Positive electrode for lithium secondary battery and lithium secondary battery comprising same

Abstract

A positive electrode for a lithium secondary battery according to one embodiment of the present invention comprises a positive electrode current collector and a positive electrode mixture layer on at least one surface of the positive electrode current collector, the positive electrode mixture layer includes a first positive electrode mixture layer on a positive electrode current collector and a second positive electrode mixture layer on the first positive electrode mixture layer, the first positive electrode mixture layer and the second positive electrode mixture layer each independently containing a lithium metal phosphorus oxide and a lithium transition metal oxide as active materials, the weight of a lithium metal phosphorus oxide contained in the first positive electrode mixture layer is greater than the weight of a lithium transition metal oxide, the first positive electrode mixture layer comprises a dot-type conductive material, and the second positive electrode mixture layer comprises a linear conductive material.

Description

Technical Field

[0001] This invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the positive electrode. Background Technology

[0002] In recent years, a great deal of research has been conducted on electric vehicles (EVs), which can replace gasoline vehicles, diesel vehicles, and other vehicles that use fossil fuels, which are one of the main causes of air pollution. As the power source for these electric vehicles (EVs), lithium secondary batteries with high discharge voltage and high power stability are mainly used.

[0003] To improve the performance of the lithium secondary battery, it is necessary to develop a technology that can improve the energy density, safety, and other properties of the positive electrode used in the lithium secondary battery. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] According to one aspect of the present invention, a positive electrode for lithium secondary batteries with improved safety can be provided.

[0006] According to another aspect of the present invention, the energy density of the positive electrode for lithium secondary batteries can be improved.

[0007] According to another aspect of the present invention, the lifespan performance of the positive electrode for lithium secondary batteries can be improved.

[0008] (II) Technical Solution

[0009] According to one embodiment, a positive electrode for a lithium secondary battery includes a positive current collector and a positive electrode mixture layer on at least one side of the positive current collector, wherein the positive electrode mixture layer includes a first positive electrode mixture layer on the positive current collector and a second positive electrode mixture layer on the first positive electrode mixture layer, the first positive electrode mixture layer and the second positive electrode mixture layer each independently contain lithium metal phosphorus oxide and lithium transition metal oxide as active materials, the weight of lithium metal phosphorus oxide contained in the first positive electrode mixture layer is greater than the weight of lithium transition metal oxide, the first positive electrode mixture layer contains a point-type conductive material, and the second positive electrode mixture layer contains a linear conductive material.

[0010] In some specific embodiments, the content of the point-type conductive material contained in the first positive electrode mixture layer can be from 0.3% to 0.5% by weight.

[0011] In some specific embodiments, the content of linear conductive material contained in the second positive electrode mixture layer can be from 0.5% by weight to 1.5% by weight.

[0012] In some specific embodiments, the first positive electrode mixture layer may further comprise a linear conductive material.

[0013] In some specific implementations, the second positive electrode mixture layer may not contain point-type conductive material.

[0014] In some specific embodiments, the weight of the linear conductive material contained in the first positive electrode mixture layer may be less than or equal to the weight of the linear conductive material contained in the second positive electrode mixture layer.

[0015] In some specific embodiments, the content of linear conductive material contained in the first positive electrode mixture layer can be from 0.5% to 0.7% by weight.

[0016] In some specific embodiments, the second positive electrode mixture layer may further include a dot-type conductive material, and the content of the dot-type conductive material included in the second positive electrode mixture layer may be from 0.1% by weight to 0.5% by weight.

[0017] In some specific implementations, the point-type conductive material may include carbon black.

[0018] In some specific implementations, the linear conductive material may include carbon nanotubes (CNTs).

[0019] A lithium secondary battery according to one specific embodiment includes a positive electrode for a lithium secondary battery as described in any one of the above specific embodiments.

[0020] (III) Beneficial Effects

[0021] According to a specific embodiment of the present invention, the stability of the positive electrode for lithium secondary batteries can be improved.

[0022] According to another specific embodiment of the present invention, a positive electrode for a lithium secondary battery with high energy density can be provided.

[0023] According to another specific embodiment of the present invention, a positive electrode for lithium secondary batteries with excellent lifespan performance can be provided. Attached Figure Description

[0024] Figure 1 This is a conceptual cross-sectional view of a positive electrode for a lithium secondary battery according to a specific embodiment.

[0025] Explanation of reference numerals in the attached figures:

[0026] 100: Positive electrode for lithium secondary batteries

[0027] 10: Positive current collector

[0028] 20: Positive electrode mixture layer

[0029] 21: First cathode mixture layer

[0030] 22: Second cathode mixture layer Detailed Implementation

[0031] The technology disclosed in this specification and its specific embodiments are described in detail below with reference to the accompanying drawings. However, the implementation of the technology can be varied in many other forms, and its scope is not limited to the specific embodiments described below. Furthermore, the technology disclosed in this specification can be applied not only by the structure of the specific embodiments described below, but also by selectively combining all or part of the various specific embodiments to make various modifications.

[0032] With the increasing demand for lithium-ion rechargeable batteries, there is a need for technologies capable of manufacturing cathodes with superior performance. Relatedly, NCM cells using lithium nickel cobalt manganese oxide (NCM)-based active materials as the cathode active material offer excellent energy density but suffer from relatively lower safety and higher cost. On the other hand, cells using lithium phosphate-based active materials with an olivine structure (e.g., lithium iron phosphate (LFP)-based active materials) as the cathode active material offer excellent safety and price competitiveness, but suffer from relatively lower energy density and lower low-temperature performance.

[0033] According to a specific embodiment of the present invention, the above-mentioned problems can be alleviated, thereby providing a positive electrode for lithium secondary batteries with excellent energy density, safety, lifespan performance, and price competitiveness. See below. Figure 1 The specific implementation schemes of the present invention will be described in detail below.

[0034] Figure 1 This is a conceptual cross-sectional view of a positive electrode for a lithium secondary battery according to a specific embodiment.

[0035] Positive electrode for lithium secondary batteries

[0036] A positive electrode 100 for a lithium secondary battery according to one embodiment includes a positive electrode current collector 10 and a positive electrode mixture layer 20 on at least one side of the positive electrode current collector, wherein the positive electrode mixture layer includes a first positive electrode mixture layer 21 on the positive electrode current collector 10 and a second positive electrode mixture layer 22 on the first positive electrode mixture layer. The first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 each independently contain lithium metal phosphorus oxide and lithium transition metal oxide as active materials. The weight of the lithium metal phosphorus oxide contained in the first positive electrode mixture layer 21 is greater than the weight of the lithium transition metal oxide. The first positive electrode mixture layer 21 contains a point-type conductive material, and the second positive electrode mixture layer 22 contains a linear conductive material.

[0037] The lithium transition metal oxide may include active materials such as lithium nickel cobalt manganese oxide (NCM) with excellent energy density, and the lithium metal phosphorus oxide may include active materials such as lithium iron phosphate (LFP) as an active material with an olivine structure and excellent structural stability.

[0038] The positive electrode 100 for the lithium secondary battery has a multilayer structure comprising lithium transition metal oxide and lithium metal phosphorus oxide as active materials (see [link]). Figure 1 This allows the disadvantages of each active material to be complemented while maximizing its advantages, and can help further improve the performance of the battery cell.

[0039] The composition of the positive electrode current collector 10 is not particularly limited. Exemplarily, the positive electrode current collector can be a plate or foil formed from one or more of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), and their alloys. In some specific embodiments, the positive electrode current collector can be aluminum foil (Al-foil).

[0040] The thickness of the positive current collector 10 is not particularly limited. For example, the thickness of the positive current collector can be from 0.1 μm to 50 μm.

[0041] In some specific embodiments, the content of lithium metal phosphorus oxide in the first positive electrode mixture layer 21, which serves as the lower layer of the positive electrode 100 for the lithium secondary battery, can be greater than or equal to the content of lithium metal phosphorus oxide in the second positive electrode mixture layer 22, which serves as the upper layer. Specifically, the weight of lithium metal phosphorus oxide in the first positive electrode mixture layer 21 can be greater than or equal to the weight of lithium metal phosphorus oxide in the second positive electrode mixture layer 22. When a mixture layer with a relatively higher content of lithium metal phosphorus oxide is used as the lower layer, i.e., the first positive electrode mixture layer 21, the positive electrode and the lithium secondary battery including the positive electrode can have excellent resistance characteristics and lifespan performance.

[0042] The content of lithium metal phosphorus oxide in the first positive electrode mixture layer 21, which is the lower layer adjacent to the positive electrode current collector 10, is greater than the content of lithium transition metal oxide. Specifically, the weight of lithium metal phosphorus oxide contained in the first positive electrode mixture layer 21 can be greater than the weight of lithium transition metal oxide. In this case, by providing a first positive electrode mixture layer 21 with a relatively higher content of lithium metal phosphorus oxide, which has excellent structural stability, in the lower layer adjacent to the positive electrode current collector 10, the safety and lifespan performance of the lithium secondary battery can be improved.

[0043] In some specific embodiments, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide in the first positive electrode mixture layer 21 can be from 70:30 to 99:1. Specifically, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide in the first positive electrode mixture layer 21 can be from 85:15 to 95:5. In this case, the energy density of the positive electrode 100 for the lithium secondary battery can be ensured, while the safety of the battery can be significantly improved.

[0044] In some specific embodiments, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide in the second positive electrode mixture layer 22 can be 30:70 to 70:30. Specifically, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide in the second positive electrode mixture layer 22 can be 40:60 to 60:40 or 45:55 to 55:45. In this case, not only can the energy density of the battery be significantly improved while ensuring the safety of the positive electrode 100 for the lithium secondary battery, but the cell resistance can also be reduced, and the battery life performance can be improved.

[0045] In some specific embodiments, the content of lithium metal phosphorus oxide contained in the first positive electrode mixture layer 21 can be from 75% to 95% by weight. Specifically, the content of lithium metal phosphorus oxide contained in the first positive electrode mixture layer 21 can be more than 80% by weight and less than 90% by weight.

[0046] In some specific embodiments, the content of lithium transition metal oxide contained in the first positive electrode mixture layer 21 can be from 1% to 20% by weight. Specifically, the content of lithium transition metal oxide contained in the first positive electrode mixture layer 21 can be more than 5% by weight and less than 10% by weight.

[0047] In some specific embodiments, the content of lithium metal phosphorus oxide contained in the second positive electrode mixture layer 22 can be from 40% to 60% by weight. Specifically, the content of lithium metal phosphorus oxide contained in the second positive electrode mixture layer 22 can be more than 45% by weight and less than 55% by weight.

[0048] In some specific embodiments, the content of lithium transition metal oxide contained in the second positive electrode mixture layer 22 can be from 40% to 60% by weight. Specifically, the content of lithium transition metal oxide contained in the second positive electrode mixture layer 22 can be more than 45% by weight and less than 55% by weight.

[0049] In some specific embodiments, in the positive electrode 100 for lithium secondary batteries, the content of lithium metal phosphorus oxide, based on the entire positive electrode mixture layer 20, can be greater than or equal to the content of lithium transition metal oxide. Specifically, the total weight of lithium metal phosphorus oxide contained in the positive electrode mixture layer 20 can be greater than or equal to the total weight of lithium transition metal oxide contained in the positive electrode mixture layer 20. In this case, both the safety and energy density of the positive electrode 100 for lithium secondary batteries can be ensured simultaneously.

[0050] In some specific embodiments, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide in the positive electrode mixture layer 20 can be from 60:40 to 80:20. Specifically, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide in the positive electrode mixture layer 20 can be from 65:35 to 75:25.

[0051] In some specific embodiments, the load weight (LW) ratio of the first positive electrode mixture layer 21 to the second positive electrode mixture layer 22 can be 30:70 to 70:30. Specifically, the load weight (LW) ratio of the first positive electrode mixture layer 21 to the second positive electrode mixture layer 22 can be 40:60 to 60:40 or 45:55 to 55:45.

[0052] In some specific embodiments, the loading weight of the first positive electrode mixture layer 21 can be 1 mg / cm³. 2 Up to 20 mg / cm 2 For example, the loading weight of the first positive electrode mixture layer 21 can be 5 mg / cm³. 2 The above, and can be 15 mg / cm³ 2 the following.

[0053] In some specific embodiments, the loading weight of the second positive electrode mixture layer 22 can be 1 mg / cm³. 2 Up to 20 mg / cm 2 For example, the loading weight of the second positive electrode mixture layer 22 can be 5 mg / cm³. 2 The above, and can be 15 mg / cm³ 2 the following.

[0054] When the load weight relationship and range of the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 are as described above, excellent life performance can be ensured while increasing the capacity of the positive electrode 100 for lithium secondary batteries.

[0055] In some specific embodiments, the lithium metal phosphorus oxide can be represented by the following chemical formula 1.

[0056] [Chemical Formula 1]

[0057] LiMePO4

[0058] In the chemical formula 1, Me is at least one element selected from Co, Ni, Fe, and Mn.

[0059] The lithium metal phosphorous oxide represented by the chemical formula 1 may be lithium metal oxide particles having an olivine structure with excellent structural stability. Exemplarily, the lithium metal phosphorous oxide may include an iron (Fe)-containing lithium iron phosphate (LFP)-based active material.

[0060] In some specific embodiments, the lithium metal phosphorous oxide may be lithium manganese x Fe 1-x PO4 (0 < x < 1)-represented lithium manganese iron phosphate (LMFP)-based active material. Specifically, in the chemical formula, 0.5 ≤ x ≤ 0.7 may be satisfied. The LMFP-based active material is an active material in a form in which a part of iron (Fe) in the LFP-based active material is substituted with manganese (Mn), and the energy density and low-temperature performance may be relatively excellent compared to the LFP-based active material. Therefore, when the positive electrode 100 for the lithium secondary battery includes the LMFP-based active material, a lithium secondary battery with a high energy density can be provided.

[0061] The particle composition of the lithium metal phosphorous oxide can be confirmed by inductively coupled plasma (ICP) analysis. Exemplarily, the particles of the lithium metal phosphorous oxide can be analyzed by ICP, and the number of phosphorus atoms can be normalized to 1 to obtain a chemical formula.

[0062] In some specific embodiments, the lithium transition metal oxide may include a layered structure or a crystal structure represented by the following chemical formula 2.

[0063] [Chemical formula 2]

[0064] Li x Ni a Co b1 Mn b2 O 2+z

[0065] In the chemical formula 2, 0.9 ≤ x ≤ 1.2, 0.6 ≤ a ≤ 0.99, 0.01 ≤ b1 + b2 ≤ 0.4, and -0.5 ≤ z ≤ 0.1 may be satisfied.

[0066] The chemical structure represented by Formula 2 indicates the bonding relationships contained in the layered or crystalline structure of the positive electrode active material, where Ni, Co, and Mn can be provided as the main active elements. That is, the chemical structure represented by Formula 2 is provided to represent the bonding relationships of the main active elements and does not exclude other additional elements. Therefore, it should be understood that Formula 2 includes the introduction and substitution of additional elements.

[0067] In some specific embodiments, in addition to the main active element, auxiliary elements may be further included to enhance the chemical stability of the positive electrode active material or the layered / crystal structure. These auxiliary elements may be incorporated into the layered / crystal structure and form bonds; this should be understood to also include the chemical structures represented by Formula 2.

[0068] The auxiliary element may include at least one of, for example, Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary element (exemplarily, Al) may function as an auxiliary active element, together with Co or Mn, to contribute to the capacity / power activity of the positive electrode active material.

[0069] In some specific embodiments, the positive electrode active material may further comprise a coating material or dopant containing coating elements or dopant elements. For example, elements substantially the same as or similar to the aforementioned auxiliary elements may be used as coating elements or dopant elements. For example, one or more combinations of the aforementioned auxiliary elements may be used as coating elements or dopant elements. In this case, the upper operating voltage limit of the lithium secondary battery can be adjusted, thereby suppressing voltage decay of the lithium secondary battery.

[0070] The coating element or dopant element may exist on the particle surface of the positive electrode active material, or may penetrate through the particle surface of the positive electrode active material and be contained within the binding structure represented by the chemical formula 2.

[0071] In some specific embodiments, the coating element can form a sea-type coating or an island-type coating.

[0072] In some specific embodiments, the content of the coating element in the particles of the positive electrode active material can be 500 ppm to 8000 ppm, 1000 ppm to 8000 ppm, or 1500 ppm to 8000 ppm by weight of all elements except lithium and oxygen. When the content of the coating element is as described above, the initial capacity reduction and resistance increase of the lithium secondary battery can be prevented, and the voltage decay of the lithium secondary battery can be further suppressed.

[0073] In some specific embodiments, the coating material can be formed by a dry coating method or a wet coating method. Exemplarily, the particles of the positive electrode active material and the coating source can be dry-mixed or wet-mixed and then heat-treated (e.g., calcined or dried) to form a coating material on the surface of the positive electrode active material particles. The coating source can be a coating source known in the art. Exemplarily, the coating source can contain B, Al, W, Zr, Ti, Mg, Co, etc.

[0074] In some specific embodiments, the Ni content in the lithium transition metal oxide (exemplarily, the mole fraction of nickel in the total moles of nickel, cobalt, and manganese) can be 0.6 or more, and can be 0.7 or less. Specifically, the Ni content in the lithium transition metal oxide (exemplarily, the mole fraction of nickel in the total moles of nickel, cobalt, and manganese) can be 0.6 or more, and can be 0.65 or less. In this case, superior thermal stability can be achieved compared to lithium transition metal oxides with a Ni content of 0.7 or more.

[0075] The particle composition of the lithium transition metal oxide can be confirmed by inductively coupled plasma (ICP) analysis. For example, the particle composition of the positive electrode active material can be analyzed by ICP to normalize the number of oxygen atoms to 1.8 to 2.2 (for example, 2), thereby obtaining the chemical formula of the positive electrode active material.

[0076] In some specific embodiments, the lithium transition metal oxide can be in the form of a single particle. Specifically, the lithium transition metal oxide can be a single particle composed of a single crystal.

[0077] The term "single-particle form" as used in this specification is used to exclude the meaning of secondary particles, such as those formed by the aggregation of multiple primary particles. For example, in the lithium transition metal oxide, secondary particle structures assembled or aggregated from primary particles (e.g., more than 10, 20, 30, 40, 50, etc.) can be excluded.

[0078] Furthermore, the term "single-particle form" as used in this specification does not exclude, for example, a range of 2 to 10 single particles that are not aggregated but simply attached or in contact and have an integral form.

[0079] When the positive electrode 100 for the lithium secondary battery contains lithium transition metal oxide in the form of single particles, the energy density and lifespan performance of the secondary battery can be further improved.

[0080] Whether the lithium transition metal oxide has a single-particle form can be determined based on ion images obtained by analyzing the cross-section of the active material particles using focused ion beam (FIB) analysis. For example, when the active material particles have a polycrystalline structure, even if they appear as a single particle in the SEM cross-sectional image, in the FIB analysis image, due to differences in crystal orientation, two or more single crystals can be observed as a particle composed of two or more crystals. Therefore, when no two or more single crystals with differences in crystal orientation are observed in the FIB analysis image, it can be determined that the active material particles have a single-particle form.

[0081] In the positive electrode 100 for the lithium secondary battery, the first positive electrode mixture layer 21, which is adjacent to the positive electrode current collector 10 and serves as the lower layer, contains a point-type conductive material, and the second positive electrode mixture layer 22, which is separated from the positive electrode current collector 10 and serves as the upper layer, contains a linear conductive material, thereby reducing the electrode resistance and improving the battery's lifespan characteristics.

[0082] In this specification, the term "sphere-type" conductive material refers to a conductive material having a spherical particle shape, and may refer to a conductive material with a relatively small aspect ratio (length / diameter ratio). For example, the aspect ratio of the sphere-type conductive material may be from 0.5 to 1.5. In some specific embodiments, the diameter (D50) of the sphere-type conductive material may be from 10 nm to 500 nm.

[0083] For example, the dot-type conductive material may include at least one of the following: graphite such as natural graphite and artificial graphite; and carbon black such as acetylene black, Ketjen black, channel black, furnace black, lampblack, and thermal cracking black. In some specific embodiments, the dot-type conductive material may include carbon black.

[0084] In this specification, the linear (needle type) conductive material is a conductive material with an elliptical particle shape, which can refer to a conductive material with a relatively large aspect ratio (length / diameter ratio). For example, the aspect ratio of the linear conductive material can be 2 or more, 10 or more, 50 or more, or 100 or more, and can be less than 10000, less than 5000, less than 3000, or less than 1000.

[0085] Exemplarily, the linear conductive material may include at least one of the following: conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes (CNTs), for example, single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); metal powders such as aluminum powder 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. In some specific embodiments, the linear conductive material may include carbon nanotubes (CNTs).

[0086] In some specific embodiments, the content of the dot-type conductive material contained in the first positive electrode mixture layer 21 can be from 0.3% to 0.5% by weight. When the content of the dot-type conductive material contained in the first positive electrode mixture layer 21 is less than 0.3% by weight, it may be difficult to obtain the performance improvement effect brought about by the addition of conductive material. When the content of the dot-type conductive material contained in the first positive electrode mixture layer 21 exceeds 0.5% by weight, electrode rolling may not be able to proceed smoothly, which may lead to a decrease in the energy density of the electrode.

[0087] In some specific embodiments, the content of the linear conductive material contained in the second positive electrode mixture layer 22 can be from 0.5% to 1.5% by weight. Specifically, the content of the linear conductive material contained in the second positive electrode mixture layer 22 can be more than 0.6% by weight and less than 1.0% by weight. When the content of the linear conductive material contained in the second positive electrode mixture layer 22 is too low, specifically, when the content of the linear conductive material contained in the second positive electrode mixture layer 22 is less than 0.5% by weight, the effect of reducing electrode resistance and improving performance may be insufficient. When the content of the linear conductive material contained in the second positive electrode mixture layer 22 is too high, specifically, when the content of the linear conductive material contained in the second positive electrode mixture layer 22 exceeds 1.5% by weight, the electrode capacity may be reduced due to the decrease in the content of the positive electrode active material.

[0088] In some specific embodiments, the first positive electrode mixture layer 21 may further comprise a linear conductive material. In these specific embodiments, the content of the linear conductive material contained in the upper second positive electrode mixture layer 22 may be greater than or equal to the content of the linear conductive material contained in the lower first positive electrode mixture layer 21.

[0089] Specifically, the weight of the linear conductive material contained in the first positive electrode mixture layer 21 can be less than or equal to the weight of the linear conductive material contained in the second positive electrode mixture layer 22. In this case, the resistance of the second positive electrode mixture layer 22, which is the upper layer, can be reduced, thereby improving the performance of the positive electrode. At the same time, the amount of linear conductive material used based on the entire positive electrode can be adjusted to an appropriate range, thereby ensuring economy.

[0090] In some specific embodiments, the content of linear conductive material in the first positive electrode mixture layer 21 can be from 0.5% to 0.7% by weight. When the content of linear conductive material in the first positive electrode mixture layer 21 is less than 0.5% by weight, the effect of reducing electrode resistance and improving performance may be insufficient. When the content of linear conductive material in the first positive electrode mixture layer 21 exceeds 0.7% by weight, the content of relatively expensive linear conductive material increases, which may lead to insufficient cost competitiveness.

[0091] In some specific embodiments, the second positive electrode mixture layer may further comprise a point-type conductive material. In these specific embodiments, the content of the point-type conductive material contained in the second positive electrode mixture layer 22 may be from 0.1% to 0.5% by weight. Specifically, the content of the point-type conductive material contained in the second positive electrode mixture layer 22 may be less than 0.4% by weight and more than 0.3% by weight.

[0092] When the content of the dot-type conductive material contained in the second positive electrode mixture layer 22 is too high, specifically, when the content of the dot-type conductive material contained in the second positive electrode mixture layer 22 exceeds 0.5% by weight, the electrode rolling may not be able to proceed smoothly, which may lead to a decrease in the energy density of the electrode.

[0093] In some specific embodiments, the second positive electrode mixture layer 22 may not contain any point-type conductive material. Specifically, the content of point-type conductive material in the second positive electrode mixture layer 22 may be 0% by weight. In this case, by increasing the content of linear conductive material in the upper second positive electrode mixture layer 22, the electrode resistance can be reduced and the performance of the positive electrode can be improved.

[0094] In some specific embodiments, the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 may further comprise an adhesive. The adhesive is not particularly limited. Exemplarily, the adhesive may comprise one or more of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), PVDF / hexafluoropropylene copolymer, polyacrylonitrile, and polymethyl methacrylate. The content of the adhesive contained in the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 is not particularly limited; exemplaryly, it may be from 0.1% by weight to 10% by weight, respectively.

[0095] The positive electrode 100 for lithium secondary batteries according to the above specific implementation scheme can be manufactured by the following method.

[0096] Manufacturing method of positive electrode for lithium secondary batteries

[0097] A method for manufacturing a positive electrode 100 for a lithium secondary battery according to a specific embodiment includes the following steps: preparing a first positive electrode slurry containing a first positive electrode active material and a second positive electrode slurry containing a second positive electrode active material; and forming a positive electrode mixture layer 20 on at least one side of the positive electrode current collector 10, wherein the positive electrode mixture layer 20 includes a first positive electrode mixture layer 21 on the positive electrode current collector 10 and a second positive electrode mixture layer 22 on the first positive electrode mixture layer 21.

[0098] The detailed descriptions of the positive electrode current collector 10, the positive electrode mixture layer 20, the first positive electrode mixture layer 21, the second positive electrode mixture layer 22, etc., are repeated above, and therefore omitted.

[0099] In some specific embodiments, the first and second positive electrode slurries can be prepared by mixing the first and second positive electrode active materials with solvents, respectively. The solvent is not particularly limited. Exemplarily, the solvent may be N-methyl-2-pyrrolidone (NMP).

[0100] In some specific embodiments, the first positive electrode slurry and the second positive electrode slurry may further include components such as binders and conductive materials. Detailed descriptions of the binders and conductive materials are repeated above and therefore omitted.

[0101] In some embodiments, the first positive electrode mixture layer 21 can be formed by coating a first positive electrode slurry onto at least one side of the positive electrode current collector 10 and then drying it. The second positive electrode mixture layer 22 can be formed by coating a second positive electrode slurry onto the first positive electrode slurry or the first positive electrode mixture layer 21 and then drying it. According to one embodiment, the second positive electrode mixture layer 22 can be formed by coating a second positive electrode slurry onto a pre-formed first positive electrode mixture layer 21 and then drying it. According to another embodiment, the first positive electrode slurry and the second positive electrode slurry can be simultaneously coated onto the surface of the positive electrode current collector 10 and then dried simultaneously, thereby simultaneously forming the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22.

[0102] There is no particular limitation on the coating method of the first positive electrode slurry and the second positive electrode slurry. For example, the first positive electrode slurry and the second positive electrode slurry can be coated by methods such as rod coating, casting, or spraying.

[0103] In some specific embodiments, the drying of the first and second positive electrode slurries can be carried out at a temperature of 100°C to 200°C. For example, the drying of the first and second positive electrode slurries can be carried out at a temperature of 130°C to 170°C.

[0104] Lithium secondary batteries

[0105] A lithium-ion secondary battery according to one embodiment includes a positive electrode 100 for a lithium-ion secondary battery as described in any of the above embodiments. Exemplarily, the lithium-ion secondary battery may include a unit cell, which includes the aforementioned positive electrode, negative electrode, and separator. The separator may be disposed between the positive and negative electrodes within the unit cell.

[0106] The negative electrode is not particularly limited. Exemplarily, the negative electrode may include a negative electrode current collector and a negative electrode mixture layer on at least one side of the negative electrode current collector.

[0107] The composition of the negative electrode current collector is not particularly limited. Exemplarily, the negative electrode current collector may be a sheet or foil formed from one or more of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), and alloys thereof. In some specific embodiments, the negative electrode current collector may be copper foil (Cu-foil).

[0108] The thickness of the negative electrode current collector is not particularly limited. For example, the thickness of the negative electrode current collector can be from 0.1 μm to 50 μm.

[0109] The negative electrode mixture layer may contain a negative electrode active material. The negative electrode active material is not particularly limited. Exemplarily, the negative electrode active material may be one or more selected from carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, carbon fibers, etc.; lithium metal; lithium alloys; silicon-containing substances and tin-containing substances.

[0110] Exemplarily, the crystalline carbon may be graphite-based carbon such as natural graphite, artificial graphite, graphitized coke, graphitized mesocarbon microbead (MCMB), graphitized mesophase pitch-based carbon fiber (MPCF), etc.

[0111] Exemplarily, the amorphous carbon may be hard carbon, soft carbon, coke, mesocarbon microbead (MCMB) or mesophase pitch-based carbon fiber (MPCF).

[0112] Exemplarily, the elements contained in the lithium alloy may be aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium.

[0113] The silicon-containing substance is not particularly limited as long as it contains silicon, and it may be an active material that can be alloyed with lithium (Li). Exemplarily, the silicon-containing substance may be one or more selected from silicon (Si), silicon oxide (SiOx; 0 < x < 2), metal-doped silicon oxide (SiOx; 0 < x < 2), silicon oxide coated with carbon (SiOx; 0 < x < 2), silicon-carbon composite (Si-C) and silicon alloy.

[0114] The negative electrode mixture layer may further contain an adhesive. The adhesive is not particularly limited. Exemplarily, the adhesive may be a rubber-based adhesive such as styrene-butadiene rubber (SBR), fluorine-based rubber, ethylene-propylene rubber, butadiene rubber, isoprene rubber, silane rubber, etc.; a cellulose-based adhesive such as carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, methyl cellulose or their alkali metal salts; and any combination thereof.

[0115] The negative electrode mixture layer may further contain a conductive material. The conductive material is not particularly limited. Exemplarily, the conductive material may be one or more selected from particulate carbon materials and fibrous carbon materials. The particulate carbon material may be carbon black such as Super-P, Super-C, acetylene black, Ketjen black, etc., and the fibrous carbon material may be carbon fiber, carbon nanotube (CNT), vapor-grown carbon fiber (VGCF), etc.

[0116] The diaphragm is not particularly limited. Exemplarily, the diaphragm may include a porous polymer membrane made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer. Alternatively, the diaphragm may also include a nonwoven fabric formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0117] In some specific embodiments, the lithium secondary battery can be manufactured by housing the aforementioned unit cell in a soft pack that serves as the battery casing and then injecting electrolyte.

[0118] The electrolyte may contain an organic solvent and a lithium salt. The organic solvent acts as a medium for the movement of ions participating in the electrochemical reaction of the battery. For example, the organic solvent may be one of the following solvents or a mixture of two or more of the following solvents: carbonate-based solvents, ester-based solvents, ether-based solvents, ketone-based solvents, alcohol-based solvents, or aprotic solvents. When using two or more solvents, the mixing ratio can be appropriately adjusted according to the desired battery performance.

[0119] The lithium salt is a substance dissolved in an organic solvent that acts as a lithium-ion supply source within the battery, enabling the basic operation of the lithium secondary battery and promoting the movement of lithium ions between the positive and negative electrodes. Known substances can be used as the lithium salt at a suitable concentration. The electrolyte may further contain known solvents and may include known additives to improve charge / discharge characteristics and flame-retardant properties, etc., as needed.

[0120] In some specific embodiments, the unit cell may not include a separator between the positive and negative electrodes, but may include a solid electrolyte. The solid electrolyte is not particularly limited; exemplarily, it may be an oxide-based solid electrolyte, a sulfide-based solid electrolyte, or a polymer-based solid electrolyte.

[0121] Example

[0122] 1. Manufacturing of positive electrode and secondary battery

[0123] 1) Manufacturing of positive electrodes for lithium secondary batteries

[0124] (1) Example 1

[0125] Prepare from chemical formula LiMn 0.6 Fe 0.4 LMFP-based active materials represented by PO4 are prepared as lithium metal phosphorus oxides, using the chemical formula LiNi. 0.6 Co 0.1 Mn 0.3O2 represents the single-particle form of NCM-based active material as lithium transition metal oxide, polyvinylidene fluoride (PVDF) is prepared as binder, and carbon black and multi-walled carbon nanotubes (MWCNTs) are prepared as point-type conductive materials and line-type conductive materials, respectively.

[0126] Based on the solids content, 87.975 wt% lithium metal phosphorus oxide, 9.775 wt% lithium transition metal oxide, 1.25 wt% binder (PVDF), 0.6 wt% linear conductive material (CNT), and 0.4 wt% dot conductive material (carbon black) were mixed with a solvent (NMP) to prepare the first positive electrode slurry. In the final first positive electrode slurry, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide was 9:1.

[0127] Furthermore, based on the solids content, 48.875 wt% lithium metal phosphorus oxide, 48.875 wt% lithium transition metal oxide, 1.25 wt% binder (PVDF), and 1 wt% linear conductive material (CNT) were mixed with a solvent (NMP) to prepare a second cathode slurry. In the final prepared second cathode slurry, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide was 5:5.

[0128] The first positive electrode slurry was prepared at 10 mg / cm³ 2 The loading weight is coated onto one side of a 12μm thick positive electrode current collector (aluminum foil), while simultaneously, the second positive electrode slurry is applied at a loading weight of 10mg / cm³. 2 The load weight is applied to the surface of the first positive electrode slurry. At this time, the first positive electrode slurry and the second positive electrode slurry are coated in such a way that the load weight ratio of the final prepared first positive electrode mixture layer (lower layer) to the second positive electrode mixture layer (upper layer) is 5:5.

[0129] Subsequently, through the aforementioned drying and rolling processes, a positive electrode of Example 1 with an electrode thickness (including the current collector) of 152 μm and an electrode density of 2.857 g / cm³ (cc) was manufactured. In the final manufactured positive electrode, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide was 7:3, based on the entire electrode.

[0130] (2) Example 2

[0131] The positive electrode of Example 2 was manufactured using the same method as in Example 1, except that the second positive electrode slurry contained 0.6% by weight of linear conductive material (CNT) and 0.4% by weight of dot conductive material (carbon black) based on solid content, instead of 1% by weight of linear conductive material (CNT).

[0132] (3) Comparative Example 1

[0133] The positive electrode of Comparative Example 1 was manufactured using the same method as in Example 1, except that the first positive electrode slurry and the second positive electrode slurry each contained only 0.6% by weight (based on solid content) of linear conductive material (CNT) as conductive material.

[0134] Specifically, based on the solids content, the first cathode slurry of Comparative Example 1 contained 88.335% by weight of lithium metal phosphorus oxide, 9.815% by weight of lithium transition metal oxide, 1.25% by weight of binder (PVDF), and 0.6% by weight of linear conductive material (CNT). Based on the solids content, the second cathode slurry of Comparative Example 1 contained 49.075% by weight of lithium metal phosphorus oxide, 49.075% by weight of lithium transition metal oxide, 1.25% by weight of binder (PVDF), and 0.6% by weight of linear conductive material (CNT).

[0135] (4) Comparative Example 2

[0136] The positive electrode of Comparative Example 2 was manufactured using the same method as in Example 2, except that the first positive electrode slurry and the second positive electrode slurry were applied in opposite ways, such that the structures of the first positive electrode mixture layer as the lower layer and the second positive electrode mixture layer as the upper layer were reversed.

[0137] (5) Comparative Example 3

[0138] Based on solids content, a third cathode slurry was prepared by mixing 68.425 wt% lithium metal phosphorus oxide, 29.325 wt% lithium transition metal oxide, 1.25 wt% PVDF binder, 0.6 wt% linear conductive material (CNT), and 0.4 wt% dot conductive material (carbon black) with a solvent (NMP). In the final prepared third cathode slurry, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide was 7:3.

[0139] Subsequently, only the third positive electrode slurry was prepared at 20 mg / cm³. 2 The load weight is coated on one side of the positive current collector (aluminum foil) to form a single layer of positive electrode mixture. Otherwise, the positive electrode of Comparative Example 3 is manufactured by the same method as in Example 1.

[0140] 2) Manufacturing of secondary batteries

[0141] A negative electrode slurry containing negative electrode active materials (artificial graphite and natural graphite) was coated onto a 6 μm thick copper foil (Cu-foil) and dried to manufacture a negative electrode for lithium secondary batteries. A polyolefin separator was placed between the positive and negative electrodes manufactured as described above to manufacture a secondary battery cell. The secondary battery cell was placed in a pouch for secondary batteries, and then an electrolyte was injected into the pouch. The electrolyte was a solution of 1 M LiPF6 dissolved in a solvent of mixed ethylene carbonate (EC) and diethyl carbonate (DEC). The pouch was then sealed to manufacture a pouch-type lithium secondary battery. The manufactured pouch-type lithium secondary batteries were used as secondary battery samples for examples and comparative examples.

[0142] [Table 1]

[0143]

[0144] 2. Evaluation of the positive electrode and secondary battery

[0145] 1) DC-IR measurement

[0146] The secondary battery samples from the examples and comparative examples were subjected to the following process twice at room temperature: charging was performed using constant current / constant voltage (CC / CV) methods at 0.3C and 4.3V, followed by discharging at 0.3C and CC methods. Afterwards, with the charging complete, the batteries were discharged at a constant current (CC) of 0.3C to 50% SOC, and the resistance at 50% SOC when discharged at 1C for 10 seconds was measured as DC-IR.

[0147] 2) Evaluation of lifespan performance

[0148] For the batteries of the examples and comparative examples, lifetime characteristics were evaluated at 45°C within the range of 94% DOD (2% to 96% SOC). The batteries were charged at 0.3C to the voltage corresponding to 96% SOC under constant current / constant voltage (CC / CV) conditions, then cut off at 0.05C, and then discharged at 0.5C to the voltage corresponding to 2% SOC under constant current (CC) conditions. The discharge capacity was measured. This process was repeated 250 times, and the discharge capacity retention rate (expressed as a percentage (%)) relative to the initial discharge capacity was measured. This was used to measure the capacity retention rate during the lifetime characteristics evaluation at 45°C, and the results are shown in Table 2 below.

[0149] [Table 2]

[0150]

[0151] Referring to Tables 1 and 2, it can be confirmed that when the cathode having a mixture of lithium metal phosphorus oxide and lithium transition metal oxide has a single-layer structure, the cell resistance is relatively high and the lifespan performance is relatively insufficient (Comparative Example 3). Furthermore, it can be confirmed that even when the cathode having a mixture of lithium metal phosphorus oxide and lithium transition metal oxide has a dual-layer structure, when the content of lithium metal phosphorus oxide and lithium transition metal oxide in the lower layer (the first cathode mixture layer) is the same, the cell resistance is relatively high and the lifespan performance is relatively insufficient (Comparative Example 2).

[0152] Furthermore, it can be confirmed that as a cathode having a mixture of lithium metal phosphorus oxide and lithium transition metal oxide, even if the content of lithium metal phosphorus oxide in the lower layer (first cathode mixture layer) is greater than the content of lithium transition metal oxide, the cell resistance is relatively high and the life performance is relatively insufficient when the lower layer (first cathode mixture layer) does not contain a point-type conductive material (carbon black) (Comparative Example 1).

[0153] Therefore, as shown in Examples 1 and 2, as a positive electrode having a bilayer structure composed of a mixture of lithium metal phosphorus oxide and lithium transition metal oxide, when the content of lithium metal phosphorus oxide in the lower layer (first positive electrode mixture layer) is greater than the content of lithium transition metal oxide, and the lower layer (first positive electrode mixture layer) contains a point-type conductive material and the upper layer (second positive electrode mixture layer) contains a linear conductive material, it is determined that a lithium secondary battery with low cell resistance and excellent life performance can be provided.

Claims

1. A positive electrode for a lithium secondary battery, comprising a positive electrode current collector and a positive electrode mixture layer on at least one side of the positive electrode current collector. in, The positive electrode mixture layer includes a first positive electrode mixture layer on the positive electrode current collector and a second positive electrode mixture layer on the first positive electrode mixture layer. The first positive electrode mixture layer and the second positive electrode mixture layer each independently contain lithium metal phosphorus oxide and lithium transition metal oxide as active materials. The weight of lithium metal phosphorus oxide contained in the first positive electrode mixture layer is greater than the weight of lithium transition metal oxide. The first positive electrode mixture layer contains a point-type conductive material. The second positive electrode mixture layer contains a linear conductive material.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein, The first positive electrode mixture layer contains a point-type conductive material at a content of 0.3% to 0.5% by weight.

3. The positive electrode for a lithium secondary battery according to claim 1, wherein, The second positive electrode mixture layer contains a linear conductive material content of 0.5% to 1.5% by weight.

4. The positive electrode for a lithium secondary battery according to claim 1, wherein, The first positive electrode mixture layer further comprises a linear conductive material.

5. The positive electrode for a lithium secondary battery according to claim 1, wherein, The second positive electrode mixture layer does not contain point-type conductive material.

6. The positive electrode for a lithium secondary battery according to claim 4, wherein, The weight of the linear conductive material contained in the first positive electrode mixture layer is less than or equal to the weight of the linear conductive material contained in the second positive electrode mixture layer.

7. The positive electrode for a lithium secondary battery according to claim 4, wherein, The first positive electrode mixture layer contains a linear conductive material content of 0.5% to 0.7% by weight.

8. The positive electrode for a lithium secondary battery according to claim 1, wherein, The second positive electrode mixture layer further comprises a dot-type conductive material, wherein the content of the dot-type conductive material in the second positive electrode mixture layer is from 0.1% to 0.5% by weight.

9. The positive electrode for a lithium secondary battery according to claim 1, wherein, The point-type conductive material includes carbon black.

10. The positive electrode for a lithium secondary battery according to claim 1, wherein, The linear conductive material includes carbon nanotubes (CNTs).

11. A lithium secondary battery comprising a positive electrode for a lithium secondary battery as described in any one of claims 1 to 10.

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