JP2019102381A - Negative electrode for lithium ion secondary battery - Google Patents

Abstract

To suppress heat generation in the event of an internal short circuit while suppressing the decrease in battery capacity.SOLUTION: A negative electrode for a lithium ion secondary battery comprises at least a current collector and a negative electrode active material layer. The negative electrode active material layer contains a graphite-based material and a lithium titanate as a negative electrode active material. The negative electrode active material layer is formed on a surface of the current collector. The negative electrode active material layer includes at least a first layer and a second layer 22. The first layer is disposed between the current collector and the second layer 22. The second layer 22 includes a first region 1 and a plurality of second regions 2. In a plan view of the second layer 22, the plurality of second regions 2 are scattered like islands in the first region 1. The lithium titanate is arranged in the first layer and in the first region 1. The graphite-based material is arranged in each second region 2.SELECTED DRAWING: Figure 2

Description

  The present disclosure relates to a negative electrode for a lithium ion secondary battery.

  Unexamined-Japanese-Patent No. 2014-199714 is disclosing arrange | positioning the layer containing a lithium titanate between the layer containing a graphite-type material, and a collector in the negative electrode for lithium ion secondary batteries.

JP, 2014-199714, A

  In general, a negative electrode for a lithium ion secondary battery (hereinafter, may be abbreviated as “negative electrode”) contains a graphite-based material as a negative electrode active material. Graphite-based materials tend to have low electrical resistance. Therefore, when an internal short circuit occurs, it is considered that the short circuit current is easily propagated through the graphite-based material. The propagation of the short circuit current is considered to increase the calorific value at the time of the internal short circuit.

  Lithium titanate (which may be abbreviated as “LTO” below) is also known as a negative electrode active material. LTO can have the property that the electrical resistance increases when lithium ions are desorbed from its structure. At the time of internal short circuit, it is thought that lithium ion is desorbed from LTO. By mixing LTO into the graphitic material, it is expected that the propagation of the short circuit current is suppressed at the time of the internal short circuit.

  However, the short circuit current is considered to selectively flow in the low resistance portion. Even if the graphitic material and LTO are mixed, it is considered that the short circuit current propagates through the continuous portion of the graphitic material. It is considered that a considerable amount of LTO is required for the graphitic material to become discontinuous by simply mixing the graphitic material and LTO. LTO has a small specific capacity (capacity per unit mass). If the mixing ratio of LTO is increased to such an extent that propagation of a short circuit current can be suppressed, the battery capacity may be significantly reduced.

  In Patent Document 1, a layer containing a graphitic material and a layer containing LTO are laminated. According to this configuration, it is expected that the propagation of the short circuit current in the thickness direction of the negative electrode is suppressed by a small amount of LTO. However, it is considered that the short circuit current can be propagated in the in-plane direction of the negative electrode (direction orthogonal to the thickness direction). Therefore, there is a possibility that heat generation suppression at the time of internal short circuit may become insufficient.

  An object of the present disclosure is to suppress heat generation at the time of an internal short circuit while suppressing a decrease in battery capacity.

  The technical configuration and effects of the present disclosure will be described below. However, the mechanism of action of the present disclosure includes presumption. The scope of the claims should not be limited by the correctness of the mechanism of action.

  The negative electrode for a lithium ion secondary battery of the present disclosure at least includes a current collector and a negative electrode active material layer. The negative electrode active material layer contains a graphite-based material and lithium titanate as a negative electrode active material. The negative electrode active material layer is formed on the surface of the current collector. The negative electrode active material layer at least includes a first layer and a second layer. The first layer is disposed between the current collector and the second layer. The second layer includes a first region and a plurality of second regions. In plan view of the second layer, each of the plurality of second regions is scattered in an island shape in the first region. Lithium titanate is disposed in the first layer and the first region. The graphitic material is disposed in each of the plurality of second regions.

  In the negative electrode of the present disclosure, the first layer is disposed between the second layer and the current collector. The LTO is disposed in the first layer. Therefore, in the negative electrode of the present disclosure, it is expected that the propagation of the short circuit current in the thickness direction of the negative electrode is suppressed.

  The second layer includes a first region and a second region. In a plan view of the second layer, the first region and the second region form a so-called sea-island structure. "Plane view" indicates a field of view in which the second layer is observed from the normal direction of the main surface of the second layer. The first region containing LTO corresponds to the sea. The second region containing the graphitic material corresponds to an island. The individual second regions (graphite-based material) are separated by the first region (LTO) in plan view. That is, the second region (graphite material) is discontinuous in the in-plane direction. Therefore, in the negative electrode of the present disclosure, it is expected that the propagation of the short circuit current in the in-plane direction of the negative electrode is also suppressed.

  Furthermore, in the negative electrode of the present disclosure, the adoption of the above-described structure efficiently suppresses the propagation of the short circuit current, and therefore, it is expected that the heat generation at the internal short circuit is suppressed by a small amount of LTO. That is, according to the present disclosure, it is considered that the heat generation at the time of the internal short circuit can be suppressed while suppressing the decrease of the battery capacity.

FIG. 1 is a cross-sectional conceptual view showing the configuration of the negative electrode for a lithium ion secondary battery of the present embodiment. FIG. 2 is a schematic plan view showing a first example of the configuration of the second layer of the present embodiment. FIG. 3 is a schematic plan view showing a second example of the configuration of the second layer of the present embodiment. FIG. 4 is a schematic plan view showing a third example of the configuration of the second layer in the present embodiment. FIG. 5 is a schematic plan view showing a fourth example of the configuration of the second layer in the present embodiment. FIG. 6 is a flowchart showing an outline of a method of manufacturing the negative electrode for a lithium ion secondary battery of the present embodiment. FIG. 7 is a conceptual diagram for explaining the method of manufacturing the negative electrode for a lithium ion secondary battery of the present embodiment. FIG. 8 is a schematic view showing an example of the configuration of a lithium ion secondary battery.

  Hereinafter, an embodiment of the present disclosure (referred to as “the embodiment” in the present specification) will be described. However, the following description does not limit the scope of the claims.

<Anode for lithium ion secondary battery>
FIG. 1 is a cross-sectional conceptual view showing the configuration of the negative electrode for a lithium ion secondary battery of the present embodiment.
The negative electrode 200 is in the form of a sheet. The negative electrode 200 at least includes a current collector 10 and a negative electrode active material layer 20. The current collector 10 may have a thickness of, for example, 5 μm to 30 μm. The current collector 10 may be, for example, a copper (Cu) foil or the like. The negative electrode active material layer 20 is formed on the surface of the current collector 10. The negative electrode active material layer 20 may have a thickness of, for example, 10 μm to 200 μm.

  The thickness of each component in the present specification can be measured, for example, by a micrometer or the like. The thickness of each configuration may be measured in a cross-sectional microscope image or the like. The thickness is measured in at least three places. At least three arithmetic means are employed.

<< Composition of negative electrode active material layer >>
The negative electrode active material layer 20 contains a graphite-based material and LTO as a negative electrode active material. The negative electrode active material layer 20 may further include, for example, a binder and a conductive material.

(Graphite based material)
Graphite-based materials are powder materials. The graphite-based material indicates a carbon material containing a graphite crystal structure or a graphite-like crystal structure. The graphite crystal structure or a crystal structure similar to graphite shows a crystal structure in which carbon hexagonal network planes are stacked. The graphite-based material may be, for example, graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon) or the like. The graphite may be natural graphite. The graphite may be artificial graphite. One type of graphitic material may be used alone. Two or more graphite-based materials may be used in combination.

  The graphitic material may also include an amorphous carbon material (amorphous carbon) as long as it includes a graphite structure or a crystal structure similar to a graphite. For example, the surface of graphite may be coated with amorphous carbon. There is no particular limitation on the particle shape in the graphitic material. The particle shape may be, for example, spherical, scaly, massive or the like.

  The graphite-based material may have an average particle size of, for example, 1 μm to 30 μm. The average particle size of the graphitic material is measured by a laser diffraction scattering method. The average particle size of the graphitic material indicates the particle size at which the volume of accumulated particles from the fine particle side becomes 50% of the total particle volume in the volume-based particle size distribution.

(Lithium titanate)
LTO is a powder material. LTO can have any composition and crystal structure known in the art as long as it contains titanium (Ti), lithium (Li) and oxygen (O). LTO may be, for example, Li ₄ Ti ₅ O ₁₂ or the like. The LTO may have an average particle size of, for example, 0.01 μm or more and 1 μm or less. The average particle size of LTO indicates the measured value of primary particles. The average particle size of LTO is measured by dynamic light scattering. The mean particle size of LTO indicates the harmonic mean particle size (diameter) according to the scattering intensity standard. The shape of primary particles in LTO should not be particularly limited. The shape of the primary particles may be, for example, spherical, plate-like, rod-like, needle-like or the like.

  In the present embodiment, a small amount of LTO can suppress the heat generation at the time of the internal short circuit. The content of LTO may be, for example, less than 20% by mass with respect to the total of the graphitic material and LTO. The content of LTO may be, for example, less than 10% by mass with respect to the total of the graphitic material and LTO. The content of LTO may be, for example, 2.7% by mass or more and 6% by mass or less with respect to the total of the graphitic material and LTO.

(Other ingredients)
The negative electrode active material layer 20 may further include a binder. The content of the binder in the negative electrode active material layer 20 may be, for example, 0.1 parts by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the negative electrode active material. The binder should not be particularly limited. The binder may be, for example, carboxymethylcellulose (CMC), styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA) or the like. One type of binder may be used alone. Two or more binders may be used in combination.

  The negative electrode active material layer 20 may further include a conductive material. The content of the conductive material in the negative electrode active material layer 20 may be, for example, 0.1 parts by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the negative electrode active material. The conductive material may be, for example, an amorphous carbon material. The conductive material may be, for example, carbon black (for example, acetylene black etc.), carbon fiber or the like. One type of conductive material may be used alone. Two or more conductive materials may be used in combination.

<< Structure of Negative Electrode Active Material Layer >>
The negative electrode active material layer 20 of the present embodiment includes a specific structure. That is, the negative electrode active material layer 20 at least includes the first layer 21 and the second layer 22. The first layer 21 is disposed between the current collector 10 and the second layer 22. The first layer 21 may be formed directly on the surface of the current collector 10, for example.

  The first layer 21 contains LTO and does not contain a graphitic material. That is, LTO is disposed in the first layer 21. The first layer 21 may further include a conductive material and a binder. It is expected that the first layer 21 suppresses the propagation of the short circuit current in the thickness direction at the time of the internal short circuit. The first layer 21 may have a thickness of, for example, 1 μm or more. The first layer 21 may have a thickness of, for example, 2 μm or more. It is expected that the heat generation suppressing effect is enhanced in the range. The first layer 21 may have a thickness of, for example, 10 μm or less. The first layer 21 may have a thickness of, for example, 5 μm or less.

  The second layer 22 may be formed directly on the surface of the first layer 21. A third layer (not shown) may be formed between the second layer 22 and the first layer 21. The third layer may comprise a graphitic material. The third layer may comprise LTO. The third layer may comprise both a graphitic material and LTO. The second layer 22 may have a thickness of, for example, 5 μm to 199 μm. The second layer 22 may have a thickness of, for example, 7 μm to 198 μm.

FIG. 2 is a schematic plan view showing a first example of the configuration of the second layer of the present embodiment.
A plan view of the second layer 22 is shown in FIG. The second layer 22 includes a first region 1 and a plurality of second regions 2. In plan view, each of the plurality of second regions 2 is scattered in an island shape in the first region 1. The first region 1 contains LTO and does not contain a graphitic material. That is, LTO is disposed in the first area 1. Each of the plurality of second regions 2 includes a graphitic material and does not include LTO. That is, the graphitic material is disposed in each of the plurality of second regions 2. The first region 1 and the second region 2 may further include a conductive material and a binder, respectively.

  In the present embodiment, the individual second regions 2 (graphite material) are separated by the first region 1 (LTO) in plan view. Therefore, it is expected that the propagation of the short circuit current in the in-plane direction is also suppressed.

  The minimum width (W1) of the first region 1 in a plan view may be, for example, 3 μm or more. The minimum width (W1) of the first area 1 indicates the width of the first area 1 at a position where the adjacent second areas 2 are closest. The minimum width (W1) of the first region 1 may be, for example, 5 μm or more. It is expected that the heat generation suppressing effect is enhanced in the range. The minimum width (W1) of the first region 1 may be, for example, 20 μm or less. The minimum width (W1) of the first region 1 may be, for example, 10 μm or less. The minimum width (W1) of the first region 1 is measured in the microscopic image of the second layer 22. The minimum width (W1) is measured at at least three locations. At least three arithmetic means are employed.

  In a plan view, the maximum width (W2) of each second region 2 may be, for example, 1.4 mm or more and 5.6 mm or less. The maximum width (W2) of each second region 2 may be, for example, 1.4 mm or more and 2.8 mm or less. The maximum width (W2) of each second region 2 may be, for example, 2.8 mm or more and 5.6 mm or less. The aspect ratio of each second region 2 in plan view may be, for example, 1 or more and 2 or less. The aspect ratio indicates the ratio of the maximum width (W 2) to the minimum width in the individual second regions 2. The minimum width used to calculate the aspect ratio indicates the minimum width in the direction orthogonal to the direction in which the second region 2 indicates the maximum width (W2). The maximum width (W2) and the aspect ratio are measured in the microscopic image of the second layer 22. The maximum width (W2) and the aspect ratio are measured in at least three second regions 2. At least three arithmetic means are employed.

(Plane pattern)
The first region 1 extends in a mesh shape in plan view. The plane pattern of the first region 1 in FIG. 2 is lattice-like. However, the planar pattern of the first region 1 should not be particularly limited as long as the individual second regions 2 can be separated. The plane pattern of the first region 1 may be regular. The planar pattern of the first region 1 may be irregular.

FIG. 3 is a schematic plan view showing a second example of the configuration of the second layer of the present embodiment.
The solid line in FIG. 3 indicates the plane pattern of the first region 1. The plane pattern of the first region 1 may be composed of, for example, concentric circles and straight lines extending radially from the center of the concentric circles. The concentric circle group may be configured by a perfect circle. The concentric circle group may be configured by an ellipse. The line group may be changed to a curve group.

FIG. 4 is a schematic plan view showing a third example of the configuration of the second layer in the present embodiment.
The solid line in FIG. 4 indicates the plane pattern of the first region 1. The plane pattern of the first region 1 may be composed of, for example, concentric arcs and straight lines extending radially from the center of the concentric arcs. The arc may be a perfect circular arc. The arc may be an oval arc. The line group may be changed to a curve group.

FIG. 5 is a schematic plan view showing a fourth example of the configuration of the second layer in the present embodiment.
The solid line in FIG. 5 indicates the plane pattern of the first region 1. The plane pattern of the first region 1 may be configured, for example, by crossing a plurality of curve groups. The curve may be, for example, a wavy line or the like.

  The planar pattern of each second region 2 is determined by the planar pattern of the first region 1. Therefore, the planar pattern of each second region 2 should not be particularly limited. The planar patterns of all the second regions 2 may be substantially identical (see, for example, FIG. 2). The planar patterns of the individual second regions 2 may be different (see, for example, FIGS. 3 to 5). The planar pattern of each second region 2 may be, for example, circular, square, rectangular, hexagonal or the like.

<Method of Manufacturing Negative Electrode for Lithium Ion Secondary Battery>
FIG. 6 is a flowchart showing an outline of a method of manufacturing the negative electrode for a lithium ion secondary battery of the present embodiment. The negative electrode 200 of the present embodiment can be manufactured by, for example, a manufacturing method including at least the following (A) to (F).
(A) A first layer 21 containing LTO is formed on the current collector 10.
(B) Preparing wet granules 3 containing a graphitic material.
(C) A plurality of second regions 2 are formed by molding the wet granules 3 into an island shape.
(D) Each of the plurality of second regions 2 is disposed on the first layer 21.
(E) Prepare a particle dispersion containing LTO.
(F) The first region 1 is formed by applying the particle dispersion liquid so as to fill the gaps between the plurality of second regions 2.
The first region 1 and the plurality of second regions 2 form a second layer 22. The first layer 21 and the second layer 22 form a negative electrode active material layer 20.

  Wet granules are aggregates of aggregate particles formed by granulation of a graphitic material (powder material). The wet granules show a state in which the solvent (liquid) is dispersed in aggregated particles (solid). Wet granules can be prepared by methods known in the art. Wet granules can be prepared, for example, by a stirring granulation method. The solid content ratio of the wet granules may be, for example, 60% by mass or more and 90% by mass or less. Solid content ratio shows the mass ratio of components (solid content) other than a solvent.

  Particle dispersions can be prepared by dispersing LTO (powder material) in a solvent. The solid content ratio of the particle dispersion may be, for example, 3% by mass or more and 30% by mass or less.

FIG. 7 is a conceptual diagram for explaining the method of manufacturing the negative electrode for a lithium ion secondary battery of the present embodiment.
The wet granules 3 can be formed into an island shape by roll forming, for example. The first rotation roll 2001, the second rotation roll 2002, and the third rotation roll 2003 are arranged such that the rotation axes are parallel to one another. Each rotating roll rotates in the direction of the arrow drawn on each rotating roll.

  The wet granules 3 are supplied to the gap between the first rotating roll 2001 and the second rotating roll 2002. On the surface of the first rotating roll 2001, a convex portion corresponding to the planar pattern of the first region 1 is formed. The wet granules 3 pass through the gap between the first rotating roll 2001 and the second rotating roll 2002 to form a plurality of second regions 2.

  The third rotating roll 2003 transports the current collector 10. The first layer 21 is formed on the surface of the current collector 10. The first layer 21 can be formed, for example, by applying a particle dispersion containing LTO to the surface of the current collector 10 and drying.

  The second rotating roll 2002 transports the plurality of second regions 2. The current collector 10 and the plurality of second regions 2 are supplied to the gap between the second rotating roll 2002 and the third rotating roll 2003. In the gap between the second rotating roll 2002 and the third rotating roll 2003, the plurality of second regions 2 are rubbed against the surface of the first layer 21. As a result, each of the plurality of second regions 2 separates from the surface of the second rotary roll 2002, and each of the plurality of second regions 2 moves to the surface of the first layer 21. That is, the second region 2 is disposed on the first layer 21.

  Furthermore, the first region 1 can be formed by applying the particle dispersion containing LTO so as to fill the spaces between the plurality of second regions 2. The first region 1 and the plurality of second regions 2 form a second layer 22. The first layer 21 and the second layer 22 form a negative electrode active material layer 20. Thereafter, the negative electrode active material layer 20 may be dried. Thus, the negative electrode 200 is manufactured.

  The negative electrode 200 can be processed to have a predetermined outside dimension according to the specification of the lithium ion secondary battery. The negative electrode 200 may be compressed. The negative electrode 200 may be cut.

  Hereinafter, embodiments of the present disclosure will be described. However, the following description does not limit the scope of the claims.

<Manufacture of negative electrode>
Example 1
A. Formation of First Layer The following materials were prepared.
LTO (powder material)
Conductive material: Acetylene black (AB)
Binder: CMC
Solvent: Water Current collector 10: Cu foil

  A particle dispersion was prepared by mixing the LTO, the conductive material and the binder. The mixing ratio of the solid content is “LTO: conductive material: binder = 93: 5: 2 (mass ratio)”. The particle dispersion was applied to the surface of the current collector 10 and dried to form a first layer 21. The first layer 21 has a thickness of 2 μm.

B. Preparation of Wet Granules The following materials were prepared.
Graphite-based material: Natural graphite (powder material)
Binder: CMC
Solvent: water

  Wet granules 3 were prepared by mixing the graphitic material, the binder and the solvent. The mixing ratio of the solid content is “graphite-based material: binder = 99: 1 (mass ratio)”.

C. Formation of Second Region As shown in FIG. 7, the wet granules 3 were formed into an island shape by roll forming. Thereby, a plurality of second regions 2 were formed. As shown in FIG. 2, in plan view, the individual second regions 2 have a square planar pattern. That is, the aspect ratio is one. The maximum width (W2) of each second region 2 is 1.4 mm. The length of one side of each second region 2 (square) is 1 mm. The individual second regions 2 are separated by grooves. The grooves extend in a grid in plan view.

D. Arrangement of Second Region As shown in FIG. 7, each of the plurality of second regions 2 was arranged on the first layer 21.

E. Preparation of Particle Dispersion Liquid A particle dispersion liquid was prepared in the same manner as the above-mentioned "A. Formation of first layer".

F. Formation of First Region The first region 1 was formed by applying the particle dispersion liquid so as to fill the gaps between each of the plurality of second regions 2. As shown in FIG. 2, in plan view, the first regions 1 extend like a grid. The minimum width (W1) of the first region 1 is 10 μm. The negative electrode active material layer 20 was formed of the first region 1 and the plurality of second regions 2. In the negative electrode active material layer 20, the content of LTO is 4.1% by mass with respect to the total of the graphitic material and LTO. The negative electrode active material layer 20 was dried. The negative electrode active material layer 20 was compressed. Thus, the negative electrode 200 was manufactured.

Examples 2-3
As shown in Table 1 below, a negative electrode 200 was manufactured in the same manner as Example 1, except that the maximum width (W2) of the second region 2 was changed. By changing the maximum width (W2), the content of LTO is also changed.

Examples 4 to 6
As shown in Table 1 below, a negative electrode 200 was manufactured in the same manner as Example 1, except that the minimum width (W1) of the first region 1 was changed. By the change of the minimum width (W1), the content of LTO is also changed.

Example 7
As shown in Table 1 below, a negative electrode 200 was manufactured in the same manner as Example 5 except that the thickness of the first layer 21 was changed. By changing the thickness of the first layer 21, the content of LTO is also changed.

Comparative Example 1
A particle dispersion was prepared by mixing the graphitic material, the binder and the solvent. The mixing ratio of the solid content is “graphite-based material: binder = 99: 1 (mass ratio)”. The particle dispersion was applied to the surface of the current collector 10 and dried to form the negative electrode active material layer 20. The negative electrode active material layer 20 was compressed. Thus, the negative electrode 200 was manufactured. The negative electrode active material layer 20 of Comparative Example 1 has a single layer structure. The negative electrode active material layer 20 of Comparative Example 1 does not contain LTO.

Comparative Example 2
A particle dispersion was prepared by mixing the graphitic material, LTO, binder and solvent. The particle dispersion was applied to the surface of the current collector 10 and dried to form the negative electrode active material layer 20. The negative electrode active material layer 20 was compressed. Thus, the negative electrode 200 was manufactured. In Comparative Example 2, the content of LTO is 20% by mass with respect to the total of the graphitic material and LTO. The negative electrode active material layer 20 of Comparative Example 2 has a single-layer structure. Comparative Example 2 is an example in which the graphitic material and LTO are simply mixed.

Comparative Example 3
A negative electrode 200 was manufactured in the same manner as Comparative Example 2 except that the content of LTO was changed as shown in Table 1 below.

Comparative Example 4
The first layer 21 was formed on the current collector 10 in the same manner as the above “A. Formation of the first layer”. The second layer 22 was formed by applying the particle dispersion of Comparative Example 1 (particle dispersion containing a graphite-based material) on the surface of the first layer 21 and drying it. Except for these, the negative electrode 200 was manufactured in the same manner as in Comparative Example 1. The negative electrode active material layer 20 of Comparative Example 4 has a two-layer structure. The negative electrode active material layer 20 of Comparative Example 4 is formed by laminating a layer containing a graphite-based material and a layer containing LTO.

<Evaluation>
<< Battery capacity >>
FIG. 8 is a schematic view showing an example of the configuration of a lithium ion secondary battery.
The positive electrode 100 was prepared. The positive electrode 100 is in the form of a sheet. The positive electrode 100 includes a positive electrode active material layer. The composition of the positive electrode active material layer is “positive electrode active material: conductive material: binder = 94.4: 4.1: 1.5 (mass ratio)”. The positive electrode active material is nickel cobalt lithium aluminate. The conductive material is AB. The binder is polyvinylidene fluoride (PVdF).

  The separator 300 was prepared. The separator 300 is a porous film made of polyethylene (PE). The separator 300 has a thickness of 15 μm. The positive electrode 100, the separator 300, and the negative electrode 200 are stacked in this order to form an electrode group 400. The electrode group 400 and the electrolyte were stored in the case 500. Case 500 was sealed. Thus, a battery 1000 (lithium ion secondary battery) was manufactured.

  The battery capacity of the battery 1000 was measured. The results are shown in Table 1 below. The values shown in the “Battery capacity” column of Table 1 below are relative values when the battery capacity of Comparative Example 1 is “100”. If the battery capacity is 90 or more, it is considered to be an acceptable level of battery capacity.

"Peeling test"
A nail sticking test of the battery 1000 was conducted. During the nail penetration test, the surface temperature of case 500 was measured. The "final temperature" in Table 1 below indicates the maximum value of the surface temperature during the nailing test. It is considered that the heat generation at the time of the internal short circuit is suppressed as the ultimate temperature is lower.

<Result>
The comparative example 1 has the largest heat generation at the time of the internal short circuit. It is considered that the negative electrode active material layer 20 does not contain LTO.

  In Comparative Examples 2 and 3, the negative electrode active material layer 20 is formed by simply mixing the graphite material and LTO. In Comparative Example 2, the content of LTO is 20% by mass. However, the heat suppression effect is slight. In Comparative Example 3, the content of LTO is 30% by mass. In the comparative example 3, the heat | fever suppression effect is recognized. However, in Comparative Example 3, the battery capacity is significantly reduced. In Comparative Example 3, the battery capacity is below the allowable level "90".

  In Comparative Example 4, the negative electrode active material layer 20 is formed by laminating a layer containing a graphite-based material and a layer containing LTO. In Comparative Example 4, heat generation is suppressed more than in Comparative Example 2 due to a small amount of LTO. However, Comparative Example 4 has a small effect of suppressing heat generation. It is considered that the short circuit current can propagate in the in-plane direction.

  Examples 1 to 7 have a large effect of suppressing heat generation. In addition to the thickness direction, it is considered that the propagation of the short circuit current is suppressed also in the in-plane direction. In Examples 1 to 7, a battery capacity of 90 or more is secured. That is, in Examples 1 to 7, it is considered that the heat generation at the time of the internal short circuit can be suppressed while suppressing the decrease of the battery capacity. In the structures of the first to seventh embodiments, since the propagation of the short circuit current is efficiently suppressed, it is considered that the heat generation is suppressed by a small amount of LTO.

  In the results of Examples 5 and 7, when the first layer 21 has a thickness of 2 μm or more, a tendency is observed that heat generation is suppressed.

  In the results of Examples 4 and 6, when the minimum width (W1) of the first region 1 is 5 μm or more, a tendency is observed that heat generation is suppressed.

  The embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The technical scope defined by the description of the claims includes all the modifications within the meaning and scope equivalent to the descriptions of the claims.

  1 first region, 2 second region, 3 wet granules, 10 current collector, 20 negative electrode active material layer, 21 first layer, 22 second layer, 100 positive electrode, 200 negative electrode (negative electrode for lithium ion secondary battery), 300 separator, 400 electrode group, 500 case, 1000 battery (lithium ion secondary battery), 2001 1st rotating roll, 2002 2nd rotating roll, 2003 3rd rotating roll.

Claims (1)

At least a current collector and a negative electrode active material layer,
The negative electrode active material layer contains a graphite material and lithium titanate as a negative electrode active material,
The negative electrode active material layer is formed on the surface of the current collector,
The negative electrode active material layer includes at least a first layer and a second layer,
The first layer is disposed between the current collector and the second layer,
The second layer includes a first region and a plurality of second regions,
In plan view of the second layer, each of the plurality of second regions is scattered in an island shape in the first region,
The lithium titanate is disposed in the first layer and the first region,
The graphitic material is disposed in each of the plurality of second regions,
Negative electrode for lithium ion secondary battery.

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