Method for manufacturing negative electrode material sheet for non-aqueous secondary battery

Abstract

To provide a method for manufacturing a negative electrode material sheet for non-aqueous secondary batteries that can suppress the occurrence of cracks in both the sheet during the manufacturing process and the sheet as a product.SOLUTION: A method for manufacturing a negative electrode material sheet for a non-aqueous secondary battery includes a primary sheet forming step of pressing a composition containing a crosslinkable resin, a crosslinking agent, and a particulate carbon material into a sheet to obtain a primary sheet, a laminate forming step of stacking a plurality of primary sheets in the thickness direction or folding or rolling the primary sheet to obtain a laminate, a crosslinking step of crosslinking the crosslinkable resin contained in the laminate to obtain a crosslinked laminate, a slicing step of slicing the crosslinked laminate at an angle of 45° or less with respect to the stacking direction to obtain a secondary sheet, and a firing step of firing the secondary sheet.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a negative electrode material sheet for a non-aqueous secondary battery. [Background technology]

[0002] Non-aqueous secondary batteries such as lithium ion secondary batteries (hereinafter sometimes simply referred to as "secondary batteries") are small, lightweight, have high energy density, and can be repeatedly charged and discharged, and are therefore used in a wide range of applications. Therefore, in recent years, improvements to battery components such as electrodes (positive and negative electrodes) have been studied with the aim of further improving the performance of secondary batteries.

[0003] Here, a negative electrode used in a secondary battery such as a lithium-ion secondary battery typically includes a current collector and an electrode mixture layer (negative electrode mixture layer) formed on the current collector. This negative electrode mixture layer (sometimes referred to as a "negative electrode active material layer") contains, for example, a negative electrode active material and a resin component such as a binder that is used as needed.

[0004] Therefore, in recent years, attempts have been made to further improve the performance of secondary batteries by improving this negative electrode composite layer. For example, Patent Document 1 discloses a secondary battery containing a particulate carbon material, in which the ratio I(110) / I(004) of the diffraction intensity of the (110) plane to the diffraction intensity of the (004) plane in X-ray diffraction of the main surface is 1.1 or more, (1) the resin content is 8 mass% or less, and (2) the density is 1.3 g / cm 3A negative electrode material sheet for a non-aqueous secondary battery that satisfies at least one of the following has been disclosed. Patent Document 2, for example, discloses a negative electrode material sheet having a first main surface and a second main surface and containing a particulate carbon material. In a thickness-direction cross-section of this negative electrode material sheet, when a first region is a region whose thickness direction distance from the first main surface is 10% to 20% of the thickness of the negative electrode material sheet and a second region is a region whose thickness direction distance from the second main surface is 10% to 20% of the thickness of the negative electrode material sheet, the average value of the angle θ1 between the thickness direction centerline of the negative electrode material sheet and the major axis line of the particulate carbon material in the first region is 40° to 80°, and the average value of the angle θ2 between the thickness direction centerline and the major axis line of the particulate carbon material in the second region is −80° to −40°, where θ1 is defined as 0° at the thickness direction centerline of the negative electrode material sheet. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022 / 102724 [Patent Document 2] Japanese Patent Publication No. 2022-190625 Summary of the Invention [Problem to be solved by the invention]

[0006] Here, when producing a negative electrode material sheet for a non-aqueous secondary battery, it is necessary that cracks are not easily generated in both the sheet during the production process and the sheet as a product. However, the above-mentioned conventional method for producing a negative electrode material sheet for a non-aqueous secondary battery has room for improvement in terms of suppressing cracks in both the sheet during the production process and the sheet as a product.

[0007] Therefore, an object of the present invention is to provide a method for producing a negative electrode material sheet for a non-aqueous secondary battery, which can suppress the occurrence of cracks in both the sheet during the production process and the sheet as a product. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to achieve the above object, and have newly discovered that a negative electrode material sheet for a nonaqueous secondary battery can be produced by compressing a composition containing a crosslinkable resin, a crosslinking agent, and a particulate carbonaceous material to form a sheet into a primary sheet, and then carrying out a crosslinking step of crosslinking the crosslinkable resin in the laminate to obtain a crosslinked laminate, a slicing step of slicing the crosslinked laminate to obtain a secondary sheet, and a firing step of firing the secondary sheet, thereby suppressing the occurrence of cracks in both the sheet during the production process and the sheet as a product, and have completed the present invention.

[0009] That is, the present invention aims to advantageously solve the above-mentioned problems, and [1] the method for producing a negative electrode material sheet for a non-aqueous secondary battery of the present invention is characterized by comprising: a primary sheet forming step of pressing a composition containing a crosslinkable resin, a crosslinking agent, and a particulate carbon material into a sheet shape to obtain a primary sheet; a laminate forming step of stacking a plurality of the primary sheets in the thickness direction or folding or rolling the primary sheets to obtain a laminate; a crosslinking step of crosslinking the crosslinkable resin contained in the laminate to obtain a crosslinked laminate; a slicing step of slicing the crosslinked laminate at an angle of 45° or less with respect to the stacking direction to obtain a secondary sheet; and a firing step of firing the secondary sheet. According to the method of the present invention for producing a negative electrode material sheet for a non-aqueous secondary battery, it is possible to suppress the occurrence of cracks in both the sheet during the production process and the sheet as a product.

[0010] [2] In the method for producing a negative electrode material sheet for a non-aqueous secondary battery according to [1] above, it is preferable that the crosslinking step is performed while the laminate is pressed by an isostatic press. By performing crosslinking while the laminate is pressed by an isostatic press in the crosslinking step, it is possible to more effectively suppress the occurrence of cracks in both the sheet during the production process and the sheet as a product.

[0011] [3] In the method for producing a negative electrode material sheet for a non-aqueous secondary battery according to the above [1] or [2], the volume fraction of the particulate carbon material in the composition to be formed in the primary sheet forming step is preferably 60% by volume or more, based on the total volume of the crosslinkable resin and the particulate carbon material. If the volume fraction of the particulate carbon material is 60% by volume or more, the density of the resulting negative electrode material sheet for a non-aqueous secondary battery can be increased, thereby increasing the negative electrode density.

[0012] [4] In the method for producing a negative electrode material sheet for a nonaqueous secondary battery according to any one of the above [1] to [3], the thickness of the primary sheet formed in the primary sheet forming step is preferably 2.0 mm or less. If the thickness of the primary sheet is equal to or less than the upper limit, the occurrence of cracks in the sheet as a product can be more effectively suppressed.

[0013] [5] In the method for producing a negative electrode material sheet for a nonaqueous secondary battery according to any one of the above [1] to [4], the crosslinkable resin may contain a liquid resin. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a method for producing a negative electrode material sheet for a non-aqueous secondary battery, which can suppress the occurrence of cracks in both the sheet during the production process and the sheet as a product. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described in detail. A sheet (hereinafter, sometimes simply referred to as "negative electrode material sheet") produced by the method for producing a negative electrode material sheet for a nonaqueous secondary battery of the present invention (hereinafter, sometimes simply referred to as "negative electrode material sheet") can be used as a negative electrode mixture layer of a negative electrode for a nonaqueous secondary battery. The obtained negative electrode material sheet can be bonded to a current collector to form a negative electrode. Naturally, such a negative electrode can be used to produce a secondary battery.

[0016] (Method of manufacturing a negative electrode material sheet for a non-aqueous secondary battery) The method for producing a negative electrode material sheet of the present invention includes: (A) a primary sheet forming step of pressurizing a composition containing a crosslinkable resin, a crosslinking agent, and a particulate carbon material to form a sheet to obtain a primary sheet; (B) a laminate forming step of stacking a plurality of primary sheets in the thickness direction or folding or rolling the primary sheet to obtain a laminate; (C) a crosslinking step of crosslinking the crosslinkable resin contained in the laminate to obtain a crosslinked laminate; (D) a slicing step of slicing the crosslinked laminate at an angle of 45° or less with respect to the stacking direction to obtain a secondary sheet; and (E) a firing step of firing the secondary sheet. The method for producing a negative electrode material sheet of the present invention may optionally further include steps other than the above steps (A) to (E).

[0017] According to the method for producing a negative electrode material sheet of the present invention, it is possible to suppress the occurrence of cracks in both the sheet during the production process and the sheet as a product.

[0018] As in the above-mentioned Patent Document 1, a method for producing a negative electrode material sheet has been attempted in the past by sintering a sheet in which the orientation of a particulate carbon material has been controlled so that the ratio I(110) / I(004), a parameter indicating the crystal orientation of the particulate carbon material, is equal to or greater than a predetermined lower limit. Here, a step of compressing the negative electrode material sheet in the thickness direction is generally performed for the purpose of increasing electrode density. However, as disclosed in Patent Document 2, when a negative electrode material sheet obtained by controlling the orientation of the particulate carbon material is compressed in the thickness direction, the particulate carbon material oriented in the thickness direction may collapse due to the pressure. As a result of the inventor's investigation, even if the density of the negative electrode material sheet is increased by compression, there is a concern that if the orientation of the particulate carbon material is disrupted (inclined in a direction oblique to the main surface of the sheet), the good conductivity resulting from controlling the orientation of the particulate carbon material may not be fully utilized. To address this concern, for example, even if the compounding ratio of particulate carbon material in the negative electrode material sheet is increased, the adhesiveness of the surface of the primary sheet constituting the laminate may decrease, which may cause delamination during lamination. If, as a countermeasure against delamination, the composition of the resin compounded in the primary sheet is adjusted by increasing the proportion of resin that is liquid at room temperature and normal pressure, thereby increasing the adhesiveness of the surface of the primary sheet, there is a concern that the strength of the entire sheet may be insufficient, making it impossible to carry out the slicing process. Therefore, the present inventors have newly discovered that compounding a crosslinkable resin and a crosslinking agent in the primary sheet can prevent delamination during lamination and improve sheet strength, thereby completing the present invention.

[0019] <(A) Primary sheet forming process> In the primary sheet forming step, a composition containing a resin and particulate carbon material is pressed into a sheet to obtain a primary sheet.

[0020] <<Composition>> The composition includes a crosslinkable resin, a crosslinking agent, and a particulate carbon material. The composition may further include a silicon active material and a fibrous carbon material. The composition may further include components (other components) other than the crosslinkable resin, crosslinking agent, particulate carbon material, silicon active material, and fibrous carbon material.

[0021] [Crosslinkable resin] The crosslinkable resin is not particularly limited, and although it depends on the types of crosslinking agent and crosslinking accelerator, for example, a crosslinkable resin that is solid at room temperature and normal pressure, or a crosslinkable resin that is liquid at room temperature and normal pressure can be used.

[0022] -Cross-linkable resin that is solid at room temperature and pressure- Examples of crosslinkable resins that are solid at room temperature and normal pressure include acrylic polymers such as poly(2-ethylhexyl acrylate), copolymers of acrylic acid and 2-ethylhexyl acrylate, polymethacrylic acid or its esters, and polyacrylic acid or its esters; styrene-acrylonitrile copolymers; acrylonitrile-butadiene-styrene copolymers (ABS resins); styrene-butadiene copolymers or hydrogenated products thereof; acrylonitrile-butadiene copolymers (nitrile rubbers); styrene-butadiene block copolymers or hydrogenated products thereof; styrene-isoprene block copolymers or hydrogenated products thereof; silicone resins such as polyimide silicone resins; and fluororesins such as vinylidene fluoride-based fluororesins, tetrafluoroethylene-propylene-based fluororesins, and tetrafluoroethylene-purovinyl ether-based fluororesins. These may be used alone or in combination. Among these, it is preferable to use acrylic polymers or acrylonitrile-butadiene copolymers. In particular, when acrylic polymers are used, high loading of particulate carbon materials may be possible.

[0023] The crosslinkable resin that is solid at room temperature and normal pressure preferably has a glass transition temperature of −60.0° C. or higher, more preferably −50.0° C. or higher, and preferably −8.0° C. or lower, more preferably −15.0° C. or lower, and even more preferably −20.0° C. or lower. If the glass transition temperature of the resin used as the solid crosslinkable resin at room temperature and normal pressure is equal to or higher than the above-mentioned lower limit, the occurrence of cracks in the sheet during the manufacturing process can be effectively suppressed. Furthermore, if the glass transition temperature of the resin used as the solid crosslinkable resin at room temperature and normal pressure is equal to or lower than the above-mentioned upper limit, the occurrence of cracks in the resulting negative electrode material sheet can be effectively suppressed.

[0024] The blending ratio of the crosslinkable resin that is solid at room temperature and normal pressure in the crosslinkable resin is preferably 25% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and particularly preferably 55% by mass or more. The crosslinkable resin may not contain any crosslinkable resin that is liquid at room temperature and normal pressure, and 100% by mass of the crosslinkable resin may be solid at room temperature and normal pressure. The upper limit of the blending ratio of the crosslinkable resin that is solid at room temperature and normal pressure in the crosslinkable resin is not particularly limited, and may be 100% by mass. When the blending ratio of the crosslinkable resin that is solid at room temperature and normal pressure in the crosslinkable resin is equal to or greater than the lower limit, the adhesiveness between the strips in both the sheet during the manufacturing process and the sheet as a product is improved, thereby more effectively suppressing the occurrence of cracks.

[0025] -Cross-linkable resin that is liquid at room temperature and pressure- The crosslinkable resin may include a crosslinkable resin that is a liquid at room temperature and normal pressure, in addition to a crosslinkable resin that is a solid at room temperature and normal pressure. Examples of the crosslinkable resin that is a liquid at room temperature and normal pressure include thermoplastic resins that are liquid at room temperature and normal pressure, such as acrylic resin, epoxy resin, silicone resin, fluororesin, acrylonitrile-butadiene copolymer, and polybutene.

[0026] The blending ratio of the crosslinkable resin that is liquid at room temperature and normal pressure in the crosslinkable resin is preferably 75% by mass or less, more preferably 70% by mass or less, even more preferably 60% by mass or less, and particularly preferably 45% by mass or less. The crosslinkable resin may not contain any crosslinkable resin that is liquid at room temperature and normal pressure. If the blending ratio of the crosslinkable resin that is liquid at room temperature and normal pressure in the crosslinkable resin is equal to or less than the above upper limit, the adhesion between the strips in both the sheet during the manufacturing process and the sheet as a product is improved, thereby more effectively suppressing the occurrence of cracks.

[0027] [Crosslinking agent] The crosslinking agent can be any agent that can undergo a crosslinking reaction with the above-mentioned crosslinkable resin and is used as a crosslinking agent for resins, and is not limited to these. Typical crosslinking agents include sulfur-based crosslinking agents, peroxides, and amine-based crosslinking agents that crosslink unsaturated bonds contained in the resin. These agents may be used alone or in combination of two or more in any ratio. In particular, when the crosslinkable resin is an acrylic polymer, it is preferable to use an amine-based crosslinking agent, and when the crosslinkable resin is an acrylonitrile-butadiene copolymer, it is preferable to use a peroxide as the crosslinking agent.

[0028] The amount of crosslinking agent is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and even more preferably 0.3 parts by mass or more, and is preferably 10.0 parts by mass or less, and more preferably 8.0 parts by mass or less, relative to 100 parts by mass of the crosslinkable resin. If the amount of crosslinking agent is within the above range, the crosslinked resin formed is sufficiently crosslinked by the crosslinking agent, which is thought to improve the adhesion between the strips in both the sheet during the manufacturing process and the sheet as a product, thereby more effectively suppressing the occurrence of cracks.

[0029] [Crosslinking accelerator] The crosslinking accelerator that can be used in the crosslinking reaction between the crosslinkable resin and the crosslinking agent is not particularly limited, and sulfenamide crosslinking accelerators and the like can be used. These may be used alone or in combination of two or more in any ratio. Among them, sulfenamide crosslinking accelerators are preferred.

[0030] The composition does not need to contain a crosslinking accelerator, but if it does, the amount of crosslinking accelerator blended is preferably 0.5 parts by mass or more, more preferably 0.7 parts by mass or more, and even more preferably 1.0 part by mass or more, and preferably 5.0 parts by mass or less, and more preferably 4.0 parts by mass or less, relative to 100 parts by mass of the crosslinkable resin. If the amount of crosslinking accelerator blended is within the above range, the crosslinked resin formed is sufficiently crosslinked by the crosslinking agent, and it is thought that the occurrence of cracks can be more effectively suppressed both in the sheet during the production process and in the sheet as a product.

[0031] [Other resins] Resins other than the crosslinkable resin are not particularly limited, and examples thereof include resins that do not undergo a crosslinking reaction with the crosslinking agent and crosslinking accelerator that contribute to the crosslinking of the crosslinkable resin. Examples of such resins include liquid resins that are liquid at room temperature and normal pressure and do not undergo a crosslinking reaction with the crosslinking agent and crosslinking accelerator that contribute to the crosslinking of the crosslinkable resin. Examples of liquid resins include thermoplastic resins that are liquid at room temperature and normal pressure. Examples of other resins include acrylic resins, epoxy resins, silicone resins, fluororesins, acrylonitrile-butadiene copolymers, and polybutene, which are thermoplastic resins that are liquid at room temperature and normal pressure and are compounds different from the above-mentioned crosslinkable resins.

[0032] [Crosslinkable resin content] The proportion of the crosslinkable resin contained in the composition is preferably 25% by volume or more, more preferably 30% by volume or more, and preferably 40% by volume or less, and more preferably 35% by volume or less, where the total volume of the crosslinkable resin and the particulate carbon material is 100% by volume. If the proportion of the resin contained in the composition is equal to or greater than the lower limit, the adhesiveness on the surface of the obtained primary sheet can be increased, and if it is equal to or less than the upper limit, the density of the particulate carbon material in the obtained negative electrode material sheet can be further increased.

[0033] [Particulate carbon materials] The particulate carbon material is a material that can absorb and release charge carriers such as lithium as a negative electrode active material for a non-aqueous secondary battery. The particulate carbon material is not particularly limited, and examples thereof include graphite such as artificial graphite, flake graphite, exfoliated graphite, natural graphite, acid-treated graphite, expandable graphite, and expanded graphite; carbon black; etc. These may be used alone or in combination of two or more.

[0034] Among the above, it is preferable to use flake graphite as the particulate carbon material. If flake graphite is used as the particulate carbon material, the rate characteristics of a secondary battery manufactured using a negative electrode including a negative electrode material sheet can be further improved. Note that, as an example of flake graphite, "UP20α" manufactured by Nippon Graphite Industries Co., Ltd. can be mentioned.

[0035] -Properties of particulate carbon materials- The volume average particle diameter of the particulate carbon material is preferably 3 μm or more, more preferably 5 μm or more, even more preferably 8 μm or more, even more preferably 12 μm or more, even more preferably 16 μm or more, particularly preferably 20 μm or more, and preferably 200 μm or less, more preferably 150 μm or less, preferably 100 μm or less, and even more preferably 50 μm or less. If the volume average particle diameter of the particulate carbon material is equal to or greater than the above-mentioned lower limit, the density of the negative electrode material sheet is appropriately reduced, facilitating the movement of charge carriers such as lithium, thereby further improving the rate characteristics of a secondary battery manufactured using a negative electrode comprising the negative electrode material sheet. On the other hand, if the volume average particle diameter of the particulate carbon material is equal to or greater than the above-mentioned lower limit, the density of the negative electrode material sheet is appropriately increased, thereby enabling a secondary battery manufactured using a negative electrode comprising the negative electrode material sheet to have a high capacity. In the present invention, the "volume average particle size" can be measured in accordance with JIS Z8825, and represents the particle size at which the cumulative volume calculated from the smallest diameter side becomes 50% in the particle size distribution (volume basis) measured by a laser diffraction method.

[0036] Furthermore, the particulate carbon material preferably has an aspect ratio (major axis / minor axis) of more than 1.2, more preferably 1.5 or more, and preferably 20 or less, more preferably 10 or less, and even more preferably 5 or less. When the aspect ratio of the particulate carbon material is within the above-mentioned range, the rate characteristics of a secondary battery produced using a negative electrode including the negative electrode material sheet can be further improved.

[0037] The content of the particulate carbon material in the composition is preferably 60% by volume or more, preferably 75% by volume or less, and more preferably 65% ​​by volume or less, where the total volume of the crosslinkable resin and the particulate carbon material is 100% by volume. If the content of the particulate carbon material in the composition is equal to or greater than the above-mentioned lower limit, the density of the produced negative electrode material sheet can be appropriately increased, improving the strength of the negative electrode material sheet and increasing the capacity of a secondary battery produced using a negative electrode comprising the negative electrode material sheet. On the other hand, if the content of the particulate carbon material in the composition is equal to or less than the above-mentioned upper limit, the density of the produced negative electrode material sheet can be appropriately reduced, further improving the rate characteristics of a secondary battery produced using a negative electrode comprising the negative electrode material sheet and improving the electrolyte injectability of the secondary battery.

[0038] [Silicon active material] The silicon active material functions as a negative electrode active material in the negative electrode mixture layer made of the negative electrode material sheet, similar to the particulate carbon material described above. By further using the silicon active material as a negative electrode active material, the capacity of a secondary battery including the negative electrode material sheet can be increased. Examples of silicon active materials include silicon (Si), silicon-containing alloys, SiO, and SiO x and a composite of a Si-containing material and conductive carbon, which is obtained by coating or compounding a Si-containing material with conductive carbon. These may be used alone or in combination of two or more.

[0039] [Carbon fiber materials] The fibrous carbon material is not particularly limited, and examples thereof include carbon nanotubes, vapor-grown carbon fibers, carbon fibers obtained by carbonizing organic fibers, and cut products thereof. These may be used alone or in combination of two or more.

[0040] [Other ingredients] The composition may further contain other components in addition to the resin, particulate carbon material, silicon active material, and fibrous carbon material described above. Examples of other components that can be used include dispersants. The dispersant is not particularly limited, and known dispersants can be used. The content of the dispersant in the composition can be adjusted within a range that achieves the desired effects of the present invention.

[0041] -Preparation of composition- The composition is not particularly limited and can be prepared by mixing the above-mentioned components. The mixing of the above-mentioned components can be carried out using known mixing devices, such as a kneader; a mixer such as a Henschel mixer, a Hobart mixer, or a high-speed mixer; a twin-screw kneader; or a roll. The mixing may also be carried out in the presence of a solvent such as ethyl acetate. The resin may be dissolved or dispersed in a solvent in advance to form a resin solution, which is then mixed with the particulate carbon material, and optionally added silicon active material, fibrous carbon material, and other components. The mixing time may be, for example, 5 minutes to 60 minutes. The mixing temperature may be, for example, 5°C to 150°C. Furthermore, it is preferable to first mix the ingredients other than the crosslinking agent, then add the crosslinking agent, and mix at a low mixing temperature (for example, 5°C to 100°C, preferably 85°C or less). This is because unintended crosslinking reactions can be prevented from progressing before the crosslinking step.

[0042] <<Molding of the composition>> The composition prepared as described above can be optionally degassed and crushed, and then pressed to form into a sheet. The sheet formed by pressing the composition in this manner can be used as a primary sheet. If a solvent is used during mixing, it is preferable to remove the solvent before forming into a sheet. For example, if degassing is performed using vacuum degassing, the solvent can be removed simultaneously during degassing.

[0043] Here, the composition can be formed into a sheet using any known forming method, such as press molding, roll molding, or extrusion molding, as long as the forming method applies pressure. Among these, roll molding is preferred. In particular, roll molding is preferred, in which the composition is passed between a first roll and a second roll having a faster peripheral speed than the first roll. The peripheral speed ratio of the second roll to the first roll ("peripheral speed of the second roll" / "peripheral speed of the first roll") is preferably 1.03 / 1 or more and 1.50 / 1 or less. The distance between the first roll and the second roll is preferably 0.5 mm or more, preferably 0.8 mm or more, preferably 2.0 mm or less, and more preferably 1.2 mm or less. The thickness of the resulting primary sheet also corresponds to the above-mentioned gap distance. By setting the roll gap, i.e., the thickness of the primary sheet, to the above-mentioned upper limit or less, the occurrence of cracks in the resulting secondary sheet can be effectively suppressed. As a result, the occurrence of cracks in the resulting negative electrode material sheet can also be effectively suppressed. By setting the roll gap, ie, the thickness of the primary sheet, to the above lower limit or more, the productivity of the thermally conductive sheet can be improved.

[0044] <(B) Laminate formation process> In the laminate formation process, a plurality of primary sheets obtained in the primary sheet molding process are stacked in the thickness direction, or the primary sheets are folded or wound, to obtain a laminate in which a plurality of primary sheets containing a resin and a particulate carbon material are formed in the thickness direction. Here, the formation of the laminate by folding the primary sheets is not particularly limited and can be performed by folding the primary sheets at a constant width using a folding machine. Furthermore, the formation of the laminate by winding the primary sheets is not particularly limited and can be performed by winding the primary sheets around an axis parallel to the short or long direction of the primary sheets. Furthermore, the formation of the laminate by stacking the primary sheets can be performed using a lamination device, not particularly limited. For example, using a sheet lamination device (manufactured by Nikkiso Co., Ltd., product name "Hi-Stacker") can prevent air from entering between the layers, thereby efficiently obtaining a good laminate.

[0045] <(C) Crosslinking step> The laminate obtained in the laminate formation process is subjected to a crosslinking process in which the crosslinkable resin contained in the laminate is crosslinked to obtain a crosslinked laminate. In the crosslinking process, the laminate is preferably heated and pressurized to promote the crosslinking reaction of the crosslinkable resin with a crosslinking agent. Furthermore, in the crosslinking process, the laminate is preferably crosslinked while being pressurized by an isostatic press. By isostatic pressing, the laminate is pressurized uniformly not only in the thickness direction but also in all directions. By promoting the crosslinking reaction in the isostatic press, a stronger crosslinked structure can be formed, thereby suppressing the occurrence of cracks in both the sheet during the manufacturing process and the sheet as a product. The crack suppression effect of the sheet during the manufacturing process is thought to be due to the formation of a strong crosslinked structure, which increases the strength of the crosslinked laminate and the secondary sheet. Furthermore, the crack suppression effect in the sheet as a product is presumably due to the crosslinked resin, which remains in a tar-like form in the negative electrode material sheet even after being burned off in the firing process. In other words, it is presumed that the cross-linked resin having a cross-linked structure that coexisted with the oriented structure of the particulate carbon material within the sheet during the manufacturing process remains in a tar-like state even after being burned off in the firing process, and functions to connect the oriented structure of the particulate carbon material at various points, thereby preventing cracks from occurring in the sheet as a finished product.

[0046] Here, the pressure when isostatically pressing the laminate in the crosslinking step is preferably 0.3 MPa or more, more preferably 0.6 MPa or more, and preferably 0.9 MPa or less, and more preferably 0.8 MPa or less. By setting the pressure when isostatically pressing the laminate in the crosslinking step within the above range, the adhesion between the strips in both the sheet during the manufacturing process and the sheet as a product can be improved, thereby more effectively suppressing the occurrence of cracks. In addition, the temperature when heating the laminate in the crosslinking step is not particularly limited, but is preferably 50°C or more, more preferably 70°C or more, and preferably 190°C or less, and more preferably 180°C or less. Furthermore, the heating time for the laminate can be, for example, 10 seconds or more and 60 minutes or less.

[0047] In the crosslinked laminate obtained through the crosslinking step, the particulate carbon material and the like are presumably oriented in a direction substantially perpendicular to the stacking direction. For example, when the particulate carbon material is flaky, the direction of the major axis of the main surface of the flaky shape is presumably substantially perpendicular to the stacking direction.

[0048] <(D) Slicing process> In the slicing step, the laminate is sliced ​​at an angle of 45° or less relative to the lamination direction to obtain secondary sheets consisting of slices of the laminate. The method for slicing the laminate is not particularly limited, and examples thereof include the multi-blade method, laser processing method, water jet method, and knife processing method. Among these, the knife processing method is preferred because it is easy to make the thickness of the secondary sheet uniform. The cutting tool used to slice the laminate is not particularly limited, and a slicing member having a smooth plate surface with a slit and a blade portion protruding from the slit portion (for example, a plane or slicer with a sharp blade) can be used.

[0049] The angle at which the laminate is sliced ​​is preferably 30° or less with respect to the stacking direction, more preferably 15° or less with respect to the stacking direction, and preferably approximately 0° with respect to the stacking direction (i.e., in the direction along the stacking direction). In the secondary sheet obtained in this manner, the particulate carbon material is well oriented in the thickness direction. For example, when the particulate carbon material is flaky, the direction of the major axis of the main surface of the flaky shape is approximately aligned with the thickness direction of the secondary sheet.

[0050] <(E) Firing process> In the firing step, the secondary sheet is fired to burn off and remove the resin contained in the secondary sheet, thereby obtaining a negative electrode material sheet. The resulting negative electrode material sheet is a sheet obtained by removing the resin from the secondary sheet (a portion of which remains as tar). Therefore, in the negative electrode material sheet, the particulate carbon material is well oriented in the thickness direction. For example, when the particulate carbon material is flaky, the direction of the major axis of the main surface of the flaky shape is approximately aligned with the thickness direction of the secondary sheet.

[0051] Here, the heating temperature when firing the secondary sheet is preferably T-50°C or higher, more preferably T-40°C or higher, even more preferably T-20°C or higher, and preferably T+2000°C or lower, more preferably T+1500°C or lower, and even more preferably T+1000°C or lower, where T°C is the temperature at which the resin contained in the secondary sheet begins to decompose. If the heating temperature when firing the secondary sheet is equal to or higher than the lower limit, the resin content in the resulting negative electrode material sheet can be reduced, further improving the rate characteristics of a secondary battery manufactured using a negative electrode including the negative electrode material sheet. On the other hand, if the heating temperature when firing the secondary sheet is equal to or lower than the upper limit, damage to the structure of the resulting negative electrode material sheet due to excessive heating can be prevented, and cracking in the negative electrode material sheet can be effectively prevented.

[0052] Furthermore, the heating temperature when firing the secondary sheet is preferably 300°C or higher, more preferably 500°C or higher, even more preferably 700°C or higher, and preferably 2000°C or lower, more preferably 1500°C or lower, and even more preferably 1200°C or lower. If the heating temperature when firing the secondary sheet is above the above-mentioned lower limit, the resin content in the produced negative electrode material sheet can be reduced, thereby further improving the rate characteristics of a secondary battery produced using a negative electrode including the negative electrode material sheet. On the other hand, if the heating temperature when firing the secondary sheet is below the above-mentioned upper limit, damage to the structure of the produced negative electrode material sheet due to excessive heating can be suppressed, and the strength of the negative electrode material sheet can be ensured to be sufficiently high. As a result, the occurrence of cracks in the negative electrode material sheet can be effectively suppressed.

[0053] The heating time when firing the secondary sheet can be adjusted depending on the heating temperature, but can be set to, for example, 30 minutes or more and 72 hours or less.

[0054] The atmosphere in which the secondary sheet is fired is not particularly limited and may be either an oxygen atmosphere or a nitrogen atmosphere, but a nitrogen atmosphere is more preferable. By firing the secondary sheet in a nitrogen atmosphere, oxidative decomposition of particulate carbon materials such as graphite can be suppressed, and tar can be efficiently generated, thereby more effectively suppressing the occurrence of cracks in the negative electrode material sheet. Furthermore, when the negative electrode material sheet is fired in an oxygen atmosphere at high temperatures (e.g., 800°C), the particulate carbon materials such as graphite tend to disappear without remaining as tar. [Example]

[0055] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified. In each example and each comparative example, various attributes were measured or evaluated by the following methods.

[0056] <Primary sheet thickness> Using a film thickness meter (manufactured by Mitutoyo, product name "Digimatic Indicator ID-C112XBS"), the thickness was measured at five points, namely, approximately the center and the four corners (squares), of the negative electrode material sheet (or the negative electrode composite layer of the coated electrode), and the average value (mm) of the measured thicknesses was taken as the thickness of the negative electrode material sheet (negative electrode composite layer).

[0057] <Density of negative electrode material sheet> The mass, area, and thickness of the negative electrode material sheet were measured, and the mass was divided by the volume (= area × thickness) to determine the density (g / cm 3 ) was calculated.

[0058] <Evaluation of crack occurrence in secondary sheet> Five secondary sheets each made by joining multiple strips (primary sheets) in parallel, produced in the examples and comparative examples, were visually observed and evaluated for crack occurrence according to the following criteria. The evaluation was based on the average value of the number of sheets observed. A: No cracks have occurred within the secondary sheet surface. B: Cracks less than 1 cm long (not penetrating) have occurred between the strips, but no breaks have occurred between the strips, and there are no breaks within the strips, or if there are breaks, they are less than 0.5 cm long. C: Cracks between strips and breaks (0.5 cm or larger) within the strip. The size of the crack is not important and the crack does not penetrate through the strip. D: Cracks between strips penetrate the sheet, splitting it apart.

[0059] <Evaluation of crack occurrence in negative electrode material sheets> A: No cracks occurred within the surface of the negative electrode material sheet. B: Cracks less than 1 cm in size have occurred between the strips, but the cracks do not penetrate through the strips. C: Cracks between strips penetrate the sheet, splitting it apart.

[0060] Example 1 <Preparation of Composition> As a crosslinkable resin, 72 parts of liquid nitrile rubber (NBR) (manufactured by Nippon Zeon Co., Ltd., trade name "Nipol 1312", decomposition onset temperature: 336 ° C, specific gravity: 1) and 108 parts of solid nitrile rubber (NBR) (manufactured by Nippon Zeon Co., Ltd., trade name "Nipol 3350", decomposition onset temperature: 375 ° C, specific gravity: 1) were used. 108 parts of flake graphite (manufactured by Nippon Graphite Industries Co., Ltd., trade name "UP20α", volume average particle diameter: 20 μm, aspect ratio = 2) were used as a particulate carbon material. 620 parts (equivalent to 60 parts by volume when the total is 100 parts by volume) were mixed and stirred at 150 ° C for 20 minutes using a pressure kneader (manufactured by Nippon Spindle). Then, 5.4 parts of dicumyl peroxide (product name "Percumyl D80") were added as a crosslinking agent at 80 ° C., and the mixture was kneaded for 5 minutes to obtain a composition.

[0061] <Primary sheet forming process> Next, 500 g of the obtained composition was rolled into a sheet using a first roll and a second roll under the following conditions: a gap between the first roll and the second roll of 1.2 mm, a roll temperature of 25°C, a sheet discharge speed (peripheral speed of the first roll) of 2 m / min, and a peripheral speed ratio of the second roll to the first roll (second roll / first roll): 1.15 / 1. The rolling was repeated while maintaining the sheet conveyance direction in the same manner. A total of 10 rolling processes were performed to obtain a primary sheet having a thickness of 1.2 mm.

[0062] <Laminate formation process> Subsequently, the obtained primary sheet was cut into a size of 150 mm length x 150 mm width x 0.8 mm thickness, and 188 sheets were stacked in the thickness direction of the primary sheet to obtain a laminate.

[0063] <Crosslinking process> The laminate was then wrapped in release PET (polyethylene terephthalate) and sealed with tape, then vacuum-packaged in a PET retort pouch. This was then placed in an autoclave (Hanida Iron Works, small autoclave "DANDELION") and heated and pressurized at 180°C and 0.8 MPa (absolute pressure) from all directions for 60 minutes to obtain a crosslinked laminate.

[0064] <Slicing process> The resulting cross-linked laminate was then pressed against the laminated side at a pressure of 0.3 MPa and sliced ​​at an angle of 0 degrees to the lamination direction (in other words, in the normal direction to the main surface of the laminated primary sheets) using a woodworking slicer (manufactured by Marunaka Iron Works Co., Ltd., product name "Super Mecha S Super Finishing Planer") to obtain a secondary sheet measuring 150 mm in length, 150 mm in width, and 0.08 mm in thickness.

[0065] <Firing process> Thereafter, the obtained secondary sheet was baked at 1000° C. for 8 hours in a nitrogen atmosphere to burn off and remove the resin component, thereby obtaining a negative electrode material sheet.

[0066] Example 2 In <Preparation of Composition>, the same operations, measurements, and evaluations as in Example 1 were carried out, except that the amounts of the various components were changed as shown in Table 1. The results are shown in Table 1.

[0067] Example 3 The same operations, measurements, and evaluations as in Example 1 were carried out, except that the roll gap in the <primary sheet forming step> was set to 2.0 mm and the thickness of the resulting primary sheet was set to 2.0 mm. The results are shown in Table 1.

[0068] Example 4 The crosslinkable resin to be blended in the <Preparation of Composition> step was an acrylic rubber (manufactured by Zeon Corporation, trade name "Nipol (registered trademark) AR-14", glass transition temperature: -42.0°C, decomposition onset temperature: 400°C, specific gravity: 1.1 g / cm), which is a solid acrylic resin at room temperature and normal pressure. 3The total weight of the mixture was changed to 220 parts, and 680 parts (equivalent to 60 parts by volume when the total volume is 100 parts by volume) of flake graphite (manufactured by Nippon Graphite Industries Co., Ltd., trade name "UP20α", volume average particle diameter: 20 μm, aspect ratio = 2) as a particulate carbon material was mixed and stirred for 20 minutes at 150°C using a pressure kneader (manufactured by Nippon Spindle). 1.1 parts of Diak#1 (manufactured by Chemours) as a crosslinking agent and 4.4 parts of Noccela DT as a crosslinking accelerator were added to the resulting composition, and the mixture was kneaded for 5 minutes at 80°C to obtain a composition. Except for using the composition obtained in this manner, the same operations, measurements, and evaluations as in Example 1 were carried out. The results are shown in Table 1.

[0069] Example 5 In <Preparation of Composition>, the same operations, measurements, and evaluations as in Example 1 were carried out, except that the amounts of the various components were changed as shown in Table 1. The results are shown in Table 1.

[0070] (Comparative Example 1) In <Preparation of Composition>, except that no crosslinking agent was added, the same operations, measurements, and evaluations as in Example 1 were carried out. The results are shown in Table 1.

[0071] (Comparative Example 2) In <Preparation of Composition>, except that no crosslinking agent was added, the same operations, measurements, and evaluations as in Example 2 were carried out. The results are shown in Table 1.

[0072] [Table 1]

[0073] Table 1 shows that in Examples 1 to 5, in which a composition containing a crosslinkable resin, a crosslinking agent, and a particulate carbon material was used to form a primary sheet and a laminate, and the laminate was crosslinked to form a crosslinked laminate, which was then sliced ​​and fired to produce a negative electrode material sheet, it was possible to suppress the occurrence of cracks in both the sheet during the manufacturing process and the sheet as a product. On the other hand, in Comparative Examples 1 and 2, in which the same procedure was carried out using a composition containing no crosslinking agent, it was found that the occurrence of cracks could not be suppressed in either the sheet during the manufacturing process or the sheet as a product. [Industrial Applicability]

[0074] According to the present invention, it is possible to provide a method for producing a negative electrode material sheet for a non-aqueous secondary battery, which can suppress the occurrence of cracks in both the sheet during the production process and the sheet as a product.

Claims

1. a primary sheet forming step of pressurizing a composition containing a crosslinkable resin, a crosslinking agent, and a particulate carbon material into a sheet to obtain a primary sheet; a laminate forming step of stacking a plurality of the primary sheets in the thickness direction or folding or rolling the primary sheet to obtain a laminate; a crosslinking step of crosslinking the crosslinkable resin contained in the laminate to obtain a crosslinked laminate; a slicing step of slicing the crosslinked laminate at an angle of 45° or less with respect to the lamination direction to obtain a secondary sheet; and a firing step of firing the secondary sheet.

2. The method for producing a negative electrode material sheet for a non-aqueous secondary battery according to claim 1 , wherein in the crosslinking step, the laminate is crosslinked while being pressed by a hydrostatic press.

3. 2. The method for producing a negative electrode material sheet for a nonaqueous secondary battery according to claim 1, wherein a volume fraction of the particulate carbon material in the composition to be formed in the primary sheet forming step is 60 volume % or more based on the total volume of the crosslinkable resin and the particulate carbon material.

4. The method for producing a negative electrode material sheet for a nonaqueous secondary battery according to claim 1 , wherein the thickness of the primary sheet formed in the primary sheet forming step is 2.0 mm or less.

5. The method for producing a negative electrode material sheet for a non-aqueous secondary battery according to any one of claims 1 to 4, wherein the crosslinkable resin comprises a liquid resin.

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