Positive electrode slurry composition, positive electrode and lithium secondary battery produced using the same

Abstract

The positive electrode slurry composition according to the present invention includes a positive electrode active material, a binder, a dispersant, and a solvent. The positive electrode active material includes lithium iron phosphate having a carbon coating layer on a surface thereof, and the binder includes polyvinylidene fluoride satisfying the following formula 1: [Formula 1] 0≦{(2A+B) / (C+D)}X100<0.2 (A, B, C and D are the same as those of the polyvinylidene fluoride.) 1 It is the integrated area of ​​each peak that appears at 11.5 ppm to 12.8 ppm, 3.9 ppm to 4.2 ppm, 2.6 ppm to 3.2 ppm, and 2.1 ppm to 2.35 ppm during H-NMR measurement.

Description

[Technical Field]

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2021-0187191 filed on December 24, 2021, and Korean Patent Application No. 10-2022-0178234 filed on December 19, 2022.

[0002] The present invention relates to a positive electrode slurry composition, a positive electrode and a lithium secondary battery prepared using the same, and more particularly to a positive electrode slurry composition having a smooth electrode coating processability, and a positive electrode and a lithium secondary battery prepared using the same. [Background technology]

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

[0004] As the positive electrode active material for lithium secondary batteries, lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, etc. are used.

[0005] Among these, lithium iron phosphate is inexpensive because it contains iron, a resource-rich and low-cost material. Furthermore, its low toxicity can reduce environmental pollution when used. Furthermore, because lithium iron phosphate has an olivine structure, its active material structure can be stably maintained at high temperatures compared to lithium transition metal oxides with a layered structure. This can improve 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, conventionally, the average particle size of lithium iron phosphate has been reduced to shorten the lithium migration path, thereby improving the mobility of lithium ions, and the surface of lithium iron phosphate has been coated with carbon to improve electrical conductivity.

[0007] However, the carbon coating layer formed on the surface of the lithium iron phosphate bonds with the functional groups of the binder, which can lead to gelation of the positive electrode slurry composition during the preparation of the positive electrode slurry. Furthermore, when the size of the lithium iron phosphate particles is small and the specific surface area of ​​the lithium iron phosphate particles is increased, the number of sites where the bonding can occur increases, making the gelation even more pronounced. This makes it difficult to coat the composition on the current collector, and even if coated, the thickness and / or surface of the positive electrode active material layer may be uneven, resulting in reduced output performance and lifespan characteristics of the manufactured battery.

[0008] Therefore, there is a need for a technique for suppressing gelation of the positive electrode slurry composition containing lithium iron phosphate and a binder. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention aims to provide a positive electrode slurry composition that suppresses gelation of a positive electrode slurry containing lithium iron phosphate, thereby enabling a smooth electrode coating process.

[0010] Another object of the present invention is to provide a positive electrode formed from the positive electrode slurry composition, and a lithium secondary battery including the positive electrode. [Means for solving the problem]

[0011] According to one embodiment of the present invention, there is provided a positive electrode slurry composition comprising a positive electrode active material, a binder, a dispersant, and a solvent, wherein the positive electrode active material comprises lithium iron phosphate having a carbon coating layer on its surface, and the binder comprises polyvinylidene fluoride satisfying Formula 1 below:

[0012] According to another embodiment of the present invention, there is provided a positive electrode comprising a positive electrode active material, a binder, and a dispersant, wherein the positive electrode active material comprises lithium iron phosphate having a carbon coating layer on its surface, and the binder comprises polyvinylidene fluoride satisfying the following formula 1:

[0013] According to another embodiment of the present invention, there is provided a lithium secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode includes a positive electrode active material, a binder, and a dispersant, the positive electrode active material includes lithium iron phosphate having a carbon coating layer on a surface thereof, and the binder includes polyvinylidene fluoride satisfying the following formula 1:

[0014] [Formula 1] 0≦{(2A+B) / (C+D)}X100<0.2

[0015] (A, B, C and D are the values ​​of the polyvinylidene fluoride.) 1 It is the integrated area of ​​the peaks that appear at 11.5 ppm to 12.8 ppm, 3.9 ppm to 4.2 ppm, 2.6 ppm to 3.2 ppm, and 2.1 ppm to 2.35 ppm when measuring H-NMR. [Effects of the Invention]

[0016] The positive electrode slurry composition according to the present invention includes polyvinylidene fluoride satisfying Formula 1 as a binder, thereby reducing hydrogen bonding between hydrogen contained in the carbon coating layer on the surface of the lithium iron phosphate and functional groups contained in the binder, thereby preventing gelation of the composition.

[0017] In addition, the positive electrode slurry composition has excellent coating processability by preventing gelation.

[0018] In addition, the positive electrode formed from the positive electrode slurry composition includes a positive electrode active material layer having minimized surface defects and a uniform thickness, and thus the output performance and life characteristics of a lithium secondary battery manufactured using the positive electrode are excellent. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a 1H NMR graph of polyvinylidene fluoride used in Examples 1 to 3. [Figure 2] 1 is a 1H NMR graph of polyvinylidene fluoride used in Comparative Example 1. [Figure 3] 1 is a graph showing the results of measuring the viscosity as a function of shear rate for the positive electrode slurry compositions prepared in Examples 1 to 3 and Comparative Example 1.

[0020] The advantages and features of the present invention, as well as methods for achieving them, will become apparent 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 so that this disclosure will be complete and will fully convey the scope of the invention to those skilled in the art. The present invention is defined solely by the claims. The same reference numerals refer to the same elements throughout the specification.

[0021] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the sense that they can be commonly understood by a person of ordinary skill in the art to which the present invention belongs. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless they are clearly and specifically defined.

[0022] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular form includes the plural form unless otherwise specified in the phrase. As used in this specification, "comprises" and / or "comprising" do not exclude the presence or addition of one or more other elements other than the elements mentioned.

[0023] In this specification, when a part is said to include a certain element, this does not mean that it excludes other elements, but that it may further include other elements, unless otherwise specified to the contrary.

[0024] In this specification, the expression "A and / or B" means A or B, or A and B.

[0025] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0026] In this specification, D 50 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, the 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.

[0027] 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 (77K) using a BELSORP-mini II manufactured by BEL Japan.

[0028] As used herein, the term "weight average molecular weight (Mw)" refers to a value measured by gel permeation chromatography (GPC) and converted to standard polystyrene. Specifically, the weight average molecular weight is a value measured by GPC under the following conditions and converted. Standard polystyrene from an Agilent system was used to prepare the calibration curve.

[0029] <Measurement conditions> Measuring instrument: Agilent GPC (Agulent 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)

[0030] In this specification, polyvinylidene fluoride (PVDF) 1 1 H NMR was measured using an NMR device (product name: Bruker 600 MHz, manufacturer: Bruker) under the conditions of ns=1 k, dl=3 s, pulse sequence zg30, temperature 298 K, and solvent DMSO-d6.

[0031] The shear viscosity of the positive electrode slurry composition herein was measured using a rheometer (product name: DHR2, manufacturer: TA Instrument) at 25°C after 10 mL of the positive electrode slurry composition was introduced into a concentric cylinder-type accessory of the rheometer.

[0032] The present invention will be specifically described below.

[0033] <Positive electrode slurry composition> A positive electrode slurry composition according to one embodiment of the present invention includes a positive electrode active material, a binder, a dispersant, and a solvent. The positive electrode active material includes lithium iron phosphate having a carbon coating layer on its surface, and the binder includes polyvinylidene fluoride satisfying the following formula 1:

[0034] [Formula 1] 0≦{(2A+B) / (C+D)}X100<0.2

[0035] (A, B, C and D are the values ​​of the polyvinylidene fluoride.) 1 It is the integrated area of ​​the peaks that appear at 11.5 ppm to 12.8 ppm, 3.9 ppm to 4.2 ppm, 2.6 ppm to 3.2 ppm, and 2.1 ppm to 2.35 ppm when measuring H-NMR.

[0036] During the preparation of a positive electrode slurry composition, a mixture containing the positive electrode active material, binder, conductive material, dispersant, and solvent is stirred. Shear force is applied to the positive electrode slurry composition during the stirring process. The carbon coating layer formed on the surface of the lithium iron phosphate bonds with the functional groups of the binder, resulting in gelation of the positive electrode slurry composition during the preparation process. Furthermore, when the size of the lithium iron phosphate particles is small and the specific surface area of ​​the lithium iron phosphate particles increases, the number of sites where the bonding can occur increases, further increasing the gelation. This makes it difficult to coat the composition onto a current collector. Even if the composition is coated, the thickness and / or surface of the positive electrode active material layer may be uneven, resulting in reduced output performance and lifespan characteristics of the resulting battery.

[0037] As a result of extensive research to solve this problem, the present inventors have found that by using a binder that satisfies certain conditions together with lithium iron phosphate, it is possible to reduce hydrogen bonding between hydrogen contained in the carbon coating layer on the surface of lithium iron phosphate and functional groups contained in the binder, thereby suppressing gelation, as will be described in detail below.

[0038] (1) Positive electrode active material The positive electrode active material includes lithium iron phosphate, which may be a compound represented by the following Chemical Formula 1. When the positive electrode active material includes the lithium iron phosphate, the stability of a positive electrode including the positive electrode active material is significantly improved, thereby significantly reducing the risk of fire in a lithium secondary battery including the positive electrode.

[0039] The lithium iron phosphate may be a compound represented by the following Chemical Formula 1:

[0040] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0041] (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.)

[0042] For example, the lithium iron phosphate may be LiFePO4.

[0043] Meanwhile, the positive electrode active material may include a carbon coating layer formed on the surface of the lithium iron phosphate. When a carbon coating layer is formed on the surface of the lithium iron phosphate, electrical conductivity may be improved, thereby improving the resistance characteristics of the positive electrode.

[0044] 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.

[0045] The thickness of the carbon coating layer may be 500 nm or less, specifically 5 nm to 400 nm, more specifically 5 nm to 300 nm. When the thickness of the carbon coating layer satisfies this range, the conductivity of the positive electrode active material may be improved, and an excessively thick carbon coating layer may be prevented from reducing the mobility of lithium ions and increasing the resistance.

[0046] The average particle size D of the lithium iron phosphate 50 The average particle diameter D of lithium iron phosphate 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. 50 When 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.

[0047] The BET specific surface area of ​​the above positive electrode active material 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.

[0048] The positive electrode active material may be contained in an amount of 91.0 wt % to 98.0 wt %, specifically 91.5 wt % to 97.0 wt %, more specifically 92.0 wt % to 97.0 wt %, based on the total solid content of the positive electrode slurry composition. When the content of the positive electrode active material satisfies the above range, the energy density per weight / volume of the positive electrode can be increased.

[0049] (2) Binder The binder functions to bond the active material to the conductive material and to aid in bonding to the current collector. The binder may include polyvinylidene fluoride (PVDF) that satisfies the following formula 1:

[0050] [Formula 1] 0≦{(2A+B) / (C+D)}X100<0.2

[0051] In the above formula 1, A, B, C and D are the values ​​of the polyvinylidene fluoride (PVDF) 1 This refers to the peak integrated area of ​​a H-NMR spectrum, where A is the integrated area of ​​the peak appearing in the region from 11.5 ppm to 12.8 ppm, B is the integrated area of ​​the peak appearing in the region from 3.9 ppm to 4.2 ppm, C is the integrated area of ​​the peak appearing in the region from 2.6 ppm to 3.2 ppm, and D is the integrated area of ​​the peak appearing in the region from 2.1 ppm to 2.35 ppm.

[0052] In this case, the range from 11.5 ppm to 12.8 ppm is the COOH functional group contained in polyvinylidene fluoride (PVDF). 1 The 3.9 ppm to 4.2 ppm range is the peak region of the OCH2 functional group contained in polyvinylidene fluoride (PVDF). 1 The 2.6 ppm to 3.2 ppm range is the peak region of the H-NMR spectrum of polyvinylidene fluoride (PVDF) monomers bonded head-to-head. 1The H-NMR peak region is from 2.1 ppm to 2.35 ppm, which corresponds to the head-to-tail bond of polyvinylidene fluoride (PVDF) monomers. 1 The peak area of ​​H-NMR is shown.

[0053] On the other hand, when polyvinylidene fluoride (PVDF) satisfies the above formula 1, it means that the polyvinylidene fluoride contains relatively few polar functional groups such as COOH and OCH2.

[0054] If the polyvinylidene fluoride contained in the binder in the positive electrode slurry composition does not satisfy Formula 1 above, the functional groups (e.g., COOH, OCH) in the binder and the hydrogen on the carbon coating layer may form multiple hydrogen bonds, which may cause gelation of the positive electrode slurry composition.

[0055] On the other hand, when the polyvinylidene fluoride contained in the binder in the positive electrode slurry composition satisfies the above formula 1, the binder contains fewer functional groups, thereby reducing the number of hydrogen bonds between the functional groups and hydrogen on the carbon coating layer. This prevents gelation of the positive electrode slurry composition, improves the coating processability of the positive electrode slurry composition, and allows the coated positive electrode active material layer to have a uniform thickness and / or surface.

[0056] In particular, the effect of preventing gelation of the positive electrode slurry composition may be even more pronounced when lithium iron phosphate is used as the positive electrode active material. Specifically, lithium iron phosphate has a smaller average particle size and a larger specific surface area than conventional positive electrode active materials such as lithium nickel cobalt manganese oxide, which increases the number of sites where hydrogen bonds can occur, making the gelation more likely to occur. For this reason, polyvinylidene fluoride contained in the binder in the positive electrode slurry composition satisfies Formula 1 above, thereby significantly reducing the likelihood of gelation occurring in the positive electrode slurry composition using lithium iron phosphate as the positive electrode active material.

[0057] Preferably, the binder may be a homopolymer when the polyvinylidene fluoride contained in the binder satisfies Formula 1. For example, when the binder is a homopolymer of polyvinylidene fluoride, the binder does not have the polar functional group described above, and therefore, hydrogen bonds are not formed between the carbon coating layer and the binder, which can prevent gelation of the positive electrode slurry composition.

[0058] The weight-average molecular weight of the binder may be 20,000 g / mol to 1,200,000 g / mol, specifically 100,000 g / mol to 1,000,000 g / mol, and more specifically 400,000 g / mol to 980,000 g / mol. When the weight-average molecular weight of the binder satisfies this range, the positive electrode slurry composition may have a viscosity suitable for a coating process, which is preferable in terms of ensuring uniformity of the positive electrode active material layer formed from the composition and improving positive electrode adhesion.

[0059] The binder may be included in an amount of 1.8 wt % to 4.0 wt %, specifically 1.8 wt % to 3.8 wt %, more specifically 2.0 wt % to 3.7 wt %, based on the total solid content of the positive electrode slurry composition. When the binder content satisfies the above range, the contact area between the binder and lithium iron phosphate is increased, thereby ensuring excellent positive electrode adhesive strength.

[0060] (3) Dispersant The dispersant prevents the lithium iron phosphate from excessively agglomerating in the positive electrode slurry composition, thereby enabling the lithium iron phosphate to be effectively dispersed and present in the prepared positive electrode active material layer.

[0061] The dispersant may include a hydrogenated nitrile copolymer, and specifically, the dispersant may be a hydrogenated nitrile copolymer.

[0062] Specifically, the hydrogenated nitrile copolymer may be a copolymer containing structural units derived from an α,β-unsaturated nitrile and structural units derived from a hydrogenated conjugated diene, or a copolymer containing structural units derived from an α,β-unsaturated nitrile, structural units derived from a conjugated diene, and structural units derived from a hydrogenated conjugated diene. Examples of the α,β-unsaturated nitrile monomer include acrylonitrile and methacrylonitrile, and these may be used alone or in combination. Examples of the conjugated diene monomer include conjugated diene monomers having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, and 2,3-methylbutadiene, and these may be used alone or in combination.

[0063] More specifically, the hydrogenated nitrile copolymer may be hydrogenated nitrile butadiene rubber (H-NBR).

[0064] The weight average molecular weight of the dispersant can be from 10,000 g / mol to 150,000 g / mol, preferably from 15,000 g / mol to 140,000 g / mol, and more preferably from 20,000 g / mol to 130,000 g / mol.

[0065] On the other hand, when the weight-average molecular weight of the dispersant satisfies the above range, the solvent wetting and dispersibility of the lithium iron phosphate particles are improved, thereby effectively suppressing the aggregation of the lithium iron phosphate particles, and thus the positive electrode slurry composition can have a low viscosity and a higher solid content than other positive electrode slurry compositions having the same viscosity.

[0066] Furthermore, when the weight-average molecular weight of the dispersant satisfies the above range, even if the conductive material aggregates in the positive electrode, it aggregates in a spherical shape, thereby minimizing the surface area of ​​the aggregated conductive material compared to when the conductive material aggregates in a linear shape. As a result, the surface area of ​​the positive electrode active material adjacent to the aggregated conductive material that cannot participate in the lithium intercalation / deintercalation reaction is minimized, thereby reducing the discharge resistance of a lithium secondary battery fabricated using the positive electrode slurry composition.

[0067] The dispersant may be contained in an amount of 0.1 wt % to 2.0 wt %, specifically 0.2 wt % to 1.8 wt %, more specifically 0.4 wt % to 1.6 wt %, based on the total solid content of the positive electrode slurry composition. 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.

[0068] The dispersant may be included in an amount of 40 parts by weight or more, specifically 45 to 60 parts by weight, more specifically 50 to 60 parts by weight, relative to 100 parts by weight of the binder in the positive electrode slurry composition. When the dispersant content in the positive electrode slurry composition 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.

[0069] (4) Conductive material Meanwhile, the positive electrode slurry composition may further contain a conductive material, if necessary, in addition to the positive electrode active material, binder, dispersant, and solvent.

[0070] 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 summer black; conductive fibers, such as carbon fiber and metal fiber; metal powders, such as carbon fluoride, aluminum, and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive materials, such as polyphenylene derivatives. Specific examples of commercially available conductive materials include acetylene black products from Chevron Chemical Company, Denka Singapore Private Limited, and Gulf Oil Company, as well as Ketjenblack, EC series (products of Armak Company), Vulcan XC-72 (products of Cabot Company), and Super P (products of Timcal).

[0071] Preferably, the conductive material is carbon nanotubes. The conductive network of carbon nanotubes can mitigate the migration of the binder during the drying process of the positive electrode slurry composition, making them particularly preferred as the conductive material contained in the positive electrode slurry composition of the present invention.

[0072] The conductive material may be contained in an amount of 0.1 wt % to 4.0 wt %, more specifically 0.2 wt % to 4.0 wt %, more specifically 0.6 wt % to 3.5 wt %, based on the total solid content of the positive electrode slurry composition. When the amount is within this range, a positive electrode conductive network is ensured, thereby improving the electrical conductivity of the positive electrode.

[0073] (5) Solvent The solvent is used to mix the positive electrode active material, binder, dispersant, and / or conductive material. The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), acetone, or water, and may be used alone or in combination of two or more thereof.

[0074] The solvent may be included in an amount that allows the positive electrode slurry composition to have a suitable viscosity and solid content. For example, the solvent may be included in an amount that allows the solid content in the composition to be 40 wt% to 75 wt%, specifically 50 wt% to 70 wt%, more specifically 55 wt% to 65 wt%. When the solid content of the positive electrode slurry composition satisfies the above range, the composition may have a viscosity that allows coating, and the positive electrode active material layer formed from the composition may have a thickness of at least a certain level, thereby ensuring energy density.

[0075] In the case of a positive electrode slurry composition according to an embodiment of the present invention, an inflection point may not appear in a flow property graph. For example, when the shear viscosity of the positive electrode slurry composition as a function of the shear rate is measured using a rheometer, -2.5 1 / s~10 0 There may be no inflection point on the graph in the shear rate range of 1 / s, which can prevent problems such as changes in the composition over time, clogging of filters during composition transfer, or gelation of the composition, which may occur when an inflection point appears on the graph.

[0076] <Positive electrode> Next, the positive electrode according to the present invention will be described.

[0077] The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, a binder, and a dispersant. The positive electrode active material includes lithium iron phosphate having a carbon coating layer on its surface, and the binder includes polyvinylidene fluoride satisfying the following formula 1:

[0078] [Formula 1] 0≦{(2A+B) / (C+D)}X100<0.2

[0079] (A, B, C and D are the values ​​of the polyvinylidene fluoride.) 1 It is the integrated area of ​​the peaks that appear at 11.5 ppm to 12.8 ppm, 3.9 ppm to 4.2 ppm, 2.6 ppm to 3.2 ppm, and 2.1 ppm to 2.35 ppm when measuring H-NMR.

[0080] Additionally, the positive electrode active material layer may further include a conductive material.

[0081] The positive electrode may be formed using the positive electrode slurry composition described above, in which the positive electrode active material, binder, dispersant, and conductive material are as described above.

[0082] 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 copper, 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.

[0083] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and may have fine irregularities on its surface to enhance adhesion to the positive electrode active material layer. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0084] The positive electrode active material layer is located on at least one surface of the positive electrode current collector and may be formed from the above-described positive electrode slurry composition.

[0085] The positive electrode can be produced by a conventional method for producing a positive electrode, except for using the positive electrode slurry composition described above. Specifically, the positive electrode can be produced by applying the positive electrode slurry composition described above onto a positive electrode current collector, followed by drying and rolling.

[0086] Alternatively, the positive electrode may be manufactured by casting the positive electrode slurry composition on a separate support, peeling the resulting film from the support, and laminating the resulting film on a positive electrode current collector.

[0087] <Lithium secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0088] The lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0089] In the lithium secondary battery, the positive electrode is as described above. For example, the positive electrode includes a positive electrode active material, a binder, and a dispersant, the positive electrode active material including lithium iron phosphate including a carbon coating layer, and the binder including polyvinylidene fluoride satisfying the following formula 1:

[0090] [Formula 1] 0≦{(2A+B) / (C+D)}X100<0.2

[0091] (A, B, C and D are the values ​​of the polyvinylidene fluoride.) 1 It is the integrated area of ​​the peaks that appear at 11.5 ppm to 12.8 ppm, 3.9 ppm to 4.2 ppm, 2.6 ppm to 3.2 ppm, and 2.1 ppm to 2.35 ppm when measuring H-NMR.

[0092] Additionally, the positive electrode may further comprise a conductive material.

[0093] 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.

[0094] 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 carbon includes soft carbon and hard carbon, while highly crystalline carbon includes natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbon such as petroleum or coal tar pitch-derived cokes. These materials may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material.

[0095] The negative electrode conductive material is used to impart conductivity to the electrode and can be any material that is electronically conductive without causing chemical changes in the resulting 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, summer black, carbon fiber, and carbon nanotubes; metal powder or fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or 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 to 30 wt %, specifically 1 to 20 wt %, and more specifically 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0096] 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 to 30 wt %, specifically 1 to 20 wt %, more specifically 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0097] 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.

[0098] The negative electrode current collector may typically have a thickness of 3 to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have fine irregularities on its surface to enhance the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0099] Meanwhile, in the lithium secondary battery, the separator can be any material commonly used as a separator in lithium secondary batteries. In particular, a material that exhibits low resistance to electrolyte ion migration and excellent electrolyte humidification is preferred. 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.

[0100] Meanwhile, in the lithium secondary battery, the electrolyte may include an organic solvent and a lithium salt that are commonly used in electrolytes, and is not particularly limited.

[0101] The organic solvent may be any solvent capable of serving as a medium through which ions involved in the electrochemical reaction of the battery can migrate. Specific examples of the organic solvent 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 dimethylcarbonate (DMC), diethylcarbonate (DEC), ethylmethylcarbonate (EMC), ethylenecarbonate (EC), and propylenecarbonate (PC).

[0102] 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 linear carbonate-based compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0103] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without particular limitation. Specifically, examples of the lithium salt include LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, and LiB(C, O) . The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0104] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric 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 to 5 wt % based on the total weight of the electrolyte.

[0105] 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 placing the resulting assembly in a battery case and sealing it.

[0106] When fabricating the lithium secondary battery of the present invention, the electrode assembly may be dried to remove one or more organic solvents used in fabricating the positive electrode, such as N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate. If an electrolyte having the same organic solvent as that used in fabricating the positive electrode is used, the step of drying the electrode assembly may be omitted.

[0107] The battery case may be any battery case commonly used in the art, and may have any shape depending on the intended use of the battery, such as a cylindrical can, a square can, a pouch, or a coin.

[0108] 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, laptops, and digital cameras, energy storage systems (ESS), and electric vehicles such as hybrid electric vehicles (HEV), etc.

[0109] 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.

[0110] Example 1: Preparation of positive electrode slurry composition Average particle size D as positive electrode active material 50 is 2 μm, and the BET specific surface area is 11 m 2LiFePO4 (LiFePO4, 0.01g / g, containing a 200nm thick carbon coating layer on its surface), carbon black as a conductive material, polyvinylidene fluoride (PVDF), a homopolymer binder with a weight average molecular weight of 630,000g / mol, and hydrogenated nitrile butadiene rubber (H-NBR) as a dispersant were added to an N-methylpyrrolidone solvent. The mixture was then stirred at 2500 rpm for 90 minutes using a homo-dispersion mixer to prepare a positive electrode slurry composition. The positive electrode active material, conductive material, binder, and dispersant were present in a weight ratio of 93.6:3.0:2.2:1.2, and the solids content of the positive electrode slurry composition was 59wt%.

[0111] Example 2: Preparation of positive electrode slurry composition A positive electrode slurry composition was prepared in the same manner as in Example 1, except that the positive electrode active material and the binder were mixed in a weight ratio of 93.3:2.5.

[0112] Example 3: Preparation of positive electrode slurry composition A positive electrode slurry composition was prepared in the same manner as in Example 1, except that the positive electrode active material, binder, and dispersant were mixed in a weight ratio of 93.9:2.2:0.9.

[0113] (Comparative Example 1: Preparation of Positive Electrode Slurry Composition) A positive electrode slurry composition was prepared in the same manner as in Example 1, except that a modified polyvinylidene fluoride (PVDF) having a weight-average molecular weight of 630,000 g / mol and not a homopolymer was used as the binder, and the solid content of the positive electrode slurry composition was 57 wt %.

[0114] [Table 1]

[0115] (Experimental Example 1 - Polyvinylidene fluoride (PVDF) 1 H NMR measurement) The polyvinylidene fluoride (PVDF) used in Examples 1 to 3 and Comparative Example 1 was analyzed by nuclear magnetic resonance (NMR). 1 1 H NMR was measured to determine whether the polyvinylidene fluoride satisfied the following formula 1.

[0116] Specifically, the polyvinylidene fluoride (PVDF) added to each composition of Examples 1 to 3 and Comparative Example 1 was measured using an NMR device (product name: Bruker 600MHz, manufacturer: Bruker) under the conditions of ns = 1k, dl = 3s, pulse sequence zg30, temperature 298K, and solvent DMSO-d6. 1 1 H NMR was measured, and the results are shown in Figures 1 and 2. In this case, Figure 1 shows the H NMR spectrum of the polyvinylidene fluoride used in Examples 1 to 3. 1 2 is a 1 H NMR graph of the polyvinylidene fluoride used in Comparative Example 1. 1 1 and 2, the integrated area value (A) of the COOH-related peak appearing between 11.5 ppm and 12.8 ppm, the integrated area value (B) of the OCH2-related peak appearing between 3.9 ppm and 4.2 ppm, the integrated area value (C) of the PVDF head-to-head-related peak appearing between 2.6 ppm and 3.2 ppm, and the integrated area value (D) of the PVDF head-to-tail-related peak appearing between 2.1 ppm and 2.35 ppm were calculated, and it was determined whether the above A, B, C, and D values ​​satisfied the following formula 1, and the results are shown in Table 2 below with O or X (O: satisfied, X: not satisfied).

[0117] [Formula 1] 0≦{(2A+B) / (C+D)}X100<0.2

[0118] (Experimental Example 2 - Measurement of Shear Viscosity of Positive Electrode Slurry Composition) The shear viscosities of the positive electrode slurry compositions prepared in Examples 1 to 3 and Comparative Example 1 were measured using a rheometer (product name: DHR2, manufacturer: TA Instrument) and the results were plotted in the graph shown in FIG. 3 below. Specifically, 10 mL of the positive electrode slurry composition was introduced into a concentric cylinder-type accessory of the rheometer at 25°C, and the shear viscosities were measured and plotted in the graph shown in FIG. 3 below. FIG. 3 is a graph showing the shear viscosity measurement results as a function of shear rate for the positive electrode slurry compositions prepared in Examples 1 to 3 and Comparative Example 1, respectively.

[0119] In this case, 10 -2.5 1 / s~10 0 The graph in FIG. 3 was checked for the presence or absence of an inflection point in the 1 / s shear rate range, and the results were indicated in Table 2 below with an O or X (O: inflection point present, X: inflection point absent). Here, an inflection point refers to a point where the shear viscosity increases with an increase in shear rate, or where there is little change in shear viscosity. Generally, positive electrode slurry compositions exhibit shear-thinning behavior, where the shear viscosity decreases with an increase in shear rate. However, if particle agglomeration is present in the positive electrode slurry composition, the shear viscosity of the positive electrode slurry composition may increase with an increase in shear rate due to the interaction of the agglomerated particles, or the shear viscosity may show little change in shear viscosity in the shear viscosity graph.

[0120] [Table 2]

[0121] As shown in Table 2 and FIG. 3, the positive electrode slurry composition of Comparative Example 1, in which the polyvinylidene fluoride contained in the binder does not satisfy the above formula 1, is different from the positive electrode slurry compositions of Examples 1 to 3. -2 1 / s~10 -1It can be seen that an inflection point appears on the shear viscosity graph in the shear rate range of 1 / s. -2 1 / s~10 -1 It can be seen that in the 1 / s shear rate range, the decrease in shear viscosity compared to the increase in shear rate is much smaller than in other shear rate ranges, which suggests that particle aggregation occurred in the positive electrode slurry composition of Comparative Example 1.

Claims

1. a positive electrode active material, a binder, a dispersant, and a solvent; the positive electrode active material includes lithium iron phosphate having a carbon coating layer on its surface; The positive electrode slurry composition, wherein the binder is a homopolymer of polyvinylidene fluoride satisfying the following formula 1: [Formula 1] 0≦{(2A+B) / (C+D)}Х100<0.2 (A, B, C and D are the values ​​of the polyvinylidene fluoride, respectively.) １ It is the integrated area of ​​each peak appearing at 11.5 ppm to 12.8 ppm, 3.9 ppm to 4.2 ppm, 2.6 ppm to 3.2 ppm, and 2.1 ppm to 2.35 ppm when measuring H-NMR.

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

3. The average particle diameter D of the lithium iron phosphate ５０ The positive electrode slurry composition of claim 1, wherein the particle size is 0.8 μm to 20.0 μm.

4. The positive electrode slurry composition of claim 1, wherein the positive electrode active material is contained in an amount of 91.0 wt % to 98.0 wt % based on the total solid content of the positive electrode slurry composition.

5. The positive electrode slurry composition of claim 1, wherein the binder has a weight average molecular weight of 20,000 g / mol to 1,200,000 g / mol.

6. The positive electrode slurry composition of claim 1 , wherein the binder is contained in an amount of 1.8 wt % to 4.0 wt % based on the total solid content of the positive electrode slurry composition.

7. 7. The positive electrode slurry composition of claim 1, wherein the dispersant is a hydrogenated nitrile butadiene rubber.

8. The positive electrode slurry composition of claim 1 , wherein the dispersant is contained in an amount of 0.1 wt % to 2.0 wt % based on the total solid content of the positive electrode slurry composition.

9. The positive electrode slurry composition of claim 1, wherein the positive electrode slurry composition has a solid content of 40 wt % to 75 wt %.

10. a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector; the positive electrode active material layer includes a positive electrode active material, a binder, and a dispersant; the positive electrode active material includes lithium iron phosphate having a carbon coating layer on its surface; The binder is a homopolymer of polyvinylidene fluoride that satisfies the following formula 1: [Formula 1] 0≦{(2A+B) / (C+D)}Х100<0.2 (A, B, C and D are the values ​​of the polyvinylidene fluoride, respectively.) １ It is the integrated area of ​​each peak appearing at 11.5 ppm to 12.8 ppm, 3.9 ppm to 4.2 ppm, 2.6 ppm to 3.2 ppm, and 2.1 ppm to 2.35 ppm when measuring H-NMR.

11. The battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. the positive electrode includes a positive electrode active material, a binder, and a dispersant; the positive electrode active material includes lithium iron phosphate having a carbon coating layer on its surface; The binder is a homopolymer of polyvinylidene fluoride that satisfies the following formula 1: [Formula 1] 0≦{(2A+B) / (C+D)}Х100<0.2 (A, B, C and D are the values ​​of the polyvinylidene fluoride, respectively.) １ It is the integrated area of ​​each peak appearing at 11.5 ppm to 12.8 ppm, 3.9 ppm to 4.2 ppm, 2.6 ppm to 3.2 ppm, and 2.1 ppm to 2.35 ppm when measuring H-NMR.

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