Negative electrode for lithium-ion secondary battery

Abstract

The present invention relates to a negative electrode. The negative electrode comprises a combination of carbon-coated artificial graphite and uncoated artificial graphite as a negative electrode active material, which shows an effect of preventing deterioration caused by deformation of the carbon-coated artificial graphite during pressing by introducing the uncoated artificial graphite. The negative electrode well maintains electrochemical characteristics, such as preventing deterioration of the carbon-coated artificial graphite, and is thus suitable for manufacturing a fast-charging battery. Meanwhile, the negative electrode of the present invention has a double-layer structure, and the upper and lower layers of the electrode can have different negative electrode active material compositions, and in particular, the upper layer can have a higher content of carbon-coated artificial graphite. In this way, a negative electrode more suitable for fast-charging characteristics can be provided. In addition, since the negative electrode is manufactured to have a double-layer structure by controlling the binder content of the negative electrode, migration of the binder in the negative electrode can be prevented, and adhesion between the electrode and the current collector can be improved.

Description

TECHNICAL FIELD

[0001] This application claims priority to Korean Patent Application No. 10-2020-0131460, filed on October 12, 2021, in Korea. The present invention relates to a negative electrode for a lithium ion secondary battery and a secondary battery comprising the same. In particular, the present invention relates to a negative electrode for a battery that can be effectively used for fast charging and a secondary battery comprising the same. BACKGROUND

[0002] As technology develops and the demand for mobile devices increases, there is an increasing demand for secondary batteries that are rechargeable, miniaturizable, and provide high capacity. In addition, among such secondary batteries, lithium secondary batteries having high energy density and operating voltage have been commercialized and widely used.

[0003] A lithium secondary battery has a structure including an electrode assembly having a positive electrode and a negative electrode each including an active material coated on an electrode current collector, and a porous separator interposed between the two electrodes, and an electrolyte containing a lithium salt injected into the electrode assembly. The electrode is obtained by coating a slurry containing an active material, a binder, and a conductive material dispersed in a solvent on a current collector, and then drying and pressing.

[0004] In addition, the basic performance characteristics of a lithium secondary battery, such as capacity, output, and lifespan, are greatly influenced by the negative electrode material. In order to maximize the performance of the battery, the negative electrode active material needs to satisfy some requirements, including an electrochemical reaction potential close to lithium metal, high reversibility of reaction with lithium ions, and a high lithium ion diffusion rate in the active material. Graphite has been frequently used as a material that satisfies such requirements.

[0005] In order to satisfy various electrochemical characteristics of the battery, combinations of various types of graphite or negative electrodes having a multi-layer structure have been proposed. In the case of a conventional electrode having a multi-layer structure, each layer contains only one type of electrode active material, such as an upper layer and a lower layer containing different negative electrode active materials from each other (upper layer material a / lower layer material b), or an upper layer and a lower layer containing the same negative electrode active material (upper layer material a / lower layer material a, or upper layer material b / lower layer material b). However, in the case of a negative electrode containing a multi-layer stack, and each layer containing a single type of material, there are problems in that the fast charging characteristics are poor or the high temperature characteristics and capacity rapidly decrease. Another problem is that cracks are generated in the electrode layer while the electrode is being dried.

[0006] In particular, when artificial graphite is used as a negative active material in a high-rate (2C rate or more) battery, there is a problem of low electrical conductivity. In order to improve the electrical conductivity, a method of coating the surface of artificial graphite with carbon (soft carbon or hard carbon) has been considered. However, in this case, the electrode active material exhibits increased strength due to the carbon coating layer, so that the electrode active material can be deformed (increase in degree of orientation, or breakage of the active material) during the manufacturing process (pressing step) of the battery, causing problems of side reactions and deterioration of high-temperature characteristics.

[0007] Meanwhile, in particular, as the thickness of the electrode active material increases, the electrode binder migrates toward the top layer as the solvent evaporates while the electrode is being dried, so that the electrode binder is mainly distributed in the top layer portion, thereby causing a problem of poor adhesion between the electrode and the current collector. SUMMARY

[0008] [TECHNICAL PROBLEM]

[0009] The present application is designed to solve the problems of the related art, and therefore the present application aims to provide a negative electrode having a multi-layer structure, which includes carbon-coated artificial graphite and uncoated artificial graphite as a negative active material. The present application also aims to provide a lithium ion secondary battery including the same. These and other objects and advantages of the present application can be understood from the following detailed description, and will be apparent from the exemplary embodiments of the present application. Furthermore, it is to be understood that the objects and advantages of the present application can be implemented by means to be shown in the appended claims and combinations thereof.

[0010] [TECHNICAL SOLUTION]

[0011] According to a first embodiment of the present application, there is provided a negative electrode for a lithium ion secondary battery, which includes a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a lower layer formed on the surface of the current collector and an upper layer formed on the lower layer, each of the lower layer and the upper layer independently includes a negative electrode mixture containing a negative active material, a conductive material, and a binder, each of the upper layer and the lower layer independently includes a negative active material a and a negative active material b, the negative active material a is artificial graphite coated with a carbonaceous material on the surface, and the negative active material b is uncoated artificial graphite.

[0012] According to a second embodiment of the present application, there is provided the negative electrode for a lithium ion secondary battery defined in the first embodiment, wherein the content of the negative active material b in the upper layer is 40 to 60% by weight of the total amount of the negative active material a and the negative active material b.

[0013] According to a third embodiment of the present invention, there is provided the negative electrode for a lithium ion secondary battery defined in the first or second embodiment, wherein the content ratio of the binder in the negative electrode mixture of the lower layer is higher than the content ratio of the binder in the negative electrode mixture of the upper layer.

[0014] According to a fourth embodiment of the present invention, there is provided a negative electrode for a lithium ion secondary battery as defined in any one of the first to third embodiments, wherein the artificial graphite of the negative electrode active material a and the artificial graphite of the negative electrode active material b each independently have an orientation degree (I of the particles) of 3-25. 110 with I 004 ratio).

[0015] According to a fifth embodiment of the present invention, there is provided a negative electrode for a lithium ion secondary battery as defined in any one of the first to fourth embodiments, wherein the artificial graphite of the negative electrode active material a and the artificial graphite of the negative electrode active material b each independently have a carbon content of 0.5-5 m 2 / g specific surface area.

[0016] According to a sixth embodiment of the present invention, there is provided the negative electrode for a lithium ion secondary battery defined in any one of the first to fifth embodiments, wherein the carbonaceous material of the negative electrode active material a comprises a low-crystallinity carbonaceous material and / or an amorphous carbonaceous material.

[0017] According to a seventh embodiment of the present invention, a negative electrode for a lithium ion secondary battery as defined in any one of the first to sixth embodiments is provided, wherein the negative electrode active material a comprises artificial graphite and a carbon coating formed on the surface of the artificial graphite, and the carbon coating is present in an amount of 1-10 weight % based on 100 weight % of the negative electrode active material a.

[0018] According to an eighth embodiment of the present invention, there is provided the negative electrode for a lithium ion secondary battery as defined in any one of the first to seventh embodiments, wherein the upper layer and the lower layer contain the same negative electrode active material a and the same negative electrode active material b.

[0019] According to a ninth embodiment of the present invention, a method for preparing a negative electrode as defined in any one of the first to eighth embodiments is provided, comprising: separately preparing a first negative electrode slurry containing a lower negative electrode mixture and a second negative electrode slurry containing an upper negative electrode mixture; applying the first negative electrode slurry and the second negative electrode slurry to a negative electrode current collector sequentially or simultaneously, and then drying.

[0020] According to a tenth embodiment of the present application, there is provided a secondary battery including the negative electrode defined in any one of the first to eighth embodiments, a positive electrode including a lithium cobalt oxide (LCO) or a lithium nickel cobalt manganese oxide (NCM), an electrolyte having an ionic conductivity of 6.5 mS / cm or more and containing a lithium salt at a concentration of 0.8-1.4 M, and a separator made of a polyethylene porous film (thickness: 3-15 μm) and optionally provided with an inorganic coating.

[0021] [Advantageous Effects]

[0022] The negative electrode of the present application contains a combination of carbon-coated artificial graphite and uncoated artificial graphite as a negative electrode active material, and thus exhibits an effect of preventing deterioration caused by deformation of the carbon-coated artificial graphite occurring at the time of pressing by introducing the uncoated artificial graphite.

[0023] The negative electrode well maintains electrochemical characteristics, such as preventing deterioration of the carbon-coated artificial graphite, and thus is suitable for manufacturing a rapid charging battery.

[0024] Meanwhile, the negative electrode of the present application has a double-layer structure in which the upper and lower layers of the electrode can have different negative electrode active material compositions, and in particular, the upper layer can have a higher content of carbon-coated artificial graphite. Thus, a negative electrode more suitable for rapid charging characteristics can be provided.

[0025] In addition, since the negative electrode is manufactured to have a double-layer structure by controlling the binder content of the negative electrode, migration of the binder in the negative electrode can be prevented, and adhesion between the electrode and the current collector can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0026] The accompanying drawings illustrate preferred embodiments of the present application and, together with the foregoing disclosure, serve to provide further understanding of the technical features of the present application, and therefore, the present application should not be construed as being limited to the accompanying drawings. Meanwhile, in order to provide a clearer description, the shape, size, scale, or ratio of certain constituent elements in the accompanying drawings can be exaggerated.

[0027] Figures 1 to 3 is a graph showing the test results of the capacity retention rate and the volume expansion rate according to the temperature and the C rate of Examples and Comparative Examples 1-3. DETAILED DESCRIPTION

[0028] Hereinafter, preferred embodiments of the present application will be described in detail with reference to the accompanying drawings. Before the description, it is to be understood that the terms used in the specification and the appended claims should not be construed as being limited to general and dictionary meanings and should be construed as having meanings and concepts meeting the technical aspects of the present application based on the principle that the inventor is allowed to define terms appropriately for best explaining the invention.

[0029] The invention relates to a negative electrode for a lithium ion secondary battery.

[0030] In one aspect of the present invention, there is provided a negative electrode for a lithium ion secondary battery, comprising a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector. The negative electrode active material layer comprises a lower layer formed on the surface of the current collector and an upper layer formed on the lower layer. The lower layer and the upper layer each independently comprise a negative electrode mixture (an upper negative electrode mixture and a lower negative electrode mixture, respectively), the negative electrode mixture containing a negative electrode active material, a conductive material and a binder, the upper layer and the lower layer each independently comprise a negative electrode active material a and a negative electrode active material b, the negative electrode active material b comprises artificial graphite, and the negative electrode active material a comprises artificial graphite and a carbon coating formed on the surface of the artificial graphite. The negative electrode active material b does not contain a carbon coating.

[0031] Here, generally speaking, artificial graphite can be prepared through a graphitization process, which involves sintering raw materials such as coal tar, coal tar pitch, and petroleum-based heavy oil at temperatures above 2500°C. After this graphitization, the resulting product is subjected to particle size adjustment, such as pulverization or secondary particle molding, so that it can be used as a negative electrode active material.

[0032] Generally speaking, artificial graphite consists of crystals randomly distributed within the particles, which have a lower sphericity and a slightly pointed shape compared to natural graphite. This artificial graphite can be provided in powder, flake, block, plate, or rod form, but preferably has crystallites with isotropic orientation to minimize lithium ion migration distance, thereby improving output characteristics. For this reason, artificial graphite can be in the form of flakes and / or plates.

[0033] In addition, the artificial graphite used herein may have an orientation degree (I of the particles) of 3-25. 110 with I 004 When the orientation degree of artificial graphite is less than 3, the particles contain excessive voids, resulting in a decrease in capacity per unit volume and an increase in irreversible capacity. When the orientation degree is greater than 25, the artificial graphite exhibits a large volume change during charge / discharge, which may undesirably degrade lifespan characteristics. According to one embodiment of the present invention, the orientation degree may be 12-25 to appropriately control porosity and irreversible capacity.

[0034] Here, the degree of orientation can be determined by the peak intensity ratio of the (110) plane to the (004) plane using X-ray diffraction (XRD). 004 Diffraction occurs on the faces stacked in the C-axis direction (longitudinal direction) of graphite, and as the diffraction increases, a higher and broader peak is formed. 100Corresponding to the A-axis direction (lateral direction). Here, the degree of orientation is evaluated as the area ratio of the two peaks. The method of measuring the degree of orientation of graphite is known to those skilled in the art, and such known method can be employed here.

[0035] According to one embodiment of the present application, the X-ray diffraction method can be performed by an X-ray diffractometer Bruker D4 Endeavor using Cu-Ka rays. At the same time, the measured values can be calibrated, if necessary, using the Topas 3 fitting program.

[0036] In particular, the XRD is performed under the following conditions.

[0037] - Objective: Cu (Kα-rays) graphite monochromator

[0038] - Slits: divergence slit = 1°, convergence slit = 0.1 mm, scatter slit = 1°

[0039] - Measurement area and step angle / measuring time:

[0040] (110) plane: 76.5° < 2Θ < 78.5°, 0.01° / 3 sec

[0041] (004) plane: 53.5° < 2Θ < 56.0°, 0.01° / 3 sec, where 2Θ denotes the diffraction angle. The above XRD analysis is a typical example, and other methods can also be used. In this way, the degree of orientation can be measured.

[0042] This will apply to the measurement of the degree of orientation of the natural graphite described below.

[0043] The artificial graphite used according to one embodiment of the present application includes commercially available mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers (MPCF), block graphitized artificial graphite, powdered graphitized artificial graphite, etc., and can be artificial graphite having a sphericity of 0.91 or less, preferably 0.6 to 0.91, more preferably 0.7 to 0.9. In addition, the particle diameter of the artificial graphite is 5 to 30 μm, preferably 10 to 25 μm.

[0044] The specific surface area of the artificial graphite can be 0.5 to 5 m 2 / g, in particular 0.6 to 4 m 2 / g. Preferably, within the above range, the specific surface area of the artificial graphite is smaller than that of the natural graphite. When the specific surface area of the artificial graphite exceeds the above range and is too small, the output characteristics during charging / discharging can decrease. On the other hand, when the specific surface area is too large, the initial efficiency can undesirably decrease.

[0045] The specific surface area of the artificial graphite can be determined by the BET (Brunauer-Emmett-Teller) method. For example, the specific surface area can be determined by the BET 6-point method based on nitrogen adsorption flow using a porosimetry analyzer (e.g., Belsorp-II mini, Bell Japan Inc.). This will apply to the determination of the specific surface area of the natural graphite described below.

[0046] The tap density of the artificial graphite can be 0.7-1.15 g / cc, particularly 0.8-1.1 g / cc. When the tap density exceeds the range defined above and is less than 0.7 g / cc, the contact area between the particles is insufficient, resulting in poor adhesion and a decrease in capacity per unit volume. When the tap density is greater than 1.15 g / cc, the tortuosity of the electrode and the wettability of the electrolyte solution can decrease, undesirably resulting in a deterioration of the output characteristics during charging / discharging.

[0047] Here, the tap density can be determined by introducing 50 g of the precursor into a 100 cc cylinder so as to be struck with a tester SEISHIN (KYT-4000) using an instrument IV-1000 obtained from COPLEY Co., and applying 3000 strokes of striking thereto. This will apply to the determination of the tap density of the natural graphite described below.

[0048] In addition, the average particle diameter (D 50 ) of the artificial graphite can be 8-30 μm, particularly 12-25 μm. When the average particle diameter (D 50 ) of the artificial graphite is less than 8 μm, the specific surface area thereof increases, resulting in a decrease in the initial efficiency of the secondary battery, making the performance of the battery poor. When the average particle diameter (D 50 ) is greater than 30 μm, the adhesion can be poor, and the bulk density can decrease, resulting in a decrease in capacity.

[0049] For example, the average particle diameter of the artificial graphite can be determined by the laser diffraction method. The laser diffraction method generally allows the determination of the particle diameter ranging from the sub-micron region to several millimeters, and provides a result with high reproducibility and high resolution. The average particle diameter (D 50 ) of the artificial graphite can be defined as the particle diameter at the point of 50% in the particle diameter distribution. For example, the average particle diameter (D 50 ) of the artificial graphite can be determined by dispersing the artificial graphite in an ethanol / water solution, introducing the resultant into a commercially available laser diffraction particle diameter analyzer (e.g., Microtrac MT 3000), irradiating the same with ultrasonic waves at an output of 60 W and a frequency of about 28 kHz, and calculating the average particle diameter (D 50 ) at the point of 50% in the particle diameter distribution determined by the analyzer.

[0050] According to the present application, the negative electrode active material b can be selected without particular limitation as long as it is artificial graphite having the above-mentioned properties.

[0051] Meanwhile, the negative electrode active material a has a core-shell structure having a core particle which is artificial graphite having the above-mentioned properties and a coating layer at least partially or completely covering the surface of the core particle, wherein the coating layer contains a carbonaceous material.

[0052] According to one embodiment of the present application, the coating layer can contain a low-crystallinity carbonaceous material and / or an amorphous carbonaceous material. According to one embodiment of the present application, the content of the low-crystallinity and / or amorphous carbonaceous material can be 70% by weight or more, 80% by weight or more, or 90% by weight or more, based on the total weight of the coating layer. The area covered by the coating layer can be 70% or more, 80% or more, or 90% or more, based on the surface area of the artificial graphite particle. In addition, according to one embodiment of the present application, the thickness of the coating layer can be 5-1000 nm, and the thickness can be appropriately controlled within the range defined above.

[0053] According to one embodiment of the present application, the low-crystallinity carbonaceous material can include at least one of soft carbon and / or a product having a low-crystallinity structure obtained by heat-treating soft carbon at a temperature of about 1000°C or less. Meanwhile, the amorphous carbonaceous material can include at least one selected from the group consisting of hard carbon, carbon black, thermal black, and acetylene black.

[0054] According to one embodiment of the present application, the coating layer can be formed by coating the artificial graphite particle with a carbon precursor material such as a polymer material or pitch, and heat-treating (carbonizing) the resulting product at a temperature of about 500-1000°C. Here, when the carbonization temperature is too high, it can affect the crystal structure of the artificial graphite. Therefore, the carbonization temperature can be controlled within the range defined above. According to another embodiment of the present application, the coating layer can be formed by directly coating a conductive carbon particle on the surface of the artificial graphite particle. Specific examples of such a conductive carbon particle include carbon black (e.g., acetylene black, thermal black, furnace black, or channel black), carbon fiber, or carbon nanotube, etc. However, the scope of the present application is not limited thereto.

[0055] Meanwhile, according to one embodiment of the present application, the upper layer contains the negative electrode active material a and the negative electrode active material b, wherein the content of the negative electrode active material b can be 40-60% by weight, based on 100% by weight of the total amount of the negative electrode active material a and the negative electrode active material b. In addition, the lower layer contains the negative electrode active material a and the negative electrode active material b, wherein the content of the negative electrode active material b can be 40-60% by weight, based on 100% by weight of the total amount of the negative electrode active material a and the negative electrode active material b.

[0056] Meanwhile, according to one embodiment of the present application, the upper layer and the lower layer can contain the same negative active material a and the same negative active material b.

[0057] Meanwhile, according to one embodiment of the present application, the carbon coating can be present in an amount of 1 to 10% by weight, based on 100% by weight of the negative active material a. For example, the carbon coating can be present in an amount of 2 to 6% by weight.

[0058] Meanwhile, according to one embodiment of the present application, the content of the carbon coating in the upper layer depends on the content of the negative active material a. For example, the content of the carbon coating can be 1 to 5% by weight, based on 100% by weight of the upper layer negative active material. In addition, the content of the carbon coating in the lower layer can be 1 to 4% by weight, based on 100% by weight of the lower layer negative active material. When the proportion of the coated negative active material in the upper layer increases, the electrical conductivity increases, which is advantageous for fast charging.

[0059] Meanwhile, according to one embodiment of the present application, the proportion of the content of the binder in the negative mixture of the lower layer can be higher than the proportion of the content of the binder in the negative mixture of the upper layer. For example, the lower layer can contain the binder in an amount of 2.4 to 3% by weight, based on 100% by weight of the lower layer negative mixture, and the content of the binder in the upper layer is designed so that it can be less than the content of the binder in the lower layer.

[0060] In the process of manufacturing the electrode, when the solvent migrates to the surface of the electrode and evaporates upon drying the electrode slurry, the binder resin migrates to the surface of the electrode along with the solvent, so that the binder resin can be located in the surface portion of the electrode. According to the present application, the ratio of the binder content of the upper layer to the lower layer is designed in the above-described manner, thus preventing such localization of the binder resin in the top layer of the electrode, thereby reducing the charge transfer resistance (Rct) on the surface of the electrode, and the binder resin remains in the lower layer, thereby preventing the adhesion between the electrode active material layer and the current collector from deteriorating.

[0061] Meanwhile, according to the present application, the negative electrode can be obtained by forming the lower layer on the negative electrode current collector and forming the upper layer on the lower layer. The method of forming the lower layer and the upper layer can include a dry-on-wet method or a wet-on-wet method. The dry-on-wet method includes applying a first negative electrode slurry containing a lower layer negative mixture to the current collector, then drying, and applying a second negative electrode slurry containing an upper layer negative mixture, then drying. The wet-on-wet method includes applying a first negative electrode slurry, applying a second negative electrode slurry before drying the first negative electrode slurry, and performing a drying step for the lower layer and the upper layer simultaneously. For example, according to one embodiment of the present application, the negative electrode can be obtained by the wet-on-wet method. In particular, the two types of slurry can be coated simultaneously using a double slot die or the like, and drying is performed, thereby forming the lower layer and the upper layer of the negative active material layer.

[0062] The method of coating the slurry is not particularly limited as long as it is conventionally used in the art. For example, a coating method using a slit die, a Mayer bar coating method, a gravure coating method, a dip coating method, a spray coating method, or the like can be used.

[0063] In the method of one embodiment of the present application, the current collector is not particularly limited as long as it has conductivity while not causing any chemical change in the corresponding battery. For example, copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy, or the like can be used.

[0064] Although the thickness of the current collector is not particularly limited, it can be currently used in a range of 3 to 500 μm.

[0065] Specific examples of the binder include various types of polymers such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylate, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and the like.

[0066] The solvent can include N-methyl-2-pyrrolidone, acetone, or water, and the like.

[0067] The conductive material is not particularly limited as long as it does not cause a chemical change in the corresponding battery and has conductivity. Specific examples of the conductive material include carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal cracking black; conductive fibers such as carbon fibers or metal fibers; SWCNT, MWCNT; fluorocarbons; metal powders such as aluminum or nickel powders; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives; and the like.

[0068] According to one embodiment of the present application, each negative electrode active material layer (i.e., the upper layer and the lower layer) can further include a thickening agent, if necessary. Specific examples of the thickening agent can include at least one selected from carboxymethyl cellulose (CMC), carboxyethyl cellulose, or polyvinylpyrrolidone, and the like.

[0069] Meanwhile, according to one embodiment of the present application, a pressing step can be further performed on the dried negative electrode. The pressing step can be performed using a method conventionally used in the art (e.g., roll pressing). Meanwhile, the pressing step can be performed under heating.

[0070] In another aspect of the present application, a lithium secondary battery including the negative electrode obtained as described above is provided. In particular, the lithium secondary battery can be obtained by injecting an electrolyte containing a lithium salt into an electrode assembly including a positive electrode, the negative electrode as described above, and a separator interposed between the two electrodes.

[0071] The positive electrode can be obtained by mixing a positive electrode active material, a conductive material, a binder, and a solvent to form a slurry, and directly coating the slurry on a metal current collector, or casting the slurry on a separate support, peeling off a positive electrode active material film from the support, and laminating the film on the metal current collector.

[0072] The positive electrode active material used in the positive electrode active material layer can be any one active material particle selected from the group consisting of LiCoO2, LiNiO2, LiMn2O4, LiCoPO4, LiFePO4, and LiNi 1-x-y-z Co x M1 y M2 z O2(wherein, M1and M2each independently represent any one selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, x, y, and z each independently represent an atomic ratio of the elements forming the oxide, and 0≤x<0.5, 0≤y<0.5, 0≤z<0.5, 0<x+y+z≤1), or a mixture of at least two thereof.

[0073] Meanwhile, the same conductive material, binder, and solvent as used in the manufacture of the negative electrode can be used.

[0074] The separator can be a conventional porous polymer film conventionally used as a separator. For example, the porous polymer film can be a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene-butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer. Such a porous polymer film can be used alone or in the form of a laminate. In addition, an insulating film having high ion permeability and mechanical strength can be used. The separator can include a safety-enhanced separator (SRS) containing a ceramic material coated to a small thickness on the surface of the separator. In addition, a conventional porous nonwoven fabric web such as a nonwoven fabric web made of high-melting-point glass fibers or polyethylene terephthalate fibers can be used, but the scope of the present application is not limited thereto.

[0075] The electrolyte has an ionic conductivity of 6.5 mS / cm or more, and includes a lithium salt as an electrolyte salt and an organic solvent for dissolving the lithium salt.

[0076] Any lithium salt conventionally used for secondary battery electrolytes can be used without particular limitation. For example, the anion of the lithium salt can be any one selected from the group consisting of F - , Cl - , Br - , 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 - . According to one embodiment of the present application, the concentration of the lithium salt in the electrolyte can be 0.8-1.4 M.

[0077] The organic solvent contained in the electrolyte can be any organic solvent conventionally used without particular limitation. Typical examples of the organic solvent include at least one selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran.

[0078] In particular, among carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are organic solvents having high viscosity and high dielectric constant, and can be preferably used since they can easily dissociate lithium salts in electrolytes. More preferably, when such cyclic carbonates are used after being mixed with linear carbonates (e.g., dimethyl carbonate or diethyl carbonate) having low viscosity and low dielectric constant, electrolytes having higher electrical conductivity can be prepared.

[0079] Optionally, the electrolyte used according to the present application can further include additives contained in conventional electrolytes, such as overcharge-preventing agents, etc.

[0080] The lithium secondary battery of the embodiments of the present application can be obtained by placing a separator between a positive electrode and a negative electrode to form an electrode assembly, introducing the electrode assembly into a pouch, a cylindrical battery case, or a prismatic battery case, and then injecting an electrolyte thereinto. In one variant, the lithium secondary battery can be obtained by stacking electrode assemblies, impregnating the stack with an electrolyte, and introducing the resulting product into a battery case, and then sealing.

[0081] According to one embodiment of the present application, the lithium secondary battery can be a stacked type, a wound type, a stacked-folded type, or a cable type battery.

[0082] The lithium secondary battery of the present application can be used for a battery cell used as a power source for a compact device, and can be preferably used as a unit cell for a medium- or large-sized battery module including a plurality of battery cells. Specific examples of such medium- or large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, or power storage systems, etc. In particular, the lithium secondary battery can be used for a hybrid electric vehicle battery requiring high output, as well as a new energy and renewable energy storage battery.

[0083] Embodiments will be described more fully below in an effort to satisfy the patent statutes. These embodiments may, however, be implemented in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the application to those skilled in the art.

[0084] (1) Example 1

[0085] 1) Preparation of negative active material a

[0086] Preparation of D 50 was 15 μm, and the specific surface area was 0.9 m 2 / g and an orientation degree of 13, and coated with pitch, and carbonized at 1100-1300°C to form a coating layer of carbonaceous material on the surface of the artificial graphite. The thickness of the coating layer was about 800 nm, and the content of the coating layer was about 4% by weight based on 100% by weight of the negative electrode active material particles a.

[0087] 2) Preparation of negative electrode active material b

[0088] Preparation D 50 was 18 μm, the specific surface area was 1.3 m 2 / g and an orientation degree of 18.1.

[0089] 3) Preparation of negative electrode

[0090] Preparation of the slurry for the lower layer

[0091] The negative electrode active material, Super C65 as the conductive material, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickening agent were introduced into distilled water in a weight ratio of 95.35:0.5:3.0:1.15 to obtain a slurry for the lower layer. In the negative electrode active material, the negative electrode active material a and the negative electrode active material b were mixed in a weight ratio of 50:50. At the same time, the total content of the carbon coating layer in the lower layer was set to 2% by weight based on 100% by weight of the negative electrode active material for the lower layer.

[0092] Preparation of the slurry for the upper layer

[0093] The negative electrode active material, Super C65 as the conductive material, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickening agent were introduced into distilled water in a weight ratio of 97.35:0.5:1.0:1.15 to obtain a slurry for the upper layer. In the negative electrode active material, the negative electrode active material a and the negative electrode active material b were mixed in a weight ratio of 50:50. At the same time, the total content of the carbon coating layer in the upper layer was set to 2% by weight based on 100% by weight of the negative electrode active material for the upper layer.

[0094] Then, the slurry for the lower layer was applied to the negative electrode current collector (copper foil, thickness 8 μm), the slurry for the upper layer was applied immediately after the application of the slurry for the lower layer, and the slurry was dried with hot air in a drier to obtain an electrode. The drier was controlled in a temperature range of about 120-130°C. The thickness of the obtained electrode was 150 μm, in which the thickness ratio of the lower layer was about 50%. In addition, the negative electrode had a porosity of 25.2% by volume and an orientation degree of 10.7.

[0095] 4) Comparative Example 1

[0096] The negative active material b, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickening agent were introduced into distilled water in a weight ratio of 95.35:0.5:3.0:1.15 to obtain a slurry for the lower layer. In addition, the negative active material a, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickening agent were introduced into distilled water in a weight ratio of 97.35:0.5:1.0:1.15 to obtain a slurry for the upper layer. Then, the lower layer slurry was applied to a negative current collector (copper foil, thickness 8 μm), the upper layer slurry was applied thereto immediately after the lower layer slurry was applied, and the slurry was dried with hot air in a drier to obtain an electrode. The drier was controlled in a temperature range of about 120-130°C. The thickness of the resulting electrode was 150 μm, in which the thickness ratio of the lower layer was about 50%. In addition, the negative electrode had a porosity of 25.2 vol% and an orientation degree of 10.7.

[0097] 5) Comparative Example 2

[0098] The spherical natural graphite (D 50 was 9 μm, and the specific surface area was 2.1 m 2 Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickening agent were introduced into distilled water in a weight ratio of 95.35:0.5:3.0:1.15 to obtain a slurry for the lower layer. In addition, the negative active material a, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickening agent were introduced into distilled water in a weight ratio of 97.35:0.5:1.0:1.15 to obtain a slurry for the upper layer. Then, the lower layer slurry was applied to a negative current collector (copper foil, thickness 8 μm), the upper layer slurry was applied thereto immediately after the lower layer slurry was applied, and the slurry was dried with hot air in a drier to obtain an electrode. The drier was controlled in a temperature range of about 120-130°C. The thickness of the resulting electrode was 150 μm, in which the thickness ratio of the lower layer was about 50%. In addition, the negative electrode had a porosity of 25.2 vol% and an orientation degree of 10.7.

[0099] 6) Comparative Example 3

[0100] The negative active material a, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickening agent were introduced into distilled water in a weight ratio of 95.35:0.5:3.0:1.15 to obtain a slurry for the lower layer. In addition, the negative active material a, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickening agent were introduced into distilled water in a weight ratio of 97.35:0.5:1.0:1.15 to obtain a slurry for the upper layer. Then, the lower layer slurry was applied to a negative current collector (copper foil, thickness 8 μm), the upper layer slurry was applied thereto immediately after the lower layer slurry was applied, and the slurry was dried with hot air in a drier to obtain an electrode. The drier was controlled at a temperature range of about 120-130°C. The thickness of the resulting electrode was 150 μm, in which the thickness ratio of the lower layer was about 50%. In addition, the negative electrode had a porosity of 25.2 vol% and an orientation degree of 10.7.

[0101] (2) Manufacture of battery

[0102] A battery was obtained using each negative electrode prepared according to Example 1 and Comparative Examples 1-3.

[0103] A positive electrode was prepared as follows.

[0104] A positive electrode active material layer forming slurry (solid content 70 wt%) was prepared by introducing a positive electrode active material LiCoO2, a binder (PVDF), and a conductive material (acetylene black) into NMP in a weight ratio of 96.5:1.5:2. The slurry was applied to an aluminum foil (thickness about 10 μm) and dried at 60°C for 6 hours to obtain a positive electrode.

[0105] A porous film (10 μm) made of polyethylene was prepared as a separator, and a positive electrode, a separator, and a negative electrode were sequentially stacked, and a lamination process of pressing the resulting stack at 80°C was performed to obtain an electrode assembly.

[0106] Then, the electrode assembly was introduced into a pouch-shaped battery case, and an electrolyte solution was injected thereinto to obtain a battery. The electrolyte solution was prepared by mixing ethylene carbonate, propylene carbonate, ethyl propionate, and propyl propionate in a weight ratio of 2:1:2.5:4.5, and introducing LiPF6 at a concentration of 1.4 M.

[0107] (3) Evaluation of capacity retention rate and swelling rate

[0108] 1) 1.5 C room temperature cycling

[0109] Each battery of Example 1 and Comparative Examples 1-3 was charged to 4.45V at 1.5C in constant current (CC) mode, and charged to a charge cutoff current of 0.005C in constant voltage (CV) mode, and then discharged to 3V at 1C in CC mode. In this way, 1000 charge / discharge cycles were repeated, and then the capacity retention rate was evaluated. The test was carried out at room temperature (25°C). The results are shown in FIG. Figure 1 The battery of Example 1 showed a higher capacity retention rate than the other batteries of Comparative Examples 1 to 3.

[0110] Meanwhile, after measuring the volume expansion ratio of each battery, it can be seen that the battery of Example 1 shows a smaller volume expansion ratio than the other batteries of Comparative Examples 1-3.

[0111] 2) 1.5°C high temperature cycle

[0112] Each battery of Example 1 and Comparative Examples 1-3 was charged to 4.45V at 1.5C in constant current (CC) mode, and charged to a charge cutoff current of 0.005C in constant voltage (CV) mode, and then discharged to 3V at 1C in CC mode. In this way, 700 charge / discharge cycles were repeated, and then the capacity retention was evaluated. The test was carried out at high temperature (45°C). The results are shown in FIG. Figure 2 The battery of Example 1 exhibited a higher capacity retention rate than the other batteries of Comparative Examples 1 to 3. Furthermore, after measuring the volume expansion rate of each battery, it was found that the battery of Example 1 exhibited a smaller volume expansion rate than the other batteries of Comparative Examples 1 to 3.

[0113] 3) 2.0C cycle

[0114] Each battery of Example 1 and Comparative Examples 1-3 was charged to 4.45V at 2.0C in constant current (CC) mode, and charged to a cutoff current of 0.005C in constant voltage (CV) mode, and then discharged to 3V at 1C in CC mode. In this way, 1000 charge / discharge cycles were repeated, and then the capacity retention was evaluated. The test was carried out at room temperature (25°C). The results are shown in FIG. Figure 3 The battery of Example 1 exhibited a higher capacity retention rate than the other batteries of Comparative Examples 1 to 3. Furthermore, after measuring the volume expansion rate of each battery, it was found that the battery of Example 1 exhibited a smaller volume expansion rate than the other batteries of Comparative Examples 1 to 3.

Claims

1. A negative electrode for a lithium ion secondary battery, comprising a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, in, The negative electrode active material layer includes a lower layer formed on the surface of the current collector and an upper layer formed on the lower layer, wherein the lower layer and the upper layer each independently contain a negative electrode mixture containing a negative electrode active material, a conductive material and a binder, and The upper layer and the lower layer each independently contain a negative electrode active material a and a negative electrode active material b, the negative electrode active material a is artificial graphite coated with a carbonaceous material, and the negative electrode active material b is uncoated artificial graphite; Wherein, in the lower layer, the content of the negative electrode active material b is 40-60 wt % based on 100 wt % of the total amount of the negative electrode active material a and the negative electrode active material b in the lower layer.

2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The content of the negative electrode active material b in the upper layer is 40 wt % to 60 wt % of the total amount of the negative electrode active material a and the negative electrode active material b in the upper layer.

3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The content ratio of the binder in the negative electrode mixture of the lower layer is higher than the content ratio of the binder in the negative electrode mixture of the upper layer.

4. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The artificial graphite of the negative electrode active material a and the artificial graphite of the negative electrode active material b each independently have an orientation degree of 3 to 25, where the orientation degree is the particle size. 004 with I 110 The ratio.

5. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The artificial graphite of the negative electrode active material a and the artificial graphite of the negative electrode active material b each independently have an orientation degree of 12 to 25, where the orientation degree is the particle size. 004 with I 110 The ratio.

6. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The artificial graphite of the negative electrode active material a and the artificial graphite of the negative electrode active material b each independently have a particle size of 0.5 to 5 m 2 / g specific surface area.

7. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The carbonaceous material of the negative electrode active material a includes a low-crystallinity carbonaceous material and / or an amorphous carbonaceous material.

8. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The negative electrode active material a includes artificial graphite and a carbon coating layer formed on a surface of the artificial graphite, and the carbon coating layer is present in an amount of 1 wt % to 10 wt % based on 100 wt % of the negative electrode active material a.

9. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The upper layer and the lower layer contain the same negative electrode active material a and the same negative electrode active material b.

10. A method for preparing the negative electrode according to claim 1, comprising: A first negative electrode slurry containing a lower negative electrode mixture and a second negative electrode slurry containing an upper negative electrode mixture are prepared respectively; the first negative electrode slurry and the second negative electrode slurry are applied to a negative electrode current collector sequentially or simultaneously, and then dried.

11. A secondary battery comprising the negative electrode according to claim 1, comprising a positive electrode comprising lithium cobalt oxide (LCO) or lithium nickel cobalt manganese oxide (NCM), using an electrolyte having an ionic conductivity of 6.5 mS / cm or more and containing a lithium salt at a concentration of 0.8-1.4 M, and comprising a separator made of a polyethylene porous film and optionally provided with an inorganic coating.

12. The secondary battery according to claim 11, wherein The thickness of the polyethylene porous membrane is 3-15 μm.

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