Polycrystalline positive electrode active material and lithium secondary battery comprising same

Abstract

The present invention relates to a novel polycrystalline positive electrode active material, and more particularly, to a novel polycrystalline positive electrode active material in which the size and number of primary particles and the size of secondary particles are determined by using the peak intensities of an H2 phase (H2 phase) and an H3 phase (H3 phase). Thus, the present invention has the effect of improving life characteristics while ensuring excellent output and capacity characteristics.

Description

Technical Field

[0001] This invention relates to a polycrystalline cathode active material and a lithium secondary battery comprising the same. More specifically, it relates to a novel polycrystalline cathode active material and a lithium secondary battery comprising the same, which exhibits lifetime, capacity, and output characteristics different from existing polycrystalline cathode active materials. Background Technology

[0002] Lithium-ion rechargeable batteries store and release energy by moving lithium ions between electrodes. Due to their rechargeable and dischargeable characteristics, they are widely used in various fields such as electronic devices, electric vehicles, and energy storage devices. In particular, compared with other metal ions, lithium ions are lighter and have a higher energy density, thus playing a crucial role in the development of high-efficiency batteries. One of the key factors determining the performance of lithium-ion rechargeable batteries is the positive electrode active material, which maintains stability during the insertion and extraction of lithium ions and directly affects the battery's capacity and lifespan.

[0003] The energy density, output, lifespan, and stability of a battery vary greatly depending on the type of positive electrode active material. Positive electrode active materials are typically composed of lithium metal oxides, with representative examples including lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium nickel cobalt manganese oxide (NCM), and lithium nickel cobalt aluminum oxide (NCA).

[0004] In recent years, medium-nickel and high-nickel cathode active materials have attracted much attention to meet the high energy density and capacity requirements of electric vehicles and other applications. Since the energy density and capacity of nickel cathode materials increase with the increase of the nickel content, they are popular in markets requiring higher-performance batteries. Representative materials include high-nickel-content NCM (nickel, cobalt, manganese) or NCA (nickel, cobalt, aluminum) materials, which are contributing to increasing the driving range of electric vehicles and improving battery energy efficiency.

[0005] Positive electrode active materials can be classified into polycrystalline and monocrystalline based on their crystal structure.

[0006] Polycrystalline cathode active materials are structures formed by the aggregation of multiple small crystal particles into a single particle. The fabrication process for polycrystalline structures is relatively easy and low-cost, and they exhibit excellent performance at specific charge / discharge rates. However, polycrystalline particles are prone to crack formation between particles during charge / discharge, and gas generation issues exist, which are detrimental to long-term lifespan. In particular, high-nickel polycrystalline cathode active materials require solutions to the expansion and stability problems caused by gas generation.

[0007] Single-crystal cathode active materials have a particle structure composed of a single crystal. The advantages of a single-crystal structure include fewer cracks generated between particles during charging / discharging, high stability, and long lifespan. However, the disadvantages of single-crystals include difficult preparation, high cost, and lower output characteristics compared to polycrystalline materials. Furthermore, when having the same nickel composition, they tend to have lower capacity than polycrystalline materials, which may lead to performance degradation.

[0008] Therefore, in order to improve the performance of lithium secondary batteries, it is necessary to develop a new type of positive electrode active material to solve the problems that arise in nickel-based positive electrode active materials and make up for the shortcomings of polycrystalline and monocrystalline positive electrode active materials.

[0009] Existing technical documents Patent documents Patent document 1: Korean Patent Publication No. 10-2023-0162830. Summary of the Invention

[0010] Technical issues The purpose of this invention is to provide a novel polycrystalline cathode active material that can ensure excellent output and capacity characteristics while improving lifetime characteristics through primary particles with an average particle size of 500 nm or more.

[0011] Another object of the present invention is to provide a lithium secondary battery comprising the polycrystalline positive electrode active material.

[0012] Technical solution One specific embodiment of the present invention provides a polycrystalline positive electrode active material comprising: primary particles having an average particle size (D50) of 500 nm to 5 μm; and secondary particles formed by the aggregation of the primary particles, wherein a portion of the primary particles is exposed on the outer surface of the secondary particles.

[0013] The average particle size (D50) of the primary and secondary particles can be determined by the H2 intensity ratio calculated using Equation 1 below.

[0014] Formula 1:

[0015] In Equation 1, the H2 intensity and H3 intensity are the peak intensities of the H2 phase and H3 phase obtained by X-ray diffraction (XRD) analysis.

[0016] The number of primary particles that form the secondary particles can be determined by the H2 intensity ratio calculated using Equation 1 below.

[0017] Formula 1:

[0018] In Equation 1, the H2 intensity and H3 intensity are the peak intensities of the H2 phase and H3 phase obtained by X-ray diffraction analysis.

[0019] When the nickel content of the primary particles is 50 mol% or more and less than 80 mol% or more, the average particle size (D50) of the primary and secondary particles can be the average particle size measured when the H2 intensity ratio is 5% or more and less than 16%.

[0020] When the nickel content of the primary particles is 50 mol% or more and less than 80 mol% or more, the number of primary particles forming the secondary particles can be the number measured when the H2 intensity ratio is 5% or more and less than 16%.

[0021] When the nickel content of the primary particles is 80 mol% or more, the average particle size (D50) of the primary and secondary particles can be the average particle size measured when the H2 intensity ratio is 10% or more and less than 25%.

[0022] When the nickel content of the primary particles is 80 mol% or more, the number of primary particles forming the secondary particles can be the number determined when the H2 intensity ratio is 10% or more and less than 25%.

[0023] The average particle size (D50) of the secondary particles can be 1.5 to 5.0 times that of the average particle size (D50) of the primary particles.

[0024] The secondary particles can be formed by the aggregation of 2 to 5 primary particles.

[0025] Another specific embodiment of the present invention provides a positive electrode slurry composition comprising the polycrystalline positive electrode active material, a conductive material, and a binder.

[0026] Another specific embodiment of the present invention provides a lithium secondary battery, wherein the positive electrode is formed by coating a current collector with the positive electrode slurry composition.

[0027] The effects of the invention The polycrystalline positive electrode active material according to embodiments of the present invention obtains secondary particles of optimal size formed by the optimal number of primary particles with an average particle size of 500 nm or more aggregated together, thereby having the advantage of improving lifetime characteristics while ensuring capacity and output characteristics. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a polycrystalline positive electrode active material according to an embodiment of the present invention.

[0029] Figures 2a to 2hTo magnify and observe the positive electrode plates of Examples 1 to 3 and Comparative Examples 1 to 5 using a scanning electron microscope (SEM).

[0030] Figures 3a-3b A graph showing the correlation between lifetime and H2 intensity ratio for Examples 1 to 3 and Comparative Examples 1 to 5.

[0031] Explanation of reference numerals in the attached figures 1: Positive electrode active material; 10: Primary particle; 20: Secondary particles. Detailed Implementation

[0032] Expressions such as “including” used in this specification should be understood as open-ended terms, implying that other technical features may be included.

[0033] As used in this specification, "as an example," "as an embodiment," and "preferred" refer to an embodiment of the invention that can provide the specified advantages under specified conditions, and are not intended to exclude other embodiments from the scope of the invention.

[0034] Figure 1 This is a schematic diagram showing the shape of the polycrystalline positive electrode active material 1 according to an embodiment of the present invention. (Reference) Figure 1 According to an embodiment of the present invention, the polycrystalline positive electrode active material 1 comprises primary particles 10 and secondary particles 20 formed by the aggregation of the primary particles 10, and provides a portion of the primary particles 10 exposed on the outer surface of the secondary particles 20.

[0035] As an example, the positive electrode active material can be a lithium composite oxide.

[0036] As a preferred example, the lithium composite oxide can be as shown in Chemical Formula 1 below.

[0037] Chemical Formula 1: Li w Ni 1-（x+y+z） Co x M1 y M2 z O2 Wherein, M1 is at least one selected from Mn and Al, and M2 is at least one selected from Mn, Ba, Ce, Hf, Ta, Cr, F, Mg, Al, Cr, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Sr, Ge, Nd, Gd, B, and Cu. M1 and M2 are distinct elements, 0.5 ≤ w ≤ 1.5, 0 ≤ x ≤ 0.50, 0 <y≤0.20,0≤z≤0.20。

[0038] As a preferred example, the positive electrode active material can be a nickel-lithium composite oxide (HNCM, High-Nickel Lithium Composite Oxide) of the following chemical formula 2.

[0039] Chemical Formula 2: LiNi x Mn y Co z O2 Where x + y + z = 1, 0.40 ≤ x ≤ 0.96, 0 <y≤0.20,0≤z≤0.20。

[0040] As an example, the primary particles can be medium-nickel cathode active materials.

[0041] As a preferred example, the nickel content relative to 100 mol% of the total primary particles can be 50 mol% or more, 53 mol% or more, 56 mol% or more, 59 mol% or more, or 60 mol% or more. Furthermore, relative to 100 mol% of the total primary particles, the nickel content can be less than 80 mol%, less than 75 mol%, or less than 70 mol%.

[0042] As an example, the primary particles can be high-nickel cathode active materials.

[0043] As a preferred example, the nickel content relative to 100 mol% of the total primary particles can be 80 mol% or more, 83 mol% or more, 86 mol% or more, 89 mol% or more, or 90 mol% or more. Furthermore, relative to 100 mol% of the total primary particles, the nickel content can be less than 110 mol%, less than 105 mol%, or less than 100 mol%.

[0044] The average particle size (D50) of the primary particles 10 can be 500 nm or more, 600 nm or more, 700 nm or more, 800 nm or more, or 900 nm or more. Furthermore, the average particle size (D50) of the primary particles 10 can be less than 5 μm, less than 4.8 μm, less than 4.6 μm, less than 4.4 μm, or less than 4.1 μm. Primary particles 10 with such average particle sizes have smoother lithium ion movement paths within the ions, resulting in improved conductivity, which is beneficial for capacity and output characteristics.

[0045] The term “average particle size (D50)” used in this invention may refer to the median particle size corresponding to 50% of the cumulative distribution (quantity baseline distribution) in which the particles are arranged in order of size.

[0046] As an example, the secondary particles 20 can be formed by the aggregation of two or more primary particles 10. In this case, the secondary particles 20 can be formed by the primary particles 10 adhering closely to each other and combining to stabilize into a larger structure. The aggregation of primary particles 10 increases the bonding force between particles, thereby reducing the occurrence of particle cracks that may occur during charging / discharging and improving the performance and lifespan characteristics of the battery. Furthermore, the aggregated secondary particles 20 help ensure the movement path of lithium ions and facilitate the smooth diffusion of lithium ions within the positive electrode active material 1.

[0047] As an example, the primary particle 10 can be in the form where a portion of the primary particle 10 is exposed on the outer surface of the secondary particle 20. The form in which the primary particle 10 is exposed refers to a structure in which a portion of the primary particle 10 is exposed on the outer surface of the secondary particle 20. That is, the primary particle 10 is not completely embedded inside the secondary particle 20, but rather a portion of the primary particle 10 is in contact with the outside, allowing lithium ions or electrons to easily access it.

[0048] As a preferred embodiment, the secondary particles 20 formed by the aggregation of the primary particles 10 may not contain isolated primary particles 10. An isolated primary particle 10 refers to a particle that is trapped inside the secondary particles 20 and does not come into contact with the outside.

[0049] As a preferred embodiment, the primary particles 10 can aggregate in numbers of 2 or more, 3 or more, or 4 or more to form secondary particles 2. Furthermore, the primary particles 10 can aggregate in numbers of 10 or fewer, 9 or fewer, 8 or fewer, 7 or fewer, 6 or fewer, or 5 or fewer to form secondary particles 20. In this case, optimal particle density and uniform size distribution are maintained, thereby effectively ensuring the movement path of lithium ions and minimizing the interaction between particles during charging / discharging.

[0050] As an example, the average particle size of the secondary particles 20 can be the average particle size measured without including isolated primary particles 10.

[0051] When a lithium-ion battery is charged, lithium ions flow out from the positive electrode, while electrons move to the negative electrode through an external circuit. During this process, the crystal structure within the positive electrode active material 1 changes, and structural deformation and phase transition occur due to the desorption of lithium ions.

[0052] In this specification, the H1 phase refers to the structure of the positive electrode active material 1 in the discharged state where lithium ions are fully intercalated, and the H2 phase refers to the intermediate charging state where lithium ions are partially intercalated and deintercalated. On the other hand, the H3 phase refers to the high-charge state where lithium ions are almost completely deintercalated and deintercalated. During charging, lithium ions flow out from the inside of the positive electrode active material 1, causing the c-axis of the positive electrode active material 1 to contract, thereby resulting in a phase transition from the H2 phase to the H3 phase.

[0053] However, when the c-axis of the positive electrode active material 1 contracts, cracks may form in the particles, resulting in isolated particles inside. These isolated particles hinder lithium-ion diffusion and therefore cannot contribute to capacity. That is, isolated particles lead to decreased charging efficiency, reduced capacity, and degraded lifetime. Furthermore, when residual H2 phase that has not transferred to the H3 phase remains, H2-H3 phase separation occurs. This H2-H3 phase separation can be confirmed by X-ray diffraction analysis, and the ratio of H2 phase to H3 phase can be used to assess the region in the positive electrode active material 1 where lithium-ion diffusion is hindered, i.e., the region containing isolated particles.

[0054] As a preferred example, the average particle size of the primary and secondary particles 20 of the positive electrode active material 1 in the charged state can be determined by X-ray diffraction analysis of the H2 and H3 phases.

[0055] As a preferred example, when the nickel content of the primary particles is 50 mol% or more and less than 80 mol%, the average particle size of the primary and secondary particles can be the particle size when the H2 intensity ratio calculated by the following formula is 5% or more, 6% or more, 7% or more or 8% or more, and 16% or less, 15% or less or 14% or less.

[0056] Formula 1:

[0057] As a preferred example, in the case of a medium-nickel cathode active material with a primary particle nickel content of 50 mol% or more but less than 80 mol% , the XRD peak intensities of the H2 and H3 phases were measured and their ratios were calculated. It was found that the more H2 phase there was, the worse the lithium ion insertion / extraction was, and therefore it could be used as an indicator of the number of isolated particles. On the other hand, when this value is 5% or more but less than 16%, there are no isolated particles, and the lifetime does not decrease. The sizes of the primary particle 10 and secondary particle 20 determined at this time can be considered as optimal sizes.

[0058] The X-ray diffraction analysis can be performed by incident Cu-Kα rays onto the positive electrode active material 1 and measuring the diffraction intensity of the diffraction angle (2θ) that appears on the (104) plane.

[0059] As a preferred example, when the nickel content of the primary particles is 80 mol% or more, the average particle size of the primary and secondary particles can be the particle size calculated by the following formula 1 when the H2 intensity ratio is 10% or more, 11% or more and 25% or less, or 23% or less.

[0060] Formula 1:

[0061] As a preferred example, in the case of a high-nickel cathode active material with a nickel content of 80 mol% or more in the primary particles, the XRD peak intensities of the H2 and H3 phases were measured and their ratios were calculated. It was found that the more H2 phase there was, the worse the lithium-ion insertion / extraction was, and therefore it could be used as an indicator of the number of isolated particles. On the other hand, when this value is above 10% and below 25%, there are no isolated particles, and the lifetime does not decrease. The sizes of the primary particles 10 and secondary particles 20 determined at this time can be considered as optimal sizes.

[0062] As an example, the average particle size (D50) of the secondary particles 20 can be more than 1.2 times, more than 1.3 times, more than 1.4 times, more than 1.5 times, or more than 1.6 times the average particle size (D50) of the primary particles 10, and less than 6.0 times, less than 5.8 times, less than 5.6 times, less than 5.4 times, less than 5.2 times, or less than 5.0 times. When the polycrystalline positive electrode active material 1 meets the above range, even if it contains primary particles 10 of 500 nm to 5 μm, its lifetime retention rate increases with increasing particle size, just like single-crystal particles.

[0063] As an example, the number of primary particles forming secondary particles can be determined by X-ray diffraction analysis of the H2 and H3 phases of the positive electrode active material 1 in the charging state.

[0064] Formula 1:

[0065] In Equation 1, the H2 intensity and H3 intensity are the peak intensities of the H2 phase and H3 phase obtained by X-ray diffraction analysis.

[0066] As a preferred example, when the nickel content of the primary particles is 50 mol% or more and less than 80 mol%, the average particle size of the primary and secondary particles can be the number of primary particles forming secondary particles when the H2 intensity ratio calculated by Formula 1 below is 5% or more, 6% or more, 7% or more or 8% or more, and 16% or less, 15% or less or 14% or less.

[0067] Formula 1:

[0068] As a preferred example, in the case of a medium-nickel cathode active material with a primary particle nickel content of 50 mol% or more but less than 80 mol% , the XRD peak intensities of the H2 and H3 phases were measured and their ratios were calculated. It was found that the more H2 phase there was, the worse the lithium ion insertion / extraction was, and therefore it could be used as an indicator of the number of isolated particles. On the other hand, when this value is 5% or more but less than 16%, there are no isolated particles, and the lifetime does not decrease. The number of primary particles forming secondary particles determined at this time can be considered the optimal number.

[0069] The X-ray diffraction analysis can be performed by incident Cu-Kα rays onto the positive electrode active material 1 and measuring the diffraction intensity of the diffraction angle (2θ) that appears on the (104) plane.

[0070] As a preferred example, when the nickel content of the primary particles is 80 mol% or more, the average particle size of the primary and secondary particles can be the number of primary particles forming secondary particles when the H2 intensity ratio calculated by Formula 1 below is 10% or more or 11% or more and 25% or less or 23% or less.

[0071] Formula 1:

[0072] As a preferred example, in the case of a high-nickel cathode active material with a nickel content of 80 mol% or more in primary particles, the XRD peak intensities of the H2 and H3 phases were measured and their ratios were calculated. It was found that the more H2 phase there was, the worse the lithium-ion insertion / extraction was, and therefore it could be used as an indicator of the number of isolated particles. On the other hand, when this value is above 10% and below 25%, there are no isolated particles, and the lifetime does not decrease. The number of primary particles forming secondary particles determined at this time can be considered the optimal number.

[0073] As a preferred embodiment, the primary particles 10 can aggregate in numbers of 2 or more, 3 or more, or 4 or more to form secondary particles 20. Furthermore, the primary particles 10 can aggregate in numbers of 10 or fewer, 9 or fewer, 8 or fewer, 7 or fewer, or 6 or fewer to form secondary particles 20.

[0074] The positive electrode slurry composition according to an embodiment of the present invention comprises the above-mentioned polycrystalline positive electrode active material, conductive material and binder.

[0075] As an example, the conductive material is used to impart conductivity to the electrode. In the constructed battery, any material can be used without special restrictions, as long as it does not cause a chemical change and has electronic conductivity. Specific examples include graphite, such as natural or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber, etc.; metal powders or metal fibers, such as copper, nickel, aluminum, silver, etc.; conductive whiskers, such as zinc oxide, potassium titanate, etc.; conductive metal oxides, such as titanium oxide, etc.; or conductive polymers, such as polystyrene derivatives, etc., and one or a mixture of two or more of these can be used. Typically, the content of the conductive material can be from 1% to 30% by weight relative to the total weight of the positive electrode slurry composition.

[0076] As an example, the adhesive enhances the adhesion between the positive electrode active material particles and the bonding force between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), PVDF-co-HFP, polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and may use one or a mixture of two or more of these. The content of the adhesive relative to the positive electrode slurry can be from 1% to 30% by weight.

[0077] As a preferred embodiment, the positive electrode slurry may further comprise a solvent. The solvent may be one commonly used in this art, including dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be one or a mixture of two or more of these. Considering the coating thickness and preparation yield of the slurry, the amount of solvent used should be sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and should be viscous to ensure excellent thickness uniformity during subsequent coating for the preparation of the positive electrode.

[0078] A lithium secondary battery according to an embodiment of the present invention includes a positive electrode formed by coating a current collector with the above-described positive electrode slurry composition.

[0079] As an example, the positive electrode can be prepared by coating a positive electrode slurry composition comprising the above-described positive electrode active material and optionally a binder and conductive material onto a positive electrode current collector, followed by drying and calendering. In this case, the type and content of the positive electrode active material, binder, and conductive material are as described above.

[0080] Furthermore, as another method, the positive electrode can also be prepared by casting the positive electrode slurry composition onto a separate support, and then laminating the film obtained by peeling it off from the support onto the positive electrode current collector.

[0081] As an example, the positive electrode current collector is not particularly limited, as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or materials that have undergone surface treatment of aluminum or stainless steel with carbon, nickel, titanium, silver, etc., can be used. Furthermore, the thickness of the positive electrode current collector can typically range from 3 μm to 500 μm, and the adhesion of the positive electrode active material can be improved by forming fine irregularities on the surface of the current collector. For example, it can be used in various forms such as thin films, sheets, foils, meshes, porous materials, foams, and non-woven fabrics.

[0082] According to another embodiment of the present invention, an electrochemical device is provided, including the positive electrode. Specifically, the electrochemical device can be a battery or a capacitor, and more specifically, it can be a lithium secondary battery.

[0083] As an example, the lithium secondary battery includes: a positive electrode; a negative electrode disposed opposite to the positive electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode is as described above. Furthermore, the lithium secondary battery may optionally include: a battery container for housing an electrode assembly composed of the positive electrode, the negative electrode, and the separator; and a sealing component for sealing the battery container.

[0084] As an example, in the lithium secondary battery, the negative electrode includes: a negative electrode current collector; and a negative electrode slurry located on the negative electrode current collector.

[0085] As an example, there are no particular limitations on the negative electrode current collector, as long as it does not cause chemical changes in the battery and has high conductivity. For example, materials such as copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or materials that have undergone surface treatment of copper or stainless steel with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. Furthermore, the thickness of the negative electrode current collector can typically be from 3 μm to 500 μm. Similar to the positive electrode current collector, the bonding force of the negative electrode active material can be enhanced by forming fine irregularities on the surface of the current collector. For example, it can be used in various forms such as thin films, sheets, foils, meshes, porous materials, foams, and non-woven fabrics.

[0086] As an example, the negative electrode slurry includes a negative electrode active material and optionally a binder and a conductive material. The negative electrode slurry can be prepared, for example, by coating a composition comprising the negative electrode active material and optionally a binder and a conductive material onto a negative electrode current collector and drying it, or by casting the composition for forming the negative electrode onto a separate support and then laminating the film peeled off from the support onto the negative electrode current collector.

[0087] As an example, compounds capable of reversibly inserting and deintercalating lithium can be used as the negative electrode active material. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; metal compounds that can alloy with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides that can be doped and dedoped with lithium, such as SiOβ (0 < β < 2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the aforementioned metal compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, where one or a mixture of two or more of these can be used. Furthermore, a thin film of metallic lithium can be used as the negative electrode active material. Additionally, low-crystallinity carbon and high-crystallinity carbon can be used as the carbon material. Representative low-crystallinity carbons include soft carbon and hard carbon, while representative high-crystallinity carbons include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, condensate graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature roasted carbons such as petroleum or coaltar pitch-derived cokes.

[0088] Furthermore, the adhesive and conductive material can be the same as those described in the positive electrode above.

[0089] On the other hand, in the lithium secondary battery, the separator is used to separate the negative electrode and the positive electrode and to provide a channel for the movement of lithium ions. Any material can be used without special restrictions, as long as it is commonly used as a separator in lithium secondary batteries. In particular, a separator with low resistance to the movement of electrolyte ions and excellent electrolyte moisture retention is preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes or laminates of two or more layers made from polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers. Furthermore, conventional porous nonwoven fabrics can also be used, such as nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Additionally, coated separators containing ceramic components or polymer materials to ensure heat resistance or mechanical strength can also be used, and can optionally be used in single-layer or multi-layer structures.

[0090] Furthermore, the electrolyte used in this invention may include, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which can be used in the preparation of lithium secondary batteries.

[0091] Specifically, the electrolyte may contain an organic solvent and a lithium salt.

[0092] As an example, the organic solvent can be any material without particular limitations, as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone can be used; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; and carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Organic solvents include: carbonates (PC), alcohols such as ethanol and isopropanol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group of C2 to C20, which may contain double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxolane compounds such as 1,3-dioxolane; or sulfolane compounds. Preferably, carbonate solvents are preferred, and more preferably, a mixture of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high dielectric constants that can improve the charge-discharge performance of the battery and linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) with low viscosity is preferred. In this case, when cyclic carbonates and linear carbonates are mixed in a volume ratio of about 1:1 to about 1:9, excellent electrolyte performance can be exhibited.

[0093] As an example, the lithium salt can be any material without special restrictions, as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably in the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within this range, the electrolyte has suitable conductivity and viscosity, thus exhibiting excellent electrolyte performance, and lithium ions can move effectively.

[0094] As an example, in addition to the electrolyte components, the electrolyte may also contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and increasing battery discharge capacity. These additives may include, for example, halogenated alkyl carbonate compounds such as ethylene difluorocarbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, glycol dimethyl ether, triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidinyl ethers, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the content of the additives relative to the total weight of the electrolyte can be from 0.1% to 5% by weight.

[0095] The lithium secondary battery containing the positive electrode active material of the present invention can be used in portable devices such as mobile phones, laptops, and digital cameras, as well as in electric vehicles such as hybrid electric vehicles (HEVs).

[0096] Therefore, according to another embodiment of the present invention, a battery module comprising the lithium secondary battery as a single cell and a battery pack comprising the same are provided.

[0097] As an example, the battery module or battery pack can be used as a power tool; an electric vehicle, including electric vehicles (EVs), hybrid electric vehicles and plug-in hybrid electric vehicles (PHEVs); or a power source for one or more medium to large-sized equipment in an energy storage system.

[0098] The present invention will now be described in more detail through embodiments. These embodiments are for illustrative purposes only and are not to be construed as limiting the scope of the invention to these embodiments.

[0099] Prepare high-nickel cathode active material Preparation Examples 1 to 3 Four liters of distilled water were added to a reactor (90 L capacity), and the temperature was maintained at 50 °C. A 3.2 mol / L transition metal solution (NiSO4, CoSO4, MnSO4 mixed to a nickel:cobalt:manganese molar ratio of 0.8:0.12:0.01) and a 28% (w / w) ammonia solution were continuously added to the reactor at rates of 300 mL / hr and 42 mL / hr, respectively. The mixture was stirred at 400 rpm. To maintain the pH, a 40% (w / w) sodium hydroxide solution was added to maintain pH 9. Precursor particles were formed by co-precipitation for 10 hours. The precursor particles were separated, washed, and then dried in an oven at 130 °C to prepare the precursor.

[0100] The precursor synthesized by co-precipitation reaction was mixed with LiOH to make the molar ratio of Li / Me (Ni, Co, Mn) 1.05, and then heat-treated for 10 hours in an oxygen atmosphere at the calcination temperature specified in Table 1 below to prepare a positive electrode active material containing secondary particles of nickel-based lithium transition metal oxide.

[0101] Table 1

[0102] Comparative Example 1 The positive electrode active material synthesized in Preparation Example 1 was pulverized at 10,000 rpm for 10 minutes using an air jet mill to prepare polycrystalline positive electrode active material particles.

[0103] Comparative Example 2 The positive electrode active material synthesized in Preparation Example 2 was pulverized at 10,000 rpm for 10 minutes using an air jet mill to prepare polycrystalline positive electrode active material particles.

[0104] Example 1 The positive electrode active material synthesized in Preparation Example 3 was pulverized once at 10,000 rpm for 10 minutes using an air jet mill, and then pulverized a second time at 17,000 rpm for 10 minutes to prepare polycrystalline positive electrode active material particles.

[0105] Preparation of nickel cathode active material Preparation Examples 4 to 8 Four liters of distilled water were added to a reactor (90 L capacity), and the temperature was maintained at 50 °C. A transition metal solution with a concentration of 3.2 mol / L (NiSO4, CoSO4, and MnSO4 mixed in a nickel:cobalt:manganese ratio as shown in Table 2 below) and a 28% (w / w) ammonia solution were continuously added to the reactor at rates of 300 mL / hr and 42 mL / hr, respectively. The mixture was stirred at 400 rpm. To maintain the pH, a 40% (w / w) sodium hydroxide solution was added to maintain pH 9. Precursor particles were formed through a co-precipitation reaction for 10 hours. The precursor particles were separated, washed, and then dried in an oven at 130 °C to prepare the precursor.

[0106] The precursor synthesized by co-precipitation reaction was mixed with LiOH to make the molar ratio of Li / Me (Ni, Co, Mn) 1.05, and then heat-treated for 10 hours in an oxygen atmosphere at the calcination temperature specified in Table 2 below to prepare a positive electrode active material containing secondary particles of nickel-based lithium transition metal oxide.

[0107] Table 2

[0108] Example 2 The positive electrode active material synthesized in Preparation Example 4 was pulverized once at 10,000 rpm for 10 minutes using an air jet mill, and then pulverized a second time at 17,000 rpm for 10 minutes to prepare polycrystalline positive electrode active material particles.

[0109] Example 3 In Preparation Example 5, the synthesized positive electrode active material was pulverized once at 10,000 rpm for 10 minutes using an air jet mill, and then pulverized a second time at 17,000 rpm for 10 minutes to prepare polycrystalline positive electrode active material particles.

[0110] Comparative Example 3 In Preparation Example 6, the synthesized positive electrode active material was pulverized at 10,000 rpm for 10 minutes using an air jet mill to prepare polycrystalline positive electrode active material particles.

[0111] Comparative Example 4 In Preparation Example 7, the synthesized positive electrode active material was pulverized at 10,000 rpm for 10 minutes using an air jet mill to prepare polycrystalline positive electrode active material particles.

[0112] Comparative Example 5 In Preparation Example 8, the synthesized positive electrode active material was pulverized at 10,000 rpm for 10 minutes using an air jet mill to prepare polycrystalline positive electrode active material particles.

[0113] Preparation of secondary batteries The polycrystalline positive electrode active material, carbon black conductive material and PVdF binder of Examples 1 to 3 and Comparative Examples 1 to 5 were mixed in N-methylpyrrolidone solvent at a weight ratio of 96:2:2 to prepare a positive electrode composite material. The composite material was coated on one side of an aluminum current collector and then dried at 100°C. The positive electrode was then prepared by calendering.

[0114] The negative electrode uses lithium metal.

[0115] A porous polyethylene membrane is inserted between the positive and negative electrodes prepared as described above to prepare an electrode assembly. The electrode assembly is placed inside a housing, and then an electrolyte is injected into the housing to prepare a battery. In this case, lithium hexafluorophosphate (LiPF6) with a concentration of 1.0 M is dissolved in an organic solvent composed of ethylene carbonate / ethyl methyl carbonate / diethyl carbonate / (EC / EMC / DEC mixed volume ratio = 3 / 4 / 3) as an electrolyte to prepare a lithium secondary battery.

[0116] Evaluation Example 1 - Determination of the intensity ratio of the H2 phase The lithium secondary battery half-cell prepared above was formed at 0.2C and then charged at 0.05C and 4.3V for positive plate analysis.

[0117] The diffraction intensity values ​​(X-ray diffraction analysis) of the diffraction angles (2) of the H2 and H3 phases appearing on the (104) surface, which were incident on the positive electrode, are shown in Table 3. Furthermore, the diffraction intensities of the measured H2 and H3 phases were calculated using Equation 1 below, and the intensity ratio of the H2 phase is shown in Table 3.

[0118] Formula 1:

[0119] Evaluation Example 2 - Lifetime Characteristics Evaluation The lithium secondary battery prepared above was repeatedly charged / discharged 500 times, and the remaining battery capacity (lifetime) relative to the initial state was measured and shown in Table 3.

[0120] Table 3

[0121] Evaluation Example 3 - Observation of Polycrystalline Particle Active Materials *167 The polycrystalline positive electrode active material particles of Examples 1 to 3 and Comparative Examples 1 to 5 were photographed using a scanning electron microscope and are shown in Figure 2.

[0122] Figure 2a To capture images of the polycrystalline positive electrode active material particles of Comparative Example 1 using a scanning electron microscope.

[0123] Figure 2b To capture images of the polycrystalline positive electrode active material particles of Comparative Example 2 using a scanning electron microscope.

[0124] Figure 2c To capture images of the polycrystalline positive electrode active material particles of Example 1 using a scanning electron microscope.

[0125] Figure 2d Images of the polycrystalline positive electrode active material particles of Example 2 were captured using a scanning electron microscope.

[0126] Figure 2e Images of the polycrystalline positive electrode active material particles of Example 3 were captured using a scanning electron microscope.

[0127] Figure 2f To capture images of the polycrystalline positive electrode active material particles of Comparative Example 3 using a scanning electron microscope.

[0128] Figure 2g To capture images of the polycrystalline positive electrode active material particles of Comparative Example 4 using a scanning electron microscope.

[0129] Figure 2h To capture images of the polycrystalline positive electrode active material particles of Comparative Example 5 using a scanning electron microscope.

[0130] Referring to Table 3, it can be confirmed that the secondary battery of Example 1, in which the H2 phase intensity ratio is 10% or more and 25% or less in the high-nickel cathode active material, has better life characteristics than the secondary batteries of Comparative Example 1 and Comparative Example 2, in which the H2 phase intensity ratio is greater than 25%.

[0131] Furthermore, refer to Figure 2c It can be confirmed that the high-nickel cathode active material of Example 1, with an H2 phase intensity ratio of 10% or more and 25% or less, does not contain isolated particles. Furthermore, it can be confirmed that there are 2 to 5 primary particles within the secondary particles.

[0132] on the other hand, Figure 3a A graph comparing the correlation between H2 phase intensity ratio and lifetime for Comparative Examples 1, 2, and Example 1. (See reference) Figure 3a It can be confirmed that, as shown in Example 1, when the H2 intensity ratio is 10% or more and 25% or less, it has excellent lifespan.

[0133] Referring to Table 3, it can be confirmed that the secondary batteries of Examples 2 and 3, in which the H2 phase intensity ratio is 5% or more and 16% or less in the medium nickel cathode active material, have better life characteristics than the secondary batteries of Comparative Examples 3 to 5, in which the H2 phase intensity ratio is greater than 16%.

[0134] Furthermore, refer to Figure 2d It can be confirmed that the nickel cathode active materials in Examples 2 and 3, where the H2 phase intensity ratio is 5% or more and 16% or less, do not contain isolated particles. Furthermore, it can be confirmed that there are 2 to 5 primary particles within the secondary particles.

[0135] on the other hand, Figure 3b A graph showing the correlation between H2 phase intensity ratio and lifetime for comparative examples 1, 2, and comparative examples 3 to 5. (See reference) Figure 3b It can be confirmed that, as shown in Example 1, excellent lifespan is achieved when the H2 intensity ratio is 5% or more and 16% or less.

[0136] On the other hand, reference Figures 2a to 2h It can be confirmed that the number of isolated particles increases with the increase of the number of primary particles within secondary particles, and the H2 intensity ratio increases with the increase of the number of isolated particles, while the lifetime characteristics decrease.

[0137] That is, according to one embodiment of the present invention, the XRD peak intensities of the H2 phase and H3 phase are measured and their ratio is calculated. It is found that the more H2 phase there is, the worse the lithium ion insertion / extraction is, and therefore it can be used as an indicator of the number of isolated particles. Furthermore, when there are no isolated particles, since it does not lead to a decrease in lifetime, it can be determined that the size and number of primary particles and the size of secondary particles at this time are the optimal conditions.

Claims

1. A polycrystalline positive electrode active material, characterized in that, Include: Primary particles, containing nickel, with an average particle size D50 of 500 nm to 5 μm; and Secondary particles are formed by the aggregation of primary particles. A portion of the primary particle is exposed on the outer surface of the secondary particle.

2. The polycrystalline positive electrode active material according to claim 1, characterized in that, The average particle size D50 of the primary and secondary particles is determined by the H2 intensity ratio calculated using Equation 1 below: Formula 1: In Equation 1, the H2 intensity and H3 intensity are the peak intensities of the H2 phase and H3 phase obtained by X-ray diffraction analysis.

3. The polycrystalline positive electrode active material according to claim 1, characterized in that, The number of primary particles forming the secondary particles is determined by the H2 intensity ratio calculated using Equation 1 below: Formula 1: In Equation 1, the H2 intensity and H3 intensity are the peak intensities of the H2 phase and H3 phase obtained by X-ray diffraction analysis.

4. The polycrystalline positive electrode active material according to claim 2, characterized in that, When the nickel content of the primary particles is 50 mol% or more but less than 80 mol% or more, the average particle size D50 of the primary and secondary particles is the average particle size measured when the H2 intensity ratio is 5% or more but less than 16%.

5. The polycrystalline positive electrode active material according to claim 3, characterized in that, When the nickel content of the primary particles is 50 mol% or more and less than 80 mol% or more, the number of primary particles forming the secondary particles is the number measured when the H2 intensity ratio is 5% or more and less than 16%.

6. The polycrystalline positive electrode active material according to claim 2, characterized in that, When the nickel content of the primary particles is 80 mol% or more, the average particle size D50 of the primary and secondary particles is the average particle size measured when the H2 intensity ratio is 10% or more and less than 25%.

7. The polycrystalline positive electrode active material according to claim 3, characterized in that, When the nickel content of the primary particles is 80 mol% or more, the number of primary particles forming the secondary particles is the number measured when the H2 intensity ratio is 10% or more and less than 25%.

8. The polycrystalline positive electrode active material according to claim 1, characterized in that, The average particle size D50 of the secondary particles is 1.5 to 5.0 times that of the average particle size D50 of the primary particles.

9. The polycrystalline positive electrode active material according to claim 1, characterized in that, The secondary particles are composed of 2 to 5 primary particles.

10. A positive electrode slurry composition, characterized in that, It comprises a polycrystalline positive electrode active material, a conductive material, and a binder according to any one of claims 1 to 9.

11. A lithium secondary battery, characterized in that, This includes a positive electrode formed by coating a current collector with the positive electrode slurry composition according to claim 10.

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