Positive electrode and lithium secondary battery including the same

Abstract

The present invention relates to a positive electrode and a lithium secondary battery manufactured using 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 weight (A) of the first positive electrode active material layer is in the range of 17% to 99% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0177477, 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 layered lithium transition metal oxides. 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, therefore, lithium iron phosphate with a small average particle size has been used to form a short lithium migration path, the surface of lithium iron phosphate has been coated with carbon to improve electrical conductivity, and an excessive amount of conductive material has been used.

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

[0008] When the positive electrode adhesive force decreases, there are problems such as detachment of the positive electrode active material layer during electrode manufacturing or charge and discharge, an increase in battery resistance, and a decrease in the capacity of the secondary battery.

[0009] As the demand for high-energy density batteries increases, in a positive electrode containing lithium iron phosphate, while ensuring a loading amount of 550 mg / 25 cm 2 or more, a technique for improving the positive electrode adhesive force is required.

Summary of the Invention

Problems to be Solved by the Invention

[0010] The present invention is for solving the above problems, and in a high-loading positive electrode, by improving the adhesive force between the positive electrode current collector and the positive electrode active material layer and improving flexibility, it is an object to provide a positive electrode in which detachment of the electrode is prevented and a lithium secondary battery manufactured using the same.

Means for Solving the Problems

[0011] According to an embodiment of the present invention, a positive electrode is provided. 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 weight (A) of the first positive electrode active material layer is in the range of 17% to 99% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer. A positive electrode is provided.

[0012] In an exemplary embodiment, the weight (A) of the first positive electrode active material layer may be in the range of 20% to 85% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer.

[0013] In an exemplary embodiment, the rubber-based binder and the rubber-based dispersant may each be hydrogenated nitrile butadiene rubber (HNBR).

[0014] 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, preferably in the range of 150,000 g / mol to 1,000,000 g / mol.

[0015] In an exemplary embodiment, the rubber-based dispersant may be hydrogenated nitrile butadiene rubber (HNBR) having a weight average molecular weight (Mw) in the range of 10,000 g / mol to 100,000 g / mol.

[0016] In an exemplary embodiment, the lithium iron phosphate contained in the first positive electrode active material layer is the first lithium iron phosphate having an average particle size D 50 of 0.5 μm to 2 μm and an average particle size D 50It can be made of a second lithium iron phosphate having a particle size of 3.5 μm to 15 μm.

[0017] At this time, the weight ratio of the first lithium iron phosphate to the second lithium iron phosphate can be 50:50 to 99.9:0.1.

[0018] In an exemplary embodiment, the average particle size D of the lithium iron phosphate contained in the second positive electrode active material layer 50 can be 0.6 μm to 3.0 μm.

[0019] In an exemplary embodiment, the lithium iron phosphate can be a compound represented by the following Chemical Formula 1.

[0020] [Chemical Formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0021] In Chemical Formula 1 above, 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.

[0022] In an exemplary embodiment, the fluorine-based binder can be contained in the first positive electrode active material layer in the range of 1.5% by weight to 3.0% by weight.

[0023] In an exemplary embodiment, the fluorine-based binder can be contained in the second positive electrode active material layer in the range of 0.8% by weight to 2.4% by weight.

[0024] In an exemplary embodiment, the fluorine-based binder can be a polyvinylidene fluoride (PVDF)-based polymer binder.

[0025] 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.

[0026] [Condition 1] 1 ≦ R2 / R1 ≦ 3

[0027] R1 represents the content (% by weight) of the rubber-based binder contained in the first positive electrode active material layer, R2 represents the content (% by weight) of the rubber-based binder contained in the second positive electrode active material layer.

[0028] In an exemplary embodiment, the rubber-based binder may be contained in the first positive electrode active material layer in the range of 0.2% to 0.9% by weight and in the second positive electrode active material layer in the range of 0.3% to 1.0% by weight.

[0029] In an exemplary embodiment, the conductive material may be a carbon nanotube.

[0030] 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 the range of 0.3% to 2.0% by weight.

[0031] The present invention also provides a lithium secondary battery including the positive electrode described above.

Advantages of the Invention

[0032] The positive electrode according to the present invention includes a first positive electrode active material layer that maximizes the adhesive force and a second positive electrode active material layer that contributes to the improvement of 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.

[0033] In addition, the positive electrode according to the present invention controls the weight ratio of the first positive electrode active material layer and the second positive electrode active material layer within a suitable range, and has an excellent effect on the adhesive force.

Embodiments for Carrying Out the Invention

[0034] Hereinafter, in order to facilitate the understanding of the present invention, the present invention will be described in more detail.

[0035] The terms and words used in this specification and the claims should not be construed as being limited to their ordinary or dictionary meanings, but rather should be construed as meanings and concepts consistent with the technical idea of the present invention, based on the principle that the inventor can appropriately define the concept of the terms in order to best explain his own invention.

[0036] The terms used in this specification are merely used to illustrate exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

[0037] In this specification, terms such as "comprising", "including", or "having" are intended to specify the presence of implemented features, numbers, steps, components, or combinations thereof, and should be understood not to preclude in advance the presence or addition possibility of one or more other features, numbers, steps, components, or combinations thereof.

[0038] In this specification, "%" means weight % unless otherwise explicitly indicated.

[0039] In this specification, the average particle size (D 50 ) can be defined as the particle size at the 50% criterion of the particle size distribution and can be measured using the laser diffraction method. Specifically, the diameter (D 50 ) of the above average particles, after dispersing the target particles in a dispersion medium, is introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac MT 3000), irradiated with ultrasonic waves of about 28 kHz at an output of 60 W, and then the average particle diameter (D 50 ) at the 50% criterion of the particle volume cumulative distribution according to the particle size in the measuring device can be calculated.

[0040] In this specification, the "specific surface area" is measured by the BET method, and specifically, it can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mino II manufactured by BEL Japan, Inc.

[0041] In this specification, "phi (φ)" represents the diameter of the measuring rod in millimeters (mm).

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

[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 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 in the longitudinal direction using double-sided tape. That is, the slide glass is attached to the region corresponding to half of the longitudinal direction of the positive electrode. Then, a roller is rotated 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 to which the slide glass is not attached to the load cell of the UTM device. Apply a force of 90° to the load cell at a speed of 100 mm / min and measure the load applied to the load cell while moving it up 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 (gf / 20 mm) of each sample.

[0045] The present invention provides a positive electrode and a lithium secondary battery manufactured using the same.

[0046] Lithium iron phosphate, which is generally used as a positive electrode active material, has 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 lithium iron phosphate particles is small, the specific surface area increases, resulting in intense particle aggregation and ineffective mixing of lithium iron phosphate and the binder, which may reduce the positive electrode adhesion force. And the fluorine-based binder used as the positive electrode binder has the characteristic of being better adhered to lithium iron phosphate particles than to the positive electrode current collector. Therefore, 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 the detachment of the positive electrode active material layer during electrode manufacturing or charge and 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 the detachment of the positive electrode active material layer due to physical external force.

[0047] Therefore, as a result of repeated research to solve such problems, the inventors of the present invention configured the positive electrode active material layer not as a single layer but as a plurality of layers, 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, and the weight (A) of the first positive electrode active material layer is in the range of 17% to 99% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer. It was discovered that while the interfacial adhesion force between the current collector and the first positive electrode active material layer is excellent, the detachment of the positive electrode active material layer is suppressed due to improved flexibility, leading to the present invention.

[0048] 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, it may not be necessary to include a 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.

[0049] The positive electrode of the present invention will be specifically described.

[0050] 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.

[0051] In an exemplary embodiment of the present invention, the weight (A) of the first positive electrode active material layer may be in the range of 17% to 99, preferably 20% to 85%, and more preferably 25% to 75% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer. When the ratio of the weight (A) of the first positive electrode active material layer satisfies the above range, while having the adhesion force required for the positive electrode, it exhibits suitable flexibility and can prevent the detachment of the positive electrode active material layer due to physical external force.

[0052] 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.

[0053] 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.

[0054] The first positive electrode active material layer and the second positive electrode active material layer can each contain a positive electrode active material. Additionally, the first positive electrode active material layer and the second positive electrode active material layer can further contain a conductive material, a binder, and a dispersant as necessary in addition to the positive electrode active material.

[0055] Hereinafter, each component included in the first positive electrode active material layer and the second positive electrode active material layer will be specifically described.

[0056] (1) Positive electrode active material

[0057] 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.

[0058] The above lithium iron phosphate can be a compound of the following Chemical Formula 1.

[0059] [Chemical Formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0060] In the above Chemical Formula 1, M contains any one or two or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X contains any one or two or more elements selected from the group consisting of F, S, and N; and a, b, and x are respectively -0.5 ≦ a ≦ 0.5, 0 ≦ b ≦ 0.1, and 0 ≦ x ≦ 0.5.

[0061] For example, the above lithium iron phosphate may be LiFePO4.

[0062] In the present invention, the "monolith structure" means a structure in which particles exist as independent phases that do not aggregate with each other morphologically. As a particle structure contrasted with such a monolith structure, there is a structure in which small-sized particles ("primary particles") are physically and / or chemically aggregated to form a relatively large-sized particle form ("secondary particles").

[0063] When lithium iron phosphate has a monolith structure composed of primary particles, compared with the case of secondary particles, the possibility of the occurrence of cracking phenomena of lithium iron phosphate particles during the rolling process is small, so it is preferable that the capacity reduction due to the detachment of cracked particles is small. Further, when lithium iron phosphate is primary particles of a monolith structure, the migration phenomenon of the binder during the drying process of the positive electrode slurry can be alleviated, so it can also be said to be preferable from the aspect of the interfacial adhesion force between the positive electrode current collector and the positive electrode active material layer.

[0064] 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.

[0065] The carbon coating layer can be formed using at least one raw material selected from the group consisting of glucose, sucrose, lactose, starch, oligosaccharides, polyoligosaccharides, fructose, cellulose, polymers of furfuryl alcohol, block copolymers of ethylene and ethylene oxide, vinyl resins, cellulose resins, phenolic resins, pitch resins, and tar resins. Specifically, the carbon coating layer can be formed through a process of heat-treating after mixing the raw material with the lithium iron phosphate.

[0066] The average particle size D of lithium iron phosphate 50 is 0.5 μm to 20.0 μm, preferably 0.5 μm to 10.0 μm, more preferably 0.6 μm to 3 μm, even more preferably 0.6 μm to 2.5 μm, and most preferably 0.7 μm to 1.5 μm. When the average particle size D of the positive electrode active material 50 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.

[0067] The BET specific surface area of lithium iron phosphate is 5 m 2 / g to 20 m 2 / g, specifically 7 m 2 / g to 18 m 2 / g, more specifically 9 m 2 / g to 16 m 2 / g. The above range corresponds to a lower value compared to ordinary lithium iron phosphate. When the above range is satisfied, the aggregation of the lithium iron phosphate can be effectively suppressed even in a positive electrode slurry composition with a relatively low content of dispersant.

[0068] The positive electrode according to an embodiment of the present invention may include a first lithium iron phosphate and a second lithium iron phosphate having different ranges of average particle size (D 50 ) in the first positive electrode active material layer. Specifically, the lithium iron phosphate contained in the first positive electrode active material layer is a first lithium iron phosphate having an average particle size D in the range of 0.5 μm to 2 μm and a second lithium iron phosphate having an average particle size D in the range of 3.5 μm to 15 μm 50 and an average particle size D in the range of 3.5 μm to 15 μm 50It may contain lithium iron phosphate having

[0069] The binder contained in the positive electrode active material layer, particularly the fluorine-based binder, tends to adhere to lithium iron phosphate rather than the current collector at the interface between the positive electrode current collector and the positive electrode active material layer. However, such a phenomenon can be more pronounced as the specific surface area of the lithium iron phosphate particles is larger. Therefore, in the first positive electrode active material layer in contact with the positive electrode current collector, the average particle size D 50 When it contains lithium iron phosphate having a large particle size and a small specific surface area, as a result of the reduction in the total specific surface area of lithium iron phosphate to which the binder can be bonded, the interfacial adhesion force between the current collector and the positive electrode active material layer can be remarkably improved.

[0070] In one embodiment, the average particle size D of the first lithium iron phosphate 50 can be 0.5 μm to 2 μm, preferably 0.55 μm to 1.8 μm, and more preferably 0.6 μm to 1.6 μm. Also, the average particle size D of the second lithium iron phosphate 50 can be 3.5 μm to 15 μm, preferably 3.8 μm to 12 μm, and more preferably 4 μm to 10 μm.

[0071] In one embodiment, the weight ratio of the first lithium iron phosphate to the second lithium iron phosphate can be 50:50 to 99.9:0.1, preferably 55:45 to 95:5, and more preferably 6:4 to 9:1. When the weight ratio of the first lithium iron phosphate to the second lithium iron phosphate satisfies the above range, it is far superior in the adhesion improvement effect and also excellent in the electrical performance of the positive electrode. When the second lithium iron phosphate having a relatively large average particle size is excessively contained, the viscosity of the positive electrode slurry can rapidly increase and the rolling performance can deteriorate, which is not preferable.

[0072] The first lithium iron phosphate can be primary particles, and the second lithium iron phosphate can be primary particles or secondary particles.

[0073] When the first positive electrode active material layer contains the first lithium iron phosphate and the second lithium iron phosphate, the lithium iron phosphate contained in the second positive electrode active material layer has an average particle size (D 50 ) that can be 0.5 μm to 20.0 μm, 0.5 μm to 10.0 μm, 0.5 μm to 2 μm, 0.55 μm to 1.8 μm, 0.6 μm to 3.0 μm, 0.6 μm to 2.5 μm, 0.6 μm to 1.6 μm, 0.7 μm to 1.5 μm. Since the second positive electrode active material layer functions to improve the flexibility of the positive electrode, the smaller the average particle size of the lithium iron phosphate contained in the second positive electrode active material layer, the more preferable it is from the aspect of the flexibility of the positive electrode.

[0074] 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 at 93.5 to 98% by weight, specifically 94 to 97.5% by weight, specifically 94.5 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 to 99% by weight, specifically 94.5 to 98.5% by weight, specifically 95 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.

[0075] (2) Binder

[0076] 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.

[0077] 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 fluoride-cotetrafluoroethylene)), PVDF-TrFE (Poly(vinylidene fluoride-co-trifluoroethylene)), and the like.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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 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 aspect of adhesion.

[0082] 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.

[0083] 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.

[0084] 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 even with the same binder content compared to a positive electrode composed of a single layer.

[0085] In an exemplary embodiment, the fluorine-based binder may be contained in the first positive electrode active material layer at 1.5 wt% to 3.0 wt%, preferably 1.6 wt% to 2.7 wt%, and more preferably 1.7 wt% to 2.5 wt%. Further, the fluorine-based binder may be contained in the second positive electrode active material layer at 0.8 wt% to 2.4 wt%, preferably 1.0 wt% to 2.2 wt%, and more preferably 1.2 wt% to 2.0 wt%.

[0086] Also, 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 fluorine-based binder may be contained at 1.2 wt% to 2.6 wt%, preferably 1.4 wt% to 2.4 wt%, and more preferably 1.6 wt% to 2.2 wt%.

[0087] 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. Also, 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.

[0088] According to an exemplary embodiment of the present invention, the rubber-based binder may be hydrogenated nitrile butadiene rubber (HNBR). The 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.

[0089] The hydrogenated nitrile butadiene rubber may have a content of repeating units derived from acrylonitrile (AN) of 20 wt% to 50 wt% based on the total weight, more preferably 25 wt% to 45 wt%, and most preferably 30 wt% to 40 wt%.

[0090] When the weight average molecular weight (Mw) of hydrogenated nitrile butadiene rubber (HNBR) is 130,000 g / mol or more, the inventors of the present invention discovered that 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 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 hydrogenated nitrile butadiene rubber is within the above range, 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 be reduced.

[0091] The rubber-based binders contained in the first positive electrode active material layer and the second positive electrode active material layer may be the same compound, and their weight average molecular weights and chemical formulas may be the same.

[0092] 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 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.

[0093] [Condition 1] 1 ≤ R2 / R1 ≤ 3

[0094] R1 represents the content (% by weight) of the rubber-based binder contained in the first positive electrode active material layer, R2 represents the content (% by weight) of the rubber-based binder contained in the second positive electrode active material layer.

[0095] 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, the rubber-based binder is contained less in the second positive electrode active material layer, and thus the flexibility of the entire positive electrode may decrease, which is not preferable.

[0096] In an exemplary embodiment, the rubber binder may be contained in the first positive electrode active material layer at 0.2 wt% to 0.9 wt%, preferably 0.3 wt% to 0.8 wt%, and more preferably 0.4 wt% to 0.7 wt%. And the rubber binder may be contained in the second positive electrode active material layer at 0.3 wt% to 1.0 wt%, preferably 0.4 wt% to 0.9 wt%, and more preferably 0.5 wt% to 0.8 wt%. And the rubber binder may be contained in the entire first positive electrode active material layer and the second positive electrode active material layer at 0.25 wt% to 0.85 wt%, preferably 0.35 wt% to 0.75 wt%, and more preferably 0.4 wt% to 0.65 wt%.

[0097] When the rubber binder is contained in each of the first positive electrode active material layer and the second positive electrode active material layer within the above ranges, 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.

[0098] The weight ratio of the fluorine-based binder to the rubber binder in the 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 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.

[0099] The weight ratio of the fluorine-based binder to the rubber 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 binder satisfies the above range, while the coating stability of the slurry is excellent, the flexibility of the positive electrode may be excellent.

[0100] In addition, the total amount of the fluorine-based binder and the rubber-based binder contained in the first and second positive electrode active material layers may be 1.8% by weight to 3.6% by weight based on the entire positive electrode active material layer.

[0101] (3) Dispersant

[0102] The first and second positive electrode active material layers constituting the positive electrode according to the present invention each contain a rubber-based dispersant.

[0103] 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.

[0104] 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 bond originally contained in the nitrile butadiene rubber (NBR) becomes a single bond.

[0105] 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 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.

[0106] Moreover, hydrogenated nitrile butadiene rubber suppresses the aggregation of the conductive material, improves the positive electrode conductive network, and aggregates the conductive material in a spherical shape rather than a linear shape when the conductive material is aggregated. As a result, the specific surface area of the aggregated conductive material is minimized as compared with the case where the conductive material is aggregated linearly. As a result, the surface area of the positive electrode active material that cannot participate in the insertion / desorption reaction of lithium adjacent to the aggregated conductive material is minimized, so that there is also an effect of reducing the discharge resistance of the lithium secondary battery.

[0107] The above dispersant can be contained in the first positive electrode active material layer in an amount of 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 can also be contained in the second positive electrode active material layer within the above range. 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.

[0108] (4) Conductive material

[0109] 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.

[0110] 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 can be used. As the conductive material of the present invention, carbon nanotubes, carbon nanofibers, and carbon black are preferred, and carbon nanotubes are most preferred. The conductive network of carbon nanotubes can alleviate the floating phenomenon of the binder 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.

[0111] A carbon nanotube is one in which a graphite sheet has a cylinder shape with a nano-sized diameter and has an sp 2 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), double-walled carbon nanotubes, and multi-walled carbon nanotubes according to the number of walls forming them, and these carbon nanotubes can be suitably selected according to the use of the dispersion liquid.

[0112] Further, the carbon nanotube may have a secondary shape formed by aggregating or arranging 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 spherical 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, it is more preferable that the carbon nanotube is a bundle type carbon nanotube.

[0113] 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.

[0114] 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 in an amount of 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 aggravated, which is not preferable. When the content of the conductive material exceeds 2.0% by weight, as a result, the content of the dispersant becomes high, and the content of the positive electrode active material may decrease, which is not preferable.

[0115] 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.

[0116] [Condition 2] 0.5 ≦ E2 / E1 ≦ 0.9

[0117] 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.

[0118] In the positive electrode satisfying 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 is favorably formed by the conductive material 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.

[0119] When satisfying Condition 2, the above conductive material may be contained in the first positive electrode active material layer in the range of 1.0 wt% to 1.5 wt%, preferably in the range of 1.1 wt% to 1.4 wt%. Further, the above conductive material may be contained in the second positive electrode active material layer in the range of 0.4 wt% to 0.95 wt%, preferably in the range of 0.6 to 0.90 wt%.

[0120] The positive electrode according to the present invention can be manufactured according to the usual 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 above positive electrode slurry composition onto a positive electrode current collector, and then drying and rolling.

[0121] Also, in another method, the above positive electrode can also be manufactured by casting the above positive electrode slurry composition onto a separate support, and then laminating the film obtained by peeling it from the support onto a positive electrode current collector.

[0122] On the other hand, according to one embodiment of the present invention, the positive electrode may have a porosity in the range of 26% to 34%, preferably in the range of 26.5% to 33%, and more preferably in the range of 27% to 30% calculated by the following Formula 1. When the porosity of the positive electrode satisfies the above range, both the flexibility and resistance characteristics of the positive electrode may be excellent.

[0123] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0124] 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 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).

[0125] The above porosity can be adjusted to the above numerical range by suitably adjusting the thickness change rate of the positive electrode active material layer before and after rolling during the rolling process in the production 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.

[0126] 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

[0127] 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.

[0128] The positive electrode according to an embodiment of the present invention includes a first positive electrode active material layer that maximizes the adhesive force and a second positive electrode active material layer that maximizes the flexibility, so that compared with the conventional single-layer positive electrode, excellent positive electrode adhesive force and flexibility can be obtained even with a small binder content.

[0129] As a result, it is possible to manufacture a high-loading positive electrode, and by preventing the detachment of the positive electrode, the cell 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.

[0130] 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 25 gf / 20 mm or more.

[0131] In addition, the positive electrode of the present invention can have a flexibility such that the maximum phi (φ) value of the measuring rod at which cracks occur during the flexibility test of lifting the positive electrode after contacting the phi-by-phi measuring rod on the positive electrode active material layer is 5 phi (φ) or less, specifically 4 phi (φ) or less, and more specifically 3 phi (φ) or less.

[0132] <Lithium secondary battery>

[0133] Next, the lithium secondary battery according to the present invention will be described.

[0134] 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.

[0135] 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 weight (A) of the first positive electrode active material layer is in the range of 17% to 99% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer.

[0136] 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.

[0137] The above rubber-based dispersant may be hydrogenated nitrile butadiene rubber (HNBR) having a weight average molecular weight (Mw) in the range of 10,000 g / mol to 100,000 g / mol.

[0138] Also, 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.

[0139] [Condition 1] 1 ≦ R2 / R1 ≦ 3

[0140] R1 represents the content (% by 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.

[0141] 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 explanations are omitted.

[0142] The above negative electrode can be produced, for example, by manufacturing a negative electrode forming composition containing a negative electrode active material, a negative electrode binder, and a negative electrode conductive material on a negative electrode current collector and then applying it onto the negative electrode current collector.

[0143] The above negative electrode active material is not particularly limited, and usually, a compound capable of reversible intercalation and deintercalation of lithium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and highly crystalline carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; or composites containing metallic compounds and carbonaceous materials. Also, 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 or a mixture of two or more can be used, and a thin film of metallic lithium can also be used as the above negative electrode active material.

[0144] 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 electronic conductivity without causing chemical changes in the configured battery. 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. One of these or a mixture of two or more thereof can be used. The above-mentioned negative electrode conductive material can usually be contained in an amount of 1 wt% to 30 wt%, specifically 1 wt% to 20 wt%, more specifically 1 wt% to 10 wt% based on the total weight of the negative electrode active material layer.

[0145] The above-mentioned negative electrode binder plays a role in improving the adhesion between negative electrode active material particles and the adhesive force between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. One of these or a mixture of two or more thereof can be used. The above-mentioned negative electrode binder can be contained in an amount of 1 wt% to 30 wt%, specifically 1 wt% to 20 wt%, more specifically 1 wt% to 10 wt% based on the total weight of the negative electrode active material layer.

[0146] On the one hand, the negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those surface-treated with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. can be used.

[0147] 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.

[0148] On the one hand, in the above lithium secondary battery, the separator can be used without particular limitation as long as it is usually used as a separator in a lithium secondary battery. In particular, those with low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention ability are preferred. Specifically, porous polymer films, for example, porous polymer films made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof can be used. Also, ordinary porous non-woven fabrics, for example, non-woven fabrics made of high-melting-point 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.

[0149] On the one hand, in the above lithium secondary battery, the electrolyte can contain organic solvents and lithium salts usually used in electrolytes and is not particularly limited.

[0150] As the above-mentioned organic solvent, any solvent can be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or 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).

[0151] Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (such as ethylene carbonate or propylene carbonate) having a high ionic conductivity and a high dielectric constant, which can enhance the charge-discharge performance of the battery, and a chain carbonate compound having a low viscosity (such as ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0152] 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%.

[0153] In addition to the constituent components of the electrolyte, the above electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, triamide hexaline, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride for the purpose of improving the life characteristics of the battery, suppressing the capacity reduction of the battery, improving the discharge capacity of the battery, etc. At this time, the above additive can be contained at 0.1% by weight to 5% by weight based on the total weight of the electrolyte.

[0154] 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 putting the above electrode assembly into 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 put into a battery case and sealed for manufacturing.

[0155] 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. When using an electrolyte having the same components as the organic solvent used during the production of the positive electrode, the step of drying the electrode assembly can be omitted.

[0156] As described above, unlike the lithium secondary battery described above, the lithium secondary battery according to another embodiment of the present invention can be an all-solid-state battery.

[0157] The battery case can be selected from those commonly used in the art, and there is no limitation on the outer shape according to the use of the battery. For example, it can be a cylindrical shape, a rectangular shape, a pouch type, or a coin type using a can.

[0158] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and thus is useful in portable devices such as mobile phones, notebook computers, and digital cameras, in the field of electric vehicles such as energy storage systems (ESS) and hybrid electric vehicles (HEV).

[0159] 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.

[0160] Example 1: Production of Positive Electrode

[0161] (1) Production of the First Positive Electrode Slurry

[0162] As a conductive material, carbon nanotubes (CNT, BET specific surface area: 250 m 2 / g) and a dispersion liquid containing hydrogenated nitrile butadiene rubber having a weight average molecular weight (Mw) of 30,000 g / mol as a rubber-based dispersant were prepared.

[0163] As lithium iron phosphate, the average particle size D 50 is 1.1 μm, and LiFePO4 which is a primary particle of a single body structure, polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 1,000,000 g / mol as a fluorine-based binder, hydrogenated nitrile butadiene rubber having a weight average molecular weight (Mw) of 310,000 g / mol as a rubber-based binder, and the above dispersion liquid were put into an N-methylpyrrolidone (NMP) solvent, and then 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 rubber-based dispersant were present in a weight ratio of 96:1.0:2.2:0.5:0.3. Also, the solid content of the first positive electrode slurry was 62 wt%.

[0164] (2) Production of the second positive electrode slurry

[0165] 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 ratio of lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant in the positive electrode slurry was changed to 96.8:1.0:1.4:0.5:0.3. Also, the solid content of the second positive electrode slurry was 67 wt%.

[0166] (3) Production of the positive electrode

[0167] Using a dual slot die coater, the first positive electrode slurry was coated on an aluminum foil at a loading amount of 450 mg / 25 cm 2 and the second positive electrode slurry was coated at a loading amount of 150 mg / 25 cm 2 and coated so that the second positive electrode slurry was laminated on the first positive electrode slurry.

[0168] 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 29%. The above porosity means the porosity calculated by the following formula 1.

[0169] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0170] Example 2: Manufacture of positive electrode

[0171] When manufacturing the positive electrode, except that the first positive electrode slurry was coated 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 a positive electrode was manufactured in the same manner as in Example 1.

[0172] Example 3: Manufacture of positive electrode

[0173] When manufacturing the positive electrode, except that the first positive electrode slurry was coated at a loading amount of 150 mg / 25 cm 2 and the second positive electrode slurry was coated at a loading amount of 450 mg / 25 cm 2 a positive electrode was manufactured in the same manner as in Example 1.

[0174] Example 4: Manufacture of positive electrode

[0175] When manufacturing the first positive electrode slurry, as the first lithium iron phosphate, LiFePO4 having an average particle size D 50 of 1.1 μm and being primary particles of a single body structure, and as the second lithium iron phosphate, LiFePO4 having an average particle size D 50 of 7 μm and being secondary particles were used at a ratio of 70:30. When manufacturing the positive electrode, except that the first positive electrode slurry was coated 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 a positive electrode was manufactured in the same manner as in Example 1.

[0176] Example 5: Production of the positive electrode

[0177] During the production of the first positive electrode slurry, LiFePO4 with an average particle size D 50 of 1.1 μm and being primary particles with a single body structure, and LiFePO4 with an average particle size D 50 of 7 μm and being secondary particles were used at a ratio of 50:50. During the production of the positive electrode, except that the first positive electrode slurry was coated 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 , the positive electrode was produced in the same manner as in Example 1.

[0178] Examples 6 - 7: Production of the positive electrode

[0179] In each of the first positive electrode slurry and the second positive electrode slurry, the weight ratios of lithium iron phosphate, conductive material, fluorine-based binder, rubber-based binder, and rubber-based dispersant were changed as shown in Table 1. During the production of the positive electrode, except that the first positive electrode slurry was coated 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 , the positive electrode was produced in the same manner as in Example 1.

[0180] Comparative Example 1: Production of the positive electrode

[0181] On the aluminum foil, the first positive electrode slurry produced in Example 1 above was coated at a loading amount of 600 mg / 25 cm 2 .

[0182] Thereafter, the positive electrode was produced in the same method as in Example 1.

[0183] Comparative Example 2: Production of the positive electrode

[0184] On the aluminum foil, the second positive electrode slurry produced in Example 1 above was coated at a loading amount of 600 mg / 25 cm 2 .

[0185] Thereafter, the positive electrode was manufactured in the same manner as in Example 1.

[0186] Comparative Examples 3 - 4: Manufacture of Positive Electrode

[0187] In each of the first positive electrode slurry and the second positive electrode slurry, the weight ratio of lithium iron phosphate, conductive material, fluorine-based binder, rubber-based binder, and rubber-based dispersant was changed as shown in Table 1. When manufacturing the positive electrode, except that the first positive electrode slurry was coated 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 , the positive electrode was manufactured in the same manner as in Example 1.

[0188] Comparative Example 5: Manufacture of Positive Electrode

[0189] When manufacturing the positive electrode, except that the first positive electrode slurry was coated at a loading amount of 60 mg / 25 cm 2 and the second positive electrode slurry was coated at a loading amount of 540 mg / 25 cm 2 , the positive electrode was manufactured in the same manner as in Example 1.

[0190]

Table 1

[0191] Experimental Example 1: Adhesion Test of Positive Electrode

[0192] After the positive electrodes manufactured in Examples 1 - 7 and Comparative Examples 1 - 5 were vacuum dried at a temperature of 130°C for 2 hours, the adhesion between the positive electrode active material layer and the positive electrode current collector was measured, and the results are shown in Table 2.

[0193] Specifically, the positive electrodes manufactured in Examples 1 to 7 and Comparative Examples 1 to 5 were each cut to a length of 150 mm and a width of 20 mm, and the surface of the positive electrode was adhered to a slide glass with 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 adhered to an area corresponding to half of the longitudinal 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.

[0194] 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 without the adhered slide glass was connected to the load cell of the UTM device. A force of 90° was applied to the load cell at a speed of 100 mm / min, 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 running section, this was repeated 5 times in total, and the average value was evaluated as the positive electrode adhesion (gf / 20 mm) of each sample.

[0195] Experimental Example 2: Flexibility Test of Positive Electrode

[0196] After the positive electrodes manufactured in Examples 1 to 7 and Comparative Examples 1 to 5 were vacuum-dried at a temperature of 130°C for 2 hours, the flexibility of each manufactured positive electrode was measured, and the results are shown in Table 2.

[0197] Specifically, the positive electrodes manufactured in Examples 1 to 7 and Comparative Examples 1 to 5 were each cut into a size of 10 cm in width and 30 cm in length. After preparing measuring rods having 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 5 N. 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.

[0198] Experimental Example 3: Measurement of Fluorine Weight

[0199] 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.

[0200] 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 solution. The absorption solution 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 solution 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.

[0201] Ion Chromatography: ICS-3000 Column: IonPac AS18 (4×250 mm) Detector: Suppressed Conductivity Detector SRS current: 76 mA Injection volume: 20 μL

[0202] The same test was repeated three times and the average value was shown.

[0203] 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.

[0204] [Table 2]

[0205] The positive electrode according to Comparative Example 1 is composed of a single-layer positive electrode having the composition of each first positive electrode active material layer of the positive electrodes according to Examples 1 to 3 and does not include a second positive electrode active material layer. Therefore, it was shown that the flexibility is far inferior to that of the positive electrodes according to the above Examples. The positive electrode according to Comparative Example 2 is composed of a single-layer positive electrode having the composition of each second positive electrode active material layer of the positive electrodes according to Examples 1 to 3 and does not include a first positive electrode active material layer. Therefore, it was shown that the adhesive force is far inferior to that of the positive electrodes according to the above embodiments.

[0206] For the positive electrode according to Comparative Example 3 in which 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 exceeds 1, it was shown that the adhesive force is low compared to each positive electrode of the Examples.

[0207] The positive electrode according to Comparative Example 4 that does not include a rubber-based binder was shown to have lower flexibility compared to each positive electrode of the Examples.

[0208] When comparing the adhesion of the positive electrodes according to Examples 1 to 3 and Comparative Example 5, it was shown that the adhesion of the positive electrode according to Comparative Example 5, where the weight (A) of the first positive electrode active material layer is 10% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer, is the lowest, and it was shown that the positive electrode according to Example 1, where the weight ratio of the first positive electrode active material layer is 75% and is the largest, is the most excellent. Therefore, in the present invention, it is preferable to increase the weight ratio of the first positive electrode active material layer in order to increase the adhesion, and in order to secure the minimum adhesion, it is considered that the weight (A) of the first positive electrode active material layer needs to exceed 10% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer.

[0209] When comparing the flexibility of the positive electrodes according to Examples 1 to 3 and Comparative Example 5, it can be seen that the smaller the ratio of the weight (A) of the first positive electrode active material layer to the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer, the more advantageous it is for flexibility. However, considering that the difference in these flexibilities is not large, in order to secure flexibility and adhesion simultaneously, it is considered preferable that the weight (A) of the first positive electrode active material layer is 25% to 75% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer.

[0210] On the other hand, the positive electrodes according to Examples 4 and 5 containing lithium iron phosphate (LiFePO₄) with an average particle diameter D 50 of 1.1 μm as the positive electrode active material in the first positive electrode active material layer and lithium iron phosphate (LiFePO₄) with an average particle diameter D 50 of 7 μm were shown to be far superior in adhesion compared to the positive electrode according to Example 2. From this, when controlling the particle diameter of the positive electrode active material in the first positive electrode active material layer facing the current collector in a bimodal form, it is expected that the effect of improving the adhesion between the positive electrode current collector and the active material layer will be maximized.

[0211] As described above, the preferred embodiments of the present invention have been explained. However, those skilled in the art or those with 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.

[0212] 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 scope of 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; a second positive electrode active material layer formed on the first positive electrode active material layer, and includes: 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; the weight (A) of the first positive electrode active material layer is in the range of 17% to 99% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer, a positive electrode.

2. The weight (A) of the first positive electrode active material layer is in the range of 20% to 85% of the total weight (A + B) of the first positive electrode active material layer and the second positive electrode active material layer, the positive electrode according to claim 1.

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

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

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

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

7. The lithium iron phosphate contained in the first positive electrode active material layer has an average particle size D ５０ is 0.5 μm to 2 μm of first lithium iron phosphate and an average particle size D ５０ is composed of second lithium iron phosphate of 3.5 μm to 15 μm, the positive electrode according to claim 1.

8. The weight ratio of the first lithium iron phosphate to the second lithium iron phosphate is 50:50 to 99.9:0.1, the positive electrode according to claim 7.

9. The average particle diameter D of the lithium iron phosphate contained in the second positive electrode active material layer ５０ is 0.6 μm to 3.0 μm, and the positive electrode according to claim 7.

10. 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 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, 0 ≦ x ≦ 0.5, the positive electrode according to claim 1.

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

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

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

14. The rubber-based binders contained in the first positive electrode active material layer and the second positive electrode active material layer respectively 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, and the positive electrode according to claim 1.

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

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

17. The conductive material is 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% by weight to 2.0% by weight, and the positive electrode according to claim 1.

18. A lithium secondary battery including the positive electrode according to any one of claims 1 to 17.

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