Positive electrode and secondary battery including the same

Abstract

The present invention relates to a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material and a conductive material, the conductive material including a linear conductive material, the positive electrode active material including a lithium composite transition metal oxide including nickel (Ni), cobalt (Co), and manganese (Mn), the lithium composite transition metal oxide being in a single particle form, and an average particle diameter (D 50 ) is 2 μm to 10 μm, and relates to a positive electrode that satisfies a specific mathematical formula for the BET specific surface area and content of the positive electrode active material and the linear conductive material.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0134107, filed on October 8, 2021, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference. The present invention relates to a positive electrode and a secondary battery including the same. [Background technology]

[0002] In recent years, with the rapid spread of battery-powered electronic devices such as mobile phones, notebook computers, and electric vehicles, the demand for secondary batteries that are small, lightweight, and have relatively high capacity has been rapidly increasing. In particular, lithium secondary batteries, which are lightweight and have high energy density, have attracted attention as a driving power source for portable electronic devices.

[0003] Generally, a lithium secondary battery includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, an electrolyte, an organic solvent, etc. The positive electrode and the negative electrode may have an active material layer including a positive electrode active material or a negative electrode active material formed on a current collector. The positive electrode generally uses a lithium transition metal oxide as the positive electrode active material, and the negative electrode generally uses a lithium-free carbon-based active material or a silicon-based active material as the negative electrode active material.

[0004] In this regard, lithium transition metal oxides have included lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMn2O4, etc.), and lithium iron phosphate compound (LiFePO4). Among these, lithium cobalt oxide (LiCoO2) is widely used due to its advantages of high operating voltage and excellent capacity characteristics, and is applied as a positive electrode active material for high voltage applications. However, rising cobalt (Co) prices and unstable supply limit its mass use as a power source in fields such as electric vehicles, and therefore there is an emerging need to develop an alternative positive electrode active material.

[0005] For this reason, nickel-cobalt-manganese-based lithium composite transition metal oxides (hereinafter simply referred to as "NCM-based lithium composite transition metal oxides") have been developed, in which some of the cobalt (Co) is replaced with nickel (Ni) and manganese (Mn). These "NCM-based lithium composite transition metal oxides" have the advantage of a superior charging speed compared to the lithium-cobalt oxides. However, because they have a larger specific surface area than the lithium-cobalt oxides, they require a larger amount of conductive material to ensure conductivity, which makes it difficult to ensure the energy density of the positive electrode. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Publication No. 10-2019-0042992 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a positive electrode having a high energy density and excellent life characteristics. Another object of the present invention is to provide a secondary battery including the above-mentioned positive electrode. [Means for solving the problem]

[0008] The present invention provides a positive electrode current collector, and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material and a conductive material, the conductive material including a linear conductive material, the positive electrode active material including a lithium composite transition metal oxide including nickel (Ni), cobalt (Co), and manganese (Mn), the lithium composite transition metal oxide being in the form of a single particle, and an average particle size (D 50 ) is 2 μm to 10 μm, and satisfies the following mathematical formula 1.

[0009] [Mathematical formula 1] 1.2≦B / A≦4.7

[0010] In the above mathematical formula 1, A is the weight percentage of the positive electrode active material relative to the BET specific surface area of ​​the positive electrode active material × the weight of the positive electrode active material layer, and B is the weight percentage of the linear conductive material relative to the BET specific surface area of ​​the linear conductive material × the weight of the positive electrode active material layer.

[0011] The present invention also provides a secondary battery including the above-described positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. [Effects of the Invention]

[0012] The positive electrode according to the present invention includes a positive electrode active material including a lithium composite transition metal oxide containing nickel, cobalt, and manganese, and a linear conductive material, wherein the positive electrode active material satisfies a specific average particle size range, the lithium composite transition metal oxide has a single particle shape, and the content of the positive electrode active material and the linear conductive material in the positive electrode active material layer and the BET specific surface area satisfy a specific mathematical formula. The linear conductive material in the positive electrode active material layer is disposed between the positive electrode active material to form a conductive network, thereby preventing an increase in resistance and a decrease in life performance due to insufficient conductivity. Furthermore, a positive electrode satisfying the specific mathematical formula 1 prevents a decrease in the energy density of the positive electrode due to the addition of an excessive amount of conductive material. Therefore, the positive electrode of the present invention has a high energy density, improved life performance, and significantly reduces gas generation. DETAILED DESCRIPTION OF THE INVENTION

[0013] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0014] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless otherwise clearly indicated in the context.

[0015] It should be understood that in this specification, the terms "comprises," "comprises," or "having" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the presence or possible addition of one or more other features, numbers, steps, components, or combinations thereof.

[0016] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured, for example, by using a laser diffraction method. The laser diffraction method is generally capable of measuring particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0017] In this specification, the BET specific surface area can be measured by the BET (Brunauer-Emmett-Teller) measurement method using an adsorbent gas such as nitrogen and a BELSORP (BET apparatus) manufactured by BEL JAPAN.

[0018] The present invention will be specifically described below. <Positive electrode> The present invention relates to a positive electrode. Specifically, the positive electrode can be preferably used as a positive electrode for a lithium secondary battery.

[0019] Specifically, the positive electrode of the present invention includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material and a conductive material, the conductive material including a linear conductive material, the positive electrode active material including a lithium composite transition metal oxide including nickel (Ni), cobalt (Co), and manganese (Mn), the lithium composite transition metal oxide being in the form of a single particle, and an average particle size (D 50 ) is 2 μm to 10 μm, and is characterized by satisfying the following mathematical formula 1.

[0020] [Mathematical formula 1] 1.2≦B / A≦4.7

[0021] In the above mathematical formula 1, A is the weight percentage of the positive electrode active material relative to the BET specific surface area of ​​the positive electrode active material × the weight of the positive electrode active material layer, and B is the weight percentage of the linear conductive material relative to the BET specific surface area of ​​the linear conductive material × the weight of the positive electrode active material layer.

[0022] The positive electrode according to the present invention includes a positive electrode active material including a lithium composite transition metal oxide containing nickel, cobalt, and manganese, and a linear conductive material, wherein the positive electrode active material satisfies a specific average particle size range, the lithium composite transition metal oxide has a single particle shape, and the content of the positive electrode active material and the linear conductive material in the positive electrode active material layer and the BET specific surface area satisfy a specific mathematical formula. The linear conductive material in the positive electrode active material layer is disposed between the positive electrode active material to form a conductive network, thereby preventing an increase in resistance and a decrease in life performance due to insufficient conductivity. Furthermore, a positive electrode satisfying the specific mathematical formula 1 prevents a decrease in the energy density of the positive electrode due to the addition of an excessive amount of conductive material. Therefore, the positive electrode of the present invention has a high energy density, improved life performance, and significantly reduces gas generation.

[0023] [Positive electrode current collector] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. Specifically, the positive electrode current collector may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, and an aluminum-cadmium alloy, and may specifically include aluminum.

[0024] The positive electrode current collector may generally have a thickness of 3 μm to 500 μm. The positive electrode current collector may have a surface with fine irregularities to enhance the bonding strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, or a nonwoven fabric.

[0025] [Cathode active material layer] The positive electrode active material layer may be disposed on at least one surface of the positive electrode current collector. More specifically, the positive electrode active material layer may be disposed on one or both surfaces of the positive electrode current collector.

[0026] The positive electrode active material layer may include a positive electrode active material and a conductive material. The positive electrode active material may include a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn). Specifically, the positive electrode active material may be a lithium composite transition metal oxide.

[0027] The lithium composite transition metal oxide may have a molar ratio (Li / M) of lithium to all metal elements (M) excluding lithium of 0.98 to 1.05, specifically 1.0 to 1.04. A range within this range is preferable in terms of improving capacity and preventing an increase in resistance due to an increase in lithium residue.

[0028] Specifically, the lithium composite transition metal oxide may include a compound represented by the following Chemical Formula 1:

[0029] [Chemical formula 1] Li 1+p [Ni 1-(x1+y1+z1) Co x1Mn y1 M a z1 1-p O2

[0030] In the chemical formula 1 above, M a is at least one element selected from the group consisting of Al, Ti, Zr, Mg, Nb, Ba, Ca, Ta, Sr, Zr, and Y, and -0.02 ≦ p ≦ 0.05, 0 < x1 < 1, 0 < y1 < 1, 0 ≦ z1 ≦ 0.1 may be satisfied.

[0031] In the lithium composite transition metal oxide of the chemical formula 1, Li may be contained in a content corresponding to 1 + p, that is, 0.98 ≦ 1 + p ≦ 1.05, 1.0 ≦ 1 + p ≦ 1.04. When in the above range, it is preferable in terms of preventing capacity degradation and preventing resistance due to an increase in Li residues.

[0032] In the lithium composite transition metal oxide of the chemical formula 1, Ni may be contained in a content corresponding to 1 - (x1 + y1 + z1), for example, 0 < 1 - (x1 + y1 + z1) < 1. More preferably, Ni may be contained in 0.5 ≦ 1 - (x1 + y1 + z1) ≦ 0.95. When contained in the above content range, it is possible to achieve high capacity and ensure excellent stability.

[0033] In the lithium composite transition metal oxide of the chemical formula 1, Co may be contained in a content corresponding to x1, that is, 0 < x1 < 1, specifically 0.1 ≦ x1 ≦ 0.4.

[0034] In the lithium composite transition metal oxide of the chemical formula 1, Mn is in a content corresponding to y1, that is, 0 < y1 < 1, specifically 0.1 ≦ y1 ≦ ≦ 0.4 may be contained. When in the above range, it is preferable in terms of improving the stability of the positive electrode active material and, as a result, being able to improve the stability of the battery.

[0035] In the lithium composite transition metal oxide of the chemical formula above, M amay be a doping element contained in the crystal structure of the lithium composite transition metal oxide, and M a may be included in an amount corresponding to z1, that is, 0≦z1≦0.1.

[0036] The lithium composite transition metal oxide or positive electrode active material according to the present invention is in the form of a single particle. Compared to a lithium composite transition metal oxide in the form of a secondary particle, a single-particle lithium composite transition metal oxide has the advantages of lower initial resistance and less particle cracking during rolling, but a higher rate of resistance increase with cycling. Therefore, when the lithium composite transition metal oxide is in the form of a single particle, the positive electrode according to the present invention is preferable in that it satisfies the mathematical formula 1 described below, thereby ensuring conductivity, improving energy density, and improving life characteristics, and preventing resistance increase with cycling. When the lithium composite transition metal oxide or positive electrode active material is in the form of a secondary particle, the specific surface area is relatively large, causing excessive electrolyte side reactions. Therefore, even if the mathematical formula 1 described above is satisfied, the desired effects of improving life performance, rate characteristics, and suppressing gas generation cannot be achieved.

[0037] In this specification, the term "single particle" refers to a primary structure of a single particle, consisting of primary particles rather than secondary particles. Furthermore, in this specification, the term "secondary particle" refers to an aggregate, i.e., a secondary structure, in which primary particles are aggregated together due to physical or chemical bonding between the primary particles, even without any intentional aggregation or granulation process of the primary particles that make up the secondary particle.

[0038] The average particle size (D 50 ) of the positive electrode active material is 2 μm to 10 μm. 50 If the average particle diameter (D) of the positive electrode active material is less than 2 μm, the specific surface area increases relatively, the side reaction of the electrolyte becomes serious, and even if the above mathematical formula 1 is satisfied, the life performance, rate characteristics, and gas generation prevention effect cannot be obtained. 50) exceeds 10 μm, the lithium ion diffusion path becomes longer as the average particle size becomes excessively large, so even if the above mathematical formula 1 is satisfied, the rate characteristics are significantly reduced and no improvement in life performance can be expected.

[0039] The average particle size (D 50 ) may be specifically 3 μm to 8 μm, more specifically 3.5 μm to 6.0 μm. When it is in the above range, the strength of the particles increases, which can prevent the particles from cracking during rolling, improve the rolling density, reduce the specific surface area, reduce lithium by-products, and reduce the amount of gas generated by side reactions with the electrolyte.

[0040] The BET specific surface area of ​​the positive electrode active material is 0.2 m 2 / g~3m 2 / g, specifically 0.4m 2 / g~1m 2 / g, more specifically 0.7m 2 / g~1m 2 When the content is in the above range, it is preferable in terms of suppressing gas generation due to side reactions on the positive electrode surface, and in particular, by satisfying mathematical formula 1 described below, it is possible to more preferably achieve the effects of ensuring the conductivity of the positive electrode, improving the energy density, and improving the life characteristics.

[0041] The positive electrode active material may be contained in the positive electrode active material layer in an amount of 90% by weight to 99% by weight, specifically 92% by weight to 98% by weight. When the amount is within this range, the capacity and energy density of the positive electrode can be improved, and by satisfying Equation 1 described below, the conductivity of the positive electrode can be ensured, and the energy density and life characteristics can be improved.

[0042] The conductive material can be used to assist and improve the conductivity of the positive electrode. The conductive material includes a linear conductive material. The term "linear conductive material" is used to distinguish it from conductive materials in the form of dots, particles, or plates. The linear conductive material is a long, fibrous conductive material that can contribute to electrical contact between positive electrode active materials and the formation of a conductive network. The formation of a conductive network by such linear conductive material can minimize the amount of conductive material in the positive electrode, thereby increasing the amount of positive electrode active material and improving the energy density of the positive electrode. In particular, according to the present invention, by satisfying Equation 1 below, it is possible to more effectively ensure the conductivity of the positive electrode, improve energy density, and improve life characteristics.

[0043] The linear conductive material may be at least one selected from conductive fibers and carbon nanotubes. More specifically, it may be carbon nanotubes. The conductive fibers may be carbon fibers, metal fibers, etc., and the carbon nanotubes may be single-walled carbon nanotubes, multi-walled carbon nanotubes, etc. More specifically, the linear conductive material may be carbon nanotubes. Carbon nanotubes have long fiber lengths and high graphitization and crystallinity, making them suitable for electrical contact with the positive electrode active material and for forming a conductive network.

[0044] The BET specific surface area of ​​the linear conductive material is 150m 2 / g~300m 2 / g, specifically 170m 2 / g~210m 2 When the specific surface area is in the above range, it is preferable in that gas generation is suppressed due to the specific surface area being reduced to a preferred level, and by satisfying mathematical formula 1 described below, it is possible to more preferably achieve the effects of ensuring the conductivity of the positive electrode, improving the energy density, and improving the life characteristics.

[0045] The BET specific surface area of ​​the linear conductive material may be 150 to 450 times, more specifically 180 to 300 times, the BET specific surface area of ​​the positive electrode active material, which is preferable in terms of realizing battery capacity while ensuring sufficient conductivity.

[0046] The pellet density of the linear conductive material may be 0.09 g / cc to 0.16 g / cc, specifically 0.095 g / cc to 0.145 g / cc, which is preferable in that the rolling density of the electrode can be adjusted to a preferred level and the energy density can be improved.

[0047] The pellet density may refer to the density measured by placing 5 g of the linear conductive material in a mold with a diameter of 22 mm and measuring it at a pressure of 2 tons using a powder resistance meter (device name: HPRM-A2, manufacturer: HANTECH).

[0048] The linear conductive material may have an average length of 1 μm to 100 μm, specifically 5 μm to 30 μm, which is preferable because a conductive network between the active materials can be smoothly maintained when the length is within the above range.

[0049] In this specification, the average length of the linear conductive material is measured by the following method. A solution (solid content: 1 wt % based on the total weight of the solution) prepared by adding linear conductive material and carboxymethyl cellulose (CMC) to water in a weight ratio of 40:60 is diluted 1,000 times with water. 20 ml of the diluted solution is then filtered through a filter, and the filter from which the linear conductive material has been filtered is dried. 100 images of the dried filter are taken with a scanning electron microscope (SEM), and the lengths of the linear conductive material are measured using the ImageJ program. The average value of the lengths is defined as the average length of the linear conductive material.

[0050] The linear conductive materials may have an average diameter of 5 nm to 30 nm, preferably 10 nm to 20 nm. When the average diameter of the linear conductive materials is within the above range, it is preferable in terms of preventing breakage of the linear conductive materials and ensuring flexibility.

[0051] In this specification, the average diameter of the linear conductive material is measured by the following method. A solution (solid content: 1 wt % based on the total weight of the solution) prepared by adding linear conductive material and carboxymethyl cellulose (CMC) to water in a weight ratio of 40:60 is diluted 1,000 times with water. One drop of the diluted solution is placed on a TEM grid, and the TEM grid is dried. The dried TEM grid is observed using a TEM device (product name: H7650, manufacturer: Hitachi) to measure the average diameter of the linear conductive material.

[0052] The linear conductive material may be included in the positive electrode active material layer in an amount of 0.50 wt % to 1.75 wt %, specifically 0.8 wt % to 1.7 wt %, more specifically 1.0 wt % to 1.5 wt %. When the amount is within this range, the conductivity of the positive electrode is maintained at a desirable level, and the aggregation of the linear conductive material due to the addition of excessive linear conductive material, which results in the formation of an uneven conductive network within the positive electrode, is prevented. The content of the positive electrode active material can be increased, thereby improving the energy density of the positive electrode.

[0053] The weight percentage of the linear conductive material relative to the total weight of the positive electrode active material layer may be 0.006 to 0.019 times, specifically 0.013 to 0.018 times, and more specifically 0.014 to 0.017 times the weight percentage of the positive electrode active material relative to the total weight of the positive electrode active material layer. When the weight percentage is within the above range, the following mathematical formula 1 can be satisfied, thereby ensuring the conductivity of the positive electrode, improving the energy density, and improving the life characteristics.

[0054] The conductive material may further include dot-like conductive materials in addition to the linear conductive materials. The dot-like conductive materials may refer to, for example, particulate conductive materials. Specifically, the dot-like conductive materials may include at least one selected from the group consisting of graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; fluorocarbon; metal powder such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0055] More specifically, the conductive material may be a linear conductive material. For example, the conductive material may include only linear conductive materials without including dot-like conductive materials. According to the present invention, excellent conductivity and improved lifespan can be achieved by satisfying the relationship of Equation 1. However, if both linear and dot-like conductive materials are included, the addition of the dot-like conductive materials may make it difficult to form a uniform conductive network, or the amount of positive electrode active material may be reduced, resulting in insufficient improvement in energy density.

[0056] The positive electrode active material layer may further include a binder. The binder is a component that aids in binding the active material and conductive material, etc., and in binding them to the current collector, and specifically may include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably at least one selected from the group consisting of polyvinylidene fluoride and carboxymethyl cellulose, more preferably polyvinylidene fluoride.

[0057] In order to ensure sufficient binding strength between components such as the positive electrode active material, the binder may be contained in the positive electrode active material layer in an amount of 0.1 wt % to 10 wt %, preferably 0.1 wt % to 3 wt %, more specifically 0.5 wt % to 2.5 wt %.

[0058] When the positive electrode active material layer further includes a binder, the binder may be included in the positive electrode active material layer in an amount remaining after excluding the positive electrode active material and the linear conductive material.

[0059] The positive electrode active material layer may further include a thickener. The thickener is used to smoothly disperse the active material, conductive material, binder, etc., and is not particularly limited as long as it is used in the relevant field. For example, the thickener may be carboxymethyl cellulose. The thickener may be contained in the positive electrode active material layer in an amount of 0.1 wt % to 10 wt %, preferably 0.1 wt % to 3 wt %.

[0060] When the positive electrode active material layer further includes the thickener, the thickener may be contained in an amount remaining after excluding the positive electrode active material and the linear conductive material. Specifically, when the positive electrode active material layer further includes the binder and the thickener, the binder and the thickener may be contained in an amount remaining after excluding the positive electrode active material and the linear conductive material.

[0061] In the present invention, the positive electrode is characterized by satisfying the following mathematical formula 1:

[0062] [Mathematical formula 1] 1.2≦B / A≦4.7

[0063] In the above mathematical formula 1, A is the weight percentage of the positive electrode active material relative to the BET specific surface area of ​​the positive electrode active material × the weight of the positive electrode active material layer, and B is the weight percentage of the linear conductive material relative to the BET specific surface area of ​​the linear conductive material × the weight of the positive electrode active material layer.

[0064] According to Equation 1, the relationship between the specific surface area and content of the positive electrode active material and the conductive material can be optimally adjusted to improve the conductivity, energy density, and lifespan of the positive electrode. If the B / A ratio is less than 1.2, the conductive material content is too low, resulting in reduced conductivity, or insufficient electrical contact of the positive electrode active material, resulting in increased positive electrode resistance and significantly reduced cycle performance. Furthermore, if the B / A ratio exceeds 4.7, the conductive material is added in an excessive amount, resulting in reduced positive electrode active material content and reduced energy density. Furthermore, aggregation of the linear conductive material can make it difficult to form a uniform conductive network within the positive electrode.

[0065] In the above mathematical formula 1, the B / A may be specifically 1.8 to 4.2, more specifically 2.7 to 4.2, and even more specifically 3.2 to 3.7. When it is in the above range, the effects of ensuring the conductivity of the positive electrode and improving the life characteristics are further improved.

[0066] In the above mathematical formula 1, the units of the weight percentage and BET specific surface area of ​​the positive electrode active material and the linear conductive material are the same. For example, the units of the weight percentage of the positive electrode active material and the linear conductive material in the above mathematical formula 1 are the same as the units of the weight percentage of the positive electrode active material and the linear conductive material. layer The unit of the BET specific surface area of ​​the positive electrode active material and the linear conductive material in the mathematical formula 1 may be "wt %" relative to the weight of the material. 2 / g".

[0067] The positive electrode may be manufactured by a typical positive electrode manufacturing method, except that it is manufactured to satisfy Equation 1 according to the present invention. For example, the positive electrode may be manufactured by dissolving or dispersing components constituting a positive electrode active material layer, i.e., a positive electrode active material, a conductive material, a binder, etc., in a solvent to prepare a positive electrode slurry, applying the positive electrode slurry to at least one surface of a positive electrode current collector, and then drying and rolling the slurry. Alternatively, the positive electrode may be manufactured by casting the positive electrode slurry on a separate support, peeling it from the support, and laminating the resulting film on the positive electrode current collector.

[0068] <Secondary battery> The present invention also provides a secondary battery including the above-described positive electrode. Specifically, the secondary battery according to the present invention includes the above-described positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The positive electrode has been described above.

[0069] The negative electrode faces the positive electrode. The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector.

[0070] The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. Specifically, the negative electrode current collector may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, and an aluminum-cadmium alloy, and may specifically include copper.

[0071] The negative electrode current collector may generally have a thickness of 3 μm to 500 μm. The negative electrode current collector may have a surface with fine irregularities to enhance the bonding strength of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, or a nonwoven fabric.

[0072] The negative electrode active material layer may be disposed on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer may be disposed on one or both surfaces of the negative electrode current collector.

[0073] The negative electrode active material layer may include a negative electrode active material. As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; SiO v (0 < v < 2), metal oxides capable of doping and undoping lithium such as SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compound and the carbonaceous material such as Si-C composite or Sn-C composite, etc. may be mentioned, and a mixture of any one or two or more of these may be used. Further, a thin film of metallic lithium may be used as the negative electrode active material. Also, as the carbon material, either low-crystalline carbon or high-crystalline carbon may be used. Representative examples of low-crystalline carbon are soft carbon and hard carbon, and representative examples of high-crystalline carbon are amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0074] In addition to the negative electrode active material described above, the negative electrode active material layer may further contain a negative electrode binder, a negative electrode conductive material, and / or a thickening agent. The negative electrode binder is a component that assists in the bonding between the active material and / or the current collector, and usually may be contained in the negative electrode active material layer at 1 wt% to 30 wt%, preferably 1 wt% to 10 wt%.

[0075] The negative electrode binder may include at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably at least one selected from polyvinylidene fluoride and styrene-butadiene rubber.

[0076] As the thickener, any thickener used in conventional lithium secondary batteries may be used, and an example thereof is carboxymethyl cellulose (CMC).

[0077] The negative electrode conductive material is a component for further improving the conductivity of the negative electrode active material, and may be contained in the negative electrode active material layer in an amount of 1 to 30% by weight, preferably 1 to 10% by weight.

[0078] The negative electrode conductive material is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. Specific examples of commercially available conductive materials include acetylene black-based products such as those manufactured by Chevron Chemical Company, Denka Singapore Private Limited, and Gulf Oil Company, as well as Ketjenblack, EC-based products manufactured by Armak Company, Vulcan XC-72 (manufactured by Cabot Company), and Super P (manufactured by Timcal).

[0079] The negative electrode may be manufactured by a conventional method commonly known in the art. For example, the negative electrode may be manufactured by dissolving or dispersing components constituting the negative electrode active material layer, i.e., the negative electrode active material, the negative electrode conductive material, and / or the negative electrode binder, in a solvent to prepare a negative electrode slurry, applying the negative electrode slurry to at least one surface of a negative electrode current collector, drying, and rolling the negative electrode slurry, or by casting the negative electrode slurry on a separate support, peeling it from the support, and laminating the resulting film on the negative electrode current collector.

[0080] Meanwhile, in the secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without particular limitations. It is particularly preferable that the separator has low resistance to ion migration of the electrolyte and excellent electrolyte humidification ability. Specifically, a porous polymer film, such as a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used, and may be selectively used as a single-layer or multi-layer structure.

[0081] On the other hand, examples of the electrolyte include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in manufacturing secondary batteries, but are not limited to these.

[0082] Specifically, the electrolyte may include an organic solvent and a lithium salt. The organic solvent may be any solvent capable of acting as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of suitable electrolyte solvents include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as Ra-CN (Ra is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred. In this case, a mixture of the cyclic carbonate and the linear carbonate at a volume ratio of approximately 1:1 to 9 can provide excellent electrolyte performance.

[0083] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without particular limitation. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The concentration of the lithium salt is preferably within a range of 0.1 M to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has suitable conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0084] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds (e.g., difluoroethylene carbonate) or pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, to improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity. In this case, the additives may be contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0085] As described above, the secondary battery including the cathode active material according to the present invention has excellent electrical characteristics and high-temperature storage properties, and can be usefully applied to portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs), etc. In particular, the secondary battery according to the present invention can be usefully used as a high-voltage battery having a voltage of 4.45 V or more.

[0086] The secondary battery according to the present invention can be used as a unit cell of a battery module, and the battery module can be applied to a battery pack. The battery module or the battery pack can be used as a power source for one or more medium- to large-sized devices including power tools, electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs), and power storage systems.

[0087] The present invention may be embodied in various different forms and should not be construed as being limited to the embodiments set forth herein, although the present invention may be embodied in various different forms and should not be construed as being limited to the embodiments set forth herein.

[0088] Examples and Comparative Examples Example 1: Production of positive electrode As the positive electrode active material, Li[Ni 0.83 Co 0.11 Mn 0.06 A single particle lithium transition metal composite oxide represented by the chemical formula: ]O2 was prepared. The BET specific surface area of ​​the positive electrode active material was 0.8 m 2 / g and the average particle size (D 50 ) was 4.4 μm.

[0089] Carbon nanotubes were prepared as linear conductive materials. The BET specific surface area of ​​the carbon nanotubes was 185 m 2 / g and the pellet density was 0.12 g / cc.

[0090] The positive electrode active material, the linear conductive material, PVdF as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed in an N-methylpyrrolidone solvent in a weight ratio of 97.00:0.68:1.20:1.12 to prepare a positive electrode slurry. The positive electrode slurry was applied to an aluminum current collector, dried, and rolled to prepare a positive electrode of Example 1.

[0091] Example 2: Production of positive electrode A positive electrode was manufactured in the same manner as in Example 1, except that the positive electrode active material, linear conductive material, binder, and thickener used in Example 1 were mixed in a weight ratio of 97.00:1.47:1.20:0.33.

[0092] Example 3: Production of positive electrode A positive electrode was manufactured in the same manner as in Example 1, except that the positive electrode active material, linear conductive material, binder, and thickener used in Example 1 were mixed in a weight ratio of 97.00:1.68:1.20:0.12.

[0093] Example 4: Production of positive electrode The positive electrode active material, linear conductive material, binder, and thickener used in Example 1, and carbon black (BET specific surface area: 135 m) as a dot-like conductive material were used. 2 A positive electrode was manufactured in the same manner as in Example 1, except that the materials (wt. / g) were mixed in a weight ratio of 97.0:1.1:1.2:0.2:0.5.

[0094] Example 5: Production of positive electrode The positive electrode active material, linear conductive material, binder, and thickener used in Example 1, and carbon black (BET specific surface area: 135 m) as a dot-like conductive material were used. 2 A positive electrode was manufactured in the same manner as in Example 1, except that the components (wt. / g) were mixed in a weight ratio of 97.00:0.75:1.20:0.05:1.00.

[0095] Comparative Example 1: Production of Positive Electrode A positive electrode was manufactured in the same manner as in Example 1, except that the positive electrode active material, linear conductive material, binder, and thickener used in Example 1 were mixed in a weight ratio of 97.00:0.42:1.20:1.38.

[0096] Comparative Example 2: Production of Positive Electrode A positive electrode was manufactured in the same manner as in Example 1, except that the positive electrode active material, linear conductive material, binder, and thickener used in Example 1 were mixed in a weight ratio of 97.00:1.89:1.00:0.11.

[0097] Comparative Example 3: Production of Positive Electrode Comparative Example 3 differs from Example 1 in that no linear conductive material is used. Specifically, the positive electrode active material, binder, and thickener were the same as those in Example 1, and carbon black (BET specific surface area: 135 m) was used as a dot-like conductive material. 2 A positive electrode was manufactured in the same manner as in Example 1, except that the materials (wt. / g) were mixed in a weight ratio of 96.6:1.2:0.2:2.0.

[0098] Comparative Example 4: Production of Positive Electrode As the positive electrode active material, Li[Ni 0.83 Co 0.11 Mn 0.06 A single particle lithium composite transition metal oxide represented by the chemical formula: ]O2 was prepared. The BET specific surface area of ​​the positive electrode active material was 1.8 m 2 / g and the average particle size (D 50 ) was 1.5 μm. The same procedure was repeated except that the positive electrode active material prepared above was used. 2 The positive electrode was produced in the same manner as above.

[0099] Comparative Example 5: Production of Positive Electrode As the positive electrode active material, Li[Ni 0.83 Co 0.11 Mn 0.06 A single particle lithium composite transition metal oxide represented by the chemical formula: ]O2 was prepared. The BET specific surface area of ​​the positive electrode active material was 1.8 m 2 / g and the average particle size (D 50 ) was 1.5 μm.

[0100] A positive electrode was manufactured in the same manner as in Example 1, except that the positive electrode active material prepared above was used and the positive electrode active material, linear conductive material, binder, and thickener were mixed in a weight ratio of 95.0:3.0:1.5:0.5.

[0101] Comparative Example 6: Production of Positive Electrode As the positive electrode active material, Li[Ni 0.83 Co 0.11 Mn 0.06 A single particle lithium composite transition metal oxide represented by the chemical formula: ]O2 was prepared. The BET specific surface area of ​​the positive electrode active material was 0.6 m 2 / g and the average particle size (D 50 ) was 12.0 μm. The same procedure was repeated except that the positive electrode active material prepared above was used. 2 The positive electrode was produced in the same manner as above.

[0102] Comparative Example 7: Production of Positive Electrode As the positive electrode active material, Li[Ni 0.83 Co 0.11 Mn 0.06 A single particle lithium composite transition metal oxide represented by the chemical formula: ]O2 was prepared. The BET specific surface area of ​​the positive electrode active material was 0.6 m 2 / g and the average particle size (D 50 ) was 12.0 μm.

[0103] A positive electrode was manufactured in the same manner as in Example 1, except that the positive electrode active material prepared above was used and the positive electrode active material, linear conductive material, binder, and thickener were mixed in a weight ratio of 96:1:2:1.

[0104] Comparative Example 8: Production of Positive Electrode As the positive electrode active material, Li[Ni 0.83 Co 0.11 Mn 0.06 A lithium composite transition metal oxide in the form of secondary particles, which is represented by the chemical formula: ]O2 and is composed of an aggregation of two or more single particles, was prepared. The BET specific surface area of ​​the positive electrode active material was 2.4 m 2 / g and the average particle size (D 50 ) was 8.0 μm. The same procedure was repeated except that the positive electrode active material prepared above was used. 2 The positive electrode was produced in the same manner as above.

[0105] [Table 1]

[0106] [Experimental Example] <Lithium secondary battery manufacturing> 1. Manufacturing the negative electrode Graphite as the negative electrode active material, carbon black as the conductive material, styrene-butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener were mixed in a weight ratio of 95.35:0.5:3:1.15 with water as the solvent to prepare a negative electrode slurry, which was then applied to a copper current collector, dried, and rolled to prepare the negative electrode.

[0107] 2. Secondary battery manufacturing An electrode assembly was prepared by interposing a porous polyethylene separator between the positive electrode and negative electrode prepared in Examples 1 to 5 and Comparative Examples 1 to 3. The electrode assembly was then placed inside a battery case, and an electrolyte was injected into the battery case to prepare half-cell lithium secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 3. The electrolyte was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent containing ethylene carbonate (EC), propylene carbonate (PC), and propylene propionate (PP) in a volume ratio of 4:4:2.

[0108] Experimental example 1: Evaluation of life characteristics The secondary batteries manufactured in Examples 1 to 5 and Comparative Examples 1 to 8 were evaluated for cycle capacity retention using an electrochemical charger / discharger.

[0109] The cycle capacity retention rate was measured by charging and discharging at 0.1C in the first and second cycles, and at 1.0C from the third cycle onwards (charge conditions: CC / CV, 4.25V / 0.005C cutoff, discharge conditions: CC, 3.0V cutoff).

[0110] The capacity retention rate was calculated as follows. Capacity retention rate (%) = {(discharge capacity at Nth cycle) / (discharge capacity at first cycle)} × 100 (In the above formula, N is an integer of 1 or more) The capacity retention rates (%) at the 150th cycle for Examples 1 to 5 and Comparative Examples 1 to 8 are shown in Table 2 below.

[0111] Experimental Example 2: Evaluation of rate characteristics The lithium secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 8 were charged at 25° C. in CC / CV mode at 0.1 C to 4.25 V, and then discharged to 3.0 V in 0.1 C CC mode, and the discharge capacity at 0.1 C discharge was determined.

[0112] Next, lithium secondary batteries of other Examples 1 to 5 and Comparative Examples 1 to 8 were prepared, and the lithium secondary batteries were charged at 25°C in CC / CV mode at 0.1 C up to 4.25 V, and then discharged to 3.0 V in 0.1 C CC mode, and the discharge capacity at 2.0 C discharge was determined.

[0113] The rate characteristics were then calculated and evaluated using the following formula. Excellent rate characteristics refer to a small decrease in normalized capacity (i.e., capacity retention) with an increase in discharge rate (C-rate). The results are shown in Table 2. Rate characteristic (%) = (discharge capacity at 2.0 C discharge / discharge capacity at 0.1 C discharge) x 100

[0114] Experimental Example 3: Evaluation of cell thickness increase rate The secondary batteries manufactured in Examples 1 to 5 and Comparative Examples 1 to 8 were subjected to 150 charge / discharge cycles using an electrochemical charger / discharger.

[0115] In the case of cycle charging and discharging, charging and discharging were performed at 0.1 C for the first and second cycles, and at 1.0 C from the third cycle onwards (charging conditions: CC / CV, 4.25 V / 0.005 C cutoff, discharging conditions: CC, 3.0 V cutoff).

[0116] The cell thickness increase rate due to cycle charge / discharge was evaluated using the following formula: In the formula, the thickness of the secondary battery was measured using an 800 gf PPHG (flat plate thickness measuring instrument). The results are shown in Table 2 below.

[0117] Cell thickness increase rate (%) = {(thickness of secondary battery at 100% SOC at the 150th cycle) / (thickness of secondary battery at 100% SOC at the first cycle)} × 100

[0118] [Table 2]

[0119] Referring to Table 2, it can be seen that the lithium secondary batteries of Examples 1 to 5, which satisfy Equation 1 according to the present invention, exhibit significantly improved capacity retention and rate characteristics and a low cell thickness increase rate, compared to Comparative Examples 1 to 3, which do not satisfy Equation 1.

[0120] On the other hand, the average particle size (D 50 In Comparative Examples 4 to 7, in which positive electrode active materials with excessively small or large values ​​of β-α and β-α were used, and in Comparative Example 8, in which a positive electrode active material that was not in a single particulate state was used, it was confirmed that even if Equation 1 was satisfied, the effects of improving capacity retention, improving rate characteristics, and reducing the rate of increase in cell thickness, which are the objectives of the present invention, could not be achieved.

Claims

1. a positive electrode current collector; and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer contains a positive electrode active material and a conductive material, the conductive material includes a linear conductive material; the positive electrode active material includes a lithium composite transition metal oxide including nickel (Ni), cobalt (Co), and manganese (Mn); the lithium composite transition metal oxide is in the form of a single particle, The average particle size (D50) of the positive electrode active material is 2 μm to 10 μm, the conductive material is made of the linear conductive material, the linear conductive material is a carbon nanotube, A positive electrode that satisfies the following mathematical formula 1. [Mathematical formula 1] 1.2≦B / A≦4.0 In Equation 1, A is the weight percentage of the positive electrode active material relative to the BET specific surface area of ​​the positive electrode active material multiplied by the weight of the positive electrode active material layer, and B is the weight percentage of the linear conductive material relative to the BET specific surface area of ​​the linear conductive material multiplied by the weight of the positive electrode active material layer.

2. The positive electrode according to claim 1 , wherein the positive electrode active material is contained in the positive electrode active material layer in an amount of 90% by weight to 99% by weight.

3. The BET specific surface area of ​​the positive electrode active material is 0.2 m ２ / g to 3m ２ / g.

4. The BET specific surface area of ​​the linear conductive material is 150 m ２ / g to 300m ２ / g.

5. 2. The positive electrode according to claim 1, wherein the linear conductive material has a pellet density of 0.09 g / cc to 0.16 g / cc (here, the pellet density is the density when the linear conductive material is placed in a mold and measured at a pressure of 2 tons using a powder resistance meter).

6. 2. The positive electrode according to claim 1, wherein the linear conductive material is contained in the positive electrode active material layer in an amount of 0.50% by weight to 1.75% by weight.

7. 2. The positive electrode according to claim 1, wherein the BET specific surface area of ​​the linear conductive material is 150 to 450 times the BET specific surface area of ​​the positive electrode active material.

8. 2. The positive electrode according to claim 1, wherein the weight percentage of the linear conductive material with respect to the total weight of the positive electrode active material layer is 0.006 to 0.018 times the weight percentage of the positive electrode active material with respect to the total weight of the positive electrode active material layer.

9. The positive electrode according to claim 1 , wherein the positive electrode active material layer further comprises a binder.

10. The positive electrode according to claim 9 , wherein the binder is contained in the positive electrode active material layer in an amount remaining after excluding the positive electrode active material and the linear conductive material.

11. The positive electrode according to any one of claims 1 to 10, a negative electrode facing the positive electrode; a separator interposed between the positive electrode and the negative electrode; Electrolytes, A secondary battery comprising:

Images