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 as an active material, at least one of the first positive electrode mixture layer and the second positive electrode mixture layer contains a lithium transition metal oxide as an active material, and a weight of a lithium metal phosphorus oxide contained in the first positive electrode mixture layer is greater than or equal to a weight of a lithium metal phosphorus oxide contained in the second positive electrode mixture layer.

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). EVs can replace vehicles that use fossil fuels, such as gasoline and diesel vehicles, which are one of the main causes of air pollution. As the power source for these 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 electrode current collector and a positive electrode mixture layer on at least one side of the positive electrode current collector, wherein 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 as an active material, at least one of the first positive electrode mixture layer and the second positive electrode mixture layer contains lithium transition metal oxide as an active material, and the weight of the lithium metal phosphorus oxide contained in the first positive electrode mixture layer is greater than or equal to the weight of the lithium metal phosphorus oxide contained in the second positive electrode mixture layer.

[0010] In some specific embodiments, when the first cathode mixture layer contains lithium transition metal oxide, the weight of lithium metal phosphorus oxide contained in the first cathode mixture layer may be greater than or equal to the weight of lithium transition metal oxide contained in the first cathode mixture layer.

[0011] In some specific embodiments, the weight ratio of the lithium metal phosphorous oxide to the lithium transition metal oxide contained in the first positive electrode mixture layer may be from 70:30 to 99:1.

[0012] In some specific embodiments, when the second positive electrode mixture layer contains a lithium transition metal oxide, the weight ratio of the lithium metal phosphorous oxide to the lithium transition metal oxide contained in the second positive electrode mixture layer may be from 30:70 to 70:30.

[0013] In some specific embodiments, the total weight of the lithium metal phosphorous oxide contained in the positive electrode mixture layer may be greater than or equal to the total weight of the lithium transition metal oxide contained in the positive electrode mixture layer.

[0014] In some specific embodiments, the weight ratio of the lithium metal phosphorous oxide to the lithium transition metal oxide contained in the positive electrode mixture layer may be from 60:40 to 80:20.

[0015] In some specific embodiments, the load weight (LW) ratio of the first positive electrode mixture layer to the second positive electrode mixture layer may be from 30:70 to 70:30.

[0016] In some specific embodiments, the lithium metal phosphorous oxide may be a lithium iron manganese phosphate (LMFP)-based active material represented by the chemical formula LiMn x Fe 1-x PO4 (0 < x < 1).

[0017] In some specific embodiments, the lithium transition metal oxide may be in the form of single particles.

[0018] A lithium secondary battery according to a specific embodiment includes the positive electrode for a lithium secondary battery according to any one of the above specific embodiments.

[0019] (III) Beneficial effects

[0020] According to a specific embodiment of the present invention, the stability of the positive electrode for a lithium secondary battery can be improved.

[0021] According to another specific embodiment of the present invention, a positive electrode for a lithium secondary battery having a high energy density can be provided.

[0022] According to another specific embodiment of the present invention, a positive electrode for a lithium secondary battery having excellent life performance can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a cross-sectional view conceptually showing a positive electrode for a lithium secondary battery according to a specific embodiment.

[0024] Explanation of reference numerals in the attached figures:

[0025] 100: Positive electrode for lithium secondary batteries

[0026] 10: Positive current collector

[0027] 20: Positive electrode mixture layer

[0028] 21: First cathode mixture layer

[0029] 22: Second cathode mixture layer Detailed Implementation

[0030] 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.

[0031] 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.

[0032] 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.

[0033] Figure 1 This is a conceptual cross-sectional view of a positive electrode for a lithium secondary battery according to a specific embodiment.

[0034] Positive electrode for lithium secondary batteries

[0035] 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 as an active material, and at least one of the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 contains lithium transition metal oxide as an active material. The weight of the lithium metal phosphorus oxide contained in the first positive electrode mixture layer 21 is greater than or equal to the weight of the lithium metal phosphorus oxide contained in the second positive electrode mixture layer 22.

[0036] 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.

[0037] The positive electrode 100 for the lithium secondary battery has a multilayer structure that simultaneously contains 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.

[0038] 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).

[0039] The thickness of the positive electrode current collector 10 is not particularly limited. Exemplarily, the thickness of the positive electrode current collector can be from 0.1 μm to 50 μm. In some specific embodiments, the thickness of the positive electrode current collector can be 5 μm or more, or 10 μm or more, and can be 30 μm or less, or 20 μm or less.

[0040] In some specific embodiments, the content of lithium metal phosphorus oxide in the lower first positive electrode mixture layer 21 of the lithium secondary battery positive electrode 100 can be greater than or equal to the content of lithium metal phosphorus oxide in the upper second positive electrode mixture layer 22. 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.

[0041] In some specific embodiments, the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 may each contain lithium transition metal oxide. When the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 each contain only lithium metal phosphorus oxide, it may result in insufficient electrode processability (achievement of viscosity and positive electrode density). Therefore, when the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 simultaneously contain both lithium metal phosphorus oxide and lithium transition metal oxide, the electrode processability can be improved.

[0042] In some specific embodiments, the first positive electrode mixture layer 21, which is adjacent to the positive current collector 10 and serves as the lower layer, may contain lithium transition metal oxide. According to one specific embodiment, the content of lithium metal phosphorus oxide in the first positive electrode mixture layer 21 may be greater than or equal to the content of lithium transition metal oxide. Specifically, when the first positive electrode mixture layer 21 contains lithium transition metal oxide, the weight of lithium metal phosphorus oxide contained in the first positive electrode mixture layer 21 may be greater than or equal to the weight of lithium transition metal oxide contained in the first positive electrode mixture layer 21.

[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 second positive electrode mixture layer 22 may contain lithium transition metal oxide. According to one embodiment, when the second positive electrode mixture layer 22 contains lithium transition metal oxide, 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, the safety of the positive electrode 100 for the lithium secondary battery can be ensured, while the energy density of the battery can be significantly 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 40% to 100% by weight. Specifically, the content of lithium metal phosphorus oxide contained in the first positive electrode mixture layer 21 can be more than 60% by weight, more than 70% by weight, more than 80% by weight, or more than 85% by weight, and can be less than 100% 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 0% to 60% by weight. Specifically, the content of lithium transition metal oxide contained in the first positive electrode mixture layer 21 can be more than 0% by weight or more than 5% by weight, and can be less than 40% by weight, less than 30% by weight, less than 20% by weight, or 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 100% by weight. Specifically, the content of lithium metal phosphorus oxide contained in the second positive electrode mixture layer 22 can be more than 40% by weight, and can be less than 80% by weight, less than 70% by weight, less than 60% by weight, or less than 50% 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 0% by weight 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 20% by weight, more than 30% by weight, or more than 40% by weight, and can be less than 50% by weight.

[0049] In some specific embodiments, when a first positive electrode mixture layer 21 with a relatively higher content of lithium metal phosphorus oxide is provided in the lower layer adjacent to the positive electrode current collector 10, and a second positive electrode mixture layer 22 with a relatively higher content of lithium transition metal oxide is provided in the upper layer, safety can be ensured while improving the capacity and energy density of the lithium secondary battery.

[0050] In some specific embodiments, in the positive electrode 100 for the lithium secondary battery, 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.

[0051] 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. In this case, the energy density can be improved without affecting the safety of the positive electrode 100 for the lithium secondary battery.

[0052] 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.

[0053] In some specific embodiments, the loading weight of the first positive electrode mixture layer 21 can be 6 mg / cm³. 2 Up to 14 mg / cm 2 For example, the loading weight of the first positive electrode mixture layer 21 can be 8 mg / cm³. 2 The above, and can be 12 mg / cm³ 2 the following.

[0054] In some specific embodiments, the loading weight of the second positive electrode mixture layer 22 can be 6 mg / cm³. 2 Up to 14 mg / cm 2 For example, the loading weight of the second positive electrode mixture layer 22 can be 8 mg / cm³. 2 The above, and can be 12 mg / cm³ 2 the following.

[0055] When the load weight relationship and range between the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 are as described above, it is possible to ensure excellent life performance while increasing the capacity of the positive electrode 100 for a lithium secondary battery.

[0056] In some specific embodiments, the lithium metal phosphoxide may be represented by the following Chemical Formula 1.

[0057] [Chemical Formula 1]

[0058] LiMePO4

[0059] In Chemical Formula 1, Me is at least one element selected from Co, Ni, Fe, and Mn.

[0060] The lithium metal phosphoxide represented by Chemical Formula 1 may be lithium metal oxide particles having an olivine structure with excellent structural stability. Exemplarily, the lithium metal phosphoxide may include an iron (Fe)-containing lithium iron phosphate (LFP)-based active material.

[0061] In some specific embodiments, the lithium metal phosphoxide may be a lithium manganese iron phosphate (LMFP)-based active material represented by the chemical formula LiMn x Fe 1-x PO4 (0 < x < 1). Specifically, in the chemical formula, 0.5 ≤ x ≤ 0.7. In this case, the performance such as the energy density of the positive electrode containing the lithium manganese iron phosphate (LMFP)-based active material can be improved.

[0062] The LMFP-based active material is an active material in which a part of iron (Fe) in the LFP-based active material is replaced by manganese (Mn). Compared with the LFP-based active material, the energy density and low-temperature performance can be relatively excellent. Therefore, when the positive electrode 100 for a lithium secondary battery contains the LMFP-based active material, a lithium secondary battery with a high energy density can be provided.

[0063] The particle composition of the lithium metal phosphoxide can be confirmed by inductively coupled plasma (ICP) analysis. Exemplarily, the particles of the lithium metal phosphoxide can be analyzed by ICP, and the number of phosphorus atoms is normalized to 1 to obtain a chemical formula.

[0064] 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.

[0065] [Chemical Formula 2]

[0066] Li x Ni a Co b1 Mn b2O 2+z

[0067] In the chemical formula 2, the values ​​can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, and -0.5≤z≤0.1.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] In some specific embodiments, the coating element can form a sea-type coating or an island-type coating.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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 include one or more of polyvinylidene fluoride, styrene-butadiene rubber (SBR), polytetrafluoroethylene, vinylidene fluoride / 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.

[0084] In some specific embodiments, the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 may further comprise conductive materials. The conductive materials may include one or more of the following: graphite such as natural or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, carbon fiber, and carbon nanotubes (CNTs); metal powders or fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. The content of the conductive materials 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.

[0085] In some specific embodiments, the first positive electrode mixture layer 21 and the second positive electrode mixture layer 22 may respectively contain carbon nanotubes (CNTs) and carbon black as conductive materials. In this case, the performance of the positive electrode 100 for the lithium secondary battery can be improved.

[0086] In some specific embodiments, the weight ratio of carbon nanotubes (CNTs) to carbon black in the first positive electrode mixture layer 21 can be 1:1 to 2:1 or 1.5 to 1.8. Furthermore, in the second positive electrode mixture layer 22, the weight ratio of carbon nanotubes (CNTs) to carbon black can be 1:1 to 2:1 or 1.5 to 1.8. The carbon black is a point-type conductive material that can improve the conductivity between active material particles. If an excessive amount of carbon black is included, although conductivity can be improved, the reduced content of the active material may lead to a decrease in capacity. On the other hand, carbon nanotubes (CNTs), as a linear conductive material, are expensive, making it difficult to increase their content. Therefore, when the weight ratio of carbon nanotubes (CNTs) to carbon black is adjusted to the above-mentioned range, the advantages of both carbon nanotubes (CNTs) and carbon black can be maximized while their disadvantages are minimized.

[0087] The positive electrode 100 for lithium secondary batteries according to the above specific implementation scheme can be manufactured by the following method.

[0088] Manufacturing method of positive electrode for lithium secondary batteries

[0089] 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.

[0090] 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.

[0091] 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).

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] Lithium secondary batteries

[0097] A lithium-ion secondary battery according to one specific embodiment includes a positive electrode for a lithium-ion secondary battery as described in any of the above specific 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.

[0098] 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 surface of the negative electrode current collector.

[0099] 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 of 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 negative electrode current collector may be a copper foil (Cu-foil).

[0100] The thickness of the negative electrode current collector is not particularly limited. Exemplarily, the thickness of the negative electrode current collector may be from 0.1 μm to 50 μm. In some specific embodiments, the thickness of the negative electrode current collector may be 1 μm or more or 5 μm or more, and may be 20 μm or less or 10 μm or less.

[0101] 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; lithium metal; lithium alloys; silicon-containing substances and tin-containing substances.

[0102] 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.

[0103] Exemplarily, the amorphous carbon may be hard carbon, soft carbon, coke, mesocarbon microbead (MCMB) or mesophase pitch-based carbon fiber (MPCF).

[0104] Exemplarily, the elements contained in the lithium alloy may be aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium.

[0105] The silicon-containing substance is not particularly limited as long as it contains silicon, and may be an active substance 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.

[0106] The negative electrode mixture layer may further comprise an adhesive. The adhesive is not particularly limited. Exemplarily, the adhesive may be a rubber-based adhesive such as styrene-butadiene rubber (SBR), fluororubber, ethylene propylene rubber, butadiene rubber, isoprene rubber, or silane rubber; a cellulose-based adhesive such as carboxymethyl cellulose (CMC), hydroxypropyl methylcellulose, methylcellulose, or their alkali metal salts; or any combination thereof.

[0107] The negative electrode mixture layer may further comprise 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 and Super-C, acetylene black, Ketjen black, etc., and the fibrous carbon material may be carbon fiber, carbon nanotubes (CNTs), vapor-grown carbon fiber (VGCF), etc.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] Example

[0114] 1. Manufacturing of positive electrode and secondary battery

[0115] 1) Manufacturing of positive electrodes for lithium secondary batteries

[0116] (1) Example 1

[0117] 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 0.3 O2 represents the single-particle form of NCM-based active material as a lithium transition metal oxide, carbon nanotubes (CNTs) and carbon black are prepared as conductive materials (CNT:carbon black weight ratio = 0.72:0.42), and polyvinylidene fluoride (PVDF) is prepared as a binder.

[0118] Based on the solids content, 87.71 wt% lithium metal phosphorus oxide, 9.75 wt% lithium transition metal oxide, 1.4 wt% binder, and 1.14 wt% conductive material were mixed with a solvent (N-methyl-2-pyrrolidone, NMP) to prepare a first cathode slurry. In the final first cathode slurry, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide was approximately 9:1.

[0119] Furthermore, based on the solids content, 48.88 wt% lithium metal phosphorus oxide, 48.88 wt% lithium transition metal oxide, 1.1 wt% binder, and 1.14 wt% conductive material 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.

[0120] 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. 2The 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.

[0121] Subsequently, drying was performed to form a first positive electrode mixture layer and a second positive electrode mixture layer on one side of the positive electrode current collector, followed by calendering to manufacture the positive electrode of Example 1 with a thickness of 150 μm and a density of 2.899 g / cm³ (cc). In the finally manufactured positive electrode, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide was approximately 7:3 based on the entire electrode.

[0122] (2) Comparative Example 1

[0123] Based on solids content, a third cathode slurry was prepared by mixing 68.33 wt% lithium metal phosphorus oxide, 29.28 wt% lithium transition metal oxide, 1.25 wt% binder, and 1.14 wt% conductive material 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 approximately 7:3.

[0124] 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 1 is manufactured by the same method as in Example 1.

[0125] (3) Comparative Example 2

[0126] Based on the solids content, 97.46% by weight of lithium metal phosphorus oxide, 1.4% by weight of binder and 1.14% by weight of conductive material were mixed with solvent (NMP) to prepare a first positive electrode slurry containing only lithium metal phosphorus oxide as the positive electrode active material.

[0127] In addition, based on the solid content, 97.96% by weight of lithium transition metal oxide, 0.9% by weight of binder and 1.14% by weight of conductive material were mixed with solvent (NMP) to prepare a second positive electrode slurry containing only lithium transition metal oxide as the positive electrode active material.

[0128] Subsequently, the first positive electrode slurry and the second positive electrode slurry were coated such that the loading weight ratio of the final prepared first positive electrode mixture layer (lower layer) to the second positive electrode mixture layer (upper layer) was 7:3. Otherwise, the positive electrode of Comparative Example 2 was manufactured by the same method as in Example 1.

[0129] (4) Comparative Example 3

[0130] Based on the solids content, 48.88% by weight of lithium metal phosphorus oxide, 48.88% by weight of lithium transition metal oxide, 1.1% by weight of binder and 1.14% by weight of conductive material were mixed with solvent (NMP) to prepare a first positive electrode slurry.

[0131] In addition, based on the solids content, 87.71% by weight of lithium metal phosphorus oxide, 9.75% by weight of lithium transition metal oxide, 1.4% by weight of binder and 1.14% by weight of conductive material were mixed with solvent (NMP) to prepare a second positive electrode slurry.

[0132] Subsequently, the first positive electrode slurry and the second positive electrode slurry were coated such that the loading weight ratio of the final prepared first positive electrode mixture layer (lower layer) to the second positive electrode mixture layer (upper layer) was 5:5. Otherwise, the positive electrode of Comparative Example 3 was manufactured by the same method as in Example 1.

[0133] 2) Manufacturing of secondary batteries

[0134] A negative electrode slurry (containing artificial graphite, natural graphite, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a weight ratio of 68.3:29.3:1.2:1.2 based on solid content) was coated onto a 6 μm thick copper foil and dried to manufacture a negative electrode for lithium-ion 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 an electrolyte in which 1.2 M of LiPF6 was dissolved in a solvent of mixed ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The pouch was then sealed to manufacture a pouch-type lithium-ion secondary battery. The manufactured pouch-type lithium-ion secondary batteries were used as secondary battery samples for examples and comparative examples.

[0135] [Table 1]

[0136]

[0137] 2. Evaluation of the positive electrode and secondary battery

[0138] 1) Comparison of positive electrode densities

[0139] The positive electrode densities (g / cm³) achievable when the positive electrodes of Examples 1 and Comparative Examples 1 to 3 are pressed to the maximum extent are shown in Table 2 below.

[0140] [Table 2]

[0141]

[0142] Referring to Tables 1 and 2, it can be confirmed that when LMFP and NCM are partially mixed as active materials in each positive electrode mixture layer and the total active material weight ratio is LMFP:NCM=7:3, an actual measured electrode mixture density of approximately 2.85 g / cm³ to 2.86 g / cm³ is achieved.

[0143] However, when comparing Example 1 and Comparative Example 2, it can be confirmed that although the total weight ratio of active materials is the same (LMFP:NCM=7:3), the first positive electrode mixture layer of Comparative Example 2 is composed only of LMFP with nanoscale size, so the actual electrode mixture density is lower than that of Example 1.

[0144] 2) Evaluation of the capacity and efficiency of positive electrode coin cells

[0145] The positive electrode half-coin cells of Example 1 and Comparative Examples 1 to 3 were subjected to two charge-discharge cycles at 25°C, following the procedure: constant current / constant voltage (CC / CV) charging at 0.1C, 4.4V, and 0.05C cut-off conditions, followed by a 10-minute rest period; and constant current (CC) discharging at 0.1C and 3.0V cut-off conditions, followed by a 10-minute rest period. The charge capacity, discharge capacity, and capacity efficiency of the first cycle are shown in Table 3 below.

[0146] [Table 3]

[0147]

[0148] Referring to Tables 1 and 3, it can be confirmed that the capacity and efficiency of Example 1 and Comparative Examples 2 to 3, which have a dual-layer electrode structure, are superior to those of Comparative Example 1, which has a single-layer electrode structure.

[0149] Furthermore, when comparing Examples 1 and Comparative Examples 2 to 3, it can be seen that the capacity and efficiency improve with the increase of the LMFP ratio in the first cathode mixture layer and the increase of the NCM ratio in the second cathode mixture layer. However, according to Table 2, it can be confirmed that in the cathode of Comparative Example 2, where the first cathode mixture layer is composed only of LMFPs with nanoscale dimensions, the actual electrode mixture density may be insufficient compared to Example 1.

[0150] Therefore, as shown in Example 1, when the first positive electrode mixture layer and the second positive electrode mixture layer respectively contain LMFP active material and NCM active material, and the proportion of LMFP in the first positive electrode mixture layer is adjusted to be relatively high and the proportion of NCM in the second positive electrode mixture layer is relatively high, it is determined that the electrode processability, electrode mixture density, battery capacity and safety performance can be improved.

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 as an active material. At least one of the first positive electrode mixture layer and the second positive electrode mixture layer contains a lithium transition metal oxide as an active material. The weight of lithium metal phosphorus oxide contained in the first positive electrode mixture layer is greater than or equal to the weight of lithium metal phosphorus oxide contained in the second positive electrode mixture layer.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein, When the first cathode mixture layer contains lithium transition metal oxide, the weight of lithium metal phosphorus oxide contained in the first cathode mixture layer is greater than or equal to the weight of lithium transition metal oxide contained in the first cathode mixture layer.

3. The positive electrode for a lithium secondary battery according to claim 2, wherein, The weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide contained in the first positive electrode mixture layer is 70:30 to 99:

1.

4. The positive electrode for a lithium secondary battery according to claim 1, wherein, When the second cathode mixture layer contains lithium transition metal oxide, the weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide contained in the second cathode mixture layer is 30:70 to 70:

30.

5. The positive electrode for a lithium secondary battery according to claim 1, wherein, The total weight of lithium metal phosphorus oxide contained in the cathode mixture layer is greater than or equal to the total weight of lithium transition metal oxide contained in the cathode mixture layer.

6. The positive electrode for a lithium secondary battery according to claim 5, wherein, The weight ratio of lithium metal phosphorus oxide to lithium transition metal oxide contained in the cathode mixture layer is 60:40 to 80:

20.

7. The positive electrode for a lithium secondary battery according to claim 1, wherein, The load weight (LW) ratio of the first positive electrode mixture layer to the second positive electrode mixture layer is 30:70 to 70:

30.

8. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lithium metal phosphorus oxide is made of the chemical formula LiMn. x Fe 1-x PO4 represents lithium manganese iron phosphate (LMFP) based active material, in which 0 <x<1。 9. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lithium transition metal oxide has a single-particle form.

10. A lithium secondary battery comprising a positive electrode for a lithium secondary battery as described in any one of claims 1 to 9.

Images