Negative electrode for secondary battery and secondary battery using same

Abstract

The present invention relates to a negative electrode for a secondary battery and a secondary battery using the same. Provided is a negative electrode containing Si-containing particles and graphite particles, said negative electrode having little swelling and sharp deterioration in performance when a secondary battery is repeatedly charged and discharged. This negative electrode for a secondary battery is provided with 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 lower layer positioned on the negative electrode collector side and an upper layer positioned on the surface side, the upper layer containing first graphite particles and first Si-containing particles as negative electrode active materials, and the lower layer containing second graphite particles and second Si-containing particles as negative electrode active materials. The average circularity (C1) of the first Si-containing particles is greater than the average circularity (C2) of the second Si-containing particles.

Description

TECHNICAL FIELD

[0001] The present application relates to a negative electrode for a secondary battery and a secondary battery using the same. BACKGROUND

[0002] In recent years, secondary batteries are suitably used for portable power sources such as personal computers, portable terminals, and the like; vehicle drive power sources such as 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 in the drive power source use of BEVs, from the viewpoint of extending the cruising distance of the vehicle, it is desired that the secondary battery be further high in capacity. As a negative electrode active material high in capacity, Si-containing particles are known, and it is known that the use of Si-containing particles enables the secondary battery to be high in capacity (for example, refer to Patent Literature 1, Patent Literature 2).

[0004] Patent Literature 1 discloses a lithium ion secondary battery in which the battery capacity and charge-discharge cycle characteristics are improved by using composite particles that are composite particles of silicon oxide particles, graphite particles, and a low-crystallinity carbon material that covers at least a part of them as a negative electrode active material, and the average circularity is 0.90 to 0.99.

[0005] Patent Literature 2 discloses a lithium ion secondary battery in which the battery capacity and cycle characteristics are improved by containing silicon compound particles containing a silicon compound (SiO x : 0.5≤x≤1.6) as a negative electrode active material, in which the average circularity of the silicon compound particles is 0.93 or more, and the proportion of particles having an average circularity of 0.85 or less is 5% or less.

[0006] Prior Art Documents

[0007] Patent Literature

[0008] Patent Literature 1: Japanese Patent Application Publication No. 2019-175851

[0009] Patent Literature 2: Japanese Patent Application Publication No. 2017-092009 SUMMARY

[0010] Problems to be Solved by the Invention

[0011] However, while the Si-containing particles improve the capacity of the secondary battery, on the other hand, the volume change due to expansion / contraction of the secondary battery during charging / discharging is large. Also, in the case where the Si-containing particles having a relatively high average circularity and graphite particles are used as the negative electrode active material, when the secondary battery is repeatedly charged and discharged, there is a problem that the internal stress becomes high due to the small remaining void and swelling of the negative electrode. Therefore, with respect to the negative electrode containing the Si-containing particles and the graphite particles, it is desirable to develop a negative electrode in which swelling of the negative electrode is small when the secondary battery is repeatedly charged and discharged. Note that the swelling of the negative electrode means that the volume of the negative electrode is larger than the initial volume in the same state of charge (for example, a state close to full charge at around 80% of SOC). Furthermore, in the case where the remaining void in the negative electrode active material layer is small, rapid deterioration of the secondary battery due to a decrease in liquid flowability also occurs.

[0012] In view of the above, an object of the present disclosure is to provide a negative electrode containing Si-containing particles and graphite particles, which achieves a decrease in the degree of swelling of the negative electrode when the secondary battery is repeatedly charged and discharged, and suppression of rapid deterioration due to a decrease in liquid flowability.

[0013] Means for solving the problem

[0014] The negative electrode disclosed herein is a negative electrode for a secondary battery that 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 lower layer on the negative electrode current collector side and an upper layer on the surface side. The upper layer contains first graphite particles and first Si-containing particles as negative electrode active materials, and the lower layer contains second graphite particles and second Si-containing particles as negative electrode active materials. Furthermore, the average circularity C1 of the first Si-containing particles is greater than the average circularity C2 of the second Si-containing particles.

[0015] According to such a configuration, in the negative electrode active material layer having a two-layer structure of the surface side and the negative electrode current collector side, the negative electrode active material is densely packed in the negative electrode active material layer on the surface side, and the negative electrode active material is sparsely packed in the negative electrode active material layer on the negative electrode current collector side. Furthermore, when charging and discharging are performed, the upper layer, which is prone to deformation, easily follows the expansion / contraction of the negative electrode active material, and thus it is possible to achieve suppression of swelling of the negative electrode due to internal stress relaxation and high capacity of the secondary battery. On the other hand, in the lower layer, which is not prone to deformation, the negative electrode active material is not easily moved, and thus it is possible to achieve suppression of swelling of the negative electrode due to prevention of disconnection of the conductive path. Furthermore, in the lower layer, even after the secondary battery is repeatedly charged and discharged, it is possible to maintain the sparse packing of the negative electrode active material, and thus it is possible to suppress rapid deterioration of the secondary battery due to a decrease in liquid flowability.

[0016] In a preferred embodiment of the negative electrode disclosed herein, C1 is 0.9 or greater and less than 1.0, and C2 is 0.5 or greater and less than 0.9. Thus, by packing the negative electrode active material more densely in the upper layer and more sparsely in the lower layer, the high capacity of the secondary battery, the inhibition of the negative electrode swelling accompanying charge and discharge, and the inhibition of the sharp deterioration of the secondary battery due to the decrease in the liquid flowability can be appropriately achieved.

[0017] In a preferred embodiment of the negative electrode disclosed herein, the average circularity D1 of the first graphite particles is 0.85 or greater and less than 1.0. Thus, by packing the negative electrode active material more densely in the upper layer, the high capacity of the secondary battery, the inhibition of the negative electrode swelling accompanying charge and discharge can be appropriately achieved.

[0018] In a preferred embodiment of the negative electrode disclosed herein, the ratio T1:T2 of the thickness T1 of the upper layer to the thickness T2 of the lower layer is 10:90 to 90:10. Thus, a state in which the negative electrode active material is packed more densely in the upper layer and more sparsely in the lower layer can be formed, and the high capacity of the secondary battery, the inhibition of the negative electrode swelling accompanying charge and discharge, and the inhibition of the sharp deterioration of the secondary battery due to the decrease in the liquid flowability can be appropriately achieved.

[0019] In a preferred embodiment of the negative electrode disclosed herein, the mass ratio N1 of the first Si-containing particles to the total mass of the first graphite particles and the first Si-containing particles in the upper layer is 10 to 60 mass%, and the mass ratio N2 of the second Si-containing particles to the total mass of the second graphite particles and the second Si-containing particles in the lower layer is 10 to 60 mass%. Thus, by packing the negative electrode active material more densely in the upper layer and more sparsely in the lower layer, the high capacity of the secondary battery, the inhibition of the negative electrode swelling accompanying charge and discharge, and the inhibition of the sharp deterioration of the secondary battery due to the decrease in the liquid flowability can be appropriately achieved.

[0020] The secondary battery disclosed herein is a secondary battery provided with a positive electrode, a negative electrode, and an electrolyte, in which the negative electrode is any one of the negative electrodes disclosed herein.

[0021] According to such a configuration, a secondary battery in which the high capacity, the inhibition of the negative electrode swelling accompanying charge and discharge, and the inhibition of the sharp deterioration due to the decrease in the liquid flowability can be achieved. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a cross-sectional view schematically showing an example of the negative electrode 60 of the present embodiment, and is a cross-sectional view along the thickness direction and the width direction.

[0023] Figure 2 is a schematic cross-sectional view showing Figure 1 particles of the negative electrode active material contained in the negative electrode active material layer 64.

[0024] Figure 3 is a view schematically showing the structure of a lithium-ion secondary battery using the negative electrode constructed in one embodiment.

[0025] Figure 4 is a schematic exploded view showing Figure 3 the structure of a wound electrode body of the lithium-ion secondary battery of

[0026] BRIEF DESCRIPTION OF DRAWINGS

[0027] 12 first graphite particles

[0028] 14 first Si-containing particles

[0029] 16 second graphite particles

[0030] 18 second Si-containing particles

[0031] 20 wound electrode body

[0032] 30 battery case

[0033] 36 safety valve

[0034] 42 positive electrode terminal

[0035] 42a positive electrode current collector

[0036] 44 negative electrode terminal

[0037] 44a negative electrode current collector

[0038] 50 positive electrode sheet (positive electrode)

[0039] 52 positive electrode current collector

[0040] 52a positive electrode active material layer non-formed portion

[0041] 54 positive electrode active material layer

[0042] 60 negative electrode sheet (negative electrode)

[0043] 62 negative electrode current collector

[0044] 62a negative electrode active material layer non-formed portion

[0045] 64 negative electrode active material layer

[0046] 70 separator sheet (separator)

[0047] 100 lithium ion secondary battery DETAILED DESCRIPTION

[0048] Hereinafter, a preferred embodiment of the technology disclosed herein will be described. Note that matters other than those mentioned below and matters necessary for the implementation of the technology disclosed herein can be understood by those skilled in the art based on the design matters of the prior art in the field. The technology disclosed herein can be implemented based on the content disclosed in this specification and the technical common sense in the field. In addition, in the drawings described in this specification, the same symbols are attached to components and parts that have the same function, and sometimes repeated description is omitted or simplified. In addition, the dimensional relationship (length, width, thickness, etc.) in each drawing does not necessarily reflect the actual dimensional relationship. In addition, A to B indicating a numerical range indicates A or more and B or less, and thus includes a numerical range greater than A and less than B.

[0049] In this specification, "secondary battery" is a term indicating all power storage devices capable of repeating charge and discharge accompanying movement of charge carriers between positive and negative electrodes, and is a concept including so-called storage batteries (chemical cells) such as lithium ion secondary batteries, sodium ion secondary batteries, and the like; and capacitors (physical cells) such as lithium ion capacitors (LIC) and the like. Hereinafter, the main constituent materials of the secondary battery of the present disclosure will be described. Note that, regarding the constituent materials of the secondary battery not described here, publicly known materials can be used.

[0050] The negative electrode disclosed herein is used for a secondary battery, and is suitable for use in a lithium ion secondary battery. Referring to Figure 1 An embodiment of the negative electrode disclosed herein will be specifically described. Figure 1 is a cross-sectional view schematically showing an example of the negative electrode 60 of the present embodiment, and is a cross-sectional view along the thickness direction and the width direction. Figure 1 The negative electrode 60 of the present embodiment shown in the drawing is a negative electrode of a lithium ion secondary battery.

[0051] 1. Negative electrode

[0052] (1) Constitution of negative electrode

[0053] As Figure 1 shown, 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 on only one surface of the negative electrode current collector 62, or can be provided on both surfaces of the negative electrode current collector 62 as Figure 1 shown. The negative electrode active material layer 64 is preferably provided on both surfaces of the negative electrode current collector 62.

[0054] The negative electrode 60 can also have components other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) can be provided adjacent to the negative electrode active material layer 64 on the negative electrode active material layer non-formed portion 62a. The insulating layer contains, for example, an inorganic filler having insulating properties and the like.

[0055] As shown in Figure 1 , the negative electrode active material layer non-formed 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-formed portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer non-formed portion 62a can function as a current collecting portion. However, the structure for collecting current from the negative electrode 60 is not limited thereto.

[0056] In Figure 1 , the negative electrode current collector 62 has a foil shape (or a sheet shape), but is not limited thereto. The negative electrode current collector 62 can have various shapes such as a rod shape, a plate shape, and a mesh shape. As the material of the negative electrode current collector 62, a metal having good conductivity (for example, copper, nickel, titanium, stainless steel, and the like) can be used as in the conventional lithium-ion secondary battery, and copper is preferable among them. A copper foil is particularly preferable as the negative electrode current collector 62.

[0057] The size of the negative electrode current collector 62 is not particularly limited and can be appropriately determined in accordance with the battery design. In the case where a copper foil is used as the negative electrode current collector 62, the thickness thereof is not particularly limited and is, for example, 5 μm or more and 35 μm or less, and is preferably 6 μm or more and 20 μm or less.

[0058] As shown in Figure 1 , the negative electrode active material layer 64 has a multilayer structure, and specifically, has a first layer 64a on the surface side of the negative electrode active material layer 64 and a second layer 64b on the negative electrode current collector 62 side. As shown in Figure 1 , the first layer 64a is an upper layer of the negative electrode active material layer 64, and the second layer 64b is a lower layer of the negative electrode active material layer 64. Note that the negative electrode active material layer 64 can also have a layer other than the first layer 64a and the second layer 64b, within a range that does not significantly hinder the effects of the present application. For example, the negative electrode active material layer 64 can have an intermediate layer in which the components of the first layer 64a and the second layer 64b are mixed together, between the first layer 64a and the second layer 64b.

[0059] The negative electrode active material layer 64 contains a negative electrode active material. A detailed description thereof will be given. Figure 2 The negative electrode active material layer 64 contains a negative electrode active material. A detailed description thereof will be given. Figure 2 is a schematic cross-sectional view showing the particles of the negative electrode active material contained in the negative electrode active material layer 64 shown in Figure 1 . Note that Figure 2 is a schematic view, and the number, distribution, and the like of the particles are not limited to those shown in Figure 2 .

[0060] As for the negative electrode active material, the first layer 64a contains the first graphite particle 12 and the first Si-containing particle 14 as the negative electrode active material. The second layer 64b contains the second graphite particle 16 and the second Si-containing particle 18 as the negative electrode active material. 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 of the Si-containing particle due to expansion / contraction accompanying charge / discharge is large, but by being used in combination with the graphite particle, the disconnection of the conductive path due to the volume change of the Si-containing particle can be suppressed.

[0061] The graphite constituting the first graphite particle 12 and the second graphite particle 16 can be natural graphite, can be artificial graphite, or can be amorphous carbon-coated graphite in which graphite is covered with amorphous carbon.

[0062] The shape of the first graphite particle 12 is preferably a spheroidized graphite particle. From the viewpoint of the packing property of the negative electrode active material, the circularity of the first graphite particle 12 is preferably 0.85 to 1, more preferably 0.88 to 1, and further preferably 0.90 to 1.

[0063] 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). 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 true sphere. The circularity can be calculated, for example, by using a commercially available static automatic image analyzer, calculating the average value of the circularity of 100 or more particles.

[0064] The average particle diameter (D50) of the first graphite particle 12 and the average particle diameter (D50) of the second graphite particle 16 are not particularly limited. The average particle diameter (D50) of the first graphite particle 12 and the average particle diameter (D50) of the second graphite particle 16 are, 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, respectively.

[0065] Note that, in the present specification, the "average particle diameter (D50)" refers to the median particle diameter (D50), and refers to the particle diameter corresponding to the cumulative frequency of 50% by volume from the side of the fine particles having a small particle diameter in the particle size distribution based on the volume basis of the laser diffraction / scattering method. The average particle diameter (D50) can be calculated using a commercially available particle size distribution measuring device of the laser diffraction / scattering type or the like.

[0066] 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.

[0067] The first Si-containing particles 14 and the second Si-containing particles 18 are, for example, particles in which Si-containing fine particles are dispersed inside a carbon material, particles in which Si-containing fine particles enter pores of a porous graphite subjected to granulation, or the like. The Si-C composite material can be particles in which Si-containing fine particles are attached to the surface of a carbon particle, particles in which carbon fine particles are attached to the surface of a Si-containing particle, or the like. From the viewpoint of suppressing the volume change of Si, particles in which Si nanoparticles are dispersed inside a carbon material, and particles in which Si nanoparticles are dispersed inside pores of a porous carbon material are preferable, and particles in which Si nanoparticles are dispersed inside pores of a porous carbon material are more preferable.

[0068] As an example of the first Si-containing particles 14 and the second Si-containing particles 18, for example, particles of a Si-C composite material can be used. The Si-C composite material typically includes a carbon domain and a Si-containing domain. Note that the first Si-containing particles 14 and the second Si-containing particles 18 can not be a Si-C composite material, and can be Si particles, Si oxide particles, or the like.

[0069] The carbon domain is, for example, a carbonized product of a carbon precursor (e.g., petroleum pitch, coal pitch, phenol resin, or the like); graphite; or the like. The carbon domain is suitably configured as a carbon matrix. Thus, the Si-C composite material is suitably 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.

[0070] The Si-containing domain contains Si, and is composed of, for example, Si, Si oxide (SiO x ), Si nitride (SiN x ), Si carbide (SiC x ), or the like. 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 fine particles. The oxygen content in the Si-containing domain is preferably 10% by mass or less.

[0071] The average particle diameter of the Si-containing region 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 region can be obtained as described below. First, the negative electrode active material layer 64 is subjected to FIB (focused ion beam) processing, and a sample for STEM (scanning transmission electron microscope) 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. The contrast and shape obtained from the BF image and the HAADF image can be used to obtain the diameter of the Si-containing region. The diameters of 10 or more Si-containing regions selected at random are obtained, and the average value thereof is used as the average particle diameter of the Si-containing region.

[0072] In this specification, the Si content ratio (S1) in the first Si-containing particle 14 and the Si content ratio (S2) in the second Si-containing particle 18 are not particularly limited. However, if these Si content ratios are too low, it can be difficult to achieve high capacity of the secondary battery. On the other hand, if these Si content ratios are too high, the volume change due to expansion / contraction of the first Si-containing particle 14 and the second Si-containing particle 18 can become too large when the secondary battery is repeatedly charged and discharged.

[0073] Therefore, for example, in the case of the first Si-containing particle 14 composed of a Si-C composite material, the Si content ratio (S1) in the particle is preferably 20% by mass to 55% by mass, and more preferably 25% by mass to 45% by mass. The Si content ratio (S2) in the second Si-containing particle 18 is preferably 45% by mass to 80% by mass, and more preferably 55% by mass to 75% by mass.

[0074] Note that the first Si-containing particle 14 and the second Si-containing particle 18 can be produced by a known method. Note that various production methods of particles of a Si-C composite material are known (for example, see Japanese Patent Application Publication No. 2015-38862, International Publication No. 2014 / 046144, and prior art documents cited in the international publication).

[0075] In this specification, the first Si-containing particle 14 and the second Si-containing particle 18 are defined by the average circularity D. The average circularity D of the first Si-containing particle 14 and the second Si-containing particle 18 can be measured by the same method as the measurement method of the average circularity described above.

[0076] The particle diameter of the first Si-containing particles 14 and the second Si-containing particles 18 is not particularly limited and can be measured by the same method as that described above for measuring the average particle diameter (D50). The average particle diameter (D50) is, for example, 1 μm to 20 μm, preferably 2 μm to 15 μm, more preferably 3 μm to 10 μm, and further preferably 4 μm to 7 μm.

[0077] Note that the negative electrode active material layer 64 can also contain components other than the negative electrode active material, and examples thereof include a binder, a conductive material, and the like. As the binder, for example, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), and the like can be used. The CMC also functions as a thickening agent. Examples of the conductive material include carbon black such as acetylene black, carbon fibers, carbon nanotubes (CNT), and the like. Of these, CNT is preferred. In the case where CNT is used as the conductive material, the negative electrode active material layer 64 can also contain a dispersant for the CNT.

[0078] (2) Inhibition of negative electrode swelling and rapid deterioration of secondary battery

[0079] In the case where the secondary battery is subjected to charge and discharge, the negative electrode active material in the vicinity of the negative electrode current collector is significantly affected by the swelling of the negative electrode active material further on the surface side, and the swelling tends to be hindered. Conversely, the negative electrode active material in the vicinity of the surface of the negative electrode active material layer has fewer factors that hinder the swelling. That is, in the negative electrode active material layer 64 in which the negative electrode active material layer is provided in a two-layer structure, the upper layer (i.e., the first layer 64a) is large in swelling at the time of repeated charge and discharge of the secondary battery. Hereinafter, from the viewpoint of inhibiting the negative electrode swelling and the rapid deterioration of the secondary battery, the average circularity, the content of the Si-C-containing particles (here, Si-C composite material) in the first layer 64a and the second layer 64b, and the thickness T of the first layer 64a and the second layer 64b will be described.

[0080] (A) Average circularity C

[0081] In the negative electrode active material layer of the present disclosure, the first Si-containing particles 14 having a higher average circularity than the second Si-containing particles are used for the upper layer, i.e., the Si-containing particles that are densely packed within the negative electrode active material layer. Thereby, in the upper layer in which the active material layer is easily deformed, the stress is alleviated by the upper layer following the expansion / contraction of the Si-containing particles accompanying charge and discharge. Moreover, high capacity of the secondary battery and inhibition of the negative electrode swelling can be achieved.

[0082] On the other hand, the lower layer uses the second Si-containing particles 18 having a smaller average circularity C than the first Si-containing particles, that is, Si-containing particles that are sparsely packed in the negative electrode active material layer. Thus, in the lower layer portion where deformation is less likely to occur, the particles are less likely to move, and the disconnection of the conductive path at the time of charge and discharge can be suppressed. Therefore, the swelling of the negative electrode accompanying the disconnection of the conductive path (swelling due to unevenness of the battery reaction, swelling due to local reaction or stress concentration, and the like) can be suppressed. Based on the above results, the negative electrode active material layer 64 as a whole can suppress the swelling of the negative electrode 60 at the time of repeated charge and discharge of the secondary battery.

[0083] Further, for the lower layer, even after charge and discharge, the sparsely packed state of the negative electrode active material in the negative electrode active material layer can be maintained, and thus the rapid deterioration of the secondary battery due to liquid depletion can be suppressed.

[0084] From the viewpoint of densely packing the first Si-containing particles 14 in the negative electrode active material layer to thereby increase the capacity of the secondary battery, the average circularity C1 of the first Si-containing particles 14 is preferably 0.9 or greater, more preferably 0.93 or greater, and particularly preferably 0.95 or greater. Note that the upper limit of the average circularity C1 of the first Si-containing particles 14 is not particularly limited, and may, for example, be 1.0 or less or 0.98 or less.

[0085] From the viewpoint of sparsely packing the second Si-containing particles 18 in the negative electrode active material layer to thereby suppress the rapid deterioration due to liquid depletion, the average circularity C2 of the second Si-containing particles 18 is preferably less than 0.9, more preferably 0.87 or less, and particularly preferably 0.83 or less. Note that if the average circularity C2 of the second Si-containing particles 18 is too small, the packing amount can be insufficient, and thus the average circularity C2 of the second Si-containing particles 18 is preferably 0.5 or greater, more preferably 0.6 or greater, and particularly preferably 0.7 or greater.

[0086] (B) Content

[0087] The content of the negative electrode active material in the first layer 64a (that is, with respect to the total mass of the first layer 64a) is preferably 90% by mass or greater, and more preferably 95% by mass or greater. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or greater and 8% by mass or less, and more preferably 0.5% by mass or greater and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or greater and 3% by mass or less, and more preferably 0.05% by mass or greater and 1% by mass or less.

[0088] 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, more preferably 95% by mass or more. The content of the binder in the negative 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. The content of the conductive material in the negative active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, more preferably 0.05% by mass or more and 1% by mass or less.

[0089] In the first layer 64a, the mass ratio N1 of the first Si-containing particle 14 relative to the total mass of the first graphite particle 12 and the first Si-containing particle 14 is preferably 10% by mass or more, more preferably 15% by mass or more, from the viewpoint of high capacity of the secondary battery. On the other hand, it is preferably 60% by mass or less, more preferably 40% by mass or less, particularly preferably 20% by mass or less, from the viewpoint of suppressing swelling of the negative electrode plate.

[0090] In the second layer 64b, the mass ratio N2 of the second Si-containing particle 18 relative to the total mass of the second graphite particle 16 and the second Si-containing particle 18 is preferably 10% by mass or more, more preferably 15% by mass or more, from the viewpoint of high capacity of the secondary battery. On the other hand, it is preferably 60% by mass or less, more preferably 40% by mass or less, particularly preferably 20% by mass or less, from the viewpoint of suppressing swelling of the negative electrode plate.

[0091] The negative active material contained in the first layer 64a can be only the first graphite particle 12 and the first Si-containing particle 14. However, the first layer 64a can also contain a negative active material other than the first graphite particle 12 and the first Si-containing particle 14, within a range that does not hinder the effects of the present application (e.g., 10% by mass or less of the total amount of the negative active material contained in the first layer 64a).

[0092] The negative active material contained in the second layer 64b can be only the second graphite particle 16 and the second Si-containing particle 18. However, the second layer 64b can also contain a negative active material other than the second graphite particle 16 and the second Si-containing particle 18, within a range that does not hinder the effects of the present application (e.g., 10% by mass or less of the total amount of the negative active material contained in the second layer 64b).

[0093] (C) Thickness T of the First Layer and the Second Layer

[0094] From the viewpoint of giving consideration to the effects of suppressing swelling of the negative electrode plate and suppressing rapid deterioration of the secondary battery due to liquid depletion, the ratio of the thickness T1 of the upper layer to the thickness T2 of the lower layer (T1 :T2) is preferably 10:90 to 90:10, more preferably 20:80 to 80:20, further preferably 30:70 to 70:30, and particularly preferably 40:60 to 60:40.

[0095] Note that the smaller the difference between the thickness T1 of the upper layer and the thickness T2 of the lower layer, the more the effects of the present disclosure can be achieved equally in each of the upper layer and the lower layer, and thus the effects of the present disclosure can be more remarkably achieved as a whole of the negative active material layer.

[0096] (4) Production of the negative electrode

[0097] The negative electrode 60 can be produced, for example, by a production method including the following steps: a step of mixing the second graphite particles 16 and the second Si-containing particles 18 in a dispersion medium to prepare a paste for forming the second layer (hereinafter also referred to as "negative electrode mixture paste for lower layer") ; a step of mixing the first graphite particles 12 and the first Si-containing particles 14 in a dispersion medium to prepare a paste for forming the first layer (hereinafter also referred to as "negative electrode mixture paste for upper layer") ; a step (hereinafter also referred to as "lower layer forming step") of coating the paste for forming the second layer on the negative electrode current collector 62 and drying to form the second layer 64b (lower layer) ; a step (hereinafter also referred to as "upper layer forming step") of coating the paste for forming the first layer on the second layer 64b and drying to form the first layer 64a (upper layer) ; and a step (hereinafter also referred to as "pressing step") of pressing the formed first layer 64a and second layer 64b.

[0098] Note that the "paste" in the present specification means 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.

[0099] As for the paste preparation step for the lower layer, the second graphite particles 16, the second Si-containing particles 18, and optional components (for example, a binder, a conductive material, and the like) can be mixed with a dispersion medium (for example, water) by using a publicly known mixing device, stirring device, or the like, according to a publicly known method.

[0100] As for the paste preparation step for the upper layer, the first graphite particles 12, the first Si-containing particles 14, and optional components (for example, a binder, a conductive material, and the like) can be mixed with a dispersion medium (for example, water) by using a publicly known mixing device, stirring device, or the like, according to a publicly known method. Note that the paste preparation step for the upper layer can be performed in parallel with the paste preparation step for the lower layer (in parallel). The paste preparation step for the upper layer can also be performed in parallel with or after the lower layer forming step.

[0101] The lower layer forming step can be performed according to a known method. Specifically, for example, it can be performed by applying a paste for lower layer formation on the negative electrode current collector 62 using a known coating device and drying. By drying, the lower layer (2nd layer 64b) is formed.

[0102] The upper layer forming step can be performed according to a known method. Specifically, for example, it can be performed by applying a paste for upper layer formation on the formed lower layer using a known coating device and drying. By drying, the upper layer (1st layer 64a) is formed, and the negative electrode active material layer 64 is formed.

[0103] The density of the negative electrode active material layer 64 is not particularly limited, and for example, is 0.7 g / cm 3 The density is preferably 1.0 g / cm 3 The density is more preferably 1.2 g / cm 3 The density is more preferably 1.2 g / cm 3 The density can be 2.0 g / cm 3 The density can be 2.0 g / cm

[0104] The pressing step can be performed according to a known method. Specifically, it can be performed by applying pressure to the above-mentioned formed upper layer and lower layer (i.e., the negative electrode active material layer 64) using a roll press or the like. By the pressing step, the negative electrode active material layer 64 is compressed to a prescribed density, whereby the negative electrode active material particles are densely packed.

[0105] 2. Secondary battery

[0106] According to the negative electrode 60 of the present embodiment, swelling of the negative electrode 60 at the time of repeated charge and discharge of the secondary battery and rapid deterioration of the secondary battery can be suppressed. In addition, the negative electrode 60 of the present embodiment uses a negative electrode active material containing Si, and thus the secondary battery can be made high-capacity.

[0107] Thus, 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 of the above-described embodiment. Hereinafter, with a lithium-ion secondary battery as an example, the structure of the lithium-ion secondary battery using the negative electrode of one embodiment will be described with reference to Figure 3 and Figure 4 One embodiment of the secondary battery disclosed herein will be described. The following configuration example is a flat square-shaped lithium-ion secondary battery having a flat square-shaped wound electrode body and a flat square-shaped battery case.

[0108] Figure 3 is a diagram schematically showing the structure of a lithium-ion secondary battery using the negative electrode of one embodiment. Figure 3The lithium ion secondary battery 100 shown is a sealed lithium ion secondary battery 100 constructed by housing a flatly shaped wound electrode body 20 and a nonaqueous electrolyte solution (not shown) in a flatly shaped battery case (i.e., an outer packaging container) 30. On the battery case 30 are provided a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 provided in a manner to release the internal pressure of the battery case 30 when the internal pressure rises to a prescribed level or more. In addition, on the battery case 30 is provided an injection port (not shown) for injecting the nonaqueous electrolyte solution. 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, for example, a lightweight and thermally conductive metal material such as aluminum is used.

[0109] Figure 4 is a schematic exploded view showing the structure of the wound electrode body of the lithium ion secondary battery of Figure 3 As shown in Figure 3 and Figure 4 , the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped with two long strip-shaped separator sheets 70 interposed therebetween and wound in the lengthwise direction. The positive electrode sheet 50 has a structure in which a positive electrode active material layer 54 is formed on one or both sides (in this case, both sides) of a long strip-shaped positive electrode current collector 52 in the lengthwise direction. The negative electrode sheet 60 has a structure in which a negative electrode active material layer 64 is formed on one or both sides (in this case, both sides) of a long strip-shaped negative electrode current collector 62 in the lengthwise direction. A positive electrode active material layer non-formed portion 52a (i.e., a portion in which the positive electrode active material layer 54 is not formed so that the positive electrode current collector 52 is exposed) and a negative electrode active material layer non-formed portion 62a (i.e., a portion in which the negative electrode active material layer 64 is not formed so that the negative electrode current collector 62 is exposed) are formed in a manner to extend outward from both ends in the winding axis direction (i.e., the sheet width direction orthogonal to the above-mentioned lengthwise direction) of the wound electrode body 20. The positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a are respectively joined to the positive electrode current collector 42a and the negative electrode current collector 44a.

[0110] 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 electrical conductivity can be cited. As the positive electrode current collector 52, an aluminum foil is preferred.

[0111] The dimensions of the positive electrode current collector 52 are 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.

[0112] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a positive electrode active material of a publicly known composition used in a lithium-ion secondary battery can be used. Specifically, for example, as the positive electrode active material, a lithium complex oxide, a lithium transition metal phosphate compound, or the like 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, or the like.

[0113] 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, or the like can be given.

[0114] Note that, in the present specification, the "lithium nickel-cobalt-manganese-based complex oxide" is a term including an oxide containing Li, Ni, Co, Mn, and O as constituent elements, and an oxide containing one or two or more kinds of additive elements other than these. As examples of the additive elements, 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, or the like can be given. In addition, the additive elements can also be semi-metal elements such as B, C, Si, and P; non-metal elements such as S, F, Cl, Br, and I. These are the same for the above-described lithium nickel-based complex oxide, lithium cobalt-based complex oxide, lithium manganese-based complex oxide, lithium nickel-manganese-based complex oxide, lithium nickel-cobalt-aluminum-based complex oxide, lithium iron-nickel-manganese-based complex oxide, and the like.

[0115] As the lithium transition metal phosphate compound, for example, lithium iron phosphate (LiFeP04), lithium manganese phosphate (LiMnP04), lithium manganese iron phosphate, or the like can be given.

[0116] These positive electrode active materials can be used alone or in combination with two or more kinds. As the positive electrode active material, from the viewpoint of excellent characteristics such as initial resistance characteristics, a lithium nickel-cobalt-manganese-based complex oxide is particularly preferable.

[0117] 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.

[0118] The positive electrode active material layer 54 can 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) can be suitably used; carbon fibers such as vapor grown carbon fiber (VGCF), carbon nanotube (CNT), and the like; and other (for example, graphite and the like) carbon materials. As the binder, for example, polyvinylidene fluoride (PVdF) and the like can be used.

[0119] 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, 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 trilithium 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.

[0120] The thickness of each single face 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.

[0121] As the negative electrode sheet 60, the above-described negative electrode 60 is used.

[0122] As the separator 70, for example, a porous sheet (film) composed of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide, and the like can be cited. 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 sides of a PE layer). A heat-resistant layer (HRL) can also be provided on the surface of the separator 70.

[0123] 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.

[0124] The nonaqueous electrolyte 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 used in the electrolyte 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 can be used in combination as two or more kinds as appropriate. As an example, the nonaqueous solvent consists only of a carbonate. As another example, the nonaqueous solvent contains a carbonate and an ester such as methyl acetate.

[0125] 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 appropriately used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0126] Note that the above nonaqueous electrolyte can contain components other than the above components, such as a film former such as vinylene carbonate (VC), an oxalate complex, or the like; a gas generator such as biphenyl (BP), cyclohexylbenzene (CHB), or the like; various additives such as a thickening agent, as long as the effects of the present disclosure are not significantly impaired.

[0127] The lithium-ion secondary battery 100 suppresses swelling of the negative electrode at the time of repeated charge and discharge, and thus has low reaction force. In addition, the lithium-ion secondary battery 100 has high capacity. The lithium-ion secondary battery 100 can be used for various uses. As appropriate uses, a driving power source mounted on 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 be used as a storage battery of a small-sized power storage device or the like. 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.

[0128] In the above, as an example, a square lithium-ion secondary battery 100 having a wound electrode body 20 of a flat shape is described. However, the lithium-ion secondary battery can also be configured as a lithium-ion secondary battery provided with 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.

[0129] In addition, the lithium-ion secondary battery 100 can be configured as a full solid lithium-ion secondary battery using a solid electrolyte instead of a nonaqueous electrolyte according to a known method.

[0130] In addition, the negative electrode 60 of the present embodiment is applicable to a negative electrode of a lithium-ion secondary battery, but can also be configured to be used as a negative electrode of another secondary battery, which can be configured according to a known method.

[0131] "Evaluation"

[0132] 1. Test Example

[0133] Hereinafter, a test example relating to the technology disclosed herein will be described, but the technology disclosed herein is not intended to be limited to the test example.

[0134] (1) Example 1

[0135] Production of Negative Electrode

[0136] The second graphite particles C (average circularity: 0.9) and the second Si-containing particles (average circularity C2: 0.8, average particle diameter M2: 7 μm) as negative electrode active materials, SWCNT as a conductive material, and CMC, PAA, and SBR as binders were weighed so that the mass ratio thereof was C: second Si-containing particles: SWCNT: CMC: PAA: SBR = 85: 15: 0.1: 1: 1: 1.5. Among them, the raw materials except for SWCNT and SBR were dry-mixed, SWCNT and a dispersion medium were mixed and dry-kneaded (solid-kneaded), and then SBR and a dispersion medium were added and diluted-mixed, whereby a negative electrode composite paste for the lower layer was produced. Note that the above dry-kneading needs to cover the binders (CMC / PAA) around the active material, and thus needs to optimize the pressure load on the paste. The ideal solid content ratio B0 of the paste for optimizing the pressure load was derived by the following equation (1). Note that the ideal solid content ratio B0 of the paste is a value depending on the conditions of the dry-kneading (for example, the shape of the stirring blade, the rotation speed, and the like).

[0137] B0 = 100 - A0 = 100 / (100 + A1) x 100 Equation (1) B0: Ideal solid content ratio [%]

[0138] A0: Water content rate [%] at which the torque required for mixing reaches a maximum

[0139] A1: Amount of moisture [mL] when the mixture is 100 g

[0140] Next, the first Si-containing particles (average circularity Cl: 0.95, particle diameter Ml: 7 μm) were used instead of the second Si-containing particles, and a negative electrode composite paste for the upper layer was produced by the same production method except for this.

[0141] Then, the above lower layer negative electrode mixture paste was applied to the negative electrode core (copper foil, 10 μm) and dried. The above upper layer negative electrode mixture paste was applied to the dried lower layer negative electrode mixture paste and dried so that the ratio T1 :T2 of the thickness T1 of the upper layer to the thickness T2 of the lower layer was 50:50, thereby providing the negative electrode active material layer of the 2-layer structure on the negative electrode core. Then, calendering and processing into a prescribed size were performed by press processing, thereby obtaining the negative electrode plate.

[0142] Production of the positive electrode

[0143] A lithium nickel cobalt manganese composite oxide (NCM) as a positive electrode active material, polyvinylidene fluoride (PVdF) as a binder, and acetylene black (AB) as a conductive material were weighed so that the mass ratio was NCM:PVdF:AB=100:1:1, and mixed in N-methyl-2-pyrrolidone (NMP), thereby preparing a positive electrode mixture paste. The positive electrode mixture paste was applied to a long strip-shaped positive electrode core (aluminum foil, thickness 15 μm) and dried. Then, calendering and processing into a prescribed size were performed by press processing, thereby obtaining the positive electrode plate.

[0144] Lead wires were attached to the above negative electrode and the above positive electrode, respectively, and the electrodes were stacked with a separator interposed therebetween, thereby producing an electrode body. The produced electrode body was inserted into an outer packaging body composed of an aluminum laminate sheet, a non-aqueous electrolyte was injected, the opening portion of the outer packaging body was sealed, and a test battery cell (laminate cell) was produced.

[0145] The non-aqueous electrolyte used was a non-aqueous electrolyte in which LiPF6 was dissolved at 1M in a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of EC:FEC:EMC:DMC=15:5:40:40.

[0146] (2) Example 2

[0147] A test battery cell was produced in the same manner as in Example 1, except that the average circularity of the first Si-containing particles was made to be 0.9 and the average circularity of the second Si-containing particles was made to be 0.85.

[0148] (3) Example 3

[0149] A test battery cell was produced in the same manner as in Example 1, except that the thickness ratio T1 :T2 of the upper layer to the lower layer was made to be 90:10.

[0150] (4) Example 4

[0151] A test battery cell was produced in the same manner as in Example 1, except that the thickness ratio T1:T2 of the upper layer to the lower layer was made to be 10:90.

[0152] (5) Example 5

[0153] A test battery cell was produced in the same manner as in Example 1, except that the thickness ratio T1:T2 of the upper layer to the lower layer was made to be 70:30.

[0154] (6) Example 6

[0155] A test battery cell was produced in the same manner as in Example 1, except that the thickness ratio T1:T2 of the upper layer to the lower layer was made to be 30:70.

[0156] (7) Example 7

[0157] A test battery cell was produced in the same manner as in Example 1, except that the average circularity of the second Si-containing particles was made to be 0.5.

[0158] (8) Comparative Example 1

[0159] A test battery cell was produced in the same manner as in Example 1, except that the negative electrode active material layer was made to be one layer containing the first Si-containing particles and the second Si-containing particles. Note that the composition of the mixture paste for forming the negative electrode active material layer was adjusted to be C: second Si-containing particles: first Si-containing particles: SWCNT: CMC: PAA: SBR = 85:7.5:7.5:0.1:1:1:1.5 in mass ratio.

[0160] (9) Comparative Example 2

[0161] A test battery cell was produced in the same manner as in Example 1, except that the lower layer was formed using a paste containing the first Si-containing particles and the upper layer was formed using a paste containing the second Si-containing particles.

[0162] (10) Comparative Example 3

[0163] A test battery cell was produced in the same manner as in Example 1, except that the negative electrode active material layer was made to be one layer using only a paste containing the first Si-containing particles.

[0164] (11) Comparative Example 4

[0165] A test battery cell was produced in the same manner as in Example 1, except that the negative electrode active material layer was made to be one layer using only a paste containing the second Si-containing particles.

[0166] (12) Comparative Example 5

[0167] A test battery cell was produced in the same manner as in Example 1, except that the average circularity of the second Si-containing particles was made to be 0.4.

[0168] 2. Evaluation Test

[0169] (1) Measurement of Average Circularities of the First Si-Containing Particles and the Second Si-Containing Particles

[0170] The average circularity in the present specification was derived from an image-type particle size distribution measuring device. The length of the circumference of a circle having the same area as the projection image of an arbitrary Si-containing particle (L0) and the length of the outer circumference of the projection image (L) were measured with the powder of the Si-containing particles as a sample, and derived by the following equation (2). Then, the average circularity of the Si-containing particles was derived from 3000 Si-containing particles.

[0171] Average circularity C = L0 / L equation (2)

[0172] (2) Evaluation of Expansion Rate of the Negative Electrode Plate

[0173] A test battery cell was produced, and charge-discharge was repeated for 250 cycles at 25°C in the environment with CCCV charging (0.4 C_4.2 V_0.1 C cutoff) - CC discharging (0.4 C_2.5 V cutoff) for 1 cycle. Then, the expansion rate of the negative electrode plate was derived from the following equation (3).

[0174] Expansion rate of the negative electrode plate = {(thickness of the test battery cell after 250 cycles / thickness of the test battery cell before 250 cycles) - 1} x 100 equation (3)

[0175] (3) Evaluation of Resistance Increase Rate

[0176] The test battery cell was subjected to CCCV charging (1.5 C_4.2 V_0.1 C cutoff) at 25°C in the environment until the SOC reached 50% at a constant current of 1.5 C. Then, after storage for 1 hour at 25°C in the environment, CC discharging (0.4 C_2.5 V cutoff) was performed at a constant current of 1.0 C for 10 seconds. The resistance value at this time was defined as the initial resistance value.

[0177] After the initial resistance value was measured, charge-discharge cycles were repeated for 50 cycles at 25°C in the environment with CCCV charging (1.5 C_4.2 V_0.1 C cutoff) and CC discharging (0.4 C_2.5 V cutoff) for 1 cycle. The test battery cell after 50 cycles was subjected to resistance evaluation at 25°C in the environment, and the resistance increase rate was derived from the following equation (4). In the present specification, the sharp deterioration of the secondary battery accompanying charge-discharge was evaluated by performing the above resistance increase rate evaluation.

[0178] Resistance increase rate = resistance value after 50 cycles / initial resistance value x 100 Equation (4)

[0179] 3. Evaluation results

[0180] The test results for each sample are summarized in Tables 1 and 2.

[0181] [Table 1]

[0182]

[0183] [Table 2]

[0184]

[0185] Examples 1 to 7 are embodiments in which the negative electrode active material layer is a two-layer structure, the negative electrode active material is more densely packed on the upper layer side, and less densely packed on the lower layer side. Therefore, it was confirmed that the expansion rate of the negative electrode plate and the high-rate cycle characteristics (resistance increase rate) were low.

[0186] The embodiments of the negative electrode active material layer of Comparative Examples 1 to 5 are outside the scope of the technical idea of the present disclosure, and therefore, it was confirmed that it was not possible to balance the suppression of negative electrode swelling and the suppression of rapid degradation of the secondary battery.

[0187] From the above results, it was confirmed that, in order to suppress negative electrode swelling and rapid degradation of the secondary battery, it is necessary to provide the negative electrode active material layer as two layers, and to arrange the negative electrode active material having a high circularity on the upper layer side and the negative electrode active material having a low circularity on the lower layer.

[0188] As described above, the present specification includes the disclosure described in each of the following items.

[0189] Item 1:

[0190] A negative electrode is a negative electrode for a secondary battery that includes a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector,

[0191] wherein the negative electrode active material layer includes a lower layer on the negative electrode current collector side and an upper layer on the surface side,

[0192] the upper layer includes first graphite particles and first Si-containing particles as negative electrode active materials,

[0193] the lower layer includes second graphite particles and second Si-containing particles as negative electrode active materials,

[0194] the average circularity C1 of the first Si-containing particles is greater than the average circularity C2 of the second Si-containing particles.

[0195] Item 2:

[0196] The negative electrode according to item 1, wherein the C1 is 0.9 or more and 1.0 or less, and the C2 is 0.5 or more and less than 0.9.

[0197] Item 3:

[0198] The negative electrode according to item 1 or 2, wherein the average circularity D1 of the first graphite particles is 0.85 or more and 1.0 or less.

[0199] Item 4:

[0200] The negative electrode according to any one of items 1 to 3, wherein a ratio T1 : T2 of the thickness T1 of the upper layer to the thickness T2 of the lower layer is 10 : 90 to 90 : 10.

[0201] Item 5:

[0202] The negative electrode according to any one of items 1 to 4, wherein a mass ratio N1 of the first Si-containing particles in the upper layer with respect to a total mass of the first graphite particles and the first Si-containing particles is 10 to 60 mass%,

[0203] a mass ratio N2 of the second Si-containing particles in the lower layer with respect to a total mass of the second graphite particles and the second Si-containing particles is 10 to 60 mass%.

[0204] Item 6:

[0205] The secondary battery is a secondary battery provided with 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.

Claims

1. A negative electrode is a secondary battery negative electrode comprising a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. in, The negative electrode active material layer comprises a lower layer located on the negative electrode current collector side and an upper layer located on the surface side. The upper layer contains first graphite particles and first Si-containing particles as negative electrode active materials. The lower layer contains a second type of graphite particles and a second type of Si-containing particles as negative electrode active materials. The average roundness C1 of the first Si-containing particle is greater than the average roundness C2 of the second Si-containing particle.

2. The negative electrode according to claim 1, wherein, The value of C1 is greater than or equal to 0.9 and less than 1.0, and the value of C2 is greater than or equal to 0.5 and less than 0.

9.

3. The negative electrode according to claim 1, wherein, The average roundness D1 of the first graphite particle is greater than 0.85 and less than 1.

0.

4. The negative electrode according to claim 1, wherein, The ratio of the thickness T1 of the upper layer to the thickness T2 of the lower layer, T1:T2, is 10:90 to 90:

10.

5. The negative electrode according to claim 1, wherein, The mass ratio N1 of the first Si-containing particle in the upper layer relative to the total mass of the first graphite particle and the first Si-containing particle is 10% to 60% by mass. The mass ratio N2 of the second Si-containing particle in the lower layer relative to the total mass of the second graphite particle and the second Si-containing particle is 10 to 60 by mass.

6. A secondary battery is a battery that has a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode according to any one of claims 1 to 5.

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