Negative active material and lithium secondary battery comprising the same

Abstract

A negative active material according to an embodiment of the present application includes first graphite particles and second graphite particles, wherein the second graphite particles have a different particle size from the first graphite particles. The ratio of the particle density of the negative active material to the tap density is 1.3 to 1.45. The particle deformation of the negative electrode is suppressed, thereby obtaining a lithium secondary battery having improved long-term performance and high energy performance.

Description

[0001] Cross-reference and priority claims of related applications

[0002] This application claims priority to Korean Patent Application No. 10-2020-0098124, filed with the Korean Intellectual Property Office (KIPO) on August 5, 2020, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] This invention relates to a negative electrode active material, a negative electrode containing the negative electrode active material for a lithium secondary battery, and a lithium secondary battery. Background Technology

[0004] With the development of information and display technologies, rechargeable and dischargeable secondary batteries have been widely used as power sources for mobile electronic devices such as portable cameras, mobile phones, and laptops. Recently, battery packs incorporating secondary batteries have also been developed and applied to environmentally friendly vehicles, such as hybrid vehicles, as their power source.

[0005] Secondary batteries include, for example, lithium-ion batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Lithium-ion batteries have attracted much attention due to their high operating voltage and energy density per unit weight, high charge rate, and compact size.

[0006] For example, a lithium secondary battery may include an electrode assembly comprising a positive electrode, a negative electrode, and a separator layer (separator); and an electrolyte immersing the electrode assembly. The lithium secondary battery may further include a housing having, for example, a pouch shape.

[0007] In early lithium-ion batteries, lithium metal was used as the negative electrode active material, but due to safety concerns during repeated charging / discharging, carbon-based materials such as graphite are replacing lithium metal.

[0008] The electrochemical reaction potential between the negative electrode active material formed by carbon-based materials and lithium ions is similar to that between lithium metal and lithium ions. Furthermore, almost no changes in the crystal structure occur in the carbon-based materials during the continuous insertion / desorption of lithium ions, thereby providing continuous charging / discharging and improving charge / discharge lifespan.

[0009] To achieve lithium-ion rechargeable batteries with higher capacity / power, negative electrode active material is coated onto the current collector and pressed under high voltage to increase the density of the active material layer and the discharge capacity per unit volume. However, it may not be easy to obtain an active material layer with a sufficiently high density from conventional carbon-based materials.

[0010] Recently, with the expansion of lithium-ion battery applications, lithium-ion batteries with higher power and higher capacity are being developed. For example, research is underway on positive or negative electrode materials that can provide higher capacity. Therefore, the application of silicon-carbon composites in the negative electrode is being investigated. However, silicon-carbon composites can cause significant volume expansion during charging and discharging.

[0011] For example, Korean Patent Publication No. 10-0326446 discloses a negative electrode active material comprising spherical carbon-based material, which may not be able to provide sufficiently high density and energy. Summary of the Invention

[0012] According to one aspect of the present invention, a negative electrode active material with improved operational stability and electrical properties is provided.

[0013] According to one aspect of the present invention, a negative electrode mixture and a negative electrode for a lithium secondary battery are provided, which have improved operational stability and electrical performance.

[0014] According to one aspect of the present invention, a lithium secondary battery with improved operational stability and electrical performance is provided.

[0015] According to an exemplary embodiment of the present invention, the negative electrode active material comprises first graphite particles and second graphite particles having a different particle size from the first graphite particles. The particle density and tap density of the negative electrode active material satisfy the relationship in Equation 1:

[0016] [Formula 1]

[0017] 1.3≤D P / D T ≤1.45

[0018] In Equation 1, D P The particle density (g / cm³) of the negative electrode active material 3 ), D T It is the tap density (g / cm³) of the negative electrode active material. 3 ),

[0019] Particle density is calculated by measuring the height difference after 1g of negative electrode active material is placed in a cylindrical granulator with a diameter of 13mm and pressurized with 3 metric tons for 10 seconds.

[0020] Tap density is determined by measuring the volume and weight of a sample obtained by filling a 10g sample of negative electrode active material into a 25ml graduated cylinder and then tapping it 3000 times with a stroke length of 10mm.

[0021] In some implementations, the second graphite particle may contain artificial graphite with a hardness higher than that of the first graphite particle.

[0022] In some implementations, the amount of the second graphite particles can range from 20% to 40% by weight, based on the total weight of the negative electrode active material.

[0023] In some implementations, the first graphite particle and the second graphite particle satisfy Equation 2:

[0024] [Equation 2]

[0025] 0.3≤D2 / D1≤0.56

[0026] In Equation 2, D1 is the average particle size of the first graphite particle (D 50 D2 is the average particle size of the second graphite particle (μm), where D2 is the average particle size of the second graphite particle (D 50 (μm).

[0027] In some implementations, the average particle size (D) of the first graphite particle 50 The diameter can range from 16μm to 19μm.

[0028] In some implementations, the average particle size (D) of the second graphite particles 50 The diameter can range from 7μm to 9μm.

[0029] In some implementations, the first graphite particle may include assembly-type artificial graphite.

[0030] According to an exemplary embodiment of the present invention, the negative electrode mixture for a lithium secondary battery comprises the negative electrode active material as described above, and has a density of 1.8 g / cm³. 3 above.

[0031] In some embodiments, the negative electrode mixture further includes a solvent, a conductive agent, and a binder.

[0032] According to an exemplary embodiment of the present invention, the negative electrode for a lithium secondary battery includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, the negative electrode active material layer including the negative electrode active material as described above.

[0033] According to an exemplary embodiment of the present invention, a lithium secondary battery includes a positive electrode and a negative electrode opposite the positive electrode and comprising the negative electrode active material as described above.

[0034] The negative electrode active material according to an exemplary embodiment of the present invention may include first graphite particles and second graphite particles, and the ratio of the particle density to the tap density of the negative electrode active material may be in the range of 1.3 to 1.45. This suppresses particle deformation of the negative electrode, thereby providing a lithium secondary battery with high energy density while ensuring long-term performance.

[0035] In some implementations, the second graphite particles may comprise high-hardness artificial graphite, and the content of the second graphite particles may be from 20% to 40% by weight, depending on the total weight of the negative electrode active material. Therefore, during the application of pressure to the electrode, such as during pressing, hard particles with low deformability can serve as a support to prevent cracking in the negative electrode active material and to achieve high-temperature storage and longevity characteristics of the lithium secondary battery. Attached Figure Description

[0036] Figure 1 and Figure 2 These are schematic top views and schematic cross-sectional views of a lithium secondary battery according to an exemplary embodiment. Detailed Implementation

[0037] According to an exemplary embodiment of the present invention, a negative electrode active material is provided, comprising first graphite particles and second graphite particles with different diameters, such that their density ratio is within a predetermined range. According to an exemplary embodiment of the present invention, a negative electrode mixture and a negative electrode formed using the negative electrode active material, and a lithium secondary battery including the negative electrode are also provided.

[0038] The invention will now be described in detail with reference to the accompanying drawings. However, those skilled in the art will understand that these embodiments described with reference to the drawings are provided to further understand the spirit of the invention and are not intended to limit the subject matter disclosed in the detailed specification and appended claims.

[0039] <Negative Electrode Active Material>

[0040] The negative electrode active material according to an embodiment of the present invention may include first graphite particles and second graphite particles with different particle sizes, and may satisfy the following formula 1.

[0041] [Formula 1]

[0042] 1.3≤D P / D T ≤1.45

[0043] In Equation 1, D P The particle density (g / cm³) of the negative electrode active material 3 ), D T It is the tap density (g / cm³) of the negative electrode active material.3 ).

[0044] To calculate the "particle density" of the negative electrode active material, 1g of the material is placed in a cylindrical granulator with a diameter of 13mm, a pressure of 3 metric tons is applied to the granulator for 10 seconds, and then the height of the granulator is measured. The particle density can be obtained using the height difference from the initial empty granulator. For example, manual type pressure can be used to apply pressure to the granulator.

[0045] In the measurement of "tap density", 10g of negative electrode active material is filled into a 25ml graduated cylinder, and then tapped 3000 times with a stroke length of 10mm. The tap density is obtained from the volume and weight of the sample. The average of the three measurements can be used as the tap density.

[0046] As indicated by Formula 1 above, in the negative electrode active material according to an exemplary embodiment of the present invention, the ratio of particle density to tap density can be 1.3 or more and 1.45 or less. Preferably, the ratio of particle density to tap density of the negative electrode active material can be 1.35 or more.

[0047] If the ratio of particle density to tap density, as shown in Equation 1, is below 1.45, deformation of the anode electrode active material and volume expansion of the anode electrode can be effectively suppressed, thus providing an electrode with high energy and high density. If the ratio of particle density to tap density exceeds 1.45, physical damage, such as cracks, will be caused to the anode active material during the pressing process, and side reactions will occur between the anode active material and the electrolyte during repeated charging and discharging.

[0048] If the ratio of particle density to tap density is less than 1.3, the density of the negative electrode mixture formed by the negative electrode may decrease, a high-density electrode may not be formed, and the rate performance of the secondary battery may decrease.

[0049] The negative electrode active material used in lithium secondary batteries can be used as a substance for absorbing and desorbing lithium ions, and graphite particles can be used as the negative electrode active material. The first graphite particles and the second graphite particles may include at least one of the following: artificial graphite, natural graphite, graphitized carbon fiber, graphitized meso-carbon microbeads, petroleum coke, sintered resin, carbon fiber, pyrolytic carbon, etc.

[0050] If the negative electrode active material used in a lithium secondary battery enables lithium ion insertion and extraction, the shapes of the first and second graphite particles are not particularly limited. In one embodiment, spherical or plate-shaped active materials can be used to improve the performance of the negative electrode active material.

[0051] In some implementations, the second graphite particles may comprise high-hardness artificial graphite, such as high-hardness crushed artificial graphite. Therefore, the second graphite particles can effectively serve as a support for the negative electrode of a lithium-ion secondary battery and can suppress particle deformation of the negative electrode active material during the pressing process. This prevents particle damage due to electrolyte penetration or decomposition reactions, thus providing long-term battery performance.

[0052] In some implementations, the content of the second graphite particles can be from 20% to 40% by weight, based on the total weight of the negative electrode active material.

[0053] If the amount of the second graphite particles is less than 20% by weight based on the total weight of the negative electrode active material, the graphite particles may be easily crushed during the pressing process, and the lithium-ion diffusion may be degraded, thereby reducing charge / discharge performance and cycle performance.

[0054] If the amount of second graphite particles exceeds 40% by weight based on the total weight of the negative electrode active material, pores may exist in the negative electrode due to insufficient pressing during the pressing process, thus preventing the formation of a high-density negative electrode mixture. Furthermore, cracks may occur in adjacent heterogeneous graphite particles, which could be detrimental to ensuring long-term performance, such as high-temperature storage performance and lifetime performance.

[0055] In some exemplary embodiments, the first graphite particle may include assembled artificial graphite.

[0056] Natural graphite can have needle-like or plate-like particle shapes and a large surface area due to its irregular structure. Therefore, when used in batteries, natural graphite can be easily damaged by electrolyte penetration or electrolyte decomposition reactions, and significant irreversible reactions may occur.

[0057] Therefore, assembled artificial graphite can be included, which can suppress changes in particle shape and promote elastic deformation and gap reduction between particles during pressing. Thus, high-density electrodes can be effectively obtained.

[0058] In an exemplary embodiment, the first graphite particle and the second graphite particle can satisfy the following equation (2).

[0059] [Equation 2]

[0060] 0.3≤D2 / D1≤0.56

[0061] In Equation 2, D1 is the average particle size of the first graphite particle (D 50 D2 is the average particle size (D, μm) of the second graphite particle. 50 (μm).

[0062] In this instruction manual, "D" 50 "Average particle size" refers to the volume average value D in the particle size distribution measured by laser diffraction. 50 (That is, the particle size when the cumulative volume is 50%), unless otherwise defined, the term "average particle size" refers to D 50 Average particle size.

[0063] As shown in Formula 2, in the negative electrode active material according to an embodiment of the present invention, the ratio of the average particle size of the second graphite particles to the average particle size of the first graphite particles can be from 0.3 to 0.56. Preferably, the ratio of the average particle size of the second graphite particles to the average particle size of the first graphite particles can be from 0.35 to 0.5. More preferably, the ratio of the average particle size of the second graphite particles to the average particle size of the first graphite particles can be from 0.4 to 0.45.

[0064] If the ratio of the average particle size of the second graphite particles to the average particle size of the first graphite particles is less than 0.3, pores may not form in the negative electrode to interfere with the impregnation of the electrolyte and lead to an increase in battery resistance and a decrease in the discharge capacity per unit volume of the battery.

[0065] If the ratio of the average particle size of the second graphite particle to the average particle size of the first graphite particle exceeds 0.56, the density of the negative electrode will decrease, thereby reducing charge / discharge performance and cycle performance.

[0066] In some embodiments, the average particle size of the first graphite particles can be from 16 μm to 19 μm, and the average particle size of the second graphite particles can be from 7 μm to 9 μm. Within this range, porosity between the negative electrode active materials can be appropriately achieved to more effectively prevent material deformation of the negative electrode. Furthermore, the negative electrode density can be more easily increased to improve charge / discharge efficiency and lifetime characteristics.

[0067] In an exemplary embodiment, as described above, particle density can be measured by measuring the volume change after pressurizing a negative electrode active material sample by placing it in a container and then applying a predetermined pressure.

[0068] For example, it can be determined according to the following equation 3 at 2000 kgf / cm 2 The particle density of the negative electrode active material was measured under pressure.

[0069] [Equation 3]

[0070] Particle density = W / [π×(13 / 2)] 2 ×(H2-H1) / 1000]

[0071] In Equation 3 above, W(g) is the weight of the sample, H2(mm) is the height of the granulator after pressurization, and H1 is the height of the granulator before pressurization.

[0072] <For negative electrodes and lithium secondary batteries>

[0073] Figure 1 and Figure 2 These are schematic top views and schematic cross-sectional views of a lithium secondary battery according to an exemplary embodiment.

[0074] Reference Figure 1 and Figure 2 The lithium secondary battery may include an electrode assembly comprising a positive electrode 100, a negative electrode 130, and a separator layer 140 between the positive and negative electrodes. The electrode assembly may be housed together with an electrolyte to be immersed therein in a housing 160.

[0075] The positive electrode 100 may include a positive electrode active material layer 110 formed by coating a positive electrode active material onto a positive electrode current collector 105.

[0076] The positive electrode current collector 105 may comprise stainless steel, nickel, aluminum, titanium, copper, or alloys thereof. Preferably, aluminum or its alloys may be used. The positive electrode current collector 105 may be surface-treated with carbon, nickel, titanium, silver, etc.

[0077] Positive electrode active materials may include compounds that enable reversible insertion and extraction of lithium ions.

[0078] In an exemplary embodiment, the positive electrode active material may include a lithium-transition metal oxide. For example, the lithium-transition metal oxide may include nickel (Ni), and may further include at least one of cobalt (Co) and manganese (Mn).

[0079] For example, lithium-transition metal oxides can be represented by the following chemical formula 1.

[0080] [Chemical Formula 1]

[0081] Li 1+a Ni 1-(x+y) Co x My O2

[0082] In the above chemical formula 1, -0.05≤a≤0.15, 0.01≤x≤0.3, 0.01≤y≤0.3, and M may include at least one element selected from Mn, Mg, Sr, Ba, B, Al, Si, Ti, Zr, and W.

[0083] The mixture can be prepared by mixing and stirring the positive electrode active material with a binder, conductive agent and / or dispersant in a solvent. The mixture can be coated onto the positive electrode current collector 105, then dried and pressed to form the positive electrode 100.

[0084] Solvents may include non-aqueous solvents. Non-limiting examples of solvents may include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc.

[0085] The adhesive may include organic-based adhesives such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc., or water-based adhesives such as styrene-butadiene rubber (SBR) that can be used with thickeners such as carboxymethyl cellulose (CMC).

[0086] For example, PVDF-based binders can be used as positive electrode binders. In this case, the amount of binder used to form the positive electrode active material layer can be reduced, while the amount of positive electrode active material can be relatively increased. As a result, the capacity and power of the lithium secondary battery can be further improved.

[0087] Conductive agents can be added to promote electron migration between active material particles. For example, conductive agents may include carbon-based materials such as graphite, carbon black, graphene, carbon nanotubes, etc., and / or metal-based materials such as tin, tin oxide, titanium oxide, perovskite minerals such as LaSrCoO3 or LaSrMnO3, etc.

[0088] The negative electrode 130 may include a negative electrode current collector 125 and a negative electrode active material layer 120 formed by coating the negative electrode current collector 125 with a negative electrode mixture containing a negative electrode active material.

[0089] In an exemplary embodiment, the density of the negative electrode mixture for a lithium secondary battery according to the exemplary embodiment can be 1.8 g / cm³. 3 Therefore, it is possible to provide lithium-ion secondary batteries that offer improved long-term storage performance while maintaining high energy density.

[0090] For example, the negative electrode active material described above can be mixed with the aforementioned binder, conductive agent, thickener, etc., and stirred to form a negative electrode mixture. The negative electrode mixture can be coated on at least one surface of the negative electrode current collector 125, dried, and pressed to form a negative electrode 130.

[0091] Solvents may include non-aqueous solvents. Non-limiting examples of solvents may include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc.

[0092] Adhesives widely used in the prior art can be used, and may include organic-based adhesives such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc., or water-based adhesives such as styrene-butadiene rubber (SBR) that can be used with thickeners such as carboxymethyl cellulose (CMC).

[0093] Considering the formation of the electrode, the amount of binder can be appropriately adjusted. In one embodiment, based on the total weight of the negative electrode active material and the binder, the amount of binder can be less than 3% by weight to enhance the resistive characteristics of the negative electrode. In another embodiment, the amount of binder can be from 0.5% to 3% by weight.

[0094] Conductive agents may include carbon-based conductive materials that are widely used in the prior art.

[0095] The negative electrode current collector 125 may comprise a metal with high conductivity and improved adhesion to the negative electrode mixture and no reactivity within the voltage range of the battery. For example, the negative electrode current collector 125 may comprise stainless steel, nickel, copper, titanium, or alloys thereof, preferably copper or copper alloys. The negative electrode current collector 125 may be surface-treated with carbon, nickel, titanium, silver, etc.

[0096] According to an embodiment of the present invention, the electrode density of the negative electrode active material layer 120 formed by coating the negative electrode active material can be 1.45 g / cm³. 3 The above points illustrate how, within these limits, battery power, lifespan, and high-temperature storage performance can be improved more effectively.

[0097] The membrane layer 140 may be located between the positive electrode 100 and the negative electrode 130. The membrane layer 140 may comprise a porous polymer membrane prepared from polymers such as polyolefin-based polymers, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, ethylene / methacrylate copolymers, etc. The membrane layer 140 may also comprise a nonwoven fabric formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0098] In some embodiments, the area and / or volume of the negative electrode 130 (e.g., the contact area with the separator layer 140) can be larger than the area and / or volume of the positive electrode 100. Therefore, lithium ions generated from the positive electrode 100 can be readily transferred to the negative electrode 130 without loss due to, for example, precipitation or sedimentation.

[0099] In an exemplary embodiment, an electrode unit may be defined by a positive electrode 100, a negative electrode 130, and a separator layer 140, and multiple electrode units may be stacked to form an electrode assembly 150 that may have, for example, a jelly roll shape. For example, the electrode assembly 150 may be formed by winding, stacking, or folding the separator layer 140.

[0100] The electrode assembly 150 and the electrolyte can be housed together in the housing 160 to define the lithium secondary battery. In an exemplary embodiment, a non-aqueous electrolyte can be used as the electrolyte.

[0101] For example, non-aqueous electrolytes can include lithium salts and organic solvents. Lithium salts commonly used in the electrolytes of lithium secondary batteries can be used, and Li... + X - express.

[0102] Lithium salt anion X - It can include, for example, 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 - (CF3CF2SO2)2N - wait.

[0103] Organic solvents may include, for example, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, tetrahydrofuran, etc. These organic solvents may be used alone or in combination.

[0104] like Figure 1 As shown, the tabs (positive and negative tabs) can protrude from the positive current collector 105 and negative current collector 125 contained in each electrode unit to one side of the housing 160. The tabs can be welded to one side of the housing 160 to form electrode leads (positive lead 107 and negative lead 127) extending or exposed outside the housing 160.

[0105] Lithium-ion batteries can be manufactured in shapes such as cylindrical (using cans), square, pouch, or coin.

[0106] The present invention is described in more detail below with reference to preferred embodiments. However, the following embodiments are given only as illustrative purposes, and those skilled in the art will clearly understand that various changes and modifications can be made within the scope and spirit of the invention. Such changes and modifications are suitably included within the appended claims.

[0107] Examples and Comparative Examples

[0108] <Negative electrode>

[0109] As shown in Table 1, assembled artificial graphite (first graphite particles) and high-hardness crushed artificial graphite (second graphite particles) are mixed to prepare negative electrode active materials.

[0110] As shown in Table 1, the particle density D pIn the calculation method, 1g of negative electrode active material is placed in a cylindrical granulator with a diameter of 13mm. A pressure of 3 metric tons is applied to the granulator manually for 10 seconds, and then the height of the granulator is measured. The particle density is obtained using the height difference from the initial empty granulator.

[0111] The tap density D shown in Table 1 T In the measurement, 10g of negative electrode active material was filled into a 25ml graduated cylinder, and the graduated cylinder was fixed to the tapping device. Simultaneous tapping and rotation were performed 3000 times, and the tapped density was measured using the volume after dropping.

[0112] [Table 1]

[0113]

[0114] The prepared negative electrode active material, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) as a thickener were mixed at a mass ratio of 97.8:1.2:1.0 and then dispersed in deionized distilled water to prepare a negative electrode mixture for lithium secondary batteries. The densities of the negative electrode mixtures including the negative electrode active materials prepared in Examples 1 to 8 and Comparative Examples 1 to 3 are shown in Table 2 below.

[0115] [Table 2]

[0116]

[0117]

[0118] The negative electrode mixture is coated on one surface of a copper foil current collector, dried, and pressed to form a negative electrode active material layer with a size of 10cm×10cm×50μm, in order to prepare the negative electrode.

[0119] Positive electrode

[0120] Li will be used as the positive electrode active material 1.0 Ni 0.6 Co 0.2 Mn 0.2 O2, acetylene black as a conductive agent, PVDF as a binder, and N-methylpyrrolidone as a solvent were mixed in a weight ratio of 46:2.5:1.5:50 to prepare a positive electrode mixture. The positive electrode mixture was coated onto an aluminum substrate, then dried and pressed to form a positive electrode.

[0121] <Battery>

[0122] The positive and negative electrodes obtained as described above are cut to suitable sizes and stacked. A separator (polyethylene, thickness: 13 μm) is inserted between the positive and negative electrodes to form an electrode unit. Each tab portion of the positive and negative electrodes is welded.

[0123] The welded positive / separator / negative electrode assembly is inserted into the flexible package, and the three sides of the package, except for the electrolyte injection side, are sealed. The tab portion is also included in the sealed portion. Electrolyte is injected through the electrolyte injection side, which is then also sealed. Subsequently, the above structure is immersed for more than 12 hours.

[0124] The solution was prepared by dissolving 1M LiPF6 in a mixed solvent of EC / EMC / DEC (25 / 45 / 30; volume ratio), followed by the addition of 1% by weight vinylene carbonate, 0.5% by weight 1,3-propenyl sulfonyl lactone (PRS) and 0.5% by weight lithium bis(oxalate)borate (LiBOB).

[0125] Subsequently, a pre-charge was performed for 36 minutes at a current corresponding to 0.25C (2.5A). After 1 hour of degassing and over 24 hours of aging, formation charging and discharging were performed (charging conditions: CC-CV 0.2C 4.2V 0.05C cutoff; discharging conditions: CC 0.2C 2.5V cutoff). Following this, standard charging and discharging were performed (charging conditions: CC-CV 0.5C 4.2V 0.05C cutoff; discharging conditions: CC 0.5C 2.5V cutoff).

[0126] Experimental Example

[0127] Measurement of initial charge / discharge capacity

[0128] According to the embodiments and comparative examples, the battery cell was charged once (CC / CV 0.1C 4.3V 0.005C cutoff) and discharged once (CC 0.1C 3.0V cutoff) to measure the initial charge capacity and initial discharge capacity (CC: constant current, CV: constant voltage).

[0129] Evaluation of initial efficiency

[0130] This will be achieved by measuring the initial... Initial discharge capacity divided by initial The percentage value obtained from the charging capacity is used as the initial efficiency for measurement.

[0131] <Evaluation of Ratio Performance>

[0132] After performing one charge (CC / CV 0.1C 4.3V 0.005C cutoff) and discharge (CC 0.1C 3.0V cutoff) cycle on the battery cells according to the embodiments and comparative examples, another charge (CC / CV 0.5C 4.3V 0.005C cutoff) and discharge (CC 4.0C 3.0V cutoff) cycle was performed. The rate performance was evaluated by converting the value obtained by dividing the 4.0C discharge capacity by the 0.1C discharge capacity into a percentage (%).

[0133] <Capacity Retention Evaluation (Lifetime Characteristics)>

[0134] According to the embodiments and comparative examples, the battery cell was subjected to 200 charge cycles (CC / CV 0.5C 4.3V 0.05C cutoff) and discharge cycles (CC 1.0C 3.0V cutoff). The capacity retention rate was evaluated as a percentage of the discharge capacity of the 200th cycle divided by the discharge capacity of the first cycle.

[0135] The results are shown in Table 3 below.

[0136] [Table 3]

[0137]

[0138]

[0139] Referring to Table 3, in the embodiment where the ratio of particle density to tap density satisfies Equation 1, a higher charge / discharge efficiency and capacity retention rate than the comparative example were obtained.

Claims

1. A negative electrode active material, comprising: First graphite particle; and The second graphite particle has a different particle size than the first graphite particle. The first graphite particle and the second graphite particle each contain artificial graphite. The particle density and tap density of the negative electrode active material satisfy the relationship in Equation 1: [Formula 1] 1.3≤D P / D T ≤1.45 In Equation 1, D P This is the particle density of the negative electrode active material, in g / cm³. 3 D T This is the tap density of the negative electrode active material, in g / cm³. 3 , The particle density is calculated using the height difference measured after placing 1g of the negative electrode active material into a cylindrical granulator with a diameter of 13mm and applying a pressure of 3 metric tons to the granulator for 10 seconds. The tap density is determined by measuring the volume and weight of a sample obtained by filling 10g of the negative electrode active material sample into a 25ml graduated cylinder and then tapping it 3000 times with a stroke length of 10mm. Furthermore, the first graphite particles and the second graphite particles satisfy Equation 2: [Equation 2] 0.3≤D2 / D1≤0.56 In Equation 2, D1 is the average particle size D of the first graphite particle. 50 The unit is μm, and D2 is the average particle size D of the second graphite particle. 50 The unit is μm.

2. The negative electrode active material according to claim 1, wherein the second graphite particle comprises artificial graphite with a hardness higher than that of the first graphite particle.

3. The negative electrode active material according to claim 1, wherein the amount of the second graphite particles is in the range of 20% to 40% by weight based on the total weight of the negative electrode active material.

4. The negative electrode active material according to claim 1, wherein the average particle size D of the first graphite particles is... 50 The thickness ranges from 16 μm to 19 μm.

5. The negative electrode active material according to claim 1, wherein the average particle size D of the second graphite particles is... 50 The size ranges from 7μm to 9μm.

6. The negative electrode active material according to claim 1, wherein the first graphite particles comprise assembled artificial graphite.

7. A negative electrode mixture for a lithium secondary battery, comprising the negative electrode active material according to claim 1, and having a density of 1.8 g / cm³. 3 above.

8. The negative electrode mixture for a lithium secondary battery according to claim 7, further comprising a solvent, a conductive agent, and a binder.

9. A negative electrode for a lithium secondary battery, comprising: Negative electrode current collector; and A negative electrode active material layer is formed on the negative electrode current collector, the negative electrode active material layer comprising the negative electrode active material according to claim 1.

10. A lithium secondary battery, comprising: positive electrode; and A negative electrode, which is opposite to the positive electrode and comprises the negative electrode active material according to claim 1.

Images