KR102270154B1 - Cathode active material layer and lithium ion secondary battery including the same - Google Patents

Abstract

[화학식 2]

[화학식 3]

According to the present invention, a positive active material containing a lithium composite oxide, a first copolymer including at least the structure of Formula 1 below, and a second copolymer including at least a structure of Formula 2 or Formula 3 below are mixed at 25:75. A positive electrode active material layer including a binder resin contained in a mass ratio of to 75:25 is provided.
[Formula 1]

[Formula 2]

[Formula 3]

Description

Positive electrode active material layer and lithium ion secondary battery {CATHODE ACTIVE MATERIAL LAYER AND LITHIUM ION SECONDARY BATTERY INCLUDING THE SAME}

The present invention relates to a positive electrode active material layer and a lithium ion secondary battery.

In recent years, with the miniaturization and weight reduction of various electronic devices, lithium ion secondary batteries are used as power sources for these electronic devices. In such a lithium ion secondary battery, a new high energy density is required. In order to increase the energy density of a lithium ion secondary battery, for example, raising the charging voltage of a lithium ion secondary battery is examined.

On the other hand, in a lithium ion secondary battery under a high potential, the oxidation power of the positive electrode active material becomes strong. For this reason, side reactions causing decomposition of the electrolyte on the surface of the positive electrode active material and structural destruction of the positive electrode active material are accelerated. As a result, the cycling characteristics and storage characteristics of a lithium ion secondary battery will fall remarkably.

Therefore, in order to improve the characteristics of the lithium ion secondary battery, various proposals have been made with respect to the binder resin included in the positive electrode active material layer.

For example, a copolymer of vinylidene fluoride and tetrafluoroethylene is used for the binder resin included in the positive electrode active material layer. Thereby, the flexibility of the positive electrode active material layer can be ensured while the positive electrode active material layer is increased in density.

In addition, a polymer having a polar functional group such as a carboxyl group, a sulfonic acid group or a phosphoric acid group is used in the binder resin. Thereby, the cycling characteristics of the lithium ion secondary battery under high voltage can be improved.

However, in the prior art, structural destruction of a positive electrode active material, which occurs in a positive electrode active material having particularly strong oxidizing power, has not been sufficiently studied. Therefore, in the prior art, it was difficult to sufficiently improve the cycle characteristics of the lithium ion secondary battery and the capacity characteristics after high temperature storage.

Accordingly, the present invention has been made in consideration of the above problems and the like. An object of the present invention is to provide a positive electrode active material layer capable of improving cycle characteristics and capacity characteristics after high temperature storage, and a lithium ion secondary battery.

In order to solve the above problems, according to one aspect of the present invention, a cathode active material containing a lithium composite oxide, a first copolymer including at least a structure of the following Chemical Formula 1, and at least a structure of the following Chemical Formula 2 or Chemical Formula 3 A positive electrode active material layer including a binder resin containing the second copolymer including the second copolymer in a mass ratio of 25:75 to 75:25 is provided.

[Formula 1]

[Formula 2]

[Formula 3]

In Formulas 2 and 3, R ₁ and R ₂ are each independently hydrogen or a methyl group, and R ₃ is hydrogen or a linear or branched alkyl group having 1 to 6 carbon atoms.

The first copolymer may include 50 mol% or more of a unit structure using vinylidene fluoride as a monomer in the polymer main chain.

The first copolymer may include 10 mol% or more and 40 mol% or less of a unit structure using tetrafluoroethylene as a monomer in the polymer main chain.

The second copolymer may include 90 mol% or more of a unit structure using acrylonitrile or methacrylonitrile as a monomer in the polymer main chain.

At least the second copolymer comprising Formula 2 may include 0.1 mol% or more and less than 10 mol% of a unit structure using acrylic acid or acrylic acid ester as a monomer in the polymer main chain.

At least, the second copolymer including Formula 3 may include 0.1 mol% or more and less than 10 mol% of a unit structure using acrylamide as a monomer in the polymer main chain.

The weight average molecular weight of the first copolymer and the second copolymer may be 300,000 or more, respectively.

The lithium composite oxide may be a compound represented by the following formula (4).

[Formula 4]

Li _a Co _x M _y O ₂

In Formula 4, M is from the group consisting of aluminum, nickel, manganese, chromium, iron, vanadium, magnesium, titanium, zirconium, niobium, molybdenum, tungsten, copper, zinc, gallium, indium, tin, lanthanum, and cerium. One or two or more metal elements selected, 0.20=a≤=1.20, 0.95≤=x<1.00, 0<y≤=0.05, and x+y=1.

According to another aspect of the present invention, there is provided a lithium ion secondary battery comprising a positive electrode including the positive electrode active material layer, a negative electrode, and a non-aqueous electrolyte.

The charging termination voltage of the lithium ion secondary battery may be greater than 4.45V based on graphite.

As described above, according to the present invention, it is possible to improve the cycle characteristics of the lithium ion secondary battery and the capacity characteristics after high temperature storage.

1 is a view schematically showing the configuration of a lithium ion secondary battery according to an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, in this specification and drawings, the same code|symbol is attached|subjected to the component which has substantially the same functional structure, and the duplicate description is abbreviate|omitted.

<1. Overview of Lithium Ion Secondary Battery>

First, an outline of a lithium ion secondary battery including a positive electrode active material layer and the positive electrode active material layer according to an embodiment of the present invention will be described.

In recent years, it has been studied to further increase the energy density of the lithium ion secondary battery. Therefore, improving the charging termination voltage of the lithium ion secondary battery to more than 4.45V based on graphite is being considered.

However, the present inventors have found that the oxidizing power of the positive electrode active material becomes strong under the high charge termination voltage environment as described above. The present inventors discovered that the decomposition reaction of electrolyte solution and structural destruction of a positive electrode active material progressed by strong oxidizing power, and the transition metal in a positive electrode active material eluted into electrolyte solution. In this case, the lithium ion secondary battery rapidly deteriorates in cycle characteristics and capacity characteristics during high temperature storage.

As a result of repeated examination of the above problems and the like, the present inventors found that two specific copolymers were used as binder resins contained in the positive electrode. The present inventors have discovered that by using two specific copolymers, the side reaction causing structural destruction of the positive electrode active material can be suppressed, and the elution of the transition metal from the positive electrode active material can be suppressed. Therefore, the present inventors discovered that the side reaction on the surface of a positive electrode active material could be suppressed also under the environment of a high charge termination voltage. According to this, the lithium ion secondary battery can improve cycle characteristics and capacity characteristics at the time of high temperature storage.

In other words, the positive electrode active material layer according to the present embodiment includes positive electrode active material particles containing lithium composite oxide and a binder resin. The binder resin contains at least a first copolymer having a structure of Formula 1 below and a second copolymer containing at least a structure of Formula 2 or Formula 3 in a mass ratio of 25:75 to 75:25.

[Formula 1]

[Formula 2]

[Formula 3]

Meanwhile, in Formulas 2 and 3, R ₁ and R ₂ are each independently hydrogen or a methyl (methyl) group. R ₃ is hydrogen or a linear or branched alkyl group having 1 to 6 carbon atoms.

In addition, the positive electrode active material layer according to the present embodiment is more effective when the lithium composite oxide containing cobalt (Co) is used as the positive electrode active material. Specifically, the positive electrode active material layer according to the present embodiment can suppress the elution of the transition metal (ie, cobalt) from the lithium composite oxide.

The lithium composite oxide containing cobalt (Co) may be specifically a compound represented by Formula 4 below.

[Formula 4]

Li _a Co _x M _y O ₂

In Formula 4, M is aluminum (Al), nickel (Ni), manganese (Mn), chromium (Cr), iron (Fe), vanadium (V), magnesium (Mg), titanium (Ti), zirconium ( Zr), niobium (Nb), molybdenum (Mo), tungsten (W), copper (Cu), zinc (Zn), gallium (Ga), indium (In), tin (Sn), lanthanum (La), and cerium (Ce) is one or more metal elements selected from the group consisting of, 0.20=a≤=1.20, 0.95≤=x<1.00, 0<y≤=0.05, and x+y=1.

In addition, the lithium ion secondary battery having the positive electrode active material layer according to the present embodiment is more effective when the final charge voltage is greater than 4.45V based on graphite. Specifically, the lithium ion secondary battery provided with the positive electrode active material layer according to the present embodiment can suppress the elution of the transition metal from the positive electrode active material. According to this, the lithium ion secondary battery provided with the positive electrode active material layer according to the present embodiment can improve the cycle characteristics and the capacity characteristics during high temperature storage.

<2. Composition of Lithium Ion Secondary Battery>

Hereinafter, with reference to FIG. 1, the configuration of the lithium ion secondary battery 10 according to the present embodiment outlined above will be described in more detail.

As shown in FIG. 1 , the lithium ion secondary battery 10 includes a positive electrode 20 , a negative electrode 30 , and a separator layer 40 impregnated with an electrolyte. The form of the lithium ion secondary battery 10 is not specifically limited. The shape of the lithium ion secondary battery 10 may be, for example, a cylindrical shape, a square shape, a laminate type, or a button type.

The positive electrode 20 includes a current collector 21 and a positive electrode active material layer 22 . The current collector 21 may be any conductor as long as it is a conductor. The current collector 21 may be, for example, aluminum, stainless steel, or nickel-plated steel.

The positive electrode active material layer 22 includes at least a positive electrode active material and a binder resin, and may further include a conductive aid. On the other hand, the ratio of the content of the positive electrode active material, the conductive auxiliary agent, and the binder resin is not particularly limited. As for the ratio of the content of these structures, it is possible to use the ratio of the content applied in a general lithium ion secondary battery.

The positive electrode active material contains a lithium composite oxide. The lithium composite oxide contained in the positive electrode active material may be a lithium composite oxide containing cobalt, and specifically, a compound represented by the following formula (4).

[Formula 4]

Li _a Co _x M _y O ₂

In Formula 4, M is aluminum (Al), nickel (Ni), manganese (Mn), chromium (Cr), iron (Fe), vanadium (V), magnesium (Mg), titanium (Ti), zirconium ( Zr), niobium (Nb), molybdenum (Mo), tungsten (W), copper (Cu), zinc (Zn), gallium (Ga), indium (In), tin (Sn), lanthanum (La), and cerium (Ce) is one or more metal elements selected from the group consisting of, 0.20=a≤=1.20, 0.95≤=x<1.00, 0<y≤=0.05, and x+y=1. On the other hand, preferably, M may be one or more metal elements selected from the group consisting of Al, Mg, Ti, and Zr.

When the positive active material includes the lithium composite oxide represented by Formula 4, the lithium ion secondary battery may be used with a charge termination voltage of greater than 4.45V based on graphite. Accordingly, since the lithium ion secondary battery can have a higher energy density, the battery capacity can be increased.

However, when the cathode active material includes the lithium composite oxide represented by Chemical Formula 4, the structure of the lithium composite oxide may be destroyed by driving under a high potential. In this case, there is a possibility that cobalt is eluted from the lithium composite oxide. In the positive electrode active material layer 22 according to the present embodiment, a binder resin containing two specific copolymers is used. Accordingly, it is possible to suppress the elution of cobalt from the lithium composite oxide. According to this, the positive electrode active material layer 22 according to the present embodiment can improve the cycle characteristics of the lithium ion secondary battery 10 and the capacity characteristics during high temperature storage.

On the other hand, it goes without saying that the positive active material may contain one or two or more other positive active materials in addition to the lithium composite oxide represented by Chemical Formula 4 above.

As another positive electrode active material, specifically, a transition metal oxide or solid solution oxide containing lithium may be exemplified. For example, as the transition metal oxide containing lithium, Li/Ni/Co/Mn-based composite oxide such as _{LiNi x} Co _y Mn _z O _{2 , Li/Ni-based composite oxide such as LiNiO 2} , or LiMn ₂ O ₄ Li/Mn-based composite oxides such as these can be exemplified. In addition, as a solid solution oxide containing lithium, Li _a Mn _x Co _y Ni _z O ₂ (1.150=a≤=1.430, 0.45≤=x≤=0.6, 0.10≤=y≤=0.15, 0.20≤=z≤= 0.28), LiMn _x Co _y Ni _z O ₂ (0.3=x≤=0.85, 0.10≤=y≤=0.3, 0.10≤=z≤=0.3), LiMn _{1 . 5} Ni _{0 . 5} O ₄ and the like can be exemplified.

The binder resin contains at least a first copolymer having a structure of the following Chemical Formula 1 and a second copolymer containing at least a structure of the following Chemical Formula 2 or 3.

[Formula 1]

[Formula 2]

[Formula 3]

Meanwhile, in Formulas 2 and 3, R ₁ and R ₂ are each independently hydrogen or a methyl group. R ₃ is hydrogen or a linear or branched alkyl group having 1 to 6 carbon atoms.

In the present embodiment, the binder resin contains, as the first copolymer, a fluorine-based copolymer containing at least the general formula (1) having a high fluorine content in the main chain and high resistance to oxidation. Accordingly, the binder resin can suppress the decomposition reaction of the electrolyte that occurs on the surface of the positive electrode active material under high potential.

However, when the binder resin contains only the fluorine-based copolymer including at least the formula (1), the dispersibility of the positive electrode active material during slurry formation is lowered. In addition, the binding property between the positive electrode active materials decreases. Thereby, the cycling characteristics of the lithium ion secondary battery 10 will fall.

Accordingly, the binder resin further contains an acrylonitrile-based copolymer containing at least Formula 2 or 3 as the second copolymer. The acrylonitrile-based copolymer has high dispersibility and binding properties of the positive electrode active material. Accordingly, in the positive electrode active material layer 22 according to the present embodiment, concerns that arise when the binder resin contains only the fluorine-based copolymer including at least Chemical Formula 1 can be improved. Accordingly, the lithium ion secondary battery 10 can improve cycle characteristics under high potential and capacity characteristics at high temperature storage.

Moreover, the binder resin contains the 1st copolymer which is a fluorine-type copolymer, and the 2nd copolymer which is an acrylonitrile-type copolymer. Accordingly, the binder resin can suppress the elution of the transition metal (specifically, cobalt) from the positive electrode active material. On the other hand, the mechanism for suppressing the elution of the transition metal from the positive electrode active material is not clear. However, it is thought that it is because the 1st copolymer which is a fluorine-type copolymer and the 2nd copolymer which is an acrylonitrile-type copolymer effectively suppresses structural destruction of a positive electrode active material.

The content ratio of the first copolymer containing at least the general formula (1) and the second copolymer containing at least the general formula (2) or (3) is 25:75 to 75:25 by mass. The content ratio with the second copolymer containing at least the formula (2) or (3) is in a mass ratio, preferably in a mass ratio, 35:65 to 65:35. When the content ratio of the first copolymer containing at least the formula (1) is less than the above range, the resistance to oxidation of the binder resin is lowered. Thereby, since it becomes difficult to suppress side reactions, such as decomposition|disassembly of electrolyte solution on the surface of a positive electrode active material, it is unpreferable. Moreover, when the content rate of the 2nd copolymer containing at least Formula 2 or 3 is less than the said range, the dispersibility of the positive electrode active material at the time of slurry preparation, and the binding property of a positive electrode active material fall. Accordingly, since the cycle characteristics of the lithium ion secondary battery 10 are lowered, it is not preferable.

Here, it is preferable that the 1st copolymer contains 50 mol% or more of the unit structure which uses vinylidene fluoride as a monomer in the polymer main chain. It is more preferable that the 1st copolymer contains 60 mol% or more of the unit structure which uses vinylidene fluoride as a monomer in the polymer main chain. On the other hand, the unit structure using vinylidene fluoride as a monomer is the left unit structure in Chemical Formula 1. When the content of the unit structure using vinylidene fluoride as a monomer is less than 50 mol%, the solubility of the first copolymer in an organic solvent is remarkably reduced. Thereby, since preparation of a slurry becomes difficult, it is unpreferable. Although the upper limit of content of the unit structure which uses vinylidene fluoride as a monomer is not specifically limited, For example, 80 mol% may be sufficient. On the other hand, the organic solvent is, for example, N-methyl-2-pyrrolidone (N-methyl-2-pyrrolidone: NMP).

Moreover, it is preferable that the 1st copolymer contains 10 mol% or more and 40 mol% or less of the unit structure which uses tetrafluoroethylene as a monomer in a polymer main chain. It is more preferable that the 1st copolymer contains 15 mol% or more and 35 mol% or less of the unit structure which uses tetrafluoroethylene as a monomer in the polymer main chain. On the other hand, the unit structure using tetrafluoroethylene as a monomer is the unit structure on the right side in Chemical Formula 1. When the content of the unit structure using tetrafluoroethylene as a monomer is less than 10 mol%, the resistance to oxidation of the first copolymer becomes low. Accordingly, it is not preferable because it becomes difficult to suppress the side reaction on the surface of the positive electrode active material. On the other hand, when the content of the unit structure using tetrafluoroethylene as a monomer is more than 40 mol%, the solubility in the organic solvent is remarkably reduced. Moreover, the dispersibility of the positive electrode active material at the time of slurry preparation falls. On the other hand, the organic solvent is, for example, N-methyl-2-pyrrolidone (N-methyl-2-pyrrolidone: NMP).

It is preferable that the second copolymer containing Formula 2 contains 90 mol% or more of a unit structure using acrylonitrile or methacrylonitrile as a monomer in the polymer main chain. It is more preferable that the 2nd copolymer contains 95 mol% or more of the unit structure which uses acrylonitrile or methacrylonitrile as a monomer in a polymer main chain. On the other hand, the unit structure using acrylonitrile or methacrylonitrile as a monomer is the left unit structure in Formula 2 or 3. When the content of the unit structure using acrylonitrile or methacrylonitrile as a monomer is less than 90 mol%, the resistance to oxidation of the second copolymer is low. Accordingly, it is not preferable because it becomes difficult to suppress the side reaction on the surface of the positive electrode active material. The upper limit of content of the unit structure which uses acrylonitrile or methacrylonitrile as a monomer is not specifically limited. However, the upper limit of content of the unit structure which uses acrylonitrile or methacrylonitrile as a monomer may be 97.5 mol%, for example.

In addition, it is preferable that the second copolymer containing Chemical Formula 2 contains 0.1 mol% or more and less than 10 mol% of a unit structure using acrylic acid or acrylic acid ester as a monomer in the polymer main chain. It is more preferable that the 2nd copolymer contains 1.0 mol% or more and 5.0 mol% or less of the unit structure which uses acrylic acid or an acrylic acid ester as a monomer in a polymer main chain. On the other hand, the unit structure using acrylic acid or acrylic ester as a monomer is the unit structure on the right side in Chemical Formula 2. When the content of the unit structure containing acrylic acid or acrylic acid ester as a monomer is less than 0.1%, the solubility of the second copolymer in an organic solvent (eg, NMP, etc.) is reduced. Thereby, since preparation of a slurry becomes difficult and binding property between positive electrode active materials falls at the same time, it is unpreferable. On the other hand, when the content of the unit structure using acrylic acid or acrylic acid ester as a monomer is 10 mol% or more, the resistance to oxidation of the second copolymer is low. Accordingly, it is not preferable because it becomes difficult to suppress the side reaction on the surface of the positive electrode active material.

On the other hand, it is preferable that the second copolymer containing Formula 3 contains 0.1 mol% or more and less than 10 mol% of the unit structure using acrylamide as a monomer in the polymer main chain. It is more preferable that the second copolymer containing Formula 3 contains 1.0 mol% or more and 5.0 mol% or less of a unit structure using acrylamide as a monomer in the polymer main chain. On the other hand, the unit structure using acrylamide as a monomer is the unit structure on the right side in Chemical Formula 3. When the content of the unit structure using acrylamide as a monomer is less than 0.1%, the solubility of the second copolymer in an organic solvent (eg, NMP, etc.) is lowered. Accordingly, it is not preferable because the slurry production becomes difficult. On the other hand, when the content of the unit structure using acrylamide as a monomer is 10 mol% or more, the resistance to oxidation of the second copolymer is low. Accordingly, it is not preferable because it becomes difficult to suppress the side reaction on the surface of the positive electrode active material.

Moreover, it is preferable that the weight average molecular weight of a 1st copolymer and a 2nd copolymer is 300,000 or more, respectively. As for the weight average molecular weight of a 1st copolymer and a 2nd copolymer, it is more preferable that it is 400,000 or more, respectively. When the weight average molecular weight of the first copolymer and the second copolymer is less than 300,000, the binding property between the positive electrode active materials and the adhesiveness of the positive electrode active material layer 22 to the current collector are reduced. Accordingly, since the cycle characteristics of the lithium ion secondary battery 10 are lowered, it is not preferable. The upper limit of the weight average molecular weight of a 1st copolymer and a 2nd copolymer is not specifically limited. However, the upper limit of the weight average molecular weight of a 1st copolymer and a 2nd copolymer may be 2,000,000, for example. In addition, the weight average molecular weight of a 1st copolymer and a 2nd copolymer can be measured by gel permeation chromatography (Gel Permeation Chromatography: GPC), for example.

In the present embodiment, the first copolymer containing at least Chemical Formula 1 and the second copolymer containing at least Chemical Formula 2 or 3 are used for the binder resin in a mass ratio of 25:75 to 75:25. Accordingly, the binder resin can suppress side reactions on the surface of the positive electrode active material. According to this, the lithium ion secondary battery 10 can suppress the elution of the metal from the positive electrode active material under high potential, and can improve cycle characteristics and capacity characteristics at high temperature storage.

Conductive aids include, for example, carbon black such as ketjen black and acetylene black, natural graphite, artificial graphite, carbon nanotubes, graphene, and Fibrous carbon, such as carbon nanofibers, or a composite_body|complex of these fibrous carbon and carbon black (carbon black), etc. can be used. However, the conductive auxiliary agent can be used without being limited to the above substances as long as it can increase the conductivity of the anode.

The positive electrode active material layer 22 may be manufactured by, for example, preparing a positive electrode slurry, coating the positive electrode slurry on the current collector 21, drying and rolling. The positive electrode slurry can be prepared by dispersing the above positive electrode active material, conductive auxiliary, and binder resin in a suitable organic solvent. A suitable organic solvent is, for example, N-methyl-2-pyrrolidone and the like. On the other hand, the density of the positive electrode active material layer 22 after rolling is not particularly limited, as long as it has a density applicable to the positive electrode active material layer of a general lithium ion secondary battery.

The negative electrode 30 includes a current collector 31 and an anode active material layer 32 . The current collector 31 may be any conductor as long as it is a conductor, and may be, for example, copper, copper alloy, aluminum, stainless steel, or nickel-plated steel.

The anode active material layer 32 contains at least an anode active material, and may further contain a conductive aid and a binder resin. On the other hand, the ratio of the content of the negative electrode active material, the conductive auxiliary agent, and the binder is not particularly limited. The ratio of the content of the negative electrode active material, the conductive aid, and the binder may be a ratio of the content applied in a general lithium ion secondary battery.

The negative electrode active material includes, for example, artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, or a graphite active material such as natural graphite coated with artificial graphite, fine particles of silicon (Si) or tin (Sn), and silicon ( Si) or fine particles of an oxide of tin (Sn), an alloy of silicon or tin, or a titanium oxide (TiO _x )-based compound such as _{Li 4} Ti ₅ O _{12 may be used.} Moreover, these mixtures can also be used. In addition, as the negative electrode active material, for example, metallic lithium or the like may be used in addition to these.

The conductive support agent may be the same as the conductive support agent used in the positive electrode active material layer 22 . Moreover, as binder resin, styrene-butadiene rubber (Styrene-Butadiene Rubber: SBR) etc. can be used, for example.

The negative electrode active material layer 32 may be prepared by, for example, preparing a negative electrode slurry, coating the negative electrode slurry on the current collector 31, drying and rolling. The negative electrode slurry can be prepared by dispersing the above negative electrode active material, conductive aid, and binder resin in a suitable solvent (eg, water, etc.). On the other hand, the density of the negative electrode active material layer 32 after rolling is not particularly limited, and may be any density applicable to the negative electrode active material layer of a general lithium ion secondary battery.

The separator layer 40 includes a separator and an electrolyte.

The separator is not particularly limited as long as it is used as a separator of a lithium ion secondary battery, and any separator may be used. As the separator, for example, a porous membrane or a nonwoven fabric exhibiting excellent high-rate discharge performance is preferably used alone or in combination. In addition, at least one surface of the separator may be coated. The coating may be performed with an inorganic material such as _{Al 2} O ₃ , SiO _{2 , or a polymer such as polyvinylidene fluoride.} The separator may contain an inorganic material such as a filler (filler), Al ₂ O ₃ or SiO _2.

As a material constituting the separator, for example, a polyolefin-based resin typified by polyethylene or polypropylene, polyethylene terephthalate or polybutylene terephthalate, etc. Polyester-based resins, polyvinylidene difluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether perfluorovinylether copolymer, vinylidene difluoride-tetrafluoroethylene copolymer, vinylidene difluoride-trifluoroethylene copolymer, vinylidene difluoride- fluoroethylene copolymer, vinylidene difluoride-hexafluoroacetone copolymer, vinylidene difluoride-ethylene copolymer, vinylidene difluoride-propylene copolymer, vinylidene difluoride-trifluoropropylene copolymer, vinylidene difluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoro Ethylene (vinylidene difluoride-ethylene- tetrafluoroethylene) copolymer You can use a sieve, etc. On the other hand, the porosity of the separator is not particularly limited, and the porosity of the separator of a general lithium ion secondary battery can be applied.

The electrolyte solution contains an electrolyte salt and a non-aqueous electrolyte.

Examples of the electrolyte salt include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , An inorganic ion salt including one of lithium (Li), sodium (Na), or potassium (K) such as KSCN may be used. Further, as the electrolyte salt, for example, LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ )(C ₄ F ₉ SO ₂ ) , LiC(CF ₃ SO ₂ ) ₃ , LiC(C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , (CH ₃ ) ₄ NBr, (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NI, (C ₃ H ₇ ) ₄ NBr, (nC ₄ H ₉ ) ₄ NClO ₄ , (nC ₄ H ₉ ) ₄ NI, (C ₂ H ₅ ) ₄ N-maleate, (C ₂ H ₅ ) ₄ N-benzoate, (C ₂ H ₅ ) ₄ N-phthalate, lithium stearyl sulfate, lithium octyl sulfate, dodecyl Organic ionic salts such as lithium dodecylbenzene sulphonate may also be used. In addition, these electrolyte salts can also be used individually or in mixture of 2 or more types. In addition, the concentration of the electrolyte salt may be any concentration applicable to a general lithium secondary battery, and there is no particular limitation. For example, the concentration of the electrolyte salt may be 0.5 mol/L to 2.0 mol/L.

Examples of the non-aqueous electrolyte include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, or vinylene carbonate. (ester), cyclic esters such as γ-butyrolactone or γ-valerolactone, dimethyl carbonate, diethyl carbonate or ethylmethyl carbonate Chain carbonates such as methyl formate, chain esters such as methyl acetate or methyl butyrate, tetrahydrofuran or a derivative thereof, 1,3-di Oxane (1,3-dioxane), 1,4-dioxane (1,4-dioxane), 1,2-dimethoxyethane (1,2-dimethoxyethane), 1,4-dibutoxyethane (1,4- Ethers such as dibutoxyethane or methyldiglyme, nitriles such as acetonitrile or benzonitrile, dioxolane or its derivatives, ethylene sulfide ), sulfolane, sultone, or derivatives thereof may be used alone or as a mixture of two or more thereof. However, the non-aqueous electrolyte is not limited thereto.

Moreover, the electrolyte solution may contain various additives, such as a negative electrode SEI (Solid Electrolyte Interface) forming agent, or surfactant.

Various additives include, for example, succinic anhydride, lithium bis (oxalate) borate, lithium tetrafluoroborate, propane sultone, butane sultone. sultone), propene sultone, 3-sulfolene, fluorinated arylether, and fluorinated methacrylate can be exemplified. In addition, it is possible to apply content of the additive in a general lithium ion secondary battery as content of each of these additives.

<3. Manufacturing method of lithium ion secondary battery>

Then, the manufacturing method of the lithium ion secondary battery 10 is demonstrated. However, the manufacturing method of the lithium ion secondary battery 10 is not limited to the following method, It is also possible to use manufacturing methods other than a well-known thing.

The anode 20 is manufactured as follows. First, materials constituting the positive electrode active material layer 22 (eg, positive electrode active material, conductive aid, and binder resin) are mixed in a desired ratio. Then, a positive electrode slurry is prepared by dispersing the mixture in an organic solvent (eg, N-methyl-2-pyrrolidone). Then, the positive electrode slurry is applied on the current collector 21 and dried to prepare the positive electrode active material layer 22 . In addition, the method of application|coating is although it does not specifically limit, For example, a knife coater (knife coater) method, a gravure coater (gravure coater) method, etc. can be used.

In addition, the positive electrode active material layer 22 is compressed to a desired density or thickness using a roll press. Accordingly, the anode 20 can be manufactured. Here, the density or thickness of the positive electrode active material layer 22 is not particularly limited, and any density or thickness of the positive electrode active material layer of a general lithium ion secondary battery may be sufficient.

The negative electrode 30 is also manufactured in the same manner as the positive electrode 20 . First, materials constituting the negative electrode active material layer 32 (eg, negative electrode active material, conductive aid, and binder resin) are mixed in a desired ratio. Then, by dispersing the mixture in a solvent (for example, water), a negative electrode slurry is prepared. Next, the negative electrode slurry is applied on the current collector 31 and dried, and the negative electrode active material layer 32 is dried. On the other hand, the method of application is not particularly limited, but, for example, a knife coater method, a gravure coater method, etc. can be used.In addition, the negative electrode active material layer 32 with a roll press machine ) is compressed to a desired density or thickness, thereby manufacturing the negative electrode 30 .

Here, the density or thickness of the anode active material layer 32 is not particularly limited, and any density or thickness of the anode active material layer of a general lithium ion secondary battery may be sufficient. On the other hand, when metallic lithium is used as the anode active material layer 32 , the anode 30 can be manufactured by attaching a metallic lithium foil to the current collector 31 .

Then, an electrode structure is manufactured by sandwiching a separator between the positive electrode 20 and the negative electrode 30 . After processing the prepared electrode structure into a shape suitable for a desired shape (eg, cylindrical, rectangular, laminated, buttoned, etc.), it is inserted into a container of the above shape. In addition, by injecting the electrolyte solution containing the above-described electrolyte salt and non-aqueous electrolyte into the container, the separator is impregnated with the electrolyte solution. Thereafter, the electrode structure and the container into which the electrolyte is injected are sealed. Accordingly, the lithium ion secondary battery 10 can be manufactured.

[Example]

Hereinafter, a lithium ion secondary battery according to the present embodiment will be described in detail with reference to Examples and Comparative Examples. In addition, the Example shown below is an example to the last, and the lithium ion secondary battery which concerns on this embodiment is not limited to the following example.

<Example 1>

In the following method, the 1st copolymer and the 2nd copolymer were prepared, and the lithium ion secondary battery which concerns on Example 1 was manufactured.

(First copolymer: synthesis of fluorine-based polymer A)

As the first copolymer, a copolymer (also referred to as fluoropolymer A) having a molar ratio of vinylidene fluoride and tetrafluoroethylene of 70 mol%:30 mol% was synthesized. Specifically, after putting 1 kg of pure water into a 4 L reaction vessel and performing nitrogen substitution, 800 g of octafluorocyclobutane was put into the reaction vessel, and the inside of the system was heated up to 45 degreeC. Then, 118 g of vinylidene fluoride and 40 g of tetrafluoroethylene were added to the reaction vessel. Then, 0.9 g of a 50 mass % methanol solution of di-n-propyl peroxydicarbonate was injected|thrown-in to the reaction container. Moreover, stirring was continued for 8 hours in the state which maintained the system internal pressure constant, supplying the mixed gas to the reaction container continuously. As the mixed gas, vinylidene fluoride/tetrafluoroethylene = 70 mol%/30 mol% was used. After the heating was stopped, the pressure was released from the reaction vessel until it reached atmospheric pressure, and the reaction product was washed with water and dried to synthesize a fluoropolymer A containing at least the formula (1).

When the weight average molecular weight of the synthesized fluorine-based polymer A was measured by GPC, it was 900,000 (in terms of polymethyl methacrylate).

(Second copolymer: synthesis of acrylonitrile-based polymer A)

As the second copolymer, a copolymer (also referred to as acrylonitrile-based polymer A) having a molar ratio of acrylic nitrile and acrylic acid of 97.5 mol%:2.5 mol% was synthesized. Specifically, 9.65 g of acrylonitrile and 0.35 g of acrylic acid (the molar ratio between acrylic nitrile and acrylic acid is 97.5:2.5) were added to 90 g of ion-exchanged water. Then, after raising the temperature to 60 degreeC in nitrogen atmosphere, by adding 1.065 g of ammonium persulfate to the mixture, the polymer was precipitation-polymerized. Accordingly, an acrylonitrile-based polymer A containing at least the formula (2) was synthesized. When the weight average molecular weight of the synthesized acrylonitrile-based polymer A was measured by GPC, it was 500,000 (in terms of polyethylene oxide).

(Production of anode)

LiCoO ₂ , carbon black, and a binder resin including the first copolymer and the second copolymer were dissolved and dispersed in N-methyl-2-pyrrolidone to form a positive electrode slurry. The mixing ratio was set as 97.6:1.2:1.2 by solid content mass ratio. On the other hand, the mixing ratio of the first copolymer and the second copolymer in the binder resin was set to 50:50 by mass ratio.

Then, the positive electrode slurry was applied to one side of the current collector made of aluminum foil so that the application amount of the slurry after drying was 20.0 mg/cm ^{2 , and it was dried.} After drying, the current collector coated with the positive electrode slurry was rolled with a roll press so that the positive electrode active material layer had a density of 4.1 g/cm ^{3 , thereby producing a positive electrode.}

(Production of cathode)

A negative electrode slurry was formed by dissolving and dispersing artificial graphite, a water dispersion of fine styrene-butadiene copolymer particles, and a sodium salt of carboxymethyl cellulose in pure water. The mixing ratio was set as 97.5:1.5:1.0 by solid content mass ratio.

Then, the negative electrode slurry was apply|coated to one side of the electrical power collector which consists of copper foil, and it dried so that the slurry application amount after drying ^{might be set to 12.5 mg/cm<2>.} After drying, the current collector coated with the negative electrode slurry was rolled with a roll press so that the negative electrode active material layer had a density of 1.6 g/cm ^{3 , thereby producing a negative electrode.}

(Production of lithium ion secondary battery cells)

An aluminum lead wire was welded to the anode fabricated above, and a nickel lead wire was welded to the cathode fabricated above. Thereafter, a flat electrode structure was produced by winding and compressing the positive electrode and the negative electrode to face each other through a polyethylene porous separator. The fabricated electrode structure was housed in a battery container formed of an aluminum laminate film in a state in which the lead wire was pulled to the outside. Further, the electrolyte solution in which LiPF ₆ was dissolved in 1mol / L in carbonate solvent was injected into the battery container. Then, by sealing the battery container under reduced pressure, a lithium ion secondary battery according to Example 1 was produced.

<Examples 2 and 3>

The mixing ratio of the first copolymer (fluoropolymer A) and the second copolymer (acrylonitrile polymer A) in the positive electrode binder resin was changed as shown in Table 1. Other than that, in the same manner as in Example 1, lithium ion secondary batteries according to Examples 2 and 3 were produced.

<Examples 4 to 6>

As the second copolymer, an acrylonitrile-based polymer B, which shows a synthesis method below, was used. In addition, the mixing ratio of the first copolymer (fluoropolymer A) and the second copolymer (acrylonitrile polymer B) in the binder resin of the positive electrode was changed as shown in Table 1. Other than that, in the same manner as in Example 1, lithium ion secondary batteries according to Examples 4 to 6 were produced.

(Second copolymer: synthesis of acrylonitrile-based polymer B)

As the second copolymer, a copolymer (also referred to as acrylonitrile polymer B) having a molar ratio of acryl nitrile and acrylamide of 95 mol%:5 mol% was synthesized. Specifically, 9.34 g of acrylonitrile and 0.66 g of acrylamide were added to 90 g of ion-exchanged water. After the temperature was raised to 60°C under a nitrogen atmosphere, 1.065 g of ammonium persulfate was added to the mixture to precipitate polymerization of the copolymer. Thus, an acrylonitrile-based polymer B containing at least the formula (3) was synthesized. When the weight average molecular weight of the synthesized acrylonitrile-based polymer B was measured by GPC, it was 500,000 (in terms of polyethylene oxide).

<Comparative Example 1>

As the binder resin for the positive electrode, only the first copolymer (fluoropolymer A) was used. Other than that, in the same manner as in Example 1, a lithium ion secondary battery according to Comparative Example 1 was produced.

<Comparative Example 2>

As the binder resin for the positive electrode, only the second copolymer (acryl nitrile polymer A) was used. Other than that, in the same manner as in Example 1, a lithium ion secondary battery according to Comparative Example 2 was produced.

<Comparative Example 3>

As the binder resin for the positive electrode, only vinylidene fluoride homopolymer (KF7200 manufactured by KUREHA Battery Materials, also referred to as fluorine-based polymer B) was used. Except that, in the same manner as in Example 1, a lithium ion secondary battery according to Comparative Example 3 was produced.

<Comparative Example 4>

As the binder resin for the positive electrode, a mixture of a fluorine-based polymer B and an acrylonitrile-based polymer A in a mass ratio of 50:50 was used. Other than that, in the same manner as in Example 1, a lithium ion secondary battery according to Comparative Example 4 was produced.

<Comparative Example 5>

As the binder resin for the positive electrode, a mixture of a fluorine-based polymer A and a fluorine-based polymer B in a mass ratio of 50:50 was used. Other than that, in the same manner as in Example 1, a lithium ion secondary battery according to Comparative Example 5 was produced.

<Comparative Examples 6-9>

The mixing ratio of the first copolymer (fluoropolymer A) and the second copolymer (acrylonitrile polymer A) in the binder resin of the positive electrode was changed as shown in Table 1. Other than that, in the same manner as in Example 1, lithium ion secondary batteries according to Comparative Examples 6 to 9 were produced.

<Evaluation of lithium ion secondary battery>

First, in a constant temperature bath at 45° C., the lithium ion secondary batteries according to Examples 1 to 6 and Comparative Examples 1 to 9 were charged with a constant current at 1/5 CA of the design capacity until they became 4.5 V. Then, constant voltage charging was performed until it became 1/20CA at 4.5V. Then, constant current discharge was performed until it became 3.0V with 1/5CA. The initial charge of the lithium ion secondary battery was performed according to this process. In addition, the discharge capacity of the initial discharge at this time was made into the initial stage discharge capacity. On the other hand, CA represents a 1-hour discharge rate.

(Evaluation method of cycle characteristics)

About each lithium ion secondary battery after initial stage charge/discharge, under the temperature of 45 degreeC, constant current charging was carried out at 1CA with the charge termination voltage of 4.5V and the discharge termination voltage of 3.0V. After that, a cycle of constant voltage charging at 1/20 CA and constant current discharging at 1 CA was repeated for 50 cycles. After 50 cycles, each lithium ion secondary battery was subjected to constant current charging at 1/5 CA, and further constant voltage charging at 1/20 CA, followed by constant current discharging at 1/5 CA to measure the discharge capacity. The capacity retention ratio was calculated by dividing the measured discharge capacity after 50 cycles by the initial discharge capacity.

(Evaluation method of cobalt elution amount)

Each lithium ion secondary battery after the above cycle characteristic evaluation (that is, after 50 cycles) was disassembled. The amount of cobalt deposited on the cathode was measured by analyzing a solution in which cobalt deposited on the cathode was dissolved by an inductively coupled plasma mass spectrometer (ICP-MS).

(Evaluation method for capacity characteristics after high temperature storage)

Each lithium ion secondary battery after the initial charge was charged at a constant current of 1/2CA at a final charge voltage of 4.5V and a final discharge voltage of 3.0V at a temperature of 25°C. Thereafter, a cycle of constant voltage charging at 1/20 CA and constant current discharging at 1/2 CA was repeated for 2 cycles. Then, under the temperature of 25 degreeC, each lithium ion secondary battery was constant-current-charged by 1/2CA to the charge termination voltage of 4.5V. Subsequently, after constant voltage charging at 1/20 CA, it was stored at a high temperature for one week in a constant temperature bath at 60°C. The charge capacity at this time was made into the charge capacity before high temperature storage.

After high-temperature storage, under a temperature of 25°C, each lithium ion secondary battery was subjected to constant current discharge at 1/2CA to 3.0V. Thereafter, the charging capacity after high temperature storage was measured by constant current charging at 1/2 CA to a charge termination voltage of 4.5 V, followed by constant voltage charging at 1/20 CA. By dividing the measured charging capacity after high-temperature storage by the charging capacity before high-temperature storage, the capacity recovery rate after high-temperature storage was calculated.

(Adhesiveness evaluation method)

On the stainless steel plate, the positive electrode plate used in each lithium ion secondary battery was fixed. An adhesive tape having a width of 1.5 cm (Cellro Tape No. 405 manufactured by Nichiban Corporation) was adhered to the surface of the fixed positive electrode plate on the positive electrode active material layer side. The adhesive tape was peeled off in a 180 degree direction using a peeling tester (SHIMAZU EZ-S manufactured by Shimadzu Corporation) to evaluate the adhesiveness of the positive electrode active material layer.

(Evaluation results)

Table 1 shows the evaluation results of the lithium ion secondary batteries according to Examples 1 to 6 and Comparative Examples 1 to 9 above.

binder resin Capacity retention rate
[%]
Cobalt Precipitation
[ppm] Capacity recovery rate
[%] adhesion
[mN/mm] (a) (b) (a):(b)
(mass ratio) Example 1 Fluoropolymer A Acrylonitrile-based polymer A 50:50 75 800 83 14 Example 2 Fluoropolymer A Acrylonitrile-based polymer A 30:70 71 880 82 18 Example 3 Fluoropolymer A Acrylonitrile-based polymer A 70:30 72 850 82 7 Example 4 Fluoropolymer A Acrylonitrile-based polymer B 50:50 74 810 83 10 Example 5 Fluoropolymer A Acrylonitrile-based polymer B 30:70 70 900 82 12 Example 6 Fluoropolymer A Acrylonitrile-based polymer B 70:30 70 860 82 5 Comparative Example 1 Fluoropolymer A - 20 1300 62 2 Comparative Example 2 Acrylonitrile polymer A - 51 1090 73 22 Comparative Example 3 Fluorine polymer B - 32 1280 64 6 Comparative Example 4 Fluorine polymer B Acrylonitrile-based polymer A 50:50 60 1150 70 16 Comparative Example 5 Fluoropolymer A Fluorine polymer B 50:50 30 1250 64 4 Comparative Example 6 Fluoropolymer A Acrylonitrile-based polymer A 10:90 38 1180 65 20 Comparative Example 7 Fluoropolymer A Acrylonitrile-based polymer A 20:80 35 1100 66 18 Comparative Example 8 Fluoropolymer A Acrylonitrile-based polymer A 80:20 26 1200 73 3 Comparative Example 9 Fluoropolymer A Acrylonitrile-based polymer A 90:10 22 1250 73 2

Referring to the results in Table 1, it can be seen that Examples 1 to 6 have higher capacity retention rates and capacity recovery rates at high temperatures than Comparative Examples 1 to 9. This is considered to be because Examples 1-6 are using the binder resin which contains the specific 2 types of copolymer in specific mass ratio. Moreover, in Examples 1-6, since the adhesiveness of a positive electrode active material layer and an electrical power collector is 5 mN/mm or more, it turns out that adhesiveness with no practical problem as a binder resin is obtained.

Moreover, in Examples 1-6, it turns out that the amount of cobalt deposited on the negative electrode after 50 cycles is suppressed to 1000 ppm or less compared with Comparative Examples 1-3. The effect of suppressing the elution of cobalt from the positive electrode active material is not obtained when the fluorine-based polymer A, the fluorine-based polymer B, or the acrylonitrile-based polymer A is used alone. Accordingly, it can be seen that this effect is obtained only when a binder resin containing two specific copolymers in a specific mass ratio is used as defined in the present embodiment.

As described above, the lithium ion secondary battery according to the present embodiment can be preferably used under a high potential having a charge termination voltage of 4.45V seconds based on graphite. Specifically, the lithium ion secondary battery according to the present embodiment can improve cycle characteristics and capacity characteristics during high temperature storage.

As mentioned above, although this invention was demonstrated in detail about preferable embodiment, referring an accompanying drawing, this invention is not limited to this example. It is clear that various changes or modifications can be made within the scope of the technical idea described in the claims by those having ordinary knowledge in the field of technology to which the present invention belongs. It is understood to be within the technical scope of the present invention.

10 Li-ion secondary battery
20 anode
21 current collector
22 cathode active material layer
30 cathode
31 current collector
32 Anode active material layer
40 separator layer

Claims (10)

A cathode active material containing lithium composite oxide, and
A binder resin containing at least a first copolymer having a structure of Formula 1 below and a second copolymer containing at least a structure of Formula 2 or Formula 3 in a mass ratio of 25:75 to 75:25,
The first copolymer is a cathode active material layer comprising a unit structure having vinylidene fluoride as a monomer in an amount of 50 mol% to 80 mol% in the polymer main chain.
[Formula 1]

[Formula 2]

[Formula 3]

In Formulas 2 and 3, R ₁ and R ₂ are each independently hydrogen or a methyl group, and R ₃ is hydrogen or a linear or branched alkyl group having 1 to 6 carbon atoms. delete delete In claim 1,
The second copolymer is a cathode active material layer comprising 90 mol% or more of a unit structure using acrylonitrile or methacrylonitrile as a monomer in the polymer main chain. In claim 1,
At least the second copolymer comprising Formula 2 is a cathode active material layer comprising 0.1 mol% or more and less than 10 mol% of a unit structure using acrylic acid or acrylic acid ester as a monomer in the polymer main chain. In claim 1,
The positive active material layer comprising at least 0.1 mol% or more and less than 10 mol% of the second copolymer comprising Formula 3 in the polymer main chain having a unit structure using acrylamide as a monomer. In claim 1,
The first copolymer and the second copolymer have a weight average molecular weight of 300,000 or more, respectively. In claim 1,
The lithium composite oxide is a positive electrode active material layer of a compound represented by the following formula (4).
[Formula 4]
Li _a Co _x M _y O ₂
In Formula 4, M is from the group consisting of aluminum, nickel, manganese, chromium, iron, vanadium, magnesium, titanium, zirconium, niobium, molybdenum, tungsten, copper, zinc, gallium, indium, tin, lanthanum, and cerium. It is one or two or more metal elements selected,
0.20=a≤=1.20, 0.95≤=x <1.00, 0 <y≤=0.05, and x+y=1. A positive electrode comprising the positive electrode active material layer according to any one of claims 1 and 4 to 8;
cathode and
non-aqueous electrolyte
A lithium ion secondary battery comprising a. In claim 9,
A lithium ion secondary battery having a charge termination voltage of the lithium ion secondary battery greater than 4.45V based on graphite.

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