous lithium transition metal oxide coating layer may be partially deposited on the cathode active material layer. If the flow rate of the argon gas is more than 100 sccm, the thickness of the coating layer becomes too thick The adhesion between the positive electrode active material layer and the positive electrode active material layer is lowered, so that the deposition of the coating layer may not be performed smoothly. In addition, if the flow rate of the oxygen gas supplied during the RF sputtering is out of the above range, it is difficult to expect the formation of the amorphous lithium transition metal oxide coating layer.

When the RF sputtering process in the step iii) is completed, an amorphous lithium transition metal oxide coating layer is deposited on the cathode active material layer. The amorphous lithium transition metal oxide, which is a component of the coating layer, is as shown in the following Formula 1, The thickness of the coating layer is preferably 150 to 200 nm as mentioned above.

In addition, the lithium secondary battery is generally manufactured using an anode, a cathode, a separator, and an electrolyte as essential components, and the present invention is also applicable to a cathode for a lithium secondary battery manufactured according to the present invention; A negative electrode containing a negative active material; A separator, and a non-aqueous electrolyte.

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

( Example )

(LiMn ₂ O ₄ ), a conductive agent (Denka Black; DB) and a binder (polyvinylidene fluoride) were mixed in a weight ratio of 92: 4: 4 and dispersed in N-methylpyrrolidone as an organic solvent The mixture was uniformly stirred with a high-speed stirrer (5,000 rpm) to obtain a slurry. The slurry was applied to an aluminum foil as a collector and dried at 80 for 8 hours to form a cathode active material layer. Then RF sputtering by using the microwave devices 100 W, 93 sccm argon gas and oxygen gas in the lithium transition metal oxide LiNi _{0 .5} and 15 sccm In the above positive electrode active material layer Mn _{1 .5} O ₄ to deposit the coating layer to the 183.6 nm thick Thereby preparing a positive electrode for a lithium secondary battery. The prepared positive electrode was used as a working electrode of a half cell, and a lithium metal foil was used as a counter electrode. Polypropylene wetted with an electrolyte was used as a separation membrane. Ethylene carbonate having 1.3 M LiPF ₆ salt dissolved therein : Ethyl-methyl carbonate (volume ratio 3: 7) was used to prepare a half-cell.

FIG. 1 is a conceptual diagram illustrating a manufacturing process of a cathode for a lithium secondary battery including an RF sputtering process according to an embodiment of the present invention.

( Comparative Example )

A half cell was fabricated in the same manner as in the above example except that the step of depositing a lithium transition metal oxide by RF sputtering was not performed.

FIG. 2 (a) is a cross-sectional view of a cathode in which an amorphous lithium transition metal oxide coating layer is formed on a cathode active material layer according to an embodiment of the present invention, and FIG. 2 (b) (SEM) photograph of the cross section of the anode which was not observed. As shown in FIG. 2 (a), LiMn ₂ O ₄ Over between the particle and the particle LiNi _{0 .5} Mn _{1 .5} O of the lithium-transition metal oxide fine particles ₄ were however found that the filling, from the FIG. 2 (b) is a cathode active material LiMn ₂ O ₄ It was confirmed that the composition of the particles was good, but the intervals between the particles were not uniform.

FIG. 3 is an X-ray diffraction (XRD) graph showing the crystal structure of the surface of the amorphous lithium transition metal oxide coating layer formed according to an embodiment of the present invention. In order to confirm the crystal structure of the surface of the deposited lithium transition metal oxide coating layer, by depositing a LiNi _{0 .5} Mn _{1 .5} O ₄ in the same manner as in RF sputtering performed in the present invention shows the results of analysis on an X-ray diffraction pattern. FIG deposition thickness is 183.6 nm, as shown in photos of a scanning electron microscope (SEM) shown with the third graph, the X-ray diffraction pattern analysis of the deposited LiNi do not show clearly any characteristic peaks attributable to the crystal structure _0.5 Mn _1.5 O ₄ was an amorphous structure.

FIG. 4 is an X-ray diffraction (XRD) graph showing the crystal structure of a cathode active material layer in which an amorphous lithium transition metal oxide coating layer is not formed according to a comparative example of the present invention. .

FIG. 5 is a graph showing the relationship between the various currents of the lithium secondary batteries manufactured in Examples and Comparative Examples of the present invention, which includes the positive electrode for a lithium secondary battery in which an amorphous lithium transition metal oxide coating layer is deposited on the positive electrode active material layer by RF sputtering according to the present invention. FIG. 5 is a graph showing a change in discharge capacity according to density. FIG. As shown in FIG. 5, the capacity reduction rates of the embodiment and the comparative example are 98.5% and 96.9% at 0.1C-rate, respectively. However, at 4C-rate, which is a relatively high rate characteristic, And 79.6%, respectively. This shows that LiNi _0.5 Mn _1.5 O ₄ containing lithium is deposited on the cathode active material layer to improve the conductivity of the lithium ion due to lithium, in addition to the stable structure.

6 is a differential scanning calorimetry (DSC) graph in which the temperature and the calorific value for ignition by raising the temperature inside the battery are measured to evaluate the high-temperature stability of the lithium secondary battery manufactured from Examples and Comparative Examples of the present invention, The ignition temperature of the battery according to the embodiment of the present invention is 300.2 and the calorific value is 815.75 J / g. In the case of the comparative example, the ignition temperature is 298.1 and the calorific value is 919.3 J / g, The lithium secondary battery including the positive electrode on which the amorphous lithium transition metal oxide coating layer is deposited by the RF sputtering method on the positive electrode active material layer manufactured according to the embodiment of the present invention is remarkably improved in thermal stability .

Claims (15)

A cathode active material layer formed by applying a slurry containing a cathode active material, a binder and a conductive agent on a current collector; And an amorphous lithium transition metal oxide coating layer deposited on the positive active material layer.
The method according to claim 1,
The positive electrode active material is a composite oxide of LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _{1 -x} Co _x O ₂ (0 <X <1), Li-Ni-Mn composite oxide, Wherein the positive electrode for lithium secondary battery is characterized in that the positive electrode
The method according to claim 1,
Wherein the binder is any one selected from the group consisting of polyethylene, polypropylene, polyvinyl alcohol, polyvinyl chloride, styrene-butadiene rubber, cellulose-based polymer, nitrile-based polymer, and fluorine-
The method according to claim 1,
Wherein the conductive agent is a conductive carbon, a conductive metal, or a conductive polymer.
The method according to claim 1,
Wherein the amorphous lithium transition metal oxide is represented by the following general formula (1)
&Lt; General Formula 1 &
Li _x -Mn _2-y -M _y -O _z (0.9? X? 1.2, 0.35? _Y? 0.85, 3.8? _Z ? 4.2)
(Wherein M is one or more selected from the group consisting of Ni, Cu, Cr, Co, Fe, Ti, V, Zn and Mo)
A positive electrode for a lithium secondary battery according to any one of claims 1 to 5;
A negative electrode containing a negative active material;
Separation membrane; And
A lithium secondary battery including a non-aqueous electrolyte
i) dispersing the cathode active material, the binder and the conductive agent in an organic solvent to obtain a slurry;
ii) applying the slurry onto a current collector and drying to form a cathode active material layer; And
and iii) depositing an amorphous lithium transition metal oxide coating layer on the positive electrode active material layer by RF sputtering.
8. The method of claim 7,
The positive electrode active material in step i) may be LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _{1 -x} Co _x O ₂ (0 <X <1) -C-based composite oxide, and a method for producing a positive electrode for a lithium secondary battery,
8. The method of claim 7,
The binder in step i) is any one selected from the group consisting of polyethylene, polypropylene, polyvinyl alcohol, polyvinyl chloride, styrene-butadiene rubber, cellulose-based polymer, nitrile-based polymer, and fluorine- Method for manufacturing positive electrode for battery
8. The method of claim 7,
A method for producing a positive electrode for a lithium secondary battery, wherein the conductive agent in step i) is a conductive carbon, a conductive metal or a conductive polymer
8. The method of claim 7,
The organic solvent in step i) is any one selected from the group consisting of water, methanol, ethanol, propanol, isopropanol, N-methylpyrrolidone, and mixtures thereof.
8. The method of claim 7,
Wherein the current collector in the step ii) is an aluminum foil.
8. The method of claim 7,
The RF sputtering method in the step iii) is carried out by charging 50 to 200 W of microwave, argon gas of 80 to 100 sccm and oxygen gas of 5 to 25 sccm for 30 to 40 minutes.
8. The method of claim 7,
Wherein the amorphous lithium transition metal oxide in step iii) is represented by the following general formula 1:
&Lt; General Formula 1 &
Li _x -Mn _2-y -M _y -O _z (0.9? X? 1.2, 0.35? _Y? 0.85, 3.8? _Z ? 4.2)
(Wherein M is one or more selected from the group consisting of Ni, Cu, Cr, Co, Fe, Ti, V, Zn and Mo)
8. The method of claim 7,
Wherein the amorphous lithium transition metal oxide coating layer in step iii) has a thickness of 150 to 200 nm.

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