Positive electrode active material, method for producing the same, and lithium secondary battery having a positive electrode comprising the same

Abstract

A positive electrode active material including: lithium transition metal oxide particles in which a part of lithium is substituted with Na and including Ni and Co atoms; and a cobalt-containing coating layer provided on a surface of the lithium transition metal oxide particles, wherein the lithium transition metal oxide particles have a concentration gradient region in which a concentration of Co atoms decreases in a direction from the surface of the particles toward a center of the particles, a method for producing the positive electrode active material, and a lithium secondary battery having a positive electrode including the positive electrode active material.

Description

Technical Field

[0001] The invention relates to a positive electrode active material with a novel composition, a method for preparing the positive electrode active material, and a lithium secondary battery having a positive electrode including the positive electrode active material.

[0002] This invention is supported by grant number P0009541 from the Ministry of Trade, Industry and Energy of Korea, entitled “Development of Ni-rich NCA (>210mAh / g, @4.3V) cathode material for medium and large lithium secondary batteries with high strength / long life / high stability”. Background Technology

[0003] Since the commercialization of lithium-ion batteries by Sony in 1991, demand for them has grown rapidly across a wide range of sectors, from small home appliances such as mobile IT products to medium and large electric vehicles and energy storage systems. In particular, while low-cost, high-energy cathode materials are essential for medium and large electric vehicles and energy storage systems, cobalt, the main raw material for single-crystal LiCoO2 (LCO), the currently commercially available cathode active material, is expensive.

[0004] Therefore, as a positive electrode active material for medium to large-sized secondary batteries, LiNi, in which a portion of the Co is replaced by other transition metals, is used. x Co y Mn z O2(NCM, x+y+z=1) and LiNi x Co y Al zNi-based cathode active materials, represented by O2(NCA, x+y+z=1), are used to replace LCO. These NCM and NCA-based cathode active materials have the advantage of low nickel cost as the main raw material and high reversible capacity. In particular, NCM and NCA with a Ni content equal to or greater than 50 mol% are of interest for their high capacity. Typically, such Ni-based cathode active materials are prepared by solid-state synthesis after mixing a transition metal compound precursor synthesized by co-precipitation with a lithium source. However, the Ni-based cathode materials synthesized in this way exist as secondary particles composed of small primary particles aggregated together. In the long run, this leads to the formation of micro-cracks within these secondary particles during charge / discharge. These micro-cracks cause side reactions at the new interface between the cathode active material and the electrolyte, resulting in battery performance degradation, such as reduced stability due to gas generation and performance degradation due to electrolyte depletion. Furthermore, the increased electrode density required to achieve high energy density (>3.3 g / cc) leads to the collapse of secondary particles and electrolyte depletion due to side reactions with the electrolyte, thus resulting in a decreased initial lifetime. Consequently, this means that Ni-based cathode active materials in the form of secondary particles synthesized via existing co-precipitation methods cannot achieve high energy density.

[0005] To address the challenges of Ni-based cathode active materials existing in the form of secondary particles, single-particle Ni-based cathode active materials have recently been investigated. Single-crystal Ni-based cathode active materials achieve excellent electrochemical performance because the particles do not collapse when the electrode density is increased (>3.3 g / cc) to achieve high energy density. However, during electrochemical evaluation, issues have arisen regarding the unstable Ni... 3+ and Ni 4+ The presence of unstable Ni ions in the structure and / or thermal structure of Ni-based cathode active materials leads to a deterioration in battery stability. Therefore, the development of high-energy lithium-ion rechargeable batteries still requires technologies to stabilize unstable Ni ions in single-crystal Ni-based cathode active materials. Summary of the Invention

[0006] Technical issues

[0007] According to one aspect, a positive electrode active material is provided, wherein unstable Ni ions in the single-crystal Ni-based positive electrode active material as described above are stabilized, and cracks in particles formed during charging and discharging are suppressed by the improved structural stability of the positive electrode active material due to the Co concentration gradient region in the positive electrode active material and the Co-containing coating disposed on the surface of the positive electrode active material, thereby achieving high energy density and improved long-life characteristics.

[0008] Technical solution

[0009] According to one aspect, a positive electrode active material is provided, comprising: lithium transition metal oxide particles, wherein a portion of Li is replaced by Na, and including Ni and Co atoms; and

[0010] A cobalt-containing coating is applied to the surface of the lithium transition metal oxide particles.

[0011] The lithium transition metal oxide particles have a concentration gradient region in which the concentration of Co atoms decreases from the surface of the particle toward the center of the particle.

[0012] According to another aspect, a method for preparing a positive electrode active material is provided, comprising: a step of preparing lithium transition metal oxide particles, wherein a portion of Li is replaced by Na, and the material includes Ni and Co atoms;

[0013] The step of mixing the lithium transition metal oxide particles with a Co-containing compound to obtain a precursor for the positive electrode active material; and

[0014] The step of sintering the precursor of the positive electrode active material to obtain the positive electrode active material.

[0015] The positive electrode active material includes a Co-containing coating on its surface, and includes a concentration gradient region in which the concentration of Co atoms decreases from the surface of the particle towards the center of the particle.

[0016] According to another embodiment, a lithium secondary battery is provided, including a positive electrode comprising a positive electrode active material; a negative electrode; and an electrolyte.

[0017] Beneficial effects

[0018] According to one aspect of the present invention, the positive electrode active material comprises: lithium transition metal oxide particles, which are single crystals and single particles, wherein a portion of the Li element in the single crystal is replaced by Na; a concentration gradient region, wherein the Co concentration in the transition metal decreases in the direction from the surface of the lithium transition metal oxide particle to the particle center; and a cobalt-containing coating on the surface of the lithium transition metal oxide particles, thereby preventing particle breakage caused by cracks in the particles formed during charging and discharging, and stabilizing unstable Ni ions present in the high Ni-based lithium transition metal oxide, thereby increasing the capacity per unit volume and improving lifetime stability. Attached Figure Description

[0019] Figure 1 These are scanning electron microscope (SEM) images of the positive electrode active materials of Example 1 and Comparative Example 1.

[0020] Figure 2A graph showing the particle size distribution of the positive electrode active materials of Example 1 and Comparative Example 1.

[0021] Figure 3 This is a high-resolution transmission electron microscopy (HR-TEM) image of the positive electrode active material of Comparative Example 1.

[0022] Figure 4 This is an HR-TEM image of the positive electrode active material of Example 1.

[0023] Figure 5a Here is an HR-TEM image of the positive electrode active material of Example 1; and Figure 5b This is an energy dispersive X-ray spectroscopy (EDX) image of the positive electrode active material of Example 1.

[0024] Figure 6a Here is an HR-TEM image of the positive electrode active material of Example 1; and Figure 6b This is a fast Fourier transform (FFT) image of the positive electrode active material of Example 1.

[0025] Figure 7 A graph showing the life retention rate of the half-cells of Example 4 and Comparative Examples 11 to 15.

[0026] Figure 8 A graph showing the life retention rate of the half-cells of Example 4 and Comparative Examples 16 to 18.

[0027] Figure 9 A graph showing the life retention rate of the half-cells of Example 5 and Comparative Example 19.

[0028] Figure 10 A graph showing the lifespan retention of the half-cells of Example 6 and Comparative Example 20.

[0029] Figure 11 This is a schematic diagram of a lithium battery according to an exemplary embodiment.

[0030] <Explanation of symbols in the main parts of the attached diagram>

[0031] 1: Lithium battery 2: Negative electrode

[0032] 3: Positive electrode 4: Separator

[0033] 5: Battery casing 6: Cover assembly Detailed Implementation

[0034] The present inventive concept described below can be applied in various modifications and can have various embodiments, and specific embodiments are shown in the accompanying drawings and described in detail in the detailed description. However, this is not intended to limit the inventive concept to the specific embodiments, and it should be understood that all modifications, equivalents, or substitutions included within the technical scope of the inventive concept are included within the inventive concept.

[0035] The terminology used below is for describing specific embodiments only and is not intended to limit the inventive concept. Unless the context clearly specifies otherwise, singular expressions include plural expressions. In the following text, terms such as “comprising” or “having” are intended to indicate the presence of features, numbers, steps, operations, components, parts, components, ingredients, materials, or combinations thereof recorded in the specification, but it should be understood that this does not preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, components, ingredients, materials, or combinations thereof. The “ / ” used below may be interpreted as “and” or “or” as appropriate.

[0036] To clearly represent the various layers and regions in the figures, the thickness is enlarged or reduced. Throughout the specification, similar reference numerals are used for similar parts. Throughout the specification, when a part such as a layer, film, region, plate, etc., is referred to as being "on" another part, this includes not only cases where it is directly on the other part, but also cases where it contains the other part. Throughout the specification, terms such as "first," "second," etc., may be used to describe various components, but components should not be limited by these terms. These terms are used only to distinguish one component from another.

[0037] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. Furthermore, it should be understood that terms defined in common dictionaries should be interpreted as having the same meaning as they have in the context of the relevant art and this disclosure, and should not be interpreted as having an idealized or overly formal meaning.

[0038] "Group" refers to a group in the periodic table of elements according to the group classification system of the International Union of Pure and Applied Chemistry ("IUPAC"), which consists of groups 1 to 18.

[0039] Although specific embodiments have been described, applicants or those skilled in the art may conceive of alternatives, modifications, variations, improvements, and substantial equivalents that are not currently anticipated or unforeseen. Therefore, the appended claims, which are filed and subject to amendment, are intended to cover all such alternatives, modifications, variations, improvements, and substantial equivalents.

[0040] In the following, a positive electrode active material according to an exemplary embodiment, a method for preparing the positive electrode active material, and a lithium secondary battery having a positive electrode including the positive electrode active material will be described in more detail.

[0041] According to one specific embodiment, the positive electrode active material may include lithium transition metal oxide particles, wherein a portion of Li is replaced by Na, and includes Ni and Co atoms; and a cobalt-containing coating disposed on the surface of the lithium transition metal oxide particles, wherein the lithium transition metal oxide particles have a concentration gradient region in which the concentration of Co atoms decreases in the direction from the surface of the particles toward the center of the particles.

[0042] Because a portion of the Li in the positive electrode active material is replaced by Na, structural deformation caused by Li ion depletion during charging is suppressed, thereby improving the long-lifetime characteristics of the positive electrode active material. Furthermore, since the lithium transition metal oxide particles include a concentration gradient region where the Co atom concentration decreases from the particle surface to the particle center, the structural stability of the crystal is further enhanced, thus suppressing crystal collapse during charging and discharging and improving lifetime characteristics. In addition, the distribution of unstable Ni(III) and Ni(IV) ions on the particle surface is reduced, while the distribution of Ni(II) ions at the particle core center is increased. Therefore, side reactions between Ni ions and the electrolyte are suppressed, and the high capacity of the positive electrode active material is obtained due to the high Ni ion content. Therefore, the positive electrode active material exhibits both high capacity and long-lifetime characteristics. Moreover, the inclusion of a cobalt-containing coating on the surface of the lithium transition metal oxide particles suppresses side reactions between the electrolyte and the lithium transition metal oxide particles, thereby inhibiting the degradation of the positive electrode active material and improving its long-lifetime characteristics.

[0043] According to one specific embodiment, the concentration gradient region may include a region extending up to 500 nm in the direction from the surface of the lithium transition metal oxide particle toward the center of the particle.

[0044] For example, the concentration gradient region may include a region extending up to 250 nm in the direction from the surface of the lithium transition metal oxide particle toward the center of the particle.

[0045] When the concentration gradient region exists within the aforementioned distance from the surface of the lithium transition metal oxide particles, the high capacity and long lifetime characteristics of the positive electrode active material are achieved.

[0046] According to one specific embodiment, in the concentration gradient region, the concentration of Ni atoms may increase in the direction from the surface of the lithium transition metal oxide particles toward the center of the particles.

[0047] According to one specific embodiment, in the concentration gradient region, the concentration of Co atoms decreases and the concentration of Ni atoms increases in the direction from the surface of the lithium transition metal oxide particles toward the center of the particles.

[0048] As the Co atom concentration gradually decreases from the surface of the lithium transition metal oxide particles, while the Ni atom concentration gradually increases, the high capacity and long lifetime characteristics of the positive electrode active material are achieved.

[0049] According to one specific embodiment, the lithium transition metal oxide particles may include lithium transition metal oxides represented by the following chemical formula 1:

[0050] [Chemical Formula 1]

[0051] Li 1-x Na x M 1-(α+β+γ) W α Mg β Ti γ O 2-a S a

[0052] M, x, α, β, γ and a in the chemical formula 1 will be described in detail below.

[0053] In the lithium transition metal oxide represented by the aforementioned chemical formula 1, a portion of Li is replaced by Na, a portion of M is replaced by W, Mg and Ti, and a portion of O is replaced by S, thereby improving the structural stability of the lithium transition metal oxide during charging / discharging of the lithium secondary battery comprising the lithium transition metal oxide, thus increasing the capacity per unit volume and improving lifetime stability.

[0054] Furthermore, in high-Ni-based lithium transition metal oxides where M includes Ni, unstable Ni ions such as Ni are present in the lithium transition metal oxides due to the substitution of small amounts of W, Mg, and Ti. 3+ and Ni 4+ Ni is reduced to Ni in the stable nickel ion form. 2+ Therefore, it suppresses the degradation of the positive electrode active material and capacity degradation caused by the side reactions of unstable nickel ions and electrolyte during charging and discharging.

[0055] In addition to having a concentration gradient region, the lithium transition metal oxide particles also contain small amounts of W, Mg, and Ti as transition metals, which can induce unstable Ni ions such as NiO distributed in the lithium transition metal oxide particles. 3+ and Ni 4+ Ni is reduced to Ni in the stable nickel ion form. 2+ This prevents the degradation of the positive electrode active material during charging and discharging, and can significantly suppress capacity degradation.

[0056] According to a specific embodiment, in the chemical formula 1, M can be one or more elements selected from alkali metal elements, alkaline earth metal elements, transition metal elements, post-transition metal elements, and nonmetal elements other than W, Mg, Ti, Na, and S.

[0057] For example, M can be one or more elements selected from K, Rb, Cs, Fr, Be, Ca, Sr, Ba, Ra, Sc, Y, La, Zr, Hf, V, Nb, Ta, Cr, Mo, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, B, Al, Ga, In, Tl, C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi, N, P, As, Sb, Bi, Se, Te, and Po.

[0058] According to a specific embodiment, M can be one or more elements selected from alkaline earth metals, transition metals, post-transition metals, and nonmetals other than W, Mg, Ti, Na, and S.

[0059] For example, M can be one or more elements selected from Be, Ca, Sr, Ba, Ra, Sc, Y, La, Zr, Hf, V, Nb, Ta, Cr, Mo, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, B, Al, Ga, In, Tl, C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi, N, P, As, Sb, Bi, Se, Te, and Po.

[0060] According to one specific embodiment, M can be one or more elements selected from Ni, Co, Mn, Al, V, Ca, Zr, B, and P.

[0061] For example, M can be one or more elements selected from Ni, Co, Mn, Al, V, Ca, Zr, B, and P.

[0062] According to a specific embodiment, x can be 0 < x ≤ 0.01. Here, x refers to the substitution molar ratio of Na to Li in the lithium transition metal oxide represented by Chemical Formula 1. Since a part of Li in the lithium transition metal oxide represented by Chemical Formula 1 is substituted by Na, the structural stability can be improved. When Li located in the lattice space is substituted by Na, due to the intervention of Na with a large ionic radius, when Li is detached in the charged state, the expansion of the crystal structure caused by the repulsive force between oxygen atoms in the lithium transition metal oxide is suppressed. As a result, even when charging is repeated, the structural stability of the lithium transition metal oxide is achieved.

[0063] According to a specific embodiment, α can be 0 < α ≤ 0.01. Here, α refers to the substitution molar ratio of W to the M element in the lithium transition metal oxide represented by Chemical Formula 1. When W is substituted within the above range, the structural stability of the lithium transition metal oxide is improved. When the substitution molar ratio of W exceeds 0.01, the structural stability decreases due to the distortion of the crystal structure, and WO3 is formed as an impurity, which may lead to deterioration of the electrochemical properties.

[0064] According to a specific embodiment, β can be 0 < β ≤ 0.005. Here, α refers to the substitution molar ratio of Mg to the M element in the lithium transition metal oxide represented by Chemical Formula 1. When the substitution molar ratio of Mg satisfies the above range, the structural expansion of the lithium transition metal oxide in the charged state can be suppressed.

[0065] According to a specific embodiment, γ can be 0 < γ ≤ 0.005. Here, γ refers to the substitution molar ratio of Ti to the M element in the lithium transition metal oxide represented by Chemical Formula 1. When the substitution molar ratio of Ti satisfies the above range, the structural expansion of the lithium transition metal oxide in the charged state can be suppressed.

[0066] When W, Mg, and Ti substitute the lithium transition metal oxide in the above molar ratio, even when Li is detached in the charged state, the structural expansion of the crystal caused by the interaction between oxygen molecules in the lithium transition metal oxide is suppressed, so the structural stability and life characteristics are improved.

[0067] According to a specific embodiment, the sum of α, β, and γ can be 0 < α + β + γ ≤ 0.02. For example, the sum of α, β, and γ can be 0 < α + β + γ ≤ 0.016. When α + β + γ satisfies the above range, the structural stability of the lithium transition metal oxide is guaranteed. When α + β + γ exceeds 0.02, impurities are formed, which not only act as resistance when Li is detached, but are more likely to cause the collapse of the crystal structure during repeated charging.

[0068] According to a specific embodiment, in Chemical Formula 1, β and γ can be 0 < β ≤ 0.003 and 0 < γ ≤ 0.003, respectively.

[0069] For example, in Chemical Formula 1, β = γ can be. When β = γ, for example, when the molar ratio of Mg and Ti is the same, the charge in the lithium transition metal oxide is balanced during charging and discharging, the collapse of the crystal structure is inhibited, and the structural stability is improved. As a result, the life characteristics are improved.

[0070] According to a specific embodiment, a can be 0 < a ≤ 0.01. For example, it can be 0 < a ≤ 0.005, 0 < a ≤ 0.003, or 0 < a ≤ 0.001. Here, a refers to the substitution molar ratio of S and O elements in the lithium transition metal oxide represented by Chemical Formula 1.

[0071] Since a part of the oxygen element is replaced by S, the binding force with the transition metal increases, the transition of the crystal structure of the lithium transition metal oxide is inhibited, and as a result, the structural stability of the lithium transition metal is improved.

[0072] On the other hand, when the substitution molar ratio of S exceeds 0.01, due to the repulsive force of S anions, the crystal structure becomes unstable and the life characteristics are even reduced.

[0073] According to a specific embodiment, the lithium transition metal oxide can be a single particle. A single particle is a concept different from a secondary particle formed by aggregating plural particles or a particle formed by aggregating plural particles and coated on the periphery of the aggregate. When the lithium transition metal oxide has a single particle form, particle breakage can be prevented even at a high electrode density. Therefore, a high energy density of the positive electrode active material including the lithium transition metal oxide can be achieved. In addition, compared with a secondary particle formed by aggregating plural single particles, breakage during pressing of the single particle can be suppressed, so a high energy density can be achieved, and deterioration of the life due to particle breakage can be prevented.

[0074] According to a specific embodiment, the lithium transition metal oxide can have a single crystal. A single crystal is a concept different from a single particle. A single particle refers to a particle formed as one particle regardless of the type and number of internal crystals, and a single crystal refers to a particle having only one crystal inside. Such a single crystal lithium transition metal oxide not only has very high structural stability, but also conducts lithium ions more easily than polycrystals, and has excellent fast charging characteristics compared with polycrystalline active materials.

[0075] According to a specific embodiment, the positive electrode active material is a single crystal and a single particle. When the positive electrode active material is formed as a single crystal and a single particle, an electrode with stable structure and high density can be achieved, and the lithium secondary battery including the positive electrode active material can simultaneously have improved life characteristics and high energy density.

[0076] According to a specific embodiment, the lithium transition metal oxide may be represented by any one of Chemical Formulas 2 to 4 below.

[0077] <Chemical Formula 2>

[0078] Li 1-x' Na x' Ni y1' Co y2' Mn y3' W α' Mg β' Ti γ' O 2-a' S a'

[0079] <Chemical Formula 3>

[0080] Li 1-x” Na x” Ni y1” Co y2” Al y3” W α” Mg β” Ti γ” O 2-a” S a”

[0081] <Chemical Formula 4>

[0082] Li 1-x”' Na x”' Ni y1”' Co y2”' W α”' Mg β”' Ti γ”' O 2-a”' S a”'

[0083] According to a specific embodiment, in Chemical Formula 2,

[0084] it may be that 0 < x' ≤ 0.01, 0 < α' ≤ 0.01, 0 < β' ≤ 0.005, 0 < γ' ≤ 0.005, 0 < a' ≤ 0.01, 0 < α' + β' + γ' ≤ 0.02, 0.48 ≤ y1' < 1, 0 < y2' ≤ 0.2, 0 < y3' ≤ 0.3, and y1' + y2' + y3' + α' + β' + γ' = 1.

[0085] In Chemical Formula 3,

[0086] It can be 0 < x” ≤ 0.01, 0 < α” ≤ 0.01, 0 < β” ≤ 0.005, 0 < γ” ≤ 0.005, 0 < a” ≤ 0.01, 0 < α” + β” + γ” ≤ 0.02, 0.73 ≤ y1” < 1, 0 < y2” ≤ 0.2, 0 < y3” ≤ 0.05, and y1” + y2” + y3” + α” + β” + γ” = 1.

[0087] And in the chemical formula 4,

[0088] It can be 0 < x”' ≤ 0.01, 0 < α”' ≤ 0.01, 0 < β”' ≤ 0.005, 0 < γ”' ≤ 0.005, 0 < a”' ≤ 0.01, 0 < α”' + β”' + γ”' ≤ 0.02, 0.78 ≤ y1”' < 1, 0 < y2”' ≤ 0.2, and y1”' + y2”' + α”' + β”' + γ”' = 1.

[0089] For example, in the chemical formula 2, it can be 0 < β' ≤ 0.003, 0 < γ' ≤ 0.003, and 0 < α” + β' + γ' ≤ 0.016. In the chemical formula 3, it can be 0 < β” ≤ 0.003, 0 < γ” ≤ 0.003, and 0 < α” + β” + γ” ≤ 0.016. In the chemical formula 4, it can be 0 < β”' ≤ 0.003, 0 < γ”' ≤ 0.003, and 0 < α”' + β”' + γ”' ≤ 0.016.

[0090] The lithium transition metal oxide satisfying the above composition can stabilize the unstable Ni ions inside it and can have high energy density and long - life stability.

[0091] In the case of a general cathode active material including high - nickel - based lithium nickel cobalt manganese oxide, the stabilization of unstable Ni ions is necessary. And when W, Mg, and Ti are introduced into a part of the positions of transition metals in the crystal, the cathode active material can have an overall charge balance, thereby suppressing the oxidation of Ni(II) ions into unstable Ni(III) or Ni(IV) ions, and the unstable Ni(III) or Ni(IV) ions can be reduced to Ni(II). On the other hand, the conductivity loss caused by replacing a part of the transition metals with W, Mg, and Ti as hetero - elements is compensated by replacing a part of O with S, and since a part of Li is replaced by Na, the reduction of the conductivity of Li due to the structural deformation during the charge - discharge process is suppressed, so a cathode active material with a stable single - crystal structure, high capacity, and long life is obtained.

[0092] According to a specific embodiment, the average particle size (D of the lithium transition metal oxide 50) can be from 0.1 μm to 20 μm. For example, the average particle size (D 50 ) can be from 0.1 μm to 15 μm, from 0.1 μm to 10 μm, from 1 μm to 20 μm, from 5 μm to 20 μm, from 1 μm to 15 μm, from 1 μm to 10 μm, from 5 μm to 15 μm, or from 5 μm to 10 μm. When the average particle size of the lithium transition metal oxide is within the above ranges, the desired energy density per unit volume can be achieved. When the average particle size of the lithium transition metal oxide exceeds 20 μm, a sharp decrease in charge-discharge capacity may occur, and when the average particle size of the lithium transition metal oxide is equal to or less than 0.1 μm, it may be difficult to obtain the desired energy density per unit volume.

[0093] According to a specific embodiment, the lithium transition metal oxide may further include a cobalt-containing coating including a cobalt compound on its surface.

[0094] For example, the cobalt compound may include cobalt and one or more elements other than cobalt, and the content of cobalt in the compound may be higher than the content of other elements. The cobalt content in the cobalt compound may be equal to or greater than 30 mol%.

[0095] According to a specific embodiment, the coating may include a cobalt compound represented by the following Chemical Formula 5:

[0096] [Chemical Formula 5]

[0097] Li x1 Co y1 M' z1 O a1

[0098] In Chemical Formula 5,

[0099] M' is one or more transition metals other than Co, and

[0100] 0.5 < x1, 0 < y1 < 1, 0 < z1 < 1, and 1 < a1 < 3.

[0101] According to a specific embodiment, in Chemical Formula 5, it may be 0.3 ≤ y1 / (y1 + z1) < 1. For example, in Chemical Formula 5, it may be 0.3 < y1 / (y1 + z1) < 1, for example, 0.5 ≤ y1 / (y1 + z1) < 1.

[0102] According to another specific embodiment, the coating may include a cobalt compound represented by the following Chemical Formula 6:

[0103] [Chemical Formula 6]

[0104] Li x1 Co y1Ni z11 M2 z12 M3 z13 O2

[0105] In the chemical formula 6,

[0106] M2 and M3 are each independently one or more transition metals selected from Mn, B, Zr, P, Ca, Al, W, Mg, V, and Ti, and

[0107] 0.5 < x1 < 1.1, 0.3 ≤ y1 < 1, 0 < z11 ≤ 0.7, 0 ≤ z12 < 1, and 0 ≤ z13 < 1.

[0108] For example, in the chemical formula 6, it may be 0.8 ≤ x1 < 1.1, 0.5 ≤ y1 < 1, 0 < z11 < 0.5, 0 < z12 < 0.5, and 0 ≤ z13 < 1.

[0109] According to a specific embodiment, the coating may be configured to cover at least a portion of the lithium transition metal oxide. For example, the coating may completely cover the surface of the lithium transition metal oxide.

[0110] According to a specific embodiment, the coating may have a thickness equal to or less than 200 nm. When the thickness of the coating is equal to or less than 200 nm, the coating has the effect of sufficiently reducing residual lithium without acting as a barrier layer to lithium movement.

[0111] According to a specific embodiment, the cobalt-containing compound may have the same crystal structure as the lithium transition metal oxide. Therefore, lithium ions are easily mobile during charging and discharging, and excellent rate characteristics are obtained despite the presence of the coating.

[0112] For example, the cobalt-containing compound may include a layered structure. For example, the cobalt-containing compound may only have a layered structure. Therefore, even during charging and discharging, a stable crystal structure is maintained to form a stable coating, thereby suppressing direct contact between the lithium transition metal oxide and the electrolyte and suppressing side reactions between the lithium transition metal oxide and the electrolyte.

[0113] Hereinafter, a method for preparing a positive electrode active material according to one aspect will be described in detail.

[0114] A method for preparing a positive electrode active material according to a specific embodiment includes: a step of preparing lithium transition metal oxide particles, in which a part of Li is substituted by Na and includes Ni and Co atoms; a step of mixing the lithium transition metal oxide particles with a compound containing a Co element to obtain a precursor of the positive electrode active material; and a step of firing the precursor of the positive electrode active material to obtain positive electrode active material particles, wherein the positive electrode active material particles include a cobalt-containing coating on their surface and include a concentration gradient region in which the concentration of Co atoms decreases in the direction from the surface of the positive electrode active material particles to the center of the particles.

[0115] According to a specific embodiment, the step of preparing the lithium transition metal oxide particles may include:

[0116] a step of obtaining a precursor of the lithium transition metal oxide by mixing a compound containing a Li element, a compound containing a Na element, a compound containing a W element, a compound containing a Mg element, a compound containing a Ti element, a compound containing a M element, and a compound containing a S element; and a step of performing heat treatment on the precursor of the lithium transition metal oxide to obtain lithium transition metal oxide particles represented by Chemical Formula 1 below:

[0117] <Chemical Formula 1>

[0118] Li 1-x Na x M 1-(α+β+γ) W α Mg β Ti γ O 2-a S a

[0119] In Chemical Formula 1,

[0120] M is one or more elements selected from alkali metal elements, alkaline earth metal elements, transition metal elements, post-transition metals, and non-metal elements other than W, Mg, Ti, Na, and S,

[0121] 0 < x ≤ 0.01, 0 < α ≤ 0.01, 0 < β ≤ 0.005, 0 < γ ≤ 0.005, 0 < a ≤ 0.01, and 0 < α + β + γ ≤ 0.02.

[0122] The specific description of Chemical Formula 1 is as described above.

[0123] The mixing step includes the mechanical mixing of the compound containing a specific element. This mechanical mixing can be performed as a dry process. Mechanical mixing refers to the process of crushing and mixing the materials to be mixed by applying mechanical force to form a homogeneous mixture. Mechanical mixing can be performed using, for example, mixing devices such as ball mills with chemically inert beads, planetary mills, stirred ball mills, and vibrating mills. In this case, to maximize the mixing effect, small amounts of alcohols such as ethanol and higher fatty acids such as stearic acid can be selectively added.

[0124] The mechanical mixing can be carried out in an oxidizing atmosphere to prevent the reduction of transition metals in the transition metal source (e.g., Ni compound) to achieve structural stability of the active material.

[0125] The lithium-containing compound may include, but is not limited to, lithium hydroxides, oxides, nitrides, carbonates, or combinations thereof. For example, the lithium precursor may be LiOH or Li₂CO₃.

[0126] The Na-containing compound may include, but is not limited to, Na hydroxides, oxides, nitrides, carbonates, or combinations thereof. For example, the Na-containing compound may be NaOH, Na₂CO₃, or combinations thereof.

[0127] The W-containing compound may include, but is not limited to, W hydroxides, oxides, nitrides, carbonates, or combinations thereof. For example, the W-containing compound may be W(OH)6, WO3, or combinations thereof.

[0128] The Mg-containing compound may include, but is not limited to, Mg hydroxides, oxides, nitrides, carbonates, or combinations thereof. For example, the Mg-containing compound may include Mg(OH)₂, MgCO₃, or combinations thereof.

[0129] The Ti-containing compound may include, but is not limited to, Ti hydroxides, oxides, nitrides, carbonates, or combinations thereof. For example, the Ti-containing compound may be Ti(OH)₂, TiO₂, or combinations thereof.

[0130] The compound containing element M may include, but is not limited to, hydroxides, oxides, nitrides, carbonates, or combinations thereof of one or more elements selected from alkali metals, alkaline earth metals, transition metals, metalloids, and nonmetals other than W, Mg, Ti, Na, and S, such as alkali metals, alkaline earth metals, transition metals, metalloids, and nonmetals. For example, the compound containing element M may be Ni. 0.8 Co 0.1 Mn0.1 (OH)2, Ni 0.9 Co 0.05 Al 0.05 (OH)2 or Ni 0.9 Co 0.1 (OH)2.

[0131] The sulfur-containing compound may include, but is not limited to, sulfur hydroxides, oxides, nitrides, carbonates, ammonium oxides, or combinations thereof. For example, the sulfur-containing compound may be (NH4)2S.

[0132] According to one specific embodiment, the step of obtaining the precursor of the positive electrode active material may further include a Co-containing compound as a mixed material.

[0133] The Co-containing compound is a compound capable of providing Co, and includes Co hydroxides, oxides, nitrides, carbonates, acetates, or combinations thereof. For example, the Co-containing compound may be cobalt acetate.

[0134] In the step of preparing the lithium transition metal oxide particles, the heat treatment step may include a first heat treatment step and a second heat treatment step. The first and second heat treatment steps may be performed continuously, or there may be a rest period after the first heat treatment step. Furthermore, the first and second heat treatment steps may be performed in the same chamber or in different chambers.

[0135] The heat treatment temperature in the first heat treatment step can be higher than the heat treatment temperature in the second heat treatment step.

[0136] The first heat treatment step can be performed at a heat treatment temperature of 800°C to 1200°C. The heat treatment temperature can be, for example, 850°C to 1200°C, 860°C to 1200°C, 870°C to 1200°C, 880°C to 1200°C, 890°C to 1200°C, or 900°C to 1200°C, but is not limited thereto, and includes all ranges formed by selecting any two points within the above ranges.

[0137] The second heat treatment step can be performed at a heat treatment temperature of 700°C to 800°C. The heat treatment temperature can be 710°C to 800°C, 720°C to 800°C, 730°C to 800°C, 740°C to 800°C, 750°C to 800°C, 700°C to 780°C, 700°C to 760°C, 700°C to 750°C, or 700°C to 730°C, but is not limited to these, and includes all ranges formed by selecting any two points within the above ranges.

[0138] According to one specific embodiment, the heat treatment time in the first heat treatment step may be shorter than the heat treatment time in the second heat treatment step.

[0139] For example, the heat treatment time in the first heat treatment step can be 3 to 5 hours, 4 to 5 hours, or 3 to 4 hours, but is not limited to this, and includes all ranges formed by selecting any two points within the above range.

[0140] For example, the heat treatment time in the second heat treatment step can be 10 to 20 hours or 10 to 15 hours, but is not limited to this, and includes all ranges formed by selecting any two points within the above range.

[0141] The first heat treatment step may include performing heat treatment at a heat treatment temperature of 800°C to 1200°C for 3 to 5 hours.

[0142] The second heat treatment step may include performing heat treatment at a heat treatment temperature of 700°C to 800°C for 10 to 20 hours.

[0143] In the first heat treatment step, lithium transition metal oxide forms a layered structure of positive electrode active material, while simultaneously inducing particle growth to form a single crystal. In this first heat treatment step, it is believed that the rapid growth of each primary particle in the secondary particle form of the lithium transition metal oxide leads to the inability to withstand inter-particle stress, causing the internal structure of the particles to be exposed and fuse together, thereby forming a single-crystal positive electrode active material for secondary batteries. The second heat treatment step is performed at a temperature lower than that in the first heat treatment step for an extended period to improve the crystallinity of the layered structure formed in the first heat treatment step. Through the first and second heat treatment steps, a single-phase, single-crystal, single-particle high-nickel-based positive electrode active material can be obtained.

[0144] According to one embodiment, the firing step can be performed at a temperature of 500°C to 900°C. For example, the firing step can be performed at a temperature of 600°C to 900°C. According to one embodiment, the firing step can be performed for 1 hour to 6 hours. For example, the firing step can be performed for 2 hours to 4 hours.

[0145] According to one specific embodiment, the firing step can be carried out at a temperature of 500°C to 900°C for 1 hour to 6 hours. By firing the precursor of the positive electrode active material at the stated firing temperature and time, a concentration gradient region in which Co atoms have a concentration gradient can be formed, and a positive electrode active material with a cobalt-containing coating formed on its surface can be obtained.

[0146] During the firing process used to form the cobalt-containing coating, the transition metal elements, other than Co atoms, included in the lithium transition metal oxide particles can diffuse to form composite particles with Co. For the description of these composite particles, see the above descriptions of Chemical Formulas 5 and 6.

[0147] According to one specific embodiment, the lithium transition metal oxide prepared by the preparation method is a single crystal or a single particle, and the single crystal may have a layered structure. For example, the average particle size of the lithium transition metal oxide may be from 0.1 μm to 20 μm.

[0148] Furthermore, in the lithium transition metal oxide prepared by the aforementioned method for preparing the positive electrode active material, W, Mg, and Ti elements replace the M element sites in the structure, S element replaces the O sites, and Na element replaces the Li sites, thereby suppressing the existing Ni 2+ The oxidation of Ni further induces unstable Ni 3+ Ions reduced to Ni 2+ Ions are used to obtain lithium transition metal oxides with structural stability and high density. Additionally, reduced Ni... 2+ ions and Li + The ions have similar ionic radii, which promotes Li / Ni disordering, allowing Ni ions to fill the empty lattice when Li detaches, thereby improving the structural stability of the crystal.

[0149] Furthermore, due to the inclusion of a Co concentration gradient region and a Co-containing coating on the surface of the positive electrode active material, the capacity and lifetime characteristics are improved compared to existing cases where only the transition metal is additionally substituted.

[0150] According to another aspect, a positive electrode comprising the above-described positive electrode active material is provided.

[0151] According to another aspect, a lithium secondary battery is provided, comprising a positive electrode, a negative electrode, and an electrolyte.

[0152] The positive electrode and the lithium secondary battery including it can be manufactured by the method described below.

[0153] First, prepare the positive electrode.

[0154] For example, a positive electrode active material composition is prepared, which contains the aforementioned positive electrode active material, conductive material, binder, and solvent. The positive electrode active material composition is then directly coated onto a metal current collector to manufacture a positive electrode plate. Alternatively, the positive electrode active material composition can be cast onto a separate support, and then a film peeled from the support is laminated onto the metal current collector to prepare a positive electrode plate. The positive electrode is not limited to the above methods and can also be prepared using methods other than those described above.

[0155] As the conductive material, graphite such as natural graphite or artificial graphite; carbon black; conductive tubes such as carbon nanotubes; conductive whiskers such as fluorocarbons, zinc oxide, potassium titanate; conductive metal oxides such as titanium oxide, etc., can be used, but are not limited thereto, and all materials that can be used as conductive materials in this art can be used.

[0156] For the adhesive, vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polytetrafluoroethylene, mixtures thereof, metal salts, or styrene-butadiene rubber-based polymers can be used, but are not limited to these, and all materials that can be used as adhesives in this art can be used. Other examples of adhesives include lithium salts, sodium salts, calcium salts, or Na salts of the above polymers.

[0157] The solvent may be N-methylpyrrolidone, acetone or water, but is not limited thereto, and may be any material that is available in this art.

[0158] The contents of the positive electrode active material, conductive material, binder, and solvent are at levels commonly used in lithium batteries. Depending on the application and composition of the lithium battery, one or more of the conductive material, binder, and solvent may be omitted.

[0159] Next, prepare the negative electrode.

[0160] For example, a negative electrode active material composition is prepared by mixing a negative electrode active material, a conductive material, a binder, and a solvent. The negative electrode active material composition is then directly coated onto a metal current collector with a thickness of 3 μm to 500 μm and dried to prepare a negative electrode plate. Alternatively, the negative electrode active material composition can be cast onto a separate support, and then a film peeled from the support is laminated onto the metal current collector to prepare a negative electrode plate.

[0161] As a negative electrode current collector, it is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity. For example, copper, nickel, and copper with carbon-treated surfaces can be used.

[0162] For the negative electrode active material, any material that can be used in this art can be used. For example, the negative electrode active material may include one or more selected from lithium metal, lithium alloyable metals, transition metal oxides, non-transition metal oxides, and carbon-based materials.

[0163] For example, the metal that can be alloyed with lithium can be Si, Sn, Al, Ge, Pb, Bi, Sb, and Si-Y alloy (where the can be an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, or a combination thereof, and is not Si), Sn-Y alloy (where Y can be an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, or a combination thereof, and is not Sn), etc. The element Y can be 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, or Te.

[0164] For example, the transition metal oxide can be lithium titanium oxide, vanadium oxide, lithium vanadium oxide, etc.

[0165] For example, the transition metal oxide can be SnO2 and SiO x (0 < x < 2), etc.

[0166] The carbon-based material can be crystalline carbon, amorphous carbon, or a mixture thereof. The crystalline carbon can be amorphous, plate-shaped, flaky, spherical, or fibrous graphite, such as natural graphite or artificial graphite, and the amorphous carbon can be soft carbon (low-temperature fired carbon), hard carbon, mesophase pitch carbide, or fired coke, etc.

[0167] In the negative electrode active material composition, for the conductive material, binder, and solvent, the same materials as those in the case of the positive electrode active material composition can be used.

[0168] The contents of the negative electrode active material, conductive material, binder, and solvent are at levels commonly used in lithium batteries. Depending on the use and composition of the lithium battery, one or more of the conductive material, binder, and solvent can be omitted.

[0169] Then, a separator to be inserted between the positive electrode and the negative electrode is prepared.

[0170] For the separator, any separator commonly used in lithium batteries can be used. A separator with low resistance to ion movement of the electrolyte and excellent electrolyte impregnation capability can be used. The separator can be a single membrane or a multilayer membrane, for example, selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene (PTFE), or combinations thereof, and can be in the form of nonwoven or woven fabric. Additionally, mixed multilayer membranes such as polyethylene / polypropylene two-layer membranes, polyethylene / polypropylene / polyethylene three-layer membranes, and polypropylene / polypropylene / polypropylene three-layer membranes can be used. For example, in lithium-ion batteries, a rollable separator of polyethylene, polypropylene, etc., can be used, and in lithium-ion polymer batteries, a separator with excellent electrolyte impregnation capability can be used. For example, the separator can be prepared according to the following method.

[0171] A membrane composition is prepared by mixing a polymer resin, filler, and solvent. The membrane composition can be directly coated onto an electrode and dried to form a membrane. Alternatively, after casting the membrane composition onto a support and drying it, a membrane film peeled from the support can be laminated onto the electrode to form a membrane.

[0172] There are no particular limitations on the polymer resin used to prepare the diaphragm; any material used as an electrode plate binder can be used. For example, vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, or mixtures thereof can be used.

[0173] Next, prepare the electrolytes.

[0174] For example, the electrolyte can be an organic electrolyte. Alternatively, the electrolyte can be a solid. For example, the electrolyte can be boron oxide or lithium oxynitride, but is not limited to these, and all materials that can be used as solid electrolytes in this art can be used. The solid electrolyte can be formed on the negative electrode using methods such as sputtering.

[0175] For example, organic electrolytes can be prepared by dissolving lithium salts in organic solvents.

[0176] For the aforementioned organic solvent, all materials that can be used as organic solvents in this technical field can be used. For example, the organic solvent can be cyclic carbonates such as propylene carbonate, ethylene carbonate, fluoroethylene carbonate, butenyl carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, isopropyl methyl carbonate, dipropyl carbonate, and dibutyl carbonate; esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone; ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane, and 2-methyltetrahydrofuran; nitriles such as acetonitrile; and amides such as dimethylformamide. The above-mentioned organic solvents can be used alone or in combination. For example, a solvent that mixes cyclic carbonates and chain carbonates can be used.

[0177] Alternatively, gel-like polymer electrolytes can be used, such as polymer electrolytes impregnated with polyethylene oxide, polyacrylonitrile, etc., or LiI, Li3N, Li x Ge y P z S α Li x Ge y P z S α X δ Inorganic solid electrolytes such as (X = F, Cl and Br).

[0178] For the lithium salt, all materials that can be used as lithium salts in this art can be used. For example, the lithium salt is LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiAlO2, LiAlCl4, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2 (where x and y are natural numbers), LiCl, LiI, or mixtures thereof.

[0179] like Figure 11 As shown, the lithium battery 1 includes a positive electrode 3, a negative electrode 2, and a separator 4. The positive electrode 3, negative electrode 2, and separator 4 are wound or folded to be housed in a battery casing 5. Next, an organic electrolyte is injected into the battery casing 5, and the battery casing 5 is sealed with a cap assembly 6, thereby completing the lithium battery 1. The battery casing 5 can be cylindrical, square, pouch-shaped, coin-shaped, thin-film, etc. For example, the lithium battery 1 can be a thin-film battery. The lithium battery 1 can be a lithium-ion battery.

[0180] A separator can be placed between the positive and negative electrodes to form a battery structure. After the battery structure is stacked into a two-porous structure, an organic electrolyte is impregnated, and the resulting product is packaged in a bag and sealed to complete the lithium-ion polymer battery.

[0181] Furthermore, multiple battery structures can be stacked to form a battery pack, which can be used in all devices requiring high capacity and high power. For example, the battery pack can be used in laptops, smartphones, and electric vehicles.

[0182] Furthermore, due to its excellent lifespan and high-speed characteristics, the lithium battery can be used in electric vehicles (EVs). For example, it can be used in hybrid vehicles, such as plug-in hybrid electric vehicles (PHEVs). Additionally, the lithium battery can be used in applications requiring large amounts of energy storage. For example, it can be used in electric bicycles, power tools, and energy storage systems.

[0183] The present invention is described in more detail through the following preparation examples, embodiments, and comparative examples. However, the embodiments are for illustrative purposes only, and the scope of the invention is not limited thereto.

[0184] (Preparation of positive electrode active materials)

[0185] Example 1

[0186] 100g of Ni 0.8 Co 0.1 Mn 0.1 (OH)₂, 41.8 g of Li₂CO₃, 3.0 g of WO₃, 0.27 g of MgCO₃, 0.24 g of TiO₂, 0.45 g of NaOH, and 0.75 g of (NH₄)₂S were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 1000 °C for 4 hours and then at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0187] Subsequently, 2g of cobalt acetate and 100g of the lithium transition metal oxide particles were mechanically mixed for 30 minutes, and the mixed powder was calcined at 800°C for 3 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0188] Example 2

[0189] 100g of Ni 0.90 Co 0.05 Al 0.05(OH)₂, 42.4 g of Li₂CO₃, 3.0 g of WO₃, 0.27 g of MgCO₃, 0.24 g of TiO₂, 0.45 g of NaOH, and 0.75 g of (NH₄)₂S were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 970 °C for 4 hours and then at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0190] Subsequently, 2g of cobalt acetate and 100g of the lithium transition metal oxide particles were mechanically mixed for 30 minutes, and the mixed powder was calcined at 800°C for 3 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0191] Example 3

[0192] 100g of Ni 0.9 Co 0.1 (OH)₂, 42.0 g of Li₂CO₃, 3.0 g of WO₃, 0.27 g of MgCO₃, 0.24 g of TiO₂, 0.45 g of NaOH, and 0.75 g of (NH₄)₂S were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 970 °C for 4 hours and then at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0193] Subsequently, 2g of cobalt acetate and 100g of the lithium transition metal oxide particles were mechanically mixed for 30 minutes, and the mixed powder was calcined at 800°C for 3 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0194] Comparative Example 1

[0195] 100g of Ni 0.8 Co 0.1 Mn 0.1 (OH)₂, 41.8 g of Li₂CO₃, 3.0 g of WO₃, 0.27 g of MgCO₃, 0.24 g of TiO₂, 0.45 g of NaOH, and 0.75 g of (NH₄)₂S were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 1,000 °C for 4 hours and then at 700 °C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0196] Comparative Example 2

[0197] 100g of Ni 0.8 Co 0.1 Mn 0.1(OH)₂, 41.8 g of Li₂CO₃, 0.27 g of MgCO₃, and 0.24 g of TiO₂ were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 1,000 °C for 4 hours and then at 700 °C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0198] Comparative Example 3

[0199] 100g of Ni 0.8 Co 0.1 Mn 0.1 (OH)₂, 41.8 g of Li₂CO₃, 0.27 g of MgCO₃, and 0.45 g of NaOH were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 1,000 °C for 4 hours and then at 700 °C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0200] Comparative Example 4

[0201] 100g of Ni 0.8 Co 0.1 Mn 0.1 (OH)₂, 41.8 g of Li₂CO₃, 3.0 g of WO₃, and 0.27 g of MgCO₃ were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 1,000 °C for 4 hours and then at 700 °C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0202] Comparative Example 5

[0203] 100g of Ni 0.8 Co 0.1 Mn 0.1 (OH)₂, 41.8 g of Li₂CO₃, and 0.75 g of (NH₄)₂S were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 1,000 °C for 4 hours and then at 700 °C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0204] Comparative Example 6

[0205] 100g of Ni 0.8 Co 0.1 Mn 0.1 (OH)2 and 41.8 g of Li2CO3 were mechanically mixed for about 15 minutes. The mixed powder was then heat-treated at 1,000 °C for 4 hours and then at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0206] Next, 2g of cobalt acetate and the lithium transition metal oxide particles were mechanically mixed and calcined at 800°C for 3 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0207] Comparative Example 7

[0208] 100g of Ni 0.8 Co 0.1 Mn 0.1 (OH)2, 41.8 g of Li2CO3, 3.0 g of WO3, and 0.27 g of MgCO3 were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 1,000 °C for 4 hours and then at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0209] Next, 2g of cobalt acetate and the lithium transition metal oxide particles were mechanically mixed and calcined at 800°C for 3 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0210] Comparative Example 8

[0211] 100g of Ni 0.8 Co 0.1 Mn 0.1 (OH)₂, 41.8 g of Li₂CO₃, 3.0 g of WO₃, 0.27 g of MgCO₃, 0.24 g of TiO₂, 0.45 g of NaOH, and 0.75 g of (NH₄)₂S were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 1,000 °C for 4 hours and then at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0212] Next, 2g of cobalt acetate was dissolved in ethanol, and then the lithium transition metal oxide particles were added and stirred for 30 minutes. The stirred solution was stirred again at 80°C for 1 hour to evaporate the ethanol solvent, and the mixed powder was calcined at 800°C to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0213] Comparative Example 9

[0214] 100g of Ni 0.9 Co 0.05 Al 0.05(OH)₂, 42.4 g of Li₂CO₃, 3.0 g of WO₃, 0.27 g of MgCO₃, 0.24 g of TiO₂, 0.45 g of NaOH, and 0.75 g of (NH₄)₂S were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 970 °C for 4 hours and then at 700 °C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0215] Comparative Example 10

[0216] 100g of Ni 0.9 Co 0.1 (OH)₂, 42.0 g of Li₂CO₃, 3.0 g of WO₃, 0.27 g of MgCO₃, 0.24 g of TiO₂, 0.45 g of NaOH, and 0.75 g of (NH₄)₂S were mechanically mixed for approximately 15 minutes. The mixed powder was then heat-treated at 970 °C for 4 hours and then at 700 °C for 10 hours to obtain lithium transition metal oxide particles. The specific composition of the obtained positive electrode active material can be confirmed from Table 1.

[0217] (Preparation of a half-cell)

[0218] Example 4

[0219] A slurry was prepared by mixing the positive electrode active material, conductive material, and binder obtained in Example 1 at a weight ratio of 94:3:3. Here, carbon black was used as the conductive material, and polyvinylidene fluoride (PVDF) was dissolved in N-methyl-2-pyrrolidone solvent to serve as the binder.

[0220] The slurry was uniformly coated onto an Al current collector and dried at 110°C for 2 hours to prepare the positive electrode. The loading on the electrode plate was 11.0 mg / cm³. 2 And the electrode density is 3.6 g / cc.

[0221] The prepared positive electrode was used as the working electrode, lithium foil was used as the counter electrode, and a liquid electrolyte was used, in which lithium salt was added to a mixed solvent of EC / EMC / DEC in a volume ratio of 3 / 4 / 3 to achieve a LiPF6 concentration of 1.3M. The CR2032 half cell was prepared according to a generally known process.

[0222] Examples 5 to 6

[0223] Half-cells were prepared in the same manner as in Example 4, except that the positive electrode active materials obtained from Examples 2 and 3 were used instead of the positive electrode active materials obtained from Example 1.

[0224] Comparative Examples 11 to 20

[0225] Half-cells were prepared in the same manner as in Example 4, except that the positive electrode active materials obtained from Comparative Examples 1 to 10 were used instead of the positive electrode active materials obtained from Example 1.

[0226] Table 1

[0227]

[0228] Evaluation Example 1: Evaluation of the composition of the positive electrode active material

[0229] The positive electrode active materials synthesized in Example 1 and Comparative Example 1 were analyzed by inductively coupled plasma (ICP) using a 700-ES (Varian) instrument. The results are shown in Table 2 below.

[0230] Referring to Table 2, based on the ICP analysis results of Comparative Example 1 and Example 1, it can be seen that when a Co concentration gradient region and a Co-containing coating are introduced onto the surface of the single-crystal Ni-based cathode active material, the Co proportion in the cathode active material increases by 1.6 mol% due to Co replacing other transition metals, while the molar number of other transition metals decreases. Furthermore, during ICP analysis, due to the influx of trace amounts of oxygen and carbon dioxide from the air, even when analyzed in a vacuum, it is difficult to analyze the stoichiometry of oxygen contained in the material.

[0231] Table 2

[0232] (molar percentage) Li Na Ni Co Mn W Mg Ti S Comparative Example 1 99 1 79.1 10.3 9.0 1 0.3 0.3 0.1 Example 1 99 1 78.2 11.9 8.3 1 0.3 0.3 0.1

[0233] Evaluation Example 2: Evaluation of Particle Size of Positive Electrode Active Material

[0234] Electron microscopy (SEM) images of the positive electrode active materials synthesized in Example 1 and Comparative Example 1 were obtained using a Verios 460 (FEI) device and displayed. Figure 1 In addition, particle size was measured using a Cilas 1090 (Scinco) instrument, and the results are shown in Table 3 and... Figure 2 middle.

[0235] Refer to Table 3 and Figure 1 and Figure 2 Although a Co concentration gradient region and a Co-containing coating were introduced into the single-particle positive electrode active material of Example 1, no significant particle size change was observed compared with the single-particle positive electrode active material of Comparative Example 1, and it was confirmed that the Co-containing compound existed on the surface of the positive electrode active material of Example 1 in particles of hundreds of nanometers in size.

[0236] Table 3

[0237] <![CDATA[D 10 (μm)]]> <![CDATA[D 50 (μm)]]> <![CDATA[D 90 (μm)]]> Comparative Example 1 3.2 6.4 10.8 Example 1 3.3 6.5 10.9

[0238] Evaluation Example 3: Evaluation of the concentration gradient region of the positive electrode active material

[0239] For the positive electrode active materials obtained in Example 1 and Comparative Example 1, high-resolution transmission electron microscopy (HR-TEM) was used to take images, and energy dispersive X-ray spectroscopy (EDX) was performed for analysis. The results are shown in Tables 4 and 5 below. Figure 3 and Figure 4 middle.

[0240] Table 4

[0241] Location Ni (mol%) Co (mol%) Mn (mol%) 1 79.1 9.9 10.0 2 79.5 10.8 9.7 3 79.7 10.3 9.8

[0242] Table 5

[0243] Location Ni (mol%) Co (mol%) Mn (mol%) 1 66.3 23.7 10.0 2 72.0 18.3 9.7 3 74.6 15.6 9.8 4 76.4 13.5 10.1 5 78.4 11.9 9.7 6 80.3 9.8 9.9

[0244] Refer to Table 4 and Figure 3 It can be seen that the concentrations of transition metals such as Ni, Co, and Mn in the positive electrode active material remain basically constant in the direction of the surface and the center of the positive electrode active material.

[0245] Refer to Table 5 and Figure 4 It was confirmed that in the positive electrode active material, the Co concentration decreases from the surface towards the center, while the Ni concentration increases in the opposite direction. It was also found that the Co concentration gradient layer is approximately 500 nm. Without being bound by any specific theory, compared to nickel ions, cobalt ions in transition metals contribute to the structural stability of positive electrode active materials with a layered structure. Therefore, it is believed that when an excess of relatively stable cobalt is included on the surface of the positive electrode active material, the structural stability of the positive electrode active material during charging and discharging is improved, and its long-lifetime characteristics are enhanced.

[0246] Evaluation Example 4: Evaluation of the coating of the positive electrode active material

[0247] The positive electrode active material obtained in Example 1 was photographed using HR-TEM and analyzed by EDX. The results are shown in Table 6 below. Figure 5a , Figure 5b , Figure 6a and Figure 6b .

[0248] Table 6

[0249] Location Ni (mol%) Co (mol%) Mn (mol%) 1 61.1 31.4 7.5 2 61.2 30.7 8.1

[0250] Refer to Table 6 and Figure 5a and Figure 5b Based on the results of EDX analysis of the particle surface in Example 1, a cobalt-containing coating of approximately 200 nm or smaller was confirmed on the surface of the lithium transition metal oxide particles. Furthermore, based on the analysis of the cobalt-containing compounds included in the coating, cobalt was confirmed as a main element comprising 30 mol% or more.

[0251] In addition, refer to Figure 6a and Figure 6b It was confirmed that the lithium transition metal oxide particles and the cobalt-containing coating in the positive electrode active material of Example 1 both have a layered structure. Since the lithium transition metal oxide particles and the coating surrounding them have the same crystalline structure, lithium ions can move easily and maintain excellent velocity characteristics even in the presence of the coating.

[0252] Furthermore, due to the structural stability of the layered structure, the coating functions as a protective film to protect lithium transition metal oxide particles from the influence of the electrolyte, thus improving lifetime stability during electrochemical evaluation.

[0253] Evaluation Example 5: Evaluation of ambient temperature lifespan

[0254] The half-cells prepared in Examples 4 to 6 and Comparative Examples 11 to 20 were left to stand for 10 hours, then charged at 0.1C in constant current (CC) mode to 4.3V, and then charged in constant voltage (CV) mode until the current reached the corresponding 0.05C. Next, the half-cells were discharged at 0.1C in CC mode to 3.0V, thereby completing the formation process.

[0255] Next, the half-cell was charged at room temperature (25°C) in 0.5C CC mode to 4.3V, and then charged in CV mode until the current reached the corresponding 0.05C. Then, the half-cell was discharged at 1C in CC mode to 3.0V, and this process was repeated 50 times.

[0256] The capacity retention rate after 50 charge-discharge cycles relative to the initial capacity is calculated, and the results are shown in Table 7 below. Furthermore, a graph showing the capacity retention rate based on the cycles is shown in... Figures 7 to 10 middle.

[0257] Table 7

[0258]

[0259] Refer to Table 7 and Figure 7 Based on the room-temperature lifetime evaluation results of Example 4 and Comparative Example 11, in the case including the concentration gradient region and the cobalt-containing coating (Example 4), the lifetime retention rate after 50 cycles was improved by approximately 2% compared to the case without the coating and concentration gradient region. This is likely because the cobalt-containing coating with a layered structure not only improves structural stability but also suppresses the degradation of the positive electrode active material by preventing direct contact between the electrolyte and lithium transition metal oxide particles. Furthermore, compared to Comparative Examples 11 to 15, which do not include the concentration gradient region and use positive electrode active materials without the introduction of one or more elements of Na, W, Mg, Ti, and S, the half-cell of Example 4 showed a lifetime retention rate improved by up to approximately 8% after 50 cycles. Furthermore, compared to Comparative Examples 16 and 17, which included a concentration gradient region and a cobalt-containing coating but did not incorporate one or more of the elements Na, W, Mg, Ti, and S, the half-cell of Example 4 showed an improvement of up to about 4% in lifetime retention after 50 cycles, and compared to Comparative Example 18, which only included a concentration gradient region but did not include a cobalt-containing coating, the half-cell of Example 4 showed an improvement of 1% in lifetime retention after 50 cycles.

[0260] These data confirm that optimal lifetime characteristics were achieved when Na, W, Mg, Ti, and S elements were applied simultaneously, along with concentration gradient regions and coatings including cobalt-containing compounds.

[0261] Furthermore, it was confirmed that, in the case of NCA (nickel-cobalt-manganese) based cathode active materials and NC (nickel-cobalt) based cathode active materials, introducing a concentration gradient region and a coating including a cobalt-containing compound can improve lifetime characteristics by approximately 4%.

[0262] The specific embodiments of the present invention have been described above with reference to the accompanying drawings and examples. However, this is only one example, and those skilled in the art will understand that various modifications and other equivalent embodiments can be made therefrom. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A positive electrode active material comprising: lithium transition metal oxide particles represented by the following Chemical Formula 2 having a layered structure; and a cobalt-containing coating layer provided on a surface of the lithium transition metal oxide particles, wherein the lithium transition metal oxide particles have a concentration gradient region in which a concentration of Co atoms decreases in a direction from the surface of the lithium transition metal oxide particles toward a center of the lithium transition metal oxide particles, <Chemical Formula 2> Li 1-x' Na x' Ni y1' Co y2' Mn y3' W α' Mg β' Ti γ' O 2-a' S a' In the Chemical Formula 2, 0 < x' ≤ 0.01, 0 < α' ≤ 0.01, 0 < β' ≤ 0.005, 0 < γ' ≤ 0.005, 0 < a' ≤ 0.01, 0 < α' + β' + γ' ≤ 0.02, 0.48 ≤ y1' < 1, 0 < y2' ≤ 0.2, 0 < y3' ≤ 0.3, and y1' + y2' + y3' + α' + β' + γ' = 1, and β' = γ', wherein the coating layer includes a cobalt-containing compound represented by the following Chemical Formula 5: [Chemical Formula 5] Li x1 Co y1 M' z1 O a1 in the Chemical Formula 5, M' is one or more transition metals other than Co, and 0.5 < x1, 0 < y1 < 1, 0 < z1 < 1, 1 < a1 < 3, and 0.5 ≤ y1 / (y1 + z1) < 1.

2. The positive electrode active material according to claim 1, wherein in the concentration gradient region, a concentration of Ni atoms increases in a direction from the surface of the lithium transition metal oxide particles toward the center of the lithium transition metal oxide particles.

3. The positive electrode active material according to claim 1, wherein the concentration gradient region includes a region extending to a distance of 500 nm in a direction from the surface of the lithium transition metal oxide particles toward the center of the lithium transition metal oxide particles.

4. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide is a single particle.

5. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide is a single crystal.

6. The positive electrode active material according to claim 1, wherein in the Chemical Formula 2, 0 < β' ≤ 0.003, 0 < γ' ≤ 0.003, and 0 < α' + β' + γ' ≤ 0.

016.

7. The positive electrode active material according to claim 1, wherein The average particle diameter D of the lithium transition metal oxide is 0.1 μm to 20 μm. 50 is 0.1 μm to 20 μm.

8. The positive electrode active material according to claim 1, wherein the coating layer includes a cobalt-containing compound represented by the following Chemical Formula 6: [Chemical Formula 6] Li x1 Co y1 Ni z11 M2 z12 M3 z13 O2 in the Chemical Formula 6, M2 and M3 are each independently one or more transition metals selected from Mn, B, Zr, P, Ca, Al, W, Mg, V, and Ti, and 0.5 < x1 < 1.1, 0.3 ≤ y1 < 1, 0 < z11 ≤ 0.7, 0 ≤ z12 < 1, and 0 ≤ z13 < 1.

9. A method for producing a positive electrode active material, comprising: a step of preparing lithium transition metal oxide particles in which a part of Li is substituted with Na, a part of O is substituted with S, has a layered structure, and includes Ni and Co atoms; a step of mixing the lithium transition metal oxide particles with a compound containing Co element to obtain a precursor of a positive electrode active material; and a step of firing the precursor of the positive electrode active material to obtain positive electrode active material particles. wherein the positive electrode active material particle includes a coating layer containing cobalt on a surface thereof, and includes a concentration gradient region in which a concentration of Co atoms decreases in a direction from the surface of the positive electrode active material particle toward a center of the positive electrode active material particle, wherein the step of preparing the lithium transition metal oxide particle includes: a step of obtaining a precursor of a lithium transition metal oxide by mixing a compound containing an element Li, a compound containing an element Na, a compound containing an element W, a compound containing an element Mg, a compound containing an element Ti, a compound containing elements Ni, Co, and Mn, and a compound containing an element S; and a step of heat-treating the precursor of the lithium transition metal oxide to obtain a positive electrode active material including a lithium transition metal oxide particle represented by the following Chemical Formula 2: <Chemical Formula 2> Li 1-x' Na x' Ni y1' Co y2' Mn y3' W α' Mg β' Ti γ' O 2-a' S a' In the Chemical Formula 2, 0 < x' ≤ 0.01, 0 < α' ≤ 0.01, 0 < β' ≤ 0.005, 0 < γ' ≤ 0.005, 0 < a' ≤ 0.01, 0 < α' + β' + γ' ≤ 0.02, 0.48 ≤ y1' < 1, 0 < y2' ≤ 0.2, 0 < y3' ≤ 0.3, and y1' + y2' + y3' + α' + β' + γ' = 1, and β' = γ', wherein the coating layer includes a cobalt-containing compound represented by the following Chemical Formula 5: [Chemical Formula 5] Li x1 Co y1 M' z1 O a1 in the Chemical Formula 5, M' is one or more transition metals other than Co, and 0.5 < x1, 0 < y1 < 1, 0 < z1 < 1, 1 < a1 < 3, and 0.5 ≤ y1 / (y1 + z1) < 1.

10. The method of claim 9, wherein, the mixing step includes a step of mechanical mixing.

11. The method according to claim 9, wherein the heat-treating step includes a first heat-treating step and a second heat-treating step, the heat-treating temperature of the first heat-treating step is higher than the heat-treating temperature of the second heat-treating step.

12. The method of claim 9, wherein, the firing step is performed at a temperature of 500°C to 900°C for 1 hour to 6 hours.

13. A lithium secondary battery comprising: a positive electrode including the positive electrode active material according to any one of claims 1 to 8; a negative electrode; and an electrolyte. ​

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