Positive electrode current collector coated with adhesion promoting layer and method of manufacturing the same, positive electrode for lithium secondary battery including the same, and lithium secondary battery

Abstract

The present invention discloses a method for producing a positive electrode current collector coated with an adhesion promotion layer, the method including: (S1) preparing an aqueous slurry in which a hydrocarbon-based water-soluble binder polymer is dissolved and polyvinylidene fluoride-based polymer particles and a first conductive material are dispersed; and (S2) coating at least one surface of a metal current collector with the aqueous slurry, and heat-treating the aqueous slurry at a temperature higher than the melting point of the polyvinylidene fluoride-based polymer particles, and drying the coated surface to form an adhesion promotion layer.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a positive electrode current collector coated with an adhesion-promoting layer, a positive electrode current collector coated with an adhesion-promoting layer manufactured thereby, a method for manufacturing a positive electrode for a lithium secondary battery, a positive electrode for a lithium secondary battery manufactured thereby, and a lithium secondary battery including the positive electrode for a lithium secondary battery.

[0002] This application claims priority based on Korean Patent Application No. 10-2021-0150120, filed on November 3, 2021, and the entire contents disclosed in the specification and drawings of that application are incorporated herein by reference. [Background technology]

[0003] Recently, with the rapid spread of battery-powered electronic devices such as mobile phones, notebook PCs, and electric vehicles, the demand for small, lightweight, and relatively high-capacity secondary batteries is rapidly increasing. In particular, lithium secondary batteries are attracting attention as a power source for portable devices due to their light weight and high energy density. As a result, research and development efforts to improve the performance of lithium secondary batteries are actively underway.

[0004] In a lithium secondary battery, an organic or polymer electrolyte is filled between a positive electrode and a negative electrode, which are made of active materials that allow the intercalation and deintercalation of lithium ions. Electrical energy is generated through oxidation and reduction reactions that occur when lithium ions are inserted into and extracted from the positive and negative electrodes.

[0005] A positive electrode of a lithium secondary battery is typically fabricated by coating a positive electrode active material slurry containing a positive electrode active material, a conductive material, a binder polymer, and a solvent onto a positive electrode current collector made of a metal such as aluminum, followed by drying to form a positive electrode active material layer. Specifically, the positive electrode is fabricated by weighing and mixing the components of the positive electrode active material slurry, coating the slurry on the positive electrode current collector, drying, and then pressing.

[0006] The fabricated cathode is then assembled into a lithium secondary battery through a post-process, but the adhesive strength between the cathode active material layer and the current collector is weak, which can lead to the cathode active material detaching. This problem becomes more severe when a lithium iron phosphate-based cathode active material is used or when the size of the active material is small.

[0007] To solve these problems, a method has been proposed in which an adhesion-promoting layer containing a binder polymer is formed on the current collector before the positive electrode active material layer is formed on the current collector. However, there is a need to develop an adhesion-promoting layer that can more firmly bond the positive electrode active material layer to the current collector and has low interfacial resistance.

[0008] When applied to a lithium secondary battery containing an electrolyte solution, it is also required to manufacture a lithium secondary battery that can maintain adhesive strength with the electrodes. Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a positive electrode current collector coated with an adhesion-promoting layer that improves adhesion between a positive electrode active material layer and a current collector and has low interfacial resistance, a method for manufacturing the same, and a positive electrode for a lithium secondary battery and a secondary battery including the same.

[0010] Another object of the present invention is to provide a positive electrode current collector coated with an adhesion-promoting layer that can maintain adhesion to an electrode when applied to a lithium secondary battery containing an electrolyte, a method for manufacturing the same, and a positive electrode for a lithium secondary battery and a secondary battery including the same. [Means for solving the problem]

[0011] In one aspect of the present invention, there is provided a method for manufacturing a positive electrode current collector coated with an adhesion-promoting layer according to the following embodiment.

[0012] The first embodiment is (S1) preparing an aqueous slurry in which a hydrocarbon-based water-soluble binder polymer is dissolved and polyvinylidene fluoride-based polymer particles and a first conductive material are dispersed; (S2) coating at least one surface of a metal current collector with the aqueous slurry, and then heat-treating the coating at a temperature higher than the melting point of the polyvinylidene fluoride-based polymer particles, followed by drying, thereby forming an adhesion-promoting layer.

[0013] The second embodiment is the first embodiment, The content of the hydrocarbon-based water-soluble binder polymer is 5 to 50 parts by weight based on 100 parts by weight of polyvinylidene fluoride-based polymer particles.

[0014] The third embodiment is the first or second embodiment, The hydrocarbon-based water-soluble binder polymer may be a hydrocarbon-based water-soluble polymer such as polyvinyl alcohol, polyvinylpyrrolidone, maleic anhydride, tannic acid, polyacrylic acid, or polyacrylamide, and may be used alone or in combination.

[0015] A fourth embodiment is any one of the first to third embodiments, The present invention relates to a method for producing a positive electrode current collector coated with an adhesion-promoting layer, wherein the melting point of the polyvinylidene fluoride-based polymer is 50 to 150°C, more specifically 70 to 150°C, and most specifically 90 to 150°C.

[0016] A fifth embodiment is any one of the first to fourth embodiments, The polyvinylidene fluoride-based polymer is a copolymer of vinylidene and hexafluoropropylene, and the method for manufacturing a positive electrode current collector coated with an adhesion-promoting layer is also disclosed.

[0017] The sixth embodiment is any one of the first to fifth embodiments, The polyvinylidene fluoride polymer has a weight-average molecular weight of 700,000 to 1,300,000, more specifically, 800,000 to 1,100,000.

[0018] The seventh embodiment is any one of the first to sixth embodiments, The present invention relates to a method for producing a positive electrode current collector coated with an adhesion-promoting layer, characterized in that the heat treatment is carried out at a temperature 10 to 80° C. higher than the melting point of the polyvinylidene fluoride-based polymer.

[0019] The eighth embodiment is: a metal current collector; an adhesion-promoting layer formed on at least one surface of the metal current collector, the adhesion-promoting layer including a hydrocarbon-based water-soluble binder polymer, a polyvinylidene fluoride-based polymer, and a first conductive material; The polyvinylidene fluoride-based polymer is dispersed in the form of islands on the surface of the metal current collector, thereby providing a positive electrode current collector coated with an adhesion promoting layer.

[0020] The ninth embodiment is the eighth embodiment, The content of the hydrocarbon-based water-soluble binder polymer is 5 to 50 parts by weight based on 100 parts by weight of polyvinylidene fluoride-based polymer particles, and the positive electrode current collector coated with the adhesion-promoting layer is also provided.

[0021] The tenth embodiment is the eighth or ninth embodiment, The hydrocarbon-based water-soluble binder polymer is at least one selected from the group consisting of polyvinyl alcohol, polyvinylpyrrolidone, maleic anhydride, tannic acid, polyacrylic acid, and polyacrylamide.

[0022] An eleventh embodiment is any one of the eighth to tenth embodiments, The present invention relates to a positive electrode current collector coated with an adhesion-promoting layer, characterized in that the melting point of the polyvinylidene fluoride-based polymer is 50 to 150°C, more specifically 70 to 150°C, and most specifically 90 to 150°C.

[0023] The twelfth embodiment is: A positive electrode for a lithium secondary battery is provided, comprising: a positive electrode current collector coated with an adhesion-promoting layer according to any one of the eighth to eleventh embodiments; and a positive electrode active material layer formed on the adhesion-promoting layer, the positive electrode active material layer including a positive electrode active material, a second conductive material, and a binder polymer.

[0024] The thirteenth embodiment is the twelfth embodiment, The present invention relates to a positive electrode for a lithium secondary battery, wherein the metal current collector is made of aluminum, and the positive electrode active material is a positive electrode active material represented by the following chemical formula 1: [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b (M is 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 is one or more elements selected from the group consisting of F, S, and N, and -0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.1.)

[0025] The fourteenth embodiment is the twelfth or thirteenth embodiment, The positive electrode for a lithium secondary battery is characterized in that the binder polymer contained in the positive electrode active material layer is a polyvinylidene fluoride-based polymer.

[0026] A fifteenth embodiment provides a lithium secondary battery including the positive electrode according to the fourteenth embodiment. [Effects of the Invention]

[0027] According to one embodiment of the present invention, the polyvinylidene fluoride-based polymer is dispersed in islands on the surface of the metal current collector, rather than covering the entire surface of the current collector, thereby improving the adhesion between the positive electrode active material layer and the current collector and providing low interfacial resistance.

[0028] In addition, the hydrocarbon-based water-soluble binder polymer contained in the adhesion-promoting layer forms hydrogen bonds with the positive electrode active material layer to further increase adhesion, and when used with an aluminum current collector, forms hydrogen bonds with the oxidized aluminum current collector to improve adhesion. Furthermore, when used in a lithium secondary battery containing an electrolyte, the binder polymer exhibits resistance to dissolution in the electrolyte, thereby maintaining good adhesion to the positive electrode active material layer. This effect is further enhanced when a polyvinylidene fluoride-based polymer having a predetermined melting point range is used as the polyvinylidene fluoride-based polymer.

[0029] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to the matters depicted in the drawings. Note that the shape, size, scale, or ratio of elements in the drawings attached to this specification may be exaggerated to emphasize a clearer description. [Brief explanation of the drawings]

[0030] [Figure 1] 1 is an SEM photograph of an adhesion-promoting layer formed in Example 1. [Figure 2] 1 is a DSC chart for polymer A used in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, the terms and words used in this specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as meanings and concepts corresponding to the technical concept of the present invention, in accordance with the principle that the inventor himself can appropriately define the concept of terms in order to best explain the invention. Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiment of the present invention and do not represent the entire technical concept of the present invention, and therefore, various equivalents and modifications that can be substituted therefor may exist at the time of filing this application.

[0032] According to one aspect of the present invention, there is provided a method for manufacturing a positive electrode current collector coated with an adhesion-promoting layer, comprising: First, an aqueous slurry is prepared in which a hydrocarbon-based water-soluble binder polymer is dissolved and polyvinylidene fluoride-based polymer particles and a first conductive material are dispersed (step S1).

[0033] The slurry for forming the adhesion-promoting layer is an aqueous slurry using water as a dispersion medium.

[0034] To the water-based slurry, a solvent such as isopropyl alcohol, acetone, ethanol, or butyl alcohol may be optionally added to lower the surface energy and improve the coatability.

[0035] The aqueous slurry contains polyvinylidene fluoride polymer particles to improve the adhesion between the metal current collector and the positive electrode active material layer. The use of particulate polyvinylidene fluoride polymer improves the resistance increase phenomenon of the adhesion-promoting layer.

[0036] The melting point of the polyvinylidene fluoride polymer is preferably 50 to 150°C. When the polyvinylidene fluoride polymer has such a melting point range, it is less likely to be dissolved in the electrolyte solution used during battery assembly, its adhesive strength with the electrode is increased, and the drying temperature during the heat treatment of the aqueous slurry, which will be described later, can be lowered, thereby reducing energy consumption. From this perspective, the melting point of the polyvinylidene fluoride polymer can be more specifically 70 to 150°C, even more specifically 90 to 150°C, and most specifically 100 to 140°C. The polyvinylidene fluoride polymer can be, for example, a vinylidene fluoride-hexafluoropropylene copolymer, but is not limited to this.

[0037] The weight-average molecular weight of the polyvinylidene fluoride polymer may be 700,000 to 1,300,000, more specifically 800,000 to 1,100,000. When the weight-average molecular weight is in this range, the adhesive strength with the positive electrode active material layer is further increased.

[0038] It is needless to say that the aqueous slurry may further contain other binder polymers in the form of particles in addition to the above-mentioned polyvinylidene fluoride polymer particles, within the limits that do not impair the object of the present invention.

[0039] The aqueous slurry also contains a dissolved hydrocarbon-based water-soluble binder polymer. The hydrocarbon-based water-soluble binder polymer can be dissolved by adding the hydrocarbon-based water-soluble binder polymer to water, if necessary, and then applying heat. The hydrocarbon-based water-soluble binder polymer forms hydrogen bonds with the positive electrode active material layer to further increase adhesion, and when used with an aluminum current collector, it also forms hydrogen bonds with the oxidized aluminum current collector to improve adhesion. Furthermore, when used in a lithium secondary battery containing an electrolyte (non-aqueous electrolyte), the hydrocarbon-based water-soluble binder polymer maintains good adhesion to the positive electrode active material layer due to its resistance to dissolution by the electrolyte.

[0040] The content of the hydrocarbon-based water-soluble binder polymer may be 5 to 50 parts by weight based on 100 parts by weight of polyvinylidene fluoride-based polymer particles, taking into consideration all of the aforementioned functions of the hydrocarbon-based water-soluble binder polymer, electrical performance degradation, and economical aspects. Examples of the hydrocarbon-based water-soluble binder polymer include polyvinyl alcohol, polyvinylpyrrolidone, maleic anhydride, tannic acid, polyacrylic acid, and polyacrylamide, which may be used alone or in combination. These types of hydrocarbon-based water-soluble polymers are excellent at improving adhesion, as described above.

[0041] The aqueous slurry contains a first conductive material to suppress an increase in the resistance of the positive electrode. The first conductive material is not particularly limited, as long as it is conductive and does not induce side reactions with other elements of the battery. Examples of the first conductive material include graphite, such as natural graphite or artificial graphite; carbon black, such as carbon black (super-p), acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers, such as carbon fiber and metal fiber; carbon nanotubes, such as MW-CNT and SW-CNT; metal powders, such as carbon fluoride, aluminum, and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive materials, such as polyphenylene derivatives. These materials may be used alone or in combination to reduce interfacial resistance.

[0042] The weight ratio of the fluoride-based polymer particles, including polyvinylidene fluoride-based polymer particles, to the first conductive material in the aqueous slurry may be, but is not limited to, 0.5:1 to 8:1. The thickness of the adhesion-promoting layer formed on one surface of the metal current collector may be 50 to 5,000 nm.

[0043] One or more thickeners may be added to the aqueous slurry to adjust the viscosity. There are no particular limitations, but cellulose-based thickeners such as polysaccharide-based carboxymethyl cellulose, hydroxymethyl cellulose, ethyl cellulose, and methyl cellulose, which are not hydrocarbon-based water-soluble polymers, may be used.

[0044] In addition to the above-mentioned components, the aqueous slurry may of course further contain other additives such as dispersants within limits that do not impair the object of the present invention.

[0045] Next, the prepared aqueous slurry is coated on at least one surface of a metal current collector, and the coating is heat-treated at a temperature higher than the melting point of the polyvinylidene fluoride-based polymer particles, followed by drying, to form an adhesion-promoting layer (Step S2).

[0046] A metal current collector such as aluminum is used as the positive electrode current collector. In particular, aluminum can be used in the form of foil. Aluminum foil is easily oxidized in air, forming a surface layer made of aluminum oxide. Therefore, a current collector made of aluminum is interpreted as including a current collector having an aluminum oxide surface layer formed by the oxidation of aluminum on the surface. The thickness of the metal current collector can typically be, but is not limited to, 3 to 500 μm.

[0047] The aqueous slurry may be applied to a metal current collector using a conventional method and apparatus. Examples of suitable methods include bar coating using a Meyer bar, gravure coating, two-roll reverse coating, vacuum slot die coating, and two-roll coating. The adhesion-promoting layer is formed on at least one surface of the metal current collector, i.e., on one or both surfaces of the metal current collector, to improve adhesion between the metal current collector and a positive electrode active material layer (described later). The thickness of the adhesion-promoting layer (based on the thickness of an adhesion-promoting layer formed on one surface of the metal current collector, not both surfaces) may be, for example, 50 to 5,000 nm, more specifically, 100 to 1,000 nm, taking into account the effect of improving adhesion and the degree of increase in positive electrode resistance, but is not limited thereto.

[0048] The metal current collector coated with the aqueous slurry is heat-treated at a temperature higher than the melting point of the polyvinylidene fluoride-based polymer particles and dried to form an adhesion-promoting layer. Specifically, the drying step can be performed at a temperature 10 to 80°C higher than the melting point of the polyvinylidene fluoride-based polymer particles, but is not limited thereto.

[0049] When the polyvinylidene fluoride-based polymer particles are heat-treated and dried at a temperature higher than the melting point of the polyvinylidene fluoride-based polymer particles, the melted polyvinylidene fluoride-based polymer particles undergo a drying process, and then solidify as the temperature drops, forming an adhesion-promoting layer on the metal current collector. In this case, the polyvinylidene fluoride-based polymer in the adhesion-promoting layer is dispersed in the form of islands on the surface of the current collector. That is, the dispersed polyvinylidene fluoride-based polymer does not cover the entire surface of the current collector. As a result, the adhesion-promoting layer improves the adhesion between the positive electrode active material layer and the current collector, resulting in low interfacial resistance.

[0050] Furthermore, as described above, it is preferable to use a polyvinylidene fluoride-based polymer having a predetermined melting point range, which allows the aqueous slurry to be dried at a relatively low temperature, thereby reducing energy consumption, and the formed adhesion-promoting layer exhibits good adhesion performance. Such an adhesion-promoting layer has good resistance to dissolution in an electrolyte solution when applied to a lithium secondary battery containing an electrolyte solution, thereby maintaining adhesion to the positive electrode.

[0051] The positive electrode current collector coated with the adhesion promoting layer manufactured by the manufacturing method according to the above-mentioned embodiment is a metal current collector; an adhesion-promoting layer formed on at least one surface of the metal current collector, the adhesion-promoting layer including a hydrocarbon-based water-soluble binder polymer, a polyvinylidene fluoride-based polymer, and a first conductive material; The polyvinylidene fluoride-based polymer is dispersed in the form of islands on the surface of the metal current collector.

[0052] The metal current collector constituting the positive electrode current collector coated with the adhesion-promoting layer, the binder component of the adhesion-promoting layer, and the conductive material component have been described above, so further description will be omitted.

[0053] After preparing a positive electrode current collector coated with an adhesion-promoting layer by the above-described method, a positive electrode active material layer including a positive electrode active material, a second conductive material, and a binder polymer is laminated on the adhesion-promoting layer and adhered to the adhesion-promoting layer to prepare a positive electrode for a lithium secondary battery. This provides a positive electrode for a lithium secondary battery including a positive electrode current collector coated with an adhesion-promoting layer and a positive electrode active material layer formed on the adhesion-promoting layer and including a positive electrode active material, a second conductive material, and a binder polymer.

[0054] The positive electrode active material may be a typical positive electrode active material used in lithium secondary batteries, such as a lithium transition metal oxide, etc. In particular, the positive electrode active material may be a positive electrode active material represented by the following Chemical Formula 1: [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b (M is 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 is one or more elements selected from the group consisting of F, S, and N, and -0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.1.)

[0055] The positive electrode active material of Formula 1 is a lithium iron phosphate-based compound, and has a problem of low adhesion to an aluminum current collector. Therefore, when applying the adhesion-promoting layer according to the present invention, there is an increasing industrial need for an improved adhesion to the current collector.

[0056] The binder polymer used to bind the positive electrode active material layers together may be a binder polymer typically used in positive electrode mixtures. Examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. More preferably, the binder polymer included in the positive electrode active material layer may be a polyvinylidene fluoride-based polymer, which further enhances interlayer adhesion through interaction with the polyvinylidene fluoride-based polymer of the adhesion-promoting layer.

[0057] The binder polymer contained in the positive electrode active material layer may be contained in an amount of, for example, 1 to 30 wt % with respect to the total weight of the positive electrode active material layer.

[0058] The second conductive material used in the positive electrode active material may be the first conductive material of the adhesion promoting layer, which may be used independently. The second conductive material may be typically included in an amount of 1 to 30 wt % based on the total weight of the positive electrode active material layer.

[0059] The method for laminating the positive electrode active material layer and adhering the adhesion promoting layer to it may be any method known in the art.

[0060] For example, a method may be used in which a composition for forming a positive electrode active material layer, including a positive electrode active material, a second conductive material, and a binder polymer, is applied onto the adhesion-promoting layer, followed by drying and rolling.

[0061] The solvent used in the composition for forming the positive electrode active material layer may be a solvent commonly used in the art. Examples include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, and water. These may be used alone or in combination. The amount of solvent used should be sufficient to dissolve the binder polymer, disperse the conductive material and positive electrode active material, and provide a viscosity that provides excellent thickness uniformity during subsequent coating for positive electrode fabrication, taking into account the coating thickness and manufacturing yield of the coating solution.

[0062] The polyvinylidene fluoride polymer contained in the adhesion-promoting layer has fluidity when heated and pressurized. For example, when heated and pressurized at a temperature higher than the glass transition temperature of the polyvinylidene fluoride polymer, in a temperature range from the melting point (Tm) of the polymer - 60°C to the melting point (Tm) of the binder polymer + 60°C, more specifically, in a temperature range from the melting point (Tm) of the binder polymer - 50°C to the melting point (Tm) of the binder polymer + 50°C, or even more specifically, in a temperature range from the melting point (Tm) of the binder polymer - 40°C to the melting point (Tm) of the binder polymer + 30°C, the binder polymer in the adhesion-promoting layer flows due to the heat and adheres to the surface of the positive electrode active material layer in contact with the adhesion-promoting layer.

[0063] In one embodiment of the positive electrode for a lithium secondary battery manufactured by the above-described manufacturing method, the thickness of the positive electrode active material layer (based on the thickness of the positive electrode active material layer formed on one side of the adhesion-promoting layer, not on both sides of the adhesion-promoting layer, after rolling) may be, but is not limited to, 40 to 200 μm.

[0064] The adhesive strength of the positive electrode active material layer is preferably 50 gf / 2 cm or more, more preferably 60 gf / 2 cm or more. The interface resistance of the positive electrode active material layer is 5 Ω cm. 2 It is desirable that the following:

[0065] The lithium secondary battery may further include a battery case that houses the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery case.

[0066] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0067] 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, and aluminum-cadmium alloys can be used. The negative electrode current collector typically has a thickness of 3 to 500 μm. As with the positive electrode current collector, the surface of the current collector can be formed with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector can be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0068] The negative electrode active material layer may include a negative electrode active material and, optionally, a binder and a conductive material. For example, the negative electrode active material may be prepared by applying a negative electrode-forming composition including the negative electrode active material and, optionally, the binder and the conductive material to a negative electrode current collector and drying the composition, or by casting the negative electrode-forming composition on a separate support, peeling the composition from the support, and laminating the resulting film on the negative electrode current collector.

[0069] The negative electrode active material may be a compound capable of reversible lithium intercalation and deintercalation. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous 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; metal oxides capable of doping and dedoping lithium, such as SiOβ(0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; and composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. A mixture of any one or more of these may be used. Alternatively, a metallic lithium thin film may be used as the negative electrode active material. Carbon materials may include both low-crystalline carbon and high-crystalline carbon. Typical low-crystalline carbons include soft carbon and hard carbon, and typical high-crystalline carbons include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbons such as petroleum or coal tar pitch-derived cokes.

[0070] The binder and conductive material may be the same as those described above for the positive electrode.

[0071] Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without limitation. It is particularly desirable for the separator to have low resistance to electrolyte ion movement and excellent electrolyte impregnation capability. Specifically, a porous polymer film, such as a porous polymer film made from 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 from high-melting-point glass fiber or polyethylene terephthalate fiber, can also be used. Separators coated with ceramic components or polymer materials to ensure heat resistance or mechanical strength can also be used, and can be selectively used in a single-layer or multi-layer structure.

[0072] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

[0073] Specifically, the electrolyte may be an electrolytic solution containing an organic solvent and a lithium salt.

[0074] The organic solvent may be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon 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-based solvents such as PC (carbonate); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a C2-C20 linear, branched, or cyclic hydrocarbon group that may contain a double-bonded aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constants, which can enhance the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred. In this case, the cyclic carbonate and linear carbonate should be mixed in a volume ratio of about 1:1 to about 1:9 to achieve excellent electrolyte performance.

[0075] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, examples of the lithium salt include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2. The lithium salt concentration is preferably within a range of 0.1 to 2.0 M. When the lithium salt concentration falls within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion mobility.

[0076] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, to improve battery life, inhibit battery capacity loss, and improve battery 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.

[0077] Such lithium secondary batteries are useful in the fields of portable devices such as mobile phones, notebook PCs, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0078] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0079] The battery module or battery pack may be used as a power source for one or more medium- to large-sized devices including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0080] While the present invention may be embodied in many different forms, it is to be understood that the invention is not limited to the specific embodiments set forth herein. [Example]

[0081] Example 1 Polymer A (Solvay, vinylidene fluoride-hexafluoropropylene copolymer, VDF:HFP=3:1 (weight ratio), average particle size: 250 nm, melting point: 100°C, Mw=1,080,000) and Denka carbon black (BET=60m 2 / g, DBP = 200ml / 100g) was dispersed in water at a weight ratio of 2:1, and then a thickener, CMC (Daicel 2200, Mw 3,000,000), was added at 1 / 5 the weight of the PVDF binder to create an aqueous dispersion to achieve the appropriate viscosity. Polyvinyl alcohol, a hydrocarbon-based water-soluble polymer, was added to water at 1 / 10 the weight of the fluoride-based polymer particles, heated to 80°C, and dissolved for one day to create a 10% aqueous solution. This was then added to the aqueous dispersion to create a final aqueous slurry with a solids content of 10%.

[0082] The aqueous slurry was then applied to both sides of a 20 μm-thick aluminum foil using a microgravure coater and dried at 120°C for 3 minutes to form an adhesion-promoting layer on the aluminum foil. LiFe(PO4), polyvinylidene fluoride (Mw=630,000) as a binder polymer, and Denka carbon black (BET=60m) as a conductive material were then coated on the formed adhesion-promoting layer. 2 / g, DBP=200ml / 100g) and a positive electrode active material slurry mixed at a ratio of 96:2:2 was applied to the substrate, dried at 140°C for 10 minutes, and then rolled to prepare a positive electrode.

[0083] Lithium metal was used as the negative electrode, and a separator (Celgard) was placed between the negative and positive electrodes to form an electrode assembly. This was punched into a coin shape, and an electrolyte solution of 1M LiPF6 dissolved in a mixed solvent of propylene carbonate (PC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) (PC:EMC:EC=3:4:3) was injected to fabricate an experimental lithium secondary battery.

[0084] Examples 2 to 4 The same procedure as in Example 1 was carried out except for the changes shown in Table 1 below.

[0085] Comparative Example 1 The same procedure as in Example 1 was carried out, except that the aqueous solution containing the hydrocarbon-based water-soluble polymer was not added and the following changes were made as shown in Table 1.

[0086] <Measuring the average particle size of polymer particles> After photographing the polymer particles with an SEM, the average length of the major axis of the primary particles was measured as the average particle size.

[0087] <Measurement of the average particle size of conductive materials> The average particle size D50 of the conductive material was measured using a laser diffraction method. After dispersing the conductive material in a dispersion medium, the average particle size D50 at 50% of the particle size distribution was calculated using a laser diffraction particle size analyzer (Microtac MT 3000).

[0088] <Measurement of Melting Point of Binder Polymer> The melting point of the binder polymer was measured using DSC. After introducing 5 - 10 mg of the sample using the DSC2500 equipment from TA Instruments, the temperature was raised from 50 - 250 °C at a heating rate of 10 °C / min under a nitrogen atmosphere, then cooled at a cooling rate of 10 °C / min, and then a thermal scan was performed in the order of raising the temperature from 50 - 250 °C at a heating rate of 10 °C / min.

[0089] Figure 2 is the DSC chart for polymer A in Example 1. Referring to Figure 2, the melting point was obtained from the endothermic reaction of the peak obtained in the second heating, and the melting point was confirmed from the temperature at the peak.

[0090] <Measurement of Weight - Average Molecular Weight of Polymer> After collecting 0.04 g of the polymer, it was dissolved in 10 g of tetrahydrofuran to prepare a sample solution. The standard sample (polystyrene) and the sample solution were filtered using a 0.45 - μm pore filter and then injected into a GPC injector. By comparing the elution time of the sample solution with the calibration curve of the standard sample, the number - average molecular weight, weight - average molecular weight, and polydispersity of the acrylic polymer were measured. The measurement was performed using GPC (Infinity II 1260, Agilent) at a flow rate of 1.00 mL / min and a column temperature of 35.0 °C.

[0091] <Adhesive Strength Evaluation> The positive electrodes produced by the examples and comparative examples were punched out to a width of 2 cm × a length of 10 cm or more using a punching machine. After using glass as the base plate (width 2.5 cm × length 7.5 cm × thickness 1T) and attaching 3M double - sided tape to the glass, the punched - out electrodes were attached in parallel. The length of the electrode attached to the tape was 6 cm, and the adhesive strength of the electrode was measured using a texture analyzer equipment (LLOYD) while maintaining a 90° angle with the base plate.

[0092] <WET Adhesive Strength Evaluation> For the evaluation of wet adhesion, the electrode-coated foil was placed in a vacuum drying oven at 130°C for 24 hours to remove moisture, then the electrode was sealed in an aluminum pouch with the electrolyte and placed in an oven at 70°C for two weeks, after which the adhesion was measured. The electrode was washed with a DMC cleaning solution to remove any remaining electrolyte, and then completely dried before measurement. The poorer the dissolution resistance of the adhesion-promoting layer compared to before the electrolyte resistance evaluation, the lower the adhesion of the adhesion-promoting layer to the electrolyte when applied to a lithium secondary battery.

[0093] <Interface resistance evaluation> The positive electrodes prepared according to the examples and comparative examples were punched out to a size of 5 cm wide x 5 cm long using a punching machine. The thickness of the punched electrode, the thickness of the aluminum foil, and the resistivity of the current collector (2.82E-06) were input using an MP tester (HIOKI Co., Ltd.), and the punched electrode was placed under a chip with a built-in probe, and the bar was lowered to measure.

[0094] [Table 1]

Claims

1. (S1) preparing an aqueous slurry in which a hydrocarbon-based water-soluble binder polymer is dissolved and polyvinylidene fluoride-based polymer particles and a first conductive material are dispersed; (S2) forming an adhesion-promoting layer by coating the aqueous slurry on at least one surface of a metal current collector, and then heat-treating the coating at a temperature higher than the melting point of the polyvinylidene fluoride-based polymer particles and drying the coating; the polyvinylidene fluoride polymer is dispersed in an island-like manner on the surface of the metal current collector; The melting point of the polyvinylidene fluoride polymer is 50 to 150°C.

2. 2. The method of claim 1, wherein the content of the hydrocarbon-based water-soluble binder polymer is 5 to 50 parts by weight based on 100 parts by weight of the polyvinylidene fluoride-based polymer particles.

3. 2. The method of claim 1, wherein the hydrocarbon-based water-soluble binder polymer is at least one selected from the group consisting of polyvinyl alcohol, polyvinylpyrrolidone, maleic anhydride, tannic acid, polyacrylic acid, and polyacrylamide.

4. 2. The method of claim 1, wherein the melting point of the polyvinylidene fluoride polymer is 70 to 150°C.

5. 2. The method for manufacturing a positive electrode current collector coated with an adhesion-promoting layer according to claim 1, wherein the polyvinylidene fluoride-based polymer is a copolymer of vinylidene and hexafluoropropylene.

6. 2. The method of claim 1, wherein the polyvinylidene fluoride polymer has a weight average molecular weight of 700,000 to 1,300,000.

7. 2. The method of claim 1, wherein the polyvinylidene fluoride polymer has a weight average molecular weight of 800,000 to 1,100,000.

8. 2. The method of claim 1, wherein the heat treatment is performed at a temperature 10 to 80° C. higher than the melting point of the polyvinylidene fluoride-based polymer.

9. a metal current collector; an adhesion-promoting layer formed on at least one surface of the metal current collector, the adhesion-promoting layer including a hydrocarbon-based water-soluble binder polymer, a polyvinylidene fluoride-based polymer, and a first conductive material; the polyvinylidene fluoride polymer is dispersed in an island-like manner on the surface of the metal current collector; The melting point of the polyvinylidene fluoride polymer is 50 to 150°C.

10. 10. The positive electrode current collector coated with an adhesion-promoting layer according to claim 9, wherein the content of the hydrocarbon-based water-soluble binder polymer is 5 to 50 parts by weight based on 100 parts by weight of the polyvinylidene fluoride-based polymer particles.

11. 10. The positive electrode current collector coated with the adhesion promoting layer according to claim 9, wherein the hydrocarbon-based water-soluble binder polymer is at least one selected from the group consisting of polyvinyl alcohol, polyvinylpyrrolidone, maleic anhydride, tannic acid, polyacrylic acid, and polyacrylamide.

12. A positive electrode current collector coated with the adhesion-promoting layer according to any one of claims 9 to 11; a positive electrode active material layer formed on the adhesion-promoting layer, the positive electrode active material layer including a positive electrode active material, a second conductive material, and a binder polymer.

13. 13. The positive electrode for a lithium secondary battery according to claim 12, wherein the metal current collector is made of aluminum, and the positive electrode active material is represented by the following Chemical Formula 1: [Chemical formula 1] Li １＋ａ Fe １－ｘ M ｘ (PO ４－ｂ )X ｂ (M is 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 is one or more elements selected from the group consisting of F, S, and N, and -0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.1.)

14. 13. The positive electrode for a lithium secondary battery according to claim 12, wherein the binder polymer contained in the positive electrode active material layer is a polyvinylidene fluoride-based polymer.

15. A lithium secondary battery comprising the positive electrode according to claim 12.

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