Positive electrode and lithium secondary battery manufactured using the same

Abstract

The positive electrode according to the present invention is a positive electrode including a positive electrode active material layer formed on a positive electrode current collector, wherein the positive electrode active material layer contains a lithium iron phosphate compound-based first positive electrode active material and a lithium nickel manganese cobalt composite oxide-based second positive electrode active material, and the second positive electrode active material is contained in an amount of 10% by weight or less based on the total weight of the first positive electrode active material and the second positive electrode active material, and the ratio (=B / A) of the average particle diameter (D 50 )A of the second positive electrode active material to the average particle diameter (D 50 )B of the first positive electrode active material is 3 or more. The positive electrode according to an embodiment of the present invention can increase the loading amount of the positive electrode due to improved rolling performance and can significantly reduce the thickness of the positive electrode active material layer through rolling, so that there is an effect of providing a lithium iron phosphate-based positive electrode with a high energy density.

Description

[Technical Field]

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2022-0147050 filed on November 7, 2022, and Korean Patent Application No. 10-2023-0152112 filed on November 6, 2023.

[0002] The present invention relates to a positive electrode and a lithium secondary battery manufactured using the same, and more particularly to a positive electrode and a lithium secondary battery having improved rolling performance in a positive electrode containing a lithium iron phosphate compound-based positive electrode active material. [Background technology]

[0003] As technological development and demand for electric vehicles and energy storage systems (ESS) increases, the demand for batteries as energy sources is rapidly increasing, and various research projects are being conducted on batteries that can meet various requirements. In particular, research on lithium secondary batteries, which have high energy density and excellent life and cycle characteristics as power sources for such devices, is being actively conducted.

[0004] Lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (LNCMO), lithium iron phosphate (LFP), etc. are used as positive electrode active materials for lithium secondary batteries.

[0005] Lithium iron phosphate is inexpensive because it contains iron, a resource-rich and low-cost material. Its low toxicity also reduces environmental pollution when used. Furthermore, because lithium iron phosphate has an olivine structure, its active material structure can be more stably maintained at high temperatures than lithium transition metal oxides, which have a layered structure. This provides the battery with excellent high-temperature stability and high-temperature life characteristics.

[0006] However, lithium iron phosphate has a relatively low tap density and poor rolling performance compared to lithium cobalt-based oxide and lithium nickel cobalt manganese-based oxide positive electrode active materials, making it difficult to significantly reduce the thickness of the positive electrode active material layer during rolling. Meanwhile, as demand for high-energy density lithium secondary batteries increases, electrode loading tends to increase, but as electrode loading increases, rolling performance decreases. Therefore, technological development to improve rolling performance is necessary to enable high loading of lithium iron phosphate positive electrodes. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to provide a lithium iron phosphate positive electrode having improved rolling performance and high energy density. [Means for solving the problem]

[0008] According to one embodiment of the present invention, there is provided a positive electrode including a positive electrode active material layer formed on a positive electrode current collector, the positive electrode active material layer including a first positive electrode active material based on a lithium iron phosphate compound and a second positive electrode active material based on a lithium nickel manganese cobalt composite oxide, the second positive electrode active material being 10 wt % or less based on a total weight of the first positive electrode active material and the second positive electrode active material, and the average particle size (D 50 ) A to the average particle size (D 50 ) B (= B / A) is 3 or more.

[0009] In an exemplary embodiment, the average particle size (D 50 ) A to the average particle size (D 50 The ratio of B (=B / A) can be 4 to 15.

[0010] In an exemplary embodiment, the average particle size (D 50 ) A to the average particle size (D 50The ratio of B (=B / A) can be 6 to 12.

[0011] In an exemplary embodiment, the second positive electrode active material may be included in an amount of 5 wt % or less based on the total weight of the first positive electrode active material and the second positive electrode active material.

[0012] In an exemplary embodiment, the first positive electrode active material has an average particle size (D 50 ) can be 0.1 μm to 3 μm.

[0013] In an exemplary embodiment, the first positive electrode active material has an average particle size (D 50 ) can be 0.5 μm to 1.5 μm.

[0014] In an exemplary embodiment, the second positive electrode active material has an average particle size (D 50 ) can be 3 μm to 20 μm.

[0015] In an exemplary embodiment, the first positive electrode active material may be a compound represented by the following Chemical Formula 1:

[0016] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0017] (In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are -0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

[0018] In an exemplary embodiment, the second positive electrode active material may be a compound represented by the following Chemical Formula 2:

[0019] [Chemical formula 2] Lia Ni 1-x-y Co x Mn y M1 z M2 w O2

[0020] (In the above chemical formula 2, M1 includes one or more elements selected from the group consisting of W, Mo, and Cr, M2 includes one or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, and Nb, and 0.95≦a≦1.5, 0 <x≦0.5、0<y≦0.5、0≦z≦0.03、0≦w≦0.02、0<x+y≦0.7である)

[0021] In an exemplary embodiment, the first positive electrode active material may further include a carbon coating layer on the surface.

[0022] In an exemplary embodiment, the first positive electrode active material can have a unitary structure consisting of primary particles.

[0023] The positive electrode according to the exemplary embodiment may have a porosity according to the following formula 1 of 24% to 30%.

[0024] [Formula 1] Porosity (%) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0025] In an exemplary embodiment, the loading amount of the positive electrode active material layer is 400 to 700 mg / 25 cm 2 It can be in the range of

[0026] According to another embodiment of the present invention, there is provided a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The positive electrode includes a positive electrode active material layer formed on a positive electrode current collector, the positive electrode active material layer including a first positive electrode active material based on a lithium iron phosphate compound and a second positive electrode active material based on a lithium nickel manganese cobalt composite oxide, the second positive electrode active material being 10 wt % or less based on the total weight of the first and second positive electrode active materials, and the average particle size (D50 ) A to the average particle size (D 50 ) The ratio of B (= B / A) is 3 or more. [Effects of the Invention]

[0027] The positive electrode according to the exemplary embodiment of the present invention may increase the loading amount of the positive electrode due to improved rolling performance, and may significantly reduce the thickness of the positive electrode active material layer through rolling, thereby providing a high-loading lithium iron phosphate-based positive electrode and a high-energy density lithium secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0028] The advantages and features of the present invention, as well as methods for achieving them, will become clear from the following detailed description of the embodiments accompanied by the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms. The present embodiments are provided solely to ensure that the disclosure of the present invention is complete and to fully convey the scope of the invention to those skilled in the art. The present invention is defined only by the scope of the claims. The same reference numerals refer to the same elements throughout the specification.

[0029] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the sense commonly understood by those skilled in the art to which the present invention pertains. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless otherwise clearly defined.

[0030] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular forms include the plural forms unless the context clearly dictates otherwise. When used in this specification, the words "comprises" and / or "comprising" do not exclude the presence or addition of one or more other elements other than the elements mentioned.

[0031] In this specification, when a part is said to include a certain component, this does not mean that it excludes other components, and that it may further include other components, unless otherwise specified.

[0032] In this specification, the expression "A and / or B" means A or B, or A and B.

[0033] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0034] In this specification, D 50 The above D means the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0035] In this specification, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mini II manufactured by BEL Japan.

[0036] In this specification, the term "weight average molecular weight (Mw)" refers to a value converted relative to standard polystyrene measured by gel permeation chromatography (GPC). Specifically, the weight average molecular weight is a value converted from a value measured using GPC under the following conditions, and standard polystyrene from an Agilent system was used to prepare a calibration curve.

[0037] <Measurement conditions> Measuring instrument: Agilent GPC (Agilent 1200 series, USA) Column: PL Mixed B 2 columns connected Column temperature: 40℃ Eluent: tetrahydrofuran Flow rate: 1.0mL / min Concentration: ~1mg / mL (100μL injection)

[0038] The present invention will be specifically described below.

[0039] <Positive electrode> A positive electrode for a lithium secondary battery according to one embodiment of the present invention is a positive electrode including a positive electrode active material layer formed on a positive electrode current collector, the positive electrode active material layer including a first positive electrode active material based on a lithium iron phosphate compound and a second positive electrode active material based on a lithium nickel manganese cobalt composite oxide, the second positive electrode active material being 10 wt % or less based on the total weight of the first positive electrode active material and the second positive electrode active material, and the average particle size (D 50 ) A to the average particle size (D 50 ) The ratio of B (= B / A) is 3 or more.

[0040] The lithium iron phosphate compound-based positive electrode active material has a drawback in that it is difficult to significantly reduce the thickness of the positive electrode active material layer during rolling, because the tap density of the lithium iron phosphate compound-based positive electrode active material is low. As a result of extensive research to solve this problem, the present inventors have found that by adding a small amount of a lithium nickel manganese cobalt composite oxide-based second positive electrode active material having a tap density superior to that of a lithium iron phosphate compound-based first positive electrode active material, the average particle size (D 50 ) A to the average particle size (D 50 )B (=B / A) is controlled to a predetermined ratio, the rolling performance of the positive electrode is surprisingly improved dramatically, leading to the present invention.

[0041] The positive electrode according to an embodiment of the present invention may have an increased loading amount due to improved rolling performance, and may have a significantly reduced thickness of the positive electrode active material layer through rolling, thereby providing a positive electrode with a high energy density.

[0042] A positive electrode according to one embodiment of the present invention may include a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector.

[0043] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the current collector may be made of stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like.

[0044] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and may have fine irregularities on its surface to enhance adhesion to the positive electrode active material layer. It may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0045] The positive electrode active material layer includes a lithium iron phosphate compound-based first positive electrode active material and a lithium nickel manganese cobalt composite oxide-based second positive electrode active material, and may further include a binder, a conductive material, a dispersant, and the like.

[0046] Hereinafter, each component contained in the positive electrode active material layer will be specifically described.

[0047] The present invention includes, as positive electrode active materials, a first positive electrode active material that is a lithium iron phosphate compound-based positive electrode active material, and a second positive electrode active material that is a lithium nickel manganese cobalt composite oxide-based positive electrode active material.

[0048] According to an exemplary embodiment, the first positive electrode active material may be a compound represented by the following Chemical Formula 1:

[0049] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0050] (In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are -0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

[0051] For example, the first positive electrode active material can be LiFePO4.

[0052] The first positive electrode active material may be primary particles or secondary particles formed by aggregation of primary particles, and preferably has a monolithic structure consisting of primary particles.

[0053] In the present invention, the term "monolith structure" refers to a morphological structure in which particles exist in independent phases and are not aggregated with each other. A particle structure that contrasts with this monolith structure is a structure in which small particles ("primary particles") are physically and / or chemically aggregated to form relatively large particles ("secondary particles").

[0054] When the first positive electrode active material has a single structure consisting of primary particles, the possibility of the lithium iron phosphate particles cracking during the rolling process is lower than when the first positive electrode active material is made of secondary particles, which is preferable because the capacity loss due to the detachment of cracked particles is reduced.

[0055] In addition, when the first positive electrode active material has a monolithic structure consisting of primary particles, the migration of the binder during the drying process of the positive electrode slurry can be alleviated, thereby improving the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer.

[0056] In an exemplary embodiment, the first positive electrode active material may further include a carbon coating layer on the surface thereof. When a carbon coating layer is formed on the surface of lithium iron phosphate, electrical conductivity may be improved, thereby improving the resistance characteristics of the positive electrode.

[0057] The carbon coating layer may be formed using at least one raw material selected from the group consisting of glucose, sucrose, lactose, starch, oligosaccharides, polyoligosaccharides, fructose, cellulose, furfuryl alcohol polymers, ethylene-ethylene oxide block copolymers, vinyl resins, cellulose resins, phenolic resins, pitch resins, and tar resins. Specifically, the carbon coating layer may be formed by mixing the raw material with the lithium iron phosphate and then heat-treating the mixture.

[0058] In an exemplary embodiment, the average particle size D of the first positive electrode active material 50 The average particle size D of the positive electrode active material may be 0.1 μm to 3.0 μm, preferably 0.5 μm to 1.5 μm, and more preferably 0.6 μm to 1.3 μm. 50 When the above range is satisfied, the mobility of lithium in the lithium phosphate is improved, and the charge / discharge characteristics of the battery can be improved.

[0059] In an exemplary embodiment, the first positive electrode active material has a BET specific surface area of ​​5 m 2 / g~20m 2 / g, specifically 7m 2 / g~18m 2 / g, more specifically 9m 2 / g~16m 2 When the above range is satisfied, aggregation of the first positive electrode active material can be effectively suppressed even in a positive electrode slurry composition with a relatively low dispersant content.

[0060] According to an exemplary embodiment, the second positive electrode active material may be a compound represented by the following Chemical Formula 2:

[0061] [Chemical formula 2] Lia Ni 1-x-y Co x Mn y M1 z M2 w O2

[0062] (In the above chemical formula 2, M1 includes one or more elements selected from the group consisting of W, Mo, and Cr, M2 includes one or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, and Nb, and 0.95≦a≦1.5, 0 <x≦0.5、0<y≦0.5、0≦z≦0.03、0≦w≦0.02、0<x+y≦0.7である)

[0063] For example, the second positive electrode active material is LiNi 0.6 Co 0.2 Mn 0.2 O2 or LiNi 0.8 Mn 0.1 Co 0.1 O2, etc., and any one or a mixture of two or more of these may be used.

[0064] The lithium nickel manganese cobalt composite oxide-based positive electrode active material represented by Chemical Formula 2 has a superior tap density compared to lithium iron phosphate compounds, and serves to improve the rolling performance of the positive electrode.

[0065] In an exemplary embodiment, the second positive electrode active material may be composed of a single primary particle or a secondary particle formed by agglomeration of a plurality of primary particles, and the primary particles may be uniform or non-uniform.

[0066] In an exemplary embodiment, the average particle size D of the second positive electrode active material 50The average particle size of the second positive electrode active material may be 3 μm to 20 μm, specifically 3 μm to 18 μm, and more specifically 4 μm to 15 μm. If the average particle size of the second positive electrode active material exceeds 20 μm, there is a risk that the second positive electrode active material particles may settle during the production of the positive electrode slurry, and if the average particle size of the second positive electrode active material is less than 3 μm, the effect of improving the rolling performance may be slight, which is undesirable.

[0067] In an exemplary embodiment, the second positive electrode active material may be 10 wt % or less, specifically 5 wt % or less, and more specifically 0.01 wt % to 1 wt %, based on the total weight of the first positive electrode active material and the second positive electrode active material.

[0068] The second positive electrode active material is added to improve rolling performance, and the effect of improving rolling performance is shown to saturate at a level of 5% to 10% by weight based on the total weight of the first and second positive electrode active materials. Therefore, when considering capacity characteristics, it is preferable to include the second positive electrode active material at a level within the above numerical range.

[0069] In an exemplary embodiment, the average particle size (D 50 ) A to the average particle size (D 50 The ratio of B to A (=B / A) may be 3 or more, specifically 4 to 15, and more specifically 6 to 12. 50 ) A to the average particle size (D 50 When the ratio of B to A (=B / A) is less than 3, the effect of improving rolling performance may be small, and therefore the value of B / A is preferably at least 3. The larger the value of B / A, the smaller the critical rolling thickness and critical rolling porosity of the positive electrode tend to be. However, when the value of B / A is 15 or greater, the reduction levels of the critical rolling thickness and critical rolling porosity converge to a certain level, and therefore the value of B / A is preferably within the above numerical range. Here, the critical rolling thickness refers to the thickness and porosity of the positive electrode active material layer when the positive electrode current collector is rolled to the maximum extent possible without causing damage such as breakage.

[0070] The positive electrode active material layer may further contain a binder, a conductive material, and a dispersant in addition to the first and second positive electrode active materials described above.

[0071] The binder is included to improve adhesion between positive electrode active material particles and between the positive electrode active material and the current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. One or more of these may be used alone or in combination. The binder may be included in an amount of 1 wt% to 30 wt% of the total weight of the positive electrode active material layer.

[0072] The conductive material is used to impart conductivity to the electrodes and is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, graphite; carbon black such as carbon black, 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 may be used. Specific examples of commercially available conductive materials include acetylene black products (such as those from Chevron Chemical Company, Denka Singapore Private Limited, and Gulf Oil Company), Ketjenblack, EC series products (such as those from Armak Company), Vulcan XC-72 (such as those from Cabot Company), and Super P (such as those from Timcal). Preferably, the conductive material is carbon nanotubes. When the conductive material is carbon nanotubes, the excellent conductive network of the carbon nanotubes can suppress binder migration, which occurs when the binder moves to the electrode surface during electrode drying, and can further improve the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer.

[0073] In an exemplary embodiment, the conductive material may be contained in the positive electrode active material layer in an amount of 0.3 wt % to 2.0 wt %, specifically 0.4 wt % to 1.5 wt %, more specifically 0.5 wt % to 1.3 wt %. When the content of the conductive material in the positive electrode active material layer satisfies the above range, a positive electrode conductive network is ensured, thereby improving the electrical conductivity of the positive electrode.

[0074] The dispersant particularly prevents the first positive electrode active material from excessively agglomerating in the positive electrode slurry, thereby enabling the first positive electrode active material and the second positive electrode active material to be effectively dispersed and present in the manufactured positive electrode active material layer.

[0075] The dispersant may include a hydrogenated nitrile copolymer, and specifically, the dispersant may be a hydrogenated nitrile copolymer.

[0076] Specifically, the hydrogenated nitrile copolymer may be a copolymer containing α,β-unsaturated nitrile-derived structural units and hydrogenated conjugated diene-derived structural units, or a copolymer containing α,β-unsaturated nitrile-derived structural units, conjugated diene-derived structural units, and hydrogenated conjugated diene-derived structural units. Examples of the α,β-unsaturated nitrile monomer include acrylonitrile and methacrylonitrile, and these may be used alone or in combination. Examples of the conjugated diene monomer include conjugated diene monomers having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, and 2,3-methylbutadiene, and these may be used alone or in combination.

[0077] More specifically, the hydrogenated nitrile copolymer may be hydrogenated nitrile butadiene rubber (H-NBR). The hydrogenated nitrile butadiene rubber may have a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, preferably 15,000 g / mol to 90,000 g / mol, and more preferably 20,000 g / mol to 50,000 g / mol. When the hydrogenated nitrile butadiene rubber satisfies the above-mentioned range, it is effective in suppressing the aggregation of the conductive material. Even if the conductive material aggregates, it aggregates into a spherical shape rather than a linear shape, thereby minimizing the specific surface area of ​​the aggregated conductive material compared to when the conductive material aggregates into a linear shape. As a result, the surface area of ​​the positive electrode active material adjacent to the aggregated conductive material that cannot participate in the lithium insertion / extraction reaction is minimized, thereby reducing the discharge resistance of the lithium secondary battery.

[0078] In an exemplary embodiment, the dispersant may be included in an amount of 0.1 wt % to 2.0 wt %, specifically 0.2 wt % to 1.8 wt %, more specifically 0.3 wt % to 1.6 wt %, based on the total weight of the positive electrode active material layer. When the content of the dispersant satisfies the above range, aggregation of the positive electrode active material may be suppressed, and gelation of the positive electrode slurry composition may be prevented.

[0079] In an exemplary embodiment, the positive electrode active material layer may contain 94% to 98% by weight of positive electrode active material, 0.5% to 2% by weight of conductive material, 1.0% to 4.0% by weight of binder, and 0.5% to 2% by weight of the dispersant. When the composition in the positive electrode active material layer satisfies the above ranges, the adhesive strength and conductivity of the electrode are ensured, and at the same time, the content of the active material is increased, resulting in excellent capacity and resistance performance of a lithium secondary battery including the positive electrode.

[0080] The positive electrode according to the present invention can be manufactured by a conventional method for manufacturing a positive electrode. Specifically, the positive electrode can be manufactured by preparing a positive electrode slurry containing the first positive electrode active material, the second positive electrode active material, a conductive material, a binder, and / or a dispersant, applying the positive electrode slurry onto a positive electrode current collector, and then drying and rolling the slurry.

[0081] Alternatively, the positive electrode may be manufactured by casting the positive electrode slurry on a separate support, peeling the resulting film from the support, and laminating the film on a positive electrode current collector.

[0082] The positive electrode active material layer according to the present invention is composed of a bimodal positive electrode active material in which the average particle diameters of the first positive electrode active material and the second positive electrode active material are different from each other. Therefore, the first positive electrode active material having a smaller average particle diameter can fill the gaps between the particles of the second positive electrode active material having a larger average particle diameter. The presence of the second positive electrode active material having a high tap density improves the rolling density, thereby enabling the realization of a high loading / high energy density positive electrode.

[0083] The positive electrode according to the exemplary embodiment may have a porosity according to the following formula 1 of 24% to 30%, specifically 24% to 29%, and more specifically 25% to 28%.

[0084] [Formula 1] Porosity (%) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0085] The porosity range is a lower level than the porosity of a positive electrode mainly including a lithium iron phosphate compound-based positive electrode active material. According to the present invention, the porosity may be within the above range because the rolling performance is improved and the density after rolling can be increased.

[0086] In an exemplary embodiment, the loading amount of the positive electrode active material layer is 400 to 700 mg / 25 cm 2 , for details, 450~700mg / 25cm 2 , more specifically 500~650mg / 25cm 2 The loading amount is based on the loading amount of the positive electrode active material layer disposed on one side of the current collector. According to the present invention, even in a positive electrode including a positive electrode active material layer with a high loading amount of the above level, the rolling performance is improved, and the porosity according to the above formula 1 can be 24% to 30%, specifically 24% to 29%, more specifically 25% to 28%, while having the above loading amount.

[0087] <Lithium secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0088] A lithium secondary battery according to an embodiment of the present invention may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0089] The positive electrode in the lithium secondary battery is as described above. For example, the positive electrode includes a positive electrode active material layer formed on a positive electrode current collector, the positive electrode active material layer includes a lithium iron phosphate compound-based first positive electrode active material and a lithium nickel manganese cobalt composite oxide-based second positive electrode active material, the second positive electrode active material is 10 wt % or less based on the total weight of the first positive electrode active material and the second positive electrode active material; The average particle size (D 50 ) A to the average particle size (D 50 ) The ratio of B (= B / A) is 3 or more.

[0090] The positive electrode has been described in detail above, so a duplicated description will be omitted.

[0091] The negative electrode can be manufactured, for example, by preparing a negative electrode-forming composition containing a negative electrode active material, a negative electrode binder, and a negative electrode conductive material on a negative electrode current collector, and then coating the composition on the negative electrode current collector.

[0092] The negative electrode active material is not particularly limited, and may generally be a compound capable of reversible lithium intercalation and deintercalation. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and highly crystalline carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and composites containing metallic compounds and carbonaceous materials. Low-crystalline carbons include soft carbon and hard carbon, while highly crystalline carbons include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbons such as petroleum or coal tar pitch-derived cokes. These may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material.

[0093] The negative electrode conductive material is used to impart conductivity to the electrode and can be any material that exhibits electronic conductivity without causing chemical changes in the battery. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powder or fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The negative electrode conductive material is typically present in an amount of 1 wt % to 30 wt %, specifically 1 wt % to 20 wt %, and more specifically 1 wt % to 10 wt %, based on the total weight of the negative electrode active material layer.

[0094] The negative electrode binder serves to improve adhesion between negative electrode active material particles and between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. One or more of these may be used alone or in combination. The negative electrode binder may be included in an amount of 1 wt% to 30 wt%, specifically 1 wt% to 20 wt%, more specifically 1 wt% to 10 wt%, based on the total weight of the negative electrode active material layer.

[0095] Meanwhile, the negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc. may be used.

[0096] The negative electrode current collector may typically have a thickness of 3 μm to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have fine irregularities on its surface to enhance the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0097] Meanwhile, in the lithium secondary battery, the separator can be any material commonly used as a separator in lithium secondary batteries. It is particularly preferred that the separator has low resistance to ion migration of the electrolyte and excellent electrolyte humidification capacity. Specifically, a porous polymer film, such as a porous polymer film made of 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, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. The separator can be a porous thin film having a pore diameter of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0098] Meanwhile, in the lithium secondary battery, the electrolyte may contain an organic solvent and a lithium salt that are commonly used in electrolytes, and is not particularly limited.

[0099] The organic solvent may be any organic solvent without particular limitation, as long as it can function as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; and carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0100] Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0101] 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 LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O), etc. The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0102] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, 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, for the purpose of improving the battery's life characteristics, suppressing battery capacity reduction, improving the battery's discharge capacity, etc. 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.

[0103] The lithium secondary battery of the present invention may be manufactured by forming an electrode assembly by disposing a separator between a positive electrode and a negative electrode, and then placing the electrode assembly in a cylindrical or prismatic battery case and injecting an electrolyte thereinto, or by stacking the electrode assemblies, impregnating them with an electrolyte, and then sealing the resulting assembly in a battery case.

[0104] When manufacturing the lithium secondary battery of the present invention, the electrode assembly may be dried to remove one or more organic solvents selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate, which were used in manufacturing the positive electrode. When an electrolyte having the same components as the organic solvent used in manufacturing the positive electrode is used, the process of drying the electrode assembly may be omitted.

[0105] Unlike the above-described lithium secondary batteries, the lithium secondary battery according to another embodiment of the present invention may be an all-solid-state battery.

[0106] The battery case may be one commonly used in the art, and the shape may be any shape depending on the intended use of the battery, such as a cylindrical shape using a can, a square shape, a pouch shape, or a coin shape.

[0107] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, energy storage systems (ESS), and electric vehicles such as hybrid electric vehicles (HEV), etc.

[0108] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0109] <Example 1: Production of positive electrode>

[0110] The first positive electrode active material has an average particle size D 50 The second positive electrode active material is LiFePO4, which is a single-body primary particle with an average particle size of 1.0 μm. 50 The secondary particles are 7μm to 8μm in size. 0.6 Mn 0.2 Co0.2 The cathode active material was prepared by mixing O2 with carbon nanotubes (CNTs) in a weight ratio of 99.5:0.5. Carbon nanotubes (CNTs) were used as a conductive material, polyvinylidene fluoride (PVDF) as a binder, and hydrogenated nitrile butadiene rubber (HNBR) as a dispersant in N-methylpyrrolidone (NMP) solvent. The mixture was mixed at 2500 rpm for 90 minutes using a homo-disperse mixer to produce cathode slurry.

[0111] In the positive electrode slurry, the positive electrode active material, conductive material, binder, and dispersant were present in a weight ratio of 95:1.2:3.0:0.8, and the solid content of the positive electrode slurry was 62 wt %.

[0112] The above positive electrode slurry was applied at 600 mg / 25 cm to a 20 μm thick aluminum thin film. 2 After coating, the positive electrode slurry was dried with hot air at 130° C. for 5 minutes so that the solid content of the positive electrode slurry was 99.0% by weight or more (the thickness of the positive electrode active material layer after drying was about 140 μm).

[0113] <Example 2: Production of positive electrode>

[0114] A positive electrode was prepared in the same manner as in Example 1, except that the weight ratio of the first positive electrode active material to the second positive electrode active material was changed to 99.9:0.1.

[0115] <Example 3: Production of positive electrode>

[0116] In Example 1, the second positive electrode active material was treated to have an average particle size D 50 LiNi 0.6 Mn 0.2 Co 0.2 A positive electrode was prepared in the same manner as in Example 1, except that O2 was used.

[0117] Example 4: Production of positive electrode

[0118] In Example 1, the second positive electrode active material was treated to have an average particle size D 50LiNi with a particle size of 11μm to 12μm 0.6 Mn 0.2 Co 0.2 A positive electrode was prepared in the same manner as in Example 1, except that O2 was used.

[0119] <Example 5: Production of positive electrode>

[0120] In Example 1, the second positive electrode active material was treated to have an average particle size D 50 LiNi with a particle size of 14μm to 15μm 0.6 Mn 0.2 Co 0.2 A positive electrode was prepared in the same manner as in Example 1, except that O2 was used.

[0121] <Example 6: Production of positive electrode>

[0122] A positive electrode was prepared in the same manner as in Example 1, except that the weight ratio of the first positive electrode active material to the second positive electrode active material was changed to 95:5.

[0123] <Example 7: Production of positive electrode>

[0124] A positive electrode was prepared in the same manner as in Example 1, except that the weight ratio of the first positive electrode active material to the second positive electrode active material was changed to 90:10.

[0125] <Comparative Example 1: Production of Positive Electrode>

[0126] A positive electrode was prepared in the same manner as in Example 1, except that the weight ratio of the first positive electrode active material to the second positive electrode active material was changed to 100:0.

[0127] <Comparative Example 2: Production of Positive Electrode>

[0128] In Example 1, the second positive electrode active material was treated to have an average particle size D 50 LiNi with a thickness of 2 μm 0.6 Mn 0.2 Co 0.2 A positive electrode was prepared in the same manner as in Example 1, except that O2 was used.

[0129] <Experimental Example: Measurement of Critical Rolling Thickness and Critical Rolling Porosity>

[0130] The critical rolled thickness and critical rolled porosity were measured for each of the positive electrodes produced in Examples 1 to 7 and Comparative Examples 1 and 2, and the results are shown in Table 1. The critical rolled thickness was measured by first rolling the positive electrode so that the porosity according to the following formula 1 reached a level of 30%, and if no breaks occurred in the current collector, the positive electrode was rolled again so that the porosity reached a level of 29%.

[0131] [Formula 1] Porosity (%) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0132] The rolling was repeated while gradually decreasing the porosity until a break occurred in the current collector. The thickness of the positive electrode active material layer measured at the rolling level just before the break occurred was defined as the critical rolling thickness, and the porosity at the rolling level just before the break occurred was defined as the critical rolling porosity. For the critical rolling porosity, the porosity of the positive electrode of Comparative Example 1 was evaluated as 1, and the porosities of the positive electrodes of Examples 1 to 7 and Comparative Example 2 were reported as a relative ratio to the porosity of the positive electrode of Comparative Example 1.

[0133] [Table 1]

[0134] The positive electrodes according to Examples 1 to 7 all have smaller critical rolling thicknesses and smaller critical rolling porosities than the positive electrode according to Comparative Example 1, which does not contain a second positive electrode active material. In addition, the positive electrodes according to Examples 1 to 7 all contain a second positive electrode active material, but the average particle size (D 50 ) A to the average particle size (D 50 )B (=B / A) is 2, the critical rolling thickness is smaller and the critical rolling porosity is also smaller.

[0135] Meanwhile, the positive electrode of Example 6, in which the weight proportion of the second positive electrode active material is 5 wt %, and the positive electrode of Example 7, in which the weight proportion of the second positive electrode active material is 10 wt %, have similar critical rolling thicknesses and critical rolling porosities. Therefore, it is analyzed that when the weight proportion of the second positive electrode active material is a certain proportion or more, the improvement effect of rolling performance converges to a certain level.

[0136] In addition, the positive electrode according to Example 4 and the positive electrode according to Example 5 have similar critical rolling thicknesses and critical rolling porosities, and therefore, the average particle size (D 50 ) A to the average particle size (D 50 It is analyzed that when the ratio of B (= B / A) exceeds a certain numerical range, the improvement level of rolling performance does not increase infinitely but converges to a certain level.

[0137] As described above, the positive electrode according to the present invention, in which the lithium iron phosphate-based positive electrode active material is 90 wt % or more as the positive electrode active material, can provide a high-loading positive electrode and a high-energy density lithium secondary battery by improving rolling performance.

Claims

1. A positive electrode including a positive electrode active material layer formed on a positive electrode current collector, the positive electrode active material layer includes a lithium iron phosphate compound-based first positive electrode active material and a lithium nickel manganese cobalt composite oxide-based second positive electrode active material, the second positive electrode active material is 10 wt % or less with respect to the total weight of the first positive electrode active material and the second positive electrode active material, The average particle size (D ５０ ) the average particle size (D ５０ ) A ratio of B (= B / A) is 4 to 15.

2. The average particle size (D ５０ ) the average particle size (D ５０ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the ratio of A to B (=B / A) is 6 to 12.

3. 2 . The positive electrode for a lithium secondary battery according to claim 1 , wherein the second positive electrode active material is 5 wt % or less with respect to the total weight of the first positive electrode active material and the second positive electrode active material.

4. The first positive electrode active material has an average particle size (D ５０ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the average particle size is 0.1 μm to 3 μm.

5. The first positive electrode active material has an average particle size (D ５０ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the average particle size is 0.5 μm to 1.5 μm.

6. The second positive electrode active material has an average particle size (D ５０ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the thickness of the first electrode is 3 μm to 20 μm.

7. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the first positive electrode active material is a compound represented by the following Chemical Formula 1: [Chemical formula 1] Li １＋ａ Fe １－ｘ M ｘ (PO ４－ｂ )X ｂ (In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

8. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the second positive electrode active material is a compound represented by the following Chemical Formula 2: [Chemical formula 2] Li ａ Ni １－ｘ－ｙ Co ｘ Mn ｙ M1 ｚ M2 ｗ O ２ (In Chemical Formula 2, M1 includes one or more elements selected from the group consisting of W, Mo, and Cr, M2 includes one or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, and Nb, and 0.95≦a≦1.5, 0<x≦0.5, 0<y≦0.5, 0≦z≦0.03, 0≦w≦0.02, and 0<x+y≦0.7).

9. The positive electrode for a lithium secondary battery according to claim 1 , wherein the first positive electrode active material further comprises a carbon coating layer on a surface thereof.

10. The positive electrode for a lithium secondary battery according to any one of claims 1 to 9, wherein the first positive electrode active material has a monolithic structure made of primary particles.

11. The positive electrode for a lithium secondary battery according to claim 1, wherein the porosity according to the following formula 1 is 24% to 30%: [Formula 1] Porosity (%)={1−(measured density of positive electrode active material layer / true density of positive electrode active material)}×100.

12. The loading amount of the positive electrode active material layer is 400 to 700 mg / 25 cm ２ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the content of the positive electrode is in the range of 0.1 to 1.

0.

13. The battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte; The positive electrode is a positive electrode active material layer formed on a positive electrode current collector, the positive electrode active material layer includes a lithium iron phosphate compound-based first positive electrode active material and a lithium nickel manganese cobalt composite oxide-based second positive electrode active material, the second positive electrode active material is 10 wt % or less with respect to the total weight of the first positive electrode active material and the second positive electrode active material, The average particle size (D ５０ ) the average particle size (D ５０ ) A lithium secondary battery in which the ratio of B (= B / A) is 4 to 15.

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