High-loading cathodes, cathode slurries, and lithium secondary batteries

Abstract

The positive electrode according to the present invention has a capacitance of 450 mg / 25 cm 2 The positive electrode active material layer includes lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material, the rubber-based binder includes a first hydrogenated nitrile-butadiene rubber having a weight average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second hydrogenated nitrile-butadiene rubber having a weight average molecular weight (Mw) of 150,000 g / mol or more, and the second hydrogenated nitrile-butadiene rubber is included in an amount of 0.2 wt % to 0.8 wt % based on the total weight of the positive electrode active material layer.

Description

[Technical Field]

[0001] This application claims priority rights based on Korean patent applications No. 10-2022-0139395, No. 10-2022-0139469, No. 10-2022-0139490, and No. 10-2022-0139418, filed on 26 October 2022.

[0002] The present invention relates to a positive electrode containing lithium iron phosphate as a positive electrode active material, a slurry for the positive electrode therefor, and a lithium secondary battery, and more particularly to a high-loading positive electrode with improved adhesive strength and flexibility, a slurry for the positive electrode therefor, and a lithium secondary battery containing the same. [Background technology]

[0003] As technological development and demand for electric vehicles and energy storage systems (ESS) increase, the demand for batteries as an energy source is rapidly rising, and accordingly, a wide range of research is being conducted on batteries that can meet diverse requirements. In particular, there is a lot of research being done on lithium-ion secondary batteries that have high energy density while also possessing excellent lifespan and cycle characteristics as power sources for such devices.

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

[0005] Lithium iron phosphate is inexpensive because it contains iron, a material that is abundant and inexpensive. Furthermore, its low toxicity can reduce environmental pollution when used. In addition, because lithium iron phosphate has an olivine structure, its active material structure can be stably maintained at high temperatures compared to layered lithium transition metal oxides. This results in superior high-temperature stability and high-temperature lifespan characteristics for batteries.

[0006] However, lithium iron phosphate has problems such as reduced lithium mobility and low electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. Therefore, conventionally, lithium iron phosphate with a small average particle size has been used to form a short lithium migration path, and the surface of lithium iron phosphate has been coated with carbon to improve electrical conductivity, and an excessive amount of conductive material has been used.

[0007] However, as the size of lithium iron phosphate particles decreases, the specific surface area increases, and lithium iron phosphate with a carbon-coated surface has reduced wettability by the solvent. As a result, severe particle aggregation of lithium iron phosphate occurs, the stability of the positive electrode slurry and the coating processability decrease, and lithium iron phosphate and the binder are not effectively mixed, so the adhesive force (hereinafter referred to as the positive electrode adhesive force) between the positive electrode current collector and the positive electrode active material layer in the manufactured positive electrode decreases. And such a phenomenon can be exacerbated in high-loading positive electrodes.

[0008] When the positive electrode adhesive force decreases, there is a problem that the positive electrode active material layer peels off during electrode manufacture or charge and discharge, the battery resistance increases, and the capacity of the secondary battery decreases.

[0009] Korean Patent Publication No. 10-2020-0008066 discloses a binder composition for a secondary battery electrode containing a copolymer containing an alkylene structural unit and a nitrile group-containing monomer unit and having a predetermined Mooney viscosity in order to disperse the conductive material well. However, when it is directly applied to a positive electrode using lithium iron phosphate as a positive electrode active material, it is not satisfactory in terms of improving adhesion and flexibility.

[0010] As the demand for high-energy density batteries increases, in a positive electrode containing lithium iron phosphate, while ensuring a loading amount of 600 mg / 25 cm 2 or more, technologies for improving the positive electrode adhesive force and flexibility are required.

Prior Art Documents

[0011] [Patent Document 1] Korean Published Patent No. 10-2020-0008066 [Overview of the project] [Problems that the invention aims to solve]

[0012] The present invention aims to provide a positive electrode and a lithium secondary battery that prevent electrode detachment, reduce battery resistance, and improve battery capacity by improving the adhesion and flexibility between the positive electrode current collector and the positive electrode active material layer in a high-loading positive electrode containing lithium iron phosphate. [Means for solving the problem]

[0013] According to one embodiment of the present invention, a positive electrode is provided. The positive electrode is a positive electrode in which a positive electrode active material layer is disposed on one or both sides of a current collector, The above positive electrode active material layer includes lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material. The above rubber-based binder contains a first-generation hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second-generation hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 150,000 g / mol or more. The above-mentioned hydrogenated nitrile butadiene rubber is included in an amount of 0.2% to 0.8% by weight based on the total weight of the positive electrode active material layer.

[0014] In a positive electrode according to one embodiment, the positive electrode active material layer comprises 450 mg / 25 cm². 2 ~700mg / 25cm 2 It has a loading amount within the range of [this range].

[0015] In one embodiment of the positive electrode, the weight-average molecular weight (Mw) of the second hydrogenated nitrile butadiene rubber is 150,000 g / mol to 1,000,000 g / mol.

[0016] In one embodiment of the positive electrode, the first hydrogenated nitrile butadiene rubber is included in an amount of 0.1 to 0.6% by weight based on the total weight of the positive electrode active material layer.

[0017] In one embodiment of the positive electrode, the lithium iron phosphate is present in an amount of 92.7% to 98.4% by weight based on the total weight of the positive electrode active material layer.

[0018] In one embodiment of the positive electrode, the fluorine-based binder is contained in the positive electrode active material layer in an amount of 1.0% to 4.0% by weight.

[0019] In one embodiment of the positive electrode, the rubber-based binder is contained in the positive electrode active material layer in an amount of 0.5% to 1.5% by weight.

[0020] In one embodiment of the positive electrode, the second hydrogenated nitrile butadiene rubber is contained in the positive electrode active material layer in an amount of 0.3% to 0.7% by weight.

[0021] In the positive electrode according to one embodiment, the amount of the second hydrogenated nitrile butadiene rubber is 33% by weight or less of the total weight of the fluorine-based binder and the second hydrogenated nitrile butadiene rubber.

[0022] In the positive electrode according to one embodiment, the lithium iron phosphate is a compound represented by the following chemical formula 1.

[0023] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0024] (In the above Chemical Formula 1, M contains 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 contains 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, and 0 ≦ x ≦ 0.5)

[0025] The positive electrode according to one embodiment has a positive electrode adhesive force of 19 gf / 20 mm or more as measured by an adhesive force test in which the positive electrode active material layer is peeled from the aluminum thin film at 90°.

[0026] In the positive electrode according to one embodiment, cracks occur with a measuring rod of 5 phi (φ) or less during a flexibility test in which the cross-section of the positive electrode is lifted after contacting a classification measuring rod on the positive electrode active material layer.

[0027] In the positive electrode according to one embodiment, the weight average molecular weight (Mw) of the fluorine-based binder is 800,000 g / mol or more.

[0028] In the positive electrode according to one embodiment, the conductive material is a carbon nanotube.

[0029] In the positive electrode according to one embodiment, the conductive material is contained in an amount of 0.3 wt% to 2.0 wt% based on the total weight of the positive electrode active material layer.

[0030] In the positive electrode according to one embodiment, the total weight of the binder contained in the positive electrode active material layer is 2.0 wt% to 4.5 wt% based on the total weight of the positive electrode active material.

[0031] In the positive electrode according to one embodiment, the average particle size D 50 of the lithium iron phosphate is 0.3 μm to 20.0 μm.

[0032] In the positive electrode according to one embodiment, the lithium iron phosphate has a single body structure composed of primary particles.

[0033] According to another embodiment of the present invention, a lithium secondary battery is provided. The lithium secondary battery comprises a positive electrode, a negative electrode, a separator membrane, and an electrolyte, wherein the positive electrode has 450 mg / 25 cm² on one or both sides of a current collector. 2 A positive electrode active material layer having the above loading amount is arranged, and the positive electrode active material layer includes lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material. The above rubber-based binder contains a first-generation hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second-generation hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 150,000 g / mol or more. The above-mentioned hydrogenated nitrile butadiene rubber is included in an amount of 0.2% to 0.8% by weight based on the total weight of the positive electrode active material layer.

[0034] According to another embodiment of the present invention, a slurry for a positive electrode is provided. The slurry for a positive electrode comprises lithium iron phosphate, a fluorine-based binder, a rubber-based binder, a conductive material, and a solvent, wherein the rubber-based binder comprises a first hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 150,000 g / mol or more. The above-mentioned hydrogenated nitrile butadiene rubber is included in an amount of 0.2% to 0.8% by weight, based on the total weight of solids in the positive electrode slurry.

[0035] In a cathode slurry according to one embodiment, the weight-average molecular weight (Mw) of the second hydrogenated nitrile butadiene rubber is 150,000 g / mol to 1,000,000 g / mol.

[0036] In a cathode slurry according to one embodiment, the weight-average molecular weight (Mw) of the fluorine-based binder is 800,000 g / mol or more.

[0037] In a cathode slurry according to one embodiment, the solid content of the cathode slurry is in the range of 50% to 75% by weight.

[0038] In one embodiment, the viscosity of the positive electrode slurry measured at 25°C and a shear rate of 2.5 / s is 5,000 cps to 25,000 cps.

[0039] In a cathode slurry according to one embodiment, the fluorine-based binder is included in an amount of 1.0% to 4.0% by weight based on the total weight of solids in the cathode slurry.

[0040] In a cathode slurry according to one embodiment, the first hydrogenated nitrile butadiene rubber is included in an amount of 0.1 to 0.6% by weight based on the total weight of solids in the cathode slurry.

[0041] In a cathode slurry according to one embodiment, the lithium iron phosphate is present in an amount of 92.7% to 98.4% by weight based on the total weight of solids in the cathode slurry.

[0042] In a cathode slurry according to one embodiment, the amount of the second hydrogenated nitrile butadiene rubber is 33% by weight or less of the total weight of the fluorine-based binder and the second hydrogenated nitrile butadiene rubber.

[0043] In a positive electrode slurry according to one embodiment, the rubber-based binder is included in an amount of 0.5% to 1.5% by weight based on the total weight of solids in the positive electrode slurry.

[0044] In a cathode slurry according to one embodiment, the lithium iron phosphate is a compound represented by the following chemical formula 1.

[0045] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0046] (In the above chemical formula 1, M comprises 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 comprises 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.)

[0047] In a cathode slurry according to one embodiment, the conductive material is a carbon nanotube.

[0048] In a positive electrode slurry according to one embodiment, the conductive material is included in an amount of 0.3% to 2.0% by weight based on the total weight of solids in the positive electrode slurry.

[0049] The sum of the above-mentioned fluorine-based binder and the above-mentioned rubber-based binder is 2.0% to 4.5% by weight, based on the total weight of solids in the above-mentioned cathode slurry. [Effects of the Invention]

[0050] According to the present invention, the loading amount of the positive electrode active material layer is 450 mg / 25 cm². 2 ~700mg / 25cm 2 This has the effect of dramatically improving the flexibility of high-level cathodes.

[0051] According to the present invention, by controlling the content of the fluorine-based binder, the first hydrogenated nitrile butadiene rubber, and the second hydrogenated nitrile butadiene rubber within a predetermined range, it is possible to simultaneously ensure excellent coating stability of the cathode slurry and excellent flexibility of the cathode.

[0052] The positive electrode according to the present invention exhibits excellent adhesion and flexibility even when the binder content in the positive electrode active material layer decreases as the adhesive strength improves, and the content of the positive electrode active material can be increased in proportion to the decrease in the binder content, thus resulting in superior energy density for secondary batteries.

[0053] According to one embodiment of the present invention, by including a fluorine-based binder and a second hydrogenated nitrile butadiene rubber in a suitable weight ratio, it is possible to manufacture a cathode slurry that is easy to mix while having excellent coating stability.

[0054] According to the present invention, it is possible to provide a positive electrode slurry in which the positive electrode active material, binder, and conductive material are well dispersed despite having a high solid content. In particular, aggregation of the positive electrode active material and conductive material is suppressed, making it possible to manufacture a positive electrode with excellent electrical conductivity. [Modes for carrying out the invention]

[0055] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and may be embodied in a variety of different forms, and these embodiments are provided merely to complete the disclosure of the present invention and to fully inform those who are ordinary skill in the art to which the invention pertains, and the present invention is defined only by the scope of the claims. Throughout the specification, the same reference numerals refer to the same components.

[0056] Unless otherwise defined, all terms used herein (including technical and scientific terms) should be used in a way that is commonly understood by those skilled in the art to which this invention pertains. Furthermore, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless explicitly defined otherwise.

[0057] The terms used herein are for illustrative purposes only and are not intended to limit the invention. In this specification, singular terms include plural terms unless otherwise specified in the statement. The terms “comprises” and / or “comprising” as used in this specification do not preclude the presence or addition of one or more other components in addition to those mentioned.

[0058] In this specification, when a part is said to include a component, this does not exclude other components, unless otherwise stated, but rather means that it may include other components.

[0059] In this specification, the term "A and / or B" means A or B, or A and B.

[0060] In this specification, "%" means weight percent unless otherwise explicitly indicated.

[0061] In this specification, D 50 This refers to the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve. 50 This can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron region to several millimeters in size, and can yield highly reproducible and high-resolution results.

[0062] In this specification, "specific surface area" is measured by the BET method, and specifically, it can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BELSORP-minoII from BEL Japan.

[0063] In this specification, "weight-average molecular weight (Mw)" refers to the converted value relative to standard polystyrene measured by gel permeation chromatography (GPC). Specifically, the above weight-average molecular weight is a converted value obtained by measuring the value using GPC under the following conditions, and standard polystyrene from the Agilent system was used to prepare the calibration curve.

[0064] <Measurement conditions> Measuring instrument: Agilent GPC (Agilent 1200 series, USA) Column: PL Mixed B (2 concatenated columns) Column temperature: 40℃ Eluent: Tetrahydrofuran Flow rate: 1.0mL / min Concentration: ~1mg / mL (100μL injection)

[0065] The positive electrode adhesion strength in this specification may be measured by the following method. A positive electrode cut to a length of 150 mm and a width of 20 mm is prepared, and the positive electrode active material layer is placed facing a slide glass measuring 75 mm in length and 25 mm in width, and the positive electrode is attached to the slide glass longitudinally using double-sided tape. That is, the slide glass is attached to a region corresponding to half of the positive electrode in the longitudinal direction. Then, a roller is rotated 10 times to ensure that the double-sided tape is uniformly attached to produce an evaluation sample. Next, the slide glass portion of the evaluation sample is fixed to the sample stage of a Universal Testing Machine (UTM) (LS5, AMETEK), and the half of the positive electrode without the slide glass attached is connected to a load cell equipped with the UTM. A force of 90° is applied to the load cell at a speed of 100 mm / min, and the load applied to the load cell is measured while moving it to a distance of 50 mm. At this time, the average value of the load measured in the 20mm to 40mm section of the running section is calculated, and this is repeated a total of 5 times. The average value is then evaluated as the positive electrode adhesion strength (gf / 20mm) of each sample.

[0066] The flexibility of the positive electrode in this specification may be measured by the following method: Prepare measuring rods for each phi (φ) and measure 600 mg / 25 cm. 2 A positive electrode with a loading capacity is cut into a 10cm wide x 30cm long section. The cut positive electrode is bent in half and the measuring rod is brought into contact with it. Then, both ends of the positive electrode are lifted at a speed of 10mm per minute. At this time, the electrode is lifted until the force measured by the UTM (Universal Testing Machine) reaches 5N. The force is measured for each phi, and the electrode is observed with an optical microscope to see if cracks occur. If no cracks are found, the test is continued with smaller phi sections.

[0067] The present invention will be described in detail below.

[0068] <Positive electrode> The positive electrode according to an embodiment of the present invention is a positive electrode in which a positive electrode active material layer is disposed on one or both sides of a current collector, The above positive electrode active material layer includes lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material. The above rubber-based binder contains a first-generation hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second-generation hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 150,000 g / mol or more. The above-mentioned hydrogenated nitrile butadiene rubber is included in an amount of 0.2% to 0.8% by weight based on the total weight of the positive electrode active material layer.

[0069] Lithium iron phosphate, compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide, has lower lithium mobility and electrical conductivity, so lithium iron phosphate with a small average particle size is mainly used as a positive electrode active material. However, when the size of lithium iron phosphate particles is small, the specific surface area increases, which causes severe particle aggregation, and the lithium iron phosphate and binder are not mixed effectively, resulting in a decrease in positive electrode adhesion. This leads to the problem of delamination of the positive electrode active material layer during electrode manufacturing or charging / discharging, increasing battery resistance and reducing the capacity of the secondary battery.

[0070] As a result of extensive research to solve these problems, the inventors have found that when a rubber-based binder contains a first-hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol and a second-hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 150,000 g / mol or more, the dispersibility of lithium iron phosphate particles is improved, suppressing their aggregation while maintaining a density of 450 mg / 25 cm 2 ~700mg / 25cm 2We discovered that it is possible to significantly improve the adhesive strength and flexibility of high-loading cathodes with a certain level of loading capacity, thus completing the present invention.

[0071] The positive electrode according to one embodiment of the present invention has a structure in which the positive electrode active material layer is in direct contact with the positive electrode current collector due to excellent interfacial adhesion between the positive electrode active material layer and the positive electrode current collector, and may not include a separate layer to improve adhesion between the positive electrode active material layer and the positive electrode current collector. In other words, the positive electrode according to one embodiment of the present invention can exhibit excellent interfacial adhesion even without including a separate layer such as a binder layer, adhesive layer, bonding layer, or primer coating layer that may be interposed between the positive electrode current collector and the positive electrode active material layer to improve adhesion.

[0072] A positive electrode according to one embodiment of the present invention may include a positive electrode active material layer. Specifically, the positive electrode according to the present invention may include a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector.

[0073] In one embodiment, the loading amount of the positive electrode active material layer is 450 mg / 25 cm². 2 The above is possible; for more details, see 450mg / 25cm 2 ~700mg / 25cm 2 For more details, see 500mg / 25cm 2 ~700mg / 25cm 2 For more details, see 550mg / 25cm 2 ~650mg / 25cm 2 This is possible. Here, the loading amount is the loading amount of the positive electrode active material layer stacked on one surface of the current collector.

[0074] The positive electrode current collector is not particularly limited, as long as it is conductive without inducing a chemical change in the battery. For example, the current collector may be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc.

[0075] The positive electrode current collector can have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance adhesion to the positive electrode active material layer. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0076] The positive electrode active material layer may contain a positive electrode active material. In addition, the positive electrode active material layer may further contain, as necessary, conductive materials, binders, and dispersants, in addition to the positive electrode active material.

[0077] The following provides a detailed explanation of each component included in the positive electrode active material layer.

[0078] (1) Positive electrode active material This invention includes lithium iron phosphate as the positive electrode active material. Since lithium iron phosphate has an olivine structure, its active material structure is stably maintained at high temperatures compared to lithium transition metal oxides with a layered structure. As a result, when lithium iron phosphate is used as the positive electrode active material, the high-temperature stability and high-temperature life characteristics of the positive electrode are significantly improved, which can reduce the risk of fire in lithium secondary batteries containing the above-mentioned positive electrode.

[0079] The lithium iron phosphate mentioned above may be the compound of the following chemical formula 1.

[0080] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b (In the above chemical formula 1, M comprises 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 comprises 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.)

[0081] For example, the lithium iron phosphate mentioned above could be LiFePO4.

[0082] The lithium iron phosphate described above may consist of secondary particles formed by the aggregation of primary particles, and preferably has a single-particle structure composed of primary particles.

[0083] In this invention, a "monolith structure" refers to a structure in which particles exist in independent phases on a morphological plane without agglomerating with each other. In contrast to such a monolith structure, an example of a particle structure is one in which small particles ("primary particles") are physically and / or chemically agglomerated to form relatively larger particle forms ("secondary particles").

[0084] When lithium iron phosphate has a single-particle structure consisting of primary particles, the likelihood of cracking of the lithium iron phosphate particles during the rolling process is smaller compared to when it is composed of secondary particles. Therefore, the reduction in capacity due to the detachment of cracked particles is less, which is preferable. Furthermore, when lithium iron phosphate is composed of primary particles with a single-particle structure, the binder migration phenomenon during the drying process of the positive electrode slurry can be mitigated, which is also preferable from the perspective of interfacial adhesion between the positive electrode current collector and the positive electrode active material layer.

[0085] Lithium iron phosphate may contain a carbon coating layer on its surface. When a carbon coating layer is formed on the surface of lithium iron phosphate, electrical conductivity can be improved, and the resistance characteristics of the positive electrode can be enhanced.

[0086] 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, polymers of furfuryl alcohol, block copolymers of ethylene and ethylene oxide, vinyl resins, cellulosic resins, phenolic resins, pitch resins, and tar resins. Specifically, the carbon coating layer may be formed through a process in which the raw material is mixed with lithium iron phosphate and then heat-treated.

[0087] Average particle size D of lithium iron phosphate 50 The average particle size D of the positive electrode active material can be 0.3 μm to 20.0 μm, more specifically 0.4 μm to 10.0 μm, and more specifically 0.5 μm to 3.0 μm. 50 When the above range is met, the mobility of lithium within lithium iron phosphate is improved, and the charge-discharge characteristics of the battery may be improved.

[0088] The BET specific surface area of ​​lithium iron phosphate is 5 m². 2 / g~20m 2 It could be / g, specifically 7m 2 / g~18m 2 / g, more specifically 9m 2 / g~16m 2 It can be as low as / g. The above range corresponds to a lower value compared to typical lithium iron phosphate. When the above range is satisfied, aggregation of lithium iron phosphate can be effectively suppressed even in cathode slurry compositions with a relatively low dispersant content.

[0089] In one embodiment of the present invention, the lithium iron phosphate has an average particle size D 50 It may contain lithium monophosphate and lithium disphosphate with different size ranges. In such cases, the lithium monophosphate particles have an average particle size D in the range of 0.5 μm to 2 μm. 50 It may have, and the lithium iron phosphate particles have an average particle size D in the range of 3.5 μm to 15 μm. 50 It may have.

[0090] The binder contained in the positive electrode active material layer, particularly PVDF-based polymer binders, tends to adhere to lithium iron phosphate rather than the current collector at the positive electrode interface. This phenomenon can be exacerbated as the specific surface area of ​​the lithium iron phosphate particles increases.

[0091] Average particle size D 50Lithium iron phosphate diphosphate particles in the range of 3.5 μm to 15 μm have a larger average particle size than the above-mentioned lithium iron phosphate monophosphate particles, and their specific surface area is smaller compared to lithium iron phosphate monophosphate particles. Therefore, when lithium iron phosphate diphosphate particles are included within a predetermined range, it has the effect of reducing the total specific surface area of ​​the lithium iron phosphate particles. As a result of this reduction in the total specific surface area of ​​the lithium iron phosphate particles, the PVDF-based binder adheres to the current collector instead of the lithium iron phosphate, thereby improving the interfacial adhesion between the current collector and the positive electrode active material layer.

[0092] The average particle size D of lithium iron monophosphate mentioned above 50 The particle size can be 0.5 μm to 2 μm, preferably 0.55 μm to 1.8 μm, and more preferably 0.6 μm to 1.6 μm. Also, the average particle size D of the above lithium diiryl phosphate 50 The particle size can be 3.5 μm to 15 μm, preferably 3.8 μm to 12 μm, and more preferably 4 μm to 10 μm.

[0093] The weight ratio of lithium iron monophosphate to lithium iron disphosphate can be 80:20 to 99.9:0.1, preferably 85:15 to 99:1, and more preferably 90:1 to 95:5. Excellent adhesion is achieved when the weight ratio of lithium iron monophosphate to lithium iron disphosphate satisfies the above range. If there is an excessive amount of lithium iron disphosphate, which has a relatively large average particle size, the viscosity of the positive electrode slurry may increase rapidly, which can reduce rolling performance and is therefore undesirable.

[0094] Lithium iron monophosphate can be a primary particle, and lithium iron disphosphate can be a primary or secondary particle.

[0095] Lithium iron phosphate may be present in the positive electrode slurry composition at a concentration of 92.7% to 98.4% by weight, specifically 93.5% to 98% by weight, and more specifically 94% to 97% by weight, based on the total solid content. When the lithium iron phosphate content satisfies the above range, the battery capacity of the positive electrode can be improved by ensuring sufficient positive electrode energy density.

[0096] (2) Binder The present invention includes both a fluorine-based binder and a rubber-based binder as the binder, wherein the rubber-based binder includes a first hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 150,000 g / mol or more.

[0097] The above-mentioned fluorine-based binders include PVDF-based polymers containing vinylidene fluoride (VDF) as a monomer. Specific examples of the above-mentioned PVDF-based polymers include PVDF monopolymer, PVDF-HFP (Poly(vinylidene fluoride-co-hexafluoropropylene)), PVDF-CTFE (Poly(vinylidene fluoride-co-chlorotrifluoroethylene)), PVDF-TFE (Poly(vinylidene tetrafluoroethylene)), and PVDF-TrFE (Poly(vinylidene trifluoroethylene)).

[0098] The fluorine-based binder, together with the rubber-based binder, provides adhesive strength between the positive electrode active material and the conductive material, and between the current collector and the positive electrode active material layer.

[0099] The weight-average molecular weight (Mw) of the fluorine-based binder according to the present invention may be 500,000 g / mol or more, more specifically in the range of 550,000 g / mol to 2,000,000 g / mol, and more specifically in the range of 600,000 g / mol to 1,500,000 g / mol. When the weight-average molecular weight of the fluorine-based binder is within the above range, the coating stability of the positive electrode slurry is superior, and the adhesion strength of the positive electrode is superior. In particular, from the perspective of the adhesion strength of the positive electrode, the weight-average molecular weight (Mw) of the fluorine-based binder is 800,000 g / mol or more, preferably 900,000 g / mol to 1,400,000 g / mol, and more preferably 950,000 g / mol to 1,200,000 g / mol.

[0100] The above-mentioned fluorine-based binder may be included in the positive electrode active material layer in an amount of 1.0% to 4.0% by weight, preferably 1.4% to 3.5% by weight, and more preferably 1.6% to 3.0% by weight, based on the total weight of the positive electrode active material layer.

[0101] Fluorine-based binders are commonly used as electrode binders due to their excellent adhesive strength and chemical stability with respect to electrolytes. However, due to their crystal structure, excessive amounts of fluorine-based binders can negatively affect the flexibility of the positive electrode. On the other hand, higher binder content leads to higher adhesive strength of the positive electrode. Therefore, improving both the adhesive strength and flexibility of the positive electrode has been a challenge that is difficult to achieve simply by adjusting the binder content. Furthermore, the loading amount of the positive electrode active material layer is 600 mg / 25 cm². 2 As described above, high-loading cathodes tend to be less flexible compared to low-loading cathodes with the same binder content but smaller loading amounts. Therefore, improving flexibility along with adhesive strength in high-loading cathodes presented an even more challenging technical problem.

[0102] Therefore, the positive electrode according to the present invention further comprises a rubber-based binder in addition to the fluorine-based binder, wherein the rubber-based binder includes a first hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol and a second hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 150,000 g / mol or more.

[0103] The above-mentioned first hydrogenated nitrile butadiene rubber not only improves the adhesion and flexibility of the positive electrode, but also improves the dispersibility of the positive electrode slurry, thereby suppressing aggregation of the conductive material and the positive electrode active material. The inventors of the present invention discovered that when the positive electrode contains only the above-mentioned first hydrogenated nitrile butadiene rubber as a rubber-based binder, the adhesion and flexibility of the positive electrode cannot be further improved beyond a certain content. However, when the positive electrode contains both the first hydrogenated nitrile butadiene rubber and the second hydrogenated nitrile butadiene rubber, the flexibility and adhesion of the positive electrode increase dramatically, leading to the present invention. As a result, the positive electrode according to the present invention is excellent in both flexibility and adhesion, and when the loading amount of the positive electrode increases, the improved flexibility results in superior rolling performance. Furthermore, the improved flexibility contributes to preventing the delamination of the positive electrode active material layer during physical impacts on the outside of the positive electrode.

[0104] The above-mentioned first hydrogenated nitrile butadiene rubber and second hydrogenated nitrile butadiene rubber (HNBR) refer to the fact that each nitrile butadiene rubber (NBR) has been subjected to a hydrogenation reaction, resulting in the double bonds originally present in the nitrile butadiene rubber (NBR) being replaced with single bonds.

[0105] The above-mentioned hydrogenated nitrile butadiene rubber may contain 20% to 50% by weight of repeating units derived from acrylonitrile (AN) relative to its total weight, more preferably 25% to 45% by weight, and most preferably 30% to 40% by weight.

[0106] The above-mentioned first hydrogenated nitrile butadiene rubber has a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, improving the solvent wettability and dispersibility of lithium iron phosphate particles and suppressing particle aggregation of lithium iron phosphate. The above-mentioned first hydrogenated nitrile butadiene rubber may have a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, preferably 10,000 g / mol to 75,000 g / mol, and more preferably 10,000 g / mol to 50,000 g / mol. When the weight-average molecular weight of the first hydrogenated nitrile butadiene rubber is within the above range, particle aggregation of lithium iron phosphate can be effectively suppressed.

[0107] Furthermore, the above-mentioned first hydrogenated nitrile butadiene rubber suppresses aggregation of conductive materials, improves the positive electrode conductive network, and even if the conductive materials aggregate, they aggregate into a spherical shape rather than a linear shape, thereby minimizing the specific surface area of ​​the aggregated conductive materials compared to the case where the conductive materials aggregate linearly. As a result, the surface area of ​​the positive electrode active material adjacent to the aggregated conductive materials that cannot participate in the lithium insertion / desorption reaction is minimized, thus lowering the discharge resistance of the lithium secondary battery.

[0108] The weight-average molecular weight (Mw) of the above-mentioned second-hydrogenated nitrile butadiene rubber is 150,000 g / mol or more, preferably 150,000 g / mol to 1,000,000 g / mol, and more preferably 200,000 g / mol to 500,000 g / mol. When the weight-average molecular weight (Mw) of the above-mentioned second-hydrogenated nitrile butadiene rubber is within the above range, it is possible to improve the adhesion and flexibility of the positive electrode active material layer, increase the rolling density of the positive electrode, and have the effect of lowering the positive electrode resistance as the rolling density increases.

[0109] The first hydrogenated nitrile butadiene rubber may be included in the positive electrode active material layer in an amount of 0.1% to 0.6% by weight, preferably 0.15% to 0.55% by weight, and more preferably 0.2% to 0.5% by weight. When the first hydrogenated nitrile butadiene rubber is included in the above range, the dispersibility of lithium iron phosphate is improved, and excellent adhesion and flexibility can be achieved without increasing the overall binder content.

[0110] The above-mentioned second hydrogenated nitrile butadiene rubber may be included in the positive electrode active material layer in an amount of 0.2% to 0.8% by weight, preferably 0.3% to 0.7% by weight, and more preferably 0.35% to 0.65% by weight. If the content of the above-mentioned second hydrogenated nitrile butadiene rubber exceeds 0.8% by weight, it may greatly increase the viscosity of the slurry and significantly reduce the mixing performance and coating performance during slurry production, which is undesirable. If the content of the above-mentioned second hydrogenated nitrile butadiene rubber is less than 0.2% by weight, it may only slightly improve the adhesive performance and flexibility of the manufactured positive electrode, which is also undesirable.

[0111] The above-mentioned rubber-based binder may be included in the positive electrode active material layer in an amount of 0.5% to 1.5% by weight, preferably 0.6% to 1.3% by weight, and most preferably 0.7% to 1.2% by weight. When the rubber-based binder is included within the above range, excellent adhesive strength and flexibility can be obtained without increasing the overall binder content.

[0112] The above-mentioned second hydrogenated nitrile butadiene rubber may be present in an amount of 40% by weight or less, preferably 14 to 33% by weight, and more preferably 15 to 30% by weight, relative to the total weight of the fluorine-based binder and the second hydrogenated nitrile butadiene rubber. When the second hydrogenated nitrile butadiene rubber is included in the above-mentioned amount in relation to the fluorine-based binder, it is possible to improve the flexibility of the positive electrode while preventing a rapid increase in the viscosity of the slurry and a decrease in coating stability.

[0113] Furthermore, the fluorine-based binder and the rubber-based binder may together be included in the positive electrode active material layer in an amount of 2.0% to 4.5% by weight, preferably 2.2% to 4.0% by weight, and most preferably 2.5% to 3.8% by weight. When the binder content satisfies the above range, the contact area between the binder and lithium iron phosphate is increased, ensuring excellent positive electrode adhesion.

[0114] (3) Conductive material The positive electrode active material layer of the present invention may further contain a conductive material.

[0115] The conductive material mentioned above is not particularly limited as long as it is conductive without inducing a chemical change in the battery. Examples of such materials include graphite; carbon black such as carbon black, acetylene black, Ketjenblack, channel black, furnace black, lamp black, and summer 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; and conductive materials such as polyphenylene derivatives. Specific examples of commercially available conductive materials include acetylene black series from Chevron Chemical Company, Denka Black (Denka Singapore Private Limited), Gulf Oil Company (products, etc.), Ketjenblack, EC series (products from Armak Company), Vulcan XC-72 (products from Cabot Company), and Super P (products from Timcal). Preferably, the conductive material may be carbon nanotubes. A conductive network of carbon nanotubes is preferable as a conductive material included in the positive electrode of the present invention because it can mitigate the binder lifting phenomenon during the drying process of the positive electrode slurry.

[0116] The conductive material may be included in the positive electrode active material layer in an amount of 0.3% to 2.0% by weight, more specifically 0.6% to 1.5% by weight, and more specifically 0.8% to 1.3% by weight. 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 a positive electrode conductive network.

[0117] According to one embodiment of the present invention, the positive electrode active material layer may contain 92.7% to 98.4% by weight of lithium iron phosphate, 1.0% to 4.0% by weight of the fluorine-based binder, 0.1% to 0.6% by weight of the first hydrogenated nitrile butadiene rubber, 0.2% to 0.8% by weight of the second hydrogenated nitrile butadiene rubber, and 0.3% to 2.0% by weight of the conductive material, based on the total weight of the positive electrode active material layer. When the composition within the positive electrode active material layer satisfies the above range, the adhesion and conductivity of the electrode are ensured, and at the same time, by increasing the active material content, the capacity and resistance performance of the lithium secondary battery may be improved, and the flexibility of the positive electrode can be dramatically improved.

[0118] The positive electrode can be manufactured according to a conventional method for manufacturing a positive electrode. Specifically, the positive electrode can be manufactured by first producing a positive electrode slurry composition containing the positive electrode active material, conductive material, binder and / or dispersant described above, then applying the positive electrode slurry composition onto a positive electrode current collector, followed by drying and rolling.

[0119] Alternatively, the positive electrode can also be manufactured by casting the positive electrode slurry composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0120] The positive electrode according to one embodiment of the present invention contains both the first hydrogenated nitrile butadiene rubber and the second hydrogenated nitrile butadiene rubber, and contains lithium iron phosphate, a fluorine-based binder, the first hydrogenated nitrile butadiene rubber, and the second hydrogenated nitrile butadiene rubber in the above weight ratio, resulting in a concentration of 550 mg / 25 cm 2 ~650mg / 25cm 2While ensuring a sufficient loading capacity, it is possible to have excellent positive electrode adhesion and flexibility. As a result, it is possible to manufacture high-loading positive electrodes, which reduces the cell resistance of secondary batteries by preventing positive electrode detachment, improving battery capacity and output characteristics, and reducing defects that occur in the manufacturing process.

[0121] The positive electrode of the present invention has an adhesive strength between the positive electrode current collector and the positive electrode active material layer measured by a 90° peel test of 19 gf / 20 mm or more, specifically in the range of 19.5 gf / 20 mm to 70 gf / 20 mm, and more specifically in the range of 20 gf / 20 mm to 67 gf / 20 mm.

[0122] Furthermore, the positive electrode of the present invention has the flexibility to crack when a measuring rod of diameter 5φ or less, specifically 2 to 4φ, is brought into contact with the positive electrode active material layer and the cross-section of the positive electrode is lifted.

[0123] <Lithium-ion secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0124] A lithium secondary battery according to one embodiment of the present invention may include a positive electrode, a negative electrode, a separation membrane interposed between the negative electrodes, and an electrolyte.

[0125] The positive electrode in the lithium secondary battery described above is as stated above. For example, the positive electrode has a positive electrode active material layer arranged on one or both sides of the current collector. The above positive electrode active material layer includes lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material. The above rubber-based binder contains a first-generation hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second-generation hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 150,000 g / mol or more. The above-mentioned hydrogenated nitrile butadiene rubber is included in an amount of 0.2% to 0.8% by weight based on the total weight of the positive electrode active material layer.

[0126] The lithium iron phosphate, fluorine-based binder, first hydrogenated nitrile butadiene rubber, second hydrogenated nitrile butadiene rubber, and conductive material have been explained in detail earlier, so redundant explanations will be omitted.

[0127] The loading amount for the positive electrode active material layer is 450 mg / 25 cm². 2 The above is possible; for more details, see 450mg / 25cm 2 ~700mg / 25cm 2 For more details, see 500mg / 25cm 2 ~700mg / 25cm 2 For more details, see 550mg / 25cm 2 ~650mg / 25cm 2 It is possible.

[0128] The above-mentioned negative electrode can be manufactured, for example, by first producing 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 to the negative electrode current collector.

[0129] The negative electrode active material is not particularly limited, and compounds capable of reversible intercalation and deintercalation of lithium can usually 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. Examples of low-crystallinity carbon include soft carbon and hard carbon, while examples of high-crystallinity 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 can be used alone or in mixtures of two or more, and a metallic lithium thin film can also be used as the negative electrode active material.

[0130] The above-mentioned negative electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitations as long as it has electronic conductivity without causing a chemical change in the battery that is constructed. 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, summer black, carbon fiber, and carbon nanotubes; 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, and one of these alone or a mixture of two or more can be used. The above-mentioned negative electrode conductive material can usually be included in an amount of 1 to 30% by weight, specifically 1 to 20% by weight, and more specifically 1 to 10% by weight, relative to the total weight of the negative electrode active material layer.

[0131] The above-mentioned negative electrode binder plays a role in improving the adhesion between negative electrode active material particles and the adhesion 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, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one or more may be used. The above-mentioned negative electrode binder may be included in an amount of 1 to 30% by weight, specifically 1 to 20% by weight, and more specifically 1 to 10% by weight, relative to the total weight of the negative electrode active material layer.

[0132] On the other 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, calcined carbon, copper or stainless steel surface treatments with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used.

[0133] Furthermore, the negative electrode current collector can typically have a thickness of 3 to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0134] On the other hand, in the lithium secondary battery described above, the separation membrane can be used without particular limitations as long as it is normally used as a separation membrane in lithium secondary batteries, and it is especially preferable that it has low resistance to ion movement of the electrolyte while having excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, for example, porous polymer films made from polyolefin 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. In addition, ordinary porous nonwoven fabrics, for example, nonwoven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc., can also be used. Furthermore, the separation membrane may 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.

[0135] On the other hand, in the lithium secondary battery described above, the electrolyte may include, and is not particularly limited to, organic solvents and lithium salts that are commonly used as electrolytes.

[0136] The above-mentioned organic solvent can be used without particular limitations, as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above organic solvents may include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; and 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).

[0137] Among these, carbonate-based solvents are preferred, and more preferably are mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate).

[0138] The lithium salt described above can be used without particular limitation as long as it is a compound capable of providing lithium ions for use in lithium secondary batteries. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. It is preferable that the lithium salt is contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0139] In addition to the components of the electrolyte, the above electrolyte may also contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the above additive may be included in an amount of 0.1 to 5% by weight relative to the total weight of the electrolyte.

[0140] The lithium secondary battery of the present invention can be manufactured by forming an electrode assembly by placing a separation membrane between the positive electrode and the negative electrode, and then injecting an electrolyte after placing the electrode assembly in a cylindrical or rectangular battery case. Alternatively, the electrode assembly can be manufactured by stacking the electrode assemblies, impregnating them with an electrolyte, and then sealing the resulting product in a battery case.

[0141] 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 during the production of the positive electrode. If an electrolyte with the same components as the organic solvent used during the production of the positive electrode is used, the step of drying the electrode assembly may be omitted.

[0142] Unlike the lithium secondary batteries described above, a lithium secondary battery according to another embodiment of the present invention may be an all-solid-state battery.

[0143] The battery case described above may be one that is commonly used in this field, and there are no restrictions on its external shape depending on the battery's application. For example, it may be cylindrical, rectangular, pouch-type, or coin-type, using a can.

[0144] The lithium secondary battery according to the present invention exhibits excellent discharge capacity, output characteristics, and capacity retention rate stably, making it useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in energy storage systems (ESS) and electric vehicles such as hybrid electric vehicles (HEVs).

[0145] <Slurry for positive electrode> According to another embodiment of the present invention, a slurry for a positive electrode is provided.

[0146] The cathode slurry according to the present invention comprises lithium iron phosphate, a fluorine-based binder, a rubber-based binder, a conductive material, and a solvent. The rubber-based binder comprises a first hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 150,000 g / mol or more. The second hydrogenated nitrile butadiene rubber is included in an amount of 0.2% to 0.8% by weight based on the total weight of solids in the cathode slurry.

[0147] In a cathode slurry according to one embodiment, the lithium iron phosphate may be present in an amount of 92.7% to 98.4% by weight, the fluorine-based binder in an amount of 1.0% to 4.0% by weight, the first hydrogenated nitrile butadiene rubber in an amount of 0.1% to 0.6% by weight, the second hydrogenated nitrile butadiene rubber in an amount of 0.2% to 0.8% by weight, and the conductive material in an amount of 0.3% to 2.0% by weight, based on the total weight of solids in the cathode slurry.

[0148] The cathode slurry according to the present invention contains a dishydrogenated nitrile butadiene rubber having a weight-average molecular weight within the above range, dramatically increasing the flexibility of the high-loading cathode, and controlling the content of the fluorine-based binder and the dishydrogenated nitrile butadiene rubber within the above range provides suitable viscosity and excellent coating stability.

[0149] Since the lithium iron phosphate, hydrogenated butadiene nitrile rubber, hydrogenated butadiene nitrile rubber, fluorine-based binders, and conductive materials mentioned above have been explained in detail previously, we will omit further explanation.

[0150] The solvent described above is for mixing the lithium iron phosphate, binder, conductive material and / or dispersant. The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), acetone, or water, of which one or more may be used.

[0151] The above solvent may be included in an amount that gives the slurry suitable viscosity and solid content for the positive electrode. For example, the solvent may be included in an amount such that the solid content in the slurry is 50% to 75% by weight, specifically 50% to 70% by weight, and more specifically 55% to 70% by weight. This corresponds to a relatively high solid content compared to conventional positive electrode slurries containing lithium iron phosphate as the positive electrode active material. When the solid content of the positive electrode slurry composition satisfies the above range, the time required for the slurry drying process during positive electrode manufacturing can be shortened, and process costs can be reduced. Furthermore, the above positive electrode slurry may have a viscosity at a level that allows for coating, and the positive electrode active material layer formed from the above composition may have a thickness of a certain level or higher, thereby ensuring excellent energy density.

[0152] The cathode slurry according to one embodiment of the present invention may have a viscosity of 5,000 cps to 25,000 cps, specifically 6,000 cps to 24,000 cps, and more specifically 6,500 cps to 23,000 cps, measured at a shear rate of 2.5 / s at 25°C. A cathode slurry having a viscosity value within the above range may have excellent storage stability and coating processability. Furthermore, since the above cathode slurry may have a higher solid content compared to conventional lithium iron phosphate cathode slurries having the same viscosity, the time required for the slurry drying process during cathode manufacturing can be shortened, and process costs can be reduced.

[0153] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and do not limit the scope of the present invention.

[0154] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and do not limit the scope of the present invention.

[0155] <Example 1: Manufacturing of a positive electrode> (1) Manufacturing of positive electrode slurry A dispersion was prepared containing carbon nanotubes (CNTs) as a conductive material and first hydrogenated nitrile butadiene rubber (first HNBR) with a weight-average molecular weight (Mw) of 30,000 g / mol as a rubber-based binder.

[0156] As lithium iron phosphate, average particle size D 50 A cathode slurry was prepared by adding 1.0 μm, single-structure primary particles of LiFePO4, carbon nanotubes (CNTs) as a conductive material, polyvinylidene fluoride (PVDF) with a weight-average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, and dihydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) of 310,000 g / mol as a rubber-based binder, and the above dispersion to N-methylpyrrolidone (NMP) solvent, and then mixing with a homo-disperse at 2500 rpm for 90 minutes.

[0157] In the above positive electrode slurry, lithium iron phosphate, conductive material, fluorine-based binder, first hydrogenated nitrile butadiene rubber, and second hydrogenated nitrile butadiene rubber were present in a weight ratio of 95.24:1.2:2.7:0.36:0.5, and the solid content of the above positive electrode slurry was 62% by weight.

[0158] (2) Manufacturing of the positive electrode A 20 μm thick aluminum thin film is coated with the above cathode slurry at a rate of 600 mg / 25 cm². 2After coating the material in this manner, the cathode slurry was hot-air dried at 130°C for 5 minutes so that its solid content was 99.0% by weight or more. Subsequently, the dried cathode slurry was rolled to produce a cathode so that the porosity of the cathode active material layer was 29%.

[0159] <Example 2: Manufacturing of the positive electrode> The positive electrode was manufactured in the same manner as in Example 1, except that the weight ratios of lithium iron phosphate, conductive material, fluorine-based binder, first hydrogenated nitrile butadiene rubber, and second hydrogenated nitrile butadiene rubber in the positive electrode slurry were changed as shown in Table 1.

[0160] <Examples 3-6: Manufacturing of the positive electrode> The cathode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) with a weight-average molecular weight (Mw) of 1,000,000 g / mol, and the weight ratios of lithium iron phosphate, conductive material, fluorine-based binder, first hydrogenated nitrile butadiene rubber, and second hydrogenated nitrile butadiene rubber in the cathode slurry were changed as shown in Table 1.

[0161] <Examples 7-10: Manufacturing of the positive electrode> The cathode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) with a weight-average molecular weight (Mw) of 1,000,000 g / mol, the second hydrogenated nitrile butadiene rubber was changed to hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 220,000 g / mol, and the weight ratio of lithium iron phosphate, conductive material, fluorine-based binder, first hydrogenated nitrile butadiene rubber, and second hydrogenated nitrile butadiene rubber in the cathode slurry was changed as shown in Table 1 (however, the solid content of the cathode slurry in Example 9 was also changed in the table).

[0162] <Example 11: Manufacturing of the positive electrode> The cathode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) with a weight-average molecular weight (Mw) of 1,000,000 g / mol, and the weight ratios of lithium iron phosphate, conductive material, fluorine-based binder, first hydrogenated nitrile butadiene rubber, and second hydrogenated nitrile butadiene rubber in the cathode slurry were changed as shown in Table 1.

[0163] <Comparative Example 1: Manufacturing of the positive electrode> (1) Manufacturing of positive electrode slurry A dispersion was prepared containing carbon nanotubes (CNTs) as a conductive material and first hydrogenated nitrile butadiene rubber (first HNBR) with a weight-average molecular weight (Mw) of 30,000 g / mol as a rubber-based binder.

[0164] As lithium iron phosphate, average particle size D 50 A cathode slurry was prepared by adding 1.0 μm, single-structure primary particles of LiFePO4, polyvinylidene fluoride (PVDF) with a weight-average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, and first hydrogenated nitrile butadiene rubber (first HNBR) with a weight-average molecular weight (Mw) of 310,000 g / mol as a rubber-based binder, and the above dispersion to N-methylpyrrolidone (NMP) solvent, and then mixing with a homo-disperse at 2500 rpm for 90 minutes.

[0165] In the above-mentioned positive electrode slurry, lithium iron phosphate, conductive material, fluorine-based binder, and primary hydrogenated nitrile butadiene rubber were present in a weight ratio of 95.24:1.2:2.7:0.86, and the solid content of the above-mentioned positive electrode slurry was 62% by weight.

[0166] (2) Manufacturing of the positive electrode A 20 μm thick aluminum thin film is coated with the above cathode slurry at a rate of 600 mg / 25 cm². 2After coating the material in this manner, the cathode slurry was hot-air dried at 130°C for 5 minutes so that its solid content was 99.0% by weight or more. Subsequently, the dried cathode slurry was rolled to produce a cathode so that the porosity of the cathode active material layer was 29%.

[0167] <Comparative Example 2: Manufacturing of the positive electrode> (1) Manufacturing of positive electrode slurry A dispersion was prepared containing carbon nanotubes (CNTs) as a conductive material and first hydrogenated nitrile butadiene rubber (first HNBR) with a weight-average molecular weight (Mw) of 30,000 g / mol as a rubber-based binder.

[0168] As lithium iron phosphate, average particle size D 50 A cathode slurry was prepared by adding 1.0 μm primary particles of LiFePO4 with a single-component structure, a rubber-based binder consisting of dihydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) of 310,000 g / mol, and the above dispersion to N-methylpyrrolidone (NMP) solvent, and then mixing with a homo-disperse at 2500 rpm for 90 minutes.

[0169] In the above positive electrode slurry, lithium iron phosphate, conductive material, first hydrogenated nitrile butadiene rubber, and second hydrogenated nitrile butadiene rubber were present in a weight ratio of 97.44:1.2:0.36:1.0, and the solid content of the above positive electrode slurry was 64% by weight.

[0170] (2) Manufacturing of the positive electrode A 20 μm thick aluminum thin film is coated with the above cathode slurry at a rate of 600 mg / 25 cm². 2 After coating the material in this manner, the cathode slurry was hot-air dried at 130°C for 5 minutes so that its solid content was 99.0% by weight or more. Subsequently, the dried cathode slurry was rolled to produce a cathode so that the porosity of the cathode active material layer was 29%.

[0171] <Comparative Examples 3-4: Manufacturing of the positive electrode> The positive electrode was manufactured in the same manner as in Example 1, except that the weight ratios of lithium iron phosphate, conductive material, fluorine-based binder, first hydrogenated nitrile butadiene rubber, and second hydrogenated nitrile butadiene rubber in the positive electrode slurry were changed as shown in Table 1.

[0172] <Comparative Example 5: Manufacturing of the positive electrode> The positive electrode was manufactured in the same manner as in Comparative Example 1, except that the weight ratios of lithium iron phosphate, conductive material, fluorine-based binder, and first hydrogenated nitrile butadiene rubber in the positive electrode slurry were changed as shown in Table 1.

[0173] <Comparative Examples 6-8: Manufacturing of the positive electrode> The cathode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) with a weight-average molecular weight (Mw) of 1,000,000 g / mol, the second hydrogenated nitrile butadiene rubber was changed to hydrogenated nitrile butadiene rubber with a weight-average molecular weight (Mw) of 220,000 g / mol, and the weight ratios of lithium iron phosphate, conductive material, fluorine-based binder, first hydrogenated nitrile butadiene rubber, and second hydrogenated nitrile butadiene rubber in the cathode slurry were changed as shown in Table 1.

[0174] <Comparative Example 9: Manufacturing of the positive electrode> The cathode was manufactured using the same method as in Comparative Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) with a weight-average molecular weight (Mw) of 530,000 g / mol.

[0175] <Comparative Example 10: Manufacturing of the positive electrode> The cathode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) with a weight-average molecular weight (Mw) of 1,000,000 g / mol, and the weight ratios of lithium iron phosphate, conductive material, fluorine-based binder, first hydrogenated nitrile butadiene rubber, and second hydrogenated nitrile butadiene rubber in the cathode slurry were changed as shown in Table 1.

[0176] <Experimental Example 1: Viscosity Measurement of Cathode Slurry> The viscosity of the positive electrode slurries produced in Examples 1-11 and Comparative Examples 1-10 was measured, and the results are shown in Table 2.

[0177] Specifically, the cathode slurries prepared in Examples 1-11 and Comparative Examples 1-10 were cooled for 1 hour at room temperature and 1% relative humidity. The viscosity of the cathode slurry composition was then measured using a Brookfield viscometer at 25°C with a shear rate of 2.5 / s. Viscosity measurements were performed within 2 hours of the preparation of the cathode slurry composition, including the cooling time.

[0178] <Experiment Example 2: Adhesion Test of the Positive Electrode> The positive electrodes manufactured in Examples 1-11 and Comparative Examples 1-10 were vacuum-dried at 130°C for 2 hours, and the adhesive strength between the positive electrode active material layer and the positive electrode current collector was measured. The results are shown in Table 2.

[0179] Specifically, the positive electrodes manufactured in Examples 1-11 and Comparative Examples 1-10 were cut to a length of 150 mm and a width of 20 mm, and the surface of the positive electrode was attached to a glass slide measuring 75 mm in length and 25 mm in width using double-sided tape in the longitudinal direction. In other words, the glass slide was attached to a region corresponding to half of the longitudinal direction of the positive electrode. Then, a roller was rotated 10 times to ensure that the double-sided tape was uniformly attached, and evaluation samples were manufactured.

[0180] Next, the slide glass portion of the evaluation sample was fixed to the sample stage of a Universal Testing Machine (UTM) (product name: LS5, manufacturer: LLOYD), and the positive electrode half, to which the slide glass was not attached, was connected to a load cell equipped with the UTM. The load cell was moved at a speed of 100 mm / min with a 90° force applied, and the load applied to the load cell was measured while moving it up to 50 mm. At this time, the average value of the load measured in the 20 mm to 40 mm section of the travel distance was calculated, and this was repeated a total of 5 times. The average value was then evaluated as the positive electrode adhesion strength (gf / 20 mm) of each sample.

[0181] <Experiment Example 3: Positive electrode flexibility test> The positive electrodes produced in Examples 1-11 and Comparative Examples 1-10 were vacuum-dried at 130°C for 2 hours, and their flexibility was measured. The results are shown in Table 2.

[0182] Specifically, the positive electrodes manufactured in Examples 1-11 and Comparative Examples 1-10 were cut to 10 cm horizontally and 30 cm vertically. Measuring rods with diameters of 2.5φ, 3φ, 4φ, 5φ, 6φ, 7φ, 8φ, 9φ, and 10φ were prepared. With the positive electrode current collector surface of each cut positive electrode facing the measuring rod, each cut positive electrode was bent in half, and both ends of the positive electrode were lifted at a speed of 10 mm per minute. The lifting was continued until the force measured by the UTM reached 5 N. Measurements were taken for each diameter, and the electrode was observed with an optical microscope to see if cracks occurred. If no cracks were found, the test was continued with smaller diameters. The diameters (φ) of the measuring rods at which cracks occurred are shown in Table 2.

[0183] [Table 1A] [Table 1B]

[0184] [Table 2]

[0185] Referring to Tables 1 and 2 above, all of the positive electrode slurries in Examples 1 to 11 have a relatively high solid content of 62% by weight. However, with the exception of the positive electrode slurry in Example 3, the viscosity of the slurries was 6,500 cps to 12,500 cps, which is very suitable for coating onto the positive electrode current collector, and it was confirmed that lithium iron phosphate and the conductive material do not aggregate linearly.

[0186] Furthermore, the positive electrodes according to Examples 1 to 11 were shown to have excellent flexibility of 4φ or less while exhibiting an adhesive strength of 20gf / 20mm or more. Therefore, it was expected that the desorption phenomenon of the positive electrode active material layer would be reduced during the manufacturing process of the positive electrode.

[0187] The positive electrodes of Comparative Examples 1, 5, and 7-9, which do not contain hydrogenated nitrile butadiene rubber, exceeded 10 during flexibility evaluation, confirming that they have poorer flexibility compared to the positive electrodes of Examples 1-11.

[0188] The positive electrode in Comparative Example 2, which did not contain a fluorine-based binder, was shown to have the worst adhesive strength. Furthermore, despite containing dihydrogenated nitrile butadiene rubber, it was shown to have inferior flexibility compared to the positive electrode in the example.

[0189] The positive electrodes of Comparative Examples 3, 4, and 6 contained a relatively higher amount of hydroxynitrile butadiene rubber and a relatively lower amount of fluorine-based binder compared to the positive electrodes of Examples 1 to 11. This resulted in excessively high viscosity of the positive electrode slurry, reduced coating stability, and difficulty in mass production. Furthermore, these positive electrodes exhibited lower adhesive strength compared to the positive electrodes of the examples. From these results, it can be confirmed that, from the perspective of viscosity characteristics of the positive electrode slurry and adhesive strength of the positive electrode, it is preferable to control the weight ratio of hydroxynitrile butadiene rubber to the total weight of fluorine-based binder and hydroxynitrile butadiene rubber within a suitable range, specifically 40% or less, and more specifically 33% or less.

[0190] The positive electrode of Comparative Example 10, lacking the first hydrogenated nitrile butadiene rubber, did not contain lithium iron phosphate and the conductive material, resulting in a viscosity so high that viscosity measurement was impossible. Consequently, the positive electrode slurry could not be coated onto the current collector, and the positive electrode could not be manufactured.

[0191] <Experimental Example 4: Measurement of Resistance of the Positive Electrode> The resistance values ​​of the positive electrodes manufactured in Examples 1 to 4 above were measured and compared.

[0192] Specifically, the Multi-Probe electrode resistance was measured for the positive electrodes manufactured in each of the above Examples 1 to 4.

[0193] The above resistances were set to the resistance of the positive electrode active material layer and the interfacial contact resistance between the positive electrode active material layer and the current collector, and the resistance was calculated from the potential difference measured between each probe.

[0194] The measurement conditions are as follows: -Current: 100μA - Speed: Slow - Voltage range: 0.5V - Current collector resistivity: Al2.82E used above -06 Ω·cm -The results are shown in Table 3 below.

[0195] [Table 3]

[0196] Referring to Tables 1 to 3, it was shown that the positive electrode according to Example 4, despite containing less binder than the positive electrode according to Example 2, had superior adhesive strength and, due to the reduced binder content, also exhibited superior resistance characteristics compared to the positive electrode according to Example 2.

[0197] The positive electrode according to Example 3, despite containing less binder than the positive electrode according to Example 1, exhibited superior adhesive strength and, due to the reduced binder content, superior resistance characteristics compared to the positive electrode according to Example 1.

[0198] Therefore, in the present invention, it is analyzed that the weight-average molecular weight of the fluorine-based binder is preferably 800,000 g / mol or more from the perspective of adhesive strength and electrical properties.

Claims

1. A positive electrode in which a positive electrode active material layer is arranged on one or both sides of the current collector, The positive electrode active material layer comprises lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material. The rubber binder comprises a first hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol or more and 100,000 g / mol or less, and a second hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 150,000 g / mol or more. The fluorine-based binder is contained in the positive electrode active material layer in an amount of 1.0% by weight or more and 4.0% by weight or less. The first hydrogenated nitrile butadiene rubber is included in an amount of 0.1% by weight or more and 0.6% by weight or less, based on the total weight of the positive electrode active material layer. The positive electrode is provided with the second hydrogenated nitrile butadiene rubber in an amount of 0.2% by weight or more and 0.8% by weight or less, based on the total weight of the positive electrode active material layer.

2. The positive electrode active material layer contains 450 mg / 25 cm². ２ Above, 700mg / 25cm ２ The positive electrode according to claim 1, having a loading amount within the following range.

3. The positive electrode according to claim 1, wherein the weight-average molecular weight (Mw) of the second hydrogenated nitrile butadiene rubber is 150,000 g / mol or more and 1,000,000 g / mol or less.

4. The positive electrode according to claim 1, wherein the first hydrogenated nitrile butadiene rubber is contained in an amount of 0.2% by weight or more and 0.5% by weight or less based on the total weight of the positive electrode active material layer.

5. The positive electrode according to claim 1, wherein the lithium iron phosphate is contained in an amount of 92.7% or more and 98.4% or less by weight, based on the total weight of the positive electrode active material layer.

6. The positive electrode according to claim 1, wherein the fluorine-based binder is contained in the positive electrode active material layer in an amount of 1.6% by weight or more and 3.0% by weight or less.

7. The positive electrode according to claim 1, wherein the rubber-based binder is contained in the positive electrode active material layer in an amount of 0.5% by weight or more and 1.5% by weight or less.

8. The positive electrode according to claim 1, wherein the second hydrogenated nitrile butadiene rubber is contained in the positive electrode active material layer in an amount of 0.3% by weight or more and 0.7% by weight or less.

9. The positive electrode according to claim 1, wherein the second hydrogenated nitrile butadiene rubber is 33% by weight or less of the total weight of the fluorine-based binder and the second hydrogenated nitrile butadiene rubber.

10. The positive electrode according to claim 1, wherein the lithium iron phosphate 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 comprises 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 comprises 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.)

11. The positive electrode according to claim 1, wherein the positive electrode adhesive strength measured in an adhesion strength test in which the positive electrode active material layer is peeled off from an aluminum thin film at a 90° angle is 19 gf / 20 mm or more.

12. The positive electrode according to claim 1, wherein, during a flexibility test in which a measuring rod of diameter 5φ or less is brought into contact with the positive electrode active material layer and the cross-section of the positive electrode is lifted, a crack occurs with a measuring rod of diameter 5φ or less.

13. The positive electrode according to claim 1, wherein the weight-average molecular weight (Mw) of the fluorine-based binder is 800,000 g / mol or more.

14. The positive electrode for a lithium secondary battery according to claim 1, wherein the conductive material is a carbon nanotube.

15. The positive electrode according to claim 12, wherein the conductive material is included in an amount of 0.3% by weight or more and 2.0% by weight or less based on the total weight of the positive electrode active material layer.

16. The positive electrode according to any one of claims 1 to 15, wherein the total weight of the binder contained in the positive electrode active material layer is 2.0% by weight or more and 4.5% by weight or less, based on the total weight of the positive electrode active material.

17. The average particle size D of lithium iron phosphate ５０ The positive electrode according to claim 1, wherein the particle size is 0.3 μm or more and 20.0 μm or less.

18. The positive electrode according to claim 1, wherein the lithium iron phosphate has a single-component structure consisting of primary particles.

19. It comprises a positive electrode, a negative electrode, a separation membrane, and an electrolyte. The aforementioned positive electrode is, A positive electrode active material layer is arranged on one or both sides of the current collector. The positive electrode active material layer comprises lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material. The rubber binder comprises a first hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol or more and 100,000 g / mol or less, and a second hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 150,000 g / mol or more. The fluorine-based binder is contained in the positive electrode active material layer in an amount of 1.0% by weight or more and 4.0% by weight or less. The first hydrogenated nitrile butadiene rubber is included in an amount of 0.1% by weight or more and 0.6% by weight or less, based on the total weight of the positive electrode active material layer. A lithium secondary battery in which the second hydrogenated nitrile butadiene rubber is contained in an amount of 0.2% to 0.8% by weight based on the total weight of the positive electrode active material layer.

20. A cathode slurry comprising lithium iron phosphate, a fluorine-based binder, a rubber-based binder, a conductive material, and a solvent, The rubber binder comprises a first hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol or more and 100,000 g / mol or less, and a second hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 150,000 g / mol or more. The fluorine-based binder is included in an amount of 1.0% by weight or more and 4.0% by weight or less, based on the total weight of solids in the positive electrode slurry. The first hydrogenated nitrile butadiene rubber is included in an amount of 0.1% by weight or more and 0.6% by weight or less, based on the total weight of solids in the positive electrode slurry. The positive electrode slurry contains the second hydrogenated nitrile butadiene rubber in an amount of 0.2% to 0.8% by weight, based on the total weight of solids in the positive electrode slurry.

21. The cathode slurry according to claim 20, wherein the weight-average molecular weight (Mw) of the second hydrogenated nitrile butadiene rubber is 150,000 g / mol or more and 1,000,000 g / mol or less.

22. The cathode slurry according to claim 20, wherein the weight-average molecular weight (Mw) of the fluorine-based binder is 800,000 g / mol or more.

23. The positive electrode slurry according to claim 20, wherein the solid content of the positive electrode slurry is in the range of 50% by weight or more and 75% by weight or less.

24. The cathode slurry according to claim 20, wherein the viscosity of the cathode slurry measured at 25°C and a shear rate of 2.5 / s is 5,000 cps or more and 25,000 cps or less.

25. The positive electrode slurry according to claim 20, wherein the fluorine-based binder is contained in an amount of 1.6% by weight or more and 3.0% by weight or less, based on the total weight of solids in the positive electrode slurry.

26. Based on the total weight of solids in the positive electrode slurry, The lithium iron phosphate is 92.7% by weight or more and 98.4% by weight or less. The cathode slurry according to claim 20, wherein the first hydrogenated nitrile butadiene rubber is contained in an amount of 0.1% by weight or more and 0.6% by weight or less.

27. The cathode slurry according to claim 20, wherein the second hydrogenated nitrile butadiene rubber is 33% by weight or less of the total weight of the fluorine-based binder and the second hydrogenated nitrile butadiene rubber.

28. The positive electrode slurry according to claim 20, wherein the rubber-based binder is contained in an amount of 0.5% by weight or more and 1.5% by weight or less, based on the total weight of solids in the positive electrode slurry.

29. The cathode slurry according to claim 20, wherein the lithium iron phosphate 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 comprises 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 comprises 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.)

30. The cathode slurry according to claim 20, wherein the conductive material is a carbon nanotube.

31. The positive electrode slurry according to claim 30, wherein the conductive material is contained in an amount of 0.3% by weight or more and 2.0% by weight or less, based on the total weight of solids in the positive electrode slurry.

32. The positive electrode slurry according to any one of claims 20 to 31, wherein the sum of the fluorine-based binder and the rubber-based binder is 2.0% by weight or more and 4.5% by weight or less, based on the total weight of solids in the positive electrode slurry.

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