Positive electrode active material and lithium secondary battery comprising 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 including the same, and more specifically, to a bimodal positive electrode active material including a first lithium composite oxide having small particles and a second lithium composite oxide having large particles, the first lithium composite oxide having different average particle sizes, which can prevent deterioration in the electrochemical properties and stability of the positive electrode active material caused by uneven distribution of a coating layer that coats at least a portion of the surface of the small particles and the large particles, a positive electrode including the positive electrode active material, and a lithium secondary battery using the positive electrode. [Background technology]

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

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

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

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

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

[0007] Furthermore, as the degree of cation mixing deepens, a large amount of Li by-products is generated, and most of these Li by-products consist of compounds of LiOH and Li2CO3, which cause gelation during the manufacture of the positive electrode paste and gas generation during charge and discharge after electrode manufacture. The remaining Li2CO3 increases the swelling phenomenon of the cell, reducing cycle life and causing the battery to swell.

[0008] Meanwhile, bimodal cathode active materials, which are a mixture of small and large particles with different average particle sizes, have recently been used to increase the capacity of lithium secondary batteries. When small and large particles are mixed, the small particles with a relatively small average particle size can fill the gaps between the large particles, improving the integration density of the lithium composite oxide within a unit volume and increasing the energy density per unit volume.

[0009] However, when small particles and large particles are mixed with a coating raw material and then fired simultaneously, the coating raw material may be unevenly distributed on the small particles, which have a relatively large specific surface area, resulting in a coating imbalance between the small particles and the large particles. This may cause deterioration in the electrochemical properties and stability of a bimodal cathode active material in which the small particles and the large particles are mixed. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Korean Patent Publication No. 10-2010-0131921 Summary of the Invention [Problem to be solved by the invention]

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

[0012] For example, lithium secondary batteries using LFP have traditionally been used primarily from the perspective of ensuring safety, but recently there has been a trend toward expanding use of nickel-based lithium composite oxides, which have a larger energy capacity per weight than LFP.

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

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

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

[0016] It is yet another object of the present invention to provide a lithium secondary battery using the positive electrode defined herein.

[0017] The object of the present invention is not limited to the object mentioned above (for example, for an electric vehicle), and other unmentioned objects and advantages of the present invention can be understood from the following description and can be more clearly understood from the examples of the present invention. Furthermore, it can be easily understood that the objects and advantages of the present invention can be realized by the means recited in the claims and combinations thereof. [Means for solving the problem]

[0018] According to one aspect of the present invention, there is provided a bimodal positive electrode active material comprising a first lithium composite oxide having small particles and a second lithium composite oxide having 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 including a first metal oxide, and a second coating layer covering at least a portion of the surface of the second lithium composite oxide and including a second metal oxide.

[0020] In this case, when the occupancy (at %) of the first coating layer on the surface of the first lithium composite oxide is referred to as r1 and the occupancy (at %) of the second coating layer on the surface of the second lithium composite oxide is referred to as r2, r1 and r2 can satisfy the following formula 1:

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

[0022] In addition, the first lithium composite oxide and the second lithium composite oxide may 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] (wherein 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, and 0.5≦w≦1.5, 0≦x≦0.50, 0≦y≦0.20, 0≦z≦0.20, and 0≦α≦0.02)

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

[0026] [Chemical formula 2] Li a M3 b O c

[0027] (wherein 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, and 0≦a≦10, 0≦b≦8, and 2≦c≦13.)

[0028] Meanwhile, in the bimodal cathode active material, the first lithium composite oxide and the second lithium composite oxide may have different grain boundary densities. The grain boundary density is an index that indirectly indicates the number of primary particles in a lithium composite oxide constituting a cathode active material in the form of secondary particles formed by aggregation of a plurality of primary particles. In the present invention, the grain boundary densities of the first lithium composite oxide and the second lithium composite oxide are defined by the following Equations 3 and 3:

[0029] Specifically, the first lithium composite oxide may be composite particles including at least one primary particle, and the first lithium composite oxide may have a grain boundary density of 0.75 or less, calculated by the following Equation 3, for primary particles arranged on an imaginary line crossing the center of the first lithium composite oxide in a cross-sectional SEM image of the first lithium composite oxide:

[0030] [Formula 3] Grain boundary density = (number of boundaries between primary particles arranged on the virtual line / number of primary particles arranged on the virtual line)

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

[0032] In addition, the second lithium composite oxide may be a composite particle including at least one primary particle, and the density of crystal grain boundaries, calculated by the following Equation 3, for the primary particles arranged on an imaginary 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:

[0033] [Formula 3] Grain boundary density = (number of boundaries between primary particles arranged on the virtual line / number of primary particles arranged on the virtual line)

[0034] In this way, in the bimodal cathode active material, the first lithium composite oxide and the second lithium composite oxide have grain boundary densities within different ranges, and the deviation between the occupancy rates of the coating layers occupying at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite oxide is reduced, thereby improving the electrochemical properties and stability of the cathode active material.

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

[0036] According to yet another aspect of the present invention, there is provided a lithium secondary battery using the positive electrode defined herein. [Effects of the Invention]

[0037] The positive electrode active materials according to various embodiments of the present invention are bimodal positive electrode active materials including a first lithium composite oxide having small particles and a second lithium composite oxide having large particles, each having different average particle diameters. Since the small particles having a relatively small average particle diameter can fill the voids between the large particles, the integration density of the lithium composite oxide within a unit volume can be improved, thereby increasing the energy density per unit volume.

[0038] In addition, according to the present invention, the deviation in the occupancy rate of the coating layer present on at least a portion of the surface of the small particles and the large particles can be reduced, thereby preventing deterioration in the electrochemical properties and stability of the bimodal cathode active material.

[0039] In particular, according to the present invention, in the bimodal cathode active material, the first lithium composite oxide and the second lithium composite oxide have grain boundary densities within different ranges, thereby further reducing the deviation between the occupancy rates of the coating layers occupying at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite oxide.

[0040] The specific effects of the present invention, together with the above-mentioned effects, will be described below while explaining specific matters for carrying out the invention. [Brief explanation of the drawings]

[0041] [Figure 1] FIG. 1 is a diagram schematically showing a cross section of a lithium composite oxide for calculating the density of crystal grain boundaries defined in the present application. [Figure 2] FIG. 1 is a diagram schematically showing a cross section of a lithium composite oxide for calculating the density of crystal grain boundaries defined in the present application. DETAILED DESCRIPTION OF THE INVENTION

[0042] In order to more readily understand the present invention, certain terms are defined herein for convenience. Unless otherwise defined herein, scientific and technical terms used herein shall have the meanings commonly understood by those of ordinary skill in the art. Furthermore, unless otherwise required by context, singular terms shall be understood to include their plural forms, and plural terms shall be understood to include their singular forms.

[0043] Hereinafter, the positive electrode active material according to the present invention and the lithium secondary battery including the positive electrode active material will be described in more detail.

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

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

[0046] The small particles refer to lithium composite oxides having an average particle size (D50) of 8 μm or less, and the large particles refer to lithium composite oxides having 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 the large particles may have an average particle size of 8.5 to 23.0 μm, for example.

[0047] In the bimodal cathode active materials according to various embodiments of the present invention, the first lithium composite oxide and the second lithium composite oxide having the above-defined average particle size (D50) may be 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 in which it fills the voids between the second lithium composite oxide, or in a form in which the first lithium composite oxide is aggregated together, or attached to the surface of the second lithium composite oxide.

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

[0050] If the ratio of the first lithium composite oxide to the second lithium composite oxide in the positive electrode active material is too high or too low, the press density of the positive electrode active material may decrease, and the effect of improving the energy density per unit volume of the positive electrode active material may become small.

[0051] In addition, the positive electrode active material may 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 above weight ratio.

[0052] Meanwhile, 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] (wherein 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, and 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 having the same composition represented by Chemical Formula 1, but are not necessarily limited thereto. For example, the first lithium composite oxide and the second lithium composite oxide may 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 may be synthesized by calcining precursors having different compositions but different average particle sizes.

[0056] Meanwhile, at least one of the first lithium composite oxide and the second lithium composite oxide represented by 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 may be determined by the content of x+y+z in Chemical Formula 1 below, as follows:

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

[0058] In the case of lithium composite oxides containing Ni, residual lithium, i.e., Li impurities such as LiOH and Li2CO3, may be formed on the surface of the lithium composite oxide due to the mixing of Li and Ni cations. Such Li impurities may cause gelation during the preparation of a paste for manufacturing a positive electrode or may cause swelling of the cell.

[0059] Such Li impurities may be formed in larger amounts in a high-Ni type cathode active material. However, as will be described later, the cathode active material according to the present invention has an advantage in that the Li impurities present on the surface of the lithium composite oxide can be removed during the process of forming a coating layer containing a metal oxide that covers at least a portion of the surface of the first lithium composite oxide and the second lithium composite oxide.

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

[0061] The metal oxide contained in the first coating layer and the second coating layer may be expressed 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, respectively, where the metal oxide represented by the following Chemical Formula 2 exists.

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

[0063] (wherein 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, and 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.

[0065] In addition, the coating layer may have a form in which different metal oxides are present simultaneously within one layer, or different metal oxides represented by Formula 2 are present 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. In addition, the metal oxide may exist in a state of forming a solid solution with the primary particles represented by Chemical Formula 1 above.

[0067] The cathode active material according to this embodiment includes a coating layer covering at least a portion of the surface of the first lithium composite oxide and the second lithium composite oxide, thereby enhancing structural stability. Furthermore, when this cathode active material is used in a lithium secondary battery, the high-temperature storage stability and lifespan of the cathode active material can be improved. Furthermore, the metal oxide reduces residual lithium on the surfaces of the first lithium composite oxide and the second lithium composite oxide, and also acts as a pathway for lithium ion migration, thereby improving the efficiency of the lithium secondary battery.

[0068] The metal oxide is a composite oxide of lithium and an element represented by M3, or an oxide of M3. 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 However, the above examples are merely given for the sake of convenience to facilitate understanding, and the metal oxides defined in the present application are not limited to the above examples.

[0069] In another embodiment, the metal oxide may be an oxide in which lithium and at least two elements represented by M3 are combined, or may further include a metal oxide in which lithium and at least two elements represented by M3 are combined. The metal oxide in which lithium and at least two elements represented by M3 are combined may be, 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 It may be, but is not necessarily limited to, the above.

[0070] Here, the metal oxide may exhibit a concentration gradient that decreases from the surface portion of the secondary particle toward the center portion of the secondary particle, whereby the concentration of the metal oxide may decrease from the outermost surface of the secondary particle toward the center portion 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 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 surface region of the positive electrode active material. Furthermore, the metal oxide prevents the overall structure of the positive electrode active material from being destroyed during electrochemical reactions.

[0072] Additionally, the coating layer may include a first oxide layer including at least one metal oxide represented by Chemical Formula 2 above, and a second oxide layer including at least one metal oxide represented by Chemical Formula 2 above, but different from the metal oxide included in the first oxide layer.

[0073] For example, the first oxide layer may be present to cover at least a portion of the exposed surface of the primary particle present at the outermost periphery of the secondary particle, and the second oxide layer may be present to cover at least a portion of the exposed surface of the primary particle not covered by the first oxide layer and the surface of the first oxide layer.

[0074] Meanwhile, when the first lithium composite oxide and the second lithium composite oxide are defined as composite particles including at least one primary particle, the coating layer not only covers at least a portion of the surface of the composite particle (e.g., secondary particle), but may also be present at the interface between multiple primary particles constituting the composite particle.

[0075] The coating layer may exist as a layer that continuously or discontinuously coats the surfaces of the primary particles and / or the secondary particles. When the coating layer exists discontinuously, the coating layer may exist in the form of islands. 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 aggregation of the primary particles.

[0076] Meanwhile, the first coating layer and the second coating layer can be obtained by mixing precursors of the first lithium composite oxide and the second lithium composite oxide with a raw material of the metal oxide represented by Chemical Formula 2, followed by a primary firing, or by primary firing the precursors of the first lithium composite oxide and the second lithium composite oxide, mixing them with a raw material of the metal oxide represented by Chemical Formula 2, followed by a secondary firing. In this case, the first lithium composite oxide and the second lithium composite oxide can be independently primary fired.

[0077] For example, when the first lithium composite oxide, the second lithium composite oxide, and the metal oxide raw material represented by Chemical Formula 2 are mixed and then coated, the first coating layer and the second coating layer may be formed on the surface of the first lithium composite oxide and the second lithium composite oxide, respectively. In this case, the coating raw material may be unevenly distributed on small particles having a relatively large specific surface area.

[0078] In this case, the deviation between the occupancy of the first coating layer on the surface of the first lithium composite oxide of small particles and the occupancy of the second coating layer on the surface of the second lithium composite oxide becomes significant, which may cause deterioration in the electrochemical properties and stability of a bimodal cathode active material in which the first lithium composite oxide and the second lithium composite oxide are mixed at a predetermined ratio.

[0079] Therefore, in a bimodal cathode active material in which the first lithium composite oxide of small particles and the second lithium composite oxide of large particles are mixed in a predetermined ratio, in order to prevent a decrease in the electrochemical properties and stability of the cathode active material due to a 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 the occupancy rate (at %) of the first coating layer on the surface of the first lithium composite oxide is referred to as r1 and the occupancy rate (at %) of the second coating layer on the surface of the second lithium composite oxide is referred to as r2, and that r1 and r2 satisfy the following formula 1:

[0080] Here, the occupancy (at%) of the first coating layer on the surface of the first lithium composite oxide and the occupancy (at%) of the second coating layer on the surface of the second lithium composite oxide may be defined as the content (at%) of the first coating layer (more specifically, the metal element specific to the metal oxide represented by Chemical Formula 2 in the first coating layer) present on the surface of the first lithium composite oxide and the content (at%) of the second coating layer (more specifically, the metal element specific to the metal oxide represented by Chemical Formula 2 in the second coating layer), respectively. In this case, the metal elements to be measured in 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 the ratio (r2 / r1) thereof can be calculated by measuring the composition of metal elements on the surfaces of the first lithium composite oxide and the second lithium composite oxide where the first coating layer and the second coating layer are not present, and then forming the first coating layer and the second coating layer on the surfaces of the first lithium composite oxide and the second lithium composite oxide, respectively, and then measuring the composition of metal elements on the surfaces, and calculating the change in the content of metal elements specifically contained in the metal oxides constituting the first coating layer and the second coating layer.

[0082] That is, the occupancy rate (r1) of the first coating layer on the surface of the first lithium composite oxide and the occupancy rate (r2) of the second coating layer on the surface of the second lithium composite oxide can mean 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 expressed in the above formula 1 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 relative to the occupancy rate of the second coating layer on the surface of the second lithium composite oxide, resulting in a gap in surface properties between the first lithium composite oxide and the second lithium composite oxide, which may degrade the electrochemical properties and stability of the positive electrode active material.

[0085] More specifically, the r1 and the r2 may satisfy the following formula 2:

[0086] [Formula 2] 0.72≦r2 / r1<1.23

[0087] If r2 / r1 expressed in Equation 2 above is 1.23 or more, the occupancy rate of the second coating layer on the surface of the second lithium composite oxide becomes excessively large relative to the occupancy rate of the first coating layer on the surface of the first lithium composite oxide, which may similarly result in a gap in surface properties between the first lithium composite oxide and the second lithium composite oxide, thereby deteriorating the electrochemical properties and stability of the positive electrode active material.

[0088] As described above, the first lithium composite oxide and the second lithium composite oxide may be composite particles including at least one primary particle capable of lithium intercalation / deintercalation. If the first lithium composite oxide and / or the second lithium composite oxide includes a plurality of primary particles, the plurality of primary particles may exist as secondary particles, which 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 a plurality of primary particles. Voids and / or grain boundaries may exist between the primary particles constituting the secondary particle.

[0090] Additionally, according to the present invention, in the bimodal cathode active material, the first lithium composite oxide and the second lithium composite oxide have grain boundary densities within different ranges, thereby further reducing the deviation between the occupancy rates of the coating layers 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 may be a composite particle including at least one primary particle, and the first lithium composite oxide may have a grain boundary density of 0.75 or less, calculated by the following Equation 3, for primary particles arranged on an imaginary line crossing the center of the first lithium composite oxide in a cross-sectional SEM image of the first lithium composite oxide:

[0092] [Formula 3] Grain boundary density = (number of boundaries between primary particles arranged on the virtual line / number of primary particles arranged on the virtual line)

[0093] 1 and 2 are schematic diagrams showing the cross section of a lithium composite oxide for calculating the density of the grain boundaries defined in the present application. The density of the grain boundaries of the lithium composite oxide calculated with reference to FIGS. 1 and 2 is shown in Table 1 below.

[0094] [Table 1]

[0095] In this case, as shown in FIG. 1, the first lithium composite oxide may have a single crystal structure in which the number of boundaries (grain boundaries) between primary particles arranged on an imaginary line is one and the number of primary particles arranged on the imaginary line is two, resulting in a grain boundary density of 0.5.

[0096] When the grain boundary density represented by Equation 3 has a value of 0.75 or less, it is possible to reduce 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.

[0097] Meanwhile, the second lithium composite oxide may be a composite particle including at least one primary particle, and the second lithium composite oxide may have a grain boundary density of 0.90 or more, calculated by the following Equation 3, for primary particles arranged on an imaginary line crossing the center of the second lithium composite oxide in a cross-sectional SEM image of the second lithium composite oxide:

[0098] [Formula 3] Grain boundary density = (number of boundaries between primary particles arranged on the virtual line / number of primary particles arranged on the virtual line)

[0099] By allowing small particles with a small average particle size to be packed, the integration density of the lithium composite oxide within a unit volume is improved, and the energy density per unit volume can be increased.

[0100] In addition, the bimodal cathode active materials according to various embodiments of the present invention may reduce the deviation in the occupancy rate of the coating layer present on at least a portion of the surface of the first lithium composite oxide and the second lithium composite oxide, thereby preventing deterioration in the electrochemical properties and stability of the bimodal cathode active materials.

[0101] In particular, according to the present invention, in the bimodal cathode active material, the first lithium composite oxide and the second lithium composite oxide have grain boundary densities within different ranges, thereby further reducing the deviation between the occupancy rates of the coating layers occupying at least a portion of the surfaces of the first lithium composite oxide and the second lithium composite oxide.

[0102] Lithium secondary battery According to another aspect of the present invention, a positive electrode may be provided, including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. Here, the positive electrode active material layer may include a positive electrode active material according to various embodiments of the present invention. Therefore, since the positive electrode active material is the same as that described above, detailed description thereof will be omitted for convenience, and only the remaining components not described above 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 surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector typically has a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. It can be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0104] The positive electrode active material layer may be prepared by coating a positive electrode slurry composition containing the positive electrode active material, a conductive material, and optionally a binder, on the positive electrode current collector.

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

[0106] The conductive material is used to impart conductivity to the electrode. Any material that does not cause chemical changes in the battery and has electronic conductivity can be used without any particular limitations. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. One or more of these materials can be used alone or in combination. The conductive material can be contained in an amount of 0.1 to 15 wt % based on the total weight of the positive electrode active material layer.

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

[0108] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode active material described above. Specifically, the positive electrode may be manufactured by dissolving or dispersing the positive electrode active material described above and, optionally, a binder and a conductive material in a solvent to prepare a positive electrode slurry composition, which is then coated on a positive electrode current collector, followed by drying and rolling.

[0109] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, taking into consideration the coating thickness of the slurry and the production yield, and to provide a viscosity that allows excellent thickness uniformity during subsequent coating for the production of a positive electrode.

[0110] In another embodiment, the positive electrode may be manufactured by casting the positive electrode slurry composition onto a separate support, peeling it off from the support, and laminating the resulting film onto a positive electrode current collector.

[0111] According to yet another aspect of the present invention, there is provided an electrochemical device including the above-described positive electrode. The electrochemical device may be, specifically, a battery, a capacitor, or the like, and more specifically, may be a lithium secondary battery.

[0112] The lithium secondary battery may specifically include a positive electrode, a negative electrode facing the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. Here, since the positive electrode is the same as that described above, detailed description thereof will be omitted for convenience, and only the remaining components not described above will be described in detail below.

[0113] The lithium secondary battery may optionally further include a battery container that houses an electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[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 and does not induce chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc., can be used. The negative electrode current collector typically has a thickness of 3 μm to 500 μm. As with the positive electrode current collector, the surface of the current collector can be formed with fine irregularities to strengthen the binding force of the negative electrode active material. The negative electrode current collector can be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0116] The negative electrode active material layer may be prepared by coating a negative electrode slurry composition containing the negative electrode active material, a conductive material, and optionally a binder, on the negative electrode current collector.

[0117] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO βExamples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide (0<β<2); or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. A mixture of two or more of these may be used. A thin film of metallic lithium may also be used as the negative electrode active material. Both low-crystalline carbon and high-crystalline carbon may be used as the carbon material. Typical low-crystalline carbons are soft carbon and hard carbon, while typical high-crystalline carbons are amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbons such as petroleum or coal tar pitch-derived cokes.

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

[0119] The binder is a component that helps bind the conductive material, active material, and current collector together, and is typically added in an amount of 0.1 to 10 wt% based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0120] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 10 wt% or less, preferably 5 wt% or less, based on the total weight of the negative electrode active material layer. The conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; 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.

[0121] In one embodiment, the negative electrode active material layer may be prepared by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent, on a negative electrode current collector and drying the coating. Alternatively, the negative electrode slurry composition may be cast on a separate support, peeled from the support, and the resulting film may be laminated on the negative electrode current collector.

[0122] In another embodiment, the negative electrode active material layer may be prepared by coating a negative electrode slurry composition prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector and drying the coating; or by casting the negative electrode slurry composition on a separate support, peeling it off from the support, and laminating the resulting film on the negative electrode current collector.

[0123] Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without any particular limitations. It is particularly preferable that the separator has low resistance to ion movement of the electrolyte and excellent electrolyte impregnation capacity. Specifically, porous polymer films, such as those made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminate structures of two or more layers thereof can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, separators coated with ceramic components or polymeric materials to ensure heat resistance or mechanical strength can also be used, and they can be selectively used in single-layer or multi-layer structures.

[0124] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

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

[0126] The organic solvent may be any solvent capable of acting as a medium for the movement of ions involved in the electrochemical reaction of the battery. Specific examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of suitable solvents include carbonate-based solvents such as ethylene carbonate (PC) and polycarbonate; alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and may contain a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can enhance the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred. In this case, the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, which can result in excellent electrolyte performance.

[0127] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without any particular limitations. Specifically, examples of the lithium salt include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2. The lithium salt is preferably used at a concentration in the range of 0.1 to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0128] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purposes of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. In this case, the additives may be contained in an amount of 0.1 to 5 wt % based on the total weight of the electrolyte.

[0129] As described above, the lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and life characteristics, and is therefore useful in portable devices such as mobile phones, notebook computers, and digital cameras, and in electric vehicles such as hybrid electric vehicles (HEVs).

[0130] The shape of the lithium secondary battery according to the present invention is not particularly limited, and may be a cylindrical shape using a can, a prismatic shape, a pouch shape, a coin shape, etc. Furthermore, the lithium secondary battery may be used as a battery cell used as a power source for a small device, and may also be preferably used as a unit battery for a medium- to large-sized battery module including a plurality of battery cells.

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

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

[0133] The present invention will be described in more detail below through examples. However, these examples are merely for the purpose of illustrating the present invention, and it should be understood that the scope of the present invention is not limited to these examples.

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

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

[0136] Separately, the second hydroxide precursor was mixed with LiOH (Li / (Ni+Co+Mn) molar ratio = 1.05 ± 0.05), and then heated to 700°C at a rate of 2°C per minute in a calcination furnace while maintaining an O2 atmosphere, and heat-treated for 12 hours (primary calcination) to obtain a 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 prepare a mixture, and then the mixture was washed with distilled water for 1 hour.

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

[0139] Finally, the mixture was heated to 700°C at a rate of 2°C per minute in an O2 atmosphere in a firing furnace for 12 hours (secondary firing), to obtain a bimodal cathode active material in which the first lithium composite oxide having small particles and the second lithium composite oxide having large particles were mixed in a predetermined ratio.

[0140] (2) Example 2 A 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 A 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 A 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 A 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 50:50 before washing with water.

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

[0145] (7) Example 7 A positive electrode active material was prepared in the same manner as in Example 3, except that 0.5 mol % of a Sr-containing compound (Sr(OH)2) was additionally mixed with each of the first hydroxide precursor and the second hydroxide precursor before primary calcination, and then heat-treated.

[0146] (8) Example 8 A positive electrode active material was prepared in the same manner as in Example 3, except that 0.5 mol % of a Mg-containing compound (Mg(OH)2) was additionally mixed with each of the first hydroxide precursor and the second hydroxide precursor before primary firing, and then heat-treated.

[0147] (9) Example 9 A 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 additionally mixed with each of the first hydroxide precursor and the second hydroxide precursor before primary calcination, and then heat-treated.

[0148] (10) Example 10 A positive electrode active material was prepared in the same manner as in Example 3, except that 0.2 mol% of an Nb-containing compound (NbO) was additionally mixed with each of the first hydroxide precursor and the second hydroxide precursor before the primary calcination, and then the mixture was heat-treated.

[0149] (11) Example 11 A positive electrode active material was prepared in the same manner as in Example 3, except that 0.2 mol % of a W-containing compound (WO) was additionally mixed with each of the first hydroxide precursor and the second hydroxide precursor before the primary calcination, and then the mixture was heat-treated.

[0150] (12) Example 12 A positive electrode active material was prepared in the same manner as in Example 3, except that 0.3 mol % of a Zr-containing compound (ZrO) was additionally mixed with each of the first hydroxide precursor and the second hydroxide precursor before the primary calcination, and then the mixture was heat-treated.

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

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

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

[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 prepare a mixture. Distilled water was added to the mixture, and the mixture was washed for 1 hour. The mixture was then dried in a vacuum dryer at 120°C for 12 hours to obtain a cathode active material. The dried cathode active material was mixed with 1.0 mol% of NbO, and the mixture was then heated to 700°C at a rate of 2°C per minute in a calcination furnace while maintaining an O atmosphere, and heat-treated for 12 hours (secondary calcination) to coat Nb. A cathode active material was prepared in the same manner as in Example 12.

[0155] (17) Example 17 The dried cathode active material was mixed with 1.0 mol % of cobalt phosphate instead of 1.0 mol % of NbO, and then heated to 700°C at a rate of 2°C per minute in an O atmosphere in a firing furnace for 12 hours (secondary firing), to prepare a cathode active material in the same manner as in Example 16, except that the cobalt phosphate coating was performed.

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

[0157] (19) Comparative Example 2 The first hydroxide precursor was mixed with LiOH (Li / (Ni+Co+Mn) molar ratio = 1.05 ± 0.05), and then heated to 700°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calcination furnace, and heat-treated for 12 hours (first calcination) to obtain a first lithium composite oxide having small particles. A 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 to prepare a mixture.

[0158] (20) Comparative Example 3 The first hydroxide precursor was mixed with LiOH (Li / (Ni+Co+Mn) molar ratio = 1.05 ± 0.05), and then heated to 700°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calcination furnace, and heat-treated for 12 hours (first calcination) to obtain a first lithium composite oxide having small particles. A 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 to prepare a mixture.

[0159] (21) Comparative Example 4 The first hydroxide precursor was mixed with LiOH (Li / (Ni+Co+Mn) molar ratio = 1.05 ± 0.05), and then heated to 700°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calcination furnace, and heat-treated for 12 hours (first calcination) to obtain a first lithium composite oxide having small particles. A 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 to prepare a mixture.

[0160] (22) Comparative Example 5 The first hydroxide precursor was mixed with LiOH (Li / (Ni+Co+Mn) molar ratio = 1.05 ± 0.05), and then heated to 700°C at a rate of 2°C per minute while maintaining an O2 atmosphere in a calcination furnace, and heat-treated for 12 hours (first calcination) to obtain a first lithium composite oxide having small particles. A 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 50:50 to prepare a mixture.

[0161] (23) Comparative Example 6 A 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 secondary batteries A positive electrode slurry was prepared by dispersing 92 wt% of the positive electrode active material prepared in Preparation Example 1, 4 wt% of artificial graphite, and 4 wt% of a PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The positive electrode slurry was uniformly applied to a 15 μm-thick aluminum foil and dried in a vacuum at 135° C. to prepare a positive electrode for a lithium secondary battery.

[0163] A coin battery was fabricated using a lithium foil as a counter electrode for the positive electrode, a porous polyethylene film (Celgard 2300, thickness: 25 μm) as a separator, and an electrolyte solution of 1.15 M LiPF6 in a solvent of ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7.

[0164] Experimental Example 1: Structural analysis of positive electrode active material (1) SEM / EDS analysis of positive electrode active material In order to measure the distribution of coating layers and the contents of coating elements on the surfaces of the first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) contained in the positive electrode active material prepared in Preparation Example 1, the surfaces of the first lithium composite oxide and the second lithium composite oxide (uncoated) were first analyzed by EDS to set a baseline.

[0165] Thereafter, the surfaces of the first and second lithium composite oxides coated by the predetermined method disclosed in Preparation Example 1 were analyzed by EDS, and the difference in the content of the 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 the positive electrode active material Cross-sectional SEM images were obtained using a FE-SEM (Bruker) for each of the first lithium composite oxide having small particles and the second lithium composite oxide having large particles contained in the positive electrode active material prepared according to Preparation Example 1, and then the average grain boundary density was calculated from the cross-sectional SEM images using the following Equation 3:

[0168] [Formula 3] Grain boundary density = (number of boundaries between primary particles arranged on an imaginary line crossing the center of the lithium composite oxide in a cross-sectional SEM image of the lithium composite oxide / number of primary particles arranged on the imaginary line)

[0169] The measurement results of the grain boundary density 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 battery prepared in Preparation Example 2 was subjected to a charge-discharge experiment using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 3.0 V to 4.25 V, and a discharge rate of 0.5 C to 4.0 C to measure the initial charge capacity, initial discharge capacity, and energy density. The energy density (Wh / L) was calculated using the following equation 4.

[0172] [Formula 4] Energy density (Wh / L) = initial discharge capacity * average voltage * press density

[0173] In addition, the lithium secondary battery manufactured by the above-mentioned method was charged and discharged 50 times at a temperature of 25°C and a driving voltage range of 3.0V to 4.25V under the condition of 1C / 1C, and then the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention rate; capacity retention) was measured.

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

[0175] [Table 4]

[0176] Referring to the results in Table 4, it can be seen that the positive electrode active materials according to Examples 1 to 17 exhibit an energy density of 2800 Wh / L or more and at the same time exhibit a lifespan characteristic of 88% or more, whereas the positive electrode active materials according to Comparative Examples 1 to 6 have lower energy densities and lifespan characteristics than the positive electrode active materials according to Examples 1 to 17.

[0177] Experimental Example 3: Evaluation of the stability of positive electrode active materials and lithium secondary batteries (1) Evaluation of the thermal stability of the positive electrode active material To evaluate the thermal stability of the cathode active materials prepared in Preparation Example 1, weight loss was measured at a heating rate of 10°C / min from 25°C to 350°C under Ar atmosphere at atmospheric pressure using a thermogravimetric analyzer (TA Instruments, Q20). 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] Referring to the results in Table 5, it was confirmed that the onset temperatures (op-set) at which the weight loss (thermal decomposition) peak appears in the positive electrode active materials according to Examples 1 to 17 are higher than those of the positive electrode active materials according to Comparative Examples 1 to 6. In other words, it can be seen that the positive electrode active materials according to Examples 1 to 17 have better thermal stability than the positive electrode active materials according to Comparative Examples 1 to 6.

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

[0181] [Table 6]

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

[0183] Although the embodiments of the present invention have been described above, a person having ordinary knowledge in the art may modify and change the present invention in various ways by adding, changing, deleting or adding components within the scope of the concept of the present invention as set forth in the claims, and this also falls within the scope of the present invention.

Claims

1. The positive electrode active material has a bimodal structure and includes a first lithium composite oxide having small particles and a second lithium composite oxide having large particles, The first lithium composite oxide has an average particle size (D50) of 8 μm or less, The second lithium composite oxide has an average particle size (D50) of 8.5 μm or more, The positive electrode active material is a first coating layer covering at least a portion of the surface of the first lithium composite oxide and including a first metal oxide; a second coating layer covering at least a portion of the surface of the second lithium composite oxide and including a second metal oxide; and the first lithium composite oxide is a composite particle including at least one primary particle, The first lithium composite oxide is a positive electrode active material, in which an average value of a grain boundary density calculated by the following Equation 3 for primary particles arranged on an imaginary line crossing a center of the first lithium composite oxide in a cross-sectional SEM image of the first lithium composite oxide is equal to or less than an average value of a grain boundary density calculated by the following Equation 3 for primary particles arranged on an imaginary line crossing a center of the second lithium composite oxide in a cross-sectional SEM image of the second lithium composite oxide: [Formula 3] Grain boundary density = (number of boundaries between primary particles arranged on the imaginary line / number of primary particles arranged on the imaginary line)

2. the first lithium composite oxide is a composite particle including at least one primary particle, 2. The positive electrode active material of claim 1, wherein the first lithium composite oxide has a grain boundary density of 0.75 or less, as calculated by Equation 3 below, for primary particles arranged on an imaginary line crossing a center of the first lithium composite oxide in a cross-sectional SEM image of the first lithium composite oxide: [Formula 3] Grain boundary density = (number of boundaries between primary particles arranged on the imaginary line / number of primary particles arranged on the imaginary 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 including at least one primary particle, 2. The positive electrode active material of claim 1, wherein the second lithium composite oxide has a grain boundary density of 0.90 or more, as calculated by Equation 3 below, for primary particles arranged on an imaginary line crossing a center of the second lithium composite oxide in a cross-sectional SEM image of the second lithium composite oxide: [Formula 3] Grain boundary density = (number of boundaries between primary particles arranged on the imaginary line / number of primary particles arranged on the imaginary line)

5. The positive electrode active material of 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 (where, 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. 2. The positive electrode active material of claim 1, wherein the first metal oxide and the second metal oxide contain at least one coating element selected from Co, Al, Mn, Zr, and Nb.

7. 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, both of which are determined by SEM / EDS analysis, is referred to as 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, both of which are determined by SEM / EDS analysis, is referred to as r2, The positive electrode active material according to claim 6 , wherein r1 and r2 satisfy the following formula 1: [Formula 1] 0.71<r2 / r1

8. The positive electrode active material according to claim 1 , wherein r1 and r2 satisfy the following formula 2: [Formula 2] 0.72≦r2 / r1<1.23

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

10. The positive electrode active material according to claim 9 , wherein M3 is at least one selected from the group consisting of Co, Al, Mn, Zr, and Nb.

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