Negative active material for lithium secondary battery and lithium secondary batteries comprising thereof

Abstract

The present invention relates to a negative electrode active material for a lithium secondary battery and a lithium secondary battery including the same, and the negative electrode active material of the present invention includes natural graphite and satisfies Formula 1 below. (NAH / NAB) × 100 ≥ 3 (In Equation 1 above, NAH and NAB represent the particle strength (MPa) and BET specific surface area (m² / g) of the cathode active material, respectively.)

Description

Technology Field

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

[0002] A lithium secondary battery generally consists of a positive electrode containing a positive active material, a negative electrode containing a negative active material, a separator, and an electrolyte, and charging and discharging are performed through the intercalation and decalation of lithium ions. Since the lithium secondary battery possesses the advantages of high energy density, high electromotive force, and the ability to exhibit high capacity, it is being applied in various fields.

[0003] Furthermore, improving high-temperature performance, such as high-temperature storage and cycling characteristics, in lithium secondary batteries is a critical challenge. For example, there is a significant problem where the high-temperature performance of the anode is likely to deteriorate if the total internal pore volume is high after the anode active material is coated onto a current collector and rolled. Therefore, it is necessary to improve high-temperature characteristics when developing anode active materials for lithium secondary batteries, such as rapid-charge batteries, by minimizing changes in electrode structure and total internal pore volume that occur during electrode rolling.

[0004] Furthermore, as technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly rising. Among secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0005] Furthermore, as interest in environmental issues grows, there is increasing interest in electric vehicles and hybrid electric vehicles that can replace fossil fuel-using vehicles, such as gasoline and diesel vehicles, which are one of the major causes of air pollution; consequently, research is actively underway to use lithium-ion batteries as a power source for the aforementioned electric vehicles and hybrid electric vehicles.

[0006] Recently, due to the rapid rise of electric vehicles (EVs), expectations for the aforementioned lithium-ion batteries are growing, and there is an increasing demand for improvements in rapid charging characteristics while preserving existing capacity. For such improvements in rapid charging, the role of the negative electrode active material, which is responsible for storing lithium ions during charging, is becoming increasingly important.

[0007] Specifically, a carbon-based negative electrode active material is used as the negative electrode active material. The carbon-based negative electrode active material exhibits excellent capacity retention characteristics and efficiency. Since the carbon-based negative electrode active material used as the negative electrode of a lithium secondary battery has a potential close to the electrode potential of lithium metal, the change in crystal structure is small during the insertion and extraction processes of ionic lithium. Furthermore, the carbon-based negative electrode active material enables continuous and repetitive oxidation and reduction reactions at the electrode, allowing the lithium secondary battery to exhibit high capacity and excellent lifespan.

[0008] Various types of materials are used as the carbon-based negative electrode active materials, such as crystalline carbon-based materials like natural graphite and artificial graphite, or amorphous carbon-based materials like hard carbon and soft carbon. Among the carbon-based negative electrode active materials, graphite-based negative electrode active materials are the most widely used because they have excellent reversibility and can improve the lifespan characteristics of lithium secondary batteries. Since the discharge voltage of the graphite-based negative electrode active material is low at -0.2 V compared to lithium, a battery using the graphite-based active material can exhibit a high discharge voltage of 3.6 V, which has an excellent advantage in terms of energy density of lithium secondary batteries.

[0009] Among these, natural graphite is characterized by having the advantages of high capacity and cost-effectiveness compared to other carbon-based materials such as synthetic graphite. However, since the natural graphite generally has low particle strength, it causes internal deformation of the particles during the rolling process accompanying cathode manufacturing. This leads to a problem of reduced pores within the cathode active material containing natural graphite, resulting in a decrease in the diffusion rate of lithium ions and reduced output characteristics. Furthermore, the natural graphite exhibits low particle strength due to the presence of numerous internal pores and has a large surface area, which promotes SEI formation and increases the expansion rate. The problem to be solved

[0010] According to one embodiment of the present invention, a negative electrode active material for a lithium secondary battery provides a negative electrode active material with improved long-term life characteristics by improving the expansion rate and suppressing internal SEI formation.

[0011] According to another embodiment of the present invention, a lithium secondary battery provides a battery comprising a negative electrode active material having the aforementioned advantages. means of solving the problem

[0012] According to one embodiment of the present invention, a negative electrode active material for a lithium secondary battery comprises natural graphite and can satisfy Formula 1 below.

[0013] <Equation 1>

[0014] NA H / NA B ≥ 3

[0015] (NA in Equation 1 above H , and NA B are the particle strength (MPa) and BET specific surface area (m²) of the cathode active material, respectively. 2 means / g)

[0016] In one embodiment, the NA, which is the particle strength of the negative electrode active material, HThe value may be 15 MPa or higher. In one embodiment, the average particle size (D50) of the negative electrode active material may be 15 to 18.5 μm. In one embodiment, the particle size (D10) of the negative electrode active material may be 8 to 11.4 μm.

[0017] In one embodiment, the particle size (D90) of the negative electrode active material may be 23 to 35 μm. In one embodiment, the SPAN value ((D90-D10) / D50) may be 0.5 to 1.5.

[0018] In one embodiment, the negative electrode active material may satisfy the following Equation 2.

[0019] <Equation 2>

[0020] NA B / NA- T ≤ 5.0

[0021] (NA in Equation 2 above B and NA- T ε₀ is the BET specific surface area (m²) of the cathode active material, respectively. 2 / g) and tap density (g / cm²) 3 It means )-

[0022] In one embodiment, the tap density NA- T g 1.0 to 1.5 g / cm³ 3 It may be. In one embodiment, the BET specific surface area NA B is 2.0 to 4.0 m 2 It may be / g. In one embodiment, the pore specific surface area according to the mercury (Hg) intrusion method is 5 to 20 m² 2 It can be / g.

[0023] In one embodiment, the negative electrode active material may satisfy the following Equation 3.

[0024] <Equation 3>

[0025] NA H / (NA B ×NA- T ) ≥ 2.5

[0026] (NA in Equation 3 aboveH , NA B and NA- T ε₀ and ε₀ are the particle strength (MPa) and BET specific surface area (m₀) of the cathode active material, respectively. 2 / g) and tap density (g / cm²) 3 It means )-

[0027] According to another embodiment of the present invention, a lithium secondary battery may include a positive electrode, a negative electrode facing the positive electrode and comprising the aforementioned negative electrode active material, a separator disposed between the negative electrode and the positive electrode, and an electrolyte. Effects of the invention

[0028] According to one embodiment of the present invention, a negative electrode active material for a lithium secondary battery can provide a negative electrode active material with improved long-term life characteristics by controlling internal porosity to improve particle strength, thereby improving the expansion rate and suppressing internal SEI formation.

[0029] According to another embodiment of the present invention, a lithium secondary battery can provide a battery comprising a negative electrode active material having the aforementioned advantages. Brief explanation of the drawing

[0030] Figure 1 shows an SEM image of Example 1 of the present invention. Figure 2 shows an SEM image of Example 2 of the present invention. Figures 3 and 4 are graphs of the lifespan evaluation of the full cells of Example 1 and Example 2 of the present invention. Specific details for implementing the invention

[0031] Terms such as first, second, and third are used to describe various parts, components, regions, layers, and / or sections, but are not limited thereto. These terms are used solely to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, the first part, component, region, layer, or section described below may be referred to as the second part, component, region, layer, or section without departing from the scope of the present invention.

[0032] The technical terms used herein are for the reference of specific embodiments only and are not intended to limit the invention. The singular forms used herein include plural forms unless phrases clearly indicate otherwise. As used in the specification, the meaning of "comprising" specifies certain characteristics, areas, integers, steps, actions, elements, and / or components, and does not exclude the presence or addition of other characteristics, areas, integers, steps, actions, elements, and / or components.

[0033] When it is stated that one part is "above" or "on" another part, it may be directly above or on the other part, or other parts may be involved in between. In contrast, when it is stated that one part is "directly above" another part, no other parts are interposed in between.

[0034] 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 this invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with relevant technical literature and the present disclosure, and are not interpreted in an ideal or highly formal sense unless otherwise defined.

[0035] Hereinafter, embodiments of the present invention will be described in detail. However, these are presented as examples and are not intended to limit the present invention, and the present invention is defined only by the scope of the claims set forth below.

[0036] According to one embodiment of the present invention, a negative electrode active material for a lithium secondary battery may include natural graphite. Specifically, the natural graphite may include various types of graphite, such as flake graphite, crystalline vein graphite, or amorphous graphite, as non-limiting examples.

[0037] In one embodiment, the negative electrode active material may satisfy the following Formula 1.

[0038] <Equation 1>

[0039] NA H / NA B ≥ 3

[0040] (NA in Equation 1 above H , and NA B represents the particle strength and BET specific surface area of ​​the cathode active material, respectively.

[0042] Equation 1 above represents a value obtained by dividing the particle strength of the cathode active material by the specific surface area, and may represent a measure of the expansion of the cathode. Equation 1 above may satisfy 5 or more, specifically 5 to 15, more specifically 5 to 11, and even more specifically 8 to 11.

[0043] By satisfying the aforementioned range of Equation 1, the electrochemical performance can be excellent and the expansion rate improved when applied to a cathode. If Equation 1 falls outside the aforementioned range, the electrochemical performance deteriorates and the expansion rate increases, making it difficult to use as an electrode.

[0044] In one embodiment, the particle strength of the negative electrode active material may be 15 MPa or higher. The particle strength represents the strength (MPa) at the point of fracture when the force is increased at a rate of about 1.3 mN / sec with an indenter for a negative electrode active material having an average particle size (D50) of 15 to 18.5 μm.

[0045] Specifically, the particle strength may be 16 MPa or higher, more specifically 16 to 100 MPa, more specifically 16 to 30 MPa, and more specifically 20 to 25 MPa. By satisfying the aforementioned range of particle strength, there is an advantage that the negative electrode active material can be continuously used without breaking even if it expands during the charging and discharging process of the electrode. In addition, by satisfying the aforementioned range of particle strength, the electrochemical performance can be excellent and the expansion rate improved when applied as a negative electrode.

[0046] In one embodiment, the particle size (D10), which is the particle size corresponding to 10% of the cumulative volume of the negative electrode active material, may be 8 to 11.4 μm. Specifically, the D10 may be 8 to 11 μm, and more specifically, 10.8 to 11.4 μm.

[0047] In one embodiment, the average particle size (D50), which is the particle size corresponding to 50% of the cumulative volume of the negative electrode active material for a lithium secondary battery, may be 15.0 to 18.5 μm. Specifically, the average particle size (D50) may be 16.0 to 18.0 μm, more specifically 17.0 to 18.0 μm, and even more specifically 17.3 to 17.5 μm.

[0048] In one embodiment, the particle size (D90) corresponding to 90% of the cumulative volume of the negative electrode active material for a lithium secondary battery may be 23 to 35 μm. Specifically, the D90 may be 25 to 33 μm, more specifically 25 to 32 μm, and even more specifically 25 to 28 μm.

[0049] If the particle size exceeds the upper limit of the aforementioned range, the relatively high separation ratio leads to a springback problem of the electrode. If the particle size exceeds the lower limit of the aforementioned range, the amount of classification increases, causing problems with process yield.

[0050] In one embodiment, the span value of the negative electrode active material for a lithium secondary battery may be 0.5 to 1.5. The span value is an indicator of the uniformity of the particles of the negative electrode active material and may be a value calculated by (particle size corresponding to 90% of the cumulative volume of the negative electrode active material (D90) - particle size corresponding to 10% of the cumulative volume of the negative electrode active material (D10)) / particle size corresponding to 50% of the cumulative volume of the negative electrode active material (D50). The span value may be 0.8 to 1.3, specifically 0.8 to 1.0.

[0051] If the above SPAN value exceeds the upper limit of the aforementioned range, the relative rejection rate increases, leading to increased springback of the electrode and causing non-uniformity issues during electrode manufacturing. If the above SPAN value exceeds the lower limit of the aforementioned range, the amount of classification increases, resulting in problems with process yield.

[0052] In one embodiment, the negative electrode active material may satisfy the following Equation 2.

[0053] <Equation 2>

[0054] NA B / NA- T ≤ 5.0

[0055] (NA in Equation 2 above B and NA- T represents the BET specific surface area and tap density of the negative electrode active material, respectively)

[0056] The above Equation 2 represents a value obtained by dividing the specific surface area of ​​the negative electrode active material by the tap density, and may be an indicator of lifespan characteristics. The above Equation 2 may be 5.0 or less, specifically, 1 to 3.5, more specifically, 1.95 to 2.57, and even more specifically, 1.95 to 2.3.

[0057] Since the above Equation 2 satisfies the aforementioned range, there is an advantage of excellent lifespan characteristics when applied as a cathode. If the above Equation 2 falls outside the aforementioned range, there is a problem of inferior lifespan characteristics when applied as a cathode.

[0058] If the value of Equation 2 above exceeds the upper limit of the aforementioned range, roughness increases, causing side reaction problems. If the value of Equation 2 above exceeds the lower limit of the aforementioned range, hardness increases, causing problems in the electrode manufacturing process.

[0059] In one embodiment, the negative electrode active material may satisfy the following Equation 3.

[0060] <Equation 3>

[0061] NA H / (NA B ×NA- T ) ≥ 2.5

[0062] (NA in Equation 3 above H , NA B and NA- T represents the particle strength, BET specific surface area, and tap density of the negative electrode active material, respectively)-

[0063] Equation 3 above represents the value obtained by dividing the particle strength of the cathode active material by the product of the specific surface area and the tap density, and may serve as an indicator of the electrochemical performance and expansion of the cathode active material containing natural graphite. By satisfying the aforementioned range, Equation 3 has the advantage of excellent lifespan characteristics of the cathode active material. If Equation 3 does not satisfy the aforementioned range, there is a problem that the lifespan characteristics of the cathode active material are inferior.

[0064] In one embodiment, NA-, which is the tap density of the negative electrode active material T g 1.0 to 1.5 g / cm³ 3 It may be. Specifically, the tap density is 1.0 to 1.2 g / cm³. 3 , specifically, 1.09 to 1.14 g / cm³ 3, more specifically, 1.10 to 1.14 g / cm³ 3 It could be.

[0065] Since the above tap density satisfies the aforementioned range, there is an advantage of excellent lifespan characteristics when applied as a cathode. If the above tap density falls outside the aforementioned range, there is a problem of inferior lifespan characteristics when applied as a cathode.

[0066] In one embodiment, the BET specific surface area NA of the negative electrode active material B 2.0 to 4.0 m 2 It may be / g. Specifically, the BET specific surface area is 2.0 to 3.0 g / cm². 3 , more specifically, 2.2 to 2.8 g / cm³ 3 , more specifically, 2.2 to 2.3 g / cm³ 3 It could be.

[0067] Since the above BET specific surface area satisfies the aforementioned range, there is an advantage of excellent lifespan characteristics when applied as a cathode. If the above tap density deviates from the aforementioned range, there is a problem of inferior lifespan characteristics when applied as a cathode.

[0068] In one embodiment, the pore specific surface area of ​​the negative electrode active material for a lithium secondary battery, measured by the mercury (Hg) indentation method, is 5 to 20 m² 2 It may be / g. Specifically, the pore specific surface area is 8 to 15 m² 2 / g, more specifically, 10 to 15 m 2 / g, more specifically, 10.3 to 13.6 m 2 / g, more specifically, 10.3 to 12.5 m 2 / g can be.

[0069] Since the above-mentioned pore specific surface area satisfies the aforementioned range, the expansion rate of the electrode is improved, and consequently, there is an advantage in that the lifespan characteristics are improved. If the above-mentioned pore specific surface area deviates from the aforementioned range, the electrode expansion rate decreases, and consequently, there is a problem of the electrode being damaged.

[0070] In one embodiment, the negative electrode active material may comprise a core portion containing natural graphite and a coating layer disposed on said core portion. The coating layer may be a carbon-based coating layer. The carbon-based coating layer may be, as a non-limiting example, a graphitic carbon coating, an amorphous carbon coating, a hard carbon coating, a soft carbon coating, a carbon nanotube and carbon nanofiber coating, or a combination thereof. By including said carbon-based coating layer, the negative electrode active material can improve electrical conductivity and enhance the structural stability of the graphite, thereby reducing irreversible capacity loss occurring during initial charge and discharge.

[0071] According to another embodiment of the present invention, a method for manufacturing a negative electrode active material for a lithium secondary battery may include the steps of preparing natural graphite particles, pressurizing the natural graphite particles, grinding the natural graphite particles after the pressurizing step, and heat-treating the ground natural graphite particles. By including the pressurizing step, the negative electrode active material of the present invention can control the internal porosity of the graphite to ensure processability for electrode manufacturing, and provide a negative electrode active material in which the expansion rate during repeated charging and discharging is minimized and side reactions are suppressed.

[0072] In one embodiment, the step of preparing natural graphite particles may be a step of preparing natural graphite particles having an average particle size (D50) of 14.0 to 18.0 μm. Specifically, the average particle size (D50) of the natural graphite particles may be controlled to 14.0 to 16.0 μm, more specifically, to 14.0 to 15.0 μm.

[0073] In one embodiment, the step of preparing natural graphite particles may further include a particle size classification step. The particle size classification step may classify the natural graphite particles so that the average particle size (D50) is controlled to the aforementioned range. The particle size classification step may remove some particles of 14.0 μm or less through air stream classification and remove some particles of 18.0 μm or more through sieve classification.

[0074] If the value exceeds the lower limit during the particle size classification stage, the overall sphericity is reduced due to fine particles; furthermore, when applied as a negative electrode active material, it can increase side reactions and reduce efficiency. If the value exceeds the upper limit during the particle size classification stage, it is difficult to control the orientation within the particles. Additionally, when applied as a negative electrode active material, the increased particle size causes lithium diffusion to take longer, leading to a problem of degraded input / output characteristics.

[0075] The step of pressurizing the natural graphite particles may be a step of applying pressure to the natural graphite particles. The step of pressurizing may be a step of reducing pores inside the natural graphite particles to impart high density to the powder.

[0076] In one embodiment, the step of pressurizing the natural graphite particles may be performed through a Cold Isostatic Press (CIP) or a Hot Isostatic Press (HIP). Specifically, the pressurizing step may pressurize the natural graphite particles using a Cold Isostatic Press.

[0077] In one embodiment, the step of pressurizing the spherical natural graphite particles may involve applying a pressure of 50 MPa or more. Specifically, the pressure may be 80 to 300 MPa, and more specifically, the pressure may be 100 to 200 MPa. In one embodiment, the step of pressurizing the natural graphite particles may include a step of pressurizing for a time of 1 to 180 minutes. Specifically, the time may be 1 minute to 60 minutes.

[0078] If the above-mentioned pressurization step exceeds the upper limit of the time, there is a problem where the structure of the spherical graphite collapses. If the above-mentioned pressurization step exceeds the lower limit of the time, there is a problem where internal pore control is difficult.

[0079] As pressure is applied within the aforementioned range, side reactions can be suppressed and the lifespan characteristics of the battery can be improved by reducing the reaction area between the graphite and the electrolyte. As the graphite becomes more densified, the tap density increases and the liquid injection performance on the high-density electrode plate can be improved. Specifically, volume expansion during the cycle can be suppressed, such as by suppressing the swelling phenomenon of the graphite particles.

[0080] After the above-mentioned pressurizing step, a grinding step may be included. The grinding step may be a step of grinding while controlling the particle size of the natural graphite particles. In one embodiment, the grinding step may be controlled by physical impact. The physical impact may utilize equipment that utilizes physical impact, such as a jet mill, an air classifier mill (or air current classification mill), or a roller mill. The jet mill directly grinds the particles by using a high-pressure airflow to collide with the particles, the air classifier grinds or classifies particles according to specific gravity using an airflow, and the roller mill grinds the particles by feeding, compressing, and grinding the particles between two or more rollers rotating in opposite directions.

[0081] In one embodiment, the grinding step may be a step of grinding the average particle size (D50) of the natural graphite particles to 15.0 to 18.5 μm. Specifically, the average particle size (D50) may be 16.0 to 18.0 μm.

[0082] If the average particle size of the natural graphite particles in the grinding step exceeds the upper limit of the aforementioned range, the relative proportion of coarse particles is high, increasing the springback of the electrode and causing non-uniformity issues during electrode manufacturing. If the average particle size of the natural graphite particles in the grinding step exceeds the lower limit of the aforementioned range, the relative proportion of fine particles is high, increasing the specific surface area of ​​the powder and causing processability issues during slurry manufacturing.

[0083] In one embodiment, the grinding step after the pressurization process may include a coarse grinding step and a fine grinding step. By including at least one grinding step, the particle size of the graphite particles can be controlled more precisely.

[0084] In one embodiment, the coarse grinding step can grind graphite lumps in a pressurized process to have an average particle size (D50) of 10 to 50 mm. Specifically, the average particle size (D50) can be ground to 10 to 30 mm. In one embodiment, the fine grinding step can grind the ground material that has undergone the coarse grinding step to an average of 10 mm or less.

[0085] The step of heat-treating the crushed natural graphite particles may be a step of heat-treating the natural graphite particles at a high temperature. Specifically, the heat-treating step may be a step of heat-treating at 1,000 to 1,500 ℃. Specifically, the temperature may be a step of heat-treating at 1,100 to 1,300 ℃. By performing heat treatment in the above temperature range, a high-strength cathode active material can be obtained. In one embodiment, the heat treatment may be performed in a gas atmosphere of hydrogen, nitrogen, argon, or a mixture thereof, as a non-limiting example.

[0086] In one embodiment, after the grinding step, the method may further include a step of coating the natural graphite particles with a carbon-based material. For example, the carbon-based material may be a graphitic carbon material, an amorphous carbon material, a hard carbon material, a soft carbon material, a carbon nanotube material, a carbon nanofiber material, or a combination thereof. For example, the carbon-based material may be a low-crystallinity carbon material.

[0087] The above low-crystallinity carbon material may be one or more selected from the group consisting of petroleum pitch, coal pitch, mesophase pitch, heavy oil, light oil, polyvinyl alcohol (PVA), polyvinyl chloride (PVC), sucrose, phenol resin, furan resin, furfuryl alcohol, polyacrylonitrile, cellulose, styrene, polyimide, epoxy resin, and glucose, as non-limiting examples.

[0088] In one embodiment, the low-crystallinity carbon material may have a softening point of 200 to 270°C. Specifically, the softening point may be 240 to 260°C. If the softening point deviates from the upper limit of the aforementioned range, there is a problem in that the hardness increases due to a high coking value. If the softening point deviates from the lower limit of the aforementioned range, there is a problem in that the specific surface area increases due to a low coking value.

[0089] By coating the above natural graphite particles with a low-crystallinity carbon material, side reactions caused by the electrolyte are suppressed when applied as a negative electrode active material, thereby reducing irreversible reactions and improving electrochemical properties.

[0090] In the step of forming a coating layer on the natural graphite particles, the content of the coating material may be included in an amount of 0.1 to 20 parts by weight based on 100 parts by weight of the natural graphite particles. When coated within the above weight range, the natural graphite particles and the coating material can effectively and easily form a bonding layer, and excellent lifespan characteristics of the lithium secondary battery can be exhibited due to reduced side reactions and improved conductivity.

[0091] In the step of forming the coating layer, a mechanical mixing method may be utilized to coat a coating material onto the surface of natural graphite particles. The mechanical mixing method may be one or more selected from the group consisting of ball milling, mechanofusion milling, shaker milling, planetary milling, atritor milling, shape milling, nauta milling, nobilta milling, high speed mixing, paddle mixing, ribbon mixing, Henschel mixing, corn type mixing, thinky mixing, homo mixing, and a stirrer.

[0092] According to another embodiment of the present invention, a lithium secondary battery may include a positive electrode comprising a positive active material, a negative electrode comprising a negative active material, and a separator and an electrolyte located between the positive electrode and the negative electrode.

[0093] The above anode is LiCoO2, LiNiO2, LiNi x Mn y O2, Li 1+z Ni x Mn y Co 1-x-y O2, LiNi x Co y Al zAny one of the group consisting of O2, LiV2O5, LiTiS2, LiMoS2, LiMnO2, LiCrO2, LiMn2O4, LiFeO2, LiFePO4, and combinations thereof, wherein x is 0.3 to 0.8, y is 0.1 to 0.45, and z is independently 0 to 0.2. More specifically, the anode may be LiFePO4, LiCoO2, NCM811, or NCM622.

[0094] The above-mentioned cathode may use the cathode active material described above, or a cathode active material manufactured through a method for manufacturing a cathode active material.

[0095] The above separator may be a porous polymer film made of a polyolefin-based polymer, such as a conventional porous polymer film conventionally used as a separator, for example, an ethylene homopolymer, a propylene homopolymer, an ethylene / propylene copolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, used alone or in a laminate thereof, or a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc., and this is a non-limiting example.

[0096] In the above electrolyte, the lithium salt that may be included as the electrolyte can be any that are commonly used in electrolytes for lithium secondary batteries without limitation, and for example, as the anion of the lithium salt, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF -, (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - It can be any one selected from the group consisting of.

[0097] In the above electrolyte, organic solvents commonly used in electrolytes for lithium secondary batteries may be used without limitation, and representatively, any one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran, or a mixture of two or more of these may be used.

[0098] The above lithium secondary battery may be placed inside a battery case. The battery case may be any one of a cylindrical, prismatic, pouch, and coin type using a can, as a non-limiting example.

[0099] Embodiments of the present invention are described below in detail so that those skilled in the art can easily implement them. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0101] (Manufacturing of cathode active material)

[0102] <Example 1>

[0103] (1) Natural graphite powder preparation step

[0104] Natural graphite powder with an average particle size (D50) of 16 μm was prepared.

[0106] (2) Grinding and coating steps

[0107] The molded sample is obtained as a graphite lump several tens of cm in size and is ground to 1 to 50 mm using a coarse grinder. The coarsely ground graphite particles are ground to less than 1 mm using a Pin Mill, and finely ground through two processes in which the adhering graphite particles are broken down using an air stream classifier. Subsequently, 7.7 parts by weight of pitch with a softening point of 250°C is mixed with the ground sample based on 100 parts by weight of the ground sample, and a coating layer is formed by performing heat treatment by mixing at 1,500 rpm for 10 minutes.

[0109] (3) Heat treatment process

[0110] Subsequently, the crushed product was heat-treated at 1,200 ℃ for 13 hours to obtain a cathode active material.

[0112] (Manufacturing of the cathode)

[0113] The negative electrode active material powder produced by the above method, the conductive material, the binder styrene butadiene rubber (SBR), and the thickener carboxymethylcellulose (CMC) were mixed in mass ratios of Coin Half Cell (CHC) - 96.3:1:1.5:1.2 and Full Cell (FC) - 95.6:1.0:1.1:2.3, and water was added to prepare a negative electrode active material slurry. The negative electrode active material slurry was coated onto a copper (Cu) foil current collector, and then dried and rolled to manufacture the respective negative electrodes. At this time, the negative electrode had a wattage of 3.95 mAh / cm² 2 A cathode for full cell evaluation was prepared by adjusting the N / P ratio to 1.1. In addition, a separator made of polyethylene was inserted between the working electrode and the counter electrode, and as the electrolyte, an ethylene carbonate (EC):ethylmethyl carbonate (EMC) = 25:75, LiPF6 1.2M electrolyte containing 1,3-propane sulfone, vinylene carbonate (VC), etc. was used.

[0115] <Example 2>

[0116] After step (1) of Example 1, the process was performed in the same manner as Example 1, except that a Cold Isostatic Pressing (CIP) process was performed.

[0117] In the above cold isostatic pressing process, natural graphite powder was filled into a molding mold, and a molded body was manufactured through a press process. At this time, the applied molding pressure was 150 MPa, and the time was 1 minute. Cold isostatic pressing was applied in the above molding process.

[0119] <Comparative Example 1>

[0120] Compared to Example 1, the procedure was performed in the same manner as Example 1, except that the cold isostatic pressing step was not performed and pitch coating was not performed in the grinding and coating steps.

[0122] <Evaluation Example 1>: Physical properties of cathode active material

[0123] Table 1 below shows the physical properties of the negative electrode active materials of Examples 1 and 2 and Comparative Example 1. The physical properties of the negative electrode active materials were measured by the following method.

[0124] D10(㎛), D50(㎛), D90(㎛), SPAN: D10, D50, and D90 represent the particle sizes corresponding to 1%, 10%, 50%, 90%, and the maximum value of the cumulative volume of the cathode active material, respectively, measured using the Laser Diffraction Method. In this case, the SPAN value represents the value of ((D90-D10) / D50).

[0125] Tap density (g / cm²) 3 ): Tap density was measured using the AT-2 model from Quantachrome. After filling the test container with a volume of 20 ml and tapping it 3,000 times, the volume and weight of the sample were measured to obtain the ratio of weight to volume.

[0126] BET(m 2 / g): It refers to the specific surface area, which is the total surface area of ​​the material per unit mass, and was measured using the BET method (Surface Area and Porosity Analyzer) (Micromeritics, ASAP2020).

[0127] Particle strength (MPa) : A single particle of the average particle size (D50) of the cathode active material was located using an optical microscope, and force was applied with an indenter to determine the force or pressure value at the point where the particle broke. Specifically, the indenter was used to observe the point of breakage while increasing the force at a rate of 1.3324 mN / sec, and the maximum value was set to 98.066 mN.

[0128] Porous specific surface area (m²) according to the Hg (mercury) intrusion method 2 / g):Using the Micromeritics (Autopore V 9620) instrument, the volume of mercury intruded into the parts identified as pores within the range of 3 nm to 2000 nm was measured, and the cumulative pore specific surface area calculated based on this was measured.

[0129] Particle size distribution (㎛) BET, AND B [I 2 / g] Tap density, NA- T [g / cm 3 ] Particle strength, NA H [MPa] Pore ​​specific surface area [m²] according to the mercury (Hg) intrusion method 2 / g] Equation 1 * Equation 2 ** Equation 3 *** D 10 D 50 D 90 SPAN Example 1 10.8 17.5 32.0 1.21 2.8 1.09 16.8 13.6 6 2.57 5.50 Example 2 11.4 17.3 25.8 0.83 2.22 1.14 22.9 10.3 10.32 1.95 9.05 Comparative Example 1 11.5 18.7 29.5 0.96 5 0.91 10.9 - 2.18 5.49 2.40 *Equation 1: NA H / NA B **Equation 2: NA B / NA- T ***Equation 3: NA H / (NA B ХNA- T )

[0130] Looking at Table 1 above, it can be seen that Example 1, Example 2, and Comparative Example 1 differ in particle size distribution, non-BET specific surface area, tap density, particle strength, and pore specific surface area, and that there are differences in the values ​​of Equations 1 to 3 derived from the aforementioned composition. In addition, it was confirmed that Example 2 has superior tap density, lower specific surface area, and lower pore specific surface area value than Example 1.

[0131] Specifically, when examining the particle strength of Example 1, Example 2, and Comparative Example 1, and checking the particle failure points of Example 1, Example 2, and Comparative Example 1, it can be seen that the particle in Comparative Example 1 fails at a lower strength compared to the Examples. When comparing Example 1 and Example 2, it can be seen that the particle strength of Example 2 is higher, so the strength at which the particle fails is higher.

[0132] Figure 1 shows an SEM image of Example 1 of the present invention.

[0133] Figure 2 shows an SEM image of Example 2 of the present invention.

[0134] Referring to Figures 1 and 2, when examining the SEM images of Example 1 and Example 2, it can be seen that the pore distribution in Example 2 is lower.

[0136] <Evaluation Example 2>: Electrochemical properties

[0137] Table 2 below shows the electrochemical characteristics in a Coin Half Cell and a Full Cell prepared from the negative electrode active material prepared by Example 1, Example 2, and Comparative Example 1.

[0138] The measurement method for the above electrochemical properties is as follows.

[0139] Evaluation of Discharge Capacity (mAh / g) and Initial Efficiency (%) of Coin Half Cells: After fabricating the lithium secondary battery half-cell, a charge-discharge test was performed after aging at 25°C for 48 hours. Using an electrochemical analyzer (WonATech), the initial capacity and efficiency were measured after performing three charge-discharge formations under conditions of 25°C, a potential range of 0.005 to 1.5 V, and a current density of 0.36 mA / cm².

[0140] Evaluation of Full Cell 25 ℃ Cycle Capacity Retention Rate (%) and 45 ℃ Cycle Capacity Retention Rate (%): After fabricating the lithium secondary battery, a life evaluation was conducted at 25°C after charging at 0.5°C and discharging at 1.0°C, and a life evaluation was conducted at 45°C after charging at 0.5°C and discharging at 0.5°C. In the aforementioned process, the lower limit voltage was 2.75 V.

[0141] In this case, the capacity retention rate refers to the value obtained by dividing the capacity after a cycle by the initial capacity and multiplying the result by 100. The 25 ℃ cycle capacity retention rate was measured after performing 3,000 cycles at 25 degrees, and the 45 ℃ cycle capacity retention rate was measured after performing 900 cycles.

[0142] 45 ℃ Electrode thickness change rate (%): After performing 3,000 cycles at 45 ℃, the cell was separated and the rate of change in the thickness of the cathode was measured.

[0143] Coin Half Cell Full Cell Discharge capacity [mAh / g] Initial efficiency[%] 25 ℃ cycle 3000 th Capacity retention rate[%] 45 ℃ cycle 900 th Capacity retention rate[%] Rate of change in electrode thickness after 45 ℃ cycled [%] Example 1 363.5 94.0 70.8 78.1 10.2 Example 2 364.1 93.9 72.3 79.6 9.3 Comparative Example 1 365.8 93.9 - - -

[0144] Looking at Table 2 above, it was confirmed that Examples 1 and 2 were superior to Comparative Example 1 in terms of the electrochemical characteristics of the battery. Specifically, it was confirmed that Examples 1 and 2 were superior to Comparative Example 1 in terms of discharge capacity and initial efficiency. In addition, when examining Examples 1 and 2, it was confirmed that in the full cell evaluation, Example 2 had a higher capacity retention rate than Example 1, and Example 1 also had a higher electrode thickness change rate than Example 2, confirming that Example 2 had superior performance.

[0145] Figures 3 and 4 are lifespan evaluation graphs of the full cells of Example 1 and Example 2 of the present invention.

[0146] Figure 3 shows the life evaluation of Example 1 and Example 2 at 25°C, and Figure 4 shows the life evaluation of Example 1 and Example 2 at 45°C. Looking at Figures 3 and 4, it can be confirmed that Example 2 has superior life characteristics compared to Example 1.

[0148] Although preferred embodiments have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements by those skilled in the art using the basic concepts defined in the following claims are also included within the scope of the present invention.

Claims

Claim 1 A negative electrode active material comprising natural graphite and satisfying the following Formula 1. <Formula 1> NA H / NA B ≥ 3(NA in Equation 1 above H , and NA B are the particle strength (MPa) and BET specific surface area (m²) of the cathode active material, respectively. 2 / g) means, and the above NA H is 16 to 30 MPa) Claim 2 delete Claim 3 A negative electrode active material according to claim 1, wherein the average particle size (D50) is 15 to 18.5 μm. Claim 4 In claim 1, the negative electrode active material has a particle size (D10) of 8 to 11.4 μm. Claim 5 In claim 1, the negative electrode active material has a particle size (D90) of 23 to 35 μm. Claim 6 A negative electrode active material according to claim 1, wherein the SPAN value ((D90-D10) / D50) is 0.5 to 1.

5. Claim 7 In claim 1, a negative electrode active material satisfying the following Equation 2. <Equation 2> NA B / NA- T ≤ 5.0(NA in Equation 2 above B and NA- T ε₀ is the BET specific surface area (m²) of the cathode active material, respectively. 2 / g) and tap density (g / cm²) 3 It means )- Claim 8 In claim 1, the tap density NA- T g 1.0 to 1.5 g / cm³ 3 Phosphorus cathode active material. Claim 9 In claim 1, the BET specific surface area NA B is 2.0 to 4.0 m 2 Cathode active material with a g content. Claim 10 In claim 1, the pore specific surface area according to the mercury (Hg) intrusion method is 5 to 20 m 2 Cathode active material with a g content. Claim 11 In claim 1, a negative electrode active material satisfying the following Equation 3. <Equation 3> NA H / (NA B ×NA- T ) ≥ 2.5(NA in Equation 3 above H , NA B and NA- T ε₀ and ε₀ are the particle strength (MPa) and BET specific surface area (m₀) of the cathode active material, respectively. 2 / g) and tap density (g / cm²) 3 It means ) Claim 12 A lithium secondary battery comprising: a positive electrode; a negative electrode facing the positive electrode and comprising a negative electrode active material according to any one of claims 1, 3 to 11; a separator disposed between the negative electrode and the positive electrode; and an electrolyte.

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