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

Abstract

To provide a manufacturing method for a positive electrode for a lithium secondary battery.SOLUTION: A manufacturing method for a positive electrode for a lithium secondary battery includes the steps of preparing a positive electrode in which a positive electrode active material layer containing lithium iron phosphate is formed on a current collector, and adsorbing an organic solvent on the positive electrode active material. The organic solvent includes one or more selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] The present invention relates to a method for producing a positive electrode for a lithium secondary battery containing a positive electrode active material of a lithium iron phosphate compound, a positive electrode for a lithium secondary battery produced using the same, and a lithium secondary battery containing the positive electrode. [Background technology]

[0003] As technological development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Lithium transition metal composite oxides are used as the positive electrode active material for lithium secondary batteries, and lithium cobalt composite metal oxides are the most commonly used due to their high operating voltage and excellent capacity characteristics. However, lithium cobalt composite metal oxides have low stability and are expensive, making mass production of lithium secondary batteries difficult.

[0004] Therefore, lithium-manganese composite metal oxides, lithium-nickel composite metal oxides, and lithium iron phosphate compounds have been developed as alternatives to lithium-cobalt composite metal oxides. Among these, lithium iron phosphate compounds with an olivinic structure have high volume density, generate high potential, and have a high theoretical capacity of approximately 170 mAh / g. Furthermore, because lithium iron phosphate compounds initially contain one electrochemically undoped Li atom per Fe atom, they are promising materials for use as positive electrode active materials in lithium secondary batteries. Furthermore, because lithium iron phosphate compounds contain iron, a resource that is abundant and inexpensive, they are less expensive than the aforementioned lithium-cobalt composite metal oxides, lithium-manganese composite metal oxides, and lithium-nickel composite metal oxides. Furthermore, they are less toxic and therefore less environmentally hazardous.

[0005] However, lithium iron phosphate compounds have a limit in that they have a low lithium insertion / extraction rate during charge / discharge, and are therefore manufactured with smaller particle sizes than positive electrode active materials of other compositions. Small particle sizes of positive electrode active materials can result in poor adhesion to the current collector, and mechanical shock applied to the electrode during the secondary battery assembly process can cause detachment of the positive electrode active material layer. Detachment of the positive electrode active material layer can result in a decrease in the actual measured capacity of the secondary battery compared to the designed capacity, and can also cause micro-short circuit defects due to the detached particles.

[0006] To solve these problems, conventional techniques have been used to improve electrode adhesion by increasing the total binder content in the positive electrode active material layer, or by mitigating binder migration by increasing the drying time during electrode coating, thereby increasing the binder content at the interface between the current collector and the active material layer.

[0007] However, a high binder content in the active material layer has drawbacks such as a decrease in the resistance characteristics and the energy density per volume of the electrode, and an increase in the drying time has a limit in that the production costs of the electrode and secondary battery increase. Summary of the Invention [Problem to be solved by the invention]

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

[0009] Another object of the present invention is to provide a positive electrode produced by the above-mentioned production method, in which the electrode active material layer has excellent adhesive strength to the electrode current collector, and a lithium secondary battery including the above-mentioned positive electrode. [Means for solving the problem]

[0010] A method for manufacturing a positive electrode for a secondary battery according to one embodiment of the present invention may include the steps of preparing a positive electrode in which a positive electrode active material layer containing a lithium iron phosphate compound is formed on a current collector, and allowing an organic solvent to be adsorbed onto the positive electrode active material layer.

[0011] In one embodiment of the present invention, the organic solvent may include one or more selected from the group consisting of N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, and ethanol.

[0012] In one embodiment of the present invention, the organic solvent may include one or more selected from the group consisting of dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0013] In one embodiment of the present invention, the lithium iron phosphate compound may be a compound represented by the following Chemical Formula 1:

[0014] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0015] In the above Chemical Formula 1, M includes any 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 any 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.

[0016] In one embodiment of the present invention, the lithium iron phosphate compound can be LiFePO4 with an olivine crystal structure.

[0017] In one embodiment of the present invention, the average particle size (D 50 ) can be 0.5 to 3 μm.

[0018] In an embodiment of the present invention, the positive electrode active material layer may further include a binder.

[0019] In one embodiment of the present invention, the binder may be one or more selected from the group consisting of polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC).

[0020] In one embodiment of the present invention, the binder content may be 5 wt % or less based on the total weight of the positive electrode active material layer.

[0021] In an embodiment of the present invention, the step of adsorbing the organic solvent into the positive electrode active material layer may include a process of directly spraying the organic solvent onto the positive electrode, or a process of sealing the positive electrode together with the organic solvent in a sealed container and adsorbing the organic solvent.

[0022] In one embodiment of the present invention, the organic solvent may be adsorbed at a ratio of 2,000 to 20,000 ppm relative to the total weight of the positive electrode active material layer.

[0023] A positive electrode according to one embodiment of the present invention includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector and containing a lithium iron phosphate compound, and the positive electrode active material layer may contain 2,000 to 20,000 ppm of an organic solvent based on the total weight of the positive electrode active material layer.

[0024] In one embodiment of the present invention, the electrode adhesive strength measured by a 90° peel test between the positive electrode active material layer and the current collector may be 10 gf / 2 cm or more.

[0025] In one embodiment of the present invention, the positive electrode active material layer may be in direct contact with the positive electrode current collector.

[0026] In one embodiment of the present invention, the lithium iron phosphate compound has an average particle size (D 50 ) can be 0.5 to 3 μm.

[0027] The lithium secondary battery according to the present invention includes the above positive electrode. [Effects of the Invention]

[0028] When the positive electrode manufactured according to the present invention is used, the adhesive strength between the electrode active material layer and the electrode current collector is high, which has the effect of suppressing defects such as capacity reduction and micro-short circuits caused by detachment of the active material during the assembly process of the secondary battery.

[0029] Specifically, in the present invention, an organic solvent that is easily adsorbed to the active material, binder, and current collector is used, and organic solvent molecules are positioned at the contact interface between the current collector and the active material layer, thereby increasing the adhesive strength by forming an attractive force between atoms / molecules.

[0030] In addition, the positive electrode according to the present invention exhibits sufficient electrode adhesion without increasing the binder content, thereby improving the resistance characteristics of the secondary battery and improving the electrode flexibility.Furthermore, since the positive electrode exhibits sufficient electrode adhesion without increasing the drying time during the manufacturing process, the manufacturing cost and time of the secondary battery can be reduced. DETAILED DESCRIPTION OF THE INVENTION

[0031] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concepts of terms in order to best describe his own invention.

[0032] As used herein, terms such as "comprise," "comprise," or "have" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the possible presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0033] In the present invention, the "particle size D n " means the particle size at n% of the volume cumulative distribution of particle size. That is, D 50 is the particle size at the 50% point of the volume cumulative distribution by particle size, and D 90 is the particle size at 90% of the volume cumulative distribution by particle size, D 10 is the particle size at 10% of the cumulative volume distribution by particle size. The above Dn can be measured using the laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac S3500). When the particles pass through a laser beam, the difference in the diffraction pattern due to particle size is measured to calculate the particle size distribution. D is calculated by calculating the particle diameters at 10%, 50%, and 90% of the cumulative volume distribution by particle size in the measuring device. 10 , D 50 and D 90 can be measured.

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

[0035] The method for producing a positive electrode for a lithium secondary battery of the present invention may include the steps of preparing a positive electrode in which a positive electrode active material layer containing a lithium iron phosphate compound is formed on a current collector, and allowing an organic solvent to be adsorbed onto the positive electrode active material layer.

[0036] Specifically, the method for manufacturing a positive electrode for a lithium secondary battery according to the present invention can prepare a positive electrode in which a composition for forming a positive electrode active material layer containing a lithium iron phosphate compound-based positive electrode active material is applied to at least one surface of a positive electrode current collector, and then dried to form a positive electrode active material layer on the current collector.

[0037] The composition for forming the positive electrode active material layer can be produced by mixing or dispersing the positive electrode active material in a solvent.

[0038] The positive electrode active material may specifically include a lithium iron phosphate compound having a composition represented by the following Chemical Formula 1, and more specifically, may include LiFePO4 having an olivine crystal structure. When a lithium iron phosphate compound is used as the positive electrode active material, it has the advantages of a high volume density of the positive electrode, the ability to generate a high potential, and a large capacity.

[0039] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0040] In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, and X includes one or more elements selected from the group consisting of F, S, and N, and -0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5.

[0041] As a result of repeated efforts to improve the adhesive strength of a positive electrode containing the above-described positive electrode active material, the inventors of the present invention discovered that the adhesive strength of the positive electrode can be dramatically increased by adding a step of adsorbing an organic solvent onto the positive electrode active material layer, leading to the present invention.

[0042] The organic solvent is not particularly limited as long as it can form an attractive force with the binder, the positive electrode active material, and the current collector contained in the positive electrode active material layer. Specifically, such an organic solvent is preferably an organic solvent used in a positive electrode slurry or an organic solvent used in an electrolyte for a lithium secondary battery.

[0043] Specific examples of the organic solvent used in the positive electrode slurry may include one or more selected from the group consisting of N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, and ethanol.

[0044] The organic solvent used in the electrolyte of a lithium secondary battery is a non-aqueous organic solvent that serves as a medium through which ions involved in the electrochemical reaction of the battery move. Specific examples of such organic solvents include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone, ether-based solvents such as dibutyl ether and tetrahydrofuran, ketone-based solvents such as cyclohexanone, aromatic hydrocarbon-based solvents such as benzene and fluorobenzene, dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of suitable solvents include carbonate solvents such as propylene carbonate (PC), alcohol solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, which may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Of these, carbonate solvents are preferred.

[0045] In the present invention, in order to increase the lithium insertion / extraction rate during charge / discharge of the lithium iron phosphate compound positive electrode active material, the average particle size (D 50 ) can be adjusted to 0.5 to 3 μm, preferably 0.5 to 2.7 μm, and more preferably 0.6 to 2.5 μm.

[0046] The positive electrode active material layer-forming composition may further contain a binder in addition to the positive electrode active material.

[0047] The binder serves to improve adhesion between particles of the positive electrode active material and between 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 polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers thereof. These may be used alone or in combination.

[0048] Among these, polyvinylidene fluoride (PVDF) is preferable as the binder for the positive electrode of the present invention because polyvinylidene fluoride (PVDF) is more advantageous than other binders in forming an intermolecular attractive force with the organic solvent of the present invention.

[0049] The binder may be contained in an amount of 5 wt % or less, preferably 1 to 5 wt %, and more preferably 2 to 3.5 wt %, based on the total weight of the solid content in the composition for forming a positive electrode active material layer. If the binder content is lower than the above range, the electrode adhesive strength may be too low, while if the binder content is higher than the above range, the resistance of the secondary battery may be too high.

[0050] The positive electrode active material layer-forming composition of the present invention may further contain one or more additives such as a conductive material, a filler, or a dispersant.

[0051] The conductive material is used to improve the conductivity of the electrode, and can be used without any particular limitation as long as it has electronic conductivity without undergoing chemical changes in the secondary battery. Specific examples include carbon powders such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite; conductive fibers such as carbon fiber, carbon nanotubes, and metal fiber; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. These may be used alone or in combination.

[0052] The conductive material may generally be contained in an amount of 0.3 to 5 wt %, preferably 0.3 to 4 wt %, and more preferably 0.5 to 3.5 wt %, based on the total weight of the solid content in the composition for forming a positive electrode active material layer.

[0053] The dispersant is used to improve the dispersibility of the lithium iron phosphate-based positive electrode active material and is not limited to any commonly used dispersant, for example, an aqueous dispersant or an organic dispersant may be used. Although not necessarily limited, hydrogenated nitrile rubber (HNBR) may be used more preferably. The hydrogenated nitrile rubber (HNBR) is a rubber obtained by hydrogenating nitrile butadiene rubber (NBR) to convert the double bonds contained in the original nitrile butadiene rubber (NBR) into single bonds.

[0054] The dispersant may be contained in an amount of 0 to 4 wt %, preferably 0 to 2 wt %, and more preferably 0.10 to 1.3 wt %, based on the total weight of the solid content in the composition for forming a positive electrode active material layer.

[0055] In the manufacturing method of the present invention, the positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used.

[0056] The positive electrode current collector may have a thickness of 8 to 20 μm, and may have fine irregularities on its surface to enhance adhesion to the positive electrode active material layer. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0057] In the manufacturing method of the present invention, the process of applying the composition for forming a positive electrode active material layer to the positive electrode current collector may be performed by a method commonly known in the art, such as uniformly dispersing the composition using a doctor blade, or by methods such as die casting, comma coating, and screen printing.

[0058] In the production method of the present invention, the composition for forming a positive electrode active material layer applied onto the positive electrode current collector can be dried according to a conventional drying method, for example, by a vacuum heating treatment in the above-mentioned temperature range or a heat treatment method such as hot air injection.

[0059] The temperature of the drying process may be 60° C. to 130° C., specifically 80° C. to 130° C., and more specifically 100° C. to 130° C. When the temperature is within this range, the moisture content in the lithium iron phosphate compound can be minimized, and volatile components contained during the process can be sufficiently removed, thereby preventing side reactions and deterioration of battery characteristics due to these components during subsequent charging and discharging of the battery.

[0060] The time required for the drying step may be 5 minutes to 3 hours, specifically 5 minutes to 20 minutes, and more specifically 5 minutes to 10 minutes. When the production method according to the present invention is followed, the time required for the drying step can be shortened to the above range.

[0061] The method according to the present invention may further include a step of adsorbing an organic solvent onto the positive electrode active material layer. The organic solvent is a material that is easily adsorbed by the active material, binder, and current collector, and the organic solvent molecules are positioned at the contact interface between the current collector and the active material layer, thereby forming an intermolecular force between atoms and molecules, thereby increasing electrode adhesion.

[0062] In one embodiment of the present invention, the organic solvent may be the organic solvent used in the positive electrode slurry, and specifically may include one or more selected from the group consisting of N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, and ethanol.

[0063] In another embodiment of the present invention, the organic solvent may be an organic solvent constituting an electrolyte solution of a lithium secondary battery, and specifically may include one or more selected from the group consisting of dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC). The carbonate-based solvent has an excellent effect of improving adhesive strength.

[0064] When the organic solvent to be adsorbed onto the positive electrode active material layer is selected to be the same as the organic solvent constituting the electrolyte of the secondary battery, there is an advantage in that the drying step of drying the organic solvent can be omitted after the organic solvent adsorption step.

[0065] In the manufacturing method of the present invention, the organic solvent can be adsorbed in a ratio of 2,000 to 20,000 ppm, preferably 2,000 to 10,000 ppm, and more preferably 2,000 to 4,000 ppm, based on the total weight of the positive electrode active material layer. If the adsorption amount is lower than the above range, the effect of improving adhesive strength is limited, while if it is higher, there is a disadvantage that the adhesive strength with the separator is reduced.

[0066] Specifically, the step of adsorbing the organic solvent into the positive electrode active material layer may be performed by directly spraying the organic solvent onto the positive electrode, or by sealing the positive electrode together with the organic solvent in a sealed container and adsorbing the organic solvent therein.

[0067] When spraying the above organic solvents with a spray nozzle, the concentration is 0.01 to 2 mg / cm 2 , preferably 0.01 to 1 mg / cm 2 , and more preferably 0.05 to 0.5 mg / cm 2 When the organic solvent is sprayed in the above-mentioned amount, it can be adsorbed in a suitable amount on the positive electrode active material layer. In addition, the positive electrode can be aged in a room temperature dry room environment so that the organic solvent applied to the surface can be completely absorbed and impregnated into the inside of the electrode.

[0068] Alternatively, the Petri dish containing the organic solvent and the positive electrode may be placed in a sealed container, sealed, and then stored for several days, so that the organic solvent evaporated in the sealed container is adsorbed onto the positive electrode active material layer.

[0069] After the laminate comprising the cathode, anode, and separator having the organic solvent adsorbed thereon is housed within the exterior of a battery, a drying process for the adsorbed organic solvent may be performed before the electrolyte is poured in to prevent a decrease in electrical characteristics due to the adsorbed organic solvent. However, if the organic solvent adsorbed on the cathode is the same as the components of the electrolyte, the organic solvent adsorbed on the cathode may serve as a medium for the migration of lithium ions, and thus the drying process may be omitted.

[0070] The positive electrode produced by the production method of the present invention contains an organic solvent in the positive electrode active material layer through a step of adsorbing the organic solvent, and the content of the organic solvent in the positive electrode active material layer is 2,000 to 20,000 ppm based on the total weight of the positive electrode active material layer.

[0071] The content of the organic solvent may be defined as the average value calculated after measuring the amount of organic solvent adsorption three times on a cathode specimen cut into a certain size using HS-GC-FID (Headspace Gas Chromatography with flame ionization detection) equipment.

[0072] When the organic solvent is an organic solvent used in positive electrode slurries, such as NMP, the content of the organic solvent is preferably 2,000 to 12,000 ppm, 2,500 to 10,000 ppm, or 3,000 to 9,000 ppm.

[0073] When the organic solvent is the organic solvent used in the electrolytic solution, the content of the organic solvent can be 2,000 to 20,000 ppm, preferably 3,000 to 15,000 ppm, and more preferably 4,000 to 12,000 ppm.

[0074] In a positive electrode according to an embodiment of the present invention, the positive electrode active material layer is in direct contact with the positive electrode current collector, and a separate layer for improving adhesion between the positive electrode active material layer and the positive electrode current collector may not be included.

[0075] The positive electrode according to the present invention includes the organic solvent in the positive electrode active material layer due to its unique manufacturing process, and the organic solvent forms an attractive force between the positive electrode active material and the current collector, thereby improving the adhesive strength at the contact interface between the positive electrode active material layer and the entire positive electrode collector. Therefore, the positive electrode according to the present invention can exhibit excellent adhesive strength, with an electrode adhesive strength of 10 gf / 2 cm or more, preferably 15 gf / 2 cm or more, as measured in a 90° peel test, even without a separate layer, such as a binder layer, adhesive layer, binding layer, or primer coating layer, that may be interposed between the positive electrode current collector and the positive electrode active material layer to improve adhesive strength.

[0076] As a result, the positive electrode of the present invention can improve the capacity and output characteristics of the battery due to the increased adhesive strength, and can reduce defects that occur during the manufacturing process.

[0077] The present invention provides a lithium secondary battery including the above-described positive electrode.

[0078] The lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, and the positive electrode is as described above.

[0079] In the lithium secondary battery, the negative electrode can be manufactured, for example, by preparing a negative electrode-forming composition containing a negative electrode active material and, optionally, additives such as a binder, a conductive material, a filler, and a dispersant on a negative electrode current collector, and then coating the composition on the negative electrode current collector.

[0080] In this case, the negative electrode active material is not particularly limited, and may generally be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and highly crystalline carbon, metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys, and composites containing metallic compounds and carbonaceous materials. Low-crystalline carbon includes soft carbon and hard carbon, while highly crystalline carbon includes natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbon such as petroleum or coal tar pitch-derived cokes. These materials may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material.

[0081] The additives such as the binder, conductive material, filler and dispersant may be the same as those previously described for the positive electrode.

[0082] Meanwhile, the negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc. may be used.

[0083] The negative electrode current collector may typically have a thickness of 3 to 500 μm, and like the positive electrode current collector, the surface of the positive electrode current collector may be formed with fine irregularities to enhance the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, nonwoven fabric, etc.

[0084] Meanwhile, in the lithium secondary battery, the separator can be any separator typically used in lithium secondary batteries, with no particular limitations. In particular, a separator that exhibits low resistance to ion migration of the electrolyte and excellent humidification of the electrolyte solution is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. The separator can be a porous thin film having a pore diameter of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0085] The electrolyte may contain an organic solvent and a lithium salt that are commonly used in electrolytes, and is not particularly limited.

[0086] The organic solvent may be any organic solvent without particular limitation, as long as it can function as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that may be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; and carbonate-based solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0087] Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0088] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without particular limitation. Specifically, examples of the lithium salt include LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(CF, SO), LiN(CF, SO), LiN(CF, SO), LiCl, LiI, and LiB(CO) . The lithium salt is preferably contained in the electrolyte at a concentration of approximately 0.6 mol% to 2 mol%.

[0089] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the battery's lifespan, suppressing battery capacity loss, and improving the battery's discharge capacity. In this case, the additives may be contained in an amount of 0.1 to 5 wt % based on the total weight of the electrolyte.

[0090] The lithium secondary battery of the present invention may be fabricated by forming an electrode assembly by disposing a separator between a positive electrode and a negative electrode, and then placing the electrode assembly in a cylindrical or prismatic battery case and injecting an electrolyte thereinto, or by stacking the electrode assemblies, impregnating them with an electrolyte, and then sealing the resulting assembly in a battery case.

[0091] When manufacturing the lithium secondary battery of the present invention, the electrode assembly may be dried to remove one or more organic solvents selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate, which were used in manufacturing the positive electrode.

[0092] If an electrolyte containing the same components as the organic solvent used in manufacturing the positive electrode is used, the step of drying the electrode assembly can be omitted.

[0093] The battery case may be any battery case commonly used in the art, and may have any shape depending on the intended use of the battery, such as a cylindrical can, a rectangular can, a pouch-type, or a coin-type.

[0094] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0095] Although the present invention may be embodied in many different forms, it is not intended to be limited to the embodiments set forth herein, and the present invention is not limited to the embodiments set forth herein.

[0096] Example 1 Average particle size (D 50 The cathode active material of LiFePO4 with a particle size of 2 μm, the carbon nanotube conductive material, the polyvinylidene fluoride (PVDF) binder, and the hydrogenated nitrile rubber (HNBR) dispersant were mixed in an N-methylpyrrolidone solvent in a weight ratio of 95.4:0.8:3.0:0.8 (solid content 60 wt%) to prepare a cathode composition. The cathode was finally fabricated on a 15 μm thick aluminum thin film and had a discharge specific capacity of 2.9 mAh / cm. 2 The cathode was fabricated by rolling the cathode active material layer to a thickness of 96 μm using a roll press. To reduce the moisture content of the fabricated cathode, the cathode was vacuum dried at 130° C. for 10 hours.

[0097] Next, propylene carbonate (PC) solvent was sprayed onto the surface of the positive electrode active material layer of the prepared positive electrode at a rate of 0.1 mg / cm using a spray nozzle. 2 The positive electrode was aged for 15 minutes in a dry room at room temperature so that the propylene carbonate (PC) applied to the surface was completely absorbed and impregnated into the electrode.

[0098] When the PC dispersed by the spray nozzle was completely adsorbed onto the positive electrode without volatilization, the amount of PC adsorbed onto the entire active material layer was 4,000 ppm.

[0099] <Example 2> A positive electrode was prepared in the same manner as in Example 1, except that acetone was used as the coating solvent.

[0100] Example 3 Average particle size (D 50 The cathode active material of LiFePO4 with a particle size of 2 μm, the carbon nanotube conductive material, the polyvinylidene fluoride (PVDF) binder, and the hydrogenated nitrile rubber (HNBR) dispersant were mixed in an N-methylpyrrolidone solvent in a weight ratio of 95.4:0.8:3.0:0.8 (solid content 60 wt%) to prepare a cathode composition. The cathode was finally fabricated on a 15 μm thick aluminum thin film and had a discharge specific capacity of 2.9 mAh / cm. 2 The cathode was fabricated by rolling the cathode active material layer to a thickness of 96 μm using a roll press. To reduce the moisture content of the fabricated cathode, the cathode was vacuum dried at 130° C. for 10 hours.

[0101] Next, the Petri dish containing the prepared cathode and N-methylpyrrolidone (NMP) solvent was placed in an airtight container, sealed, and then stored for one day so that the NMP solvent evaporated in the airtight container was adsorbed onto the cathode active material layer.

[0102] The positive electrode of Example 3 was prepared in a size of 50 mm × 50 mm, and the amount of adsorption of the NMP solvent was measured three times using HS-GC-FID (Headspace Gas Chromatography with flame ionization detection) equipment. The average value was calculated to be 2,600 ppm with respect to the entire positive electrode active material layer.

[0103] Example 4 A positive electrode was prepared in the same manner as in Example 3, except that the prepared positive electrode and a Petri dish containing N-methylpyrrolidone (NMP) solvent were placed in an airtight container, sealed, and then stored for 5 days.

[0104] The positive electrode of Example 4 was prepared in a size of 50 mm × 50 mm, and the amount of adsorption of the NMP solvent was measured three times using HS-GC-FID (Headspace Gas Chromatography with flame ionization detection) equipment. The average value was calculated to be 8,600 ppm with respect to the entire positive electrode active material layer.

[0105] <Example 5> A positive electrode was prepared in the same manner as in Example 1, except that dimethyl carbonate was used as the coating solvent.

[0106] <Comparative Example 1> Average particle size (D 50 A cathode active material of LiFePO4 with a particle size of 2 μm, a carbon nanotube conductive material, a polyvinylidene fluoride (PVDF) binder, and a hydrogenated nitrile rubber (HNBR) dispersant were mixed in N-methylpyrrolidone solvent in a weight ratio of 95.4:0.8:3.0:0.8 (solids content 60 wt%) to prepare a cathode composition. The cathode was then uniformly coated onto a 15 μm-thick aluminum foil so that the cathode active material layer in the final cathode was 96 μm thick. The cathode was then vacuum dried at 130°C for 10 hours to reduce its moisture content.

[0107] The positive electrode of Comparative Example 1 was prepared to a size of 50 mm × 50 mm, and the amount of adsorption of the NMP solvent was measured three times using HS-GC-FID (Headspace Gas Chromatography with flame ionization detection) equipment. The average value was calculated to be 160 ppm relative to the entire positive electrode active material layer.

[0108] <Comparative Example 2> A positive electrode was prepared in the same manner as in Example 1, except that distilled water was used as the coating solvent.

[0109] The positive electrode of Comparative Example 1 was prepared to a size of 50 mm x 50 mm, and the amount of water adsorption was measured three times using a Karl Fischer titration moisture meter (Metrohm). The average value was calculated to be 9500 ppm relative to the entire positive electrode active material layer.

[0110] [Experimental example: Adhesion strength evaluation] The positive electrodes produced in Examples 1 to 5 and Comparative Examples 1 and 2 were compared in terms of adhesive strength between the positive electrode active material layer and the positive electrode current collector.

[0111] Specifically, the positive electrodes prepared in Examples 1 to 5 and Comparative Examples 1 and 2 were cut to a size of 150 mm long and 20 mm wide, and the electrode surface was attached longitudinally to a 75 mm long and 25 mm wide glass slide using double-sided tape. That is, the glass slide was attached to an area corresponding to half of the positive electrode in the longitudinal direction. Then, evaluation samples were prepared by rubbing the surface with a 2 kg load roller 10 times to ensure uniform adhesion of the double-sided tape. The glass slide portion of the evaluation sample was fixed to the sample stage of a Universal Testing Machine (UTM) (LF Plus, LLOYD), and the half of the positive electrode not attached to the glass slide was connected to the load cell of the UTM equipment. The load cell was moved up to 50 mm at a speed of 100 mm / min, and the load applied to the load cell was measured. The minimum load measured in the 20 mm to 40 mm section of the travel section was measured as the electrode adhesive strength (gf / 2 cm) of each sample. Each positive electrode was evaluated five times in total, and the average values ​​are shown in Table 1 below.

[0112] [Table 1]

[0113] As a result of the experiment, the positive electrodes of Examples 1 to 5 exhibited significantly higher adhesive strength than the positive electrodes of Comparative Examples 1 and 2. It can be seen that the electrode adhesive strength was poor in Comparative Example 1 because, unlike the present invention, no organic solvent was adsorbed on the positive electrode active material layer. The distilled water of Comparative Example 2 exhibited low intermolecular attraction with the PVDF binder, which has a relatively low polarity, which is interpreted as the reason for its poor electrode adhesive strength.

Claims

1. preparing a positive electrode having a positive electrode active material layer including a lithium iron phosphate compound formed on a current collector; a step of adsorbing an organic solvent onto the positive electrode active material layer, The method for producing a positive electrode for a lithium secondary battery includes adsorbing the organic solvent at a ratio of 2,000 to 20,000 ppm based on the total weight of the positive electrode active material layer.

2. 2. The method for producing a positive electrode for a lithium secondary battery according to claim 1, wherein the organic solvent comprises one or more selected from the group consisting of N-methyl-2-pyrrolidone, dimethyl sulfoxide, isopropyl alcohol, acetone, and ethanol.

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

4. The lithium iron phosphate compound is LiFePO having an olivine crystal structure. ４ 2. The method for producing a positive electrode for a lithium secondary battery according to claim 1,

5. The average particle size (D ５０ 2. The method for producing a positive electrode for a lithium secondary battery according to claim 1, wherein the thickness of the first electrode is 0.5 to 3 μm.

6. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1 , wherein the positive electrode active material layer further comprises a binder.

7. 7. The method for producing a positive electrode for a lithium secondary battery according to claim 6, wherein the binder is at least one selected from the group consisting of polyvinylidene fluoride, styrene butadiene rubber, and carboxymethyl cellulose.

8. 7. The method for producing a positive electrode for a lithium secondary battery according to claim 6, wherein the binder content is 5% by weight or less based on the total weight of the positive electrode active material layer.

9. 2. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein the step of adsorbing the organic solvent into the positive electrode active material layer comprises directly spraying the organic solvent onto the positive electrode or sealing the positive electrode together with the organic solvent in a sealed container to adsorb the organic solvent.

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