Positive electrode active material, positive electrode for lithium secondary battery, and lithium secondary battery

Abstract

A positive electrode active material according to one embodiment of the present invention comprises a first active material and a second active material, in which the first active material comprises an olivine-structured active material, and the second active material comprises a lithium-rich oxide, the weight of a first active material contained in the positive electrode active material is greater than or equal to the weight of a second active material contained in the positive electrode active material. According to one embodiment of the present invention, a positive electrode active material having improved overvoltage characteristics can be provided.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material, a positive electrode for a lithium secondary battery, and a lithium secondary battery. Background Art

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

[0003] In order to improve the performance of the lithium secondary battery, it is necessary to develop a technology that can improve the energy density and life performance of the positive electrode for the lithium secondary battery. Summary of the Invention

[0004] (1) Technical issues to be resolved

[0005] According to one aspect of the present invention, the overvoltage characteristics of a positive electrode active material can be improved.

[0006] According to another aspect of the present invention, a positive electrode for a lithium secondary battery having improved lifespan performance can be provided.

[0007] According to another aspect of the present invention, the energy density of a positive electrode for a lithium secondary battery can be improved.

[0008] (2) Technical solution

[0009] According to a specific embodiment of the present invention, the positive electrode active material includes a first active material and a second active material, wherein the first active material includes an active material with an olivine structure, the second active material includes a lithium-rich oxide, and the weight of the first active material included in the positive electrode active material is greater than or equal to the weight of the second active material included in the positive electrode active material.

[0010] In some specific embodiments, the weight ratio of the first active material to the second active material in the positive electrode active material may be 65:35 to 85:15.

[0011] In some specific embodiments, the average particle size (D50) of the second active material can be larger than the average particle size (D50) of the first active material.

[0012] In some specific embodiments, the average particle size (D50) of the first active material may be 0.3 μm to 5 μm.

[0013] In some specific embodiments, the average particle size (D50) of the second active material can be 2 μm to 9 μm.

[0014] In some embodiments, the first active material can include a carbon coating on the surface.

[0015] In some specific embodiments, based on the total weight of the positive electrode active material, the content of the carbon coating layer may be greater than 0.25 wt % and less than 2 wt %.

[0016] In some specific embodiments, the olivine-structured active material may be represented by the following Chemical Formula 1.

[0017] [Chemical Formula 1]

[0018] LiMePO4

[0019] In the Chemical Formula 1, Me is at least one element selected from the group consisting of Co, Ni, Fe, and Mn.

[0020] In some specific embodiments, the lithium-rich oxide may be represented by the following Chemical Formula 2.

[0021] [Chemical Formula 2]

[0022] Li a [M x Ni y Mn z ]O b

[0023] In Chemical Formula 2, M is at least one element selected from Co, Mg, Ti, Al, Fe, Ru, Zr, W, Sn, Sr, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, 0≤x≤0.9, 0≤y≤0.9, x+y>0, 0.1≤z≤0.9, 1.8≤a+x+y+z≤2.2, 1.05≤a / (x+y+z)≤1.95, and 1.8≤b≤2.2.

[0024] In some specific embodiments, the positive active material may be represented by the following Chemical Formula 3.

[0025] [Chemical Formula 3]

[0026] Li a Ni b Mn c Fe d P e O f

[0027] In the chemical formula 3, 1.002≤a≤1.196, 0.003≤b≤0.196, 0.6≤c≤0.649, 0.204≤d≤0.396, 0.51≤e≤0.99, and 3.02≤f≤3.98.

[0028] A positive electrode for a lithium secondary battery according to one embodiment includes the positive electrode active material described in any one of the above embodiments.

[0029] A lithium secondary battery according to one embodiment includes the positive electrode for a lithium secondary battery according to any one of the above embodiments.

[0030] (3) Beneficial effects

[0031] According to one embodiment of the present invention, a positive electrode active material having improved overvoltage characteristics can be provided.

[0032] According to another embodiment of the present invention, the lifespan performance of a positive electrode for a lithium secondary battery can be improved.

[0033] According to another embodiment of the present invention, a positive electrode for a lithium secondary battery having excellent energy density can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a graph showing charge and discharge curves of lithium secondary batteries including positive electrodes according to Examples and Comparative Examples.

[0035] Figure 2 Graphs showing room-temperature lifetime characteristic evaluation results of lithium secondary batteries including positive electrodes according to Examples and Comparative Examples. DETAILED DESCRIPTION

[0036] The technology disclosed in this specification and its specific embodiments are described in detail below with reference to the accompanying drawings. However, the technology can be implemented in various other forms, and its scope is not limited to the specific embodiments described below. Furthermore, the technology disclosed in this specification is not limited to the specific embodiments described below, but can also be constructed by selectively combining all or part of the specific embodiments, thereby realizing various variations.

[0037] As the demand for lithium secondary batteries increases, a technology that can manufacture positive electrodes with excellent performance is needed. In this regard, according to a specific embodiment, by applying an olivine-structured active material with excellent structural stability as a positive electrode active material, the life performance of the positive electrode for lithium secondary batteries can be improved. The active material with an olivine structure has excellent thermal stability, so when the active material is applied as a positive electrode active material, even if the battery undergoes continuous charge / discharge cycles in a high temperature environment, the degree of degradation of the material can be less, but the capacity characteristics may be relatively insufficient. For example, when a lithium manganese iron phosphate (LMFP)-based active material with an olivine structure is applied as a positive electrode active material, due to the poor overvoltage characteristics of the LMFP-based active material during the charge / discharge process of the battery, the actual capacity of the positive electrode active material (about 150 mAh / g) may be lower than the theoretical capacity (170 mAh / g).

[0038] In addition, due to the structural characteristics of the active material having an olivine structure, the ionic conductivity and electrical conductivity of the active material having an olivine structure are poor compared to active materials with other structures, and when manganese (Mn) is contained in the olivine structure, such as the LMFP-based active material, this characteristic may be further exacerbated. According to a specific embodiment, the electrical conductivity can be improved by reducing the particle size of the active material having an olivine structure. However, in this case, in order to ensure the same energy density, more active material particles are required, and the total volume of the active material powder may increase due to the gaps between the particles. Therefore, the compaction density of the electrode may be reduced, and the energy density of the secondary battery cell may be reduced.

[0039] According to a specific embodiment of the present invention, by alleviating the above problems, a positive electrode active material for lithium secondary batteries having excellent overvoltage characteristics, capacity characteristics, etc. can be provided. Figure 1 and Figure 2 , the specific implementation scheme of the present invention is described in detail. Figure 1 is a graph showing charge and discharge curves of lithium secondary batteries including positive electrodes according to Examples and Comparative Examples. Figure 2 Graphs showing room-temperature lifetime characteristic evaluation results of lithium secondary batteries including positive electrodes according to Examples and Comparative Examples.

[0040] positive electrode active material

[0041] According to a specific embodiment of the present invention, the positive electrode active material includes a first active material and a second active material, wherein the first active material includes an active material with an olivine structure, the second active material includes a lithium-rich oxide, and the weight of the first active material included in the positive electrode active material is greater than or equal to the weight of the second active material included in the positive electrode active material.

[0042] In some specific embodiments, the first active material may include lithium metal oxide particles as an olivine-structured active material having excellent structural stability. Specifically, the olivine-structured active material may be represented by the following Chemical Formula 1.

[0043] [Chemical Formula 1]

[0044] LiMePO4

[0045] In the Chemical Formula 1, Me is at least one element selected from the group consisting of Co, Ni, Fe, and Mn.

[0046] For example, the first active material may be a lithium iron phosphate (LFP)-based active material containing iron (Fe) or a lithium manganese iron phosphate (LMFP)-based active material in which part of the iron (Fe) in the LFP-based active material is replaced by manganese (Mn).

[0047] The particle composition of the first active material represented by Chemical Formula 1 can be confirmed by inductively coupled plasma (ICP) analysis. For example, the particles of the first active material are analyzed using ICP and the number of oxygen atoms is normalized to 4, thereby obtaining the chemical formula of the first active material.

[0048] In some specific embodiments, the first active material may include a carbon coating on the surface. When a carbon coating is formed on the surface of the first active material, the electronic conductivity of the first active material can be improved. In some specific embodiments, the content of the carbon coating may be more than 0.25 wt % and less than 2 wt % based on the total weight of the positive active material. Exemplarily, the content of the carbon coating may be more than 0.7 wt % or more than 0.9 wt % based on the total weight of the positive active material, and may be less than 1.5 wt % or less than 1.3 wt %. The analysis of the carbon content can be measured using a CS analyzer.

[0049] In some specific embodiments, the second active material may include lithium-rich oxide particles having a high discharge capacity. Specifically, the lithium-rich oxide may be represented by the following Chemical Formula 2.

[0050] [Chemical Formula 2]

[0051] Li a [M x Ni y Mn z O b

[0052] In the chemical formula 2, M is at least one element selected from Co, Mg, Ti, Al, Fe, Ru, Zr, W, Sn, Sr, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, 0 ≤ x ≤ 0.9, 0 ≤ y ≤ 0.9, x + y > 0, 0.1 ≤ z ≤ 0.9, 1.8 ≤ a + x + y + z ≤ 2.2, 1.05 ≤ a / (x + y + z) ≤ 1.95, 1.8 ≤ b ≤ 2.2.

[0053] The positive electrode active material represented by the chemical formula 2 is a lithium-rich oxide particle having a high discharge capacity, and can be a composite of 1) a lithium-rich phase oxide such as Li2MnO3 and 2) a layered structure oxide such as an NCM-based positive electrode active material. Therefore, different from the active materials containing only lithium-rich phase oxides such as Li2M1O2 (where M1 can be Ni, Co, Fe, Mn, Zn, Mg, Ca, Cu, etc.) and Li2M2O3 (where M2 can be Mn, Sn, Mo, Ru, Ir, etc.), the positive electrode active material can also contain a layered structure oxide. Therefore, compared with the above active materials containing only lithium-rich phase oxides, the positive electrode active material can have more excellent stability of crystal structure and long-term life performance.

[0054] In some specific embodiments, in the chemical formula 2, it can be 0 < x ≤ 0.9, 0.05 ≤ x ≤ 0.9, 0.1 ≤ x ≤ 0.9, 0 < x ≤ 0.8, 0.05 ≤ x ≤ 0.8, or 0.1 ≤ x ≤ 0.8.

[0055] In some specific embodiments, in the chemical formula 2, it can be 0 < y ≤ 0.9, 0.05 ≤ y ≤ 0.9, 0.1 ≤ y ≤ 0.9, 0 < y ≤ 0.8, 0.05 ≤ y ≤ 0.8, or 0.1 ≤ y ≤ 0.8.

[0056] In some specific embodiments, in the chemical formula 2, it can be 1.1 ≤ a / (x + y + z) ≤ 1.95, 1.15 ≤ a / (x + y + z) ≤ 1.95, 1.2 ≤ a / (x + y + z) ≤ 1.95, or 1.3 ≤ a / (x + y + z) ≤ 1.95.

[0057] In some specific embodiments, in the lithium-rich oxide particles, the molar fraction of manganese relative to all elements except lithium and oxygen may be 0.5 to 0.75. For example, in Chemical Formula 2, 0.5≤z / (x+y+z)≤0.75.

[0058] In some specific embodiments, in the Chemical Formula 2, 0.25≤(x+y) / (x+y+z)≤0.5.

[0059] In some specific embodiments, in the lithium-rich oxide particles, the molar fraction of cobalt relative to all elements except lithium and oxygen may be 0 to 0.02. For example, the lithium-rich oxide particles may not contain cobalt.

[0060] In some specific embodiments, in the Chemical Formula 2, 1.9≤b≤2.1 or 1.95≤b≤2.05 may be used.

[0061] The chemical structure represented by the chemical formula 2 represents the binding relationship contained in the structure of the positive electrode active material, and does not exclude other additional elements. For example, M may include Co, and Co may be provided as the main active element (main active element) of the positive electrode active material together with Ni and Mn. The chemical formula 2 is provided to represent the binding relationship of the main active elements, and it should be understood that the chemical formula 2 is a formula including the introduction and substitution of additional elements.

[0062] In some specific embodiments, in addition to the main active element, an auxiliary element for enhancing the chemical stability of the positive electrode active material structure may be further included. The auxiliary element may be mixed into the positive electrode active material to form a bond, and it should be understood that this situation is also included in the chemical structure represented by Chemical Formula 2.

[0063] For example, the auxiliary element may include at least one of 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 may function as an auxiliary active element that contributes to the capacity / power activity of the positive electrode active material together with Co or Mn, such as Al.

[0064] In some specific embodiments, the positive electrode active material may further include a coating material or a doping material containing a coating element or a doping element. Exemplarily, an element substantially identical or similar to the above-mentioned auxiliary element may be used as a coating element or a doping element. Exemplarily, a combination of one or more of the above-mentioned elements may be used as a coating element or a doping element. In this case, the operating upper limit voltage of the lithium secondary battery can be adjusted, thereby suppressing the voltage drop (voltage decay) of the lithium secondary battery.

[0065] The coating element or the doping element may be present on the particle surface of the positive active material, or may be permeated through the particle surface of the positive active material and included in the bonding structure represented by Chemical Formula 2.

[0066] In some specific embodiments, the coating material can form a sea-type coating or an island-type coating.

[0067] In some specific embodiments, in the particles of the positive electrode active material, the content of the coating element can be 500-8000ppm, 1000-8000ppm or 1500-8000ppm of the total weight of all elements except lithium and oxygen. When the content of the coating element is as described above, the reduction of the initial capacity and the increase of the resistance of the lithium secondary battery can be prevented, and the voltage drop of the lithium secondary battery can be further suppressed.

[0068] In some specific embodiments, the coating material can be formed by a dry coating method or a wet coating method. For example, the particles of the positive electrode active material and the coating source can be dry mixed or wet mixed, and heat treated (for example, calcined or dried) to form a coating material on the surface of the particles of the positive electrode active material. The coating source can use a coating source well known in the art. For example, the coating source can contain B, Al, W, Zr, Ti, Mg, Co, etc.

[0069] The particle composition of the positive electrode active material represented by Chemical Formula 2 can be confirmed by inductively coupled plasma (ICP) analysis. For example, the particles of the positive electrode active material are analyzed using ICP, and the number of oxygen atoms is normalized to 1.8 to 2.2 (for example, 2), thereby obtaining the chemical formula of the positive electrode active material.

[0070] In some specific embodiments, the positive electrode active material may include a lithium manganese iron phosphate (LMFP)-based active material as a first active material, and may include a lithium-rich manganese (LMR)-based active material as a second active material. The LMFP-based active material has stable structural characteristics at high voltages above 4V, thereby improving the life performance of the positive electrode and the secondary battery, and the LMR-based active material has a relatively high oxidation voltage (operating voltage) range and capacity, thereby compensating for the relatively low capacity characteristics of the LMR-based active material. In particular, compared with NCM (nickel-cobalt-manganese)-based active materials, the LMR-based active material has a relatively high oxidation voltage range and capacity. Therefore, when the LMFP-based active material and the LMR-based active material, which have stable characteristics at high voltages, are mixed, the relatively low capacity characteristics of the LMFP-based active material can be better compensated compared to the NCM-based active material.

[0071] In some specific embodiments, the positive active material may be represented by the following Chemical Formula 3.

[0072] [Chemical Formula 3]

[0073] Li a Ni b Mn c Fe d P e O f

[0074] In the chemical formula 3, 1.002≤a≤1.196, 0.003≤b≤0.196, 0.6≤c≤0.649, 0.204≤d≤0.396, 0.51≤e≤0.99, and 3.02≤f≤3.98.

[0075] The weight of the first active material contained in the positive electrode active material is greater than or equal to the weight of the second active material contained in the positive electrode active material. Specifically, the weight of the first active material contained in the positive electrode active material may be more than 50 weight%, more than 60 weight%, more than 70 weight% or more than 80 weight%, and may be less than 100 weight%, less than 99 weight%, less than 95 weight% or less than 90 weight%. The weight of the second active material contained in the positive electrode active material may be more than 0 weight%, more than 1 weight%, more than 5 weight% or more than 10 weight%, and may be less than 50 weight%, less than 40 weight%, less than 30 weight% or less than 20 weight%. In some specific embodiments, the weight ratio of the first active material to the second active material contained in the positive electrode active material may be 65:35 to 85:15.

[0076] When the contents of the first active material and the second active material in the positive electrode active material are adjusted as described above, the life performance of the positive electrode active material can be improved, and a high-capacity positive electrode active material for lithium secondary batteries with excellent overvoltage characteristics can be provided.

[0077] In some specific embodiments, the average particle size (D50) of the second active material may be greater than the average particle size (D50) of the first active material. Specifically, the positive electrode active material may improve the compaction density of the positive electrode by including the first active material and the second active material in a bimodal structure. More specifically, the positive electrode active material may include small particles instead of large particles with an average particle size (D50) of about 10 μm, and may include even smaller particles instead of small particles with an average particle size (D50) of about 3 μm, thereby having a bimodal structure with a finer structure. Exemplarily, the average particle size (D50) of the first active material may be 0.3 μm to 5 μm, and the average particle size (D50) of the second active material may be 2 μm to 9 μm. The measuring method of the average particle size (D50) is not particularly limited. For example, the average particle size (D50) may be measured using a laser diffraction analyzer (Microtrac, MT 3000, etc.) according to a laser diffraction method.

[0078] The olivine-structured active material contained in the first active material has poor electrical conductivity due to its material properties. Therefore, as the particle size increases, this may reduce the performance (capacity, power life, etc.) of the lithium secondary battery. Therefore, in a bimodal structure where the average particle size (D50) of the first active material is larger than the average particle size (D50) of the second active material, this problem may occur.

[0079] On the other hand, when the positive electrode active material includes the first active material and the second active material in a bimodal structure in which the average particle size (D50) of the second active material is larger than the average particle size (D50) of the first active material, these problems can be prevented from occurring and the compaction density of the positive electrode can be further improved.

[0080] Positive electrode for lithium secondary battery

[0081] A positive electrode for a lithium secondary battery according to one embodiment includes the positive electrode active material described in any of the above embodiments. For example, the positive electrode for a lithium secondary battery may include: a positive electrode current collector; and a positive electrode mixture layer disposed on at least one side of the positive electrode current collector. The positive electrode mixture layer may include the positive electrode active material described in any of the above embodiments.

[0082] The composition of the positive electrode current collector is not particularly limited. For example, 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 alloys thereof. In some specific embodiments, the positive electrode current collector can be aluminum foil (Al-foil).

[0083] The thickness of the positive electrode current collector is not particularly limited. For example, the thickness of the positive electrode current collector may be 0.1 μm to 50 μm.

[0084] The structure of the positive electrode mixture layer is not particularly limited. For example, the positive electrode mixture layer may have a single-layer structure or a multi-layer structure including two or more mixture layers.

[0085] The content of the positive electrode active material in the positive electrode mixture layer is not particularly limited. For example, the content of the positive electrode active material in the positive electrode mixture layer may be 80 wt % to 99 wt %.

[0086] In some specific embodiments, the positive electrode mixture layer may further include a binder. The binder is not particularly limited. For example, the binder may include one or more of styrene-butadiene rubber (SBR), polytetrafluoroethylene (Polytetrafluoroethylene), polyvinylidene fluoride (Polyvinylidenefluoride), vinylidene fluoride / hexafluoropropylene copolymer, polyacrylonitrile and polymethyl methacrylate, etc. The content of the binder included in the positive electrode mixture layer is not particularly limited. For example, the content of the binder included in the positive electrode mixture layer may be 0.1% by weight to 10% by weight, respectively.

[0087] In some specific embodiments, the positive electrode mixture layer may further include a conductive material. The conductive material is not particularly limited. For example, the conductive material may include one or more of the following substances: graphite such as natural graphite or artificial graphite; carbon-based substances such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanotubes (CNTs); metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. The content of the conductive material contained in the positive electrode mixture layer is not particularly limited. For example, the content of the conductive material contained in the positive electrode mixture layer may be 0.1% by weight to 10% by weight.

[0088] In some specific embodiments, the compaction density of the positive electrode for lithium secondary batteries can be 2.0g / cubic centimeter (cc) to 3.0g / cubic centimeter. Specifically, the compaction density of the positive electrode for lithium secondary batteries can be more than 2.2g / cubic centimeter, 2.3g / cubic centimeter or more, 2.4g / cubic centimeter or more, 2.45g / cubic centimeter or more, or 2.5g / cubic centimeter or more, and can be 2.9g / cubic centimeter or less, 2.8g / cubic centimeter or less, 2.7g / cubic centimeter or less, 2.6g / cubic centimeter or less, or less than 2.55g / cubic centimeter. The compaction density can refer to the mixture density of the positive electrode mixture layer.

[0089] The method for manufacturing the positive electrode for a lithium secondary battery according to the above-mentioned specific embodiment is not particularly limited. For example, the positive electrode for a lithium secondary battery can be manufactured by coating a positive electrode slurry on at least one side of a positive electrode current collector and then drying it, wherein the positive electrode slurry contains the positive electrode active material described in any one of the above-mentioned specific embodiments. The coating method of the positive electrode slurry is not particularly limited. For example, the positive electrode slurry can be coated using a method such as rod coating, casting or spraying. The drying temperature of the positive electrode slurry is not particularly limited. For example, the drying of the positive electrode slurry can be carried out at 100°C to 200°C.

[0090] lithium secondary batteries

[0091] A lithium secondary battery according to one embodiment includes the positive electrode for a lithium secondary battery described in any of the above embodiments. For example, the lithium secondary battery may include a single cell, each cell including the positive electrode for a lithium secondary battery, a negative electrode, and a separator. The separator may be disposed between the positive electrode and the negative electrode within the single cell.

[0092] The negative electrode is not particularly limited. For example, the negative electrode may include: a negative electrode current collector; and a negative electrode mixture layer, wherein the negative electrode mixture layer is disposed on at least one side of the negative electrode current collector.

[0093] The composition of the negative electrode current collector is not particularly limited. For example, the negative electrode current collector can 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 can be copper foil (Cu-foil).

[0094] The thickness of the negative electrode current collector is not particularly limited. For example, the thickness of the negative electrode current collector may be 0.1 μm to 50 μm.

[0095] 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 the following materials: 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.

[0096] 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.

[0097] Exemplarily, the amorphous carbon may be hard carbon, soft carbon, coke, mesocarbon microbead (MCMB) or mesophase pitch-based carbon fiber (MPCF).

[0098] Exemplarily, the elements contained in the lithium alloy may be aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium.

[0099] The silicon-containing substance is not particularly limited as long as it contains silicon, and the silicon-containing substance 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 (SiO x , 0 < x < 2), metal-doped silicon oxide (SiO x , 0 < x < 2), carbon-coated silicon oxide (SiO x , 0 < x < 2), silicon-carbon composite (Si-C) and silicon alloy.

[0100] 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, silyl rubber, etc.; a cellulose-based adhesive such as carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, methyl cellulose or their alkali metal salts; and any one of their combinations.

[0101] 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.

[0102] The separator is not particularly limited. For example, the separator may include a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer. Alternatively, the separator may include a non-woven fabric formed of high-melting-point glass fiber, polyethylene terephthalate fiber, or the like.

[0103] In some specific embodiments, the lithium secondary battery can be manufactured by placing the above-mentioned single battery cell in a soft pack as a battery case and then injecting an electrolyte.

[0104] The electrolyte may include an organic solvent and a lithium salt. The organic solvent acts as a medium for the migration of ions involved in the electrochemical reaction of the battery. For example, the organic solvent may be a single solvent or a mixture of two or more of the following solvents: a carbonate-based solvent, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent, or an aprotic solvent. When two or more solvents are mixed, the mixing ratio may be appropriately adjusted according to the desired battery performance.

[0105] The lithium salt is a substance that dissolves in an organic solvent and acts as a source of lithium ions within the battery, enabling the basic operation of the lithium secondary battery and promoting the migration of lithium ions between the positive electrode and the negative electrode. As the lithium salt, a known substance can be used at a concentration suitable for the purpose. The electrolyte may further contain a known solvent as needed, and may contain known additives to improve charge and discharge characteristics, flame retardancy, etc.

[0106] In some specific embodiments, the single cell may include a solid electrolyte between the positive electrode and the negative electrode without a separator. The solid electrolyte is not particularly limited, and illustratively, the solid electrolyte may be an oxide-based solid electrolyte, a sulfide-based solid electrolyte, or a polymer-based solid electrolyte.

[0107] Example

[0108] 1. Manufacturing of positive electrode and secondary battery

[0109] 1) Manufacturing of positive electrodes for lithium secondary batteries

[0110] (1) Preparation of positive electrode active material

[0111] Prepare lithium manganese iron phosphate (LMFP) based active material (chemical formula LiMn 0.6 Fe 0.4 PO4 and an olivine structure compound with a carbon coating formed on the surface) as the first active material (average particle size D50: about 1 μm, carbon coating content: about 2 wt%), and a lithium-rich manganese (LMR)-based active material (represented by the chemical formula Li1.2 Ni 0.4 Mn 0.6 O2) as the second active material (average particle size D50: about 3 μm). For each example and comparative example, the first active material and the second active material were mixed (blending) in different ratios and used as each positive electrode active material. In addition, in the case of comparative example 4, the active material represented by the chemical formula LiNi 0.6 Co 0.1 Mn 0.3 An NCM (Ni—Co—Mn) based active material represented by O 2 (average particle size D50: about 3 μm) was used as the second active material.

[0112] The positive electrode active material was subjected to inductively coupled plasma (ICP) analysis using an Agilent 700s inductively coupled plasma-optical emission spectrometer (ICP-OES). The obtained mixing ratios of the positive electrode active materials are shown in Table 1 below. Specifically, the ICP analysis was performed by the following method:

[0113] 1. Add 0.3 g of sample and an equal amount of ultrapure water into a platinum crucible.

[0114] 2. Add concentrated nitric acid and hydrogen peroxide and heat to decompose.

[0115] 3. Dry the sample completely to remove the hydrofluoric acid component in the sample.

[0116] 4. Add a small amount of nitric acid to the residue, heat to dissolve, and then dilute to 10 mL with ultrapure water.

[0117] 5. Start measuring.

[0118] In addition, the weight (carbon content) of the carbon coating layer based on the total weight of the positive electrode active material was measured using a CS analyzer (CS844), and the results are shown in the following Table 1. Specifically, the analysis using the CS analyzer was performed by the following method:

[0119] 1. Considering the carbon (C) and sulfur (S) contents in the sample, add 0.1-1.0 g of the sample to a ceramic crucible and measure the weight.

[0120] 2. Add 1 spoon of combustion aid (LECOCEL).

[0121] 3. Add 1 tbsp of iron chips.

[0122] (2) Manufacturing of positive electrode

[0123] Based on the solid content, 93 wt% of the positive electrode active material, 2 wt% of the binder (polyvinylidene fluoride, PVDF) and 5 wt% of the conductive material (carbon black) were mixed with the solvent (N-methyl-2-pyrrolidone (NMP)) to prepare the positive electrode slurry. The positive electrode slurry was heated at 10-12 mg / cm 2 A positive electrode mixture layer was formed on one side of a 20 μm thick positive electrode current collector (aluminum foil) using a coating of 100 μm and dried at 100°C for 12 hours to produce a positive electrode. The dried positive electrode was rolled using a roller press. The compacted density (mixture density) of the resulting positive electrode is shown in Table 1 below.

[0124] 2) Manufacturing of lithium secondary batteries (manufacturing of lithium secondary battery positive electrode half-cells)

[0125] The positive electrode prepared as described above is cut into a circle with a diameter of 14 mm. Subsequently, the positive electrode is placed in the shell of a coin-cell of 2032 specifications, and a polyolefin separator with a diameter of 18 mm is set thereon, and an electrolyte is injected into the shell. The electrolyte is an electrolyte of 1M LiPF6 dissolved in a solvent mixed with ethylene carbonate (EC) and diethyl carbonate (DEC), and then a lithium foil (Li-foil) with a thickness of 0.4 mm and a diameter of 16 mm is placed on the separator to assemble into a coin-type battery. The lithium secondary battery positive electrode half-cell as a coin-type battery prepared as described above is used as the secondary battery sample of the embodiment and the comparative example.

[0126] 2. Evaluation of lithium secondary batteries

[0127] 1) Capacity characteristics

[0128] At 25°C, the lithium secondary battery samples prepared as described above were subjected to CC / CV charging (0.1C 4.6V, 0.05C cut-off (CUT-OFF)) and CC discharge (0.1C 2.5V cut-off), and the discharge capacity was measured. The measurement results are shown in Table 1 below. At this time, the charge and discharge curves of the lithium secondary batteries of Example 2 and Example 4 and Comparative Examples 1 to Comparative Examples 4 are as follows: Figure 1 shown.

[0129] 2) Overvoltage characteristics

[0130] The lithium secondary battery samples prepared as described above were charged at 25°C at 0.1C (4.6V), with a cutoff of 0.05C. The data recording condition was set to 10 mV, and the voltage change (Delta Voltage; dV) within 10 minutes after charging was measured. The measurement results are shown in Table 1 below. If the voltage change of a secondary battery is larger within the same time period after charging under the same conditions, according to Ohm's law (V=IR), it means that the current (I) value remains the same, but the voltage change (dV) is larger, and therefore it can be judged that the internal resistance (R) of the battery has increased.

[0131] 3) Life characteristics at room temperature

[0132] At 25°C, the lithium secondary battery sample prepared as described above was subjected to CC / CV charging (0.1C 4.6V, 0.05C cut-off) and CC discharge (0.1C 2.5V cut-off) 2 times, and then the evaluation of the life characteristics was started. The life evaluation conditions were set to CC / CV charging (0.5C 4.6V, 0.05C cut-off) and CC discharge (1C2.5V cut-off) 100 times for the lithium secondary battery sample at 25°C. The normal temperature life capacity retention rate (%) is defined as the capacity of the 100th time / the capacity of the 1st time, and the results are shown in Table 2 below. In addition, the changes in the capacity retention rate of the lithium secondary batteries of Example 2, Example 4 and Comparative Example 3 according to life are shown as follows Figure 2 shown.

[0133] [Table 1]

[0134]

[0135] [Table 2]

[0136]

[0137] As shown in Tables 1 and 2, in Comparative Example 1, which contains only an olivine-structured LMFP-based active material as the positive electrode active material, the compacted density and capacity values are low, and the dV value is high, confirming that the battery exhibits relatively poor overvoltage characteristics and energy density. Furthermore, in the case of Comparative Example 2, which contains only an LMR-based active material as the positive electrode active material, the battery life performance (capacity retention) is expected to be relatively low. This is particularly true when considering the following results: in the case of Comparative Example 3, which contains a mixture of an olivine-structured LMFP-based active material and an LMR-based active material, but with the proportion of the LMR-based active material exceeding that of the LMFP-based active material, the battery life performance (capacity retention) is relatively low.

[0138] In the case of Comparative Example 4, which contains an NCM-based active material as the positive electrode active material instead of the LMR-based active material, the capacity and oxidation voltage range are relatively low, and therefore it is judged to be inferior in energy density.

[0139] On the other hand, in the case of Examples 1 to 4, which contain a positive electrode active material comprising a mixture of an LMFP-based active material and an LMR-based active material having an olivine structure, and in which the weight of the LMFP-based active material is greater than or equal to the weight of the LMR-based active material, the compaction density and capacity values are high, and the dV value is low. Therefore, it is judged that the positive electrode active material has not only excellent overvoltage characteristics and energy density, but also relatively excellent life characteristics.

Claims

1. A positive electrode active material, comprising a first active material and a second active material, in, The first active material comprises an olivine-structured active material, The second active material comprises a lithium-rich oxide, The weight of the first active material included in the positive electrode active material is greater than or equal to the weight of the second active material included in the positive electrode active material.

2. The positive electrode active material according to claim 1, wherein The weight ratio of the first active material to the second active material in the positive electrode active material is 65:35 to 85:

15.

3. The positive electrode active material according to claim 1, wherein The average particle size D50 of the second active material is greater than the average particle size D50 of the first active material.

4. The positive electrode active material according to claim 1, wherein The average particle size D50 of the first active material is 0.3 μm to 5 μm.

5. The positive electrode active material according to claim 1, wherein The average particle size D50 of the second active material is 2 μm to 9 μm.

6. The positive electrode active material according to claim 1, wherein The first active material includes a carbon coating on a surface.

7. The positive electrode active material according to claim 6, wherein The content of the carbon coating layer is greater than 0.25 wt % and less than 2 wt % based on the total weight of the positive electrode active material.

8. The positive electrode active material according to claim 1, wherein The olivine structure active material is represented by the following chemical formula 1: [Chemical Formula 1] LiMePO4 In the Chemical Formula 1, Me is at least one element selected from the group consisting of Co, Ni, Fe, and Mn.

9. The positive electrode active material according to claim 1, wherein The lithium-rich oxide is represented by the following chemical formula 2: [Chemical Formula 2] Li a [M x Ni y Mr z ]O b In Chemical Formula 2, M is at least one element selected from Co, Mg, Ti, Al, Fe, Ru, Zr, W, Sn, Sr, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, 0≤x≤0.9, 0≤y≤0.9, x+y>0, 0.1≤z≤0.9, 1.8≤a+x+y+z≤2.2, 1.05≤a / (x+y+z)≤1.95, and 1.8≤b≤2.

2.

10. The positive electrode active material according to claim 1, wherein The positive electrode active material is represented by the following chemical formula 3: [Chemical Formula 3] Li a Ni b Mr c Feb d Q e O f In the chemical formula 3, 1.002≤a≤1.196, 0.003≤b≤0.196, 0.6≤c≤0.649, 0.204≤d≤0.396, 0.51≤e≤0.99, and 3.02≤f≤3.

98. 11 . A positive electrode for a lithium secondary battery, comprising the positive electrode active material according to claim 1 . 12 . A lithium secondary battery comprising the positive electrode for a lithium secondary battery according to claim 11 .

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