Negative electrode active material and secondary battery containing the same

Abstract

To provide a negative-electrode active material that is dynamically balanced and the life property of which is improved, and a secondary cell including the same.SOLUTION: A negative-electrode active material includes large-grain-size artificial graphite and small-grain-size natural graphite, and an average grain size ratio between a small grain size and a large grain size is in the range of 1:1.5 to 1:5. A secondary cell includes the negative-electrode active material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material and a secondary battery including the same, and more particularly to a negative electrode active material having improved life characteristics and a secondary battery including the same.

[0002] This application claims priority based on Korean Patent Application No. 10-2016-0015158, filed on February 5, 2016, and the entire contents of the specification and drawings of that application are incorporated herein by reference. [Background technology]

[0003] Factors that affect the lifespan characteristics of a battery can be divided into intrinsic property factors of the material and design factors. Intrinsic property factors are the intrinsic properties of the core materials that make up the battery (positive electrode, negative electrode, separator, and electrolyte), while design factors are related to the cell design, and particularly include the design balance between the positive electrode and negative electrode.

[0004] If the battery life is poor due to the core materials that make up the battery, recovery is impossible. However, as the temperature of the battery rises, the degradation of the materials accelerates, causing the life characteristics to deteriorate even more rapidly. In such cases, the fundamental characteristics of the materials must be improved. Meanwhile, deterioration of life due to design factors is mostly caused by a failure to achieve a thermodynamic or electrochemical balance between the positive and negative electrodes. In fact, even if a battery is designed by selecting a positive electrode with excellent performance and a negative electrode with excellent performance, it does not necessarily show a good life. Furthermore, using a positive electrode and a negative electrode with poor performance does not necessarily result in poor life characteristics. In other words, the battery The design of a battery requires consideration of various factors, most of which are determined empirically. This is why battery design seems difficult.

[0005] In particular, the anode is constantly under dynamic conditions due to rapid contraction and expansion during charging and discharging. When using graphite in the design of a lithium secondary battery, it is necessary to consider the degree of volume expansion due to structural changes in graphite with the degree of charging. Failure to do so can result in electrode warping due to expansion of the anode during charging, significantly reducing the battery's lifespan and performance.

[0006] However, natural graphite, which is currently used as an anode active material, has problems such as high consumption of surface coating agents, and the amount of lithium ions that enter and exit per unit area increases as the capacity increases, resulting in a relatively poor rate-limiting characteristic. It also has problems with poor lifespan due to its large volume expansion during charging and discharging.

[0007] In addition, artificial graphite does not have pores inside it when graphitized through powder graphitization, which results in poor rolling characteristics during electrode manufacturing. Also, its low surface area increases the charge transfer resistance of lithium ions, resulting in poor output characteristics.

[0008] Therefore, a technology for manufacturing electrodes by mixing natural graphite and artificial graphite has been developed. However, because the dynamic properties of the materials are different, the dynamic properties in the negative electrode battery do not match, and a synergistic effect cannot be obtained. Therefore, the current situation is that technological development is still required. Summary of the Invention [Problem to be solved by the invention]

[0009] In order to solve the above problems, an object of the present invention is to provide a negative electrode active material that is dynamically balanced and has improved life characteristics, and a secondary battery including the same. [Means for solving the problem]

[0010] In order to solve the above problems, according to one aspect of the present invention, there is provided a negative electrode active material according to the following embodiment.

[0011] Embodiment 1 relates to an anode active material comprising large-particle-size artificial graphite and small-particle-size natural graphite, wherein the average particle size ratio of the small-particle-size graphite to the large-particle-size graphite is 1:1.5 to 1:5, the large-particle-size artificial graphite and the small-particle-size natural graphite each further comprising a coating layer containing amorphous carbon, the coating layer amounting to 1 to 10 parts by weight per 100 parts by weight of the large-particle-size artificial graphite and the small-particle-size natural graphite, and the content ratio of the large-particle-size artificial graphite to the small-particle-size natural graphite being 1:9 to 4:6.

[0012] Embodiment 2 relates to the negative electrode active material of Embodiment 1, wherein the large particle size artificial graphite has an average particle size of 18 to 30 μm.

[0013] Embodiment 3 relates to the negative electrode active material of Embodiment 1 or 2, wherein the small particle size natural graphite has an average particle size of 5 to 13 μm.

[0014] According to another aspect of the present invention, there is provided a negative electrode according to the following embodiment.

[0015] Embodiment 4 relates to a negative electrode including a current collector and a negative electrode active material layer formed on at least one surface of the current collector and including any one of the negative electrode active materials of Embodiments 1 to 3.

[0016] Embodiment 5 is the negative electrode active material layer of Embodiment 4, wherein the negative electrode active material layer is a binder, a conductive material, and a dispersant. The present invention relates to a negative electrode further comprising at least one of the above.

[0017] Embodiment 6 relates to the negative electrode of Embodiment 4 or 5, wherein the negative electrode active material layer has a packing density of 1 to 2.

[0018] According to still another aspect of the present invention, there is provided a secondary battery according to the following embodiment.

[0019] Embodiment 7 relates to a secondary battery including a positive electrode, a negative electrode according to any one of Embodiments 4 to 6, and a separator interposed between the positive electrode and the negative electrode.

[0020] Embodiment 8 relates to the secondary battery of Embodiment 7, wherein the secondary battery is a lithium secondary battery. [Effects of the Invention]

[0021] The present invention uses a dynamically balanced mixture of large particle size artificial graphite and small particle size natural graphite in an optimized ratio, which reduces charge transfer resistance and liquid diffusion resistance, thereby improving fast charging performance, and the lower resistance also improves lifespan.

[0022] In addition, by mixing small particle size natural graphite, which has excellent rolling properties, with large particle size artificial graphite, rolling properties are improved, which is advantageous in that a secondary battery with excellent capacity can be realized.

[0023] Furthermore, by using large particle size artificial graphite and small particle size natural graphite with amorphous carbon coated on their surfaces, it is possible to suppress exfoliation caused by the electrolyte, which has the advantage of improving low temperature output. [Brief explanation of the drawings]

[0024] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to the matters described in the drawings. [Figure 1] 1 is a graph showing comparative measurements of charge transfer resistance in Example 2-1 and Comparative Examples 2-1 to 2-3. [Figure 2] 1 is a graph showing a comparative measurement of resistance depending on the surface coating content of Examples 2-2 to 2-4 and Comparative Examples 2-4 and 2-5. [Figure 3] 1 is a graph showing comparative measurements of cycle characteristics depending on the ratio of large particle size artificial graphite to small particle size natural graphite in Examples 2-1, 2-5, and 2-6, and Comparative Examples 2-6 and 2-7. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be described in detail below. Prior to this, the terms and words used in the specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as meanings and concepts corresponding to the technical ideas of the present invention, in accordance with the principle that the inventor himself can appropriately define the concepts of terms in order to best explain the invention. Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and do not represent the entire technical ideas of the present invention, and therefore, various equivalents and modifications that can be substituted therefor may exist at the time of filing this application.

[0026] The negative electrode active material of the present invention includes large particle size artificial graphite and small particle size natural graphite.

[0027] Graphite crystals are sd 2 - Carbon atoms in hybrid orbitals are bonded to each other in a hexagonal plane to form a graphene layer, and the π electrons located above and below the graphene layer This is achieved by bonding hexagonal mesh planes from the carbon. π electrons can move relatively freely between the carbon hexagonal mesh planes, which gives graphite good electronic conductivity. The π bonds connecting these graphite layers are weak van der Waals bonds, but the bonds within the carbon hexagonal mesh planes are very strong covalent bonds, which show anisotropy, allowing lithium ions to be inserted and removed between these graphite layers.

[0028] There are two types of graphite: natural graphite, which is naturally produced and mined, and artificial synthetic (pyrolytic) graphite, which is produced by carbonizing coal-based or petroleum-based pitch at temperatures above 2,500°C.

[0029] The artificial graphite applicable to one embodiment of the present invention may be any artificial graphite manufactured in the relevant technical field, and non-limiting examples include artificial graphite obtained by calcining MCMB (mesophase carbon microbeads) and mosaic cokes at 2800 to 3000°C.

[0030] In addition, the natural graphite contained in the negative electrode active material of the present invention may be any natural graphite used in the art, and may be converted into a smooth surface shape through post-treatment such as a spheroidization process in order to reduce irreversible reactions and improve the processability of the electrode.

[0031] In this case, the negative electrode active material of the present invention contains a mixture of large particle size artificial graphite and small particle size natural graphite, thereby achieving a dynamic balance.

[0032] In this specification, the kinetic balance refers to the balance of resistance that occurs at the interface and within the particles when lithium ions are charged and discharged. Natural graphite, which has small particles, has reduced charge transfer resistance, while mosaic-shaped artificial graphite, which has large particles, has reduced charge transfer resistance due to the isotropic edge directionality of the crystal planes, thereby achieving a balance.

[0033] In addition, the solid diffusion resistance within the particles is also reduced in natural graphite, which has small particles, because the crystal length is short, while in artificial graphite, the isotropically grown crystals are shorter than the anisotropically grown crystals, so the resistance is reduced and a dynamic balance can be achieved.

[0034] Previously, artificial graphite could not be produced using a spheroidization process, resulting in irregular patterns. Manufacturing electrodes using only large-particle artificial graphite required large amounts of binders (such as carboxymethyl cellulose (CMC) or styrene butadiene rubber (SBR)), which could act as resistance within the battery and degrade battery performance. Conversely, using only small-particle spherical natural graphite could result in increased surface side reactions due to the small particles and large specific surface area, potentially degrading battery performance and long-life characteristics. Therefore, combining large-particle artificial graphite with small-particle natural graphite can solve the problems that arise when using each material alone.

[0035] In this case, the average particle size ratio of the small particle size to the large particle size is 1:1.5 to 1:5, preferably 1:2 to 1:4, and more preferably 1:2 to 1:3. If this average particle size ratio range is not satisfied, the dynamic balance is not maintained, and lithium metal precipitates on the surface of the negative electrode during high current charging, resulting in a rapid deterioration in cell performance.

[0036] More specifically, the average particle size of the large particle size artificial graphite applicable to the present invention may be, for example, 18 to 30 μm, preferably 20 to 25 μm, and more preferably 23 μm. If it exceeds 30 μm, there is a problem that the charge transfer resistance increases significantly and the output characteristics of the cell decrease, and if it is less than 18 μm, there is a problem that the energy density of the battery decreases due to low initial efficiency and capacity decrease.

[0037] Furthermore, the average particle size of the small particle size natural graphite applicable to the present invention may be, for example, 5 to 13 μm, and preferably 8 to 11 μm. As with the large particle size artificial graphite described above, if the average particle size exceeds 13 μm, there is a problem that the charge transfer resistance increases significantly and the output characteristics of the cell decrease, and if it is less than 5 μm, there is a problem that the energy density of the battery decreases due to low initial efficiency and capacity decrease.

[0038] The large particle size artificial graphite and the small particle size natural graphite are mixed and used in a weight ratio of 1:9 to 4:6, preferably 2:8 to 3:7. If the weight ratio exceeds the above range, the dynamic balance is lost, and lithium metal precipitates on the surface of the negative electrode during high current charging, resulting in a rapid deterioration in the performance of the secondary battery.

[0039] In the negative electrode active material of the present invention, the large particle size artificial graphite and the small particle size natural graphite each further comprise a coating layer containing amorphous carbon.

[0040] In the past, propylene carbonate (PC) was added to the electrolyte to improve the low-temperature output of batteries. However, when lithium ions form and move into lithium ion clusters, the energy barrier for desolvation is high, which causes problems such as exfoliation when the clusters are inserted into the graphite.

[0041] However, the present invention provides a coating layer containing amorphous carbon on the surface of artificial graphite and natural graphite, which can adsorb lithium ions and solve the above-mentioned conventional problems. In addition, the present invention improves solid-state diffusion performance, thereby improving fast charging performance.

[0042] The amount of the coating layer containing amorphous carbon is 1 to 10 parts by weight, preferably 2 to 8 parts by weight, and more preferably 3 to 5 parts by weight, per 100 parts by weight of the large-particle size artificial graphite and small-particle size natural graphite. If the amount of the coating layer is less than 1 part by weight, peeling occurs due to the electrolyte, and in the case of natural graphite, the specific surface area increases, and functional groups on the edge surface react directly with the electrolyte, resulting in a decrease in initial efficiency. Conversely, if the amount exceeds 10 parts by weight, the coating layer on the particle surface results in low initial efficiency, reducing discharge capacity per unit weight and battery capacity, and the hardness of the coating layer containing amorphous carbon also reduces rollability.

[0043] According to one aspect of the present invention, there is provided a negative electrode including a current collector and a negative electrode active material layer formed on one surface of the current collector, the negative electrode active material layer including the above-described negative electrode active material.

[0044] The negative electrode active material according to the present invention may be manufactured into a negative electrode by a manufacturing method commonly used in the art. As a non-limiting example, the negative electrode active material according to the present invention may be mixed with a binder, a solvent, and optionally a conductive material and a dispersant, and stirred to prepare a slurry, which may then be applied to one or both sides of a current collector and compressed to manufacture a negative electrode.

[0045] The current collector may be any current collector applicable in the art without limitation, and non-limiting examples thereof include stainless steel; aluminum; nickel; titanium; sintered carbon; copper; stainless steel surface-treated with carbon, nickel, titanium, or silver; and aluminum-cadmium alloy.

[0046] The binder may be any component known in the art that aids in binding between the electrode active material and the conductive material, and between the electrode current collector, and is preferably at least one selected from the group consisting of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylonitrile, and polymethyl methacrylate. The binder may typically be added in an amount of 1 to 20 wt % based on the total weight of the mixture including the electrode active material.

[0047] Non-limiting examples of the solvent include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, water, or a mixture thereof, etc. Such a solvent provides an appropriate level of viscosity so that a desired degree of slurry coating layer is formed on the current collector surface.

[0048] The conductive material is not particularly limited as long as it does not induce chemical changes and has conductivity in the art. Non-limiting examples include carbon black compounds 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. Typically, the conductive material may be added in an amount of 1 to 20 wt % based on the total weight of the mixture including the mixed electrode active material.

[0049] In addition, the dispersing agent may be N-methyl-2-pyrrolidone, diacetone alcohol, dimethylformaldehyde, propylene glycol monomethyl ether, methyl cellosolve, ethyl cellosolve, butyl cellosolve, isopropyl cellosolve, acetylacetone, methyl isobutyl ketone, n-butyl acetate, cellosolve acetate, toluene, xylene, or the like, which may be used alone or in combination.

[0050] The slurry may be applied continuously or discontinuously using various methods such as slot die coating, slide coating, and curtain coating. After the slurry is applied to the current collector, the solvent is finally dried to manufacture a negative electrode having a negative electrode active material layer applied to the current collector.

[0051] In this case, the negative electrode active material layer has a packing density of 1 to 2, preferably 1.55 to 1.65, more preferably 1.575 to 1.625, and even more preferably 1.59 to 1.62, which is an excellent packing density.

[0052] This is the result of using large particle size artificial graphite, which has poor rolling properties during electrode manufacturing due to the loss of pores during graphitization through powder graphitization, while simultaneously mixing small particle size natural graphite, thereby providing a battery with high capacity through excellent packing density.

[0053] According to yet another aspect of the present invention, there is provided a secondary battery including a positive electrode, the above-described negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0054] The positive electrode applicable to the present invention may be any positive electrode used in the art, and may have a form in which a positive electrode active material layer is coated on a positive electrode current collector.

[0055] The positive electrode current collector may be stainless steel, aluminum, nickel, titanium, or calcined carbon; or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like.

[0056] The positive electrode active material layer may include a positive electrode active material, a binder, a conductive material, a dispersant, and the like.

[0057] The positive electrode active material may be any material commonly used in the art, and may be, but is not limited to, a lithium-containing transition metal oxide. For example, Li x CoO2(0.5 <x<1.3)、Li x NiO2(0.5 <x<1.3)、Li x MnO2(0.5 <x<1.3)、Li x Mn2O4(0.5 <x<1.3)、Li x (Ni a Co b Mn c)O2 (0.5 < x < 1.3, 0 < a < 1, 0 < b < 1, 0 < c < 1, a + b + c = 1), Li x Ni 1-y Co y O2 (0.5 < x < 1.3, 0 < y < 1), Li x Co 1-y Mn y O2 (0.5 < x < 1.3, 0 ≤ y < 1), Li x Ni 1-y Mn y O2 (0.5 < x < 1.3, 0 ≤ y < 1), Li x (Ni a Co b Mn c )O4 (0.5 < x < 1.3, 0 < a < 2, 0 < b < 2, 0 < c < 2, a + b + c = 2), Li x [[ID=,28]]Mn 2-z Ni z O4 (0.5 < x < 1.3, 0 < z < 2), Li x Mn 2-z Co z O4 (0.5 < x < 1.3, 0 < z < 2), Li x CoPO4 (0.5 < x < 1.3) and Li x Any one selected from the group consisting of FePO4 (0.5 < x < 1.3) or a mixture of two or more of these can be used, and the lithium-containing transition metal oxide may be coated with a metal such as aluminum (Al) or a metal oxide. In addition to the lithium-containing transition metal oxide, sulfides, selenides, halides, etc. can also be used.

[0058] The binder, conductive material, and dispersant contained in the positive electrode active material layer can be the same as those used in the negative electrode active material layer described above. Although repeated explanations are omitted, it is of course possible to use all of the binder, conductive material, and dispersant used in the negative electrode active material layer.

[0059] The separator applicable to the present invention may be made of any material known in the art for preventing short circuits between a positive electrode and a negative electrode. Non-limiting examples of the separator include a porous polymer substrate, or a porous polymer substrate and a porous coating layer formed on one or both sides of the porous polymer substrate and including inorganic particles and a binder polymer.

[0060] In this case, the porous polymer substrate may be a porous polymer film substrate or a porous polymer nonwoven fabric substrate. A non-limiting example of the porous polymer film substrate may be a porous polymer film made of polyolefin such as polyethylene or polypropylene. Such polyolefin porous polymer film substrates may exhibit a shutdown function at a temperature of, for example, 80 to 130°C.

[0061] In this case, the polyolefin porous polymer film can be formed by using a polyolefin polymer such as polyethylene (e.g., high density polyethylene, linear low density polyethylene, low density polyethylene, ultra-high molecular weight polyethylene), polypropylene, polybutylene, polypentene, etc., either alone or in combination of two or more thereof.

[0062] In addition, the porous polymer film substrate can be manufactured by molding various polymers such as polyester in addition to polyolefin into a film. The porous polymer film substrate can also be formed into a structure in which two or more film layers are laminated, and each film layer can be formed from the above-mentioned polymers such as polyolefin and polyester alone or a mixture of two or more of them.

[0063] In addition to the polyolefins described above, the porous polymer film substrate and the porous nonwoven fabric substrate can be formed from polymers such as polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalate, either alone or in combination.

[0064] The thickness of the porous substrate is not particularly limited, but is preferably 5 to 50 μm. The size of the pores in the porous substrate should be smaller than the binder polymer fibers of the electrode adhesive layer, preferably 0.001 to 50 μm, and the porosity is preferably 0.1 to 99%.

[0065] The porous coating layer may include inorganic particles and a binder polymer, and the inorganic particles are packed and in contact with each other and bound to each other by the binder polymer, thereby forming interstitial volumes between the inorganic particles, and the interstitial volumes between the inorganic particles may become empty spaces to form pores.

[0066] That is, the binder polymer adheres the inorganic particles to each other so that they remain bound together, e.g., the binder polymer can connect and fix the inorganic particles together. In one embodiment, the pores of the porous coating layer are pores formed by the interstitial volume between the inorganic particles, which may be spaces limited by the inorganic particles that are substantially adjacent to each other in a packed structure (closed packed or densely packed) of the inorganic particles. The pores of the porous coating layer can provide paths for the movement of lithium ions, which are essential for battery operation.

[0067] In addition, to form a packed structure of inorganic particles that forms a pore structure due to the interstitial volume, the weight ratio of inorganic particles to binder polymer contained in the porous coating layer is preferably in the range of 50:50 to 99:1, and more preferably 70:30 to 95:5. An inorganic particle content of 50:50 or more contributes to the thermal stability of the separator. The formation of interstitial volume between inorganic particles contributes to ensuring the porosity and pore size of the porous coating layer. If the inorganic particle content exceeds 99 parts by weight, the relatively low binder polymer content may weaken the peel resistance of the porous coating layer.

[0068] The secondary battery may be a lithium secondary battery.

[0069] The present invention will be described in detail below with reference to examples. However, the examples according to the present invention can be modified into many other forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0070] [Example 1-1]

[0071] The experiment used GT from Zichen as artificial graphite and AGP8 from BTR as natural graphite. The average particle size of GT was 23 μm, and AGP8 was 11 μm. They were mixed in a weight ratio of 3:7, and 2 parts by weight of amorphous carbon was coated on 100 parts by weight of the total artificial graphite and natural graphite to prepare a negative electrode active material.

[0072] [Example 1-2]

[0073] A negative electrode active material was prepared in the same manner as in Example 1-1, except that 5 parts by weight of amorphous carbon was used for coating.

[0074] [Examples 1-3]

[0075] A negative electrode active material was prepared in the same manner as in Example 1-1, except that 1 part by weight of amorphous carbon was used for coating.

[0076] [Examples 1-4]

[0077] A negative electrode active material was prepared in the same manner as in Example 1-1, except that 10 parts by weight of amorphous carbon was used for coating.

[0078] [Examples 1-5]

[0079] A negative electrode active material was prepared in the same manner as in Example 1-1, except that the weight ratio of the artificial graphite to the natural graphite was 1:9.

[0080] [Examples 1-6]

[0081] A negative electrode active material was prepared in the same manner as in Example 1-1, except that the weight ratio of the artificial graphite to the natural graphite was 4:6.

[0082] [Comparative Example 1-1]

[0083] The experiment used Zichen GT as artificial graphite and BTR AGP8 as natural graphite. The average particle size of GT was 23 μm, that of AGP8 was 11 μm, and the weight ratio was 3:7 to prepare a mixed negative electrode active material.

[0084] [Comparative Example 1-2]

[0085] A negative active material was prepared in the same manner as in Example 1-1, except that the amount of amorphous carbon to be coated was 2 parts by weight per 100 parts by weight of artificial graphite, and only the large particle size artificial graphite was coated with amorphous carbon.

[0086] [Comparative Example 1-3]

[0087] A negative active material was prepared in the same manner as in Example 1-1, except that the amount of amorphous carbon to be coated was 2 parts by weight per 100 parts by weight of natural graphite, and only the small particle size natural graphite was coated with amorphous carbon.

[0088] [Comparative Example 1-4]

[0089] A negative electrode active material was prepared in the same manner as in Example 1-1, except that 0.5 parts by weight of amorphous carbon was coated.

[0090] [Comparative Example 1-5]

[0091] 10 .5 A negative electrode active material was prepared in the same manner as in Example 1-1, except that parts by weight of amorphous carbon was coated.

[0092] [Comparative Examples 1-6]

[0093] A negative electrode active material was prepared in the same manner as in Example 1-1, except that the weight ratio of the artificial graphite to the natural graphite was 0.5:9.5.

[0094] [Comparative Example 1-7]

[0095] A negative electrode active material was prepared in the same manner as in Example 1-1, except that the weight ratio of the artificial graphite to the natural graphite was 5:5.

[0096] [Example 2-1]

[0097] Anode manufacturing

[0098] The negative electrode active material of Example 1-1, CMC and SBR as binders, and carbon black as a conductive material were added in amounts of 96 parts by weight, 1 part by weight, 2 parts by weight, and 1 part by weight, respectively, to triple distilled water to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to both sides of a 10 μm-thick copper foil serving as a negative electrode current collector, dried, and roll-pressed to prepare a negative electrode.

[0099] Cathode manufacturing

[0100] LiMn with an average diameter of 2 μm1 / 3 Ni 1 / 3 Co 1 / 3 O2 and LiMn with an average diameter of 20 μm 1 / 3 Ni 1 / 3 Co 1 / 3 A cathode mixture slurry was prepared by adding 92 parts by weight of a cathode active material (a mixture of 1:3 weight ratio of O2 and 4 parts by weight of carbon black as a conductive material) and 4 parts by weight of polyvinylidene fluoride (PVDF) as a binder polymer to N-methyl-2-pyrrolidone (NMP) as a solvent. The cathode mixture slurry was applied to both sides of a 20 μm-thick aluminum (Al) thin film cathode current collector, dried, and roll-pressed to prepare a cathode.

[0101] Separator manufacturing

[0102] A binder polymer solution was prepared by dissolving polyvinylidene fluoride (PVdF) and epoxy in acetone. Alumina (Al2O3) was added to the binder polymer solution at a weight ratio of PVdF / Al2O3 = 7.15 / 92.5. The alumina (Al2O3) was crushed and dispersed using a ball mill for over three hours to prepare a slurry. The particle size of the alumina (Al2O3) in the slurry can be controlled by the size of the beads used in the ball mill and the ball milling time; in this experiment, the alumina was crushed to approximately 400 mm to prepare the slurry. The resulting slurry was coated on one side of a 12 μm-thick polyethylene porous polymer film (porosity 45%) and dried.

[0103] Battery manufacturing

[0104] The negative electrode, positive electrode, and separator prepared as described above were stacked in the order of positive electrode / separator / negative electrode / separator / positive electrode and pressurized at high temperature to fabricate a unit bicell. 21 unit bicells were arranged on one side of a separator and folded to fabricate an electrode assembly. An electrolyte solution of 1 M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate and ethyl methyl carbonate (EC / EMC = 1:2, volume ratio) was injected into the assembled battery to fabricate a lithium secondary battery.

[0105] [Example 2-2]

[0106] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Example 1-2 was used.

[0107] [Example 2-3]

[0108] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Example 1-3 was used.

[0109] [Example 2-4]

[0110] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Example 1-4 was used.

[0111] [Example 2-5]

[0112] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Example 1-5 was used.

[0113] [Example 2-6]

[0114] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Example 1-6 was used.

[0115] [Comparative Example 2-1]

[0116] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Comparative Example 1-1 was used.

[0117] [Comparative Example 2-2]

[0118] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Comparative Example 1-2 was used.

[0119] [Comparative Example 2-3]

[0120] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Comparative Example 1-3 was used.

[0121] [Comparative Example 2-4]

[0122] A battery was prepared in the same manner as in Example 2-1, except that the negative electrode active material of Comparative Example 1-4 was used. Manufactured.

[0123] [Comparative Example 2-5]

[0124] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Comparative Example 1-5 was used.

[0125] [Comparative Example 2-6]

[0126] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Comparative Example 1-6 was used.

[0127] [Comparative Example 2-7]

[0128] A battery was produced in the same manner as in Example 2-1, except that the negative electrode active material of Comparative Example 1-7 was used.

[0129] Performance evaluation

[0130] Charge transfer resistance (related to Figure 1)

[0131] After setting the SOC to 50%, the charge transfer resistance values ​​of the batteries of Example 2-1 and Comparative Examples 2-1 to 2-3 were measured using EIS (Electrochemical Impedance Spectroscopy) under conditions of an amplitude of 10 mV and a frequency of 100 kHz to 50 mHz, and the results are shown in Figure 1.

[0132] In the case of Example 2-1, in which the negative electrode active material coated with both large particle size artificial graphite and small particle size natural graphite was used, the charge transfer resistance was significantly reduced when measured by EIS.

[0133] On the other hand, in the case of Comparative Example 2-1, in which neither the large-particle-size artificial graphite nor the small-particle-size natural graphite was coated, the resistance was the highest, and it was confirmed that the case of Example 2-1, in which both the large-particle-size artificial graphite and the small-particle-size natural graphite were coated, was more effective in reducing resistance than Comparative Examples 2-2 and 2-3, in which only one of the large-particle-size artificial graphite and the small-particle-size natural graphite was coated.

[0134] Resistance by surface coating content (related to Figure 2)

[0135] After setting the SOC to 50%, a current of 2.5 C was applied for 30 seconds, and the resistance of the batteries of Examples 2-2, 2-3, and 2-4 and Comparative Examples 2-4 and 2-5 was calculated based on the voltage change. The results are shown in FIG.

[0136] In Examples 2-3, 2-2, and 2-4, in which the content of the coating layer containing amorphous carbon was 1, 5, and 10 parts by weight per 100 parts by weight of the large particle size artificial graphite and the small particle size natural graphite, the effect of reducing resistance was observed.

[0137] In Comparative Example 2-4, in which the content of the amorphous carbon coating layer was 0.5 parts by weight, which was less than 1 part by weight, and Comparative Example 2-5, in which the content of the amorphous carbon coating layer was 10.5 parts by weight, which was more than 10 parts by weight, it was confirmed that the resistance increased significantly and the kinetic balance was disrupted.

[0138] If the amorphous carbon layer is less than 1 part by weight, the amorphous carbon cannot completely surround the surface of the negative electrode, increasing its reactivity with the electrolyte and forming a thick SEI (solid electrolyte interface) film on the surface, slowing the movement of lithium ions. If the amorphous carbon layer is more than 10 parts by weight, the amorphous carbon coating layer is formed too thick, resulting in low initial efficiency, increasing the movement distance of lithium ions, and increasing resistance.

[0139] Cycle characteristics depending on the ratio of large particle size artificial graphite to small particle size natural graphite (related to Figure 3)

[0140] The 2C life performance of the batteries of Examples 2-1, 2-5, and 2-6 and Comparative Examples 2-6 and 2-7 was measured at SOCs of 0 to 100%, and the results are shown in FIG.

[0141] When the weight ratio of large particle size artificial graphite to small particle size natural graphite was 1:9 (Example 2-5), 3:7 (Example 2-1), and 4:6 (Example 2-6), excellent results were observed in terms of life performance while maintaining dynamic balance.

[0142] On the other hand, when the weight ratio of large particle size artificial graphite to small particle size natural graphite was 0.5:9.5 (Comparative Example 2-6) and 5:5 (Comparative Example 2-7), which was outside the range of 1:9 to 4:6, the dynamic balance was lost and a deterioration in life occurred.

[0143] That is, when the weight ratio exceeds the range of 1:9 to 4:6, the difference between the rate at which lithium ions are accepted and the rate at which lithium ions move within the graphite particles becomes large, accelerating deterioration of the battery life. When the weight ratio is less than 1:9 (when the ratio of large-particle-size artificial graphite is less than 1), the rate at which lithium ions are accepted is fast, but the rate at which lithium ions move within the graphite particles is slow, accelerating lithium deposition. When the weight ratio is more than 4:6 (when the ratio of large-particle-size artificial graphite is more than 4), the rate at which lithium ions are accepted cannot keep up with the rate at which lithium ions move within the graphite particles, resulting in a large lithium ion concentration gradient and accelerating degradation.

[0144] As described above, the present invention has been described using limited embodiments and drawings, but the present invention is not limited thereto, and it goes without saying that various modifications and variations can be made by a person having ordinary skill in the art to which the present invention pertains within the technical spirit of the present invention and the equivalent scope of the claims.

Claims

1. It includes large particle size artificial graphite and small particle size natural graphite, The ratio of the average particle size of the small particle size natural graphite to the large particle size artificial graphite is 1:2 to 1:5, The large particle size artificial graphite and the small particle size natural graphite each further comprise a coating layer containing amorphous carbon, The coating layer is 1 to 10 parts by weight based on 100 parts by weight of the total of the large particle size artificial graphite and the small particle size natural graphite, The weight ratio of the large particle size artificial graphite to the small particle size natural graphite is 1:9 to 3:

7.

2. 2. The negative electrode active material according to claim 1, wherein the large particle size artificial graphite has an average particle size of 18 to 30 μm.

3. 2. The negative electrode active material according to claim 1, wherein the small particle size natural graphite has an average particle size of 5 to 13 μm.

4. 10. A negative electrode comprising: a current collector; and a negative electrode active material layer formed on at least one surface of the current collector, the negative electrode active material layer comprising the negative electrode active material according to claim 1.

5. The negative electrode of claim 4 , wherein the negative electrode active material layer further comprises at least one of a binder, a conductive material, and a dispersant.

6. A secondary battery comprising a positive electrode, the negative electrode according to claim 4, and a separator interposed between the positive electrode and the negative electrode.

7. 7. The secondary battery according to claim 6, wherein the secondary battery is a lithium secondary battery.

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