Self-supporting electrode film and its manufacturing method

Abstract

A self-supporting electrode film capable of effectively improving the output performance of a secondary battery and a method for manufacturing the same are provided. The technology disclosed in this specification relates to a self-supporting electrode film. The self-supporting electrode film includes positive electrode active material particles. Furthermore, the self-supporting electrode film includes a first polymer. Further, the self-supporting electrode film includes a second polymer. The first polymer has lithium-ion conductivity. The second polymer has a fibrillated morphology.

Description

Technical Field

[0001] The technology disclosed in this specification relates to self-supporting electrode films and their manufacturing methods. Background Technology

[0002] In recent years, self-supporting electrode films have begun to be manufactured using dry processes. For example, a self-supporting electrode film containing polytetrafluoroethylene (PTFE) is known (Japanese Patent Application Laid-Open No. 2024-26272). In this method, PTFE is used as the main binder, and to improve adhesion, a composite binder combining polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), and other components is used. The method describes the preparation of a mixture of active material, conductive additive, and composite binder, applying shear force to fibrillate the PTFE, and then pressing it to form a self-supporting electrode film. Summary of the Invention

[0003] In the method described in Japanese Patent Application Publication No. 2024-26272, PTFE is used as the main binder, and PVDF and CMC are used to improve the bonding performance. However, this polymer has low lithium-ion conductivity, resulting in a decrease in the battery's output power.

[0004] This specification provides a self-supporting electrode film that can effectively improve the output performance of a secondary battery and its manufacturing method.

[0005] The technology disclosed in this specification is embodied in the following self-supporting electrode film and its manufacturing method.

[0006] The self-supporting electrode film according to the first aspect of this disclosure comprises:

[0007] Positive electrode active material particles,

[0008] Conductive additives

[0009] The first polymer with lithium-ion conductivity, and

[0010] The second polymer has a fibrillated form.

[0011] In the self-supporting electrode film according to the first aspect of this disclosure,

[0012] The first polymer can also be polyethylene oxide with a crystallinity of more than 10% and less than 50%.

[0013] In the self-supporting electrode film according to the first aspect of this disclosure,

[0014] Part of the surface of the positive electrode active material particles can also be covered by the first polymer.

[0015] In the self-supporting electrode film according to the first aspect of this disclosure,

[0016] The second polymer may also contain polytetrafluoroethylene.

[0017] The electrode involved in the second aspect of this disclosure may also have the self-supporting electrode film described in the first aspect above.

[0018] The secondary battery according to the third aspect of this disclosure may also have the electrodes described in the second aspect above.

[0019] The fourth aspect of this disclosure relates to a method for manufacturing an electrode sheet for a self-supporting electrode film, comprising the following steps:

[0020] A first mixture is prepared by mixing positive electrode active material particles, conductive additives, and a first polymer that is lithium-ion conductive and not fibrillated, in a fluidized state of the first polymer.

[0021] The first mixture is mixed with a fibrillable second polymer to prepare a second mixture having fibrillated the second polymer; and

[0022] The electrode sheet was made using the second mixture.

[0023] In the method for manufacturing an electrode sheet for a self-supporting electrode film according to the fourth aspect of this disclosure, the first polymer may also contain polyethylene oxide with a crystallinity of 10% or more and 50% or less.

[0024] The fifth aspect of this disclosure relates to a method for manufacturing an electrode having a self-supporting electrode film, comprising the following steps:

[0025] A first mixture is prepared by mixing positive electrode active material particles having an electronically conductive coating and containing olivine-type positive electrode material, a conductive additive, and a first polymer that is not fibrillated, in a fluidized state.

[0026] The first mixture is mixed with a second polymer capable of fibrillation to prepare a second mixture having the second polymer fibrillated.

[0027] The second mixture is used to fabricate electrode sheets for the self-supporting electrode film; and

[0028] The electrode is fabricated by bonding the electrode sheet to a current collector to form the electrode having the self-supporting electrode film.

[0029] According to the self-supporting electrode film, the first polymer has lithium-ion conductivity, thus enabling the construction of a self-supporting electrode film through fibrillation without reducing the lithium-ion conductivity of the positive electrode.

[0030] Furthermore, according to the manufacturing method of the electrode sheet, the first polymer has lithium-ion conductivity, thus enabling the active material to be mixed without reducing the lithium-ion conductivity of the electrode film, thereby improving the adhesion of the self-supporting electrode film resulting from fibrillation.

[0031] When preparing the first mixture, the first polymer is prepared in a fluidized state. Therefore, the first polymer and the positive electrode active material particles are in sufficient contact, and the first polymer covers a portion of the surface of the positive electrode active material particles. Furthermore, when preparing the second mixture, the first polymer, while fully mixed with the positive electrode active material particles, is bound and integrated by the fibrillated second polymer. Therefore, it is possible to manufacture electrode sheets and self-supporting electrode films while maintaining lithium-ion conductivity. Attached Figure Description

[0032] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention are described below with reference to the accompanying drawings, wherein the same reference numerals denote the same elements.

[0033] Figure 1 This is a diagram that schematically represents an example of a cell in a lithium-ion secondary battery.

[0034] Figure 2 This is a cross-sectional view schematically illustrating an example of the structure of the positive electrode disclosed in this specification.

[0035] Figure 3 This is a schematic diagram illustrating an example of the existing form of active material particles, etc., in the electrode film disclosed in this specification.

[0036] Figure 4 This is a process diagram schematically illustrating an example of a method for manufacturing an electrode sheet disclosed in this specification. Detailed Implementation

[0037] Hereinafter, with appropriate reference to the accompanying drawings, a method for manufacturing a self-supporting electrode film and electrode, as well as an electrode sheet for a self-supporting electrode film, will be described. Furthermore, in this specification, the self-supporting electrode film and electrode are, for example, the self-supporting electrode film and positive electrode constituting the positive electrode of a lithium-ion secondary battery.

[0038] Self-supporting electrode film and electrode

[0039] Figure 1 This schematically illustrates an example of unit 2 of a lithium-ion secondary battery (hereinafter referred to as a secondary battery). For example... Figure 1As shown, unit 2, as a unit structure of a secondary battery, includes a positive electrode 4, a separator 6, and a negative electrode 8. Unit 2 also includes a positive current collector 10 and a negative current collector 12. Secondary batteries typically have a structure in which multiple units 2 are stacked. The separator 6 may, for example, hold a liquid or gel-like electrolyte. The separator 6 is made of known materials such as a polyolefin-based microporous membrane with fine pores. A liquid electrolyte containing lithium salts such as lithium hexafluorophosphate is impregnated in the separator 6 using organic solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) as a medium. Alternatively, a solid electrolyte layer may be used instead. The negative electrode 8 has a negative electrode binder layer 40.

[0040] Figure 2 This is an example of the cross-section of the positive electrode 4 of a secondary battery. The positive electrode 4 has a self-supporting electrode film (hereinafter also simply referred to as the electrode film) 20 and a current collector 10. The current collector 10 is not particularly limited, and examples include aluminum foil, nickel foil, etc.

[0041] like Figure 3 As shown, the electrode film 20 includes active material particles 22, conductive additive 26, first polymer 30, and second polymer 32.

[0042] Active substance particles

[0043] The active material particles 22 can be known positive electrode active material particles. For example, olivine-type compounds and layered rock salt-type oxides can be used. Examples of olivine-type compounds include lithium iron phosphate (LiFePO4, LFP) and lithium iron manganese phosphate (LiMn). （1-d） F d Lithium manganese phosphate (LiMnPO4, LMP), etc., can be used as active material particles 22. One or more of these can be used. Active material particles 22 can have any average particle size. Active material particles 22 can also have a morphology formed by secondary aggregation of individual particles.

[0044] The active material particles 22 may also have an electronically conductive material coating (hereinafter referred to as coating) 24 covering at least a portion of their surface. The coating 24 helps to improve the electronic conductivity of the active material particles 22. The electronically conductive material contained in the coating 24 may appropriately use known materials. Examples of electronically conductive materials include carbon-based materials such as graphite and semi-graphite, and nanoscale metal particle materials.

[0045] The coating 24 is formed in such a way that it covers at least a portion of the surface of the active material particle 22. Furthermore, even if the coating 24 covers the entire surface of the active material particle 22, it is acceptable as long as lithium-ion insertion and extraction are possible. The shape of the coating 24 is not particularly limited. When the active material particle 22 serves as the core, it can be a generally uniform coating, or it can be a carbon-based material of any shape attached to the surface of the active material particle 22. Examples of the former include carbon-based material coatings obtained by carbonizing organic matter applied to the surface of the active material particle 22. Examples of the latter include coatings obtained by attaching carbon-based materials to the surface of the active material particle 22 through mechanochemical mixing or the like.

[0046] The thickness of the film 24 is not particularly limited. For example, it can be 0.1 nm or more and 10 nm or less, or 0.5 nm or more and 3 nm or less.

[0047] Conductive additives

[0048] The conductive additive 26 is an additive used to improve the electronic conductivity in the electrode film 20. The conductive additive 26 is dispersedly maintained in the electrode film 20, for example, it may be dispersed and maintained in a manner between the active material particles 22 or attached to their surface.

[0049] There are no particular limitations on the conductive additive 26, and various known conductive additives can be used. Examples of conductive additive 26 include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fibers and metal fibers; carbon materials such as graphene and carbon nanotubes; fluorinated carbon; metal powders such as aluminum and nickel powders; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. As for carbon nanotubes, single-walled carbon nanotubes (SW-CNTs) and multi-walled carbon nanotubes (MW-CNTs), including double-walled carbon nanotubes, are sometimes preferred. One or more of these conductive additives can be used.

[0050] The content of conductive additive 26 in electrode film 20 is not particularly limited, but may be, for example, 0.01% by mass or more and 5% by mass or less of the total mass of electrode film 20. Alternatively, it may be, for example, 1.0% by mass or more and 3.0% by mass or more and 2.0% by mass or less.

[0051] Polymer 1

[0052] The first polymer 30 is a polymer with lithium-ion conductivity. There is no particular limitation on the first polymer 30; examples include polyethylene oxide (PEO), polypropylene oxide, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, polyvinylidene fluoride, ethylene carbonate, polyethylene, polypropylene, polystyrene, ABS resin, polyacrylate, polymethyl methacrylate, polyvinyl alcohol, and polycarbonate. One or more of these can be used. The first polymer 30 functions as a binder in the electrode film 20.

[0053] Here, each of the aforementioned polymers comprises, for example, homopolymers and copolymers, which are respectively composed of basic structural units based on the name of the polymer and structural units modified from the basic structural units. For example, PEO comprises structural units derived from ethylene oxide and / or homopolymers and copolymers having modified structural units thereon, and can have various forms such as linear, comb-like, and cross-linked. Such polymers are well known to those skilled in the art, and can be appropriately obtained or manufactured by those skilled in the art.

[0054] For example, as the first polymer 30, a homopolymer of ethylene oxide, such as a homopolymer, can be used, which is a structural unit of ethylene oxide with a number average molecular weight of 3,000 or more and 10,000,000 or less. The number average molecular weight of this PEO is, for example, 3,000 or more and 50,000 or less, 8,000 or more and 50,000 or less, or 10,000 or more and 30,000 or less.

[0055] The first polymer 30 may also be one or more selected from PEO, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, and polyvinylidene fluoride. These polymers have lithium-ion conductivity and can suppress the decrease in lithium-ion conductivity.

[0056] PEO is preferably used as the first polymer 30. The crystallinity of PEO is preferably 50% or less. If the crystallinity is, for example, 40% or less, 30% or less, or 20% or less, the lithium-ion diffusion resistance can be further reduced. The crystallinity is preferably, for example, 5% or more, 10% or more, 20% or more, or 30% or more. The range of crystallinity is preferably, for example, 10% or more and 50% or less, 20% or more and 50% or less, 10% or more and 30% or less, or 10% or more and 20% or less. If it falls within this range, the lithium-ion diffusion resistance can be reliably reduced.

[0057] Furthermore, the crystallinity of the polymer can be determined using Raman spectroscopy and XRD.

[0058] The first polymer 30 can be contained in the electrode membrane 20 in a form that covers the active material particles 22 and / or the coating 24. The first polymer 30 can cover part or all of the coating 24 without impairing its function.

[0059] The first polymer 30, having a morphology that covers the active material particles 22 and / or the coating 24, is capable of suppressing damage to the active material particles 22 in addition to suppressing damage to the coating 24 within the electrode film 20. By including the first polymer 30 in such a morphology, the decrease in powder resistance of the electrode film 20 can be suppressed.

[0060] The first polymer 30 is an unfibrillated polymer. Furthermore, the first polymer 30 preferably has a melting point of 60°C or higher and 260°C or lower. Having a melting point within this temperature range allows the first polymer 30 to easily flow and mix with the active material particles 22, etc.

[0061] The melting point of the first polymer 30 is, for example, above 70°C, above 75°C, above 80°C, or above 90°C, and below 240°C, below 230°C, below 220°C, or below 200°C. In addition to the above-mentioned ranges, the melting point range of the first polymer 30 can also be set by appropriately combining the aforementioned lower and upper temperature limits. For example, it can be above 60°C and below 240°C, above 60°C and below 200°C, above 60°C and below 180°C, or above 60°C and below 120°C.

[0062] The content of the first polymer 30 in the electrode film 20 is not particularly limited, as long as it can suppress damage to the coating 24. If the content of the first polymer 30 is, for example, 0.1% by mass or more and 3.0% by mass or less relative to the total mass of the electrode film 20, it is effective in reducing friction during the fabrication of the electrode film 20, protecting the coating 24, and suppressing the decrease in powder resistance in the electrode film 20. Other examples include 0.5% by mass or more, 0.7% by mass or more, 0.8% by mass or more, 1.0% by mass or more, 1.5% by mass or more, and 2.0% by mass or more. Additionally, it can be 2.5% by mass or less, 2.0% by mass or less, 1.5% by mass or less, and 1.2% by mass or less. In addition to the above ranges, the content of the first polymer 30 can also be set by appropriately combining their lower and upper limits. For example, it is 0.7% or more and 3.0% or less, 0.8% or more and 2.0% or less, 0.8% or more and 1.5% or less, or 0.8% or more and 1.2% or less.

[0063] Polymer 2

[0064] The electrode membrane 20 contains a second polymer 32. In the electrode membrane 20, the second polymer 32 has a fibrillated morphology. For example, the second polymer 32, in a fibrillated state, exists on the outside of the active material particles 22, which can also be covered by the membrane 24, in a fibrous winding manner.

[0065] As the second polymer 32, a fibrillable polymer is used. If shear force is applied to the second polymer 32, the second polymer 32 becomes fibrillated. The fibrillated second polymer 32 wraps around and constrains the active material particles 22 and the conductive additive 26, giving the electrode film 20 mechanical strength.

[0066] The second polymer 32 can be used without particular limitation as long as it is a fibrillable polymer. Fibrous polymers are well known to those skilled in the art. Examples of the second polymer 32 include polytetrafluoroethylene (PTFE), polyolefins, polyalkylene polymers, polyethers, styrene-butadiene, polysiloxanes and copolymers of polysiloxanes, branched polyethers, polyethylene ethers, and copolymers thereof. Cellulose containing carboxymethyl cellulose (CMC) can also be used as the second polymer 32. One or more of these can be used as the second polymer 32. For example, PTFE is sometimes preferred as the second polymer 32 because of its excellent fibrillability and adhesive properties.

[0067] The second polymer 32 is preferably capable of fibrillation at a temperature above 60°C and below 260°C. This temperature range coincides with the temperature range of the preferred melting point of the first polymer 30. Since the second polymer 32 can fibrillate within the flow range of the first polymer 30, the lubricating effect provided by the first polymer 30 can be anticipated when the active material particles 22 of the second polymer 32 are incorporated. Furthermore, the second polymer 32 preferably has heat resistance at the flow temperature of the first polymer 30. From this viewpoint, the second polymer 32 preferably has, for example, a melting point exceeding 260°C.

[0068] The content of the second polymer 32 in the electrode film 20 is not particularly limited, as long as it can constrain the electrode film 20 and achieve integration. The content of the second polymer 32 is, for example, 1.0% by mass or more and 10% by mass or less relative to the total mass of the electrode film 20. Alternatively, it may be 1.5% by mass or more, 2.0% by mass or more, 2.5% by mass or more, or 3.0% by mass or more. Alternatively, it may be 8.0% by mass or less, 6.0% by mass or less, or 5.0% by mass or less. In addition to the above ranges, the content of the second polymer 32 can also be set by appropriately combining their lower and upper limits. For example, it may be 1.5% by mass or more and 5.0% by mass or less, 2.0% by mass or more and 5.0% by mass or less, or 2.5% by mass or more and 4.5% by mass or less.

[0069] Furthermore, the electrode film 20 may, as needed, contain a polymer that acts as a binder. This polymer, for example, in addition to the first polymer 30 and the second polymer 32 described above, may also be any known polymer suitable for use in this electrode. These polymers may also be in particulate form, etc.

[0070] The electrode film 20 described above has a thickness of, for example, between 10 μm and 500 μm, and together with the current collector 10 forms the positive electrode 4.

[0071] According to this positive electrode 4, a first polymer with excellent lithium-ion conductivity is included in the electrode film 20, thus the lithium-ion diffusion in the secondary battery using the electrode film 20 is excellent. Furthermore, the adhesive function of the first polymer 30 improves the integrity of the electrode film 20. Additionally, the first polymer 30, being located on the surface of the active material particles 22, effectively enhances lithium-ion conductivity. Furthermore, damage to the active material particles 22 is suppressed.

[0072] Furthermore, in the electrode film 20, the second polymer 32 is dispersed substantially uniformly relative to the active material particles 22 and the conductive additive 26. This allows the active material particles 22 to be constrained from their outer side in a fibrous manner. From this perspective, the increase in powder resistance is also suppressed.

[0073] Next, refer to the appropriate Figure 4 The manufacturing method of the electrode sheet used to manufacture such an electrode film 20 will be described. Furthermore, the electrode sheet is stacked on the current collector 10 and appropriately integrated by hot pressing or the like, thereby forming the electrode film 20. Figure 4 An example illustrating a method for manufacturing an electrode sheet.

[0074] The manufacturing method includes: a step S10 of preparing a first mixture; a step S20 of preparing a second mixture using the first mixture; and a step S30 of making an electrode sheet for electrode film 20 using the second mixture.

[0075] Step S10 of preparing the first mixture

[0076] Step S10 includes mixing the active material particles 22, the conductive additive 26, and the unfibrillated first polymer 30 to prepare a first mixture. The first mixture preferably does not contain a fibrillable second polymer 32. The presence of the second polymer 32 results in a decrease in the protective effect of the first polymer 30 on the film 24 and in the overall mixing lubricity.

[0077] The first mixture may be a mixture comprising active material particles 22, conductive additive 26, and a first polymer 30, having at least a surface in which the first polymer 30 covers the surface of the active material particles 22 (or, in the case of a coating 24, covers the coating 24). In step S10, the first mixture can be obtained by mixing the materials to be mixed in a fluidized state with the first polymer 30. For example, when using a polymer with a melting point of 60°C or higher and a temperature of 260°C or lower, mixing can be performed under appropriate shear force at the temperature at which the polymer melts and flows, i.e., 60°C or higher and 260°C or lower, thereby obtaining a suitable first mixture.

[0078] By melting and fluidizing the first polymer 30, the friction between the active material particles 22 is reduced. As a result, the first polymer 30 is uniformly dispersed relative to the active material particles 22 and the conductive additive 26, adhering to the surface of the active material particles 22. By fluidizing the first polymer 30, it can function as a lubricant in the mixing of the active material particles 22 and the conductive additive 26, even without the use of a solvent, making mixing easier. As a result, damage to the active material particles 22 and the film 24 during mixing with the conductive additive 26 can be suppressed.

[0079] As long as the first mixture can be obtained, step S10 can also include multiple stages. For example, the active material particles 22 and the first polymer 30 can be mixed in advance in a fluidized state to form a shape in which the first polymer 30 covers the surface of the coating 24, and then, while maintaining the fluidized state of the first polymer 30, the conductive additive 26 is added and mixed. Thus, when the coating 24 is present, strong frictional waves can be effectively suppressed from affecting the coating 24.

[0080] When preparing the first mixture, the conductive additive 26 and the first polymer 30 can be used within the range of contents described above relative to the total mass of the final second mixture. Additionally, the amount of active material particles 22 can be used such that it is the remainder other than the conductive additive 26, the first polymer 30, and the second polymer 32 relative to the total mass of the second mixture.

[0081] Step S10 can be carried out using a known mixing apparatus. The mixing apparatus, for example, is a rotating body. Besides various kneaders such as pressure kneaders equipped with rollers, rotors, paddles, blades, gears, and screws, as well as Banbury mixers, Henschel mixers, and twin-screw extruders, examples include various mills such as jet mills, roller mills, and hammer mills, which are capable of using known shear mixing. Those skilled in the art can appropriately set mixing conditions such as shear force and mixing time to obtain a suitable mixing state between the active material particles 22 and the conductive additive 26.

[0082] Step S20 of preparing the second mixture

[0083] Step S20 includes mixing a first mixture with a fibrillable second polymer 32 to prepare a second mixture having the fibrillated state of the second polymer 32.

[0084] The second mixture is a mixture that, in addition to the active material particles 22, conductive additive 26, and first polymer 30 from the first mixture, also has the second polymer 32 in a fibrillated state. The second mixture can be obtained by mixing these materials to be mixed in a manner that fibrillates the second polymer 32.

[0085] By adding the second polymer 32 after preparing the first mixture and mixing it in a fibrillated manner, a second mixture having a structure of electrode film 20 or a structure similar thereto can be obtained. That is, in the second mixture, the first polymer 30 has a membrane 24 covering the active material particles 22, and the second polymer 32 has a fibrillated form. Furthermore, the second polymer 32 is used to constrain the active material particles 22 by fibrous winding from its outside.

[0086] Similar to step S10, step S20 is preferably performed in a molten and fluidized state of the first polymer 30. That is, mixing is preferably carried out at a temperature of 60°C or higher and 260°C or lower, based on the melting point of the first polymer 30. Thus, in step S20, the first polymer 30 also functions as a lubricant, making mixing in step S20 easier and suppressing damage to the film 24.

[0087] When preparing the second mixture, it can be used in a manner that allows the second polymer to reach the range of the above-mentioned content, relative to the total mass of the final second mixture to be obtained.

[0088] Step S20, like step S10, can be carried out using a known mixing apparatus. Those skilled in the art can appropriately set mixing conditions such as shear force and mixing time to obtain fibrillation of the second polymer 32 and a suitable mixing state of the materials to be mixed.

[0089] Step S30 for fabricating electrode sheets for electrode films

[0090] Step S30 includes fabricating an electrode sheet for manufacturing the electrode film 20 using the second mixture. Here, the thickness of the electrode sheet is approximately 10 μm or more and 2000 μm or less, or 10 μm or more and 1000 μm or less. There are no particular limitations on the sheet forming process. For example, without using a support, the second mixture can be supplied to rotating rollers for hot pressing and sheet forming calendering. Alternatively, the electrode mixture can be supplied to a suitable forming die for hot pressing and stamping.

[0091] The resulting electrode sheet has active material particles 22, conductive additives 26, first polymer 30, and second polymer 32 of the same morphology as those in the electrode film 20, and has a predetermined mechanical strength. Therefore, it is possible to subsequently construct an electrode film 20 that is self-supporting without a support.

[0092] Furthermore, such an electrode sheet can also be used to fabricate the positive electrode 4 and the secondary battery. First, the current collector 10 and the electrode sheet are stacked. Then, by applying pressure or the like at a predetermined temperature, the electrode sheet is compacted to form a thickness of about 10 μm to 500 μm, thereby forming an electrode film 20. Simultaneously with obtaining the electrode film 20, a positive electrode 4 can be obtained in which the electrode film 20 and the current collector 10 are integrated. Furthermore, by integrating the separator 6, the negative electrode binder layer 40, and the negative electrode current collector 12 relative to the positive electrode 4, a secondary battery can be obtained. The secondary battery can be fabricated using known methods.

[0093] In the electrode film 20 thus obtained, lithium-ion conductivity is imparted by the first polymer 30, so lithium-ion diffusion does not decrease, resulting in an electrode film with excellent integrity. Therefore, by using this electrode film 20, a positive electrode 4 that can exhibit good electrode characteristics and a secondary battery having the positive electrode 4 can be obtained.

[0094] In the above description, the electrode film 20, etc., have been explained. However, according to this specification, in addition to the electrode sheet that serves as a precursor to the electrode film 20, a positive electrode 4 having the electrode film 20 and a secondary battery having the positive electrode 4 are also provided. Furthermore, in the above description, the manufacturing method of the electrode sheet used for the electrode film 20 has been explained. However, according to this specification, a method for manufacturing an electrode film mixture is also provided, which includes a step of preparing a first mixture and a step of preparing a second mixture. Additionally, a method for manufacturing an electrode (positive electrode 4) is provided, which, in addition to the steps in the electrode sheet manufacturing method, includes a step of fabricating the electrode film 20 using the electrode sheet. A method for manufacturing a secondary battery is also provided, which, in addition to the electrode manufacturing method, includes a secondary battery manufacturing step.

[0095] The following describes embodiments that embody the disclosure of this specification, but these embodiments are for illustrative purposes only and are not limiting.

[0096] In this embodiment, the mixture used to prepare the electrode film for the positive electrode of the lithium-ion secondary battery is prepared by the following method, an electrode sheet is made, and its powder resistance is measured.

[0097] In the preparation of the mixture, LFP (lithium iron phosphate) is used as olivine-type compound particles with carbon coating, MW-CNT is used as a conductive additive, PEO1 (20% crystallinity, melting point about 60°C) and PEO2 (50% crystallinity, melting point about 60°C) are used as the first polymer, and PTFE is used as the second polymer.

[0098] Example 1

[0099] In Example 1, a first mixture containing active material particles, a conductive additive, and a first polymer but excluding a second polymer was prepared for the electrode film. Subsequently, a second mixture containing the second polymer was further prepared as a mixture for the electrode film. Furthermore, the mass ratio of the materials in the second mixture was set such that the active material particles / conductive additive / PEO1 / PTFE ratio was 94.1 / 1.5 / 1.0 / 3.4.

[0100] First, the active material particles (LFP), conductive additive (MW-CNT), and first polymer (PEO) are mixed in the above mass ratio using a kneader under mixing conditions of 80°C, 60 rpm, and 600 seconds to obtain the first mixture.

[0101] Next, the second polymer (PTFE) is added to the first mixture in the manner described above in terms of the mass ratio, and the mixture is mixed using the kneader at 160°C, 60 rpm and 30 seconds to obtain the second mixture as a mixture for electrode film.

[0102] The second mixture was further calendered using a calendering apparatus at 160°C, a roll gap of 340 μm, a roll 1 speed of 1.8 m / s, and a roll 2 speed of 2.4 m / s to obtain an electrode sheet. For a sample prepared from this electrode sheet using a predetermined method, the impedance was measured according to conventional methods, thereby obtaining the lithium-ion diffusion resistance. The results are shown in Table 1.

[0103] Example 2

[0104] In the first mixture, PEO2 was used instead of PEO1 as the first polymer, and the operation was otherwise the same as in Example 1 to obtain the lithium-ion diffusion resistance.

[0105] Comparative Example 1

[0106] As Comparative Example 1, instead of using the first polymer (PEO) in the first mixture, the mass ratio of active material particles was increased to replace the first polymer. All other things being done under the same mixing conditions as in Example 1, the first mixture, the second mixture, and the electrode sheet of Comparative Example 1 were obtained, and the lithium-ion diffusion resistance was obtained in the same manner as in Example 1. Furthermore, in Comparative Example 1, the mass ratio of each material in the second mixture (active material particles / conductive additive / PTFE) was 95.1 / 1.5 / 3.4. The results are shown in Table 1.

[0107] Table 1

[0108]

[0109] As shown in Table 1, Example 1, which used PEO1 with a crystallinity of 20%, exhibited the lowest lithium-ion diffusion resistance, and Example 2, which used PEO2 with a crystallinity of 50%, also showed a low lithium-ion diffusion resistance. In contrast, Comparative Example 1, which did not use PEO, showed the highest lithium-ion diffusion resistance.

[0110] As can be seen from the above, using lithium-ion conductive polymers, and further using PEO with a crystallinity of 20% or more and 50% or less, is effective in suppressing the lithium-ion diffusion resistance in the electrode sheet.

[0111] Furthermore, in Example 1, the cross-section of the electrode sheet was observed using a transmission electron microscope. The results showed that PEO1, as the first polymer, was composited within the electrode film by covering a portion of the surface of the active material particles. Additionally, PTFE, as the second polymer, was dispersed substantially uniformly relative to the carbon-coated active material particles and CNTs, which served as a conductive additive. Furthermore, PTFE constrained the active material particles from the outside in a fibrous winding manner.

[0112] As described above, a first mixture is prepared by premixing active material particles and conductive additives using a first polymer such as PEO, which has lithium-ion conductivity and can be melted and flowable. Then, a second mixture is prepared by adding a second polymer such as PTFE, which can be fibrillated, to form an electrode film mixture, thus improving the lithium-ion conductivity of the electrode sheet. In other words, this manufacturing method can achieve a composite form that effectively utilizes the lithium-ion conductivity of PEO. As a result, a self-supporting electrode film can be effectively formed even without increasing the amount of binder, and the lithium-ion diffusion in the electrode sheet can be improved.

[0113] Furthermore, it has been found that by keeping the crystallinity of the first polymer with lithium-ion conductivity within a predetermined range, for example, PEO content of 50% or less, the lithium-ion diffusion resistance is reduced. In particular, it has been found that a crystallinity of PEO of 20% or less is more effective.

[0114] The technical elements described in this specification or drawings are technically useful individually or in various combinations, but are not limited to the combinations described in the claims at the time of application.

Claims

1. A self-supporting electrode film, comprising: Positive electrode active material particles, Conductive additives The first polymer with lithium-ion conductivity, and The second polymer has a fibrillated form.

2. The self-supporting electrode film according to claim 1, The first polymer is polyethylene oxide with a crystallinity of more than 10% and less than 50%.

3. The self-supporting electrode film according to claim 2, Part of the surface of the positive electrode active material particles is covered by the first polymer.

4. The self-supporting electrode film according to claim 3, The second polymer comprises polytetrafluoroethylene.

5. An electrode comprising a self-supporting electrode film according to any one of claims 1 to 4.

6. A secondary battery comprising the electrode as described in claim 5.

7. A method for manufacturing an electrode sheet for a self-supporting electrode film, comprising the following steps: A first mixture is prepared by mixing positive electrode active material particles, conductive additives, and a first polymer that is lithium-ion conductive and not fibrillated, in a fluidized state of the first polymer. The first mixture is mixed with a fibrillable second polymer to prepare a second mixture having fibrillated the second polymer; and The electrode sheet was made using the second mixture.

8. The method for manufacturing the electrode sheet according to claim 7, The first polymer comprises polyethylene oxide with a crystallinity of more than 10% and less than 50%.

9. A method for manufacturing an electrode with a self-supporting electrode film, comprising the following steps: A first mixture is prepared by mixing positive electrode active material particles, conductive additives, and an unfibrillated first polymer in a fluidized state. The first mixture is mixed with a fibrillable second polymer to prepare a second mixture having the second polymer fibrillated. The second mixture is used to fabricate electrode sheets for the self-supporting electrode film; and The electrode is fabricated by bonding the electrode sheet to the current collector, thereby forming the electrode having the self-supporting electrode film.

Images