Electrode manufacturing method and electricity storage device manufacturing method

Abstract

To provide a method for manufacturing an electrode with reduced resistance.SOLUTION: A manufacturing method disclosed herein includes: a first mixing step (S1) of mixing a first electrode active material with a pore-forming agent to obtain a mixture; a second mixing step (S2) of mixing the mixture with a binder to obtain an electrode mixture; an application step (S3) of applying the electrode mixture onto a current collector; and a heat treatment step (S6) of heat-treating the electrode mixture on the current collector to remove the pore-forming agent.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing an electrode and a method for manufacturing an electricity storage device. [Background technology]

[0002] Patent Documents 1 to 3 are examples of prior art documents related to electricity storage devices such as lithium-ion secondary batteries. For example, Patent Document 1 describes a method for producing a sheet-like negative electrode, which includes, in this order, a step of mixing a negative electrode active material, a binder, and a soluble pore-forming agent together and pressing the resulting negative electrode composite into a sheet, a step of removing the pore-forming agent from the sheet-like negative electrode composite using a solvent, and a step of removing the solvent. Patent Document 1 also describes that desired pores can be formed in the sheet-like negative electrode by removing the pore-forming agent using a solvent. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-146581 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-103052 [Patent Document 3] Patent No. 7121449 Summary of the Invention [Problem to be solved by the invention]

[0004] As a result of intensive research by the present inventors, it was found that there is room for further improvement when the above technology is applied to, for example, an electricity storage device that requires both high capacity and high input / output. In particular, in an electricity storage device of a high input / output type that is mounted on a moving body such as a vehicle, it is particularly required to reduce the resistance of the electrodes and improve the input / output characteristics.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing an electrode with reduced resistance and a method for manufacturing an electricity storage device. [Means for solving the problem]

[0006] The present invention provides a method for manufacturing an electrode, including a first mixing step of mixing a first electrode active material with a pore-forming agent to obtain a mixture, a second mixing step of mixing the mixture with a binder to obtain an electrode composite, an application step of applying the electrode composite onto a current collector, and a heat treatment step of heat-treating the electrode composite on the current collector to remove the pore-forming agent.

[0007] In the manufacturing method disclosed herein, in the first mixing step, the first electrode active material and a pore-forming agent are mixed first, and the pore-forming agent is disposed around the first electrode active material. This allows voids to be suitably formed around the first electrode active material when the pore-forming agent is removed in the heat treatment step. As a result, the permeability of the electrolyte around the first electrode active material is improved, allowing for smooth movement (liquid flow) of the electrolyte during charge and discharge (especially during high-rate charge and discharge). This allows for the manufacture of a low-resistance electrode. Ultimately, this allows for the realization of an electricity storage device with excellent input / output characteristics. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view schematically illustrating an electricity storage device according to one embodiment. [Figure 2] FIG. 2 is a schematic vertical cross-sectional view taken along line II-II in FIG. [Figure 3] FIG. 3 is a schematic diagram showing the configuration of a wound electrode body. [Figure 4] FIG. 4 is a flowchart of a method for producing a negative electrode according to one embodiment. [Figure 5] FIG. 5 is a schematic diagram of the negative electrode composite before and after the heat treatment step according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Preferred embodiments of the technology disclosed herein will be described below with reference to the drawings. It should be noted that matters other than those specifically mentioned in this specification that are necessary for implementing the technology disclosed herein (for example, the general configuration and manufacturing process of an electricity storage device that does not characterize the present invention) can be understood as design matters for those skilled in the art based on prior art in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field.

[0010] In this specification, the term "electricity storage device" refers to a general device that can be repeatedly charged and discharged by the movement of charge carriers between a positive electrode and a negative electrode via an electrolyte. The concept of an electricity storage device encompasses secondary batteries such as lithium ion secondary batteries and nickel-metal hydride batteries, and capacitors such as lithium ion capacitors and electric double layer capacitors. In addition, in this specification, the expression "A to B" indicating a range not only means A or more and B or less, but also encompasses the meanings of "greater than A (exceeds A)" and "smaller than B (less than B)." Hereinafter, one embodiment will be described using a lithium ion secondary battery as an example.

[0011] 1. Structure of the electricity storage device First, an energy storage device 100 manufactured by the manufacturing method disclosed herein will be described. FIG. 1 is a perspective view of the energy storage device 100. FIG. 2 is a schematic longitudinal sectional view taken along line II-II in FIG. 1. In the following description, the symbol X indicates the "depth direction," the symbol Y indicates the "width direction" perpendicular to the depth direction, and the symbol Z indicates the "height direction" perpendicular to the depth and width directions. Furthermore, the symbol F in the depth direction X indicates the "front," and the symbol Rr indicates the "rear." In the width direction Y, the symbol L indicates the "left," and the symbol R indicates the "right." In the height direction Z, the symbol U indicates the "upper," and the symbol D indicates the "lower." However, these directions are merely used for the convenience of explanation and do not limit the installation form of the energy storage device 100 disclosed herein.

[0012] As shown in FIG. 2, the electricity storage device 100 includes an electrode assembly 40 having a positive electrode 10 and a negative electrode 20, a battery case 50, a positive electrode terminal 60, and a negative electrode terminal 65. Although not shown, the electricity storage device 100 further includes an electrolyte. The electricity storage device 100 is preferably a nonaqueous electrolyte secondary battery. The electricity storage device 100 is preferably a lithium ion secondary battery. The electricity storage device 100 is characterized by including the positive electrode 10 and / or the negative electrode 20 disclosed herein, and other configurations may be the same as conventional devices.

[0013] The battery case 50 is a housing that houses the electrode assembly 40 and the electrolyte. As shown in FIG. 1, the battery case 50 has a flat, bottomed, rectangular parallelepiped (square) outer shape. Any conventionally known material can be used for the battery case 50 without any particular restrictions. The battery case 50 is preferably made of metal. Examples of materials for the battery case 50 include aluminum, aluminum alloy, iron, and iron alloy. As shown in FIGS. 1 and 2, the battery case 50 includes an exterior body 52 and a sealing plate 54. The battery case 50 is preferably a prismatic battery including the exterior body 52 and the sealing plate 54. However, the battery case 50 may also be in the shape of a bag made of, for example, a laminate film.

[0014] As shown in FIG. 2, the exterior body 52 is a flat, bottomed, rectangular container having an opening 52h on its top surface. As shown in FIG. 1, the exterior body 52 includes a bottom wall 52a that is substantially rectangular in plan view, a pair of long side walls 52b extending upward in the height direction Z from the long sides of the bottom wall 52a, and a pair of short side walls 52c extending upward in the height direction Z from the short sides of the bottom wall 52a. The sealing plate 54 is a plate-like member that closes the opening 52h of the exterior body 52. ​​The sealing plate 54 has a substantially rectangular shape in plan view. The periphery of the sealing plate 54 is joined (e.g., welded) to the opening 52h of the exterior body 52. ​​This hermetically seals (hermetically seals) the battery case 50. The sealing plate 54 is provided with a liquid injection hole 55, a gas release valve 57, and two terminal insertion holes 58 and 59. The liquid injection hole 55 is a through-hole for injecting the electrolyte into the battery case 50 after the sealing plate 54 is attached to the exterior body 52. ​​After the electrolyte is injected, the liquid injection hole 55 is sealed with a sealing member 56. The gas release valve 57 is a thin-walled portion designed to rupture (open) when the pressure inside the battery case 50 reaches or exceeds a predetermined value, thereby releasing the gas to the outside.

[0015] The positive electrode terminal 60 and the negative electrode terminal 65 are attached to both ends of the sealing plate 54 in the width direction Y. As shown in FIG. 2 , the positive electrode terminal 60 and the negative electrode terminal 65 extend from the inside to the outside of the sealing plate 54 through terminal insertion holes 58, 59. A resin gasket 90 is attached to each of the terminal insertion holes 58, 59 of the sealing plate 54. This insulates the positive electrode terminal 60 and the negative electrode terminal 65 inserted through the terminal insertion holes 58, 59 from the sealing plate 54. As shown in FIGS. 1 and 2 , the positive electrode terminal 60 is connected to a plate-shaped positive electrode external conductive member 62 on the outer surface of the sealing plate 54. The negative electrode terminal 65 is connected to a plate-shaped negative electrode external conductive member 67. The positive electrode external conductive member 62 and the negative electrode external conductive member 67 are each insulated from the sealing plate 54 by a resin external insulating member 92. The positive electrode external conductive member 62 and the negative electrode external conductive member 67 are connected to other electricity storage devices or external equipment via external connection members (bus bars, etc.).

[0016] As shown in Fig. 2, the lower end 60c of the positive electrode terminal 60 is connected to a positive electrode current collector 70 inside the exterior body 52. ​​The positive electrode terminal 60 is connected to the positive electrode 10 (see Fig. 3) of the electrode assembly 40 via the positive electrode current collector 70. The lower end 65c of the negative electrode terminal 65 is connected to a negative electrode current collector 75 inside the exterior body 52. ​​The negative electrode terminal 65 is connected to the negative electrode 20 (see Fig. 3) of the electrode assembly 40 via the negative electrode current collector 75.

[0017] FIG. 3 is a schematic diagram showing the configuration of an electrode assembly 40. As shown in FIG. 3, the electrode assembly 40 has a flat outer shape. The electrode assembly 40 is a wound electrode assembly formed by stacking and winding a strip-shaped separator 32, a strip-shaped positive electrode 10, a strip-shaped separator 34, and a strip-shaped negative electrode 20. However, the electrode assembly 40 may also be a laminated electrode assembly formed by stacking a square-shaped (typically rectangular) positive electrode and a square-shaped (typically rectangular) negative electrode in an insulated state. The positive electrode 10 and / or the negative electrode 20 are examples of the "electrode" disclosed herein. As can be seen from FIGS. 2 and 3, the electrode assembly 40 is disposed inside the battery case 50 with the winding axis WL (see FIG. 3) oriented parallel to the width direction Y of the electricity storage device 100. The specific configuration of the electrode assembly 40 will be described below.

[0018] As shown in FIG. 3 , the positive electrode 10 is a strip-shaped member. The positive electrode 10 includes a strip-shaped positive electrode current collector 12, and a positive electrode active material layer 14 and a protective layer 16 fixed to at least one surface of the positive electrode current collector 12. From the viewpoint of achieving high capacity, the positive electrode active material layer 14 is preferably formed on both sides of the positive electrode current collector 12. Note that the protective layer 16 is not essential and may be omitted in other embodiments. Furthermore, when the negative electrode 20 is an electrode disclosed herein, the configuration of the positive electrode 10 is not particularly limited and may be the same as that of a conventional electrode. A metal foil having a predetermined conductivity can preferably be used as the positive electrode current collector 12. The positive electrode current collector 12 is preferably made of, for example, aluminum or an aluminum alloy. When the positive electrode 10 is an electrode disclosed herein, the positive electrode current collector 12 is an example of a "current collector."

[0019] In the positive electrode 10, a positive electrode tab 12t protrudes outward (to the left in FIG. 3 ) from one end edge in the width direction TD. The positive electrode tab 12t is a portion where the positive electrode active material layer 14 is not formed and where the positive electrode current collector 12 is exposed (current collector exposed portion). As shown in FIG. 3 , multiple positive electrode tabs 12t are stacked at one end (the left end in FIG. 3 ) of the electricity storage device 100 in the width direction Y to form a positive electrode tab group 42. As shown in FIG. 2 , a positive electrode current collecting member 70 is attached to the positive electrode tab group 42. The positive electrode tab group 42 is connected to the positive electrode terminal 60 via the positive electrode current collecting member 70.

[0020] 3, the positive electrode active material layer 14 is provided in a strip shape along the longitudinal direction of the positive electrode current collector 12. Although not particularly limited, the thickness (average thickness) of the positive electrode active material layer 14 per side is typically 10 μm or more, for example, 20 μm or more, or 50 μm or more, and is typically 300 μm or less, for example, 200 μm or less. The positive electrode active material layer 14 contains a positive electrode active material and a positive electrode binder.

[0021] The positive electrode active material may be any material capable of reversibly absorbing and releasing charge carriers, and materials conventionally used as positive electrode active materials can be used as appropriate. These materials may be used alone or in combination of two or more. The positive electrode active material preferably contains a lithium transition metal composite oxide. A suitable example of a lithium transition metal composite oxide is a lithium transition metal composite oxide represented by the general formula LiMO2 (where M is one or more transition metal elements other than Li). As the M, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn is preferred, a lithium transition metal composite oxide containing Ni is more preferred, and a lithium nickel cobalt manganese composite oxide containing Ni, Co, and Mn is particularly preferred.

[0022] The positive electrode active material is typically in the form of particles, bound to one another by a positive electrode binder, and may be fixed to the positive electrode current collector 12. Although not particularly limited, the average particle size D1 of the positive electrode active material is typically 1 μm or more, for example, 5 μm or more, 10 μm or more, and typically 30 μm or less, for example, 20 μm or less, 15 μm or less. In this specification, the term "average particle size" refers to the particle size (D50, also referred to as the median diameter) corresponding to a cumulative frequency of 50 volume % from the side of fine particles with small particle sizes in a volume-based particle size distribution based on a general laser diffraction / scattering method.

[0023] As the positive electrode binder, resin binders conventionally used as positive electrode binders can be used appropriately. Specific examples include vinyl halide resins such as polyvinylidene fluoride (PVdF) and polyalkylene oxides such as polyethylene oxide (PEO). These materials may be used alone or in combination of two or more. Although not particularly limited, the content of the positive electrode binder is typically 0.1 parts by mass or more, preferably 0.5 parts by mass or more, for example 1 part by mass or more, relative to 100 parts by mass of the positive electrode active material, and typically 10 parts by mass or less, preferably 8 parts by mass or less, for example 5 parts by mass or less.

[0024] In addition to the positive electrode active material and the positive electrode binder, the positive electrode active material layer 14 may further contain any optional components such as a conductive material. When the positive electrode 10 is an electrode disclosed herein, a pore-forming agent may remain in the positive electrode active material layer 14 due to the manufacturing method described below. In this case, the proportion of the pore-forming agent in the entire positive electrode active material layer 14 is preferably reduced to approximately 3 mass % or less, and preferably 1 mass % or less.

[0025] Examples of conductive materials include carbon blacks such as acetylene black (AB) and Ketjen black, carbon nanotubes (CNT), carbon fibers, carbon nanofibers, activated carbon, and other carbon materials. These materials may be used alone or in combination of two or more. Among these, acetylene black is preferred. Although not particularly limited, the content of the conductive material is typically 0.1 parts by mass or more, preferably 0.5 parts by mass or more, for example, 1 part by mass or more, relative to 100 parts by mass of the positive electrode active material, and typically 10 parts by mass or less, preferably 8 parts by mass or less, for example, 5 parts by mass or less.

[0026] The protective layer 16 is a layer configured to have lower electrical conductivity than the positive electrode active material layer 14. The protective layer 16 is provided in a region adjacent to the edge of the positive electrode 10 on the positive electrode tab 12t side. The protective layer 16 is formed in a strip shape along the longitudinal direction of the positive electrode current collector 12. The protective layer 16 preferably contains insulating ceramic particles such as alumina. The protective layer 16 may further contain a protective layer binder.

[0027] As shown in FIG. 3 , the negative electrode 20 is a strip-shaped member. The negative electrode 20 includes a strip-shaped negative electrode current collector 22 and a negative electrode active material layer 24 fixed to at least one surface of the negative electrode current collector 22. From the viewpoint of achieving high capacity, the negative electrode active material layer 24 is preferably formed on both sides of the negative electrode current collector 22. When the positive electrode 10 is an electrode disclosed herein, the configuration of the negative electrode 20 is not particularly limited and may be the same as that of a conventional negative electrode. A metal foil having a predetermined conductivity can be preferably used as the negative electrode current collector 22. The negative electrode current collector 22 is preferably made of, for example, copper or a copper alloy. When the positive electrode 10 is an electrode disclosed herein, the negative electrode current collector 22 is an example of a "current collector."

[0028] In the negative electrode 20, a negative electrode tab 22t protrudes outward (the right side in FIG. 3) from one end side in the width direction TD. The negative electrode tab 22t is a portion (current collector exposed portion) where the negative electrode current collector 22 is exposed without the formation of the negative electrode active material layer 24. As shown in FIG. 3, the plurality of negative electrode tabs 22t are laminated at one end portion in the width direction Y of the power storage device 100 (the right end portion in FIG. 3) to form a negative electrode tab group 44. As shown in FIG. 2, a negative electrode current collector member 75 is attached to the negative electrode tab group 44. The negative electrode tab group 44 is connected to the negative electrode terminal 65 via the negative electrode current collector member 75.

[0029] As shown in FIG. 3, the negative electrode active material layer 24 is provided in a strip shape along the longitudinal direction of the negative electrode current collector 22. Although not particularly limited, the thickness (average thickness) per one side of the negative electrode active material layer 24 is typically preferably 10 μm or more, for example, 20 μm or more, 50 μm or more, and typically preferably 300 μm or less, for example, 200 μm or less. The negative electrode active material layer 24 contains a negative electrode active material and a negative electrode binder.

[0030] The negative electrode active material may be any material that can reversibly occlude and release charge carriers, and materials conventionally used as negative electrode active materials can be appropriately used. These materials may be used alone or in combination of two or more. From the viewpoint of increasing the capacity, etc., the negative electrode active material preferably contains a Si (silicon)-containing material. Preferred examples of the Si-containing material include Si, SiC composites, silicon oxide represented by SiOa (where 0.05 < a < 1.95), silicon carbide represented by SiCb (0 < b < 1), silicon nitride represented by SiNc (0 < c < 4 / 3), and those in which nano-Si particles are dispersed in porous particles, etc. Among them, SiC composites are preferred. Although not particularly limited, when the total amount of the negative electrode active material is 100% by mass, the content ratio of the Si-containing material is preferably 10 to 60% by mass.

[0031] From the viewpoint of achieving a high level of various properties (for example, high capacity and high-rate cycle characteristics), the negative electrode active material preferably contains graphite such as artificial graphite or natural graphite in addition to the Si-containing material. In this case, the mass ratio of the Si-containing material to the graphite is preferably Si-containing material:graphite=10:90 to 60:40. The negative electrode active material may contain negative electrode active materials other than the Si-containing material and graphite. Specific examples of negative electrode active materials other than the Si-containing material and graphite include carbon materials such as hard carbon, soft carbon, and amorphous carbon.

[0032] The negative electrode active material is typically in the form of particles, bound to one another by a negative electrode binder, and may be fixed to the negative electrode current collector 22. Although not particularly limited, the average particle size D1 of the negative electrode active material is typically 1 μm or more, for example, 3 μm or more, 5 μm or more, and typically 30 μm or less, for example, 20 μm or less, 10 μm or less.

[0033] As the negative electrode binder, any resin binder conventionally used as a negative electrode binder can be used as appropriate. Specific examples include rubbers such as styrene butadiene rubber (SBR), celluloses such as carboxymethyl cellulose (CMC), and acrylic resins (resins obtained by polymerizing a monomer having an acryloyl group) such as polyacrylic acid (PAA). The negative electrode binder preferably contains SBR, CMC, and PAA. Although not particularly limited, the content of the negative electrode binder is typically 0.5 parts by mass or more, preferably 1 part by mass or more, for example, 2 parts by mass or more, relative to 100 parts by mass of the negative electrode active material, and typically 10 parts by mass or less, preferably 8 parts by mass or less, for example, 5 parts by mass or less.

[0034] The negative electrode active material layer 24 may further contain optional components such as a conductive material and a dispersant in addition to the negative electrode active material and the negative electrode binder. When the negative electrode 20 is an electrode disclosed herein, a pore-forming agent may remain in the negative electrode active material layer 24 due to the manufacturing method described below. In this case, the proportion of the pore-forming agent in the entire negative electrode active material layer 24 is preferably reduced to approximately 3 mass % or less, and preferably 1 mass % or less.

[0035] The conductive material may be one or more of the carbon materials exemplified as optional components that may be contained in the positive electrode active material layer 14. Among these, fibrous carbon materials (fibrous carbon) are preferred. Specifically, carbon fibers, carbon nanofibers, carbon nanotubes (CNTs), etc. are preferred, with single-walled carbon nanotubes (SWCNTs) being more preferred. Although not particularly limited, the content of the conductive material is typically 0.1 parts by mass or more, preferably 0.5 parts by mass or more, for example 1 part by mass or more, and typically 10 parts by mass or less, preferably 8 parts by mass or less, for example 5 parts by mass or less, relative to 100 parts by mass of the negative electrode active material.

[0036] As shown in FIG. 3, the separators 32 and 34 are strip-shaped members. The separators 32 and 34 are disposed between the positive electrode 10 and the negative electrode 20. The separators 32 and 34 are insulating sheets each having a plurality of fine through-holes through which charge carriers can pass. The separators 32 and 34 may have the same configuration or different configurations. Any separator conventionally used can be used as the separators 32 and 34 without any particular limitations. Suitable separators 32 and 34 are porous resin sheets made of polyolefin resins such as polyethylene (PE) and polypropylene (PP). The surfaces of the separators 32 and 34 may be provided with functional layers, such as an adhesive layer containing a binder or a heat-resistant layer (HRL) containing an inorganic filler.

[0037] The electrolyte may be the same as conventional ones and is not particularly limited. The electrolyte is, for example, a non-aqueous electrolyte containing a non-aqueous solvent (organic solvent) and a supporting salt (electrolyte salt). The electrolyte is preferably a non-aqueous electrolyte. Examples of non-aqueous solvents include aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones. Among them, carbonates, for example, cyclic carbonates such as ethylene carbonate (EC) and monofluoroethylene carbonate (FEC), and chain carbonates such as dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), are preferred. An example of the supporting salt is a fluorine-containing lithium salt such as lithium hexafluorophosphate (LiPF6). The electrolyte may further contain additives as needed.

[0038] <2. Negative electrode manufacturing method> An example of the method for manufacturing an electrode disclosed herein will be described in detail below using the negative electrode 20 as an example, but the positive electrode 10 can also be manufactured in a similar manner. In that case, the "negative electrode" below can be appropriately read as the "positive electrode."

[0039] 4 is a flowchart of a method for manufacturing a negative electrode according to one embodiment. As shown in FIG. 4, the negative electrode 20 can be manufactured by a manufacturing method including, for example, a first mixing step (S1), a second mixing step (S2), an application step (S3), a drying step (S4), a pressing step (S5), and a heat treatment step (S6), in this order. However, the drying step (S4) and the pressing step (S5) are not essential and may be omitted in other embodiments. Furthermore, the manufacturing method disclosed herein may further include other steps at any stage.

[0040] As shown in FIG. 4, the first mixing step (S1) is a step of mixing a first negative electrode active material (first electrode active material) with a pore-forming agent to obtain a mixture. The method for mixing the first negative electrode active material with the pore-forming agent is not particularly limited, and a conventionally known dry mixing method or wet mixing method can be appropriately adopted. From the viewpoints of simplicity and cost, the dry mixing method (dry mix) is preferred. Mixing can be performed using, for example, an agitator granulator, a mortar, a ball mill, a jet mill, a planetary mixer, a disperser, or the like. This step may be performed in an environment of approximately 40°C or less, typically room temperature (e.g., 25±10°C, preferably 25±5°C).

[0041] As the first negative electrode active material, one of the above-mentioned materials can be used alone or two or more of them can be used in combination as appropriate. Among these, it is preferable that the first negative electrode active material contains a Si-containing material, and it is more preferable that the first negative electrode active material is made of a Si-containing material. According to the inventor's research, while Si-containing materials have a larger specific capacity than carbon materials such as graphite, they tend to expand and contract significantly during charge and discharge, making their conductive paths more susceptible to disconnection. Therefore, conductive paths are likely to be disconnected during charge and discharge cycles (especially high-rate cycles), leading to a deterioration in cycle characteristics. Therefore, when a Si-containing material is used as the first negative electrode active material, the technology disclosed herein is particularly effective.

[0042] The pore-forming agent is a component that vaporizes and burns out in the heat treatment step (S6) described below. By adding the pore-forming agent in this step, voids can be suitably formed around the first negative electrode active material in the negative electrode active material layer 24 after the heat treatment step (S6). The pore-forming agent preferably has a higher thermal decomposition temperature than the dispersion solvent used in the second mixing step (S2). In order to improve burn-out in the heat treatment step (S6), the pore-forming agent preferably has a lower thermal decomposition temperature than the binder used in the second mixing step (S2). The thermal decomposition temperature of the pore-forming agent is preferably approximately 150°C or higher, for example, 200°C or higher, or 300°C or higher, and is preferably approximately 500°C or lower, or 400°C or lower.

[0043] In this specification, the term "thermal decomposition temperature" refers to the temperature at which a pore-forming agent vaporizes due to an increase in temperature. The thermal decomposition temperature can be determined by conventionally known differential thermal analysis (DTA). For example, in resin materials, thermal decomposition may occur stepwise as the temperature increases. In such cases, the lowest temperature is taken as the thermal decomposition temperature.

[0044] The pore-forming agent is preferably particulate. The shape of the pore-forming agent is not particularly limited, but is preferably spherical or approximately spherical from the viewpoint of forming uniform voids in the negative electrode active material layer 24. In this specification, the term "approximately spherical" is a term that also encompasses spherical, rugby ball-shaped, polygonal, and the like, and refers to, for example, an average aspect ratio (the ratio of the length in the long axis direction to the length in the shortest rectangle circumscribing the particle) of approximately 1 to 2, for example, 1 to 1.5.

[0045] The pore-forming agent is preferably insoluble or poorly soluble in the dispersion solvent (e.g., water or NMP (N-methyl-2-pyrrolidone)) used in the second mixing step (S2). In this specification, "insoluble" means that the solubility in the dispersion solvent (e.g., water or NMP) at 20°C is 1 g / 100 g or less, and "poorly soluble" means that the solubility in the dispersion solvent (e.g., water or NMP) at 20°C is less than 10 g / 100 g.

[0046] The pore-forming agent preferably has a smaller swelling rate with respect to the dispersion solvent (e.g., water or NMP) used in the second mixing step (S2) than the negative electrode binder (binder) described below. The pore-forming agent preferably maintains a particulate shape (e.g., spherical shape) in the negative electrode composite paste prepared in the second mixing step (S2).

[0047] The pore-forming agent is preferably a resin material, more preferably a thermoplastic resin. Specific examples of the pore-forming agent include polymer materials, such as methacrylic resins (resins obtained by polymerizing monomers having methacrylic groups) such as polymethyl methacrylate (PMMA), styrene resins such as polystyrene (PS), and polyolefin resins such as polypropylene (PP). The polymer material is typically a synthetic polymer (synthetic resin), but may also be a natural polymer (natural resin). These may be used alone or in combination of two or more. Furthermore, the polymer material may contain various additives (e.g., antioxidants) that are known to be added to conventional polymer materials, as needed. Among these, it is preferable to contain at least one of PMMA, PS, and PP. Furthermore, from the viewpoint of reducing battery resistance, it is more preferable to contain a methacrylic resin. In particular, PMMA obtained by polymerizing at least methyl methacrylate monomer as a raw material is preferred.

[0048] Although not particularly limited, the mixing ratio (mass ratio) of the first negative electrode active material (particularly, the Si-containing material) and the pore-forming agent is preferably 95:5 to 88:12 (first negative electrode active material:pore-forming agent). By setting the ratio of the pore-forming agent to a predetermined value or more, it becomes easier to ensure a suitable porosity around the first negative electrode active material in the negative electrode active material layer 24. As a result, the permeability of the electrolyte around the first negative electrode active material is improved, and the movement (liquid flow) of the electrolyte during charge and discharge is smooth. This reduces the resistance of the negative electrode 20 and improves input / output characteristics. Furthermore, by setting the ratio of the pore-forming agent to a predetermined value or less, excessive voids around the first negative electrode active material are prevented. As a result, for example, the expansion of the Si-containing material during charge improves the packing property of the negative electrode active material layer 24, facilitating the formation of conductive paths, thereby improving cycle characteristics. Furthermore, by utilizing the voids, expansion and contraction of the negative electrode active material layer 24 (particularly in the thickness direction) can be suppressed.

[0049] In a preferred embodiment, the first negative electrode active material and the pore-forming agent are each particulate. While not particularly limited, the average particle size D1 of the first negative electrode active material is preferably 1 to 30 μm, more preferably 5 to 20 μm, and even more preferably 5 to 10 μm. The average particle size D2 of the pore-forming agent is preferably 0.1 to 10 μm, and more preferably 1 to 5 μm. In this process, the particle size ratio (D2 / D1) of the average particle size D2 of the pore-forming agent to the average particle size D1 of the first negative electrode active material is typically 2 or less, for example, 1.5 or less, preferably approximately 0.1 to 1, more preferably 0.1 to 0.5, and particularly preferably 0.4 to 0.5. This facilitates the formation of voids of an appropriate size around the first negative electrode active material in the negative electrode active material layer 24. As a result, for example, the expansion of the Si-containing material during charging improves the packing ability of the negative electrode active material layer 24, making it easier to form conductive paths, thereby improving cycle characteristics. Furthermore, the voids can be utilized to suppress expansion and contraction of the negative electrode active material layer 24 (particularly in the thickness direction).

[0050] As shown in FIG. 4, the second mixing step (S2) is a step in which the mixture obtained in the first mixing step (S1) is mixed with a negative electrode binder (binder) to obtain a negative electrode composite (electrode composite). The method for mixing the mixture and the negative electrode binder is not particularly limited, and a conventionally known dry mixing method or wet mixing method can be appropriately adopted. Mixing can be performed, for example, using an agitator granulator, mortar, ball mill, jet mill, planetary mixer, disperser, or the like. This step may be performed at a temperature of approximately 40°C or lower, typically at room temperature (e.g., 25±10°C, preferably 25±5°C).

[0051] In this step, components other than the negative electrode binder may be further mixed and contained in the negative electrode composite within a range that does not significantly impair the effects of the present invention. Examples include a second negative electrode active material (second electrode active material), a conductive material, a dispersion solvent, and the like, as shown in FIG. 4. To improve the coatability in the application step (S3), the negative electrode composite is preferably prepared in a paste form (including a slurry form and an ink form) containing a dispersion solvent. In a preferred embodiment, this step includes a pre-paste preparation step (step S21), a pre-mixing step (step S22), and a paste preparation step (step S23).

[0052] The preliminary paste preparation step (step S21) is a step of adding a dispersion solvent to the mixture obtained in the first mixing step (S1) to prepare a paste. The dispersion solvent may be an aqueous solvent or a non-aqueous solvent. In particular, when producing the negative electrode 20, from the viewpoint of reducing the environmental load, the dispersion solvent is preferably water or a mixed solvent mainly composed of water, and more preferably water. However, when producing, for example, the positive electrode 10, the dispersion solvent may be a non-aqueous solvent (organic solvent) such as NMP.

[0053] The pre-mixing step (step S22) is a step of dry-mixing the second negative electrode active material and a powdered negative electrode binder to obtain a powdered pre-mixture. This can improve the homogeneity and integrity of the negative electrode active material layer 24. As the second negative electrode active material, one type alone or a mixture of two or more types from the materials described above can be used as appropriate. In particular, the second negative electrode active material preferably contains graphite, and more preferably consists of graphite. It is preferable that the second negative electrode active material does not substantially contain a Si-containing material (the Si-containing material accounts for less than 5 mass% of the entire second negative electrode active material).

[0054] The negative electrode binder is preferably soluble in a dispersion solvent (e.g., water or NMP). In this specification, "soluble" means that the solubility in the dispersion solvent (e.g., water or NMP) at 20°C is 10 g / 100 g or more. When the dispersion solvent is an aqueous solvent (e.g., water), the binder preferably contains a cellulose such as CMC or an acrylic resin such as PAA. When the dispersion solvent is a non-aqueous solvent (e.g., NMP), the binder preferably contains a vinyl halide resin such as PVdF. The negative electrode binder preferably has a swelling rate in the dispersion solvent greater than that of the pore-forming agent (e.g., PMMA).

[0055] Although not particularly limited, the amount of the pore-forming agent added is typically 0.01 to 10 parts by mass, preferably 0.05 to 5 parts by mass, more preferably 0.1 to 3 parts by mass, and particularly preferably 0.5 to 2 parts by mass, relative to 100 parts by mass of the total amount of the negative electrode active material (here, the sum of the first negative electrode active material and the second negative electrode active material). By adding a pore-forming agent in an amount equal to or greater than a predetermined value, it becomes easier to ensure a suitable porosity around the first negative electrode active material in the negative electrode active material layer 24. As a result, the permeability of the electrolyte around the first electrode active material is improved, and the movement (liquid flow) of the electrolyte during charge and discharge is smoother. This reduces the resistance of the negative electrode 20 and improves input / output characteristics. Furthermore, by adding a pore-forming agent in an amount equal to or less than a predetermined value, it becomes possible to prevent the area around the first negative electrode active material from becoming too porous. As a result, for example, the Si-containing material expands during charging, improving the packing of the negative electrode active material layer 24 and facilitating the formation of conductive paths, thereby improving cycle characteristics. In addition, the voids can be utilized to suppress expansion and contraction of the negative electrode active material layer 24 (particularly in the thickness direction).

[0056] The paste preparation step (step S23) is a step of preparing a negative electrode composite paste by mixing the paste-like mixture obtained in the first mixing step with the preliminary mixture obtained in the second mixing step, adding a conductive material and a liquid negative electrode binder (e.g., SBR), and diluting and mixing with a dispersion solvent. As the conductive material, one type alone or a mixture of two or more types from the materials described above can be used as appropriate. In particular, it is preferable that the conductive material contains fibrous carbon (e.g., SWCNT). The solid content concentration of the negative electrode composite paste can be appropriately determined depending on, for example, the application method in the application step (S3).

[0057] The application step (S3) is a step of applying the negative electrode composite (electrode composite) obtained in the second mixing step (S2) onto the negative electrode current collector 22 (current collector). The method of applying the negative electrode composite is not particularly limited and may be the same as conventional methods. The negative electrode composite can be applied (coated) onto the surface (one or both sides) of the current collector using a coating device such as a gravure coater, slit coater, die coater, comma coater, or dip coater. The amount of coating may be determined appropriately depending on, for example, the solids concentration of the negative electrode composite so that the negative electrode active material layer 24 has the desired properties (thickness, etc.).

[0058] The drying step (S4) is a step of drying the negative electrode mixture (electrode mixture) on the negative electrode current collector 22 (current collector) to remove the dispersion solvent. Here, this is a step of removing moisture contained in the negative electrode mixture. In this specification, "remove" means to remove at least a portion (all or a part). The drying method is not particularly limited, and a conventionally known heat drying method, reduced pressure drying method, or the like can be appropriately adopted. From the viewpoint of production efficiency, etc., the heat drying method is preferred. It is preferable to set the drying conditions (drying temperature and drying time) so that the dispersion solvent is removed while components other than the dispersion solvent (for example, a pore-forming agent, a negative electrode binder, etc.) remain.

[0059] The drying temperature may be appropriately determined depending on the components of the negative electrode mixture (e.g., the type of dispersion solvent, the type of pore-forming agent, the type of negative electrode binder), etc. The drying temperature is set to a temperature at which at least a portion of the dispersion solvent is vaporized (particularly, volatilized). The drying temperature may be, for example, a temperature equal to or higher than the thermal decomposition temperature (or boiling point) of the dispersion solvent. On the other hand, the drying temperature is preferably a temperature at which components other than the dispersion solvent (e.g., the pore-forming agent, the negative electrode binder, etc.) are not decomposed or altered. The drying temperature is typically lower than the heat treatment temperature in the heat treatment step (S6) described below. The drying temperature may be, for example, a temperature lower than the thermal decomposition temperature of the pore-forming agent or the negative electrode binder. In one example, the drying temperature can be set to approximately 40 to 200°C, or 40 to 150°C. The drying time may be appropriately determined depending on, for example, the type and content of the dispersion solvent, the drying temperature, etc.

[0060] The pressing step (S5) is a step of pressing the negative electrode composite on the negative electrode current collector 22. The pressing method and pressing conditions are not particularly limited and may be the same as conventional methods. In one example, the pressing may be performed using a pressing machine such as a roll press. The pressing conditions (e.g., pressure, holding time, etc.) may be appropriately set so that the negative electrode active material layer has the desired properties, such as thickness and density. Pressing may be performed at room temperature or while heating (at a high temperature).

[0061] The heat treatment step (S6) is a step of heating the negative electrode composite on the negative electrode current collector 22 to remove the pore-forming agent. The heat treatment method is not particularly limited, and can be performed using a conventionally known heating device. The heat treatment is preferably performed until the content of the pore-forming agent in the negative electrode composite is approximately 3% by mass or less, preferably 1% by mass or less. The heat treatment is preferably performed in an inert gas atmosphere or under reduced pressure. Examples of the inert gas atmosphere include a nitrogen (N2) atmosphere and a rare gas atmosphere such as argon (Ar). The heat treatment conditions (heat treatment temperature and heat treatment time) are preferably set so that the pore-forming agent is removed while components other than the pore-forming agent (in other words, components of the negative electrode active material layer 24, such as the negative electrode binder) remain. The heat treatment conditions can be easily determined, for example, by conducting a preliminary experiment using a thermal desorption gas analyzer.

[0062] The heat treatment temperature may be appropriately determined depending on the composition of the negative electrode composite after the pressing step (S5) (e.g., the type of pore-forming agent, the type of negative electrode binder), etc. The heat treatment temperature is set to a temperature at which at least a portion of the pore-forming agent is vaporized (particularly, volatilized). The heat treatment temperature is typically a temperature higher than the drying temperature in the drying step (S4). The heat treatment temperature may be, for example, 10°C or higher, 20°C or higher, 30°C or higher, or even 50°C or higher, or 100°C or higher than the drying temperature. The heat treatment temperature may be, for example, a temperature higher than the thermal decomposition temperature of the pore-forming agent (the lowest temperature when multiple types of pore-forming agents are included). The heat treatment temperature may be, for example, 10°C or higher, 20°C or higher, 30°C or higher, or even 50°C or higher than the thermal decomposition temperature of the pore-forming agent. On the other hand, the heat treatment temperature is preferably a temperature at which the constituent components of the negative electrode active material layer 24 (e.g., the negative electrode binder, etc.) do not decompose or deteriorate. The heat treatment temperature is preferably lower than the thermal decomposition temperature of the negative electrode binder (the lowest temperature when multiple types of negative electrode binders are included). The heat treatment temperature is preferably lower than the thermal decomposition temperature of the negative electrode binder by 10°C or more, 20°C or more, 30°C or more, or even 50°C or more. In a preferred embodiment, the heat treatment temperature is preferably 200 to 400°C, more preferably 250 to 350°C.

[0063] The heat treatment time may be appropriately determined depending on, for example, the type and content of the pore-forming agent, the heat treatment temperature, etc. Therefore, it is not particularly limited, but is, for example, 1 minute or more, preferably 5 minutes or more, and more preferably 10 minutes or more. On the other hand, from the viewpoint of production efficiency, etc., the heat treatment time is preferably 1 hour or less, more preferably 30 minutes or less.

[0064] FIG. 5 is a schematic diagram of a negative electrode composite material before and after this step according to one embodiment. In the manufacturing method disclosed herein, in the first mixing step (S1), a Si-containing material (first negative electrode active material) and a pore-forming agent are first mixed. Therefore, as shown in the upper diagram of FIG. 5, before heat treatment, the pore-forming agent is distributed around the Si-containing material, for example, in a larger amount than the negative electrode binder. In other words, the pore-forming agent is unevenly distributed around the Si-containing material. Therefore, when the pore-forming agent burns through in this step, voids are preferably formed around the Si-containing material, as shown in the lower diagram of FIG. 5. This improves the permeability of the electrolyte around the Si-containing material, facilitating smooth movement (liquid flow) of the electrolyte during charge and discharge. This reduces the resistance of the negative electrode 20 and improves input / output characteristics. Furthermore, the expansion of the Si-containing material during charge and discharge improves the packing of the negative electrode active material layer 24, facilitating the formation of conductive paths, thereby improving cycle characteristics. Furthermore, the voids can be utilized to suppress expansion and contraction (particularly in the thickness direction) of the negative electrode active material layer 24. In this manner, the negative electrode 20 (electrode) can be produced.

[0065] <3. Method for manufacturing an electricity storage device> The positive electrode 10 and / or negative electrode 20 produced as described above can be suitably used to produce an electricity storage device 100 according to a known method.

[0066] <4. Uses of electricity storage devices> The electricity storage device 100 can be used for a variety of purposes, but because of its reduced resistance and excellent input / output characteristics, it can be particularly suitably used as a power source (driving power source) for a motor mounted on a moving body, for example, a vehicle such as a passenger car, a truck, etc. The type of vehicle is not particularly limited, and examples thereof include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and a battery electric vehicle (BEV).

[0067] Examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples.

[0068] <Test Example I: Method of manufacturing negative electrode> In this test example, a negative electrode was produced by the production method disclosed herein, and was placed opposite a conventional positive electrode to construct an electricity storage device, and the characteristics of the device were then evaluated.

[0069] <<Fabrication of Negative Electrode>> First, negative electrodes were fabricated in which the negative electrode active material layer contained the following components in common. Negative electrode active material: SiC composite, graphite Conductive material: fibrous carbon (specifically, SWCNT) Negative electrode binder: CMC, PAA, SBR

[0070] (Example 1) First, using an agitation granulator, SiC composite particles (SiC, average particle size D1: 7 μm) as a first negative electrode active material and polymethyl methacrylate particles (PMMA, average particle size D2: 3 μm) as a pore-forming agent were dry-mixed to obtain a first mixture (first mixing step). Next, a dispersion solvent (water) was added to the obtained mixture, and a paste was prepared using a disperser. Next, using an agitation granulator, graphite particles as a second negative electrode active material and carboxymethyl cellulose particles (CMC) and polyacrylic acid particles (PAA) as a negative electrode binder were dry-mixed to obtain a powdery premix. Next, the premix obtained in the second mixing step was mixed with the paste-like mixture obtained in the first mixing step, and further fibrous carbon (specifically, SWCNT) as a conductive material and styrene-butadiene rubber (SBR) as a negative electrode binder were added, and the mixture was diluted and mixed with the dispersion solvent (water) to prepare a negative electrode composite paste (second mixing step). In the negative electrode composite paste, the blending ratio of graphite particles as the second negative electrode active material to SiC composite particles as the first negative electrode active material was 85:15. The blending ratio of other components relative to the total amount (100 parts by mass) of the negative electrode active material was PMMA:SWCNT:CMC:PAA:SBR=1:0.1:1:1:1.5 (parts by mass).

[0071] Next, the prepared negative electrode composite paste was applied to a Cu foil (thickness: 10 μm) as a negative electrode current collector (application step). Next, the negative electrode composite paste on the negative electrode current collector was heated and dried (50°C, 3 min) while applying hot air to remove the dispersion solvent (drying step). Next, the negative electrode composite on the negative electrode current collector was pressed to a predetermined thickness to densify the negative electrode composite (pressing step). Next, the negative electrode composite on the negative electrode current collector was heat-treated (300°C, 20 min) in an inert gas atmosphere to remove the pore-forming agent (heat treatment step). Then, by processing to predetermined dimensions, a negative electrode having a negative electrode active material layer on the negative electrode current collector was fabricated.

[0072] (Examples 2) to (Example 9) Negative electrodes were prepared in the same manner as in Example 1, except for the conditions shown in Table 1.

[0073] Comparative Example 1: A negative electrode was fabricated in the same manner as in Example 1, except that a negative electrode composite paste was prepared without adding a pore-forming agent. Specifically, first, SiC composite particles and graphite particles as the negative electrode active material, and CMC and PAA as the negative electrode binder were dry-mixed using a stirring granulator to obtain a mixture. Next, SWCNT as the conductive material and SBR as the negative electrode binder were added to the obtained mixture, and the mixture was diluted and mixed with a dispersion solvent (water) to prepare a negative electrode composite paste.

[0074] (Comparative Example 2) A negative electrode was fabricated in the same manner as in Example 1, except that the negative electrode composite paste was prepared without premixing the SiC composite particles and pore-forming agent particles. First, using a stirring granulator, SiC composite particles and graphite particles as the negative electrode active material, CMC and PAA as the negative electrode binder, and PMMA as the pore-forming agent were dry-mixed all at once to obtain a mixture. Next, SWCNT as the conductive material and SBR as the negative electrode binder were added to the obtained mixture, and the mixture was diluted and mixed with a dispersion solvent (water) to prepare a negative electrode composite paste.

[0075] <<Preparation of Positive Electrode>> Next, a positive electrode containing the following components in the positive electrode active material layer was prepared. Positive electrode active material: Lithium nickel cobalt manganese composite oxide (NCM) Conductive material: AB Positive electrode binder: PVdF Specifically, first, NCM as the positive electrode active material, AB as the conductive material, and PVdF as the positive electrode binder were mixed in a mass ratio of NCM:AB:PVdF = 100:1:1. Next, the fluidity was adjusted with a dispersion solvent (NMP) to prepare a positive electrode composite paste. Next, the prepared positive electrode composite paste was applied to an Al foil (thickness: 15 μm) as a positive electrode current collector, dried, and pressed to a predetermined thickness. Then, by processing to the predetermined dimensions, a positive electrode having a positive electrode active material layer on the positive electrode current collector was fabricated.

[0076] <Construction of Test Cell> First, the negative electrode and positive electrode prepared above were stacked with a separator interposed therebetween to prepare an electrode assembly. Next, the prepared electrode assembly was inserted into an exterior body made of an aluminum laminate sheet, a non-aqueous electrolyte was poured, and the opening of the exterior body was sealed to construct a test cell (laminated cell). The non-aqueous electrolyte used was a mixed solvent of EC, FEC, EMC, and DMC in a volume ratio of 15:5:40:40, in which LiPF6 as a supporting salt (Li salt) was dissolved at a concentration of 1.0 mol / L.

[0077] <Evaluation of Battery Resistance> At 25°C, the battery was charged at a constant current of 0.3 C until the state of charge (SOC) reached 50%, followed by constant voltage charging at 0.02 C at 50% SOC. After storage at 25°C for 1 hour, the battery was discharged at a constant current of 1 C for 10 seconds. DC resistance (Ω) was calculated by dividing the difference between the open circuit voltage (OCV) and the closed circuit voltage (CCV) 10 seconds after discharge by the discharge current 10 seconds after discharge, as shown in the following formula. The results are shown in Table 1. DC resistance = (OCV-CCV) / Discharge current value 10 seconds after discharge

[0078] <Evaluation of cycle characteristics> In a 25°C environment, 250 charge / discharge cycles were performed, with one cycle consisting of CCCV charging (constant current charging at a constant current of 0.4 C up to 4.2 V, followed by constant voltage charging until the current value reached 0.1 C), followed by CC discharging (constant current discharging at a constant current of 0.4 C down to 2.5 V). The capacity retention rate (%) was calculated using the following formula from the discharge capacity at the first cycle and the discharge capacity at the 250th cycle. The results are shown in Table 1. Capacity retention rate = (discharge capacity at 250th cycle / discharge capacity at 1st cycle) x 100

[0079] <Evaluation of Negative Electrode Swelling Rate> After the cycle test, the test cell (discharged to 2.5 V) was disassembled under an inert gas atmosphere, and the thickness of the negative electrode was measured. The swelling rate (%) was calculated using the following formula from the thickness of the negative electrode when the test cell was constructed (before the cycle test) and the thickness of the negative electrode after the cycle test. The results are shown in Table 1. Swelling rate = (thickness of negative electrode after cycle test / thickness of negative electrode before cycle test) × 100

[0080] [Table 1]

[0081] <Evaluation Results> As shown in Table 1, Example 1, in which a pore-forming agent was added to the negative electrode and the first negative electrode active material and the pore-forming agent were premixed in the first mixing step, was compared with Comparative Examples 1 and 2. In Example 1, the battery resistance was significantly reduced and the capacity retention rate after cycle testing was also significantly improved. This is believed to be because first mixing the SiC composite and the pore-forming agent in the first mixing step allowed the pore-forming agent to be optimally positioned around the SiC composite. Furthermore, removing the pore-forming agent in the heat treatment step allowed for optimal void formation around the SiC composite, thereby suppressing conductive path disconnection during expansion and contraction of the SiC composite. Furthermore, in Example 1, the expansion rate of the negative electrode was significantly reduced. This is believed to be because the voids formed around the SiC composite absorbed the expansion and contraction of the SiC composite, thereby alleviating internal stress in the negative electrode.

[0082] Comparing Examples 1, 4, 5, and 9, which were heat-treated at different temperatures, it was found that the heat-treatment temperature was preferably 200 to 400° C., and more preferably 250 to 350° C. This is thought to be because by setting the heat-treatment temperature to a predetermined value or higher, the pore-forming agent could be suitably removed, making it easier to secure suitable voids around the SiC composite.

[0083] Comparing Examples 1 to 3 and 6, which differ in the amount of pore-forming agent added, it was found that the amount of pore-forming agent added is preferably 0.1 to 3 parts by mass, and more preferably 1 to 2 parts by mass. This is thought to be because, by setting the amount of pore-forming agent added to a predetermined value or less, the area around the SiC composite does not become too void, making it easier to create a conductive path.

[0084] Comparing Examples 1 to 3, 7, and 8, which have different particle size ratios (D2 / D1), it was found that the particle size ratio (D2 / D1) is preferably 0.1 to 1, and more preferably 0.4 to 0.5. This is thought to be because setting the particle size ratio (D2 / D1) within a predetermined range makes it easier to form voids of an appropriate size around the first negative electrode active material.

[0085] <Test Example II: Method of manufacturing positive electrode> In this test example, a positive electrode was produced by the production method disclosed herein, and an electricity storage device was constructed by placing it opposite a conventional negative electrode, and the characteristics of the device were then evaluated.

[0086] (Example 10) First, using an agitation granulator, lithium nickel cobalt manganese composite oxide particles (NCM, average particle size: 15 μm) as a positive electrode active material and polymethyl methacrylate particles (PMMA, average particle size: 3 μm) as a pore-forming agent were dry-mixed to obtain a mixture (first mixing step). Next, a dispersion solvent (NMP) was added to the obtained mixture, and a paste was prepared using a disperser. Next, using an agitation granulator, a dispersion liquid of polyvinylidene fluoride (PVdF) as a positive electrode binder and acetylene black (AB) as a conductive material were mixed to obtain a premix. Next, the premix was mixed with the paste-like mixture obtained in the first mixing step, and the mixture was diluted and mixed with the dispersion solvent (NMP) to prepare a positive electrode composite paste (second mixing step). Note that the mass ratio of each component in the positive electrode composite paste was NCM:PMMA:AB:PVdF = 100:1:1:1.

[0087] Next, the prepared positive electrode composite paste was applied to an Al foil (thickness: 15 μm) as a positive electrode current collector (application step). Next, the positive electrode composite paste on the positive electrode current collector was heated and dried (100 to 120°C, 2 to 3 minutes) while applying hot air to remove the dispersion solvent (drying step). Next, the positive electrode composite on the positive electrode current collector was pressed to a predetermined thickness to densify the positive electrode composite (pressing step). Next, the positive electrode composite on the positive electrode current collector was heat-treated (300°C, 20 minutes) in an inert gas atmosphere to remove the pore-forming agent (heat treatment step). Then, by processing to a predetermined dimension, a positive electrode having a positive electrode active material layer on a positive electrode current collector was fabricated.

[0088] Comparative Example 3: A positive electrode was fabricated in the same manner as in Example 10, except that the lithium-nickel-cobalt-manganese composite oxide particles and pore-forming agent particles were not premixed, and a positive electrode composite paste was prepared as follows. Specifically, first, using a stirring granulator, NCM as the positive electrode active material, PMMA as the pore-forming agent, AB as the conductive material, and PVdF as the positive electrode binder were dry-mixed all at once to obtain a mixture. The resulting mixture was then diluted and mixed with a dispersion solvent (NMP) to prepare a positive electrode composite paste.

[0089] <Construction and Evaluation of Test Cell> A test cell was fabricated in the same manner as in Test Example I, using the negative electrode of Comparative Example 1 of Test Example I. Then, the battery resistance was evaluated in the same manner as in Test Example I. Specifically, the DC resistance (Ω) was calculated from the above formula. The results are shown in Table 2. Table 2 also shows the battery resistance of Comparative Example 1, in which no pore-forming agent was added to the positive electrode.

[0090] [Table 2]

[0091] <Evaluation Results> As shown in Table 2, in Example 10, in which a pore-forming agent was added to the positive electrode and the positive electrode active material and the pore-forming agent were premixed in the first mixing step, the battery resistance was relatively lower and the capacity retention rate after the cycle test was relatively higher compared to Comparative Examples 1 and 3. Furthermore, the expansion rate of the negative electrode was kept low. Therefore, it was found that the technology disclosed herein can be suitably applied not only to the fabrication of negative electrodes but also to the fabrication of positive electrodes. In other words, it was found that the effects of the technology disclosed herein are exhibited regardless of the types of components of the active material layer (e.g., active material, binder, conductive material). It was also found that the particle size ratio (D2 / D1) of the average particle size D2 of the pore-forming agent to the average particle size D1 of the positive electrode active material is more preferably 0.1 to 0.3.

[0092] Although several embodiments of the present invention have been described above, the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiments. For example, it is possible to replace part of the above-described embodiments with other modified embodiments, and it is also possible to add other modified embodiments to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

[0093] For example, in the embodiment of FIG. 4 described above, two types of electrode active materials, i.e., the first electrode active material and the second electrode active material, are added and mixed at different times. However, this is not limited to this. For example, only one type of electrode active material may be used. Alternatively, the two types of electrode active materials may be added together in the first mixing step. Alternatively, no electrode active material may be added in the second mixing step. In this case, it is preferable that the content of the pore-forming agent in the electrode mixture be 0.1 parts by mass or more and 3 parts by mass or less, relative to 100 parts by mass of the first electrode active material.

[0094] 4 is applied to the production of the positive electrode 10, the preferred range of the particle size ratio (D2 / D1) may differ from that of the negative electrode due to differences in the preferred range of the average particle size D1 between the positive electrode active material and the negative electrode active material. Specifically, the particle size ratio (D2 / D1) of the average particle size D2 of the pore-forming agent to the average particle size D1 of the positive electrode active material is typically 1.5 or less, for example 1 or less, and is preferably approximately 0.1 to 1, more preferably 0.1 to 0.5, and particularly preferably 0.1 to 0.3.

[0095] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A method for manufacturing an electrode, comprising: a first mixing step of mixing a first electrode active material with a pore-forming agent to obtain a mixture; a second mixing step of mixing the mixture with a binder to obtain an electrode mixture; an application step of applying the electrode mixture onto a current collector; and a heat treatment step of heat-treating the electrode mixture on the current collector to remove the pore-forming agent. Item 2: The manufacturing method according to Item 1, further comprising a pressing step of pressing the electrode mixture on the current collector after the applying step and before the heat treatment step. Item 3: The manufacturing method according to Item 1 or 2, wherein the pore-forming agent contains a thermoplastic resin, and the heat treatment step is carried out at a heat treatment temperature of 200°C or higher and 400°C or lower. Item 4: The manufacturing method according to any one of Items 1 to 3, wherein the first electrode active material and the pore-forming agent are each particulate, and in the first mixing step, a particle size ratio (D2 / D1) of an average particle size D2 of the particulate pore-forming agent to an average particle size D1 of the particulate first electrode active material is 0.1 or more and 1 or less. Item 5: The manufacturing method according to any one of Items 1 to 4, wherein in the second mixing step, a second electrode active material is further mixed to form the electrode mixture, and when the total amount of the first electrode active material and the second electrode active material in the electrode mixture is 100 parts by mass, the amount of the pore-forming agent added is 0.1 parts by mass or more and 3 parts by mass or less. Item 6: The manufacturing method according to any one of Items 1 to 4, wherein the content of the pore-forming agent in the electrode mixture is 0.1 parts by mass or more and 3 parts by mass or less when the first electrode active material is taken as 100 parts by mass. Item 7: The manufacturing method according to any one of Items 1 to 6, wherein the electrode is a negative electrode. Item 8: The manufacturing method according to any one of Items 1 to 7, wherein the first electrode active material comprises a Si-containing material. Item 9: The manufacturing method according to any one of Items 1 to 6, wherein the electrode is a positive electrode. Item 10: Using an electrode manufactured by the manufacturing method according to any one of items 1 to 9. A method for manufacturing an electricity storage device. [Explanation of symbols]

[0096] 10 Positive electrode (electrode) 12 Positive electrode current collector (current collector) 14 Cathode active material layer 20 negative electrode 22 Negative electrode current collector (current collector) 24 Negative electrode active material layer 40 Electrode body 100 Energy storage device S1 1st mixing process S2 2nd mixing process S3 application process S4 Drying process S5 Pressing Process S6 Heat treatment process

Claims

1. a first mixing step of dry-mixing a first electrode active material and a pore-forming agent to obtain a mixture; a second mixing step of mixing the mixture with a binder to obtain an electrode mixture; an application step of applying the electrode mixture onto a current collector; a heat treatment step of heat treating the electrode mixture on the current collector to remove the pore-forming agent and cause voids to be unevenly distributed around the first electrode active material; The method for producing an electrode for a non-aqueous electrolyte secondary battery includes the steps of:

2. further comprising a pressing step of pressing the electrode mixture on the current collector after the applying step and before the heat treatment step. The method of claim 1.

3. the pore-forming agent includes a thermoplastic resin, In the heat treatment step, the heat treatment temperature is set to 200°C or higher and 400°C or lower. The method according to claim 1 or 2.

4. the first electrode active material and the pore-forming agent are each in particulate form; In the first mixing step, a particle size ratio (D2 / D1) of an average particle size D2 of the particulate pore-forming agent to an average particle size D1 of the particulate first electrode active material is set to 0.1 or more and 1 or less. The method according to claim 1 or 2.

5. In the second mixing step, a second electrode active material is further mixed to form the electrode mixture, In the electrode mixture, when the total amount of the first electrode active material and the second electrode active material is 100 parts by mass, the amount of the pore-forming agent added is 0.1 parts by mass or more and 3 parts by mass or less. The method according to claim 1 or 2.

6. In the electrode mixture, the amount of the pore-forming agent added is 0.1 parts by mass or more and 3 parts by mass or less when the first electrode active material is taken as 100 parts by mass. The method according to claim 1 or 2.

7. The electrode is a negative electrode, The first electrode active material includes a Si-containing material. The method according to claim 1 or 2.

8. In the second mixing step, a second electrode active material is further mixed to form the electrode mixture, The second electrode active material includes graphite. The method of claim 7.

9. The electrode is a positive electrode. The method according to claim 1 or 2.

10. An electrode manufactured by the manufacturing method according to claim 1 or 2 and a non-aqueous electrolyte solution are used. A method for manufacturing an electricity storage device.

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