Positive electrode active material and lithium secondary battery comprising same

Abstract

Provided is a positive electrode active material including a first positive electrode active material and a second positive electrode active material The first positive electrode active material includes lithium iron phosphate-based particles having an olivine structure and having a coating layer formed on the surface thereof. The second positive electrode active material includes lithium transition metal oxide-based particles having a layered structure. The lithium iron phosphate-based particles have an average particle size (D50) of less than 1 µm. The coating layer contains 1.8 wt% or more and less than 5 wt% of carbon on the basis of the total weight of the lithium iron phosphate-based particles. The lithium iron phosphate-based particles are nanoscale particles, and by adjusting the content of carbon in the carbon coating layer to a specific level, powder resistance can be reduced and rolling density increased when the lithium iron phosphate-based particles are mixed with the lithium transition metal oxide-based particles which do not have a carbon coating layer.

Description

positive electrode active material and lithium secondary battery containing the same

[0001] The present invention relates to a positive electrode active material and a lithium secondary battery comprising the same. Specifically, the present invention relates to a positive electrode active material comprising a mixture of lithium iron phosphate-based particles having an olivine structure and lithium transition metal oxide-based particles having a layered structure, and a lithium secondary battery comprising the same.

[0002] With the recent rapid proliferation of battery-powered electronic devices such as mobile phones, laptop computers, and electric vehicles, the demand for small, lightweight, and relatively high-capacity rechargeable batteries is increasing rapidly. In particular, lithium-ion batteries are gaining prominence as power sources for portable devices due to their lightweight nature and high energy density. Consequently, active research and development efforts are underway to improve the performance of lithium-ion batteries.

[0003] In a lithium secondary battery, electrical energy is produced by oxidation and reduction reactions when lithium ions are inserted into or removed from the positive and negative electrodes, which are composed of active materials capable of lithium ion intercalation and deintercalation, with an organic or polymer electrolyte charged between them.

[0004] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMn2O4, etc.), and lithium iron phosphate compounds (LiFePO4) have been used as cathode active materials for lithium-ion batteries. Among these, lithium cobalt oxide (LiCoO2) is widely used and applied as a cathode active material for high-voltage applications due to its advantages of high operating voltage and excellent capacity characteristics. However, due to the rising price and supply instability of cobalt (Co), there are limitations to its mass use as a power source in fields such as electric vehicles, leading to the emergence of a need for the development of cathode active materials that can replace it.

[0005] Accordingly, a nickel-cobalt-manganese-based lithium composite transition metal oxide (hereinafter simply referred to as 'NCM-based lithium composite transition metal oxide') was developed in which a portion of the cobalt (Co) was substituted with nickel (Ni) and manganese (Mn).

[0006] However, conventionally developed NCM-based lithium composite transition metal oxides generally exist in the form of secondary particles formed by the aggregation of primary particles. Due to their large specific surface area, low particle strength, and high lithium byproduct content, they exhibit problems such as high gas generation and reduced stability during cell operation. In particular, in the case of high-nickel (High-Ni) NCM-based lithium composite transition metal oxides in which the nickel (Ni) content is increased to over 65 mol% to secure high capacity, structural and chemical stability are further degraded, and securing thermal stability becomes even more difficult.

[0007] Accordingly, there is still a need to develop a cathode active material with secured stability capable of realizing high capacity, while overcoming the disadvantages of high-nickel (High-Ni) NCM-based lithium composite transition metal oxides.

[0008] [Prior Art Literature]

[0009] [Patent Literature]

[0010] (Patent Document 1) Korean Published Patent Application No. 2018-0013512

[0011] The present invention aims to provide a positive electrode active material comprising a mixture of lithium iron phosphate-based particles with an olivine structure and lithium transition metal oxide-based particles with a layered structure, wherein a carbon coating layer is introduced to the lithium iron phosphate-based particles to control powder resistance and rolling density, and a lithium secondary battery comprising the same.

[0012] According to the first aspect of the present invention,

[0013] The present invention provides a positive electrode active material comprising a first positive electrode active material and a second positive electrode active material.

[0014] In one embodiment of the present invention, the first positive electrode active material comprises olivine-structured lithium iron phosphate-based particles having a coating layer formed on their surface, and the second positive electrode active material comprises layered-structured lithium transition metal oxide-based particles.

[0015] In one embodiment of the present invention, the lithium iron phosphate-based particles have an average particle size (D 50 ) is less than 1㎛, and the coating layer is 1.8% by weight or more and less than 5% by weight based on the total weight of the lithium iron phosphate-based particles.

[0016] In one embodiment of the present invention, the lithium iron phosphate-based particles comprise a compound represented by the following chemical formula 1:

[0017] [Chemical Formula 1]

[0018] Li 1+a Fe 1-x-y Mn x M y PO4

[0019] In the above chemical formula 1,

[0020] M is at least one element selected from the group consisting of V, Al, Ni, Co, Mg, Nb, Mo, W, Ti, Sr, Zr, B, Y and Zn, and

[0021] -0.1≤a≤0.1, 0<x≤0.7, 0≤y≤0.1.

[0022] In one embodiment of the present invention, in the above formula 1, x is 0.2≤x≤0.6.

[0023] In one embodiment of the present invention, the lithium transition metal oxide-based particles comprise a compound represented by the following chemical formula 2:

[0024] [Chemical Formula 2]

[0025] Li 1+a (Ni x Co y Mn 1-x-y-z M' z )O2

[0026] In the above chemical formula 2,

[0027] M' is at least one element selected from the group consisting of V, Al, Mg, Nb, Mo, W, Ti, Sr, Zr, B, Y and Zn, and

[0028] -0.1≤a≤0.1, 0≤x≤1, 0≤y≤0.5, 0≤z≤0.05 and x+y+z≤1.

[0029] In one embodiment of the present invention, in the above formula 2, x is 0.6≤x≤1.

[0030] In one embodiment of the present invention, the lithium iron phosphate-based particles have an average particle size (D) of 200 nm to 300 nm. 50 has ).

[0031] In one embodiment of the present invention, the lithium transition metal oxide-based particles have an average particle size (D) of 2 μm to 20 μm. 50 has ).

[0032] In one embodiment of the present invention, the lithium iron phosphate-based particles have an aspect ratio of 1.2 to 1.51.

[0033] In one embodiment of the present invention, the lithium iron phosphate-based particles are in the form of primary particles, and the lithium transition metal oxide-based particles are in the form of secondary particles.

[0034] In one embodiment of the present invention, the weight of the first positive active material in the positive active material is greater than the weight of the second positive active material.

[0035] In one embodiment of the present invention, the first positive active material is included in the positive active material in an amount of 60% to 90% by weight based on the total weight of the positive active material.

[0036] In one embodiment of the present invention, the second positive active material is included in the positive active material in an amount of 20 to 60 parts by weight based on 100 parts by weight of the first positive active material.

[0037] In one embodiment of the present invention, the positive active material has a rolled density of 2.6 g / cc to 3.0 g / cc.

[0038] In one embodiment of the present invention, the positive active material has a powder resistance of 5 Ω·m to 100 Ω·m under 2-ton pressure.

[0039] According to a second aspect of the present invention,

[0040] The present invention provides a positive electrode for a lithium secondary battery comprising the positive electrode active material described above.

[0041] According to the third aspect of the present invention,

[0042] The present invention provides a lithium secondary battery comprising the aforementioned positive electrode active material.

[0043] A positive electrode active material according to one embodiment of the present invention comprises lithium iron phosphate-based particles with an olivine structure having a carbon coating layer formed on their surface and lithium transition metal oxide-based particles with a layered structure. The lithium iron phosphate-based particles are nano-scale particles, and by controlling the carbon content within the carbon coating layer to a specific level, the powder resistance can be lowered and the rolling density increased when mixed with lithium transition metal oxide-based particles without a carbon coating layer. When applied to a battery, the positive electrode active material can significantly improve performance, such as discharge energy density at high rates.

[0044] Figure 1 is a segmentation image of the first positive active material of Preparation Example 1 obtained according to Experimental Example 1.

[0045] Figure 2 is a segmentation image of the first positive active material of Comparative Manufacturing Example 1 obtained according to Experimental Example 1.

[0046] Figure 3 is a segmentation image of the first positive active material of Comparative Manufacturing Example 2 obtained according to Experimental Example 1.

[0047] Figure 4 is a relatively low-magnification segmentation image to confirm the physical properties of the secondary particles for the second positive active material of the example obtained according to Experimental Example 2.

[0048] Figure 5 is a relatively high-magnification segmentation image to confirm the physical properties of the primary particles for the second positive electrode active material of the example obtained according to Experimental Example 2.

[0049] FIG. 6 is a high rate (4.0C) discharge profile for Example 1 and Comparative Example 3 obtained according to Experimental Example 4.

[0050] FIG. 7 is a high rate (4.0C) discharge profile for Comparative Example 1 and Comparative Example 4 obtained according to Experimental Example 4.

[0051] FIG. 8 is a high rate (4.0C) discharge profile for Comparative Example 2 and Comparative Example 5 obtained according to Experimental Example 4.

[0052] All embodiments provided according to the present invention can be achieved by the following description. It should be understood that the following description describes preferred embodiments of the present invention and that the present invention is not necessarily limited thereto.

[0053] Where measurement conditions and methods are not specifically described for the physical properties described in this specification, said physical properties are measured according to measurement conditions and methods generally used by a person skilled in the art in the relevant technical field.

[0054]

[0055] <Cathode Active Material>

[0056]

[0057] The present invention provides a positive electrode active material comprising a first positive electrode active material and a second positive electrode active material. The first positive electrode active material comprises lithium iron phosphate-based particles having an olivine structure with a carbon coating layer formed on their surface. The second positive electrode active material comprises lithium transition metal oxide-based particles having a layered structure. To compensate for low electronic conductivity, lithium iron phosphate-based particles must have a nanoscale particle size of less than 1 μm, and a carbon coating layer is essentially required. However, the carbon coating layer can hinder particle growth and cause a decrease in rolling density, and when mixed with other positive electrode active materials, the reduced rolling density can lower the interfacial contact rate between particles and impair rate characteristics. According to one embodiment of the present invention, the lithium iron phosphate-based particles are nanoscale particles, and by controlling the carbon content within the carbon coating layer to a specific level, the powder resistance can be lowered and the rolling density increased when mixed with lithium transition metal oxide-based particles without a carbon coating layer. A mixed positive electrode active material according to one embodiment of the present invention can significantly improve performance, such as discharge energy density at high rates, when applied to a battery.

[0058] The first positive electrode active material comprises olivine-structured lithium iron phosphate particles having a coating layer formed on their surface. According to one embodiment of the present invention, the lithium iron phosphate particles comprise a compound represented by the following chemical formula 1. The compound represented by the following chemical formula 1 is an internal component of the lithium iron phosphate particles excluding the coating layer.

[0059] [Chemical Formula 1]

[0060] Li 1+a Fe 1-x-y Mn x M y PO4

[0061] In the above chemical formula 1,

[0062] M is at least one element selected from the group consisting of V, Al, Ni, Co, Mg, Nb, Mo, W, Ti, Sr, Zr, B, Y and Zn, and

[0063] -0.1≤a≤0.1, 0<x≤0.7, 0≤y≤0.1. Specifically, x can be 0.2≤x≤0.6 and y can be 0≤y≤0.05.

[0064] Typically, so-called LFP series cathode active materials, such as lithium iron phosphate (LiFePO4), have high structural stability against volume changes caused by charging and discharging of the battery compared to other types of cathode active materials, such as nickel-cobalt-manganese (NCM) ternary cathode active materials, because oxygen atoms and phosphorus atoms form strong covalent bonds (P=O) in the PO4 tetrahedral structure forming an olivine structure. Furthermore, they exhibit excellent characteristics in maintaining high thermal stability because oxygen atoms are not easily removed by thermal decomposition. Therefore, LFP series cathode active materials are materials with excellent stability, exhibiting minimal capacity reduction due to the collapse of the crystal structure caused by overcharging and generating little gas, which can secure the stability required, especially for large lithium-ion batteries.

[0065] However, the aforementioned olivine-structured LFP-based cathode active materials have a problem in that the movement of lithium ions is not smooth because oxygen atoms are strongly bonded in a hexagonal close-packed structure, and the flow of electrons is also not smooth because the electrical conductivity is relatively low. To solve this problem, a method of mixing NCM-based cathode active materials with high energy capacity with LFP-based cathode active materials is being studied.

[0066] However, when the aforementioned LFP-based cathode active material and NCM-based cathode active material are mixed, another problem arises, such as a decrease in output due to a voltage drop caused by the difference in operating voltage between the two cathode active materials.

[0067] In this regard, the present invention provides a mixed cathode active material that can prevent output degradation due to voltage drop and secure thermal stability by mixing a first cathode active material comprising an olivine structure in which a portion of iron (Fe) in a pure lithium iron phosphate (LFP) cathode active material is substituted with manganese (Mn), etc., and a second cathode active material comprising a layered structure of lithium transition metal oxide, thereby reducing the difference in operating voltage between the first cathode active material and the second cathode active material. However, if the content of manganese substituted in the cathode active material of the olivine structure represented by Chemical Formula 1 is excessive, there may be a problem in which the structural stability of the battery is reduced in the charged state due to manganese distortion within the olivine structure lattice, so the content of manganese needs to be appropriately controlled.

[0068] When the mixed cathode active material according to the present invention is applied to the cathode of a lithium secondary battery, the electrical conductivity can be improved compared to when an LFP-based cathode active material is used alone, and the pellet density under pressure can be high, thereby improving the problem of low electrode density of LFP-based cathode active materials, and thus enabling the high capacity of the lithium secondary battery.

[0069] The second positive electrode active material mixed with the first positive electrode active material comprises lithium transition metal oxide-based particles having a layered structure. The lithium transition metal oxide-based particles are not particularly limited as long as they are materials generally used in the relevant technical field, and unlike lithium iron phosphate-based particles, they do not include a separate coating layer on their surface. According to one embodiment of the present invention, the lithium transition metal oxide-based particles comprise a compound represented by the following chemical formula 2.

[0070] [Chemical Formula 2]

[0071] Li 1+a (Ni x Co y Mn 1-x-y-z M' z )O2

[0072] In the above chemical formula 2,

[0073] M' is at least one element selected from the group consisting of V, Al, Mg, Nb, Mo, W, Ti, Sr, Zr, B, Y and Zn, and

[0074] -0.1≤a≤0.1, 0≤x≤1, 0≤y≤0.5, 0≤z≤0.05, and x+y+z≤1. Specifically, x can be 0.6≤x≤1.

[0075] Since the second positive active material has different physical properties from the first positive active material, it is necessary to appropriately adjust the physical properties of the first positive active material or the second positive active material in order for the first positive active material and the second positive active material to be effectively mixed.

[0076] The lithium iron phosphate-based particles constituting the first positive electrode active material may be particles having a nanoscale particle size, and the nanoscale is the average particle size (D 50 ) means that it is less than 1㎛. The lower limit may be 1nm. In this specification, the average particle size (D 50 ) is a particle size corresponding to 50% of the volume accumulation in the particle size measuring device, and this can also be interpreted as the average value of the particle size. The above particle size is based on primary particles. Specifically, the average particle size (D of the lithium iron phosphate-based particles) 50 ) is less than 1㎛, 900nm or less, 800nm ​​or less, 700nm or less, 600nm or less, 500nm or less, 400nm or less, 300nm or less, and may be 1nm or more, 50nm or more, 100nm or more, 150nm or more, 200nm or more. According to one embodiment of the present invention, the lithium iron phosphate-based particles have an average particle size (D) of 200nm to 300nm. 50 It has ). The average particle size (D) of the lithium iron phosphate-based particles. 50) can be effectively mixed with micro-scale lithium transition metal oxide-based particles while including a carbon coating layer within the aforementioned range.

[0077] The lithium transition metal oxide-based particles constituting the second positive electrode active material may be particles having a micro-scale particle size, and the micro-scale is the average particle size (D 50 It means that ) is 1㎛ or more. The upper limit may be 100㎛. Specifically, the average particle size (D) of the lithium transition metal oxide-based particles. 50 ) is 1㎛ or more, 1.5㎛ or more, 2㎛ or more, 2.5㎛ or more, 3㎛ or more, and 100㎛ or less, 90㎛ or less, 80㎛ or less, 70㎛ or less, 60㎛ or less, 50㎛ or less, 40㎛ or less, 30㎛ or less, 20㎛ or less, and 15㎛ or less. According to one embodiment of the present invention, the lithium transition metal oxide-based particles have an average particle size (D) of 2㎛ to 20㎛. 50 It has ). The average particle size (D) of the lithium transition metal oxide-based particles above. 50 ) may be commonly used in the relevant technical field, and in the present invention, compatibility with various lithium transition metal oxide particles can be improved by controlling physical properties such as the size and shape of lithium iron phosphate particles rather than lithium transition metal oxide particles.

[0078] The above lithium transition metal oxide-based particles may be in the form of secondary particles formed by the aggregation of primary particles. Unlike the above lithium iron phosphate-based particles, the lithium transition metal oxide-based particles have a relatively regular secondary particle shape, and this secondary particle shape can be maintained even after being mixed with the lithium iron phosphate-based particles. On the other hand, the lithium iron phosphate-based particles may be in the form of primary particles. Although the above lithium iron phosphate-based particles may exist with some aggregation of primary particles, this is not regular compared to the lithium transition metal oxide-based particles and therefore cannot be defined as a specific shape; thus, in this specification, the lithium iron phosphate-based particles are defined as primary particle shapes, and the particulate characteristics of the above-described lithium iron phosphate-based particles are based on the primary particles. When the above lithium transition metal oxide-based particles are in the form of secondary particles, the above-described average particle size (D 50 ) is the average secondary particle size (D 50 It means ).

[0079] In the case of lithium transition metal oxide-based particles in the form of secondary particles, primary particles and secondary particles can be distinguished in terms of aspect ratio. Primary particles have a relatively large aspect ratio, but the aspect ratio may decrease as they aggregate into secondary particles. According to one embodiment of the present invention, in the lithium transition metal oxide-based particles, the aspect ratio of the primary particles is greater than the aspect ratio of the secondary particles. Specifically, the aspect ratio of the primary particles may be greater than 1.5 and less than or equal to 2.0, 1.6 to 1.95, or 1.7 to 1.9. In addition, the aspect ratio of the secondary particles may be greater than 1.0 and less than or equal to 1.5, 1.05 to 1.4, or 1.1 to 1.3. The aspect ratio characteristics of the primary and secondary particles described above can help improve the performance of the battery by mixing with the first positive electrode active material.

[0080] A coating layer formed on the surface of lithium iron phosphate-based particles contains carbon. The carbon coating layer can improve the electronic conductivity of lithium iron phosphate-based particles that have low electronic conductivity. However, since the rolling density may decrease along with particle growth due to the formation of the carbon coating layer, it is necessary to appropriately control the carbon content within the coating layer. According to one embodiment of the present invention, the coating layer contains 1.8 weight% or more and less than 5 weight% of carbon based on the total weight of the lithium iron phosphate-based particles. Specifically, the carbon content may be 1.8 wt% or more, 1.81 wt% or more, 1.82 wt% or more, 1.83 wt% or more, 1.84 wt% or more, 1.85 wt% or more, less than 5 wt%, 4.5 wt% or less, 4.0 wt% or less, 3.5 wt% or less, 3.0 wt% or less, 1.8 wt% or more and less than 5 wt%, 1.83 wt% to 4.5 wt%, and 1.85 wt% to 4.0 wt%. By controlling the carbon content in the coating layer to the range described above, the particles may have a size and shape that can be effectively mixed with lithium transition metal oxide particles while appropriately imparting electronic conductivity to the lithium iron phosphate-based particles.

[0081] The above coating layer may be an amorphous carbon layer. Here, "amorphous" means not exhibiting a distinct crystal structure. The above amorphous carbon layer not only imparts electronic conductivity to the lithium iron phosphate-based particles but also prevents adverse reactions between the lithium iron phosphate-based particles and the electrolyte, thereby improving stability. The above amorphous carbon may include, for example, a material selected from soft carbon (low-temperature calcined carbon), hard carbon, pitch carbide, mesophase pitch carbide, calcined coke, and combinations thereof.

[0082] To effectively mix the above lithium iron phosphate-based particles with lithium transition metal oxide-based particles, the shape of the particles as well as the particle size can be controlled. The above lithium iron phosphate-based particles are not perfectly spherical, and it may be desirable for them to have a specific aspect ratio. The aspect ratio refers to a value obtained by dividing the length of the major axis of the particle by the length of the minor axis, and the value is 1 or greater, and if the value is 1, it indicates a spherical shape. According to one embodiment of the present invention, the above lithium iron phosphate-based particles have an aspect ratio of 1.2 to 1.51. Specifically, the aspect ratio of the above lithium iron phosphate-based particles is 1.2 or greater, 1.25 or greater, 1.3 or greater, 1.51 or less, 1.505 or less, 1.5 or less, and may be 1.2 to 1.51, 1.25 to 1.505, or 1.3 to 1.5. The aspect ratio of the above-mentioned lithium iron phosphate-based particles can be effectively mixed with lithium transition metal oxide-based particles when adjusted to the range described above.

[0083] The content of the first positive active material and the second positive active material in the above positive active material can be appropriately controlled. Since the positive active material according to one embodiment of the present invention is basically based on the first positive active material, it contains the first positive active material in the largest proportion. According to one embodiment of the present invention, the weight of the first positive active material in the above positive active material is greater than the weight of the second positive active material. According to one embodiment of the present invention, the first positive active material is included in the positive active material in an amount of 60% to 90% by weight based on the total weight of the positive active material. Specifically, the content of the first positive active material may be 60% by weight or more, 65% by weight or more, 70% by weight or more, 90% by weight or less, 85% by weight or less, 80% by weight or less, 60% by weight to 90% by weight, 65% by weight to 85% by weight, and 70% by weight to 80% by weight. Since the above-mentioned positive active material contains the first positive active material in the largest proportion, it basically depends on the functionality of the first positive active material, and by adjusting the physical characteristics of the first positive active material, compatibility with other positive active materials, such as the second positive active material, can be improved.

[0084] According to one embodiment of the present invention, the second positive active material is included in the positive active material in an amount of 20 to 60 parts by weight based on 100 parts by weight of the first positive active material. The second positive active material is an essential component of the positive active material according to one embodiment of the present invention together with the first positive active material, and the content of the second positive active material complements the functionality lacking in the first positive active material. Specifically, the content of the second positive active material may be 20 parts by weight or more, 25 parts by weight or more, 30 parts by weight or more, 35 parts by weight or more, 40 parts by weight or more, 60 parts by weight or less, 55 parts by weight or less, 50 parts by weight or less, or 20 to 60 parts by weight, 30 to 55 parts by weight, or 40 to 50 parts by weight. According to one embodiment of the present invention, when a first positive electrode active material with controlled physical properties is mixed with a second positive electrode active material within the range described above, the effect of improving functionality due to the addition of the second positive electrode active material may be excellent.

[0085] A positive electrode active material according to one embodiment of the present invention exhibits excellent properties even after mixing a first positive electrode active material, which is a lithium iron phosphate-based particle, with a second positive electrode active material, which is a lithium transition metal oxide-based particle.

[0086] According to one embodiment of the present invention, the anode active material has a rolled density of 2.6 g / cc to 3.0 g / cc when measured under pressure of 3 tons. Generally, a carbon coating layer can lower the rolled density, but in the present invention, the rolled density can be significantly increased by controlling the physical properties of the first anode active material. Specifically, the rolled density may be 2.6 g / cc or more, 2.61 g / cc or more, 2.62 g / cc or more, 3.0 g / cc or less, 2.95 g / cc or less, 2.9 g / cc or less, 2.85 g / cc or less, 2.8 g / cc or less, and 2.6 g / cc to 3.0 g / cc, 2.61 g / cc to 2.9 g / cc, or 2.62 g / cc to 2.8 g / cc.

[0087] According to one embodiment of the present invention, the positive electrode active material has a powder resistance of 5 Ω·m to 100 Ω·m when measured under pressure of 2 tons. Generally, high powder resistance can cause overvoltage due to a decrease in ionic conductivity at the interface, and if carbon-uncoated lithium transition metal oxide particles are mixed with lithium iron phosphate-based particles having such high powder resistance, a rapid decrease in ionic conductivity that was not observed in a single active material may occur. In the present invention, the powder resistance can be significantly lowered by controlling the physical properties of the first positive electrode active material. Specifically, the powder resistance is 5 Ω·m or more, 10 Ω·m or more, 15 Ω·m or more, 20 Ω·m or more, 100 Ω·m or less, 90 Ω·m or less, 80 Ω·m or less, 70 Ω·m or less, 60 Ω·m or less, and may be 5 Ω·m to 100 Ω·m, 10 Ω·m to 80 Ω·m, and 20 Ω·m to 60 Ω·m.

[0088] In the positive electrode active material according to one embodiment of the present invention, the low powder resistance and high rolling density can improve the performance of the battery when applied to the battery, and in particular, can significantly improve the discharge energy density at high rate speeds.

[0089]

[0090] <Bipolar>

[0091]

[0092] The present invention provides a positive electrode comprising the positive electrode active material described above. The positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer comprises the positive electrode active material described above. The positive electrode may be manufactured, for example, by the following method.

[0093] A composition for forming an anode active material layer comprising the aforementioned anode active material, conductive material, and binder is prepared. An anode slurry is prepared by mixing the composition with a solvent, and then the anode slurry is directly coated and dried onto an anode current collector to produce an anode electrode plate. Alternatively, the anode slurry may be cast onto a separate support, and then a film obtained by peeling off from the support is laminated onto the anode current collector to produce an anode electrode plate.

[0094] The binder used in the composition for forming the positive electrode active material layer is a component that assists in the bonding of the active material and the conductive material, as well as in the bonding to the current collector. Examples of such binders include polyvinylidene fluoride, vinylidene fluoride / hexafluoropropylene copolymer, polyacrylonitrile, polymethyl methacrylate polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluororubber, and various copolymers. The content of the binder may be 1% to 30% by weight based on the total weight of the composition for forming the positive electrode active material layer. When the content of the binder is within the above range, the bonding strength of the active material layer to the current collector is good.

[0095] The conductive material used in the composition for forming the positive electrode active material layer is used to impart conductivity to the electrode, and can be used without special restrictions as long as it possesses electronic conductivity without causing chemical changes in the battery being formed. 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, or silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used. The content of the conductive material may be 1% to 30% by weight based on the total weight of the composition for forming the positive electrode active material layer. When the content of the conductive material is within the above range, the conductivity within the active material layer is good.

[0096] When mixing the above-mentioned composition for forming the positive electrode active material layer with a solvent to form a slurry, the solvent used may include N-methylpyrrolidone (NMP), acetone, water, etc. The content of the solvent is used in an amount of 1 to 10 parts by weight based on 100 parts by weight of the composition for forming the positive electrode active material layer. When the solvent content is within the above range, the process of forming the active material layer is easy.

[0097] The positive current collector, to which the positive slurry is coated or laminated, has a thickness of about 3 to 500 μm and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the positive current collector may be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treated with carbon, nickel, titanium, silver, etc. The positive current collector may also have fine irregularities formed on its surface to increase the adhesion of the positive active material, and various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics are possible.

[0098] An anode is manufactured by directly coating and drying an anode slurry onto an anode current collector, or by laminating an anode film made of a separate film onto an anode current collector and then pressing it.

[0099]

[0100] Lithium secondary battery

[0101]

[0102] The present invention provides a lithium secondary battery comprising the anode described above. The lithium secondary battery comprises the anode described above; a cathode; a separator interposed between the anode and the cathode; and an electrolyte. Additionally, the lithium secondary battery may optionally further comprise a battery container housing an electrode assembly of the anode, cathode, and separator described above, and a sealing member sealing the battery container. The lithium secondary battery may be manufactured, for example, by the following method.

[0103] The anode and cathode are manufactured by applying and drying an anode slurry or a cathode slurry, respectively, onto a current collector. The manufacturing of the anode is as described above.

[0104] To manufacture a cathode, a cathode slurry for forming a cathode is prepared by mixing a cathode active material, a binder, a conductive material, and a solvent.

[0105] The above-mentioned cathode active material is not specifically limited to those generally used in the field, but more specifically, lithium metal, metals capable of alloying with lithium, transition metal oxides, materials capable of doping and undoping lithium, materials capable of reversibly inserting and extracting lithium ions, etc. may be used.

[0106] The above transition metal oxide may be, for example, tungsten oxide, molybdenum oxide, titanium oxide, lithium titanium oxide, vanadium oxide, lithium vanadium oxide, etc. The material capable of doping and dedoping the lithium is, for example, Si, SiO x (0<x≤2), Si-Y alloy (where Y is an alkali metal, alkaline earth metal, group 13 element, group 14 element, transition metal, rare earth element, or a combination thereof, and is not Si), Sn, SnO2, Sn-Y (where Y is an alkali metal, alkaline earth metal, group 13 element, group 14 element, transition metal, rare earth element, or a combination thereof, and is not Sn), etc., and at least one of these may be mixed with SiO2 and used. The above element Y may be Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, Te, Po, or a combination thereof.

[0107] The material capable of reversibly inserting and extracting the above lithium ions is a carbon-based material, and any carbon-based negative electrode active material commonly used in lithium batteries can be used. For example, it is crystalline carbon, amorphous carbon, or a mixture thereof. The crystalline carbon is, for example, amorphous, plate-like, flake-like, spherical, or fibrous natural graphite; or artificial graphite, and the amorphous carbon is, for example, soft carbon (low-temperature calcined carbon) or hard carbon, mesophase pitch carbide, calcined coke, etc.

[0108] The conductive material, binder, and solvent used in the cathode slurry may be the same as those used for the anode. In some cases, it is also possible to form pores inside the electrode plate by adding a plasticizer to the anode slurry and the cathode slurry. The content of the cathode active material, conductive material, binder, and solvent is at a level typically used in lithium secondary batteries.

[0109] The negative electrode current collector is generally made with a thickness of 3 to 500 μm. Such a negative electrode current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and 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 alloy may be used. In addition, similar to the positive electrode current collector, fine irregularities may be formed on the surface to strengthen the bonding strength of the negative electrode active material, and it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

[0110] Similar to the manufacture of the anode, the prepared cathode slurry can be directly coated and dried onto a cathode current collector to manufacture a cathode plate. Alternatively, the cathode slurry can be cast onto a separate support, and then the film obtained by peeling off from the support can be laminated onto a cathode current collector to manufacture a cathode plate.

[0111] The above-mentioned positive and negative electrodes may be separated by a separator, and any separator commonly used in lithium secondary batteries may be used. In particular, it is suitable to have low resistance to ion movement of the electrolyte and excellent electrolyte wettability. For example, it may be a material selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or combinations thereof, and may be in the form of a non-woven fabric or a woven fabric. The separator is used with a pore diameter of 0.01 μm to 10 μm and a thickness generally of 5 μm to 300 μm.

[0112] Lithium salt-containing non-aqueous electrolytes consist of a non-aqueous electrolyte and lithium. Non-aqueous electrolytes include non-aqueous electrolytes, solid electrolytes, and inorganic solid electrolytes.

[0113] As the above-mentioned non-aqueous electrolyte, for example, non-protic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyl lactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, ethyl propionate, etc. may be used.

[0114] The above organic solid electrolyte may be, for example, a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, a polyester sulfide, a polyvinyl alcohol, polyvinylidene fluoride, a polymer containing an ionic dissociator, etc.

[0115] As the above-mentioned inorganic solid electrolyte, for example, nitrides, halides, sulfates of Li such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, and Li3PO4-Li2S-SiS2 may be used.

[0116] Any lithium salt commonly used in lithium secondary batteries may be used, and as a substance that dissolves well in the above-mentioned non-aqueous electrolyte, examples include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10 One or more materials such as LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, (CF3SO2)2NLi, lithium chloroborate, lithium lower aliphatic carboxylate, and lithium 4-phenylborate may be used.

[0117] In addition to the above electrolyte components, various additives may be used in the above electrolyte for purposes such as improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity.

[0118] The above additives include imide-based salts such as lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethylsulfonyl)imide; borate-based salts such as lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalate)borate (LiOdFB), and tris(trimethylsilyl)borate (TMSB); phosphate-based salts such as difluorophosphate and tris(trimethylsilyl)phosphate; and haloalkylene carbonate-based compounds such as difluoroethylene carbonate. Alternatively, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride may be included, and the above additives may be used alone or in combination. In this case, the above additives may each be included in an amount of 0.1% to 10% by weight based on the total weight of the electrolyte.

[0119] In addition to existing applications such as mobile phones and portable computers, the above-mentioned lithium secondary battery is suitable for applications requiring high capacity, high output, and high-temperature operation, such as electric vehicles, and can also be used in hybrid vehicles by combining it with existing internal combustion engines, fuel cells, and supercapacitors. Furthermore, the above-mentioned lithium secondary battery can be used in all other applications requiring high output, high voltage, and high-temperature operation.

[0120]

[0121] Hereinafter, preferred embodiments are presented to aid in understanding the present invention, but the following embodiments are provided only to facilitate a better understanding of the present invention and do not limit the present invention thereto.

[0122]

[0123] Preparation Example (Preparation of 1st positive active material)

[0124]

[0125] Preparation Example 1

[0126] Li2CO3, MnCO3, FePO4, and NH4H2PO4 were prepared in a molar ratio of 1.02:0.3:0.7:1 and mixed with water to achieve a solid content concentration of 30 wt%. The mixture was wet-milled using a bead mill to obtain an average particle size of solids (D 50 A slurry having a wavelength of 200 nm was prepared. Subsequently, 8 parts by weight of glucose was mixed into the slurry based on a total of 100 parts by weight of Li2CO3, MnCO3, FePO4, and NH4H2PO4. The mixture was dried using a spray drying device (inlet temperature: 230°C, outlet temperature: 95°C) and then calcined at 740°C for 10 hours under a nitrogen atmosphere to produce LiMn 0.3 Fe 0.7 Lithium iron phosphate-based particles having an olivine structure with a composition of PO4 and a carbon-containing coating layer formed thereon (average particle size (D 50 A coating layer was prepared with a thickness of 247 nm and an aspect ratio of 1.495. The carbon content in the coating layer was 1.86 wt% based on the total weight of the lithium iron phosphate particles.

[0127]

[0128] Comparative Manufacturing Example 1

[0129] Li2CO3, MnCO3, FePO4, and NH4H2PO4 were prepared in a molar ratio of 1.02:0.3:0.7:1 and mixed with water to achieve a solid content concentration of 30 wt%. The mixture was wet-milled using a bead mill to obtain an average particle size of solids (D 50 A slurry having a wavelength of 200 nm was prepared. Subsequently, 7 parts by weight of glucose was mixed into the slurry based on a total of 100 parts by weight of Li2CO3, MnCO3, FePO4, and NH4H2PO4. The mixture was dried using a spray drying device (inlet temperature: 230°C, outlet temperature: 95°C) and then calcined at 700°C for 8 hours under a nitrogen atmosphere to produce LiMn0.3 Fe 0.7 Lithium iron phosphate-based particles having an olivine structure with a composition of PO4 and a carbon-containing coating layer formed thereon (average particle size (D 50 A coating layer was prepared with a thickness of 191 nm (aspect ratio: 1.516). The carbon content in the coating layer was 1.78 wt% based on the total weight of the lithium iron phosphate particles.

[0130]

[0131] Comparative Manufacturing Example 2

[0132] Li2CO3, MnCO3, FePO4, and NH4H2PO4 were prepared in a molar ratio of 1.02:0.6:0.4:1 and mixed with water to achieve a solid content concentration of 30 wt%. The mixture was wet-milled using a bead mill to obtain an average particle size of solids (D 50 A slurry having a wavelength of 200 nm was prepared. Subsequently, 6 parts by weight of glucose was mixed into the slurry based on a total of 100 parts by weight of Li2CO3, MnCO3, FePO4, and NH4H2PO4. The mixture was dried using a spray drying device (inlet temperature: 230°C, outlet temperature: 95°C) and then calcined at 750°C for 8 hours under a nitrogen atmosphere to produce LiMn 0.6 Fe 0.4 Lithium iron phosphate-based particles having an olivine structure with a composition of PO4 and a carbon-containing coating layer formed thereon (average particle size (D 50 A coating layer was prepared with a thickness of 462 nm and an aspect ratio of 1.605. The carbon content in the coating layer was 1.63 wt% based on the total weight of the lithium iron phosphate particles.

[0133]

[0134] Example (Preparation of positive electrode active material)

[0135]

[0136] When manufacturing the positive electrode active material, the second positive electrode active material mixed with the first positive electrode active material is Li 1.07 Ni 0.86 Co 0.05 Mn 0.07 Al0.02 Layered lithium transition metal oxide-based particles with a composition of O2 (containing trace amounts of Zr) (average secondary particle size (D 50 It was manufactured by the following method with a particle size of 9.9㎛, secondary particle aspect ratio: 1.20, and primary particle aspect ratio: 1.79.

[0137] Li 1.07 Ni 0.88 Co 0.05 Mn 0.07 To produce O2, a homogeneous solution was prepared by adjusting the stoichiometric ratio of Ni, Co, and Mn sulfate hydrates (Ni:Co:Mn = 0.88:0.05:0.07), co-precipitating it with ammonia water to a pH of 11, followed by washing and drying at 150°C for 6 hours. Then, LiOH was mixed with Al(OH)32 mol% and ZrO23,000 ppm in proportions, ground, and calcined at 800°C for 20 hours to produce a second cathode active material.

[0138]

[0139] Example 1

[0140] A first positive active material prepared in Preparation Example 1 was mixed with a second positive active material prepared by the method described above in a weight ratio of 7:3 to prepare a final positive active material mixed with heterogeneous active materials.

[0141]

[0142] Comparative Example 1

[0143] A first positive active material prepared in Comparative Preparation Example 1 was mixed with a second positive active material prepared by the method described above in a weight ratio of 7:3 to prepare a final positive active material mixed with heterogeneous active materials.

[0144]

[0145] Comparative Example 2

[0146] A first positive active material prepared in Comparative Preparation Example 2 was mixed with a second positive active material prepared by the method described above in a weight ratio of 7:3 to prepare a final positive active material mixed with heterogeneous active materials.

[0147]

[0148] Comparative Example 3

[0149] The final positive active material was composed solely of the first positive active material prepared in Preparation Example 1.

[0150]

[0151] Comparative Example 4

[0152] The final positive active material was composed solely of the first positive active material prepared in Comparative Manufacturing Example 1.

[0153]

[0154] Comparative Example 5

[0155] The final positive active material was composed solely of the first positive active material prepared in Comparative Manufacturing Example 2.

[0156]

[0157] Experimental Example

[0158]

[0159] Experimental Example 1 (Evaluation of primary particle structure of the first positive electrode active material)

[0160] Scanning electron microscope (SEM, Manufacturer: FEI, Product Name: Inspect F) was used to obtain SEM images (approx. 3K magnification) of the first cathode active materials prepared in Preparation Example 1 and Comparative Preparation Examples 1 and 2, and the average particle size of the primary particles (D) was processed using an image processing program (Manufacturer: LG Chem, Product Name: DX Program). 50 ) and aspect ratio were measured. Specifically, a two-dimensional segmentation image was obtained by dividing the boundaries of primary particles present in the SEM image and displaying them in random colors, and the average particle size (D) of the primary particles from the segmentation image was calculated. 50The ) and aspect ratio were measured. A segmentation image of the particles according to Preparation Example 1 is shown in FIG. 1, a segmentation image of the particles according to Comparative Preparation Example 1 is shown in FIG. 2, and a segmentation image of the particles according to Comparative Preparation Example 2 is shown in FIG. 3. In addition, the average particle size (D) of the primary particles measured therefrom 50 The values ​​of ) and aspect ratio are shown in Table 1 below.

[0161]

[0162] Average particle size (D 50 , nm) Aspect Ratio Preparation Example 1247 1.495 Comparative Preparation Example 119 1.516 Comparative Preparation Example 246 21.605

[0163]

[0164] According to Table 1 above, the first cathode active materials according to Preparation Example 1 and Comparative Examples 1 and 2 were prepared on a nanoscale to compensate for the low electronic conductivity of lithium iron phosphate-based particles. All of the first cathode active materials include a carbon coating layer, which affects the growth and shape of the particles. Compared to the first cathode active materials according to Comparative Examples 1 and 2, the first cathode active material according to Preparation Example 1 has a medium size and a lower aspect ratio, and this feature can improve the performance of the following cathode active materials and batteries.

[0165]

[0166] Experimental Example 2 (Evaluation of Primary Particle Structure of Second Anode Active Material)

[0167] For the second cathode active material described in the example, SEM images (approx. 2K magnification) were obtained using a scanning electron microscope (SEM, Manufacturer: FEI, Product Name: Inspect F), and the average particle size (D) of the secondary particles was calculated using an image processing program (Manufacturer: LG Chem, Product Name: DX Program). 50) and aspect ratio were measured. Specifically, a two-dimensional segmentation image was obtained by dividing the boundaries of secondary particles present in the SEM image and displaying them in random colors, and the average particle size (D) of the secondary particles was obtained from the segmentation image. 50 The ) and aspect ratio were measured. A segmentation image of the second cathode active material particle described in the example is shown in FIG. 4, and the average particle size (D) of the secondary particle measured therefrom was 50 ) was 9.9㎛, and the aspect ratio was 1.20.

[0168] In addition, the aspect ratio of the primary particles was measured through a two-dimensional segmentation image by increasing the magnification of the SEM image to 30K. The segmentation image is shown in Fig. 5, and the aspect ratio of the primary particles measured from it was 1.79.

[0169] Unlike the first cathode active material, the second cathode active material has a more clearly distinguishable secondary particle shape, and this shape is maintained even when mixed with the first cathode active material, affecting the compression density of the entire cathode active material. The second cathode active material has a high aspect ratio in its primary particles, but the aspect ratio decreases as it aggregates into secondary particles. These physical characteristics of the second cathode active material can help improve battery performance by increasing the rate characteristics while enhancing compression density when mixed with the first cathode active material.

[0170]

[0171] Experimental Example 3 (Evaluation of Compression Density and Powder Resistance of Anode Active Material)

[0172] 2.5 g each of the positive active materials prepared in Example 1 and Comparative Examples 1 and 2 were placed into a dedicated mold of 22Φ, and the powder resistance at a pressure of 2 tons was measured using an automatic powder resistance meter (Manufacturer: HANTECH, Product Name: HPRM-FA-L). In addition, 1.3 g each of the positive active materials were placed into a dedicated mold of 13Φ, and the thickness was measured after applying a pressure of 3 tons with a hydraulic pump to calculate the rolled density. The measurement results for powder resistance and rolled density are shown in Table 2 below.

[0173]

[0174] Powder resistance (2 ton, Ω·m) Rolled density (3 ton, g / cc) Example 1 46 2.62 Comparative Example 1 16 9 2.55 Comparative Example 2 39 25 2.30

[0175]

[0176] According to Table 2 above, it was confirmed that the anode active material according to Example 1 has a powder resistance of 100 Ω·m or less, which is significantly lower than the powder resistance of the anode active materials according to Comparative Examples 1 and 2. In addition, it was confirmed that the anode active material according to Example 1 has a rolled density of 2.6 g / cc or more, which is significantly higher than the rolled density of the anode active materials according to Comparative Examples 1 and 2.

[0177] To compensate for the low electronic conductivity of lithium iron phosphate-based particles, a carbon coating layer having a nanoscale particle size is essential; however, such a carbon coating layer can hinder particle growth and cause a decrease in rolling density. Since a decrease in rolling density in mixed active materials lowers the interfacial contact rate between particles and impairs rate characteristics, a cathode active material capable of solving this problem is required. The cathode active material according to Example 1 can maintain a high rolling density while introducing a carbon coating layer, and thus can significantly improve the performance of the battery when applied to a battery.

[0178]

[0179] Experimental Example 4 (Battery Performance Evaluation)

[0180] To evaluate the performance of a battery using the positive electrode active material prepared in Example 1 and Comparative Examples 1 to 5, a battery was manufactured in the following manner.

[0181] A cathode composite material was prepared by mixing the cathode active material of Example 1 and Comparative Examples 1 to 5, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in N-methylpyrrolidone (NMP) solvent in a weight ratio of 90:5:5. This was coated onto one side of an aluminum foil (Al poil) with a thickness of 20 μm, dried at 130°C, and then rolled with a roll press to produce a cathode with a porosity of 30%. Lithium metal was used as the anode.

[0182] An electrode assembly was manufactured by interposing a porous polyethylene separator between the anode and cathode prepared by the method described above, and the electrode assembly was placed inside a case, after which an electrolyte was injected into the case to manufacture a lithium secondary battery. At this time, the electrolyte was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent composed of ethylene carbonate / ethyl methyl carbonate / diethyl carbonate (mixed volume ratio of EC:EMC:DEC = 1:2:1).

[0183] For each lithium secondary battery half cell manufactured by the method described above, charging in CC / CV mode and discharging in CC mode were performed at 25°C. Specifically, after performing activation with 0.1C charging / discharging, the charging and discharging capacities of two cycles at 0.1C (normal rate capacity) were recorded. For high-rate discharge, the discharge capacity was recorded by charging at 0.2C until 4.25V was reached and discharging to 2.5V with a constant current of 4.0C. For high-rate charging, the charge capacity was recorded by charging at 2.0C until 4.25V was reached. Stabilization was performed with one 0.1C charging / discharging cycle between high-rate discharge and high-rate charge.

[0184] The discharge capacity ratio of high rate speed (4.0C) to standard rate speed (0.1C), the discharge energy density of high rate speed (4.0C), and the charge capacity ratio of high rate speed (2.0C) to standard rate speed (0.1C) are shown in Table 3 below. In addition, by pairing corresponding mixed cathode active materials with single cathode active materials, the high rate speed (4.0C) discharge profiles of Example 1 and Comparative Example 3 are shown in FIG. 6, the high rate speed (4.0C) discharge profiles of Comparative Example 1 and Comparative Example 4 are shown in FIG. 7, and the high rate speed (4.0C) discharge profiles of Comparative Example 2 and Comparative Example 5 are shown in FIG. 8. Here, the discharge energy density (Wh / g) is the value obtained by integrating the discharge curve in the capacity-voltage (Li vs. Li+) profile. The 2.0C charge capacity refers to the capacity recorded just before entering the CV region due to overvoltage.

[0185]

[0186] 4.0C / 0.1C Discharge Capacity Ratio (%) 4.0C Discharge Energy Density (Wh / g) 2.0C / 0.1C Charge Capacity Ratio (%) Example 1 84.25 12.08 4.2 Comparative Example 1 82.54 89.58 2.1 Comparative Example 2 51.32 24.74.5 Comparative Example 3 84.24 37.18 3.2 Comparative Example 4 82.64 13.78 0.2 Comparative Example 5 85.84 52.26 9.2

[0187]

[0188] According to Table 3 above, the positive electrode active material according to Example 1 showed stable and high charge and discharge capacities even at high rate speeds compared to general rate speeds. In particular, it was confirmed that the positive electrode active material according to Example 1 had the highest discharge energy density at high rate speeds. The difference in effect can be more clearly confirmed when compared to using only the first positive electrode active material without mixing the first positive electrode active material and the second positive electrode active material.

[0189] The positive active material of Example 1 is compared with the positive active material of Comparative Example 3, which uses the same first positive active material but does not mix the second positive active material, and the results can be confirmed through Table 3 as well as Figure 6. The positive active material of Comparative Example 1 is compared with the positive active material of Comparative Example 4, which uses the same first positive active material but does not mix the second positive active material, and the results can be confirmed through Table 3 as well as Figure 7. The positive active material of Comparative Example 2 is compared with the positive active material of Comparative Example 5, which uses the same first positive active material but does not mix the second positive active material, and the results can be confirmed through Table 3 as well as Figure 8.

[0190] When lithium iron phosphate-based particles corresponding to the first positive electrode active material have high powder resistance, their performance does not particularly degrade even at high rates, but when lithium transition metal oxide-based particles corresponding to the second positive electrode active material are mixed, a clear degradation in performance at high rates is observed. High powder resistance can cause overvoltage due to a decrease in ion conductivity at the interface, and when lithium transition metal oxide-based particles without carbon coating are mixed, a rapid decrease in ion conductivity that was not observed in the single active material may occur.

[0191] When comparing the positive electrode active materials of Example 1 and Comparative Example 3, it can be confirmed that the first positive electrode active material in Example 1 and Comparative Example 3 has low powder resistance and high rolling density, and that the discharge energy density at high rate is significantly improved when mixed with the second positive electrode active material. Specifically, the discharge energy density at high rate in the positive electrode active material of Example 1 was 512.0 Wh / g, which is 74.9 Wh / g higher than that of the positive electrode active material of Comparative Example 3, which was 437.1 Wh / g. This trend can be confirmed through Figure 6.

[0192] On the other hand, as the powder resistance of the first positive electrode active material increased and the rolling density decreased, the discharge energy density at high rate when mixed with the second positive electrode active material showed a lower value compared to the example or even decreased. Specifically, the discharge energy density at high rate of the positive electrode active material of Comparative Example 1 was 489.5 Wh / g, which is 75.8 Wh / g higher than that of the positive electrode active material of Comparative Example 4, which was 413.7 Wh / g. This trend can be confirmed through Fig. 7. Although the discharge energy density at high rate of the positive electrode active material of Comparative Example 1 increased to a level similar to that of the positive electrode active material of Example 1, the discharge energy density at high rate of the positive electrode active material of Comparative Example 3 is higher than that of the positive electrode active material of Comparative Example 4; therefore, the discharge energy density at high rate of the positive electrode active material of Example 1 appears significantly higher than that of the positive electrode active material of Comparative Example 1. In addition, the discharge energy density at high rate of the cathode active material of Comparative Example 2 was 224.7 Wh / g, which is 227.5 Wh / g lower than that of the cathode active material of Comparative Example 5, which was 452.2 Wh / g. This trend can be confirmed through Fig. 8.

[0193]

[0194] All simple variations or modifications of the present invention fall within the scope of the present invention, and the specific scope of protection of the present invention will be clarified by the appended claims.

Claims

1. A positive active material comprising a first positive active material and a second positive active material, The first positive active material comprises lithium iron phosphate-based particles with an olivine structure having a coating layer formed on the surface, and The above second positive active material comprises lithium transition metal oxide-based particles with a layered structure, and The above lithium iron phosphate-based particles have an average particle size (D 50 A positive electrode active material having a carbon content of 1.8% by weight or more and less than 5% by weight based on the total weight of the lithium iron phosphate-based particles.

2. In Claim 1, A positive electrode active material characterized in that the above lithium iron phosphate-based particles comprise a compound represented by the following chemical formula 1: [Chemical Formula 1] Li 1+a Fe 1-x-y Mn x M y PO4 In the above chemical formula 1, M is at least one element selected from the group consisting of V, Al, Ni, Co, Mg, Nb, Mo, W, Ti, Sr, Zr, B, Y and Zn, and -0.1≤a≤0.1, 0<x≤0.7, 0≤y≤0.

1.

3. In Claim 2, A positive active material characterized in that, in the above chemical formula 1, x is 0.2 ≤ x ≤ 0.

6.

4. In Claim 1, A positive electrode active material characterized in that the above lithium transition metal oxide-based particles comprise a compound represented by the following chemical formula 2: [Chemical Formula 2] Li 1+a (Ni x Co y Mr 1-x-y-z M' z )O2 In the above chemical formula 2, M' is at least one element selected from the group consisting of V, Al, Mg, Nb, Mo, W, Ti, Sr, Zr, B, Y and Zn, and -0.1≤a≤0.1, 0≤x≤1, 0≤y≤0.5, 0≤z≤0.05 and x+y+z≤1.

5. In Claim 4, A positive active material characterized in that, in the above chemical formula 2, x is 0.6 ≤ x ≤ 1.

6. In Claim 1, The above lithium iron phosphate-based particles have an average particle size of 200 nm to 300 nm (D 50 A positive electrode active material characterized by having ).

7. In Claim 1, The above lithium transition metal oxide-based particles have an average particle size (D) of 2㎛ to 20㎛. 50 A positive electrode active material characterized by having ).

8. In Claim 1, The above lithium iron phosphate-based particles are characterized by having an aspect ratio of 1.2 to 1.

51.

9. In Claim 1, A positive electrode active material characterized in that the lithium iron phosphate-based particles are in the form of primary particles, and the lithium transition metal oxide-based particles are in the form of secondary particles.

10. In Claim 1, A positive active material characterized in that the weight of the first positive active material is greater than the weight of the second positive active material.

11. In Claim 10, The above-mentioned first positive active material is characterized by being included in the positive active material in an amount of 60% to 90% by weight based on the total weight of the positive active material.

12. In Claim 1, The above-mentioned second positive active material is characterized by being included in the positive active material in an amount of 20 to 60 parts by weight based on 100 parts by weight of the first positive active material.

13. In Claim 1, The above positive active material is characterized by having a rolled density of 2.6 g / cc to 3.0 g / cc under 3-ton pressure.

14. In Claim 1, The above positive active material is characterized by having a powder resistance of 5 Ω·m to 100 Ω·m under 2-ton pressure.

15. A positive electrode for a lithium secondary battery comprising a positive electrode active material according to Claim 1.

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