KR102609878B1 - Positive electrode and secondary battery comprising the same - Google Patents

Abstract

The present invention includes a positive electrode current collector, and a positive electrode active material layer formed on the positive electrode current collector and including a lithium iron phosphate-based active material and carbon nanotubes, wherein the carbon nanotubes have an average length of 3 μm to 15 μm. , a positive electrode having a weight ratio of the lithium iron phosphate-based active material and the carbon nanotubes of 30:1 to 98:1, and a secondary battery including the same are provided.

Description

Positive electrode and secondary battery containing the same {POSITIVE ELECTRODE AND SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a positive electrode and a secondary battery including the same.

As the electronics, communications, and space industries develop, the demand for secondary batteries as an energy power source is rapidly increasing. In particular, as the importance of global eco-friendly policies is emphasized, the electric vehicle market is growing rapidly, and research and development on secondary batteries is being actively conducted at home and abroad.

Among various secondary batteries, lithium secondary batteries with high discharge voltage and energy density are widely used as an energy source for various mobile devices and various electronic products.

Lithium transition metal composite oxide is used as a positive electrode active material for lithium secondary batteries, and among these, lithium cobalt composite metal oxide of LiCoO ₂ , which has a high operating voltage and excellent capacity characteristics, is mainly used. However, because LiCoO ₂ has low stability and is expensive, it is difficult to mass-produce lithium secondary batteries.

Accordingly, as materials to replace LiCoO ₂ , lithium manganese composite metal oxide, lithium iron phosphate, and lithium nickel composite metal oxide were developed. Among these, lithium iron phosphate (LiFePO ₄ ) with an olivine structure has a high volume density of 3.6 g/cm ³ and a theoretical capacity of about 170 mAh/g.

However, due to the low electrical conductivity of lithium iron phosphate (LiFePO ₄ ), when LiFePO ₄ is used as a positive electrode active material, the internal resistance of the battery increases, which causes the problem of deterioration of initial capacity and lifespan characteristics. In order to improve the electrical conductivity of LiFePO ₄ , a method of mixing point-shaped conductive materials such as carbon black was introduced, but the point-shaped conductive material had the problem of blocking the pores between LiFePO ₄ particles and preventing the movement of lithium ions.

The present invention provides a positive electrode with improved initial capacity and lifespan characteristics and a secondary battery including the same.

The positive electrode according to the present invention includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including a lithium iron phosphate-based active material and carbon nanotubes, and the average length of the carbon nanotubes is 3 μm to 3 μm. It is 15㎛, and the weight ratio of the lithium iron phosphate-based active material and the carbon nanotube is 30:1 to 98:1.

In one embodiment of the present invention, the lithium iron phosphate-based active material may be a compound represented by Formula 1 below.

[Formula 1]

Li _1+a Fe _1-x M _x (PO _4-b )Y _b

In Formula 1, M is one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, and Zn, and Y is F, It is one or more elements selected from the group consisting of S and N, and a, b, x are -0.5≤a≤0.5, 0≤b≤0.1, 0≤x≤0.5

In one embodiment of the present invention, the lithium iron phosphate-based active material may be LiFePO ₄ .

In one embodiment of the present invention, the lithium iron phosphate-based active material may include a carbon coating layer formed on the surface.

In one embodiment of the present invention, the carbon nanotubes may be single-walled carbon nanotubes.

In one embodiment of the present invention, the average diameter of the carbon nanotubes may be 1.0 nm to 6.0 nm.

In one embodiment of the present invention, the content of the carbon nanotubes may be 1% to 3% by weight based on the total weight of the positive electrode active material layer.

In one embodiment of the present invention, a primer layer formed between the positive electrode current collector and the positive electrode active material layer may be further included.

In one embodiment of the present invention, the primer layer may include a thickener and a binder.

The secondary battery according to the present invention includes the positive electrode, the negative electrode, and a separator interposed between the positive electrode and the negative electrode.

The present invention includes carbon nanotubes having a length within a specific range as a conductive material in the positive electrode active material layer, and by including lithium iron phosphate-based active material and carbon nanotubes in a specific ratio, the secondary battery including the positive electrode active material layer has an initial capacity of and has the effect of improving lifespan characteristics.

In addition, the present invention has the effect of improving the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer by providing a primer layer containing a binder and a thickener between the positive electrode current collector and the positive electrode active material layer.

The structural or functional descriptions of the embodiments disclosed in the present specification or application are merely illustrative for the purpose of explaining embodiments according to the technical idea of the present invention, and the embodiments according to the technical idea of the present invention are described in this specification. Alternatively, it may be implemented in various forms other than the embodiments disclosed in the application, and the technical idea of the present invention is not to be construed as being limited to the embodiments described in this specification or application.

Below, a positive electrode according to the present invention and a secondary battery including the same will be described.

<Anode>

The positive electrode according to the present invention is formed on the positive electrode current collector and includes a positive electrode active material layer containing a lithium iron phosphate-based active material and carbon nanotubes, and the average length of the carbon nanotubes is 3㎛ to 15㎛. and the weight ratio of the lithium iron phosphate-based active material and carbon nanotubes is 30:1 to 98:1.

The positive electrode according to the present invention includes a positive electrode current collector. The positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the secondary battery, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon on the surface of aluminum or stainless steel. , those surface treated with nickel, titanium, silver, etc. can be used. The thickness of the positive electrode current collector may be adjusted depending on the required product, and may be, for example, 10 μm to 500 μm, 10 μm to 300 μm, or 20 μm to 300 μm.

The positive electrode according to the present invention is formed on a positive electrode current collector and includes a positive electrode active material layer including a lithium iron phosphate-based active material and carbon nanotubes.

The lithium iron phosphate-based active material may be a compound represented by the following formula (1).

[Formula 1]

Li _1+a Fe _1-x M _x (PO _4-b )Y _b

In Formula 1, M is one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, and Cu, and Y is F, It is one or more elements selected from the group consisting of S and N, and a, b, and x are -0.5≤a≤0.5, 0≤b≤0.1, 0≤x≤0.5.

For example, the lithium iron phosphate-based active material may be LiFePO ₄ .

The lithium iron phosphate-based active material may include a carbon coating layer formed on the surface.

The carbon coating layer includes a carbon-based material, and the carbon-based material includes carbon black, carbon fiber or metal fiber, metal powder, conductive whisker, conductive metal, activated carbon, polyphenylene derivative, natural graphite, artificial graphite, and super. Any one selected from the group consisting of Super-P, acetylene black, Ketjen black, channel black, pinnace black, lamp black, summer black, Denka black, aluminum powder, nickel powder, zinc oxide, gallium titanate and titanium oxide. It could be more than that.

The carbon coating layer may include a carbon-based material having an intercrystalline spacing d002 of 3.345 Å to 3.550 Å. The spacing d002 between crystal planes of a carbon-based material means the distance at which each carbon hexagonal network plane is stacked. The spacing between crystal planes of carbon-based materials can be measured by XRD experiments, and specifically by X-ray diffraction wavelength using CuKα. The intercrystal spacing d002 of the carbon-based material may be 3.345 Å to 3.550 Å, 3.345 Å to 3.548 Å, or 3.425 Å to 3.486 Å. When the above range is satisfied, insertion and desorption of lithium ions can easily occur, and the collapse of the structure of the lithium iron phosphate-based active material can be delayed. Because of this, the initial capacity and lifespan characteristics of the secondary battery can be improved.

The carbon coating layer may have hydrophilic functional groups introduced. The hydrophilic functional group may be chemically bonded to a carbon-based material and may be one or more selected from the group consisting of -COOH, -OH, and -COO ^- M ⁺ . The hydrophilic functional group may be introduced from a metal salt containing a metal functional group, and may be one or more types selected from the group consisting of salts of metal ions such as Li ⁺ , B ^{+ ,} and Al ⁺ . More specifically, the hydrophilic functional group may be introduced from borate such as Mg ₂ B ₂ O ₅ , CaAlB ₃ O ₇ , and Li ₆ B ₄ O ₉ .

The lithium iron phosphate-based active material may have a contact angle reduction time. The contact angle reduction time can be measured by the following method. First, the lithium iron phosphate-based active material is placed flat on a tray. Thereafter, when a water droplet with a volume of about 10 μl is dropped on the lithium iron phosphate-based active material contained in the tray, the contact angle over time can be measured. The contact angle reduction time is the time until the contact angle becomes about 5°. The contact angle over time was measured using a contact angle measuring device such as a Phoenix 300 analyzer. The contact angle reduction time may be less than about 2 seconds. The contact angle reduction time may be less than about 1.9 seconds. The contact angle reduction time may be less than about 1.8 seconds.

As the hydrophilic functional group is introduced into the carbon coating layer, the electrolyte impregnation property can be improved, the ion diffusivity of the lithium iron phosphate-based active material particles can be improved, and the deterioration of the lithium iron phosphate-based active material particles can be delayed, increasing the initial capacity of the secondary battery. and life characteristics can be further improved.

Lithium iron phosphate-based active materials have superior stability compared to nickel cobalt-based metal oxide active materials. However, due to the low electrical conductivity of lithium iron phosphate-based active materials, when used as a positive electrode active material, the internal resistance of the secondary battery increases, which causes a decrease in initial capacity and lifespan characteristics. A method of mixing point-shaped conductive materials such as carbon black has been introduced to improve the electrical conductivity of lithium iron phosphate-based active materials, but the point-shaped conductive material has the problem of blocking the pores between particles of lithium iron phosphate-based active materials, preventing the movement of lithium ions. . In addition, a method of mixing linear conductive materials such as carbon nanotubes has been introduced to improve the electrical conductivity of lithium iron phosphate-based active materials and facilitate the movement of lithium ions. However, as the length of carbon nanotubes increases, the surface area increases. However, there is a problem that the stability and lifespan characteristics of the secondary battery are reduced due to side reactions caused by the electrolyte solution under high temperature conditions.

Accordingly, the positive electrode according to the present invention includes carbon nanotubes with an average length of 3㎛ to 15㎛ as a conductive material in order to improve the electrical conductivity of the lithium iron phosphate-based active material and at the same time prevent side reactions caused by the electrolyte solution under high temperature conditions. do. The average length of carbon nanotubes may be 3㎛ to 15㎛, 4㎛ to 15㎛, or 4㎛ to 13㎛. If the average length of the carbon nanotubes exceeds 15㎛, the surface area of the carbon nanotubes increases, and side reactions due to the electrolyte solution may occur under high temperature conditions, thereby reducing the stability and lifespan characteristics of the secondary battery. If the average length of the carbon nanotubes is less than 3㎛, the initial capacity and lifespan characteristics of the secondary battery may be reduced because a conductive network cannot be sufficiently formed between the lithium iron phosphate-based active materials. The average length is a value calculated by measuring the length of 100 carbon nanotubes in the positive electrode active material layer using SEM and then calculating their average.

The average diameter of carbon nanotubes may be 1.0 nm to 6.0 nm, 1.5 nm to 6.0 nm, or 1.5 nm to 5.0 nm. When the above range is satisfied, it is easy to disperse in the positive electrode active material slurry, and a conductive network can be effectively formed between lithium iron phosphate-based active materials. The average diameter is a value obtained by measuring the diameters of 100 carbon nanotubes in the positive electrode active material layer using SEM and then calculating their average.

The carbon nanotube may be a single-walled carbon nanotube. Single-walled carbon nanotubes can significantly improve electrical conductivity even with a lower content compared to multi-walled carbon nanotubes. The anode may further include multi-walled carbon nanotubes as a conductive material. Multi-walled carbon nanotubes refer to carbon nanotubes containing multiple single-walled carbon nanotubes. Multi-walled carbon nanotubes are more economical than single-walled carbon nanotubes, and can be used as a conductive material along with single-walled carbon nanotubes, contributing to forming a conductive network between lithium iron phosphate-based active materials.

Meanwhile, in order to improve the electrical conductivity of the lithium iron phosphate-based active material, even if carbon nanotubes with an average length of 3㎛ to 15㎛ are included as a conductive material, the secondary battery's Initial capacity and life characteristics may vary. Accordingly, the positive electrode according to the present invention includes a lithium iron phosphate-based active material and carbon nanotubes with a weight ratio of 30:1 to 98:1. The weight ratio of the lithium iron phosphate-based active material and the carbon nanotubes may be 30:1 to 98:1, 40:1 to 98:1, or 45:1 to 98:1. If the weight of the lithium iron phosphate-based active material exceeds 98 times the weight of the carbon nanotube, the effect of improving the conductivity of the lithium iron phosphate-based active material may be insufficient, and the mobility of lithium ions may be reduced. If the weight of the lithium iron phosphate-based active material is less than 30 times the weight of the carbon nanotube, the initial capacity of the secondary battery may decrease.

The content of carbon nanotubes may be 1% by weight to 3% by weight, 1% by weight to 2.8% by weight, or 1.5% by weight to 2.8% by weight based on the total weight of the positive electrode active material layer. When the above range is satisfied, the amount of carbon nanotubes used can be reduced without deteriorating the lifespan characteristics of the secondary battery, thereby improving manufacturing economics through cost reduction.

The positive active material layer may further include a dot-shaped conductive material. The point-like conductive material may be carbon black, Super-P, or acetylene black. The point-shaped conductive material can be disposed in the space formed between carbon nanotubes, which are linear conductive materials, to improve the dispersibility of the carbon nanotubes. The weight ratio of the carbon nanotubes and the point-like conductive material may be 1:0.5 to 1:4.0, 1:1.0 to 1:3.0, or 1:1.5 to 1:2.0.

The positive active material layer may include a binder. It is desirable to use a binder that has good adhesion to the positive electrode current collector, for example, polyvinylidene fluoride, carboxymethyl cellulose, styrene butadiene rubber, polyimide, polyamideimide, polyvinyl alcohol, and hydroxypropyl cellulose. , polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyethylene, polypropylene, acrylated styrene - One or more types selected from the group consisting of butadiene rubber and epoxy resin may be used.

The positive electrode according to the present invention may further include a primer layer formed between the positive electrode current collector and the positive electrode active material layer. The primer layer can improve the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer, thereby improving the electrode detachment problem. In addition, the anode provided with a primer layer is advantageous in securing an electron transfer path, so electronic conductivity can be improved.

The primer layer may include a thickener and binder.

The thickener can improve the problem of cracks on the anode surface by strengthening the cohesion of the binder. The thickener may include one or more selected from the group consisting of carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, methyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose, methyl ethyl hydroxyethyl cellulose, and cellulose gum. . The content of the thickener may be 0.05% to 3.00% by weight, 0.10% to 2.50% by weight, or 0.10% to 2.00% by weight based on the total weight of the primer layer. When the above range is satisfied, it is possible to prevent process defects such as slip of the primer layer from occurring. If necessary, the primer layer can be made of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, carbon fiber, metal fiber, fluorinated carbon, and aluminum powder to improve conductivity. , nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives.

The binder can improve the adhesion between the positive electrode current collector and the positive active material layer. The binder may include styrene butadiene rubber. The content of the binder may be 10% to 40% by weight, 10% to 35% by weight, or 10% to 30% by weight based on the total weight of the primer layer. When the above range is satisfied, the resistance of the secondary battery does not increase, and the adhesion between the positive electrode current collector and the positive active material layer can be improved.

<Secondary battery>

The secondary battery according to the present invention includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

The secondary battery includes a positive electrode, and the positive electrode can be used in the same manner as the positive electrode active material, positive electrode current collector, positive electrode active material layer, etc. described in relation to the positive electrode described above.

In addition to the positive electrode of the present invention, the secondary battery includes a negative electrode and a separator. The negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector.

The negative electrode current collector is not particularly limited as long as it is conductive without causing chemical changes in the secondary battery. For example, it can be used on the surface of copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel. Surface treatment with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, the bonding power of the negative electrode active material can be strengthened by forming fine irregularities on the surface, and it can be used in various forms such as films, sheets, foils, nets, porous materials, foams, and non-woven fabrics.

The negative electrode active material layer may include a negative electrode active material.

The type of anode active material is not particularly limited, and usually a compound capable of reversible intercalation and deintercalation of lithium can be used. For example, carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and highly crystalline carbon; Metallic compounds that can be alloyed with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy, can be used. Low-crystalline carbon includes soft carbon and hard carbon, and high-crystalline carbon includes natural graphite, kish graphite, pyrolytic carbon, and liquid crystalline carbon. Examples include high-temperature calcined carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and petroleum or coal tar pitch derived cokes. The negative electrode active material may be a composite containing a metallic compound and a carbonaceous material, if necessary.

A separator may be interposed between the cathode and the anode. The separator is designed to prevent electrical short-circuiting between the cathode and anode and to generate ion flow. The separator may include a porous polymer film or a porous non-woven fabric. Here, the porous polymer film is ethylene polymer, propylene polymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate. ) It may be composed of a single layer or multiple layers containing polyolefin-based polymers such as copolymers. The porous nonwoven fabric may include high melting point glass fibers and polyethylene terephthalate fibers. However, it is not limited to this, and depending on the embodiment, the separator may be a highly heat-resistant separator (CCS; Ceramic Coated Separator) containing ceramic.

The anode, cathode, and separator can be manufactured into an electrode assembly by winding, lamination, folding, or zigzag stacking processes. Additionally, the electrode assembly can be provided with an electrolyte to manufacture a secondary battery according to the present invention. The secondary battery may be any one of a cylindrical, square, pouch, or coin type using a can, but is not limited thereto.

The electrolyte may be a non-aqueous electrolyte. The electrolyte solution may include a lithium salt and an organic solvent. The organic solvent includes propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), dipropyl carbonate (DPC), Vinylene carbonate (VC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, gamma-butyrolactone ), propylene sulfide, or tetrahydrofuran.

Below, the present invention will be described in more detail based on examples and comparative examples. However, the following examples and comparative examples are only examples to explain the present invention in more detail, and the present invention is not limited by the following examples and comparative examples.

Example

Example 1

<Primer manufacturing>

A primer slurry was prepared by mixing carboxymethyl cellulose and styrene butadiene rubber at a weight ratio of 1:99.

<Manufacture of positive electrode active material>

LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride (PVdF) at a weight ratio of 93:1.5:2.5:3.0. A slurry was prepared by mixing.

<Anode manufacturing>

The prepared primer slurry was applied to the aluminum thin film, and the positive electrode active material was applied on the primer slurry. Afterwards, it was dried under vacuum at 130°C for 1 hour to prepare a positive electrode equipped with a primer layer and a positive electrode active material layer.

<Cathode manufacturing>

A negative electrode active material slurry was prepared by mixing soft carbon negative electrode active material, which is amorphous carbon with an average particle diameter of 10㎛, acetylene black, and polyvinylidene fluoride, in N-methyl pyrrolidone solvent at a weight ratio of 85:5:10. Afterwards, the negative electrode active material slurry was applied to the copper thin film and dried for 1 hour under vacuum at 110°C to prepare a negative electrode.

<Secondary battery manufacturing>

After manufacturing the electrode assembly by interposing a separator made of polyethylene with a thickness of 20 ㎛ between the anode and the cathode, the electrode assembly is stored in a case, and ethylene carbonate: methyl ethyl carbonate is mixed inside the case at a volume ratio of 2:1. A secondary battery was manufactured by adding an electrolyte solution containing 1M LiPF ₆ and 3% by weight of vinyl carbonate to the prepared solvent.

Example 2

When manufacturing the positive electrode active material, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride (PVdF) were used at 93:1.5:2.5. : Instead of preparing a slurry by mixing at a weight ratio of 3.0, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 8.0 ㎛), Super-b, and polyvinylidene fluoride A secondary battery was manufactured in the same manner as in Example 1, except that (PVdF) was mixed at a weight ratio of 93:1.8:2.2:3.0 to prepare a slurry.

Example 3

Example except that when manufacturing a positive electrode, the prepared primer slurry is applied to an aluminum thin film, and instead of applying the positive electrode active material on the primer slurry, the positive electrode active material is applied on the aluminum thin film to manufacture a positive electrode without a primer layer. A secondary battery was manufactured in the same manner as in 1.

Example 4

When manufacturing the positive electrode active material, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride (PVdF) were used at 93:1.5:2.5. : Instead of preparing a slurry by mixing at a weight ratio of 3.0, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 14.0 ㎛), Super-b, and polyvinylidene fluoride A secondary battery was manufactured in the same manner as Example 1, except that (PVdF) was mixed at a weight ratio of 90:1:5:4 to prepare a slurry.

Example 5

When manufacturing the positive electrode active material, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride (PVdF) were used at 93:1.5:2.5. : Instead of preparing a slurry by mixing at a weight ratio of 3.0, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), and polyvinylidene fluoride (PVdF) were mixed at 98:1: A secondary battery was manufactured in the same manner as Example 1, except that a slurry was prepared by mixing at a weight ratio of 1.

Comparative Example 1

When manufacturing the positive electrode active material, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride (PVdF) were used at 93:1.5:2.5. : Instead of preparing a slurry by mixing at a weight ratio of 3.0, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 16.0 ㎛), Super-b, and polyvinylidene fluoride A secondary battery was manufactured in the same manner as in Example 1, except that (PVdF) was mixed at a weight ratio of 93:1.5:2.5:3 to prepare a slurry.

Comparative Example 2

When manufacturing the positive electrode active material, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride (PVdF) were used at 93:1.5:2.5. : Instead of preparing a slurry by mixing at a weight ratio of 3.0, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 2.0 ㎛), Super-b, and polyvinylidene fluoride A secondary battery was manufactured in the same manner as in Example 1, except that (PVdF) was mixed at a weight ratio of 93:1.5:2.5:3 to prepare a slurry.

Comparative Example 3

When manufacturing the positive electrode active material, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride (PVdF) were used at 93:1.5:2.5. : Instead of preparing a slurry by mixing at a weight ratio of 3.0, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride A secondary battery was manufactured in the same manner as Example 1, except that (PVdF) was mixed at a weight ratio of 88:1:5:6 to prepare a slurry.

Comparative Example 4

When manufacturing the positive electrode active material, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride (PVdF) were used at 93:1.5:2.5. : Instead of preparing a slurry by mixing at a weight ratio of 3.0, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 6.0 ㎛), and polyvinylidene fluoride (PVdF) were mixed at 99:0.5: A secondary battery was manufactured in the same manner as in Example 1, except that a slurry was prepared by mixing at a weight ratio of 0.5.

Comparative Example 5

When manufacturing the positive electrode active material, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 4.0 ㎛), Super-b, and polyvinylidene fluoride (PVdF) were used at 93:1.5:2.5. : Instead of preparing a slurry by mixing at a weight ratio of 3.0, LiFePO ₄ with a carbon coating layer, single-walled carbon nanotubes (average diameter 1.8 nm, average length 18.0 ㎛), and polyvinylidene fluoride (PVdF) were mixed at 99:0.5: A secondary battery was manufactured in the same manner as in Example 1, except that a slurry was prepared by mixing at a weight ratio of 0.5.

Experiment example

Experimental Example 1 - Coulombic efficiency evaluation

Each secondary battery manufactured in Examples 1 to 5 and Comparative Examples 1 to 5 was charged at room temperature in CCCV mode and 0.2C until 4.4V, and then discharged to 3.0V at a constant current of 0.2C for 20 days. The process was carried out three times, and the charge capacity and discharge capacity in the first cycle were measured to calculate the coulombic efficiency, and the results are shown in Table 1 below.

Experimental Example 2 - Evaluation of lifespan characteristics

Each secondary battery manufactured in Examples 1 to 5 and Comparative Examples 1 to 5 was charged at room temperature in CCCV mode and 0.2C until 4.4V, and then discharged to 3.0V at a constant current of 0.2C for 100 The process was carried out three times, and the maintenance rate of discharge capacity compared to the first cycle was measured after the 100th charge and discharge, and the results are shown in Table 1 below.

coulombic efficiency Discharge capacity maintenance rate Example 1 92.4% 96.8% Example 2 92.3% 96.5% Example 3 91.1% 95.1% Example 4 92.3% 96.2% Example 5 92.2% 96.3% Comparative Example 1 90.8% 88.4% Comparative Example 2 90.9% 89.9% Comparative Example 3 87.8% 94.5% Comparative Example 4 88.8% 94.6% Comparative Example 5 88.2% 89.1%

As can be seen in Table 1, the secondary batteries of Examples 1 to 5 manufactured to meet the average length range of the carbon nanotubes of the present invention and the weight ratio of the lithium iron phosphate-based active material and the carbon nanotubes were Comparative Example 1. It was confirmed that the initial capacity and lifespan characteristics were improved compared to 5.

Claims (5)

anode;
cathode;
A separator disposed between the anode and the cathode; and
It is a secondary battery containing a non-aqueous electrolyte mixed with 1M LiPF ₆ and 3% by weight of vinyl carbonate in a solvent mixed with ethylene carbonate:methylethylcarbonate at a volume ratio of 2:1,
The anode is,
positive electrode current collector;
A positive electrode active material layer formed on the positive electrode current collector and including a lithium iron phosphate-based active material and a single-walled carbon nanotube; and
It includes a primer layer formed between the positive electrode current collector and the positive electrode active material layer,
The positive electrode active material layer is manufactured from positive electrode active material slurry,
The primer layer is prepared from a primer slurry, and the primer slurry includes carboxymethyl cellulose and styrene butadiene rubber at a weight ratio of 1:99,
The primer layer and the positive electrode active material layer are manufactured by applying the positive electrode active material slurry on the primer slurry and drying it in a vacuum at 130 ° C. for 1 hour,
The lithium iron phosphate-based active material includes a carbon coating layer formed on the surface,
The carbon coating layer includes a carbon-based material having an intercrystalline spacing d002 of 3.345 Å to 3.550 Å,
The carbon coating layer is introduced with a hydrophilic functional group,
The average length of the single-walled carbon nanotubes is 3㎛ to 15㎛,
The average diameter of the single-walled carbon nanotubes is 1.0 nm to 6.0 nm,
The content of the single-walled carbon nanotube is 1% by weight to 1.8% by weight based on the total weight of the positive electrode active material layer,
A secondary battery wherein the weight ratio of the lithium iron phosphate-based active material and the single-walled carbon nanotube is 30:1 to 98:1. According to paragraph 1,
A secondary battery wherein the lithium iron phosphate-based active material is a compound represented by the following formula (1):
[Formula 1]
Li _1+a Fe _1-x M _x (PO _4-b )Y _b
In Formula 1,
M is one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In and Zn,
Y is any one or more elements selected from the group consisting of F, S and N,
a, b, x are -0.5≤a≤0.5, 0≤b≤0.1, 0≤x≤0.5. delete delete delete