Method of manufacturing a positive electrode for a lithium secondary battery, positive electrode manufactured using the same, and lithium secondary battery including the same

Abstract

The present invention provides a manufacturing method for a positive electrode for a lithium secondary battery, including: a step of preparing a positive electrode in which a positive electrode active material layer containing lithium iron phosphate is formed on a current collector; and a step of adsorbing an organic solvent into the positive electrode active material layer, the organic solvent including one or more selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, methyl ethyl carbonate, ethylene carbonate, and dimethyl carbonate.

Description

Technical Field

[0001] This application claims priority based on Korean Patent Application No. 10-2021-0146518, filed on October 29, 2021, and Korean Patent Application No. 10-2022-0136064, filed on October 21, 2022.

[0002] The present invention relates to a method for manufacturing a positive electrode for a lithium secondary battery comprising a lithium iron phosphate positive electrode active material, a positive electrode for a lithium secondary battery manufactured therefrom, and a lithium secondary battery comprising said positive electrode. Background Technology

[0003] With technological advancements and increasing demand for mobile devices, the need for lithium-ion batteries as an energy source is rapidly growing. Lithium transition metal composite oxides are used as positive electrode active materials in lithium-ion batteries, and among them, lithium-cobalt composite metal oxides, which possess high operating voltage and excellent capacity characteristics, are commonly employed. However, due to the low stability and high cost of lithium-cobalt composite metal oxides, large-scale production of lithium-ion batteries is difficult.

[0004] Accordingly, lithium manganese composite metal oxides, lithium nickel composite metal oxides, lithium iron phosphate, or similar materials have been developed as alternatives to lithium cobalt composite metal oxides. Among them, lithium iron phosphate, with its olivinic structure, exhibits high bulk density, high potential, and a high theoretical capacity of approximately 170 mAh / g. Furthermore, since lithium iron phosphate in its initial state includes an electrochemically doped Li atom for each Fe atom, it is a promising material for positive electrode activity in lithium secondary batteries. Moreover, because lithium iron phosphate contains iron, which is a resource-rich and low-cost material, it is less expensive and less toxic than the aforementioned lithium cobalt composite metal oxides, lithium manganese composite metal oxides, or lithium nickel composite metal oxides, thus offering the advantage of less environmental pollution.

[0005] However, lithium iron phosphate (LFP) is limited by its low lithium insertion / extraction ratio during charging / discharging, and therefore it is manufactured with smaller particle sizes than positive electrode active materials with other compositions. When the particle size of the positive electrode active material is small, there is a problem of reduced adhesion strength with the current collector, and the positive electrode active material layer may deintercalate due to mechanical vibrations applied to the electrode during the assembly process of the secondary battery. When deintercalation of the positive electrode active material layer occurs, the actual measured capacity of the secondary battery is lower than the designed capacity, and there is a problem of fine short-circuit defects that may occur due to the deintercalated particles. Conventionally, to solve the above problems, techniques for improving electrode adhesion strength include increasing the total binder content in the positive electrode active material layer, or adjusting the binder content at the interface between the current collector and the active material layer to a higher level by mitigating binder migration through increasing the drying time during electrode coating.

[0006] However, increasing the binder content in the active material layer leads to drawbacks such as reduced electrode resistance and decreased energy density per volume. Furthermore, increased drying time limits the production cost of both the electrode and the secondary battery. Summary of the Invention

[0007] Technical issues

[0008] The present invention relates to a method for manufacturing a positive electrode for a lithium secondary battery, which can increase the content of binder included in the positive electrode active material layer when manufacturing a positive electrode containing lithium iron phosphate positive electrode active material with small particle size, and increase the adhesion strength of the electrode without increasing the drying time of the electrode.

[0009] In addition, there is a positive electrode manufactured by the above manufacturing method and having excellent adhesion of the electrode active material layer to the electrode current collector, and a lithium secondary battery including the positive electrode.

[0010] Technical solution

[0011] The method for manufacturing a positive electrode for a secondary battery according to the present invention may include: the step of preparing a positive electrode on a current collector in which a positive electrode active material layer containing lithium iron phosphate is formed; and the step of adsorbing an organic solvent into the positive electrode active material layer.

[0012] In one embodiment of the invention, the organic solvent may include one or more of the group consisting of N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, and ethanol.

[0013] In one embodiment of the invention, the organic solvent may include one or more of the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0014] In one embodiment of the present invention, the lithium iron phosphate may be a compound represented by the following formula 1:

[0015] [Formula 1]

[0016] Li 1+a Fe 1-x M x (PO 4-b )X b

[0017] (Wherein, in the formula, 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.)

[0018] In one embodiment of the present invention, the lithium iron phosphate may be LiFePO4 having an olivine crystal structure.

[0019] In one embodiment of the present invention, the average particle diameter (D) of the lithium iron phosphate is... 50 The diameter can range from 0.5 μm to 3 μm.

[0020] In one embodiment of the present invention, the positive electrode active material layer may further include an adhesive.

[0021] In one embodiment of the invention, the adhesive may be one or more of the group consisting of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC).

[0022] In one embodiment of the present invention, the binder content relative to the total weight of the positive electrode active material layer may be 5% by weight or less.

[0023] In one embodiment of the present invention, the step of adsorbing organic solvent into the positive electrode active material layer may include spraying the organic solvent directly onto the positive electrode or sealing the positive electrode and the organic solvent in a sealed container to adsorb the organic solvent.

[0024] In one embodiment of the invention, the organic solvent may be adsorbed at a ratio of 2,000 ppm to 20,000 ppm relative to the total weight of the positive electrode active material layer.

[0025] According to one embodiment of the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on at least one side of the positive electrode current collector and comprising lithium iron phosphate, and the positive electrode active material layer may comprise an organic solvent in a proportion of 2,000 ppm to 20,000 ppm relative to the total weight of the positive electrode active material layer.

[0026] In one embodiment of the present invention, the electrode adhesion strength measured by a 90° peel test between the positive electrode active material layer and the positive electrode current collector can be 10 gf / 2cm or greater.

[0027] In one embodiment of the present invention, the positive electrode active material layer may be in direct contact with the positive electrode current collector.

[0028] In one embodiment of the present invention, the average particle diameter (D) of the lithium iron phosphate is... 50 The diameter can range from 0.5 μm to 3 μm.

[0029] The lithium secondary battery according to the present invention includes the positive electrode.

[0030] Beneficial effects

[0031] When using the positive electrode manufactured according to the present invention, due to the high adhesion strength between the positive electrode active material layer and the electrode current collector, there is an effect of preventing defects such as capacity reduction and fine short circuits caused by the deintercalation of active material during the secondary battery assembly process.

[0032] Specifically, in this invention, by using an organic solvent that is well adsorbed onto the active material, adhesive, and current collector, organic solvent molecules are inserted at the interface between the current collector and the active material layer, and the adhesion strength is increased by forming an attraction between atoms / molecules.

[0033] In addition, since the positive electrode according to the present invention exhibits sufficient electrode adhesion strength without increasing the binder content, the resistance characteristics and electrode flexibility of the secondary battery are improved. Furthermore, since sufficient electrode adhesion strength is exhibited during the manufacture of the positive electrode without increasing the drying time, the manufacturing cost and manufacturing time of the secondary battery can be reduced. Detailed Implementation

[0034] The terms and words used in this specification and claims should not be construed as limited to their common or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical scope of the invention, based on the principle that the inventors have appropriately defined the terms and concepts in order to best describe the invention.

[0035] The terms “comprising,” “including,” and “having” as used herein specify the presence of the features, quantities, steps, components, or elements, or combinations thereof, as described in the specification, and it should be understood that the possibility of the presence or addition of one or more other features, quantities, steps, components, elements, or combinations thereof is not excluded in advance.

[0036] In this invention, the "particle diameter D" of the positive electrode active material n "" represents the particle diameter at the n% point of the volumetric cumulative distribution according to particle diameter. That is, D 50 It is the particle diameter at the 50% point in the volumetric cumulative distribution according to particle diameter. D 90 It is the particle diameter at the 90th percentile point in the volumetric cumulative distribution according to particle diameter. D 10 It is the particle diameter at the 10% point in the volumetric cumulative distribution according to particle diameter. (D) n The laser diffraction method can be used for measurement. Specifically, particle size distribution is calculated by dispersing the powder to be measured in a dispersion medium, introducing it into a commercially available laser diffraction particle size measurement device (e.g., Microtrac S3500), and measuring the differences in diffraction patterns according to particle size as the particles pass through the laser beam. 10 D 50 and D 90 The particle diameter can be measured by calculating the particle diameter at the 10%, 50%, and 90% points of the volumetric cumulative distribution according to the particle diameter in the measuring device.

[0037] The invention will be described in detail below.

[0038] The method for manufacturing a positive electrode for a lithium secondary battery according to the present invention may include: a step of preparing a positive electrode on a current collector in which a positive electrode active material layer containing lithium iron phosphate is formed; and a step of adsorbing an organic solvent into the positive electrode active material layer.

[0039] Specifically, according to the method for manufacturing a positive electrode for a lithium secondary battery according to the present invention, the positive electrode in which a positive electrode active material layer is formed on a current collector can be prepared by applying a composition containing a lithium iron phosphate-based positive electrode active material for forming a positive electrode active material layer to at least one side of the positive electrode current collector and drying it.

[0040] The composition for forming the positive electrode active material layer can be prepared by mixing or dispersing the positive electrode active material in a solvent.

[0041] Specifically, the positive electrode active material may include lithium iron phosphate having a composition represented by Formula 1 below. More specifically, it may include LiFePO4 having an olivine crystal structure. When lithium iron phosphate is used as the positive electrode active material, it has the advantages of high volumetric density, high potential, and large capacity.

[0042] [Formula 1]

[0043] Li 1+a Fe 1-x M x (PO 4-b )X b

[0044] (Wherein, in the formula, 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.)

[0045] As a result of repeated efforts to improve the adhesion strength of a positive electrode, including a positive electrode active material, the inventors of this invention discovered that the adhesion strength of the positive electrode is significantly increased by adding a step of adsorbing organic solvent into the positive electrode active material layer, which led to this invention.

[0046] There are no restrictions on the type of organic solvent used, as long as it is an organic solvent capable of attracting the binder, positive electrode active material, and current collector included in the positive electrode active material layer. Specifically, organic solvents used in the slurry for the positive electrode or the electrolyte for lithium secondary batteries are preferred.

[0047] Specific examples of organic solvents used in slurries for the positive electrode may include one or more selected from the group consisting of N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, and ethanol.

[0048] The organic solvent used in the electrolyte of a lithium-ion secondary battery is a non-aqueous organic solvent that acts as a medium for ion movement associated with the electrochemical reactions of the battery. Specific examples of such organic solvents include ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents, such as dibutyl ether or tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic hydrocarbon solvents, such as benzene and fluorobenzene; and carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate. PC); alcohol solvents, such as ethanol and isopropanol; nitriles, such as R-CN (R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, and may include an aromatic ring with a double bond or ether linkage); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; and sulfolane. In the above examples, carbonate-based solvents are preferred.

[0049] In this invention, in order to accelerate the lithium insertion / extraction ratio of the lithium iron phosphate cathode active material during charging / discharging, the average particle diameter (D) of the cathode active material is... 50 The thickness can be from 0.5 μm to 3 μm, preferably from 0.5 μm to 2.7 μm, and more preferably from 0.6 μm to 2.5 μm.

[0050] In addition, the composition used to form the positive electrode active material layer may further include an adhesive in addition to the positive electrode active material.

[0051] Adhesives are used to improve the adhesion between positive electrode active material particles and the adhesion strength between the positive electrode active material and the positive electrode 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 (EPDM), sulfonated EPDM, and styrene-butadiene rubber, fluororubber, or various copolymers thereof, and they may be used alone or in mixtures of two or more thereof.

[0052] In the examples above, polyvinylidene fluoride (PVDF) is more advantageous than other adhesives in forming an attraction between the organic solvent and the molecules of the present invention, thus PVDF is preferred as the positive electrode adhesive of the present invention.

[0053] The binder may be included in an amount of 5% by weight or less, preferably 1% to 5% by weight, and more preferably 2% to 3.5% by weight, relative to the total weight of the solids in the composition used to form the positive electrode active material layer. When the binder content is less than the above range, there is a problem of too low electrode adhesion strength, while when the binder content is greater than the above range, there is a problem of too high resistance of the secondary battery.

[0054] The compositions of the present invention for forming a positive electrode active material layer may further include one or more additives such as conductive additives, fillers, or dispersants.

[0055] Conductive additives are used to improve the conductivity of electrodes and can be used without restriction as long as they do not cause chemical changes in the secondary battery and are electronically conductive. Specific examples include, for instance, carbon powders such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lampblack, or thermal black; graphite powders such as natural graphite, artificial graphite, or graphite; conductive fibers such as carbon fibers, carbon nanotubes, and metal fibers; conductive powders such as fluorinated carbon powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc., and they can be used alone or in mixtures of two or more of them.

[0056] The conductive additive may conventionally be included in an amount of 0.3% to 5% by weight, preferably 0.3% to 4% by weight, and more preferably 0.5% to 3.5% by weight, relative to the total weight of the solids content in the composition used to form the positive electrode active material layer.

[0057] Dispersants are used to improve the dispersibility of lithium iron phosphate-based cathode active materials, and there are no limitations on their use, as long as they are conventionally used dispersants, and for example, aqueous or organic dispersants can be used. While there are no limitations, hydrogenated nitrile butadiene rubber (HNBR) is more preferably used. Hydrogenated nitrile butadiene rubber (HNBR) means that the double bonds originally included in nitrile rubber (NBR) are converted into single bonds through hydrogenation of the nitrile butadiene rubber (NBR).

[0058] The dispersant may be included in an amount of 0% to 4% by weight, preferably 0% to 2% by weight, and more preferably 0.10% to 1.3% by weight, relative to the total weight of the solids content in the composition used to form the positive electrode active material layer.

[0059] In the manufacturing method of the present invention, there are no particular limitations on the positive current collector, as long as it is conductive and does not cause chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or other surfaces treated with carbon, nickel, titanium, silver, etc. can be used.

[0060] In addition, the positive current collector can have a thickness of 8 μm to 20 μm, and the adhesion strength to the positive active material layer can be increased by forming fine irregularities on the surface of the positive current collector. For example, it can be used in various forms such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0061] In the manufacturing method of the present invention, the step of applying the composition for forming the positive electrode active material layer to the positive electrode current collector can be performed by methods conventionally used in the industry, but it can be performed by, for example, uniformly dispersing using a doctor blade, or by methods such as die casting, commacoating, screen printing, etc.

[0062] In the manufacturing method of the present invention, the drying process of the composition for forming the positive electrode material layer applied to the positive electrode current collector can be performed by conventional drying methods, such as heat treatment such as vacuum heating treatment in the above temperature range, or hot air injection.

[0063] Here, the drying process temperature can be 60°C to 130°C, specifically 80°C to 130°C, or more specifically 100°C to 130°C. When the temperature is within the above range, the moisture content in lithium iron phosphate can be minimized, and volatile components included in the manufacturing process are sufficiently removed, thereby preventing side reactions and battery performance degradation caused by these components during battery charging / discharging.

[0064] In addition, the required drying time can be from 5 minutes to 3 hours, specifically from 5 minutes to 20 minutes, or more specifically from 5 minutes to 10 minutes. The required drying time can be shortened to the above ranges by the manufacturing method according to the invention.

[0065] Next, the manufacturing method of the present invention may include an adsorption step of organic solvent to the positive electrode active material layer. Organic solvent is a material that can be well adsorbed onto the active material, binder, and current collector, and it can increase the electrode adhesion strength by forming atomic / molecular attraction through the insertion of organic solvent molecules at the contact interface between the current collector and the active material layer.

[0066] In one embodiment of the invention, the organic solvent may be an organic solvent used for positive electrode slurry, and more specifically, it may include one or more of the group consisting of N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, and ethanol.

[0067] In another embodiment of the invention, the organic solvent may be an organic solvent constituting the electrolyte of a lithium secondary battery, and more specifically, it may include one or more of the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC). Carbonate-based solvents have excellent effects on improving adhesion strength.

[0068] When an organic solvent with the same composition as the electrolyte of a secondary battery is selected as the organic solvent to be adsorbed onto the positive electrode active material layer, there is an advantage that the step of drying the organic solvent after the adsorption step can be omitted.

[0069] In the manufacturing method of the present invention, the organic solvent can be adsorbed at a ratio of 2,000 ppm to 20,000 ppm, preferably 2,000 ppm to 10,000 ppm, and more preferably 2,000 ppm to 4,000 ppm relative to the total weight of the positive electrode active material layer. When the adsorption amount is below the above range, the effect of improving adhesion strength is limited, while when the adsorption amount is high, there is a disadvantage of reduced adhesion strength with the separation membrane.

[0070] Specifically, the step of adsorbing organic solvents onto the positive electrode active material layer may include spraying the organic solvent directly onto the positive electrode or sealing the positive electrode and the organic solvent together in a closed container and adsorbing the organic solvent.

[0071] When organic solvents are sprayed through a spray nozzle, they can reach 0.01 mg / cm³. 2 Up to 2 mg / cm 2 0.01 mg / cm³ is preferred. 2 Up to 1 mg / cm 2 More preferably 0.05 mg / cm 2 Up to 0.5 mg / cm 2 The appropriate amount of organic solvent is sprayed. When sprayed at the above amount, it can be adsorbed into the positive electrode active material layer. In addition, the positive electrode can be aged in a dry chamber environment at room temperature, so that the organic solvent applied to the surface can be completely adsorbed and penetrated into the electrode.

[0072] Alternatively, the Petri dish containing the organic solvent and the positive electrode can be placed in a sealed container, sealed, and stored for several days, so that the organic solvent volatilized in the sealed container is adsorbed into the positive electrode active material layer.

[0073] As described above, after the laminate containing the positive electrode, negative electrode, and separation membrane, in which organic solvents have been adsorbed, is stored inside the external material of the battery, the organic solvents can undergo a drying process before the electrolyte is injected to prevent the electrical properties from being degraded by the adsorbed organic solvents. However, when the organic solvents adsorbed into the positive electrode have the same composition as the electrolyte, the drying process can be omitted because the organic solvents adsorbed into the positive electrode can act as a medium for ion movement.

[0074] The positive electrode manufactured according to the method of the present invention includes an organic solvent in its positive electrode active material layer as it undergoes a step of adsorbing an organic solvent. The content of the organic solvent included in the positive electrode active material layer relative to the total weight of the positive electrode active material layer is from 2,000 ppm to 20,000 ppm.

[0075] The content of organic solvent relative to a cathode sample cut to a certain size is defined as the average value obtained by measuring the amount of organic solvent adsorbed three times using headspace gas chromatography with flameionization detection (HS-GC-FID).

[0076] When the organic solvent is the same organic solvent used in the cathode slurry, such as NMP, it is preferred that the content of the organic solvent is 2,000 ppm to 12,000 ppm, 2,500 ppm to 10,000 ppm, or 3,000 ppm to 9,000 ppm.

[0077] When the organic solvent is the same organic solvent used in the electrolyte, it is preferred that the content of the organic solvent can be from 2,000 ppm to 20,000 ppm, more preferably from 3,000 ppm to 15,000 ppm, and even more preferably from 4,000 ppm to 12,000 ppm.

[0078] According to one embodiment of the present invention, the positive electrode has a structure in which the positive electrode active material layer directly contacts the positive electrode current collector, and it may not include a separate layer for improving the adhesion strength between the positive electrode active material layer and the positive electrode current collector.

[0079] Due to its unique manufacturing process, the positive electrode of the present invention includes an organic solvent in the positive electrode active material layer, and the organic solvent forms an attractive force between the positive electrode active material and the current collector, thereby improving the adhesion strength of the contact interface between the positive electrode active material layer and the positive electrode current collector. Therefore, although there is no separate layer such as an adhesive layer, bonding layer, coupling layer, or primer coating inserted between the positive electrode current collector and the positive electrode active material layer to improve adhesion strength, the positive electrode of the present invention exhibits excellent electrode adhesion strength, measured by a 90° peel test to be 10 gf / 2 cm or greater, preferably 15 gf / 2 cm or greater.

[0080] In summary, the positive electrode of the present invention can improve the capacity and output characteristics of the battery due to the increased adhesion strength, and reduce defects that occur during the manufacturing process.

[0081] The present invention provides a lithium secondary battery including the above-described positive electrode.

[0082] A lithium secondary battery includes a positive electrode, a negative electrode, a separator inserted between the positive and negative electrodes, and a non-aqueous electrolyte, wherein the positive electrode is as described above.

[0083] The negative electrode in a lithium-ion secondary battery can be manufactured by applying a composition for forming the negative electrode, including a negative electrode active material and optional additives such as binders, conductive agents, fillers, and dispersants, to a negative electrode current collector. There are no particular limitations on the negative electrode active material, as long as the compound can reversibly insert and extract lithium. Specific examples include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers, amorphous carbon, and highly crystalline carbon; metal compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; or composites comprising metal compounds and carbonaceous materials. In addition, soft carbon and hard carbon can be used as low-crystallinity carbons; natural graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and petroleum or coal tar pitch-derived cokes can be used as high-crystallinity carbons. These can be used alone or in mixtures of two or more of them, and lithium metal films can be used as anode active materials.

[0084] In addition, additives such as binders, conductive additives, fillers, and dispersants may be the same as those described in the positive electrode section.

[0085] Meanwhile, there are no particular restrictions on the negative electrode current collector, as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or copper / stainless steel and aluminum-cadmium alloys or similar materials with surface treatments of carbon, nickel, titanium, silver or the like can be used.

[0086] In addition, the negative electrode current collector can typically have a thickness of 3 μm to 500 μm, and similar to the positive electrode current collector, the adhesion strength of the negative electrode active material can be increased by forming fine irregularities on the surface of the positive electrode current collector. For example, it can be used in various forms such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0087] Meanwhile, in lithium secondary batteries, separators can be used without restriction, as long as they are already used as separators in conventional secondary batteries. In particular, low resistance to ion movement of the electrolyte and excellent electrolyte hiding ability are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof. Alternatively, conventional porous nonwoven fabrics can be used, such as nonwoven fabrics made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, the separator can be a porous film with a pore diameter of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0088] In addition, electrolytes may include organic solvents and lithium salts commonly used as electrolytes, and there are no particular limitations.

[0089] Organic solvents can be used without particular restrictions, as long as they can serve as a medium for the movement of ions involved in the electrochemical reactions of the battery.

[0090] Specifically, as organic solvents, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone can be used; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; and carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC) can be used.

[0091] Among them, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate, propylene carbonate, etc.) with high ionic conductivity and high dielectric constant, and low viscosity linear carbonate-based compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) that can increase the charge / discharge performance of the battery are more preferred.

[0092] As lithium salts, any compound capable of providing lithium ions used in lithium secondary batteries can be used without particular limitation. Specifically, as lithium salts, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc., can be used. The lithium salt is preferably included in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0093] In addition to the electrolyte components mentioned above, the electrolyte may further include one or more additives, such as pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-ethylene glycol dimethyl ether (glyme), triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolium ketones, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. The additives may be included in the electrolyte in an amount from 0.1% to 5% by weight relative to the total weight of the electrolyte.

[0094] The lithium secondary battery of the present invention can be manufactured by inserting a separator between the positive and negative electrodes to form an electrode assembly, and the electrode assembly can be formed by inserting it into a cylindrical battery casing or a prismatic battery casing and then injecting an electrolyte. Alternatively, after stacking the electrode assembly, it can be manufactured by impregnating it with an electrolyte, placing the resulting product into a battery casing and sealing it.

[0095] When manufacturing the lithium secondary battery of the present invention, one or more organic solvents selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, methyl ethyl carbonate, ethylene carbonate, and dimethyl carbonate used in manufacturing the positive electrode can be removed by drying the electrode assembly.

[0096] If an electrode with the same composition as the organic solvent used in the manufacture of the positive electrode is used as the electrolyte, the step of drying the electrode assembly can be omitted.

[0097] The battery casing can be of the type commonly used in the art, and there are no restrictions on its appearance depending on the purpose of the battery. For example, it can be cylindrical, prismatic, pouch-shaped, or coin-shaped.

[0098] Because the lithium secondary battery of the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention, it can be used in portable devices such as mobile phones, laptops, and digital cameras, as well as in electric vehicles such as hybrid electric vehicles (HEVs).

[0099] The embodiments of the present invention will be described in detail below, so that they can be easily implemented by those skilled in the art. However, the present invention can be implemented in many different forms and is not limited to the embodiments described below.

[0100] Example

[0101] Example 1

[0102] The positive electrode is manufactured through the following process: by mixing particles with an average particle diameter (D) in an N-methylpyrrolidone solvent at a weight ratio of 95.4:0.8:3.0:0.8. 50 A composition (60% by weight solids) for preparing the cathode was formed by using 2 μm LiFePO4 positive electrode active material, carbon nanotube conductive material, polyvinylidene fluoride (PVDF) binder, and hydrogenated nitrile butadiene rubber (HNBR) dispersant; this composition was uniformly applied to an aluminum film with a thickness of 15 μm, resulting in a specific discharge capacity of 2.9 mAh / cm² for the cathode. 2 The cathode active material layer was then rolled to a thickness of 96 μm. To reduce the moisture content of the prepared cathode, it was vacuum dried at 130°C for 10 hours.

[0103] Next, spray at 0.1 mg / cm³ through the spray nozzle. 2 A certain amount of propylene carbonate (PC) solvent was sprayed onto the surface of the positive electrode active material layer of the prepared positive electrode to ensure uniform application. The positive electrode was aged in a dry chamber environment at room temperature for 15 minutes, thereby allowing all the organic solvent applied to the surface to be adsorbed and penetrated into the electrode.

[0104] When all the PC dispersed through the spray nozzle is adsorbed into the positive electrode without volatilization, the adsorption capacity of the entire active material layer is 4,000 ppm.

[0105] Example 2

[0106] The positive electrode was prepared in the same manner as in Example 1, except that acetone was used as the coating solvent.

[0107] Example 3

[0108] The positive electrode is manufactured through the following process: by mixing particles with an average particle diameter (D) in an N-methylpyrrolidone solvent at a weight ratio of 95.4:0.8:3.0:0.8. 50 A composition (60% by weight solids) for preparing the cathode was formed by using 2 μm LiFePO4 positive electrode active material, carbon nanotube conductive material, polyvinylidene fluoride (PVDF) binder, and hydrogenated nitrile butadiene rubber (HNBR) dispersant. This composition was then uniformly applied to a 15 μm thick aluminum film, resulting in a specific discharge capacity of 2.9 mAh / cm² for the cathode. 2 The cathode active material layer was then rolled to a thickness of 96 μm. To reduce the moisture content of the prepared cathode, it was vacuum dried at 130°C for 10 hours.

[0109] Next, the prepared positive electrode and N-methylpyrrolidone (NMP) solvent were placed in a Petri culture dish in a sealed container, sealed, and stored for 1 day, so that the NMP solvent volatilized in the sealed container was adsorbed into the positive electrode active material layer.

[0110] After preparing the cathode of Example 3 with a size of 50 mm × 50 mm and measuring the adsorption amount of NMP solvent three times using headspace gas chromatography with flame ionization detection (HS-GC-FID), the average value relative to the total cathode active material layer was 2,600 ppm.

[0111] Example 4

[0112] The positive electrode was prepared in the same manner as in Example 3, except that the prepared positive electrode and Petri dishes containing N-methylpyrrolidone (NMP) solvent were placed in a sealed container, sealed, and stored for 5 days.

[0113] After preparing the cathode of Example 4 with a size of 50 mm × 50 mm and measuring the adsorption amount of NMP solvent three times using headspace gas chromatography with flame ionization detection (HS-GC-FID), the average value relative to the total cathode active material layer was 8,600 ppm.

[0114] Example 5

[0115] The positive electrode was prepared in the same manner as in Example 1, except that dimethyl carbonate was used as the coating solvent.

[0116] Comparative Example 1

[0117] The positive electrode is manufactured through the following process: by mixing particles with an average particle diameter (D) in an N-methylpyrrolidone solvent at a weight ratio of 95.4:0.8:3.0:0.8. 50 A composition (60% by weight solids) for preparing the cathode was formed by combining 2 μm LiFePO4 positive electrode active material, carbon nanotube conductive material, polyvinylidene fluoride (PVDF) binder, and hydrogenated nitrile butadiene rubber (HNBR) dispersant. This composition was then uniformly applied to a 15 μm thick aluminum film, resulting in a final cathode active material layer thickness of 96 μm. To reduce the moisture content of the prepared cathode, it was vacuum dried at 130°C for 10 hours.

[0118] After preparing the positive electrode of Comparative Example 1 with a size of 50 mm × 50 mm and measuring the adsorption amount of NMP solvent three times using headspace gas chromatography with flame ionization detection (HS-GC-FID), the average value relative to the entire positive electrode active material layer was 160 ppm.

[0119] Comparative Example 2

[0120] The positive electrode was prepared in the same manner as in Comparative Example 1, except that distilled water was used as the coating solvent.

[0121] After preparing the positive electrode of Comparative Example 1 with a size of 50 mm × 50 mm and measuring the adsorption amount of NMP solvent three times using a Karl Fischer titration moisture analyzer (Metrohm Co.), the average value relative to the total positive electrode active material layer was 9500 ppm.

[0122] [Experimental Example: Evaluation of Adhesion Strength]

[0123] The adhesion strength between the positive electrode active material layer and the positive electrode current collector of the prepared positive electrode was compared in Examples 1 to 5 and Comparative Examples 1 to 2.

[0124] Specifically, the positive electrodes prepared in Examples 1 to 5 and Comparative Examples 1 to 2 were cut into pieces 150 mm long and 20 mm wide, and the surface of the electrode was attached longitudinally to a glass slide 75 mm long and 25 mm wide. That is, the glass slide was attached to half of the region corresponding to the longitudinal position of the positive electrode. The sample for evaluation was then prepared by uniformly attaching double-sided tape using a 2 kg load friction roller 10 times. The portion of the sample with the attached glass slide was fixed to the sample stage of a Universal Testing Machine (UTM) (LF Plus LLOYD Co.), and the half of the positive electrode without the attached glass slide was connected to the load sensor of the UTM device.

[0125] The load applied to the load sensor was measured while moving it at a speed of 100 mm / min up to 50 mm. The minimum load measured during this period, within intervals of 20 mm to 40 mm throughout the entire drive process, was used to measure the electrode adhesion strength (gf / 2cm) for each sample. The average values ​​after a total of 5 evaluations for each positive electrode are shown in Table 1 below.

[0126] [Table 1]

[0127]

[0128] The experimental results show that the positive electrodes of Examples 1 to 5 have significantly higher adhesion strength compared to the positive electrodes of Comparative Examples 1 to 2. In the case of Comparative Example 1, unlike the present invention, the organic solvent was not adsorbed into the positive electrode active material layer, resulting in poor electrode adhesion strength. The distilled water in Comparative Example 2 has low intermolecular attraction with the PVDF binder, thus contributing to low electrode adhesion strength.

Claims

1. A method for manufacturing a positive electrode for a lithium secondary battery, comprising: The step of preparing a positive electrode on a current collector by forming a positive electrode active material layer therein containing lithium iron phosphate, wherein the composition for forming the positive electrode active material layer is prepared by mixing or dispersing the positive electrode active material in a solvent, wherein the positive electrode active material layer further comprises a binder; and The step of adsorbing organic solvents into the positive electrode active material layer. The step of adsorbing the organic solvent into the positive electrode active material layer includes either spraying the organic solvent directly onto the positive electrode or sealing the positive electrode and the organic solvent in a sealed container to adsorb the organic solvent. The organic solvent comprises one or more selected from the group consisting of N-methyl-2-pyrrolidone, dimethyl sulfoxide, isopropanol, acetone, and ethanol, and the organic solvent is adsorbed at a ratio of 3,000 ppm to 12,000 ppm relative to the total weight of the positive electrode active material layer, or The organic solvent comprises one or more selected from the group consisting of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl methyl carbonate, ethylene carbonate, and propylene carbonate, and the organic solvent is adsorbed at a ratio of 4,000 ppm to 12,000 ppm relative to the total weight of the positive electrode active material layer.

2. The method for manufacturing the positive electrode of a lithium secondary battery according to claim 1, wherein the lithium iron phosphate is a compound represented by the following formula 1: [Formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b in, In the formula, 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, and 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 method for manufacturing the positive electrode for a lithium secondary battery according to claim 1, wherein the lithium iron phosphate is LiFePO4 having an olivine crystal structure.

4. The method for manufacturing the positive electrode of a lithium secondary battery according to claim 1, wherein the average particle diameter D of the lithium iron phosphate is... 50 The range is from 0.5 μm to 3 μm.

5. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein the adhesive comprises one or more selected from the group consisting of polyvinylidene fluoride, styrene-butadiene rubber, and carboxymethyl cellulose.

6. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein the binder content relative to the total weight of the positive electrode active material layer is 5% by weight or less.

7. A positive electrode for a lithium secondary battery, manufactured using the manufacturing method according to claim 1, wherein the positive electrode for the lithium secondary battery comprises: Positive current collector; and A positive electrode active material layer, including lithium iron phosphate, is formed on at least one side of the positive electrode current collector; The positive electrode active material layer comprises an organic solvent in a proportion of 3,000 ppm to 12,000 ppm relative to the total weight of the positive electrode active material layer, and the organic solvent comprises one or more selected from the group consisting of N-methyl-2-pyrrolidone, dimethyl sulfoxide, isopropanol, acetone, and ethanol. or, The organic solvent is adsorbed at a ratio of 4,000 ppm to 12,000 ppm relative to the total weight of the positive electrode active material layer, and the organic solvent comprises one or more of the group consisting of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl methyl carbonate, ethylene carbonate, and propylene carbonate.

8. The positive electrode for a lithium secondary battery according to claim 7, wherein the positive electrode active material layer is in direct contact with the positive electrode current collector.

9. The positive electrode for a lithium secondary battery according to claim 7, wherein the average particle diameter of the lithium iron phosphate is 0.5 μm to 3 μm.

10. A lithium secondary battery comprising the positive electrode as described in claim 7.

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