Negative electrode and secondary battery including the same

Abstract

The present invention relates to a negative electrode comprising: a negative electrode current collector; and a negative electrode active material layer disposed on the negative electrode current collector and containing a negative electrode active material, wherein the negative electrode active material contains natural graphite particles, and the particle strength of the negative electrode active material during plastic deformation is 40 MPa to 200 MPa.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0120429, filed on September 18, 2020, the entire contents of which are incorporated herein by reference.

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

[0003] With the depletion of fossil fuels causing rising energy prices and increasing concerns about environmental pollution, environmentally friendly alternative energy sources have become an essential element for future life.

[0004] In particular, with the development of technology and increasing demand for mobile devices, the demand for secondary batteries as an environmentally friendly alternative energy source is rapidly increasing.

[0005] Recently, with the increasing interest in environmental issues, much research has been conducted on electric vehicles (EVs) and hybrid electric vehicles (HEVs) that can be used as an alternative to vehicles that use fossil fuels such as gasoline and diesel, which are one of the main causes of air pollution. Lithium secondary batteries, which have high energy density, high discharge voltage, and stable output, are mainly being researched and used as the power source for such electric vehicles (EVs) and hybrid electric vehicles (HEVs).

[0006] In the past, lithium metal was used as the negative electrode in secondary batteries. However, there was a risk of battery short circuit due to the formation of dendrites, which could lead to explosion. Therefore, the use of carbon-based active materials, which allow reversible intercalation and deintercalation of lithium ions and maintain structural and electrical properties, has emerged.

[0007] The carbon-based active material is available in various forms, including artificial graphite, natural graphite, and hard carbon, among which graphite-based active materials are the most widely used, as they have excellent reversibility and can ensure the life characteristics of lithium secondary batteries. Because the graphite-based active material has a lower discharge voltage of -0.2 V compared to lithium, batteries using the graphite-based active material can exhibit a high discharge voltage of 3.6 V, offering many advantages in terms of the energy density of lithium batteries.

[0008] Among these, natural graphite is known to exhibit higher output and capacity than other carbon-based active materials such as artificial graphite. However, natural graphite generally has low particle strength, which can cause internal deformation of the particles during the rolling process involved in the manufacture of the negative electrode, resulting in a decrease in the pores within the natural graphite or the negative electrode. This can result in a decrease in the diffusion rate of lithium ions and a decrease in output characteristics.

[0009] Therefore, when natural graphite is used as the negative electrode, it is necessary to develop a negative electrode that can exhibit high output characteristics.

[0010] Japanese Patent No. 4403327 discloses graphite powder for the negative electrode of a lithium ion secondary battery, but fails to provide an alternative solution to the above-mentioned problems. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 4403327 Summary of the Invention [Problem to be solved by the invention]

[0012] An object of the present invention is to provide a negative electrode having high output characteristics and high-temperature storage performance.

[0013] Another object of the present invention is to provide a method for producing the above-mentioned negative electrode.

[0014] A further object of the present invention is to provide a secondary battery including the above-mentioned negative electrode. [Means for solving the problem]

[0015] The present invention provides a negative electrode comprising: a negative electrode current collector; and a negative electrode active material layer disposed on the negative electrode current collector and containing a negative electrode active material, wherein the negative electrode active material contains natural graphite particles, and the particle strength of the negative electrode active material during plastic deformation is 40 MPa to 200 MPa.

[0016] The present invention also provides a secondary battery including the above-mentioned negative electrode, a positive electrode facing the negative electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte. [Effects of the Invention]

[0017] The anode of the present invention includes an anode active material that includes natural graphite particles and has a specific range of particle strength during plastic deformation. Because the anode active material has a desirable level of particle strength during plastic deformation, internal deformation of the active material within the anode can be prevented and the pores of the natural graphite can be maintained. Therefore, the anode of the present invention can smoothly exhibit the excellent capacity characteristics of the natural graphite particles and prevent a reduction in lithium diffusion paths, thereby improving the output characteristics and high-temperature storage performance of the anode and secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0018] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0019] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0020] In this specification, the terms "comprise," "include," "comprise," or "have" are intended to indicate the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the presence or additional possibility of one or more other features, numbers, steps, components, or combinations thereof.

[0021] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0022] The present invention will be specifically described below.

[0023] negative electrode The present invention relates to a negative electrode, and more particularly to a negative electrode for a lithium secondary battery.

[0024] Specifically, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector and including a negative electrode active material, the negative electrode active material includes natural graphite particles, and the particle strength of the negative electrode active material during plastic deformation is 40 MPa to 200 MPa.

[0025] Conventionally, natural graphite particles have been known to have superior capacity compared to artificial graphite particles but have low particle strength. Generally, a negative electrode requires a rolling process to form a negative electrode active material layer on a negative electrode current collector. Therefore, when conventional natural graphite particles are used as a negative electrode active material, the rolling process causes internal deformation of the particles, resulting in a decrease in pore size within the negative electrode. Therefore, a negative electrode using conventional natural graphite particles as an active material cannot exhibit the excellent capacity characteristics of natural graphite particles, and suffers from a decrease in capacity at high output due to a decrease in lithium diffusion paths.

[0026] To address these issues, the anode of the present invention includes a negative electrode active material that includes natural graphite particles and has a particle strength during plastic deformation within the aforementioned range. Because the negative electrode active material has a desirable level of particle strength during plastic deformation, internal deformation of the active material is prevented and the pores of the natural graphite are maintained even when a rolling process is performed during the manufacture of the negative electrode. Therefore, the anode of the present invention smoothly exhibits the excellent capacity characteristics of the natural graphite particles and prevents a reduction in lithium diffusion paths, thereby improving the output characteristics and high-temperature storage performance of the anode and secondary battery.

[0027] The negative electrode current collector may be any negative electrode current collector commonly used in the art, and may be any current collector that does not cause chemical changes in the lithium secondary battery and has high conductivity. For example, the negative electrode current collector may include at least one selected from copper, stainless steel, aluminum, nickel, titanium, sintered carbon, and an aluminum-cadmium alloy, and preferably copper.

[0028] The negative electrode current collector may have fine irregularities on its surface to strengthen the binding force of the negative electrode active material, and may be used in various forms such as a film, sheet, foil, mesh, porous body, foam, or nonwoven fabric.

[0029] The negative electrode current collector generally has a thickness of 3 μm to 500 μm.

[0030] The negative electrode active material layer is disposed on the negative electrode current collector.

[0031] The negative electrode active material includes natural graphite particles. Natural graphite particles generally have more pores than artificial graphite particles, which provides the advantage of high output characteristics. Meanwhile, the natural graphite particles according to the present invention have a high level of particle strength during plastic deformation, as described below. This prevents internal deformation within the negative electrode and maintains the pore structure within the negative electrode, ensuring smooth lithium diffusion paths and improving output characteristics and high-temperature storage characteristics.

[0032] The negative electrode active material may further include a carbon coating layer disposed on the natural graphite particles, in addition to the natural graphite particles. The carbon coating layer may improve the structural stability and particle strength of the natural graphite particles and may help prevent side reactions between the negative electrode active material and the electrolyte.

[0033] The carbon coating layer may be included in the negative electrode active material in an amount of 1 wt % to 15 wt %, preferably 2 wt % to 5 wt %. The presence of the carbon coating layer can improve the particle strength of the negative electrode active material, but excessive formation of the carbon coating layer can reduce the pores inside the negative electrode active material, increasing the diffusion resistance of lithium ions and increasing side reactions in the electrolyte, which can reduce thermal stability and degrade output characteristics. Therefore, it is preferable to form the carbon coating layer in an amount within the above range.

[0034] The carbon coating layer may include amorphous carbon. For example, the carbon coating layer may be formed by providing the natural graphite particles with at least one carbon coating precursor selected from the group consisting of coal-tar pitch, rayon, and polyacrylonitrile resin, and then heat-treating the resulting mixture. The heat-treatment process for forming the carbon coating layer may be performed at a temperature ranging from 900°C to 1,500°C to ensure uniform formation of the carbon coating layer.

[0035] The particle strength of the negative electrode active material during plastic deformation is 40 MPa to 200 MPa. By having the particle strength during plastic deformation within the above range, the negative electrode active material according to the present invention can maintain the pore structure within the negative electrode without internal deformation of the negative electrode active material, thereby reducing the lithium ion diffusion resistance of the negative electrode, improving output characteristics, and improving high-temperature storage performance.

[0036] If the particle strength of the negative electrode active material during plastic deformation is less than 40 MPa, the internal deformation of the negative electrode active material may be severe during the rolling process, which is always required during negative electrode manufacturing. This may result in a decrease in the internal pores of the negative electrode, significantly reducing output characteristics, and impairing lithium ion diffusion, leading to lithium precipitation, significantly reducing life characteristics and storage performance. If the particle strength of the negative electrode active material during plastic deformation is more than 200 MPa, particle cracking may occur due to particle contact or collision during negative electrode rolling, thereby reducing the pore structure within the negative electrode and impairing lithium output characteristics.

[0037] The particle strength of the negative electrode active material during plastic deformation may be preferably 70 MPa to 150 MPa, more preferably 90 MPa to 130 MPa. When the particle strength is within the above range, the effects of preventing structural deformation of the active material and maintaining the pore structure are maximized, and particle cracking due to particle contact, etc. is prevented, thereby significantly improving the output characteristics and life characteristics of the negative electrode.

[0038] The particle strength of the negative electrode active material during plastic deformation can be achieved by controlling the execution and conditions of cold isostatic pressing (CIP) or the content of the carbon coating layer during the preparation of the natural graphite particles, specifically, by controlling the execution and conditions of cold isostatic pressing (CIP).

[0039] As used herein, "plasticity" refers to the property of an object that, when an external force is applied to the object, changes its shape without changing its state of motion, and the object's shape remains permanently deformed even after the external force is removed. On the other hand, elasticity refers to the property of an object that, when an external force is applied to the object, changes its shape without changing its state of motion, and the object returns to its original state when the external force is removed, and is a concept distinct from plasticity. As used herein, "particle strength upon plastic deformation" may be defined as the particle strength at the point when an external force is applied to the object, changes its shape without changing its state of motion, and the object's shape remains permanently deformed even after the external force is removed. Specifically, when a load is applied to a negative electrode active material using a powder resistance measuring device or the like and a stress-strain curve is plotted, if the stress and strain are proportional to each other, it can be defined as an elastic region, and if only the strain changes without a change in stress, it can be defined as a plastic region. The particle strength during plastic deformation can be defined as the stress at the point where the stress changes from the elastic region to the plastic region on the stress-strain curve.

[0040] The compressive fracture strength of the negative electrode active material may be 350 MPa to 1,000 MPa, preferably 400 MPa to 900 MPa, more preferably 480 MPa to 900 MPa, and even more preferably 650 MPa to 800 MPa. In this specification, the compressive fracture strength may be defined as the maximum compressive stress that can be applied to an object without fracture. The compressive fracture strength is a concept distinct from "particle strength during plastic deformation," which measures particle strength during permanent deformation of the external shape of an object. When the negative electrode active material according to the present invention has a compressive fracture strength within the above range, the effects of preventing structural deformation of the active material and maintaining the pore structure are maximized, and particle cracking due to particle contact, etc., is prevented, thereby significantly improving the output characteristics and life characteristics of the negative electrode.

[0041] The compressive fracture strength of the negative electrode active material can be achieved by controlling the execution and conditions of cold isostatic pressing (CIP) or the content of the carbon coating layer during the preparation of the natural graphite particles, specifically, by controlling the execution and conditions of cold isostatic pressing (CIP).

[0042] The negative electrode active material may be spherical. When the negative electrode active material is spherical, the negative electrode active material smoothly maintains the pore structure inside the negative electrode, thereby ensuring a diffusion path for lithium ions and improving the output characteristics of the negative electrode. In this specification, the term "spherical" refers to a shape that is not only perfectly spherical, but also includes a shape that is substantially spherical even if it is somewhat crushed.

[0043] The average particle size (D 50 ) can be 10 μm to 25 μm, preferably 15 μm to 20 μm. When it is within the above range, it is preferable in terms of simultaneously improving the output characteristics and life characteristics.

[0044] The negative electrode active material may be contained in the negative electrode active material layer in an amount of 80% by weight to 99% by weight, preferably 88% by weight to 98% by weight.

[0045] The negative electrode active material layer may further contain a binder, a conductive material, and / or a thickener in addition to the above-mentioned negative electrode active material.

[0046] The binder is a component that facilitates bonding between the active material and / or the current collector, and can usually be contained in the negative electrode active material layer in an amount of 1 to 30% by weight, preferably 1 to 10% by weight.

[0047] The binder can include at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably at least one selected from polyvinylidene fluoride and styrene-butadiene rubber.

[0048] As the thickener, any thickener conventionally used in lithium secondary batteries can be used, and an example thereof is carboxymethyl cellulose (CMC).

[0049] The conductive material is a component for further improving the conductivity of the negative electrode active material, and can be contained in the negative electrode active material layer in an amount of 1 to 30% by weight, preferably 1 to 10% by weight.

[0050] The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and is conductive, and examples of such conductive materials 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; metal powders such as carbon fluoride, 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. Specific examples of commercially available conductive materials include acetylene black-based materials (manufactured by Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, etc.), Ketjenblack, EC-based materials (manufactured by Armak Company), Vulcan XC-72 (manufactured by Cabot Company), and Super P (manufactured by Timcal).

[0051] The negative electrode active material layer may be manufactured by mixing the above-described negative electrode active material and at least one selected from a binder, a conductive material, and a thickener in a solvent to prepare a negative electrode slurry, and then applying the negative electrode slurry to the negative electrode current collector, rolling, and drying the slurry.

[0052] The solvent may include water or an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that provides a desired viscosity when the negative electrode active material and, optionally, a binder and a conductive material are included. For example, the solvent may be included so that the concentration of solids including the negative electrode active material and, optionally, at least one selected from the binder, thickener, and conductive material is 50 wt % to 95 wt %, preferably 70 wt % to 90 wt %.

[0053] The porosity of the negative electrode may be 20% to 45%, preferably 25% to 35%, which is preferable in that the life performance and output performance can be improved simultaneously when the porosity is within this range.

[0054] In this specification, the porosity of the negative electrode can be calculated by the following mathematical formula 1.

[0055] [Mathematical formula 1] Porosity of negative electrode (%) = {1 - (electrode density of negative electrode / true density of negative electrode)} × 100

[0056] In Equation 1, the true density of the negative electrode is the density of the negative electrode active material layer measured when a negative electrode is cut into a predetermined size and pressed in a press until the thickness of the negative electrode no longer changes, and the electrode density of the negative electrode is the density of the negative electrode active material layer measured when a negative electrode is cut into a predetermined size.

[0057] The pore resistance of the negative electrode can be 15Ω or less, preferably 3Ω to 14Ω, and more preferably 9.5Ω to 11.5Ω.

[0058] The pore resistance can be defined as the resistance value obtained by performing electrochemical impedance spectroscopy (EIS) on a symmetric cell fabricated using the negative electrode as both the working electrode and the counter electrode, injecting an electrolyte solution containing lithium ions into the symmetric cell. Because the pore resistance is measured by EIS on a symmetric cell, only lithium ions originating from the electrolyte can be present, allowing for an objective measurement of the diffusion resistance of lithium ions within the negative electrode.

[0059] By having a pore resistance within the above range, the negative electrode can minimize the diffusion paths of lithium ions in the negative electrode, thereby improving the output performance of the battery. Furthermore, by having a pore resistance within the above range, the diffusion paths of lithium ions can be minimized, effectively preventing the problem of lithium plating on the surface that can occur during high-rate charging, and preventing side reactions on the surface of the negative electrode. Therefore, the negative electrode of the present invention, having a pore resistance within the above range, can achieve excellent output and life characteristics.

[0060] The above-mentioned range of pore resistance can be adjusted by, for example, adjusting the particle strength, compressive fracture strength, structure, size, etc. during plastic deformation of the negative electrode active material.

[0061] secondary battery The present invention also provides a secondary battery, more specifically a lithium secondary battery, including the above-described negative electrode.

[0062] The secondary battery may include the above-described negative electrode, a positive electrode facing the negative electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte.

[0063] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector.

[0064] The positive electrode current collector can be used without limitation with any negative electrode current collector commonly used in the art. For example, it is not particularly limited as long as it does not cause chemical changes in the secondary battery and has high conductivity. For example, the positive electrode current collector can include at least one selected from copper, stainless steel, aluminum, nickel, titanium, fired carbon, and an aluminum-cadmium alloy, preferably aluminum.

[0065] The positive electrode current collector can also form fine irregularities on its surface to strengthen the binding force of the positive electrode active material, and can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, non-woven bodies, etc.

[0066] Generally, the positive electrode current collector can have a thickness of 3 μm to 500 μm.

[0067] The positive electrode active material layer can contain a positive electrode active material.

[0068] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium. Specifically, it can include a lithium composite metal oxide containing lithium and one or more metals such as cobalt, manganese, nickel, or aluminum. More specifically, the lithium composite metal oxide is a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiNiO2, etc.), a lithium-nickel-manganese-based oxide (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-z Ni z O4 (where 0 < Z < 2), etc.), a lithium-nickel-cobalt-based oxide (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), a lithium-manganese-cobalt-based oxide (e.g., LiCo 1-Y2 Mn Y2O2 (where 0 < Y2 < 1), LiMn 2-z1 Co z1 O4 (where 0 < Z1 < 2), etc.), lithium-nickel-manganese-cobalt-based oxides (e.g., Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2), etc.), or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni p2 Co q2 Mn r3 M s2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are the atomic fractions of independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + r3 + s2 = 1), etc.), etc. can be mentioned, and any one or two or more of these compounds can be included. Among them, in terms of being able to enhance the capacity characteristics and stability of the battery, the lithium composite metal oxide is LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (e.g., Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), or lithium nickel cobalt aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.), etc. can be, considering the remarkable improvement effect by controlling the types and content ratios of the constituent elements forming the lithium composite metal oxide, the lithium composite metal oxide is Li(Ni 0.6 Mn 0.2 Co 0.2)O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1 ) O2, etc., and any one or a mixture of two or more of these can be used.

[0069] The positive electrode active material may be contained in the positive electrode active material layer in an amount of 80% by weight to 99% by weight.

[0070] The positive electrode active material layer may further contain at least one selected from the group consisting of a binder and a conductive material, in addition to the positive electrode active material.

[0071] The binder is a component that facilitates bonding between the active material and the conductive material and between the active material and the current collector, and is typically added in an amount of 1 to 30 wt % based on the total weight of the positive electrode mixture. Examples of such binders include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber.

[0072] The binder may be contained in the positive electrode active material layer in an amount of 1% by weight to 30% by weight.

[0073] The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and is conductive, and examples thereof include graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, 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. Specific examples of commercially available conductive materials include acetylene black-based materials (manufactured by Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, etc.), ketjen black, EC-based materials (manufactured by Armak Company), Vulcan XC-72 (manufactured by Cabot Company), and Super P (manufactured by Timcal).

[0074] The conductive material may be added in an amount of 1% by weight to 30% by weight in the positive electrode active material layer.

[0075] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without particular limitations. In particular, a separator that exhibits low resistance to electrolyte ion movement and excellent electrolyte humidification is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of 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 can be used in a single-layer or multi-layer structure.

[0076] Furthermore, examples of the electrolyte used in the present invention include 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 production of lithium secondary batteries, but are not limited to these.

[0077] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0078] The organic solvent can be any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (R is a C2-C20 linear, branched, or cyclic hydrocarbon group that may contain a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of a battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, thereby providing excellent electrolyte performance.

[0079] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, examples of the lithium salt include LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, and LiB(C, O) . The lithium salt is preferably used in a concentration range of 0.1 to 2.0 M. When the lithium salt concentration falls within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0080] As described above, the lithium secondary battery according to the present invention exhibits excellent discharge capacity, rapid charge characteristics, and stable capacity retention, and is therefore useful in portable devices such as mobile phones, laptop computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs), and is particularly suitable as a component battery of medium- to large-sized battery modules. Accordingly, the present invention also provides a medium- to large-sized battery module including the above-described secondary battery as a unit cell.

[0081] Such a medium- to large-sized battery module can be preferably applied to power sources that require high output and large capacity, such as electric vehicles, hybrid electric vehicles, and power storage devices.

[0082] While the present invention may be embodied in various different forms, it is to be understood that the invention is not limited to the specific embodiments set forth herein, and that the invention may be embodied in various different forms without departing from the spirit or scope of the present invention.

[0083] Example Example 1: Preparation of negative electrode (1) Manufacturing of negative electrode active material Natural graphite raw material was extracted from graphite ore using floatation, and the natural graphite raw material was treated with an acid or base to remove impurities, washed, and dried to produce flake natural graphite. The obtained flake natural graphite was spheroidized using a vortex flow pulverizer, and impurities were removed with sulfuric acid, followed by drying to produce spherical natural graphite.

[0084] The spherical natural graphite was filled into a mold, and then pressed and crushed by cold isostatic pressing (CIP) at a pressure of 90 MPa for 15 minutes.

[0085] The compressed spherical natural graphite was mixed with pitch, and the mixture was dry-heat-treated in an inert atmosphere at 1,250°C for 24 hours to form an amorphous carbon coating layer on the spherical natural graphite, which was used as the negative electrode active material of Example 1. The amorphous carbon coating layer was contained in the negative electrode active material at a content of 3 wt %.

[0086] (2) Manufacturing of the negative electrode The negative electrode active material, Super C65 as a conductive material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed in a weight ratio of 96.6:1.0:1.3:1.1, and water was added to prepare a negative electrode slurry. The negative electrode slurry was applied to a copper negative electrode current collector, vacuum dried at about 130°C for 10 hours, and rolled to form a negative electrode active material layer, which was used as the negative electrode of Example 1. The negative electrode loading was 3.61 mAh / cm. 2 It was manufactured to be.

[0087] Example 2: Preparation of negative electrode (1) Manufacturing of negative electrode active material The negative electrode active material of Example 2 was produced in the same manner as in Example 1, except that the pressure applied by CIP was 50 MPa.

[0088] (2) Manufacturing of the negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the negative electrode active material prepared above was used.

[0089] Example 3: Preparation of negative electrode (1) Manufacturing of negative electrode active material The negative electrode active material of Example 3 was produced in the same manner as in Example 1, except that the pressure applied by CIP was 175 MPa.

[0090] (2) Manufacturing of the negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the negative electrode active material prepared above was used.

[0091] Example 4: Preparation of negative electrode (1) Manufacturing of negative electrode active material The negative electrode active material of Example 4 was prepared in the same manner as in Example 1, except that the weight ratio of the spherical natural graphite and the pitch was adjusted so that the amorphous carbon coating layer was contained in the negative electrode active material at 10 wt %.

[0092] (2) Manufacturing of the negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the negative electrode active material prepared above was used.

[0093] Comparative Example 1: Production of negative electrode (1) Manufacturing of negative electrode active material The negative electrode active material of Comparative Example 1 was produced in the same manner as in Example 1, except that pressure was not applied by CIP.

[0094] (2) Manufacturing of the negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the negative electrode active material prepared above was used.

[0095] Comparative Example 2: Production of negative electrode (1) Manufacturing of negative electrode active material The negative electrode active material of Comparative Example 2 was produced in the same manner as in Example 1, except that the pressure applied during CIP was set to 375 MPa.

[0096] (2) Manufacturing of the negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the negative electrode active material prepared above was used.

[0097] The properties of the negative electrodes and negative electrode active materials of Examples 1 to 4 and Comparative Examples 1 and 2 prepared above were evaluated as shown in Table 1 below.

[0098] [Table 1]

[0099] The particle strength, compressive fracture strength, and rolled density of the negative electrode active material during plastic deformation, as well as the pore resistance and porosity of the negative electrode, were measured by the following methods.

[0100] (1) Particle strength during plastic deformation The particle strength of the negative electrode active materials of the examples and comparative examples during plastic deformation was measured using a powder resistivity tester (instrument name: PI88 SEM Picoindenter, manufacturer: HYSITRON). Specifically, a load was applied to the negative electrode active material, a stress-strain curve was plotted, and the particle strength was measured at the point where the material changed from the elastic region to the plastic region, i.e., when only the strain changed without a change in stress.

[0101] (2) Compression fracture strength The compressive fracture strength of the negative electrode active materials of the examples and comparative examples was measured using a powder resistivity tester (instrument name: PI88 SEM Picoindenter, manufacturer: HYSITRON). Specifically, a load was applied to the negative electrode active material, a stress-strain curve was plotted, and the maximum compressive stress that could be applied without breaking the negative electrode active material was measured.

[0102] (3) Pore resistance The negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were used as the working and counter electrodes, and a polyethylene separator was interposed between the working and counter electrodes to prepare an electrode assembly. An electrolyte solution prepared by dissolving 1M LiPF6 in a solvent containing ethylene carbonate (EC) and diethylene carbonate (EMC) in a volume ratio of 1:4 was poured into the electrode assembly to prepare a control cell.

[0103] The symmetric cell was measured in an electrochemical impedance analyzer over a frequency range of 10 6 The frequency was set from 0.05 Hz to 0.05 Hz, the impedance was measured, and the electrolyte resistance and the pore resistance were separated to measure the pore resistance.

[0104] (4) Porosity The porosity of the negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 and 2 was calculated using the following mathematical formula 1.

[0105] [Mathematical formula 1] Porosity of negative electrode (%) = {1 - (electrode density of negative electrode / true density of negative electrode)} × 100

[0106] In Equation 1, the true density of the negative electrode is the density of the negative electrode active material layer measured when a negative electrode is cut into a predetermined size and pressed in a press until the thickness of the negative electrode no longer changes, and the electrode density of the negative electrode is the density of the negative electrode active material layer measured when a negative electrode is cut into a predetermined size.

[0107] Experimental example <Manufacturing coin-shaped half cells> The negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were punched to the size of coin cells. A polyolefin separator was then placed between the negative electrodes and lithium foil, which served as the counter electrode. An electrolyte solution in which 1M LiPF6 was dissolved in a solvent made by mixing ethylene carbonate (EC) and ethyl methyl carbonate (DEC) in a volume ratio of 50:50 was then injected to prepare coin-shaped half cells for Examples 1 to 4 and Comparative Examples 1 and 2.

[0108] Experimental example 1: Evaluation of output characteristics The coin-type half cells of Examples 1 to 4 and Comparative Examples 1 and 2 were cycle-charged and discharged at a high current for 50 cycles, and the capacity retention rate was measured to evaluate the output characteristics.

[0109] Specifically, the coin-type half-cells of Examples 1 to 4 and Comparative Examples 1 and 2 were charged and discharged up to the 50th cycle under the conditions of charge (CC / CV mode, 2C charge, cut-off at 0.005V and 0.005C) and discharge (CC mode, 2C discharge, cut-off at 1.5V).

[0110] The capacity retention rate after 50 cycles was evaluated using the following formula, and the results are shown in Table 2.

[0111] Capacity retention rate (%) = {(discharge capacity at 50th cycle) / (discharge capacity at first cycle)} × 100

[0112] [Table 2]

[0113] Referring to Table 2, it can be seen that the negative electrodes of Examples 1 to 4, which contain negative electrode active materials satisfying the particle strength during plastic deformation of the present invention, have superior output characteristics and life characteristics compared to the comparative examples, which do not.

[0114] Experimental Example 2: Evaluation of high-temperature storage characteristics For the coin-type half-cells of Examples 1 to 3 and Comparative Examples 1 and 2, the coin-type half-cells were charged to an SOC of 95% and stored at high temperature (60°C) for 4 weeks. The remaining capacity retention rate (%) was measured by the following method and is shown in Table 3 below.

[0115] <Remaining capacity maintenance rate (%)> After storing the lithium secondary battery at high temperature for 4 weeks, it was set at room temperature and then charged / discharged three times at 0.1 C. During the third cycle, the battery was discharged to 5% SOC to determine the discharge capacity. The remaining capacity retention rate was calculated as the ratio (%) of the discharge capacity to the charge capacity at the initial SOC of 95%.

[0116] [Table 3]

[0117] Referring to Table 3, it can be seen that the negative electrodes of Examples 1 to 3, which contain negative electrode active materials satisfying the particle strength during plastic deformation of the present invention, have superior high-temperature storage performance compared to the comparative example.

Claims

1. a negative electrode current collector; a negative electrode active material layer disposed on the negative electrode current collector and including a negative electrode active material, the negative electrode active material contains natural graphite particles, The particle strength of the negative electrode active material during plastic deformation is 40 MPa to 200 MPa; The pore resistance of the negative electrode is 15 Ω or less, The average particle size (D50) of the negative electrode active material is 18 μm to 20 μm, the negative electrode active material further comprises a carbon coating layer disposed on the natural graphite particles, The carbon coating layer is contained in the negative electrode active material in an amount of 2% by weight to 5% by weight.

2. 2. The negative electrode according to claim 1, wherein the negative electrode active material has a compressive fracture strength of 350 MPa to 1,000 MPa.

3. The negative electrode according to claim 1 , wherein the negative electrode active material is spherical.

4. The negative electrode according to claim 1 , wherein the carbon coating layer comprises amorphous carbon.

5. The negative electrode according to claim 1 , wherein the negative electrode has a pore resistance of 13Ω or less.

6. 6. The negative electrode according to claim 1, wherein the negative electrode active material layer has a porosity of 20% to 45%.

7. The negative electrode according to any one of claims 1 to 6, a positive electrode facing the negative electrode; a separator interposed between the negative electrode and the positive electrode; and an electrolyte.

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