JP2024167674A - Anode for secondary battery, method for producing said anode, and secondary battery using said anode - Google Patents

Abstract

To provide a negative electrode containing Si-containing particles and graphite particles and capable of suppressing capacity degradation in repetitive charge and discharge of a secondary battery.SOLUTION: A negative electrode disclosed herein includes a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles. A Si content ratio in the first Si-containing particles is higher than a Si content ratio in the second Si-containing particles. The first Si-containing particles each are coated with a first resin binder, and the second Si-containing particles each are coated with a second resin binder. The Tg of the first resin binder is lower than the Tg of the second resin binder. The Tg of the first resin binder is less than 80°C.SELECTED DRAWING: Figure 2

Description

The present invention relates to a negative electrode for a secondary battery and a method for manufacturing the same. The present invention also relates to a secondary battery using the negative electrode.

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 a negative electrode that contains Si-containing particles and graphite particles and that can suppress capacity degradation when a secondary battery is repeatedly charged and discharged.

The negative electrode 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 graphite particles, first Si-containing particles, and second Si-containing particles. The Si content of the first Si-containing particles is higher than the Si content of the second Si-containing particles. The first Si-containing particles are coated with a first resin binder, and the second Si-containing particles are coated with a second resin binder. The Tg of the first resin binder is lower than the Tg of the second resin binder. The Tg of the first resin binder is less than 80°C.

With this configuration, it is possible to provide a negative electrode 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 method for manufacturing a negative electrode of a secondary battery disclosed herein includes the steps of preparing first Si-containing particles coated with a first resin binder and second Si-containing particles coated with a second resin binder, where the Si content in the first Si-containing particles is higher than the Si content in the second Si-containing particles, the Tg of the first resin binder is lower than the Tg of the second resin binder, and the Tg of the first resin binder is less than 80°C; mixing the first Si-containing particles coated with the first resin binder, the second Si-containing particles coated with the second resin binder, and graphite particles in a dispersion medium to prepare a negative electrode paste; applying the negative electrode paste onto a negative electrode current collector; and drying the applied negative electrode paste.

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

From another aspect, the secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode described above.

This configuration makes it possible to provide a secondary battery that has excellent resistance to capacity degradation when repeatedly charged and discharged.

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. 2 is a cross-sectional view that illustrates a schematic configuration of particles contained in a negative electrode active material layer of the negative electrode of FIG. 1. 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 disclosed herein is used in a secondary battery, and is preferably used in a lithium ion secondary battery. One embodiment of the negative electrode disclosed herein will be specifically described with reference to FIG. 1. FIG. 1 is a cross-sectional view that shows a schematic example of a negative electrode 60 according to this embodiment, and is a cross-sectional view along the thickness direction and width direction. The negative electrode 60 according to this embodiment shown in FIG. 1 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. At least graphite particles, first Si-containing particles with a high Si content, and second Si-containing particles with a low Si content are used as the negative electrode active material. This will be described in detail with reference to FIG. 2. FIG. 2 is a schematic cross-sectional view showing particles contained in the negative electrode active material layer 64 shown in FIG. 1. As shown in FIG. 2, the negative electrode active material layer 64 contains graphite particles 12, first Si-containing particles 14 with a high Si content, and second Si-containing particles 16 with a low Si content.

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.

The shape of the graphite particles 12 is not particularly limited and may be flaky, spherical, etc. The graphite particles 12 are preferably spheroidized graphite particles. When the graphite particles 12 are spherical, 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.

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 using a commercially available static automatic image analyzer to determine the circularity of 100 or more particles and calculating the average value.

The average particle diameter (D50) of the graphite particles 12 is not particularly limited. The average particle diameter (D50) of the graphite particles 12 is, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and even more preferably 12 μm to 20 μm.

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 content of the graphite particles relative to the total of the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16 is preferably 40% by mass to 90% by mass, more preferably 45% by mass to 85% by mass, and even more preferably 50% by mass to 80% by mass.

The first Si-containing particle 14 and the second Si-containing particle 16 may be, for example, particles of a Si-C composite material. A Si-C composite material typically contains a carbon domain and a Si-containing domain. Note that the first Si-containing particle 14 and the second Si-containing particle 16 do not have to be a Si-C composite material, and may be Si particles, Si oxide particles, etc.

The carbon domains are, for example, carbonized products of carbon precursors (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite, etc. The carbon domains preferably constitute a carbon matrix. Thus, the Si-C composite material is preferably a material in which multiple Si-containing domains are dispersed in a carbon matrix. In this case, it is advantageous because the carbon matrix can mitigate volumetric changes due to expansion/contraction of the Si-containing domains.

The Si-containing domain 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 is preferably composed of at least one of Si and Si oxide (SiO _x ). The Si-containing domain may be a fine particle.

The average particle diameter of the Si-containing domain is, for example, 50 nm or less, and may be 5 nm to 50 nm. The "average particle diameter of the Si-containing domain" can be determined as follows. First, the negative electrode active material layer 64 is processed with a focused ion beam (FIB) to prepare a sample for observation with a scanning transmission electron microscope (STEM). Then, the sample is subjected to elemental analysis by EDX element mapping, and 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 domain 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 are determined, and the average value is defined as the "average particle diameter of the Si-containing domain" here.

Examples of Si-C composite materials include those in which fine particles containing Si are dispersed inside a carbon material; those in which fine particles containing Si are embedded in the pores of granulated porous graphite; etc.

In this embodiment, the Si content (S1) in the first Si-containing particle 14 is higher than the Si content (S2) in the second Si-containing particle 16. The Si content (S1) in the first Si-containing particle 14 and the Si content (S2) in the second Si-containing particle 16 are not particularly limited as long as they satisfy this relationship. If these Si content rates are too low, the effect of improving cycle characteristics may be small and the capacity effect of the secondary battery may be reduced. On the other hand, if these Si content rates are too high, the volume change due to expansion/contraction of the first Si-containing particle 14 and the second Si-containing particle 16 may be too large when the secondary battery is repeatedly charged and discharged.

Therefore, the Si content (S1) in the first Si-containing particle 14 is preferably 45% by mass to 80% by mass, and more preferably 55% by mass to 75% by mass. The Si content (S2) in the second Si-containing particle 16 is preferably 20% by mass to 55% by mass, and more preferably 25% by mass to 45% by mass.

The ratio (S1/S2) of the Si content (S1) in the first Si-containing particle 14 to the Si content (S2) in the second Si-containing particle 16 is preferably 1.2 or more, more preferably 1.5 or more, and even more preferably 1.8 or more. The ratio (S1/S2) may be 4 or less, 3.5 or less, 3 or less, or 2.5 or less.

The mass ratio of the first Si-containing particles 14 to the second Si-containing particles 16 is not particularly limited as long as the effects of the present invention are obtained. In order to obtain a better filling state between the first Si-containing particles 14 and the second Si-containing particles 16, the mass ratio of the first Si-containing particles 14 to the second Si-containing particles 16 is preferably 10:90 to 60:40, more preferably 15:85 to 55:45, and even more preferably 20:80 to 45:55.

The total content of the 1Si-containing particles 14 and the second Si-containing particles 16 relative to the total of the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16 is preferably 10% by mass to 60% by mass, more preferably 15% by mass to 55% by mass, and even more preferably 20% by mass to 50% by mass.

The average particle diameter (D50) of the first Si-containing particle 14 and the second Si-containing particle 16 is not particularly limited. The average particle diameter (D50) of the first Si-containing particle 14 and the second Si-containing particle 16 is, for example, 1 μm to 15 μm, preferably 2 μm to 10 μm, and more preferably 4 μm to 10 μm.

As shown in FIG. 2, the first Si-containing particles 14 are coated with a first resin binder 15. The second Si-containing particles 16 are coated with a second resin binder 17. In this specification, the term "resin binder" refers to a resin component that bonds the negative electrode active material particles together and between the negative electrode active material particles and the negative electrode current collector 62.

In the illustrated example, the first Si-containing particle 14 and the second Si-containing particle 16 are entirely coated with the first resin binder 15 and the second resin binder 17, respectively. Thus, the first resin binder 15 and the second resin binder 17 form a coating layer. However, the first Si-containing particle 14 and the second Si-containing particle may be partially coated with the first resin binder 15 and the second resin binder 17, respectively. The coverage of the first Si-containing particle 14 by the first resin binder 15 is preferably 50% to 100%, more preferably 70% to 100%. The coverage of the second Si-containing particle 16 by the second resin binder 17 is preferably 50% to 100%, more preferably 70% to 100%. This coverage is the ratio of the coverage area of the resin binder to the surface area of the Si-containing particle. The coverage rate can be determined by obtaining a cross-sectional electron microscope image of the Si-containing particle and calculating it as a percentage of the total length of the resin binder-coated portion on the surface of the Si-containing particle relative to the circumferential length of the Si-containing particle. The average coverage rate of five or more arbitrarily selected particles can be used as the "coverage rate" here.

In this embodiment, the Tg (glass transition temperature) of the first resin binder 15 is lower than the Tg of the second resin binder 17. In addition, the Tg of the first resin binder 15 is less than 80°C. Therefore, the Tg of the second resin binder 17 is 80°C or higher.

In this way, by using first Si-containing particles 14 and second Si-containing particles 16 with different Si contents in addition to graphite particles 12, coating the first Si-containing particles 14 with a high Si content with a first resin binder 15 with a low Tg, and coating the second Si-containing particles 16 with a low Si content with a second resin binder 17 with a high Tg, it is possible to suppress capacity degradation when the secondary battery is repeatedly charged and discharged. The reason for this is thought to be as follows.

The cause of the capacity degradation when a secondary battery is repeatedly charged and discharged is that the volume change due to the expansion/contraction of the Si-containing particles when the secondary battery is repeatedly charged and discharged is large, which causes the displacement of the Si-containing particles and the breakage of the conductive path. Here, Tg is the temperature at which the molecular chain interaction weakens and the molecular chain segments start micro-Brownian motion when the resin is heated (i.e., when thermal energy is applied to the resin). Therefore, the higher the Tg, the stronger the interaction between the molecular chains, and therefore Tg is an index of how easily/difficult the resin is to deform under stress. A resin binder with a Tg of less than 80°C is easily deformed under stress due to the expansion/contraction of Si, and a resin binder with a Tg of 80°C or higher is difficult to deform under stress due to the expansion/contraction of Si. Therefore, by covering the first Si-containing particles 14 with a high Si content with the first resin binder 15 with a Tg of less than 80°C, the first resin binder 15 can follow the expansion/contraction of Si, and the displacement of the first Si-containing particles 14 due to internal stress caused by volume change can be suppressed. Furthermore, by covering the second Si-containing particles 16 with a low Si content with a resin binder with a Tg of 80°C or more, the expansion/contraction of Si can be suppressed and the occurrence of conductive path breakage can be suppressed. This makes it possible to suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

Examples of resin binders having a Tg of less than 80°C used for the first resin binder 15 include polyvinylidene fluoride, polyvinyl alcohol, polyethylene oxide, polylactic acid, etc. From the viewpoint of higher cycle characteristics of a secondary battery using the negative electrode 60, the Tg of the first resin binder 15 is preferably 70°C or less, more preferably 60°C or less, and even more preferably 50°C or less. The Tg of the first resin binder 15 may be -100°C or more, or 0°C or more.

Examples of resin binders having a Tg of 80°C or higher used in the second resin binder 17 include polyacrylic acid, carboxymethyl cellulose, polyamideimide, polyacrylonitrile, tetrafluoroethylene, polyvinylpyrrolidone, etc. From the viewpoint of higher cycle characteristics of a secondary battery using the negative electrode 60, the Tg of the second resin binder 17 is preferably 100°C or higher, more preferably 150°C or higher, and even more preferably 200°C or higher. The Tg of the second resin binder 17 may be 400°C or lower, or 350°C or lower.

The Tg of the first resin binder 15 and the second resin binder 17 can be determined by differential scanning calorimetry (DSC measurement).

The amount of the first Si-containing particle 14 coated with the first resin binder 15 is not particularly limited. Since the first resin binder 15 is usually insulating, if the amount of coating is too large, the battery resistance may increase. On the other hand, if the amount of coating is too small, the effect of the present invention may be reduced. Therefore, the amount of the first Si-containing particle 14 coated with the first resin binder 15 is preferably 5% by mass to 75% by mass, more preferably 10% by mass to 50% by mass, and even more preferably 15% by mass to 40% by mass. Similarly, the amount of the second Si-containing particle 16 coated with the second resin binder 17 is not particularly limited, but is preferably 5% by mass to 75% by mass, more preferably 10% by mass to 50% by mass, and even more preferably 15% by mass to 40% by mass. The amount of coating is the ratio (%) of the mass of the resin binder to the mass of the Si-containing particle.

The first Si-containing particle 14 and the second Si-containing particle 16 can be produced according to a known method. Various methods for producing particles of Si-C composite material are known (see, for example, JP 2015-38862 A, WO 2014/046144 A, and prior art documents cited in the WOs).

The negative electrode active material layer 64 may contain components other than the negative electrode active material, examples of which include a third resin binder and a conductive material. By using the third resin binder, it is possible to improve the adhesion between the negative electrode active material particles and the negative electrode current collector 62, and improve the adhesion between the resin binders that cover the Si-containing particles. As the third resin binder, for example, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), etc. can be used. CMC also functions as a thickener. Examples of conductive materials include carbon black such as acetylene black, carbon fiber, carbon nanotubes (CNT), etc. Among them, CNT is preferable. 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 third resin 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, 0.7 g/cm ³ or more, preferably 1.0 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 is prepared by a process (hereinafter also referred to as a "coated particle preparation process") in which a first Si-containing particle 14 coated with a first resin binder 15 and a second Si-containing particle 16 coated with a second resin binder 17 are prepared, in which the Si content in the first Si-containing particle 14 is higher than the Si content in the second Si-containing particle 16, the Tg of the first resin binder 15 is lower than the Tg of the second resin binder 17, and the Tg of the first resin binder 15 is less than 80°C; The negative electrode paste can be suitably manufactured by a manufacturing method including a process for preparing a negative electrode paste by mixing the first Si-containing particles 14 coated with the resin binder 15, the second Si-containing particles 16 coated with the second resin binder 17, and the graphite particles 12 in a dispersion medium (hereinafter also referred to as the "paste preparation process"); a process for applying the negative electrode paste onto the negative electrode current collector 62 (hereinafter also referred to as the "application process"); and a process for drying the applied negative electrode paste (hereinafter also referred to as the "drying process").

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.

In the coated particle preparation process, first Si-containing particles 14 with a high Si content and second Si-containing particles 16 with a low Si content are prepared. In addition, a first resin binder with a Tg of less than 80°C and a second resin binder with a Tg of 80°C or higher are prepared.

A known coating method can be used to coat the first Si-containing particles 14 and the second Si-containing particles 16 with the first resin binder and the second resin binder. For example, a solution in which the first resin binder 15 is dissolved in a solvent and a solution in which the second resin binder 17 is dissolved in a solvent are prepared, and the first Si-containing particles 14 and the second Si-containing particles 16 are added thereto, respectively, and the solvent is removed by drying, if necessary, to perform coating.

The paste preparation process can be carried out according to a known method by mixing the graphite particles 12, the first Si-containing particles 14 coated with the first resin binder 15, the second Si-containing particles 16 coated with the second resin binder 17, and optional components (e.g., conductive material, third resin binder, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc.

The coating process can be carried out according to a known method. Specifically, for example, the coating process can be carried out by applying the obtained negative electrode paste onto the negative electrode current collector 62 using a coating device such as a gravure coater, a comma coater, a slit coater, or a 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.

After the drying step, a step of pressing the negative electrode active material layer 64 may be further carried out. The pressing step may be carried out according to a known method. Specifically, the pressing step may be carried out by applying pressure to the negative electrode active material layer 64 formed as above using a roller press or the like. The pressing step may allow the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16 contained in the negative electrode active material layer 64 to be densely packed. In this manner, the negative electrode 60 may be obtained.

The negative electrode 60 according to this embodiment can provide the secondary battery with excellent resistance to capacity degradation when repeatedly charged and discharged. In addition, the negative electrode 60 according to this embodiment uses a negative electrode active material containing Si, so the capacity of the secondary battery can be increased. Therefore, a secondary battery using the negative electrode 60 according to this embodiment has a high capacity and excellent cycle characteristics.

Therefore, from another perspective, the secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-mentioned embodiment. Below, one embodiment of the secondary battery disclosed herein will be described with reference to Figures 3 and 4, taking a lithium-ion secondary battery as an example. The following configuration example 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 above-mentioned negative electrode 60 is used as the negative electrode sheet 60.

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 constructed and used as a negative electrode in other secondary batteries, and the 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>
Example 1
The following negative electrode active materials were prepared. The Si content of the first Si-containing particles and the second Si-containing particles was measured using a commercially available ICP analyzer. The average particle diameter (D50) of the first Si-containing particles, the second Si-containing particles, and the graphite particles was measured using a commercially available laser diffraction/scattering type particle size distribution analyzer.
First Si-containing particles: Si-C composite material, Si content ratio = 65% by mass, average particle diameter (D50) = 6 μm
Second Si-containing particles: Si-C composite material, Si content ratio = 35% by mass, average particle diameter (D50) = 7 μm
Graphite particles: average particle size (D50) = 14 μm

Polyvinyl alcohol (Tg = 40°C) was prepared as the first resin binder. Polyamideimide (Tg = 280°C) was prepared as the second resin binder. These were each dissolved in a solvent to obtain a solution. The Tg of the first resin binder and the second resin binder were measured using a commercially available differential scanning calorimeter.

The first Si-containing particles, the first resin binder solution, and the solvent were mixed using a disper at a rotation speed of 3000 rpm. In this way, particles in which the surfaces of the first Si-containing particles are coated with the first resin binder were obtained. Similarly, the second Si-containing particles, the second resin binder solution, and the solvent were mixed using a disper at a rotation speed of 3000 rpm. In this way, particles in which the surfaces of the second Si-containing particles are coated with the second resin binder were obtained.

Single-walled carbon nanotubes (SWCNT) were prepared as the conductive material. The SWCNT was prepared in the form of a dispersion liquid. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared as binders.

A negative electrode paste containing graphite particles, first Si-containing particles, second Si-containing particles, first resin binder, second resin binder, CMC, PAA, SBR, and SWCNT in a mass ratio of 60:12:28:3:7:1:1:1.5:0.1 was prepared by the following procedure.

First, graphite particles, CMC, and PAA were dry-blended using a planetary mixer. To the resulting mixture, SWCNT dispersion and a dispersion medium were added, and the mixture was kneaded in the planetary mixer. To the resulting mixture, the first Si-containing particles coated with a first resin binder, the second Si-containing particles coated with a second resin binder, and a dispersion medium were added, and the mixture was mixed in the planetary mixer. Furthermore, SBR and a dispersion medium were added to the planetary mixer, and the mixture was diluted and mixed to obtain a negative electrode paste.

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.

Example 2
A negative electrode sheet of Example 2 was obtained in the same manner as in Example 1, except that the first resin binder was changed to polylactic acid (Tg = 60 ° C.) and the second resin binder was changed to polyacrylic acid (Tg = 110 ° C.).

Comparative Example 1
A negative electrode sheet of Comparative Example 1 was obtained in the same manner as in Example 1, except that the first resin binder was changed to polyamideimide (Tg = 280 ° C.) and the second resin binder was changed to polyvinyl alcohol (Tg = 40 ° C.).

Comparative Example 2
A negative electrode sheet of Comparative Example 2 was obtained in the same manner as in Example 1, except that the second resin binder was changed to polyvinyl alcohol (Tg = 40°C).

Comparative Example 3
A negative electrode sheet of Comparative Example 3 was obtained in the same manner as in Example 1, except that the first resin binder was changed to polyamideimide (Tg = 280°C).

Comparative Example 4
A negative electrode sheet of Comparative Example 4 was obtained in the same manner as in Example 1, except that the mass ratio of the solid contents of the negative electrode paste was changed to graphite particles:first Si-containing particles:second Si-containing particles:first resin binder:second resin binder:CMC:PAA:SBR:SWCNT=60:40:0:10:0:1:1:1.5:0.1.

Comparative Example 5
A negative electrode sheet of Comparative Example 5 was obtained in the same manner as in Example 1, except that the mass ratio of the solid contents of the negative electrode paste was changed to graphite particles:first Si-containing particles:second Si-containing particles:first resin binder:second resin binder:CMC:PAA:SBR:SWCNT=60:0:40:0:10:1:1:1.5:0.1.

<Preparation of Lithium-Ion Secondary Battery for Evaluation>
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 250 times. The discharge capacity after 250 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 250 charge/discharge cycles/initial capacity) x 100. The results are shown in Table 1.

The results in Table 1 show that when the Si content in the first Si-containing particle is higher than the Si content in the second Si-containing particle, the first Si-containing particle is coated with a resin binder having a Tg of less than 80°C, and the second Si-containing particle is coated with a resin binder having a Tg of 80°C or higher, the capacity retention rate after 250 charge/discharge cycles is significantly high. Therefore, it can be seen that the negative electrode disclosed herein can suppress capacity degradation 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 of the secondary battery, the manufacturing method thereof, and the secondary battery disclosed herein are the following items [1] to [7].
[1] A negative electrode comprising 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 graphite particles, first Si-containing particles, and second Si-containing particles,
a Si content in the first Si-containing particle is higher than a Si content in the second Si-containing particle,
the first Si-containing particles are coated with a first resin binder, and the second Si-containing particles are coated with a second resin binder;
The Tg of the first resin binder is lower than the Tg of the second resin binder,
The first resin binder has a Tg of less than 80° C.
Negative electrode.
[2] The negative electrode according to item [1], wherein the second resin binder has a Tg of more than 100° C.
[3] The negative electrode according to item [1], wherein the first resin binder has a Tg of 50° C. or less, and the second resin binder has a Tg of 200° C. or more.
[4] The graphite particles, the first Si-containing particles, and the second Si-containing particles are contained in a total of 40% by mass to 90% by mass of the graphite particles, and the mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 60:40. The negative electrode according to any one of items [1] to [3].
[5] The negative electrode according to any one of items [1] to [4], wherein the first Si-containing particles and the second Si-containing particles are particles of a Si-C composite material.
[6] A step of preparing first Si-containing particles coated with a first resin binder and second Si-containing particles coated with a second resin binder, wherein a Si content in the first Si-containing particles is higher than a Si content in the second Si-containing particles, a Tg of the first resin binder is lower than a Tg of the second resin binder, and the Tg of the first resin binder is less than 80° C.;
mixing the first Si-containing particles coated with the first resin binder, the second Si-containing particles coated with the second resin binder, and graphite particles in a dispersion medium to prepare a negative electrode paste;
applying the negative electrode paste onto a negative electrode current collector; and drying the applied negative electrode paste.
A method for producing a negative electrode comprising the steps of:
[7] A positive electrode, a negative electrode, and an electrolyte;
A secondary battery comprising:
The negative electrode is the negative electrode according to any one of items [1] to [5].

12 Graphite particle 14 First Si-containing particle 15 First resin binder 16 Second Si-containing particle 17 Second resin binder 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 (7)

A negative electrode comprising 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 graphite particles, first Si-containing particles, and second Si-containing particles,
a Si content in the first Si-containing particle is higher than a Si content in the second Si-containing particle,
the first Si-containing particles are coated with a first resin binder, and the second Si-containing particles are coated with a second resin binder;
The Tg of the first resin binder is lower than the Tg of the second resin binder,
The first resin binder has a Tg of less than 80° C.
Negative electrode. The negative electrode according to claim 1, wherein the Tg of the second resin binder is greater than 100°C. The negative electrode according to claim 1, wherein the Tg of the first resin binder is 50°C or less, and the Tg of the second resin binder is 200°C or more. The negative electrode according to claim 1, wherein the content of the graphite particles relative to the total of the graphite particles, the first Si-containing particles, and the second Si-containing particles is 40% by mass to 90% by mass, and the mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 60:40. The negative electrode according to claim 1, wherein the first Si-containing particles and the second Si-containing particles are each particles of a Si-C composite material. a step of preparing first Si-containing particles coated with a first resin binder and second Si-containing particles coated with a second resin binder, wherein a Si content in the first Si-containing particles is higher than a Si content in the second Si-containing particles, a Tg of the first resin binder is lower than a Tg of the second resin binder, and a Tg of the first resin binder is less than 80° C.;
mixing the first Si-containing particles coated with the first resin binder, the second Si-containing particles coated with the second resin binder, and graphite particles in a dispersion medium to prepare a negative electrode paste;
applying the negative electrode paste onto a negative electrode current collector; and drying the applied negative electrode paste.
A method for producing a negative electrode comprising the steps of: A positive electrode, a negative electrode, an electrolyte,
A secondary battery comprising:
A secondary battery, wherein the negative electrode is the negative electrode according to claim 1.

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