Spheroidized carbonaceous negative electrode active material, method for producing the same, and negative electrode and lithium secondary battery containing the same

Abstract

A method for manufacturing a spheroidized carbon-based negative electrode active material is proposed, the method comprising the following steps: mixing particulate flaky graphite and coarse-grained flaky graphite having an average diameter larger than that of the particulate flaky graphite, and then spheroidizing to prepare spheroidized granulated particles; carbon coating the spheroidized granulated particles; and crushing the carbon-coated spheroidized granulated particles.

Description

Technical Field

[0001] The present disclosure relates to a spheroidized carbonaceous negative electrode active material, a preparation method thereof, and a negative electrode and a lithium secondary battery comprising the same. More particularly, the present disclosure relates to a spheroidized carbonaceous negative electrode active material with reduced inner pores and improved sphericity, a preparation method thereof, and a negative electrode and a lithium secondary battery comprising the same.

[0002] This application claims priority to Korean Patent Application No. 10-2019-0123397 filed in Korea on October 4, 2019, the disclosure of which is incorporated herein by reference. Background Art

[0003] As the technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources for such mobile devices is increasing. Among such secondary batteries, lithium secondary batteries with high energy density and operating voltage, long cycle life and low discharge rate have been commercialized and widely used.

[0004] A lithium secondary battery has a structure including an electrode assembly and a lithium salt-containing electrolyte injected into the electrode assembly, wherein the electrode assembly has a positive electrode and a negative electrode each including an active material coated on an electrode current collector, and a porous separator interposed between the two electrodes. The electrode is obtained by applying a slurry including an active material, a binder, and a conductive material dispersed in a solvent to a current collector, followed by drying and pressing.

[0005] Lithium metal has been used as the negative electrode of secondary batteries in the past. However, since lithium metal is known to cause battery short circuit due to the formation of lithium dendrites, resulting in the risk of explosion, it has been replaced by carbonaceous compounds that can reversibly insert / extract lithium ions while maintaining structural and electrical properties.

[0006] Such a carbonaceous compound has a significantly low discharge potential of about -3 V based on the standard hydrogen electrode potential, and exhibits excellent electrode cycle life due to a significantly reversible charge / discharge behavior brought about by the uniaxial orientation of the graphene layer. In addition, the carbonaceous compound exhibits 0 V Vs.Li / Li upon lithium ion insertion. + The electrode potential of lithium metal is similar to that of pure lithium metal. Therefore, when lithium metal is combined with an oxide positive electrode to form a battery, higher energy can be obtained.

[0007] Various types of carbonaceous materials including artificial graphite, natural graphite and hard carbon have been used as the carbonaceous compound. Among the carbonaceous compounds, graphite is currently the most widely used.

[0008] Among various types of graphite, natural graphite is used after being converted into graphite with a smooth surface shape through a post-treatment process such as a spheroidization process to reduce irreversible reactions and improve the processability of electrodes. In addition, when natural graphite is surface-coated by heat treatment of low-crystalline carbon such as pitch, the edge face of graphite can be prevented from being directly exposed, damage caused by electrolyte can be prevented, and irreversible reactions can be reduced. Negative electrode material manufacturers adopt a method of coating spherical natural graphite with low-crystalline carbon to prepare negative electrode active materials.

[0009] However, the negative electrode active material obtained by the above method is prepared by spheroidizing natural graphite having a flaky particle shape, and the spheroidized graphite particles contain a large number of voids therein. Such voids reduce the density of the negative electrode active material, making it difficult to manufacture a high-density negative electrode plate. In addition, since the low-crystalline carbon coating is destroyed during the process of densifying the negative electrode active material on the collector, the graphite edge surface is exposed, resulting in damage caused by the electrolyte and problems of irreversible reaction.

[0010] In addition, natural graphite has the disadvantage of higher electrode expansion compared to artificial graphite. Moreover, the inner pores generated during the spheroidization of natural graphite are larger than those of artificial graphite. The coating layer generated by such a larger number of inner pores may cause the generation of gas due to side reactions at high temperatures and the deterioration of high temperature storage characteristics. Summary of the invention

[0011] Technical issues

[0012] The present disclosure aims to solve the problems of the prior art, and thus the present disclosure aims to provide a spheroidized carbonaceous negative electrode active material with reduced inner pores and improved sphericity, a preparation method thereof, and a negative electrode and a lithium secondary battery comprising the same.

[0013] The present disclosure also relates to providing a negative electrode including the negative electrode active material and a lithium secondary battery including the negative electrode.

[0014] Technical Solution

[0015] According to the present disclosure, there are provided a spheroidized carbonaceous negative electrode active material according to the following embodiments, a preparation method thereof, and a negative electrode and a lithium secondary battery including the same.

[0016] According to a first embodiment of the present disclosure, there is provided a method for preparing a spheroidized carbonaceous negative electrode active material, comprising the following steps:

[0017] Mixing fine-particle flaky graphite with coarse-particle flaky graphite having an average particle size larger than that of the fine-particle flaky graphite, and spheroidizing the resulting mixture to prepare spheroidized granulated particles;

[0018] Carbon coating the spheroidized granulated particles; and

[0019] The carbon-coated spheroidized granulated particles are disintegrated.

[0020] According to a second embodiment of the present disclosure, a method for preparing a spheroidized carbonaceous negative electrode active material as defined in the first embodiment is provided, wherein the average particle size of the micro-particle flaky graphite is 20 μm-50 μm, and the average particle size of the coarse-particle flaky graphite is 50 μm-100 μm.

[0021] According to a third embodiment of the present disclosure, there is provided a method for preparing a spheroidized carbonaceous negative electrode active material as defined in the first embodiment or the second embodiment, wherein the coarse-grained flaky graphite and the fine-grained flaky graphite are used in a weight ratio of 70:30-40:60.

[0022] According to a fourth embodiment of the present disclosure, a spheroidized carbonaceous negative electrode active material is provided, the specific surface area of ​​which is 1.5 m 2 / g-2.8m 2 / g, the total pore volume is 1.0e -2 cm 3 / g-1.8e -2 cm 3 / g, and the pores with a size of 24 nm or more in the negative electrode active material have a size of 0.1 m 2 / g-0.8m 2 / g specific surface area.

[0023] According to a fifth embodiment of the present disclosure, there is provided a spheroidized carbonaceous negative electrode active material as defined in the fourth embodiment, wherein the spheroidized carbonaceous negative electrode active material has a carbon content of 1.8 m 2 / g-2.5m 2 / g specific surface area.

[0024] According to a sixth embodiment of the present disclosure, there is provided a spheroidized carbonaceous negative electrode active material as defined in the fourth or fifth embodiment, wherein the total pore volume of the spheroidized carbonaceous negative electrode active material is 1.19e -2 cm 3 / g to 1.57e -2 cm 3 / g.

[0025] According to a seventh embodiment of the present disclosure, there is provided a spheroidized carbonaceous negative electrode active material as defined in any one of the fourth to sixth embodiments, wherein the specific surface area of ​​pores having a size of 24 nm or more in the negative electrode active material is 0.3 m 2 / g-0.7m 2 / g.

[0026] According to an eighth embodiment of the present disclosure, there is provided a spheroidized carbonaceous negative electrode active material as defined in any one of the fourth to seventh embodiments, wherein the spheroidized carbonaceous negative electrode active material has an average particle size of 10 μm to 20 μm.

[0027] According to a ninth embodiment of the present disclosure, there is provided a spheroidized carbonaceous negative electrode active material as defined in any one of the fourth to eighth embodiments, wherein the sphericity of the spheroidized carbonaceous negative electrode active material is 0.82-0.98.

[0028] According to a tenth embodiment of the present disclosure, there is provided a negative electrode comprising a current collector and a negative electrode active material layer disposed on at least one surface of the current collector, wherein the negative electrode active material layer comprises a spheroidized carbonaceous negative electrode active material as defined in any one of the fourth to ninth embodiments.

[0029] According to an eleventh embodiment of the present disclosure, there is provided a lithium secondary battery including the anode as defined in the tenth embodiment.

[0030] Beneficial Effects

[0031] According to an embodiment of the present disclosure, instead of spheroidizing using a flaky graphite according to the prior art, flaky graphite having a larger average particle size is mixed with flaky graphite having a smaller average particle size and then spheroidized. In this way, a spheroidized carbonaceous negative electrode active material with improved sphericity and reduced inner pores can be provided. When the negative electrode active material is applied to the negative electrode of a secondary battery, a secondary battery can be provided that exhibits reduced internal stress and improved expansion characteristics and has a high capacity retention rate during high temperature storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the foregoing disclosure, are used to provide a further understanding of the technical features of the present disclosure; therefore, the present disclosure is not to be construed as being limited to the accompanying drawings.

[0033] Figure 1 Schematic diagram illustrating a spheroidization step of a method for preparing a spheroidized carbonaceous negative electrode active material according to an embodiment of the present disclosure.

[0034] Figure 2 The schematic diagram illustrates a spheroidization step of a method for preparing a spheroidized carbonaceous negative electrode active material according to the prior art. DETAILED DESCRIPTION

[0035] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terms used in the specification and the appended claims should not be interpreted as limited to the general meaning and dictionary meaning, but should be interpreted based on the meaning and concept corresponding to the technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to appropriately define the terms for the best interpretation.

[0036] In one aspect of the present disclosure, there is provided a method for preparing a spheroidized carbonaceous negative electrode active material, comprising the following steps:

[0037] Mixing fine-particle flaky graphite with coarse-particle flaky graphite having an average particle size larger than that of the fine-particle flaky graphite, and spheroidizing the resulting mixture to prepare spheroidized granulated particles;

[0038] Carbon coating the spheroidized granulated particles; and

[0039] The carbon-coated spheroidized granulated particles are disintegrated.

[0040] When spheroidization is performed by using flaky graphite having a single average particle size according to the prior art, a large number of inner pores are generated in the spheroidized particles, and side reactions occur in such inner pores. In order to solve the above problems, according to the present disclosure, two types of flaky graphites having different average particle sizes, i.e., particulate flaky graphite and coarse-grained flaky graphite having a larger average particle size than the particulate graphite (the coarse-grained flaky graphite has a larger average particle size, and the particulate flaky graphite has a smaller average particle size) are mixed with each other and then spheroidized. In this way, the present disclosure aims to provide a spheroidized carbonaceous negative electrode active material having reduced inner pores and improved sphericity, which suppresses electrode expansion when used in a battery electrode and provides the battery with improved high-temperature storage characteristics.

[0041] Below, each step of the method will be described in detail.

[0042] First, fine-particle flaky graphite is mixed with coarse-particle flaky graphite having an average particle size larger than that of the fine-particle flaky graphite, and the resulting mixture is spheroidized to prepare spheroidized granulated particles.

[0043] In this step, fine flaky graphite and coarse flaky graphite having an average particle size larger than that of the fine flaky graphite are prepared in a predetermined weight ratio, and then mixed and spheroidized to prepare spheroidized granulated particles.

[0044] In this step, while the coarse-grained flaky graphite particles are in contact with each other to form granulated particles, the fine-grained flaky graphite particles are inserted into the empty spaces (voids) formed between the coarse-grained flaky graphite particles so that the empty spaces (voids) can be filled with the fine-grained flaky graphite. In this way, the inner pores can be reduced and more dense spheroidized particles can be provided.

[0045] In this step, a mixture of fine-particle flaky graphite and coarse-particle flaky graphite can be used as a starting material and subjected to a spheroidization process generally known to those skilled in the art, such as applying a mechanical treatment including impact compression, friction or shearing force. The mechanical treatment can be carried out by using a spheroidization device generally known to those skilled in the art, and specific examples of such devices include a mill, such as a back jet mill (Hosokawa Micron Co., Ltd., Japan), an ACM pulverizer (Hosokawa Micron Co., Ltd., Japan) or a fluid mill (Nissin Co., Ltd., Japan); a granulator, such as SARARA (Kawasaki Heavy Industries, Ltd., Japan), GRANUREX (Freund Co., Ltd., Japan), New-GraMachine (Seishin Co., Ltd., Japan) or Acromaster (Hosokawa Micron Co., Ltd., Japan); a kneader, such as a dispersion kneader or a double roll mill; a compression shear processing device, such as a mechanical micro system, an extruder, a ball mill, a planetary mill, a mechanical fusion system, Nobilta, a mixer (Hybridation) or a rotary ball mill, etc.

[0046] According to one embodiment of the present disclosure, the mixture is introduced into a spheroidizing device that applies a mechanical shear force to form a granulation particle core, and then spheroidized granulation particles can be formed, wherein the spheroidized granulation particles have one or more surface layers that are stacked toward the surface portion of the granulation particle core in a concentric circle direction and combined in a spherical manner. The granulation particle core and the surface layer are formed simultaneously, thereby forming spheroidized granulation particles.

[0047] According to one embodiment of the present disclosure, a mixture of micro-particle flaky graphite and coarse-grained flaky graphite is repeatedly processed using a rotary processing machine to obtain spheroidized granulated particles. As a result of repeated rotation, the micro-particle flaky graphite and coarse-grained flaky graphite are granulated by crushing caused by collision of the mixture of micro-particle flaky graphite and coarse-grained flaky graphite with the inner surface of the processing machine, friction between graphite particles, and shearing caused by shear stress. Finally, spheroidized granulated particles can be obtained. Here, the crushing time and speed can be controlled within an appropriate range according to the amount of graphite introduced into the machine.

[0048] Furthermore, this step may further include a step of isotropically pressurizing the obtained spheroidized granulated particles to improve the contact between the fine flaky graphite and the coarse flaky graphite contained in the spheroidized granulated particles.

[0049] Here, "isotropic pressing" means that the spheroidized granulated particles are three-dimensionally uniformly pressed. To isotropically pressurize the spheroidized granulated particles, water or argon may be used as a medium at room temperature, or cold isotropic pressing may be performed at room temperature.

[0050] Furthermore, although the pressure applied during the isotropic pressurization of the spheroidized granulated particles is not particularly limited, a pressure of 50 atm to 100 atm is preferred, and a pressure of 100 atm to 200 atm is more preferred.

[0051] The term "flaky graphite" refers to natural graphite having a flaky particle shape, and can be obtained by pulverizing natural graphite in the form of flakes, plates, crushed shapes, sheets, etc. into a desired particle size.

[0052] According to one embodiment of the present disclosure, the micro-particle flaky graphite may have an average particle size of 20 μm-50 μm or 25 μm-45 μm, and the coarse-grained flaky graphite may have an average particle size of 50 μm-100 μm or 55 μm-90 μm. When the average particle size of each of the micro-particle flaky graphite and the coarse-grained flaky graphite is within the range defined above, the inner pores are reduced, and therefore, the side reactions and gas generation of the resulting coating layer at high temperatures caused by the inner pores can be reduced, and the high-temperature storage characteristics can be improved.

[0053] According to one embodiment of the present disclosure, the weight ratio of coarse-grained flaky graphite to fine-grained flaky graphite may be 70:30-40:60, 70:30-45:55, 70:30-50:50, or 50:50-45:55. When the weight ratio of coarse-grained flaky graphite to fine-grained flaky graphite satisfies the above-defined range, the inner pores may be advantageously controlled.

[0054] Referring to the embodiment of the present disclosure Figure 1 , the fine flaky graphite 110 is mixed with the coarse flaky graphite 120, and then the resulting mixture is subjected to the above-mentioned spheroidization treatment to obtain the spheroidized granulated particles 100. Here, in the spheroidized granulated particles 100, the empty spaces formed between the coarse flaky graphite particles 120 are filled with the fine flaky graphite 110, so that the inner pores 130 are significantly reduced.

[0055] On the other hand, referring to the prior art Figure 2 , only the coarse-grained flaky graphite 210 is used and spheroidized. In this case, the spheroidized granulated particles 200 can be obtained, but the empty spaces formed between the coarse-grained flaky graphite particles 210 still remain in the spheroidized granulated particles 200 to provide a large number of inner pores 220.

[0056] Next, the spheroidized granulated particles are coated with carbon.

[0057] In the carbon coating step, the spheroidized granulated particles prepared by the previous step are uniformly mixed with a carbon coating material so that the carbon coating material can adhere to the surface of the spheroidized granulated particles, and then the resulting product is carbonized to form a carbon coating on the surface of the spheroidized granulated particles. The carbon material forms a coating on the surface of the spheroidized granulated particles so that the fine flaky graphite and the coarse flaky graphite forming the spheroidized granulated particles can further be combined with each other. In this way, the deterioration of the stability of the spheroidized granulated particles that may occur during repeated charge / discharge cycles can be prevented.

[0058] The carbon coating material can be prepared from a carbon precursor, which includes sucrose, phenolic resin, naphthalene resin, polyvinyl alcohol resin, furfuryl alcohol resin, polyacrylonitrile resin, polyamide resin, furan resin, cellulose resin, styrene resin, polyimide resin, epoxy resin or vinyl chloride resin, coal tar, petroleum asphalt, polyvinyl chloride, mesophase asphalt, tar, block copolymer, low molecular weight heavy oil or a mixture thereof.

[0059] Here, based on 100 parts by weight of the spheroidized granulated particles, the amount of the carbon coating material may be 1 part by weight to 10 parts by weight or 3 parts by weight to 6 parts by weight. When the amount of the carbon coating meets the above-defined range, it is possible to prevent the problem of reduced capacity per unit weight due to the formation of an overly thick coating, reduced initial efficiency due to irreversibility, increased side reactions due to increased specific surface area of ​​the active material due to the formation of an overly thin coating, and reduced life efficiency due to coating peeling during charge / discharge cycles. It can also contribute to the formation of an initial SEI layer, thereby improving the stability of the spheroidized granulated particles during repeated charge / discharge cycles.

[0060] The method for uniformly mixing the surface of the spheroidized granulated particles with the carbon coating material is not particularly limited, and any method generally known to those skilled in the art can be used. For example, the mixing can be performed by using mechanical and chemical methods, such as a kneading machine such as a double roll mill, a paddle, a mechanical micro system, an extruder, a ball mill, a planetary mill, a mechanical fusion system, a Nobilta, a mixer or a rotary ball mill, or by using a spray drying method or an emulsification method.

[0061] After the carbon coating material is uniformly mixed with the spheroidized granulated particles as described above, the resulting mixture is carbonized at a temperature of 900° C. to 1300° C. for 12 hours to 48 hours, so that a carbon coating can be formed on the spheroidized granulated particles. The formed carbon coating may contain amorphous carbon or crystalline carbon. When the carbonization conditions are met, the carbon coating material can be fully stabilized, impurities in the carbon coating material can be completely removed, and the coating surface performance of the carbon coating material can be prevented from deteriorating at high temperatures.

[0062] Then, the carbon-coated spheroidized granulated particles are disintegrated.

[0063] The spheroidized granulated particles obtained by carbonization in the carbon coating step of the spheroidized granulated particles may be agglomerated with each other. Therefore, the agglomerated particles are subjected to a disintegration process.

[0064] In the disintegration step, the agglomerated particles can be easily disintegrated by applying only a slight level of shear force to the agglomerated spheroidized granulated particles. The disintegration process is not particularly limited. For example, the disintegration can be performed by using a stirrer with a stirring blade, or by using a known mill (e.g., a conventional jet mill, a vibration mill, a pin mill, a hammer mill, etc.).

[0065] In another aspect of the present disclosure, a spheroidized carbonaceous negative electrode active material is provided, which has a 1.5 m 2 / g-2.8m 2 / g specific surface area and 1.0e -2 cm 3 / g to 1.8e -2 cm 3 / g of total pore volume, and the pores with a size of 24 nm or more in the negative electrode active material have a 0.1m 2 / g-0.8m 2 / g specific surface area.

[0066] According to one embodiment of the present disclosure, the spheroidized carbonaceous negative electrode active material can be obtained by the above-mentioned method for preparing a spheroidized carbonaceous negative electrode active material.

[0067] The specific surface area of ​​the negative electrode active material is 1.5 m 2 / g-2.8m 2 According to one embodiment of the present disclosure, the specific surface area of ​​the negative electrode active material may be 1.8 m 2 / g-2.5m 2 When the specific surface area of ​​the negative electrode active material satisfies the above-defined range, a side reaction with the electrolyte may be reduced to advantageously provide improved high temperature storage characteristics.

[0068] Here, the specific surface area is determined by the BET method. Specifically, the specific surface area can be calculated by nitrogen adsorption at liquid nitrogen temperature (77K) using BELSOR mino II available from BEL Corporation of Japan.

[0069] The total pore volume of the negative electrode active material is 1.0e -2 cm 3 / g to 1.8e -2 cm 3 According to one embodiment of the present disclosure, the total pore volume of the negative electrode active material may be 1.19e -2 cm 3 / g to 1.57e -2 cm 3 When the total pore volume of the negative electrode active material satisfies the above-defined range, a side reaction with the electrolyte may be reduced to advantageously provide improved high temperature storage characteristics.

[0070] Here, similarly to the measurement of the specific surface area, the total pore volume of the negative electrode active material may be measured by the BET method by using the same system as that for the measurement of the specific surface area.

[0071] In addition, the pores with a size of 24 nm or more in the negative electrode active material have a size of 0.1 m 2 / g-0.8m 2 According to an embodiment of the present disclosure, the specific surface area may be 0.3 m 2 / g-0.7m 2 When the specific surface area of ​​pores having a size of 24 nm or more in the negative electrode active material satisfies the above-defined range, side reactions with the electrolyte may be advantageously reduced.

[0072] Similar to the measurement of the specific surface area, the specific surface area of ​​pores having a size of 24 nm or more in the negative electrode active material may be measured by the BET method and by using the same system as that for the measurement of the specific surface area.

[0073] In particular, it is important to satisfy all the conditions: the specific surface area of ​​the negative electrode active material is 1.5 m 2 / g-2.8m 2 / g; the total pore volume of the negative electrode active material is 1.0e -2 cm 3 / g to 1.8e -2 cm 3 / g; and in the negative electrode active material, the pores having a size of 24 nm or more have a size of 0.1 m 2 / g-0.8m 2 / g of specific surface area, as this reduces irreversibility, suppresses side reactions, and suppresses expansion caused by gas generation during long-term storage.

[0074] According to one embodiment of the present disclosure, the average particle size of the spheroidized carbonaceous negative electrode active material may be 10 μm-20 μm or 11 μm-18 μm.

[0075] The average particle size D 50 It is the particle size at the 50% point in the cumulative distribution of the number of particles as a function of particle size. For example, D 90 It refers to the particle size at the 90% point in the cumulative distribution of the number of particles as a function of particle size, D 10 It refers to the particle size at the 10% point in the cumulative distribution of the number of particles as a function of particle size.

[0076] The average particle size can be determined using a laser diffraction method. Specifically, the powder to be analyzed is dispersed in a dispersion medium and introduced into a commercial laser diffraction particle size analyzer (e.g., Microtrac S3500) to measure the difference in diffraction patterns depending on the particle size when the particles pass through a laser beam, and then calculate the particle size distribution. Then, D 10 , D 50 and D 90 It can be determined by respectively calculating the particle sizes at the 10%, 50% and 90% points in the cumulative distribution of the particle numbers according to the particle sizes in the analyzer system.

[0077] The sphericity of the spherical carbonaceous negative electrode active material may be 0.82-0.98 or 0.88-0.92.

[0078] The sphericity may be a value obtained by dividing the circumference of a circle having the same area as the projected image of the negative electrode active material by the circumference of the projected image, and specifically, may be represented by the following Mathematical Formula 1. The sphericity may be determined by using a particle shape analyzer (such as Sysmex FPIA3000 available from Malvern Corporation).

[0079] [Mathematical formula 1]

[0080] Sphericity = circumference of a circle with the same area as the projected image of the active material / circumference of the projected image

[0081] In another aspect of the present invention, a negative electrode including the negative electrode active material is provided.

[0082] In particular, the negative electrode according to an embodiment of the present disclosure includes a current collector and a negative electrode active material layer provided on at least one surface of the current collector and including the negative electrode active material according to an embodiment of the present disclosure.

[0083] The electrode layer may be formed by coating a slurry for a negative active material layer prepared by dispersing the negative active material according to the present disclosure, a binder, and a conductive material in a solvent on at least one surface of a current collector, followed by drying and pressing.

[0084] The current collector is not particularly limited as long as it has conductivity and does not cause any chemical changes in the corresponding battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. Although the thickness of the current collector is not particularly limited, its thickness can be 3-500 μm.

[0085] The negative electrode active material may be used in an amount of 80 to 99 wt % based on the total weight of the negative electrode slurry composition.

[0086] The binder is a component that helps the bonding between the conductive material and the active material or the bonding with the collector, and is generally used in an amount of 0.1-20% by weight based on the total weight of the negative electrode slurry composition. Specific examples of the binder include: polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylate, styrene-butadiene rubber (SBR), lithium polyacrylate (Li-PAA), etc. More particularly, compared with other binders (such as SBS / CMC), lithium polyacrylate can provide higher adhesion when used for a negative electrode containing an active material with a high silicon content of about 80%. With the help of the above characteristics, when lithium polyacrylate is used for a silicon-based negative electrode, the advantage of lithium polyacrylate is that a high capacity retention rate can be achieved during charge / discharge.

[0087] The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the corresponding battery. Specific examples of the conductive material include: carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal black; conductive fibers such as carbon fibers or metal fibers; metal powders such as fluorocarbons, aluminum or nickel powders; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. The conductive material may be added in an amount of 0.1-20 wt % based on the total weight of the negative electrode slurry composition.

[0088] The dispersion medium may include water or an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount such that the negative electrode paste containing the negative electrode active material and optionally a binder and a conductive material may have a desired viscosity level.

[0089] In addition, there is no particular limitation in the coating process of the negative electrode paste as long as it is a method commonly used in the art. For example, a coating process using a slot die may be used. In addition to this, a Mayer rod coating process, a gravure coating process, a dip coating process, a spray coating process, etc. may also be used.

[0090] In another aspect of the present disclosure, a lithium secondary battery including the negative electrode is provided. In particular, the lithium secondary battery may be obtained by injecting an electrolyte containing a lithium salt into an electrode assembly including a positive electrode, the above negative electrode, and a separator inserted between the positive electrode and the negative electrode.

[0091] The positive electrode may be obtained by mixing a positive electrode active material, a conductive material, a binder, and a solvent to form a paste, and directly coating the paste onto a metal current collector; or casting the paste onto a separate carrier, peeling off the positive electrode active material film from the carrier, and laminating the film onto a metal current collector.

[0092] The positive electrode active material used in the positive electrode may be selected from the group consisting of LiCoO 2 , LiNiO 2 , LiMn 2 O 4 , LiCoPO 4 , LiFePO 4 , and LiNi 1-x-y-z Co x M1 y M2 z O 2 (wherein M1 and M2 each independently represent any one selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, x, y, and z each independently represent the atomic ratio of the elements forming the oxide, and 0 ≤ x < 0.5, 0 ≤ y < 0.5, 0 ≤ z < 0.5 and 0 < x + y + z ≤ 1) of any active material particles in the group, or a mixture of at least two of them.

[0093] On the other hand, the same conductive material, binder, and solvent as those used in manufacturing the negative electrode may be used.

[0094] The diaphragm can be a conventional porous polymer film conventionally used as a diaphragm. For example, the porous polymer film can be a porous polymer film made of a polyolefin polymer (e.g., ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene / hexene copolymer or ethylene / methacrylate copolymer). Such porous polymer films can be used alone or in the form of a laminate. In addition, an insulating film with high ion permeability and mechanical strength can be used. The diaphragm may include a safety reinforced diaphragm (SRS), which includes a ceramic material coated to a small thickness on the surface of the diaphragm. In addition, a conventional porous non-woven fabric web, such as a non-woven fabric web made of high melting point glass fiber or polyethylene terephthalate fiber, can be used, but the scope of the present disclosure is not limited thereto.

[0095] The electrolyte includes a lithium salt as an electrolyte salt and an organic solvent for dissolving the lithium salt.

[0096] Any lithium salt conventionally used for electrolytes for secondary batteries may be used without particular limitation. For example, the anion of the lithium salt may be any one selected from the following groups: - , Cl - Br - ,I - 、NO 3 - 、N(CN) 2 - , BF 4 - , ClO 4 - PF 6 - ,(CF 3 ) 2 PF 4 - ,(CF 3 ) 3 PF 3 - ,(CF 3 ) 4 PF 2 - ,(CF 3 ) 5 PF - ,(CF 3 ) 6 P - CF 3 SO 3 - CF 3 CF 2 SO 3 - ,(CF 3 SO2 ) 2 N - 、(FSO 2 ) 2 N - CF 3 CF 2 (CF 3 ) 2 CO - ,(CF 3 SO 2 ) 2 CH - , (SF 5 ) 3 C - ,(CF 3 SO 2 ) 3 C - CF 3 (CF 2 ) 7 SO 3 - CF 3 CO 2 - , CH 3 CO 2 - 、SCN - and (CF 3 CF 2 SO 2 ) 2 N - .

[0097] The organic solvent contained in the electrolyte may be any organic solvent commonly used without particular limitation. Typical examples of the organic solvent include at least one selected from the following group: propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, cyclopentane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran.

[0098] Especially, in the carbonate organic solvent, ethylene carbonate and propylene carbonate as cyclic carbonate are organic solvents with high viscosity and high dielectric constant, and because they can easily dissociate the lithium salt in the electrolyte, therefore can preferably be used.When such cyclic carbonate is mixed with the linear carbonate (such as dimethyl carbonate or diethyl carbonate) with low viscosity and low dielectric constant and used, it is more preferable to prepare the electrolyte with higher conductivity.

[0099] Optionally, the electrolyte used according to the present disclosure may further contain additives contained in conventional electrolytes, such as an overcharge preventing agent and the like.

[0100] The lithium secondary battery according to the embodiment of the present disclosure can be obtained by inserting a separator between the positive electrode and the negative electrode to form an electrode assembly, introducing the electrode assembly into a bag, a cylindrical battery shell or a prismatic battery shell, and then injecting the electrolyte therein. In one variation, the lithium secondary battery can be obtained by stacking the electrode assemblies, impregnating the stack with the electrolyte and introducing the resulting product into a battery shell, and then sealing.

[0101] According to one embodiment of the present disclosure, the lithium secondary battery may be a stacked type battery, a wound type battery, a stacked and folded type battery, or a cable type battery.

[0102] The lithium secondary battery according to the present disclosure can be used as a battery cell used as a power source for a compact device, and can preferably be used as a unit battery of a medium-to-large battery module containing a plurality of battery cells. Specific examples of such medium-to-large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc. In particular, the lithium secondary battery can be used for batteries of hybrid electric vehicles requiring high output and new energy and renewable energy storage batteries.

[0103] Embodiments will be described more fully below so that the present disclosure can be easily understood. However, the following embodiments can be embodied in many different forms and should not be construed as being limited to the exemplary embodiments set forth therein. On the contrary, these exemplary embodiments are provided so that the present disclosure will be comprehensive and complete, and will fully convey the scope of the present disclosure to those skilled in the art.

[0104] Example 1

[0105] (Preparation of spheroidized carbonaceous negative electrode active materials)

[0106] Coarse-grained flaky graphite with an average particle size of 75 μm and fine-grained flaky graphite with an average particle size of 35 μm were prepared at a weight ratio of 70:30, mixed using a ball mill, and spheroidized using a back jet mill (Hosokawa Micron, Japan) to obtain spheroidized granulated particles. Then, 100 parts by weight of the obtained spheroidized granulated particles were mixed with 5 parts by weight of asphalt (solid asphalt) as a carbon coating material, and the obtained mixture was carbonized at a temperature of 1500° C. for 24 hours for carbon coating, and then the carbon-coated spheroidized granulated particles were crushed to obtain a spheroidized carbonaceous negative electrode active material.

[0107] (Manufacturing of negative electrode)

[0108] The spheroidized carbonaceous negative electrode active material obtained as described above, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed in a weight ratio of 96.6:1:1.3:1.1, and water was added thereto to prepare a negative electrode slurry. The resulting slurry was applied to a copper foil and vacuum dried at a temperature of about 130°C for 10 hours to obtain a negative electrode slurry with an area of ​​1.4875 cm 2 Here, at 3.61 mAh / cm 2 The negative electrode is prepared with a loading amount of .

[0109] (Manufacturing of coin-type secondary battery)

[0110] The negative electrode obtained as described above was used as a working electrode and cut into 1.7671 cm 2 The lithium metal of the working electrode was used as the counter electrode. In addition, a porous polyethylene separator was inserted between the working electrode and the counter electrode to form an electrode assembly. Then, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 1:4, and 0.5 wt% of vinyl carbonate (VC) as an additive for non-aqueous electrolyte and 1M LiPF were added. 6 The nonaqueous electrolyte is added to the mixed solvent to prepare the nonaqueous electrolyte. The electrode assembly is housed in a coin-type case, and the nonaqueous electrolyte is injected therein to obtain a coin-type half-cell secondary battery.

[0111] Example 2

[0112] A spheroidized carbonaceous negative electrode active material was obtained in the same manner as in Example 1 except that coarse-grained flaky graphite having an average particle size of 75 μm and fine-grained flaky graphite having an average particle size of 35 μm were prepared at a weight ratio of 50:50.

[0113] Furthermore, a secondary battery was obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0114] Example 3

[0115] A spheroidized carbonaceous negative electrode active material was obtained in the same manner as in Example 1 except that coarse-grained flaky graphite having an average particle size of 75 μm and fine-grained flaky graphite having an average particle size of 35 μm were prepared at a weight ratio of 45:55.

[0116] Furthermore, a lithium secondary battery was obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0117] Comparative Example 1

[0118] A spheroidized carbonaceous negative electrode active material was obtained in the same manner as in Example 1 except that coarse-grained flaky graphite having an average particle size of 75 μm was used alone instead of fine-grained flaky graphite having an average particle size of 35 μm.

[0119] Furthermore, a secondary battery was obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0120] Comparative Example 2

[0121] A spheroidized carbonaceous negative electrode active material was obtained in the same manner as in Example 1, except that the coarse-grained flaky graphite having an average particle size of 75 μm was not used, and the fine-grained flaky graphite having an average particle size of 35 μm was used alone.

[0122] Furthermore, a secondary battery was obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0123] Test methods and results

[0124] The specific surface area of ​​the negative electrode active material, the total pore volume of the negative electrode active material, and the size of the pores in the negative electrode active material are Surface area of ​​pores larger than 24 nm

[0125] Each negative electrode active material according to Examples 1 to 3 and Comparative Examples 1 and 2 was measured in terms of specific surface area, total pore volume, and specific surface area of ​​pores having a size of 24 nm or more of the negative electrode active material by using the BET method. In particular, the specific surface area, total pore volume, and specific surface area of ​​pores having a size of 24 nm or more were calculated by nitrogen adsorption at liquid nitrogen temperature (77 K) using BELSOR minoII available from BEL Corporation of Japan.

[0126] Average particle size of negative electrode active material

[0127] Each negative electrode active material according to Examples 1 to 3 and Comparative Examples 1 and 2 was dispersed in water as a dispersion medium, and then introduced into a laser diffraction particle size analyzer (Microtrac S3500) to measure the difference in diffraction patterns according to particle size when the negative electrode active material particles passed through a laser beam, and then the particle size distribution was calculated. Then, D 50 That is, the average particle size is determined by calculating the particle size at the 50% point in the cumulative distribution of the particle numbers according to the particle sizes in the analyzer system.

[0128] Sphericity of negative electrode active materials

[0129] The sphericity of each negative electrode active material according to Examples 1 to 3 and Comparative Examples 1 and 2 is defined by the following Mathematical Formula 1. The sphericity is measured by using a Sysmex FPIA 3000 particle shape analyzer available from Malvern Corporation.

[0130] [Mathematical formula 1]

[0131] Sphericity = circumference of a circle with the same area as the projected image of the active material / circumference of the projected image

[0132] Expansion characteristics

[0133] The expansion ratio was determined after each of the secondary batteries according to Examples 1 to 3 and Comparative Examples 1 and 2 was subjected to 30 charge / discharge cycles at a charge / discharge current of 0.1 C and a charge / discharge voltage of 5 mV to 1.5 V. The results are shown in Table 1 below.

[0134] Here, the expansion ratio (%) is defined by the following formula.

[0135] Expansion ratio (%) = [(electrode thickness after charge / discharge cycle - initial electrode thickness) / (initial electrode thickness)] × 100

[0136] High temperature storage characteristics

[0137] After each of the secondary batteries according to Examples 1 to 3 and Comparative Examples 1 and 2 was stored at 60° C. at 100% SOC (state of charge) for 4 weeks, it was charged / discharged at room temperature under the conditions of a charge / discharge current of 0.1 C and a charge / discharge voltage of 5 mV to 1.5 V to determine the capacity retention rate. The results are shown in Table 1 below.

[0138] Here, the capacity retention rate (%) is defined by the following formula.

[0139] Capacity retention rate (%) = [(capacity after high temperature storage) / (initial capacity)] × 100

[0140] Table 1

[0141]

[0142] As can be seen from Table 1, each of the secondary batteries using the negative electrode active materials according to Examples 1 to 3 showed improved results of expansion characteristics and high temperature storage capacity retention characteristics compared to the secondary batteries using the negative electrode active materials according to Comparative Examples 1 and 2.

Claims

1. A method for preparing a spheroidized carbonaceous negative electrode active material for a lithium secondary battery, the method The following steps are involved: Mixing fine flaky graphite with coarse flaky graphite having an average particle size larger than that of the fine flaky graphite, and then spheroidizing the resulting mixture to prepare spheroidized granulated particles in which empty spaces formed between the coarse flaky graphite particles are filled with the fine flaky graphite; Carbon coating the spheroidized granulated particles; and breaking up the carbon-coated spheroidized granulated particles, The average particle size of the micro-scale flaky graphite is 20 μm-50 μm, the average particle size of the coarse-grained flaky graphite is 50 μm-100 μm, and wherein the coarse-grained flaky graphite and the fine-grained flaky graphite are used in a weight ratio of 70:30-40:60, The average particle size D 50 It refers to the particle size at the 50% point in the cumulative distribution of the number of particles as a function of the particle size, and the average particle size is measured using a laser diffraction method.

2. A spheroidized carbonaceous negative electrode active material for a lithium secondary battery, the spheroidized carbonaceous negative electrode active material comprising fine flaky graphite and coarse flaky graphite having an average particle size greater than that of the fine flaky graphite, wherein empty spaces formed between the coarse flaky graphite particles are filled with the fine flaky graphite, The spheroidized carbonaceous negative electrode active material has a 1.5 m 2 / g-2.8m 2 / g specific surface area and 1.0e -2 cm 3 / g to 1.8e - 2 cm 3 / g of total pore volume, and the pores with a size of 24 nm or more in the negative electrode active material have a pore volume of 0.1 m 2 / g-0.8m 2 / g specific surface area, The average particle size D 50 It refers to the particle size at the 50% point in the cumulative distribution of the number of particles as a function of the particle size, and the average particle size is measured using a laser diffraction method.

3. The spheroidized carbonaceous negative electrode active material for lithium secondary battery according to claim 2, wherein the spheroidized carbonaceous negative electrode active material has a 1.8 m 2 / g-2.5m 2 / g specific surface area.

4. The spheroidized carbonaceous negative electrode active material for lithium secondary batteries according to claim 2, wherein the total pore volume of the spheroidized carbonaceous negative electrode active material is 1.19e -2 cm 3 / g to 1.57e -2 cm 3 / g.

5. The spheroidized carbonaceous negative electrode active material for a lithium secondary battery according to claim 2, wherein the pores having a size of 24 nm or more in the negative electrode active material have a size of 0.3 m 2 / g-0.7m 2 / g specific surface area. 6 . The spheroidized carbonaceous negative electrode active material for a lithium secondary battery according to claim 2 , wherein the average particle size of the spheroidized carbonaceous negative electrode active material is 10 μm to 20 μm. 7 . The spheroidized carbonaceous negative electrode active material for a lithium secondary battery according to claim 2 , wherein the spheroidized carbonaceous negative electrode active material has a sphericity of 0.82-0.

98.

8. A negative electrode for a lithium secondary battery, the negative electrode comprising a current collector and a negative electrode active material layer arranged on at least one surface of the current collector, wherein the negative electrode active material layer comprises the spheroidized carbonaceous negative electrode active material for a lithium secondary battery according to any one of claims 2 to 7. 9 . A lithium secondary battery, comprising the negative electrode for a lithium secondary battery according to claim 8 .

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