JP7731386B2 - secondary battery - Google Patents

Description

The present invention relates to a secondary battery.

Secondary batteries such as lithium-ion secondary batteries are suitable for use as portable power sources for personal computers, mobile devices, and other devices, as well as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The negative electrodes used in such secondary batteries generally have a configuration in which a negative electrode active material layer containing a negative electrode active material is disposed on a negative electrode current collector.

In recent years, the use of Si-based materials as negative electrode active materials has been investigated with the aim of increasing the capacity of secondary batteries (see, for example, Patent Documents 1 and 2). Patent Document 1 discloses a negative electrode active material containing a first active material powder made of Si-based material and a second active material powder made of plate-like graphite particles. Patent Document 2 discloses a secondary battery in which the negative electrode contains carbon particles containing multiple types of graphite particles and an Si-based material, and the particle size distribution of the multiple types of graphite particles is adjusted.

Patent No. 6385749 Patent No. 6596815

Si-based materials have a higher specific capacity than carbon materials such as graphite particles, but they also expand and contract significantly during charging and discharging, making their conductive paths more susceptible to breakage. Therefore, while the use of Si-based materials can increase capacity, the cycle characteristics of the secondary battery are likely to deteriorate. In particular, when the content of Si-based materials is increased to increase capacity, the cycle characteristics of the secondary battery deteriorate significantly. Therefore, there is still room for improvement in improving the cycle characteristics of secondary batteries when Si-based materials are used as the negative electrode active material.

The present invention was made in consideration of these points, and aims to provide a secondary battery equipped with a negative electrode containing a Si-based material as the negative electrode active material, which has excellent cycle characteristics.

The secondary battery disclosed herein is a secondary battery equipped with an electrode assembly having a positive electrode and a negative electrode. The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer contains graphite particles and Si-containing particles as negative electrode active materials. The Si-containing particles include first Si-containing particles and second Si-containing particles. The compressive modulus of the first Si-containing particles is smaller than the compressive modulus of the second Si-containing particles, and the compressive modulus of the first Si-containing particles is 250 MPa or more and 2000 MPa or less. In the secondary battery, the weight ratio of the first Si-containing particles to the second Si-containing particles is 50:50 to 90:10.

The first Si-containing particles, which have a low compressive modulus, are easily deformed in response to expansion and contraction during charging and discharging, and are able to alleviate the stress of such expansion and contraction. On the other hand, the second Si-containing particles, which have a high compressive modulus, are less likely to deform during charging and discharging, and are able to maintain a consistent conductive path like a wedge. By including Si-containing particles with different compressive moduli in a specified ratio, the conductive path is less likely to break even during repeated charging and discharging, improving the cycle characteristics of the secondary battery.

FIG. 1 is a diagram schematically illustrating the internal structure of a secondary battery according to one embodiment. FIG. 2 is a diagram schematically illustrating the configuration of an electrode assembly according to one embodiment. FIG. 3 is a diagram schematically illustrating a negative electrode according to one embodiment.

Embodiments of the technology disclosed herein are described below with reference to the drawings. Matters necessary for implementing the technology disclosed herein that are not specifically mentioned in this specification (for example, the general configuration and manufacturing process of secondary batteries that do not characterize the technology disclosed herein) can be understood as design matters for those skilled in the art based on conventional technology in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. The drawings are schematic, and dimensional relationships (length, width, thickness, etc.) do not necessarily reflect actual dimensional relationships. In the drawings described below, components and parts that perform the same function are designated by the same reference numerals, and redundant descriptions may be omitted or simplified. In this specification, the notation "A to B" (A and B are arbitrary numbers) indicating a range means A or greater and B or less.

In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged, and is a term that encompasses so-called storage batteries and electricity storage elements such as electric double-layer capacitors. Furthermore, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

Figure 1 is a diagram schematically illustrating the internal structure of a secondary battery 100 according to one embodiment. As shown in Figure 1, the secondary battery 100 includes an electrode assembly 20 having a positive electrode 50 and a negative electrode 60, a non-aqueous electrolyte (not shown), and a battery case 30 that houses the electrode assembly 20 and the non-aqueous electrolyte. The secondary battery 100 shown in Figure 1 is a lithium-ion secondary battery. The negative electrode 60 disclosed herein is preferably used as a negative electrode for a lithium-ion secondary battery.

The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 that is designed to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 also has an inlet (not shown) for injecting non-aqueous electrolyte. The positive terminal 42 is electrically connected to the positive current collector plate 42a. The negative terminal 44 is electrically connected to the negative current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

Figure 2 is a schematic diagram showing the configuration of the electrode assembly 20. Here, the electrode assembly 20 is a flat-shaped wound electrode assembly. As shown in Figure 2, the electrode assembly 20 has a configuration in which a long sheet-shaped positive electrode 50 (hereinafter also referred to as "positive electrode sheet 50") and a long sheet-shaped negative electrode 60 (hereinafter also referred to as "negative electrode sheet 60") are stacked with two long separators 70 interposed between them and wound longitudinally. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed longitudinally on one or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed longitudinally on one or both sides (both sides in this case) of a long negative electrode current collector 62. As shown in FIGS. 1 and 2, the positive electrode current collector exposed portion 52a (i.e., the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54) and the negative electrode current collector exposed portion 62a (i.e., the portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64) are formed to protrude outward from both ends of the electrode body 20 in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a are joined to the positive electrode current collector plate 42a and the negative electrode current collector plate 44a, respectively.

The positive electrode current collector 52 constituting the positive electrode sheet 50 may be any known positive electrode current collector used in lithium-ion secondary batteries, and is not particularly limited. Examples include sheets or foils made of metals with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52. The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on 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 to 35 μm, and preferably 7 μm to 20 μm.

The positive electrode active material layer 54 contains a positive electrode active material. The positive electrode active material may be a positive electrode active material of a known composition used in lithium ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds (e.g., lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ )). The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

Preferably, the lithium composite oxide is a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element. Specific examples include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These positive electrode active materials may be used alone or in combination of two or more. Of these, lithium nickel cobalt manganese composite oxide is preferably used as the positive electrode active material.

In this specification, the term "lithium nickel cobalt manganese composite oxide" encompasses oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a non-metal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

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

Although not particularly limited, the proportion of the conductive material is preferably 0.1 to 10 parts by weight, and more preferably 1 to 5 parts by weight, per 100 parts by weight of the positive electrode active material. Furthermore, the proportion of the binder is preferably 0.1 to 10 parts by weight, and more preferably 1 to 5 parts by weight, per 100 parts by weight of the positive electrode active material.

The thickness of each side of the positive electrode active material layer 54 is not particularly limited, but is, for example, 20 μm or more, and preferably 50 μm or more. On the other hand, the thickness is, for example, 300 μm or less, and preferably 200 μm or less.

The separator 70 can be made of various conventional microporous sheets, such as microporous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such microporous resin sheets may have a single-layer structure or a multi-layer 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). The separator 70 may also have a heat-resistant layer (HRL).

The nonaqueous electrolyte can be the same as conventional ones, and for example, a nonaqueous electrolyte solution containing a supporting salt in an organic solvent (nonaqueous solvent) can be used. As the nonaqueous solvent, aprotic solvents such as carbonates, esters, ethers, etc. can be used. Among them, carbonates, such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc., can be preferably used. Alternatively, fluorine-based solvents such as fluorinated carbonates, such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC), can be preferably used. Such nonaqueous solvents can be used alone or in appropriate combinations of two or more. As the supporting salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ , and LiClO ₄ can be preferably used. The concentration of the supporting electrolyte is not particularly limited, but is preferably about 0.7 mol/L or more and 1.3 mol/L or less.
The nonaqueous electrolyte may contain components other than the nonaqueous solvent and supporting salt described above, as long as the effects of the present technology are not significantly impaired. For example, the nonaqueous electrolyte may contain various additives such as a gas generating agent, a film-forming agent, a dispersant, and a thickener.

The negative electrode 60 of the secondary battery disclosed herein will now be described. Figure 3 is a schematic diagram showing the negative electrode 60 of the secondary battery 100 disclosed herein. As shown in Figure 3, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 disposed on the negative electrode current collector 62. The negative electrode current collector 62 may be any conventionally known material and is not particularly limited. Examples include sheets or foils made of metals such as copper, nickel, titanium, and stainless steel. When copper foil is used as the negative electrode current collector 62, its average thickness is not particularly limited, but is, for example, 5 μm to 30 μm, preferably 5 μm to 20 μm, and more preferably 5 μm to 15 μm.

The negative electrode active material layer 64 contains at least a negative electrode active material. The negative electrode active material layer 64 contains graphite particles 66 and Si-containing particles 68 as the negative electrode active material. The Si-containing particles 68 include first Si-containing particles 68a and second Si-containing particles 68b, which have different compressive moduli. The compressive moduli of the first Si-containing particles 68a are smaller than the compressive moduli of the second Si-containing particles 68b. The weight ratio of the first Si-containing particles 68a to the second Si-containing particles 68b is adjusted to 50:50 to 90:10. The higher the compressive modulus (also known as Young's modulus), the more resistant the material to deformation. In other words, the negative electrode 60 disclosed herein contains a predetermined ratio of easily deformable first Si-containing particles 68a and relatively less deformable second Si-containing particles 68b, thereby improving the cycle characteristics of the secondary battery 100.

While not intending to limit the technology disclosed herein, the reason for this effect is presumed to be as follows. Si-containing particles have a higher specific capacity than graphite particles, but also a higher expansion and contraction rate during charging and discharging. Therefore, repeated charging and discharging easily breaks the conductive path, resulting in a decline in cycle characteristics. The first Si-containing particles 68a, which have a low compressive modulus, easily deform in response to expansion and contraction during charging and discharging, thereby mitigating the stress of such expansion and contraction. Meanwhile, the second Si-containing particles 68b, which have a high compressive modulus, are less likely to deform during charging and discharging, allowing them to maintain a consistent conductive path like a wedge. By including the first Si-containing particles 68a, which have a low compressive modulus, and the second Si-containing particles 68b, which have a high compressive modulus, in a predetermined ratio, the first Si-containing particles 68a are presumed to be positioned so as to fill gaps in the conductive path maintained by the second Si-containing particles 68b. This makes the conductive path less likely to break, even with repeated charging and discharging, thereby favorably improving the cycle characteristics of the secondary battery 100.

Examples of graphite particles 66 that can be used include artificial graphite and natural graphite. Graphite particles 66 may have a coating layer of amorphous carbon on their surfaces. While not particularly limited, graphite particles 66 are preferably approximately spherical. In this specification, the term "approximately spherical" encompasses shapes such as spherical and rugby ball shapes, and refers to particles with an average aspect ratio (the ratio of the length in the superaxial direction to the length in the minor axis direction in the smallest rectangle circumscribing the particle) of, for example, 1 to 2 (preferably, 1 to 1.5).

The average particle diameter ( _D50 particle diameter) of the graphite particles 66 is not particularly limited, but is preferably 5 μm or more and 30 μm or less, and more preferably 10 μm or more and 25 μm or less. In this specification, the term "average particle diameter" refers to the particle diameter ( _D50 particle diameter) corresponding to a cumulative 50% from the fine particle side in a volume-based particle size distribution measured by particle size distribution measurement based on a laser diffraction/light scattering method.

Although not particularly limited, the compressive modulus of the graphite particles 66 is preferably 10 MPa or more and 250 MPa or less, and more preferably 50 MPa or more and 180 MPa or less. Graphite particles 66 with a compressive modulus within this range can deform favorably in response to expansion and contraction. Note that, in this specification, "compressive modulus" can be measured using a micro-compression testing machine in an environment of 25°C. First, a single particle is placed in the micro-compression testing machine and compressed vertically (in the direction of gravity). The compressive displacement and compressive stress are measured. The measured compressive displacement is then divided by the average particle diameter of the particles (compressive displacement/average particle diameter) to calculate the compressive strain. The compressive modulus is then calculated by dividing the compressive stress by the compressive strain (compressive stress/compressive strain). The compressive modulus is calculated in the same manner for multiple particles (e.g., 5 to 10 particles) with approximately the same average particle diameter, and the average of these values is used as the compressive modulus.

The Si-containing particles 68 are not particularly limited as long as they contain Si. As long as they contain Si, the Si-containing particles 68 may contain components other than Si. Examples of Si-containing particles 68 include SiOx, Si-C composites, and porous particles with nano-Si particles dispersed within them. Among these, Si-C composites are preferably used as the Si-containing particles 68. Si-C composites are particles containing at least Si and C. Si-C composites can be formed, for example, by supporting Si metal, Si oxide, or the like on a carbon material (graphite, amorphous carbon, etc.).

Although not particularly limited, the weight ratio of the graphite particles 66 to the Si-containing particles 68 is preferably adjusted to be between 90:10 and 40:60, and may be adjusted to be between 90:10 and 60:40. By adjusting the weight ratio of the graphite particles 66 to the Si-containing particles 68 to be within the above range, it is possible to achieve both high capacity and improved cycle characteristics.

As described above, the Si-containing particles 68 include first Si-containing particles 68a and second Si-containing particles 68b, which have different compressive moduli. The compressive moduli of the first Si-containing particles 68a are not particularly limited, as long as they are set to be smaller than the compressive moduli of the second Si-containing particles 68b. The compressive moduli of the first Si-containing particles 68a are preferably, for example, 2000 MPa or less, more preferably 1500 MPa or less, even more preferably 1200 MPa or less, and may even be 1000 MPa or less. First Si-containing particles 68a having such compressive moduli can suitably adapt to expansion and contraction associated with charging and discharging. This can alleviate stress caused by expansion and contraction during charging and discharging, and prevent disconnection of the conductive paths within the negative electrode active material layer. The lower limit of the compressive moduli of the first Si-containing particles 68a is not particularly limited, but is preferably, for example, 250 MPa or more. The compressive elastic modulus of the first Si-containing particles 68a is preferably, for example, 250 MPa or more and 2000 MPa or less, and more preferably 250 MPa or more and 1500 MPa or less.

A Si-C composite is preferably used as the first Si-containing particle 68a. The first Si-containing particle 68a is preferably a Si-C composite in which Si particles smaller than the carbon particles are dispersed within carbon particles having an amorphous carbon coating on the surface. C domains and Si domains exist within such Si-C composite particles, and the average diameter of the Si domains observed with a transmission electron microscope (TEM) is preferably 50 nm or less. Although not particularly limited, the average diameter of the Si domains can be, for example, 5 nm or more. The average diameter of the Si domains refers to the arithmetic mean of the diameters of at least 10 Si domains.

Although not particularly limited, when the first Si-containing particles 68a are Si-C composites as described above, the oxygen content of the Si-C composites may be, for example, 7 wt% or less, when the entire Si-C composite is taken as 100 wt%. The oxygen content can be measured using an oxygen analyzer.

The first Si-containing particles 68a may have voids therein. The porosity of the first Si-containing particles 68a is preferably, for example, 5 vol% or more. There is no particular upper limit to the porosity, but it is preferably, for example, 60 vol% or less. The "porosity" can be calculated based on the formula: porosity (%) = 1 - bulk density of the first Si-containing particles / true density of the first Si-containing particles × 100.

The first Si-containing particles 68a may have any shape, as long as their compressive modulus is adjusted to fall within the above-mentioned range. For example, the first Si-containing particles 68a may have a substantially spherical shape. The average particle diameter ( _D50 particle diameter) of the first Si-containing particles 68a is not particularly limited, but is preferably 1 μm or more and 15 μm or less, and more preferably 2 μm or more and 10 μm or less.

The compressive modulus of the second Si-containing particles 68b is not particularly limited, as long as it is set to be greater than the compressive modulus of the first Si-containing particles 68a. A Si-C composite containing at least Si and C is preferably used as the second Si-containing particles 68b. The compressive modulus of the second Si-containing particles 68b is preferably, for example, greater than 2000 MPa, more preferably 2500 MPa or greater, and even more preferably 3500 MPa or greater. Second Si-containing particles 68b with such a compressive modulus are less likely to deform during expansion and contraction associated with charging and discharging, and can maintain a consistent conductive path. By appropriately arranging the easily deformable first Si-containing particles 68a between the second Si-containing particles 68b while maintaining a consistent conductive path, the conductive path is less likely to be broken, improving cycle characteristics. The upper limit of the compressive modulus of the second Si-containing particles 68b is not particularly limited, but is preferably, for example, 5000 MPa or less. The compressive elastic modulus of the second Si-containing particles 68b is preferably greater than 2000 MPa and less than 5000 MPa, and more preferably greater than 2500 MPa and less than 5000 MPa.

The second Si-containing particles 68b may have any shape, as long as their compressive modulus is adjusted to fall within the above-mentioned range. For example, the second Si-containing particles 68b may have a substantially spherical shape. The average particle diameter ( _D50 particle diameter) of the second Si-containing particles 68b is not particularly limited, but is preferably 1 μm or more and 15 μm or less, and more preferably 2 μm or more and 10 μm or less.

The compressive modulus of the first Si-containing particles 68a and the second Si-containing particles 68b can be adjusted as appropriate, for example, by adjusting the porosity, surface coating, type of carbon particles, etc. Alternatively, the first Si-containing particles 68a and the second Si-containing particles 68b may be prepared by purchasing commercially available Si-containing particles whose compressive modulus falls within the above-mentioned range.

The negative electrode 60 disclosed herein contains first Si-containing particles 68a and second Si-containing particles 68b in a weight ratio of 50:50 to 90:10. More preferably, the content of the first Si-containing particles 68a is greater than the content of the second Si-containing particles 68b. This allows the first Si-containing particles 68a to more effectively mitigate the stress caused by expansion and contraction. From this perspective, the weight ratio of the first Si-containing particles 68a to the second Si-containing particles 68b is more preferably 60:40 to 90:10, and even more preferably 65:35 to 90:10.

Although not particularly limited, it is preferable that the second Si-containing particles 68b be arranged adjacent to the first Si-containing particles 68a and/or graphite particles 66 in the negative electrode active material layer 64. This more effectively relieves stress caused by expansion and contraction, making the conductive paths less likely to be broken.

The negative electrode active material layer 64 may contain Si-containing particles (third Si-containing particles) other than the first Si-containing particles 68a and the second Si-containing particles 68b described above, as long as the effects of the present technology are not significantly impaired. Examples of the third Si-containing particles include SiOx and porous particles in which nano-Si particles are dispersed.

The negative electrode active material layer 64 may contain components other than the above-described negative electrode active material (graphite particles 66 and Si-containing particles 68), such as a conductive material, a binder, etc. Conventional conductive materials can be used. Examples of conductive materials that can be used include carbon nanotubes such as single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT), carbon black such as acetylene black (AB), and carbon fiber. Among these, carbon nanotubes are preferred, and single-walled carbon nanotubes are more preferred. Using carbon nanotubes as a conductive material can more effectively maintain the conductive path and more effectively improve the cycle characteristics of the secondary battery 100.

The proportion of the conductive material may be, for example, 0.01 parts by weight or more, and 0.05 parts by weight or more, per 100 parts by weight of the negative electrode active material. Furthermore, the proportion of the conductive material may be 2 parts by weight or less, and may be 1 part by weight or less, 0.5 parts by weight or less, or 0.2 parts by weight or less, per 100 parts by weight of the negative electrode active material.

Conventional binders can be used. Examples of binders include carboxymethyl cellulose (CMC), polyacrylic acid (PAA), styrene butadiene rubber (SBR), and polyvinylidene fluoride (PVDF). Of these, CMC, PAA, and SBR are preferred. Although not particularly limited, it is more preferable to use a combination of CMC, PAA, and SBR.

The total proportion of the binder is, for example, 1 part by weight or more, preferably 3 parts by weight or more, and more preferably 3.5 parts by weight or more, based on 100 parts by weight of the negative electrode active material. The total proportion of the binder is, for example, 10 parts by weight or less, preferably 8 parts by weight or less, and more preferably 5 parts by weight or less, based on 100 parts by weight of the negative electrode active material.

The thickness of each surface of the negative electrode active material layer 64 is not particularly limited, but is, for example, 20 μm or more, and preferably 50 μm or more. On the other hand, the thickness is, for example, 300 μm or less, and preferably 200 μm or less.

Although not particularly limited, the proportion of the negative electrode active material in the entire negative electrode active material layer 64 is, for example, 80% by mass or more, preferably 90% by mass or more, and more preferably 95% by mass or more. Furthermore, although not particularly limited, the proportion of the negative electrode active material in the entire negative electrode active material layer 64 may be, for example, 98% by mass or less.

The negative electrode active material layer 64 can be formed by dispersing graphite particles 66 and Si-containing particles 68 as negative electrode active materials, as well as other materials (e.g., conductive materials and binders) used as needed, in an appropriate solvent (e.g., water) to prepare a paste (or slurry) composition, applying the composition to the surface of the negative electrode current collector 62, and drying it. The thickness and density of the negative electrode active material layer 64 can then be adjusted by pressing as needed.

The configuration of the negative electrode 60 and the secondary battery 100 according to one embodiment have been described above. The negative electrode 60 is suitable for use in nonaqueous electrolyte secondary batteries. Such a negative electrode 60 effectively prevents the conductive path from being broken due to expansion and contraction caused by repeated charge and discharge. This results in a secondary battery 100 with improved cycle characteristics. Such a secondary battery 100 can be used for a variety of purposes, and is suitable, for example, as a power source (driving power source) for motors mounted on vehicles such as passenger cars and trucks. The type of vehicle is not particularly limited, and examples include plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), and battery electric vehicles (BEVs). The secondary battery 100 can also be suitable for use in the construction of assembled batteries.

In addition, in the secondary battery 100 described above, a wound electrode body is exemplified as the electrode body 20, but this is not limited to this. For example, the electrode body may be a laminated electrode body, which is an electrode body in which multiple approximately rectangular positive electrodes and multiple approximately rectangular negative electrodes are alternately stacked with separators interposed between them.

Test examples related to the present invention are described below, but the present invention is not intended to be limited to those shown in the following test examples.

Example 1
First, graphite particles (compressive modulus: 180 MPa, _D50 particle diameter: 13 μm), first Si-containing particles (Si/C composite particles, compressive modulus: 1000 MPa, _D50 particle diameter: 8 μm), and second Si-containing particles (Si/C composite particles, compressive modulus: 3500 MPa, _D50 particle diameter: 7 μm) were prepared as negative electrode active materials. Furthermore, single-walled carbon nanotubes (SWCNTs) were prepared as conductive materials. Finally, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared as binders. These were kneaded with water as a solvent so as to have a weight ratio of graphite particles:first Si-containing particles:second Si-containing particles:SWCNT:CMC:PAA:SBR=60:32:8:0.1:1:1:1.5, thereby preparing a slurry for forming a negative electrode active material layer.

Specifically, the mixing and kneading of the slurry for forming the negative electrode active material layer was carried out as follows. First, graphite particles, first Si-containing particles, second Si-containing particles, CMC, and PAA were dry-mixed. Next, the dry-mixed powder mixture was kneaded with a paste of SWCNT (solid content 2%) and a dispersion medium. The solid content at the time of kneading was 61%. SBR and a solvent (water) were further added to the kneaded mixture and mixed. In this way, the slurry for forming the negative electrode active material layer was prepared. This slurry was applied in strips to both sides of copper foil (thickness 10 μm). The slurry on the copper foil was then dried, pressed to the specified thickness, and processed to the specified dimensions to produce a negative electrode sheet.

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

The prepared negative electrode sheet and positive electrode sheet were stacked with a separator interposed therebetween to produce a laminated electrode body. Current collecting leads were attached to the positive electrode sheet and negative electrode sheet, respectively, and the laminated electrode body was inserted into an exterior body composed of an aluminum laminate sheet. A nonaqueous electrolyte was injected into the interior of the exterior body, and the opening of the exterior body was sealed to produce the evaluation battery of Example 1. A porous polyolefin sheet having a three-layer structure of PP/PE/PP was used as the separator. The nonaqueous electrolyte was a mixed solvent of ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:FEC:EMC:DMC = 15:5:40:40, in which _LiPF6 was dissolved as a supporting electrolyte at a concentration of 1 mol/L.

Example 2
An evaluation battery of Example 2 was fabricated in the same manner as in Example 1, except that the first Si-containing particles had a compressive modulus of elasticity of 1200 MPa and the second Si-containing particles had a compressive modulus of elasticity of 3000 MPa.

Example 3
An evaluation battery of Example 3 was fabricated in the same manner as in Example 1, except that the compounding ratio of each material was changed to a weight ratio of graphite particles:first Si-containing particles:second Si-containing particles:SWCNT:CMC:PAA:SBR=60:26:14:0.1:1:1:1.5.

<Comparative Example 1>
An evaluation battery of Comparative Example 1 was fabricated in the same manner as in Example 1, except that the compounding ratio of each material was changed to a weight ratio of graphite particles:first Si-containing particles:second Si-containing particles:SWCNT:CMC:PAA:SBR=60:14:26:0.1:1:1:1.5.

<Comparative Example 2>
An evaluation battery for Comparative Example 2 was fabricated in the same manner as in Example 1, except that the blending ratio of each material was changed to a weight ratio of graphite particles:first Si-containing particles:second Si-containing particles:SWCNT:CMC:PAA:SBR=60:40:0:0.1:1:1:1.5. That is, Comparative Example 2 did not contain second Si-containing particles.

<Comparative Example 3>
An evaluation battery of Comparative Example 3 was fabricated in the same manner as in Example 1, except that the blending ratio of each material was changed to a weight ratio of graphite particles:first Si-containing particles:second Si-containing particles:SWCNT:CMC:PAA:SBR=60:0:40:0.1:1:1:1.5. That is, Comparative Example 3 did not contain the first Si-containing particles.

<Measurement of compressive elastic modulus>
The compressive modulus of the first Si-containing particles was measured as follows. The compressive modulus was measured using a microcompression tester (MCT-211, manufactured by Shimadzu Corporation) in an environment of 25°C. One first Si-containing particle was compressed in the vertical direction (gravity direction) at a constant compression rate (2.6 mN/sec), and the compressive displacement and compressive stress were measured. Next, the measured compressive displacement was divided by the average particle diameter ( _D50 particle diameter) of the first Si-containing particles (compressive displacement/average particle diameter) to calculate the compressive strain. The compressive stress was divided by the calculated compressive strain (compressive stress/compressive strain) to calculate the compressive modulus. Five first Si-containing particles with approximately the same average particle diameter were selected, and the compressive modulus was calculated as described above. The average value of the compressive modulus of the five particles was used as the value of the compressive modulus here. The compressive modulus of the second Si-containing particles and graphite particles was also calculated in the same manner.

<Evaluation of cycle capacity retention rate>
A cycle test was conducted in which 250 charge-discharge cycles were repeated in a 25°C environment, with CCCV charging (0.4C rate up to 4.2V, then 0.1C cut) followed by CC discharging (0.4C rate, 2.5V cut). The discharge capacity at the first cycle (initial capacity) and the discharge capacity at the 250th cycle were measured, and the cycle capacity retention rate was calculated using the following formula (1). The higher the cycle capacity retention rate, the better the cycle characteristics of the secondary battery. The results are shown in Table 1.
Cycle capacity retention rate (%)=((discharge capacity at 250th cycle)/(discharge capacity at 1st cycle))×100 Formula (1)

As shown in Table 1, the evaluation batteries of Examples 1 to 3 had a capacity retention rate of 85% or more. These results demonstrate that a secondary battery with excellent cycle characteristics is achieved by including graphite particles, first Si-containing particles, and second Si-containing particles as the negative electrode active material, the compressive modulus of the first Si-containing particles being smaller than the compressive modulus of the second Si-containing particles, the compressive modulus of the first Si-containing particles being 250 MPa or more and 2000 MPa or less, and the weight ratio of the first Si-containing particles to the second Si-containing particles being 50:50 to 90:10.

Although several embodiments of the present invention have been described above, these are merely examples. The present invention can be implemented in a variety of other forms. The present invention can be implemented based on the content disclosed in this specification and the common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and variations of the embodiments exemplified above. For example, it is possible to replace part of the above-described embodiments with other modified forms, and it is also possible to add other modified forms to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

As described above, specific aspects of the technology disclosed herein include those described in the following sections.
Item 1: A secondary battery including an electrode assembly having a positive electrode and a negative electrode, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer including, as a negative electrode active material, graphite particles and Si-containing particles, the Si-containing particles including first Si-containing particles and second Si-containing particles, the compressive modulus of the first Si-containing particles being smaller than the compressive modulus of the second Si-containing particles, the compressive modulus of the first Si-containing particles being 250 MPa or more and 2000 MPa or less, and the weight ratio of the first Si-containing particles to the second Si-containing particles being 50:50 to 90:10.
Item 2: The secondary battery according to Item 1, wherein the second Si-containing particles have a compressive elastic modulus of more than 2000 MPa and not more than 5000 MPa.
Item 3: The secondary battery according to Item 1 or 2, wherein the graphite particles have a compressive modulus of elasticity of 10 MPa or more and 250 MPa or less.
Item 4: The secondary battery according to any one of Items 1 to 3, wherein the weight ratio of the graphite particles to the Si-containing particles is 90:10 to 40:60.
Item 5: The secondary battery according to any one of Items 1 to 4, wherein the first Si-containing particles have a porosity of 5 vol % or more.

20 Electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector plate 44 Negative electrode terminal 44a Negative electrode current collector plate 50 Positive electrode (positive electrode sheet)
52 Positive electrode current collector 52a Positive electrode current collector exposed portion 54 Positive electrode active material layer 60 Negative electrode (negative electrode sheet)
62 Negative electrode current collector 62a Negative electrode current collector exposed portion 64 Negative electrode active material layer 66 Graphite particles 68 Si-containing particles 68a First Si-containing particles 68b Second Si-containing particles 70 Separator 100 Secondary battery

Claims (5)

A secondary battery including an electrode assembly having a positive electrode and a negative electrode,
the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector,
the negative electrode active material layer contains, as a negative electrode active material, graphite particles and Si-containing particles,
the Si-containing particles include first Si-containing particles and second Si-containing particles,
the compressive elastic modulus of the first Si-containing particles is smaller than the compressive elastic modulus of the second Si-containing particles;
the compressive elastic modulus of the first Si-containing particles is 250 MPa or more and 2000 MPa or less;
A secondary battery, wherein a weight ratio of the first Si-containing particles to the second Si-containing particles is 50:50 to 90:10. The secondary battery described in claim 1, wherein the compressive elastic modulus of the second Si-containing particles is greater than 2000 MPa and less than or equal to 5000 MPa. The secondary battery described in claim 1 or 2, wherein the compressive elastic modulus of the graphite particles is 10 MPa or more and 250 MPa or less. The secondary battery according to claim 1 or 2, wherein the weight ratio of the graphite particles to the Si-containing particles is 90:10 to 40:60. The secondary battery according to claim 1 , wherein the first Si-containing particles have a porosity of 5 vol % or more.