Positive electrode for lithium secondary batteries and lithium secondary batteries containing the same

Abstract

To provide a positive electrode for a lithium secondary battery with high energy density.SOLUTION: With regard to positive electrode for a lithium secondary battery and a lithium secondary battery containing the same, a positive electrode includes a dry positive electrode active material layer containing large-grain lithium iron phosphate particles and small-grain lithium iron phosphate particles as positive electrode active materials and a polytetrafluoroethylene binder.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a positive electrode for lithium secondary batteries and lithium secondary batteries containing the same. [Background technology]

[0002] Lithium-ion batteries are batteries that exhibit excellent discharge voltage and high energy density.

[0003] The electrodes for such lithium secondary batteries can be formed using either wet or dry processes, and research is progressing on the dry process, which offers higher capacity and is simpler. [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] One embodiment provides a positive electrode for a lithium secondary battery having a high energy density.

[0005] Another embodiment is to provide a lithium secondary battery including the positive electrode.

[0006] One embodiment provides a positive electrode for a lithium secondary battery, comprising a positive electrode active material consisting of large lithium iron phosphate particles and small lithium iron phosphate particles, and a dry positive electrode active material layer containing a polytetrafluoroethylene binder.

[0007] Another embodiment provides a lithium secondary battery comprising the positive electrode; a non-aqueous electrolyte.

[0008] Other specific details of embodiments of the present invention are included in the following detailed description.

[0009] A positive electrode for a lithium secondary battery according to one embodiment can exhibit a high energy density. [Brief explanation of the drawing]

[0010] [Figure 1] It is a cross-sectional view schematically showing a lithium secondary battery according to an embodiment. [Figure 2] It is a cross-sectional view schematically showing a lithium secondary battery according to an embodiment. [Figure 3] It is a cross-sectional view schematically showing a lithium secondary battery according to an embodiment. [Figure 4] It is a cross-sectional view schematically showing a lithium secondary battery according to an embodiment.

Embodiments for Carrying out the Invention

[0011] Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example, and the present invention is not limited thereby, and the present invention is only defined by the scope of the claims described later. The terms used herein are merely used to explain exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

[0012] As used herein, "these combinations" can mean mixtures, laminates, composites, copolymers, alloys, blends, reaction products, etc. of components.

[0013] Here, terms such as "comprising", "including" or "having" are used to specify the presence of implemented features, numbers, steps, components or combinations thereof, and it should be understood that the presence or addition possibility of one or more other features, numbers, steps, components or combinations thereof is not precluded in advance.

[0014] To clearly represent multiple layers and regions in the drawings, thicknesses are shown enlarged, and similar parts throughout the specification are given the same drawing reference numerals. When a part such as a layer, film, region, or plate is said to be "on top of" or "above" another part, this includes not only when it is "directly on top" of the other part, but also when there is another part in between. Conversely, when a part is said to be "directly on top" of another part, it means that there is no other part in between.

[0015] Furthermore, the term "layer" here includes not only the shapes formed on the entire surface when observed in a plan view, but also the shapes formed on a portion of the surface.

[0016] Here, "or" is not interpreted as having an exclusive meaning; for example, "A or B" is interpreted as including A, B, A+B, etc.

[0017] Unless otherwise defined herein, particle size or size may refer to average particle size. Average particle size refers to the average particle size (D50), which is the diameter of the particle whose cumulative volume in the particle size distribution is 50% by volume. Average particle size (D50) can be measured by methods widely known to those skilled in the art, such as by a particle size analyzer, or by transmission electron microscope (SEM) or scanning electron microscope (SEM) imaging. Alternatively, it can be measured using a dynamic light-scattering apparatus, the data can be analyzed, and the average particle size (D50) value can be calculated after counting the number of particles for each particle size range.

[0018] A positive electrode for a lithium secondary battery according to one embodiment includes a positive electrode active material consisting of large lithium iron phosphate particles and small lithium iron phosphate particles, and a dry positive electrode active material layer containing a polytetrafluoroethylene binder.

[0019] Such a positive electrode refers to one manufactured using a dry process rather than a wet process, without the use of solvents during manufacturing.

[0020] In one embodiment, the positive electrode active material is lithium iron phosphate particles, and includes two types of lithium iron phosphate particles that are different in appearance from each other. For example, the positive electrode active material may consist of large lithium iron phosphate particles and small lithium iron phosphate particles. Large lithium iron phosphate particles may be larger than the small lithium iron phosphate particles. The size of the lithium iron phosphate particles may be the average particle size, and this average particle size may be the average particle size (D50). Since the positive electrode active material contains both large and small lithium iron phosphate particles, a positive electrode suitable for dry cell batteries can be provided, thereby enabling the provision of a positive electrode with high energy density. When forming the positive electrode active material layer using a wet process with a solvent using lithium iron phosphate particle positive electrode active material, the content of coating-compatible lithium iron phosphate particles is low, making it impossible to improve the loading level, and consequently, high energy density cannot be obtained. The positive electrode according to one embodiment is a dry positive electrode, so the loading level can be improved, and a positive electrode with high energy density can be obtained.

[0021] If only large or small lithium iron phosphate particles are used as the positive electrode active material, the adhesion to the current collector may decrease when manufacturing a dry positive electrode together with a polytetrafluoroethylene binder, and an uneven coating may be formed when manufacturing a positive electrode with a large surface area.

[0022] The mixing ratio of large lithium iron phosphate particles to small lithium iron phosphate particles can be 99:1 to 60:40 by weight, 95:5 to 65:35 by weight, 80:20 to 60:40 by weight, or 80:20 to 70:30 by weight. When the mixing ratio of large lithium iron phosphate particles to small lithium iron phosphate particles falls within the above range, the dry positive electrode active material layer can be formed more effectively, and a positive electrode with a higher energy density can be obtained.

[0023] In one embodiment, the size of the large lithium iron phosphate particles can be larger than the size of the small lithium iron phosphate particles. For example, the ratio of the size of the large lithium iron phosphate particles to the size of the small lithium iron phosphate particles can be 1.5:1 to 19:1, 1.5:1 to 10:1, or 2:1 to 10:1. When the ratio of the size of the large lithium iron phosphate particles to the size of the small lithium iron phosphate particles is within the above range, a positive electrode with a very high current density, for example, 5 mAh / cm², can be achieved. 2 A positive electrode with the above current density can be manufactured, and an ultra-thick film dry positive electrode can be manufactured.

[0024] The size of the large lithium iron phosphate particles can be 2.5 μm to 20 μm, and can also be 2.5 μm to 15 μm, or 5 μm to 15 μm.

[0025] The size of the small lithium iron phosphate particles can be 0.1 μm to 2.5 μm, and can also be 0.1 μm to 2 μm, or 0.5 μm to 2.0 μm.

[0026] When the size ratio of large lithium iron phosphate particles to small lithium iron phosphate particles satisfies the aforementioned range, and the angle between the large lithium iron phosphate particles and the small lithium iron phosphate particles satisfies the aforementioned range, sufficient shear stress can be transmitted to the binder used in the dry cathode manufacturing process, thereby enabling the production of a dry cathode. This makes it possible to obtain a cathode with high loading and high energy density even when using lithium iron phosphate cathode active materials, which generally have low energy density.

[0027] In one embodiment, the large lithium iron phosphate particles and the small lithium iron phosphate particles may be the same or different. The lithium iron phosphate particles are represented by the following chemical formula 1.

[0028] [Chemical formula 1] Li a Fe 1-xM x PO4 (In the above chemical formula 1, 0.90 ≤ a ≤ 1.8, 0 ≤ x ≤ 0.7, (M is Mg, Co, Ni, Mn, or a combination of these.)

[0029] In the positive electrode active material layer of one embodiment, the polytetrafluoroethylene binder can be fibrillated polytetrafluoroethylene. When a shear force is applied to the polytetrafluoroethylene binder, it is fibrillated into a fibrous form, for example. When the binder is mixed with the positive electrode active material and a shear force is applied, the fibrillated binder and the positive electrode active material intertwine to form a positive electrode active material layer, thus enabling the manufacture of a dry positive electrode. Such a dry positive electrode is possible when large lithium iron phosphate particles and small lithium iron phosphate particles of different sizes are used together as the positive electrode active material. If only small lithium iron phosphate particles are used as the positive electrode active material, fibrillation cannot be achieved, and a dry positive electrode cannot be manufactured. Furthermore, when only large lithium iron phosphate particles are used as the positive electrode active material, sufficient fibrillation is difficult, and excessive particle cracking and electrolyte side reactions occur during charging and discharging, which is unsuitable.

[0030] In one embodiment, the content of the positive electrode active material may be 95% to 99.9% by weight, 97% to 99.5% by weight, or 97.4% to 99.4% by weight, based on 100% by weight of the dry positive electrode active material layer.

[0031] The polytetrafluoroethylene binder content may be 0.1% to 5% by weight, 0.5% to 3.0% by weight, or 0.6% to 2.6% by weight, based on 100% by weight of the entire dry positive electrode active material layer. When the polytetrafluoroethylene binder content falls within these ranges, the dry positive electrode active material layer can be effectively formed.

[0032] The positive electrode active material layer may further selectively contain a conductive material. When the positive electrode active material layer further contains a conductive material, the content of the positive electrode active material relative to 100% by weight of the positive electrode active material layer may be 90% to 99% by weight, 94% to 99% by weight, or 96% to 99% by weight, the binder content may be 0.5% to 5% by weight, 0.5% to 3% by weight, or 0.5% to 2% by weight, and the conductive material content may be 0.5% to 5% by weight, 0.5% to 3% by weight, or 0.5% to 2% by weight.

[0033] The conductive material is used to impart conductivity to the electrodes, and any conductive material that does not undergo chemical changes in the battery can be used. Examples of conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fibers; metallic materials such as metal powders or metal fibers of copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or conductive materials containing mixtures thereof. The carbon black may include, for example, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or combinations thereof. Al can be used as the current collector, but is not limited to it.

[0034] In one embodiment, the positive electrode can exhibit an energy density of 400 Wh / L to 490 Wh / L. In another embodiment, the energy density of the positive electrode may be 400 Wh / L to 450 Wh / L, or 450 Wh / L to 480 Wh / L. Thus, the positive electrode according to one embodiment can exhibit a high energy density.

[0035] Furthermore, the loading level of the dry positive electrode active material layer is 40 mg / cm³. 2 ~90 mg / cm³ 2 It can be 40 mg / cm³ 2 ~80 mg / cm³ 2It can be. Thus, the positive electrode active material layer according to one embodiment can exhibit a high loading level.

[0036] In addition, the positive electrode can have a high current density of 5 mAh / cm 2 or more. The current density of the positive electrode can be 5 mAh / cm 2 to 8 mAh / cm 2 and can also be 6 mAh / cm 2 to 8 mAh / cm 2

[0037] <Method for manufacturing the positive electrode> The positive electrode according to one embodiment can be manufactured as follows.

[0038] Large lithium iron phosphate particles and small lithium iron phosphate particles are mixed to produce a positive electrode active material. The positive electrode active material, a polytetrafluoroethylene binder, and optionally a conductive material are dry-mixed, and the positive electrode active material layer is produced in the step of extruding this mixture. The dry-mixing step refers to a step that does not use a solvent. The dry-mixing step can be performed at a speed of 1000 rpm to 10000 rpm, 1500 rpm to 8000 rpm, or 2000 rpm to 7000 rpm, and can be performed for 1 minute to 30 minutes at this speed. In this dry-mixing step, the polytetrafluoroethylene binder can be fibrillated.

[0039] The positive electrode active material layer can be in sheet form and can be a self-standing film.

[0040] The positive electrode is manufactured by attaching the positive electrode active material layer to a current collector. This attachment step may be a lamination step and is not limited thereto.

[0041] <Lithium secondary battery> Another lithium secondary battery according to one embodiment includes the positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0042] <Negative electrode> ​The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector.

[0043] The negative electrode active material layer contains a negative electrode active material.

[0044] The negative electrode active material includes a material capable of reversibly inserting / desorbing lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and undoping lithium, or a transition metal oxide.

[0045] Examples of the material capable of reversibly inserting / desorbing lithium ions include carbon-based negative electrode active materials, which can include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon can include graphite such as amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon can include soft carbon or hard carbon, mesophase pitch carbide, fired coke, and the like.

[0046] As the alloy of lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0047] As the material capable of doping and undoping lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material can be used. The Si-based negative electrode active material can be silicon, a silicon-carbon composite, SiO x (0 < x ≤ 2), a Si-Q alloy (where Q is selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements (excluding Si), group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof), or a combination thereof. The Sn-based negative electrode active material can be Sn, SnO2, a Sn-based alloy, or a combination thereof.

[0048] The silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in the form of silicon particles and amorphous carbon coated on the surface of the silicon particles. For example, it may include secondary particles (core) assembled from silicon particles and an amorphous carbon coating layer (shell) located on the surface of these secondary particles. The amorphous carbon may also be located between the silicon particles, for example, silicon primary particles may coat the amorphous carbon. The secondary particles may be dispersed in the amorphous carbon matrix.

[0049] The silicon-carbon composite may further contain crystalline carbon. For example, the silicon-carbon composite may include a core containing crystalline carbon and silicon particles and an amorphous carbon coating layer located on the surface of this core.

[0050] The silicon particles may also be silicon nanoparticles.

[0051] The particle size of the silicon nanoparticles may be between 10 nm and 1,000 nm, or, according to another embodiment, between 10 nm and 200 nm, or between 20 nm and 150 nm. When the particle size of the silicon nanoparticles falls within the above range, excessive volume expansion during charging and discharging can be suppressed, and the disruption of the conductive path due to particle fragmentation during charging and discharging can be prevented.

[0052] In one embodiment of the negative electrode active material, the silicon-carbon composite of the core may include silicon nanoparticles and an amorphous carbon coating layer located on the surface of the silicon nanoparticles. The silicon-carbon composite may also include an assembly of at least one silicon nanoparticle and an amorphous carbon coating layer located on the surface of this assembly.

[0053] In the amorphous carbon coating layer, the amorphous carbon may be soft carbon, hard carbon, mesophase pitch carbide, calcined coke, or a combination thereof. The thickness of the amorphous carbon coating layer may be 1 nm to 2 μm, 1 nm to 500 nm, 10 nm to 300 nm, or 20 nm to 200 nm. When the thickness of the amorphous carbon coating layer falls within the above range, silicon volume expansion during charging and discharging can be effectively suppressed.

[0054] The crystalline carbon may be amorphous, plate-like, flake-like, spherical, or fibrous natural graphite, artificial graphite, or a combination thereof.

[0055] When the silicon-carbon composite includes silicon nanoparticles and an amorphous carbon coating layer, the content of the silicon nanoparticles can be 30% to 70% by weight or 40% to 65% by weight, relative to 100% by weight of the entire silicon-carbon composite. The content of the amorphous carbon coating layer can also be 30% to 70% by weight or 35% to 60% by weight, relative to 100% by weight of the entire silicon-carbon composite.

[0056] If the silicon-carbon composite further contains crystalline carbon, the content of silicon nanoparticles may be 20% to 70% by weight or 25% to 65% by weight relative to 100% by weight of the entire silicon-carbon composite. The content of amorphous carbon may be 25% to 70% by weight or 25% to 60% by weight relative to 100% by weight of the entire silicon-carbon composite, and the content of crystalline carbon may be 1% to 20% by weight or 5% to 15% by weight.

[0057] In the negative electrode active material layer, the content of the negative electrode active material may be 95% to 98% by weight relative to 100% by weight of the entire negative electrode active material layer.

[0058] The negative electrode active material layer may contain a binder and may further contain a conductive material. The binder content may be 1% to 5% by weight based on 100% by weight of the entire negative electrode active material layer. The conductive material content may be 1% to 5% by weight based on 100% by weight of the entire negative electrode active material layer.

[0059] The binder plays a role in ensuring that the negative electrode active material particles adhere well to each other and that the negative electrode active material adheres well to the current collector. The binder can be a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof.

[0060] Examples of the non-aqueous binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, or combinations thereof.

[0061] The aqueous binder can be selected from styrene-butadiene rubber, (meth)acrylate styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0062] The binder for the negative electrode may be a cellulosic compound, and this cellulosic compound may be used together with the aqueous binder. The cellulosic compound can be a mixture of one or more carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, or alkali metal salts thereof. As the alkali metal, Na, K, or Li can be used. The cellulosic compound can act as a binder and can also act as a thickener that imparts viscosity. The cellulosic compound can be used in an appropriate amount within the binder content, for example, 0.1 to 3 parts by weight per 100 parts by weight of the negative electrode active material.

[0063] The dry binder is a polymeric substance that can be formed into fibers, and can be, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0064] The conductive material is used to impart conductivity to the electrodes, and any conductive material that does not undergo chemical changes can be used in the battery that is constructed. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, carbon fibers, carbon nanofibers, and carbon nanotubes; metallic materials in the form of metal powders or metal fibers containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0065] As the current collector, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with a conductive metal, and combinations thereof can be selected.

[0066] <Electrolyte> The electrolyte comprises a non-aqueous organic solvent and a lithium salt.

[0067] The aforementioned non-aqueous organic solvent acts as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0068] The non-aqueous organic solvent can be a carbonate, ester, ether, ketone, alcohol, or aprotic solvent, or a combination thereof.

[0069] Suitable carbonate solvents include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), and butylene carbonate (BC). Suitable ester solvents include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, and caprolactone. Suitable ether solvents include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran. Furthermore, cyclohexanone and the like can be used as ketone solvents. Ethyl alcohol and isopropyl alcohol can be used as alcohol solvents, and as aprotic solvents, nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, and can include double bonds, aromatic rings, or ether groups); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane and 1,4-dioxolane; and sulfolanes can be used.

[0070] The aforementioned non-aqueous organic solvents can be used alone or in combination of two or more.

[0071] Furthermore, when using carbonate-based solvents, cyclic carbonates and linear carbonates can be mixed, and these can be mixed in a volume ratio of 1:1 to 1:9.

[0072] The aforementioned lithium salts dissolve in organic solvents and act as a source of lithium ions within the battery, enabling the operation of basic lithium secondary batteries and facilitating the movement of lithium ions between the positive and negative electrodes. Typical examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N(lithium bis(fluorosulfonyl)imide (LiFSI), LiC4F9SO3, and LiN(C x F2 x+1 SO2)(C y F 2y+1 It may contain one or more selected from SO2) (where x and y are integers from 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalate)phosphate (LiDFOB), and lithium bis(oxalate)borate (LiBOB).

[0073] [Separator] Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. Such separators can be made of polyethylene, polypropylene, polyvinylidene fluoride, or multilayer films of two or more layers of these materials. Mixed multilayer films such as polyethylene / polypropylene two-layer separators, polyethylene / polypropylene / polyethylene three-layer separators, and polypropylene / polyethylene / polypropylene three-layer separators can also be used.

[0074] The separator may include a porous substrate and a coating layer containing organic, inorganic, or a combination thereof located on one or both sides of the porous substrate.

[0075] The porous substrate may be a polymer film formed from one polymer selected from polyethylene, polyolefins such as polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyetherketone, polyallyl etherketone, polyetherimide, polyamideimide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cycloolefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon®, and polytetrafluoroethylene, or from copolymers or mixtures of two or more of these polymers.

[0076] The aforementioned organic material may include polyvinylidene fluoride polymers or (meth)acrylic polymers.

[0077] The inorganic material may include, but is not limited to, inorganic particles selected from Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0078] The organic and inorganic materials can exist mixed together in a single coating layer, or in a form where a coating layer containing organic materials and a coating layer containing inorganic materials are stacked on top of each other.

[0079] Lithium-ion secondary batteries can be classified into lithium-ion batteries, lithium-ion polymer batteries, and lithium polymer batteries depending on the type of separator and electrolyte used. They can also be classified by shape into cylindrical, prismatic, coin-type, pouch-type, etc., and by size into bulk type and thin-film type. The structure and manufacturing methods of these batteries are widely known in this field, so a detailed explanation will be omitted.

[0080] Lithium secondary batteries can be classified into cylindrical, prismatic, pouch-type, coin-type, and other shapes depending on their form. Figures 1 to 4 are schematic diagrams showing a lithium secondary battery according to one embodiment, with Figure 1 being circular, Figure 2 being prismatic, and Figures 3 and 4 being pouch-type batteries. Referring to Figures 1 to 4, the lithium secondary battery 100 can include an electrode assembly 40 with a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a case 50 in which the electrode assembly 40 is housed. The positive electrode 10, negative electrode 20, and separator 30 can be impregnated with an electrolyte (not shown). The lithium secondary battery 100 can include a sealing member 60 that seals the case 50, as shown in Figure 1. Also, in Figure 2, the lithium secondary battery 100 can include a positive electrode lead tab 11 and a positive electrode terminal 12, a negative electrode lead tab 21 and a negative electrode terminal 22. As shown in Figures 3 and 4, the lithium secondary battery 100 may include electrode tabs 70, namely a positive electrode tab 71 and a negative electrode tab 72, which serve as electrical pathways for guiding the current formed in the electrode assembly 40 to the outside.

[0081] A lithium secondary battery according to one embodiment of the present invention can be applied to automobiles, mobile phones, and / or various forms of electrical devices, and the present invention is not limited thereto.

[0082] The following describes examples and comparative examples of the present invention. These examples are merely one embodiment of the present invention, and the present invention is not limited to these examples.

[0083] (Example 1) A positive electrode active material was manufactured by mixing large LiFePO4 particles with an average particle size (D50) of 5 μm and small LiFePO4 particles with an average particle size (D50) of 0.5 μm in a weight ratio of 6:4.

[0084] 90% by weight of the positive electrode active material, 5% by weight of polytetrafluoroethylene binder, and 5% by weight of Ketjenblack conductive material were dry-mixed at a speed of 5000 rpm for 10 minutes, and this mixture was extruded to produce a sheet-type self-supporting positive electrode active material layer.

[0085] The positive electrode was manufactured by laminating the positive electrode active material layer onto an aluminum foil current collector. In the manufactured positive electrode, the loading level of the positive electrode active material layer was 40 mg / cm². 2 The polytetrafluoroethylene binder was fibrillated polytetrafluoroethylene.

[0086] A slurry of 98% by weight of natural graphite negative electrode active material, 1% by weight of styrene-butadiene rubber, and 1% by weight of carboxymethylcellulose was mixed in an aqueous solvent to produce a negative electrode active material layer slurry. The slurry of the negative electrode active material layer slurry was applied to a copper foil current collector, dried, and rolled to produce a negative electrode.

[0087] A full cell with a volume of 0.96 L was produced using the positive electrode, the negative electrode, and the electrolyte. The electrolyte used was a mixed solvent (2:1:7 volume ratio) of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in which 1 M LiPF6 was dissolved, to which fluoroethylene carbonate was added at a rate of 3.5% by weight relative to the total weight of the mixed solvent.

[0088] (Example 2) A positive electrode and a full cell with a volume of 0.96 L were manufactured in the same manner as in Example 1, except that large LiFePO4 particles with an average particle size (D50) of 5 μm and small LiFePO4 particles with an average particle size (D50) of 0.5 μm were mixed in a weight ratio of 65:35. In the manufactured positive electrode, the loading level of the positive electrode active material layer was 60 mg / cm³. 2The polytetrafluoroethylene binder was fibrillated polytetrafluoroethylene.

[0089] (Example 3) A lithium secondary battery with a positive electrode and a volume of 0.96 L was manufactured in the same manner as in Example 1, except that large LiFePO4 particles with an average particle size (D50) of 10 μm and small LiFePO4 particles with an average particle size (D50) of 1.0 μm were used. In the manufactured positive electrode, the loading level of the positive electrode active material layer was 70 mg / cm³. 2 The polytetrafluoroethylene binder was fibrillated polytetrafluoroethylene.

[0090] (Example 4) A lithium secondary battery with a positive electrode and a volume of 0.96 L was manufactured in the same manner as in Example 1, except that large LiFePO4 particles with an average particle size (D50) of 10 μm and small LiFePO4 particles with an average particle size (D50) of 2.0 μm were used. In the manufactured positive electrode, the loading level of the positive electrode active material layer was 80 mg / cm². 2 The polytetrafluoroethylene binder was fibrillated polytetrafluoroethylene.

[0091] (Comparative Example 1) A positive electrode active material was manufactured by mixing large LiFePO4 particles with an average particle size (D50) of 5 μm and small LiFePO4 particles with an average particle size (D50) of 0.5 μm in a weight ratio of 60:40.

[0092] A cathode active material layer slurry was prepared by mixing 98% by weight of the aforementioned cathode active material, 1% by weight of polytetrafluoroethylene binder, and 1% by weight of Ketjenblack in an N-methylpyrrolidone solvent.

[0093] The positive electrode was manufactured by coating an aluminum foil current collector with the aforementioned positive electrode active material slurry, drying it, and rolling it. In the manufactured positive electrode, the loading level of the positive electrode active material layer was 30 mg / cm². 2 That was the case.

[0094] A lithium secondary battery with a volume of 0.96 L was manufactured using the aforementioned positive electrode in the same manner as in Example 1.

[0095] (Comparative Example 2) A lithium secondary battery with a positive electrode and a volume of 0.96 L was manufactured in the same manner as in Comparative Example 1, except that a polyvinylidene fluoride binder was used instead of a polytetrafluoroethylene binder, and large LiFePO4 particles with an average particle size (D50) of 20 μm and small LiFePO4 particles with an average particle size (D50) of 2.5 μm were used. In the manufactured positive electrode, the loading level of the positive electrode active material layer was 35 mg / cm². 2 That was the case.

[0096] (Comparative Example 3) A lithium secondary battery with a positive electrode and a volume of 0.96 L was manufactured in the same manner as in Example 1, except that only large LiFePO4 particles with an average particle size (D50) of 30 μm were used as the positive electrode active material. In the manufactured positive electrode, the loading level of the positive electrode active material layer was 38 mg / cm³. 2 That was the case.

[0097] (Comparative Example 4) A lithium secondary battery with a positive electrode and a volume of 0.96 L was manufactured in the same manner as in Example 1, except that a small-grained LiFePO4 positive electrode active material with an average particle size (D50) of 0.05 μm was used. In the manufactured positive electrode, the loading level of the positive electrode active material layer was 25 mg / cm². 2 That was the case.

[0098] Experimental Example 1) Current Density (mAh / cm³) 2 )evaluation The current density of the positive electrodes manufactured according to Examples 1 to 4 and Comparative Examples 1 to 5 was determined using the following formula 1. For example, when the current density of Example 1 was calculated using the above formula 1, it was 40 mg / cm³. 2 )*90(%)*140(mAh / g)=5(mAh / cm 2 ) was.

[0099] [Formula 1] Current density = Loading level (mg / cm²) 2 )*Active material ratio (%)*LFP capacity (mAh / g)

[0100] In Equation 1 above, the LFP capacity is the experimental capacity of LiFePO4, which is 140 mAh / g.

[0101] Experimental Example 2) Evaluation of Energy Density (Wh / L) The energy density for half cells in Examples 1 to 4 and Comparative Examples 1 to 4 was determined using Equation 2 below. The results are shown in Table 1 below. For example, calculating the energy density of Example 1 using Equation 2 below yielded 3. 2(V)*135(Ah) / 0.96(L)=450(Wh / L).

[0102] [Formula 2] Energy density (Wh / L) = Theoretical voltage (Nominal Voltage, V) * Battery capacity (Ah) / Battery volume (L)

[0103] In Equation 2, the battery capacity is a value obtained from the current density value obtained in Example 1.

[0104] [Table 1]

[0105] As shown in Table 1 above, the batteries of Examples 1 to 4 have a concentration of 40 mg / cm³. 2 Since it is possible to manufacture cathodes with a loading level of 5mAh / cm², 2 A positive electrode with a high current density can be manufactured. Furthermore, a positive electrode with a high energy density of 450 Wh / L or more can be obtained, resulting in a high-capacity battery of 135 Ah or more.

[0106] On the other hand, the cathodes of Comparative Examples 1 and 2, which were manufactured using a wet process, had a concentration of 30 mg / cm³. 2 and 35 mg / cm³ 2A positive electrode with a low loading level was manufactured, resulting in lower current density, energy density, and battery capacity.

[0107] At the same time, even when manufactured using a dry process, in Comparative Example 3, which used only large LiFePO4 particles, the loading level increased slightly, but was still 38 mg / cm³. 2 As a result, a positive electrode was manufactured that still had a lower loading level compared to the examples. In addition, the current density, energy density, and battery capacity were all lower compared to the examples. Furthermore, in the case of Comparative Example 4, which used only small LiFePO4 particles, the load was 25 mg / cm³. 2 Various positive electrodes were manufactured with extremely low loading levels, resulting in significantly lower current density, energy density, and battery capacity.

[0108] The present invention is not limited to the embodiments described above, and can be manufactured in a variety of different forms. A person with ordinary skill in the art to which the present invention pertains will understand that the invention can be implemented in other specific forms without altering the technical idea or essential features of the present invention. Accordingly, the embodiments described above should be understood to be illustrative and not limiting in all respects.

Claims

1. The positive electrode active material includes a dry positive electrode active material layer comprising large lithium iron phosphate particles and small lithium iron phosphate particles, and a polytetrafluoroethylene binder, wherein the polytetrafluoroethylene binder is fibrillated polytetrafluoroethylene. The ratio of the size of the large lithium iron phosphate particles to the size of the small lithium iron phosphate particles is 1.5:1 to 19:

1. Positive electrode for lithium secondary batteries.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein the mixing ratio of the large lithium iron phosphate particles and the small lithium iron phosphate particles is 99:1 to 60:40 by weight.

3. The positive electrode for a lithium secondary battery according to claim 1, wherein the size of the large lithium iron phosphate particles is larger than the size of the small lithium iron phosphate particles.

4. The positive electrode for a lithium secondary battery according to claim 1, wherein the size of the large lithium iron phosphate particles is 2.5 μm to 20 μm.

5. The positive electrode for a lithium secondary battery according to claim 1, wherein the size of the small lithium iron phosphate particles is 0.1 μm to 2.5 μm.

6. The positive electrode for a lithium secondary battery according to claim 1, wherein the large lithium iron phosphate particles and the small lithium iron phosphate particles are the same or different from each other and are represented by the following chemical formula 1. [Chemical formula 1] Li a Fe 1-x M x PO 4 (In the above chemical formula 1, 0.90 ≤ a ≤ 1.8, 0 ≤ x ≤ 0.7, and M is Mg, Co, Ni, Mn, or a combination thereof.)

7. The positive electrode for a lithium secondary battery according to claim 1, wherein the content of the polytetrafluoroethylene binder is 0.1% by weight to 5.0% by weight based on 100% by weight of the dry positive electrode active material layer.

8. The positive electrode for a lithium secondary battery according to claim 1, wherein the dry positive electrode active material layer further comprises a conductive material.

9. The positive electrode for a lithium secondary battery according to claim 1, wherein the energy density of the positive electrode is 400 Wh / L to 490 Wh / L.

10. The loading level of the dry positive electrode active material layer is 40 mg / cm². 2 ~90 mg / cm³ 2 The positive electrode for a lithium secondary battery according to claim 1.

11. The positive electrode has a capacitance of 5 mAh / cm². 2 The positive electrode for a lithium secondary battery according to claim 1, having the above current density.

12. The positive electrode has a current density of 5 mAh / cm 2 to 8 mAh / cm 2 The positive electrode for a lithium secondary battery according to claim 1, which has such a current density.

13. The positive electrode according to any one of claims 1 to 12; Negative electrode; and A lithium secondary battery containing a non-aqueous electrolyte.

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