Positive electrode active material, method for preparing the same, and lithium secondary battery including positive electrode comprising the same

Abstract

A positive electrode active material, a method for producing the same, and a lithium secondary battery having a positive electrode including the same are provided. The positive electrode active material includes lithium transition metal oxide particles in which part of Li is substituted with Na and including Ni and Co atoms, and the lithium transition metal oxide particles include a concentration gradient region in which the concentration of Co atoms decreases from the surface toward the center of the particles.

Description

Technical Field

[0001] The invention relates to a novel positive electrode active material, its preparation method, and a lithium secondary battery having a positive electrode comprising the positive electrode active material.

[0002] This invention was achieved with funding from the Ministry of Trade, Industry and Energy of South Korea under the project titled "Research and Development of Ni-rich NCA (>210mAh / g, @4.3V) Cathode Material with High Strength, Long Lifespan, and High Safety for Large and Medium-Sized Lithium Secondary Batteries" (Project No. P0009541). Background Technology

[0003] Since Sony commercialized lithium-ion batteries in 1991, demand for them has been increasing across various sectors, from small home appliances like mobile IT products to large and medium-sized electric vehicles and energy storage systems. Especially for large and medium-sized electric vehicles and energy storage systems, inexpensive, high-energy cathode materials are essential. However, cobalt, the main raw material for single-crystal LiCoO2 (LCO), currently the commercially available cathode active material, is relatively expensive.

[0004] Therefore, in recent years, LiNi, which replaces part of the Co in LCO by other transition metals, has been used as the positive electrode active material for large and medium-sized secondary batteries. 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), have the advantage of being inexpensive and possessing high reversible capacity as raw materials for such NCM and NCA-based cathode active materials. In particular, NCM and NCA with a Ni molar ratio greater than or equal to 50 mol% are of interest for high capacity applications. Typically, these Ni-based cathode active materials are prepared by mixing transition metal compound precursors synthesized via co-precipitation with a lithium source and then performing solid-phase synthesis. However, the Ni-based cathode materials synthesized as described above exist as secondary particles formed from the aggregation of small primary particles, leading to micro-cracks within the material during prolonged charge / discharge cycles. These micro-cracks cause new interfaces in the cathode active material and side reactions in the electrolyte, resulting in decreased safety due to gas generation and battery performance degradation due to electrolyte depletion. Furthermore, increasing the electrode density (>3.3 g / cc) is necessary to achieve high energy density, but this triggers the collapse of secondary particles, leading to electrolyte depletion due to side reactions with the electrolyte, resulting in a sharp drop in initial lifetime. Consequently, this means that Ni-based cathode active materials in the form of secondary particles synthesized via the original co-precipitation method cannot achieve high energy density.

[0005] To address the issues associated with secondary particle-type Ni-based cathode active materials, recent research has focused on single-particle Ni-based cathode active materials. When increasing electrode density (>3.3 g / cc) to achieve high energy density, single-crystal Ni-based cathode active materials do not experience particle collapse, thus achieving excellent electrochemical performance. However, during the electrochemical evaluation of these single-crystal Ni-based cathode active materials, the unstable Ni... 3+ and Ni 4+ The structural and / or thermal instabilities caused by ions, leading to a decrease in battery safety, have been identified. Therefore, the technological need to stabilize unstable Ni ions in monocrystalline Ni-based cathode active materials remains for the development of high-energy lithium-ion secondary batteries. Summary of the Invention

[0006] Technical issues

[0007] According to one aspect, a positive electrode active material as described above is provided, which improves the structural stability of the positive electrode active material by stabilizing the Ni ions in the single-crystal Ni-based positive electrode active material and the Co concentration gradient region in the positive electrode active material, thereby suppressing particle cracking during charging and discharging, achieving high energy density and improving long lifetime 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 includes Ni and Co atoms.

[0010] The lithium transition metal oxide particles include a concentration gradient region in which the concentration of Co atoms decreases from the surface toward the center of the particle.

[0011] According to another aspect, a method for preparing a positive electrode active material is provided, which includes: preparing lithium transition metal oxide particles, wherein a portion of Li is replaced by Na and includes Ni and Co atoms;

[0012] A precursor for the positive electrode active material is obtained by mixing the lithium transition metal oxide particles and a Co-containing compound; and

[0013] The positive electrode active material is obtained by calcining the precursor of the positive electrode active material.

[0014] The positive electrode active material includes a concentration gradient region in which the concentration of Co atoms decreases from the surface of the lithium transition metal oxide particles toward the center of the particles.

[0015] According to another aspect, a lithium secondary battery is provided, comprising: a positive electrode including the positive electrode active material; a negative electrode; and an electrolyte.

[0016] Beneficial effects

[0017] According to one aspect of the present invention, the positive electrode active material comprises lithium transition metal oxide particles consisting of single crystals and single particles, wherein a portion of the Li in the single crystal is replaced by Na, and includes a concentration gradient region in which the Co concentration in the transition metal decreases from the surface of the lithium transition metal oxide particles towards the center of the particles, thereby preventing particle breakage caused by cracks during charging and discharging, and stabilizing unstable Ni ions present in high-Ni lithium transition metal oxides, thereby improving the capacity and lifetime stability per volume. Attached Figure Description

[0018] Figure 1 These are SEM images of the positive electrode active materials in Example 1 and Comparative Example 1.

[0019] Figure 2 This is a graph showing the particle size distribution of the positive electrode active material in Example 1 and Comparative Example 1.

[0020] Figure 3 This is a high-resolution transmission electron microscopy (HR-TEM) image of the positive electrode active material in Comparative Example 1.

[0021] Figure 4This is a high-resolution transmission electron microscopy (HR-TEM) image of the positive electrode active material in Example 1.

[0022] Figure 5 This is a graph showing the half-cell life retention rate in Examples 4 and Comparative Examples 8 to 12.

[0023] Figure 6 This is a graph showing the half-cell life retention rate in Example 5 and Comparative Example 13.

[0024] Figure 7 This is a graph showing the half-cell life retention rate in Example 6 and Comparative Example 14.

[0025] Figure 8 This is a schematic view of a lithium battery according to an exemplary embodiment.

[0026] <Explanation of Figure Markers>

[0027] 1: Lithium battery 2: Negative electrode

[0028] 3: Positive electrode 4: Separator

[0029] 5: Battery casing 6: Cover assembly Detailed Implementation

[0030] The inventive concept described below can be modified and has various embodiments, with specific embodiments 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 scope of the inventive concept are included therein.

[0031] The terminology used below is for describing specific embodiments only and is not intended to limit the inventive concept. Singular expressions include plural expressions unless the context clearly specifies otherwise. Terms such as "comprising" or "having" used below are intended to indicate the presence of features, quantities, steps, operations, constituent elements, components, components, materials, or combinations thereof described in the specification, and should be understood not to preclude the possibility of the presence or addition of one or more other features, quantities, steps, operations, constituent elements, components, materials, or combinations thereof. The " / " used below may be interpreted as "and" or "or" depending on the context.

[0032] To clearly show the individual layers and regions, the thicknesses in the accompanying drawings are shown in enlarged or reduced form. Similar reference numerals are used for similar portions throughout the specification. Throughout the specification, when indicating that a part of a layer, film, region, plate, etc., is located "on" or "above" other parts, this includes cases where it is directly on top of other parts, as well as cases where other parts exist in between. Throughout the specification, terms such as "first," "second," etc., may be used to describe various constituent elements, but the constituent elements are not limited by the terms. Terms are used only to distinguish one constituent element from another.

[0033] All terms used in this specification (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains, unless otherwise defined. Furthermore, it should be understood that terms defined in common dictionaries shall be interpreted as having the same meaning as they have in the relevant technical context and in the context of this disclosure, and shall not be idealized or interpreted as having overly formal meanings.

[0034] "Group" refers to a group of elements in the periodic table according to the International Union of Pure and Applied Chemistry ("IUPAC") classification system of groups 1 to 18.

[0035] Although specific embodiments have been described, alternatives, modifications, alterations, improvements, and substantial equivalents may occur to the applicant or a person skilled in the art that are currently unforeseeable or unpredictable. Therefore, the appended claims, which can be amended post-filing, are intended to cover all such alternatives, modifications, alterations, improvements, and substantial equivalents.

[0036] The following describes in more detail the positive electrode active material according to an exemplary embodiment, its preparation method, and a lithium secondary battery including a positive electrode comprising the active material.

[0037] The positive electrode active material according to one embodiment may include: lithium transition metal oxide particles, wherein a portion of Li is replaced by Na and includes Ni and Co atoms, wherein the lithium transition metal oxide particles include a concentration gradient region in which the concentration of Co atoms decreases from the surface toward the center of the particles.

[0038] Because some Li in the positive electrode active material is replaced by Na, the structural deformation caused by Li ion insertion / extraction 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 surface towards the particle center, the stability of the crystal structure is further improved. Therefore, by suppressing crystal collapse during charging and discharging, the lifetime characteristics are improved. Moreover, by reducing the distribution of unstable Ni(III) and Ni(IV) ions on the surface and increasing the distribution of Ni(II) ions at the core center, not only are side reactions with the Ni ion electrolyte suppressed, but the high Ni ion content also results in a high capacity of the positive electrode active material. Therefore, the positive electrode active material exhibits both high capacity and long lifetime characteristics. Additionally, as will be explained in detail below, by further doping with transition metals such as W, Mg, and Ti, the unstable Ni(III) and Ni(IV) are reduced, thereby further improving structural stability and significantly enhancing the lifetime characteristics.

[0039] According to one embodiment, the concentration gradient region may include a region extending 500 nm from the surface of the lithium transition metal oxide particles toward the center.

[0040] For example, the concentration gradient region may include a region extending 250 nm from the surface of the lithium transition metal oxide particles toward the center.

[0041] The high capacity and long lifetime characteristics of the positive electrode active material are achieved by the presence of the concentration gradient region at the distance from the surface of the lithium transition metal oxide particles.

[0042] According to one embodiment, in the concentration gradient region, the concentration of Ni atoms can increase from the surface of the lithium transition metal oxide particles toward the center of the particles.

[0043] According to one embodiment, in the concentration gradient region, from the surface of the lithium transition metal oxide particles toward the particle center, the concentration of Co atoms can decrease, while the concentration of Ni atoms can increase.

[0044] By gradually decreasing the Co atom concentration and gradually increasing the Ni atom concentration starting 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.

[0045] According to one embodiment, the lithium transition metal oxide particles may include lithium transition metal oxides represented by the following chemical formula 1.

[0046] [Chemical Formula 1]

[0047] Li 1-x Na x M 1-(α+β+γ) Wα Mg β Ti γ O 2-a S a

[0048] M, x, α, β, γ, and a in the chemical formula 1 will be described in detail later.

[0049] The lithium transition metal oxide represented by the aforementioned chemical formula 1 has some Li replaced by Na, some M replaced by W, Mg and Ti, and some O replaced by S. Therefore, lithium secondary batteries containing this oxide have increased structural safety of lithium transition metal oxide during charging and discharging, thereby improving capacity and lifespan stability per unit volume.

[0050] Furthermore, for high-Ni lithium transition metal oxides where M includes Ni, the substitution of small amounts of W, Mg, and Ti can induce the reduction of unstable nickel ions present in the lithium transition metal oxide, for example, by substituting Ni... 3+ Ni 4+ Reduced to the stable nickel ion form Ni 2+ This suppresses the degradation and capacity reduction of the positive electrode active material caused by side reactions of unstable nickel ions and electrolyte during charging and discharging.

[0051] Outside of the concentration gradient region, the lithium transition metal oxide particles, through the inclusion of small amounts of W, Mg, and Ti as transition metals, cause the reduction of unstable nickel ions distributed within the lithium transition metal oxide particles, for example, reducing Ni... 3+ Ni 4+ Reduced to the stable nickel ion form Ni 2+ This prevents the degradation of the positive electrode active material during charging and discharging, and significantly inhibits capacity decline.

[0052] According to one embodiment, in the chemical formula 1, M can be at least one element selected from alkali metal elements other than W, Mg, Ti, Na and S, alkaline earth metal elements, transition metal elements, post-transition metal elements and non-metal elements.

[0053] For example, M can be at least one element 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.

[0054] According to one embodiment, M may be at least one element selected from alkaline earth metal elements, transition metal elements, post-transition metals, and non-metal elements other than W, Mg, Ti, Na, and S.

[0055] For example, M may be at least one element 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.

[0056] According to one embodiment, M may be at least one element selected from Ni, Co, Mn, Al, V, Ca, Zr, B, and P.

[0057] For example, M may be at least one element selected from Ni, Co, Mn, Al, V, Ca, Zr, B, and P. For example, M may be Ni and at least one element selected from Co, Mn, and Al.

[0058] According to one embodiment, 0 < x ≤ 0.01, where x refers to the substitution molar ratio of Na to Li in the lithium transition metal oxide represented by Chemical Formula 1. A part of Li in the lithium transition metal oxide represented by Chemical Formula 1 is replaced by Na, thereby improving the structural stability. When Na is substituted in the lattice space where Li is located, when lithium is deintercalated due to the intervention of large Na with an ionic radius larger than that of lithium in the charged state, the expansion of the crystal structure is suppressed due to the repulsive force between oxygen atoms in the lithium transition metal oxide. As a result, the structural stability of the lithium transition metal oxide can also be achieved during repeated charging.

[0059] According to one embodiment, 0 < α ≤ 0.01, where 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 is greater than 0.01, the structural stability decreases due to the distortion of the crystal structure and impurities WO3 are formed, so it may cause a reduction in electrochemical properties.

[0060] According to one embodiment, 0 < β ≤ 0.005, where β 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.

[0061] According to an 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.

[0062] When W, Mg, and Ti are substituted in the lithium transition metal oxide at the above molar ratios, during lithium deintercalation in the charged state, the structural stability is also improved because the interaction between oxygen in the lithium transition metal oxide suppresses the structural expansion of the crystal, thereby improving the life characteristics.

[0063] According to an 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 can be ensured. When α + β + γ is greater than 0.02, an impurity phase is formed, which not only acts as an impedance during lithium deintercalation, but may also cause the collapse of the crystal structure during repeated charging.

[0064] According to an embodiment, in Chemical Formula 1, β and γ can be 0 < β ≤ 0.003 and 0 < γ ≤ 0.003, respectively.

[0065] For example, in Chemical Formula 1, it can be β = γ. When β = γ, for example, when the molar ratios of Mg and Ti are the same, the charge in the lithium transition metal oxide during charging and discharging is balanced, suppressing the collapse of the crystal structure, thereby improving the structural stability and ultimately improving the life characteristics.

[0066] According to an 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, α refers to the substitution molar ratio of S to the O element in the lithium transition metal oxide represented by Chemical Formula 1.

[0067] Since part of the oxygen element is substituted by S, the binding activity with the transition metal increases, and the transformation of the crystal structure of the lithium transition metal oxide is suppressed. As a result, the structural stability of the lithium transition metal oxide is improved.

[0068] On the other hand, when the substitution molar ratio of S is greater than 0.01, the crystal structure is unstable due to the repulsive force of S anions, and the life characteristics are instead reduced.

[0069] According to one embodiment, the lithium transition metal oxide can be a single particle. The concept of a single particle differs from secondary particles formed by aggregating multiple particles, or particles formed by aggregating multiple particles and coating their outer surface. Because the lithium transition metal oxide has the form of a single particle, particle breakage can be prevented at high electrode densities. Therefore, high energy density of the cathode active material including the lithium transition metal oxide can be achieved. Furthermore, compared to secondary particles formed by aggregating multiple single particles, breakage during rolling is suppressed, thus enabling high energy density and preventing lifetime degradation caused by particle breakage.

[0070] According to one embodiment, the lithium transition metal oxide may be monocrystalline. Monocrystalline has a different concept from a single particle. A single particle refers to a particle formed as a single grain regardless of the type and number of internal crystals, while monocrystalline refers to a grain containing only one crystal. The monocrystalline lithium transition metal oxide exhibits very high structural stability and, compared to polycrystalline materials, facilitates lithium-ion conductivity, thus exhibiting superior high-speed charging characteristics.

[0071] According to one embodiment, the positive electrode active material is a single crystal or a single particle. By forming it as a single crystal or a single particle, its structure is stable and a high-density electrode can be achieved. Therefore, the lithium secondary battery including it can simultaneously have improved lifespan characteristics and high energy density.

[0072] According to one embodiment, the lithium transition metal oxide can be represented by any one of the following chemical formulas 2 to 4.

[0073] <Chemical Formula 2>

[0074] Li 1-x' Na x' Ni y1' Co y2' Mn y3' W α' Mg β' Ti γ' O 2-a' S a

[0075] <Chemical Formula 3>

[0076] Li 1-x” Na x” Ni y1” Co y2” Al y3” W α” Mg β” Ti γ” O 2-a” S a”

[0077] <Chemical Formula 4>

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

[0079] According to one embodiment, in chemical formula 2,

[0080] 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、0.48≤y1'<1、0<y2'≤0.2、0<y3'≤0.3、y1'+y2'+y3'+α'+β'+γ'=1。

[0081] In the chemical formula 3,

[0082] 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、y1”+y2”+y3”+α”+β”+γ”=1。

[0083] In the chemical formula 4,

[0084] 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、y1”'+y2”'+α”'+β”'+γ”'=1。

[0085] For example, in chemical formula 2, it can be 0<β'≤0.003, 0<γ'≤0.003, 0<α'+β'+γ'≤0.016; in chemical formula 3, it can be 0<β”≤0.003, 0<γ”≤0.003, 0<α”+β”+γ”≤0.016; and in chemical formula 4, it can be 0<β”'≤0.003, 0<γ”'≤0.003, 0<α”'+β”'+γ”'≤0.016.

[0086] The lithium transition metal oxide that meets the above composition can stabilize the unstable Ni ions inside and has high energy density and long lifetime stability.

[0087] Common cathode active materials, including high-nickel lithium nickel cobalt manganese oxides, must stabilize unstable Ni ions. By introducing W, Mg, and Ti into some transition metal positions in the crystal, the overall charge balance of the cathode active material is achieved, thereby suppressing the oxidation of Ni(II) ions to unstable Ni(III) or Ni(IV) ions, and the unstable Ni(III) or Ni(IV) ions can be reduced back to Ni(II). On the other hand, the conductivity loss caused by the substitution of some transition metals by different elements W, Mg, and Ti is compensated by the substitution of some O by S, and the decrease in Li conductivity caused by structural deformation during charging and discharging is suppressed by the substitution of some Li by Na. This results in a single-crystal cathode active material with stable structure, high capacity, and long lifetime.

[0088] According to one embodiment, the average particle size (D) of the lithium transition metal oxide 50 The average particle size (D) can range from 0.1 μm to 20 μm. For example, the average particle size (D) 50 The average particle size of the lithium transition metal oxide can be 0.1 μm to 15 μm, 0.1 μm to 10 μm, 1 μm to 20 μm, 5 μm to 20 μm, 1 μm to 15 μm, 1 μm to 10 μm, 5 μm to 15 μm, or 5 μm to 10 μm. When the average particle size of the lithium transition metal oxide is within these ranges, the desired energy density per volume can be achieved. When the average particle size of the lithium transition metal oxide is greater than 20 μm, a sharp drop in charge / discharge capacity occurs; when it is less than or equal to 0.1 μm, it is difficult to obtain the desired energy density per volume.

[0089] The preparation method of the positive electrode active material is described in detail below.

[0090] A method for preparing a positive electrode active material according to one embodiment includes: preparing lithium transition metal oxide particles, wherein a portion of Li is replaced by Na and includes Ni and Co atoms; obtaining a positive electrode active material precursor by mixing the lithium transition metal oxide particles and a Co-containing compound; and obtaining a positive electrode active material by calcining the positive electrode active material precursor, wherein the positive electrode active material includes a concentration gradient region in which the Co atom concentration decreases from the surface towards the particle center.

[0091] According to one embodiment, the step of preparing lithium transition metal oxide particles may include:

[0092] A lithium transition metal oxide precursor is obtained by mixing compounds containing Li, Na, W, Mg, Ti, M, and S; and lithium transition metal oxide particles represented by the following chemical formula 1 are obtained by heat-treating the lithium transition metal oxide precursor.

[0093] <Chemical Formula 1>

[0094] Li 1-x Na x M 1-(α+β+γ) W α Mg β Ti γ O 2-a S a

[0095] In the chemical formula 1,

[0096] M is at least one element selected from alkali metals, alkaline earth metals, transition metals, post-transition metals, and nonmetals, excluding W, Mg, Ti, Na, and S.

[0097] And 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。

[0098] For a detailed description of chemical formula 1, please refer to the above description.

[0099] The mixing step may include mechanically mixing the compound containing the specific element. This mechanical mixing is performed using a dry method. Mechanical mixing refers to the process of applying mechanical force to pulverize and mix the substances to be mixed to form a homogeneous mixture. Mechanical mixing can be performed using mixing devices, such as ball mills using chemically inert beads, planetary mills, stirred ball mills, vibrating mills, etc. In this case, small amounts of alcohols such as ethanol and higher fatty acids such as stearic acid may be selectively added to maximize the mixing effect.

[0100] The mechanical mixing is performed in an oxidizing environment to prevent the reduction of the transition metal in the transition metal supply source (e.g., Ni compound), thereby achieving structural stability of the active material.

[0101] 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.

[0102] The Na-containing compound may include, but is not limited to, Na hydroxides, oxides, nitrides, carbonates, or combinations thereof. For example, it may be NaOH, Na₂CO₃, or combinations thereof.

[0103] The W-containing compound may include, but is not limited to, W hydroxides, oxides, nitrides, carbonates, or combinations thereof. For example, it may be W(OH)6, WO3, or combinations thereof.

[0104] The Mg-containing compound may include, but is not limited to, Mg hydroxides, oxides, nitrides, carbonates, or combinations thereof. For example, it may be Mg(OH)₂, MgCO₃, or combinations thereof.

[0105] The Ti-containing compound may include, but is not limited to, Ti hydroxides, oxides, nitrides, carbonates, or combinations thereof. For example, it may be Ti(OH)₂, TiO₂, or combinations thereof.

[0106] The M-containing compound may include, but is not limited to, hydroxides, oxides, nitrides, carbonates, or combinations thereof of at least one of the following elements: alkali metals other than W, Mg, Ti, Na, and S; alkaline earth metals; transition metals; metalloids; and nonmetals. For example, it may be Ni. 0.8 Co 0.1 Mn 0.1 (OH)2, Ni 0.9 Co 0.05 Al 0.05 (OH)2 or Ni 0.9 Co 0.1 (OH)2.

[0107] The sulfur-containing compound may include, but is not limited to, sulfur hydroxides, oxides, nitrides, carbonates, ammonides, or combinations thereof. For example, it may be (NH4)2S.

[0108] According to one embodiment, the step of obtaining the positive electrode active material precursor may further include a Co-containing compound as a mixed material.

[0109] The Co-containing compound is a compound capable of providing Co, including Co hydroxides, oxides, nitrides, carbonates, acetates, or combinations thereof. For example, it could be cobalt acetate.

[0110] In the step of preparing lithium transition metal oxide particles, the heat treatment step may include a first heat treatment step and a second heat treatment step. The first heat treatment step and the second heat treatment step may be performed continuously, or there may be a rest period after the first heat treatment step. In addition, the first heat treatment step and the second heat treatment step may be performed in the same or different chambers.

[0111] The heat treatment temperature in the first heat treatment step can be higher than the heat treatment temperature in the second heat treatment step.

[0112] 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; it includes all ranges formed by selecting any two points within the range.

[0113] 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 thereto; it includes all ranges formed by selecting any two points within the range.

[0114] According to one embodiment, the heat treatment time in the first heat treatment step can be shorter than the heat treatment time in the second heat treatment step.

[0115] For example, the heat treatment time in the first heat treatment step may 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 range.

[0116] 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 range.

[0117] The first heat treatment step may include heat treatment at a heat treatment temperature of 800°C to 1200°C for 3 to 5 hours.

[0118] The second heat treatment step may include heat treatment at a heat treatment temperature of 700°C to 800°C for 10 to 20 hours.

[0119] The first heat treatment step forms a layered structure of lithium transition metal oxide as the positive electrode active material, while simultaneously inducing particle growth to achieve a single-crystal shape. In this first heat treatment step, it is assumed that each primary particle in the secondary-particle-shaped lithium transition metal oxide grows rapidly, thus becoming unable to withstand inter-particle stress and exposing the interior of the primary particles, which then fuse together, forming a single-crystal positive electrode active material for secondary batteries. Compared to the first heat treatment step, the second heat treatment step is performed at a lower temperature for a longer duration, thereby increasing the crystallinity of the layered structure generated by the first heat treatment step. Both the first and second heat treatment steps yield single-phase, single-crystal, and single-particle high-nickel-based positive electrode active materials.

[0120] According to one embodiment, in the step of obtaining the positive electrode active material precursor, the Co-containing compound may be contained in an organic solvent. For example, the organic solvent may be a volatile solvent. For example, the organic solvent may be a solvent that is volatile at a temperature below or equal to 80°C, such as methanol or ethanol.

[0121] 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.

[0122] According to one embodiment, the firing step can be performed at a temperature of 500°C to 900°C for 1 to 6 hours. By firing the positive electrode active material precursor under the stated firing temperature and time conditions, a positive electrode active material forming a concentration gradient region with a concentration gradient of Co atoms can be obtained.

[0123] According to one embodiment, the concentration gradient region may include a region less than or equal to 500 nm extending from the surface of the lithium transition metal oxide particles towards the center. It is assumed that the Co concentration gradient region is formed during the sintering of the positive electrode active material precursor, as Co penetrates into the surface of the lithium transition metal oxide particles and diffuses inward.

[0124] According to one embodiment, the lithium transition metal oxide prepared by the aforementioned method is a single crystal, a single particle, and the single crystal may have a layered structure. Furthermore, the average particle size of the lithium transition metal oxide may be from 0.1 μm to 20 μm.

[0125] Furthermore, the lithium transition metal oxide prepared by the aforementioned method for preparing the positive electrode active material suppresses the original Ni by substituting W, Mg, and Ti elements into the M element position, S element into the O position, and Na element into the Li position. 2+ The oxidation of Ni and the initiation of the originally unstable Ni 3+ Ions reduced to Ni 2+ The reduction of Ni ions yields a structurally stable and high-density lithium transition metal oxide. Additionally, the reduced Ni... 2+ Ions and Ni + The similarity of ionic radii promotes Li / Ni disorder, and Ni ions fill empty lattice during Li insertion / extraction, thereby achieving the structural stability of the crystal.

[0126] Furthermore, compared to the original transition metal being simply replaced, the structural stability is improved by further including a Co concentration gradient region, and the capacity and lifetime characteristics are enhanced.

[0127] According to another aspect, a positive electrode is provided, which includes the aforementioned positive electrode active material.

[0128] According to another aspect, a secondary battery is provided, which includes the positive electrode, the negative electrode, and the electrolyte.

[0129] The positive electrode and the lithium secondary battery including it can be prepared by the following methods.

[0130] First, prepare the positive electrode.

[0131] For example, a positive electrode active material composition is prepared by mixing the above-mentioned positive electrode active material, conductive material, binder, and solvent. A positive electrode plate is prepared by directly coating the positive electrode active material composition onto a metal current collector. Alternatively, a positive electrode plate can be prepared by casting the positive electrode active material composition onto a separate carrier and then laminating a film peeled from the support onto the metal current collector. The positive electrode is not limited to the forms listed above; it can be in forms other than those described.

[0132] The conductive material can be natural graphite, artificial graphite, carbon black, carbon nanotubes, fluorocarbons, zinc oxide, potassium titanate, titanium oxide, etc., but is not limited to these. Any material used as a conductive material in the relevant technical field can be used.

[0133] The adhesive may be made of vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polytetrafluoroethylene, mixtures thereof, metal salts, or polymers based on styrene-butadiene rubber, but is not limited thereto; any adhesive used in the art may be employed. Another adhesive may be a lithium salt, sodium salt, calcium salt, or sodium salt of the above polymers.

[0134] The solvent may be N-methylpyrrolidone, acetone, or water, but is not limited to these; any solvent applicable to the relevant technical field may be used.

[0135] The content of the positive electrode active material, conductive material, binder, and solvent is at the level commonly used in lithium batteries. One or more of the conductive material, binder, and solvent may be omitted depending on the application and configuration of the lithium battery.

[0136] Then, prepare the negative electrode.

[0137] 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 plate is then prepared by directly coating the negative electrode active material composition onto a metal current collector having a thickness of 3 μm to 500 μm and drying it. Alternatively, the negative electrode plate can be prepared by casting the negative electrode active material composition onto a separate carrier and then laminating a film peeled from the support onto the metal current collector.

[0138] The negative electrode current collector is not particularly limited as long as it is a material that does not cause chemical changes to the battery in the relevant technical field and is conductive. For example, copper, nickel, or materials with carbon surface treatment on the copper can be used.

[0139] Any material that can be used as a negative electrode active material in the relevant technical field can be used as the negative electrode active material. For example, it may include at least one selected from the group consisting of lithium metal, metals that can be alloyed with lithium, transition metal oxides, non-transition metal oxides, and carbon-based materials.

[0140] For example, the metal that can be alloyed with lithium can be Si, Sn, Al, Ge, Pb, Bi, Sb, Si-Y alloy (where Y is an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, or a combined element thereof, but not Si), Sn-Y alloy (where Y is an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, or a combined element thereof, but 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.

[0141] For example, the transition metal oxide can be lithium titanium oxide, vanadium oxide, lithium vanadium oxide, etc.

[0142] For example, the non-transition metal oxide can be SnO2, SiO x (0 < x < 2), etc.

[0143] The carbon-based material can be crystalline carbon, amorphous carbon, or a mixture thereof. The crystalline carbon can be graphite such as amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite; the amorphous carbon can be soft carbon (low-temperature fired carbon), hard carbon, mesophase pitch carbide, and fired coke, etc.

[0144] The conductive material, binder, and solvent in the negative electrode active material composition can use the same materials as those in the positive electrode active material composition.

[0145] The contents of the negative electrode active material, conductive material, binder, and solvent are at the levels conventionally used in lithium batteries. One or more of the conductive material, binder, and solvent can be omitted according to the use and configuration of the lithium battery.

[0146] Then, a separator to be inserted between the positive electrode and the negative electrode is prepared.

[0147] Materials conventionally used in this technical field can be used as the separator. The separator can be made of materials with low resistance to electrolyte movement and excellent electrolyte permeability. The separator can be a single-layer or multi-layer membrane, for example, it can be selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene (PTFE), or combinations thereof, and can be in non-woven or woven fabric form. Alternatively, mixed multilayer membranes such as polyethylene / polypropylene two-layer membranes, polyethylene / polypropylene / polyethylene three-layer membranes, and polypropylene / polypropylene / polypropylene three-layer membranes can also be used. For example, rollable separators such as polyethylene and polypropylene can be used in lithium-ion batteries, and separators with excellent organic electrolyte impregnation properties can be used in lithium-ion polymer batteries. For example, the separator can be prepared by the following methods.

[0148] A membrane composition is prepared by mixing a polymer resin, a filler, and a solvent. The membrane is formed by directly coating the membrane composition onto an electrode and drying it. Alternatively, a membrane is formed by casting the membrane composition onto a support, drying it, and then laminating a membrane layer peeled from the support onto the electrode.

[0149] There are no particular limitations on the number of polymers used to prepare the diaphragm, and any material used for the electrode plate bonding material can be used. For example, vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, or mixtures thereof can be used.

[0150] Next, prepare the electrolytes.

[0151] For example, the electrolyte can be an organic electrolyte. Alternatively, the electrolyte can be a solid. For example, it can be boron oxide and lithium oxynitride, but is not limited to these; any material that can be used as a solid electrolyte in the art can be used. The solid electrolyte can be formed on the negative electrode by methods such as sputtering.

[0152] For example, organic electrolytes can be prepared by dissolving lithium salts in organic solvents.

[0153] Any organic solvent applicable within the relevant technical field can be used as the stated organic solvent. Examples include 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, methyl isopropyl 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. These can be used individually or in combination. For example, a mixed solvent of cyclic and chain carbonates can be used.

[0154] Alternatively, gel-like polymer electrolytes, in which the electrolyte is impregnated with polymer electrolytes such as polyethylene oxide and polyacrylonitrile, 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, Br)

[0155] Any material that can be used as a lithium salt in the relevant technical field can also be used as the lithium salt mentioned above. Examples include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiAlO2, LiAlCl4, and LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2 (but x and y are natural numbers), LiCl, LiI, or mixtures thereof, etc.

[0156] As shown in Figure 11, 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 housed in a battery casing (5) by winding or folding. Then, an organic electrolyte is injected into the battery casing (5) and sealed with a cap assembly (6) to complete the lithium battery (1). The battery casing (5) can be cylindrical, square, pouch-shaped, coin-shaped, or film-shaped. For example, the lithium battery (1) can be film-shaped. The lithium battery (1) can be a lithium-ion battery.

[0157] A battery structure can be formed by placing a separator between the positive and negative electrodes. After the battery structure is stacked into a bicell structure, it is immersed in an organic electrolyte, and the resulting product is contained in a bag and sealed, thereby completing a lithium-ion polymer battery.

[0158] Furthermore, multiple battery structures can be stacked to form a battery pack, which can be used in all devices requiring high capacity and high output. For example, it can be used in laptops, smartphones, and electric vehicles.

[0159] Furthermore, the lithium battery, due to its excellent lifespan and high-speed characteristics, can be used in electric vehicles (EVs). For example, it can be used in plug-in hybrid electric vehicles (PHEVs) and other hybrid vehicles. Additionally, it can be used in fields requiring the storage of large amounts of electricity, such as electric bicycles, power tools, and systems for storing electricity.

[0160] The present invention will be further described in 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.

[0161] (Preparation of positive electrode active material)

[0162] Example 1

[0163] 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 at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0164] Then, in a solution of 2g cobalt acetate dissolved in ethanol, 100g of the lithium transition metal oxide particles were added and stirred for 30 minutes. The mixture was then placed at 80°C to allow the ethanol to evaporate. The resulting powder was then calcined at 800°C for 3 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0165] Example 2

[0166] 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 at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0167] Then, in a solution of 2g cobalt acetate dissolved in ethanol, 100g of the lithium transition metal oxide particles were added and stirred for 30 minutes. The mixture was then placed at 80°C to allow the ethanol to evaporate. The resulting powder was then calcined at 800°C for 3 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0168] Example 3

[0169] 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 at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0170] Then, in a solution of 2g cobalt acetate dissolved in ethanol, 100g of the lithium transition metal oxide particles were added and stirred for 30 minutes. The mixture was then placed at 80°C to allow the ethanol to evaporate. The resulting powder was then calcined at 800°C for 3 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0171] Comparative Example 1

[0172] 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 at 700 °C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0173] Comparative Example 2

[0174] 100g of Ni0.8 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 1000 °C for 4 hours and at 700 °C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0175] Comparative Example 3

[0176] 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 1000°C for 4 hours and at 700°C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0177] Comparative Example 4

[0178] 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 1000°C for 4 hours and at 700°C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0179] Comparative Example 5

[0180] 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 about 15 minutes. The mixed powder was then heat-treated at 1000 °C for 4 hours and at 700 °C for 10 hours to obtain lithium transition metal oxide particles.

[0181] Then, in a solution of 2g cobalt acetate dissolved in ethanol, 100g of the lithium transition metal oxide particles were added and stirred for 30 minutes. The mixture was then placed at 80°C to allow the ethanol to evaporate. The resulting powder was then calcined at 800°C for 3 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0182] Comparative Example 6

[0183] 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 at 700 °C for 10 hours to obtain the positive electrode active material. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0184] Comparative Example 7

[0185] 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 at 700 °C for 10 hours to obtain lithium transition metal oxide particles. The specific composition of the obtained positive electrode active material is shown in Table 1.

[0186] (Preparation of half-cells)

[0187] Example 4

[0188] A slurry was prepared by mixing the positive electrode active material obtained in Example 1, the conductive material, and the binder in a weight ratio of 94:3:3. Carbon black was used as the conductive material, and polyvinylidene fluoride (PVdF) dissolved in N-methyl-2-pyrrolidone was used as the binder.

[0189] 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 level of the electrode plate was 11.0 mg / cm². 2 The electrode density is 3.6 g / cc.

[0190] The prepared positive electrode was used as the working electrode, the lithium foil was used as the counter electrode, and a CR2032 half cell was prepared by adding 1.3M of LiPF6 as a lithium salt to a mixed solvent of EC / EMC / DEC in a volume ratio of 3 / 4 / 3, according to generally known procedures.

[0191] Examples 5 to 6

[0192] Half-cells were prepared using the same method as in Example 4, except that the positive electrode active material obtained in Examples 2 to 3 was used instead of the positive electrode active material obtained in Examples 2 to 3.

[0193] Comparative Examples 8 to 14

[0194] Half-cells were prepared using the same method as in Example 4, except that the positive electrode active material obtained by Comparative Examples 1 to 7 was used instead of the positive electrode active material obtained by Comparative Examples 1 to 7.

[0195] [Table 1]

[0196]

[0197] Evaluation Example 1: Composition Evaluation of Positive Electrode Active Material

[0198] The positive electrode active materials synthesized by 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.

[0199] Referring to Table 2, it can be seen that the ICP analysis results of Comparative Example 1 and Example 1 show that, due to the substitution of other transition metals in the positive electrode active material, the introduction of the Co concentration gradient region into the surface of the single-particle Ni-based positive electrode active material increases the proportion of Co element in the positive electrode active material by about 1 mol%, and reduces the molar number of other transition metals. Furthermore, even when performing ICP analysis in a vacuum environment, the introduction of trace amounts of oxygen and carbon dioxide from the atmosphere makes it difficult to analyze the stoichiometry of oxygen contained in the material.

[0200] [Table 2]

[0201] (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 78.5 11.1 8.8 1 0.3 0.3 0.1

[0202] Evaluation Example 2: Particle Size Evaluation of Positive Electrode Active Material

[0203] SEM images of the appearance of the positive electrode active materials synthesized in Example 1 and Comparative Example 1 were obtained using a Verios 460 (FEI) instrument, and are shown below. Figure 1 Additionally, particle size distribution was measured using a Cilas 1090 (Scinco) instrument, as shown in Table 3 below. Figure 2 .

[0204] Refer to Table 3 and Figures 1 to 2 The single-particle positive electrode active material of Example 1 introduced a Co concentration gradient region, but no larger particle size change was observed compared to the single-particle positive electrode active material of Comparative Example 1. This indicates that the Co-containing compound formed a concentration gradient region by penetrating into the lithium transition metal oxide particles.

[0205] [Table 3]

[0206] <![CDATA[D 10 (μm)]]> <![CDATA[D 50 (μm)]]> <![CDATA[D 90 (μm)]]> Comparative Example 1 3.0 5.9 10.1 Example 1 2.9 5.8 9.9

[0207] Evaluation Example 3: Evaluation of the Concentration Gradient Region of the Positive Electrode Active Material

[0208] The positive electrode active materials obtained in Example 1 and Comparative Example 1 were imaged using high-resolution transmission electron microscopy (HR-TEM) and analyzed by energy dispersive X-ray spectroscopy (EDX). The results are shown in Tables 4 and 5 below. Figure 3 , Figure 4 .

[0209] [Table 4]

[0210] 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

[0211] [Table 5]

[0212] Location Ni (mol%) Co (mol%) Mn (mol%) 1 52.8 27.4 9.8 2 59.9 20.4 9.7 3 67.8 22.3 9.9 4 77.9 12.0 10.1 5 79.8 10.4 9.8

[0213] Refer to Table 4 and Figure 3 It can be seen that the concentration of transition metals (e.g., Ni, Co, and Mn) in the positive electrode active material is actually maintained constant from the surface of the positive electrode active material towards the center.

[0214] Refer to Table 5 and Figure 4 It was confirmed that in the transition metals within the positive electrode active material, the Co concentration decreases from the surface towards the center, while the Ni concentration increases in the opposite direction. Furthermore, a Co concentration gradient layer of approximately 500 nm was observed. Without being constrained by specific theories, cobalt ions, compared to nickel ions in the transition metals, contribute to the structural stability of the layered positive electrode active material. Therefore, it is believed that by including a relatively stable excess of cobalt on the surface of the positive electrode active material, the structural stability of the positive electrode active material during charge and discharge is improved, thereby enhancing its long-lifespan characteristics.

[0215] Evaluation Example 4: Lifetime Evaluation at Normal Temperature

[0216] The half-cells prepared by Examples 4 to 6 and Comparative Examples 8 to 14 were left to stand for 10 hours, then charged to 4.3V at 0.1C in CC mode, and then charged to a current corresponding to 0.05C in CV mode. The formation process was then terminated by discharging to 3.0V at 0.1C in CC mode.

[0217] Then, at room temperature (25°C), it is charged to 4.3V at 0.5C using CC mode, and then charged to the current corresponding to 0.05C using CV mode. Then, it is discharged to 3.0V at 1C using CC mode, and this process is repeated 50 times.

[0218] The capacity retention rate after 50 charge and discharge cycles relative to the initial capacity was calculated, and the results are shown in Table 7 below. Additionally, a graph showing the capacity retention rate based on cycles is presented in... Figures 5 to 7 .

[0219] [Table 6]

[0220]

[0221] Refer to Table 6 and Figure 5 Based on the room-temperature lifetime results of Examples 4 and Comparative Examples 8, the cell including the concentration gradient region (Example 4) showed an approximately 3% improvement in lifetime retention after 50 cycles compared to the cell without the concentration gradient region. This is believed to be because the relatively excessive distribution of cobalt, which contributes to the structural stabilization of the layered structure, on the surface of the positive electrode active material suppresses the degradation of the positive electrode active material. Furthermore, taking the half-cell of Example 4 as an example, which does not include the concentration gradient region, it showed a maximum improvement of approximately 7% in lifetime retention after 50 cycles compared to Comparative Examples 9 to 11, which used a positive electrode active material without at least one of Na, W, Mg, Ti, and S. Even further, compared to examples including the concentration gradient region but without at least one of Na, W, Mg, Ti, and S, for example, compared to Comparative Example 12, which used a positive electrode active material without the introduction of Ti, it showed an approximately 3% improvement in lifetime retention.

[0222] The data confirms that excellent lifetime characteristics are achieved through the synergistic effect of elements including Na, W, Mg, Ti, and S, as well as in the concentration gradient region.

[0223] In addition, it was confirmed that when NCA (nickel-cobalt-manganese) based cathode active materials and NC (nickel-cobalt) based cathode active materials also include a concentration gradient region, the lifetime characteristics are improved by about 2% to 3%.

[0224] The preferred embodiments of the present invention have been described above with reference to the accompanying drawings and examples. However, these are merely exemplary, and those skilled in the art will understand that various modifications and equivalent embodiments can be made based on them. 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, and having an average particle diameter D 50 of 5 μm to 20 μm: Chemical Formula 2 Li1+xNi0.5-yMny0.5-zMz02-2x (wherein 0≤x≤0.2, 0.3≤y≤0.7, 0≤z≤0.1, <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, y1' + y2' + y3' + α' + β' + γ' = 1, and β' = γ', wherein the lithium transition metal oxide particle includes a concentration gradient region in which a Co atomic concentration decreases from a surface toward a center of the particle, and wherein the concentration gradient region includes a region of less than or equal to 500 nm from the surface toward the center of the lithium transition metal oxide particle.

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

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

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

5. 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.

6. A method for producing a positive electrode active material, comprising: a lithium transition metal oxide precursor is obtained by mixing a Li element-containing compound, a Na element-containing compound, a W element-containing compound, a Mg element-containing compound, a Ti element-containing compound, a Ni, Co, Mn element-containing compound, and an S element-containing compound; The positive electrode active material including the lithium transition metal oxide particles represented by the following Chemical Formula 2 is obtained by heat-treating the lithium transition metal oxide precursor, and the average particle diameter D 50 is 5 μm to 20 μm; a positive electrode active material precursor is obtained by mixing the lithium transition metal oxide particle and a Co element-containing compound; and the positive electrode active material is obtained by firing the positive electrode active material precursor, wherein the positive electrode active material includes a concentration gradient region in which a Co atomic concentration decreases from a surface toward a center of the particle, and wherein the concentration gradient region includes a region of less than or equal to 500 nm from the surface toward the center of the lithium transition metal oxide particle. <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, y1' + y2' + y3' + α' + β' + γ' = 1, and β' = γ'.

7. The method for producing a positive electrode active material according to claim 6, wherein the mixing step includes a mechanical mixing step.

8. The method for producing a positive electrode active material according to claim 6, wherein the heat treatment step includes a first heat treatment step and a second heat treatment step, the heat treatment temperature in the first heat treatment step is higher than the heat treatment temperature in the second heat treatment step.

9. The method for producing a positive electrode active material according to claim 6, wherein in the step of obtaining a positive electrode active material precursor, the Co element-containing compound is included in an organic solvent.

10. The method for producing a positive electrode active material according to claim 6, wherein the step of firing is performed at a temperature of 500°C to 900°C for 1 hour to 6 hours.

11. A lithium secondary battery comprising: a positive electrode including the positive electrode active material according to any one of claims 1 to 5; a negative electrode; and an electrolyte.

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