Positive electrode active material, method of preparing the same, and rechargeable lithium battery including the same

Abstract

Provided are a positive electrode active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same, the positive electrode active material for a rechargeable lithium battery including a lithium nickel-based composite oxide, wherein the positive electrode active material includes: secondary particles in which a plurality of primary particles are aggregated, wherein at least a portion of the primary particles are radially arranged; a boron coating layer disposed on a surface of the secondary particles and including a lithium borate; and a boron-doped layer inside the primary particles exposed to the surface of the secondary particles.

Description

Technical Field

[0001] A positive electrode active material for a rechargeable lithium battery, a method for preparing the active material thereto, and a rechargeable lithium battery including the active material thereto are disclosed. Background Technology

[0002] Portable information devices such as mobile phones, laptops, and smartphones, as well as electric vehicles, already use rechargeable lithium batteries with high energy density and portability as their power source. Recently, research has been actively underway to use rechargeable lithium batteries with high energy density as a power source or energy storage source for hybrid or electric vehicles.

[0003] Various positive electrode active materials have been studied to realize rechargeable lithium batteries for the aforementioned applications. Among these positive electrode active materials, lithium nickel oxides, lithium nickel manganese cobalt composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium cobalt oxides are mainly used. However, these positive electrode active materials suffer from structural collapse or cracking during repeated charging and discharging, resulting in problems such as deterioration of the long-term cycle life of rechargeable lithium batteries and increased resistance, thus failing to exhibit satisfactory capacity characteristics. Accordingly, there is a need to develop novel positive electrode active materials that ensure long-term cycle life characteristics and achieve high capacity and high energy density. Summary of the Invention

[0004] Positive electrode active material for rechargeable lithium batteries, a method for preparing the same, and a rechargeable lithium battery including the same are provided, wherein the positive electrode active material achieves high capacity while having improved cycle life characteristics.

[0005] In an embodiment, the positive electrode active material for a rechargeable lithium battery includes a lithium-nickel composite oxide, wherein the positive electrode active material includes: secondary particles in which a plurality of primary particles are aggregated, wherein at least a portion of the primary particles are arranged radially; a boron coating disposed on the surface of the secondary particles and comprising lithium borate; and a boron doping layer inside the primary particles exposed to the surface of the secondary particles.

[0006] In another embodiment, a method for preparing a positive electrode active material for a rechargeable lithium battery includes mixing a nickel hydroxide, a lithium feedstock, and a boron feedstock and then heat-treating the resulting product.

[0007] In another embodiment, the rechargeable lithium battery includes a positive electrode containing the above-described positive electrode active material, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte.

[0008] The positive electrode active material for a rechargeable lithium battery according to the embodiments and the rechargeable lithium battery including the positive electrode active material can exhibit excellent cycle life characteristics while achieving high capacity and high energy density. Attached Figure Description

[0009] Figure 1 A schematic diagram illustrating the shape of the plate-like primary particles.

[0010] Figure 2 This is a view used to explain the definition of radial direction in secondary particles.

[0011] Figure 3 This is a schematic diagram illustrating the cross-sectional structure of the secondary particles according to an embodiment.

[0012] Figure 4 A cross-sectional view of a rechargeable lithium battery according to an embodiment is shown for illustrative purposes.

[0013] Figure 5 This is a scanning electron microscope (SEM) image of the cross-section of the positive electrode active material precursor prepared in Example 1.

[0014] Figure 6 This is a SEM image of the cross-section of the positive electrode active material prepared in Example 1.

[0015] Figure 7 This is a transmission electron microscope (TEM) image of primary particles exposed on the surface of secondary particles in the positive electrode active material of Example 1.

[0016] Figure 8 for Figure 7 The image shows TEM-EELS (Transmission Electron Microscopy-Electron Energy Loss Spectroscopy) analysis of a total of 10 points, from point 1 to point 10.

[0017] Figure 9 This is a ToF-SIMS analysis image of the surface of the positive electrode active material in Example 1.

[0018] Figure 10 for Figure 9 Mass spectrometry results from ToF-SIMS analysis.

[0019] Figure 11 This is a graph showing the boron content of the boron coating (external) and the boron-coated grain boundary portion (internal) based on the amount of boron added, as measured by ICP emission spectroscopy.

[0020] Figure 12 The graph shows the initial charge / discharge capacity and capacity retention rate after 50 cycles of the battery cells of Example 1 and Comparative Examples 1-3 at high temperature.

[0021] Figure 13 To illustrate the lifetime characteristics of the battery cells of Example 1 and Comparative Examples 1-3 at high temperature after 50 cycles of capacity retention.

[0022] <Explanation of Figure Markers>

[0023] 11: Secondary particles 12: Internal part of secondary particles

[0024] 13: Primary particles 14: External portion of secondary particles

[0025] 100: Rechargeable lithium battery; 112: Negative electrode

[0026] 113: Diaphragm; 114: Positive electrode

[0027] 120: Battery casing; 140: Sealing component Detailed Implementation

[0028] Specific implementation methods will be described in detail below so that those skilled in the art can readily implement them. However, this disclosure may be embodied in many different forms and is not to be construed as limited to the exemplary embodiments set forth herein.

[0029] The terminology used herein is for descriptive purposes only and is not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0030] As used herein, “combination of” refers to mixtures, laminates, complexes, copolymers, alloys, blends, and reaction products of the components.

[0031] Here it should be understood that terms such as “comprising,” “including,” or “having” are intended to indicate the presence of the features, quantities, steps, elements, or combinations thereof embodied, but do not preclude the possibility of the presence or addition of one or more other features, quantities, steps, elements, or combinations thereof.

[0032] In the accompanying drawings, for clarity, the thicknesses of layers, films, panels, regions, etc., are enlarged, and throughout the specification, the same reference numerals denote the same elements. It will be understood that when an element such as a layer, film, region, or substrate is referred to as "on" another element, the element may be directly on the other element, or an intervening element may be present. Conversely, when an element is referred to as "directly on" another element, no intervening element is present.

[0033] Furthermore, when viewed from a plan view, the “layers” here include not only shapes formed on the entire surface, but also shapes formed on parts of the surface.

[0034] Additionally, the average particle size can be measured using methods well known to those skilled in the art, such as by a particle size analyzer, or by transmission electron microscopy or scanning electron microscopy. Alternatively, the average particle size value can be obtained by measuring using dynamic light scattering methods, performing data analysis, counting the number of particles in each particle size range, and thereby calculating. Unless otherwise specified, the average particle size (D50) may mean the diameter of the particles that constitute 50% of the total volume in the particle size distribution.

[0035] Positive electrode active material

[0036] In an embodiment, the positive electrode active material for a rechargeable lithium battery includes a lithium-nickel composite oxide, wherein the positive electrode active material includes: secondary particles in which a plurality of primary particles are aggregated, wherein at least a portion of the primary particles are arranged radially; a boron coating disposed on the surface of the secondary particles and comprising lithium borate; and a boron doping layer inside the primary particles exposed to the surface of the secondary particles.

[0037] Generally, conventional methods for coating boron onto positive electrode active materials involve first preparing a lithium metal composite oxide, then mixing it with a boron raw material using a wet or dry method, and finally heat-treating the mixture. However, boron acts as a resistor on the surface of the positive electrode active material, thus degrading capacity and cycle life. In contrast, embodiments can provide a secondary particulate positive electrode active material in which at least a portion of the primary particles are radially arranged by adding both lithium and boron raw materials to a specific positive electrode active material precursor (such as nickel hydroxide), and then heat-treating the mixture under specific conditions. In this secondary particulate positive electrode active material, a boron coating and a boron doping layer can be formed simultaneously. When appropriate amounts of both the boron coating and the boron doping layer are formed simultaneously, boron no longer acts as a resistor, but instead improves the structural stability of the positive electrode active material, and simultaneously improves the capacity characteristics and high-temperature cycle life characteristics of the rechargeable lithium battery.

[0038] The boron coating forms on the surface of the secondary particles and can be uniformly applied thereto. The lithium borate of the boron coating can be represented as lithium boron oxide, for example, LiBO2, Li3B7O. 12 Li6B4O9, Li3B 11 O 18 Li₂B₄O₇, Li₃BO₃, Li₈B₆O 13 Li5B3O7, Li4B2O5, Li 10 B4O 11 Li8B2O7 or any combination thereof.

[0039] In the boron coating, the content of lithium borate can be from about 0.02 wt% to about 0.5 wt%, for example, about 0.03 wt% to about 0.4 wt%, about 0.04 wt% to about 0.3 wt%, or about 0.05 wt% to about 0.2 wt%, etc., based on the total weight of the positive electrode active material. When this content range is met, the boron coating does not act as a resistor, but rather improves the structural stability of the positive electrode active material, and thus improves the cycle life characteristics.

[0040] The boron-doped layer can be formed inside the secondary particles, and specifically, inside the primary particles exposed on the surface of the secondary particles. The primary particles exposed on the surface of the secondary particles can be the outermost primary particles of the secondary particles. For example, the boron-doped layer can be located at a depth of approximately 10 nm from the outer surface of the primary particles exposed on the surface of the secondary particles. When the distance from the outer surface of the primary particles exposed on the surface of the secondary particles is approximately 0 nm, the boron-doped layer can exist at a depth of approximately 0 nm to approximately 10 nm from the surface. In other words, the boron-doped layer can also be considered to be located at a depth of approximately 10 nm from the surface of the secondary particles. When the distance from the surface of the secondary particles is approximately 0 nm, the boron-doped layer can exist at a depth of approximately 0 nm to approximately 10 nm from the surface.

[0041] The boron-doped layer may be located, for example, at a depth of about 9 nm, about 8 nm, about 7 nm, about 6 nm, about 5 nm, about 4 nm, about 3 nm, or about 2.5 nm from the outer surface of the primary particle exposed to the surface of the secondary particle. This boron-doped layer differs from a boron coating and also from a grain boundary boron coating described later, and contributes to the structural stability of the positive electrode active material.

[0042] Simultaneously, the positive electrode active material may further include a grain boundary boron coating portion on the surface of the primary particles within the secondary particles, comprising lithium borate. This grain boundary boron coating portion exists within the internal portion of the secondary particles rather than on the surface, and it is coated along the interface of the primary particles within the internal portion of the secondary particles. Here, the internal portion of the secondary particles means the entire interior except for the surface, for example, it may mean the entire interior from a depth of approximately 2 μm from the outer surface. The internal portion of the secondary particles may also be defined as the portion inaccessible to distilled water when the secondary particles of the positive electrode active material are washed with distilled water.

[0043] According to embodiments, the weight of the boron coating can be greater than the weight of the grain boundary boron coating portion. For example, based on the total amount of the boron coating and the grain boundary boron coating portion, the grain boundary boron coating portion can be in the range of about 2 wt% to about 30 wt%, and specifically, in the range of about 3 wt% to about 25 wt% or about 5 wt% to about 20 wt%. Additionally, the boron coating can be included in the range of about 70 wt% to about 98 wt%, about 75 wt% to about 97 wt%, or about 80 wt% to about 95 wt%, etc. For example, the boron coating and the grain boundary boron coating portion can have a weight ratio of about 70:30 to about 98:2 (e.g., about 75:25 to about 97:3, or about 80:20 to about 95:5). When the boron coating and the grain boundary boron coating portion are included in these proportions, boron can improve performance in the positive electrode active material instead of acting as a resistor, thereby improving the capacity characteristics and cycle life characteristics of the rechargeable lithium battery.

[0044] Based on the total weight of the positive electrode active material, the content of the boron coating (i.e., the content of lithium borate in the boron coating) can be, for example, about 0.02 wt% to about 0.5 wt%, about 0.03 wt% to about 0.4 wt%, about 0.04 wt% to about 0.3 wt%, or about 0.05 wt% to about 0.2 wt%. Based on the total weight of the positive electrode active material, the content of the grain boundary boron coating (i.e., the content of lithium borate in the grain boundary boron coating) can be, for example, about 0.001 wt% to about 0.05 wt%, about 0.001 wt% to about 0.04 wt%, about 0.002 wt% to about 0.03 wt%, or about 0.003 wt% to about 0.02 wt%, but is not limited thereto. When the content of the boron coating and the grain boundary boron coating based on the total weight of the positive electrode active material are within this range, both the capacity characteristics and cycle life characteristics of the rechargeable lithium battery can be improved.

[0045] Based on the total amount of the positive electrode active material, the total amount of boron coating and grain boundary boron coating can be 0.1 mol% to 5 mol% (e.g., about 0.1 mol% to about 3 mol%, about 0.1 mol% to about 2.5 mol%, about 0.1 mol% to about 2 mol%, about 0.1 mol% to about 1.5 mol%, or about 0.5 mol% to about 1.3 mol%). When the total amount of boron coating and grain boundary boron coating exceeds the predetermined content, the initial discharge capacity will decrease, and the cycle life characteristics will deteriorate. In particular, when the content of boron coating on the surface of secondary particles becomes excessive, boron can act as a resistor, and thus greatly reduce the initial discharge capacity of the rechargeable lithium battery.

[0046] On the other hand, the positive electrode active material includes secondary particles in which at least two primary particles are aggregated, and at least a portion of the primary particles has a radially arranged structure. At least some of the primary particles may have a plate shape. The plate-shaped primary particles may have a thickness less than the length of their major axis. Here, the length of the major axis refers to the maximum length relative to the widest surface of the primary particle. That is, the primary particles may have a structure in which the length (t) in one axial direction (i.e., the thickness direction) is less than the length (a) of the major axis in another direction (i.e., the planar direction).

[0047] Figure 1 This is a schematic diagram illustrating the plate shape of the primary particles of the first positive electrode active material. (Reference) Figure 1 The primary particles according to the embodiments have various detailed shapes, while having a basic plate structure, such as (A) polygonal nanoplate shapes (e.g., hexagonal), (B) nanodisc shapes, and (C) rectangular parallelepiped shapes. Figure 1 In this context, "a" refers to the length of the major axis of the primary particle, "b" refers to the length of the minor axis of the primary particle, and "t" represents the thickness of the primary particle. The thickness t of the primary particle can be less than the lengths a and b in the planar direction. Among the lengths in the planar direction, a can be longer than or equal to b. The direction defining the thickness t in the primary particle is defined as the thickness direction, and the directions having lengths a and b are defined as the planar direction.

[0048] In the positive electrode active material, at least a portion of the primary particles may have a radially arranged structure, and for example, the long axis of the primary particles may be arranged in the radial direction. Figure 2 This is a view used to explain the radial definition in secondary particles according to an embodiment. In the embodiment, as... Figure 2 As shown, a radially arranged structure means that the thickness (t) direction of the primary particles is perpendicular to the direction (R) from the center of the secondary particles to the surface, or within an angle of approximately ±5° perpendicular to that direction (R).

[0049] The average length of the primary particles in the secondary particles can be from about 0.01 μm to about 5 μm (e.g., about 0.01 μm to about 2 μm, about 0.01 μm to about 1 μm, about 0.02 μm to about 1 μm, about 0.05 μm to about 0.5 μm, or about 150 nm to about 500 nm). Here, when the primary particles are plate-shaped, "average length" means the average length of the major axis (a) in the planar direction, and when the primary particles are spherical, "average length" means the average particle size.

[0050] When the primary particles are plate-shaped, the average thickness of the primary particles can be, for example, greater than or equal to about 50 nm, greater than or equal to about 100 nm, greater than or equal to about 200 nm, greater than or equal to about 300 nm, greater than or equal to about 400 nm, greater than or equal to about 500 nm, greater than or equal to about 600 nm, greater than or equal to about 700 nm, greater than or equal to about 800 nm, or greater than or equal to about 900 nm, and for example, less than or equal to about 5 μm, less than or equal to about 4 μm, less than or equal to about 3 μm, less than or equal to about 2 μm, less than or equal to about 1 μm, less than or equal to about 900 nm, less than or equal to about 800 nm, less than or equal to about 700 nm, less than or equal to about 600 nm, less than or equal to about 500 nm, or less than or equal to about 200 nm. For example, the average thickness of the primary particles can be from about 100 nm to about 200 nm. In addition, in the primary particles, the ratio of average thickness to average length can be about 1:1 to about 1:10 (e.g., about 1:1 to about 1:8, about 1:1 to about 1:6 or about 1:2 to about 1:5).

[0051] As described above, when the average length, average thickness, and ratio of average thickness to average length of the primary particles meet the above ranges, and at least a portion of the primary particles are radially arranged, there can be relatively more lithium diffusion pathways between grain boundaries on the surface side, and a large number of lithium crystal faces can be exposed to the outside, thereby improving lithium diffusion and ensuring high initial efficiency and capacity. Furthermore, when the primary particles are radially arranged, the pores exposed on the surface point towards the center of the secondary particles, thereby promoting lithium diffusion. Due to the radially arranged primary particles, uniform contraction and expansion are possible during lithium deintercalation and / or intercalation, and when lithium deintercalation occurs, the pores exist in the (001) direction, which is the direction of particle expansion, making them act as buffers. Additionally, due to the size and arrangement of the primary particles, the possibility of cracking during the contraction and expansion of the active material can be reduced, and the internal pores further mitigate volume changes to reduce cracking between primary particles during charging and discharging, thereby improving the cycle life characteristics of the rechargeable lithium battery and reducing resistance increase.

[0052] The positive electrode active material may have an irregular porous structure in at least one of the internal and external portions of the secondary particles. The term "irregular porous structure" refers to a structure in which the pore size and shape are irregular and non-uniform. For example, the secondary particles may include an internal portion containing an irregular porous structure and an external portion having a radially arranged structure. That is, unlike the primary particles in the external portion, the primary particles in the internal portion may be arranged irregularly. The internal portion containing the irregular porous structure includes primary particles, just like the external portion.

[0053] The ratio of the radius of the inner portion of the secondary particle to the radius of the secondary particle (the distance from the center of the secondary particle to the surface) can be approximately 40% to approximately 60% of its length, and such a region can be defined as the inner portion. The outer portion can refer to the region extending from the outermost surface of the secondary particle to approximately 40% to approximately 60% of its radius. Furthermore, the ratio of the volume of the outer portion to the volume of the secondary particle can be less than or equal to approximately 90% by volume, less than or equal to approximately 87% by volume, or less than or equal to approximately 85% by volume.

[0054] The secondary particles of the positive electrode active material may include radially arranged outer portions and an inner portion having an irregular porous structure, wherein the inner portion of the secondary particles may have pores larger than those of the outer portion. For example, in the positive electrode active material, the inner portion may have a pore size of about 150 nm to about 1 μm, and the outer portion may have a pore size of less than about 150 nm. Thus, compared to secondary particles where the inner and outer portions have the same pore size, when the pore size of the inner portion is larger than that of the outer portion, it has the advantage of shortening the diffusion distance of lithium in the active material, and lithium can be easily intercalated from the outside. Additionally, it can have the effect of mitigating volume changes during charging and discharging. Here, pore size refers to the average diameter when the pore is spherical or circular, and the length of the major axis when the pore is elliptical.

[0055] The secondary particles of the positive electrode active material may have openings on the surface and facing the center of the internal portion, and the size of the openings may be less than about 150 nm (e.g., about 10 nm to about 148 nm). The openings are exposed pores through which material can enter and exit. The openings may be formed at a depth of less than or equal to about 150 nm (e.g., about 0.001 nm to about 100 nm, for example, about 1 nm to about 50 nm) from the surface of the secondary particles.

[0056] Closed pores can exist in the internal portion of secondary particles, and closed pores and / or open pores can exist in the external portion. Closed pores can exclude or largely exclude electrolytes, while open pores can contain electrolytes. Closed pores are independent pores that are not connected to other pores because all the walls of the pore are formed in a closed structure, while open pores are continuous pores that are connected to the outside of the particle because at least some of the walls of the pore are formed in an open structure.

[0057] Figure 3 This is a schematic diagram showing the cross-sectional structure of the secondary particles of the positive electrode active material. (Reference) Figure 3According to the embodiment, the secondary particles 11 of the positive electrode active material have an outer portion 14 (the outer portion 14 has a structure in which plate-shaped primary particles are arranged radially) and an inner portion 12 (in the inner portion 12, primary particles 13 are irregularly arranged). The inner portion 12 may have more empty space between the primary particles than the outer portion. Furthermore, the pore size and porosity of the inner portion are larger and more irregular compared to those of the outer portion. Figure 3 In the image, the arrows indicate the direction of lithium ion movement.

[0058] In the secondary particles, the internal portion has a porous structure, which reduces the diffusion distance of lithium ions into the interior, while the external portion has a radial structure, facilitating the embedding of lithium ions into the surface. Furthermore, the small size of the primary particles ensures efficient lithium transfer pathways between grains. Additionally, the small size of the primary particles and the porosity between them mitigate volume changes during charging and discharging, minimizing stress caused by these volume changes. This positive electrode active material can reduce the resistance of rechargeable lithium batteries and improve capacity and cycle life characteristics.

[0059] On the other hand, in the secondary particles, multiple primary particles can have a radial arrangement structure by arranging them toward a "single (1)" center to make surface contact along the thickness direction of the primary particles, or alternatively, the secondary particles can have a "multi-center" radial arrangement structure with multiple centers. Thus, when the secondary particles have a single-center or multi-center radial arrangement structure, lithium is easily de-intercalated and / or intercalated into the center of the secondary particles.

[0060] Secondary particles may include radial primary particles and non-radial primary particles. Based on the total weight of radial and non-radial primary particles, the content of non-radial primary particles may be less than or equal to about 30 wt%, for example, from about 0.01 wt% to about 30 wt%, specifically from about 0.1 wt% to about 20 wt%.

[0061] Positive electrode active materials include lithium-nickel composite oxides. Based on the total amount of metals other than lithium, the nickel content in lithium-nickel composite oxides can be greater than or equal to about 30 mol% (e.g., greater than or equal to about 40 mol%, greater than or equal to about 50 mol%, greater than or equal to about 60 mol%, greater than or equal to about 70 mol%, greater than or equal to about 80 mol%, or greater than or equal to about 90 mol%), and less than or equal to about 99.9 mol% or less than or equal to about 99 mol%. For example, the nickel content in lithium-nickel composite oxides can be higher than the content of each of the other metals (such as cobalt, manganese, and aluminum). When the nickel content meets the above range, the positive electrode active material can exhibit excellent battery performance while achieving high capacity.

[0062] The lithium-nickel composite oxide can be represented by Chemical Formula 1.

[0063] [Chemical Formula 1]

[0064] Li a1 Ni x1 M 1 y1 M 2 1-x1-y1 O 2-z X z

[0065] In Chemical Formula 1, 0.9 ≤ a1 ≤ 1.8, 0.3 ≤ x1 ≤ 1, 0 ≤ y1 ≤ 0.7, and 0 ≤ z ≤ 0.1, M 1 and M 2 are each independently Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W, Zr, or any combination thereof, and X is F, P, S, or any combination thereof.

[0066] In Chemical Formula 1, 0.4 ≤ x1 ≤ 1 and 0 ≤ y1 ≤ 0.6, 0.5 ≤ x1 ≤ 1 and 0 ≤ y1 ≤ 0.5, 0.6 ≤ x1 ≤ 1 and 0 ≤ y1 ≤ 0.4, or 0.7 ≤ x1 ≤ 1 and 0 ≤ y1 ≤ 0.3, 0.8 ≤ x1 ≤ 1 and 0 ≤ y1 ≤ 0.2, or 0.9 ≤ x1 ≤ 1 and 0 ≤ y1 ≤ 0.1.

[0067] The lithium-nickel composite oxide can be represented, for example, by Chemical Formula 2.

[0068] [Chemical Formula 2]

[0069] Li a2 Ni x2 Co y2 M 3 1-x2-y2 O 2-z X z

[0070] In Chemical Formula 2, 0.9 ≤ a2 ≤ 1.8, 0.3 ≤ x2 < 1, 0 < y2 ≤ 0.7, and 0 ≤ z ≤ 0.1, M 3 is Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W, Zr, or a combination thereof, and X is F, P, S, or any combination thereof.

[0071] In chemical formula 2, 0.3≤x²≤0.99 and 0.01≤y²≤0.7, 0.4≤x²≤0.99 and 0.01≤y²≤0.6, 0.5≤x²≤0.99 and 0.01≤y²≤0.5, 0.6≤x²≤0.99 and 0.01≤y²≤0.4, 0.7≤x²≤0.99 and 0.01≤y²≤0.3, 0.8≤x²≤0.9 and 0.01≤y²≤0.2, or 0.9≤x²≤0.99 and 0.01≤y²≤0.1.

[0072] Lithium-nickel composite oxides can be represented, for example, by chemical formula 3.

[0073] [Chemical Formula 3]

[0074] Li a3 Ni x3 Co y3 M 4 z3 M 5 1-x3-y3-z3 O 2-z X z

[0075] In chemical formula 3, 0.9 ≤ a³ ≤ 1.8, 0.3 ≤ x³ ≤ 0.98, 0.01 ≤ y³ ≤ 0.69, 0.01 ≤ z³ ≤ 0.69, 0 ≤ z ≤ 0.1, M 4 For Al, Mn, or any combination thereof, M 5 X is B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sr, Ti, V, W, Zr or any combination thereof, and X is F, P, S or any combination thereof.

[0076] In chemical formula 3, 0.4≤x3≤0.98, 0.01≤y3≤0.59 and 0.01≤z3≤0.59, 0.5≤x3≤0.98, 0.01≤y3≤0.49 and 0.01≤z3≤0.49, 0.6≤x3≤0.98, 0.01≤y3≤0.39 and 0.01≤z3≤0.39, 0.7≤x3≤0.98, 0.01≤y3≤0.29 and 0.01≤z3≤0.29, 0.8≤x3≤0.98, 0.01≤y3≤0.19 and 0.01≤z3≤0.19, or 0.9≤x3≤0.98, 0.01≤y3≤0.09 and 0.01≤z3≤0.09.

[0077] Generally speaking, as the nickel content in the positive electrode active material increases, due to the increase in Ni content... 2+The mixing of cations occupying lithium sites also increases, thus reducing capacity. Alternatively, impurities (such as NiO) can hinder lithium-ion diffusion, leading to a deterioration in battery cycle life. Furthermore, the positive electrode active material undergoes structural collapse and breakage during charging and discharging, resulting in side reactions with the electrolyte, which further reduces battery cycle life and poses safety risks. To address these issues, in conventional methods, when boron is simply coated on the surface of the active material, boron acts as a resistor, actually reducing capacity and degrading cycle life. Conversely, the positive electrode active material according to the embodiments can appropriately include a boron coating and a boron doping layer, even when using high-nickel materials. This mitigates the problems caused by high nickel concentrations, achieving high capacity while simultaneously improving cycle life characteristics without degrading initial discharge capacity.

[0078] Methods for preparing positive electrode active materials

[0079] In one embodiment, a method for preparing a positive electrode active material for a rechargeable lithium battery includes mixing a nickel hydroxide, a lithium raw material, and a boron raw material and then heat-treating the resulting product.

[0080] Conventionally, when coating boron onto a positive electrode active material, lithium raw material is typically mixed with a nickel hydroxide, followed by heat treatment of the result to prepare a lithium-nickel composite oxide. A boron source is then mixed with it using a wet or dry process, followed by another heat treatment. In this case, only the surface of the positive electrode active material is coated with boron, thus the boron acts as a resistor, reducing capacity and cycle life. On the other hand, according to the preparation method of the embodiment, secondary particles in which at least a portion of the primary particles are radially arranged are produced, and simultaneously, a boron coating and a boron doping layer are appropriately formed, so that the boron does not act as a resistor, thereby obtaining a positive electrode active material with stable structure and excellent cycle life characteristics.

[0081] In the preparation method, nickel hydroxide is a precursor of the positive electrode active material, and can be represented as nickel metal complex hydroxide or nickel transition metal complex hydroxide, and can be prepared by co-precipitation method, etc.

[0082] Nickel hydroxides can be represented, for example, by chemical formula 11.

[0083] [Chemical Formula 11]

[0084] Ni x11 M 11 y11 M 12 1-x11-y11 (OH)2

[0085] In chemical formula 11, 0.3 ≤ x11 ≤ 1, 0 ≤ y11 ≤ 0.7, and M 11 and M12 Each of the following is independently Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, or any combination thereof.

[0086] As a specific example, nickel hydroxides can be represented by chemical formula 12 or chemical formula 13.

[0087] [Chemical Formula 12]

[0088] Ni x12 Co y12 M 13 1-x12-y12 (OH)2

[0089] In chemical formula 12, 0.3 ≤ x12 < 1, 0 <y12≤0.7,M 13 It is Al, B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr or any combination thereof.

[0090] [Chemical Formula 13]

[0091] Ni x13 Co y13 M 14 z13 M 15 1-x13-y13-z13 (OH)2

[0092] In chemical formula 13, 0.3 ≤ x13 ≤ 0.98, 0.01 ≤ y13 ≤ 0.69, 0.01 ≤ z13 ≤ 0.69, M 14 For Al, Mn, or any combination thereof, and M 15 It is B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr or any combination thereof.

[0093] The lithium raw material can be, for example, lithium hydroxide, and can be mixed with 1 mole of nickel hydroxide in a ratio of about 0.8 moles to about 1.8 moles or about 0.9 moles to about 1.2 moles.

[0094] Boron raw materials can be boron-containing compounds, such as H3BO3, HBO2, B2O3, C6H5B(OH)2, (C6H5O)3B, [CH3(CH2)3O]3B, (C3H7O)3B, C3H9B3O6, C 13 H 19 BO3 or any combination thereof.

[0095] Based on 100 mol parts of nickel hydroxide, the boron content can be from about 0.1 mol parts to about 5 mol parts, for example, about 0.1 mol parts to about 4 mol parts, about 0.1 mol parts to about 3 mol parts, about 0.1 mol parts to about 2.9 mol parts, about 0.1 mol parts to about 2.5 mol parts, about 0.1 mol parts to about 2 mol parts, about 0.1 mol parts to about 1.5 mol parts, or about 0.5 mol parts to about 1.3 mol parts. When the boron content meets the above ranges, boron does not act as a resistor in the positive electrode active material and can be used to improve the performance of rechargeable lithium batteries, thereby improving capacity and cycle life characteristics. When the boron content is excessive, the boron coating content increases excessively, and boron acts as a resistor in the positive electrode active material, thereby reducing battery capacity and cycle life.

[0096] The heat treatment can be performed at temperatures ranging from approximately 650°C to approximately 850°C or from approximately 690°C to approximately 780°C. Under these conditions, a positive electrode active material for a rechargeable lithium battery with a stable structure comprising both a boron coating and a boron doping layer can be prepared.

[0097] In addition, the heat treatment can be carried out for approximately 5 to approximately 25 hours, for example, approximately 8 to approximately 12 hours. In this case, a positive electrode active material for a rechargeable lithium battery with a stable structure including both a boron coating and a boron doped layer is prepared.

[0098] Rechargeable lithium battery

[0099] Another embodiment provides a rechargeable lithium battery, which includes a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte.

[0100] Figure 4 This is a schematic diagram illustrating a rechargeable lithium battery according to an embodiment. (Reference) Figure 4 According to an embodiment, a rechargeable lithium battery 100 includes a battery cell, a battery casing 120 housing the battery cell, and a sealing member 140 sealing the battery casing 120. The battery cell includes: a positive electrode 114; a negative electrode 112 facing the positive electrode 114; a separator 113 located between the positive electrode 114 and the negative electrode 112; and an electrolyte for the rechargeable lithium battery, which impregnates the positive electrode 114, the negative electrode 112, and the separator 113.

[0101] positive electrode

[0102] The positive electrode for a rechargeable lithium battery according to an embodiment may include a current collector and a layer of positive electrode active material on the current collector. The positive electrode active material layer may include a positive electrode active material and may further include a binder and / or a conductive material.

[0103] The adhesive improves the adhesion properties between the positive electrode active material particles and between them and the current collector. Examples of adhesives include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon.

[0104] Based on the total weight of the positive electrode active material layer, the content of the binder in the positive electrode active material layer can be approximately 1 wt% to approximately 5 wt%.

[0105] Conductive materials are included to provide electrode conductivity. Any electrically conductive material may be used as a conductive material unless it causes a chemical change. Examples of conductive materials may include: carbon-based materials, such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fibers, and carbon nanotubes; metallic materials, including metal powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive polymers, such as polyphenylene derivatives; or mixtures thereof.

[0106] Based on the total weight of the positive electrode active material layer, the content of conductive material in the positive electrode active material layer can be approximately 1 wt% to approximately 5 wt%.

[0107] Aluminum foil can be used as a current collector, but is not limited to this.

[0108] negative electrode

[0109] The negative electrode for a rechargeable lithium battery may include, for example, a current collector and a layer of negative electrode active material on the current collector. The negative electrode active material layer may include a negative electrode active material and may further include a binder and / or a conductive material.

[0110] The negative electrode active material may include materials that can reversibly insert / deintercalate lithium ions, lithium metal, lithium metal alloys, materials that can be doped / dedoped with lithium, or transition metal oxides.

[0111] Materials capable of reversibly inserting / deintercalating lithium ions can include, for example, crystalline carbon, amorphous carbon, or any combination thereof as active materials for carbon-based negative electrodes. Crystalline carbon can be shapeless or in the form of flakes, sheets, spheres, or fibers, such as natural or artificial graphite. Amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbonization products, and calcined coke, etc.

[0112] Lithium metal alloys include alloys of lithium and metals selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn.

[0113] The substance capable of doping / undoping lithium may be a Si-based negative electrode active material or a Sn-based negative electrode active material. The Si-based negative electrode active material may include silicon, a silicon-carbon composite, SiO x (0 < x < 2), a Si-Q alloy (where Q is an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element other than Si, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, or any combination thereof), and the Sn-based negative electrode active material may include Sn, SnO2, a Sn-R alloy (where R is an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element other than Sn, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, or any combination thereof). At least one of these substances may be mixed with SiO2. The elements Q and R may be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn (R does not include Sn), In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, or any combination thereof.

[0114] The silicon-carbon composite may be, for example, a silicon-carbon composite including a core containing crystalline carbon and silicon particles and an amorphous carbon coating provided on the surface of the core. The crystalline carbon may be artificial graphite, natural graphite, or any combination thereof. The amorphous carbon precursor may be coal-based pitch, mesophase pitch, petroleum-based pitch, coal-based oil, petroleum-based heavy oil, or a polymer resin (such as phenolic resin, furan resin, or polyimide resin). In this case, based on the total weight of the silicon-carbon composite, the content of silicon may be about 10 wt% to about 50 wt%. Additionally, based on the total weight of the silicon-carbon composite, the content of crystalline carbon may be about 10 wt% to about 70 wt%, and based on the total weight of the silicon-carbon composite, the content of amorphous carbon may be about 20 wt% to about 40 wt%. Additionally, the thickness of the amorphous carbon coating may be about 5 nm to about 100 nm. The average particle size (D50) of the silicon particles may be about 10 nm to about 20 μm. The average particle size (D50) of the silicon particles may preferably be about 10 nm to about 200 nm. The silicon particles may exist in an oxidized form, and in this case, the ratio of the atomic content of Si:O in the silicon particles indicating the degree of oxidation may be about 99:1 to about 33:67. The silicon particles may be SiO x particles, and in this case, x in SiO x may have a range greater than about 0 and less than about 2. As used herein, when no other definition is provided, the average particle size (D50) indicates the diameter of the particles at which the cumulative volume in the particle distribution is about 50 volume%.

[0115] Si-based or Sn-based negative electrode active materials can be mixed with carbon-based negative electrode active materials. When mixing and using Si-based or Sn-based negative electrode active materials and carbon-based negative electrode active materials, the mixing ratio can be approximately 1:99 to approximately 90:10 by weight.

[0116] In the negative electrode active material layer, the amount of negative electrode active material included, based on the total weight of the negative electrode active material layer, can be approximately 95 wt% to approximately 99 wt%.

[0117] In an embodiment, the negative electrode active material layer may further include a binder, and optionally further include a conductive material. Based on the total weight of the negative electrode active material layer, the binder content in the negative electrode active material layer may be from about 1 wt% to about 5 wt%. Alternatively, when further including a conductive material, the negative electrode active material layer may include about 90 wt% to about 98 wt% of the negative electrode active material, about 1 wt% to about 5 wt% of the binder, and about 1 wt% to about 5 wt% of the conductive material.

[0118] The adhesive is used to ensure good adhesion between particles of the negative electrode active material, and also to adhere the negative electrode active material to the current collector. The adhesive may be a water-insoluble adhesive, a water-soluble adhesive, or any combination thereof.

[0119] Examples of water-insoluble adhesives include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, ethylene propylene copolymers, polystyrene, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, or any combination thereof.

[0120] Water-soluble adhesives may include rubber adhesives or polymeric resin adhesives. Rubber adhesives may be selected from styrene-butadiene rubber, acrylated styrene-butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, or any combination thereof. Polymeric resin adhesives may be selected from polyethylene oxide, polyvinylpyrrolidone, polyepoxychloropropane, polyphosphazene, polyacrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, or any combination thereof.

[0121] When a water-soluble binder is used as a negative electrode binder, it may further include a cellulose-based compound capable of imparting viscosity as a thickener. As a cellulose-based compound, one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, and their alkali metal salts may be mixed and used. As an alkali metal, Na, K, or Li may be used. Based on 100 parts by weight of the negative electrode active material, the amount of this thickener may be from about 0.1 parts by weight to about 3 parts by weight.

[0122] Conductive materials are included to provide electrode conductivity. Any electrically conductive material can be used as a conductive material unless it causes a chemical change. Examples of conductive materials include: carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fibers, and carbon nanotubes; metallic materials including metal powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive polymers, such as polyphenylene derivatives; or mixtures thereof.

[0123] The current collector may include one selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and any combination thereof.

[0124] electrolytes

[0125] Electrolytes include non-aqueous organic solvents and lithium salts.

[0126] Non-aqueous organic solvents are used as media for transporting ions involved in the electrochemical reactions of a battery. Non-aqueous organic solvents can be carbonates, esters, ethers, ketones, alcohols, or aprotic solvents. Carbonate solvents can include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), etc. Ester solvents can include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolactone, valerate, mevalonolactone, caprolactone, etc. Ether solvents can include dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. Ketone solvents can include cyclohexanone, etc. In addition, alcohol solvents may be ethanol, isopropanol, etc., and aprotic solvents may be nitriles (such as R-CN, where R is a C2 to C20 straight chain, branched chain or cyclic hydrocarbon group, and may include double bonds, aromatic rings or ether bonds), amides (such as dimethylformamide), dioxolane (such as 1,3-dioxolane), sulfolane, etc.

[0127] Non-aqueous organic solvents can be used alone or in mixtures. When organic solvents are used in mixtures, the mixing ratio can be controlled according to the desired battery performance.

[0128] Alternatively, in the case of carbonate solvents, a mixture of cyclic carbonates and chain carbonates can be used. In this case, when cyclic carbonates and chain carbonates are mixed in a volume ratio of about 1:1 to about 1:9, the electrolyte exhibits excellent performance.

[0129] In addition to carbonate solvents, non-aqueous organic solvents may further include aromatic hydrocarbon organic solvents. In this case, carbonate solvents and aromatic hydrocarbon organic solvents can be mixed in a volume ratio of about 1:1 to about 30:1.

[0130] As an aromatic hydrocarbon solvent, aromatic hydrocarbon compounds represented by chemical formula I can be used.

[0131] [Chemical Formula I]

[0132]

[0133] In chemical formula I, R 4 ~R 9 They may be the same or different, and are selected from hydrogen, halogen, C1-C10 alkyl, C1-C10 haloalkyl or any combination thereof.

[0134] Specific examples of aromatic hydrocarbon solvents include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene, and 2... 3-Difluorotoluene, 2,4-Difluorotoluene, 2,5-Difluorotoluene, 2,3,4-Trifluorotoluene, 2,3,5-Trifluorotoluene, Chlorotoluene, 2,3-Dichlorotoluene, 2,4-Dichlorotoluene, 2,5-Dichlorotoluene, 2,3,4-Trichlorotoluene, 2,3,5-Trichlorotoluene, Iodotoluene, 2,3-Diiodotoluene, 2,4-Diiodotoluene, 2,5-Diiodotoluene, 2,3,4-Triiodotoluene, 2,3,5-Triiodotoluene, Xylene, or any combination thereof.

[0135] To improve battery cycle life, the electrolyte may further include vinylene carbonate or ethylene carbonate compounds of formula II.

[0136] [Chemical Formula II]

[0137]

[0138] In chemical formula II, R 10 and R 11 The same or different, and selected from hydrogen, halogen, cyano, nitro and fluorinated C1 to C5 alkyl groups, under the condition that R 10 and R 11 At least one of them is selected from halogen, cyano, nitro and fluorinated C1-C5 alkyl groups, but R 10 and R 11 None of them are hydrogen.

[0139] Examples of ethylene carbonate compounds may be difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, or fluoroethylene carbonate. The amount of additives used to improve cycle life may be used within appropriate limits.

[0140] Lithium salts dissolved in non-aqueous organic solvents supply lithium ions in the battery, enabling the rechargeable lithium battery to perform basic operations and improving lithium ion transport between the positive and negative electrodes.

[0141] Examples of lithium salts include at least one carrier salt selected from the following: LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2 (where x and y are natural numbers, for example, integers ranging from 1 to 20), lithium difluoro(bis(oxalate)phosphate), LiCl, LiI, LiB(C2O4)2 (lithium bis(oxalate)borate; LiBOB) and lithium difluoro(oxalate)borate (LiDFOB).

[0142] Lithium salts can be used in concentration ranges from about 0.1 M to about 2.0 M. When lithium salts are included in the above concentration range, the electrolyte can exhibit excellent performance and lithium-ion mobility due to optimal electrolyte conductivity and viscosity.

[0143] diaphragm

[0144] Separator 113 separates the positive electrode 114 and the negative electrode 112, and provides a transport channel for lithium ions. It can be any commonly used separator in lithium-ion batteries. In other words, separator 113 can have low ion transport resistance and excellent electrolyte impregnation properties. For example, the separator material can be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or any combination thereof. The separator can be in the form of a non-woven or woven fabric. For example, in lithium-ion batteries, polyolefin polymer separators (such as polyethylene and polypropylene) can be used primarily. To ensure heat resistance or mechanical strength, coated separators including ceramic components or polymer materials can be used. Optionally, the separator can have a single-layer or multi-layer structure.

[0145] Rechargeable lithium batteries can be classified into lithium-ion batteries, lithium-ion polymer batteries, and lithium polymer batteries based on the type of separator and electrolyte used. They can also be classified by shape (cylindrical, prismatic, coin-shaped, and pouch-shaped, etc.) and by size (bulk and thin-film types). Because the structure and manufacturing methods of these batteries are well known in the art, detailed descriptions will be omitted.

[0146] Due to its high capacity, improved storage stability and cycle life characteristics at high temperatures, and high rate capability, the rechargeable lithium battery according to the embodiments can be used in electric vehicles (EVs) or hybrid vehicles (such as plug-in hybrid electric vehicles (PHEVs)).

[0147] The following describes embodiments and comparative examples of the present invention. However, it should be understood that these embodiments are for illustrative purposes and should not be construed as limiting the invention.

[0148] Example 1

[0149] 1. Preparation of positive electrode active material precursor

[0150] The precursor for the positive electrode active material was prepared by the following co-precipitation method. Nickel sulfate, cobalt sulfate, and aluminum nitrate were used as metal raw materials.

[0151] [First step: 1.5kW / m] 3 [NH4OH 0.35M, pH 11.5–11.7 and reaction time 6 hours]

[0152] First, 0.35M ammonia solution is added to the reactor. At a power output of 1.5 kW / m³... 3 The reaction was initiated at 50°C with the addition of the metal raw material and the complexing agent at 85 ml / min and 9 ml / min, respectively, under stirring power. NaOH was added to maintain the pH while the reaction proceeded for 6 hours. As a result, the average size of the obtained core particles was confirmed to be in the range of approximately 6.5 μm to 7.5 μm, and the second step was carried out as follows.

[0153] [Second step: 1.0kW / m] 3 [NH4OH 0.40M, pH 11.5–11.7 and reaction time 16 hours]

[0154] The metal raw material and complexing agent were added at rates of 107 ml / min and 13 ml / min, respectively, while the reaction temperature was maintained at 50°C to keep the complexing agent concentration at 0.40 M. NaOH was added to maintain the pH, and the reaction proceeded for 16 hours. At this point, the stirring power was reduced to 1.0 kW / m. 3(This is lower than the stirring power in the first step), and the reaction continues. By carrying out this reaction, the average size of the product particles containing the core and intermediate layer was confirmed to be 10.5 μm to 11.5 μm, and the third step was carried out as follows.

[0155] [Third step: 0.5kW / m] 3 [NH4OH 0.40M, pH 11.5–11.7 and reaction time 5 hours]

[0156] While maintaining a reaction temperature of 50°C, the metal raw material and complexing agent were added at rates of 142 ml / min and 17 ml / min, respectively, to keep the complexing agent concentration the same as in the second step. NaOH was added to maintain the pH, and the reaction proceeded for 5 hours, resulting in an average particle size of 14 μm. At this point, the stirring power was reduced to 0.5 kW / m. 3 (This is lower than the stirring power in the second step), and the reaction continues.

[0157] [Post-processing]

[0158] After washing, the product was dried in hot air at approximately 150°C for 24 hours to obtain nickel hydroxide (Ni) as a precursor for the positive electrode active material. 0.945 Co 0.04 Al 0.015 (OH)2). Figure 5 This is a SEM image of the cross-section of the positive electrode active material precursor prepared in Example 1.

[0159] 2. Preparation of positive electrode active material

[0160] The obtained nickel hydroxide and LiOH were mixed in a 1:1 molar ratio, and 1.0 molar of boric acid based on 100 molar parts of nickel hydroxide was added. Then, the mixture was heat-treated at 725°C for 10 hours under an oxygen atmosphere to obtain the positive electrode active material LiNi. 0.945 Co 0.04 Al 0.015 O2. Figure 6 This is a SEM image of the cross-section of the positive electrode active material prepared in Example 1. (Reference) Figure 6 The positive electrode active material has a structure in which a plurality of primary particles are aggregated into secondary particles, wherein the structure has a porous center inside and the primary particles are arranged radially on its outside.

[0161] 3. Manufacturing of the positive electrode

[0162] 96 wt% of the obtained positive electrode active material, 2 wt% of polyvinylidene fluoride, 2 wt% of carbon nanotubes, and N-methylpyrrolidone as a solvent were mixed in a mixer to prepare a slurry for the positive electrode active material layer. The slurry for the positive electrode active material layer was coated onto aluminum foil to form an electrode plate, then dried at 135°C for at least 3 hours, rolled, and vacuum dried to manufacture the positive electrode.

[0163] 4. Manufacturing of individual battery cells

[0164] A coin-shaped half-cell was fabricated using a positive electrode and a lithium metal counter electrode. A separator (approximately 16 μm thick) made of porous polyethylene membrane was inserted between the positive electrode and the lithium metal counter electrode, and an electrolyte solution was injected into it. The electrolyte solution was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a 3:5 volume ratio and dissolving 1.1 M LiPF6 in the mixed solvent.

[0165] Comparative Example 1

[0166] In addition to using LiNi composed of ordinary secondary particles with a particle size of approximately 14 μm. 0.945 Co 0.04 Al 0.015 Except for O2, which was used as the positive electrode active material in Comparative Example 1, the positive electrode and battery cell were manufactured in a manner substantially the same as that in Example 1.

[0167] Comparative Example 2

[0168] By using the positive electrode active material (LiNi) of Comparative Example 1 0.945 Co 0.04 Al 0.015 O2) was mixed with 1.0 mol% boric acid based on 100 moles of nickel hydroxide, and then the mixture was heat-treated at 350°C for 8 hours under an oxygen atmosphere to prepare a positive electrode active material coated with a boron compound using conventional methods. Subsequently, the positive electrode and battery cell were manufactured in substantially the same manner as in Example 1, except that this positive electrode active material of Comparative Example 2 was used.

[0169] Comparative Example 3

[0170] Without the addition of boric acid, the positive electrode active material (LiNi) of Comparative Example 1 was used. 0.945 Co 0.04 Al 0.015 The positive electrode and battery cell were heat-treated at 350°C for 8 hours in an oxygen atmosphere (O2). Subsequently, the positive electrode and battery cell were manufactured in essentially the same manner as in Example 1, except that the positive electrode active material of Comparative Example 3 was used.

[0171] Evaluation Example 1: Confirmation of Boron Doped Layer by TEM-EELS Analysis

[0172] Figure 7 TEM images of primary particles (i.e., the outermost portion of the primary particles exposed on the surface of the secondary particles in the positive electrode active material of Example 1). Figure 7 The electron energy loss spectroscopy (EELS) was performed at a total of 10 points from point 1 to point 10, and the results are shown in Figure 8 As shown in [the image]. Figure 8 In the example, ① refers to the analysis diagram at point 1. (See reference) Figure 8 In the image below, cobalt is detected from point 4 upwards (i.e., from point 4 to point 10). Therefore, the surface of the active material begins approximately from point 4. (Reference) Figure 8 In the above figure, boron was detected from point 3 to point 8. Here, the boron detected at point 3 can be considered as a boron coating present on the surface of the active material, but the boron from point 4 to point 8 can be considered as present inside the active material and as a very thin boron doping layer.

[0173] Evaluation Example 2: Coating Confirmation

[0174] Figure 9 The image shows a ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) analysis of the surface of the positive electrode active material in Example 1. Figure 10 for Figure 9 Mass spectrometry results from ToF-SIMS analysis. Figure 9 In this process, boron is uniformly distributed on the surface of the positive electrode active material, and... Figure 10 A large amount of BO2 was found in the sample. Therefore, lithium borate was uniformly coated on the surface of the positive electrode active material according to Example 1, wherein lithium borate (such as LiBO2) was found to be a major component.

[0175] Evaluation Example 3: Evaluation of boron content in boron coatings and grain boundary boron coatings

[0176] The boron content was measured by ICP (Inductively Coupled Plasma) emission spectroscopy analysis of the positive electrode active material prepared in Example 1. Then, 10 g of each positive electrode active material was added to 100 g of distilled water, stirred for 30 minutes, and filtered to obtain the positive electrode active material. This washing process completely removed boron from the surface of the positive electrode active material. The recovered positive electrode active material was dried at 130°C for 24 hours, and the remaining boron content was measured again by ICP emission spectroscopy analysis. This boron content is expressed as the boron content of the "internal portion," which refers to the content of the boron coating at the grain boundaries. Additionally, the difference obtained by subtracting the boron content after washing from the boron content before washing (i.e., the boron content removed by washing) is expressed as the boron content of the "external portion," which represents the content of the boron coating.

[0177] Except for changing the boric acid content (1.0 mol in Example 1) to 0 mol, 0.5 mol, 1.5 mol, 3.0 mol, and 5.0 mol, the positive electrode active material was prepared using the same method as in Example 1. Then, the internal and external boron content was analyzed using the same method as described above, and the results were... Figure 11 As shown in [the image]. Figure 11 In Chinese, the unit ppm means 10 -4 wt%, which refers to the weight ratio of boron to the total weight of each positive electrode active material.

[0178] refer to Figure 11 In Example 1, where the boron input was 1.0 molar, the boron content of the boron coating was approximately 1000 ppm (i.e., 0.1 wt% based on the total amount of the positive electrode active material), and the boron content of the grain boundary boron coating was approximately 50 ppm (i.e., 0.005 wt% based on the total amount of the positive electrode active material). Here, the weight ratio of external boron to internal boron was calculated to be approximately 95:5. Furthermore, in Figure 11 In this study, when the boron input was 1.5 moles, the calculated weight ratio of external boron to internal boron was approximately 92:8. (Reference) Figure 11 Boron compounds (i.e., lithium salts, etc.) are coated not only on the surface of the secondary particles of the positive electrode active material, but also on the internal grain boundaries, wherein the boron content on the surface is 4 times or more than the boron content on the internal grain boundaries.

[0179] Evaluation Example 4: Evaluation of Capacity and High-Temperature Cycling Life Characteristics

[0180] The battery cells according to the embodiments and comparative examples were charged at 25°C with a constant current of 0.2C to an upper limit voltage of 4.25V, and discharged at 0.2C to a discharge cutoff voltage of 3.0V. The initial charge and initial discharge capacities were then measured, and the results were... Figure 12 As shown in [the image]. Figure 12In the graph, gray bars represent the initial charge capacity, and black bars represent the initial discharge capacity. After the initial charge and discharge, the battery cells were repeatedly charged 50 times or more at 1C within a voltage range of 3.0V to 4.3V at 45°C, and then discharged at 1C. The high-temperature cycle life characteristics were then evaluated, and the results were presented in [the graph is missing from the original text]. Figure 12 and Figure 13 As shown in the image. Figure 12 The dashed line graph represents the capacity retention rate at the 50th cycle.

[0181] refer to Figure 12 and Figure 13 Compared with Comparative Example 1, which uses ordinary secondary particulate positive electrode active material, Comparative Example 2, which uses positive electrode active material coated with boron by conventional methods, and Comparative Example 3, which uses positive electrode active material that has been heat-treated but not coated with boron, Example 1 exhibits high initial charge capacity and high initial discharge capacity, and at the same time its high-temperature cycle life characteristics are improved.

[0182] While this disclosure has been described in conjunction with exemplary embodiments now considered practical, it should be understood that the invention is not limited to the disclosed embodiments. Rather, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A positive electrode active material for a rechargeable lithium battery, said positive electrode active material comprising a lithium-nickel composite oxide, in, The positive electrode active material comprises: secondary particles in which a plurality of primary particles are aggregated, wherein at least a portion of the primary particles are radially arranged; a boron coating disposed on the surface of the secondary particles and comprising lithium borate; and a boron-doped layer disposed within the primary particles exposed to the surface of the secondary particles, wherein the boron-doped layer is disposed at a depth of 10 nm from the outer surface of the primary particles exposed to the surface of the secondary particles. The positive electrode active material further includes a grain boundary boron coating portion, which is disposed on the surface of the primary particle inside the secondary particle along the interface of the primary particle and includes lithium borate.

2. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, The boron-doped layer is disposed at a depth of 5 nm from the outer surface of the primary particle exposed to the surface of the secondary particle.

3. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, The lithium borate in the boron coating includes LiBO2 and Li3B7O. 12 Li6B4O9, Li3B 11 O 18 Li₂B₄O₇, Li₃BO₃, Li₈B₆O 13 Li5B3O7, Li4B2O5, Li 10 B4O 11 Li8B2O7 or any combination thereof.

4. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, Based on the positive electrode active material, the content of lithium borate in the boron coating is 0.02 wt% to 0.5 wt%.

5. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, The weight of the boron coating is greater than the weight of the boron-coated portion at the grain boundaries.

6. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, The weight of the boron coating is at least four times the weight of the boron-coated portion at the grain boundaries.

7. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, Based on the total amount of the boron coating and the grain boundary boron coating portion, the amount of the boron coating is 70 wt% to 98 wt%, and the amount of the grain boundary boron coating portion is 2 wt% to 30 wt%.

8. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, Based on the positive electrode active material, the content of the boron coating is 0.02 wt% to 0.5 wt%, and the content of the grain boundary boron coating is 0.001 wt% to 0.05 wt%.

9. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, The primary particles have a plate shape, and at least a portion of the plate-shaped primary particles are arranged radially.

10. The positive electrode active material for a rechargeable lithium battery as described in claim 9, wherein, The plate-shaped primary particles have an average major axis length of 150 nm to 500 nm, an average thickness of 100 nm to 200 nm, and a ratio of average thickness to average major axis length of 1:2 to 1:

5.

11. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, The secondary particles include an inner portion and an outer portion surrounding the inner portion, the inner portion including an irregular porous structure, and the outer portion including a radially arranged structure.

12. The positive electrode active material for a rechargeable lithium battery as described in claim 11, wherein, The internal portion of the secondary particle has pores larger than those of the external portion. The internal portion has a pore size of 150 nm to 1 μm, and The outer portion has a pore size of less than 150 nm.

13. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, The secondary particle includes an opening on the surface having a size of less than 150 nm and facing the center of the secondary particle.

14. The positive electrode active material for a rechargeable lithium battery as described in claim 1, wherein, The lithium-nickel composite oxide is represented by chemical formula 1: [Chemical Formula 1] Li a1 Ni x1 M 1 y1 M 2 1-x1-y1 O 2-z X z In chemical formula 1, 0.9 ≤ a1 ≤ 1.8, 0.3 ≤ x1 ≤ 1, 0 ≤ y1 ≤ 0.7, and 0 ≤ z ≤ 0.1, M 1 and M 2 Each of them is independently Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W, Zr or any combination thereof, and X is F, P, S or any combination thereof.

15. A method for preparing a positive electrode active material for a rechargeable lithium battery, the method comprising mixing a nickel hydroxide, a lithium raw material and a boron raw material and subjecting the result to a heat treatment to obtain the positive electrode active material for a rechargeable lithium battery as described in any one of claims 1 to 14, wherein the heat treatment is performed at a temperature of 650 °C to 850 °C for 5 to 25 hours.

16. The method of claim 15, wherein, Based on 100 molar parts of the nickel hydroxide, the boron raw material content is 0.1 molar parts to 5 molar parts.

17. A rechargeable lithium battery, said rechargeable lithium battery comprising: The positive electrode includes the positive electrode active material for a rechargeable lithium battery as described in any one of claims 1 to 14. negative electrode; A diaphragm located between the positive electrode and the negative electrode; and an electrolyte.

Images