Anode material, anode of secondary battery using said anode material, and method of manufacturing the same

Abstract

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

Description

[Technical Field]

[0001] The present invention relates to a negative electrode material. The present invention also relates to a negative electrode of a secondary battery using the negative electrode material. The present invention further relates to a method for producing the negative electrode of the secondary battery. [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 material 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 material disclosed herein contains Si-containing particles and graphite particles. The ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1 to 8. The mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 10% by mass to 60% by mass. When 1 g of the Si-containing particles is uniaxially pressed at 25°C and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting compact is 0.9 g / cm. 3 That's all.

[0008] According to this configuration, it is possible to provide a negative electrode material that contains Si-containing particles and graphite particles and that can suppress capacity degradation when a secondary battery is repeatedly charged and discharged.

[0009] From another aspect, the present disclosure provides a negative electrode for a secondary battery 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 the above-described negative electrode material.

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

[0011] From another aspect, a method for manufacturing a negative electrode of a secondary battery disclosed herein includes the steps of applying a negative electrode paste containing the above-described negative electrode material to a negative electrode current collector, drying the applied negative electrode paste to form a negative electrode active material layer, and pressing the negative electrode active material layer.

[0012] The negative electrode obtained by such a configuration can provide the secondary battery with excellent resistance to capacity degradation when repeatedly charged and discharged. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram of an example of a negative electrode material according to an embodiment of the present invention. [Figure 2] 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 3] 1 is a cross-sectional view schematically illustrating 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] 4 is a schematic exploded view showing the configuration of a wound electrode body of the lithium ion secondary battery of FIG. 3. 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 material according to this embodiment contains Si-containing particles and graphite particles. The ratio of the average particle diameter D50 of the graphite particles to the average particle diameter D50 of the Si-containing particles is 1 to 8. The mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 10% by mass to 60% by mass. When 1 g of the Si-containing particles is uniaxially pressed at 25°C and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting compact is 0.9 g / cm. 3 That's all.

[0017] Fig. 1 is a diagram schematically illustrating an example of the negative electrode material according to this embodiment used in a negative electrode. In the example shown in Fig. 1, the negative electrode material 10 contains graphite particles 12 and Si-containing particles 14. In the example shown in Fig. 1, the graphite particles 12 and the Si-containing particles 14 are mixed. The negative electrode material 10 is filled and arranged on a negative electrode current collector 62.

[0018] 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.

[0019] 1, particles of a Si-C composite material are used as the Si-containing particles 14. The Si-containing particles 14 have carbon domains 14a and Si-containing domains 14b. The Si-containing domains 14b are dispersed in the carbon domains 14a, and thus the carbon domains 14a form a matrix.

[0020] The carbon domains 14a are, for example, carbonized products of carbon precursors (eg, petroleum pitch, coal pitch, phenolic resin, etc.); graphite, etc.

[0021] The Si-containing domain 14b contains Si, for example, Si, Si oxide (SiO x ), Si nitride (SiNx), Si carbide (SiCx), etc. The Si-containing domain 14b is preferably composed of Si and Si oxide (SiO x ) The Si-containing domains 14b may be fine particles. The number of Si-containing domains 14b in the Si-containing particle 14 is not limited to that shown in the figure.

[0022] The average particle diameter of the Si-containing domains 14b is, for example, 50 nm or less, and may be 5 nm to 50 nm. The "average particle diameter of the Si-containing domains 14b" can be determined as follows. First, a sample for scanning transmission electron microscope (STEM) observation of the negative electrode material 10 is prepared. For example, a negative electrode active material layer containing the negative electrode material 10 is processed with a focused ion beam (FIB) to prepare a sample for STEM observation. 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 annular dark-field image) are obtained. The diameter of the Si-containing domains 14b can be determined from the contrast and shape obtained from the BF image and the HAADF image. The diameters of 10 or more arbitrarily selected Si-containing domains 14b are determined, and the average value thereof is defined as the "average particle diameter of the Si-containing domains 14b" herein.

[0023] The Si content in the Si-containing particles 14 is not particularly limited. From the viewpoint of achieving a higher capacity of the secondary battery, the Si content in the Si-containing particles 14 is preferably 20 mass % or more, more preferably 30 mass % or more, and even more preferably 40 mass % or more. From the viewpoint of suppressing excessive volume change of the Si-containing particles 14, the Si content in the Si-containing particles 14 is preferably 60 mass % or less, more preferably 50 mass % or less. Furthermore, the oxygen (O) content in the Si-containing particles 14 is not particularly limited, but is preferably 10 mass % or less. The Si content and O content in the Si-containing particles 14 can be determined by analysis based on high-frequency inductively coupled plasma (ICP) emission spectroscopy.

[0024] The Si-containing particles 14 are, for example, particles containing Si dispersed inside a carbon material; particles containing Si embedded in the pores of spherically granulated porous graphite; or the like.

[0025] The Si-containing particles 14 can be obtained by a known method. For example, they can be obtained by mixing fine particles of Si, Si oxide, or the like with the carbon precursor, followed by carbonization and spheroidization. Alternatively, they can be obtained by mixing spherically granulated porous graphite with fine particles of Si, Si oxide, or the like in a dispersion medium, drying the mixture, and disposing the fine particles in the pores of the porous graphite.

[0026] However, the Si-containing particles 14 are not limited to those shown in the figure. The Si-containing particles may be carbon particles with Si-containing fine particles attached to their surfaces, or Si-containing particles with carbon fine particles attached to their surfaces. As shown in the figure, in a Si-C composite material, the carbon domains 14a form a matrix and multiple Si-containing domains 14b are dispersed within the carbon domains 14a. This is advantageous because the carbon domains 14a can mitigate volume changes due to expansion / contraction of the Si-containing domains 14b. Furthermore, the Si-containing particles may be composited with elements other than carbon, or may be metal Si particles, Si oxide particles, or the like that are not composited with other elements.

[0027] In this embodiment, when 1 g of the Si-containing particles 14 is pressed uniaxially at 25° C. and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting compact is 0.9 g / cm 3 The higher the density value of the compact, the higher the packing density of the particles. When 1 g of conventionally used Si-containing particles is compacted in the same manner, the density of the compact is usually 0.9 / cm 3Therefore, in this embodiment, Si-containing particles 14 having a higher packing density than conventionally used Si-containing particles are used. By using such Si-containing particles 14 having a higher packing density together with graphite particles 12 at a specific content ratio and controlling the ratio of the average particle diameters (D50) of the Si-containing particles 14 and the graphite particles 12 to fall within a specific range, it is possible to suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

[0028] The density of the compact is preferably 0.95 g / cm 3 More preferably, it is 1.0 g / cm or more. 3 On the other hand, the upper limit of the density of the compact is 2.3 g / cm 3 Below 2.0g / cm 3 Below 1.8g / cm 3 or less, or 1.5g / cm 3 It may be the following:

[0029] The density of the compact can be determined by weighing 1 g of Si-containing particles 14 into a 20 mm diameter mold in a temperature environment of 25° C., applying a pressure of 60 MPa in one axial direction (i.e., a direction perpendicular to the radial direction) to obtain a tablet-shaped compact, and measuring the bulk density of the compact. The density of the compact can be easily measured, for example, using an automatic powder resistivity measurement system (e.g., "MCP-PD600" manufactured by Nitto Seiko Analytech Co., Ltd.) and a 20 mm diameter probe (corresponding to the mold).

[0030] The density of the compact is affected by the circularity of the Si-containing particles 14. When the circularity of the Si-containing particles 14 is increased, the density of the compact tends to increase. Therefore, when the circularity of the Si-containing particles 14 is set to 0.85 to 1 (particularly 0.90 to 1), the density of the compact becomes 0.9 g / cm 3In 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.

[0031] Furthermore, the particle size of the Si-containing particles 14 also affects the density of the compact. When the average particle size of the Si-containing particles 14 is set to 2 μm to 10 μm (particularly, 5 μm to 10 μm), the density of the compact becomes 0.9 g / cm 3 The true density of the Si-containing particles 14 also affects the density of the compact. Therefore, by adjusting the composition of the Si-containing particles 14 (the content ratio of the constituent elements), the density of the compact can be finely adjusted.

[0032] There is no particular limitation on the circularity of the graphite particles 12. Spheroidized graphite particles are preferred as the graphite particles 12, and therefore the circularity of the graphite particles 12 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0033] The ratio of the average particle size (D50) of the graphite particles 12 to the average particle size (D50) of the Si-containing particles 14 (D50 of graphite-containing particles / D50 of Si-containing particles) is 1 to 8. If the ratio (D50 of graphite-containing particles / D50 of Si-containing particles) is less than 1, the packing property decreases due to the expansion / contraction of the Si-containing particles 14 during charging / discharging of the secondary battery, and the effect of improving the cycle characteristics of the secondary battery cannot be obtained. On the other hand, if the ratio (D50 of graphite-containing particles / D50 of Si-containing particles) exceeds 8, the dispersibility of the graphite particles 12 decreases, causing aggregation of the Si-containing particles 14, which decreases the packing property. As a result, the effect of improving the cycle characteristics of the secondary battery cannot be obtained. The ratio (D50 of graphite-containing particles / D50 of Si-containing particles) is preferably 1.0 to 5.0, more preferably 1.2 to 3.0, and even more preferably 1.4 to 2.5.

[0034] 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.

[0035] The average particle diameter (D50) of the graphite particles 12 is not particularly limited as long as the ratio (D50 of graphite-containing particles / D50 of Si-containing particles) is 1 to 8. The average particle diameter (D50) of the graphite particles 12 is preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and even more preferably 12 μm to 20 μm.

[0036] The average particle diameter (D50) of the Si-containing particles 14 is not particularly limited as long as the ratio (D50 of graphite-containing particles / D50 of Si-containing particles) is 1 to 8. The average particle diameter (D50) of the Si-containing particles 14 is preferably 2 μm to 10 μm, more preferably 4 μm to 10 μm, and even more preferably 6 μm to 9 μm.

[0037] The mass proportion of the Si-containing particles 14 to the total of the Si-containing particles 14 and the graphite particles 12 is 10% by mass to 60% by mass. If the mass proportion of the Si-containing particles 14 is less than 10% by mass, the capacity of the secondary battery is insufficient. If the mass proportion of the Si-containing particles 14 exceeds 60% by mass, the effect of volume change of the Si-containing particles 14 becomes large, and the effect of suppressing capacity degradation when the secondary battery is repeatedly charged and discharged becomes insufficient. The mass proportion of the Si-containing particles 14 is preferably 15% by mass to 55% by mass, and more preferably 20% by mass to 50% by mass.

[0038] The negative electrode material 10 can be obtained by mixing Si-containing particles 14 and graphite particles 12 according to a known method.

[0039] The negative electrode material 10 may further contain materials used in the negative electrode of a secondary battery, such as a binder and a conductive material.

[0040] The negative electrode material 10 according to this embodiment can suppress capacity degradation during repeated charge and discharge of a secondary battery. That is, when a negative electrode is fabricated using the negative electrode material 10 according to this embodiment and a secondary battery is fabricated using this negative electrode, the secondary battery has excellent resistance to capacity degradation during repeated charge and discharge, and therefore excellent cycle characteristics. Furthermore, since the negative electrode material 10 according to this embodiment contains Si, the capacity of the secondary battery can be increased. Furthermore, since the negative electrode material 10 according to this embodiment contains Si-containing particles 14 with high packing properties, the packing property of the negative electrode active material layer can be improved, and the energy density of the secondary battery can be increased.

[0041] Therefore, from another aspect, the negative electrode according to this embodiment includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer contains the negative electrode material according to the above-described embodiment. Such a negative electrode can impart excellent resistance to capacity degradation during repeated charge and discharge to a secondary battery. Furthermore, such a negative electrode can increase the capacity of the secondary battery. Furthermore, such a negative electrode can increase the energy density of the secondary battery.

[0042] The negative electrode according to this embodiment will be specifically described with reference to Fig. 2. Fig. 2 is a cross-sectional view schematically showing 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. 2 is a negative electrode for a lithium-ion secondary battery.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] The negative electrode active material layer 64 contains a negative electrode active material, and the negative electrode material 10 according to the above-described embodiment is used as this negative electrode active material. Thus, the negative electrode active material layer 64 includes the negative electrode material 10 according to the above-described embodiment.

[0048] The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as a binder and a conductive material. Examples of binders that can be used include styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a thickener. Examples of conductive materials include carbon nanotubes (CNTs). When CNTs are used as the conductive material, the negative electrode active material layer 64 may contain a dispersant for the CNTs.

[0049] 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.

[0050] 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.

[0051] The density of the negative electrode active material layer 64 is not particularly limited, but is, for example, 0.9 g / cm 3 or more, preferably 1.1 g / cm 3 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:

[0052] 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.

[0053] The negative electrode 60 can be suitably manufactured by a manufacturing method including a step of applying a negative electrode paste containing the negative electrode material according to the above-described embodiment to a negative electrode current collector 62 (hereinafter also referred to as a "coating step"), a step of drying the applied negative electrode paste to form a negative electrode active material layer 64 (hereinafter also referred to as a "drying step"), and a step of pressing the negative electrode active material layer 64 (hereinafter also referred to as a pressing step).

[0054] The coating process can be performed according to a known method, except for using the negative electrode material according to the embodiment described above. Specifically, for example, the negative electrode material according to the embodiment described above and optional components (e.g., binder, conductive material, etc.) are mixed with a dispersion medium (e.g., water) using a known mixer, stirrer, etc. to prepare a negative electrode paste.

[0055] 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.

[0056] The coating step can be carried out by applying the negative electrode paste onto the negative electrode current collector 62 using a coating device such as a gravure coater, a comma coater, a slit coater, or a die coater.

[0057] 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.

[0058] 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 allows the graphite particles 12 and Si-containing particles 14 contained in the negative electrode material 10 to be densely packed. This results in the negative electrode 60.

[0059] The negative electrode 60 can be used in a secondary battery according to a known method. A secondary battery typically includes a positive electrode, a negative electrode, and an electrolyte, and the negative electrode is the above-described negative electrode 60. An example of the configuration of a secondary battery using the negative electrode 60 will be described in detail below with reference to FIGS. 3 and 4. The example configuration is a flat prismatic lithium ion secondary battery having a flat wound electrode body and a flat battery case.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0068] 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.

[0069] 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.

[0070] 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).

[0071] 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.

[0072] 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.

[0073] The negative electrode sheet 60 is a negative electrode 60 using the above-mentioned negative electrode material 10 as the negative electrode active material.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] Furthermore, although the negative electrode 60 according to this embodiment is suitable for use as a negative electrode for a lithium-ion secondary battery, it can also be used as a negative electrode for other secondary batteries, and these other secondary batteries can be constructed according to known methods.

[0083] 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.

[0084] <Preparing the negative electrode material> Example 1 Particles of a Si-C composite material having an average particle size (D50) of 8 μm, a Si content of 40 mass %, and a circularity of 0.98 were prepared as Si-containing particles.

[0085] The density of the compact was measured by the method described below when 1 g of these Si-containing particles was pressed uniaxially at 25°C and 60 MPa to form a tablet with a diameter of 20 mm. The value of the density of the compact was 1.3 g / cm 3 .

[0086] Graphite particles with an average particle diameter (D50) of 18 μm were prepared. The negative electrode material of Example 1 was obtained by mixing the graphite particles and the Si-containing particles at a mass ratio of 60:40.

[0087] [Examples 2 to 3 and Comparative Examples 1 to 3] Particles of the Si-C composite material having the average particle diameter (D50), Si content, and circularity shown in Table 1 were prepared as the Si-containing particles. The density of the compact was measured by the method described below when 1 g of the Si-containing particles was pressed uniaxially at 25°C and 60 MPa to form a tablet with a diameter of 20 mm. The density of the compact is shown in Table 1.

[0088] In addition, graphite particles having the average particle diameter (D50) shown in Table 1 were prepared. The negative electrode materials of Examples 2 to 3 and Comparative Examples 1 to 3 were obtained by mixing the graphite particles and the Si-containing particles at a mass ratio of 60:40. In each of the examples and comparative examples, the average particle diameter (D50) of the Si-containing particles and the graphite particles, the Si content in the Si-containing particles, and the circularity of the Si-containing particles were determined by the following methods.

[0089] [Measurement of the average particle diameter (D50) of Si-containing particles and graphite particles] Using a commercially available laser diffraction type particle size distribution measuring device, the particle size distributions of the Si-containing particles and the graphite particles were measured on a volume basis, respectively, and the particle diameter corresponding to a cumulative frequency of 50% by volume from the side of the fine particles with a small particle diameter was determined as the average particle diameter (D50) of the Si-containing particles and the graphite particles.

[0090] [Measurement of the Si content of Si-containing particles] Using a commercially available ICP analyzer, the Si content in the Si-containing particles was determined as mass%.

[0091] <Measurement of Circularity of Si-Containing Particles> Using a commercially available image-based particle size distribution measuring device, the circularity of Si-containing particles was determined as the perimeter of a perfect circle having the same area as the projected area of the particle / the perimeter of the particle projection image.

[0092] <Measurement of Density of Formed Body of Si-Containing Particles> 1 g of Si-containing particles was weighed as a powder measurement sample and set on a probe (diameter 20 mm) of an automatic powder resistance measurement system “MCP-PD600” (manufactured by Nitto Seiko Analytic Co., Ltd.). At 25 °C, using this automatic powder resistance measurement system, the load and displacement when pressurized in the uniaxial direction were measured. Based on this, the bulk density of the formed body at a pressure of 60 MPa was determined.

[0093] <Fabrication of Negative Electrode and Lithium Ion Secondary Battery for Evaluation> As binders, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared. Also, a dispersion of single-walled carbon nanotubes (SWCNT) as a conductive material was prepared. The negative electrode material, CMC, and PAA of each example and each comparative example were dry-blended using a planetary mixer. The obtained mixture, the SWCNT dispersion, and the dispersion medium were kneaded using a planetary mixer. To this, SBR and further the dispersion medium were added and uniformly mixed to prepare a negative electrode paste. In the negative electrode paste, the mass ratio of negative electrode material:CMC:PAA:SBR:SWCNT was 100:1:1:1.5:0.1.

[0094] The prepared negative electrode paste was applied to the surface of a copper foil with a thickness of 10 μm and dried to form a negative electrode active material layer. After roll-pressing the negative electrode active material layer, the obtained sheet was processed into a predetermined size to obtain a negative electrode sheet.

[0095] LiNi as the positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3A 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.

[0096] 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.

[0097] <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.

[0098] 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.

[0099] [Table 1]

[0100] From the results in Table 1, it can be seen that the ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1 to 8, the mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 10% by mass to 60% by mass, and when 1 g of the Si-containing particles is pressed uniaxially at 25°C and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting compact is 0.9 g / cm 3 It can be seen that when the capacity retention rate is above this level, the capacity retention rate after 200 charge / discharge cycles is significantly high. This shows that the negative electrode material disclosed herein can suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

[0101] 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.

[0102] That is, the negative electrode material, the negative electrode of a secondary battery, and the method for producing the same disclosed herein are the following items [1] to [8]. [1] A negative electrode material containing Si-containing particles and graphite particles, a ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1 to 8; a mass ratio of the Si-containing particles to the total mass of the Si-containing particles and the graphite particles is 10 mass% to 60 mass%; When 1 g of the Si-containing particles is pressed uniaxially at 25°C under 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting compact is 0.9 g / cm 3 That's it for the negative electrode material. [2] The negative electrode material according to item [1], wherein the Si-containing particles have an average particle size (D50) of 2 μm to 10 μm, and the graphite particles have an average particle size (D50) of 5 μm to 25 μm. [3] The negative electrode material according to item [1] or [2], wherein the ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1.2 to 3.0. [4] The negative electrode material according to any one of items [1] to [3], wherein the mass ratio of the Si-containing particles to the total of the Si-containing particles and the graphite particles is 20 mass % to 50 mass %. [5] The negative electrode material according to any one of items [1] to [4], wherein the Si-containing particles are particles of a Si-C composite material in which Si-containing domains are dispersed in a carbon matrix. [6] The negative electrode material according to item [5], wherein the Si content in the Si-containing particles is 20% by mass or more and 60% by mass or less. [7] A negative electrode current collector; a negative electrode active material layer supported on the negative electrode current collector; A negative electrode of a secondary battery comprising: The negative electrode of a secondary battery, wherein the negative electrode active material layer contains the negative electrode material according to any one of items [1] to [6]. [8] A step of applying a negative electrode paste containing the negative electrode material according to any one of items [1] to [6] to a negative electrode current collector; drying the applied negative electrode paste to form a negative electrode active material layer; pressing the negative electrode active material layer; A method for manufacturing a negative electrode of a secondary battery, comprising: [Explanation of symbols]

[0103] 10. Anode materials 12 Graphite particles 14 Si-containing particles 14a Carbon domain 14b Si-containing domain 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 material containing Si-containing particles and graphite particles, the Si-containing particles are particles of a Si-C composite material in which Si-containing domains are dispersed in a carbon matrix; the Si-containing domain is composed of at least one of Si and a Si oxide; the Si-containing domains are fine particles, the average particle size of the Si-containing domains is 5 nm to 50 nm; the circularity of the Si-containing particles is 0.90 to 1; The graphite particles have a circularity of 0.85 to 1; The Si content in the Si-containing particles is 40% by mass or more and 50% by mass or less, a ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the Si-containing particles is 1.4 to 2.5; the Si-containing particles have an average particle size (D50) of 6 μm to 9 μm, and the graphite particles have an average particle size (D50) of 12 μm to 20 μm; a mass ratio of the Si-containing particles to the total mass of the Si-containing particles and the graphite particles is 20 mass% to 50 mass%; The negative electrode material, wherein when 1 g of the Si-containing particles is uniaxially pressed at 25° C. and 60 MPa to form a tablet having a diameter of 20 mm, the density of the resulting tablet is 1.0 g / cm 3 or more and 1.5 g / cm 3 or less.

2. The negative electrode material described in claim 1, wherein the circularity of the graphite particles is 0.90 to 1.

3. a negative electrode current collector; a negative electrode active material layer supported on the negative electrode current collector; A negative electrode of a secondary battery comprising:

10. A negative electrode for a secondary battery, wherein the negative electrode active material layer contains the negative electrode material according to claim 1.

4. a step of applying a negative electrode paste containing the negative electrode material according to claim 1 to a negative electrode current collector; drying the applied negative electrode paste to form a negative electrode active material layer; pressing the negative electrode active material layer; A method for manufacturing a negative electrode of a secondary battery, comprising:

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