JP2024159006A - Anode material, anode of secondary battery using said anode material, and method of manufacturing same - Google Patents

Abstract

To provide an anode material that contains Si-containing particles and graphite particles, and that is capable of suppressing capacity degradation when a secondary battery is repeatedly charged and discharged.SOLUTION: An anode material disclosed herein contains Si-containing particles and graphite particles. The ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1 to 8. The mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 10 mass% to 60 mass%. When 1 g of the Si-containing particles is uniaxially pressed at 25°C and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting molded body is 0.9 g/cm3 or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to a negative electrode material. The present invention also relates to a negative electrode of a secondary battery using the negative electrode material. The present invention further relates to a method for producing the negative electrode of the secondary battery.

In recent years, secondary batteries have been 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), and plug-in hybrid electric vehicles (PHEVs).

In applications as a power source for driving vehicles, particularly BEVs, secondary batteries are desired to have a higher capacity from the viewpoint of extending the driving range of the vehicle. Si-containing particles are known as a high-capacity negative electrode active material, and it is known that the Si-containing particles can increase the capacity of secondary batteries (see, for example, Patent Document 1). Patent Document 1 discloses a technology that uses a combination of Si-containing particles and graphite particles such as natural graphite as a negative electrode active material.

JP 2015-38862 A

However, while the Si-containing particles have a high capacity, they undergo a large change in volume due to expansion/contraction when the secondary battery is charged and discharged. For this reason, when Si-containing particles and graphite particles are used in combination, particularly when the proportion of Si-containing particles is large, the packing properties of these particles decrease when the secondary battery is repeatedly charged and discharged, which may result in breakage of the conductive path and the generation of internal stress. For this reason, when Si-containing particles and graphite particles are used in combination, there is a problem that the cycle characteristics of the secondary battery are reduced, specifically, there is a problem that the capacity of the secondary battery is significantly deteriorated when the secondary battery is repeatedly charged and discharged.

In view of the above circumstances, the present invention aims to provide an anode material that contains Si-containing particles and graphite particles and that can suppress capacity degradation during repeated charging and discharging of a secondary battery.

The negative electrode material disclosed herein contains Si-containing particles and graphite particles. The ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1 to 8. The mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 10 mass% to 60 mass%. When 1 g of the Si-containing particles is uniaxially pressed at 25°C and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting molded body is 0.9 g/ ^cm3 or more.

This configuration makes it possible to provide an anode material that contains Si-containing particles and graphite particles and that can suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

From another aspect, the negative electrode of the secondary battery disclosed herein comprises a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer contains the above-mentioned negative electrode material.

A negative electrode with this configuration can provide the secondary battery with excellent resistance to capacity degradation when repeatedly charged and discharged.

From another perspective, the method for manufacturing the negative electrode of the secondary battery disclosed herein includes a step of applying a negative electrode paste containing the above-mentioned negative electrode material to a negative electrode current collector, a step of drying the applied negative electrode paste to form a negative electrode active material layer, and a step of pressing the negative electrode active material layer.

The negative electrode obtained by this configuration can provide the secondary battery with excellent resistance to capacity degradation when repeatedly charged and discharged.

FIG. 2 is a schematic diagram of an example of a negative electrode material according to an embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating a schematic configuration of a negative electrode of a secondary battery according to one embodiment of the present invention. FIG. 1 is a cross-sectional view showing a schematic configuration of a lithium ion secondary battery constructed using a negative electrode of a secondary battery according to one embodiment of the present invention. FIG. 4 is a schematic exploded view showing the configuration of a wound electrode body of the lithium ion secondary battery of FIG. 3.

Below, the embodiments of the present invention will be described with reference to the drawings. Note that matters not mentioned in this specification but necessary for implementing the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. In addition, in the following drawings, the same reference numerals are used to describe members and parts that perform the same function. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships. Note that the numerical ranges expressed as "A to B" in this specification include A and B.

In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged. 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.

The negative electrode material according to the present embodiment contains Si-containing particles and graphite particles. The ratio of the average particle diameter D50 of the graphite particles to the average particle diameter D50 of the Si-containing particles is 1 to 8. The mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 10 mass% to 60 mass%. When 1 g of the Si-containing particles is uniaxially pressed at 25°C and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting molded body is 0.9 g/ ^cm3 or more.

Figure 1 is a schematic diagram showing an example of the negative electrode material according to this embodiment used in a negative electrode. In the example shown in Figure 1, the negative electrode material 10 contains graphite particles 12 and Si-containing particles 14. In the example shown in Figure 1, the graphite particles 12 and the Si-containing particles 14 are mixed. The negative electrode material 10 is filled and arranged on the negative electrode current collector 62.

The graphite constituting the graphite particles 12 may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which the graphite is coated with an amorphous carbon material.

In the example shown in FIG. 1, particles of a Si-C composite material are used as the Si-containing particles 14. The Si-containing particles 14 have carbon domains 14a and Si-containing domains 14b. A plurality of Si-containing domains 14b are dispersed in the carbon domains 14a, and thus the carbon domains 14a form a matrix.

The carbon domain 14a is, for example, a carbonized material of a carbon precursor (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite, etc.

The Si-containing domain 14b contains Si and is composed of, for example, Si, Si oxide (SiO _x ), Si nitride (SiN x ), Si carbide (SiC x ), etc. The Si-containing domain 14b is preferably composed of at least one of Si and Si oxide (SiO _x ). The Si-containing domain 14b may be a fine particle. The number of Si-containing domains 14b in the Si-containing particle 14 is not limited to that shown in the figure.

The average particle diameter of the Si-containing domains 14b is, for example, 50 nm or less, and may be 5 nm to 50 nm. The "average particle diameter of the Si-containing domains 14b" can be determined as follows. First, a sample for scanning transmission electron microscope (STEM) observation of the negative electrode material 10 is prepared. For example, a negative electrode active material layer containing the negative electrode material 10 is processed with FIB (focused ion beam) to prepare a sample for STEM observation. Then, the sample is subjected to elemental analysis by EDX element mapping, and then a BF image (bright field image) and a HAADF image (high angle scattering annular dark field image) are obtained. The diameter of the Si-containing domains 14b can be determined from the contrast and shape obtained from the BF image and the HAADF image. The diameters of 10 or more arbitrarily selected Si-containing domains 14b are determined, and the average value is defined as the "average particle diameter of the Si-containing domains 14b" here.

The Si content in the Si-containing particles 14 is not particularly limited. From the viewpoint of a higher capacity of the secondary battery, the Si content in the Si-containing particles 14 is preferably 20 mass% or more, more preferably 30 mass% or more, and even more preferably 40 mass% or more. From the viewpoint of suppressing excessive volume change of the Si-containing particles 14, the Si content in the Si-containing particles 14 is preferably 60 mass% or less, more preferably 50 mass% or less. In addition, the oxygen (O) content in the Si-containing particles 14 is not particularly limited, but is preferably 10 mass% or less. The Si content and O content in the Si-containing particles 14 can be determined by analysis based on high-frequency inductively coupled plasma (ICP) emission spectroscopy.

The Si-containing particles 14 are, for example, particles containing Si dispersed inside a carbon material; particles containing Si embedded in the pores of spherically granulated porous graphite; etc.

The Si-containing particles 14 can be obtained according to known methods. For example, they can be obtained by mixing fine particles of Si, Si oxide, etc. with the carbon precursor, followed by carbonization and spheroidization. Alternatively, they can be obtained by mixing spherically granulated porous graphite with fine particles of Si, Si oxide, etc. in a dispersion medium, drying, and disposing the fine particles in the pores of the porous graphite.

However, the Si-containing particles 14 are not limited to those shown in the figure. The Si-containing particles may be carbon particles with fine particles containing Si attached to the surface; particles containing Si with fine carbon particles attached to the surface, etc. As in the illustrated example, in a Si-C composite material, when the carbon domains 14a form a matrix and multiple Si-containing domains 14b are dispersed in the carbon domains 14a, this is advantageous because the carbon domains 14a can mitigate the volume change caused by the expansion/contraction of the Si-containing domains 14b. In addition, the Si-containing particles may be composited with elements other than carbon, or may be metal Si particles, Si oxide particles, etc. that are not composited with other elements.

In this embodiment, when 1 g of the Si-containing particles 14 is pressed in a uniaxial direction at 25° C. and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting molded body is 0.9 g/cm ³ or more. The higher the density value of this molded body, the higher the particle packing. When 1 g of conventionally used Si-containing particles is molded in the same manner, the density of the molded body is usually less than 0.9/cm ^3. Therefore, in this embodiment, Si-containing particles 14 having a higher packing than conventionally used Si-containing particles are used. By using such Si-containing particles 14 having a higher packing property together with graphite particles 12 at a specific content ratio and controlling the ratio of the average particle diameter (D50) of the Si-containing particles 14 to the graphite particles 12 to a specific range, it is possible to suppress the capacity deterioration when the secondary battery is repeatedly charged and discharged.

The density of the molded body is preferably 0.95 g/cm ³ or more, more preferably 1.0 g/cm ³ or more. On the other hand, the upper limit of the density of the molded body may be 2.3 g/cm ³ or less, 2.0 g/cm ³ or less, 1.8 g/cm ³ or less, or 1.5 g/cm ³ or less.

The density of the compact can be determined by weighing out 1 g of Si-containing particles 14 into a 20 mm diameter mold in a temperature environment of 25°C, applying a press pressure in one axial direction (i.e., a direction perpendicular to the radial direction) until a tablet-shaped compact is obtained, and measuring the bulk density of the compact. The density of the compact can be easily measured, for example, by using an automatic powder resistivity measuring system (e.g., "MCP-PD600" manufactured by Nitto Seiko Analytech Co., Ltd.) and a 20 mm diameter probe (corresponding to the mold).

The density of the compact is affected by the circularity of the Si-containing particles 14. When the circularity of the Si-containing particles 14 is increased, the density of the compact tends to increase. Therefore, when the circularity of the Si-containing particles 14 is set to 0.85 to 1 (particularly 0.90 to 1), the density of the compact is likely to be 0.9 g/cm ³ or more. In this specification, the "circularity" refers to the ratio of the perimeter of a perfect circle having the same area as the projected area of the particle to the perimeter of the projected image of the particle (i.e., circularity = perimeter of a perfect circle having the same area as the projected area of the particle / perimeter of the projected image of the particle). Therefore, the closer the circularity is to 1, the closer the projected image of the particle is to a perfect circle, and the closer the particle is to a perfect sphere. The circularity can be determined, for example, by determining the circularity of 100 or more particles using a commercially available static automatic image analyzer and calculating the average value.

Furthermore, the particle size of the Si-containing particles 14 also affects the density of the compact. When the average particle size of the Si-containing particles 14 is set to 2 μm to 10 μm (particularly, 5 μm to 10 μm), the density of the compact is likely to be 0.9 g/cm ³ or more. The true density of the Si-containing particles 14 also affects the density of the compact. Therefore, by adjusting the composition of the Si-containing particles 14 (the content ratio of the constituent elements), the density of the compact can be finely adjusted.

The circularity of the graphite particles 12 is not particularly limited. As the graphite particles 12, spheroidized graphite particles are preferred, and therefore the circularity of the graphite particles 12 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

The ratio of the average particle diameter (D50) of the graphite particles 12 to the average particle diameter (D50) of the Si-containing particles 14 (D50 of graphite-containing particles/D50 of Si-containing particles) is 1 to 8. If the ratio (D50 of graphite-containing particles/D50 of Si-containing particles) is less than 1, the packing property is reduced due to the expansion/contraction of the Si-containing particles 14 during charging/discharging of the secondary battery, and the effect of improving the cycle characteristics of the secondary battery cannot be obtained. On the other hand, if the ratio (D50 of graphite-containing particles/D50 of Si-containing particles) exceeds 8, the dispersibility of the graphite particles 12 is reduced, and the aggregation of the Si-containing particles 14 occurs, which reduces the packing property. As a result, the effect of improving the cycle characteristics of the secondary battery cannot be obtained. The ratio (D50 of graphite-containing particles/D50 of Si-containing particles) is preferably 1.0 to 5.0, more preferably 1.2 to 3.0, and even more preferably 1.4 to 2.5.

In this specification, the term "average particle size (D50)" refers to the median size (D50), which is the particle size that corresponds to a cumulative frequency of 50 volume percent from the small particle side in a volume-based particle size distribution based on a laser diffraction/scattering method. The average particle size (D50) can be determined using a commercially available laser diffraction/scattering type particle size distribution measuring device, etc.

The average particle size (D50) of the graphite particles 12 is not particularly limited as long as the above ratio (D50 of graphite-containing particles/D50 of Si-containing particles) is between 1 and 8. The average particle size (D50) of the graphite particles 12 is preferably between 5 μm and 25 μm, more preferably between 10 μm and 23 μm, and even more preferably between 12 μm and 20 μm.

The average particle size (D50) of the Si-containing particles 14 is not particularly limited as long as the above ratio (D50 of graphite-containing particles/D50 of Si-containing particles) is between 1 and 8. The average particle size (D50) of the Si-containing particles 14 is preferably between 2 μm and 10 μm, more preferably between 4 μm and 10 μm, and even more preferably between 6 μm and 9 μm.

The mass ratio of the Si-containing particles 14 to the total of the Si-containing particles 14 and the graphite particles 12 is 10% by mass to 60% by mass. If the mass ratio of the Si-containing particles 14 is less than 10% by mass, the capacity of the secondary battery is insufficient. If the mass ratio of the Si-containing particles 14 exceeds 60% by mass, the effect of the volume change of the Si-containing particles 14 becomes large, and the effect of suppressing capacity degradation when the secondary battery is repeatedly charged and discharged becomes insufficient. The mass ratio of the Si-containing particles 14 is preferably 15% by mass to 55% by mass, and more preferably 20% by mass to 50% by mass.

The negative electrode material 10 can be obtained by mixing Si-containing particles 14 and graphite particles 12 according to a known method.

The negative electrode material 10 may further contain materials used in the negative electrodes of secondary batteries, such as binders and conductive materials.

The negative electrode material 10 according to this embodiment can suppress the capacity degradation when the secondary battery is repeatedly charged and discharged. That is, when a negative electrode is produced using the negative electrode material 10 according to this embodiment and a secondary battery is produced using this negative electrode, the secondary battery has excellent resistance to capacity degradation when repeatedly charged and discharged, and therefore has excellent cycle characteristics. In addition, since the negative electrode material 10 according to this embodiment contains Si, the secondary battery can have a high capacity. Furthermore, since the negative electrode material 10 according to this embodiment contains Si-containing particles 14 with high packing properties, the packing properties of the negative electrode active material layer can be increased, and the energy density of the secondary battery can be increased.

From another perspective, the negative electrode according to this embodiment includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer contains the negative electrode material according to the above-mentioned embodiment. Such a negative electrode can impart excellent resistance to capacity degradation when repeatedly charged and discharged to the secondary battery. In addition, such a negative electrode can increase the capacity of the secondary battery. Furthermore, such a negative electrode can increase the energy density of the secondary battery.

The negative electrode according to this embodiment will be specifically described with reference to FIG. 2. FIG. 2 is a cross-sectional view showing a schematic example of a negative electrode 60 according to this embodiment, the cross-sectional view being taken along the thickness direction and the width direction. The negative electrode 60 according to this embodiment shown in FIG. 2 is a negative electrode for a lithium ion secondary battery.

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

As shown in the illustrated example, a negative electrode active material layer non-forming portion 62a where the negative electrode active material layer 64 is not provided may be provided at one end in the width direction of the negative electrode 60. In the negative electrode active material layer non-forming portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer non-forming portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

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

The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately depending on the battery design. When copper foil is used as the negative electrode current collector 62, the thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 6 μm to 20 μm.

The negative electrode active material layer 64 contains a negative electrode active material, and the negative electrode material 10 according to the above-described embodiment is used as the negative electrode active material. Therefore, the negative electrode active material layer 64 contains the negative electrode material 10 according to the above-described embodiment.

The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as a binder and a conductive material. Examples of binders that can be used include styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), and the like. CMC also functions as a thickener. Examples of conductive materials include carbon nanotubes (CNT). When CNT is used as the conductive material, the negative electrode active material layer 64 may contain a dispersant for CNT.

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

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

The density of the negative electrode active material layer 64 is not particularly limited, but is, for example, 0.9 g/cm ³ or more, preferably 1.1 g/cm ³ or more, and more preferably 1.2 g/cm ³ or more. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g/cm ³ or less, and may be 2.0 g/cm ³ or less.

The negative electrode 60 may include members other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) adjacent to the negative electrode active material layer 64 may be provided on the negative electrode active material layer non-forming portion 62a. The insulating layer contains, for example, an insulating inorganic filler.

The negative electrode 60 can be suitably manufactured by a manufacturing method including a process of applying a negative electrode paste containing the negative electrode material according to the above-described embodiment to the negative electrode current collector 62 (hereinafter also referred to as the "applying process"), a process of drying the applied negative electrode paste to form a negative electrode active material layer 64 (hereinafter also referred to as the "drying process"), and a process of pressing the negative electrode active material layer 64 (hereinafter also referred to as the pressing process).

The coating process can be carried out according to a known method, except for using the negative electrode material according to the above-mentioned embodiment. Specifically, for example, the negative electrode material according to the above-mentioned embodiment and optional components (e.g., binder, conductive material, etc.) are mixed with a dispersion medium (e.g., water) using a known mixer, stirrer, etc. to prepare a negative electrode paste.

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

The coating process can be carried out by applying the negative electrode paste onto the negative electrode current collector 62 using a coating device such as a gravure coater, comma coater, slit coater, or die coater.

The drying step can be carried out according to a known method. Specifically, for example, the dispersion medium is removed from the negative electrode current collector 62 on which the negative electrode paste is applied, using a drying device such as a drying oven, to form the negative electrode active material layer 64. This allows the drying step to be carried out. The drying temperature and drying time can be appropriately determined according to the solid content concentration of the negative electrode paste, and are not particularly limited. The drying temperature is, for example, 60°C or higher and 200°C or lower, and preferably 70°C or higher and 150°C or lower. The drying time is, for example, 10 seconds or higher and 30 minutes or lower, and preferably 30 seconds or higher and 10 minutes or lower.

The pressing step can be carried out according to a known method. Specifically, the pressing step can be carried out by applying pressure to the negative electrode active material layer 64 formed above using a roller press or the like. The pressing step can densely pack the graphite particles 12 and Si-containing particles 14 contained in the negative electrode material 10. This results in the negative electrode 60.

The negative electrode 60 can be used in a secondary battery according to a known method. A secondary battery typically includes a positive electrode, a negative electrode, and an electrolyte, and the negative electrode is the above-mentioned negative electrode 60. Below, an example of the configuration of a secondary battery using the negative electrode 60 will be described in detail with reference to Figures 3 and 4. The example configuration is a flat prismatic lithium ion secondary battery having a flat wound electrode body and a flat battery case.

The lithium ion secondary battery 100 shown in FIG. 3 is a sealed lithium ion secondary battery 100 constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., an outer container) 30. 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 the non-aqueous electrolyte. The positive terminal 42 is electrically connected to the positive electrode current collector 42a. The negative terminal 44 is electrically connected to the negative electrode current collector 44a. The material of the battery case 30 is, for example, a lightweight metal material with good thermal conductivity, such as aluminum.

As shown in Figures 3 and 4, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are stacked with two long separator sheets 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 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. The positive electrode active material layer non-forming portion 52a (i.e., the portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer non-forming portion 62a (i.e., the portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) are formed so as to protrude outward from both ends of the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the wound electrode body 20. The positive electrode active material layer non-forming portion 52a and the negative electrode active material layer non-forming portion 62a are respectively joined to the positive electrode current collector 42a and the negative electrode current collector 44a.

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, examples of which include a sheet or foil made of a metal with good electrical conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

The dimensions of the positive electrode collector 52 are not particularly limited and may be determined appropriately according to the battery design. When aluminum foil is used as the positive electrode collector 52, the 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 of a known composition used in lithium ion secondary batteries may be used. Specifically, for example, a lithium composite oxide, a lithium transition metal phosphate compound, or the like may be used as the positive electrode active material. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, 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.

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.

Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ ), and lithium manganese iron phosphate.

These positive electrode active materials may be used alone or in combination of two or more. As the positive electrode active material, lithium nickel cobalt manganese composite oxide is particularly preferred because of its excellent properties such as initial resistance.

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

The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, 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.) can be suitably used. As the binder, for example, polyvinylidene fluoride (PVdF) can be used.

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

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

The negative electrode sheet 60 is a negative electrode 60 that uses the above-mentioned negative electrode material 10 as the negative electrode active material.

The separator 70 may be a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which a PP layer is laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

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

The non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones, which are used in the electrolyte of a general lithium ion secondary battery, can be used without any particular limitation. Among them, carbonates are preferable, and specific examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combination of two or more. As an example, the non-aqueous solvent consists of only carbonates. As another example, the non-aqueous solvent contains carbonates and esters such as methyl acetate.

As the supporting salt, for example, a lithium salt such as LiPF ₆ , LiBF ₄ , or lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF ₆ ) can be suitably used. The concentration of the supporting salt is preferably 0.7 mol/L or more and 1.3 mol/L or less.

The nonaqueous electrolyte may contain various additives other than those mentioned above, such as film-forming agents such as vinylene carbonate (VC) and oxalate complexes; gas generators such as biphenyl (BP) and cyclohexylbenzene (CHB); and thickeners, as long as the effects of the present invention are not significantly impaired.

The lithium ion secondary battery 100 has a high capacity and is suppressed from deteriorating in capacity when repeatedly charged and discharged. The lithium ion secondary battery 100 can be used for various applications. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs). The lithium ion secondary battery 100 can also be used as a storage battery for small power storage devices. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of multiple batteries connected in series and/or parallel.

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

In addition, the lithium ion secondary battery 100 can be configured as an all-solid-state lithium ion secondary battery using a solid electrolyte instead of a nonaqueous electrolyte, according to known methods.

The negative electrode 60 according to this embodiment is suitable for use as a negative electrode in a lithium-ion secondary battery, but can also be used as a negative electrode in other secondary batteries, which can be constructed according to known methods.

The following describes in detail examples of the present invention, but it is not intended that the present invention be limited to those examples.

<Preparation of negative electrode material>
Example 1
Particles of a Si-C composite material having an average particle size (D50) of 8 μm, a Si content of 40 mass %, and a circularity of 0.98 were prepared as Si-containing particles.

The Si-containing particles (1 g) were pressed uniaxially at 25° C. and 60 MPa to form a tablet having a diameter of 20 mm, and the density of the formed body was measured by the method described below. The density of the formed body was 1.3 g/cm ³ .

Graphite particles with an average particle size (D50) of 18 μm were prepared. The graphite particles and Si-containing particles were mixed in a mass ratio of 60:40 to obtain the negative electrode material of Example 1.

[Examples 2 to 3 and Comparative Examples 1 to 3]
Particles of Si-C composite material having the average particle size (D50), Si content, and circularity shown in Table 1 were prepared as Si-containing particles. 1 g of the Si-containing particles was uniaxially pressed at 25°C and 60 MPa to form a tablet having a diameter of 20 mm. The density of the formed body was measured by the method described below. The density of the formed body is shown in Table 1.

Graphite particles with the average particle size (D50) shown in Table 1 were prepared. The graphite particles and the Si-containing particles were mixed in a mass ratio of 60:40 to obtain the negative electrode materials of Examples 2 to 3 and Comparative Examples 1 to 3. In each Example and Comparative Example, the average particle size (D50) of the Si-containing particles and the graphite particles, the Si content in the Si-containing particles, and the circularity of the Si-containing particles were determined by the following method.

<Measurement of average particle size (D50) of Si-containing particles and graphite particles>
Using a commercially available laser diffraction type particle size distribution measuring device, the particle size distribution of the Si-containing particles and the graphite particles was measured on a volume basis, and the particle size corresponding to a cumulative frequency of 50 volume % from the fine particle side with a small particle size was determined as the average particle size (D50) of the Si-containing particles and the graphite particles.

<Measurement of Si content in Si-containing particles>
The Si content in the Si-containing particles was determined in terms of mass % using a commercially available ICP analyzer.

<Measurement of circularity of Si-containing particles>
The circularity of the Si-containing particles was determined by using a commercially available image type particle size distribution measuring device, as the perimeter of a perfect circle having the same area as the projected area of the particle/perimeter of the projected image of the particle.

<Density measurement of compact of Si-containing particles>
1 g of the Si-containing particles was weighed out as a powder measurement sample and set on the probe (diameter 20 mm) of an automatic powder resistivity measurement system "MCP-PD600" (manufactured by Nitto Seiko Analytech Co., Ltd.). At 25°C, the load and displacement when pressure was applied in one axial direction were measured using this automatic powder resistivity measurement system. Based on this, the bulk density of the compact at a pressure of 60 MPa was calculated.

<Preparation of negative electrode and lithium ion secondary battery for evaluation>
Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared as binders. A dispersion of single-walled carbon nanotubes (SWCNT) was also prepared as a conductive material. The negative electrode material of each example and each comparative example, CMC, and PAA were dry-blended using a planetary mixer. The obtained mixture, SWCNT dispersion, and dispersion medium were kneaded using a planetary mixer. SBR and further dispersion medium were added to this and mixed uniformly to prepare a negative electrode paste. In the negative electrode paste, the mass ratio of the negative electrode material: CMC: PAA: SBR: SWCNT was 100: 1: 1: 1.5: 0.1.

The negative electrode paste was applied to the surface of a 10 μm-thick copper foil and dried to form a negative electrode active material layer. After the negative electrode active material layer was roll-pressed, the resulting sheet was processed to a specified size to obtain a negative electrode sheet.

A positive electrode paste was prepared by mixing LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM) as a positive electrode active material powder, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder with N-methylpyrrolidone (NMP) in a mass ratio of NCM:AB:PVdF=100:1:1. This paste was applied to the surface of an aluminum foil having a thickness of 15 μm and dried to form a positive electrode active material layer. After the positive electrode active material layer was roll-pressed, the obtained sheet was processed to a predetermined size to obtain a positive electrode sheet.

A separator made of porous polyolefin was prepared. A lead was attached to each of the negative electrode sheet and the positive electrode sheet prepared above, and the sheets were laminated through a separator to prepare an electrode body. This was housed in a case made of an aluminum laminate film together with a non-aqueous electrolyte. The non-aqueous electrolyte was a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 15:5:40:40, in which LiPF ₆ as a supporting salt was dissolved at a concentration of 1.0 mol/L. The case was then sealed to obtain a lithium ion secondary battery for evaluation.

<Cycle characteristic evaluation>
Each of the lithium ion secondary batteries for evaluation prepared above was placed in an environment of 25° C. Each of the lithium ion secondary batteries for evaluation was charged at a constant current of 0.4 C up to 4.2 V, and then charged at a constant voltage until the current value reached 0.1 C. Next, each of the lithium ion secondary batteries for evaluation was discharged at a constant current of 0.4 C up to 2.5 V. The discharge capacity at this time was measured to obtain the initial capacity.

The above charge/discharge cycle was repeated 200 times. The discharge capacity after 200 cycles was determined in the same manner as the initial capacity. As an index of cycle characteristics, the capacity retention rate (%) was calculated by (discharge capacity after 200 charge/discharge cycles/initial capacity) x 100. The results are shown in Table 1.

From the results in Table 1, it can be seen that the capacity retention rate after 200 charge/discharge cycles is significantly high when the ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1 to 8, the mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 10 mass% to 60 mass%, and when ¹ g of the Si-containing particles is pressed in a uniaxial direction at 25°C and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting molded body is 0.9 g/cm3 or more. Therefore, it can be seen that the negative electrode material disclosed herein can suppress capacity deterioration when the secondary battery is repeatedly charged and discharged.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

That is, the negative electrode material, the negative electrode of a secondary battery, and the method for producing the same disclosed herein are the following items [1] to [8].
[1] A negative electrode material containing Si-containing particles and graphite particles,
a ratio of an average particle size (D50) of the graphite particles to an average particle size (D50) of the Si-containing particles is 1 to 8;
a mass ratio of the Si-containing particles to a total of the Si-containing particles and the graphite particles is 10 mass% to 60 mass%;
A negative electrode material, wherein when 1 g of the Si-containing particles is uniaxially pressed at 25° C. under 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting tablet is 0.9 g/cm ³ or more.
[2] The negative electrode material according to item [1], wherein the average particle diameter (D50) of the Si-containing particles is 2 μm to 10 μm, and the average particle diameter (D50) of the graphite particles is 5 μm to 25 μm.
[3] The negative electrode material according to item [1] or [2], wherein the ratio of the average particle diameter (D50) of the graphite particles to the average particle diameter (D50) of the Si-containing particles is 1.2 to 3.0.
[4] The mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 20% by mass to 50% by mass. The negative electrode material according to any one of [1] to [3].
[5] The negative electrode material according to any one of items [1] to [4], wherein the Si-containing particles are particles of a Si-C composite material in which Si-containing domains are dispersed in a carbon matrix.
[6] The negative electrode material according to item [5], wherein the Si content in the Si-containing particles is 20% by mass or more and 60% by mass or less.
[7] a negative electrode current collector;
a negative electrode active material layer supported on the negative electrode current collector;
A negative electrode of a secondary battery comprising:
The negative electrode active material layer contains the negative electrode material according to any one of items [1] to [6].
[8] A step of applying a negative electrode paste containing the negative electrode material according to any one of items [1] to [6] to a negative electrode current collector;
drying the applied negative electrode paste to form a negative electrode active material layer;
pressing the negative electrode active material layer;
A method for manufacturing a negative electrode of a secondary battery, comprising the steps of:

10 Negative electrode material 12 Graphite particle 14 Si-containing particle 14a Carbon domain 14b Si-containing domain 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode)
52 Positive electrode current collector 52a Positive electrode active material layer non-forming portion 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-forming portion 64 Negative electrode active material layer 70 Separator sheet (separator)
100 Lithium-ion secondary battery

Claims (8)

A negative electrode material containing Si-containing particles and graphite particles,
a ratio of an average particle size (D50) of the graphite particles to an average particle size (D50) of the Si-containing particles is 1 to 8;
a mass ratio of the Si-containing particles to a total of the Si-containing particles and the graphite particles is 10 mass% to 60 mass%;
A negative electrode material, wherein when 1 g of the Si-containing particles is uniaxially pressed at 25° C. under 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting tablet is 0.9 g/cm ³ or more. The negative electrode material according to claim 1, wherein the average particle diameter (D50) of the Si-containing particles is 2 μm to 10 μm, and the average particle diameter (D50) of the graphite particles is 5 μm to 25 μm. The negative electrode material according to claim 1, wherein the ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1.2 to 3.0. The negative electrode material according to claim 1, wherein the mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 20 mass% to 50 mass%. The negative electrode material according to claim 1, wherein the Si-containing particles are particles of a Si-C composite material in which Si-containing domains are dispersed in a carbon matrix. The negative electrode material according to claim 5, wherein the Si content in the Si-containing particles is 20% by mass or more and 60% by mass or less. A negative electrode current collector;
a negative electrode active material layer supported on the negative electrode current collector;
A negative electrode of a secondary battery comprising:
The negative electrode of a secondary battery, wherein the negative electrode active material layer contains the negative electrode material according to claim 1 . A step of applying a negative electrode paste containing the negative electrode material according to claim 1 to a negative electrode current collector;
drying the applied negative electrode paste to form a negative electrode active material layer;
pressing the negative electrode active material layer;
A method for manufacturing a negative electrode of a secondary battery, comprising the steps of:

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