Positive electrode active material for lithium secondary battery, method for producing the same, and lithium secondary battery including the same

Abstract

To provide a cathode active material for a lithium secondary battery, capable of improving a long life characteristic and a thermal stability characteristic while enhancing an energy density of a high-nickel system cathode active material, and a manufacturing method of them, and the lithium secondary battery containing them.SOLUTION: A cathode active material for a lithium secondary battery, according to an embodiment, is a cathode active material for a lithium secondary battery, containing: a first cathode active material of a secondary particle formation formed by correcting a plurality of primary particles; and a second cathode active material of a single particle formation. Each of the first and second cathode active materials contains a lithium-nickel system complex oxide of which a content of nickel against a whole element excluding lithium and oxide is 70 mol% or more, and the first cathode active material contains a coating part existed in a film formation onto a front surface of the secondary particles, and the second cathode active material contains the coating part existed in the film formation onto the front surface of the single particles. Each of the coating part of the first cathode active material and the coating part of the second cathode active material contains a lithium cobalt oxide and a cobalt oxyhydroxide.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same. [Background technology]

[0002] Lithium secondary batteries, which have high energy density yet are easy to carry, are widely used as power sources for mobile information terminals such as mobile phones, laptops, and smartphones. Recently, active research has been conducted into using high-energy-density lithium secondary batteries as power sources for hybrid and electric vehicles or as power storage sources.

[0003] Various positive electrode active materials have been investigated to realize lithium secondary batteries suitable for these applications. Among these, lithium nickel oxide, lithium nickel manganese cobalt composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium cobalt oxide are commonly used as positive electrode active materials. However, these positive electrode active materials suffer from structural collapse or cracking as they are repeatedly charged and discharged, resulting in reduced long-term life of lithium secondary batteries and increased resistance, resulting in unsatisfactory capacity characteristics. Therefore, there is a need for the development of new positive electrode active materials that can achieve high capacity and high energy density while maintaining long-term life characteristics. Summary of the Invention [Problem to be solved by the invention]

[0004] It increases the energy density of high-nickel cathode active materials while improving their long life and thermal stability. [Means for solving the problem]

[0005] In one embodiment, the present invention provides a positive electrode active material for a lithium secondary battery, comprising: a first positive electrode active material in the form of secondary particles formed by agglomerating a plurality of primary particles; and a second positive electrode active material in the form of single particles. The first positive electrode active material and the second positive electrode active material each comprise a lithium-nickel-based composite oxide in which the nickel content relative to the total elements excluding lithium and oxygen is 70 mol % or more. The first positive electrode active material comprises a coating portion present in the form of a film on the surface of the secondary particles, and the second positive electrode active material comprises a coating portion present in the form of a film on the surface of the single particles. The coating portion of the first positive electrode active material and the coating portion of the second positive electrode active material comprise lithium cobalt oxide and cobalt oxyhydroxide, respectively.

[0006] In another embodiment, there is provided a method for producing a positive electrode active material for a lithium secondary battery, the method comprising: introducing a first positive electrode active material in the form of secondary particles formed by agglomeration of a plurality of primary particles, the first positive electrode active material containing a lithium-nickel-based composite oxide having a nickel content of 70 mol % or more relative to the total elements excluding lithium and oxygen; a second positive electrode active material in the form of single particles, the second positive electrode active material containing a lithium-nickel-based composite oxide having a nickel content of 70 mol % or more relative to the total elements excluding lithium and oxygen; cobalt sulfate and sodium hydroxide into a solvent and mixing them; removing the solvent; adding a lithium source to the obtained material; and heat-treating the resulting material to obtain the positive electrode active material.

[0007] In yet another embodiment, a lithium secondary battery is provided, which includes a positive electrode containing the positive electrode active material, a negative electrode, and an electrolyte. [Effects of the Invention]

[0008] A positive electrode active material for a lithium secondary battery according to an embodiment of the present invention has high capacity and high energy density while enhancing stability, and a lithium secondary battery including the positive electrode active material has improved high-temperature long-life characteristics and thermal stability. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment. [Figure 2] 1 is an X-ray diffraction (XRD) analysis graph of the positive electrode active material prepared in Example 1, a positive electrode plate before its lifespan, and a positive electrode plate after its lifespan. [Figure 3] This is an enlarged view of the graph in the vicinity of 20° in Figure 2. [Figure 4] This is an enlarged view of the graph around 40° in Figure 2. [Figure 5] 1 is an X-ray diffraction analysis graph for Co(OH)2, CoOOH, and Co3O4. [Figure 6] 1 is an image highlighting cobalt element in a fracture surface of a first positive electrode active material of Example 1 by SEM-EDS analysis. [Figure 7] 1 is an image highlighting cobalt element in a fracture surface of a second positive electrode active material of Example 1 by SEM-EDS analysis. [Figure 8] 1 is a graph showing evaluation results of life characteristics for coin half-cells manufactured in Example 1, Comparative Example 1, and Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0010] Although the present invention may be embodied in many different forms, it is not limited to the embodiments set forth herein.

[0011] The terms used herein are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0012] As used herein, "combinations thereof" refers to mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0013] It should be understood that the terms "comprises," "comprises," or "has" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the possible presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0014] In the drawings, the thickness of various layers and regions is exaggerated for clarity, and similar parts are designated by the same reference numerals throughout the specification. When a layer, film, region, plate, or other part is said to be "on" or "above" another part, this includes not only the case where it is "directly on" another part, but also the case where there is another part between them. Conversely, when a part is said to be "directly on" another part, it means that there is no other part between them.

[0015] Furthermore, the term "layer" as used herein includes not only a shape formed on the entire surface when observed in a plan view, but also a shape formed on a portion of the surface.

[0016] The average particle size can be measured by methods well known to those skilled in the art, such as by using a particle size analyzer or by using a transmission electron microscope or scanning electron microscope. Alternatively, the average particle size can be calculated by measuring using a dynamic light scattering method, counting the number of particles in each particle size range, and then performing data analysis. Unless otherwise defined, the average particle size can refer to the diameter (D50) of particles whose cumulative volume is 50% by volume in the particle size distribution.

[0017] Here, "or" is not to be construed in an exclusive sense; for example, "A or B" is to be construed as including A, B, A+B, etc.

[0018] positive electrode active material In one embodiment, a positive electrode active material for a lithium secondary battery is provided, which includes a first positive electrode active material in the form of secondary particles formed by agglomerating a plurality of primary particles, and a second positive electrode active material in the form of single particles.

[0019] The first and second positive electrode active materials each contain a lithium-nickel composite oxide in which the nickel content relative to the total elements excluding lithium and oxygen is 70 mol % or more, i.e., the first and second positive electrode active materials can be considered high-nickel positive electrode active materials.

[0020] The first positive electrode active material includes a coating portion existing in the form of a film on the surface of the secondary particles, and the second positive electrode active material includes a coating portion existing in the form of a film on the surface of the single particles, wherein the coating portion of the first positive electrode active material and the coating portion of the second positive electrode active material include lithium cobalt oxide (LiCoO2) and cobalt oxyhydroxide (CoOOH), respectively.

[0021] In the coating, the cobalt oxyhydroxide has an R-3m crystal structure, which is less structurally heterogeneous with the lithium nickel-based composite oxide, facilitating lithium migration and thereby improving the performance of the lithium secondary battery.

[0022] The coating can be said to include a composite phase of lithium cobalt oxide and cobalt oxyhydroxide. This composite phase can strengthen the surface structure of the positive electrode active material, thereby suppressing side reactions between the positive electrode active material and the electrolyte, thereby improving the life, overcharge, and high-temperature storage characteristics of the lithium secondary battery. The composite phase overcomes the unstable structure of high-nickel positive electrode active materials, ensuring stable characteristics comparable to those of low-nickel positive electrode active materials.

[0023] The composite phase can be confirmed by X-ray diffraction analysis of the positive electrode active material or the positive electrode plate. For example, the positive electrode active material may exhibit peaks at 19.5° to 20.5° and 39° to 40° in X-ray diffraction analysis, which may indicate the presence of cobalt oxyhydroxide.

[0024] The coating portion exists in the form of a film, which may be in the form of a continuous coating layer or an island, which is different from the coating portion existing in the form of particles.

[0025] The thickness of each of the coatings of the first and second positive electrode active materials may be 1 nm to 500 nm, for example, 1 nm to 400 nm, 1 nm to 300 nm, 1 nm to 200 nm, 1 nm to 100 nm, 5 nm to 50 nm, or 5 nm to 40 nm. By forming the coatings within this thickness range, the structure of the positive electrode active material can be stabilized and side reactions with the electrolyte can be suppressed, improving the performance of the lithium secondary battery, without acting as a resistor or degrading battery performance. The thickness of the coatings can be measured by TOF-SIMS, XPS, or EDS, for example, by TEM-EDS line profile analysis.

[0026] In the positive electrode active material, the cobalt content of the coating portion may be about 0.5 mol % to 5 mol %. Specifically, in the positive electrode active material as a whole, the surface cobalt content relative to 100 mol % of elements excluding lithium and oxygen in the lithium nickel-based composite oxide may be 0.5 mol % to 5 mol %, for example, 0.5 mol % to 4 mol %, or 1 mol % to 3 mol %. By coating with cobalt at this content, the structure of the positive electrode active material can be stabilized without reducing capacity, thereby improving the efficiency and lifespan of the battery.

[0027] Meanwhile, according to one embodiment, the ratio of the cobalt content (at %) relative to the total amount of nickel and cobalt in the coating portion of the second positive electrode active material to the cobalt content (at %) relative to the total amount of nickel and cobalt in the coating portion of the first positive electrode active material may be 1.45 to 1.60. When a mixed positive electrode active material, consisting of a first positive electrode active material in the form of secondary particles and a second positive electrode active material in the form of single particles, is fully coated, a coating imbalance may occur between the two particles. For example, when two types of particles are mixed and simultaneously wet-coated with cobalt, a larger amount of coating material tends to react and coat single particles with a relatively large specific surface area. As a result, the secondary particles may not achieve sufficient coating effect, resulting in structural collapse and side reactions on the surface, accelerating degradation. The uneven distribution of coating particles on the surface may induce gas generation. Excessive coating on single particles may act as resistance, adversely affecting battery performance. On the other hand, according to one embodiment, by suppressing excessive coating on single particles while strengthening the coating on secondary particles, i.e., by appropriately adjusting the coating content relationship between the two types of particles, lithium secondary battery performance, such as high-temperature long-term life characteristics, can be improved. For example, when the ratio of the cobalt coating content of the first positive electrode active material to the cobalt coating content of the second positive electrode active material is 1.45 to 1.60, high capacity and high energy density can be achieved while simultaneously improving high-temperature long-term life characteristics, suppressing gas generation during high-temperature storage, and improving initial charge / discharge efficiency. The ratio may be, for example, 1.45 to 1.55 or 1.50 to 1.60.

[0028] First positive electrode active material The first positive electrode active material has a polycrystalline form and includes secondary particles formed by agglomerating at least two or more primary particles. According to one embodiment, the first positive electrode active material includes a coating portion existing in the form of a film along the surface of the secondary particles, the coating portion including a composite phase of lithium cobalt oxide and cobalt oxyhydroxide.

[0029] The cobalt content of the cobalt coating portion of the first positive electrode active material relative to the total amount of nickel and cobalt may be 55 at% to 70 at%, for example, 55 at% to 68 at%, 55 at% to 65 at%, 55 at% to 63 at%, 57 at% to 70 at%, 59 at% to 70 at%, 60 at% to 70 at%, 60 at% to 65 at%, or 61 at% to 63 at%. When the cobalt coating content of the first positive electrode active material satisfies this range and is 1.45 to 1.60 times the cobalt coating content of the second positive electrode active material, uneven coating between particles is eliminated and a uniform coating is formed on the surface of the first positive electrode active material, suppressing gas generation and improving initial charge / discharge efficiency and long-term life characteristics.

[0030] According to one embodiment, the first positive electrode active material may further include a grain boundary cobalt coating portion located on the surface of the primary particles within the secondary particles. The grain boundary coating portion includes lithium cobalt oxide and cobalt oxyhydroxide. The grain boundary cobalt coating portion is present inside the secondary particles, not on the surface, and can be said to be coated along the interface of the primary particles within the secondary particles, thereby being referred to as being coated at the grain boundary. Here, the "inside" of the secondary particles refers to the entire interior excluding the surface, and may refer to, for example, a region from a depth of approximately 10 nm from the outer surface to the entire interior, or from a depth of 10 nm to a depth of approximately 2 μm. According to one embodiment, the first positive electrode active material further includes a grain boundary cobalt coating portion, which enhances structural stability, induces a uniform and even coating on the surface, and appropriately controls the coating content on the surface, thereby improving initial charge / discharge efficiency and life characteristics without increasing resistance.

[0031] The average particle size of the first positive electrode active material, i.e., the average particle size of the secondary particles, may be 5 μm to 20 μm. For example, it may be 7 μm to 20 μm, 10 μm to 20 μm, or 12 μm to 18 μm. The average particle size of the secondary particles of the first positive electrode active material may be larger than the average particle size of the second positive electrode active material, which is a single particle, described below. According to one embodiment, the positive electrode active material may be a mixture of a first positive electrode active material that is a polycrystalline, large particle and a second positive electrode active material that is a single, small particle. This may improve the mixture density and achieve high capacity and high energy density. The average particle size of the first positive electrode active material may be determined by measuring the particle size of approximately 20 secondary particle active materials in an electron microscope photograph of the positive electrode active material, and then determining the diameter (D50) of the particles that make up 50% of the cumulative volume in the particle size distribution.

[0032] The first positive electrode active material may be a high-nickel-based positive electrode active material containing a high content of nickel. In the lithium-nickel-based composite oxide, the nickel content may be 70 mol% or more, based on the total amount of elements excluding lithium and oxygen, for example, 75 mol% or more, 80 mol% or more, 85 mol% or more, or 90 mol% or more, or 99.9 mol% or less, or 99 mol% or less. Such a high-nickel-based first positive electrode active material can achieve high capacity and high performance.

[0033] Second positive electrode active material The second positive electrode active material may be in the form of a single particle, meaning that it does not have a grain boundary within the particle and exists alone, or may have a monolithic structure or a single structure or non-aggregated particles, meaning that the particles are morphologically present in an independent phase without aggregation, and may be a single crystal. By including the second positive electrode active material in the form of a single particle, the positive electrode active material according to one embodiment may achieve high capacity, high energy density, and improved life characteristics.

[0034] According to one embodiment, the second positive electrode active material includes a coating portion in the form of a film formed along the surface of a single particle, the coating portion including a composite phase of lithium cobalt oxide and cobalt oxyhydroxide.

[0035] The cobalt content of the coating portion of the second positive electrode active material relative to the total amount of nickel and cobalt may be 39 at% to 45 at%, for example, 39 at% to 44 at%, 39 at% to 43 at%, 39 at% to 42 at%, 40 at% to 45 at%, or 41 at% to 45 at%. When the cobalt coating content of the second positive electrode active material satisfies this range and the ratio of the cobalt coating content of the first positive electrode active material to the second positive electrode active material is 1.45 to 1.60, uneven coating between particles is eliminated, excessive coating on the second positive electrode active material is suppressed, and a uniform coating is induced, reducing resistance and improving initial charge / discharge efficiency and long-term life characteristics.

[0036] In one embodiment, the cobalt content relative to the total amount of nickel and cobalt on the surface of the positive electrode active material may be measured by quantitatively determining the nickel and cobalt contents of the surface of the positive electrode active material using scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS), and then calculating the ratio of the cobalt content to the total. In addition to SEM-EDS, other methods for measuring the cobalt content include inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0037] The average particle size of the second positive electrode active material, i.e., the average particle size of a single particle, may be 0.1 μm to 10 μm, for example, 0.1 μm to 7 μm, 0.5 μm to 6 μm, or 1 μm to 5 μm. The particle size of the second positive electrode active material may be smaller than that of the first positive electrode active material, thereby further increasing the density of the positive electrode active material. Here, the average particle size of the second positive electrode active material may be determined by measuring the particle size of approximately 20 single-particle active material particles randomly selected from an electron microscope photograph of the positive electrode active material, and taking the diameter (D50) of the particle at 50% of the cumulative volume in the particle size distribution as the average particle size.

[0038] The second positive electrode active material may be a high-nickel-based positive electrode active material containing a high content of nickel. In the lithium-nickel-based composite oxide, the nickel content may be 70 mol% or more, based on the total amount of elements excluding lithium and oxygen, for example, 75 mol% or more, 80 mol% or more, 85 mol% or more, or 90 mol% or more, or 99.9 mol% or less, or 99 mol% or less. Such a high-nickel-based second positive electrode active material can achieve high capacity and high performance.

[0039] Specifically, the first positive electrode active material and the second positive electrode active material may each independently include a lithium nickel-based composite oxide represented by the following Chemical Formula 1:

[0040] [Chemical formula 1] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1

[0041] In the above chemical formula 1, 0.9≦a1≦1.8, 0.7≦x1≦1, 0≦y1≦0.3, 0≦z1≦0.2, 0.9≦x1+y1+z1≦1.1, and 0≦b1≦0.1; M 1 and M 2are each independently one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0042] In the above Chemical Formula 1, 0.8≦x1≦1, 0≦y1≦0.2, and 0≦z1≦0.15, or 0.9≦x1≦1, 0≦y1≦0.1, and 0≦z1≦0.1 may be satisfied.

[0043] For example, the first positive electrode active material and the second positive electrode active material may each independently include a lithium nickel-based composite oxide represented by the following Chemical Formula 2. The compound represented by Chemical Formula 2 can be said to be a lithium nickel cobalt-based composite oxide.

[0044] [Chemical formula 2] Li a2 Ni x2 Co y2 M 3 z2 O 2-b2 X b2

[0045] In the above chemical formula 2, 0.9≦a2≦1.8, 0.7≦x2<1, 0 <y2≦0.3、0≦z2≦0.2、0.9≦x2+y2+z2≦1.1、および0≦b2≦0.1であり、M 3 is one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0046] In Chemical Formula 2, 0.8≦x2≦0.99, 0.01≦y2≦0.2, and 0.01≦z2≦0.15, or 0.9≦x2≦0.99, 0.01≦y2≦0.1, and 0.01≦z2≦0.1 may be satisfied.

[0047] For example, the first positive electrode active material and the second positive electrode active material may each independently include a lithium nickel-based composite oxide represented by the following Chemical Formula 3. The compound of Chemical Formula 3 may be lithium nickel-cobalt-aluminum oxide or lithium nickel-cobalt-manganese oxide.

[0048] [Chemical formula 3] Li a3 Ni x3 Co y3 M 4 z3 M 5 w3 O 2-b3 X b3

[0049] In the above chemical formula 3, 0.9≦a3≦1.8, 0.7≦x3≦0.98, 0.01≦y3≦0.29, 0.01≦z3≦0.29, 0≦w3≦0.19, 0.9≦x3+y3+z3+w3≦1.1, and 0≦b3≦0.1; and M 4 is one or more elements selected from the group consisting of Al and Mn, 5 is one or more elements selected from the group consisting of B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sr, Ti, V, W, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0050] In Chemical Formula 3, 0.85≦x3≦0.98, 0.01≦y3≦0.14, 0.01≦z3≦0.14, and 0≦w3≦0.14, or 0.9≦x3≦0.98, 0.01≦y3≦0.09, 0.01≦z3≦0.09, and 0≦w3≦0.09 may be satisfied.

[0051] In one embodiment, the cathode active material may contain 50 wt% to 90 wt% of the first cathode active material and 10 wt% to 50 wt% of the second cathode active material relative to the total weight of the first and second cathode active materials. The first cathode active material may be contained in an amount of, for example, 60 wt% to 90 wt% or 70 wt% to 90 wt%, and the second cathode active material may be contained in an amount of 10 wt% to 40 wt% or 10 wt% to 30 wt%. When the content ratios of the first and second cathode active materials are as described above, the cathode active material containing these materials can achieve high capacity, have improved composite density, and exhibit high energy density.

[0052] Method for producing positive electrode active material In one embodiment, there is provided a method for producing a positive electrode active material for a lithium secondary battery, the method including: introducing a first positive electrode active material in the form of secondary particles formed by agglomeration of a plurality of primary particles, the first positive electrode active material containing a lithium-nickel-based composite oxide having a nickel content of 70 mol % or more relative to the total elements excluding lithium and oxygen; a second positive electrode active material in the form of single particles, the second positive electrode active material containing a lithium-nickel-based composite oxide having a nickel content of 70 mol % or more relative to the total elements excluding lithium and oxygen; cobalt sulfate; and sodium hydroxide (NaOH) into a solvent and mixing them; removing the solvent; adding a lithium source to the obtained material; and heat-treating the resulting material to obtain the positive electrode active material.

[0053] The method can provide a cathode active material that includes a first cathode active material including a coating portion present in the form of a film on the surface of the secondary particles and a second cathode active material including a coating portion present in the form of a film on the surface of the single particles, wherein the coating portion includes a composite phase of lithium cobalt oxide and cobalt oxyhydroxide.

[0054] The lithium nickel-based composite oxide is the same as that described above, and therefore will not be described further. The solvent may be an aqueous solvent such as water or an alcohol-based solvent.

[0055] The first positive electrode active material and the second positive electrode active material may be mixed in a weight ratio of 9:1 to 5:5, for example, 8:2 to 6:4, which is advantageous for achieving high capacity and high energy density.

[0056] The cobalt sulfate can be considered as a coating raw material added to manufacture a cathode active material according to an embodiment. The cobalt sulfate may be mixed so that the total cobalt content relative to elements excluding lithium and oxygen in the lithium-nickel-based composite oxide of the first cathode active material and the lithium-nickel-based composite oxide of the second cathode active material is 0.5 to 5 molar parts, for example, 0.5 to 4 molar parts, or 1 to 3 molar parts. The sodium hydroxide may function as a precipitant and / or pH adjuster. If sodium hydroxide is not added, the coating may not be formed effectively, the desired amount of cobalt may not be coated, and the CoOOH phase may not appear in the coating on the surface of the final positive electrode active material. The sodium hydroxide may be added so that the sodium content is 1 to 10 molar parts, for example, 3 to 9 molar parts, or 5 to 8 molar parts, per 100 molar parts of all elements excluding lithium and oxygen in the lithium-nickel-based composite oxide of the first positive electrode active material and the lithium-nickel-based composite oxide of the second positive electrode active material. When the content of sodium added satisfies the above range, a film-like coating containing LiCoO2 and CoOOH is effectively formed. Furthermore, the ratio of the molar content of cobalt in the cobalt sulfate to the molar content of sodium in the sodium hydroxide may be 1:1.1 to 1:5, for example, 1:1.5 to 1:4, or 1:2 to 1:3. When the molar ratio of the added cobalt to sodium satisfies the above range, it is advantageous to form a coating portion in the form of a film containing LiCoO2 and CoOOH.

[0057] After removing the solvent, a lithium source is added to the resulting material, where the lithium source may be, for example, Li2CO3, LiOH, a hydrate thereof, or a combination thereof. The lithium source may be added so that the total lithium content, excluding lithium and oxygen, in the lithium-nickel-based composite oxide of the first positive electrode active material and the lithium-nickel-based composite oxide of the second positive electrode active material is 0.1 to 10 molar parts, for example, 0.1 to 8 molar parts, or 1 to 6 molar parts. The process of washing and coating the positive electrode active material with a solvent can damage the surface of the lithium-nickel-based composite oxide particles, resulting in reduced capacity and rate characteristics and increased resistance during high-temperature storage. However, adding a lithium source during heat treatment can repair the surface damage and improve capacity and rate characteristics.

[0058] The heat treatment can be carried out in an oxidizing gas atmosphere such as oxygen or air, at a temperature of 650°C to 900°C or 650°C to 800°C. The heat treatment time varies depending on the heat treatment temperature, but can be, for example, 5 to 30 hours or 10 to 24 hours. By carrying out the heat treatment under these conditions, a coating portion containing a composite phase of lithium cobalt oxide and cobalt oxyhydroxide can be formed.

[0059] According to one embodiment, the process of adding the second positive electrode active material, cobalt sulfate, and sodium hydroxide to a solvent and mixing them may be performed sequentially by first adding the second positive electrode active material, cobalt sulfate, and sodium hydroxide to the solvent and mixing them, followed by a second adding the first positive electrode active material, cobalt sulfate, and sodium hydroxide to the solvent and mixing them. This process prevents excessive coating of the single particles and strengthens the coating of the secondary particles, thereby appropriately controlling the coating content relationship between the two types of particles, thereby improving the performance of lithium secondary batteries, including high-temperature long-term life characteristics, initial charge / discharge efficiency, and high-temperature storage performance. For example, this sequential process may produce a positive electrode active material in which the ratio of the cobalt content (at %) relative to the total amount of nickel and cobalt in the cobalt-coated portion of the second positive electrode active material to the cobalt content (at %) relative to the total amount of nickel and cobalt in the cobalt-coated portion of the first positive electrode active material is 1.45 to 1.60.

[0060] The ratio of the time (minutes) required for the first step to the time (minutes) required for the second step may be 50:50 to 75:25 or 65:35 to 75:25. By designing such a time ratio, the coating content of each of the first and second positive electrode active materials can be optimized. The time required for the first step is approximately 10 to 100 minutes, and the time required for the second step is approximately 10 to 100 minutes.

[0061] positive electrode A positive electrode for a lithium secondary battery may include a current collector and a positive electrode active material layer disposed on the current collector. The positive electrode active material layer includes a positive electrode active material and may further include a binder and / or a conductive material.

[0062] The binder serves to firmly adhere the positive electrode active material particles to each other and to firmly adhere the positive electrode active material to the current collector. Representative examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon.

[0063] The content of the binder in the positive electrode active material layer may be approximately 1 wt % to 5 wt % based on the total weight of the positive electrode active material layer.

[0064] The conductive material is used to impart conductivity to the electrodes, and any material that does not cause a chemical change in the constructed battery and is electron-conductive can be used. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; and conductive materials containing mixtures thereof.

[0065] The content of the conductive material in the positive electrode active material layer may be 1 wt % to 5 wt % based on the total weight of the positive electrode active material layer.

[0066] The positive electrode current collector may be, but is not limited to, aluminum foil.

[0067] negative electrode A negative electrode for a lithium secondary battery includes a current collector and a negative electrode active material layer disposed on the current collector. The negative electrode active material layer includes a negative electrode active material and may further include a binder and / or a conductive material.

[0068] The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, a lithium metal alloy, a material capable of being doped with and dedoped from lithium, or a transition metal oxide.

[0069] The material capable of reversibly intercalating / deintercalating lithium ions may be a carbon-based negative electrode active material, such as crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flake-like, spherical, or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon, hard carbon, mesophase pitch carbide, and calcined coke.

[0070] As the lithium metal alloy, an alloy of lithium and a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0071] The material capable of doping and dedoping lithium may be a Si-based negative electrode active material or a Sn-based negative electrode active material. Examples of the Si-based negative electrode active material include silicon, silicon-carbon composites, and SiO x(0 < x < 2), Si-Q alloy (where Q is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof, and is not Si), examples of the Sn-based negative electrode active material include Sn, SnO2, Sn-R alloy (where R is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof, and is not Sn), etc., and at least one of these can also be used by mixing with SiO2. As the elements Q and R, 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, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and those selected from the group consisting of combinations thereof can be used.

[0072] The silicon-carbon composite may include, for example, a silicon-carbon composite including a core containing crystalline carbon and silicon particles, and an amorphous carbon coating layer disposed on the core surface. The crystalline carbon may be artificial graphite, natural graphite, or a 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. The silicon content may be 10 wt% to 50 wt% of the total weight of the silicon-carbon composite. The crystalline carbon content may be 10 wt% to 70 wt% of the total weight of the silicon-carbon composite, and the amorphous carbon content may be 20 wt% to 40 wt% of the total weight of the silicon-carbon composite. The amorphous carbon coating layer may have a thickness of 5 nm to 100 nm. The silicon particles may have an average particle size (D50) of 10 nm to 20 μm. The silicon particles may have an average particle size (D50) of preferably 10 nm to 200 nm. The silicon particles are present in an oxidized state, and at this time, the atomic ratio of Si:O in the silicon particles, which indicates the degree of oxidation, may be 99:1 to 33:67. x In this case, SiO x In this specification, x may be in the range of more than 0 and less than 2. Unless otherwise defined, the average particle size (D50) refers to the diameter of particles whose cumulative volume is 50% by volume in the particle size distribution.

[0073] The Si-based or Sn-based negative electrode active material can be mixed with a carbon-based negative electrode active material, and when the Si-based or Sn-based negative electrode active material and the carbon-based negative electrode active material are mixed, the mixing ratio by weight may be 1:99 to 90:10.

[0074] The content of the negative electrode active material in the negative electrode active material layer may be 95 wt % to 99 wt % based on the total weight of the negative electrode active material layer.

[0075] In one embodiment, the negative electrode active material layer may further include a binder and, optionally, a conductive material. The content of the binder in the negative electrode active material layer may be 1 wt% to 5 wt% based on the total weight of the negative electrode active material layer. When the conductive material is further included, the negative electrode active material layer may include 90 wt% to 98 wt% of the negative electrode active material, 1 wt% to 5 wt% of the binder, and 1 wt% to 5 wt% of the conductive material.

[0076] The binder serves to firmly adhere the negative active material particles to each other and to the current collector, and may be a water-insoluble binder, a water-soluble binder, or a combination thereof.

[0077] Examples of the water-insoluble binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, ethylene propylene copolymers, polystyrene, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, and combinations thereof.

[0078] The water-soluble binder may be a rubber-based binder or a polymer resin binder. The rubber-based binder may be selected from styrene butadiene rubber, acrylated styrene butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, and combinations thereof. The polymer resin binder may be selected from polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0079] When a water-soluble binder is used as the negative electrode binder, a cellulose-based compound capable of imparting viscosity may be further included. The cellulose-based compound may be a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li. The amount of the thickener used may be 0.1 to 3 parts by weight per 100 parts by weight of the negative electrode active material.

[0080] The conductive material is used to impart conductivity to the electrode, and any material that does not cause a chemical change in the constructed battery and is electron-conductive can be used. Examples of conductive materials that can be used include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and conductive materials including mixtures thereof.

[0081] The negative electrode current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0082] Lithium secondary battery Another embodiment provides a lithium secondary battery including a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte.

[0083] 1 is a schematic diagram illustrating a lithium secondary battery according to one embodiment. Referring to FIG. 1, the lithium secondary battery 100 according to one embodiment includes a battery cell including a positive electrode 114, a negative electrode 112 facing the positive electrode 114, a separator 113 disposed between the positive electrode 114 and the negative electrode 112, and a lithium secondary battery electrolyte impregnated in the positive electrode 114, the negative electrode 112, and the separator 113; a battery container 120 housing the battery cell; and a sealing member 140 sealing the battery container 120.

[0084] The electrolyte includes a non-aqueous organic solvent and a lithium salt.

[0085] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can migrate. Examples of the non-aqueous organic solvent include carbonates, esters, ethers, ketones, alcohols, and aprotic solvents. Examples of carbonate solvents 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), and butylene carbonate (BC). Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, etc. Examples of the ether solvent include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. Examples of the ketone solvent include cyclohexanone, etc. In addition, examples of the alcohol solvent that can be used include ethyl alcohol and isopropyl alcohol, and examples of the aprotic solvent that can be used include nitriles such as R-CN (where R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and may contain a double bond, an aromatic ring, or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes.

[0086] The non-aqueous organic solvents may be used alone or in combination of two or more thereof. When two or more thereof are used in combination, the mixing ratio may be appropriately adjusted depending on the desired battery performance, which should be widely understood by those skilled in the art.

[0087] In addition, in the case of the carbonate-based solvent, a cyclic carbonate and a chain carbonate can be mixed and used. In this case, the performance of the electrolyte is excellent when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9.

[0088] The non-aqueous organic solvent may further include an aromatic hydrocarbon-based organic solvent in addition to the carbonate-based solvent, and the carbonate-based solvent and the aromatic hydrocarbon-based organic solvent may be mixed in a volume ratio of about 1:1 to about 30:1.

[0089] The aromatic hydrocarbon solvent may be an aromatic hydrocarbon compound represented by the following formula I:

[0090] [ka]

[0091] In the above chemical formula I, R 4 ~R 9 are the same or different and are selected from the group consisting of hydrogen, halogen, alkyl groups having 1 to 10 carbon atoms, haloalkyl groups having 1 to 10 carbon atoms, and combinations thereof.

[0092] Specific examples of the aromatic hydrocarbon solvent 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, fluorobenzene, fluoroisopropyl ether ... The fluorotoluene may be selected from the group consisting of fluorotoluene, 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 a combination thereof.

[0093] The electrolyte may further include vinylene carbonate or an ethylene carbonate-based compound of the following formula II as a life-enhancing additive to improve battery life.

[0094] [ka]

[0095] In the above chemical formula II, R 10 and R 11 are the same or different and are selected from the group consisting of hydrogen, a halogen group, a cyano group, a nitro group, and a fluorinated alkyl group having 1 to 5 carbon atoms; 10 and R 11At least one of R is selected from the group consisting of a halogen group, a cyano group, a nitro group, and a fluorinated alkyl group having 1 to 5 carbon atoms, with the proviso that R 10 and R 11 But it's not all hydrogen.

[0096] Representative examples of the ethylene carbonate-based compound include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, fluoroethylene carbonate, etc. When such a life-improving additive is further used, the amount used can be appropriately adjusted.

[0097] The lithium salt is dissolved in a non-aqueous organic solvent and serves as a source of lithium ions in the battery, enabling basic operation of a lithium secondary battery, and promoting the movement of lithium ions between the positive electrode and the negative electrode.

[0098] Representative examples of lithium salts include 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, and LiN(C x F 2x+1 SO2)(C y F 2y+1SO2) (where x and y are natural numbers, for example, integers of 1 to 20), lithium difluoro(bisoxalato)phosphate, LiCl, LiI, LiB(C2O4)2 (lithium bis(oxalato)borate; LiBOB), and lithium difluoro(oxalato)borate (LiDFOB).

[0099] The lithium salt concentration is preferably within the range of 0.1 M to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0100] The separator 113 separates the positive electrode 114 and the negative electrode 112 and provides a path for lithium ions to move. Any separator commonly used in lithium ion batteries can be used. That is, a separator that has low resistance to electrolyte ion movement and excellent electrolyte humidification capability can be used. For example, the separator 113 can include glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof, and can be in the form of a nonwoven or woven fabric. For example, polyolefin-based polymer separators such as polyethylene and polypropylene are commonly used in lithium ion batteries. Coated separators containing ceramic components or polymer materials can also be used to ensure heat resistance or mechanical strength, and can be used in either a single-layer or multi-layer structure.

[0101] Lithium secondary batteries can be classified into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries depending on the type of separator and electrolyte used, into cylindrical, prismatic, coin, pouch, etc. depending on the shape, and into bulk and thin film types depending on the size. The structure and manufacturing method of these batteries are widely known in this field, so a detailed description will be omitted.

[0102] The lithium secondary battery according to an embodiment achieves high capacity and has excellent storage stability, life characteristics, and high rate characteristics at high temperatures, and can be used in electric vehicles (EVs), hybrid vehicles such as plug-in hybrid electric vehicles (PHEVs), and portable electronic devices.

[0103] Examples of the present invention and comparative examples are described below. The following examples are merely examples of the present invention, and the present invention is not limited to the following examples. [Example]

[0104] Example 1 1. Production of primary lithium nickel composite oxide in the form of secondary particles The metal raw materials, nickel sulfate (NiSO4·6H2O), cobalt sulfate (CoSO4·7H2O), and aluminum sulfate (Al2(SO4)3·18H2O), were dissolved in distilled water as a solvent in a molar ratio of 91:8:1 to prepare a mixed solution, and ammonia water (NH4OH) and sodium hydroxide (NaOH) were prepared as a precipitant to form the complex compound.

[0105] After the ammonia water dilution solution is introduced into the continuous reactor, the metal raw material mixed solution is continuously introduced, and sodium hydroxide is introduced to maintain the pH inside the reactor.

[0106] The reaction is carried out slowly for about 80 hours, and after the reaction is stabilized, the overflowing product is collected and washed and dried to obtain the final precursor. As a result, the primary particles are aggregated to form secondary particles of the first nickel hydroxide (Ni 0.91 Co 0.08 Al 0.01 (OH)2) is obtained, washed and dried.

[0107] The first nickel-based hydroxide and LiOH were mixed so that the molar ratio of lithium to the total amount of metal in the first nickel-based hydroxide was 1.04, and the mixture was subjected to a first heat treatment at about 750°C for 15 hours in an oxygen atmosphere, thereby obtaining a first lithium-nickel-based oxide (LiNi 0.91 Co 0.08 Al 0.01 O2) is obtained. The average particle size of the obtained first lithium nickel-based oxide is approximately 15 μm, and the primary particles are aggregated to form secondary particles.

[0108] 2. Preparation of the second lithium nickel composite oxide in single particle form A mixed solution is prepared by dissolving nickel sulfate, cobalt sulfate, and manganese sulfate (MnSO4·H2O) in distilled water as a solvent in a molar ratio of 95:4:1. A diluted solution of ammonia water (NH4OH) and sodium hydroxide (NaOH) as a precipitant are prepared to form a complex compound. Then, the mixed solution of metal raw materials, ammonia water, and sodium hydroxide are each placed into the reactor. The reaction is then carried out for approximately 20 hours while stirring. After that, the slurry solution in the reactor is filtered and washed with high-purity distilled water, and then dried for 24 hours to obtain a second nickel hydroxide (Ni 0.95 Co 0.04 Mn 0.01 The resulting second nickel hydroxide powder has an average particle size of about 4.0 μm and a specific surface area measured by the BET method of about 15 m 2 / g.

[0109] The resulting second nickel-based hydroxide was mixed with LiOH satisfying a Li / (Ni+Co+Mn)=1.05 ratio and placed in a calcination furnace where a second heat treatment was performed at 820°C for 10 hours in an oxygen atmosphere. The resulting mixture was then pulverized for approximately 30 minutes to separate and disperse it into a number of second lithium-nickel-based oxide particles having a single particle shape. The average particle size of the resulting second lithium-nickel-based oxide particles was approximately 3.7 μm.

[0110] 3. Cobalt coating and final cathode active material production The first step involves adding distilled water solvent and cobalt sulfate (CoSO4·7H2O) to a mixer, followed by the second lithium-nickel-based composite oxide, which is then mixed. The cobalt sulfate is added so that the cobalt content relative to the total elements excluding lithium and oxygen in the second lithium-nickel-based composite oxide is 3.0 molar parts. After the second lithium-nickel-based composite oxide is added in the first step, sodium hydroxide, which acts as a precipitant and pH adjuster, is added and mixed. The sodium hydroxide is added so that the sodium content relative to 100 molar parts of the total elements excluding lithium and oxygen in the second lithium-nickel-based composite oxide is 6.0 molar parts.

[0111] After the first step is performed for 30 minutes, cobalt sulfate is added thereto, followed by the first lithium-nickel-based composite oxide, which is then mixed in the second step. The first lithium-nickel-based composite oxide is added so that the weight ratio of the first lithium-nickel-based composite oxide to the previously added second lithium-nickel-based composite oxide is 7:3. The cobalt sulfate is added so that the cobalt content relative to the total elements excluding lithium and oxygen in the first lithium-nickel-based composite oxide is 3.0 molar parts. In the second step, after the first lithium-nickel-based composite oxide is added, sodium hydroxide is also added and mixed. The sodium hydroxide is added so that the sodium content relative to 100 molar parts of the total elements excluding lithium and oxygen in the first lithium-nickel-based composite oxide is 6.0 molar parts.

[0112] After performing the second process for 30 minutes, the mixture is filtered and dried at 200°C for 10 hours. The resulting material is then mixed with lithium hydroxide and placed in a calcination furnace, where it is heat-treated in an oxygen atmosphere at approximately 700°C for 15 hours. The lithium hydroxide is added so that the lithium content is 6 parts by mole relative to the total elements in the resulting material, excluding lithium and oxygen. The calcination furnace is then cooled to room temperature, yielding a final positive electrode active material that is a mixture of a first positive electrode active material in which a film-like coating is formed on the surface of secondary particles made of a first lithium-nickel-based composite oxide, and a second positive electrode active material in which a film-like coating is formed on the surface of single particles made of a second lithium-nickel-based composite oxide.

[0113] 4. Fabrication of the Positive Electrode A positive electrode active material slurry was prepared by mixing 95 wt% of the final positive electrode active material, 3 wt% of polyvinylidene fluoride binder, and 2 wt% of carbon nanotube conductive material in N-methylpyrrolidone solvent. The positive electrode active material slurry was applied to an aluminum current collector, dried, and then rolled to prepare a positive electrode.

[0114] 5. Lithium secondary battery manufacturing A coin half-cell is fabricated using the prepared positive electrode and lithium metal counter electrode, with a polyethylene-polypropylene multilayer separator between them, and an electrolyte solution of 1.0 M LiPF6 lithium salt added to a solvent of ethylene carbonate and diethyl carbonate mixed in a 50:50 volume ratio.

[0115] Comparative Example 1 In "3. Preparation of cobalt coating and final cathode active material" of Example 1, a cathode active material, a cathode, and a coin half-cell were prepared in the same manner as in Example 1, except that sodium hydroxide, which acts as a precipitant and a pH adjuster, was not added in the first and second steps.

[0116] Comparative Example 2 In "3. Cobalt Coating and Preparation of Final Cathode Active Material" of Example 1, dry coating was performed instead of wet coating. That is, the first lithium-nickel-based composite oxide, the second lithium-nickel-based composite oxide, cobalt sulfate, and lithium hydroxide were mixed in a calcination furnace and heat-treated at 700°C for 15 hours. At this time, cobalt sulfate was added so that the cobalt content relative to the total elements excluding lithium and oxygen in the first lithium-nickel-based composite oxide and the second lithium-nickel-based composite oxide was 3.0 molar parts, and lithium hydroxide was added so that the lithium content was 6.0 molar parts. The cathode active material, cathode, and coin half-cell were otherwise prepared in the same manner as in Example 1.

[0117] Evaluation example 1: X-ray diffraction analysis X-ray diffraction analysis was performed on (i) the final positive electrode active material produced in Example 1, (ii) the positive electrode plate obtained by disassembling the coin half-cell of Example 1 immediately before operating the battery, and (iii) the positive electrode plate obtained by disassembling the coin half-cell of Example 1 after 150 cycles. The results are shown in Figure 2.

[0118] In the above (iii), the coin half-cell of Example 1 was initially charged under constant current (0.2 C) and constant voltage (4.25 V, 0.05 C cut-off) conditions, and after a 10-minute rest, initially discharged to 3.0 V under constant current (0.2 C) conditions, and then repeatedly charged and discharged 150 times at 45°C and 0.5 C / 0.5 C.

[0119] Figure 3 is an enlarged view of the graph in the vicinity of 20° in Figure 2, and Figure 4 is an enlarged view of the graph in the vicinity of 40° in Figure 2. For comparison, Figure 5 shows X-ray diffraction analysis graphs for Co(OH)2, CoOOH, and Co3O4.

[0120] 2 to 5, the cathode active material according to Example 1 exhibits peaks at 19.5° to 20.5° and at 39° to 40°, confirming the presence of CoOOH on the surface. That is, it can be seen that the cathode active material according to Example 1 has a composite phase of LiCoO2-like structures and CoOOH in the surface coating. This composite phase is captured not only in the cathode active material as prepared, but also after it is fabricated into a cathode plate and after the battery is operated.

[0121] X-ray diffraction analysis was also performed on Comparative Examples 1 and 2. However, in Comparative Example 1, quantitative coating was not performed, and the CoOOH phase was not observed in the X-ray diffraction graph. In Comparative Example 2, continuous coating was not performed on the surface of the positive electrode active material, and the CoOOH phase was not observed in the X-ray diffraction graph.

[0122] Evaluation example 2: SEM-EDS analysis of cross section of positive electrode active material Figure 6 is an SEM-EDS analysis image of a cross section of a particle corresponding to the first positive electrode active material in the final positive electrode active material prepared in Example 1. Figure 7 is an SEM-EDS analysis image of a cross section of a particle corresponding to the second positive electrode active material in the final positive electrode active material prepared in Example 1. The highlighted areas in Figures 6 and 7 indicate cobalt.

[0123] Referring to Figure 6, it can be seen that a cobalt-containing coating is present in the form of a film on the surface of the secondary particles, and that the grain boundaries, which are the surfaces of the primary particles present inside the secondary particles, are also coated with cobalt. Referring to Figure 7, it can be seen that a cobalt-containing coating is also formed in the form of a film on the surface of the single particles.

[0124] Evaluation Example 3: Analysis of cobalt coating content of positive electrode active material Through the SEM-EDS analysis performed in Evaluation Example 2, the cobalt content (Co / (Ni+Co), at %) on the surface of each of the first and second positive electrode active materials was measured, and the results are shown in Table 1 below.

[0125] [Table 1]

[0126] Evaluation example 4: Battery performance evaluation Three samples were prepared for each of the lithium secondary batteries manufactured in Example 1, Comparative Example 1, and Comparative Example 2. They were initially charged at a constant current (0.2 C) and constant voltage (4.25 V, 0.05 C cut-off), rested for 10 minutes, and then discharged to 3.0 V under a constant current (0.2 C) condition. The battery life was evaluated by repeating charge and discharge 50 times at 0.5 C / 0.5 C at 45°C. The capacity retention rate at each cycle relative to the initial discharge capacity is shown in FIG. 8.

[0127] Referring to FIG. 8, it can be seen that Example 1 has better life characteristics than Comparative Examples 1 and 2.

[0128] Although the preferred embodiments have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concepts defined in the following claims also fall within the scope of the present invention. [Explanation of symbols]

[0129] 100: Lithium secondary battery 112: Negative electrode 113: Separator 114: Positive electrode 120:Battery container 140: Sealing member

Claims

1. a first positive electrode active material in the form of secondary particles formed by agglomeration of a plurality of primary particles; and A positive electrode active material for a lithium secondary battery, comprising a second positive electrode active material in the form of a single particle, the first positive electrode active material and the second positive electrode active material each contain a lithium-nickel composite oxide in which the content of nickel relative to all elements excluding lithium and oxygen is 70 mol % or more; The first positive electrode active material includes a coating portion that exists in the form of a film on the surface of the secondary particle, the second positive electrode active material includes a coating portion that exists in the form of a film on the surface of the single particle, the coating portion of the first positive electrode active material and the coating portion of the second positive electrode active material each include lithium cobalt oxide and cobalt oxyhydroxide; the lithium cobalt oxide and the cobalt oxyhydroxide exist in a composite phase in the coating portion of the first positive electrode active material and the coating portion of the second positive electrode active material; a ratio of a cobalt content (at %) relative to the total amount of nickel and cobalt in the coating portion of the first positive electrode active material to a cobalt content (at %) relative to the total amount of nickel and cobalt in the coating portion of the second positive electrode active material is 1.45 to 1.

60.

2. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the positive electrode active material for a lithium secondary battery exhibits peaks at 19.5° to 20.5° and 39° to 40° in X-ray diffraction analysis.

3. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the coating portion of the first positive electrode active material and the coating portion of the second positive electrode active material each have a thickness of 1 nm to 500 nm.

4. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the lithium nickel-based composite oxide has a surface cobalt content of 0.5 mol % to 5 mol % relative to 100 mol % of elements excluding lithium and oxygen in the entire positive electrode active material for a lithium secondary battery.

5. 10. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the content of cobalt in the coating portion of the first positive electrode active material is 55 at % to 70 at % based on the total amount of nickel and cobalt.

6. 10. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the content of cobalt in the coating portion of the second positive electrode active material is 39 at % to 45 at % based on the total amount of nickel and cobalt.

7. the first positive electrode active material further includes a grain boundary coating portion located on a surface of the primary particle inside the secondary particle, The positive electrode active material for a lithium secondary battery according to claim 1 , wherein the grain boundary coating portion comprises lithium cobalt oxide and cobalt oxyhydroxide.

8. The first positive electrode active material has an average particle size of 5 μm to 20 μm, 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the second positive electrode active material has an average particle size of 0.1 μm to 10 μm.

9. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the first positive electrode active material is contained in an amount of 50% by weight to 90% by weight and the second positive electrode active material is contained in an amount of 10% by weight to 50% by weight, based on the total amount of the first positive electrode active material and the second positive electrode active material.

10. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the lithium nickel-based composite oxide of the first positive electrode active material and the lithium nickel-based composite oxide of the second positive electrode active material are each independently represented by the following chemical formula 1: [Chemical formula 1] Li ａ１ Ni ｘ１ M １ ｙ１ M ２ ｚ１ O ２－ｂ１ X ｂ１ In the above chemical formula 1, 0.9≦a1≦1.8, 0.7≦x1≦1, 0≦y1≦0.3, 0≦z1≦0.2, 0.9≦x1+y1+z1≦1.1, and 0≦b1≦0.1; M １ and M ２ are each independently one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

11. A first positive electrode active material in the form of secondary particles formed by agglomeration of a plurality of primary particles, the first positive electrode active material containing a lithium-nickel-based composite oxide having a nickel content of 70 mol % or more relative to the total elements excluding lithium and oxygen, a second positive electrode active material in the form of single particles, the second positive electrode active material containing a lithium-nickel-based composite oxide having a nickel content of 70 mol % or more relative to the total elements excluding lithium and oxygen, cobalt sulfate, and sodium hydroxide are introduced into a solvent and mixed; removing the solvent; Adding lithium source to the obtained material; Heat treated; obtaining a cathode active material according to any one of claims 1 to 10; Adding the first positive electrode active material, the second positive electrode active material, cobalt sulfate, and sodium hydroxide to a solvent and mixing them includes: A method for producing a positive electrode active material for a lithium secondary battery, the method comprising: performing a first step of adding a second positive electrode active material, cobalt sulfate, and sodium hydroxide to a solvent and mixing them; and then sequentially performing a second step of adding a first positive electrode active material, cobalt sulfate, and sodium hydroxide to the solvent and mixing them.

12. 12. The method of claim 11, wherein the first and second positive electrode active materials are mixed in a weight ratio of 9:1 to 5:

5.

13. 12. The method for producing a positive electrode active material for a lithium secondary battery according to claim 11, wherein the cobalt sulfate is mixed so that the total content of cobalt relative to elements excluding lithium and oxygen in the lithium-nickel-based composite oxide of the first positive electrode active material and the lithium-nickel-based composite oxide of the second positive electrode active material is 0.5 to 5 parts by mole.

14. 12. The method for producing a positive electrode active material for a lithium secondary battery according to claim 11, wherein the sodium hydroxide is mixed so that a content of sodium is 1 to 10 molar parts per 100 molar parts of all elements excluding lithium and oxygen in the lithium nickel-based composite oxide of the first positive electrode active material and the lithium nickel-based composite oxide of the second positive electrode active material.

15. 12. The method for producing a positive electrode active material for a lithium secondary battery according to claim 11, wherein a ratio of a molar content of cobalt in the cobalt sulfate to a molar content of sodium in the sodium hydroxide is 1:1.1 to 1:

5.

16. 12. The method for producing a positive electrode active material for a lithium secondary battery according to claim 11, wherein the lithium raw material is added so that the total amount of lithium in the lithium-nickel-based composite oxide of the first positive electrode active material and the lithium-nickel-based composite oxide of the second positive electrode active material is 0.1 to 10 parts by mole based on elements excluding lithium and oxygen.

17. The method for producing a positive electrode active material for a lithium secondary battery according to claim 11, wherein the heat treatment is carried out at a temperature in the range of 650°C to 900°C.

18. 12. The method for producing a positive electrode active material for a lithium secondary battery according to claim 11, wherein the ratio of the time (minutes) required for the first step to the time (minutes) required for the second step is 50:50 to 75:

25.

19. A lithium secondary battery comprising a positive electrode containing the positive electrode active material for lithium secondary batteries according to any one of claims 1 to 10, a negative electrode, and an electrolyte.

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