阅读说明：本技术 正极和包括所述正极的二次电池 (Positive electrode and secondary battery comprising same ) 是由 白柱烈 林俊默 李昌周 吴一根 金帝映 崔相勳 于 2019-02-19 设计创作，主要内容包括：本发明涉及一种正极,包括集电器和设置在所述集电器上的正极活性材料层,其中所述正极活性材料层包括正极活性材料、碳纳米管和粘合剂,所述粘合剂包括重均分子量为720,000g/mol至980,000g/mol的聚偏二氟乙烯,所述碳纳米管的BET比表面积为140m<Sup>2</Sup>/g至195m<Sup>2</Sup>/g,并且正极满足下式1：[式1]1.3≤B/A≤3.4在式1中,B是正极活性材料层中的聚偏二氟乙烯的量(重量％),并且A是正极活性材料层中的碳纳米管的量(重量％)。(The present invention relates to a positive electrode for a lithium secondary battery,comprising a current collector and a positive electrode active material layer disposed on the current collector, wherein the positive electrode active material layer comprises a positive electrode active material, carbon nanotubes and a binder, the binder comprises polyvinylidene fluoride having a weight average molecular weight of 720,000 to 980,000g/mol, and the carbon nanotubes have a BET specific surface area of 140m 2 G to 195m 2 And the positive electrode satisfies the following formula 1: [ formula 1]1.3. ltoreq. B/A. ltoreq.3.4 in formula 1, B is the amount (wt%) of polyvinylidene fluoride in the positive electrode active material layer, and A is the amount (wt%) of carbon nanotubes in the positive electrode active material layer.)

1. A positive electrode comprising a current collector and a positive electrode active material layer disposed on the current collector,

wherein the positive electrode active material layer includes a positive electrode active material, carbon nanotubes, and a binder,

the binder comprises polyvinylidene fluoride having a weight average molecular weight of 720,000 to 980,000g/mol,

the BET specific surface area of the carbon nano tube is 140m²G to 195m²Per g, and

the positive electrode satisfies the following formula 1:

[ formula 1]

1.3≤B/A≤3.4

In formula 1, B is an amount (wt%) of polyvinylidene fluoride in the positive electrode active material layer, and a is an amount (wt%) of the carbon nanotube in the positive electrode active material layer.

2. The cathode according to claim 1, wherein the carbon nanotube is a bundle carbon nanotube comprising a plurality of carbon nanotube units.

3. The cathode according to claim 2, wherein the carbon nanotube unit has an average diameter of 1nm to 30 nm.

4. The positive electrode according to claim 1, wherein the positive electrode satisfies the following formula 2:

[ formula 2]

1.3≤B/A≤1.85

In formula 2, B is an amount (wt%) of polyvinylidene fluoride in the positive electrode active material layer, and a is an amount (wt%) of the carbon nanotube in the positive electrode active material layer.

5. The cathode according to claim 1, wherein the cathode active material comprises Li [ Ni [ ]_x1Mn_y1Co_z1]O₂(0.40-0.70-0. 1, 0.15-0.30-0. 1, 0.15-0. 1-0.30, and 1-x 1+ y1+ z 1).

6. The positive electrode according to claim 1, wherein the positive electrode active material is included in the positive electrode active material layer at 95.6 wt% to 99.0 wt%.

7. The positive electrode according to claim 1, wherein the positive electrode is activeAverage particle diameter (D) of the material₅₀) Is 3 μm to 20 μm.

8. The cathode of claim 1, wherein the binder comprises a non-fluorine based binder.

9. The cathode according to claim 8, wherein the non-fluorine-based binder is at least one of nitrile rubber and hydrogenated nitrile rubber.

10. A secondary battery comprising:

the positive electrode according to any one of claims 1 to 9;

a negative electrode;

a separator disposed between the positive electrode and the negative electrode; and

an electrolyte.

Technical Field

Cross Reference to Related Applications

The present application claims the rights of korean patent application No. 10-2018-0019487 filed at 19.2018 in the korean intellectual property office and korean patent application No. 10-2019-0018735 filed at 18.2019 in the korean intellectual property office, the disclosures of which are incorporated herein by reference in their entireties.

Background

Recently, as the technical development and demand for mobile devices have increased, the demand for batteries as an energy source has also increased. Therefore, diversified studies have been made on batteries satisfying various requirements. In particular, research into lithium secondary batteries having high energy density, excellent life and cycle characteristics as power sources for such devices is actively being conducted.

A lithium secondary battery is a battery including an electrode assembly and a lithium ion-containing nonaqueous electrolyte, the electrode assembly including: a positive electrode including a positive electrode active material capable of intercalating and deintercalating lithium ions; an anode including an anode active material capable of intercalating and deintercalating lithium ions; and a microporous separator disposed between the positive electrode and the negative electrode.

In the positive electrode and/or the negative electrode, the positive active material layer may include a conductive agent to improve conductivity. Conventionally, dot type conductive agents such as carbon black are mainly used. However, if the amount of the conductive agent is increased in order to improve conductivity, the relative amount of the positive or negative electrode active material decreases, and the capacity of the battery decreases, or the positive or negative electrode binder decreases to deteriorate the adhesiveness. In particular, in the positive electrode, the conductivity of the positive electrode active material itself is low, and therefore the above-described defect becomes large.

To solve these drawbacks, a method using a linear conductive agent such as carbon nanotubes has been reported. The carbon nanotube has a relatively longer length than the particle-type conductive agent, and can achieve an improvement effect of conductivity and an improvement of a binding force of a constituent material of the positive electrode active material layer in a smaller amount.

In addition, in order to improve the adhesion between the positive electrode active material layer and the current collector (positive electrode adhesion), the positive electrode active material layer may include a binder. The binder generally contributes little to improving the conductivity of the positive electrode active material layer. Therefore, even if carbon nanotubes are used, improvement of conductivity is limited by use with a binder.

Therefore, research is being conducted to improve the performance of the binder or the carbon nanotube.

Disclosure of Invention

Technical problem

An aspect of the present invention provides a cathode in which the viscosity of a cathode slurry can be maintained at a preferred level and advantages in consideration of workability and manufacturing costs can be obtained, and which has both excellent cathode adhesion and improved conductivity, and a secondary battery including the same.

Technical scheme

According to the practice of the inventionIn another aspect, there is provided a positive electrode including a current collector and a positive electrode active material layer disposed on the current collector, wherein the positive electrode active material layer includes a positive electrode active material, carbon nanotubes, and a binder, the binder includes polyvinylidene fluoride having a weight average molecular weight of 720,000g/mol to 980,000g/mol, and the carbon nanotubes have a BET specific surface area of 140m²G to 195m²And the positive electrode satisfies the following formula 1:

[ formula 1]

1.3≤B/A≤3.4

In formula 1, B is the amount (wt%) of polyvinylidene fluoride in the positive electrode active material layer, and a is the amount (wt%) of carbon nanotubes in the positive electrode active material layer.

According to another embodiment of the present invention, there is provided a secondary battery including the positive electrode.

Advantageous effects

According to the present invention, by controlling the BET specific surface area of the carbon nanotube, the content ratio of polyvinylidene fluoride to the carbon nanotube, and the weight average molecular weight of polyvinylidene fluoride, the conductivity of the positive electrode and the adhesion of the positive electrode can be improved, and an excessive increase or an excessive decrease of the positive electrode slurry during the manufacture of the positive electrode can be suppressed. Therefore, there are advantages in terms of workability and manufacturing cost during manufacturing of the positive electrode, application of the positive electrode slurry is easy, and the positive electrode active material layer can be uniformly formed. In addition, the output of the battery and the life characteristics at high temperatures can be further improved.

Detailed Description

Hereinafter, the present invention will be described in more detail to help understanding of the present invention. In this case, it will be understood that the terms or words used in the specification and claims should not be construed as meanings defined in a general dictionary, and it will be further understood that the terms or words should be construed as having meanings consistent with the technical idea of the present invention and the meanings in the context of the related art, based on the principle that the inventor can properly define the meanings of the terms or words to best explain the present invention.

Positive electrode pack according to embodiment of the present inventionComprising a current collector and a positive electrode active material layer disposed on the current collector, wherein the positive electrode active material layer comprises a positive electrode active material, carbon nanotubes and a binder, the binder comprises polyvinylidene fluoride having a weight average molecular weight of 720,000 to 980,000g/mol, and the carbon nanotubes have a BET specific surface area of 140m²G to 195m²(ii)/g, and satisfies the following formula 1:

[ formula 1]

1.3≤B/A≤3.4

In formula 1, B is the amount (wt%) of polyvinylidene fluoride in the positive electrode active material layer, and a is the amount (wt%) of carbon nanotubes in the positive electrode active material layer.

The current collector may be any one as long as it has conductivity without causing chemical changes in the battery, and is not particularly limited. For example, the current collector may use copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. In particular, transition metals such as copper and nickel, which adsorb carbon well, may be used as the current collector.

The positive electrode active material layer may be disposed on the current collector. The positive electrode active material layer may be disposed on one side or both sides of the current collector.

The positive active material layer may include a positive active material, carbon nanotubes, and a binder. Specifically, the positive electrode active material layer may be composed of a positive electrode active material, carbon nanotubes, and a binder.

The positive electrode active material may include Li [ Ni ]_x1Mn_y1Co_z1]O₂(0.40-0.70-0. 1, 0.15-0.30-0. 1, 0.15-0. 1-0.30, and 1-x 1+ y1+ z 1). The positive active material has high energy density and allows the manufacture of a battery having high capacity. Specifically, the positive electrode active material may include Li (Ni)_x2Mn_y2Co_z2)O₂(0.56<x2<0.68,0.16<y2<0.22,0.16<z2<0.22, and x2+ y2+ z2 ═ 1). The positive active material has high energy density and excellent safety.

Average particle diameter (D) of positive electrode active material₅₀) Can be 3 μm to 20 μm, specifically 6 μm to 18 μm, more specifically 9 μm to 18 μm16 μm. If the above range is satisfied, the life characteristics at high temperature and the output characteristics of the battery can be improved. In the present disclosure, in the particle size distribution curve of the particles, the average particle diameter (D)₅₀) It may be defined as the particle size corresponding to 50% of the cumulative volume. The average particle diameter (D) can be measured using, for example, a laser diffraction method₅₀). By the laser diffraction method, particle diameters from a submicron (submicron) region to the order of several millimeters can be generally measured, and results of high reproducibility and high resolution can be obtained.

The positive electrode active material may be 95.6 wt% to 99.0 wt% in the positive electrode active material layer, and specifically, may be 97.0 wt% to 98.0 wt%. When the above range is satisfied, the adhesion of the positive electrode and the conductivity of the positive electrode can be improved at the same time.

In the present invention, carbon nanotubes may be used as the positive electrode conductive agent. Specifically, the positive electrode active material layer may include a positive electrode conductive agent composed of carbon nanotubes. For example, if the positive electrode active material layer includes a particle phase conductive agent such as acetylene black or a plate conductive agent in addition to the carbon nanotubes, the conductivity of the positive electrode may be reduced as compared to that of the positive electrode of the present invention.

The carbon nanotubes may be bundle carbon nanotubes. The bundled carbon nanotube may include a plurality of carbon nanotube units. In particular, "bundle type" used herein refers to a bundle-like or rope-like secondary form in which a plurality of carbon nanotube units are arranged such that axes in the length direction of the carbon nanotube units are arranged in parallel or wound together in substantially the same direction, unless otherwise specified. In the carbon nanotube unit, the graphite sheet (graphite sheet) has a nano-size diameter and sp²The cylindrical shape of the key structure. In this case, conductor or semiconductor characteristics can be exhibited depending on the curling angle and structure of the graphite sheet. The carbon nanotube unit may be classified into a single-walled carbon nanotube (SWCNT) unit, a double-walled carbon nanotube (DWCNT) unit, and a multi-walled carbon nanotube (MWCNT) unit. In particular, carbon nano-meterThe tube unit may be a multi-walled carbon nanotube unit. Multi-walled carbon nanotube units are preferred because of the lower energy required for dispersion and the easily controllable dispersion conditions compared to single-walled carbon nanotube units and double-walled carbon nanotube units.

The carbon nanotube unit may have an average diameter of 1nm to 30nm, specifically 3nm to 26nm, more specifically 5nm to 22 nm. If the above range is satisfied, the carbon nanotubes are easily dispersed in the positive electrode slurry, and the conductivity of the positive electrode can be improved. The average diameter can be determined by TEM or SEM.

The BET specific surface area of the carbon nanotubes may be 140m²G to 195m²/g, in particular 145m²G to 195m²Per g, more particularly 160m²G to 190m²(ii) in terms of/g. If the BET specific surface area of the carbon nanotubes is less than 140m²In the case of the positive electrode, the viscosity of the positive electrode slurry during the manufacture of the positive electrode is too low, and thus, the coating and drying processability of the positive electrode slurry is lowered, and the manufacturing cost is excessively increased. In addition, the conductive path (path) decreases due to the decrease in BET specific surface area, and the conductivity of the positive electrode greatly decreases. Meanwhile, if the BET specific surface area of the carbon nanotube is more than 195m²In the case of the positive electrode slurry, the viscosity of the positive electrode slurry is excessively increased, and it is very difficult to apply the positive electrode slurry to a current collector. Therefore, the cathode slurry may not be uniformly applied and the cathode active material layer formed thereby is not uniform. In addition, the dispersibility of the carbon nanotubes may be reduced due to the increase of the BET specific surface area, and the conductivity of the positive electrode may be greatly reduced. The BET specific surface area can be measured by a nitrogen absorption BET method.

The binder may comprise polyvinylidene fluoride (PVdF).

The weight average molecular weight of the polyvinylidene fluoride can be 720,000 to 980,000g/mol, specifically 750,000 to 950,000g/mol, more specifically 800,000 to 920,000 g/mol. If the weight average molecular weight is less than 720,000g/mol, the viscosity of the cathode slurry formed during the manufacture of the cathode is excessively low and the application of the cathode slurry is difficult, the workability during drying is excessively deteriorated, and the manufacturing cost is excessively increased even if formula 1 described later is satisfied. In addition, the adhesion of the positive electrode is excessively reduced, and a defect in which the positive electrode active material is detached may occur. Further, if the weight average molecular weight is more than 980,000g/mol, a defect that the resistance of the positive electrode and the battery excessively increases may occur. In addition, the viscosity of the cathode slurry excessively increases, for example, to 50,000cp or more at room temperature, and the application of the cathode slurry itself may become difficult, and the cathode active material layer thus formed may become uneven, and thus the battery performance may be deteriorated. The weight average molecular weight of polyvinylidene fluoride may be an optimum range satisfying improvements in processability and manufacturing cost, uniformity of the positive electrode active material layer, improvement in conductivity of the positive electrode, and improvement in adhesion of the positive electrode, simultaneously, with a limited composition.

The binder may further include a non-fluorine-based binder. The non-fluorine-based binder may be at least one of Nitrile rubber (NBR) and Hydrogenated-Nitrile rubber (H-NBR), and may be specifically Hydrogenated Nitrile rubber.

The weight ratio of polyvinylidene fluoride to non-fluorine based binder may be 23:1 to 1:1, specifically 20:1 to 3: 1. If the above range is satisfied, the effect of improving the adhesion of the positive electrode and the dispersibility of the carbon nanotubes can be obtained.

The positive electrode of the present invention may satisfy the following formula 1:

[ formula 1]

1.3≤B/A≤3.4

In formula 1, B is the amount (wt%) of polyvinylidene fluoride in the positive electrode active material layer, and a is the amount (wt%) of carbon nanotubes in the positive electrode active material layer.

If B/a is less than 1.3, the adhesion of the positive electrode is too weak, and detachment of the positive electrode active material may occur during the manufacture of the positive electrode. In addition, the viscosity of the cathode slurry may excessively increase, the application of the cathode slurry itself may become difficult, and the cathode active material layer may become non-uniform, and thus, the battery performance may be deteriorated. Further, if B/a is greater than 3.4, the adhesion of the positive electrode may be excellent, but the powder resistance may excessively increase, and the battery resistance may excessively increase. Therefore, the output characteristics of the battery are greatly reduced. In addition, the viscosity of the cathode slurry formed during the manufacture of the cathode may be excessively reduced, and the application of the cathode slurry is difficult, the workability during drying is excessively reduced, and the manufacturing cost is excessively increased.

The positive electrode may satisfy the following formula 2:

[ formula 2]

1.3≤B/A≤1.85

If formula 2 is satisfied, the powder resistance and adhesion of the positive electrode can be further improved. The reason for this result is that the viscosity of the positive electrode slurry increases to some extent as long as the uniformity of the positive electrode active material layer is not impaired and the migration (migration) of the binder in the positive electrode slurry is suppressed. Therefore, since the adhesion of the positive electrode is improved and the adhesive is not applied non-uniformly, the conductivity covering the entire positive electrode can be further improved.

A secondary battery according to another embodiment of the present invention may include a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, and the positive electrode is the same as the positive electrode of the above embodiment. Therefore, the description of the positive electrode will be omitted.

The negative electrode may include: a negative electrode current collector; and an anode active material layer disposed on one or both sides of the anode current collector.

The anode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like may be used as the anode current collector. In particular, transition metals such as copper and nickel, which adsorb carbon well, may be used as the current collector.

The negative electrode active material layer may include a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder.

The negative active material may be graphite-based active material particles or silicon-based active material particles. The graphite-based active material particles may be used selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, and graphitizationOne or more of the group consisting of carbon microspheres, and particularly, in the case of using artificial graphite, rate capability can be improved. The silicon-based active material particles can be selected from Si and SiO_x(0<x<2) The present invention relates to a battery including a Si-C composite material, a Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, transition metals, group 13 elements, group 14 elements, rare earth elements, and combinations thereof), and a battery having a high capacity, in particular, in the case of using Si, can be obtained.

The negative binder may include at least one selected from the group consisting of: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (polyvinylidene fluoride), polyacrylonitrile (polyacrylonitrile), polymethyl methacrylate (polymethyl methacrylate), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid (polyacrylic acid), and a material obtained by substituting hydrogen therein with Li, Na, or Ca, and may include various copolymers thereof.

The negative electrode conductive agent may be any one without limitation as long as it has conductivity without causing chemical changes in the battery. For example, graphite, such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or a conductive material such as a polyphenylene derivative.

The separator separates the negative electrode and the positive electrode and provides a transport path of lithium ions, wherein any separator may be used as the separator without particular limitation as long as it is generally used for a secondary battery, and particularly, a separator having high moisture retention ability to an electrolyte and low resistance to transport of electrolyte ions is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, or an ethylene/methacrylate copolymer, or a laminated structure having two or more layers thereof may be used. In addition, a typical porous nonwoven fabric, for example, a nonwoven fabric formed of high-melting glass fibers or polyethylene terephthalate fibers, may be used. In addition, a coated separator including a ceramic component or a polymer material may be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure may be selectively used.

The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a melt-type inorganic electrolyte, which may be used in manufacturing a lithium secondary battery, but the present invention is not limited thereto.

Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

As the nonaqueous organic solvent, for example, there can be used: aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ -butyrolactone, 1, 2-dimethoxyethane, tetrahydrofuran (franc), 2-methyltetrahydrofuran, dimethylsulfoxide, 1, 3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1, 3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate.

In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates, among carbonate organic solvents are high viscosity organic solvents having a high dielectric constant and well dissociating lithium salts, and thus may be preferably used. If such a cyclic carbonate is mixed with a linear carbonate having a low viscosity and a low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte having high conductivity can be prepared, which is more preferable.

The metal salt may use a lithium salt, and the lithium salt is a material that can be easily dissolved in the nonaqueous electrolyte, and as the anion of the lithium salt, for example, one or more selected from the group consisting of: f^-、Cl^-、I^-、NO₃ ^-、N(CN)₂ ^-、BF₄ ^-、ClO₄ ^-、PF₆ ^-、(CF₃)₂PF₄ ^-、(CF₃)₃PF₃ ^-、(CF₃)₄PF₂ ^-、(CF₃)₅PF^-、(CF₃)₆P^-、CF₃SO₃ ^-、CF₃CF₂SO₃ ^-、(CF₃SO₂)₂N^-、(FSO₂)₂N^-、CF₃CF₂(CF₃)₂CO^-、(CF₃SO₂)₂CH^-、(SF₅)₃C^-、(CF₃SO₂)₃C^-、CF₃(CF₂)₇SO₃ ^-、CF₃CO₂ ^-、CH₃CO₂ ^-、SCN^-And (CF)₃CF₂SO₂)₂N^-。

In order to improve the life characteristics of the battery, suppress the decrease in the capacity of the battery, and increase the discharge capacity of the battery, at least one additive of the following may be further added to the electrolyte in addition to the electrolyte components: for example, halogenated alkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, N-ethylene glycol dimethyl ether (glyme), hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

According to another embodiment of the present invention, there are provided a battery module including the secondary battery as a unit cell, and a battery pack including the battery module. The battery module and the battery pack include a secondary battery having high capacity, high rate performance, and high cycle characteristics, and may be used as a power source for middle-and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

Hereinafter, various embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.