Positive electrode slurry composition for secondary battery, positive electrode prepared by using the same, and secondary battery comprising the same

Abstract

This invention relates to a positive electrode slurry composition for secondary batteries and a positive electrode and a lithium secondary battery prepared using the same. The positive electrode slurry composition comprises a lithium iron phosphate positive electrode active material, a binder, a conductive material, a dispersant, and a solvent. The dispersant comprises a nitrile copolymer as a first dispersant and a polymeric dispersant containing more than 60% by weight of oxyalkylene units as a second dispersant.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2021-0036166, filed on March 19, 2021, the disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a positive electrode paste composition for a secondary battery, a positive electrode prepared by using the same, and a secondary battery including the positive electrode. More specifically, the present invention relates to a positive electrode paste composition including a lithium iron phosphate-based positive electrode active material, a positive electrode prepared by using the same, and a secondary battery. Background Art

[0003] With the technological development and increasing demand for mobile devices, the demand for secondary batteries as an energy source has increased significantly. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharge rate have been commercialized and widely used.

[0004] As a positive electrode active material for lithium secondary batteries, lithium metal oxides such as LiCoO2, LiFePO4, and LiNi a Co b M c O2 (where M is at least one of manganese (Mn) and aluminum (Al), and 0 < a < 1, 0 < b < 1, and 0 < c < 1) have been used. Among them, lithium iron phosphate-based positive electrode active materials such as LiFePO4 have a voltage of about 3.5 V with respect to lithium, a high packing density of about 3.6 g / cm 3 and a theoretical capacity of about 170 mAh / g, and have the advantages of low price and excellent high-temperature stability. Lithium iron phosphate-based positive electrode active materials are positive electrode active materials with very stable structures, but have the disadvantages of low conductivity and ion conductivity. Therefore, lithium iron phosphate-based positive electrode active materials are used by coating carbon on the surface of the lithium iron phosphate-based positive electrode active materials to improve conductivity and reducing the particle size of the lithium iron phosphate-based positive electrode active materials to improve ion conductivity. However, as the particle size of the positive electrode active material becomes smaller, the specific surface area increases, and aggregation between the positive electrode active material particles becomes severe, reducing the dispersibility. As a result, there is a problem that the viscosity of the positive electrode paste composition increases, reducing the coating processability during the preparation of the positive electrode. The viscosity can be reduced by decreasing the solid content of the positive electrode paste composition, but in this case, in order to form the positive electrode active material layer to a desired thickness, multiple coating processes must be performed, thereby reducing productivity. In addition, the efficiency of the drying process for removing the solvent contained in the paste may decrease, and the adhesion of the electrode may be reduced due to the migration of the binder.

[0005] Therefore, there is a need to develop a cathode slurry composition using lithium iron phosphate cathode active materials, which has low viscosity even when the solid content is high. Summary of the Invention

[0006] Technical issues

[0007] One aspect of the present invention provides a cathode slurry composition comprising lithium iron phosphate cathode active material, wherein the cathode slurry composition has low viscosity characteristics and less aggregation of lithium iron phosphate particles even when the solid content is high.

[0008] Another aspect of the present invention provides a positive electrode prepared by using the above-described positive electrode slurry composition and a lithium secondary battery comprising the positive electrode.

[0009] Technical solution

[0010] According to one aspect of the present invention, a positive electrode slurry composition for a secondary battery is provided, the positive electrode slurry composition comprising a lithium iron phosphate positive electrode active material, a binder, a conductive material, a dispersant, and a solvent, wherein the dispersant comprises a nitrile copolymer as a first dispersant and a polymeric dispersant containing more than 60% by weight of oxyalkylene units as a second dispersant.

[0011] According to another aspect of the present invention, a positive electrode prepared by using a positive electrode slurry composition for secondary batteries according to the present invention and a lithium secondary battery comprising said positive electrode are provided.

[0012] Beneficial effects

[0013] The cathode slurry composition according to the present invention exhibits lower viscosity characteristics than conventional cathode slurry compositions containing lithium iron phosphate cathode active materials by simultaneously using two specific types of dispersants.

[0014] Lithium iron phosphate (LiFePO4) cathode active materials are structurally very stable, but they suffer from low conductivity and ionic conductivity. Therefore, LiFePO4 cathode active materials are used to improve conductivity by coating the surface with carbon and to improve ionic conductivity by reducing the particle size. However, as the particle size of the cathode active material decreases, as described above, the aggregation between particles becomes severe, increasing the viscosity of the cathode slurry composition. Consequently, the coating processability during cathode preparation decreases, and it becomes difficult to form a uniform cathode active material layer. Therefore, viscosity is typically reduced by decreasing the solids content of the cathode slurry composition using LiFePO4 cathode active materials. However, with a low solids content in the cathode slurry composition, multiple coating processes are required to prepare a cathode with the desired thickness, and there is a problem of reduced electrode adhesion due to binder migration.

[0015] This invention addresses the aforementioned problems by enabling the suppression of particle aggregation even when using lithium iron phosphate (LiFePO4) cathode active materials with small particle sizes, and achieving low viscosity characteristics through the use of two specific types of dispersants that improve the dispersibility of LiFePO4 cathode active materials in cathode slurry compositions. According to this invention, low viscosity can be achieved even when the solids content in the cathode slurry composition is higher than conventionally, and improved coating properties and electrode adhesion can be obtained during cathode preparation. Attached Figure Description

[0016] Figure 1 These are photographs showing the state of the filter after passing through the positive electrode slurry composition of Example 1;

[0017] Figure 2 This is a photograph showing the state of the filter after passing through the positive electrode slurry composition of Comparative Example 1. Detailed Implementation

[0018] It should be understood that the words or terms used in the specification and claims should not be interpreted as having the meaning defined in a common dictionary, and should be further understood that, based on the principle that the inventors can appropriately define the meaning of the words or terms to best interpret the invention, the words or terms should be interpreted as having a meaning consistent with their meaning in the context of the relevant field and technical ideas of the invention.

[0019] It will be further understood that the terms "comprising," "including," or "having" in this specification mean the presence of the features, values, steps, elements, or combinations thereof that are expressly described, but do not exclude the presence or addition of one or more other features, values, steps, elements, or combinations thereof.

[0020] In this specification, "weight-average molecular weight" refers to the equivalent value of standard polystyrene determined by gel permeation chromatography (GPC). Specifically, the weight-average molecular weight is obtained by converting values ​​measured using GPC under the following conditions, and a standard curve is plotted using standard polystyrene from an Agilent system.

[0021] <Measurement Conditions>

[0022] Measuring instrument: Agilent GPC (Agulent 1200 series, USA)

[0023] Column: Two PL hybrid B-columns connected in series

[0024] Column temperature: 40℃

[0025] Eluent: Tetrahydrofuran

[0026] Flow rate: 1.0 mL / min

[0027] Concentration: ~1 mg / mL (100 μL injection)

[0028] The "amine value" stated in this instruction manual can be measured using the following method. First, take 0.5 g to 1.5 g of the polymer dispersant sample, place it in a 100 mL beaker, weigh it accurately, and then dissolve it in 50 mL of acetic acid to prepare the sample. Then, using an automatic titrator equipped with a pH electrode, perform neutralization titration on the sample using a 0.1 mol / L HClO4 acetic acid solution to obtain a titration pH curve. Use the inflection point of the titration pH curve as the titration endpoint, and calculate the amine value using the following formula.

[0029] Amine value [mg KOH / g] = (561 × V) / (W × S)

[0030] In the above formula, W is the weight (g) of the polymer dispersant sample.

[0031] V is the titration volume (mL) at the titration endpoint.

[0032] S represents the solids content (by weight) of the polymer dispersant sample.

[0033] The term "average particle size (D)" in this specification is used to describe the particle size distribution. 50 The value was measured by wet laser diffraction scattering, where it was obtained by diluting 1 ml of positive electrode slurry composition 20 times using a Malvern Mastersizer 3000.

[0034] The present invention will be described in detail below.

[0035] Positive electrode slurry composition

[0036] First, the positive electrode paste composition according to the present invention will be described.

[0037] The positive electrode paste composition according to the present invention contains (1) a lithium iron phosphate-based positive electrode active material, (2) a binder, (3) a conductive material, (4) a dispersant, and (5) a solvent, wherein the dispersant contains (4-1) a nitrile copolymer as a first dispersant and (4-2) a polymer dispersant containing 60% by weight or more, preferably 60% to 99% by weight, of oxyalkylene units as a second dispersant.

[0038] Hereinafter, various components of the positive electrode paste composition according to the present invention will be described in detail.

[0039] (1) Positive electrode active material

[0040] The positive electrode paste composition according to the present invention contains a lithium iron phosphate-based positive electrode active material as the positive electrode active material.

[0041] Specifically, the lithium iron phosphate-based positive electrode active material can be represented by the following [Formula 1].

[0042] [Formula 1]

[0043] Li 1+a Fe 1-x M x PO 4-b A b

[0044] In Formula 1, <00**********118>M is at least one selected from Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn, A is at least one selected from S, Se, F, Cl, and I, and -0.5 < a < 0.5, 0 ≤ x < 0.5, and 0 ≤ b ≤ 0.1.

[0046] For example, the lithium iron phosphate-based positive electrode active material can be LiFePO4. In addition, the lithium iron phosphate-based positive electrode active material can be coated with a carbon-based material on the particle surface to improve conductivity.

[0047] The lithium iron phosphate-based positive electrode active material can have an average particle diameter (D 50 ) of 5 μm or less, preferably from 0.5 μm to 5 μm, and more preferably from 0.5 μm to 3 μm. When the average particle diameter (D 50 ) of the lithium iron phosphate-based positive electrode active material satisfies the above range, the effect of improving lithium ion conductivity can be obtained.

[0048] Based on the total solid content of 100 parts by weight in the positive electrode slurry composition, the content of the positive electrode active material can be from 85 parts by weight to 99.7 parts by weight, preferably from 90 parts by weight to 99 parts by weight, and more preferably from 90 parts by weight to 97 parts by weight. When the amount of positive electrode active material meets the above range, a positive electrode with excellent capacity characteristics can be prepared.

[0049] (2) Adhesive

[0050] Adhesives are used to ensure adhesion between positive electrode active material particles or between the positive electrode active material and the current collector, wherein adhesives commonly used in the art can be used, and there are no particular limitations on their type.

[0051] The adhesive may include, for example, polyvinylidene fluoride (PVdF), polyvinylidene 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 (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more thereof may be used. Preferably, the adhesive may be polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), or a mixture thereof.

[0052] Based on the total solid content of 100 parts by weight in the positive electrode slurry composition, the binder content can be from 0.1 parts by weight to 10 parts by weight, preferably from 0.5 parts by weight to 8 parts by weight, and more preferably from 1 part by weight to 5 parts by weight. When the amount of binder meets the above range, excellent electrode adhesion can be achieved while minimizing the increase in electrode resistance.

[0053] (3) Conductive materials

[0054] Conductive materials are used to provide conductivity to the electrodes. Any conductive material can be used without particular restrictions, as long as it has suitable conductivity and does not cause adverse chemical changes in the secondary battery.

[0055] For example, as a conductive material, at least one selected from the following can be used: graphite such as natural or artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, carbon nanotubes, and carbon fibers; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. Preferably, the conductive material can be a carbon material such as carbon nanotubes or carbon black.

[0056] Based on the total solid content of 100 parts by weight in the positive electrode slurry composition, the content of conductive material can be from 0.1 parts by weight to 5 parts by weight, preferably from 0.5 parts by weight to 5 parts by weight, and more preferably from 1 part by weight to 3 parts by weight. When the amount of conductive material meets the above range, an electrode with low resistance and excellent output characteristics can be prepared while minimizing the decrease in positive electrode capacity.

[0057] (4) Dispersant

[0058] The cathode slurry composition according to the present invention comprises two dispersants to prevent the aggregation of lithium iron phosphate cathode active materials. Specifically, the cathode slurry composition according to the present invention comprises a nitrile copolymer as a first dispersant and a polymeric dispersant containing oxyalkylene units as a second dispersant.

[0059] (4-1) First dispersant

[0060] The first dispersant is a nitrile copolymer, specifically, a copolymer having α,β-unsaturated nitrile-derived units and conjugated diene-derived units. In this case, the conjugated diene-derived units may be partially or completely hydrogenated. The hydrogenation of the conjugated diene can be carried out by hydrogenation reactions known in the art, for example, by catalytic hydrogenation reactions using catalyst systems such as Rh, Ru, Pd, and Ir, and the hydrogenation rate can be adjusted by controlling the amount of catalyst, the hydrogen pressure of the reaction, and the reaction time.

[0061] Nitrile copolymers can be prepared by copolymerizing α,β-unsaturated nitrile monomers with conjugated diene monomers, followed by hydrogenation of the C=C double bonds in the copolymer. The polymerization and hydrogenation of the monomers can be carried out using conventional methods.

[0062] As an α,β-unsaturated nitrile monomer, acrylonitrile or methacrylonitrile can be used, and any one of them or a mixture of two or more thereof can be used.

[0063] As a conjugated diene monomer, conjugated diene monomers having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, or 2,3-methylbutadiene, may be used, and any one of them or a mixture of two or more thereof may be used.

[0064] Nitrile copolymers may contain α,β-unsaturated nitrile derivative units and conjugated diene derivative units in amounts such that the weight ratio of α,β-unsaturated nitrile derivative units to conjugated diene derivative units is in the range of 10–50:50–90, preferably 20–40:60–80, and more preferably 25–40:60–75. When the amounts of various units in the nitrile copolymer meet the above ranges, excellent dispersibility and high-temperature properties are observed. In this document, the amount of α,β-unsaturated nitrile derivative units can be measured as the median value of the amount of nitrogen produced by the furnace grinding method according to JIS K 6364 and quantified by converting the amount of α,β-unsaturated nitrile derivative units into their binding amount based on the molecular weight of the α,β-unsaturated nitrile, and the amount of conjugated diene derivative units can be calculated by subtracting the weight of the α,β-unsaturated nitrile derivative units from the total weight of the copolymer.

[0065] The hydrogenation rate of the conjugated diene units in the nitrile copolymers of the present invention is 80% or more, for example, 90%. This is because, when using a dispersant with residual unhydrogenated conjugated diene units, the reactivity with the electrolyte is increased due to the double bonds in the conjugated diene, thereby deteriorating the high-temperature properties.

[0066] Specifically, the nitrile copolymer may contain repeating units represented by [Equation 2] and repeating units represented by [Equation 3].

[0067] [Equation 2]

[0068]

[0069] [Formula 3]

[0070]

[0071] In this case, the amount of repeating unit represented by [Equation 2] can be in the range of 10 wt% to 50 wt%, preferably 20 wt% to 40 wt%, and more preferably 25 wt% to 40 wt%, and the amount of repeating unit represented by [Equation 3] can be in the range of 50 wt% to 90 wt%, preferably 60 wt% to 80 wt%, and more preferably 60 wt% to 75 wt%.

[0072] More specifically, the first dispersant may be a hydrogenated acrylonitrile-butadiene copolymer.

[0073] The weight-average molecular weight of the nitrile copolymer can be from 10,000 g / mol to 100,000 g / mol, preferably from 20,000 g / mol to 100,000 g / mol, and more preferably from 20,000 g / mol to 50,000 g / mol. When using nitrile copolymers with the above-mentioned weight-average molecular weight, lithium iron phosphate cathode active materials can be uniformly dispersed with a small amount of dispersant, and the effect of reducing the viscosity of the cathode slurry composition is even better.

[0074] (4-2) Second dispersant

[0075] The second dispersant is used in conjunction with the first dispersant to improve the dispersibility of lithium iron phosphate cathode active materials, wherein it is a polymeric dispersant containing 60% by weight or more, preferably 60% by weight to 99% by weight, alkylene units.

[0076] The oxyalkylene unit can be represented by the following formula 4.

[0077] [Formula 4]

[0078]

[0079] In Formula 4, R1 is an alkylene group having 1 to 20 carbon atoms, preferably an alkylene group having 2 to 10 carbon atoms, and more preferably an alkylene group having 2 to 5 carbon atoms.

[0080] Based on the total weight of the polymer dispersant, the content of alkylene units is 60% by weight or more, preferably 60% by weight to 99% by weight, and more preferably 60% by weight to 98% by weight. According to the inventors' research, when a polymer dispersant in which the amount of alkylene units is 60% by weight or more is used with a nitrile copolymer dispersant, the viscosity of the cathode slurry composition decreases significantly; however, when a polymer dispersant in which the amount of alkylene units is less than 60% by weight is used, the viscosity of the cathode slurry composition is found to increase.

[0081] The polymer dispersant may also contain units other than oxyalkylene units. For example, the polymer dispersant may also contain units derived from at least one compound selected from carbamates and phosphates, but the invention is not limited thereto.

[0082] Polymer dispersants can be prepared using methods well-known in the art for preparing copolymers. For example, polymer dispersants can be prepared by mixing a compound capable of derivatizing alkylene units with a compound capable of derivatizing units other than alkylene units, followed by polymerization or an acid-base reaction. In this case, as compounds capable of derivatizing alkylene units, examples such as polyalkylene glycols like polyethylene glycol and polypropylene glycol, or alkylene oxide compounds like (poly)ethylene oxide and (poly)propylene oxide, can be used, but the invention is not limited thereto.

[0083] The weight-average molecular weight of the polymer dispersant can be from 800 g / mol to 20,000 g / mol, preferably from 800 g / mol to 10,000 g / mol, and more preferably from 1,000 g / mol to 10,000 g / mol. When the weight-average molecular weight of the polymer dispersant meets the above range, it has an excellent effect on reducing the viscosity of the positive electrode slurry composition.

[0084] Furthermore, the amine value of the polymer dispersant can be from 30 mg KOH / g to 90 mg KOH / g, preferably from 36 mg KOH / g to 80 mg KOH / g, and more preferably from 40 mg KOH / g to 80 mg KOH / g. When the amine value of the polymer dispersant meets the above range, the dispersibility of the lithium iron phosphate cathode active material is improved because the hydrogen bonds formed between the polymer dispersant and organic functional groups such as -OH, -COOH, and -C=O on the surface of the lithium iron phosphate cathode active material increase, and the viscosity of the cathode slurry composition is effectively reduced.

[0085] In this invention, the nitrile copolymer and the polymeric dispersant may be contained in a weight ratio of 1:1 to 5:1, preferably 1:1 to 4:1, and more preferably 2:1 to 4:1. When the mixing ratio of the nitrile copolymer to the polymeric dispersant meets the above range, the effect of improving viscosity is particularly excellent.

[0086] Based on the total solids content of 100 parts by weight in the cathode slurry composition, the total amount of dispersant in the combination of the first and second dispersants can be in the range of 0.1 parts by weight to 3 parts by weight, preferably 0.1 parts by weight to 2 parts by weight, and more preferably 0.5 parts by weight to 2 parts by weight. When the amount of dispersant meets the above range, the viscosity of the cathode slurry composition can be effectively reduced. If the amount of dispersant is too small, the effect of improving viscosity is not obvious, while if the amount of dispersant is too large, physical properties such as capacity may deteriorate.

[0087] (5) Solvent

[0088] The solvent is used to uniformly mix the various components in the positive electrode slurry composition and to carry out the coating process, wherein solvents commonly used in the art for positive electrode slurry compositions can be used without limitation.

[0089] For example, the solvent can be water, an organic solvent, or a mixture thereof. For instance, the organic solvent may include: amide-based polar organic solvents such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 2-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; and diols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or... Hexanediol; polyols such as glycerol, trimethylolpropane, pentaerythritol, or sorbitol; glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, or tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, or cyclopentanone; and esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone, and any one or a mixture of two or more thereof may be used, but the invention is not limited thereto.

[0090] The solvent may be included in an amount such that the solid content in the positive electrode slurry composition is 55% by weight or more, preferably 55% by weight to 85% by weight, and more preferably 55% by weight to 80% by weight. When the solid content of the electrode slurry is less than 55% by weight, the electrode loading may be reduced, increasing processing costs; binder migration may occur, reducing electrode adhesion; and coating defects may occur.

[0091] The positive electrode slurry composition of the present invention can be prepared by mixing the above-mentioned components. In this case, the mixing of the various components can be carried out simultaneously or sequentially.

[0092] For example, the positive electrode slurry composition of the present invention can be prepared by simultaneously adding positive electrode active material, conductive material, binder and dispersant to a solvent and then dispersing them. It can also be prepared by adding conductive material, binder and solvent after pre-dispersing by mixing positive electrode active material and dispersant in a solvent, or by adding active material, dispersant, binder and solvent after pre-dispersing by mixing conductive material in a solvent.

[0093] The cathode slurry composition according to the present invention, as described above, has low viscosity characteristics. Specifically, the viscosity of the cathode slurry composition of the present invention, measured at 1 rpm and 25°C, is less than 20 Pa·s, preferably from 1 Pa·s to 20 Pa·s, and more preferably from 5 Pa·s to 18 Pa·s.

[0094] positive electrode

[0095] Next, the positive electrode according to the present invention will be described.

[0096] The positive electrode according to the present invention is prepared using the above-described positive electrode slurry composition of the present invention. As described above, in the positive electrode slurry composition according to the present invention, because the dispersibility of the lithium iron phosphate positive electrode active material is improved, the binder can be uniformly distributed in the positive electrode active material, thus improving the adhesion of the electrode when preparing the positive electrode using the positive electrode slurry composition according to the present invention. Furthermore, because the migration of the binder that occurs during solvent evaporation of the slurry can be reduced with the increase of the solid content of the final positive electrode slurry, the adhesion between the current collector and the positive electrode active material layer is improved.

[0097] Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector and formed by using the above-described slurry composition for the positive electrode.

[0098] There are no particular restrictions on the positive electrode current collector, as long as it is conductive and will not cause adverse chemical changes in the battery. It can be made of materials such as stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. Furthermore, the thickness of the positive electrode current collector can typically range from 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0099] In addition to using the above-described positive electrode slurry composition, a positive electrode can be prepared according to typical methods for preparing a positive electrode. Specifically, the above-described positive electrode slurry composition is coated onto a positive electrode current collector, and then the positive electrode can be prepared by drying and calendering the coated positive electrode current collector.

[0100] Alternatively, as another method, the positive electrode can be prepared by casting the above-mentioned slurry composition for the positive electrode onto a separate carrier, and then pressing the film layer separated from the carrier onto the positive electrode current collector.

[0101] Electrochemical device

[0102] Next, an electrochemical device according to the present invention will be described. The electrochemical device includes the positive electrode of the present invention, and specifically may be a battery or capacitor, more specifically a lithium secondary battery.

[0103] Specifically, a lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive and negative electrodes, and an electrolyte, wherein the positive electrode is the same as described above. Additionally, the lithium secondary battery may optionally further include: a battery container housing an electrode assembly containing the positive electrode, negative electrode, and separator; and a sealing member for sealing the battery container.

[0104] In a lithium secondary battery, the negative electrode includes a negative electrode current collector and a layer of negative electrode active material disposed on the negative electrode current collector.

[0105] There are no particular restrictions on the negative electrode current collector, as long as it has high conductivity and does not cause adverse chemical changes in the battery. Examples of suitable materials include: copper; stainless steel; aluminum; nickel; titanium; calcined carbon; copper or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc.; and aluminum-cadmium alloys. Furthermore, the negative electrode current collector can typically have a thickness of 3μm to 500μm, and similar to the positive electrode current collector, fine irregularities can be formed on its surface to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0106] In addition to the negative electrode active material, the negative electrode active material layer may optionally include a binder and a conductive material.

[0107] Compounds capable of reversibly inserting and deintercalating lithium can be used as anode active materials. Specific examples of anode active materials include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; (semi-)metallic materials capable of forming alloys with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and (semi-)metal oxides that can be doped or undoped with lithium, such as SiO2. β (0<β<2), SnO2, vanadium oxide and lithium vanadium oxide; or composite materials containing (semi-)metallic materials and carbonaceous materials such as Si-C composite materials or Sn-C composite materials, and any one of them or a mixture of two or more thereof can be used. Additionally, lithium metal films can be used as negative electrode active materials. Furthermore, low-crystallinity carbon and high-crystallinity carbon can be used as carbon materials. Typical examples of low-crystallinity carbon can be soft carbon and hard carbon, and typical examples of high-crystallinity carbon can be irregular, planar, sheet-like, spherical or fibrous natural or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, mesophase pitch and high-temperature sintered carbon such as coke derived from petroleum or coal tar pitch.

[0108] Furthermore, the binders and conductive materials can be the same as those described above for the positive electrode slurry composition.

[0109] The negative electrode can be prepared by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing an optional binder and conductive material and a negative electrode active material in a solvent, onto a negative electrode current collector and drying the coated negative electrode current collector, or by casting the negative electrode slurry composition onto a separate carrier and then pressing the film layer separated from the carrier onto the negative electrode current collector.

[0110] In lithium-ion secondary batteries, the separator separates the negative and positive electrodes and provides a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is commonly used in lithium-ion secondary batteries. In particular, separators with high electrolyte retention capacity and low resistance to electrolyte ion transfer can be used. Specifically, porous polymer membranes can be used, such as porous polymer membranes prepared from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers; or laminated structures having two or more layers. Furthermore, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. Additionally, separators coated with ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can optionally be used.

[0111] Furthermore, the electrolyte used in this invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes that can be used in the preparation of lithium secondary batteries, but this invention is not limited thereto.

[0112] Specifically, the electrolyte may contain organic solvents and lithium salts.

[0113] Any organic solvent can be used without particular limitation, as long as it serves as a medium through which ions participating in the battery electrochemical reaction can move. Specifically, the following substances can be used as the organic solvent: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched, or cyclic C2-C20 hydrocarbon group and may contain double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxolane such as 1,3-dioxolane; or sulfolane. Among these solvents, carbonate solvents can be used, such as mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate).

[0114] Lithium salts can be used without particular restrictions, as long as they are compounds capable of providing lithium ions used in lithium secondary batteries. Specifically, the following substances can be used as lithium salts: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. Lithium salts can be used in concentrations ranging from 0.1 M to 4.0 M. If the concentration of the lithium salt is included in the above range, excellent electrolyte performance can be obtained because the electrolyte can have appropriate conductivity and viscosity, and lithium ions can move efficiently.

[0115] To improve battery life characteristics, suppress battery capacity reduction, and increase battery discharge capacity, additives can be included in the electrolyte in addition to the electrolyte components. In this case, the additive content can be from 0.1% to 5% by weight, based on the total weight of the electrolyte.

[0116] As described above, because the lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics and capacity retention, the lithium secondary battery is suitable for use in: portable devices such as mobile phones, laptops and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs).

[0117] Preferred implementation scheme

[0118] In the following description, examples of the invention will be presented in a manner that allows those skilled in the art to readily implement it. However, the invention may be embodied in many different forms and should not be construed as limited to the examples set forth herein.

[0119] The specifications of the various components used in the following examples and comparative examples are as follows.

[0120] (A) Positive electrode active material: using an average particle size D 50 It is 2.2 μm LiFePO4.

[0121] (B) Conductive material: using a conductive material with a conductivity of 185m 2 / g of Brunauer-Emmett-Teller (BET) carbon nanotubes with specific surface area.

[0122] (C) Adhesive: PVdF with a weight-average molecular weight (Mw) of 630,000 g / mol was used.

[0123] (D) First dispersant: Hydrogenated acrylonitrile-butadiene rubber (H-NBR, acrylonitrile unit: hydrogenated butadiene unit weight ratio = 34:66) with a weight average molecular weight (Mw) of 34,000 g / mol.

[0124] (E) Second dispersant: The following polymer dispersants (E1) to (E7) are used as second dispersants, and the acid value, amine value, weight-average molecular weight and weight ratio of various units in the polymers of polymer dispersants (E1) to (E7) are shown in Table 1 below.

[0125] The amount of oxyalkylene units in the polymer dispersant was measured by nuclear magnetic resonance (NMR).

[0126] (E1) Use BYK's DISPERBYK-9076 (amine value 44 mg KOH / g).

[0127] (E2) Use BYK's DISPERBYK-145 (amine value 71 mg KOH / g).

[0128] (E3) Use BYK's DISPERBYK-2155 (amine value 48 mg KOH / g).

[0129] (E4) Use BYK's BYK Synergist-2100, a copper phthalocyanine compound.

[0130] (E5) Polyacrylic acid (weight average molecular weight of 5,000 g / mol) from Wako Pure Chemical Corporation was used.

[0131] (E6) Use SIGMA-ALDRICH tannins.

[0132] (E7) Use BYK's DISPERBYK-187 (amine value 35 mg KOH / g).

[0133] [Table 1]

[0134]

[0135] Example 1

[0136] After adding positive electrode active material (A), conductive material (B), binder (C), first dispersant (D), and second dispersant (E1) to N-methyl-2-pyrrolidone at a weight ratio of 95.06:1.04:3.00:0.675:0.225 to make the solid content 65% by weight, 75g of the solution was weighed and placed in a 250mL container. The mixture was then mixed at 3,000rpm for 15 minutes using a 700mm diameter saw-shaped impeller (VMA-GETZMANN GMBH, DISPERMAT CN20) to prepare the positive electrode slurry composition.

[0137] Example 2

[0138] The positive electrode slurry composition was prepared in the same manner as in Example 1, except that (E2) was used instead of (E1) as the second dispersant.

[0139] Example 3

[0140] The positive electrode slurry composition was prepared in the same manner as in Example 1, except that (E3) was used instead of (E1) as the second dispersant.

[0141] Comparative Example 1

[0142] The positive electrode slurry composition was prepared in the same manner as in Example 1, except that the second dispersant (E1) was not added and the positive electrode active material (A), conductive material (B), binder (C) and first dispersant (D) were added in a weight ratio of 95.06:1.04:3.00:0.9.

[0143] Comparative Example 2

[0144] The positive electrode slurry composition was prepared in the same manner as in Example 1, except that (E4) was used instead of (E1) as the second dispersant.

[0145] Comparative Example 3

[0146] The positive electrode slurry composition was prepared in the same manner as in Example 1, except that (E5) was used instead of (E1) as the second dispersant.

[0147] Comparative Example 4

[0148] The positive electrode slurry composition was prepared in the same manner as in Example 1, except that (E6) was used instead of (E1) as the second dispersant.

[0149] Comparative Example 5

[0150] The positive electrode slurry composition was prepared in the same manner as in Example 1, except that (E7) was used instead of (E1) as the second dispersant.

[0151] Experimental Example 1

[0152] The viscosity of the positive electrode slurry compositions prepared in the examples and comparative examples was measured using a viscometer (TOKI SANGYO CO., LTD., viscometer TV-22) at 1 rpm and 25°C.

[0153] Furthermore, based on the viscosity of the positive electrode slurry composition of Comparative Example 1 without the use of the second dispersant, the viscosity reduction rate of the positive electrode slurry compositions of Examples 1 to 3 and Comparative Examples 2 to 5 was calculated. Specifically, the viscosity reduction rate is the value obtained by subtracting the viscosity of the various positive electrode slurry compositions prepared in Examples 1 to 3 and Comparative Examples 2 to 5 from the viscosity of the positive electrode slurry composition of Comparative Example 1, dividing it by the viscosity of the positive electrode slurry composition of Comparative Example 1, and then multiplying by 100.

[0154] The measurement results are shown in Table 2 below.

[0155] [Table 2]

[0156] Viscosity (Pa·s) Viscosity reduction rate (%) Comparative Example 1 22.74 - Example 1 14.28 37.20 Example 2 15.11 33.66 Example 3 14.7 36.36 Comparative Example 2 Unpredictable - Comparative Example 3 Unpredictable - Comparative Example 4 46.36 -103.83 Comparative Example 5 28.44 -26.07

[0157] As shown in Table 2, the cathode slurry compositions of Examples 1 to 3, in which a nitrile copolymer dispersant and a polymer dispersant containing more than 60% by weight of oxyalkylene units are used together, exhibit lower viscosity characteristics than the cathode slurry composition of Comparative Example 1, which uses a nitrile copolymer dispersant alone.

[0158] Comparative Examples 2 to 4, in which a nitrile copolymer and a polymeric dispersant without oxyalkylene units are used together, exhibit higher viscosity characteristics than the cathode slurry composition of Comparative Example 1.

[0159] Furthermore, even when a polymer dispersant containing oxyalkylene units is used, as in Comparative Example 5, and the amount of oxyalkylene units is less than 60% by weight, it can be confirmed that the viscosity of the cathode slurry composition actually increases.

[0160] Experiment Example 2

[0161] After preparing 250g of each of the positive electrode slurry compositions prepared in Examples 1 to 3 and Comparative Examples 1 to 4 and passing them through a 100-mesh filter using a vacuum pump, the number of large particles remaining on the filter was checked using a KEYENCE CORPORATION VR-500, and the passability of each positive electrode slurry composition was evaluated according to the following criteria. The evaluation results are shown in Table 3 below.

[0162] OK: Cross-sectional area is 0.5mm² 2 The above cases are when the number of large particles is less than 10.

[0163] NG: Cross-sectional area is 0.5mm² 2 The above cases involve a number greater than 10 large particles.

[0164] Furthermore, photographs showing the state of the filter after passing through the positive electrode slurry composition of Example 1 are presented. Figure 1 The photographs showing the state of the filter after passing through the positive electrode slurry composition of Comparative Example 1 are presented in the image. Figure 2 middle.

[0165] Experimental Example 3

[0166] After the various positive electrode slurry compositions prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were coated onto aluminum current collectors to a thickness of 100 μm using a rod coater, the presence of surface line defects was visually observed, and the coatability was evaluated according to the following criteria. The evaluation results are shown in [Table 3].

[0167] OK: No linear defects were observed after coating.

[0168] NG: A defect in which a portion of the current collector is observed to be exposed in a linear pattern after coating.

[0169] [Table 3]

[0170] Whether to pass through the filter Coating properties Example 1 OK OK Example 2 OK OK Example 3 OK OK Comparative Example 1 NG NG Comparative Example 2 NG NG Comparative Example 3 NG NG Comparative Example 4 NG NG Comparative Example 5 NG NG

[0171] According to [Table 3] and Figure 1For the cathode slurry compositions of Examples 1-3, which use a nitrile copolymer dispersant and a polymer dispersant containing 60% by weight or more oxyalkylene units, the cross-sectional area is confirmed to be 0.5 mm² because there is little particle aggregation in the cathode slurry compositions. 2 The number of large particles is as small as less than 10, and the coating quality is excellent.

[0172] Conversely, as shown in [Table 3] and Figure 2 As shown, in the positive electrode slurry compositions of Comparative Examples 1 to 5, aggregation occurs, thereby increasing the amount of large particles, and surface line defects are generated after coating due to the large particles during slurry coating.

Claims

1. A positive electrode slurry composition for secondary batteries, said positive electrode slurry composition comprising a lithium iron phosphate positive electrode active material, a binder, a conductive material, a dispersant, and a solvent. The dispersant comprises a nitrile copolymer as a first dispersant and a polymeric dispersant containing more than 60% by weight of oxyalkylene units as a second dispersant. The polymeric dispersant has an amine value of 40 mg KOH / g to 80 mg KOH / g.

2. The positive electrode slurry composition for secondary batteries according to claim 1, wherein the polymer dispersant has a weight-average molecular weight of 800 g / mol to 20,000 g / mol.

3. The positive electrode slurry composition for secondary batteries according to claim 1, wherein the nitrile copolymer is a hydrogenated acrylonitrile-butadiene copolymer.

4. The positive electrode slurry composition for secondary batteries according to claim 1, wherein the nitrile copolymer has a weight-average molecular weight of 10,000 g / mol to 100,000 g / mol.

5. The positive electrode slurry composition for secondary batteries according to claim 1, wherein the nitrile copolymer and the polymer dispersant are contained in a weight ratio of 1:1 to 5:

1.

6. The positive electrode slurry composition for secondary batteries according to claim 1, wherein based on a total solid content of 100 parts by weight in the positive electrode slurry composition, the positive electrode slurry composition comprises: The lithium iron phosphate cathode active material comprises 85 to 99.7 parts by weight; 0.1 parts by weight to 10 parts by weight of the adhesive; 0.1 to 5 parts by weight of the conductive material; and 0.1 to 3 parts by weight of the dispersant.

7. The positive electrode slurry composition for secondary batteries according to claim 1, wherein the positive electrode slurry composition has a solid content of 60% to 85% by weight.

8. The positive electrode slurry composition for secondary batteries according to claim 1, wherein the positive electrode slurry composition has a viscosity of less than 20 Pa·s as measured at 1 rpm and 25°C.

9. A positive electrode, said positive electrode being prepared using the positive electrode slurry composition for secondary batteries according to any one of claims 1 to 8.

10. A lithium secondary battery, the lithium secondary battery comprising the positive electrode as described in claim 9.

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