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 during repeated charging and discharging of a secondary battery. This negative electrode for a secondary battery is provided with a negative electrode current collector and a negative electrode active material layer that is supported by the negative electrode current collector and contains a negative electrode active material. The negative electrode active material layer is provided with an upper layer relatively positioned on the surface side and a lower layer relatively positioned on the negative electrode current collector side. The negative electrode active material contains at least Si-containing particles obtained by compounding carbon and Si, and graphite particles. The reaction area A1 of the negative electrode active material contained in the upper layer is smaller than the reaction area A2 of the negative electrode active material contained in the lower layer, and the Si amount Q1 of the first Si-containing particles contained in the upper layer is smaller than the Si amount Q2 of the second Si-containing particles contained in the lower layer.

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 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 (for example, refer to Patent Documents 1 and 2).

[0004] PRIOR ART DOCUMENTS

[0005] PATENT DOCUMENTS

[0006] Patent Document 1: Japanese Patent Application Publication No. 2019-175851

[0007] Patent Document 2: Japanese Patent Application Publication No. 2017-92009 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] However, while Si-containing particles are high in capacity, on the other hand, the volume change due to expansion / contraction at the time of charge and discharge of the secondary battery is large. Also, in the case where Si-containing particles and graphite particles are used together as a negative electrode active material, there is a problem that the internal stress becomes high due to swelling of the negative electrode at the time of repeated charge and discharge of the secondary battery. Therefore, for a negative electrode containing Si-containing particles and graphite particles, it is desired to develop a negative electrode that swells less at the time of repeated charge and discharge of the secondary battery. In addition, the swelling rate of the negative electrode is not dependent on the surface area of the negative electrode active material, but is dependent on the area of the region where the chemical reaction actually occurs, that is, where charge and discharge is performed. Note that the swelling of the negative electrode refers to the fact that the volume of the negative electrode is larger than the initial volume at the same state of charge (for example, a state close to full charge at around 80% of SOC).

[0010] In view of the above-described actual circumstances, an object of the present application is to provide a negative electrode for a secondary battery containing Si-containing particles and graphite particles, which swells less at the time of repeated charge and discharge of the secondary battery.

[0011] MEANS FOR SOLVING THE PROBLEMS

[0012] The negative electrode disclosed herein is a negative electrode for a secondary battery having a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector and containing a negative electrode active material. The negative electrode active material layer has an upper layer located on the surface side and a lower layer located on the negative electrode current collector side. The negative electrode active material contains at least Si-containing particles in which carbon and Si are compounded, and graphite particles. The reaction area Al of the negative electrode active material contained in the upper layer is smaller than the reaction area A2 of the negative electrode active material contained in the lower layer, and the Si amount Ql of the first Si-containing particles contained in the upper layer is smaller than the Si amount Q2 of the second Si-containing particles contained in the lower layer.

[0013] In the negative electrode active material layer having a two-layer structure of the upper layer on the surface side and the lower layer on the negative electrode current collector side, the upper layer on the surface side is more likely to swell. In the negative electrode disclosed herein, the first Si-containing particles are used in the upper layer that is likely to swell, and the second Si-containing particles are used in the lower layer that is not likely to swell. Further, the reaction area and the Si amount of the first Si-containing particles are smaller than those of the second Si-containing particles, and thus the first Si-containing particles are less likely to swell. As a result, swelling is suppressed as a whole of the negative electrode active material layer, and high capacity due to Si is achieved. As described above, according to the technology disclosed herein, a negative electrode for a secondary battery containing Si-containing particles and graphite particles, which has less swelling during repeated charge and discharge of a secondary battery and is high in capacity, can be provided.

[0014] In one preferred embodiment of the negative electrode disclosed herein, the ratio Al / A2 of the reaction area Al of the negative electrode active material contained in the upper layer to the reaction area A2 of the negative electrode active material contained in the lower layer is 0.6 or more and less than 1.0. Thus, suppression of swelling of the negative electrode and high capacity of the secondary battery can be more significantly achieved.

[0015] In one preferred embodiment of the negative electrode disclosed herein, the ratio Ql / Q2 of the Si amount Ql of the first Si-containing particles to the Si amount Q2 of the second Si-containing particles is 0.4 or more and less than 1.0. Thus, suppression of swelling of the negative electrode and high capacity of the secondary battery can be more significantly achieved.

[0016] In one preferred embodiment of the negative electrode disclosed herein, the ratio Tl:T2 of the thickness Tl of the upper layer to the thickness T2 of the lower layer is 10:90 to 90:10. Thus, suppression of swelling of the negative electrode and high capacity of the secondary battery can be more significantly achieved.

[0017] In one preferred embodiment of the negative electrode disclosed herein, the Si amount Ql of the first Si-containing particles is 20 to 55% by mass, and the Si amount Q2 of the second Si-containing particles is 45 to 80% by mass. Thus, suppression of swelling of the negative electrode and high capacity of the secondary battery can be more significantly achieved.

[0018] The secondary battery disclosed herein is a secondary battery provided with a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode disclosed herein.

[0019] According to such a configuration, a secondary battery in which swelling of the negative electrode accompanying charge and discharge is suppressed and high capacity is achieved can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0020] 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.

[0021] Figure 2 is a schematic cross-sectional view showing Figure 1 the particles of the negative electrode active material contained in the negative electrode active material layer 64 shown in

[0022] Figure 3 is a view schematically showing the structure of a lithium ion secondary battery using the negative electrode structure of an embodiment.

[0023] Figure 4 is a schematic exploded view showing Figure 3 the structure of the wound electrode body of the lithium ion secondary battery of

[0024] REFERENCE NUMERALS

[0025] 12 first graphite particles

[0026] 14 first Si-containing particles

[0027] 16 second graphite particles

[0028] 18 second Si-containing particles

[0029] 20 wound electrode body

[0030] 30 battery case

[0031] 36 safety valve

[0032] 42 positive electrode terminal

[0033] 42a positive electrode current collector

[0034] 44 negative electrode terminal

[0035] 44a negative electrode current collector

[0036] 50 positive electrode sheet (positive electrode)

[0037] 52 positive electrode current collector

[0038] 52a positive electrode current collector exposed portion

[0039] 54 positive electrode active material layer

[0040] 60 negative electrode sheet (negative electrode)

[0041] 62 negative electrode current collector

[0042] 62a negative electrode current collector exposed portion

[0043] 64 negative electrode active material layer

[0044] 70 separator sheet (separator)

[0045] 100 lithium ion secondary battery DETAILED DESCRIPTION

[0046] Hereinafter, a preferred embodiment of the technology disclosed herein will be described. Note that matters other than those specifically mentioned in this specification and matters necessary for the implementation of the technology disclosed herein can be understood as design matters based on the conventional technology in the field by those skilled in the art. 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.

[0047] 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 batteries) such as lithium ion secondary batteries, sodium ion secondary batteries, and the like, and capacitors (physical batteries) 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 herein, conventionally known materials can be used.

[0048] 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 FIG. 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 FIG. 1 is a negative electrode of a lithium ion secondary battery.

[0049] 1. Negative electrode

[0050] (1) Constitution of negative electrode

[0051] AsFigure 1 As shown, the negative electrode 60 includes a negative current collector 62 and a negative active material layer 64 supported by the negative current collector 62. In other words, the negative electrode 60 includes a negative current collector 62 and a negative active material layer 64 disposed on the negative current collector 62. The negative active material layer 64 may be disposed only on one side of the negative current collector 62, or it may be disposed as follows: Figure 1 The negative electrode current collector 62 is shown to be disposed on both sides. The negative electrode active material layer 64 is preferably disposed on both sides of the negative electrode current collector 62. Furthermore, as detailed below, the negative electrode active material layer 64 contains a negative electrode active material.

[0052] like Figure 1 As shown, a negative electrode current collector exposed portion 62a can be provided at one end of the negative electrode 60 in the width direction, exposing the negative electrode current collector 62. This negative electrode current collector exposed portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0053] The negative electrode 60 may include components other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) may be provided on the surface of the negative electrode current collector 62 in the region adjacent to the negative electrode active material layer 64. This insulating layer may contain, for example, an insulating inorganic filler.

[0054] The negative electrode current collector 62 is in the shape of a foil (or sheet) in the illustrated example, but is not limited to this. The negative electrode current collector 62 can be in various shapes such as rod, plate, or sieve. As for the material of the negative electrode current collector 62, similar to conventional lithium-ion secondary batteries, a metal with good conductivity (such as copper, nickel, titanium, stainless steel, etc.) can be used, with copper being preferred.

[0055] The size of the negative electrode current collector 62 is not particularly limited and can be determined appropriately according to the battery design. When copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, for example, it is 5μm or more and 35μm or less, preferably 6μm or more and 20μm or less.

[0056] like Figure 1 As shown, the negative electrode active material layer 64 has a multilayer structure. Specifically, the negative electrode active material layer 64 has an upper layer 64a located opposite the surface side and a lower layer 64b located opposite the negative electrode current collector 62 side. It should be noted that, without significantly hindering the effects of the present invention, the negative electrode active material layer 64 may also have layers other than the upper layer 64a and the lower layer 64b. For example, the negative electrode active material layer 64 may have an intermediate layer between the upper layer 64a and the lower layer 64b, in which the components of these layers are mixed together (hybridized).

[0057] The negative electrode active material layer 64 contains a negative electrode active material. It should be noted that the negative electrode active material layer 64 may also contain components other than the negative electrode active material. Examples of components other than the negative electrode active material include adhesives and conductive materials. Examples of adhesives include styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a thickener. Examples of conductive materials include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are preferred. When using CNTs as the conductive material, the negative electrode active material layer 64 may also contain a CNT dispersant.

[0058] Next, use Figure 2 The negative electrode active material of the negative electrode 60 disclosed herein is described in detail. Figure 2 It is shown 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. This is for illustrative purposes. Figure 2 This is a schematic diagram; the number and distribution of particles are not limited to... Figure 2 As shown.

[0059] The negative electrode active material contains at least graphite particles and Si particles. In the following description, the graphite particles and Si particles in the upper layer are referred to as "first graphite particles" and "first Si particles," respectively, and the graphite particles and Si particles in the lower layer are referred to as "second graphite particles" and "second Si particles," respectively. Therefore, in the upper layer 64a, at least first graphite particles 12 and first Si particles 14 are used as the negative electrode active material. In the lower layer 64b, at least second graphite particles 16 and second Si particles 18 are used as the negative electrode active material. The volume change of these Si particles due to expansion / contraction during charging and discharging is large, but by using them in conjunction with graphite particles, the disconnection of the conductive path caused by the volume change of the Si particles can be suppressed.

[0060] The graphite that constitutes the graphite particles (i.e., the first graphite particle 12 and the second graphite particle 16) in the negative electrode active material can be natural graphite, artificial graphite, or graphite covered by amorphous carbon material in the form of amorphous carbon-coated graphite.

[0061] The shapes of the first graphite particle 12 and the second graphite particle 16 are not particularly limited, and can be flake-like, spherical, etc. The first graphite particle 12 and the second graphite particle 16 are preferably spherical graphite particles. When the first graphite particle 12 and the second graphite particle 16 are spherical, the roundness of the first graphite particle 12 and the second graphite particle 16 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0062] 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 determined, for example, by using a commercially available static automatic image analyzer, calculating the average value of the circularity of 100 or more particles.

[0063] The average particle diameter of the first graphite particles 12 and the average particle diameter of the second graphite particles 16 are not particularly limited. The average particle diameter of the first graphite particles 12 and the average particle diameter of the second graphite particles 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.

[0064] Note that in the present specification, the "average particle diameter" refers to the median particle diameter (D50), which refers to the D50 particle diameter corresponding to the cumulative frequency of 50% by volume from the side of fine particles having a small particle diameter in the particle size distribution based on the volume basis of the laser diffraction / scattering method. The D50 particle diameter can be determined using a commercially available particle size distribution measuring device of the laser diffraction / scattering type or the like.

[0065] 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. It is particularly preferable to use the same graphite particles as the first graphite particles 12 and the second graphite particles 16.

[0066] The Si-containing particles are a composite of carbon and Si. For example, the Si-containing particles can be particles in which fine particles containing Si are dispersed inside a carbon material, particles in which fine particles containing Si enter the pores of a porous graphite subjected to granulation, particles in which fine particles containing Si are attached to the surface of a carbon particle, particles in which fine carbon particles are attached to the surface of a particle containing Si, and 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 the pores of a porous carbon material are preferable, and particles in which Si nanoparticles are dispersed inside the pores of a porous carbon material are more preferable. As one example of the Si-containing particles, 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.

[0067] The carbon region is, for example, a carbonization product of a carbon precursor (e.g., petroleum pitch, coal pitch, phenol resin, etc.); graphite; or the like. The carbon region is suitably configured as a carbon matrix. Thus, the Si-C composite is suitably a material in which a plurality of Si-containing regions are dispersed in a carbon matrix. At this time, the carbon matrix is able to moderate the volume change caused by the expansion / contraction of the Si-containing regions, and is thus advantageous.

[0068] The Si-containing region contains Si, and is configured of, for example, Si, a Si oxide (SiO x ), a Si nitride (SiN x ), a Si carbide (SiC x ), or the like. The Si-containing region is preferably configured of at least either one of Si and a Si oxide (SiO x ). The Si-containing region can be a fine particle. The oxygen content in the Si-containing region is preferably 10% by mass or less.

[0069] 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 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, after the sample is subjected to elemental analysis by EDX elemental mapping, 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 make it possible 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 taken as the "average particle diameter of the Si-containing region" herein.

[0070] Note that the first Si-containing particle 14 and the second Si-containing particle 18 can be produced in accordance with a known method. Note that various production methods of the particles of the Si-C composite are known (for example, refer to Japanese Patent Application Publication No. 2015-38862, International Publication No. 2014 / 046144, and the prior art documents listed in the international publication).

[0071] In the present specification, the first Si-containing particle 14 and the second Si-containing particle 18 are specified in terms of the reaction area (hereinafter sometimes referred to as the capacitance) and the Si amount. Note that these values are independent of each other, and make it possible to show the degree of expansion of the Si-containing particles before and after charge and discharge from different perspectives.

[0072] The reaction area of the negative electrode active material makes it possible to show the degree of expansion caused by the region in which the chemical reaction actually occurs. For example, the reaction area of the negative electrode active material (the first graphite particle and the first Si-containing particle) in the upper layer can be derived in the following order.

[0073] A negative electrode plate was obtained by coating the upper layer negative electrode mixture paste described below on a Cu foil having a thickness of 10 μm and drying, pressing, and processing to achieve a prescribed thickness and size. A lead was attached to the negative electrode plate, and the electrode was stacked with a separator therebetween to produce an electrode body containing the first graphite particles and the first Si-containing particles. The produced electrode body was inserted into an outer package body composed of an aluminum laminate sheet, a non-aqueous electrolyte was injected, and the opening of the outer package body was sealed to thereby produce a negative electrode / negative electrode symmetric battery cell. Then, impedance measurement of the produced negative electrode / negative electrode symmetric battery cell was performed at 25°C, and the capacitance was derived from the measured value. Then, the reaction area [F / g] of the negative electrode active material can be derived by the following formula (1). The reaction area is measured based on the resistance value in the actual charge / discharge reaction, and thus is a value independent of the surface area of the negative electrode active material, and can also be referred to as the area in which the chemical reaction actually occurs. Note that the reaction area of the graphite particles alone or the Si-containing particles alone can also be derived by changing the composition of the mixture paste forming the negative electrode active material layer.

[0074] Reaction area = (capacitance) / (amount of addition of negative electrode active material)

[0075] Comparing secondary batteries using silicon or carbon as the negative electrode active material, it is known that the theoretical capacity when using silicon is more than 10 times higher. Therefore, there is a tendency for the reaction area from Si materials to be larger than that from carbon. Also, in the case of using Si materials as the negative electrode active material, a high-capacity secondary battery can be achieved. Note that in lithium ion secondary batteries, an SEI coating film is formed when the electrolyte contacts the active material surface at the time of initial charging. Due to the accumulation of the SEI coating film, the negative electrode swells, and thus the smaller the reaction area of the negative electrode active material, the more the swelling of the negative electrode accompanying charge / discharge can be suppressed. In summary, silicon has a large volume expansion at the time of charging compared to carbon, and also has difficulty in returning to the original volume at the time of discharging. Therefore, when using Si materials as the negative electrode active material, swelling of the negative electrode accompanying charge / discharge is likely to occur.

[0076] Next, in the present specification, the Si amount Q refers to the weight % concentration of silicon when the total weight of the Si-containing particles is taken as 100 wt%, and shows a constant value even if the particles deform with charge / discharge. Also, the degree of swelling of the negative electrode before and after charging can be shown from a different perspective from the above reaction area. Hereinafter, the relationship between the Si amount and the reaction area will be described.

[0077] As described above, the reaction area can also be referred to as the area in which the chemical reaction actually occurs. For example, due to the influence of the internal structure such as the voids within the Si-containing particles, in the case where the electrolyte is difficult to impregnate into the Si-containing particles, even if the Si amount is relatively large, the reaction area can be a relatively small value.

[0078] Further, even in the Si-containing particles in which the Si amount is relatively large and the reaction area is relatively small as described above, there is a case where the Si-containing particles are deformed with charge and discharge, the impregnation property of the electrolytic solution changes, and the reaction area increases. That is, the reaction area differs from the Si amount, and sometimes changes due to deformation of the particles with charge and discharge.

[0079] Here, the swelling of the negative electrode of a secondary battery is described using a lithium ion secondary battery as an example. In the entire lithium ion secondary battery, a reversible chemical reaction in which lithium ions move back and forth between the positive electrode and the negative electrode mainly occurs with charge and discharge. However, a reaction other than the reaction associated with charge and discharge (defined as a side reaction in the present specification) also occurs, and further, this side reaction can be a main cause of the swelling of the negative electrode. As described above, it can be said that in the case of Si-containing particles in which the reaction area is relatively small and the Si amount is relatively small, the side reaction is less likely to occur in the negative electrode, and the swelling is suppressed. Note that the description of the mechanism of action of the present technology described above is a presumption, and the present technology is not limited thereto.

[0080] (2) Inhibition of Swelling of the Negative Electrode Plate

[0081] In the case where a secondary battery is subjected to charge and discharge, the negative electrode active material in the vicinity of the negative electrode current collector is affected by the swelling of the negative electrode active material further on the surface side, and tends to hinder the swelling. In contrast, the negative electrode active material in the vicinity of the surface of the negative electrode active material layer has less factors that hinder the swelling. Further, the particles in the lower layer 64b are less likely to move, and thus the disconnection of the conduction path at the time of charge and discharge can be suppressed. Therefore, in the lower layer 64b, the swelling of the negative electrode associated with the disconnection of the conduction path (swelling due to unevenness of the battery reaction, local reaction, or stress concentration, etc.) is easily suppressed. 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 is large in swelling at the time of repeated charge and discharge of the secondary battery. Hereinafter, the inhibition of the swelling of the negative electrode plate is described from the viewpoint of the reaction area and the Si amount, the average particle diameter, the content of the Si-containing-C particles in the upper layer 64a and the lower layer 64b, and the thickness T of the upper layer 64a and the lower layer 64b.

[0082] (A) Reaction Area A and Si Amount Q

[0083] In the negative electrode active material layer 64 of the present disclosure, the reaction area Al of the negative electrode active material of the upper layer 64a is smaller than the reaction area A2 of the negative electrode active material of the lower layer 64b, and the Si amount Ql of the first Si-containing particle 14 contained in the upper layer is less than the Si amount Q2 of the second Si-containing particle 18 contained in the lower layer. Thus, the swelling of the upper layer 64a, in which the swelling is more likely to occur, can be suppressed, and thus the swelling of the entire negative electrode active material layer 64 can be effectively suppressed. On the other hand, in the lower layer 64b, in which the swelling is less likely to occur, the second Si-containing particle 18 having a larger reaction area and a larger Si amount is used. Thus, the high capacity of the secondary battery can be facilitated while the swelling of the entire negative electrode active material layer 64 is suppressed. Based on the above results, the swelling of the negative electrode 60 at the time of repeated charge and discharge can be significantly suppressed while the sufficient capacity of the entire negative electrode active material layer 64 is ensured.

[0084] From the viewpoint of the high capacity of the secondary battery, the ratio (Al / A2) of the reaction area Al of the negative electrode active material contained in the upper layer to the reaction area A2 of the negative electrode active material contained in the lower layer is preferably 0.6 or more, more preferably 0.65 or more, and particularly preferably 0.7 or more. On the other hand, from the viewpoint of preventing the disconnection of the conductive path at the time of charge and discharge and suppressing the swelling of the negative electrode, Al / A2 is preferably less than 1.0, more preferably 0.9 or less, and particularly preferably 0.8 or less.

[0085] From the viewpoint of the high capacity of the secondary battery, the ratio (Ql / Q2) of the Si amount Ql of the first Si-containing particle to the Si amount Q2 of the second Si-containing particle is preferably 0.4 or more, more preferably 0.5 or more, and particularly preferably 0.6 or more. On the other hand, from the viewpoint of preventing the disconnection of the conductive path at the time of charge and discharge and suppressing the swelling of the negative electrode, Ql / Q2 is preferably less than 1.0, more preferably 0.8 or less, and particularly preferably 0.7 or less.

[0086] From the viewpoint of the high capacity of the secondary battery, the Si amount Ql of the first Si-containing particle in the upper layer is preferably 20% by mass or more, more preferably 30% by mass or more, and particularly preferably 40% by mass or more. On the other hand, from the viewpoint of suppressing the swelling of the negative electrode, it is preferably 55% by mass or less, more preferably 50% by mass or less, and particularly preferably 45% by mass or less.

[0087] From the viewpoint of the high capacity of the secondary battery, the Si amount Q2 of the second Si-containing particle in the lower layer is preferably 45% by mass or more, more preferably 50% by mass or more, and particularly preferably 55% by mass or more. On the other hand, from the viewpoint of suppressing the swelling of the negative electrode, it is preferably 80% by mass or less, more preferably 70% by mass or less, and particularly preferably 60% by mass or less.

[0088] (B) Average particle diameter M

[0089] As described above, in the present specification, the inhibition of the expansion of the negative electrode is achieved by defining the reaction area A of the Si-containing particles and the Si amount Q. Therefore, the particle diameter of the first Si-containing particles 14 and the second Si-containing particles 18 of the present disclosure is not particularly limited. Note that the average particle diameter of the Si-containing particles can be measured by the same method as the method described above for measuring the average particle diameter. The average particle diameter of the first Si-containing particles 14 and the second Si-containing particles 18 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.

[0090] Note that, regarding the particle diameter of the Si-containing particles, it is preferable to adjust in consideration of the relationship with the particle diameter of the graphite particles. For example, the ratio of the average particle diameter of the first graphite particles 12 to the average particle diameter of the first Si-containing particles 14 (D50 of the first graphite particles 12 / D50 of the first Si-containing particles 14) is not particularly limited. From the viewpoint of high packing property, the ratio (D50 of the first graphite particles 12 / D50 of the first Si-containing particles 14) is preferably 1.0 to 8.0, more preferably 1.0 to 5.0, further preferably 1.2 to 3.0, and particularly preferably 1.4 to 2.5.

[0091] On the other hand, the ratio of the average particle diameter of the second graphite particles 16 to the average particle diameter of the second Si-containing particles 18 (D50 of the second graphite particles 16 / D50 of the second Si-containing particles 18) is not particularly limited. From the viewpoint of high packing property, the ratio (D50 of the second graphite particles 16 / D50 of the second Si-containing particles 18) is preferably 1.0 to 8.0, more preferably 1.0 to 5.0, further preferably 1.2 to 3.0, and particularly preferably 1.4 to 2.5.

[0092] (C) Content

[0093] The content of the negative electrode active material in the upper layer 64a (i.e., relative to the total mass of the upper layer 64a) is preferably 90% by mass or more, and more preferably 95% by mass or more. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more 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 more and 3% by mass or less, and more preferably 0.05% by mass or more and 1% by mass or less.

[0094] The content of the negative active material in the lower layer 64b (i.e., relative to the total mass of the lower 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.

[0095] From the viewpoint of high capacity of the secondary battery, the mass ratio N1 of the content of the first Si-containing particle 14 in the upper layer 64a relative to the negative active material (i.e., the total mass of the first graphite particle 12 and the first Si-containing particle 14) is preferably 10% by mass or more, particularly preferably 15% by mass or more. On the other hand, from the viewpoint of suppressing swelling of the negative electrode plate, it is preferably 60% by mass or less, more preferably 40% by mass or less, and further preferably 20% by mass or less.

[0096] From the viewpoint of high capacity of the secondary battery, the mass ratio N2 of the content of the second Si-containing particle 18 in the lower layer 64b relative to the negative active material (i.e., the total mass of the second graphite particle 16 and the second Si-containing particle 18) is preferably 10% by mass or more, particularly preferably 15% by mass or more. On the other hand, from the viewpoint of suppressing swelling of the negative electrode plate, it is preferably 60% by mass or less, more preferably 40% by mass or less, and particularly preferably 20% by mass or less.

[0097] The negative active material contained in the upper layer 64a can be only the first graphite particle 12 and the first Si-containing particle 14. However, within a range that does not hinder the effects of the present application (for example, 10% by mass or less of the total amount of the negative active material contained in the upper layer 64a), the upper layer 64a can also contain a negative active material other than the first graphite particle 12 and the first Si-containing particle 14.

[0098] The negative active material contained in the lower layer 64b can be only the second graphite particle 16 and the second Si-containing particle 18. However, within a range that does not hinder the effects of the present application (for example, 10% by mass or less of the total amount of the negative active material contained in the lower layer 64b), the lower layer 64b can also contain a negative active material other than the second graphite particle 16 and the second Si-containing particle 18.

[0099] (D) Thickness T of Upper Layer and Lower Layer

[0100] In the present specification, the thickness of the upper layer refers to the maximum thickness of the negative electrode active material layer containing only the first graphite particles and the first Si-containing particles as the negative electrode active material. Note that, similarly, the thickness of the lower layer refers to the maximum thickness of the negative electrode active material layer containing only the second graphite particles and the second Si-containing particles as the negative electrode active material. In one preferred embodiment of the negative electrode, the upper layer 64a is formed in a manner that the thickness is thin among the multiple layers provided in the negative electrode active material layer. Thus, the swelling in the upper layer 64a is appropriately suppressed, and as a result, the swelling of the entire negative electrode 60 can be suppressed.

[0101] On the other hand, the lower layer 64b is formed in a manner that the thickness is thick. As described above, the lower layer 64b is relatively less likely to swell. By setting the lower layer 64b to be relatively thick, the swelling of the negative electrode 60 as a whole can be suppressed. In addition, since the lower layer 64b contains the second Si-containing particles 18 having a high capacity, the secondary battery can be made high-capacity by thickening the lower layer 64b. Based on the above results, the swelling of the negative electrode 60 during repeated charge and discharge of the secondary battery and the high-capacity of the secondary battery can be significantly achieved in the entire negative electrode active material layer 64.

[0102] From the viewpoint of suppressing the swelling of the negative electrode plate, 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 10:90 to 50:50, and particularly preferably 20:80 to 40:60.

[0103] (3) Manufacture of the Negative Electrode

[0104] The negative electrode 60 is suitably manufactured, for example, by a manufacturing 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 negative electrode composite paste for the 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 negative electrode composite paste for the upper layer; a step of applying the negative electrode composite paste for the lower layer on the negative electrode current collector 62 and drying to form the lower layer 64b; a step of applying the negative electrode composite paste for the upper layer on the lower layer 64b and drying to form the upper layer 64a; and a step of pressing the formed upper layer 64a and lower layer 64b (hereinafter also referred to as "pressing step").

[0105] Note that, in the present specification, "paste" refers to a mixture in which a part or all of the solid components are dispersed in a dispersion medium, including so-called "slurry", "ink", and the like.

[0106] In the lower layer negative electrode mixture paste preparation step, the second graphite particles 16, the second Si-containing particles 18, and optional components (e.g., a binder, a conductive material, etc.) are mixed to prepare a lower layer paste. This step can be performed by mixing with a dispersion medium (e.g., water) using a known mixing device, a stirring device, etc. according to a known method.

[0107] In the upper layer negative electrode mixture paste preparation step, the first graphite particles 12, the first Si-containing particles 14, and optional components (e.g., a binder, a conductive material, etc.) are mixed to prepare an upper layer paste. This step can be performed by mixing with a dispersion medium (e.g., water) using a known mixing device, a stirring device, etc. according to a known method. Note that the upper layer negative electrode mixture paste preparation step can be performed in parallel with the lower layer negative electrode mixture paste preparation step. The upper layer negative electrode mixture paste preparation step can also be performed in parallel with or after the lower layer formation step.

[0108] In the lower layer formation step, the lower layer negative electrode mixture paste is applied to the negative electrode current collector 62 and dried. In this step, the paste can be applied and dried by using a known application device. Thus, the lower layer (lower layer 64b) is formed.

[0109] In the upper layer formation step, the upper layer negative electrode mixture paste is applied to the lower layer 64b and dried. In this step, the paste can be applied and dried by using a known application device. Thus, the upper layer (upper layer 64a) is formed, and the negative electrode active material layer 64 is formed.

[0110] In the pressing step, the upper layer and the lower layer (i.e., the negative electrode active material layer 64) formed above are pressed to a prescribed density. In this step, the upper layer and the lower layer can be pressed to a prescribed density by applying a pressure using a known pressing device such as a roll press or the like. By the pressing step, the negative electrode active material layer 64 is compressed to a prescribed density, and thus the negative electrode active material particles are densely packed.

[0111] Note that the density of the negative electrode active material layer 64 after pressing is not particularly limited, and is, for example, 0.7 g / cm 3 The density is preferably 1.0 g / cm 3 The density is more preferably 1.2 g / cm 3 or more. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm 3 The density can also be 2.0 g / cm 3 or more.

[0112] 2. Secondary battery

[0113] The secondary battery disclosed herein has a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 of the above-described embodiment. According to the negative electrode 60 of 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, since the negative electrode 60 of the present embodiment uses a negative electrode active material containing Si, the secondary battery can be made high-capacity. Referring to Figure 3 and Figure 4 An embodiment of the secondary battery disclosed herein will be described. Note that the following structural example is a flat square lithium-ion secondary battery having a flat square shape wound electrode body and a flat square battery case.

[0114] Figure 3 is a diagram schematically showing the structure of a lithium-ion secondary battery using the negative electrode structure of an embodiment. Figure 3 The lithium-ion secondary battery 100 shown is constructed by housing a flat square shape wound electrode body 20 and a nonaqueous electrolyte solution (not shown) in a flat square battery case (i.e., an outer packaging container) 30. On the battery case 30, a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 set in a manner to release the internal pressure of the battery case 30 when the internal pressure rises to a prescribed level or more are provided. In addition, an injection port (not shown) for injecting the nonaqueous electrolyte solution is provided on the battery case 30. 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 metal material that is lightweight and has good thermal conductivity, such as aluminum, is used.

[0115] Figure 4 is a diagram schematically showing the structure of the wound electrode body of the lithium-ion secondary battery of Figure 3 . As shown in Figure 3 and Figure 4As shown, in the wound electrode body 20, the positive electrode sheet 50 and the negative electrode sheet 60 are overlapped with two long strip-shaped separator sheets 70 interposed therebetween. Further, the positive electrode sheet 50 and the negative electrode sheet 60 have a form wound along the length direction. The positive electrode sheet 50 has a structure in which a positive electrode active material layer 54 is formed on one face or both faces (in this case, both faces) of a long strip-shaped positive electrode current collector 52 along the length direction. The negative electrode sheet 60 has a structure in which a negative electrode active material layer 64 is formed on one face or both faces (in this case, both faces) of a long strip-shaped negative electrode current collector 62 along the length direction. Positive electrode current collector exposed portions 52a (i.e., portions in which the positive electrode current collector 52 is exposed since the positive electrode active material layer 54 is not formed) and negative electrode current collector exposed portions 62a (i.e., portions in which the negative electrode current collector 62 is exposed since the negative electrode active material layer 64 is not formed) 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-described length direction) of the wound electrode body 20. The positive electrode current collector exposed portions 52a and the negative electrode current collector exposed portions 62a are joined to the positive electrode current collector plate 42a and the negative electrode current collector plate 44a, respectively.

[0116] 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 given. As the positive electrode current collector 52, an aluminum foil is preferable.

[0117] 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.

[0118] 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 given. 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.

[0119] 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 given.

[0120] Note that in the present specification, "lithium nickel cobalt manganese-based complex oxide" refers to 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. Examples of the additive 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 additive 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-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, and lithium iron-nickel-manganese-based complex oxide.

[0121] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFeP04), lithium manganese phosphate (LiMnP04), and lithium manganese iron phosphate.

[0122] These positive electrode active materials can be used alone or in combination with two or more kinds. From the viewpoint of excellent characteristics such as initial resistance characteristics, a lithium nickel cobalt manganese-based complex oxide is particularly preferable as the positive electrode active material.

[0123] The average particle diameter 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.

[0124] 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 appropriately used. As the binder, for example, polyvinylidene fluoride (PVdF) and the like can be used.

[0125] 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.

[0126] The thickness of each 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.

[0127] As the negative electrode sheet 60, the above-described negative electrode 60 is used.

[0128] 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 given. 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 be provided on the surface of the separator 70.

[0129] 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, which is obtained by the Gurley test method, is not particularly limited, and is preferably 350 seconds / 100 cc or less.

[0130] 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.

[0131] 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.

[0132] Note that the above nonaqueous electrolyte 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, as long as the effects of the present disclosure are not significantly impaired.

[0133] The lithium-ion secondary battery 100 suppresses swelling of the negative electrode at the time of repeated charge and discharge, and thus has a low reaction force. In addition, the lithium-ion secondary battery 100 has a high capacity. The lithium-ion secondary battery 100 can be used for various purposes. As appropriate purposes, 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 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 batteries are connected in series and / or in parallel.

[0134] In the above, as an example, a square lithium-ion secondary battery 100 having a wound electrode body 20 with a flat shape has been 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.

[0135] In addition, the lithium-ion secondary battery 100 can also be configured as a full solid lithium-ion secondary battery using a solid electrolyte instead of a nonaqueous electrolyte, according to a known method.

[0136] 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 for use as a negative electrode of another secondary battery, which can be configured according to a known method.

[0137] "Evaluation"

[0138] 1. Test Example

[0139] 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.

[0140] (1) Example 1

[0141] [Production of Negative Electrode]

[0142] The reaction area of the negative electrode active material disclosed below is a conversion value when the reaction area Al of the upper layer negative electrode active material of Example 1 is set to 100. First, a negative electrode composite paste for the lower layer is produced. Specifically, graphite particles C and second Si-containing particles (Si amount Q2: 60 wt%, particle diameter M2: 4 μm) as negative electrode active materials (reaction area A2: 140), SWCNT as a conductive material, and CMC, PAA, and SBR as binders are prepared. Then, each material is weighed so that the mass ratio of each material is C: second Si-containing particles: SWCNT: CMC: PAA: SBR = 85: 15: 0.1: 1: 1: 1: 1.5. Among them, the raw materials except for SWCNT and SBR are dry-mixed, then SWCNT and the dispersion medium are mixed and dry-kneaded (solid kneading), and then SBR and the dispersion medium are added and diluted mixed, whereby the negative electrode composite paste for the lower layer is produced. It is to be noted that, regarding the above dry-kneading, in order to cover the binders (CMC / PAA) around the active material, the pressure load on the paste needs to be optimized. The ideal solid content ratio B0 of the paste for optimizing the pressure load is derived by the following formula (1). It is to be noted 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, the stirring time, etc.).

[0143] B0 = 100 - A0 = 100 / (100 + Al) x 100 Formula (1) B0: Ideal solid content ratio [%]

[0144] A0: Moisture content [%] at which the torque required for mixing reaches a maximum

[0145] Al: Moisture amount [mL] when the mixture is set to 100 g

[0146] Next, a negative electrode mixture paste for the upper layer was prepared. The negative electrode mixture paste for the upper layer was prepared in the same manner as the negative electrode mixture paste for the lower layer described above, except that the first Si-containing particles (Si amount Q1: 40 wt%, particle diameter M1: 7 μm) were used instead of the second Si-containing particles and the reaction area A1 was 100.

[0147] Then, the negative electrode mixture paste for the lower layer described above was applied to the negative electrode core (copper foil, 10 μm) and dried. The negative electrode mixture paste for the upper layer described above was applied to the negative electrode mixture paste for the lower layer after drying 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, whereby a negative electrode active material layer having a two-layer structure was provided on the negative electrode core (reaction area ratio A1 / A2 = 0.7, Si amount ratio Q1 / Q2 = 0.7, particle diameter ratio = 1.8). Then, calendering and processing into a prescribed size were performed by press processing, whereby a negative electrode plate was obtained.

[0148] [Production of a positive electrode]

[0149] A lithium nickel cobalt manganese-based complex 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 of NCM:PVdF:AB = 100:1:1, and mixed in N-methyl-2-pyrrolidone (NMP), whereby a positive electrode mixture paste was prepared. 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, whereby a positive electrode plate was obtained.

[0150] A lead was attached to each of the above-described negative electrode and the above-described positive electrode, and the electrodes were stacked with a separator interposed therebetween, whereby an electrode body was produced. The produced electrode body was inserted into an outer packaging body composed of an aluminum laminate sheet, a nonaqueous electrolyte was injected, the opening portion of the outer packaging body was sealed, and a test battery cell (laminate battery cell) was produced.

[0151] The nonaqueous electrolyte solution used a nonaqueous electrolyte solution in which LiPF6 was dissolved at 1 M 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.

[0152] (2) Example 2

[0153] A test battery cell was produced in the same manner as in Example 1, except that the Si amount Q1 of the first Si-containing particles was 45, the Si amount Q2 of the second Si-containing particles was 55, and the ratio Q1 / Q2 of the Si amounts was 0.8.

[0154] (3) Example 3

[0155] A test battery cell was produced in the same manner as in Example 1, except that the Si amount Q1 of the first Si-containing particles was made to be 20, the Si amount Q2 of the second Si-containing particles was made to be 45, and the ratio Q1 / Q2 of the Si amounts was made to be 0.4.

[0156] (4) Example 4

[0157] A test battery cell was produced in the same manner as in Example 1, except that the Si amount Q1 of the first Si-containing particles was made to be 55, the Si amount Q2 of the second Si-containing particles was made to be 80, and the ratio Q1 / Q2 of the Si amounts was made to be 0.7.

[0158] (5) Example 5

[0159] A test battery cell was produced in the same manner as in Example 1, except that the thickness ratio T1:T2 of the thickness T1 of the upper layer to the thickness T2 of the lower layer was made to be 70:30.

[0160] (6) Example 6

[0161] A test battery cell was produced in the same manner as in Example 1, except that the thickness ratio T1:T2 of the thickness T1 of the upper layer to the thickness T2 of the lower layer was made to be 30:70.

[0162] (7) Example 7

[0163] A test battery cell was produced in the same manner as in Example 1, except that the reaction area A1 of the negative electrode active material of the upper layer was made to be 130, the reaction area A2 of the negative electrode active material of the lower layer was made to be 140, and the ratio A1 / A2 of the reaction areas was made to be 0.9.

[0164] (8) Comparative Example 1

[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 contain the first Si-containing particles and the second Si-containing particles in one layer. 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.

[0166] (9) Comparative Example 2

[0167] A test battery cell was produced in the same manner as in Example 1, except that the paste containing the first Si-containing particles was applied to the lower layer and the paste containing the second Si-containing particles was applied to the upper layer.

[0168] (10) Comparative Example 3

[0169] A test battery cell was produced in the same manner as in Example 1, except that the reaction area Al of the upper layer of the negative active material was 140, the reaction area A2 of the lower layer of the negative active material was 100, and the ratio Al / A2 of the reaction areas was 1.4.

[0170] 2. Evaluation Test

[0171] (1) Measurement of the reaction areas Al, A2 of the negative active materials in the upper and lower layers

[0172] The upper layer negative electrode mixture paste described above was applied to a Cu foil having a thickness of 10 μm and dried, and then pressed to a prescribed thickness. After being processed to a prescribed size, a negative electrode plate was obtained. A lead was attached to the negative electrode plate, and the electrode was stacked with a separator interposed therebetween, to produce an electrode body containing the negative active material (the first graphite particles and the first Si-containing particles). The electrode body produced was inserted into an outer packaging body composed of an aluminum laminate sheet, a nonaqueous electrolyte was injected, and the opening of the outer packaging body was sealed, to produce a negative electrode / negative electrode symmetrical battery cell. Then, the resistance of the negative electrode / negative electrode symmetrical battery cell produced was measured at 25°C, and the capacitance was derived from the measured value. Then, the reaction area of the negative active material in the upper layer was derived from the following formula (1). As for the negative active material in the lower layer, the reaction area was derived in the same manner, except that the lower layer negative electrode mixture paste was used.

[0173] Reaction area = (capacitance) / (amount of addition of the negative active material) Formula (1)

[0174] (2) Evaluation of the expansion rate of the negative electrode plate

[0175] A test battery cell was produced, and charge and discharge were repeated 250 cycles at 25°C, with 1 cycle of CC-CV charge (0.4 C_4.2 V_0.1 C cutoff)-CC discharge (0.4 C_2.5 V cutoff). Then, the expansion rate of the negative electrode plate was derived from the following formula (2).

[0176] 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 Formula (2)

[0177] 3. Evaluation Results

[0178] The test results for each sample are summarized in Tables 1 and 2.

[0179]

[0180] [Table 2]

[0181]

[0182] From the above results, it was confirmed that Examples 1 to 7 are an embodiment in which the negative electrode active material layer is a two-layer structure and the upper layer uses particles in which the side reaction of the Si-containing particles is less likely to occur, and thus the expansion rate of the negative electrode plate is relatively low.

[0183] Although Comparative Example 1 contains two kinds of Si-containing particles in the negative electrode active material layer, the structure of the negative electrode active material layer is one layer, and thus the effect of suppressing the expansion rate of the negative electrode plate cannot be appropriately achieved.

[0184] Although Comparative Examples 2 and 3 are two-layer structures of the negative electrode active material layer, they are an embodiment in which the upper layer uses particles in which the side reaction of the Si-containing particles is more likely to occur, and thus the effect of reducing the expansion rate of the negative electrode plate cannot be appropriately achieved.

[0185] From the above results, it was confirmed that, in order to reduce the expansion rate of the negative electrode plate, it is necessary to provide an embodiment in which the negative electrode active material layer is two layers and the upper layer side is less likely to expand.

[0186] As described above, the present specification includes the disclosure described in each of the following items.

[0187] Item 1:

[0188] A negative electrode is a negative electrode for a secondary battery including a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector and containing a negative electrode active material,

[0189] wherein the negative electrode active material layer includes an upper layer located on the surface side and a lower layer located on the negative electrode current collector side,

[0190] the negative electrode active material contains at least Si-containing particles in which carbon and Si are complexed, and graphite particles,

[0191] the reaction area A1 of the negative electrode active material contained in the upper layer is smaller than the reaction area A2 of the negative electrode active material contained in the lower layer, and

[0192] the amount Q1 of Si of the first Si-containing particles contained in the upper layer is less than the amount Q2 of Si of the second Si-containing particles contained in the lower layer.

[0193] Item 2:

[0194] The negative electrode according to Item 1, wherein the ratio A1 / A2 of the reaction area A1 of the negative electrode active material contained in the upper layer to the reaction area A2 of the negative electrode active material contained in the lower layer is 0.6 or more and less than 1.0.

[0195] Item 3:

[0196] The negative electrode according to any one of items 1 to 3, wherein a ratio Q1 / Q2 of the Si amount Q1 of the first Si-containing particles to the Si amount Q2 of the second Si-containing particles is 0.4 or more and less than 1.0.

[0197] Item 4:

[0198] The negative electrode according to any one of items 1 to 3, wherein a ratio T1:T2 of the average thickness T1 of the upper layer to the average thickness T2 of the lower layer is 10:90 to 90:10.

[0199] Item 5:

[0200] The negative electrode according to any one of items 1 to 4, wherein the Si amount Q1 of the first Si-containing particles is 20 to 55 mass%, and the Si amount Q2 of the second Si-containing particles is 45 to 80 mass%.

[0201] Item 6:

[0202] 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, which is a negative electrode for a secondary battery provided with a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector and containing a negative electrode active material, the negative electrode active material layer being provided with an upper layer located on the surface side and a lower layer located on the negative electrode current collector side, the negative electrode active material containing at least Si-containing particles composed of carbon and Si in combination and graphite particles, the reaction area Al of the negative electrode active material contained in the upper layer being smaller than the reaction area A2 of the negative electrode active material contained in the lower layer, and the Si amount Ql of the first Si-containing particles contained in the upper layer being less than the Si amount Q2 of the second Si-containing particles contained in the lower layer. wherein The ratio Al / A2 of the reaction area Al of the negative electrode active material contained in the upper layer to the reaction area A2 of the negative electrode active material contained in the lower layer is 0.6 or greater and less than 1.

0. The ratio Ql / Q2 of the Si amount Ql of the first Si-containing particles to the Si amount Q2 of the second Si-containing particles is 0.4 or greater and less than 1.

0. The ratio Tl:T2 of the average thickness Tl of the upper layer to the average thickness T2 of the lower layer is 10:90 to 90:

10. The Si amount Ql of the first Si-containing particles is 20 to 55% by mass, and the Si amount Q2 of the second Si-containing particles is 45 to 80% by mass.

2. The negative electrode according to claim 1, wherein The negative electrode is the negative electrode described in any one of claims 1 to 5.

3. The negative electrode according to claim 1, wherein The negative electrode is the negative electrode described in any one of claims 1 to 5.

4. The negative electrode according to claim 1, wherein ​ 5. The negative electrode according to claim 1, wherein ​ 6. A secondary battery, which is a secondary battery provided with a positive electrode, a negative electrode, and an electrolyte, wherein ​

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