Positive electrode and lithium secondary battery including the same

Abstract

The present invention relates to a positive electrode and a lithium secondary battery including the same. The positive electrode includes a positive electrode current collector, a first positive electrode active material layer formed on one or both surfaces of the positive electrode current collector, and a second positive electrode active material layer formed on the first positive electrode active material layer. The first positive electrode active material layer and the second positive electrode active material layer each contain lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant. The ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, and the porosity calculated by the following formula 1 is 25% to 28%. [Formula 1] Porosity = {1 - (measured density of the positive electrode active material layer / true density of the positive electrode active material)} × 100

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0177463, filed on December 16, 2022.

[0002] The present invention relates to a positive electrode and a lithium secondary battery manufactured using the same.

Background Art

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

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

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

[0006] However, lithium iron phosphate has problems such as reduced lithium mobility and low electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. Conventionally, in order to form a short lithium migration path, lithium iron phosphate with a small average particle size was used, and the surface of 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 lithium iron phosphate with a carbon-coated surface has reduced wettability by the solvent. As a result, severe particle aggregation of lithium iron phosphate occurs, the stability of the positive electrode slurry and the coating processability decrease, and lithium iron phosphate and the binder are not effectively mixed, so the adhesive force (hereinafter referred to as the positive electrode adhesive force) between the positive electrode current collector and the positive electrode active material layer in the manufactured positive electrode decreases. And such a phenomenon can be aggravated in a high-loading positive electrode.

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

[0009] As the demand for high-energy density batteries increases, in the positive electrode containing lithium iron phosphate, while ensuring a loading amount of 550 mg / 25 cm 2 or more, technologies for improving the positive electrode adhesive force and flexibility are required. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] An object of the present invention is to provide a positive electrode and a lithium secondary battery that improve the adhesive force between a positive electrode current collector and a positive electrode active material layer in a high-loading positive electrode, improve the flexibility of the positive electrode, prevent the electrode from peeling off, and improve the resistance characteristics of the battery. MEANS FOR SOLVING THE PROBLEMS

[0011] According to an embodiment of the present invention, there is provided a positive electrode current collector, a first positive electrode active material layer formed on one or both surfaces of the positive electrode current collector, and a second positive electrode active material layer formed on the first positive electrode active material layer. The first positive electrode active material layer and the second positive electrode active material layer each contain lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant. The ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, and a positive electrode having a porosity of 25% to 28%, preferably 25.5% to 27.5%, calculated by the following formula 1 is provided.

[0012] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0013] In an exemplary embodiment, the rubber-based binder may be hydrogenated nitrile butadiene rubber (HNBR) having a weight average molecular weight (Mw) of 130,000 g / mol or more.

[0014] In an exemplary embodiment, the rubber-based dispersant may be hydrogenated nitrile butadiene rubber (HNBR) in the range of a weight average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol.

[0015] In an exemplary embodiment, the fluorine-based binder may be contained in the first positive electrode active material layer in the range of 1.5% by weight to 3.0% by weight.

[0016] In an exemplary embodiment, the fluorine-based binder may be contained in the second positive electrode active material layer in the range of 0.8% by weight to 2.4% by weight.

[0017] In an exemplary embodiment, the fluorine-based binder may be a polyvinylidene fluoride (PVDF)-based polymer binder.

[0018] In an exemplary embodiment, the rubber-based binders contained in the first positive electrode active material layer and the second positive electrode active material layer may satisfy the following condition 1.

[0019] [Condition 1] 1 ≦ R2 / R1 ≦ 3

[0020] R1 represents the content (% by weight) of the rubber-based binder contained in the first positive electrode active material layer, and R2 represents the content (% by weight) of the rubber-based binder contained in the second positive electrode active material layer.

[0021] In an exemplary embodiment, the rubber-based binder may be contained in the first positive electrode active material layer in a range of 0.2% to 0.9% by weight and in the second positive electrode active material layer in a range of 0.3% to 1.0% by weight.

[0022] In an exemplary embodiment, the rubber-based binder may have a weight average molecular weight (Mw) in the range of 150,000 g / mol to 1,000,000 g / mol.

[0023] In an exemplary embodiment, the conductive material may be carbon nanotubes.

[0024] In an exemplary embodiment, the conductive material may be contained in each of the first positive electrode active material layer and the second positive electrode active material layer in a range of 0.3% to 2.0% by weight.

[0025] In an exemplary embodiment, the lithium iron phosphate may be a compound represented by the following Chemical Formula 1.

[0026] [Chemical Formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0027] (In the above Chemical Formula 1, M contains any one or two or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X contains any one or two or more elements selected from the group consisting of F, S, and N; and a, b, and x satisfy -0.5 ≦ a ≦ 0.5, 0 ≦ b ≦ 0.1, and 0 ≦ x ≦ 0.5, respectively.)

[0028] In an exemplary embodiment, the average particle size D of the above lithium iron phosphate 50 can be 0.6 μm to 3.0 μm.

[0029] In an exemplary embodiment, the above fluorine-based binder can be a polyvinylidene fluoride (PVDF)-based polymer binder.

[0030] Further, the present invention provides a lithium secondary battery including the above-described positive electrode.

Advantages of the Invention

[0031] The positive electrode according to the present invention includes a first positive electrode active material layer that maximizes adhesion and a second positive electrode active material layer that maximizes flexibility. Thus, compared with a conventional single-layer positive electrode, it can have excellent positive electrode adhesion and flexibility even with a low binder content.

[0032] In addition, the positive electrode according to the present invention controls the porosity of the positive electrode active material layer within an optimal range, increases the electrical conductivity of the positive electrode without inhibiting the ionic conductivity, and has the effect of improving the resistance characteristics of the battery.

[0033] Thereby, the positive electrode according to the present invention can provide a battery that is excellent in terms of capacity and electrical resistance while realizing a high-loading positive electrode.

Modes for Carrying Out the Invention

[0034] The advantages and features of the present invention, and the method of achieving them, will be clear by referring to the embodiments described in detail below. However, the present invention is not limited to the embodiments disclosed below, and can be embodied in various different forms, and these embodiments are merely provided so that the disclosure of the present invention is complete, and to fully inform those skilled in the art to which the present invention pertains of the scope of the invention. The present invention is only defined by the scope of the claims. The same reference numerals throughout the specification refer to the same components.

[0035] Unless otherwise defined, all terms (including technical and scientific terms) used in this specification may be used as having a meaning commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Also, terms defined in commonly used dictionaries are not ideally or overly interpreted unless specifically defined otherwise.

[0036] The terms used in this specification are for the purpose of describing embodiments and are not intended to limit the present invention. In this specification, the singular form also includes the plural form unless otherwise specifically stated in the phrase. The terms "comprises" and / or "comprising" used in the specification do not exclude the presence or addition of one or more other components in addition to the recited components.

[0037] In this specification, when a certain part includes a certain component, this means that, unless otherwise stated to the contrary, it does not exclude other components and may further include other components.

[0038] In this specification, the description of "A and / or B" means A or B, or A and B.

[0039] In this specification, "%" means weight % unless otherwise explicitly indicated.

[0040] In this specification, D 50means the particle size corresponding to 50% of the volume cumulative amount in the particle size distribution curve of the particles. The above D 50 For example, it can be measured using the laser diffraction method. The above laser diffraction method can generally measure particle sizes in the range from the submicron region to about several millimeters, and high reproducibility and high resolution results can be obtained.

[0041] In this specification, the "specific surface area" is measured by the BET method. Specifically, it can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mini II of BEL Japan.

[0042] In this specification, the "weight average molecular weight (Mw)" means the converted numerical value with respect to standard polystyrene measured by Gel Permeation Chromatography (GPC). Specifically, the above weight average molecular weight is the value obtained by converting the value measured under the following conditions using GPC. For the preparation of the calibration curve, standard polystyrene of the Agilent system was used.

[0043] <Measurement conditions> Measuring instrument: Agilent GPC (Agilent 1200 series, USA) Columns: Two PL Mixed B columns connected in series Column temperature: 40 °C Eluent: Tetrahydrofuran Flow rate: 1.0 mL / min Concentration: ~1 mg / mL (100 μL injection)

[0044] The positive electrode adhesion force in this specification can be measured by the following method. Prepare a positive electrode cut to a length of 150 mm and a width of 20 mm, and attach the positive electrode active material layer to a slide glass with a length of 75 mm and a width of 25 mm facing each other, using double-sided tape to attach the positive electrode to the slide glass in the longitudinal direction. That is, the slide glass is attached to the region corresponding to half of the longitudinal direction of the positive electrode. Then, roll the roller 10 times so that the double-sided tape is evenly attached to produce an evaluation sample. Next, fix the slide glass part of the evaluation sample to the sample stage of a Universal Testing Machine (UTM) (LS5, AMETEK), and connect the half of the positive electrode without the attached slide glass to the load cell of the UTM equipment. Apply a force to the load cell at a speed of 100 mm / min at an angle of 90°, and measure the load applied to the load cell while moving it to 50 mm. At this time, obtain the average value of the load measured in the 20 mm to 40 mm section of the running section. Repeat this 5 times in total, and evaluate the average value as the positive electrode adhesion force (gf / 20 mm) of each sample.

[0045] In this specification, the flexibility of the positive electrode can be measured by the following method. Fabricate measuring rods according to different phi (φ), and cut a positive electrode with a loading amount of 600 mg / 25 cm 2 to a size of 10 cm in width and 30 cm in length. After bending the cut positive electrode in half and bringing it into contact with the measuring rod, lift both ends of the positive electrode at a speed of 10 mm per minute. At this time, lift it until the force measured by the UTM reaches 5 N. Measure according to different phi, and observe whether cracks occur in the electrode with an optical microscope. If there are no cracks, proceed with the test with a smaller phi.

[0046] In this specification, "phi (φ)" represents the diameter of the measuring rod in millimeters (mm).

[0047] In this specification, the porosity of the positive electrode active material layer can be defined by the following formula 1. In the following formula 1, the "measured density of the positive electrode active material layer" is a value calculated by density by measuring the weight and volume of the positive electrode active material layer separated from the positive electrode current collector of the rolled positive electrode. The "true density of the positive electrode active material" is the density with respect to the volume of only the positive electrode active material particles excluding pores, and can be measured using a Pycnometer (AccuPycII 1340).

[0048] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0049] The present invention provides a positive electrode and a lithium secondary battery manufactured using the same.

[0050] Lithium iron phosphate generally used as a positive electrode active material has a lower lithium mobility and lower electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. Therefore, lithium iron phosphate with a small average particle size is mainly used as the positive electrode active material. However, when the size of the lithium iron phosphate particles is small, the specific surface area increases, resulting in severe particle aggregation and ineffective mixing of the lithium iron phosphate and the binder. As a result, the positive electrode adhesion may decrease. In addition, the fluorine-based binder used as the positive electrode binder has a characteristic of being better adhered to the lithium iron phosphate particles than the positive electrode current collector. When the positive electrode active material is lithium iron phosphate, the interfacial adhesion force between the positive electrode current collector and the positive electrode active material layer may decrease compared to the case where the positive electrode active material is lithium nickel cobalt manganese oxide. As a result, a positive electrode containing lithium iron phosphate as the positive electrode active material has problems such as detachment of the positive electrode active material layer during electrode manufacturing or charge / discharge, an increase in battery resistance, and a decrease in the capacity of the secondary battery. In addition, a positive electrode containing lithium iron phosphate as the positive electrode active material has poor brittleness due to the characteristics of the positive electrode active material compared to a positive electrode containing lithium nickel cobalt manganese oxide, and is vulnerable to detachment of the positive electrode active material layer by physical external force.

[0051] Therefore, as a result of repeated research to solve such problems, the inventors of the present invention have found that the positive electrode active material layer is composed of a plurality of layers instead of a single layer, and the first positive electrode active material layer and the second positive electrode active material layer each contain lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant. The ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less. When the porosity calculated by the following formula 1 is 25% to 28%, while having excellent interfacial adhesion between the current collector and the first positive electrode active material layer, the desorption of the positive electrode active material layer is suppressed due to improved flexibility, the ionic conductivity of the positive electrode active material layer is not inhibited, and the electrical conductivity is improved, thereby improving the resistance characteristics of the battery. Thus, the present invention has been achieved.

[0052] [Formula 1] Porosity = {1 - (measured density of the positive electrode active material layer / true density of the positive electrode active material)} × 100

[0053] Hereinafter, the positive electrode according to the present invention and the lithium secondary battery manufactured using the same will be described in detail.

[0054] [Positive Electrode] The positive electrode according to an embodiment of the present invention includes a positive electrode current collector, a first positive electrode active material layer formed on one or both surfaces of the positive electrode current collector, and a second positive electrode active material layer formed on the first positive electrode active material layer. The first positive electrode active material layer and the second positive electrode active material layer each contain lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant. The ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, and the porosity calculated by the following formula 1 can be 25% to 28%, preferably 25.5% to 27.5%.

[0055] [Formula 1] Porosity = {1 - (measured density of the positive electrode active material layer / true density of the positive electrode active material)} × 100

[0056] When the porosity is less than 25%, the flexibility of the positive electrode may decrease, which may not be preferable. When the porosity exceeds 28%, the resistance of the battery may increase, which is not preferable.

[0057] The above porosity means the porosity of the positive electrode active material layer including both the first positive electrode active material layer and the second positive electrode active material layer. In the above formula 1, the "measured density of the positive electrode active material layer" is a value calculated by density by measuring the weight and volume of the positive electrode active material layer separated from the positive electrode current collector of the rolled positive electrode, and the "true density of the positive electrode active material" is the density with respect to the volume of only the positive electrode active material particles excluding pores, and can be measured using a Pycnometer (AccuPycII 1340).

[0058] The above porosity can be adjusted to the above numerical range by preferably adjusting the thickness change rate of the positive electrode active material layer before and after rolling during the rolling process in the manufacturing process of the positive electrode. Specifically, the larger the thickness change rate before and after rolling, the smaller the above porosity can be, and conversely, the smaller the thickness change rate before and after rolling, the larger the porosity of the positive electrode active material layer can be. The thickness change rate can be calculated as follows.

[0059] Thickness change rate = (thickness of the positive electrode active material layer before rolling - thickness of the positive electrode active material layer after rolling) / thickness of the positive electrode active material layer before rolling

[0060] The above thickness change rate can be adjusted by the linear pressure applied to the positive electrode during the rolling process of the positive electrode. That is, the larger the linear pressure, the larger the thickness change rate before and after rolling, and the smaller the linear pressure, the smaller the thickness change rate before and after rolling can be.

[0061] The positive electrode according to the present invention has a porosity that satisfies the above range, which is a range slightly lower than the conventional numerical range of the porosity of a positive electrode containing lithium iron phosphate as a positive electrode active material. The resistance is affected by the ion conductivity and the electric conductivity, but the positive electrode according to the present invention is rolled by applying a high linear pressure for improving the interfacial adhesion force during rolling and rolling to such an extent that the ion conductivity does not decrease, so that the resistance characteristics are improved by rolling to such an extent that the porosity satisfies the above numerical range.

[0062] The positive electrode according to an exemplary 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, and due to the excellent interfacial adhesion force between the positive electrode active material layer and the positive electrode current collector, there may be no separate layer for improving the adhesion force between the positive electrode active material layer and the positive electrode current collector. That is, the positive electrode according to an embodiment of the present invention can exhibit excellent interfacial adhesion force without including a separate layer such as a binder layer, an adhesive layer, a bonding layer, or a primer coating layer that may be interposed between the positive electrode current collector and the positive electrode active material layer for improving the adhesion force.

[0063] The positive electrode of the present invention will be specifically described.

[0064] The 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, a first positive electrode active material layer formed on the current collector, and a second positive electrode active material layer formed on the first positive electrode active material layer. At this time, the first positive electrode active material layer and the second positive electrode active material layer may be formed on one or both surfaces of the current collector.

[0065] The positive electrode according to the present invention has a positive electrode composition in which the first positive electrode active material layer, which is the lower layer region, increases the interfacial adhesion force between the positive electrode current collector and the positive electrode active material layer, and the second positive electrode active material layer, which is the upper layer region, has a positive electrode composition for improving the flexibility of the electrode. As a result, the positive electrode according to the present invention has the effect of improving both the adhesion force and the flexibility compared to a single-layer positive electrode.

[0066] The positive electrode current collector may be any material that has conductivity without inducing a chemical change in the battery and is not particularly limited. For example, as the current collector, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface-treating the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used.

[0067] The positive electrode current collector can have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance the adhesive force to the positive electrode active material layer. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

[0068] The positive electrode active material layer may contain a positive electrode active material. Additionally, the positive electrode active material layer may further contain a conductive material, a binder, and a dispersant as necessary in addition to the positive electrode active material.

[0069] Hereinafter, each component included in the positive electrode active material layer will be specifically described.

[0070] (1) Positive electrode active material The present invention includes lithium iron phosphate as the positive electrode active material. Since lithium iron phosphate has an olivine structure, the active material structure is stably maintained at high temperatures compared to layered lithium transition metal oxides. As a result, when lithium iron phosphate is used as the positive electrode active material, the high-temperature stability and high-temperature life characteristics of the positive electrode are significantly improved, and thus the risk of ignition of the lithium secondary battery including the above positive electrode can be reduced.

[0071] The above lithium iron phosphate can be a compound of the following Chemical Formula 1.

[0072] [Chemical Formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0073] In the above Chemical Formula 1, M includes any one or two or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, X includes any one or two or more elements selected from the group consisting of F, S, and N, and a, b, and x are -0.5 ≦ a ≦ 0.5, 0 ≦ b ≦ 0.1, and 0 ≦ x ≦ 0.5, respectively.

[0074] For example, the above lithium iron phosphate can be LiFePO4.

[0075] In the present invention, the "monolith structure" means a structure in which particles exist as independent phases that are not aggregated with each other morphologically. As a particle structure contrasted with such a monolith structure, there is a structure in which small particles ("primary particles") are physically and / or chemically aggregated to form a relatively large particle form ("secondary particles").

[0076] When lithium iron phosphate has a monolith structure composed of primary particles, compared with the case where it is secondary particles, the possibility of the occurrence of the cracking phenomenon of lithium iron phosphate particles during the rolling process is low, so the capacity reduction due to the detachment of cracked particles is small and preferable. Further, when lithium iron phosphate is primary particles having a monolith structure, the migration phenomenon of the binder can be alleviated during the drying process of the positive electrode slurry, so it can be said to be preferable also from the aspect of the interfacial adhesion force between the positive electrode current collector and the positive electrode active material layer.

[0077] Lithium iron phosphate may include a carbon coating layer on its surface. When a carbon coating layer is formed on the surface of lithium iron phosphate, the electrical conductivity is improved and the resistance characteristics of the positive electrode can be improved.

[0078] The carbon coating layer can be formed using at least one or more raw material substances selected from the group consisting of glucose, sucrose, lactose, starch, oligosaccharide, polyoligosaccharide, fructose, cellulose, polymer of furfuryl alcohol, block copolymer of ethylene and ethylene oxide, vinyl resin, cellulose resin, phenol resin, pitch resin, and tar resin. Specifically, the carbon coating layer can be formed through a process of heat-treating after mixing the raw material substances with the lithium iron phosphate.

[0079] The average particle size D of lithium iron phosphate 50can be from 0.5 μm to 20.0 μm, preferably from 0.5 μm to 10.0 μm, more preferably from 0.6 μm to 3 μm, even more preferably from 0.6 μm to 2.5 μm, and most preferably from 0.7 μm to 1.5 μm. The average particle diameter D of the positive electrode active material 50 When satisfies the above range, the mobility of lithium in lithium iron phosphate can be improved, and the charge-discharge characteristics of the battery can be improved.

[0080] The BET specific surface area of lithium iron phosphate is 5 m 2 / g to 20 m 2 / g, specifically can be 7 m 2 / g to 18 m 2 / g, more specifically can be 9 m 2 / g to 16 m 2 / g. The above range corresponds to a lower value compared to ordinary lithium iron phosphate. When satisfying the above range, the aggregation of the above lithium iron phosphate can be effectively suppressed even in a positive electrode slurry composition with a relatively low content of the dispersant.

[0081] The lithium iron phosphate of the present invention can be contained in the first positive electrode active material layer and the second positive electrode active material layer in the same content or different contents. In a specific example, lithium iron phosphate can be contained in the first positive electrode active material layer in the range of 93.5% by weight to 98% by weight, specifically 94% by weight to 97.5% by weight, more specifically 94.5% by weight to 97% by weight, based on the total weight of the first positive electrode active material layer. Also, lithium iron phosphate can be contained in the second positive electrode active material layer in the range of 94% by weight to 99% by weight, specifically 94.5% by weight to 98.5% by weight, more specifically 95% by weight to 98% by weight, based on the total weight of the second positive electrode active material layer. When the content of lithium iron phosphate satisfies the above range, the battery capacity of the positive electrode can be improved by ensuring a sufficient positive electrode energy density.

[0082] (2) Binder The first positive electrode active material layer and the second positive electrode active material layer constituting the positive electrode according to the present invention each contain both a fluorine-based binder and a rubber-based binder as binders.

[0083] The fluorine-based binder may include a PVDF-based polymer containing vinylidene fluoride (VDF) as a monomer. Specific examples of the PVDF-based polymer include PVDF single polymer, PVDF-HFP (Poly(vinylidene fluoride-co-hexafluoropropylene)), PVDF-CTFE (Poly(vinylidene fluoride-co-chlorotrifluoroethylene)), PVDF-TFE (Poly(vinylidene tetrafluoroethylene)), PVDF-TrFE (Poly(vinylidene trifluoroethylene)), and the like.

[0084] The fluorine-based binder, together with the rubber-based binder, imparts an adhesive force between the positive electrode active material and the conductive material, and an adhesive force between the current collector and the positive electrode active material layer.

[0085] The weight average molecular weight (Mw) of the fluorine-based binder can be 300,000 to 2,000,000, preferably 400,000 to 1,500,000, and most preferably 500,000 to 1,300,000. When the weight average molecular weight of the fluorine-based binder satisfies the above numerical range, it is easy for the viscosity of the positive electrode slurry to be in a preferable range, and the positive electrode adhesive force can be maximized in a small content range.

[0086] The fluorine-based binders contained in the first positive electrode active material layer and the second positive electrode active material layer are the same compound, and their weight average molecular weights and chemical formulas can be the same.

[0087] Regarding the positive electrode according to an exemplary embodiment of the present invention, the content (by weight) of the fluorine-based binder contained in the lower first positive electrode active material layer is the same as or greater than the content (by weight) of the fluorine-based binder contained in the upper second positive electrode active material layer, and the ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, specifically, it can be 0.5 to 1, preferably 0.52 to 0.95, and more preferably 0.53 to 0.85. When the ratio (P2 / P1) of P2 to P1 exceeds 1, it is not preferable from the perspective of adhesion.

[0088] The fact that the weight P1 of fluorine contained in the first positive electrode active material layer and the weight P2 of fluorine contained in the second positive electrode active material layer satisfy the above ratio range is due to the difference in the content of the fluorine-based binder contained in the first positive electrode active material layer and the second positive electrode active material layer, respectively.

[0089] That is, although the fluorine-based binder contains fluorine elements, lithium iron phosphate, rubber-based binder, rubber-based dispersant, and conductive material that constitute the positive electrode of the present invention do not contain fluorine components. Therefore, the weight of fluorine contained in the first positive electrode active material layer and the second positive electrode active material layer, respectively, is proportional to the content (by weight) of the fluorine-based binder contained in the first positive electrode active material layer and the second positive electrode active material layer, respectively.

[0090] The fluorine-based binder has the advantage of showing very excellent adhesion, but since it has a crystalline structure, it is inferior in flexibility compared to the rubber-based binder. Therefore, when increasing the content of the fluorine-based binder with excellent adhesion in the first positive electrode active material layer in contact with the positive electrode current collector and decreasing the content of the fluorine-based binder in the second positive electrode active material layer where flexibility is required, it has the effect of improving the interfacial adhesion of the positive electrode and preventing the electrode from detaching from the surface of the electrode during an external impact. Thus, by controlling the content of the fluorine-based binder contained in the first positive electrode active material layer and the second positive electrode active material layer as described above, the positive electrode according to the present invention is superior in adhesion and flexibility even with the same binder content compared to a positive electrode composed of a single layer.

[0091] The above fluorine-based binder may be contained in the first positive electrode active material layer at 1.5% to 3.0% by weight, preferably 1.6% to 2.7% by weight, and more preferably 1.7% to 2.5% by weight. Further, the above fluorine-based binder may be contained in the second positive electrode active material layer at 0.8% to 2.4% by weight, preferably 1.0% to 2.2% by weight, and more preferably 1.2% to 2.0% by weight.

[0092] Further, within the entire positive electrode active material layer including the first positive electrode active material layer and the second positive electrode active material layer, the above fluorine-based binder may be contained at 1.2% to 2.6% by weight, preferably 1.4% to 2.4% by weight, and more preferably 1.6% to 2.2% by weight.

[0093] On the other hand, the rubber-based binder improves the flexibility of the positive electrode, increases the loading amount of the positive electrode, and makes the rolling performance excellent. Further, at the time of an external physical impact, the improved flexibility suppresses a part of the positive electrode active material layer from peeling off, so that a positive electrode having excellent capacity can be provided.

[0094] According to an exemplary embodiment of the present invention, the above rubber-based binder may be hydrogenated nitrile butadiene rubber (HNBR). The above hydrogenated nitrile butadiene rubber (HNBR) means that nitrile butadiene rubber (NBR) is subjected to a hydrogenation reaction, and the double bonds originally contained in the nitrile butadiene rubber (NBR) become single bonds.

[0095] The above hydrogenated nitrile butadiene rubber may have a content of repeating units derived from acrylonitrile (AN) of 20% to 50% by weight, more preferably 25% to 45% by weight, and most preferably 30% to 40% by weight with respect to the total weight.

[0096] The inventors of the present invention have discovered that when the weight-average molecular weight (Mw) of hydrogenated nitrile butadiene rubber (HNBR) is 130,000 g / mol or more, the brittleness of the positive electrode containing lithium iron phosphate as the positive electrode active material is remarkably improved, and thus completed the present invention. The above hydrogenated nitrile butadiene rubber may have a weight-average molecular weight (Mw) of 130,000 g / mol or more, preferably 150,000 g / mol to 1,000,000 g / mol, and more preferably 200,000 g / mol to 500,000 g / mol. When the weight-average molecular weight (Mw) of the above second hydrogenated nitrile-based butadiene rubber is 130,000 g / mol or more, due to the improved flexibility of the positive electrode active material layer, the rolling density of the positive electrode can be increased, and as the rolling density increases, the positive electrode resistance can have the effect of decreasing.

[0097] The rubber-based binders contained in the first positive electrode active material layer and the second positive electrode active material layer respectively may be the same compound, and their weight-average molecular weights and chemical formulas may be the same.

[0098] On the other hand, the rubber-based binders contained in the first positive electrode active material layer and the second positive electrode active material layer respectively may satisfy the following condition 1. The value of the following R2 / R1 may preferably be 1 to 2.9, and more preferably 1.1 to 2.8.

[0099] [Condition 1] 1 ≤ R2 / R1 ≤ 3

[0100] R1 represents the content (weight%) of the rubber-based binder contained in the first positive electrode active material layer, R2 represents the content (weight%) of the rubber-based binder contained in the second positive electrode active material layer.

[0101] When the value of R2 / R1 exceeds 3, as a result, an excessive amount of the rubber-based binder is contained in the second positive electrode active material layer, and thus the coating performance may deteriorate due to an increase in the viscosity of the slurry for the second positive electrode active material layer, which is not preferable. Conversely, when it is less than 1, as a result, an excessive amount of the rubber-based binder is contained in the second positive electrode active material layer, and thus the flexibility of the entire positive electrode may deteriorate, which is not preferable.

[0102] The above rubber-based binder may be contained in the first positive electrode active material layer at 0.2% by weight to 0.9% by weight, preferably 0.3% by weight to 0.8% by weight, and more preferably 0.4% by weight to 0.7% by weight. And the above rubber-based binder may be contained in the second positive electrode active material layer at 0.3% by weight to 1.0% by weight, preferably 0.4% by weight to 0.9% by weight, and more preferably 0.5% by weight to 0.8% by weight. And the above rubber-based binder may be contained in the entire first positive electrode active material layer and the second positive electrode active material layer at 0.25% by weight to 0.85% by weight, preferably 0.35% by weight to 0.75% by weight, and more preferably 0.4% by weight to 0.65% by weight.

[0103] When the above rubber-based binder is contained in each of the first positive electrode active material layer and the second positive electrode active material layer within the above range, the flexibility of the positive electrode is improved, the coating stability of the slurry is also excellent, and each of the first positive electrode active material layer and the second positive electrode active material layer can be uniformly coated.

[0104] The weight ratio of the fluorine-based binder to the rubber-based binder in the above first positive electrode active material layer can be 90:10 to 75:25, preferably 8:1 to 4:1, and more preferably 7:1 to 4:1. When the weight ratio of the fluorine-based binder to the rubber-based binder satisfies the above range, while the coating stability of the slurry is excellent, the interfacial adhesion between the positive electrode current collector and the first positive electrode active material layer may be excellent.

[0105] The weight ratio of the fluorine-based binder to the rubber-based binder in the second positive electrode active material layer can be 80:20 to 60:40, preferably 75:25 to 60:40. When the weight ratio of the fluorine-based binder to the rubber-based binder satisfies the above range, while the coating stability of the slurry is excellent, the flexibility of the positive electrode may be excellent.

[0106] In addition, the total of the fluorine-based binder and the rubber-based binder contained in each of the above first positive electrode active material layer and the second positive electrode active material layer can be 1.8% by weight to 3.6% by weight based on the entire positive electrode active material layer.

[0107] (3) Dispersant The first positive electrode active material layer and the second positive electrode active material layer constituting the positive electrode according to the present invention each contain a rubber-based dispersant.

[0108] The dispersant is used to increase the dispersibility of the components constituting the active material layer. In particular, it increases the dispersibility of the conductive material. On the other hand, the rubber-based dispersant of the present invention also plays a role in adjusting the viscosity of the positive electrode slurry to a suitable range.

[0109] The rubber-based dispersant of the present invention can be hydrogenated nitrile butadiene rubber (HNBR). The above hydrogenated nitrile butadiene rubber (HNBR) means that nitrile butadiene rubber (NBR) is subjected to a hydrogenation reaction, and the double bonds originally contained in the nitrile butadiene rubber (NBR) become single bonds.

[0110] The rubber-based dispersant of the present invention can be hydrogenated nitrile butadiene rubber (HNBR). The above hydrogenated nitrile butadiene rubber may have a weight average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, preferably 10,000 g / mol to 80,000 g / mol, and more preferably 10,000 g / mol to 50,000 g / mol. When the weight average molecular weight of the hydrogenated nitrile butadiene rubber (HNBR) as the dispersant satisfies the above range, the solvent wettability and dispersibility of the lithium iron phosphate particles can be improved, and the particle aggregation of the lithium iron phosphate can be suppressed. In addition, it can contribute to effectively and well mixing the binder and the lithium iron phosphate particles, and improving the interfacial adhesion force between the positive electrode current collector and the first positive electrode active material layer.

[0111] Moreover, hydrogenated nitrile butadiene rubber suppresses the aggregation of the conductive material, improves the positive electrode conductive network, and causes the conductive material to aggregate in a spherical shape rather than linearly. As a result, the specific surface area of the aggregated conductive material is minimized compared to the case where the conductive material aggregates linearly. Consequently, the surface area of the positive electrode active material that cannot participate in the lithium insertion / desorption reaction adjacent to the aggregated conductive material is minimized, so that the discharge resistance of the lithium secondary battery can be reduced.

[0112] The above dispersant may be contained in the first positive electrode active material layer at 0.01% by weight to 0.7% by weight, preferably 0.05% by weight to 0.6% by weight, and more preferably 0.1% by weight to 0.5% by weight. Further, the above dispersant may also be contained within the above range in the second positive electrode active material layer. When the content of the dispersant satisfies the above range, aggregation of the positive electrode active material can be suppressed and gelation of the positive electrode slurry composition can be prevented.

[0113] (4) Conductive material The first positive electrode active material layer and the second positive electrode active material layer constituting the positive electrode according to the present invention each contain a conductive material.

[0114] The above conductive material is for improving the conductivity of the electrode and is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. can be used. Among these, carbon nanotubes, carbon nanofibers, and carbon black are preferred as the conductive material of the present invention, and carbon nanotubes are most preferred. The conductive network of carbon nanotubes can alleviate the binder floating phenomenon during the drying process of the positive electrode slurry, so it is most preferred as the conductive material contained in the positive electrode of the present invention.

[0115] A carbon nanotube has a graphite sheet with a nanosized diameter in a cylindrical shape and has an sp2 bonding structure, and exhibits conductor or semiconductor characteristics depending on the angle and structure by which the graphite sheet is wound. Carbon nanotubes can be classified into single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes according to the number of bonds forming the wall, and these carbon nanotubes can be suitably selected according to the use of the dispersion liquid.

[0116] In addition, the carbon nanotube may have a secondary shape formed by aggregation or arrangement of a plurality of carbon nanotubes. For example, a bundle or rope-shaped bundle type carbon nanotube in which a plurality of carbon nanotubes are arranged or aligned in a certain direction, or a sphere or potato-shaped entangled type carbon nanotube in which a plurality of carbon nanotubes are entangled without a certain directionality. From the aspect of dispersibility, the carbon nanotube is more preferably a bundle type carbon nanotube.

[0117] The BET specific surface area of the carbon nanotube is 100 m 2 / g to 1000 m 2 / g, 150 m 2 / g to 800 m 2 / g, 150 m 2 / g to 500 m 2 / g, 150 m 2 / g to 300 m 2 / g, or 150 m 2 / g to 200 m 2 / g.

[0118] When the conductive material is a carbon nanotube, the conductive material can be contained in each of the first positive electrode active material layer and the second positive electrode active material layer at 0.3% by weight to 2.0% by weight, 0.6% by weight to 1.5% by weight, and more specifically 0.7% by weight to 1.3% by weight. When the content of the conductive material is less than 0.3% by weight, the phenomenon of binder floating during drying of the electrode may be further enlarged, which is not preferable. When the content of the conductive material exceeds 2.0% by weight, as a result, the contents of the dispersant both become high, and the content of the positive electrode active material may decrease, which is not preferable.

[0119] The conductive material may be contained in the first positive electrode active material layer and the second positive electrode active material layer in the same content, or may be contained in different contents. When the contents of the conductive material contained in the first positive electrode active material layer and the second positive electrode active material layer are different from each other, the following Condition 2 may be satisfied.

[0120] [Condition 2] 0.5 ≦ E2 / E1 ≦ 0.9

[0121] E1 represents the content (% by weight) of the linear conductive material contained in the first active material layer, E2 represents the content (% by weight) of the linear conductive material contained in the second active material layer.

[0122] In the positive electrode that satisfies the above Condition 2, the content of the conductive material in the first positive electrode active material layer in surface contact with the positive electrode current collector is larger than the content of the conductive material in the second positive electrode active material layer, and a conductive network by the conductive material is well formed in the first positive electrode active material layer. Such a conductive network suppresses the binder migration phenomenon and improves the interfacial adhesion force between the positive electrode current collector and the first positive electrode active material layer. And the improved interfacial adhesion force can suppress the volume change during charge and discharge, so that the interfacial resistance of the positive electrode can be reduced, and the resistance characteristics of the positive electrode can be improved.

[0123] When Condition 2 is satisfied, the conductive material may be included in the first positive electrode active material layer in the range of 1.0 wt% to 1.5 wt%, preferably 1.1 wt% to 1.4 wt%. Further, the conductive material may be included in the second positive electrode active material layer in the range of 0.4 wt% to 0.95 wt%, preferably 0.6 to 0.90 wt%.

[0124] The positive electrode according to the present invention can be manufactured according to a normal method for manufacturing a positive electrode. Specifically, the positive electrode can be manufactured by producing a positive electrode slurry composition containing the above positive electrode active material, conductive material, binder, and dispersant, then applying the positive electrode slurry composition onto a positive electrode current collector, and then drying and rolling it.

[0125] In another method, the positive electrode can also be manufactured by casting the positive electrode slurry composition onto a separate support and then laminating the film obtained by peeling it from the support onto the positive electrode current collector.

[0126] The positive electrode according to an embodiment of the present invention includes a first positive electrode active material layer that maximizes adhesive force and a second positive electrode active material layer that maximizes flexibility. Thus, compared with a conventional single-layer positive electrode, it can have excellent positive electrode adhesive force and flexibility even with a small binder content. Further, it controls the porosity within an optimal range and has the effect of improving the resistance characteristics of the battery.

[0127] As a result, it is possible to manufacture a high-loading positive electrode, and by preventing the detachment of the positive electrode, the internal resistance of the secondary battery is reduced, the capacity and output characteristics of the battery are improved, and defects generated in the manufacturing process can be reduced.

[0128] The positive electrode of the present invention can have an adhesive force between the positive electrode current collector and the positive electrode active material layer measured by a 90-degree peel test (90° peel test) of 20 gf / 20 mm or more, specifically 23 gf / 20 mm or more.

[0129] Further, the positive electrode of the present invention may have flexibility such that when the positive electrode is lifted during a flexibility test in which a classification measurement rod is brought into contact with the positive electrode active material layer and then lifted, the maximum phi (φ) value of the measurement rod at which cracks occur is 5 phi (φ) or less, specifically 4 phi (φ) or less, and more specifically 3 phi (φ) or less.

[0130] <Lithium secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0131] The lithium secondary battery according to an embodiment of the present invention may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0132] In the above lithium secondary battery, the positive electrode is as described above. For example, the positive electrode includes a positive electrode current collector, a first positive electrode active material layer formed on one or both surfaces of the positive electrode current collector, and a second positive electrode active material layer formed on the first positive electrode active material layer. The first positive electrode active material layer and the second positive electrode active material layer each contain lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant. The ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, and the porosity calculated by the following formula 1 is 25% to 28%, preferably 25.5% to 27.5%.

[0133] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0134] The above rubber-based binder may be hydrogenated nitrile butadiene rubber (HNBR) having a weight average molecular weight (Mw) of 130,000 g / mol or more.

[0135] The above rubber-based dispersant may be hydrogenated nitrile butadiene rubber (HNBR) in the range of a weight average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol.

[0136] In addition, the rubber-based binders contained in the first positive electrode active material layer and the second positive electrode active material layer may satisfy the following Condition 1.

[0137] [Condition 1] 1 ≦ R2 / R1 ≦ 3

[0138] R1 represents the content (weight %) of the rubber-based binder contained in the first positive electrode active material layer, R2 represents the content (weight %) of the rubber-based binder contained in the second positive electrode active material layer.

[0139] Since the lithium iron phosphate, fluorine-based binder, rubber-based binder, and rubber-based dispersant contained in the first positive electrode active material layer and the second positive electrode active material layer have been described in detail above, duplicate descriptions are omitted.

[0140] The negative electrode can be manufactured, for example, by preparing a negative electrode forming composition containing a negative electrode active material, a negative electrode binder, and a negative electrode conductive material on a negative electrode current collector and then coating it on the negative electrode current collector.

[0141] The negative electrode active material is not particularly limited, and usually, a compound capable of reversible intercalation and deintercalation of lithium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and highly crystalline carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; or composites containing metallic compounds and carbonaceous materials. In addition, examples of low-crystalline carbon include soft carbon and hard carbon, and examples of highly crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes. One of these alone or a mixture of two or more can be used, and a thin film of metallic lithium can also be used as the negative electrode active material.

[0142] The above-mentioned negative electrode conductive material is used to impart conductivity to the electrode, and can be used without particular limitation as long as it has electron conductivity without causing chemical changes in the battery being configured. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based substances such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotube; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. Among these, one kind alone or a mixture of two or more kinds can be used. The above-mentioned negative electrode conductive material can typically be contained in an amount of 1% by weight to 30% by weight, specifically 1% by weight to 20% by weight, and more specifically 1% by weight to 10% by weight based on the total weight of the negative electrode active material layer.

[0143] The above-mentioned negative electrode binder serves to improve the adhesion between negative electrode active material particles and the adhesive force between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. Among these, one kind alone or a mixture of two or more kinds can be used. The above-mentioned negative electrode binder can be contained in an amount of 1% by weight to 30% by weight, specifically 1% by weight to 20% by weight, and more specifically 1% by weight to 10% by weight based on the total weight of the negative electrode active material layer.

[0144] On the one hand, the negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those with surface treatment such as carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. can be used.

[0145] Also, the negative electrode current collector can usually have a thickness of 3 μm to 500 μm. Similar to the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to strengthen the binding force of the negative electrode active material. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

[0146] On the one hand, in the above lithium secondary battery, the separator can be used without particular limitation as long as it is usually used as a separator in a lithium secondary battery. In particular, those with low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention ability are preferred. Specifically, porous polymer films, for example, porous polymer films made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof can be used. Also, typical porous non-woven fabrics, for example, non-woven fabrics made of high-melting glass fibers, polyethylene terephthalate fibers, etc. can also be used. Also, the separator can be a porous thin film having a pore diameter of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0147] On the one hand, in the above lithium secondary battery, the electrolyte can contain an organic solvent and a lithium salt typically used in electrolytes and is not particularly limited.

[0148] As the above-mentioned organic solvent, any solvent can be used without particular limitation as long as it can serve as a medium in which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0149] Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant, which can enhance the charge and discharge performance of the battery, and a linear carbonate compound with low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, etc.) is more preferred.

[0150] The above lithium salt can be used without particular limitation as long as it is a compound that can provide lithium ions used in a lithium secondary battery. Specifically, the above lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The above lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0151] In addition to the constituent components of the above electrolyte, for the purpose of improving the life characteristics of the battery, suppressing the reduction of the battery capacity, improving the discharge capacity of the battery, etc., the above electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. At this time, the above additive can be contained at 0.1% by weight to 5% by weight based on the total weight of the electrolyte.

[0152] The lithium secondary battery of the present invention can be manufactured by arranging a separator between the positive electrode and the negative electrode to form an electrode assembly, and then injecting an electrolyte after placing the above electrode assembly in a cylindrical battery case or a rectangular battery case. Or, after laminating the above electrode assemblies, the resultant obtained by impregnating them with an electrolyte can be placed in a battery case and sealed for manufacturing.

[0153] When manufacturing the lithium secondary battery of the present invention, the electrode assembly is dried to remove one or more organic solvents selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate used during the production of the positive electrode. If an electrolyte having the same components as the organic solvent used during the production of the positive electrode is used as the electrolyte, the step of drying the electrode assembly may be omitted.

[0154] As described above, unlike the lithium secondary battery described above, the lithium secondary battery according to another embodiment of the present invention may be an all-solid-state battery.

[0155] The battery case may be one typically used in the art and is not limited to an outer shape according to the use of the battery. For example, it may be a cylindrical shape using a can, a square shape, a pouch type, or a coin type.

[0156] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and thus is useful in mobile devices such as mobile phones, notebook computers, and digital cameras, in the field of electric vehicles such as energy storage systems (ESS) and hybrid electric vehicles (HEV).

[0157] Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are for illustrative purposes of the present invention, and the scope of the present invention is not limited thereto.

[0158] Example 1: Production of Positive Electrode (1) Production of First Positive Electrode Slurry

[0159] A dispersion liquid containing carbon nanotubes (CNT, BET specific surface area; 250 m 2 / g) as a conductive material and hydrogenated nitrile butadiene rubber having a weight average molecular weight (Mw) of 30,000 g / mol as a rubber-based dispersant was prepared.

[0160] As lithium iron phosphate, the average particle size D 50 is 1.0 μm, LiFePO4 which is a primary particle with a single structure, polyvinylidene fluoride (PVDF) with a weight average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, hydrogenated nitrile butadiene rubber with a weight average molecular weight (Mw) of 310,000 g / mol as a rubber-based binder, and after putting the above dispersion into an N-methylpyrrolidone (NMP) solvent, it was mixed at 2500 rpm for 90 minutes using Homo-disperse to produce a first positive electrode slurry. In the above first positive electrode slurry, lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a dispersant were present at a weight ratio of 96.26:0.8:2.2:0.5:0.24.

[0161] (2) Production of the second positive electrode slurry The second positive electrode slurry was produced in the same manner as the production method of the first positive electrode slurry, except that the weight ratios of lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a dispersant in the positive electrode slurry were changed as shown in Table 1.

[0162] (3) Production of the positive electrode Using a dual slot die coater, the first positive electrode slurry was coated on an aluminum foil at a loading amount of 300 mg / 25 cm 2 , and the second positive electrode slurry was coated at a loading amount of 300 mg / 25 cm 2 , and the coating was performed such that the second positive electrode slurry was laminated on the first positive electrode slurry.

[0163] Thereafter, it was dried with hot air at 130 °C for 5 minutes so that the solid content of the above positive electrode slurry became 99.0% by weight or more. Thereafter, a positive electrode was manufactured by rolling so that the porosity of the positive electrode active material layer became 26.6%. The above porosity means the porosity calculated by the following formula 1. In the following formula 1, the "measured density of the positive electrode active material layer" is a value obtained by measuring the weight and volume of the positive electrode active material layer separated from the positive electrode current collector of the rolled positive electrode and calculating the density, and the "true density of the positive electrode active material" is the density with respect to the volume of only the positive electrode active material particles excluding pores, and can be measured using a Pycnometer (AccuPycII 1340).

[0164] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0165] Examples 2 to 3, Comparative Examples 1 to 6: Manufacture of positive electrode The weight ratios of lithium iron phosphate, conductive material, fluorine-based binder, rubber-based binder, and dispersant in the first positive electrode slurry and the second positive electrode slurry were changed as shown in Table 1, and the positive electrodes were manufactured in the same manner as in Example 1 except that they were each rolled so that the porosity of the positive electrode became as shown in Table 1.

[0166] Comparative Example 7

[0167] As the conductive material, a dispersion containing carbon nanotubes (CNT, BET specific surface area; 250 m 2 / g) and hydrogenated nitrile butadiene rubber having a weight average molecular weight (Mw) of 30,000 g / mol as a rubber-based dispersant was prepared.

[0168] As lithium iron phosphate, the average particle size D 50LiFePO4, which is a primary particle with a size of 1.0 μm and a single structure, polyvinylidene fluoride (PVDF) with a weight average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, and the above dispersion were put into an N-methylpyrrolidone (NMP) solvent, and then mixed at 2500 rpm for 90 minutes using Homo-disperse to produce a positive electrode slurry. In the above positive electrode slurry, lithium iron phosphate, conductive material, fluorine-based binder, and dispersant were present in a weight ratio of 96.96:0.8:3.0:0.24.

[0169] The positive electrode slurry was coated on an aluminum foil at a loading amount of 600 mg / 25 cm 2 . It was dried with hot air at 130 °C for 5 minutes so that the solid content of the above positive electrode slurry was 99.0 wt% or more. Then, it was rolled to produce a positive electrode so that the porosity of the positive electrode active material layer was 29.2%.

[0170]

Table 1

[0171] Experimental Example 1: Viscosity Measurement of Positive Electrode Slurry

[0172] The viscosities of the positive electrode slurries produced in Examples 1 to 3 and Comparative Examples 1 to 7 were measured, and the results are shown in Table 2.

[0173] Specifically, after the positive electrode slurries produced in Examples 1 to 3 and Comparative Examples 1 to 7 were cooled for 1 hour at room temperature and a relative humidity of 1%, the viscosity of the positive electrode slurry composition was measured at 25 °C and a shear rate of 2.5 / s using a viscometer (Brookfield). The viscosity measurement was carried out within 2 hours including the cooling time after the production of the positive electrode slurry composition.

[0174] Experimental Example 2: Adhesion Test of Positive Electrode

[0175] After the positive electrodes manufactured in Examples 1 to 3 and Comparative Examples 1 to 7 were vacuum-dried at a temperature of 130°C for 2 hours, the adhesive force between the positive electrode active material layer and the positive electrode current collector was measured, and the results are shown in Table 2.

[0176] Specifically, the positive electrodes manufactured in Examples 1 to 3 and Comparative Examples 1 to 7 were cut into pieces with a length of 150 mm and a width of 20 mm, and the surface of the positive electrode was attached to a slide glass with a length of 75 mm and a width of 25 mm in the length direction using double-sided tape. That is, the slide glass was attached to a region corresponding to half of the length direction of the positive electrode. Then, a roller was rotated 10 times so that the double-sided tape adhered uniformly to produce an evaluation sample.

[0177] Next, the slide glass portion of the evaluation sample was fixed to the sample stage of a universal testing machine (UTM) (product name: LS5, manufacturer: LLOYD), and the half of the positive electrode to which the slide glass was not attached was connected to the load cell of the UTM device. A force was applied to the load cell at a speed of 100 mm / min at an angle of 90°, and the load applied to the load cell was measured while moving it up to 50 mm. At this time, after obtaining the average value of the load measured in the 20 mm to 40 mm section of the traveling section, this was repeated 5 times in total, and the average value was evaluated as the positive electrode adhesive force (gf / 20 mm) of each sample.

[0178] Experimental Example 3: Flexibility Test of Positive Electrode

[0179] After the positive electrodes manufactured in Examples 1 to 3 and Comparative Examples 1 to 7 were vacuum-dried at a temperature of 130°C for 2 hours, the flexibility of the manufactured positive electrodes was measured, and the results are shown in Table 2.

[0180] Specifically, the positive electrodes manufactured in Example 1 to Example 3 and Comparative Example 1 to Comparative Example 7 were each cut into a size of 10 cm in width and 30 cm in length. After preparing measuring rods with diameters of 2.5φ, 3φ, 4φ, 5φ, 6φ, 7φ, 8φ, 9φ, and 10φ, each of the cut positive electrodes was bent in half with the positive electrode current collector surface of each cut positive electrode facing the above-mentioned measuring rod, and both ends of the positive electrode were lifted at a speed of 10 mm per minute. At this time, lift until the force measured by the UTM reaches 5N. Measure by file, observe whether cracks occur in the electrode with an optical microscope, and if there are no cracks, proceed with the test with a smaller file. As a result, the diameter (φ) of the measuring rod at which cracks occur is shown in Table 2.

[0181] Experimental Example 4: Measurement of Fluorine Weight

[0182] In each of the positive electrodes according to the examples and comparative examples, after scraping the first positive electrode active material layer and the second positive electrode active material layer and collecting 100 mg of samples respectively, the weight of fluorine contained in each sample was measured using a combustion reaction technique, and the ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer was calculated, and the results are shown in Table 2.

[0183] To measure the weight of fluorine contained in each sample, the gas generated while completely burning the sample of the first positive electrode active material layer and the sample of the second positive electrode active material layer under high-pressure oxygen conditions (40 atm) was collected in an absorption liquid. The absorption liquid for gas collection is a 1 L solution containing 2.52 g of NaHCO3 and 2.52 g of Na2CO3. When fluoride ions were absorbed or dissolved in the above absorption liquid for gas collection, the fluorine weight of each sample was measured using ion chromatography. The column and analysis conditions of ion chromatography are as follows.

[0184] Ion Chromatography: ICS-3000 Column: IonPac AS18 (4×250 mm) Detector: Suppressed conductivity detector SRS current: 76 Ma Injection volume: 20 μL

[0185] The same test was repeated three times to show the average value.

[0186] Based on the analysis results of ion chromatography, Table 2 shows the ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer.

[0187]

Table 2

[0188] The positive electrodes according to Examples 1 to 3 satisfy the condition that the ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less. As a result, it was shown that the positive electrodes are far superior in adhesive strength compared to the positive electrode according to Comparative Example 3 that does not satisfy the above conditions.

[0189] The positive electrodes according to Examples 1 to 3 containing a rubber-based binder were shown to be far superior in flexibility compared to the positive electrode of Comparative Example 4 that does not contain a rubber-based binder.

[0190] It was shown that the positive electrode according to Comparative Example 1 with a porosity of less than 25% is inferior in flexibility compared to the positive electrode according to the Example, and the positive electrode according to Comparative Example 2 with a porosity exceeding 28% is inferior in adhesive strength compared to the positive electrode according to the Example. Therefore, it can be understood that it is preferable to control the porosity to 25% - 28% from the aspects of flexibility and adhesive strength.

[0191] On the other hand, in the case of Comparative Example 5, since the first positive electrode slurry and the second positive electrode slurry do not contain a rubber-based dispersant, the aggregation of lithium iron phosphate and the conductive material contained in the slurry was severe, and the electrode could not be manufactured.

[0192] Experimental Example 5: Resistance Evaluation of Battery Using the positive electrodes manufactured in Examples 1 to 3, Comparative Examples 1 to 2, and Comparative Examples 6 to 7 respectively, each lithium secondary battery was manufactured.

[0193] On the other hand, the negative electrode constituting the above lithium secondary battery is a mixture of artificial graphite (GT), natural graphite (AGP8), and SiO (KSC6027D) (85:10:5, wt%) as the negative electrode active material, a mixture of carbon black (Super-C65) and CNT (NCL-H5) as the conductive material, SBR (BM-L302) as the binder, and CMC (Daicel 2200) as the thickener in a weight ratio of 95.1:1.5:2.3:1.1, and added to distilled water as the solvent to produce a negative electrode slurry. The negative electrode slurry was coated on one side of a copper foil with a thickness of 6 μm, and dried and rolled under the same conditions as the above positive electrode to produce a negative electrode.

[0194] Also, the electrolytic solution is a non-aqueous electrolytic solution in which 0.1 wt% of tetravinylsilane (VS2), 1 wt% of ethylene sulfate (ESa), 0.5 wt% of 1,3-propene sultone (PS), 1 wt% of lithium difluorophosphate (DFP), and 0.2 wt% of LiBF4 are mixed in an organic solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a composition of 30:70 (volume ratio), and 0.7M LiPF6 and 0.5M LiFSI are dissolved. After interposing a separator (DB0905 / BA1, 8 μm) between the positive electrode and the negative electrode, the above electrolytic solution was injected to manufacture a lithium secondary battery.

[0195] The resistance of each secondary battery including the positive electrodes manufactured in Examples 1 to 3, Comparative Examples 1 to 2, and Comparative Examples 6 to 7 respectively was measured by the following method. It is discharged at a current of 0.2C from SOC100% to SOC30% at 25°C. Then, a pulse is applied by applying a current of 2.0C for 10 seconds in the SOC30% state. DCIR is calculated according to the following formula, and the results are shown in Table 3.

[0196]

Table 3

[0197] Since the initial resistance of the battery including the positive electrodes of Examples 1 to 3 where the porosity satisfies 25% to 28% is lower than that of the batteries including the positive electrodes of Comparative Examples 1, 2, 6, and 7 where the porosity does not satisfy the above range, the lithium secondary battery including the positive electrode according to the present invention is expected to have improved resistance characteristics.

[0198] As described above, the preferred embodiments of the present invention have been described. However, those skilled in the art or those having ordinary knowledge in the technical field can understand that the present invention can be variously modified and changed without departing from the spirit and technical scope of the present invention described in the claims.

[0199] Therefore, the technical scope of the present invention is not limited to the content described in the summary of the invention in the specification, but can be defined by the claims.

Claims

1. A positive electrode current collector, a first positive electrode active material layer formed on one or both surfaces of the positive electrode current collector, and a second positive electrode active material layer formed on the first positive electrode active material layer, wherein the first positive electrode active material layer and the second positive electrode active material layer each contain lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant, the ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, a positive electrode having a porosity of 25% to 28% calculated by the following formula 1. [Formula 1] Porosity = {1 - (measured density of the positive electrode active material layer / true density of the positive electrode active material)} × 100

2. The positive electrode according to claim 1, wherein the porosity calculated by the formula 1 is 25.5% to 27.5%.

3. The positive electrode according to claim 1, wherein the rubber-based binder and the rubber-based dispersant are each hydrogenated nitrile butadiene rubber (HNBR).

4. The positive electrode according to claim 3, wherein the rubber-based binder has a weight average molecular weight (Mw) of 130,000 g / mol or more.

5. The positive electrode according to claim 3, wherein the rubber-based binder has a weight average molecular weight (Mw) in the range of 150,000 g / mol to 1,000,000 g / mol.

6. The positive electrode according to claim 3, wherein the rubber-based dispersant has a weight average molecular weight (Mw) in the range of 10,000 g / mol to 100,000 g / mol.

7. The fluorine-based binder is contained in the first positive electrode active material layer in the range of 1.5% by weight to 3.0% by weight, and is contained in the second positive electrode active material layer in the range of 0.8% by weight to 2.4% by weight. The positive electrode according to claim 1.

8. The positive electrode according to claim 1, wherein the rubber-based binders contained in the first positive electrode active material layer and the second positive electrode active material layer each satisfy the following condition 1. [Condition 1] 1 ≤ R2 / R1 ≤ 3 R1 represents the content (% by weight) of the rubber-based binder contained in the first positive electrode active material layer, and R2 represents the content (% by weight) of the rubber-based binder contained in the second positive electrode active material layer.

9. The rubber-based binder is contained in the first positive electrode active material layer in the range of 0.2% by weight to 0.9% by weight, and is contained in the second positive electrode active material layer in the range of 0.3% by weight to 1.0% by weight. The positive electrode according to claim 1.

10. The positive electrode according to claim 1, wherein the conductive material is a carbon nanotube.

11. The conductive material is included in each of the first positive electrode active material layer and the second positive electrode active material layer in the range of 0.3% by weight to 2.0% by weight, and the positive electrode according to claim 1.

12. The lithium iron phosphate is a compound represented by the following Chemical Formula 1, and the positive electrode according to claim 1. [Chemical Formula 1] Li １＋ａ Fe １－ｘ M ｘ (PO ４－ｂ )X ｂ (In the Chemical Formula 1, M contains any one or two or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X contains any one or two or more elements selected from the group consisting of F, S, and N; and a, b, and x are each -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.1, and 0 ≤ x ≤ 0.5)

13. The average particle size D of the lithium iron phosphate ５０ is 0.6 μm to 3.0 μm, and the positive electrode according to claim 1.

14. The fluorine-based binder is a polyvinylidene fluoride (PVDF)-based polymer binder, and the positive electrode according to claim 1.

15. A lithium secondary battery including the positive electrode according to any one of claims 1 to 14.

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