WO2014112619A1

WO2014112619A1 - Copper foil, anode for lithium ion battery, and lithium ion secondary battery

Info

Publication number: WO2014112619A1
Application number: PCT/JP2014/050942
Authority: WO
Inventors: 健作篠崎; 鈴木　昭利; 季実子藤澤; 健繪面
Original assignee: 古河電気工業株式会社
Priority date: 2013-01-18
Filing date: 2014-01-20
Publication date: 2014-07-24
Also published as: JPWO2014112619A1; TW201448338A

Abstract

[Problem] To provide a copper foil that does not easily wrinkle at the time of press working or at the time of charging or discharging, that is capable of maintaining a high degree of adhesion to carbon-based and Si-based anode active materials, and that has mechanical strength (tensile strength) and hardness, and to provide an anode for a lithium ion battery using the same, and a lithium ion secondary battery using the same. [Solution] A copper foil that has a first copper layer having a tensile strength of at least 550MPa and a second copper layer that is provided on at least one side of the first copper layer and that has a hardness less than that of the first copper layer. The tensile strength of the copper foil measured at room temperature after one hour of heat processing at 300ºC is at least 400MPa.

Description

Copper foil, negative electrode for lithium ion battery and lithium ion secondary battery

The present invention relates to a copper foil, a negative electrode for a lithium ion secondary battery using the copper foil, and a lithium ion secondary battery using the copper foil, and in particular, mechanical strength (tensile strength) suitable for a negative electrode for a lithium ion secondary battery. And a copper foil having hardness, a negative electrode for a lithium ion secondary battery using the same, and a lithium ion secondary battery using the same.

In recent years, the development of next-generation negative electrode active materials having charge / discharge capacities far exceeding the theoretical capacity of carbon materials has been promoted as negative electrode active materials for lithium ion secondary batteries. For example, a material containing a metal that can be alloyed with lithium (Li) such as silicon (Si), germanium (Ge), or tin (Sn) is expected.

In particular, when Si, Sn, or the like is used as an active material, these materials have a large volume change due to insertion / extraction of Li during charge / discharge, and thus maintain a good adhesion state between the current collector and the active material It is difficult. In addition, these materials have a very large volume change rate due to Li insertion and desorption, and are repeatedly expanded and contracted by the charge / discharge cycle, so that the active material particles are pulverized or desorbed. Has the disadvantage of being very large.

In order to eliminate such drawbacks, a proposal has been made to use a polyimide binder in order to improve the adhesion between the active material and the current collector. However, since the curing temperature of the polyimide binder is about 300 ° C., when the conventional electrolytic copper foil is used as a current collector, the electrolytic copper foil is annealed at a temperature of about 300 ° C., and recrystallization proceeds and softens. The active material cannot withstand volume expansion and may break.
For this reason, there is a need for an electrolytic copper foil that is less softened even when heat treatment at about 300 ° C. is performed and that has high mechanical strength such as high tensile strength.
Hereinafter, the mechanical strength refers to tensile strength and the like.

For example, Patent Document 1 has a rough surface with a rough surface roughness Rz of 2.0 μm or less and a uniform low roughness as an optimal copper foil for a negative electrode current collector for a secondary battery, at 180 ° C. A low rough surface electrolytic copper foil having an elongation of 10.0% or more is described.
The above-mentioned electrolytic copper foil is obtained by using a sulfuric acid-copper sulfate aqueous solution as an electrolytic solution and adding polyethyleneimine or a derivative thereof, a sulfonate salt of an active organic sulfur compound, and chloride ions.

Further, in Patent Document 2, the rough surface roughness Rz of the electrolytic copper foil is 2.5 μm or less, and the tensile strength at 25 ° C. measured within 20 minutes from the completion of electrodeposition is 820 MPa or more. An electrolytic copper foil is described in which the rate of decrease in tensile strength at 25 ° C. measured after 300 minutes from the completion of electrodeposition with respect to tensile strength at 25 ° C. measured within 20 minutes from the completion time is 10% or less. .
The electrolytic copper foil is obtained by adding hydroxyethyl cellulose, polyethyleneimine, sulfonic acid salt of active organic sulfur compound, acetylene glycol, and chloride ion using sulfuric acid-copper sulfate aqueous solution as an electrolytic solution.

Further, Patent Document 3 discloses an electrodeposited copper foil having a grain structure having no columnar grains and twin boundaries and having an average grain size of up to 10 μm, and the grain structure is substantially uniform and randomly. A controlled low profile electrodeposited copper foil is described which is an oriented grain structure.
This electrodeposited copper foil has a maximum tensile strength at 23 ° C. of 87,000 to 120,000 psi (600 MPa to 827 MPa) and a maximum tensile strength at 180 ° C. of 25,000 to 35,000 psi (172 MPa to 172 MPa). 241 MPa).

The conventional high-strength electrolytic copper foils disclosed in Patent Documents 1 to 3 have a high mechanical strength in a normal state, and the mechanical strength hardly changes even when heated at around 180 ° C. However, when such a foil is heated at about 300 ° C., it is annealed and recrystallization proceeds, so that it softens rapidly and the mechanical strength decreases.

On the other hand, for example, Patent Documents 4 and 5 describe a method of producing an electrolytic copper foil with an electrolytic solution in which W is added to a sulfuric acid-copper sulfate electrolytic solution, and further glue and chlorine ions are added. It is described that it is possible to manufacture a copper foil having a hot elongation rate of 3% or more, a rough surface having a large roughness, and less pinholes.
However, the copper foil produced by adding the above W has a high mechanical strength (tensile strength) at room temperature (RT), but the mechanical strength cannot be maintained when heat-treated at 300 ° C. for 1 hour.
As a result of analyzing this copper foil, it was found that W was not co-deposited in the electrodeposited copper.

Patent Document 6 describes a current collector having a surface layer portion that is easily plastically deformed on the surface of the current collector. Among them, it is described that the surface layer portion preferably has a Vickers hardness of 200 or less, preferably 160 or less.
In Patent Document 6, it is described as a current collector having a surface layer portion that is easily plastically deformed on the surface of the current collector, but in reality, the plastic deformation of the surface is not sufficient, and the shape of the active material is limited. Need arises. In Patent Document 6, the above problem is solved by increasing the roughness of the surface layer of the current collector, forming the active material layer into columnar particles, and providing voids between the active materials. Of course, the battery capacity can be reduced by providing a gap between the active materials, but when current flows locally, stress is concentrated on a part of the active material and the current collector, resulting in deterioration of cycle characteristics. A problem occurs.

Non-Patent Document 1 shows the correlation between Vickers hardness (HV) and hardness by nanoindentation (H _IT ), and describes that there is a relationship of HV = 0.945H _IT .
Therefore, when converting the hardness by nano-indenter of Patent Document 6 from the correlation of the non-patent document 1, 2116mgf / μm ² or less, it can be said that preferably has said 1693mgf / μm ² or less of the hardness is preferred.

When the negative electrode current collector of a lithium ion secondary battery is made using the above-mentioned conventional copper foil, if the mechanical strength (tensile strength) of the copper foil is low, press working in the electrode forming process of the lithium ion secondary battery There are disadvantages that can easily lead to wrinkles.
Moreover, when the mechanical strength (tensile strength) of the copper foil is low, there is a disadvantage that wrinkles easily occur as the secondary battery is repeatedly charged and discharged.
In addition, when the hardness of the copper foil is high, for example, a carbon-based or Si-based negative electrode active material is difficult to bite into the copper foil, the adhesion between the active material and the copper foil tends to be low, and the battery capacity and cycle characteristics are reduced. There is a disadvantage that it is easy to do.
For this reason, lithium having a mechanical strength (tensile strength) and hardness that is difficult to cause wrinkles even during press working and during charge and discharge and that can maintain high adhesion between the Si-based negative electrode active material and the copper foil. There is a need for a copper foil suitable for a negative electrode current collector of an ion secondary battery.

Japanese Patent No. 4120806 Japanese Patent No. 4273309 Japanese Patent No. 3,270,637 Japanese Patent No. 3238278 JP-A-9-67693 JP 2008-98157 A

The object of the present invention is to provide mechanical strength (tensile strength) and surface hardness that are difficult to cause wrinkles and deformation during pressing and charging / discharging and that can maintain high adhesion between the negative electrode active material and the copper foil. It is intended to provide a copper foil having a negative electrode for a lithium ion secondary battery excellent in cycle characteristics using the copper foil and a lithium ion secondary battery using the same.

As a result of diligent research, the present inventor has mechanical strength (see FIG. 5) that is resistant to wrinkles even during press working and charge / discharge, and that can maintain high adhesion between a carbon-based and Si-based negative electrode active material and copper foil ( We have succeeded in developing a copper foil having tensile strength and surface hardness.

The copper foil according to the present invention includes a first copper layer having a tensile strength of 550 MPa or more at room temperature, and a second copper layer having a lower hardness than the first copper layer on at least one surface of the first copper layer. Have.

The copper foil of the present invention preferably has a total thickness of 7 to 12 μm including the first copper layer and the second copper layer.

In the above-described copper foil of the present invention, the thickness of the second copper layer is preferably 0.3 to 3 μm.

In the copper foil of the present invention, preferably, the thickness of the first copper layer is equal to or greater than the thickness of the second copper layer.

The copper foil of the present invention preferably has a Young's modulus at room temperature of the first copper layer of 75 to 130 GPa.

The copper foil of the present invention preferably has a tensile strength at room temperature of 500 MPa or more.

The copper foil of the present invention preferably has a Young's modulus at room temperature of 70 to 120 GPa.

The copper foil of the present invention preferably has a surface hardness of 180 to 370 mgf / μm ² at room temperature.

The copper foil of the present invention preferably has a tensile strength of 400 MPa or more measured at room temperature after heat treatment at 300 ° C. for 1 hour.

The copper foil of the present invention preferably has a Young's modulus of 65 GPa or more measured at room temperature after heat treatment at 300 ° C. for 1 hour.

The negative electrode for a lithium ion secondary battery of the present invention is a negative electrode using the copper foil described above as a current collector.

In the lithium ion secondary battery of the present invention, the negative electrode for a lithium ion secondary battery described above is used as the negative electrode.

According to the copper foil of the present invention, by providing a second copper layer having a lower hardness than the first copper layer on at least one surface of the first copper layer having a tensile strength of 550 MPa or more at room temperature, Copper having mechanical strength (tensile strength) and surface hardness that is resistant to wrinkles even during press working and charging and discharging, and that can maintain high adhesion between carbon-based and Si-based negative electrode active materials and copper foil. A foil can be provided.

The negative electrode for a lithium ion secondary battery of the present invention is a current collector constituting the negative electrode, and is not easily wrinkled at the time of pressing and charging / discharging, and a carbon-based and Si-based negative electrode active material and a copper foil Since the copper foil which has the mechanical strength (tensile strength) and hardness which hold | maintains high adhesiveness is used, the negative electrode for lithium ion secondary batteries excellent in charging / discharging characteristics can be provided.

The lithium ion secondary battery of the present invention can provide a battery having excellent charge / discharge characteristics by using the negative electrode.

Hereinafter, embodiments of a copper foil according to the present invention, a negative electrode for a lithium ion secondary battery using the copper foil, and a lithium ion secondary battery using the copper foil will be described in detail.

<First Embodiment>
[Composition of copper foil]
The copper foil according to this embodiment includes a first copper layer having a tensile strength of 550 MPa or more at room temperature, a second copper layer having a lower hardness than the first copper layer provided on at least one surface of the first copper layer, Have
As the first copper layer, for example, an electrolytic copper foil (including an electrolytic copper alloy foil, the same applies hereinafter) or a rolled copper foil (including a rolled copper alloy foil, the same applies hereinafter) can be used.

The second copper layer can be formed on the surface of the first copper layer by any method such as electrolytic treatment such as electrolytic plating, electroless treatment such as sputtering or chemical vapor deposition, and lamination (cladding). it can.

In the copper foil of this embodiment, the first copper layer is a copper layer having a tensile strength of 550 MPa or more at room temperature, and the second copper layer is a copper layer having a lower hardness than the first copper layer.
By adopting the copper foil of the present embodiment as a current collector for a negative electrode for a lithium ion secondary battery, the copper foil is less likely to be wrinkled even during press working and charge / discharge, and carbon-based and Si The mechanical strength (tensile strength) and hardness that can maintain high adhesion between the negative electrode active material and the copper foil (current collector) can be achieved.

The copper foil of this embodiment preferably has a smooth surface, and Ra is preferably 1.0 μm or less and Rz is preferably 4.0 μm or less (JIS B 0601: 1994). If the surface of the copper foil has large irregularities, the contact area of the active material is reduced, or the active material cannot enter the valleys of the irregularities, and the adhesion between the active material and the copper foil is likely to decrease, and the cycle characteristics are improved. It is because it falls.

In the copper foil of this embodiment, the first copper layer is preferably made of an electrolytic copper alloy foil or a rolled copper alloy foil containing at least tungsten (W) or molybdenum (Mo).
By using a copper foil containing at least tungsten or molybdenum, a tensile strength at room temperature of 550 MPa or more can be realized.
The first copper layer is not limited to the above-described electrolytic copper alloy foil containing tungsten or molybdenum, and may be used as long as the electrolytic copper foil has a tensile strength at room temperature of 550 MPa or more. It is.

In the copper foil of this embodiment, when used as a current collector for a lithium ion secondary battery, the total thickness of the copper foil including the first copper layer and the second copper layer is preferably 7 to 12 μm.
If the thickness is less than 7 μm, the handleability of the copper foil is lowered, and if it exceeds 12 μm, the energy density is lowered and it is not suitable for the negative electrode current collector for lithium ion secondary batteries. However, when used for other uses such as printed wiring boards and suspension materials, other foil thicknesses can be used.

In the copper foil of this embodiment, the thickness per one side of the second copper layer provided on the surface of the first copper layer (the thickness of one side when the second copper layer is provided on both sides) is 0.3 to 3 μm. Is preferred.
This is because if the thickness is less than 0.3 μm, the hardness of the surface of the copper foil may be inappropriate, and if it exceeds 3 μm, there is almost no effect of improving the adhesion to the active material. For this reason, the thickness of the second copper layer provided on one side of the first copper layer is practically 3 μm or less. In order to obtain the maximum effect of the present invention, the thickness of the second copper layer is preferably thinner than the thickness of the first copper layer.

It can be set as the structure which balanced the mechanical strength (tensile strength) and surface hardness of copper foil because the thickness of a 1st copper layer is more than the thickness of a 2nd copper layer.

In the copper foil of this embodiment, the Young's modulus of the first copper layer at room temperature is preferably 75 to 130 GPa. This is because when the Young's modulus is less than 75 GPa, even if the breaking strength (tensile strength) of the copper foil is high, the copper foil is easily deformed by a low stress, and the cycle characteristics are liable to deteriorate when the current collector is used. Moreover, when the copper foil whose Young's modulus is higher than 130 GPa is used as a negative electrode (current collector),
Breakage may occur in the current collector (copper foil) due to stress applied by expansion of the active material during charging, and the above range is preferable.
Note that the Young's modulus of the copper layer in which the thickness of the first copper layer is equal to or greater than the thickness of the second copper layer greatly contributes to the Young's modulus of the first copper layer. Is desirable.

In the copper foil in which the thickness of the first copper layer of the present embodiment is equal to or greater than the thickness of the second copper layer, the tensile strength of the copper foil is preferably 500 MPa or more at room temperature.
In the copper foil of this embodiment, the Young's modulus of the copper foil is preferably 70 to 120 GPa at room temperature.
In the copper foil of this embodiment, the hardness of the surface layer of the copper foil at room temperature is preferably 180 to 370 mgf / μm ² .
The copper foil of this embodiment has the above mechanical strength (tensile strength) and hardness, so that it is difficult for wrinkles to occur during press working when forming a negative electrode using the copper foil as a current collector. Even when charging and discharging the negative electrode, it is difficult for soot to enter, and high adhesion can be maintained between the carbon-based or Si-based negative electrode active material and the copper foil (current collector).

The copper foil of this embodiment preferably has a tensile strength of 400 MPa or more measured at room temperature after heat treatment at 300 ° C. for 1 hour.
In addition, the copper foil of the present embodiment preferably has a Young's modulus of 65 GPa or more measured at room temperature after heat treatment at 300 ° C. for 1 hour.
This is because when the Young's modulus of the copper foil is lower than 65 GPa, the copper foil is deformed by stress generated by expansion of the active material during charging, and the negative electrode may be short-circuited with the positive electrode.

If there is no problem such as a local battery or a significant increase in resistance between the first copper layer and the second copper layer, another metal layer, oxide layer, or organic layer may be appropriately interposed. it can.
It is also effective to appropriately provide another metal layer, oxide layer, organic material layer, or the like on the surface of the second copper layer.

The 1st copper layer of the copper foil of this embodiment can be manufactured by the electroplating using the plating bath which added the below-mentioned various additives, for example in the following copper plating bath basic composition and plating conditions.
Copper plating bath basic composition and plating conditions for first copper foil foil Copper concentration: 50 to 100 g / l
Sulfuric acid concentration: 40-100g / l
Liquid temperature: 40-70 ° C
Current density: 35-60 A / dm ²

Additive (A)
Chlorine, tungsten (W) or molybdenum (Mo) is added to the above copper plating bath basic composition.
Chlorine (Cl): 2 ppm or less W concentration: 20-300 ppm
Mo concentration: 10-100ppm
By electrolytic plating using the above plating bath, W or Mo is contained in the copper foil, and the first copper foil that becomes the first copper layer having high mechanical strength (tensile strength) can be formed.
In addition, you may contain 2 or more types of said metals of W or Mo. Moreover, you may add another metal suitably as needed.
The concentration of W or Mo may be added in excess of the above upper limit, but even if the concentration exceeds the upper limit, the influence on the improvement of the mechanical strength (tensile strength) of the first copper foil is small. In view of manufacturing costs, the above is preferably set as the upper limit.

Additive (B)
Chlorine (Cl), an organic additive, and a metal (W) are added to the above copper plating bath basic composition.
Cl concentration: 10-60ppm
Thioureas: 2-20ppm
W concentration: 10-200ppm
As thioureas, for example, thiourea (TU), ethylenethiourea (ETU), and tetramethylthiourea (TMTU) can be used.
By electrolytic plating using the above plating bath, a first copper layer made of copper containing W and having high mechanical strength (tensile strength) can be formed.

Additive (C)
Chlorine (Cl) and an organic additive are added to the above copper plating bath basic composition.
Cl concentration: 10-60ppm
Thioureas: 2-20ppm
HEC (hydroxyethyl cellulose): 0 to 20 ppm
As thioureas, for example, thiourea (TU), ethylenethiourea (ETU), and tetramethylthiourea (TMTU) can be used. Furthermore, glue may be added.
By electrolytic plating using the above plating bath, a first copper foil (first copper layer) made of Cu and having high mechanical strength (tensile strength) can be formed.

Moreover, you may use a rolled copper foil, for example as a 1st copper layer of the copper foil which concerns on this embodiment.
As the rolled copper foil, for example, a copper foil containing 0.03% of Zr with respect to Cu can be used.

As the first copper layer, a copper foil having a tensile strength of 550 MPa or more at room temperature can be appropriately used even if it is manufactured by a method other than the above.

The second copper layer of the copper foil according to this embodiment can be formed on at least one surface of the first copper layer by, for example, electrolytic treatment under the following copper plating bath basic composition and plating conditions.
Copper plating bath composition and plating conditions for the second copper foil foil Copper concentration: 50 to 100 g / l
Sulfuric acid concentration: 40-100g / l
Liquid temperature: 40-70 ° C
Current density: 35-60 A / dm ²

Also, the second copper layer formed on at least one surface of the first copper layer can be provided by, for example, electroless treatment such as sputtering or chemical vapor deposition, or lamination (cladding).

The thickness of the copper foil of this embodiment is 7 to 12 μm for the entire copper foil including the first copper layer and the second copper layer for the current collector of the lithium ion secondary battery. The thickness of the layer is preferably 0.3 to 3 μm, and the thickness of the first copper layer is preferably equal to or greater than the thickness of the second copper layer. However, this is not the case when used for applications such as printed wiring boards and suspensions.

At the time of the press work at the time of processing copper foil as a negative electrode by setting it as the composition whose tensile strength in the normal temperature of the 1st copper layer is 550 MPa or more, and the hardness of the 2nd copper layer becomes lower than the hardness of the 1st copper layer And mechanical strength (tensile strength) and hardness that can keep high adhesion between the carbon-based and Si-based negative electrode active material and the copper foil, even during charging and discharging of the battery. It can be set as the copper foil which was compatible, and such a copper foil can be manufactured with the said manufacturing method.

In the negative electrode coated with the Si-based active material, the volume of the active material changes to three times or more due to charge / discharge. In contrast to this large change, when a W-containing copper foil or a Mo-containing copper foil is used, the tensile strength is high, but there is a disadvantage that the elongation is small.
For this reason, the negative electrode incorporated in the lithium ion secondary battery, that is, the collector heated and pressurized after applying the active material, has a tensile strength of 400 MPa or more and can follow the volume change of the active material. It is desirable that the 0.2% proof stress is 340 MPa or more and the Young's modulus is 65 GPa or more.

By the way, a copper foil having a tensile strength of 650 MPa or more generally has a brittle property.
The present inventors, for example, form a first copper layer having a tensile strength of 550 MPa or more at room temperature and a second copper layer with a thickness of 0.3 to 3 μm on at least one surface, and have a surface hardness of 180 μm. By setting it to ˜370 mgf / μm ² , the brittleness of the copper foil can be overcome, and a copper foil suitable for the negative electrode current collector of a lithium ion secondary battery using carbon-based and Si-based active materials can be produced. , And got the knowledge.
This is because the second copper layer has a hardness lower than that of the first copper layer and the thickness is set to 0.3 to 3 μm, and is applied to the surface of the second copper layer and heated under pressure at 300 ° C. for 1 hour. The Si-based active material to be treated can be made to penetrate into the surface of the second copper layer, and even if the hardness of the first copper layer is high, the second copper layer having a lower hardness is Si-based, etc. This is considered to be because the volume change of the active material can be followed.

In the copper foil of this embodiment, since the hardness of the copper foil surface (2nd copper layer surface) which contacts an active material is low when it press-processes in order to set it as the negative electrode of a lithium ion secondary battery, the copper foil surface is Due to the processing pressure, it deforms along the shape of the active material, improves the adhesion between the active material and the copper foil, and increases the contact area, so that a negative electrode with high conductivity can be obtained.

When using the rolled copper foil as the first copper layer, the surface of the rolled copper foil is processed to be hardened, and the second copper layer is provided as described above so that the surface hardness at room temperature is 180 to 370 mgf / μm ² . A foil can be created and the same effects as described above can be obtained.

Second Embodiment
The present embodiment relates to a lithium ion secondary battery.
The negative electrode for a lithium ion secondary battery according to this embodiment uses the copper foil of the first embodiment as a negative electrode current collector.

In the lithium ion secondary battery according to the present embodiment, a negative electrode using the copper foil as a negative electrode current collector is a negative electrode for a lithium ion secondary battery.
According to the above-described embodiment, the copper foil constituting the negative electrode is resistant to wrinkles during press working and charge / discharge, and maintains high adhesion between the negative electrode active material such as Si and the copper foil. Since it has mechanical strength (tensile strength) and hardness, it can provide a negative electrode for a lithium ion secondary battery that is particularly excellent in charge / discharge characteristics and a lithium ion secondary battery using the same.

Hereinafter, the characteristics of the copper foil according to the present invention will be described in more detail with reference to examples.

1. Preparation of first copper layer Examples 1 to 23
Using the copper plating bath having the composition shown in Table 1, the first copper layer according to Examples 1 to 9 was produced by the method of adding the additive (A).
In addition, using the copper plating bath having the composition shown in Table 1, the first copper layer according to Examples 10 to 17 was prepared by the method of adding the additive (B).
In addition, using the copper plating bath having the composition shown in Table 1, the first copper layer according to Examples 18 to 22 was produced by the method of adding the additive (C).
Moreover, the Example using the rolled copper foil shown in Table 1 as a 1st copper layer was made into Example 23. FIG.
Table 1 summarizes the tensile strength (MPa), Young's modulus (GPa), hardness (nanoindenter hardness, mgf / μm ² ), and standard deviation (σ) thereof at room temperature for each of the above examples.
As shown in Table 1, A to W were used as electrode numbers when Examples 1 to 23 were used as the first copper layer of the negative electrode current collector of the lithium ion secondary battery.

Formation of Comparative Examples 1 to 10 First copper layers according to Comparative Examples 1 to 3 were produced by the method of adding the additive (A) shown in the above table.
Further, first copper layers according to Comparative Examples 4 to 7 were prepared by the method of adding the additive (B) shown in the table.
Moreover, the 1st copper layer which concerns on the comparative examples 8 and 9 was produced by the method of adding the additive (C) shown to a table | surface.
Moreover, the example which used rolled copper foil (TPC: tough pitch copper) as a 1st copper layer was made into the comparative example 10.
Table 1 summarizes the tensile strength (MPa), Young's modulus (GPa), hardness (nanoindenter hardness, mgf / μm ² ), and standard deviation (σ) of each of the comparative examples.
As shown in Table 1, the electrode numbers a to j were used when Comparative Examples 1 to 10 were used as the first copper layer of the negative electrode current collector of the lithium ion secondary battery.

As shown in Table 1, in Examples 1 to 23, the tensile strength of the first copper foil at room temperature was 550 MPa or more, but in Comparative Examples 1 to 10, the tensile strength was less than 550 MPa.

2. Formation of second copper foil (second copper layer) and assembly of lithium ion secondary battery Examples 24 to 34
The surface of the first copper foil selected from Examples 1 to 23 is subjected to electrolytic plating according to a copper plating bath and a plating condition for the second copper foil having the following composition, or by electroless plating or a second copper by a clad method. Layers were formed as Examples 24 to 34, and mechanical characteristics and the like were measured. A lithium ion secondary battery using the copper foil as a negative electrode was assembled, and the battery characteristics were measured.
Table 2 shows the thickness (μm) of the first copper layer and the thickness (μm) of the second copper layer.

Copper plating bath composition and plating conditions when forming the second copper foil by electrolytic plating Copper concentration: 50 to 100 g / l
Sulfuric acid concentration: 40-100g / l
Liquid temperature: 40-70 ° C
Current density: 35-60 A / dm ²

When forming the second copper foil by electroless plating, plating was performed using a plating solution OPC-700 manufactured by Okuno Pharmaceutical Co., Ltd. until a predetermined foil thickness was obtained.

When forming (stacking) the second copper foil by the clad method, a general rolling method was used, and after removing the degreasing and oxide film, clad rolling was used.

As mechanical properties of each of the above examples, surface roughness (Rz, Ra) at normal temperature (RT), tensile strength (MPa) and 0.2% proof stress (MPa), Young's modulus (GPa), and hardness (nano The indenter hardness, mgf / μm ² ) and its standard deviation (σ) are summarized in Table 2. The hardness was measured by measuring the number of samples n = 30 at an indentation depth of 1 μm, and using the average value and standard deviation.
Further, the tensile strength (MPa) and 0.2% proof stress (MPa), Young's modulus (GPa) and hardness (nanoindenter hardness, mgf / μm ² ) measured at room temperature after heat treatment at 300 ° C. for 1 hour in each example. ) And its standard deviation (σ) are summarized in Table 2.

here,
・ Nanoindenter hardness is measured using an ultra-fine indentation hardness tester ENT-2100 manufactured by Elionix Co., Ltd. The measurement was performed at room temperature at a tensile speed of 10 mm / min.
-Tensile strength and 0.2% proof stress were measured based on IPC-TM-650, and surface roughness was measured according to JIS B 0601: 1994.
In general, Vickers hardness is often used for measuring the hardness of a metal material. However, as described in JIS Z2244, the minimum thickness of a sample is determined to be 1.5 times or more of the diagonal of the depression, The measurement of thin copper foil as in the present invention is not suitable because the foil is torn. Also, in order to measure the amount of dents by pressing the indenter of a regular square pyramid, the copper foil of the two-layer structure is affected by the strength of the first layer (center material) as in the present invention. Since it was difficult to measure the hardness, the measurement was performed using a nanoindenter.

For Examples 24 to 34, the active material adhesion in a roll press under a pressing condition of a linear pressure of 500 kg / cm and 130 to 150 ° C. and the deformation of the foil were examined. In this example, the active material was continuously applied to one side of the current collector. In this case, pressure is applied to the coated portion of the active material during pressing, and pressure is not applied to the uncoated portion during pressing, so that the thickness of the coated portion is reduced and deformation occurs. In each of Examples 24 to 34, the active material adhesion was good, and no deformation of the foil was observed.
In addition, the evaluation of the adhesion of the active material is performed by a 90-degree peel test, where ○ indicates that the active material layer is agglomerated and broken, and part of the active material layer remains on the surface of the current collector. △, and what completely peeled off at the interface was marked with ×. In addition, deformation of the foil after pressing is less than 1 mm (0.1%) when the deformation of the coated part is less than 1 mm (0.1%) relative to 1 m of the uncoated part. 0.3%) or more were evaluated as x, and the evaluation results are shown in Table 2.

Comparative examples 11-14
By using the same electrolytic plating or electroless plating as in Examples 24 to 34, the first copper foil (first copper layer) selected from Examples 1 to 23 or Comparative Examples 1 to 10 was subjected to the second copper under the conditions shown in Table 2. A layer was formed to prepare a copper foil, and Comparative Examples 11 to 14 were obtained. Here, the comparative example 11 does not form the second copper layer. About each comparative example, while measuring a mechanical characteristic etc., the lithium ion secondary battery which uses this copper foil as a negative electrode was assembled, and the battery characteristic was measured.
Table 2 shows the thickness (μm) of the first copper layer and the thickness of the second copper layer.

As mechanical characteristics of each of the above comparative examples, surface roughness (Rz, Ra) at normal temperature (RT), tensile strength (MPa) and 0.2% proof stress (MPa), Young's modulus (GPa), and hardness (nano The indenter hardness, mgf / μm ² ) and its standard deviation (σ) are summarized in Table 2. The hardness was measured by measuring the number of samples n = 30 at an indentation depth of 1 μm, and using the average value and standard deviation.
In addition, tensile strength (MPa) and 0.2% proof stress (MPa), Young's modulus (GPa), and hardness (nanoindenter hardness, mgf / μm ² ) measured at room temperature after heat treatment at 300 ° C. for 1 hour in each comparative example. ) And its standard deviation (σ) are summarized in Table 2.

For Comparative Examples 11 to 14, the active material adhesion in a roll press under a pressing condition of a linear pressure of 500 kg / cm and 130 to 150 ° C., and deformation of the foil were examined. In all of Comparative Examples 11 to 14, there was a defect in either the active material adhesion or the deformation of the foil (indicated by x in Table 2).

In addition, Table 2 also shows the capacity retention ratio during charging and discharging when using a Si—C-based active material.
In this test, the capacity maintenance rate during 50 cycles of charge and discharge was confirmed at a charge and discharge rate of 0.2 C using a hybrid active material of Si and C blended to 2500 mAh / g. The electrodes were prepared by baking at 300 ° C. using a polyimide binder manufactured by Hitachi Chemical as the binder. If the capacity retention rate after 50 cycles is 70% or more, it is practical, but more preferably 80% or more.

As shown in Table 2, the overall tensile strength of the copper foils having the first copper layer and the second copper layer of Examples 24 to 33 was 500 MPa or more at room temperature.
Further, the entire Young's modulus of the copper foil having the first copper layer and the second copper layer of Examples 24 to 33 was 70 to 120 GPa at room temperature.
In addition, the nanoindenter hardness of the surface of the second copper layer of the copper foil having the first copper layer and the second copper layer in Examples 24 to 34 was 180 to 370 mgf / μm ² .
Further, the overall tensile strength of the copper foils having the first copper layer and the second copper layer of Examples 24 to 34 was 400 MPa or more as measured at room temperature after heat treatment at 300 ° C. for 1 hour.
The total Young's modulus of the copper foils having the first copper layer and the second copper layer of Examples 24 to 34 was 65 GPa or more as measured at room temperature after heat treatment at 300 ° C. for 1 hour.
Further, as shown in Table 2, the samples of Examples 24 to 34 were evaluated to have a capacity retention rate of 50% or more or 70% or more after 50 cycles.
As described above, Examples 24 to 34 were copper foils having good adhesion to the active material, no wrinkles when the active material was mounted, and good battery characteristics.

Comparison with Examples 24 to 34 Comparative Example 11 is a sample that does not have a second copper layer, and since the surface hardness (nanoindenter hardness) was high, the adhesion with the active material was inferior, and The discharge characteristics could not be satisfied.
Comparative Examples 12 to 14 have low tensile strength and Young's modulus at room temperature, low tensile strength and Young's modulus even after heat treatment at 300 ° C. for 1 hour, and the foil fills the foil when the active material is mounted and satisfies charge / discharge characteristics. I couldn't.
Further, as shown in Table 2, the capacity retention rate after 50 cycles was evaluated to be less than 70% in all of Comparative Examples 11 to 14.

As described above, according to the present example, the active material adhesion is good and the foil is not deformed. High adhesion between the substance and the copper foil can be maintained, and a copper foil having mechanical strength (tensile strength) and hardness can be provided.

Claims

A copper foil having a first copper layer having a tensile strength at room temperature of 550 MPa or more and a second copper layer having a lower hardness than the first copper layer provided on at least one surface of the first copper layer.
2. The copper foil according to claim 1, wherein the total thickness of the copper foil including the first copper layer and the second copper layer is 7 to 12 μm.
The copper foil according to claim 1 or 2, wherein the thickness of the second copper layer is 0.3 to 3 µm.
The copper foil according to any one of claims 1 to 3, wherein a thickness of the first copper layer is equal to or greater than a thickness of the second copper layer.
The copper foil according to any one of claims 1 to 4, wherein the first copper layer has a Young's modulus at room temperature of 75 to 130 GPa.
6. The copper foil according to claim 1, having a tensile strength at room temperature of 500 MPa or more.
The copper foil according to any one of claims 1 to 6, which has a Young's modulus at room temperature of 70 to 120 GPa.
The copper foil according to any one of claims 1 to 7, having a surface hardness of 180 to 370 mgf / µm 2 at room temperature.
The copper foil according to any one of claims 1 to 8, wherein the copper foil has a tensile strength of 400 MPa or more measured at room temperature after heat treatment at 300 ° C for 1 hour.
The copper foil according to any one of claims 1 to 9, wherein the Young's modulus of the copper foil measured at room temperature after heat treatment at 300 ° C for 1 hour is 65 GPa or more.
A negative electrode for a lithium ion secondary battery, wherein the negative electrode current collector is the copper foil according to any one of claims 1 to 10.
A lithium ion secondary battery, wherein the negative electrode is a negative electrode for a lithium ion secondary battery according to claim 11.