Positive electrode active material, and positive electrode and lithium secondary battery comprising same

Abstract

The present invention relates to a positive electrode active material, and a positive electrode and a lithium secondary battery comprising the same, the positive electrode active material comprising a lithium transition metal oxide in the form of a single particle, in which the lithium transition metal oxide satisfies at least one of conditions 1 to 3 described in the specification.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of priority to Korean Patent Application Nos. 10-2023-0043514, 10-2023-0043515, and 10-2023-0043517, filed on April 3, 2023, the disclosures of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a positive electrode active material for a lithium secondary battery, and a positive electrode and a lithium secondary battery containing the same. Background Art

[0004] Recently, with the rapid popularization of electronic devices such as mobile phones, laptop computers and electric vehicles using batteries, the demand for secondary batteries with relatively high capacity, small size and light weight has grown rapidly. In particular, due to the light weight and high energy density of lithium secondary batteries, lithium secondary batteries have attracted much attention as driving power sources for portable devices. Therefore, research and development efforts have been actively carried out to improve the performance of lithium secondary batteries.

[0005] In a lithium secondary battery in a state in which an organic electrolyte or a polymer electrolyte is filled between a positive electrode and a negative electrode respectively formed of active materials capable of intercalating and deintercalating lithium ions, electric energy is generated by oxidation and reduction reactions when lithium ions are intercalated / deintercalated in / from the positive electrode and the negative electrode.

[0006] Lithium cobalt oxide (LiCoO2), nickel-based lithium transition metal oxides, lithium manganese oxide (LiMnO2 or LiMn2O4) or lithium iron phosphate compounds (LiFePO4) have been used as positive electrode active materials for lithium secondary batteries. Among them, lithium cobalt oxide (LiCoO2) is widely used due to its advantages of high operating voltage and excellent capacity characteristics, and has been used as a positive electrode active material for high voltage. However, due to rising prices and unstable supply of cobalt (Co), there are limitations in the large-scale use of lithium cobalt oxide as a power source for applications such as electric vehicles, and therefore there is a need to develop a positive electrode active material that can replace lithium cobalt oxide.

[0007] Therefore, a nickel-based lithium transition metal oxide in which a portion of cobalt (Co) is replaced with nickel (Ni) or the like, represented by a nickel-cobalt-manganese-based lithium transition metal oxide, has been developed.

[0008] Conventionally developed nickel-based lithium transition metal oxides are agglomerated with an average particle size (D 50) in the form of secondary particles of tiny primary particles, wherein they have a large surface area and low particle strength. Therefore, when an electrode is prepared using a positive electrode active material comprising secondary particles agglomerated with fine primary particles and then rolled, there is a problem in that the amount of gas generated during battery operation is large due to severe particle cracking, and the stability is poor. In particular, for high-nickel (high-Ni) lithium transition metal oxides in which the amount of nickel (Ni) is increased to ensure high capacity, due to the above-mentioned structural problems, the chemical stability is further reduced, and it is also difficult to ensure thermal stability.

[0009] In order to improve the above-mentioned shortcomings of conventional nickel-based lithium transition metal oxides in the form of secondary particles in which fine primary particles are agglomerated, a method is proposed in which a secondary particle having an average particle size (D 50 ) Nickel-based lithium transition metal oxide positive electrode active material in the form of secondary particles of large giant primary particles.

[0010] Since the nickel-based lithium transition metal oxide positive electrode active material in the form of secondary particles in which giant primary particles are agglomerated minimizes interfaces between secondary particles, problems such as thermal stability, shortened life due to side reactions during electrochemical reactions, and gas generation are improved.

[0011] High-Ni lithium transition metal oxide cathode active materials are typically subjected to a washing process to reduce the amount of lithium impurities remaining on the surface. While this washing process is beneficial in reducing gas generation because it removes lithium byproducts from the surface, it is disadvantageous in terms of lifespan due to surface damage to the cathode active material particles. In particular, nickel-based lithium transition metal oxide cathode active materials in the form of secondary particles containing agglomerated fine primary particles inherently suffer from poor lifespan characteristics. Washing further shortens these lifespan characteristics, and their resistance increases with charge and discharge cycles.

[0012] [Prior art literature]

[0013] [Patent Document]

[0014] (Patent Document 1) Korean Patent Application Publication No. 10-2014-0093529 Summary of the Invention

[0015] Technical issues

[0016] An object of the present invention is to provide a lithium secondary battery having reduced gas generation and improved energy density by developing a positive electrode active material having a single particle form and suppressed particle cracking.

[0017] Technical Solution

[0018] In order to solve the above problems, the present invention provides a positive electrode active material, a positive electrode and a lithium secondary battery.

[0019] (1) The present invention provides a positive electrode active material comprising a lithium transition metal oxide in the form of single particles,

[0020] The lithium transition metal oxide satisfies at least one of conditions 1 to 3.

[0021] [Condition 1] The average cross-sectional area change rate (ΔX) of the lithium transition metal oxide according to the mathematical formula (1) 11 ) is 5% to 20%;

[0022] Mathematical formula (1)

[0023]

[0024] In mathematical formula (1),

[0025] X 30≤ is the average cross-sectional area [μm] of lithium transition metal oxide particles contained in an electrode containing lithium transition metal oxide and not subjected to rolling. 2 ],and

[0026] X 11 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 11%. 2 ],

[0027] [Condition 2] The C of the lithium transition metal oxide according to the mathematical formula (2) 11 [%] The ratio of the number of particles greater than 6% to the total number of particles of the lithium transition metal oxide contained in the electrode is greater than 3%;

[0028] Mathematical formula (2)

[0029]

[0030] In mathematical formula (2),

[0031] When an electrode comprising a lithium transition metal oxide is rolled so as to have a porosity of x%,

[0032] a is the cross-sectional area of ​​cracks formed in the lithium transition metal oxide particles contained in the electrode [μm 2 ],

[0033] b is the cross-sectional area of ​​the lithium transition metal oxide particles contained in the electrode [μm 2 ],and

[0034] [Condition 3] When an electrode including a lithium transition metal oxide is rolled so as to have a porosity of 11%, the ratio of the number of particles of the lithium transition metal oxide having cracks to the total number of particles of the lithium transition metal oxide included in the electrode is 78% or less.

[0035] (2) The present invention provides the positive electrode active material described in (1) above, wherein the average cross-sectional area change rate (ΔX 11 ) is 8% to 15%.

[0036] (3) The present invention provides a positive electrode active material as described in (1) or (2) above, wherein the average cross-sectional area change rate (ΔX) of the lithium transition metal oxide according to the mathematical formula (3) is 20 ) is 1% to 15%.

[0037] Mathematical formula (3)

[0038]

[0039] In mathematical formula (3),

[0040] X 30≤ is the average cross-sectional area [μm] of lithium transition metal oxide particles contained in an electrode containing lithium transition metal oxide and not subjected to rolling. 2 ],and

[0041] X 20 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 20%. 2 ].

[0042] (4) The present invention provides the positive electrode active material according to any one of (1) to (3) above, wherein when an electrode comprising a lithium transition metal oxide is rolled so as to have a porosity of 11%, the cross-sectional area is 3 μm 2 to 4 μm 2 The ratio of the number of particles of the lithium transition metal oxide to the total number of particles of the lithium transition metal oxide contained in the electrode is 40% or less.

[0043] (5) The present invention provides the positive electrode active material described in any one of (1) to (4) above, wherein X 11 At 5 μm 2 to 10 μm 2 within the range.

[0044] (6) The present invention provides a positive electrode active material as described in any one of (1) to (5) above, wherein the lithium transition metal oxide has a C according to the mathematical formula (2) 20[%] The ratio of the number of particles greater than 6% to the total number of particles of the lithium transition metal oxide contained in the electrode is 0.5% or more.

[0045] (7) The present invention provides a positive electrode active material as described in any one of (1) to (6) above, wherein the lithium transition metal oxide has a C according to the mathematical formula (2) 11 [%] The ratio of the number of particles having a range of 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode is 40% or less.

[0046] (8) The present invention provides a positive electrode active material as described in any one of (1) to (7) above, wherein the lithium transition metal oxide has a C according to the mathematical formula (2) 20 [%] The ratio of the number of particles having a range of 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode is 45% or less.

[0047] (9) The present invention provides a positive electrode active material as described in any one of (1) to (8) above, wherein the lithium transition metal oxide has a C according to the mathematical formula (2) 11 [%] The ratio of the number of particles of 1% to 2% to the total number of particles of the lithium transition metal oxide contained in the electrode is 20% or less.

[0048] (10) The present invention provides a positive electrode active material as described in any one of (1) to (9) above, wherein the lithium transition metal oxide has a C according to the mathematical formula (2) 20 [%] The ratio of the number of particles of 1% to 2% to the total number of particles of the lithium transition metal oxide contained in the electrode is 20% or less.

[0049] (11) The present invention provides a positive electrode active material as described in any one of (1) to (10) above, wherein, when an electrode containing a lithium transition metal oxide is rolled so that the porosity is 20%, the ratio of the number of particles of the lithium transition metal oxide having cracks to the total number of particles of the lithium transition metal oxide contained in the electrode is less than 70%.

[0050] (12) The present invention provides the positive electrode active material described in any one of (1) to (11) above, wherein the ratio of the number of particles having cracks to the total number of particles of the lithium transition metal oxide is 50% or less.

[0051] (13) The present invention provides a positive electrode active material as described in any one of (1) to (12) above, wherein, when an electrode containing a lithium transition metal oxide is rolled so that the porosity is 11%, the difference between the ratio of the number of particles of the lithium transition metal oxide having cracks to the total number of particles of the lithium transition metal oxide contained in the electrode before and after rolling is 20% to 80%.

[0052] (14) The present invention provides the positive electrode active material according to any one of the above (1) to (13), wherein the average particle diameter (D 50 ) of the lithium transition metal oxide is 1.5 μm to 6 μm.

[0053] (15) The present invention provides the positive electrode active material according to any one of the above (1) to (14), wherein the lithium transition metal oxide is represented by Chemical Formula 1.

[0054] [Chemical Formula 1]

[0055] Li(Ni 1-x-y-z Co x Mn y M z )O2

[0056] In Chemical Formula 1,

[0057] M is at least one selected from the following: aluminum (Al), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo), and

[0058] x, y, and z are the atomic fractions of the independent elements,

[0059] where 0 < x ≤ 0.25, 0 < y ≤ 0.25, 0 ≤ z < 0.1, and 0 < x + y + z ≤ 0.5.

[0060] (16) The present invention provides a positive electrode for a lithium secondary battery, which contains the positive electrode active material according to any one of the above (1) to (15).

[0061] (17) The present invention provides a lithium secondary battery, which contains the positive electrode according to the above (16).

[0062] Advantageous Effects

[0063] Since the positive electrode active material according to the present invention can reduce the generation of fine particles due to particle cracking when used as the positive electrode active material in a lithium secondary battery, a high single particle formation degree or a low crack particle ratio can be achieved, and the gas generation phenomenon of the battery can be reduced. Therefore, the battery life can be significantly improved, and excellent battery performance can be exhibited. Description of the Drawings

[0064] Figure 1 is a segmented image of a cross-section of the positive electrode containing the positive electrode active material prepared in Example 1.

[0065] Figure 2 is a segmented image of a cross-section of the positive electrode containing the positive electrode active material prepared in Example 2.

[0066] Figure 3 is a segmented image of a cross section of a positive electrode including the positive electrode active material prepared in Comparative Example 1.

[0067] Figure 4 4(A) and 4(B) are scanning electron microscope (SEM) images of the positive electrode active materials prepared in Example 1 and Comparative Example 1, respectively.

[0068] Figure 5 (A) to 5(C) are SEM images of the positive electrode active materials prepared in Example 1, Example 2, and Comparative Example 1, respectively.

[0069] Figure 6 The graph shows changes in the particle size distribution of the lithium transition metal oxide contained in the positive electrode when the positive electrode containing the positive electrode active material prepared in Example 1 is pressurized.

[0070] Figure 7 The graph shows changes in the particle size distribution of the lithium transition metal oxide contained in the positive electrode when the positive electrode containing the positive electrode active material prepared in Comparative Example 1 is pressurized.

[0071] Figure 8 Graph showing the change in gas generation amount (ml) over time (weeks) for batteries containing the positive electrode active materials prepared in Example 1, Example 2, and Comparative Example 1. DETAILED DESCRIPTION

[0072] Hereinafter, the present invention will be described in more detail for a clearer understanding of the present invention.

[0073] It should be understood that the words or terms used in this specification and claims should not be interpreted as the meanings defined in commonly used dictionaries. In addition, it should be understood that based on the principle that the inventor can appropriately define the meanings of words or terms in order to best interpret the present invention, the words or terms should be interpreted as having meanings consistent with their meanings in the context of the relevant technology and the technical spirit of the present invention.

[0074] Hereinafter, the present invention will be described in detail.

[0075] In the present invention, the expression "primary particle" means the smallest particle unit that is distinguished as a bulk when a cross section of a positive electrode active material is observed by a scanning electron microscope (SEM), wherein it may be composed of one crystallite or a plurality of crystallites. In the present invention, the average particle size (Dv) of the primary particles can be measured using a scanning electron microscope (SEM) (JEOL Co., Ltd., JSM-7900F). 50). Specifically, the average particle size can be defined as the particle size at 50% cumulative volume in the particle size distribution curve (particle size distribution curve diagram) of each particle obtained using a scanning electron microscope (SEM). After the volume of a sphere with a radius of half the primary particle diameter obtained using a scanning electron microscope (SEM) is set as the volume of the primary particle, the average particle size can be measured by calculating the particle size at 50% of the volume cumulative distribution according to the particle size in the primary particle volume calculation result. In the present invention, the area of ​​each primary particle is calculated by utilizing the number of pixels corresponding to each of the n primary particles present in the SEM image, and the particle size of each primary particle present in the SEM image is calculated using the radius of a circle having the same area as each primary particle, the particle size of the primary particles can be calculated.

[0076] In the present invention, "single particle" is a term used to distinguish from the conventionally used positive electrode active material particles in the form of secondary particles formed by the agglomeration of tens to hundreds of primary particles, where it is a concept including single particles composed of one primary particle and agglomerated particles composed of 50 or less primary particles.

[0077] In the present invention, the average particle size (D 50 )" can be defined as the particle size at 50% in the volume-based cumulative particle size distribution, and can be measured by using a laser diffraction method. Specifically, after dispersing the target particles in a dispersion medium, the dispersion medium is introduced into a commercially available laser diffraction particle size measuring instrument (e.g., Microtrac MT 3000) and irradiated with an ultrasonic wave of about 28 kHz with an output of 60 W, and then the average particle size D based on 50% in the volume-based cumulative distribution of the particle size in the measuring instrument can be calculated. 50 .

[0078] In the present invention, the "single particle formation degree" is determined by the average particle size (Dv) of the primary particles constituting the single particles. 50 ) and the average particle size (D 50 ) is adjusted, wherein the average particle size (Dv 50 ) is closer to the average particle size of lithium transition metal oxides (D 50 ), the higher the single particle formation degree.

[0079] In the present invention, "single crystal" refers to a crystal in a state where no grain boundaries are included in the particles.

[0080] In the present invention, when preparing an electrode containing a lithium transition metal oxide, the "average cross-sectional area change rate" refers to the average cross-sectional area change rate in which the average cross-sectional area of ​​the lithium transition metal oxide particles contained in the electrode changes as a percentage when rolled so that the porosity is x% compared to the electrode without rolling. The "average cross-sectional area" can be measured by calculating the arithmetic mean of the "cross-sectional area".

[0081] In the present invention, the cross section of the positive electrode is obtained by cutting the positive electrode using an ion milling method in which the positive electrode is irradiated with an argon (Ar) ion beam using an ion milling device (JEOL Co., Ltd., CP-09IB19520CCP, accelerating voltage: 6 kV), and after obtaining a scanning electron microscope (SEM) image of the positive electrode cross section, the "cross-sectional area" can be confirmed by image processing of the image. Specifically, the cross-sectional area can be measured from a two-dimensional segmented image obtained by processing the image with an image processing program and segmented by particle units. After preparing a positive electrode containing a positive electrode active material to be tested, the positive electrode is cut using an ion milling method to obtain a cross section of the positive electrode, and a scanning electron microscope (SEM) image of the cross section is obtained. Then, a segmented image can be obtained by inputting the obtained scanning electron microscope image into a deep learning program to identify multiple objects contained in the image, and then segmenting the SEM image into particle units based on the multiple objects. The "cross-sectional area" can be calculated using the number of pixels corresponding to each of the n particles present in the segmented image.

[0082] In the present invention, "crack" may refer to a rupture (crack) area on the surface of a particle, which is distinguished from a boundary between particles having a linear shape larger than a predetermined size by learning boundary information in an image observed with a scanning electron microscope (SEM). In addition, "crack" may refer to an area that appears black in an image observed with a scanning electron microscope (SEM) or an area that appears darker than adjacent parts. The area that appears darker is determined by contrast with the surrounding area, and only areas that are not boundaries between particles having a linear shape larger than a predetermined size are identified as cracks. On the contrary, in the absence of a crack, it may appear white or a color brighter than the cracks in the adjacent parts.

[0083] In the present invention, a cross section of the positive electrode is obtained by cutting the positive electrode using an ion milling method in which an argon (Ar) ion beam is irradiated to the positive electrode using an ion milling device (JEOL Co., Ltd., CP-09IB19520CCP, accelerating voltage: 6 kV), and after obtaining a scanning electron microscope (SEM) image of the positive electrode cross section, the "cross-sectional area of ​​the crack" and the "cross-sectional area of ​​the particle" can be confirmed by image processing. Specifically, the cross-sectional area of ​​the crack and the cross-sectional area of ​​the particle can be measured from a two-dimensional segmented image obtained by processing the image using an image processing program, which is segmented by particle units. After preparing a positive electrode containing a positive electrode active material to be tested, the positive electrode is cut using an ion milling method to obtain a cross section of the positive electrode, and a scanning electron microscope (SEM) image of the cross section is obtained. Then, a segmented image can be obtained by inputting the obtained scanning electron microscope image into a deep learning program to identify multiple objects contained in the image, and then segmenting the SEM image into particle units based on the multiple objects. The "cross-sectional area of ​​the crack" can be calculated using the number of pixels corresponding to an area appearing black or an area appearing darker than adjacent parts in a particle present in the segmented image. The "cross-sectional area of ​​the particle" can be calculated using the number of pixels corresponding to the particle, in which case it can include the cross-sectional area of ​​the crack when a crack exists in the particle.

[0084] In the present invention, the number of particles with cracks can be determined by the presence or absence of cracks. The presence or absence of cracks is determined by visually observing the segmented image, wherein black areas or areas with a relatively darker color than adjacent areas are determined to have cracks, and white areas or areas with a relatively lighter color than adjacent areas are determined to have no cracks.

[0085] positive electrode active material

[0086] Hereinafter, the positive electrode active material according to the present invention will be described.

[0087] The positive electrode active material of the present invention is characterized in that it contains a lithium transition metal oxide, wherein the lithium transition metal oxide is in the form of a single particle and satisfies at least one of the following conditions 1 to 3.

[0088] [Condition 1] The average cross-sectional area change rate (ΔX) of the lithium transition metal oxide according to the mathematical formula (1) 11 ) is 5% to 20%;

[0089] Mathematical formula (1)

[0090]

[0091] In mathematical formula (1),

[0092] X 30≤is the average cross-sectional area [μm] of lithium transition metal oxide particles contained in an electrode containing lithium transition metal oxide and not subjected to rolling. 2 ],and

[0093] X 11 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 11%. 2 ],

[0094] [Condition 2] The C of the lithium transition metal oxide according to the following mathematical formula (2) 11 [%] The ratio of the number of particles greater than 6% to the total number of particles of the lithium transition metal oxide contained in the electrode is greater than 3%;

[0095] Mathematical formula (2)

[0096]

[0097] In mathematical formula (2),

[0098] When an electrode comprising a lithium transition metal oxide is rolled so as to have a porosity of x%,

[0099] a is the cross-sectional area of ​​cracks formed in the lithium transition metal oxide particles contained in the electrode [μm 2 ],

[0100] b is the cross-sectional area of ​​the lithium transition metal oxide particles contained in the electrode [μm 2 ],and

[0101] [Condition 3] When an electrode including a lithium transition metal oxide is rolled so as to have a porosity of 11%, the ratio of the number of particles of the lithium transition metal oxide having cracks to the total number of particles of the lithium transition metal oxide included in the electrode is 78% or less.

[0102] The lithium transition metal oxide is a single particle form consisting of 50 or less primary particles. That is, the lithium transition metal oxide is a single particle or a single particle form in which 2 to 50 particles are agglomerated. Specifically, the lithium transition metal oxide can be a single particle form consisting of 2 to 40 primary particles, 2 to 30 primary particles, 2 to 20 primary particles, 2 to 10 primary particles, and can preferably be a single particle form consisting of 2 to 10 primary particles. The single particle form is different from the secondary particles in which more than 50 primary particles are agglomerated.

[0103] Lithium transition metal oxides, which serve as secondary particles, are susceptible to particle cracking during the electrode rolling process. This results in an increase in the specific surface area of ​​the active material, exacerbating degradation of storage and lifespan performance at high temperatures. In particular, particle cracking during rolling for preparing the positive electrode is a common problem for lithium metal oxides containing high nickel content. In this case, side reactions between the lithium metal oxide and the electrolyte may increase, and the physical properties of the secondary battery may deteriorate.

[0104] The present invention improves the problems of lithium transition metal oxides in the form of secondary particles by developing a single-particle lithium transition metal oxide. In particular, when the positive electrode active material satisfies any of conditions 1 to 3 of the present invention, a secondary battery can be achieved in which particle cracking is suppressed while in the form of a single particle and high-temperature life characteristics and high-temperature storage performance are excellent.

[0105] [Condition 1]

[0106] In order to evaluate and quantitatively represent the degree of particle cracking of the lithium transition metal oxide in the present invention before and after rolling, the average cross-sectional area change rate (ΔX x ) concept, wherein, when preparing an electrode comprising a lithium transition metal oxide, the average cross-sectional area change rate (ΔX x ) is an indicator of how much the average cross-sectional area of ​​the lithium transition metal oxide particles contained in the electrode changes relative to the average cross-sectional area of ​​the lithium transition metal oxide particles contained in the electrode that has not been rolled when the electrode is rolled so that the porosity is x%.

[0107] The porosity (x%) was calculated using the ratio of the electrode density to the true density of the total solid content of the positive electrode active material.

[0108] Specifically, the prepared positive electrode active material, carbon black conductive material, and PVdF binder were mixed in an N-methylpyrrolidone solvent at a weight ratio of 95:2:3 to prepare a positive electrode material mixture (viscosity: 5,000 mPa·s). The positive electrode material mixture was applied to one surface of an aluminum current collector and then dried at 130°C to prepare a positive electrode. In this case, the porosity of the positive electrode active material layer in the positive electrode was 30% or more, and the porosity of the positive electrode active material layer was adjusted by rolling to a thickness that satisfied the porosity calculated by the following equation 1.

[0109] [Equation 1]

[0110] Porosity (P) = [(True density of positive electrode active material layer (T) - positive electrode density (D)) / True density of positive electrode active material layer (T)] × 100

[0111] After punching each positive electrode to a size of 14 phi (ø), the mass and thickness of each punched positive electrode were measured, and the mass and thickness of the aluminum current collector were subtracted to obtain the mass (M) and thickness (H) of the positive electrode active material layer.

[0112] The positive electrode density (D) is obtained by dividing the mass (M) of the positive electrode active material layer by (positive electrode area (S)×thickness (H) of the positive electrode active material layer) (D=M / (S×H)).

[0113] The true density (T) of each positive electrode active material layer is obtained as the ratio of each component of the positive electrode material mixture × the sum of the true density of each component (T = [(0.95 × true density of positive electrode active material) + (0.02 × true density of carbon black conductive material) + (0.03 × true density of PVDF binder)]).

[0114] Average cross-sectional area change rate (ΔX x ) is low means that when the electrode containing the lithium transition metal oxide is rolled at a constant strength, the cross-sectional area of ​​the particles changes little because the lithium transition metal oxide particles rarely crack.

[0115] The positive electrode active material of the present invention comprises a lithium transition metal oxide, wherein the lithium transition metal oxide is in the form of a single particle, and the average cross-sectional area change rate (ΔX 11 ) is 5% to 20%.

[0116] Mathematical formula (1)

[0117]

[0118] In mathematical formula (1),

[0119] X 30≤ is the average cross-sectional area [μm] of lithium transition metal oxide particles contained in an electrode containing lithium transition metal oxide and not subjected to rolling. 2 ],and

[0120] X 11 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 11%. 2 ].

[0121] According to the average cross-sectional area change rate (ΔX 11) is a value determined based on the change in cross-sectional area when an electrode containing a lithium transition metal oxide is rolled so that the porosity is 11%, wherein, since the pressure level during the positive electrode preparation is similar to or higher than the rolling pressure level, it can be understood as a clearer standard for evaluating the particle cracking phenomenon when used in a lithium secondary battery, and in particular, the trend when the rolling pressure increases can be confirmed in more detail.

[0122] In the present invention, the average cross-sectional area change rate (ΔX 11 ) is 5% to 20%, more specifically, the average cross-sectional area change rate (ΔX 11 ) can be more than 5%, more than 6%, more than 7%, more than 8%, more than 9% or more than 10%, can be less than 12%, less than 13%, less than 14%, less than 15%, less than 16%, less than 17%, less than 18%, less than 19% or less than 20%, preferably it can be in the range of 8% to 15%.

[0123] When lithium transition metal oxides meet the above physical properties, particle strength is improved, thereby suppressing particle cracking during rolling, and by reducing the amount of lithium byproducts, side reactions with the electrolyte can be suppressed. In addition, excellent physical properties such as high-temperature life characteristics and storage performance can be achieved in lithium secondary batteries.

[0124] Meanwhile, when the electrode including the lithium transition metal oxide is rolled so as to have a porosity of 11%, the cross-sectional area is 3 μm 2 to 4 μm 2 The ratio of the number of particles to the total number of particles of the lithium transition metal oxide contained in the electrode can be less than 40%, specifically less than 40%, less than 39%, less than 38%, less than 37%, less than 36%, less than 35%, less than 34% or less than 33%.

[0125] Furthermore, when the electrode comprising the lithium transition metal oxide is rolled so as to have a porosity of 11%, the cross-sectional area is 4 μm 2 to 5 μm 2 The ratio of the number of particles of the lithium transition metal oxide to the total number of particles of the lithium transition metal oxide contained in the electrode may be 22% or less, specifically 22% or less, 21% or less, or 20% or less.

[0126] Furthermore, when the electrode comprising the lithium transition metal oxide is rolled so as to have a porosity of 11%, the cross-sectional area is 10 μm 2 The ratio of the above number of particles to the total number of particles of the lithium transition metal oxide contained in the electrode may be 5% or more or 6% or more.

[0127] The fact that lithium transition metal oxides satisfy the above physical properties means that the distribution of the ratio of the cross-sectional area of ​​the particles before and after rolling is small, because the cracking of the particles is suppressed after rolling. In the case where the particles are prone to cracking, the ratio of the particles with a reduced cross-sectional area tends to increase, and the ratio of the particles with a cross-sectional area of ​​10 μm is 0. 2 The proportion of particles with larger cross-sectional areas tends to decrease.

[0128] X 11 Can be 5 μm 2 to 10 μm 2 More specifically, X 11 Can be 5.0 μm 2 Above or 5.5 μm 2 above, and can be 6 μm 2 Below, 7 μm 2 Below, 8 μm 2 Below, 9 μm 2 Below or 10 μm 2 the following.

[0129] In addition, in the present invention, even if the rolling strength is changed, the average cross-sectional area change rate (ΔX x ) is also lower than the average cross-sectional area change rate (ΔX x ).

[0130] For example, the average cross-sectional area change rate (ΔX 20 ) can be from 1% to 15%.

[0131] Mathematical formula (3)

[0132]

[0133] In mathematical formula (3),

[0134] X 30≤ is the average cross-sectional area [μm] of lithium transition metal oxide particles contained in an electrode containing lithium transition metal oxide and not subjected to rolling. 2 ],and

[0135] X 20 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 20%. 2 ].

[0136] In addition, according to the average cross-sectional area change rate (ΔX 20 ) can be more than 2%, and can be less than 12%, less than 11% or less than 10%.

[0137] In addition, according to ΔX x According to the definition of the average cross-sectional area change rate (ΔX 14 ) can be 1% to 5%. Specifically, the average cross-sectional area change rate (ΔX 14 ) can be more than 1%, more than 1.2% or more than 1.3%, and can be less than 5%, less than 4%, less than 3%, less than 2% or less than 1.8%.

[0138] In addition, according to ΔX x According to the definition of the average cross-sectional area change rate (ΔX 16 ) can be 3% to 15%. Specifically, the average cross-sectional area change rate (ΔX 16 ) can be more than 3%, more than 5%, more than 7% or more than 8%, and can be less than 15%, less than 12% or less than 10%.

[0139] Satisfying all the average cross-sectional area change rates for various rolling strengths means that, due to its stable structure and the absence of cracking, it can improve the battery's performance degradation and high-temperature stability problems caused by particle cracking and gas generation. Furthermore, by maximizing the porosity to a level where particle cracking is eliminated, a high electrode density can be achieved, which can be advantageous in terms of the energy density per unit volume of the secondary battery.

[0140] [Condition 2]

[0141] In order to evaluate and quantitatively express the degree of cracking of the lithium transition metal oxide particles before and after rolling, the cross-sectional area of ​​the cracks in the lithium transition metal oxide is used. 2 ] ratio (C x [%]). When an electrode comprising a lithium transition metal oxide is rolled so as to have a porosity of x%, C x [%] represents the ratio of the cross-sectional area of ​​cracks generated in the lithium transition metal oxide particles contained in the electrode, which is an indicator of the strength and rolling resistance of the lithium transition metal oxide and the degree of crack generation and particle cracking.

[0142] For example, the large C x The [%] value means that cracks with large areas are generated in the particles of lithium transition metal oxide due to rolling, and C x [%] The ratio of particles with a value greater than a predetermined value (y) (A x,y ) means that many particles have cracks due to rolling. The generation of cracks in particles due to rolling is different from the generation of particle cracking. The generation of cracks in particles means that cracks are generated when the particles resist rolling without cracking, so it can be explained as Cx The greater the number of particles having a high [%] value, the higher the strength of the lithium transition metal oxide particles.

[0143] In the present invention, C is defined x [%] is the ratio of particles with a predetermined value (y) (A x,y ). Due to C x The [%] value is the ratio of the cross-sectional area of ​​cracks formed in the lithium transition metal oxide particles contained in the electrode when the electrode containing the lithium transition metal oxide is rolled so that the porosity is x%. Therefore, C x [%] is the ratio of particles with a constant value (y) (A x,y ) represents the number of lithium transition metal oxide particles in the total number of particles when the total number of particles is taken as the number of particles. x [%] is a concept representing the ratio of the number of particles to a constant value (y).

[0144] The positive electrode active material of the present invention comprises a lithium transition metal oxide, wherein the lithium transition metal oxide has a C according to the following mathematical formula (2): 11 [%] The ratio of the number of particles greater than 6% to the total number of particles of the lithium transition metal oxide contained in the electrode is 3% or more.

[0145] Mathematical formula (2)

[0146]

[0147] In mathematical formula (2),

[0148] When an electrode comprising a lithium transition metal oxide is rolled so as to have a porosity of x%,

[0149] a is the cross-sectional area of ​​cracks formed in the lithium transition metal oxide particles contained in the electrode [μm 2 ],

[0150] b is the cross-sectional area of ​​the lithium transition metal oxide particles contained in the electrode [μm 2 ].

[0151] According to the mathematical formula (2), C 11 The percentage (%) is a value determined based on the cross-sectional area of ​​cracks generated in each particle of the lithium transition metal oxide contained in the electrode when rolled to an 11% porosity. Because the pressure level during positive electrode preparation is similar to or higher than the rolling pressure level, this value can be understood as a more specific metric for evaluating particle cracking during use in lithium secondary batteries, particularly allowing for more detailed identification of trends as the rolling pressure increases.

[0152] In the present invention, the lithium transition metal oxide according to the mathematical formula (2) C11 [%] The ratio of the number of particles greater than 6% to the total number of lithium transition metal oxide particles contained in the electrode is 3% or more. Specifically, the ratio may be 3% or more, 3.5% or more, 4.0% or more, or 4.5% or more. This means that cracks are generated in a predetermined percentage or more of the lithium transition metal oxide particles due to rolling. Specifically, this means that there are many particles having a large number of cracks so wide that C 11 [%] Cracks greater than 6%.

[0153] As described above, the lithium transition metal oxide of the present invention generates cracks by resisting the pressure caused by rolling, thereby suppressing particle cracking and suppressing side reactions with the electrolyte by reducing the amount of lithium by-products. In addition, excellent physical properties such as high-temperature life characteristics and storage performance can be achieved in lithium secondary batteries.

[0154] In the present invention, the lithium transition metal oxide according to the mathematical formula (2) C 11 [%] The ratio of the number of particles of 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode can be 40% or less, more specifically, the ratio can be 25% or more, 28% or more, or 30% or more, and can be 40% or less.

[0155] In the present invention, the lithium transition metal oxide according to the mathematical formula (2) C 11 [%] The ratio of the number of particles of 1% to 2% to the total number of particles of the lithium transition metal oxide contained in the electrode can be less than 20%. More specifically, the ratio can be more than 5%, more than 6%, more than 7%, more than 8%, more than 9%, more than 10% or more than 11%, and can be less than 20%.

[0156] In addition, when the positive electrode of the present invention is prepared without rolling, if the ratio of the crack cross-sectional area according to the mathematical formula (2) is defined as C 30≤ [%], then according to the mathematical formula (2) C 11 [%] is the ratio of the number of particles from 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode (A 11≤,0-1 ) can be higher than C 30≤ [%] is the ratio of the number of particles from 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode (A 30≤,0-1 ). In addition, according to the mathematical formula (2) C 11 [%] Ratio of the number of particles greater than 6% to the total number of particles of lithium transition metal oxide contained in the electrode (A 11≤,6< ) can be higher than C 30≤ [%] Ratio of the number of particles greater than 6% to the total number of particles of lithium transition metal oxide contained in the electrode (A30≤,6< ).

[0157] In particular, with the lithium transition metal oxide of the present invention, since crack generation (rather than particle cracking) increases during rolling, the ratio of the number of particles having small-area cracks as described above is high.

[0158] Specifically, according to the mathematical formula (2), C 11 [%] is the ratio of the number of particles from 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode (A 11,0-1 ) and C 30≤ [%] is the ratio of the number of particles from 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode (A 30≤,0-1 ) can be more than 15%, specifically more than 15%, more than 18% or more than 20%.

[0159] Specifically, according to the mathematical formula (2), C 11 [%] Ratio of the number of particles greater than 6% to the total number of particles of lithium transition metal oxide contained in the electrode (A 11≤,6< ) and C 30≤ [%] Ratio of the number of particles greater than 6% to the total number of particles of lithium transition metal oxide contained in the electrode (A 30≤,6< ) can be more than 3%, specifically more than 3%, more than 3.5%, more than 4.0% or more than 4.5%.

[0160] This indicates that by rolling the electrode containing the lithium transition metal oxide, cracks are generated even in the particles of the lithium transition metal oxide contained in the positive electrode active material of the present invention, which have no cracks when not rolled. Therefore, it can be understood that cracks are generated in the particles at a high frequency (rather than a particle cracking phenomenon).

[0161] In the present invention, the lithium transition metal oxide according to the mathematical formula (2) C 20 [%] The ratio of the number of particles ranging from 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode can be 45% or less, more specifically, the ratio can be 20% or more or 25% or more, and can be 45% or less, 40% or less, 39% or less, 38% or less, 37% or less, 36% or less, 35% or less or 34% or less.

[0162] In the present invention, the lithium transition metal oxide according to the mathematical formula (2) C 20[%] The ratio of the number of particles of 1% to 2% to the total number of particles of the lithium transition metal oxide contained in the electrode can be less than 20%. More specifically, the ratio can be more than 5%, more than 6%, more than 7%, more than 8%, more than 9%, more than 10%, more than 11%, more than 12% or more than 15%, and can be less than 20%.

[0163] In the present invention, the lithium transition metal oxide according to the mathematical formula (2) C 20 [%] The ratio of the number of particles greater than 6% to the total number of particles of the lithium transition metal oxide contained in the electrode may be 0.5% or more, and the ratio may be 0.6% or more or 0.8% or more, and may be 1.5% or less.

[0164] In particular, with the lithium transition metal oxide of the present invention, since crack generation (rather than particle cracking) increases during rolling, the ratio of the number of particles having small-area cracks as described above is high.

[0165] In addition, for the lithium transition metal oxide in the present invention, C 11 [%] is the ratio of 1% to 2% of the number of particles to the total number of particles of the lithium transition metal oxide contained in the electrode (A 11,1-2 ) can be higher than C 20 [%] is the ratio of 1% to 2% of the number of particles to the total number of particles of the lithium transition metal oxide contained in the electrode (A 20,1-2 ).

[0166] This indicates that when the rolling strength is further increased, the proportion of particles with cracks in the lithium transition metal oxide contained in the positive electrode active material of the present invention is further increased, where it can be understood that even under a certain level of high pressure, cracks are generated in the particles at a high frequency (rather than particle cracking phenomenon).

[0167] [Condition 3]

[0168] In order to evaluate and quantitatively indicate the degree of particle cracking in the lithium transition metal oxide of the present invention before and after rolling, the concept of the cracked particle ratio [%] of the lithium transition metal oxide contained in the electrode is used. The cracked particle ratio [%] represents the ratio (in percentage) of the number of particles having cracks to the total number of particles of the lithium transition metal oxide contained in the electrode. Since the proportion of particles having cracks in the lithium transition metal oxide of the present invention is low when the electrode is rolled, this suppresses side reactions with the electrolyte and can prevent problems such as gas generation and reduced battery cycle characteristics.

[0169] The cracked particle ratio [%] can be calculated as the ratio of the number of particles with cracks (c) to the total number of lithium transition metal oxide particles (d) contained in the electrode. After preparing an electrode (positive electrode) containing a positive electrode active material containing lithium transition metal oxide particles, the electrode is cut using an ion milling method to obtain a cross-section of the positive electrode. An SEM image of the positive electrode cross-section is obtained using an SEM. The presence or absence of cracks can be determined using an image processing program to calculate c and d.

[0170] The cracked particle ratio is a value determined based on whether cracks are generated in each particle when an electrode containing a lithium transition metal oxide is rolled so that the porosity is 11%. Since the pressure level during the positive electrode preparation is similar to or higher than the rolling pressure level, it can be understood as a clearer standard for evaluating the particle cracking phenomenon when used in a lithium secondary battery, and in particular, the trend when the rolling pressure increases can be confirmed in more detail.

[0171] In the present invention, when an electrode containing a lithium transition metal oxide is rolled to a porosity of 11%, the ratio of the number of lithium transition metal oxide particles having cracks to the total number of lithium transition metal oxide particles contained in the electrode is 78% or less. Specifically, the ratio can be 78% or less or 77% or less. This means that the proportion of lithium transition metal oxide particles having cracks is low even after rolling, and that particle cracking is improved.

[0172] As described above, the lithium transition metal oxide of the present invention can suppress particle cracking and can suppress side reactions with the electrolyte by reducing the amount of lithium byproducts. In addition, excellent physical properties such as high-temperature life characteristics and storage performance can be achieved in lithium secondary batteries.

[0173] In addition, in the present invention, when an electrode containing a lithium transition metal oxide is rolled so that the porosity is 20%, the ratio of the number of particles of the lithium transition metal oxide having cracks to the total number of particles of the lithium transition metal oxide contained in the electrode can be 70% or less, more specifically, the ratio can be 70% or less, 69% or less or 68% or less, and can be 50% or more or 52% or more.

[0174] In the present invention, the ratio of the number of particles having cracks to the total number of particles of the lithium transition metal oxide may be 50% or less, more specifically, the ratio may be 5% or more, 10% or more, 12% or more, or 15% or more, and may be 50% or less, 45% or less, 40% or less, 35% or less, or 30% or less.

[0175] In the present invention, when the electrode containing a lithium transition metal oxide is rolled to a porosity of 11%, the difference in the ratio of the number of cracked particles of the lithium transition metal oxide to the total number of particles of the lithium transition metal oxide contained in the electrode before and after rolling can be 20% to 80%. More specifically, the difference can be more than 20%, more than 30% or more than 40%, and can be less than 80%, less than 70%, less than 65% or less than 60%.

[0176] In the present invention, the average particle diameter (D 50 ) of the lithium transition metal oxide can be 1.5 μm or more, specifically 1.5 μm or more, 2 μm or more, 3 μm or more, or 3.5 μm or more, and can be 6 μm or less, 4.5 μm or less, or 4 μm or less.

[0177] Since the positive electrode active material of the present invention is in the form of single particles, compared with conventional secondary particles in which primary particles are aggregated, it has the advantages of reducing gas generation and suppressing particle cracking. In addition, when the average particle diameter of the single particles is within the above range, performance degradation due to a decrease in rolling density can be prevented, and an increase in resistance can be suppressed.

[0178] In the present invention, the lithium transition metal oxide can be represented by the following Chemical Formula 1.

[0179] [Chemical Formula 1]

[0180] Li(Ni 1-x-y-z Co x Mn y M z )O2

[0181] In Chemical Formula 1,

[0182] M is at least one selected from the following: aluminum (Al), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo), and

[0183] x, y, and z are the atomic fractions of the respective elements,

[0184] where 0 < x ≤ 0.25, 0 < y ≤ 0.25, 0 ≤ z < 0.1, and 0 < x + y + z ≤ 0.5.

[0185] M is an element substituted at the transition metal site in the oxide represented by Chemical Formula 1, and can contain at least one selected from the following: Al, Fe, V, Cr, Ti, Ta, Mg, and Mo.

[0186] 1 - x - y - z represents the molar ratio of nickel in the metal components other than lithium in the lithium transition metal oxide represented by Chemical Formula 1, where 1 - x - y - z can satisfy 0.5 ≤ 1 - x - y - z < 1, preferably 0.8 ≤ 1 - x - y - z ≤ 0.95.

[0187] x represents the molar ratio of cobalt in the metal components other than lithium in the lithium transition metal oxide represented by Chemical Formula 1, where x can satisfy 0 < x ≤ 0.25, preferably 0.025 ≤ x ≤ 0.1.

[0188] y represents the molar ratio of manganese in the metal components other than lithium in the lithium transition metal oxide represented by Chemical Formula 1, where y can satisfy 0 < y ≤ 0.25, preferably 0.005 ≤ x ≤ 0.1.

[0189] z represents the molar ratio of M in the metal components other than lithium in the lithium transition metal oxide represented by Chemical Formula 1, where z can satisfy 0 ≤ z < 0.1, preferably 0 ≤ z ≤ 0.02.

[0190] In addition, the positive electrode active material may include a coating formed on its surface, and the coating preferably may include B (boron).

[0191] Since the coating blocks the contact between the positive electrode active material and the electrolyte contained in the lithium secondary battery, thereby suppressing the occurrence of side reactions, it can further improve the life characteristics, and in addition, can increase the packing density of the positive electrode active material.

[0192] The coating may be formed on the entire surface of the positive electrode active material, or may be partially formed. Specifically, in the case where the coating is partially formed on the surface of the positive electrode active material, the coating may be formed in 20% or more to less than 100% of the total specific surface area of the positive electrode active material. In the case where the specific surface area of the coating is less than 20%, the effects of improving the life characteristics and the packing density caused by forming the coating may not be significant.

[0193] In addition, the positive electrode active material of the present invention can be prepared by the following steps:

[0194] (S1) Prepare a solid-phase raw material mixture containing a lithium raw material powder, a nickel raw material powder, a cobalt raw material powder, and a manganese raw material powder such that the molar ratio of lithium to all transition metals is in the range of 0.90 to 1.10;

[0195] (S2) Perform primary sintering and grinding on the solid-phase raw material mixture to prepare a primary sintered product; and

[0196] (S3) Mix the lithium raw material powder with the primary sintered product such that the total molar ratio of lithium to all transition metals is in the range of 0.95 to 1.10, and perform secondary sintering.

[0197] In the above method, the temperature of the secondary sintering may be in the range of 500°C to 900°C, 600°C to 850°C, or 700°C to 800°C.

[0198] Furthermore, in step (S1), the solid-phase raw material mixture may further comprise M raw material powder (wherein M is at least one selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo).

[0199] Method for preparing positive electrode active material

[0200] The lithium transition metal oxide of the present invention is prepared by a solid phase synthesis method using a solid phase raw material mixture, wherein the solid phase synthesis method has the advantage of being able to produce lithium transition metal oxides on a large scale through a relatively simple synthesis process, and is particularly advantageous for preparing high nickel NCM with a high nickel content.

[0201] Furthermore, since the raw material particles are made smaller in size and uniformly mixed through the grinding and mixing processes, the particles grow smoothly at a lower temperature, and thus, excellent single particle formation and sphericity can be exhibited.

[0202] In contrast, when a process is used that first prepares a cathode active material precursor and then uses it to prepare a single-particle cathode active material slurry, single-particle formation may not proceed sufficiently due to poor particle growth, and an over-sintering process may be required. Consequently, when the battery is used, the capacity may be reduced due to factors such as the formation of a rock salt phase. Furthermore, due to the coexistence of secondary particles of the precursor, the sphericity and single-particle formation may not reach the desired levels of the present invention.

[0203] In the present invention, as the lithium raw material powder, for example, lithium-containing carbonates (e.g., lithium carbonate), lithium-containing hydrates (e.g., lithium hydroxide hydrate (LiOH·H2O), etc.), lithium-containing hydroxides (e.g., lithium hydroxide), lithium-containing nitrates (e.g., lithium nitrate (LiNO3), etc.), and lithium-containing chlorides (e.g., lithium chloride (LiCl)) can be used. Preferably, at least one selected from the following can be used as the first lithium raw material: lithium hydroxide, lithium hydroxide hydrate, and lithium carbonate.

[0204] In the present invention, the nickel raw material powder refers to a raw material powder containing only nickel in order to provide nickel as a transition metal. The nickel raw material powder may be at least one selected from the following: nickel oxide, nickel carbonate, nickel sulfate, nickel hydroxide, nickel phosphate, and nickel nitrate.

[0205] The cobalt raw material powder may be at least one selected from the group consisting of cobalt oxide, cobalt carbonate, cobalt sulfate, cobalt hydroxide, and cobalt phosphate.

[0206] The manganese raw material powder may be at least one selected from the group consisting of manganese dioxide, manganese carbonate, manganese sulfate, and manganese nitrate.

[0207] In the solid-phase raw material mixture prepared in step (S1), lithium raw material powder, nickel raw material powder, cobalt raw material powder and manganese raw material powder are mixed in such an amount that the molar ratio of lithium to all transition metals is within a range of 0.90 to 1.10.

[0208] If the molar ratio of lithium is less than 0.90, problems may occur when forming a composite transition metal phase according to solid phase synthesis, and the lack of lithium may reduce the discharge efficiency and may significantly increase the surface resistance, and if the mixing molar ratio is greater than 1.10, problems of increased residual lithium and reduced performance of the positive electrode active material may occur.

[0209] The molar ratio of lithium may be a condition controlled to prepare a cathode active material having excellent performance because residual lithium is not excessive while being in the form of single particles with a high degree of single particle formation, as in the present invention.

[0210] In the present invention, the solid-phase raw material mixture is sintered once and then ground.

[0211] The primary sintering can be performed at a temperature of 400°C to 900°C. When the primary sintering temperature is within the above range, it is possible to prevent an increase in unreacted residual lithium due to low reactivity between the lithium raw material powder and the transition metal raw material powders in the solid-phase raw material mixture, or a decrease in battery capacity and life due to a lack of lithium in the positive electrode active material. Furthermore, it is possible to prevent a decrease in battery capacity and life due to a decrease in the performance of the positive electrode active material caused by localized over-sintering due to uneven reactions during the primary sintering.

[0212] In the present invention, the solid raw material mixture is sintered once and then ground. By grinding, the average particle size (D 50 ) can be adjusted to the range of 2 μm to 4 μm or 3 μm to 4 μm, and the maximum particle size (D max ) is adjusted to less than 30 μm or less than 20 μm. This grinding process is conducive to preparing the positive electrode active material in the form of single particles of the desired size, and ultimately single particles with uniform composition can be prepared.

[0213] The cathode active material of the present invention can be prepared by mixing lithium raw material powder with the product of step (S2) so that the total molar ratio of lithium to all transition metals is in the range of 0.95 to 1.10 and performing a secondary sintering step.

[0214] If the total molar ratio of lithium to transition metal after mixing the lithium raw material powder is less than 0.95, problems may occur when forming a composite transition metal phase according to solid phase synthesis, and the lack of lithium may reduce the discharge efficiency and may significantly increase the surface resistance, while if the total molar ratio of lithium is greater than 1.10, problems such as increased residual lithium and decreased performance of the positive electrode active material may occur.

[0215] In the present invention, the secondary sintering temperature may be in the range of 500 to 900° C., 600 to 850° C., or 700 to 800° C. In addition, the secondary sintering time may be in the range of 5 to 15 hours or 7 to 12 hours.

[0216] Since the secondary sintering is carried out within the above range, insufficient particle growth during the primary sintering can be compensated by supplementing additional lithium raw materials according to the thermal energy and environment, and fine particles generated during the grinding process can be grown and absorbed to prepare a positive electrode active material with an appropriate particle size and a low particle crack rate.

[0217] In the present invention, sintering can be performed in an oxygen or air atmosphere. When sintering in such an atmosphere, the local oxygen partial pressure increases, thereby improving the crystallinity of the positive electrode active material and facilitating surface phase control. In contrast, when sintering in an inert gas atmosphere or a non-oxidizing atmosphere other than the aforementioned atmospheres, oxygen desorption during sintering causes a decrease in crystallinity and uneven surface phase formation, making it difficult to control the phases present on the surface.

[0218] In addition, the present invention may further include the step of forming a coating layer. Preferably, the coating layer may contain an element such as B or cobalt (Co), but the present invention is not limited thereto.

[0219] For example, the coating element may form a coating on the surface of the positive electrode active material through heat treatment.

[0220] The heat treatment for forming the coating layer may be performed within a temperature range suitable for applying the coating material to the surface of the positive electrode active material, and specifically, may be performed within a temperature range of 100°C to 800°C.

[0221] Furthermore, the present invention also provides a positive electrode for a lithium secondary battery, which includes a positive electrode active material.

[0222] Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector and including the positive electrode active material.

[0223] There are no particular limitations on the positive electrode current collector, as long as it has conductivity without causing adverse chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used. In addition, the thickness of the positive electrode current collector can generally be 3 μm to 500 μm, and fine concavoconvexity can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various shapes, such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabrics, and the like.

[0224] The positive electrode active material layer may include a conductive material and a binder as well as the positive electrode active material.

[0225] In this case, the content of the positive electrode active material may be 80 wt % to 99 wt %, more specifically 85 wt % to 98 wt % based on the total weight of the positive electrode active material layer. When the content of the positive electrode active material is within the above range, excellent capacity characteristics may be obtained.

[0226] In this case, a conductive material is used to provide conductivity to the electrode, wherein any conductive material can be used without particular limitation as long as it has suitable electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of the conductive material can be: graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; powder or fiber of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one thereof or a mixture of two or more thereof can be used. The content of the conductive material can be 1% by weight to 30% by weight based on the total weight of the positive electrode active material layer.

[0227] The binder improves the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples of the binder can be: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-to-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber or its various copolymers, and any one thereof or a mixture of two or more thereof can be used. Based on the gross weight of the positive electrode active material layer, the content of the binder can be 1 wt % to 30 wt %.

[0228] The positive electrode can be prepared according to a typical method for preparing a positive electrode, except that the above-mentioned positive electrode active material is used. Specifically, a positive electrode material mixture prepared by dissolving or dispersing the positive electrode active material and an optional binder and a conductive material in a solvent is coated on a positive electrode current collector, and then the positive electrode can be prepared by drying and rolling the coated positive electrode current collector. In this case, the type and amount of the positive electrode active material, binder and conductive material are the same as those described above.

[0229] The solvent may be a commonly used solvent in the art and may include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone or water, and any one thereof or a mixture of two or more thereof may be used. Considering the coating thickness and manufacturing yield of the slurry, if the solvent can dissolve or disperse the positive electrode active material, the conductive material and the binder, and can allow for a viscosity that can provide excellent thickness uniformity during subsequent coating for preparing the positive electrode, the amount of the solvent may be sufficient.

[0230] Furthermore, as another method, the positive electrode can be prepared by casting the positive electrode material mixture on a separate support and then laminating the film separated from the support on a positive electrode current collector.

[0231] Furthermore, in the present invention, an electrochemical device including a positive electrode can be prepared. Specifically, the electrochemical device can be a battery or a capacitor, and more specifically, a lithium secondary battery.

[0232] The lithium secondary battery specifically includes a positive electrode, a negative electrode disposed to face the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, and since the positive electrode is the same as above, its detailed description will be omitted, and only the remaining configuration will be described in detail below.

[0233] In addition, the lithium secondary battery may optionally further include a battery container that houses an electrode assembly of a positive electrode, a negative electrode, and a separator, and a sealing member that seals the battery container.

[0234] In a lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector.

[0235] There are no particular limitations on the negative electrode current collector, as long as it has high conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like, and aluminum-cadmium alloys can be used. Furthermore, the thickness of the negative electrode current collector can generally be 3 μm to 500 μm, and similar to the positive electrode current collector, fine concavo-convexities can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various shapes, such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabrics, and the like.

[0236] The negative electrode active material layer optionally contains a binder and a conductive material in addition to the negative electrode active material.

[0237] Compounds that can reversibly intercalate and deintercalate lithium can be used as negative electrode active materials. Specific examples of negative electrode active materials can be: carbonaceous materials, such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; (semi)metallic materials that can form alloys with lithium, such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloys, Sn alloys, or Al alloys; (semi)metallic oxides that can be doped or undoped with lithium, such as SiO β (0<β<2), SnO2, vanadium oxide and lithium vanadium oxide; or a composite material comprising a (semi)metallic material and a carbonaceous material, such as a Si-C composite material or a Sn-C composite material, and any one thereof or a mixture of two or more thereof can be used. In addition, a metallic lithium film can be used as a negative electrode active material. In addition, both low-crystalline carbon and high-crystalline carbon can be used as carbon materials. Typical examples of low-crystalline carbon can be soft carbon and hard carbon, and typical examples of high-crystalline carbon can be irregular, planar, flaky, spherical or fibrous natural graphite or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-type carbon fibers, mesophase carbon microbeads, mesophase pitch, and high-temperature sintered carbon such as coke derived from petroleum or coal tar pitch.

[0238] The content of the negative electrode active material may be 80 wt % to 99 wt % based on the total weight of the negative electrode active material layer.

[0239] The binder is a component that helps to bind the conductive material, the active material and the current collector, wherein the amount of the binder added is generally 0.1 to 10 parts by weight based on 100 parts by weight of the total weight of the negative electrode active material layer. Examples of the binder can be polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluororubber and various copolymers thereof.

[0240] The conductive material is a component used to further improve the conductivity of the negative electrode active material. The amount of the conductive material added may be 10% by weight or less, preferably 5% by weight or less, based on the total weight of the negative electrode active material layer. There is no particular limitation on the conductive material, as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, conductive materials such as graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers, such as carbon fibers or metal fibers; fluorocarbons; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives may be used.

[0241] For example, the negative electrode active material layer can be prepared by coating a negative electrode material mixture prepared by dissolving or dispersing an optional binder and a conductive material, and a negative electrode active material in a solvent on a negative electrode collector and drying the coated negative electrode collector, or can be prepared by casting the negative electrode material mixture on a separate support and then laminating the film separated from the support on the negative electrode collector.

[0242] In lithium secondary batteries, the diaphragm separates the negative electrode from the positive electrode and provides a path for the movement of lithium ions, wherein any diaphragm can be used as a diaphragm without particular limitation, as long as it is commonly used in lithium secondary batteries, in particular, preferably has a high moisture retention capacity for electrolytes and a diaphragm with low resistance to the movement of electrolyte ions. Specifically, a porous polymer film can be used, for example, a porous polymer film prepared by polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers and ethylene / methacrylate copolymers, or a laminated structure having more than two layers thereof. In addition, a typical porous non-woven fabric can be used, for example, a non-woven fabric formed of high melting point glass fiber or polyethylene terephthalate fiber. In addition, a coated diaphragm comprising a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a diaphragm with a single layer or multilayer structure can be optionally used.

[0243] In addition, the electrolyte used in the present invention may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte that can be used to prepare a lithium secondary battery, but the present invention is not limited thereto.

[0244] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0245] Any organic solvent can be used as an organic solvent without particular limitation, as long as it can be used as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, there can be used: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; or carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitrile such as R-CN (wherein R is a linear, branched or cyclic C2-C20 hydrocarbon group, and may contain a double bond, an aromatic ring or an ether bond); amide such as dimethylformamide; dioxolane such as 1,3-dioxolane; or cyclopentane sulfone. Among these solvents, carbonate solvents are preferred, and more preferably a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) that can increase the charge / discharge performance of the battery. In this case, when cyclic carbonate and chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte can be excellent.

[0246] Lithium salts can be used without particular limitation as long as they are compounds that can provide lithium ions used in lithium secondary batteries. Specifically, as lithium salts, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI or LiB(C2O4)2 can be used. The lithium salt can be used in a concentration range of 0.1 M to 2.0 M. When the concentration of the lithium salt is within the above range, since the electrolyte can have appropriate conductivity and viscosity, excellent electrolyte performance can be obtained, and lithium ions can be effectively moved.

[0247] In order to improve the life characteristics of the battery, suppress the reduction of the battery capacity and improve the discharge capacity of the battery, in addition to the electrolyte components, the electrolyte may further include at least one additive, such as a halogenated alkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, (condensed) glycol dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum chloride. In this case, the content of the additive may be 0.1 to 5 parts by weight based on 100 parts by weight of the total weight of the electrolyte.

[0248] As described above, since the lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics and life characteristics, the lithium secondary battery is suitable for portable devices such as mobile phones, notebook computers and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs).

[0249] Therefore, according to another embodiment of the present invention, a battery module including a lithium secondary battery as a unit cell and a battery pack including the battery module are provided.

[0250] The battery module or battery pack can be used as a power source for at least one of the following medium to large devices: a power tool; an electric vehicle, including an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0251] The shape of the lithium secondary battery of the present invention is not particularly limited, but a cylindrical type using a can, a prismatic type, a pouch type, or a coin type can be used.

[0252] The lithium secondary battery according to the present invention can be used not only in a battery cell used as a power source for small devices but also as a unit battery in a medium or large battery module including a plurality of battery cells.

[0253] Preferred embodiments

[0254] Examples and Comparative Examples

[0255] The present invention will be described in more detail below based on examples. However, the following examples are only for illustrating the present invention, and the scope of the present invention is not limited thereto.

[0256] Example 1

[0257] Lithium (Li), nickel (Ni), Co, manganese (Mn), and Al raw materials were added to a mixer in an amount such that the molar ratio of Li:Ni:Co:Mn:Al was 0.97:0.93:0.05:0.01:0.01, and mixed and ground using a high-energy grinding device. In this case, LiOH·H2O, Ni(OH)2, Co3O4, MnO2, and Al3O4 were used as raw materials.

[0258] The mixed and ground raw materials were filled into a crucible at a density of 0.7 g / cc or more, and after primary sintering at 810°C in an oxygen atmosphere for 10 hours, the primary sintered product thus obtained was ground using an air grinder. In this case, the size of the primary sintered product was adjusted so that D 50 In the range of 2 μm to 4 μm and D max >20 μm.

[0259] After the ground primary sintered product was mixed with LiOH·H 2 O so that the molar ratio of lithium to transition metal was 1.00, the mixture was secondarily sintered at 750° C. for 5 hours and then pulverized with an air mill to prepare a positive electrode active material (lithium transition metal oxide).

[0260] Example 2

[0261] A positive electrode active material (lithium transition metal oxide) was prepared in the same manner as in Example 1, except that primary sintering was performed at a temperature of 830°C.

[0262] Comparative Example 1

[0263] Nickel cobalt manganese hydroxide, Li raw material and Al raw material were added to the mixer in an amount such that the molar ratio of Li:Ni:Co:Mn:Al was 0.97:0.93:0.05:0.01:0.01 and mixed. 0.94 Co 0.05 Mn 0.01 (OH)2 and Al2O3 are used as raw materials.

[0264] The mixed raw materials were filled into a crucible at a density of 0.7 g / cc or more, and after primary sintering at 760°C in an oxygen atmosphere for 12 hours, the primary sintered product thus obtained was ground using an air grinder. In this case, the size of the primary sintered product was adjusted so that D 50 In the range of 2 μm to 4 μm and D max >20 μm.

[0265] Experimental example

[0266] Experimental Example 1: Regarding Condition 1

[0267] (1) Measurement of cross-sectional area and calculation of average cross-sectional area

[0268] After preparing an electrode (positive electrode) comprising each positive electrode active material prepared in the Examples and Comparative Examples, the electrode was cut by ion milling to obtain a cross section of the positive electrode, and an SEM image (about 5K magnification) of the cross section of the positive electrode was obtained using SEM (JEOL Co., Ltd., JSM-7900F). The cross-sectional area of ​​the particles was measured using an image processing program, and the average cross-sectional area was calculated, as shown in Tables 1 to 3 below.

[0269] Specifically, 95% by weight of each positive electrode active material prepared in the Examples and Comparative Examples, 2.0% by weight of carbon black as a conductive material, and 3.0% by weight of polyvinylidene fluoride (PVDF) as a binder were mixed in an N-methylpyrrolidone (NMP) solvent to prepare each positive electrode slurry. The positive electrode slurry prepared above was applied to one surface of an aluminum current collector and then dried at 130°C to prepare a positive electrode. Rolling was then performed without rolling or with rolling to achieve a porosity of x%.

[0270] After obtaining a cross section of the positive electrode by cutting the positive electrode using an ion milling method in which the positive electrode is irradiated with an argon (Ar) ion beam using an ion milling apparatus (JEOL Co., Ltd., CP-09IB19520CCP, accelerating voltage: 6 kV), an SEM image (about 5K magnification) of the cross section of the positive electrode is obtained using an SEM (JEOL Co., Ltd., JSM-7900F). Thereafter, an image processing program (deep learning program, Mask-RCNN model) is used to obtain a two-dimensional segmented image in which the boundaries of the particles present in the SEM image are segmented and displayed in random colors, as shown in FIG. Figures 1 to 3 As shown in .

[0271] For reference, the image processing operation includes a method of direct classification by researchers and an image classification method through deep learning, and the image classification method through deep learning is used in the present invention, but the present invention is not limited thereto.

[0272] The cross-sectional area of ​​the particles is calculated using the number of pixels corresponding to each of n (about 100 to 200) particles present in the segmented image and the total number of particles of the lithium transition metal oxide contained in the electrode (total number of particles [number]), and the ratio (B) of the number of particles present per cross-sectional area (z) to the total number of particles of the lithium transition metal oxide contained in the electrode is calculated. x,z [%]) and the average cross-sectional area of ​​particles present in the segmented image (X x [μm 2 ]), as shown in Tables 1 to 3 below. However, the cross-sectional area captured in the resulting image is less than 3 μm2 Particles with large or no overall shape observed in the image were excluded as they could compromise the representativeness of the sample.

[0273] Figure 1 is a segmented image of a cross section of a positive electrode including the positive electrode active material prepared in Example 1.

[0274] Figure 2 is a segmented image of a cross section of a positive electrode including the positive electrode active material prepared in Example 2.

[0275] Figure 3 is a segmented image of a cross section of a positive electrode including the positive electrode active material prepared in Comparative Example 1.

[0276] according to Figures 1 to 3 , it was confirmed that the positive electrode active materials prepared in Examples 1 and 2 were in the form of single particles with a high degree of single particle formation, while the positive electrode active material prepared in Comparative Example 1 had a low degree of single particle formation. In addition, it was confirmed that the positive electrode active materials prepared in Examples 1 and 2 were in the form of single particles composed of 2 to 10 primary particles, and compared with the positive electrode active material prepared in Comparative Example 1, due to a smaller number of grain boundaries and a higher sphericity, it was confirmed that the particle strength was high and particle cracking during electrode rolling was extremely rare.

[0277] (2) Calculation of average cross-sectional area change rate

[0278] For the positive electrode active materials of Examples and Comparative Examples, the average cross-sectional area was calculated using the same method as in (1) above by changing the unrolled or rolled conditions during electrode preparation, and the average cross-sectional area change rate [%] was calculated by the following method.

[0279]

[0280] X 30≤ is the average cross-sectional area [μm] of lithium transition metal oxide particles contained in an electrode containing lithium transition metal oxide and not subjected to rolling. 2 ],

[0281] X 11 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 11%. 2 ],

[0282] X 14 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 14%. 2 ],

[0283] X 16is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 16%. 2 ],

[0284] X 20 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 20%. 2 ].

[0285]

[0286]

[0287]

[0288] According to Tables 1 to 3, the average cross-sectional area change rate (ΔX 11 ) is 8% to 15%, and the average cross-sectional area change rate (ΔX 20 ) is 1% to 15%. In addition, when the electrode containing the lithium transition metal oxide was rolled so that the porosity was 11%, it was confirmed that the cross-sectional area of ​​the positive electrode active materials of Examples 1 and 2 was 3 μm 2 to 4 μm 2 The ratio of the number of particles of lithium transition metal oxide to the total number of particles of lithium transition metal oxide contained in the electrode (B 11,3-4 ) is below 40%, and confirmed X 11 At 5 μm 2 to 10 μm 2 within the range.

[0289] On the contrary, it was confirmed that the average cross-sectional area change rate (ΔX 11 ) is greater than 20%, and according to the average cross-sectional area change rate (ΔX 20 ) is greater than 15%. In addition, when the electrode containing the lithium transition metal oxide is rolled so that the porosity is 11%, the cross-sectional area of ​​the positive electrode active material of Comparative Example 1 is 3 μm 2 to 4 μm 2 The ratio of the number of particles of lithium transition metal oxide to the total number of particles of lithium transition metal oxide contained in the electrode (B 11,3-4 ) is greater than 40%, and confirms that X 11 Less than 5 μm 2 .

[0290] This means that the cathode active material according to the present invention exhibits a low cross-sectional area change rate due to an improved particle cracking phenomenon during rolling.

[0291] Experimental Example 2: Regarding Condition 2

[0292] After preparing an electrode (positive electrode) containing each positive electrode active material prepared in Examples and Comparative Examples, the electrode was cut by ion milling to obtain a cross section of the positive electrode, and an SEM image (about 5K magnification) of the cross section of the positive electrode was obtained using SEM (JEOL Co., Ltd., JSM-7900F). Thereafter, the cross-sectional area of ​​the cracks and the cross-sectional area of ​​the particles were measured using an image processing program, and C was calculated. x and A x,y , as shown in Tables 4 to 6 below.

[0293] Specifically, as in Experimental Example 1, a positive electrode was prepared, a cross section of the positive electrode was photographed, and a two-dimensional segmented image was obtained using an image processing program. A region that appeared black or darker than an adjacent portion in each of n (about 100 to 200) lithium transition metal oxide particles present in the segmented image was defined as a crack. The cross-sectional area [μm] of the crack formed in the lithium transition metal oxide particle was calculated using the number of pixels corresponding to the crack. 2 ], the cross-sectional area of ​​the lithium transition metal oxide particle is calculated using the number of pixels corresponding to the cross-sectional area of ​​the lithium transition metal oxide particle including the crack, and the percentage of the cross-sectional area of ​​the crack formed in the lithium transition metal oxide particle relative to the cross-sectional area of ​​the lithium transition metal oxide particle is calculated (C x [%]), and according to the mathematical formula (2) described in this specification, C x The ratio (A) of the number of particles of the predetermined value (y) to the total number of particles of the lithium transition metal oxide contained in the electrode x,y [%]), the number of cracked particles [number], and the total number of particles [number] are shown in Tables 4 to 6 below.

[0294]

[0295]

[0296]

[0297] According to Tables 4 to 6, the C values ​​of the positive electrode active materials of Examples 1 and 2 according to the mathematical formula (2) described in this specification were confirmed. 11 [%] Ratio of the number of particles greater than 6% to the total number of particles of lithium transition metal oxide contained in the electrode (A 11,6< ) is more than 3%.

[0298] Furthermore, it was confirmed that C 20 [%] Ratio of the number of particles greater than 6% to the total number of particles of lithium transition metal oxide contained in the electrode (A 20,6< ) is 0.5% or more,

[0299] According to the mathematical formula (2) described in this specification, C 11 [%] is the ratio of the number of particles from 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode (A 11,0-1 ) is below 40%,

[0300] According to the mathematical formula (2) described in this specification, C 20 [%] is the ratio of the number of particles from 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode (A 20,0-1 ) is below 45%,

[0301] According to the mathematical formula (2) described in this specification, C 11 [%] is the ratio of 1% to 2% of the number of particles to the total number of particles of the lithium transition metal oxide contained in the electrode (A 11,1-2 ) is less than 20%, and

[0302] According to the mathematical formula (2) described in this specification, C 20 [%] is the ratio of 1% to 2% of the number of particles to the total number of particles of the lithium transition metal oxide contained in the electrode (A 20,1-2 ) is below 20%.

[0303] On the contrary, it was confirmed that the positive electrode active material of Comparative Example 1 had a C 11 [%] Ratio of the number of particles greater than 6% to the total number of particles of lithium transition metal oxide contained in the electrode (A 11,6< ) is less than 3%.

[0304] In addition, it was confirmed that C 20 [%] Ratio of the number of particles greater than 6% to the total number of particles of lithium transition metal oxide contained in the electrode (A 20,6< ) is less than 0.5%,

[0305] According to the mathematical formula (2) described in this specification, C 11 [%] is the ratio of the number of particles from 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode (A 11,0-1 ) is greater than 40%,

[0306] According to the mathematical formula (2) described in this specification, C 20[%] is the ratio of the number of particles from 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode (A 20,0-1 ) is greater than 45%,

[0307] According to the mathematical formula (2) described in this specification, C 11 [%] is the ratio of 1% to 2% of the number of particles to the total number of particles of the lithium transition metal oxide contained in the electrode (A 11,1-2 ) is greater than 20%, and

[0308] According to the mathematical formula (2) described in this specification, C 20 [%] is the ratio of 1% to 2% of the number of particles to the total number of particles of the lithium transition metal oxide contained in the electrode (A 20,1-2 ) is greater than 20%.

[0309] This means that for the lithium transition metal oxide contained in the positive electrode active material of the present invention, the proportion of the number of particles with small-area cracks is high due to the increase in crack generation (rather than particle cracking) during rolling, which indicates that when the rolling strength is further increased, the proportion of particles with cracks in the lithium transition metal oxide contained in the positive electrode active material further increases, which means that even under a certain level of high pressure, cracks are generated in the particles at a high frequency (rather than particle cracking phenomenon).

[0310] Experimental Example 3: Regarding Conditions 2 and 3

[0311] After each of the positive electrode active materials prepared in Examples and Comparative Examples was placed in a circular mold and pressed at a pressure of 0 to 9 tons, the compacted pellets were pulverized with a mortar to examine the distribution of particles.

[0312] Figure 6 The figure shows changes in the particle size distribution of the lithium transition metal oxide contained in the positive electrode when the positive electrode containing the positive electrode active material prepared in Example 1 is pressurized, specifically after pressurization at pressures of 0 tons, 3 tons, 6 tons, and 9 tons. Specifically, the changes in the particle size distribution are expressed as a graph of the cumulative distribution function (Q [%]) versus the particle size [μm].

[0313] Figure 7 The following table shows changes in the particle size distribution of the lithium transition metal oxide contained in the positive electrode when the positive electrode containing the positive electrode active material prepared in Comparative Example 1 is pressurized, specifically after pressurization at pressures of 0 tons, 3 tons, 6 tons, and 9 tons. Specifically, the changes in particle size distribution are expressed as a graph of the cumulative distribution function (Q [%]) versus the particle size [μm].

[0314] according to Figure 6 and 7When the pressure increased, the distribution curve itself was hardly shifted due to particle cracking in the positive electrode active material prepared in Example 1. However, since particle cracking occurred in the positive electrode active material prepared in Comparative Example 1 as the pressure increased, it was confirmed that the particle distribution shifted to the left as a whole. As a result, it can be understood that when the positive electrode active material according to the present invention is pressurized, cracks rather than particle cracking occur.

[0315] Experimental Example 4: Regarding Condition 3

[0316] After preparing an electrode (positive electrode) containing each positive electrode active material prepared in Examples and Comparative Examples, the electrode was cut by ion milling to obtain a cross section of the positive electrode, and an SEM image (about 5K magnification) of the cross section of the positive electrode was obtained using SEM (JEOL Co., Ltd., JSM-7900F). Thereafter, the number of lithium transition metal oxide particles having cracks was determined using an image processing program, and the crack particle ratio was calculated, as shown in Tables 7 to 9 below.

[0317] Specifically, as in Experimental Example 1, a positive electrode is prepared, a cross section of the positive electrode is photographed, and a two-dimensional segmented image is obtained using an image processing program. The presence or absence of cracks is confirmed by visually observing each of n (about 100 to 200) lithium transition metal oxide particles present in the segmented image, wherein a black area or an area with a relatively darker color than an adjacent area is determined to have cracks, and a white area or an area with a relatively brighter color than an adjacent area is determined to have no cracks. When an electrode containing a lithium transition metal oxide is rolled so that the porosity is x%, the total number of particles of the lithium transition metal oxide contained in the electrode (d [number]) and the number of particles with cracks (c [number]) are calculated, and the percentage of the number of particles with cracks (c [number]) relative to the total number of particles of the lithium transition metal oxide (d [number]) is calculated (cracked particle ratio (D x [%])) are shown in Tables 7 to 9 below.

[0318]

[0319]

[0320]

[0321] According to Tables 7 to 9, when the electrode containing the lithium transition metal oxide was rolled so that the porosity was 11%, it was confirmed that the ratio of the number of particles having cracks of the positive electrode active materials of Examples 1 and 2 to the total number of particles of the lithium transition metal oxide contained in the electrode (D 11 ) is below 78%.

[0322] Furthermore, when the electrode containing the lithium transition metal oxide was rolled so as to have a porosity of 20%, the ratio (D 20 ) is 70% or less, and the ratio of the number of particles having cracks to the total number of particles of the lithium transition metal oxide is 50% or less.

[0323] In contrast, when the electrode containing the lithium transition metal oxide was rolled so as to have a porosity of 11%, it was confirmed that the ratio of the number of particles having cracks of the positive electrode active material of Comparative Example 1 to the total number of particles of the lithium transition metal oxide contained in the electrode (D 11 ) is greater than 78%.

[0324] Furthermore, when the electrode containing the lithium transition metal oxide was rolled so as to have a porosity of 20%, the ratio (D 20 ) is greater than 70%, and the ratio of the number of particles having cracks to the total number of particles of the lithium transition metal oxide is greater than 50%.

[0325] This indicates that the ratio of particles having cracks in the lithium transition metal oxide contained in the positive electrode active material increases relatively less when the rolling strength is further increased, which means that particle cracking does not often occur even at a certain level of high pressure.

[0326] Experimental Example 5

[0327] SEM images of the positive electrode active materials prepared in Examples and Comparative Examples were obtained using SEM (JEOL Co., Ltd., JSM-7900F). Figure 4 and 5 As shown in .

[0328] Figure 4 4(A) and 4(B) are SEM images of the positive electrode active materials prepared in Example 1 and Comparative Example 1, respectively.

[0329] Figure 5 (A) to 5(C) are SEM images of the positive electrode active materials prepared in Example 1, Example 2, and Comparative Example 1, respectively.

[0330] according to Figure 4 and 5, it was confirmed that the positive electrode active material prepared in Example 1 was in the form of single particles with a high degree of single particle formation, while the positive electrode active material prepared in Comparative Example 1 had a low degree of single particle formation. In addition, it was confirmed that the positive electrode active material prepared in Example 1 was in the form of single particles composed of 2 to 10 primary particles, and compared with the positive electrode active material prepared in Comparative Example 1, due to a smaller number of grain boundaries and higher sphericity, it was confirmed that the particle strength was high and particle cracking during electrode rolling was extremely rare.

[0331] Experimental Example 6

[0332] Each positive electrode active material prepared in the Examples or Comparative Examples, a conductive material (Danka Black), and a binder (PVDF) were mixed in an N-methyl-2-pyrrolidone (NMP) solvent at a weight ratio of 95:2:3 to prepare a positive electrode slurry. An aluminum current collector was coated with the positive electrode slurry, dried, and then rolled to prepare a positive electrode.

[0333] Next, the negative electrode active material (natural graphite), conductive material (carbon black), and binder (SBR + CMC) were mixed in water at a weight ratio of 95.6:1.0:3.4 to prepare a negative electrode slurry. A copper current collector was coated with the negative electrode slurry, dried, and then rolled to prepare a negative electrode.

[0334] After preparing an electrode assembly by placing a separator between the positive electrode and the negative electrode, the electrode assembly was placed in a battery case and then injected with electrolyte to prepare two single cells, each with an electrode size of 3 cm × 4 cm. In this case, an electrolyte solution of 0.7 M LiPF6 and 0.3 M LiFSI dissolved in an organic solvent mixed with ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7 was used as the electrolyte.

[0335] After charging two single cells to 4.2 V at 25°C with a constant current of 0.33 C and a cutoff of 0.05 C, the positive electrode was separated. The separated positive electrode was placed in a battery bag, and the electrolyte was further injected, and then the battery bag was sealed to prepare a sample. The gas generation amount was measured while the sample was stored at 60°C for 12 weeks, and the gas generation amount for 12 weeks is shown in the following Tables 10 and Figure 8 As shown in .

[0336] Figure 8 Graph showing the change in gas generation amount (ml) over time (weeks) for batteries containing the positive electrode active materials prepared in Example 1, Example 2, and Comparative Example 1.

[0337]

[0338] According to Table 10, it was confirmed that the secondary battery using the cathode active material according to the present invention had an excellent effect of reducing gas generation.

Claims

1. A positive electrode active material, the positive electrode active material comprising a lithium transition metal oxide in the form of single particles, wherein the lithium transition metal oxide satisfies at least one of Conditions 1 to 3: [Condition 1] The average cross-sectional area change rate (ΔX) of the lithium transition metal oxide according to the mathematical formula (1) is 11 ) is 5% to 20%; Mathematical formula (1) in, In Mathematical formula (1), X 30≤ is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode containing the lithium transition metal oxide and not subjected to rolling. 2 ],and X 11 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 11%. 2 ], [Condition 2] The lithium transition metal oxide has a C value according to formula (2): 11 [%] The ratio of the number of particles greater than 6% to the total number of particles of the lithium transition metal oxide contained in the electrode is greater than 3%; Mathematical formula (2) wherein, in Mathematical formula (2), when the electrode containing the lithium transition metal oxide is rolled to a porosity of x%, a is the cross-sectional area of ​​cracks formed in the lithium transition metal oxide particles contained in the electrode [μm 2 ], b is the cross-sectional area of ​​the lithium transition metal oxide particles contained in the electrode [μm 2 ], [Condition 3] When the electrode containing the lithium transition metal oxide is rolled to a porosity of 11%, the ratio of the number of cracked particles of the lithium transition metal oxide to the total number of particles of the lithium transition metal oxide contained in the electrode is 78% or less.

2. The positive electrode active material according to claim 1, wherein the average cross-sectional area change rate (ΔX 11 ) is 8% to 15%.

3. The positive electrode active material according to claim 1, wherein the average cross-sectional area change rate (ΔX 20 ) is 1% to 15%: Mathematical formula (3) in, In Mathematical formula (3), X 30≤ is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode containing the lithium transition metal oxide and not subjected to rolling. 2 ],and X 20 is the average cross-sectional area [μm] of the lithium transition metal oxide particles contained in the electrode when the electrode is rolled so that the porosity is 20%. 2 ].

4. The positive electrode active material according to claim 1, wherein When the electrode including the lithium transition metal oxide is rolled so as to have a porosity of 11%, the cross-sectional area is 3 μm 2 to 4 μm 2 The ratio of the number of particles of the lithium transition metal oxide to the total number of particles of the lithium transition metal oxide contained in the electrode is 40% or less.

5. The positive electrode active material according to claim 1, wherein the X 11 At 5 μm 2 to 10 μm 2 within the range.

6. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide has a C 20 [%] The ratio of the number of particles greater than 6% to the total number of particles of the lithium transition metal oxide contained in the electrode is greater than or equal to 0.5%.

7. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide has a C 11 [%] A ratio of the number of particles having a range of 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode is 40% or less.

8. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide has a C 20 [%] A ratio of the number of particles having a range of 0% to 1% to the total number of particles of the lithium transition metal oxide contained in the electrode is 45% or less.

9. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide has a C 11 [%] The ratio of the number of particles of 1% to 2% to the total number of particles of the lithium transition metal oxide contained in the electrode is 20% or less.

10. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide has a C 20 [%] The ratio of the number of particles of 1% to 2% to the total number of particles of the lithium transition metal oxide contained in the electrode is 20% or less.

11. The positive electrode active material according to claim 1, wherein when the electrode containing the lithium transition metal oxide is rolled to a porosity of 20%, the ratio of the number of cracked particles of the lithium transition metal oxide to the total number of particles of the lithium transition metal oxide contained in the electrode is 70% or less.

12. The positive electrode active material according to claim 1, wherein the ratio of the number of cracked particles of the lithium transition metal oxide to the total number of particles is 50% or less.

13. The positive electrode active material according to claim 1, wherein When the electrode containing the lithium transition metal oxide is rolled to a porosity of 11%, the difference in the ratio of the number of cracked particles of the lithium transition metal oxide to the total number of particles of the lithium transition metal oxide contained in the electrode before and after the rolling is 20% to 80%.

14. The positive electrode active material according to claim 1, wherein the average particle size (D 50 ) is 1.5 μm to 6 μm.

15. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide is represented by Chemical formula (1): [Chemical formula (1)] Li (Ni 1-x-y-z Co x Mr y M z )O2 in, In Chemical formula (1), M is at least one selected from the following: Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and x, y, and z are atomic fractions of independent elements, respectively, where 0 < x ≤ 0.25, 0 < y ≤ 0.25, 0 ≤ z < 0.1, and 0 < x + y + z ≤ 0.

5.

16. A positive electrode for a lithium secondary battery, the positive electrode comprising the positive electrode active material according to any one of claims 1 to 15.

17. A lithium secondary battery, the lithium secondary battery comprising the positive electrode for a lithium secondary battery according to claim 16.

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