Energy storage devices

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 active material composite particles containing at least two types of 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 pores). 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 diameter particles located in the gaps between the large particle diameter particles, thereby achieving improved charge-discharge cycle characteristics (particularly, suppression of a rapid capacity decrease during charge-discharge cycles at high temperatures and improvement of high-rate characteristics).

[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, each having different particle size distributions. The publication states that this configuration allows the high capacity characteristic of silicon-based negative electrode active materials to be exhibited, while the electrode density can be increased 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] Japanese Patent Application Laid-Open No. 2010-33830 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-9727 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. However, 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 internal stress in the negative electrode active material layer. In view of this situation, the present inventors wish to suppress an increase in the thickness of an electricity storage device that includes a negative electrode active material layer containing a negative electrode active material containing silicon, which increases with charging and discharging. [Means for solving the problem]

[0006] The electricity storage device disclosed herein includes a negative electrode active material layer containing a negative electrode active material. The negative electrode active material includes first particles and second particles. The first particles are particles containing silicon. The second particles are particles containing silicon. Here, the silicon content C2 of the second particles is smaller than the silicon content C1 of the first particles. Furthermore, 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 including a negative electrode active material layer containing a negative electrode active material containing silicon, an increase in the thickness of the electricity storage device due to charging and discharging can be suppressed. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a cross-sectional view of an electricity storage device 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 sheet 60. DETAILED DESCRIPTION OF THE INVENTION

[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 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 will 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."

[0009] 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. 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 will be described in which a lithium ion secondary battery is used as an example of the above-mentioned electricity storage device.

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

[0011] Fig. 2 is a schematic diagram of the electrode assembly 20. As shown in Fig. 1 and Fig. 2, the electrode assembly 20 is a wound electrode assembly in which a long, sheet-like positive electrode sheet 50 and a long, sheet-like negative electrode sheet 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 sheet 50 and the exposed region 62a of the negative electrode sheet 60 protrude outward from both ends in the lateral direction perpendicular to the longitudinal direction.

[0012] As shown in FIGS. 1 and 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, 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 strip shape along the longitudinal direction on one or both sides (both sides in this example) of the positive electrode collector foil 52. The positive electrode active material layer 54 is not provided at the end in the short direction (the end on the left side in the figure). Here, the exposed region 52a is a strip-shaped region at the end in the short direction (the end on the left side in the figure). As shown in FIG. 1, a 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 04 and lithium transition metal phosphate compounds such as LiFePO4. 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 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 a negative electrode sheet 60. FIG. 3 shows a partially enlarged cross-sectional structure of a negative electrode current collector foil 62 and a negative electrode active material layer 64 in the negative electrode sheet 60. 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, for example, provided in a strip shape along the longitudinal direction on one or both sides (both sides in this example) of the negative electrode current collector foil 62. The negative electrode active material layer 64 is not provided on the end portion in the lateral direction perpendicular to the longitudinal direction (the end portion on the right side in the figure). 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 includes a negative electrode active material 68, which will be described later (see FIG. 3). As shown in FIG. 3, the negative electrode active material 68 includes 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 differ from the first particles 681 in the silicon content and average particle diameter of the particles.

[0016] The silicon content C2 of the second particles 682 is smaller than the silicon content C1 of the first particles 681. In this specification, "the silicon content C1 of the first particles 681" refers to the silicon content (% by mass) when the entire first particles 681 are taken as 100% by mass. Furthermore, "the silicon content C2 of the second particles 682" refers to the silicon content (% by mass) when the entire second particles 682 are taken as 100% by mass. The content C2 is smaller than the content C1 and is not limited as long as the effects of the technology disclosed herein are achieved. 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 viewpoints of realizing the effects of the technology disclosed herein and suppressing a decrease in the capacity retention rate of the electricity 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 one 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 effects of the technology disclosed herein, the content C1 is, for example, 30% by mass or more, and may be 45% by mass or more, preferably more than 55% by mass, and more preferably 60% by 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% by mass or less, and preferably 80% by 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% by mass or more, preferably 15% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. On the other hand, from the viewpoint of realizing the effects of the technology disclosed herein, the content C2 is, for example, less than 70% by mass, preferably 65% ​​by mass or less, more preferably 60% by mass or less, and even more preferably 55% by 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 / D2). For example, the ratio (D1 / D2) of the average particle diameter D1 to 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 a decrease in the capacity retention rate when the electricity storage device 100 is charged and discharged is achieved. From the viewpoint of better achieving both of these effects, it is particularly preferable to set the ratio (D1 / D2) to 0.07 to 0.6, or 0.3 to 0.6.

[0019] The average particle diameter D1 is preferably set to approximately 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 within this range, in addition to the above-mentioned effects, it is possible to achieve the effect of suppressing a decrease in the capacity retention rate when the electricity storage device 100 is charged and discharged. Furthermore, the average particle diameter D2 is preferably set to approximately 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, for example, silicon-containing particles. Here, the silicon-containing particles are first particles 681 and second particles 682. In a bimodal particle size distribution, for example, two mutually independent peaks are observed, one on the small diameter side and one on the large diameter side. In this way, for the silicon-containing particles, the negative electrode active material layer 64 is configured to have two mutually independent peaks, so that the effects of the particles that are the origin of each of the two peaks can be more efficiently exhibited. For the silicon-containing particles, the negative electrode active material layer 64 has peaks on the small diameter side and the large diameter side, respectively, so that the packing 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 is derived from the first particles 681. Meanwhile, the peak on the large diameter side is derived from the second particles 682. Note that the peak on the small diameter side (here, the peak derived from the first particles 681) is preferably in the range of 1 μm to 5 μm. The peak on the larger diameter side (here, the peak derived from the second particles 682) is preferably in the range of 6 μm to 12 μm.

[0021] The first particles 681 and the second particles 682 may both be, for example, composite particles of silicon and carbon. 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 particles 681 and the second particles 682 are Si / C particles, it is possible to mitigate the expansion and contraction of silicon that accompanies charging and discharging of the electricity storage device 100, and ultimately to suppress an increase in the thickness of the electricity 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 that accompanies the charging and discharging of the electricity storage device 100. Therefore, when the carbon material of the Si / C particles is a porous carbon material, an increase in the thickness of the electricity storage device 100 can be suppressed. The silicon may be supported, for example, 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 ultimately, an increase in the thickness of the electricity storage device 100 can be suppressed. Note that, 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), etc.

[0025] The mixing step is, for example, a step of mixing the raw materials prepared in the preparation step. By performing the mixing step, a mixture of the raw materials, ie, the SiO-C composite and the metal reducing agent, is obtained. Here, the raw materials may be mixed using 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 the 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, for example, a rare gas atmosphere such as an argon atmosphere, or an inert atmosphere such as a nitrogen atmosphere. The heating temperature condition in the heating step may be, for example, 200°C to 500°C. The heating time may be, for example, 0.1 to 10 hours.

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

[0028] There is no particular limitation on the ratio (mass ratio) of first particles 681 to second particles 682 as long as the effects of the technology disclosed herein are realized. The ratio of first particles 681 to 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. Furthermore, the graphite particles 683 expand and contract to a lesser extent during charging and discharging of the electricity storage device 100 compared to the first particles 681 and the second particles 682, which contain 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 more effectively suppress the expansion and contraction of the negative electrode sheet 60 during charging and discharging of the electricity storage device 100, and ultimately more efficiently suppress an increase in the thickness of the electricity storage device 100.

[0030] The graphite particles 683 may be, for example, artificial graphite or natural graphite. The graphite particles 683 may have a coating layer of amorphous carbon on their surfaces. Although not particularly limited, the graphite particles 683 may have, for example, a substantially spherical shape. In this specification, the term "substantially spherical" in relation to the graphite particles 683 refers to an average aspect ratio of the graphite particles 683 based on observation with a scanning electron microscope (SEM) of 1 to 2 (preferably 1 to 1.5). The average aspect ratio can be determined, for example, by obtaining a planar SEM image of the graphite particles 683, randomly selecting a plurality of graphite particles 683 (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 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, in order to achieve 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% by mass, the proportion of the graphite particles 683 is preferably approximately 20% by mass to 80% by mass (preferably 40% by mass to 75% by mass, and more preferably 50% by mass to 70% by mass).

[0032] 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. Of these, carbon nanotubes are preferred, and single-walled carbon nanotubes are more preferred. By using carbon nanotubes as the conductive material, the conductive path can be more suitably maintained, and the cycle characteristics of the electricity storage device 100 can 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 preferably 80% by mass or more, more preferably 90% to 99% by mass, and may be 95% 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 0.01% to 1% by mass, for example.

[0034] 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, 1% by mass to 10% by mass.

[0035] In producing the negative electrode sheet 60, the negative electrode active material and optional materials (such as conductive materials and binders) are dispersed in an appropriate solvent (such as water) to prepare a paste (or slurry) composition. This composition is then 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] Examples of separator 70 include porous sheets (films) made of resin materials such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such porous sheets 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 separator 70.

[0037] 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 sheet 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 sheet 60 of the electrode assembly 20 via a current collector 44a. The safety valve 36 is, for example, a thin-walled portion configured to release internal pressure when the internal pressure of the case 30 rises above a predetermined level. The inlet is, for example, a portion through which the nonaqueous electrolyte 80 is injected 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 is preferably, for example, 0.7 mol / L to 1.3 mol / L. The non-aqueous solvent can 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 can be used alone or in combination of two or more.

[0039] The power storage device 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 power storage device 100 can also be used as a storage battery for small-sized 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 containing a negative electrode active material. The negative electrode active material includes first particles 681 and second particles 682. The first particles 681 are particles containing silicon. The second particles 682 are particles containing 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 configured as described above, for example, high capacity is achieved by including, as the negative electrode active material, first particles 681 having a relatively high silicon content and a relatively small average particle diameter. The electricity storage device 100 also includes, as the negative electrode active material, second particles 682 having a relatively low silicon content and a relatively large average particle diameter. This can mitigate the change in volume of silicon in the negative electrode active material that expands and contracts during charging and discharging of the electricity storage device 100 (particularly, the increase in volume due to expansion). This can suppress the expansion and contraction of the negative electrode sheet 60 that accompanies charging and discharging of the electricity storage device 100, and ultimately 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 producing the negative electrode sheet, first, materials were prepared. First particles, second particles, and graphite particles were prepared as negative electrode active materials. 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. Single-walled carbon nanotubes (SWCNTs) were prepared as the conductive material. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared as binders. These were kneaded with water as a solvent so as to achieve a weight ratio of 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 preparing the negative electrode mixture paste, first, the first particles, the second particles, a paste of SWCNT (solid content 2%), and a dispersion medium were placed in a kneader and dispersed and mixed at 3000 rpm using a disperser to prepare a first paste. Next, graphite particles, CMC, and PAA were dry-mixed using a stirring granulator. Then, the first paste, the mixed powder obtained by dry mixing, and a dispersion medium (water) were kneaded together. The solid content during the kneading was 65%. SBR and a dispersion medium (water) were further added to the kneaded mixture and mixed. In this way, a negative electrode mixture paste was prepared. This paste was applied in a strip shape to both sides of a 10 μm-thick copper foil. The paste on the copper foil was then dried, pressed to a predetermined thickness, and processed to a predetermined dimension to prepare a negative electrode sheet.

[0045] Next, lithium nickel cobalt manganese composite oxide (NCM) was prepared as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder. These were mixed with N-methylpyrrolidone (NMP) as the solvent in a mass ratio of NCM:AB:PVDF = 100:1:1 to prepare a positive electrode composite paste. This paste was applied in strips to both sides of a 15 μm-thick aluminum foil. The paste on the aluminum foil was then dried, pressed to a predetermined thickness, and processed to the predetermined dimensions to prepare a positive electrode sheet.

[0046] A current collecting lead was attached to each of the positive electrode sheet and negative electrode sheet obtained as described above, and the sheets were stacked with a separator interposed therebetween to produce a laminated electrode assembly. The laminated electrode assembly was then inserted into an exterior body made of an aluminum laminate sheet, a nonaqueous electrolyte was injected into the interior of the exterior body, and the opening of the exterior body was sealed to produce a test cell of Example 1. A porous polyolefin sheet having a three-layer structure of PP / PE / PP was used as the separator. The nonaqueous electrolyte was a mixed solvent 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 was dissolved as a supporting electrolyte at a concentration of 1 mol / L.

[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 test cell of this example was produced using the same materials and procedures as in Example 1, except that the first particles used had an average particle diameter D1 of 6 μm.

[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 test cell of this example was fabricated using the same materials and procedures as in Example 1, except that the first particles used had an average particle diameter D1 of 0.7 μm.

[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 were first particles having an average particle diameter D1 of 6 μm. The second particles used were second particles having an average particle diameter D2 of 6 μm. Except for this, the test cell of this example was produced using the same materials and procedures as in Example 1.

[0053] -Comparative Example 3- The negative electrode active material used was a negative electrode active material containing graphite particles and second particles, but not containing first particles. The mass ratio of the components 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 fabricated using the same materials and procedures as in Example 1.

[0054] <Evaluation of thickness increase rate> The initial thickness T0 of each test cell prepared as described above was measured. Specifically, arbitrary points were 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 these three points, and the arithmetic mean value was defined as the initial thickness T0. Next, the test cell of each example was subjected to 200 cycles of charge-discharge, with one cycle consisting of CCCV charging (0.4C rate up to 4.2V, then 0.1C cut) in a 25°C environment, followed by CC discharging (0.4C rate, 2.5V cut). After 200 cycles of charge-discharge, the thickness of each test cell of each example was measured at the same three points as when measuring the initial thickness T0, and the arithmetic mean value was defined as the thickness after 200 cycles T0. 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) The thickness increase rate (%) of the test cell for each example was measured based on the above. 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 during charging and discharging was suppressed. The "center" of the wide surface is the center of the wide surface. The "upper part" of the wide surface is the region closer to one end than the center of the wide surface. The "lower part" of the wide surface is the region closer to the one end than 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 CCCV charging (0.4C rate up to 4.2V, then 0.1C cut) was followed by CC discharging (0.4C rate, 2.5V cut) under the above conditions, with one cycle consisting of charging and discharging. The discharge capacity at the first cycle (initial capacity) and the discharge capacity at the 200th cycle were measured, and the following formula (B) was used: Capacity retention rate (%) = (discharge capacity at 200th cycle / initial capacity) × 100 Formula (B) The capacity retention rate (%) of the test cell of each example was measured based on the results shown in the corresponding column in Table 1.

[0056] [Table 1]

[0057] As described above, the test cells of Examples 1 to 5 each include a negative electrode active material layer containing a negative electrode active material including first particles and second particles. 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. Furthermore, 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, the test cells of Examples 1 to 5 configured as described above exhibited a more suppressed increase in the thickness increase rate than 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 first particles comprising silicon; second particles comprising silicon; It contains where: the silicon content C2 of the second particles is smaller than the silicon content C1 of the first particles; The second particles have an average particle diameter D2 larger than the average particle diameter D1 of the first particles. Section 2: Item 2. The electricity storage device according to item 1, wherein the 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% by mass or more and 55% by mass or less. Section 4: 4. The electricity storage device according to any one of items 1 to 3, wherein the 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 diameter 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: Item 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 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 embodiments of the technology disclosed herein have been described above, it is not intended that the technology disclosed herein be limited to the above-described embodiments. The technology disclosed herein may also be implemented in other embodiments. The technology described in the claims includes various modifications and alterations of the above-described exemplary embodiments. For example, it is possible to replace part of the above-described embodiments with other modified embodiments, and it is also possible to add other modified embodiments to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate. [Explanation of symbols]

[0060] 20 Electrode body 30 cases 50 positive electrode sheet 60 negative electrode sheet 62 Negative electrode current collecting foil 64 Negative electrode active material layer 68 Negative electrode 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 first particles comprising silicon; second particles comprising silicon; It contains where: each of the first particle and the second particle includes a composite particle of a porous carbon material having pores and silicon supported in the pores of the porous carbon material; the silicon content C2 of the second particles is smaller than the silicon content C1 of the first particles; the content C1 is the content (mass%) of silicon in the first particles when the entire first particles are taken as 100 mass%, and the content C2 is the content (mass%) of silicon in the second particles when the entire second particles are taken as 100 mass%, an average particle diameter D2 of the second particles being larger than an average particle diameter D1 of the first particles;

2. The electricity 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 electricity storage device according to claim 1 , wherein the content C2 is 5 mass % or more and 55 mass % or less.

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 electricity storage device according to claim 1 , wherein the average particle diameter D2 is 5 μm or more and 15 μm or less.

6. The electricity 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 electricity storage device according to any one of claims 1 to 6, wherein the negative electrode active material layer further contains graphite particles.

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