Electrode manufacturing method and power 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 electric storage devices such as lithium ion secondary batteries. For example, Patent Document 1 describes a method for producing a sheet-shaped 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-shaped negative electrode composite using a solvent, and a step of removing the solvent. Patent Document 1 describes that desired pores can be formed in the sheet-shaped negative electrode by removing the pore-forming agent using a solvent. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-146581 A [Patent Document 2] JP 2014-103052 A [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 is required to have both high capacity and high input / output. In particular, in a high input / output type electricity storage device 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 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.

[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 arranged around the first electrode active material. In this way, when the pore-forming agent is removed in the heat treatment step, voids are suitably formed around the first electrode active material. As a result, the permeability of the electrolyte is improved around the first electrode active material, and the movement (liquid flow) of the electrolyte becomes smooth during charging and discharging (especially during high-rate charging and discharging). Therefore, a low-resistance electrode can be manufactured. In turn, an electricity storage device with excellent input and output characteristics can be realized. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view that illustrates a schematic diagram of an electricity accumulation device according to an embodiment. [Diagram 2] FIG. 2 is a schematic vertical cross-sectional view taken along line II-II in FIG. [Diagram 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. [Diagram 5] FIG. 5 is a schematic diagram of the negative electrode composite material before and after a heat treatment step according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, preferred embodiments of the technology disclosed herein will be described with reference to the drawings. Note that matters other than those specifically mentioned in this specification and 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 of a person skilled in the art based on the prior art in the field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense in the field.

[0010] In this specification, the term "electricity storage device" refers to a device in general 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 includes secondary batteries such as lithium ion secondary batteries and nickel-hydrogen batteries, and capacitors such as lithium ion capacitors and electric double layer capacitors. In this specification, the expression "A to B" indicating a range includes not only A or more and B or less, but also "greater than A (exceeding 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, the electric storage device 100 manufactured by the manufacturing method disclosed herein will be described. FIG. 1 is a perspective view of the electric 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 a "depth direction", the symbol Y indicates a "width direction" perpendicular to the depth direction, and the symbol Z indicates a "height direction" perpendicular to the depth direction and the width direction. The symbol F in the depth direction X indicates a "front", and the symbol Rr indicates a "rear". The symbol L in the width direction Y indicates a "left", and the symbol R indicates a "right". The symbol U in the height direction Z indicates a "top", and the symbol D indicates a "bottom". However, these are merely directions for the convenience of explanation, and do not limit the installation form of the electric storage device 100 disclosed herein in any way.

[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 non-aqueous 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 similar to those of conventional devices.

[0013] The battery case 50 is a housing that contains the electrode body 40 and the electrolyte. As shown in FIG. 1, the battery case 50 has a flat, bottomed rectangular parallelepiped (square) outer shape. The battery case 50 can be made of any conventionally known material 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 square battery including the exterior body 52 and the sealing plate 54. However, the battery case 50 may 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 the upper surface. As shown in FIG. 1, the exterior body 52 includes a bottom wall 52a having a substantially rectangular shape in a plan view, a pair of long side walls 52b extending upward in the height direction Z from the long side of the bottom wall 52a, and a pair of short side walls 52c extending upward in the height direction Z from the short side 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 a plan view. The periphery of the sealing plate 54 is joined (for example, welded) to the opening 52h of the exterior body 52. ​​This hermetically seals (closes) the battery case 50. The sealing plate 54 is provided with a liquid injection hole 55, a gas exhaust 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. ​​The liquid injection hole 55 is sealed with a sealing member 56 after the electrolyte is injected. The gas release valve 57 is a thin-walled portion designed to break (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 the terminal insertion holes 58 and 59. A resin gasket 90 is attached to the terminal insertion holes 58 and 59 of the sealing plate 54. As a result, the positive electrode terminal 60 and the negative electrode terminal 65 inserted into the terminal insertion holes 58 and 59 are insulated from the sealing plate 54. As shown in FIG. 1 and FIG. 2, the positive electrode terminal 60 is connected to the 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 the plate-shaped negative electrode external conductive member 67. The positive electrode external conductive member 62 and the negative electrode external conductive member 67 are insulated from the sealing plate 54 by the 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, a lower end 60c of the positive electrode terminal 60 is connected to a positive electrode current collecting member 70 inside the exterior body 52. ​​The positive electrode terminal 60 is connected to a positive electrode 10 (see Fig. 3) of the electrode body 40 via the positive electrode current collecting member 70. A lower end 65c of the negative electrode terminal 65 is connected to a negative electrode current collecting member 75 inside the exterior body 52. ​​The negative electrode terminal 65 is connected to a negative electrode 20 (see Fig. 3) of the electrode body 40 via the negative electrode current collecting member 75.

[0017] FIG. 3 is a schematic diagram showing the configuration of the electrode body 40. As shown in FIG. 3, the electrode body 40 has a flat outer shape. The electrode body 40 is a wound electrode body 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 body 40 may be a laminated electrode body 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 an example of the "electrode" disclosed herein. As can be seen from FIG. 2 and FIG. 3, the electrode body 40 is disposed inside the battery case 50 with the winding axis WL (see FIG. 3) parallel to the width direction Y of the electricity storage device 100. The specific configuration of the electrode body 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 collector 12, and a positive electrode active material layer 14 and a protective layer 16 fixed on at least one surface of the positive electrode collector 12. From the viewpoint of increasing capacity, the positive electrode active material layer 14 is preferably formed on both sides of the positive electrode collector 12. The protective layer 16 is not essential and may be omitted in other embodiments. In addition, 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 be preferably used for the positive electrode collector 12. The positive electrode collector 12 is preferably composed of, for example, aluminum or an aluminum alloy. When the positive electrode 10 is an electrode disclosed herein, the positive electrode collector 12 is an example of a "collector".

[0019] In the positive electrode 10, a positive electrode tab 12t protrudes outward (to the left in FIG. 3) from one end side 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 the positive electrode current collector 12 is exposed (current collector exposed portion). As shown in FIG. 3, the multiple positive electrode tabs 12t are stacked at one end (the left end in FIG. 3) in the width direction Y of the electricity storage device 100 to form a positive electrode tab group 42. As shown in FIG. 2, a positive electrode current collector 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 collector 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, 50 μm or more, and 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 any material conventionally used as a positive electrode active material may be appropriately used. 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 the lithium transition metal composite oxide is a lithium transition metal composite oxide represented by the general formula LiMO2 (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 preferable, a lithium transition metal composite oxide containing Ni is more preferable, and a lithium nickel cobalt manganese composite oxide containing Ni, Co, and Mn is particularly preferable.

[0022] The positive electrode active material is typically in the form of particles, bound to each other 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 "average particle size" refers to a particle size (D50, also called the median size) corresponding to a cumulative frequency of 50 volume % from the fine particle side with a small particle size in a volume-based particle size distribution based on a general laser diffraction / scattering method.

[0023] As the positive electrode binder, a resin binder that has been conventionally used as a positive electrode binder can be appropriately used. 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 ratio 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, 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 positive electrode active material.

[0024] The positive electrode active material layer 14 may further contain any component such as a conductive material in addition to the positive electrode active material and the positive electrode binder. 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, preferably 1 mass % or less.

[0025] Examples of the conductive material include carbon black such as acetylene black (AB) and Ketjen black, carbon nanotubes (CNT), carbon fibers, carbon nanofibers, and carbon materials such as activated carbon. These materials may be used alone or in combination of two or more. Among them, acetylene black is preferable. Although not particularly limited, the content ratio 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 positive electrode active material.

[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 end side 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 collector 22 and a negative electrode active material layer 24 fixed on at least one surface of the negative electrode collector 22. From the viewpoint of increasing capacity, the negative electrode active material layer 24 is preferably formed on both sides of the negative electrode 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 electrode. A metal foil having a predetermined conductivity can be preferably used for the negative electrode collector 22. The negative electrode 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 collector 22 is an example of a "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 negative electrode active material layer 24 being formed. As shown in FIG. 3, a 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 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 combining various characteristics (for example, high capacity and high rate cycle characteristics) at a high level, the negative electrode active material more 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 a negative electrode active material other than the Si-containing material and graphite. Specific examples of the negative electrode active material 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 each other 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, a resin binder that has been conventionally used as a negative electrode binder can be appropriately used. Specific examples include rubbers such as styrene butadiene rubber (SBR), celluloses such as carboxymethyl cellulose (CMC), and acrylic resins such as polyacrylic acid (PAA) (resins obtained by polymerizing monomers having an acryloyl group). The negative electrode binder preferably contains SBR, CMC, and PAA. Although not particularly limited, the content ratio 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, 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.

[0034] The negative electrode active material layer 24 may further contain any component such as a conductive material, a dispersant, etc., 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, preferably 1 mass % or less.

[0035] As the conductive material, one or more of the carbon materials exemplified as optional components that can be contained in the positive electrode active material layer 14 can be used. Among them, a carbon material in a fibrous form (fibrous carbon) is preferable. Specifically, carbon fiber, carbon nanofiber, carbon nanotube (CNT), etc. are preferable, and single-walled carbon nanotube (SWCNT) is more preferable. Although not particularly limited, the content ratio 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 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. As the separators 32 and 34, any separator that has been used in the past can be used without any particular restrictions. As the separators 32 and 34, a porous resin sheet made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP) is suitable. In addition, the surfaces of the separators 32 and 34 may be provided with a functional layer such as an adhesive layer containing a binder or a heat-resistant layer (Heat Resistance Layer: HRL) containing an inorganic filler.

[0037] The electrolyte may be the same as that used in the past, 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 the non-aqueous solvent 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 an additive as necessary.

[0038] <2. Method of manufacturing the negative electrode> An example of the method for producing 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 produced in a similar manner. In that case, the "negative electrode" in the following description can be read as "positive electrode" as appropriate.

[0039] Fig. 4 is a flow chart of a method for producing a negative electrode according to an embodiment. As shown in Fig. 4, the negative electrode 20 can be produced by a production 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 can be omitted in other embodiments. In addition, the production 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 of 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 viewpoint of simplicity, cost, etc., the dry mixing method (dry mix) is preferable. The mixing can be performed, for example, with an agitation granulator, a mortar, a ball mill, a jet mill, a planetary mixer, a disperser, etc. This step may be performed in an environment of about 40° C. or less, typically room temperature (for example, 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 them, 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 study by the present inventors, while the Si-containing material has a large specific capacity compared to carbon materials such as graphite, it has a large expansion and contraction during charging and discharging, and tends to easily break the conductive path. Therefore, the conductive path is broken with the charge and discharge cycle (especially the high-rate cycle), and the cycle characteristics are likely to deteriorate. Therefore, when the 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 the 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 about 150°C or higher, for example, 200°C or higher, 300°C or higher, and about 500°C or lower, 400°C or lower.

[0043] In this specification, the term "thermal decomposition temperature" refers to the temperature at which the pore-forming agent vaporizes due to a rise in temperature. The thermal decomposition temperature can be determined by a conventionally known differential thermal analysis (DTA). For example, in a resin material, thermal decomposition may occur stepwise as the temperature rises, and in that case, 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 from the viewpoint of forming uniform voids in the negative electrode active material layer 24, it is preferably spherical or approximately spherical. In this specification, the term "approximately spherical" includes a spherical shape, a rugby ball shape, a polygonal shape, 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 short axis direction in the smallest 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 mixture paste prepared in the second mixing step (S2).

[0047] The pore-forming agent is preferably a resin material, and 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). Any one of these may be used alone, or two or more may be used in combination. The polymer material may also contain various additives (e.g., antioxidants) that are known to be added to conventional polymer materials, as necessary. Among them, it is preferable to contain at least one of PMMA, PS, and PP. From the viewpoint of reducing the battery resistance, it is more preferable to contain a methacrylic resin. In particular, PMMA obtained by polymerization using at least methyl methacrylate monomer as a raw material is preferable.

[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 first negative electrode active material:pore-forming agent=95:5 to 88:12. 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 is improved around the first electrode active material, and the movement (liquid flow) of the electrolyte during charging and discharging becomes smooth. Therefore, the resistance of the negative electrode 20 can be reduced, and the input / output characteristics can be improved. In addition, by setting the ratio of the pore-forming agent to a predetermined value or less, it is possible to prevent the periphery of the first negative electrode active material from becoming too void. As a result, for example, the Si-containing material expands during charging, improving the filling property of the negative electrode active material layer 24 and making it easier to form a conductive path, thereby improving the cycle characteristics. In addition, by utilizing the voids, the expansion and contraction (particularly in the thickness direction) of the negative electrode active material layer 24 can be suppressed.

[0049] In a preferred embodiment, the first negative electrode active material and the pore former are each particulate. Although not particularly limited, the average particle diameter 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 diameter D2 of the pore former is preferably 0.1 to 10 μm, and more preferably 1 to 5 μm. In this step, the particle diameter ratio (D2 / D1) of the average particle diameter D2 of the pore former to the average particle diameter D1 of the first negative electrode active material is typically 2 or less, for example 1.5 or less, preferably about 0.1 to 1, more preferably 0.1 to 0.5, and particularly preferably 0.4 to 0.5. This makes it easier to form voids of suitable size around the first negative electrode active material in the negative electrode active material layer 24. As a result, for example, the Si-containing material expands during charging, improving the filling property of the negative electrode active material layer 24 and making it easier to form a conductive path, 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).

[0050] As shown in FIG. 4, the second mixing step (S2) is a step of mixing the mixture obtained in the first mixing step (S1) with a negative electrode binder (binder) to obtain a negative electrode mixture (electrode mixture). The method of 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. The mixing can be performed, for example, with an agitation granulator, a mortar, a ball mill, a jet mill, a planetary mixer, a disperser, or the like. This step may be performed at a temperature of about 40° C. or lower, typically at room temperature (for example, 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 mixture within a range that does not significantly impair the effects of the present invention. As an example, as shown in FIG. 4, a second negative electrode active material (second electrode active material), a conductive material, a dispersion solvent, etc. are included. In order to improve the coatability in the application step (S3), it is preferable that the negative electrode mixture is 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 preparing the negative electrode 20, the dispersion solvent is preferably water or a mixed solvent mainly composed of water, and more preferably water, from the viewpoint of reducing the environmental load. However, for example, when preparing the positive electrode 10, the dispersion solvent may be a non-aqueous solvent (organic solvent) such as NMP.

[0053] The premixing step (step S22) is a step of dry-mixing the second negative electrode active material with a powdered negative electrode binder to obtain a powdered premix. This can improve the homogeneity or 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 among the materials described above can be appropriately used. 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 is 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 a 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 that is greater than that of the pore-forming agent (e.g., PMMA).

[0055] Although not particularly limited, when the total amount of the negative electrode active material (here, the total of the first negative electrode active material and the second negative electrode active material) is 100 parts by mass, 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. By setting the amount 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 electrode active material is improved, and the movement (liquid flow) of the electrolyte during charging and discharging becomes smooth. Therefore, the resistance of the negative electrode 20 can be reduced, and the input / output characteristics can be improved. In addition, by setting the amount of the pore-forming agent added to a predetermined value or less, it is possible to prevent the periphery of the first negative electrode active material from becoming too void. 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 the 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 pre-mixture obtained in the second mixing step with the paste-like mixture obtained in the first 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 among the materials described above can be appropriately used. 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 may 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 mixture (electrode mixture) obtained in the second mixing step (S2) onto the negative electrode current collector 22 (current collector). The method of applying the negative electrode mixture is not particularly limited and may be the same as in the past. The negative electrode mixture can be applied (coated) onto the surface (one side or both sides) of the current collector using a coating device such as a gravure coater, a slit coater, a die coater, a comma coater, or a dip coater. The coating amount may be appropriately determined depending on, for example, the solid content concentration of the negative electrode mixture 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, it is a step of removing moisture contained in the negative electrode mixture. In this specification, "remove" means to remove at least a part (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 preferable. 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 (for example, 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 part 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 (for example, 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 later. 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 about 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 material on the negative electrode current collector 22. The pressing method and pressing conditions are not particularly limited and may be the same as those of the conventional method. 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 mass % or less, preferably 1 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 performing a preliminary experiment using a temperature rise desorption gas analyzer.

[0062] The heat treatment temperature may be appropriately determined depending on the composition of the negative electrode composite material after the pressing step (S5) (for example, 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 part 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, a temperature higher than the drying temperature by 10°C or more, 20°C or more, 30°C or more, or even 50°C or more, or 100°C or more. 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, a temperature higher than the thermal decomposition temperature of the pore-forming agent by 10°C or more, 20°C or more, 30°C or more, or even 50°C or more. 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 (for example, the negative electrode binder, etc.) do not decompose or change in quality. The heat treatment temperature may be, for example, a temperature 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 may be, for example, a temperature 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 the 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), first, the Si-containing material (first negative electrode active material) and the pore-forming agent are mixed. Therefore, as shown in the upper diagram of FIG. 5, before the heat treatment, the pore-forming agent is arranged around the Si-containing material, for example, more 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 suitably 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, and the movement (liquid flow) of the electrolyte during charging and discharging becomes smooth. Therefore, the resistance of the negative electrode 20 can be reduced, and the input / output characteristics can be improved. In addition, the Si-containing material expands during charging and discharging, improving the filling of the negative electrode active material layer 24 and making it easier to form a conductive path, thereby improving the 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 electricity storage device The positive electrode 10 and / or negative electrode 20 produced as described above can be suitably used for producing 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 various 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 or truck. 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: Manufacturing method of negative electrode> In this test example, a negative electrode was produced by the production method disclosed herein, and an electricity storage device was constructed by placing it opposite a conventional positive electrode, and the characteristics of the device were then evaluated.

[0069] <<Preparation of Negative Electrode>> First, negative electrodes were prepared 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-like mixture 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 negative electrode binders were dry-mixed to obtain a powdered preliminary mixture. Next, the preliminary mixture 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 a dispersion solvent (water) to prepare a negative electrode composite paste (second mixing step). In the negative electrode composite paste, the compounding ratio of the graphite particles as the second negative electrode active material to the SiC composite particles as the first negative electrode active material was 85:15. The compounding ratio of the other components was PMMA:SWCNT:CMC:PAA:SBR=1:0.1:1:1:1.5 (parts by mass) relative to the total amount (100 parts by mass) of the negative electrode active material.

[0071] Next, the prepared negative electrode composite paste was applied onto 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 a predetermined dimension, a negative electrode having a negative electrode active material layer on the negative electrode current collector was produced.

[0072] (Example 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 produced in the same manner as in Example 1, except that a negative electrode composite paste was prepared without adding a pore-forming agent. That is, first, SiC composite particles and graphite particles as a negative electrode active material, and CMC and PAA as a negative electrode binder were dry-mixed using a stirring granulator to obtain a mixture. Next, SWCNT as a conductive material and SBR as a 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 produced in the same manner as in Example 1, except that the SiC composite particles and the pore-forming agent particles were not premixed, and the negative electrode composite paste was prepared. That is, 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 a positive electrode active material, AB as a conductive material, and PVdF as a 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 onto an Al foil (thickness: 15 μm) as a positive electrode current collector, dried, and pressed to a predetermined thickness. Then, by processing to a predetermined dimension, a positive electrode having a positive electrode active material layer on the positive electrode current collector was produced.

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

[0077] <Evaluation of Battery Resistance> In an environment of 25°C, the battery was charged at a constant current of 0.3C until the state of charge (SOC) reached 50%, and then charged at a constant voltage until the current value reached 0.02C at SOC 50%. Next, the battery was stored in an environment of 25°C for 1 hour, and then discharged at a constant current of 1C for 10 seconds. Then, the 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 value 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 repeated, 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 up to 2.5 V). The capacity retention rate (%) was calculated from the discharge capacity at the first cycle and the discharge capacity at the 250th cycle using the following formula. 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 the swelling rate of the negative electrode> 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 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 using the following formula. The results are shown in Table 1. Swelling ratio = (thickness of negative electrode after cycle test / thickness of negative electrode before cycle test) x 100

[0080] [Table 1]

[0081] <Evaluation Results> As shown in Table 1, in 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, compared with Comparative Examples 1 and 2, the battery resistance in Example 1 was significantly reduced, and the capacity retention rate after the cycle test was also significantly improved. This is believed to be because the pore-forming agent was suitably arranged around the SiC composite by first mixing the SiC composite and the pore-forming agent in the first mixing step, and voids were suitably formed around the SiC composite by removing the pore-forming agent in the heat treatment step, which resulted in suppressing disconnection of the conductive path during expansion and contraction of the SiC composite. Furthermore, in Example 1, the expansion rate of the negative electrode was also significantly suppressed. This is believed to be because the voids formed around the SiC composite were utilized to absorb the expansion and contraction of the SiC composite, thereby alleviating the internal stress of the negative electrode.

[0082] Comparing Examples 1, 4, 5, and 9, which differ in the heat treatment temperature, it was found that the heat treatment temperature is preferably 200 to 400° C., and more preferably 250 to 350° C. This is believed to be because setting the heat treatment temperature to a predetermined value or higher allows the pore-forming agent to be suitably removed, making it easier to ensure suitable voids around the SiC composite.

[0083] Comparing Examples 1 to 3 and 6, in which different amounts of pore-forming agent were added, it was found that the amount of pore-forming agent added was 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 did not become too void, making it easier to create a conductive path.

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

[0085] <Test Example II: Manufacturing method of 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 opposing it to 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-like mixture 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 preliminary mixture. Next, the preliminary mixture 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). In addition, in the positive electrode composite paste, the mass ratio of each component was NCM:PMMA:AB:PVdF=100:1:1:1.

[0087] Next, the prepared positive electrode composite paste was applied onto 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 min) 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 min) 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 the positive electrode current collector was produced.

[0088] (Comparative Example 3) A positive electrode was produced in the same manner as in Example 10, except that the lithium nickel cobalt manganese composite oxide particles and the pore former particles were not premixed, and the positive electrode composite paste was prepared as follows. That is, first, NCM as the positive electrode active material, PMMA as the pore former, AB as the conductive material, and PVdF as the positive electrode binder were dry-mixed all at once using a stirring granulator to obtain a mixture. Next, the obtained mixture was 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 suppressed and the capacity retention rate after the cycle test was relatively high compared to Comparative Examples 1 and 3. Furthermore, the expansion rate of the negative electrode was suppressed to a low level. Therefore, it was found that the technology disclosed herein can be suitably applied not only to the preparation of a negative electrode but also to the preparation of a positive electrode. In other words, it was found that the effect of the technology disclosed herein is exerted regardless of the type 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 technical common sense in the field. The technology described in the claims includes various modifications and changes to the above-exemplified embodiments. For example, it is possible to replace a part of the above-mentioned embodiment with another modified form, and it is also possible to add another modified form to the above-mentioned embodiment. Furthermore, if a technical feature is not described as essential, it is also possible to delete it 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, the electrode active material used may be only one type. Also, the two types of electrode active materials may be added together in the first mixing step. Also, the electrode active material may not be added in the second mixing step. In this case, when the first electrode active material is taken as 100 parts by mass in the electrode mixture, the content ratio of the pore-forming agent is preferably 0.1 parts by mass or more and 3 parts by mass or less.

[0094] 4 is applied to the manufacture of the positive electrode 10, the preferred range of the particle size ratio (D2 / D1) may differ from that of the negative electrode side 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 about 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 method according to item 1, further comprising a pressing step of pressing the electrode mixture on the current collector after the application step and before the heat treatment step. Item 3: The method according to item 1 or 2, wherein the pore-forming agent contains a thermoplastic resin, and the heat treatment step is performed 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 100 parts by mass. Item 7: The method according to any one of items 1 to 6, wherein the electrode is a negative electrode. Item 8: The method according to any one of items 1 to 7, wherein the first electrode active material comprises a Si-containing material. Item 9: The 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 12 Positive electrode 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 Press Process S6 Heat treatment process

Claims

1. a first mixing step of 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; a 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; Including, A method for manufacturing an electrode.

2. The method further includes a pressing step of pressing the electrode mixture on the current collector after the application 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 more and 400° C. or less. The method according to claim 1 or 2.

4. the first electrode active material and the pore-forming agent are each particulate; 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 obtain 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 100 parts by mass. The method according to claim 1 or 2.

7. The electrode is a negative electrode. The method according to claim 1 or 2.

8. the first electrode active material comprises a Si-containing material; The method according to claim 1 or 2.

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

10. Using an electrode manufactured by the manufacturing method according to claim 1 or 2, A method for manufacturing an electricity storage device.

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