KR20120138344A - Cathode active material and lithium secondary battery containing the same - Google Patents

Abstract

PURPOSE: A positive electrode active material is provided to prevent internal short due to precipitation of lithium metal on a negative electrode surface because a part of lithium cobalt oxide includes aluminum in a specific shape, thereby being capable of obtaining batter stability and remarkably excellent cycle performance. CONSTITUTION: A positive electrode active material comprises a lithium cobalt oxide and the lithium cobalt oxide comprises aluminum. The aluminum is located inside and on the lithium cobalt oxide. A part of the aluminum inside the lithium cobalt oxide is in the state of impurities and the other comprises a shape substituting Co. The content of the aluminum on the lithium cobalt oxide is 0.05-0.25 weight% based on the total weight of the positive electrode active material.

Description

Cathode Active Material and Lithium Secondary Battery Containing the Same

The present invention relates to a positive electrode active material for a secondary battery and a lithium secondary battery including the same. More specifically, the positive electrode active material includes lithium cobalt oxide, the lithium cobalt oxide includes aluminum, and the aluminum is lithium Located in and on the surface of the cobalt oxide, respectively, a part of the aluminum located inside is included in the form of impurities and the rest in the form of substituting Co, the positive electrode active material for secondary batteries having excellent cycle characteristics, and the same It relates to a lithium secondary battery.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing, and the most actively researched fields are power generation and storage using electrochemical reactions.

A representative example of an electrochemical device using such electrochemical energy is a secondary battery, and its use area is gradually increasing.

Recently, as the development and demand for portable devices such as portable computers, portable telephones, cameras, and the like, the demand for secondary batteries is rapidly increasing, and these secondary batteries exhibit high energy density and operating potential, and have a cycle life. Many studies have been conducted on this long, low self-discharge rate lithium battery and are commercially available and widely used.

In addition, as interest in environmental issues grows, researches on electric vehicles and hybrid electric vehicles, which can replace vehicles using fossil fuel, such as gasoline and diesel vehicles, which are one of the main causes of air pollution, are being conducted. . As a power source of such electric vehicles and hybrid electric vehicles, nickel-metal hydride secondary batteries are mainly used, but researches using lithium secondary batteries with high energy density and discharge voltage have been actively conducted and some commercialization stages are in progress.

Conventionally, a typical lithium secondary battery uses graphite as a negative electrode active material, and charging and discharging are performed while repeating a process in which lithium ions of a positive electrode are inserted into and detached from a negative electrode.

However, in such a repeated charging and discharging process, there is a lack of space inside the negative electrode into which the excess lithium ions detached from the positive electrode can be inserted, thereby causing lithium ions to precipitate as lithium metal on the negative electrode surface. This precipitation of lithium metal is more severe at high temperature storage, which leads to a decrease in the remaining capacity and recovery capacity of the battery.

Accordingly, various methods for preventing internal short circuits due to dendrite growth due to deposition of lithium metal on the surface of the cathode have been proposed.

For example, Japanese Patent No. 2,771,406 discloses a secondary battery in which an anode surface of at least the surface facing the cathode is coated with an insulator, a semiconductor, a composite film of an insulator and a semiconductor capable of transmitting ions involved in battery reaction. And cyclodextrins are exemplified as components of such membranes. However, the coating of the insulator film or the like on the surface of the positive electrode greatly reduces the lithium ion mobility inside the positive electrode, and further causes a decrease in conductivity at the positive electrode with high resistance, thereby seriously deteriorating the performance of the battery such as the rate characteristic. There is a problem.

Accordingly, there is a high need for a lithium secondary battery capable of exhibiting excellent charge / discharge cycle characteristics even at high temperature and high voltage while solving the above problems.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve the above-mentioned problems of the prior art and the technical problems required from the past.

The inventors of the present application, after extensive research and various experiments, as described later, a cathode active material for a secondary battery including lithium cobalt oxide, in which aluminum substitutes for a form of impurities and Co specific to a part of the lithium cobalt oxide. To develop a positive electrode active material for a secondary battery included in the form, and when manufacturing a secondary battery using the positive electrode active material, not only can improve the safety of the battery, but also found excellent cycle characteristics, the present invention Came to complete.

Accordingly, the cathode active material for a secondary battery according to the present invention is a cathode active material including lithium cobalt oxide, and the lithium cobalt oxide contains aluminum, and the aluminum is located on the inside and the surface of the lithium cobalt oxide, respectively. It provides a cathode active material for a secondary battery, characterized in that part of the aluminum is located in the form of impurities and the rest is included in the form of substituting Co.

Conventional typical lithium secondary battery, as described above, there is a problem that the lithium detached from the positive electrode in the charge and discharge process is precipitated on the surface of the negative electrode.

Thus, the positive electrode active material according to the present invention may include aluminum in a specific form inside and on the surface of the lithium cobalt oxide, thereby increasing resistance of the positive electrode, thereby preventing excessive movement of lithium ions from the positive electrode active material to the negative electrode, As a result, precipitation of lithium metal at the negative electrode can be prevented, and the cycle characteristics can be remarkably improved while ensuring the safety of the battery.

The content of aluminum located on the surface of the lithium cobalt oxide is greater than the amount located inside, and the content of aluminum located on the surface of the lithium cobalt oxide may be 0.05 to 0.25 wt% based on the total weight of the positive electrode active material. If the content of aluminum is too small, it may be difficult to exhibit the effect as intended in the present invention. Conversely, if the content is too large, the anode resistance may excessively increase, which may lead to deterioration in overall battery performance.

The aluminum may be located on the surface of the lithium cobalt oxide, preferably in the form of an oxide. Specifically, the aluminum may be coated on the surface of the lithium cobalt oxide, and may be physically adsorbed and / or chemically bonded to the surface of the lithium cobalt oxide.

The amount of aluminum included in the form of impurities in the lithium cobalt oxide may be, for example, 0.05 to 0.25 wt% based on the total weight of aluminum.

In addition, the aluminum may be located inside the lithium cobalt oxide in the form of an oxide.

In the cathode active material according to the present invention, the lithium cobalt oxide may be lithium cobalt oxide having a composition of Formula 1 in one preferred example.

Li _{1 + x} Co _{1 -x- y} A _y O ₂ (1)

In the above formula, -0.1 <x <+0.3, 0 <y <0.2.

Since the aluminum is mainly present on the surface of the lithium cobalt oxide, when aluminum is present both on the surface and inside of the lithium cobalt oxide, the aluminum is present in an amount of from 0.25 wt% on the surface of the lithium cobalt oxide to 0.01 And can exhibit an abrupt concentration profile in weight%.

For example, a cathode active material according to the present invention is prepared by mixing lithium precursor and cobalt precursor and sintering in an oxidizing atmosphere to produce lithium cobalt oxide particles, and then applying the lithium cobalt oxide particles to an aluminum precursor and performing heat treatment. It can be prepared by.

The lithium precursor may be, for example, at least one selected from the group consisting of lithium carbonate and carbonate hydroxide.

The cobalt precursor may be, for example, at least one selected from the group consisting of cobalt oxide, cobalt sulfate and cobalt nitrate.

The sintering may be performed for 1 to 20 hours at a temperature range of 900 to 1100 ° C., but may vary depending on various factors such as composition, and is not limited thereto.

The aluminum precursor may be, for example, aluminum oxide. A slurry containing the aluminum precursor may be coated on the lithium cobalt oxide particles and heat-treated to prepare a cathode active material containing aluminum.

The heat treatment may be preferably performed for 8 to 24 hours at a temperature range of 500 to 1000 ° C., but is not limited thereto.

The positive electrode active material according to the present invention may be optionally mixed with a conductive agent, a binder, a filler, or the like to constitute a positive electrode mixture.

The conductive material is usually added in an amount of 1 to 30% by weight 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 a chemical change in the battery, and includes, for example, 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 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. Specific examples of commercially available conductive materials include Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, Ketjenblack and EC, which are acetylene black series. Family (Armak Company), Vulcan XC-72 (manufactured by Cabot Company) and Super P (manufactured by Timcal).

The binder is added to the binder in an amount of 1 to 30% by weight, based on the total weight of the mixture containing the cathode active material, as a component that assists in bonding between the active material and the conductive agent and bonding to the current collector. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, and various copolymers.

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 fibers and carbon fibers are used.

The present invention provides a secondary battery positive electrode including the positive electrode active material.

The positive electrode according to the present invention may be prepared by applying a slurry prepared by mixing a positive electrode mixture including the positive electrode active material to a solvent such as NMP on a positive electrode current collector, followed by drying and rolling.

The cathode current collector is generally made to have a thickness of 3 to 500 mu m. Such a positive electrode collector is not particularly limited as long as it has conductivity without causing chemical change to the battery, and may be formed on the surface of stainless steel, aluminum, nickel, titanium, sintered carbon or aluminum or stainless steel 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 present invention also provides a lithium secondary battery comprising the positive electrode, the negative electrode, the separator, and a non-aqueous electrolyte containing a lithium salt.

The negative electrode is prepared, for example, by coating a negative electrode mixture containing a negative electrode active material on a negative electrode collector and then drying the mixture. The negative electrode mixture may contain the above-described components as required.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such a negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver, and the like on the surface, aluminum-cadmium alloy and 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 separation membrane 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 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The lithium salt-containing nonaqueous electrolyte solution is composed of an electrolyte solution and a lithium salt. As the electrolyte solution, a nonaqueous organic solvent, an organic solid electrolyte, and an inorganic solid electrolyte may be used.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane , Acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate Nonionic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like may be used.

As the inorganic solid electrolyte, for example, 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 material that is readily soluble 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, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene carbonate, PRS (propene sultone), FPC (fluoro-propylene carbonate), and the like.

The secondary battery according to the present invention may not only be used in a battery cell used as a power source for a small device, but also preferably used as a unit battery in a medium and large battery module including a plurality of battery cells used as a power source for a medium and large device. . In particular, the excellent rate characteristics and cycle characteristics may be more preferably used for medium and large battery modules requiring high power and long term use depending on operating conditions.

Preferred examples of the above medium to large devices include a power tool that is powered by an electric motor and moves; Electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like; Electric motorcycles including electric bicycles (E-bikes) and electric scooters (E-scooters); An electric golf cart, and the like, but the present invention is not limited thereto.

As described above, the positive electrode active material for a secondary battery including the lithium cobalt oxide according to the present invention includes aluminum in a specific form in part of the lithium cobalt oxide, thereby preventing internal short circuit due to precipitation of lithium metal on the surface of the negative electrode. Since it can prevent, while ensuring the safety of a battery, it can exhibit remarkably excellent cycling characteristics.

1 is a graph showing the results of confirming the cycle characteristics for the batteries including the lithium cobalt oxide of Example 1 and Comparative Example 1 as the positive electrode active material in Experimental Example 1;
2 is a graph showing the results of checking the charge and discharge capacity according to the cycle for the batteries including the lithium cobalt oxide of Example 1 and Comparative Example 1 as the positive electrode active material in Experimental Example 1.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example 1

Li ₂ CO ₃ and Co ₃ O ₄ were mixed in a stoichiometric ratio, and sintered for about 10 hours in an oxidizing atmosphere at 900 ° C. to produce lithium cobalt oxide particles. The lithium cobalt oxide particles thus prepared were coated with a slurry containing aluminum oxide, and then heat-treated to prepare lithium cobalt oxide containing 2000 ppm of aluminum.

Comparative Example 1

In the same manner as in Example 1, lithium cobalt oxide particles were prepared. The lithium cobalt oxide particles thus prepared were coated with a slurry containing aluminum oxide, and then heat-treated to prepare lithium cobalt oxide containing 500 ppm of aluminum.

[Experimental Example 1]

After mixing the lithium cobalt oxide prepared by the method of Example 1 and Comparative Example 1 and carbon black 1.0% by weight as a conductive material and 3% by weight of PVdF was stirred with a solvent NMP, It was coated on aluminum foil, which is a metal current collector. This was dried in a vacuum oven at about 120 ° C. for at least 2 hours to prepare a positive electrode.

In addition, an electrode assembly was prepared using the cathode prepared above, a cathode coated with MCMB artificial graphite on copper foil, and a porous separator made of polypropylene. The electrode assembly was placed in a cylindrical cylindrical can, and a volume ratio of 1: 1 ethylene carbonate (EC) and dimethyl carbonate (DMC) solution in which 1 M of LiPF ₆ salt was dissolved was injected into the electrolyte, and then the open top of the can was opened. A lithium secondary battery was assembled by attaching a cap assembly such as a CID.

The life characteristics of the battery were measured for the lithium secondary battery thus prepared. Charge and discharge are shown in Figure 1 by performing a 300 cycle at 25 ℃ under 0.8C / 0.5C charge / discharge conditions. As shown in Figure 1, the battery of Example 1 shows excellent cycle characteristics compared to the battery of Comparative Example 1.

[Experimental Example 2]

After mixing the lithium cobalt oxide prepared by the method of Example 1 and Comparative Example 1 and carbon black 1.0% by weight as a conductive material and 3% by weight of PVdF was stirred with a solvent NMP, It was coated on aluminum foil, which is a metal current collector. This was dried in a vacuum oven at about 120 ° C. for at least 2 hours to prepare a positive electrode.

In addition, an electrode assembly was prepared using the cathode prepared above, a cathode coated with MCMB artificial graphite on copper foil, and a porous separator made of polypropylene. The electrode assembly was placed in a cylindrical cylindrical can, and a volume ratio of 1: 1 ethylene carbonate (EC) and dimethyl carbonate (DMC) solution in which 1 M of LiPF ₆ salt was dissolved was injected into the electrolyte, and then the open top of the can was opened. A lithium secondary battery was assembled by attaching a cap assembly such as a CID.

For the lithium secondary battery thus manufactured, based on the results of Experimental Example 1, the charge and discharge capacity according to the potential change in 1 cycle and 300 cycle was measured and shown in FIG. As shown in FIG. 2, the battery of Example 1 and the battery of Comparative Example 1 showed equivalent performance at the beginning of the cycle, but at 300 cycles, the battery of Example 1 had a higher charge / discharge capacity than the battery of Comparative Example 1. Is showing.

The results of Experimental Examples 1 and 2 indicate that aluminum oxide is positioned on the surface of lithium cobalt oxide in a specific amount, thereby preventing excessive lithium ion migration from the positive electrode active material to the negative electrode active material, thereby preventing the precipitation of lithium ions at the negative electrode. It is assumed that this is because.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (14)

A cathode active material for a secondary battery including lithium cobalt oxide, wherein the lithium cobalt oxide includes aluminum, and the aluminum is located on the inside and the surface of the lithium cobalt oxide, and some of the aluminum located therein is in the form of impurities. It contains and the remainder is contained in the form of substituting the positive electrode active material for secondary batteries. The cathode active material of claim 1, wherein the aluminum content on the surface of the lithium cobalt oxide is 0.05 to 0.25 wt% based on the total weight of the cathode active material. The cathode active material of claim 1, wherein the aluminum is positioned on the surface of the lithium cobalt oxide in the form of an oxide. The cathode active material of claim 1, wherein the aluminum content in the form of impurities in the lithium cobalt oxide is 0.05 to 0.25 wt% based on the total weight of the aluminum. The cathode active material of claim 1, wherein the aluminum is positioned inside the lithium cobalt oxide in the form of an oxide. The cathode active material of claim 1, wherein the lithium cobalt oxide has a composition of Formula 1 by aluminum:
Li _{1 + x} Co _{1 -x- y} A _y O ₂ (1)
In the above formula, -0.1 <x <+0.3, 0 <y <0.2. The cathode active material of claim 1, wherein the aluminum has a rapid concentration profile from 0.25 wt% to 0.01 wt% in the lithium cobalt oxide surface based on the total weight of the lithium cobalt oxide. The method of claim 1, wherein the positive electrode active material is prepared by mixing lithium precursor and cobalt precursor and sintering in an oxidizing atmosphere to produce lithium cobalt oxide particles, and then applying the lithium cobalt oxide particles with an aluminum precursor and heat treatment. A cathode active material for a secondary battery. The method of claim 8, wherein the lithium precursor is at least one selected from the group consisting of lithium carbonate and carbonate hydroxide, and the cobalt precursor is at least one selected from the group consisting of cobalt oxide, cobalt sulfate and cobalt nitrate Cathode active material for battery. The cathode active material of claim 9, wherein the sintering is performed for 1 to 20 hours in a temperature range of 900 to 1100 ° C. 11. The cathode active material of claim 8, wherein the aluminum precursor is aluminum oxide, and the slurry including the aluminum precursor is coated on lithium cobalt oxide particles to heat treatment. The cathode active material of claim 11, wherein the heat treatment is performed for 8 to 24 hours in a temperature range of 500 to 1000 ° C. 13. The cathode for a secondary battery, comprising the cathode active material according to any one of claims 1 to 12. A lithium secondary battery comprising the positive electrode for a secondary battery according to claim 13.

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