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

Abstract

The present embodiment relates to a cathode active material for a lithium secondary battery, a method for preparing the same, and a lithium secondary battery including the cathode active material. A positive electrode active material for a lithium secondary battery according to an embodiment of the present invention comprises: a core part; the shell part is positioned on the surface of the inner core part; the positive electrode active material for a lithium secondary battery includes a shell portion including needle-shaped particles arranged in a direction from the core portion to the shell portion, and is represented by the following Chemical Formula 1. [Chemical Formula 1] Li1 + q (NixCoyMnz) 1-w (ZraNbb) wO2 (In Chemical Formula 1, q, x, y, z, a, b, and w are-0.1 < = q < = 0.2, 0.0032 lt; wlt; 0.013, 0.7 < = xlt; 1.0, 0 lt; yt; Yt; 0, 0, 0.3, 0; zlt, zlt; x + y + z = 1, 0.2 < = a < = 0.7, 0.3 < = b < = 0.8, and a + b = 1.

Description

Technical Field

[0001] This embodiment relates to a positive electrode active material for a lithium secondary battery, a method for preparing the same, and a lithium secondary battery including the positive electrode active material. Background Art

[0002] Recently, due to the demand for electric vehicles with a driving range of more than 600 km per single charge, secondary batteries with high capacity and high energy density are being actively developed worldwide. To fabricate excellent secondary batteries, the positive electrode material is very important. In the case of NCM-based layered materials with increased nickel content for achieving high capacity, it is necessary to improve performance in terms of initial efficiency, lifespan, output, impedance, and stability. One of the methods to improve these performances is to use a concentration gradient type positive electrode material, that is, increasing the nickel concentration in the core and decreasing the nickel concentration in the shell, thereby greatly improving the stability of the shell directly contacting the electrolyte. However, in the preparation of the precursor, since the feeding tanks of the solution are connected in series to change the coprecipitation composition in real time, there are problems of long precipitation time, low productivity, and rising prices.

[0003] Therefore, the development of a positive electrode active material for a lithium secondary battery with improved secondary battery performance and a method for preparing the same through a simple process is an urgent need at present. Summary of the Invention

[0004] Technical Problem

[0005] An object of the present invention is to provide a positive electrode active material for a lithium secondary battery, a method for preparing the same, and a lithium secondary battery including the positive electrode active material, the positive electrode active material having a structure that can easily intercalate lithium ions on the surface of the positive electrode material without using a conventional concentration gradient structure.

[0006] Technical Solution

[0007] The positive electrode active material for a lithium secondary battery according to an embodiment of the present invention includes: a core part; and

[0008] a shell part located on the surface of the core part; the shell part includes needle-like particles arranged along the direction from the core part to the shell part, and the positive electrode active material for a lithium secondary battery is represented by the following Chemical Formula 1.

[0009] [Chemical Formula 1]

[0010] Li 1+q (Ni x Co y Mn z ) 1-w (Zr a Nb b )w O2

[0011] In the chemical formula 1, q, x, y, z, a, b, and w are such that 0 ≤ q ≤ 0.5, 0.0032 < w < 0.013, 0.7 ≤ x < 1.0, 0 < y < 0.3, 0 < z < 0.3, x + y + z = 1, 0.2 ≤ a ≤ 0.7, 0.3 ≤ b ≤ 0.8, and a + b = 1.

[0012] The aspect ratio of the acicular particles may be 4 to 15, and the thickness of the shell portion may be 0.5 μm to 3 μm.

[0013] In the lithium secondary battery using the positive electrode active material, in the fully charged state, the core portion does not change, and the shell portion can be transformed into a comb shape, with pore channels formed in the direction from the core portion to the shell portion.

[0014] In addition, the exterior of the housing may further include a boron (B) coating.

[0015] The surface of the positive electrode active material may further include rod-shaped primary particles.

[0016] The average length of the rod-shaped primary particles is 300 nm to 800 nm, and the average width is 50 nm to 200 nm.

[0017] The average particle diameter (D50) of the positive electrode active material for the lithium secondary battery may be 10 μm to 16 μm.

[0018] A method for preparing a positive electrode active material for a lithium secondary battery according to another embodiment of the present invention includes: a step of preparing a metal hydroxide containing nickel, manganese, and cobalt; a step of mixing the metal hydroxide, lithium hydroxide, niobium (Nb) oxide, and zirconium (Zr) oxide to obtain a mixture; and a step of heat-treating the mixture to form a shell portion; wherein the niobium (Nb) oxide raw material includes two or more different crystal structures.

[0019] The niobium (Nb) oxide may include Nb2O5 having a monoclinic structure and Nb 16.8 O 42 .

[0020] Wherein, the molar ratio of Nb2O5 to the Nb 16.8 O 42 may be 2:1 to 4:1.

[0021] The molar ratio of niobium (Nb) in the niobium (Nb) oxide to zirconium (Zr) in the zirconium (Zr) oxide can be from 0.5:1 to 4.5:1.

[0022] The step of heat-treating the mixture to form the shell part may include a first heat-treating step performed at a temperature range of 400 °C to 600 °C for 1 hour to 5 hours; and a second heat-treating step performed at a temperature range of 700 °C to 800 °C for 12 hours to 24 hours.

[0023] In the step of preparing the metal hydroxide containing nickel, manganese, and cobalt, a metal hydroxide having an average particle diameter (D50) of 14 μm to 16 μm may be prepared.

[0024] The positive electrode for a lithium secondary battery according to another embodiment of the present invention includes: a current collector; and a positive electrode active material layer for a lithium secondary battery located on at least one surface of the current collector and containing the positive electrode active material described above.

[0025] The lithium secondary battery according to another embodiment of the present invention includes: the positive electrode for a lithium secondary battery.

[0026] Advantageous Effects

[0027] According to an embodiment of the present invention, a positive electrode active material for a lithium secondary battery with improved performance can be provided. It has a structure in which lithium ions can be easily intercalated on the surface of the positive electrode active material by a simple method without using a concentration gradient structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematically shows a method for preparing a positive electrode active material for a lithium secondary battery according to an embodiment of the present invention.

[0029] Figure 2 Shows a surface SEM image of the positive electrode active material for a lithium secondary battery prepared according to Example 1.

[0030] Figure 3 Shows a surface SEM image of the positive electrode active material for a lithium secondary battery prepared according to Comparative Example 1.

[0031] Figure 4 Shows a cross-sectional FIB image of the positive electrode active material for a lithium secondary battery prepared according to Example 1.

[0032] Figure 5 Shows a cross-sectional FIB image of the positive electrode active material for a lithium secondary battery prepared according to Comparative Example 1.

[0033] Figure 6Shows the cross-sectional SEM image of the positive electrode active material in the discharged state after 300 charge-discharge cycles of a coin-type half-cell using the positive electrode active material for a lithium secondary battery prepared according to Example 1.

[0034] Figure 7 Shows the cross-sectional SEM image of the positive electrode active material in the discharged state after 300 charge-discharge cycles of a coin-type half-cell using the positive electrode active material for a lithium secondary battery prepared according to Comparative Example 1.

[0035] Figure 8 Shows the cross-sectional SEM image of the electrode and the positive electrode active material in the charged state after 300 charge-discharge cycles of a coin-type half-cell using the positive electrode active material for a lithium secondary battery prepared according to Example 1.

[0036] Figure 9 Shows the cross-sectional SEM image of the electrode and the positive electrode active material in the charged state after 300 charge-discharge cycles of a coin-type half-cell using the positive electrode active material for a lithium secondary battery prepared according to Comparative Example 1. Detailed Description

[0037] The terms first, second, third, etc. are used to describe various parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, and / or segment from another part, component, region, layer, and / or segment. Thus, without departing from the scope of the present invention, the first part, component, region, layer, and / or segment described below can also be described as the second part, component, region, layer, and / or segment.

[0038] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. Unless the context clearly dictates otherwise, the singular forms used herein are also intended to include the plural forms. The "including" used in the specification can specifically refer to a certain characteristic, field, integer, step, action, element, and / or component, but does not exclude the existence or addition of other characteristics, fields, integers, steps, actions, elements, components, and / or groups.

[0039] Although not otherwise defined, the meanings of all terms (including technical terms and scientific terms) used herein are the same as those commonly understood by those of ordinary skill in the art to which the present invention pertains. For terms defined in a dictionary, they should be interpreted as having a meaning consistent with the relevant technical literature and the content disclosed herein, and should not be interpreted in an idealized or overly formal sense.

[0040] Hereinafter, embodiments of the present invention will be described in detail. However, these embodiments are merely examples, and the present invention is not limited thereto. The present invention is only determined by the scope of the claims.

[0041] The positive electrode active material for a lithium secondary battery according to an embodiment of the present invention includes a core part containing nickel, manganese, and cobalt, and a shell part located on the surface of the core part. The shell part includes needle-like particles in the direction from the core part to the shell part, and the major axis of the needle-like particles can be radially arranged in the direction from the core part to the shell part.

[0042] The positive electrode active material for a lithium secondary battery according to an embodiment of the present invention can be represented by the following Chemical Formula 1.

[0043] [Chemical Formula 1]

[0044] Li 1+q (Ni x Co y Mn z ) 1-w (Zr a Nb b ) w O2

[0045] In Chemical Formula 1, q, x, y, z, a, b, and w respectively satisfy -0.1 ≤ q ≤ 0.2, 0.0032 < w < 0.013, 0.7 ≤ x < 1.0, 0 < y < 0.3, 0 < z < 0.3, x + y + z = 1, 0.2 ≤ a ≤ 0.7, 0.3 ≤ b ≤ 0.8, and a + b = 1.

[0046] When two first transition metal oxides having different crystal structures are mixed in the above ratio, at full charge, the shell part forms a comb-like structure, forming pore channels that serve as convenient mobile channels for lithium ion insertion and extraction, which is beneficial to improving the battery capacity and initial efficiency, reducing the initial resistance, and suppressing the generation of micro cracks in the positive electrode active material during charge and discharge, thereby facilitating the extension of the life of the secondary battery.

[0047] On the other hand, the needle-like particles can be radially and uniformly distributed in the direction from the center of the core part to the surface, and the aspect ratio of the needle-like particles can be in the range of 2 to 30, specifically, in the range of 4 to 15. When the aspect ratio of the needle-like primary particles is in the above range, at full charge, the shell part easily forms a comb-like structure, which is beneficial to the efficient progress of lithium ion intercalation and deintercalation.

[0048] In the present invention, the aspect ratio can be calculated as the ratio of the major axis length to the minor axis length (Length / Width ratio) of the needle-shaped primary particles. When the major axis represents the direction of the relatively longer region of the primary particles, the minor axis represents the length of the relatively shorter region of the primary particles. At this time, the minor axis can be in a direction perpendicular to and intersecting the major axis. The "aspect ratio" of the primary particles can be calculated by the ratio of the major axis to the minor axis measured from the cross-section of the primary particles. Specifically, in the needle-shaped primary particles, the major axis length refers to the longest length among the needle-shaped primary particles observed in the SEM image, and the minor axis length refers to the longest length among the lengths perpendicular to the major axis length.

[0049] The thickness of the shell part can be in the range of 0.5 μm to 3 μm, specifically in the range of 0.5 μm to 2 μm.

[0050] On the other hand, based on the total moles of nickel, manganese, and cobalt, the nickel content in the core part can be 70 mol% or more, specifically 80 mol% or more.

[0051] According to an embodiment of the present invention, in a half-cell using the positive electrode active material as the positive electrode and lithium metal as the negative electrode, when charged at a constant current of 0.5C to 4.2V, the core part of the positive electrode active material does not change, and the shell part can be transformed into a comb shape forming pore channels. Specifically, the pore channels can be formed radially from the center of the core part towards the surface direction. This is conducive to the effective intercalation and deintercalation of lithium ions, conducive to reducing the resistance of the battery and improving the battery performance.

[0052] In addition, the outer side of the shell part can further include a boron (B) coating. In the present invention, as long as the desired battery capacity, initial efficiency, resistance, and lifespan are satisfied, the content of boron is not particularly limited.

[0053] On the other hand, according to an embodiment of the present invention, rod-shaped primary particles can be located on the outer surface of the positive electrode active material for a lithium secondary battery. The average diameter of the rod-shaped primary particles can be in the range of 100 nm to 1000 nm, specifically in the range of 300 nm to 800 nm. In addition, the average width of the rod-shaped primary particles can be in the range of 10 nm to 300 nm, specifically in the range of 50 nm to 200 nm.

[0054] In the present invention, the rod-shaped primary particles may be the particles on the surface of the positive electrode active material observed in the SEM image. The average diameter and average width of the rod-shaped primary particles may be the average values of the diameters and widths of 20 to 50 primary particles observed in the SEM image.

[0055] Meanwhile, the diameter of the rod-shaped primary particle refers to the longest length in the cross-section of the primary particle observed in the SEM image, and the width refers to the longest length among the lengths perpendicular to the diameter.

[0056] Meanwhile, the average particle size (D50) of the positive electrode active material for a lithium secondary battery may be in the range of 5 μm to 20 μm, and specifically may be in the range of 10 μm to 16 μm.

[0057] In the present invention, the average particle size of the positive electrode active material for a lithium secondary battery may be the average value of the particle sizes of 10 to 30 positive electrode active material particles observed in the SEM image. Meanwhile, the particle size of the positive electrode active material particle refers to the longest length in the cross-section observed in the SEM image.

[0058] Figure 1 Schematically shows a method for preparing a positive electrode active material for a lithium secondary battery according to an embodiment of the present invention.

[0059] Refer to Figure 1 , a method for preparing a positive electrode active material for a lithium secondary battery according to another embodiment of the present invention includes the following steps: preparing a metal hydroxide containing nickel, manganese, and cobalt; mixing the metal hydroxide, lithium hydroxide, niobium (Nb) oxide, and zirconium (Zr) oxide to obtain a mixture; and performing heat treatment on the mixture to form a shell part.

[0060] First, prepare a metal hydroxide containing nickel, manganese, and cobalt. In this embodiment, after preparing a metal salt solution containing a nickel raw material substance, a manganese raw material substance, a cobalt raw material substance, and water, the metal salt solution is supplied to a co-precipitation reactor, and a metal hydroxide is obtained according to a method for preparing a positive electrode active material precursor known in the art.

[0061] In addition, the average particle size (D50) of the positive electrode active material precursor may be 10 μm or more, and specifically may be in the range of 14 μm to 16 μm. When the particle size of the precursor satisfies the above range, a shell part composed of rod-shaped primary particles can be effectively formed on the surface of the precursor, and by mixing with small particle sizes, when preparing the electrode plate, the pores can be minimized, thereby preparing a positive electrode active material with improved performance, which has its advantages.

[0062] Next, a step of mixing the metal hydroxide, lithium hydroxide, niobium (Nb) oxide, and zirconium (Zr) oxide to obtain a mixture can be carried out.

[0063] The niobium (Nb) oxide may include a monoclinic crystal structure and an orthorhombic crystal structure. Specifically, the niobium (Nb) oxide may include Nb2O5 having a monoclinic crystal structure and Nb 16.8 O 42 and Nb2O5 may be mixed with Nb 16.8 O 42 in a molar ratio of 2:1 to 4:1.

[0064] Moreover, the molar ratio of niobium (Nb) in the niobium (Nb) oxide to zirconium (Zr) in the zirconium (Zr) oxide may be in the range of 0.5:1 to 4.5:1.

[0065] Subsequently, a step of heat-treating the mixture to form a shell portion can be carried out.

[0066] The heat-treatment step may specifically include: a primary heat-treatment step of performing for 1 hour to 5 hours in a temperature range of 400°C to 600°C; and a secondary heat-treatment step of performing for 10 hours to 20 hours in a temperature range of 700°C to 800°C.

[0067] At this time, the heating rate for raising the temperature to the heat-treatment temperature may be in the range of 1°C / minute to 10°C / minute, specifically in the range of 3°C / minute to 7°C / minute. This is beneficial for the uniform mixing of transition metal oxides with different crystal structures to form a uniform shell portion.

[0068] On the other hand, the step of heat-treating the mixture to form a shell portion may be carried out in an atmosphere with continuous oxygen supply. The oxygen may be supplied at a flow rate of 100 mL / minute to 500 mL / minute, specifically at a flow rate of 100 mL / minute to 300 mL / minute.

[0069] The positive electrode active material obtained through the heat treatment can be subjected to post-treatment processes such as cooling, crushing, and classification.

[0070] In addition, after washing and drying, dry mixing heat treatment of H3BO3 (Aldrich) can be carried out to further form a boron (B) coating.

[0071] In another embodiment, a positive electrode for a lithium secondary battery may be provided, including: a current collector; and a positive electrode active material layer located on at least one surface of the current collector and containing the positive electrode active material prepared according to the above embodiment.

[0072] The characteristics of the positive electrode active material constituting the positive electrode active material layer are the same as those described above. Therefore, the specific description of the positive electrode active material will be omitted.

[0073] The positive electrode active material layer may contain a binder and a conductive agent.

[0074] The binder can enhance the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. If it is a binder for preparing a positive electrode of a lithium secondary battery, its type is not particularly limited, and one or more binders can be used. The binder may be contained in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

[0075] The conductive agent is used to impart conductivity to the electrode. In the formed battery, as long as it does not cause chemical changes and has electronic conductivity, its type is not particularly limited. The conductive agent is usually contained in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

[0076] In addition to using the positive electrode active material, the positive electrode can be prepared according to a general positive electrode preparation method.

[0077] In another embodiment, a lithium secondary battery including the positive electrode is provided.

[0078] The lithium secondary battery specifically includes a positive electrode, a negative electrode positioned opposite to the positive electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is as described above. In addition, the lithium secondary battery may optionally include a battery container for accommodating the electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing assembly for sealing the battery container.

[0079] In addition to using the positive electrode active material, the lithium secondary battery can be prepared according to a general lithium secondary battery preparation method, and is not particularly limited.

[0080] Embodiments of the Invention

[0081] Hereinafter, the present invention will be described in more detail by way of examples. However, these examples are only intended to illustrate the present invention, and the present invention is not limited to the examples described herein. The present invention is only defined by the scope of the following claims.

[0082] (Prepare a metal hydroxide containing nickel, manganese, and cobalt)

[0083] A metal hydroxide containing nickel, manganese, and cobalt was synthesized by a conventional co - precipitation method. Specifically, NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O were dissolved in deionized water (DI water) to prepare a 2.5 M metal salt aqueous solution.

[0084] To prevent the oxidation of metal ions during the co - precipitation reaction, nitrogen was introduced and the temperature inside the reactor was maintained at 50 °C.

[0085] The metal salt aqueous solution and NH4OH used as a co - precipitation chelating agent were added to a co - precipitation reactor (100 L), and NaOH was added to adjust the pH value of the mixed solution inside the reactor. After the average particle size (D50) of the finally formed metal hydroxide precursor reached 14 μm, the finally formed metal hydroxide precursor was filtered and separated, washed with deionized water (DI water), and then dried in an oven at 100 °C for 24 hours. Finally, a metal hydroxide precursor with the composition (Ni 0.88 Co 0.10 Mn 0.02 )(OH)2 was obtained.

[0086] (Example 1: Preparation of the positive electrode active material)

[0087] Based on 1 mole of the prepared metal hydroxide precursor, 1.05 moles of LiOH·H2O (ternary chemistry, battery grade) were mixed, and ZrO2 (Aldrich, 4N) was added to make Zr reach 0.0022 moles. At the same time, niobium oxides with a monoclinic structure of Nb2O5 and an orthorhombic structure of Nb 16.8 O 42 were mixed. These oxides were mixed in a mass ratio of 76:24 to finally make niobium reach 0.005 moles.

[0088] Then the mixture was loaded into a tube furnace (inner diameter 50 mm, length 1000 mm) and sintered under an oxygen flow of 200 mL / min. The specific sintering process was as follows: heated to 480 °C at a heating rate of 5 °C / min, held at this temperature for 5 hours, then heated to 740 - 780 °C at a heating rate of 5 °C / min, and held at this temperature for 16 hours. Then, after the sintered body was cooled to room temperature, it was crushed / graded to prepare a Zr - Nb - doped NCM positive electrode active material.

[0089] 100 g of the prepared positive electrode active material was added to 100 g of deionized water (DI water), stirred for 10 minutes, and then filtered to recover the positive electrode active material. The recovered positive electrode active material was dried in a chamber at a temperature above 100 °C and then dry-mixed with H3BO3 (Aldrich), and then heat-treated at a temperature of 300 - 350 °C for 5 hours in an air atmosphere to prepare the final positive electrode active material.

[0090] (Examples 2 to 4 and Comparative Examples 1 to 2: Preparation of positive electrode active material)

[0091] Except for adjusting the niobium oxide mixed with monoclinic Nb2O5 and orthorhombic Nb 16.8 O 42 in a mass ratio of 76:24 so that the final NbO is as shown in Table 1, the positive electrode active material was prepared in the same manner as in Example 1.

[0092]

Table 1

[0093]

[0094] (Characteristic analysis 1 - SEM analysis)

[0095] The surfaces of the positive electrode active materials prepared in Example 1 and Comparative Example 1 were analyzed by SEM, and the results are as Figure 2 and Figure 3 shown.

[0096] Referring to Figure 2 , it can be confirmed that rod-shaped primary particles were formed on the surface of the positive electrode active material prepared in Example 1, and the width of the primary particles was approximately in the range of 50 to 200 nm, and the length was approximately in the range of 300 to 800 nm.

[0097] Referring to Figure 3 , it can be confirmed that angular particles were formed on the surface of the positive electrode active material prepared in Comparative Example 1. Specifically, primary particles in the shape of a square or rectangle were formed.

[0098] The horizontal width of the primary particles was approximately in the range of 200 to 600 nm, and the vertical length was in the range of 200 to 600 nm.

[0099] (Characteristic analysis 2 - FIB analysis)

[0100] The cross-sectional FIB analysis results of the positive electrode active materials prepared in Example 1 and Comparative Example 1 are as Figure 4 and Figure 5 shown.

[0101] Refer to Figure 4, it can be confirmed that in the region with a thickness of about 1 μm from the surface towards the core of the positive electrode active material prepared in Example 1, acicular particles are uniformly distributed. In addition, it can be confirmed that the long axes of the acicular particles are radially distributed from the surface towards the center of the core part.

[0102] The length of the acicular particles is about 0.8 μm to 1.0 μm, and the width is about 80 nm to 200 nm.

[0103] Reference Figure 5 , it can be confirmed that in the cross-section of the positive electrode active material prepared in Comparative Example 1, square or rectangular shaped square particles are formed. In addition, it can be confirmed that the lateral width of the square particles is about 200 nm to 600 nm, and the longitudinal length is about 200 nm to 600 nm.

[0104] (Preparing a coin-type half-cell)

[0105] Using the positive electrode active materials prepared through the above-mentioned examples and comparative examples, CR2032 coin-type batteries were prepared. After electrochemical evaluation, the results are summarized in Table 2 below.

[0106] Specifically, the positive electrode active material: conductive agent (carbon black, denka black): binder (PVDF, KF1100) was mixed at a weight ratio of 92.5:3.5:4, and NMP (N-Methyl-2-pyrrolidone) was added until the solid content was about 30%, and the viscosity of the slurry was adjusted to prepare a slurry for the electrode plate. The prepared slurry was coated on a 15-μm thick aluminum foil using a doctor blade, dried and then calendered. The electrode loading was about 14 mg / cm 2 , and the calendered density was about 3.4 g / cm 3 . The electrolyte was a mixture of 1 M LiPF6 and EC:DMC:EMC = 3:4:3 (volume %), and then 1.5 wt% of vinylene carbonate (VC) was added. After preparing a coin-type half-cell using a PP separator membrane and a lithium negative electrode (200 μm, Honzo metal), it was aged at room temperature for 10 hours.

[0107] The prepared coin-type half-cell was used for charge-discharge testing.

[0108] Figure 6 Shows the cross-sectional SEM image of the positive electrode active material in the discharged state of a coin-type half-cell using the positive electrode active material for lithium secondary batteries prepared according to Example 1 after 300 charge-discharge cycles. Figure 7Shows the cross-sectional SEM image of the positive electrode active material in the discharged state after 300 charge-discharge cycles of a coin-type half-cell using the positive electrode active material for a lithium secondary battery prepared according to Comparative Example 1.

[0109] Referring to Figure 6 , for the coin-type half-cell using the positive electrode active material for a lithium secondary battery of Example 1, after 300 charge-discharge cycles, in the discharged state, almost no micro-cracks were found in the positive electrode active material itself except for the cracks generated during electrode preparation.

[0110] In contrast, referring to Figure 7 , for the coin-type half-cell using the positive electrode active material for a lithium secondary battery of Comparative Example 1, after 300 charge-discharge cycles, serious cracks appeared in the positive electrode active material particles in the discharged state.

[0111] Figure 8 Shows the cross-sectional SEM image of the electrode and the positive electrode active material in the charged state after 300 charge-discharge cycles of a coin-type half-cell using the positive electrode active material for a lithium secondary battery prepared according to Example 1.

[0112] Specifically, Figure 8 (a) shows the cross-sectional SEM image of the positive electrode in the charged state after 300 charge-discharge cycles of the coin-type half-cell according to Example 1, Figure 8 (b) shows the cross-sectional SEM image of the positive electrode active material. Referring to Figure 8 , it can be confirmed that for the positive electrode active material for a lithium secondary battery according to Example 1, after charge and discharge, even in the charged state with a large volume expansion, the cracking phenomenon does not occur significantly. In addition, in the charged state, the core part of the positive electrode active material maintains its shape unchanged, and when lithium ions are released, the shell part forms a shape similar to a comb. The core part that maintains its shape firmly grasps the shell part that has become a comb shape, thereby suppressing the generation of micro-cracks. Therefore, it is expected that the shortening of the battery life due to the generation of micro-cracks can be prevented.

[0113] Figure 9 Shows the cross-sectional SEM image of the electrode and the positive electrode active material in the charged state after 300 charge-discharge cycles of a coin-type half-cell using the positive electrode active material for a lithium secondary battery prepared according to Comparative Example 1.

[0114] Referring to Figure 9 , it can be confirmed that for the positive electrode active material for a lithium secondary battery according to Comparative Example 1, after charge and discharge, in the charged state with a large volume expansion, serious cracks appear in the electrode, and the interval between the primary particles of the positive electrode active material is large and the strength is weak.

[0115] The charge-discharge test results of the coin-type half-cells using the positive electrode active materials according to Examples 1 to 4 and Comparative Examples 1 to 3 are summarized in Table 2 below.

[0116] [Table 2]

[0117]

[0118]

[0119] Referring to Table 2, it can be confirmed that the coin-type half-cells using the positive electrode active materials according to Examples 1 to 4 have high initial charge-discharge capacities and initial efficiencies, and excellent life characteristics. Therefore, it can be judged that the generation of microcracks during the charge-discharge process of the positive electrode active material prepared according to the embodiments of the present invention is reduced, thereby improving the life and cycle characteristics of the battery.

[0120] The present invention is not limited to the above embodiments, but can be implemented in various different forms. Those skilled in the art should understand that the present invention can be implemented in other specific forms without changing the technical idea or basic characteristics of the present invention. Therefore, it should be understood that the above embodiments are exemplary in all aspects and not restrictive.

Claims

1. A positive electrode active material for a lithium secondary battery, comprising: a core part; and a shell part located on the surface of the core part; the shell part includes acicular particles arranged along the direction from the core part to the shell part, the positive electrode active material for the lithium secondary battery is represented by the following Chemical Formula 1, [Chemical Formula 1] Li 1+q (Ni x Co y Mn z ) 1-w (Zr a Nb b ) w O₂ In Chemical Formula 1, q, x, y, z, a, b, and w are such that 0 ≤ q ≤ 0.5, 0.0032 < w < 0.013, 0.7 ≤ x < 1.0, 0 < y < 0.3, 0 < z < 0.3, x + y + z = 1, 0.2 ≤ a ≤ 0.7, 0.3 ≤ b ≤ 0.8, and a + b = 1.

2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the aspect ratio of the acicular particles is 4 to 15.

3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the thickness of the shell part is 0.5 μm to 3 μm.

4. The positive electrode active material for a lithium secondary battery according to claim 1, wherein in a lithium secondary battery using the positive electrode active material, in a fully charged state, the core part does not change, and the shell part transforms into a comb shape, and pore channels are formed in the direction from the core part to the shell part.

5. The positive electrode active material for a lithium secondary battery according to claim 1, wherein a boron (B) coating is further included outside the shell.

6. The positive electrode active material for a lithium secondary battery according to claim 5, wherein rod-shaped primary particles are further included on the surface of the positive electrode active material.

7. The positive electrode active material for a lithium secondary battery according to claim 6, wherein the average length of the rod-shaped primary particles is 300 nm to 800 nm, and the average width is 50 nm to 200 nm.

8. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the average particle diameter (D50) of the positive electrode active material for the lithium secondary battery is 10 μm to 16 μm.

9. A method for preparing a positive electrode active material for a lithium secondary battery, comprising: a step of preparing a metal hydroxide containing nickel, manganese, and cobalt; a step of mixing the metal hydroxide, lithium hydroxide, niobium (Nb) oxide, and zirconium (Zr) oxide to obtain a mixture; and a step of heat-treating the mixture to form a shell part; wherein the niobium (Nb) oxide raw material includes two or more different crystal structures.

10. The method for preparing a positive electrode active material for a lithium secondary battery according to claim 9, wherein The niobium (Nb) oxide includes Nb2O5 with a monoclinic crystal structure and Nb 16.8 O 42 .

11. The method for preparing a positive electrode active material for a lithium secondary battery according to claim 10, wherein The molar ratio of the Nb2O5 to the Nb 16.8 O 42 is from 2:1 to 4:

1.

12. The method for preparing a positive electrode active material for a lithium secondary battery according to claim 9, wherein the molar ratio of niobium (Nb) in the niobium (Nb) oxide to zirconium (Zr) in the zirconium (Zr) oxide is 0.5:1 to 4.5:

1.

13. The method for preparing a positive electrode active material for a lithium secondary battery according to claim 9, wherein the step of heat-treating the mixture to form a shell part includes: a first heat-treatment step of performing heat treatment at a temperature range of 400°C to 600°C for 1 hour to 5 hours; and A secondary heat treatment step is carried out at a temperature in the range of 700 °C to 800 °C for 12 hours to 24 hours.

14. The method for preparing a positive electrode active material for a lithium secondary battery according to claim 9, wherein, In the step of preparing a metal hydroxide containing nickel, manganese and cobalt, a metal hydroxide with an average particle size (D50) in the range of 14 μm to 16 μm is prepared.

15. A positive electrode for a lithium secondary battery, comprising: A current collector; and A positive electrode active material layer for a lithium secondary battery located on at least one surface of the current collector and containing the positive electrode active material for a lithium secondary battery according to any one of claims 1 to 8.

16. A lithium secondary battery, comprising: The positive electrode for a lithium secondary battery according to claim 15.

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