KR20130128505A - Cathode for lithium secondary battery - Google Patents

Abstract

The present invention relates to a cathode for a lithium secondary battery which includes first lithium-transition metal composite oxide with relatively high charge selectivity and second lithium-transition metal composite oxide with relatively low charge selectivity and has morphology which is charged with the first lithium-transition metal composite oxide via the second lithium-transition metal composite oxide. [Reference numerals] (AA) Before improvement;(BB) After improvement

Description

Cathode for Lithium Secondary Battery {Cathode for Lithium Secondary Battery}

The present invention relates to a positive electrode for a lithium secondary battery, and more particularly, to a first lithium transition metal composite oxide having a relatively high charge selectivity and a second lithium transition metal composite oxide having a relatively low charge selectivity under fast charging conditions. The present invention relates to a positive electrode for a lithium secondary battery, which is included as an active material and has a morphology filled with the first lithium transition metal composite oxide via the second lithium transition metal composite oxide.

Lithium secondary batteries dominate the mobile power supply market because they have advantages such as high voltage, long life and high energy density compared to competitive secondary batteries based on the wide potential window of electrolyte. In recent years, convergence of IT devices is rapidly progressing, and technologies such as electric vehicles are attracting attention, and thus, there is a growing demand for a market for increasing product capacity and shortening charge / discharge time.

In particular, since lithium secondary batteries used in electric vehicles must be able to be used for more than 10 years under severe conditions, they should have superior safety and long-life characteristics than conventional small lithium secondary batteries, and can exhibit high energy density and large output in a short time. Certain characteristics are inevitably required.

Therefore, in such an atmosphere, the development direction that has become a major issue in the recent lithium secondary battery field includes high capacity, improved rate characteristics, and safety. Up to now, research for high capacity has taken up most of the open development, and accordingly, improvement of electrode density, high voltage cell development, and high capacity material development have been made.

Lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as a positive electrode active material of a conventional lithium secondary battery. In addition, lithium-containing manganese oxides such as LiMnO _{2 in a} layered crystal structure and LiMn ₂ O _{4 in a} spinel crystal structure, and lithium-containing The use of nickel oxide (LiNiO ₂ ) is also contemplated.

Among them, LiMn ₂ O ₄ may be suitable as a cathode active material of a lithium secondary battery for an electric vehicle because it has lower raw material prices, higher thermal stability, and higher capacity than LiCoO ₂ and LiNiO ₂ . In particular, LiMn ₂ O ₄ can react with a high current as lithium ions are effectively detached under fast charging conditions, thereby providing a cathode having high charging efficiency.

However, LiMn ₂ O ₄ has a problem in that capacity decreases rapidly at the beginning of the cycle and manganese is eluted at a high temperature. In other words, the capacity decreases at the beginning of the cycle due to the loss of oxygen during charging, and as Mn ions dissolve from the anode, LiMn ₂ O ₄ It causes a phase transition phenomenon and Mn ions dissolved from the anode are electrodeposited on the cathode to promote deterioration.

In order to solve this problem, some prior arts propose a technique of coating or surface treating a material of an active material surface of a positive electrode, and in addition, techniques such as specific surface area control, dissimilar metal doping, and blending with other positive electrode materials have been attempted. . However, an anode that can fundamentally solve the above problems has not been developed yet.

Accordingly, there is a great need for an electrode material which can maximize the capacity of the anode and at the same time improve safety and improve the quick charge characteristics.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

After in-depth study and various experiments, the inventors of the present application have a specific morphology of the first lithium transition metal composite oxide having high charge selectivity and the second lithium transition metal composite oxide having low charge selectivity, as described later. When used in the positive electrode, by charging the first lithium transition metal composite oxide via the second lithium transition metal composite oxide during rapid charging, it is possible to increase the charging efficiency and to confirm that the side reactions occurring on the surface is suppressed The present invention has been completed.

Accordingly, the present invention includes, as a positive active material, a first lithium transition metal composite oxide having a relatively high charge selectivity and a second lithium transition metal composite oxide having a relatively low charge selectivity under a fast charge condition, and a second lithium transition metal. Provided is a morphology in which a first lithium transition metal composite oxide is charged via a composite oxide.

The positive electrode according to the present invention includes two kinds of lithium transition metal composite oxides having different charge selectivity as positive electrode active materials.

In general, when two kinds of lithium transition metal composite oxides having different charge selectivities are included, each lithium transition metal composite oxide reacts competitively at the time of rapid charging, and as a result, the lithium transition metal composite oxide is not evenly charged and the charge selectivity is increased. This high lithium transition metal composite oxide selectively participates in the reaction.

In the present invention, the first lithium transition metal composite oxide is charged through the second lithium transition metal composite oxide having a lower selectivity to charge than the first lithium transition metal composite oxide when the positive electrode active material in the form of the mixture is charged, and sequentially charged. As the reaction occurs, the efficiency of the anode can be increased.

The fast charge condition may be preferably a charge rate condition of 2C to 10C, more preferably a charge rate condition of 2.5C to 4C.

The charging selectivity may be expressed in terms of ease of charging, for example, the degree of charging at the same voltage or current conditions.

As described above, the positive electrode including the first lithium transition metal composite oxide having a relatively high charge selectivity and the second lithium transition metal composite oxide having a relatively low charge selectivity according to the present invention is a second battery under fast charging conditions. Since the first lithium transition metal composite oxide is filled via the lithium transition metal composite oxide, not only the rate characteristic is improved but also side effects are suppressed on the surface.

1 is a schematic diagram showing a cathode active material for a lithium secondary battery prepared according to the present invention and a cathode active material for a lithium secondary battery of the prior art.

As defined above, the charge selectivity of the first lithium transition metal composite oxide is higher than the charge selectivity of the second lithium transition metal composite oxide. Preferably, the charge selectivity of the first lithium transition metal composite oxide is the second lithium transition metal composite. It can be 1.5 to 5 times the charge selectivity of the oxide.

In one preferred example, the first lithium transition metal complex oxide may be a compound represented by the following formula (1).

Li _{1 + x} Mn _2-xy M _y O _{4 + d} (1)

Wherein 0 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.5, 0 ≦ d ≦ 0.05, and M is selected from the group consisting of Al, Mg, Ti, Cr, Co, Zr, V, Ni, Ga, and Gd Is one or more.)

In one preferred example, the second lithium transition metal complex oxide may be a compound represented by the following formula (2).

Li _x Me _y O ₂ (2)

Where

Me is Ni _a Mn _b Co _c (0≤a≤0.9, 0≤b≤0.9, 0≤c≤0.5, 0.85≤a + b + c≤1.05), 0.95≤x≤1.15 as x + y = 2 to be.

Since the compound of Formula 1 and the compound of Formula 2 may be easily prepared by those skilled in the art based on the compositional formula, description of such a manufacturing method is omitted herein.

As defined above, the positive electrode of the present invention has a morphology in which the first lithium transition metal complex oxide is charged via the second lithium transition metal complex oxide during rapid charging, and thus the specific morphology may be implemented in various forms. Can be.

In a first specific example, the particles of the second lithium transition metal composite oxide may have a structure in contact with the surface of the particle of the first lithium transition metal composite oxide.

In this case, the particle size of the second lithium transition metal composite oxide may be in the range of 0.05 to 3 μm, and the particle size of the first lithium transition metal composite oxide may be in the range of 5 to 20 μm.

Particles of the second lithium transition metal complex oxide may preferably surround 50% or more of the particle surface of the first lithium transition metal complex oxide.

In a second specific example, the second lithium transition metal composite oxide may have a structure coated on the particle surface of the first lithium transition metal composite oxide.

In this case, the coating thickness of the second lithium transition metal composite oxide may be in the range of 0.01 to 2 μm. If the coating thickness is too small, it may be difficult to achieve the desired effect in the present invention, on the contrary, too large is not preferable because it may adversely affect the battery performance by lowering the output characteristics.

However, in spite of the examples described above, the positive electrode morphology that can achieve the desired effect in the present invention may vary, and all of them should be construed as being included in the scope of the present invention.

In the positive electrode of the present invention, a mixture containing a positive electrode active material, a conductive agent, and a binder is added to a predetermined solvent such as water and NMP, for example, to form a slurry, and then the slurry is applied onto the positive electrode current collector. It can be prepared by drying and rolling. If necessary, the positive electrode mixture may further contain at least one substance selected from the group consisting of a viscosity adjusting agent and a filler.

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. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can 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 agent is a component for further enhancing the conductivity of the electrode active material and is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery. Examples of the conductive agent 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; Carbon fibers such as carbon nanotubes and fullerene; conductive fibers such as carbon fibers and metal fibers; 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.

The binder is a component that assists in bonding of the active material and the conductive agent to the binder and in binding 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 cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose, polyvinylidene fluoride and the like, but are not limited thereto. In some cases, the above-described solvent may play a role as a viscosity adjusting agent.

The filler is optionally used as an auxiliary component for suppressing the expansion of the electrode. The filler is not particularly limited as long as it is a fibrous material without causing chemical change in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The present invention also provides a lithium secondary battery comprising the positive electrode, a negative electrode containing a negative electrode active material of a carbon-based material, a porous separator interposed between the positive electrode and the negative electrode, and an electrolyte containing a lithium salt and an electrolyte compound. do.

The negative electrode is manufactured by coating and drying a negative electrode active material on a negative electrode current collector, and if necessary, components such as a conductive agent and a binder as described above may be further included.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti which can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

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 conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, 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 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 non-aqueous electrolyte is composed of a non-aqueous 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, gamma-butyl Low lactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxolon, aceto Nitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative Aprotic organic solvents such as tetrahydrofuran derivatives, ethers, methyl propionate and ethyl propionate can be used.

Examples of the organic solid electrolytes include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, 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, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, 4-phenyl lithium borate, and imide Can be.

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, 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), FEC (fluoro-ethylene carbonate), and the like.

In addition to including the secondary battery according to the present invention as a unit cell may be a device comprising a battery module as a power source.

Preferred examples of the device include an electric power tool, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric motorcycle, an electric golf cart, or a power storage device. It is not limited to this.

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 (15)

Under fast charging conditions, a first lithium transition metal composite oxide having a relatively high charge selectivity and a second lithium transition metal composite oxide having a relatively low charge selectivity are included as the positive electrode active material, and the second lithium transition metal composite oxide is A lithium secondary battery positive electrode having a morphology filled with the first lithium transition metal complex oxide. The positive electrode for a lithium secondary battery according to claim 1, wherein the fast charge condition is a charge rate condition of 2C to 10C. The positive electrode for a lithium secondary battery according to claim 2, wherein the fast charge condition is a charge rate condition of 2.5C to 4C. The positive electrode for a rechargeable lithium battery of claim 1, wherein the charge selectivity of the first lithium transition metal composite oxide is 1.5 to 5 times that of the charge selectivity of the second lithium transition metal composite oxide. The positive electrode for a lithium secondary battery according to claim 1, wherein the first lithium transition metal composite oxide is represented by the following Chemical Formula 1:
Li _{1 + x} Mn _2-xy M _y O _{4 + d} (1)
In this formula,
0≤x≤0.2, 0≤y≤0.5, 0≤d≤0.05,
M is at least one selected from the group consisting of Al, Mg, Ti, Cr, Co, Zr, V, Ni, Ga, and Gd. The positive electrode for a lithium secondary battery according to claim 1, wherein the second lithium transition metal composite oxide is represented by the following Chemical Formula 2:
Li _x Me _y O ₂ (2)
In this formula,
Me is Ni _a Mn _b Co _c (0≤a≤0.9, 0≤b≤0.9, 0≤c≤0.5, 0.85≤a + b + c≤1.05),
x + y = 2, where 0.95 ≦ x ≦ 1.15. The positive electrode for a lithium secondary battery according to claim 1, wherein the particles of the second lithium transition metal composite oxide are in contact with each other while covering the particle surface of the first lithium transition metal composite oxide. The method of claim 7, wherein the particle size of the second lithium transition metal complex oxide is in the range of 0.05 to 3 ㎛, the particle size of the first lithium transition metal complex oxide is in the range of 5 to 20 ㎛ Secondary battery positive electrode. The cathode of claim 7, wherein the particles of the second lithium transition metal composite oxide surround 50% or more of the surface of the particles of the first lithium transition metal composite oxide. The positive electrode for a lithium secondary battery according to claim 1, wherein the second lithium transition metal composite oxide is coated on the particle surface of the first lithium transition metal composite oxide. The positive electrode for a lithium secondary battery according to claim 10, wherein the coating thickness of the second lithium transition metal composite oxide is in a range of 0.01 to 2 μm. 12. A positive electrode according to any one of claims 1 to 11, a negative electrode containing a negative electrode active material of a carbon-based material, a porous separator interposed between the positive electrode and the negative electrode, and an electrolyte containing a lithium salt and an electrolyte solution compound. Lithium secondary battery characterized in that. A battery module comprising the lithium secondary battery according to claim 12 as a unit cell. A device comprising the battery module according to claim 13 as a power source. 15. The device of claim 14, wherein the device is an electric power tool, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric motorcycle, an electric golf cart, or a power storage device. Device.

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