WO2024048732A1

WO2024048732A1 - Secondary battery negative electrode, secondary battery, and method for manufacturing secondary battery negative electrode

Info

Publication number: WO2024048732A1
Application number: PCT/JP2023/031854
Authority: WO
Inventors: 洋輝中土; 敬光田下; 晶大加藤木; 諒玉川; 幸代金子
Original assignee: パナソニックエナジー株式会社
Priority date: 2022-08-31
Filing date: 2023-08-31
Publication date: 2024-03-07

Abstract

This secondary battery negative electrode comprises a negative electrode mixture layer of thickness T that is supported on a negative electrode current collector. A negative electrode active material in the negative electrode mixture layer contains graphite particles. The graphite particles contain graphite particles A having an internal porosity greater than 5%, and graphite particles B having internal porosity of 5% or less. The negative electrode mixture layer has a first layer, a second layer, and a third layer in order from the negative electrode current collector side. The second layer includes a 2a-th layer and a 2b-th layer disposed respectively in a first region and a second region of the first layer. The proportions (mass%) of the graphite particles B in the total of the graphite particles A and the graphite particles B in the first layer, 2a-th layer, 2b-th layer, and third layer are MB1, MB2a, MB2b, and MB3, respectively, where MB1, MB2a, MB2b, and MB3 are such that 0 ≤ MB1 ≤ MB2a < 50 < MB2b ≤ MB3 ≤ 100. The thickness of the third layer is greater than 0.05T.

Description

Secondary battery negative electrode, secondary battery, and manufacturing method of secondary battery negative electrode

The present disclosure relates to a negative electrode for a secondary battery, a secondary battery, and a method for manufacturing a negative electrode for a secondary battery.

Nonaqueous electrolyte secondary batteries, especially lithium ion secondary batteries, have high voltage and high energy density, and are widely used as power sources for small electronic devices such as mobile devices.

Patent Document 1 discloses an electrode active material layer for a non-aqueous electrolyte secondary battery formed on a current collector, in which the porosity is graded or continuous in the thickness direction from the current collector side to the surface side. An electrode active material layer for nonaqueous electrolyte secondary batteries has been proposed in which the degree of porosity gradient in the thickness direction is 3.5 to 4.0%/10 μm. .

Japanese Patent Application Publication No. 2015-037008

There is a need for improved rapid charging characteristics and cycle characteristics for secondary batteries.

In view of the above, one aspect of the present disclosure includes a negative electrode mixture layer containing a negative electrode active material, and a negative electrode current collector supporting the negative electrode mixture layer, the negative electrode active material containing graphite particles, The graphite particles include graphite particles A having an internal porosity of more than 5% and graphite particles B having an internal porosity of 5% or less, and the negative electrode mixture layer is arranged on the side of the negative electrode current collector. The second layer includes a second layer, a second layer, and a second layer, which are arranged in the first region and the second region of the first layer, respectively, in this order. The ratio (mass%) of the graphite particles B to the total of the graphite particles A and the graphite particles B in the first layer, the second a layer, the second b layer, and the third layer. , respectively, where MB ₁ , MB _2a , MB _2b _, and _{MB 3} _are 0≦MB ₁ ≦MB _2a _< ₅₀ < The present invention relates to a negative electrode for a secondary battery that satisfies the relationship MB _2b ≦MB ₃ ≦100, and where the thickness of the negative electrode mixture layer is T, the thickness of the third layer is greater than 0.05T.

Another aspect of the present disclosure relates to a secondary battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte, where the negative electrode is the negative electrode for a secondary battery described above.

Yet another aspect of the present disclosure includes a first step of preparing a negative electrode slurry containing graphite particles, and a second step of applying the negative electrode slurry to the surface of a negative electrode current collector and drying it to form a negative electrode mixture layer. and a third step of compressing the laminate of the negative electrode mixture layer and the negative electrode current collector, and in the first step, as the negative electrode slurry, a first negative electrode slurry, a second negative electrode slurry, and a third negative electrode slurry are compressed. 3 negative electrode slurry is prepared, and in the second step, the first negative electrode slurry, the second negative electrode slurry, and the third negative electrode slurry are applied to the surface of the negative electrode current collector in this order, and the negative electrode slurry is applied to the surface of the negative electrode current collector. The negative electrode mixture layer is formed of a first layer, a second layer, and a third layer in this order from the side, and the second negative electrode slurry includes a 2a negative electrode slurry and a 2b negative electrode slurry, and the In the step of forming two layers, a 2a negative electrode slurry and a 2b negative electrode slurry are applied to a first region and a second region of the first negative electrode slurry application region, respectively, and a 2a layer and a 2b negative electrode slurry are formed as the second layer. the first negative electrode slurry; The proportion (mass %) of the graphite particles B in the total of the graphite particles A and the graphite particles B in the second a negative electrode slurry, the second b negative electrode slurry, and the third negative electrode slurry is MB ₁ , When MB _2a , MB _2b , and MB ₃ are MB ₁ , MB _2a , MB _2b , and MB ₃ , 0≦MB ₁ ≦MB _2a <50<MB _2b ≦MB ₃ ≦ The present invention relates to a method for manufacturing a negative electrode for a secondary battery, in which the third layer has a thickness greater than 0.05T, where the relationship of 100 is satisfied and the thickness of the negative electrode mixture layer is T.

According to the present disclosure, the rapid charging characteristics and cycle characteristics of a secondary battery can be improved.

While the novel features of the invention are set forth in the appended claims, the invention is further understood by the following detailed description, taken together with the drawings, both as to structure and content, as well as other objects and features of the invention. It will be well understood.

FIG. 1 is a cross-sectional view schematically showing an example of a negative electrode for a secondary battery according to an embodiment of the present disclosure. It is a figure which shows an example of the arrangement pattern of the 2nd a layer and the 2nd b layer. It is a figure which shows the modification of the arrangement pattern of the 2nd a layer and the 2nd b layer. It is a figure which shows another modification of the arrangement pattern of the 2nd a layer and the 2nd b layer. It is a figure which shows yet another modification of the arrangement pattern of the 2nd a layer and the 2nd b layer. FIG. 2 is a cross-sectional view schematically showing graphite particles that are a negative electrode active material. FIG. 1 is a partially cutaway schematic perspective view of a secondary battery according to an embodiment of the present disclosure.

Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values and materials may be illustrated, but other numerical values and materials may be applied as long as the effects of the present disclosure can be obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "more than or equal to numerical value A and less than or equal to numerical value B." In the following explanation, when lower limits and upper limits are given as examples for numerical values such as specific physical properties or conditions, any of the illustrated lower limits and any of the illustrated upper limits can be arbitrarily combined, as long as the lower limit is not greater than the upper limit. . When a plurality of materials are exemplified, one type may be selected from them and used alone, or two or more types may be used in combination.

Furthermore, the present disclosure includes combinations of matters recited in two or more claims arbitrarily selected from a plurality of claims recited in the appended claims. In other words, unless a technical contradiction occurs, matters described in two or more claims arbitrarily selected from the plurality of claims described in the appended claims can be combined.

(Negative electrode for secondary batteries)
A negative electrode for a secondary battery according to an embodiment of the present disclosure includes a negative electrode mixture layer containing a negative electrode active material and a negative electrode current collector supporting the negative electrode mixture layer. The negative electrode active material includes graphite particles, and the graphite particles include graphite particles A whose internal porosity is greater than 5% and graphite particles B whose internal porosity is 5% or less. The negative electrode mixture layer includes, in order from the negative electrode current collector side, a first layer, a second layer, and a third layer. The second layer includes a 2a layer and a 2b layer arranged in the first region and the second region of the first layer, respectively. The first region and the second region do not overlap with each other. The proportion (mass%) of graphite particles B in the graphite particles (total of graphite particles A and graphite particles B) in the first layer, the second a layer, the second b layer, and the third layer is MB ₁ and MB, respectively. _2a , MB _2b , and MB ₃ . At this time, MB ₁ , MB _2a , MB _2b , and MB ₃ satisfy the relationship 0≦MB ₁ ≦MB _2a <50<MB _2b ≦MB ₃ ≦100. When the thickness of the negative electrode mixture layer is T, the thickness of the third layer is greater than 0.05T.

By using a negative electrode for a secondary battery that satisfies the above configuration, the cycle characteristics and rapid charging characteristics of the secondary battery can be improved.

A layer 2a and a layer 2b are respectively formed as second layers between the first layer and the third layer. The second a layer (the first region of the first layer) may be divided into a plurality of parts and arranged. The 2b layer (the second region of the first layer) may be divided into a plurality of parts and arranged. The second a layer (the first region of the first layer) and the second b layer (the second region of the first layer) may be arranged in an alternating pattern. One of the 2A layer and the 2B layer (the first region and the second region of the first layer) may be dispersed and arranged in the form of islands in the other sea. When the second layer is viewed from the normal direction of the main surface of the negative electrode current collector, the ratio of the total area of the 2b layer to the total area of the 2a layer and the 2b layer is, for example, It may be 10% or more and 75% or less, 40% or more and 55% or less, or about 50%.

The first layer is the layer disposed closest to the negative electrode current collector among the negative electrode mixture layers. The third layer is a layer disposed closest to the surface of the negative electrode among the negative electrode mixture layers. The second layer is a layer placed between the first layer and the third layer. The negative electrode mixture layer may further include another layer (fourth layer) other than the first to third layers as long as the effects of the first to third layers are not impaired. The fourth layer may be arranged between the first layer and the second layer, for example. Such a fourth layer may be, for example, a layer that is composed of one layer, unlike the second layer, and, unlike the first layer, the ratio of graphite particles B to the graphite particles is more than 50% by mass. Can be mentioned. The thickness T of the negative electrode mixture layer is the total thickness of the first layer, the second layer, and the third layer, and when the negative electrode mixture layer includes the fourth layer, the thickness T of the negative electrode mixture layer excluding the fourth layer. It means thickness.

0≦MB ₁ ≦MB _2a <50, and the first layer and the second a layer are layers in which the ratio of graphite particles A to graphite particles is large. 50<MB _2b ≦MB ₃ ≦100, and the third layer and the second b layer are layers in which the ratio of graphite particles B to graphite particles is large.

Graphite particles A are easily deformed when the negative electrode is compressed, and in the first layer containing a large amount of graphite particles A, the voids between the particles are small. By arranging such a first layer on the negative electrode current collector side, the adhesion between the negative electrode mixture layer and the negative electrode current collector is improved, and the cycle characteristics are improved.
Furthermore, a part of the second layer is constituted by the second a layer. The 2nd a layer contains many graphite particles A, which are more flexible and easily deformed than graphite particles B. By arranging such a second a layer inside the negative electrode mixture layer, the stress generated in the negative electrode mixture layer due to expansion and contraction of the negative electrode active material during charging and discharging is alleviated, and the cycle characteristics are improved.

Graphite particles B are hard and are not easily deformed when the negative electrode is compressed, and in the third layer containing a large amount of graphite particles B, the voids between the particles are large, making it easy to form a diffusion path for the electrolyte. By arranging such a third layer on the surface side of the negative electrode (negative electrode mixture layer), the electrolyte can easily penetrate into the negative electrode mixture layer.
Furthermore, a part of the second layer is constituted by the second b layer. The 2nd b layer contains many graphite particles B, and the voids between the particles are large. By arranging such a second b layer inside the negative electrode mixture layer, the electrolyte can easily diffuse into the interior from the surface of the negative electrode mixture layer, and the electrolyte permeability of the entire negative electrode (negative electrode mixture layer) is greatly improved. This improves quick charging characteristics.

When 0≦MB ₁ ≦MB _2a <50, the above-mentioned effects due to the arrangement of the first layer and the second a layer can be efficiently obtained. When 50<MB _2b ≦MB ₃ ≦100 is satisfied, the above effects due to the arrangement of the second b layer and the third layer can be efficiently obtained.
However, if the thickness of the third layer is 0.05T or less, the effect of forming the third layer may be insufficient and the rapid charging characteristics may deteriorate.

From the viewpoint of improving cycle characteristics, MB _2a is 0% by mass or more and less than 50% by mass, preferably 0% by mass or more and 40% by mass or less. From the viewpoint of improving rapid charging characteristics, MB _2b is more than 50% by mass and less than 100% by mass, preferably more than 60% by mass and less than 100% by mass. From the viewpoint of improving cycle characteristics and rapid charging characteristics, MB _2a /MB _2b is preferably 0 or more and 2/3 or less, more preferably 0 or more and 1/4 or less.

The first layer (or the first layer and the second a layer) may contain only graphite particles A as graphite particles. The third layer (or the third layer and the second b layer) may contain only graphite particles B as graphite particles.

FIG. 1 is a cross-sectional view schematically showing an example of a negative electrode for a secondary battery according to an embodiment of the present disclosure. FIG. 2 is a diagram showing an example of the arrangement pattern of the second a layer and the second b layer. FIG. 2 is a diagram of the second layer viewed from the normal direction of the main surface of the negative electrode current collector. The shaded area in FIG. 2 is the second b layer.

The negative electrode 30 includes a sheet-like (foil-like) negative electrode current collector 31, a negative electrode mixture layer 32 supported on one surface of the negative electrode current collector 31, and a negative electrode mixture layer 32 supported on the other surface of the negative electrode current collector 31. and a negative electrode mixture layer 33. The negative electrode mixture layer 32 and the negative electrode mixture layer 33 have the same configuration. The negative electrode mixture layer 32 will be described in detail below.

The negative electrode mixture layer 32 contains graphite particles as a negative electrode active material. The graphite particles include graphite particles A whose internal porosity is greater than 5% and graphite particles B whose internal porosity is 5% or less. The negative electrode mixture layer 32 includes, in order from the negative electrode current collector 31 side, a first layer 41 , a second layer 42 , and a third layer 43 . The second layer 42 includes a second a layer 42 a disposed in the first region of the first layer 41 and a second b layer 42 b disposed in the second region of the first layer 41 .

The ratio MB ₁ (mass %) of graphite particles B to the graphite particles in the first layer 41, the ratio MB _2a (mass %) of graphite particles B to the graphite particles in the second a layer 42a, and the second b layer 42b The ratio MB _2b (mass %) of the graphite particles B to the graphite particles in the third layer 43 and the ratio MB ₃ (mass %) of the graphite particles B to the graphite particles in the third layer 43 are 0≦MB ₁ ≦MB _2a The following relationship is satisfied: <50<MB _2b ≦MB ₃ ≦100.

The negative electrode mixture layer 32 has a thickness T. The thickness T is the total thickness of the thickness T ₁ of the first layer 41 , the thickness T ₂ of the second layer 42 , and the thickness T ₃ of the third layer 43 . The thickness T of the negative electrode mixture layer 32 is, for example, 50 μm or more and 300 μm or less. The thickness T ₃ of the third layer 43 is greater than 0.05T. The thickness ratio of the second layer 42 to the third layer 43: T ₂ /T ₃ is, for example, 1/3 or more and 5 or less. From the viewpoint of easy arrangement of the first to third layers in a well-balanced manner, the thickness T ₂ of the second layer 42 is preferably 0.1T or more (or 0.2T or more) and 0.5T or less. Similarly, the thickness T ₃ of the third layer 43 is preferably 0.1T or more and 0.3T or less. Similarly, the thickness _T1 of the first layer 41 is, for example, 0.25T or more and 0.75T or less.

The second a layer 42a and the second b layer 42b are arranged in an alternating pattern. The 2nd a layer 42a and the 2nd b layer 42b are arranged in a stripe shape in the width direction (Y direction) of the negative electrode current collector, and the 2nd a layer 42a and the 2nd b layer 42b have approximately the same width. There is. The plurality of second a layers 42a and second b layers 42b each have approximately the same width and are arranged at regular intervals. The second layer preferably has a regularly repeated pattern, as shown in FIG. 2 and FIGS. 3A to 3C, which will be described later. In this case, the effects of the second a layer and the second b layer can be stably obtained throughout the negative electrode.

The arrangement pattern of the second a layer and the second b layer is not limited to the arrangement pattern shown in FIG. The second a layer and the second b layer may be arranged in stripes in the length direction (X direction) of the negative electrode current collector, and may have different widths from each other.

The arrangement pattern of the second a layer and the second b layer may be a checkerboard arrangement pattern shown in FIG. 3A. In FIG. 3A, the rectangular second a layer 42a and second b layer 42b (shaded areas in FIG. 3A) are arranged in a pattern that is alternately repeated in the length direction (X direction) and width direction (Y direction) of the negative electrode current collector. It is located in

The arrangement pattern of the second a layer and the second b layer may be a honeycomb arrangement pattern shown in FIG. 3B. In FIG. 3B, the hexagonal second a layer 42a is lined up in a row in the width direction (Y direction) of the negative electrode current collector, and the hexagonal second b layer 42b is aligned in the width direction (Y direction) of the negative electrode current collector. The portions lined up in a row (hatched portions in FIG. 3B) are arranged in an alternating pattern.

Furthermore, the 2B layer (or 2A layer) may be dispersed and arranged in the form of islands in the sea of the 2A layer (or 2B layer). The shape of the island is not particularly limited, and may be circular or polygonal such as a quadrangle. Preferably, the plurality of islands have substantially the same shape and size and are regularly arranged at regular intervals. In this case, the effects of the second a layer and the second b layer can be stably obtained throughout the negative electrode.

For example, as shown in FIG. 3C, circular island portions (hatched portions in FIG. 3C) of the second b layer 42b may be dispersed and arranged in the sea portion of the second a layer 42a. The island portions of the plurality of second b layers 42b have approximately the same size and are regularly arranged at regular intervals. The sea portion and the island portion may be the 2b layer and the 2a layer, respectively.

(graphite particles)
The negative electrode mixture layer contains graphite particles as a negative electrode active material. The graphite particles include graphite particles A whose internal porosity is greater than 5% and graphite particles B whose internal porosity is 5% or less. The internal porosity of graphite particles A is preferably 8% or more and 20% or less, more preferably 12% or more and 18% or less. The internal porosity of graphite particles B is preferably 1% or more (or 2% or more) and 5% or less. When the internal porosity of graphite particles A and graphite particles B is within the above range, cycle characteristics and rapid charging characteristics are likely to be improved, and it is also advantageous for increasing capacity. The internal porosity of graphite particles A and graphite particles B can be controlled within the above range by the production method described below.

Here, FIG. 4 is a diagram schematically showing a cross section of a graphite particle.
The graphite particles 20 have an internal void 21 closed inside the graphite particle 20 and an external void 22 connected to a space outside the graphite particle 20 . The external voids 22 are included in the voids between the negative electrode active material particles in the negative electrode mixture layer. Graphite particles A have more internal voids 21 and more external voids 22 than graphite particles B.

Here, the internal porosity of the graphite particle 20 means the ratio of the area S2 of the internal void 21 to the area S1 of the graphite particle 20 in the cross section of the graphite particle 20, and is determined by (S2/S1)×100. The area S1 of the graphite particle 20 is the area of the region surrounded by the outer periphery of the graphite particle 20, and means the combined area of the hatched portion in FIG. 2 and the internal void 21. When there are multiple internal voids 21, the area S2 of the internal voids 21 means the total area of the multiple internal voids 21.

The internal porosity of graphite particles can be determined by the following method.
Disassemble the initial battery (unused battery or battery that has been charged and discharged several times), take out the negative electrode, sample a part of the negative electrode, and use an ion milling device etc. to obtain a sample cross section of the negative electrode mixture layer. . As the ion milling device, for example, a device name “IM4000PLUS” manufactured by Hitachi High-Tech Corporation can be used. Graphite particles to be included in a negative electrode slurry used in producing a negative electrode for a secondary battery may be obtained as a sample. A sample of graphite particles may be embedded in an epoxy resin or the like and polished to obtain cross sections of a plurality of graphite particles, which may be used as sample cross sections. A backscattered electron image (magnification: 3,000 to 5,000 times) of the cross section of the sample is taken using a scanning electron microscope (SEM) to obtain a cross-sectional image of the sample. The cross-sectional image of the sample obtained above is imported into a computer and subjected to binarization processing. For the binarization process of the cross-sectional image, for example, image analysis software "ImageJ" manufactured by the National Institutes of Health can be used. In the binarization process, in the cross-sectional image, negative electrode active material particles (including voids inside the particles) are represented in black, and voids between particles are represented in white.

Graphite particles with a maximum diameter of 5 μm or more and 50 μm or less are arbitrarily selected using the binarized cross-sectional image. The area S1 of the graphite particles and the area S2 of internal voids of the graphite particles are calculated. It should be noted that it may be difficult to distinguish between minute voids having a maximum diameter of 3 μm or less as either internal voids or external voids in image analysis. Therefore, voids with a maximum diameter of 3 μm or less are considered internal voids. Further, when the negative electrode active material contains graphite particles and Si-based material, the graphite particles and the particles of the Si-based material can be distinguished using the SEM image (backscattered electron image) of the cross section of the sample obtained above. In the SEM image of the cross section of the sample, graphite particles appear black, and Si-based material particles (SiOx particles, etc.) appear gray.

Using the area S1 of the graphite particles and the area S2 of the internal voids obtained above, the internal porosity (%) of the graphite particles is calculated from the following formula.
Internal porosity of graphite particles = (S2/S1) x 100
Graphite particles having an internal porosity of more than 5% are referred to as graphite particles A, and graphite particles having an internal porosity of 5% or less are referred to as graphite particles B. The average value of internal porosity of 10 graphite particles A is determined. The average value of internal porosity of 10 graphite particles B is determined.

Graphite particles A can be produced, for example, by the following method.
The main raw material, coke (precursor), is pulverized, a binder is added to the pulverized material, and the pulverized material is agglomerated. The obtained aggregate is pressure-molded to obtain a block-shaped compact, which is then fired and graphitized. The firing temperature is, for example, 2600°C or higher, and may be 2600°C or higher and 3000°C or lower. The density of the molded body is, for example, 1.6 g/cm ³ or more and 1.9 g/cm ³ or less. The obtained block-shaped graphitized material is crushed and sieved to obtain graphite particles A of a desired size.

The average particle diameter (D50) of the crushed coke is, for example, 12 μm or more and 20 μm or less. In addition, in this specification, the average particle diameter (D50) means the particle diameter (volume average particle diameter) at which the volume integrated value is 50% in the particle size distribution measured by laser diffraction scattering method. As the measuring device, for example, "LA-750" manufactured by Horiba, Ltd. can be used.

The internal porosity of graphite particles A may be controlled by the particle size of crushed coke or aggregates, etc. When a volatile component is added to the block-shaped molded body during firing, the internal porosity of the graphite particles A may be controlled by the amount of the volatile component added to the block-shaped molded body. When part of the binder added to the crushed coke is volatilized during firing, the binder may also serve as a volatile component added to the molded body. In this case, the internal porosity may be controlled by the amount of binder added. Pitch is exemplified as such a binder. For example, by increasing the amount of pitch added to the pulverized material in the process of producing graphite particles A than the amount of pitch added to the pulverized product in the process of producing graphite particles B, the internal porosity is larger than that of graphite particles B. Graphite particles A may also be produced.

Graphite particles B can be produced, for example, by the following method.
The main raw material, coke (precursor), is pulverized, a binder is added to the pulverized material, and the pulverized material is agglomerated. The average particle diameter (D50) of the crushed coke is, for example, 12 μm or more and 20 μm or less. The obtained aggregate is fired, graphitized, and sieved to obtain graphite particles B of a desired size. The firing temperature is, for example, 2600°C or higher, and may be 2600°C or higher and 3000°C or lower.

The internal porosity of graphite particles B can be controlled by, for example, the particle size of crushed coke or aggregates. For example, in order to reduce the internal porosity, the particle size of crushed coke or aggregates may be increased. Furthermore, if part of the binder contained in the aggregate evaporates during firing, the internal porosity may be controlled by the amount of binder added. Pitch is exemplified as such a binder.

(Method for manufacturing negative electrode for secondary battery)
A method for manufacturing a negative electrode for a secondary battery according to an embodiment of the present disclosure includes first to third steps. In the first step, a negative electrode slurry containing graphite particles is prepared. In the second step, a negative electrode slurry is applied to the surface of the negative electrode current collector and dried to form a negative electrode mixture layer. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector, or may be formed on both surfaces of the negative electrode current collector. In the third step, the laminate of the negative electrode mixture layer and the negative electrode current collector is compressed.

In the first step, for example, a negative electrode mixture is dispersed in a dispersion medium to prepare a negative electrode slurry. The negative electrode mixture contains graphite particles, which are negative electrode active materials, as an essential component, and may also contain a binder, a conductive agent, a thickener, etc. as optional components. As the binder and the like, those exemplified below can be used. In the second step, a negative electrode slurry may be applied to one surface of a sheet (foil)-shaped negative electrode current collector, or a negative electrode slurry may be formed on both surfaces of the negative electrode current collector. In the third step, for example, the long laminate is roll pressed.

In the first step, a first slurry, a second slurry, and a third slurry are prepared as negative electrode slurries. In the second step, the surface of the negative electrode current collector is coated with a first slurry, a second slurry, and a third slurry in order, and the first layer, second layer, and third layer are applied in order from the negative electrode current collector side. Form a negative electrode mixture layer consisting of: Drying in the second step may be performed each time the first slurry to third slurry are applied, or may be performed after sequentially applying the first slurry to third slurry.

The second slurry includes a second a slurry and a second b slurry. The second layer forming step includes applying a 2a slurry and a 2b slurry to the first and second areas of the first slurry application area, respectively, to form a 2a layer and a 2b layer as the second layer. Including process.

The graphite particles include graphite particles A whose internal porosity is greater than 5% and graphite particles B whose internal porosity is 5% or less. The proportion (mass%) of graphite particles B in the graphite particles (total of graphite particles A and graphite particles B) in the first slurry, second a slurry, second b slurry, and third slurry is MB 1 and MB ₁ , respectively. _2a , MB _2b , and MB ₃ . At this time, MB ₁ , MB _2a , MB _2b , and MB ₃ satisfy the relationship 0≦MB ₁ ≦MB _2a <50<MB _2b ≦MB ₃ ≦100. When the thickness of the negative electrode mixture layer is T, the thickness of the third layer is greater than 0.05T.

The proportion of graphite particles B in the above graphite particles may be adjusted by separately producing graphite particles A and graphite particles B and including them in a slurry at a predetermined mass ratio. The thickness of each layer may be adjusted by the amount of each slurry applied.

Examples of the coating method in the second step include a die coating method, an inkjet printing method, a reverse coating method (transfer method), and the like. After applying one of the 2A slurry and the 2B slurry, the other of the 2A slurry and the 2B slurry may be applied. The application of the second a slurry and the second b slurry may be performed simultaneously. The 2a slurry and the 2b slurry are mixed so that the 2a layer and the 2b layer formed in the second step have the arrangement pattern exemplified in the negative electrode for a secondary battery (the arrangement pattern shown in FIGS. 2 and 3A to 3C). may be applied in a pattern.

(Secondary battery)
A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. This negative electrode is the negative electrode for secondary batteries described above. The secondary battery will be explained in detail below.

[Negative electrode]
The negative electrode includes a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. The negative electrode mixture layer contains a negative electrode active material as an essential component, and may also contain a binder, a conductive agent, a thickener, etc. as optional components.

The negative electrode active material contains at least graphite particles. As the graphite particles, particles of natural graphite, artificial graphite, etc. can be used. Artificial graphite particles are preferred from the viewpoint of easy control of internal porosity. Particles of natural graphite can be used as graphite particles A. The graphite particles may partially contain amorphous carbon, graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). The average particle diameter (D50) of the graphite particles is, for example, 1 μm or more and 30 μm or less.

Graphite is a carbon material with a developed graphite-type crystal structure. The interplanar spacing d002 of the (002) planes of graphite particles measured by X-ray diffraction may be, for example, 0.340 nm or less, or 0.3354 nm or more and 0.340 nm or less. Further, the crystallite size Lc (002) of the graphite particles measured by X-ray diffraction may be, for example, 5 nm or more, 5 nm or more and 300 nm or less, or 5 nm or more (or 10 nm or more). , 200 nm or less. The crystallite size Lc (002) is measured, for example, by the Scherrer method. When the interplanar spacing d002 of the (002) planes of the graphite particles and the crystallite size Lc (002) are within the above ranges, high capacity can be easily obtained.

From the viewpoint of increasing capacity, the negative electrode active material may include a Si-based material. The average particle size (D50) of the particles of the Si-based material is, for example, 1 μm or more and 25 μm or less. Examples of Si-based materials include simple silicon, alloys containing silicon, and composite materials containing silicon. Alloys containing silicon include, for example, silicon (Si), tin (Sn), nickel (Ni), iron (Fe), copper (Cu), titanium (Ti), manganese (Mn), and aluminum (Al). and at least one element selected from the group.

A composite material containing silicon may include an ion-conducting phase (matrix phase) and a silicon phase (particulate Si phase) dispersed within the phase. Examples of the ion conductive phase include a silicon oxide phase, a silicate phase (for example, a lithium silicate phase), a carbon phase, and the like.

The silicon oxide phase is, for example, a SiO ₂ phase containing 95% by mass or more of silicon dioxide. Particles of a composite material in which a silicon phase is dispersed within a SiO ₂ phase are represented by SiO _x , where x satisfies, for example, 0.8≦x≦1.6. The lithium silicate phase may have, for example, a composition represented by the formula: Li _2z SiO _2+z (0<z<2). z may be 1/2 or 1. Particles of a composite material in which a silicon phase is dispersed within a silicate phase can be obtained, for example, by grinding a mixture of silicate and raw silicon with stirring in a ball mill or the like to form fine particles, and then heat-treating the mixture in an inert atmosphere. Can be done. The carbon phase includes, for example, amorphous carbon with low crystallinity.

From the viewpoint of improving conductivity, at least a portion of the surface of the particles of the composite material may be coated with a conductive layer. The conductive layer includes a conductive material such as conductive carbon. The coating amount of the conductive layer is, for example, 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the composite material particles and the conductive layer. Composite particles having a conductive layer on the surface can be obtained, for example, by mixing coal pitch or the like with particles of a composite material and heat-treating the mixture in an inert atmosphere.

Si-based materials have a larger capacity density than graphite particles. Graphite particles have a smaller degree of expansion and contraction during charging and discharging than Si-based materials. When graphite particles and Si-based material are used together, the proportion of Si-based material in the negative electrode active material (total of graphite particles and Si-based material) is preferably 1% by mass or more and 10% by mass or less, 3% by mass or more, More preferably, it is 7% by mass or less. In this case, it is easy to simultaneously improve cycle characteristics and increase capacity.

As the binder, resin materials such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resins; polyimide resins such as polyimide and polyamideimide; ; Acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymers; Vinyl resins such as polyacrylonitrile and polyvinyl acetate; Polyvinylpyrrolidone; Polyethersulfone; Styrene-butadiene copolymer rubber (SBR) Examples include rubber-like materials such as. One type of binder may be used alone, or two or more types may be used in combination.

Examples of the conductive agent include carbon such as acetylene black; conductive fibers such as carbon fiber and metal fiber; and metal powder such as aluminum. One type of conductive agent may be used alone, or two or more types may be used in combination.

Examples of thickeners include carboxymethylcellulose (CMC) and its modified products (including salts such as Na salt), cellulose derivatives such as methylcellulose (cellulose ethers, etc.); Examples include chemical substances. One type of thickener may be used alone, or two or more types may be used in combination.

The dispersion medium is not particularly limited, but for example, water, alcohol such as ethanol, ether such as tetrahydrofuran, amide such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof may be used. .

As the negative electrode current collector, a non-porous conductive substrate (metal foil, etc.) or a porous conductive substrate (mesh body, net body, punched sheet, etc.) is used. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm.

[Positive electrode]
The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry in which the positive electrode mixture is dispersed in a dispersion medium onto the surface of the positive electrode current collector and drying the slurry. The dried coating film may be rolled if necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the sheet-like positive electrode current collector. The positive electrode mixture includes a positive electrode active material as an essential component, and may also include a binder, a conductive agent, etc. as optional components. NMP or the like is used as a dispersion medium for the positive electrode slurry.

As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal such as Ni, Co, or Mn can be used. For example, Li _a CoO ₂ , Li _a NiO ₂ , Li _a MnO ₂ , Li _a Co _b Ni _1-b O ₂ , Li _a Co _b Me _1-b O _c , Li _a Ni _1-b Me _b O _c , Li _a Mn ₂ O ₄ , Li _a Mn _2-b Me _b O ₄ , LiMePO ₄ , Li ₂ MePO ₄ F (Me is Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, at least one selected from the group consisting of Al, Cr, Pb, Sb, and B). Here, 0<a≦1.2, 0<b≦0.9, and 2.0≦c≦2.3. Note that the a value indicating the molar ratio of lithium increases or decreases due to charging and discharging.

Among them, Li _a Ni _b M _1-b O ₂ (M is at least one selected from the group consisting of Mn, Co and Al, 0<a≦1.2, 0.3≦b≦ 1) is preferred. From the viewpoint of increasing capacity, it is more preferable to satisfy 0.85≦b≦1. From the viewpoint of crystal structure stability, Li _a Ni _b Co _c Al _d O ₂ (0<a≦1.2, 0.85≦b<1, 0<c<0. 15, 0<d≦0.1, b+c+d=1) is more preferable.

As the binder and conductive agent, those similar to those exemplified for the negative electrode can be used. As the conductive agent, graphite such as natural graphite or artificial graphite may be used.

The shape and thickness of the positive electrode current collector can be selected from a shape and range similar to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, and the like.

[Nonaqueous electrolyte]
The non-aqueous electrolyte includes a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol/L or more and 2 mol/L or less.

As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylic acid ester, a chain carboxylic ester, etc. are used. Examples of the cyclic carbonate include propylene carbonate and ethylene carbonate. A small amount of a cyclic carbonate having an unsaturated bond such as vinylene carbonate, a cyclic carbonate having a fluorine atom such as fluoroethylene carbonate, etc. may be included in the nonaqueous electrolyte. Examples of chain carbonate esters include diethyl carbonate, ethylmethyl carbonate, dimethyl carbonate, and the like. Examples of the cyclic carboxylic acid ester include γ-butyrolactone and γ-valerolactone. Examples of the chain carboxylic acid ester include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and the like. The non-aqueous solvents may be used alone or in combination of two or more.

Examples of lithium salts include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiB ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, LiCl , LiBr, LiI, borates, imide salts, and the like. As borates, bis(1,2-benzenediolate (2-)-O,O') lithium borate, bis(2,3-naphthalenediolate (2-)-O,O') boric acid Lithium, bis(2,2'-biphenyldiolate(2-)-O,O') lithium borate, bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O') lithium borate etc. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO ₂ ) ₂ ), lithium bistrifluoromethanesulfonate imide (LiN(CF ₃ SO ₂ ) ₂ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ )), lithium bispentafluoroethanesulfonic acid imide (LiN(C ₂ F ₅ SO ₂ ) ₂ ), and the like. One type of lithium salt may be used alone, or two or more types may be used in combination.

[Separator]
Usually, it is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability, appropriate mechanical strength, and insulation properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc. can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

The secondary battery may include, for example, a wound-type electrode group configured by winding a positive electrode and a negative electrode with a separator in between, or a stacked electrode group configured by laminating a positive electrode and a negative electrode with a separator in between. A type of electrode group may be provided. The secondary battery may have any form, such as a cylindrical shape, a square shape, a coin shape, a button shape, a laminate shape, etc., for example.

Hereinafter, the structure of a square secondary battery will be described as an example of a secondary battery according to the present disclosure with reference to FIG. 5. FIG. 5 is a partially cutaway schematic perspective view of a secondary battery according to an embodiment of the present disclosure.

The battery includes a rectangular battery case 4 with a bottom, an electrode group 1 housed in the battery case 4, and a non-aqueous electrolyte (not shown). The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding a negative electrode, a positive electrode, and a separator around a flat core, and then removing the core.

One end of the negative electrode lead 3 is attached to the negative electrode current collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate (not shown). The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of a positive electrode lead 2 is attached to the positive electrode current collector by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. That is, the positive electrode lead 2 is electrically connected to the battery case 4 which also serves as a positive electrode terminal. The insulating plate isolates the electrode group 1 and the sealing plate 5 as well as the negative electrode lead 3 and the battery case 4. The peripheral edge of the sealing plate 5 fits into the open end of the battery case 4, and the fitting portion is laser welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. A non-aqueous electrolyte injection hole provided in the sealing plate 5 is closed with a sealing plug 8 .

《Additional notes》
The following techniques are disclosed by the description of the above embodiments.
(Technology 1)
comprising a negative electrode mixture layer containing a negative electrode active material and a negative electrode current collector supporting the negative electrode mixture layer,
The negative electrode active material includes graphite particles,
The graphite particles include graphite particles A having an internal porosity of more than 5% and graphite particles B having an internal porosity of 5% or less,
The negative electrode mixture layer includes, in order from the negative electrode current collector side, a first layer, a second layer, and a third layer,
The second layer includes a 2a layer and a 2b layer arranged in a first region and a second region of the first layer, respectively,
The proportion (mass%) of the graphite particles B in the total of the graphite particles A and the graphite particles B in the first layer, the second a layer, the second b layer, and the third layer is MB ₁ , MB _2a , MB _2b , and MB ₃ ,
The MB ₁ , the MB _2a , the MB _2b , and the MB ₃ are:
0≦MB ₁ ≦MB _2a <50<MB _2b ≦MB ₃ ≦100
satisfies the relationship of
When the thickness of the negative electrode mixture layer is T,
In the negative electrode for a secondary battery, the thickness of the third layer is greater than 0.05T.
(Technology 2)
The negative electrode for a secondary battery according to technique 1, wherein the graphite particles A have an internal porosity of 8% or more and 20% or less.
(Technology 3)
The thickness of the second layer is 0.1T or more and 0.5T or less,
The negative electrode for a secondary battery according to

technology

1 or 2, wherein the third layer has a thickness of 0.1T or more and 0.3T or less.
(Technique 4)
The MB _2a is 0% by mass or more and 40% by mass or less,
The negative electrode for a secondary battery according to any one of Techniques 1 to 3, wherein the MB _2b is 60% by mass or more and 100% by mass or less.
(Technology 5)
The negative electrode for a secondary battery according to any one of Techniques 1 to 4, wherein the negative electrode active material includes a Si-based material.
(Technology 6)
The negative electrode for a secondary battery according to any one of techniques 1 to 5, wherein the second a layer and the second b layer are arranged in an alternating pattern on the first layer.
(Technology 7)
The negative electrode for a secondary battery according to technique 6, wherein the second a layer and the second b layer are arranged in a stripe shape, a checkerboard shape, or a honeycomb shape on the first layer.
(Technology 8)
On the first layer, one of the second a layer and the second b layer is a secondary layer according to any one of techniques 1 to 7, wherein one of the second a layer and the second b layer is arranged in an island-like manner dispersed in the other sea. Negative electrode for batteries.
(Technology 9)
Comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte,
A secondary battery, wherein the negative electrode is a negative electrode for a secondary battery according to any one of Techniques 1 to 8.
(Technology 10)
A first step of preparing a negative electrode slurry containing graphite particles;
a second step of applying the negative electrode slurry on the surface of the negative electrode current collector and drying it to form a negative electrode mixture layer;
a third step of compressing the laminate of the negative electrode mixture layer and the negative electrode current collector;
including;
In the first step, a first slurry, a second slurry, and a third slurry are prepared as the negative electrode slurry,
In the second step, the first slurry, the second slurry, and the third slurry are applied to the surface of the negative electrode current collector in this order, and the first layer, the second slurry are applied in this order from the negative electrode current collector side. forming the negative electrode mixture layer composed of a layer and a third layer,
The second slurry includes a second a slurry and a second b slurry,
In the step of forming the second layer, the second a slurry and the second b slurry are respectively applied to a first region and a second region of the application region of the first slurry, and the second layer is formed as the second layer. 2b layer forming step,
The graphite particles include graphite particles A having an internal porosity of more than 5% and graphite particles B having an internal porosity of 5% or less,
The proportion (mass%) of the graphite particles B in the total of the graphite particles A and the graphite particles B in the first slurry, the second a slurry, the second b slurry, and the third slurry is MB ₁ , MB _2a , MB _2b , and MB ₃ ,
The MB ₁ , the MB _2a , the MB _2b , and the MB ₃ are:
0≦MB ₁ ≦MB _2a <50<MB _2b ≦MB ₃ ≦100
satisfies the relationship of
When the thickness of the negative electrode mixture layer is T,
A method for manufacturing a negative electrode for a secondary battery, wherein the thickness of the third layer is greater than 0.05T.
(Technology 11)
The method for producing a negative electrode for a secondary battery according to technique 10, wherein the graphite particles A have an internal porosity of 8% or more and 20% or less.
(Technology 12)
The thickness of the second layer is 0.1T or more and 0.5T or less,
The method for manufacturing a negative electrode for a secondary battery according to technology 10 or 11, wherein the third layer has a thickness of 0.1 T or more and 0.3 T or less.
(Technology 13)
The MB _2a is 0% by mass or more and 40% by mass or less,
The method for producing a negative electrode for a secondary battery according to any one of Techniques 10 to 12, wherein the MB _2b is 60% by mass or more and 100% by mass or less.

Hereinafter, the present disclosure will be specifically described based on Examples and Comparative Examples, but the present disclosure is not limited to the following Examples.

《Examples 1 to 6 and Comparative Examples 3 to 4, 6 to 7》
(Preparation of graphite particles A)
Coke (precursor) serving as the main raw material was pulverized to obtain a pulverized product with an average particle size (D50) of 15 μm. Pitch is added to the pulverized material as a binder to agglomerate the pulverized material, and the resulting agglomerate is pressure-molded to form a block having a density of 1.6 g/cm ³ to 1.9 g/cm ³ I got a body. The obtained block-shaped molded body was fired at a temperature of 2800°C to graphitize it. The obtained block-shaped graphitized material was crushed and sieved to obtain graphite particles A having an average particle diameter (D50) of 23 μm.

(Preparation of graphite particles B)
Coke (precursor) serving as the main raw material was pulverized to obtain a pulverized product with an average particle size (D50) of 12 μm. Pitch was added to the pulverized material as a binder to agglomerate the pulverized material to obtain an agglomerate having an average particle size (D50) of 18 μm. The aggregate was calcined at a temperature of 2800°C to graphitize it. The obtained graphitized product was sieved to obtain graphite particles B having an average particle diameter (D50) of 23 μm.

The internal porosity of graphite particles A determined by the method described above was within the range of 8% to 20%, and the average value of the internal porosity was 15%. The internal porosity of graphite particles B determined by the method described above was within the range of 1% to 5%, and the average value of the internal porosity was 3%.
The internal porosity of graphite particles A and B was adjusted by adjusting the amount of pitch added during the production process of graphite particles A and B. Note that the larger the pitch amount, the larger the internal porosity.

(Preparation of negative electrode slurry)
An appropriate amount of water was added to the negative electrode mixture to obtain a negative electrode slurry. The negative electrode mixture used was a mixture of a negative electrode active material, styrene-butadiene copolymer rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener. A mixture of graphite particles and Si-based material was used as the negative electrode active material. In the negative electrode active material, the mass ratio of graphite particles to Si-based material was 95:5. The mass ratio of the negative electrode active material, SBR, and CMC was 100:1:1. As the Si-based material, SiO _x particles (0.8≦x≦1.6, average particle diameter (D50) 5 μm) whose surface was coated with a conductive layer containing conductive carbon were used as a composite material. The coating amount of the conductive layer was 5 parts by mass per 100 parts by mass of the SiO _x particles and the conductive layer.

A first slurry, a second slurry, and a third slurry were prepared as negative electrode slurries. As the second slurry, a 2nd a slurry and a 2nd b slurry were prepared. The mixing ratio of graphite particles A and graphite particles B was adjusted appropriately, and the proportion (mass%) of graphite particles B in the graphite particles (total of graphite particles A and graphite particles B) contained in each slurry was determined as shown in Table 1. The values shown are as follows.

(Preparation of negative electrode)
A copper foil (thickness: 10 μm) was prepared as a negative electrode current collector. Apply the first slurry, second slurry, and third slurry to both sides of the negative electrode current collector in this order, and dry to form the first layer, second layer, and third layer in this order from the negative electrode current collector side. did. In this way, a negative electrode mixture layer consisting of the first to third layers was formed. A 2nd a slurry and a 2nd b slurry were used as the second slurry. In the second layer forming step, a 2a slurry and a 2b slurry were applied to the first and second areas of the first slurry application area, respectively, to form a 2a layer and a 2b layer as the second layer. . Each slurry was applied by a die coating method. The second a layer and the second b layer were arranged in stripes in the pattern shown in FIG. The widths of the 2nd a layer and the 2nd layer (the width dimension in the X direction in FIG. 2) were substantially the same. The coating amount of each slurry was adjusted as appropriate, and the thicknesses (T ₁ to T ₃ ) of the first to third layers were set to the values shown in Table 1, respectively. The thickness T of the negative electrode mixture layer per side was 80 μm.

A laminate of a negative electrode current collector and negative electrode mixture layers formed on both sides of the negative electrode current collector was rolled. In this way, a negative electrode was obtained.

(Preparation of positive electrode)
An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture to obtain a positive electrode slurry.
The positive electrode mixture used was a mixture of a lithium-containing composite oxide as a positive electrode active material, graphite as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder. LiNi _0.88 Co _0.09 Al _0.03 O ₂ was used as the lithium-containing composite oxide. In the positive electrode mixture, the mass ratio of the lithium-containing composite oxide, graphite, and PVDF was 100:1:0.9.

A positive electrode slurry was applied to both sides of an aluminum foil (thickness: 15 μm) serving as a positive electrode current collector, and the coating was dried and rolled to form a positive electrode mixture layer to obtain a positive electrode. A doctor blade method was used to apply the positive electrode slurry.

(Preparation of non-aqueous electrolyte)
Add vinylene carbonate (VC) to a nonaqueous solvent that is a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1:3, and dissolve LiPF ₆ to prepare a nonaqueous electrolyte. did. The content of VC in the non-aqueous electrolyte was 5% by mass based on the entire non-aqueous electrolyte. The concentration of LiPF ₆ in the non-aqueous electrolyte was 1.5 mol/L.

(Preparation of secondary battery)
An aluminum positive electrode lead was attached to the exposed portion of the positive electrode current collector of the positive electrode, and a nickel negative electrode lead was attached to the exposed portion of the negative electrode current collector of the negative electrode. Thereafter, the positive electrode and the negative electrode were wound together with a separator interposed therebetween to produce a wound electrode group. A microporous polyethylene membrane was used as the separator. The electrode group was housed inside the battery case. At this time, an upper insulating plate and a lower insulating plate made of resin were placed above and below the electrode group, respectively. The negative electrode lead was welded to the inner bottom of the battery case. The positive electrode lead was welded to a metal sealing body that also served as a positive electrode terminal. Thereafter, a nonaqueous electrolyte was injected into the battery case, and the opening of the battery case was closed using a sealing member. At this time, a resin gasket was interposed between the open end of the battery case and the sealing body. In this way, a cylindrical secondary battery (nominal capacity 4600 mAh) was obtained.
In Table 1, E1 to E6 are the batteries of Examples 1 to 6, and R3 to R4 and R6 to R7 are the batteries of Comparative Examples 3 to 4 and 6 to 7.

《Comparative example 1》
Using the first slurry and the third slurry, the negative electrode material mixture layer was composed of the first layer and the third layer. The proportion MB ₁ of graphite particles B in the graphite particles in the first slurry and the proportion MB ₃ of graphite particles B in the graphite particles in the third slurry were set to the values shown in Table 1. The thickness T ₁ of the first layer and the thickness T ₃ of the third layer were set to the values shown in Table 1. A battery R1 was produced in the same manner as the battery E1 of Example 1 except for the above.

《Comparative example 2》
Using the first slurry and the second slurry (the 2nd a slurry and the 2nd b slurry), the negative electrode mixture layer was composed of the first layer and the second layer (the 2nd a layer and the 2nd b layer). The proportion of graphite particles B in the graphite particles in the first slurry MB ₁ , the proportion of graphite particles B in the graphite particles in the second slurry MB _2a , and the proportion of graphite particles B in the graphite particles in the second slurry MB _2b was set to the value shown in Table 1. The thickness T ₁ of the first layer and the thickness T ₂ of the second layer were set to the values shown in Table 1. A battery R2 was produced in the same manner as the battery E1 of Example 1 except for the above.

《Comparative Example 5》
Using the second slurry (2a slurry and 2b slurry) and the third slurry, the negative electrode mixture layer was composed of the second layer (2a layer and 2b layer) and the third layer. The thickness T ₂ of the second layer and the thickness T ₃ of the third layer were set to the values shown in Table 1. A battery R5 was produced in the same manner as the battery E1 of Example 1 except for the above.

The following evaluations were performed for each battery produced above.

[Evaluation 1: Cycle capacity maintenance rate]
In a 25°C environment, perform constant current charging with a current of 0.3C (1380mA) until the voltage reaches 4.2V, then perform constant voltage charging with a voltage of 4.2V until the current reaches 0.02C. Ta. After resting for 10 minutes, constant current discharge was performed at a current of 0.5C until the voltage reached 2.5V. This charging and discharging was performed for 300 cycles, and the ratio (percentage) of the discharge capacity at the 300th cycle to the discharge capacity at the 1st cycle was determined as the capacity retention rate at the 300th cycle.

[Evaluation 2: Electrolyte permeability of negative electrode]
3 μl of propylene carbonate was dropped as a solvent onto one surface of the negative electrode (negative electrode mixture layer), and the time from when the solvent was dropped until all of the solvent permeated into the negative electrode mixture layer was measured. The numerical value of electrolyte permeability in Table 1 indicates the time (seconds) measured above, and when the numerical value is small, the rapid charging characteristics are good.

The evaluation results are shown in Table 1.

In batteries E1 to E6, the electrolyte permeability values were small and excellent rapid charging characteristics were obtained. In addition, batteries E1 to E6 had high capacity retention rates and excellent cycle characteristics.

In battery R1, the negative electrode mixture layer was formed only with the first layer and the third layer, and the second layer was not formed, so the electrolyte permeability value was large and the capacity retention rate was reduced. In battery R2, the electrolyte permeability value increased significantly because the third layer was not formed. In battery R3, since the second layer was composed of one layer in which the ratio of graphite particles B to the graphite particles was 50% by mass, the electrolyte permeability value increased and the capacity retention rate decreased.

In battery R4 in which the thickness of the third layer was 0.05T, the effect of the third layer was insufficient, and the electrolyte permeability value increased. In battery R5, since the first layer was not formed, the adhesion between the negative electrode mixture layer and the negative electrode current collector decreased, and the capacity retention rate decreased. In battery R6, since the first layer not containing graphite particles A was formed, the adhesion between the negative electrode mixture layer and the negative electrode current collector decreased, and the capacity retention rate decreased. In battery R7, since the third layer not containing graphite particles B was formed, the electrolyte permeability value increased.

The negative electrode for secondary batteries according to the present disclosure is suitably used in secondary batteries that require excellent rapid charging characteristics and cycle characteristics.

Although the invention has been described in terms of presently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and alterations will no doubt become apparent to those skilled in the art to which this invention pertains after reading the above disclosure. It is, therefore, intended that the appended claims be construed as covering all changes and modifications without departing from the true spirit and scope of the invention.

1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug, 20: graphite particle, 21: internal void, 22 : external void, 30: negative electrode, 31: negative electrode current collector, 32, 33: negative electrode mixture layer, 41: first layer, 42: second layer, 42a: second a layer, 42b: second b layer, 43: 3rd layer

Claims

comprising a negative electrode mixture layer containing a negative electrode active material and a negative electrode current collector supporting the negative electrode mixture layer,
The negative electrode active material includes graphite particles,
The graphite particles include graphite particles A having an internal porosity of more than 5% and graphite particles B having an internal porosity of 5% or less,
The negative electrode mixture layer includes, in order from the negative electrode current collector side, a first layer, a second layer, and a third layer,
The second layer includes a 2a layer and a 2b layer arranged in a first region and a second region of the first layer, respectively,
The proportion (mass%) of the graphite particles B in the total of the graphite particles A and the graphite particles B in the first layer, the second a layer, the second b layer, and the third layer is MB 1 , MB 2a , MB 2b , and MB 3 ,
The MB 1 , the MB 2a , the MB 2b , and the MB 3 are:
0≦MB 1 ≦MB 2a <50<MB 2b ≦MB 3 ≦100
satisfies the relationship of
When the thickness of the negative electrode mixture layer is T,
In the negative electrode for a secondary battery, the thickness of the third layer is greater than 0.05T.
The negative electrode for a secondary battery according to claim 1, wherein the graphite particles A have an internal porosity of 8% or more and 20% or less.
The thickness of the second layer is 0.1T or more and 0.5T or less,
The negative electrode for a secondary battery according to claim 1, wherein the third layer has a thickness of 0.1 T or more and 0.3 T or less.
The MB 2a is 0% by mass or more and 40% by mass or less,
The negative electrode for a secondary battery according to claim 1, wherein the MB 2b is 60% by mass or more and 100% by mass or less.
The negative electrode for a secondary battery according to claim 1, wherein the negative electrode active material includes a Si-based material.
The negative electrode for a secondary battery according to claim 1, wherein the second a layer and the second b layer are arranged in an alternating pattern on the first layer.
The negative electrode for a secondary battery according to claim 6, wherein the second a layer and the second b layer are arranged in a stripe shape, a checkerboard shape, or a honeycomb shape on the first layer.
The negative electrode for a secondary battery according to claim 1, wherein on the first layer, one of the second a layer and the second b layer is arranged in an island-like manner dispersed in the other sea.
Comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte,
A secondary battery, wherein the negative electrode is the negative electrode for a secondary battery according to claim 1.
A first step of preparing a negative electrode slurry containing graphite particles;
a second step of applying the negative electrode slurry on the surface of the negative electrode current collector and drying it to form a negative electrode mixture layer;
a third step of compressing the laminate of the negative electrode mixture layer and the negative electrode current collector;
including;
In the first step, a first slurry, a second slurry, and a third slurry are prepared as the negative electrode slurry,
In the second step, the first slurry, the second slurry, and the third slurry are applied to the surface of the negative electrode current collector in this order, and the first layer, the second slurry are applied in this order from the negative electrode current collector side. forming the negative electrode mixture layer composed of a layer and a third layer,
The second slurry includes a second a slurry and a second b slurry,
In the step of forming the second layer, the second a slurry and the second b slurry are respectively applied to a first region and a second region of the application region of the first slurry, and the second layer is formed as the second layer. 2b layer forming step,
The graphite particles include graphite particles A having an internal porosity of more than 5% and graphite particles B having an internal porosity of 5% or less,
The proportion (mass%) of the graphite particles B in the total of the graphite particles A and the graphite particles B in the first slurry, the second a slurry, the second b slurry, and the third slurry is MB 1 , MB 2a , MB 2b , and MB 3 ,
The MB 1 , the MB 2a , the MB 2b , and the MB 3 are:
0≦MB 1 ≦MB 2a <50<MB 2b ≦MB 3 ≦100
satisfies the relationship of
When the thickness of the negative electrode mixture layer is T,
A method for manufacturing a negative electrode for a secondary battery, wherein the thickness of the third layer is greater than 0.05T.
The method for manufacturing a negative electrode for a secondary battery according to claim 10, wherein the graphite particles A have an internal porosity of 8% or more and 20% or less.
The thickness of the second layer is 0.1T or more and 0.5T or less,
The method for manufacturing a negative electrode for a secondary battery according to claim 10, wherein the third layer has a thickness of 0.1T or more and 0.3T or less.
The MB 2a is 0% by mass or more and 40% by mass or less,
The method for manufacturing a negative electrode for a secondary battery according to claim 10, wherein the MB 2b is 60% by mass or more and 100% by mass or less.