KR20170098146A - Positive electrode and wounding element for rechargeable lithium battery, and rechargeable lithium battery - Google Patents

Abstract

The present invention provides a positive electrode for a lithium secondary battery, a winding element for a lithium secondary battery, and a lithium secondary battery. The positive electrode comprises: a positive active material; and a mixed binder including a first binder, a second binder, and a third binder. The first binder comprises any one or more selected from polyvinylidene fluoride, acid-modified polyvinylidene fluoride, and a copolymer including the acid-modified polyvinylidene fluoride. The mixed binder comprises 30 to 60 wt% of the first binder on the basis of the total weight of the mixed binder. At the same time, a tensile modulus is 200 to 600 MPa.

Description

TECHNICAL FIELD [0001] The present invention relates to a positive electrode for a lithium secondary battery, a winding element for a lithium secondary battery, and a lithium secondary battery,

A positive electrode for a lithium secondary battery, a winding element for a lithium secondary battery, and a lithium secondary battery.

2. Description of the Related Art Recently, with the miniaturization of information processing apparatuses such as mobile phones and notebook computers, it is required to increase the energy density of non-aqueous electrolyte secondary batteries that can be used as a power source for these information processing apparatuses.

For example, Japanese Patent Application Laid-Open Publication Nos. 2002-146590 and 2015-109154 disclose contents in which the cathode active material layer is densified to improve the characteristics (capacity, cycle characteristics) of the nonaqueous electrolyte secondary battery.

More specifically, JP-A-2012-146590 discloses a composition in which a plurality of kinds of active material particles having different average particle diameters are compounded at a predetermined blending ratio, and carbon black and expanded graphite are blended at a predetermined blending ratio ,

Japanese Patent Application Laid-Open No. 2015-109154 discloses a configuration in which a specific conductive material is compounded on the surface of the positive electrode active material particles and the tensile modulus of the binder is adjusted to a value within a specific range.

However, the characteristics of the nonaqueous electrolyte secondary battery can not be sufficiently improved by simply densifying the positive electrode active material layer as described above. Accordingly, a method of thickening the positive electrode active material layer while densifying the positive electrode active material layer has been proposed.

However, when the positive electrode active material layer is thickened while increasing the density of the positive electrode active material layer, the flexibility of the positive electrode active material layer is lowered, thereby possibly damaging the positive electrode in the manufacture of the wound nonaqueous electrolyte secondary battery. In particular, the closer the distance between the anode and the center of the winding element is, the smaller the radius of curvature of the anode becomes.

Therefore, there is a strong demand for a technique capable of improving the characteristics of the nonaqueous electrolyte secondary battery while maintaining the flexibility of the positive electrode active material layer.

As a technique for ensuring the flexibility of the positive electrode active material layer, it is considered to use a binder having a low elastic modulus as a binder for the positive electrode active material layer. However, the binder having a low modulus of elasticity may be a factor for lowering the characteristics of the non-aqueous electrolyte secondary battery, particularly the cycle characteristics. Therefore, when the binder having a low modulus of elasticity is used, the characteristics of the nonaqueous electrolyte secondary battery can not be improved even when the positive electrode active material layer is formed thick.

As another method for securing the flexibility of the cathode active material, WO2011 / 052126 discloses a method of forming a cathode active material layer on the front and back surfaces of a current collector and a binder contained in the cathode active material layer on the front surface and a cathode active material layer A configuration in which a binder having a different elastic modulus is used as a binder is described. However, this problem could not fundamentally solve the above problem.

One embodiment of the present invention provides a positive electrode for a lithium secondary battery capable of improving the characteristics of a nonaqueous electrolyte secondary battery while maintaining the flexibility of the positive electrode active material layer.

Another embodiment is to provide a winding element for a lithium secondary battery including the positive electrode.

Another embodiment provides a lithium secondary battery comprising the positive electrode.

According to one embodiment, the positive electrode active material; And a mixed binder including a first binder, a second binder, and a third binder, wherein the first binder is at least one selected from the group consisting of polyvinylidene fluoride (PVdF), acid denatured polyvinylidene fluoride, and acid denatured polyvinylidene And a fluoride-containing copolymer, wherein the mixed binder includes the first binder in a proportion of 30% by weight to 60% by weight based on the total weight of the mixed binder, the tensile modulus A positive electrode for a lithium secondary battery having a capacity of 200 MPa to 600 MPa.

The copolymer comprising the acid-modified polyvinylidene fluoride may be any one or more of monomers selected from the group consisting of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and chlorotrifluoroethylene (CTFE) . ≪ / RTI >

The second binder may be hydrogenated acrylonitrile butadiene rubber (hydrogenated NBR). The mixed binder may include the second binder in a proportion of 10% by weight to 40% by weight based on the total weight of the mixed binder.

The third binder may be a copolymer comprising vinylidene fluoride. The third binder may have a tensile elastic modulus of 150 MPa to 600 MPa.

The copolymer containing vinylidene fluoride may be at least one selected from the group consisting of vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride- Fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-acrylate copolymer, vinylidene fluoride-hexafluoropropylene-acrylate copolymer, vinylidene fluoride-tetrafluoroethylene- Acrylate copolymer, and a vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene-acrylate copolymer.

According to another embodiment, there is provided a winding element for a lithium secondary battery including the positive electrode.

According to another embodiment, there is provided a lithium secondary battery including the winding element.

The positive electrode for a lithium secondary battery according to an embodiment can improve the characteristics of the lithium secondary battery while maintaining the flexibility of the positive electrode active material layer.

1 is a plan sectional view showing a schematic configuration of a lithium secondary battery according to one embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited thereto, and the present invention is only defined by the scope of the following claims.

In the specification and drawings, the same reference numerals are given to constituent elements having substantially the same functional configuration, and redundant description will be omitted.

<1. Configuration of Non-aqueous Electrolyte Secondary Battery>

First, the configuration of a lithium secondary battery according to one embodiment will be described with reference to FIG.

Fig. 1 shows a plan view of the winding element 1a and an enlarged view of the area A of the winding element 1a.

The lithium secondary battery includes a winding element 1a, a nonaqueous electrolyte, and a sheathing material 40.

The winding element 1a is constituted such that the strip-shaped anode 10, the separator 20, the strip-shaped cathode 30 and the separator 20 are wound in the longitudinal direction in the stacked order and compressed in the direction of the arrow B It is. Of course, the stacking order of the respective components is not limited thereto.

The strip type positive electrode 10 (hereinafter referred to as &quot; positive electrode 10 &quot;) includes a positive electrode collector 11 and a positive electrode active material layer 12.

The cathode current collector 11 is not particularly limited, but may be aluminum (Al), stainless steel, nickel-plated steel, or the like.

A positive terminal may be connected to the positive electrode collector 11.

The positive electrode active material layer 12 includes a positive electrode active material and a mixed binder, and may further include a conductive material.

The positive electrode active material is not particularly limited as long as it is a material capable of reversibly intercalating and deintercalating lithium ions, and examples thereof include lithium-containing transition metal oxides, nickel sulfide, copper sulfide, sulfur, iron oxide and vanadium oxide.

Examples of the lithium-containing transition metal oxide include lithium cobalt oxide (LCO), lithium nickel oxide, lithium nickel cobalt oxide, lithium nickel cobalt aluminum oxide (hereinafter referred to as NCA), lithium nickel cobalt manganese oxide Quot;), lithium manganese oxide, lithium iron phosphate, and the like. These cathode active materials may be used alone or in combination of two or more.

Among them, the lithium-containing transition metal oxide is suitable, and the lithium-containing transition metal oxide having a layered salt salt type structure is more suitable.

The positive electrode active material may be subjected to a surface treatment to suppress side reactions with an electrolyte at a high voltage.

The mean particle size of the positive electrode active material is suitably from 10 탆 to 30 탆 from the viewpoints of the safety and the packing of the positive electrode active material.

When the cathode active material is in the form of secondary particles in which primary particles are assembled, the average particle size may be a particle size of the secondary particles. In this case, the average particle size of the cathode active material is a 50% volume integrated value (D50 value) of the diameter distribution when the secondary particles aggregated by the primary particles of the cathode active material are regarded as spheres, Can be measured by a scattering method.

The content (for example, bulk density) of the positive electrode active material in the positive electrode active material layer 12 is not particularly limited and may be any amount as long as the content is applied to the positive electrode active material layer of the conventional lithium secondary battery.

The mixed binder may combine the positive electrode active material and the conductive material, and may combine the positive electrode active material and the conductive material with the positive electrode collector 11.

The mixed binder may include at least first to third binders described later. As described above, since the first to third binders are used in combination, the characteristics of the lithium secondary battery 1 can be improved while maintaining the flexibility of the positive electrode active material layer 12. [

The first binder may include at least one selected from polyvinylidene fluoride (PVdF), an acid-modified polyvinylidene fluoride, and a copolymer comprising an acid-modified polyvinylidene fluoride.

The mixed binder may include a first binder in a proportion of 30% by weight to 60% by weight based on the total weight of the mixed binder.

The acid-modified polyvinylidene fluoride may include a monomer having a carboxylic acid group such as acrylic acid or maleic acid or a carboxylic anhydride group such as maleic anhydride as an acid-modified monomer.

Further, the copolymer comprising the acid-modified polyvinylidene fluoride may be any one selected from the group consisting of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and chlorotrifluoroethylene (CTFE) Or more of the above monomers. For example, the copolymer comprising an acid-modified polyvinylidene fluoride may be an acid-modified PVdF-HFP copolymer.

The first binder can bond the positive electrode active material, the conductive material and the positive electrode collector 11 firmly. That is, the first binder is a binder capable of increasing the peeling strength of the positive electrode active material layer 12 with respect to the positive electrode collector 11. In particular, the acid-denatured binder has a strong binding force and can reduce the total amount of the binder, which is suitable for increasing the densification of the thick film of the anode 10.

Further, since the first binder has high electrochemical stability, the deterioration of the battery characteristics is small even when the first binder is used.

However, since the first binder is less flexible than the second and third binders, if the first binder is used in excess, the flexibility of the cathode active material layer 12 may be reduced. In consideration of this, it is appropriate that the first binder is included in the mixed binder in the above ratio.

The second binder may be hydrogenated acrylonitrile butadiene rubber (hydrogenated NBR).

The mixed binder may include the second binder in a proportion of 10% by weight to 40% by weight based on the total weight of the mixed binder.

The second binder can increase the flexibility of the positive electrode active material layer 12. [ In addition, since the second binder has a high elongation property, the anchor effect can increase the binding strength between the positive electrode active material and the conductive material and the positive electrode collector 11.

However, since the second binder has a lower electrochemical stability than the first binder, there is a possibility that the battery characteristics may deteriorate if the second binder is used excessively. Accordingly, it is appropriate that the second binder is included in the mixed binder in the above ratio.

The third binder may be a copolymer comprising vinylidene fluoride. The third binder may have a tensile elastic modulus of 150 MPa to 600 MPa. According to one embodiment, the tensile modulus of elasticity of the third binder may be 200 MPa to 350 MPa.

The mixed binder may include the third binder at a ratio of 15% by weight to 35% by weight based on the total weight of the mixed binder.

The copolymer comprising vinylidene fluoride may be at least one selected from the group consisting of vinylidene fluoride-tetrafluoroethylene (VdF-TFE) copolymer, vinylidene fluoride-hexafluoropropylene (VdF-HFP) (VdF-CTFE) copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene (VdF-TFE-HFP) copolymer, vinylidene fluoride-acrylate Copolymers, vinylidene fluoride-hexafluoropropylene-acrylate copolymers, vinylidene fluoride-tetrafluoroethylene-acrylate copolymers, and vinylidene fluoride-tetrafluoroethylene-hexafluoro Propylene-acrylate copolymer, and a propylene-acrylate copolymer.

The third binder can increase the flexibility of the cathode active material layer 12. [

In addition, the third binder is a flexible binder and has high electrochemical stability. Therefore, even if the third binder is used, the deterioration of the battery characteristics is insignificant.

However, since the third binder has a lower drawability than the second binder, the binding force due to the anchor effect is low. Therefore, if the third binder is used in excess, there is a possibility that the peeling strength of the positive electrode active material layer 12 to the positive electrode collector 11 is lowered.

However, when the weight ratio of the first and second binders is within the above-mentioned range, the weight ratio of the third binder may also be included in an appropriate range.

When the mixed binder is composed of only the first to third binders, the total weight% is 100% by weight.

The tensile elastic modulus of the mixed binder is suitably from 200 MPa to 600 MPa, more preferably from 250 MPa to 450 MPa.

When the mixed binder satisfies the above requirements, the characteristics of the lithium secondary battery 1 can be improved while maintaining the flexibility of the positive electrode active material layer 12. [

In one embodiment, the first binder is a harder binder than the second binder and the third binder. In one embodiment, the second binder and the third binder are added to the first binder in an appropriate amount (specifically, And the mixture was mixed to appropriately adjust the tensile elastic modulus of the mixed binder.

The content of the mixed binder in the positive electrode active material layer 12 is not particularly limited, but is preferably 0.3% by weight to 5% by weight based on the weight of the positive electrode active material layer 12, % Is more appropriate.

When the content of the mixed binder is within the above range, the characteristics of the lithium secondary battery 1 can be improved while maintaining the flexibility of the positive electrode active material layer 12.

Examples of the conductive material include carbon black such as ketjen black and acetylene black, natural graphite, artificial graphite, carbon nanotubes, and graphene. However, the conductive material is not particularly limited as long as the conductivity of the anode is increased. It does not.

The thickness of the positive electrode active material layer 12 is not particularly limited and may be at least as thick as that of a conventional lithium secondary battery. In one embodiment, since the positive electrode active material layer 12 has excellent flexibility, it is possible to make the positive electrode active material layer 12 thicker than before.

The porosity of the positive electrode active material layer 12 is not particularly limited, but may be 10% by volume to 20% by volume.

The separator 20, the strip-shaped negative electrode 30 (hereinafter referred to as &quot; negative electrode 30 &quot;), the electrolytic solution, and the exterior member can be arbitrarily used as those usable in general lithium secondary batteries.

These will be outlined below.

The separator 20 is not particularly limited, and any separator may be used as long as it is generally used as a separator of a lithium secondary battery. For example, a porous film or nonwoven fabric exhibiting excellent high-rate discharge performance as a separator can be used singly or in combination.

The separator may be coated with an inorganic material such as Al ₂ O ₃ , Mg (OH) ₂ , SiO ₂ or the like.

Examples of the material constituting the separator include polyolefin resins typified by polyethylene, polypropylene, etc., polyethylene terephthalate, polybutylene terephthalate, and the like A polyvinylidene difluoride, a vinylidene fluoride-hexafluoropropylene copolymer, a vinylidene fluoride-perfluorovinyl ether copolymer, a vinylidene fluoride- Vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone copolymer , Vinylidene fluoride-ethylene copolymer, vinylidene fluoride-pro Vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, vinylidene fluoride- Can be used.

The porosity of the separator is not particularly limited, and the porosity of the separator of the lithium secondary battery can be arbitrarily applied.

The negative electrode 30 includes a negative electrode current collector 31 and a negative electrode active material layer 32.

The anode current collector 31 may include, for example, copper (Cu), nickel (Ni), or the like.

The negative electrode active material layer 32 may be any negative electrode active material layer of a lithium secondary battery. For example, the negative electrode active material layer 32 may include a negative electrode active material, and may further include a negative electrode binder.

The negative electrode active material may be, for example, a graphite active material (such as artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite), silicon (Si) or tin (Sn) And a graphite active material, fine particles of silicon or tin, an alloy containing silicon or tin as a base material, Li ₄ Ti ₅ O ₁₂ , a titanium oxide (TiO _x ) -based compound, or the like. The silicon oxide may be represented by SiO _x (0? _X ? 2).

As the negative electrode active material, for example, metal lithium or the like may also be used.

The binder for the negative electrode may be, for example, polyvinylidene difluoride, ethylene-propylene-diene terpolymer, styrene-butadiene rubber (SBR), acrylonitrile Acrylonitrile-butadiene rubber, fluoro elastomer, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, and the like. .

The binder for the negative electrode is not particularly limited as long as it can bind the negative electrode active material and the conductive material on the negative electrode current collector 31.

The content of the binder for the negative electrode is not particularly limited and may be any value as long as it is applied to the negative electrode active material layer of the lithium secondary battery.

The nonaqueous electrolytic solution which can be conventionally used for a lithium secondary battery is not particularly limited and can be used.

The electrolytic solution has a composition containing an electrolytic salt in a non-aqueous solvent.

Examples of the non-aqueous solvent include cyclic carbonates such as propylene carbonate, ethylene carbonate, buthylene carbonate, chloroethylene carbonate, and vinylene carbonate, Esters Cyclic esters such as? -Butyrolactone,? -Valerolactone, etc. Dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, etc. Carbonates in the form of linear or branched esters such as methyl formate, methyl acetate or methyl butyrate tetrahydrofuran or its derivatives 1,3-dioxane (1, 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyl Diglyme iglyme etc. nitriles such as acetonitrile and benzonitrile dioxolane or derivatives thereof ethylene sulfide, sulfolane, sultone (sulphonate) sultone or a derivative thereof may be used singly or in combination of two or more kinds.

The electrolytic salt may be, for example, 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 LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ containing one kind of lithium (Li), sodium (Na) or potassium (K) _{, 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 stearyl sulfate), lithium octyl sulfate (lithium octyl sulfate, and lithium dodecylbenzene sulphonate. These ionic compounds may be used singly or in combination of two or more.

The concentration of the electrolyte salt may be the same as that of the non-aqueous electrolyte used in a conventional lithium secondary battery, and is not particularly limited.

In one embodiment, an electrolytic solution containing an appropriate lithium compound (electrolyte salt) at a concentration of about 0.5 mol / L to 2.0 mol / L can be used.

The exterior material 40 may be, for example, an aluminum laminate, but a metal exterior material may also be used.

<2. Manufacturing Method of Lithium Secondary Battery>

Hereinafter, a method for producing a lithium secondary battery will be described.

(Manufacturing method of strip-shaped anode)

The anode 10 is manufactured, for example, by the following method.

First, the positive electrode active material layer 12 is formed on the positive electrode collector 11. In other words, the material of the positive electrode active material layer 12 is dispersed in an organic solvent or water to form a positive electrode material mixture slurry, and the positive electrode material mixture slurry is coated on the positive electrode collector 11 to form a coating layer.

Then, the coated layer is dried. The cathode active material layer 12 is formed on the cathode current collector 11 according to this step.

The coating method is not particularly limited. For example, a doctor blade method, a slot die method, a knife coater method, and a gravure coater method may be used.

(Method for manufacturing strip type negative electrode)

The cathode 30 is fabricated, for example, by the following method.

The negative electrode active material layer is dispersed in a solvent (for example, water) to form a negative electrode mixture slurry, and the negative electrode mixture slurry is coated on the current collector to form a coating layer. Subsequently, the coated layer is dried, and the dried coated layer is rolled together with the negative electrode collector 31. The cathode 30 is produced according to this process.

(Winding device and manufacturing method of battery)

The positive electrode 10, the separator 20, the negative electrode 30, and the separator 20 are laminated in this order to produce an electrode laminate. The electrode laminate is wound (wound) to produce a wound element 1a. Then, the winding element 1a is pressed in the direction of arrow B, for example, to crush the winding element 1a.

Next, the flat type rolling element 1a is inserted into an external body (for example, a laminate film) 40 together with the non-aqueous electrolyte, and the external body is sealed to manufacture the non-aqueous electrolyte secondary battery 1. At this time, terminals that are electrically connected to the respective current collectors can be protruded to the outside of the external body when sealing the external body.

But the present invention is not limited thereto. For example, a stacked type lithium secondary battery, specifically, a cylindrical lithium ion secondary battery, a cylindrical lithium ion secondary battery, a laminate type lithium ion secondary battery, a laminate type lithium ion secondary battery, It is possible.

Hereinafter, examples and comparative examples of the present invention will be described. The following embodiments are only examples of the present invention, and the present invention is not limited to the following embodiments.

[Example]

<1. Example 1>

A lithium secondary battery (1) according to Example 1 was produced in the following process.

(1-1. Production of anode)

Lithium cobalt oxide, carbon black, and a mixed binder were dissolved and dispersed in N-methylpyrrolidone (NMP) at a solid weight ratio of 97.8: 1.2: 1.0 to prepare a positive electrode mixture slurry.

As the mixed binder, as shown in the following Table 1, acrylic acid modified polyvinylidene fluoride (PVdF, first binder), hydrogenated NBR (second binder), and vinylidene fluoride-tetrafluoroethylene-hexafluoro Propylene (VdF-TFE-HFP) copolymer (third binder) at a weight ratio of 0.4: 0.3: 0.3 was used.

At this time, the tensile modulus of the acrylic acid-modified PVdF was 1200 MPa. The tensile modulus of the hydrogenated NBR was 180 MPa, the tensile modulus of the VdF-TFE-HFP copolymer was 250 MPa, and the tensile modulus of the mixed binder was 420 MPa.

Here, the tensile modulus of elasticity of each binder was measured by the following method.

First, a cast film of a binder was prepared, and a dumbbell-shaped test piece having a test width of 5 mm was produced using this cast film. Then, the test piece was uniaxially stretched at a test speed of 2 mm / min in Autograph AGS-100NX manufactured by Shimadzu Corporation, and the stress and warpage were measured. The tensile modulus of elasticity was calculated according to the measured value obtained.

The positive electrode material mixture slurry was coated on both sides of a 12 mu m-thick aluminum foil current collector to prepare a coated layer. Subsequently, the anode 10 was rolled so that the solid content of the applied layer was 4.1 g / cc.

Next, an aluminum lead wire was welded to the anode end portion.

(1-2. Preparation of cathode)

Graphite, styrene butadiene rubber (SBR) and sodium salt of carboxymethyl cellulose were dissolved and dispersed in a water solvent at a weight ratio of 98: 1: 1 to prepare a negative electrode mixture slurry.

Subsequently, the negative electrode material mixture slurry was applied to both sides of a copper foil current collector (anode current collector 31) having a thickness of 6 m and dried to prepare a coating layer. Subsequently, the dried coating layer was rolled to prepare a negative electrode. Then, a nickel lead wire was welded to the end of the cathode 30.

(1-3. Production of winding device)

An anode, a separator (ND314 manufactured by ASAHI KASEI E-MATERIALS CO., LTD.), A negative electrode and a separator were laminated in this order, and this laminate was wound in the longitudinal direction using a core of 3 cm in diameter. The ends were fixed with a tape, wicks were removed, a cylindrical electrode winding element was sandwiched between two metal plates having a thickness of 3 cm and held for 3 seconds to produce a flat-type winding element.

(1-4. Production of lithium secondary battery)

The electrode winding element was vacuum-sealed with a laminated film composed of three layers of polypropylene / aluminum / nylon so that two lead wires led out of the laminate film to produce a lithium secondary battery.

At this time, 10 vol% of FEC (fluoroethylene carbonate) and 1.3M of LiPF ₆ were dissolved in a solvent in which ethylene carbonate / dimethyl carbonate was mixed at 3: 7 (volume ratio) as the electrolyte solution.

The lithium secondary battery was sandwiched between two metal plates having a thickness of 3 cm heated to 90 占 폚 and held for 5 minutes.

A lithium secondary battery (1) was produced according to the above process.

(1-5. Flexibility evaluation test of anode)

Flexibility of the positive electrode was evaluated by bending the prepared positive electrode by 180 °.

When the flexibility of the positive electrode is low, the result is that the positive electrode current collector 11 is broken after 180 ° bending, and the positive electrode in which the positive electrode current collector 11 is broken has the same fracture at the time of manufacturing the winding element, Can not be produced.

Therefore, in this test, whether or not there is a break (including a pinhole) of the positive electrode collector 11 after 180 ° bending is visually checked. If the breakage can not be confirmed, the flexibility is evaluated as "0" Flexibility was evaluated as "X".

On the other hand, when the flexibility becomes &quot; X &quot;, since the battery can not be manufactured, the cycle test described later is not performed.

(1-6 cycle test)

First, in the first cycle, CC-CV charging (constant current constant voltage charging) is performed at 0.1 C until the voltage becomes 4.4 V, and CC discharge (constant current discharge) is performed at 0.1 C until the voltage becomes 2.75 V Respectively. Subsequently, in the second cycle, CC-CV charging was performed at 0.2 C until the voltage became 4.4 V, and CC discharge was performed at 0.2 C until the voltage reached 2.75 V. In the third and subsequent cycles, CC-CV charging was performed at 1.0 C until the voltage reached 4.4 V, and CC discharging was performed at 1.0 C until the voltage reached 3.00 V.

And the discharge capacity at the 300th cycle divided by the discharge capacity at the third cycle was defined as the capacity retention rate.

The results of the flexibility and capacity retention as measured by the above method are shown in Table 1 below.

<2. Example 2>

The procedure of Example 1 was repeated except that the third binder was changed to a VdF-acrylate copolymer (tensile modulus of elasticity: 200 MPa). At this time, the tensile modulus of elasticity of the mixed binder was 400 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<3. Example 3 >

The procedure of Example 1 was repeated except that the first binder was changed to an acrylic acid modified PVdF-HFP copolymer (tensile elastic modulus: 1100 MPa). At this time, the tensile modulus of elasticity of the mixed binder was 350 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<4. Example 4>

The procedure of Example 1 was repeated except that the weight ratio of the first binder, the second binder, and the third binder was changed to 0.33: 0.33: 0.33. At this time, the tensile modulus of the mixed binder was 250 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<5. Example 5>

The procedure of Example 1 was repeated except that the weight ratio of the first binder, the second binder, and the third binder was changed to 0.5: 0.35: 0.15. At this time, the tensile modulus of the mixed binder was 450 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<6. Example 6:

The procedure of Example 1 was repeated except that the weight ratio of the first binder, the second binder, and the third binder was changed to 0.5: 0.15: 0.35. At this time, the tensile modulus of the mixed binder was 450 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<7. Example 7:

The third binder was changed to vinylidene fluoride-tetrafluoroethylene (VdF-TFE) copolymer (tensile elastic modulus: 350 MPa), and the weight ratio of the first binder, the second binder, and the third binder was changed to 0.4: 0.35 : 0.25. &Lt; tb &gt; &lt; TABLE &gt; At this time, the tensile modulus of the mixed binder was 420 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<8. Example 8 >

The third binder was changed to a vinylidene fluoride-hexafluoropropylene (VdF-HFP) -acrylate copolymer (tensile modulus of elasticity: 200 MPa), and the weight ratio of the first binder, the second binder, and the third binder 0.5: 0.3: 0.2. &Lt; tb &gt; &lt; TABLE &gt; At this time, the tensile modulus of the mixed binder was 440 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<9. Example 9>

The first binder is PVdF, the third binder is VdF-HFP-acrylate copolymer (tensile modulus: 200 MPa), and the third binder is VdF-HFP-acrylate copolymer And the weight ratio of the first binder, the second binder and the third binder was changed to 0.58: 0.21: 0.21. At this time, the tensile modulus of the mixed binder was 500 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<10. Comparative Example 1>

The procedure of Example 1 was repeated except that the weight ratio of the first binder, the second binder, and the third binder was changed to 0.4: 0.6: 0. That is, in Comparative Example 1, the third binder was not used. At this time, the tensile elastic modulus of the mixed binder was 300 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<11. Comparative Example 2>

The procedure of Example 1 was repeated except that the weight ratio of the first binder, the second binder, and the third binder was changed to 0.4: 0: 0.6. That is, in Comparative Example 2, the second binder was not used. At this time, the tensile modulus of elasticity of the mixed binder was 400 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<12. Comparative Example 3>

The procedure of Example 2 was repeated except that the weight ratio of the first binder, the second binder, and the third binder was changed to 0.4: 0: 0.6. That is, in Comparative Example 3, the second binder was not used. At this time, the tensile elastic modulus of the mixed binder was 300 MPa.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

<13. Comparative Example 4>

The procedure of Example 1 was repeated except that the weight ratio of the first binder, the second binder, and the third binder was changed to 0.8: 0.1: 0.1. That is, in Comparative Example 4, the weight percentage of the first binder was set to 80 wt%, which is larger than 60 wt%. At this time, the tensile modulus of the mixed binder was 900 MPa because the weight ratio of the first binder was large.

Flexibility and capacity retention were measured in the same manner as in Example 1, and the results are shown in Table 1 below.

The first binder The second binder Third binder The elastic modulus of the third binder
(MPa) Binder Mixing Ratio (Weight Ratio) (First Binder / Second Binder / Third Binder) Modulus of elasticity of mixed binder
(MPa) Capacity retention rate (%) flexibility Example 1 Acrylic acid-modified PVdF Hydroxide
NBR VdF-TFE-HFP copolymer 250 0.4 / 0.3 / 0.3 420 85 ○ Example 2 Acrylic acid-modified PVdF Hydroxide
NBR VdF-acrylate copolymer 200 0.4 / 0.3 / 0.3 400 84 ○ Example 3 Acrylic acid modified PVdF-HFP copolymer Hydroxide
NBR VdF-TFE-HFP copolymer 250 0.4 / 0.3 / 0.3 350 84 ○ Example 4 Acrylic acid modified PVdF Hydroxide
NBR VdF-TFE-HFP copolymer 250 0.33 / 0.33 / 0.33 250 81 ○ Example 5 Acrylic acid-modified PVdF Hydroxide
NBR VdF-TFE-HFP copolymer 250 0.5 / 0.35 / 0.15 450 83 ○ Example 6 Acrylic acid-modified PVdF Hydroxide
NBR VdF-TFE-HFP copolymer 250 0.5 / 0.15 / 0.35 450 87 ○ Example 7 Acrylic acid-modified PVdF Hydroxide
NBR VdF-TFE copolymer 350 0.4 / 0.35 / 0.25 420 83 ○ Example 8 Acrylic acid-modified PVdF Hydroxide
NBR VdF-HFP-acrylate copolymer 200 0.5 / 0.3 / 0.2 440 84 ○ Example 9 PVdF Hydroxide
NBR VdF-HFP-acrylate copolymer 200 0.58 / 0.21 / 0.21 500 87 ○ Comparative Example 1 Acrylic acid-modified PVdF Hydroxide
NBR - - 0.4 / 0.6 / 0 300 20 ○ Comparative Example 2 Acrylic acid-modified PVdF - VdF-TFE-HFP copolymer 250 0.4 / 0 / 0.6 400 20 ○ Comparative Example 3 Acrylic acid-modified PVdF - VdF-acrylate copolymer 200 0.4 / 0 / 0.6 300 20 ○ Comparative Example 4 Acrylic acid-modified PVdF Hydroxide
NBR VdF-TFE-HFP copolymer 250 0.8 / 0.1 / 0.1 900 Not rated X

The results of Table 1 show that Examples 1 to 9 improve the characteristics of the nonaqueous electrolyte secondary battery 1 while maintaining the flexibility of the positive electrode active material layer 12.

On the other hand, in the batteries of Comparative Examples 1 to 3 in which the second binder or the third binder was not included in the mixed binder, the flexibility of the mixed binder was obtained at the same level as in Examples 1 to 9, lost. However, the characteristics of the batteries according to Comparative Examples 1 to 3 were remarkably lowered.

It can be seen that the mixed binder used in Comparative Example 4 is excessively hard, and thus the result of fracture of the positive electrode collector 11 in the flexibility evaluation test is obtained.

It will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims. And it is understood that it belongs to the technical scope of the present invention.

[Description of Symbols]

1: Non-aqueous electrolyte secondary battery

1a: winding element

10: anode

11: anode current collector

12: cathode active material layer

20: Separator

30: cathode

31: cathode collector

32: anode active material layer

Claims (9)

Cathode active material; And
And a mixed binder including a first binder, a second binder, and a third binder,
Wherein the first binder includes at least one selected from the group consisting of polyvinylidene fluoride, acid-denatured polyvinylidene fluoride, and acid-denaturated polyvinylidene fluoride,
Wherein the mixed binder includes the first binder in a proportion of 30% by weight to 60% by weight based on the total weight of the mixed binder, and the tensile elastic modulus is 200 MPa to 600 MPa. The method according to claim 1,
Wherein the copolymer comprising the acid-modified polyvinylidene fluoride comprises at least one monomer selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, and chlorotrifluoroethylene. . The method according to claim 1,
Wherein the second binder is hydrogenated acrylonitrile butadiene rubber. The method according to claim 1,
Wherein the mixed binder comprises the second binder in a proportion of 10% by weight to 40% by weight based on the total weight of the mixed binder. The method according to claim 1,
Wherein the third binder is a copolymer comprising vinylidene fluoride. The method according to claim 1,
And the third binder has a tensile modulus of 150 MPa to 600 MPa. 6. The method of claim 5,
The copolymer containing vinylidene fluoride may be at least one selected from the group consisting of vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride- Fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-acrylate copolymer, vinylidene fluoride-hexafluoropropylene-acrylate copolymer, vinylidene fluoride-tetrafluoroethylene- Acrylate copolymer, and a vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene-acrylate copolymer. The positive electrode for a lithium secondary battery according to claim 1, A winding element for a lithium secondary battery, comprising a positive electrode according to any one of claims 1 to 7. A lithium secondary battery comprising the winding element of claim 8.

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