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 based on 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 having improved rolling performance in a positive electrode containing a lithium iron phosphate compound-based positive electrode active material and a lithium secondary battery.

Background Art

[0003] As the technology development and demand for electric vehicles and energy storage systems (ESS) increase, the demand for batteries as an energy source has been rapidly increasing, and various studies on batteries that can meet various requirements have been conducted accordingly. In particular, studies on lithium secondary batteries having high energy density and excellent life and cycle characteristics as a power source for such devices have been actively carried out.

[0004] As the positive electrode active material of the lithium secondary battery, lithium cobalt-based oxide (LCO), lithium nickel cobalt manganese-based oxide (LNCMO), lithium iron phosphate (LFP), etc. are used.

[0005] Lithium iron phosphate is a low-cost material because it contains iron, which is resource-rich and low-cost. In addition, since lithium iron phosphate has low toxicity, environmental pollution can be reduced when using lithium iron phosphate. Furthermore, since lithium iron phosphate has an olivine structure, the active material structure can be stably maintained at high temperatures compared to layered lithium transition metal oxides. Thereby, it has the advantage of excellent high-temperature stability and high-temperature life characteristics of the battery.

[0006] However, lithium iron phosphate has a relatively low tap density and inferior rolling performance compared to the cathode active materials of lithium cobalt-based oxides and lithium nickel cobalt manganese-based oxides. Therefore, it has the disadvantage that it is difficult to significantly reduce the thickness of the cathode active material layer during rolling. On the other hand, with the increasing demand for high energy density lithium secondary batteries, the loading amount of the electrode tends to increase. However, when the loading amount of the electrode increases, the rolling performance deteriorates. Therefore, in order to increase the loading of the lithium iron phosphate cathode, it is necessary to develop a technology to improve the rolling performance.

Summary of the Invention

Problems to be Solved by the Invention

[0007] The present invention aims to provide a lithium iron phosphate cathode with improved rolling performance and high energy density.

Means for Solving the Problems

[0008] According to an embodiment of the present invention, there is provided a cathode including a cathode active material layer formed on a cathode current collector, wherein the cathode active material layer includes a lithium iron phosphate compound-based first cathode active material and a lithium nickel manganese cobalt composite oxide-based second cathode active material, the second cathode active material is 10% by weight or less based on the total weight of the first cathode active material and the second cathode active material, and the ratio (=B / A) of the average particle size (D 50 )A of the first cathode active material to the average particle size (D 50 )B of the second cathode active material is 3 or more. A cathode for a lithium secondary battery is provided.

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

[0010] In an exemplary embodiment, the ratio (=B / A) of the average particle size (D 50 )A of the first cathode active material to the average particle size (D 50)The value of the 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 a proportion of 5% by weight 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 ) that 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 ) that 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 ) that 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 any one or two 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 any one or two 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 contains any one or two or more elements selected from the group consisting of W, Mo, and Cr, M2 contains any one or two or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, and Nb, 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)

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

[0022] In an exemplary embodiment, the first positive electrode active material may have a single body structure composed of primary particles.

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

[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 may be in the range of 400 to 700 mg / 25 cm 2 of the range.

[0026] According to another embodiment of the present invention, 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 is provided. 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% 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 average particle size (D of the first positive electrode active material50 )The average particle size (D) of the second positive electrode active material with respect to A 50 )The value of the ratio of B(=B / A) is 3 or more. [Advantages of the Invention]

[0027] The positive electrode according to an exemplary 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. Therefore, there is an effect of providing a high-loading lithium iron phosphate-based positive electrode and a lithium secondary battery with a high energy density. [Modes for Carrying Out the Invention]

[0028] The advantages and features of the present invention, and the methods for achieving them, will be clear by referring to the embodiments described in detail below together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and can be embodied in various different forms. Merely, these embodiments are provided so that the disclosure of the present invention is complete and that those with ordinary knowledge in the technical field to which the present invention pertains are fully informed of the scope of the invention. The present invention is only defined by the scope of the claims. The same reference numerals throughout the specification refer to the same components.

[0029] Unless otherwise defined, all terms (including technical and scientific terms) used in this specification can be used with the meaning commonly understood by those with ordinary knowledge in the technical field to which the present invention pertains. Also, terms defined in commonly used dictionaries are not ideally or overly interpreted unless specifically defined otherwise.

[0030] The terms used in this specification are for explaining the embodiments and are not intended to limit the present invention. In this specification, the singular form also includes the plural form unless specifically mentioned otherwise in the context. The "comprises" and / or "comprising" used in the specification do not exclude the presence or addition of one or more other components in addition to the recited components.

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

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

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

[0034] In this specification, D 50 means the particle size corresponding to 50% of the volume cumulative amount in the particle size distribution curve of the particles. The above D 50 can be measured, for example, using the laser diffraction method. The above laser diffraction method can generally measure particle sizes in the range from the submicron region to about several millimeters, and can obtain highly reproducible and highly resolvable results.

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

[0036] In this specification, the "weight average molecular weight (Mw)" means the conversion numerical value with respect to standard polystyrene measured by Gel Permeation Chromatography (GPC). Specifically, the above weight average molecular weight is a value obtained by converting the value measured under the following conditions using GPC, and standard polystyrene of the Agilent system was used for the preparation of the calibration curve.

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

[0038] Hereinafter, the present invention will be specifically described.

[0039] <Positive electrode> The positive electrode for a lithium secondary battery according to an embodiment of 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 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 the second positive electrode active material is 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 average particle size (D 50 ) A of the second positive electrode active material to the average particle size (D 50 ) B of the first positive electrode active material (= B / A) has a value of 3 or more.

[0040] The lithium iron phosphate compound-based positive electrode active material has a low tap density, and the lithium iron phosphate positive electrode has a disadvantage in that it is difficult to significantly reduce the thickness of the positive electrode active material layer during rolling. As a result of repeated studies to solve such problems, the present inventors added 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 the lithium iron phosphate compound-based first positive electrode active material, and the average particle size (D 50 ) A of the first positive electrode active material to the average particle size (D 50 ) B of the second positive electrode active material (= B / A) was controlled to a predetermined ratio, and surprisingly, it was discovered that the rolling performance of the positive electrode was dramatically improved, leading to the present invention.

[0041] 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 it has the effect of providing a positive electrode with a high energy density.

[0042] The positive electrode according to an 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 may be any material that has conductivity without inducing a chemical change in the battery and is not particularly limited. For example, as the current collector, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface-treating the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used.

[0044] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector to enhance the adhesion to the positive electrode active material layer. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

[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, etc.

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

[0047] The present invention includes, as the positive electrode active material, 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 contains any 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 contains any one or more elements selected from the group consisting of F, S, and N; and a, b, and x satisfy -0.5 ≦ a ≦ 0.5, 0 ≦ b ≦ 0.1, and 0 ≦ x ≦ 0.5, respectively.)

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

[0052] The first positive electrode active material may be primary particles or secondary particles in which the primary particles are aggregated with each other, and preferably may have a single body structure composed of primary particles.

[0053] In the present invention, the "monolith structure" means a structure in which particles exist as independent phases that are not aggregated with each other in terms of morphology. Examples of the particle structure contrasted with such a monolith structure include a structure in which small-sized particles ("primary particles") are physically and / or chemically aggregated to form a relatively large-sized particle form ("secondary particles").

[0054] When the first positive electrode active material has a single body structure composed of primary particles, compared with the case of secondary particles, the possibility of the occurrence of cracking phenomenon of lithium iron phosphate particles in the rolling process is low, so it is preferable that the capacity reduction due to the detachment of cracked particles is small.

[0055] Also, when the first positive electrode active material has a single body structure composed of primary particles, the migration phenomenon of the binder can be alleviated in the drying process of the positive electrode slurry, so the interfacial adhesion force between the positive electrode current collector and the positive electrode active material layer can be improved.

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

[0057] The carbon coating layer can be formed using at least one or more raw material substances selected from the group consisting of glucose, sucrose, lactose, starch, oligosaccharide, polyoligosaccharide, fructose, cellulose, polymer of furfuryl alcohol, block copolymer of ethylene and ethylene oxide, vinyl resin, cellulose resin, phenolic resin, pitch resin, and tar resin. Specifically, the carbon coating layer can be formed through a process of heat-treating after mixing the raw material substances with the lithium iron phosphate.

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

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

[0060] According to an exemplary embodiment, the second positive electrode active material can 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 contains any one or two or more elements selected from the group consisting of W, Mo, and Cr, M2 contains any one or two or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, and Nb, 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)

[0063] For example, the above 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 or the like, and any one or a mixture of two or more thereof can be used.

[0064] The lithium nickel manganese cobalt composite oxide-based positive electrode active material represented by the above Chemical Formula 2 is excellent in tap density compared to the lithium iron phosphate compound, and plays a role in improving the rolling performance of the positive electrode.

[0065] In an exemplary embodiment, the above second positive electrode active material can also consist of a single primary particle, or can consist of secondary particles in which a plurality of primary particles are aggregated. At this time, the primary particles may be uniform or non-uniform.

[0066] In an exemplary embodiment, the average particle size D of the above second positive electrode active material 50It can be 3 μm to 20 μm, specifically 3 μm to 18 μm, and more specifically 4 μm to 15 μm. When the average particle size of the second positive electrode active material exceeds 20 μm, sedimentation of the second positive electrode active material particles may occur during the production of the positive electrode slurry. When 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 negligible, which is not preferable.

[0067] In an exemplary embodiment, the second positive electrode active material can be 10% by weight or less, specifically 5% by weight or less, and more specifically 0.01% by weight to 1% by weight 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 the rolling performance, and it is shown that the effect of improving the rolling performance saturates at a level of 5% by weight to 10% by weight based on the total weight of the first positive electrode active material and the second positive electrode active material. Therefore, when considering the capacity characteristics, it is preferable to include the second positive electrode active material at the level within the above numerical range.

[0069] In an exemplary embodiment, the ratio (= B / A) of the average particle size (D 50 )B of the second positive electrode active material to the average particle size (D 50 )A of the first positive electrode active material can be 3 or more, specifically 4 to 15, and more specifically 6 to 12. When the ratio (= B / A) of the average particle size (D 50 )A of the first positive electrode active material to the average particle size (D 50 )B of the second positive electrode active material is less than 3, the effect of improving the rolling performance may be small. Therefore, the value of B / A is preferably at least 3 or more. And, the larger the value of B / A, the smaller the limiting rolling thickness and the limiting rolling porosity of the positive electrode tend to be. However, when the value of B / A is 15 or more, the reduction levels of the limiting rolling thickness and the limiting rolling porosity converge to a certain level. Therefore, the value of B / A is preferably within the above numerical range. Here, the limiting rolling thickness means the thickness of the positive electrode active material layer and the porosity of the positive electrode active material layer when rolled to the maximum within the range where no damage such as disconnection occurs in the positive electrode current collector.

[0070] The above-mentioned positive electrode active material layer may further contain a binder, a conductive material, and a dispersant together with the above-mentioned first positive electrode active material and second positive electrode active material.

[0071] The above-mentioned binder contains a binder to improve the adhesion between positive electrode active material particles and the adhesive force between the positive electrode active material and the current collector. Specific examples of the above-mentioned 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, or various copolymers thereof, etc. Among these, one kind alone or a mixture of two or more kinds can be used. The above-mentioned binder may be contained in an amount of 1% by weight to 30% by weight based on the total weight of the positive electrode active material layer.

[0072] The above conductive material is used to impart conductivity to the electrode, and is not particularly limited as long as it has conductivity without inducing chemical changes in the battery. For example, graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers and metal fibers; 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; conductive materials such as polyphenylene derivatives can be used. Specific examples of commercially available conductive materials include acetylene black series products (products manufactured by Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, etc.), Ketjenblack, EC series (products manufactured by Armak Company), Vulcan XC-72 (products manufactured by Cabot Company), and Super P (products manufactured by Timcal). Preferably, the above conductive material can be a carbon nanotube. When the conductive material is a carbon nanotube, the excellent conductive network of the carbon nanotube can suppress the binder migration phenomenon in which the binder on the electrode surface tries to move during the drying of the electrode, and has the effect of further improving the interfacial adhesion force between the positive electrode current collector and the positive electrode active material layer.

[0073] In an exemplary embodiment, the above conductive material may be contained in the positive electrode active material layer at 0.3 wt% to 2.0 wt%, specifically 0.4 wt% to 1.5 wt%, and 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, the electrical conductivity of the positive electrode can be improved by ensuring the positive electrode conductive network.

[0074] The above dispersant can suppress, in particular, the phenomenon that the first positive electrode active material aggregates excessively in the positive electrode slurry, so that the first positive electrode active material and the second positive electrode active material can be effectively dispersed and present in the manufactured positive electrode active material layer.

[0075] The above dispersant may contain a hydrogenated nitrile copolymer. Specifically, the above dispersant may be a hydrogenated nitrile copolymer.

[0076] Specifically, the above hydrogenated nitrile copolymer is a copolymer containing a structural unit derived from α,β-unsaturated nitrile and a structural unit derived from hydrogenated conjugated diene, or may be a copolymer containing a structural unit derived from α,β-unsaturated nitrile, a structural unit derived from conjugated diene, and a structural unit derived from hydrogenated conjugated diene. As the above α,β-unsaturated nitrile monomer, for example, acrylonitrile or methacrylonitrile can be used, and one kind alone or a mixture of two or more kinds thereof can be used. As the above conjugated diene monomer, for example, conjugated diene monomers having 4 to 6 carbon atoms such as 1,3-butadiene, isoprene, or 2,3-methylbutadiene can be used, and one kind alone or a mixture of two or more kinds thereof can be used.

[0077] More specifically, the above hydrogenated nitrile copolymer can be hydrogenated nitrile butadiene rubber (H-NBR). At this time, 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 numerical range, it is excellent in the effect of suppressing the aggregation of the conductive material. Even if the conductive material aggregates, it aggregates into a spherical shape rather than a linear shape. Compared with the case where the conductive material aggregates linearly, the specific surface area of the aggregated conductive material can be minimized. As a result, the surface area of the positive electrode active material that cannot participate in the lithium insertion / desorption reaction adjacent to the aggregated conductive material is minimized, so that the discharge resistance of the lithium secondary battery can be lowered.

[0078] In an exemplary embodiment, the dispersant may be contained in an amount of 0.1% by weight to 2.0% by weight, specifically 0.2% by weight to 1.8% by weight, and more specifically 0.3% by weight to 1.6% by weight 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 can be suppressed and gelation of the positive electrode slurry composition can be prevented.

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

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

[0081] Also, in another method, the positive electrode can also be manufactured by casting the positive electrode slurry onto a separate support and then laminating a film obtained by peeling the support from the positive electrode current collector.

[0082] Since the positive electrode active material layer according to the present invention is composed of a positive electrode active material in a bimodal form in which the average particle size of the first positive electrode active material and the average particle size of the second positive electrode active material are different, the first positive electrode active material having a small average particle size can be filled in the voids between the second positive electrode active material particles having a large average particle size, and it has an improved rolling density due to the presence of the second positive electrode active material having a high tap density. As a result, it is possible to realize a high loading / high energy density of the 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 the positive electrode active material layer / true density of the positive electrode active material)} × 100

[0085] The numerical range of the above porosity is a level decreased from the porosity of a positive electrode mainly containing a lithium iron phosphate compound-based positive electrode active material as the positive electrode active material. According to the present invention, since the rolling performance is improved and the density after rolling can be increased, the porosity can be within the above numerical range.

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

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

[0088] The 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 above 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 above 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, The above second positive electrode active material is 10% by weight 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 of the first positive electrode active material to the average particle size (D 50 )B of the second positive electrode active material (=B / A) has a value of 3 or more.

[0090] Since the above positive electrode has been described in detail previously, duplicate explanations are omitted.

[0091] The above negative electrode can be produced, for example, by manufacturing 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 applying it on the negative electrode current collector.

[0092] The above negative electrode active material is not particularly limited, and usually, a compound capable of reversible intercalation and deintercalation of lithium can be used. 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, or Al alloys; or composites containing metallic compounds and carbonaceous materials. Further, examples of low-crystalline carbon include soft carbon and hard carbon, and examples of highly crystalline carbon include natural 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. One of these alone or a mixture of two or more can be used, and a thin film of metallic lithium can also be used as the above negative electrode active material.

[0093] The above-mentioned negative electrode conductive material is used to impart conductivity to the electrode, and can be used without particular limitation as long as it has electron conductivity without causing chemical changes in the battery being constructed. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based substances such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotube; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. Among these, one kind alone or a mixture of two or more kinds can be used. The above-mentioned negative electrode conductive material can usually be contained 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.

[0094] The above-mentioned negative electrode binder plays a role in improving the adhesion between negative electrode active material particles and the adhesive force 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, or various copolymers thereof. Among these, one kind alone or a mixture of two or more kinds can be used. The above-mentioned negative electrode binder can be contained 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] On the one hand, the negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those surface-treated with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. can be used.

[0096] Also, the negative electrode current collector can usually have a thickness of 3 μm to 500 μm. Similar to the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to strengthen the binding force of the negative electrode active material. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven bodies, etc.

[0097] On the one hand, in the above lithium secondary battery, the separator can be used without particular limitation as long as it is usually used as a separator in a lithium secondary battery. In particular, those with low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention ability are preferred. Specifically, porous polymer films, for example, porous polymer films made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof can be used. Also, ordinary porous non-woven fabrics, for example, non-woven fabrics made of high-melting glass fibers, polyethylene terephthalate fibers, etc. can also be used. Also, 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] On the one hand, in the above lithium secondary battery, the electrolyte can contain organic solvents and lithium salts usually used in electrolytes and is not particularly limited.

[0099] As the above-mentioned organic solvent, any solvent can be used without particular limitation as long as it can serve 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; carbonate 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 solvents are preferred. A mixture of a cyclic carbonate (such as ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can enhance the charge-discharge performance of the battery, and a linear carbonate compound having low viscosity (such as ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0101] The above lithium salt can be used without particular limitation as long as it is a compound that can provide lithium ions used in a lithium secondary battery. Specifically, the above lithium salt can 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, etc. The above lithium salt is preferably contained in the above electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0102] In addition to the constituent components of the above electrolyte, the above electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, triamide hexaline, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride for the purpose of improving the life characteristics of the battery, suppressing the capacity reduction of the battery, improving the discharge capacity of the battery, etc. At this time, the above additive can be contained at 0.1% by weight to 5% by weight based on the total weight of the electrolyte.

[0103] The lithium secondary battery of the present invention can be manufactured by arranging a separator between the positive electrode and the negative electrode to form an electrode assembly, and injecting the electrolyte after placing the above electrode assembly in a cylindrical battery case or a rectangular battery case. Or, after laminating the above electrode assemblies, the resultant obtained by impregnating them with an electrolyte can be placed in a battery case and sealed for manufacturing.

[0104] When manufacturing the lithium secondary battery of the present invention, the electrode assembly can 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 used during the production of the positive electrode. When using an electrolyte having the same components as the organic solvent used during the production of the positive electrode, the step of drying the electrode assembly can be omitted.

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

[0106] The battery case can be selected from those commonly used in the art, and there is no limitation on the outer shape according to the use of the battery. For example, it can be a cylindrical shape using a can, a square shape, a pouch type, or a coin type.

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

[0108] Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are for illustrative purposes only, and the scope of the present invention is not limited thereto.

[0109] <Example 1: Production of Positive Electrode>

[0110] As the first positive electrode active material, LiFePO4 having an average particle size D 50 of 1.0 μm and being primary particles with a single body structure, and as the second positive electrode active material, LiNi having an average particle size D 50 of 7 μm to 8 μm and being secondary particles 0.6 Mn 0.2 Co0.2 A positive electrode active material in which O2 was mixed at a weight ratio of 99.5:0.5 was prepared. Carbon nanotubes (CNT) were used as the conductive material, polyvinylidene fluoride (PVDF) was used as the binder, and hydrogenated nitrile butadiene rubber (HNBR) was used as the dispersant and added to an N-methylpyrrolidone (NMP) solvent. The positive electrode slurry was produced by mixing at 2500 rpm for 90 minutes using a Homo-disperse.

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

[0112] The above positive electrode slurry was applied to a 20-μm-thick aluminum thin film at 600 mg / 25 cm 2 and then heat-dried at 130 °C for 5 minutes so that the solid content of the above 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 produced 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 in Example 1 was changed to 99.9:0.1.

[0115] <Example 3: Production of Positive Electrode>

[0116] In Example 1 above, the second positive electrode active material was changed to LiNi 50 with an average secondary particle diameter D 0.6 Mn 0.2 Co 0.2 O2 of 4 μm to 5 μm, and a positive electrode was produced in the same manner as in Example 1, except for this change.

[0117] <Example 4: Production of Positive Electrode>

[0118] In Example 1 above, the second positive electrode active material was changed to an average secondary particle diameter D 50LiNi with a diameter 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 the above 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.

[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 active material to the second positive 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 active material to the second positive 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 active material to the second positive active material was changed to 100:0.

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

[0128] In the above 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 Limiting Rolling Thickness and Limiting Rolling Porosity>

[0130] For each of the positive electrodes produced in Examples 1 to 7 and Comparative Examples 1 to 2, the limiting rolling thickness and the limiting rolling porosity were measured, and the results are shown in Table 1. The method for measuring the limiting rolling thickness is as follows: First, roll so that the porosity according to the following formula (1) reaches a level of 30%. If no disconnection occurs in the current collector, roll again so that the porosity reaches 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] Roll while gradually decreasing the porosity in this way, and repeat such a process until disconnection occurs in the current collector. Define the thickness of the positive electrode active material layer measured at the rolling level immediately before disconnection as the limiting rolling thickness, and define the porosity at the rolling level immediately before disconnection as the limiting rolling porosity. In the case of the limiting rolling porosity, the porosity of the positive electrode according to Comparative Example 1 was evaluated as 1, and the porosities of the positive electrodes according to Examples 1 to 7 and Comparative Example 2 were described as relative ratios to the porosity of the positive electrode according to Comparative Example 1.

[0133]

Table 1

[0134] All of the positive electrodes according to Examples 1 to 7 have a smaller limiting rolling thickness and a smaller limiting rolling porosity than the positive electrode according to Comparative Example 1 that does not contain the second positive electrode active material. Also, all of the positive electrodes according to Examples 1 to 7 contain the second positive electrode active material, but compared with the positive electrode according to Comparative Example 2 in which the ratio (= B / A) of the average particle diameter (D 50 )A of the first positive electrode active material to the average particle diameter (D 50 )B of the second positive electrode active material is 2, they also have a smaller limiting rolling thickness and a smaller limiting rolling porosity.

[0135] On the one hand, the positive electrode according to Example 6 in which the weight ratio of the second positive electrode active material is 5% by weight and the positive electrode according to Example 7 in which the weight ratio of the second positive electrode active material is 10% by weight have similar limiting rolling thickness and limiting rolling porosity. Therefore, it is analyzed that when the weight ratio of the second positive electrode active material is equal to or more than a certain ratio, the improvement effect of the rolling performance converges to a certain level.

[0136] Also, since the positive electrode according to Example 4 and the positive electrode according to Example 5 have similar limiting rolling thickness and limiting rolling porosity, when the ratio (= B / A) of the average particle size (D 50 )A of the first positive electrode active material to the average particle size (D 50 )B of the second positive electrode active material exceeds a certain numerical range, the improvement level of the rolling performance does not increase infinitely but converges to a certain level.

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

Claims

1. A positive electrode including a positive electrode active material layer formed on a positive electrode current collector, wherein 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 the second positive electrode active material is 10% by weight 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 of the first positive electrode active material) A to the average particle size (D ５０ of the second positive electrode active material) B ratio (= B / A) value is 3 or more, a positive electrode for a lithium secondary battery.

2. The average particle size (D ５０ ) A of the first positive electrode active material to the average particle size (D ５０ ) B of the second positive electrode active material has a ratio (= B / A) value of 4 to 15. The positive electrode for a lithium secondary battery according to claim 1.

3. The average particle size (D ５０ ) A of the first positive electrode active material to the average particle size (D ５０ ) B of the second positive electrode active material has a ratio (= B / A) value of 6 to 12. The positive electrode for a lithium secondary battery according to claim 2.

4. The positive electrode for a lithium secondary battery according to claim 1, wherein the second positive electrode active material is 5% by weight or less based on the total weight of the first positive electrode active material and the second positive electrode active material.

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

6. The first positive electrode active material has an average particle diameter (D ５０ ), which is 0.5 μm to 1.5 μm. The positive electrode for a lithium secondary battery according to claim 1.

7. The second positive electrode active material has an average particle size (D ５０ ), which is 3 μm to 20 μm. The positive electrode for a lithium secondary battery according to claim 1.

8. 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 chemical formula 1, M includes any one or two 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 any one or two or more elements selected from the group consisting of F, S, and N; and a, b, and x are each -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.1, 0 ≤ x ≤ 0.5).

9. 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 the chemical formula 2, M1 includes any one or two or more elements selected from the group consisting of W, Mo, and Cr; M2 includes any one or two or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, and Nb; 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).

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

11. The positive electrode for a lithium secondary battery according to any one of claims 1 to 10, wherein the first positive electrode active material has a single body structure composed of primary particles.

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

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

14. Including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is 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% by weight 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 of the second positive electrode active material with respect to that (A) of the first positive electrode active material is such that the ratio (= B / A) of the average particle size (D ５０ ) B is 3 or more. A lithium secondary battery.

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