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

Abstract

The present embodiment relates to a positive electrode active material for a lithium secondary battery and a lithium secondary battery including the same. [Solution] According to one embodiment, a positive electrode active material for a lithium secondary battery includes a metal oxide having a single particle form and a layered structure, and a coating layer located on the surface of the metal oxide and having a layered structure, and the average inter-plane distance of the layered structure included in the coating layer may be smaller than the average inter-plane distance of the layered structure included in the metal oxide.

Description

[Technical Field]

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

[0002] Recently, driven by the explosive demand for electric vehicles and the demand for increased driving distance, the development of secondary batteries having high capacity and high energy density to meet these demands has been actively pursued worldwide.

[0003] To meet these requirements, a technology using high-nickel NCM (nickel cobalt manganese) cathode material with a high Ni content has been proposed. At the same time, to improve the density of the electrodes, which are the components of the cell, the material must be constructed in a bimodal form, with a certain ratio of large and small particles blended together.

[0004] However, in the case of cathode materials that are composed of secondary particles formed by agglomeration of primary particles ranging in size from several tens of nanometers to several micrometers, the specific surface area of ​​the powder is large, resulting in a large area in contact with the electrolyte, which increases the possibility of gas generation and reduces the lifespan.

[0005] To solve this problem, methods have been proposed to increase the size of primary particles using sintering agents or fluxes, but this method has the problem of reducing the electrochemical performance of the positive electrode active material due to the formation of rocksalt structures on the particle surface.

[0006] Therefore, there is a need to develop a positive electrode active material that has excellent electrochemical performance while increasing the size of the primary particles. Summary of the Invention [Problem to be solved by the invention]

[0007] In accordance with the present invention, a positive electrode active material having a single particle form and excellent electrochemical performance, and a lithium secondary battery including the same, are provided. [Means for solving the problem]

[0008] According to one embodiment, a positive electrode active material for a lithium secondary battery includes a metal oxide having a single particle form and a layered structure; and a coating layer located on a surface of the metal oxide and having a layered structure. The average inter-plane distance of the layered structure included in the coating layer may be smaller than the average inter-plane distance of the layered structure included in the metal oxide.

[0009] A lithium secondary battery according to an embodiment may include the positive electrode, a negative electrode, and an electrolyte. [Effects of the Invention]

[0010] In accordance with an embodiment, a positive electrode active material for a lithium secondary battery may be provided by modifying a surface structure such that the interplanar distance of a layered structure included in a coating layer located on a surface of a metal oxide is shorter than the interplanar distance of a layered structure included in the metal oxide, thereby realizing a lithium secondary battery with excellent electrochemical performance. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a SEM image of the positive electrode active material prepared in Example 2, measured at a magnification of 20,000 times. [Figure 2] FIG. 2 is a SEM image of the positive electrode active material prepared in Example 4, measured at a magnification of 20,000 times. [Figure 3] FIG. 3 is a SEM image of the positive electrode active material prepared according to Comparative Example 3, measured at a magnification of 20,000 times. [Figure 4]Fig. 4(a) is a cross-sectional image of the positive electrode active material produced in Example 2 after milling with a FIB (Focused Ion Beam, SEIKO 3050SE). Fig. 4(b), Fig. 4(c), and Fig. 4(d) are diagrams showing the results of analysis using an EDS (Energy Dispersed Spectroscopy, Oxford X_max 100TLE) analyzer to identify elements present inside the positive electrode active material of Fig. 4(a). [Figure 5] Figure 5(a) is an image obtained by measuring the cathode active material prepared in Example 2 using a high-resolution transmission electron microscope (HRTEM). Figure 5(b) is an enlarged HRTEM image of the rectangular area indicated by the dotted line in Figure 5(a). [Figure 6] Figure 6(a) shows the results of measuring the interplanar distances of 10 layers in the layered structure of Area A (coating layer) in 5(b). Figure 6(b) shows the results of measuring the interplanar distances of 10 layers in the layered structure of Area B (single particle base material) in 5(b). [Figure 7] Fig. 7(a) is an image obtained by measuring the positive electrode active material prepared according to Comparative Example 3 using a high resolution transmission electron microscope (HRTEM). Fig. 7(b) is an enlarged HRTEM image of the rectangular area indicated by the dotted line in Fig. 7(a). [Figure 8] Figure 8(a) shows the results of measuring the interplanar distances of 10 layers in the layered structure of Area A (coating layer) in 7(b). Figure 8(b) shows the results of measuring the interplanar distances of 10 layers in the layered structure of Area B (single particle base material) in 7(b). [Figure 9] FIG. 9 is a SEM image of the positive electrode active material prepared in Example 9, measured at 5,000 times magnification. DETAILED DESCRIPTION OF THE INVENTION

[0012] Terms such as "first," "second," and "third" are used to describe various parts, components, regions, layers, and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Therefore, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0013] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. As used in the specification, the term "comprising" embodies certain properties, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other properties, regions, integers, steps, operations, elements, and / or components.

[0014] When a part is referred to as being "on" another part, it may be directly on top of the other part, or there may be other parts between them. In contrast, when a part is referred to as being "directly on top" of another part, there are no other parts between them.

[0015] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless otherwise defined. "Positive electrode active material for lithium secondary batteries" As described above, when the size of primary particles is increased, a rocksalt structure is formed on the surface, which reduces the electrochemical performance of the positive electrode active material. However, in this embodiment, this problem is solved by providing a positive electrode active material having a surface structure modified so that the average interplanar distance of the layered structure of the coating layer located on the surface of the metal oxide is smaller than the average interplanar distance of the layered structure of the metal oxide.

[0016] That is, a positive electrode active material for a lithium secondary battery according to an embodiment may include a metal oxide having a single particle form and a layered structure, and a coating layer located on a surface of the metal oxide and having a layered structure, wherein an average inter-plane distance of the layered structure included in the coating layer may be smaller than an average inter-plane distance of the layered structure included in the metal oxide.

[0017] In this case, the average interplanar distance of the layered structure contained in the coating layer may be 0.48 nm or less, more specifically, in the range of 0.45 nm to 0.48 nm.

[0018] When the average interplanar distance of the layered structure included in the coating layer satisfies the above range, the electrochemical performance of a lithium secondary battery using the positive electrode active material according to the present embodiment can be significantly improved.

[0019] The positive electrode active material may have a Li / Ni cation mixing ratio of 1.5% or less, more specifically, 1.1 to 1.4%. If the Li / Ni cation mixing ratio is too high, the Li layer may easily collapse, which may significantly reduce the battery's lifespan. If the Li / Ni cation mixing ratio is too low, the irreversible sites in the bulk portion of the positive electrode active material may increase, reducing the mobility of lithium ions and degrading the resistance and output characteristics. Therefore, when the Li / Ni cation mixing ratio satisfies the above range, a positive electrode active material with low resistance and improved lifespan can be realized, which is advantageous.

[0020] In this specification, the Li / Ni cation mixing ratio means the amount of Ni substituted for the Li site.

[0021] In this embodiment, the coating layer can contain at least one of Co, Al, W, V, Ti, Nb, Ce, B, and P. Since the coating layer contains at least one of these elements, the surface structure of the positive electrode active material of this embodiment can be modified.

[0022] The coating layer may contain Co, and the content of Co contained in the coating layer may be higher than the content of Co contained in the metal oxide. When the coating layer contains Co in this way and the content of Co is higher than that of the metal oxide, a positive electrode active material with excellent life and resistance characteristics can be realized.

[0023] In this embodiment, the coating layer may have an average thickness in the range of 30 nm to 60 nm. When the coating layer has a thickness in this range, a positive electrode active material having excellent room temperature resistance and high temperature life characteristics and a significantly reduced rate of increase in high temperature resistance can be provided.

[0024] The Co content in the coating layer may be in the range of 0.5 mol % to 3.5 mol % based on the entire coating layer. Since the coating layer contains Co in this range, the surface structure of the positive electrode active material of this embodiment can be modified.

[0025] The metal oxide may contain nickel, cobalt, and manganese, and the content of nickel in the entire metal oxide may be greater than the sum of the contents of cobalt and manganese.

[0026] More specifically, the nickel content in the metal oxide particles may be 0.8 mol or more per mol of the nickel, cobalt, and manganese, and more specifically, the nickel content may be in the range of 0.8 to 0.99, 0.85 to 0.99, or 0.88 to 0.99.

[0027] When the nickel content in the lithium metal oxide is 0.8 moles or more, as in this embodiment, a cathode active material with high output characteristics can be realized. The cathode active material of this embodiment having such a composition has a high energy density per volume, which can improve the capacity of the battery to which it is applied, making it highly suitable for use in electric vehicles.

[0028] The metal oxide may further comprise a doping element, and the doping element may comprise at least one of Al, Zr, Nb, Mo, W, Ti, Ce, Mg, B, P, V, Sr, and B.

[0029] The content of the doping element may be in the range of 0.0005 mol to 0.04 mol or 0.001 mol to 0.03 mol, where the sum of the nickel, cobalt, manganese and the doping element is 1 mol. Here, the doping element refers to the doping amount of the doping element contained in the final cathode active material.

[0030] In order to ensure a long life and various electrochemical performances of a positive electrode active material, it is important to select a doping element. In the present embodiment, various doping elements are used as described above to improve the characteristics of the positive electrode active material.

[0031] In this embodiment, the doping elements may include Zr and Al.

[0032] Zr ions occupy the Li site, acting as a kind of pillar, mitigating the contraction of the lithium ion path during the charge and discharge process and stabilizing the layered structure. This phenomenon reduces cation mixing, increases the lithium diffusion coefficient, and can extend the cycle life.

[0033] Furthermore, Al ions migrate to tetragonal lattice sites, preventing the layered structure from deteriorating into a spinel structure in which lithium ions are less likely to migrate smoothly.

[0034] The Zr content may be 0.001 mol to 0.01 mol, more specifically, 0.001 mol to 0.006 mol, 0.001 mol to 0.005 mol, or 0.0015 mol to 0.005 mol, based on 1 mol of the total of the nickel, cobalt, manganese, and doping element. When the Zr doping amount satisfies the above range, it is possible to reduce the rate of increase in high-temperature resistance and ensure excellent life characteristics.

[0035] The Al content may be 0.001 mol to 0.03 mol, more specifically, 0.004 mol to 0.025 mol, 0.0045 mol to 0.025 mol, or 0.005 mol to 0.025 mol, based on 1 mol of the total of the nickel, cobalt, manganese, and doping element. When the Al doping amount satisfies the above range, high-temperature life and thermal stability can be further improved.

[0036] Next, the average crystal grain size of the metal oxide may be 200 nm or more, more specifically, in the range of 200 nm to 350 nm or 220 nm to 300 nm.

[0037] When the average crystal grain size is within this range, the particles can be defined as single particles. Furthermore, when the average crystal grain size is within this range, the crystallization has progressed well, which can reduce residual lithium on the surface of the positive electrode active material and further improve the life characteristics of the lithium secondary battery.

[0038] In this specification, the average crystal grain size is defined as being measured by the following method. 1. Sample structure analysis using Rigaku smart lab equipment 2. 45kV, 200mA (9kW) applied to the Cu anode to generate X-rays 3. The optical system has an incident slit of 1 / 2 deg and a receiving slit of 8.0mm. 4. Scan from 10 to 80°, step 0.02°, 10° / min. 5. Calculate crystal size using SmartLab Studio II v4.2.82.0S / W 6. Calculations are performed using WPPF (Whole powder pattern fitting) in software. 7. Use Layered structure for WPPF progression and set Profile fitting to FP method 8. Receiving optic is set to Graphite (002) and Soller slit 3.8 9. Set Shape to Spherical, Strain to 0, Fix, Refine and calculate crystal size

[0039] The average particle size (D50) of the positive electrode active material may be 2.5 μm or more, more specifically, in the range of 3.0 μm to 5.0 μm. In this embodiment, a single-particle positive electrode active material having such an average particle size can be produced without the need for an expensive crushing device or multiple crushing processes. That is, even when using a conventional crushing device, a single-particle positive electrode active material having a uniform particle size distribution with very little fine or large powder can be produced. Therefore, when the average particle size of the positive electrode active material of this embodiment satisfies the above range, a lithium secondary battery with excellent electrochemical properties can be realized.

[0040] Meanwhile, the positive electrode active material of the present embodiment may further include a positive electrode active material containing a metal oxide in the form of secondary particles formed by aggregation of primary particles.

[0041] That is, the cathode active material may include a cathode active material containing the metal oxide in the form of single particles, and a cathode active material containing the metal oxide in the form of secondary particles, the average particle size (D50) of which is larger than the average particle size (D50) of the cathode active material containing the metal oxide in the form of single particles. When the cathode active material containing the metal oxide in the form of single particles and the cathode active material containing the metal oxide in the form of secondary particles are mixed and used in a bimodal manner as described above, the density of the electrode mixture can be advantageously increased.

[0042] In this bimodal cathode active material, the mixing ratio of the cathode active material containing a metal oxide in single particle form and the cathode active material containing a metal oxide in secondary particle form may be in the range of 30:70 to 10:90 or 25:75 to 15:85 by weight (single particle:secondary particle). When the cathode active materials containing metal oxides in single particle and secondary particle forms are mixed and used in such a weight ratio, the density of the electrode mixture can be increased.

[0043] In this case, the positive electrode active materials including the metal oxide in the form of single particles and the metal oxide in the form of secondary particles may have the same composition or different compositions. Specifically, both the positive electrode active materials including the metal oxide in the form of single particles and the metal oxide in the form of secondary particles may include nickel, cobalt, manganese, and a doping element.

[0044] For example, the positive electrode active material containing the metal oxide in the form of secondary particles may contain nickel, cobalt, and manganese, and the content of the nickel in the entire metal oxide in the form of secondary particles may be greater than the sum of the contents of the cobalt and manganese.

[0045] More specifically, the content of nickel in the metal oxide particles in the form of secondary particles may be 0.8 mol or more based on 1 mol of the nickel, cobalt, and manganese, and more specifically, the content of nickel may be in the range of 0.8 to 0.99, 0.85 to 0.99, or 0.88 to 0.99.

[0046] The metal oxide in secondary particle form further comprises a doping element, and the doping element may comprise at least one of Al, Zr, Nb, Mo, W, Ti, Ce, Mg, B, P, V, Sr, and B.

[0047] The specific description and content of the doping element are the same as those of the positive electrode active material containing the single particle metal oxide described above, and therefore will not be repeated here.

[0048] In addition, in this embodiment, the average particle size (D50) of the cathode active material containing the metal oxide in the form of secondary particles may be in the range of 10 μm to 20 μm, or 12 μm to 17 μm. When the average particle size of the cathode active material containing the metal oxide in the form of secondary particles satisfies this range, large particles and small particles can be appropriately distributed in the bimodal cathode active material, thereby improving the energy density of the lithium secondary battery.

[0049] "Lithium secondary battery" In yet another embodiment of the present invention, there is provided a lithium secondary battery including a positive electrode including the positive electrode active material according to the embodiment of the present invention described above, a negative electrode, and an electrolyte disposed between the positive electrode and the negative electrode.

[0050] Specifically, the positive electrode includes a current collector and a positive electrode active material layer formed on the current collector, and the characteristics of the positive electrode active material constituting the positive electrode active material layer are the same as those described above. Therefore, a detailed description of the positive electrode active material will be omitted.

[0051] The current collector may be, for example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like.

[0052] Meanwhile, the positive electrode active material layer may include a binder and a conductive material.

[0053] The binder serves to firmly adhere the positive electrode active material particles to each other and to the current collector.

[0054] The conductive material is used to impart electrical conductivity to the electrode, and any material can be used as long as it is electron-conductive without causing chemical changes in the battery that is constructed.

[0055] The positive electrode is fabricated by mixing an active material, a conductive material, and a binder in a solvent to prepare an active material composition, and then coating the composition on a current collector. Since this electrode fabrication method is well known in the art, a detailed description thereof will be omitted. Examples of the solvent include, but are not limited to, N-methylpyrrolidone.

[0056] The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector, and the negative electrode active material layer includes a negative electrode active material.

[0057] The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, a lithium metal alloy, a material capable of doping and dedoping lithium, or a transition metal oxide.

[0058] As the substance capable of reversibly intercalating / deintercalating lithium ions, a carbon substance can be used, and any of the carbon-based negative electrode active materials generally used in lithium ion secondary batteries can be used. Representative examples thereof include crystalline carbon, amorphous carbon, or both of these can be used.

[0059] For the alloy of the lithium metal, an alloy of lithium and a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn can be used.

[0060] As the substance capable of doping and undoping lithium, Si, SiO x (0 < x < 2), Si-Y alloy (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof, and is not Si), Sn, SnO2, Sn-Y (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof, and is not Sn), etc. can be mentioned.

[0061] Examples of the transition metal oxide include vanadium oxide, lithium vanadium oxide, etc.

[0062] The negative electrode active material layer also contains a binder and may further selectively contain a conductive material.

[0063] The binder may be, but is not limited to, polyvinyl alcohol, carboxymethyl cellulose / styrene-butadiene rubber, hydroxypropylene cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, or polypropylene, and may be mixed in an amount of 1 wt % to 30 wt % of the total amount of the composition for forming the negative electrode active material layer.

[0064] The conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery, and specific examples include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. The conductive material can be mixed in an amount of 0.1 wt % to 30 wt % with respect to the total amount of the composition for forming the negative electrode active material layer.

[0065] The current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0066] The negative electrode is fabricated by mixing an active material, a conductive material, and a binder in a solvent to prepare an active material composition, and then coating the composition on a current collector. Since this electrode fabrication method is well known in the art, a detailed description thereof will be omitted. Examples of the solvent include, but are not limited to, N-methylpyrrolidone.

[0067] In addition, in the lithium secondary battery, examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used when manufacturing a lithium secondary battery.

[0068] Specifically, the organic liquid electrolyte may include an organic solvent and a lithium salt.

[0069] The organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0070] The lithium salt is a substance that dissolves in an organic solvent and acts as a source of lithium ions in the battery, enabling basic operation of a lithium secondary battery, and facilitating the movement of lithium ions between the positive electrode and the negative electrode.

[0071] Depending on the type of lithium secondary battery, a separator may be present between the positive electrode and the negative electrode. Such a separator can be made of polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more of these layers. Of course, mixed multilayer films such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator can also be used.

[0072] Lithium secondary batteries are classified into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries depending on the type of separator and electrolyte used, and may be classified into cylindrical, prismatic, coin, pouch, etc. depending on the shape, and into bulk and thin film types depending on the size. The structure and manufacturing method of these batteries are widely known in the field, so detailed description will be omitted. [Example]

[0073] The following detailed description of the present invention is provided by way of example only and does not limit the scope of the present invention, which is defined solely by the scope of the claims that follow.

[0074] "Example 1 - Positive electrode active material in single particle form, heat treated at 600°C for 6 hours" (1) Preparation of precursor The precursor was prepared by a conventional coprecipitation method.

[0075] Specifically, NiSO4·6H2O was used as the nickel source material, CoSO4·7H2O was used as the cobalt source material, and MnSO4·H2O was used as the manganese source material. These raw materials were dissolved in distilled water to produce metal salt aqueous solutions.

[0076] After preparing the coprecipitation reactor, N2 was purged to prevent oxidation of metal ions during the coprecipitation reaction, and the reactor temperature was maintained at 50°C.

[0077] NH4(OH) was added as a chelating agent to the co-precipitation reactor, and NaOH was used to adjust the pH. The precipitate obtained by the co-precipitation process was filtered, washed with distilled water, and dried in an oven at 100°C for 24 hours to prepare a cathode active material precursor.

[0078] The composition of the produced precursor was (Ni 0.98 Co 0.01 Mn 0.01 )(OH)2, and the average particle size (D50) was approximately 4 μm.

[0079] (2) Manufacturing of positive electrode active material 684 g of the precursor prepared in (1) was weighed with 315 g of LiOH·H2O, 2.94 g of Al(OH)3, and 1.47 g of ZrO2, and then uniformly mixed and fired at 830-890°C for 24 hours in a box-type firing furnace with an oxygen inflow of 1,000 mL / min to synthesize a metal oxide cathode material. The mixture was then crushed using a jet mill to prepare a cathode active material containing single particle metal oxide.

[0080] Approximately 1.927 g of Co(OH)2 was mixed with 100 g of the single particle metal oxide, and the mixture was heat-treated at 600°C for 6 hours in an oxygen atmosphere to prepare a cathode active material having a coating layer formed thereon.

[0081] "Examples 2 to 8" A positive electrode active material having a coating layer formed thereon was prepared in the same manner as in Example 1, except that the heat treatment conditions during the formation of the coating layer were adjusted as shown in Table 1 below.

[0082] [Table 1]

[0083] "Example 8 (1) Preparation of precursor" The composition is (Ni 0.92 Co 0.04 Mn 0.04 A precursor was produced in the same manner as in Example 1, except that the average particle size (D50) was about 14.5 μm.

[0084] (2) Manufacturing of large particle size positive electrode active material To 684 g of the precursor prepared in (1), 325 g of LiOH·H2O, 11.4 g of Al(OH)3, and 3.2 g of ZrO2 were weighed, mixed uniformly, and fired at 830-890°C for 24 hours in a box-type firing furnace with an oxygen inflow rate of 1,000 mL / min to synthesize a metal oxide cathode material.

[0085] Thereafter, the powder was crushed using an ACM (Air Classifying Mill), washed with water to remove residual lithium on the surface, and then dried for 12 hours.

[0086] For post-treatment, approximately 0.35 g of boric acid (H3BO3) was mixed with 100 g of the positive electrode material, and the mixture was heat-treated at 250 to 300°C for 5 hours in an air atmosphere.

[0087] (3) Manufacturing of bimodal cathode active material The positive electrode active material prepared in (2) and the positive electrode active material prepared in Example 4 were mixed in a weight ratio of 8:2 to prepare a bimodal positive electrode active material.

[0088] "Comparative Example 1" (1) Preparation of precursor A precursor was produced in the same manner as in Example 1.

[0089] (2) Manufacturing of positive electrode active material 684 g of the precursor prepared in (1) was weighed with 315 g of LiOH·H2O, 2.94 g of Al(OH)3, and 1.47 g of ZrO2, and then uniformly mixed and fired at 830-890°C for 24 hours in a box-type firing furnace with an oxygen inflow of 1,000 mL / min to synthesize a metal oxide cathode material. The mixture was then crushed using a jet mill to prepare a cathode active material containing single particle metal oxide.

[0090] Comparative Example 2 The positive electrode active material prepared in Comparative Example 1 was additionally heat-treated at 660° C. for 6 hours to prepare a positive electrode active material according to Comparative Example 2.

[0091] "Comparative Example 3 - Co coating layer formed by wet method" A positive electrode active material was produced in the same manner as in Comparative Example 1.

[0092] Next, 0.03 grams (g) of LiNO3 and 11.74 g of Co(NO3)2·6H2O were dissolved in 100 g of H2O heated to 60°C, and 100 g of the cathode material of Comparative Example 1 was added. The resulting slurry was stirred for 30 minutes and then spray-dried to obtain a powder. The powder was placed in an alumina crucible and heated to approximately 450°C at a rate of approximately 5°C per minute. The temperature was maintained at approximately 450°C for approximately 1 hour, and then the temperature was increased by approximately 2°C per minute to approximately 700°C and maintained for approximately 2 hours. The sample was then cooled to room temperature to produce the cathode active material of Comparative Example 2.

[0093] "Experimental Preparation - Manufacturing a Coin-Type Half Cell" For the positive electrode active materials prepared in Examples 1 to 8 and Comparative Examples 1 to 3, coin-type half cells were prepared as follows to evaluate the physical properties and electrochemical properties.

[0094] Specifically, the positive electrode active material, polyvinylidene fluoride binder (product name: KF1120), and carbon black conductive material were mixed in a weight ratio of 96.5:1.5:2, and this mixture was added to N-methyl-2-pyrrolidone solvent so that the solid content was approximately 30 wt % to produce a positive electrode active material slurry.

[0095] The slurry was coated on an aluminum foil (thickness: 15 μm) as a positive electrode current collector using a doctor blade, dried, and then rolled to prepare a positive electrode. The loading amount of the positive electrode was about 15 mg / cm. 2 The electrode thickness is approximately 65 μm and the rolling density is approximately 3.4 g / cm 3 That was all.

[0096] A 2032 coin-type half-cell was fabricated using the cathode, a lithium metal anode (300 μm thick, MTI), an electrolyte, and a polypropylene separator. The electrolyte was a mixed solution of 1M LiPF6 dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate (EMC) (mixing ratio EC:DMC:EMC = 3:4:3 vol%).

[0097] (Experimental Example 1) Electrochemical Performance Evaluation (1) Measurement of residual lithium and particle size Residual lithium was measured using a METTLER TOLEDO T50 model, and particle size was measured using a microtrac S3500 model.

[0098] (2) Capacity evaluation The coin-type half cell prepared according to the experimental preparation was aged at room temperature (25° C.) for 10 hours, and then a charge / discharge test was carried out.

[0099] The capacity evaluation was performed with 200mAh / g as the reference capacity, and the charge / discharge conditions were CC / CV 2.5 to 4.25V with a 1 / 20C cut-off. The initial capacity was measured with a 0.2C charge / 0.2C discharge.

[0100] (3) Measurement of life characteristics The lifespan characteristics were measured 50 times at high temperature (45°C) under the conditions of 0.5C charge / 1.0C discharge.

[0101] (4) Measurement of resistance characteristics The room temperature initial resistance (DC-IR (Direct current internal resistance)) was calculated by charging the battery at 0.2C and discharging it at 0.2C once at 25°C under constant current-constant voltage conditions of 2.5V to 4.25V with a 1 / 20C cutoff, and measuring the voltage 60 seconds after applying the discharge current at 4.25V, 100% charge.

[0102] The resistance increase rate was measured by comparing the resistance measured initially at high temperature (45°C) (initial resistance at room temperature) with the resistance measured after 30 cycles in the same manner as the initial resistance measurement, and the increase rate was converted into a percentage (%).

[0103] (Experimental Example 2) X-ray diffraction evaluation The crystalline size of the positive electrode active materials produced in Examples 1 to 8 and Comparative Examples 1 to 3 was measured using an X'Pert powder (PANalytical) XRD device with CuKα radiation as the target radiation.

[0104] The X-ray diffraction measurement test was carried out under the following measurement conditions: 2θ = 10° to 130°, scan speed (° / S) = 0.328, and step size = 0.026° / step.

[0105] (Experimental Example 3) Method for measuring the Cation Minxing ratio 1. Sample structure analysis using Rigaku smart lab equipment 2. 45kV, 200mA (9kW) applied to the Cu anode to generate X-rays 3. The optical system has an incident slit of 1 / 2 deg and a receiving slit of 8.0mm. 4. Scan from 10 to 80°, step 0.02°, 10° / min. 5. Calculate crystal size using SmartLab Studio II v4.2.82.0S / W 6. Calculations are performed using WPPF (Whole powder pattern fitting) in software. 7. Use Layered structure for WPPF progression and set Profile fitting to FP method 8. Receiving optic is set to Graphite (002) and Soller slit 3.8 9. Set Shape to Spherical, Strain to 0, Fix, Refine and calculate crystal size 10.In the Crystal structure tab, use the Occupancy of Ni1 as the Cation mixing value.

[0106] [Table 2]

[0107] Referring to Table 2, it can be seen that Examples 1 to 8, which use a positive electrode active material containing a single-particle metal oxide on which a Co coating layer is formed, have increased discharge capacity and excellent high-temperature life and resistance characteristics compared to Comparative Examples 1 and 2, which do not have a Co coating layer. Also, in Comparative Example 3, in which the coating layer is formed by a wet method, the high-temperature resistance increase rate increases significantly, resulting in a significant decrease in resistance characteristics.

[0108] In particular, the positive electrode active material according to Example 4, which was prepared under the condition that the heat treatment temperature in the coating layer formation process was 660° C., exhibited the best 0.2C discharge capacity, lifespan, and high temperature resistance increase rate characteristics.

[0109] (Experimental Example 4) Analysis of Positive Electrode Active Material Structure The results of SEM image measurement of the positive electrode active materials prepared in Examples 2, 4, and Comparative Example 3 are shown in FIGS. 1 to 3, respectively.

[0110] Specifically, FIG. 1 is an SEM image of the cathode active material prepared according to Example 2, magnified 20,000 times, FIG. 2 is an SEM image of the cathode active material prepared according to Example 4, magnified 20,000 times, and FIG. 3 is an SEM image of the cathode active material prepared according to Comparative Example 3, magnified 20,000 times.

[0111] 1 to 3, it can be seen that the surface roughness of the positive electrode active material shown in FIGS. 1 and 2, which shows an example in which a coating layer is formed by a dry method, is different from that shown in FIG. 3, which shows an example in which a coating layer is formed by a wet method.

[0112] Next, FIG. 4(a) is a diagram showing a cross-sectional image of the positive electrode active material produced in Example 2 after milling treatment with a FIB (Focused Ion Beam, SEIKO 3050SE).

[0113] 4(b), 4(c), and 4(d) are diagrams showing the results of analysis using an EDS (Energy Dispersed Spectroscopy, Oxford X_max 100TLE) analyzer to identify elements present inside the positive electrode active material of FIG. 4(a).

[0114] Referring to Figures 4(a) to 4(d), it can be seen that the Co content in the coating portion increases.

[0115] FIG. 5(a) is an image of the cathode active material prepared in Example 2 measured using a high resolution transmission electron microscope (HRTEM), and FIG. 5(b) is an enlarged HRTEM image of the rectangular area indicated by the dotted line in FIG. 5(a).

[0116] Figure 6(a) shows the results of measuring the interplanar distances of 10 layers in the layered structure of Area A (coating layer) in 5(b), and Figure 6(b) shows the results of measuring the interplanar distances of 10 layers in the layered structure of Area B (single particle base material) in 5(b). The interplanar distances were measured by line profile analysis of HR-TEM images using the Gatan DigitalMicrograph program.

[0117] 6(a) and 6(b), the total distance in Area A is 4.67 nm, and the total distance in Area B is 4.82 nm. Therefore, the average interplanar distance of the 10 layers in Area A (coating layer) is 0.467 nm, and the average interplanar distance of the 10 layers in Area B is 0.482 nm.

[0118] That is, it can be seen that the metal oxide of the cathode active material prepared in Example 2 and the coating layer located on the surface of the metal oxide all have a layered structure, but the average inter-plane distance of the coating layer is smaller than the average inter-plane distance of the metal oxide.

[0119] FIG. 7(a) is an image of the cathode active material prepared according to Comparative Example 3 measured using a high resolution transmission electron microscope (HRTEM), and FIG. 7(b) is an enlarged HRTEM image of the rectangular area indicated by the dotted line in FIG. 7(a).

[0120] Figure 8(a) shows the results of measuring the interplanar distances of 10 layers in the layered structure of Area A (coating layer) in 7(b), and Figure 8(b) shows the results of measuring the interplanar distances of 10 layers in the layered structure of Area B (single particle base material) in 7(b). The interplanar distances were measured by line profile analysis of HR-TEM images using the Gatan DigitalMicrograph program.

[0121] 8(a) and 8(b), the total distance in Area A is 4.82 nm, and the total distance in Area B is 4.81 nm. Therefore, the average interplanar distance of the 10 layers in Area A (coating layer) is 0.482 nm, and the average interplanar distance of the 10 layers in Area B is 0.481 nm.

[0122] In other words, it can be confirmed that the metal oxide of the positive electrode active material prepared in Comparative Example 3 and the coating layer located on the surface of the metal oxide all have a layered structure, but the average interplanar distance of the coating layer and the metal oxide is the same.

[0123] Next, Figure 9 is an SEM image of the cathode active material prepared in Example 9, measured at 5,000 times magnification. That is, it is related to the cathode active material prepared in a bimodal form.

[0124] Referring to FIG. 9, it can be seen that large grain sizes of polycrystals and small grain sizes of single grains are uniformly mixed.

[0125] The cathode active material according to an embodiment has a single particle form, and has a surface structure modified so that a coating layer having a stripe shape is located on the surface, thereby providing a cathode active material with improved discharge capacity, room temperature resistance, high temperature life, and high temperature resistance characteristics.

[0126] The present invention is not limited to the above-described embodiments, and can be manufactured in various different forms, and those skilled in the art will understand that the present invention can be embodied in other specific forms without changing the technical spirit or essential characteristics of the present invention. Therefore, it should be understood that the above-described embodiments are illustrative in all respects and not limiting.

Claims

1. a metal oxide in a single particle form and including a layered structure; a coating layer located on the surface of the metal oxide and including a layer structure; Including, a layer structure contained in the coating layer having a smaller average interplanar distance than a layer structure contained in the metal oxide;

2. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein an average interplanar distance of the layered structure contained in the coating layer is 0.48 nm or less.

3. The positive electrode active material for a lithium secondary battery according to claim 1 , wherein the positive electrode active material has a Li / Ni cation mixing ratio of 1.5% or less.

4. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the coating layer contains at least one of Co, Al, W, V, Ti, Nb, Ce, B, and P.

5. the coating layer contains Co; The positive electrode active material for a lithium secondary battery according to claim 4 , wherein the content of Co contained in the coating layer is higher than the content of Co contained in the metal oxide.

6. 6. The positive electrode active material for a lithium secondary battery according to claim 5, wherein the content of Co contained in the coating layer is in the range of 0.5 mol % to 3.5 mol % based on the entire coating layer.

7. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the coating layer has an average thickness in the range of 30 nm to 60 nm.

8. the metal oxide comprises nickel, cobalt, and manganese; 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the content of the nickel in the entire metal oxide is greater than the total content of the cobalt and manganese.

9. The metal oxide further comprises a doping element; 9. The positive electrode active material for a lithium secondary battery according to claim 8, wherein the doping element comprises at least one of Al, Zr, Nb, Mo, W, Ti, Ce, Mg, B, P, V, Sr, and B.

10. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the metal oxide has a crystal grain size of 200 nm or more.

11. The positive electrode active material for a lithium secondary battery according to claim 1 , further comprising a metal oxide in the form of secondary particles formed by aggregation of primary particles.

12. 12. The positive electrode active material for a lithium secondary battery according to claim 11, wherein the average particle size (D50) of the positive electrode active material containing the metal oxide in the form of secondary particles is larger than the average particle size (D50) of the positive electrode active material containing the metal oxide in the form of single particles.

13. the metal oxide in secondary particle form comprises nickel, cobalt, and manganese; The positive electrode active material for a lithium secondary battery according to claim 11 , wherein the content of the nickel in the entire metal oxide in the form of secondary particles is greater than the total content of the cobalt and manganese.

14. The metal oxide in the form of secondary particles further comprises a doping element; 14. The positive electrode active material for a lithium secondary battery according to claim 13, wherein the doping element comprises at least one of Al, Zr, Nb, Mo, W, Ti, Ce, Mg, B, P, V, Sr, and B.

15. The positive electrode active material for a lithium secondary battery according to claim 11 , wherein the positive electrode active material including the metal oxide in the form of single particles and the metal oxide in the form of secondary particles has the same or different composition.

16. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 15; a negative electrode; Electrolytes, A lithium secondary battery comprising:

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