Power storage device

Abstract

To suppress an increase in the thickness of a power storage device.SOLUTION: A power storage device 100 includes a negative electrode active material layer 64 that includes a negative electrode active material. The negative electrode active material includes first particles 681 and second particles 682. The first particles 681 are particles that include silicon. The second particles 682 are particles that include silicon. Here, a silicon content C2 of the second particles 682 is smaller than a silicon content C1 of the first particles 681. An average particle diameter D2 of the second particles 682 is larger than an average particle diameter D1 of the first particles 681.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to an electricity storage device. [Background technology]

[0002] The negative electrode disclosed in JP 2010-33830 A has a current collector and a negative electrode active material. The negative electrode active material has at least two types of active material composite particles containing elemental silicon, a silicon compound, and carbon, each having a different particle size distribution, and the specific surface area of ​​the active material composite particles is 5 m 2 / g or more 50m 2 / g or less. The publication states that, with this configuration, the presence of carbon contained in the active material composite particles can improve the specific surface area of ​​the active material composite particles (i.e., the active material composite particles can have many voids). Furthermore, the publication states that, as a result, even if the silicon-based negative electrode active material expands during charging, the electrolyte is retained in the small particle size particles located in the gaps between the large particle size particles, and improvement in charge-discharge cycle characteristics (particularly, suppression of a rapid capacity decrease during charge-discharge cycles at high temperatures, and improvement in high-rate characteristics) can be realized.

[0003] The negative electrode disclosed in JP 2009-9727 A is characterized by having a negative electrode active material made of at least two types of composite particles containing elemental silicon and a silicon compound with different particle size distributions. The publication states that this configuration allows the electrode to exhibit the high capacity that is characteristic of silicon-based negative electrode active materials, while increasing the electrode density by mixing at least two types of composite particles with different particle size distributions, thereby achieving even higher capacity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2010-33830 A [Patent Document 2] JP 2009-9727 A Summary of the Invention [Problem to be solved by the invention]

[0005] By using a negative electrode active material containing silicon, the capacity of an electricity storage device can be increased. On the other hand, when the electricity storage device is repeatedly charged and discharged, the negative electrode active material containing silicon tends to expand and contract to a large extent, which increases the internal stress in the negative electrode active material layer. In view of this situation, the present inventors would like to suppress the increase in the thickness of an electricity storage device that is accompanied by charging and discharging, for an electricity storage device that includes a negative electrode active material layer that contains a negative electrode active material containing silicon. [Means for solving the problem]

[0006] The electricity storage device disclosed herein includes a negative electrode active material layer that includes a negative electrode active material. The negative electrode active material includes first particles and second particles. The first particles are particles that include silicon. The second particles are particles that include silicon. Here, the silicon content C2 of the second particles is smaller than the silicon content C1 of the first particles. Also, the average particle diameter D2 of the second particles is larger than the average particle diameter D1 of the first particles. With this configuration, for an electricity storage device that includes a negative electrode active material layer that includes a negative electrode active material that includes silicon, an increase in the thickness of the electricity storage device that accompanies charging and discharging can be suppressed. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a cross-sectional view of an electricity storage device 100. As shown in FIG. [Diagram 2] FIG. 2 is a schematic diagram of the electrode body 20. [Diagram 3] FIG. 3 is a schematic cross-sectional view of the negative electrode sheet 60. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] An embodiment of the technology disclosed herein will be described below. The embodiment described herein is not intended to limit the technology disclosed herein. The technology disclosed herein is not limited to the embodiment described herein unless otherwise specified. The drawings are drawn typically and do not necessarily reflect the actual product. In addition, the same reference numerals are appropriately used for members and parts that perform the same function, and duplicated explanations are omitted. In addition, the notation "A to B" indicating a numerical range means "A or more and B or less" unless otherwise specified, and also includes the meaning of "greater than A and less than B."

[0009] In this specification, the term "electricity storage device" refers to a device in which charging and discharging occurs by the movement of charge carriers between a pair of electrodes (positive and negative electrodes) via an electrolyte. Such 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. In the following, an embodiment in which a lithium ion secondary battery is used as an example of the above-mentioned electricity storage device will be described.

[0010] Fig. 1 is a cross-sectional view of an electricity storage device 100. Fig. 1 is a cross-sectional view along the widest surface of the electricity storage device 100. As shown in Fig. 1, the electricity storage device 100 includes an electrode body 20, a case 30, and a non-aqueous electrolyte solution 80.

[0011] Fig. 2 is a schematic diagram of the electrode body 20. As shown in Fig. 1 and Fig. 2, the electrode body 20 is a wound electrode body in which a long sheet-like positive electrode sheet 50 and a long sheet-like negative electrode sheet 60 are overlapped with a long sheet-like separator 70 interposed therebetween and wound in the sheet longitudinal direction (hereinafter also simply referred to as the "longitudinal direction"). In the electrode body 20, an exposed region 52a in the positive electrode sheet 50 and an exposed region 62a in the negative electrode sheet 60 protrude outward from both ends in the short direction perpendicular to the longitudinal direction.

[0012] As shown in FIG. 1 and FIG. 2, the positive electrode sheet 50 includes a long sheet-like positive electrode collector foil 52 and a positive electrode active material layer 54. The positive electrode collector foil 52 is, for example, an aluminum foil. In this embodiment, the positive electrode collector foil 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 foil 52 is exposed. The positive electrode active material layer 54 is provided, for example, in a band shape along the longitudinal direction on one side or both sides (both sides here) of the positive electrode collector foil 52. The positive electrode active material layer 54 is not provided at the end in the short side direction (the end on the left side in the figure). Here, the exposed region 52a is a band-shaped region at the end in the short side direction (the end on the left side in the figure). As shown in FIG. 1, the current collector plate 42a is attached to the exposed region 52a.

[0013] The positive electrode active material layer 54 contains, for example, a positive electrode active material. The positive electrode active material is, for example, a lithium nickel cobalt manganese composite oxide (NCM) (for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 Examples of the positive electrode active material layer 54 include lithium transition metal oxides such as LiFePO4, and lithium transition metal phosphate compounds such as LiFePO4. The positive electrode active material layer 54 may contain a conductive material, a binder, and the like in addition to the positive electrode active material. Examples of the conductive material include carbon black such as acetylene black (AB), and other carbon materials such as graphite. Examples of the binder include polyvinylidene fluoride (PVDF).

[0014] FIG. 3 is a schematic cross-sectional view of the negative electrode sheet 60. In FIG. 3, the cross-sectional structure of the negative electrode current collector foil 62 and the negative electrode active material layer 64 in the negative electrode sheet 60 is partially enlarged. As shown in FIGS. 1 to 3, the negative electrode sheet 60 includes a long sheet-like negative electrode current collector foil 62 and a negative electrode active material layer 64. The negative electrode current collector foil 62 is, for example, a copper foil. In this embodiment, the negative electrode current collector foil 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 in a strip shape along the longitudinal direction on one or both sides (both sides in this case) of the negative electrode current collector foil 62, for example. The negative electrode active material layer 64 is not provided on the end (the right end in the figure) in the short direction perpendicular to the longitudinal direction. The exposed area 62a is a strip-shaped area at the end in the short direction (the end on the right side in the drawing). As shown in Fig. 1, the current collector plate 44a is attached to the exposed area 62a.

[0015] The negative electrode active material layer 64 contains a negative electrode active material 68 described below (see FIG. 3). As shown in FIG. 3, the negative electrode active material 68 here contains first particles 681 and second particles 682. The first particles 681 are particles containing silicon. The second particles 682 are also particles containing silicon, but are different from the first particles 681. The second particles 682 are different from the first particles 681 in the silicon content and average particle diameter of the particles.

[0016] The silicon content C2 in the second particle 682 is smaller than the silicon content C1 in the first particle 681. In this specification, the "silicon content C1 in the first particle 681" refers to the silicon content (mass%) when the entire first particle 681 is taken as 100 mass%. Also, the "silicon content C2 in the second particle 682" refers to the silicon content (mass%) when the entire second particle 682 is taken as 100 mass%. The content C2 is smaller than the content C1 and is not limited as long as the effect of the technology disclosed herein is realized. The ratio (C2 / C1) of the content C1 to the content C2 is generally 0.05 or more, preferably 0.07 or more, more preferably 0.1 or more, even more preferably 0.3 or more, and particularly preferably 0.5 or more, from the viewpoint of realizing the effect of the technology disclosed herein and suppressing a decrease in the capacity maintenance rate of the power storage device. From the same viewpoint, the ratio (C2 / C1) is generally less than 1, preferably 0.9 or less, and more preferably 0.8 or less. In a preferred embodiment, the ratio (C2 / C1) is 0.07 to 0.8. In another preferred embodiment, the ratio (C2 / C1) is 0.6 to 0.8.

[0017] From the viewpoint of realizing the effect of the technique disclosed herein, the content C1 is, for example, 30 mass% or more, and may be 45 mass% or more, preferably more than 55 mass%, and more preferably 60 mass% or more. On the other hand, from the viewpoint of realizing the effect of increasing the capacity of the power storage device 100 by including silicon, the content C1 is, for example, 90 mass% or less, and preferably 80 mass% or less. Also, from the viewpoint of realizing the effect of increasing the capacity of the power storage device 100 by including silicon, the content C2 is, for example, 5 mass% or more, preferably 15 mass% or more, more preferably 30 mass% or more, and even more preferably 40 mass% or more. On the other hand, from the viewpoint of realizing the effect of the technique disclosed herein, the content C2 is, for example, less than 70 mass%, preferably 65 mass% or less, more preferably 60 mass% or less, and even more preferably 55 mass% or less.

[0018] The average particle diameter D2 of the second particles 682 is larger than the average particle diameter D1 of the first particles 681. In this specification, the term "average particle diameter" refers to the particle diameter (D 50 The average particle diameter (D1 / D2) is defined as the average particle diameter D1 and the average particle diameter D2. For example, the ratio (D1 / D2) of the average particle diameter D1 and the average particle diameter D2 is preferably set to 0.01 or more and less than 1. From the viewpoint of better achieving the effects of the technology disclosed herein, the ratio (D1 / D2) is preferably 0.03 or more, more preferably 0.05 or more, and even more preferably 0.07 or more. From the same viewpoint, the ratio (D1 / D2) is preferably 0.9 or less, preferably 0.8 or less, more preferably 0.7 or less, and even more preferably 0.6 or less. By setting the ratio (D1 / D2) in this range, in addition to the above-mentioned effects, the effect of suppressing the decrease in the capacity maintenance rate when the power storage device 100 is charged and discharged is achieved. From the viewpoint of better achieving both of these two effects, it is particularly preferable that the ratio (D1 / D2) is set to 0.07 to 0.6, or 0.3 to 0.6.

[0019] The average particle diameter D1 is preferably set to about 0.1 μm to 20 μm. From the viewpoint of realizing the effects of the technology disclosed herein, the average particle diameter D1 is, for example, 0.3 μm or more, preferably 0.5 μm or more, and more preferably 0.7 μm or more. From the same viewpoint, the average particle diameter D1 is, for example, 15 μm or less, preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 6 μm or less. By setting the average particle diameter D1 in this range, in addition to the above-mentioned effects, it is possible to realize the effect of suppressing the decrease in the capacity maintenance rate when the power storage device 100 is charged and discharged. In addition, the average particle diameter D2 is preferably set to about 1 μm to 30 μm. From the viewpoint of realizing the effects of the technology disclosed herein, the average particle diameter D2 is, for example, 3 μm or more, preferably 5 μm or more, and more preferably 7 μm or more. From the same viewpoint, the average particle diameter D2 is, for example, 25 μm or less, preferably 20 μm or less, and more preferably 15 μm or less.

[0020] The negative electrode active material layer 64 may have a bimodal particle size distribution, for example, for the particles containing silicon. The particles containing silicon are the first particles 681 and the second particles 682. In the bimodal particle size distribution, for example, two mutually independent peaks are observed on the small diameter side and the large diameter side. In this way, for the particles containing silicon, the negative electrode active material layer 64 is configured to have two mutually independent peaks, so that the effect of the particles from which each of the two peaks originates can be more efficiently exhibited. For the particles containing silicon, the negative electrode active material layer 64 has peaks on the small diameter side and the large diameter side, respectively, so that the filling property of the negative electrode active material layer 64 during charging and discharging of the power storage device 100 can be improved, and the cycle characteristics can be improved. Here, the peak on the small diameter side originates from the first particles 681. Moreover, the peak on the large diameter side originates from the second particles 682. The peak on the small diameter side (here, the peak originating from the first particles 681) may be in the range of 1 μm to 5 μm. The peak on the larger diameter side (here, the peak derived from the second particle 682) is preferably in the range of 6 μm to 12 μm.

[0021] The first particle 681 and the second particle 682 may both be composite particles of silicon and carbon, for example. The composite particles of silicon and carbon are, for example, particles in which silicon and carbon are integrated and behave like a single particle. Hereinafter, the composite particles of silicon and carbon are also referred to as "Si / C particles." When the first particle 681 and the second particle 682 are Si / C particles, it is possible to mitigate the expansion and contraction of silicon that accompanies charging and discharging of the power storage device 100, and as a result, it is possible to suppress an increase in the thickness of the power storage device 100.

[0022] The Si / C particles contain silicon, for example, on the surface and inside of the carbon material. The carbon material may be, for example, a porous carbon material. Here, the porous carbon material refers to a carbon material having pores. The pores of the porous carbon material can contribute to mitigating the expansion and contraction of silicon accompanying the charging and discharging of the power storage device 100. For this reason, when the carbon material of the Si / C particles is a porous carbon material, it is possible to suppress an increase in the thickness of the power storage device 100. For example, the silicon may be supported in the pores of the porous carbon material. When the silicon is supported in the pores of the porous carbon material, for example, when the silicon expands, the pores can absorb the expansion, and thus it is possible to suppress an increase in the thickness of the power storage device 100. Although not particularly limited, the porous carbon material is preferably fibrous or granular.

[0023] The Si / C particles can be produced, for example, by the method described below. However, the method for producing the Si / C particles is not limited to the method described below. The method for producing the Si / C particles includes, for example, a preparation step, a mixing step, and a heating step.

[0024] The preparation step is, for example, a step of preparing a composite of a porous carbon material and SiO (SiO-C composite) as a raw material, and a metal reducing agent. As the metal reducing agent, any metal reducing agent used for this type of application can be used without any particular limitation. The metal reducing agent may be, for example, magnesium (Mg), aluminum (Al), or the like.

[0025] The mixing step is, for example, a step of mixing the raw materials prepared in the preparation step. By carrying out the mixing step, a mixture of the raw materials, SiO-C composite and a metal reducing agent, is obtained. Here, the raw materials may be mixed by a conventionally known mixing means such as a mortar.

[0026] The heating step is, for example, a step of heating the mixture obtained in the mixing step. By carrying out the heating step, a reduction reaction by a metal reducing agent can occur. As a result, for example, SiO in the SiO-C composite is reduced to Si, and silicon (Si) is placed in the pores of the porous carbon material. The heating step is preferably carried out in an inert atmosphere, for example, a rare gas atmosphere such as an argon atmosphere, or a nitrogen atmosphere. The temperature condition of the heating step may be, for example, 200°C to 500°C. The heating time may be, for example, 0.1 hours to 10 hours.

[0027] When the first particle 681 and the second particle 682 are Si / C particles, for example, by appropriately adjusting the amount of SiO in the SiO-C composite as a raw material, the desired silicon (Si) contents C1 and C2 for the first particle 681 and the second particle 682 can be realized. In addition, by appropriately adjusting the size of the SiO-C composite as a raw material, the desired average particle diameters D1 and D2 for the first particle 681 and the second particle 682 can be realized.

[0028] The ratio (mass ratio) of the first particles 681 to the second particles 682 is not particularly limited as long as the effects of the technology disclosed herein are realized. The ratio of the first particles 681 to the second particles 682 (first particles 681:second particles 682) is generally 10:90 to 90:10, preferably 20:80 to 40:60, and more preferably 25:75 to 75:25.

[0029] In the embodiment shown in FIG. 3, the negative electrode active material layer 64 further includes graphite particles 683. In addition to the first particles 681 and the second particles 682, the graphite particles 683 can also function as a negative electrode active material. In addition, the graphite particles 683 have a smaller degree of expansion and contraction accompanying charging and discharging of the power storage device 100 than the first particles 681 and the second particles 682 containing silicon. Therefore, by including the graphite particles 683 in the negative electrode active material layer 64, the graphite particles 683 can play a part of the role of the negative electrode active material in the negative electrode active material layer 64. This can better suppress the expansion and contraction of the negative electrode sheet 60 due to charging and discharging of the power storage device 100, and by extension, can more efficiently suppress an increase in the thickness of the power storage device 100.

[0030] The graphite particles 683 may be, for example, artificial graphite, natural graphite, etc. The graphite particles 683 may have a coating layer of amorphous carbon on the surface. Although not particularly limited, the graphite particles 683 are, for example, substantially spherical. In this specification, the term "substantially spherical" with respect to the graphite particles 683 means that the average aspect ratio of the graphite particles 683 based on the observation with an electron microscope (SEM) is 1 to 2 (preferably 1 to 1.5). The average aspect ratio is obtained, for example, by obtaining a planar SEM observation image of the graphite particles 683, randomly selecting a plurality of graphite particles 683 (for example, 10 to 100) from the SEM observation image, calculating the aspect ratio of each, and calculating the arithmetic average value. The average particle diameter of the graphite particles 683 may be, for example, 5 μm to 30 μm, or may be 10 μm to 20 μm.

[0031] When the negative electrode active material contains graphite particles 683, from the viewpoint of realizing the above-mentioned effects, when the total of the first particles 681, the second particles 682, and the graphite particles 683 is taken as 100 mass%, the proportion of the graphite particles 683 is preferably approximately 20 mass% to 80 mass% (preferably 40 mass% to 75 mass%, and more preferably 50 mass% to 70 mass%).

[0032] The negative electrode active material layer 64 may contain a conductive material in addition to the negative electrode active material. As the conductive material, for example, 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); carbon fibers; and the like may be used. Among them, carbon nanotubes are preferable, and single-walled carbon nanotubes are more preferable. By using carbon nanotubes as the conductive material, the conductive path is more preferably maintained, and the cycle characteristics of the electricity storage device 100 may be further improved.

[0033] The proportion of the negative electrode active material in the negative electrode active material layer 64 taken as 100% by mass is, for example, preferably 80% by mass or more, more preferably 90% by mass to 99% by mass, and may be 95% by mass to 99% by mass. The proportion of the conductive material in the negative electrode active material layer 64 taken as 100% by mass may be, for example, 0.01% by mass to 1% by mass.

[0034] The negative electrode active material layer 64 may contain a binder in addition to the negative electrode active material. Examples of the binder include carboxymethyl cellulose (CMC), polyacrylic acid (PAA), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), and the like. Among them, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) can be preferably used. The ratio of the binder when the entire negative electrode active material layer 64 is taken as 100% by mass may be, for example, 1% by mass to 10% by mass.

[0035] In producing the negative electrode sheet 60, the negative electrode active material and materials used as necessary (such as conductive materials and binders) are dispersed in an appropriate solvent (such as water) to prepare a paste (or slurry) composition. Next, this composition is applied to the surface of the negative electrode current collector foil 62 and dried. Then, by pressing as necessary, the negative electrode sheet 60 is produced in which the negative electrode active material layer 64 is provided on the surface of the negative electrode current collector foil 62.

[0036] The separator 70 may be, for example, a porous sheet (film) made of a resin material such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Such a 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 a PP layer is laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0037] The case 30 is, for example, an outer container that houses the electrode body 20 and the non-aqueous 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 a liquid injection hole (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 sheet 50 of the electrode body 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 sheet 60 of the electrode body 20 via a current collector 44a. The safety valve 36 is, for example, a thin-walled portion that is set to release the internal pressure when the internal pressure of the case 30 rises to a predetermined level or higher. The liquid injection hole is, for example, a portion for injecting the non-aqueous electrolyte 80 into the case 30.

[0038] The non-aqueous electrolyte 80 contains, for example, an electrolyte salt, a non-aqueous solvent, and a scavenger. The electrolyte salt can be, for example, LiPF6. The concentration of the electrolyte salt in the non-aqueous electrolyte 80 may be, for example, 0.7 mol / L to 1.3 mol / L. The non-aqueous solvent may be, for example, a carbonate such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), or trifluorodimethyl carbonate (TFDMC). These may be used alone or in combination of two or more.

[0039] The power storage device 100 can be used for various 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 power storage device 100 can also be used as a storage battery for small power storage devices and the like. The power storage device 100 can also be used in the form of a battery pack in which a plurality of the power storage devices 100 are connected in series and / or parallel.

[0040] As described above, the power storage device 100 includes the negative electrode active material layer 64 that includes a negative electrode active material. The negative electrode active material includes first particles 681 and second particles 682. The first particles 681 are particles that include silicon. The second particles 682 are particles that include silicon. Here, the silicon content C2 of the second particles 682 is smaller than the silicon content C1 of the first particles 681. The average particle diameter D2 of the second particles 682 is larger than the average particle diameter D1 of the first particles 681.

[0041] In the electricity storage device 100 having such a configuration, for example, the negative electrode active material contains first particles 681 having a relatively large silicon content and a relatively small average particle diameter, thereby realizing a high capacity. The electricity storage device 100 also contains second particles 682 having a relatively small silicon content and a relatively large average particle diameter, as the negative electrode active material. This makes it possible to reduce the change in volume of silicon in the negative electrode active material that expands and contracts when the electricity storage device 100 is charged and discharged (particularly, the increase in volume due to expansion). This makes it possible to suppress the expansion and contraction of the negative electrode sheet 60 accompanying the charging and discharging of the electricity storage device 100, and ultimately to suppress an increase in the thickness of the electricity storage device 100.

[0042] Test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to those shown in the following test examples.

[0043] <Preparation of test cell> Example 1 In the preparation of the negative electrode sheet, first, materials were prepared. As the negative electrode active material, first particles, second particles, and graphite particles were prepared. The first particles were Si / C particles with a silicon content (content C1) of 70 mass% when the entire first particles were taken as 100 mass% and an average particle diameter D1 of 3 μm. The second particles were Si / C particles with a silicon content (content C2) of 40 mass% when the entire second particles were taken as 100 mass% and an average particle diameter D2 of 10 μm. The graphite particles were graphite particles with an average particle diameter of 15 μm. As the conductive material, single-walled carbon nanotubes (SWCNT) were prepared. As the binder, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared. These were mixed with water as a solvent in a weight ratio of the first particles:second particles:graphite particles:SWCNT:CMC:PAA:SBR=7:28:65:0.1:1:1:1.5 to prepare a negative electrode mixture paste.

[0044] In the preparation of the negative electrode mixture paste, first, the first and second particles, the SWCNT paste (solid content rate 2%), and the dispersion medium were put into a kneader, and the first paste was prepared by dispersing and mixing at 3000 rpm using a disperser. Next, the graphite particles, CMC, and PAA were dry-mixed using a stirring granulator. Then, the first paste, the mixed powder obtained by dry mixing, and the dispersion medium (water) were kneaded and kneaded. The solid content rate during the kneading and kneading was 65%. SBR and the dispersion medium (water) were further added to the kneaded and kneaded mixture. In this way, the negative electrode mixture paste was prepared. This paste was applied in a strip shape on both sides of a copper foil having a thickness of 10 μm. Then, the paste on the copper foil was dried, pressed to a predetermined thickness, and then processed to a predetermined dimension to prepare a negative electrode sheet.

[0045] Next, lithium nickel cobalt manganese composite oxide (NCM) as a positive electrode active material, 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 so that the mass ratio of NCM:AB:PVDF was 100:1:1 to prepare a positive electrode composite paste. This paste was applied in a strip shape to both sides of an aluminum foil with a thickness of 15 μm. Then, the paste on the aluminum foil was dried, pressed to a predetermined thickness, and processed to a predetermined dimension to prepare a positive electrode sheet.

[0046] A current collecting lead was attached to each of the positive electrode sheet and the negative electrode sheet obtained as described above, and the sheets were laminated with a separator interposed therebetween to prepare a laminated electrode body. Next, the laminated electrode body was inserted into an exterior body made of an aluminum laminate sheet, a non-aqueous electrolyte was injected into the interior of the exterior body, and the opening of the exterior body was sealed to prepare a test cell of Example 1. As the separator, a porous polyolefin sheet having a three-layer structure of PP / PE / PP was used. As the non-aqueous electrolyte, a mixture of 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, in which LiPF6 as a supporting salt was dissolved at a concentration of 1 mol / L was used.

[0047] Example 2 The second particles used had a silicon content (content C2) of 55 mass% when the entire second particles were taken as 100 mass%. Except for this, the test cell of this example was produced using the same materials and procedures as in Example 1.

[0048] Example 3 The first particles used had an average particle diameter D1 of 6 μm. Except for this, the test cell of this example was produced using the same materials and procedures as in Example 1.

[0049] Example 4 The second particles used had a silicon content (content C2) of 5 mass% when the entire second particles were taken as 100 mass%. Except for this, the test cell of this example was produced using the same materials and procedures as in Example 1.

[0050] --Example 5-- The first particles used had an average particle diameter D1 of 0.7 μm. Except for this, the test cell of this example was produced using the same materials and procedures as in Example 1.

[0051] -Comparative Example 1- The second particles used had a silicon content (content C2) of 70 mass% when the entire second particles were taken as 100 mass%. Except for this, the test cell of this example was produced using the same materials and procedures as in Example 1.

[0052] -Comparative Example 2- The first particles used had an average particle diameter D1 of 6 μm. The second particles used had an average particle diameter D2 of 6 μm. The test cell of this example was produced using the same materials and procedures as in Example 1 except for this.

[0053] --Comparative Example 3-- As the negative electrode active material, a negative electrode active material containing graphite particles and second particles but not containing first particles was used. The mass ratio of each component in the negative electrode active material of this example was graphite particles: first particles: second particles = 65:0:35. A test cell of this example was produced using the same materials and procedures as in Example 1 except for this.

[0054] <Evaluation of thickness increase rate> The initial thickness T0 of each test cell prepared as described above was measured. Specifically, an arbitrary point was selected from the top, center, and bottom of the wide surface of each test cell, and 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 200 cycles of charge and discharge in a 25°C environment, in which CCCV charging (rate 0.4C up to 4.2V, then 0.1C cut) was performed, and CC discharging (rate 0.4C cut to 2.5V) was performed as one cycle. After 200 cycles of charge and discharge, the thickness of each test cell of each example was measured at the same three points as when the initial thickness T0 was measured, and the arithmetic average value was defined as the thickness after 200 cycles T 200 The initial thickness T0 and the thickness after 200 cycles T 200 From this, the following formula (A): Thickness increase rate (%) = (Thickness after 200 cycles T 200 / initial thickness T0) × 100...Formula (A) Based on this, the thickness increase rate (%) of the test cell of each example was measured. The results are shown in the corresponding column in Table 1. Examples in which the thickness increase rate (%) was 20% or less were evaluated as examples in which the increase in thickness of the test cell due to charging and discharging was suppressed. Furthermore, the "center" of the wide surface is the center of the wide surface. The "upper part" of the wide surface is the region on the one end side of the center of the wide surface. The "lower part" of the wide surface is the region on the opposite side to the one end from the center of the wide surface.

[0055] <Evaluation of capacity retention rate> A cycle test was carried out on each test cell in an environment of 25°C, in which one cycle was CCCV charging (0.4C rate up to 4.2V, then cut off at 0.1C) followed by CC discharging (0.4C rate cut off at 2.5V) until 200 cycles were reached. The discharge capacity (initial capacity) at the first cycle and the discharge capacity at the 200th cycle were measured, and the following formula (B): Capacity retention rate (%) = (200th cycle discharge capacity / initial capacity) × 100 Formula (B) The capacity retention rate (%) of the test cell of each example was measured based on the above. The results are shown in the corresponding column in Table 1.

[0056] [Table 1]

[0057] As described above, the test cells of Examples 1 to 5 have a negative electrode active material layer containing a negative electrode active material containing first particles and second particles. Here, in the test cells of Examples 1 to 5, the silicon content C2 of the second particles is smaller than the silicon content C1 of the first particles. Also, in the test cells of Examples 1 to 5, the average particle diameter D2 of the second particles is larger than the average particle diameter D1 of the second particles. As shown in Table 1, in the test cells of Examples 1 to 5 having the above configuration, the increase in the thickness increase rate was suppressed more than in the test cells of Comparative Examples 1 to 3.

[0058] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Section 1: An electricity storage device including a negative electrode active material layer containing a negative electrode active material, The negative electrode active material is a first particle comprising silicon; second particles comprising silicon; Contains Where: the silicon content C2 of the second particles is smaller than the silicon content C1 of the first particles, an average particle diameter D2 of the second particles is larger than an average particle diameter D1 of the first particles, Section 2: Item 2. The electricity storage device according to item 1, wherein a ratio (C2 / C1) of the content C1 to the content C2 is 0.05 or more and 0.9 or less. Section 3: The content C1 is more than 55% by mass and not more than 90% by mass, Item 3. The electricity storage device according to item 1 or 2, wherein the content C2 is 5 mass % or more and 55 mass % or less. Section 4: 4. The electricity storage device according to any one of items 1 to 3, wherein a ratio (D1 / D2) of the average particle diameter D1 to the average particle diameter D2 is 0.05 or more and 0.8 or less. Section 5: The average particle size D1 is 0.5 μm or more and 10 μm or less, 5. The electricity storage device according to any one of items 1 to 4, wherein the average particle diameter D2 is 5 μm or more and 15 μm or less. Item 6: 6. The electricity storage device according to any one of items 1 to 5, wherein the negative electrode active material layer has a bimodal particle size distribution for particles containing silicon. Section 7: 7. The electricity storage device according to any one of items 1 to 6, wherein the first particles and the second particles are both composite particles of silicon and carbon. Section 8: 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.

[0059] Although the embodiment of the technology disclosed herein has been described above, it is not intended that the technology disclosed herein be limited to the above embodiment. The technology disclosed herein may be implemented in other embodiments. The technology described in the claims includes various modifications and changes to the above-exemplified embodiments. For example, it is possible to replace a part of the above-described embodiment with another modified embodiment, and it is also possible to add another modified embodiment to the above-described embodiment. Furthermore, if a technical feature is not described as essential, it is also possible to delete it as appropriate. [Explanation of symbols]

[0060] 20 Electrode body 30 cases 50 Positive electrode sheet 60 Negative electrode sheet 62 Negative electrode current collector foil 64 Negative electrode active material layer 68 Anode active material 681 1st particle 682 2nd particle 683 Graphite particles 100 Energy storage device

Claims

1. An electricity storage device including a negative electrode active material layer containing a negative electrode active material, The negative electrode active material is a first particle comprising silicon; second particles comprising silicon; Contains Where: The silicon content C2 of the second particle is smaller than the silicon content C1 of the first particle, an average particle diameter D2 of the second particles is larger than an average particle diameter D1 of the first particles.

2. The power storage device according to claim 1 , wherein a ratio (C2 / C1) of the content C1 to the content C2 is equal to or greater than 0.05 and equal to or less than 0.

9.

3. The content C1 is more than 55% by mass and not more than 90% by mass, The power storage device according to claim 1 , wherein the content C2 is equal to or greater than 5 mass % and equal to or less than 55 mass %.

4. The electricity storage device according to claim 1 , wherein a ratio (D1 / D2) of the average particle diameter D1 to the average particle diameter D2 is 0.05 or more and 0.8 or less.

5. The average particle diameter D1 is 0.5 μm or more and 10 μm or less, The power storage device according to claim 1 , wherein the average particle diameter D2 is 5 μm or more and 15 μm or less.

6. The power storage device according to claim 1 , wherein the negative electrode active material layer has a bimodal particle size distribution for particles containing silicon.

7. The power storage device according to claim 1 , wherein the first particles and the second particles are both composite particles of silicon and carbon.

8. The power storage device according to any one of claims 1 to 7, wherein the negative electrode active material layer further contains graphite particles.

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