JP7745587B2 - Anode material, anode of secondary battery using said anode material, and method of manufacturing the same - Google Patents

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 manufacturing the negative electrode of the secondary battery.

In recent years, secondary batteries have been used favorably as portable power sources for personal computers, mobile devices, etc., and as power sources for vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

When used as a power source for driving vehicles, particularly BEVs, secondary batteries are desired to have even higher capacities in order to extend the vehicle's driving range. Si-containing particles are known as a high-capacity negative electrode active material, and it is known that 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 Si-containing particles have a high capacity, they also undergo large volume changes due to expansion and contraction when the secondary battery is charged and discharged. Therefore, when Si-containing particles and graphite particles are used together, particularly when the proportion of Si-containing particles is high, the packing ability of these particles decreases when the secondary battery is repeatedly charged and discharged, which can lead to conductive path breakage, internal stress, and other problems. Therefore, when Si-containing particles and graphite particles are used together, the cycle characteristics of the secondary battery decrease; specifically, there is a problem of significant capacity degradation when the secondary battery is repeatedly charged and discharged.

In light of the above circumstances, the present invention aims to provide an anode material containing Si-containing particles and graphite particles 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% by mass to 60% by 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 compact is 0.9 g/ ^cm3 or more.

This configuration makes it possible to provide a negative electrode material containing Si-containing particles and graphite particles, which can suppress capacity degradation when a 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 during repeated charge and discharge.

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

A negative electrode obtained with this configuration can provide the secondary battery with excellent resistance to capacity degradation during repeated charge and discharge.

FIG. 1 is a schematic diagram of an example of a negative electrode material according to an embodiment of the present invention. 1 is a cross-sectional view schematically illustrating the configuration of a negative electrode of a secondary battery according to one embodiment of the present invention. 1 is a cross-sectional view schematically showing the 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.

Embodiments of the present invention will be described below with reference to the drawings. Matters not mentioned in this specification but necessary for implementing the present invention can be understood as design matters for those 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 common technical knowledge in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function are denoted by the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships. Note that in this specification, numerical ranges expressed as "A to B" include A and B.

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

The negative electrode material according to this 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% by mass to 60% by 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 compact 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 together. The negative electrode material 10 is filled and disposed on the negative electrode current collector 62.

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

In the example shown in Figure 1, particles of 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. Multiple Si-containing domains 14b are dispersed within the carbon domains 14a, and therefore the carbon domains 14a form a matrix.

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

The Si-containing domains 14b contain Si and are composed of, for example, Si, Si oxide (SiO _x ), Si nitride (SiN x ), Si carbide (SiC x ), etc. The Si-containing domains 14b are preferably composed of at least one of Si and Si oxide (SiO _x ). The Si-containing domains 14b may be fine particles. 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 of the negative electrode material 10 for observation with a scanning transmission electron microscope (STEM) is prepared. For example, a negative electrode active material layer containing the negative electrode material 10 is processed with a focused ion beam (FIB) to prepare a sample for STEM observation. The sample is then subjected to elemental analysis using EDX element mapping, after which a BF image (bright-field image) and a HAADF image (high-angle 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" herein.

The Si content in the Si-containing particles 14 is not particularly limited. From the viewpoint of achieving a higher capacity secondary battery, the Si content in the Si-containing particles 14 is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. From the viewpoint of suppressing excessive volume change in the Si-containing particles 14, the Si content in the Si-containing particles 14 is preferably 60% by mass or less, more preferably 50% by mass or less. Furthermore, the oxygen (O) content in the Si-containing particles 14 is not particularly limited, but is preferably 10% by 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) optical 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.

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 described above, 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 Si-containing microparticles attached to the surface; Si-containing particles with carbon microparticles attached to the surface, etc. In a Si-C composite material, as in the example shown, carbon domains 14a form the matrix and multiple Si-containing domains 14b are dispersed within the carbon domains 14a, which is advantageous because the carbon domains 14a can mitigate volume changes due to the expansion/contraction of the Si-containing domains 14b. Furthermore, 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 Si-containing particles 14 is uniaxially pressed at 25°C under 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting compact is 0.9 g/ ^cm³ or more. The higher the density value of this compact, the higher the particle packing. When 1 g of conventionally used Si-containing particles is similarly molded, the density of the compact is usually less than 0.9 g/ ^cm³ . Therefore, in this embodiment, Si-containing particles 14 having a packing property higher than that of conventionally used Si-containing particles are used. By using such Si-containing particles 14 having a packing property higher than that of conventionally used Si-containing particles in a specific content ratio together with graphite particles 12 and controlling the ratio of the average particle diameters (D50) of the Si-containing particles 14 and the graphite particles 12 within a specific range, it is possible to suppress capacity degradation during repeated charge and discharge of the secondary battery.

The density of the molded body is preferably 0.95 g/cm or ^more , more preferably 1.0 g/cm ^{or more} . On the other hand, the upper limit of the density of the molded body may be 2.3 g/cm ^or less, 2.0 g/cm ^or less, 1.8 g/cm ^{or less} , or 1.5 g/cm ^{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 25°C environment, applying a pressure of 60 MPa in one axial direction (i.e., perpendicular to the radial direction) to obtain a tablet-shaped compact, and measuring the bulk density of this compact. The density of the compact can be easily measured, for example, using an automatic powder resistivity measurement system (e.g., the "MCP-PD600" manufactured by Nitto Seiko Analytech Co., Ltd.) and a 20 mm diameter probe (equivalent to the mold).

The density of the compact is affected by the circularity of the Si-containing particles 14. Increasing the circularity of the Si-containing particles 14 tends to increase the density of the compact. 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 higher. In this specification, "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 measuring 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 higher. 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. Spheroidized graphite particles are preferred as the graphite particles 12, 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 this ratio (D50 of graphite-containing particles/D50 of Si-containing particles) is less than 1, the packing strength decreases 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 this ratio (D50 of graphite-containing particles/D50 of Si-containing particles) exceeds 8, the dispersibility of the graphite particles 12 decreases, causing aggregation of the Si-containing particles 14, which decreases the packing strength. 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, "average particle size (D50)" refers to the median size (D50), which is the particle size corresponding to a cumulative frequency of 50 volume percent from the smallest particle size 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 particle size distribution measuring device, etc.

The average particle diameter (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 diameter (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 diameter (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 diameter (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% 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 will not be sufficiently increased. If the mass ratio of the Si-containing particles 14 exceeds 60% by mass, the effect of volume change of the Si-containing particles 14 will be greater, and the effect of suppressing capacity degradation when the secondary battery is repeatedly charged and discharged will be insufficient. The mass ratio of the Si-containing particles 14 is preferably 15% to 55% by mass, and more preferably 20% to 50% by mass.

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

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 capacity degradation during repeated charge and discharge of a secondary battery. That is, when a negative electrode is fabricated using the negative electrode material 10 according to this embodiment and a secondary battery is fabricated using this negative electrode, the secondary battery has excellent resistance to capacity degradation during repeated charge and discharge, and therefore excellent cycle characteristics. Furthermore, because the negative electrode material 10 according to this embodiment contains Si, the capacity of the secondary battery can be increased. Furthermore, because the negative electrode material 10 according to this embodiment contains Si-containing particles 14 with high packing properties, the packing property of the negative electrode active material layer can be improved, 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-described embodiment. Such a negative electrode can provide the secondary battery with excellent resistance to capacity degradation during repeated charge and discharge. Furthermore, 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 described in detail with reference to Figure 2. Figure 2 is a cross-sectional view schematically illustrating an example of a negative electrode 60 according to this embodiment, taken along the thickness and width directions. The negative electrode 60 according to this embodiment shown in Figure 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, as shown in the example, may be provided on both sides of the negative electrode current collector 62. It is preferable that the negative electrode active material layer 64 be provided on both sides of the negative electrode current collector 62.

As shown in the illustrated example, one end of the negative electrode 60 in the width direction may have a negative electrode active material layer non-forming portion 62a where the negative electrode active material layer 64 is not provided. 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 current collector 62 is foil (or sheet), but is not limited to this. The negative electrode current collector 62 may be in various forms, such as a rod, plate, or mesh. As with conventional lithium-ion secondary batteries, the material for the negative electrode current collector 62 can be a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.), with copper being preferred. Copper foil is particularly preferred for the negative electrode current 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, its 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 this 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), and polyvinylidene fluoride (PVDF). 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 the 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 or ^more , preferably 1.1 g/cm ^{or more} , and more preferably 1.2 g/cm or ^more . On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g/cm ^or less, and may be 2.0 g/cm ^or less.

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

The negative electrode 60 can be suitably manufactured by a manufacturing method including the steps 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 "coating step"), drying the applied negative electrode paste to form the negative electrode active material layer 64 (hereinafter also referred to as the "drying step"), and pressing the negative electrode active material layer 64 (hereinafter also referred to as the pressing step).

The coating process can be carried out according to known methods, except for using the negative electrode material according to the above-described embodiment. Specifically, for example, the negative electrode material according to the above-described 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, "paste" refers to a mixture in which some or all of the solids are dispersed in a dispersion medium, and includes so-called "slurry," "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 process can be carried out using known methods. Specifically, for example, the dispersion medium is removed from the negative electrode current collector 62 coated with the negative electrode paste using a drying device such as a drying oven, thereby forming the negative electrode active material layer 64. This completes the drying process. The drying temperature and drying time can be determined appropriately depending on the solids 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 known methods. Specifically, the pressing step can be carried out by applying pressure to the formed negative electrode active material layer 64 using a roller press or the like. The pressing step allows the graphite particles 12 and Si-containing particles 14 contained in the negative electrode material 10 to be densely packed. This results in the negative electrode 60.

The negative electrode 60 can be used in a secondary battery according to known methods. A secondary battery typically includes a positive electrode, a negative electrode, and an electrolyte, with the negative electrode being the above-described negative electrode 60. Below, an example of the configuration of a secondary battery using the negative electrode 60 will be described in detail using 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 constructed by housing a flat wound electrode assembly 20 and a nonaqueous electrolyte (not shown) in a flat, rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, as well as a thin-walled safety valve 36 designed to release internal pressure if the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 also has an inlet (not shown) for injecting the nonaqueous electrolyte. The positive terminal 42 is electrically connected to the positive current collector plate 42a. The negative terminal 44 is electrically connected to the negative current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

As shown in Figures 3 and 4, the wound electrode assembly 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 between them 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. The positive electrode active material layer-free portions 52a (i.e., portions 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-free portions 62a (i.e., portions 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 wound electrode body 20 in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode active material layer-free portions 52a and the negative electrode active material layer-free portions 62a are joined to the positive electrode current collector 42a and the negative electrode current collector 44a, respectively.

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

The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

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

Preferably, the lithium composite oxide is a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element. Specific examples include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

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

Examples of lithium transition metal phosphate compounds 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 due to 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. 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).

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 typically 10 μm or more, and preferably 20 μm or more. On the other hand, this thickness is typically 400 μm or less, and preferably 300 μm or less.

The negative electrode sheet 60 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 PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

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 as measured by the Gurley test method is not particularly limited, but is preferably 350 seconds/100 cc or less.

Non-aqueous electrolytes typically contain a non-aqueous solvent and a supporting salt (electrolyte salt). The non-aqueous solvent can be any organic solvent commonly used in lithium-ion secondary battery electrolytes, including carbonates, ethers, esters, nitriles, sulfones, and lactones. Carbonates are preferred, including 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). These non-aqueous solvents can be used alone or in combination. For example, the non-aqueous solvent may consist solely of 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.

Lithium-ion secondary battery 100 exhibits high capacity and is resistant to capacity degradation during repeated charging and discharging. Lithium-ion secondary battery 100 can be used in a variety of applications. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Lithium-ion secondary battery 100 can also be used as a storage battery for small-sized power storage devices. 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 assembly 20. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries equipped with a stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked). Lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, laminated case lithium-ion secondary batteries, etc.

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

Furthermore, although the negative electrode 60 according to this embodiment is suitable as a negative electrode for a lithium-ion secondary battery, it can also be used as a negative electrode for other secondary batteries, and these other secondary batteries 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 an Si—C composite material having an average particle size (D50) of 8 μm, an Si content of 40 mass %, and a circularity of 0.98 were prepared as Si-containing particles.

1 g of the Si-containing particles was pressed uniaxially at 25°C under 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 was 1.3 g/ ^cm3 .

Graphite particles with an average particle diameter (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 pressed uniaxially 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. Graphite particles and Si-containing particles were mixed in a mass ratio of 60:40 to obtain the negative electrode materials of Examples 2-3 and Comparative Examples 1-3. In each Example and Comparative Example, the average particle size (D50) of the Si-containing particles and graphite particles, the Si content in the Si-containing particles, and the circularity of the Si-containing particles were determined using the following methods.

<Measurement of average particle size (D50) of Si-containing particles and graphite particles>
Using a commercially available laser diffraction particle size distribution analyzer, 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% by volume from the fine particles with smaller particle sizes 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 mass % using a commercially available ICP analyzer.

<Measurement of circularity of Si-containing particles>
Using a commercially available image particle size distribution analyzer, the circularity of the Si-containing particles was determined as the perimeter of a perfect circle having the same area as the projected area of the particle divided by the perimeter of the projected image of the particle.

<Measurement of Density of Molded Body of Si-Containing Particles>
1 g of Si-containing particles was weighed 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.). Using this automatic powder resistivity measurement system, the load and displacement when pressed in one axial direction were measured at 25°C. 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 materials of each example and comparative example, CMC, and PAA were dry-blended using a planetary mixer. The resulting mixture, the SWCNT dispersion, and a dispersion medium were kneaded using a planetary mixer. SBR and the dispersion medium were added to this mixture and mixed uniformly to prepare a negative electrode paste. The mass ratio of the negative electrode material:CMC:PAA:SBR:SWCNT in the negative electrode paste was 100:1:1:1.5:0.1.

The prepared negative electrode paste was applied to the surface of a 10 μm thick copper foil and dried to form a negative electrode active material layer. After the negative electrode active material layer was roll-pressed, the resulting sheet was cut to the specified dimensions 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 a 15 μm thick aluminum foil and dried to form a positive electrode active material layer. The positive electrode active material layer was then roll-pressed, and the resulting sheet was processed to a predetermined size to obtain a positive electrode sheet.

A porous polyolefin separator was prepared. Leads were attached to each of the prepared negative electrode sheet and positive electrode sheet, and the sheets were stacked via a separator to prepare an electrode assembly. This was housed in a case made of 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, with LiPF ₆ dissolved as a supporting electrolyte 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 subjected to constant current charging at a current value of 0.4 C up to 4.2 V, and then constant voltage charging until the current value reached 0.1 C. Next, each of the lithium-ion secondary batteries for evaluation was subjected to constant current discharge at a current value of 0.4 C down to 2.5 V. The discharge capacity at this time was measured to determine 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 multiplying (discharge capacity after 200 charge/discharge cycles/initial capacity) by 100. The results are shown in Table 1.

The results in Table 1 show 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% by mass to 60% by mass, and 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. Therefore, it is clear that the negative electrode material disclosed herein can suppress capacity degradation when a 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 exemplified 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 the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1 to 8;
a mass ratio of the Si-containing particles to the total mass 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/ ^cm3 or more.
[2] 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 [1].
[3] 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. The negative electrode material according to item [1] or [2].
[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. [1] The negative electrode material according to any one of [3] to [4].
[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:

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 plate 44 Negative electrode terminal 44a Negative electrode current collector plate 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 (4)

A negative electrode material containing Si-containing particles and graphite particles,
the Si-containing particles are particles of a Si-C composite material in which Si-containing domains are dispersed in a carbon matrix;
the Si-containing domain is composed of at least one of Si and a Si oxide;
the Si-containing domains are fine particles,
the average particle size of the Si-containing domains is 5 nm to 50 nm;
the circularity of the Si-containing particles is 0.90 to 1;
The graphite particles have a circularity of 0.85 to 1;
The Si content in the Si-containing particles is 40% by mass or more and 50% by mass or less,
a ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1.4 to 2.5 ;
the Si-containing particles have an average particle size (D50) of 6 μm to 9 μm, and the graphite particles have an average particle size (D50) of 12 μm to 20 μm;
a mass ratio of the Si-containing particles to the total mass of the Si-containing particles and the graphite particles is 20 mass% to 50 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 1.0 g/cm3 ^or more and ^1.5 g/cm3 or less . 2. The negative electrode material according to claim 1, wherein the graphite particles have a circularity of 0.90 to 1 . 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:
10. A negative electrode for 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: