Negative electrode for secondary battery and secondary battery using the negative electrode

Abstract

The present invention provides a negative electrode containing Si-containing particles and graphite particles, which exhibits minimal swelling when repeatedly charged and discharged in a secondary battery. [Solution] The negative electrode of the secondary battery of this disclosure comprises 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 comprises an upper layer located relatively on the surface side and a lower layer located relatively on the negative electrode current collector side. The negative electrode active material contains at least graphite particles and Si-containing particles which are a composite of carbon and Si. 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 amount of Si Q1 of the first Si-containing particles contained in the upper layer is less than the amount of Si Q2 of the second Si-containing particles contained in the lower layer.

Description

Technical Field

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

Background Art

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

[0003] In the application of power sources for vehicle drive, particularly in the application of power sources for BEV drive, from the viewpoint of extending the cruising range of vehicles, further increase in the capacity of secondary batteries is desired. As a high-capacity negative electrode active material, Si-containing particles are known (see, for example, Patent Documents 1 and 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, while Si-containing particles have high capacity, they exhibit significant volume changes due to expansion / contraction during charging and discharging of secondary batteries. Furthermore, when Si-containing particles and graphite particles are used together as the negative electrode active material, repeated charging and discharging of the secondary battery leads to increased internal stress due to the expansion of the negative electrode. Therefore, there is a need to develop a negative electrode containing Si-containing particles and graphite particles that exhibits less expansion when repeatedly charged and discharged in a secondary battery. The expansion rate of the negative electrode depends not on the surface area of ​​the negative electrode active material, but on the area of ​​the region where the chemical reaction actually occurs, i.e., where charging and discharging take place. Note that this expansion of the negative electrode refers to the volume of the negative electrode becoming larger than its initial volume in the same charge state (for example, a state close to full charge around 80% SOC).

[0006] In view of the above circumstances, the present disclosure aims to provide a negative electrode for a secondary battery that contains Si-containing particles and graphite particles, and which exhibits minimal swelling when the secondary battery is repeatedly charged and discharged. [Means for solving the problem]

[0007] The negative electrode disclosed herein is a negative electrode for a secondary battery comprising 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 comprises an upper layer located relatively on the surface side and a lower layer located relatively on the negative electrode current collector side. The negative electrode active material contains at least graphite particles and Si-containing particles which are a composite of carbon and Si. 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 amount of Si Q1 of the first Si-containing particles contained in the upper layer is less than the amount of Si Q2 of the second Si-containing particles contained in the lower layer.

[0008] In a negative electrode active material layer having a two-layer structure consisting of an upper layer on the surface side and a lower layer on the negative electrode current collector side, the upper layer on the surface side is relatively prone to swelling. In the negative electrode disclosed herein, first Si-containing particles are used in the upper layer, which is prone to swelling, and second Si-containing particles are used in the lower layer, which is less prone to swelling. Since the reaction area and Si content of the first Si-containing particles are smaller than those of the second Si-containing particles, they are relatively less prone to swelling. As a result, swelling of the negative electrode active material layer as a whole is suppressed, and high capacity derived from Si is achieved. As described above, the technology disclosed herein provides a negative electrode for a secondary battery containing Si-containing particles and graphite particles, which exhibits less swelling when the secondary battery is repeatedly charged and discharged, and has high capacity.

[0009] In one preferred embodiment of the negative electrode disclosed herein, 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. This makes it possible to more significantly suppress the expansion of the negative electrode and increase the capacity of the secondary battery.

[0010] In one preferred embodiment of the negative electrode disclosed herein, the ratio Q1 / Q2 of the Si content Q1 of the first Si-containing particle to the Si content Q2 of the second Si-containing particle is 0.4 or more and less than 1.0. This makes it possible to more significantly suppress the expansion of the negative electrode and increase the capacity of the secondary battery.

[0011] In one 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. This makes it possible to more significantly suppress the expansion of the negative electrode and increase the capacity of the secondary battery.

[0012] In one preferred embodiment of the negative electrode disclosed herein, the Si content Q1 of the first Si-containing particles is 20 to 55 mass%, and the Si content Q2 of the second Si-containing particles is 45 to 80 mass%. This makes it possible to more significantly suppress the expansion of the negative electrode and increase the capacity of the secondary battery.

[0013] The secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode disclosed herein.

[0014] This configuration suppresses the expansion of the negative electrode during charging and discharging, and enables the realization of a high-capacity secondary battery. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a schematic cross-sectional view showing an example of a negative electrode 60 according to one embodiment, and is a cross-sectional view along the thickness direction and the width direction. [Figure 2] 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. [Figure 3] Figure 3 is a schematic diagram showing the configuration of a lithium-ion secondary battery constructed using a negative electrode according to one embodiment. [Figure 4] Figure 4 is a schematic exploded view showing the configuration of the wound electrode body of the lithium-ion secondary battery shown in Figure 3. [Modes for carrying out the invention]

[0016] Preferred embodiments of the technology disclosed herein are described below. Matters other than those specifically mentioned herein but necessary for implementing the technology disclosed herein can be understood as design matters for those skilled in the art based on prior art. The technology disclosed herein can be implemented based on the content disclosed herein and common technical knowledge in the art. Furthermore, in the drawings described herein, the same reference numerals are used to denote members and parts that perform the same function, and redundant explanations may be omitted or simplified. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not necessarily reflect actual dimensional relationships. Furthermore, A to B indicating a numerical range means A or greater and B or less, and therefore includes numerical ranges greater than A and less than B.

[0017] As used herein, the term "secondary battery" refers to all rechargeable energy storage devices capable of repeated charge and discharge with the movement of charge carriers between the positive and negative electrodes, including so-called storage batteries (chemical batteries) such as lithium-ion secondary batteries and sodium-ion secondary batteries, and capacitors (physical batteries) such as lithium-ion capacitors (LICs). Hereinafter, the main constituent materials of the secondary battery according to the present disclosure will be described. For the constituent materials of the secondary battery not described herein, those known in the art can be used.

[0018] The negative electrode disclosed herein is used in a secondary battery, and preferably used in a lithium-ion secondary battery. One embodiment of the negative electrode disclosed herein will be specifically described with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically showing an example of the negative electrode 60 according to the present embodiment, which is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to the present embodiment shown in FIG. 1 is the negative electrode of a lithium-ion secondary battery.

[0019] 1. Negative Electrode (1) Structure of Negative Electrode As shown in FIG. 1, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided only on one side of the negative electrode current collector 62, or may be provided on both sides of the negative electrode current collector 62 as shown in FIG. 1. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62. Further, as will be described in detail later, the negative electrode active material layer 64 contains a negative electrode active material.

[0020] As shown in FIG. 1, a negative electrode current collector exposed portion 62a where the negative electrode current collector 62 is exposed may be provided at one end in the width direction of the negative electrode 60. This negative electrode current collector exposed portion 62a can function as a current collecting portion. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0021] 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 a region adjacent to the negative electrode active material layer 64. This insulating layer may contain, for example, an insulating inorganic filler.

[0022] The shape of the negative electrode current collector 62 is foil-like (or sheet-like) in the illustrated example, but is not limited to this. The negative electrode current collector 62 may take various forms such as rod-like, plate-like, or mesh-like. As with conventional lithium-ion secondary batteries, a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.) can be used as the material for the negative electrode current collector 62, and copper is preferred among these.

[0023] The dimensions of the negative electrode current collector 62 are not particularly limited and can be determined as appropriate according to the battery design. When copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 6 μm or more and 20 μm or less.

[0024] As shown in Figure 1, 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 relatively on the surface side and a lower layer 64b located relatively on the negative electrode current collector 62 side. The negative electrode active material layer 64 may have further layers other than the upper layer 64a and the lower layer 64b, as long as this does not significantly impair the effects of the present invention. 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.

[0025] The negative electrode active material layer 64 contains the negative electrode active material. 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 binders and conductive materials. Examples of binders include styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a thickening agent. Examples of conductive materials include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are particularly preferred. When CNTs are used as the conductive material, the negative electrode active material layer 64 may contain a CNT dispersant.

[0026] Next, the negative electrode active material of the negative electrode 60 disclosed herein will be described in detail with reference to Figure 2. 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 diagram, and the number and distribution of particles are not limited to those shown in Figure 2.

[0027] The negative electrode active material contains at least graphite particles and Si-containing particles. In the following description, the graphite particles and Si-containing particles in the upper layer will be referred to as "first graphite particles" and "first Si-containing particles," respectively, and the graphite particles and Si-containing particles in the lower layer will be referred to as "second graphite particles" and "second Si-containing particles," respectively. Therefore, in the upper layer 64a, at least first graphite particles 12 and first Si-containing particles 14 are used as the negative electrode active material. In the lower layer 64b, at least second graphite particles 16 and second Si-containing particles 18 are used as the negative electrode active material. These Si-containing particles undergo large volume changes due to expansion / contraction associated with charging and discharging. However, by using them in combination with graphite particles, it is possible to suppress the interruption of the conductive path caused by the volume change of the Si-containing particles.

[0028] The graphite constituting the graphite particles in the negative electrode active material (i.e., the first graphite particles 12 and the second graphite particles 16) may be natural graphite or artificial graphite, or it may be amorphous carbon-coated graphite in which the graphite is coated with an amorphous carbon material.

[0029] The shape of the first graphite particles 12 and the second graphite particles 16 is not particularly limited and may be flaky, spherical, or the like. The first graphite particles 12 and the second graphite particles 16 are preferably spheroidized graphite particles. When the first graphite particles 12 and the second graphite particles 16 are spherical, the circularity of the first graphite particles 12 and the second graphite particles 16 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0030] In this specification, "circularity" refers to the ratio of the circumference of a perfect 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 perfect 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 perfect circle, and the closer the particle is to a perfect sphere. Circularity can be determined, for example, by using a commercially available static automatic image analyzer to determine the circularity of 100 or more particles and calculating the average value.

[0031] 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 even more preferably 12 μm to 20 μm.

[0032] In this specification, "average particle diameter" refers to the median diameter (D50), which is the D50 particle diameter corresponding to a cumulative frequency of 50% by volume from the smaller particle diameter side in a volume-based particle size distribution based on laser diffraction and scattering. The D50 particle diameter can be determined using commercially available laser diffraction and scattering particle size distribution analyzers.

[0033] The first graphite particles 12 and the second graphite particles 16 may be of the same type, or different types of graphite particles may be used. It is particularly preferable to use the same graphite particles for the first graphite particles 12 and the second graphite particles 16.

[0034] Si-containing particles are composites of carbon and Si. For example, Si-containing particles may be those in which fine particles containing Si are dispersed inside a carbon material; those in which fine particles containing Si are embedded in the pores of granulated porous graphite; those in which fine particles containing Si are attached to the surface of carbon particles; or those in which carbon fine particles are attached to the surface of Si-containing particles. From the viewpoint of suppressing volume changes of Si, it is preferable that Si nanoparticles are dispersed inside a carbon material, and that Si nanoparticles are dispersed in the pores of a porous carbon material, and it is more preferable that Si nanoparticles are dispersed in the pores of a porous carbon material. As an example of Si-containing particles, for example, particles of Si-C composite material can be used. Si-C composite material typically contains carbon domains and Si-containing domains.

[0035] Carbon domains are, for example, carbonized forms of carbon precursors (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite, etc. Preferably, carbon domains constitute a carbon matrix. Therefore, a Si-C composite material is preferably a material in which multiple Si-containing domains are dispersed within a carbon matrix. In this case, the carbon matrix is ​​advantageous because it can mitigate volume changes due to the expansion / contraction of the Si-containing domains.

[0036] The Si-containing domain contains Si, for example, Si, Si oxide (SiO x It is composed of Si, Si nitride (SiNx), Si carbide (SiCx), etc. The Si-containing domain is preferably Si and Si oxide (SiO x It consists of at least one of the following. The Si-containing domains may be fine particles. The oxygen content in the Si-containing domains is preferably 10% by mass or less.

[0037] The average particle diameter of the Si-containing domains is, for example, 50 nm or less, and may be between 5 nm and 50 nm. The "average particle diameter of the Si-containing domains" can be determined as follows: First, the negative electrode active material layer 64 is processed with FIB (Focused Ion Beam) to prepare a sample for scanning transmission electron microscopy (STEM) observation. Then, the sample is subjected to elemental analysis by EDX elemental mapping, and BF images (bright-field images) and HAADF images (high-angle scattering annular dark-field images) are obtained. The diameter of the Si-containing domains can be determined from the contrast and shape obtained from the BF and HAADF images. The diameters of 10 or more arbitrarily selected Si-containing domains are determined, and their average value is taken as the "average particle diameter of the Si-containing domains" here.

[0038] The first Si-containing particles 14 and the second Si-containing particles 18 can be manufactured according to known methods. Various methods for manufacturing particles of Si-C composite materials are known (see, for example, Japanese Patent Publication No. 2015-38862, International Publication No. 2014 / 046144, and the prior art documents cited in said International Publication).

[0039] In this specification, the first Si-containing particle 14 and the second Si-containing particle 18 are defined by their reaction area (hereinafter sometimes referred to as capacitance) and Si content. These values ​​are independent of each other and can indicate the degree of expansion of the Si-containing particles before and after charging and discharging from different perspectives.

[0040] The reaction area of ​​the negative electrode active material can indicate the degree of expansion caused by the region where the chemical reaction is actually occurring. For example, the reaction area of ​​the negative electrode active material (first graphite particles and first Si-containing particles) in the upper layer can be derived using the following procedure. The negative electrode composite paste for the upper layer, described later, is applied to a 10 μm thick Cu foil, dried, and pressed and processed to the desired thickness and dimensions to obtain a negative electrode plate. Leads are attached to the negative electrode plate and laminated with separators in between to produce an electrode body containing first graphite particles and first Si-containing particles. The produced electrode body is inserted into an outer casing made of aluminum laminate sheet, then a non-aqueous electrolyte is injected, and the opening of the outer casing is sealed to produce a negative electrode / negative electrode symmetric cell. Subsequently, the impedance of the produced negative electrode / negative electrode symmetric cell is measured in a 25°C environment, and the capacitance is derived from the measured value. Then, the reaction area [F / g] of the negative electrode active material can be derived using the following equation (1). Since this reaction area is measured based on the resistance value in the actual charge-discharge reaction, it is not a value that depends on the surface area of ​​the negative electrode active material, but can be rephrased as the region where the chemical reaction is actually occurring. Furthermore, by changing the composition of the composite paste that forms the negative electrode active material layer, it is also possible to derive the reaction area of ​​individual graphite particles or individual Si-containing particles. Reaction area = (Capacitance) / (Amount of negative electrode active material added) Equation (1)

[0041] When comparing secondary batteries using silicon or carbon as the negative electrode active material, it is known that the theoretical capacity is more than 10 times higher when silicon is used. This suggests that the reaction area derived from Si material tends to be larger than that derived from carbon. Furthermore, when Si material is used as the negative electrode active material, high-capacity secondary batteries can be realized. In lithium-ion secondary batteries, an SEI film is formed when the electrolyte comes into contact with the active material surface during the initial charge. As the accumulation of the SEI film promotes expansion of the negative electrode, the smaller the reaction area of ​​the negative electrode active material, the more the expansion of the negative electrode during charging and discharging can be suppressed. Therefore, silicon expands in volume more during charging than carbon, and it is more difficult for it to return to its original volume during discharging. For this reason, when Si material is used as the negative electrode active material, expansion of the negative electrode during charging and discharging is likely to occur.

[0042] Next, in this specification, the Si content Q refers to the weight percentage concentration of silicon when the total weight of the Si-containing particles is taken as 100% by weight, and it maintains a constant value even if the particles deform during charging and discharging. Furthermore, from a different perspective than the reaction area described above, it can indicate the degree of expansion of the negative electrode before and after charging. The relationship between the Si content and the reaction area will be explained below.

[0043] As mentioned above, the reaction area can be rephrased as the region where the chemical reaction is actually taking place. For example, if the electrolyte does not easily penetrate the Si-containing particles due to the influence of internal structures such as voids within the Si-containing particles, the reaction area may be relatively small even if the amount of Si is relatively large.

[0044] Furthermore, even with Si-containing particles that have a relatively high Si content and a relatively small reaction area, as described above, the reaction area may increase as the Si-containing particles deform during charging and discharging, changing the impregnation properties of the electrolyte. In other words, unlike the Si content, the reaction area may fluctuate due to the deformation of the particles during charging and discharging.

[0045] Here, we will explain the anode swelling of secondary batteries using lithium-ion secondary batteries as an example. In lithium-ion secondary batteries as a whole, reversible chemical reactions mainly occur as lithium ions move back and forth between the positive and negative electrodes during charging and discharging. However, reactions other than those related to charging and discharging (defined as side reactions in this specification) also occur, and these side reactions can be a factor in anode swelling. Therefore, it can be said that when the reaction area of ​​Si-containing particles is relatively small and the amount of Si is relatively small, side reactions are less likely to occur in the anode and swelling is suppressed. Note that the above explanation of the mechanism of action of this technology is a hypothesis and does not limit this technology.

[0046] (2) Suppression of negative electrode plate expansion When a secondary battery is charged and discharged, the negative electrode active material near the negative electrode current collector is affected by the expansion of the negative electrode active material located closer to the surface, and its expansion tends to be inhibited. Conversely, the negative electrode active material located near the surface of the negative electrode active material layer has fewer factors that inhibit its expansion. Furthermore, since particles do not move easily in the lower layer 64b, the interruption of the conductive path during charging and discharging is suppressed. For this reason, the swelling of the negative electrode due to the interruption of the conductive path (such as swelling due to uneven battery reaction and localized concentration of reaction and stress) is easily suppressed in the lower layer 64b. In other words, in a negative electrode active material layer 64 with a two-layer structure, the upper layer is the one that expands the most when the secondary battery is repeatedly charged and discharged. Below, the suppression of negative electrode plate expansion will be explained from the perspective of the reaction area and Si content of Si-C-containing particles in the upper layer 64a and lower layer 64b, the average particle diameter, the content, and the thickness T of the upper layer 64a and lower layer 64b.

[0047] (A) Reaction area A and Si amount Q In the negative electrode active material layer 64 according to this disclosure, the reaction area A1 of the negative electrode active material in the upper layer 64a is smaller than the reaction area A2 of the negative electrode active material in the lower layer 64b, and the amount of Si Q1 of the first Si-containing particles 14 in the upper layer is less than the amount of Si Q2 of the second Si-containing particles 18 in the lower layer. As a result, swelling of the upper layer 64a, which is relatively prone to swelling, can be suppressed, and thus the swelling of the entire negative electrode active material layer 64 can be efficiently suppressed. On the other hand, the lower layer 64b, which is relatively less prone to swelling, uses second Si-containing particles 18 with a relatively large reaction area and Si amount. As a result, while suppressing the swelling of the entire negative electrode active material layer 64, it is possible to significantly suppress the swelling of the negative electrode 60 when repeated charging and discharging is performed, while ensuring sufficient capacity for the entire negative electrode active material layer 64.

[0048] 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 preferably 0.6 or higher, more preferably 0.65 or higher, and particularly preferably 0.7 or higher, from the viewpoint of increasing the capacity of the secondary battery. On the other hand, from the viewpoint of preventing conductive path breakage during charging and discharging and suppressing negative electrode swelling, it is preferably less than 1.0, more preferably 0.9 or lower, and particularly preferably 0.8 or lower.

[0049] The ratio (Q1 / Q2) of the Si content Q1 of the first Si-containing particles to the Si content Q2 of the second Si-containing particles is preferably 0.4 or higher, more preferably 0.5 or higher, and particularly preferably 0.6 or higher, from the viewpoint of increasing the capacity of the secondary battery. On the other hand, from the viewpoint of preventing conductive path breakage during charging and discharging and suppressing negative electrode swelling, it is preferably less than 1.0, more preferably 0.8 or lower, and particularly preferably 0.7 or lower.

[0050] From the viewpoint of increasing the capacity of the secondary battery, the Si content Q1 of the first Si-containing particles 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 negative electrode swelling, it is preferably 55% by mass or less, more preferably 50% by mass or less, and particularly preferably 45% by mass or less.

[0051] From the viewpoint of increasing the capacity of the secondary battery, the Si content Q2 of the second Si-containing particles 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 negative electrode swelling, it is preferably 80% by mass or less, more preferably 70% by mass or less, and particularly preferably 60% by mass or less.

[0052] (B) Average particle diameter M As described above, this specification suppresses negative electrode swelling by defining the reaction area A and Si content Q of the Si-containing particles. Therefore, the particle diameters of the first Si-containing particles 14 and the second Si-containing particles 18 in this disclosure are not particularly limited. The average particle diameter of the Si-containing particles can be measured by the same method as the method for measuring the average particle diameter described above. 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 even more preferably 4 μm to 7 μm.

[0053] Furthermore, it is preferable that the particle size of the Si-containing particles is adjusted in consideration of its relationship with the particle size of the graphite particles. For example, the ratio of the average particle size of the first graphite particles 12 to the average particle size 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 performance, 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, even more preferably 1.2 to 3.0, and particularly preferably 1.4 to 2.5.

[0054] 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 performance, 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, even more preferably 1.2 to 3.0, and particularly preferably 1.4 to 2.5.

[0055] (C) Content 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.

[0056] The content of the negative electrode 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, 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.

[0057] The mass ratio N1 of the first Si-containing particles 14 to the content of the negative electrode active material in the upper layer 64a (i.e., the total mass of the first graphite particles 12 and the first Si-containing particles 14) is preferably 10% by mass or more, and particularly preferably 15% by mass or more, from the viewpoint of increasing the capacity of the secondary battery. On the other hand, from the viewpoint of suppressing swelling of the negative electrode plate, it is preferably 60% by mass or less, preferably 40% by mass or less, and more preferably 20% by mass or less.

[0058] The mass ratio N2 of the second Si-containing particles 18 to the content of the negative electrode active material in the lower layer 64b (i.e., the total mass of the second graphite particles 16 and the second Si-containing particles 18) is preferably 10% by mass or more, and particularly preferably 15% by mass or more, from the viewpoint of high capacity secondary batteries. 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.

[0059] The negative electrode active material contained in the upper layer 64a may consist only of the first graphite particles 12 and the first Si-containing particles 14. However, the upper layer 64a may further contain negative electrode active material other than the first graphite particles 12 and the first Si-containing particles 14, within a range that does not impede the effects of the present invention (for example, 10% by mass or less of the total amount of negative electrode active material contained in the upper layer 64a).

[0060] The negative electrode active material contained in the lower layer 64b may consist only of the second graphite particles 16 and the second Si-containing particles 18. However, the lower layer 64b may further contain negative electrode active material other than the second graphite particles 16 and the second Si-containing particles 18, within a range that does not impede the effects of the present invention (for example, 10% by mass or less of the total amount of negative electrode active material contained in the lower layer 64b).

[0061] (D) Thickness T of the upper and lower layers In this specification, the thickness of the upper layer refers to the maximum thickness of the negative electrode active material layer containing only first graphite particles and first Si-containing particles as the negative electrode active material. Similarly, the thickness of the lower layer refers to the maximum thickness of the negative electrode active material layer containing only second graphite particles and second Si-containing particles as the negative electrode active material. In one preferred embodiment of the negative electrode according to one embodiment, the thickness of the upper layer 64a of the multilayer negative electrode active material layer is formed to be relatively thin. This effectively suppresses swelling in the upper layer 64a, and as a result, it is possible to suppress swelling of the entire negative electrode 60.

[0062] On the other hand, the lower layer 64b is formed to be relatively thick. As described above, the lower layer 64b is relatively resistant to swelling. By making this lower layer 64b relatively thicker, the overall swelling of the negative electrode 60 can be suppressed. In addition, since the lower layer 64b contains high-capacity second Si-containing particles 18, the capacity of the secondary battery can be increased by making this lower layer 64b thicker. As a result, the entire negative electrode active material layer 64 can significantly suppress the swelling of the negative electrode 60 when the secondary battery is repeatedly charged and discharged, and increase the capacity of the secondary battery.

[0063] The ratio of the thickness of the lower layer T2 to the thickness of the upper layer T1 (T1:T2) is preferably 10:90 to 90:10, more preferably 10:90 to 50:50, and particularly preferably 20:80 to 40:60, from the viewpoint of suppressing swelling of the negative electrode plate.

[0064] (3) Manufacturing of the negative electrode The negative electrode 60 can be suitably manufactured by a manufacturing method comprising, for example, the steps of: preparing a lower layer negative electrode composite paste by mixing second graphite particles 16 and second Si-containing particles 18 in a dispersion medium; preparing an upper layer negative electrode composite paste by mixing first graphite particles 12 and first Si-containing particles 14 in a dispersion medium; coating the lower layer negative electrode composite paste onto the negative electrode current collector 62 and drying it to form a lower layer 64b; coating the upper layer negative electrode composite paste onto the lower layer 64b and drying it to form an upper layer 64a; and pressing the formed upper layer 64a and lower layer 64b (hereinafter also referred to as the "pressing step").

[0065] In this specification, "paste" refers to a mixture in which some or all of the solid components are dispersed in a dispersion medium, and includes so-called "slurry," "ink," etc.

[0066] The lower layer negative electrode composite paste preparation step involves mixing the second graphite particles 16, the second Si-containing particles 18, and optional components (e.g., binder, conductive material, etc.) to prepare the lower layer paste. This step can be carried out by mixing with a dispersion medium (e.g., water) using known mixing equipment, stirring equipment, etc., according to known methods.

[0067] The upper layer negative electrode paste preparation step involves mixing the first graphite particles 12, the first Si-containing particles 14, and optional components (e.g., binder, conductive material, etc.) to prepare the upper layer paste. This step can be carried out by mixing with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method. The upper layer negative electrode paste preparation step may be carried out in parallel with the lower layer negative electrode paste preparation step. The upper layer negative electrode paste preparation step may be carried out in parallel with or after the lower layer formation step.

[0068] The lower layer formation process involves coating the negative electrode composite paste for the lower layer onto the negative electrode current collector 62 and drying it. This process can be carried out by coating the paste using a known coating apparatus and then drying it. This forms the lower layer (lower layer 64b).

[0069] The upper layer formation process involves coating the lower layer 64b with a negative electrode composite paste for the upper layer and drying it. This process can be carried out by coating the paste using a known coating apparatus and then drying it. This forms the upper layer (upper layer 64a) and the negative electrode active material layer 64.

[0070] The pressing process applies pressure to the formed upper and lower layers (i.e., the negative electrode active material layer 64) to compress them to a predetermined density. In this process, the upper and lower layers can be compressed to a predetermined density by applying pressure using a known pressing device, such as a roller press. The pressing process compresses the negative electrode active material layer 64 to a predetermined density, thereby densely filling it with negative electrode active material particles.

[0071] The density of the negative electrode active material layer 64 after pressing is not particularly limited, but for example, 0.7 g / cm³ 3 The above is preferable, preferably 1.0 g / cm³. 3 The above is more preferable, or 1.2 g / cm³. 3 That concludes the explanation. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm³. 3 The following is the case: 2.0 g / cm³ 3 The following is acceptable:

[0072] 2. Secondary battery The secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-described embodiment. According to the negative electrode 60 of this embodiment, swelling of the negative electrode 60 when the secondary battery is repeatedly charged and discharged can be suppressed. Furthermore, since the negative electrode 60 of this embodiment uses a negative electrode active material containing Si, the capacity of the secondary battery can be increased. One embodiment of the secondary battery disclosed herein will be described with reference to Figures 3 and 4. The following example configuration is a flat rectangular lithium-ion secondary battery having a flat wound electrode body and a flat battery case.

[0073] Figure 3 is a schematic diagram showing the configuration of a lithium-ion secondary battery constructed using a negative electrode according to one embodiment. The lithium-ion secondary battery 100 shown in Figure 3 is constructed by housing a flat-shaped wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 is also provided with an inlet (not shown) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight metal material with good thermal conductivity such as aluminum is used.

[0074] Figure 4 is a schematic exploded view showing the configuration of the wound electrode body of the lithium-ion secondary battery shown in Figure 3. As shown in Figures 3 and 4, the wound electrode body 20 consists of a positive electrode sheet 50 and a negative electrode sheet 60 superimposed on each other via two elongated separator sheets 70. The positive electrode sheet 50 and the negative electrode sheet 60 are wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated negative electrode current collector 62. The exposed positive electrode current collector portion 52a (i.e., the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed) and the exposed negative electrode current collector portion 62a (i.e., the portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed) are formed to protrude outward from both ends of the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction perpendicular to the longitudinal direction). The exposed positive electrode current collector portion 52a and the exposed negative electrode current collector portion 62a are joined to the positive electrode current collector portion 42a and the negative electrode current collector portion 44a, respectively.

[0075] 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 may be used, and examples include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0076] The dimensions of the positive electrode current collector 52 are not particularly limited and can be determined as appropriate according to the battery design. When aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 7 μm or more and 20 μm or less.

[0077] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known composition of positive electrode active material used in lithium-ion secondary batteries may be used. Specifically, for example, lithium composite oxides, lithium transition metal phosphate compounds, etc., can be used as the positive electrode active material. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, spinel structure, olivine structure, etc.

[0078] Preferably, lithium transition metal composite oxides are lithium transition metal composite oxides containing at least one of Ni, Co, and Mn as a transition metal element. Specific examples include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel-manganese-based composite oxides, lithium nickel-cobalt-manganese-based composite oxides, lithium nickel-cobalt-aluminum-based composite oxides, and lithium iron-nickel-manganese-based composite oxides.

[0079] In this specification, "lithium nickel cobalt manganese composite oxide" is a term that encompasses not only oxides whose constituent elements are Li, Ni, Co, Mn, and O, but also oxides that contain one or more additive elements other than these. Examples of such additive elements include transition metal elements and main group metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additive elements may also be metalloid elements such as B, C, Si, and P, or nonmetallic elements such as S, F, Cl, Br, and I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, lithium iron nickel manganese composite oxide, etc.

[0080] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0081] These positive electrode active materials may be used individually or in combination of two or more. Lithium nickel cobalt manganese composite oxides are particularly preferred as positive electrode active materials due to their excellent properties, such as initial resistance characteristics.

[0082] The average particle size of the positive electrode active material is not particularly limited, but is, for example, 0.05 μm or more and 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

[0083] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, conductive material, binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB); carbon fibers such as vapor-processed carbon fiber (VGCF) and carbon nanotubes (CNT); and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVdF), etc.

[0084] The content of positive electrode active material in the positive electrode active material layer 54 (i.e., the content of positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more and 99% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 15% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less. The content of conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.3% by mass or more and 15% by mass or less. The content of binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.4% by mass or more and 15% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less.

[0085] The thickness of the positive electrode active material layer 54 per side is not particularly limited, but is usually 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is usually 400 μm or less, and preferably 300 μm or less.

[0086] The negative electrode sheet 60 used is the negative electrode 60 described above.

[0087] Examples of the separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such porous sheets may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0088] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm. The air permeability obtained by the Gurley test method of the separator 70 is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0089] Non-aqueous electrolytes typically contain a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents commonly used in lithium-ion secondary battery electrolytes, such as carbonates, ethers, esters, nitriles, sulfones, and lactones, can be used without particular limitation. Among these, carbonates are preferred, with specific examples including 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), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used individually or in appropriate combinations of two or more. For example, the non-aqueous solvent may consist solely of carbonates. As another example, non-aqueous solvents include carbonates and esters such as methyl acetate.

[0090] Suitable supporting salts include lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI), preferably LiPF6. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0091] Furthermore, the above-mentioned non-aqueous electrolyte may contain components other than those described above, such as film-forming agents like vinylene carbonate (VC) and oxalat complexes; gas-generating agents like biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and other additives, as long as they do not significantly impair the effects of the present disclosure.

[0092] The lithium-ion secondary battery 100 has suppressed negative electrode swelling during repeated charging and discharging, and therefore exhibits low reaction force. Furthermore, the lithium-ion secondary battery 100 has high capacity. The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power supplies for vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium-ion secondary battery 100 can also be used as a storage battery for small-scale power storage devices. Typically, the lithium-ion secondary battery 100 can also be used in the form of a battery pack, where multiple batteries are connected in series and / or parallel.

[0093] The above describes a rectangular lithium-ion secondary battery 100 equipped with a flattened wound electrode body 20 as an example. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries equipped with a stacked electrode body (i.e., an electrode body in which multiple positive electrodes and multiple negative electrodes are stacked alternately). Furthermore, lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, laminated case type lithium-ion secondary batteries, and the like.

[0094] Furthermore, according to known methods, the lithium-ion secondary battery 100 can also be configured as an all-solid-state lithium-ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte.

[0095] Furthermore, while the negative electrode 60 according to this embodiment is suitable as the negative electrode of a lithium-ion secondary battery, it can be constructed and used as the negative electrode of other secondary batteries, and other secondary batteries can be constructed according to known methods.

[0096] ≪Rating≫ 1. Example Test The following describes test examples relating to the technology disclosed herein, but it is not intended that the technology disclosed herein is limited to these test examples.

[0097] (1) Example 1 [Creating the negative electrode] The reaction area of ​​the negative electrode active material disclosed below is a converted value when the reaction area A1 of the negative electrode active material in the upper layer of Example 1 is set to 100. First, a negative electrode composite paste for the lower layer was prepared. Specifically, graphite particles C and secondary Si-containing particles (Si content Q2: 60 wt%, particle size M2: 4 μm) were prepared as the negative electrode active material (reaction area A2: 140), SWCNT as the conductive material, and CMC, PAA, and SBR as binders. The materials were then weighed so that the mass ratio of each material was C:secondary Si-containing particles:SWCNT:CMC:PAA:SBR = 85:15:0.1:1:1:1.5. Of these, the raw materials other than SWCNT and SBR were dry-mixed, then SWCNT and the dispersion medium were mixed and kneaded into a solid paste, and then SBR and the dispersion medium were added and diluted and mixed to prepare a negative electrode composite paste for the upper layer. Furthermore, the above-mentioned solid mixing process requires optimizing the pressure load on the paste in order to coat the active material with a binder (CMC / PAA). To optimize this pressure load, the ideal solid content B0 of the paste was derived using the following equation (1). Note that the ideal solid content B0 of the paste is a value that depends on the conditions of the solid mixing process (e.g., shape of the stirring blade, rotation speed, stirring time, etc.). B0 = 100 - A0 = 100 / (100 + A1) × 100 Equation (1) B0: Ideal solid content rate [%] A0: Moisture content [%] at which the torque required for mixing is maximized. A1: Moisture content [mL] when the mixture is 100g

[0098] Next, a negative electrode mixture paste for the upper layer was prepared. The negative electrode mixture paste for the upper layer was prepared using the same method as the negative electrode mixture paste for the lower layer, except that first Si-containing particles (Si content Q1: 40 wt%, particle size M1: 7 μm) were used instead of second Si-containing particles, and the reaction area A1 was set to 100.

[0099] Next, the lower layer negative electrode composite paste was applied to the negative electrode core (copper foil, 10 μm) and dried. After drying, the upper layer negative electrode composite paste was applied to the lower layer negative electrode composite paste so that the ratio of the thickness T1 of the upper layer to the thickness T2 of the lower layer, T1:T2, was 50:50, and the mixture was dried to create a two-layer negative electrode active material layer on the negative electrode core (reaction area ratio A1 / A2 = 0.7, Si content ratio Q1 / Q2 = 0.7, particle size ratio = 1.8). Subsequently, the material was rolled by press working and processed to the specified dimensions to obtain a negative electrode plate.

[0100] [Creating the positive electrode] A cathode composite paste was prepared by weighing lithium nickel cobalt manganese composite oxide (NCM) as the cathode active material, polyvinylidene fluoride (PVdF) as the binder, and acetylene black (AB) as the conductive material, in a mass ratio of NCM:PVdF:AB = 100:1:1, and mixing them in N-methyl-2-pyrrolidone (NMP). This cathode composite paste was applied to a long, strip-shaped cathode core (aluminum foil, 15 μm thick) and dried. Subsequently, it was rolled by press processing and processed to the specified dimensions to obtain a cathode plate.

[0101] Leads were attached to the negative electrode and positive electrode, and the electrodes were stacked with separators in between to create an electrode body. The fabricated electrode body was inserted into an outer casing made of aluminum laminate sheet, a non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to create a test cell (laminate cell). The non-aqueous electrolyte was prepared by dissolving LiPF6 in a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:FEC:EMC:DMC = 15:5:40:40 to a concentration of 1 M.

[0102] (2) Example 2 A test cell was prepared in the same manner as in Example 1, except that the Si content Q1 = 45 for the first Si-containing particle, the Si content Q2 = 55 for the second Si-containing particle, and the Si content ratio Q1 / Q2 = 0.8.

[0103] (3) Example 3 A test cell was prepared in the same manner as in Example 1, except that the Si content Q1 = 20 for the first Si-containing particle, the Si content Q2 = 45 for the second Si-containing particle, and the Si content ratio Q1 / Q2 = 0.4.

[0104] (4) Example 4 A test cell was prepared in the same manner as in Example 1, except that the Si content Q1 = 55 for the first Si-containing particle, the Si content Q2 = 80 for the second Si-containing particle, and the Si content ratio Q1 / Q2 = 0.7.

[0105] (5) Example 5 A test cell was prepared in the same manner as in Example 1, except that the thickness ratio T1:T2 between the thickness of the upper layer T1 and the thickness of the lower layer T2 was set to 70:30.

[0106] (6) Example 6 A test cell was prepared in the same manner as in Example 1, except that the thickness ratio T1:T2 between the thickness of the upper layer T1 and the thickness of the lower layer T2 was set to 30:70.

[0107] (7) Example 7 A test cell was prepared in the same manner as in Example 1, except that the reaction area of ​​the negative electrode active material in the upper layer was A1 = 130, the reaction area of ​​the negative electrode active material in the lower layer was A2 = 140, and the ratio of the reaction areas A1 / A2 was 0.9.

[0108] (8) Comparative Example 1 A test cell was prepared in the same manner as in Example 1, except that the negative electrode active material layer consisted of a single layer containing first Si-containing particles and second Si-containing particles. The composition of the composite paste forming the negative electrode active material layer was adjusted so that the mass ratio was C:second Si-containing particles:first Si-containing particles:SWCNT:CMC:PAA:SBR = 85:7.5:7.5:0.1:1:1:1.5.

[0109] (9) Comparative Example 2 A test cell was prepared in the same manner as in Example 1, except that a paste containing the first Si-containing particles was applied to the lower layer and a paste containing the second Si-containing particles was applied to the upper layer.

[0110] (10) Comparative Example 3 A test cell was prepared in the same manner as in Example 1, except that the reaction area of ​​the negative electrode active material in the upper layer was A1 = 140, the reaction area of ​​the negative electrode active material in the lower layer was A2 = 100, and the ratio of the reaction areas A1 / A2 was 1.4.

[0111] 2. Evaluation Test (1) Measurement of the reaction areas A1 and A2 of the negative electrode active material in the upper and lower layers. The aforementioned upper layer negative electrode composite paste was applied to a 10 μm thick Cu foil, dried, and pressed to a predetermined thickness. After processing to the predetermined dimensions, a negative electrode plate was obtained. Leads were attached to the negative electrode plate, and the plates were laminated with separators in between to create an electrode body containing negative electrode active material (first graphite particles and first Si-containing particles). The fabricated electrode body was inserted into an outer casing made of aluminum laminate sheet, a non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to create a negative electrode / negative electrode symmetric cell. Subsequently, the impedance of the fabricated negative electrode / negative electrode symmetric cell was measured in a 25°C environment, and the capacitance was derived from the measured values. Then, the reaction area of ​​the negative electrode active material in the upper layer was derived from the following equation (1). The reaction area of ​​the negative electrode active material in the lower layer was also derived using the same method, except that a lower layer negative electrode composite paste was used. Reaction area = (Capacitance) / (Amount of negative electrode active material added) Equation (1)

[0112] (2) Evaluation of negative electrode plate expansion rate A test cell was prepared, and 250 cycles of CCCV charging (0.4C_4.2V_0.1C cut) - CC discharge (0.4C_2.5V cut) were performed in a 25°C environment. The negative electrode plate expansion rate was then derived from the following equation (2). Negative electrode plate expansion coefficient = {(thickness of test cell after 250 cycles) / (thickness of test cell before 250 cycles) - 1} × 100 Equation (2)

[0113] 3. Evaluation Results The test results for each sample are summarized in Tables 1 and 2.

[0114] [Table 1]

[0115] [Table 2]

[0116] From the above results, it was confirmed that in Examples 1 to 7, the negative electrode active material layer has a two-layer structure, and the upper layer uses particles that are relatively less prone to side reactions of Si-containing particles, resulting in a relatively low negative electrode plate expansion rate. Although Comparative Example 1 contained two types of Si-containing particles in the negative electrode active material layer, the negative electrode active material layer had a single-layer structure, and therefore the effect of suppressing the expansion rate of the negative electrode plate was not adequately achieved. In Comparative Examples 2 and 3, although the negative electrode active material layer has a two-layer structure, the upper layer uses particles that are relatively prone to side reactions involving Si-containing particles. As a result, the effect of reducing the negative electrode plate expansion coefficient was not successfully achieved.

[0117] Based on these results, it was confirmed that in order to reduce the expansion rate of the negative electrode plate, the negative electrode active material layer must consist of two layers, and the upper layer must be configured to be less prone to expansion.

[0118] As described above, this specification includes the disclosures set forth in the following sections. Section 1: Negative electrode current collector and Supported by the aforementioned negative electrode current collector, the negative electrode active material layer includes a negative electrode active material, A negative electrode for a secondary battery, The aforementioned negative electrode active material layer is The upper layer, which is located relatively on the surface side, The lower layer located relatively on the negative electrode current collector side and Equipped with, The negative electrode active material is at least Graphite particles and, Si-containing particles, which are a composite of carbon and Si. It contains, 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 amount of Si Q1 in the first Si-containing particles contained in the upper layer is less than the amount of Si Q2 in the second Si-containing particles contained in the lower layer. Negative electrode.

[0119] Section 2: 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.

[0120] Section 3: The negative electrode according to item 1 or 2, wherein the ratio Q1 / Q2 of the amount of Si in the first Si-containing particle to the amount of Si in the second Si-containing particle is 0.4 or more and less than 1.0.

[0121] Section 4: The negative electrode according to any one of items 1 to 3, wherein the 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.

[0122] Section 5: The Si content Q1 of the first Si-containing particle is 20-55% by mass. The negative electrode according to any one of items 1 to 4, wherein the Si content Q2 of the second Si-containing particle is 45 to 80 mass%.

[0123] Item 6: Positive electrode, negative electrode, electrolyte, A secondary battery equipped with The negative electrode is the negative electrode described in any one of items 1 to 5. Secondary battery. [Explanation of Symbols]

[0124] 12. First Graphite Particles 14 1st Si-containing particles 16. Second-order graphite particles 18 2nd Si-containing particles 20 Wound electrode body 30 Battery Cases 36 Safety valve 42 Positive terminal 42a Positive electrode current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheets (positive electrode) 52 Positive electrode current collector 52a Exposed part of positive electrode current collector 54 Cathode active material layer 60 Negative electrode sheets (negative electrode) 62 Negative electrode current collector 62a Exposed part of negative electrode current collector 64 Negative electrode active material layer 70 Separator Sheets (Separators) 100 Lithium-ion rechargeable batteries

Claims

1. Negative electrode current collector and Supported by the aforementioned negative electrode current collector, a negative electrode active material layer containing a negative electrode active material, A negative electrode for a secondary battery, The aforementioned negative electrode active material layer is The upper layer, which is located relatively on the surface side, The lower layer located relatively on the negative electrode current collector side and Equipped with, The negative electrode active material is at least Graphite particles and, Si-containing particles and It contains, 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 amount of Si Q1 in the first Si-containing particles in the upper layer is less than the amount of Si Q2 in the second Si-containing particles in the lower layer. Negative electrode.

2. The negative electrode according to claim 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.

3. The negative electrode according to claim 1, wherein the ratio Q1 / Q2 of the amount of Si in the first Si-containing particle to the amount of Si in the second Si-containing particle is 0.4 or more and less than 1.

0.

4. The negative electrode according to claim 1, wherein the 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.

5. The Si content Q1 of the first Si-containing particles is 20 to 55% by mass. The negative electrode according to claim 1, wherein the Si content Q2 of the second Si-containing particle is 45 to 80% by mass.

6. Positive electrode, negative electrode, electrolyte, A secondary battery equipped with The negative electrode is the negative electrode described in any one of claims 1 to 5. Secondary battery.

Images