Positive electrode active material for lithium secondary battery and lithium secondary battery comprising same

Abstract

The present embodiment relates to a lithium secondary battery positive electrode active material and a lithium secondary battery comprising the same. A lithium secondary battery positive electrode active material according to one embodiment comprises: a metal oxide in the form of a single particle; and a coating layer on the surface of the metal oxide, the lithium concentration in a 2 / 5 to 3 / 5 thickness region may have a value lower than the lithium concentration of the metal oxide, based on the entire thickness of the coating layer.

Description

Technical Field

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

[0002] Recently, with the explosive growth in the demand for electric vehicles and the requirement to increase the driving range, high-capacity and high-energy-density secondary batteries that meet the requirements are being actively developed globally.

[0003] As a solution to meet these requirements, a technology using a high-nickel NCM (nickel cobalt manganese) positive electrode material with a high Ni content has been proposed. At the same time, in order to increase the plate density of the electrode as a battery component, it is necessary to construct a bimodal form in which large particles and small particles are mixed at a certain fraction.

[0004] However, for a positive electrode material in the form of secondary particles formed by aggregating primary particles sized from several tens of nanometers to several micrometers, since the specific surface area of the powder is large and the contact area with the electrolyte is wide, the possibility of generating gas is high, and there is a problem of deteriorated life characteristics.

[0005] To solve these problems, a method of increasing the size of primary particles using a sintering agent or a flux has been proposed. However, in this case, a rocksalt structure is formed on the surface portion of the particles, and there is a problem of a decrease in the electrochemical performance of the positive electrode active material.

[0006] Therefore, it is necessary to develop a positive electrode active material that increases the size of primary particles and has excellent electrochemical performance. Summary of the Invention

[0007] Technical Problem to be Solved

[0008] An object of this embodiment is to provide a positive electrode active material having a single-particle form and excellent electrochemical performance, and a lithium secondary battery including the same.

[0009] Technical Solution

[0010] A positive electrode active material for a lithium secondary battery according to an embodiment includes: a metal oxide in a single-particle form; and a coating on the surface of the metal oxide, and a lithium concentration in a 2 / 5 to 3 / 5 thickness region based on the entire thickness of the coating may have a value lower than the lithium concentration of the metal oxide.

[0011] A lithium secondary battery according to an embodiment may include the positive electrode; a negative electrode; and an electrolyte.

[0012] Advantageous Effects

[0013] The positive electrode active material of a lithium secondary battery according to an embodiment is surface-modified by forming a coating on the surface of a metal oxide and controlling the lithium concentration in a certain thickness region based on half of the coating thickness, so that the electrochemical performance of a lithium secondary battery using the positive electrode active material in the form of single particles can be significantly improved. Brief Description of the Drawings

[0014] Figure 1 It is a SEM image detected after magnifying the positive electrode active material prepared according to Example 2 by 20,000 times.

[0015] Figure 2 It is a SEM image detected after magnifying the positive electrode active material prepared according to Example 4 by 20,000 times.

[0016] Figure 3 It is a SEM image detected after magnifying the positive electrode active material prepared according to Comparative Example 2 by 20,000 times.

[0017] Figure 4 It shows a cross-sectional TEM image after grinding and processing the positive electrode active material prepared according to Example 4 with FIB (Focused Ion Beam, SEIKO 3050SE).

[0018] Figures 5 to 7 It shows the results of element mapping of the positive electrode active material prepared according to Example 4 using an EDS (Energy Dispersive Spectroscopy, Oxford X_max 100TLE) analysis device respectively.

[0019] Figure 8 It shows Figure 4 of the TEM-EDS line scan graph.

[0020] Figure 9 It shows Figure 4 at measurement points 1, 2, and 3 in the EELS graph.

[0021] Figure 10 It shows a cross-sectional TEM image after grinding and processing the positive electrode active material prepared according to Comparative Example 2 with FIB (Focused Ion Beam, SEIKO 3050SE).

[0022] Figures 11 to 13 It shows the results of element mapping of the positive electrode active material prepared according to Comparative Example 2 using an EDS (Energy Dispersive Spectroscopy, Oxford X_max 100TLE) analysis device respectively.

[0023] Figure 14 It shows Figure 10 of the TEM-EDS line scan graph.

[0024] Figure 15 Show Figure 10 The EELS diagrams at measuring points 1, 2, and 3.

[0025] Figure 16 It is an SEM image detected after magnifying the positive electrode active material prepared in Example 8 by 5,000 times. Detailed implementation manner

[0026] The terms first, second, third, etc. are used to describe various parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, and / or segment from another part, component, region, layer, and / or segment. Therefore, without departing from the scope of the present invention, the first part, component, region, layer, and / or segment described below can also be described as the second part, component, region, layer, and / or segment.

[0027] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. Unless otherwise clearly indicated to the contrary in the context, the singular forms used are also intended to include the plural forms. It should also be understood that the term "comprising" used in the specification can specifically refer to a certain characteristic, field, integer, step, action, element, and / or component, but does not exclude the existence or addition of other characteristics, fields, integers, steps, actions, elements, and / or components.

[0028] If a part is described as being above another part, there may be other parts directly above or between the other part. When a part is described as being directly above another part, there will be no other parts therebetween.

[0029] Unless otherwise defined, the meanings of all terms (including technical terms and scientific terms) used herein are the same as those commonly understood by those of ordinary skill in the art to which the present invention pertains. For terms defined in a dictionary, they should be interpreted as having a meaning consistent with the relevant technical literature and the content disclosed herein, and should not be interpreted in an idealized or overly formal sense.

[0030] Positive electrode active material for lithium secondary battery

[0031] As described above, when the primary particle size is increased, a Rocksalt structure is formed on the surface, and the electrochemical performance of the positive electrode active material will decline. However, in this embodiment, a positive electrode active material with surface structure modification is achieved by controlling the lithium concentration in a certain thickness region based on half of the coating thickness in the coating on the surface of the metal oxide, thereby solving the above problems.

[0032] That is, the positive electrode active material of a lithium secondary battery according to an embodiment includes: a metal oxide in the form of single particles; and a coating on the surface of the metal oxide. Based on the entire thickness of the coating, the lithium concentration in the 2 / 5 to 3 / 5 thickness region may have a value lower than the lithium concentration in the metal oxide.

[0033] In this embodiment, the average thickness of the coating may be 100 nm or less, and more specifically, it may be in the range of 18 nm to 75 nm or 25 nm to 65 nm. When the thickness of the coating satisfies the above range, a positive electrode active material with excellent room temperature resistance, high temperature life characteristics, and a significantly reduced high temperature resistance increase rate can be provided.

[0034] In the EDS analysis of the positive electrode active material of this embodiment, the concentration of nickel in the coating may increase from the coating surface toward the metal oxide. When the nickel concentrations of the coating and the metal oxide in the positive electrode active material are characterized in the above form, excellent capacity of the lithium secondary battery can be ensured.

[0035] On the other hand, the positive electrode active material may have a Li / Ni cation mixing ratio of 1.5% or less, and more specifically, it may be 1.1 to 1.4%. If the Li / Ni cation mixing ratio is too large, the Li layer is likely to disintegrate, which may cause a significant reduction in the life characteristics of the battery. In addition, if the Li / Ni cation mixing ratio is too small, the irreversible sites in the bulk part of the positive electrode active material become larger, the lithium ion mobility decreases, and the resistance characteristics and output characteristics may deteriorate. Therefore, when the Li / Ni cation mixing ratio satisfies the above range, a positive electrode active material with low resistance and increased life can be achieved, which has an advantageous effect.

[0036] In this specification, the Li / Ni cation mixing ratio refers to the amount of Li sites replaced by Ni.

[0037] In this embodiment, the coating may include at least one of Co, Al, W, V, Ti, Nb, Ce, B, and P. At this time, based on the entire coating, the content of the element contained in the coating may be in the range of 0.5 mol% to 3.5 mol%. Since the coating includes at least one of the above elements, modification of the surface structure of the positive electrode active material of this embodiment can be achieved.

[0038] In addition, the coating contains Co. Based on the entire thickness of the coating, the Co concentration in the 2 / 5 to 3 / 5 thickness region may be higher than the Co concentrations in the coating surface and the metal oxide. When the Co concentrations of the coating and the metal oxide in the positive electrode active material are characterized in the above form, a positive electrode active material with excellent battery capacity, efficiency, and high temperature life characteristics can be achieved.

[0039] The metal oxide contains nickel, cobalt, and manganese, and the content of nickel in the entire metal oxide can be greater than the sum of the contents of cobalt and manganese.

[0040] More specifically, based on 1 mole of nickel, cobalt, and manganese, the content of nickel in the metal oxide particles can be 0.8 moles or more. More specifically, the content of nickel can be in the range of 0.8 to 0.99, 0.85 to 0.99, or 0.88 to 0.99.

[0041] As described in this embodiment, when the content of nickel in the metal of the lithium metal oxide is 0.8 moles or more, a positive electrode active material having high output characteristics can be achieved. For the positive electrode active material of this embodiment having such a composition, the energy density per unit volume is high, so the capacity of a battery using the positive electrode active material can be increased, and it is also very suitable for electric vehicles.

[0042] In the positive electrode active material of this embodiment, the concentration difference of manganese from the halfway point of the coating thickness to the center of the metal oxide can be 1 mol% or less. That is to say, the concentration of manganese can be uniformly formed from about half of the coating thickness to the center of the metal oxide. Thus, when the concentration of manganese is uniform, a lithium secondary battery having excellent stability can be provided.

[0043] The metal oxide may further contain a doping element, and the doping element may include at least one of Al, Zr, Nb, Mo, W, Ti, Ce, Mg, B, P, V, Sr, and B.

[0044] Regarding the content of the doping element, based on 1 mole of the total of nickel, cobalt, manganese, and the doping element, it can be in the range of 0.0005 moles to 0.04 moles or 0.001 moles to 0.03 moles. At this time, the doping element refers to the doping amount of the doping element contained in the finally obtained positive electrode active material.

[0045] In the positive electrode active material, in order to ensure the lifespan and various electrochemical properties, the selection of the doping element is important. In this embodiment, by adopting various doping elements as described above, the characteristics of the positive electrode active material can be improved.

[0046] In this embodiment, the doping element may include Zr and Al.

[0047] Since Zr ions occupy Li sites, Zr acts as a kind of pillar, alleviating the shrinkage of the lithium ion path during charge and discharge, thus realizing the stability of the layered structure. This phenomenon reduces cation mixing and increases the lithium diffusion coefficient, thereby increasing the cycle life.

[0048] In addition, Al ions migrate to the tetragonal lattice site, thus inhibiting the degradation of the layered structure into a spinel structure where the migration of lithium ions is relatively less smooth.

[0049] Based on 1 mole of the total of the nickel, cobalt, manganese and doping elements, the content of Zr can be from 0.001 mole to 0.01 mole, and more specifically can be in the range of 0.0016 mole to 0.005 mole. When the Zr doping amount meets the above range, the high-temperature resistance increase rate can be reduced, and at the same time excellent life characteristics can be ensured.

[0050] Based on 1 mole of the total of the nickel, cobalt, manganese and doping elements, the content of Al can be from 0.001 mole to 0.04 mole, and more specifically can be in the range of 0.004 mole to 0.028 mole, 0.0045 mole to 0.027 mole or 0.0055 mole to 0.025 mole. When the Al doping amount meets the above range, the high-temperature life and thermal stability can be further improved.

[0051] Secondly, the average grain size of the metal oxide can be 200 nm or more.

[0052] When having such an average grain size, it can be defined as single particles. In addition, if the average grain size meets the above range, the crystallization progresses well, the residual lithium on the surface of the positive electrode active material can be reduced, and the life characteristics of the lithium secondary battery can be further improved.

[0053] In this specification, the average grain size is defined as the grain size detected by the following method.

[0054] 1. Analyze the sample structure using a Rigaku smart lab device.

[0055] 2. To generate X-rays, apply 45 kV and 200 mA (9 kW) to the Cu anode.

[0056] 3. The optical device is set such that the incident slit is 1 / 2 degree and the receiving slit is 8.0 mm.

[0057] 4. XRD detection is performed with a scan range of 10 - 80°, a step of 0.02°, and a rate of 10° / minute.

[0058] 5. The grain size is calculated using SmartLab Studio II v4.2.82.0 software.

[0059] 6. For the calculation, the whole powder pattern fitting (WPPF) in the software is utilized.

[0060] 7. When performing WPPF, the structure adopts a layered structure, and the profile fitting is set to the FP method.

[0061] 8. The receiving optic is set to graphite (002), and the Soller slit is set to 3.8.

[0062] 9. The shape is set to spherical, the strain is fixed at 0, and the grain size is calculated by refinement.

[0063] The average particle size (D50) of the positive electrode active material is 3 μm or more, and more specifically, it can be in the range of 3.5 μm to 4.5 μm. In this embodiment, in order to prepare a positive electrode active material in the form of single particles having the above-mentioned average particle size, no additional expensive pulverization equipment or multiple pulverization processes are required. That is, using a conventional pulverization device, a positive electrode active material in the form of single particles with a very small amount of fine powder and coarse powder and a uniform particle size distribution can be prepared. Therefore, when the average particle size of the positive electrode active material in this embodiment satisfies the above range, a lithium secondary battery with excellent electrochemical characteristics can be achieved.

[0064] On the other hand, the positive electrode active material of this embodiment may also include a positive electrode active material in the form of secondary particles formed by aggregation of primary particles and containing a metal oxide.

[0065] That is to say, it may include a positive electrode active material containing the metal oxide in the form of single particles and a positive electrode active material containing the metal oxide in the form of secondary particles with an average particle size (D50) larger than that of the positive electrode active material containing the metal oxide in the form of single particles. As described above, when the positive electrode active material containing the metal oxide in the form of single particles and the positive electrode active material containing the metal oxide in the form of secondary particles are mixed and used in the bimodal form as described above, the density of the electrode mixture can be increased, so there is an advantage.

[0066] As described above, in the positive electrode active material in the bimodal form, in terms of the weight ratio (single particle: secondary particle), the mixing ratio of the positive electrode active material containing the metal oxide in the form of single particles and the positive electrode active material containing the metal oxide in the form of secondary particles can be in the range of 30:70 to 10:90 or 25:75 to 15:85. When the positive electrode active materials containing the metal oxides in the form of single particles and secondary particles are mixed and used in the weight ratio as described above, the density of the electrode mixture can be increased.

[0067] At this time, the positive electrode active materials containing the metal oxides in the form of single particles and secondary particles may have the same composition or different compositions. Specifically, the positive electrode active materials containing the metal oxides in the form of single particles and secondary particles can both contain nickel, cobalt, manganese, and doping elements.

[0068] For example, the positive electrode active material containing the metal oxide in the form of secondary particles contains nickel, cobalt, and manganese, and the content of nickel in the entire metal oxide in the form of secondary particles can be greater than the sum of the contents of cobalt and manganese.

[0069] More specifically, based on 1 mole of nickel, cobalt, and manganese, the content of nickel in the metal oxide particles in the form of secondary particles can be 0.8 mole or more. More specifically, the content of nickel can be in the range of 0.8 to 0.99, 0.85 to 0.99, 0.88 to 0.99.

[0070] The metal oxide in the form of secondary particles further contains a doping element, and the doping element can include at least one of Al, Zr, Nb, Mo, W, Ti, Ce, Mg, B, P, V, Sr, and B.

[0071] The specific description and content of the doping element are the same as those of the positive electrode active material containing the metal oxide in the form of single particles described above, and will not be elaborated here.

[0072] In addition, in the present embodiment, the average particle diameter (D50) of the positive electrode active material containing the metal oxide in the form of the secondary particles may be in the range of 10 μm to 20 μm, or 12 μm to 17 μm. When the average particle diameter of the positive electrode active material containing the metal oxide in the form of the secondary particles satisfies the above range, large particles and small particles can be appropriately distributed in the positive electrode active material in a bimodal form, and thus the energy density of the lithium secondary battery can be improved.

[0073] Lithium secondary battery

[0074] Another embodiment of the present invention provides a lithium secondary battery, which includes: a positive electrode containing the positive electrode active material according to one embodiment of the present invention as described above; a negative electrode; and an electrolyte disposed between the positive electrode and the negative electrode.

[0075] Specifically, the positive electrode includes a current collector and a positive electrode active material layer formed on the current collector, and the positive electrode active material constituting the positive electrode active material layer has the same characteristics as those described above. Therefore, the specific description of the positive electrode active material is omitted.

[0076] For the current collector, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or a material obtained by surface-treating the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. can be used.

[0077] On the other hand, the positive electrode active material layer may include a binder and a conductive material.

[0078] The binder serves to bond the positive electrode active material particles to each other well and bond the positive electrode active material to the current collector well.

[0079] The conductive material is used to impart conductivity to the electrode, and any conductive material can be used as long as it does not cause a chemical change in the formed battery.

[0080] For the positive electrode, the active material, the conductive material, and the binder are mixed in a solvent to form an active material composition, and the composition is coated on the current collector to prepare the positive electrode. Such an electrode preparation method is well known in the art, and thus the detailed description is omitted in this specification. As the solvent, N-methylpyrrolidone, etc. can be used, but it is not limited thereto.

[0081] 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.

[0082] As the negative electrode active material, a substance capable of reversibly inserting / extracting lithium ions, lithium metal, an alloy of lithium metal, a substance capable of doping and dedoping lithium, or a transition metal oxide can be used.

[0083] As the material capable of reversibly inserting / extracting lithium ions, any carbonaceous negative electrode active material generally used in lithium ion secondary batteries as a carbon material can be used. As typical examples thereof, crystalline carbon, amorphous carbon, or a combination thereof can be used.

[0084] As the alloy of the lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0085] As the material capable of doping and dedoping lithium, Si, SiO x (0 < x < 2), Si-Y alloy (wherein Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, but not Si), Sn, SnO2, Sn-Y (wherein Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, but not Sn), etc. can be cited.

[0086] As the transition metal oxide, vanadium oxide, lithium vanadium oxide, etc. can be cited.

[0087] The negative electrode active material layer also contains a binder, and may optionally further contain a conductive material.

[0088] As the binder, polyvinyl alcohol, carboxymethyl cellulose / styrene-butadiene rubber, hydroxypropyl cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, etc. can be used, but are not limited thereto. The binder can be mixed in an amount of 1 wt% to 30 wt% relative to the total amount of the composition for forming the negative electrode active material layer.

[0089] There is no particular limitation on the conductive material as long as it has conductivity and does not cause chemical changes in the battery. Specifically, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powders; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polybenzene derivatives, etc. can be used. The conductive material can be mixed in an amount of 0.1 wt% to 30 wt% relative to the total amount of the composition for forming the negative electrode active material layer.

[0090] As the current collector, a material selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer material coated with a conductive metal, and combinations thereof can be used.

[0091] For the negative electrode, an active material composition is prepared by mixing an active material, a conductive material, and a binder in a solvent, and the composition is coated on a current collector to prepare the negative electrode. Such an electrode preparation method is well known in the art, and thus detailed description is omitted in this specification. As the solvent, N-methylpyrrolidone or the like can be used, but is not limited thereto.

[0092] In addition, in the lithium secondary battery, as the electrolyte, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used when preparing a lithium secondary battery can be cited, but is not limited thereto.

[0093] Specifically, the organic liquid electrolyte may contain an organic solvent and a lithium salt.

[0094] The organic solvent serves as a medium through which ions participating in the electrochemical reaction of the battery can migrate.

[0095] The lithium salt is a substance that plays the following role: it dissolves in the organic solvent, serves as a lithium ion source in the battery, can ensure the basic operation of the lithium secondary battery, and promotes the migration of lithium ions between the positive electrode and the negative electrode.

[0096] Depending on the type of lithium secondary battery, a separator may also be present between the positive electrode and the negative electrode. As such a separator, polyethylene, polypropylene, polyvinylidene fluoride, or a multi-layer film of at least two layers thereof can be used. Of course, a mixed multi-layer film such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, a polypropylene / polyethylene / polypropylene three-layer separator, etc. can be used.

[0097] For lithium secondary batteries, depending on the type of separator and electrolyte used, they can be classified into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries. Depending on the shape, they can be classified into cylindrical, prismatic, coin-shaped, pouch-shaped, etc. Depending on the size, they can be classified into block-shaped and thin film-shaped. The structures and preparation methods of these batteries are well known in the art, and thus will not be elaborated further.

[0098] Modes for Carrying Out the Invention

[0099] Hereinafter, embodiments of the present invention will be described in detail. However, the following embodiments are merely examples, and the present invention is not limited to the following embodiments. The scope of the present invention is defined by the claims.

[0100] Example 1 - Positive electrode active material in the form of single particles, heat-treated at 600 °C for 6 hours

[0101] (1) Preparation of the precursor

[0102] The precursor was prepared by a general co - precipitation method.

[0103] Specifically, NiSO4·6H2O was used as the nickel raw material substance, CoSO4·7H2O was used as the cobalt raw material substance, and MnSO4·H2O was used as the manganese raw material substance. These raw materials were dissolved in distilled water to make a metal salt aqueous solution.

[0104] After preparing the co - precipitation reactor, it was purged with N2 to prevent the oxidation of metal ions during the co - precipitation reaction, and the reactor temperature was maintained at 50 °C.

[0105] NH4(OH) was added as a chelating agent to the co - precipitation reactor, and NaOH was used to adjust the pH. The precipitate obtained according to the co - precipitation process was filtered, washed with distilled water, and then dried in an oven at 100 °C for 24 hours to prepare the precursor of the positive electrode active material.

[0106] The composition of the prepared precursor was (Ni 0.98 Co 0.01 Mn 0.01 )(OH)2, and the average particle size (D50) was about 4 μm.

[0107] (2) Preparation of the positive electrode active material

[0108] After separately weighing 315 g of LiOH·H2O, 2.94 g of Al(OH)3, and 1.47 g of ZrO2, they were uniformly mixed with 684 g of the precursor prepared in (1) above, and calcined at 830 - 890 °C for 24 hours in a box - type calcination furnace with oxygen being introduced at a rate of 1000 mL / minute to synthesize the positive electrode material in the form of metal oxide. Then, it was disintegrated using a jet - Mill to prepare the positive electrode active material containing metal oxide in the form of single particles.

[0109] Approximately 1.927 g of Co(OH)2 was mixed per 100 of the metal oxide in the form of single particles, and heat - treated at 600 °C for 6 hours in an oxygen atmosphere to prepare the positive electrode active material with a coating formed.

[0110] Examples 2 to 7

[0111] Except that the heat - treatment conditions during coating formation were adjusted as shown in Table 1 below, the positive electrode active material with a coating formed was prepared by the same method as in Example 1.

[0112]

Table 1

[0113]

[0114] Example 8 (1) Preparation of the precursor

[0115] Except that the components are (Ni 0.92 Co 0.04 Mn 0.04 )(OH)2 and the average particle size (D50) is made to reach about 14.5 μm, a precursor was prepared by the same method as in Example 1.

[0116] (2) Preparation of large particle size positive electrode active material

[0117] After weighing 325 g of LiOH·H2O, 11.4 g of Al(OH)3, and 3.2 g of ZrO2 respectively, they were uniformly mixed with 684 g of the precursor prepared in (1) above, and calcined at 830 - 890 °C for 24 hours in a box-type calcination furnace with oxygen being introduced at a rate of 1000 mL / minute to synthesize a positive electrode material in the form of a metal oxide.

[0118] Then, it was disintegrated using an ACM (air classification mill), and then subjected to a washing treatment to remove residual lithium on the surface, and then dried for 12 hours.

[0119] Next, a mixture of about 0.35 g of boric acid (H3BO3) per 100 of the dried positive electrode material was heat-treated at 250 - 300 °C for 5 hours in an air atmosphere.

[0120] (3) Preparation of bimodal positive electrode active material

[0121] The positive electrode active material prepared in (2) above and the positive electrode active material prepared according to Example 4 were mixed at a weight ratio of 8:2 to prepare a bimodal positive electrode active material.

[0122] Comparative Example 1

[0123] (1) Preparation of precursor

[0124] A precursor was prepared by the same method as in Example 1.

[0125] (2) Preparation of positive electrode active material

[0126] After weighing 315 g of LiOH·H2O, 2.94 g of Al(OH)3, and 1.47 g of ZrO2 respectively, they were uniformly mixed with 684 g of the precursor prepared in (1) above, and calcined at 830 - 890 °C for 24 hours in a box-type calcination furnace with oxygen being introduced at a rate of 1000 mL / minute to synthesize a positive electrode material in the form of a metal oxide. Then, it was disintegrated using a jet-mill to prepare a positive electrode active material containing metal oxide in the form of single particles.

[0127] Comparative Example 2

[0128] The positive electrode active material prepared according to Comparative Example 1 was further heat-treated at 660 °C for 6 hours to prepare the positive electrode active material according to Comparative Example 2.

[0129] Comparative Example 3 - The case of forming a Co coating by a wet method

[0130] The positive electrode active material was prepared by the same method as in Comparative Example 1.

[0131] Next, 0.03 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 positive electrode 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, heated to about 450 °C at a rate of about 5 °C per minute, held at about 450 °C for about 1 hour, then heated to about 700 °C at a rate of about 2 °C per minute, and then held for about 2 hours. Subsequently, the sample was cooled to room temperature to prepare the positive electrode active material according to Comparative Example 2.

[0132] Experimental Example 1: Electrochemical performance evaluation

[0133] (1) Preparation of coin-type half-cell

[0134] To evaluate the physical properties and electrochemistry of the positive electrode active materials prepared according to Examples 1 to 8 and Comparative Examples 1 to 3, coin-type half-cells were prepared as described below.

[0135] Specifically, the positive electrode active material, polyvinylidene fluoride binder (trade name: KF1120), and carbon black conductive material were mixed at a weight ratio of 96.5:1.5:2, and the mixture was added to N-methyl-2-pyrrolidone solvent to make the solid content reach about 30 wt% to prepare a positive electrode active material slurry.

[0136] The slurry was coated on an aluminum foil (Al foil, thickness: 15 μm) as the positive electrode current collector using a doctor blade, dried and pressed to form a positive electrode. The loading of the positive electrode was about 15 mg / cm 2 , the electrode thickness was about 65 μm, and the pressing density was about 3.4 g / cm 3 above.

[0137] Using the said positive electrode, lithium metal negative electrode (with a thickness of 300 μm, MTI), electrolyte, and polypropylene separator, a 2032 coin-type half-cell was prepared by a conventional method. For the electrolyte, 1 M of LiPF6 was dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate (EMC) (mixing ratio EC:DMC:EMC = 3:4:3 vol%) to prepare a mixed solution. After preparing the half-cell, it was aged at room temperature for 10 hours.

[0138] (2) Detecting residual lithium and particle size

[0139] For residual lithium, detection was carried out using the T50 model of METTLER TOLEDO company. For particle size, detection was carried out using the S3500 model of microtrac company.

[0140] (3) Evaluating capacity

[0141] After the coin-type half-cell prepared according to the experimental preparation was aged at room temperature (25 °C) for 10 hours, charge-discharge tests were then carried out.

[0142] For capacity evaluation, 200 mAh / g was used as the reference capacity, and the charge-discharge conditions were CC / CV 2.5 - 4.25 V, 1 / 20C cut-off. For the initial capacity, detection was carried out by charging at 0.2C / discharging at 0.2C.

[0143] (4) Detecting life characteristics

[0144] For life characteristics, detection was carried out 50 times under the conditions of charging at 0.5C / discharging at 1.0C at high temperature (45 °C).

[0145] (5) Detecting resistance characteristics

[0146] For the initial resistance at room temperature (direct current internal resistance: DC-IR), at 25 °C, under the conditions of constant current - constant voltage from 2.5 V to 4.25 V, 1 / 20C cut-off, the battery was subjected to one 0.2C charge and 0.2 discharge, and the voltage value after applying a discharge current for 60 seconds when charged to 100% at 4.25 V was detected, and then calculated.

[0147] For the resistance increase rate, the initial resistance (initial resistance at room temperature) was detected at high temperature (45 °C), and the resistance after 30 cycles was detected by the same method as the initial resistance detection method, and its increase rate was converted into a percentage (%).

[0148] Experimental Example 2: X-ray diffraction evaluation

[0149] - Analyze the sample structure using Rigaku smart lab equipment.

[0150] - To generate X-rays, apply 45 kV and 200 mA (9 kW) to the Cu anode.

[0151] - Set the optic equipment with an incident slit of 1 / 2 degree and a receiving slit of 8.0 mm.

[0152] - Conduct XRD detection with a scan range of 10 - 80°, a step of 0.02°, and a rate of 10° / min.

[0153] Experimental Example 3: Cation Mixing Ratio Detection Method

[0154] 1. Analyze the sample structure using Rigaku smart lab equipment.

[0155] 2. To generate X-rays, apply 45 kV and 200 mA (9 kW) to the Cu anode.

[0156] 3. Set the optic equipment with an incident slit of 1 / 2 degree and a receiving slit of 8.0 mm.

[0157] 4. Conduct XRD detection with a scan range of 10 - 80°, a step of 0.02°, and a rate of 10° / min.

[0158] 5. Calculate the grain size using SmartLab Studio II v4.2.82.0 software.

[0159] 6. For the calculation, utilize the Whole Powder Pattern Fitting (WPPF) in the software.

[0160] 7. When performing WPPF, adopt a layered structure for the structure and set the profile fitting to the FP method.

[0161] 8. Set the receiving optic to graphite (002) and the Soller slit to 3.8.

[0162] 9. Set the shape to spherical, fix the strain at 0, and calculate the grain size by refinement.

[0163] 10. In the Crystal structure tap, the occupancy of Ni1 is used as the cation mixing value.

[0164]

Table 2

[0165]

[0166] Referring to Table 2, it can be confirmed that for Examples 1 to 8 of the positive electrode active material containing metal oxide in the form of single particles with a Co coating formed, compared with Comparative Examples 1 and 2 without a Co coating formed, excellent electrochemical properties such as increased discharge capacity and improved life performance are shown. In particular, for the positive electrode active material according to Example 4 prepared under the condition that the heat treatment temperature of the coating formation process is 660 °C, its 0.2C discharge capacity, life, and high-temperature resistance increase rate characteristics are the most excellent.

[0167] Experimental Example 4: Structural Analysis of Positive Electrode Active Material

[0168] The SEM image detection results of the positive electrode active materials prepared according to Example 2, Example 4, and Comparative Example 3 are respectively shown in Figures 1 to 3 .

[0169] Specifically, Figure 1 is the SEM image detected after magnifying the positive electrode active material prepared according to Example 2 by 20,000 times, Figure 2 is the SEM image detected after magnifying the positive electrode active material prepared according to Example 4 by 20,000 times, Figure 3 is the SEM image detected after magnifying the positive electrode active material prepared according to Comparative Example 3 by 20,000 times.

[0170] Referring to Figures 1 to 3 it can be confirmed that in the Figure 1 and Figure 2 showing the positive electrode active materials of Examples 1 and 4 where the coating is formed by the dry method, wrinkles are observed on the surface, while in the case of the positive electrode active material of Comparative Example 3 showing Comparative Example 3 where the coating is formed by the wet method, the surface is in a smooth form.

[0171] As described above, by forming the coating using the wet method, the positive electrode active material with wrinkles on the surface has excellent electrochemical characteristics.

[0172] Secondly, Figure 4 shows the cross-sectional TEM image after grinding the positive electrode active material prepared according to Example 4 with FIB (Focused Ion Beam, SEIKO 3050SE).

[0173] Figures 5 to 7The results of element mapping using an EDS (Energy Dispersive Spectroscopy, Oxford X_max 100TLE) analyzer after polishing the positive electrode active material prepared according to Example 4 with FIB (Focused Ion Beam, SEIKO 3050SE) are shown respectively.

[0174] In addition, Figure 8 shown Figure 4 is the TEM-EDS line scan image of Figure 9 shown Figure 4 is the TEM-EELS image at measurement points 1, 2, and 3 in

[0175] Referring to Figures 5 to 7 it can be confirmed that cobalt, nickel, and oxygen are present in the coating of the positive electrode active material of Example 4.

[0176] In addition, referring to Figure 8 it can be seen that the coating thickness of the positive electrode active material in Example 4 is about 80 nm, and the cobalt content is the highest at 50 nm from the surface. That is, based on the entire thickness of the coating in Example 4, the Co concentration in the 2 / 5 to 3 / 5 thickness region is higher than the Co concentration in the coating surface and the metal oxide.

[0177] Referring to Figure 9 it can be seen that the lithium concentration in the positive electrode active material of Example 4 is lower than the lithium concentration in the metal oxide at 1 / 2 of the entire coating thickness or at the highest Co content. Specifically, it can be confirmed that the lithium content increases from the coating surface towards the metal oxide (base material). In addition, it can be seen that the nickel concentration in the positive electrode active material increases from the surface towards the metal oxide.

[0178] Figure 10 The cross-sectional TEM image after polishing the positive electrode active material prepared according to Comparative Example 2 with FIB (Focused Ion Beam, SEIKO 3050SE) is shown.

[0179] Figures 11 to 13 The results of element mapping of the positive electrode active material prepared according to Comparative Example 2 using an EDS (Energy Dispersive Spectroscopy, Oxford X_max 100TLE) analyzer are shown respectively.

[0180] In addition, Figure 14 shown Figure 10 is the TEM-EDS line scan image of Figure 15 shown Figure 10 is the EELS image at measurement points 1, 2, and 3 in

[0181] Referring to Figure 14, in the case of Comparative Example 3 in which the coating is formed by a wet process, the coating thickness is 80 nm, and based on the total thickness of the coating, the cobalt concentration is higher in the region from the surface to the region exceeding 2 / 5 to 3 / 5 of the thickness.

[0182] In addition, referring to Figure 15 it can be seen that the lithium concentration in the coating is higher than that in the metal oxide.

[0183] Figure 16 is an SEM image detected after magnifying the positive electrode active material prepared in Example 8 by 5,000 times. That is to say, this relates to the positive electrode active material in a bimodal form.

[0184] Referring to Figure 16 it can be confirmed that the large particle size of the polycrystal and the small particle size of the single particle are uniformly mixed.

[0185] The positive electrode active material according to one embodiment is in the form of single particles, and the surface structure is modified such that the coating is located on the surface and the lithium concentration in a certain thickness region is controlled based on half of the total thickness of the coating, so that a positive electrode active material with improved discharge capacity, room temperature resistance, high temperature life, and high temperature resistance characteristics can be achieved. In this regard, it has a very advantageous effect.

[0186] The present invention can be implemented in various different ways and is not limited to the described embodiments. Those of ordinary skill in the technical field to which the present invention pertains can understand that the present invention can be implemented in other specific ways without changing the technical idea or essential features of the present invention. Therefore, it should be understood that the above embodiments are exemplary in all aspects and are not used to limit the present invention.

Claims

1. A positive electrode active material for a lithium secondary battery, the positive electrode active material for a lithium secondary battery comprising: a metal oxide in the form of single particles; and a coating on the surface of the metal oxide, Based on the entire thickness of the coating, the lithium concentration in the 2 / 5 to 3 / 5 thickness region has a value lower than the lithium concentration of the metal oxide.

2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, The average thickness of the coating is 100 nm or less.

3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, In the EDS analysis of the positive electrode active material, The concentration of nickel in the positive electrode active material increases from the coating surface towards the metal oxide.

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

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

6. The positive electrode active material for a lithium secondary battery according to claim 5, wherein, The coating contains Co, Based on the entire thickness of the coating, the Co concentration in the 2 / 5 to 3 / 5 thickness region is higher than the Co concentration contained in the coating surface and the metal oxide.

7. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, The metal oxide contains nickel, cobalt, and manganese, The content of nickel in the entire metal oxide is greater than the sum of the contents of cobalt and manganese.

8. The positive electrode active material for a lithium secondary battery according to claim 7, wherein, The concentration difference of manganese from the 1 / 2 of the coating thickness to the center of the metal oxide is 1 mol% or less.

9. The positive electrode active material for a lithium secondary battery according to claim 7, wherein, The metal oxide further contains a doping element, The doping element contains at least one of Al, Zr, Nb, Mo, W, Ti, Ce, Mg, B, P, V, Sr, and B.

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

11. The positive electrode active material for a lithium secondary battery according to claim 10, wherein, The positive electrode active material further contains a positive electrode active material containing a metal oxide in the form of secondary particles formed by aggregation of primary particles.

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 greater than the average particle size (D50) of the positive electrode active material containing the metal oxide in the form of single particles.

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

14. The positive electrode active material for a lithium secondary battery according to claim 13, wherein, The metal oxide in the form of secondary particles further contains a doping element, The doping element includes 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 components of the positive electrode active material containing the single particles and the metal oxide in the form of secondary particles are the same or different.

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

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