Self-supporting electrode film and method for manufacturing the same

Abstract

A technique is provided to suppress or avoid the rise of powder resistance in a self-supporting electrode film. The self-supporting electrode film (4) comprises: active material particles (6) containing olivine-type positive electrode material having an electronically conductive material film 8, a conductive additive (10), a first polymer having a morphology covering the electronically conductive material film, and a second polymer having a fibrillated morphology.

Description

Technical Field

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

[0002] In recent years, it has been known to manufacture self-supporting electrode films using dry processes. For example, a self-supporting electrode film containing a composite binder of polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO) in addition to polytetrafluoroethylene (PTFE) is known (Patent Document 1). Patent Document 1 describes the preparation of a mixture for an electrode film containing a composite binder, active material particles, and conductive additives, followed by the addition of a small amount of solvent to the mixture, the application of shear force to fibrillate the PTFE, and then pressing to form a self-supporting electrode film.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2024-26272 Summary of the Invention

[0006] Problems solved by the invention

[0007] However, although the method in Patent Document 1 is described as a dry process, a solvent is used. Furthermore, in the dry process, when mixing the binder, active material particles, and conductive additives to prepare the electrode film mixture, large frictional forces sometimes occur between the active material particles. When the active material particles have a coating of electronically conductive materials, such coating damage occurs, and as a result, the powder resistance of the electrode film sometimes increases.

[0008] This specification provides techniques for suppressing or avoiding the rise in powder resistance in self-supporting electrode films.

[0009] Methods for solving problems

[0010] One technology disclosed in this specification relates to a self-supporting electrode film. The self-supporting electrode film comprises: active material particles containing an olivine-type positive electrode material having an electronically conductive material coating, a conductive additive, a first polymer having a morphology covering the electronically conductive material coating, and a second polymer having a fibrillated morphology.

[0011] According to the self-supporting electrode film, by giving the first polymer a morphology that covers the electronically conductive material film, damage to the electronically conductive material film can be suppressed or avoided. By giving the second polymer a fibrillated morphology, the active material particles and the conductive additives are bound together and integrated. As a result, the increase in the powder resistance of the self-supporting electrode film is suppressed or improved.

[0012] In addition, another technology disclosed in this specification is a method for manufacturing an electrode sheet for a self-supporting electrode film. The manufacturing method includes the following steps: mixing active material particles containing an olivine-type positive electrode material having an electronically conductive coating, a conductive additive, and a first polymer that does not fibrillate, wherein the first polymer is in a fluidized state, to prepare a first mixture; mixing the first mixture with a second polymer that can fibrillate, to prepare a second mixture having a state in which the second polymer is fibrillated; and using the second mixture to fabricate the electrode sheet.

[0013] When preparing the first mixture, it is prepared in a fluidized state. Therefore, the frictional force between the active material particles can be reduced, and damage to the electronically conductive material film on the surface of the active material particles can be suppressed or reduced. Furthermore, when preparing the second mixture, the active material particles with suppressed damage to the electronically conductive material and the conductive additive are bound together by the fibrillated second polymer, thus achieving integration. Therefore, the increase in powder resistance of the electrode sheet and the self-supporting electrode film can be suppressed or improved.

[0014] Furthermore, when preparing the first mixture, mixing can be performed in a fluidized state of the first polymer, thus avoiding the use of solvents. Attached Figure Description

[0015] Figure 1 A cross-sectional view illustrating an example of the structure of the electrode disclosed in this specification.

[0016] Figure 2 A process diagram illustrating an example of the method for manufacturing the electrode sheet disclosed in this specification. Detailed Implementation

[0017] The self-supporting electrode films and their manufacturing methods disclosed in this specification are as described above, and their disclosure can be carried out in the following ways.

[0018] Another aspect of the self-supporting electrode film includes the first polymer having a melting point above 60°C and below 260°C. By using such a first polymer, in the manufacturing process of the self-supporting electrode film, the first polymer melts, flows easily, and readily reduces the frictional forces between active material particles. Such a first polymer readily forms a morphology that covers the electronically conductive material.

[0019] Another embodiment of the self-supporting electrode film includes a first polymer selected from one or more of the following: polyethylene oxide, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, polyvinylidene fluoride, ethylene carbonate, polyethylene, polypropylene, polystyrene, ABS resin, polyacrylate, polymethyl methacrylate, polyvinyl alcohol, and polycarbonate. These polymers exhibit fluidity at temperatures above 60°C and below 260°C. The first polymer can be selected from one or more of the following: polyethylene oxide, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, and polyvinylidene fluoride. These polymers possess ionic conductivity and can sometimes suppress the reduction in lithium-ion conductivity.

[0020] Another embodiment of the self-supporting electrode film includes a first polymer comprising 0.1% by mass and less than 3.0% by mass relative to the total mass of the self-supporting electrode film. This effectively reduces the frictional force between the active material particles and suppresses or prevents an increase in the powder resistance of the self-supporting electrode film.

[0021] In another embodiment of the self-supporting electrode membrane, the second polymer may contain polytetrafluoroethylene. Thus, the fibrillated second polymer effectively binds the self-supporting electrode membrane, thereby integrating it.

[0022] This specification discloses electrodes having any of the self-supporting electrode films described herein. Furthermore, this specification discloses secondary batteries having said electrodes.

[0023] Another method for manufacturing the electrode sheet includes the first polymer having a melting point of 60°C or higher and 260°C or lower. By giving the first polymer a melting point within the aforementioned temperature range, heating the first polymer can fluidize it, thereby easily reducing the frictional force between the active material particles. Furthermore, in this method, the step of preparing the second mixture may include mixing the first mixture with the second polymer while the first polymer is fluidized. Thus, when preparing the second mixture, the first polymer fluidizes, reducing the frictional force between the active material particles, thereby suppressing damage to the electronically conductive material film.

[0024] This specification also discloses a method for manufacturing an electrode having a self-supporting electrode film. More specifically, the method may include the following steps: mixing active material particles containing an olivine-type positive electrode material having an electronically conductive material coating, a conductive additive, and a first polymer that does not fibrillate, wherein the first polymer is in a fluidized state, to prepare a first mixture; mixing the first mixture with a second polymer that can fibrillate, to prepare a second mixture wherein the second polymer is in a fibrillated state; using the second mixture to prepare an electrode sheet; and bonding the electrode sheet to a current collector to prepare the electrode having the self-supporting electrode film.

[0025] The following description, with reference to appropriate accompanying drawings, explains the self-supporting electrode film and electrode, as well as the manufacturing method of the electrode sheet for the self-supporting electrode film. In this specification, the self-supporting electrode film and electrode are, for example, a self-supporting electrode film and positive electrode constituting the positive electrode of a lithium-ion secondary battery.

[0026] (Self-supporting electrode film and electrode)

[0027] Figure 1 An example of a cross-section of the positive electrode 2 of a lithium-ion secondary battery (hereinafter referred to as a secondary battery) is shown. The positive electrode 2 has a self-supporting electrode film (hereinafter referred to as an electrode film) 4 and a current collector 20. The current collector 20 is not particularly limited, and examples include aluminum foil, nickel, etc. In addition, although not shown, the secondary battery has a separator containing an electrolyte and a negative electrode. The separator can be made of known materials such as a polyolefin-based microporous membrane with fine pores. The separator contains a liquid electrolyte impregnated with an organic solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), or diethyl carbonate (DEC) as a medium and containing a lithium salt such as lithium hexafluorophosphate. Alternatively, a solid electrolyte layer can be used instead. The negative electrode of the secondary battery can have an electrode film and a current collector formed of suitable known materials and structures.

[0028] like Figure 1 As shown, the electrode film 4 contains active material particles 6, conductive additive 10, first polymer 12, and second polymer 14.

[0029] (Active substance particles)

[0030] The active material particles 6 can be particles containing olivine-type cathode materials that are already known to be used. Examples of olivine-type cathode materials include lithium iron phosphate (LiFePO4, LFP) and lithium iron manganese phosphate (LiMn). （1－d） F dLithium manganese phosphate (LiMnPO4, LMP), etc., can be used as active material particles 6. One or more of these types can be used. Active material particles 6 can have any average particle size. Active material particles 6 can also have a morphology formed by secondary aggregation of individual particles.

[0031] (Electrically conductive material coating)

[0032] The active material particle 6 has an electronically conductive material coating (hereinafter referred to as the coating) 8 covering at least a portion of its surface. The coating 8 contributes to improving the electronic conductivity of the active material particle 6. The electronically conductive material contained in the coating 8 can be a known material. Examples of electronically conductive materials include carbon-based materials such as graphite and semi-graphite, and nanoscale metal particle materials.

[0033] The coating 8 is formed to cover a portion of the surface of the active material particle 6. Alternatively, if the coating 8 covers the entire surface of the active material particle 6, any method that allows for lithium ion insertion and removal is acceptable. The shape of the coating 8 is not particularly limited. When the active material particle 6 serves as the core, it can be a generally uniform coating or a coating formed by attaching a carbon-based material of any shape to the surface of the active material particle 6. Examples of the former include carbon-based material coatings obtained by carbonizing organic matter attached to the surface of the active material particle 6. Examples of the latter include coatings obtained by attaching carbon-based materials to the surface of the active material particle 6 through mechanochemical mixing or the like.

[0034] The thickness of the film 8 is not particularly limited. For example, it can be 0.1 nm or more and 10 nm or 0.5 nm or more and 3 nm or less.

[0035] (Conductive additive)

[0036] The conductive additive 10 is an additive used to improve the electronic conductivity of the electrode film 4. The conductive additive 10 is dispersed and held in the electrode film 4, for example, it may have a form that is interposed between the active material particles 6 coated by the first polymer 12, or it may be attached to and dispersed on their surface.

[0037] There are no particular limitations on the conductive additive 10, and various known conductive additives can be used. Examples of conductive additive 10 include graphite (natural graphite, artificial graphite, etc.); carbon black such as acetylene black, Ketjen black, channel black, furnace black, lampblack, and Sama 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. 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.

[0038] The content of conductive additive 10 in electrode film 4 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 4. Alternatively, it may be, for example, 1.0% by mass or more and 3.0% by mass or 1.0% by mass or 2.0% by mass or less.

[0039] (Polymer 1)

[0040] The first polymer 12 may be contained in the electrode film 4 in a form that covers the coating 8. The first polymer 12 may cover part or all of the coating 8 without impairing its function.

[0041] By having the first polymer 12 in a form that covers the coating 8, damage to the electrode film 4 can be suppressed not only for the coating 8 but also for the active material particles 6. This can be understood as follows: by including the first polymer 12 in the electrode film 4 in this form, when mixed with the conductive additive 10, the frictional force between the active material particles 6 can be reduced, thereby suppressing damage to the coating 8. Furthermore, damage to the active material particles 6 themselves can also be suppressed. Therefore, by including the first polymer 12 in this form, the decrease in the powder resistance of the electrode film 4 can be suppressed.

[0042] Since the first polymer 12 is a film 8 covering the surface of the active material particles 6, it is a non-fibrillated polymer. Furthermore, the first polymer 12 preferably has a melting point of 60°C or higher and 260°C or lower. Having a melting point within this temperature range facilitates the flowability of the first polymer 12, effectively reducing the frictional forces between the active material particles 6.

[0043] Furthermore, the melting point of the first polymer 12 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. Regarding the range of the melting point of the first polymer 12, in addition to the ranges mentioned above, it can be set by appropriately combining the aforementioned lower and upper limits of temperature. 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.

[0044] The first polymer 12 is not particularly limited as long as it can be fluidized, for example, in a manner that reduces the friction between the active material particles 6 by covering them with a film 8. Examples of the first polymer 12 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 12 can also function as a binder in the electrode film 4.

[0045] The first polymer 12 can be one or more selected from PEO, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, and polyvinylidene fluoride. These polymers have ionic conductivity and can suppress the reduction of lithium-ion conductivity.

[0046] Here, the aforementioned polymers comprise, for example, homopolymers and copolymers formed from basic structural units based on the polymer name and structural units modified from those basic structural units. For example, PEO comprises homopolymers and copolymers having structural units derived from ethylene oxide and / or modified structural units thereof, which 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 suitably obtained or manufactured by them.

[0047] For example, as the first polymer 12, a homopolymer of ethylene oxide structural units, such as PEO, can be used, with a number average molecular weight of 3,000 or more and 10,000,000 or less. Furthermore, 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.

[0048] The content of the first polymer 12 in the electrode film 4 is not particularly limited, as long as it can suppress damage to the coating 8. If the content of the first polymer 12 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 4, it is sometimes effective in reducing friction during the fabrication of the electrode film 4, protecting the coating 8, and suppressing the decrease in powder resistance of the electrode film 4. Furthermore, its content is, for example, 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, its content is 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. Regarding the range of the content of the first polymer 12, in addition to the above ranges, their lower and upper limits can be appropriately combined and set. For example, 0.7% by mass or more and 3.0% by mass or less, 0.8% by mass or more and 2.0% by mass or less, 0.8% by mass or more and 1.5% by mass or less, and 0.8% by mass or more and 1.2% by mass or less.

[0049] (Polymer 2)

[0050] The electrode membrane 4 contains a second polymer 14. In the electrode membrane 4, the second polymer 14 has a fibrillated morphology. For example, the second polymer 14 exists in a fibrillated state on the outside of the active material particles 6 coated by the membrane 8 in a fibrous interweaving manner.

[0051] As the second polymer 14, a fibrillable polymer is used. If a shear force is applied to the second polymer 14, the second polymer 14 undergoes fibrillation. The fibrillated second polymer 14 interweaves the active material particles 6 and the conductive additive 10 together, thereby binding them together and imparting mechanical strength to the electrode film 4.

[0052] Regarding the second polymer 14, any polymer capable of fibrillation can be used without limitation. Fibrous polymers are well known to those skilled in the art. Examples of the second polymer 14 include, for instance, polytetrafluoroethylene (PTFE), polyolefins, polyalkylene compounds, polyethers, styrene-butadiene copolymers, polysiloxanes and copolymers of polysiloxanes, branched polyethers, polyvinyl ethers, and copolymers thereof. Cellulose containing carboxyalkyl cellulose such as carboxymethyl cellulose (CMC) can also be listed as the second polymer 14. One or more of these can be used as the second polymer 14. For example, PTFE is sometimes preferred for the second polymer 14. This is because PTFE has excellent fibrillability and adhesive properties.

[0053] The second polymer 14 is preferably a polymer capable of fibrillation at temperatures above 60°C and below 260°C. This temperature range coincides with the preferred melting point temperature range of the first polymer 12. Since the second polymer 14 can fibrillate within the flow range of the first polymer 12, a lubricating effect from the first polymer 12 can sometimes be expected when using the active material particles 6 of the second polymer 14 in a composite. Furthermore, the second polymer 14 preferably has heat resistance at the flow temperature of the first polymer 12. From this viewpoint, the second polymer 14 preferably has a melting point exceeding 260°C.

[0054] The content of the second polymer 14 in the electrode film 4 is not particularly limited, as long as it can bind the electrode film 4 together and form an integral unit. Regarding the content of the second polymer 14, for example, it is 1.0% by mass or more and 10% by mass or less relative to the total mass of the electrode film 4. Alternatively, its content is, for example, 1.5% by mass or more, 2.0% by mass or more, 2.5% by mass or more, and 3.0% by mass or more. Alternatively, its content is 8.0% by mass or less, 6.0% by mass or less, and 5.0% by mass or less. Regarding the range of the content of the second polymer 14, in addition to the above ranges, their lower and upper limits can be appropriately combined and set. For example, its content range is 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, and 2.5% by mass or more and 4.5% by mass or less.

[0055] In addition, the electrode film 4 may contain, as needed, a polymer that acts as a binder between the active material particles 6, the conductive additive 10, and the current collector 20. Regarding this polymer, for example, in addition to the first polymer 12 and the second polymer 14 described above, known polymers suitable for this type of electrode can be used. These polymers may be in particulate form, etc.

[0056] The electrode film 4 described above has a thickness of approximately 10 μm or more and 500 μm or less, and together with the current collector 20, it constitutes the positive electrode 2.

[0057] According to the positive electrode 2, in the electrode film 4, the first polymer 12 has a morphology that covers the coating 8. This morphology can suppress damage to the coating 8 and inhibit the increase in powder resistance. It is understood that, due to this morphology of the first polymer 12, when the mixture for the electrode film 4 is prepared, the first polymer 12 exists in the form of covering the coating 8, which can reduce the frictional force between the active material particles 6. Therefore, even if the electrode film 4 is obtained by a dry process using a fibrillated second polymer 14, damage to the coating 8 can be suppressed in this process.

[0058] Furthermore, in the electrode film 4, the second polymer 14 is dispersed substantially uniformly relative to the active material particles 6 and the conductive additive 10 having the coating 8. Additionally, the coating 8 of the active material particles 6 can be bound from its outer side in a fibrous interweaving manner. This suppresses the increase in powder resistance.

[0059] The following is a suitable reference. Figure 2 The method for manufacturing the electrode sheet used to manufacture such an electrode film 4 will be described. Furthermore, the electrode sheet is laminated onto the current collector 20 and integrated through appropriate hot pressing or the like, thereby forming the electrode film 4. Figure 2 An example of a method for manufacturing an electrode sheet is shown.

[0060] 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 an electrode film 4 using the second mixture.

[0061] (Step S10: Preparation of the first mixture)

[0062] Step S10 includes: mixing active material particles 6 having a coating 8, conductive additive 10, and a non-fibrillated first polymer 12 to prepare a first mixture. Preferably, the first mixture does not contain a fibrillable second polymer 14. If the second polymer 14 is present, the protective and lubricating properties of the coating 8 provided by the first polymer 12 may be reduced. As a result, the first polymer 12 may have difficulty forming the morphology of the coating 8, and damage to the coating 8 may easily occur. Furthermore, the dispersion uniformity of the second polymer 14 and the conductive additive 10 may be reduced.

[0063] The first mixture contains active material particles 6, conductive additive 10, and a first polymer 12, and is a mixture having at least the first polymer 12 covering the surface of the active material particles 6, i.e., the surface of the electronically conductive material film 8. In step S10, the first mixture can be obtained by mixing these materials in a fluidized state while the first polymer 12 is in a fluidized state. For example, when using a polymer with a melting point of 60°C or higher and a temperature of 260°C or lower for the first polymer 12, the first mixture can be obtained by mixing 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.

[0064] By melting and fluidizing the first polymer 12, the friction between the active material particles 6 is reduced. This effect is believed to be due to the first polymer 12's morphology during mixing, which covers the film 8. By fluidizing the first polymer 12, it acts as a lubricant, facilitating mixing even without a solvent, when mixing the active material particles 6 and the conductive additive 10. Consequently, damage to the film 8 can be suppressed when mixed with the conductive additive 10.

[0065] Step S10 can include multiple stages as long as the first mixture can be obtained. For example, the active material particles 6 and the first polymer 12 can be mixed in advance while the first polymer 12 is in a fluidized state to form a shape in which the first polymer 12 covers the surface of the film 8. Then, while the first polymer 12 is in a fluidized state, the conductive additive 10 is added and mixed. This can effectively suppress the strong frictional force applied to the film 8.

[0066] When preparing the first mixture, the conductive additive 10 and the first polymer 12 can be used within the aforementioned content range relative to the total mass of the final second mixture. Furthermore, the amount of active material particles 6 can be used such that, relative to the total mass of the second mixture, it constitutes the remainder other than the conductive additive 10, the first polymer 12, and the second polymer 14.

[0067] Step S10 can be carried out using a known mixing apparatus. Examples of such mixing apparatus include, in addition to various kneaders such as pressure kneaders (which have rollers, rotors, blades, gears, screws, etc. as rotating bodies), Banbury mixers, Henschel mixers, twin-screw extruders, and other known mixing apparatuses suitable for shear mixing, including jet mills, roller mills, hammer mills, and other mills. Those skilled in the art can appropriately set the shear force and mixing time to obtain a suitable mixing state between the active material particles 6 and the conductive additive 10.

[0068] (Step S20 of preparing the second mixture)

[0069] Step S20 includes the step of mixing the first mixture with a fibrillable second polymer 14 to modulate the second polymer 14 into a fibrillated state, resulting in a second mixture.

[0070] The second mixture is a mixture in which the second polymer 14 is in a fibrillated state, except that it contains the active material particles 6, the conductive additive 10, and the first polymer 12 from the first mixture. The second mixture can be obtained by mixing these materials in a manner that fibrillates the second polymer 14.

[0071] After preparing the first mixture, the second polymer 14 is added and mixed in a fibrillating manner to obtain a second mixture having the structure in or similar to that of the electrode film 4. That is, in the second mixture, the first polymer 12 has a film 8 covering the active material particles 6, and the second polymer 14 has a fibrillated form. Furthermore, the second polymer 14 binds the active material particles 6 from their outer side in a fibrous interweaving manner.

[0072] Step S20 is preferably carried out in the same state as step S10, where the first polymer 12 is molten and fluidized. That is, based on the melting point of the first polymer 12, mixing is preferably carried out at a temperature of 60°C or higher and 260°C or lower. Thus, in step S20, the first polymer 12 also functions as a lubricant, which facilitates mixing in step S20 and can suppress damage to the coating 8.

[0073] When preparing the second mixture, the second polymer can be used in a manner that falls within the aforementioned content range, relative to the total mass of the final second mixture to be obtained.

[0074] Step S20 is carried out using a known mixing apparatus, just like step S10. Those skilled in the art can appropriately set mixing conditions such as shear force and mixing time to obtain the fibrillation of the second polymer 14 and a suitable mixing state of the mixed materials.

[0075] (S30: Fabrication process of electrode sheet for electrode film)

[0076] Step S30 includes the step of using the second mixture to prepare an electrode sheet for manufacturing the electrode film 4. The electrode sheet has a thickness of approximately 10 μm or more and 2000 μm or less, or approximately 10 μm or more and 1000 μm or less. There are no particular limitations on the sheet-forming process. For example, a calendering process can be used where the second mixture is fed to rotating rollers without a support, and sheet-forming is performed while hot pressing. Alternatively, a hot pressing process can be used where the electrode mixture is fed to a suitable forming die.

[0077] The resulting electrode sheet has the same active material particles 6, conductive additive 10, first polymer 12, and second polymer 14 as the aforementioned electrode film 4, and possesses a specified mechanical strength. Therefore, it can itself constitute an electrode film 4 that is self-supporting without a support.

[0078] Furthermore, using such an electrode sheet, a positive electrode 2 and a secondary battery can be fabricated. First, the current collector 20 is stacked with the electrode sheet. Then, the electrode sheet is compacted by applying pressure at a specified temperature, etc., to make the thickness approximately 10 μm or more and 500 μm or less, thereby forming an electrode film 4. Simultaneously with obtaining the electrode film 4, a positive electrode 2 integrating the electrode film 4 and the current collector 20 can be obtained. The secondary battery having the positive electrode 2 can be fabricated using appropriate known methods.

[0079] The electrode film 4 obtained in this way can suppress damage to the coating 8 on the surface of the active material particles 6 and suppress the loss of electronic conductivity. Therefore, a positive electrode 2 that can perform well and a secondary battery having a positive electrode 2 can be obtained.

[0080] Although the electrode film 4 has been described above, according to this specification, in addition to the electrode sheet that serves as a precursor to the electrode film 4, a positive electrode 2 having the electrode film 4 and a secondary battery having the positive electrode 2 are also provided. Furthermore, while the manufacturing method of the electrode sheet for the electrode film 4 has been described above, according to this specification, a method for manufacturing a mixture for the electrode film, including a step of preparing a first mixture and a step of preparing a second mixture, is also provided. Additionally, a method for manufacturing an electrode, including a step of using the electrode sheet to fabricate the electrode film 4, is also provided, in addition to the steps in the electrode sheet manufacturing method. Besides the electrode manufacturing method, a method for manufacturing a secondary battery, including a secondary battery fabrication step, is also provided.

[0081] This specification explicitly includes the following components.

[0082] [1] A self-supporting electrode film, which has the following characteristics:

[0083] Active material particles containing olivine-type cathode material with an electronically conductive coating,

[0084] Conductive additives

[0085] A first polymer having a morphology covering the electronically conductive material film, and

[0086] The second polymer has a fibrillated morphology.

[0087] [2] As described in [1], the first polymer has a melting point of 60°C or higher and 260°C or lower.

[0088] [3] As described in [1] or [2], the first polymer is one or more selected from polyoxyethylene, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, polyvinylidene fluoride, ethylene carbonate, polyethylene, polypropylene, polystyrene, ABS resin, polyacrylate, polyvinyl alcohol and polycarbonate.

[0089] [4] The self-supporting electrode film as described in any one of [1] to [3] contains, relative to the total mass of the self-supporting electrode film, 0.1% by mass and 3.0% by mass of the first polymer.

[0090] [5] The self-supporting electrode membrane as described in any one of [1] to [4], wherein the second polymer contains polytetrafluoroethylene.

[0091] [6] An electrode having the self-supporting electrode film described in any one of [1] to [5].

[0092] [7] A secondary battery having the electrodes described in [6].

[0093] [8] A method for manufacturing an electrode sheet for a self-supporting electrode film, the method comprising the following steps:

[0094] The process of preparing a first mixture by mixing active material particles containing olivine-type positive electrode material with an electronically conductive material coating, a conductive additive, and a non-fibrillated first polymer in a fluidized state.

[0095] The process of mixing the first mixture with a fibrillable second polymer to prepare a second mixture in which the second polymer is in a fibrillated state; and

[0096] The process of making the electrode sheet using the second mixture.

[0097] [9] The manufacturing method as described in [8], wherein the first polymer has a melting point of 60°C or higher and 260°C or lower.

[0098]

[10] The manufacturing method as described in [8] or [9] includes the step of preparing the second mixture by mixing the first mixture with the second polymer while the first polymer is in a fluidized state.

[0099]

[11] A method for manufacturing an electrode having a self-supporting electrode film, the method comprising the following steps:

[0100] The process of preparing a first mixture by mixing active material particles containing olivine-type positive electrode material with an electronically conductive material coating, a conductive additive, and a non-fibrillated first polymer in a fluidized state.

[0101] The process of mixing the first mixture with a second polymer capable of fibrillation to prepare a second mixture in which the second polymer is in a fibrillated state;

[0102] The process of using the second mixture to fabricate an electrode sheet for the self-supporting electrode film; and

[0103] The process of bonding the electrode sheet to a current collector to fabricate the electrode having the self-supporting electrode film.

[0104] Example

[0105] The following describes embodiments that embody the disclosure of this specification. These embodiments are used to illustrate the disclosure of this specification, but do not limit the disclosure of this specification.

[0106] In this embodiment, the electrode film mixture for the positive electrode of a lithium-ion secondary battery is prepared by the following method to fabricate an electrode sheet, and its powder resistance is measured.

[0107] When preparing the mixture, LFP (lithium iron phosphate) is used as the olivine-type cathode material (active material particles) with carbon coating, MW-CNT is used as the conductive additive, PEO (number average molecular weight of about 20,000, melting point of 60°C) is used as the first polymer, and PTFE is used as the second polymer.

[0108] (Example 1)

[0109] As the electrode film mixture of Example 1, a first mixture containing active material particles, a conductive additive, and a first polymer but not a second polymer was prepared, and then a second mixture further containing the second polymer was prepared as the electrode film mixture. Furthermore, the mass ratio of each material in the second mixture was set as follows: active material particles / conductive additive / PEO / PTFE = 94.1 / 1.5 / 1.0 / 3.4.

[0110] First, the active material particles (LFP), conductive additive (MW-CNT), and first polymer (PEO) are combined according to the above mass ratio and mixed using a kneader under mixing conditions of 80°C, 60 rpm, and 600 seconds to obtain the first mixture.

[0111] 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.

[0112] The second mixture was then formed using a calendering apparatus at 160°C, a roll gap of 340 μm, a roll speed of 1.8 m / s, and a roll speed of 2.4 m / s to obtain an electrode sheet. A sample prepared from this electrode sheet using a prescribed method was then used to determine the volume resistivity (Ω·cm) according to conventional methods for powder resistivity measurement. The results are shown in Table 1. For reference, the total electrical force required to obtain the first and second mixtures in the kneader is also shown as the processing energy.

[0113] (Comparative Example 1)

[0114] As Comparative Example 1, the first polymer (PEO) was not used in the first mixture. Instead, the mass ratio of active material particles was increased. All other things being done, the same mixing conditions as in Example 1 were applied to obtain the first mixture, the second mixture, and the electrode sheet of Comparative Example 1. The volume resistivity was measured 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.

[0115] (Comparative Example 2)

[0116] As Comparative Example 2, the active material particles / conductive additives / PEO / PTFE of all materials were mixed from the beginning at the mass ratio of the second mixture (94.1 / 1.5 / 1.0 / 3.4), under the mixing conditions of the first mixture (80°C, 60 rpm, and 600 seconds) and the subsequent mixing conditions of the second mixture (160°C, 60 rpm, and 30 seconds). Electrode sheets were obtained using the same method as in Example 1. The volume resistivity was measured in the same manner as in Example 1. The results are shown in Table 1.

[0117] In addition, in Examples 1, 1, and 2, the mixing was carried out at 80°C, which is above the melting point of the first polymer (PEO), so the first polymer (PEO) was in a molten and fluidized state throughout the entire mixing process.

[0118] Table 1

[0119]

[0120] As shown in Table 1, Example 1 exhibits the lowest volume resistivity and requires the least processing energy. Furthermore, in Example 1, cross-sections of the electrode sheet observed using transmission electron microscopy revealed that PEO, the first polymer, has a carbon coating covering the active material particles, while PTFE, the second polymer, is dispersed approximately uniformly relative to the carbon-coated active material particles and CNTs, which serve as a conductive additive. Moreover, PTFE binds the active material particles from the outside in a fibrous, interwoven manner.

[0121] In contrast, in Comparative Example 1, the volume resistivity increased significantly, and the processing energy also increased. This indicates that in the absence of the first polymer (PEO), the mixing generates large shear forces, and the carbon coating in the active material particles is damaged by the phased modulation of the second mixture.

[0122] Furthermore, in Comparative Example 2, the volume resistivity became even higher than in Comparative Example 1, and the processing energy also increased. This is believed to be because, when using the first polymer (PEO), by mixing it together with the second polymer (PTFE), the lubricating function of the first polymer's fluidization could not be fully utilized, which promoted damage to the carbon coating on the surface of the active material particles. In addition, the dispersibility of the conductive additives was also reduced.

[0123] As can be seen from the above, by using a first polymer such as PEO that can be melted and flowed, pre-mixing the active material particles with conductive additives to prepare a first mixture, and then adding a second polymer such as PTFE that can be fibrillated to prepare a second mixture, a mixture for electrode film can be formed. This can suppress damage to the carbon coating and obtain electrode sheets and electrode films that ensure the dispersibility of conductive additives.

[0124] The technical elements described in this specification or drawings may exert their technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of application. Furthermore, the technologies exemplified in this specification or drawings are technologies that simultaneously achieve multiple objectives, and the solution for achieving one of these objectives is itself technically useful.

[0125] Explanation of reference numerals in the attached figures

[0126] 2 Positive electrode, 4 Self-supporting electrode film, 6 Active material particles, 8 Electron-conductive material film, 10 Conductive additive, 12 First polymer, 14 Second polymer, 20 Current collector

Claims

1. A self-supporting electrode film, comprising: Active material particles containing olivine-type cathode material with an electronically conductive coating, Conductive additives A first polymer having a morphology covering the electronically conductive material film, and The second polymer has a fibrillated morphology.

2. The self-supporting electrode film as claimed in claim 1, wherein the first polymer has a melting point of 60°C or higher and 260°C or lower.

3. The self-supporting electrode film as described in claim 2, wherein the first polymer is selected from one or more of the following: polyoxyethylene, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, polyvinylidene fluoride, ethylene carbonate, polyethylene, polypropylene, polystyrene, ABS resin, polyacrylate, polyvinyl alcohol, and polycarbonate.

4. The self-supporting electrode film as claimed in claim 1, wherein the first polymer comprises 0.1% by mass and 3.0% by mass or more relative to the total mass of the self-supporting electrode film.

5. The self-supporting electrode film as claimed in claim 1, wherein the second polymer contains polytetrafluoroethylene.

6. An electrode having the self-supporting electrode film of claim 1.

7. A secondary battery having the electrode as described in claim 6.

8. A method for manufacturing an electrode sheet for a self-supporting electrode film, the method comprising the following steps: The process of preparing a first mixture by mixing active material particles containing olivine-type positive electrode material with an electronically conductive material coating, a conductive additive, and a non-fibrillated first polymer in a fluidized state. The process of mixing the first mixture with a fibrillable second polymer to prepare a second mixture in which the second polymer is in a fibrillated state; and The process of making the electrode sheet using the second mixture.

9. The manufacturing method according to claim 8, wherein the first polymer has a melting point of 60°C or higher and 260°C or lower.

10. The manufacturing method of claim 8, wherein the step of preparing the second mixture comprises mixing the first mixture with the second polymer while the first polymer is in a fluidized state.

11. A method for manufacturing an electrode having a self-supporting electrode film, the method comprising the following steps: The process of preparing a first mixture by mixing active material particles containing olivine-type positive electrode material with an electronically conductive material coating, a conductive additive, and a non-fibrillated first polymer in a fluidized state. The process of mixing the first mixture with a fibrillable second polymer to prepare a second mixture having a fibrillated state; The process of using the second mixture to fabricate an electrode sheet for the self-supporting electrode film; and The process of bonding the electrode sheet to a current collector to fabricate the electrode having the self-supporting electrode film.

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