Anodes and secondary batteries

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 claims the benefit of the filing date of Korean Patent Application No. 10-2022-0132407 filed with the Korean Intellectual Property Office on October 14, 2022, and Korean Patent Application No. 10-2023-0133971 filed with the Korean Intellectual Property Office on October 10, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

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

[0003] Secondary batteries are widely used not only in portable devices but also in electric vehicles (EVs) and hybrid electric vehicles (HEVs), which are powered by electrical sources.

[0004] Such secondary batteries have the primary advantage of dramatically reducing the use of fossil fuels, as well as the advantage of not producing any by-products from energy use, making them environmentally friendly and drawing attention as a new energy source for improving energy efficiency.

[0005] In general, a secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, etc. In addition, the electrodes such as the positive electrode and the negative electrode may have an electrode active material layer formed on a current collector.

[0006] As secondary batteries become more widely used, various battery performance requirements are being met. Attempts have been made to improve battery performance by incorporating additives into the active material layer. However, depending on the type of additive, some battery performance may be improved, while other performance may be degraded. Therefore, research is needed to determine the selection or combination of materials contained in the electrode that can improve the required performance of secondary batteries. Summary of the Invention [Problem to be solved by the invention]

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

[0008] One embodiment of the present invention comprises: 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, the first negative electrode active material layer contains isostatically pressed natural graphite, The second negative electrode active material layer provides a negative electrode for a secondary battery, which contains secondary particulate artificial graphite and single particulate artificial graphite.

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

[0010] Another embodiment of the present invention provides a method for manufacturing a negative electrode comprising the steps of: forming a first negative electrode active material layer on a current collector; 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 isostatically pressed natural graphite or graphite having a BET specific surface area of ​​1 m 2 / g~3m 2 / g of natural graphite, The second negative electrode active material layer contains secondary particulate artificial graphite and single particulate artificial graphite. [Effects of the Invention]

[0011] The negative electrode of the present invention uses isostatically pressed natural graphite and single-particle artificial graphite together with secondary-particle artificial graphite in the negative electrode active material layer, thereby relatively reducing the orientation within the electrode and reducing side reactions with the electrolyte, thereby improving the electrode expansion performance. Furthermore, by forming the isostatically pressed natural graphite at the bottom of the negative electrode active material layer, the adhesive strength of the negative electrode active material layer can be maximized, resulting in improved rapid filling. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in more detail below to facilitate understanding of the present invention. The present invention can be embodied in various different forms and is not limited to the embodiments described herein. In this regard, the terms and words used in this specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concepts of terms in order to best describe his or her invention.

[0013] In this specification, the terms "comprises," "has," "comprises," and the like are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and are understood not to preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0014] Furthermore, when a part such as a layer is said to be "above" another part, this does not only mean that it is "directly above" that part, but also includes cases where there is another part between them. Conversely, when a part is said to be "directly above" another part, it means that there is no other part between them. Note that being "above" a reference part refers to being located above or below the reference part, and does not necessarily mean being located "above" 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 a laser diffraction method. For example, the average particle size (D50) of the positive electrode active material can be measured by dispersing particles of the positive electrode active material in a dispersion medium, introducing the dispersion into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), and irradiating the dispersion with ultrasonic waves of about 28 kHz at an output of 60 W. The average particle size (D50) corresponding to 50% of the cumulative volume in the analyzer can then be calculated.

[0016] In this specification, a description simply referring to an "active material layer" without the terms "first" or "second" can apply to both the first active material layer and the second active material layer.

[0017] According to one embodiment of the present specification, there is provided a negative electrode for a secondary battery, comprising: a current collector; a first negative electrode active material layer disposed on the current collector; and a second negative electrode active material layer disposed on the first negative electrode active material layer, wherein the first negative electrode active material layer comprises isostatically pressed natural graphite, and the second negative electrode active material layer comprises secondary-particle artificial graphite and single-particle artificial graphite. In other words, the negative electrode for a secondary battery comprises two negative electrode active material layers, each containing secondary-particle artificial graphite, isostatically pressed natural graphite, and single-particle artificial graphite, with the isostatically pressed natural graphite disposed at the bottom of the negative electrode active material layer, thereby achieving a negative electrode with excellent expansion performance and adhesive strength.

[0018] According to another embodiment, a method for manufacturing a negative electrode for a secondary battery includes 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 pressed natural graphite or graphite having a BET specific surface area of ​​1 m 2 / g~3m 2 / g of natural graphite, and the second negative electrode active material layer contains secondary particle-like artificial graphite and single particle-like artificial graphite.

[0019] In this specification, the term "single particle" refers to a particle consisting of 10 or less primary particles, in contrast to a secondary particle formed by agglomeration of tens to hundreds of primary particles. Specifically, in the present invention, the single particle may be a single particle consisting of one primary particle, or may be a particle formed by agglomeration of multiple primary particles.

[0020] In this specification, the term "primary particle" refers to the smallest particle unit that can be recognized when observing an active material through a scanning electron microscope, and the term "secondary particle" refers to a secondary structure formed by aggregation of several tens to several 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" with crystalline forms measured in units of several tens of nanometers. Further magnification reveals distinct regions in which atoms form a lattice structure in a specific direction, which are called "crystal grains." The size of particles observed by XRD is defined as the crystal grain size. Crystal grain size can be quantitatively determined using the Scherrer equation from XRD data.

[0022] According to one embodiment of the present invention, the first negative electrode active material layer includes isostatically pressed natural graphite. The isostatically pressed natural graphite can be obtained by a method such as cold isostatic pressing (CIP). Specifically, size-controlled spheroidized coarse particles obtained using an airflow mill are loaded into the mold of a cold isostatic pressing apparatus, the entire surface of the mold is filled with water, and cold isostatic pressing is performed for 100 seconds at a molding pressure of 90 MPa. The cold isostatically pressed spheroidized coarse particles are then crushed. Isostatically pressed natural graphite can be obtained by mixing 100 parts by weight of the crushed spheroidized coarse particles with 5 parts by weight of pitch (solid phase pitch) as a carbon coating material, carbonizing the mixture at a temperature of 1,250°C for 24 hours, and carbon coating the mixture. However, the method for producing isostatically pressed natural graphite is not limited to the above.

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

[0024] According to one embodiment of the present invention, the first negative electrode active material layer includes isostatically pressed natural graphite, which reduces the orientation within the electrode, reduces side reactions with the electrolyte, improves the expansion performance of the electrode, and increases the adhesive strength of the negative electrode active material layer.

[0025] According to one embodiment of the present invention, the BET specific surface area of ​​the natural graphite is 1 m 2 / g~3m 2 / g. By using natural graphite having a BET specific surface area in the above range, there is an advantage that the durability of the cell is improved. Specifically, 2 If it exceeds 1m / g, durability will decrease due to side reactions of the electrolyte. 2 If it is less than 1.2m / g, it is difficult to realize the material and the electrolyte penetration is hindered. 2 / g~2.8m 2 / g, and 1.2m 2 / g~2.8m 2 / g, and 1.2m 2 / g~2.6m 2 / g, and 1.4m 2 / g~2.4m 2 / g.

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

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

[0028] The average particle size (D50) of the isostatically pressed natural graphite can be adjusted through a pulverization process using an airflow pulverizer. Specifically, flake graphite having an average particle size (D50) of 200 μm is prepared and pulverized using an airflow pulverizer to adjust the size. The pulverized flake graphite can be subjected to a spheroidizing process to obtain spheroidized coarse particles, but the methods for obtaining isostatically pressed natural graphite and natural graphite with an adjusted average particle size are not limited to those described above.

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

[0030] According to one embodiment of the present invention, the average particle size (D50) of the monoparticulate 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 secondary particle-like artificial graphite may have an average particle size (D50) of 16 μm to 22 μm.

[0032] In one embodiment of the present invention, methods such as pulverization using a jet mill followed by post-pulverization heat treatment can be used to obtain single-particle artificial graphite and secondary-particle artificial graphite having an average particle size (D50) within the above range. Specifically, needle coke can be pulverized using a jet mill and then sieved to obtain a powder with an adjusted size. Furthermore, the powder can be heat-treated (graphitized) at 3,000°C for 20 hours in an inert argon (Ar) gas atmosphere to produce primary artificial graphite particles (or single particles). The primary artificial graphite particles (single particles) can be mixed with a binder such as petroleum pitch and then heat-treated at 1,000°C or higher for 10 hours to produce secondary artificial graphite particles in which the primary artificial graphite particles are aggregated.

[0033] In one embodiment of the present invention, the OI(004 / 110) of the monoparticulate artificial graphite is 35 or less, 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 and can be obtained using a common 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-particle artificial graphite of the second negative electrode active material layer is a value obtained by X-ray diffraction analysis and can be obtained by a common method of X-ray diffraction analysis, for example, using a Bruker D4 Endeavor X-ray diffraction analyzer. The OI value of the negative electrode may 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 graphite crystals in 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 graphite crystals are less oriented, the battery expands randomly during charging rather than expanding in a specific direction, improving the battery's expansion performance.

[0037] According to one embodiment of the present invention, the adhesive strength of the first negative electrode active material layer to the current collector is 18 gF / cm or more. Since the negative electrode of the present invention is formed so that the first negative electrode active material layer and the current collector are in contact with each other, the adhesive strength of the isostatically pressed natural graphite, which is the material of the first negative electrode active material layer, affects the adhesive strength 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 isostatically pressed natural graphite contained in the first negative electrode active material layer is 18 gF / cm or more. The adhesive strength of the isostatically pressed natural graphite may be adjusted depending on the particle size distribution.

[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 particle artificial graphite based on 100 parts by weight of the negative electrode active material. When the content is in this range, it is advantageous 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 the single-particle artificial graphite based on 100 parts by weight of the negative electrode active material. When the content is within this range, it is advantageous in improving the expansion performance and energy density of the battery.

[0041] According to one embodiment of the present specification, each of the negative electrode active material layers (first negative electrode active material layer, second negative electrode active material layer) may further contain a negative electrode active material other than the above-mentioned isostatically pressed natural graphite, secondary particle artificial graphite, and single particle artificial graphite, and the type thereof is not limited.

[0042] In one embodiment of the present specification, the negative electrode active material may be contained 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, in 100 parts by weight of each negative electrode active material layer.

[0043] According to a further embodiment of the present specification, the first negative electrode active material layer may further include a negative electrode binder in addition to the isostatically pressed natural graphite, and the second negative electrode active material layer may further include a negative electrode binder in addition to the secondary particle artificial graphite and the single particle artificial graphite.

[0044] The negative electrode binder may improve adhesion between negative electrode active material particles and between the negative electrode active material particles and the negative electrode current collector. The negative electrode binder may be any binder known in the art, and non-limiting examples thereof may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, 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, and substances in which hydrogen is substituted with Li, Na, or Ca, or various copolymers thereof.

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

[0046] The negative electrode active material layer may not contain a conductive material, but may further contain a conductive material if necessary. The conductive material contained in the negative electrode active material layer is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; 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. 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 include a thickener such as Na-CMC (sodium carboxymethyl cellulose), Li-CMC (carboxymethyl cellulose lithium), or CNF (cellulose nanofiber).

[0048] In one embodiment of the present specification, the first and second negative electrode active material layers may each have a thickness of 5 μm or more and 500 μm or less.

[0049] According to one embodiment of the present 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 the present specification, the negative electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes 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 whose surface is treated with carbon, nickel, titanium, silver, or the like. Specifically, a transition metal that effectively adsorbs carbon, such as copper or nickel, can be used as the current collector. The thickness of the current collector can be 1 μm to 500 μm, but is not limited thereto.

[0051] A further embodiment herein provides a secondary battery comprising an anode, a cathode and a separator according to the previous embodiment.

[0052] In one embodiment of the present 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 positive electrode active material layer may have a thickness of 20 μm to 500 μm.

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

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

[0055] In one embodiment, the positive electrode active material may be contained 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, in 100 parts by weight of the positive electrode active material layer.

[0056] According to a further embodiment of the present specification, the positive electrode active material layer according to the aforementioned embodiment may further include a positive electrode binder and a conductive material.

[0057] The positive electrode binder may serve to improve adhesion between positive electrode active material particles and between the positive electrode active material particles and the positive electrode current collector. The positive electrode binder may be any binder known in the art, and non-limiting examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These binders may be used alone or in combination.

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

[0059] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode, and can be any material that does not undergo chemical changes in the battery and has electronic conductivity. Specific examples include graphite such as natural graphite and 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 powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These may be used alone or in combination of two or more.

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

[0061] The positive and negative electrodes can be fabricated according to conventional methods for fabricating positive and negative electrodes, except for using the positive and negative electrode active materials described above. Specifically, they can be fabricated by applying an active material layer-forming composition containing the active material and, optionally, 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. These may be used alone or in combination. The amount of solvent used is sufficient to dissolve or disperse the active material, conductive material, and binder, taking into account the coating thickness of the slurry and production yield, and to provide a viscosity that allows excellent thickness uniformity when applied to produce positive and negative electrodes. Alternatively, the positive and negative electrodes may be fabricated by casting the active material layer-forming composition on a separate support, peeling it off from the support, and laminating the resulting film on a current collector.

[0063] A further embodiment of the present specification provides a method for producing a negative electrode for a secondary battery according to the above-described embodiment.

[0064] The method includes forming a first negative electrode active material layer on a current collector; A method for manufacturing a negative electrode for a secondary battery, comprising 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 isostatically pressed natural graphite or graphite having a BET specific surface area of ​​1 m 2 / g~3m 2 / g of natural graphite, The second negative electrode active material layer may contain secondary particle artificial graphite and single particle artificial graphite.

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

[0066] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without limitation. It is particularly preferable that the separator has low resistance to electrolyte ion movement and excellent electrolyte humidification capability. Specifically, a porous polymer film, such as a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and it can be used in a single-layer or multi-layer structure.

[0067] Examples of the electrolyte 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 manufacturing lithium secondary batteries.

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

[0069] Examples of the non-aqueous organic solvent that may be used include 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, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0070] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferably used because they are high-viscosity organic solvents with high dielectric constants and can dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte with high electrical conductivity can be produced, and therefore, these cyclic carbonates are more preferably used.

[0071] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, the anion of the lithium salt may be 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 selected from the group consisting of:

[0072] In addition to the electrolyte components, the electrolyte may further include one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0073] A further embodiment of the present invention provides a battery module including the aforementioned secondary battery as a unit cell, and a battery pack including the same. The battery module and battery pack include the secondary battery having high capacity, excellent rate-limiting characteristics, and excellent cycle characteristics, and can be used as a power source for medium- to large-sized 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 stably exhibits excellent discharge capacity, output characteristics, and cycle performance, and can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, as well as portable devices such as mobile phones, laptops, and digital cameras. For example, the battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0075] In the following, preferred embodiments are presented to aid in understanding the present invention, but these embodiments are merely illustrative of the present invention, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the present invention and the technical spirit of the present invention. Such changes and modifications are naturally intended to fall within the scope of the appended claims.

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

[0077] Example 2 A negative electrode was prepared in the same manner as in Example 1, except that isostatically pressed natural graphite having an average particle size (D50) of 9 μm and an adhesive strength of 22 gF / cm was used.

[0078] Example 3 A negative electrode was manufactured in the same manner as in Example 1, except that natural graphite that had been isostatically pressed at a pressure of 90 MPa was used.

[0079] Example 4 A negative electrode was manufactured in the same manner as in Example 1, except that the negative electrode active material of the second negative electrode slurry was a mixture of secondary particle artificial graphite and single particle artificial graphite in a ratio of 70:30.

[0080] Comparative Example 1 A 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.

[0081] Comparative Example 2 A negative electrode was prepared in the same manner as in Example 1, except that a single-layer negative electrode active material layer containing (mixing) secondary particle artificial graphite having a particle size (D50) of 18 μm and single particle artificial graphite having a particle size (D50) of 18 μm was formed as the negative electrode active material layer.

[0082] Comparative Example 3 A negative electrode was fabricated in the same manner as in Example 1, except that a single-layer negative electrode active material layer containing (mixing) secondary particle artificial graphite having a particle size (D50) of 18 μm and non-isostatically pressed natural graphite having a particle size (D50) of 9 μm was formed as the negative electrode active material layer.

[0083] Comparative Example 4 A negative electrode was fabricated in the same manner as in Example 1, except that a first negative electrode active material layer containing non-isostatically pressed natural graphite having a particle size (D50) of 9 μm and a second negative electrode active material layer containing secondary particulate artificial graphite having a particle size (D50) of 18 μm were formed as the negative electrode active material layers.

[0084] Comparative Example 5 A negative electrode was manufactured in the same manner as in Example 1, except that a single-layer negative electrode active material layer containing (a mixture of) isostatically pressed natural graphite having a particle size (D50) of 9 μm, secondary particle-like artificial graphite having a particle size (D50) of 18 μm, and single particle-like artificial graphite having a particle size (D50) of 9 μm was formed as the negative electrode active material layer.

[0085] Comparative Example 6 A negative electrode was prepared in the same manner as in Example 1, except that a single-layer negative electrode active material layer containing (mixing) secondary particle artificial graphite having a particle size (D50) of 18 μm and single particle artificial graphite having a particle size (D50) of 9 μm was formed as the negative electrode active material layer.

[0086] Comparative Example 7 The negative electrode was fabricated in the same manner as in Example 1, except that the copper foil electrode collector was surface-coated with the second negative electrode slurry to form a second negative electrode active material layer, and the first negative electrode slurry was coated on 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 prepared negative electrode and the cycle expansion performance of the batteries prepared using the negative electrodes prepared in the Examples and Comparative Examples were measured and are shown in Table 1 below.

[0088] (1) Method for measuring adhesive strength (gF / cm) of negative electrode active material layer: Each negative electrode manufactured in the examples and comparative examples was rolled, dried in a vacuum oven at 130°C for 8 hours, and then punched out to a certain size. The negative electrode was then fixed to the center of a slide glass using tape, and the negative electrode current collector was peeled off using a UTM (Universal Testing Machine, a peel strength tester) to measure the peel strength.

[0089] (2) Cycle expansion performance (Swelling ratio (%) @ 30 th Measurement method for cycle (charge): LiCoO2 as the positive electrode active material, Li-435 (Denka) as the conductive material, binder, and thickener were mixed in a weight ratio of 96:2:2 (positive electrode active material: conductive material: binder + thickener), and N-methylpyrrolidone (NMP) solvent was added to prepare 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 produce a 1.7671 cm2 cathode slurry. 2 At this time, the loading of the positive electrode was 3.4 mAh / cm 2A polyethylene separator was interposed between the negative electrode and the positive electrode prepared in each of the Examples and Comparative Examples, and an electrolyte was then injected to fabricate full-cell secondary batteries using the negative electrodes of each of the Examples and Comparative Examples. The electrolyte was prepared by dissolving LiPF6 to a non-aqueous electrolyte solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:4, adding vinylene carbonate (VC) at 0.5 wt% relative to the solvent, and dissolving LiPF6 to a concentration of 1M. The coin full-cell batteries were charged and discharged from SOC 0 to SOC 95 at 0.1 C for the first cycle, 0.2 C for the second cycle, and 0.5 C for the third to 30th cycles. The swelling ratio was then measured using the following mathematical formula 1.

[0090] [Formula 1] Swelling ratio (%) = {(t2-t1) / t1} × 100 (t1 is the thickness of the negative electrode for a secondary battery before the first charge-discharge cycle, and t2 is the thickness of the negative electrode for a 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 cycle swelling performance of the battery in Examples 1 to 4 were superior to the adhesive strength of the negative electrode active material layer and the cycle swelling performance of the battery in Comparative Examples 1 to 7.

Claims

1. A current collector; a first negative electrode active material layer provided on the current collector; a second negative electrode active material layer provided on the first negative electrode active material layer, the first negative electrode active material layer contains isostatically pressed natural graphite; The second negative electrode active material layer comprises secondary particle-like artificial graphite and single particle-like artificial graphite.

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

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

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

5. 2. 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. 2. 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. 2. The negative electrode for a secondary battery according to claim 1, wherein the second negative electrode active material layer comprises 30 to 90 parts by weight of the secondary particle-like artificial graphite based on 100 parts by weight of the negative electrode active material.

8. 2. The negative electrode for a secondary battery according to claim 1, wherein the second negative electrode active material layer comprises 10 to 30 parts by weight of the single-particle artificial graphite based on 100 parts by weight of the negative electrode active material.

9. A current collector; a first negative electrode active material layer provided on the current collector; 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 m ２ / g to 3m ２ / g of natural graphite, The second negative electrode active material layer comprises secondary particle-like artificial graphite and single particle-like artificial graphite.

10. A secondary battery comprising the negative electrode for secondary batteries according to any one of claims 1 to 9, a positive electrode, and a separator.

11. A method for producing a negative electrode for a secondary battery, comprising: forming a first negative electrode active material layer on a current collector; 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 isostatically pressed natural graphite or graphite having a BET specific surface area of ​​1 m ２ / g to 3m ２ / g of natural graphite, The second negative electrode active material layer comprises secondary particle-like artificial graphite and single particle-like artificial graphite.

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