Cathode material, cathode containing same, and lithium secondary battery

Abstract

The present invention relates to a cathode material comprising: a first cathode active material in the form of secondary particles formed by aggregation of multiple grains, at least a portion of which has an oriented structure in which the long axes of the grains are aligned from the center toward the surface of the secondary particles, and in which the cobalt concentration at the grain boundaries, which are the interfaces between the grains, is higher than the cobalt concentration within the grains; a central portion having the form of at least one of a single particle consisting of a single nodule and a quasi-single particle, which is a composite of 30 or fewer nodules; and a second cathode active material formed on the central portion and including a coating layer containing cobalt.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0183385, filed December 13, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a cathode material, a cathode and a lithium secondary battery including the same, and more specifically to a cathode material that has little particle cracking during rolling, generates little gas at high temperatures, has excellent life characteristics, and has low initial resistance, and a cathode and a lithium secondary battery including the cathode material. [Background technology]

[0003] A lithium secondary battery generally comprises a positive electrode, a negative electrode, a separator, and an electrolyte, and the positive electrode and the negative electrode contain active materials capable of intercalating and deintercalating lithium ions.

[0004] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMnO4, etc.), and lithium iron phosphate (LiFePO4) have been used as positive electrode active materials for lithium secondary batteries. Among these, lithium cobalt oxide has the advantages of high operating voltage and excellent capacity characteristics, but the high cost and unstable supply of cobalt, the raw material, make its commercial application in large-capacity batteries difficult. Lithium nickel oxide has poor structural stability and is difficult to achieve sufficient life characteristics. On the other hand, lithium manganese oxide has excellent stability but poor capacity characteristics. Therefore, to address the issues of lithium transition metal oxides containing only Ni, Co, or Mn, lithium composite transition metal oxides containing two or more transition metals have been developed. Among these, lithium nickel cobalt manganese oxide containing Ni, Co, and Mn is widely used in the field of electric vehicle batteries.

[0005] Conventional lithium nickel cobalt manganese oxides are generally in the form of spherical secondary particles formed by agglomeration of tens to hundreds of primary particles. However, lithium nickel cobalt manganese oxides in the form of secondary particles formed by agglomeration of many primary particles are prone to particle cracking, where primary particles break off during the rolling process during positive electrode production, and internal cracks occur during charge and discharge. When particles of the positive electrode active material break or crack, the contact area with the electrolyte increases, which increases gas generation and degradation of the active material due to side reactions with the electrolyte, resulting in reduced life characteristics.

[0006] To solve the above problems, single particle cathode active materials consisting of a single particle have been developed. Single particle cathode active materials have the advantage of high particle strength, less particle cracking during rolling, and excellent life characteristics. However, the lithium diffusion resistance inside the particle is high, and increasing the particle size leads to poor resistance and output characteristics. Therefore, currently, in the case of single particles, D 50 It is currently used in the form of small particles with a size of around 4 μm. However, when such small particle cathode active materials are used alone, they have poor electrode processability, so a technology has been proposed in which they are used in combination with secondary particles and single particles. However, when secondary particles and single particles are used in combination, the difference in particle strength between the single particles and secondary particles causes the secondary particles to crack more severely during electrode rolling, resulting in little effect in suppressing gas generation and improving lifespan, and therefore has not been commercialized. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention is intended to solve the above problems, and provides a cathode material that uses a mixture of secondary particles and a single particle cathode active material, in which the secondary particles having a specific orientation structure and composition are mixed with the single particle cathode active material having a cobalt coating layer formed on the surface, thereby reducing particle cracking during rolling and gas generation at high temperatures, thereby achieving excellent life characteristics and low initial resistance.

[0008] The present invention also provides a positive electrode and a lithium secondary battery containing the above-mentioned positive electrode material. [Means for solving the problem]

[0009] According to one embodiment, the present invention provides a cathode material comprising: a first cathode active material in the form of secondary particles formed by agglomeration of a plurality of grains, at least a portion of the secondary particles having an oriented structure in which the major axes of the grains are aligned from the center toward the surface of the secondary particles, and a cobalt concentration at the grain boundaries, which are the interfaces between the grains, that is higher than the cobalt concentration within the grains; a central portion having the form of at least one of a single particle consisting of a single nodule and a quasi-single particle, which is a composite of 30 or fewer nodules; and a second cathode active material formed on the central portion and including a coating layer containing cobalt.

[0010] According to another embodiment, the present invention provides a positive electrode comprising the positive electrode material according to the present invention, and a lithium secondary battery comprising the positive electrode. [Effects of the Invention]

[0011] The cathode material according to the present invention includes a first cathode active material in the form of secondary particles having a specific orientation structure and composition, and a second cathode active material in the form of a single particle having a cobalt coating layer formed on the surface thereof. Compared to conventional cathode materials that combine secondary particles and single particles, the cathode material exhibits less particle cracking during rolling, less gas generation at high temperatures, and has excellent life characteristics and low initial resistance.

[0012] The first positive electrode active material included in the positive electrode material according to the present invention has an oriented structure in which the long axis of the grains is aligned from the center toward the surface of the secondary particles. This shortens the lithium diffusion path within the particles, thereby achieving excellent lithium mobility and low resistance. However, as described above, positive electrode active materials with an oriented structure tend to experience more severe particle cracking during electrode rolling than positive electrode active materials in the form of non-oriented secondary particles. This cracking is particularly severe when mixed with single particles, which have high particle strength. Particle cracking of the positive electrode active material increases side reactions with the electrolyte, resulting in reduced lifespan and increased gas generation. However, when a high concentration of cobalt is formed at the grain boundaries of secondary particles with an oriented structure, as in the first positive electrode active material according to the present invention, the cobalt concentrated at the grain boundaries increases particle strength, reducing particle cracking even when mixed with single particles, thereby minimizing degradation of the positive electrode active material and gas generation.

[0013] The second positive electrode active material included in the positive electrode material according to the present invention has high particle strength and is less likely to crack during rolling. The cobalt coating layer formed on the surface minimizes the electrically inactive rock salt phase, resulting in low surface resistance. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional SEM photograph of the lithium transition metal composite oxide produced in Production Example 1. [Figure 2] 1 is a cross-sectional SEM photograph of the lithium transition metal composite oxide produced in Production Example 3. [Figure 3] 1 is an SEM photograph of the lithium transition metal composite oxide produced in Production Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0015] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0016] In the present invention, the term "grain" refers to the smallest particle unit that can be distinguished as a single mass without apparent grain boundaries when observed at a magnification of 5,000 to 20,000 times using a scanning electron microscope, and may consist of a single crystallite or multiple crystallites. In the present invention, the average grain size can be measured by measuring the size of each particle distinguished in cross-sectional SEM data of the positive electrode active material particles and then calculating the arithmetic mean value of these particle sizes.

[0017] In the present invention, the term "secondary particle" refers to a secondary structure formed by the aggregation of a plurality of grains.

[0018] In the present invention, "D 50 " means the particle size at 50% of the volume cumulative particle size distribution of the positive electrode active material powder. 50 can be measured using a laser diffraction method. For example, the positive electrode active material powder is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000). Ultrasonic waves of about 28 kHz are irradiated at an output of 60 W, and a volume cumulative particle size distribution graph is obtained. The particle size can be measured by determining the particle size corresponding to 50% of the volume cumulative amount.

[0019] In the present invention, the "oriented structure" refers to a structure in which the long axes of the grains are aligned from the center of the secondary particle toward the surface. Here, "the long axes of the grains are aligned from the center of the secondary particle toward the surface" means that the angle between the long axis of the grain and the shortest line segment connecting the center and surface of the secondary particle while passing through the grain is in the range of -15° to 15°.

[0020] In the present invention, "aspect ratio of a grain" means the ratio of the long axis length to the short axis length of a grain, and "average aspect ratio" means the arithmetic mean value of the aspect ratios of the grains in the region.

[0021] In the present invention, a "single particle" refers to a particle consisting of one single nodule. In the present invention, a "quasi-single particle" refers to a particle that is a complex formed by 30 or fewer nodules.

[0022] In the present invention, the term "nodule" refers to a particle unit body constituting a single particle or a quasi-single particle. The nodule may be a single crystal lacking a crystalline grain boundary, or may be a polycrystal lacking any apparent grain boundary when observed at a magnification of 5,000 to 20,000 times using a scanning electron microscope (SEM) or electron backscatter diffraction (EBSD). The average particle size of the nodule refers to the arithmetic mean value calculated after measuring the particle size of the nodule observed using SEM or EBSD.

[0023] The term "particle" as used in the present invention can include any one or all of single particles, quasi-single particles, primary particles, nodules, and secondary particles.

[0024] The present invention will be specifically described below.

[0025] <Cathode material> The cathode material according to the present invention includes a first cathode active material and a second cathode active material, wherein the first cathode active material is in the form of secondary particles formed by agglomeration of a plurality of grains, and at least a portion of the secondary particles has an oriented structure in which the major axes of the grains are aligned from the center of the secondary particles toward the surface thereof, and the cobalt concentration at the grain boundaries, which are the interfaces between the grains, is higher than the cobalt concentration within the grains. The second cathode active material includes a core portion having the form of at least one of a single particle consisting of one nodule and a quasi-single particle, which is a composite of 30 or fewer nodules, and a coating layer containing cobalt formed on the core portion.

[0026] Preferably, the cathode material according to the present invention may have a bimodal particle size distribution in which the particle size of the first cathode active material is larger than the particle size of the second cathode active material. When the cathode material according to the present invention has a bimodal particle size distribution, a high electrode density can be achieved, thereby improving battery capacity characteristics. However, in the case of the second cathode active material, which is a single-particle cathode active material, a large particle size can lengthen the lithium diffusion path within the particle, resulting in a decrease in resistance and output characteristics. Therefore, it is preferable to minimize the decrease in resistance and output characteristics and achieve a high electrode density by making the particle size of the first cathode active material, which is in the form of secondary particles, larger than the particle size of the second cathode active material.

[0027] Specifically, the first positive electrode active material D 50 The D of the second positive electrode active material may be 8 μm to 20 μm, preferably 8 μm to 18 μm, and more preferably 8 μm to 15 μm. 50 The D of the first positive electrode active material and the second positive electrode active material can be 2 μm to 7 μm, preferably 2.5 μm to 7 μm, and more preferably 3 μm to 7 μm. 50 When the above range is satisfied, the capacitance characteristic, resistance characteristic and output characteristic are further improved.

[0028] On the one hand, the first positive electrode active material and the second positive electrode active material can have the same or different compositions. For example, the first positive electrode active material and the second positive electrode active material can each independently contain a nickel-based lithium composite transition metal oxide represented by the following [Chemical Formula 1].

[0029] [Chemical Formula 1] Li X [Ni a Co b M 1 c M 2 d O 2-y A y

[0030] In the above [Chemical Formula 1], the M 1 can be one or more elements selected from the group consisting of Mn and Al, and for example, can be Mn or a combination of Mn and Al.

[0031] The M 2 can be one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo.

[0032] Also, the A can be one or more elements selected from the group consisting of F, Cl, Br, I, At, and S.

[0033] The x represents the ratio of the number of moles of Li to the total number of moles of transition metals, and can be 0.98 ≤ x ≤ 1.20, preferably 0.99 ≤ x ≤ 1.10, more preferably 1.0 ≤ x ≤ 1.10.

[0034] The a represents the ratio of the number of moles of Ni to the total number of moles of the remaining metals other than lithium, and can be 0 < a < 1, preferably 0.3 ≤ a < 1, more preferably 0.6 ≤ a < 1, still more preferably 0.8 ≤ a < 1, and still more preferably 0.85 ≤ a < 1. <000021_{2} b represents the ratio of the number of moles of Co to the total number of moles of the remaining metals other than lithium, where 0 < b < 1, preferably 0 < b < 0.7, more preferably 0 < b < 0.4, still more preferably 0 < b < 0.2, and even more preferably 0 < b ≤ 0.1.

[0036] c represents the ratio of the number of moles of M to the total number of moles of the remaining metals other than lithium 1 and 0 < c < 1, preferably 0 < c < 0.7, more preferably 0 < c < 0.4, still more preferably 0 < c < 0.2, and even more preferably 0 < c ≤ 0.1.

[0037] d represents the ratio of the number of moles of M to the total number of moles of the remaining metals other than lithium 2 and 0 ≤ d ≤ 0.2, preferably 0 ≤ d ≤ 0.15, and more preferably 0 ≤ d ≤ 0.10.

[0038] y represents the ratio of the number of moles of element A substituted at the oxygen site, and 0 ≤ y ≤ 0.2, preferably 0 ≤ y ≤ 0.15, and more preferably 0 ≤ y ≤ 0.10.

[0039] On the other hand, the first positive electrode active material and the second positive electrode active material can be included in a weight ratio of 90:10 to 50:50, preferably 80:20 to 50:50, and more preferably 70:30 to 50:50. When the mixing ratio of the first positive electrode active material and the second positive electrode active material satisfies the above range, a high electrode density can be achieved.

[0040] When using both a first positive electrode active material having an orientation structure in which grains are arranged from the center to the surface direction of secondary particles and having a composition with a higher cobalt concentration at grain boundaries than inside the grains, and a single-particle type second positive electrode active material having a cobalt coating layer formed on the surface, as in the positive electrode material of the present invention, during rolling, not only are there few cracks in the secondary particles, but even when cracks occur in the particles, the surface of the grains is coated with cobalt, and there are few side reactions with the electrolytic solution, so the amount of gas generation at high temperatures is small and the life characteristics are excellent.

[0041] Next, the first and second positive electrode active materials constituting the positive electrode material of the present invention will be described in more detail.

[0042] First positive electrode active material The first positive electrode active material is in the form of secondary particles formed by agglomeration of multiple grains, and at least a portion of the secondary particles has an oriented structure in which the major axes of the grains are aligned from the center to the surface of the secondary particle. Here, "the major axes of the grains are aligned from the center to the surface of the secondary particle" means that the angle between the major axis of the grain and the shortest line segment connecting the center and surface of the secondary particle while passing through the grain is in the range of -15° to 15°. The major axis of the grain refers to the longest line segment connecting two points on the surface of the grain while passing through the center of the grain. The interfaces between grains within the secondary particles of the positive electrode active material serve as diffusion paths for lithium ions. When the grains are aligned from the center to the surface of the secondary particle, the diffusion path of lithium ions within the secondary particle is shortened, increasing lithium mobility and thereby improving output and / or resistance characteristics.

[0043] Meanwhile, the grains in which the long axes of the grains are aligned from the center of the secondary particle toward the surface may form an angle of -15° to 15°, preferably -10° to 10°, between the long axes and the a-axis direction of the crystal structure. Since lithium ions move in the a-axis direction within the grains, when the long axes of the grains form an angle of -15° to 15° between the long axes of the grains and the a-axis direction of the crystal structure, insertion and desorption of lithium ions becomes easy, thereby obtaining the effect of improving output and / or resistance characteristics.

[0044] On the other hand, the grains in which the major axes of the grains are aligned from the center of the secondary particle toward the surface can have an aspect ratio of 1.5 to 15, preferably 2 to 15, and more preferably 4 to 15. When the aspect ratio of the grains satisfies the above range, the contraction and expansion of the grains during charge and discharge occurs mainly in the direction of the minor axis perpendicular to the orientation direction, and the occurrence of cracks due to non-uniform contraction and expansion of the grains within the secondary particles can be effectively suppressed.

[0045] Meanwhile, the first positive electrode active material may have a core-shell structure including, but not limited to, a core portion in which grains are aggregated without any particular orientation, and a shell portion in which the major axes of the grains are aligned from the center of the secondary particle toward the surface.

[0046] The core region is a region where grains are randomly aggregated without any particular orientation and is formed at the center of the secondary particle. The core region may be a portion formed as a seed during the coprecipitation reaction for forming the positive electrode active material precursor, and may be, for example, a region located at a distance of 1 / 3R or 1 / 4R from the center of the secondary particle, where R is the radius of the secondary particle from the center of the secondary particle.

[0047] The grains in the core may have a nearly spherical shape, and may have an aspect ratio of 0.7 to 1.3, preferably 0.8 to 1.2.

[0048] The shell portion is formed outside the core portion and is a region in which grains are arranged in an oriented structure. The shell portion may be a portion formed as the particles grow during a coprecipitation reaction for forming a precursor for a positive electrode active material. For example, the shell portion may be a region from 1 / 3R to R of the secondary particle, or a region from 1 / 4R to R of the secondary particle, where R is the radius of the secondary particle from the center of the secondary particle.

[0049] The grains in the shell portion may be rod-shaped, and may have an aspect ratio of 1.5 to 15, preferably 2 to 15, and more preferably 4 to 15.

[0050] Meanwhile, in the first positive electrode active material of the present invention, the average grain size may be 0.05 μm to 4 μm, preferably 0.1 μm to 3 μm, and more preferably 0.1 μm to 2 μm. If the average grain size is too large, a rock salt phase may be formed, which may reduce the resistance characteristics and life characteristics. If the average grain size is too small, the contact area with the electrolyte may increase, which may cause rapid deterioration.

[0051] Meanwhile, the first positive electrode active material may include a nickel-based lithium composite transition metal oxide, for example, a nickel-based lithium composite transition metal oxide represented by the following [Chemical Formula 1-1].

[0052] [Chemical formula 1-1] Li X1 [Ni a1 Co b1 M 1 c1 M 2 d1 ]O 2-y1 A y1

[0053] In the above [Chemical Formula 1-1], the M 1 can be one or more elements selected from the group consisting of Mn and Al, for example, Mn or a combination of Mn and Al.

[0054] Said M 2 can be one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo.

[0055] Furthermore, A can be one or more elements selected from the group consisting of F, Cl, Br, I, At and S.

[0056] The x1 represents the ratio of the number of moles of Li to the total number of moles of transition metals, and can be 0.98≦x1≦1.20, preferably 0.99≦x1≦1.10, and more preferably 1.0≦x1≦1.10.

[0057] The a1 represents the ratio of the number of moles of Ni to the total number of moles of the remaining metals other than lithium, and can be 0.6≦a1≦0.9, preferably 0.7≦a1≦0.9, more preferably 0.8≦a1≦0.9, and even more preferably 0.8≦a1≦0.86.

[0058] The b1 represents the ratio of the number of moles of Co to the total number of moles of the remaining metals other than lithium, and can be 0.01≦b1<0.4, preferably 0.01≦b1<0.3, more preferably 0.01≦b1<0.2, and still more preferably 0.01≦b1≦0.1.

[0059] The c1 is M relative to the total number of moles of the remaining metals other than lithium. 1 and may be 0.01≦c1<0.4, preferably 0.01≦c1<0.3, more preferably 0.01≦c1<0.2, and even more preferably 0.01≦c1≦0.1.

[0060] The d1 is M relative to the total number of moles of the remaining metals other than lithium. 2 and can be 0≦d1≦0.2, preferably 0≦d1≦0.15, and more preferably 0≦d1≦0.10.

[0061] The y1 indicates the ratio of the number of moles of the A element substituted at the oxygen position, and can be 0≦y1≦0.2, preferably 0≦y1≦0.15, and more preferably 0≦y1≦0.10.

[0062] Meanwhile, the first positive electrode active material according to the present invention is characterized in that the cobalt concentration at the grain boundary, which is the interface between grains, is higher than the cobalt concentration inside the grain. Here, the cobalt concentration refers to the molar ratio of cobalt to the remaining metals other than lithium. That is, the first positive electrode active material has cobalt enriched at the grain boundaries. When cobalt is enriched at the grain boundaries, even though the first positive electrode active material has an oriented structure, particle cracking during rolling is reduced, and even if particle cracking occurs during rolling, side reactions with the electrolyte are minimized. Therefore, when the first positive electrode active material is used, superior high-temperature characteristics can be achieved compared to conventional positive electrode active materials having an oriented structure.

[0063] Meanwhile, the first positive electrode active material may be prepared by adding a lithium composite transition metal oxide, which is in the form of secondary particles formed by agglomeration of a plurality of grains and has an oriented structure in which the major axes of the grains are aligned from the center of the secondary particles to the surface thereof, to a coating solution containing cobalt element, followed by heat treatment.

[0064] The lithium composite transition metal oxide having an oriented structure may be prepared by purchasing a commercially available product or by using a method for producing a lithium composite transition metal oxide well known in the art. For example, the lithium composite transition metal oxide having an oriented structure may be produced by mixing a precursor for a positive electrode active material with a lithium source material and then calcining the mixture.

[0065] To produce a cathode active material having an oriented structure, as in the present invention, a cathode active material precursor having an oriented structure in which primary particles are aligned from the center toward the surface of secondary particles must be used. The aggregation morphology of the primary particles of the cathode active material precursor is affected by factors such as the pH, stirring speed, and reaction temperature during the coprecipitation reaction. Therefore, the cathode active material precursor having an oriented structure can be prepared by appropriately adjusting the pH, stirring speed, and reaction temperature during the preparation of the cathode active material precursor, and a lithium transition metal composite oxide having an oriented structure can be produced using the obtained precursor.

[0066] The lithium source material and the cathode active material precursor may be mixed so that the molar ratio of Li:total transition metals in the precursor is 1:1 to 1.2:1, preferably 1:1 to 1.1:1. When the mixing ratio of the lithium source material and the transition metals in the cathode active material precursor satisfies this range, the crystalline structure of the cathode active material is well developed, and a cathode active material with excellent capacity characteristics and structural stability can be produced.

[0067] Examples of the lithium source material include lithium-containing carbonates (e.g., lithium carbonate, etc.), hydrates (e.g., lithium hydroxide hydrate (LiOH·H2O)), hydroxides (e.g., lithium hydroxide, etc.), nitrates (e.g., lithium nitrate (LiNO3), etc.), and chlorides (e.g., lithium chloride (LiCl)), and any of these may be used alone or in combination.

[0068] The calcination may be carried out at an appropriate temperature taking into consideration the composition of the lithium composite transition metal oxide, for example, at 600° C. to 1000° C., preferably 700° C. to 900° C. The calcination time may be, for example, 5 to 30 hours, preferably 8 to 15 hours, but is not limited thereto.

[0069] Next, the lithium composite transition metal oxide having the oriented structure is post-heat-treated in a coating solution containing cobalt element, so that cobalt is concentrated at the grain boundaries of the lithium composite transition metal oxide.

[0070] The coating solution containing the cobalt element may be prepared by dissolving at least one selected from the group consisting of Co(NO3)2, Co(NO3)2·6H2O, CoCl2, CoSO4, Co(OCOCH3)2·4H2O, and Co(OH)2 in a solvent such as water or ethanol.

[0071] As described above, when wet coating is performed by mixing a lithium composite transition metal oxide into a coating solution and then heat-treating it, the cobalt element contained in the coating solution penetrates not only into the surfaces of the secondary particles of the lithium composite transition metal oxide but also into the interfaces (grain boundaries) between the grains, making it possible to form a higher cobalt concentration at the grain boundaries than inside the grains.

[0072] Meanwhile, the heat treatment can be carried out at 300°C to 800°C, preferably 400°C to 700°C, and more preferably 500°C to 650°C. When the heat treatment temperature is within this range, the cobalt element contained in the coating solution is concentrated at the grain boundaries, making the cobalt concentration at the grain boundaries higher than within the grains. If the first heat treatment temperature is too low, the coating will not proceed smoothly, and if the first heat treatment temperature is too high, the cobalt will diffuse into the grains and not concentrate at the grain boundaries.

[0073] Second positive electrode active material The second positive electrode active material includes a core in the form of a single particle and / or a quasi-single particle, and a coating layer containing cobalt formed on the core.

[0074] The core is a single particle consisting of one nodule and / or a quasi-single particle which is a complex of 30 or less, preferably 2 to 20, more preferably 2 to 10 nodules.

[0075] The second positive electrode active material, whose core is in the form of a single particle and / or pseudo-single particle, has higher particle strength and is less likely to crack during rolling than existing positive electrode active materials in the form of secondary particles, which are composed of tens to hundreds of aggregated primary particles. Furthermore, since the second positive electrode active material has a small number of nodules constituting the positive electrode active material particles, there is less change in the volume of the nodules due to expansion and contraction during charge and discharge, which significantly reduces the occurrence of cracks inside the particles.

[0076] Meanwhile, the core may include a nickel-based lithium composite transition metal oxide represented by the following [Chemical Formula 1-2].

[0077] [Chemical formula 1-2] Li X2 [Ni a2 Co b2 M 1 C2 M 2 d2 ]O 2-y2 A y2

[0078] In the above Chemical Formula 1-2, the M 1 can be one or more elements selected from the group consisting of Mn and Al, preferably Mn, Al or a combination thereof, and more preferably Mn or Mn and Al.

[0079] Said M 2 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and is preferably one or more elements selected from the group consisting of Zr, Y, Mg, and Ti, and more preferably Zr, Y, or a combination thereof. 2 Although the elements are not essential, when they are contained in an appropriate amount, they can promote grain growth during firing or improve the stability of the crystal structure.

[0080] Said x2 represents the molar ratio of lithium in the lithium nickel-based oxide, and can be 0.8 ≦ a ≦ 1.2, 0.85 ≦ a ≦ 1.15, or 0.9 ≦ a ≦ 1.2. When the molar ratio of lithium satisfies the above range, the crystal structure of the positive electrode active material can be stably formed.

[0081] Said a2 represents the molar ratio of nickel among all the metals other than lithium, and can be 0.8 ≦ a2 < 1, 0.82 ≦ a2 < 1, or 0.83 ≦ a2 < 1. When the molar ratio of nickel satisfies the above range, it exhibits a high energy density and enables the realization of a high capacity.

[0082] Said b2 represents the cobalt molar ratio among all the metals other than lithium, and can be 0 < b2 < 0.2, 0 < b2 < 0.18, or 0.01 ≦ b2 ≦ 0.17. When the cobalt molar ratio satisfies the above range, good resistance characteristics and output characteristics can be realized.

[0083] Said c2 represents the molar ratio of element M among all the metals other than lithium 1 and can be 0.01 ≦ c2 < 0.2, 0.01 ≦ c2 < 0.18, or 0.01 ≦ d ≦ 0.17. When the molar ratio of element M 1 satisfies the above range, the positive electrode active material has excellent structural stability.

[0084] Said d2 represents the ratio of the number of moles of M to the total number of moles of the remaining metals other than lithium 2 and can be 0 ≦ d2 ≦ 0.2, preferably 0 ≦ d2 ≦ 0.15, and more preferably 0 ≦ d2 ≦ 0.10.

[0085] Said y2 represents the ratio of the number of moles of element A substituted at the oxygen site, and can be 0 ≦ y2 ≦ 0.2, preferably 0 ≦ y2 ≦ 0.15, and more preferably 0 ≦ y2 ≦ 0.10.

[0086] On the other hand, the second positive electrode active material according to the present invention includes a cobalt-containing coating layer on the central portion including the single particles and / or pseudo-single particles.

[0087] Typically, single-particle and / or quasi-single-particle positive electrode active materials are prepared by firing at higher temperatures than secondary-particle positive electrode active materials. However, high firing temperatures increase the amount of electrically inactive rock salt phase on the particle surface, resulting in increased resistance of the positive electrode active material. However, when a cobalt-containing coating layer is formed on the surface of the single particles and / or quasi-single particles as in the present invention, recrystallization occurs due to a reaction with cobalt during the coating layer formation process, reducing the amount of rock salt phase on the surface of the single particles and / or quasi-single particles, thereby improving resistance characteristics.

[0088] Specifically, the coating layer may be an oxide containing lithium and cobalt, and may have a composition represented by the following [Chemical Formula 2].

[0089] [Chemical formula 2] Li z Co 1-w M 3 w O2

[0090] In the above [Chemical Formula 2], the M 3 may be one or more selected from the group consisting of Ni, Mn, Al, W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and may satisfy 0.8≦z≦1.2 and 0≦w≦0.2. Preferably, the M 3 can be one or more selected from the group consisting of Ni, Mn, Al, Ti, Zr, and Mg, and 0.8≦z≦1.1, 0≦w≦0.1.

[0091] When the coating layer has the above composition, it is possible to obtain an effect of improving the initial resistance characteristics and the high-temperature life characteristics.

[0092] The shape and area of ​​the coating layer are not particularly limited. For example, the coating layer may be in the form of a continuous film that surrounds the entire surface of the monoparticles and / or quasi-monoparticles that make up the core, or in the form of particles that are discontinuously distributed on the surface of the monoparticles and / or quasi-monoparticles. The area of ​​the coating layer may be 10% to 100%, 10% to 80%, or 20% to 70% of the total surface area of ​​the core.

[0093] Meanwhile, the second positive electrode active material according to the present invention may have an average nodule particle size of 0.5 μm to 3 μm, preferably 0.8 μm to 2.5 μm, and more preferably 0.8 μm to 1.5 μm. When the average nodule particle size satisfies this range, a single-particle and / or quasi-single-particle positive electrode active material with excellent electrochemical properties can be formed. If the average nodule particle size is too small, the number of agglomerates forming the center increases, reducing the effect of suppressing particle cracking during rolling. If the average nodule particle size is too large, the lithium diffusion path within the nodule becomes longer, increasing resistance and potentially reducing output characteristics.

[0094] On the other hand, the second positive electrode active material has an average particle size D 50 The D of the second positive electrode active material can be 2 μm to 7 μm, preferably 2.5 μm to 7 μm, and more preferably 3 μm to 7 μm. 50 If the value is too small, aggregation of the slurry occurs, making it difficult to manufacture the electrode, and the electrochemical properties deteriorate due to the decrease in electrolyte impregnation. 50 If is too large, the resistance increases and the output characteristics deteriorate.

[0095] Additionally, the second positive electrode active material may have an average crystallite size of 150 nm to 300 nm, 200 nm to 280 nm, or 200 nm to 250 nm. When the average crystallite size is within this range, the formation of a rock salt phase during the preparation of the lithium-nickel-based oxide is reduced, resulting in the production of a single-particle and / or quasi-single-particle positive electrode active material with excellent resistance characteristics. Generally, single-particle and / or quasi-single-particle positive electrode active materials are prepared by increasing the sintering temperature to increase the nodule size. However, increasing the nodule size while the crystallite size is small can result in the formation of a rock salt phase on the surface of the nodules, resulting in increased resistance. However, increasing both the average crystallite size and the average particle size of the nodules can minimize the formation of the rock salt phase and suppress the increase in resistance.

[0096] Meanwhile, the second positive electrode active material may be prepared by mixing a precursor for a positive electrode active material and a lithium source material, calcining the mixture to form single particles and / or quasi-single particles, mixing the single particles and / or quasi-single particles with a Co-containing coating source material, and then heat treating the mixture.

[0097] Here, the precursor for the positive electrode active material may be a commercially available precursor such as a nickel-cobalt-manganese hydroxide, or may be prepared by a precursor preparation method well known in the art, such as a coprecipitation method.

[0098] Preferably, the positive electrode active material precursor used in the present invention is a transition metal hydroxide containing nickel and cobalt, with the Ni content being 80 mol% or more of the total transition metals, and more preferably, nickel-cobalt-manganese hydroxide with the Ni content being 80 mol% or more. When the nickel content in the transition metal precursor satisfies this range, high capacity characteristics can be achieved.

[0099] The lithium source material may be a lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or oxyhydroxide, such as Li2CO3, LiNO3, LiNO2, LiOH, LiOH·H2O, LiH, LiF, LiCl, LiBr, LiI, CH3COOLi, Li2O, Li2SO4, CH3COOLi, Li3C6H5O7, or a mixture thereof.

[0100] Meanwhile, the lithium source material and the cathode active material precursor may be mixed so that the molar ratio of Li:total metals in the precursor is 1:1 to 1.1:1, preferably 1.02:1 to 1.05:1. When the mixing ratio of the lithium source material and the metals in the cathode active material precursor satisfies this range, the layered crystal structure of the cathode active material is well developed, and a cathode active material with excellent capacity characteristics and structural stability can be produced.

[0101] Meanwhile, the calcination is performed at a temperature that allows the formation of monoparticles and / or quasi-monoparticles. To form monoparticles and / or quasi-monoparticles, calcination must be performed at a temperature higher than that used in the preparation of conventional secondary particle-form cathode active materials. For example, when the precursor composition is the same, calcination must be performed at a temperature 30°C to 100°C higher than that used in the preparation of conventional secondary particle-form cathode active materials. The calcination temperature for the formation of monoparticles and / or quasi-monoparticles may vary depending on the metal composition of the precursor. For example, when a high-nickel (High-Ni) lithium nickel-based oxide having a nickel (Ni) content of 80 mol% or more is formed as monoparticles and / or quasi-monoparticles, the primary calcination temperature may be about 800°C to 1000°C, preferably 800°C to 950°C, and more preferably 800°C to 900°C. When the calcination temperature satisfies this range, monoparticles and / or quasi-monoparticles with excellent electrochemical properties can be produced. If the calcination temperature is less than 800°C, a positive electrode active material in the form of secondary particles is produced, and if the calcination temperature is more than 1000°C, the calcination is excessive, the layered crystal structure is not properly formed, and the electrochemical properties are deteriorated.

[0102] The calcination can be carried out in an oxygen atmosphere for 6 to 35 hours, preferably 6 to 20 hours, and more preferably 6 to 12 hours. When the calcination time is within this range, single particles and / or quasi-single particles can be formed. If the calcination time is too short, particle growth is insufficient, resulting in the formation of secondary particle-form lithium nickel oxide. If the calcination time is too long, a rock salt phase may form, deteriorating the electrochemical properties of the active material. In this specification, an oxygen atmosphere refers to an atmosphere containing sufficient oxygen for calcination, including the air atmosphere. It is particularly preferable to perform calcination in an atmosphere with an oxygen partial pressure higher than that of the air atmosphere.

[0103] Meanwhile, the Co-containing coating raw material may be, but is not limited to, cobalt hydroxide.

[0104] The method for mixing the single particles and / or quasi-single particles with the Co-containing coating raw material is not particularly limited, and various coating methods well known in the art, such as dry coating and wet coating, can be applied.

[0105] On the other hand, the heat treatment can be carried out, for example, at 300° C. to 800° C., preferably 600° C. to 750° C., for 5 hours to 20 hours, preferably 10 hours to 12 hours.

[0106] <Positive electrode> Next, the positive electrode according to the present invention will be described.

[0107] The positive electrode includes a positive electrode material according to the present invention. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer includes the positive electrode material according to the present invention.

[0108] The cathode material is as described above, so a detailed description thereof will be omitted, and only the remaining components will be described in detail below.

[0109] The positive electrode current collector may include a highly conductive metal, and is not particularly limited as long as it is easily adhered to the positive electrode active material layer and is non-reactive within the battery voltage range. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, heat-treated carbon, and aluminum or stainless steel whose surfaces are surface-treated with carbon, nickel, titanium, silver, etc. The positive electrode current collector typically has a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0110] The positive electrode active material layer may optionally contain a conductive material and a binder in addition to the positive electrode material, as required.

[0111] Here, the positive electrode material may be contained in a content 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 when contained in this content range, excellent capacity characteristics can be exhibited.

[0112] The conductive material is used to impart conductivity to the electrode. Any conductive material can be used without particular limitations, as long as it does not cause chemical changes in the battery and has electronic conductivity. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber, such as copper, nickel, aluminum, and silver; conductive tubes, such as carbon nanotubes; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials can be used alone or in combination. The conductive material can be present in an amount of 0.1 to 15 wt % based on the total weight of the positive electrode active material layer.

[0113] The binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, polymers in which hydrogen is substituted with Li, Na, or Ca, or various copolymers thereof. These may be used alone or in combination. The binder may be contained in an amount of 0.1 to 15% by weight based on the total weight of the positive electrode active material layer.

[0114] The positive electrode can be manufactured by a conventional method for manufacturing a positive electrode, except that the positive electrode material of the present invention is used. Specifically, the positive electrode can be manufactured by dissolving or dispersing the positive electrode material and, optionally, a binder, a conductive material, and a dispersant in a solvent to prepare a positive electrode slurry composition, which is then applied to a positive electrode current collector, followed by drying and rolling.

[0115] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), dimethylformamide (DMF), 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, binder, and dispersant, taking into consideration the coating thickness of the slurry and the manufacturing yield, and to provide a viscosity that allows excellent thickness uniformity during subsequent coating for manufacturing a positive electrode.

[0116] Alternatively, the positive electrode can be produced by casting the positive electrode slurry composition on a separate support, peeling the composition from the support, and laminating the resulting film on a positive electrode current collector.

[0117] <Electrochemical element> The present invention can provide an electrochemical device including the positive electrode. The electrochemical device can be, for example, a battery or a capacitor, and more specifically, a lithium secondary battery.

[0118] 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. Since the positive electrode is as described above, detailed description thereof will be omitted, and only the remaining components will be described in detail below.

[0119] The lithium secondary battery may further include a battery container that houses the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0120] In the lithium secondary battery, the negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0121] The negative electrode current collector may be any material that does not cause chemical changes in the battery and has high conductivity, and examples of such materials include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloys. The negative electrode current collector typically has a thickness of 3 μm to 500 μm. As with the positive electrode current collector, the current collector may have fine irregularities on its surface to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0122] The negative electrode active material layer may optionally contain a binder and a conductive material in addition to the negative electrode active material.

[0123] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples thereof 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. βExamples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide (0<β<2); and composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. These may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material. The carbon material may be either low-crystalline carbon or high-crystalline carbon. Typical low-crystalline carbons are soft carbon and hard carbon, while typical high-crystalline carbons are 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.

[0124] The negative electrode active material may be contained in an amount of 80% by weight to 99% by weight based on the total weight of the negative electrode active material layer.

[0125] The binder is a component that helps bind the conductive material, active material, and current collector together, and is typically added in an amount of 0.1 to 10% by weight 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, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0126] 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. The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity. Examples of the 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; carbon fluoride; metal powder such as 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.

[0127] The negative electrode active material layer can be manufactured by coating a negative electrode slurry composition, which is 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 can be cast on a separate support, peeled from the support, and then laminating the resulting film on the negative electrode current collector.

[0128] 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. In particular, a separator that exhibits low resistance to electrolyte ion movement and excellent electrolyte humidification capability is preferred. 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 also be used, and can be selectively used in a single-layer or multi-layer structure.

[0129] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type 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.

[0130] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0131] 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 solvents that can be used 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 hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and 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 a high dielectric constant, which can improve 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.

[0132] The lithium salt can be any compound that can provide lithium ions used in lithium secondary batteries without any particular limitations. Specifically, the anion of the lithium salt can be F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - The lithium salt may be at least one selected from the group consisting of: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO2, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The concentration of the lithium salt is preferably within 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, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0133] As described above, the lithium secondary battery including the cathode active material according to the present invention exhibits excellent capacity and life characteristics and can be usefully used in various fields such as portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles.

[0134] Hereinafter, the present invention will be described in detail with reference to examples. However, the examples according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the examples detailed below. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0135] [Manufacturing Example 1] An aqueous transition metal solution was prepared by mixing NiSO4, CoSO4, and MnSO4 in distilled water in amounts such that the molar ratio of nickel:cobalt:manganese was 8:1:1.

[0136] Next, deionized water was added to the reactor, and then nitrogen gas was purged into the reactor to remove dissolved oxygen in the water. NaOH was then added to maintain the pH of the reactor at 11.

[0137] Next, a coprecipitation reaction was carried out for 30 hours under the conditions of a reaction temperature of 50°C, pH 11, and a stirring speed of 400 rpm while adding a transition metal aqueous solution, a NaOH aqueous solution, and an NH4OH aqueous solution to the reactor, and the average particle size (D 50 ) is 12 μm. 0.8 Co 0.1 Mn 0.1 (OH)2 was prepared, where the molar ratio of transition metal ions to NH4OH was 1:1.10.

[0138] The precursor for the positive electrode active material and LiOH were mixed so that the molar ratio of Li:transition metal (Ni+Co+Mn) was 1.02:1, and the mixture was fired at 730°C for 20 hours, washed with water, and dried to prepare a lithium composite transition metal oxide.

[0139] A scanning electron microscope (SEM) image of a cross section of the lithium composite transition metal oxide prepared as described above is shown in Figure 1. Referring to Figure 1, it can be seen that the lithium composite transition metal oxide prepared by the method described above is in the form of secondary particles in which multiple grains are aggregated, and that the major axes of the grains are aligned from the center of the secondary particle toward the surface.

[0140] [Manufacturing Example 2] Co(NO3)2·6H2O was dissolved in deionized water (DI water) to prepare a Co-containing coating solution.

[0141] The lithium composite transition metal oxide prepared in Preparation Example 1 was added to the coating solution, stirred at 500 rpm for 3 hours, and then heat-treated at 600° C. to prepare a positive electrode active material.

[0142] [Manufacturing Example 3] A lithium transition metal composite oxide was produced in the same manner as in Production Example 1, except that when producing the precursor for the positive electrode active material, a transition metal aqueous solution, a NaOH aqueous solution, and an NH4OH aqueous solution were added so that the molar ratio of transition metal to NH4OH was 1:0.8, and a coprecipitation reaction was carried out for 30 hours under conditions of a reaction temperature of 60°C, pH 11, and a stirring speed of 300 rpm, and calcination was carried out at 750°C.

[0143] A scanning electron microscope (SEM) image of a cross section of the lithium composite transition metal oxide prepared as described above is shown in Figure 2. Referring to Figure 2, it can be seen that the lithium composite transition metal oxide prepared by the method described above is in the form of secondary particles in which multiple grains are aggregated, and the grains are randomly arranged without any particular direction.

[0144] Co(NO3)2·6H2O was dissolved in deionized water (DI water) to prepare a Co-containing coating solution, and the lithium transition metal composite oxide prepared above was added to the coating solution. The solution was stirred at 500 rpm for 3 hours and then heat-treated at 600°C to prepare a cathode active material.

[0145] [Manufacturing Example 4] Transition metal precursor Ni 0.83 Co 0.11 Mn 0.06(OH)2 and LiOH·H2O were mixed so that the weight ratio of transition metals (Ni+Co+Mn):Li was 1:1.03, and then the mixture was fired at 900°C for 10 hours and milled to produce a nickel-lithium composite transition metal oxide.

[0146] A scanning electron microscope (SEM) image of the nickel-based lithium composite transition metal oxide prepared by the above method is shown in Figure 3. Referring to Figure 3, it can be seen that the lithium composite transition metal oxide prepared by the above method has a single particle and / or quasi-single particle morphology.

[0147] [Manufacturing Example 5] The nickel-based lithium composite transition metal oxide prepared in Preparation Example 4 and Co(OH)2 were mixed in a weight ratio of 100:0.2 and heat-treated at 700°C for 10 hours to prepare a positive electrode active material having a cobalt coating layer.

[0148] [Example] The positive electrode active material prepared in Preparation Example 2 and the positive electrode active material prepared in Preparation Example 5 were mixed in a weight ratio of 6:4 to prepare a positive electrode material.

[0149] [Comparative Example 1] The positive electrode active material prepared in Preparation Example 3 and the positive electrode active material prepared in Preparation Example 5 were mixed in a weight ratio of 6:4 to prepare a positive electrode material.

[0150] Comparative Example 2 The positive electrode active material prepared in Preparation Example 2 and the positive electrode active material prepared in Preparation Example 4 were mixed in a weight ratio of 6:4 to prepare a positive electrode material.

[0151] Comparative Example 3 The positive electrode active material prepared in Preparation Example 1 and the positive electrode active material prepared in Preparation Example 5 were mixed in a weight ratio of 6:4 to prepare a positive electrode material.

[0152] <Manufacturing lithium secondary batteries> The cathode materials prepared in Examples and Comparative Examples 1 to 3, conductive material (Denka Black), and binder (PVDF) were mixed in a weight ratio of 96:2:2 in N-methyl-2-pyrrolidone (NMP) solvent to prepare cathode slurry. The cathode slurry was applied to one side of an aluminum current collector, dried, and then rolled to prepare cathodes.

[0153] Next, the negative electrode active material (natural graphite), conductive material (carbon black), and binder (PVDF) were mixed in a weight ratio of 96:2:2 in N-methylpyrrolidone solvent to prepare a negative electrode slurry. The negative electrode slurry composition was applied to a copper current collector, dried, and then rolled to prepare a negative electrode.

[0154] An electrode assembly was fabricated by interposing a separator between the positive and negative electrodes, and then placed inside a battery case. An electrolyte solution was then injected to fabricate a lithium secondary battery. Here, the electrolyte solution used was a 1.0 M LiPF6 solution dissolved in an organic solvent containing ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7.

[0155] [Experimental example 1: High temperature life characteristics] Each of the lithium secondary batteries prepared above was charged at 45°C at a constant current of 1.0 C up to 4.2 V, and then discharged at a constant current of 0.5 C down to 2.5 V, and the capacity retention rate was measured after 50 charge-discharge cycles. The measurement results are shown in Table 1 below.

[0156] [Experimental Example 2: Gas generation during high temperature storage] Each of the lithium secondary batteries prepared above was charged to an SOC of 100 and then stored at 60°C for 8 weeks. The batteries were then pierced in a vacuum chamber to evacuate the gas inside the batteries and collect it in the vacuum chamber. The gas generated in the chamber was analyzed using a gas chromatography-flame ionization detector (GC-FID). The ratio of the gas generated by the lithium secondary batteries using the cathode materials of Comparative Examples 1 to 3 to the gas generated by the lithium secondary batteries using the cathode materials of the Examples is shown in Table 1 below.

[0157] [Experimental Example 3: Resistance Evaluation] Each of the lithium secondary batteries prepared above was charged and discharged for one cycle at 25°C under the condition of 0.2 / 0.2C, and then in the next cycle, a current of 0.2C was applied to charge and discharge the batteries to SOC 50 and SOC 10 based on the discharge capacity. Then, a 1C pulse was applied for 10 seconds to measure the DCIR resistance. The measurement results are shown in Table 1.

[0158] [Experimental Example 4: Evaluation of cracking rate of secondary particles] Three grams of each of the cathode material powders from Example and Comparative Examples 1 to 3 were collected, and a volume cumulative particle size distribution graph was measured. The collected cathode material powders were then placed in a holder with a diameter of 1.3 cm, and a pressure of 6 tons was applied, after which the volume cumulative particle size distribution graph was measured again. In the volume cumulative particle size distribution graph before pressure application, the range of particle sizes larger than the particle size at the point where dY / dX (where X is particle size and Y is volume) is minimum was defined as the secondary particle region, and the value calculated in the secondary particle region using the following mathematical formula was evaluated as the secondary particle cracking rate.

[0159]

number

[0160] In the above mathematical formula, X0 and Y0 are the particle size and volume in the volume cumulative particle size distribution graph before pressing, respectively, and X p and Yp is the particle size and volume in the volume cumulative particle size distribution graph after 6 ton pressure.

[0161] [Table 1]

[0162] Referring to Table 1, it can be seen that the cathode materials of Examples had less particle cracking after pressure application than the cathode materials of Comparative Examples 1 to 3, and that the lithium secondary batteries employing the cathode materials of Examples had better high-temperature life characteristics and less gas generation after high-temperature storage, thereby exhibiting better high-temperature characteristics, compared to the lithium secondary batteries employing the cathode materials of Comparative Examples 1 to 3.

Claims

1. a first positive electrode active material in the form of secondary particles formed by aggregation of a plurality of grains, at least a portion of the secondary particles having an oriented structure in which the major axes of the grains are aligned from the center toward the surface of the secondary particles, and a cobalt concentration at grain boundaries, which are boundaries between the grains, is higher than a cobalt concentration inside the grains; A positive electrode material comprising: a core having the form of at least one of a single particle consisting of one nodule and a quasi-single particle which is a composite of 30 or less nodules; and a second positive electrode active material formed on the core and including a coating layer containing cobalt.

2. the cathode material has a bimodal particle size distribution; The cathode material according to claim 1 , wherein the particle size of the first cathode active material is larger than the particle size of the second cathode active material.

3. D of the first positive electrode active material ５０ is 8 μm or more and 20 μm or less, D of the second positive electrode active material ５０ The cathode material according to claim 1, wherein the average particle size is 2 μm or more and 7 μm or less.

4. The positive electrode material according to claim 1 , wherein the first positive electrode active material and the second positive electrode active material each independently contain a nickel-based lithium composite transition metal oxide represented by the following [Chemical Formula 1]: [Chemical formula 1] Li ｘ [Ni ａ Co ｂ M １ ｃ M ２ ｄ ]O ２－ｙ A ｙ In the above [Chemical Formula 1], Said M １ is one or more elements selected from the group consisting of Mn and Al, Said M ２ is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; A is one or more elements selected from the group consisting of F, Cl, Br, I, At, and S, 98≦x≦1.20, 0<a<1, 0<b<1, 0<c<1, 0≦d≦0.2, 0≦y≦0.

2.

5. The cathode material according to claim 1 , wherein the first cathode active material and the second cathode active material are contained in a weight ratio of 90:10 to 50:

50.

6. The positive electrode material according to claim 1, wherein the first positive electrode active material comprises a nickel-based lithium composite transition metal oxide represented by the following [Chemical Formula 1-1]: [Chemical formula 1-1] Li ｘ１ [Ni ａ１ Co ｂ１ M １ ｃ１ M ２ ｄ１ ]O ２－ｙ１ A ｙ１ In the above [Chemical Formula 1-1], Said M １ is one or more elements selected from the group consisting of Mn and Al, Said M ２ is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; A is one or more elements selected from the group consisting of F, Cl, Br, I, At, and S, 98≦x1≦1.20, 0.6≦a1≦0.9, 0.01≦b1<0.4, 0.01≦c1<0.4, 0≦d1≦0.2, 0≦y1≦0.

2.

7. 2. The cathode material according to claim 1, wherein in the first cathode active material, the long axes of the grains arranged from the center of the secondary particles toward the surface form an angle of −15° to 15° between the long axes and an a-axis direction of the crystal structure.

8. 2 . The cathode material according to claim 1 , wherein in the first cathode active material, the grains whose major axes are aligned from the center of the secondary particle toward the surface have an aspect ratio of 1.5 or more and 15 or less.

9. 2. The cathode material according to claim 1, wherein the first cathode active material comprises a core portion in which grains are randomly aggregated, and a shell portion formed outside the core portion and in which grains are arranged in an oriented structure.

10. The cathode material according to claim 9 , wherein the grains in the core portion have an aspect ratio of 0.8 or more and 1.2 or less.

11. The cathode material according to claim 9 , wherein the grains in the shell portion have an aspect ratio of 1.5 or more and 15 or less.

12. The cathode material according to claim 1 , wherein the average particle size of the grains of the first cathode active material is 0.05 μm or more and 4 μm or less.

13. The positive electrode material according to claim 1, wherein a central portion of the second positive electrode active material comprises a nickel-based lithium composite transition metal oxide represented by the following [Chemical Formula 1-2]: [Chemical formula 1-2] Li ｘ２ [Ni ａ２ Co ｂ２ M １ ｃ２ M ２ ｄ２ ]O ２－ｙ２ A ｙ２ In the above [Chemical Formula 1-2], Said M １ is one or more elements selected from the group consisting of Mn and Al, Said M ２ is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; A is one or more elements selected from the group consisting of F, Cl, Br, I, At, and S, 0.8≦x2≦1.20, 0.8≦a2<1, 0<b2<0.2, 0.01≦c2<0.2, 0≦d2≦0.2, 0≦y2≦0.

2.

14. The cathode material according to claim 1 , wherein the coating layer of the second cathode active material has a composition represented by the following [Chemical Formula 2]: [Chemical formula 2] Li ｚ Co １－ｗ M ３ ｗ O ２ In the above [Chemical Formula 2], the M ３ is one or more elements selected from the group consisting of Ni, Mn, Al, W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and 0.8≦z≦1.2, 0≦w≦0.

2.

15. The cathode material according to claim 1 , wherein the average particle size of the nodules of the second cathode active material is 0.5 μm or more and 3.5 μm or less.

16. The cathode material according to claim 9 , wherein the grains in the shell portion are rod-shaped.

17. A positive electrode comprising the positive electrode material of any one of claims 1 to 16.

18. A lithium secondary battery comprising the positive electrode according to claim 17.

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