secondary batteries

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

[Technical Field]

[0001] The present invention relates to a secondary battery. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are suitably used as portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc. 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.

[0003] 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 (e.g., Patent Documents 1 and 2). Patent Document 1 discloses a negative electrode active material containing 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 contains 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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6385749 [Patent Document 2] Patent No. 6596815 Summary of the Invention [Problem to be solved by the invention]

[0005] Although Si-based materials have a larger specific capacity than carbon materials such as graphite particles, they also tend to expand and contract significantly during charging and discharging, resulting in easily broken conductive paths. Therefore, while the use of Si-based materials can increase capacity, the cycle characteristics of secondary batteries tend to deteriorate. In particular, when the content of Si-based materials is increased to increase capacity, the cycle characteristics of secondary batteries deteriorate significantly. Therefore, there is still room for improvement in improving the cycle characteristics of secondary batteries when Si-based materials are used as negative electrode active materials.

[0006] The present invention has been made in view of the above points, and aims to provide a secondary battery having a negative electrode containing a Si-based material as a negative electrode active material, and having excellent cycle characteristics. [Means for solving the problem]

[0007] The secondary battery disclosed herein is 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 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, and the compressive modulus of the first Si-containing particles is smaller than that 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.

[0008] The first Si-containing particles, which have a small compressive modulus, easily deform in response to expansion and contraction during charging and discharging, thereby alleviating 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 Si-containing particles with different compressive moduli in a predetermined ratio, the conductive path is less likely to break even during repeated charging and discharging, thereby improving the cycle characteristics of the secondary battery. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram schematically illustrating the internal structure of a secondary battery according to one embodiment. [Figure 2] FIG. 2 is a diagram schematically illustrating the configuration of an electrode assembly according to one embodiment. [Figure 3] FIG. 3 is a diagram schematically illustrating a negative electrode according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. Matters necessary for implementing the technology disclosed herein, other than those specifically mentioned in this specification (e.g., the general configuration and manufacturing process of a secondary battery that do not characterize the technology disclosed herein), can be understood as design matters for a person 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. Each drawing is a schematic representation, and dimensional relationships (e.g., length, width, thickness) 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.

[0011] 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. In addition, 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.

[0012] FIG. 1 is a diagram schematically illustrating the internal structure of a secondary battery 100 according to one embodiment. As shown in FIG. 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 accommodates the electrode assembly 20 and the non-aqueous electrolyte. The secondary battery 100 shown in FIG. 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.

[0013] The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is configured to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 is also provided with an inlet (not shown) for injecting a non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0014] FIG. 2 is a diagram schematically illustrating the configuration of an electrode assembly 20. Here, the electrode assembly 20 is a flat-shaped wound electrode assembly. As shown in FIG. 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 together with two long separators 70 interposed therebetween 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 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 along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62. 1 and 2, the positive electrode current collector exposed portion 52a (i.e., a 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., a portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64) are formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the electrode body 20. A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are joined to the positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a, respectively.

[0015] The positive electrode current collector 52 constituting the positive electrode sheet 50 may be a known positive electrode current 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 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.

[0016] 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 (LiFePO4), lithium manganese phosphate (LiMnPO4)). 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.

[0017] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, 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 is preferably used as the positive electrode active material.

[0018] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to 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 nonmetal 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.

[0019] 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. 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).

[0020] 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, based on 100 parts by weight of the positive electrode active material, and the proportion of the binder is preferably 0.1 to 10 parts by weight, and more preferably 1 to 5 parts by weight, based on 100 parts by weight of the positive electrode active material.

[0021] The thickness of the positive electrode active material layer 54 per side 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.

[0022] As the separator 70, various conventional microporous sheets 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). Separator 70 may also have a heat-resistant layer (HRL).

[0023] Conventional nonaqueous electrolytes can be used, for example, nonaqueous electrolytes containing a supporting salt in an organic solvent (nonaqueous solvent). Nonaqueous solvents include aprotic solvents such as carbonates, esters, and ethers. Among these, carbonates, such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), are preferred. 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), are preferred. These nonaqueous solvents can be used alone or in appropriate combinations of two or more. Lithium salts, such as LiPF, LiBF, and LiClO, are preferred as supporting salts. 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. 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.

[0024] The negative electrode 60 of the secondary battery disclosed herein will now be described. 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 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 a conventionally known one and is not particularly limited. Examples include a sheet or foil made of a metal such as copper, nickel, titanium, or 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.

[0025] The negative electrode active material layer 64 includes at least a negative electrode active material. The negative electrode active material layer 64 includes 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 having different compressive elastic moduli. The compressive elastic moduli of the first Si-containing particles 68a are smaller than those 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 referred to as Young's modulus), the more resistant the material is to deformation. In other words, the negative electrode 60 disclosed herein includes 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.

[0026] 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 larger specific capacity than graphite particles, but a higher expansion / contraction rate during charging / discharging. Therefore, repeated charging / discharging easily breaks the conductive path, resulting in a deterioration in cycle characteristics. Here, the first Si-containing particles 68a, which have a small compressive modulus, easily deform in response to expansion / contraction during charging / discharging, thereby alleviating the stress of such expansion / contraction. On the other hand, the second Si-containing particles 68b, which have a large compressive modulus, are less likely to deform during charging / 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 modulus, and the second Si-containing particles 68b, which have a large compressive modulus, are contained in a predetermined ratio, thereby allowing the first Si-containing particles 68a to be arranged 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 after repeated charging / discharging, thereby favorably improving the cycle characteristics of the secondary battery 100.

[0027] 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, the graphite particles 66 are preferably substantially spherical. In this specification, the term "substantially spherical" encompasses spherical shapes, rugby ball shapes, etc., and refers to particles having 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).

[0028] The average particle diameter of graphite particles 66 (D 50 The particle diameter 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 (D) corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by particle size distribution measurement based on the laser diffraction / light scattering method. 50 This refers to the particle size.

[0029] 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 having a compressive modulus within this range can deform favorably in response to 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 placed in the micro-compression tester, and the particle is compressed in the vertical direction (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.

[0030] The Si-containing particles 68 are not particularly limited as long as they contain Si. The Si-containing particles 68 may contain components other than Si as long as they contain Si. Examples of the Si-containing particles 68 include SiOx, Si-C composites, and porous particles in which nano-Si particles are dispersed. Among these, Si-C composites are preferably used as the Si-containing particles 68. The Si-C composites are particles containing at least Si and C. The Si-C composites can be formed, for example, by supporting Si metal, Si oxide, or the like on a carbon material (graphite, amorphous carbon, or the like).

[0031] 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 fall within the above range, both high capacity and improved cycle characteristics can be suitably achieved.

[0032] As described above, the Si-containing particles 68 include first Si-containing particles 68a and second Si-containing particles 68b, which have different compressive elastic moduli. The compressive elastic moduli of the first Si-containing particles 68a are not particularly limited, as long as they are set to be smaller than the compressive elastic moduli of the second Si-containing particles 68b. The compressive elastic 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. The first Si-containing particles 68a having such compressive elastic moduli can suitably follow expansion and contraction associated with charge and discharge. This can alleviate stress due to expansion and contraction associated with charge and discharge, thereby preventing disconnection of the conductive paths in the negative electrode active material layer. The lower limit of the compressive elastic 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.

[0033] The first Si-containing particles 68a are preferably Si-C composites. The first Si-containing particles 68a are preferably, for example, Si-C composites in which Si particles smaller than carbon particles are dispersed within carbon particles having an amorphous carbon coating on their surfaces. 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.

[0034] 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.

[0035] 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. The upper limit of the porosity is not particularly limited, 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 × 100.

[0036] The first Si-containing particles 68a are not particularly limited in terms of their shape, etc., as long as their compressive elastic 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 (D 50 The particle size is not particularly limited, but is preferably, for example, from 1 μm to 15 μm, and more preferably from 2 μm to 10 μm.

[0037] 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 more, and even more preferably 3500 MPa or more. The second Si-containing particles 68b having such a compressive modulus are less likely to deform during expansion and contraction associated with charge and discharge, and can maintain a constant conductive path. While the second Si-containing particles 68b maintain a constant conductive path, the easily deformable first Si-containing particles 68a are suitably arranged between the second Si-containing particles 68b, thereby making the conductive path less likely to be broken and 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, for example, more than 2000 MPa and not more than 5000 MPa, and more preferably 2500 MPa or more and not more than 5000 MPa.

[0038] The second Si-containing particles 68b are not particularly limited in terms of their shape, etc., as long as their compressive elastic 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 (D 50 The particle size is not particularly limited, but is preferably, for example, from 1 μm to 15 μm, and more preferably from 2 μm to 10 μm.

[0039] The compressive elastic modulus of the first Si-containing particle 68a and the second Si-containing particle 68b can be adjusted appropriately by, for example, the porosity, the surface coating, the type of carbon particle, 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 range.

[0040] 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 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.

[0041] 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 suitably relieves the stress of expansion and contraction, making the conductive paths less likely to be broken.

[0042] The negative electrode active material layer 64 may contain Si-containing particles (third Si-containing particles) other than the first Si-containing particles 68 a and the second Si-containing particles 68 b as long as the effect of the present technology is not significantly impaired. Examples of the third Si-containing particles include SiOx and porous particles in which nano Si particles are dispersed.

[0043] 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. A conventionally known conductive material can be used. Examples of the conductive material that can be used include carbon nanotubes such as single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT), carbon black such as acetylene black (AB), and carbon fibers. 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.

[0044] The proportion of the conductive material may be, for example, 0.01 parts by weight or more, such as 0.05 parts by weight or more, or 2 parts by weight or less, such as 1 part by weight or less, 0.5 parts by weight or less, or 0.2 parts by weight or less, based on 100 parts by weight of the negative electrode active material.

[0045] As the binder, conventionally known binders 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 can be preferably used. Although not particularly limited, it is more preferable to use CMC, PAA, and SBR in combination.

[0046] The total amount of 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, per 100 parts by weight of the negative electrode active material, and is 10 parts by weight or less, preferably 8 parts by weight or less, and more preferably 5 parts by weight or less, per 100 parts by weight of the negative electrode active material.

[0047] The thickness of the negative electrode active material layer 64 per side 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.

[0048] 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. 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 mass % or less.

[0049] 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 (for example, a conductive material or a binder) in an appropriate solvent (for example, 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.

[0050] The configuration of the negative electrode 60 and the configuration of the secondary battery 100 according to one embodiment have been described above. The negative electrode 60 is preferably used in a nonaqueous electrolyte secondary battery. The negative electrode 60 is preferably prevented from breaking a conductive path due to expansion and contraction caused by repeated charge and discharge. This results in a secondary battery 100 with improved cycle characteristics. The secondary battery 100 can be used for various purposes, and is preferably used, for example, 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.

[0051] Furthermore, in the above-described 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, which is an electrode body in which a plurality of approximately rectangular positive electrodes and a plurality of approximately rectangular negative electrodes are alternately stacked with separators interposed therebetween.

[0052] Test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to those shown in the following test examples.

[0053] Example 1 First, graphite particles (compressive modulus: 180 MPa, D 50Particle diameter: 13 μm) and the first Si-containing particle (Si / C composite particle, compressive modulus: 1000 MPa, D 50 Particle diameter: 8 μm) and second Si-containing particles (Si / C composite particles, compressive modulus: 3500 MPa, D 50 Particle diameter: 7 μm) were prepared. Single-walled carbon nanotubes (SWCNT) were also prepared as a conductive material. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were also prepared as binders. These were kneaded with water as a solvent so that the weight ratio of graphite particles: first Si-containing particles: second Si-containing particles: SWCNT: CMC: PAA: SBR was 60: 32: 8: 0.1: 1: 1: 1.5, to prepare a slurry for forming a negative electrode active material layer.

[0054] 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 mixed powder was kneaded with a paste of SWCNTs (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 a predetermined thickness, and processed to a predetermined dimension to produce a negative electrode sheet.

[0055] 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 an aluminum foil (thickness 15 μm). The slurry on the aluminum foil was then dried, pressed to a predetermined thickness, and processed to the predetermined dimensions to produce a positive electrode sheet.

[0056] The prepared negative electrode sheet and positive electrode sheet were stacked with a separator interposed therebetween to produce a laminated electrode assembly. Current-collecting leads were attached to the positive electrode sheet and negative electrode sheet, respectively, and the laminated electrode assembly was inserted into an exterior housing made of an aluminum laminate sheet. A nonaqueous electrolyte was poured into the interior of the exterior housing, and the opening of the exterior housing was sealed to produce the evaluation battery of Example 1. A porous polyolefin sheet with a three-layer structure of PP / PE / PP was used as the separator. The nonaqueous electrolyte was prepared by dissolving LiPF6 as a supporting electrolyte at a concentration of 1 mol / L in 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.

[0057] <Example 2> An evaluation battery of Example 2 was fabricated in the same manner as 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.

[0058] Example 3 The evaluation battery of Example 3 was fabricated in the same manner as Example 1, except that the compounding ratio of each material was changed to obtain 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.

[0059] <Comparative Example 1> An evaluation battery for 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.

[0060] <Comparative Example 2> An evaluation battery for Comparative Example 2 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=60:40:0:0.1:1:1:1.5. That is, Comparative Example 2 did not contain second Si-containing particles.

[0061] <Comparative Example 3> An evaluation battery for Comparative 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=60:0:40:0.1:1:1:1.5. That is, Comparative Example 3 did not contain the first Si-containing particles.

[0062] <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 speed (2.6 mN / sec), and the compressive displacement and compressive stress were measured. The measured compressive displacement was then used as the average particle diameter (D 50 The compressive strain was calculated by dividing the compressive stress by the calculated compressive strain (compressive stress / compressive strain). The compressive modulus was calculated by selecting five first Si-containing particles with approximately the same average particle size, calculating the compressive modulus as described above, and the average value of the compressive moduli of the five particles was used as the compressive modulus value. The compressive moduli of the second Si-containing particles and graphite particles were calculated in the same manner.

[0063] <Evaluation of cycle capacity retention rate> A cycle test was conducted in which 250 charge-discharge cycles were repeated at 25°C, 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)

[0064] [Table 1]

[0065] As shown in Table 1, the capacity retention rates of the test batteries of Examples 1 to 3 were 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.

[0066] 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 common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiments. For example, it is possible to replace part of the above-described embodiments with other modified embodiments, and it is also possible to add other modified embodiments to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

[0067] 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 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 modulus smaller than that of the second Si-containing particles, the first Si-containing particles having a compressive 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 modulus of elasticity 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. [Explanation of symbols]

[0068] 20 Electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode (positive electrode sheet) 52 Positive electrode current collector 52a Exposed part of positive electrode current collector 54 Cathode active material layer 60 Negative electrode (negative electrode sheet) 62 Negative electrode current collector 62a Exposed part of negative electrode current collector 64 Negative electrode active material layer 66 graphite particles 68 Si-containing particles 68a 1st Si-containing particle 68b 2nd Si-containing particles 70 Separator 100 Secondary battery

Claims

1. 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.

2. The secondary battery according to claim 1 , wherein the second Si-containing particles have a compressive modulus of elasticity of more than 2000 MPa and not more than 5000 MPa.

3. 3. The secondary battery according to claim 1, wherein the graphite particles have a compressive modulus of elasticity of 10 MPa or more and 250 MPa or less.

4. 3. The secondary battery according to claim 1, wherein a weight ratio of the graphite particles to the Si-containing particles is 90:10 to 40:

60.

5. 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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