JP2018125077A - Negative electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the worsening of the cycle durability of a lithium ion secondary battery that has a negative electrode including a silicon oxide.SOLUTION: A negative electrode 101 for a lithium ion secondary battery comprises: a current collector 1; and a negative electrode active material layer 100. The negative electrode active material layer 100 is disposed on the surface of the current collector 1. The negative electrode active material layer 100 includes a first layer 10 and a second layer 20. The first layer 10 and the second layer 20 are laminated in a thickness direction of the negative electrode active material layer 100. The first layer 10 includes conglobated graphite 11 and does not include silicon oxide 22. The second layer 20 includes flake-like crystalline graphite 21 and silicon oxide 22 and does not include conglobated graphite 11. The silicon oxide 22 accounts for a mass percentage of 1 mass% or more and 15 mass% or less to a total mass of the conglobated graphite 11, the flake-like crystalline graphite 21 and the silicon oxide 22.SELECTED DRAWING: Figure 1

Description

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

  Japanese Unexamined Patent Publication No. 2002-373653 (Patent Document 1) discloses a negative electrode for a lithium ion secondary battery (hereinafter sometimes abbreviated as “negative electrode”) containing silicon oxide coated with a conductive material and graphite. Is disclosed.

JP 2002-373653 A

  Silicon oxide has been studied as a negative electrode active material for lithium ion secondary batteries (hereinafter sometimes abbreviated as “battery”). Silicon oxide has a large charge / discharge capacity. The use of silicon oxide is expected to increase the battery capacity.

  However, silicon oxide expands greatly during charging. When silicon oxide expands, the entire negative electrode active material layer containing silicon oxide also expands. It is considered that the cycle durability of the battery is lowered by the expansion of the entire negative electrode active material layer. Silicon oxide has low electronic conductivity. The low electronic conductivity of silicon oxide is also considered to be a factor that reduces cycle durability.

  The objective of this indication is in suppressing the fall of the cycle durability of the lithium ion secondary battery which contains a silicon oxide in a negative electrode.

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

  The negative electrode for a lithium ion secondary battery includes a current collector and a negative electrode active material layer. The negative electrode active material layer is disposed on the surface of the current collector. The negative electrode active material layer includes a first layer and a second layer. The first layer and the second layer are laminated in the thickness direction of the negative electrode active material layer. The first layer contains spheroidized graphite and does not contain silicon oxide. The second layer contains scaly graphite and silicon oxide and does not contain spheroidized graphite. Silicon oxide has a mass ratio of 1% by mass to 15% by mass with respect to the total of spheroidized graphite, scaly graphite, and silicon oxide.

  In the negative electrode active material layer of the present disclosure, silicon oxide is localized. That is, the second layer contains silicon oxide, and the first layer does not contain silicon oxide. The second layer further includes flaky graphite. Scaly graphite has a bulky structure. Therefore, many voids are formed in the second layer. The expansion of silicon oxide is expected to be absorbed in the voids in the second layer. This is expected to suppress the expansion of the second layer containing silicon oxide, and thus the expansion of the entire negative electrode active material layer.

  The first layer made of spheroidized graphite has good electron conductivity. By including the first layer in the negative electrode active material layer, it is expected that a decrease in electronic conductivity due to the silicon oxide contained in the negative electrode active material layer is suppressed. By synergizing the above actions, it is expected that a decrease in cycle durability of the lithium ion secondary battery is suppressed.

  Silicon oxide has a mass ratio of 1% by mass to 15% by mass with respect to the total of spheroidized graphite, scaly graphite, and silicon oxide. When the mass ratio of silicon oxide is less than 1% by mass, it is difficult to increase the capacity of the battery. When the mass ratio of silicon oxide exceeds 15% by mass, the effect of suppressing a decrease in cycle durability may be reduced.

FIG. 1 is a conceptual cross-sectional view illustrating a configuration of a negative electrode for a lithium ion secondary battery according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram showing the configuration of the electrode group. FIG. 3 is a schematic diagram showing the configuration of the lithium ion secondary battery.

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

<Anode for lithium ion secondary battery>
FIG. 1 is a conceptual cross-sectional view illustrating a configuration of a negative electrode for a lithium ion secondary battery according to an embodiment of the present disclosure. The negative electrode 101 includes the current collector 1 and the negative electrode active material layer 100.

<Current collector>
The current collector 1 may be, for example, a copper (Cu) foil. The Cu foil may be a pure Cu foil or a Cu alloy foil. The current collector 1 may have a thickness of 5 to 30 μm, for example.

<Negative electrode active material layer>
The negative electrode active material layer 100 is disposed on the surface of the current collector 1. The negative electrode active material layer 100 can be formed, for example, by applying a paste containing a negative electrode active material to the surface of the current collector 1. The negative electrode active material layer 100 may be disposed on both surfaces (front and back surfaces) of the current collector 1. The negative electrode active material layer 100 may have a thickness of 10 to 100 μm, for example.

  The negative electrode active material layer 100 includes a two-layer structure. That is, the negative electrode active material layer 100 includes the first layer 10 and the second layer 20. The first layer 10 and the second layer 20 are stacked in the thickness direction of the negative electrode active material layer 100. The first layer 10 and the second layer 20 may be laminated in the order of the current collector 1, the first layer 10 and the second layer 20, or the current collector 1, the second layer 20 and the first layer 10. They may be laminated in the order. In any stacking order, it is considered that the effect of suppressing the decrease in cycle durability is exhibited.

<< First layer >>
The first layer 10 includes spheroidized graphite 11. The first layer 10 does not contain silicon oxide 22. The spheroidized graphite 11 functions as a negative electrode active material. “Sphericalized graphite” refers to flaky graphite that has been subjected to spheroidizing treatment. The spheroidization process indicates a process of bringing the outer shape of the scaly graphite closer to a sphere by, for example, friction in airflow and pulverization. Spherical graphite is expected to show excellent cycle durability because the active edge surface is less exposed to the electrolyte. The surface of the spheroidized graphite 11 may be covered with amorphous carbon. The spheroidized graphite 11 may have an average particle diameter of 1 to 30 μm, for example. The “average particle size” in the present specification indicates a particle size of 50% cumulative from the fine particle side in the volume-based particle size distribution measured by the laser diffraction scattering method.

  As long as the first layer 10 includes the spheroidized graphite 11, it may further include, for example, a binder. The binder may be, for example, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polyacrylic acid (PAA), polyimide, or the like. One kind of binder may be used alone, or two or more kinds of binders may be used in combination. The binder may have a mass ratio of 0.5 to 5% by mass with respect to the spheroidized graphite 11, for example.

  The first layer 10 composed of the spheroidized graphite 11 has good electron conductivity. By including the first layer 10 in the negative electrode active material layer 100, it is expected that a decrease in electronic conductivity due to the silicon oxide 22 being included in the negative electrode active material layer 100 is suppressed. The first layer 10 may be disposed directly on the surface of the current collector 1. It is expected that electronic conduction between the negative electrode active material layer 100 and the current collector 1 is promoted by arranging the first layer 10 directly on the surface of the current collector 1.

<< Second layer >>
The second layer 20 includes flaky graphite 21 and silicon oxide 22. The second layer 20 does not contain the spheroidized graphite 11. The flaky graphite 21 has a bulky structure. A number of voids are formed in the second layer 20. The expansion of the silicon oxide 22 is expected to be absorbed by the voids in the second layer 20. This is expected to suppress the expansion of the second layer 20 and thus the entire negative electrode active material layer 100. Since the spheroidized graphite 11 is low in volume, if the second layer 20 includes the spheroidized graphite 11, the effect of suppressing the expansion of the entire negative electrode active material layer 100 may be reduced.

  The flaky graphite 21 functions as a negative electrode active material and a conductive material. The flaky graphite 21 may have an average particle diameter of 1 to 30 μm, for example. The flaky graphite 21 may have, for example, an oil absorption of 100 ml / 100 g or more and 400 ml / 100 g or less, an oil absorption of 200 ml / 100 g or more and 400 ml / 100 g or less, or 300 ml / 100 g or more. It may have an oil absorption of 400 ml / 100 g or less. This is expected to increase the effect of suppressing the expansion of the entire negative electrode active material layer 100. The “oil absorption amount” in the present specification is measured by a method based on “JIS 6217-4 (carbon black for rubber—basic characteristics—part 4: determination of oil absorption amount (including compressed sample))”. The absorption amount of dibutyl phthalate (DBP) is shown.

  In addition to the scaly graphite 21, the second layer 20 may further include a bulky negative electrode active material, a bulky conductive material, and the like. This is expected to increase the effect of suppressing the expansion of the entire negative electrode active material layer 100. Examples of the bulky negative electrode active material include scaly graphite and exfoliated graphite. Examples of the bulky conductive material include graphene flakes, furnace black, acetylene black, thermal black, vapor grown carbon fiber (VGCF), and the like. These materials may be used singly or in combination of two or more. These materials may also have an oil absorption of, for example, 100 ml / 100 g or more and 400 ml / 100 g or less.

  The silicon oxide 22 has a larger charge / discharge capacity than the spheroidized graphite 11 and the flaky graphite 21. When the second layer 20 contains the silicon oxide 22, it is expected that the battery has a higher capacity. The silicon oxide 22 expands greatly during charging. In the present embodiment, it is expected that the expansion of the silicon oxide 22 is absorbed by the voids in the second layer 20.

The silicon oxide 22 contains silicon (Si) and oxygen (O). The silicon oxide 22 can have any conventionally known atomic ratio. The silicon oxide 22 may be represented by, for example, a composition formula SiO _x (where x satisfies 0.5 ≦ x ≦ 1.5). Silicon oxide 22 may have an average particle diameter of 0.1 to 30 μm, for example.

  The silicon oxide 22 has a mass ratio of 1% by mass or more and 15% by mass or less with respect to the total of the spheroidized graphite 11, the scaly graphite 21, and the silicon oxide 22. When the mass ratio of the silicon oxide 22 is less than 1% by mass, it is difficult to increase the capacity of the battery. When the mass ratio of silicon oxide exceeds 15% by mass, the effect of suppressing a decrease in cycle durability may be reduced.

  The silicon oxide 22 may have a mass ratio of 10% by mass or less with respect to the total of the spheroidized graphite 11, the flaky graphite 21, and the silicon oxide 22. As a result, it is expected that the effect of suppressing a decrease in cycle durability is further increased. The silicon oxide 22 may have a mass ratio of 5 mass% or more with respect to the total of the spheroidized graphite 11, the flaky graphite 21, and the silicon oxide 22. As a result, higher capacity is expected.

  The silicon oxide 22 may have a mass ratio of 2.2 mass% or more and 25 mass% or less with respect to the total of the scaly graphite 21 and the silicon oxide 22. Thereby, it is expected that the effect of suppressing the expansion of the negative electrode active material layer 100 is increased. The silicon oxide 22 may have a mass ratio of 20% by mass or less or a mass ratio of 18.2% by mass or less with respect to the total of the flaky graphite 21 and the silicon oxide 22. You may have a mass ratio of 10 mass% or less.

  Similarly to the first layer 10, the second layer 20 may include a binder. As the binder, a binder similar to the binder described in the first layer 10 can be used. The binder may have a mass ratio of, for example, 0.5 to 5% by mass with respect to the total of the scaly graphite 21 and the silicon oxide 22.

  Examples will be described below. However, the following examples do not limit the scope of the present disclosure.

<Example 1>
1. Production of negative electrode The following materials were prepared.
Negative electrode active material: spheroidized graphite, scaly graphite, silicon oxide Binder: CMC, SBR
Solvent: water Current collector: Cu foil (thickness = 10 μm)

  The spheroidized graphite, the binder and the solvent were mixed by the mixer. Thereby, the first paste was prepared. The solid content composition of the first paste was “spheroidized graphite: CMC: SBR = 98.6: 0.7: 0.7” by mass ratio.

  The flake graphite, silicon oxide, binder and solvent were mixed by a mixer. Thereby, the 2nd paste was prepared. The solid content composition of the second paste was “(flaky graphite + silicon oxide): CMC: SBR = 98.6: 0.7: 0.7” by mass ratio.

  The 1st paste was apply | coated to the surface (front and back both surfaces) of the electrical power collector 1 with the die coater, and it dried. Thereby, the first layer 10 was formed. The second paste was applied to the surface of the first layer 10 by a die coater and dried. Thereby, the second layer 20 was formed, and the negative electrode active material layer 100 was formed. In the negative electrode active material layer 100, the mass ratio of silicon oxide was 5% by mass with respect to the total of spheroidized graphite, scaly graphite, and silicon oxide.

  The negative electrode active material layer 100 was compressed by a roll mill. Thereby, the negative electrode 101 was manufactured. The negative electrode 101 was cut into a predetermined dimension. The configuration of the negative electrode 101 is as follows.

Negative electrode active material layer width: 122 mm
Length of negative electrode active material layer: 5835mm
Negative electrode thickness: 144 μm

2. Production of positive electrode The following materials were prepared: Positive electrode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂
Conductive material: Acetylene black Binder: PVdF
Solvent: N-methyl-2-pyrrolidone Current collector: Aluminum (Al) foil (thickness = 15 μm)

  The positive electrode active material, the conductive material, the binder, and the solvent were mixed by the mixer. This prepared a paste. The solid content composition was “positive electrode active material: conductive material: binder = 93: 4: 3” by mass ratio. The paste was applied to the surface (both front and back surfaces) of the current collector by a die coater and dried. As a result, a positive electrode active material layer was formed. The positive electrode active material layer was compressed by a roll mill. This produced a positive electrode. The positive electrode was cut to a predetermined size. The configuration of the positive electrode is as follows.

Width of positive electrode active material layer: 117 mm
Length of positive electrode active material layer: 5681 mm
Positive electrode thickness: 142 μm

3. Assembly A separator having a thickness of 20 μm was prepared. The separator is configured by laminating a polypropylene porous film, a polyethylene porous film, and a polypropylene porous film in this order.

FIG. 2 is a schematic diagram showing the configuration of the electrode group. The positive electrode 201, the separator 300, and the negative electrode 101 were laminated so that the positive electrode 201 and the negative electrode 101 face each other with the separator 300 interposed therebetween, and were further wound in a spiral shape. Thereby, a cylindrical electrode group was formed. A cylindrical electrode group was formed into a flat shape by a flat plate press. The molding pressure was 4 kN / cm ² . The molding time was 2 minutes. Thereby, the electrode group 400 was manufactured.

  FIG. 3 is a schematic diagram showing the configuration of the lithium ion secondary battery. An outer package 500 was prepared. The exterior body 500 has a square shape. The outer package 500 is made of an Al alloy. The electrode group 400 was accommodated in the outer package 500. An external terminal was connected to the electrode group 400. 120 ml of the electrolytic solution was injected into the outer package 500. The electrolytic solution has the following composition.

Solute: LiPF ₆ (1.0 mol / l)
Solvent: [EC: DMC: EMC = 3: 4: 3 (volume ratio)]
EC represents ethylene carbonate, DMC represents dimethyl carbonate, and EMC represents ethyl methyl carbonate.

  The outer package 500 was sealed. Thus, the battery 1000 was manufactured. Battery 1000 is designed to have a rated capacity of 35 Ah in the voltage range of 3.0-4.1V.

<Examples 2, 4 to 6>
The same as Example 1 except that the coating amount of the first layer and the second layer and the solid content composition of the second paste are changed so that the silicon oxide has the mass ratio shown in Table 1 below. The negative electrode was manufactured by the manufacturing method, and the battery was manufactured.

<Example 3>
A negative electrode was manufactured and a battery was manufactured by the same manufacturing method as in Example 1 except that the current collector, the second layer, and the first layer were laminated in this order.

<Comparative Example 1>
Spherical graphite, scaly graphite, silicon oxide, a binder and a solvent were mixed by a mixer. This prepared a paste. The solid content composition of the paste was “(spheroidized graphite + flaky graphite + silicon oxide): CMC: SBR = 98.6: 0.7: 0.7” by mass ratio. The mass ratio of silicon oxide was 5 mass% with respect to the total of spheroidized graphite, scaly graphite and silicon oxide. The paste was applied to the surface of the current collector by a die coater and dried. As a result, a single-layer negative electrode active material layer was formed. Except for this, the negative electrode was manufactured and the battery was manufactured by the same manufacturing method as in Example 1.

<Comparative example 2>
The flake graphite, silicon oxide, binder and solvent were mixed by a mixer. This prepared a paste. The solid content composition of the paste was “(flaky graphite + silicon oxide): CMC: SBR = 98.6: 0.7: 0.7” by mass ratio. The mass ratio of silicon oxide was 10% by mass with respect to the total of the scaly graphite and silicon oxide. Except for this, the negative electrode was manufactured and the battery was manufactured by the same manufacturing method as Comparative Example 1.

<Evaluation of cycle durability>
One cycle of the following constant current charging and constant current discharging was taken as one cycle, and 500 cycles were carried out.

Constant current charging: Current = 70A, end voltage = 4.1V
Constant current discharge: Current = 70A, end voltage = 3.0V

  The capacity retention rate was calculated by dividing the discharge capacity after 500 cycles by the initial discharge capacity. The results are shown in Table 1 below. In Table 1 below, the value shown in the column of “cycle durability” is a value obtained by dividing the capacity retention rate of each example by the capacity retention rate of Comparative Example 1. The larger the value, the lower the cycle durability is suppressed.

<Result>
Comparative Example 1 has low cycle durability. In Comparative Example 1, the negative electrode active material layer is a single layer. Since spheroidized graphite, scaly graphite and silicon oxide coexist in one layer, it is considered that voids are hardly formed around silicon oxide.

  In Comparative Example 2, the cycle durability is further low. Since there is no layer (first layer) composed of spheroidized graphite, it is considered that the electronic conductivity of the entire negative electrode active material layer is low.

  In the example, a decrease in cycle durability is suppressed as compared with the comparative example. The following reasons can be considered. Since the voids are formed in the second layer by the scaly graphite, the expansion of silicon oxide is absorbed in the voids. Since the electronic conductivity of the first layer containing the spheroidized graphite is good, a decrease in electronic conductivity due to the silicon oxide contained in the negative electrode active material layer is suppressed.

  The above embodiments and examples are to be considered in all respects as illustrative and not restrictive. The scope of the present disclosure is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Current collector, 10 1st layer, 11 Spheroidized graphite, 20 2nd layer, 21 Scale-like graphite, 22 Silicon oxide, 100 Negative electrode active material layer, 101 Negative electrode, 201 Positive electrode, 300 Separator, 400 Electrode group, 500 Exterior Body, 1000 batteries.

Claims (1)

Including a current collector and a negative electrode active material layer,
The negative electrode active material layer is disposed on the surface of the current collector,
The negative electrode active material layer includes a first layer and a second layer,
The first layer and the second layer are laminated in the thickness direction of the negative electrode active material layer,
The first layer includes spheroidized graphite and does not include silicon oxide;
The second layer contains scale-like graphite and the silicon oxide, and does not contain the spheroidized graphite,
The silicon oxide has a mass ratio of 1% by mass to 15% by mass with respect to the total of the spheroidized graphite, the flaky graphite, and the silicon oxide.
Negative electrode for lithium ion secondary battery.

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