KR102147495B1 - Cathode active material with improved safety and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention includes a core part comprising a lithium metal oxide; A first outer layer surrounding the core portion in a shell shape and including LiFePO ₄ particles and a conductive material; And a second outer layer made of polyethylene and surrounding the first outer layer in a shell shape, and has excellent nail penetration and safety.

Description

Positive electrode active material with improved safety and lithium secondary battery comprising the same {Cathode active material with improved safety and lithium secondary battery comprising the same}

The present invention relates to a positive electrode active material with improved safety and a lithium secondary battery including the same.

With the rapid development of the communication and computer industries, camcorders, mobile phones, notebook PCs, and the like are continuing remarkable development, and the demand for lithium secondary batteries as a power source capable of driving these portable electronic communication devices is increasing day by day. In particular, research and development are being actively conducted in Japan, Europe, and the United States, as well as in Korea, in connection with the application of electric vehicles, uninterruptible power supplies, power tools, and satellites as eco-friendly power sources. Moreover, as the commercialization of lithium secondary batteries is expanded in recent years, the problem of increasing the capacity and safety of lithium secondary batteries has been further raised.

Meanwhile, lithium cobalt oxide (LiCoO ₂ ) was mainly used as a positive electrode active material for lithium secondary batteries, but now lithium nickel oxide (Li(Ni-Co-Al)O ₂ ) and lithium composite metal oxide as other layered positive electrode active materials. (Li(Ni-Co-Mn)O ₂ ) and the like are also used, and in addition, spinel-type lithium manganese oxide (LiMn ₂ O ₄ ) and olivine-type lithium iron phosphate compound (LiFePO ₄ ) with low cost stability are also attracting attention.

However, lithium secondary batteries using lithium cobalt oxide, lithium nickel oxide, lithium composite metal oxide, etc., have excellent basic battery characteristics, but safety, especially thermal safety, overcharge characteristics, etc. are not sufficient, and meet the demand for high capacity. If the filling property of the self-cathode active material is increased, there is a problem that the safety is further lowered.

In addition, spinel-type lithium manganese batteries have the advantages of low price and high safety, and in recent years, they are being actively studied as positive electrode active materials for lithium secondary batteries used as power sources of electric vehicles, but high energy density due to low electric capacity. There is a problem in not meeting the demand for.

In addition, although the olivine-type lithium iron phosphate compound has low cost and high safety characteristics, it is difficult to expect excellent battery characteristics due to its very low electronic conductivity, and it is not able to meet the demand for high capacity due to its low average operating potential.

Accordingly, various studies have been conducted to solve the above problems, but an effective solution has not been suggested until now.

Accordingly, the problem to be solved by the present invention is to provide a positive electrode active material having excellent safety, particularly thermal stability, and a lithium secondary battery including the same, without deteriorating basic battery characteristics.

According to an embodiment of the present invention, a core part comprising a lithium metal oxide; A first outer layer surrounding the core portion in a shell shape and including LiFePO ₄ particles and a conductive material; And a second outer layer surrounding the first outer layer in a shell shape and comprising polyethylene, wherein the lithium metal oxide of the core portion is LiCoO ₂ , Li(Ni _a Co _b Al _c )O ₂ (0<a<1, 0<b<1, 0<c<1, a+b+c=1), Li(Ni _a Co _b Mn _c )O ₂ (0<a<1, 0<b< 1, 0<c<1, a+b+c=1), Li(Li _x Ni _a Co _b Mn _c )O ₂ (0<x<0.5), 0<a<1, 0<b<1, 0<c<1, a+b+c=1) and LiMn ₂ O ₄ Any one or two selected from the group consisting of A positive electrode active material that is a mixture of the above is provided.

LiFePO ₄ The particles may be used in an amount of 0.05 to 5 parts by weight based on 100 parts by weight of the core part.

The polyethylene may be low density polyethylene or high density polyethylene.

The second outer layer may have a thickness of 10 to 30 μm.

According to another embodiment of the present invention, a lithium secondary battery using a positive electrode including the positive electrode active material is provided.

The positive electrode active material for a lithium secondary battery according to an aspect of the present invention includes a core portion comprising a lithium metal oxide; A first outer layer comprising LiFePO ₄ particles and a conductive material; And by having a structure in which the second outer layer made of polyethylene is sequentially coated, excellent nail penetrability and thermal safety may be simultaneously exhibited.

1 is a schematic cross-sectional view of a positive active material according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail. The terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

According to an embodiment of the present invention, a core part comprising a lithium metal oxide; A first outer layer surrounding the core portion in a shell shape and including LiFePO ₄ particles and a conductive material; And a second outer layer comprising polyethylene and surrounding the first outer layer in a shell shape.

Lithium metal oxide that can be used as a core part in the present invention is LiFePO ₄ Except for oxides, lithium metal oxides used as positive electrode active materials for lithium secondary batteries in the art may be used without limitation. For example, LiCoO ₂ , Li(Ni _a Co _b Al _c )O ₂ (0<a<1, 0<b<1, 0<c<1, a+b+c=1), Li(Ni _a Co _b Mn _c )O ₂ (0<a<1, 0<b< 1, 0<c<1, a+b+c=1), Li(Li _x Ni _a Co _b Mn _c )O ₂ (0<x<0.5), 0<a<1, 0<b<1, 0<c<1, a+b+c=1) and LiMn ₂ O _{4 It} may be any one or a mixture of two or more selected from the group consisting of, but is not limited thereto.

The lithium metal oxide particles used as the core part in the present invention are in the form of secondary particles in which lithium metal oxide primary particles are agglomerated, and may be prepared by a conventional method known in the art, and as an embodiment, the following coprecipitation method is used. I can.

Metal hydroxides use respective raw materials according to the target positive electrode active material. As the main raw material, the metal salt may be sulfate, nitrate, acetate, or the like. An aqueous solution containing such a metal salt is continuously added in a coprecipitation environment, and a slurry containing a metal hydroxide is continuously taken, and a metal hydroxide is produced by washing with water, filtration, and drying.

As described above, in the manufacture of lithium metal oxide, when such metal hydroxide is used, the inflow of impurities contained in each metal salt is limited, the composition can be controlled at the atomic level, and the addition of heterogeneous elements introduced in trace amounts. The effect can be maximized, and lithium metal oxide having a uniform crystal structure with almost no impurities can be easily manufactured.

As described above, the metal hydroxide prepared by the hydroxide coprecipitation method from the raw material precursor is heat-treated at a certain temperature and mixed with various lithium salts to obtain a desired composition, and then calcined under normal firing conditions to form the core of the positive electrode active material of the present invention. The corresponding lithium metal oxide can be prepared. The lithium metal oxide thus prepared is obtained as lithium metal oxide secondary particles in which lithium metal oxide primary particles are aggregated.

The average particle diameter of the primary particles constituting the secondary particles may vary in various ways depending on the coprecipitation environment and firing conditions according to the composition ratio of the metal hydroxide, and is not limited to a certain range. In addition, the average particle diameter of the secondary particles may be variously changed according to use and manufacturing environment, and may be, for example, 7 to 15 μm, but is not limited thereto.

The shape of the secondary particles is not particularly limited, and it is preferable that the secondary particles have a spherical shape in terms of process efficiency of coating a layer (first outer layer) including LiFePO ₄ particles and a conductive material in a shell shape.

The first outer layer not only suppresses the contact between the lithium metal oxide used as the core part and the electrolyte solution, but also suppresses heat generation, and LiFePO ₄ itself has excellent thermal stability, so nail penetration and overcharge safety of the battery are improved. Improves.

The average particle diameter of the LiFePO ₄ particles is not particularly limited as long as it meets the object of the present invention, but is preferably 1 μm or less. When the average particle diameter of the LiFePO ₄ particles exceeds 1 μm, coating uniformity, coating process efficiency and reproducibility of the first outer layer are deteriorated, which is not preferable.

LiFePO ₄ The amount of particles may be appropriately determined according to the specific type of battery in which the positive electrode active material is used, and may be, for example, 0.05 to 5 parts by weight based on 100 parts by weight of the core part, but is not limited thereto. If the content is less than 0.05 parts by weight, the purpose of coating the first outer layer, that is, the effect of improving overcharge characteristics is insignificant, and if it exceeds 5 parts by weight, the discharge voltage can be lowered, which is not preferable.

Meanwhile, LiFePO ₄ Since the compound has very low conductivity, a conductive material is further included in the first outer layer to compensate for this.

The conductive material may be any one or a mixture of two or more selected from the group consisting of a conductive metal, a conductive polymer, and a conductive carbon, but is not limited thereto. Specific examples of the conductive carbon may include carbon nanotubes, Katchen black, acetylene black, Super-P, graphite, activated carbon, etc., but are not limited thereto. It is preferable that the average particle diameter of the conductive material that can be used is 1 µm or less. If the average particle diameter of the conductive material exceeds 1 µm, it may be an obstacle to forming a uniform coating layer. For example, the average particle diameter of the conductive material particles is LiFePO ₄ It may be similar to the average particle diameter of the secondary particles.

The content of the conductive material may be appropriately determined according to the specific type of battery in which the positive electrode active material is used, and for example, may be included in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of the core material, but is limited thereto. no. If the content of the conductive material is less than 0.1 parts by weight, the effect of using the conductive material is insignificant, and even if more than 10 parts by weight is used, the effect of improving the conductivity is not large.

The first outer layer may have a thickness of 5 to 50 μm. If the thickness of the first outer layer is thinner than 5 μm, it is difficult to secure nail penetration and overcharge safety of the battery, and if it is thicker than 50 μm, the capacity of the battery may decrease.

The first outer layer is completely surrounded by a second outer layer made of polyethylene.

As the polyethylene constituting the second outer layer, low density polyethylene (LDPE) or high density polyethylene (HDPE) may be used, but is not limited thereto. The low density polyethylene has a melting point of approximately 115 °C, and the high density polyethylene has a melting point of 135 °C.

Therefore, when the battery is at an abnormally high temperature, the polyethylene constituting the second layer melts and blocks the conductive network formed by the conductive material, thereby increasing the resistance, and as a result, the safety of the battery is secured. I can.

The second outer layer may have a thickness of 10 to 30 μm. When the thickness of the second outer layer is less than 10 μm, the stability of the battery may not be sufficiently secured, and when the thickness of the second outer layer is larger than 30 μm, lithium ions of the positive electrode active material may not move smoothly.

One aspect of the positive electrode active material according to the present invention is shown in FIG. 1. Referring to FIG. 1, the positive electrode active material includes a core part 1 including a predetermined lithium metal oxide; A first outer layer (2) surrounding the core portion in a shell shape and including LiFePO ₄ particles and a conductive material; And a second outer layer 3 made of polyethylene and surrounding the first outer layer in the form of a shell.

According to another embodiment of the present invention, for preparing the positive electrode active material, (S1) preparing a lithium metal oxide secondary particle core portion in which a plurality of lithium metal oxide primary particles are aggregated by firing a metal hydroxide and a lithium salt; (S2) forming a first outer layer by dry coating and heat treating LiFePO ₄ particles and conductive material particles on the surface of the core part; And (S3) forming a second outer layer surrounding the first outer layer; there is provided a method of manufacturing a cathode active material for a lithium secondary battery comprising.

The coating in step (S3) may be performed by a physical or chemical method. These chemical methods include polymerization methods such as catalytic polymerization using a catalyst on the surface of the first outer layer, a method of bonding polyethylene constituting the second outer layer by surface treatment of the first outer layer, and chemical vapor deposition (CVD). ), such as, but is not limited thereto. In addition, as a physical method, melt-spinning coating, solution-spinning coating or electrospinning coating of coating a polyethylene polymer constituting the second outer layer on the surface of the first outer layer, bonding method by thermal fusion, or physical vapor growth method (physical vapor deposition) deposition, PVD), and the like.

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

In addition to the positive electrode active material, the positive electrode mixture may optionally include a conductive material, a binder, and a filler.

The positive electrode current collector is generally made to have a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface treatment with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, etc. may be used. The positive electrode current collector may strengthen the bonding force of the positive electrode active material by forming fine irregularities on the surface, and may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

The conductive material is typically added in an amount of 1 to 30% by weight based on the total weight of the mixture including the positive electrode active material.

Such a conductive material is not particularly limited as long as it has conductivity without causing chemical changes to the battery, for example, graphite carbon black such as natural graphite or artificial graphite, acetylene black, Ketjen black, channel black, furnace black , Carbon black such as lamp black and thermal black Conductive fiber such as carbon fiber or metal fiber Conductive fiber such as carbon fiber or metal fiber Metal powder such as fluorocarbon, aluminum and nickel powder Conductive whiskey such as zinc oxide and potassium titanate Conductive metal oxide such as titanium oxide Polyphenylene derivative Conductive materials such as, etc. may be used.

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

The filler is selectively used as a component that suppresses the expansion of the positive electrode, and is not particularly limited as long as it is a fibrous material without causing chemical changes to the battery, for example, glass of an olefin-based polymerizer such as polyethylene or polypropylene. Fibrous materials such as fibers and carbon fibers are used.

Representatively, isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, and the like may be used as the dispersion.

The method of evenly applying the electrode material paste to the metal material may be selected from known methods or may be performed by a new suitable method in consideration of the properties of the material. For example, after distributing the paste over the current collector, it can be uniformly dispersed using a doctor blade or the like. In some cases, it is also possible to use a method of performing the distribution and dispersion processes as a single process. In addition, die casting, comma coating, screen printing, or the like may be selected, or the current collector by pressing or lamination after molding on a separate substrate. And can also be joined.

It is preferable to dry the paste applied on the metal plate within 1 day in a vacuum oven at 50 to 200°C.

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

The negative electrode current collector is generally made to have a thickness of 3 to 500 μm. Such a negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes to the battery, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. For the surface treatment with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloys, etc. may be used. In addition, like the positive electrode current collector, it is possible to enhance the bonding strength of the negative electrode active material by forming fine irregularities on the surface thereof, and may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

The negative electrode active material is, for example, carbon such as non-graphitized carbon and graphite-based carbon LixFe ₂ O ₃ (0≤x≤1), Li _x WO ₂ (0≤x≤1), Sn _x Me _{1 - x} Me ' _y O _z (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, elements of groups 1, 2 and 3 of the periodic table, halogen 0<x≤1;1≤y≤3; 1≤z≤8) metal complex oxide lithium metal lithium alloy silicon-based alloy tin-based alloy SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , Metal oxides such as Bi ₂ O ₅ A conductive polymer Li-Co-Ni-based material such as polyacetylene may be further included. .

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally 0.01 to 10 µm, and the thickness is generally 5 to 300 µm. As such a separator, for example, a sheet or nonwoven kraft paper made of olefin-based polymer glass fiber such as polypropylene or polyethylene having chemical resistance and hydrophobicity is used. Representative examples currently on the market include Celgard R2400, 2300 (Hoechest Celanese Corp.), polypropylene separator (Ube Industries Ltd. or Pall RAI), and polyethylene (Tonen or Entek).

In some cases, a gel polymer electrolyte may be coated on the separator to increase the stability of the battery. Representative examples of such gel polymers include polyethylene oxide, polyvinylidene fluoride, and polyacrylonitrile.

When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The lithium salt-containing non-aqueous electrolyte is composed of a non-aqueous electrolyte and lithium. As the non-aqueous electrolyte, a non-aqueous electrolyte solution, a solid electrolyte, an inorganic solid electrolyte, or the like is used.

Examples of the non-aqueous electrolyte solution include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, gamma-butyllolactone, 1,2 -Dimethoxy ethane, 1,2-diethoxy ethane, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolone, 4-methyl-1,3-dioxene, diethyl ether, formamide, Dimethylformamide, dioxolone, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate tryster, trimethoxy methane, dioxolone derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imine An aprotic organic solvent such as dazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl pyropionate, ethyl propionate, etc. may be used.

As the organic solid electrolyte, for example, a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, a poly agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer or the like containing an ionic dissociating group 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, and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ may be used.

The lithium salt is a material 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, LiSCN, LiC (CF 3 SO 2) 3, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, Lithium 4 phenyl borate, imide, and the like may be used.

In addition, for the purpose of improving charge/discharge properties and flame retardancy, the electrolyte solution includes, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, and nitro. Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, aluminum trichloride, etc. may be added. . In some cases, in order to impart non-flammability, a halogen-containing solvent such as carbon tetrachloride and ethylene trifluoride may be further included, and carbon dioxide gas may be further included in order to improve high-temperature storage characteristics, and FEC (fluoro-ethylene carbonate), PRS (propene sultone), FPC (fluoro-propylene carbonate), etc. may be further included.

The secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also can be preferably used as a unit cell in a medium or large battery module including a plurality of battery cells.

In addition, the present invention provides a battery pack including the battery module as a power source for a medium and large device, and the medium and large device includes an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in hybrid. Electric vehicles and power storage devices including electric vehicles (Plug-in Hybrid Electric Vehicle, PHEV), etc. may be mentioned, but the present invention is not limited thereto.

Claims (5)

A core portion made of lithium metal oxide;
A first outer layer surrounding the core portion in a shell shape, formed on the surface of the core portion, and including LiFePO ₄ particles and a conductive material; And
Including; a second outer layer surrounding the first outer layer in a shell shape and comprising polyethylene,
The lithium metal oxide of the core portion is LiCoO ₂ , Li(Ni _a Co _b Al _c )O ₂ (0<a<1, 0<b<1, 0<c<1, a+b+c=1), Li (Ni _a Co _b Mn _c )O ₂ (0<a<1, 0<b<1, 0<c<1, a+b+c=1), Li(Li _x Ni _a Co _b Mn _c )O ₂ (0<x<0.5), 0<a<1, 0<b<1, 0<c<1, a+b+c=1) and LiMn ₂ O ₄ Any one or 2 selected from the group consisting of A mixture of more than one species
Positive electrode active material.
The method of claim 1,
LiFePO ₄ The positive electrode active material, characterized in that the particles are used in an amount of 0.05 to 5 parts by weight based on 100 parts by weight of the core part.
The method of claim 1,
The polyethylene is a positive electrode active material, characterized in that the low-density polyethylene or high-density polyethylene.
The method of claim 1,
The positive electrode active material, wherein the second outer layer has a thickness of 10 to 30 μm.
A lithium secondary battery using a positive electrode containing the positive electrode active material according to any one of claims 1 to 4.

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