Negative electrode of a secondary battery, method for manufacturing the same, and secondary battery using the same

Abstract

To provide a negative electrode containing Si-containing particles and graphite particles and capable of suppressing capacity degradation in repetitive charge and discharge of a secondary battery.SOLUTION: A negative electrode disclosed herein includes a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles. A Si content ratio in the first Si-containing particles is higher than a Si content ratio in the second Si-containing particles. Each of the first Si-containing particles and each of the second Si-containing particles are coated with a conductive material, respectively. A ratio of a coating amount (mass %) of a conductive material with respect to the first Si-containing particle, with respect to a coating amount (mass %) of a conductive material with respect to the second Si-containing particle is 3.0 or more.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a negative electrode of a secondary battery and a method for manufacturing the same. The present invention also relates to a secondary battery using the negative electrode.

Background Art

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

[0003] In the application of power sources for driving vehicles, particularly in the application of power sources for driving BEVs, from the viewpoint of extending the cruising range of vehicles, further increase in the capacity of secondary batteries is desired. As a high-capacity negative electrode active material, Si-containing particles are known, and it is known that the capacity of secondary batteries can be increased by Si-containing particles (see, for example, Patent Document 1). Patent Document 1 discloses a technique of using, as a negative electrode active material, Si-containing particles and graphite particles such as natural graphite in combination.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, while Si-containing particles have a high capacity, they have a large volume change due to expansion / contraction when the secondary battery is charged and discharged. Therefore, when Si-containing particles and graphite particles are used in combination, especially when the proportion of Si-containing particles is large, the fillability of these particles decreases when the secondary battery is repeatedly charged and discharged, resulting in disconnection of the conductive path and generation of internal stress. Therefore, when Si-containing particles and graphite particles are used in combination, there is a problem that the cycle characteristics of the secondary battery deteriorate. Specifically, there is a problem that the capacity degradation is large 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 the secondary battery is repeatedly charged and discharged.

Means for Solving the Problems

[0007] The 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 Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles. The first Si-containing particles and the second Si-containing particles are each coated with a conductive material. The ratio of the coating amount (mass%) of the conductive material with respect to the first Si-containing particles to the coating amount (mass%) of the conductive material with respect to the second Si-containing particles is 3.0 or more.

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

[0009] From another aspect, the method for manufacturing a negative electrode of a secondary battery disclosed herein includes the steps of preparing first Si-containing particles coated with a first conductive material and second Si-containing particles coated with a second conductive material, where the Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles, and the ratio of the coating amount (mass %) of the first conductive material with respect to the first Si-containing particles to the coating amount (mass %) of the second conductive material with respect to the second Si-containing particles is 3.0 or more; mixing the first Si-containing particles coated with the first conductive material, the second Si-containing particles coated with the second conductive material, and graphite particles in a dispersion medium to prepare a negative electrode paste; coating the negative electrode paste on a negative electrode current collector; and drying the coated negative electrode paste.

[0010] According to the negative electrode obtained with such a configuration, excellent resistance to capacity degradation can be imparted when the secondary battery is repeatedly charged and discharged.

[0011] From another aspect, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the above-described negative electrode.

[0012] According to such a configuration, a secondary battery having excellent resistance to capacity degradation when repeatedly charged and discharged can be provided.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out 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 the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and the common general knowledge in the relevant field. Also, in the following drawings, members and parts having the same function are denoted by the same reference numerals for explanation. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. In this specification, the numerical range expressed as "A to B" includes A and B.

[0015] In this specification, the "secondary battery" refers to a rechargeable power storage device. Also, in this specification, the "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and realizes charge and discharge by the movement of charges associated with lithium ions between the positive and negative electrodes.

[0016] The negative electrode disclosed herein is used in a secondary battery, and 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 showing an example of the negative electrode 60 according to this embodiment, and is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to this embodiment shown in FIG. 1 is the negative electrode of 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 a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided only on 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. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62.

[0018] As shown in the figure example, a non-formed portion 62a of the negative electrode active material layer where the negative electrode active material layer 64 is not provided may be provided at one end in the width direction of the negative electrode 60. In the non-formed portion 62a of the negative electrode active material layer, the negative electrode current collector 62 is exposed, and the non-formed portion 62a of the negative electrode active material layer can function as a current collecting portion. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0019] The shape of the negative electrode current collector 62 is, in the illustrated example, in the form of a foil (or sheet), but is not limited thereto. The negative electrode current collector 62 may be in various forms such as rod-shaped, plate-shaped, mesh-shaped, etc. As the material of the negative electrode current collector 62, a metal with good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be used, as in a conventional lithium-ion secondary battery. Among them, copper is preferable. As the negative electrode current collector 62, a copper foil is particularly preferable.

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

[0021] The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, at least graphite particles, first Si-containing particles with a high Si content, and second Si-containing particles with a low Si content are used. This will be described in detail with reference to FIG. 2. FIG. 2 is a schematic cross-sectional view showing the particles contained in the negative electrode active material layer 64 shown in FIG. 1. As shown in FIG. 2, the negative electrode active material layer 64 contains graphite particles 12, first Si-containing particles 14 with a high Si content, and second Si-containing particles 16 with a low Si content.

[0022] The graphite constituting the graphite particles 12 may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in a form where the 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, or the like. The graphite particles 12 are preferably spheroidized graphite particles. When the graphite particles 12 are spherical, the circularity of the graphite particles 12 is preferably from 0.85 to 1, more preferably from 0.88 to 1, and still more preferably from 0.90 to 1.

[0024] In this specification, the "circularity" refers to the ratio of the circumference of a true circle having the same area as the projected area of the particle to the circumference of the particle projection image (that is, circularity = circumference of a true circle having the same area as the projected area of the particle / circumference of the particle projection image). Therefore, the closer the circularity is to 1, the closer the particle projection image is to a true 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 thereof.

[0025] The average particle diameter (D50) of the graphite particles 12 is not particularly limited. The average particle diameter (D50) of the graphite particles 12 is, for example, from 1 μm to 30 μm, preferably from 5 μm to 25 μm, more preferably from 10 μm to 23 μm, and still more preferably from 12 μm to 20 μm.

[0026] In this specification, the "average particle diameter (D50)" refers to the median diameter (D50), and in the volume-based particle size distribution based on the laser diffraction / scattering method, it refers to the particle diameter corresponding to a cumulative frequency of 50 volume% from the side of fine particles with a small particle diameter. The average particle diameter (D50) can be determined using a commercially available laser diffraction / scattering type particle size distribution measuring device or the like.

[0027] The content ratio of the graphite particles with respect to the total of the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16 is preferably from 40% by mass to 90% by mass, more preferably from 45% by mass to 85% by mass, and still more preferably from 50% by mass to 80% by mass.

[0028] As the first Si-containing particles 14 and the second Si-containing particles 16, for example, particles of an Si-C composite material can be used. The Si-C composite material typically includes a carbon domain and an Si-containing domain. Note that the first Si-containing particles 14 and the second Si-containing particles 16 do not have to be an Si-C composite material, and may be Si particles, Si oxide particles, or the like.

[0029] The carbon domain is, for example, a carbonized product of a carbon precursor (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite or the like. The carbon domain preferably constitutes a carbon matrix. Therefore, the Si-C composite material is preferably a material in which a plurality of Si-containing domains are dispersed in a carbon matrix. In this case, it is advantageous because the carbon matrix can relieve the volume change due to the expansion / contraction of the Si-containing domain.

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

[0031] The average particle diameter of the Si-containing domain is, for example, 50 nm or less, and may be 5 nm to 50 nm. The "average particle diameter of the Si-containing domain" can be obtained as follows. First, the negative electrode active material layer 64 is processed by FIB (focused ion beam) to prepare a sample for scanning transmission electron microscope (STEM) observation. Then, after element analysis of the sample by EDX element mapping, a BF image (bright field image) and a HAADF image (high angle annular dark field image) are acquired. From the contrast and shape obtained by the BF image and the HAADF image, the diameter of the Si-containing domain can be determined. 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 domain" here.

[0032] The Si-C composite material is, for example, one in which fine particles containing Si are dispersed inside a carbon material; one in which fine particles containing Si have entered into the pores of granulated porous graphite; and the like.

[0033] In this embodiment, the Si content ratio (S1) in the first Si-containing particles 14 is higher than the Si content ratio (S2) in the second Si-containing particles 16. The Si content ratio (S1) in the first Si-containing particles 14 and the Si content ratio (S2) in the second Si-containing particles 16 are not particularly limited as long as this relationship is satisfied. If these Si content ratios are too low, the effect of improving the cycle characteristics becomes small and the effect of increasing the capacity of the secondary battery may be low. On the other hand, if these Si content ratios are too high, when the secondary battery is repeatedly charged and discharged, the volume change due to the expansion / contraction of the first Si-containing particles 14 and the second Si-containing particles 16 may become too large.

[0034] Therefore, the Si content ratio (S1) in the first Si-containing particles 14 is preferably 45% by mass to 80% by mass, more preferably 55% by mass to 75% by mass. The Si content ratio (S2) in the second Si-containing particles 16 is preferably 20% by mass to 55% by mass, more preferably 25% by mass to 45% by mass.

[0035] Also, the ratio (S1 / S2) of the Si content ratio (S1) in the first Si-containing particles 14 to the Si content ratio (S2) in the second Si-containing particles 16 is preferably 1.2 or more, more preferably 1.5 or more, and even more preferably 1.8 or more. The ratio (S1 / S2) may be 4 or less, 3.5 or less, 3 or less, or 2.5 or less.

[0036] The mass ratio between the first Si-containing particles 14 and the second Si-containing particles 16 is not particularly limited as long as the effects of the present invention can be obtained. In order to obtain a better filling state between the first Si-containing particles 14 and the second Si-containing particles 16, the mass ratio of the first Si-containing particles 14 to the second Si-containing particles 16 is preferably 10:90 to 60:40, more preferably 15:85 to 55:45, and even more preferably 20:80 to 45:55.

[0037] The total content ratio of the first Si-containing particles 14 and the second Si-containing particles 16 to the total of the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16 is preferably 10% by mass to 60% by mass, more preferably 15% by mass to 55% by mass, and even more preferably 20% by mass to 50% by mass.

[0038] The average particle diameter (D50) of the first Si-containing particles 14 and the second Si-containing particles 16 is not particularly limited. The average particle diameter (D50) of the first Si-containing particles 14 and the second Si-containing particles 16 is, for example, 1 μm to 15 μm, preferably 2 μm to 10 μm, and more preferably 4 μm to 10 μm, respectively.

[0039] The first Si-containing particles 14 and the second Si-containing particles 16 are coated with a conductive material. As shown in FIG. 2, the first Si-containing particles 14 are coated with the first conductive material 15, and the second Si-containing particles 16 are coated with the second conductive material 17. In the illustrated example, the entire first Si-containing particles 14 and the entire second Si-containing particles 16 are coated with the first conductive material 15 and the second conductive material 17, respectively. However, the first Si-containing particles 14 and the second Si-containing particles may be partially coated with the first conductive material 15 and the second conductive material 17, respectively.

[0040] Examples of the conductive material include carbon black such as acetylene black, carbon fiber, carbon nanotube (CNT), etc. Among them, CNT is preferred. The first conductive material 15 and the second conductive material 17 are preferably the same.

[0041] When using CNT as the conductive material, its type is not particularly limited. For example, single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), multi-walled carbon nanotubes (MWCNT), etc. can be used. These can be used alone or in combination of two or more. Since more conductive paths can be formed between the negative electrode active materials, SWCNT is preferred as the CNT. The CNT may be manufactured by an arc discharge method, a laser ablation method, a chemical vapor deposition method, etc.

[0042] The average length of the CNT is not particularly limited. If the average length of the CNT is too long, the CNT tends to aggregate and the dispersibility decreases. Therefore, the average length of the CNT is preferably 15 μm or less, more preferably 8.0 μm or less, and even more preferably 5.0 μm or less. On the other hand, if the average length of the CNT is too short, it tends to be difficult to form a conductive path between the negative electrode active materials. Therefore, the average length of the CNT is preferably 0.3 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more.

[0043] The average diameter of the CNT is not particularly limited and is, for example, 0.1 nm to 50 nm, preferably 0.3 nm to 30 nm.

[0044] Note that the average length and average diameter of the CNT can be obtained by taking an electron micrograph of the CNT and using the average values of the lengths and diameters of 100 or more CNTs, respectively. Specifically, for example, after diluting the CNT dispersion and drying it, a measurement sample is prepared. Scanning electron microscope (SEM) observation is performed on this sample to obtain the lengths and diameters of 100 or more CNTs and calculate the average values. At this time, when the CNT is re-aggregated, the lengths and diameters are obtained for the bundles of the aggregated CNTs.

[0045] In this embodiment, the coating amount (C1) (mass %) of the first Si-containing particles 14 with the first conductive material 15 is larger than the coating amount (C2) (mass %) of the second Si-containing particles 16 with the second conductive material 17. Note that the coating amount (C1) is the ratio (%) of the mass of the first conductive material 15 to the mass of the first Si-containing particles 14, and the coating amount (C2) is the ratio (%) of the mass of the second conductive material 17 to the mass of the second Si-containing particles 16.

[0046] And the ratio (C1 / C2) of the coating amount (C1) (mass %) of the first conductive material to the coating amount (C2) (mass %) of the second conductive material is 3.0 or more.

[0047] Thus, in addition to the graphite particles 12, the first Si-containing particles 14 and the second Si-containing particles 16 having different Si contents are used in combination, and the first Si-containing particles 14 having a high Si content are coated with a conductive material in a larger amount than the second Si-containing particles 16 having a low Si content at a specific ratio or more, whereby it is possible to suppress capacity deterioration when the secondary battery is repeatedly charged and discharged. The reason is considered as follows.

[0048] The first Si-containing particles having a high Si content have a larger volume change due to expansion / contraction when the secondary battery is repeatedly charged and discharged than the second Si-containing particles 16 having a low Si content. Therefore, near the first Si-containing particles 14 having a high Si content, disconnection of the conductive path is likely to occur, but by coating the first Si-containing particles 14 with a large amount of conductive material, disconnection of the conductive path accompanying this expansion / contraction can be suppressed. On the other hand, by coating the second Si-containing particles 16 having a low Si content with a small amount of conductive material, side reactions during charge and discharge can be suppressed. Thereby, capacity deterioration when the secondary battery is repeatedly charged and discharged can be suppressed.

[0049] The ratio (C1 / C2) of the coating amounts is preferably 3.2 or more, more preferably 3.5 or more, still more preferably 5.0 or more, and particularly preferably 8.0 or more. The ratio (C1 / C2) of the coating amounts may be 50 or less, 30 or less, 20 or less, or 15 or less.

[0050] The content of the conductive material in the negative electrode active material layer 64 is not particularly limited as long as the effects of the present invention can be obtained. The amount of the conductive material is, for example, 0.01% by mass to 10% by mass with respect to the negative electrode active material, and may be appropriately determined according to the type of the conductive material. When the conductive material is CNT, the content of the conductive material in the negative electrode active material layer 64 is preferably 0.02% by mass to 1.0% by mass, more preferably 0.1% by mass to 1.0% by mass, with respect to the negative electrode active material.

[0051] Therefore, the coating amount (C1) of the first Si-containing particles 14 with the first conductive material 15 and the coating amount (C2) of the second Si-containing particles 16 with the second conductive material 17 may be appropriately set in consideration of the content of the conductive material in the negative electrode active material layer 64 described above, the ratio (C1 / C2) of the coating amounts, and the mass ratio of the first Si-containing particles 14 and the second Si-containing particles 16.

[0052] When the first conductive material 15 is CNT, the coating amount (C1) of the first Si-containing particles 14 with the first conductive material 15 is, for example, 0.5% by mass to 10% by mass, preferably 1% by mass to 5% by mass. When the second conductive material 17 is CNT, the coating amount (C2) of the second Si-containing particles 16 with the second conductive material 17 is, for example, 0.05% by mass to 4.0% by mass, preferably 0.1% by mass to 1.0% by mass.

[0053] The ratio of the total mass of the first conductive material 15 that coats the first Si-containing particles 14 to the total mass of the second conductive material 17 that coats the second Si-containing particles 16 is preferably greater than 1, more preferably 1.5 or more, still more preferably 2 or more, and particularly preferably 4 or more.

[0054] Note that a dispersant for CNT may be attached to the first conductive material 15 and the second conductive material 17. As the dispersant for CNT, a known dispersant for CNT can be used.

[0055] Incidentally, the first Si-containing particles 14 and the second Si-containing particles 16 can be produced according to known methods. Incidentally, various production methods of the particles of the Si-C composite material are known (for example, refer to Japanese Patent Application Laid-Open No. 2015-38862, International Publication No. 2014 / 046144, and prior art documents cited in the said international publication).

[0056] The negative electrode active material layer 64 may contain components other than the negative electrode active material, and examples thereof include a binder. As the binder, for example, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), etc. can be used. CMC also functions as a thickening agent.

[0057] The content of the negative electrode active material in the negative electrode active material layer 64 (that is, with respect to the total mass of the negative electrode active material layer 64) is preferably 90% by mass or more, more preferably 95% by mass or more. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less.

[0058] The thickness of the negative electrode active material layer 64 is not particularly limited, but for example, it is 10 μm or more and 400 μm or less, preferably 20 μm or more and 300 μm or less.

[0059] The density of the negative electrode active material layer 64 is not particularly limited, but for example, it is 0.7 g / cm 3 or more, preferably 1.0 g / cm 3 or more, more preferably 1.2 g / cm 3 or more. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm 3 or less, and may be 2.0 g / cm 3 or less.

[0060] 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 or the like.

[0061] The negative electrode 60 includes a step of preparing first Si-containing particles 14 coated with a first conductive material 15 and second Si-containing particles 16 coated with a second conductive material 17 (hereinafter also referred to as the "coated particle preparation step"). Here, the Si content ratio in the first Si-containing particles 14 is higher than the Si content ratio in the second Si-containing particles 16, and the ratio of the coating amount (C1) (mass %) of the first conductive material 15 with respect to the first Si-containing particles 14 to the coating amount (C2) (mass %) of the second conductive material 17 with respect to the second Si-containing particles 16 is 3.0 or more; a step of mixing the first Si-containing particles 14 coated with the first conductive material 15, the second Si-containing particles 16 coated with the second conductive material 17, and the graphite particles 12 in a dispersion medium to prepare a negative electrode paste (hereinafter also referred to as the "paste preparation step"); a step of coating the negative electrode paste on a negative electrode current collector 62 (hereinafter also referred to as the "coating step"); and a step of drying the coated negative electrode paste (hereinafter also referred to as the "drying step"). It can be preferably manufactured by a manufacturing method including these steps.

[0062] In this specification, the "paste" refers to a mixture in which part or all of the solid content is dispersed in a dispersion medium, and includes so-called "slurry", "ink", etc.

[0063] In the coated particle preparation step, first Si-containing particles 14 with a high Si content ratio and second Si-containing particles 16 with a low Si content ratio are prepared. Also, a conductive material is prepared. For the coating of the first Si-containing particles 14 and the second Si-containing particles 16 with the conductive material, known coating methods can be adopted. For example, a method of dispersing Si-containing particles and a conductive material in a dispersion medium and then drying by heating, reduced pressure, etc. as necessary; a method of spray-drying a slurry in which Si-containing particles and a conductive material are dispersed in a dispersion medium; a method of subjecting Si-containing particles and a conductive material to mechanofusion treatment; a method of subjecting Si-containing particles and a conductive material to mechanical milling treatment; a method of depositing a conductive material on Si-containing particles by chemical vapor deposition (CDV), etc. can be mentioned.

[0064] In addition, the coating of the first Si-containing particles and the second Si-containing particles with the conductive material is performed such that the ratio of the coating amounts (C1 / C2) is 3.0 or more.

[0065] The paste preparation step can be carried out by mixing graphite particles 12, first Si-containing particles 14 coated with a first conductive material 15, second Si-containing particles 16 coated with a second conductive material 17, and optional components (e.g., binder, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc.

[0066] The coating step can be carried out according to a known method. Specifically, for example, the obtained negative electrode paste can be coated on the negative electrode current collector 62 using a coating device such as a gravure coater, comma coater, slit coater, die coater, etc., thereby performing the coating step.

[0067] The drying step can be carried out according to a known method. Specifically, for example, the dispersion medium is removed from the negative electrode current collector 62 coated with the negative electrode paste using a drying device such as a drying furnace, thereby forming the negative electrode active material layer 64. Thereby, the drying step can be carried out. The drying temperature and drying time may be appropriately determined according to the solid content concentration of the negative electrode paste and are not particularly limited. The drying temperature is, for example, 60°C or higher and 200°C or lower, preferably 70°C or higher and 150°C or lower. The drying time is, for example, 10 seconds or longer and 30 minutes or shorter, preferably 30 seconds or longer and 10 minutes or shorter.

[0068] After the drying step, a step of pressing the negative electrode active material layer 64 may be further carried out. The pressing step can be carried out according to a known method. Specifically, the pressing step can be carried out by applying pressure to the formed negative electrode active material layer 64 using a roller press or the like. By the pressing step, the graphite particles 12, first Si-containing particles 14, and second Si-containing particles 16 contained in the negative electrode active material layer 64 can be densely filled. In this way, the negative electrode 60 can be obtained.

[0069] According to the negative electrode 60 according to the present embodiment, excellent resistance to capacity deterioration can be imparted when the secondary battery is repeatedly charged and discharged. In addition, since the negative electrode 60 according to the present embodiment uses a negative electrode active material containing Si, the secondary battery can be made to have a high capacity. Therefore, the secondary battery using the negative electrode 60 according to the present embodiment has a high capacity and excellent cycle characteristics.

[0070] 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, an 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 rectangular lithium-ion secondary battery having a flat wound electrode body and a flat battery case.

[0071] The lithium-ion secondary battery 100 shown in FIG. 3 is a sealed lithium-ion secondary battery 100 constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., an exterior 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 safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. In addition, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous 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. As the material of the battery case 30, for example, a lightweight and highly thermally conductive metal material such as aluminum is used.

[0072] As shown in FIGS. 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 via two long separator sheets 70 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 (here, both sides) 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 (here, both sides) of a long negative electrode current collector 62. The non-formed part 52a of the positive electrode active material layer (that is, the part where the positive electrode current collector 52 is exposed without the formation of the positive electrode active material layer 54) and the non-formed part 62a of the negative electrode active material layer (that is, the part where the negative electrode current collector 62 is exposed without the formation of the negative electrode active material layer 64) are formed so as to protrude outward from both ends in the winding axis direction of the wound electrode body 20 (that is, the sheet width direction orthogonal to the longitudinal direction). A positive electrode current collecting plate 42a and a negative electrode current collecting plate 44a are joined to the non-formed part 52a of the positive electrode active material layer and the non-formed part 62a of the negative electrode active material layer, respectively.

[0073] As the positive electrode current collector 52 constituting the positive electrode sheet 50, a known positive electrode current collector used in a lithium-ion secondary battery may be used. Examples thereof include a sheet or foil made of a metal having good conductivity (for example, aluminum, nickel, titanium, stainless steel, etc.). As the positive electrode current collector 52, an aluminum foil is preferable.

[0074] The dimensions of the positive electrode current collector 52 are not particularly limited and may be appropriately determined according to the battery design. When an aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0075] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a positive electrode active material having a known composition used in a lithium-ion secondary battery may be used. Specifically, for example, a lithium composite oxide, a lithium transition metal phosphate compound, etc. can be used as the positive electrode active material. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, a spinel structure, an olivine structure, etc.

[0076] As the lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferable. Specific examples thereof include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel manganese-based composite oxides, lithium nickel cobalt manganese-based composite oxides, lithium nickel cobalt aluminum-based composite oxides, lithium iron nickel manganese-based composite oxides, and the like.

[0077] In addition, in this specification, the term "lithium nickel cobalt manganese-based composite oxide" includes, in addition to the oxide composed of Li, Ni, Co, Mn, and O as constituent elements, an oxide containing one or more additional elements other than these. 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, Sn, etc. The additional element may also be a semi-metal element such as B, C, Si, P, etc. or a non-metal element such as S, F, Cl, Br, I, etc. This also applies to the above-mentioned lithium nickel-based composite oxide, lithium cobalt-based composite oxide, lithium manganese-based composite oxide, lithium nickel manganese-based composite oxide, lithium nickel cobalt aluminum-based composite oxide, lithium iron nickel manganese-based composite oxide, etc.

[0078] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium manganese iron phosphate, and the like.

[0079] These cathode active materials may be used alone or in combination of two or more. As the cathode active material, a lithium nickel cobalt manganese-based composite oxide is particularly preferable because of its excellent various properties such as initial resistance characteristics.

[0080] The average particle diameter (D50) of the positive electrode active material is not particularly limited, but for example, it is 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.

[0081] 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, and the like. As the conductive material, for example, carbon black such as acetylene black (AB); carbon fibers such as vapor grown carbon fiber (VGCF) and carbon nanotube (CNT); and other carbon materials (e.g., graphite, etc.) can be preferably used. As the binder, for example, polyvinylidene fluoride (PVdF) and the like can be used.

[0082] The content of the positive electrode active material in the positive electrode active material layer 54 (that is, the content of the positive electrode active material with respect to the total mass of the positive electrode active material layer 54) is not particularly limited, but 70% by mass or more is preferable, more preferably 80% by mass or more, and still 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 0.1% by mass or more and 15% by mass or less is preferable, and 0.2% by mass or more and 10% by mass or less is more preferable. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but 0.1% by mass or more and 20% by mass or less is preferable, and 0.3% by mass or more and 15% by mass or less is more preferable. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but 0.4% by mass or more and 15% by mass or less is preferable, and 0.5% by mass or more and 10% by mass or less is more preferable.

[0083] The thickness per side of the positive electrode active material layer 54 is not particularly limited, but it is usually 10 μm or more, preferably 20 μm or more. On the other hand, the thickness is usually 400 μm or less, preferably 300 μm or less.

[0084] As the negative electrode sheet 60, the above-described negative electrode 60 is used.

[0085] Examples of the separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and 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.

[0086] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less. The air permeability of the separator 70 obtained by the Gurley test method is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0087] The non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones, which are used in the electrolytes of general lithium-ion secondary batteries, can be used without particular limitation. Among them, 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). Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As an example, the non-aqueous solvent consists only of carbonates. As another example, the non-aqueous solvent contains carbonates and esters such as methyl acetate.

[0088] As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) can be preferably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0089] In addition, as long as the effects of the present invention are not significantly impaired, the non-aqueous electrolyte may contain components other than the above-described components, for example, film-forming agents such as vinylene carbonate (VC) and oxalato complex; gas generators such as biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and various other additives.

[0090] The lithium-ion secondary battery 100 has suppressed capacity deterioration when charge and discharge are repeated, and also has a high capacity. The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power sources for driving mounted on vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Further, the lithium-ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium-ion secondary battery 100 can typically be used in the form of a battery pack in which a plurality of batteries are connected in series and / or in parallel.

[0091] As described above, as an example, the prismatic lithium-ion secondary battery 100 including the flat wound electrode body 20 has been described. However, the lithium-ion secondary battery can also be configured as a lithium-ion secondary battery including a laminated electrode body (that is, an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). Further, the lithium-ion secondary battery can be configured as a cylindrical lithium-ion secondary battery, a laminated case type lithium-ion secondary battery, or the like.

[0092] Further, according to a known method, the lithium-ion secondary battery 100 can be configured as an all-solid-state lithium-ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte.

[0093] In addition, the negative electrode 60 according to the present embodiment is suitable for the negative electrode of a lithium-ion secondary battery, but can be constructed and used as the negative electrode of other secondary batteries, and the other secondary batteries can be configured according to a known method.

[0094] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not intended to be limited to those shown in such embodiments.

[0095] <Fabrication of Negative Electrode> [Example 1] As the negative electrode active material, the following were prepared. The Si content ratios of the first Si-containing particles and the second Si-containing particles were measured using a commercially available ICP analyzer. The average particle diameters (D50) of the first Si-containing particles, the second Si-containing particles, and the graphite particles were measured using a commercially available laser diffraction / scattering particle size distribution measuring device. First Si-containing particles: Si-C composite material, Si content ratio = 65% by mass, average particle diameter (D50) = 6 μm Second Si-containing particles: Si-C composite material, Si content ratio = 35% by mass, average particle diameter (D50) = 7 μm Graphite particles: average particle diameter (D50) = 14 μm

[0096] As the conductive material, single-walled carbon nanotubes (SWCNTs) were prepared. The SWCNTs were prepared in the form of a dispersion. The first Si-containing particles, the SWCNT dispersion, and the dispersion medium were mixed using a disperser at a rotational speed of 3000 rpm. Thus, particles in which the surface of the first Si-containing particles was coated with SWCNTs (the first conductive material) were obtained. Similarly, the second Si-containing particles, the SWCNT dispersion, and the dispersion medium were mixed using a disperser at a rotational speed of 3000 rpm. Thus, particles in which the surface of the second Si-containing particles was coated with SWCNTs (the second conductive material) were obtained.

[0097] As the binder, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared.

[0098] A negative electrode paste containing graphite particles, first Si-containing particles, second Si-containing particles, the first conductive material, the second conductive material, CMC, PAA, and SBR in a mass ratio of 60:14:26:0.5:0.1:1:1:1.5 was prepared by the following procedure.

[0099] First, graphite particles, CMC, and PAA were dry-blended using a planetary mixer. The obtained mixture, the first Si-containing particles coated with the first conductive material, and the second Si-containing particles coated with the second conductive material were charged into a planetary mixer together with a dispersion medium and kneaded. Further, SBR and an additional dispersion medium were charged into the planetary mixer and diluted and mixed to obtain a negative electrode paste.

[0100] 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.

[0101] [Example 2] A negative electrode sheet of Example 2 was obtained in the same manner as in Example 1, except that the mass ratio of the first conductive material was changed to 0.4 and the mass ratio of the second conductive material was changed to 0.2.

[0102] [Comparative Example 1] A negative electrode sheet of Comparative Example 1 was obtained in the same manner as in Example 1, except that the mass ratio of the first conductive material was changed to 0.1 and the mass ratio of the second conductive material was changed to 0.5.

[0103] [Comparative Example 2] A negative electrode sheet of Comparative Example 2 was obtained in the same manner as in Example 1, except that the mass ratio of the first conductive material was changed to 0.1 and the mass ratio of the second conductive material was changed to 0.1.

[0104] [Comparative Example 3] A negative electrode sheet of Comparative Example 3 was obtained in the same manner as in Example 1, except that the mass ratio of the first conductive material was changed to 0.5 and the mass ratio of the second conductive material was changed to 0.5.

[0105] [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 content of the negative electrode paste was changed to graphite particles:first Si-containing particles:second Si-containing particles:first conductive material:second conductive material:CMC:PAA:SBR = 60:40:0:0.5:0:1::1:1.5.

[0106] <Fabrication of Lithium-Ion Secondary Battery for Evaluation> LiNi as the positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder were mixed with N-methylpyrrolidone (NMP) at a mass ratio of NCM:AB:PVdF = 100:1:1 to prepare a positive electrode paste. This paste was applied to the surface of an aluminum foil with a thickness of 15 μm and dried to form a positive electrode active material layer. After roll-pressing the positive electrode active material layer, the obtained sheet was processed into a predetermined size to obtain a positive electrode sheet.

[0107] A separator made of porous polyolefin was prepared. Leads were attached to each of the negative electrode sheet and the positive electrode sheet prepared above, and they were laminated through the separator to fabricate an electrode assembly. This was housed together with a non-aqueous electrolyte in a case made of an aluminum laminate film. As the non-aqueous electrolyte, a solution in which LiPF6 as a supporting salt was dissolved at a concentration of 1.0 mol / L in 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 was used. Then, the case was sealed to obtain a lithium-ion secondary battery for evaluation.

[0108] <Cycle Performance Evaluation> Each of the fabricated lithium-ion secondary batteries for evaluation was placed in an environment at 25°C. Each lithium-ion secondary battery for evaluation was subjected to constant current charging up to 4.2 V at a current value of 0.4C, and then constant voltage charging was performed until the current value reached 0.1C. Next, each lithium-ion secondary battery for evaluation was discharged at a constant current of 0.4C to 2.5 V. And the discharge capacity at this time was measured to obtain the initial capacity.

[0109] The charge and discharge described above were repeated 200 cycles with one cycle of charge and discharge. 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 determined from (discharge capacity after 200 cycles of charge and discharge / initial capacity) × 100. The results are shown in Table 1.

[0110]

Table 1

[0111] From the results in Table 1, it can be seen that the Si content ratio in the first Si-containing particles is higher than that in the second Si-containing particles, and the first Si-containing particles and the second Si-containing particles are each coated with a conductive material. When the ratio of the coating amount (mass %) of the conductive material for the first Si-containing particles to the coating amount (mass %) of the conductive material for the second Si-containing particles is 3.0 or more, the capacity retention rate after 200 cycles of charge and discharge is significantly high. Therefore, according to the negative electrode disclosed herein, it can be seen that capacity degradation can be suppressed when the secondary battery is repeatedly charged and discharged.

[0112] As described above, specific examples of the present invention have been described in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples exemplified above.

[0113] That is, the negative electrode of the secondary battery, its manufacturing method, and the secondary battery disclosed herein are as follows in items [1] to [7]. [1] A negative electrode comprising a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector, The negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles, The Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles, The first Si-containing particles and the second Si-containing particles are each coated with a conductive material, The ratio of the coating amount (mass %) of the conductive material on the first Si-containing particles to the coating amount (mass %) of the conductive material on the second Si-containing particles is 3.0 or more. Negative electrode. [2] The ratio of the Si content ratio in the first Si-containing particles to the Si content ratio in the second Si-containing particles is 1.2 or more. The negative electrode according to item [1]. [3] The 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% by mass to 90% by mass, and the mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 60:40. The negative electrode according to item [1] or [2]. [4] The conductive material is a carbon nanotube. The negative electrode according to any one of items [1] to [3]. [5] The first Si-containing particles and the second Si-containing particles are each particles of an Si-C composite material. The negative electrode according to any one of items [1] to [4]. [6] A step of preparing first Si-containing particles coated with a first conductive material and second Si-containing particles coated with a second conductive material, where the Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles, and the ratio of the coating amount (mass %) of the first conductive material on the first Si-containing particles to the coating amount (mass %) of the second conductive material on the second Si-containing particles is 3.0 or more. A step of mixing the first Si-containing particles coated with the first conductive material, the second Si-containing particles coated with the second conductive material, and graphite particles in a dispersion medium to prepare a negative electrode paste. A step of coating the negative electrode paste on a negative electrode current collector, and A step of drying the coated negative electrode paste A method for manufacturing a negative electrode comprising the steps. [7] A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode according to any one of items [1] to [5]. A secondary battery.

Explanation of symbols

[0114] 12 Graphite particles 14 First Si-containing particles 15 First conductive material 16 Second Si-containing particles 17 Second conductive material 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector 44 Negative terminal 44a Negative current collector 50 Positive electrode sheet (positive electrode) 52 Positive current collector 52a Non-formation part of positive electrode active material layer 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative current collector 62a Non-formation part of negative electrode active material layer 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, wherein the negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles, the Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles, the first Si-containing particles and the second Si-containing particles are each coated with a conductive material, and the ratio of the coating amount (mass %) of the conductive material for the first Si-containing particles to the coating amount (mass %) of the conductive material for the second Si-containing particles is 3.0 or more. Negative electrode.

2. The negative electrode according to claim 1, wherein the ratio of the Si content ratio in the first Si-containing particles to the Si content ratio in the second Si-containing particles is 1.2 or more.

3. The negative electrode according to claim 1, wherein the 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% by mass to 90% by mass, and the mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 60:

40.

4. The negative electrode according to claim 1, wherein the conductive material is a carbon nanotube.

5. The negative electrode according to claim 1, wherein the first Si-containing particles and the second Si-containing particles are each a particle of an Si—C composite material.

6. A step of preparing first Si-containing particles coated with a first conductive material and second Si-containing particles coated with a second conductive material, where the Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles, and the ratio of the coating amount (mass %) of the first conductive material for the first Si-containing particles to the coating amount (mass %) of the second conductive material for the second Si-containing particles is 3.0 or more, a step of mixing the first Si-containing particles coated with the first conductive material, the second Si-containing particles coated with the second conductive material, and graphite particles in a dispersion medium to prepare a negative electrode paste, a step of coating the negative electrode paste on a negative electrode current collector, and a step of drying the coated negative electrode paste A method for manufacturing a negative electrode comprising these steps.

7. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode according to claim 1. ​

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