Self-supporting electrode film and method for manufacturing the same

Abstract

To provide a self-supporting electrode that effectively improves the output performance of secondary batteries. [Solution] A self-supporting electrode film is provided, comprising positive electrode active material particles, a first polymer having lithium ion conductivity, and a second polymer having a fibrillated form.

Description

Technical Field

[0001] The technology disclosed in this specification relates to a self-supporting electrode film and a method for manufacturing the same.

Background Art

[0002] In recent years, self-supporting electrode films have been manufactured by dry processes. For example, a self-supporting electrode film containing polytetrafluoroethylene (PTFE) is known (Patent Document 1). In Patent Document 1, while using PTFE as the main binder, a composite binder combining polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), etc. is used to improve the binding performance. In this method, it is described that a mixture of an active material, a conductive assistant, and the composite binder is prepared, shear force is applied to fibrillate PTFE, and then it is pressed into a self-supporting electrode film.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the method of Patent Document 1, while using PTFE as the main binder, PVDF or CMC is used to improve the binding performance. However, this type of polymer has low lithium ion conductivity, and the output of the battery may decrease.

[0005] This specification provides a self-supporting electrode that can effectively improve the output performance of a secondary battery and a method for manufacturing the same.

Means for Solving the Problems

[0006] The technologies disclosed herein are embodied in the following self-supporting electrodes and methods for manufacturing the same. [1] Positive electrode active material particles and Conductive additives, A first polymer having lithium ion conductivity, A second polymer having a fibrillated form, A self-supporting electrode film equipped with the following features. [2] The self-supporting electrode film according to [1], wherein the first polymer is polyethylene oxide having a crystallinity of 10% or more and 50% or less. [3] The self-supporting electrode film according to [1] or [2], wherein a portion of the positive electrode active material particles is coated with the first polymer. [4] The second polymer comprises polytetrafluoroethylene, and the self-supporting electrode film is as described in any one of [1] to [3]. An electrode comprising a self-supporting electrode film as described in any of [5][1] to [4]. A secondary battery comprising the electrodes described in [6][5]. [7] A method for manufacturing an electrode sheet for a self-supporting electrode film, The process involves: preparing a first mixture by mixing positive electrode active material particles, a conductive additive, and a first polymer that has lithium ion conductivity and does not fibrillate, while the first polymer is in a fluid state; preparing a second mixture by mixing the first mixture with a second polymer that can be fibrillated, in which the second polymer is in a fibrillated state; and manufacturing the electrode sheet using the second mixture. A manufacturing method that includes the following features. [8] The first polymer comprises polyethylene oxide with a crystallinity of 10% or more and 50% or less, as described in [7]. [9] A method for manufacturing an electrode comprising a self-supporting electrode film, A step of preparing a first mixture by mixing positive electrode 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, while the first polymer is in a fluid state, A step of mixing the first mixture with a fibrillable second polymer to prepare a second mixture having the second polymer in a fibrillated state, A step of preparing an electrode sheet for the self-supporting electrode film using the second mixture, A manufacturing method comprising the step of joining the electrode sheet to a current collector to produce the electrode having the self-supporting electrode film.

[0007] According to the self-supporting electrode film, since the first polymer has lithium ion conductivity, a self-supporting electrode film can be constructed by fibrillation without reducing the lithium ion conductivity of the positive electrode.

[0008] Furthermore, according to the method for manufacturing the electrode sheet, since the first polymer has lithium ion conductivity, it is possible to knead active materials and the like without reducing the lithium ion conductivity of the electrode film, thereby improving the binding properties in the self-supporting electrode film due to fibrillation.

[0009] During the preparation of the first mixture, the first polymer is prepared in a fluid state. As a result, the first polymer and the positive electrode active material particles come into good contact, and the first polymer coats a portion of the surface of the positive electrode active material particles. Furthermore, during the preparation of the second mixture, the first polymer is bound and integrated with the positive electrode active material particles by the fibrillated second polymer, which is well mixed with the positive electrode active material particles. As a result, electrode sheets and self-supporting electrode films can be manufactured while maintaining lithium ion conductivity. [Brief explanation of the drawing]

[0010] [Figure 1] This diagram schematically shows an example of a lithium-ion secondary battery cell. [Figure 2] This is a schematic cross-sectional view showing an example of the structure of a positive electrode disclosed herein. [Figure 3] This figure schematically illustrates an example of the form in which active material particles and the like exist in an electrode film disclosed herein. [Figure 4]This is a process diagram schematically showing an example of a method for manufacturing an electrode sheet disclosed in this specification.

Embodiments for Carrying Out the Invention

[0011] Hereinafter, with appropriate reference to the drawings, a self-supporting electrode film and an electrode, and a method for manufacturing an electrode sheet for the self-supporting electrode film will be described. In this specification, the self-supporting electrode film and the electrode are, for example, the self-supporting electrode film and the positive electrode constituting the positive electrode of a lithium-ion secondary battery.

[0012] (Self-supporting electrode film and electrode) FIG. 1 schematically shows an example of a cell 2 of a lithium-ion secondary battery (hereinafter simply referred to as a secondary battery) 100. As shown in FIG. 1, the cell 2 as a unit structure of the secondary battery 100 includes a positive electrode 4, a separator 6, and a negative electrode 8. The cell 2 further includes a positive electrode current collector 10 and a negative electrode current collector 12. The secondary battery 100 usually has a structure in which a large number of cells 2 are stacked. The separator 6 may hold, for example, a liquid or gel-like electrolyte. The separator 6 is made of a known material such as a polyolefin-based microporous membrane in which fine pores are formed. The separator 6 is impregnated with a liquid electrolyte containing a lithium salt such as lithium hexafluorophosphate with an organic solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), or diethyl carbonate (DEC) as a medium. Note that a solid electrolyte layer may be used instead of these. The negative electrode 8 has a negative electrode composite material layer 40.

[0013] FIG. 2 shows an example of a cross-section of the positive electrode 4 of the secondary battery 100. The positive electrode 4 includes a self-supporting electrode film (hereinafter also simply referred to as an electrode film) 20 and a current collector 10. The current collector 10 is not particularly limited, and examples thereof include aluminum foil and nickel foil.

[0014] As shown in FIG. 3, the electrode film 20 includes active material particles 22, a conductive assistant 26, a first polymer 30, and a second polymer 32.

[0015] (Active material particles) The active material particles 22 can use known positive electrode active material particles. For example, olivine-type compounds and layered rock salt-type oxides can be used. As the olivine-type compound, lithium iron phosphate (LiFePO4, LFP), lithium manganese iron phosphate (LiMn (1-d) F d PO4, LMFP), lithium manganese phosphate (LiMnPO4, LMP), etc. can be mentioned. As the active material particles 22, one or more of these can be used. The active material particles 22 can have an arbitrary average particle diameter. The active material particles 22 may have a form in which each particle is secondarily aggregated.

[0016] The active material particles 22 may be provided with an electron conductive material film (hereinafter, also simply referred to as a film) 24 that covers at least a part of the surface thereof. The film 24 contributes to improving the electron conductivity of the active material particles 22. As the electron conductive material contained in the film 24, known materials can be appropriately used. Examples of the electron conductive material include carbon-based materials such as graphite and semi-graphite, and nano-sized metal particle materials.

[0017] The film 24 is formed so as to cover at least a part of the surface of the active material particles 22. Even when the film 24 covers the entire surface of the active material particles 22, it is sufficient if lithium ion insertion and desorption are possible. The form of the film 24 is not particularly limited. When the active material particles 22 are used as a core, it may be a substantially uniform coating layer, or may be formed by attaching a carbon-based material of an arbitrary shape to the surface of the active material particles 22. The former includes, for example, a carbon-based material film obtained by carbonization of an organic substance applied to the surface of the active material particles 22. The latter includes a film obtained by attaching a carbon-based material to the surface of the active material particles 22 by mechanochemical mixing or the like.

[0018] The film 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.

[0019] (Conductive aid) The conductive additive 26 is an additive for improving the electronic conductivity of the electrode film 20. The conductive additive 26 is dispersed and held in the electrode film 20, but may also be in a form where it is interposed between active material particles 22 or attached to their surfaces and dispersed therein.

[0020] The conductive additive 26 is not particularly limited, and various known conductive additives can be used. Examples of conductive additives 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; carbon fluoride; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. As for carbon nanotubes, for example, single-wall carbon nanotubes (SW-CNTs) and multi-wall carbon nanotubes (MW-CNTs) including double-wall carbon nanotubes may be preferred. One or more of these can be used as the conductive additive 26.

[0021] The content of the conductive additive 26 in the electrode film 20 is not particularly limited, but for example, it is 0.01% by mass or more and 5% by mass or less of the total mass of the electrode film 20. Alternatively, for example, it may be 1.0% by mass or more and 3.0% by mass or 1.0% by mass or more and 2.0% by mass or less.

[0022] (First polymer) The first polymer 30 is a polymer having lithium ion conductivity. The first polymer 30 is not particularly limited, but examples include polyethylene oxide (PEO), polypropylene oxide, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, polyvinylidene fluoride, ethylene carbonate, polyethylene, polypropylene, polystyrene, ABS resin, polyacrylate, polymethacrylate, 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.

[0023] Here, each of the above polymers includes, for example, homopolymers and copolymers consisting of a basic structural unit based on the name of the polymer and structural units modified from the basic structural unit. For example, PEO includes homopolymers and copolymers having structural units derived from ethylene oxide and / or modified structural units thereof, and can have various forms such as linear, comb-shaped, and crosslinked. Such polymers are known to those skilled in the art and can be obtained or manufactured as appropriate.

[0024] For example, as the first polymer 30, a PEO such as a homopolymer of structural units derived from ethylene oxide with a number average molecular weight of 3,000 to 10,000,000 can be used. The number average molecular weight of such a PEO may also be, for example, 3,000 to 50,000, 8,000 to 50,000, or 10,000 to 30,000.

[0025] The first polymer 30 may be one or more selected from the group consisting of PEO, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, and polyvinylidene fluoride. These polymers have lithium ion conductivity and can suppress the decrease in lithium ion conductivity.

[0026] It is sometimes preferable to use PEO as the first polymer 30. It is sometimes preferable that the PEO has a crystallinity of 50% or less. It is also possible to further reduce the lithium ion diffusion resistance if the crystallinity is, for example, 40% or less, 30% or less, or 20% or less. It is also preferable that the crystallinity is, for example, 5% or more, 10% or more, 20% or more, or 30% or more. It is also preferable that the range of crystallinity is, for example, 10% to 50%, 20% to 50%, 10% to 30%, or 10% to 20%. Within this range, the lithium ion diffusion resistance can be reliably reduced.

[0027] The crystallinity of the polymer can be obtained by measuring it using Raman spectroscopy and XRD.

[0028] The first polymer 30 may be included in the electrode film 20 in a form that covers the active material particles 22 and / or the coating 24. The first polymer 30 may cover part or all of the coating 24 to the extent that it does not impair the function of the coating 24.

[0029] The first polymer 30 has a form that covers the active material particles 22 and / or the coating 24, which may suppress damage to the active material particles 22 as well as the coating 24 in the electrode film 20. When the first polymer 30 is included in this form, the decrease in the powder resistance of the electrode film 20 is suppressed.

[0030] The first polymer 30 is a non-fibrillating polymer. Furthermore, it is preferable that the first polymer 30 has a melting point between 60°C and 260°C. Having a melting point within this temperature range allows the first polymer 30 to easily become fluid and mix with the active material particles 22, etc.

[0031] The melting point of the first polymer 30 is, for example, 70°C or higher, 75°C or higher, 80°C or higher, 90°C or higher, and also 240°C or lower, 230°C or lower, 220°C or lower, and 200°C or lower. The range of the melting point of the first polymer 30 can be set by appropriately combining the lower and upper temperature limits described above, in addition to the above ranges. For example, 60°C or higher and 240°C or lower, 60°C or higher and 200°C or lower, 60°C or higher and 180°C or lower, and 60°C or higher and 120°C or lower.

[0032] The content of the first polymer 30 in the electrode film 20 is not particularly limited and should be such that it can suppress damage to the coating 24. For example, if the content of the first polymer 30 is 0.1% by mass or more and 3.0% by mass or less relative to the total mass of the electrode film 20, it may be effective in reducing the frictional force 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. Also, 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 be set by appropriately combining these lower and upper limits. For example, the content is 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.

[0033] (Second polymer) The electrode film 20 contains a second polymer 32. In the electrode film 20, the second polymer 32 has a fibrillated form. For example, in its fibrillated state, the second polymer 32 is present in a fibrous manner, intertwined with the outside of the active material particles 22, which may be coated with the film 24.

[0034] A fibrillable polymer is used as the second polymer 32. When a shear force is applied to the second polymer 32, it fibrillates. The fibrillated second polymer 32 entangles and restrains the active material particles 22 and the conductive additive 26, thereby imparting mechanical strength to the electrode film 20.

[0035] The second polymer 32 can be any fibrillable polymer without particular limitations. Fibrillable polymers are well known to those skilled in the art. Examples of the second polymer 32 include polytetrafluoroethylene (PTFE), polyolefins, polyalkylenes, polyethers, styrene-butadiene, polysiloxanes and polysiloxane copolymers, branched polyethers, polyvinyl ethers, and copolymers thereof. Another example of the second polymer 32 is cellulose containing carboxyalkyl cellulose, such as carboxymethylcellulose (CMC). One or more of these can be used as the second polymer 32. For example, the second polymer 32 may preferably be PTFE because it has excellent fibrillability and binder properties.

[0036] The second polymer 32 is preferably capable of fibrillation at a temperature of 60°C to 260°C. This temperature range coincides with the preferred melting point range of the first polymer 30. By allowing the second polymer 32 to fibrillate within the fluidization range of the first polymer 30, a lubricating effect from the first polymer 30 may be expected when the second polymer 32 is compounded with active material particles 22, etc. It is preferable that the second polymer 32 has heat resistance at the fluidization temperature of the first polymer 30. From this viewpoint, it is preferable that the second polymer 32 has a melting point exceeding 260°C, for example.

[0037] The content of the second polymer 32 in the electrode film 20 is not particularly limited, and it is sufficient as long as it can restrain and integrate the electrode film 20. The content of the second polymer 32 is, for example, 1.0% by mass or more and 10% by mass or less based on 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, 3.0% by mass or more, or 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 range of the second polymer 32 can be set by appropriately combining these lower and upper limits. For example, 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.

[0038] Furthermore, the electrode film 20 may also contain a polymer that functions as a binder, if necessary. Such polymers may include, for example, the first polymer 30 and the second polymer 32 described above, as well as other known polymers that can be used in this type of electrode. These polymers may be in particulate form or the like.

[0039] The electrode film 20 described above has a thickness of, for example, 10 μm to 500 μm, and together with the current collector 10, constitutes the positive electrode 4.

[0040] According to this positive electrode 4, the electrode film 20 contains a first polymer with excellent lithium ion conductivity, resulting in excellent lithium ion diffusion in secondary batteries using the electrode film 20. Furthermore, the binder function of the first polymer 30 improves the integrity of the electrode film 20. In addition, the interposition of the first polymer 30 on the surface of the active material particles 22 allows for the effective expression of lithium ion conductivity. Moreover, damage to the active material particles 22 is suppressed.

[0041] Furthermore, in the electrode film 20, the second polymer 32 is dispersed fairly uniformly among the active material particles 22 and the conductive additive 26. In addition, the active material particles 22 can be restrained by being entangled in a fibrous manner from the outside. From this point of view as well, the increase in powder resistance is suppressed.

[0042] Next, a method for manufacturing an electrode sheet for producing such an electrode film 20 will be described with reference to Figure 4 as appropriate. The electrode sheet is laminated onto the current collector 10 and integrated by heat pressing or other means as appropriate to constitute the electrode film 20. Figure 4 shows an example of a method for manufacturing an electrode sheet.

[0043] This manufacturing method comprises 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 producing an electrode sheet for the electrode film 20 using the second mixture.

[0044] (Step S10: Preparation of the first mixture) Step S10 includes mixing active material particles 22, a conductive additive 26, and a non-fibrillating first polymer 30 to prepare a first mixture. Preferably, the first mixture does not contain a fibrillating second polymer 32. The presence of the second polymer 32 may result in reduced protection of the coating 24 by the first polymer 30 and reduced overall mixed lubricity.

[0045] The first mixture may comprise active material particles 22, a conductive additive 26, and a first polymer 30, wherein at least the first polymer 30 covers the surface of the active material particles 22 (or the coating 24, if present). In step S10, the first mixture can be obtained by mixing the materials to be mixed in a state in which the first polymer 30 becomes fluid. For example, when using a polymer with a melting point of 60°C to 260°C, a suitable first mixture can be obtained by mixing under appropriate shear force at a temperature of 60°C to 260°C, which is the temperature at which the polymer melts and becomes fluid.

[0046] As the first polymer 30 melts and becomes fluid, the frictional force between the active material particles 22 is reduced. As a result, the first polymer 30 disperses uniformly with respect to the active material particles 22 and the conductive additive 26, and adheres to the surface of the active material particles 22. By using the first polymer 30 in a fluid state, it functions as a lubricant in the mixing of the active material particles 22 and the conductive additive 26 without the need for a solvent, thus facilitating mixing. As a result, damage to the active material particles 22 and the coating 24 during mixing with the conductive additive 26 may also be suppressed.

[0047] Step S10 may include multiple steps, as long as the first mixture is obtained. For example, the active material particles 22 and the first polymer 30 may be mixed in advance in a state in which the first polymer 30 is fluid to form a form in which the surface of the coating 24 is covered with the first polymer 30, and then the conductive additive 26 may be added and mixed while maintaining the fluid state of the first polymer 30. In this way, if the coating 24 is present, strong frictional force acting on the coating 24 can be effectively suppressed.

[0048] In preparing the first mixture, the conductive additive 26 and the first polymer 30 can be used within the aforementioned content range relative to the total mass of the second mixture to be finally obtained. Furthermore, the amount of active material particles 22 can be used such that it constitutes the remainder of the total mass of the second mixture, excluding the conductive additive 26, the first polymer 30, and the second polymer 32.

[0049] Step S10 can be carried out using a known mixing apparatus. Examples of known mixing apparatuses that can be used for shear mixing include various kneaders such as pressure kneaders equipped with rotating bodies such as rolls, rotors, paddles, blades, gears, and screws, as well as Banbury mixers, Henschel mixers, twin-screw extruders, and various mills such as jet mills, roller mills, and hammer mills. A person skilled in the art can appropriately set the mixing conditions such as shear force and mixing time to obtain an appropriate mixing state of the active material particles 22 and the conductive additive 26.

[0050] (Step S20: Preparation of the second mixture) Step S20 includes mixing the first mixture with a fibrillable second polymer 32 to prepare a second mixture having the second polymer 32 in a fibrillated state.

[0051] The second mixture is a mixture having active material particles 22 derived from the first mixture, a conductive additive 26, and a first polymer 30, in addition to a second polymer 32 in a fibrillated state. The second mixture can be obtained by mixing these materials in such a way that the second polymer 32 becomes fibrillated.

[0052] After preparing the first mixture, the second polymer 32 is added and mixed in a way that allows for fibrillation, thereby obtaining a second mixture having the structure of the electrode film 20 or a structure similar thereto. That is, in the second mixture, the first polymer 30 has a form that covers the film 24 of the active material particles 22, and the second polymer 32 has a fibrillated form. Furthermore, the second polymer 32 restrains the active material particles 22 by intertwining them in a fibrous manner from the outside.

[0053] Step S20 is preferably carried out in a state where the first polymer 30 is melted and fluid, similar to step S10. That is, it is preferable to mix the polymer at a temperature of 60°C to 260°C, based on the melting point of the first polymer 30. In this way, the first polymer 30 also functions as a lubricant in step S20, facilitating the mixing in step S20 and suppressing damage to the coating 24.

[0054] In preparing the second mixture, the second polymer can be used in such a way that its content falls within the range described above relative to the total mass of the second mixture to be ultimately obtained.

[0055] Step S20 can be carried out using a known mixing apparatus, similar to step S10. A person skilled in the art can appropriately set the mixing conditions, such as shear force and mixing time, to obtain fibrillation of the second polymer 32 and a proper mixing state of the materials to be mixed.

[0056] (Step S30: Manufacturing an electrode sheet for the electrode film) Step S30 includes preparing an electrode sheet for manufacturing the electrode film 20 using the second mixture. The thickness of the electrode sheet is approximately 10 μm to 2000 μm, or 10 μm to 1000 μm. The sheet formation process is not particularly limited. For example, the second mixture can be supplied between rotating rollers without using a support and formed into a sheet under heat pressure by calendering. Alternatively, the electrode mixture can be supplied to a suitable mold and pressed under heat by press forming.

[0057] The resulting electrode sheet comprises active material particles 22, a conductive additive 26, a first polymer 30, and a second polymer 32 in the same form as those in the electrode film 20 described above, and possesses a predetermined mechanical strength. Therefore, it is possible to subsequently form an electrode film 20 that can stand on its own without a support.

[0058] Furthermore, a positive electrode 4 and a secondary battery 100 can also be manufactured using these electrode sheets. First, the current collector 10 and the electrode sheet are laminated together. Then, by applying pressure at a predetermined temperature, the electrode sheet is compacted to a thickness of approximately 10 μm to 500 μm to form an electrode film 20. Simultaneously with obtaining the electrode film 20, a positive electrode 4 can be obtained in which the electrode film 20 is integrated with the current collector 10. Furthermore, a secondary battery 100 can be obtained by integrating the separator 6, the negative electrode composite layer 40, and the negative electrode current collector 12 with the positive electrode 4. The secondary battery 100 can be manufactured by any known method as appropriate.

[0059] In the electrode film 20 obtained in this way, lithium ion conductivity is imparted by the first polymer 30, so lithium ion diffusion is not reduced, and the electrode film has excellent integration properties. Therefore, by using this electrode film 20, a positive electrode 4 and a secondary battery 100 equipped with the positive electrode 4 can be obtained, which exhibit good electrode characteristics.

[0060] In the above description, the electrode film 20 and the like have been described, but according to this specification, in addition to an electrode sheet which is a precursor of the electrode film 20, a positive electrode 4 equipped with the electrode film 20 and a secondary battery 100 equipped with the positive electrode 4 are also provided. Furthermore, in the above description, a method for manufacturing an electrode sheet for the electrode film 20 has been described, but according to this specification, a method for manufacturing a mixture for an electrode film is also provided, comprising the steps of preparing a first mixture and preparing a second mixture. In addition, a method for manufacturing an electrode (positive electrode 4) is also provided, comprising the steps of manufacturing an electrode sheet plus the step of manufacturing an electrode film 20 using the electrode sheet. A method for manufacturing a secondary battery 100 is also provided, comprising the steps of manufacturing a secondary battery 100 in addition to the method for manufacturing an electrode. It will be done. [Examples]

[0061] The following describes embodiments that embody the disclosures of this specification, but these embodiments are for illustrative purposes only and are not limiting.

[0062] In this embodiment, a mixture for the electrode film of the positive electrode of a lithium-ion secondary battery was prepared by the following method, an electrode sheet was fabricated, and its powder resistance was measured.

[0063] For the preparation of the mixture, LFP (lithium iron phosphate) was used as olivine-type compound particles with a carbon coating, MW-CNT as a conductive additive, PEO1 (crystallinity 20%, melting point approximately 60°C) and PEO2 (crystallinity 50%, melting point approximately 60°C) as the first polymer, and PTFE as the second polymer.

[0064] (Example 1) In Example 1, the electrode film mixture contained active material particles, a conductive additive, and a first polymer. A first mixture was prepared without the second polymer, and then a second mixture containing the second polymer was prepared as the electrode film mixture. In the second mixture, the mass ratio of each material was set to 94.1 / 1.5 / 1.0 / 3.4 for active material particles / conductive additive / PEO1 / PTFE.

[0065] First, active material particles (LFP), conductive additive (MW-CNT), and first polymer (PEO) were blended in 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.

[0066] Next, the second polymer (PTFE) was added to the first mixture in the above-mentioned mass ratio, and the mixture was mixed using the kneader under mixing conditions of 160°C, 60 rpm, and 30 seconds to obtain the second mixture, which is a mixture for electrode films.

[0067] The second mixture was further molded using a calender at 160°C, with a roll clearance of 340 μm, a roll 1 speed of 1.8 m / sec, and a roll 2 speed of 2.4 m / sec to obtain an electrode sheet. The lithium ion diffusion resistance was obtained by measuring the impedance of samples prepared from this electrode sheet using a predetermined method, according to a standard procedure. The results are shown in Table 1.

[0068] (Example 2) Lithium ion diffusion resistance was obtained by following the same procedure as in Example 1, except that PEO2 was used in the first mixture instead of PEO1 as the first polymer.

[0069] (Comparative Example 1) As Comparative Example 1, the first polymer (PEO) was not used in the first mixture, and the mass ratio of the active material particles was increased in place of the first polymer. The same mixing conditions as in Example 1 were used, and the first mixture, second mixture, and electrode sheet of Comparative Example 1 were obtained. The lithium ion diffusion resistance was obtained in the same manner as in Example 1. In Comparative Example 1, the mass ratio of each material (active material particles / conductive additive / PTFE) in the second mixture was 95.1 / 1.5 / 3.4. The results are also shown in Table 1. [Table 1]

[0070] As shown in Table 1, Example 1, which used PEO1 with a crystallinity of 20%, showed the lowest lithium-ion diffusion resistance, and Example 2, which used PEO2 with a crystallinity of 50%, also showed low lithium-ion diffusion resistance. In contrast, Comparative Example 1, which did not use PEO, showed the highest lithium-ion diffusion resistance.

[0071] From the above, it was found that using a lithium-ion conductive polymer, and furthermore, using PEO with a crystallinity of 20% to 50%, is effective in suppressing lithium-ion diffusion resistance in the electrode sheet.

[0072] Furthermore, in Example 1, when the cross-section of the obtained electrode sheet was observed with a transmission electron microscope, it was found that the first polymer, PEO1, was interposed on the surface of the active material particles and compounded within the electrode film, covering a portion of the surface of the active material particles. In addition, the second polymer, PTFE, was dispersed fairly uniformly among the carbon-coated active material particles and the conductive additive, CNTs. Moreover, the PTFE constrained the active material particles by intertwining them in a fibrous manner from the outside.

[0073] From the above, it was found that preparing a first mixture by pre-mixing a first polymer with lithium ion conductivity, such as PEO, which can be melted and fluidized, with active material particles and a conductive additive, and then preparing a second mixture by adding a second polymer that can be fibrillated, such as PTFE, to form an electrode film mixture, contributes to improving the lithium ion conductivity of the electrode sheet. In other words, it was found that this manufacturing method allows for obtaining a composite form that effectively exhibits the lithium ion conductivity of PEO, and as a result, a self-supporting electrode film can be effectively formed without increasing the amount of binder, and moreover, lithium ion diffusion in the electrode sheet can be improved.

[0074] Furthermore, it was found that the lithium-ion diffusion resistance could be reduced by setting the crystallinity of the lithium-ion conductive first polymer to a predetermined range, for example, 50% or less for PEO. In particular, it was found that a crystallinity of 20% or less for PEO was even more effective.

[0075] The technical elements described herein or in the drawings demonstrate technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technologies illustrated herein or in the drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives constitutes technical usefulness in itself. [Explanation of Symbols]

[0076] 2 cells, 4 positive electrodes, 6 separators, 8 negative electrodes, 10 positive electrode current collectors, 12 negative electrode current collectors, 20 self-supporting electrode films, 22 active material particles, 24 electronically conductive material coatings, 26 conductive additives, 30 first polymer, 32 second polymer, 40 negative electrode composite layer, 100 lithium-ion secondary battery

Claims

1. Positive electrode active material particles, Conductive additives, A first polymer having lithium ion conductivity, A second polymer having a fibrillated form, A self-supporting electrode film equipped with the following features.

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

3. The self-supporting electrode film according to claim 2, wherein a portion of the positive electrode active material particles is coated with the first polymer.

4. The self-supporting electrode film according to claim 3, wherein 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 self-supporting electrode described in claim 5.

7. A method for manufacturing an electrode sheet for a self-supporting electrode film, A step of preparing a first mixture by mixing positive electrode active material particles, a conductive additive, and a first polymer that has lithium ion conductivity and does not fibrillate, while the first polymer is in a fluid state, The process involves mixing the first mixture with a fibrillable second polymer to prepare a second mixture having the second polymer in a fibrillated state, and then producing the electrode sheet using the second mixture. A manufacturing method that includes the following features.

8. The manufacturing method according to claim 7, wherein the first polymer comprises polyethylene oxide having a crystallinity of 10% or more and 50% or less.

9. A method for manufacturing an electrode equipped with a self-supporting electrode film, A step of preparing a first mixture by mixing positive electrode active material particles, a conductive additive, and a first polymer that does not fibrillate, while the first polymer is in a fluid state, A step of mixing the first mixture with a fibrillable second polymer to prepare a second mixture having the second polymer in a fibrillated state, A step of preparing an electrode sheet for the self-supporting electrode film using the second mixture, A manufacturing method comprising the step of joining the electrode sheet to a current collector to produce the electrode having the self-supporting electrode film.

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