Electricity storage device

Abstract

The present invention addresses the problem of achieving a more preferable specific surface area in a silicon-containing negative electrode active material layer. A power storage device (100) is provided with a negative electrode active material layer (64) containing a negative electrode active material (68). The negative electrode active material (68) contains first particles (681) that contain silicon and second particles (682) that contain silicon and are different from the first particles (681). The average circularity (CR1) of the first particles (681) is 0.5 to 0.9. The second particles (682) have an average circularity, CR2, greater than 0.9. The ratio (D2 / D1) of the average particle diameter (D1) of the first particles (681) to the average particle diameter (D2) of the second particles (682) is at least 0.1 and less than 1.

Description

Technical Field

[0001] This disclosure relates to energy storage devices. Background Technology

[0002] The negative electrode active material disclosed in Japanese Patent Application Publication No. 2017-92009 comprises negative electrode active material particles, wherein the negative electrode active material particles contain silicon compound particles comprising silicon compound (SiOx: 0.5 ≤ x ≤ 1.6). The silicon compound particles contain at least one of Li₂SiO₃ and Li₄SiO₄. The loose packing density BD of the negative electrode active material particles is 0.5 g / cm³. 3 ~0.9g / cm 3 The tapped bulk density (TD) is 0.7 g / cm³. 3 ~1.2g / cm 3 Furthermore, the compression ratio specified in (TD-BD) / TD is 25% or less. This publication describes how, for negative electrode active material particles containing silicon compound particles, the SiO2 component in the silicon compound, which becomes unstable during lithium insertion and extraction during battery charging and discharging, is partially modified into lithium silicate, thereby reducing irreversible capacity generated during charging. This publication also describes how the negative electrode active material particles meet specified loose packing density, tap density, and compression ratio, thereby improving battery cycle characteristics. Furthermore, it describes how using such negative electrode active material particles can improve electrode fillability, thereby improving the cycle characteristics of the secondary battery.

[0003] Japanese Patent Application Publication No. 2019-175851 discloses a negative electrode active material comprising composite particles in which at least a portion of the surface of silicon oxide particles and graphite particles is coated with a low-crystallinity carbon material with lower crystallinity than graphite particles. The composite particles contain 50 to 500 parts by mass of silicon oxide particles and 200 to 2000 parts by mass of graphite particles, relative to 100 parts by mass of the low-crystallinity carbon material. For the composite particles, in the differential curve of the thermogravimetric analysis obtained under an oxygen-containing atmosphere, two peaks with weight reduction accompanying heating are observed, and the peak observed on the lower temperature side of the two peaks is observed in the temperature range of 500 to 600°C. This publication describes a negative electrode active material for lithium-ion secondary batteries that can provide a lithium-ion secondary battery with a large initial discharge capacity and excellent charge-discharge cycle characteristics.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2017-92009

[0007] Patent Document 2: Japanese Patent Application Publication No. 2019-175851 Summary of the Invention

[0008] The inventors aim to achieve a more desirable specific surface area in a silicon-containing negative electrode active material layer.

[0009] The energy storage device disclosed herein includes a negative electrode active material layer comprising a negative electrode active material. The negative electrode active material comprises a first particle and a second particle containing silicon. The second particle is a particle different from the first particle. The average sphericity CR1 of the first particle is 0.5 to 0.9. The average sphericity CR2 of the second particle is greater than 0.9. The ratio (D2 / D1) of the average particle size D1 of the first particle to the average particle size D2 of the second particle is 0.1 or more and less than 1. According to this configuration, a more preferred specific surface area can be achieved in the silicon-containing negative electrode active material layer in the energy storage device. Attached Figure Description

[0010] Figure 1 This is a schematic cross-sectional view of the energy storage device 100.

[0011] Figure 2 This is a schematic diagram of electrode 20.

[0012] Figure 3 This is a schematic cross-sectional view of the negative electrode sheet 60.

[0013] Symbol Explanation

[0014] 20 Electrode Body

[0015] 30. Housing

[0016] 50 positive electrode sheet

[0017] 60 negative electrode sheet

[0018] 62 Negative electrode current collector foil

[0019] 64 Negative Electrode Active Material Layer

[0020] 68 Negative Electrode Active Material

[0021] 681 First Particle

[0022] 682 Second Particle

[0023] 683 graphite particles

[0024] 100 storage devices Detailed Implementation

[0025] The following describes one embodiment of the technology disclosed herein. The embodiment described herein is not specifically intended to limit the technology disclosed herein. Unless otherwise specified, the technology disclosed herein is not limited to the embodiment described herein. The accompanying drawings are schematic and do not necessarily reflect actual objects. Furthermore, components and parts that perform the same function are appropriately labeled with the same symbols, and repeated descriptions are omitted. Additionally, unless otherwise specified, the expression "A~B" indicating a numerical range means "above A and below B," and also includes the meaning of "higher than A and lower than B."

[0026] In this specification, "energy storage device" refers to a device that generates charge and discharge through the movement of a charge carrier between a pair of electrodes (positive and negative electrodes) via an electrolyte. Such energy storage devices include secondary batteries such as lithium-ion batteries, nickel-metal hydride batteries, and nickel-cadmium batteries; and capacitors such as lithium-ion capacitors and double-layer capacitors. Hereinafter, as an example of the aforementioned energy storage device, an embodiment using a lithium-ion secondary battery will be described.

[0027] Figure 1 This is a schematic cross-sectional view of the energy storage device 100. Figure 1 This is a schematic cross-sectional view along the widest section of the energy storage device 100. (Example) Figure 1 As shown, the energy storage device 100 includes an electrode body 20, a housing 30, and a non-aqueous electrolyte 80.

[0028] Figure 2 This is a schematic diagram of electrode body 20. (For example...) Figure 1 and Figure 2 As shown, the electrode body 20 is a wound electrode body consisting of an elongated positive electrode sheet 50 and an elongated negative electrode sheet 60 sandwiching an elongated separator 70, which are overlapped and wound along the length direction of the sheets (hereinafter also referred to as the "length direction"). In the electrode body 20, the exposed areas 52a of the positive electrode sheet 50 and 62a of the negative electrode sheet 60 extend outward from both ends in the short side direction orthogonal to the length direction.

[0029] like Figure 1 and Figure 2 As shown, the positive electrode sheet 50 includes a strip-shaped positive electrode current collector foil 52 and a positive electrode active material layer 54. The positive electrode current collector foil 52 is, for example, an aluminum foil. In this embodiment, the positive electrode current collector foil 52 has a region where the positive electrode active material layer 54 is disposed and an exposed region 52a where the surface of the positive electrode current collector foil 52 is exposed without the positive electrode active material layer 54. The positive electrode active material layer 54 is disposed, for example, in a strip-like manner along the length direction on one or both sides (in this case, both sides) of the positive electrode current collector foil 52. The positive electrode active material layer 54 is not disposed at the end in the short side direction (the left end in the figure). The exposed region 52a is, in this case, a strip-like region at the end in the short side direction (the left end in the figure). Figure 1As shown, a current collector 42a is installed in the exposed area 52a.

[0030] The positive electrode active material layer 54 contains, for example, a positive electrode active material. Examples of positive electrode active materials include lithium nickel cobalt manganese composite oxide (NCM) (e.g., LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2), LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 Lithium transition metal oxides such as O4; lithium transition metal phosphate compounds such as LiFePO4; etc. The positive electrode active material layer 54 may contain conductive materials, binders, etc., in addition to the positive electrode active material. Examples of conductive materials include carbon black such as acetylene black (AB); other carbon materials such as graphite. Examples of binders include polyvinylidene fluoride (PVDF).

[0031] Figure 3 This is a schematic cross-sectional view of the negative electrode sheet 60. Figure 3 The magnified view shows 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. (See image below.) Figures 1-3 As shown, the negative electrode sheet 60 includes a strip-shaped 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 disposed and an exposed region 62a where the surface of the negative electrode active material layer 64 is exposed. The negative electrode active material layer 64 is disposed, for example, in a strip-like manner along the length direction on one or both sides (in this case, both sides) of the negative electrode current collector foil 62. The negative electrode active material layer 64 is not disposed at the end in the short side direction orthogonal to the length direction (the right end in the figure). The exposed region 62a is, in this case, a strip-like region at the end in the short side direction (the right end in the figure). Figure 1 As shown, a current collector 44a is installed in the exposed area 62a.

[0032] However, for example, to achieve high capacity in energy storage devices, silicon-containing anode active materials are sometimes used. According to the inventors' research, if only anode active materials with high average sphericity are used, the filling density of the anode active material layer can be increased, but the internal voids within the anode active material are insufficient, thus easily increasing internal stress. According to the inventors' research, if only anode active materials with low average sphericity are used, voids can be appropriately provided within the anode active material layer, but the anode active material is prone to cracking due to the stress of pressing the anode active material layer during the manufacturing process. The cracking of the anode active material increases the specific surface area of ​​the anode active material layer, becoming a significant factor in the reduction of energy storage device capacity. The inventors believe that, for example, by suppressing the increase in specific surface area before and after pressing during the manufacturing process, a more desirable specific surface area can be achieved in the silicon-containing anode active material layer. The inventors believe that this can suppress the reduction of energy storage device capacity and achieve a high-density anode active material layer that appropriately mitigates internal stress caused by expansion and contraction. It should be noted that "anode active material with high average roundness" refers to anode active material with an average roundness greater than 0.9. "Anode active material with low average roundness" refers to anode active material with an average roundness less than 0.9.

[0033] like Figure 3 As shown, the negative electrode active material layer 64 includes a negative electrode active material 68. The negative electrode active material 68 here includes first particles 681 and second particles 682. The first particles 681 are silicon-containing particles. The second particles 682 are silicon-containing particles different from the first particles 681. The first particles 681 and the second particles 682 differ from each other, for example, in terms of average sphericity and average particle size.

[0034] The average sphericity CR1 of the first particle 681 is, for example, 0.5 to 0.9. From the viewpoint of increasing the density of the negative electrode active material layer 64, the average sphericity CR1 is preferably 0.6 or more, more preferably 0.7 or more, and even more preferably 0.75 or more. From the viewpoint of providing appropriate voids in the negative electrode active material layer 64, the average sphericity CR1 is preferably 0.85 or less, more preferably 0.8 or less. The average sphericity CR2 of the second particle 682 is, for example, greater than 0.9. From the viewpoint of increasing the density of the negative electrode active material layer 64, the average sphericity CR2 is preferably 0.91 or more, more preferably 0.92 or more, and even more preferably 0.93 or more. From the viewpoint of providing appropriate voids in the negative electrode active material layer 64, the average sphericity CR2 is preferably less than 1, preferably 0.98 or less, more preferably 0.96 or less, and even more preferably 0.95 or less.

[0035] In this specification, the “average roundness” of a particle refers to the arithmetic mean of the roundness of more than 1,000 (e.g., 3,000) randomly selected particles. The roundness referred to herein is calculated by performing image analysis on the particle image and using the following formula (A) to calculate the perimeter (L) of the particle and the perimeter (L0) of a circle with the same projected area as the particle.

[0036] Circularity = L0 / L... (A)

[0037] As an image analysis device for calculating average roundness, an image-based particle size distribution measuring device for this purpose can be used without particular restrictions.

[0038] The second particle 682 is preferably a particle with a smaller size than the first particle 681. This allows the smaller second particle 682 to enter the gaps between the larger first particles 681, increasing the density of the negative electrode active material layer 64. From the viewpoint of achieving this effect, the ratio (D2 / D1) of the average particle size D1 of the first particle 681 to the average particle size D2 of the second particle 682 can be 0.1 or more and less than 1, preferably 0.8 or less, more preferably 0.6 or less. From the viewpoint of suppressing the decrease in the capacity retention rate of the energy storage device 100, the ratio (D2 / D1) is further preferably 0.5 or less, particularly preferably 0.4 or less. To prevent excessive reduction of the void space in the negative electrode active material layer 64 due to the second particles 682 entering the gaps between the first particles 681, the ratio (D2 / D1) is preferably 0.15 or more, more preferably 0.2 or more.

[0039] The average particle size D1 and average particle size D2 are not particularly limited as long as they achieve the effect of the technology disclosed herein. The average particle size D1 can be approximately 3 μm to 25 μm. From the viewpoint of appropriately setting voids within the negative electrode active material layer 64, the average particle size D1 is preferably 5 μm or more, more preferably 7.5 μm or more, or preferably 20 μm or less, more preferably 15 μm or less. The average particle size D2 can be approximately 0.3 μm to 10 μm. From the viewpoint of easily allowing the second particle to enter the voids between the first particles 681, the average particle size D2 is preferably 8 μm or less, more preferably 6 μm or less. From the viewpoint of achieving the effect of suppressing the reduction of capacity retention rate, it is further preferably 5 μm or less, and particularly preferably 4 μm or less. In order to prevent the voids in the negative electrode active material layer 64 from being excessively reduced due to the second particle 682 entering the voids between the first particles 681, the average particle size D2 is preferably 0.5 μm or more, more preferably 0.7 μm or more. In addition to this effect, from the viewpoint of achieving the effect of suppressing the decrease in capacity retention, the average particle size D2 is further preferably 1 μm or more, and particularly preferably 2 μm or more. It should be noted that, in this specification, regarding particles, "average particle size" refers to the particle size (D2) that represents 50% of the cumulative particle size from the microparticle side in a volume-based particle size distribution determined by particle size distribution measurement based on laser diffraction and light scattering. 50 Particle size).

[0040] To provide adequate spacing, the negative electrode active material layer 64 may contain more first particles 681 than second particles 682. The mass ratio (C2 / C1) of the content C1 of first particles 681 to the content C2 of second particles 682 in the negative electrode active material 68 is not particularly limited as long as the effect of the disclosed technology is achieved. The ratio (C2 / C1) can be approximately 0.03 or more and less than 1. From the viewpoint of achieving the effect of the disclosed technology, the ratio (C2 / C1) is preferably 0.05 or more, more preferably 0.08 or more, and even more preferably 0.1 or more. In addition to this effect, from the viewpoint of suppressing the decrease in the capacity retention rate of the energy storage device 100, the ratio (C2 / C1) is preferably 0.8 or less, more preferably 0.7 or less, even more preferably 0.5 or less, and particularly preferably 0.3 or less.

[0041] The first particle 681 and the second particle 682 can both be, for example, composite particles of silicon and carbon. A silicon-carbon composite particle is, for example, a particle in which silicon and carbon are integrated and appear as a single particle. Hereinafter, the silicon-carbon composite particle will also be referred to as a "Si / C particle". By using Si / C particles for the first particle 681 and the second particle 682, the degree of deformation of the first particle 681 and the second particle 682 caused by the manufacturing process of the energy storage device 100 and charging / discharging can be reduced. Therefore, a suitable specific surface area can be easily achieved in the negative electrode active material layer 64.

[0042] Si / C particles, for example, contain silicon on the surface and inside of the carbon material. The carbon material can be, for example, a porous carbon material. Porous carbon material here refers to a carbon material with fine pores. The fine pores of the porous carbon material can, for example, help mitigate the expansion and contraction of silicon caused by the charging and discharging of the energy storage device 100. Therefore, by using a porous carbon material with Si / C particles, the internal stress of the negative electrode active material layer 64 can be reduced. Silicon can, for example, be supported in the fine pores of the porous carbon material. Through the silicon supported in the fine pores of the porous carbon material, for example, when silicon expands, the fine pores can absorb its expanded portion. It should be noted that, without particular limitation, the porous carbon material is preferably fibrous or granular.

[0043] Si / C particles are manufactured, for example, by the methods described below. However, the methods for manufacturing Si / C particles are not limited to those described below. For example, the methods for manufacturing Si / C particles include a preparation step, a mixing step, and a heating step.

[0044] The preparation process includes, for example, preparing a porous carbon material and a SiO composite (SiO-C composite) as raw materials, as well as a metal reducing agent. As the metal reducing agent, any metal reducing agent used in this application can be used without restriction. Examples of metal reducing agents include magnesium (Mg) and aluminum (Al).

[0045] The mixing process is, for example, the process of mixing the raw materials prepared in the preparation process. By performing the mixing process, a mixture of SiO-C composite material and metal reducing agent is obtained. Here, conventionally known mixing methods such as mortars and pestles can be used to mix the raw materials.

[0046] The heating process is, for example, a process of heating the mixture obtained in the mixing process. By performing the heating process, a reduction reaction using a metal reducing agent can occur. Thus, for example, the SiO in the SiO-C composite is reduced to Si, and silicon (Si) is disposed in the pores of the porous carbon material. The heating process is preferably performed, for example, in a rare gas atmosphere such as argon, or in an inactive atmosphere such as nitrogen. The heating temperature in the heating process can be, for example, 200°C to 500°C. Furthermore, the heating time can be, for example, 0.1 hours to 10 hours.

[0047] It should be noted that when the first particle 681 and the second particle 682 are Si / C particles, the desired average particle size D1 and average particle size D2 can be achieved for the first particle 681 and the second particle 682, for example, by appropriately adjusting the size of the SiO-C composite used as a raw material. By appropriately adjusting the amount of SiO in the SiO-C composite used as a raw material, the amount of silicon (Si) contained in the first particle 681 and the second particle 682 can be adjusted.

[0048] exist Figure 3 In the form shown, 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, compared to the silicon-containing first particles 681 and the second particles 682, the graphite particles 683 exhibit less expansion and contraction due to the charging and discharging of the energy storage device 100. Therefore, by including graphite particles 683 in the negative electrode active material layer 64, the graphite particles 683 can perform part of the function of the negative electrode active material in the negative electrode active material layer 64. This better suppresses the expansion and contraction of the negative electrode sheet 60 due to the charging and discharging of the energy storage device 100.

[0049] The graphite particles 683 can be, for example, artificial graphite, natural graphite, etc. The graphite particles 683 may have an amorphous carbon coating on their surface. Without particular limitation, the graphite particles 683 are, for example, substantially spherical. In this specification, "substantially spherical" refers to the graphite particles 683 having an average aspect ratio of 1 to 2 (preferably 1 to 1.5) based on electron microscopy (SEM) observation. It should be noted that the average aspect ratio is determined, for example, by obtaining a planar SEM image of the graphite particles 683, randomly selecting multiple (e.g., 10 to 100) graphite particles 683 from the SEM image, calculating their respective aspect ratios, and then calculating their arithmetic mean. The average particle size of the graphite particles 683 can be, for example, 5 μm to 30 μm, or 10 μm to 20 μm.

[0050] When the negative electrode active material contains graphite particles 683, from the viewpoint of achieving the above-mentioned effect, when the total of the first particle 681, the second particle 682 and the graphite particles 683 is set to 100% by mass, the proportion of graphite particles 683 can be approximately 20% to 80% by mass (preferably 40% to 75% by mass, more preferably 50% to 70% by mass).

[0051] In addition to the negative electrode active material, the negative electrode active material layer 64 may contain a conductive material. As a conductive material, examples include carbon nanotubes such as single-layer carbon nanotubes (SWCNTs), double-layer carbon nanotubes (DWCNTs), and multi-layer carbon nanotubes (MWCNTs); carbon black such as acetylene black (AB); carbon fibers; etc. Among these, carbon nanotubes are preferred, and single-layer carbon nanotubes are more preferred. By using carbon nanotubes as a conductive material, the conductive path is better maintained, and the cycle characteristics of the energy storage device 100 can be further improved.

[0052] When the negative electrode active material layer 64 is set to 100% by mass, the proportion of the negative electrode active material is preferably 80% by mass or more, more preferably 90% to 99% by mass, and may also be 95% to 99% by mass. When the negative electrode active material layer 64 is set to 100% by mass, the proportion of the conductive material may be, for example, 0.01% to 1% by mass.

[0053] The negative electrode active material layer 64 may contain an adhesive in addition to the negative electrode active material. Examples of adhesives 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 preferred. The proportion of adhesive when the negative electrode active material layer 64 is 100% by mass can be, for example, 1% to 10% by mass.

[0054] In the fabrication of the negative electrode sheet 60, a paste-like (or slurry-like) composition is prepared by dispersing the negative electrode active material and, as needed, materials (conductive materials, adhesives, etc.) in a suitable solvent (e.g., water). Next, this composition is coated onto the surface of the negative electrode current collector foil 62 and dried. Then, as needed, it is pressed to fabricate the negative electrode sheet 60, on which a negative electrode active material layer 64 is disposed, on the surface of the negative electrode current collector foil 62.

[0055] Examples of insulating materials 70 include porous sheets (membranes) made of resin materials such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. The porous sheet can be a single-layer structure or a multi-layer structure (e.g., a three-layer structure with PP layers laminated on both sides of a PE layer). A heat-resistant layer (HRL) can also be provided on the surface of the insulating material 70.

[0056] The housing 30 is, for example, an outer container for housing the electrode body 20 and the non-aqueous electrolyte 80. Here, the housing 30 is a flat, square housing. Figure 1As shown, the housing 30 has a positive terminal 42, a negative terminal 44, a safety valve 36, and an injection port (not shown). The positive terminal 42 is, for example, an external connection terminal on the positive side. The positive terminal 42 is electrically connected to the positive electrode sheet 50 of the electrode body 20 via a current collector 42a. The negative terminal 44 is, for example, an external connection terminal on the negative side. The negative 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 configured to release internal pressure when the internal pressure of the housing 30 rises above a predetermined level. The injection port is, for example, a location for injecting a non-aqueous electrolyte 80 into the housing 30.

[0057] The non-aqueous electrolyte 80 may contain, for example, an electrolyte salt, a non-aqueous solvent, and a trapping agent. Examples of electrolyte salts include 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. Non-aqueous solvents may be carbonates 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), and difluorotrifluoromethyl carbonate (TFDMC). These may be used alone or in combination of two or more.

[0058] The energy storage device 100 can be used for various purposes. Suitable applications include power supplies for driving vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The energy storage device 100 can also be used as a battery for small-scale energy storage devices. Multiple energy storage devices 100 can also be connected in series and / or parallel to form a battery pack.

[0059] As described above, the energy storage device 100 includes a negative electrode active material layer 64 comprising a negative electrode active material 68. The negative electrode active material 68 comprises first particles 681 containing silicon and second particles 682 containing silicon and different from the first particles 681. The average sphericity CR1 of the first particles 681 is 0.5 to 0.9. The average sphericity CR2 of the second particles 682 is greater than 0.9. The ratio (D2 / D1) of the average particle size D1 of the first particles 681 to the average particle size D2 of the second particles 682 is 0.1 or more and less than 1.

[0060] In the energy storage device 100 configured in this way, a negative electrode active material 68 comprising first silicon particles 681 and second silicon particles 682 is used, thereby achieving high capacity. The average particle size D1 of the first particles 681, which has a lower average sphericity, is greater than the average particle size D2 of the second particles 682, which has a higher average sphericity. Therefore, the second particles 682 can easily enter the voids generated between the first particles 681. As a result, the increase in the specific surface area of ​​the negative electrode active material layer 64 can be suppressed, achieving a more desirable specific surface area. In addition to the effect of improving the specific surface area of ​​the negative electrode active material layer 64, appropriate voids can also be maintained within the negative electrode active material layer 64. Therefore, even if the negative electrode active material 68 expands and contracts with the charging and discharging of the energy storage device 100, the expansion and contraction of the negative electrode active material 68 will be eliminated by these voids. As a result, the increase in internal stress of the negative electrode active material layer 64 and the interruption of the conductive path can be suppressed.

[0061] The following describes experimental examples related to the present invention, but it is not intended to limit the present invention to the contents shown in the following experimental examples.

[0062] <Fabrication of Experimental Single Cells>

[0063] Example 1

[0064] In the fabrication of the negative electrode sheet, the materials are first prepared. As the negative electrode active material, first particles, second particles, and graphite particles are prepared. The first particle is a Si / C particle with an average sphericity of 0.64 and an average particle size of 10 μm. The second particle is a Si / C particle with an average sphericity of 0.95 and an average particle size of 2 μm. The graphite particles are graphite particles with an average particle size of 15 μm. As the conductive material, single-layer carbon nanotubes (SWCNTs) are prepared. As the binder, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) are prepared. These are mixed with water as a solvent in a mass ratio of first particle: second particle: graphite particle: SWCNT: CMC: PAA: SBR = 31.5: 3.5: 65: 0.1: 1: 1: 1.5 to prepare the negative electrode paste.

[0065] In the preparation of the negative electrode paste, firstly, first and second particles, SWCNT paste (2% solids content), and a dispersion medium are added to a mixer and dispersed at 3000 rpm using a disperser to prepare the first paste. Next, graphite particles, CMC, and PAA are dry-mixed using a stirred granulator. Then, the first paste, the mixed powder obtained by dry mixing, and the dispersion medium (water) are solid-state kneaded. The solids content during solid-state kneading is 65%. SBR and the dispersion medium (water) are further added to the solid-state kneaded mixture and mixed. This prepares the negative electrode paste. The negative electrode paste is then coated in strips onto both sides of a 10 μm thick copper foil. The paste on the copper foil is then dried, pressed to a specified thickness, and processed into specified dimensions to produce the negative electrode sheet.

[0066] Next, lithium nickel cobalt manganese composite oxide (NCM) as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder are prepared. These are mixed with N-methylpyrrolidone (NMP) as a solvent in a mass ratio of NCM:AB:PVDF = 100:1:1 to prepare a positive electrode composite paste. This paste is then coated in strips onto both sides of an aluminum foil with a thickness of 15 μm. The paste on the aluminum foil is then dried, pressed to a specified thickness, and processed into specified dimensions to produce the positive electrode sheet.

[0067] Current collector leads were installed on the positive and negative electrode sheets obtained as described above, and then stacked in a spacer to form a stacked electrode body. Next, the stacked electrode body was inserted into an outer casing made of aluminum laminate, a non-aqueous electrolyte was injected into the outer casing, and the opening of the outer casing was sealed, thereby fabricating the experimental single-cell battery of Example 1. It should be noted that a porous polyolefin sheet with a three-layer structure of PP / PE / PP was used as the spacer. The non-aqueous electrolyte was obtained by dissolving LiPF6, the supporting salt, at a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate (EC), fluorinated ethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:FEC:EMC:DMC = 15:5:40:40.

[0068] It should be noted that the average roundness CR1 of the first particle and the average roundness CR2 of the second particle are calculated by image analysis of the particle images of each particle. In this image analysis, 3000 particles were tested. At this time, the roundness of each particle was calculated using the perimeter (L) of the particle, the perimeter (L0) of the circle with the same projected area as the particle, and the following formula (A).

[0069] Circularity = L0 / L... (A)

[0070] Then, the sum of the roundness of the 3000 tested particles is divided by the number of tested particles (3000 in this case) to calculate the average roundness CR1 and average roundness CR2. It should be noted that in this experimental example, an image-based particle size distribution measuring device is used to measure the average roundness CR1 and average roundness CR2.

[0071] Example 2

[0072] For the negative electrode active material, the first particle, the second particle, and the graphite particles were prepared in a mass ratio of first particle: second particle: graphite particles = 26.25:87:25:65. Otherwise, the experimental single-cell battery of this example was fabricated using the same materials and procedures as in Example 1.

[0073] Example 3 -

[0074] As the second particle, a second particle with an average particle size D2 of 4 μm was used. Otherwise, the experimental single cell for this example was fabricated using the same materials and procedures as in Example 1.

[0075] Example 4

[0076] As the second particle, a second particle with an average particle size D2 of 6 μm was used. Otherwise, the experimental single cell for this example was fabricated using the same materials and procedures as in Example 1.

[0077] Example 5 -

[0078] For the negative electrode active material, the first particle, the second particle, and the graphite particles were prepared in a mass ratio of first particle: second particle: graphite particles = 21:14:65. Otherwise, the experimental single-cell battery of this example was fabricated using the same materials and procedures as in Example 1.

[0079] - Comparative Example 1 -

[0080] As the second particle, a second particle with an average particle size D2 of 0.7 μm was used. Otherwise, the experimental single cell for this example was fabricated using the same materials and procedures as in Example 1.

[0081] - Comparative Example 2 -

[0082] For the negative electrode active material, the first particle, the second particle, and the graphite particles were prepared in a mass ratio of first particle: second particle: graphite particles = 35:0:65. Otherwise, the experimental single-cell battery of this example was fabricated using the same materials and procedures as in Example 1.

[0083] - Comparative Example 3 -

[0084] For the negative electrode active material, the first particle, the second particle, and the graphite particles were prepared in a mass ratio of first particle: second particle: graphite particles = 0:35:65. Otherwise, the experimental single-cell battery of this example was fabricated using the same materials and procedures as in Example 1.

[0085] <Evaluation of the rate of increase in specific surface area>

[0086] For each test cell, the specific surface area of ​​the negative electrode sheet before and after pressing was calculated in the above manufacturing process to obtain the specific surface area increase rate. Here, for samples obtained by cutting negative electrode sheets before and after pressing to specified dimensions, the amount of gas adsorbed was measured by gas adsorption method using a commercially available specific surface area and pore size measuring device. Nitrogen was used as the adsorbed gas. The specific surface area was the value obtained by the BET method. Then, the specific surface area increase rate (%) was calculated using the following formula (B).

[0087] Specific surface area increase rate (%)

[0088] = [{(specific surface area after compression) - (specific surface area before compression)} / (specific surface area before compression)] × 100 ... Formula (B)

[0089] The results are shown in this column of Table 1.

[0090] <Evaluation of Capacity Maintenance Rate>

[0091] The test cells for each example were subjected to the following cycle test: A charge-discharge cycle consisting of CCCV charging (0.4C to 4.2V, then 0.1C cutoff) followed by CC discharge (0.4C cutoff to 2.5V) at 25°C was considered one cycle. This charge-discharge cycle was repeated until 200 cycles were reached. The discharge capacity (initial capacity) of the first cycle and the discharge capacity of the 200th cycle were then measured. The capacity retention rate (%) of each test cell was determined based on the following formula (C).

[0092] Capacity retention rate (%)

[0093] = (Discharge capacity in the 200th cycle / Initial capacity) × 100 ... Formula (C)

[0094] The results are shown in this column of Table 1.

[0095]

[0096] As shown in Table 1, the test cells of Examples 1-5 contained first and second silicon particles as negative electrode active materials. In the test cells of Examples 1-5, the average sphericity CR1 of the first particle was 0.5-0.9, the average sphericity CR2 of the second particle was greater than 0.9, and the ratio (D2 / D1) of the average particle size D1 of the first particle to the average particle size D2 of the second particle was 0.1 or more and less than 1. This indicates that the increase rate of specific surface area before and after pressing in the negative electrode sheet used in the test cells of Examples 1-5 was lower than that in the comparative examples. It can be seen that the negative electrode sheet used in the test cells of Examples 1-5 is less affected by pressing than the negative electrode sheet of Comparative Examples 1-3, and achieves a more desirable specific surface area.

[0097] As described above, the following items can be cited as specific examples of the technology disclosed herein.

[0098] Item 1:

[0099] An energy storage device comprising a negative electrode active material layer containing a negative electrode active material.

[0100] The above-mentioned negative electrode active material includes:

[0101] The first particle containing silicon, and

[0102] A second particle containing silicon and different from the first particle mentioned above;

[0103] in,

[0104] The average roundness CR1 of the first particle mentioned above is 0.5 to 0.9.

[0105] The average roundness CR2 of the second particle mentioned above is greater than 0.9.

[0106] The ratio (D2 / D1) of the average particle size D1 of the first particle to the average particle size D2 of the second particle is greater than 0.1 and less than 1.

[0107] Item 2:

[0108] According to the energy storage device of item 1, the ratio (C2 / C1) of the content of the first particle C1 to the content of the second particle C2 in the negative electrode active material is 0.5 or less.

[0109] Item 3:

[0110] According to the energy storage device described in item 1 or 2, the ratio (D2 / D1) of the average particle size D1 to the average particle size D2 of the second particle is 0.5 or less.

[0111] Item 4:

[0112] The energy storage device according to any one of items 1 to 3, wherein the average particle size D2 is 1 μm to 5 μm.

[0113] Item 5:

[0114] The energy storage device according to any one of items 1 to 4, wherein the first particle and the second particle are both composite particles of silicon and carbon.

[0115] Item 6:

[0116] The energy storage device according to any one of claims 1 to 5, wherein the aforementioned negative electrode active material layer further comprises graphite particles.

[0117] The embodiments of the technology disclosed herein have been described above, but it is not intended to limit the technology disclosed herein to the above embodiments. The technology disclosed herein can also be implemented in other embodiments. The technology described in the scope of the patent claim includes technology obtained by various modifications and alterations to the embodiments illustrated above. For example, a part of the above embodiments may be replaced with other modifications, or other modifications may be added to the above embodiments. In addition, if a technical feature is not described as an essential feature, it may be appropriately deleted.

Claims

1. An electric storage device comprising a negative electrode active material layer containing a negative electrode active material, The negative electrode active material comprises: a first particle comprising silicon, and a second particle containing silicon and different from the first particle; in, The average circularity CR1 of the first particles is 0.5 to 0.9, The average circularity CR2 of the second particles is greater than 0.9, The ratio of the average particle size D2 of the second particles to the average particle size D1 of the first particles, that is, D2 / D1, is greater than or equal to 0.1 and less than 1.

2. The power storage device according to claim 1, wherein The ratio of the content C2 of the second particles to the content C1 of the first particles in the negative electrode active material, namely C2 / C1, is 0.5 or less.

3. The power storage device according to claim 2, wherein: The ratio of the average particle diameter D2 of the second particles to the average particle diameter D1, that is, D2 / D1, is 0.5 or less.

4. The power storage device according to claim 1, wherein The average particle size D2 is 1 μm to 5 μm.

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

6. The power storage device according to any one of claims 1 to 5, wherein The negative electrode active material layer further includes graphite particles.

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