Power storage device and method of manufacturing power storage device

Abstract

To provide a technique for improving cycle characteristics of a power storage device comprising a negative electrode including a negative electrode active material layer containing Si.SOLUTION: In a power storage device, a negative electrode active material layer 64 contains a graphite substrate having a cavity, Si-containing particles which are composite particles with silicon disposed in the cavity of the graphite substrate, and graphite particles as negative electrode active material particles. The negative electrode active material layer 64 is divided in a thickness direction on a boundary having ruggedness. The negative electrode active material layer 64 includes a first layer 641 and a second layer 642. The first layer 641 is a layer which is provided on the side of a negative electrode collector 62 and includes a recess 6411 on the side of the boundary. The second layer 642 is a layer which is provided on the first layer 641 and includes a projection 6421 on the side of the boundary. The projection of the second layer 642 is stored in the recess of the first layer 641. An Si content C1 in the first layer 641 and an Si content C2 in the second layer 642 are different from each other.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] Japanese Patent Publication No. 2022-133086 discloses a method for manufacturing an electrode. This method includes the steps of: preparing a wet powder formed of aggregated particles containing at least an electrode active material, a binder resin, and a solvent; using the wet powder to form a coating film made of the wet powder on an electrode current collector while leaving the gas phase of the coating film; forming a concave-convex shape on the surface of the coating film in a predetermined pattern and at a constant pitch; applying a coating material containing at least one inorganic compound to the coating film with the concave-convex shape formed; and drying the coating film and coating material formed on the current collector to form an electrode comprising an electrode active material layer made of the coating film and a coating material made of the coating material disposed in the recesses of the concave-convex shape of the active material layer. The publication states that this manufacturing method can form a desired concave-convex shape in the coating film, thereby increasing the surface area of ​​the active material layer and arranging the coating material in the thickness direction.

[0004] Japanese Patent Laid-Open Publication No. 2004-103474 discloses a nonaqueous electrolyte battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode comprises a negative electrode active material layer containing at least one negative electrode active material selected from Groups 11, 12, 13, 14, and 15 elements on a major surface of a negative electrode current collector. The negative electrode has a predetermined pattern of voids adjacent to the negative electrode active material layer, where the major surface of the negative electrode current collector is exposed. The voids relieve stress caused by interference between the negative electrode active material layers as they expand and contract during charging and discharging. The publication states that this configuration can suppress deterioration of the negative electrode, which would otherwise occur if the negative electrode active material layer cracked due to stress caused by interference between the negative electrode active material layers as they expand and contract during charging and discharging.

[0005] The electrode disclosed in JP 2004-127561 A has a current collector made of a material that does not alloy with Li, and a thin film made of a metal that alloys with Li or an alloy containing this metal formed on the current collector. The thin film is selectively formed in a predetermined pattern on the current collector. The publication states that this configuration makes it possible to mitigate the volumetric expansion of active material particles during charge and discharge, and effectively suppresses the occurrence of cracks in the electrode and peeling between the current collector and the active material. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-133086 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-103474 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-127561 Summary of the Invention [Problem to be solved by the invention]

[0007] The present inventors wish to improve the cycle characteristics of an electricity storage device that includes a negative electrode having a negative electrode active material layer containing Si. [Means for solving the problem]

[0008] The electricity storage device disclosed herein includes 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 substrate having voids, Si-containing particles, which are composite particles of silicon and disposed in the voids of the graphite substrate, and graphite particles. The negative electrode active material layer is divided in the thickness direction by a boundary having concave and convex portions. The negative electrode active material layer includes a first layer and a second layer. The first layer is provided on the negative electrode current collector side and has concave portions on the boundary side. The second layer is provided on the first layer and has convex portions on the boundary side. The convex portions of the second layer are accommodated in the concave portions of the first layer. The Si content C1 of the first layer and the Si content C2 of the second layer are different from each other. This configuration can improve the cycle characteristics of an electricity storage device including a negative electrode having a negative electrode active material layer containing Si.

[0009] The method for manufacturing an electricity storage device disclosed herein includes providing a coating of a first paste on a negative electrode current collector; forming recesses on the surface of the coating in a predetermined pattern at a constant pitch; applying a second paste to the coating with the recesses; and pressing the coating formed on the negative electrode current collector and the coating of the second slurry together to form a negative electrode active material layer. The first slurry and the second slurry each contain, as negative electrode active material particles, Si-containing particles, which are composite particles of a voided graphite substrate and silicon disposed in the voids of the graphite substrate, and graphite particles, and each have a different Si content. This configuration can improve the cycle characteristics of an electricity storage device equipped with a negative electrode having a Si-containing negative electrode active material layer. [Brief explanation of the drawings]

[0010] [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 diagram of a cross section of the negative electrode active material layer 64 taken along the thickness direction. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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."

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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 0.1Mn 0.1 O2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 O4 and the like can be preferably used.

[0020] 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 %.

[0021] 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.

[0022] 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 and graphite 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.

[0023] 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.

[0024] The use of a negative electrode active material containing silicon (Si) can further increase the capacity and energy density of an electricity storage device. However, it is known that a negative electrode containing silicon (Si) undergoes large expansion and contraction during charging and discharging, and the stress caused by the expansion and contraction can cause the conductive paths in the negative electrode active material layer to be broken, resulting in a decrease in capacity. The present inventors have investigated a configuration for a negative electrode active material layer containing silicon (Si) that reduces the stress caused by the expansion and contraction and suppresses the disconnection of the conductive paths in the negative electrode active material layer by controlling the degree of expansion and contraction during charging and discharging. The present inventors believe that this can improve the cycle characteristics of an electricity storage device having a negative electrode active material layer containing silicon (Si).

[0025] Furthermore, according to the inventors' findings, as the content of Si-containing particles in the negative electrode increases, the load applied during pressing tends to become uneven between the relatively hard Si-containing particles and the relatively soft graphite particles. In such cases, if stress concentrates at a certain location in the negative electrode active material layer, cracks may occur in the negative electrode active material layer at that location. Such cracks are undesirable because they can lead to a deterioration in cycle performance. Therefore, the inventors further investigated a configuration for producing a silicon (Si)-containing negative electrode active material layer that can suppress the occurrence of cracks and the deterioration of cycle performance.

[0026] 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. 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).

[0027] The graphite particles serving 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. The graphite particles may have, for example, a substantially spherical shape, although this is not particularly limited. In this specification, the term "substantially spherical" refers to graphite particles having an average aspect ratio of 1 to 2 (preferably 1 to 1.5) based on scanning electron microscope (SEM) observation. 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.

[0028] In the entire negative electrode active material layer 64, when the total of the Si-containing particles and graphite is taken as 100% by mass, the content of the Si-containing particles is generally 5% by mass or more, for example 10% by mass or more, preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and particularly preferably 30% by mass. On the other hand, when the total of the Si-containing particles and graphite is taken as 100% by mass, the content of the Si-containing particles is generally 70% by mass or less, for example 60% by mass or less, preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less.

[0029] 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.

[0030] 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 %.

[0031] 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.

[0032] FIG. 3 is a schematic cross-sectional view of the negative electrode 60. FIG. 3 schematically illustrates a partial cross-sectional structure of the negative electrode 60. As illustrated in FIG. 3, the negative electrode active material layer 64 is divided in the thickness direction by a boundary having an uneven surface. The negative electrode active material layer 64 has a first layer 641 and a second layer 642. The first layer 641 is disposed on the negative electrode current collector 62 side and has a recessed portion 6411. In this embodiment, the surface of the first layer 641 that is relatively far from the negative electrode current collector 62 at the boundary between the first layer 641 and the second layer 642 is defined as a reference surface 641B. In the embodiment illustrated in FIG. 3, the recessed portion 6411 is a portion recessed from the reference surface 641B toward the negative electrode current collector 62. The second layer 642 is disposed on the first layer 641 and has a protruding portion 6421 and a base portion 6422. The protrusion 6421 protrudes from the base 6422. In this embodiment, for the second layer 642, a surface at the boundary between the first layer 641 and the second layer 642 that is relatively far from the negative electrode current collector 62 is defined as a reference surface 642B. In the embodiment shown in FIG. 3, the protrusion 6421 is a portion that protrudes from the reference surface 642B toward the negative electrode current collector 62. In this embodiment, the reference surface 641B of the first layer 641 and the reference surface 642B of the second layer 642 are in contact with each other.

[0033] 3, the convex portion 6421 of the second layer 642 is housed in the concave portion 6411 of the first layer 641. In this embodiment, the tip of the convex portion 6421 is in contact with the bottom surface of the concave portion 6411. The side surface of the convex portion 6421 is in contact with the inner wall surface of the concave portion 6411.

[0034] In this embodiment, the presence of the first layer 641 and the second layer 642 is confirmed by using a scanning electron microscope (SEM). For example, when an SEM observation image of a cross section along the thickness direction of the negative electrode active material layer 64 (hereinafter simply referred to as a "cross-sectional SEM observation image") is obtained, the first layer 641 on the negative electrode current collector 62 side and the second layer 642 on the first layer 641 can be confirmed in the cross-sectional SEM observation image. In addition, at the boundary between the first layer 641 and the second layer 642, irregularities due to recesses 6411 in the first layer 641 and protrusions 6421 in the second layer 642 can be confirmed.

[0035] From the viewpoint of suppressing stress concentration in the negative electrode active material layer 64 due to expansion and contraction accompanying charge and discharge, the width A (see FIG. 3 ) of the tip of the protrusion 6421 is preferably set to, for example, 10 times or less the average particle diameter of the Si-containing particles. From this viewpoint, the width A is preferably 7 times or less, more preferably 6 times or less, and even more preferably 5 times or less. From the viewpoint of formability of the negative electrode active material layer 64, the width A is preferably equal to or greater than the average particle diameter of the Si-containing particles, and is preferably 1.3 times or more, more preferably 1.5 times or more, and even more preferably 1.7 times or more. The width A may be calculated, for example, by analyzing a cross-sectional SEM observation image using image analysis software.

[0036] To prevent stress concentration in the negative electrode active material layer 64 due to expansion and contraction during charging and discharging, the length B (see FIG. 3 ) between two adjacent protrusions 6421 of the second layer 642 is preferably equal to or greater than the average particle diameter of the Si-containing particles. From this perspective, the length B is, for example, at least two times the average particle diameter of the Si-containing particles, preferably at least three times, more preferably at least four times, and even more preferably at least five times. The length B is not particularly limited, but is generally no greater than 20 times the average particle diameter of the Si-containing particles, for example, no greater than 15 times, preferably no greater than 13 times, and more preferably no greater than 10 times. The length B may be calculated, for example, by analyzing a cross-sectional SEM image using image analysis software.

[0037] From the viewpoint of suppressing stress concentration on the negative electrode active material layer 64 due to expansion and contraction accompanying charge and discharge, the ratio (D / C) of the thickness C of the protrusion 6421 (see FIG. 3) to the thickness D of the base 6422 (see FIG. 3) is preferably 1 or less, and more preferably 0.9 or less. From the same viewpoint, the ratio (D / C) is, for example, 0.1 or more, preferably 0.3 or more, and more preferably 0.5 or more. The thicknesses C and D may be calculated, for example, by analyzing a cross-sectional SEM observation image using image analysis software.

[0038] For example, the Si content C1 in the first layer 641 and the Si content C2 in the second layer 642 are different from each other. In this specification, "the Si content C1 in the first layer 641 and the Si content C2 in the second layer 642 are different from each other" means that when the Si element in samples obtained from the first layer 641 and the second layer 642 is measured by ICP analysis, different content ratios (mass%) are obtained between the two samples.

[0039] In this embodiment, the Si content C1 is smaller than the Si content C2. When the Si content C2 is taken as 1, the Si content C1 is preferably approximately 0.1 to 0.95. When the Si content C2 is taken as 1, the Si content C1 is, for example, 0.9 or less, preferably 0.85 or less, more preferably 0.8 or less, and even more preferably 0.75 or less. When the Si content C2 is taken as 1, the Si content C1 is, for example, 0.15 or more, preferably 0.2 or more, more preferably 0.25 or more, even more preferably 0.3 or more, and particularly preferably 0.4 or more.

[0040] The difference between the Si content C1 and the Si content C2 is, for example, the difference between the content ratio of Si-containing particles in the first layer 641 and the content ratio of Si-containing particles in the second layer 642. When the total amount of Si-containing particles in the negative electrode active material layer 64 is taken as 100 mass%, the content of the Si-containing particles in the first layer 641 is, for example, less than 50 mass%, preferably 48 mass% or less, and more preferably 45 mass% or less. When the total amount of Si-containing particles in the negative electrode active material layer 64 is taken as 100 mass%, the content of the Si-containing particles in the first layer 641 is, for example, 10 mass% or more, preferably 15 mass% or more, more preferably 20 mass% or more, and even more preferably 25 mass% or more.

[0041] FIG. 4 is a schematic diagram of a cross section of the negative electrode active material layer 64 taken along the thickness direction. FIG. 4 is a three-dimensional model of the negative electrode active material layer 64 obtained by the present inventor through computer simulation. FIG. 4 shows an enlarged view of the configuration near the boundary between the first layer 641 and the second layer 642. As shown in FIG. 4, both the first layer 641 and the second layer 642 contain graphite particles 64A and Si-containing particles 64B. The proportion of the Si-containing particles 64B in the first layer 641 is greater than the proportion of the Si-containing particles 64B in the second layer 642. The proportion of the graphite particles 64A in the first layer 641 is smaller than the proportion of the graphite particles 64A in the second layer 642.

[0042] For example, by using an analytical method such as scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX), it is possible to distinguish between the graphite component (here, the graphite base material of the graphite particles and the Si-containing particles) and the Si component (here, Si contained in the Si-containing particles) in each layer, and further to make the above-mentioned comparison.

[0043] 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 recess formation step, a second application step, a second drying step, and a second pressing step.

[0044] 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, Si-containing particles, a conductive material, and a binder. The raw materials listed here are as described above.

[0045] The first mixing step is, for example, a step of dry-mixing graphite particles, Si-containing particles, and a first binder. In this embodiment, in the first mixing step, the graphite particles, Si-containing particles, and the first binder are dry-mixed to prepare a first mixed powder. Similarly, the graphite particles, Si-containing particles, and the first binder are dry-mixed to prepare a second mixed powder. The first mixed powder and the second mixed powder have different contents of Si-containing particles. 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 any particular limitation.

[0046] 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 material and a dispersion medium. In this embodiment, in the second mixing step, the first mixed powder, the conductive material, and water are thick-kneaded to obtain a first kneaded product. Similarly, the second mixed powder, the conductive material, 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.

[0047] 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. Here, the first paste is a paste for forming a first layer 641 (see FIG. 3). Similarly, the second kneaded material, the second binder, and water are mixed to obtain a second paste. Here, the second paste is a paste for forming a second layer 642 (see FIG. 3). 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 particular limitation.

[0048] The first application step is, for example, a step of applying the first paste obtained in the third mixing step to the negative electrode current collector 62 to provide a coating film of the first paste. In this embodiment, the first 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 any 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 first paste applied to the negative electrode current collector 62 in the first application step. The drying conditions are not particularly limited, and the conditions used when producing this type of negative electrode may be appropriately adopted (the same applies to the second drying step).

[0049] The recess forming step is, for example, a step of forming recesses 6411 on the surface of the coating film of the first paste (here, the coating film after the first drying step). In this step, for example, forming recesses in the coating film and pressing may be performed simultaneously. In this embodiment, a mold with protrusions is used, and recesses 6411 are formed in the coating film at a predetermined pattern and a constant pitch. As the mold, for example, a mold with protrusions that can form desired recesses 6411 in the coating film is used. By placing this mold on the coating film and pressing it, recesses 6411 are formed in the coating film to form the first layer 641. The pressing conditions are not particularly limited, and the conditions used when producing this type of negative electrode may be appropriately adopted (the same applies to the second pressing step).

[0050] The second application step is, for example, a step of applying a second paste to the first layer 641 provided through the recess formation step. In this embodiment, the second paste is applied to the first layer 641 formed through the recess formation step. The second drying step is, for example, a step of drying the second paste applied to the first layer in the second application step. The second pressing step is a step of pressing the coating of the second paste (here, the coating after the second drying step) to obtain the second layer 642. Therefore, it should be noted that the above-described method for manufacturing the negative electrode 60 is merely an example. The method for manufacturing the negative electrode 60 may not include some of the above-described steps, as necessary, or may include any steps.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] As described above, 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 includes, as negative electrode active material particles, a graphite substrate having voids, Si-containing particles, which are composite particles of silicon and silicon disposed in the voids of the graphite substrate, and graphite particles. The negative electrode active material layer 64 is divided in the thickness direction by a boundary having an uneven surface. The negative electrode active material layer 64 includes a first layer 641 and a second layer 642. The first layer 641 is a layer provided on the negative electrode current collector 62 side and has a recess 6411 on the boundary side. The second layer 642 is a layer provided on the first layer 641 and has a protrusion 6421 on the boundary side. The protrusion 6421 of the second layer 642 is accommodated in the recess 6411 of the first layer 641. The Si content C1 in the first layer 641 and the Si content C2 in the second layer 642 are different from each other.

[0056] 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 preventing an increase in the thickness of the negative electrode 60. The negative electrode active material layer 64 further contains graphite particles as the negative electrode active material, thereby enhancing the conductivity of the negative electrode active material layer 64. The negative electrode active material layer 64 is divided in the thickness direction by an uneven boundary into a first layer 641 on the negative electrode current collector side and a second layer 642 on the first layer 641. The first layer 641 has a recess 6411 on the boundary side. The second layer 642 has a protrusion 6421 on the boundary side. At the boundary between the two layers, the convex portions 6421 of the second layer 642 are accommodated in the concave portions 6411 of the first layer 641. Furthermore, the Si content of the first layer 641 and the Si content of the second layer 642 are different from each other. As a result, expansion and contraction of the convex portions 6421 of the second layer 642 that accompany, for example, charging and discharging of the lithium-ion secondary battery 100 is alleviated by the concave portions 6411 of the first layer 641. Furthermore, the Si content of the first layer 641 and the Si content of the second layer 642 are different from each other, and the degree of expansion and contraction during charging and discharging differs between the first layer 641 and the second layer 642. Therefore, the expansion and contraction of the convex portions 6421 are more effectively alleviated by the concave portions 6411, and localized load application to the negative electrode active material layer 64 can be suppressed. This makes it possible to suppress damage to the negative electrode active material layer 64, for example, and in turn to obtain the effect of improving the cycle characteristics of the lithium ion secondary battery 100.

[0057] The Si content C1 may be smaller than the Si content C2. In other words, the second layer 642 may contain more Si than the first layer 641. In this case, the degree of expansion and contraction of the second layer 642 may be large. In particular, the degree of expansion and contraction of the protrusions 6422 may be significant in the planar direction of the negative electrode active material layer 64. This expansion is absorbed by the recesses 6411, and the load of the expansion is more effectively alleviated, thereby enhancing the effect of suppressing disconnection of the conduction path in the negative electrode active material layer 64, and ultimately, the effect of improving the cycle characteristics of the lithium-ion secondary battery 100 can be more effectively obtained.

[0058] This embodiment may be achieved by making the content ratio of the Si-containing particles in the second layer 642 greater than the content ratio of the Si-containing particles in the first layer 641. This increases the hardness of the protrusions 6421, thereby improving the uniformity of the load applied in the second pressing step, for example. This, for example, makes it possible to prevent cracks from occurring in the negative electrode active material layer 64 during the manufacturing process. Furthermore, the hardness of the recesses 6411 can be made lower than that of the protrusions 6421, thereby better mitigating the expansion and contraction of the protrusions 6422.

[0059] In the cross-sectional SEM image, width A of the tip of protrusion 6421 of second layer 642 may be 1 to 7 times the average particle diameter of the Si-containing particles, thereby more effectively alleviating the expansion and contraction of protrusion 6421 by recess 6411 and more easily producing negative electrode active material layer 64.

[0060] In a cross-sectional SEM image, the length B between two adjacent protrusions 6421 of the second layer 642 may be four times or more the average particle diameter of the Si-containing particles, thereby enabling the recesses 6411 to more effectively mitigate the expansion and contraction of the protrusions 6421.

[0061] When the total of the Si-containing particles and graphite particles contained in the entire negative electrode active material layer 64 is taken as 100 mass %, the negative electrode active material layer 64 may contain 10 mass % to 60 mass % of the Si-containing particles, which can more effectively achieve the effects of the technology disclosed herein.

[0062] In this embodiment, the method for manufacturing the lithium-ion secondary battery 100 includes providing a coating of a first paste on the negative electrode current collector 62 side; forming recesses 6411 in a predetermined pattern and at a constant pitch on the surface of the coating; applying a second paste to the coating with the recesses 6411 formed therein; and pressing the coating formed on the negative electrode current collector 62 and the coating of the second paste together to form the negative electrode active material layer 64. The first paste and the second paste each contain, as negative electrode active material particles, a graphite base having voids, Si-containing particles which are composite particles of silicon and disposed in the voids of the graphite base, and graphite particles, and have different Si contents from each other.

[0063] The manufacturing method of the lithium-ion secondary battery 100 includes the production (manufacturing) of the negative electrode active material layer 64 having two layers: a layer (here, the first layer 641) that is a coating of the first paste; and a layer (here, the second layer) that is a coating of the second paste. The method comprises forming recesses 6411 on the surface of the coating of the first paste, and applying the second paste onto the coating. Then, the laminate of the coating of the first paste and the applied material of the second paste is pressed (here, the second pressing step). At this time, the laminate is pressed with the second paste having a different Si content filling the recesses 6411. Alternatively, the second paste fills the recesses 6411 by pressing. This allows the load on the laminate to be more uniform during pressing. Therefore, for example, the occurrence of cracks in the negative electrode active material layer 64 can be suppressed, thereby improving the cycle characteristics.

[0064] The first paste may contain fewer Si-containing particles than the second paste, thereby achieving the above-mentioned effects more effectively.

[0065] In forming the recesses 6411, the diameter of the bottoms of the recesses 6411 may be 1 to 7 times the average particle diameter of the Si-containing particles. This makes it possible to form the protrusions 6421 whose tips have the same size. This makes it possible to more effectively realize the above-mentioned effects and to enhance the effect of mitigating the expansion and contraction of the protrusions 6421 during charge and discharge.

[0066] The recesses 6411 may be formed so that the distance between two adjacent recesses 6411 is four times or more the average particle diameter of the Si-containing particles. This allows the distance between two adjacent protrusions 6421 to be the same. This makes it possible to more effectively achieve the above-mentioned effects and to enhance the effect of mitigating the expansion and contraction of the protrusions 6421 during charge and discharge.

[0067] Second Embodiment In the first embodiment, the Si content in the first layer 641 is set to be smaller than the Si content in the second layer 642. However, the technology disclosed herein is not limited to this. The Si content in the first layer 641 may be larger than the Si content in the second layer 642. This configuration can also achieve the effect of improving cycle characteristics. Furthermore, the degree of expansion and contraction on the surface of the negative electrode active material layer 64 opposite the negative electrode current collector 62 is reduced, thereby improving the liquid flow of the electrolyte in this area. This can achieve the effect of improving high-rate characteristics.

[0068] Although not particularly limited, the configuration of this embodiment can be realized, for example, by making the content ratio of Si-containing particles in the first layer 641 greater than the content ratio of Si-containing particles in the second layer 642. For example, it is preferable to make the content ratio of Si-containing particles in the first paste greater than the content ratio of Si-containing particles in the second paste.

[0069] Regarding this embodiment, the configurations described in the first embodiment can be adopted for the configurations other than those described above, and therefore the description here will be omitted.

[0070] <Other embodiments> In the above-described embodiment, the content ratio of the Si-containing particles in the first layer 641 and the content ratio of the Si-containing particles in the second layer 642 are made different from each other, thereby making the Si content C1 in the first layer 641 and the Si content C2 in the second layer 642 different. However, the technology disclosed herein is not limited to this. For example, the content ratio of the Si-containing particles in the first layer 641 and the content ratio of the Si-containing particles in the second layer 642 may be the same. In this case, by making the Si content of the Si-containing particles contained in the first layer 641 and the Si content of the Si-containing particles contained in the second layer 642 different from each other, the Si content C1 in the first layer 641 and the Si content C2 in the second layer 642 can be made different from each other.

[0071] In the above-described embodiment, the negative electrode active material layer 64 has a two-layer structure of the first layer 641 and the second layer 642. However, the technology disclosed herein is not limited to this. For example, as long as the effects of the technology disclosed herein can be achieved, an arbitrary layer (preferably a layer containing a negative electrode active material) may be provided between the first layer 641 and the negative electrode current collector 62. An arbitrary layer (preferably a layer containing a negative electrode active material) may be provided on the second layer 642.

[0072] 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.

[0073] [Manufacturing test cells] Example 1 Si-containing particles and graphite particles were prepared as negative electrode active materials. The Si-containing particles were Si / C particles with an average particle diameter of 6 μm. The graphite particles had an average particle diameter of 15 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 prepare a first negative electrode mixture paste and a second negative electrode mixture paste. The first negative electrode mixture paste was prepared so that the mass ratio of graphite particles / Si-containing particles / SWCNT / CMC / PAA / SBR was 80 / 20 / 0.1 / 1 / 1 / 1.5. The second negative electrode mixture paste was prepared so as to have a mass ratio of graphite particles / Si-containing particles / SWCNT / CMC / PAA / SBR=50 / 50 / 0.1 / 1 / 1 / 1.5.

[0074] The first negative electrode mixture paste and the second negative electrode mixture paste were prepared as follows. First, graphite particles, Si-containing particles, CMC, and PAA were dry-mixed to obtain a mixed powder. SWCNT (a water-soluble paste with a solid content of 2%) and water were added and kneaded. Next, SBR and water were added to the mixed powder and kneaded. In this way, the first negative electrode mixture paste and the second negative electrode mixture paste were prepared.

[0075] Next, the first negative electrode mixture paste was applied to both sides of a 10 μm thick copper foil and dried to produce a first layer. Next, a convex mold and a roll press were used to create an uneven surface on the first layer. Next, the second negative electrode mixture paste was applied to the first layer and dried to produce a second 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.

[0076] The uneven shape was provided so that, in a cross-sectional SEM observation image along the thickness direction of the negative electrode active material layer, the width A at the boundary between the first layer and the second layer was 1.7 times the average particle diameter of the Si-containing particles, the length B was 10 times the average particle diameter of the Si-containing particles, and the length D was 0.5 times the length C.

[0077] 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 15 μm thick aluminum foil, dried, and then pressed to the specified thickness and processed to the specified dimensions to obtain a positive electrode plate.

[0078] 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.

[0079] <Example 2> The first negative electrode mixture paste was prepared to have a mass ratio of graphite particles / Si-containing particles / SWCNT / CMC / PAA / SBR = 70 / 30 / 0.1 / 1 / 1 / 1.5. The second negative electrode mixture paste was prepared to have a mass ratio of graphite particles / Si-containing particles / SWCNT / CMC / PAA / SBR = 60 / 40 / 0.1 / 1 / 1 / 1.5. In a cross-sectional SEM image along the thickness direction of the negative electrode active material layer, the uneven shape was such that at the boundary between the first and second layers, the width A was 3.4 times the average particle diameter of the Si-containing particles, the length B was 7 times the average particle diameter of the Si-containing particles, and the length D was 0.7 times the length C. Otherwise, the test cell of this example was prepared using the same materials and procedures as in Example 1.

[0080] Example 3 The uneven shape was provided so that the width A at the boundary between the first layer and the second layer was five times the average particle diameter of the Si-containing particles in a cross-sectional SEM image of the negative electrode active material layer along the thickness direction. Except for this, the test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0081] Example 4 The uneven shape was provided so that the length B at the boundary between the first layer and the second layer was 5 times the average particle diameter of the Si-containing particles in a cross-sectional SEM image of the negative electrode active material layer along the thickness direction. Except for this, the test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0082] <Example 5> The uneven shape was provided so that, in a cross-sectional SEM image of the negative electrode active material layer taken along the thickness direction, the length D at the boundary between the first layer and the second layer was 0.9 times the length C. Except for this, the test cell of this example was produced using the same materials and procedures as in Example 1.

[0083] Example 6 The uneven shape was provided so that the width A at the boundary between the first layer and the second layer was 3.4 times the average particle diameter of the Si-containing particles in a cross-sectional SEM image of the negative electrode active material layer along the thickness direction. Except for this, the test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0084] Example 7 The uneven shape was provided so that the length B at the boundary between the first layer and the second layer was 7 times the average particle diameter of the Si-containing particles in a cross-sectional SEM image of the negative electrode active material layer along the thickness direction. Except for this, the test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0085] Example 8 The first negative electrode mixture paste was prepared to have a mass ratio of graphite particles / Si-containing particles / SWCNT / CMC / PAA / SBR = 60 / 40 / 0.1 / 1 / 1 / 1.5. The second negative electrode mixture paste was prepared to have a mass ratio of graphite particles / Si-containing particles / SWCNT / CMC / PAA / SBR = 70 / 30 / 0.1 / 1 / 1 / 1.5. In a cross-sectional SEM image along the thickness direction of the negative electrode active material layer, the uneven shape was such that at the boundary between the first and second layers, the width A was 3.4 times the average particle diameter of the Si-containing particles, the length B was 7 times the average particle diameter of the Si-containing particles, and the length D was 0.9 times the length C. Otherwise, the test cell of this example was prepared using the same materials and procedures as in Example 1.

[0086] <Comparative Example 1> A negative electrode mixture paste was prepared with a mass ratio of graphite particles / Si-containing particles / SWCNT / CMC / PAA / SBR=70 / 30 / 0.1 / 1 / 1 / 1 / 1.5. A single layer of negative electrode active material was formed using this negative electrode mixture paste. The test cell of this example was fabricated using the same materials and procedures as in Example 1.

[0087] <Comparative Example 2> The test cell of this example was fabricated using the same materials and procedures as in Example 1, except that no unevenness was provided at the boundary between the first and second layers.

[0088] <Cycle test> The test cell was placed in an environment of 25°C. The test cell was subjected to CCCV charging (at a rate of 0.4C up to 4.2V, then cut off at 0.1C). Then, CC discharging (at a rate of 0.4C and cut off at 2.5V) was performed. The capacity at the time of this charge-discharge cycle, which consisted of CCCV charging and CC discharging, was defined as the initial capacity. Furthermore, this charge-discharge cycle was performed 200 times, and the capacity retention rate after 200 cycles was calculated. The capacity retention rate was calculated using the following formula (X): Capacity retention rate (%) = (capacity after 200 cycles / initial capacity) × 100 … (X) The results are shown in the corresponding columns in Table 1.

[0089] Although not specifically stated in the table, the effect of improving cycle characteristics can be evaluated according to the following criteria. Particularly excellent effect: capacity retention rate of over 85% Excellent performance: Capacity retention rate is between 80% and 85% Good effect: Capacity retention is between 75% and 80% Poor performance: Less than 75% capacity retention

[0090] [Table 1]

[0091] As shown in Table 1, Examples 1 to 8 had higher capacity retention rates after charge / discharge cycling than Comparative Examples 1 and 2. In the test cells of Examples 1 to 8, the negative electrode active material layer contained, as negative electrode active material particles, a graphite substrate having voids, Si-containing particles, which are composite particles of silicon disposed in the voids of the graphite substrate, and graphite particles. The negative electrode active material layer was divided in the thickness direction by a boundary having irregularities. The negative electrode active material layer had a first layer on the negative electrode current collector side, which had concave portions on the boundary side. The negative electrode active material layer had a second layer on the first layer, which had convex portions on the boundary side. The convex portions of the second layer were accommodated in the concave portions of the first layer. The Si content C1 of the first layer was different from the Si content C2 of the second layer. The results shown in Table 1 demonstrate that such a configuration can improve the cycle characteristics of an electricity storage device.

[0092] 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 is The negative electrode active material particles include a graphite base material having voids, Si-containing particles which are composite particles of silicon disposed in the voids of the graphite base material, and graphite particles, It is divided in the thickness direction by uneven boundaries, a first layer having a recess on the boundary side on the negative electrode current collector side; a second layer having a convex portion on the boundary side is provided on the first layer, the protrusions of the second layer are accommodated in the recesses of the first layer, The electricity storage device, wherein the Si content C1 in the first layer and the Si content C2 in the second layer are different from each other. Section 2: Item 2. The electricity storage device according to item 1, wherein the Si content C1 is smaller than the Si content C2. Section 3: Item 2. The electricity storage device according to item 1, wherein the Si content C1 is greater than the Si content C2. Section 4: 4. The electricity storage device according to any one of items 1 to 3, wherein in an electron microscope image of a cross section along the thickness direction, the width of the tips of the convex portions of the second layer is 1 to 7 times the average particle diameter of the Si-containing particles. Section 5: 5. The electricity storage device according to any one of items 1 to 4, wherein in the electron microscope image, the distance between two adjacent convex portions of the second layer is four times or more the average particle diameter. Item 6: Item 6. The electricity storage device according to any one of Items 1 to 5, wherein the negative electrode active material layer contains 10 mass % or more and 60 mass % or less of the Si-containing particles, when the total of the Si-containing particles and the graphite particles contained in the entire negative electrode active material layer is taken as 100 mass %. Section 7: 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 a coating film of a first paste on the negative electrode current collector side; forming recesses in a predetermined pattern and at a constant pitch on the surface of the coating film; applying a second paste to the coating film in which the recesses are formed; pressing the coating film and the second paste applied on the negative electrode current collector to form the negative electrode active material layer; It encompasses The first paste and the second paste are The negative electrode active material particles include Si-containing particles, which are composite particles of a graphite base material having voids and silicon disposed in the voids of the graphite base material, and graphite particles, A method for manufacturing electricity storage devices having different Si contents. Section 8: Item 8. The manufacturing method according to Item 7, wherein the amount of the Si-containing particles contained in the first paste is less than the amount of the Si-containing particles contained in the second paste. Section 9: Item 8. The manufacturing method according to Item 7, wherein the Si-containing particles contained in the first paste are more than the Si-containing particles contained in the second paste. Section 10: Item 10. The manufacturing method according to any one of Items 7 to 9, wherein the recesses have a bottom diameter that is 1 to 7 times the average particle diameter of the Si-containing particles. Section 11: Item 11. The manufacturing method according to any one of Items 7 to 10, wherein the recesses are formed so that the distance between two adjacent recesses is four times or more the average particle diameter.

[0093] 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]

[0094] 20 Electrode body 30 cases 42 Positive terminal 44 Negative terminal 50 positive electrode 60 negative electrode 62 Negative electrode current collector 64 Negative electrode active material layer 641 1st layer 642 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 is The negative electrode active material particles include a graphite base material having voids, Si-containing particles which are composite particles of silicon and silicon disposed in the voids of the graphite base material, and graphite particles, It is divided in the thickness direction by uneven boundaries, a first layer on the negative electrode current collector side, the first layer having a recess on the boundary side; a second layer having a convex portion on the boundary side is provided on the first layer, the protrusions of the second layer are accommodated in the recesses of the first layer, An electricity storage device, wherein the Si content C1 in the first layer and the Si content C2 in the second layer are different from each other.

2. The power storage device according to claim 1 , wherein the Si content C1 is smaller than the Si content C2.

3. The power storage device according to claim 1 , wherein the Si content C1 is greater than the Si content C2.

4. 4. The electricity storage device according to claim 1, wherein, in an electron microscope image of a cross section along the thickness direction, a width of a tip of the convex portion of the second layer is 1 to 7 times an average particle diameter of the Si-containing particles.

5. The electricity storage device according to claim 4 , wherein in the electron microscope image, the length between two adjacent convex portions of the second layer is four times or more the average particle diameter.

6. 4. The electricity storage device according to claim 1, wherein the negative electrode active material layer contains 10 mass% or more and 60 mass% or less of the Si-containing particles, where the total of the Si-containing particles and the graphite particles contained in the entire negative electrode active material layer is taken as 100 mass%.

7. 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 a coating film of a first paste on the negative electrode current collector side; forming recesses in a predetermined pattern and at a constant pitch on the surface of the coating film; applying a second paste to the coating film in which the recesses are formed; pressing the coating film and the second paste applied onto the negative electrode current collector to form the negative electrode active material layer; It encompasses The first paste and the second paste are The negative electrode active material particles include Si-containing particles, which are composite particles of a graphite base material having voids and silicon disposed in the voids of the graphite base material, and graphite particles, A method for manufacturing electricity storage devices having different Si contents.

8. The manufacturing method according to claim 7 , wherein the first paste contains fewer Si-containing particles than the second paste.

9. The manufacturing method according to claim 7 , wherein the first paste contains more Si-containing particles than the second paste.

10. The method according to any one of claims 7 to 9, wherein in forming the recesses, the diameter of the bottom of the recesses is 1 to 7 times the average particle diameter of the Si-containing particles.

11. The method according to claim 10 , wherein the recesses are formed so that the distance between two adjacent recesses is four times or more the average particle diameter.

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