Anode active material, method of manufacturing the same and lithium secondary battery comprising the same

Abstract

One aspect of the present invention provides a method for manufacturing a cathode active material. The method comprises the steps of: preparing primary particles made of natural graphite; mixing the primary particles and pitch with a high-speed mixer to obtain secondary particles formed from the primary particles; and heat-treating the secondary particles, wherein in the step of obtaining the secondary particles, the primary particles and the pitch may be stirred at a speed of 2,000 to 3,000 rpm.

Description

Technology Field

[0001] The present invention relates to a negative electrode active material, a method for manufacturing the same, and a lithium secondary battery including the same. Background Technology

[0002] As the market for electronic devices such as mobile phones, laptops, and PCs grows, the market for lithium-ion batteries, their power source, is also expanding rapidly. Furthermore, as interest in environmental issues grows and the demand for eco-friendly vehicles like electric cars increases, there is a growing trend of research into lithium-ion batteries capable of meeting various applications.

[0003] Among the components of a lithium-ion battery, the negative electrode active material stores lithium ions during charging and plays a crucial role in determining factors such as charging speed and battery capacity. Among these negative electrode active materials, various forms of carbon-based materials, such as artificial graphite, natural graphite, and hard carbon, have been applied as raw materials for carbon-based active materials.

[0004] Among these, natural graphite is generally processed into a spherical shape and utilized as an active material, characterized by the ability to secure energy density and battery performance. Additionally, forming a carbon coating layer on the surface of the spherical natural graphite can enhance the battery properties of the active material.

[0005] However, there was a problem in that performance deterioration occurred due to increased resistance caused by internal SEI and irreversible reactions, as a large number of internal pores were generated during the process of processing natural graphite into spheres. In addition, there was also a disadvantage that the manufacturing process was somewhat cumbersome because an additional process of forming a coating layer after sphericalizing the natural graphite had to be performed.

[0006] Therefore, there is a need for research on cathode active materials that utilize natural graphite and can be manufactured through a simple process while minimizing performance degradation caused by pores. The problem to be solved

[0007] One aspect of the present invention is to provide a negative electrode active material capable of minimizing performance degradation caused by pores, a method for manufacturing the same, and a lithium secondary battery including the same.

[0009] The problems of the present invention are not limited to those described above. Additional problems of the present invention are described throughout the specification, and a person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the contents described in the specification. means of solving the problem

[0010] One aspect of the present invention provides a method for manufacturing a cathode active material. The method comprises the steps of: preparing primary particles made of natural graphite; mixing the primary particles and pitch with a high-speed mixer to obtain secondary particles formed from the primary particles; and heat-treating the secondary particles, wherein in the step of obtaining the secondary particles, the primary particles and the pitch may be stirred at a speed of 2,000 to 3,000 rpm.

[0011] In the above-described method, the step of obtaining the secondary particles may include: a step of heating the high-speed mixer to an assembly temperature of 100 to 200°C by primary stirring at a speed of 2500 to 3000 rpm; and a step of aggregating the primary particles by secondary stirring at a speed of 2000 to 2500 rpm at the assembly temperature.

[0012] In one of the methods described above, the heating step may be performed for 15 to 35 minutes, and the assembling step may be performed for 5 to 15 minutes.

[0013] In one of the methods described above, the softening point of the pitch may be 100 to 200°C.

[0014] In one of the methods described above, the average particle size (D50) of the primary particles may be 5 to 25 μm.

[0015] In one of the methods described above, the step of preparing the primary particles can be performed by grinding the natural graphite.

[0016] In one of the methods described above, the step of classifying the heat-treated secondary particles may be further included.

[0017] In one of the methods described above, the average particle size (D50) of the cathode active material may be 10 to 20 μm.

[0018] In one of the methods described above, the total pore volume of the negative electrode active material may be 0.013 cm³ / g or less.

[0020] Another aspect of the present invention is to provide a negative electrode active material. The negative electrode active material may be manufactured according to any one of the methods described above.

[0022] Another aspect of the present invention is to provide a lithium secondary battery. The lithium secondary battery comprises a negative electrode; a positive electrode; and an electrolyte, and the negative electrode may comprise the aforementioned negative electrode active material.

[0024] Another aspect of the present invention is to provide a negative electrode active material. The negative electrode active material comprises primary particles made of natural graphite; and secondary particles assembled by mixing the primary particles and pitch, and the total pore volume of the secondary particles may be 0.013 cm³ / g or less.

[0025] In the aforementioned cathode active material, the average particle size (D50) of the primary particles may be 5 to 25 μm.

[0026] In one of the aforementioned cathode active materials, the average particle size (D50) of the secondary particles may be 10 to 20 μm. Effects of the invention

[0027] According to the present invention, by performing a process of assembling natural graphite, excessive formation of internal pores in the cathode active material can be prevented, and performance degradation caused by pores can also be improved.

[0028] In addition, according to the present invention, since the mixing and assembly processes of natural graphite and pitch are performed simultaneously, a cathode active material can be manufactured through a simplified process.

[0029] In addition, according to the present invention, as natural graphite and pitch are stirred at high speed, heat generation due to frictional heat is generated, so the aggregation of natural graphite can be performed without separate external heating.

[0031] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention. Brief explanation of the drawing

[0032] A brief description of each drawing is provided to help to better understand the drawings cited in the detailed description of the invention. Figure 1 is a scanning electron microscope (SEM) image of an example and a comparative example. Figure 2 is a scanning electron microscope (SEM) image of the cross-sections of the example and comparative example. Specific details for implementing the invention

[0033] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the terms used herein are for describing the present invention and are not intended to limit the present invention. Additionally, singular forms used herein include plural forms unless the relevant definitions expressly indicate otherwise.

[0034] In this specification, the term “includes” is used to indicate that other components may be included, rather than excluding other components, unless specifically stated otherwise.

[0035] In addition, unless otherwise specifically defined in the specification of the present invention, the % unit means weight %.

[0036] In addition, in this specification, “Dn” refers to the particle size distribution and may refer to the particle size at the n% point of the cumulative distribution of the number of particles according to particle size. For example, D50 is the particle size (average particle size) at the 50% point of the cumulative distribution of the number of particles according to particle size, D90 is the particle size at the 90% point of the cumulative distribution of the number of particles according to particle size, and D10 is the particle size at the 10% point of the cumulative distribution of the number of particles according to particle size. Meanwhile, the particle size distribution can be measured using the laser diffraction method. Specifically, the particle size distribution can be calculated by dispersing the powder to be measured in a dispersion medium, introducing it into a laser diffraction particle size measuring device, and measuring the difference in diffraction patterns according to particle size as the particles pass through the laser beam.

[0037] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms defined in advance are interpreted to have meanings consistent with relevant technical literature and the presently disclosed content.

[0039] Hereinafter, a method for manufacturing a negative electrode active material according to an embodiment of the present invention will be described. A method for manufacturing a negative electrode active material according to an embodiment of the present invention may include the steps of: preparing primary particles; obtaining secondary particles; and heat treatment.

[0041] [Preparation of Primary Particles]

[0042] Primary particles made of natural graphite can be prepared. Here, primary particles may refer to a single particle, that is, a single particle. Natural graphite has the advantage of having a larger capacity and lower raw material costs compared to other carbon-based active materials such as artificial graphite, and excellent adhesion, which allows for the reduction of binder usage and the realization of a high-capacity, high-density cathode.

[0043] For example, the step of preparing the primary particles can be performed by grinding natural graphite. The particle size of the primary particles can be appropriately controlled through the grinding of natural graphite. By controlling the particle size of the primary particles to an appropriate range, a negative electrode active material having appropriate physical properties and excellent electrochemical characteristics can be realized.

[0044] For example, the grinding of natural graphite can be performed using physical impact. For instance, the grinding of natural graphite can be performed using equipment that utilizes physical impact, such as a jet mill, an air classifier mill, or a roller mill. The jet mill grinds particles directly by utilizing collisions between particles, the air classifier grinds particles using airflow, and the roller mill grinds particles by feeding and compressing them between two or more rollers rotating in opposite directions. However, equipment utilizing physical impact is not limited to these, and various types of equipment commonly used in the industry, such as Raymond mills, vertical roller mills, jaw crushers, ball mills, or pin mills, may be used.

[0045] For example, the average particle size (D50) of the primary particles may be 5 to 25 μm. If the average particle size of the primary particles is less than 5 μm, the aggregation of the primary particles may not be properly performed. In addition, if the average particle size of the primary particles exceeds 25 μm, there may be a problem in that the particle size of the secondary particles increases excessively. That is, the average particle size of the primary particles may be 5 to 25 μm, more specifically 7 to 23 μm, and even more specifically 10 to 20 μm.

[0047] [Obtaining secondary particles]

[0048] Secondary particles can be obtained by mixing the primary particles and pitch prepared in the aforementioned step. Here, secondary particles may refer to aggregates formed by assembling multiple primary particles through an intentional assembly or bonding process. Specifically, secondary particles can be obtained by placing pitch on at least a portion of the primary particles.

[0049] Generally, there was a problem in that performance deterioration occurred due to increased resistance caused by internal SEI and irreversible reactions, as a large number of internal pores were generated during the process of processing natural graphite into spherical shapes. In addition, there was also a disadvantage that the manufacturing process was somewhat cumbersome because an additional process of forming a coating layer after sphericalizing the natural graphite had to be performed.

[0050] The present invention is characterized by preventing the excessive formation of internal pores in the cathode active material and improving performance degradation caused by pores by performing a process of assembling natural graphite without sphericalizing it. In addition, the present invention has the advantage of simplifying the manufacturing process by performing the mixing and assembling processes of natural graphite and pitch simultaneously.

[0051] The softening point of the pitch may be 100 to 200°C. In this case, unlike coal tar, liquid pitch, etc., it exists as a solid at room temperature, so it prevents sticking to mixing equipment or inside packaging containers, which has the advantage of being efficient in terms of manufacturing cathode active materials and the production and management of pitch. For example, solid pitches commonly used in the industry, such as petroleum pitch, synthetic pitch, and wood tar pitch, can be used.

[0052] For example, the average particle size of the pitch may be 1 to 7 µm. If the average particle size of the pitch is less than 1 µm, the manufacturing cost may increase significantly, or due to the rapid melting reaction, it may adhere to the surface of the equipment rather than the primary particles, and the yield of the secondary particles may decrease. In addition, if the average particle size of the pitch exceeds 7 µm, the time for the pitch to melt increases, and it may adhere unevenly to the surface of the primary particles, which may degrade the binder properties. That is, the average particle size of the pitch may be 1 to 7 µm, more specifically 1 to 6 µm, and even more specifically 2 to 5 µm.

[0053] For example, pitch may be mixed in an amount of 5 to 20 weight% based on 100 weight% of primary particles. If pitch is mixed in an amount of less than 5 weight%, it may not sufficiently fill the pores of the primary particles, resulting in the formation of numerous pores in the secondary particles and causing adverse reactions with the electrolyte. Additionally, if pitch is mixed in an amount exceeding 20 weight%, the content of graphitic material derived from the pitch increases, which may reduce the capacity of the negative electrode active material. That is, the pitch content may be 5 to 20 weight%, more specifically 7 to 15 weight%, and even more specifically 9 to 12 weight%.

[0055] In the step of obtaining secondary particles, the primary particles and pitch can be mixed using a high-speed mixer. For example, in the step of obtaining secondary particles, the primary particles and pitch can be high-speed stirred at a speed of 2,000 to 3,000 rpm. When the primary particles and pitch are high-speed stirred using a high-speed mixer, heat generation due to frictional heat occurs, and the temperature can reach a level above the softening point of the pitch. Accordingly, the pitch acts as a binder without separate external heating, and the aggregation of the primary particles can be achieved.

[0056] For example, the step of obtaining secondary particles may include the step of heating a high-speed mixer through primary stirring; and the step of assembling primary particles through secondary stirring. The stirring speed and / or stirring time of the primary stirring and the secondary stirring may be the same or different from each other. Specifically, the stirring speed and / or stirring time of the primary stirring and the secondary stirring may be different.

[0057] In the heating step, the high-speed mixer can be heated by primarily rotating it at high speed. By heating the high-speed mixer to an aggregation temperature above the softening point of the pitch through high-speed rotation, the pitch can act as a binder during the aggregation of primary particles. For example, the high-speed mixer can be heated to an aggregation temperature of 100 to 200°C through primary stirring.

[0058] For example, the heating step may be performed by primary stirring the primary particles and pitch at a speed of 2500 to 3000 rpm, more specifically at a speed of 2550 to 2950 rpm, and even more specifically at a speed of 2600 to 2900 rpm. Additionally / alternatively, the heating step may be performed for 15 to 35 minutes, more specifically for 16 to 34 minutes, and even more specifically for 17 to 33 minutes. By adjusting the speed and / or duration of the primary stirring to the above ranges, the temperature inside the high-speed mixer can be controlled to an appropriate range and the primary particles and pitch can be uniformly dispersed.

[0059] In the assembly step, the high-speed mixer can be secondarily rotated at high speed at the assembly temperature to bond the primary particles and the pitch to each other. Specifically, as the pitch, which acts as a binder by reaching a temperature above the softening point, is placed between the primary particles, multiple primary particles can be assembled to obtain secondary particles.

[0060] For example, the assembly step may be performed by secondary stirring of primary particles and pitch at a speed of 2000 to 2500 rpm, more specifically at a speed of 2050 to 2450 rpm, and even more specifically at a speed of 2100 to 2400 rpm. Additionally / alternatively, the assembly step may be performed for 5 to 15 minutes, more specifically for 5 to 13 minutes, and even more specifically for 5 to 10 minutes. By adjusting the speed and / or duration of the secondary stirring to the above ranges, the pitch can be uniformly distributed among the primary particles, and the porosity of the secondary particles can be controlled to an appropriate range.

[0062] [Heat treatment of secondary particles]

[0063] The secondary particles obtained in the aforementioned step can be heat-treated. Through the heat-treatment step, the particle size of the secondary particles can be secured to an appropriate level, thereby improving the electrochemical properties of the cathode active material.

[0064] The heat treatment step can be performed at 900 to 1300°C. When the heat treatment temperature satisfies the above range, the crystallinity of the secondary particles becomes excellent, so a negative electrode active material with excellent battery performance can be manufactured. On the other hand, if the heat treatment temperature deviates from the above range, the electrochemical properties of the negative electrode active material deteriorate, and problems such as thermal shock to the equipment may occur. That is, the heat treatment temperature may be 900 to 1300°C, more specifically 1000 to 1300°C, and even more specifically 1100 to 1250°C.

[0065] The heat treatment step may be performed for at least one hour, more specifically at least two hours. For example, the heat treatment step may be performed in an inert atmosphere. Specifically, the heat treatment step may be performed in a nitrogen, argon, and / or helium gas atmosphere, but is not limited thereto.

[0066] The heat treatment step can be performed using various types of high-temperature furnaces known in the industry, such as the Etchison furnace. Since the heat treatment temperature conditions, such as the heating rate and the holding time at the maximum temperature, can be adjusted differently depending on the type and characteristics of the heat treatment equipment, the heat treatment step is not limited to the above.

[0068] [Classification]

[0069] For example, after the heat treatment step, a step of classifying the heat-treated secondary particles may be further included. If the powders fuse together and clump due to the heat treatment, they can be appropriately separated through classification. For example, a small amount of excess particles formed by aggregation during the heat treatment process can be removed by sieving. By separating the excess particles through the classification step, it is possible to prevent them from affecting the processing performance of the material (e.g., slurry stability, coating performance, etc.).

[0071] Hereinafter, a negative electrode active material according to an embodiment of the present invention will be described. The negative electrode active material according to an embodiment of the present invention may be manufactured by the manufacturing method described above.

[0072] The cathode active material may include primary particles made of natural graphite; and secondary particles. Here, the primary particles may refer to a single particle, that is, a single particle. Natural graphite has the advantage of having a larger capacity and lower raw material costs compared to other carbon-based active materials such as artificial graphite, and excellent adhesion, which allows for a reduction in the amount of binders used and enables the realization of a high-capacity, high-density cathode.

[0073] Secondary particles may be assembled by mixing primary particles and pitch. Specifically, secondary particles may be obtained by placing pitch on at least a portion of the primary particles.

[0074] In the present invention, by performing a process of assembling natural graphite without sphericalizing it, excessive formation of internal pores in the cathode active material is prevented, and an appropriate level of particle size can be secured.

[0075] For example, the total pore volume of the secondary particles and / or negative electrode active material may be 0.013 cm³ / g or less, more specifically 0.0130 cm³ / g or less, and even more specifically 0.0128 cm³ / g or less. Here, the total pore volume may be measured using a porosimetry analyzer, but is not limited thereto. As the total pore volume of the secondary particles and / or negative electrode active material is controlled to the above range, performance degradation caused by pores can be improved. Since a lower total pore volume is superior in terms of battery performance, a lower limit value is not separately specified.

[0076] For example, the average particle size (D50) of the secondary particles and / or negative electrode active material may be 10 to 20 μm, more specifically 10.0 to 20.0 μm, and even more specifically 11.0 to 19.0 μm. As the average particle size of the secondary particles and / or negative electrode active material is controlled to the above range, output characteristics and lifespan characteristics can be improved.

[0078] Hereinafter, a lithium secondary battery according to an embodiment of the present invention will be described. A lithium secondary battery according to an embodiment of the present invention may include a negative electrode; a positive electrode; and an electrolyte.

[0080] The cathode may include a cathode active material layer comprising the aforementioned cathode active material; and a current collector.

[0081] The current collector can serve the role of imparting conductivity. The materials that can be used as current collectors are not specifically limited, and any material that is conductive without causing chemical changes in the battery may be used. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used as current collectors.

[0082] The negative active material layer may be disposed on at least one surface of the current collector, specifically on one surface or both surfaces. For example, the negative active material may be included in an amount of 80 to 98 weight percent relative to the total weight of the negative active material layer.

[0083] The negative electrode active material layer may further include a binder and / or a conductive material. The binder may serve to attach the particles forming the negative electrode active material to each other and improve the adhesion performance of the negative electrode active material to the current collector. For example, the binder may be included in an amount of 1 to 5 weight percent relative to the total weight of the negative electrode active material layer.

[0084] For example, the binder may include a non-aqueous binder, an aqueous binder, or a combination thereof. For example, the non-aqueous binder may include one or more selected from the group consisting of ethylene / propylene copolymer, polyacrylonitrile (PAN), polystyrene (PS), polyvinyl chloride (PVC), carboxylated polyvinyl chloride, poly(vinylidene fluoride) (PVDF), polyurethane, polytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene (PP), polyamide-imide (PAI), and polyimide (PI). However, it is not limited thereto.

[0085] Water-based binders include styrene-butadiene rubber (SBR), acrylated styrene-butadiene rubber, nitrile-butadiene rubber (NBR), acrylic rubber, butyl rubber, fluororubber, polymers including ethylene oxide, polyvinylpyrrolidone (PVP), polyepichlorohydrin, polyphosphazenes, ethylene-propylene-diene copolymer (EPDM), poly(vinyl pyridine), chlorosulfonated polyethylene (CSM), latex, and polyester resin. It may include one or more selected from the group consisting of resin, acrylic resin, phenol resin, epoxy resin, and polyvinyl alcohol (PVA). However, it is not limited thereto.

[0086] For example, when an aqueous binder is used as the binder, the negative electrode active material layer may further include a thickening agent capable of imparting viscosity. The thickening agent may include a cellulose-based compound. For example, the cellulose-based compound may include one or more selected from the group consisting of carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, and alkali metal salts in which the hydrogens thereof are substituted with Na, K, or Li. For example, the thickening agent may be included in an amount of 0.1 to 3 weight% relative to the total weight of the negative electrode active material layer.

[0087] A conductive material may be used to impart conductivity to an electrode. The materials that can be used as conductive materials are not specifically limited, and materials that are conductive without causing chemical changes in the battery may be used. For example, the conductive material may include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjenblack, carbon fiber; metal-based materials such as metal powder or metal fibers such as copper, nickel, aluminum, silver; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0088] A cathode can be obtained by preparing an active material composition by mixing a binder and / or a conductive material together with a cathode active material in a solvent, and applying this active material composition to a current collector. For example, the solvent may include water. Since methods commonly used in the industry can be applied to the method of manufacturing such a cathode, a detailed description is omitted in this specification.

[0090] The positive electrode comprises a current collector; and a positive electrode active material layer formed on at least one surface of the current collector and comprising a positive electrode active material. For example, the positive electrode active material may be included in an amount of 80 to 98 weight percent with respect to the total weight of the positive electrode active material layer.

[0091] The current collector can serve the role of imparting conductivity. The materials that can be used as current collectors are not specifically limited, and any materials that are conductive without causing chemical changes in the battery may be used. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc., may be used as current collectors.

[0092] The cathode active material may include a compound capable of reversible intercalation and deintercalation of lithium (a rethated intercalation compound). The cathode active material may be a conventionally used cathode active material and, for example, may include one or more composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof. Specifically, the cathode active material may include layered compounds such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; lithium iron oxides such as LiFe3O4; and compounds with the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4 (0≤c1≤0.33), LiMnO3, LiMn2O3, LiMnO2, etc.; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7, etc.; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented by O2 (wherein M is one or more selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, satisfying 0.01≤c2≤0.3); chemical formula LiMn 2-c3 M c3Lithium manganese composite oxides represented by O2 (wherein M is one or more selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, satisfying 0.01≤c3≤0.1) or Li2Mn3MO8 (wherein M is one or more selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4 in which a portion of the Li in the chemical formula is substituted with alkaline earth metal ions, etc., may be used, but are not limited thereto. The anode may also be Li-metal.

[0093] For example, the above compound may have a coating layer on its surface, or the above compound and the compound having the coating layer may be mixed and used. The coating layer may include one or more coating element compounds selected from the group consisting of oxides of coating elements, hydroxides of coating elements, oxyhydroxides of coating elements, oxycarbonates of coating elements, and hydroxycarbonates of coating elements. The compound forming the coating layer may be amorphous or crystalline. As for the coating elements included in the coating layer, one or more selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, and Zr may be used. For the coating layer formation process, any coating method may be used as long as the compound can be coated using these elements in a way that does not adversely affect the physical properties of the cathode active material (e.g., spray coating, immersion method, etc.), and since methods commonly used in the industry can be applied, a detailed description thereof is omitted in this specification.

[0094] The positive active material layer may further include a binder and / or a conductive material. The binder may serve to attach the particles forming the positive active material to each other and improve the adhesion performance of the positive active material to the current collector. For example, the binder may be included in an amount of 1 to 5 weight percent relative to the total weight of the positive active material layer.

[0095] The binder is poly(vinylidene fluoride; PVDF), poly(vinylidene fluoride-co-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), carboxymethyl cellulose, starch, hydroxypropyl methylcellulose, regenerated cellulose, polyvinyl chloride (PVC), carboxylated polyvinyl chloride, polyvinyl pyrrolidone (PVP), polytetrafluoroethylene (PTFE), polyethylene (PE), and polypropylene (PP). It may include one or more selected from the group consisting of ethylene-propylene-diene copolymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof.

[0096] Conductive materials may be used to impart conductivity to the electrode. The materials that can be used as conductive materials are not specifically limited, and materials that are conductive without causing chemical changes in the battery may be used. For example, conductive materials may include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskies such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used.

[0097] A positive electrode can be obtained by preparing an active material composition by mixing a positive electrode active material with a binder and / or a conductive material in a solvent, and applying this active material composition to a current collector. For example, the solvent may include water. Since methods commonly used in the industry can be applied to the method of manufacturing such a positive electrode, a detailed description is omitted in this specification.

[0099] The electrolyte may include, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes that are commonly used in the industry when manufacturing lithium secondary batteries. Specifically, the electrolyte may include non-aqueous organic solvents and / or lithium salts.

[0100] Non-aqueous organic solvents can serve as a medium through which ions involved in the electrochemical reaction of a battery can move. For example, non-aqueous organic solvents may include one or more selected from the group consisting of carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, and aprotic solvents.

[0101] The above carbonate-based solvents may include dimethyl carbonate, diethyl carbonate, dicaprylyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, or butylene carbonate. The above-mentioned ester-based solvents may include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, propyl propionate, decanolide, mevalonolactone, or caprolactone. The above-mentioned ether-based solvents may include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Cyclohexanone, etc. may be used as the above-mentioned ketone-based solvent. Ethanol or isopropyl alcohol, etc. may be used as the above-mentioned alcohol-based solvent.The above-mentioned aprotic solvent may be a nitrile such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double bond-directing ring or ether bond), an amide such as dimethylformamide, a dioxolane such as 1,3-dioxolane, or a sulfolane.

[0102] The above lithium salt is dissolved in an organic solvent and acts as a source of lithium ions within the battery, enabling the basic operation of a lithium secondary battery and facilitating the movement of lithium ions between the positive and negative electrodes. As an anion of the above lithium salt, F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more types selected from the group consisting of can be used.

[0103] For example, the concentration of the lithium salt may be 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance, and accordingly, lithium ions can move more effectively.

[0105] For example, depending on the type of lithium secondary battery, the lithium secondary battery may further include a separator formed between the positive electrode and the negative electrode. The separator separates the negative electrode and the positive electrode and provides a pathway for the movement of lithium ions; any separator used as a secondary battery separator in the relevant industry may be used without special restrictions. For example, polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof may be used as the separator. However, it is not limited thereto, and the separator may be a mixed multilayer film such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator.

[0106] The above lithium secondary battery may be placed inside a battery case. The shape of the battery case may be one or more types selected from the group consisting of cylindrical, prismatic, pouch, and coin types using a can. However, the shape of the battery case is not limited thereto and may have various types of shapes used in the industry.

[0108] The present invention will be explained in more detail below through examples. However, it should be noted that the following examples are intended merely to illustrate and explain the invention in more detail, and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0110] (Example 1)

[0111] Natural graphite was crushed to prepare primary particles having an average particle size (D50) of 15 μm. Then, the primary particles were fed into a high-speed mixer along with 10 wt% of pitch (average particle size (D50): 3 μm, softening point: 150°C) based on 100 wt% of the primary particles. The high-speed mixer was rotated at a speed of 2800 rpm for 24 minutes to heat the internal temperature of the high-speed mixer to 150°C. Subsequently, the primary particles and pitch were stirred at a speed of 2200 rpm for 6 minutes to obtain secondary particles in which the primary particles were aggregated. Finally, the secondary particles were heat-treated at 1200°C and then classified by a 325 mesh sieve to produce the negative electrode active material of Example 1.

[0113] (Comparative Examples 1 and 2)

[0114] Spherical natural graphite (model name: C3B, POSCO Future M Co.) was prepared as the negative electrode active material of Comparative Example 1. In addition, spherical natural graphite (model name: CP9L, POSCO Future M Co.) was prepared as the negative electrode active material of Comparative Example 2.

[0116] (Experimental Example 1: Measurement of Physical Properties of Cathode Active Material)

[0117] The particle size distribution, specific surface area (BET), and total pore volume of the above examples and comparative examples were measured, and the results are shown in Table 2.

[0118] The particle size distribution was measured using the laser diffraction method. Specifically, each sample was dispersed in a dispersion medium and introduced into a laser diffraction particle size measuring device. The particle size distribution was calculated by measuring the difference in diffraction patterns according to particle size as the particles passed through the laser beam. In this case, Dmax represents the maximum particle size.

[0119] The specific surface area (BET) was measured using adsorbed gases such as nitrogen. Specifically, 2g of each sample was placed in a container, dried at 80°C for 30 minutes, and then pretreated under vacuum or lower conditions. Subsequently, nitrogen and helium gases were adsorbed onto the samples to measure the specific surface area.

[0120] The total pore volume was measured using a porosimetry analyzer.

[0122] division PSD(㎛) Specific surface area (㎡ / g) Total pore volume (cm³ / g) D10 D50 D90 Dmax Example 1 6.9 14.7 24.4 45.0 2.9 0.0124 Comparative Example 1 11.0 16.5 24.8 36.0 2.7 0.0195 Comparative Example 2 9.8 14.9 22.0 32.0 2.1 0.0132

[0124] Referring to Table 1, it was found that Example 1, which satisfies the manufacturing conditions proposed in the present invention, secured an appropriate level of particle size distribution, specific surface area, and total pore volume.

[0125] It was confirmed that Comparative Examples 1 and 2, which underwent a spheroidization process rather than the assembly process proposed in the present invention, had a high total pore volume. That is, it was found that pores were excessively formed in Comparative Examples 1 and 2 as they underwent the spheroidization process.

[0127] Meanwhile, scanning electron microscope (SEM) images of the outer surface and cross-section of the example and comparative example are shown in FIGS. 1 and 2. Specifically, FIGS. 1 and 2 (a) is an SEM image of the outer surface and cross-section of Example 1, FIGS. 1 and 2 (b) is an SEM image of the outer surface and cross-section of Comparative Example 1, and FIGS. 1 and 2 (c) is an SEM image of the outer surface and cross-section of Comparative Example 2.

[0128] Referring to Figures 1 and 2 (a), it was found that pore formation was minimized as Example 1 performed an assembly process rather than a spherical process.

[0129] In addition, referring to (b) and (c) of Figures 1 and 2, it was found that in Comparative Examples 1 and 2, pores were excessively formed as a result of undergoing the sphericalization process.

[0131] (Experimental Example 2: Measurement of Electrochemical Properties of Cathode Active Material)

[0132] To evaluate the electrochemical characteristics of the above examples and comparative examples, lithium half-cells were prepared using each negative electrode active material.

[0133] Specifically, 96% by weight of each cathode active material was mixed in a distilled water solvent to prepare a cathode active material slurry. The cathode active material slurry was coated onto a copper current collector, dried at 80°C for 5 minutes, compressed in a roll press, and vacuum dried to produce a cathode. After vacuum drying, the electrode density of the cathode was set to 1.55 g / cc.

[0134] Lithium metal was used as the counter electrode, and a solution of 1 mole of LiPF6 dissolved in EC / EMC (2:8) + VC 0.5 wt% was used as the electrolyte. Using each of the above components, a 2032 coin cell type half-cell was manufactured according to a conventional manufacturing method.

[0136] For each half-cell prepared as an example and a comparative example, the initial capacity, initial efficiency, and capacity retention rate were measured and are shown in Table 2 below.

[0137] After applying current to each battery until it reached 0.05V, it was charged to a constant potential of 0.01C and discharged to 1.5V with a constant current of 0.1C. At this time, the initial discharge capacity was measured, and the initial efficiency was measured as the percentage of the initial discharge capacity relative to the initial charge capacity.

[0138] After repeating the charging (CC / CV mode, 0.5C charging, 0.05C and 0.05V or less cut-off) and discharging (CC mode, 0.5C discharging, 1.5V or more cut-off) cycles for each battery 50 times, the capacity retention rate at 50 cycles was calculated using the following formula.

[0139] Capacity Retention Rate (%) = [(Discharge Capacity at the 50th Cycle) / (Discharge Capacity at the 1st Cycle)] × 100

[0141] division Initial capacity (mAh / g) Initial efficiency (%) Capacity retention rate (%) Example 1 354.6 89.0 84.0 Comparative Example 1 363.4 93.1 78.7 Comparative Example 2 359.7 93.6 79.9

[0143] Referring to Table 2, it was confirmed that Example 1, satisfying the manufacturing conditions proposed in the present invention, secured an appropriate level of initial capacity, initial efficiency, and capacity retention rate. In other words, it was found that Example 1 improved performance degradation caused by pores because pore formation was minimized through the assembly process.

[0144] Meanwhile, it was confirmed that Comparative Examples 1 and 2 measured a low capacity retention rate. That is, it was found that the lifespan characteristics were degraded because, as Comparative Examples 1 and 2 underwent a sphericalization process, pores were excessively formed, leading to performance degradation due to increased resistance caused by internal SEI and irreversible reactions.

[0146] The above embodiments are merely examples, and the present invention is not limited thereto. Any configuration having substantially the same structure as the technical concept described in the claims of the present invention and achieving the same functional effect is included within the technical scope of the present invention.

Claims

Claim 1 A method for manufacturing a negative electrode active material, comprising: a step of preparing a primary particle made of natural graphite; a step of mixing the primary particle and pitch with a high-speed mixer to obtain a secondary particle in which the primary particle is assembled; and a step of heat-treating the secondary particle to manufacture a negative electrode active material, wherein in the step of obtaining the secondary particle, the primary particle and the pitch are stirred at a speed of 2,000 to 3,000 rpm, and the total pore volume of the negative electrode active material is 0.013 cm³ / g or less. Claim 2 A method for manufacturing a cathode active material according to claim 1, wherein the step of obtaining the secondary particles comprises: a step of heating the high-speed mixer to an assembly temperature of 100 to 200°C by primary stirring at a speed of 2500 to 3000 rpm; and a step of assembling the primary particles by secondary stirring at a speed of 2000 to 2500 rpm at the assembly temperature. Claim 3 A method for manufacturing a cathode active material according to claim 2, wherein the heating step is performed for 15 to 35 minutes and the assembling step is performed for 5 to 15 minutes. Claim 4 A method for manufacturing a negative electrode active material according to claim 1, wherein the softening point of the pitch is 100 to 200°C. Claim 5 A method for manufacturing a negative electrode active material according to claim 1, wherein the average particle size (D50) of the primary particles is 5 to 25 μm. Claim 6 A method for manufacturing a cathode active material according to claim 1, wherein the step of preparing the primary particles is performed by grinding the natural graphite. Claim 7 A method for manufacturing a cathode active material according to claim 1, further comprising the step of classifying the heat-treated secondary particles. Claim 8 A method for manufacturing a negative electrode active material according to claim 1, wherein the average particle size (D50) of the negative electrode active material is 10 to 20 μm. Claim 9 delete Claim 10 delete Claim 11 delete Claim 12 A negative electrode active material comprising: primary particles made of natural graphite; and secondary particles assembled by mixing the primary particles and pitch; wherein the total pore volume of the secondary particles is 0.013 cm³ / g or less. Claim 13 In claim 12, the negative active material wherein the average particle size (D50) of the primary particles is 5 to 25 μm. Claim 14 In claim 12, the negative electrode active material wherein the average particle size (D50) of the secondary particles is 10 to 20 μm. Claim 15 A lithium secondary battery comprising a negative electrode; a positive electrode; and an electrolyte, wherein the negative electrode comprises a negative electrode active material according to claim 12.

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