Manufacturing method for negative electrode, negative electrode, and power storage device equipped with the negative electrode

Abstract

To provide a technology to suppress resistance increase after charge-discharge cycles in power storage devices containing silicon-containing negative electrode active materials.SOLUTION: A manufacturing method for a negative electrode for a power storage device including a non-aqueous electrolyte is provided. The manufacturing method includes mixing carbon nanotubes and flake graphite to obtain a first mixture, mixing the first mixture with a negative electrode active material, which is a silicon-containing graphite particle, to obtain a second mixture, and coating a negative electrode slurry containing the second mixture onto the surface of a negative electrode current collector to form a negative electrode active material layer on the surface of the negative electrode current collector.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a negative electrode, a negative electrode, and an electricity storage device including the negative electrode. [Background technology]

[0002] An example of an electricity storage device is a secondary battery such as a lithium-ion secondary battery. In recent years, this type of secondary battery has been suitably used as a portable power source for personal computers, mobile terminals, etc., and as a power source for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] Japanese Patent Application Laid-Open Publication No. 2017-50142 discloses an anode active material for lithium-ion secondary batteries. This anode active material includes a plurality of flake graphite particles and flake silicon particles whose surfaces are coated with carbon. The plurality of flake graphite particles are aggregated to form particles. Silicon particles are present between the plurality of flake graphite particles. The publication states that this configuration can provide an anode active material for lithium-ion secondary batteries that has high capacity and long life.

[0004] International Publication No. 2023 / 053773 discloses a lithium-ion secondary battery including a positive electrode, a negative electrode, a separator separating the positive electrode and the negative electrode, and an electrolyte. In this lithium-ion secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer contains a negative electrode active material having a discharge capacity of 400 mAh / g to 750 mAh / g and a conductive agent. The negative electrode active material contains a carbon-based material and a silicon-based material. The negative electrode current collector has a thickness of 4 μm to 12 μm and a 1% proof stress of 300 MPa to 700 MPa. The conductive agent contains carbon nanotubes. The publication also describes that this configuration enables the lithium-ion secondary battery to have a high capacity and suppresses the decrease in battery capacity due to repeated charge and discharge.

[0005] Japanese Patent Laid-Open Publication No. 2014-177722 discloses a battery electrode containing composite carbon fibers or composite carbon fiber aggregates. The composite carbon fibers contain multi-walled carbon nanotubes, carbon particles, and graphitized carbon nanofibers. The multi-walled carbon nanotubes account for 99% or more of the fiber diameter between 5 nm and 40 nm. The carbon particles have a primary particle diameter between 20 nm and 100 nm. The graphitized carbon nanofibers account for 99% or more of the fiber diameter between 50 nm and 300 nm. Furthermore, in the composite carbon fiber, the multi-walled carbon nanotubes are uniformly dispersed among the graphitized carbon nanofibers and the carbon particles. The publication also describes that the composite carbon fibers, when added to a matrix such as a resin, can be easily dispersed without leaving any agglomerates while maintaining a high aspect ratio, and that a smaller amount can reduce electrical resistance. According to this publication, the use of such composite carbon fibers improves battery characteristics such as the capacity retention rate of lithium ion secondary batteries. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-50142 [Patent Document 2] International Publication No. 2023 / 053773 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-177722 Summary of the Invention [Problem to be solved by the invention]

[0007] The present inventors wish to suppress an increase in resistance after charge-discharge cycling in an electricity storage device that includes a negative electrode active material containing silicon. [Means for solving the problem]

[0008] The technology disclosed herein provides a method for manufacturing a negative electrode of an electricity storage device including a nonaqueous electrolyte. This manufacturing method includes the steps of: mixing carbon nanotubes and flake graphite to obtain a first mixture; mixing the first mixture with a negative electrode active material, which is silicon-containing graphite particles, to obtain a second mixture; and applying a negative electrode slurry containing the second mixture to the surface of a negative electrode current collector to form a negative electrode active material layer on the surface of the negative electrode current collector. This configuration can suppress an increase in resistance after charge / discharge cycling in an electricity storage device including a silicon-containing negative electrode active material.

[0009] The technology disclosed herein provides a negative electrode for an electricity storage device equipped with a nonaqueous electrolyte. The negative electrode includes silicon-containing graphite particles as a negative electrode active material and a conductive additive that is a composite of carbon nanotubes and flake graphite. This configuration can suppress an increase in resistance after charge / discharge cycling in an electricity storage device that includes a silicon-containing negative electrode active material.

[0010] The presently disclosed technology provides an electricity storage device including the above-described negative electrode and a nonaqueous electrolyte solution. With this configuration, it is possible to suppress an increase in resistance after charge-discharge cycling in an electricity storage device including a silicon-containing negative electrode active material. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic cross-sectional view of a lithium-ion secondary battery 100. As shown in FIG. [Figure 2] FIG. 2 is a schematic diagram of the electrode body 20. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the energy storage device disclosed herein will be described below. The embodiment described herein does not particularly limit the technology disclosed herein. The technology disclosed herein is not limited to the embodiment described herein, unless otherwise specified. The drawings are schematic and do not necessarily reflect the actual product. Furthermore, members and parts that perform the same function are appropriately designated by the same reference numerals, and redundant explanations may be omitted. Furthermore, the notation "A to B" indicating a numerical range means "greater than or equal to A and less than or equal to B" unless otherwise specified, and also encompasses the meaning of "greater than A and less than B."

[0013] In this specification, the term "electricity storage device" refers to a device in which charging and discharging occur by the movement of charge carriers between a pair of electrodes (positive and negative electrodes) via an electrolyte. Electricity storage devices include secondary batteries such as lithium ion secondary batteries; and capacitors such as lithium ion capacitors and electric double layer capacitors. Below, an embodiment in which the electricity storage device is a lithium ion secondary battery will be described.

[0014] Fig. 1 is a schematic cross-sectional view of a lithium-ion secondary battery 100. Fig. 2 is a schematic diagram of an electrode assembly 20. As shown in Fig. 1, the lithium-ion secondary battery 100 includes the electrode assembly 20, a case 30, and a non-aqueous electrolyte solution 80.

[0015] 1 and 2, the electrode assembly 20 is a wound electrode assembly in which a long, sheet-like positive electrode 50 and a long, sheet-like negative electrode 60 are stacked together with a long, sheet-like separator 70 interposed therebetween and wound in the sheet longitudinal direction (hereinafter simply referred to as the "longitudinal direction"). In the electrode assembly 20, the exposed region 52a of the positive electrode 50 and the exposed region 62a of the negative electrode 60 protrude outward from both ends in the lateral direction perpendicular to the longitudinal direction.

[0016] As shown in FIGS. 1 and 2, the positive electrode 50 includes a long sheet-like positive electrode collector 52 and a positive electrode active material layer 54. The positive electrode collector 52 is, for example, aluminum foil. In this embodiment, the positive electrode collector 52 has a region where the positive electrode active material layer 54 is provided and an exposed region 52a where the positive electrode active material layer 54 is not provided and the surface of the positive electrode collector 52 is exposed. The positive electrode active material layer 54 is provided, for example, in a strip shape along the longitudinal direction on one or both sides (both sides here) of the positive electrode collector 52. The positive electrode active material layer 54 is not provided at an end (the left end in the drawing) in the short-side direction of the sheet (hereinafter simply referred to as the "short-side direction"). Here, the exposed region 52a is a strip-shaped region at the end (the left end in the drawing) in the short-side direction. As shown in FIG. 1, the current collector plate 42a is attached to the exposed region 52a.

[0017] The positive electrode active material layer 54 contains, for example, a positive electrode active material. The positive electrode active material is not particularly limited as long as the effects of the technology disclosed herein are realized, and any positive electrode active material having a conventionally known composition used for this type of application can be used. The positive electrode active material may be, for example, a lithium composite oxide, a lithium transition metal phosphate compound, or the like. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0018] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element. Examples of the lithium transition metal composite oxide include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These positive electrode active materials may be used alone or in combination of two or more.

[0019] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of the additional elements include transition metal elements and typical metal elements, such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional elements may also be metalloid elements, such as B, C, Si, and P; or nonmetallic elements, such as S, F, Cl, Br, and I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0020] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate. Examples of the positive electrode active material include LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 O4 and the like can be preferably used.

[0021] The positive electrode active material layer 54 may contain, in addition to the positive electrode active material, a conductive material, a binder, and the like. Examples of conductive materials include carbon black such as acetylene black (AB) and other carbon materials such as graphite. Examples of binders include polyvinylidene fluoride (PVDF). The content of the positive electrode active material relative to the entire positive electrode active material layer 54 is, for example, preferably 70 mass % or more, more preferably 80 mass % to 98 mass %, and even more preferably 85 mass % to 95 mass %. The content of the conductive material relative to the entire positive electrode active material layer 54 is, for example, 0.1 mass % to 20 mass %. The content of the binder relative to the entire positive electrode active material layer 54 is, for example, 0.5 mass % to 15 mass %.

[0022] As shown in FIGS. 1 and 2, the negative electrode 60 includes a long, sheet-like negative electrode current collector 62 and a negative electrode active material layer 64. The negative electrode current collector 62 is, for example, copper foil. In this embodiment, the negative electrode current collector 62 has a region where the negative electrode active material layer 64 is provided and an exposed region 62a where the negative electrode active material layer 64 is not provided and the surface of the negative electrode active material layer 64 is exposed. The negative electrode active material layer 64 is provided, for example, in a strip shape along the longitudinal direction on one or both sides (both sides in this case) of the negative electrode current collector 62. The negative electrode active material layer 64 is not provided at the end in the short side direction (the end on the right side in the figure). Here, the exposed region 62a is a strip-shaped region at the end in the short side direction (the end on the right side in the figure). As shown in FIG. 1, a current collector 44a is attached to the exposed region 62a.

[0023] The negative electrode active material layer 64 includes, for example, a negative electrode active material. In this embodiment, the negative electrode active material includes silicon-containing graphite particles. The silicon-containing graphite particles may be, for example, graphite particles containing silicon-containing particles therein. In this case, the silicon-containing graphite particles may include, for example, graphite particles having voids and silicon-containing particles disposed within the voids. The voided graphite particles may function as substrate particles for the silicon-containing particles. Although not particularly limited, from the viewpoint of including a larger number of silicon-containing particles and realizing a higher capacity, a higher energy density, etc. of the lithium-ion secondary battery 100, the voided graphite particles are preferably porous graphite particles. Alternatively, in another embodiment, the voided graphite particles may be aggregates of flake graphite.

[0024] The average particle diameter of the silicon-containing graphite particles is generally 0.1 μm to 20 μm. From the viewpoint of realizing the effects of the technology disclosed herein, the average particle diameter is, for example, 0.3 μm or more, preferably 0.5 μm or more, more preferably 1 μm or more, and even more preferably 2 μm or more. From the viewpoint of increasing the degree of filling in the negative electrode active material layer 64, the average particle diameter is, for example, 15 μm or less, preferably 10 μm or less, and more preferably 5 μm or less. In this specification, the "average particle diameter" of particles refers to the particle diameter (D 50 The average particle size of the silicon-containing graphite particles may be, for example, the same as the average particle size of the porous graphite particles described above.

[0025] The silicon-containing particles contained in the silicon-containing graphite particles may be, for example, particles containing silicon and capable of functioning as a negative electrode active material. The silicon-containing particles may be, for example, silicon particles or silicon oxide particles. In this embodiment, the particle diameter of the silicon-containing particles is 1 μm or less. Although not particularly limited, from the viewpoint of containing more silicon-containing particles inside the graphite particles (for example, within the voids of the graphite particles), the particle diameter of the silicon-containing particles is generally 500 nm or less, for example, 250 nm or less, preferably 150 nm or less, more preferably 100 nm or less, and even more preferably 50 nm or less. From the same viewpoint, the particle diameter of the silicon-containing particles is, for example, 1 nm or more, preferably 3 nm or more, more preferably 5 nm or more, and even more preferably 10 nm or more. The particle diameter of the silicon-containing particles can be determined, for example, by observing the plane of the silicon-containing particles with an electron microscope (SEM) to obtain an SEM image, randomly selecting a plurality of silicon-containing particles (e.g., 10 to 100) from the SEM image, calculating the particle diameter (circle-equivalent diameter) of each, and then calculating the arithmetic mean value thereof. Alternatively, the nominal value provided by the manufacturer or the like may be used.

[0026] Silicon-containing graphite particles can be obtained by known methods. For example, they can be obtained by mixing silicon-containing particles with a carbon precursor (e.g., petroleum pitch, coal pitch, phenolic resin, etc.), followed by carbonization and spheroidization. Alternatively, they can be obtained by mixing spherically granulated graphite substrate and silicon-containing particles in a dispersion medium, drying, and disposing the silicon-containing particles in the pores of the graphite particles as the substrate.

[0027] In this embodiment, the negative electrode active material further contains other graphite particles (hereinafter simply referred to as "other graphite particles"). The other graphite particles are, for example, graphite particles that are substantially free of silicon (preferably, graphite particles that are free of silicon). With respect to the other graphite particles, "substantially free of silicon" means that the silicon content in the other graphite particles is 1% by mass or less, preferably 0.5% by mass or less, and more preferably 0.3% by mass or less. The silicon content can be calculated by a conventional method such as ICP analysis. The other graphite particles may be, for example, artificial graphite or natural graphite. The other graphite particles may have an amorphous carbon coating layer on their surfaces. Although not particularly limited, the other graphite particles have, for example, a substantially spherical shape. In this specification, with respect to the other graphite particles, the term "substantially spherical shape" means that the other graphite particles have an average aspect ratio of 1 to 2 (preferably 1 to 1.5) based on SEM observation. The average aspect ratio can be determined, for example, by obtaining a planar SEM image of the other graphite particles, randomly selecting a plurality of graphite particles (e.g., 10 to 100) from the SEM image, calculating the aspect ratio of each, and then calculating the arithmetic mean value thereof. The average particle diameter of the other graphite particles may be, for example, 5 μm to 30 μm, or may be 15 μm to 25 μm.

[0028] Although not particularly limited, from the viewpoint of achieving a high capacity and a high energy density of the lithium-ion secondary battery 100, the negative electrode active material layer 64 may contain, for example, 0.3% by mass or more of the silicon-containing graphite particles, preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 1.5% by mass or more, when the total of the silicon-containing graphite particles and the graphite particles is taken as 100% by mass. On the other hand, from the viewpoint of suppressing expansion and contraction of the negative electrode 60 during charging and discharging, the negative electrode active material layer 64 may contain, for example, 10% by mass or less of the silicon-containing graphite particles, preferably 7.5% by mass or less, and more preferably 5% by mass or less, when the total of the silicon-containing graphite particles and the graphite particles is taken as 100% by mass.

[0029] In this embodiment, the negative electrode active material layer 64 contains a conductive additive. The conductive additive has, for example, the function of maintaining the conductive path in the negative electrode 60 in good condition. When a silicon-containing negative electrode active material is used as the negative electrode active material, carbon nanotubes (CNTs) are preferably used from the viewpoint of preventing the negative electrode active material from being isolated within the negative electrode active material layer due to expansion and contraction of the negative electrode active material accompanying charging and discharging of the power storage device. However, if the effects of CNTs are not properly exerted, for example, the conductive path in the negative electrode may be broken during charging and discharging of the power storage device, leading to an increase in resistance. Therefore, the present inventors investigated the composition of a conductive additive containing CNTs and hoped to suppress the increase in resistance after charging and discharging cycles of the power storage device by appropriately arranging the CNTs between the negative electrode active material layers and appropriately utilizing the CNTs' function as a conductive additive.

[0030] In this embodiment, the conductive additive includes a composite of CNTs and flaky graphite. Here, the "composite of CNTs and flaky graphite" refers to an integrated product of CNTs and flaky graphite. In this case, the composite of CNTs and flaky graphite may be one that includes at least two types of particles, CNT particles and flaky graphite particles, and behaves as a single particle. The composite of CNTs and flaky graphite may be, for example, a particle in which CNTs are bonded to flaky graphite.

[0031] Examples of CNTs include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). While not particularly limited, MWCNTs are preferred for use as CNTs, for example, from the viewpoint of ease of production and production costs of the negative electrode 60. While not particularly limited, the length of the CNTs is preferably approximately 0.1 μm to 10 μm. To better realize the effects of the technology disclosed herein, the length of the CNTs is preferably 0.3 μm to 5 μm, more preferably 0.5 μm to 3 μm. The "CNT length" here is calculated, for example, based on an SEM image. For example, first, CNTs are suspended in a solvent to prepare a CNT dispersion with a concentration of 0.0025%. Next, this CNT dispersion is applied to a glass substrate using, for example, a spin coater, and the glassy coating is then dried to prepare a sample in which CNTs are dispersed on the glass substrate. The sample is then observed using an SEM to obtain images of 100 randomly selected individual CNTs. The observed image is processed using, for example, image processing software based on the contrast ratio in the observed image to obtain the long diameter of the CNTs. The arithmetic mean value of the long diameters obtained from 100 CNTs is taken as the length of the CNTs.

[0032] The flaky graphite is, for example, flat graphite particles. Examples of flaky graphite include flake graphite and graphene. The average particle diameter of the flaky graphite is preferably approximately 0.1 μm to 20 μm. From the viewpoint of appropriately disposing the conductive additive between the negative electrode active materials, the average particle diameter of the flaky graphite is, for example, 0.3 μm or more, preferably 0.5 μm or more, and more preferably 1 μm or more. From the viewpoint of increasing the packing density of the negative electrode active material layer 64, the average particle diameter of the flaky graphite is, for example, 15 μm or less, preferably 10 μm or less, and more preferably 5 μm or less.

[0033] As long as the effects of the technology disclosed herein can be achieved, the conductive additive may contain other conductive additives (hereinafter simply referred to as "other conductive additives") different from the conductive additive that is a composite of CNT and flaky graphite. Examples of other conductive additives include carbon black such as acetylene black (AB); carbon fiber; and the like. Alternatively, the other conductive additive may include CNT that is not in a composite with flaky graphite, or flaky graphite that is not in a composite with CNT. The content of the composite of CNT and flaky graphite in the entire conductive additive is, for example, 70% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, and the closer to 100% by mass the better.

[0034] The proportion of the negative electrode active material relative to the entire negative electrode active material layer 64 is, for example, 70 mass % or more, preferably 80 mass % or more, more preferably 85 mass % to 99 mass %, and may be 90 mass % to 95 mass %. The proportion of the conductive additive relative to the entire negative electrode active material layer 64 may be, for example, 0.01 mass % to 3 mass %.

[0035] The negative electrode active material layer 64 may contain a binder in addition to the negative electrode active material. Examples of binders include carboxymethyl cellulose (CMC), polyacrylic acid (PAA), styrene butadiene rubber (SBR), and polyvinylidene fluoride (PVDF). Among these, carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) are preferably used. The proportion of the binder when the entire negative electrode active material layer 64 is taken as 100% by mass may be, for example, 0.5% by mass to 10% by mass.

[0036] The method for producing the negative electrode 60 (manufacturing method) includes, for example, a preparation step, a first mixing step, a second mixing step, a third mixing step, a fourth mixing step, a coating step, a drying step, and a pressing step.

[0037] The preparation step is, for example, a step of preparing raw materials for the negative electrode active material layer 64. In this embodiment, the preparation step includes silicon-containing graphite particles, other graphite particles, CNTs, flaky graphite, and a binder. The raw materials listed here are as described above.

[0038] The first mixing step is, for example, a step of mixing CNTs and exfoliated graphite to obtain a first mixture. By performing the first mixing step, for example, a first mixture containing a composite of CNTs and exfoliated graphite can be obtained. In this embodiment, a CNT dispersion may be prepared prior to the first mixing step. For example, water may be preferably used as the dispersion medium. In the first mixing step, for example, the first mixture may be obtained by adding exfoliated graphite to a CNT dispersion and mixing them. Although not particularly limited, from the viewpoint of improving the productivity of the composite of CNTs and exfoliated graphite, the mixing in the first mixing step is preferably performed using a kneader (the same applies to the second to fourth mixing steps described below). From the viewpoint of more efficiently obtaining a composite of CNTs and exfoliated graphite, the first mixing step (here, kneading) is preferably performed at a rotation speed of 20 rpm to 100 rpm. In the first mixing step and other mixing steps described below, any commercially available kneader used for this type of application may be used without particular limitation, but preferably the "Hibismix 2P-1" manufactured by PRIMIX Co., Ltd. The first mixing step is preferably carried out at a temperature of 50°C or less (for example, 20°C to 30°C) from the viewpoint of preventing excessive aggregation of the raw materials.

[0039] The second mixing step is, for example, a step of mixing the first mixture with silicon-containing graphite particles to obtain a second mixture. By carrying out the second mixing step, for example, a composite of CNT and flaky graphite can be coordinated on the surface of the silicon-containing graphite particles. In this embodiment, in the second mixing step, the silicon-containing graphite particles are preferably placed in a kneader that contains the first mixture after the first mixing step, and kneaded. The second mixing step (here, kneading) is preferably carried out at a rotation speed of 20 rpm to 100 rpm, from the viewpoint of more efficiently coordinating the composite of CNT and flaky graphite on the surface of the silicon-containing graphite particles. The second mixing step is preferably carried out at a temperature of 50°C or less (for example, 20°C to 30°C), from the viewpoint of suppressing aggregation of the composite of CNT and flaky graphite.

[0040] The third mixing step is, for example, a step of mixing the second mixture with other graphite particles to obtain a third mixture. In this embodiment, a mixture A containing other graphite particles, a first binder, and water may be prepared prior to the third mixing step. As the first binder, for example, carboxymethyl cellulose (CMC) may be used. In the third mixing step, the third mixture may be obtained by adding the mixture A to the second mixture and mixing them. In this embodiment, in the third mixing step, the mixture A may be placed in a kneader that accommodates the second mixture after the second mixing step, and kneaded. In order to more uniformly disperse the silicon-containing graphite particles and the other graphite particles, the third mixing step (here, kneading) may be performed at a rotation speed of, for example, 20 rpm to 100 rpm, preferably 30 rpm to 80 rpm. The third mixing step is preferably carried out at a temperature of 50° C. or less (for example, 20° C. to 30° C.) from the viewpoint of suppressing aggregation of the composite of CNT and flaky graphite, and consequently aggregation of the silicon-containing graphite particles.

[0041] The fourth mixing step is, for example, a step of mixing the third mixture with a second binder to obtain a fourth mixture. The second binder may be, for example, styrene butadiene rubber (SBR). In this embodiment, the fourth mixing step may be performed by adding SBR to the third mixture and mixing them to obtain the fourth mixture. In the fourth mixing step, the third mixture obtained after the third mixing step may be kneaded by adding SBR and, if necessary, a dispersion medium (water) to a kneader that contains the third mixture obtained after the third mixing step. The rotation speed in the fourth mixing step is not particularly limited and may be set appropriately. The fourth mixing step may be performed at a temperature of 50°C or less (for example, 20°C to 30°C) to prevent aggregation of the composite of CNT and flaky graphite, and thereby the aggregation of the silicon-containing graphite particles.

[0042] The coating step is, for example, a step of coating the surface of the negative electrode current collector 62 with negative electrode slurry. The negative electrode slurry is, for example, a slurry (mixture) containing the second mixture. In this embodiment, the negative electrode slurry is the fourth mixture. In this embodiment, in the coating step, the negative electrode slurry is coated in a strip shape on a copper foil serving as the negative electrode current collector 62. The coating method is not particularly limited, and a conventionally known method may be employed. The drying step is, for example, a step of drying the negative electrode slurry coated on the negative electrode current collector 62 in the coating step to obtain a dried film. The drying conditions are not particularly limited, and conditions used in producing this type of negative electrode may be appropriately employed. The pressing step is, for example, a step of pressing the dried film obtained in the drying step to obtain the negative electrode active material layer 64. The pressing conditions are not particularly limited, and conditions used in producing this type of negative electrode may be appropriately employed.

[0043] The separator 70 may be a porous sheet (film) made of a resin material such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. The porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0044] The case 30 is, for example, an outer container that houses the electrode assembly 20 and the nonaqueous electrolyte 80. Here, the case 30 is a flat, rectangular case. As shown in FIG. 1 , the case 30 has a positive electrode terminal 42, a negative electrode terminal 44, a safety valve 36, and an inlet (not shown). The positive electrode terminal 42 is, for example, a terminal for external connection on the positive electrode side. Here, the positive electrode terminal 42 is electrically connected to the positive electrode 50 of the electrode assembly 20 via a current collector 42a. The negative electrode terminal 44 is, for example, a terminal for external connection on the negative electrode side. Here, the negative electrode terminal 44 is electrically connected to the negative electrode 60 of the electrode assembly 20 via a current collector 44a. The safety valve 36 is, for example, a thin-walled portion that is configured to release internal pressure in the case 30 when the internal pressure rises above a predetermined level. The inlet is, for example, a portion through which the nonaqueous electrolyte 80 is injected into the case 30.

[0045] The nonaqueous electrolyte 80 includes, for example, a nonaqueous solvent and a supporting salt. Examples of the nonaqueous solvent include organic solvents such as various carbonates, ethers, esters, nitriles, sulfones, and lactones commonly used in this type of application. Among these, carbonates are preferred. Examples of carbonates include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC) (preferably monofluoroethylene carbonate), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). As the nonaqueous solvent, one type of nonaqueous solvent may be used alone, or two or more types of nonaqueous solvents may be used in combination. Examples of the supporting salt include lithium salts such as LiPF6, LiBF4, and LiClO4. The concentration of the supporting electrolyte may be, for example, 0.7 mol / L to 1.4 mol / L. The nonaqueous electrolyte 80 may contain additives used in this type of application, as needed. Examples of additives include film-forming agents such as LiB(C2O4)2 (LiBOB) and LiBF2 (C2O4); gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; etc.

[0046] The lithium ion secondary battery 100 can be used for a variety of purposes. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion secondary battery 100 can be used, for example, as a storage battery for small-sized power storage devices. The lithium ion secondary battery 100 can also be used, for example, in the form of an assembled battery in which a plurality of batteries are connected in series and / or parallel.

[0047] As described above, a method for manufacturing a negative electrode 60 is disclosed, which is used in a lithium-ion secondary battery 100 including a nonaqueous electrolyte solution 80 and includes a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the surface of the negative electrode current collector 62. This manufacturing method includes the steps of mixing CNTs and flaky graphite to obtain a first mixture (here, a first mixing step), mixing the first mixture with a negative electrode active material that is silicon-containing graphite particles to obtain a second mixture (here, a second mixing step), and applying a negative electrode slurry (here, a fourth mixture) containing the second mixture to the surface of the negative electrode current collector 62 to provide the negative electrode active material layer 64 on the surface of the negative electrode current collector 62 (here, an application step).

[0048] In this manufacturing method, CNTs and flaky graphite are mixed to form a composite of CNTs and flaky graphite as a conductive additive. Next, a first mixture containing the composite of CNTs and flaky graphite is mixed with silicon-containing graphite particles, thereby efficiently aligning the composite of CNTs and flaky graphite on the surface of the silicon-containing graphite particles. A negative electrode active material layer 64 containing silicon-containing graphite particles with a conductive additive coordinated on the surface can be formed by applying a negative electrode slurry containing a second mixture containing silicon-containing graphite particles with a composite of CNTs and flaky graphite coordinated on the surface to a negative electrode current collector 62. This allows for the manufacture of a negative electrode 60 in which conductive path breakage is unlikely to occur even when the silicon-containing graphite particles expand and contract during charge and discharge. This ultimately allows for the suppression of resistance increases after charge and discharge cycles in a lithium-ion secondary battery 100 containing a silicon-containing negative electrode active material.

[0049] The carbon nanotubes may be multi-walled carbon nanotubes, which, in addition to the above-mentioned effects, can improve the productivity of the negative electrode 60 and the lithium-ion secondary battery 100 and reduce the manufacturing costs thereof.

[0050] The flake graphite may be flake graphite or graphene, which can better realize the effects of the technology disclosed herein.

[0051] The average particle size of the flaky graphite may be 0.5 μm to 10 μm. When the average particle size of the flaky graphite is within a predetermined range, the composite of CNT and flaky graphite can be disposed in a preferable state between the negative electrode active materials. In this embodiment, it is possible to prevent the composite of CNT and flaky graphite from partially or entirely entering the voids between the graphite particles in the silicon-containing graphite particles. Therefore, it is possible to provide a negative electrode 60 with an excellent conductive path.

[0052] The silicon-containing graphite particles may include graphite particles having voids and silicon-containing particles disposed within the voids. This allows the voids in the graphite particles to mitigate expansion and contraction of the silicon-containing particles during charge and discharge. This prevents expansion and contraction of the entire negative electrode active material layer 64, and ultimately the entire negative electrode 60, and thus prevents disconnection of the conductive path.

[0053] The particle diameter of the silicon-containing particles may be 1 μm or less. This allows the silicon-containing particles to be contained in the silicon-containing graphite particles at a high ratio. This improves the electrical conductivity of the negative electrode active material while achieving high output.

[0054] This manufacturing method may further include mixing the second mixture with another negative electrode active material, graphite particles substantially free of silicon, to obtain a negative electrode slurry, which can further improve the conductivity of the negative electrode active material layer 64 and, ultimately, the negative electrode 60.

[0055] The technology disclosed herein provides an anode 60 having an anode active material layer 64 for use in an electricity storage device (here, a lithium-ion secondary battery 100) including a non-aqueous electrolyte solution 80. The anode 60 contains silicon-containing graphite particles as the anode active material, and a conductive additive that is a composite of carbon nanotubes and flake graphite.

[0056] By including silicon-containing graphite particles as the negative electrode active material in the negative electrode 60, it is possible to achieve high capacity and high energy density in an electricity storage device (here, a lithium-ion secondary battery 100) including the negative electrode 60. By including a composite of CNT and flaky graphite as a conductive additive in the negative electrode 60, it is possible to achieve a state in which the composite of CNT and flaky graphite is coordinated on the surface of the silicon-containing graphite particles. This makes it possible to provide a negative electrode 60 in which conductive path breakage is unlikely to occur even when the silicon-containing graphite particles expand and contract during charge and discharge. This in turn makes it possible to suppress an increase in resistance after charge and discharge cycles of the lithium-ion secondary battery 100.

[0057] The technology disclosed herein provides a lithium ion secondary battery 100 including a negative electrode 60 and a nonaqueous electrolyte solution 80. In the lithium ion secondary battery 100, by including the negative electrode 60, an increase in resistance after charge / discharge cycles can be suppressed, as described above.

[0058] Test examples relating to the technology disclosed herein will be described below, but it is not intended that the technology disclosed herein be limited to those shown in the following test examples.

[0059] [Manufacturing test cells] Example 1 LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (LNCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were prepared. These were combined with N-methylpyrrolidone (NMP) as a solvent in a mass ratio of LNCM:AB:PVdF = 92:5:3 and kneaded using a stirring granulator to prepare a positive electrode slurry. The positive electrode slurry was applied to both sides of a 15 μm thick aluminum foil and dried. The foil was then pressed to the specified thickness and processed to the specified dimensions to obtain a positive electrode.

[0060] Graphite particles (graphite particles substantially free of silicon) and silicon-containing graphite particles were prepared as negative electrode active materials. The graphite particles had an average particle diameter of 22 μm and a BET specific surface area of ​​1.4 m. 2 The graphite particles were 1 / g. The silicon-containing graphite particles were particles containing voided graphite particles with an average particle diameter of 3 μm and silicon particles arranged in the voids. The particle diameter of the silicon particles was 30 nm. As the conductive additive, flake graphite with an average particle diameter of 4 μm and multi-walled carbon nanotubes (MWCNT) with a fiber length of 1.2 μm were prepared. As the binder, carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) were prepared. A negative electrode slurry was prepared with a mass ratio of graphite particles / silicon-containing graphite particles / conductive additive / CMC / SBR=93:2:1:1:3. The mass ratio of the conductive additive was based on the total amount of flake graphite and MWCNT.

[0061] In preparing the negative electrode slurry, flake graphite was first added to an aqueous dispersion of MWCNT and kneaded at 30 rpm to obtain a first mixture. A composite of flake graphite and MWCNT was obtained in the first mixture. Silicon-containing graphite particles were then added to the first mixture and kneaded at 30 rpm to obtain a second mixture. Separately, a mixture A containing graphite particles (graphite particles substantially free of silicon), CMC, and water was prepared. The second mixture and mixture A were then kneaded at 50 rpm to obtain a third mixture. SBR and water were then added to the third mixture and kneaded (mixed) to obtain a fourth mixture (negative electrode slurry). The negative electrode slurry was then applied to both sides of a 10 μm-thick copper foil, dried, and pressed to a predetermined thickness. The negative electrode was then processed to the predetermined dimensions to obtain a negative electrode. The basis weight of the negative electrode slurry was 215 g / m on each side. 2 The packing density was 1.6 g / cc. The packing density Z was calculated using the following formula (P): Packing density Z(g / cc)= {Active material layer weight X (g / m 2 )} / {active material layer thickness Y (μm)} (P) The kneading machine used for the above-mentioned kneading and mixing was "Hivismix 2P-1" manufactured by PRIMIX Co., Ltd. The negative electrode slurry was prepared at room temperature.

[0062] A separator was prepared, which consisted of a porous polyolefin sheet (20 μm thick) with a three-layer structure of PP / PE / PP and a heat-resistant layer (4 μm thick). Leads were attached to each of the positive and negative electrodes, and the electrodes were stacked with the separator interposed between them to prepare an electrode assembly. The electrode assembly was inserted into an exterior body of an aluminum laminate sheet, a nonaqueous electrolyte was poured, and the opening of the exterior body was sealed to prepare a test cell for each example. The composition (volume ratio) of the nonaqueous electrolyte was EC / FEC / EMC / DMC = 15:5:40:40. LiPF6 was dissolved in the nonaqueous electrolyte as a supporting electrolyte at a concentration of 1 mol / L.

[0063] <Example 2> Graphene having a particle diameter of 2 μm was used as a conductive additive, and a composite of graphene and MWCNT was formed in the preparation of the negative electrode slurry. The test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0064] <Comparative Examples 1 to 3> The conductive additive used was the one shown in the corresponding column in Table 1. The first mixture was not prepared. To obtain the second mixture, silicon-containing graphite particles were added to the aqueous dispersion of the conductive additive, and the mixture was kneaded under the conditions described above. The test cell of this example was otherwise prepared using the same materials and procedures as in Example 1.

[0065] [Initial capacity measurement] The test cell was subjected to one charge-discharge cycle in an environment of 25°C, in which constant current charging (CC charging) was performed at a charge rate of 0.05 C until the voltage between the positive and negative electrodes reached 4.2 V, and then constant current discharging (CC discharging) was performed at a discharge rate of 0.05 C until the voltage between the positive and negative electrodes reached 2.5 V. The discharge capacity at this time was measured and used as the initial capacity.

[0066] [Resistance rating] The initial capacity obtained as described above was taken as SOC 100%, and the test cell was adjusted to SOC 50% in a thermostatic chamber at 25°C. The test cell in this state was discharged for 10 seconds at current values ​​of 1C, 1.5C, 2C, and 3C, and the battery voltage was measured in each case. The current values ​​and battery voltage were plotted to determine the IV characteristics during discharge. The IV resistance during discharge was calculated from the slope of the resulting line. The ratio of each example to Comparative Example 1, which was taken as 100%, was calculated. The results are shown in the corresponding column in Table 1.

[0067] [Cycle characteristic evaluation] The test cell was placed in a thermostatic chamber at 25°C and subjected to CC charging at 0.33 C up to 4.15 V, followed by CV charging until the current reached 0.1 C, and then fully charged. It was then CC discharged at 0.33 C down to 3 V. This cycle constitutes one charge-discharge cycle, and 200 charge-discharge cycles were performed. The following formula (X): Capacity retention rate (%) = (discharge capacity after 200 cycles / initial capacity) x 100 (X) The capacity retention rate after 200 cycles was calculated for each test cell based on the above formula. The results are shown in the corresponding column in Table 1.

[0068] [Table 1]

[0069] As described above, the test cell of Example 1 and the test cell of Example 2 were produced by carrying out a production method including: mixing CNTs and flake graphite to obtain a first mixture; mixing the first mixture with a negative electrode active material, which is silicon-containing graphite particles, to obtain a second mixture; and applying a negative electrode slurry containing the second mixture to the surface of a negative electrode current collector to provide a negative electrode active material layer on the surface of the negative electrode current collector. As shown in Table 1, it was found that the test cell of Example 1 and the test cell of Example 2 obtained by carrying out the production method configured as described above exhibited suppressed resistance increase after charge-discharge cycling.

[0070] The techniques disclosed herein may include the techniques described in the following sections. Section 1: A method for manufacturing a negative electrode used in an electricity storage device including a non-aqueous electrolyte, the negative electrode including a negative electrode current collector and a negative electrode active material layer provided on a surface of the negative electrode current collector, the method comprising: Mixing carbon nanotubes and flake graphite to obtain a first mixture; mixing the first mixture with a negative electrode active material that is silicon-containing graphite particles to obtain a second mixture; applying a negative electrode slurry containing the second mixture to a surface of the negative electrode current collector to provide the negative electrode active material layer on the surface of the negative electrode current collector; The manufacturing method includes the steps of: Section 2: Item 2. The method according to item 1, wherein the carbon nanotubes are multi-walled carbon nanotubes. Section 3: Item 3. The method according to item 1 or 2, wherein the flake graphite is flake graphite or graphene. Section 4: Item 4. The method according to any one of Items 1 to 3, wherein the average particle size of the flaky graphite is 0.5 μm or more and 10 μm or less. Section 5: 5. The method according to any one of items 1 to 4, wherein the silicon-containing graphite particles include graphite particles having voids and silicon-containing particles disposed in the voids. Item 6: Item 6. The method according to any one of Items 1 to 5, wherein the silicon-containing particles have a particle size of 1 μm or less. Section 7: Item 7. The method according to any one of Items 1 to 6, further comprising mixing the second mixture with graphite particles that are substantially free of silicon, which is another negative electrode active material, to obtain the negative electrode slurry. Section 8: A negative electrode including a negative electrode active material layer, which is used in an electricity storage device including a non-aqueous electrolyte solution, The negative electrode active material layer is silicon-containing graphite particles as a negative electrode active material; a conductive additive which is a composite of carbon nanotubes and flake graphite; a negative electrode comprising: Section 9: Item 9. The negative electrode according to Item 8, wherein the carbon nanotubes are multi-walled carbon nanotubes. Section 10: Item 10. The negative electrode according to item 8 or 9, wherein the flake graphite is flake graphite or graphene. Section 11: Item 11. The negative electrode according to any one of items 8 to 10, wherein the average particle size of the flaky graphite is 0.5 μm or more and 10 μm or less. Section 12: Item 12. The negative electrode according to any one of items 8 to 11, wherein the silicon-containing graphite particles include graphite particles having voids and silicon-containing particles disposed in the voids. Section 13: Item 13. The negative electrode according to any one of items 8 to 12, wherein the silicon-containing particles have a particle size of 1 μm or less. Section 14: Item 14. The negative electrode according to any one of items 8 to 13, further comprising graphite particles that are substantially free of silicon as another negative electrode active material. Section 15: 15. An electricity storage device comprising the negative electrode according to any one of items 8 to 14 and a non-aqueous electrolyte solution.

[0071] Although the embodiments of the technology disclosed herein have been described above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Explanation of symbols]

[0072] 20 Electrode body 30 cases 42 Positive terminal 44 Negative terminal 50 positive electrode 60 negative electrode 70 Separator 80 Nonaqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. A method for manufacturing a negative electrode used in an electricity storage device including a non-aqueous electrolyte, the negative electrode including a negative electrode current collector and a negative electrode active material layer provided on a surface of the negative electrode current collector, the method comprising: Mixing carbon nanotubes and flake graphite to obtain a first mixture; mixing the first mixture with a negative electrode active material that is silicon-containing graphite particles to obtain a second mixture; applying a negative electrode slurry containing the second mixture to a surface of the negative electrode current collector to provide the negative electrode active material layer on the surface of the negative electrode current collector; The manufacturing method includes the steps of:

2. The method according to claim 1 , wherein the carbon nanotubes are multi-walled carbon nanotubes.

3. The method according to claim 1 or 2, wherein the flake graphite is flake graphite or graphene.

4. The method according to claim 1 or 2, wherein the average particle size of the flake graphite is 0.5 μm or more and 10 μm or less.

5. The method according to claim 1 or 2, wherein the silicon-containing graphite particles comprise graphite particles having voids and silicon-containing particles disposed within the voids.

6. The method according to claim 5 , wherein the silicon-containing particles have a particle size of 1 μm or less.

7. The method according to claim 1 or 2, further comprising mixing the second mixture with graphite particles that are substantially free of silicon, which is another negative electrode active material, to obtain the negative electrode slurry.

8. A negative electrode including a negative electrode active material layer, which is used in an electricity storage device including a non-aqueous electrolyte solution, The negative electrode active material layer is silicon-containing graphite particles as a negative electrode active material; a conductive additive which is a composite of carbon nanotubes and flake graphite; a negative electrode comprising:

9. The negative electrode according to claim 8 , wherein the carbon nanotubes are multi-walled carbon nanotubes.

10. The negative electrode according to claim 8 or 9, wherein the flake graphite is flake graphite or graphene.

11. 10. The negative electrode according to claim 8, wherein the average particle size of the flake graphite is 0.5 μm or more and 10 μm or less.

12. 10. The negative electrode according to claim 8, wherein the silicon-containing graphite particles comprise graphite particles having voids and silicon-containing particles disposed in the voids.

13. The negative electrode according to claim 12 , wherein the silicon-containing particles have a particle size of 1 μm or less.

14. The negative electrode according to claim 8 or 9, further comprising graphite particles that are substantially free of silicon as another negative electrode active material.

15. An electricity storage device comprising the negative electrode according to claim 8 or 9 and a non-aqueous electrolyte solution.

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