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

Abstract

The present embodiment relates to a positive electrode active material, a method for preparing the same, and a lithium secondary battery comprising the same. The positive electrode active material according to one embodiment has a single particle structure having a D50 of 2 to 7 [mu] m, and the number of crystal grains measured in one particle during ASTAR analysis may be 20 or less.

Description

Technical Field

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

[0002] Recently, as environmental problems have become increasingly serious, electric vehicles have received attention as one of the solutions to overcome this problem. Thanks to the explosive demand for electric vehicles and the requirement to increase the driving range, the world is actively developing secondary batteries with high capacity and high energy density that meet the requirements.

[0003] To meet these demands, research on NCM positive electrode materials capable of ensuring high capacity, especially NCM with a high Ni content, is very active.

[0004] However, as the Ni content in the NCM positive electrode material increases, problems such as increased resistance, reduced output, and reduced lifespan have emerged.

[0005] Therefore, there is a need to develop technologies for NCM positive electrode materials with a high Ni content and excellent electrochemical performance. Summary of the Invention

[0006] Technical Problem to be Solved

[0007] The purpose of this embodiment is to provide a positive electrode active material with excellent lifespan characteristics, reduced resistance increase rate, and reduced gas generation rate, a method for preparing the same, and a lithium secondary battery including the positive electrode active material.

[0008] Technical Solution

[0009] The positive electrode active material according to an embodiment has a single particle structure with a D50 of 2 to 7 μm, and the number of crystal grains measured in one particle during ASTAR analysis is 20 or less.

[0010] Specifically, in this embodiment, the number of crystal grains measured in one particle during ASTAR analysis may be 5 to 20.

[0011] The grain size of the positive electrode active material may be in the range of 500 nm to 5 μm.

[0012] In addition, the positive electrode active material may include: metal oxide particles containing nickel, cobalt, and manganese; and two or more doping elements doped in the metal oxide particles.

[0013] The doping elements may include at least two of Zr, Al, Nb, B, Ti, Ta, V, W, and Mo.

[0014] The doping amount of the Zr may range from 0.0005 mole to 0.005 mole relative to 1 mole of the total amount of nickel, cobalt, manganese, and the doping element.

[0015] The doping amount of the Al may range from 0.001 mole to 0.008 mole relative to 1 mole of the total amount of nickel, cobalt, manganese, and the doping element.

[0016] A method for preparing a positive electrode active material according to another embodiment includes: a step of preparing an aqueous metal salt solution containing a nickel raw material substance, a cobalt raw material substance, a manganese raw material substance, and water; a step of supplying the aqueous metal salt solution to a coprecipitation reactor to obtain metal hydroxides; and a step of mixing the metal hydroxide particles, a lithium raw material substance, and a doping raw material substance and then performing calcination to obtain a lithium metal oxide, where the calcination may include a step of performing a first calcination process; and a step of continuously performing a second calcination process at a temperature range 50°C to 150°C lower than the first calcination process.

[0017] The first calcination process may be performed at a range of 800°C to 890°C for 3 to 5 hours.

[0018] The second calcination process may be performed at a range of 750°C to 800°C for 9 to 20 hours.

[0019] The first calcination process and the second calcination process may be performed as a continuous process maintaining a temperature difference of 50°C to 80°C.

[0020] In addition, a coating process of forming a coating on the surface of the positive electrode active material after the second calcination process may be further included.

[0021] The coating process may be performed to include 1 mole to 2 moles of a coating element based on the entire positive electrode active material.

[0022] The coating element included in the coating may include Co and Al.

[0023] A lithium secondary battery according to still another embodiment may include a positive electrode including the positive electrode active material according to one embodiment.

[0024] Advantageous Effects

[0025] According to one embodiment, the single particle degree can be detected by an ASTAT image and then the single particle formation can be defined.

[0026] In addition, when the single particle degree is high based on the definition in this embodiment, a positive electrode active material with excellent life characteristics and reduced resistance increase rate and gas generation rate can be achieved. Description of the Drawings

[0027] Figure 1They are TEM images and ASTAR images of the positive electrode active material prepared according to Example 1.

[0028] Figure 2 They are TEM images and ASTAR images of the positive electrode active material prepared according to Example 2.

[0029] Figure 3 They are TEM images and ASTAR images of the positive electrode active material prepared according to Example 3.

[0030] Figure 4 They are TEM images and ASTAR images of the positive electrode active material prepared according to Comparative Example 1. Detailed implementation manners

[0031] 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.

[0032] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. Unless the context clearly gives the opposite meaning, 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.

[0033] If a part is described as being above another part, there may be other parts directly above the other part or there may be other parts therebetween. When a part is described as being directly above another part, there will be no other parts therebetween.

[0034] Although not otherwise defined, the meanings of all terms (including technical terms and scientific terms) used herein are the same as those commonly understood by those of ordinary skill in the art to which the present invention pertains. For terms defined in a dictionary, they should be interpreted as having a meaning consistent with the relevant technical literature and the content disclosed herein, and should not be interpreted in an idealized or overly formal sense.

[0035] The positive electrode active material of a lithium secondary battery according to an embodiment has a single-particle structure with a D50 of 2 to 7 μm, and the number of crystal grains measured in one particle during ASTAR analysis can be 20 or less. More specifically, the number of crystal grains measured in one particle during ASTAR analysis can be in the range of 6 to 20 or 5 to 15.

[0036] The positive electrode active material with a single-particle structure in which the number of crystal grains measured in one particle during ASTAR analysis satisfies the above range has a better lifespan than the positive electrode active material with a polycrystalline structure. In addition, the gas generation rate can be significantly reduced.

[0037] In particular, when the number of crystal grains measured in one particle during ASTAR analysis exceeds the above range, the average diameter D50 size of the particles increases, and at the same time, the diffusion distance of electrons increases, so the capacity of the positive electrode active material decreases.

[0038] Therefore, in this embodiment, a positive electrode active material that maintains the D50 size and has a large crystal grain size is provided, so that a lithium secondary battery with excellent electrochemical performance and improved lifespan retention rate can be realized.

[0039] That is to say, in this embodiment, when the number of crystal grains measured in one particle during ASTAR analysis satisfies the above range, it is defined as excellent single-particle degree. For the NCM positive electrode material with excellent single-particle degree, relatively few cracks are generated, so the lifespan characteristics are excellent.

[0040] When a lithium secondary battery is manufactured using the positive electrode active material with excellent single-particle degree as described above, compared with using a polycrystalline positive electrode active material, not only can equivalent or higher lifespan characteristics be ensured, but also the resistance increase rate can be reduced.

[0041] On the other hand, the crystal grain size of the positive electrode active material during ASTAR analysis is 500 nm to 5 μm, and more specifically, it can be in the range of 1 μm to 3 μm. When the crystal grain size satisfies the above range, a positive electrode active material with a single-particle structure that can maintain an appropriate D50 and has a lifespan similar to that of a polycrystalline positive electrode active material can be realized.

[0042] If the crystal grain size is less than 500 nm, the surface area of the metal oxide particles becomes wider, so the gas generation rate will increase. In addition, if the crystal grain size is greater than 5 μm, the diffusion distance of lithium ions increases, and there is a problem of reduced capacity.

[0043] In this specification, the ASTAR analysis shows the results obtained by using the JEOL JEM-2100F equipment equipped with an ASTAR device from NanoMEGAS.

[0044] In this embodiment, the positive electrode active material may include: metal oxide particles containing nickel, cobalt, and manganese; and two or more doping elements doped in the metal oxide particles.

[0045] The doping elements may include at least two of Zr, Al, Nb, B, Ti, Ta, V, W, and Mo.

[0046] In order to ensure the lifespan and various electrochemical properties by doping lithium metal oxides, the selection of doping elements is important. As currently known doping elements, for example, there are Ag + , Na + and other mono-valent ions, and Co 2+ , Cu 2+ , Mg 2+ , Zn 2+ , Ba 2+ , Al 3+ , Fe 3+ , Cr 3+ , Ga 3+ , Zr 4+ , Ti 4+ and other multi-valent ions with a valence of two or more. These elements each have different effects on the lifespan and output characteristics of the battery.

[0047] In this embodiment, by including at least two of Zr, Al, Nb, B, Ti, Ta, V, W, and Mo among these doping elements, high capacity can be ensured, and the lifespan characteristics and thermal stability at room temperature and high temperature can be improved, and the initial resistance characteristics and resistance increase rate can be significantly reduced.

[0048] Specifically, since Zr ions occupy the Li sites, Zr 4+ plays a kind of pillar role, alleviating the contraction of the lithium ion path during charge and discharge, thereby realizing the stability of the layered structure. This phenomenon reduces cation mixing and increases the lithium diffusion coefficient, thereby increasing the cycle lifespan.

[0049] In addition, for Al 3+ , Al ions migrate to the tetragonal lattice site, thereby inhibiting the degradation of the layered structure into the spinel structure. The layered structure is beneficial to the insertion and extraction of Li ions, but the spinel structure is not conducive to the migration of Li ions.

[0050] In this embodiment, relative to 1 mole of the total amount of nickel, cobalt, manganese, and doping elements, the doping amount of Zr is 0.0005 mole to 0.005 mole, and more specifically, it can be in the range of 0.001 mole to 0.005 mole. When the Zr doping amount satisfies the above range, the lifespan and room-temperature lifespan characteristics of the lithium secondary battery can be significantly improved.

[0051] Relative to 1 mole of the total amount of nickel, cobalt, manganese, and doping elements, the doping amount of Al is 0.001 mole to 0.008 mole, and more specifically, it can be in the range of 0.002 mole to 0.006 mole. When the content of Al satisfies the above range, a lithium secondary battery with excellent lifespan characteristics and a reduced resistance increase rate can be achieved.

[0052] A method for preparing a positive electrode active material according to another embodiment includes: a step of preparing an aqueous metal salt solution containing a nickel raw material substance, a cobalt raw material substance, a manganese raw material substance, and water; a step of supplying the aqueous metal salt solution to a co-precipitation reactor to obtain metal hydroxides; and a step of mixing the metal hydroxide particles, a lithium raw material substance, and a doping raw material substance and then performing calcination to obtain a lithium metal oxide, where the calcination may include a step of performing a first calcination process; and a step of continuously performing a second calcination process in a temperature range 20°C to 100°C lower than the first calcination process.

[0053] In this embodiment, the first calcination process and the second calcination process are performed as a continuous process as described above, so that the production time of the positive electrode active material can be shortened. Therefore, more positive electrode active materials can be prepared in the same time, and thus the economy can be improved.

[0054] In addition, since the temperatures of the first calcination process and the second calcination process, which are continuous processes as described above, are controlled, the temperature setting is very important.

[0055] Specifically, the first calcination process is a step of increasing the grain size of the positive electrode active material with a single particle structure. The first calcination process controls the temperature range to perform heat treatment at a temperature about 50 degrees to 150 degrees higher than the temperature at which the layered structure is stable so that the grains can fully grow. However, if such a temperature is maintained for a long time, the layered structure will be damaged, resulting in a decrease in the electrochemical performance of the positive electrode active material. Therefore, the first calcination process tries to be maintained for a short time.

[0056] Secondly, the second calcination process is used to restore the layered structure damaged during the first calcination process. Therefore, the second calcination process can be performed in a temperature range about 50°C to 150°C lower than the first calcination process. More specifically, the first calcination process and the second calcination process can be performed as a continuous process maintaining a temperature difference of 50°C to 80°C.

[0057] Secondly, the method for preparing the positive electrode active material according to the present embodiment may further include a coating process of forming a coating on the surface of the positive electrode active material after the second calcination process. By performing such a coating process, the residual lithium content present on the surface of the positive electrode active material is controlled, thereby further improving the electrochemical performance of the positive electrode active material of the present embodiment.

[0058] Specifically, the coating process may be performed to include 1 mole to 2 moles of coating elements based on the entire positive electrode active material, and the coating elements included in the coating may include Co and Al.

[0059] Another embodiment of the present invention provides a lithium secondary battery, which includes: a positive electrode including the positive electrode active material according to one embodiment of the present invention as described above; a negative electrode including a negative electrode active material; and an electrolyte located between the positive electrode and the negative electrode.

[0060] The description of the positive electrode active material is the same as that of one embodiment of the present invention described above, and thus will not be repeated.

[0061] The positive electrode active material layer may include a binder and a conductive material.

[0062] The binder serves to bond the positive electrode active material particles to each other well and to bond the positive electrode active material well to the current collector.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] As the substance capable of reversibly inserting / extracting lithium ions, any carbon-based negative electrode active material generally used in lithium ion secondary batteries as a carbon substance can be used, and as typical examples thereof, crystalline carbon, amorphous carbon, or a combination thereof can be used.

[0067] As the alloy of 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.

[0068] As the substance capable of doping and dedoping lithium, Si, SiOx (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.

[0069] As the transition metal oxide, vanadium oxide, lithium vanadium oxide, etc. can be cited. The negative electrode active material layer also contains a binder and may optionally further contain a conductive material.

[0070] The binder serves to bond the negative electrode active material particles well to each other and to bond the negative electrode active material well to the current collector.

[0071] 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 battery formed.

[0072] As the current collector, materials selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer materials coated with a conductive metal, and combinations thereof can be used.

[0073] For the negative electrode and 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, thereby preparing the negative electrode and the positive 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, etc. can be used, but it is not limited thereto.

[0074] As the electrolyte, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc. that can be used in the preparation of lithium secondary batteries can be cited, but it is not limited thereto.

[0075] Specifically, the organic liquid electrolyte may contain an organic solvent and a lithium salt.

[0076] The organic solvent serves as a medium through which ions participating in the electrochemical reaction of the battery can migrate.

[0077] The lithium salt is a substance that dissolves in the organic solvent and 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.

[0078] 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 multilayer film of at least two layers thereof can be used. Of course, hybrid multilayer films 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.

[0079] 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 type and thin film type. The structures and preparation methods of these batteries are well known in the art and will not be elaborated here.

[0080] Modes for Carrying Out the Invention

[0081] 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.

[0082] Example 1 - Single-Particle Positive Electrode Active Material

[0083] (1) Preparation of Precursor

[0084] The precursor was prepared by a general co-precipitation method.

[0085] 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 form a metal salt aqueous solution.

[0086] After preparing the co-precipitation reactor, it was purged with N2 to prevent oxidation of metal ions during the co-precipitation reaction, and the reactor temperature was maintained at 50°C.

[0087] 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 positive electrode active material precursor.

[0088] The composition of the prepared precursor was (Ni 0.955 Co 0.02 Mn 0.02 Al 0.5 )(OH)2, and the average particle size (D50) was about 4 μm.

[0089] (2) Preparation of Positive Electrode Active Material

[0090] LiOH·H2O (Sanda Chemical, battery grade), ZrO2 (Aldrich, 4N), Al(OH)3 (Aldrich, 4N) were uniformly mixed with the precursor prepared in (1) above, and then the mixture was calcined in a box-type calciner with oxygen being introduced at a rate of 1000 mL / minute.

[0091] For the calcination, the first calcination process was carried out at 850 °C for 3.75 hours, and then the second calcination process was carried out at 790 °C for 9.75 hours.

[0092] At this time, the molar ratio of lithium (Li) to the total metal (Me) other than lithium (Li / Me) was designed to be 1.01, and the doping amount was based on LiNi without doping metal elements 0.955 Co 0.02 Mn 0.02 Al 0.5 O2, expressed as M = Ni 0.955 Co 0.02 Mn 0.02 Al 0.5 , and the addition amount of the doping raw material was adjusted so that the sum of M and the doping amount was 1 mol. That is, it had the structure of Li(M) 1-x (D) x O2 (M = NCMA, D = doping material).

[0093] Next, the calcined product was crushed into a powder form using a rotor mill or a jet mill (Rotor mill or Jet-mill), etc., and then Al and Co raw materials were mixed and heat-treated at 680 to 700 degrees for 5 hours to prepare a cathode active material with a coating formed.

[0094] The cathode active material of Example 1 prepared in this way had a single-particle structure defined in the present invention, and the overall composition was Li(M) 0.995 Zr 0.001 Al 0.004 O2.

[0095] Example 2

[0096] A cathode active material was prepared by the same method as in Example 1, except that the first calcination process was carried out at 890 degrees and the second calcination process was carried out at 790 degrees.

[0097] Example 3

[0098] A cathode active material was prepared by the same method as in Example 1, except that the first calcination process was carried out at 850 degrees and the second calcination process was carried out at 820 degrees.

[0099] Comparative Example 1

[0100] The mixture obtained by uniformly mixing the precursor, lithium raw material, and doping raw material prepared in Example 1 was calcined in a roller hearth kiln (hereinafter referred to as RHK) with oxygen being introduced at a rate of 1000 mL / min. The calcination conditions were to hold at 765 °C for 12 hours with a heating rate of 3 °C / min.

[0101] As the lithium raw material used, LiOH·H2O (Sanda Chemical, battery grade) was used, and as the doping raw materials, ZrO2 (Aldrich, 3N) and Al(OH)3 (Aldrich, 3N) were used.

[0102] At this time, the molar ratio of lithium (Li) to the total metal (Me) other than lithium (Li / Me) was designed to be 1.03, and the doping amount was based on LiNi without doped metal elements 0.92 Co 0.04 Mn 0.04 O2 and expressed as M = Ni 0.92 Co 0.04 Mn 0.04 , and the addition amount of the doping raw material was adjusted so that the sum of M and the doping amount was 1 mol. That is, it had a Li(M) 1-x (D) x O2 (M = NCMA, D = doping material) structure. For the positive electrode active material of Comparative Example 1 prepared in this way, the overall composition was Li(M) 0.993 Zr 0.002 Al 0.005 O2, which was small particles with a polycrystalline structure.

[0103] Comparative Example 2

[0104] Except that the precursor was prepared by controlling the average particle size (D50) to be about 15 μm through the precursor preparation process, the precursor was prepared by the same method as in Example 1.

[0105] Next, a large-particle positive electrode active material with a polycrystalline structure was prepared from the precursor by the same method as in Comparative Example 1.

[0106] Experimental Example 1 - Detection of Life Characteristics

[0107] (1) Fabrication of coin-type half-cell

[0108] After fabricating a CR2032 coin battery using the positive electrode active material prepared as above, electrochemical evaluation was carried out.

[0109] Specifically, a positive electrode active material, a conductive material (Denka black), and a polyvinylidene fluoride binder (trade name: KF1120) are mixed at a weight ratio of 96.25:1.65:2.1, and the mixture is added to an N-methyl-2-pyrrolidone solvent to make the solid content reach about 30% by weight to prepare a positive electrode active material slurry.

[0110] The slurry is coated on an aluminum foil (Al foil, thickness: 10 μm) serving as a positive electrode current collector using a doctor blade, dried and then pressed to make a positive electrode. The loading amount of the positive electrode is about 15 mg / cm 2 , and the pressing density is about 3.5 g / cm 3 or more.

[0111] Using the positive electrode, a lithium metal negative electrode (thickness 300 μm, MTI), an electrolyte, and a polypropylene separator, a 2032 coin-type half cell is prepared by a conventional method. For the electrolyte, 1 M LiPF6 is 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, and then 3% by weight of vinylene carbonate (VC) is added.

[0112] (2) Evaluate charge and discharge characteristics

[0113] After aging the coin-type half cell prepared in (1) at room temperature (25 °C) for 10 hours, a charge and discharge test is carried out.

[0114] For capacity evaluation, 200 mAh / g is used as a reference capacity, and the charge and discharge conditions are constant current (CC) / constant voltage (CV) from 2.5 to 4.25 V, 1 / 20C cut-off. For the initial capacity, it is detected by 0.1C charge / 0.1C discharge.

[0115] (3) Evaluate life characteristics

[0116] For the life characteristics of the positive electrode active materials prepared according to Examples 1 to 3, Comparative Examples 1 and 2, they are detected 30 times and 50 times respectively under the conditions of 0.5C charge / 1C discharge at high temperature (45 °C).

[0117] (4) Detect resistance characteristics

[0118] For the initial room temperature resistance (DC internal resistance: DC-IR), at a constant current-constant voltage of 2.5 V to 4.25 V at 25°C and a cut-off condition of 1 / 20C, the battery is subjected to one 0.1C charge and 0.1C discharge, and the voltage value after applying a discharge current for 60 seconds when charged to 100% at 4.25 V is detected, and then the calculation is carried out.

[0119] For the resistance increase rate, the initial resistance (room temperature initial resistance) is detected at a high temperature (45°C), and the resistances after 30 and 50 cycles are respectively detected by the same method as the initial resistance detection method, and the increase rate is converted into a percentage (%).

[0120] [Table 1]

[0121]

[0122] Referring to Table 1, it can be confirmed that although the positive electrode active materials prepared according to Examples 1 to 3 have a single particle structure, they have excellent life characteristics compared with the positive electrode active materials of Comparative Examples 1 and 2 having a polycrystalline structure. In addition, when compared with the positive electrode active materials of Comparative Examples 1 and 2, it can be confirmed that the resistance increase rate of the positive electrode active materials prepared according to Examples 1 to 3 is significantly reduced.

[0123] Experimental Example 2 - Grain Size Detection

[0124] For the positive electrode active materials prepared according to Examples 1 to 3 and Comparative Examples 1 to 2, the grain size is detected using the JEOL JEM-2100F equipment equipped with an ASTAR device from NanoMEGAS, and the results are shown in Table 2 below.

[0125] [Table 2]

[0126]

[0127]

[0128] Experimental Example 3 - Structure Analysis of Positive Electrode Active Material

[0129] The detection results of the TEM images and ASTAR images of the positive electrode active materials prepared according to Examples 1 to 1 and Comparative Example 1 are respectively shown in Figures 1 to 4 as follows.

[0130] Specifically, Figure 1 are the TEM image and ASTAR image of the positive electrode active material prepared according to Example 1, Figure 2 are the TEM image and ASTAR image of the positive electrode active material prepared according to Example 2,Figure 3 are the TEM image and ASTAR image of the positive electrode active material prepared according to Example 3. In addition, Figure 4 are the TEM image and ASTAR image of the positive electrode active material prepared according to Comparative Example 1.

[0131] On the TEM image, the distinction of the crystal grains present in the particles is not clear, and it is difficult to determine the size and number of the crystal grains. However, if the ASTAR technique is used, such a distinction will become clear, so the comparison of the size and number of the crystal grains becomes very easy. Refer to Figures 1 to 3 From the ASTAR image of [], it can be confirmed that the number of crystal grains measured in one particle is very small. In contrast, referring to Figure 4 From the ASTAR image of [], it can be seen that there are more crystal grains compared with each of the embodiments.

[0132]

Table 3

[0133] Classification The number of crystal grains measured in one particle Example 1 1.75 Example 2 2.33 Example 3 6.25 Comparative Example 1 39 or more

[0134] 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, wherein, the positive electrode active material has a single particle structure with a D50 of 2 to 7 μm, the number of crystal grains measured in one particle during ASTAR analysis is 20 or less.

2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, the number of crystal grains measured in one particle during ASTAR analysis is 5 to 20.

3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, the crystal grain size of the positive electrode active material is 500 nm to 5 μm.

4. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, the positive electrode active material comprises: metal oxide particles containing nickel, cobalt and manganese; and two or more doping elements doped into the metal oxide particles.

5. The positive electrode active material for a lithium secondary battery according to claim 4, wherein, the doping elements comprise at least two of Zr, Al, Nb, B, Ti, Ta, V, W and Mo.

6. The positive electrode active material for a lithium secondary battery according to claim 5, wherein, the doping amount of Zr is 0.0005 mol to 0.005 mol relative to 1 mol of the total amount of nickel, cobalt, manganese and doping elements.

7. The positive electrode active material for a lithium secondary battery according to claim 5, wherein, the doping amount of Al is 0.001 mol to 0.008 mol relative to 1 mol of the total amount of nickel, cobalt, manganese and doping elements.

8. A method for preparing a positive electrode active material, comprising: a step of preparing an aqueous metal salt solution containing a nickel raw material substance, a cobalt raw material substance, a manganese raw material substance and water; a step of supplying the aqueous metal salt solution to a co-precipitation reactor to obtain metal hydroxide; and a step of mixing the metal hydroxide particles, a lithium raw material substance and a doping raw material substance and then performing calcination to obtain a lithium metal oxide, the calcination comprising: a step of performing a first calcination process; and a step of continuously performing a second calcination process in a temperature range 50 °C to 150 °C lower than the first calcination process.

9. The method for preparing a positive electrode active material according to claim 8, wherein, the first calcination process is performed in the range of 800 °C to 890 °C for 3 to 5 hours.

10. The method for preparing a positive electrode active material according to claim 8, wherein, the second calcination process is performed in the range of 750 °C to 800 °C for 9 to 20 hours.

11. The method for preparing a positive electrode active material according to claim 8, wherein, the first calcination process and the second calcination process are performed as a continuous process maintaining a temperature difference of 50 °C to 80 °C.

12. The method for preparing a positive electrode active material according to claim 8, further comprising a coating process of forming a coating on the surface of the positive electrode active material after the second calcination process.

13. The method for preparing a positive electrode active material according to claim 12, wherein, the coating process is performed to include 1 mol to 2 mol of coating elements based on the entire positive electrode active material.

14. The method for preparing a positive electrode active material according to claim 12, wherein, The coating elements included in the coating include Co and Al.

15. A lithium secondary battery comprising: a positive electrode including the positive electrode active material according to any one of claims 1 to 7.

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