Positive electrode slurry composition and lithium secondary battery manufactured using the same

Abstract

The positive electrode slurry composition according to the present invention includes a positive electrode active material, a dispersant, a conductive material, a binder, and a solvent, the positive electrode active material including lithium iron phosphate, and the dispersant having a weight average molecular weight of 10,000 g / mol to 150,000 g / mol.

Description

[Technical Field]

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2021-0187722 filed on December 24, 2021, and Korean Patent Application No. 10-2022-0179775 filed on December 20, 2022.

[0002] The present invention relates to a positive electrode slurry composition and a lithium secondary battery manufactured using the same, and more particularly to a positive electrode slurry composition including a low molecular weight dispersant and a lithium secondary battery manufactured 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 allows for reduced 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 provides advantages such as excellent high-temperature stability and high-temperature life characteristics.

[0006] However, lithium iron phosphate has the problem of lower lithium mobility and lower electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. Therefore, conventionally, 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, the specific surface area of ​​the lithium iron phosphate increases, and the lithium iron phosphate with a carbon-coated surface exhibits reduced wettability with solvents. As a result, significant particle agglomeration of the lithium iron phosphate occurs, degrading the stability and coating processability of the positive electrode slurry. Furthermore, when an excessive amount of conductive material is used, excessive particle agglomeration between the conductive material particles occurs in the positive electrode slurry. The greater the amount of agglomerated conductive material, the fewer lithium iron phosphate particles can participate in the cell's charge / discharge reactions, which can result in increased charge / discharge resistance of the lithium secondary battery.

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

[0009] An object of the present invention is to provide a positive electrode slurry composition having a relatively low viscosity and a high solids content by improving the dispersibility of a positive electrode active material and / or a conductive material in the positive electrode slurry composition and suppressing particle aggregation.

[0010] Another object of the present invention is to provide a lithium secondary battery with reduced discharge resistance by improving the dispersibility of the positive electrode active material and / or conductive material in the positive electrode and suppressing particle aggregation. [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 dispersant, a conductive material, a binder, and a solvent, wherein the positive electrode active material comprises lithium iron phosphate, and the dispersant has a weight-average molecular weight of 10,000 g / mol to 150,000 g / mol.

[0012] According to another embodiment of the present invention, there is provided a lithium secondary battery including a positive electrode, the positive electrode including a positive electrode active material, a dispersant, a conductive material, and a binder, the positive electrode active material including lithium iron phosphate, and in a cross section of the positive electrode, a maximum major axis length of an agglomerated region of the conductive material is 10 μm. [Effects of the Invention]

[0013] The positive electrode slurry composition of the present invention includes a dispersant having a low weight-average molecular weight, which improves the solvent wetting and dispersibility of the lithium iron phosphate particles contained in the positive electrode slurry composition, thereby suppressing particle aggregation of the lithium iron phosphate. As a result, the positive electrode slurry composition has a low viscosity and a higher solids content than other positive electrode slurry compositions having the same viscosity.

[0014] In addition, the positive electrode slurry composition of the present invention may suppress particle aggregation of lithium iron phosphate, thereby improving the stability of the positive electrode slurry composition and the coating processability.

[0015] In addition, since the positive electrode slurry composition of the present invention has a relatively high solid content, the time required for the slurry drying process during the preparation of a positive electrode can be shortened, thereby reducing process costs.

[0016] In addition, since the weight-average molecular weight of the dispersant is in a low range, the conductive material in the positive electrode is agglomerated into a spherical shape, which minimizes the surface area of ​​the agglomerated conductive material compared to when the conductive material is agglomerated into a linear shape. As a result, the surface area of ​​the positive electrode active material adjacent to the agglomerated conductive material, which 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. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is an SEM image of a cross section of a positive electrode in a lithium secondary battery of Example 1. [Figure 2] 1 is an SEM image of a cross section of a positive electrode in a lithium secondary battery of Example 2. [Figure 3] 10 is an SEM image of a cross section of a positive electrode in a lithium secondary battery of Example 3. [Figure 4] 1 is an SEM image of a cross section of a positive electrode in a lithium secondary battery of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] In this specification, the expression "A and / or B" means A or B, or A and B.

[0023] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0024] 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 allows measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0025] 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.

[0026] In this specification, the viscosity of the positive electrode slurry composition is measured by cooling the positive electrode slurry composition for 1 hour under conditions of room temperature and relative humidity of 1%, and then measuring the viscosity at 25°C using a viscometer (Brookfield) at 10 -2 The viscosity was measured at a shear rate of 100 rpm. The viscosity was measured within 2 hours after the preparation of the positive electrode slurry composition, including the cooling time.

[0027] In this specification, SOC50 discharge resistance refers to the voltage drop value that appears when a discharge pulse is applied for 10 seconds at a current of 2.5 C at a state of charge (SOC) of 50% divided by the current value.

[0028] The present invention will be specifically described below.

[0029] <Positive electrode slurry composition> A positive electrode slurry composition according to one embodiment of the present invention is for forming a positive electrode active material layer, and includes a positive electrode active material, a dispersant, a conductive material, a binder, and a solvent, wherein the positive electrode active material includes lithium iron phosphate, and the dispersant has a weight-average molecular weight of 10,000 g / mol to 150,000 g / mol.

[0030] In the case of conventional positive electrode slurry compositions containing lithium iron phosphate, the small particle size of the lithium iron phosphate and the carbon coating layer on the surface cause excessive particle agglomeration of the lithium iron phosphate, resulting in problems such as reduced stability of the positive electrode slurry and reduced coating processability.Furthermore, the use of an excessive amount of conductive material to improve the electrical conductivity of the positive electrode causes significant particle agglomeration between the conductive material, degrading the conductive network of the positive electrode and resulting in increased charge / discharge resistance of the lithium secondary battery.

[0031] As a result of extensive research aimed at solving these problems, the inventors have found that when a positive electrode slurry composition contains a dispersant with a small weight-average molecular weight, the dispersibility of the positive electrode active material and / or conductive material in the positive electrode slurry composition is improved, and particle aggregation can be suppressed. This finding will be described in detail herein.

[0032] (1) Positive electrode active material The positive electrode active material may include lithium iron phosphate. When the positive electrode active material includes lithium iron phosphate, the stability of the 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.

[0033] The lithium iron phosphate may be a compound represented by the following formula 1:

[0034] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0035] (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.)

[0036] For example, the lithium iron phosphate may be LiFePO4.

[0037] 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.

[0038] 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.

[0039] 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. 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.

[0040] 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.

[0041] The lithium iron phosphate may be contained in an amount of 93% to 98% by weight, specifically 93.5% to 98% by weight, and more specifically 94% to 97% by weight, based on the total solid content of the positive electrode slurry composition. When the content of the lithium iron phosphate satisfies the above range, sufficient positive electrode energy density can be ensured, thereby improving the battery capacity of the positive electrode.

[0042] (2) Dispersant The dispersant prevents excessive aggregation of lithium iron phosphate in the positive electrode slurry composition, thereby allowing the lithium iron phosphate to be effectively dispersed and present in the prepared positive electrode active material layer.

[0043] The dispersant may include a hydrogenated nitrile copolymer, and specifically, the dispersant may be a hydrogenated nitrile copolymer.

[0044] 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.

[0045] More specifically, the hydrogenated nitrile copolymer may be hydrogenated nitrile butadiene rubber (H-NBR).

[0046] The weight-average molecular weight of the dispersant may be 10,000 g / mol to 150,000 g / mol, preferably 15,000 g / mol to 140,000 g / mol, and more preferably 20,000 g / mol to 130,000 g / mol, which is smaller than the weight-average molecular weight of dispersants contained in conventional positive electrode slurry compositions.

[0047] If the weight-average molecular weight of the dispersant is less than 10,000 g / mol, the dispersibility of the lithium iron phosphate decreases, resulting in the problem of dissolving the dispersant during electrode production.If the weight-average molecular weight of the dispersant is more than 150,000 g / mol, the positive electrode slurry composition will have a high viscosity, which may reduce the stability and coating processability of the positive electrode slurry composition and cause the conductive material to aggregate in a linear form, which is undesirable in terms of the resistance of the lithium secondary battery.

[0048] On the other hand, when the weight-average molecular weight of the dispersant satisfies the above range, the wetting and dispersibility of the lithium iron phosphate particles in the solvent are improved, thereby suppressing particle aggregation of the lithium iron phosphate, 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.

[0049] Furthermore, when the weight-average molecular weight of the dispersant satisfies the above range, the conductive material in the positive electrode can be agglomerated into a spherical shape, thereby minimizing the surface area of ​​the agglomerated conductive material compared to when the conductive material is agglomerated into a linear shape. As a result, the surface area of ​​the positive electrode active material adjacent to the agglomerated conductive material, which cannot participate in the lithium intercalation / deintercalation reaction, can be minimized, thereby reducing the discharge resistance of a lithium secondary battery fabricated using the positive electrode slurry composition.

[0050] The dispersant may be contained in an amount of 0.2 wt % to 1.0 wt %, specifically 0.2 wt % to 0.9 wt %, more specifically 0.3 wt % to 0.8 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 conductive material in the positive electrode active material layer can be suppressed, thereby improving the conductive network of the positive electrode.

[0051] (3) Binder The binder functions to bind the positive electrode active material and conductive material together, and to aid in binding to the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers thereof, and these may be used alone or in combination.

[0052] The binder may be contained in an amount of 2.0 wt % to 4.0 wt %, specifically 2.2 wt % to 3.8 wt %, and more specifically 2.3 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.

[0053] (4) Conductive material The conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples 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; Ketjenblack, EC series (products of Armak Company), Vulcan XC-72 (products of Cabot Company), and Super P (products of Timcal). Preferably, the conductive material is carbon nanotubes. The conductive network of carbon nanotubes is particularly preferable as a conductive material contained in the positive electrode slurry composition of the present invention because it can alleviate the migration phenomenon of the binder during the drying process of the positive electrode slurry composition.

[0054] The conductive material may be contained in an amount of 0.1 wt % to 3.0 wt %, specifically 0.2 wt % to 2.0 wt %, more specifically 0.6 wt % to 1.2 wt %, based on the total solid content of the positive electrode slurry composition. When the content of the conductive material satisfies the above range, the conductive network of the positive electrode is ensured, thereby improving the electrical conductivity of the positive electrode.

[0055] (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.

[0056] The solvent may be included in an amount that allows the positive electrode slurry composition to have a suitable viscosity and solids content. For example, the solvent may be included in an amount that allows the composition to have a solids content of 40 wt% to 75 wt%, specifically 50 wt% to 70 wt%, more specifically 55 wt% to 70 wt%. This corresponds to a relatively high solids content compared to other conventional positive electrode slurry compositions. When the solids content of the positive electrode slurry composition satisfies the above range, the time required for the slurry drying process during positive electrode production can be shortened, thereby reducing process costs. In addition, the composition may have a viscosity that allows coating, and the positive electrode active material layer formed from the composition can have a certain level of thickness or more, thereby ensuring excellent energy density.

[0057] In the case of a positive electrode slurry composition according to one embodiment of the present invention, -2 The viscosity of the composition measured at a shear rate of 100 rpm may be 5,000 cps to 20,000 cps, specifically 6,000 cps to 15,000 cps, and more specifically 8,000 cps to 15,000 cps. A positive electrode slurry composition having a viscosity within this range may have excellent storage stability and coating processability. In addition, the positive electrode slurry composition may have a higher solids content than other positive electrode slurry compositions having the same viscosity, thereby shortening the time required for the slurry drying process during positive electrode production and reducing process costs.

[0058] <Positive electrode> Next, the positive electrode according to the present invention will be described.

[0059] 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. In this case, the positive electrode active material layer includes a positive electrode active material, a conductive material, a binder, and a dispersant, the positive electrode active material includes lithium iron phosphate, and the weight-average molecular weight of the dispersant is 10,000 g / mol to 150,000 g / mol. The positive electrode can be formed using the above-mentioned positive electrode slurry composition. The positive electrode active material, binder, dispersant, and conductive material are as described above.

[0060] The positive electrode according to the present invention includes a hydrogenated nitrile-butadiene rubber as a dispersant, and the weight-average molecular weight of the dispersant is in the range of 10,000 g / mol to 150,000 g / mol, thereby causing the conductive material to aggregate into spherical shapes within the positive electrode. In this positive electrode, the surface area of ​​the aggregated conductive material can be minimized compared to when the conductive material aggregates into linear shapes. As a result, the surface area of ​​the positive electrode active material adjacent to the aggregated conductive material, which cannot participate in the lithium insertion / extraction reaction, is minimized, thereby reducing the discharge resistance of the lithium secondary battery.

[0061] In the positive electrode according to the present invention, the conductive material is aggregated into spherical shapes, so that the maximum major axis length of the aggregated regions of the conductive material in the cross section of the positive electrode is 10 μm or less, and the ratio of the minor axis length to the major axis length of the aggregated regions of the conductive material can be 0.1 to 1, preferably 0.2 to 1, and more preferably 0.3 to 1.

[0062] When the agglomerated region of the conductive material satisfies the above numerical range, the discharge resistance of the lithium secondary battery can be further improved.

[0063] The conductive material agglomeration regions in the cross section of the positive electrode can be confirmed by observing the cross section of the positive electrode with a scanning electron microscope (SEM). In an SEM image or backscattered electron (BSE) image obtained by photographing the cross section of the positive electrode, lithium iron phosphate appears as a bright contrast, and the conductive material agglomeration regions appear as a dark contrast. The long and short axis lengths of the conductive material agglomeration regions that appear as dark contrast can be measured.

[0064] In the present invention, a region of 50 μm×50 μm area in the cross section of the positive electrode was observed by SEM, and the major axis length and minor axis length of the region appearing as a dark shadow in the SEM image or BSE image were measured.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] The positive electrode can be produced by a conventional method for producing a positive electrode, except for using the positive electrode slurry composition. Specifically, the positive electrode can be produced by applying the positive electrode slurry composition to a positive electrode current collector, followed by drying and rolling.

[0069] Alternatively, the positive electrode can be manufactured by casting the positive electrode slurry composition on a support, peeling it off from the support, and laminating the resulting film on a positive electrode current collector.

[0070] <Lithium secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0071] A lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0072] In the lithium secondary battery, the positive electrode is as described above. For example, the positive electrode includes a positive electrode active material, a conductive material, a binder, and a dispersant, the positive electrode active material includes lithium iron phosphate, and the dispersant has a weight average molecular weight of 10,000 g / mol to 150,000 g / mol.

[0073] 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.

[0074] 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 may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material.

[0075] 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 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 wt % to 30 wt %, preferably 1 wt % to 20 wt %, and more preferably 1 wt % to 10 wt %, based on the total weight of the negative electrode active material layer.

[0076] 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%, preferably 1 wt% to 20 wt%, and more preferably 1 wt% to 10 wt% of the total weight of the negative electrode active material layer.

[0077] 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.

[0078] The negative electrode current collector may typically have a thickness of 3 μm 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.

[0079] 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.

[0080] 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.

[0081] 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).

[0082] 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.

[0083] 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%.

[0084] 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, 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 wt % to 5 wt % based on the total weight of the electrolyte.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] In the case of the lithium secondary battery according to the present invention, the cell resistance (SOC50 discharge resistance) obtained by dividing the voltage drop observed when a discharge pulse is applied for 10 seconds at a current of 2.5 C at a state of charge (SOC) of 50% by the current value may be 1.7 mΩ or less, specifically 1.1 mΩ to 1.7 mΩ, and more specifically 1.1 mΩ to 1.6 mΩ.

[0091] A cell resistance value satisfying the above numerical range can be achieved because the positive electrode active material in the lithium secondary battery includes lithium iron phosphate and the weight-average molecular weight of the dispersant is 10,000 g / mol to 150,000 g / mol. Specifically, when a low-molecular-weight dispersant is included in the positive electrode slurry composition, the conductive material in the positive electrode aggregates into a spherical shape, thereby minimizing the surface area of ​​the aggregated conductive material compared to when the conductive material aggregates into 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 cell resistance of a lithium secondary battery manufactured using the positive electrode slurry composition to the above range.

[0092] 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.

[0093] Example 1 (1) Preparation of Positive Electrode Slurry Composition Average particle size D as positive electrode active material 50 is 1 μm, and the BET specific surface area is 11 m 2 / g LiFePO4, carbon nanotubes (CNT) as a conductive material, polyvinylidene fluoride (PVdF) as a binder, and hydrogenated nitrile butadiene rubber (H-NBR) with a weight average molecular weight (Mw) of 40,000 g / mol as a dispersant were added to an N-methylpyrrolidone (NMP) solvent and mixed at 2500 rpm for 60 minutes using Homo-disperse to prepare a positive electrode slurry composition.

[0094] In the positive electrode slurry composition, the positive electrode active material, the conductive material, the binder, and the dispersant were present in a weight ratio of 95.68:0.8:3.0:0.52, and the solids content of the positive electrode slurry composition was 60 wt %.

[0095] (2) Manufacturing of the positive electrode The cathode slurry composition was applied to a 15 μm-thick aluminum foil using a slot die coater and then vacuum dried at 130°C for 10 hours. The cathode was then rolled to prepare a cathode active material layer with a porosity of 28%. The cathode active material layer had a thickness of 98 μm, a width of 33 mm, and a length of 50 mm, and a loading capacity of 3.6 mAh / cm. 2 It was.

[0096] (3) Manufacture of lithium secondary batteries First, artificial graphite as the negative electrode active material, super C as the conductive material, and SBR / CMC as the binder were mixed in a weight ratio of 96:1:3 to prepare a negative electrode slurry. This was then applied to one side of a copper current collector, dried at 130°C, and rolled to a porosity of 29% to produce a negative electrode measuring 34 mm in width and 51 mm in length.

[0097] Next, an 18 μm-thick polypropylene separator was interposed between the fabricated positive and negative electrodes to fabricate an electrode assembly. The electrode assembly was placed in an aluminum pouch-type battery case, and 230 g of an electrolyte solution consisting of 1.0 M LiPF6 and 2 wt% vinylene carbonate (VC) dissolved in an organic solvent (EC / EMC / DMC = 3:3:4 volume ratio) was poured into the case, followed by vacuum sealing. The electrolyte was aged for one day, activated for three hours at 7.9 mAh, and then aged for three more days. Finally, a degassing process was performed to fabricate a lithium secondary battery.

[0098] <Examples 2 and 3> Positive electrode slurry compositions and lithium secondary batteries were prepared in the same manner as in Example 1, except that hydrogenated nitrile-butadiene rubbers (H-NBR) with weight-average molecular weights (Mw) as shown in Table 1 were used as dispersants.

[0099] <Comparative Example 1> A positive electrode slurry composition and a lithium secondary battery were prepared in the same manner as in Example 1, except that a hydrogenated nitrile butadiene rubber (H-NBR) having a weight average molecular weight (Mw) of 220,000 g / mol was used as a dispersant and the solid content of the positive electrode slurry composition was 57 wt %.

[0100] <Comparative Example 2> A positive electrode slurry composition and a lithium secondary battery were prepared in the same manner as in Comparative Example 1, except that the solid content of the positive electrode slurry composition was 60 wt %.

[0101] [Table 1]

[0102] <Experimental Example 1 - Viscosity Measurement> The viscosity values ​​of the positive electrode slurry compositions produced in Examples 1 to 3 and Comparative Examples 1 and 2 were measured, and the measurement results are shown in Table 2 below.

[0103] Specifically, the positive electrode slurry compositions prepared in Examples 1 to 3 and Comparative Examples 1 and 2 were cooled for 1 hour at room temperature and a relative humidity of 1%, and then measured at 25°C for 10 minutes using a Brookfield viscometer. -2 The viscosity of the positive electrode slurry composition was measured at a shear rate of 100 rpm within 2 hours after the preparation of the positive electrode slurry composition, including the cooling time.

[0104] <Experimental Example 2 - Confirmation of the aggregation form of conductive material on the cross section of the positive electrode> A 50 μm×50 μm area of ​​the cross section of each of the positive electrodes produced in Examples 1 to 3 and Comparative Example 1 was observed by SEM to confirm the aggregation morphology of the conductive material on the cross section of the positive electrode. The SEM images are shown in FIGS. 1 to 4, respectively, and the aggregation morphology of the conductive material on the cross section of the positive electrode is shown in Table 2 below.

[0105] FIG. 1 is an SEM image of a cross section of a positive electrode in the lithium secondary battery of Example 1, FIG. 2 is an SEM image of a cross section of a positive electrode in the lithium secondary battery of Example 2, FIG. 3 is an SEM image of a cross section of a positive electrode in the lithium secondary battery of Example 3, and FIG. 4 is an SEM image of a cross section of a positive electrode in the lithium secondary battery of Comparative Example 1.

[0106] In Figures 1 to 4, the lithium iron phosphate appears as a bright area, and the agglomerated areas of the conductive material appear as dark shades.

[0107] As shown in Figures 1 and 2, no linearly aggregated agglomerated regions of conductive material were found in the cross sections of the positive electrodes of Examples 1 and 2, and it was confirmed that the conductive material was spherically aggregated. As shown in Figure 3, the cross section of the positive electrode of Example 3 contained a mixture of spherically aggregated agglomerated regions of conductive material and linearly aggregated regions of conductive material, but even in the linearly aggregated agglomerated regions of conductive material, the maximum major axis length was 10 μm or less. On the other hand, as shown in Figure 4, the cross section of the positive electrode of Comparative Example 1 contained linearly aggregated agglomerated regions of conductive material with a major axis length of 1 μm or more, and three agglomerated regions of conductive material with a major axis length exceeding 10 μm were observed.

[0108] <Experimental Example 3 - Measurement of cell resistance of lithium secondary battery> The cell resistance values ​​of the lithium secondary batteries produced in Examples 1 to 3 and Comparative Examples 1 and 2 were measured, and the measurement results are shown in Table 2 below.

[0109] Specifically, for the lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 and 2, respectively, a discharge pulse was applied at a current of 2.5 C for 10 seconds at a state of charge (SOC) of 50%, and the cell resistance (SOC50 discharge resistance) was measured as a value obtained by dividing the voltage drop value that appeared when the discharge pulse was applied by the current value.

[0110] [Table 2]

[0111] From Table 2, it can be seen that the positive electrode slurry composition of Comparative Example 2, in which the weight-average molecular weight of the dispersant exceeds 150,000 g / mol, has a significantly higher viscosity compared to the positive electrode slurry compositions of Examples 1 to 3 at the same solid content.

[0112] In Comparative Example 2, the viscosity of the positive electrode slurry composition was so high that the transfer pipe connected to the slot die coater was clogged with the positive electrode slurry composition, making it impossible to discharge the slurry composition from the slot die coater at a normal pump pressure. Therefore, a high pump pressure had to be applied to the slot die coater to discharge the positive electrode slurry composition from the slot die coater. Furthermore, as a result of the application of excessive pump pressure, the positive electrode slurry composition was not applied at the same loading level as in Example 1, making it impossible to fabricate a positive electrode. As a result, it was impossible to measure the cell resistance of the lithium secondary battery.

[0113] The positive electrode slurry composition of Comparative Example 1, in which the weight-average molecular weight of the dispersant exceeds 150,000 g / mol, has a low viscosity. However, since the solid content is even lower than that of the positive electrode slurry composition of Example 1, the drying process of the composition during positive electrode production takes a long time, resulting in a problem of high process costs.

[0114] It can also be seen that the lithium secondary battery of Comparative Example 1 has a higher cell resistance than the lithium secondary battery of Example 1. This result is believed to be due to the fact that in Comparative Example 1, the conductive material aggregates in a linear shape in the positive electrode as shown in Figure 4, increasing the surface area of ​​the aggregated conductive material, which increases the surface area of ​​the positive electrode active material adjacent to the aggregated conductive material and which cannot participate in the lithium intercalation / deintercalation reaction.

Claims

1. A positive electrode slurry composition comprising a positive electrode active material, a dispersant, a conductive material, a binder, and a solvent, the positive electrode active material contains lithium iron phosphate, The weight average molecular weight of the dispersant is 10,000 g / mol to 40,000 g / mol; the dispersant is a hydrogenated nitrile-butadiene rubber, The positive electrode slurry composition, wherein the dispersant is contained in an amount of 0.2 to 0.8 parts by weight per 100 parts by weight of a solid content in the positive electrode slurry composition.

2. 2. The positive electrode slurry composition 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 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 positive electrode slurry composition according to claim 1, wherein the positive electrode slurry composition has a solid content of 40% by weight to 75% by weight.

4. 25℃, 10 －２ 2. The positive electrode slurry composition according to claim 1, wherein the viscosity of the positive electrode slurry composition measured at 1000 rpm is 5,000 cps to 20,000 cps.

5. The positive electrode slurry composition according to claim 1, wherein the conductive material is contained in an amount of 0.1 to 3.0 parts by weight based on 100 parts by weight of a solid content in the positive electrode slurry composition.

6. The positive electrode slurry composition of claim 1, wherein the binder is contained in an amount of 2.0 to 4.0 parts by weight based on 100 parts by weight of a solid content in the positive electrode slurry composition.

7. a positive electrode, the positive electrode includes a positive electrode active material, a dispersant, a conductive material, and a binder; the positive electrode active material contains lithium iron phosphate, In a cross section of the positive electrode, the maximum value of the major axis length of the agglomeration region of the conductive material is 10 μm or less, the dispersant is a hydrogenated nitrile butadiene rubber; the dispersant is included in an amount of 0.2 parts by weight to 0.8 parts by weight based on 100 parts by weight of the total of the positive electrode active material, dispersant, conductive material, and binder; The weight average molecular weight of the dispersant is 10,000 g / mol to 40,000 g / mol.

8. 8. The lithium secondary battery according to claim 7, wherein the SOC50 discharge resistance of the lithium secondary battery is 1.7 mΩ or less.

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