Method for preparing lithium iron phosphate positive electrode active material

Abstract

The present invention relates to a method for preparing a lithium iron phosphate-based positive electrode active material, and more particularly, to a method for preparing a lithium iron phosphate-based positive electrode active material, the method comprising the steps of: (A) dry-grinding iron phosphate (FePO4) and a carbon-containing coating material, respectively; (B) performing dry mixing on a lithium-containing raw material, the ground iron phosphate and the ground carbon-containing coating raw material to prepare a reaction mixture; (C) compressing the reaction mixture to be processed into a pellet shape; and (D) calcining the granular reaction mixture to produce a calcined product.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2023-0140629, filed on October 19, 2023, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] This invention relates to a method for preparing lithium iron phosphate-based positive electrode active materials. Background Technology

[0004] With technological advancements and increasing demands for mobile devices, the need for secondary batteries as an energy source is rapidly growing. Among secondary batteries, lithium-ion batteries, characterized by high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0005] Lithium-ion batteries consist of four main components: the positive electrode, the negative electrode, the separator, and the electrolyte. Among these, the positive electrode active material plays a major role in determining the battery's capacity, output, and lifespan. Currently used positive electrode active materials include NCM-type positive electrode active materials containing nickel, cobalt, manganese, and / or aluminum, and LFP (lithium iron phosphate)-type positive electrode active materials. Furthermore, to achieve high energy density, output, and lifespan in lithium-ion batteries, improving the performance of the positive electrode active material is essential; therefore, extensive research has recently been conducted to develop high-performance positive electrode active materials.

[0006] LFP-type cathode active materials are olivine-structured cathode active materials. While their capacity is lower than NCM-type cathode active materials, they possess advantages such as excellent thermal stability and price competitiveness. However, LFP-type cathode active materials suffer from high resistance and low lithium-ion diffusion. To address these issues, research has been conducted on carbon coating and particle size adjustment to several hundred nanometers. However, to achieve carbon coating and particle size adjustment to several hundred nanometers, wet grinding processes such as bead milling and spray drying are unavoidable. This not only requires high energy for the synthesis of cathode active materials but also increases costs due to reduced yield. Therefore, a low-energy and low-cost dry synthesis method is needed that avoids wet grinding and spray drying processes. However, dry synthesis methods have been found to have the disadvantage of high resistance due to uneven carbon coating. Summary of the Invention

[0007] Technical issues

[0008] The purpose of this invention is to provide a method for preparing lithium iron phosphate (LFP) cathode active materials, which can achieve uniform carbon coating and low resistance even through dry synthesis.

[0009] Technical solution

[0010] This invention provides a method for preparing lithium iron phosphate-based positive electrode active materials.

[0011] (1) The present invention provides a method for preparing lithium iron phosphate positive electrode active materials, the method comprising the following steps: (A) dry grinding iron phosphate (FePO4) and carbon-containing coating raw material respectively; (B) dry mixing lithium-containing raw material, ground iron phosphate and ground carbon-containing coating raw material to prepare a reaction mixture; (C) compressing the reaction mixture to be processed into granular shape; and (D) calcining the granular reaction mixture to prepare a calcined product.

[0012] (2) The present invention provides a method for preparing lithium iron phosphate positive electrode active materials according to (1) above, wherein the carbon-containing coating raw material is at least one selected from sucrose, glucose, polyethylene glycol, polyvinyl alcohol and polyvinyl acetate.

[0013] (3) The present invention provides a method for preparing lithium iron phosphate positive electrode active materials according to (1) or (2) above, wherein, in preparing the reaction mixture, a raw material containing M is further mixed (M is at least one selected from V, Ti, Mn, Zn, Mo, Mg and N).

[0014] (4) The present invention provides a method for preparing lithium iron phosphate positive electrode active material according to any one of (1) to (3) above, wherein, in step (A), the iron phosphate is ground by air jet milling.

[0015] (5) The present invention provides a method for preparing lithium iron phosphate positive electrode active materials according to any one of (1) to (4) above, wherein, in step (A), the grinding of the carbon-containing coating raw material is performed by mechanical grinding.

[0016] (6) The present invention provides a method for preparing lithium iron phosphate positive electrode active materials according to any one of (1) to (5) above, wherein the average particle size (D) of the milled iron phosphate is... 50 The thickness ranges from 1.0 μm to 2.0 μm.

[0017] (7) The present invention provides a method for preparing lithium iron phosphate cathode active materials according to any one of (1) to (6) above, wherein the bulk density of the milled carbon-containing coating raw material is 500 kg / m³. 3 Up to 700kg / m 3 .

[0018] (8) The present invention provides a method for preparing lithium iron phosphate positive electrode active material according to any one of (1) to (7) above, wherein the compression in step (C) is carried out at a pressure of 0.10 tons / cm² to 1.0 tons / cm².

[0019] (9) The present invention provides a method for preparing lithium iron phosphate positive electrode active materials according to any one of (1) to (8) above, wherein the calcination is carried out at a temperature of 600°C to 800°C.

[0020] (10) The present invention provides a method for preparing lithium iron phosphate positive electrode active material according to any one of (1) to (9) above, wherein the method further includes step (E): grinding the calcined product.

[0021] Beneficial effects

[0022] In the method for preparing lithium iron phosphate cathode active materials according to the present invention, iron phosphate (FePO4) and carbon-containing coating raw materials are first dry-milled separately, then mixed with lithium-containing raw materials to prepare a reaction mixture, which is then compressed and processed into granules before calcination. This process not only prevents the raw materials from solidifying but also improves their reactivity. The resulting lithium iron phosphate cathode active material has a uniform carbon coating and exhibits low powder resistance. Attached Figure Description

[0023] Figure 1 This is a SEM image of the lithium iron phosphate positive electrode active material of Example 1.

[0024] Figure 2 This is a SEM image of the lithium iron phosphate cathode active material of Comparative Example 1.

[0025] Figure 3 This is a STEM-EDS mapping image of the lithium iron phosphate positive electrode active material of Example 1. Detailed Implementation

[0026] The invention will be described in more detail below to aid in understanding. The terms or words used in the specification and claims of this application should not be construed as limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts consistent with the technical spirit of the invention, based on the principle that the inventor can adequately define the terms and concepts to best describe his invention.

[0027] The terminology used herein is for describing exemplary embodiments only and is not intended to limit the invention. Unless the context clearly implies otherwise, singular expressions include plural expressions.

[0028] It should be understood that the terms “comprising,” “including,” and “having” as used herein are intended to indicate the presence of a feature, number, step, component, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0029] In this specification, "D" n "" refers to the particle size at the n% point of the volumetric cumulative distribution based on particle size. That is, D 50 It is the particle size at the 50% point of the volumetric cumulative distribution of particle size, D. 90 It is the particle size at the 90% point of the volumetric cumulative distribution of particle size, D. 10 It is the particle size at the 10% point of the volumetric cumulative distribution of particle size. D n Laser diffraction can be used for measurement. Specifically, the powder of the object to be measured is dispersed in a dispersion medium (xylene) and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Malvern Panalytical's Mastersizer 3000). The difference in the diffraction pattern corresponding to the particle size as it passes through the laser beam is measured, thereby calculating the particle size distribution. The particle size distribution (D) can be measured by calculating the particle size at points representing the 10%, 50%, and 90% of the cumulative volume distribution corresponding to the particle size in the measuring device. 10 D 50 and D 90 .

[0030] In this specification, bulk density is measured according to ISO 60 method.

[0031] Methods for preparing lithium iron phosphate cathode active materials

[0032] The method for preparing lithium iron phosphate cathode active materials according to the present invention includes the following steps: (A) dry grinding iron phosphate (FePO4) and carbon-containing coating raw materials respectively; (B) dry mixing lithium-containing raw materials, ground iron phosphate and ground carbon-containing coating raw materials to prepare a reaction mixture; (C) compressing the reaction mixture to be processed into granular shape; and (D) calcining the granular reaction mixture to prepare a calcined product.

[0033] The method for preparing lithium iron phosphate cathode active materials may also include a step of grinding the calcined product (E).

[0034] In the method for preparing lithium iron phosphate cathode active materials according to the present invention, iron phosphate (FePO4) and carbon-containing coating raw materials are first dry-milled separately, then mixed with lithium-containing raw materials to prepare a reaction mixture. This mixture is then compressed, processed into granules, and calcined. This process not only prevents the raw materials from solidifying but also improves their reactivity. The resulting lithium iron phosphate cathode active material has a uniform carbon coating and exhibits low powder resistivity.

[0035] The method for preparing lithium iron phosphate cathode active materials according to the present invention will be described step by step below.

[0036] Step (A)

[0037] Step (A) involves dry grinding the ferric phosphate (FePO4) and the carbon-containing coating material separately. That is, it is a step of preparing ground ferric phosphate and ground carbon-containing coating material by separately dry grinding ferric phosphate (FePO4) and the carbon-containing coating material. Step (A) involves grinding each material before mixing them, which increases the surface area of ​​the materials and improves their reactivity. On the other hand, if the materials are mixed without first grinding and then the positive electrode active material is manufactured, there is a problem of uneven mixing and fixation of the materials, sometimes resulting in partial compositional inhomogeneity, thus increasing the resistance.

[0038] Dry grinding is a grinding method that does not use water or liquids. Compared with wet grinding, it can significantly improve process costs and yield.

[0039] According to the present invention, the carbon-containing coating raw material may be at least one selected from sucrose, glucose, polyethylene glycol, polyvinyl alcohol and polyvinyl acetate.

[0040] Specifically, step (A) is to grind ferric phosphate (FePO4) and carbon-containing coating raw materials separately and independently.

[0041] According to the present invention, the grinding of ferric phosphate in step (A) can be performed by air jet milling. Air jet milling is a method of grinding powder by impact while forming an airflow, and can be performed using an air jet mill. The grinding of ferric phosphate can be carried out for 0.1 to 2 hours under jet air of 1 to 10 bar and grinding air of 0.5 to 2.0 bar. Specifically, ferric phosphate can be injected at a rate of 5 g / min using a single screw feeder, and grinding can be performed under the above conditions.

[0042] According to the present invention, in step (A), the grinding of the carbon-containing coating material can be performed by mechanical grinding. Mechanical grinding is a method of grinding powder by collision with a rotating device, and can be a rotary mill, hammer mill, etc. The grinding of the carbon-containing coating material can be performed at 500 to 2000 rpm for 0.5 to 2 hours.

[0043] According to the present invention, the average particle size (D) of the ground iron phosphate is 50 The average particle size (D) of the ground ferric phosphate ranges from 1.0 μm to 2.0 μm. Specifically, the average particle size of the ground ferric phosphate is... 50 The particle size can be 1.0 μm or larger, 1.10 μm or larger, 1.20 μm or larger, 1.30 μm or larger, 1.40 μm or larger, 1.50 μm or larger, 1.60 μm or larger, 1.70 μm or larger, 1.80 μm or larger, or 1.85 μm or larger, and can be less than 1.90 μm, less than 1.95 μm, or less than 2.0 μm. The average particle size (D) of the ground ferric phosphate... 50 Within the above range, due to the small particle size and high reactivity with lithium, the electrochemical properties of the manufactured positive electrode active material are excellent.

[0044] According to the present invention, the ground carbon-containing coating raw material can have a strength of 500 kg / m³. 3 Up to 700kg / m 3 The bulk density. Specifically, the bulk density of the ground carbon-containing coating material can be 500 kg / m³. 3 Above, 510kg / m 3 Above, 520kg / m 3 Above, 530kg / m 3 Above, 540kg / m 3 Above or 550kg / m 3 And it can be 600 kg / m 3 Below, 610kg / m 3 Below, 620kg / m 3 Below, 630kg / m 3 Below, 640kg / m 3 Below, 650kg / m 3 Below, 660kg / m 3 Below, 670kg / m 3 Below, 680kg / m 3 Below, 690kg / m 3 Below, or 700kg / m 3 The following applies. If the bulk density of the ground carbon-containing coating material is within the above range, the surface area increases, the contact area with iron phosphate and lithium becomes wider, and thus uniform coating is possible. Therefore, it has the advantage of improved conductivity of the manufactured positive electrode active material. The bulk density is determined according to the ISO 60 method, for example, by dropping the powder of the ground carbon-containing coating material into a 3.16 cc measuring container and measuring its weight.

[0045] Step (B)

[0046] Step (B) is a step of preparing a reaction mixture by dry mixing lithium-containing raw material, ground iron phosphate and ground carbon-containing coating raw material.

[0047] According to the present invention, when preparing the reaction mixture, a raw material containing M (M is at least one selected from V, Ti, Mn, Zn, Mo, Mg and N) may be further mixed.

[0048] According to the present invention, the lithium-containing raw material may be Li2S, Li2S2, Li2S4, Li2S6, LiCl, LiBr, Li, Li3PS4, LiOH, Li2CO3 or a combination thereof, but is not limited thereto.

[0049] The lithium-containing raw materials can be mixed in an amount such that the resulting lithium iron phosphate-based compound has the composition represented by Chemical Formula 1 as described herein. For example, the lithium-containing raw materials can be injected in such a way that the molar ratio (Li:Fe) of lithium (Li) contained in the lithium-containing raw materials to the iron (Fe) contained in the ground iron phosphate is 1.0 to 1.1:1, specifically 1.0 to 1.05:1. In the case of further mixing the M-containing raw materials, the lithium-containing raw materials can be injected in such a way that the molar ratio (Li:Fe+M) of lithium (Li) contained in the lithium-containing raw materials to the sum of the molar numbers of iron contained in the ground iron phosphate and M contained in the M-containing raw materials is 1.0 to 1.1:1, specifically 1.0 to 1.05:1.

[0050] According to the present invention, the carbon-containing coating raw material may be at least one selected from sucrose, glucose, polyethylene glycol, polyvinyl alcohol and polyvinyl acetate.

[0051] Based on the total weight of the lithium-containing raw material and the ground iron phosphate, the carbon-containing raw material can be mixed in an amount of 8% to 13% by weight. In this case, by appropriately forming a carbon-containing coating on the surface of the cathode active material thus manufactured, it is possible to improve the conductivity while maintaining the energy density of the cathode active material.

[0052] According to the present invention, the M-containing raw material (selected from at least one of V, Ti, Mn, Zn, Mo, Mg and N) may be V2O5, TiO2, MnO, ZnO, MoO, MgO, NH4OH or combinations thereof, but is not limited thereto. The M-containing raw materials may be mixed in an amount such that the resulting lithium iron phosphate compound has the composition represented by chemical formula 1 as described herein.

[0053] When the reaction mixture contains a raw material containing M (selected from at least one of V, Ti, Mn, Zn, Mo, Mg and N), the charge-discharge capacity and rate characteristics can be further improved by doping M into the resulting positive electrode active material.

[0054] The mixing in step (A) can be performed mechanically. Mechanical mixing is a method of mixing powders using a rotating device, such as a rotary mill or hammer mill. Mixing can be carried out at 500 to 2000 rpm for 0.5 to 2 hours. In this case, the raw materials can be mixed more uniformly.

[0055] Step (C)

[0056] Step (C) is the step of compressing the reaction mixture to process it into a granular shape. For example, the reaction mixture can be compressed by a press to granulate it. In this case, the raw materials contained in the reaction mixture are densified, thus increasing the contact area between the particles and therefore having the advantage of improved reactivity between the raw materials.

[0057] According to the present invention, the compression in step (C) can be 0.10 tons / cm³. 2 Up to 1.0 ton / cm 2 It can be carried out under pressure. Specifically, it can be done at 0.10 tons / cm². 2 Above, 0.15 tons / cm 2 Above, 0.20 tons / cm 2 Above, 0.25 tons / cm 2 Above, 0.30 tons / cm 2 Above, 0.35 tons / cm 2 Above, 0.40 tons / cm 2 Above, 0.45 tons / cm 2 Above, or 0.50 tons / cm 2 Above, and 0.80 tons / cm 2 Below, 0.85 tons / cm 2 Below, 0.85 tons / cm 2 Below, 0.95 tons / cm 2 Below, 0.95 tons / cm 2 Below, or 1.0 ton / cm 2 Compression is performed under the following pressure. In this case, the pellet shape remains good even after the pellets are removed from the mold, resulting in a more uniform outcome.

[0058] Step (D)

[0059] Step (D) is the step of calcining the reaction mixture in granular form.

[0060] According to the present invention, calcination can be carried out under an inert atmosphere to prevent side reactions with oxygen and moisture. For example, calcination can be carried out under an argon atmosphere, a nitrogen atmosphere, or a helium atmosphere.

[0061] According to the present invention, calcination can be carried out at a temperature of 600°C to 800°C. More specifically, the calcination temperature can be above 600°C, above 610°C, above 620°C, above 630°C, above 640°C, or above 650°C, or below 750°C, below 760°C, below 770°C, below 780°C, below 790°C, or below 800°C. By keeping the calcination temperature within the above range, a suitable primary particle size can be obtained, thereby improving the electrochemical properties.

[0062] The temperature holding time for the calcination step can be from 5 to 20 hours. In this case, the powder resistivity and rate characteristics are improved due to the uniform carbon coating.

[0063] Step (E)

[0064] The method for preparing lithium iron phosphate cathode active materials may also include (D) the step of grinding the calcined product.

[0065] To improve the reactivity with lithium, a grinding and calcination step can be performed to reduce the average particle size (D) of the resulting lithium iron phosphate cathode active material. 50 The thickness ranges from 0.5 μm to 3.0 μm.

[0066] According to the present invention, the grinding in step (E) can be performed by air jet milling. Air jet milling is a method of grinding powder by impact while forming an airflow, and can also be performed using an air jet pulverizer. The grinding of the calcined product can be carried out for 0.1 to 2 hours under jet air of 1 to 10 bar and grinding air of 0.5 to 2.0 bar. Specifically, the calcined product can be injected at a rate of 5 g / min using a single screw feeder, and grinding can be performed under the above conditions.

[0067] According to the present invention, the obtained lithium iron phosphate cathode active material comprises a lithium iron phosphate compound; and a carbon-containing coating formed on the lithium iron phosphate compound; and can be a lithium iron phosphate cathode active material with a powder resistivity of less than 500 Ω×cm.

[0068] Powder resistance testing involves placing 5g of powder into a 22mm diameter circular 4-point probe (gold needle) mold and applying a pressure of 2000kgf / cm. 2 The volume resistivity was measured using a Hioki Rm3545 resistance measuring device under pressure.

[0069] According to the present invention, lithium iron phosphate compounds may have a composition represented by the following chemical formula 1: [Chemical Formula 1] LiFe 1-x M x PO4 Where M is selected from at least one of V, Ti, Mn, Zn, Mo, Mg and N, and 0 ≤ x < 1.

[0070] Lithium iron phosphate compounds can be doped with M. In this case, the lattice structure and spacing of the iron phosphate are altered, thereby increasing the diffusion rate of lithium ions and thus improving the electrochemical characteristics of batteries containing positive electrode active materials.

[0071] According to the present invention, the average particle size (D) of lithium iron phosphate cathode active materials is... 50 The average particle size (D) of lithium iron phosphate cathode active materials can range from 0.5 μm to 3.0 μm. 50 The average particle size (D) of lithium iron phosphate cathode active materials can be ≥0.5 μm, ≥0.6 μm, ≥0.7 μm, ≥0.8 μm, ≥0.9 μm, ≥1.0 μm, ≥1.1 μm, ≥1.2 μm, ≥1.3 μm, ≥1.4 μm, or ≥1.5 μm, or it can be ≤2.0 μm, ≤2.1 μm, ≤2.2 μm, ≤2.3 μm, ≤2.4 μm, ≤2.5 μm, ≤2.6 μm, ≤2.7 μm, ≤2.8 μm, ≤2.9 μm, or ≤3.0 μm. 50 Within the above range, due to their small particle size and high reactivity with lithium, they have the advantage of excellent electrochemical properties.

[0072] According to the present invention, the above-mentioned coating can also be uniformly applied to the surface of lithium iron phosphate compounds. During the charging and discharging process of a battery containing a positive electrode active material, the coating can improve electronic and ionic conductivity when electrons move. On the other hand, without the coating, the resistance of the positive electrode active material is high, leading to problems such as lithium ion diffusion and electron movement.

[0073] To further improve electronic and ionic conductivity, the carbon content in the coating can be from 1% to 3% by weight relative to the total weight of the lithium iron phosphate cathode active material. Alternatively, the coating can consist solely of carbon, in which case the carbon content can be from 1% to 3% by weight relative to the total weight of the lithium iron phosphate cathode active material.

[0074] The powder resistivity of the lithium iron phosphate cathode active material of the present invention is 500 Ω×cm or less. Specifically, the powder resistivity of the lithium iron phosphate cathode active material can be 50 Ω×cm or more, 100 Ω×cm or more, 150 Ω×cm or more, 200 Ω×cm or more, 250 Ω×cm or more, 300 Ω×cm or more, or 350 Ω×cm or more, and can be 390 Ω×cm or less, 400 Ω×cm or less, 410 Ω×cm or less, 420 Ω×cm or less, 430 Ω×cm or less, 440 Ω×cm or less, 450 Ω×cm or less, 460 Ω×cm or less, 470 Ω×cm or less, 480 Ω×cm or less, 490 Ω×cm or less, or 500 Ω×cm or less. In this case, the cathode active material has low resistance and excellent conductivity, thus improving electrochemical characteristics. On the other hand, when the powder resistance of the positive electrode active material exceeds 500 Ω×cm, the high resistance of the positive electrode active material leads to problems with lithium ion diffusion and electron movement.

[0075] The lithium iron phosphate compounds of the present invention can exist in the form of primary particles, rather than in the form of secondary particles formed by the aggregation of primary particles. That is, the primary particles can exist independently without aggregation. Therefore, the lithium iron phosphate cathode active material of the present invention can also have a coating formed on the primary particle form of the lithium iron phosphate compound. In this case, the coating can be in the form of a thin film.

[0076] positive electrode

[0077] The present invention provides a cathode comprising the above-mentioned lithium iron phosphate cathode active material.

[0078] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, wherein the positive electrode active material layer may include a positive electrode active material.

[0079] The positive electrode current collector can contain a highly conductive metal, as long as the positive electrode active material layer can easily adhere to it and it is non-reactive within the battery's voltage range; there are no particular limitations. Examples of positive electrode current collectors include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatments such as carbon, nickel, titanium, or silver. Furthermore, the thickness of the positive electrode current collector is typically from 3 μm to 500 μm. To improve the adhesion of the positive electrode active material, fine irregularities can be formed on the surface of the current collector. For example, it can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0080] If desired, the positive electrode active material layer may optionally include a conductive material and a binder together with the positive electrode active material. In this case, based on the total weight of the positive electrode active material layer, the positive electrode active material may be included in an amount of 80% to 99% by weight, more specifically 85% to 98.5% by weight, and may exhibit excellent capacity characteristics within this range.

[0081] Conductive materials are used to impart conductivity to electrodes. They can be any material as long as they possess electronic conductivity without causing a chemical change in the constructed battery; there are no particular limitations. Specific examples may include graphite, such as natural or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive tubes such as carbon nanotubes; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, or any one or a mixture of two or more of them. Based on the total weight of the positive electrode active material layer, the conductive material may be included in an amount from 0.1% to 15% by weight.

[0082] The adhesive is used to improve the bonding between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, polymers wherein hydrogen is substituted by Li, Na, or Ca, or various copolymers thereof, and any one or a mixture of two or more thereof may be used. The adhesive may be included in an amount of 0.1% to 15% by weight based on the total weight of the positive electrode active material layer.

[0083] In addition to using the aforementioned positive electrode active material, the positive electrode can be prepared according to conventional positive electrode preparation methods. Specifically, the aforementioned positive electrode active material and, as needed, a binder, conductive material, and dispersant can be dissolved or dispersed in a solvent to prepare a composition for forming the positive electrode active material layer (positive electrode slurry). This composition is then coated onto a positive electrode current collector, dried, and rolled to prepare the positive electrode. Alternatively, the positive electrode can be prepared by casting the composition for forming the positive electrode active material layer onto a separate support, peeling off the support to obtain a film, and then laminating the film onto the positive electrode current collector.

[0084] The solvent can be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), dimethylformamide (DMF), acetone, water, etc., and any one or a mixture of two or more of them can be used. Considering the coating thickness and preparation yield of the slurry, the solvent can be used in an amount sufficient to dissolve or disperse the positive electrode active material, conductive material, binder, and dispersant, and allows the slurry to have a viscosity that exhibits excellent thickness uniformity when coated in subsequent positive electrode preparation.

[0085] Lithium secondary batteries

[0086] The present invention provides a lithium secondary battery having the above-mentioned positive electrode.

[0087] A lithium secondary battery may include: a positive electrode; a negative electrode; and a separator and an electrolyte between the positive electrode and the negative electrode. Furthermore, a lithium secondary battery may optionally include a battery container for housing an electrode assembly formed by the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.

[0088] The negative electrode may include a negative electrode current collector and a layer of negative electrode active material disposed on the negative electrode current collector.

[0089] The negative electrode current collector is not particularly limited in its properties as long as it does not cause chemical changes in the battery and possesses high conductivity. Examples include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and copper or stainless steel or aluminum-cadmium alloys that have undergone surface treatment with carbon, nickel, titanium, silver, etc. Furthermore, the negative electrode current collector typically has a thickness ranging from 3 μm to 500 μm and, similar to the positive electrode current collector, can have fine irregularities formed on its surface to increase the adhesion of the negative electrode active material. For example, it can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0090] The negative electrode active material layer may optionally include an adhesive and a conductive material together with the negative electrode active material.

[0091] As negative electrode active materials, compounds capable of reversibly inserting and deintercalating lithium can be used. Specifically, examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; metal compounds that can be alloyed with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and metal oxides capable of doping and dedoping lithium, such as SiO₂. β(0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composite materials containing metal compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or more mixtures thereof. Alternatively, lithium metal films can be used as the negative electrode active material. Additionally, low-crystallinity carbon, high-crystallinity carbon, etc., can be used as the carbon material. Representative examples of low-crystallinity carbon can include soft carbon and hard carbon, and representative examples of high-crystallinity carbon can include amorphous, plate-like, sheet-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, mesophase pitch, and high-temperature calcined carbon such as coke from petroleum or coal tar pitch. The negative electrode active material can be included in an amount of 80% to 99% by weight based on the total weight of the negative electrode active material layers.

[0092] The binder for the negative electrode active material layer is a component that facilitates the bonding between the conductive material, the active substance, and the current collector, and is typically added in an amount of 0.1% to 10% by weight of the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile rubber, fluororubber, and various copolymers thereof.

[0093] The conductive material in the negative electrode active material layer is a component that further improves the conductivity of the negative electrode active material. It can be added in an amount of 10% by weight or less, preferably 5% by weight or less, relative to the total weight of the negative electrode active material layer. As for the conductive material, there are no particular restrictions as long as it is conductive without causing a chemical change in the battery. Examples include natural graphite, artificial graphite, etc.; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; fluorinated carbon; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0094] The negative electrode active material and optionally a binder and conductive material can be dissolved or dispersed in a solvent to prepare a composition (negative electrode slurry) for forming a negative electrode active material layer. This composition can be applied to a negative electrode current collector and dried to prepare a negative electrode. Alternatively, the negative electrode can be prepared by casting a composition for forming a negative electrode active material layer onto a separate support, peeling the film off the support, and then stacking the film layer onto the negative electrode current collector.

[0095] The separator separates the negative and positive electrodes and provides a channel for the movement of lithium ions. Any separator can be used, as long as it is commonly used in lithium secondary batteries, without particular limitation. In particular, separators with low resistance to electrolyte ion migration and excellent electrolyte moisture retention are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes composed of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures having two or more layers. Alternatively, conventional porous nonwoven fabrics can also be used, such as nonwoven fabrics composed of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, to ensure heat resistance or mechanical strength, coated separators containing ceramic components or polymer materials can also be used, and can optionally be used in single-layer or multi-layer structures.

[0096] Electrolytes can be organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which can be used to prepare lithium secondary batteries, but are not limited to these. As a specific example, electrolytes can include organic solvents and lithium salts.

[0097] As an organic solvent, any solvent can be used as long as it can serve as a medium through which the ions involved in the electrochemical reaction of the battery can move; there are no particular restrictions. Specifically, organic solvents can be ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; ether solvents, such as dibutyl ether or tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic hydrocarbon solvents, such as benzene or fluorine; carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (PC); alcohol solvents, such as ethanol or isopropanol; nitriles, such as R-CN (R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, and may include double bonds, aromatic rings, or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; or sulfolane. Preferably, the solvent is a carbonate-based solvent, and more preferably, it is a mixture of cyclic carbonates (such as ethylene carbonate, propylene carbonate, etc.) with high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery and low viscosity linear carbonate compounds (such as ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.).

[0098] As a lithium salt, there are no particular restrictions as long as it can provide lithium ions for use in lithium secondary batteries; it can be any compound. Specifically, the anion of the lithium salt can be selected from F... - Cl - ,Br - I - NO3- N(CN)2 - BF4 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - At least one of the following lithium salts is used: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The lithium salt is preferably used at a concentration of 0.1 M to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance, and lithium ions can move efficiently.

[0099] To improve battery life characteristics, suppress battery capacity reduction, and increase battery discharge capacity, in addition to the electrolyte components mentioned above, the electrolyte may also contain one or more additives, such as halogenated alkylene carbonates like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, N-glycol dimethyl ether, triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additives may be included in an amount from 0.1% to 5% by weight, based on the total weight of the electrolyte.

[0100] Lithium secondary batteries incorporating the positive electrode active material of the present invention exhibit excellent capacity and resistance characteristics, and therefore can be used in portable devices such as mobile phones, laptops, and digital cameras, as well as electric vehicles such as hybrid electric vehicles (HEVs) and electric vehicles (EVs).

[0101] The shape of the lithium secondary battery of the present invention is not particularly limited, and it can be cylindrical, prismatic, pouch-shaped or coin-shaped.

[0102] The lithium secondary battery according to the present invention can be used not only as a battery cell for powering small devices, but also preferably as a unit cell in medium to large battery modules containing multiple battery cells.

[0103] Therefore, a battery module including a lithium secondary battery as a unit cell and a battery pack including the battery module are provided.

[0104] Battery modules or battery packs can be used as a power source for any one or more medium and large-sized devices in power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, plug-in hybrid electric vehicles (PHEVs); or systems for energy storage.

[0105] Invention Embodiments

[0106] Hereinafter, embodiments of the present invention will be described in detail to enable those skilled in the art to readily implement the invention. However, the present invention can be implemented in various forms and is not limited to the examples described herein.

[0107] Examples and Comparative Examples

[0108] Example 1

[0109] FePO4 (Yacheng Corporation, G01X, China) was injected at a rate of 5 g / min using a single screw feeder and ground for 1 hour using an air jet mill (Isaac E&C, 4” size jet mill) under 5 bar jet air and 1.0 bar grinding air to prepare ground FePO4 (average particle size (D)). 50 (1.86 μm).

[0110] Sucrose (Duksan Pharmaceutical, 99.6% purity) was ground at 2000 rpm for 1 hour using a mixer (Shinil Electronics, SFN-C656CS) to prepare ground sucrose (bulk density: 590 kg / m³). 3 ).

[0111] Li₂CO₃ (Tianqi) and ground FePO₄ were injected into a mixer (Shinil Electronics, SFN-C656CS) such that the molar ratio of Li:Fe was 1.05:1. Ground sucrose was also injected, making up 12% by weight of the total weight of Li₂CO₃ and ground FePO₄ injected into the mixer. The injected raw materials were mixed at 2000 rpm for about 1 minute to prepare the reaction mixture.

[0112] 3g of the reaction mixture was injected into a 31mm diameter pellet mold (Carver, Carver-3902), and a vacuum was applied while simultaneously applying a force of 4 tons (pressure: 0.53 tons / cm²). 2 It is pressed and processed into granular shape.

[0113] Then, the granular reaction mixture was calcined at 700°C for 10 hours under a nitrogen atmosphere to prepare a calcined product. The calcined product was then injected at a rate of 5 g / min using a single screw feeder and milled for 1 hour using an air jet mill (Isaac E&C, 4” size jet mill) with 5 bar jet air and 1.0 bar grinding air to prepare a lithium iron phosphate cathode active material containing a carbon coating.

[0114] Example 2

[0115] The granular reaction mixture from Example 1 was calcined at 700°C for 8 hours under a nitrogen atmosphere. Otherwise, lithium iron phosphate positive electrode active material was prepared in the same manner as in Example 1.

[0116] Example 3

[0117] 3g of the reaction mixture was injected into a 31mm diameter pellet mold (Carver, Carver-3902) and subjected to a force of 6 tons (pressure: 0.79 tons / cm²). 2 While applying pressure, vacuum processing is applied to form granules. Otherwise, lithium iron phosphate positive electrode active material is prepared in the same manner as in Example 1.

[0118] Comparative Example 1

[0119] Sucrose (Duksan Pharmaceutical, 99.6% purity) was ground at 2000 rpm for 1 hour using a mixer (Shinil Electronics, SFN-C656CS) to prepare ground sucrose (bulk density: 590 kg / m³). 3 ).

[0120] Li2CO3 (Tianqi) and FePO4 (Yacheng, G01X) (average particle size (D)) were used. 50 Li:Fe molar ratio of 1.05:1 was injected into a mixer (Shinil Electronics, SFN-C656CS) at a density of 2.34 μm. Additionally, milled sucrose was injected, making up 12% by weight of the total weight of Li2CO3 and FePO4 injected into the mixer. The injected raw materials were mixed at 2000 rpm for about 1 minute to prepare the reaction mixture.

[0121] 3g of the reaction mixture was injected into a 31mm diameter pellet mold (Carver, Carver-3902) and processed into pellet shape by applying vacuum and pressing with a force of 4 tons.

[0122] Then, the granular reaction mixture was calcined at 700°C for 10 hours under a nitrogen atmosphere to prepare a lithium iron phosphate positive electrode active material with a carbon coating.

[0123] Comparative Example 2

[0124] Li2CO3 (Tianqi) and FePO4 (Yacheng, G01X) (average particle size (D)) were used. 50 Li:Fe molar ratio of 1.05:1 was achieved by injecting 2.34 μm of sucrose into a mixer (Shinil Electronics, SFN-C656CS), and sucrose (Duksan Pharmaceutical, 99.6% purity) (bulk density: 760 kg / m³) was also injected. 3 The mixture is prepared by mixing the injected raw materials at 2000 rpm for about 1 minute to make 12% by weight of the total weight of Li2CO3 and FePO4 injected into the mixer.

[0125] 3g of the reaction mixture was injected into a 31mm diameter pellet mold (Carver, Carver-3902) and processed into pellet shape by applying vacuum and pressing with a force of 4 tons.

[0126] Then, the granular reaction mixture was calcined at 700°C for 10 hours under a nitrogen atmosphere to prepare a lithium iron phosphate positive electrode active material with a carbon coating.

[0127] Comparative Example 3

[0128] FePO4 (Yacheng Corporation, G01X, China) was injected at a rate of 5 g / min using a single screw feeder and ground for 1 hour using an air jet mill (Isaac E&C, 4” size jet mill) under 5 bar jet air and 1.0 bar grinding air to prepare ground FePO4 (average particle size (D)). 50 (1.86 μm).

[0129] Sucrose (Duksan Pharmaceutical, 99.6% purity) was ground at 2000 rpm for 1 hour using a mixer (Shinil Electronics, SFN-C656CS) to prepare ground sucrose (bulk density: 590 kg / m³). 3 ).

[0130] Li₂CO₃ (Tianqi) and ground FePO₄ were injected into a mixer (Shinil Electronics, SFN-C656CS) such that the molar ratio of Li:Fe was 1.05:1. Ground sucrose was also injected, making up 12% by weight of the total weight of Li₂CO₃ and ground FePO₄ injected into the mixer. The injected raw materials were mixed at 2000 rpm for about 1 minute to prepare the reaction mixture.

[0131] The reaction mixture was calcined at 700°C for 10 hours under a nitrogen atmosphere to prepare a lithium iron phosphate positive electrode active material with a carbon coating.

[0132] Comparative Example 4

[0133] Li2CO3 (Tianqi) and FePO4 (Yacheng, G01X) (average particle size (D)) were used. 50 Li:Fe molar ratio of 1.05:1 was achieved by injecting 2.34 μm of sucrose into a mixer (Shinil Electronics, SFN-C656CS), and sucrose (Duksan Pharmaceutical, 99.6% purity) (bulk density: 760 kg / m³) was also injected. 3 The mixture is prepared by mixing the injected raw materials at 2000 rpm for about 1 minute to make 12% by weight of the total weight of Li2CO3 and FePO4 injected into the mixer.

[0134] The reaction mixture was calcined at 700°C for 10 hours under a nitrogen atmosphere to prepare a lithium iron phosphate cathode active material with a carbon-containing coating.

[0135] Table 1 below shows whether the raw materials were ground before mixing and whether the reaction mixture was processed into granular shape in the examples and comparative examples.

[0136] [Table 1]

[0137] Experimental Example

[0138] Experimental Example 1: Evaluation of Powder Resistance

[0139] Five g of each of the lithium iron phosphate positive electrode active material prepared in the above examples and comparative examples were placed into a circular 4-point probe (gold needle) mold with a diameter of 22 mm, and an application of 2000 kgf / cm² was applied. 2The pressure was measured. Under this condition, the volume resistivity was measured using a Hiokki Rm3545 resistivity measuring device, and the results are shown in Table 1 below as powder resistivity.

[0140] Experimental Example 2: Evaluation of Particle Size Distribution

[0141] 20 mg of each of the lithium iron phosphate positive electrode active materials prepared in the above examples and comparative examples were dispersed in 5 mL of Triton X-100 (Sigma Aldrich) and then introduced into a laser diffraction particle size analyzer (Malvern Panalytical, Mastersizer 3000) to measure the average particle size (D) of the positive electrode active materials. 50 The results are shown in Table 1 below.

[0142] [Table 2]

[0143] Referring to Table 2 above, it can be confirmed that the powder resistance of the positive electrode active materials of Examples 1 to 3 is significantly lower than that of the positive electrode active materials of Comparative Examples 1 to 4. This is because, in preparing the positive electrode active materials of Examples 1 to 3, iron phosphate (FePO4) and carbon-containing coating raw materials were first dry-milled separately, and then mixed with lithium-containing raw materials to prepare a reaction mixture. After being compressed into granules, the mixture was calcined, thereby preventing the solidification of the raw materials and significantly improving their reactivity.

[0144] Experiment Example 3: Measurement of SEM Images

[0145] SEM images of the lithium iron phosphate cathode active materials prepared in Example 1 and Comparative Example 1 were measured using a SEM (FEI QUANTA 600), and the results are shown below. Figure 1 and Figure 2 .

[0146] Figure 1 This is a SEM image of the lithium iron phosphate cathode active material from Example 1. Figure 2 This is a SEM image of the lithium iron phosphate cathode active material of Comparative Example 1.

[0147] Reference Figure 1 and Figure 2 It can be confirmed that the particle size of the lithium iron phosphate positive electrode active material in Example 1 is smaller than that of the lithium iron phosphate positive electrode active material in Comparative Example 1, because ground FePO4 was used in the preparation of the positive electrode active material.

[0148] Experiment Example 4: STEM-EDS Analysis

[0149] STEM-EDS mapping images of the lithium iron phosphate cathode active material prepared in Example 1 above were obtained using STEM-EDS (ThermoFisher, Heilos 4UX) and are shown below. Figure 3 .

[0150] Figure 3 This is a STEM-EDS mapping image of the lithium iron phosphate positive electrode active material of Example 1.

[0151] Reference Figure 3 It can be confirmed that a carbon coating is uniformly formed on the surface of the lithium iron phosphate positive electrode active material of Example 1.

[0152] Experimental Example 5: Evaluation of Battery Characteristics

[0153] The lithium iron phosphate positive electrode active material prepared in the above examples and comparative examples, carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in N-methylpyrrolidone (NMP) solvent at a weight ratio of 96:2:2 to prepare a positive electrode slurry. The prepared positive electrode slurry was coated on one surface of an aluminum current collector, dried at 130°C, and then rolled to prepare the positive electrode.

[0154] An electrode assembly was prepared by using a lithium metal electrode as the negative electrode and inserting a porous polyethylene membrane between the positive and negative electrodes. The electrode assembly was placed inside a battery casing, and an electrolyte obtained by dissolving 1.0 M LiPF6 in an organic solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 3:4:3 was injected to prepare a half-cell.

[0155] Each half-cell prepared as described above was charged to 3.65V at 25°C in CC (0.1C)-CV (cutoff current: 0.1C) mode, and then discharged to 2.5V at 0.1C. The initial charge / discharge capacity and DCIR resistance were measured and are shown in Table 3 below.

[0156] [Table 3]

[0157] Referring to Table 3 above, it can be confirmed that, compared with batteries containing the positive electrode active materials of Comparative Examples 1 to 4, batteries containing the positive electrode active materials of Examples 1 to 3 not only have significantly superior initial charge-discharge capacity, but also significantly reduced resistance. This is because, as described above, the positive electrode active materials of Examples 1 to 3 were prepared by the method for preparing positive electrode active materials of the present invention, and therefore have a uniform carbon coating and low powder resistance.

Claims

1. A method for preparing lithium iron phosphate-based positive electrode active materials, the method comprising the following steps: (A) Dry grinding of ferric phosphate (FePO4) and carbon-containing coating raw materials respectively; (B) A reaction mixture is prepared by dry mixing lithium-containing raw materials, ground iron phosphate, and ground carbon-containing coating raw materials; (C) Compress the reaction mixture to be processed into granular form; and (D) Calcination of the granular reaction mixture to prepare the calcined product.

2. The method for preparing lithium iron phosphate-based positive electrode active materials according to claim 1, wherein, The carbon-containing coating material is selected from at least one of sucrose, glucose, polyethylene glycol, polyvinyl alcohol, and polyvinyl acetate.

3. The method for preparing lithium iron phosphate-based positive electrode active materials according to claim 1, wherein, In preparing the reaction mixture, a raw material containing M is further mixed (M is at least one selected from V, Ti, Mn, Zn, Mo, Mg and N).

4. The method for preparing lithium iron phosphate-based positive electrode active materials according to claim 1, wherein, In step (A), the ferric phosphate is ground by air jet milling.

5. The method for preparing lithium iron phosphate-based positive electrode active materials according to claim 1, wherein, In step (A), the grinding of the carbon-containing coating material is carried out by mechanical grinding.

6. The method for preparing lithium iron phosphate-based positive electrode active materials according to claim 1, wherein, The average particle size (D) of the ground ferric phosphate 50 The thickness ranges from 1.0 μm to 2.0 μm.

7. The method for preparing lithium iron phosphate-based positive electrode active materials according to claim 1, wherein, The bulk density of the ground carbon-containing coating raw material is 500 kg / m³. 3 Up to 700 kg / m 3 .

8. The method for preparing lithium iron phosphate-based positive electrode active materials according to claim 1, wherein, The compression in step (C) is carried out at a pressure of 0.10 tons / cm² to 1.0 tons / cm².

9. The method for preparing lithium iron phosphate-based positive electrode active materials according to claim 1, wherein, The calcination is carried out at a temperature of 600°C to 800°C.

10. The method for preparing lithium iron phosphate-based positive electrode active materials according to claim 1, wherein, The method further includes step (E): grinding the calcined product.

Images