Energy storage device and method for manufacturing the same

Abstract

To provide a technique capable of reducing a resistance increase ratio while suppressing an increase in electrode plate expansion ratio after charge and discharge cycles of an energy storage device having a negative electrode including Si.SOLUTION: According to the technology disclosed herein, there is disclosed an energy storage device including: a negative electrode current collector 62; and a negative electrode active material layer 64 on the negative electrode current collector 62, the negative electrode active material layer 64 including graphite substrates having voids and Si-containing particles that are composite particles of silicon disposed in the voids of the graphite substrates, the Si-containing particles serving as negative electrode active material particles, and the hardness of the Si-containing particles contained in at least a part of layers divided in a thickness direction of the negative electrode active material layer 64 being lower than the hardness of the Si-containing particles contained in other layers.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to an electricity storage device and a method for manufacturing an electricity storage device. [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] The negative electrode active material disclosed in Japanese Patent No. 6385749 contains a mixed powder of a first active material powder consisting of at least one selected from the group consisting of Si, Si compounds, Sn, and Sn compounds, and a second active material powder consisting of plate-like graphite particles having a thickness of 0.3 nm to 100 nm and a length in the major axis direction of 0.1 μm to 500 μm. The plate-like graphite particles are characterized by having an aromatic vinyl copolymer containing a vinyl aromatic monomer unit represented by the following formula (1): -(CH2-CHX)- (1) adsorbed on their surfaces. In formula (1), X represents a phenyl group, naphthyl group, anthracenyl group, or pyrenyl group, and these groups may have a substituent. The publication states that this configuration improves the cycle characteristics and initial efficiency of the power storage device, while also enabling a high capacity.

[0004] Japanese Patent No. 5522817 discloses a negative electrode active material composition for a lithium secondary battery. This composition contains a negative electrode active material, a polyimide precursor compound, and a polymer having a glass transition temperature of 50°C or lower. The polyimide precursor compound is a polyamic acid. The polymer is polyvinylidene fluoride. The negative electrode active material is SnO, SnO2, SiO, or SiOx (0 < x < 2). The content of the polyimide precursor compound is 4.95 to 15% by weight. The content of the polymer in the composition is 0.05 to 3% by weight. This publication describes that by using this composition, the bending phenomenon of the electrode plate can be prevented, flexibility can be imparted to the electrode plate, and the capacity and life characteristics of the lithium secondary battery can be improved.

[0005] Japanese Unexamined Patent Application Publication No. 2006-196447 discloses a negative electrode for a lithium ion secondary battery. This negative electrode includes a current collector and an active material layer supported on the current collector. The active material layer includes a first layer and a second layer alternately laminated in the thickness direction of the active material layer. This publication describes that by achieving both high lithium ion conductivity and electron conductivity, the high rate charge and discharge characteristics can be improved, the expansion rate of the active material during the reaction with lithium ions can be reduced, and the stress due to expansion can be dispersed and relaxed throughout the active material layer, thereby improving the cycle characteristics.

[0006] International Publication No. 2020 / 031869 discloses a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and an electrolytic solution. The negative electrode has a negative electrode mixture layer containing a first negative electrode active material and a negative electrode current collector to which the negative electrode mixture layer is attached. The first negative electrode active material includes a first lithium silicate phase containing lithium, silicon, and oxygen, and first silicon particles dispersed in the first lithium silicate phase. The atomic ratio A1:O / Si of oxygen to silicon in the first lithium silicate phase satisfies the relationship 2 < A1 ≤ 3. The proportion of the first negative electrode active material in the negative electrode mixture layer is larger on the surface side of the negative electrode than on the negative electrode current collector side. This publication describes that with such a configuration, the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6385749 [Patent Document 2] Patent No. 5522817 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-196447 [Patent Document 4] International Publication No. 2020 / 031869 Summary of the Invention [Problem to be solved by the invention]

[0008] It is known that negative electrodes containing silicon (Si) expand significantly during charging and discharging, hardening the negative electrode plate and resulting in reduced repulsive force characteristics. As described in Patent Documents 1 to 4, research and development efforts have been actively conducted to address this issue, but there is still room for improvement in order to soften negative electrode plates with a high Si content without degrading the performance of the electricity storage device.

[0009] In view of the above circumstances, the present inventors wished to reduce the rate of increase in resistance of an electricity storage device having a negative electrode containing Si, while suppressing an increase in the rate of expansion of the electrode plate after charge-discharge cycling. [Means for solving the problem]

[0010] The technology disclosed herein provides an electricity storage device including a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector. The negative electrode active material layer includes a graphite substrate having voids and Si-containing particles, which are composite particles of silicon disposed within the voids of the graphite substrate, as negative electrode active material particles. The Si-containing particles contained in at least some layers separated in the thickness direction of the negative electrode active material layer have a lower hardness than the Si-containing particles contained in other layers. This configuration can reduce the resistance increase rate of an electricity storage device having a Si-containing negative electrode while suppressing an increase in the electrode plate expansion rate after charge / discharge cycling.

[0011] The presently disclosed technology provides a method for manufacturing an electricity storage device including a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector. The method includes providing a negative electrode active material layer containing Si-containing particles as the negative electrode active material on the negative electrode current collector. Providing the negative electrode active material layer includes providing a first layer using a first paste containing Si-containing particles with a relatively low hardness and providing a second layer using a second paste containing Si-containing particles with a relatively high hardness. The Si-containing particles are composite particles of a graphite substrate having voids and silicon disposed in the voids of the graphite substrate. This configuration enables an electricity storage device having a negative electrode containing Si to have a reduced resistance increase rate while suppressing an increase in the electrode plate expansion rate after charge / discharge cycles. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a vertical 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. [Figure 3] FIG. 3 is a schematic cross-sectional view of the negative electrode 60. As shown in FIG. [Figure 4] FIG. 4 is a schematic cross-sectional view of the negative electrode 260. DETAILED DESCRIPTION OF THE INVENTION

[0013] 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."

[0014] 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, nickel-metal hydride batteries, and nickel-cadmium 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.

[0015] First Embodiment Fig. 1 is a longitudinal 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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 97 mass %, and even more preferably 85 mass % to 96 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 %.

[0023] 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.

[0024] The negative electrode active material layer 64 includes, for example, a negative electrode active material. In this embodiment, the negative electrode active material includes Si-containing particles. The Si-containing particles are, for example, composite particles of a graphite substrate having voids and silicon disposed in the voids of the graphite substrate (in the following description, such particles are also referred to as "Si / C particles"). The graphite substrate may be, for example, porous and may be fibrous or granular. In this embodiment, the composite particles of the graphite substrate and silicon refer to particles in which the graphite substrate and silicon are integrated and behave like a single particle.

[0025] The Si-containing particles can be obtained by known methods. For example, they can be obtained by mixing fine particles of Si, Si oxide, or the like with a carbon precursor (e.g., petroleum pitch, coal pitch, phenolic resin, or the like), followed by carbonization and spheroidization. Alternatively, they can be obtained by mixing spherically granulated graphite substrate and fine particles of Si, Si oxide, or the like in a dispersion medium, drying the mixture, and disposing the fine particles in the pores of the graphite substrate.

[0026] By using a negative electrode active material containing silicon (Si), it is possible to achieve even higher capacity and higher energy density of an electricity storage device. However, it is known that a negative electrode active material containing Si significantly expands and contracts during charge and discharge of the electricity storage device, hardening the negative electrode plate, which reduces the durability of the electricity storage device. The present inventors have conducted extensive research into a configuration for a negative electrode of an electricity storage device that includes a negative electrode active material containing Si, which suppresses expansion and contraction during charge and discharge and also suppresses an increase in plate hardness.

[0027] The hardness of the Si-containing particles contained in at least some of the layers divided in the thickness direction of the negative electrode active material layer 64 is, for example, lower than the hardness of the Si-containing particles contained in the other layers. In this embodiment, the negative electrode active material layer 64 has at least two layers. One of the two layers contains Si-containing particles with a relatively low hardness. The other of the two layers contains Si-containing particles with a relatively high hardness. In the following description, the Si-containing particles with a relatively low hardness may be referred to as "first Si-containing particles," and the Si-containing particles with a relatively high hardness may be referred to as "second Si-containing particles." The layer structure of the negative electrode active material layer 64 will be described further below.

[0028] In this embodiment, the hardness of the first Si-containing particles and the second Si-containing particles is determined by the compressive modulus of each particle. The compressive modulus of the second Si-containing particles is generally 1.5 times or more, for example, 1.7 times or more, relative to the compressive modulus of the first Si-containing particles. From the viewpoint of better achieving the effects of the technology disclosed herein, it is preferably 2 times or more, and more preferably 2.2 times or more. From the same viewpoint, the compressive modulus of the second Si-containing particles is generally 5 or less, for example, 4.5 or less, preferably 4 or less, and more preferably 3.5 or less, relative to the compressive modulus of the first Si-containing particles. The compressive modulus of the first Si-containing particles and the second Si-containing particles can be adjusted to the desired compressive modulus by, for example, the porosity of the graphite substrate, the type of graphite substrate, the surface coating, etc.

[0029] The compressive elastic modulus of the first Si-containing particles is preferably approximately 250 MPa or more and less than 2000 MPa. From the viewpoint of better suppressing expansion and contraction of the negative electrode 60 during charge and discharge of the lithium-ion secondary battery 100, the compressive elastic modulus of the first Si-containing particles is, for example, 1800 MPa or less, preferably 1600 MPa or less, and more preferably 1400 MPa or less. From the viewpoint of realizing appropriate hardness in the negative electrode active material layer 64, the compressive elastic modulus of the first Si-containing particles is, for example, 500 MPa or more, preferably 750 MPa or more, and more preferably 1000 MPa or more. The compressive elastic modulus of the second Si-containing particles is preferably approximately 2000 MPa to 5000 MPa or less. From the viewpoint of suppressing deformation of the negative electrode 60 during charge and discharge of the lithium-ion secondary battery 100, the compressive elastic modulus of the second Si-containing particles is, for example, 4500 MPa or less, preferably 4000 MPa or less, and more preferably 3500 MPa or less. From the viewpoint of suppressing disconnection of the conductive path in the negative electrode active material layer 64 during charging and discharging of the lithium ion secondary battery 100, the compressive elastic modulus of the second Si-containing particles is, for example, 2200 MPa or more, preferably 2400 MPa or more, more preferably 2600 MPa or more, and even more preferably 2800 MPa or more.

[0030] The compressive modulus of the Si-containing particles (hereinafter, when there is no particular distinction between the first Si-containing particles and the second Si-containing particles, they will also be simply referred to as "Si-containing particles") can be measured, for example, by using a commercially available testing machine. A preferred example of a commercially available testing machine is the "MCT-211" micro-compression testing machine manufactured by Shimadzu Corporation. In a method for measuring the compressive modulus of the Si-containing particles, first, a single Si-containing particle is compressed in the vertical direction using the testing machine, and the displacement (compressive displacement) and stress (compressive stress) during compression are measured. Next, the average particle diameter of the Si-containing particles is divided by the compressive displacement to calculate the compressive strain. Then, the following formula (A): Compressive modulus (MPa) = Compressive stress (MPa) / Compressive strain (A) The compressive modulus of each Si-containing particle is calculated using the above formula. In this embodiment, the compressive modulus of at least five Si-containing particles is calculated, and the arithmetic average value is obtained to be the compressive modulus of the Si-containing particle. Here, the at least five Si-containing particles have a particle diameter that is 90% to 110% of the average particle diameter. The particle diameter of the Si-containing particles can be measured when measuring the compressive modulus using the above-mentioned testing machine. The average particle diameter of the Si-containing particles will be described later.

[0031] The average particle diameter of the Si-containing particles is generally 0.5 μm to 20 μm. From the viewpoint of realizing the effects of the technology disclosed herein, the average particle diameter is, for example, 1 μm or more, preferably 3 μm or more, and more preferably 5 μm or more. From the same viewpoint, the average particle diameter is, for example, 15 μm or less, preferably 12 μm or less, and more preferably 10 μm or less. The average particle diameter of the first Si-containing particles and the average particle diameter of the second Si-containing particles may be the same or different. In this specification, the "average particle diameter" of particles refers to the particle diameter (D) corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by particle size distribution measurement based on the laser diffraction / light scattering method. 50 particle size).

[0032] The negative electrode active material 64 may further include graphite particles as the negative electrode active material. The graphite particles as the negative electrode active material may be, for example, artificial graphite or natural graphite. The graphite particles may have a coating layer of amorphous carbon on their surfaces. Although not particularly limited, the graphite particles may have, for example, a substantially spherical shape. In this specification, the term "substantially spherical" in relation to graphite particles refers to an average aspect ratio of 1 to 2 (preferably 1 to 1.5) based on observation with a scanning electron microscope (SEM). The average aspect ratio can be determined, for example, by obtaining a planar SEM image of the graphite particles, randomly selecting a plurality of graphite particles (for example, 10 to 100) from the SEM image, calculating the aspect ratio of each, and then calculating the arithmetic mean value. The average particle diameter of the graphite particles may be, for example, 5 μm to 30 μm, or may be 10 μm to 20 μm. The compressive elastic modulus of the graphite particles may be, for example, 100 MPa to 300 MPa, and preferably 150 MPa to 200 MPa.

[0033] The content of the graphite particles in the entire negative electrode active material is approximately 20% by mass to 80% by mass, preferably 40% by mass to 75% by mass, and more preferably 50% by mass to 70% by mass.

[0034] The negative electrode active material layer 64 may contain a conductive material in addition to the negative electrode active material. Examples of the conductive material that can be used include carbon nanotubes such as single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT); carbon black such as acetylene black (AB); and carbon fibers. Among these, from the viewpoint of improving the cycle characteristics of the lithium-ion secondary battery 100, carbon nanotubes are preferred as the conductive material, and single-walled carbon nanotubes are more preferred.

[0035] The ratio of the negative electrode active material to the entire negative electrode active material layer 64 is, for example, 70 mass % or more, preferably 80 mass % or more, more preferably 90 mass % to 99 mass %, and may be 95 mass % to 99 mass %. The ratio of the conductive material to the entire negative electrode active material layer 64 may be, for example, 0.01 mass % to 1 mass %.

[0036] 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), polyacrylic acid (PAA), 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.

[0037] FIG. 3 is a schematic cross-sectional view of the negative electrode 60. FIG. 3 schematically shows a partial cross-sectional structure of the negative electrode 60. As shown in FIG. 3, the negative electrode active material layer 64 has a first region R1 and a second region R2. In this embodiment, when the negative electrode active material layer 64 is divided into two in the thickness direction (the vertical direction in FIG. 3), the first region R1 is a region relatively closer to the negative electrode current collector 62. When the negative electrode active material layer 64 is divided into two in the thickness direction, the second region R2 is a region farther from the negative electrode current collector 62.

[0038] In the embodiment shown in FIG. 3, the negative electrode active material layer 64 has a first layer 641 and a second layer 642. In this embodiment, the first layer 641 is provided in the second region R2. The second layer 642 is provided in the first region R1. As shown in FIG. 3, the first layer 641 is provided on the negative electrode current collector 62. In this embodiment, the second layer 642 is provided in the first region R1. The second layer 642 is provided on the first layer 641, and here, is on the surface layer side of the negative electrode active material layer 64. The ratio (T1:T2) of the thickness T1 of the first layer 641 to the thickness T2 of the second layer 642 is not particularly limited. The ratio (T1:T2) is, for example, 10:90 to 90:10.

[0039] In this embodiment, the first layer 641 contains first Si-containing particles. The second layer 642 contains second Si-containing particles. In this embodiment, the fact that the first layer 641 contains the first Si-containing particles and the second layer 642 contains the second Si-containing particles can be explained, for example, by the following procedure. First, a lithium-ion secondary battery 100 to be inspected is prepared. Next, the lithium-ion secondary battery 100 is disassembled, and the negative electrode 60 is removed. Next, the negative electrode active material layer 64 is evenly divided into 3 to 20 layers in the thickness direction from the surface of the negative electrode active material layer 64 to the negative electrode current collector 62, and the compressive elastic modulus of the Si-containing particles contained in each layer is measured using the procedure described above. By measuring the compressive elastic modulus, it can be determined, for example, whether each layer corresponds to either the first layer 641 or the second layer 642. The negative electrode active material layer 64 contains both graphite particles and Si-containing particles, and these can be distinguished from each other by using an analytical method such as scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX).

[0040] A method for producing (manufacturing) the negative electrode 60 includes, for example, providing a first layer 641 on a negative electrode current collector 62 using a first paste containing Si-containing particles (first Si-containing particles) with a relatively low hardness, and providing a second layer 642 using a second paste containing Si-containing particles (second Si-containing particles) with a relatively high hardness. By performing this method, the negative electrode 60 having the above-described characteristics can be produced, and ultimately, an electricity storage device (lithium ion secondary battery 100) including the negative electrode 60 can be produced.

[0041] In this embodiment, the method for producing (manufacturing) the negative electrode 60 includes a preparation step, a first mixing step, a second mixing step, a third mixing step, a first application step, a first drying step, a first pressing step, a second application step, a second drying step, and a second pressing step.

[0042] 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 preparing graphite particles, first Si-containing particles, second Si-containing particles, a conductive agent, and a binder. The raw materials listed here are as described above.

[0043] The first mixing step is, for example, a step of dry-mixing graphite particles, first Si-containing particles or second Si-containing particles, and a first binder. In this embodiment, in the first mixing step, the graphite particles, the first Si-containing particles, and the first binder are dry-mixed to prepare a first mixed powder. Similarly, the graphite particles, the second Si-containing particles, and the first binder are dry-mixed to prepare a second mixed powder. The first binder is, for example, carboxymethyl cellulose and polyacrylic acid. For the dry mixing in the first mixing step, for example, a conventionally known mixing device used for this type of application can be used without particular limitation.

[0044] The second mixing step is, for example, a step of thick-kneading the first mixed powder or the second mixed powder prepared in the first mixing step with a conductive agent and a dispersion medium. In this embodiment, in the second mixing step, the first mixed powder, the conductive agent, and water are thick-kneaded to obtain a first kneaded product. Similarly, the second mixed powder, the conductive agent, and water are thick-kneaded to obtain a second kneaded product. The solid content of the first kneaded product and the solid content of the second kneaded product are approximately 50% to 80%, and preferably 60% to 70%. For thick-kneading in the second mixing step, for example, a conventionally known mixing device used for this type of application can be used without any particular limitation.

[0045] The third mixing step is, for example, a step of mixing the first kneaded material or the second kneaded material obtained in the second mixing step with a second binder and a dispersion medium. In this embodiment, in the third mixing step, the first kneaded material, the second binder, and water are mixed to obtain a first paste. Similarly, the second kneaded material, the second binder, and water are mixed to obtain a second paste. The second binder is, for example, styrene-butadiene rubber. For the mixing in the third mixing step, for example, a conventionally known mixing device used for this type of application can be used without any particular limitation.

[0046] The first application step is, for example, a step of applying the first paste or the second paste obtained in the third mixing step to the negative electrode current collector 62. In this embodiment, the second paste is applied in a strip shape to copper foil serving as the negative electrode current collector 62. The application method is not particularly limited, and a conventionally known method may be adopted (the same applies to the second application step). The first drying step is, for example, a step of drying the second paste applied to the negative electrode current collector 62 in the first application step to obtain a dried film. The drying conditions are not particularly limited, and the conditions used in producing this type of negative electrode may be appropriately adopted (the same applies to the second drying step). The first pressing step is, for example, a step of pressing the dried film obtained in the first drying step to obtain the second layer 642. The pressing conditions are not particularly limited, and the conditions used in producing this type of negative electrode may be appropriately adopted (the same applies to the second pressing step).

[0047] The second application step is, for example, a step of applying the first paste or the second paste obtained in the third mixing step to the dried film obtained after the first pressing step. In this embodiment, the first paste is applied to the second layer 642 formed by the first pressing step. The second drying step is, for example, a step of drying the first paste applied to the second layer 642 in the second application step to obtain a dried film. The second pressing step is, for example, a step of pressing the dried film obtained in the second drying step to obtain the first layer 641. Note that the above-described method for manufacturing the negative electrode 60 is merely an example. The method for manufacturing the negative electrode 60 may, for example, not include some of the above-described steps as necessary, or may include any steps. In the above method, the first paste is applied and dried, and then the second paste is applied. However, the second paste may be applied after the first paste is applied and before drying, or the first paste and the second paste may be applied simultaneously. Alternatively, the first paste may be applied and dried, and then the second paste may be applied and dried, and then pressed.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] In this embodiment, the lithium-ion secondary battery 100 includes a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 contains Si-containing particles as negative electrode active material particles. The Si-containing particles are composite particles of a graphite substrate having voids and silicon disposed in the voids of the graphite substrate. The hardness of the Si-containing particles contained in at least some of the layers partitioned in the thickness direction of the negative electrode active material layer 64 is lower than the hardness of the Si-containing particles contained in the other layers.

[0053] In the lithium-ion secondary battery 100, the negative electrode active material layer 64 contains Si-containing particles as the negative electrode active material. The Si-containing particles contain a porous graphite substrate and Si disposed within the voids of the graphite substrate. This allows the voids to mitigate the Si expansion caused by charge and discharge, thereby suppressing an increase in the thickness of the negative electrode 60. The negative electrode active material layer 64 has, in the thickness direction, at least a layer containing Si-containing particles with a relatively low hardness and a layer containing Si-containing particles with a relatively high hardness. The layer containing Si-containing particles with a relatively low hardness is more likely than other layers to follow the shape change of the electrode plate during expansion and contraction during charge and discharge. This allows an increase in the electrode plate expansion rate after charge and discharge cycles to be suppressed. On the other hand, the layer containing Si-containing particles with a relatively high hardness is less likely to deform during expansion and contraction during charge and discharge, making it less likely to cause disconnection of the conductive path. This allows an increase in the electrode plate expansion rate and a reduction in the resistance increase rate after charge and discharge cycles to be suppressed. That is, in the lithium ion secondary battery 100, high capacity and high energy density are realized, the increase in the expansion rate of the electrode plates due to expansion and contraction caused by charge and discharge cycles is suppressed, and the rate of increase in resistance is reduced.

[0054] When the negative electrode active material layer 64 is divided in half in the thickness direction, with the region relatively closer to the negative electrode current collector 62 designated as a first region R1 and the region relatively farther from the negative electrode current collector 62 designated as a second region R2, the first layer 641 containing Si-containing particles (first Si-containing particles) with relatively low hardness may be provided in the second region R2. In the negative electrode active material layer 64, the second region R2, which is farther from the negative electrode current collector 62, is more likely to expand and contract and to deform. Therefore, by providing the first layer 641 containing the first Si-containing particles in the second region R2, it is possible to follow the expansion and contraction of the negative electrode 60 and alleviate stress due to the expansion and contraction, thereby further enhancing the effect of suppressing an increase in the electrode plate expansion rate after charge-discharge cycles.

[0055] The compressive elastic modulus of the second Si-containing particle may be at least twice as large as the compressive elastic modulus of the first Si-containing particle, where the compressive elastic modulus of the first Si-containing particle is 1. By setting the compressive elastic modulus of the first Si-containing particle and the compressive elastic modulus of the second Si-containing particle as described above, the effects of the technology disclosed herein can be more effectively achieved.

[0056] The compressive modulus of the first Si-containing particles may be 250 MPa or more and less than 2000 MPa. By setting the compressive modulus of the first Si-containing particles in this range, the effects of the technology disclosed herein can be more effectively realized. The compressive modulus of the second Si-containing particles may be 2000 MPa or more and 5000 MPa or less. By setting the compressive modulus of the second Si-containing particles in this range, the effects of the technology disclosed herein can be more effectively realized.

[0057] The ratio (T1:T2) of the thickness T1 of the first layer 641 to the thickness T2 of the second layer 642 may be 10:90 to 90:10. By setting the ratio of the thickness T1 to the thickness T2 in this range, the effects of the technology disclosed herein can be more effectively achieved.

[0058] The negative electrode active material layer 64 may further contain graphite particles. Graphite particles are less likely to expand and contract during charging and discharging of the lithium-ion secondary battery 100 than Si-containing particles. Therefore, when the negative electrode active material layer 64 contains graphite particles, the graphite particles fulfill part of the function of the negative electrode active material, thereby suppressing the degree of expansion and contraction.

[0059] The manufacturing method for the lithium-ion secondary battery 100 is a method for manufacturing an electricity storage device including a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The manufacturing method includes providing the negative electrode active material layer 64 containing Si-containing particles as a negative electrode active material on the negative electrode current collector 62. The providing of the negative electrode active material layer 64 includes providing a first layer 641 using a first paste containing Si-containing particles (first Si-containing particles) with a relatively low hardness, and providing a second layer 642 using a second paste containing Si-containing particles (second Si-containing particles) with a relatively high hardness. The Si-containing particles are composite particles of a graphite substrate having voids and silicon disposed in the voids of the graphite substrate. In the lithium-ion secondary battery 100 manufactured by carrying out this manufacturing method, the increase in the electrode plate expansion rate of the negative electrode 60 after charge / discharge cycles is suppressed, and the rate of increase in resistance is reduced.

[0060] In this manufacturing method, the preparation of the first paste and the preparation of the second paste may include dry-mixing the first Si-containing particles or the second Si-containing particles with a first binder to obtain a first mixed powder or a second mixed powder; kneading the first mixed powder or the second mixed powder with a conductive agent and a dispersion medium to obtain a first kneaded material or a second kneaded material; and mixing the first kneaded material or the second kneaded material with the second binder and the dispersion medium. This allows the preparation of a first paste and a second paste that are suitable for achieving the effects of the technology disclosed herein.

[0061] This manufacturing method may include applying the second paste onto negative electrode current collector 62 to form second layer 642, and applying the first paste onto second layer 642 to form first layer 641. This makes it possible to fabricate negative electrode 60 having a configuration suitable for achieving the effects of the technology disclosed herein.

[0062] Second Embodiment FIG. 4 is a schematic cross-sectional view of the negative electrode 260. FIG. 4 schematically illustrates a partial cross-sectional structure of the negative electrode 260. As illustrated in FIG. 4, the negative electrode active material layer 264 has a first layer 2641 containing first Si-containing particles and a second layer 2642 containing second Si-containing particles. In this embodiment, the first layer 2641 is provided in the first region R1. The second layer 2642 is provided in the second region R2. As illustrated in FIG. 3, the first layer 2641 is provided on the negative electrode current collector 262. In this embodiment, the second layer 2642 is provided on the first layer 2641, and here, is on the surface layer side of the negative electrode active material layer 264. In this embodiment, the first layer 2641 containing Si-containing particles (first Si-containing particles) with relatively high hardness is provided in the first region R1, which is relatively close to the negative electrode current collector 262. This improves the liquid flow of the electrolyte on the side of the negative electrode active material layer 264 opposite to the negative electrode current collector 262. Therefore, with this configuration, of the effects of the technology disclosed herein, the effect of reducing the rate of increase in resistance after charge / discharge cycles can be particularly effectively achieved.

[0063] In this embodiment, the method for manufacturing lithium ion secondary battery 100 includes applying a first paste onto negative electrode current collector 262 to form first layer 2641, and applying a second paste onto first layer 2641 to form second layer 2642. This makes it possible to manufacture lithium ion secondary battery 100 that particularly effectively achieves the effect of reducing the rate of increase in resistance after charge / discharge cycles, among the effects of the technology disclosed herein.

[0064] 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.

[0065] [Manufacturing test cells] <Example 1> First Si-containing particles, second Si-containing particles, and graphite particles were prepared as negative electrode active materials. The first Si-containing particles were Si / C particles with a compressive modulus of 1100 MPa and an average particle diameter of 7 μm. The second Si-containing particles were Si / C particles with a compressive modulus of 3400 MPa and an average particle diameter of 8 μm. The graphite particles had a compressive modulus of 180 MPa and an average particle diameter of 14 μm. Single-walled carbon nanotubes (SWCNTs) were prepared as a conductive material. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared as binders. The prepared materials were mixed with water as a dispersion medium to produce a first paste in a mass ratio of graphite particles:first Si-containing particles:SWCNTs:CMC:PAA:SBR=60:40:0.1:1:1:2. The graphite particles, second Si-containing particles, SWCNT, CMC, PAA, and SBR were kneaded with water as a solvent so as to have a mass ratio of 60:40:0.1:1:1:2 to prepare a second paste.

[0066] The procedure for preparing the first paste and the second paste is as follows. First, graphite particles, first Si-containing particles or second Si-containing particles, CMC, and PAA were dry-mixed to obtain a mixed powder. Next, SWCNTs (a water-soluble paste with a solid content of 2%) and water were added to this mixed powder and kneaded. SBR and water were further added to the kneaded mixture and mixed. In this way, the first paste and the second paste were prepared.

[0067] Next, the second paste was applied to both sides of a 10 μm-thick copper foil and dried to form the second layer. The first paste was then applied to the second layer and dried to form the first layer. The first and second layers on the copper foil were pressed to a predetermined thickness and processed to the predetermined dimensions to obtain a negative electrode plate. The ratio of the thickness T1 of the first layer to the thickness T2 of the second layer (T1:T2) was 50:50.

[0068] 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 mixed with N-methylpyrrolidone (NMP) as a solvent in a mass ratio of LNCM:AB:PVdF = 100:1:1 to prepare a positive electrode paste. The positive electrode paste was applied to both sides of a 10 μm thick aluminum foil, dried, and then pressed to the specified thickness and processed to the specified dimensions to obtain a positive electrode plate.

[0069] A separator sheet was prepared, consisting of a porous polyolefin sheet with a three-layer structure of PP / PE / PP and an HRL. Leads were attached to each of the negative and positive electrodes prepared as described above, and the electrodes were stacked via a separator to produce an electrode assembly. This electrode assembly was inserted into an exterior body made of an aluminum laminate sheet, a nonaqueous electrolyte was injected, and the exterior of the exterior body was sealed to produce a test cell of this example. The nonaqueous electrolyte was a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:FEC:EMC:DMC=15:5:40:40, with LiPF6 dissolved as a supporting electrolyte at a concentration of 1 mol / L.

[0070] <Example 2> The first Si-containing particles were Si / C particles with a compressive modulus of 1300 MPa. The second Si-containing particles were Si / C particles with a compressive modulus of 2900 MPa. The test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0071] <Example 3> The ratio (T1:T2) of the thickness T1 of the first layer to the thickness T2 of the second layer was 90: 10. Except for this, the test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0072] <Example 4> The ratio (T1:T2) of the thickness T1 of the first layer to the thickness T2 of the second layer was set to 10:90. Except for this, the test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0073] <Example 5> A first paste containing first Si-containing particles was applied to a copper foil to form a first layer. A second paste containing second Si-containing particles was applied to the first layer to form a second layer. The test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0074] <Example 6> The graphite particles, first Si-containing particles, second Si-containing particles, SWCNTs, CMC, PAA, and SBR were mixed with water as a dispersion medium to obtain a mass ratio of 60:20:20:0.1:1:1:2 to prepare a paste of Example 6. The procedure for preparing the paste of Example 6 is as follows. First, the graphite particles, first Si-containing particles, second Si-containing particles, CMC, and PAA were dry-mixed to obtain a mixed powder. Next, SWCNTs (a water-soluble paste with a solid content of 2%) and water were added to this mixed powder and kneaded. SBR and water were then added to the kneaded mixture and mixed. In this way, the paste of Example 6 was prepared. A test cell of this example was prepared using the same materials and procedures as in Example 1, except for the above.

[0075] <Example 7> The first paste was applied to both sides of a copper foil, dried, and then pressed to a predetermined thickness and processed to a predetermined size to obtain a negative electrode plate. A test cell for this example was fabricated using the same materials and procedures as in Example 1.

[0076] <Example 8> The second paste was applied to both sides of the copper foil, dried, and then pressed to a predetermined thickness and processed to a predetermined size to obtain a negative electrode plate. The test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0077] The compressive modulus of elasticity of the first Si-containing particles and the second Si-containing particles used in Examples 1 to 8 was measured using a commercially available tester (a micro-compression tester "MCT-211" manufactured by Shimadzu Corporation) according to the equipment manual. First, using the tester, one particle was compressed in the vertical direction, and the displacement (compressive displacement) and stress (compressive stress) during compression were measured. Next, the average particle diameter of the Si-containing particles was divided by the compressive displacement to calculate the compressive strain. Next, the following formula (A): Compressive modulus (MPa) = Compressive stress (MPa) / Compressive strain (A) The compressive modulus of each Si-containing particle was calculated using the above formula. Here, the compressive modulus of at least five Si-containing particles was calculated, and the arithmetic average value was obtained to determine the compressive modulus of the Si-containing particle. The at least five Si-containing particles here were Si-containing particles having a particle diameter of 90% to 110% of the average particle diameter. The particle diameter of the Si-containing particles was measured when measuring the compressive modulus using a testing machine.

[0078] [Measurement of electrode plate hardness] The electrode plate hardness (spring constant) of each negative electrode was measured. A commercially available testing machine (precision universal testing machine "AGX-10kNVD" manufactured by Shimadzu Corporation) was used for the measurement. First, the negative electrode plate was cut to a predetermined size, and a load was applied in the stacking direction of the negative electrode plate to measure the amount of displacement. Next, the following formula (B): Spring constant (kN / mm) = Load (kN) / Displacement in thickness direction (mm)...(B) The spring constant (hardness of the electrode plate) of each negative electrode was measured based on the results shown in the corresponding column in Table 1.

[0079] [Measurement of resistance increase rate] <Initial resistance measurement> The test cell was placed in a 25°C environment. The test cell was charged at a constant current and constant voltage (CCCV) until the state of charge (SOC) reached 50%. The test cell was then left in a 25°C environment for 1 hour. Then, a constant current (CC) discharge was performed at a constant current of 1 C for 10 seconds. The following equation (C): Initial resistance = [Open circuit voltage (OCV) - Closed circuit voltage (CCV)] / Discharge current (C) The initial resistance of each test cell was measured based on the following formula: The closed circuit voltage in the above formula is the voltage value 10 seconds after the start of discharge. The discharge current value is the current value 10 seconds after the start of discharge.

[0080] <High-rate cycle test> The test cell was placed in a 25°C environment. The test cell was CCCV charged until the state of charge (SOC) reached 50%. The test cell was then CC charged at a current of 1.5C for 400 seconds, followed by CC discharge at a current of 0.75C for 800 seconds, for a total of 400 cycles.

[0081] <Measurement of resistance increase rate> The battery resistance of the test cell after 400 cycles was calculated in the same manner as described above. Resistance increase rate (%) = [battery resistance at 400th cycle / initial resistance] x 100 (D) The resistance increase rate of the test cell for each example was calculated based on the above. The results are shown in the corresponding column in Table 1.

[0082] [Measurement of electrode plate expansion rate] First, the initial thickness T0 of each test cell was measured. Three randomly selected points were selected from the top, center, and bottom of the wide surface of each test cell. The thickness of the test cell was measured at each of the three points, and the arithmetic average value was defined as the initial thickness T0. Next, the test cell of each example was subjected to 250 charge-discharge cycles, with CCCV charging (0.4C rate up to 4.2V, then 0.1C cut) followed by CC discharge (0.4C rate, 2.5V cut) in a 25°C environment. Each test cell was then discharged to 2.5V, disassembled under an argon atmosphere, and the negative electrode was removed. The negative electrode was immersed in DMC, washed, and dried. The thickness was measured at the same three points as when measuring the initial thickness T0. The arithmetic average value was defined as the thickness after 250 cycles, T250. The initial thickness T0 and the thickness after 250 cycles, T250, were calculated using the following formula (E): Plate expansion rate (%) = (thickness after 250 cycles T250 / initial thickness T0) × 100 Formula (E) The electrode plate expansion rate (%) of each test cell was measured based on the above data. The results are shown in the corresponding column in Table 1. Note that test cells with an electrode plate expansion rate (%) of 50% or less were evaluated as examples in which the electrode plate expansion rate of the test cell during charging and discharging was suppressed. The "center" of the wide surface is the center of the wide surface. The "upper portion" of the wide surface is the region closer to one end than the center of the wide surface. The "lower portion" of the wide surface is the region closer to the one end than the center of the wide surface.

[0083] [Table 1]

[0084] Regarding Examples 1 to 8, in the test cells of Examples 1 to 5, the negative electrode active material layer contains a graphite substrate having voids and Si-containing particles (Si / C particles), which are composite particles of silicon, disposed within the voids of the graphite substrate. The hardness of the Si-containing particles contained in at least one layer (here, the first layer) divided in the thickness direction of the negative electrode active material layer is lower than the hardness of the Si-containing particles contained in the other layer (here, the second layer). As shown in Table 1, in Examples 1 to 5 having such a configuration, the increase in the electrode plate expansion rate after charge / discharge cycling is suppressed and the rate of increase in resistance is reduced compared to Examples 6 to 8, which have a single-layer structure.

[0085] In the above-described embodiment, the negative electrode active material layer has a two-layer structure consisting of a first layer and a second layer. However, the technology disclosed herein is not limited to the above-described embodiment, as long as the negative electrode active material layer has a layer (first layer) containing Si-containing particles with a relatively low hardness. For example, in the negative electrode active material layer, the first layer may be sandwiched between the second layer and another second layer.

[0086] The techniques disclosed herein may include the techniques described in the following sections. Section 1: An electricity storage device including a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, the negative electrode active material layer includes, as negative electrode active material particles, a graphite base material having voids and Si-containing particles which are composite particles of silicon disposed in the voids of the graphite base material; The negative electrode active material layer is divided in a thickness direction thereof, and the hardness of the Si-containing particles contained in at least some of the layers is lower than the hardness of the Si-containing particles contained in other layers. Section 2: Item 2. The electricity storage device according to Item 1, wherein the negative electrode active material layer is divided into two in a thickness direction, with a region relatively closer to the negative electrode current collector as a first region and a region relatively farther from the negative electrode current collector as a second region, and the first layer containing Si-containing particles having a relatively low hardness is provided in the second region. Section 3: Item 2. The electricity storage device according to Item 1, wherein the negative electrode active material layer is divided into two in a thickness direction, with a region relatively closer to the negative electrode current collector as a first region and a region relatively farther from the negative electrode current collector as a second region, and the first layer containing Si-containing particles having a relatively low hardness is provided in the first region. Section 4: 4. The electricity storage device according to any one of items 1 to 3, wherein when the compressive modulus of the Si-containing particles having a relatively low hardness is taken as 1, the compressive modulus of the Si-containing particles having a relatively high hardness is at least twice as high. Section 5: 5. The electricity storage device according to any one of items 1 to 4, wherein the compressive modulus of the Si-containing particles having a relatively low hardness is 250 MPa or more and less than 2000 MPa. Item 6: 6. The electricity storage device according to any one of items 1 to 5, wherein the compressive modulus of the Si-containing particles having a relatively high hardness is 2000 MPa or more and 5000 MPa or less. Section 7: the negative electrode active material layer includes a first layer including the Si-containing particles having a relatively low hardness and a second layer including the Si-containing particles having a relatively high hardness, 7. The electricity storage device according to any one of items 1 to 6, wherein a ratio (T1:T2) of a thickness T1 of the first layer to a thickness T2 of the second layer is 10:90 to 90:10. Section 8: Item 8. The electricity storage device according to any one of items 1 to 7, wherein the negative electrode active material layer further contains graphite particles as the negative electrode active material. Section 9: A method for manufacturing an electricity storage device including a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, the method comprising: providing the negative electrode active material layer containing Si-containing particles as a negative electrode active material on the negative electrode current collector, The provision of the negative electrode active material layer is providing a first layer using a first paste including Si-containing particles having a relatively low hardness; providing a second layer using a second paste including Si-containing particles having a relatively high hardness; Including, The Si-containing particles are composite particles of a graphite substrate having voids and silicon disposed in the voids of the graphite substrate. Section 10: The preparation of the first paste and the preparation of the second paste include: dry-mixing the Si-containing particles having a relatively low hardness or the Si-containing particles having a relatively high hardness with a first binder to obtain a first mixed powder or a second mixed powder; kneading the first mixed powder or the second mixed powder, a conductive agent, and a dispersion medium to obtain a first kneaded product or a second kneaded product; and mixing the first kneaded material or the second kneaded material with a second binder and a dispersion medium; Item 10. The method for producing a medicament according to Item 9, comprising: Section 11: applying the second paste onto the negative electrode current collector to form a second layer; applying the first paste onto the second layer to form a first layer; Item 11. The method according to Item 9 or 10, comprising: Section 12: applying the first paste onto the negative electrode current collector to form a first layer; applying the second paste onto the first layer to form a second layer; Item 11. The method according to Item 9 or 10, comprising: Section 13: Item 13. The method according to any one of Items 9 to 12, further comprising preparing Si-containing particles having a compressive modulus of at least twice that of the Si-containing particles having a relatively low hardness, when the compressive modulus of the Si-containing particles having a relatively low hardness is taken as 1. Section 14: Item 14. The manufacturing method according to any one of Items 9 to 13, wherein the compressive modulus of the Si-containing particles having a relatively low hardness is 250 MPa or more and less than 2000 MPa. Section 15: Item 15. The manufacturing method according to any one of Items 9 to 14, wherein the compressive modulus of the Si-containing particles having a relatively high hardness is 2000 MPa or more and 5000 MPa or less. Section 16: Item 16. The method according to any one of items 9 to 15, wherein the negative electrode active material layer further contains graphite particles as the negative electrode active material.

[0087] 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]

[0088] 20 Electrode body 30 cases 42 Positive terminal 44 Negative terminal 50 positive electrode 60,260 negative electrode 62,262 Negative electrode current collector 64,264 Negative electrode active material layer 641,2641 1st layer 642,2642 2nd layer 70 Separator 80 Nonaqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. An electricity storage device including a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, the negative electrode active material layer includes, as negative electrode active material particles, a graphite base material having voids and Si-containing particles which are composite particles of silicon disposed in the voids of the graphite base material; The negative electrode active material layer is divided in a thickness direction thereof, and the hardness of the Si-containing particles contained in at least some of the layers is lower than the hardness of the Si-containing particles contained in the other layers.

2. 2. The electricity storage device according to claim 1, wherein when the negative electrode active material layer is divided into two in a thickness direction, a region relatively closer to the negative electrode current collector is defined as a first region, and a region relatively farther from the negative electrode current collector is defined as a second region, the first layer containing Si-containing particles having a relatively low hardness is provided in the second region.

3. 2. The electricity storage device according to claim 1, wherein when the negative electrode active material layer is divided into two in a thickness direction, a region relatively closer to the negative electrode current collector is defined as a first region, and a region relatively farther from the negative electrode current collector is defined as a second region, the first layer containing Si-containing particles having a relatively low hardness is provided in the first region.

4. 4. The electricity storage device according to claim 1, wherein the compressive modulus of the Si-containing particles having a relatively high hardness is 2 times or more when the compressive modulus of the Si-containing particles having a relatively low hardness is 1.

5. 4. The electricity storage device according to claim 1, wherein the compressive modulus of the Si-containing particles having a relatively low hardness is 250 MPa or more and less than 2000 MPa.

6. 4. The electricity storage device according to claim 1, wherein the compressive modulus of the Si-containing particles having a relatively high hardness is 2000 MPa or more and 5000 MPa or less.

7. the negative electrode active material layer includes a first layer including the Si-containing particles having a relatively low hardness and a second layer including the Si-containing particles having a relatively high hardness, The electricity storage device according to any one of claims 1 to 3, wherein a ratio (T1:T2) of a thickness T1 of the first layer to a thickness T2 of the second layer is 10:90 to 90:

10.

8. 4. The power storage device according to claim 1, wherein the negative electrode active material layer further contains graphite particles as the negative electrode active material.

9. A method for manufacturing an electricity storage device including a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, the method comprising: providing the negative electrode active material layer containing Si-containing particles as a negative electrode active material on the negative electrode current collector, The provision of the negative electrode active material layer is providing a first layer using a first paste including Si-containing particles having a relatively low hardness; providing a second layer using a second paste including Si-containing particles having a relatively high hardness; Including, The Si-containing particles are composite particles of a graphite substrate having voids and silicon disposed in the voids of the graphite substrate.

10. The preparation of the first paste and the preparation of the second paste include: dry-mixing the Si-containing particles having a relatively low hardness or the Si-containing particles having a relatively high hardness with a first binder to obtain a first mixed powder or a second mixed powder; kneading the first mixed powder or the second mixed powder, a conductive agent, and a dispersion medium to obtain a first kneaded product or a second kneaded product; and mixing the first kneaded material or the second kneaded material with a second binder and a dispersion medium; The method of claim 9, comprising:

11. applying the second paste onto the negative electrode current collector to form a second layer; applying the first paste onto the second layer to form a first layer; The method of claim 9, comprising:

12. applying the first paste onto the negative electrode current collector to form a first layer; applying the second paste onto the first layer to form a second layer; The method of claim 9, comprising:

13. The manufacturing method according to any one of claims 9 to 12, comprising preparing Si-containing particles having a compressive modulus of at least twice as high as that of the Si-containing particles having a relatively low hardness, where the compressive modulus is set to 1.

14. The method according to any one of claims 9 to 12, wherein the compressive modulus of the Si-containing particles having a relatively low hardness is 250 MPa or more and less than 2000 MPa.

15. The method according to any one of claims 9 to 12, wherein the compressive modulus of the Si-containing particles having a relatively high hardness is 2000 MPa or more and 5000 MPa or less.

16. The method according to any one of claims 9 to 12, wherein the negative electrode active material layer further contains graphite particles as the negative electrode active material.

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