KR101662641B1 - Cathode for lithium secondary battery and lithium secondary battery including the same - Google Patents

Abstract

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same, and more particularly to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same. And a positive electrode active material layer formed on at least one surface of the positive electrode collector, wherein the positive electrode active material has an average particle diameter of 7 to 13 占 퐉, and Li (Ni _a Mn _b Co _{c ) O 2 (0.40≤a≤0.50, 0.30≤b≤0.40,} 0.15≤c≤0.25 and a + b + c = 1) of the first cathode active material and the average particle size of from 2 to 6 and ㎛, Li (Ni _x Mn _y Co _z ) O ₂ (0.35? X? 0.45, 0.25? Y? 0.35, 0.25? Z? 0.35 and x + y + z = 1), wherein the first cathode active material And the weight ratio of the second cathode active material is 6: 4 to 8: 2. The present invention also relates to a lithium secondary battery including the same.
According to an embodiment of the present invention, the output and cycle life characteristics of the lithium secondary battery can be improved by improving the reaction rate due to an increase in the total surface area of the cathode active material.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same,

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery comprising the same, and more particularly, to a lithium secondary battery including a positive electrode active material having a different composition and an average particle size, An anode, and a lithium secondary battery including the same.

As technology development and demand for mobile devices have increased, there has been a rapid increase in demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential, long cycle life, Batteries have been commercialized and widely used.

In recent years, there has been a growing interest in environmental issues, and as a result, electric vehicles (EVs) and hybrid electric vehicles (HEVs), which can replace fossil-fueled vehicles such as gasoline vehicles and diesel vehicles, And the like.

Such electric vehicles (EV) and hybrid electric vehicles (HEV) use nickel metal hydride (Ni-MH) secondary batteries as a power source or lithium secondary batteries having high energy density, high discharge voltage and output stability. Is required to be used for an electric vehicle for 10 years or more under severe conditions in addition to high energy density and characteristics capable of exhibiting large output in a short period of time. Therefore, it is inevitable that safety and long- . In addition, a secondary battery used in an electric vehicle (EV), a hybrid electric vehicle (HEV), and the like requires excellent rate characteristics and power characteristics depending on operating conditions of the vehicle.

At present, as a cathode active material of a lithium ion secondary battery, a lithium-containing cobalt oxide such as LiCoO ₂ of a layered structure, a lithium-containing nickel oxide such as LiNiO ₂ of a layered structure, LiMn ₂ O ₄ of a spinel crystal structure And lithium-containing manganese oxides such as lithium manganese oxide and lithium manganese oxide.

LiCoO ₂ has excellent properties such as excellent cycle characteristics and is widely used at present. However, LiCoO ₂ is low in safety, is expensive due to the resource limit of cobalt as a raw material, and has a limitation in mass use as a power source for fields such as electric vehicles. LiNiO ₂ is difficult to apply to actual mass production process at a reasonable cost due to its characteristics depending on its manufacturing method.

On the other hand, lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ have the advantage of using manganese which is rich in resources and environment-friendly as a raw material, and thus attracts much attention as a cathode active material capable of replacing LiCoO ₂ . However, these lithium manganese oxides also have a disadvantage of poor cycle characteristics and the like. LiMnO ₂ has a disadvantage in that the initial capacity is small and dozens of charge-discharge cycles are required until a certain capacity is reached. In addition, LiMn ₂ O ₄ has a disadvantage in that the cycle characteristics are seriously deteriorated as the cycle continues, and in particular, the cycle characteristics are drastically lowered due to decomposition of the electrolytic solution and elution of manganese at a temperature of 50 ° C or higher.

In such a situation, a lithium nickel-manganese composite oxide or a lithium nickel-manganese-cobalt composite oxide has been proposed in order to overcome or minimize the drawbacks of each cathode active material. However, Technology development for active materials is required.

Accordingly, an object of the present invention is to provide a novel composition for a lithium secondary battery having high output and high cycle life characteristics, and a lithium secondary battery including the same, which solves the above- will be.

According to an aspect of the present invention, there is provided a cathode current collector comprising: a cathode current collector; And a positive electrode active material layer formed on at least one surface of the positive electrode collector, wherein the positive electrode active material has an average particle diameter of 7 to 13 占 퐉, and Li (Ni _a Mn _b Co _{c ) O 2 (0.40≤a≤0.50, 0.30≤b≤0.40,} 0.15≤c≤0.25 and a + b + c = 1) of the first cathode active material and the average particle size of from 2 to 6 and ㎛, Li (Ni _x Mn _y Co _z ) O ₂ (0.35? X? 0.45, 0.25? Y? 0.35, 0.25? Z? 0.35 and x + y + z = 1), wherein the first cathode active material Wherein the weight ratio of the first cathode active material to the second cathode active material is 6: 4 to 8: 2, more preferably 7: 3 to 8: 2.

Here, the cathode active material may further include a third cathode active material of Li ₂ MnO ₄ .

In this case, the third cathode active material may include 25 to 50 wt% based on the total weight of the cathode active material.

The average particle diameter of the third cathode active material may be 10 to 15 탆.

On the other hand, there is provided a lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for the lithium secondary battery of the present invention.

According to an embodiment of the present invention, the output and cycle life characteristics of the lithium secondary battery can be improved by improving the reaction rate due to an increase in the total surface area of the cathode active material.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the invention and, together with the description of the invention, It should not be construed as limited.
1 is a graph comparing output characteristics of a lithium secondary battery including a positive electrode according to an embodiment of the present invention and one comparative example.
2 is a graph comparing cycle life characteristics of a lithium secondary battery including a positive electrode according to an embodiment of the present invention and one comparative example.

Hereinafter, the present invention will be described in detail. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

A positive electrode for a lithium secondary battery according to the present invention includes: a positive electrode collector; And a positive electrode active material layer formed on at least one surface of the positive electrode collector, wherein the positive electrode active material has an average particle diameter of 7 to 13 占 퐉, and Li (Ni _a Mn _b Co _{c ) O 2 (0.40≤a≤0.50, 0.30≤b≤0.40,} 0.15≤c≤0.25 and a + b + c = 1) of the first cathode active material and the average particle size of from 2 to 6 and ㎛, Li (Ni _x Mn _y Co _z ) O ₂ (0.35? X? 0.45, 0.25? Y? 0.35, 0.25? Z? 0.35 and x + y + z = 1), wherein the first cathode active material And the weight ratio of the second cathode active material is 6: 4 to 8: 2, more preferably 7: 3 to 8: 2.

Since the positive electrode active material of the present invention contains lithium nickel-manganese-cobalt oxide, it is possible to simultaneously achieve the improvement of capacity and energy density through nickel and the improvement of stability through manganese. Particularly, By including two or more different kinds of the cathode active materials, the output of the lithium secondary battery can be improved by improving the reaction rate due to the increase of the total surface area of the cathode active material, and by including two or more kinds of cathode active materials having different voltage profiles, The cycle life characteristics of the lithium secondary battery can be improved.

At this time, if the ratio of the second cathode active material is higher than the above value, the thickness expansion may be increased due to the increase of the side reaction, and the cell performance may be deteriorated.

Here, the cathode active material may further include a third cathode active material of Li ₂ MnO ₄ . The third cathode active material is very inexpensive and is effective in improving the output due to the high voltage profile.

At this time, the third cathode active material may include 25 to 50% by weight based on the total weight of the cathode active material. By satisfying these numerical ranges, the output and safety of the lithium secondary battery can be improved.

The size of the average particle size of the third cathode active material is not limited, but may be 10 to 15 탆 which is generally used because it is generally used.

According to another aspect of the present invention, there is provided a lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is a positive electrode for a lithium secondary battery according to the present invention A lithium secondary battery is provided.

At this time, the anode is prepared by applying a mixture of a cathode active material, a conductive material and a binder on a cathode current collector, followed by drying and pressing, and if necessary, a filler may be further added to the mixture.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel A surface treated with carbon, nickel, titanium, silver or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The conductive material may be typically added in an amount of 1 to 50 wt% based on the total weight of the mixture including the cathode active material.

Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey 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 binder is a component which assists in bonding of the positive electrode active material and the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 50% by weight based on the total weight of the mixture containing the positive electrode active material. Examples of such binders include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, poly Polytetrafluoroethylene, polypropylene, polyacrylic acid, and polytetrafluoroethylene (PTFE) may be used alone or in combination of two or more selected from the group consisting of polyvinyl pyrrolidone, polyethylene, tetrafluoroethylene, polytetrafluoroethylene, polytetrafluoroethylene and polytetrafluoroethylene.

The filler is not particularly limited as long as it is a fibrous material that is used selectively as a component for suppressing the expansion of the anode and does not cause chemical change in the battery. Examples of the filler include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fiber, carbon fiber and the like can be used.

On the other hand, the negative electrode is prepared by applying a mixture of a negative electrode active material, a conductive material and a binder on a negative electrode current collector, followed by drying and pressing. If necessary, a filler may be further added to the mixture.

The negative electrode active material may be a conventional negative electrode active material that can be used for a negative electrode of an electrochemical device. Examples of the negative electrode active material include lithium metal, lithium alloy, carbon, petroleum coke, activated carbon, carbon, graphite or other carbon-based materials.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples of the anode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, a surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

The conductive material, the binder and the filler may be the same as those used in the anode.

Meanwhile, the separator used in the present invention can be any porous polymer substrate commonly used in the art. For example, a polyolefin porous membrane or a porous polymer nonwoven fabric may be used, but the separator is particularly limited thereto It is not.

Examples of the polyolefin-based porous film include polyolefin-based polymers such as polyethylene, polypropylene, polybutylene, and polypentene, such as high-density polyethylene, linear low density polyethylene, low density polyethylene and ultra high molecular weight polyethylene, One membrane can be mentioned.

Examples of the porous polymer nonwoven fabric include polyolefin-based nonwoven fabrics such as polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyolefins such as polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, and polyethylenenaphthalene can be used alone or in combination. Or a nonwoven fabric formed of a polymer mixed with these. The structure of the nonwoven fabric may be a spun bond nonwoven fabric or a meltblown nonwoven fabric composed of long fibers.

The thickness of the porous polymer base material is not particularly limited but may be in the range of 5 탆 to 50 탆. The pore size and porosity present in the porous polymer base material are also not particularly limited, but may be 0.01 탆 to 50 탆 and 10-95% have.

In order to improve the mechanical strength of the separator and to improve the safety of the lithium secondary battery, the porous polymer substrate may further include a porous coating layer including inorganic particles and a polymeric binder on at least one surface of the porous polymer substrate.

Here, the inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles usable in the present invention are not particularly limited as long as the oxidation and / or reduction reaction does not occur in the operating voltage range of the applied lithium secondary battery (for example, 0 to 5 V based on Li / Li ⁺ ). Particularly, when inorganic particles having a high dielectric constant are used as the inorganic particles, the dissociation of the electrolyte salt, for example, the lithium salt in the liquid electrolyte, can be increased, and the ion conductivity of the electrolyte can be improved.

The polymer binder may be at least one selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-co chlorotrifluoroethylene, polyvinylidene fluoride-co-trichlorethylene, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinyl But are not limited to, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene, polyethylene oxide, polyarylate, cellulose acetate cellulose acetate, cellulose acetate butyrate, Cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan (which may be referred to as " pullulan, and carboxyl methyl cellulose, or a mixture of two or more thereof, but is not limited thereto.

In the porous coating layer, the polymeric binder is coated on a part or the entire surface of the inorganic particles, and the inorganic particles are connected and fixed to each other by the polymeric binder in an adhered state, and an interstitial volume an interstitial volume between the inorganic particles is formed, and an interstitial volume between the inorganic particles becomes an empty space to form pores. This void space becomes the pores of the porous coating layer, and it is preferable that these pores are equal to or smaller than the average particle diameter of the inorganic particles.

Meanwhile, the electrolyte salt included in the nonaqueous electrolyte solution which can be used in the present invention is a lithium salt. The lithium salt can be used without limitation as those conventionally used in an electrolyte for a lithium secondary battery. For example is the above lithium salt anion ^{F -, Cl -, Br -} , I -, NO 3 -, N (CN) 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF _{^{4 -, (CF 3) 3}} PF 3 -, (CF 3) 4 PF 2 -, (CF 3) 5 PF -, (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 - _{, (CF 3 SO 2) 2} N -, (FSO 2) 2 N -, CF 3 CF 2 (CF 3) 2 CO -, (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, ( CF ₃ SO ₂ ) ₃ C ^- , CF ₃ (CF ₂ ) ₇ SO ₃ ^- , CF ₃ CO ₂ ^- , CH ₃ CO ₂ ^- SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- .

Examples of the organic solvent included in the above-mentioned non-aqueous electrolyte include those commonly used in electrolytic solutions for lithium secondary batteries, such as ether, ester, amide, linear carbonate, cyclic carbonate, etc., Can be mixed and used.

Among them, a carbonate compound which is typically a cyclic carbonate, a linear carbonate, or a mixture thereof may be included.

Specific examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, Propylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halides thereof, or a mixture of two or more thereof. Examples of such halides include, but are not limited to, fluoroethylene carbonate (FEC) and the like.

Specific examples of the linear carbonate compound include any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate and ethyl propyl carbonate And mixtures of two or more of them may be used as typical examples, but the present invention is not limited thereto.

In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates in the carbonate-based organic solvent, are high-viscosity organic solvents having a high dielectric constant and can dissociate the lithium salt in the electrolyte more easily. In addition, such cyclic carbonates can be used as dimethyl carbonate and diethyl carbonate When a low viscosity, low dielectric constant linear carbonate is mixed in an appropriate ratio, an electrolyte having a higher electric conductivity can be produced.

As the ether in the organic solvent, any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether and ethyl propyl ether or a mixture of two or more thereof may be used , But is not limited thereto.

Examples of the ester in the organic solvent include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate,? -Butyrolactone,? -Valerolactone,? -Caprolactone,? -Valerolactone, ε-caprolactone, or a mixture of two or more thereof, but the present invention is not limited thereto.

The injection of the non-aqueous electrolyte may be performed at an appropriate stage of the production process of the lithium secondary battery, depending on the manufacturing process and required properties of the final product. That is, it can be applied before assembling the lithium secondary battery or at the final stage of assembling the lithium secondary battery.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example

(1) Preparation of positive electrode

On one surface of the current collector of aluminum foil of a sheet-like, the Li is 10 ㎛ average particle diameter as a positive electrode active material _{(Ni 0 .45 Mn 0 .35 Co} 0 .2) of O ₂ and an average particle diameter of 4 ㎛ Li (Ni _{0 .4} the one, as the conductive material Denka black, and a mixture of 80% by weight PVdF as a binder is 5% by weight and 15% by weight of the NMP solvent mixture in a weight ratio of 3: Mn _{0 .3 0 .3} Co) ₂ O 7 The dispersed cathode active material slurry was coated and dried to prepare a cathode for a lithium secondary battery.

(2) Production of lithium secondary battery

An electrode assembly was fabricated by interposing a polyethylene separator between the positive electrode for a lithium secondary battery manufactured in Example (1) and the negative electrode made of lithium foil. The electrode assembly was placed in a battery case, and an electrolytic solution to which 1 M LiPF ₆ was added was added to a nonaqueous solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 2, to prepare a coin type half cell.

Comparative Example

The use of that mixed in a weight ratio of 3: As a positive electrode active material has a mean particle diameter of 10 ㎛ of _{Li (Ni 0 .45 Mn 0 .35} Co 0 .2) O 2 with a mean particle diameter of 12 ㎛ of Li ₂ MnO ₄ 7 A coin-shaped half-cell was produced in the same manner as in Example.

Experimental study of lithium secondary battery

FIG. 1 is a graph comparing output characteristics of a lithium secondary battery including a positive electrode according to one embodiment of the present invention and one comparative example. FIG. 2 is a graph showing the output characteristics of the positive electrode according to one embodiment of the present invention and one comparative example Which is a graph showing the cycle life characteristics of a lithium secondary battery.

Referring to FIGS. 1 and 2, it can be seen that the output and cycle life characteristics of the battery are further improved compared to the comparative example.

The foregoing description is merely illustrative of the technical idea of the present invention, and various changes and modifications may be made by those skilled in the art without departing from the essential characteristics of the present invention. Therefore, the embodiments disclosed in the present invention are intended to illustrate rather than limit the scope of the present invention, and the scope of the technical idea of the present invention is not limited by these embodiments. The scope of protection of the present invention should be construed according to the following claims, and all technical ideas within the scope of equivalents should be construed as falling within the scope of the present invention.

Claims (6)

Anode collector; And
And a positive electrode active material layer formed on at least one surface of the positive electrode collector,
Wherein the cathode active material has an average particle size of 7 to 13 占 퐉 and Li (Ni _a Mn _b Co _c ) O ₂ (0.40? A? 0.50, 0.30? B? 0.40, 0.15? C? 0.25 and a + b + c = 1) and a first positive electrode active material having an average particle diameter of 2 to 6 占 퐉 and Li (Ni _x Mn _y Co _z ) O ₂ (0.35? X? 0.45, 0.25? Y? 0.35, 0.25? Z? y + z = 1), and the second positive electrode active material,
Wherein the weight ratio of the first cathode active material to the second cathode active material is from 7: 3 to 8: 2. delete delete delete delete A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode,
The lithium secondary battery according to claim 1, wherein the positive electrode is a positive electrode for a lithium secondary battery.

Images