Positive electrode active material, and positive electrode and lithium secondary battery comprising same

Abstract

The present invention relates to a positive electrode active material comprising a lithium transition metal oxide in a single particle form, and a positive electrode and a lithium secondary battery comprising the same, and relates to a single particle type positive electrode active material, wherein the lithium transition metal oxide in the form of a single particle comprises an outer boundary forming an outer contour of the particle and an inner boundary formed in the particle, and 0 lt is satisfied; the length of the inner boundary / the length lt of the outer boundary; and 0.4 part.

Description

Technical Field

[0001] Cross-references to related applications This application claims priority to Korean Patent Application No. 10-2023-0071877, filed on June 2, 2023, the disclosure of which is incorporated herein by reference. Technical Field

[0002] This invention relates to a positive electrode active material for lithium secondary batteries, as well as a positive electrode and a lithium secondary battery containing the same. Background Technology

[0003] Recently, with the development of electric vehicle technology, the demand for high-capacity rechargeable batteries is increasing. Correspondingly, research is being actively conducted on cathodes using high-nickel (high-Ni) cathode active materials with excellent capacity characteristics.

[0004] Because high-nickel cathode active materials are prepared using a co-precipitation method, the prepared high-nickel cathode active materials have a secondary particle form in which primary particles are aggregated. However, the secondary particle form of the active material leads to side reactions due to microcracks generated in the secondary particles during long-term charge-discharge processes. Furthermore, when increasing the electrode density to improve energy density, a disadvantage of secondary particles is that the secondary particle structure collapses, resulting in a decrease in lifetime characteristics and energy density due to the reduction of active material and electrolyte.

[0005] To address these issues with high-nickel cathode active materials in the form of secondary particles, single-crystal nickel-based cathode active materials have recently been developed. The advantage of single-crystal nickel-based cathode active materials is that the particles do not collapse even when electrode density is increased for higher energy density. However, because the preparation of single-crystal nickel-based cathode active materials requires relatively high sintering temperatures, a phase transformation occurs as the R-3m layered structure cannot be properly maintained and lithium leaves the crystal structure, resulting in an Fm-3m rock salt structure such as NiO. Furthermore, as the crystallinity of the cathode active material decreases, the proportion of NiO on the surface of the prepared single-crystal particles increases. Therefore, the resistance increases with increasing NiO content, and energy density and output decrease. Moreover, when the sintering temperature is lowered, the presence of over-sintered secondary particles means that the improvement in lifetime and gas generation does not reach the levels desired by single particles.

[0006] Therefore, there is still a need to develop a positive electrode active material that exhibits excellent lifetime and output characteristics while having high electrode density.

[0007] [Existing Technical Documents] [Patent Literature] (Patent Document 1) KR 2019-0094529 A1 Summary of the Invention

[0008] Technical issues One aspect of the present invention provides a positive electrode active material with excellent lifetime characteristics.

[0009] Technical solution To address the aforementioned problems, this invention provides a positive electrode active material, a positive electrode, and a lithium secondary battery.

[0010] (1) The present invention provides a positive electrode active material, wherein the positive electrode active material comprises lithium transition metal oxide in the form of single particles. The single-particle form of lithium transition metal oxide includes an outer boundary forming the outer contour of the particle and an inner boundary forming within the particle, and satisfies mathematical equation 1. [Mathematical Expression 1] 0 < length of inner boundary / length of outer boundary < 0.4 The length of the internal boundary is obtained by subtracting the length of the boundary measured from the EBSD band contrast map from the length of the boundary measured from the electron backscatter diffraction (EBSD)-inverse pole figure (IPF) of lithium transition metal oxide particles. The length of the outer boundary is the length of the outer boundary of the transition metal oxide particle measured by scanning electron microscopy (SEM) image segmentation.

[0011] (2) The present invention provides the positive electrode active material described in (1) above, wherein the lithium transition metal oxide in the form of single particles further satisfies mathematical formula 2.

[0012] [Mathematical Expression 2] The length of the boundary measured from the EBSD IPF map / the length of the boundary measured from the EBSD band contrast map ≤ 1.4 The length of the outer boundary is the length of the outer boundary of the transition metal oxide particles measured by SEM image segmentation.

[0013] (3) The present invention provides the positive electrode active material described in (1) or (2) above, wherein the lithium transition metal oxide in single-particle form further satisfies mathematical formula 3.

[0014] [Mathematical Expression 3] 0 ≤ (Length of inner boundary / Length of outer boundary) - (Length of inner boundary / Length of boundary measured from EBSD with contrast map) ≤ 1 (4) The present invention provides a positive electrode active material according to any one of (1) to (3) above, wherein the length of the boundary measured from the electron backscatter diffraction (EBSD)-IPF pattern includes: the length of the weak boundary contained in the lithium transition metal oxide particles in the form of single particles, which differs only in atomic arrangement and whose crystallinity has not collapsed, and the length of the strong boundary formed by the collapse of the layered structure.

[0015] (5) The present invention provides a positive electrode active material according to any one of (1) to (4) above, wherein the length of the boundary measured by EBSD band contrast image includes: the length of the strong boundary contained in the lithium transition metal oxide particles in the form of single particles, formed by the collapse of the layered structure.

[0016] (6) The present invention provides a positive electrode active material according to any one of (1) to (5) above, wherein the lithium transition metal oxide in single-particle form contains 2 to 50 particles.

[0017] (7) The present invention provides a positive electrode active material according to any one of (1) to (6) above, wherein the lithium transition metal oxide is a lithium composite transition metal oxide containing nickel, cobalt and manganese.

[0018] (8) The present invention provides a positive electrode active material according to any one of (1) to (7) above, wherein the lithium transition metal oxide is a lithium composite transition metal oxide represented by chemical formula 1: [Chemical Formula 1] Li a Ni b Co c Mn d M 1 e O2 In chemical formula 1, M 1 It is selected from at least one of aluminum (Al), zirconium (Zr), boron (B), tungsten (W), molybdenum (Mo), chromium (Cr), niobium (Nb), magnesium (Mg), hafnium (Hf), tantalum (Ta), lanthanum (La), titanium (Ti), strontium (Sr), barium (Ba), cerium (Ce), tin (Sn), yttrium (Y), zinc (Zn), fluorine (F), phosphorus (P), and sulfur (S), and 0.9≤a≤1.1, 0.6≤b<1, 0 <c<0.4,0<d<0.4,0≤e<0.1,b+c+d+e=1。

[0019] (9) The present invention provides a positive electrode active material according to any one of (1) to (8) above, wherein the lithium transition metal oxide is a lithium composite transition metal oxide represented by chemical formula 2: [Chemical Formula 2] Lia Ni b Co c Mn d O2 In chemical formula 2, 0.9≤a≤1.1, 0.6≤b<1, 0 <c<0.4,0<d<0.4,b+c+d=1。

[0020] (10) The present invention provides a positive electrode active material according to any one of (1) to (9) above, wherein the positive electrode active material further comprises having a smaller average particle size (D) than that of a single-particle lithium transition metal oxide. 50 It is a second lithium transition metal oxide in single-particle form and has a bimodal particle size distribution.

[0021] (11) The present invention provides the positive electrode active material described in (10) above, wherein the second lithium transition metal oxide is a lithium composite transition metal oxide represented by chemical formula 3.

[0022] [Chemical Formula 3] Li a3 Ni b3 Co c3 Mn d3 M 3 e3 O2 In chemical formula 3, M 3 It is selected from at least one of Al, Zr, B, W, Mo, Cr, Nb, Mg, Hf, Ta, La, Ti, Sr, Ba, Ce, Sn, Y, Zn, F, P, and S, and 0.9≤a³≤1.1, 0.6≤b³<1, 0 <c3<0.4,0<d3<0.4,0≤e3<0.1,b3+c3+d3+e3=1。

[0023] (12) The present invention provides the positive electrode active material described in (10) or (11) above, wherein the weight ratio of lithium transition metal oxide to second lithium transition metal oxide is in the range of 1:1 to 9:1.

[0024] (13) The present invention provides a positive electrode active material according to any one of (10) to (12) above, wherein the press density is 3.50 g / cm³. 2 Up to 3.90 g / cm 2 Within the range.

[0025] (14) The present invention provides a positive electrode comprising any one of the positive electrode active materials described in (1) to (13) above.

[0026] (15) The present invention provides a lithium secondary battery, wherein the lithium secondary battery comprises the positive electrode described in (14) above.

[0027] Beneficial effects The positive electrode active material of the present invention is a positive electrode active material comprising lithium transition metal oxide in the form of single particles, wherein, since the lithium transition metal oxide in the form of single particles contains a certain amount of internal boundaries, it can exhibit excellent lifetime characteristics. Attached Figure Description

[0028] Figure 1 The images show scanning electron microscope (SEM) images, electron backscatter diffraction (EBSD) band comparison diagrams, EBSD-inverse pole figure (IPF) diagrams, and IPF boundaries of the positive electrode active material of Example 1.

[0029] Figure 2 The images show SEM images, EBSD band comparison images, EBSD-IPF images, and IPF boundaries of the positive electrode active material in Comparative Example 4. Detailed Implementation

[0030] The invention will be described in more detail below to allow for a clearer understanding of it.

[0031] It should be understood that the words or terms used in the specification and claims should not be interpreted as having the meaning defined in a common dictionary, but should be further understood as having a meaning consistent with its meaning in the context of the relevant technical field and the technical spirit of the invention, based on the principle that the inventor can appropriately define the meaning of the words or terms to best interpret the invention.

[0032] In this invention, the term "primary particle" refers to the smallest particle unit that is classified as a block when the cross-section of the positive electrode active material is observed by a scanning electron microscope (SEM), which may consist of multiple grains.

[0033] In this invention, the term "secondary particle" refers to a secondary structure formed by the aggregation of multiple primary particles. The average particle size of the secondary particles can be measured using a particle size analyzer.

[0034] In this invention, the term "single-particle form" can be used interchangeably with "single-crystallinity" or "single-particle type," referring to a form contrasted with the secondary-particle form formed by the aggregation of hundreds of primary particles prepared by conventional methods. Furthermore, the terms "single-particle type cathode active material" or "single-particle form lithium transition metal oxide" in this invention are concepts contrasted with the secondary-particle form of cathode active materials formed by the aggregation of hundreds of primary particles prepared by conventional methods, referring to cathode active materials or lithium transition metal active materials composed of 50 or fewer single-crystal particles.

[0035] Generally, a single-crystal particle refers to a particle in which the entire sample consists of only one grain or grain region. The single-particle type cathode active material or single-particle form of lithium transition metal oxide in this invention refers to a single-particle type cathode active material or single-particle form of lithium transition metal oxide containing 1 to 50 particles, specifically 1 to 30 particles, which can exhibit properties similar to those of a single-crystal particle by including a small number of single-crystal particles. "Single-crystal particle" refers to the smallest particle unit identified when observing the cathode active material using a scanning electron microscope, and "grain" or "grain region" refers to a region in which atoms in the sample are continuously and periodically arranged in one direction. Grains can be analyzed by obtaining EBSD IPF (inverse pole figure) diagrams using an electron backscatter diffraction (EBSD) analyzer.

[0036] The term "average particle size (D)" in this invention 50 ")" indicates the particle size at 50% of the volumetric cumulative distribution based on particle size. After dispersing the target powder in a dispersion medium, the dispersion medium is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). The particle size distribution is calculated by measuring the difference in the diffraction pattern caused by particle size as particles pass through the laser beam, and D 50 The particle size can be measured by using this measuring instrument to calculate the particle size at 50% of the volumetric cumulative distribution based on the particle size.

[0037] The positive electrode active material of the present invention is a positive electrode active material comprising a lithium transition metal oxide in the form of a single particle, wherein the lithium transition metal oxide in the form of a single particle comprises an outer boundary forming the outer contour of the particle and an inner boundary formed in the particle, and satisfies the following mathematical formula 1.

[0038] [Mathematical Expression 1] 0 < length of inner boundary / length of outer boundary < 0.4 The length of the internal boundary is obtained by subtracting the length of the boundary measured from the EBSD band contrast (BC) plot from the length of the boundary measured from the electron backscatter diffraction (EBSD)-inverse pole figure (IPF) plot of lithium transition metal oxide particles. The length of the outer boundary is the length of the outer boundary of the transition metal oxide particles measured by SEM image segmentation.

[0039] In this invention, the internal boundary can refer to the boundary formed by the contact between grains contained in a single-particle lithium transition metal oxide particle. Because lithium ions readily move at the internal boundary (the boundary formed by the contact between grains), degradation preferentially occurs with charge / discharge cycles, and lifetime characteristics may decrease due to the presence of numerous easily accessible migration paths for lithium ions. The amount of internal boundary can be quantitatively determined by the length of the internal boundary contained within the single-particle lithium transition metal oxide particle.

[0040] If an EBSD IPF map of a single-particle lithium transition metal oxide particle is obtained using the EBSD IPF measurement method, the grains contained within that single-particle lithium transition metal oxide particle can be distinguished. Therefore, the boundary formed by the contact between one grain and another within a single-particle lithium transition metal oxide particle can be identified from the EBSD IPF map.

[0041] However, the boundaries identified by the EBSD IPF map include not only grain boundaries where the atomic orientation changes only according to the boundary (weak boundaries), but also boundaries where the layered structure of the transition metal oxide layer collapses along with the change in atomic orientation (especially the layered structure collapse of the NiO layer) (strong boundaries). Therefore, in order to obtain only the grain boundaries, it is necessary to exclude the boundaries where the layered structure of the transition metal oxide layer collapses.

[0042] If a BC diagram of a single-particle lithium transition metal oxide particle is obtained using EBSD, it is impossible to distinguish weak boundaries within single-particle lithium transition metal oxide particles that differ only in atomic arrangement and whose crystallinity has not collapsed. However, it is possible to distinguish strong boundaries where the layered structure of the transition metal oxide has collapsed. These collapsed boundaries can be the boundaries formed between one single crystal particle and another within a single-particle lithium transition metal oxide particle.

[0043] The length of the boundary measured from the EBSD IPF plot can include: the length of the boundary between grains contained in a single-particle lithium transition metal oxide particle, the length of the boundary formed by the collapse of the layered structure, and the length of the outer boundary of the particle.

[0044] Furthermore, the length of the boundary measured from the EBSD BC diagram can include: the length of the outer boundary, and the length of the boundary formed by the collapse of the layered structure contained in the lithium transition metal oxide particles in single-particle form.

[0045] Therefore, the length of the inner boundary is obtained by subtracting the length of the boundary measured from the EBSD BC diagram from the length of the boundary measured from the EBSD IPF diagram of the lithium transition metal oxide particle. Since the length of the outer contour (outer boundary) of the lithium transition metal oxide particle in single-particle form measured in the EBSD IPF diagram and the EBSD BC diagram is the same, the length of the inner boundary can be expressed as follows when the length of the outer boundary is excluded.

[0046] Length of internal boundary =[Length of boundary measured from EBSD IPF plot] - [Length of boundary measured from EBSD BC plot] =[Length of strong boundary + Length of weak boundary] - [Length of strong boundary] =Length of the weak boundary Furthermore, to determine the length of the outer boundary of a single-particle lithium transition metal oxide particle, an SEM image of the single-particle lithium transition metal oxide particle is obtained, and the length of the outer boundary forming the outer surface of the lithium transition metal oxide particle can be obtained by segmenting the SEM image.

[0047] The above mathematical formula 1 is the value obtained by dividing the length of the inner boundary by the length of the outer boundary, which quantifies the extent to which the inner boundary of a single-particle lithium transition metal oxide particle is contained.

[0048] The decrease in the value obtained by dividing the length of the line formed inside a circular or similar shaped object by the length of the object's outer contour, calculated using two-dimensional images obtained by EBSD and SEM, reflects the decrease in the length of the weak boundary inside the single-particle lithium transition metal oxide particle. For the single-particle positive electrode active material of the present invention, the length of the inner boundary / the length of the outer boundary in Formula 1 is less than 0.4. Since the single-particle positive electrode active material satisfies the above value, it can exhibit excellent lifetime characteristics by reducing the phenomenon that preferentially occurs in the movement path through which lithium ions can easily move.

[0049] The value of the ratio of the inner boundary length to the outer boundary length in Equation 1 can specifically satisfy a value greater than 0 and equal to or less than 0.3, more specifically greater than 0.01 and equal to or less than 0.2. If the value of Equation 1 is too large, the improvement effect on the lifetime characteristics of the single-particle positive electrode active material may decrease. Conversely, if the value of Equation 1 is 0, the improvement effect on the capacity and lifetime characteristics of the single-particle positive electrode active material may decrease.

[0050] Therefore, when the lithium transition metal oxide particles in the single-particle form of the present invention satisfy the value of mathematical formula 1, they can exhibit a better effect in improving lifetime characteristics.

[0051] The degree of single-crystal formation of lithium transition metal oxide particles in single-particle form can be determined by the value of mathematical formula 1. For example, when the lithium transition metal oxide particles in single-particle form are single crystals, the value of mathematical formula 1 is 0, and when the lithium transition metal oxide particles in single-particle form have a low degree of single-crystal formation by containing a large number of crystals, the value of mathematical formula 1 increases.

[0052] In addition, lithium transition metal oxides in single-particle form can also satisfy the following mathematical formula 2.

[0053] [Mathematical Expression 2] The length of the boundary measured from the EBSD IPF map / the length of the boundary measured from the EBSD band contrast map ≤ 1.4 Formula 2 is obtained by dividing the sum of the lengths of the strong boundaries and the weak boundaries measured from the EBSD IPF map by the length of the strong boundaries measured from the EBSD BC map.

[0054] The value of Equation 2 decreases as the proportion of the length of weak boundaries, in which the boundaries of lithium transition metal oxides in single-particle form differ only in atomic arrangement and whose crystallinity has not collapsed, decreases, or the value of Equation 2 decreases as the proportion of strong boundaries formed by the collapse of layered structures increases relative to weak boundaries.

[0055] For the positive electrode active material of the present invention, the length of the boundary measured from the EBSD IPF plot / the length of the boundary measured from the EBSD band contrast plot in Formula 2 can specifically satisfy the range of 1 to 1.4, more specifically 1 to 1.3.

[0056] When the value of mathematical formula 2 meets the above range, it can appropriately exhibit excellent lifetime characteristics. When the value of mathematical formula 2 is too large, the lifetime characteristics of the positive electrode active material may decrease.

[0057] Therefore, when the single-particle form of the lithium transition metal oxide of the present invention satisfies the value of mathematical formula 1 while the value of mathematical formula 2 satisfies the above-mentioned range, it can exhibit better lifetime characteristics.

[0058] The length of the outer boundary is the length of the outer boundary of the transition metal oxide particles measured by SEM image segmentation.

[0059] To determine the length of the outer boundary of a single-particle lithium transition metal oxide particle, an SEM image of the single-particle lithium transition metal oxide particle is obtained, and the length of the outer boundary forming the outer surface of the lithium transition metal oxide particle can be obtained by segmenting the SEM image.

[0060] Furthermore, the positive electrode active material of the present invention can satisfy the following mathematical formula 3, which is the difference between the value of mathematical formula 2 and the value of mathematical formula 1.

[0061] [Mathematical Expression 3] 0 ≤ (Length of inner boundary / Length of outer boundary) - (Length of inner boundary / Length of boundary measured from EBSD with contrast map) ≤ 1 For the positive electrode active material of the present invention, mathematical formula 3 can specifically satisfy the range of 0 to 0.8, 0 to 0.7, or 0 to 0.5, more specifically 0 to 0.3. When mathematical formula 3 satisfies the above range, it can exhibit excellent lifetime characteristics while also exhibiting appropriate capacity expression.

[0062] In a positive electrode active material according to one embodiment of the present invention, the average particle size (D) of the particles contained in the positive electrode active material is... 50 The average particle size can be from 0.1 μm to 10 μm. When the average particle size of the particles contained in the positive electrode active material meets the above range, it may have advantages in terms of rolling yield or electrode porosity when they agglomerate to form single-particle positive electrode active material or single-particle lithium transition metal oxide. However, when the average particle size is less than or greater than the above range, the performance in terms of electrode capacity, lifetime characteristics or resistance may deteriorate.

[0063] In the positive electrode active material according to one embodiment of the present invention, the lithium transition metal oxide in single-particle form can be a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co) and manganese (Mn).

[0064] Specifically, lithium transition metal oxides in single-particle form can be lithium complex transition metal oxides represented by the following chemical formula 1.

[0065] [Chemical Formula 1] Li a Ni b Co c Mn d M 1 e O2 In chemical formula 1, M 1 It is selected from at least one of aluminum (Al), boron (B), barium (Ba), cerium (Ce), chromium (Cr), fluorine (F), magnesium (Mg), vanadium (V), titanium (Ti), iron (Fe), zirconium (Zr), zinc (Zn), silicon (Si), yttrium (Y), niobium (Nb), gallium (Ga), tin (Sn), molybdenum (Mo), tungsten (W), phosphorus (P), sulfur (S), strontium (Sr), tantalum (Ta), lanthanum (La), and hafnium (Hf), and 0.9≤a≤1.1, 0.6≤b<1, 0 <c<0.4,0<d<0.4,0≤e<0.1,b+c+d+e=1。

[0066] In addition, specifically, lithium transition metal oxides can be positive electrode active materials, which are lithium composite transition metal oxides represented by the following chemical formula 2.

[0067] [Chemical Formula 2] Li a Ni b Co c Mn d O2 In chemical formula 2, 0.9≤a≤1.1, 0.6≤b<1, 0 <c<0.4,0<d<0.4,b+c+d=1。

[0068] Considering the specific surface area and the density of the cathode material mixture, the average particle size (D) of the cathode active material 50 The particle size can range from 1 μm to 50 μm, with an average particle size (D). 50 Specifically, the average particle size can be from 2 μm to 20 μm. When the average particle size of the positive electrode active material meets the above range, it can have advantages in terms of rolling yield or electrode porosity, while when the average particle size is less than or greater than the above range, the performance in terms of electrode capacity, lifetime characteristics, or resistance may deteriorate.

[0069] In one embodiment of the present invention, the positive electrode active material may be composed of 2 to 50 particles, 2 to 40 particles, 2 to 30 particles, 2 to 20 particles, 2 to 15 particles, 2 to 10 particles, or 2 to 5 particles.

[0070] In one embodiment of the present invention, lithium transition metal oxide particles in single-particle form can be prepared by mixing a transition metal oxide precursor with a lithium raw material and performing a first sintering, and then crushing the pre-sintered product prepared by the first sintering and performing a second sintering.

[0071] Specifically, lithium transition metal oxide particles in single-particle form can be prepared by a method including the following steps: (A) A mixture is prepared by mixing a positive electrode active material precursor containing Ni, Co and Mn with a first lithium-containing raw material; (B) A one-time sintered product is prepared by sintering the mixture once at a temperature of 800°C to 950°C; and (C) A secondary sintered product is prepared by mixing a second lithium-containing raw material with a primary sintered product and performing a secondary sintering at a temperature of 680°C to 850°C.

[0072] If the mixture is sintered in one step at a temperature between 800℃ and 950℃, a single-particle sintered product is prepared simultaneously with the primary particle agglomeration of the positive electrode active material precursor. Specifically, the primary sintering temperature can be above 800℃, 810℃, 820℃, 830℃, 840℃, or 850℃, or below 900℃, 910℃, 920℃, 930℃, 940℃, or 950℃. When the primary sintering temperature is within the above range, a structurally stable single-particle sintered product is prepared simultaneously with the primary particle agglomeration of the positive electrode active material precursor. However, if the primary sintering temperature is below 800℃, there is a problem of insufficient primary particle agglomeration. If the primary sintering temperature is above 950℃, there is a problem of preparing a sintered product with unstable structure and low crystallinity.

[0073] To prevent lithium transition metal oxides from degenerating into rock salt structures, a single sintering process can be carried out in an oxygen atmosphere.

[0074] In terms of primary particle agglomeration and improving the crystallinity of primary sintering products, primary sintering can be carried out for 3 to 12 hours, specifically 3 hours, 4 hours, 5 hours, more than 6 hours, 9 hours, 10 hours, 11 hours or less than 12 hours.

[0075] If a primary sintering product is sintered a second time at a temperature between 680°C and 850°C, a secondary sintering product is prepared simultaneously with lithium intercalation into the primary sintering product. In this case, the secondary sintering product is a single-particle lithium composite transition metal oxide. The specific secondary sintering temperature can be 680°C, 700°C, 720°C, 740°C, 760°C, 780°C, above 800°C, 840°C, or below 850°C. When the secondary sintering temperature is within the above range, the layered structure is restored while lithium intercalates into the rock salt structure that can form on the surface of the primary sintering product due to the high temperature during primary sintering, and lithium byproducts are reduced. When the secondary sintering temperature is below 680°C, there is a problem of a low lithium intercalation rate due to the low temperature; when the secondary sintering temperature is above 850°C, there is a problem of the surface of the primary sintering product degenerating into a rock salt structure due to the high temperature, and lithium byproducts remaining.

[0076] Secondary sintering can be carried out in an oxygen atmosphere to prevent lithium transition metal oxides from degenerating into rock salt structures.

[0077] In terms of improving the crystallinity of the internal crystal structure of the positive electrode active material, the secondary sintering can be carried out for 3 to 12 hours, specifically 3 hours, 4 hours, 5 hours, 6 hours, more than 9 hours, 9 hours, 10 hours, 11 hours or less than 12 hours.

[0078] Furthermore, the method for preparing the positive electrode active material according to the present invention may further include a step (B') of grinding the primary sintering product before step (C). In order to prevent an increase in initial resistance, step (B') may grind the primary sintering product to reduce the average particle size (D). 50 The range is from 3 μm to 20 μm.

[0079] Furthermore, the method for preparing the positive electrode active material according to the present invention may further include a step (C') of grinding the secondary sintering product. Also in order to prevent an increase in initial resistance, step (C') may grind the secondary sintering product to reduce the average particle size (D). 50 The range is from 3 μm to 20 μm.

[0080] The grinding in steps (B') and (C') can be performed using a pin mill, an air classifier (ACM), or a jet mill. For a pin mill, grinding can be performed at 18,000 rpm; for an ACM, using equipment from Hosokawa Micron Corporation, classification can be performed at 6,000 rpm and main grinding at 12,000 rpm; and for a jet mill, using equipment from ZM Solution, classification can be performed at 3,500 rpm, and grinding can be performed at a pressure of 6 bar. Under these conditions, a particle size (D) with the desired average particle size can be easily obtained. 50 ) positive electrode active material.

[0081] The positive electrode active material according to the present invention is prepared by adding lithium-containing raw materials in two parts. That is, the lithium-containing raw materials can be added separately before the first sintering and before the second sintering. In this case, the advantage is that lithium intercalation can be formed in the rock salt structure on the surface, which is beneficial to the restoration of the layered structure. Where the lithium-containing raw materials are added all at once before the first sintering, there is a problem of decreased electrochemical performance due to the increase of lithium by-products, and if the lithium-containing raw materials are not added in the second sintering, there is a problem of requiring high temperature and long time due to low reaction rate.

[0082] When the lithium-containing raw material is added in two parts, in step (A), the first lithium-containing raw material can be mixed such that the ratio (M:Li) of the total number of moles of transition metals (M) contained in the cathode active material precursor to the number of moles of lithium (Li) contained in the first lithium-containing raw material is 1:0.98, 1:0.99, 1:1.00, 1:1.01, 1:1.02 or more, 1:1.04 or less, or 1:1.05. In step (C), the second lithium-containing raw material can be mixed such that the ratio (M:Li) of the total number of moles of transition metals (M) contained in the cathode active material precursor in step (A) to the number of moles of lithium (Li) contained in the second lithium-containing raw material is 1:0.01 or more, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09 or less, or 1:1.10.

[0083] Furthermore, the present invention provides a positive electrode active material, which further comprises a smaller average particle size (D0) than that of a single-particle lithium transition metal oxide (hereinafter referred to as the first lithium transition metal oxide). 50 It is a second lithium transition metal oxide in single-particle form and has a bimodal particle size distribution.

[0084] Because the positive electrode active material with a bimodal particle size distribution contains two lithium transition metal oxides of different sizes, with the second lithium transition metal oxide filling the spaces between the first lithium transition metal oxides, it achieves a high rolling density, allowing the desired electrode thickness to be achieved without applying high pressure. Furthermore, during the rolling process for electrode fabrication, particle breakage is prevented because the stress applied to the first lithium transition metal oxide is dispersed.

[0085] In the case where the positive electrode active material according to the invention has a bimodal particle size distribution, the average particle size (D) of the first lithium transition metal oxide is... 50 The average particle size (D) of the first lithium transition metal oxide can range from 5 μm to 9 μm. Specifically, the average particle size (D) of the first lithium transition metal oxide... 50 The particle size can be 5 μm, 5.5 μm, 6 μm, or larger than 6.5 μm, or smaller than 7.5 μm, 8 μm, 8.5 μm, or 9 μm. Additionally, the average particle size (D) of the second lithium transition metal oxide... 50 The average particle size (D) of the second lithium transition metal oxide can range from 1 μm to 4 μm. Specifically, the average particle size (D) of the second lithium transition metal oxide... 50 The size can be 1 μm, 1.5 μm, 2 μm, or larger than 2.5 μm, or smaller than 2.5 μm, 3 μm, 3.5 μm, or 4 μm.

[0086] The average particle size (D) of the first lithium transition metal oxide and the second lithium transition metal oxide in them 50Within the aforementioned range, the filling rate can be excellent because the second lithium transition metal oxide is appropriately distributed among the first lithium transition metal oxides.

[0087] The ratio of the average particle size of the first lithium transition metal oxide to the average particle size of the second lithium transition metal oxide can be in the range of 2:1 to 5:1. When the ratio of the average particle size of the first lithium transition metal oxide to the average particle size of the second lithium transition metal oxide is within the above range, not only is the filling rate excellent, but there is also the advantage of improving the rolling density.

[0088] According to the present invention, the second lithium transition metal oxide may be a lithium complex transition metal oxide represented by the following chemical formula 3.

[0089] [Chemical Formula 3] Li a3 Ni b3 Co c3 Mn d3 M 3 e3 O2 In chemical formula 3, M 3 It is selected from at least one of Al, Zr, B, W, Mo, Cr, Nb, Mg, Hf, Ta, La, Ti, Sr, Ba, Ce, Sn, Y, Zn, F, P, and S, and 0.9≤a³≤1.1, 0.6≤b³<1, 0 <c3<0.4,0<d3<0.4,0≤e3<0.1,b3+c3+d3+e3=1。

[0090] According to the present invention, the weight ratio of the first lithium transition metal oxide to the second lithium transition metal oxide can be in the range of 1:1 to 9:1. When the weight ratio of the first lithium transition metal oxide to the second lithium transition metal oxide is within the above range, the particle packing density can be increased, and as a result, the battery capacity can be increased when the positive electrode active material is used in the battery.

[0091] In the case where the positive electrode active material according to the present invention has a bimodal particle size distribution, the calendering density of the positive electrode active material can be 3.50 g / cm³. 2 Up to 3.90 g / cm 2 Specifically, the calendering density of the positive electrode active material can be 3.50 g / cm³. 2 3.55 g / cm 2 3.60 g / cm 2 Above, 3.80 g / cm 2 3.85 g / cm 2 Or 3.90 g / cm 2The following applies: When the rolling density of the positive electrode active material is within the above-mentioned range, the energy density per unit volume can be increased.

[0092] positive electrode According to another embodiment of the present invention, a positive electrode comprising the above-described positive electrode active material is provided.

[0093] 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 containing the aforementioned positive electrode active material.

[0094] There are no particular restrictions on the positive electrode current collector, as long as it is conductive and does not cause adverse chemical changes in the battery. For example, it can be made of stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. Furthermore, the thickness of the positive electrode current collector can typically range from 3 μm to 500 μm, and fine irregularities can be formed on its surface to improve the adhesion of the positive electrode active material. The positive electrode current collector can be used in various shapes, such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0095] In addition to the aforementioned positive electrode active material, the positive electrode active material layer may also include conductive materials and adhesives.

[0096] In this context, a conductive material is used to provide conductivity to the electrode. Any conductive material can be used without particular limitation, as long as it has suitable electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of conductive materials include: graphite, such as natural or artificial graphite; carbonaceous materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fibers; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one or a mixture of two or more of them can be used. Based on the total weight of the positive electrode active material layer, the content of the conductive material can typically be from 1% to 30% by weight.

[0097] The adhesive improves the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples of adhesives can be: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more thereof can be used. Based on the total weight of the positive electrode active material layer, the adhesive content can be from 1% by weight to 30% by weight.

[0098] The positive electrode can be prepared according to typical methods for preparing positive electrodes, the difference being the use of the aforementioned positive electrode active material. Specifically, a positive electrode active material layer forming composition, prepared by dissolving or dispersing the aforementioned positive electrode active material, along with optional binders and conductive materials, in a solvent, is coated onto a positive electrode current collector. The positive electrode can then be prepared by drying and calendering the coated positive electrode current collector. In this case, the types and amounts of the positive electrode material, binder, and conductive material are the same as those described above.

[0099] The solvent can be any solvent commonly used in the art. Solvents may include dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, or water, and any one or a mixture of two or more thereof may be used. Considering the coating thickness and manufacturing yield of the slurry, the amount of solvent used may be sufficient if it can dissolve or disperse the positive electrode active material, conductive material, and binder, and allows for a viscosity that provides excellent thickness uniformity during subsequent coating for positive electrode preparation.

[0100] Alternatively, as another method, the positive electrode can be prepared by casting a layer of positive electrode active material onto a separate support to form a composition, and then stacking a membrane separated from the support onto the positive electrode current collector.

[0101] Lithium secondary batteries According to another embodiment of the present invention, an electrochemical device including the positive electrode is provided. Specifically, the electrochemical device can be a battery or a capacitor, and more specifically, a lithium secondary battery.

[0102] A lithium secondary battery specifically includes a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive and negative electrodes, and an electrolyte, wherein the positive electrode is the same as described above. Additionally, a lithium secondary battery may optionally include a battery container housing the electrode assembly containing the positive electrode, negative electrode, and separator, and a sealing member for sealing the battery container.

[0103] In a lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector.

[0104] There is no particular limitation on the negative electrode current collector as long as it has high electrical conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy can be used. In addition, the thickness of the negative electrode current collector can generally be 3 μm to 500 μm, and similar to the positive electrode current collector, fine concavities and convexities can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. The negative electrode current collector can be used in various shapes such as a film, sheet, foil, net, porous body, foam, non-woven fabric body, etc.

[0105] In addition to the negative electrode active material, the negative electrode active material layer optionally further contains a binder and a conductive material.

[0106] Compounds capable of reversibly inserting and extracting lithium can be used as the negative electrode active material. Specific examples of the negative electrode active material can be: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; (semi) metallic materials capable of alloying with lithium such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloy, Sn alloy or Al alloy; (semi) metal oxides capable of doping and undoping lithium such as SiO x (0 < x < 2), SnO2, vanadium oxides, and lithium vanadium oxides; or composite materials containing (semi) metallic materials and carbonaceous materials such as Si-C composite materials or Sn-C composite materials, and any one of them or a mixture of two or more of them can be used. In addition, a thin film of metallic lithium can be used as the negative electrode active material. In addition, both low-crystalline carbon and high-crystalline carbon can be used as the carbon material. Typical examples of low-crystalline carbon can be soft carbon and hard carbon, and typical examples of high-crystalline carbon can be irregular, planar, sheet-like, spherical, or fibrous natural graphite or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbeads, mesophase pitch, and high-temperature sintered carbon such as coke derived from petroleum or coal tar pitch.

[0107] In addition, the binder and the conductive material can be the same as those described for the positive electrode above.

[0108] As an example, the negative electrode active material layer can be prepared by coating a negative electrode active material layer forming composition prepared by dissolving or dispersing the optional binder and conductive agent and the negative electrode active material in a solvent onto the negative electrode current collector and drying the coated negative electrode current collector, or by casting the negative electrode active material layer forming composition onto a separate support and then laminating a film separated from the support onto the negative electrode current collector.

[0109] In lithium-ion secondary batteries, the separator separates the negative and positive electrodes and provides a pathway for lithium ion movement. Any separator can be used without particular limitation, as long as it is commonly used in lithium-ion secondary batteries. Specifically, separators with high electrolyte retention capacity and low resistance to electrolyte ion movement can be used. More specifically, porous polymer membranes can be used, such as those prepared from polyolefin polymers like ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or those with two or more layers. Additionally, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. Furthermore, separators coated with ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can optionally be used.

[0110] Furthermore, the electrolyte used in this invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes that can be used to prepare lithium secondary batteries, but this invention is not limited thereto.

[0111] Specifically, the electrolyte may contain organic solvents and lithium salts.

[0112] Any organic solvent can be used without particular limitation, as long as it serves as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, the following substances can be used as organic solvents: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents, such as dibutyl ether or tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic solvents, such as benzene and fluorobenzene; or carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents, such as ethanol and isopropanol; nitriles, such as R-CN (where R is a linear, branched, or cyclic C2-C20 hydrocarbon group and may contain double bonds, aromatic rings, or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; or sulfolane. Among these solvents, carbonate solvents are preferred, and more preferably, mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant, which can improve the charge / discharge performance of the battery, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate). In this case, when cyclic carbonates and linear carbonates are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte can be excellent.

[0113] The use of lithium salts is not particularly restricted, as long as they are compounds capable of providing lithium ions for use in lithium secondary batteries. Specifically, the following lithium salts can be used: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. Lithium salts can be used in concentration ranges from 0.1 M to 2.0 M. When the concentration of the lithium salt is within the above range, excellent electrolyte performance can be obtained because the electrolyte can have suitable conductivity and viscosity, and lithium ions can move efficiently.

[0114] To improve battery life characteristics, suppress battery capacity degradation, and improve battery discharge capacity, in addition to the electrolyte component, the electrolyte may also contain at least one additive, such as alkylene carbonate halide compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted sulfadiazine ketones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive content can be from 0.1% by weight to 5% by weight, based on the total weight of the electrolyte.

[0115] As described above, since lithium secondary batteries containing the positive electrode active material according to the present invention stably exhibit excellent discharge capacity, output characteristics and lifespan characteristics, these lithium secondary batteries are suitable for use in portable devices, such as mobile phones, laptops and digital cameras, as well as electric vehicles, such as hybrid electric vehicles (HEVs).

[0116] Therefore, according to another embodiment of the present invention, a battery module comprising a lithium secondary battery as a unit cell and a battery pack comprising the battery module are provided.

[0117] Battery modules or battery packs can be used as a power source for at least one medium to large-sized device: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or energy storage systems.

[0118] There are no particular limitations on the shape of the lithium secondary battery of the present invention, but it can be used in the form of a cylindrical can, prismatic can, bag, or coin.

[0119] The lithium secondary battery according to the present invention can be used not only in battery cells used as power sources for small devices, but also as unit cells in medium and large battery modules comprising multiple battery cells.

[0120] Preferred Implementation Example In the following description, embodiments of the invention will be described in detail in a manner that enables those skilled in the art to readily implement the invention. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments described herein.

[0121] Example 1 The positive electrode active material precursor [composition: Ni] 0.95 Co 0.03 Mn 0.02 (OH)2, average particle size (D)50 The pre-sintered product was prepared by mixing [10.0 μm] and LiOH as lithium raw material at a molar ratio of 1:1.05 and sintering at 850°C for 6 hours in an oxygen atmosphere. The pre-sintered product was then ground to an average particle size (D). 50 The particle size was 5.0 μm, and then the particles were sintered a second time at 750 °C for 9 hours in an oxygen atmosphere to prepare the sintered product. Subsequently, the sintered product was mixed with 2 mol% Co(OH)2 and heat-treated at 700 °C for 6 hours to prepare lithium transition metal oxide in single-particle form.

[0122] Example 2 The positive electrode active material precursor [composition: Ni] 0.86 Co 0.05 Mn 0.09 (OH)2, average particle size (D) 50 The pre-sintered product was prepared by mixing [10.0 μm] and LiOH as lithium raw material at a molar ratio of 1:1.05 and sintering at 870°C for 6 hours in an oxygen atmosphere. The pre-sintered product was then ground to an average particle size (D). 50 The particle size was 4.0 μm, and then the product was sintered again at 780 °C for 9 hours in an oxygen atmosphere to prepare the sintered product. Subsequently, the sintered product was mixed with 2 mol% Co(OH)2 and heat-treated at 700 °C for 6 hours to prepare lithium transition metal oxide in single-particle form.

[0123] Example 3 The positive electrode active material precursor [composition: Ni] 0.83 Co 0.05 Mn 0.12 (OH)2, average particle size (D) 50 The pre-sintered product was prepared by mixing [10.0 μm] and LiOH as lithium raw material at a molar ratio of 1:1.05 and sintering at 890°C for 6 hours in an oxygen atmosphere. The pre-sintered product was then ground to an average particle size (D). 50 The particle size was 4.0 μm, and then the product was sintered again at 800 °C for 9 hours in an oxygen atmosphere to prepare the sintered product. Subsequently, the sintered product was mixed with 2 mol% Co(OH)2 and heat-treated at 700 °C for 6 hours to prepare lithium transition metal oxide in single-particle form.

[0124] Example 4 The positive electrode active material precursor [composition: Ni] 0.6 Co 0.1 Mn 0.3 (OH)2, average particle size (D) 50The pre-sintered product was prepared by mixing [4 μm] and LiOH as lithium raw material at a molar ratio of 1:1.05 and sintering at 970°C for 9 hours in an oxygen atmosphere. The pre-sintered product was then ground to an average particle size (D). 50 The particle size was 4 μm, and then the particles were sintered a second time at 900 °C for 9 hours in an oxygen atmosphere to prepare the sintered product. Subsequently, the sintered product was mixed with 3 mol% Co(OH)2 and heat-treated at 790 °C for 6 hours to prepare lithium transition metal oxide in single-particle form.

[0125] Comparative Example 1 The positive electrode active material precursor [composition: Ni] 0.95 Co 0.03 Mn 0.02 (OH)2, average particle size (D) 50 The pre-sintered product was prepared by mixing [5.0 μm] and LiOH as lithium raw material at a molar ratio of 1:1.07 and sintering at 850°C for 6 hours in an oxygen atmosphere. The pre-sintered product was then ground to an average particle size (D). 50 The particle size was 5.0 μm, and then the particles were sintered a second time at 750 °C for 9 hours in an oxygen atmosphere to prepare the sintered product. Subsequently, the sintered product was mixed with 2 mol% Co(OH)2 and heat-treated at 700 °C for 6 hours to prepare lithium transition metal oxide in single-particle form.

[0126] Comparative Example 2 The positive electrode active material precursor [composition: Ni] 0.86 Co 0.05 Mn 0.09 (OH)2, average particle size (D) 50 The pre-sintered product was prepared by mixing [4.0 μm] and LiOH as lithium raw material at a molar ratio of 1:1.07 and sintering at 900°C for 9 hours in an oxygen atmosphere. The pre-sintered product was then ground to an average particle size (D). 50 The particle size was 4.0 μm, and then the product was sintered again at 780 °C for 9 hours in an oxygen atmosphere to prepare the sintered product. Subsequently, the sintered product was mixed with 2 mol% Co(OH)2 and heat-treated at 700 °C for 6 hours to prepare lithium transition metal oxide in single-particle form.

[0127] Comparative Example 3 The positive electrode active material precursor [composition: Ni] 0.83 Co 0.05 Mn 0.12 (OH)2, average particle size (D) 50The pre-sintered product was prepared by mixing LiOH (6.0 μm) and LiOH (as a lithium raw material) at a molar ratio of 1:1.07 and sintering at 910 °C for 9 hours in an oxygen atmosphere. The pre-sintered product was then ground to an average particle size (D). 50 The particle size was 4.0 μm, and then the product was sintered again at 800 °C for 9 hours in an oxygen atmosphere to prepare the sintered product. Subsequently, the sintered product was mixed with 2 mol% Co(OH)2 and heat-treated at 700 °C for 6 hours to prepare lithium transition metal oxide in single-particle form.

[0128] Comparative Example 4 The positive electrode active material precursor [composition: Ni] 0.95 Co 0.03 Mn 0.02 (OH)2, average particle size (D) 50 The pre-sintered product was prepared by mixing [5.0 μm] and LiOH as lithium raw material at a molar ratio of 1:1.03 and sintering at 850°C for 6 hours in an oxygen atmosphere. The pre-sintered product was then ground to an average particle size (D). 50 The particle size was 5.0 μm, and then the particles were sintered again at 750 °C for 9 hours in an oxygen atmosphere to prepare single-particle lithium transition metal oxide.

[0129] Comparative Example 5 The positive electrode active material precursor [composition: Ni] 0.95 Co 0.03 Mn 0.02 (OH)2, average particle size (D) 50 The pre-sintered product was prepared by mixing [10.0 μm] and LiOH as lithium raw material at a molar ratio of 1:1.03 and sintering at 830°C for 9 hours in an oxygen atmosphere. The pre-sintered product was then ground to an average particle size (D). 50 The particle size was 5.0 μm, and then the particles were sintered again at 750 °C for 9 hours in an oxygen atmosphere to prepare single-particle lithium transition metal oxide.

[0130] Experimental Example 1: Analysis of Positive Electrode Active Materials Samples of each cross section were prepared by ion milling Ar ions for 2 hours (accelerating voltage: 6 kV) on each of the powdered positive electrode active materials in the form of single particles prepared in Examples 1 to 4 and Comparative Examples 1 to 5 using an ion milling system (JBOL, IB19520CCP).

[0131] 1) EBSD band contrast maps were created for cross-sectional samples of single-particle powdered positive electrode active materials using field emission scanning electron microscopy (SEM, JEOL JSM-7900F w / Oxford symmetric EBSD detector) (accelerating voltage: 20 kV). Aztec Crystal from OXFORD Instruments was used for image processing and EBSD quantitative analysis.

[0132] 2) Field emission scanning electron microscopy (SEM, JEOL JSM-7900F w / Oxford symmetric EBSD detector) (accelerating voltage: 20 kV) was used to measure and analyze the cross-sectional area of ​​each section of the single-particle powdered positive electrode active material. EBSD IPF maps were created using OXFORD Instruments' AztecCrystal image processing-EBSD quantitative analysis software.

[0133] 3) Use a scanning electron microscope (SEM, JEOL JSM-7900F) to capture SEM images of the surface of cross-sectional samples of powdered positive electrode active materials in single-particle form.

[0134] Experiment Example 2: Measurement of Boundary Length (1) Calculate the length of the boundary using the EBSD IPF plot. Boundary images are obtained by detecting boundaries using an EBSD IPF map and the imageJ program, and the length of the boundary measured from the EBSD IPF map is calculated.

[0135] (2) Use EBSD with comparison chart to calculate the length of the boundary. The length of the boundary was calculated using the EBSD with a comparison plot in the same manner as in (1) above.

[0136] (3) Using the SEM image, obtain the boundary image by separating the interior and exterior of the particles in the same way as in (1) above, and calculate the length of the SEM outer boundary.

[0137] SEM images, EBSD band comparison images, EBSD IPF images, and IPF boundaries of the positive electrode active materials of Example 1 and Comparative Example 4 are shown in the figures below. Figure 1 and Figure 2 middle.

[0138] Reference Figure 1For the positive electrode active material of Example 1, boundaries formed inside the particles of the positive electrode active material, which were not observed in the SEM image of the positive electrode of Comparative Example 4, were observed in the EBSD BC image. Furthermore, boundaries not observed in the EBSD IPF image were also observed. As described above, the boundaries not observed in the SEM image but observed in the EBSD BC image correspond to strong boundaries formed by the collapse of the layered structure of the NiO layer, while the boundaries observed only in the EBSD IPF image correspond to weak boundaries where only the atomic arrangement differs and the crystallinity has not collapsed.

[0139] Reference Figure 2 For the positive electrode active material of Comparative Example 4, no boundary inside the particles was observed in any of the SEM images, EBSD BC images and EBSD IPF images. Therefore, for the positive electrode active material of Comparative Example 4, it can be confirmed that neither strong boundary nor weak boundary is formed inside the particles.

[0140] Experimental Example 3: Evaluation of Electrochemical Performance Preparation of positive electrode and half cell The positive electrode active material prepared in Example 1, carbon black (Denka Company Limited, Denka Black) as a conductive material, and PVdF (Kureha Corporation, KF1300) as a binder were added to a solvent (DAEJUNG CHEMICALS & METALSCO., LTD., N-methylpyrrolidone (NMP)) in a weight ratio of 97.5:1:1.5 (positive electrode active material: conductive material: binder) to prepare a composition for forming a positive electrode active material layer.

[0141] The positive electrode active material layer prepared above was used to coat one surface of a 12 μm thick aluminum foil current collector with the composition, and dried at 135°C for 3 hours to form the positive electrode active material layer. Subsequently, the positive electrode active material layer was calendered by calendering, and after calendering, a positive electrode with a porosity of 24% was prepared.

[0142] The positive electrode was prepared in the same manner as described above by using the positive electrode active materials of Examples 2 to 4 and Comparative Examples 1 to 5 instead of the positive electrode active material prepared in Example 1.

[0143] A half-cell was prepared by using lithium metal as the negative electrode and together with the positive electrode prepared above.

[0144] Electrochemical performance evaluation methods Each of the coin half-cells prepared above was charged at 25°C with a constant current (CC) of 0.2C to 4.25 V, and then charged with a constant voltage (CV) until the charging current reached 0.05C (cutoff current), thereby measuring the charging capacity. Afterward, each coin half-cell was allowed to stand for 20 minutes, and then discharged with a constant current (CC) of 0.2C to 2.5 V, thereby measuring the discharge capacity in the first cycle.

[0145] After completing one cycle, the battery was transferred to a 45°C chamber and repeatedly charged and discharged at 0.33C for 50 cycles. The discharge capacity in the 50th cycle was measured, and the capacity retention was evaluated by calculating the discharge capacity in the 50th cycle relative to the discharge capacity in the 1st cycle.

[0146] [Table 1]

[0147] Referring to Table 1, it can be confirmed that the positive electrode active materials of Examples 1 to 4 exhibit improved lifetime characteristics by demonstrating a higher 50-cycle lifetime than the positive electrode active materials of Comparative Examples 1 to 5.

Claims

1. A positive electrode active material, said positive electrode active material comprising lithium transition metal oxide in single-particle form, The single-particle form of the lithium transition metal oxide comprises an outer boundary forming the outer contour of the particle and an inner boundary forming within the particle, and satisfies mathematical formula 1. [Mathematical Expression 1] 0 < length of inner boundary / length of outer boundary < 0.4 The length of the inner boundary is obtained by subtracting the length of the boundary measured from the EBSD band contrast diagram from the length of the boundary measured from the electron backscatter diffraction (EBSD)-inverse pole figure (IPF) of the lithium transition metal oxide particles. The length of the outer boundary is the length of the outer boundary of the transition metal oxide particle measured by scanning electron microscopy (SEM) image segmentation.

2. The positive electrode active material according to claim 1, wherein the single-particle form of the lithium transition metal oxide further satisfies mathematical formula 2. [Mathematical Expression 2] The length of the boundary measured from the EBSD IPF plot / the length of the boundary measured from the EBSD band contrast plot ≤ 1.

4.

3. The positive electrode active material according to claim 1, wherein the single-particle form of the lithium transition metal oxide further satisfies mathematical formula 3. [Mathematical Expression 3] 0 ≤ (length of inner boundary / length of outer boundary) - (length of inner boundary / length of boundary measured from EBSD with contrast map) ≤ 1.

4. The positive electrode active material according to claim 1, wherein the length of the boundary measured from the electron backscatter diffraction (EBSD)-IPF pattern comprises: the length of the weak boundary contained in the single-particle lithium transition metal oxide particles, which differs only in atomic arrangement and whose crystallinity has not collapsed, and the length of the strong boundary formed by the collapse of the layered structure.

5. The positive electrode active material according to claim 1, wherein the length of the boundary measured by EBSD band contrast includes: the length of the boundary contained in the single-particle lithium transition metal oxide particles formed by the collapse of the layered structure.

6. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide in single-particle form comprises 2 to 50 single crystal particles.

7. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide is a lithium composite transition metal oxide containing nickel, cobalt and manganese.

8. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide is a lithium composite transition metal oxide represented by chemical formula 1: [Chemical Formula 1] Li a Ni b Co c Mr d M 1 e O2 in, In chemical formula 1, M 1 It is selected from at least one of Al, Zr, B, W, Mo, Cr, Nb, Mg, Hf, Ta, La, Ti, Sr, Ba, Ce, Sn, Y, Zn, F, P, and S, and 0.9≤a≤1.1, 0.6≤b<1, 0 <c<0.4,0<d<0.4,0≤e<0.1,b+c+d+e=1。 9. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide is a lithium composite transition metal oxide represented by chemical formula 2: [Chemical Formula 2] Li a Ni b Co c Mr d O2 in, In chemical formula 2, 0.9≤a≤1.1, 0.6≤b<1, 0 <c<0.4,0<d<0.4,b+c+d=1。 10. The positive electrode active material according to claim 1, wherein the positive electrode active material further comprises a smaller average particle size (D0) than that of the single-particle lithium transition metal oxide. 50 The second lithium transition metal oxide in single-particle form, and It has a bimodal particle size distribution.

11. The positive electrode active material according to claim 10, wherein the second lithium transition metal oxide is a lithium composite transition metal oxide represented by chemical formula 3: [Chemical Formula 3] Li a3 Ni b3 Co c3 Mr d3 M 3 e3 O2 in, In chemical formula 3, M 3 It is selected from at least one of Al, Zr, B, W, Mo, Cr, Nb, Mg, Hf, Ta, La, Ti, Sr, Ba, Ce, Sn, Y, Zn, F, P, and S, and 0.9≤a³≤1.1, 0.6≤b³<1, 0 <c3<0.4,0<d3<0.4,0≤e3<0.1,b3+c3+d3+e3=1。 12. The positive electrode active material according to claim 10, wherein the weight ratio of the lithium transition metal oxide to the second lithium transition metal oxide is in the range of 1 to 9:

1.

13. The positive electrode active material according to claim 10, wherein the calendering density is 3.50 g / cm³. 2 Up to 3.90 g / cm 2 Within the range.

14. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 13.

15. A lithium secondary battery comprising the positive electrode as described in claim 14.

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