JP2024115207A - Secondary battery - Google Patents

Abstract

To provide a secondary battery which is equipped with an anode including a si-based material as an anode active material, and which has excellent cycle characteristics.SOLUTION: A secondary battery disclosed herein is equipped with an electrode body having a cathode and an anode 60. The anode 60 includes an anode collector 62 and an anode active material layer 64 which is located on the anode collector. The anode active material layer 64 contains a graphite particle 66 and a Si-containing particle 68 as anode active materials, and the Si-containing particle 68 contains a first Si-containing particle 68a and a second Si-containing particle 68b. The compressive modulus of the first Si-containing particle 68a is smaller than the compressive modulus of the second Si-containing particle 68b, and the compressive modulus of the first Si-containing particle 68a is 250 MPa to 2000 MPa inclusive. In such a second battery, the weight ratio of the first Si-containing particle 68a to the second Si-containing particle 68b is 50:50 to 90:10.SELECTED DRAWING: Figure 3

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 terminals, and the like, and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid vehicles (HEVs), and plug-in hybrid 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 considered in order to increase the capacity of secondary batteries (for example, Patent Documents 1 and 2). Patent Document 1 discloses a negative electrode active material that includes a first active material powder made of a 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 includes carbon particles containing multiple types of graphite particles and a 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 larger specific capacity than carbon materials such as graphite particles, but they expand and contract significantly during charging and discharging, and their conductive paths tend to be easily broken. For this reason, when Si-based materials are used, the capacity can be increased, but the cycle characteristics of the secondary battery are likely to deteriorate. In particular, when the content of Si-based materials is increased to increase the capacity, the deterioration of the cycle characteristics of the secondary battery is significant. 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 has been made in consideration of these points, and aims to provide a secondary battery having a negative electrode containing a Si-based material as the negative electrode active material, and having excellent cycle characteristics.

The secondary battery disclosed herein is a secondary battery equipped with an electrode body having a positive electrode and a negative electrode, and 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 includes 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, and the compressive elastic modulus of the first Si-containing particles is smaller than the compressive elastic modulus of the second Si-containing particles, and the compressive elastic 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 small compressive modulus, are easily deformed in response to expansion and contraction during charging and discharging, and can alleviate the stress of such expansion and contraction. On the other hand, the second Si-containing particles, which have a large compressive modulus, are less likely to deform during charging and discharging, and can maintain a constant conductive path like a wedge. By including a predetermined ratio of Si-containing particles with different compressive moduli, the conductive path is less likely to break even when charging and discharging are repeated, and the cycle characteristics of the secondary battery can be improved.

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

The following describes embodiments of the technology disclosed herein with reference to the drawings. Note that matters other than those specifically mentioned in this specification and necessary for implementing the technology disclosed herein (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 of a person skilled in the art based on the conventional technology in the field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense in the field. Note that each drawing is drawn diagrammatically, and the dimensional relationships (length, width, thickness, etc.) do not necessarily reflect the actual dimensional relationships. In addition, in the drawings described below, the same reference numerals are used for members and parts that perform the same function, and duplicated descriptions may be omitted or simplified. In addition, the notation "A to B" (A and B are arbitrary numbers) indicating a range in this specification means A or more 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 includes so-called storage batteries and electricity storage elements such as electric double-layer capacitors. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and realizes charging and discharging by the transfer of charge associated with lithium ions between the positive and negative electrodes.

Figure 1 is a diagram showing a schematic 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 contains the electrode assembly 20 and the non-aqueous electrolyte. The secondary battery 100 shown in Figure 1 is a lithium-ion secondary battery here. 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 set to release the internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or higher. The battery case 30 is also provided with an injection port (not shown) for injecting a non-aqueous electrolyte. The positive terminal 42 is electrically connected to the positive current collector 42a. The negative terminal 44 is electrically connected to the negative current collector 44a. The material of the battery case 30 is, for example, a lightweight metal material with good thermal conductivity, such as aluminum.

2 is a schematic diagram showing the configuration of the electrode body 20. Here, the electrode body 20 is a flat-shaped wound electrode body. As shown in FIG. 2, the electrode body 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 via two long separators 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 (both sides here) of a long positive electrode 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 (both sides here) of a long negative electrode collector 62. As shown in FIG. 1 and FIG. 2, the positive electrode collector exposed portion 52a (i.e., the portion where the positive electrode collector 52 is exposed without the positive electrode active material layer 54) and the negative electrode collector exposed portion 62a (i.e., the portion where the negative electrode collector 62 is exposed without the negative electrode active material layer 64) are formed so as 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 collector exposed portion 52a and the negative electrode collector exposed portion 62a are joined to the positive electrode collector plate 42a and the negative electrode collector plate 44a, respectively.

The positive electrode collector 52 constituting the positive electrode sheet 50 may be a known positive electrode collector used in lithium ion secondary batteries, and is not particularly limited. For example, a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.) can be used. Aluminum foil is preferable as the positive electrode collector 52. The dimensions of the positive electrode collector 52 are not particularly limited and may be appropriately determined according to the battery design. When aluminum foil is used as the positive electrode 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. As the positive electrode active material, a positive electrode active material having a known composition used in lithium ion secondary batteries may be used. Specifically, for example, as the positive electrode active material, a lithium composite oxide, a lithium transition metal phosphate compound (e.g., lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ )), or the like may be used. 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.

As the lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferable, and specific examples thereof 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. Among them, lithium nickel cobalt manganese composite oxide can be preferably used as the positive electrode active material.

In this specification, the term "lithium nickel cobalt manganese composite oxide" includes oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements other than those. 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 semimetal element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the above-mentioned 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, a binder, etc. As the conductive material, for example, 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, etc.) may be suitably used. As the binder, for example, polyvinylidene fluoride (PVdF) may be used.

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

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.

As the separator 70, various types of microporous sheets similar to those used conventionally can be used, 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 non-aqueous electrolyte can be the same as that used in the past, and for example, a non-aqueous electrolyte solution containing a supporting salt in an organic solvent (nonaqueous solvent) can be used. As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, etc. can be used. Among them, carbonates, for example, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. can be preferably adopted. 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 non-aqueous solvents can be used alone or in appropriate combination 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 salt is not particularly limited, but is preferably about 0.7 mol/L or more and 1.3 mol/L or less.
In addition, the nonaqueous electrolyte may contain components other than the nonaqueous solvent and supporting salt described above, so 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 dispersing agent, and a thickening agent.

The negative electrode 60 of the secondary battery disclosed herein will be described below. FIG. 3 is a schematic diagram showing the negative electrode 60 of the secondary battery 100 disclosed herein. As shown in FIG. 3, the negative electrode 60 includes a negative electrode collector 62 and a negative electrode active material layer 64 disposed on the negative electrode collector 62. The negative electrode collector 62 may be a conventionally known one, and is not particularly limited. For example, a sheet or foil made of a metal such as copper, nickel, titanium, or stainless steel may be used. When copper foil is used as the negative electrode collector 62, its average thickness is not particularly limited, but is, for example, 5 μm or more and 30 μm or less, preferably 5 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less.

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 contain first Si-containing particles 68a and second Si-containing particles 68b having different compressive elastic moduli, and the compressive elastic modulus of the first Si-containing particles 68a is smaller than that 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 larger the compressive elastic modulus (also called Young's modulus), the more difficult the material is to deform. In other words, the negative electrode 60 disclosed herein contains the easily deformable first Si-containing particles 68a and the relatively less deformable second Si-containing particles 68b in a predetermined ratio, thereby improving the cycle characteristics of the secondary battery 100.

Although there is no intention to limit the technology disclosed herein, the reason for obtaining such an effect is presumed to be as follows. Si-containing particles have a large specific capacity compared to graphite particles, but a high expansion and contraction rate due to charging and discharging. Therefore, repeated charging and discharging easily breaks the conductive path, and the cycle characteristics tend to deteriorate. Here, the first Si-containing particles 68a, which have a small compressive elastic modulus, are easily deformed in accordance with the expansion and contraction during charging and discharging, and can alleviate the stress of such expansion and contraction. On the other hand, the second Si-containing particles 68b, which have a large compressive elastic modulus, are difficult to deform during charging and discharging, and can maintain a constant conductive path like a wedge. It is presumed that the first Si-containing particles 68a, which have a small compressive elastic modulus, and the second Si-containing particles 68b, which have a large compressive elastic modulus, are contained in a predetermined ratio, and the first Si-containing particles 68a are arranged so as to fill the gaps in the conductive path maintained by the second Si-containing particles 68b. As a result, the conductive path is less likely to break even if charging and discharging are repeated, and the cycle characteristics of the secondary battery 100 can be suitably improved.

As the graphite particles 66, for example, artificial graphite, natural graphite, etc. are used. The graphite particles 66 may have a coating layer of amorphous carbon on their surfaces. Although not particularly limited, it is preferable that the graphite particles 66 are approximately spherical. In this specification, the term "approximately spherical" includes a spherical shape, a rugby ball shape, etc., and refers to a shape with an average aspect ratio (the ratio of the superaxial length to the short-axis length in the smallest rectangle circumscribing the particle) of, for example, 1 to 2 (preferably 1 to 1.5).

The average particle size ( _D50 particle size) 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 size" refers to the particle size ( _D50 particle size) corresponding to 50% cumulatively from the fine particle side in the 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. If the graphite particles 66 have a compressive modulus in this range, they can be deformed appropriately in accordance with expansion and contraction. In this specification, the "compressive modulus" can be measured using a micro-compression tester in an environment of 25°C. First, one particle is set in the micro-compression tester, and the particle is compressed in the vertical direction (gravity direction), and the compressive displacement and compressive stress at this time are measured. Next, the measured compressive displacement is divided by the average particle diameter of the particles (compressive displacement/average particle diameter) to calculate the compressive strain. The compressive stress is divided by the compressive strain (compressive stress/compressive strain) to calculate the compressive modulus. 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 value of these values is taken as the compressive modulus here.

The Si-containing particles 68 are not particularly limited as long as they are particles that contain Si. The Si-containing particles 68 may contain components other than Si, so long as they contain Si. Examples of the Si-containing particles 68 include SiOx, Si-C composites, and nano-Si particles dispersed in porous particles. Among these, the Si-containing particles 68 are preferably Si-C composites. The Si-C composites are particles that contain at least Si and C. The Si-C composites can be formed, for example, by supporting Si metal, Si oxide, etc. 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 90:10 to 40:60, and may be adjusted to 90:10 to 60:40. By adjusting the weight ratio of the graphite particles 66 to the Si-containing particles 68 to the above range, it is possible to favorably achieve both high capacity and improved cycle characteristics.

As described above, the Si-containing particles 68 include the first Si-containing particles 68a and the second Si-containing particles 68b, which have different compressive elastic moduli. The compressive elastic modulus of the first Si-containing particles 68a is not particularly limited as long as it is set to be smaller than the compressive elastic modulus of the second Si-containing particles 68b. The compressive elastic modulus of the first Si-containing particles 68a is preferably, for example, 2000 MPa or less, more preferably 1500 MPa or less, even more preferably 1200 MPa or less, and may be 1000 MPa or less. If the first Si-containing particles 68a have such a compressive elastic modulus, they can suitably follow the expansion and contraction associated with charging and discharging. This makes it possible to alleviate the stress of expansion and contraction due to charging and discharging, and to suppress the disconnection of the conductive path in the negative electrode active material layer. The lower limit of the compressive elastic modulus 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 particle 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.

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

Although not particularly limited, when the first Si-containing particle 68a is a Si-C composite as described above, the oxygen content of the Si-C composite 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 particle 68a may have voids therein. The porosity of the first Si-containing particle 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 particle / true density of the first Si-containing particle) x 100.

The first Si-containing particles 68a may have any shape, as long as the compressive elastic modulus is adjusted to be within the above 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 elastic modulus of the second Si-containing particle 68b is not particularly limited as long as it is set to be greater than the compressive elastic modulus of the first Si-containing particle 68a. As the second Si-containing particle 68b, a Si-C composite containing at least Si and C is preferably used. The compressive elastic modulus of the second Si-containing particle 68b is preferably, for example, more than 2000 MPa, more preferably 2500 MPa or more, and even more preferably 3500 MPa or more. If the second Si-containing particle 68b has such a compressive elastic modulus, it is difficult to deform during expansion and contraction accompanying charging and discharging, and a constant conductive path can be maintained. While the second Si-containing particle 68b maintains a constant conductive path, the first Si-containing particle 68a, which is easily deformed, is suitably arranged between the second Si-containing particles 68b, so that the conductive path is less likely to be broken, and the cycle characteristics are improved. The upper limit of the compressive elastic modulus of the second Si-containing particle 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, for example, 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 be adjusted so that the compressive elastic modulus is set within the above-mentioned range, and the outer shape is not particularly limited. For example, the second Si-containing particles 68b may be substantially spherical. 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 elastic modulus of the first Si-containing particle 68a and the second Si-containing particle 68b described above can be adjusted as appropriate, for example, by the porosity, the surface coating, the type of carbon particles, etc. Alternatively, the first Si-containing particle 68a and the second Si-containing particle 68b may be prepared by purchasing commercially available Si-containing particles whose compressive elastic modulus satisfies the above-mentioned range.

The negative electrode 60 disclosed herein contains the first Si-containing particles 68a and the 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 relieve the stress caused by expansion and contraction. From this perspective, it is more preferable that the weight ratio of the first Si-containing particles 68a and the second Si-containing particles 68b is 60:40 to 90:10, and even more preferably 65:35 to 90:10.

Although not particularly limited, in the negative electrode active material layer 64, the second Si-containing particles 68b are preferably arranged adjacent to the first Si-containing particles 68a and/or the graphite particles 66. This more effectively relieves the stress of expansion and contraction, making the conductive path 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 effect of the present technology is not significantly impaired. Examples of the third Si-containing particles include SiOx and nano Si particles dispersed in porous particles.

The negative electrode active material layer 64 may contain components other than the above-mentioned negative electrode active material (graphite particles 66 and Si-containing particles 68), such as a conductive material, a binder, etc. As the conductive material, a conventionally known material can be used. As the conductive material, for example, carbon nanotubes such as single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT), carbon black such as acetylene black (AB), carbon fibers, etc. can be used. Among these, carbon nanotubes are preferred, and single-walled carbon nanotubes are more preferred. By using carbon nanotubes as the conductive material, the conductive path can be more suitably maintained, and the cycle characteristics of the secondary battery 100 can be more suitably improved.

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 when the negative electrode active material is 100 parts by weight. The proportion of the conductive material may be, for example, 2 parts by weight or less and 1 part by weight or less, 0.5 parts by weight or less, or 0.2 parts by weight or less when the negative electrode active material is 100 parts by weight.

As the binder, a conventionally known binder can be used. Examples of binders include carboxymethyl cellulose (CMC), polyacrylic acid (PAA), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), etc. Among them, CMC, PAA, and SBR are preferably used. Although not particularly limited, it is more preferable to use CMC, PAA, and SBR in combination.

The proportion of the total 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, when the negative electrode active material is 100 parts by weight. The proportion of the total 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, when the negative electrode active material is 100 parts by weight.

The thickness of each side 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 mass% or more, preferably 90 mass% or more, and more preferably 95 mass% or more. In addition, 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 mass% or less.

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

The above describes the configuration of the negative electrode 60 and the configuration of the secondary battery 100 according to one embodiment. The negative electrode 60 is preferably used in a non-aqueous electrolyte secondary battery. The negative electrode 60 is preferably suppressed from breaking the conductive path due to expansion and contraction caused by repeated charging and discharging. Therefore, a secondary battery 100 with improved cycle characteristics is realized. The secondary battery 100 can be used for various purposes, and can be preferably used as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car or truck. The type of vehicle is not particularly limited, and examples include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and an electric vehicle (BEV). The secondary battery 100 can also be preferably used in the construction of a battery pack.

In addition, in the above-mentioned secondary battery 100, a wound electrode body is exemplified as the electrode body 20, but this is not limited thereto, and the electrode body 20 may be, for example, a laminated electrode body in which multiple approximately rectangular positive electrodes and multiple approximately rectangular negative electrodes are alternately stacked with separators interposed therebetween.

Test examples relating 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 size: 13 μm), first Si-containing particles (Si/C composite particles, compressive modulus: 1000 MPa, _D50 particle size: 8 μm), and second Si-containing particles (Si/C composite particles, compressive modulus: 3500 MPa, _D50 particle size: 7 μm) were prepared as negative electrode active materials. Single-walled carbon nanotubes (SWCNTs) were also prepared as conductive materials. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were also prepared as binders. These were mixed 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, the graphite particles, the first Si-containing particles, the second Si-containing particles, the CMC, and the PAA were dry-mixed. Next, the dry-mixed mixed powder, the SWCNT paste (solid content rate 2%), and the dispersion medium were kneaded and kneaded. The solid content rate during the kneading and kneading was 61%. SBR and a solvent (water) were further added to the kneaded and kneaded mixture and mixed. In this way, the slurry for forming the negative electrode active material layer was prepared. This slurry was applied in a strip shape to both sides of a copper foil (thickness 10 μm). Then, the slurry on the copper foil was dried, pressed to a predetermined thickness, and processed to a predetermined dimension 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 so that the weight ratio of NCM:AB:PVDF was 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 an aluminum foil (thickness 15 μm). The slurry on the aluminum foil was then dried, pressed to a specified thickness, and processed to the specified dimensions to produce a positive electrode sheet.

The negative electrode sheet and the positive electrode sheet prepared above were laminated with a separator interposed therebetween to prepare a laminated electrode body. A current collecting lead was attached to each of the positive electrode sheet and the negative electrode sheet, and the laminated electrode body was inserted into an exterior body made of an aluminum laminate sheet. A non-aqueous electrolyte was injected into the interior of the exterior body, and the opening of the exterior body was sealed to prepare an evaluation battery of Example 1. As the separator, a porous polyolefin sheet having a three-layer structure of PP/PE/PP was used. As the non-aqueous electrolyte, a mixed solvent in which ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed to a volume ratio of EC:FEC:EMC:DMC=15:5:40:40 was used, in which LiPF ₆ as a supporting salt was dissolved at a concentration of 1 mol/L.

Example 2
An evaluation battery of Example 2 was produced 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 produced in the same manner as in Example 1, except that the compounding ratio of each material was changed so that the weight ratio of graphite particles: first Si-containing particles: second Si-containing particles: SWCNT: CMC: PAA: SBR was 60: 26: 14: 0.1: 1: 1: 1.5.

<Comparative Example 1>
An evaluation battery for Comparative Example 1 was prepared 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>
A battery for evaluation of Comparative Example 2 was produced 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: 40: 0: 0.1: 1: 1: 1.5. That is, Comparative Example 2 does not contain the second Si-containing particles.

<Comparative Example 3>
A battery for evaluation of Comparative Example 3 was produced 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: 0: 40: 0.1: 1: 1: 1.5. That is, Comparative Example 3 does not contain the first Si-containing particles.

<Measurement of Compressive Elastic Modulus>
The compressive modulus of the first Si-containing particle 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 speed (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 (D ₅₀ particle diameter) of the first Si-containing particle (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 having approximately the same average particle diameter were selected, and the compressive modulus was calculated as described above, and the average value of the compressive modulus of the five particles was taken as the value of the compressive modulus here. The compressive modulus of the second Si-containing particle and the graphite particle was also calculated in the same manner.

<Evaluation of cycle capacity retention rate>
A cycle test was performed in which 250 cycles of charge and discharge were repeated, with one cycle being CCCV charging (0.4C rate up to 4.2V, then 0.1C cut) and then CC discharging (0.4C rate, 2.5V cut). The discharge capacity (initial capacity) at the first cycle and the discharge capacity at the 250th cycle were measured, and the cycle capacity retention rate was calculated by the following formula (1). It can be said that the higher the cycle capacity retention rate, the higher 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 have a capacity retention rate of 85% or more. These results show that a secondary battery having excellent cycle characteristics is realized 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, the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. The technology described in the claims includes various modifications and changes to the above-exemplified embodiments. For example, it is possible to replace part of the above-mentioned embodiments with other modified forms, and it is also possible to add other modified forms to the above-mentioned embodiments. Furthermore, if a technical feature is not described as essential, it can also 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, the negative electrode including 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 graphite particles and Si-containing particles as a negative electrode active material, the Si-containing particles including first Si-containing particles and second Si-containing particles, the first Si-containing particles having a compressive elastic modulus smaller than the compressive elastic modulus of the second Si-containing particles, the first Si-containing particles having a compressive elastic modulus of 250 MPa or more and 2000 MPa or less, and a 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 elastic modulus 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 a 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 particle 68 Si-containing particle 68a First Si-containing particle 68b Second Si-containing particle 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 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 elastic modulus of the first Si-containing particle is smaller than the compressive elastic modulus of the second Si-containing particle,
The compressive elastic modulus of the first Si-containing particle 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 according to 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 according to 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.

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