Negative electrode for secondary battery and secondary battery using same

Abstract

The present disclosure relates to a negative electrode of a secondary battery. The present disclosure also relates to a secondary battery using the negative electrode. Provided is a negative electrode which contains Si-containing particles and graphite particles, and which has little swelling during repeated charging and discharging of a secondary battery. A negative electrode of a secondary battery according to the present disclosure 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 includes a first layer located on the surface layer portion side and a second layer located on the negative electrode current collector side. The first layer contains first graphite particles and first Si-containing particles. The second layer contains second graphite particles and second Si-containing particles. The content ratio of Si in the first Si-containing particles is smaller than the content ratio of Si 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 second Si-containing particles to the coating amount (mass%) of the conductive material with respect to the first Si-containing particles is 2.5 or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a negative electrode of a secondary battery. The present disclosure also relates to a secondary battery using the negative electrode. BACKGROUND

[0002] In recent years, secondary batteries are suitably used for mobile power sources of personal computers, portable terminals, and the like, vehicle drive power sources of electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like.

[0003] In the vehicle drive power source use, particularly the drive power source use of BEVs, from the viewpoint of extending the cruising distance of the vehicle, further high capacity of the secondary battery is desired. As a negative electrode active material with high capacity, Si-containing particles are known, and it is known that the secondary battery can be made high capacity by using the Si-containing particles (for example, refer to Patent Literature 1). In Patent Literature 1, a technique of using Si-containing particles and graphite particles such as natural graphite as a negative electrode active material is disclosed.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Publication No. 2015-38862 SUMMARY

[0007] However, the Si-containing particles have high capacity, and on the other hand, have large volume change due to expansion / contraction of the secondary battery at the time of charge and discharge. Also, in the case where the Si-containing particles and the graphite particles are used together as a negative electrode active material, if the secondary battery is repeatedly charged and discharged, there is a problem that internal stress increases due to expansion of the negative electrode. Therefore, for the negative electrode containing the Si-containing particles and the graphite particles, it is desired to develop a negative electrode with small expansion at the time of repeated charge and discharge of the secondary battery. Note that the expansion of the negative electrode means that the volume of the negative electrode becomes larger than the initial volume in the same state of charge (for example, the discharged state).

[0008] In view of the above-described actual circumstances, an object of the present disclosure is to provide a negative electrode containing Si-containing particles and graphite particles, which has small expansion at the time of repeated charge and discharge of a secondary battery.

[0009] The negative electrode of the secondary battery of the present disclosure 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 includes a first layer on the side of a surface layer portion, and a second layer on the side of the negative electrode current collector. The first layer contains first graphite particles and first Si-containing particles. The second layer contains second graphite particles and second Si-containing particles. The Si content ratio in the first Si-containing particles is smaller than the Si content ratio in the second Si-containing particles. Each of the first Si-containing particles and the second Si-containing particles is coated with an electrically conductive material. The ratio of the coating amount of the electrically conductive material with respect to the second Si-containing particles (mass %) to the coating amount of the electrically conductive material with respect to the first Si-containing particles (mass %) is 2.5 or greater.

[0010] According to such a configuration, it is possible to provide a negative electrode, which is a negative electrode containing Si-containing particles and graphite particles, that has small swelling when 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, it is possible to provide a secondary battery that has small swelling of the negative electrode when repeatedly charged and discharged, even if a negative electrode containing Si-containing particles and graphite particles is used. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 A cross-sectional view schematically showing the configuration of the negative electrode of the secondary battery according to an embodiment of the present disclosure.

[0014] Figure 2 A cross-sectional view schematically showing the configuration of the negative electrode active material layer contained in the negative electrode active material of Figure 1

[0015] Figure 3 A cross-sectional view schematically showing the configuration of a lithium-ion secondary battery using the negative electrode of the secondary battery according to an embodiment of the present disclosure.

[0016] Figure 4 A cross-sectional view schematically showing the configuration of the wound electrode body of the lithium-ion secondary battery of Figure 3 DETAILED DESCRIPTION

[0017] ​​Embodiments according to the present disclosure will be described below with reference to the accompanying drawings. It should be noted that matters not mentioned in the present specification, which are required for implementing the present disclosure, can be understood by those skilled in the art as design matters based on the prior art in the field. The present disclosure can be implemented based on the content disclosed in the present specification and technical common sense in the field. In addition, in the following drawings, the same reference numerals are assigned to components that serve the same function, and description will be given. In addition, the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship. It should be noted that in the present specification, a numerical range expressed as "A to B" includes A and B.

[0018] It should be noted that in the present specification, "secondary battery" refers to a power storage device that can be repeatedly charged and discharged. In addition, in the present specification, "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and realizes charge and discharge by movement of charges associated with lithium ions between positive and negative electrodes.

[0019] The negative electrode disclosed herein is used for a secondary battery, preferably a lithium-ion secondary battery. Referring to Figure 1 An embodiment of the negative electrode disclosed herein will be described specifically. Figure 1 A cross-sectional view schematically showing an example of the negative electrode 60 according to the present embodiment is a cross-sectional view along the thickness direction and the width direction. Figure 1 The negative electrode 60 according to the present embodiment shown in the drawing is a negative electrode of a lithium-ion secondary battery.

[0020] As illustrated, 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 can be provided only on one surface of the negative electrode current collector 62, or can be provided on both surfaces of the negative electrode current collector 62 as illustrated in the example. The negative electrode active material layer 64 is preferably provided on both surfaces of the negative electrode current collector 62.

[0021] As illustrated in the example, a negative electrode active material layer non-formation portion 62a in which the negative electrode active material layer 64 is not provided can be provided at one end portion in the width direction of the negative electrode 60. In the negative electrode active material layer non-formation portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer non-formation portion 62a can function as a current collecting portion. However, the configuration for collecting current from the negative electrode 60 is not limited thereto.

[0022] The negative electrode current collector 62 is shown in the example as a foil (or sheet), but is not limited to this. The negative electrode current collector 62 can be in various forms, such as rod-shaped, plate-shaped, or mesh-shaped. As with conventional lithium-ion secondary batteries, a metal with good conductivity (such as copper, nickel, titanium, stainless steel, etc.) can be used as the material for the negative electrode current collector 62, with copper being preferred. Copper foil is particularly preferred as the negative electrode current collector 62.

[0023] There is no particular limitation on the size of the negative electrode current collector 62, which can be appropriately determined according to the battery design. When copper foil is used as the negative electrode current collector 62, there is no particular limitation on its thickness, for example, it can be 5μm or more and 35μm or less, preferably 6μm or more and 20μm or less.

[0024] like Figure 1 As shown, the negative electrode active material layer 64 has a multilayer structure, specifically, it has a first layer 64a located on the surface side of the negative electrode active material layer 64 and a second layer 64b located on the side of the negative electrode current collector 62. Figure 1 As shown, the first layer 64a is the upper layer of the negative electrode active material layer 64, and the second layer 64b is the lower layer of the negative electrode active material layer 64. It should be noted that the negative electrode active material layer 64 may further have layers other than the first layer 64a and the second layer 64b, to a extent that does not significantly impair the effects of this disclosure. For example, the negative electrode active material layer 64 may have an intermediate layer between the first layer 64a and the second layer 64b, which mixes the components of these layers.

[0025] The negative electrode active material layer 64 contains negative electrode active material. For this purpose, [the following is used]... Figure 2 A detailed explanation. Figure 2 To show Figure 1 The diagram shows a schematic cross-sectional view of the particles of the negative electrode active material contained in the negative electrode active material layer 64. It should be noted that... Figure 2 For illustrative purposes only; therefore, the number, distribution, etc., of particles are not limited to... Figure 2 The content shown.

[0026] Regarding the negative electrode active material, the first layer 64a contains a first graphite particle 12 and a first Si-containing particle 14. The second layer 64b contains a second graphite particle 16 and a second Si-containing particle 18. Therefore, in the first layer 64a, at least the first graphite particle 12 and the first Si-containing particle 14 are used as the negative electrode active material, and in the second layer 64b, at least the second graphite particle 16 and the second Si-containing particle 18 are used as the negative electrode active material. The volume change caused by the expansion / contraction associated with charging and discharging of the Si-containing particles is large, but by using them in conjunction with graphite particles, it is possible to suppress the disconnection of the conductive path caused by the volume change of the Si-containing particles.

[0027] The graphite constituting the first graphite particles 12 and the second graphite particles 16 can be natural graphite, can be artificial graphite, or can be amorphous carbon-coated graphite in which graphite is coated with amorphous carbon.

[0028] The shape of the first graphite particles 12 and the second graphite particles 16 is not particularly limited, and can be flaky, spherical, or the like. The first graphite particles 12 and the second graphite particles 16 are preferably spheroidized graphite particles. In the case where the first graphite particles 12 and the second graphite particles 16 are spherical, the circularity of the first graphite particles 12 and the second graphite particles 16 is preferably 0.85 to 1, more preferably 0.88 to 1, and further preferably 0.90 to 1.

[0029] Note that, in the present 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 (i.e., circularity = circumference of a true circle having the same area as the projected area of the particle / circumference of the particle projection image). Thus, 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 true sphere. The circularity can be found, for example, by using a commercially available static automatic image analyzer, finding the circularity for 100 or more particles, and calculating the average value thereof.

[0030] The average particle diameter (D50) of the first graphite particles 12 and the second graphite particles 16 is not particularly limited. The average particle diameter (D50) of the first graphite particles 12 and the second graphite particles 16 is each, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and further preferably 12 μm to 20 μm.

[0031] Note that, in the present specification, the "average particle diameter (D50)" refers to the median diameter (D50), and refers to the particle diameter corresponding to the cumulative frequency of 50% by volume from the fine particle side in a particle size distribution based on the volume basis of the laser diffraction-scattering method. The average particle diameter (D50) can be found using a commercially available particle size distribution measuring device of the laser diffraction-scattering type or the like.

[0032] The same graphite particles can be used as the first graphite particles 12 and the second graphite particles 16, or different graphite particles can be used. The same graphite particles are preferably used as the first graphite particles 12 and the second graphite particles 16.

[0033] As the first Si-containing particles 14 and the second Si-containing particles 18, for example, particles of Si-C composite material can be used. The Si-C composite material typically contains carbon domains and domains containing Si. Note that the first Si-containing particles 14 and the second Si-containing particles 18 can not be Si-C composite material, and can be Si particles, Si oxide particles, or the like.

[0034] The carbon domain is, for example, a carbonization product of a carbon precursor (e.g., petroleum pitch, coal pitch, phenol resin, etc.); graphite, etc. The carbon domain preferably constitutes a carbon matrix. Thus, 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, the carbon matrix is able to moderate the volume change caused by the expansion / contraction of the Si-containing domains, and is thus advantageous.

[0035] The Si-containing domain contains Si, and is, for example, composed of Si, Si oxide (SiO x ), Si nitride (SiN x ), Si carbide (SiC x ), etc. The Si-containing domain is preferably composed of at least any one of Si, and Si oxide (SiO x ). The Si-containing domain can be a fine particle. The oxygen content in the Si-containing domain is preferably 10% by mass or less.

[0036] The average particle diameter of the Si-containing domain is, for example, 50 nm or less, and can be 5 nm to 50 nm. Note that the "average particle diameter of the Si-containing domain" can be obtained as follows. First, the negative electrode active material layer 64 is subjected to FIB (focused ion beam) processing, and a sample for scanning transmission electron microscope (STEM) observation is prepared. Then, the sample is subjected to elemental analysis using EDX elemental mapping, and a BF image (bright field image) and a HAADF image (high angle annular dark field image) are obtained. From the contrast and shape obtained using the BF image and the HAADF image, the diameter of the Si-containing domain can be obtained. The diameters of 10 or more Si-containing domains selected at random are obtained, and the average value thereof is set as the "average particle diameter of the Si-containing domain" herein.

[0037] The Si-C composite material is, for example, a material in which fine particles containing Si are dispersed in a carbon material; a material in which fine particles containing Si have entered the pores of granulated porous graphite; etc. The Si-C composite material can be a material in which fine particles containing Si are attached to the surface of carbon particles; a material in which carbon fine particles are attached to the surface of particles containing Si; etc. From the viewpoint of suppressing the volume change of Si, a material in which Si nanoparticles are dispersed in a carbon material, and a material in which Si nanoparticles are dispersed in the pores of a porous carbon material are preferred, and a material in which Si nanoparticles are dispersed in the pores of a porous carbon material is more preferred.

[0038] In this embodiment, the Si content (S1) in the first Si-containing particle 14 is smaller than the Si content (S2) in the second Si-containing particle 18. There are no particular limitations on the Si content (S1) in the first Si-containing particle 14 and the Si content (S2) in the second Si-containing particle 18, as long as this relationship is satisfied. However, if these Si content proportions are too low, the effect of suppressing the swelling of the negative electrode during repeated charging and discharging of the secondary battery may be reduced. On the other hand, if these Si content proportions are too high, the volume change caused by the expansion / contraction of the first Si-containing particle 14 and the second Si-containing particle 18 during repeated charging and discharging of the secondary battery may become excessively large.

[0039] Therefore, the Si content (S1) in the first Si-containing particle 14 is preferably 20% to 55% by mass, more preferably 25% to 45% by mass. The Si content (S2) in the second Si-containing particle 18 is preferably 45% to 80% by mass, more preferably 55% to 75% by mass.

[0040] Furthermore, the ratio (S1 / S2) of the Si content ratio (S1) in the first Si-containing particle 14 to the Si content ratio (S2) in the second Si-containing particle 18 is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, and even more preferably 0.40 to 0.75.

[0041] There is no particular limitation on the average particle size (D50) of the first Si-containing particle 14 and the second Si-containing particle 18. The average particle size (D50) of the first Si-containing particle 14 and the second Si-containing particle 18 is, for example, 1 μm to 15 μm, preferably 2 μm to 10 μm, and more preferably 4 μm to 10 μm.

[0042] It should be noted that the first Si-containing particle 14 and the second Si-containing particle 18 can be manufactured according to known methods. It should be noted that various methods for manufacturing particles of Si-C composite materials are known (for example, see Japanese Patent Application Publication No. 2015-38862, International Publication No. 2014 / 046144, and other prior art documents listed in that international publication).

[0043] like Figure 2 As shown, the first layer 64a contains a first conductive material 15. In the first layer 64a, first Si-containing particles 14 are coated with the first conductive material 15. Furthermore, the second layer 64b contains a second conductive material 19. In the second layer 64b, second Si-containing particles 18 are coated with the second conductive material 19.

[0044] In the illustrated example, the first Si-containing particles 14 as a whole and the second Si-containing particles 18 as a whole are coated with the first conductive material 15 and the second conductive material 19, respectively. Thus, the first conductive material 15 and the second conductive material 19 form a coating layer, respectively. However, the first Si-containing particles 14 and the second Si-containing particles 18 can be partially coated with the first conductive material 15 and the second conductive material 19, respectively.

[0045] As examples of the conductive material (i.e., the first conductive material 15 and the second conductive material 19), carbon black such as acetylene black, carbon fibers, carbon nanotubes (CNT), and the like can be given. Among them, CNT is preferred. Note that the first conductive material 15 and the second conductive material 19 are preferably the same.

[0046] In the case where CNT is used as the conductive material, the kind thereof is not particularly limited, and for example, single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), multi-walled carbon nanotubes (MWCNT), and the like can be used. These can be used alone or in combination of two or more. Since the conductive path between the negative electrode active materials can be formed more, as the CNT, SWCNT is preferred. The CNT can be a substance produced by an arc discharge method, a laser ablation method, a chemical vapor deposition method, or the like.

[0047] The average length of the CNT is not particularly limited. If the average length of the CNT is too long, the CNT aggregates, and the dispersibility tends to decrease. Thus, the average length of the CNT is preferably 15 μm or less, more preferably 8.0 μm or less, and further preferably 5.0 μm or less. On the other hand, if the average length of the CNT is too short, the conductive path between the negative electrode active materials tends to be difficult to form. Thus, the average length of the CNT is preferably 0.3 μm or more, more preferably 0.5 μm or more, and further preferably 1.0 μm or more.

[0048] The average diameter of the CNT is not particularly limited, and for example, is 0.1 nm to 50 nm, and is preferably 0.3 nm to 30 nm.

[0049] Note that, in terms of the average length and the average diameter of the CNT, an electron microscope photograph of the CNT can be taken, and the average values of the length and the diameter of 100 or more CNTs can be found, respectively. Specifically, for example, a CNT dispersion liquid is diluted and dried to prepare a measurement sample. For the sample, scanning electron microscope (SEM) observation is performed, and the length and the diameter of 100 or more CNTs are found, and the average values are calculated. At this time, in the case where the CNTs re-aggregate, the length and the diameter of the aggregate CNT bundle are found.

[0050] In the present embodiment, the amount of coating (C1) (mass %) of the first Si-containing particles 14 with the first conductive material 15 is less than the amount of coating (C2) (mass %) of the second Si-containing particles 18 with the second conductive material 19. Note that the amount of coating (C1) is the ratio (%) of the mass of the first conductive material 15 with respect to the mass of the first Si-containing particles 14, and the amount of coating (C2) is the ratio (%) of the mass of the second conductive material 19 with respect to the mass of the second Si-containing particles 18.

[0051] Further, the ratio (C2 / C1) of the amount of coating (C2) of the second Si-containing particles with the conductive material to the amount of coating (C1) of the first Si-containing particles with the conductive material is 2.5 or more.

[0052] Thus, in the first layer 64a, which is the upper layer of the negative electrode active material layer 64, the low-Si-content first Si-containing particles 14 coated with the first conductive material 15 are used in addition to the first graphite particles 12, and in the second layer 64b, which is the lower layer of the negative electrode active material layer 64, the high-Si-content second Si-containing particles 18 coated with the second conductive material 19 in an amount 2.5 times or more of the first Si-containing particles 14 are used in addition to the second graphite particles 16. Thereby, the swelling of the negative electrode 60 upon repeated charge and discharge of the secondary battery can be significantly suppressed. The reason is considered as follows.

[0053] That is, in the negative electrode active material layer 64, the swelling upon repeated charge and discharge of the secondary battery is large in the upper layer (i.e., the first layer 64a). Therefore, the Si-containing particles having a low Si content, which have a small expansion and contraction, are used in the upper layer, and further, the particles are coated with a relatively small amount of the conductive material. Thereby, the expansion and contraction of the Si-containing particles and the side reactions caused by charge and discharge can be suppressed.

[0054] On the other hand, the Si-containing particles having a high Si content, which have a large expansion and contraction, are used in the lower layer (i.e., the second layer 64b), and further, the particles are coated with a relatively large amount of the conductive material. Thereby, the disconnection of the conductive path upon charge and discharge of the secondary battery can be suppressed. Therefore, the swelling of the negative electrode (i.e., the battery reaction becomes uneven, the swelling caused by local reaction, stress concentration, etc.) accompanying the disconnection of the conductive path can be suppressed. Thus, the negative electrode active material layer 64 as a whole can significantly suppress the swelling of the negative electrode 60 upon repeated charge and discharge of the secondary battery.

[0055] The ratio (C2 / C1) of the amounts of coating is preferably 2.8 or more, more preferably 3.0 or more, further preferably 5.0 or more, and particularly preferably 7.0 or more. The ratio (C2 / C1) of the amounts of coating can be 50 or less, 30 or less, 20 or less, or 15 or less.

[0056] The content of the conductive material in the negative electrode active material layer 64 (i.e., the first layer 64a and the second layer 64b) is not particularly limited as long as the effects of the present disclosure are obtained. The amount of the conductive material is, for example, 0.02 to 10 mass% with respect to the negative electrode active material, and can be appropriately determined depending on the kind of the conductive material. In the case where the conductive material is CNT, the content of the conductive material in the negative electrode active material layer 64 is preferably 0.02 to 3.0 mass%, more preferably 0.05 to 1.0 mass% with respect to the negative electrode active material.

[0057] Therefore, the coating amount (C1) of the first Si-containing particle 14 with the first conductive material 15 and the coating amount (C2) of the second Si-containing particle 18 with the second conductive material 19 can be appropriately set in consideration of the content of the conductive material in the negative electrode active material layer 64 described above, the ratio of the coating amounts (C2 / C1), and the mass ratio of the first Si-containing particle 14 to the second Si-containing particle 18.

[0058] In the case where the first conductive material 15 is CNT, the coating amount (C1) of the first Si-containing particle 14 with the first conductive material 15 is, for example, 0.05 to 2.0 mass%, preferably 0.05 to 1.0 mass%, more preferably 0.05 to 0.25 mass%, particularly preferably 0.07 to 0.24 mass%. In the case where the second conductive material 19 is CNT, the coating amount (C2) of the second Si-containing particle 18 with the second conductive material 19 is, for example, 0.10 to 3.0 mass%, preferably 0.20 to 1.5 mass%, more preferably 0.25 to 0.70 mass%, particularly preferably 0.27 to 0.55 mass%.

[0059] The ratio of the total mass of the second conductive material 19 coating the second Si-containing particle 18 to the total mass of the first conductive material 15 coating the first Si-containing particle 14 is preferably greater than 1, more preferably 1.5 or greater, further preferably 2 or greater, particularly preferably 4 or greater.

[0060] Note that the dispersant of CNT can be attached to the first conductive material 15 and the second conductive material 19. As the dispersant of CNT, a publicly known dispersant of CNT can be used.

[0061] In the first layer 64a, the mass proportion of the first Si-containing particle 14 with respect to the total of the first graphite particle 12 and the first Si-containing particle 14 is preferably 10 to 60 mass%, more preferably 15 to 50 mass%, further preferably 20 to 40 mass%.

[0062] In the second layer 64b, the mass proportion of the second Si-containing particles 18 with respect to the total of the second graphite particles 16 and the second Si-containing particles 18 is preferably 10 to 60 mass%, more preferably 15 to 50 mass%, and further preferably 20 to 40 mass%. Note that the mass proportion of the first Si-containing particles 14 in the first layer 64a and the mass proportion of the second Si-containing particles 18 in the second layer 64b can be the same or different.

[0063] The negative electrode active material contained in the first layer 64a can be only the first graphite particles 12 and the first Si-containing particles 14. However, the first layer 64a can further contain a negative electrode active material other than the first graphite particles 12 and the first Si-containing particles 14, within a range that does not hinder the effects of the present disclosure (e.g., 10 mass% or less of the total amount of the negative electrode active material contained in the first layer 64a).

[0064] The negative electrode active material contained in the second layer 64b can be only the second graphite particles 16 and the second Si-containing particles 18. However, the second layer 64b can further contain a negative electrode active material other than the second graphite particles 16 and the second Si-containing particles 18, within a range that does not hinder the effects of the present disclosure (e.g., 10 mass% or less of the total amount of the negative electrode active material contained in the second layer 64b).

[0065] In the negative electrode active material layer 64, the ratio (T2 / T1) of the thickness (T2) of the second layer 64b to the thickness (T1) of the first layer 64a is not particularly limited as long as the effects of the present disclosure are obtained, and is, for example, 5 / 95 to 95 / 5. From the viewpoint of further suppressing the swelling of the negative electrode during repeated charge and discharge of the secondary battery, the ratio (T2 / T1) is preferably 10 / 90 to 90 / 10, more preferably 10 / 90 to 80 / 20, and further preferably 30 / 70 to 60 / 40.

[0066] The negative electrode active material layer 64 can contain components other than the negative electrode active material and the conductive material, and as an example thereof, a binder can be cited. As the binder, for example, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), or the like can be used. The CMC also functions as a thickening agent.

[0067] The content of the negative electrode active material in the first layer 64a (i.e., with respect to the total mass of the first layer 64a) is preferably 90 mass% or more, and more preferably 95 mass% or more. The content of the binder in the first layer 64a is preferably 0.1 mass% or more and 8 mass% or less, and more preferably 0.5 mass% or more and 5 mass% or less.

[0068] The content of the negative active material in the second layer 64b (i.e., relative to the total mass of the second layer 64b) is preferably 90% by mass or more, and more preferably 95% by mass or more. The content of the binder in the second layer 64b is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less.

[0069] The thickness of the negative active material layer 64 is not particularly limited, and is, for example, 10 μm or more and 400 μm or less, and is preferably 20 μm or more and 300 μm or less.

[0070] The density of the negative active material layer 64 is not particularly limited, and is, for example, 0.7 g / cm 3 or more, and is preferably 1.0 g / cm 3 or more, and is more preferably 1.2 g / cm 3 or more. On the other hand, the density of the negative active material layer 64 is, for example, 2.3 g / cm 3 or less, and is preferably 2.0 g / cm 3 or less.

[0071] The negative electrode 60 can include members other than the negative electrode current collector 62 and the negative active material layer 64. For example, an insulating layer (not shown) that is adjacent to the negative active material layer 64 can be provided on the negative active material layer non-forming portion 62a. The insulating layer contains, for example, an inorganic filler that is insulating or the like.

[0072] The negative electrode 60 can be suitably manufactured, for example, by a manufacturing method including the following steps: a step of preparing the first Si-containing particles 14 coated with the first conductive material 15 and the second Si-containing particles 18 coated with the second conductive material 19, respectively (hereinafter also referred to as a "coated particle preparation step"), in which the Si content ratio (S1) in the first Si-containing particles 14 is smaller than the Si content ratio (S2) in the second Si-containing particles 18, and the ratio of the coating amount (mass%) of the conductive material with respect to the second Si-containing particles 18 to the coating amount (mass%) of the conductive material with respect to the first Si-containing particles 14 is 2.5 or more; a step of mixing the second Si-containing particles 18 coated with the second conductive material 19 and the second graphite particles 16 in a dispersion medium to prepare a lower layer-forming paste (hereinafter also referred to as a "lower layer-forming paste preparation step"); a step of mixing the first Si-containing particles 14 coated with the first conductive material 15 and the first graphite particles 12 in a dispersion medium to prepare an upper layer-forming paste (hereinafter also referred to as an "upper layer-forming paste preparation step"); a step of applying and drying the lower layer-forming paste on the negative electrode current collector 62 to form a lower layer (hereinafter also referred to as a "lower layer formation step"), and a step of applying and drying the upper layer-forming paste on the lower layer to form an upper layer (hereinafter also referred to as an "upper layer formation step").

[0073] Note that, in the present specification, the "paste" refers to a mixture in which a part or all of solid components are dispersed in a dispersion medium, and includes so-called "slurry", "ink", and the like.

[0074] The coating particle preparation step can be performed according to a known method. Specifically, for example, the first Si-containing particle 14 having a low Si content ratio and the second Si-containing particle 18 having a high Si content ratio are prepared. In addition, the conductive material (the first conductive material and the second conductive material) is prepared. For the coating of the first Si-containing particle 14 and the second Si-containing particle 18 with the conductive material, a known coating method can be used. For example, a method in which the Si-containing particle and the conductive material are dispersed in a dispersion medium, and then dried as needed by heating, reduced pressure, or the like; a method in which a slurry in which the Si-containing particle and the conductive material are dispersed in a dispersion medium is spray-dried; a method in which the Si-containing particle and the conductive material are mechanically fused; a method in which the Si-containing particle and the conductive material are mechanically ground; a method in which the conductive material is deposited on the Si-containing particle by chemical vapor deposition (CVD); and the like can be exemplified.

[0075] Note that the coating of the first Si-containing particle 14 and the second Si-containing particle 18 with the conductive material is performed in such a manner that the ratio of the coating amounts (C2 / C1) becomes 2.5 or more.

[0076] The lower layer formation paste preparation step can be performed according to a known method by mixing the second graphite particle 16, the second Si-containing particle 18 coated with the second conductive material 19, and an optional component (for example, a binder or the like) with a dispersion medium (for example, water) using a known mixing device, a stirring device, or the like.

[0077] The upper layer formation paste preparation step can be performed according to a known method by mixing the first graphite particle 12, the first Si-containing particle 14 coated with the first conductive material 15, and an optional component (for example, a binder or the like) with a dispersion medium (for example, water) using a known mixing device, a stirring device, or the like. Note that the upper layer formation paste preparation step can be performed in parallel with the lower layer formation paste preparation step. The upper layer formation paste preparation step can be performed in parallel with or after the lower layer formation step.

[0078] The lower layer formation step can be performed according to a known method. Specifically, for example, the lower layer formation paste can be applied and dried on the negative electrode current collector 62 using a known coating device. By drying, the lower layer (the second layer 64b) is formed.

[0079] The upper layer forming step can be performed according to a known method. Specifically, for example, the upper layer forming paste can be applied to the formed lower layer by using a known coating device, and dried to perform. By drying, the upper layer (first layer 64a) is formed, and the negative electrode active material layer 64 is formed.

[0080] After the drying step, a step of pressing the negative electrode active material layer 64 can be further performed. The pressing step can be performed according to a known method.

[0081] According to the negative electrode 60 related to the present embodiment, the swelling of the negative electrode 60 at the time of repeated charge and discharge of the secondary battery can be suppressed. In addition, the negative electrode 60 related to the present embodiment can increase the capacity of the secondary battery because the Si-containing negative electrode active material is used.

[0082] 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 related to the above-described embodiment. Hereinafter, a lithium ion secondary battery will be exemplified, and the secondary battery will be described with reference to Figure 3 and Figure 4 An embodiment of the secondary battery disclosed herein will be described. The following configuration example is a flat square lithium ion secondary battery having a flat square shape of a wound electrode body and a flat square battery case.

[0083] Figure 3 The lithium ion secondary battery 100 illustrated is a sealed lithium ion secondary battery 100 configured by housing a flat square wound electrode body 20 and a non-aqueous electrolyte (not illustrated) in a flat square 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-walled safety valve 36 configured to open the internal pressure of the battery case 30 when the internal pressure of the battery case 30 rises to a predetermined level or more. In addition, the battery case 30 is provided with an injection port (not illustrated) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to the positive electrode current collector 42a. The negative electrode terminal 44 is electrically connected to the negative electrode current collector 44a. As the material of the battery case 30, a lightweight metal material having good thermal conductivity, such as aluminum, is used.

[0084] As Figure 3 and Figure 4As shown, the wound electrode body 20 has a configuration in which the positive electrode sheet 50 and the negative electrode sheet 60 are overlapped via two long separator sheets 70 and wound in the length direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed in the length direction on one face or both faces (in this case, both faces) 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 in the length direction on one face or both faces (in this case, both faces) of a long negative electrode current collector 62. The positive electrode active material layer non-formed portion 52a (i.e., the portion of the positive electrode current collector 52 exposed without the positive electrode active material layer 54 formed thereon) and the negative electrode active material layer non-formed portion 62a (i.e., the portion of the negative electrode current collector 62 exposed without the negative electrode active material layer 64 formed thereon) are formed in a manner that they protrude outward from both ends in the winding axis direction (i.e., the sheet width direction orthogonal to the above-mentioned length direction) of the wound electrode body 20. The positive electrode current collector plate 42a and the negative electrode current collector plate 44a are respectively joined to the positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a.

[0085] As the positive electrode current collector 52 constituting the positive electrode sheet 50, a known positive electrode current collector used in lithium ion secondary batteries can be used, and as examples thereof, a sheet or foil made of a metal (e.g., aluminum, nickel, titanium, stainless steel, etc.) having good conductivity can be listed. As the positive electrode current collector 52, an aluminum foil is preferable.

[0086] The size of the positive electrode current collector 52 is not particularly limited, and can be appropriately determined in accordance with the battery design. In the case where an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited, and is, for example, 5 μm or more and 35 μm or less, and is preferably 7 μm or more and 20 μm or less.

[0087] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known positive electrode active material used in lithium ion secondary batteries can be used. Specifically, for example, as the positive electrode active material, a lithium complex oxide, a lithium transition metal phosphate compound, etc. can be used. The crystal structure of the positive electrode active material is not particularly limited, and can be a layered structure, a spinel structure, an olivine structure, etc.

[0088] As the lithium complex oxide, a lithium transition metal complex oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferable, and as specific examples thereof, a lithium nickel-based complex oxide, a lithium cobalt-based complex oxide, a lithium manganese-based complex oxide, a lithium nickel-manganese-based complex oxide, a lithium nickel-cobalt-manganese-based complex oxide, a lithium nickel-cobalt-aluminum-based complex oxide, a lithium iron-nickel-manganese-based complex oxide, etc. can be listed.

[0089] Note that, in the present specification, the "lithium nickel cobalt manganese complex oxide" is a term including oxides containing one or more kinds of added elements other than Li, Ni, Co, Mn, and O, in addition to oxides in which Li, Ni, Co, Mn, and O are constituent elements. Examples of the added elements include transition metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, Sn, and typical metal elements. In addition, the added elements can be semimetal elements such as B, C, Si, and P, and nonmetal elements such as S, F, Cl, Br, and I. The same applies to the above-described lithium nickel complex oxide, lithium cobalt complex oxide, lithium manganese complex oxide, lithium nickel manganese complex oxide, lithium nickel cobalt aluminum complex oxide, and lithium iron nickel manganese complex oxide.

[0090] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFeP04), lithium manganese phosphate (LiMnP04), and lithium manganese iron phosphate.

[0091] The positive electrode active material can be used alone or in combination with two or more kinds. The lithium nickel cobalt manganese complex oxide is particularly preferable as the positive electrode active material because of excellent initial resistance characteristics and other characteristics.

[0092] The average particle diameter (D50) of the positive electrode active material is not particularly limited and 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.

[0093] The positive electrode active material layer 54 can contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, and a binder. 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 such as graphite can be preferably used. As the binder, for example, polyvinylidene fluoride (PVdF) can be used.

[0094] 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 with respect to the total mass of the positive electrode active material layer 54) is not particularly limited, and is preferably 70% by mass or more, more preferably 80% by mass or more, and further preferably 85% by mass or more and 99% by mass or less. The content of lithium phosphate in the positive electrode active material layer 54 is not particularly limited, and 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, and 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, and 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.

[0095] The thickness of each surface of the positive electrode active material layer 54 is not particularly limited, and is typically 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is typically 400 μm or less, and preferably 300 μm or less.

[0096] As the negative electrode sheet 60, the above-described negative electrode 60 is used.

[0097] As the separator 70, for example, a porous sheet (film) composed of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide, or the like can be exemplified. The porous sheet can be a single layer structure, or a laminated structure of two or more layers (for example, a three-layer structure in which a PP layer is laminated on both surfaces of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the separator 70.

[0098] The thickness of the separator 70 is not particularly limited, and is, for example, 5 μm or more and 50 μm or less, and is 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, and is preferably 350 seconds / 100 cc or less.

[0099] The nonaqueous electrolyte solution typically contains a nonaqueous solvent and a supporting salt (electrolyte salt). As the nonaqueous solvent, an organic solvent such as a carbonate, an ether, an ester, a nitrile, a sulfone, a lactone, or the like, which is used in the electrolyte solution of a general lithium-ion secondary battery, can be used without particular limitation. Among them, a carbonate is preferred, and as specific examples thereof, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), and the like can be exemplified. Such a nonaqueous solvent can be used alone as one kind, or two or more kinds can be used in appropriate combination. As an example, the nonaqueous solvent contains only a carbonate. As another example, the nonaqueous solvent contains a carbonate and an ester such as methyl acetate.

[0100] As the supporting salt, for example, a lithium salt such as LiPF6, LiBF4, lithium bis(fluorosulfonyl)imide (LiFSI), or the like (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.

[0101] Note that the above nonaqueous electrolyte solution can contain components other than the above components, such as a film-forming agent such as vinylene carbonate (VC), an oxalate complex, a gas generating agent such as biphenyl (BP), cyclohexylbenzene (CHB), various additives such as a thickening agent, and the like, without significantly impairing the effects of the present disclosure.

[0102] The swelling of the negative electrode at the time of repeated charge and discharge of the lithium-ion secondary battery 100 is suppressed, and thus the reaction force is low. In addition, the lithium-ion secondary battery 100 is high in capacity. The lithium-ion secondary battery 100 can be used for various purposes. As suitable purposes, a power source for driving a vehicle such as an electric vehicle (BEV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or the like can be exemplified. In addition, the lithium-ion secondary battery 100 can also be used as a storage battery for a small-sized power storage device or the like. The lithium-ion secondary battery 100 can also be typically used in the form of a battery pack in which a plurality of cells are connected in series and / or in parallel.

[0103] In the above, as an example, a square lithium-ion secondary battery 100 including a flat wound electrode body 20 was described. However, the lithium-ion secondary battery can also be configured as a lithium-ion secondary battery including a stacked electrode body (i.e., an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked). In addition, the lithium-ion secondary battery can also be configured as a cylindrical lithium-ion secondary battery, a laminated case type lithium-ion secondary battery, or the like.

[0104] In addition, the lithium-ion secondary battery 100 can be configured as an all-solid lithium-ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte, according to a known method.

[0105] In addition, the negative electrode 60 according to the present embodiment is applicable to a negative electrode of a lithium-ion secondary battery, but can also be configured as a negative electrode of another secondary battery, which can be configured according to a known method.

[0106] The following describes embodiments related to the present disclosure in detail, but is not intended to limit the present disclosure to the content shown in the embodiments.

[0107] <Manufacture of Negative Electrode>

[0108] [Example 1]

[0109] As the negative electrode active material, the following negative electrode active materials were prepared. Note that the Si content ratio of the first Si-containing particles and the second Si-containing particles was measured using a commercially available ICP-OES device. The average particle diameter (D50) of each particle was measured using a commercially available laser diffraction-scattering type particle size distribution measuring device.

[0110] First Si-containing particles: Si-C composite, Si content ratio = 35 mass%, average particle diameter (D50) = 8 μm

[0111] Second Si-containing particles: Si-C composite, Si content ratio = 65 mass%, average particle diameter (D50) = 7 μm

[0112] Graphite particles (first and second graphite particles): average particle diameter (D50) = 15 μm

[0113] A paste for upper layer formation containing graphite particles, the first Si-containing particles, the first conductive material, CMC, PAA, and SBR in a mass ratio of 70:30:0.1:1:1:1 was prepared using the following procedure. In addition, a paste for lower layer formation containing graphite particles, the second Si-containing particles, the second conductive material, CMC, PAA, and SBR in a mass ratio of 80:20:0.1:1:1:1 was prepared using the following procedure.

[0114] As the conductive material, single-walled carbon nanotubes (SWCNT) were prepared. The SWCNT were prepared in the form of a dispersion liquid. The first Si-containing particles, the SWCNT dispersion liquid, and the dispersion medium were mixed using a disperser at a rotation speed of 3000 rpm. In this way, particles in which the surface of the first Si-containing particles was coated with SWCNT (first conductive material) were obtained. Similarly, the second Si-containing particles, the SWCNT dispersion liquid, and the dispersion medium were mixed using a disperser at a rotation speed of 3000 rpm. In this way, particles in which the surface of the second Si-containing particles was coated with SWCNT (second conductive material) were obtained.

[0115] As the binder, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared.

[0116] The graphite particles, CMC, and PAA were dry-mixed using a planetary mixer. The resulting dry mixture and the first Si-containing particles coated with the first conductive material were put into the planetary mixer together with a dispersion medium, and kneaded. Further, SBR and additional dispersion medium were put into the planetary mixer, and diluted mixed, thereby obtaining the paste for the upper layer formation.

[0117] The graphite particles, CMC, and PAA were dry-mixed using a planetary mixer. The resulting dry mixture and the second Si-containing particles coated with the second conductive material were put into the planetary mixer together with a dispersion medium, and kneaded. Further, SBR and additional dispersion medium were put into the planetary mixer, and diluted mixed, thereby obtaining the paste for the lower layer formation.

[0118] The lower layer of the negative electrode active material layer was formed by coating the prepared paste for the lower layer formation on the surface of a copper foil having a thickness of 10 μm, and drying. Further, the prepared paste for the upper layer formation was coated on the lower layer, and dried, thereby forming the upper layer. Thus, the negative electrode active material layer having a multilayer structure was formed. After the negative electrode active material layer was roll-pressed, the resulting sheet was processed into a prescribed size, thereby obtaining the negative electrode sheet.

[0119] [Example 2]

[0120] The negative electrode sheet of Example 2 was obtained by the same method as in Example 1, except that the first Si-containing particles and the second Si-containing particles were changed to the following substances, and the blending mass ratio of the first conductive material in the upper layer was changed from 0.1 to 0.2.

[0121] First Si-containing particles: Si-C composite, Si content ratio = 40 mass%, average particle diameter (D50) = 7 μm

[0122] Second Si-containing particles: Si-C composite, Si content ratio = 60 mass%, average particle diameter (D50) = 6 μm

[0123] [Example 3]

[0124] The negative electrode sheet of Example 3 was obtained by the same method as in Example 1, except that the ratio (T2 / T1) of the thickness (T2) of the upper layer to the thickness (T1) of the lower layer was changed to 10 / 90.

[0125] [Example 4]

[0126] A negative electrode sheet of Example 4 was obtained in the same manner as in Example 1 except that the ratio (T2 / T1) of the thickness (T2) of the upper layer to the thickness (T1) of the lower layer was changed to 90 / 10.

[0127] [Example 5]

[0128] A negative electrode sheet of Example 5 was obtained in the same manner as in Example 1 except that the ratio (T2 / T1) of the thickness (T2) of the upper layer to the thickness (T1) of the lower layer was changed to 20 / 80.

[0129] [Example 6]

[0130] A negative electrode sheet of Example 6 was obtained in the same manner as in Example 1 except that the ratio (T2 / T1) of the thickness (T2) of the upper layer to the thickness (T1) of the lower layer was changed to 80 / 20.

[0131] [Example 7]

[0132] A negative electrode sheet of Example 7 was obtained in the same manner as in Example 1 except that the compounding mass ratio of the second electrically conductive material in the lower layer was changed from 0.5 to 0.3.

[0133] [Example 8]

[0134] A negative electrode sheet of Example 8 was obtained in the same manner as in Example 1 except that the compounding mass ratio of the first electrically conductive material in the upper layer was changed from 0.1 to 0.2 and the compounding mass ratio of the second electrically conductive material in the lower layer was changed from 0.5 to 0.4.

[0135] [Comparative Example 1]

[0136] A negative electrode active material layer-forming paste was prepared by mixing the lower layer-forming paste and the upper layer-forming paste in such a manner that the mass ratio of the solid components thereof became 1:1. The paste was applied to the surface of a copper foil having a thickness of 10 μm and dried, thereby forming a negative electrode active material layer. The negative electrode active material layer was roll-pressed, and the resulting sheet was processed into a prescribed size, thereby obtaining a negative electrode sheet of Comparative Example 1. Note that the thickness of the negative electrode sheet of Comparative Example 1 was the same as that of Example 1.

[0137] [Comparative Example 2]

[0138] A negative electrode sheet of Comparative Example 2 was obtained in the same manner as in Example 1 except that the lower layer was formed using the upper layer-forming paste and the upper layer was formed using the lower layer-forming paste. Thus, in Comparative Example 2, the first Si-containing particles coated with the first electrically conductive material and the second Si-containing particles coated with the second electrically conductive material were used in exchange.

[0139] [Comparative Example 3]

[0140] The negative electrode sheet of Comparative Example 3 was obtained in the same manner as in Example 1, except that the compounding mass ratio of the first electrically conductive material in the upper layer was changed from 0.1 to 0.5.

[0141] [Comparative Example 4]

[0142] The negative electrode sheet of Comparative Example 4 was obtained in the same manner as in Example 1, except that the compounding mass ratio of the second electrically conductive material in the lower layer was changed from 0.5 to 0.1.

[0143] [Comparative Example 5]

[0144] The negative electrode active material layer was formed by applying the paste for the lower layer to the surface of a copper foil having a thickness of 10 μm, and drying. After the negative electrode active material layer was roll-pressed, the resulting sheet was processed to a prescribed size, to obtain the negative electrode sheet of Comparative Example 5. Note that the thickness of the negative electrode sheet of Comparative Example 5 was the same as that of Example 1.

[0145] <Evaluation of expansion rate of electrode plate>

[0146] The thickness of the negative electrode of each of the examples and the comparative examples was measured. This thickness was taken as the initial thickness (To). Using this negative electrode, a lithium-ion secondary battery for evaluation was produced as described below.

[0147] LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM), acetylene black (AB) as an electrically conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed at a mass ratio of NCM:AB:PVdF = 100:1:1 with N-methylpyrrolidone (NMP), to prepare a positive electrode paste. The paste was applied to the surface of an aluminum foil having a thickness of 15 μm, and dried, to form a positive electrode active material layer. After the positive electrode active material layer was roll-pressed, the resulting sheet was processed to a prescribed size, to obtain a positive electrode sheet.

[0148] A separator made of porous polyolefin was prepared. The negative electrode sheet and the positive electrode sheet produced as described above were each fitted with a lead wire, and were laminated via the separator, to produce an electrode body. This was housed in an aluminum laminate film case together with a nonaqueous electrolyte solution. As the nonaqueous electrolyte solution, a liquid obtained by dissolving LiPF6 as a supporting salt in a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of 15:5:40:40 at a concentration of 1.0 mol / L was used. Then, the case was sealed, to obtain a lithium-ion secondary battery for evaluation.

[0149] Next, each of the above prepared lithium-ion secondary batteries for evaluation was placed in an environment of 25°C. Each of the lithium-ion secondary batteries for evaluation was subjected to constant-current charging at a current value of 0.4 C until 4.2 V, and then subjected to constant-voltage charging until the current value became 0.1 C. Next, each of the lithium-ion secondary batteries for evaluation was subjected to constant-current discharging at a current value of 0.4 C until 2.5 V.

[0150] The above charge-discharge was repeated as one cycle of charge-discharge for 250 cycles. Each of the lithium-ion secondary batteries for evaluation was disassembled under an argon atmosphere, and the negative electrode was washed by immersion in DMC and then dried. Then, the thickness of the negative electrode was measured, and the thickness after the charge-discharge cycle (Tc) was obtained. The change rate (%) of the thickness of the negative electrode before and after the charge-discharge cycle was calculated by (Tc / T0-1) x 100. The results are shown in Table 1.

[0151]

Table 1

[0152]

[0153] As is apparent from the results in Table 1, in the case where the low-Si-content first Si-containing particles coated with the first conductive material are used in the upper layer of the negative electrode active material layer in addition to the first graphite particles, and the high-Si-content second Si-containing particles coated with the second conductive material at a coating amount of 2.5 times or more of the first Si-containing particles are used in the lower layer of the negative electrode active material layer in addition to the second graphite particles, the expansion rate of the electrode plate is very small. Therefore, it is apparent that the negative electrode according to the present disclosure is small in swelling of the negative electrode during repeated charge-discharge of the secondary battery, although the negative electrode containing the Si-containing particles and the graphite particles is used.

[0154] The above detailed description of the specific examples of the present disclosure is merely illustrative and does not limit the claims. The technology recited in the claims includes technology in which various modifications and changes have been made to the above-described specific examples.

[0155] That is, the negative electrode of the secondary battery and the secondary battery disclosed herein are the items [1] to [9] below.

[0156] [1] A negative electrode that is a negative electrode including a negative electrode current collector, and a negative electrode active material layer supported by the negative electrode current collector,

[0157] the negative electrode active material layer includes a first layer on the side of the surface layer portion, and a second layer on the side of the negative electrode current collector,

[0158] the first layer contains first graphite particles and first Si-containing particles,

[0159] the second layer contains second graphite particles and second Si-containing particles,

[0160] the Si content ratio in the first Si-containing particles is smaller than the Si content ratio in the second Si-containing particles,

[0161] each of the first Si-containing particles and the second Si-containing particles is coated with an electrically conductive material,

[0162] a ratio of a coated amount of the electrically conductive material with respect to the second Si-containing particles (mass%) to a coated amount of the electrically conductive material with respect to the first Si-containing particles (mass%) is 2.5 or more.

[0163] [2] The negative electrode according to item [1], wherein the electrically conductive material is a carbon nanotube.

[0164] [3] The negative electrode according to item [2], wherein the coated amount of the electrically conductive material with respect to the first Si-containing particles is 0.05 mass% to 2.0 mass%, and the coated amount of the electrically conductive material with respect to the second Si-containing particles is 0.10 mass% to 3.0 mass%.

[0165] [4] The negative electrode according to any one of items [1] to [3], wherein a ratio of a thickness of the second layer to a thickness of the first layer is 10 / 90 to 90 / 10.

[0166] [5] The negative electrode according to any one of items [1] to [4], wherein a ratio of the Si content ratio in the first Si-containing particles to the Si content ratio in the second Si-containing particles is 0.10 to 0.90.

[0167] [6] The negative electrode according to any one of items [1] to [5], wherein the Si content ratio in the first Si-containing particles is 20 mass% to 55 mass%, and the Si content ratio in the second Si-containing particles is 45 mass% to 80 mass%.

[0168] [7] The negative electrode according to any one of items [1] to [6], wherein each of the first Si-containing particles and the second Si-containing particles is a particle of Si-C composite material.

[0169] [8] The negative electrode according to any one of items [1] to [7], wherein, in the first layer, a mass ratio of the first Si-containing particles with respect to a total of the first graphite particles and the first Si-containing particles is 10 mass% to 60 mass%,

[0170] in the second layer, a mass ratio of the second Si-containing particles with respect to a total of the second graphite particles and the second Si-containing particles is 10 mass% to 60 mass%.

[0171] [9] A secondary battery which is a secondary battery including a positive electrode, a negative electrode, and an electrolyte, the negative electrode being the negative electrode according to any one of items [1] to [8].

Claims

1. A negative electrode, comprising a negative electrode current collector and a layer of negative electrode active material supported by the negative electrode current collector. The negative electrode active material layer includes a first layer located on the surface side and a second layer located on the negative electrode current collector side. The first layer contains first graphite particles and first Si-containing particles. The second layer contains second graphite particles and second Si-containing particles. The proportion of Si in the first Si-containing particle is smaller than the proportion of Si in the second Si-containing particle. The first Si-containing particle and the second Si-containing particle are each coated with a conductive material. The ratio of the coating amount (mass%) of the second Si-particle-containing conductive material to the coating amount (mass%) of the first Si-particle-containing conductive material is 2.5 or more.

2. The negative electrode according to claim 1, wherein, The conductive material is carbon nanotubes.

3. The negative electrode according to claim 2, wherein, The coating amount relative to the first Si-particle-containing conductive material is 0.05% to 2.0% by mass, and the coating amount relative to the second Si-particle-containing conductive material is 0.10% to 3.0% by mass.

4. The negative electrode according to claim 1, wherein, The ratio of the thickness of the second layer to the thickness of the first layer is 10 / 90 to 90 / 10.

5. The negative electrode according to claim 1, wherein, The ratio of the Si content in the first Si-containing particle to the Si content in the second Si-containing particle is 0.10 to 0.

90.

6. The negative electrode according to claim 1, wherein, The first Si-containing particle contains 20% to 55% Si by mass, and the second Si-containing particle contains 45% to 80% Si by mass.

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

8. The negative electrode according to claim 1, wherein, In the first layer, the mass percentage of the first Si-containing particles relative to the total of the first graphite particles and the first Si-containing particles is 10% to 60% by mass. In the second layer, the mass ratio of the second Si-containing particles relative to the total of the second graphite particles and the second Si-containing particles is 10% to 60% by mass.

9. 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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