The negative electrode of a secondary battery, and a secondary battery using the said negative electrode

Abstract

The present invention provides a negative electrode containing Si-containing particles, in which the decrease in capacity retention rate is suppressed when repeated charging and discharging is performed. [Solution] The negative electrode of the secondary battery of this disclosure comprises a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer contains first Si-containing particles and second Si-containing particles. When the expansion rate S2 of the second Si-containing particles after charge A compared to before charge A is set to 1, the expansion rate S1 of the first Si-containing particles after charge A compared to before charge A is greater than 0.3 and less than or equal to 0.9. Charging A is a constant current charge performed at a current value of 0.01C up to 4.2V in a 25℃ environment, followed by constant voltage charging until the current value is 0.005C. When the average particle diameter (D50) of the first Si-containing particles is set to 1, the average particle diameter (D50) of the second Si-containing particles is greater than 0.1 and less than or equal to 1.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode of a secondary battery. The present disclosure also relates to a secondary battery using such a negative electrode.

Background Art

[0002] In recent years, secondary batteries have been suitably used for portable power sources such as personal computers and mobile terminals, and power sources for vehicle driving such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] In the application of power sources for vehicle driving, particularly in the application of power sources for driving BEVs, from the viewpoint of extending the cruising range of vehicles, further higher capacity of secondary batteries is desired. As a high-capacity negative electrode active material, Si-containing particles are known, and it is known that secondary batteries can be made to have a higher capacity according to the Si-containing particles. Patent Document 1 discloses a negative electrode used in a non-aqueous electrolyte secondary battery including a negative electrode, a positive electrode, and a lithium-ion conductive non-aqueous electrolyte. This negative electrode has a negative electrode active material composed of at least two types of composite particles containing elemental silicon and silicon compounds having different particle size distributions. The publication describes that with such a configuration, in a non-aqueous electrolyte secondary battery, the current collection property can be improved, the charge-discharge efficiency in the first charge-discharge can be increased, and good cycle characteristics with a high energy density can be realized.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, Si-containing particles have the property of expanding and contracting significantly during charging and discharging. Secondary batteries using Si-containing particles have the problem that the negative electrode expands and contracts repeatedly during repeated charging and discharging, leading to a decrease in cycle performance. Therefore, there is a need to develop a negative electrode that suppresses the decrease in capacity retention rate during repeated charging and discharging.

[0006] In view of the above circumstances, the present disclosure aims to provide a negative electrode containing Si-containing particles in which the decrease in capacity retention rate is suppressed when repeated charging and discharging is performed. [Means for solving the problem]

[0007] The negative electrode of the secondary battery of this disclosure comprises a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is supported by the negative electrode current collector. The negative electrode active material layer contains first Si-containing particles and second Si-containing particles. When the expansion rate S2 of the second Si-containing particles after charge A compared to before charge A is set to 1, the expansion rate S1 of the first Si-containing particles after charge A compared to before charge A is greater than 0.3 and less than or equal to 0.9. Charging A is a constant current charge performed at a current value of 0.01C up to 4.2V in a 25°C environment, followed by constant voltage charging until the current value is 0.005C. When the average particle diameter (D50) of the first Si-containing particles is set to 1, the average particle diameter (D50) of the second Si-containing particles is greater than 0.1 and less than or equal to 1. With this configuration, it is possible to suppress the decrease in capacity retention rate when charging and discharging is repeated.

[0008] From another perspective, the secondary battery of this disclosure comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode described above. This configuration makes it possible to suppress the decrease in capacity retention rate when repeated charging and discharging is performed. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic cross-sectional view showing the configuration of the negative electrode of a secondary battery according to one embodiment of the present disclosure. [Figure 2] Figure 1 is a schematic cross-sectional view showing the composition of particles contained in the negative electrode active material layer of the negative electrode. [Figure 3] This is a schematic cross-sectional view showing the configuration of a lithium-ion secondary battery constructed using the negative electrode of a secondary battery according to one embodiment of the present disclosure. [Figure 4] Figure 3 is a schematic exploded view showing the configuration of the wound electrode body of a lithium-ion secondary battery. [Modes for carrying out the invention]

[0010] Embodiments relating to this disclosure will be described below with reference to the drawings. Matters not mentioned herein but necessary for the implementation of this disclosure can be understood as design matters for those skilled in the art based on prior art in the relevant field. This disclosure can be implemented based on the contents disclosed herein and common technical knowledge in the relevant field. In the following drawings, members and parts that perform the same function are denoted by the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships. In this specification, the numerical range expressed as "A~B" includes A and B.

[0011] In this specification, "secondary battery" refers to an energy storage device that can be repeatedly charged and discharged. Furthermore, in this specification, "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and achieves charging and discharging through the transfer of charge associated with lithium ions between the positive and negative electrodes.

[0012] The negative electrode disclosed herein is used in secondary batteries, and preferably in lithium-ion secondary batteries. One embodiment of the negative electrode disclosed herein will be specifically described with reference to Figure 1. Figure 1 is a schematic cross-sectional view showing an example of the negative electrode 60 according to this embodiment, and is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to this embodiment shown in Figure 1 is the negative electrode of a lithium-ion secondary battery.

[0013] As shown in the figure, the negative electrode 60 comprises a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 comprises a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided on only one side of the negative electrode current collector 62, or it may be provided on both sides of the negative electrode current collector 62 as in the illustrated example. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62.

[0014] As shown in the illustrated example, a portion 62a without a negative electrode active material layer 64 may be provided at one end of the negative electrode 60 in the width direction. In the portion 62a without a negative electrode active material layer, the negative electrode current collector 62 is exposed, and the portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0015] The shape of the negative electrode current collector 62 is foil-like (or sheet-like) in the illustrated example, but is not limited thereto. The negative electrode current collector 62 may take various forms such as rod-like, plate-like, or mesh-like. As with conventional lithium-ion secondary batteries, a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.) can be used as the material for the negative electrode current collector 62, and copper is particularly preferred. Copper foil is especially preferred as the negative electrode current collector 62.

[0016] The dimensions of the negative electrode current collector 62 are not particularly limited and can be determined as appropriate according to the battery design. When copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 6 μm or more and 20 μm or less.

[0017] The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, for example, first Si-containing particles and second Si-containing particles are used. This will be described using FIG. 2. FIG. 2 is a schematic cross-sectional view showing the particles contained in the negative electrode active material layer 64 shown in FIG. 1. As shown in FIG. 2, the negative electrode active material layer 64 contains first Si-containing particles 12 and second Si-containing particles 14. Note that FIG. 2 is a schematic diagram, and thus the number, distribution, etc. of the particles are not limited to those shown in FIG. 2.

[0018] The first Si-containing particles 12 and the second Si-containing particles 14 may be, for example, particles of a Si-C composite material. The Si-C composite material has, for example, a carbon domain and a Si-containing domain. Note that the first Si-containing particles 12 and the second Si-containing particles 14 only need to contain Si and may not be a Si-C composite material. The first Si-containing particles 12 and the second Si-containing particles 14 may be, for example, Si particles, Si oxide particles, or the like.

[0019] The carbon domain is, for example, a carbonized product of a carbon precursor (e.g., petroleum pitch, coal pitch, phenol resin, etc.); graphite or the like. The carbon domain preferably constitutes a carbon matrix. Thus, the Si-C composite material is preferably a material in which Si-containing domains are dispersed in a carbon matrix. The Si-C composite material may have a plurality of Si-containing domains in the carbon matrix, for example. In this case, it is advantageous because the carbon matrix can relieve volume changes due to expansion and contraction of the Si-containing domains.

[0020] The Si-containing domain contains Si and is composed of, for example, Si, Si oxide (SiO x ), Si nitride (SiNx), Si carbide (SiCx), or the like. The Si-containing domain is preferably composed of at least one of Si and Si oxide (SiO x ). The Si-containing domain may be nanoparticles. The oxygen content in the Si-containing domain is preferably 10 mass% or less.

[0021] The average particle diameter of the Si-containing domains is, for example, 50 nm or less, and may be between 5 nm and 50 nm. The "average particle diameter of the Si-containing domains" can be determined as follows: First, the negative electrode active material layer 64 is processed with FIB (Focused Ion Beam) to prepare a sample for scanning transmission electron microscopy (STEM) observation. Then, the sample is subjected to elemental analysis by EDX elemental mapping, and BF images (bright-field images) and HAADF images (high-angle scattering annular dark-field images) are obtained. The diameter of the Si-containing domains can be determined from the contrast and shape obtained from the BF and HAADF images. The diameters of 10 or more arbitrarily selected Si-containing domains are determined, and their average value is taken as the "average particle diameter of the Si-containing domains" here.

[0022] A Si-C composite material includes, for example, a carbon material and Si-containing nanoparticles dispersed within the carbon material (hereinafter also referred to as "Si-containing nanoparticles"). From the viewpoint of suppressing volume changes of Si, the Si-C composite material preferably includes a carbon material having voids and Si-containing nanoparticles disposed within the voids, and more preferably includes a porous carbon material and Si-containing nanoparticles disposed within the voids of the porous carbon material. Alternatively, the Si-C composite material may include carbon particles and Si-containing nanoparticles attached to the surface of the carbon particles, or it may include Si-containing particles and carbon nanoparticles attached to the surface of the Si-containing particles.

[0023] When the expansion rate S2 of the second Si-containing particle 14 is set to 1, the expansion rate S1 of the first Si-containing particle 12 is, for example, greater than 0.3 and less than or equal to 0.9, and from the viewpoint of realizing the effects of the technology disclosed herein, 0.4 to 0.8 is preferred. Here, the expansion rate S1 and the expansion rate S2 refer to the volume expansion rate after charging A compared to before charging A. The expansion rate S1 and the expansion rate S2 can be appropriately changed, for example, by changing the Si content ratio in the first Si-containing particle 12 and the second Si-containing particle 14, the amount of voids in the carbon material, etc.

[0024] Charging A, in this context, is a constant current charge performed at a current value of 0.01C up to 4.2V in a 25°C environment, followed by constant voltage charging until the current value becomes 0.005C. Charging A is performed on an evaluation secondary battery (e.g., a lithium-ion secondary battery) equipped with a negative electrode containing only first Si-containing particles or second Si-containing particles as the negative electrode active material. When performing Charging A, it is preferable that the evaluation secondary battery is in a state of charge (SOC) of 0%. Although not particularly limited, it is preferable that Charging A is performed, for example, before the initial charging of the evaluation secondary battery. Such an evaluation secondary battery may have the same configuration as lithium-ion secondary battery 100 (see Figure 3), except that the negative electrode active material layer contains only first Si-containing particles or second Si-containing particles as the negative electrode active material.

[0025] The expansion coefficient S1 can be determined, for example, as follows. First, a cross-section along the thickness direction of the negative electrode active material layer containing only first Si-containing particles as the negative electrode active material is processed for observation with an electron microscope (SEM). This processing is carried out using a cross polisher under the conditions of a voltage of 4kV to 6kV and a processing time of 6 to 8 hours. The voltage conditions and processing time can be appropriately changed depending on the thickness of the negative electrode active material layer and the type of negative electrode active material. Next, an SEM image of the processed surface is obtained. The observation magnification at this time should be set to, for example, 1000x to 4000x. Next, an evaluation secondary battery equipped with this negative electrode active material layer is constructed and charged A is performed on it. Next, the evaluation secondary battery is disassembled and the negative electrode is removed. Next, an SEM observation of the processed surface, for which an SEM image was obtained before charging A, is performed, and an SEM image after charging A is obtained. Then, using image analysis software (for example, "ImageJ"), the area P1 of 100 randomly selected first Si-containing particles is measured from the SEM observation image before charge A, and the area P2 of the same first Si-containing particles is measured from the SEM observation image after charge A. Then, for each particle, the following equation (A) is used: Expansion rate (%) = [√{(Area P2) / (Area P1)}] 3 ×100 (A) The expansion coefficient is calculated using [a specific method / tool]. The arithmetic mean of the expansion coefficients of the 100 particles obtained here is calculated and designated as the expansion coefficient S1 of the first Si-containing particle.

[0026] The expansion rate S1 is, for example, smaller than the expansion rate S2. The expansion rate S1 is, for example, 100% or more, preferably less than 200%, and more preferably less than 230%. Although not particularly limited, the expansion rate S1 may be 110% or more, 120% or more, 190% or less, or 180% or less.

[0027] To determine the expansion coefficient S2, first, a cross-section along the thickness direction of the negative electrode active material layer containing only secondary Si-containing particles is prepared for observation by SEM. The expansion coefficient S2 is determined, for example, by measuring the area Q1 of 100 random secondary Si-containing particles from the SEM observation image before charge A, and then measuring the area Q2 of the same secondary Si-containing particles from the SEM observation image after charge A. Then, for each particle, the following equation (B) is used: Expansion rate (%)=[√{(Area Q2) / (Area Q1)}] 3 ×100 (B) The expansion coefficient is calculated using [a specific method]. The arithmetic mean of the expansion coefficients of the 100 particles obtained here is calculated and designated as the expansion coefficient S2 of the second Si-containing particle. Note that the procedure for determining the expansion coefficient S2 may be the same as the procedure for determining the expansion coefficient S1. Therefore, the explanation of the procedure for determining the expansion coefficient S2 is omitted as appropriate.

[0028] The expansion rate S2 is, for example, greater than the expansion rate S1. The expansion rate S2 is generally 180% or more, for example 200% or more, and may be 420% or less, preferably 400% or less. Although not particularly limited, the expansion rate S2 may be 210% or more, or 380% or less, 360% or less, or 350% or less.

[0029] When the average particle diameter (D50) of the first Si-containing particle 12 is set to 1, the average particle diameter (D50) of the second Si-containing particle 14 is, for example, greater than 0.1 and less than or equal to 1, and from the viewpoint of realizing the effects of the technology disclosed herein, 0.3 to 1 or 0.3 to 0.9 is preferred. In this specification, "average particle diameter (D50)" means median diameter (D50), and refers to the particle size corresponding to a cumulative frequency of 50% by volume from the fine particle side in a volume-based particle size distribution based on the laser diffraction-scattering method. The average particle diameter (D50) can be determined using a commercially available laser diffraction-scattering particle size distribution analyzer or the like.

[0030] The average particle diameter (D50) of the first Si-containing particles 12 is, for example, the same as the average particle diameter (D50) of the second Si-containing particles 14, or larger than the average particle diameter (D50) of the second Si-containing particles 14. The average particle diameter (D50) of the first Si-containing particles 12 is, for example, 1 μm or more, and preferably 25 μm or less. Although not particularly limited, the average particle diameter (D50) of the first Si-containing particles 12 is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 7 μm or more. On the other hand, the average particle diameter (D50) of the first Si-containing particles 12 is preferably 20 μm or less, more preferably 15 μm or less, and even more preferably 13 μm or less.

[0031] The average particle diameter (D50) of the second Si-containing particles 14 is, for example, smaller than the average particle diameter (D50) of the first Si-containing particles 12. The average particle diameter (D50) of the second Si-containing particles 14 is, for example, 1 μm or more, and preferably 10 μm or less. Although not particularly limited, the average particle diameter (D50) of the second Si-containing particles 14 is preferably 2 μm or more, and more preferably 3 μm or more. On the other hand, the average particle diameter (D50) of the second Si-containing particles 14 is preferably 9 μm or less, and more preferably 8 μm or less.

[0032] The Si content in the first Si-containing particle 12 and the Si content in the second Si-containing particle 14 may be the same or different. While not particularly limited, it is preferable that the Si content in the first Si-containing particle 12 and the Si content in the second Si-containing particle 14 be approximately 20% by mass to 80% by mass.

[0033] The mass ratio of the first Si-containing particles 12 and the second Si-containing particles 14 in the negative electrode active material layer 64 is not particularly limited as long as the effects of the disclosed technology are obtained, and is, for example, 5:95 to 95:5. The mass ratio (first Si-containing particles 12: second Si-containing particles 14) is preferably 10:90 to 90:10, and more preferably 15:85 to 85:10.

[0034] The first Si-containing particles 12 and the second Si-containing particles 14 can be manufactured according to known methods. Various manufacturing methods for Si-C composite material particles are known (see, for example, Japanese Patent Publication No. 2015-38862, International Publication No. 2014 / 046144, and the prior art documents cited in said International Publication).

[0035] As shown in Figure 3, the negative electrode active material layer 64 may contain graphite particles 16 as the negative electrode active material, from the viewpoint of improving conductivity at the negative electrode 60. The graphite particles 16 are, for example, substantially free of Si. The Si content in the graphite particles 16 is generally 10% by mass or less, for example 7% by mass or less, preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less or 0.5% by mass or less, and the closer to 0% by mass, the better. The Si content can be calculated, for example, by conventionally known methods such as ICP analysis.

[0036] The graphite constituting the graphite particles 16 may be natural graphite or artificial graphite, or it may be amorphous carbon-coated graphite in which the graphite is coated with an amorphous carbon material.

[0037] The shape of the graphite particles 16 is not particularly limited and may be flaky, spherical, or the like. Preferably, the graphite particles 16 are spheroidized graphite particles. When the graphite particles 16 are spherical, the circularity of the graphite particles 16 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0038] In this specification, "circularity" refers to the ratio of the circumference of a perfect circle having the same area as the projected area of ​​the particle to the circumference of the particle projection image (i.e., circularity = circumference of a perfect circle having the same area as the projected area of ​​the particle / circumference of the particle projection image). Therefore, the closer the circularity is to 1, the closer the particle projection image is to a perfect circle, and the closer the particle is to a perfect sphere. Circularity can be determined, for example, by using a commercially available static automatic image analyzer to determine the circularity of 100 or more particles and calculating the average value.

[0039] The average particle size (D50) of the graphite particles 16 is not particularly limited. The average particle size (D50) of the graphite particles 16 is, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and even more preferably 12 μm to 20 μm.

[0040] The ratio of graphite particles 16 to the total of first Si-containing particles 12, second Si-containing particles 14, and graphite particles 16 is preferably 40% to 90% by mass, more preferably 45% to 85% by mass, and even more preferably 50% to 80% by mass. The ratio of the total of first Si-containing particles 12 and second Si-containing particles 14 to the total of first Si-containing particles 12, second Si-containing particles 14, and graphite particles 16 is preferably 10% to 60% by mass, more preferably 15% to 55% by mass, and even more preferably 20% to 50% by mass.

[0041] The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as binders and conductive materials. Examples of binders include styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a thickening agent. Examples of conductive materials include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are particularly preferred. When CNTs are used as the conductive material, the negative electrode active material layer 64 may contain a CNT dispersant.

[0042] The content of the negative electrode active material in the negative electrode active material layer 64 (i.e., relative to the total mass of the negative electrode active material layer 64) is preferably 90% by mass or more, and more preferably 95% by mass or more. The content of the binder in the negative electrode active material layer 64 is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, and more preferably 0.05% by mass or more and 1% by mass or less.

[0043] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm or more and 400 μm or less, and preferably 20 μm or more and 300 μm or less.

[0044] The negative electrode 60 may include components other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) adjacent to the negative electrode active material layer 64 may be provided on the portion 62a where the negative electrode active material layer is not formed. This insulating layer may contain, for example, an insulating inorganic filler.

[0045] The negative electrode 60 can be suitably manufactured by a manufacturing method comprising the steps of: preparing a negative electrode mixture paste containing first Si-containing particles 12, second Si-containing particles 14, graphite particles 16 as needed, and a dispersion medium (hereinafter also referred to as the "paste preparation step"); coating the prepared negative electrode mixture paste onto the negative electrode current collector 62 (hereinafter also referred to as the "coating step"); drying the coated negative electrode mixture paste to form a negative electrode active material layer 64 (hereinafter also referred to as the "drying step"); and pressing the negative electrode active material layer 64 (hereinafter also referred to as the "pressing step").

[0046] In this specification, "paste" refers to a mixture in which some or all of the solid components are dispersed in a dispersion medium, and includes so-called "slurry," "ink," etc.

[0047] The paste preparation process can be carried out by mixing the first Si-containing particles 12, the second Si-containing particles 14, graphite particles 16 if necessary, and optional components (e.g., binder, conductive material, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method.

[0048] The coating process can be carried out according to known methods. Specifically, for example, the coating process can be carried out by coating the obtained negative electrode mixture paste onto the negative electrode current collector 62 using a coating device such as a gravure coater, comma coater, slit coater, or die coater.

[0049] The drying process can be carried out according to a known method. For example, the negative electrode active material layer 64 can be formed by removing the dispersion medium from the negative electrode current collector 62 coated with the negative electrode mixture paste using a drying apparatus such as a drying oven. This allows the drying process to be carried out. The drying temperature and drying time can be appropriately determined according to the solid content concentration of the negative electrode paste and are not particularly limited. The drying temperature is, for example, 60°C to 200°C, preferably 70°C to 150°C. The drying time is, for example, 10 seconds to 30 minutes, preferably 30 seconds to 10 minutes.

[0050] The pressing process can be carried out according to a known method. Specifically, the pressing process can be performed by applying pressure to the formed negative electrode active material layer 64 using a roller press or the like. The pressing process compresses the negative electrode active material layer 64 to a predetermined density, thereby densely packing the first Si-containing particles 12, the second Si-containing particles 14, and the graphite particles 16. In this way, the negative electrode 60 is obtained.

[0051] As described above, the negative electrode 60 for the secondary battery comprises a negative electrode current collector 62 and a negative electrode active material layer 64. The negative electrode active material layer 64 is supported by the negative electrode current collector 62. The negative electrode active material layer 64 contains first Si-containing particles 12 and second Si-containing particles 14. When the expansion rate S2 of the second Si-containing particles 14 after charging A compared to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles 12 after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.9. Charging A is a constant current charge performed at a current value of 0.01C up to 4.2V in a 25℃ environment, followed by constant voltage charging until the current value is 0.005C. When the average particle diameter (D50) of the first Si-containing particles 12 is set to 1, the average particle diameter (D50) of the second Si-containing particles 14 is greater than 0.1 and less than or equal to 1.

[0052] In other words, in the negative electrode 60, the negative electrode active material layer 64 contains first Si-containing particles 12 having a relatively small expansion rate and the same or relatively large average particle diameter (D50), and second Si-containing particles 14 having a relatively large expansion rate and the same or relatively small average particle diameter (D50). By including two types of Si-containing particles with the same or mutually different average particle diameters (D50) in the negative electrode active material layer 64, the packing is improved and the conductive path during charging and discharging is maintained in a favorable state. Furthermore, in the negative electrode 60, with respect to the Si-containing particles contained in the negative electrode active material layer 64, the expansion rate of the first Si-containing particles 12 having the same or relatively large average particle diameter (D50) is set to be relatively small, while the expansion rate of the second Si-containing particles 14 having the same or relatively small average particle diameter (D50) is set to be relatively large. This suppresses the movement of Si-containing particles during charging and discharging, thereby maintaining the conductive path in an optimal state during charging and discharging, and consequently suppressing the decrease in capacity retention rate when charging and discharging is repeated.

[0053] The expansion rate S1 of the first Si-containing particles 12 may be 100% or more and less than 230%. This sets the expansion rate of the second Si-containing particles 14 within a desirable range, thereby suppressing excessive expansion and contraction of the negative electrode active material layer 64 during charging and discharging.

[0054] The expansion rate S2 of the second Si-containing particles 14 may be between 180% and 420%. This sets the expansion rate of the first Si-containing particles 12 within a favorable range. As a result, the movement of the negative electrode active material in the negative electrode active material layer 64 during charging and discharging can be more effectively suppressed.

[0055] The average particle diameter (D50) of the first Si-containing particles 12 may be between 1 μm and 25 μm. This sets the average particle diameter (D50) of the second Si-containing particles 14 to a preferred range. As a result, the second Si-containing particles 14 can more easily fill the gaps between particles, and the conductive path of the negative electrode active material layer 64 can be better maintained during charging and discharging.

[0056] The average particle diameter (D50) of the second Si-containing particles 14 may be between 1 μm and 10 μm. This sets the average particle diameter (D50) of the first Si-containing particles 12 to a preferred range. As a result, an appropriate gap can be created between the first Si-containing particles 12, allowing the second Si-containing particles 14 to be properly positioned and better maintaining the conductive path of the negative electrode active material layer 64 during charging and discharging.

[0057] The first Si-containing particles 12 and the second Si-containing particles 14 may each contain a carbon material having voids and Si-containing nanoparticles disposed within the voids. This allows the voids in the carbon material to mitigate the expansion of the Si-containing nanoparticles during charging. Therefore, the expansion rate S1 of the first Si-containing particles 12 and the expansion rate S2 of the second Si-containing particles 14 can be set to a suitable range.

[0058] The negative electrode active material layer 64 may further contain graphite particles 16 that are substantially free of Si. This can improve the conductivity of the negative electrode active material layer 64.

[0059] Therefore, from another perspective, the secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-described embodiment. Hereinafter, an embodiment of the secondary battery disclosed herein will be described with reference to Figures 3 and 4, using a lithium-ion secondary battery as an example. The following configuration example is a flat rectangular lithium-ion secondary battery having a flat wound electrode body and a flat battery case.

[0060] The lithium-ion secondary battery 100 shown in Figure 3 is a sealed lithium-ion secondary battery 100 constructed by housing a flat-shaped wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 is also provided with an inlet (not shown) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight metal material with good thermal conductivity such as aluminum is used.

[0061] As shown in Figures 3 and 4, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are superimposed on each other via two elongated separator sheets 70 and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated negative electrode current collector 62. The portion 52a where the positive electrode active material layer is not formed (i.e., the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed) and the portion 62a where the negative electrode active material layer is not formed (i.e., the portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed) are formed to protrude outward from both ends of the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode current collector plate 42a and the negative electrode current collector plate 44a are joined to the positive electrode active material layer not formed portion 52a and the negative electrode active material layer not formed portion 62a, respectively.

[0062] As the positive electrode current collector 52 constituting the positive electrode sheet 50, a known positive electrode current collector used in lithium-ion secondary batteries may be used, and examples include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0063] The dimensions of the positive electrode current collector 52 are not particularly limited and can be determined as appropriate according to the battery design. When aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 7 μm or more and 20 μm or less.

[0064] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known composition of positive electrode active material used in lithium-ion secondary batteries may be used. Specifically, for example, lithium composite oxides, lithium transition metal phosphate compounds, etc., can be used as the positive electrode active material. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, spinel structure, olivine structure, etc.

[0065] Preferably, lithium transition metal composite oxides are lithium transition metal composite oxides containing at least one of Ni, Co, and Mn as a transition metal element. Specific examples include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel-manganese-based composite oxides, lithium nickel-cobalt-manganese-based composite oxides, lithium nickel-cobalt-aluminum-based composite oxides, and lithium iron-nickel-manganese-based composite oxides.

[0066] In this specification, "lithium nickel cobalt manganese composite oxide" is a term that encompasses not only oxides whose constituent elements are Li, Ni, Co, Mn, and O, but also oxides that contain one or more additive elements other than these. Examples of such additive elements include transition metal elements and main group metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additive elements may also be metalloid elements such as B, C, Si, and P, or nonmetallic elements such as S, F, Cl, Br, and I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, lithium iron nickel manganese composite oxide, etc.

[0067] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0068] These positive electrode active materials may be used individually or in combination of two or more. Lithium nickel cobalt manganese composite oxides are particularly preferred as positive electrode active materials due to their excellent properties, such as initial resistance characteristics.

[0069] The average particle size (D50) of the positive electrode active material is not particularly limited, but is, for example, 0.05 μm or more and 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

[0070] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, conductive material, binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB); carbon fibers such as vapor-processed carbon fiber (VGCF) and carbon nanotubes (CNT); and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVdF), etc.

[0071] The content of positive electrode active material in the positive electrode active material layer 54 (i.e., the content of positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more and 99% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 15% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less. The content of conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.3% by mass or more and 15% by mass or less. The content of binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.4% by mass or more and 15% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less.

[0072] The thickness of the positive electrode active material layer 54 per side is not particularly limited, but is usually 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is usually 400 μm or less, and preferably 300 μm or less.

[0073] The negative electrode sheet 60 used is the negative electrode 60 described above.

[0074] Examples of the separator 70 include porous sheets (films) made of resins 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 the separator 70.

[0075] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm. The air permeability obtained by the Gurley test method of the separator 70 is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0076] A non-aqueous electrolyte contains, for example, a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents commonly used in lithium-ion secondary battery electrolytes, such as carbonates, ethers, esters, nitriles, sulfones, and lactones, can be used without particular limitation. Among these, carbonates are preferred, and specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used individually or in appropriate combinations of two or more. As an example, the non-aqueous solvent may consist solely of carbonates. As another example, non-aqueous solvents include carbonates and esters such as methyl acetate.

[0077] Suitable supporting salts include lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI), preferably LiPF6. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0078] Furthermore, the above-mentioned non-aqueous electrolyte may contain components other than those described above, such as film-forming agents like vinylene carbonate (VC) and oxalat complexes; gas-generating agents like biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and other additives, as long as they do not significantly impair the effects of the present disclosure.

[0079] The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power supplies for vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium-ion secondary battery 100 can also be used as a storage battery for small-scale power storage devices. The lithium-ion secondary battery 100 can also be used in the form of a battery pack, for example, by connecting multiple batteries in series and / or parallel.

[0080] The above describes a rectangular lithium-ion secondary battery 100 equipped with a flattened wound electrode body 20 as an example. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries equipped with a stacked electrode body (i.e., an electrode body in which multiple positive electrodes and multiple negative electrodes are stacked alternately). Furthermore, lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, laminated case type lithium-ion secondary batteries, and the like.

[0081] Furthermore, according to known methods, the lithium-ion secondary battery 100 can also be configured as an all-solid-state lithium-ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte.

[0082] Furthermore, while the negative electrode 60 according to this embodiment is suitable as the negative electrode of a lithium-ion secondary battery, it can be constructed and used as the negative electrode of other secondary batteries, and other secondary batteries can be constructed according to known methods.

[0083] The following describes examples relating to this disclosure in detail, but this disclosure is not intended to be limited to those shown in these examples. In the following description, unless otherwise specified, "average particle size" refers to the "average particle size (D50)" described above.

[0084] <Fabrication of the negative electrode> [Example 1] As negative electrode active materials, we prepared a first particle, a second particle, and graphite particles. The first particle was a Si-C composite material with an expansion coefficient of 153% and an average particle diameter of 10 μm. The second particle was a Si-C composite material with an expansion coefficient of 261% and an average particle diameter of 4 μm. The graphite particles were 15 μm graphite particles. These graphite particles were substantially Si-free. As a conductive material, we prepared single-walled carbon nanotubes (SWCNTs). As binders, we prepared carboxymethylcellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR).

[0085] The expansion rate of the first particle was obtained by the following procedure. A negative electrode with the same configuration as in this example, except that only the first particle was used as the negative electrode active material, was fabricated using the same procedure as in this example. A cross-section along the thickness direction of this negative electrode was processed for observation by SEM, and an SEM image of the processed surface was obtained. The observation magnification at this time was 2500x. Next, a measurement battery, which is a lithium-ion secondary battery with the same configuration as in this example except that this negative electrode was used, was fabricated using the same procedure as in this example. The measurement battery was placed in a 25°C environment and charged with a constant current of 0.01C up to 4.2V, and then charged with a constant voltage until the current value was 0.005C. After that, the measurement battery was disassembled and the negative electrode was removed. Then, the processed surface, from which an SEM image was obtained before charging, was observed by SEM, and an SEM image was obtained after charging. Then, using the image analysis software "ImageJ", 100 first particles were randomly selected from the SEM image before charging, and the area A1 of the selected particles was measured. Next, the area A2 of the same first particle was measured in the SEM image after charging. Then, for each particle, the following equation (X): Expansion rate (%) = [√{(Area A2) / (Area A1)}×100] 3 (X) The expansion rate was calculated using [a specific method]. The arithmetic mean of the expansion rates of the 100 particles obtained was calculated and used as the expansion rate of the first particle. The expansion rate of the second particle was calculated using a similar procedure.

[0086] The average particle diameter of the first particle and the average particle diameter of the second particle were both particle diameters corresponding to the cumulative 50% from the finest particles in the volume-based particle size distribution measured by laser diffraction and light scattering.

[0087] The materials described above were mixed with water as a solvent to prepare a negative electrode mixture paste, with the weight ratio of graphite particles / first particle / second particle / SWCNT / CMC / PAA / SBR being 65 / 28 / 7 / 0.1 / 1 / 1 / 1.5.

[0088] The negative electrode mixture paste was prepared by carrying out the following two steps. In the first step, the first and second particles, paste-like SWCNT (solid content 2%), and a dispersion medium were first put into a kneader and dispersed and mixed at 3000 rpm using a disperser to prepare the first paste. In the second step, graphite particles, CMC, and PAA were dry mixed using a stirring granulator. The first paste, the mixed powder obtained by dry mixing, and the dispersion medium (water) were kneaded together. The solid content during kneading was 65%. SBR and the dispersion medium (water) were further added to the kneaded mixture and mixed. In this way, the negative electrode mixture paste was prepared.

[0089] The prepared negative electrode paste was applied to the surface of a 10 μm thick copper foil and dried to form a negative electrode active material layer. After roll pressing the negative electrode active material layer, the resulting sheet was processed to a predetermined size to obtain the negative electrode sheet of this example.

[0090] [Example 2] As the second particle, a Si-C composite material with an expansion coefficient of 342% and an average particle diameter of 3 μm was used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0091] [Example 3] As the first particle, a Si-C composite material with an expansion coefficient of 126% and an average particle diameter of 10 μm was used. As the second particle, a Si-C composite material with an expansion coefficient of 342% and an average particle diameter of 3 μm was used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0092] [Example 4] As the first particle, a Si-C composite material with an expansion coefficient of 173% and an average particle diameter of 9 μm was used. As the second particle, a Si-C composite material with an expansion coefficient of 219% and an average particle diameter of 8 μm was used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0093] [Example 5] As the second particle, a Si-C composite material with an expansion coefficient of 261% and an average particle diameter of 10 μm was used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0094] [Comparative Example 1] As the first particle, a Si-C composite material with an expansion coefficient of 153% and an average particle diameter of 6 μm was used. As the second particle, a Si-C composite material with an expansion coefficient of 261% and an average particle diameter of 10 μm was used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0095] [Comparative Example 2] As the first particle, a Si-C composite material with an expansion coefficient of 261% and an average particle diameter of 10 μm was used. As the second particle, a Si-C composite material with an expansion coefficient of 261% and an average particle diameter of 4 μm was used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0096] [Comparative Example 3] As the first particle, a Si-C composite material with an expansion coefficient of 153% and an average particle diameter of 3 μm was used. As the second particle, a Si-C composite material with an expansion coefficient of 261% and an average particle diameter of 10 μm was used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0097] [Comparative Example 4] As the first particle, a Si-C composite material with an expansion coefficient of 121% and an average particle diameter of 10 μm was used. As the second particle, a Si-C composite material with an expansion coefficient of 415% and an average particle diameter of 3 μm was used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0098] [Comparative Example 5] As the second particle, a Si-C composite material with an expansion coefficient of 261% and an average particle diameter of 1 μm was used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0099] [Comparative Example 6] As the negative electrode active material, the first particle and graphite particles were used, but the second particle was not used. Otherwise, the same materials and procedure as in Example 1 were used to obtain the negative electrode sheet of this example.

[0100] <Fabrication of lithium-ion secondary batteries for evaluation> LiNi as a positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode mixture paste was prepared by mixing O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder with N-methylpyrrolidone (NMP) in a mass ratio of NCM:AB:PVdF = 100:1:1. This paste was applied to the surface of a 15 μm thick aluminum foil and dried to form a positive electrode active material layer. After roll pressing the positive electrode active material layer, the resulting sheet was processed to a predetermined size to obtain a positive electrode sheet.

[0101] A separator made of porous polyolefin was prepared. Leads were attached to the negative electrode sheet and positive electrode sheet prepared above, and the electrodes were stacked via the separator to create an electrode body. This was then placed in an aluminum laminate film case along with a non-aqueous electrolyte. The non-aqueous electrolyte was a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 15:5:40:40, with LiPF6 dissolved as a supporting salt at a concentration of 1.0 mol / L. The case was then sealed to obtain a lithium-ion secondary battery for evaluation.

[0102] <Cycle Characteristics Evaluation> Each of the lithium-ion secondary batteries prepared for evaluation was placed in a 25°C environment. Each evaluation lithium-ion secondary battery was charged with a constant current of 0.4C up to 4.2V, and then charged with a constant voltage until the current value was 0.1C. Next, each evaluation lithium-ion secondary battery was discharged with a constant current of 0.4C up to 2.5V. The discharge capacity at this time was measured to determine the initial capacity.

[0103] The above charge-discharge cycle was repeated 200 times, with each cycle being considered one. The discharge capacity after 200 cycles was determined using the same method as the initial capacity. As an indicator of cycle characteristics, the capacity retention rate (%) was calculated using the formula: (discharge capacity after 200 charge-discharge cycles / initial capacity) × 100. The initial capacity was the discharge capacity of the first cycle of the above charge-discharge process. A capacity retention rate closer to 100% is considered good, and a rate of 80% or higher was evaluated as suppressing the decrease in capacity retention rate after charge-discharge cycles.

[0104] [Table 1]

[0105] As shown in Table 1, in Examples 1 to 4, when the expansion rate of the second particle is set to 1, the expansion rate of the first particle is greater than 0.3 and less than or equal to 0.9. When the average particle diameter of the first particle is set to 1, the average particle diameter of the second particle is greater than 0.1 and less than or equal to 1. Examples 1 to 4, which have a negative electrode with such a configuration, were shown to suppress the decrease in capacity retention rate after charge-discharge cycles.

[0106] The specific examples of this disclosure have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples described above.

[0107] In other words, the negative electrode of the secondary battery and the secondary battery of this disclosure are as described in the following items [1] to [8]. [1] Negative electrode current collector and The negative electrode active material layer supported by the negative electrode current collector, The negative electrode of a secondary battery, The negative electrode active material layer contains first Si-containing particles and second Si-containing particles. Under a 25°C environment, constant current charging is performed at a current of 0.01C up to 4.2V, followed by constant voltage charging until the current value reaches 0.005C. When the expansion rate S2 of the second Si-containing particles after charging A compared to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.9. The negative electrode is such that, when the average particle diameter (D50) of the first Si-containing particles is 1, the average particle diameter (D50) of the second Si-containing particles is greater than 0.1 and less than or equal to 1. [2] The negative electrode according to item [1], wherein the expansion coefficient S1 of the first Si-containing particle is 100% or more and less than 230%. [3] The negative electrode according to item [1] or [2], wherein the expansion coefficient S2 of the secondary Si-containing particles is 180% or more and 420% or less. [4] The negative electrode according to any one of items [1] to [3], wherein the average particle size (D50) of the first Si-containing particles is 1 μm or more and 25 μm or less. [5] The negative electrode according to any one of items [1] to [4], wherein the average particle diameter (D50) of the second Si-containing particles is 1 μm or more and 10 μm or less. [6] The negative electrode according to any one of the items [1] to [5], wherein the first Si-containing particles and the second Si-containing particles each comprise a carbon material having voids and nanoparticles containing Si disposed within the voids. [7] The negative electrode according to any one of the items [1] to [6], wherein the negative electrode active material layer further contains substantially Si-free graphite particles. [8] Positive electrode and, The negative electrode and, Electrolytes, A secondary battery equipped with, A secondary battery wherein the negative electrode is the negative electrode described in any one of items [1] to [7]. [Explanation of symbols]

[0108] 12 1st Si-containing particles 14 2nd Si-containing particles 16 Graphite Particles 20 Wound electrode body 30 Battery Cases 36 Safety valve 42 Positive terminal 42a Positive electrode current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheets (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheets (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator Sheets (Separators) 100 Lithium-ion rechargeable batteries

Claims

1. Negative electrode current collector and The negative electrode active material layer supported by the negative electrode current collector, The negative electrode of a secondary battery, The negative electrode active material layer contains first Si-containing particles and second Si-containing particles. Under a 25°C environment, constant current charging is performed at a current value of 0.01C up to 4.2V, followed by constant voltage charging until the current value reaches 0.005C. When the expansion rate S2 of the second Si-containing particles after charging A compared to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.

9. The negative electrode is such that, when the average particle diameter (D50) of the first Si-containing particles is 1, the average particle diameter (D50) of the second Si-containing particles is greater than 0.1 and less than or equal to 1.

2. The negative electrode according to claim 1, wherein the expansion coefficient S1 of the first Si-containing particle is 100% or more and less than 230%.

3. The negative electrode according to claim 2, wherein the expansion coefficient S2 of the second Si-containing particle is 180% or more and 420% or less.

4. The negative electrode according to any one of claims 1 to 3, wherein the average particle diameter (D50) of the first Si-containing particles is 1 μm or more and 25 μm or less.

5. The negative electrode according to claim 4, wherein the average particle diameter (D50) of the second Si-containing particles is 1 μm or more and 10 μm or less.

6. The negative electrode according to any one of claims 1 to 3, wherein the first Si-containing particles and the second Si-containing particles each comprise a carbon material having voids and nanoparticles containing Si disposed within the voids.

7. The negative electrode according to any one of claims 1 to 3, wherein the negative electrode active material layer further contains graphite particles that are substantially free of Si.

8. Positive electrode and, The negative electrode and, Electrolytes, A secondary battery equipped with, A secondary battery wherein the negative electrode is the negative electrode described in claim 1.

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