Negative electrode for secondary battery, method for manufacturing the negative electrode, and secondary battery using the negative electrode

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. The first Si-containing particles have an aspect ratio larger than an aspect ratio of the second Si-containing particles. The aspect ratio of the first Si-containing particles is 4.0 to 10.0, and the aspect ratio of the second Si-containing particles is 1.0 to 3.0. A mass ratio between the first Si-containing particles and the second Si-containing particles is 10:90 to 50:50.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a negative electrode for a secondary battery and a method for producing the same, and also to a secondary battery using the negative electrode. [Background technology]

[0002] In recent years, secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, and the like, and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] 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 in which Si-containing particles and graphite particles such as natural graphite are used in combination as a negative electrode active material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-38862 Summary of the Invention [Problem to be solved by the invention]

[0005] However, while Si-containing particles have a high capacity, they undergo large volume changes due to expansion / contraction when a secondary battery is charged and discharged. Therefore, when Si-containing particles and graphite particles are used in combination, particularly when the proportion of Si-containing particles is high, the packing of these particles decreases when the secondary battery is repeatedly charged and discharged, which can lead to breakage of the conductive path and the generation of internal stress. Therefore, when Si-containing particles and graphite particles are used in combination, the cycle characteristics of the secondary battery deteriorate, specifically, the capacity deteriorates significantly when the secondary battery is repeatedly charged and discharged.

[0006] In view of the above circumstances, an object of the present invention is to provide a negative electrode containing Si-containing particles and graphite particles, which can suppress capacity degradation when a secondary battery is repeatedly charged and discharged. [Means for solving the problem]

[0007] The negative electrode disclosed herein 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 graphite particles, first Si-containing particles, and second Si-containing particles. The first Si-containing particles have a larger aspect ratio than the second Si-containing particles. The aspect ratio of the first Si-containing particles is 4.0 to 10.0, and the aspect ratio of the second Si-containing particles is 1.0 to 3.0. The mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 50:50.

[0008] According to 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.

[0009] From another aspect, the present disclosure provides a method for manufacturing a negative electrode for a secondary battery, comprising the steps of: preparing a negative electrode paste containing graphite particles, first Si-containing particles, second Si-containing particles, and a dispersion medium; applying the negative electrode paste 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. The first Si-containing particles have a larger aspect ratio than the second Si-containing particles. The aspect ratio of the first Si-containing particles is 4.0 to 10.0, and the aspect ratio of the second Si-containing particles is 1.0 to 3.0. The mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 50:50.

[0010] The negative electrode obtained by such a configuration can provide the secondary battery with excellent resistance to capacity degradation when repeatedly charged and discharged.

[0011] From another aspect, the present disclosure provides a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode described above.

[0012] According to this configuration, it is possible to provide a secondary battery that has excellent resistance to capacity degradation when repeatedly charged and discharged. [Brief explanation of the drawings]

[0013] [Figure 1] 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. [Figure 2] 2 is a cross-sectional view schematically showing the configuration of particles contained in the negative electrode active material layer of the negative electrode of FIG. 1. FIG. [Figure 3] 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. [Figure 4] FIG. 4 is a schematic exploded view showing the configuration of a wound electrode body of the lithium ion secondary battery of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described 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 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. 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 drawing do not reflect the actual dimensional relationships. In this specification, a numerical range expressed as "A to B" includes A and B.

[0015] 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.

[0016] 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 schematically illustrating an example of a negative electrode 60 according to this embodiment, taken along the thickness direction and the width direction. The negative electrode 60 according to this embodiment shown in FIG. 1 is a negative electrode for a lithium-ion secondary battery.

[0017] 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 the negative electrode current collector 62 and the 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.

[0018] As shown in the illustrated example, a negative electrode active material layer-free portion 62a where no negative electrode active material layer 64 is provided may be provided at one end in the width direction of the negative electrode 60. In the negative electrode active material layer-free portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer-free portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0019] In the illustrated example, the shape of the negative electrode current collector 62 is foil (or sheet), but is not limited thereto. The negative electrode current collector 62 may have various shapes such as a rod, a plate, or a mesh. As with conventional lithium-ion secondary batteries, the material of the negative electrode current 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 current collector 62.

[0020] The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately depending on the battery design. When a copper foil is used as the negative electrode current collector 62, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 6 μm to 20 μm.

[0021] The negative electrode active material layer 64 contains a negative electrode active material. The negative electrode active material includes at least graphite particles, first Si-containing particles with a high aspect ratio, and second Si-containing particles with a low aspect ratio. 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 aspect ratio, and second Si-containing particles 16 with a low aspect ratio.

[0022] The graphite constituting the graphite particles 12 may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

[0023] The shape of the graphite particles 12 is not particularly limited and may be flaky, spherical, etc. The graphite particles 12 are preferably spherical 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.

[0024] 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.

[0025] There are no particular limitations on the average particle diameter (D50) of the graphite particles 12. 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.

[0026] In this specification, the term "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 analyzer or the like.

[0027] 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 mass% to 90 mass%, more preferably 45 mass% to 85 mass%, and even more preferably 50 mass% to 80 mass%.

[0028] The first Si-containing particles 14 and the second Si-containing particles 16 can be, for example, particles of a Si-C composite material. The Si-C composite material typically contains carbon domains and Si-containing domains. The first Si-containing particles 14 and the second Si-containing particles 16 do not have to be Si-C composite materials and may be Si particles, Si oxide particles, or the like.

[0029] 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. Therefore, the Si-C composite material is preferably a material in which multiple Si-containing domains are dispersed in a carbon matrix. In this case, the carbon matrix is ​​advantageous because it can mitigate volume changes due to the expansion / contraction of the Si-containing domains.

[0030] The Si-containing domain contains Si, for example, Si, Si oxide (SiO x ), Si nitride (SiNx), Si carbide (SiCx), etc. The Si-containing domain is preferably composed of Si and Si oxide (SiO x The Si-containing domain may be a fine particle.

[0031] The average particle diameter of the Si-containing domains is, for example, 50 nm or less, and may be 5 nm to 50 nm. The "average particle diameter of the Si-containing domains" 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). The sample is then subjected to elemental analysis by EDX element mapping, and 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 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 thereof is defined as the "average particle diameter of the Si-containing domains" herein.

[0032] The Si-C composite material is, for example, a material in which fine particles containing Si are dispersed inside a carbon material; or a material in which fine particles containing Si are embedded in the pores of granulated porous graphite;

[0033] The Si content in the first Si-containing particles 14 and the Si content in the second Si-containing particles 16 are not particularly limited. The Si content in the first Si-containing particles 14 and the Si content in the second Si-containing particles 16 are each preferably 20% by mass to 80% by mass, more preferably 30% by mass to 70% by mass, and even more preferably 40% by mass to 60% by mass. The ratio of the Si content in the first Si-containing particles 14 to the Si content in the second Si-containing particles 16 is preferably 1.0 or less.

[0034] To obtain a suitable packing 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 10:90 to 50:50, preferably 15:85 to 40:60, more preferably 18:82 to 35:65, and even more preferably 20:80 to 30:70.

[0035] The total content of the first Si-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 mass% to 60 mass%, more preferably 15 mass% to 55 mass%, and even more preferably 20 mass% to 50 mass%.

[0036] The aspect ratio of the first Si-containing particles 14 is larger than that of the second Si-containing particles 16. The aspect ratio of the first Si-containing particles 14 is 4.0 to 10.0. The aspect ratio of the second Si-containing particles 16 is 1.0 to 3.0. Therefore, the second Si-containing particles 16 have a spherical or nearly spherical shape.

[0037] By using such first Si-containing particles 14 and second Si-containing particles 16 together with graphite particles at a predetermined mass ratio, it is possible to suppress capacity degradation during repeated charge and discharge of the secondary battery. The reason for this is believed to be as follows.

[0038] When such first Si-containing particles 14 and second Si-containing particles 16 are contained in the negative electrode active material layer 64, the first Si-containing particles 14 with a high aspect ratio fill the gaps between the second Si-containing particles 16 with a low aspect ratio (i.e., approximately spherical) like wedges, as shown in FIG. 2. As a result, the Si-containing particles are less likely to deform during charging and discharging of the secondary battery, which relieves internal stress during expansion and contraction and suppresses displacement of the Si-containing particles. Furthermore, disconnection of the conductive paths can be suppressed. This suppresses capacity degradation during repeated charging and discharging of the secondary battery.

[0039] Therefore, if the aspect ratio of the first Si-containing particles 14 is too small, the first Si-containing particles 14 will have difficulty entering the gaps between the second Si-containing particles 16. On the other hand, if the aspect ratio of the first Si-containing particles 14 is too large, the filling ability will decrease. Therefore, the aspect ratio of the first Si-containing particles 14 is 4.0 to 10.0, preferably 4.5 to 9.0, more preferably 5.0 to 9.0, and even more preferably 5.0 to 7.0. The first Si-containing particles 14 are preferably Si-C composite particles in which Si-containing domains are introduced into flake graphite or flake graphite granules.

[0040] If the second Si-containing particles 16 are too non-spherical, their packing properties will decrease. Therefore, the aspect ratio of the second Si-containing particles 16 is 1.0 to 3.0, preferably 1.0 to 2.0, more preferably 1.0 to 1.5, and even more preferably 1.0 to 1.3. The second Si-containing particles 16 are preferably Si-C composite particles in which Si-containing domains are introduced into spherical graphite granules.

[0041] In this specification, the aspect ratio of a particle refers to the ratio of the particle's major axis diameter to its minor axis diameter (major axis diameter / minor axis diameter). The aspect ratios of the first Si-containing particle 14 and the second Si-containing particle 16 can be determined by acquiring images of the first Si-containing particle 14 and the second Si-containing particle 16, determining the ratio of the major axis diameter to the minor axis diameter (major axis diameter / minor axis diameter) for 100 or more arbitrarily selected particles, and calculating the average value. The aspect ratio can be easily measured using an image particle size distribution analyzer.

[0042] 2 is a schematic diagram conceptually illustrating the packed state of the first Si-containing particles 14 and the second Si-containing particles 16, the arrangement of the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16 in the negative electrode active material layer 64 is not limited to the illustrated example. All of the first Si-containing particles 14 do not have to be wedged between the second Si-containing particles 16.

[0043] There are no particular limitations on the sizes of the first Si-containing particles 14 and the second Si-containing particles 16. The major axis diameter (D1) of the first Si-containing particles 14 is, for example, 2 μm to 15 μm, and preferably 4 μm to 12 μm. The major axis diameter (D2) of the second Si-containing particles 16 is, for example, 2 μm to 10 μm, and preferably 4 μm to 8 μm.

[0044] Here, the cycle characteristics of the secondary battery can be further improved by ensuring that the ratio (D1 / D2) of the major axis diameter (D1) of the first Si-containing particles 14 with a high aspect ratio to the major axis diameter (D2) of the second Si-containing particles 16 with a low aspect ratio is not too long. This is thought to be because the number of first Si-containing particles filling the gaps between the second Si-containing particles 16 like wedges increases. Therefore, the ratio (D1 / D2) of the major axis diameter (D1) of the first Si-containing particles 14 to the major axis diameter (D2) of the second Si-containing particles 16 is preferably 2 or less, more preferably 1.5 or less, and even more preferably 1.0 or less. The ratio (D1 / D2) may be 0.5 or more, 0.7 or more, or 0.8 or more.

[0045] The major axis diameter (D1) of the first Si-containing particle 14 and the major axis diameter (D2) of the second Si-containing particle 16 can be determined by acquiring images of the first Si-containing particle 14 and the second Si-containing particle 16, determining the major axis diameters of 100 or more arbitrarily selected particles, and calculating the average value. The major axis diameter (D1) and the major axis diameter (D2) can be easily measured using an image particle size distribution analyzer.

[0046] The first Si-containing particles 14 and the second Si-containing particles 16 can be produced by a known method. Various methods for producing particles of Si-C composite materials are known (see, for example, JP 2015-38862 A, WO 2014 / 046144 A, and prior art documents cited in the WOs).

[0047] The negative electrode active material layer 64 may contain components other than the negative electrode active material, examples of which include a binder, a conductive material, etc. Examples of binders that can be used include styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), etc. CMC also functions as a thickener. Examples of conductive materials include carbon black such as acetylene black, carbon fiber, and carbon nanotubes (CNT). Among these, CNT is preferred. When CNT is used as the conductive material, the negative electrode active material layer 64 may contain a dispersant for the CNT.

[0048] 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 mass% or more, and more preferably 95 mass% or more. The content of the binder in the negative electrode active material layer 64 is preferably 0.1 mass% to 8 mass% or less, and more preferably 0.5 mass% to 5 mass% or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01 mass% to 3 mass% or less, and more preferably 0.05 mass% to 1 mass% or less.

[0049] 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.

[0050] 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 More preferably, it is 1.2 g / cm or more. 3 On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm 3 is less than or equal to 2.0 g / cm 3 It may be the following:

[0051] 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.

[0052] The negative electrode 60 can be suitably produced by a production method including the steps of preparing a negative electrode paste containing graphite particles 12, first Si-containing particles 14, second Si-containing particles 16, and a dispersion medium (hereinafter also referred to as the "paste preparation step"); applying the prepared negative electrode paste to a negative electrode current collector 62 (hereinafter also referred to as the "coating step"); drying the applied negative electrode paste to form a 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 first Si-containing particles 14 have a larger aspect ratio than the second Si-containing particles 16, with the aspect ratio of the first Si-containing particles 14 being 4.0 to 10.0 and the aspect ratio of the second Si-containing particles 16 being 1.0 to 3.0. The mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 50:50.

[0053] In this specification, the term "paste" refers to a mixture in which a part or all of the solid content is dispersed in a dispersion medium, and includes so-called "slurry," "ink," and the like.

[0054] In the paste preparation step, first, first Si-containing particles 14 having an aspect ratio of 4.0 to 10.0 and second Si-containing particles 16 having an aspect ratio of 1.0 to 3.0 are prepared. At this time, these are weighed so that the mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 50:50.

[0055] The paste can be prepared by a known method by mixing the graphite particles 12, the first Si-containing particles 14, the second Si-containing particles 16, and optional components (e.g., binder, conductive material, etc.) with a dispersion medium (e.g., water) using a known mixer, stirrer, etc. Here, the first Si-containing particles 14 and the second Si-containing particles 16 are preferably premixed by dry blending, and the resulting premix is ​​used to prepare the negative electrode paste. Premixing the first Si-containing particles 14 and the second Si-containing particles 16 improves the packing state of the first Si-containing particles 14 and the second Si-containing particles 16 in the negative electrode active material layer 64, thereby further improving the cycle characteristics. Premixing is preferably performed so that the first Si-containing particles 14 and the second Si-containing particles 16 are uniformly mixed.

[0056] The first Si-containing particles 14 and the second Si-containing particles 16 can be premixed by dry blending according to a known method. For example, the first Si-containing particles 14 and the second Si-containing particles 16 can be mixed uniformly using a mixer equipped with a stirring blade (e.g., a Disper mixer, a tumbler mixer, a Henschel mixer, a ribbon mixer, etc.) or a Nauta mixer. The resulting premix, graphite particles, and optional components (e.g., a binder, a conductive material, etc.) can be mixed with a dispersion medium (e.g., water) to obtain a negative electrode paste.

[0057] The coating step can be performed according to a known method. Specifically, for example, the coating step can be performed 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.

[0058] The drying step can be performed according to a known method. 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 allows the drying step to be performed. The drying temperature and drying time can be appropriately determined depending on 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.

[0059] The pressing step can be performed according to a known method. Specifically, the pressing step can be performed by applying pressure to the formed negative electrode active material layer 64 using a roller press or the like. The pressing step densely packs the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16, increasing the number of first Si-containing particles filling the gaps between the second Si-containing particles 16 like wedges. In this way, the negative electrode 60 is obtained.

[0060] The negative electrode 60 according to this embodiment can provide a secondary battery with excellent resistance to capacity degradation during repeated charge and discharge. Furthermore, the negative electrode 60 according to this embodiment uses a negative electrode active material containing Si, which can increase the capacity of the secondary battery. Therefore, a secondary battery using the negative electrode 60 according to this embodiment has a high capacity and excellent cycle characteristics.

[0061] Therefore, from another aspect, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-described embodiment. Hereinafter, one embodiment of the secondary battery disclosed herein will be described with reference to FIGS. 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.

[0062] 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 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 electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is designed to release internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or higher. The battery case 30 also has an inlet (not shown) for injecting the nonaqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0063] 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 overlapped 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 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 portion 52a (i.e., a 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-free portion 62a (i.e., a 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 in 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-free portion 52a and the negative electrode active material layer-free portion 62a are joined to the positive electrode current collector 42a and the negative electrode current collector 44a, respectively.

[0064] The positive electrode current collector 52 constituting the positive electrode sheet 50 may be a known positive electrode current collector used in lithium ion secondary batteries, and examples thereof include 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.

[0065] The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0066] 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, a spinel structure, an olivine structure, or the like.

[0067] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0068] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0069] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0070] 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.

[0071] 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.

[0072] 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).

[0073] 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.

[0074] The thickness of the positive electrode active material layer 54 per side 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.

[0075] As the negative electrode sheet 60, the above-mentioned negative electrode 60 is used.

[0076] 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.

[0077] 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 measured by the Gurley test method is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0078] The nonaqueous electrolyte typically contains a nonaqueous solvent and a supporting salt (electrolyte salt). As the nonaqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones commonly used in electrolytes for lithium-ion secondary batteries can be used without any particular limitation. Among these, carbonates are preferred, 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). One of these nonaqueous solvents can be used alone, or two or more can be used in appropriate combination. For example, the nonaqueous solvent consists solely of carbonates. As another example, non-aqueous solvents include carbonates and esters such as methyl acetate.

[0079] As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) 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.

[0080] The nonaqueous electrolyte may contain various additives other than the above-mentioned components, such as film-forming agents such as vinylene carbonate (VC) and oxalate complexes; gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); and thickeners, as long as the effects of the present invention are not significantly impaired.

[0081] The lithium ion secondary battery 100 exhibits reduced capacity degradation during repeated charge and discharge and has a high capacity. The lithium ion secondary battery 100 can be used for 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). The lithium ion secondary battery 100 can also be used as a storage battery for small power storage devices and the like. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of a plurality of batteries connected in series and / or parallel.

[0082] The above describes, as an example, a rectangular lithium ion secondary battery 100 equipped with a flat wound electrode assembly 20. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery equipped with a stacked electrode assembly (i.e., an electrode assembly 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 lithium ion secondary battery, etc.

[0083] Furthermore, 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 non-aqueous electrolyte according to known methods.

[0084] Furthermore, although the negative electrode 60 according to this embodiment is suitable as a negative electrode for a lithium ion secondary battery, it can be constructed and used as a negative electrode for other secondary batteries, and the other secondary batteries can be constructed according to known methods.

[0085] Examples of the present invention will be described in detail below, but it is not intended that the present invention be limited to those shown in these examples.

[0086] <Preparation of negative electrode> Example 1 The following negative electrode active materials were prepared. The major axis lengths and aspect ratios of the first Si-containing particles and the second Si-containing particles were measured using an image particle size distribution analyzer. The Si contents of the first Si-containing particles and the second Si-containing particles were measured using a commercially available ICP analyzer. First Si-containing particle: Si-C composite material, aspect ratio = 7, major axis length = 6 μm, Si content = 48 mass% Second Si-containing particles: Si-C composite material, aspect ratio = 1.2, major axis length = 6 μm, Si content = 53 mass% Graphite particles: average particle size (D50) = 13 μm

[0087] Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared as binders, and a dispersion of single-walled carbon nanotubes (SWCNTs) was prepared as a conductive material.

[0088] A negative electrode paste containing graphite particles, first Si-containing particles, second Si-containing particles, CMC, PAA, SBR, and SWCNTs in a mass ratio of 60:8:32:1:1:2:0.1 was prepared as follows: First, the first Si-containing particles and the second Si-containing particles were dry-blended using a disper at a rotation speed of 3000 rpm to uniformly premix them.

[0089] The resulting premix, graphite particles, CMC, and PAA were dry-blended using a planetary mixer at a rotation speed of 60 rpm. The resulting mixture, SWCNT dispersion, and dispersion medium were kneaded using the planetary mixer. SBR and the dispersion medium were added to this mixture and mixed uniformly to prepare a negative electrode paste.

[0090] 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. The negative electrode active material layer was roll-pressed, and the resulting sheet was cut to a predetermined size to obtain a negative electrode sheet.

[0091] Example 2 The negative electrode sheet of Example 2 was obtained in the same manner as in Example 1, except that Si-C composite particles with an aspect ratio of 5, a major axis length of 5 μm, and a Si content of 50 mass% were used as the first Si-containing particles, and 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:CMC:PAA:SBR:SWCNT=60:12:28:1:1:2:0.1.

[0092] Example 3 The negative electrode sheet of Example 3 was obtained in the same manner as in Example 1, except that Si-C composite particles with an aspect ratio of 7, a major axis length of 10 μm, and a Si content of 52 mass% were used as the first Si-containing particles.

[0093] Example 4 The negative electrode sheet of Example 4 was obtained in the same manner as in Example 1, except that when preparing the negative electrode paste, the first Si-containing particles and the second Si-containing particles were not premixed, and the graphite particles, the first Si-containing particles, the second Si-containing particles, CMC, and PAA were dry-blended using a planetary mixer at a rotation speed of 60 rpm.

[0094] Comparative Example 1 A negative electrode sheet of Comparative Example 1 was obtained in the same manner as in Example 1, except that Si-C composite particles with an aspect ratio of 2, a major axis length of 7 μm, and a Si content of 51 mass % were used as the first Si-containing particles.

[0095] Comparative Example 2 The negative electrode sheet of Comparative Example 2 was obtained in the same manner as in Example 1, except that Si-C composite particles with an aspect ratio of 5, a major axis length of 5 μm, and a Si content of 50 mass% were used as the first Si-containing particles, and 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:CMC:PAA:SBR:SWCNT=60:2:38:1:1:2:0.1.

[0096] Comparative Example 3 The negative electrode sheet of Comparative Example 3 was obtained in the same manner as in Example 1, except that Si-C composite particles with an aspect ratio of 5, a major axis length of 5 μm, and a Si content of 50 mass% were used as the first Si-containing particles, and 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:CMC:PAA:SBR:SWCNT=60:24:16:1:1:2:0.1.

[0097] 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:CMC:PAA:SBR:SWCNT=60:40:0:1:1:2:0.1.

[0098] 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 content of the negative electrode paste was changed to graphite particles:first Si-containing particles:second Si-containing particles:CMC:PAA:SBR:SWCNT=60:0:40:1:1:2:0.1.

[0099] <Preparation of Lithium-ion Secondary Batteries for Evaluation> LiNi as a positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode paste was prepared by mixing O2 (NCM), 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. After roll pressing the positive electrode active material layer, the resulting sheet was cut to the specified dimensions to obtain a positive electrode sheet.

[0100] A porous polyolefin separator was prepared. Leads were attached to the negative electrode sheet and positive electrode sheet prepared above, and they were stacked with a separator interposed between them 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 used 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 LiPF6 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.

[0101] <Cycle characteristic evaluation> Each of the lithium-ion secondary batteries for evaluation prepared above was placed in an environment of 25°C. Each lithium-ion secondary battery 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 lithium-ion secondary battery 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.

[0102] 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 the discharge capacity after 200 charge / discharge cycles by the initial capacity by 100. The results are shown in Table 1.

[0103] [Table 1]

[0104] The results in Table 1 show that when the aspect ratio of the first Si-containing particles is 4.0 to 10.0, the aspect ratio of the second Si-containing particles is 1.0 to 3.0, and the mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 50:50, the capacity retention rate after 200 charge / discharge cycles is significantly high. This shows that the negative electrode disclosed herein can suppress capacity degradation during repeated charge / discharge of a secondary battery.

[0105] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0106] 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 [9]. [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; the first Si-containing particles have a larger aspect ratio than the second Si-containing particles; the first Si-containing particles have an aspect ratio of 4.0 to 10.0; the aspect ratio of the second Si-containing particles is 1.0 to 3.0; a mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 50:50; Negative electrode. [2] The negative electrode according to item [1], wherein the ratio of the major axis diameter of the first Si-containing particle to the major axis diameter of the second Si-containing particle is 1.5 or less. [3] The negative electrode according to item [1], wherein the ratio of the major axis diameter of the first Si-containing particle to the major axis diameter of the second Si-containing particle is 1.0 or less. [4] The negative electrode according to any one of items [1] to [3], wherein the first Si-containing particles have a major axis length of 4 μm to 12 μm, and the second Si-containing particles have a major axis length of 2 μm to 10 μm. [5] The negative electrode according to any one of items [1] to [4], wherein the aspect ratio of the first Si-containing particles is 5.0 to 9.0, and the aspect ratio of the second Si-containing particles is 1.0 to 2.0. [6] The negative electrode according to any one of items [1] to [5], wherein a content ratio of the graphite particles to the total of the graphite particles, the first Si-containing particles, and the second Si-containing particles is 40 mass% to 90 mass%. [7] preparing a negative electrode paste containing graphite particles, first Si-containing particles, second Si-containing particles, and a dispersion medium; applying the negative electrode paste to a negative electrode current collector; a step of drying the applied negative electrode paste to form a negative electrode active material layer; and pressing the negative electrode active material layer; Equipped with the first Si-containing particles have a larger aspect ratio than the second Si-containing particles; the first Si-containing particles have an aspect ratio of 4.0 to 10.0; the aspect ratio of the second Si-containing particles is 1.0 to 3.0; a mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 50:50; A method for manufacturing a negative electrode of a secondary battery. [8] The manufacturing method according to item [7], wherein in the step of preparing the negative electrode paste, the first Si-containing particles and the second Si-containing particles are premixed by dry blending, and the obtained premix is ​​used to prepare the negative electrode paste. [9] 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 any one of items [1] to [6]. [Explanation of symbols]

[0107] 12 Graphite particles 14 1st Si-containing particles 16 2nd Si-containing particles 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator sheet (separator) 100 Lithium-ion secondary battery

Claims

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, the first Si-containing particles have a larger aspect ratio than the second Si-containing particles; the first Si-containing particles have an aspect ratio of 4.0 to 10.0; the aspect ratio of the second Si-containing particles is 1.0 to 3.0; the first Si-containing particles have a major axis length of 4 μm to 12 μm, and the second Si-containing particles have a major axis length of 2 μm to 10 μm; a mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 50:50; Negative electrode.

2. 2 . The negative electrode according to claim 1 , wherein a ratio of a major axis diameter of the first Si-containing particle to a major axis diameter of the second Si-containing particle is 1.5 or less.

3. 2 . The negative electrode according to claim 1 , wherein a ratio of a major axis diameter of the first Si-containing particle to a major axis diameter of the second Si-containing particle is 1.0 or less.

4. 2. The negative electrode according to claim 1, wherein the aspect ratio of the first Si-containing particles is 5.0 to 9.0, and the aspect ratio of the second Si-containing particles is 1.0 to 2.

0.

5. 2. The negative electrode according to claim 1, wherein a content ratio of the graphite particles to a total of the graphite particles, the first Si-containing particles, and the second Si-containing particles is 40% by mass to 90% by mass.

6. preparing a negative electrode paste containing graphite particles, first Si-containing particles, second Si-containing particles, and a dispersion medium; applying the negative electrode paste to a negative electrode current collector; a step of drying the applied negative electrode paste to form a negative electrode active material layer; and pressing the negative electrode active material layer; Equipped with the first Si-containing particles have a larger aspect ratio than the second Si-containing particles; the first Si-containing particles have an aspect ratio of 4.0 to 10.0; the aspect ratio of the second Si-containing particles is 1.0 to 3.0; the first Si-containing particles have a major axis length of 4 μm to 12 μm, and the second Si-containing particles have a major axis length of 2 μm to 10 μm; a mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 50:50; A method for manufacturing a negative electrode of a secondary battery.

7. 7. The manufacturing method according to claim 6, wherein in the step of preparing the negative electrode paste, the first Si-containing particles and the second Si-containing particles are premixed by dry blending, and the obtained premix is ​​used to prepare the negative electrode paste.

8. 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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