TW201448338A - Copper foil, anode for lithium ion battery, and lithium ion secondary battery - Google Patents

Abstract

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. 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 300 C is at least 400MPa.

Description

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

The present invention relates to a copper foil, a negative electrode for a lithium ion secondary battery using the same, and a lithium ion secondary battery using the negative electrode for a lithium ion secondary battery, and more particularly to a lithium ion secondary A copper foil having a mechanical strength (tensile strength) and hardness of a negative electrode for a battery, a negative electrode for a lithium ion secondary battery using the copper foil, and a lithium ion secondary battery using the negative electrode for a lithium ion secondary battery.

In recent years, as a negative electrode active material of a lithium ion secondary battery, a new generation of negative electrode active material having a charge and discharge capacity far exceeding the theoretical capacity of a carbon material has been developed. For example, a material containing a metal such as bismuth (Si), germanium (Ge), or tin (Sn) which can form an alloy with lithium (Li) is expected.

In particular, when Si or Sn is used for the active material, the volume change of these materials due to the absorption and release of Li during charge and discharge is large, and thus it is difficult to maintain the adhesion state between the current collector and the active material. In addition, the volume change rate of these materials due to the insertion and extraction of Li is very large, and as the charge and discharge cycles repeatedly expand and contract, the active material particles form fine powder or desorb, so the deterioration of the cycle characteristics is very large. Disadvantages.

In order to solve such a disadvantage, there has been proposed a method of improving the adhesion of an active material to a current collector by using a polyimide conjugate. However, Jujua The curing temperature of the amine binder is about 300 ° C. When the conventional electrolytic copper foil is used as the current collector, the electrolytic copper foil is annealed at a temperature of about 300 ° C, recrystallized and softened, and thus may not be able to withstand the volume of the active material. Swell, causing a break. Therefore, there has been a demand for an electrolytic copper foil which has a softening property even if it is subjected to heat treatment at about 300 ° C and has high mechanical strength such as high tensile strength.

Hereinafter, the mechanical strength means tensile strength or the like.

For example, Patent Document 1 describes a low-thickness electrolytic copper foil which is a copper foil which is most suitable for a negative electrode current collector for a secondary battery, and has a rough surface roughness Rz of 2.0 μm or less and uniformly achieves low roughness. The rough surface and the elongation at 180 ° C are 10.0% or more. Further, the above-mentioned electrolytic copper foil can be obtained by using a sulfuric acid-copper sulfate aqueous solution as an electrolytic solution and adding polyethyleneimine or a derivative thereof, a sulfonate of an active organic sulfur compound, and chloride ions.

Further, Patent Document 2 describes an electrolytic copper foil having a rough surface roughness Rz of 2.5 μm or less and a tensile strength of 820 MPa or more at 25 ° C measured within 20 minutes from the completion of plating, with respect to the plating. The tensile strength at 25 ° C measured within 20 minutes after completion was 10% or less from the tensile strength at 25 ° C measured after 300 minutes from the completion of plating.

Moreover, the literature states that the above electrolysis can be obtained by using a sulfuric acid-copper sulfate aqueous solution as an electrolyte and adding hydroxyethyl cellulose, polyethyleneimine, a sulfonate of an active organic sulfur compound, acetylene glycol, and chloride ions. Copper foil.

Further, Patent Document 3 describes a controlled low-roughness electroplated copper foil which has no columnar particles and twin boundaries and has an average particle size. A particle structure having a maximum of 10 μm, the particle structure being substantially identical and randomly oriented particle structures.

The maximum tensile strength of the electroplated copper foil at 23 ° C is in the range of 87,000 to 120,000 psi (600 MPa to 827 MPa), and the maximum tensile strength at 180 ° C is 25,000 to 35,000 psi (172 MPa to 241 MPa).

The conventional high-strength electrolytic copper foil disclosed in the above Patent Documents 1 to 3 has a large mechanical strength in a normal state, and the mechanical strength hardly changes after heating at about 180 °C. However, when such a foil is heated at about 300 ° C, annealing occurs and recrystallization is performed, so that it softens rapidly and the mechanical strength is lowered.

Further, for example, Patent Documents 4 and 5 describe a method for producing an electrolytic copper foil by adding W to a sulfuric acid-copper sulfate electrolyte solution and adding a gel and a chloride ion, and the effect is that it can be produced. The thermal elongation at 180 ° C is 3% or more, the roughness of the rough surface is large, and the pinholes generate less copper foil.

However, the copper foil produced by the above-mentioned addition of W has a high mechanical strength (tensile strength) at normal temperature (RT), but it cannot maintain mechanical strength after heat treatment at 300 ° C for 1 hour.

Analysis of the copper foil revealed that there was no eutectoid W in the electrodeposited copper.

Further, Patent Document 6 describes a current collector in which a surface layer portion which is easily plastically deformed is formed on the surface of a current collector. It is described that the Vickers hardness of the surface layer portion is 200 or less, preferably 160 or less.

Further, although Patent Document 6 describes a current collector in which a surface layer portion which is easily plastically deformed is formed on the surface of the current collector, the plastic deformation of the surface is not sufficient, and the shape of the active material must be limited. Patent Document 6 The above problem is solved by increasing the roughness of the surface layer of the current collector and forming the active material layer into columnar particles to form voids between the active materials. Although the voids are formed between the active materials, the battery capacity is of course reduced. However, due to the local flow current, the stress concentrates on the active material and the localized portion of the current collector, and the cycle characteristics are lowered.

Non-Patent Document 1 discloses that Vickers hardness (HV) and nanoindentation hardness (H _IT ) are related, and the relationship is HV=0.0945H _IT .

Therefore, when the nanoindentation hardness of Patent Document 6 is converted according to the relationship of Non-Patent Document 1, it is considered to be preferably 2116 mgf/μm ² or less, and more preferably 1693 mgf/μm ² or less.

When the conventional copper foil is used as the negative electrode current collector of the lithium ion secondary battery, if the mechanical strength (tensile strength) of the copper foil is too low, it is easy to be pressed during the electrode formation process of the lithium ion secondary battery. It is not advantageous to produce wrinkles.

Further, when the mechanical strength (tensile strength) of the copper foil is too low, wrinkles are likely to occur when the secondary battery is repeatedly charged and discharged, which is not advantageous.

In addition, when the hardness of the copper foil is too high, for example, it is difficult to embed the carbon-based and Si-based negative electrode active material in the copper foil, and the adhesion between the active material and the copper foil is likely to be lowered, and the capacity and cycle characteristics of the battery are liable to be lowered, which is not advantageous.

Therefore, there is a demand for a copper foil suitable for a negative electrode current collector of a lithium ion secondary battery, which has mechanical strength (tensile strength) and hardness which are less likely to cause wrinkles during press working, and are difficult to be charged and discharged. The wrinkles are generated, and the high adhesion between the Si-based negative electrode active material and the copper foil can be maintained.

Prior art literature Patent literature

Patent Document 1: Japanese Patent No. 4120806

Patent Document 2: Japanese Patent No. 4273309

Patent Document 3: Japanese Patent No. 3270637

Patent Document 4: Japanese Patent No. 3238278

Patent Document 5: Japanese Patent Laid-Open No. Hei 9-67693

Patent Document 6: Japanese Patent Laid-Open Publication No. 2008-98157

Non-patent literature

Non-Patent Document 1: INTERNATIONALSTANDARDISO 14577-1 Metallic Materials - Indentation Technique Test and Material Parameters of Hardness

An object of the present invention is to provide a copper foil, 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 negative electrode for a lithium ion secondary battery, the copper foil having It is difficult to cause wrinkles and deformation during press working, and it is difficult to cause wrinkles and deformation even during charge and discharge, and the mechanical strength (tensile strength) and surface hardness of the high adhesion between the negative electrode active material and the copper foil can be maintained.

The inventors have intensively studied and finally succeeded in developing a copper foil which has the ability to produce wrinkles even during press processing, even in charge and discharge. It is also difficult to produce wrinkles, and the mechanical strength (tensile strength) and surface hardness of the high adhesion between the carbon-based and Si-based negative electrode active materials and the copper foil can be maintained.

The copper foil of the present invention has a first copper layer having a tensile strength at a normal temperature of 550 MPa or more, and a second copper layer formed on at least one surface of the first copper layer and having a hardness lower than that of the first copper layer.

In the copper foil of the present invention, it is preferable that the total thickness of the copper foil added to the first copper layer and the second copper layer is 7 to 12 μm.

In the copper foil of the present invention, it is preferable that the thickness of the second copper layer is 0.3 to 3 μm.

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

In the copper foil of the present invention, it is preferable that the first copper layer has a Young's modulus at a normal temperature of 75 to 130 GPa.

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

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

In the copper foil of the present invention, it is preferable that the surface hardness of the copper foil at room temperature is 180 to 370 mgf/μm ² .

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

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

In the negative electrode for a lithium ion secondary battery of the present invention, the copper foil according to any one of the above aspects is used as a negative electrode of a current collector.

The lithium ion secondary battery of the present invention uses the negative electrode for a lithium ion secondary battery described above as a negative electrode.

According to the copper foil of the present invention, the second copper layer having a hardness lower than that of the first copper layer is provided on at least one surface of the first copper layer having a tensile strength of 550 MPa or more at normal temperature, so that a copper can be provided. The foil has a mechanical strength (tensile strength) which is less likely to cause wrinkles during press working, and which is less likely to wrinkle even during charge and discharge, and which can maintain high adhesion between the carbon-based and Si-based negative electrode active material and the copper foil. And surface hardness.

In the negative electrode for a lithium ion secondary battery of the present invention, the copper foil used as the current collector constituting the negative electrode has a wrinkle that is less likely to be generated during press working, and is less likely to wrinkle even during charge and discharge, and can retain carbon. Since the mechanical strength (tensile strength) and hardness of the high-adhesion property between the Si-based negative electrode active material and the copper foil are high, it is possible to provide a negative electrode for a lithium ion secondary battery having excellent charge and discharge characteristics.

In the lithium ion secondary battery of the present invention, it is possible to provide a battery excellent in charge and discharge characteristics by using the above negative electrode.

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

<First embodiment> [Composition of copper foil]

The copper foil of the present embodiment has a first copper layer having a tensile strength of 550 MPa or more at normal temperature, and a second copper layer formed on at least one surface of the first copper layer and having a hardness lower than that of the first copper layer.

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 may be formed on the surface of the first copper layer by any one of electroless treatment such as electroplating, electroless treatment such as sputtering or chemical vapor deposition, or bonding (cladding).

In the copper foil of the present embodiment, the first copper layer is composed of a copper layer having a tensile strength at normal temperature of 550 MPa or more, and the second copper layer is composed of a copper layer having a hardness lower than that of the first copper layer.

By using the copper foil of the above-described embodiment as the current collector of the negative electrode for a lithium ion secondary battery, the copper foil can be used to prevent wrinkles from occurring during press working, and it is difficult to cause wrinkles even during charge and discharge. The mechanical strength (tensile strength) and hardness of the high adhesion between the carbon-based and Si-based negative electrode active materials and the copper foil (current collector) are maintained.

The copper foil of the present embodiment preferably has a smooth surface, and Ra is 1.0 μm or less, and preferably Rz is 4.0 μm or less (JIS B0601: 1994). The reason for this is that if the surface unevenness of the copper foil is too large, the contact area of the active material is reduced, or the active material cannot enter the valley of the unevenness, the adhesion between the active material and the copper foil is liable to lower, and the cycle characteristics are lowered.

In the copper foil of the present 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, the tensile strength at normal temperature can be 550 MPa or more.

In addition, the first copper layer is not limited to the above-mentioned electrolytic copper alloy foil containing tungsten or molybdenum, and an electrolytic copper foil having a tensile strength of 550 MPa or more at normal temperature can of course be used.

When the copper foil of the present embodiment is used as a current collector for a lithium ion secondary battery, it is preferable that the total thickness of the copper foil added to the first copper layer and the second copper layer is 7 to 12 μm.

When the thickness is less than 7 μm, the handleability of the copper foil is lowered. When the thickness exceeds 12 μm, the energy density is lowered, and it is not suitable for use as a negative electrode current collector for a lithium ion secondary battery. However, other foil thicknesses can be used for other applications such as printed wiring boards and suspension materials.

In the copper foil of the present embodiment, it is preferable that the thickness of each surface of the second copper layer formed on the surface of the first copper layer (the thickness of one surface when the second copper layer is formed on both sides) is 0.3 to 3 μm.

The reason for this is that if the thickness is less than 0.3 μm, the hardness of the surface of the copper foil may not be suitable, and even if it exceeds 3 μm, the adhesion to the active material is hardly improved. Therefore, it is sufficient that the thickness of the second copper layer formed on one surface of the first copper layer is 3 μm or less. In order to obtain the effect of the present invention to the utmost extent, the thickness of the second copper layer is preferably smaller than the thickness of the first copper layer.

When the thickness of the first copper layer is equal to or greater than the thickness of the second copper layer, the mechanical strength (tensile strength) of the copper foil is balanced with the surface hardness.

In the copper foil of the present embodiment, the Young's modulus of the first copper layer at a normal temperature is preferably 75 to 130 GPa. The reason for this is that if 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 low stress, and the cycle characteristics are liable to lower when used as a current collector. Further, when a copper foil having a Young's modulus higher than 130 GPa is used as a negative electrode (current collector), the current collector (copper foil) may be broken due to stress generated by expansion of the active material during charging, and therefore it is preferred. For the above range.

Further, in 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, the Young's modulus of the first copper layer is large, so the Young's modulus of the first copper layer is preferably used. The amount is set to the above range.

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 normal temperature.

Further, in the copper foil of the present embodiment, it is preferable that the Young's modulus of the copper foil is 70 to 120 GPa at normal temperature.

Further, in the copper foil of the present embodiment, the hardness of the surface layer of the copper foil at a normal temperature is preferably 180 to 370 mgf/μm ² .

The copper foil of the present embodiment has the above mechanical strength (tensile strength) and hardness, and is capable of forming wrinkles during press working of a negative electrode using a copper foil as a current collector, even after forming a secondary battery negative electrode. It is also difficult to generate wrinkles during charge and discharge, and it is possible to maintain high adhesion between the carbon-based and Si-based negative electrode active materials and the copper foil (current collector).

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

Further, in the copper foil of the present embodiment, the Young's modulus measured at room temperature after heat treatment at 300 ° C for 1 hour is preferably 65 GPa or more.

The reason for this is that if the Young's modulus of the copper foil is lower than the above-mentioned 65 GPa, the stress caused by the expansion of the active material during charging causes the copper foil to be deformed, which may cause a short circuit between the negative electrode and the positive electrode.

When there is no problem such as a significant increase in the local battery or the resistance between the first copper layer and the second copper layer, it is possible to adopt a configuration in which another metal layer, an oxide layer, an organic layer or the like is interposed in the middle.

Further, it is also effective to appropriately form another metal layer, an oxide layer, an organic layer or the like on the surface of the second copper layer.

The first copper layer of the copper foil of the present embodiment can be produced by electroplating using, for example, a plating bath having the following various additives, under the basic composition and plating conditions of the following copper plating bath.

Basic composition and plating conditions of copper plating bath for first copper foil foil

Copper concentration: 50~100g/l

Sulfuric acid concentration: 40~100g/l

Liquid temperature: 40~70°C

Current density: 35~60A/dm ²

Additive (A)

Chlorine, tungsten (W) or molybdenum (Mo) is added to the basic composition of the above copper plating bath.

Chlorine (Cl): 2ppm or less

W concentration: 20~300ppm

Mo concentration: 10~100ppm

By using the plating bath described above, the first copper foil containing W or Mo in the copper foil and having a high mechanical strength (tensile strength) first copper layer can be formed.

Further, the metal of W or Mo may be contained in two or more kinds. In addition, other metals may be added as needed.

Although the concentration of the above W or Mo may exceed the above upper limit at the time of addition, 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, so that the manufacturing cost is preferable. Limit the above concentration to the upper limit.

Additive (B)

Chlorine (Cl), an organic additive, and a metal (W) are added to the basic composition of the above copper plating bath.

Cl concentration: 10~60ppm

Thiourea: 2~20ppm

W concentration: 10~200ppm

As the thiourea, for example, thiourea (TU), ethylene thiourea (ETU), and tetramethylthiourea (TMTU) can be used.

By electroplating using the above plating bath, a first copper layer composed 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 basic composition of the above copper plating bath.

Cl concentration: 10~60ppm

Thiourea: 2~20ppm

HEC (hydroxyethyl cellulose): 0~20ppm

As the thiourea, for example, thiourea (TU), ethylene thiourea (ETU), and tetramethylthiourea (TMTU) can be used. Additional glue can be added.

By electroplating using the above plating bath, a first copper foil (first copper layer) composed of Cu and having high mechanical strength (tensile strength) can be formed.

Further, as the first copper layer of the copper foil of the present embodiment, for example, a rolled copper foil can be used.

As the rolled copper foil, for example, a copper foil containing 0.03% of Zr with respect to Cu can be used.

The first copper layer can be suitably used as long as it is a copper foil having a tensile strength of 550 MPa or more at normal temperature, unless it is produced by the above method.

The second copper layer of the copper foil of the present embodiment can be formed on at least one surface of the first copper layer by, for example, electrolytic treatment of a basic composition of a copper plating bath described below and plating conditions.

Composition and plating conditions of copper plating bath for second copper foil foil

Copper concentration: 50~100g/l

Sulfuric acid concentration: 40~100g/l

Liquid temperature: 40~70°C

Current density: 35~60A/dm ²

Further, the second copper layer formed on at least one surface of the first copper layer may be formed by a method such as electroless treatment or bonding (cladding) such as sputtering or chemical vapor deposition.

When it is used for a current collector of a lithium ion secondary battery, the copper foil of the present embodiment preferably has a total thickness of 7 to 12 in addition to the first copper layer and the second copper layer. Μm, the thickness of the second copper layer is 0.3 to 3 μm, and the thickness of the first copper layer is equal to or greater than the thickness of the second copper layer. However, it is not limited to this when used for printed wiring boards and suspensions.

The tensile strength at a normal temperature of the first copper layer is 550 MPa or more, and the hardness of the second copper layer is lower than the hardness of the first copper layer, thereby obtaining a copper foil which can both When the copper foil is processed into a negative electrode, wrinkles are less likely to occur during the press working, and it is difficult to generate wrinkles even during charge and discharge after the battery is formed, and mechanical strength capable of maintaining high adhesion between the carbon-based and Si-based negative electrode active material and the copper foil can be achieved. (tensile strength) and hardness, and such a copper foil can be produced by the above production method.

In the negative electrode coated with the Si-based active material, the volume of the active material changes by a factor of three or more due to charge and discharge. With respect to this large change, when a W-containing copper foil or a Mo-containing copper foil is used, although the tensile strength is high, the elongation is small, which is not advantageous.

Therefore, with respect to the negative electrode incorporated in the lithium ion secondary battery, that is, the current collector subjected to heating and pressurization after application of the active material, the tensile strength is preferably 400 MPa or more, and the 0.2% endurance is 340 MPa or more and Young's The modulus is 65 GPa or more to follow the volume change of the active material.

However, the properties of copper foil having a tensile strength of 650 MPa or more are generally brittle.

The present inventors have found that, for example, a first copper layer having a tensile strength of 550 MPa or more at normal temperature and a second copper layer having a thickness of 0.3 to 3 μm are formed on at least one side thereof, and the surface hardness thereof is 180 to 370 mgf/ Μm ² can overcome the brittleness of the copper foil and form a copper foil suitable for a negative electrode current collector of a lithium ion secondary battery using a carbon-based or Si-based active material.

It is considered that the hardness of the second copper layer is lower than the hardness of the first copper layer, and the thickness is 0.3 to 3 μm, so that the surface of the second copper layer can be applied and heated at 300 ° C. An active substance such as Si is embedded in 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 hardness lower than that of the first copper layer can follow the volume change of the active material such as Si.

In the copper foil of the present embodiment, when the press processing is performed as a negative electrode of a lithium ion secondary battery, the hardness of the surface of the copper foil (the surface of the second copper layer) which is in contact with the active material is low, and therefore the surface of the copper foil is Since the shape of the active material is deformed by the processing pressure, the contact area is increased by increasing the adhesion between the active material and the copper foil, so that a negative electrode having a high electrical conductivity can be obtained.

When a rolled copper foil is used as the first copper layer, by performing a process of hardening the surface of the rolled copper foil and forming the second copper layer as described above, the hardness at a normal temperature of the surface can be 180 to 370 mgf/μm ² . Copper foil, the same effect as described above was obtained.

<Second embodiment>

This embodiment relates to a lithium ion secondary battery.

In the negative electrode for a lithium ion secondary battery of the present embodiment, the copper foil of the first embodiment is used as the negative electrode current collector.

In the lithium ion secondary battery of the present embodiment, the negative electrode using the copper foil as the negative electrode current collector is used as a negative electrode for a lithium ion secondary battery.

According to the embodiment, the copper foil constituting the negative electrode is less likely to wrinkle during press working, and is less likely to be pleated even during charge and discharge. The wrinkle can maintain the high mechanical strength (tensile strength) and hardness of the negative electrode active material such as a Si-based material and the copper foil. Therefore, it is possible to provide a negative electrode for a lithium ion secondary battery which is excellent in charge and discharge characteristics, and the like. A lithium ion secondary battery of a negative electrode for a lithium ion secondary battery.

Hereinafter, the characteristics of the copper foil of the present invention will be described in further detail by way of examples.

Example 1. Production of the first copper layer . Example 1~23

The first copper layers of Examples 1 to 9 were produced by using the copper plating bath having the composition shown in Table 1 and adding the above additive (A).

Further, the first copper layers of Examples 10 to 17 were produced by using the copper plating bath having the composition shown in Table 1 and adding the above additive (B).

Further, the first copper layers of Examples 18 to 22 were produced by using the copper plating bath having the composition shown in Table 1 and adding the above additive (C).

Further, an example in which the rolled copper foil shown in Table 1 was used as the first copper layer was used as Example 23.

The tensile strength (MPa), Young's modulus (GPa), hardness (nanoindentation hardness, mgf/μm ² ) and the standard deviation (σ) thereof at normal temperature in each of the above examples are shown in Table 1.

As shown in Table 1, the electrode numbers when the above-mentioned Examples 1 to 23 were used as the first copper layer of the negative electrode current collector of the lithium ion secondary battery were A to W.

. Formation of Comparative Examples 1 to 10

The first copper layer of Comparative Examples 1 to 3 was prepared by adding the additive (A) shown in the above table.

Further, the first copper layers of Comparative Examples 4 to 7 were prepared by adding the additive (B) shown in the table.

Further, the first copper layers of Comparative Examples 8 and 9 were prepared by adding the additive (C) shown in the table.

Further, as an example of Comparative Example 10, a rolled copper foil (TPC: Toughened Copper) was used as an example of the first copper layer.

The tensile strength (MPa), Young's modulus (GPa), hardness (nanoindentation hardness, mgf/μm ² ) and the standard deviation (σ) of the above Comparative Examples are shown in Table 1.

As shown in Table 1, the electrode numbers when the above Comparative Examples 1 to 10 were used as the first copper layer of the negative electrode current collector of the lithium ion secondary battery were a to j.

As shown in Table 1, in Examples 1 to 23, the tensile strength at room temperature of the first copper foil 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 second copper is formed on the surface of the first copper foil selected from the examples 1 to 23 by using a copper plating bath for the second copper foil having the following composition and plating conditions, or by electroless plating or cladding. The layers were used as Examples 24 to 34, and mechanical properties and the like were measured, and a lithium ion secondary battery using the copper foil as a negative electrode was assembled, and battery characteristics were measured.

The thickness (μm) of the first copper layer and the thickness (μm) of the second copper layer are shown in Table 2.

Copper bath composition and plating conditions when forming a second copper foil by electroplating

Copper concentration: 50~100g/l

Sulfuric acid concentration: 40~100g/l

Liquid temperature: 40~70°C

Current density: 35~60A/dm ²

When the second copper foil is formed by electroless plating, the plating solution OPC-700 produced by Okuno Pharmaceutical Co., Ltd. is used to form a predetermined foil thickness.

When the second copper foil is formed (laminated) by the cladding method, a general rolling method is employed, and after the degreasing and removal of the oxide film, cladding rolling is performed.

As the mechanical properties of the above respective examples, the surface roughness (Rz, Ra), tensile strength (MPa), and 0.2% proof stress (MPa), Young's modulus (GPa), and hardness at room temperature (RT) are The nanoindentation hardness, mgf/μm ² ) and its standard deviation (σ) are shown together in Table 2. Further, for the hardness, n = 30 samples were measured at a depth of 1 μm, and the average value and standard deviation were used.

Further, tensile strength (MPa) and 0.2% proof stress (MPa), Young's modulus (GPa), and hardness (nanoindentation hardness, mgf/μm) measured at room temperature after heat treatment at 300 ° C for 1 hour in each example. ² ) and its standard deviation (σ) are shown together in Table 2.

Here,

. The nanoindentation hardness was measured using Elionix's ultra-small hardness tester ENT-2100.

. The Young's modulus was measured at a normal temperature using an AUTOGRAPHAG-IS (500N) manufactured by Shimadzu Corporation at a distance between the punctuation of 50 mm and a tensile speed of 10 mm/min.

. Tensile strength and 0.2% endurance were measured based on IPC-TM-650, and surface roughness was measured in accordance with JIS B0601:1994. In the hardness measurement of metal materials, the Vickers hardness is generally used. However, as described in JIS Z2244, the Vickers hardness defines the minimum thickness of the sample to be 1.5 times or more of the diagonal of the indentation, and the thin copper foil similar to the present invention is measured. The foil will be damaged and therefore not applicable. In addition, since the amount of depression is measured by pressing the indenter of the square pyramid, the copper foil of the two-layer structure similar to the present invention is affected by the strength of the first layer (center material), and it is difficult to measure the surface hardness of the copper foil and Hardness, so the measurement was carried out using a nanoindenter.

For Examples 24 to 34, the adhesion of the active material and the deformation of the foil at the time of pressing under the pressing conditions of 500 kg/cm and 130 to 150 °C were examined. In this embodiment, the active material is continuously applied to one side of the current collector. at this time, The coated portion of the active material is subjected to pressure at the time of pressing, and the uncoated portion is not subjected to pressure at the time of pressing, so that the thickness of the coated portion is thinned and deformed. The active materials of Examples 24 to 34 all had good adhesion and no foil deformation.

Further, when the adhesion of the active material was evaluated, a 90-degree peeling test was used, and the occurrence of aggregation failure inside the active material layer was evaluated as ○, and the portion of the active material layer remaining on the surface of the current collector was evaluated as Δ, which was completely peeled off at the interface. The evaluation is ×. Further, regarding the deformation of the foil after pressing, the deformation of the coated portion was less than 1 mm (0.1%) with respect to the uncoated portion 1m, and the evaluation of the foil was less than 3 mm (0.3%), and the evaluation was Δ, 3 mm (0.3). %) The above evaluation was ×, and the evaluation results are shown in Table 2.

. Comparative Example 11~14

Formed on the first copper foil (first copper layer) selected from Examples 1 to 23 or Comparative Examples 1 to 10 under the conditions shown in Table 2 by the same plating or electroless plating as in Examples 24-34. The second copper layer was made into a copper foil, and this was made into the comparative example 11-14. Here, in Comparative Example 11, the second copper layer was not formed. For each comparative example, mechanical properties and the like were measured, and a lithium ion secondary battery using the copper foil as a negative electrode was assembled, and battery characteristics were measured.

The thickness (μm) of the first copper layer and the thickness of the second copper layer are shown in Table 2.

As the mechanical properties of the above comparative examples, the surface roughness (Rz, Ra), tensile strength (MPa), and 0.2% proof stress (MPa), Young's modulus (GPa), and hardness at room temperature (RT) were The nanoindentation hardness, mgf/μm ² ) and its standard deviation (σ) are shown together in Table 2. Further, for the hardness, n = 30 samples were measured at a depth of 1 μm, and the average value and standard deviation were used.

Further, tensile strength (MPa) and 0.2% proof stress (MPa), Young's modulus (GPa), and hardness (nanoindentation hardness, mgf/μm) measured at room temperature after heat treatment at 300 ° C for 1 hour in each comparative example. ² ) and its standard deviation (σ) are shown together in Table 2.

In Comparative Examples 11 to 14, the adhesiveness of the active material and the deformation of the foil at the time of pressing under the pressing conditions of 500 kg/cm and 130 to 150 ° C were examined. In Comparative Examples 11 to 14, all of the defects of the active material adhesion or the foil deformation occurred (shown as × in Table 2).

Further, the capacity retention ratio at the time of charge and discharge using the Si-C type active material is also shown in Table 2.

In this test, a mixed active material of Si and C which was compatible with 2500 mAh/g was used, and the capacity retention rate at the charge and discharge rate of 0.2 C at 50 cycles was confirmed. Further, an electrode was prepared by performing a melting at 300 ° C using a polyhedral imide binder produced by Hitachi Chemical Co., Ltd. as a binder. Although the capacity retention after 50 cycles is 70% or more, it is practical, but it is more preferably 80% or more.

As shown in Table 2, the tensile strength of the entire copper foil having the first copper layer and the second copper layer of Examples 24 to 33 was 500 MPa or more at normal temperature.

Further, the Young's modulus of the entire copper foil having the first copper layer and the second copper layer of Examples 24 to 33 was 70 to 120 GPa at normal temperature.

Further, the surface of the second copper layer of the copper foil having the first copper layer and the second copper layer of Examples 24 to 34 had a nanoindentation hardness of 180 to 370 mgf/μm ² .

Further, the whole of the copper foil having the first copper layer and the second copper layer of Examples 24 to 34 was heat-treated at 300 ° C for 1 hour, and the tensile strength value measured at room temperature was 400 MPa or more.

Further, the whole of the copper foil having the first copper layer and the second copper layer of Examples 24 to 34 was heat-treated at 300 ° C for 1 hour, and the Young's modulus value measured at room temperature was 65 GPa or more.

Further, as shown in Table 2, the samples of Examples 24 to 34 were evaluated such that the capacity retention after 50 cycles was 80% or more or 70% or more.

As described above, in the copper foils of Examples 24 to 34, the adhesion to the active material was good, and wrinkles did not occur when the active material was provided, and the battery characteristics were also good.

Compared with the above examples 24 to 34

In Comparative Example 11, a sample having no second copper layer was used, and since the hardness (nanoindentation hardness) of the surface was hard, the adhesion to the active material was poor, and the charge and discharge characteristics could not be satisfied.

In Comparative Examples 12 to 14, the tensile strength and Young's modulus at normal temperature were low, and the tensile strength and Young's modulus were also low after heat treatment at 300 ° C for 1 hour, and wrinkles were formed on the foil when the active material was placed. Meet the charge and discharge characteristics.

Further, as shown in Table 2, all of Comparative Examples 11 to 14 were evaluated as having a capacity retention ratio of less than 70% after 50 cycles.

As described above, according to the present embodiment, since the active material has good adhesion and no foil deformation, it is possible to provide a copper foil which is capable of generating wrinkles during press working, and which is difficult to wrinkle even during charge and discharge. The mechanical strength (tensile strength) and hardness of the high adhesion between the Si-based negative electrode active material and the copper foil are maintained.

Claims (12)

A copper foil having a first copper layer having a tensile strength at a normal temperature of 550 MPa or more, and a second copper layer formed on at least one surface of the first copper layer and having a hardness lower than that of the first copper layer. The copper foil according to claim 1, wherein the total thickness of the copper foil added to 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 the thickness of the first copper layer is equal to or greater than the 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 a normal temperature of 75 to 130 GPa. The copper foil according to any one of claims 1 to 5, wherein the tensile strength at normal temperature is 500 MPa or more. The copper foil according to any one of claims 1 to 6, wherein the Young's modulus at room temperature is 70 to 120 GPa. The copper foil according to any one of claims 1 to 7, wherein the surface hardness at room temperature is 180 to 370 mgf/μm ² . The copper foil according to any one of claims 1 to 8, wherein the copper foil is heat-treated at 300 ° C for 1 hour, and the tensile strength measured at room temperature is 400 MPa or more. The copper foil according to any one of claims 1 to 9, wherein the copper foil has a Young's modulus of 65 GPa or more measured at room temperature after heat treatment at 300 ° C for 1 hour. A negative electrode for a lithium ion secondary battery, the negative electrode current collector of which is a copper foil according to any one of claims 1 to 10. A lithium ion secondary battery, the negative electrode of which is the negative electrode for a lithium ion secondary battery according to claim 11 of the patent application.