Positive electrode active material and lithium secondary battery containing the same

Abstract

The present invention relates to a positive electrode active material and a lithium secondary battery including the same; more specifically, the present invention relates to a positive electrode active material that includes a lithium composite oxide containing at least nickel and cobalt, and that has a concentration gradient of a predetermined thickness in which the cobalt concentration decreases from the surface region toward the center region of the lithium composite oxide relative to the average radius of the lithium composite oxide, thereby enabling improvement in particle stability not only in the surface region of the lithium composite oxide but also in the center region; a positive electrode including the positive electrode active material; and a lithium secondary battery using the positive electrode.

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material and a lithium secondary battery including the same; more specifically, the present invention relates to a positive electrode active material that includes a lithium composite oxide containing at least nickel and cobalt, and that has a concentration gradient of a predetermined thickness in which the cobalt concentration decreases from the surface region toward the center region of the lithium composite oxide relative to the average radius of the lithium composite oxide, thereby enabling improvement in particle stability not only in the surface region of the lithium composite oxide but also in the center region; a positive electrode including the positive electrode active material; and a lithium secondary battery using the positive electrode. [Background technology]

[0002] Batteries store electricity by using materials capable of electrochemical reaction at the positive and negative electrodes. A typical example of such batteries is a lithium secondary battery, which stores electrical energy by utilizing the difference in chemical potential when lithium ions are intercalated / deintercalated at the positive and negative electrodes.

[0003] The lithium secondary battery is manufactured by using a material capable of reversible intercalation / deintercalation of lithium ions as a positive electrode and a negative electrode active material, and filling an organic electrolyte solution or a polymer electrolyte solution between the positive electrode and the negative electrode.

[0004] Lithium composite oxides are used as the positive electrode active material for lithium secondary batteries, and composite oxides such as LiCoO2, LiMn2O4, LiNiO2, and LiMnO2 are being researched.

[0005] Among the positive electrode active materials, LiCoO2 is the most widely used due to its excellent life characteristics and charge / discharge efficiency. However, it has a drawback in that it is expensive due to the limited availability of cobalt as a raw material, limiting its price competitiveness.

[0006] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of excellent thermal safety and low cost, but suffer from low capacity and poor high-temperature characteristics. LiNiO2-based positive electrode active materials exhibit high discharge capacity, but are difficult to synthesize due to the problem of cation mixing between Li and transition metals, resulting in significant problems with rate characteristics.

[0007] Furthermore, as the degree of cation mixing deepens, a large amount of Li by-products is generated, and most of these Li by-products consist of compounds of LiOH and Li2CO3, which can cause gelling during the manufacture of the positive electrode paste and gas generation during charge and discharge after electrode manufacture. The remaining Li2CO3 not only increases the swelling phenomenon of the cell, reducing cycle life, but also causes the battery to swell.

[0008] The lithium composite oxide contained in the positive electrode active material undergoes volume changes due to the intercalation / deintercalation of lithium ions into the lithium composite oxide during charge / discharge. Typically, lithium composite oxides are in the form of secondary particles formed by the aggregation of multiple primary particles. However, if the primary particles undergo a sudden volume change during charge / discharge or if stress accumulates due to repeated charge / discharge, cracks may occur within the secondary particles, or the crystalline structure may collapse or change (phase transition) in the crystalline structure.

[0009] These problems ultimately lead to a decrease in the stability and reliability of the positive electrode active material, and therefore, various studies are being conducted to alleviate the volume change of the lithium composite oxide during charge and discharge, minimize the stress caused by the volume change, and prevent damage to the particles. Summary of the Invention [Problem to be solved by the invention]

[0010] In the lithium secondary battery market, the growth of lithium secondary batteries for electric vehicles is driving the market, and the demand for positive electrode active materials used in lithium secondary batteries is also continuously changing.

[0011] For example, lithium secondary batteries using LFP have traditionally been used primarily for reasons such as ensuring safety, but recently there has been a trend toward the use of nickel-based lithium composite oxides, which have a higher energy capacity per weight than LFP.

[0012] Therefore, the positive electrode active material used in higher specification lithium secondary batteries must adequately meet all the expected stability and reliability even under more severe operating conditions.

[0013] In the past, in order to alleviate volumetric changes in lithium composite oxides during charge and discharge or to minimize stress caused by volumetric changes and prevent particle damage, certain voids have been formed between primary particles to disperse stress caused by volumetric changes in primary particles. However, such lithium composite oxides have a limitation in that they have a low energy density per unit volume.

[0014] Furthermore, since degradation of lithium composite oxides occurs primarily at the particle surface or interparticle interface, coating the particle surface and / or interface can alleviate volumetric changes in the lithium composite oxide during charge and discharge and reduce stress caused by volumetric changes. Generally, lithium composite oxides are in the form of secondary particles formed by the aggregation of numerous primary particles. In this case, sufficient coating of the surfaces and / or interfaces of the primary particles requires an increased amount of raw materials for coating. This increases the amount of coating elements in the final product, which can lead to a decrease in the nickel content and, therefore, a decrease in the charge and discharge capacity of the positive electrode active material.

[0015] Therefore, the present invention aims to provide a cathode active material with a low degree of particle aggregation so that sufficient particle protection effect can be exerted not only in the surface region but also in the center region of the lithium composite oxide even when a relatively small amount of coating raw material is used.

[0016] Another object of the present invention is to provide a positive electrode active material that can improve particle stability not only in the surface region of the lithium composite oxide but also in the center region by forming a concentration gradient of a predetermined thickness in which the cobalt concentration decreases from the surface region toward the center region of the lithium composite oxide relative to the average radius of the lithium composite oxide.

[0017] Another object of the present invention is to provide a positive electrode comprising the positive electrode active material defined herein.

[0018] It is yet another object of the present invention to provide a lithium secondary battery using the positive electrode defined herein. [Means for solving the problem]

[0019] According to one aspect of the present invention, there is provided a cathode active material comprising a lithium composite oxide containing at least nickel and cobalt, wherein a concentration gradient is formed at a predetermined thickness in which the cobalt concentration decreases from the surface portion toward the center of the lithium composite oxide relative to the average radius of the lithium composite oxide, with the cross section of the lithium composite oxide as the reference. This improves particle stability not only in the surface portion of the lithium composite oxide but also in the center portion.

[0020] Specifically, the lithium composite oxide of the positive electrode active material may be divided into a first section in which a concentration gradient is formed in which the cobalt concentration decreases from the surface portion to the center portion of the lithium composite oxide based on the cross section of the lithium composite oxide, and a second section inside the first section in which the cobalt concentration is maintained within a predetermined range.

[0021] In this case, when the average radius measured from the cross section of the lithium composite oxide is referred to as d, the ratio d1 / d of the average radius d to the thickness d1 of the first section is within a range of 0.08 to 0.27, thereby minimizing the amount of coating raw material used for surface and / or interface coating of the lithium composite oxide and providing sufficient protection effect for the surface and core particles of the lithium composite oxide.

[0022] In one embodiment, after the lithium composite oxide is subjected to cross-section processing, a cross-section of the lithium composite oxide is photographed using a scanning electron microscope (SEM). In the cross-sectional SEM image obtained, the density of crystal grain boundaries calculated by the following formula 1 for crystallites arranged on a virtual line crossing the center of the lithium composite oxide in a minor axis direction may be 0.50 or less.

[0023] [Formula 1] Grain boundary density = (number of boundary surfaces between crystallites arranged on the virtual line / number of crystallites arranged on the virtual line)

[0024] In this case, the lithium composite oxide may be a non-aggregated single particle consisting of a single crystallite, and in this case, the density of the crystal grain boundaries calculated from the lithium composite oxide according to Equation 1 is 0.

[0025] When the lithium composite oxide is a non-aggregated single particle consisting of a single crystallite, the cross section of the single particle may be divided into a first section having a concentration gradient in which the cobalt concentration decreases from the surface portion toward the center portion of the single particle, and a second section inside the first section in which the cobalt concentration is maintained within a predetermined range.

[0026] The positive electrode active material may also be an aggregate of a plurality of lithium composite oxides, i.e., the positive electrode active material may be provided as an aggregate of a plurality of lithium composite oxides having the same or different grain boundary densities calculated by Equation 1.

[0027] In this case, it is preferable that, in a cross-sectional SEM image obtained by photographing a cross section of the lithium composite oxide using a scanning electron microscope (SEM) after cross-section processing of the lithium composite oxide in the positive electrode active material, the proportion of lithium composite oxide having a grain boundary density of 0.50 or less, calculated by the above formula 1, relative to crystallites arranged on a virtual line crossing the center of the lithium composite oxide in the minor axis direction, is 30% or more.

[0028] In another embodiment, a coating layer may be provided to cover at least a portion of the surface of the lithium composite oxide in order to enhance particle stability on the surface of the lithium composite oxide.

[0029] According to another aspect of the present invention, there is provided a positive electrode comprising the positive electrode active material defined herein.

[0030] According to yet another aspect of the present invention, there is provided a lithium secondary battery using the positive electrode defined herein. [Effects of the Invention]

[0031] As described above, the lithium composite oxide contained in the positive electrode active material inevitably undergoes volumetric changes due to the intercalation / deintercalation of lithium ions into the lithium composite oxide during charge / discharge. While there are various methods for mitigating the volumetric change of the lithium composite oxide during charge / discharge or minimizing the stress caused by the volumetric change to prevent particle damage, it is difficult to say that the degradation problem of lithium secondary batteries caused by damage to the lithium composite oxide contained in the positive electrode active material has been fully resolved.

[0032] However, according to the present invention, a lithium composite oxide containing at least nickel and cobalt is included, and a concentration gradient in which the cobalt concentration decreases from the surface portion toward the center portion of the lithium composite oxide relative to the average radius of the lithium composite oxide is formed at a predetermined thickness, thereby making it possible to improve the particle stability not only in the surface portion of the lithium composite oxide but also in the center portion.

[0033] Therefore, when the positive electrode active material according to the present invention is used, it is possible to delay the performance degradation of the lithium secondary battery caused by the positive electrode active material. DETAILED DESCRIPTION OF THE INVENTION

[0034] For convenience, certain terms are defined herein to make the present invention more readily understandable. Unless otherwise defined herein, scientific and technical terms used herein shall have the meanings that are commonly understood by those of ordinary skill in the art. Furthermore, unless otherwise indicated by context, singular terms shall be understood to include their plural forms, and plural terms shall be understood to include their singular forms.

[0035] Hereinafter, the positive electrode active material according to the present invention and the lithium secondary battery including the positive electrode active material will be described in more detail.

[0036] positive electrode active material According to one aspect of the present invention, there is provided a positive electrode active material comprising a lithium composite oxide containing at least nickel and cobalt, wherein the lithium composite oxide is a composite metal oxide containing lithium in addition to nickel and cobalt and having a layered crystal structure capable of intercalating and deintercalating lithium ions.

[0037] The cobalt in the lithium composite oxide may have a concentration gradient that decreases from the surface to the center of the lithium composite oxide. The concentration of the transition metal in the lithium composite oxide may be measured by various known methods. For example, after cross-section processing of the lithium composite oxide, the change in the concentration of the target transition metal may be measured using EDS mapping in a line scanning mode. That is, the change in the concentration of the target transition metal may be observed from the surface to the center of the lithium composite oxide.

[0038] In addition, the acceleration voltage V of the electron beam irradiated onto the surface of the lithium composite oxide acc There is an Energy Profiling-Energy Dispersive X-ray Spectroscopy (EP-EDS) method in which the concentration of a target transition metal is measured at a specific depth penetrated by an electron beam at each accelerating voltage by changing the accelerating voltage.

[0039] In this case, the cobalt concentration gradient may be a concentration gradient in which the cobalt concentration decreases continuously or discontinuously from the surface portion to the center portion of the lithium composite oxide.

[0040] In other words, when the concentration of cobalt decreases from the start point to the end point within any interval in which the concentration of cobalt is measured, the cobalt can be referred to as having a concentration gradient that decreases from the start point to the end point within the interval.

[0041] A region in the cross section of the lithium composite oxide where a concentration gradient is formed in which the cobalt concentration decreases from the surface portion toward the center of the lithium composite oxide can be defined as a first section. In this case, the surface portion of the lithium composite oxide, which is the starting point of the first section, corresponds to the outermost periphery of the lithium composite oxide. In addition, the end point of the first section is the same as the starting point of the second section, which will be described later.

[0042] Within the first section, there exists a region in which the cobalt concentration in the lithium composite oxide is maintained within a predetermined range, and this region can be defined as a second section.

[0043] The second section can be defined as a region where the change in cobalt concentration within the second section is insignificant when compared with the slope of the change in cobalt concentration between the start point and the end point of the first section, and exhibits a slope smaller than the slope of the change in cobalt concentration between the start point and the end point of the first section.

[0044] In addition, a region in which the cobalt concentration change rate in the lithium composite oxide is 10 mol% or less may be defined as a second region. In this case, the cobalt concentration change rate may be a change rate calculated by comparing the cobalt concentration at the start point and the end point of an arbitrary region, or may refer to the difference between the average cobalt concentration in the second region and the cobalt concentration at an arbitrary point in the second region.

[0045] In other words, the lithium composite oxide included in the cathode active material according to the present invention is divided into a first section in which a concentration gradient is formed in which the cobalt concentration decreases from the surface portion to the center portion of the lithium composite oxide based on the cross section of the lithium composite oxide, and a second section inside the first section in which the cobalt concentration is maintained within a predetermined range.

[0046] In this case, when the average radius measured from the cross section of the lithium composite oxide is referred to as d, it is preferable that the ratio d1 / d of the average radius d to the thickness d1 of the first section is 0.08 to 0.27, and the ratio d2 / d of the average radius d to the thickness d2 of the second section is 0.73 to 0.92.

[0047] If the ratio d1 / d of the average radius d to the thickness d1 of the first section is less than 0.08, the cobalt in the lithium composite oxide is located too close to the outermost periphery of the lithium composite oxide, making it difficult to sufficiently prevent particle damage such as crack generation, collapse of the crystalline structure, or change in the crystalline structure (phase transition) inside the lithium composite oxide.

[0048] In addition, when the ratio d1 / d of the average radius d to the thickness d1 of the first section is greater than 0.27, the cobalt content in the core of the lithium composite oxide becomes excessively high, which reduces the nickel content and reduces the charge / discharge capacity of a lithium secondary battery using the lithium composite oxide.

[0049] It is preferable that the average molar ratio of Co / Ni in the first section is 0.25 to 0.39, and the ratio of the average Co concentration c1 in the first section to the average Co concentration c measured by ICP analysis of the lithium composite oxide is 1.70 to 2.60.

[0050] If the molar ratio of Co / Ni in the first interval is less than 0.25 or the ratio of the average Co concentration c1 in the first interval to the average Co concentration c measured based on ICP analysis of the lithium composite oxide is less than 1.70, this means that not only is there insufficient cobalt present in the first interval, but also that the cobalt content in the second interval, which has a lower cobalt content than the first interval, is excessively low.

[0051] In this case, it is difficult to sufficiently prevent damage to the particles present on the surface and center of the lithium composite oxide (for example, collapse of the crystal structure, change in the crystal structure (phase transition)).

[0052] In other words, when the lithium composite oxide is a secondary particle formed by agglomeration of a plurality of primary particles, the protective effect on the surface portion of the secondary particle (or the surface of the primary particle located on the surface portion of the secondary particle and exposed to the outside air) and the surface of the primary particle located in the center of the secondary particle is insufficient, which may lead to damage to the primary particles located on the surface portion and in the center of the secondary particle (for example, collapse of the crystal structure, change in the crystal structure (phase transition)).

[0053] On the other hand, when the average molar ratio of Co / Ni in the first section is greater than 0.39 or the ratio of the average Co concentration c1 in the first section to the average Co concentration c measured based on ICP analysis of the lithium composite oxide is greater than 2.60, this means that the cobalt content in the second section is insufficient or that the cobalt is present in an excessive state in the first section due to excessive cobalt concentration in the first section.

[0054] In this case, excessive concentration of cobalt in the first section may result in a reverse effect of lowering the charge / discharge capacity of a lithium secondary battery using the lithium composite oxide, or an insufficient cobalt content in the second section may make it difficult to sufficiently prevent particle damage, such as collapse of the crystalline structure or change in the crystalline structure (phase transition), in the central portion of the lithium composite oxide (the region corresponding to the second section).

[0055] In addition, the average molar ratio of Co / Ni in the second section may vary depending on the composition of the target lithium composite oxide, but is preferably less than 0.090 to improve the charge / discharge capacity of a lithium secondary battery using the lithium composite oxide. If the average molar ratio of Co / Ni in the second section is 0.089, the average molar ratio of Co / Ni at least at the end of the first section is 0.089 or more.

[0056] As the molar ratio of Co / Ni in the second range increases, the charge / discharge capacity of a lithium secondary battery using the lithium composite oxide may decrease.

[0057] In addition, a larger Co / Ni molar ratio in the second region may mean that the cobalt content in the first region is relatively insufficient, assuming that the same amount of cobalt-containing raw material is used to prepare the lithium composite oxide, resulting in insufficient particle protection effect on the surface of the lithium composite oxide.

[0058] In one embodiment, the lithium composite oxide may be represented by the following Chemical Formula 1:

[0059] [Chemical formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2 z O2

[0060] (where, M1 is at least one selected from Mn and Al; M2 is at least one selected from P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu; M1 and M2 are different from each other, 0.5≦w≦1.5, 0≦x≦0.50, 0≦y≦0.20, 0≦z≦0.20)

[0061] In the lithium composite oxide represented by Chemical Formula 1, x corresponding to the concentration of cobalt in the first interval and x corresponding to the concentration of cobalt in the second interval have different values. In addition, since the concentration of cobalt is different between the first interval and the second interval, the concentration of at least one metal element selected from nickel, M1, and M2 may also be different.

[0062] The lithium composite oxide can be expressed as an average composition measured by ICP analysis, even if the concentration of at least one metal element including cobalt differs between the first and second sections.

[0063] The lithium composite oxide may be a high-Ni type lithium composite oxide in which the concentrations (mol %) of Ni, Co, M1, and M2 in Chemical Formula 1 satisfy the following Relational Formula 1:

[0064] [Equation 1] Ni / (Ni+Co+M1+M2)≧80.0

[0065] The lithium composite oxide may be a high-Ni / low-Co type lithium composite oxide in which the concentrations (mol %) of Ni, Co, M1, and M2 in Chemical Formula 1 satisfy Relational Formula 1, and the Co content is 10 mol % or less, preferably 5 mol % or less.

[0066] That is, in the lithium composite oxide, the concentrations (mol %) of Ni, Co, M1, and M2 in Chemical Formula 1 can satisfy the following Relational Formula 2.

[0067] [Equation 2] Co / (Ni+Co+M1+M2)≦5.0

[0068] It is generally known that in lithium composite oxides containing at least nickel and cobalt, an increase in the Ni content can lead to structural instability of the lithium composite oxide due to Li / Ni cation mixing. Also, it is known that a decrease in the cobalt content in lithium composite oxides containing at least nickel and cobalt increases the initial overpotential (resistance), which inevitably leads to a decrease in rate characteristics.

[0069] However, the lithium composite oxide included in the cathode active material according to an embodiment of the present invention has a concentration gradient of a predetermined thickness in which the cobalt concentration decreases from the surface to the center of the lithium composite oxide relative to the average radius of the lithium composite oxide, thereby mitigating and / or preventing the structural instability and deterioration of rate characteristics of high-Ni type or high-Ni / low-Co type lithium composite oxide.

[0070] The lithium composite oxide contained in the positive electrode active material defined herein may be a secondary particle including at least one primary particle, which may be expressed as a crystallite.

[0071] Here, "secondary particles containing at least one primary particle" should be interpreted as including both "particles formed by agglomeration of multiple primary particles" and "non-agglomerated single particles consisting of a single crystallite."

[0072] The primary particles and the secondary particles can each independently have a rod-like, ellipsoidal, and / or irregular shape.

[0073] When the average major axis length is used as an index showing the size of the primary particles and the secondary particles, the average major axis length of the primary particles constituting the lithium composite oxide may be 0.1 μm to 5 μm, and the average major axis length of the secondary particles may be 1 μm to 30 μm. The average major axis length of the secondary particles may vary depending on the number of the primary particles constituting the secondary particles, and the positive electrode active material may contain particles having various average major axis lengths.

[0074] When the lithium composite oxide is a "non-aggregated single particle consisting of a single crystallite" or a "particle formed by aggregating a relatively small number of primary particles," the size (average particle size) of the primary particles contained in the "non-aggregated single particle consisting of a single crystallite" or the "particle formed by aggregating a relatively small number of primary particles" may be larger than the size (average particle size) of the primary particles contained in the "secondary particle formed by aggregating tens to hundreds or more primary particles."

[0075] Lithium composite oxides that are "non-aggregated single particles consisting of a single crystallite" or "particles formed by aggregating a relatively small number of primary particles" generally require stronger heat treatment conditions (high heat treatment temperature / long heat treatment time) than those that produce "secondary particles formed by aggregating tens to hundreds or more primary particles." It is known that when heat treatment is performed at a relatively high temperature (e.g., 800°C or higher) for a long period of time, particle growth (crystal growth) is promoted, increasing the size of the single particles while also producing a cathode active material with a low degree of particle agglomeration.

[0076] For example, when the lithium composite oxide is "a non-aggregated single particle consisting of a single crystallite" or "a particle formed by aggregating a relatively small number of primary particles," the primary particles may have an average major axis length in the range of 0.5 μm to 20 μm. On the other hand, when the lithium composite oxide is "a particle formed by aggregating a plurality of (tens to hundreds or more) primary particles," the primary particles may have an average major axis length in the range of 0.1 μm to 5 μm.

[0077] As described above, the cathode active material according to an embodiment of the present invention has a low degree of particle (crystallite) aggregation so that sufficient particle protection effect can be achieved not only at the surface but also at the center of the lithium composite oxide even when a relatively small amount of coating raw material is used.

[0078] The degree of aggregation of the lithium composite oxide can be measured by the density of crystal grain boundaries as defined herein.

[0079] Specifically, the density of the crystal grain boundaries can be calculated by substituting the number of crystallites arranged on a virtual line crossing the center of the lithium composite oxide in the minor axis direction and the number of boundary surfaces between crystallites into the following formula 1 in a cross-sectional SEM image obtained by photographing a cross section of the lithium composite oxide using a scanning electron microscope (SEM) after cross-section processing of the lithium composite oxide.

[0080] [Formula 1] Grain boundary density = (number of boundary surfaces between crystallites arranged on the virtual line / number of crystallites arranged on the virtual line)

[0081] In this case, the lithium composite oxide may have a grain boundary density calculated by the formula 1 of 0.50 or less.

[0082] For example, when the lithium composite oxide is a non-aggregated single particle consisting of a single crystallite, the number of crystallites arranged on a virtual line crossing the center of the lithium composite oxide in a cross-sectional SEM image of the lithium composite oxide is one, and therefore, no boundary surface exists between the crystallites, and the density of the crystal grain boundaries calculated by the above formula 1 is zero.

[0083] Furthermore, when the lithium composite oxide is a particle formed by agglomeration of a relatively small number of primary particles, the number of crystallites (primary particles) arranged on a virtual line crossing the center of the lithium composite oxide in a cross-sectional SEM image of the lithium composite oxide is two, and as a result, there is one boundary surface between crystallites, and the density of the crystal grain boundaries calculated by the above formula 1 is 0.5.

[0084] Generally, lithium composite oxides are in the form of secondary particles formed by the aggregation of numerous primary particles. In this case, to sufficiently coat the surfaces and / or interfaces of the primary particles, the amount of raw material required for the coating also increases, which may increase the amount of coating elements in the final product and reduce the charge / discharge capacity or reversible efficiency of the positive electrode active material. Furthermore, assuming the same amount of coating raw material is used, as the number of primary particles in the secondary particles increases, the relative amount of coating raw material infiltrating into the secondary particles increases, which may reduce the amount of coating elements present on the surfaces of the secondary particles.

[0085] Therefore, the lithium composite oxide according to the present invention, which satisfies the above-mentioned definition of grain boundary density, has an advantage that it can exhibit sufficient particle protection effect not only at the surface portion but also at the center portion of the lithium composite oxide, even when a relatively small amount of coating raw material is used.

[0086] The positive electrode active material may be an aggregate of a plurality of lithium composite oxides, i.e., the positive electrode active material may be provided as an aggregate of a plurality of lithium composite oxides having the same or different grain boundary densities calculated by Equation 1.

[0087] In this case, it is preferable that the proportion of lithium composite oxide having a grain boundary density of 0.50 or less, calculated by Equation 1, relative to crystallites arranged on a virtual line crossing the center of the lithium composite oxide in the minor axis direction, is 30% or more in a cross-sectional SEM image obtained by subjecting the lithium composite oxide in the positive electrode active material to cross-section processing and then photographing a cross section of the lithium composite oxide using a scanning electron microscope (SEM).

[0088] In the positive electrode active material, the proportion of lithium composite oxides having a grain boundary density of 0.50 or less as calculated by Equation 1 can be calculated by calculating the grain boundary densities of each of the lithium composite oxides observed in the cross-sectional SEM image, and then calculating the proportion of the number of lithium composite oxides having a grain boundary density of 0.50 or less to the total number of lithium composite oxides observed in the cross-sectional SEM image.

[0089] When the proportion of the lithium composite oxide having a grain boundary density of 0.50 or less, calculated by Equation 1, in the positive electrode active material is less than 30%, it means that an excessive amount of polycrystalline lithium composite oxide is present in the positive electrode active material.

[0090] This ultimately means that an excessive amount of cobalt-containing raw material must be added to achieve the thickness of the first section and a predetermined cobalt concentration in the first section relative to the average radius of the lithium composite oxide defined herein. In this case, the average cobalt content in the positive electrode active material provided as an aggregate of multiple lithium composite oxides increases, which may result in a lower Ni content and a lower charge / discharge capacity of the positive electrode active material. Furthermore, the increased amount of cobalt-containing raw material used to produce the positive electrode active material increases the production cost of the positive electrode active material.

[0091] Furthermore, a coating layer may be formed to cover at least a portion of the surface of the lithium composite oxide. The coating layer contains at least one metal oxide represented by the following chemical formula 2. That is, the coating layer can be defined as a region on the surface of the lithium composite oxide where the oxide represented by the following chemical formula 2 is present.

[0092] [Chemical formula 2] Li a M3 b O c

[0093] (where, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd; (0≦a≦10, 0≦b≦8, 2≦c≦13)

[0094] The coating layer may have a structure in which different types of metal oxides are present simultaneously in one layer, or different types of metal oxides are present in separate layers.

[0095] The metal oxide represented by Chemical Formula 2 may be in a state of being physically and / or chemically bonded to the lithium composite oxide represented by Chemical Formula 1. In addition, the metal oxide may exist in a state of forming a solid solution with the primary particles represented by Chemical Formula 1.

[0096] When the lithium composite oxide is a non-aggregated single particle consisting of a single crystallite, the metal oxide may be present partially or entirely on the surface of the single particle.

[0097] On the other hand, when the lithium composite oxide is a secondary particle formed by agglomeration of a plurality of primary particles, the metal oxide may be present partially or entirely at the interface between the plurality of primary particles and on the surface of the secondary particles.

[0098] When the metal oxide is partially present on the surface of the particle, the coating layer may be present in the form of islands.

[0099] Lithium secondary battery According to yet another aspect of the present invention, a positive electrode may be provided, including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. Here, the positive electrode active material layer may include a positive electrode active material according to various embodiments of the present invention. Therefore, since the positive electrode active material layer is similar to those described above, detailed description thereof will be omitted for brevity, and only the remaining components not described above will be described below.

[0100] The positive electrode active material layer may be prepared by coating a positive electrode slurry composition containing the positive electrode active material, a conductive material, and optionally a binder, on the positive electrode current collector.

[0101] In this case, the positive electrode active material may be included in an amount of 80 to 99 wt %, more specifically, 85 to 98.5 wt %, based on the total weight of the positive electrode active material layer, and excellent capacity characteristics can be exhibited when the amount is within this range, but the amount is not necessarily limited thereto.

[0102] The conductive material is used to impart conductivity to the electrode and can be any material that provides electronic conductivity without causing chemical changes in the resulting battery. Specific examples include graphite (e.g., natural graphite or artificial graphite); carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon-based materials (e.g., carbon fiber); metal powder or metal fiber (e.g., copper, nickel, aluminum, or silver); conductive whiskers (e.g., zinc oxide or potassium titanate); conductive metal oxides (e.g., titanium oxide); and conductive polymers (e.g., polyphenylene derivatives). These materials may be used alone or in combination. The conductive material may be present in an amount of 0.1 to 15 wt % based on the total weight of the positive electrode active material layer.

[0103] The binder serves to improve adhesion between positive electrode active material particles and between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder may be included in an amount of 0.1 to 15 wt % based on the total weight of the positive electrode active material layer.

[0104] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode active material. Specifically, the positive electrode may be manufactured by dissolving or dispersing the positive electrode active material and, optionally, a binder and a conductive material in a solvent to prepare a positive electrode slurry composition, coating the composition on a positive electrode current collector, and then drying and rolling the composition.

[0105] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of the solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and to provide a viscosity that allows excellent thickness uniformity when the slurry is applied to produce a positive electrode, taking into consideration the coating thickness of the slurry and the production yield.

[0106] In another embodiment, the positive electrode may be manufactured by casting the positive electrode slurry composition on a separate support, peeling it off from the support, and laminating the resulting film on a positive electrode current collector.

[0107] According to yet another aspect of the present invention, there may be provided an electrochemical device including the above-described positive electrode. The electrochemical device may be, for example, a battery or a capacitor, and more specifically, a lithium secondary battery.

[0108] Specifically, the lithium secondary battery may include a positive electrode, a negative electrode facing the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. Here, since the positive electrode is as described above, detailed description thereof will be omitted for convenience, and only the remaining components not described above will be described in detail below.

[0109] The lithium secondary battery may optionally further include a battery container that houses an electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0110] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0111] The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy may be used. The negative electrode current collector may typically have a thickness of 3 μm to 500 μm. As with the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to strengthen the binding force of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0112] The negative electrode active material layer may be prepared by coating a negative electrode slurry composition containing the negative electrode active material, a conductive material, and optionally a binder, on the negative electrode current collector.

[0113] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO. β Examples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide, or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. One or a mixture of two or more of these may be used. A thin film of metallic lithium may also be used as the negative electrode active material. Both low-crystalline carbon and high-crystalline carbon may also be used as the carbon material. Typical low-crystalline carbons include soft carbon and hard carbon, while typical high-crystalline carbons include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbons such as petroleum or coal tar pitch-derived cokes.

[0114] The negative electrode active material may be contained in an amount of 80 to 99 wt % based on the total weight of the negative electrode active material layer.

[0115] The binder, which aids in bonding between the conductive material, active material, and current collector, may typically be added in an amount of 0.1 to 10 wt% based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0116] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 10 wt % or less, preferably 5 wt % or less, based on the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of such a conductive material 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.

[0117] In one embodiment, the negative electrode active material layer may be prepared by coating a negative electrode slurry composition prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector and drying the coating. Alternatively, the negative electrode slurry composition may be cast on a separate support, peeled from the support, and then laminating the resulting film on the negative electrode current collector.

[0118] In another embodiment, the negative electrode active material layer may be fabricated by coating a negative electrode slurry composition prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector and drying the coating; or by casting the negative electrode slurry composition on a separate support, peeling it off from the support, and laminating the resulting film on the negative electrode current collector.

[0119] Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without particular limitations. It is particularly preferable that the separator has low resistance to ion migration and excellent electrolyte humidification ability. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can be used, and it can be selectively used in a single-layer or multi-layer structure.

[0120] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

[0121] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0122] The organic solvent may be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone, ether solvents such as dibutyl ether and tetrahydrofuran, ketone solvents such as cyclohexanone, aromatic hydrocarbon solvents such as benzene and fluorobenzene, dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of suitable solvents include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constants, which can enhance the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred. In this case, the cyclic carbonate and the chain carbonate may be mixed in a volume ratio of about 1:1 to about 1:9 to produce excellent electrolyte performance.

[0123] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without particular limitation. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3), LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The lithium salt is preferably used at a concentration in the range of 0.1 to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0124] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purposes of improving battery life characteristics, suppressing battery capacity reduction, improving battery discharge capacity, etc. In this case, the additives may be contained in an amount of 0.1 to 5 wt % based on the total weight of the electrolyte.

[0125] As described above, a lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and life characteristics, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0126] The external shape of the lithium secondary battery according to the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, etc. Furthermore, the lithium secondary battery can be used not only as a battery cell used as a power source for a small device, but also as a unit battery for a medium- to large-sized battery module including a plurality of battery cells.

[0127] According to yet another aspect of the present invention, there may be provided a battery module including the lithium secondary battery as a unit cell and / or a battery pack including the same.

[0128] The battery module or the battery pack can be used as a power source for one or more medium- to large-sized devices, such as a power tool, an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV), or a power storage system.

[0129] The present invention will be described in more detail below with reference to examples. However, these examples are for the purpose of illustrating the present invention and are not to be construed as limiting the scope of the present invention.

[0130] Production Example 1: Production of positive electrode active material Example 1 (a) NiCoAl(OH) hydroxide precursor (Ni:Co:Al = 91:8:1 (at%)) was synthesized using a known co-precipitation method using nickel sulfate, cobalt sulfate, and aluminum sulfate. The synthesized hydroxide precursor was oxidized by heating at a rate of 10°C per minute up to 450°C and then calcined at 450°C for 6 hours.

[0131] (b) The oxide precursor was mixed with LiOH (Li / (Ni+Co+Al) molar ratio=1.03) and NaOH (Na / (Ni+Co+Al) molar ratio=0.01), and then heated to 820°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calciner. A first heat treatment was performed at 820°C for 12 hours to obtain a lithium composite oxide.

[0132] (c) An aqueous solution containing 3.0 mol% cobalt sulfate based on the lithium composite oxide was prepared, and the lithium composite oxide, a complexing agent, and a pH adjuster were added to the cobalt sulfate aqueous solution to prepare a mixture, which was then stirred for 1 hour. The mixture was then dehydrated and dried at 120°C for 12 hours.

[0133] (d) The lithium composite oxide is heated to 700°C at a rate of 2°C per minute in a firing furnace while maintaining an O2 atmosphere, and then subjected to a second heat treatment at 700°C for 8 hours to obtain Li 1.0 Ni 0.876 Co 0.109 Al 0.011 Na 0.004 A lithium composite oxide having an O2 composition was obtained, and the composition of the lithium composite oxide was confirmed through ICP analysis.

[0134] Example 2 A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature in step (b) was 850°C.

[0135] Example 3 A positive electrode active material was prepared in the same manner as in Example 1, except that in step (b), the oxide precursor was mixed with LiOH (Li / (Ni+Co+Al) molar ratio=1.03), NaOH (Na / (Ni+Co+Al) molar ratio=0.01), and KCl (K / (Ni+Co+Al) molar ratio=0.01), and the mixture was heated to 900°C at a rate of 2°C per minute in a firing furnace while maintaining an O atmosphere, and subjected to a first heat treatment at 900°C for 12 hours to obtain a lithium composite oxide.

[0136] The composition of the lithium composite oxide obtained in Example 3 was confirmed by ICP analysis. 1.0 Ni 0.871 Co 0.108 Al 0.011 Na 0.003 K 0.007 It was O2.

[0137] Example 4 A positive electrode active material was prepared in the same manner as in Example 1, except that the second heat treatment in step (d) was performed for 6 hours.

[0138] Example 5 A positive electrode active material was prepared in the same manner as in Example 1, except that the second heat treatment in step (d) was performed for 10 hours.

[0139] Example 6 (a) NiCoAl(OH) hydroxide precursor (Ni:Co:Al = 91:8:1 (at%)) was synthesized using a known co-precipitation method using nickel sulfate, cobalt sulfate, and aluminum sulfate. The synthesized hydroxide precursor was oxidized by heating at a rate of 10°C per minute up to 450°C and then calcined at 450°C for 6 hours.

[0140] (b) The oxide precursor was mixed with LiOH (Li / (Ni+Co+Al) molar ratio=1.03) and NaOH (Na / (Ni+Co+Al) molar ratio=0.01), and then heated to 820°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calciner. A first heat treatment was performed at 820°C for 12 hours to obtain a lithium composite oxide.

[0141] (c) An aqueous solution containing 3.0 mol% cobalt sulfate and 1.0 mol% aluminum sulfate based on the lithium composite oxide was prepared, and the lithium composite oxide, complexing agent, and pH adjuster were added to the cobalt sulfate aqueous solution to prepare a mixture, which was then stirred for 1 hour. The mixture was then dehydrated and dried at 120°C for 12 hours.

[0142] (d) The lithium composite oxide is heated to 700°C at a rate of 2°C per minute in a firing furnace while maintaining an O2 atmosphere, and then subjected to a second heat treatment at 700°C for 8 hours to obtain Li 1.0 Ni 0.865 Co 0.108 Al 0.023 Na 0.004 A lithium composite oxide having an O2 composition was obtained, and the composition of the lithium composite oxide was confirmed through ICP analysis.

[0143] Comparative Example 1 (a) NiCoAl(OH) hydroxide precursor (Ni:Co:Al = 91:8:1 (at%)) was synthesized using a known co-precipitation method using nickel sulfate, cobalt sulfate, and aluminum sulfate. The synthesized hydroxide precursor was oxidized by heating at a rate of 10°C per minute up to 450°C and then calcined at 450°C for 6 hours.

[0144] (b) The oxide precursor was mixed with LiOH (Li / (Ni+Co+Al) molar ratio = 1.03), and then the mixture was heated to 720°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calciner, and subjected to a first heat treatment at 720°C for 12 hours to obtain a lithium composite oxide.

[0145] (c) An aqueous solution containing 3.0 mol% cobalt sulfate based on the lithium composite oxide was prepared, and the lithium composite oxide, a complexing agent, and a pH adjuster were added to the cobalt sulfate aqueous solution to prepare a mixture, which was then stirred for 1 hour. The mixture was then dehydrated and dried at 120°C for 12 hours.

[0146] (d) The lithium composite oxide is heated to 700°C at a rate of 2°C per minute in a firing furnace while maintaining an O2 atmosphere, and then subjected to a second heat treatment at 700°C for 8 hours to obtain Li 1.0 Ni 0.879 Co 0.109 Al 0.012 A lithium composite oxide having an O2 composition was obtained, and the composition of the lithium composite oxide was confirmed through ICP analysis.

[0147] Comparative Example 2 (a) NiCoAl(OH) hydroxide precursor (Ni:Co:Al = 91:8:1 (at%)) was synthesized using a known co-precipitation method using nickel sulfate, cobalt sulfate, and aluminum sulfate. The synthesized hydroxide precursor was oxidized by heating at a rate of 10°C per minute up to 450°C and then calcined at 450°C for 6 hours.

[0148] (b) The oxide precursor was mixed with LiOH (Li / (Ni+Co+Al) molar ratio=1.03) and NaOH (Na / (Ni+Co+Al) molar ratio=0.01), and then heated to 820°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calciner. A first heat treatment was performed at 820°C for 12 hours to obtain a lithium composite oxide.

[0149] (c) The lithium composite oxide was added to distilled water to prepare a mixture, which was then stirred for 1 hour, dehydrated, and dried at 120°C for 12 hours.

[0150] (d) The lithium composite oxide is heated to 700°C at a rate of 2°C per minute in a firing furnace while maintaining an O2 atmosphere, and then subjected to a second heat treatment at 700°C for 8 hours to obtain Li 1.0 Ni 0.905 Co 0.081 Al 0.011 Na 0.003 A lithium composite oxide having an O2 composition was obtained, and the composition of the lithium composite oxide was confirmed through ICP analysis.

[0151] Comparative Example 3 (a) NiCoAl(OH) hydroxide precursor (Ni:Co:Al = 91:8:1 (at%)) was synthesized using a known co-precipitation method using nickel sulfate, cobalt sulfate, and aluminum sulfate. The synthesized hydroxide precursor was oxidized by heating at a rate of 10°C per minute up to 450°C and then calcined at 450°C for 6 hours.

[0152] (b) The oxide precursor was mixed with LiOH (Li / (Ni+Co+Al) molar ratio = 1.03), and then the mixture was heated to 820°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calciner, and subjected to a first heat treatment at 820°C for 12 hours to obtain a lithium composite oxide.

[0153] (c) An aqueous solution containing 3.0 mol% cobalt sulfate based on the lithium composite oxide was prepared, and the lithium composite oxide, a complexing agent, and a pH adjuster were added to the cobalt sulfate aqueous solution to prepare a mixture, which was then stirred for 1 hour. The mixture was then dehydrated and dried at 120°C for 12 hours.

[0154] (d) The lithium composite oxide is heated to 700°C at a rate of 2°C per minute in a firing furnace while maintaining an O2 atmosphere, and then subjected to a second heat treatment at 700°C for 8 hours to obtain Li 1.0 Ni 0.880 Co 0.109 Al 0.011 A lithium composite oxide having the composition (O2) was obtained. The composition of the lithium composite oxide was confirmed by ICP analysis.

[0155] Comparative Example 4 (a) NiCoAl(OH) hydroxide precursor (Ni:Co:Al = 91:8:1 (at%)) was synthesized using a known co-precipitation method using nickel sulfate, cobalt sulfate, and aluminum sulfate. The synthesized hydroxide precursor was oxidized by heating at a rate of 10°C per minute up to 450°C and then calcined at 450°C for 6 hours.

[0156] (b) The oxide precursor was mixed with LiOH (Li / (Ni+Co+Al) molar ratio=1.03) and NaOH (Na / (Ni+Co+Al) molar ratio=0.01), and then heated to 820°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calciner. A first heat treatment was performed at 820°C for 12 hours to obtain a lithium composite oxide.

[0157] (c) An aqueous solution containing 3.0 mol% cobalt sulfate based on the lithium composite oxide was prepared, and the lithium composite oxide, a complexing agent, and a pH adjuster were added to the cobalt sulfate aqueous solution to prepare a mixture, which was then stirred for 1 hour. The mixture was then dehydrated and dried at 120°C for 12 hours.

[0158] (d) The lithium composite oxide is heated to 700°C at a rate of 2°C per minute in a firing furnace while maintaining an O2 atmosphere, and then subjected to a second heat treatment at 700°C for 18 hours to obtain Li 1.0 Ni 0.874 Co 0.111 Al 0.011 Na 0.004 A lithium composite oxide having an O2 composition was obtained, and the composition of the lithium composite oxide was confirmed through ICP analysis.

[0159] Manufacturing Example 2: Manufacturing of lithium secondary batteries A positive electrode slurry was prepared by dispersing 92 wt% of the positive electrode active material prepared in Preparation Example 1, 4 wt% of artificial graphite, and 4 wt% of a PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The positive electrode slurry was uniformly coated on a 15 μm-thick aluminum foil and dried under vacuum at 135° C. to prepare a positive electrode for a lithium secondary battery.

[0160] A coin battery was fabricated using a lithium foil as a counter electrode for the positive electrode, a porous polyethylene film (Celgard 2300, thickness: 25 μm) as a separator, and an electrolyte solution of 1.15 M LiPF in a solvent of ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7.

[0161] Experimental Example 1: Structural analysis of positive electrode active material In order to measure the density of the crystal grain boundaries of the lithium composite oxide contained in the positive electrode active material prepared in Preparation Example 1, the degree of aggregation of primary particles within secondary particles observed in a cross-sectional SEM image was confirmed.

[0162] First, the lithium composite oxides contained in the positive electrode active material prepared in Preparation Example 1 were separated, and then cross-sections of the lithium composite oxides were processed using an FIB (Ga-ion source), and cross-sectional SEM images were taken using a scanning electron microscope.

[0163] Next, for each of the multiple particles observed in the cross-sectional SEM image, the number of crystallites arranged on a virtual line crossing the center of the particle in the short axis direction and the number of boundary surfaces between the crystallites were substituted into the following equation 1 to calculate the density of the crystal grain boundaries.

[0164] [Formula 1] Grain boundary density = (number of boundary surfaces between crystallites arranged on the virtual line / number of crystallites arranged on the virtual line)

[0165] In addition, the percentage of particles having a grain boundary density of 0.5 or less calculated by Equation 1 among the multiple particles observed in the cross-sectional SEM image was calculated, and the calculation results are shown in Table 1 below.

[0166] [Table 1]

[0167] Experimental Example 2: Composition analysis of positive electrode active material In order to confirm the change in the concentration of cobalt in the lithium composite oxide contained in the positive electrode active material prepared in Preparation Example 1, SEM / EDX analysis was carried out.

[0168] First, the lithium composite oxides contained in the positive electrode active material prepared in Preparation Example 1 were separated, and then cross-sections of the lithium composite oxides were processed using an FIB (Ga-ion source), and cross-sectional SEM images were taken using a scanning electron microscope.

[0169] Next, 10 particles were selected from the plurality of particles observed in the cross-sectional SEM image, and the target transition metal cobalt was mapped through EDS analysis of the selected particles. The change in cobalt concentration from the surface to the center of the lithium composite oxide was confirmed through line scanning.

[0170] In this case, a region in which a concentration gradient was formed in which the cobalt concentration decreased from the surface portion toward the center of the lithium composite oxide based on the cross-sectional SEM image of the lithium composite oxide was defined as a first section, and a region inside the first section in which the cobalt concentration was maintained within a predetermined range (the rate of change in cobalt concentration was 10 mol% or less) was defined as a second section.

[0171] The analysis results are shown in Tables 2 and 3 below, and the results shown in Table 2 represent average values ​​measured for multiple lithium composite oxides among the positive electrode active materials according to each of the Examples and Comparative Examples.

[0172] [Table 2]

[0173] [Table 3] d: Average radius of lithium composite oxide d1: The thickness of the first section ascertained from the line sum spectrum graph c1: Average concentration of Co in the first section (mol%) n1: Average concentration of Ni in the first section (mol%) c: Average Co concentration measured based on ICP analysis for lithium composite oxides c2: Average concentration of Co in the second section (mol%) n2: Average concentration of Ni in the second section (mol%)

[0174] Experimental Example 3: Evaluation of the electrochemical properties of lithium secondary batteries A charge-discharge experiment was carried out on the lithium secondary battery (coin cell) manufactured in Manufacturing Example 2 using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 3.0V to 4.25V, and a discharge rate of 0.2C to measure the charge and discharge capacities.

[0175] In addition, the same lithium secondary battery was charged and discharged 50 times at 25°C and 1C / 1C within a driving voltage range of 3.0V to 4.25V, and then the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured.

[0176] The measurement results are shown in Table 4 below.

[0177] [Table 4]

[0178] Experimental Example 4: Evaluation of the stability of lithium secondary batteries A 50 mm x 65 mm pouch-shaped lithium secondary battery was manufactured in the same manner as in Manufacturing Example 2. The lithium secondary battery was charged to 4.3 V at a constant current of 0.2 C and then stored at high temperature (70°C) for 14 days to measure the volume change of the lithium secondary battery due to gas generation inside the lithium secondary battery. The volume change rate was calculated by measuring the volume before and after high-temperature storage and converting it into a percentage.

[0179] The volume change rates of the lithium secondary batteries measured by the above-mentioned method are shown in Table 5 below.

[0180] [Table 5]

[0181] Although the embodiments of the present invention have been described above, a person having ordinary knowledge in the art may modify and change the present invention in various ways by adding, changing, deleting or adding components within the scope of the concept of the present invention as set forth in the claims, and this also falls within the scope of the present invention.

Claims

1. A positive electrode active material comprising a lithium composite oxide containing at least nickel and cobalt, The cross section of the lithium composite oxide is divided into a first section in which the cobalt concentration decreases from the surface portion of the lithium composite oxide toward the center portion thereof, and a second section inside the first section in which the cobalt concentration is maintained within a predetermined range; When an average radius measured from a cross section of the lithium composite oxide is referred to as d, a ratio d1 / d of the average radius d to a thickness d1 of the first section is 0.08 to 0.27, The average molar ratio of Co / Ni in the second section is less than 0.090; The positive electrode active material has an average Co / Ni molar ratio of 0.25 to 0.39 in the first range.

2. 2. The cathode active material according to claim 1, wherein a ratio of the average Co concentration c1 in the first section to the average Co concentration c measured based on ICP analysis of the lithium composite oxide is 1.70 to 2.

60.

3. 2. The positive electrode active material according to claim 1, wherein a ratio d2 / d of the average radius d to the thickness d2 of the second section is 0.73 to 0.

92.

4. The positive electrode active material according to claim 1 , wherein the lithium composite oxide is represented by the following chemical formula 1: [Chemical formula 1] Li ｗ Ni １－（ｘ＋ｙ＋ｚ） Co ｘ M1 ｙ M2 ｚ O ２ (where, M1 is at least one selected from Mn and Al; M2 is at least one selected from P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, Cu, Na, and K; M1 and M2 are different from each other, 0.5≦w≦1.5, 0<x≦0.50, 0≦y≦0.20, 0≦z≦0.20)

5. 2. The positive electrode active material according to claim 1, wherein in a cross-sectional SEM image obtained by photographing a cross section of the lithium composite oxide using a scanning electron microscope (SEM) after cross-section processing of the lithium composite oxide, a density of crystal grain boundaries calculated by the following Equation 1 for crystallites arranged on a virtual line crossing a center of the lithium composite oxide in a minor axis direction is 0.50 or less: [Formula 1] Grain boundary density = (number of boundary surfaces between crystallites arranged on the imaginary line / number of crystallites arranged on the imaginary line)

6. The positive electrode active material is an aggregate of a plurality of lithium composite oxides, 2. The positive electrode active material according to claim 1, wherein in a cross-sectional SEM image obtained by photographing a cross section of the lithium composite oxide using a scanning electron microscope (SEM) after cross-section processing of the lithium composite oxide in the positive electrode active material, a proportion of lithium composite oxide having a grain boundary density of 0.50 or less, calculated by the following Equation 1, with respect to crystallites arranged on a virtual line crossing a center of the lithium composite oxide in a minor axis direction, is 30% or more: [Formula 1] Grain boundary density = (number of boundary surfaces between crystallites arranged on the imaginary line / number of crystallites arranged on the imaginary line)

7. the lithium composite oxide is a non-aggregated single particle consisting of a single crystallite, 2. The positive electrode active material of claim 1, wherein a cross section of the single particle is divided into a first section having a concentration gradient in which the cobalt concentration decreases from the surface portion toward the center portion of the single particle, and a second section inside the first section in which the cobalt concentration is maintained within a predetermined range.

8. further comprising a coating layer covering at least a portion of the surface of the lithium composite oxide; The positive electrode active material according to claim 1 , wherein the coating layer contains at least one metal oxide represented by the following Chemical Formula 2: [Chemical formula 2] Li ａ M3 ｂ O ｃ (where, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd; 0<a≦10, 0≦b≦8, 2≦c≦13)

9. A positive electrode comprising the positive electrode active material according to claim 1 .

10. A lithium secondary battery using the positive electrode according to claim 9.

Images