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 larger 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 greater 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 driving vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] In the application of power sources for driving vehicles, particularly in the application of power sources for driving BEVs, from the viewpoint of extending the cruising range of vehicles, secondary batteries are desired to have a further higher capacity. 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] <舍

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Since the above-mentioned Si-containing particles have a relatively high capacity, the reaction resistance hardly increases even at low temperatures, and lithium precipitation hardly occurs. However, due to their high capacity, reactions other than the reactions accompanying charge and discharge (side reactions) are also likely to occur, and as a result, the capacity retention rate tends to decrease easily during storage. Therefore, in a lithium-ion secondary battery, it can be said that suppressing the increase in reaction resistance in a low-temperature environment and improving the capacity retention rate during storage are in a trade-off relationship with each other. [[ID=I44]]

[0006] In view of the above circumstances, the present disclosure aims to provide a negative electrode for a secondary battery containing Si-containing particles and graphite particles, which suppresses the increase in reaction resistance in low-temperature environments and improves the capacity retention rate during storage. [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, and 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 larger 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 greater 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, which is located further away from the current collector, is relatively more susceptible to chemical reactions. In the negative electrode disclosed herein, first Si-containing particles are used in the upper layer where chemical reactions are more likely to occur, and second Si-containing particles are used in the lower layer where chemical reactions are less likely to occur. Since the reaction area and Si content of the first Si-containing particles are larger than those of the second Si-containing particles, chemical reactions occur relatively more easily. In other words, first Si-containing particles, which are more susceptible to chemical reactions (superior in suppressing the increase in reaction resistance in low-temperature environments), are used in the upper layer where chemical reactions are more likely to occur (superior in improving the capacity retention rate during storage), and second Si-containing particles, which are less susceptible to chemical reactions (superior in improving the capacity retention rate during storage), are used in the lower layer where chemical reactions are less likely to occur (superior in improving the capacity retention rate during storage). Based on the above, a negative electrode for a secondary battery is provided, which has a negative electrode active material layer in both the upper and lower layers that can suppress the increase in reaction resistance in low-temperature environments and improve the capacity retention rate during storage.

[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 greater than 1.0 and less than or equal to 1.6. This allows for a more significant suppression of the increase in reaction resistance at low temperatures by increasing the reaction area within the negative electrode active material layer on the surface side. Furthermore, by further reducing the reaction area within the negative electrode active material layer on the negative electrode current collector side, a more significant improvement in capacity retention during storage can be achieved.

[0010] In one preferred embodiment of the negative electrode disclosed herein, the ratio Q1 / Q2 of the Si content Q1 of the first S-containing particles to the Si content Q2 of the second Si-containing particles is greater than 1.0 and less than or equal to 3.0. This allows for a more significant suppression of the increase in reaction resistance at low temperatures by further increasing the Si content in the negative electrode active material layer on the surface side. Furthermore, by further reducing the Si content in the negative electrode active material layer on the negative electrode current collector side, a more significant improvement in capacity retention during storage can be achieved.

[0011] In a preferred embodiment of the negative electrode disclosed herein, the ratio T1:T2 of the thickness T1 of the upper layer to the thickness T2 of the lower layer is 10:90 to 90:10. This allows for a more significant suppression of the increase in reaction resistance at low temperatures by increasing the reaction area and the total amount of Si in the negative electrode active material layer on the surface side. Furthermore, by further reducing the reaction area and the total amount of Si in the negative electrode active material layer on the negative electrode current collector side, a more significant improvement in the capacity retention rate during storage can be achieved.

[0012] In one preferred embodiment of the negative electrode disclosed herein, the Si content Q1 of the first Si-containing particles is 45-70% by mass, and the Si content Q2 of the second Si-containing particles is 20-55% by mass. This allows for a more significant suppression of the increase in reaction resistance at low temperatures by further increasing the Si content in the negative electrode active material layer on the surface side. Furthermore, by further reducing the Si content in the negative electrode active material layer on the negative electrode current collector side, a more significant improvement in capacity retention during storage can be achieved.

[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] With this configuration, by using the above-mentioned negative electrode, it is possible to realize a secondary battery that suppresses the increase in reaction resistance in low-temperature environments and improves the capacity retention rate during storage. [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] In this specification, 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 (LIC). 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, conventionally known materials can be used.

[0018] The negative electrode disclosed herein is used in a secondary battery, preferably 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 on only 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, although it 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 current collection 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. Si-containing particles are particles in which carbon and Si are composited. 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 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 is composed 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 serve as indicators of reaction resistance under low-temperature conditions and capacity retention during storage from different perspectives.

[0040] The reaction area of ​​the negative electrode active material can indicate 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 the theoretical capacities of secondary batteries using silicon or carbon as the negative electrode active material, it is known that the capacity of batteries using silicon is more than 10 times higher. This suggests that the reaction area derived from Si material tends to be larger than that derived from carbon. Therefore, when Si material is used as the negative electrode active material, high-capacity secondary batteries can be realized.

[0042] In this specification, Si amount Q refers to the weight percentage concentration of silicon when the total weight of Si-containing particles is taken as 100% by weight. This Si amount Q remains constant even if the particles deform due to charging and discharging. Furthermore, Si amount Q can indicate the degree of suppression of reaction resistance under low-temperature conditions and the degree of improvement in capacity retention during storage, from a different perspective than the reaction area described above. The suppression of the increase in reaction resistance under low-temperature conditions and the improvement in capacity retention during storage will be explained below from the perspectives of reaction area, Si amount, and the reactivity of Si-containing particles in the negative electrode active material layer.

[0043] In lithium-ion secondary batteries, the primary reversible chemical reaction that occurs during charging and discharging involves lithium ions moving back and forth between the positive and negative electrodes. Reaction resistance is an indicator of how easily this chemical reaction occurs. Therefore, an increase in reaction resistance is more likely to occur in low-temperature environments where the viscosity of the electrolyte increases and the movement of lithium ions is suppressed. Furthermore, increasing the reaction area and amount of Si-containing particles can make the above chemical reaction easier to occur, thereby suppressing the increase in reaction resistance. On the other hand, lithium-ion secondary batteries also undergo reactions other than those related to charging and discharging (defined as side reactions in this specification), and these side reactions can contribute to a decrease in capacity retention during storage.

[0044] As described above, in low-temperature environments, the migration rate of Li ions decreases, and it takes a relatively long time for them to diffuse into the negative electrode active material layer. As a result, the concentration of Li ions increases near the surface of the negative electrode active material layer, which can easily lead to a decrease in reaction resistance. Therefore, when the negative electrode active material layer has a multilayer structure, the negative electrode active material located closer to the surface of the current collector may become more reactive. Thus, the negative electrode active material layer on the surface side of the current collector is advantageous in suppressing the increase in reaction resistance in low-temperature environments, but disadvantageous in improving the capacity retention rate during storage. Furthermore, as you move towards the current collector side, the above-mentioned superiority / inferiority relationship tends to reverse.

[0045] Based on the above, by increasing the reaction area of ​​the first Si-containing particles 14 used in the upper layer 64a (superior in suppressing the increase in reaction resistance under low temperature conditions) and increasing the amount of Si (superior in suppressing the increase in reaction resistance under low temperature conditions), the suppression of the increase in reaction resistance under low temperature conditions can be suitably achieved. Furthermore, by decreasing the reaction area of ​​the Si-containing particles 16 used in the lower layer 64b (superior in improving the capacity retention rate during storage) and decreasing the amount of Si (superior in improving the capacity retention rate during storage), the improvement in the capacity retention rate during storage can be suitably achieved. As a result, the negative electrode active material layer 64 as a whole can suitably achieve suppression of the increase in reaction resistance under low temperature conditions and improvement in the capacity retention rate during storage. 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 the increase in reaction resistance in low-temperature environments and improvement of the volume retention rate during storage. (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 larger 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 contained in the upper layer is greater than the amount of Si Q2 of the second Si-containing particles 18 contained in the lower layer. This makes it possible to suppress the increase in reaction resistance in the upper layer 64a, where chemical reactions are relatively less likely to occur, under low-temperature conditions. On the other hand, the lower layer 64b, where chemical reactions are relatively more likely to occur, uses second Si-containing particles 18 with a relatively small reaction area and Si amount. This makes it possible to improve the capacity retention rate during storage. As a result, the negative electrode active material layer as a whole can provide a negative electrode for a secondary battery that suppresses the increase in reaction resistance under low-temperature conditions and improves the capacity retention rate during storage.

[0047] The ratio (A1 / A2) of the reaction area A2 of the negative electrode active material in the lower layer to the reaction area A1 of the negative electrode active material in the upper layer is preferably greater than 1.0, more preferably 1.1 or greater, and particularly preferably 1.2 or greater, from the viewpoint of suppressing the increase in reaction resistance at low temperatures. On the other hand, from the viewpoint of improving the volume retention rate during storage, it is preferably 1.6 or less, more preferably 1.5 or less, and particularly preferably 1.4 or less.

[0048] The ratio of the amount of Si in the second Si-containing particle (Q1 / Q2) to the amount of Si in the first Si-containing particle (Q1) is preferably greater than 1.0, more preferably 1.2 or greater, and particularly preferably 1.5 or greater, from the viewpoint of suppressing the increase in reaction resistance in low-temperature environments. On the other hand, from the viewpoint of improving the volume retention rate during storage, it is preferably 3.0 or less, more preferably 2.5 or less, and particularly preferably 2.0 or less.

[0049] The Si content Q1 of the first Si-containing particles in the upper layer is preferably 45% by mass or more, more preferably 50% by mass or more, and particularly preferably 55% by mass or more, from the viewpoint of suppressing the increase in reaction resistance in low-temperature environments. On the other hand, from the viewpoint of improving the volume retention rate during storage, it is preferably 70% by mass or less, more preferably 65% ​​by mass or less, and particularly preferably 60% by mass or less.

[0050] The Si content Q2 of the secondary Si-containing particles in the lower layer is preferably 20% by mass or more, more preferably 30% by mass or more, and particularly preferably 40% by mass or more, from the viewpoint of suppressing the increase in reaction resistance in low-temperature environments. On the other hand, from the viewpoint of improving the volume retention rate during storage, it is preferably 55% by mass or less, more preferably 50% by mass or less, and particularly preferably 45% by mass or less.

[0051] (B) Average particle diameter M As described above, this specification defines the reaction area A and Si content Q of the Si-containing particles to suppress the increase in reaction resistance under low-temperature conditions and improve the volume retention rate during storage. 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.

[0052] 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.

[0053] 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.

[0054] (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.

[0055] 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.

[0056] 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 suppressing reaction resistance in low-temperature environments. On the other hand, from the viewpoint of improving the volume retention rate during storage, it is preferably 60% by mass or less, preferably 40% by mass or less, and more preferably 20% by mass or less.

[0057] 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 suppressing reaction resistance in low-temperature environments. On the other hand, from the viewpoint of improving the volume retention rate during storage, it is preferably 60% by mass or less, more preferably 40% by mass or less, and particularly preferably 20% by mass or less.

[0058] 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).

[0059] 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).

[0060] (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.

[0061] 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 successfully suppressing the increase in reaction resistance in low-temperature environments and improving the volume retention rate during storage. The effects of the present invention can be suitably realized by setting T1 and T2 to arbitrary values. For example, if the effect of suppressing the increase in reaction resistance in low-temperature environments is to be more pronounced, the thickness of the upper layer T1 can be made relatively thicker, or if the effect of improving the volume retention rate during storage is to be more pronounced, the thickness of the lower layer T2 can be made relatively thicker.

[0062] (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").

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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 64b.

[0067] 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 64a and the negative electrode active material layer 64.

[0068] 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.

[0069] 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:

[0070] 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. The negative electrode 60 according to this embodiment provides a negative electrode for a secondary battery that suppresses the increase in reaction resistance in low-temperature environments and improves the capacity retention rate during storage. 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

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

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] The negative electrode sheet 60 used is the negative electrode 60 described above.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] ≪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.

[0095] (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 A2 of the negative electrode active material in the lower 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: 40 wt%, particle size M2: 7 μm) were prepared as the negative electrode active material (reaction area A2: 100), 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 dispersion medium were mixed and kneaded into a solid paste, and then SBR and dispersion medium were added and diluted and mixed to prepare a negative electrode composite paste for the lower 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

[0096] 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: 60 wt%, particle size M1: 4 μm) were used instead of second Si-containing particles, and the reaction area A1 was set to 140 (a converted value when the reaction area A2 of the negative electrode active material in the lower layer is set to 100).

[0097] 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.

[0098] [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.

[0099] 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.

[0100] (2) Example 2 A test cell was prepared in the same manner as in Example 1, except that the reaction area A1 = 140 for the first Si-containing particle, the reaction area A2 = 130 for the second Si-containing particle, and the reaction area ratio A1 / A2 was 1.1.

[0101] (3) Example 3 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 = 45 for the second Si-containing particle, and the Si content ratio Q1 / Q2 was 1.2.

[0102] (4) Example 4 A test cell was prepared in the same manner as in Example 1, except that the Si content Q1 = 45 in the first Si-containing particle, the Si content Q2 = 20 in the second Si-containing particle, and the Si content ratio Q1 / Q2 = 2.3.

[0103] (5) Example 5 A test cell was prepared in the same manner as in Example 1, except that the Si content Q1 = 70 for the first Si-containing particle, the Si content Q2 = 55 for the second Si-containing particle, and the Si content ratio Q1 / Q2 = 1.3.

[0104] (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 70:30.

[0105] (7) Example 7 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.

[0106] (8) Comparative Example 1 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.

[0107] (9) Comparative Example 2 A test cell was prepared in the same manner as in Example 1, except that the negative electrode active material layer was 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.

[0108] (10) Comparative Example 3 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 Si-containing particles (reaction area: 100, Si content: 55).

[0109] (11) Comparative Example 4 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 Si-containing particles (reaction area: 140, Si content: 55).

[0110] 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 prepared 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 prepared negative electrode / negative electrode symmetric cell was measured in a 25°C environment, and the capacitance was measured 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)

[0111] (2) Evaluation of volume retention rate during storage After adjusting the test cells prepared using the above procedure to a SOC of 90%, they were stored at 60°C for 8 weeks. Volume measurements were then performed, and the volume retention rate was evaluated using the following formula (2). Capacity retention rate = (Volume of test cell after 8 weeks of storage / Volume of test cell before storage begins) × 100 Equation (2)

[0112] (3) Evaluation of reaction resistance under low-temperature conditions The test cell prepared using the above procedure was kept at -10°C for 24 hours. Afterward, impedance measurements were performed under the same conditions, and the reaction resistance was calculated from the diameter of the arc in the Nyquist plot.

[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 Examples 1 to 7 have a two-layer structure for the negative electrode active material layer, with the upper layer using particles that have a large reaction area and a high Si content, and the lower layer using particles that have a small reaction area and a low Si content. From this, it was confirmed that the negative electrode active material layer as a whole has low reaction resistance at low temperatures and high capacity retention during storage. Comparative Example 1 contains two types of Si-containing particles in the negative electrode active material layer, but the upper layer uses particles with a small reaction area and low Si content, while the lower layer uses particles with a large reaction area and high Si content. As a result, it was confirmed that neither the suppression of the increase in reaction resistance at low temperatures nor the improvement of the volume retention rate during storage was achieved. Although Comparative Example 2 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 increase in reaction resistance at low temperatures was not adequately achieved. Comparative Examples 3 and 4 contained one type of Si-containing particles in the negative electrode active material layer, and the negative electrode active material layer had a single-layer structure. As a result, neither the suppression of the increase in reaction resistance at low temperatures nor the improvement of the capacity retention rate during storage was favorably achieved.

[0117] Based on the above results, it was confirmed that in order to provide a negative electrode for secondary batteries that suppresses the increase in reaction resistance in low-temperature environments and improves capacity retention, it is necessary to have two negative electrode active material layers, with Si-containing particles having a relatively large reaction area and a relatively high Si content used in the upper layer.

[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, 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, 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 larger 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 greater than the amount of Si Q2 in the second Si-containing particles 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 greater than 1.0 and less than or equal to 1.6.

[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 greater than 1.0 and less than or equal to 3.0.

[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 particles is 45-70% 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 20 to 55 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 larger 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 greater than the amount of Si Q2 in the second Si-containing particles contained 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 greater than 1.0 and less than or equal to 1.

6.

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 greater than 1.0 and less than or equal to 3.

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 45 to 70% by mass. The negative electrode according to claim 1, wherein the Si content Q2 of the second Si-containing particle is 20 to 55% 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.

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