Negative electrode and secondary battery

Abstract

The present invention relates to a secondary battery and a method for manufacturing a negative electrode for a secondary battery, the battery comprising: a current collector; a first negative electrode active material layer provided on the current collector; and a second negative electrode active material layer provided on the first negative electrode active material layer, wherein the first negative electrode active material layer contains isostatically pressed natural graphite, and the second negative electrode active material layer contains secondary particle-state artificial graphite and single particle-state artificial graphite.

Description

[Technical Field]

[0001] This specification asserts the benefit as of the filing date of Korean Patent Application No. 10-2022-0132407, filed with the Korean Intellectual Property Office on 14 October 2022, and Korean Patent Application No. 10-2023-0133971, filed with the Korean Intellectual Property Office on 10 October 2023, and all content disclosed in the documents of said Korean patent applications is incorporated herein by reference.

[0002] This invention relates to a negative electrode for a secondary battery and a secondary battery including the same. [Background technology]

[0003] Rechargeable batteries are widely used not only in portable devices but also in electric vehicles (EVs) and hybrid electric vehicles (HEVs) that are powered by an electrical drive source.

[0004] Such rechargeable batteries offer not only the primary benefit of dramatically reducing the use of fossil fuels, but also the advantage of producing no by-products from energy use. Therefore, they are attracting attention as a new energy source that is environmentally friendly and improves energy efficiency.

[0005] Generally, a secondary battery includes a positive electrode, a negative electrode, a separator membrane interposed between the positive and negative electrodes, and an electrolyte. Furthermore, electrodes such as the positive and negative electrodes may have an electrode active material layer provided on a current collector.

[0006] As the use of rechargeable batteries increases, various battery performance characteristics are required. Attempts have been made to improve battery performance by adding additives to the active material layer. However, depending on the type of additive, some battery performance may improve, while others may actually deteriorate. Therefore, research is needed on the selection or combination of materials included in electrodes that can improve the required performance of rechargeable batteries. [Overview of the project]

Problems to be Solved by the Invention

[0007] An object of the present invention is to provide a negative electrode for a secondary battery with improved adhesive strength and expansion performance, and a secondary battery including the same.

Means for Solving the Problems

[0008] One embodiment of the present invention is a current collector; a first negative electrode active material layer provided on the current collector; and a second negative electrode active material layer provided on the first negative electrode active material layer including wherein the first negative electrode active material layer contains isotropically pressed natural graphite, and the second negative electrode active material layer provides a negative electrode for a secondary battery containing secondary particle-shaped artificial graphite and single particle-shaped artificial graphite.

[0009] Another embodiment of the present invention provides a secondary battery including the negative electrode for a secondary battery, a positive electrode, and a separator.

[0010] Another embodiment of the present invention includes the step of forming a first negative electrode active material layer on a current collector; including the step of forming a second negative electrode active material layer on the first negative electrode active material layer, wherein the first negative electrode active material layer contains isotropically pressed natural graphite or natural graphite having a BET specific surface area of 1 m , / g to 3 m 2 / g, and the second negative electrode active material layer contains secondary particle-shaped artificial graphite and single particle-shaped artificial graphite, and provides a method for manufacturing a negative electrode for a secondary battery.

Advantages of the Invention

[0011] The negative electrode of the present invention uses isotropically treated natural graphite and single-particle artificial graphite together with secondary particulate artificial graphite in the negative electrode active material layer. This results in relatively low orientation within the electrode, reduced side reactions with the electrolyte, and improved electrode expansion performance. Furthermore, by forming isotropically treated natural graphite at the lower end of the negative electrode active material layer, the adhesion strength of the negative electrode active material layer is maximized, resulting in an improved rapid filling effect. [Modes for carrying out the invention]

[0012] The present invention will be described in further detail below to aid in understanding the present invention. The present invention can be embodied in a variety of different forms and is not limited to the embodiments described herein. In this regard, terms or words used herein and in the claims shall not be interpreted to be limited to their usual or dictionary meanings, but rather in a sense and concept consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0013] In this specification, terms such as “includes,” “equip,” and “have” are intended to specify the presence of implemented features, figures, stages, components, or combinations thereof, and are understood not to preemptively exclude the possibility of the presence or addition of one or more other features, figures, stages, components, or combinations thereof.

[0014] Furthermore, when a layer or other part is said to be "on top of" another part, this includes not only the case where it is "directly above" the other part, but also the case where there is another part in between. Conversely, when a part is said to be "directly above" another part, it means that there is no other part in between. Note that being "on top of" a reference part means being located above or below the reference part, and does not necessarily mean being located "up" in the opposite direction of gravity.

[0015] In this specification, particle size refers to the average particle size represented by D50. D50 can be defined as the particle size at 50% of the particle size distribution and can be measured using the laser diffraction method. For example, the method for measuring the average particle size (D50) of the positive electrode active material involves dispersing the particles of the positive electrode active material in a dispersion medium, introducing them into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), irradiating them with ultrasound at approximately 28 kHz at an output of 60 W, and then calculating the average particle size (D50) corresponding to 50% of the cumulative volume in the measuring device.

[0016] In this specification, any description that refers only to the “active material layer” without the terms “first” or “second” may apply to both the first and second active material layers.

[0017] A negative electrode for a secondary battery according to one embodiment of this specification includes a current collector; a first negative electrode active material layer provided on the current collector; and a second negative electrode active material layer provided on the first negative electrode active material layer, wherein the first negative electrode active material layer includes isotropically treated natural graphite, and the second negative electrode active material layer includes secondary particulate artificial graphite and single-particulate artificial graphite. In other words, the negative electrode for a secondary battery includes two negative electrode active material layers and includes secondary particulate artificial graphite, isotropically treated natural graphite, and single-particulate artificial graphite, while the isotropically treated natural graphite is placed at the lower end of the negative electrode active material layer to obtain a negative electrode with excellent expansion performance and adhesion.

[0018] A method for manufacturing a negative electrode for a secondary battery according to another embodiment includes the steps of: forming a first negative electrode active material layer on a current collector; and forming a second negative electrode active material layer on the first negative electrode active material layer, wherein the first negative electrode active material layer is made of isostatically treated natural graphite or a BET specific surface area of ​​1 m². 2 / g~3m 2 The second negative electrode active material layer contains natural graphite at a concentration of / g, and includes secondary particulate artificial graphite and single particulate artificial graphite.

[0019] In this specification, "single particle" is a concept contrasted with secondary particle form, which is formed by the aggregation of tens to hundreds of primary particles, and means consisting of 10 or fewer primary particles. Specifically, in the present invention, a single particle may be a single particle consisting of one primary particle, or it may be a particulate matter formed by the aggregation of multiple primary particles.

[0020] In this specification, "primary particle" refers to the smallest unit of particle recognized when an active material is observed through a scanning electron microscope, and "secondary particle" refers to a secondary structure formed by the aggregation of tens to hundreds of primary particles.

[0021] In this specification, "particle" refers to a particle measured in micrometers, which, when observed under magnification, can be divided into "grains" having a crystalline form on the order of tens of nanometers. Further magnification reveals a segmented region in which atoms form a lattice structure in a specific direction; this is called a "crystal grain." The size of a particle observed by XRD is defined as the crystal grain size. The crystal grain size can be quantitatively determined using XRD data through Scherrer's equation.

[0022] According to one embodiment of the present invention, the first negative electrode active material layer contains isostatically treated natural graphite. The isostatically treated natural graphite can be obtained by methods such as cold isostatic pressing (CIP). Specifically, size-adjusted spheroidal coarse particles obtained by an air-jet pulverizer are filled into a molding mold of a cold isostatic molding apparatus, the entire surface of the molding mold is filled with water, and then cold isostatic molding is performed for 100 seconds at a molding pressure of 90 MPa. After that, the cold isostatically molded spheroidal coarse particles are crushed. Isostatically treated natural graphite can be obtained by mixing 100 parts by weight of the crushed spheroidal coarse particles with 5 parts by weight of pitch (solid phase pitch), which is a carbon coating material, and carbonizing the mixture at a temperature of 1,250°C for 24 hours to carbonize it, but the method for producing isostatically treated natural graphite is not limited to the above description.

[0023] As described above, when a method such as the cold isostatic pressing method (Cold Isostatic Press, CIP) is used to obtain isostatically pressed natural graphite, since pressure can be applied isotropically to the raw material particles from all directions, the raw material particles in the spheroidized aggregate particles have the advantages that the internal voids are reduced and the specific surface area is reduced.

[0024] According to an embodiment of the present invention, since the first negative electrode active material layer contains isostatically pressed natural graphite, the orientation in the electrode becomes relatively low, side reactions with the electrolyte are reduced, the expansion performance of the electrode can be improved, and the adhesion force of the negative electrode active material layer can be increased.

[0025] According to an embodiment of the present invention, the BET specific surface area of the natural graphite may be 1 m 2 / g to 3 m 2 / g. By using natural graphite having a BET specific surface area within the above range, there is an advantage that the durability of the cell is improved. Specifically, when it exceeds 3 m 2 / g, the durability decreases due to side reactions of the electrolyte, and when it is less than 1 m 2 / g, it is difficult to implement and it will prevent the penetration of the electrolyte. For example, it may be 1.2 m 2 / g to 2.8 m 2 / g, it may be 1.2 m 2 / g to 2.8 m 2 / g, it may be 1.2 m 2 / g to 2.6 m 2 / g, it may be 1.4 m 2 / g to 2.4 m 2 / g.

[0026] In this specification, the BET specific surface area can be measured by the BET (Brunauer - Emmett - Teller) measurement method using BEL JAPAN's BELSORP (BET equipment) with an adsorbing gas such as nitrogen.

[0027] According to one embodiment of the present invention, the average particle size (D50) of the isotropically treated natural graphite may be 6 μm to 9 μm, 7 μm to 9 μm, or 8 μm to 9 μm. By using isotropically treated natural graphite having an average particle size (D50) within the above range, the effects of including isotropically treated natural graphite in the negative electrode active material layer (improvement of electrode expansion performance, increase in adhesion strength of the negative electrode active material layer) can be maximized.

[0028] The average particle size (D50) of the isostatically treated natural graphite can be adjusted through a process of crushing using an air-jet pulverizer. Specifically, flake graphite with an average particle size (D50) of 200 μm can be prepared and crushed using an air-jet pulverizer to adjust the size. The crushed flake graphite can then be subjected to a spheroidization process to obtain spheroidized coarse particles. However, the method for obtaining natural graphite with isostatic treatment and adjusted average particle size is not limited to the above description.

[0029] According to one embodiment of the present invention, the second negative electrode active material layer comprises secondary particulate artificial graphite and single particulate artificial graphite.

[0030] According to one embodiment of the present invention, the average particle size (D50) of the single-particulate artificial graphite may be 6 μm to 9 μm, 7 μm to 9 μm, or 8 μm to 9 μm.

[0031] According to one embodiment of the present invention, the average particle size (D50) of the secondary particulate artificial graphite may be 16 μm to 22 μm.

[0032] In one embodiment of the present invention, to obtain single-particle artificial graphite having an average particle size (D50) within the range and the secondary-particle artificial graphite, methods such as grinding using a jet mill and then heat-treating the material after grinding can be used. Specifically, after grinding needle coke using a jet mill, a powder with adjusted size can be obtained by sieving. Furthermore, the powder can be heat-treated (graphitized) at 3,000°C for 20 hours under an inert argon (Ar) gas atmosphere to produce primary artificial graphite particles (or single particles). After adding the primary artificial graphite particles (single particles) and a binder such as petroleum-based pitch, the material can be heat-treated at 1,000°C or higher for 10 hours to obtain secondary artificial graphite particles in which the primary artificial graphite particles have aggregated.

[0033] In one embodiment of the present invention, the OI(004 / 110) of the single-particulate artificial graphite is 35 or less, or 25 or less.

[0034] The OI(004 / 110) of the second negative electrode active material layer is a value obtained through X-ray diffraction analysis, which can be obtained using a general method of X-ray diffraction analysis, such as JIS K 0131-1996. The OI value of the negative electrode can be expressed as C004 / C110, where C004 represents the characteristic diffraction peak area of ​​the (004) crystal plane and C110 represents the characteristic diffraction peak area of ​​the (110) crystal plane.

[0035] The OI(004 / 110) of the single-particulate artificial graphite in the second negative electrode active material layer is a value obtained through X-ray diffraction analysis, which can be obtained using a general method of X-ray diffraction analysis, such as an X-ray diffraction analyzer Bruker D4 Endeavor. The OI value of the negative electrode may also be expressed as I004 / I110, where I004 represents the characteristic diffraction peak area of ​​the (004) crystal plane and I110 represents the characteristic diffraction peak area of ​​the (110) crystal plane.

[0036] These characteristics indicate the degree of orientation of the graphite crystals within the second negative electrode active material layer. A higher OI value indicates higher graphite orientation, while a lower OI value indicates lower orientation. When the orientation of the graphite crystals is low, the expansion performance of the battery can be improved by allowing the battery to expand randomly or at random during charging, rather than expanding only in a specific direction.

[0037] According to one embodiment of the present invention, the adhesive force of the first negative electrode active material layer to the current collector is 18 gF / cm or more. In the negative electrode of the present invention, the first negative electrode active material layer and the current collector are in contact, so the adhesive force of the isotropically treated natural graphite, which is the material of the first negative electrode active material layer, affects the adhesive force of the first negative electrode active material layer to the current collector.

[0038] According to one embodiment of the present invention, the adhesive strength of the isotropically treated natural graphite contained in the first negative electrode active material layer is 18 gF / cm or more. The adhesive strength of the isotropically treated natural graphite can be adjusted according to the particle size distribution value.

[0039] In one embodiment of the present invention, the second negative electrode active material layer may contain 30 to 90 parts by weight of the secondary particulate artificial graphite based on 100 parts by weight of the negative electrode active material. When this range is met, there are advantages in improving the expansion performance and energy density of the battery.

[0040] In one embodiment of the present invention, the second negative electrode active material layer may contain 10 to 30 parts by weight of single-particulate artificial graphite based on 100 parts by weight of the negative electrode active material. When this range is met, there are advantages in improving the expansion performance and energy density of the battery.

[0041] According to one embodiment of this specification, each of the negative electrode active material layers (the first negative electrode active material layer, the second negative electrode active material layer) may further contain negative electrode active materials other than the isotropically treated natural graphite, secondary particulate artificial graphite, and single particulate artificial graphite described above, and the type thereof is not limited.

[0042] In one embodiment of this specification, the negative electrode active material in each 100 parts by weight of the negative electrode active material layer may be present in an amount of 80 parts by weight or more and 99.9 parts by weight or less, preferably 90 parts by weight or more and 99.9 parts by weight or less, more preferably 95 parts by weight or more and 99.9 parts by weight or less, and even more preferably 98 parts by weight or more and 99.9 parts by weight or less.

[0043] According to further embodiments of this specification, the first negative electrode active material layer may further contain a negative electrode binder in addition to isotropically treated natural graphite, and the second negative electrode active material layer may further contain a negative electrode binder in addition to secondary particulate artificial graphite and single particulate artificial graphite.

[0044] The negative electrode binder can serve to improve adhesion between negative electrode active material particles and adhesion between negative electrode active material particles and negative electrode current collector. The negative electrode binder can be any known in the art, and non-limiting examples include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which their hydrogen atoms are substituted with Li, Na, or Ca, and may also include various copolymers thereof.

[0045] The negative electrode binder may be included in an amount of 0.1 parts by weight or more and 20 parts by weight or less based on 100 parts by weight of the negative electrode active material layer, for example, preferably 0.3 parts by weight or more and 20 parts by weight or less, and more preferably 0.5 parts by weight or more and 10 parts by weight or less.

[0046] The negative electrode active material layer does not necessarily have to contain a conductive material, but may further contain a conductive material as needed. The conductive material contained in the negative electrode active material layer is not particularly limited as long as it does not induce a chemical change in the battery and is conductive, and for example, graphite such as natural graphite or artificial graphite; carbon black such as 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 fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used. The content of the conductive material in the negative electrode active material layer may be 0.01 to 20 parts by weight, preferably 0.03 to 18 parts by weight, per 100 parts by weight of the negative electrode active material layer.

[0047] The negative electrode active material layer may further contain thickeners such as Na-CMC (Sodium carboxymethyl cellulose), Li-CMC (Carboxymethyl cellulose lithium), and CNF (Cellulose nanofiber).

[0048] In one embodiment of this specification, the thickness of the first and second negative electrode active material layers may be 5 μm or more and 500 μm or less, respectively.

[0049] According to one embodiment of this specification, the thickness ratio of the first and second negative electrode active material layers (thickness of the first negative electrode active material layer:thickness of the second negative electrode active material layer) may be 3:7 to 5:5.

[0050] In one embodiment of this specification, the negative electrode current collector is not particularly limited as long as it is conductive without inducing a chemical change in the battery. For example, the current collector can be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. Specifically, transition metals that effectively adsorb carbon, such as copper and nickel, can be used as the current collector. The thickness of the current collector may be 1 μm to 500 μm, but is not limited thereto.

[0051] Further embodiments of this specification provide a secondary battery comprising a negative electrode, a positive electrode, and a separator membrane according to the embodiments described above.

[0052] In one embodiment of this specification, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material. The thickness of the positive electrode active material layer may be 20 μm or more and 500 μm or less.

[0053] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc., can be used. The positive electrode current collector may also have a thickness of 1 to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0054] In one embodiment of this specification, the positive electrode may include a lithium-compound transition metal compound comprising nickel (Ni) and cobalt (Co) as an active material. The lithium-compound transition metal compound may further include at least one of manganese and aluminum. The lithium-compound transition metal compound may contain 80 mol% or more, for example, 80 mol% or more but less than 100 mol%, of nickel among the metals excluding lithium.

[0055] In one embodiment, the positive electrode active material in the 100 parts by weight of the positive electrode active material layer may be present in an amount of 80 parts by weight or more and 99.9 parts by weight or less, preferably 90 parts by weight or more and 99.9 parts by weight or less, more preferably 95 parts by weight or more and 99.9 parts by weight or less, and even more preferably 98 parts by weight or more and 99.9 parts by weight or less.

[0056] According to further embodiments of this specification, the positive electrode active material layer according to the above-described embodiment may further include a positive electrode binder and a conductive material.

[0057] The positive electrode binder can play a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material particles and the positive electrode current collector. As the positive electrode binder, any known material in the art can be used, and non-limiting examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used.

[0058] The positive electrode binder may be included in an amount of 0.1 parts by weight or more and 50 parts by weight or less based on 100 parts by weight of the positive electrode active material layer, for example, preferably in an amount of 0.3 parts by weight or more and 35 parts by weight or less, and more preferably in an amount of 0.5 parts by weight or more and 20 parts by weight or less.

[0059] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode and can be used without particular limitations as long as it has electronic conductivity without undergoing chemical changes within the battery. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more may be used.

[0060] Specifically, in one embodiment, the conductive material may include one or more single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The conductive material may be included in an amount of 0.1 parts by weight or more and 2 parts by weight or less per 100 parts by weight of the positive electrode active material layer composition, for example, preferably 0.3 parts by weight or more and 1.5 parts by weight or less, and more preferably 0.5 parts by weight or more and 1.2 parts by weight or less.

[0061] The positive electrode and the negative electrode can be manufactured according to a conventional method for manufacturing positive and negative electrodes, except that the positive and negative electrode active materials described above are used. Specifically, they can be manufactured by coating a composition for forming an active material layer, which includes the active material and, selectively, a binder and a conductive material, onto a current collector, followed by drying and rolling. In this case, the types and contents of the positive and negative electrode active materials, binder, and conductive material are as described above.

[0062] The active material layer forming composition may further contain a solvent. Specifically, the solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more may be used. The amount of solvent used is sufficient to dissolve or disperse the active material, conductive material, and binder, taking into consideration the coating thickness of the slurry and the manufacturing yield, and to have a viscosity that allows for excellent thickness uniformity when applied for the manufacture of the positive and negative electrodes. Alternatively, the positive and negative electrodes may be manufactured by casting the active material layer forming composition onto another support, peeling it off the support, and then laminating the resulting film onto a current collector.

[0063] Further embodiments of this specification provide a method for manufacturing a negative electrode for a secondary battery according to the embodiments described above.

[0064] The manufacturing method includes the step of forming a first negative electrode active material layer on a current collector; A method for manufacturing a negative electrode for a secondary battery, comprising the step of forming a second negative electrode active material layer on the first negative electrode active material layer, The first negative electrode active material layer is made of isotropically treated natural graphite or a BET specific surface area of ​​1 m². 2 / g~3m 2 Contains natural graphite / g The second negative electrode active material layer may contain secondary particulate artificial graphite and single particulate artificial graphite.

[0065] A secondary battery according to one embodiment of the present invention comprises an assembly including a positive electrode, a negative electrode, a separator membrane, and an electrolyte, which may be a lithium secondary battery.

[0066] The separation membrane separates the negative and positive electrodes and provides a pathway for lithium ions to move. It can be used without particular limitations as long as it is the type typically used as a separation membrane in secondary batteries. Particularly preferred is a membrane that exhibits low resistance to electrolyte ion movement while maintaining excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof, may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, coated separation membranes containing ceramic components or polymeric substances may be used to ensure heat resistance or mechanical strength, and may be selectively used in single-layer or multi-layer structures.

[0067] Examples of the aforementioned electrolytes include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0068] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0069] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

[0070] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferred because they are high-viscosity organic solvents with high dielectric constants that effectively dissociate lithium salts. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in appropriate proportions, an electrolyte with high electrical conductivity can be created, and this mixture is even more preferable.

[0071] The metal salt can be a lithium salt, and the lithium salt is a substance that dissolves easily in the non-aqueous electrolyte. For example, the anion of the lithium salt is 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 (CF3CF2SO2)2N - One or more types selected from the group consisting of the following can be used.

[0072] In addition to the electrolyte components, the electrolyte may further contain one or more additives for purposes such as improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0073] Further embodiments of the present invention provide a battery module and a battery pack containing the aforementioned secondary battery as a unit cell. Since the battery module and battery pack include the secondary battery having high capacity, high rate-limiting characteristics and cycle characteristics, they can be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems.

[0074] The secondary battery according to the embodiment of the present invention exhibits excellent discharge capacity, output characteristics, and cycle performance stably, and can therefore be used as a power source not only for portable devices such as mobile phones, laptop computers, and digital cameras, but also for medium- and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems. For example, the battery module or battery pack can be used as a power source for one or more medium- and large-sized devices from among power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0075] Preferred embodiments are presented below to aid in understanding the present invention, but these embodiments are illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the present description and the technical concept. Such variations and modifications naturally fall within the scope of the appended claims.

[0076] Example 1 Natural graphite with an average particle size (D50) of 9 μm, treated with isotropy at a pressure of 80 MPa (adhesion strength: 19 gF / cm), SBR as a binder, CMC as a thickener, and carbon black as a conductive material were weighed in a weight ratio of 96:1:2:1 (natural graphite:SBR:CMC:carbon black), and then distilled water was added and mixed to prepare the first negative electrode slurry. Next, the second negative electrode slurry was prepared by weighing out secondary particulate artificial graphite with an average particle size (D50) of 18 μm, single particulate artificial graphite (OI(004 / 110):25) with an average particle size (D50) of 8.5 μm as the negative electrode active material, SBR as a binder, CMC as a thickener, and carbon black as a conductive material in a weight ratio of 95:1:3:1 (artificial graphite (secondary particulate artificial graphite:single particulate artificial graphite = 80:20):SBR:CMC:carbon black), and then mixing with distilled water. The prepared first negative electrode slurry was coated onto a copper foil electrode current collector to form the first negative electrode active material layer, and the prepared second negative electrode slurry was coated onto the surface of the first negative electrode active material layer to form the second negative electrode active material layer, thereby manufacturing the negative electrode. In this case, the weight ratio (natural graphite:artificial graphite) of natural graphite contained in the first negative electrode active material layer to artificial graphite contained in the second negative electrode active material layer is 30:70, and the weight ratio (secondary particles:single particles) of secondary particulate artificial graphite to single particulate artificial graphite is 80:20.

[0077] Example 2 The negative electrode was manufactured in the same manner as in Example 1, except that isotropically treated natural graphite with an average particle size (D50) of 9 μm and an adhesive strength of 22 gF / cm was used.

[0078] Example 3 The negative electrode was manufactured in the same manner as in Example 1, except that natural graphite treated with isotropy at a pressure of 90 MPa was used.

[0079] Example 4 The anode was manufactured in the same manner as in Example 1, except that the mixing ratio of secondary particle artificial graphite to single particle artificial graphite used as the anode active material in the second anode slurry was 70:30.

[0080] Comparative Example 1 The negative electrode was manufactured in the same manner as in Example 1, except that only a negative electrode active material layer containing secondary particulate artificial graphite with a particle size (D50) of 18 μm was formed as the negative electrode active material layer.

[0081] Comparative Example 2 The negative electrode was fabricated in the same manner as in Example 1, except that a single layer of negative electrode active material was formed containing (mixed) secondary particulate artificial graphite with a particle size (D50) of 18 μm and single particulate artificial graphite with a particle size (D50) of 18 μm as the negative electrode active material layer.

[0082] Comparative Example 3 The negative electrode was fabricated in the same manner as in Example 1, except that a single-layer negative electrode active material layer was formed containing (mixed) secondary particulate artificial graphite with a particle size (D50) of 18 μm and untreated natural graphite with a particle size (D50) of 9 μm.

[0083] Comparative Example 4 The anode was manufactured in the same manner as in Example 1, except that a first anode active material layer was formed containing untreated natural graphite with a particle size (D50) of 9 μm, and a second anode active material layer was formed containing secondary particulate artificial graphite with a particle size (D50) of 18 μm.

[0084] Comparative Example 5 The negative electrode was manufactured in the same manner as in Example 1, except that a single layer of negative electrode active material was formed, containing (mixed) isotropically treated natural graphite with a particle size (D50) of 9 μm, secondary particulate artificial graphite with a particle size (D50) of 18 μm, and single particulate artificial graphite with a particle size (D50) of 9 μm.

[0085] Comparative Example 6 The negative electrode was fabricated in the same manner as in Example 1, except that a single layer of negative electrode active material was formed containing (mixed) secondary particulate artificial graphite with a particle size (D50) of 18 μm and single particulate artificial graphite with a particle size (D50) of 9 μm.

[0086] Comparative Example 7 The negative electrode was fabricated in the same manner as in Example 1, except that a second negative electrode slurry was surface-coated onto the copper foil electrode current collector to form a second negative electrode active material layer, and a first negative electrode slurry was coated onto the surface of the second negative electrode active material layer to form a first negative electrode active material layer.

[0087] The adhesive strength between the negative electrode active material layer and the current collector of the manufactured negative electrode, and the cycle expansion performance of batteries manufactured using the negative electrodes manufactured in the above examples and comparative examples were measured and are shown in Table 1 below.

[0088] (1) Method for measuring the adhesive strength (gF / cm) of the negative electrode active material layer: After drying each negative electrode manufactured in the examples and comparative examples in a vacuum oven at 130°C for 8 hours after rolling, they were punched out to a certain size, fixed to the center of a glass slide with tape, and the peel strength was measured by peeling off the negative electrode current collector using a UTM (Universal Testing Machine).

[0089] (2) Cycle expansion performance (Swelling ratio (%) @ 30 th Cycle (Charge) Measurement Method: LiCoO2 was used as the positive electrode active material, Li-435 (manufactured by Denka) as the conductive material, and a binder and thickener were mixed in a weight ratio of 96:2:2 (positive electrode active material: conductive material: binder + thickener). N-methylpyrrolidone (NMP) solvent was added to produce a positive electrode slurry. The positive electrode slurry was applied to aluminum foil, vacuum dried at approximately 130°C for 8 hours, and rolled to cover an area of ​​1.7671 cm². 2 The positive electrode was manufactured. At this time, the loading of the positive electrode was ~3.4mAh / cm². 2The batteries were manufactured to meet the following criteria. A polyethylene separation membrane was interposed between the negative electrode manufactured in the Examples and Comparative Examples and the positive electrode manufactured above, and then an electrolyte was injected to produce full-cell secondary batteries using the respective negative electrodes of the Examples and Comparative Examples. The electrolyte used was a non-aqueous electrolyte solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:4, adding vinylene carbonate (VC) at 0.5% by weight relative to the solvent, and dissolving LiPF6 in 1M. The coin full-cell batteries were charged and discharged with a charging range of SOC0 to SOC95, with the first cycle at 0.1C, the second cycle at 0.2C, and the third to 30th cycles at 0.5C. After that, the swelling ratio was measured using the following mathematical formula 1.

[0090] [Formula 1] Swing ratio (%) = {(t2-t1) / t1} × 100 (The t1 is the thickness of the negative electrode for the secondary battery before the first charge-discharge cycle, and the t2 is the thickness of the negative electrode for the secondary battery after the 30th charge-discharge cycle.)

[0091] [Table 1]

[0092] As shown in Table 1 above, it was confirmed that the adhesive strength of the negative electrode active material layer and the battery cycle expansion performance of Examples 1 to 4 were superior to those of Comparative Examples 1 to 7.

Claims

1. Current collector and, A first negative electrode active material layer provided on the current collector, The present invention includes a second negative electrode active material layer provided on the first negative electrode active material layer, The first negative electrode active material layer contains isotropically treated natural graphite. The second negative electrode active material layer comprises secondary particulate artificial graphite and single particulate artificial graphite. The second negative electrode active material layer contains 30 to 90 parts by weight of the secondary particulate artificial graphite based on 100 parts by weight of the negative electrode active material. The second negative electrode active material layer contains 10 to 30 parts by weight of the single-particulate artificial graphite based on 100 parts by weight of the negative electrode active material, and is a negative electrode for a secondary battery.

2. The BET specific surface area of ​​the aforementioned natural graphite is 1 m² ２ / g~3m ２ The negative electrode for a secondary battery according to claim 1, wherein the value is / g.

3. The negative electrode for a secondary battery according to claim 1, wherein the average particle size (D50) of the natural graphite is 6 μm to 9 μm.

4. The negative electrode for a secondary battery according to claim 1, wherein the average particle size (D50) of the single-particulate artificial graphite is 6 μm to 9 μm.

5. The negative electrode for a secondary battery according to claim 1, wherein the OI(004 / 110) of the single-particulate artificial graphite is 35 or less.

6. The negative electrode for a secondary battery according to claim 1, wherein the adhesive strength of the first negative electrode active material layer to the current collector is 18 gF / cm or more.

7. Current collector and, A first negative electrode active material layer provided on the current collector, The present invention includes a second negative electrode active material layer provided on the first negative electrode active material layer, The first negative electrode active material layer has a BET specific surface area of ​​1 m². ２ / g~3m ２ Contains natural graphite / g, The second negative electrode active material layer comprises secondary particulate artificial graphite and single particulate artificial graphite. The second negative electrode active material layer contains 30 to 90 parts by weight of the secondary particulate artificial graphite based on 100 parts by weight of the negative electrode active material. The second negative electrode active material layer contains 10 to 30 parts by weight of the single-particulate artificial graphite based on 100 parts by weight of the negative electrode active material, and is a negative electrode for a secondary battery.

8. A secondary battery comprising a negative electrode, a positive electrode, and a separator membrane according to any one of claims 1 to 7.

9. A method for manufacturing a negative electrode for a secondary battery, The steps include forming a first negative electrode active material layer on the current collector, The process includes the step of forming a second negative electrode active material layer on the first negative electrode active material layer, The first negative electrode active material layer is made of isotropically treated natural graphite or BET with a specific surface area of ​​1 m². ２ / g~3m ２ Contains natural graphite / g, The second negative electrode active material layer comprises secondary particulate artificial graphite and single particulate artificial graphite. The second negative electrode active material layer contains 30 to 90 parts by weight of the secondary particulate artificial graphite based on 100 parts by weight of the negative electrode active material. A method for manufacturing a negative electrode for a secondary battery, wherein the second negative electrode active material layer contains 10 to 30 parts by weight of the single-particulate artificial graphite based on 100 parts by weight of the negative electrode active material.

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