High loading positive electrodes, positive electrode 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 the benefit of priority from Korean Patent Applications Nos. 10-2022-0139395, 10-2022-0139469, 10-2022-0139490, and 10-2022-0139418, filed on October 26, 2022.

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

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

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

[0005] Lithium iron phosphate is low-cost because it contains iron, which is an abundant and low-cost material. In addition, lithium iron phosphate has low toxicity, so its use can reduce environmental pollution. Furthermore, because lithium iron phosphate has an olivine structure, the active material structure can be stably maintained at high temperatures compared to lithium transition metal oxides with a layered structure. This provides the battery with excellent high-temperature stability and high-temperature life characteristics.

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

[0007] However, as the size of lithium iron phosphate particles decreases, the specific surface area increases, and the lithium iron phosphate with a carbon-coated surface has reduced wettability with a solvent. As a result, lithium iron phosphate particles are severely aggregated, the stability and coating processability of the positive electrode slurry are reduced, and the lithium iron phosphate and the binder are not effectively mixed, resulting in a decrease in the adhesive strength between the positive electrode current collector and the positive electrode active material layer in the manufactured positive electrode (hereinafter, positive electrode adhesive strength). This phenomenon can be exacerbated in high-loading positive electrodes.

[0008] If the adhesive strength of the positive electrode is reduced, the positive electrode active material layer may be detached during the manufacture of the electrode or during charging and discharging, which may lead to an increase in battery resistance and a decrease in the capacity of the secondary battery.

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

[0010] As the demand for high energy density batteries increases, lithium iron phosphate-containing cathodes with a density of 600 mg / 25 cm 2 There is a demand for a technology that can improve the adhesion and flexibility of the positive electrode while still ensuring the above loading amount. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Korean Patent Publication No. 10-2020-0008066 Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention aims to provide a positive electrode and a lithium secondary battery in which electrode detachment is prevented, battery resistance is reduced, and battery capacity is improved by improving the adhesive strength and flexibility between a positive electrode current collector and a 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 comprising a positive electrode active material layer disposed on one or both sides of a current collector, the positive electrode active material layer contains 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, 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.

[0014] In the positive electrode according to one embodiment, the positive electrode active material layer has a density of 450 mg / 25 cm 2 ~700mg / 25cm 2 The loading amounts range from 0.01 to 0.01.

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

[0016] In the positive electrode according to one embodiment, the first hydrogenated nitrile butadiene rubber is contained 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 the positive electrode according to one embodiment, the lithium iron phosphate is contained in an amount of 92.7% by weight to 98.4% by weight based on the total weight of the positive electrode active material layer.

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

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

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

[0021] In the positive electrode according to one embodiment, the second hydrogenated nitrile butadiene rubber is 33% by weight or less based on 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 includes any one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, X includes any 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.)

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

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

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

[0028] In one embodiment of the positive electrode, 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% by weight to 4.5% by weight based on the total weight of the positive electrode active material.

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

[0032] In one embodiment of the positive electrode, the lithium iron phosphate has a monolithic structure made of primary particles.

[0033] According to another embodiment of the present invention, there is provided a lithium secondary battery. The lithium secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode has a current collector with a thickness of 450 mg / 25 cm on one or both sides. 2 a positive electrode active material layer having the above loading amount is disposed, the positive electrode active material layer including 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, 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.

[0034] According to another embodiment of the present invention, there is provided a positive electrode slurry, the positive electrode slurry comprising lithium iron phosphate, a fluorine-based binder, a rubber-based binder, a conductive material and a solvent, the rubber-based binder comprising 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% by weight to 0.8% by weight based on the total weight of the solid content in the positive electrode slurry.

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

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

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

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

[0039] In the positive electrode slurry according to one embodiment, the fluorine-based binder is included in an amount of 1.0 wt % to 4.0 wt % based on the total weight of the solid content in the positive electrode slurry.

[0040] In the positive electrode slurry according to one embodiment, the first hydrogenated nitrile butadiene rubber is contained in an amount of 0.1 to 0.6% by weight based on the total weight of the solid content in the positive electrode slurry.

[0041] In the positive electrode slurry according to one embodiment, the lithium iron phosphate is contained in an amount of 92.7 wt % to 98.4 wt % based on the total weight of the solid content in the positive electrode slurry.

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

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

[0044] In the positive electrode 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 includes any one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, X includes any 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 the positive electrode slurry according to one embodiment, the conductive material is carbon nanotubes.

[0048] In the positive electrode slurry 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 solid content in the positive electrode slurry.

[0049] The total amount of the fluorine-based binder and the rubber-based binder is 2.0% by weight to 4.5% by weight based on the total weight of the solid content in the positive electrode slurry. Effect of the Invention

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

[0051] According to the present invention, the contents of the fluorine-based binder, the first hydrogenated nitrile butadiene rubber, and the second hydrogenated nitrile butadiene rubber are controlled within a predetermined range, so that excellent coating stability of the positive electrode slurry and excellent flexibility of the positive electrode can be secured at the same time.

[0052] The positive electrode according to the present invention has excellent adhesiveness and flexibility even when the content of the binder contained in the positive electrode active material layer is reduced due to the improved adhesive strength, and the content of the positive electrode active material can be increased as the binder content is reduced, resulting in excellent energy density of the secondary battery.

[0053] According to an embodiment of the present invention, a fluorine-based binder and a dihydrogenated nitrile butadiene rubber are contained in a suitable weight ratio, so that it is possible to prepare a positive electrode slurry that is easy to mix and has 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 the high solid content, and in particular, it is possible to manufacture a positive electrode having excellent electrical conductivity by suppressing the aggregation phenomenon of the positive electrode active material and conductive material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] The advantages and features of the present invention, and the methods for achieving them, will become clear from the detailed description of the embodiments below in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms, and the present embodiments are provided solely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the present invention of the scope of the invention, and the present invention is only defined by the scope of the claims. The same reference numerals refer to the same elements throughout the specification.

[0056] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used as commonly understood by those having ordinary skill in the art to which the present invention belongs. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless otherwise clearly defined.

[0057] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular form includes the plural form unless otherwise specified in the phrase. The words "comprises" and / or "comprising" used in the specification do not exclude the presence or addition of one or more other components other than the components mentioned.

[0058] In this specification, when a part includes a certain component, this does not mean that it excludes other components, and it means that it may further include other components, unless specifically stated to the contrary.

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

[0060] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0061] In this specification, D 50 D means the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve. 50 can be measured, for example, by using a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0062] In this specification, the "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 manufactured by BEL Japan.

[0063] In the present specification, the term "weight average molecular weight (Mw)" refers to a value converted to standard polystyrene measured by gel permeation chromatography (GPC). Specifically, the weight average molecular weight is a value converted from a value measured under the following conditions using GPC, and standard polystyrene from an Agilent system was used to create a calibration curve.

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

[0065] The positive electrode adhesive strength in this specification can 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 glass slide of a length of 75 mm and a width of 25 mm, and the positive electrode is attached to the glass slide in the longitudinal direction using double-sided tape. That is, the glass slide is attached to an area corresponding to half of the positive electrode in the longitudinal direction. Then, a roller is rolled 10 times so that the double-sided tape is evenly attached to prepare an evaluation sample. Next, the glass slide 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 to which the glass slide is not attached is connected to a load cell of the UTM equipment. The load cell is applied with a force of 90° at a speed of 100 mm / min and moved to 50 mm to measure the load applied to the load cell. At this time, the average load measured in a 20 mm to 40 mm section of the running section is calculated, and this is repeated five times in total, and the average value is evaluated as the positive electrode adhesive strength (gf / 20 mm) of each sample.

[0066] The flexibility of the positive electrode in this specification can be measured by the following method. A measuring rod with a diameter of 600 mg / 25 cm 2 A positive electrode with a loading of 10 ...

[0067] The present invention will be specifically described below.

[0068] <Positive electrode> The positive electrode according to the embodiment of the present invention is a positive electrode having a positive electrode active material layer disposed on one or both sides of a current collector, the positive electrode active material layer contains 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, 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.

[0069] Lithium iron phosphate has a lower lithium mobility and lower electrical conductivity than lithium transition metal oxides such as lithium nickel cobalt manganese oxide, so lithium iron phosphate with a small average particle size is mainly used as a positive electrode active material. However, when the size of the 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 effectively mixed, resulting in a decrease in the positive electrode adhesive strength. As a result, there is a problem that the positive electrode active material layer is detached during electrode manufacturing or charging and discharging, increasing the battery resistance and reducing the capacity of the secondary battery.

[0070] The present inventors have conducted extensive research to solve such problems, and as a result, have found that when a rubber binder contains 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 dispersibility of lithium iron phosphate particles is improved to suppress their aggregation, while still achieving a particle size distribution of 450 mg / 25 cm. 2 ~700mg / 25cm 2The inventors have discovered that the adhesive strength and flexibility of a high-loading positive electrode having a high-loading level can be greatly improved, and have completed 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 for improving adhesion between the positive electrode active material layer and the positive electrode current collector. That is, the positive electrode according to one embodiment of the present invention may 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 for improving adhesion.

[0072] A positive electrode according to an 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 cm 2 More than 450mg / 25cm 2 ~700mg / 25cm 2 For details, see 500mg / 25cm 2 ~700mg / 25cm 2 , more specifically 550mg / 25cm 2 ~650mg / 25cm 2 Here, the loading amount is a loading amount of a positive electrode active material layer laminated on one surface of a current collector.

[0074] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the current collector may be stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like.

[0075] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and may have fine irregularities on its surface to enhance adhesion to the positive electrode active material layer. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0076] The positive electrode active material layer may include a positive electrode active material. Additionally, the positive electrode active material layer may further include a conductive material, a binder, and a dispersant, if necessary, in addition to the positive electrode active material.

[0077] Hereinafter, each component contained in the positive electrode active material layer will be specifically described.

[0078] (1) Positive electrode active material The present invention includes lithium iron phosphate as a positive electrode active material. Since lithium iron phosphate has an olivine structure, the active material structure is more stable at high temperatures than lithium transition metal oxides having a layered structure. As a result, when lithium iron phosphate is used as a positive electrode active material, the high temperature stability and high temperature life characteristics of the positive electrode are significantly improved, thereby reducing the risk of fire in a lithium secondary battery including the positive electrode.

[0079] The lithium iron phosphate may be a compound represented by the following 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 includes any one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, X includes any 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 may be LiFePO4.

[0082] The lithium iron phosphate may be in the form of secondary particles formed by agglomeration of primary particles, and preferably has a monolithic structure consisting of primary particles.

[0083] In the present invention, the term "monolith structure" refers to a structure in which particles are not aggregated with each other in terms of morphology and exist in independent phases. A particle structure that is contrasted with this monolith structure is a structure in which small particles ("primary particles") are physically and / or chemically aggregated to form relatively large particles ("secondary particles").

[0084] When lithium iron phosphate has a monolithic structure made of primary particles, the possibility of the lithium iron phosphate particles cracking during rolling is small compared to when it is made of secondary particles, which is preferable because capacity loss due to detachment of the cracked particles is small. Also, when lithium iron phosphate is made of monolithic primary particles, it can be said to be preferable in terms of interfacial adhesion between the positive electrode current collector and the positive electrode active material layer because it can mitigate the migration of the binder during the drying process of the positive electrode slurry.

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

[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, furfuryl alcohol polymers, ethylene and ethylene oxide block copolymers, vinyl resins, cellulose resins, phenol resins, pitch resins, and tar resins. Specifically, the carbon coating layer may be formed by mixing the raw material with the lithium iron phosphate and then heat treating the mixture.

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

[0088] The BET specific surface area of ​​lithium iron phosphate is 5m 2 / g~20m 2 / g, specifically 7m 2 / g~18m 2 / g, more specifically 9m 2 / g~16m 2 / g. This range corresponds to a lower value compared to typical lithium iron phosphate. When this range is satisfied, aggregation of the lithium iron phosphate can be effectively suppressed even in a positive electrode slurry composition having a relatively low content of a dispersant.

[0089] In one embodiment of the present invention, the lithium iron phosphate has an average particle size D 50 In such a case, the first lithium iron phosphate particles may have an average particle size D in the range of 0.5 μm to 2 μm. 50 The divalent lithium iron phosphate particles may have an average particle size D in the range of 3.5 μm to 15 μm. 50 may have:

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

[0091] Average particle size D 50The second lithium iron phosphate particles having a particle size in the range of 3.5 μm to 15 μm have a larger average particle size than the first lithium iron phosphate particles, and a smaller specific surface area than the first lithium iron phosphate particles. Therefore, when the second lithium iron phosphate particles are contained in a predetermined range, the total specific surface area of ​​the lithium iron phosphate particles is reduced, and the PVDF-based binder is attached to the current collector instead of the lithium iron phosphate to the extent that the total specific surface area of ​​the lithium iron phosphate particles is reduced, thereby improving the interfacial adhesion between the current collector and the positive electrode active material layer.

[0092] The average particle diameter D of the above-mentioned first lithium iron phosphate 50 The average particle diameter D of the lithium iron phosphate dibasic may 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. 50 The thickness may 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 the first lithium iron phosphate and the second lithium iron phosphate may be 80:20 to 99.9:0.1, preferably 85:15 to 99:1, and more preferably 90:1 to 95:5. When the weight ratio of the first lithium iron phosphate and the second lithium iron phosphate satisfies the above range, the adhesive strength is excellent. When an excessive amount of the second lithium iron phosphate having a relatively large average particle size is contained, the viscosity of the positive electrode slurry may increase rapidly and the rolling performance may decrease, which is not preferable.

[0094] The lithium iron phosphate may be a primary particle, and the lithium iron phosphate may be a primary particle or a secondary particle.

[0095] The lithium iron phosphate may be contained in an amount of 92.7% by weight to 98.4% by weight, specifically 93.5% by weight to 98% by weight, more specifically 94% by weight to 97% by weight, based on the total solid content of the positive electrode slurry composition. When the content of the lithium iron phosphate satisfies the above range, sufficient positive electrode energy density is ensured, thereby improving the battery capacity of the positive electrode.

[0096] (2) Binder The present invention includes both a fluorine-based binder and a rubber-based binder as the binder, and 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 fluorine-based binder includes a PVDF-based polymer containing vinylidene fluoride (VDF) as a monomer. Specific examples of the PVDF-based polymer include a PVDF single polymer, 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, imparts adhesive strength between the positive electrode active material and the conductive material, and adhesive strength 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, specifically 550,000 g / mol to 2,000,000 g / mol, more specifically 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 excellent, and the adhesive strength of the positive electrode is excellent. In particular, from the aspect of the adhesive 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, more preferably 950,000 g / mol to 1,200,000 g / mol.

[0100] The fluorine-based binder may be included in the positive electrode active material layer in an amount of 1.0 wt % to 4.0 wt %, preferably 1.4 wt % to 3.5 wt %, and more preferably 1.6 wt % to 3.0 wt %, 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 against electrolytes. However, due to their crystal structure, if the amount of fluorine-based binder is excessive, it can have a negative effect on the flexibility of the positive electrode. On the other hand, the higher the binder content, the higher the adhesive strength of the positive electrode, so improving the adhesive strength and flexibility of the positive electrode is a difficult task to achieve by simply adjusting the binder content. In addition, when the loading amount of the positive electrode active material layer is 600 mg / 25 cm, 2 The high-loading positive electrode described above tends to have inferior flexibility compared to a low-loading positive electrode having the same binder content and a smaller loading amount, so improving the flexibility as well as the adhesive strength in a high-loading positive electrode has been a difficult technical challenge.

[0102] Therefore, the positive electrode according to the present invention contains a rubber-based binder in addition to the fluorine-based binder, and the rubber-based binder contains 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 first hydrogenated nitrile butadiene rubber not only improves the adhesive strength and flexibility of the positive electrode, but also improves the dispersibility of the positive electrode slurry to suppress the aggregation of the conductive material and the positive electrode active material. The inventors of the present invention have discovered that when the positive electrode contains only the first hydrogenated nitrile butadiene rubber as a rubber-based binder, the adhesive strength and flexibility of the positive electrode cannot be further improved at a certain content or more, but when the positive electrode contains the second hydrogenated nitrile butadiene rubber together with the first hydrogenated nitrile butadiene rubber, the flexibility and adhesive strength of the positive electrode are dramatically increased, leading to the present invention. As a result, the positive electrode according to the present invention is excellent in both flexibility and adhesive strength, and when the loading amount of the positive electrode increases, the improved flexibility provides excellent rolling performance. In addition, the improved flexibility contributes to preventing the positive electrode active material layer from being detached when a physical impact is applied to the outside of the positive electrode.

[0104] The first hydrogenated nitrile butadiene rubber and the second hydrogenated nitrile butadiene rubber (HNBR) are made by hydrogenating each nitrile butadiene rubber (NBR) so that the double bonds originally contained in the nitrile butadiene rubber (NBR) are converted into single bonds.

[0105] The hydrogenated nitrile butadiene rubber may have a repeating unit content derived from acrylonitrile (AN) of 20% by weight to 50% by weight, more preferably 25% by weight to 45% by weight, and most preferably 30% by weight to 40% by weight, based on the total weight.

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

[0107] In addition, the first hydrogenated nitrile butadiene rubber inhibits the agglomeration of the conductive material, improves the positive electrode conductive network, and even if the conductive material is agglomerated, the conductive material is agglomerated in a spherical shape rather than a linear shape, thereby minimizing the specific surface area of ​​the agglomerated conductive material compared to when the conductive material is agglomerated in a linear shape. As a result, the surface area of ​​the positive electrode active material adjacent to the agglomerated conductive material that cannot participate in the lithium insertion / extraction reaction is minimized, thereby reducing the discharge resistance of the lithium secondary battery.

[0108] The weight average molecular weight (Mw) of the second hydrogenated nitrile butadiene rubber is 150,000 g / mol or more, preferably 150,000 g / mol to 1,000,000 g / mol, more preferably 200,000 g / mol to 500,000 g / mol. When the weight average molecular weight (Mw) of the second hydrogenated nitrile butadiene rubber is within the above range, the adhesive strength and flexibility of the positive electrode active material layer can be improved, and the rolling density of the positive electrode can be increased, and the higher the rolling density, the lower the positive electrode resistance can be obtained.

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

[0110] The second hydrogenated nitrile butadiene rubber may be contained in the positive electrode active material layer in an amount of 0.2 wt% to 0.8 wt%, preferably 0.3 wt% to 0.7 wt%, more preferably 0.35 wt% to 0.65 wt%. If the content of the second hydrogenated nitrile butadiene rubber exceeds 0.8 wt%, the viscosity of the slurry is significantly increased, which is undesirable since it may significantly reduce the mixing performance and coating performance during the preparation of the slurry, and if the content of the second hydrogenated nitrile butadiene rubber is less than 0.2 wt%, the improvement effect on the adhesive performance and flexibility of the prepared positive electrode may be slight, which is undesirable.

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

[0112] The second hydrogenated nitrile butadiene rubber may be in the range of 40% by weight or less, preferably 14 to 33% by weight, more preferably 15 to 30% by weight, based on the total weight of the fluorine-based binder and the second hydrogenated nitrile butadiene rubber. When the second hydrogenated nitrile butadiene rubber is contained in the above content in relation to the fluorine-based binder, it is possible to prevent a sudden increase in the viscosity of the slurry and a decrease in coating stability, while improving the flexibility of the positive electrode.

[0113] The fluorine-based binder and the rubber-based binder may be contained in the positive electrode active material layer in a total amount of 2.0% by weight to 4.5% by weight, preferably 2.2% by weight to 4.0% by weight, and most preferably 2.5% by weight to 3.8% by weight. When the binder content satisfies the above range, the contact area between the binder and lithium iron phosphate becomes large, and excellent positive electrode adhesive strength can be ensured.

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

[0115] The conductive material is not particularly limited as long as it is conductive without inducing a chemical change in the battery, and may be, for example, graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. Specific examples of commercially available conductive materials include acetylene black series products of Chevron Chemical Company, Denka Black (Denka Singapore Private Limited), Gulf Oil Company (Gulf Oil Company products, etc.), Ketjenblack, EC series (Armak Company products), Vulcan XC-72 (Cabot Company products), and Super P (Timcal product). Preferably, the conductive material may be carbon nanotubes. A conductive network of carbon nanotubes can alleviate the binder lift-off phenomenon during the drying process of the positive electrode slurry, and is therefore preferable as the conductive material contained in the positive electrode of the present invention.

[0116] The conductive material may be contained in the positive electrode active material layer at 0.3 wt% to 2.0 wt%, specifically 0.6 wt% to 1.5 wt%, more specifically 0.8 wt% to 1.3 wt%. When the content of the conductive material in the positive electrode active material layer satisfies the above range, the positive electrode conductive network is ensured, thereby improving the electrical conductivity of the positive electrode.

[0117] According to one embodiment of the present invention, the positive electrode active material layer may contain 92.7 wt% to 98.4 wt% of lithium iron phosphate, 1.0 wt% to 4.0 wt% of the fluorine-based binder, 0.1 to 0.6 wt% of the first hydrogenated nitrile butadiene rubber, 0.2 wt% to 0.8 wt% of the second hydrogenated nitrile butadiene rubber, and 0.3 wt% to 2.0 wt% of the conductive material, based on the total weight of the positive electrode active material layer. When the composition in the positive electrode active material layer satisfies the above range, the adhesive force and conductivity of the electrode are ensured, and at the same time, the content of the active material is increased, so that the capacity and resistance performance of the lithium secondary battery can be improved, and the flexibility of the positive electrode can be significantly 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 preparing a positive electrode slurry composition containing the above-mentioned positive electrode active material, conductive material, binder and / or dispersant, applying the positive electrode slurry composition onto a positive electrode current collector, and then drying and rolling the composition.

[0119] Alternatively, the positive electrode may be prepared by casting the positive electrode slurry composition on a separate support, peeling the composition from the support, and laminating the resulting film on a 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, and has a capacitance of 550 mg / 25 cm. 2 ~650mg / 25cm 2As a result, a high-loading positive electrode can be manufactured, and the cell resistance of the secondary battery can be reduced by preventing the positive electrode from being detached, thereby improving the capacity and output characteristics of the battery and reducing defects that occur during the manufacturing process.

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

[0122] Furthermore, the positive electrode of the present invention has such flexibility that, during a flexibility test in which a φ measuring rod is brought into contact with the positive electrode active material layer and then the cross section of the positive electrode is lifted, a crack occurs with a measuring rod of 5 φ or less, specifically 2 to 4 φ.

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

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

[0125] The positive electrode in the lithium secondary battery is as described above. For example, the positive electrode has a positive electrode active material layer disposed on one or both sides of a current collector, the positive electrode active material layer contains 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, 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.

[0126] The lithium iron phosphate, the fluorine-based binder, the first hydrogenated nitrile-butadiene rubber, the second hydrogenated nitrile-butadiene rubber and the conductive material have been described in detail above, so that a duplicated description will be omitted.

[0127] The loading amount of the positive electrode active material layer is 450 mg / 25 cm 2 More than 450mg / 25cm 2 ~700mg / 25cm 2 For details, see 500mg / 25cm 2 ~700mg / 25cm 2 , more specifically 550mg / 25cm 2 ~650mg / 25cm 2 It could be.

[0128] The negative electrode can be manufactured, for example, by preparing a composition for forming a negative electrode, which contains a negative electrode active material, a negative electrode binder, and a negative electrode conductive material, on a negative electrode current collector, and then coating the composition on the negative electrode current collector.

[0129] The negative electrode active material is not particularly limited, and can generally use a compound capable of reversible intercalation and deintercalation of lithium.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 alloy, Sn alloy, or Al alloy; or a composite containing a metallic compound and a carbonaceous material. In addition, examples of low crystalline carbon include soft carbon and hard carbon, and examples of high crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature fired carbon such as petroleum or coal tar pitch derived cokes. Among these, one type alone or a mixture of two or more types may be used, and a metallic lithium thin film may also be used as the negative electrode active material.

[0130] The negative electrode conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it has electronic conductivity without causing chemical changes in the battery that is constructed. Specific examples include graphite such as natural graphite and 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 powder or metal fiber 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 or more of these may be used alone or in combination. The negative electrode conductive material may be generally contained in an amount of 1 to 30% by weight, specifically 1 to 20% by weight, and more specifically 1 to 10% by weight, based on the total weight of the negative electrode active material layer.

[0131] The negative electrode binder plays a role in improving the adhesion between the negative electrode active material particles and the adhesive strength between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used alone or in combination. The negative electrode binder may be included in an amount of 1 to 30% by weight, specifically 1 to 20% by weight, more specifically 1 to 10% by weight, based on the total weight of the negative electrode active material layer.

[0132] Meanwhile, the negative electrode current collector is not particularly limited as long as it has high conductivity without inducing a chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc. may be used.

[0133] In addition, the negative electrode current collector may typically have a thickness of 3 to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have fine irregularities on its surface to enhance the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0134] Meanwhile, in the lithium secondary battery, the separator can be any one that is generally used as a separator in a lithium secondary battery, and is particularly preferred to have low resistance to ion migration of the electrolyte and excellent electrolyte humidification ability. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting point glass fiber, polyethylene terephthalate fiber, etc. can also be used. In addition, the separator can be a porous thin film having a pore diameter of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0135] Meanwhile, in the lithium secondary battery, the electrolyte may include an organic solvent and a lithium salt that are commonly used in electrolytes, and is not particularly limited.

[0136] The organic solvent may be any organic solvent without particular limitation, so long as it can function as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; and carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

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

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

[0139] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing the decrease in the capacity of the battery, improving the discharge capacity of the battery, etc. In this case, the additives may be contained in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

[0140] The lithium secondary battery of the present invention may be manufactured by forming an electrode assembly by disposing a separator between a positive electrode and a negative electrode, and then inserting the electrode assembly into a cylindrical or prismatic battery case and then injecting an electrolyte into the battery case. Alternatively, the electrode assemblies may be stacked, impregnated with an electrolyte, and then inserted into a battery case and sealed.

[0141] When manufacturing the lithium secondary battery of the present invention, the electrode assembly may be dried to remove at least one organic solvent used in manufacturing the positive electrode, selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate. If an electrolyte having the same composition as the organic solvent used in manufacturing the positive electrode is used as the electrolyte, the process of drying the electrode assembly may be omitted.

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

[0143] The battery case may be one commonly used in the art, and the shape of the battery case is not limited depending on the application of the battery. For example, the battery case may be a cylindrical type using a can, a square type, a pouch type, a coin type, or the like.

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

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

[0146] The positive electrode slurry according to the present invention includes lithium iron phosphate, a fluorine-based binder, a rubber-based binder, a conductive material, and a solvent, 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 solids in the positive electrode slurry.

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

[0148] The slurry for positive electrode according to the present invention contains the d-hydrogenated nitrile butadiene rubber having the weight average molecular weight in the above range, and thus significantly increases the flexibility of the high loading positive electrode. The content of the fluorine-based binder and the d-hydrogenated nitrile butadiene rubber is controlled within the above range, and thus the slurry has a suitable viscosity and excellent coating stability.

[0149] The lithium iron phosphate, the first hydrogenated nitrile butadiene rubber, the second hydrogenated nitrile butadiene rubber, the fluorine-based binder and the conductive material have been described in detail above, so that a duplicate description will be omitted.

[0150] The solvent 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, water, etc., and one or more of these may be used alone or in combination.

[0151] The solvent may be included in an amount that allows the slurry for the positive electrode to have a suitable viscosity and solid content. For example, the solvent may be included in an amount that allows the solid content in the slurry to be 50% by weight to 75% by weight, specifically 50% by weight to 70% by weight, more specifically 55% by weight to 70% by weight. This corresponds to a relatively high solid content compared to a slurry for the positive electrode that contains lithium iron phosphate as a conventional 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 the preparation of the positive electrode can be shortened, and the process cost can be reduced. In addition, the positive electrode slurry may have a viscosity at a level that allows coating, and the positive electrode active material layer formed from the composition has a thickness of a certain level or more, thereby ensuring excellent energy density.

[0152] The slurry for a positive electrode according to an embodiment of the present invention may have a composition viscosity of 5,000 cps to 25,000 cps, specifically 6,000 cps to 24,000 cps, more specifically 6,500 cps to 23,000 cps, measured at 25° C. and a shear rate of 2.5 / s. The slurry for a positive electrode having a viscosity value within the above range may have excellent storage stability and coating processability. In addition, the slurry for a positive electrode may have a higher solid content than a conventional lithium iron phosphate slurry for a positive electrode having the same viscosity, thereby shortening the time required for a slurry drying process during the preparation of a positive electrode, thereby reducing process costs.

[0153] The present invention will be described in more detail below with reference to examples. However, the following examples are merely for the purpose of illustrating the present invention, and the scope of the present invention is not limited to these examples alone.

[0154] The present invention will be described in more detail below with reference to examples. However, the following examples are merely for the purpose of illustrating the present invention, and the scope of the present invention is not limited to these examples alone.

[0155] <Example 1: Production of positive electrode> (1) Preparation of positive electrode slurry A dispersion liquid was prepared that contained 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] Average particle size D as lithium iron phosphate 50 The cathode slurry was prepared by adding LiFePO4, which is a primary particle having a single structure with a particle size of 1.0 μm, carbon nanotubes (CNT) as a conductive material, polyvinylidene fluoride (PVDF) with a weight average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, secondary hydrogenated nitrile butadiene rubber (secondary HNBR) with a weight average molecular weight (Mw) of 310,000 g / mol as a rubber-based binder, and the above dispersion to an N-methylpyrrolidone (NMP) solvent and then mixing at 2500 rpm for 90 minutes using Homo-disperse.

[0157] In the positive electrode slurry, the lithium iron phosphate, the conductive material, the fluorine-based binder, the first hydrogenated nitrile butadiene rubber, and the 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 positive electrode slurry was 62 wt %.

[0158] (2) Manufacturing of positive electrodes The above positive electrode slurry was applied at 600 mg / 25 cm on a 20 μm thick aluminum thin film. 2After coating, the positive electrode slurry was dried with hot air at 130° C. for 5 minutes so that the solid content of the positive electrode slurry was 99.0 wt % or more. Then, the dried positive electrode slurry was rolled to prepare a positive electrode so that the porosity of the positive electrode active material layer was 29%.

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

[0160] <Examples 3 to 6: Production of Positive Electrodes> A positive electrode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 1,000,000 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 positive electrode slurry was changed as shown in Table 1.

[0161] <Examples 7 to 10: Production of positive electrodes> A positive electrode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) having 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 having 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 positive electrode slurry was changed as shown in Table 1 (however, the solid content of the positive electrode slurry in Example 9 was also changed as shown in Table 1).

[0162] <Example 11: Production of positive electrode> A positive electrode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 1,000,000 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 positive electrode slurry was changed as shown in Table 1.

[0163] <Comparative Example 1: Production of Positive Electrode> (1) Preparation of positive electrode slurry A dispersion liquid was prepared that contained 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] Average particle size D as lithium iron phosphate 50 LiFePO4, which is a primary particle having a single structure with a particle size of 1.0 μm, polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, first hydrogenated nitrile butadiene rubber (first HNBR) having a weight average molecular weight (Mw) of 310,000 g / mol as a rubber-based binder, and the above dispersion were added to an N-methylpyrrolidone (NMP) solvent, and then mixed for 90 minutes at 2500 rpm using Homo-disperse to prepare a positive electrode slurry.

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

[0166] (2) Manufacturing of positive electrodes The above positive electrode slurry was applied at 600 mg / 25 cm on a 20 μm thick aluminum thin film. 2After coating, the positive electrode slurry was dried with hot air at 130° C. for 5 minutes so that the solid content of the positive electrode slurry was 99.0 wt % or more. Then, the dried positive electrode slurry was rolled to prepare a positive electrode so that the porosity of the positive electrode active material layer was 29%.

[0167] <Comparative Example 2: Production of Positive Electrode> (1) Preparation of positive electrode slurry A dispersion liquid was prepared that contained 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] Average particle size D as lithium iron phosphate 50 LiFePO4, which is a primary particle with a single structure and a particle size of 1.0 μm, 2nd hydrogenated nitrile butadiene rubber (2nd HNBR) with a weight average molecular weight (Mw) of 310,000 g / mol as a rubber binder, and the above dispersion were added to an N-methylpyrrolidone (NMP) solvent, and then mixed at 2500 rpm for 90 minutes using Homo-disperse to prepare a positive electrode slurry.

[0169] In the positive electrode slurry, the lithium iron phosphate, the conductive material, the first hydrogenated nitrile butadiene rubber, and the 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 positive electrode slurry was 64 wt %.

[0170] (2) Manufacturing of positive electrodes The above positive electrode slurry was applied at 600 mg / 25 cm on a 20 μm thick aluminum thin film. 2 After coating, the positive electrode slurry was dried with hot air at 130° C. for 5 minutes so that the solid content of the positive electrode slurry was 99.0 wt % or more. Then, the dried positive electrode slurry was rolled to prepare a positive electrode so that the porosity of the positive electrode active material layer was 29%.

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

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

[0173] <Comparative Examples 6 to 8: Production of Positive Electrodes> A positive electrode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 1,000,000 g / mol, the second hydrogenated nitrile butadiene rubber was changed to a hydrogenated nitrile butadiene rubber having 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 positive electrode slurry was changed as shown in Table 1.

[0174] <Comparative Example 9: Production of Positive Electrode> A positive electrode was prepared in the same manner as in Comparative Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 530,000 g / mol.

[0175] <Comparative Example 10: Production of Positive Electrode> A positive electrode was manufactured in the same manner as in Example 1, except that the fluorine-based binder was changed to polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 1,000,000 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 positive electrode slurry was changed as shown in Table 1.

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

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

[0178] <Experimental Example 2: Positive electrode adhesion test> The positive electrodes produced in Examples 1 to 11 and Comparative Examples 1 to 10 were each vacuum-dried at 130° C. for 2 hours, and then 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 produced in Examples 1 to 11 and Comparative Examples 1 to 10 were cut to a length of 150 mm and a width of 20 mm, and the positive electrode surface was attached to a slide glass having a length of 75 mm and a width of 25 mm in the longitudinal direction using double-sided tape. That is, the slide glass was attached to an area corresponding to half of the longitudinal direction of the positive electrode. Then, a roller was rotated 10 times to uniformly attach the double-sided tape to produce an evaluation sample.

[0180] Next, the glass slide 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 glass slide was not attached was connected to the load cell of the UTM equipment. The load cell was subjected to a 90° force at a speed of 100 mm / min, and the load applied to the load cell was measured as it was moved up to 50 mm. The average load measured in the 20 mm to 40 mm section of the travel section was calculated, and this was repeated five times, and the average value was evaluated as the positive electrode adhesive strength (gf / 20 mm) of each sample.

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

[0182] Specifically, each of the positive electrodes manufactured in Examples 1 to 11 and Comparative Examples 1 to 10 was cut into a size of 10 cm wide and 30 cm long, and measuring rods with diameters of 2.5φ, 3φ, 4φ, 5φ, 6φ, 7φ, 8φ, 9φ, and 10φ were prepared. Then, with the positive electrode 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. At this time, the positive electrode was lifted until the force measured by the UTM reached 5 N. Measurements were made 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 a smaller diameter. The diameter (φ) of the measuring rod at which cracks occurred was shown in Table 2.

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

[0184] [Table 2]

[0185] Referring to Tables 1 and 2, the positive electrode slurries according to Examples 1 to 11 all have a relatively high solid content of 62 wt %, but the viscosities of the slurries, except for the positive electrode slurry of Example 3, are 6,500 cps to 12,500 cps, which is a very suitable viscosity for coating on a positive electrode current collector, and it was confirmed that the lithium iron phosphate and the conductive material did not aggregate in a linear shape.

[0186] The positive electrodes according to Examples 1 to 11 were shown to have excellent flexibility of 4φ or less while having an adhesive strength of 20 gf / 20 mm or more. Therefore, it was expected that the detachment 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 Example 1, Comparative Example 5, and Comparative Examples 7 to 9, which do not contain the second hydrogenated nitrile butadiene rubber, exceeded 10 in the flexibility evaluation, and therefore it can be confirmed that they have poor flexibility compared to the positive electrodes of Examples 1 to 11.

[0188] The positive electrode according to Comparative Example 2 did not contain a fluorine-based binder and was shown to have the poorest adhesive strength. In addition, although it contained dihydrogenated nitrile butadiene rubber, it was also shown to have poorer flexibility than the positive electrodes of the Examples.

[0189] The positive electrodes according to Comparative Example 3, Comparative Example 4 and Comparative Example 6 contain a relatively large amount of the second hydrogenated nitrile butadiene rubber and a relatively small amount of the fluorine-based binder compared to the positive electrodes of Examples 1 to 11, and therefore the viscosity of the positive electrode slurry is too high, coating stability is reduced, and it is difficult to apply to mass production. In addition, it is shown that the adhesive strength of these positive electrodes is lower than that of the positive electrodes of the Examples. From these results, it can be confirmed that it is preferable to control the weight ratio of the second hydrogenated nitrile butadiene rubber to the total weight of the fluorine-based binder and the second hydrogenated nitrile butadiene rubber to a suitable range, specifically 40% or less, specifically 33% or less, from the viewpoint of the viscosity characteristics of the positive electrode slurry and the adhesive strength of the positive electrode.

[0190] The positive electrode of Comparative Example 10 did not contain the first hydrogenated nitrile butadiene rubber, and therefore the lithium iron phosphate and the conductive material were not dispersed, and the viscosity was so high that it was impossible to measure the viscosity. As a result, the positive electrode slurry could not be coated on the current collector, and the positive electrode could not be manufactured.

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

[0192] Specifically, the multi-probe electrode resistance was measured for each of the positive electrodes produced in Examples 1 to 4 above.

[0193] The resistance was 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 the respective probes.

[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 together, the positive electrode of Example 4 contains less binder than the positive electrode of Example 2, but has better adhesion than the positive electrode of Example 2. Due to the reduced binder content, the positive electrode of Example 4 also has better resistance characteristics than the positive electrode of Example 2.

[0197] Although the positive electrode according to Example 3 contains less binder than the positive electrode according to Example 1, it has better adhesion than the positive electrode according to Example 1, and due to the reduced binder content, it has better resistance characteristics than 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 in terms of adhesive strength and electrical properties.

Claims

1. A positive electrode having a positive electrode active material layer disposed on one or both surfaces of a current collector, the positive electrode active material layer contains 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 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 second hydrogenated nitrile butadiene rubber is contained in an amount of 0.2 wt % or more and 0.8 wt % or less based on the total weight of the positive electrode active material layer.

2. The positive electrode active material layer has a thickness of 450 mg / 25 cm ２ Above, 700mg / 25cm ２ 2. The positive electrode of claim 1 having a loading in the following range:

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

4. 2. The positive electrode of claim 1, wherein the first hydrogenated nitrile butadiene rubber is included in an amount of 0.1 wt % to 0.6 wt % based on a 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 wt % or more and 98.4 wt % or less 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 1.0% by weight or more and 4.0% 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. 2. 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. 2. The positive electrode according to claim 1, wherein the second hydrogenated nitrile butadiene rubber is 33% by weight or less based on a 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 includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are -0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

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

12. The positive electrode according to claim 1 , wherein a crack occurs with a measuring rod of 5 phi (φ) or less during a flexibility test in which a cross section of the positive electrode is lifted after contacting the positive electrode active material layer with the measuring rod.

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

14. 2. 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 contained in an amount of 0.3 wt % or more and 2.0 wt % 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 wt% or more and 4.5 wt% or less based on the total weight of the positive electrode active material.

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

18. 2. The positive electrode of claim 1, wherein the lithium iron phosphate has a unitary structure consisting of primary particles.

19. The battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode is A positive electrode active material layer is disposed on one or both sides of the current collector, the positive electrode active material layer contains 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 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 second hydrogenated nitrile butadiene rubber is contained in an amount of 0.2 wt % to 0.8 wt % based on the total weight of the positive electrode active material layer.

20. A positive electrode slurry comprising lithium iron phosphate, a fluorine-based binder, a rubber-based binder, a conductive material, and a solvent, The rubber-based binder includes 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 second hydrogenated nitrile butadiene rubber is contained in an amount of 0.2 wt % to 0.8 wt % based on the total weight of solids in the positive electrode slurry.

21. The slurry for positive electrode 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 positive electrode slurry according to claim 20, wherein the fluorine-based binder has a weight average molecular weight (Mw) of 800,000 g / mol or more.

23. 21. The positive electrode slurry according to claim 20, wherein the positive electrode slurry has a solids content in the range of 50% by weight to 75% by weight.

24. The positive electrode slurry according to claim 20, wherein the viscosity of the positive electrode 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 included in an amount of 1.0 wt % to 4.0 wt % based on a total weight of solids in the positive electrode slurry.

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

27. 21. The positive electrode slurry according to claim 20, wherein the second hydrogenated nitrile butadiene rubber is 33% by weight or less based on a 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 included in an amount of 0.5 wt % to 1.5 wt % based on a total weight of solids in the positive electrode slurry.

29. The positive electrode 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 includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are -0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

30. The positive electrode 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 wt % or more and 2.0 wt % or less based on a 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 wt% or more and 4.5 wt% or less based on the total weight of the solid content in the positive electrode slurry.

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