Positive electrode and lithium secondary battery manufactured using the same

Abstract

According to the present invention, there is provided a positive electrode including a 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, wherein the first positive electrode active material layer and the second positive electrode active material layer each include lithium iron phosphate, a fluorine-based binder, a first hydrogenated nitrile butadiene rubber having a weight average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, a second hydrogenated nitrile butadiene rubber having a weight average molecular weight (Mw) of 130,000 g / mol or more, and a conductive material, and a ratio (P2 / P1) of a weight P1 of fluorine contained in the first positive electrode active material layer to a weight P2 of fluorine contained in the second positive electrode active material layer is 1 or less.

Description

[Technical Field]

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2022-0139726 filed on October 26, 2022 and Korean Patent Application No. 10-2023-0143944 filed on October 25, 2023.

[0002] The present invention relates to a positive electrode and a lithium secondary battery manufactured using the same, and more particularly to a high-loading positive electrode having excellent flexibility and adhesive strength and a lithium secondary battery including the same. [Background technology]

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

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

[0005] Lithium iron phosphate is inexpensive because it contains iron, a resource-rich and low-cost material. Its low toxicity also reduces environmental pollution when used. Furthermore, because lithium iron phosphate has an olivine structure, its active material structure can be more stably maintained at high temperatures than lithium transition metal oxides, which have a layered structure. This provides the battery with excellent high-temperature stability and high-temperature life characteristics.

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

[0007] However, as the size of lithium iron phosphate particles decreases, their specific surface area increases, and the lithium iron phosphate with a carbon-coated surface exhibits reduced wettability with solvents. As a result, the lithium iron phosphate particles aggregate severely, reducing the stability and coating processability of the positive electrode slurry. Furthermore, the lithium iron phosphate and binder are not effectively mixed, resulting in a decrease in the adhesive strength between the positive electrode current collector and the positive electrode active material layer (hereinafter referred to as "positive electrode adhesive strength"). This phenomenon can worsen in high-loading positive electrodes.

[0008] If the positive electrode adhesive strength is reduced, the positive electrode active material layer may be detached during electrode manufacturing or during charge and discharge, resulting in an increase in battery resistance and a decrease in the capacity of the secondary battery.

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

[0010] Therefore, there is a need for a technique to improve the adhesion and flexibility of high-loading positive electrodes containing lithium iron phosphate. Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention aims to provide a positive electrode and a lithium secondary battery that, in a positive electrode containing lithium iron phosphate as the positive electrode active material, improves the adhesive strength between the positive electrode current collector and the positive electrode active material layer, and improves the flexibility of the positive electrode, thereby preventing electrode detachment, reducing battery resistance, and improving battery capacity. [Means for solving the problem]

[0012] According to one embodiment of the present invention, there is provided a positive electrode comprising a 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, wherein the first positive electrode active material layer and the second positive electrode active material layer each comprise lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material, the rubber-based binder comprising a first hydrogenated nitrile-butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second hydrogenated nitrile-butadiene rubber having a weight-average molecular weight (Mw) of 130,000 g / mol or more, and a ratio (P2 / P1) of a weight of fluorine contained in the second positive electrode active material layer (P2) to a weight of fluorine contained in the first positive electrode active material layer (P1) is 1 or less.

[0013] In one embodiment, 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 0.5 to 1.

[0014] In one embodiment, the first positive electrode active material layer contains 0.9 wt % or less of the second hydrogenated nitrile butadiene rubber based on the weight of the first positive electrode active material layer.

[0015] In one embodiment, a weight ratio of the fluorine-based binder to the second hydrogenated nitrile butadiene rubber contained in the first positive electrode active material layer is 90:10 to 75:25; The weight ratio of the fluorine-based binder to the second hydrogenated nitrile butadiene rubber contained in the second positive electrode active material layer is 80:20 to 60:40.

[0016] In one embodiment, the first positive electrode active material layer has a weight ratio of, based on the weight of the first positive electrode active material layer, 93.5% by weight to 97.99% by weight of the lithium iron phosphate, 1.5% by weight to 3.0% by weight of the fluorine-based binder, 0.01% by weight to 0.7% by weight of the first hydrogenated nitrile butadiene rubber, 0.2% by weight to 0.8% by weight of the second hydrogenated nitrile butadiene rubber, The conductive material is contained in an amount of 0.3% by weight to 2.0% by weight.

[0017] In one embodiment, the second positive electrode active material layer has a weight ratio of, based on the weight of the second positive electrode active material layer, 94.2% by weight to 98.49% by weight of the lithium iron phosphate, 0.9% by weight to 2.2% by weight of the fluorine-based binder, 0.01% by weight to 0.7% by weight of the first hydrogenated nitrile butadiene rubber, 0.3% by weight to 0.9% by weight of the second hydrogenated nitrile butadiene rubber, The conductive material is contained in an amount of 0.3% by weight to 2.0% by weight.

[0018] In one embodiment, the weight average molecular weight (Mw) of the second hydrogenated nitrile butadiene rubber is 150,000 g / mol to 1,000,000 g / mol.

[0019] In one embodiment, the fluorine-based binder contained in the first positive electrode active material layer and the fluorine-based binder contained in the second positive electrode active material layer are the same compound.

[0020] In one embodiment, the ratio (=H2 / H1) of the weight % (H2) of the second hydrogenated nitrile butadiene rubber contained in the second positive electrode active material layer to the weight % (H1) of the second hydrogenated nitrile butadiene rubber contained in the first positive electrode active material layer is 1 to 3.

[0021] In one embodiment, the lithium iron phosphate is a compound represented by the following Chemical Formula 1:

[0022] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0023] (In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are -0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

[0024] In one embodiment, the conductive material is carbon nanotubes.

[0025] In one embodiment, the ratio (C2 / C1) of the weight % C2 of the conductive material contained in the second positive electrode active material layer to the weight % C1 of the conductive material contained in the first positive electrode active material layer is 0.5-2.

[0026] In one embodiment, the total loading amount of the first positive electrode active material layer and the second positive electrode active material layer is 400 mg / 25 cm 2 ~700mg / 25cm 2 is in the range.

[0027] In one embodiment, the positive electrode has a positive electrode adhesive strength of 22 gf / 20 mm or more, as measured in an adhesive strength test in which the first positive electrode active material layer is peeled off at an angle of 90° from an aluminum thin film.

[0028] In one embodiment of the positive electrode, cracks occur with a measuring rod of 5 phi (φ) or less during a flexibility test in which a measuring rod of different diameters is brought into contact with the positive electrode active material layer and then the cross section of the positive electrode is lifted.

[0029] According to another embodiment of the present invention, there is provided a lithium secondary battery including the above positive electrode. [Effects of the Invention]

[0030] The positive electrode according to the present invention is configured with multiple layers, each layer containing a fluorine-based binder, a first hydrogenated nitrile-butadiene rubber, and a second hydrogenated nitrile-butadiene rubber having different weight-average molecular weights. The ratio (P2 / P1) of the weight of fluorine contained in the second positive electrode active material layer to the weight of fluorine contained in the first positive electrode active material layer (P1) is controlled to be 1 or less, thereby achieving superior positive electrode adhesion and flexibility even with a lower binder content compared to conventional single-layer positive electrodes. DETAILED DESCRIPTION OF THE INVENTION

[0031] The advantages and features of the present invention, as well as methods for achieving them, will become clear from the following detailed description of the embodiments accompanied by the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms. The present embodiments are provided solely to ensure that the disclosure of the present invention is complete and to fully convey the scope of the invention to those skilled in the art. The present invention is defined only by the scope of the claims. The same reference numerals refer to the same elements throughout the specification.

[0032] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the sense commonly understood by those skilled in the art to which the present invention pertains. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless otherwise clearly defined.

[0033] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular forms include the plural forms unless the context clearly dictates otherwise. When used in this specification, the words "comprises" and / or "comprising" do not exclude the presence or addition of one or more other elements other than the elements mentioned.

[0034] In this specification, when a part is said to include a certain component, this does not mean that it excludes other components, and that it may further include other components, unless otherwise specified.

[0035] In this specification, the expression "A and / or B" means A or B, or A and B.

[0036] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0037] In this specification, D 50 The above D means the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0038] In this specification, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mini II manufactured by BEL Japan.

[0039] In this specification, the term "weight average molecular weight (Mw)" refers to a value converted relative to standard polystyrene measured by gel permeation chromatography (GPC). Specifically, the weight average molecular weight is a value converted from a value measured using GPC under the following conditions, and standard polystyrene from an Agilent system was used to prepare a calibration curve.

[0040] <Measurement conditions> Measuring instrument: Agilent GPC (Agilent 1200 series, USA) Column: PL Mixed B 2 columns connected Column temperature: 40℃ Eluent: tetrahydrofuran Flow rate: 1.0mL / min Concentration: ~1mg / mL (100μL injection)

[0041] The positive electrode adhesive strength in this specification can be measured by the following method. A positive electrode cut to a length of 150 mm and a width of 20 mm was prepared. The positive electrode active material layer was placed facing a 75 mm long and 25 mm wide glass slide, and the positive electrode was attached to the glass slide in the longitudinal direction using double-sided tape. That is, the glass slide was attached to an area corresponding to half of the positive electrode in the longitudinal direction. Then, a test sample was prepared by rolling the test sample with a roller 10 times to ensure uniform adhesion of the double-sided tape. Next, the glass slide portion of the test sample was fixed to the sample stage of a Universal Testing Machine (UTM) (LS5, AMETEK), and the half of the positive electrode not attached to the glass slide was connected to the load cell of the UTM. The load cell was moved 50 mm at a speed of 100 mm / min, applying a 90° force, and the load applied to the load cell was measured. The average load measured over a 20 mm to 40 mm section of the running section was calculated. This is repeated a total of five times, and the average value is evaluated as the positive electrode adhesive strength (gf / 20 mm) of each sample.

[0042] The flexibility of the positive electrode in this specification can be measured by the following method. 2 A positive electrode with a loading of 10 ...

[0043] The present invention will be specifically described below.

[0044] <Positive electrode> The positive electrode for a lithium secondary battery according to the present invention will now be described.

[0045] A positive electrode according to an embodiment of the present invention is a positive electrode including a 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, wherein the first positive electrode active material layer and the second positive electrode active material layer each include lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material, and the rubber-based binder includes a first hydrogenated nitrile-butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol and a second hydrogenated nitrile-butadiene rubber having a weight-average molecular weight (Mw) of 130,000 g / mol or more, and a ratio (P2 / P1) of a weight P1 of fluorine contained in the first positive electrode active material layer to a weight P2 of fluorine contained in the second positive electrode active material layer is 1 or less.

[0046] Lithium iron phosphate, which has a small average particle size, is commonly used as a positive electrode active material because it has lower lithium mobility and electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. However, when the lithium iron phosphate particle size is small, the specific surface area increases, which leads to severe particle aggregation and ineffective mixing of the lithium iron phosphate and binder, resulting in reduced positive electrode adhesion. This can lead to detachment of the positive electrode active material layer during electrode fabrication or charge / discharge, resulting in increased battery resistance and reduced secondary battery capacity.

[0047] As a result of extensive research into solving these problems, the inventors have discovered that when a positive electrode active material layer is composed of two layers, the binders contained in each layer include a fluorine-based binder, the first hydrogenated nitrile-butadiene rubber, and the second hydrogenated nitrile-butadiene rubber, and the ratio (P2 / P1) of the weight of fluorine contained in the second positive electrode active material layer to the weight of fluorine contained in the first positive electrode active material layer (P1) is 1 or less, the interfacial adhesion strength between the current collector and the first positive electrode active material layer is dramatically increased, the flexibility of the entire positive electrode active material layer is improved, and the resistance of the positive electrode is reduced, thereby completing the present invention.

[0048] A positive electrode according to an embodiment of the present invention has a structure in which the positive electrode active material layer is in direct contact with a positive electrode current collector, and due to excellent interfacial adhesion between the positive electrode active material layer and the positive electrode current collector, it may not include a separate layer for improving adhesion between the positive electrode active material layer and the positive electrode current collector. That is, a positive electrode according to an embodiment of the present invention may exhibit excellent interfacial adhesion without including a separate layer, such as a binder layer, adhesive layer, bonding layer, or primer coating layer, that may be interposed between the positive electrode current collector and the positive electrode active material layer for improving adhesion.

[0049] The positive electrode of the present invention will now be described in detail.

[0050] A positive electrode according to one embodiment of the present invention may include a positive electrode active material layer. Specifically, the positive electrode according to the present invention may include a positive electrode current collector, 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. In this case, the first positive electrode active material layer and the second positive electrode active material layer may be formed on one or both sides of the current collector.

[0051] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the current collector may be made of stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like.

[0052] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and may have fine irregularities on its surface to enhance adhesion to the positive electrode active material layer. It may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0053] The first and second positive electrode active material layers may each contain a positive electrode active material. Additionally, the first and second positive electrode active material layers may further contain, as needed, a conductive material, a binder, and a dispersant in addition to the positive electrode active material.

[0054] Hereinafter, each component contained in the positive electrode active material layer will be specifically described.

[0055] (1) Positive electrode active material The present invention includes lithium iron phosphate as a positive electrode active material. Specifically, the first positive electrode active material layer and the second positive electrode active material layer each include lithium iron phosphate as a positive electrode active material. Because lithium iron phosphate has an olivine structure, the active material structure is maintained more stably at high temperatures than lithium transition metal oxides with a layered structure. As a result, when lithium iron phosphate is used as a positive electrode active material, the high-temperature stability and high-temperature life characteristics of the positive electrode are significantly improved, thereby reducing the risk of fire in a lithium secondary battery including the positive electrode.

[0056] The lithium iron phosphate may be a compound represented by the following formula 1:

[0057] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0058] (In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are -0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

[0059] For example, the lithium iron phosphate may be LiFePO4.

[0060] The lithium iron phosphate may be in the form of secondary particles formed by agglomeration of primary particles, and preferably has a monolithic structure consisting of primary particles.

[0061] In the present invention, the term "monolith structure" refers to a structure in which particles are morphologically present in independent phases that do not aggregate with each other. A particle structure that contrasts with this monolith structure is a structure in which small particles ("primary particles") are physically and / or chemically aggregated to form relatively large particles ("secondary particles").

[0062] When lithium iron phosphate has a monolithic structure consisting of primary particles, it is less likely to crack during the rolling process compared to secondary particles, which is preferable because capacity loss due to detachment of broken particles is reduced.Furthermore, when lithium iron phosphate has a monolithic structure consisting of primary particles, it can mitigate binder migration during the drying process of the positive electrode slurry, which is also preferable in terms of interfacial adhesion between the positive electrode current collector and the positive electrode active material layer.

[0063] The lithium iron phosphate may include a carbon coating layer on its surface. When a carbon coating layer is formed on the surface of the lithium iron phosphate, electrical conductivity is improved, which can improve the resistance characteristics of the positive electrode.

[0064] The carbon coating layer may be formed using at least one raw material selected from the group consisting of glucose, sucrose, lactose, starch, oligosaccharides, polyoligosaccharides, fructose, cellulose, furfuryl alcohol polymers, ethylene-ethylene oxide block copolymers, vinyl resins, cellulose resins, phenolic resins, pitch resins, and tar resins. Specifically, the carbon coating layer may be formed by mixing the raw material with the lithium iron phosphate and then heat-treating the mixture.

[0065] Average particle size D of lithium iron phosphate 50 The average particle size D of the positive electrode active material may be 0.8 μm to 20.0 μm, specifically 0.9 μm to 10.0 μm, and more specifically 0.9 μm to 3.0 μm. 50When the above range is satisfied, the mobility of lithium in the lithium iron phosphate is improved, and the charge / discharge characteristics of the battery can be improved.

[0066] The BET specific surface area of ​​lithium iron phosphate is 5m 2 / g~20m 2 / g, specifically 7m 2 / g~18m 2 / g, more specifically 9m 2 / g~16m 2 / g. This range corresponds to a lower value compared to typical lithium iron phosphate. When this range is satisfied, aggregation of the lithium iron phosphate can be effectively suppressed even in a positive electrode slurry composition with a relatively low dispersant content.

[0067] In one embodiment, the first positive electrode active material layer may contain 93.5 wt % to 97.99 wt %, specifically 94 wt % to 97.5 wt %, and more specifically 94.4 wt % to 97 wt %, of the lithium iron phosphate based on the weight of the first positive electrode active material layer.

[0068] In one embodiment, the second positive electrode active material layer may contain the lithium iron phosphate in an amount of 94.2 wt % to 98.49 wt %, specifically 94.5 wt % to 98 wt %, and more specifically 95 wt % to 97.5 wt %, based on the weight of the second positive electrode active material layer.

[0069] When the content of the lithium iron phosphate satisfies the above range, a sufficient positive electrode energy density is ensured, thereby making it possible to improve the battery capacity of the positive electrode.

[0070] (2) Binder The first positive electrode active material layer and the second positive electrode active material layer of the present invention each contain, as binders, both a fluorine-based binder and a rubber-based binder, and the rubber-based binder contains a first hydrogenated nitrile-butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second hydrogenated nitrile-butadiene rubber having a weight-average molecular weight (Mw) of 130,000 g / mol or more.

[0071] In this case, the content (wt%) of the fluorine-based binder contained in the first lower positive electrode active material layer is equal to or greater than the content (wt%) of the fluorine-based binder contained in the second upper 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, preferably 0.5 to 1, and more preferably 0.52 to 0.95. If the ratio (P2 / P1) of P2 to P1 exceeds 1, this is undesirable from the viewpoint of adhesive strength.

[0072] 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 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.

[0073] That is, although the fluorine-based binder contains elemental fluorine, the lithium iron phosphate, the first hydrogenated nitrile-butadiene rubber, the second hydrogenated nitrile-butadiene rubber and the conductive material that constitute the positive electrode of the present invention do not contain a fluorine component, and therefore the weight of fluorine contained in each of the first positive electrode active material layer and the second positive electrode active material layer is proportional to the content (weight) of the fluorine-based binder contained in each of the first positive electrode active material layer and the second positive electrode active material layer.

[0074] The fluorine-based binder contained in the first positive electrode active material layer and the fluorine-based binder contained in the second positive electrode active material layer may be the same compound having the same weight average molecular weight and chemical formula.

[0075] The fluorine-based binder includes a polyvinylidene fluoride (PVDF)-based polymer containing vinylidene fluoride (VDF) as a monomer. Specific examples of the PVDF-based polymer include PVDF homopolymer, PVDF-HFP (poly(vinylidene fluoride-co-hexafluoropropylene)), PVDF-CTFE (poly(vinylidene fluoride-co-chlorotrifluoroethylene)), PVDF-TFE (poly(vinylidene tetrafluoroethylene)), and PVDF-TrFE (poly(vinylidene trifluoroethylene)).

[0076] In one specific example, the fluorine-based binder may be included in the first positive electrode active material layer in an amount of 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 %, based on the weight of the first positive electrode active material layer. The fluorine-based binder may be included in the second positive electrode active material layer in an amount of 0.9 wt % to 2.2 wt %, preferably 1.0 wt % to 2.1 wt %, and more preferably 1.2 wt % to 2.0 wt %, based on the weight of the second positive electrode active material layer.

[0077] In addition, in 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 in an amount of 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 %.

[0078] When the content of the fluorine-based binder is within the above range, the viscosity and coating stability of the positive electrode slurry are excellent, and the adhesive strength and flexibility of the positive electrode are doubled.

[0079] The weight-average molecular weight (Mw) of the fluorine-based binder may 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 is within the above range, the viscosity of the positive electrode slurry tends to fall within a preferred range, and the positive electrode adhesive strength can be maximized even with a small content range.

[0080] Fluorine-based binders are commonly used as electrode binders due to their excellent adhesive strength and chemical stability against electrolytes. However, due to their crystalline structure, excessive fluorine-based binder content can negatively affect the flexibility of the positive electrode. On the other hand, the higher the binder content, the stronger the adhesive strength of the positive electrode. Therefore, improving the adhesive strength and flexibility of the positive electrode has been a challenge that is difficult to achieve simply by adjusting the binder content. Furthermore, the loading amount of the positive electrode active material layer was 600 mg / 25 cm. 2 The high-loading positive electrode described above tends to have poorer flexibility compared to a low-loading positive electrode with the same binder content but a smaller loading amount. Therefore, improving the flexibility as well as the adhesive strength of a high-loading positive electrode has been a more difficult technical challenge.

[0081] Therefore, the positive electrode according to the present invention contains a rubber-based binder in addition to the fluorine-based binder, and the rubber-based binder contains a first hydrogenated nitrile-butadiene rubber having a weight average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol and a second hydrogenated nitrile-butadiene rubber having a weight average molecular weight (Mw) of 130,000 g / mol or more.

[0082] The first hydrogenated nitrile butadiene rubber not only improves the adhesive strength and flexibility of the positive electrode, but also improves the dispersibility of the positive electrode slurry, thereby suppressing the aggregation of the conductive material and the positive electrode active material. The inventors of the present invention discovered that when a positive electrode contains only the first hydrogenated nitrile butadiene rubber as a rubber binder, the adhesive strength and flexibility of the positive electrode cannot be further improved above a certain content. However, when the positive electrode contains both the first hydrogenated nitrile butadiene rubber and the second hydrogenated nitrile butadiene rubber, the flexibility and adhesive strength of the positive electrode are dramatically improved, leading to the present invention. As a result, the positive electrode according to the present invention has excellent flexibility and adhesive strength, and when the loading of the positive electrode increases, the improved flexibility provides excellent rolling performance. Furthermore, the improved flexibility contributes to preventing detachment of the positive electrode active material layer when subjected to physical impact from outside the positive electrode.

[0083] The first hydrogenated nitrile-butadiene rubber and the second hydrogenated nitrile-butadiene rubber (HNBR) are obtained by subjecting each nitrile-butadiene rubber (NBR) to a hydrogenation reaction, and thereby converting the double bonds originally contained in the nitrile-butadiene rubber (NBR) into single bonds.

[0084] The hydrogenated nitrile butadiene rubber may have a repeating unit content derived from acrylonitrile (AN) of 20% by weight to 50% by weight, more preferably 25% by weight to 45% by weight, and most preferably 30% by weight to 40% by weight, based on the total weight.

[0085] The rubber binder of the present invention contains a first hydrogenated nitrile-butadiene rubber and a second hydrogenated nitrile-butadiene rubber having different weight average molecular weight ranges.

[0086] The first hydrogenated nitrile-butadiene rubber may have a weight average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, preferably 10,000 g / mol to 80,000 g / mol, and more preferably 10,000 g / mol to 50,000 g / mol, and improves the solvent wetting and dispersibility of lithium iron phosphate particles and suppresses particle aggregation of lithium iron phosphate.

[0087] In addition, the first hydrogenated nitrile butadiene rubber inhibits the aggregation of the conductive material, improves the positive electrode conductive network, and causes the conductive material to aggregate in a spherical rather than linear form, thereby minimizing the specific surface area of ​​the aggregated conductive material compared to when the conductive material aggregates in a linear form. As a result, the surface area of ​​the positive electrode active material adjacent to the aggregated conductive material that cannot participate in the lithium insertion / extraction reaction is minimized, thereby reducing the discharge resistance of the lithium secondary battery.

[0088] The weight average molecular weight (Mw) of the second hydrogenated nitrile-butadiene rubber is 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 second hydrogenated nitrile-butadiene rubber is 130,000 g / mol or more, the adhesive strength and flexibility of the positive electrode active material layer can be improved, and the rolling density of the positive electrode can be increased. As the rolling density increases, the positive electrode resistance can be reduced.

[0089] In one specific example, the first hydrogenated nitrile-butadiene rubber may be contained in the first positive electrode active material layer in an amount of 0.01 wt % to 0.7 wt %, preferably 0.05 wt % to 0.6 wt %, more preferably 0.1 wt % to 0.5 wt %, and the second positive electrode active material layer may also contain the first hydrogenated nitrile-butadiene rubber in the above range.

[0090] When the content of the first hydrogenated nitrile butadiene rubber is within the above range, the positive electrode slurry can have a viscosity suitable for coating, and the lithium iron phosphate, conductive material, and binder in the positive electrode slurry can be well dispersed.

[0091] In one specific example, the second hydrogenated nitrile-butadiene rubber is contained in the first positive electrode active material layer in an amount of 0.9 wt % or less, preferably 0.2 wt % to 0.8 wt %, and more preferably 0.3 wt % to 0.6 wt %, based on the weight of the first positive electrode active material layer. Also, the second hydrogenated nitrile-butadiene rubber is contained in the second positive electrode active material layer in an amount of 0.9 wt % or less, preferably 0.3 wt % to 0.9 wt %, and more preferably 0.5 wt % to 0.8 wt %, based on the weight of the second positive electrode active material layer.

[0092] In addition, in the entire positive electrode active material layer including the first positive electrode active material layer and the second positive electrode active material layer, the total weight percent of the second hydrogenated nitrile butadiene rubber may be 0.9 weight percent or less, preferably 0.25 weight percent to 0.85 weight percent, more preferably 0.35 weight percent to 0.75 weight percent, and most preferably 0.4 weight percent to 0.65 weight percent.

[0093] If the second hydrogenated nitrile butadiene rubber is included in an excessively large amount outside the above range, it exerts a synergistic effect with the fluorine-based binder, dramatically increasing the viscosity of the positive electrode slurry and resulting in poor coating properties. As a result, it may become impossible to manufacture a positive electrode. Also, if the second hydrogenated nitrile butadiene rubber is included in an excessively small amount outside the above range, the effect of improving the flexibility and adhesive strength of the positive electrode may be small. Therefore, it is preferable that the second hydrogenated nitrile butadiene rubber be included in the above range.

[0094] In one specific example, the ratio (H2 / H1) of the weight percent (H1) of the second hydrogenated nitrile-butadiene rubber contained in the first positive electrode active material layer to the weight percent (H2) of the second hydrogenated nitrile-butadiene rubber contained in the second positive electrode active material layer may be 1 to 3, preferably 1 to 2.9, and more preferably 1.1 to 2.8. Here, weight percent may refer to the weight ratio of the second hydrogenated nitrile-butadiene rubber contained in the first positive electrode active material layer based on 100 parts by weight of the first positive electrode active material layer, or the weight ratio of the second hydrogenated nitrile-butadiene rubber contained in the second positive electrode active material layer based on 100 parts by weight of the second positive electrode active material layer.

[0095] If the ratio (H2 / H1) of the weight percent (H1) of the second hydrogenated nitrile butadiene rubber contained in the first positive electrode active material layer to the weight percent (H2) of the second hydrogenated nitrile butadiene rubber contained in the second positive electrode active material layer exceeds 3, the second positive electrode active material layer contains too much second hydrogenated nitrile butadiene rubber, which increases the viscosity of the slurry for the second positive electrode active material layer and can reduce coating performance, which is undesirable. Conversely, if the ratio is less than 1, the second positive electrode active material layer contains too little second hydrogenated nitrile butadiene rubber, which can reduce the flexibility of the entire positive electrode, which is undesirable.

[0096] The first hydrogenated nitrile butadiene rubber contained in the first positive electrode active material layer and the first hydrogenated nitrile butadiene rubber contained in the second positive electrode active material layer may be the same compound, and may have the same weight average molecular weight and chemical formula.

[0097] In addition, the second hydrogenated nitrile butadiene rubber contained in the first positive electrode active material layer and the second hydrogenated nitrile butadiene rubber contained in the second positive electrode active material layer may be the same compound, and may have the same weight average molecular weight and chemical formula.

[0098] In one specific example, the weight ratio of the fluorine-based binder to the second hydrogenated nitrile-butadiene rubber contained in the first positive electrode active material layer may 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 second hydrogenated nitrile-butadiene rubber contained in the first positive electrode active material layer satisfies the above range, it can be said that the coating stability of the slurry is excellent and the interfacial adhesion between the current collector and the first positive electrode active material layer is excellent.

[0099] The weight ratio of the fluorine-based binder to the second hydrogenated nitrile-butadiene rubber contained in the second positive electrode active material layer may be 80:20 to 60:40, and preferably 75:25 to 60:40. When the weight ratio of the fluorine-based binder to the second hydrogenated nitrile-butadiene rubber contained in the second positive electrode active material layer satisfies the above range, it can be said that the coating stability of the slurry is excellent and the flexibility of the positive electrode is excellent.

[0100] In addition, the total amount of the fluorine-based binder, the first hydrogenated nitrile-butadiene rubber, and the second hydrogenated nitrile-butadiene rubber contained in the first and second positive electrode active material layers may be 1.8 wt % to 3.3 wt % based on the entire positive electrode active material layer.

[0101] (3) Conductive material The first positive electrode active material layer and the second positive electrode active material layer of the present invention each contain a conductive material.

[0102] The conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of such conductive materials include graphite; carbon black, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers, such as carbon fiber and metal fiber; metal powders, such as carbon fluoride, aluminum, and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive materials, such as polyphenylene derivatives. Specific examples of commercially available conductive materials include acetylene black-based products (such as those from Chevron Chemical Company, Denka Singapore Private Limited, and Gulf Oil Company), Ketjenblack, EC-based products (products from Armak Company), Vulcan XC-72 (products from Cabot Company), and Super P (products from Timcal). Preferably, the conductive material is carbon nanotubes. The conductive network of carbon nanotubes can mitigate the binder lift-off phenomenon during the drying process of the positive electrode slurry, making them a preferred conductive material for the positive electrode of the present invention.

[0103] In one specific example, the first positive electrode active material layer may contain the conductive material in an amount of 0.3 wt% to 2.0 wt%, specifically 0.6 wt% to 1.5 wt%, and more specifically 0.8 wt% to 1.3 wt%, based on the weight of the first positive electrode active material layer. The second positive electrode active material layer may contain the conductive material in an amount of 0.3 wt% to 2.0 wt%, specifically 0.6 wt% to 1.5 wt%, and more specifically 0.8 wt% to 1.3 wt%, based on the weight of the second positive electrode active material layer. When the content of the conductive material in the first positive electrode active material layer and the second positive electrode active material layer satisfies the above range, a positive electrode conductive network is ensured, thereby improving the electrical conductivity of the positive electrode.

[0104] In a preferred embodiment of the present invention, the ratio (C2 / C1) of the weight % C2 of the conductive material contained in the second positive electrode active material layer to the weight % C1 of the conductive material contained in the first positive electrode active material layer may be 0.1 to 3, preferably 0.5 to 2, and more preferably 0.5 to 1. When the weight % of the conductive material contained in the first positive electrode active material layer in contact with the current collector is the same as or greater than the weight % of the conductive material contained in the second positive electrode active material layer, the floating phenomenon of the conductive material in the first positive electrode active material layer is alleviated, thereby further improving the interfacial adhesion between the first positive electrode active material layer and the current collector.

[0105] The positive electrode may be manufactured according to a typical positive electrode manufacturing method, except for using the above-described positive electrode active material. Specifically, the positive electrode may be manufactured by preparing a positive electrode slurry for the first positive electrode active material layer and a positive electrode slurry for the second positive electrode active material layer, each containing the above-described positive electrode active material, a conductive material, and a binder, applying the slurry to the positive electrode current collector using a dual slot die, and then drying and rolling the slurry.

[0106] Alternatively, the positive electrode may be manufactured by casting the positive electrode slurry composition on a separate support, peeling it off from the support, and laminating the resulting film on a positive electrode current collector.

[0107] According to one embodiment of the present invention, the positive electrode slurry for the first positive electrode active material layer and the positive electrode slurry for the second positive electrode active material layer may each have a viscosity of 5,000 cps to 25,000 cps, specifically 6,000 cps to 24,000 cps, and more specifically 6,500 cps to 23,000 cps, measured at 25° C. and a shear rate of 2.5 s. A positive electrode slurry having a viscosity within the above range may have excellent storage stability and coating processability.

[0108] The positive electrode according to one embodiment of the present invention is a high-loading positive electrode, and the total loading amount of the first positive electrode active material layer and the second positive electrode active material layer is 400 mg / 25 cm. 2 ~700mg / 25cm2 , for details, see 500mg / 25cm 2 ~650mg / 25cm 2 , more specifically 550mg / 25cm 2 ~650mg / 25cm 2 Here, the above loading amount is measured based on the first and second positive electrode active material layers stacked on one side of the current collector, and when positive electrode active material layers are formed on both sides of the current collector, the above range is doubled.

[0109] A positive electrode according to an embodiment of the present invention may have excellent positive electrode adhesion and flexibility even with a low binder content compared to conventional single-layer positive electrodes. As a result, a high-loading positive electrode can be manufactured, and detachment of the positive electrode is prevented, thereby reducing the cell resistance of the secondary battery, improving the capacity and output characteristics of the battery, and reducing defects occurring during the manufacturing process.

[0110] In the positive electrode of the present invention, the adhesive strength between the positive electrode current collector and the positive electrode active material layer, as measured by a 90° peel test, is 22 gf / 20 mm or more, specifically 23 gf / 20 mm to 80 gf / 20 mm, more specifically in the range of 35 gf / 20 mm to 65 gf / 20 mm.

[0111] Furthermore, the positive electrode of the present invention has flexibility such that cracks occur with a measuring rod of 5 phi (φ) or less, specifically 4 phi (φ) or less, more specifically 1.5 phi (φ) to 3.5 phi (φ) during a flexibility test in which a measuring rod of different diameters is brought into contact with the positive electrode active material layer and then the cross section of the positive electrode is lifted.

[0112] <Lithium secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0113] A lithium secondary battery according to an embodiment of the present invention may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0114] The positive electrode of the lithium secondary battery is as described above. For example, the positive electrode includes a 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, the first positive electrode active material layer and the second positive electrode active material layer each including lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material, the rubber-based binder including a first hydrogenated nitrile-butadiene rubber having a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol and a second hydrogenated nitrile-butadiene rubber having a weight-average molecular weight (Mw) of 130,000 g / mol or more, and the ratio (P2 / P1) of the weight of fluorine contained in the second positive electrode active material layer to the weight of fluorine contained in the first positive electrode active material layer (P1) can be 1 or less, preferably 0.5 to 1.

[0115] The positive electrode has been described in detail above, so a duplicated description will be omitted.

[0116] 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 the composition on the negative electrode current collector.

[0117] The negative electrode active material is not particularly limited, and may generally be a compound capable of reversible lithium intercalation and deintercalation. 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, and Al alloys; and composites containing metallic compounds and carbonaceous materials. Low-crystalline carbons include soft carbon and hard carbon, while highly crystalline carbons include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbons such as petroleum or coal tar pitch-derived cokes. These may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material.

[0118] The negative electrode conductive material is used to impart conductivity to the electrode and can be any material that exhibits electronic conductivity without causing chemical changes in the battery. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The negative electrode conductive material is typically present in an amount of 1 wt % to 30 wt %, specifically 1 wt % to 20 wt %, and more specifically 1 wt % to 10 wt %, based on the total weight of the negative electrode active material layer.

[0119] The negative electrode binder serves to improve adhesion between negative electrode active material particles and 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, and various copolymers thereof. One or more of these may be used alone or in combination. The negative electrode binder may be included 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.

[0120] Meanwhile, the negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc. may be used.

[0121] The negative electrode current collector may have a thickness of typically 3 μm to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have a surface with fine irregularities to enhance the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0122] Meanwhile, in the lithium secondary battery, the separator can be any material commonly used as a separator in lithium secondary batteries. It is particularly preferred that the separator has low resistance to ion migration of the electrolyte and excellent electrolyte humidification capacity. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. 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.

[0123] Meanwhile, in the lithium secondary battery, the electrolyte may contain an organic solvent and a lithium salt that are commonly used in electrolytes, and is not particularly limited.

[0124] The organic solvent may be any organic solvent without particular limitation, as long as it can function as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; and carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0125] Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of the battery, and a low-viscosity chain carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0126] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without particular limitation. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O), etc. The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0127] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the battery's life characteristics, suppressing battery capacity reduction, improving the battery's discharge capacity, etc. In this case, the additives may be contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0128] The lithium secondary battery of the present invention may be manufactured by forming an electrode assembly by disposing a separator between a positive electrode and a negative electrode, and then placing the electrode assembly in a cylindrical or prismatic battery case and injecting an electrolyte thereinto, or by stacking the electrode assemblies, impregnating them with an electrolyte, and then sealing the resulting assembly in a battery case.

[0129] When manufacturing the lithium secondary battery of the present invention, the electrode assembly may be dried to remove one or more organic solvents selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate, which were used in manufacturing the positive electrode. When an electrolyte having the same components as the organic solvent used in manufacturing the positive electrode is used, the process of drying the electrode assembly may be omitted.

[0130] Unlike the above-described lithium secondary batteries, the lithium secondary battery according to another embodiment of the present invention may be an all-solid-state battery.

[0131] The battery case may be one commonly used in the art, and the shape may be any shape depending on the intended use of the battery, such as a cylindrical shape using a can, a square shape, a pouch shape, or a coin shape.

[0132] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, energy storage systems (ESS), and electric vehicles such as hybrid electric vehicles (HEV), etc.

[0133] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0134] <Examples 1 to 6 and Comparative Examples 1 to 5>

[0135] (1) Preparation of positive electrode slurry for first positive electrode active material layer A dispersion liquid containing carbon nanotubes (CNTs) as a conductive material and first hydrogenated nitrile butadiene rubber (first HNBR) having a weight average molecular weight (Mw) of 30,000 g / mol as a rubber binder was prepared.

[0136] Average particle size D as lithium iron phosphate 50 A positive electrode slurry for a first positive electrode active material layer was prepared by adding LiFePO4, which is a primary particle having a monolithic structure with a particle size of 1.0 μm, carbon nanotubes (CNT) as a conductive material, polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, second hydrogenated nitrile butadiene rubber (second HNBR) having a weight average molecular weight (Mw) of 310,000 g / mol as a rubber-based binder, and the dispersion liquid to an N-methylpyrrolidone (NMP) solvent and mixing them at 2500 rpm for 90 minutes using Homo-disperse.

[0137] In each of the positive electrode slurries for the first positive electrode active material layer of Examples 1 to 6 and Comparative Examples 1 to 5, the weight ratios of the lithium iron phosphate, conductive material, fluorine-based binder, first hydrogenated nitrile-butadiene rubber, and second hydrogenated nitrile-butadiene rubber were adjusted as shown in Table 1 to prepare the positive electrode slurries for the first positive electrode active material layer.

[0138] (2) Preparation of positive electrode slurry for second positive electrode active material layer Positive electrode slurries for the second positive electrode active material layer of Examples 1 to 6 and Comparative Examples 1 to 5 were manufactured using the same method as the method for manufacturing the positive electrode slurry for the first positive electrode active material layer, except that the weight ratios of the lithium iron phosphate, the conductive material, the fluorine-based binder, the first hydrogenated nitrile-butadiene rubber, and the second hydrogenated nitrile-butadiene rubber in the positive electrode slurry were changed as shown in Table 1.

[0139] (3) Manufacturing of the positive electrode The positive electrode slurry for the first positive electrode active material layer was applied to an aluminum foil at a rate of 300 mg / 25 cm using a dual slot die coater. 2 , 300 mg / 25 cm of the positive electrode slurry for the second positive electrode active material layer2 and the positive electrode slurry for the second positive electrode active material layer was coated on the positive electrode slurry for the first positive electrode active material layer.

[0140] Thereafter, the positive electrode slurry was dried with hot air at 130°C for 5 minutes so that the solid content of the positive electrode slurry was 99.0 wt% or more, and then the dried positive electrode slurry was rolled to prepare a positive electrode so that the porosity of the positive electrode active material layer was 29%.

[0141] [Table 1A] [Table 1B]

[0142] <Experimental Example 1: Measurement of fluorine weight>

[0143] The first and second positive electrode active material layers according to the examples and comparative examples were each scraped to obtain 100 mg samples. The weight of fluorine contained in each sample was measured using a combustion reaction technique, and the ratio (P2 / P1) of the weight of fluorine contained in the second positive electrode active material layer (P2) to the weight (P1) of fluorine contained in the first positive electrode active material layer (P1) was calculated. The results are shown in Table 2.

[0144] To measure the weight of fluorine contained in each sample, the first and second positive electrode active material layer samples were completely combusted under high-pressure oxygen conditions (40 atm), and the generated gas was collected in an absorbent. The gas collection absorbent was a 1 L solution containing 2.52 g NaHCO3 and 2.52 g Na2CO3. Once fluorine ions were absorbed or dissolved in the gas collection absorbent, the weight of fluorine in each sample was measured using ion chromatography. The ion chromatography column and analytical conditions were as follows:

[0145] Ion chromatography: ICS-3000 Column: IonPac AS18 (4 x 250 mm) Detector: suppressed conductivity detector SRS current:76Ma Injection volume: 20uL

[0146] The same test was repeated three times and the average values ​​were shown.

[0147] 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 based on the results of ion chromatography analysis.

[0148] <Experimental Example 2: Positive electrode adhesive strength test>

[0149] The positive electrodes produced in the examples and comparative examples were vacuum dried at 130°C for 2 hours, and then the adhesive strength between each positive electrode active material layer and the positive electrode current collector was measured. The results are shown in Table 2.

[0150] Specifically, the positive electrodes produced in the above Examples and Comparative Examples were cut to a length of 150 mm and a width of 20 mm, and the positive electrode surface was attached longitudinally to a 75 mm long and 25 mm wide glass slide using double-sided tape. That is, the glass slide was attached to an area corresponding to half of the positive electrode in the longitudinal direction. Then, a roller was rolled 10 times to ensure uniform adhesion of the double-sided tape, producing an evaluation sample.

[0151] Next, the glass slide portion of the evaluation sample was fixed to the sample stage of a Universal Testing Machine (UTM) (product name: LS5, manufacturer: LLOYD), and the half of the positive electrode without the glass slide attached was connected to the load cell of the UTM device. The load cell was moved 50 mm at a speed of 100 mm / min, applying a 90° force, and the load applied to the load cell was measured. The average value of the load measured in the 20 mm to 40 mm section of the travel section was calculated. This was repeated five times, and the average value was evaluated as the positive electrode adhesive strength (gf / 20 mm) of each sample.

[0152] <Experimental Example 3: Positive electrode flexibility test>

[0153] The positive electrodes produced in the above Examples and Comparative Examples were dried in vacuum at 130° C. for 2 hours, and then the flexibility of each positive electrode was measured. The results are shown in Table 2.

[0154] Specifically, the positive electrodes prepared in Examples 1 to 6 and Comparative Examples 1 to 5 were cut into 10 cm wide x 30 cm long pieces. Measuring rods with diameters of 2.5, 3, 4, 5, 6, 7, 8, 9, and 10 φ were prepared. Then, with the positive electrode current collector surface facing the measuring rod, each cut positive electrode was bent in half and both ends of the positive electrode were lifted at a rate of 10 mm per minute. The force measured by UTM was increased to 5 N. Measurements were made at each diameter, and the electrodes were observed under an optical microscope to determine whether cracks had occurred. If no cracks were found, the test was continued with a smaller diameter. The diameters (φ) of the measuring rods at which cracks occurred are listed in Table 2.

[0155] <Experimental Example 4: Viscosity of Positive Electrode Slurry for First Positive Electrode Active Material Layer and Positive Electrode Slurry for Second Positive Electrode Active Material Layer>

[0156] The viscosities of the positive electrode slurries for the first and second positive electrode active material layers prepared in the above Examples and Comparative Examples were measured. The results are shown in Table 2.

[0157] Specifically, the positive electrode slurry for the first positive electrode active material layer and the positive electrode slurry for the second positive electrode active material layer were each cooled for 1 hour at room temperature and a relative humidity of 1%, and then the viscosity of the positive electrode slurry was measured at 25°C and a shear rate of 2.5 / s using a Brookfield viscometer. The viscosity measurement was performed within 2 hours, including the cooling time, after the preparation of the positive electrode slurry for the first positive electrode active material layer and the positive electrode slurry for the second positive electrode active material layer.

[0158] [Table 2]

[0159] Referring to Table 2 above, the positive electrode of Comparative Example 1, 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, has inferior adhesive strength compared to the positive electrodes of the Examples.

[0160] The positive electrode according to Comparative Example 2 does not contain the second hydrogenated nitrile butadiene rubber, and therefore is inferior in flexibility to the positive electrodes according to the Examples.

[0161] In the positive electrode according to Comparative Example 3, the positive electrode slurry for the first positive electrode active material layer and the positive electrode slurry for the second positive electrode active material layer did not contain the first hydrogenated nitrile butadiene rubber, and therefore the lithium iron phosphate and the conductive material contained in the slurries severely aggregated, making it impossible to manufacture the positive electrode using a dual slot die coater.

[0162] In the positive electrode according to Comparative Example 4, the viscosity of the positive electrode slurry for the second positive electrode active material layer was too high, which reduced the coating performance and made it impossible to manufacture the positive electrode using a dual slot die coater. In the positive electrode according to Comparative Example 5, the viscosity of the positive electrode slurry for the first positive electrode active material layer was too high, making it impossible to manufacture the positive electrode.

[0163] As described above, the positive electrode according to the present invention includes a first positive electrode active material layer and a second positive electrode active material layer. The first positive electrode active material layer and the second positive electrode active material layer each contain a fluorine-based binder, a first hydrogenated nitrile-butadiene rubber, a second hydrogenated nitrile-butadiene rubber, and a conductive material in a predetermined weight ratio, thereby achieving a positive electrode active material layer having a thickness of 550 mg / 25 cm. 2 ~750mg / 25cm 2 The cathode can have excellent adhesion and flexibility while maintaining a high level of loading. As a result, high-loading cathodes can be manufactured, and detachment of the cathode is prevented, reducing the cell resistance of the secondary battery, improving the capacity and output characteristics of the battery, and reducing defects that occur during the manufacturing process.

Claims

1. A positive electrode including a 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, the first positive electrode active material layer and the second positive electrode active material layer each contain lithium iron phosphate, a fluorine-based binder, a rubber-based binder, and a conductive material; the rubber binder comprises a first hydrogenated nitrile-butadiene rubber having a weight average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, and a second hydrogenated nitrile-butadiene rubber having a weight average molecular weight (Mw) of 130,000 g / mol or more; a ratio (P2 / P1) of a weight P2 of fluorine contained in the second positive electrode active material layer to a weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less; the lithium iron phosphate, the first hydrogenated nitrile butadiene rubber, the second hydrogenated nitrile butadiene rubber, and the conductive material constituting the positive electrode do not contain a fluorine component, the fluorine-based binder contained in the first positive electrode active material layer and the fluorine-based binder contained in the second positive electrode active material layer are the same compound; The first hydrogenated nitrile-butadiene rubber and the second hydrogenated nitrile-butadiene rubber each have a repeating unit content derived from acrylonitrile (AN) of 20 wt% to 50 wt% based on the total weight of the rubber, the first hydrogenated nitrile butadiene rubber is contained in the first positive electrode active material layer at 0.01 wt % to 0.7 wt % and in the second positive electrode active material layer at 0.01 wt % to 0.7 wt %; the second hydrogenated nitrile butadiene rubber is contained in the first positive electrode active material layer at a content of 0.9 wt % or less, based on the weight of the first positive electrode active material layer, and in the second positive electrode active material layer at a content of 0.9 wt % or less, based on the weight of the second positive electrode active material layer.

2. 2. The positive electrode of claim 1, wherein a ratio (P2 / P1) of a weight P2 of fluorine contained in the second positive electrode active material layer to a weight P1 of fluorine contained in the first positive electrode active material layer is 0.5 to 1.

3. a weight ratio of the fluorine-based binder to the second hydrogenated nitrile butadiene rubber contained in the first positive electrode active material layer is 90:10 to 75:25; 2. The positive electrode of claim 1, wherein a weight ratio of the fluorine-based binder to the second hydrogenated nitrile butadiene rubber contained in the second positive electrode active material layer is 80:20 to 60:

40.

4. The first positive electrode active material layer has a weight ratio of, based on the weight of the first positive electrode active material layer, 93.5% by weight to 97.99% by weight of the lithium iron phosphate, 1.5% by weight to 3.0% by weight of the fluorine-based binder, 0.01% by weight to 0.7% by weight of the first hydrogenated nitrile butadiene rubber, 0.2% by weight to 0.8% by weight of the second hydrogenated nitrile butadiene rubber, 2. The positive electrode of claim 1, comprising 0.3% to 2.0% by weight of the conductive material.

5. The second positive electrode active material layer has a weight ratio of, based on the weight of the second positive electrode active material layer, 94.2% by weight to 98.49% by weight of the lithium iron phosphate, 0.9% by weight to 2.2% by weight of the fluorine-based binder, 0.01% by weight to 0.7% by weight of the first hydrogenated nitrile butadiene rubber, 0.3% by weight to 0.9% by weight of the second hydrogenated nitrile butadiene rubber, 2. The positive electrode of claim 1, comprising 0.3% to 2.0% by weight of the conductive material.

6. 2. The positive electrode according to claim 1, wherein the weight average molecular weight (Mw) of the second hydrogenated nitrile butadiene rubber is 150,000 g / mol to 1,000,000 g / mol.

7. 2. The positive electrode according to claim 1, wherein a ratio (=H2 / H1) of the weight % (H2) of the second hydrogenated nitrile butadiene rubber contained in the second positive electrode active material layer to the weight % (H1) of the second hydrogenated nitrile butadiene rubber contained in the first positive electrode active material layer is 1 to 3.

8. 2. The positive electrode of claim 1, wherein the lithium iron phosphate is a compound represented by the following chemical formula 1: [Chemical formula 1] Li １＋ａ Fe １－ｘ M ｘ (PO ４－ｂ )X ｂ (In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of S and N; and a, b, and x are −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

9. The positive electrode of claim 1 , wherein the conductive material is a carbon nanotube.

10. 2. The positive electrode according to claim 1, wherein a ratio (C2 / C1) of the weight % C2 of the conductive material contained in the second positive electrode active material layer to the weight % C1 of the conductive material contained in the first positive electrode active material layer is 0.5 to 2.

11. The total loading amount of the first positive electrode active material layer and the second positive electrode active material layer is 400 mg / 25 cm ２ ~700mg / 25cm ２ The positive electrode according to claim 1 , wherein the ionic liquid is in the range of 12. The positive electrode of claim 1, wherein the positive electrode adhesion strength measured in an adhesion test in which the first positive electrode active material layer is peeled off from the current collector at a 90° angle is 22 gf / 20 mm or more, as measured by the following method: (Method for measuring positive electrode adhesive strength) A positive electrode cut to a length of 150 mm and a width of 20 mm was prepared. The positive electrode active material layer was placed facing a glass slide measuring 75 mm in length and 25 mm in width, and the positive electrode was attached to the glass slide in the longitudinal direction using double-sided tape. That is, the glass slide was attached to an area corresponding to half of the positive electrode in the longitudinal direction. Then, a roller was rolled 10 times to ensure uniform adhesion of the double-sided tape, producing an evaluation sample. Next, the glass slide portion of the evaluation sample was fixed to the sample stage of a Universal Testing Machine (UTM), and the half of the positive electrode without the glass slide attached was connected to the load cell of the UTM equipment. The load cell was moved up to 50 mm at a speed of 100 mm / min, applying a 90° force, and the load applied to the load cell was measured. The average load measured over a 20 mm to 40 mm section of the travel section was calculated. This was repeated a total of five times, and the average value was evaluated as the positive electrode adhesive strength (gf / 20 mm) of each sample.

13. The positive electrode of claim 1, wherein cracks occur with a measuring rod of 5 phi (φ) or less during a flexibility test in which a cross section of the positive electrode is lifted after contacting the measuring rod with a different phi on the positive electrode active material layer, the flexibility test being carried out by the following method: (Flexibility test method) Measuring rods were prepared for each diameter, and a positive electrode with a loading of 600 mg / 25 cm2 was cut into a 10 cm wide x 30 cm long piece. The cut positive electrode was bent in half to contact the measuring rod, and both ends of the positive electrode were then lifted at a rate of 10 mm per minute until the force measured by the Universal Testing Machine (UTM) reached 5 N. After measuring each diameter, the electrode was observed under an optical microscope to see if cracks had occurred. If no cracks were found, the test was continued with a smaller diameter.

14. A lithium secondary battery comprising the positive electrode according to any one of claims 1 to 13.

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