Positive electrode active material and lithium secondary battery containing the same

Abstract

To provide a positive electrode active material with high energy density.SOLUTION: A bimodal-type positive electrode active material includes small particles of a first lithium composite oxide, and large particles of a second lithium composite oxide. The positive electrode active material comprises: a first coating layer covering at least a part of the surface of the first lithium composite oxide and comprising a first metal oxide; and a second coating layer covering at least a part of the surface of the second lithium composite oxide and comprising a second metal oxide. The first lithium composite oxide is composite particles each comprising at least one primary particle. The average of grain boundary density of the first lithium composite oxide with respect to the primary particles lying on an imaginary straight line crossing the center of the first lithium composite oxide in a cross-sectional SEM image of the first lithium composite oxide, where the grain boundary density is (the number of interfaces between primary particles lying on an imaginary straight line) / (the number of primary particles lying on the imaginary straight line), is equal to or less than the average of grain boundary density of the second lithium composite oxide.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material and a lithium secondary battery containing the same, and more specifically, to a positive electrode active material in a bimodal form comprising a first lithium composite oxide with small particles and a second lithium composite oxide with large particles having different average particle sizes, which prevents a decrease in the electrochemical properties and stability of the positive electrode active material caused by the uneven distribution of a coating layer that coats at least a portion of the surfaces of the small and large particles, a positive electrode containing the positive electrode active material, and a lithium secondary battery using the positive electrode. [Background technology]

[0002] A battery stores electricity by using electrochemically reactive materials at its positive and negative electrodes. A typical example of such a battery is the lithium-ion secondary battery, which stores electrical energy through the difference in chemical potential that occurs when lithium ions are intercalated / deintercalated at the positive and negative electrodes.

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

[0004] Lithium composite oxides are used as positive electrode active materials in lithium secondary batteries, and examples of composite oxides such as LiCoO2, LiMn2O4, LiNiO2, and LiMnO2 are being studied.

[0005] Of the aforementioned positive electrode active materials, LiCoO2 is the most widely used due to its excellent lifespan characteristics and charge / discharge efficiency. However, it has the disadvantage of being expensive due to the resource limitations of cobalt used as a raw material, thus limiting its price competitiveness.

[0006] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of excellent thermal safety and low cost, but they have problems such as small capacity and poor high-temperature characteristics. In addition, LiNiO2-based cathode active materials exhibit battery characteristics with high discharge capacity, but synthesis is difficult due to the cation mixing problem between Li and transition metals, and thus there are significant problems with rate characteristics.

[0007] In addition, a large amount of Li by-products will be generated according to the degree of deepening of such cation mixing. Since most of these Li by-products consist of compounds of LiOH and Li2CO3, there are problems of gelation during the production of the cathode paste and the cause of gas generation due to charge and discharge during the production of the electrode. Residual Li2CO3 increases the swelling phenomenon of the cell, reduces the cycle, and causes the battery to bulge.

[0008] On the other hand, recently, for the purpose of increasing the capacity of lithium secondary batteries, a bimodal form of cathode active material in which small particles and large particles with different average particle sizes are mixed is often used. When mixing small particles and large particles, the voids between large particles can be filled with small particles with relatively small average particle sizes, so that the accumulation density of lithium composite oxides in a unit volume is improved, and the energy density per unit volume can be increased.

[0009] However, when the small particles and large particles are mixed with the coating raw material and then fired simultaneously, the coating raw material may be unevenly distributed on the small particles with a relatively large specific surface area, resulting in a problem of coating imbalance between the small particles and large particles, which can act as a cause for reducing the electrochemical characteristics and stability of the bimodal form of cathode active material in which small particles and large particles are mixed.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

[0011] In the lithium-ion battery market, the growth of lithium-ion batteries for electric vehicles is driving the market, and the demand for cathode materials used in lithium-ion batteries is also continuously changing.

[0012] For example, conventionally, lithium secondary batteries using LFPs have been mainly used from the perspective of ensuring safety, but recently, there has been a growing trend to use nickel-based lithium composite oxides, which have a higher energy capacity per unit weight compared to LFPs.

[0013] In line with these trends in cathode materials, the present invention aims to provide a cathode active material with high energy density by proposing a bimodal cathode active material comprising a first lithium composite oxide with small particles and a second lithium composite oxide with large particles having different average particle sizes.

[0014] In particular, the present invention relates to a positive electrode active material that can improve electrochemical properties and stability by reducing the phenomenon of uneven distribution of coating layers that coat at least a portion of the surfaces of small and large particles in the bimodal positive electrode active material.

[0015] Another object of the present invention is to provide a positive electrode comprising a positive electrode active material as defined in this application.

[0016] Another object of the present invention is to provide a lithium secondary battery using the positive electrode as defined in this application.

[0017] The objects of the present invention are not limited to those mentioned above (e.g., for electric vehicles), and other objects and advantages of the present invention not mentioned can be understood from the following description and may be more clearly understood from the embodiments of the present invention. It can also be readily seen that the objects and advantages of the present invention can be realized by the means and combinations set forth in the claims. [Means for solving the problem]

[0018] According to one aspect of the present invention, a bimodal positive electrode active material is provided, comprising a first lithium composite oxide with small particles and a second lithium composite oxide with large particles.

[0019] Here, the positive electrode active material may include a first coating layer covering at least a portion of the surface of the first lithium composite oxide and containing a first metal oxide, and a second coating layer covering at least a portion of the surface of the second lithium composite oxide and containing a second metal oxide.

[0020] In this case, when the occupancy rate (at%) of the first coating layer on the surface of the first lithium composite oxide is denoted as r1, and the occupancy rate (at%) of the second coating layer on the surface of the second lithium composite oxide is denoted as r2, then r1 and r2 can satisfy the following equation 1.

[0021] [Formula 1] 0.71 <r2 / r1

[0022] Furthermore, the first lithium composite oxide and the second lithium composite oxide can be represented by the following chemical formula 1.

[0023] [Chemical formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2 z O 2+α

[0024] (Here, M1 is at least one selected from Mn and Al, M2 is at least one selected from P, Sr, Ba, B, Ce, Cr, Mn, Mo, Na, K, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu, M1 and M2 are different from each other, 0.5 ≦ w ≦ 1.5, 0 ≦ x ≦ 0.50, 0 ≦ y ≦ 0.20, 0 ≦ z ≦ 0.20, 0 ≦ α ≦ 0.02)

[0025] The first metal oxide and the second metal oxide can each independently be represented by the following Chemical Formula 2.

[0026] [Chemical Formula 2] Li a M3 b O[[ID=15 c

[0027] (Here, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd, 0 ≦ a ≦ 10, 0 ≦ b ≦ 8, 2 ≦ c ≦ 13)

[0028] On the other hand, in the bimodal cathode active material, the density of the grain boundaries between the first lithium composite oxide and the second lithium composite oxide may be different from each other. The density of the grain boundaries is generally an index indirectly indicating the number of primary particles in the lithium composite oxide constituting the cathode active material in the form of secondary particles aggregated from a plurality of primary particles. In the present invention, the density of the grain boundaries of the first lithium composite oxide and the second lithium composite oxide is defined by the following Formula 3 and Formula 3.

[0029] Specifically, the first lithium composite oxide is a composite particle containing at least one primary particle, and the density of the grain boundaries calculated by the following Formula 3 for the primary particles arranged on a virtual straight line crossing the center of the first lithium composite oxide in the cross-sectional SEM image of the first lithium composite oxide can be 0.75 or less.

[0030] [Formula 3] The density of grain boundaries = (number of interface surfaces between primary particles arranged on the imaginary straight line / number of primary particles arranged on the imaginary straight line)

[0031] In this case, the first lithium composite oxide may have a single crystal structure.

[0032] Furthermore, the second lithium composite oxide is a composite particle containing at least one primary particle, and the density of the grain boundaries calculated by Equation 3 below for a primary particle located on a hypothetical straight line crossing the center of the second lithium composite oxide in a cross-sectional SEM image of the second lithium composite oxide may be 0.90 or higher.

[0033] [Formula 3] The density of grain boundaries = (number of interface surfaces between primary particles arranged on the imaginary straight line / number of primary particles arranged on the imaginary straight line)

[0034] In this way, the electrochemical properties and stability of the positive electrode active material can be improved by reducing the deviation between the occupancy rates of the coating layer that occupies at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite oxide, and by having the grain boundary densities of the first lithium composite oxide and the second lithium composite oxide within different ranges in the bimodal positive electrode active material.

[0035] Furthermore, according to another aspect of the present invention, a positive electrode comprising a positive electrode active material as defined herein is provided.

[0036] Furthermore, according to yet another aspect of the present invention, a lithium secondary battery using the positive electrode defined in this application is provided. [Effects of the Invention]

[0037] The positive electrode active material according to various embodiments of the present invention is a bimodal positive electrode active material containing a first lithium composite oxide with small particles and a second lithium composite oxide with large particles having different average particle sizes. By allowing the voids between the large particles to be filled with the relatively smaller particles, the accumulation density of the lithium composite oxide per unit volume is improved, and the energy density per unit volume can be increased.

[0038] Furthermore, according to the present invention, it is possible to reduce the deviation in the occupancy rate of the coating layer present on at least a portion of the surfaces of the small particles and the large particles, thereby preventing a decrease in the electrochemical properties and stability of the bimodal positive electrode active material.

[0039] In particular, according to the present invention, by having the density of grain boundaries in the bimodal positive electrode active material be within different ranges for the first lithium composite oxide and the second lithium composite oxide, it is possible to further mitigate the deviation between the occupancy rates of the coating layer occupying at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite oxide.

[0040] Along with the effects described above, the specific effects of the present invention will be described below, along with a detailed explanation of the specific matters for carrying out the invention. [Brief explanation of the drawing]

[0041] [Figure 1] This figure schematically shows a cross-section of a lithium composite oxide used to calculate the density of grain boundaries as defined in this application. [Figure 2] This figure schematically shows a cross-section of a lithium composite oxide used to calculate the density of grain boundaries as defined in this application. [Modes for carrying out the invention]

[0042] For the sake of easier understanding of the present invention, certain terms are defined herein for convenience. Unless otherwise defined herein, the scientific and technical terms used herein have meanings that are generally understood by a person of ordinary skill in the art. Furthermore, unless otherwise specified in the context, singular terms are to be understood as including their plural forms, and plural terms are to be understood as including their singular forms.

[0043] The positive electrode active material and the lithium secondary battery containing the positive electrode active material according to the present invention will be described in more detail below.

[0044] positive electrode active material According to one aspect of the present invention, a bimodal positive electrode active material is provided, comprising a first lithium composite oxide with small particles and a second lithium composite oxide with large particles.

[0045] In this application, the range of average particle size (D50) for small and large particles is not particularly limited; however, in order to distinguish whether any lithium composite oxide consists of small or large particles, the following reference ranges for average particle size (D50) for small and large particles may be determined.

[0046] Small particles refer to lithium composite oxides with an average particle size (D50) of 8 μm or less, and large particles refer to lithium composite oxides with an average particle size (D50) of 8.5 μm or more. There is no upper limit to the average particle size (D50) of the large particles, but for example, the large particles may have an average particle size of 8.5 to 23.0 μm.

[0047] In various embodiments of the present invention, the bimodal positive electrode active material can exist in a state in which the first lithium composite oxide and the second lithium composite oxide exhibiting the above-defined average particle size (D50) are mixed in a weight ratio of 5:95 to 50:50.

[0048] In this case, the first lithium composite oxide may exist in a form that fills the voids between the second lithium composite oxides, or it may exist in a form that is attached to the surface of the second lithium composite oxide, or it may exist in a form that is aggregated with the first lithium composite oxides.

[0049] On the other hand, it is preferable that the first lithium composite oxide and the second lithium composite oxide are present in the positive electrode active material in a weight ratio of 5:95 to 50:50.

[0050] If the proportion of the first lithium composite oxide to the second lithium composite oxide in the positive electrode active material is excessively high or excessively low, the effect of improving the energy density per unit volume of the positive electrode active material may become negligible as the press density of the positive electrode active material decreases.

[0051] Furthermore, the positive electrode active material can have a press density of 3.63 g / cc when the first lithium composite oxide and the second lithium composite oxide are present in the weight ratio described above.

[0052] On the other hand, the first lithium composite oxide and the second lithium composite oxide may be lithium composite oxides represented by the following chemical formula 1.

[0053] [Chemical formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2 z O 2+α

[0054] (Here, M1 is at least one selected from Mn and Al, and M2 is at least one selected from P, Sr, Ba, B, Ce, Cr, Mn, Mo, Na, K, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu, and M1 and M2 are distinct from each other, with 0.5 ≤ w ≤ 1.5, 0 ≤ x ≤ 0.50, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.20, and 0 ≤ α ≤ 0.02)

[0055] Furthermore, the first lithium composite oxide and the second lithium composite oxide may be lithium composite oxides represented by chemical formula 1 and having the same composition, but are not necessarily limited to this. For example, the first lithium composite oxide and the second lithium composite oxide can be synthesized by calcining precursors having the same composition but different average particle sizes, or the first lithium composite oxide and the second lithium composite oxide can be synthesized by calcining precursors having different compositions but different average particle sizes.

[0056] On the other hand, at least one of the first lithium composite oxide and the second lithium composite oxide represented by the chemical formula 1, preferably both the first lithium composite oxide and the second lithium composite oxide, may be a High-Ni type lithium composite oxide having a Ni content (molar ratio) of 80% or more. In this case, the Ni content in the first lithium composite oxide and the second lithium composite oxide can be determined by the content of x+y+z in the following chemical formula 1, as shown below.

[0057] Ni(mol%) / (Ni+Co+M1+M2)(mol%)≧80

[0058] In the case of lithium composite oxides containing Ni, the mixing of Li and Ni cations can lead to the formation of residual lithium, i.e., Li impurities such as LiOH and Li2CO3, on the surface of the lithium composite oxide. Such Li impurities can act as a cause of gelation during the production of paste for manufacturing the cathode, or cause cell swelling.

[0059] Such Li impurities may form in even larger quantities in high-Ni type cathode active materials. However, as will be described later, the cathode active material according to the present invention has the advantage of being able to cover at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite, and to remove Li impurities present on the surface of the lithium composite oxide during the process of forming a coating layer containing a metal oxide.

[0060] More specifically, the positive electrode active material may include a first coating layer covering at least a portion of the surface of the first lithium composite oxide and containing a first metal oxide, and a second coating layer covering at least a portion of the surface of the second lithium composite oxide and containing a second metal oxide.

[0061] Furthermore, the metal oxide contained in the first coating layer and the second coating layer may be represented by the following chemical formula 2. In this case, the first coating layer and the second coating layer may be defined as regions on the surfaces of the first lithium composite oxide and the second lithium composite oxide where the metal oxide represented by the following chemical formula 2 exists.

[0062] [Chemical formula 2] Li a M3 b O c

[0063] (Here, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd, where 0 ≤ a ≤ 10, 0 ≤ b ≤ 8, and 2 ≤ c ≤ 13)

[0064] The metal oxides contained in the first coating layer and the second coating layer may be the same or different metal oxides.

[0065] Furthermore, the coating layer may be in a form in which different metal oxides exist simultaneously within a single layer, or in which different metal oxides represented by the chemical formula 2 exist in separate layers.

[0066] The metal oxide represented by chemical formula 2 above may be physically and / or chemically bonded to the primary particles represented by chemical formula 1 above. Furthermore, the metal oxide may exist in a solid solution with the primary particles represented by chemical formula 1 above.

[0067] The positive electrode active material according to this embodiment can have enhanced structural stability by including a coating layer that covers at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite oxide. Furthermore, when such a positive electrode active material is used in a lithium secondary battery, the high-temperature storage stability and life characteristics of the positive electrode active material can be improved. In addition, the metal oxide can influence the improvement of the efficiency characteristics of the lithium secondary battery by reducing residual lithium on the surfaces of the first lithium composite oxide and the second lithium composite oxide, while simultaneously acting as a lithium ion transport pathway.

[0068] Furthermore, the metal oxide is either an oxide in which lithium and an element represented by M3 are combined, or an oxide of M3, and the metal oxide is, for example, Li a W b O c Li a Zr b O c Li a Ti b O c Li a Ni b O c Li a B b O c Li a Co b O c Li a Al b O c Co b O c , Al b O c , W b O c , Zr b O c Ti bO c Or, B b O c These may include other examples, but the examples given above are merely for convenience to aid understanding, and the metal oxides as defined in this application are not limited to the examples given above.

[0069] In other embodiments, the metal oxide may be an oxide composed of lithium and at least two elements represented by M3, or may further comprise a metal oxide composed of lithium and at least two elements represented by M3. A metal oxide composed of lithium and at least two elements represented by M3 is, for example, Li a (W / Ti) b O c Li a (W / Zr) b O c Li a (W / Ti / Zr) b O c Li a (W / Ti / B) b O c These are possible, but not necessarily limited to them.

[0070] Here, the metal oxide can exhibit a concentration gradient that decreases from the surface of the secondary particle toward the center of the secondary particle. As a result, the concentration of the metal oxide can decrease from the outermost surface of the secondary particle toward the center of the secondary particle.

[0071] As described above, the metal oxide exhibits a concentration gradient that decreases from the surface of the secondary particles toward the center of the secondary particles, thereby effectively reducing the residual lithium present on the surface of the positive electrode active material and preventing side reactions caused by unreacted residual lithium. Furthermore, the metal oxide prevents a decrease in crystallinity in the inner region of the surface of the positive electrode active material. In addition, the metal oxide prevents the overall structure of the positive electrode active material from collapsing during the electrochemical reaction.

[0072] Additionally, the coating layer may include a first oxide layer containing at least one metal oxide represented by the above chemical formula 2, and a second oxide layer containing at least one metal oxide represented by the above chemical formula 2, and a metal oxide different from the metal oxide contained in the first oxide layer.

[0073] For example, the first oxide layer may exist so as to cover at least a portion of the exposed surface of the primary particles that are present on the outermost edge of the secondary particles, and the second oxide layer may exist so as to cover at least a portion of the exposed surface of the primary particles that are not covered by the first oxide layer and the surface of the first oxide layer.

[0074] On the other hand, when the first lithium composite oxide and the second lithium composite oxide are defined as composite particles containing at least one primary particle, the coating layer can not only cover at least a portion of the surface of the composite particle (e.g., a secondary particle), but can also be present at the interfaces between the multiple primary particles constituting the composite particle.

[0075] Furthermore, the coating layer may exist as a layer that continuously or discontinuously coats the surface of the primary particles and / or the secondary particles. When the coating layer exists discontinuously, it may exist in the form of an island. In other cases, the coating layer may exist in the form of a solid solution that does not form a boundary with the primary particles and / or the secondary particles formed by the aggregation of the primary particles.

[0076] On the other hand, the first coating layer and the second coating layer can be obtained by mixing the precursors of the first lithium composite oxide and the second lithium composite oxide with the raw material of the metal oxide represented by chemical formula 2, and then performing primary calcination, or by performing primary calcination of the precursors of the first lithium composite oxide and the second lithium composite oxide, mixing with the raw material of the metal oxide represented by chemical formula 2, and then performing secondary calcination. In this case, the first lithium composite oxide and the second lithium composite oxide can also be performed primary calcination independently of each other.

[0077] For example, when the precursors of the first lithium composite oxide and the second lithium composite oxide are obtained by first calcining the precursors of the first lithium composite oxide and the second lithium composite oxide independently, and then the raw materials of the metal oxide represented by chemical formula 2 are mixed and a coating process is performed, the first coating layer and the second coating layer may be formed on the surfaces of the first lithium composite oxide and the second lithium composite oxide, respectively. However, in this case, the coating raw materials may be unevenly distributed among the small particles, which have a relatively large specific surface area.

[0078] In this case, the deviation between the occupancy rate of the first coating layer on the surface of the small-particle first lithium composite oxide and the occupancy rate of the second coating layer on the surface of the second lithium composite oxide becomes significant, which can act as a cause to degrade the electrochemical properties and stability of the bimodal positive electrode active material in which the first lithium composite oxide and the second lithium composite oxide are mixed in a predetermined ratio.

[0079] In a bimodal positive electrode active material in which small-particle first lithium composite oxide and large-particle second lithium composite oxide are mixed in a predetermined ratio, in order to prevent a decrease in the electrochemical properties and stability of the positive electrode active material due to the deviation between the occupancy rate of the first coating layer on the surface of the first lithium composite oxide and the occupancy rate of the second coating layer on the surface of the second lithium composite oxide, it is preferable that r1 and r2 satisfy the following formula 1, where r1 is the occupancy rate (at%) of the first coating layer on the surface of the first lithium composite oxide and r2 is the occupancy rate (at%) of the second coating layer on the surface of the second lithium composite oxide.

[0080] Here, the occupancy rate (at%) of the first coating layer on the surface of the first lithium composite oxide and the occupancy rate (at%) of the second coating layer on the surface of the second lithium composite oxide can be defined as the content (at%) of the first coating layer present on the surface of the first lithium composite oxide (more specifically, the content (at%) of the metal element specific to the metal oxide represented by chemical formula 2 in the first coating layer) and the content (at%) of the second coating layer present on the surface of the second lithium composite oxide (more specifically, the content (at%) of the metal element specific to the metal oxide represented by chemical formula 2 in the second coating layer). In this case, the metal elements to be measured for r1 and r2 may be the same.

[0081] For example, the occupancy rate (r1) of the first coating layer on the surface of the first lithium composite oxide, the occupancy rate (r2) of the second coating layer on the surface of the second lithium composite oxide, and their ratio (r2 / r1) can be calculated by measuring the composition of metal elements on the surfaces of the first and second lithium composite oxides where the first and second coating layers are not present, and then, after forming the first and second coating layers on the surfaces of the first and second lithium composite oxides, respectively, measuring the composition of metal elements on the surfaces and calculating the changes in the content of metal elements specifically contained in the metal oxides constituting the first and second coating layers.

[0082] That is, the occupancy rate of the first coating layer on the surface of the first lithium composite oxide (r1) and the occupancy rate of the second coating layer on the surface of the second lithium composite oxide (r2) can represent the area ratio of the first coating layer and the second coating layer covering the surfaces of the first lithium composite oxide and the second lithium composite oxide, respectively, but can also be understood as the content ratio of the first coating layer and the second coating layer present on the surfaces of the first lithium composite oxide and the second lithium composite oxide.

[0083] [Formula 1] 0.71 <r2 / r1

[0084] If r2 / r1, as shown in Equation 1 above, is 0.71 or less, the occupancy rate of the first coating layer on the surface of the first lithium composite oxide becomes excessively large compared to the occupancy rate of the second coating layer on the surface of the second lithium composite oxide, which may cause a gap in the surface properties of the first lithium composite oxide and the second lithium composite oxide, potentially degrading the electrochemical properties and stability of the positive electrode active material.

[0085] More specifically, r1 and r2 can satisfy the following equation 2.

[0086] [Formula 2] 0.72 ≤ r² / r1 < 1.23

[0087] If r2 / r1, as shown in Equation 2 above, is 1.23 or greater, then as the occupancy rate of the second coating layer on the surface of the second lithium composite oxide becomes excessively large compared to the occupancy rate of the first coating layer on the surface of the first lithium composite oxide, a gap in the surface properties of the first lithium composite oxide and the second lithium composite oxide will occur, thereby reducing the electrochemical properties and stability of the positive electrode active material.

[0088] Furthermore, as described above, the first lithium composite oxide and the second lithium composite oxide may be composite particles containing at least one primary particle capable of lithium intercalation / deintercalation. If the first lithium composite oxide and / or the second lithium composite oxide contain a plurality of primary particles, the plurality of primary particles may exist as secondary particles that are aggregates of each other.

[0089] The primary particle refers to a single crystal grain (grain or crystallite), and the secondary particle refers to an aggregate formed by the aggregation of multiple primary particles. Voids and / or grain boundaries may exist between the primary particles that make up the secondary particle.

[0090] Additionally, according to the present invention, by having the density of grain boundaries in the bimodal positive electrode active material be within different ranges for the first lithium composite oxide and the second lithium composite oxide, it is possible to further reduce the deviation between the occupancy rates of the coating layer occupying at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite oxide.

[0091] Specifically, the first lithium composite oxide is a composite particle consisting of at least one primary particle, and the density of grain boundaries of the first lithium composite oxide, calculated by Equation 3 below for primary particles arranged on a hypothetical straight line crossing the center of the first lithium composite oxide in a cross-sectional SEM image of the first lithium composite oxide, may be 0.75 or less.

[0092] [Formula 3] The density of grain boundaries = (number of interface surfaces between primary particles arranged on the imaginary straight line / number of primary particles arranged on the imaginary straight line)

[0093] Figures 1 and 2 schematically show cross-sections of lithium composite oxides used to calculate the grain boundary density as defined in this application. The grain boundary densities of lithium composite oxides calculated with reference to Figures 1 and 2 are shown in Table 1 below.

[0094] [Table 1]

[0095] In this case, as shown in Figure 1, the first lithium composite oxide can have a single crystal structure with a grain boundary density of 0.5, because there is one interface (grain boundary) between primary particles arranged on a virtual straight line, and two primary particles arranged on the virtual straight line.

[0096] By having a grain boundary density of 0.75 or less, as shown in formula 3, it is possible to mitigate the difference between the occupancy rate of the first coating layer on the surface of the first lithium composite oxide and the occupancy rate of the second coating layer on the surface of the second lithium composite oxide.

[0097] On the other hand, the second lithium composite oxide is a composite particle consisting of at least one primary particle, and the density of the grain boundaries of the second lithium composite oxide, calculated by Equation 3 below for a primary particle located on a hypothetical straight line crossing the center of the second lithium composite oxide in a cross-sectional SEM image of the second lithium composite oxide, may be 0.90 or more.

[0098] [Formula 3] The density of grain boundaries = (number of interface surfaces between primary particles arranged on the imaginary straight line / number of primary particles arranged on the imaginary straight line)

[0099] By enabling the inclusion of small particles with a smaller average particle size, the density of lithium composite oxides per unit volume can be increased, thereby raising the energy density per unit volume.

[0100] Furthermore, the bimodal positive electrode active material according to various embodiments of the present invention can reduce the deviation in the occupancy rate of the coating layer present on at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite oxide, thereby preventing a decrease in the electrochemical properties and stability of the bimodal positive electrode active material.

[0101] In particular, according to the present invention, by having the density of grain boundaries in the bimodal positive electrode active material be within different ranges for the first lithium composite oxide and the second lithium composite oxide, it is possible to further mitigate the deviation between the occupancy rates of the coating layer occupying at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite oxide.

[0102] Lithium-ion battery According to another aspect of the present invention, a positive electrode may be provided comprising a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. Here, the positive electrode active material layer may include positive electrode active materials according to various embodiments of the present invention. Since the positive electrode active material is the same as that described above, for convenience, a detailed explanation will be omitted, and only the remaining undescribed components will be described below.

[0103] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc., can be used. The positive electrode current collector can also typically have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, it can be used in a variety of forms such as film, sheet, foil, net, porous material, foam, nonwoven fabric, etc.

[0104] The positive electrode active material layer may be manufactured by applying a positive electrode slurry composition, which includes a conductive material and optionally a binder, together with the positive electrode active material, to the positive electrode current collector.

[0105] In this case, the positive electrode active material may be included in an amount of 80-99 wt%, more specifically 85-98.5 wt%, relative to the total weight of the positive electrode active material layer. When included within the above-mentioned content range, excellent capacity characteristics can be observed, but the material is not necessarily limited to this range.

[0106] The conductive material is used to impart conductivity to the electrodes and can be used in the battery without special restrictions as long as it does not cause chemical changes 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 powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more can be used. The conductive material may be included in an amount of 0.1 to 15 wt% relative to the total weight of the positive electrode active material layer.

[0107] The binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used. The binder may be included in an amount of 0.1 to 15 wt% relative to the total weight of the positive electrode active material layer.

[0108] The positive electrode can be manufactured by conventional methods for manufacturing positive electrodes, except for the use of the positive electrode active material described above. Specifically, it can be manufactured by applying a positive electrode slurry composition, prepared by dissolving or dispersing the positive electrode active material and, selectively, a binder and a conductive material in a solvent, onto a positive electrode current collector, followed by drying and rolling.

[0109] The solvent may be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one or more of these may be used individually or in mixtures of two or more. The amount of solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, taking into consideration the coating thickness and production yield of the slurry, and to have a viscosity that allows for excellent thickness uniformity when applied for the manufacture of the positive electrode thereafter.

[0110] In other embodiments, the positive electrode may also be manufactured by casting the positive electrode slurry composition onto another support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0111] Furthermore, according to yet another aspect of the present invention, an electrochemical element including the positive electrode described above may be provided. The electrochemical element may specifically be a battery, a capacitor, etc., and more specifically, a lithium secondary battery.

[0112] The lithium secondary battery may specifically include a positive electrode, a negative electrode positioned opposite the positive electrode, and a separator membrane and electrolyte interposed between the positive and negative electrodes. Here, since the positive electrode is the same as described above, for convenience, a detailed explanation will be omitted, and only the remaining components not mentioned above will be described in detail below.

[0113] The lithium secondary battery may further selectively include a battery container for housing the electrode assembly comprising the positive electrode, the negative electrode, and the separator membrane, and a sealing member for sealing the battery container.

[0114] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0115] The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. The negative electrode current collector can also typically have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in a variety of forms such as film, sheet, foil, net, porous material, foam, nonwoven fabric, etc.

[0116] The negative electrode active material layer can be manufactured by applying a negative electrode slurry composition, which includes a conductive material and, if necessary, a binder, together with the negative electrode active material to the negative electrode current collector.

[0117] As the negative electrode active material, compounds capable of reversible intercalation and deintercalation of lithium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and SiO2. βExamples include metallic oxides that can be doped and dedoped with lithium, such as (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compound and carbonaceous material, such as Si-C composites or Sn-C composites, and any one or more mixtures of these may be used. A metallic lithium thin film may also be used as the negative electrode active material. Furthermore, all carbon materials, including low-crystallinity carbon and high-crystallinity carbon, may be used. Typical examples of low-crystalline carbon include soft carbon and hard carbon, while typical examples of high-crystalline carbon include amorphous, plate-like, flaky, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0118] The aforementioned negative electrode active material may be present in an amount of 80-99 wt% based on the total weight of the negative electrode active material layer.

[0119] The binder is a component that assists in bonding between the conductive material, active material, and current collector, and can usually be added at a concentration of 0.1 to 10 wt% based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0120] The conductive material is a component for further improving the conductivity of the negative electrode active material and can be added in an amount of 10 wt% or less, preferably 5 wt% or less, based on the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it is conductive without inducing a chemical change in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Kechen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used.

[0121] In one embodiment, the negative electrode active material layer may be manufactured by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing a negative electrode active material and a binder and conductive material selectively in a solvent, onto a negative electrode current collector and drying the mixture; or by casting the negative electrode slurry composition onto another support, peeling it off the support, and laminating the resulting film onto the negative electrode current collector.

[0122] In other embodiments, the negative electrode active material layer may also be manufactured by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing a negative electrode active material and a binder and conductive material selectively in a solvent, onto a negative electrode current collector and drying it, or by casting the negative electrode slurry composition onto another support, peeling it off the support, and laminating the resulting film onto the negative electrode current collector.

[0123] On the other hand, in the lithium secondary battery, the separation membrane separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any membrane typically used as a separation membrane in lithium secondary batteries can be used without special limitations, and it is particularly preferable that the membrane has low resistance to ion movement of the electrolyte and excellent electrolyte impregnation ability. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof, can be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, coated separation membranes containing ceramic components or polymeric substances to ensure heat resistance or mechanical strength can be used, and they can be selectively used in single-layer or multi-layer structures.

[0124] Furthermore, the electrolytes used in the present invention include, but are not limited to, 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 manufacture of lithium secondary batteries.

[0125] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0126] The aforementioned organic solvent can be used without special limitations, as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the aforementioned organic solvents include 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 hydrocarbon solvents such as benzene and fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (propylene Carbonate solvents such as carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a hydrocarbon group with 2 to 20 carbon atoms in a linear, branched, or cyclic structure, and may include a double-bonded aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, mixing the cyclic carbonate and the linear carbonate in a volume ratio of about 1:1 to about 1:9 can bring out the excellent performance of the electrolyte.

[0127] The lithium salt can be any compound capable of providing lithium ions for use in a lithium secondary battery, without any special limitations. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably within the range of 0.1 to 2.0 M. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0128] In addition to the electrolyte components, the electrolyte may also contain one or more additives for purposes such as improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be present in an amount of 0.1 to 5 wt% relative to the total weight of the electrolyte.

[0129] As described above, the lithium secondary battery containing the positive electrode active material according to the present invention exhibits excellent discharge capacity, output characteristics, and life characteristics in a stable manner, making it useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the electric vehicle field, such as hybrid electric vehicles (HEVs).

[0130] The external shape of the lithium secondary battery according to the present invention is not particularly limited, but it can be cylindrical, rectangular, pouch-shaped, or coin-shaped, using a can. Furthermore, the lithium secondary battery can be used as a battery cell for powering small devices, and can also be preferably used as a unit battery in medium-to-large battery modules containing a large number of battery cells.

[0131] According to yet another aspect of the present invention, a battery module and / or a battery pack including the lithium secondary battery as a unit cell may be provided.

[0132] The battery module or battery pack is used as a power source for one or more medium-to-large devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0133] The present invention will be described in more detail below through examples. However, these examples are merely illustrative and should not be interpreted as limiting the scope of the present invention.

[0134] Manufacturing Example 1. Manufacturing of positive electrode active material (1) Example 1 NiCoMn(OH)2 hydroxide precursors (Ni:Co:Mn=91:8:1(at%)) of small-particle first lithium composite oxide and large-particle second lithium composite oxide were synthesized using a known co-precipitation method with nickel sulfate, cobalt sulfate, and manganese sulfate. The average particle size (D50) of the hydroxide precursor of the first lithium composite oxide (first hydroxide precursor) was 3.0 μm, and the average particle size (D50) of the hydroxide precursor of the second lithium composite oxide (second hydroxide precursor) was 18.0 μm.

[0135] Next, the first hydroxide precursor was mixed with LiOH (Li / (Ni+Co+Mn)mol ratio=1.05±0.05), and then heat-treated (primary calcination) in a calcination furnace at a rate of 2°C per minute up to 850°C while maintaining an O2 atmosphere for 12 hours to obtain small-particle first lithium composite oxide.

[0136] Separately, the second hydroxide precursor was mixed with LiOH (Li / (Ni+Co+Mn)mol ratio=1.05±0.05), and then subjected to heat treatment (primary calcination) for 12 hours in a calcination furnace, maintaining an O2 atmosphere, and increasing the temperature to 700°C at a rate of 2°C per minute to obtain small-particle second lithium composite oxide.

[0137] Next, the first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 10:90 to produce a mixture, and then distilled water was added to the mixture and it was washed for 1 hour.

[0138] Next, 3.0 mol% cobalt sulfate was added to the mixture while stirring, thereby coating the surfaces of the first lithium composite oxide and the second lithium composite oxide in the mixture with Co. The mixture was then dried in a vacuum dryer at 120°C for 12 hours.

[0139] Finally, the material was heat-treated (secondary firing) in a firing furnace at a rate of 2°C per minute up to 700°C while maintaining an O2 atmosphere, yielding a bimodal cathode active material in which small-particle first lithium composite oxide and large-particle second lithium composite oxide were mixed in a predetermined ratio.

[0140] (2) Example 2 The positive electrode active material was prepared in the same manner as in Example 1, except that the first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 20:80 before washing with water.

[0141] (3) Example 3 The positive electrode active material was prepared in the same manner as in Example 1, except that the first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 30:70 before washing with water.

[0142] (4) Example 4 The positive electrode active material was prepared in the same manner as in Example 1, except that the first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 40:60 before washing with water.

[0143] (5) Example 5 The positive electrode active material was prepared in the same manner as in Example 1, except that the first lithium composite oxide and the second lithium composite oxide were mixed in a 50:50 weight ratio before washing with water.

[0144] (6) Example 6 The positive electrode active material was prepared in the same manner as in Example 3, except that 0.5 mol% of the Ba-containing compound (Ba(OH)2) was added to the first hydroxide precursor and the second hydroxide precursor, respectively, before primary calcination, followed by heat treatment.

[0145] (7) Example 7 The positive electrode active material was prepared in the same manner as in Example 3, except that 0.5 mol% of the Sr-containing compound (Sr(OH)2) was added to the first hydroxide precursor and the second hydroxide precursor, respectively, before primary calcination, followed by heat treatment.

[0146] (8) Example 8 The positive electrode active material was prepared in the same manner as in Example 3, except that 0.5 mol% of the Mg-containing compound (Mg(OH)2) was added to the first hydroxide precursor and the second hydroxide precursor, respectively, before primary calcination, followed by heat treatment.

[0147] (9) Example 9 The positive electrode active material was prepared in the same manner as in Example 3, except that 0.5 mol% of a Na-containing compound (Na(NO)3) was added to the first hydroxide precursor and the second hydroxide precursor, respectively, before primary calcination, followed by heat treatment.

[0148] (10) Example 10 The positive electrode active material was prepared in the same manner as in Example 3, except that 0.2 mol% of Nb-containing compound (Nb2O5) was added to the first hydroxide precursor and the second hydroxide precursor, respectively, before primary calcination, followed by heat treatment.

[0149] (11) Example 11 The positive electrode active material was prepared in the same manner as in Example 3, except that 0.2 mol% of W-containing compound (WO3) was added to the first hydroxide precursor and the second hydroxide precursor, respectively, before primary calcination, followed by heat treatment.

[0150] (12) Example 12 The positive electrode active material was prepared in the same manner as in Example 3, except that 0.3 mol% of a Zr-containing compound (ZrO2) was added to the first hydroxide precursor and the second hydroxide precursor, respectively, before primary calcination, followed by heat treatment.

[0151] (13) Example 13 The positive electrode active material was prepared in the same manner as in Example 12, except that instead of adding 3.0 mol% cobalt sulfate to the mixture of the first lithium composite oxide and the second lithium composite oxide, 3.0 mol% aluminum sulfate was added to coat the mixture with Al.

[0152] (14) Example 14 The positive electrode active material was prepared in the same manner as in Example 12, except that instead of adding 3.0 mol% cobalt sulfate to the mixture of the first lithium composite oxide and the second lithium composite oxide, 3.0 mol% manganese sulfate was added to coat the Mn.

[0153] (15) Example 15 The positive electrode active material was prepared in the same manner as in Example 12, except that instead of adding 3.0 mol% cobalt sulfate to the mixture of the first lithium composite oxide and the second lithium composite oxide, 3.0 mol% zirconium nitrate was added to coat Zr.

[0154] (16) Example 16 The first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 30:70 to produce a mixture. After adding distilled water to the mixture and washing it for 1 hour, the mixture was dried in a vacuum dryer at 120°C for 12 hours to obtain a positive electrode active material. The dried positive electrode active material was mixed with 1.0 mol% Nb2O5, and then heat-treated (secondary firing) in a firing furnace at a rate of 2°C per minute up to 700°C for 12 hours while maintaining an O2 atmosphere to produce a coating of Nb. The positive electrode active material was produced in the same manner as in Example 12.

[0155] (17) Example 17 The positive electrode active material was manufactured in the same manner as in Example 16, except that 1.0 mol% of cobalt phosphate was added to the dried positive electrode active material instead of 1.0 mol% of Nb2O5, and then the material was heat-treated (secondary firing) in a firing furnace at a rate of 2°C per minute up to 700°C while maintaining an O2 atmosphere for 12 hours to coat it with cobalt phosphate.

[0156] (18) Comparative Example 1 The cathode active material was produced in the same manner as in Example 1, except that LiOH (Li / (Ni+Co+Mn)mol ratio=1.05±0.05) was mixed with the first hydroxide precursor, and then the mixture was heat-treated (primary calcination) in a calcination furnace at a rate of 2°C per minute up to 700°C for 12 hours while maintaining an O2 atmosphere, to obtain small-particle first lithium composite oxide.

[0157] (19) Comparative Example 2 The positive electrode active material was manufactured in the same manner as in Example 1, except that LiOH (Li / (Ni+Co+Mn)mol ratio=1.05±0.05) was mixed with the first hydroxide precursor, and then the mixture was heat-treated (primary calcination) for 12 hours in a calcination furnace at a rate of 2°C per minute up to 700°C while maintaining an O2 atmosphere to obtain small-particle first lithium composite oxide, and the first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 20:80 to produce a mixture.

[0158] (20) Comparative Example 3 The positive electrode active material was manufactured in the same manner as in Example 1, except that LiOH (Li / (Ni+Co+Mn)mol ratio=1.05±0.05) was mixed with the first hydroxide precursor, and then heat-treated (primary calcination) for 12 hours in a calcination furnace at a rate of 2°C per minute up to 700°C while maintaining an O2 atmosphere to obtain small-particle first lithium composite oxide, and the first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 30:70 to produce a mixture.

[0159] (21) Comparative Example 4 The positive electrode active material was manufactured in the same manner as in Example 1, except that LiOH (Li / (Ni+Co+Mn)mol ratio=1.05±0.05) was mixed with the first hydroxide precursor, and then the mixture was heat-treated (primary calcination) for 12 hours in a calcination furnace at a rate of 2°C per minute up to 700°C while maintaining an O2 atmosphere to obtain small-particle first lithium composite oxide, and the first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 40:60 to produce a mixture.

[0160] (22) Comparative Example 5 The positive electrode active material was manufactured in the same manner as in Example 1, except that LiOH (Li / (Ni+Co+Mn)mol ratio=1.05±0.05) was mixed with the first hydroxide precursor, and then the mixture was heat-treated (primary calcination) for 12 hours in a calcination furnace at a rate of 2°C per minute up to 700°C while maintaining an O2 atmosphere to obtain small-particle first lithium composite oxide, and the first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 50:50 to produce a mixture.

[0161] (23) Comparative Example 6 The positive electrode active material was prepared in the same manner as in Example 1, except that the first lithium composite oxide and the second lithium composite oxide were mixed in a weight ratio of 3:97 before washing with water.

[0162] Manufacturing Example 2: Manufacturing of Lithium-ion Rechargeable Batteries A cathode slurry was prepared by dispersing 92 wt% each of the cathode active materials produced by Production Example 1, 4 wt% of artificial graphite, and 4 wt% of PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The cathode slurry was uniformly coated onto a 15 μm thick aluminum thin film and vacuum-dried at 135°C to produce a cathode for a lithium secondary battery.

[0163] A coin cell was manufactured using a lithium wheel as the counter electrode for the positive electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separation membrane, and an electrolyte containing ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7 with LiPF6 present at a concentration of 1.15 M.

[0164] Experimental Example 1. Structural Analysis of Cathode Active Material (1) SEM / EDS analysis of cathode active material To measure the distribution of the coating layer and the content of coating elements on the surfaces of the small-particle lithium-1 and large-particle lithium-2 composite oxides contained in the cathode active material manufactured by Manufacturing Example 1, the surfaces of the lithium-1 and lithium-2 composite oxides in their uncoated state were first subjected to EDS analysis to establish a baseline.

[0165] Subsequently, the surfaces of the first lithium composite oxide and the second lithium composite oxide coated by the predetermined method disclosed in Manufacturing Example 1 were subjected to EDS analysis, and the difference in the content of coating elements before and after coating was calculated to measure the occupancy rate of the first coating layer (r1) and the occupancy rate of the second coating layer (r2).

[0166] [Table 2]

[0167] (2) Cross-sectional SEM analysis of positive electrode active material After obtaining cross-sectional SEM images of the small-particle first lithium composite oxide and the large-particle second lithium composite oxide contained in the cathode active material manufactured according to Manufacturing Example 1 using FE-SEM (Bruker), the average value of the grain boundary density was calculated from the cross-sectional SEM images using Equation 3 below.

[0168] [Formula 3] Grain boundary density = (Number of interface surfaces between primary particles located on a hypothetical straight line crossing the center of the lithium composite oxide in a cross-sectional SEM image of the lithium composite oxide / Number of primary particles located on the said hypothetical straight line)

[0169] The measurement results for the density of the aforementioned grain boundaries are shown in Table 3 below.

[0170] [Table 3]

[0171] Experimental Example 2. Evaluation of the electrochemical properties of the positive electrode active material. (1) Evaluation of the battery capacity and life characteristics of lithium secondary batteries The lithium secondary batteries manufactured according to Manufacturing Example 2 were subjected to charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, with a voltage range of 3.0V to 4.25V and a discharge rate of 0.5C to 4.0C. The initial charge capacity, initial discharge capacity, and energy density were measured. The energy density (Wh / L) was calculated using Equation 4 below.

[0172] [Formula 4] Energy density (Wh / L) = Initial discharge capacity * Average voltage * Press density

[0173] Furthermore, the lithium secondary batteries manufactured using the method described above were subjected to 50 charge-discharge cycles at a temperature of 25°C and a driving voltage range of 3.0V to 4.25V under 1C / 1C conditions. The ratio of the discharged capacity at the 50th cycle to the initial capacity (cycle capacity retention) was then measured.

[0174] The results of the electrochemical property evaluation of the lithium secondary battery measured by the above method are shown in Table 4 below.

[0175] [Table 4]

[0176] Based on the results in Table 4, it can be confirmed that the positive electrode active materials from Examples 1 to 17 exhibit an energy density of 2800 Wh / L or higher and a lifetime of 88% or higher, while the positive electrode active materials from Comparative Examples 1 to 6 have lower energy density and lifetime characteristics compared to those from Examples 1 to 17.

[0177] Experimental Example 3. Evaluation of the stability of the positive electrode active material and lithium secondary battery. (1) Evaluation of the thermal stability of the positive electrode active material The thermal stability of the cathode active material produced by Production Example 1 was evaluated using a thermogravimetric analyzer (TA Instruments, Q20). Weight loss was measured from 25°C to 350°C at a heating rate of 10°C / min under atmospheric pressure and an Ar atmosphere. The onset temperature (op-set) at which the weight loss (thermal decomposition) peak appeared for each cathode active material is shown in Table 5 below.

[0178] [Table 5]

[0179] Based on the results in Table 5, it was confirmed that the onset temperature (op-set) at which the weight loss (thermal decomposition) peak appears in the positive electrode active materials of Examples 1 to 17 is higher than that of the positive electrode active materials of Comparative Examples 1 to 6. In other words, the thermal stability of the positive electrode active materials of Examples 1 to 17 is even better than that of the positive electrode active materials of Comparative Examples 1 to 6.

[0180] (2) Measurement of gas generation amount of lithium secondary battery The lithium secondary battery manufactured according to Manufacturing Example 2 was charged to 4.25V with a constant current of 0.2C, and then stored at 60°C for 14 days. The volume change of the lithium secondary battery due to gas generation inside the battery was measured. The measurement results of the volume change are shown in Table 6 below.

[0181] [Table 6]

[0182] Based on the results in Table 6, it can be confirmed that the volume change of lithium secondary batteries using the positive electrode active materials in Examples 1 to 17 is smaller than the volume change of lithium secondary batteries using the positive electrode active materials in Comparative Examples 1 to 6.

[0183] Although embodiments of the present invention have been described above, a person with ordinary skill in the art can modify and change the present invention in various ways by adding, changing, deleting, or adding components, without departing from the spirit of the invention as described in the claims, and this can also be said to be within the scope of the rights of the present invention.

Claims

1. The positive electrode active material is in a bimodal form, containing small-particle first lithium composite oxide and large-particle second lithium composite oxide. The average particle size (D50) of the first lithium composite oxide is 8 μm or less. The average particle size (D50) of the second lithium composite oxide is 8.5 μm or larger. The positive electrode active material is The first coating layer covers at least a portion of the surface of the first lithium composite oxide and includes a first metal oxide, The second coating layer covers at least a portion of the surface of the second lithium composite oxide and includes a second metal oxide, The first lithium composite oxide is a composite particle containing at least one primary particle, The first lithium composite oxide is defined such that the average value of the grain boundary density calculated by the following equation 3 for primary particles arranged on a hypothetical straight line crossing the center of the first lithium composite oxide in a cross-sectional SEM image of the first lithium composite oxide is less than or equal to the average value of the grain boundary density calculated by the following equation 3 for primary particles arranged on a hypothetical straight line crossing the center of the second lithium composite oxide in a cross-sectional SEM image of the second lithium composite oxide. The first metal oxide and the second metal oxide each comprise at least one coating element selected from Co, Al, Mn, Zr, and Nb. When the difference (at%) between the content of the coating element present on the surface of the first lithium composite oxide and the content of the coating element present on the surface of the first coating layer, as determined by SEM / EDS analysis, is called r1, and the difference (at%) between the content of the coating element present on the surface of the second lithium composite oxide and the content of the coating element present on the surface of the second coating layer, as determined by SEM / EDS analysis, is called r2, The aforementioned r1 and r2 are positive electrode active materials that satisfy the following formula 2. [Formula 3] The density of grain boundaries = (number of interface surfaces between primary particles arranged on the imaginary straight line / number of primary particles arranged on the imaginary straight line) [Formula 2] 0.72≦r2 / r1<1.23

2. The first lithium composite oxide is a composite particle containing at least one primary particle, The positive electrode active material according to claim 1, wherein the first lithium composite oxide has a grain boundary density calculated by the following formula 3 for primary particles arranged on a hypothetical straight line crossing the center of the first lithium composite oxide in a cross-sectional SEM image of the first lithium composite oxide, which is 0.75 or less. [Formula 3] The density of grain boundaries = (number of interface surfaces between primary particles arranged on the imaginary straight line / number of primary particles arranged on the imaginary straight line)

3. The positive electrode active material according to claim 2, wherein the first lithium composite oxide has a single crystal structure.

4. The second lithium composite oxide is a composite particle containing at least one primary particle, The positive electrode active material according to claim 1, wherein the second lithium composite oxide has a grain boundary density of 0.90 or more, calculated by the following formula 3 for primary particles arranged on a hypothetical straight line crossing the center of the second lithium composite oxide in a cross-sectional SEM image of the second lithium composite oxide. [Formula 3] The density of grain boundaries = (number of interface surfaces between primary particles arranged on the imaginary straight line / number of primary particles arranged on the imaginary straight line)

5. The positive electrode active material according to claim 1, wherein the first lithium composite oxide and the second lithium composite oxide are represented by the following chemical formula 1. [Chemical formula 1] Li ｗ Ni １－（ｘ＋ｙ＋ｚ） Co ｘ M1 ｙ M2 ｚ O ２＋α (Here, M1 is at least one selected from Mn and Al. M2 is at least one selected from P, Sr, Ba, B, Ce, Cr, Mn, Mo, Na, K, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu. M1 and M2 are different from each other. (0.5 ≤ w ≤ 1.5, 0 ≤ x ≤ 0.50, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.20, 0 ≤ α ≤ 0.02)

6. The positive electrode active material according to claim 1, wherein the first metal oxide and the second metal oxide are each independently represented by the following chemical formula 2. [Chemical formula 2] Li ａ M3 ｂ O ｃ (Here, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd. (0 ≤ a ≤ 10, 0 ≤ b ≤ 8, 2 ≤ c ≤ 13)

7. The positive electrode active material according to claim 6, wherein M3 is at least one selected from Co, Al, Mn, Zr, and Nb.

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