JP2013143314A - Rustproof metal sheet for lithium ion secondary battery negative electrode, negative electrode and method for manufacturing the same, and battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium ion secondary battery, high in strength and large in discharge capacity.SOLUTION: A rustproof metal sheet for supporting a negative electrode active material of a lithium ion secondary battery is a rustproof metal sheet 20 which includes copper coating layers 7 on both sides of a steel sheet 6 and a rust inhibitor layer 8 on a surface of each of the copper coating layers 7. In the rustproof metal sheet 20, the total thickness t of the steel sheet 6 and the copper coating layers 7 on both sides is 3-100 μm, and each of the copper coating layers 7 on both sides has an average film thickness tof 0.02-5.0 μm and t/t of 0.3 or less. The rust inhibitor layer 8 is more preferably obtained by rolling the rust inhibitor layer 8, which is stuck on a surface of a copper coated steel sheet 10 serving as an intermediate product, together with the copper coated steel sheet 10 to be reduced in thickness. A rust inhibitor, for example blended with benzotriazole, can be used.

Description

  The present invention relates to a metal sheet used for a negative electrode of a lithium ion secondary battery, and in particular, to a surface of a copper-coated steel foil subjected to rust prevention treatment. Further, the present invention relates to a negative electrode of a lithium ion secondary battery in which an active material is supported on the surface of the metal sheet, a manufacturing method thereof, and a lithium ion secondary battery using the negative electrode.

  Lithium ion secondary batteries are widely used as small storage batteries such as portable electronic devices. In the future, demand for lithium ion secondary batteries is expected as a relatively large storage battery that can be used as a power source for driving hybrid vehicles and electric vehicles. In the future, lithium-ion secondary batteries can also be used to store new energy. For these reasons, it is strongly desired to increase the capacity of lithium ion secondary batteries in recent years.

  Conventionally, a general lithium ion secondary battery is formed by alternately stacking a positive electrode in which a positive electrode active material is supported on the surface of an aluminum foil and a negative electrode in which a negative electrode active material is supported on the surface of a copper foil via a separator. It has a structure in which the “electrode laminate” is disposed in a lithium ion electrolyte. When classified by the electrode stacking method, a type in which a belt-like positive electrode and a negative electrode are wound (for example, Patent Document 1), and a number of sheet-like positive electrodes and negative electrodes are alternately overlapped and welded at the tab lead portions of each electrode. (For example, Patent Documents 2 to 4).

  The copper foil used in the negative electrode has a surface electrical resistance (hereinafter referred to as “oxidized film” formed on the surface during the storage period after the active material containing paint is applied to the surface after being adjusted to a predetermined thickness. (Referred to as “surface resistance”) is likely to increase. An increase in surface resistance leads to an increase in battery internal resistance. In addition, the oxide film on the surface is a factor that reduces the adhesion of the active material layer. Such a problem due to the formation of an oxide film can be avoided to some extent by subjecting it to the active material supporting process as soon as possible after the production of the copper foil, but a great restriction is imposed on the process control. Also, the surface resistance tends to vary depending on the storage period and storage environment before carrying the active material. On the other hand, after copper foil manufacture, the method of carrying out the antirust process on the surface with a antirust agent is also employ | adopted (for example, patent documents 5, 6). The rust prevention treatment of the copper foil is accompanied by a slight increase in surface resistance due to the formation of a rust prevention agent layer, similar to the rust prevention treatment performed for general copper product materials.

JP 2001-57243 A JP 2002-75324 A JP 2010-16043 A JP 2010-73408 A JP-A-10-21928 JP 2010-282957 A

  Aluminum foil and copper foil used in lithium ion secondary batteries are low in strength, so the foil is likely to be deformed in the production line where the active material is applied, and advanced management is required to obtain an electrode with high shape accuracy. The If the management is insufficient, the band of the foil-like metal sheet may break in the line. In addition, in the case of battery products, especially in the case of “laminate type” lithium ion secondary batteries in which the battery contents are sealed with a laminate pack, it is advantageous for increasing the size in terms of excellent heat dissipation characteristics, but locally from the outside of the battery. When an external force is applied, the electrode laminate is deformed and the electrodes are easily damaged. Furthermore, the volume of the active material increases or decreases depending on the charge / discharge cycle due to the use of the battery product, but the strength of the foil-like metal sheet is low because it is difficult to completely arrange the electrode laminate in the battery. The electrode is easily damaged at the strain concentrated part. In this specification, a sheet having a thickness of 100 μm or less is called “foil”.

  On the other hand, in order to increase the capacity of the battery, it is desired that the discharge capacity per unit volume of the electrode is large. For this purpose, it is advantageous that the active material is present at a high density on the surface of the foil-like metal sheet. In order to increase the density of the active material layer, it is effective to strongly press the coating film formed by applying the active material-containing paint by a roll press or the like. However, as described below, it is difficult to further increase the density of the active material layer with the current electrode using aluminum foil or copper foil.

  In FIG. 1, the state of the material cross section at the time of forming an active material layer with a roll press method is shown typically. A coating film 2 containing an active material is formed on the surface of a foil-like metal sheet 1 that is a core material of the current collector. By pressing with a rotating roll 3, the thickness of the coating film 2 is reduced, A material layer 4 is formed. Usually, the foil-like metal sheet 1 is an aluminum foil in the case of a positive electrode and a copper foil in the case of a negative electrode. In FIG. 1, the thicknesses of the foil-like metal sheet 1, the coating film 2, and the active material layer 4 are exaggerated, and the thickness ratios do not necessarily reflect actual dimensions.

  FIG. 2 schematically shows the state of the material cross section when passing through the roll as viewed from the direction A in FIG. 1 when an appropriate rolling force is applied when forming the active material layer by the roll press method. If the rolling force of the roll 3 is appropriate, the active material layer 4 is formed with almost no deformation of the foil-like metal sheet 1. In FIG. 2, the thicknesses of the foil-like metal sheet 1 and the active material layer 4 are exaggerated.

  FIG. 3 schematically shows the state of the material cross section when passing through the roll as viewed from the direction A in FIG. 1 when an excessive reduction force is applied when forming the active material layer by the roll press method. In this case, the rolling force of the roll 3 is larger than in the case of FIG. As the reduction force increases, the active material layer 4 becomes more dense. However, since the foil-like metal sheet 1 is an aluminum foil or a copper foil, its strength is low, and plastic deformation may occur at the central portion in the width direction, resulting in a so-called “medium elongation” state. When the non-application part 5 is provided in the width direction edge part (edge) vicinity of the foil-shaped metal sheet 1, the difference of the thickness of an edge and a center part tends to become still more remarkable. When the middle elongation occurs, a shape defect as a current collector material or a decrease in dimensional accuracy becomes a problem. Therefore, the rolling force of the roll 3 is limited to a range in which the aluminum foil and the copper foil are not deformed, and this is a limitation for increasing the density of the active material layer 4.

  The present invention provides a metal sheet for a negative electrode having higher strength, higher durability, and good rust prevention as one of elemental technologies leading to higher capacity of a lithium ion secondary battery, Is intended to provide a negative electrode having a large discharge capacity. Moreover, it aims at providing the lithium ion secondary battery using the same.

The above object is a rust-proof metal sheet having a copper coating layer on both surfaces of a steel sheet and having a rust preventive layer on the surface of the copper coating layer, the total thickness of the steel sheet and the copper coating layers on both surfaces. A lithium ion secondary battery having a thickness t of 3 to 100 μm, an average film thickness t _Cu of 0.02 to 5.0 μm, and a t _Cu / t of 0.3 or less. This is achieved by a rust-proof metal sheet for supporting a negative electrode active material.

  More preferably, the rust preventive agent layer is a thin layer obtained by rolling a rust preventive agent layer attached to the surface of a copper-coated steel sheet as an intermediate product together with the copper-coated steel sheet. Examples of the rust preventive include those containing benzotriazole. Examples of the copper coating layer include those formed through electrolytic copper plating, and those formed through clad rolling of a steel sheet and a copper sheet.

  As a steel sheet which is a core material for forming a copper coating layer, a cold-rolled steel sheet, austenitic or ferritic stainless steel sheet can be used as a raw material. As a standard product, in the case of plain steel, for example, a material made of a cold-rolled steel sheet (including a steel strip) defined in JIS G3141: 2009 can be applied. In the case of stainless steel, for example, one having an austenitic or ferrite chemical composition defined in JIS G4305: 2005 can be applied.

Specific content ranges of the constituent elements constituting the steel sheet are exemplified below.
[Regular steel]
In mass%, C: 0.001 to 0.15%, Si: 0.001 to 0.1%, Mn: 0.005 to 0.6%, P: 0.001 to 0.05%, S: 0.001 to 0.5%, Al: 0.001 to 0.5%, Ni: 0.001 to 1.0%, Cr: 0.001 to 1.0%, Cu: 0 to 0.1% , Ti: 0 to 0.5%, Nb: 0 to 0.5%, N: 0 to 0.05%, balance Fe and inevitable impurities.

[Austenitic stainless steel]
By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 4.0%, Mn: 0.001 to 2.5%, P: 0.001 to 0.045%, S: 0.0005 to 0.03%, Ni: 6.0 to 28.0%, Cr: 15.0 to 26.0%, Mo: 0 to 7.0%, Cu: 0 to 3.5%, Nb : 0-1.0%, Ti: 0-1.0%, Al: 0-0.1%, N: 0-0.3%, B: 0-0.01%, V: 0-0. 5%, W: 0 to 0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0 to 0.1%, balance Fe and inevitable impurities.

[Ferrite stainless steel]
By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.2%, Mn: 0.001 to 1.2%, P: 0.001 to 0.04%, S: 0.0005 to 0.03%, Ni: 0 to 0.6%, Cr: 11.5 to 32.0%, Mo: 0 to 2.5%, Cu: 0 to 1.0%, Nb: 0 -1.0%, Ti: 0-1.0%, Al: 0-0.2%, N: 0-0.025%, B: 0-0.01%, V: 0-0.5% , W: 0 to 0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0 to 0.1%, remaining Fe and inevitable impurities.

  Here, the element whose lower limit is 0% is an arbitrary element. Foil-like metal sheets employing these steel sheets exhibit higher strength than copper foil, which is a conventional negative electrode material. In particular, a metal sheet having a tensile strength adjusted to 450 to 900 MPa is advantageous in improving the durability of the electrode, and more preferably adjusted to more than 600 to 900 MPa.

Moreover, in this invention, the negative electrode of the lithium ion secondary battery which formed the active material layer for lithium ion secondary battery negative electrodes on the at least one surface of said rust prevention metal sheet is provided. Furthermore, a lithium ion secondary battery using the negative electrode is provided. In addition, what is necessary is just to select the density of the active material layer according to the objective in the range of 1.0 g / cm < ³ > or more when applying a carbon type active material. In particular, for applications in which input / output characteristics using non-graphite active materials are important, for example, 1.0 to 1.5 g / cm ³ may be selected, but in applications where energy density using graphite active materials is important. Then, it is desirable to set it as 1.50 g / cm < ³ > or more like the past. In order to enhance the effect of increasing the discharge capacity, the density of the active material layer is more preferably 1.80 g / cm ³ or more, and even more preferably 2.00 g / cm ³ or more.

Moreover, as a manufacturing method of the negative electrode of a lithium ion secondary battery,
Forming a coating film containing a lithium ion secondary battery negative electrode active material on the surface of at least one copper coating layer of the rust-proof metal sheet,
A step of densifying the coating film by reducing the coating thickness by 30 to 70% by a roll press after the coating film is dried;
Is provided.
When using a carbon-based active material, it is desirable to adjust the density of the coating film to a predetermined density of the above-described active material layer by a roll press.

  According to the present invention, it is possible to provide a foil-like metal sheet for a negative electrode of a lithium ion secondary battery having higher strength than before. For this reason, durability of a battery improves and it can respond to the needs of the area increase or thickness reduction of a negative electrode. Plastic deformation of the foil-like metal sheet due to the volume change of the negative electrode active material during charge / discharge is suppressed, which is advantageous for extending the life of the battery. Since the metal sheet is not easily deformed in the roll press process for supporting the active material, an electrode with high dimensional accuracy can be realized. In particular, it becomes easy to increase the density of the active material layer more than before. Moreover, since this metal sheet is excellent in rust prevention, the surface resistance increases due to the formation of an oxide film during the storage period from the production of the foil-like metal sheet to the application of the active material-containing paint, and the active material layer The problem of reduced adhesion is solved. The increase in surface resistance due to the presence of the anticorrosive layer is often small compared to the increase in surface resistance due to the formation of an oxide film during a long storage period. In particular, when the rust preventive layer is thinned by rolling, an increase in surface resistance due to the presence of the rust preventive layer is further suppressed, which is particularly effective for increasing the discharge capacity.

The figure which showed typically the state of the material cross section at the time of forming an active material layer on the foil-shaped metal sheet surface by the roll press method in the electrode manufacturing process of a lithium ion secondary battery. The figure which showed typically the state of the material cross section at the time of the roll passage seen from the A direction of FIG. 1 at the time of providing the appropriate rolling force when forming an active material layer by the roll press method. The figure which showed typically the state of the material cross section at the time of the roll passage seen from the A direction of FIG. 1 at the time of applying excessive rolling force when forming an active material layer by the roll press method. The figure which showed typically the cross-section of the antirust metal sheet for negative electrode active material carrying | support of this invention. The figure which showed typically the cross-section of the negative electrode of this invention.

FIG. 4 schematically shows a cross-sectional structure of the rust-proof metal sheet for supporting the negative electrode active material of the lithium ion secondary battery of the present invention. A metal sheet in which both surfaces of the steel sheet 6 are coated with the copper coating layer 7 is referred to as a copper-coated steel sheet in this specification, and is indicated by reference numeral 10 in FIG. The rust preventive metal sheet 20 according to the present invention has a rust preventive agent layer 8 on the surface of the copper-coated steel sheet 10. In FIG. 4, the thicknesses of the copper coating layer 7 and the rust preventive agent layer 8 are exaggerated, and the thickness ratios do not necessarily reflect actual dimensions (the same applies in FIG. 5 described later). The copper coating layer 7 is adjusted to have an average film thickness t _Cu per side of 0.02 to 5.0 μm on both sides, and the average thickness t of the copper coated steel sheet 10 including the copper coating layer 7 is 3 to 100 μm. It is in the range. The average film thickness t _Cu of the copper coating layer 7 is not necessarily the same on both surfaces, but t _Cu / t needs to satisfy 0.3 or less on each surface side.

The copper coating layer 7 can be suitably formed using, for example, an electrolytic copper plating method as described later. In order to improve the adhesion of the electrolytic copper plating layer, it is advantageous to form a base plating layer. For example, when the steel sheet 6 is plain steel, it is desirable to perform copper strike plating, and when the steel sheet 6 is stainless steel, it is desirable to perform nickel strike plating. Two or more base plating layers may be applied. When the copper coating layer 7 is formed by performing electrolytic copper plating (main plating) directly on the copper strike plating layer, the copper strike plating layer is considered to constitute a part of the copper coating layer 7. When performing base plating with a metal other than copper, such as nickel strike plating, assuming that the total average film thickness of the base plating layer formed under the copper coating layer 7 is t _M , (t _M + t _Cu ) is 5. It is desirable that it is 0 μm or less.

  When the average thickness t of the copper-coated steel sheet 10 is smaller than 3 μm, it is difficult to ensure sufficient strength as an electrode and a necessary amount of active material even if a high strength steel sheet 6 is applied. . You may manage in the range of 5 micrometers or more, or 7 micrometers or more. On the other hand, if t exceeds 100 μm, it will not meet the requirements for battery size reduction and capacity increase. In general, the range is preferably 50 μm or less, and may be controlled to 25 μm or less, or 15 μm or less.

If the average film thickness t _Cu per side of the copper coating layer 7 is smaller than 0.02 μm, the absolute amount of copper having high conductivity in the copper coated steel sheet 10 is reduced, and the film thickness of the copper coating layer 7 is reduced. Due to an increase in defects such as penetrating pinholes, it is difficult to stably maintain a high discharge capacity. t _Cu may be controlled to 0.03 μm or more, or 0.05 μm or more. On the other hand, when t _Cu exceeds 5.0 μm, when the rolling force is increased in the roll press process, plastic deformation of the copper coating layer 7 is likely to occur, and the active material layer is densified while maintaining high dimensional accuracy. It becomes difficult to plan. Moreover, the cost of copper plating also increases. When importance is attached to the dimensional accuracy of the electrode and the increase in the density of the active material layer, it is more preferable that t _{Cu be} in the range of 1.0 μm or less (or less than 1.0 μm).

When the average thickness t of the copper-coated steel sheet 10 is reduced to, for example, about 20 μm or less, if the average film thickness t _Cu of the copper coating layer 7 is not reduced accordingly, the copper coating layer 7 in the roll press is not reduced. It becomes difficult to suppress plastic deformation. As a result of various studies, if t _Cu / t is 0.3 or less in each of the copper coating layers 7 on both sides, the deformation of the copper coating layer 7 is effectively restrained by the steel sheet 6 and current collection with high dimensional accuracy is performed. It is advantageous in obtaining the body. t _Cu / t is more preferably 0.2 or less, or even 0.1 or less.

  The copper-coated steel sheet 10 using the steel sheet 6 as a core material has a significantly higher strength than the copper foil used in the conventional negative electrode. In order to stably and significantly improve the durability of the negative electrode in the state of being incorporated in the battery and the shape maintaining property (medium elongation inhibiting performance) in the roll press process for forming the negative electrode active material layer 7, It is more effective to set the tensile strength of the coated steel sheet 10 to 450 to 900 MPa. You may manage to 500 Mpa or more. In particular, the copper-coated steel sheet 10 adjusted to a strength level exceeding 600 MPa or further to a strength level of 650 MPa or more is extremely advantageous for improving the reliability of the negative electrode. The tensile strength of the copper-coated steel sheet 10 can be controlled by the chemical composition of the steel sheet 6 and the cold rolling rate. Even if the tensile strength exceeds 900 MPa, further improvement in durability and shape maintainability cannot be expected so much, and conversely, the disadvantage of increased cost due to an increase in the cold rolling rate increases.

  The rust preventive agent layer 8 is a rust preventive coating having an action of suppressing a phenomenon that an oxide coating is formed on the surface of the copper-based material after cold rolling. Various rust preventive agents conventionally used for rust prevention of copper-based materials can be applied. There are also commercially available rust inhibitors for copper-based materials called corrosion inhibitors and anti-discoloration materials, and these may be applied. Specific examples include those containing benzotriazole (BTA) or a compound thereof. In addition, there are rust preventives containing thiadiazole, sorbitan ester, sulfonate of alkali metal or alkaline earth metal, salicylate of alkali metal or alkaline earth metal, chromium oxide and the like. Moreover, the rust preventive agent layer 8 may have a coupling treatment film on its surface layer portion. For example, you may form the rust preventive agent layer 8 by passing through the film formation process and coupling process using the said rust preventive agent. As the coupling agent, known ones such as silane, aluminum, titanium, and zirconium can be applied.

  Since the rust preventive agent layer 8 itself has an electric resistance larger than that of the metal material, it is advantageous to reduce the film thickness of the rust preventive agent layer 8 in order to suppress an increase in the surface resistance of the rust preventive metal sheet 20 as much as possible. It becomes. In general, the rust prevention treatment is performed by immersing the material in a treatment liquid containing a rust prevention agent or by applying the treatment liquid by a roll or spray. Moreover, by adding an appropriate amount of a rust inhibitor to various treatment liquids used in the production process, it is possible to carry out a rust prevention treatment simultaneously with passing through the process. Specifically, the treatment liquid includes rolling oil in the rolling process, cleaning agent or degreasing liquid in the degreasing process of rolling oil, or washing water. According to the study by the inventors, when the copper-coated steel sheet 10 is stored in a normal atmospheric environment and then subjected to the step of applying the active material-containing paint, the rust prevention formed by the above general rust prevention treatment. The film thickness of the agent layer is not necessary, and it has been found that a large rust prevention effect can be obtained even when the agent layer is thinned. In that case, since the electrical resistance of the rust preventive agent layer itself is reduced by thinning the rust preventive agent layer, the surface resistance of the rust preventive metal sheet 20 can be reduced as a result. That is, it is more preferable that the rust preventive agent layer 8 is obtained by rolling and thinning the rust preventive agent layer attached to the surface of the copper-coated steel sheet as an intermediate product together with the copper-coated steel sheet. It is effective if the rolling rate for thinning is 30 to 98%.

  FIG. 5 schematically shows a cross-sectional structure of the negative electrode of the lithium ion secondary battery according to the present invention. On the surface of the rust preventive metal sheet 20 in which the rust preventive agent layer 8 is formed on the surface of the copper-coated steel sheet 10, a negative electrode active material layer 40 densified by a roll press or the like is formed. In this figure, a case where the negative electrode active material layer 40 is formed on both surfaces of the copper-coated steel sheet 10 is illustrated, but a negative electrode current collector in which the negative electrode active material layer 40 is formed only on one surface may be employed. is there. For example, in the negative electrode located at the end of the electrode laminate, the negative electrode active material layer 40 may be formed on one side.

The average thickness of the densified negative electrode active material layer 40 is preferably 5 to 150 μm per side, and more preferably 20 to 100 μm. In the case of the negative electrode active material layer 40 containing a carbon-based active material (described later), the density (including the fine voids in the layer) of the negative electrode active material layer 40 is 1.50 g / cm ^{3 in} applications where energy density is important. The above is desirable. Further, according to the steps described later, the density of the carbon-based active material layer can be 1.80 g / cm ³ or more, or even 2.00 g / cm ³ or more. Thus, when the negative electrode active material layer is densified, the strength of the metal sheet is increased with respect to the conventional negative electrode (for example, the density of the active material layer: about 1.50 to 1.75 g / cm ³ ). In addition to the effect of improving the durability, the effect of improving the discharge capacity per unit volume of the active material layer can be obtained. On the other hand, if the density of the active material layer becomes too high, the electrolytic solution is less likely to penetrate into the layer, which may be a factor that inhibits charge transfer. If the theoretical density of graphite is considered to be the 2.26 g / cm ^3, the density of the carbon-based active material layer is preferably set to 2.20 g / cm ³ or less in the range, 2.15 g / cm ³ or less in the range You may manage. The density of the active material layer is calculated from the average thickness of the active material layer obtained by microscopic observation of the current collector cross section and the average mass per unit area of the active material layer.

[Example of manufacturing process of rust-proof metal sheet]
The following can be illustrated as a process for manufacturing the antirust metal sheet of this invention.
[] Is the name of the material at each stage. () Is a process that can be performed as necessary.
A. [Cold rolled steel sheet] → Rolling → (Under plating) → Copper plating → [Copper coated steel sheet] → Rust prevention treatment → [Rust prevention metal sheet]
B. [Cold rolled steel sheet] → Rolling → (Under plating) → Copper plating → [Copper coated steel sheet] → Rust prevention treatment → Further rolling → [Rust prevention metal sheet]
C. [Cold rolled steel sheet] → (Undercoat) → Copper plating → [Copper coated steel sheet] → Rust prevention treatment → Rolling → [Rust prevention metal sheet]
D. [Cold rolled steel sheet] → Clad bonding with copper sheet → [Copper coated steel sheet] → Antirust treatment → Rolling → [Antirust metal sheet]
E. [Cold rolled steel sheet] → Rolling → Clad bonding with copper sheet → [Copper coated steel sheet] → Rust prevention treatment → [Rust prevention metal sheet]
Among the above, in the steps B, C, and D, the rust preventive agent layer can be thinned by rolling after the rust prevention treatment.

[Example of negative electrode manufacturing process]
Using the rust-proof metal sheet obtained as described above, a negative electrode of a lithium ion secondary battery can be produced, for example, by the following steps.
[Rust-proof metal sheet] → Active material-containing coating application → Coating film drying → Roll press → Cutting, etc. → [Negative electrode]

[Steel sheet]
As the steel sheet (symbol 6 in FIG. 4) that is the core material of the copper-coated steel sheet used in the present invention, stainless steel can be used in addition to ordinary steel. Since stainless steel is excellent in corrosion resistance, it is suitable for applications where high durability and reliability are important. The specific chemical composition range is as described above.

[Copper plating]
As a method for forming the copper coating layer, a copper plating method can be used as exemplified in the above-described A, B, and C steps. Various known copper plating techniques such as electroplating, chemical plating, and vapor phase plating can be used. Examples of the chemical plating include electroless plating, and examples of the vapor phase plating include sputtering and ion plating. Among these, the electrolytic copper plating method is suitable for mass production because a plating layer can be formed relatively quickly and economically and the plating thickness can be easily controlled.

Electrolytic copper plating;
Various known electrolytic copper plating methods can be employed. If the conditions of electrolytic copper plating when using a sulfuric acid bath are exemplified, for example, using a plating bath of copper sulfate: 200 to 250 g / L, sulfuric acid: 30 to 75 g / L, liquid temperature: 20 to 50 ° C., cathode current density: it can be 1 to 20A / dm ^2. However, the adhesion amount of copper plating varies greatly depending on whether the copper coating is rolled to a predetermined thickness after copper plating or a copper coating layer having a target film thickness is directly formed by copper plating. In the former case, it is necessary to form a copper plating layer having a thickness calculated backward from the target film thickness of the copper coating layer according to the rolling rate in the subsequent process. If the required copper plating layer thickness cannot be obtained by one pass of the copper plating line, the passing of the copper plating line may be performed a plurality of times.

Pretreatment of electrolytic copper plating;
In the case of performing electrolytic copper plating, copper strike plating or nickel strike plating can be performed as a pretreatment for forming a base plating layer. In particular, when the steel sheet is stainless steel, nickel strike plating is extremely effective for improving the adhesion of copper plating.

The conditions for copper strike plating are, for example, copper pyrophosphate: 65-105 g / L, potassium pyrophosphate: 240-450 g / L, total pyrophosphate ion concentration (g / L) relative to total copper ion concentration (g / L) Ratio (P ratio): 6.4 to 8.0, aqueous ammonia: 1 to 6 mL / L, liquid temperature: 50 to 60 ° C., pH: 8.2 to 9.2, cathode current Density: It can be set in a range of 1 to 7 A / dm ² or less.
The conditions of nickel strike plating are, for example, a normal temperature plating bath of nickel chloride: 230 to 250 g / L, hydrochloric acid: 125 ml / L, pH: 1 to 1.5, and cathode current density: 1 to 10 A / dm ^2. It can be.

Vapor phase plating;
The copper coating layer can also be formed by a known vapor phase plating method such as vapor deposition, sputtering, or ion plating. A manufacturing method using sputtering is exemplified. First, as a pretreatment, a wet cleaning line having steps of “methylene chloride cleaning → drying → isopropyl alcohol cleaning → water cleaning → drying” on a steel sheet as a base material (plating original plate). Apply degreasing cleaning. Next, a copper coating layer is formed by passing the steel foil after degreasing and washing through a continuous sputtering line. The continuous sputtering line can be configured, for example, by arranging a set of a coil dispensing device, a high-frequency magnetron sputtering device, and a winding device in a vacuum chamber.

Specifically, for example, sputtering can be performed by the following method. The argon partial pressure in the chamber is adjusted to about 0.1 Pa, and reverse sputtering is performed at an output of about 100 W to activate the surface of the steel sheet. Next, a copper coating layer having an average film thickness t _Cu of about 0.05 μm is formed on one surface of the steel sheet by performing film formation sputtering with an output of about 300 W using pure copper as a target. At that time, the average film thickness t _Cu may be adjusted by controlling the plate passing speed of the continuous sputtering line. By repeating such an operation by reversing the front and back of the steel sheet, a copper-coated steel sheet having a steel sheet as a core and having a copper coating layer on both sides is obtained.

[Clad]
As another method for producing a copper-coated steel sheet, a technique in which a copper sheet is clad-bonded to both surfaces of the steel sheet can be employed. As the cladding method, a hot pressure welding method, a cold pressure welding method, an explosion method, and the like are known. In particular, the cold welding method is suitable for mass production because of excellent thickness accuracy and good productivity.

A method for producing a copper-coated steel foil using the cold welding method in the above-described production process D will be exemplified. As a raw material, one strip steel sheet (for example, a cold-rolled steel strip) whose thickness is adjusted so that the above-mentioned t _Cu / t becomes a predetermined value and two strip copper sheets (for example, strip-shaped copper foil) are used. prepare. Examples of the copper sheet include tough pitch copper, oxygen-free copper, and alloy copper. After removing the rolling oil by passing through a degreasing and cleaning line respectively, cold rolling is performed by continuously performing cold rolling on the laminated material having a three-layer structure in which both surfaces of the belt-shaped steel sheet are sandwiched between the strip-shaped copper sheets. By pressure welding, a clad material in which these three layers are integrated by clad bonding is manufactured. If the cold rolling rate during pressure welding is too low, the generation of a new surface at the interface between the steel sheet and the copper sheet is reduced, and the joining strength tends to be insufficient. If the cold rolling rate is too high, the rolling load becomes excessive and the rolling shape deteriorates, or the tensile load becomes excessive and a trouble such as breaking in the line is likely to occur. As a result of various studies, the cold rolling rate for pressure welding can be set in the range of approximately 10 to 75%, but when the steel sheet is ordinary steel, it is 40 to 50%, and when stainless steel is 15 to 40%. More preferably. A copper-coated steel sheet can be obtained by subjecting the obtained clad material to cold rolling with a foil rolling machine.

  Moreover, like the above-mentioned manufacturing process E, a cold-rolled steel plate is rolled to foil in advance to obtain a foil-like steel sheet, and the foil-like steel sheet and the foil-like copper sheet are clad joined by cold welding. Thus, a copper-coated steel sheet can be obtained.

〔rolling〕
In the rolling in the manufacturing processes A to E described above, a rolling mill capable of imparting high pressure underforce such as a general Sendzimir type rolling mill or a cluster type rolling mill may be used. Here, if the plate thickness before rolling is t _in and the plate thickness after rolling is t _out , the rolling rate r is expressed by the following equation.
Rolling ratio r (%) = (1−t _out / t _in ) × 100

  In order to optimize the strength level of the copper-coated steel sheet according to the specifications of the battery, cold rolling (cladding joint) that is received until the steel material after final annealing becomes the core material of the final copper-coated steel foil It is effective to appropriately control the total rolling ratio (including cold rolling at the time). As a result of various studies, in order to obtain a copper-coated steel sheet having a particularly high strength level, it is extremely effective to set the total rolling rate to 90% or more, and if it is desired to further increase the strength, it may be set to 95% or more. . The upper limit of the total rolling rate is restricted mainly by the ability of the rolling mill to be used, but excessively high strength becomes uneconomical. Usually, the total rolling rate may be 99% or less, and may be set in a range of 98% or less in consideration of economy and productivity.

[Rust prevention treatment]
A rust preventive treatment is performed on the surface of the copper-coated steel sheet using the aforementioned rust preventive agent to form a rust preventive agent layer. As the method of rust prevention treatment, the same technique as that of a general strip-shaped copper-based material can be applied. That is, there are a method of immersing a material in a processing solution containing a rust inhibitor and then pulling it up, a method of applying the rust inhibitor by a roll or spray, and the like. Further, a treatment for forming a film of the above-described coupling agent may be performed. When thinning the rust preventive layer formed by the rust preventive treatment to reduce the electrical resistance of the rust preventive agent layer, as in the above-mentioned steps B, C, D, to obtain a rust preventive metal sheet It is desirable that the copper-coated steel sheet of the intermediate product is subjected to a rust prevention treatment at an intermediate stage, and then thinned using rolling for adjusting the thickness of the copper-coated steel sheet. It is more effective to set the rolling rate of rolling accompanied by thinning of the rust preventive agent layer to 30 to 98% in terms of elongation of the copper-coated steel sheet, and it is more effective to set the rolling rate to 50 to 98%. is there. As described above, the rust-proof metal sheet of the present invention is obtained.

[Formation of active material-containing coating film]
The negative electrode according to the present invention is composed of the above-mentioned rust-proof metal sheet and an active material layer formed on the surface thereof. The active material layer has a void that allows the electrolyte solution to permeate and allows charge transfer by lithium ions, and includes a negative electrode active material, a conductive additive, a binder, and the like. Any negative electrode active material may be used as long as it can insert and desorb lithium ions. For example, as carbon-based active materials, pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites, glassy carbons, organic polymer fired bodies (furan resin, etc.) are fired at an appropriate temperature. And carbonized), carbon fiber, activated carbon and the like. As the conductive agent, for example, graphite, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, metal fiber and the like can be used. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethylene, polypropylene, a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and a vinylidene fluoride-hexafluoropropylene copolymer. Coalescence etc. can be used.

  The negative electrode active material using the carbon material as described above is referred to as a “carbon-based active material” in this specification. A negative electrode active material layer using a carbon-based active material is referred to as a “carbon-based active material layer”. As a procedure for forming the carbon-based active material layer, for example, as described above, a process of “active material-containing coating application → coating film drying → roll press” can be employed. In this case, first, a paint containing a carbon-based active material for the negative electrode of the lithium ion secondary battery as described above (active material-containing paint) is prepared, and this is applied onto the surface of the copper coating layer of the copper-coated steel foil. Apply by coater method. Thereafter, the coating film is dried. The film thickness of the dried coating film is determined by calculating backward from the target active material layer thickness in anticipation of a reduction rate of the coating film thickness by a roll press described later. When the active material layers are formed on both sides, it is desirable that the coating thicknesses on both sides are substantially uniform.

[High density active material layer]
In order to increase the discharge capacity of the electrode, it is effective to increase the density of the active material layer. As a method for increasing the density of the active material layer, generally, a method of reducing the thickness of the above-described dry coating film by a roll press is employed. According to the present invention, a high-strength copper-coated steel sheet is used, so that the rust-proof metal sheet (foil-like metal sheet in FIGS. 1 and 2) hardly undergoes plastic deformation even when the rolling force by the roll press is increased. For this reason, the rolling force by a roll press can be raised conventionally.

Specifically, it is preferable to increase the density by reducing the thickness of the dried coating film by 30% or more by a roll press. This coating thickness reduction rate is determined by the following equation (1).
[Coating thickness reduction rate (%)] = (h ₀ −h ₁ ) / h ₀ × 100 (1)
Here, h ₀ is the average coating thickness (μm) per side before roll pressing, and h ₁ is the average coating thickness (μm) after roll-pressing the coating. When emphasizing higher density of the active material layer, it is more effective to set the coating thickness reduction rate to 35% or more, more preferably 40% or more. However, if the rolling force is too large, the coating film density becomes excessive and the electrolytic solution does not easily penetrate into the coating film, and there is a possibility that sufficient gaps necessary for charge transfer cannot be secured. Moreover, it becomes a factor which causes the nonuniform deformation | transformation of metal foil. As a result of various studies, the reduction rate of the coating film thickness by roll press is desirably in the range of 70% or less, and may be controlled in the range of 60% or less. Thus, the negative electrode according to the present invention is obtained.

[Lithium ion secondary battery]
The negative electrode having a high-density active material layer on the surface of the rust-proof metal sheet is combined with the positive electrode via a separator to form an “electrode stack”, which constitutes a lithium ion secondary battery together with the electrolyte To do. As the positive electrode, the separator, and the electrolytic solution, a known material used for a lithium ion secondary battery or a new material that can be used as an alternative can be applied.

As an example of the electrolytic solution, examples of the solvent include nonaqueous solvents such as ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate, butylene carbonate, dimethyl carbonate, sulfolane, dimethoxyethane, tetrahydrofuran, and dioxolane. These may be used alone or in combination of two or more. Examples of the solute include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiCF ₃ (CF ₂ ) ₃ SO _{3 and the} like may be mentioned. These may be used alone or in combination of two or more. .

  Examples of the shape of the case material for hermetically storing the electrode laminate and the electrolyte include a coin shape, a cylindrical shape, a rectangular parallelepiped shape, and a laminate sheet pack shape. Examples of the material of the case material include aluminum or its alloy, titanium or its alloy, nickel or its alloy, copper or its alloy, stainless steel, ordinary steel, nickel-plated steel plate, copper-plated steel plate, galvanized steel plate and the like. In the case of a laminate sheet pack type, for example, a laminate foil obtained by laminating a resin film having heat sealability on a metal foil such as an aluminum foil or a stainless steel foil is used.

Example 1
Various copper-coated steel sheets were prepared by the methods of Examples a to g below, and the surface was subjected to rust prevention treatment to obtain a rust prevention metal sheet. Here, in order to make the influence of the film thickness of the rust preventive agent layer constant, the thinning after the rust preventive treatment is not performed. Tensile strength was measured for some copper-coated steel sheets. Each rust-proof metal sheet was subjected to a corrosion resistance test in an electrolytic solution. Moreover, the negative electrode of the lithium ion secondary battery was produced using each antirust metal sheet, and the discharge capacity was evaluated.

[Preparation of copper-coated steel sheet]
<Example a>
A copper-coated steel sheet was prepared by a process in which the rust prevention treatment was omitted in the process C.
A plurality of cold-rolled steel strips (annealing materials) having a sheet thickness of 0.3 mm and a sheet width of 200 mm having the following chemical composition were prepared.
Chemical composition:% by mass, C: 0.003%, Al: 0.038%, Si: 0.003%, Mn: 0.12%, P: 0.012%, S: 0.122%, Ni : 0.02%, Cr: 0.02%, Cu: 0.01%, Ti: 0.073%, N: 0.0027%, balance Fe and inevitable impurities

Copper strike plating and electrolytic copper plating (main plating) were performed on both surfaces of the steel strip in a continuous electroplating line to prepare copper-plated steel strips having copper plating layers of various thicknesses. In one copper-plated steel strip, the thicknesses of the copper plating layers on both sides were substantially uniform. Thereafter, it is cold-rolled by a foil rolling machine, and various stages with an average thickness t including the copper plating layers on both sides of 20 μm and an average thickness t _Cu per side of the copper coating layer of 0.9 to 0.005 μm. A copper-coated steel sheet was obtained. At that time, the thickness of the copper plating layer formed on the surface of the steel strip having a thickness of 0.3 mm (the total amount of plating of the copper strike plating and the main plating) is the average thickness of the copper-coated steel sheet after cold rolling. When the thickness t was set to 20 μm, the average thickness t _Cu of the copper coating layer was determined so as to be a predetermined thickness in the range of 0.9 to 0.005 μm. For example, when a copper-coated steel sheet having a t _Cu of 0.5 μm is obtained, the amount of copper plating attached per side is 7.9 μm.

The copper strike plating bath includes copper pyrophosphate: 80 g / L, potassium pyrophosphate: 300 g / L, ammonia water: 3 mL / L, P ratio: 7, liquid temperature: 55 ° C., pH: 9 copper strike plating bath And a plating adhesion amount of 0.3 μm thick per side under the condition of a cathode current density of 5 A / dm ² .
The electrolytic copper plating (main plating) was performed under the conditions of a cathode current density of 10 A / dm ² using a copper plating bath containing 210 g / L of copper sulfate and 45 g / L of sulfuric acid and having a liquid temperature of 40 ° C.

  In addition, in order to confirm whether or not a predetermined copper coating layer thickness was obtained, after ion milling cross-section polishing of the copper coated steel foil produced with a target thickness of the copper coating layer of 0.5 μm, an electron microscope was used. Observe and measure the copper coating layer thickness. The measurement sample collection positions were three places every 5 m in the rolling longitudinal direction, and three visual fields were observed for each sample. As a result, the minimum value of the measurement data of 3 visual fields × 3 locations = total 9 visual fields was 0.42 μm, the maximum value was 0.55 μm, and the average value was 0.48 μm. That is, it was confirmed that the foil rolling performed here was able to realize highly accurate rolling almost as intended.

<Example b>
A copper-coated steel sheet was prepared by a process in which the rust prevention treatment was omitted in the process A.
Commercially available SUS304 and SUS430 cold-rolled steel strip (both annealed materials corresponding to JIS G4305: 2005) were cold-rolled by a foil rolling machine to obtain a steel foil having a thickness of 20 μm. On both surfaces of the steel foil, at the electroplating equipment, by performing nickel strike plating and electrolytic copper plating, 0.5 [mu] m average thickness t _Cu of the copper coating layer per one side or 0.05μm copper coated steel, A sheet was produced. The adhesion amount of nickel strike plating is about 0.2 μm per side. In one copper-coated steel sheet, the thicknesses of the copper coating layers on both sides were substantially uniform.

The nickel strike plating is performed using a normal temperature plating bath of nickel chloride: 230 to 250 g / L, hydrochloric acid: 125 ml / L, pH: 1 to 1.5, under conditions of a cathode current density of 1 to 10 A / dm ² . went.
The electrolytic copper plating was performed under the conditions of a cathode current density of 10 A / dm ² using a copper plating bath containing copper sulfate: 210 g / L, sulfuric acid: 45 g / L, and liquid temperature: 40 ° C.

<Example c>
A copper-coated steel sheet was prepared by a process in which the rust prevention treatment was omitted in the process B.
A commercially available cold rolled steel strip of SUS304 (an annealed material corresponding to JIS G4305: 2005) was cold-rolled by a foil rolling machine to obtain a steel foil having a thickness of 20 μm. A copper-coated steel sheet (intermediate product) having an average thickness of the copper coating layer on one side of 0.5 μm was applied to both surfaces of the steel foil by nickel strike plating and electrolytic copper plating in an electroplating facility. . The adhesion amount of nickel strike plating is about 0.2 μm per side. This copper-coated steel sheet is further rolled by a foil rolling machine to obtain an average thickness t including the copper coating layer of 8.0 μm and an average thickness t _Cu of the copper coating layer per side of 0.2 μm. A sheet was obtained. The thickness of the copper coating layers on both sides was uniform. The plating conditions are the same as in Example b.

<Example d>
A copper-coated steel sheet was produced by the process in which the rust prevention treatment was omitted in the process D.
A ferritic stainless steel cold-rolled steel strip (annealing material) equivalent to SUS430 having a thickness of 0.684 mm and a width of 300 mm having the following chemical composition was prepared.
Chemical composition:% by mass, C: 0.058%, Al: 0.009%, Si: 0.56%, Mn: 0.31%, P: 0.021%, S: 0.005%, Ni : 0.20%, Cr: 16.7%, Mo: 0.32%, Cu: 0.031%, N: 0.030%, balance Fe and unavoidable impurities Also, the thickness having the following chemical composition Two rolled copper foil strips having a diameter of 18 μm and a width of 300 mm were prepared.
Chemical composition:% by mass, O: 0.0003%, P: 0.0002%, remaining Cu and inevitable impurities

After the stainless steel cold-rolled steel strip and the rolled copper foil strip are passed through a degreasing cleaning line to remove the rolling oil, the stainless steel cold-rolled steel strip is stacked so as to be sandwiched between the rolled copper foil strip from the front and back. They were arranged in three layers and passed through a continuous cold-welded clad production line. A three-layer clad material having a thickness of 0.36 mm was produced by pressure welding at a cold rolling rate of 50%. This was further cold-rolled with a foil rolling machine to obtain a copper-coated steel sheet having an average thickness t including the copper coating layer of 20 μm and an average thickness t _Cu of the copper coating layer per side of 0.5 μm. .

<Example e>
A copper-coated steel sheet was produced by the process in which the rust prevention treatment was omitted in the process D.
A ferritic stainless steel cold-rolled steel strip and rolled copper foil strip equivalent to SUS430 having the same composition as in Example d above were prepared. The stainless steel cold-rolled steel strip has a plate thickness of 1.8 mm and a plate width of 300 mm, and the rolled copper foil strip has a thickness of 38 μm and a width of 300 mm. A three-layer clad material was produced at a cold rolling rate of 50% by the same method as in Example d above. This was further cold-rolled with a foil rolling machine to obtain a copper-coated steel sheet having an average thickness t including the copper coating layer of 100 μm and an average thickness t _Cu of one side of the copper coating layer of 2 μm.

<Example f>
A copper-coated steel sheet was produced by the process in which the rust prevention treatment was omitted in the process D.
A ferritic stainless steel cold-rolled steel strip and rolled copper foil strip equivalent to SUS430 having the same composition as in Example d above were prepared. The stainless steel cold-rolled steel strip has a plate thickness of 0.5 mm and a plate width of 300 mm, and the rolled copper foil strip has a thickness of 63 μm and a width of 300 mm. A three-layer clad material was produced at a cold rolling rate of 50% by the same method as in Example d above. This was further cold-rolled with a foil rolling machine to obtain a copper-coated steel sheet having an average thickness t including the copper coating layer of 50 μm and an average thickness tCu of the copper coating layer per side of 5 μm.

<Example g>
A copper-coated steel sheet was prepared by a process in which the rust prevention treatment was omitted in the process E.
A ferritic stainless steel strip (annealed material) and rolled copper foil strip corresponding to SUS430 having the same composition as in Example d above were prepared. The stainless steel cold-rolled steel strip has a plate thickness of 0.6845 mm and a plate width of 300 mm, and the rolled copper foil strip has a thickness of 12 μm and a width of 300 mm. The stainless steel cold-rolled steel strip was cold-rolled with a foil rolling machine to obtain a steel foil strip having a thickness of 15 μm. With both surfaces of the steel foil strip sandwiched between the rolled copper foil strips, the steel sheet is pressed at a cold rolling rate of 38% in a continuous cold welding clad production line, and the average thickness t including the copper coating layer is A copper-coated steel sheet having a thickness of 15 μm and an average thickness t _Cu of the copper coating layer per side of 4.5 μm was obtained.

[Tensile test]
Some copper-coated steel sheets (t _Cu = 0.5 μm prepared by Examples a and b), which are the subject of the present invention, and two types of copper sheets corresponding to conventional lithium ion secondary battery negative electrode materials (copper Foil) and an aluminum sheet (aluminum foil) corresponding to a conventional lithium ion secondary battery positive electrode material were subjected to a tensile test using a universal precision tensile tester. The test piece dimensions were 12.7 mm width and 175 mm length, and the rolling direction was the longitudinal direction. An initial chuck distance was set to 125 mm, a tensile test was performed until the fracture occurred at a tensile speed of 2 mm / min, and the tensile strength was obtained by dividing the maximum load by the initial cross-sectional area (actual value) of the test piece. Each material was tested with n = 3 and the average value was taken as the tensile strength of the material. The results are shown in Table 1.

  It can be seen that the copper-coated steel sheet, which is the subject of the present invention, has extremely high strength compared to the copper sheet used for the negative electrode of conventional lithium ion secondary batteries and the aluminum sheet used for the positive electrode. The strength of the copper-coated steel sheet can be controlled at various levels depending on the cold rolling rate during the production process. The tensile strength of the copper-coated steel sheet shown in Table 1 is an example, but can be adjusted in the range of 450 to 900 MPa. According to the study by the inventors, it is possible to obtain a copper-coated steel sheet having a tensile strength exceeding 600 MPa or even 650 MPa or more by using existing rolling technology when various steel types are used as the steel sheet. It is.

[Rust prevention treatment]
The copper-coated steel sheet obtained in each of the above examples was subjected to a rust prevention treatment after the final rolling. The rust prevention treatment was performed by a method of immersing the metal sheet to be treated in an aqueous solution in which 2 g / L of benzotriazole (BTA) was dissolved for 3 seconds and then pulling it up and drying it with warm air. For comparison, a copper sheet (copper foil) having a thickness of 20 μm was prepared and subjected to rust prevention treatment by the same method. As described above, a rust-proof metal sheet was obtained.

[Corrosion resistance test in electrolyte]
About each rust prevention metal sheet | seat, the corrosion resistance in electrolyte solution was investigated. A test piece of 30 × 50 mm size cut out from each rust-proof metal sheet was used. The steel substrate of the steel sheet is exposed on the end face of the test piece. As an electrolyte for a lithium ion secondary battery, a solution was prepared by dissolving LiPF ₆ at a concentration of 1 mol / L in a solvent in which ethylene carbonate (EC) and diethylene carbonate (DEC) were mixed at a volume ratio of 1: 1. Using a glove box with a gas circulation purifier, the test piece was immersed in the above electrolytic solution at 25 ° C. for 4 weeks in a glove box in which oxygen and water concentrations were kept at 1 ppm or less. Corrosion resistance evaluation was performed by mass measurement of the test piece before and after the immersion test and ICP-AES quantitative analysis of Fe and Cu dissolved in the electrolytic solution.

  As a result, no significant mass change was observed before and after the immersion test for each test piece. In addition, the Fe and Cu concentrations in the electrolytic solution in which each test piece was immersed were both below the detection limit of ICP-AES analysis (less than 1 ppm), and could not be quantified. That is, dissolution of Fe and Cu in the electrolytic solution was not recognized. From this, it was confirmed that each said copper covering steel foil exhibits favorable corrosion resistance in the electrolyte solution for lithium ion secondary batteries.

[Preparation of negative electrode sample]
90 parts by mass of graphite powder as a negative electrode active material, 5 parts by mass of acetylene black as a conductive auxiliary agent, and 5 parts by mass of polyvinylidene fluoride as a binder are mixed, and this mixture is dispersed in N-methyl-2-pyrrolidone to form a slurry. Thus, an active material-containing paint was obtained. This paint was applied to one side of each of the rust preventive metal sheets to form a carbon-based active material-containing coating film. After the coating film was dried, in order to improve the density of the active material layer, a roll press was performed to form a carbon-based active material layer, and a negative electrode sample was obtained. The roll press was carried out under two conditions of a load per unit length (referred to as “linear pressure”) of 980 kN / m and 1960 kN / m applied to the material from the roll. Here, a negative electrode sample having an active material layer only on one side of a metal sheet was obtained, but even when an active material layer is formed on both sides, the influence of linear pressure on the density of the active material layer is formed only on one side. This is basically the same as the case. Table 2 shows the linear pressure and the reduction rate of the coating thickness determined by the above equation (1).

[Measurement of active material layer density]
After the ion milling cross-section polishing of the negative electrode sample, it was observed with an optical microscope equipped with a CCD camera, and the thickness of the carbon-based active material layer was measured based on a digital image of the cross-sectional structure taken with this CCD camera. The average thickness of the active material layer was calculated by observing 3 fields per sample. Further, a circular sample having a diameter of 35 mm was punched from the negative electrode sample, and the mass of the circular sample was measured. Thereafter, the circular sample was immersed in an N-methyl-2-pyrrolidone solution for 1 week to completely peel the carbon-based active material layer on the sample surface, and the mass of the peeled test piece was measured. The density of the active material layer was determined using the mass difference before and after peeling and the measured value of the average thickness of the active material layer. The results are shown in Table 2.

[Evaluation of discharge capacity]
A circular piece having a diameter of 15.958 mm (area: 2 cm ² ) was punched out from each of the negative electrode samples, and this was used as a test piece for measuring discharge capacity. A general three-electrode test cell having a working electrode, a reference electrode, and a counter electrode was constructed in a glove box in which oxygen and moisture concentrations were maintained at 1 ppm or less using a glove box with a gas circulation purifier. HS-3E manufactured by Hosen Co., Ltd. was used for the test cell casing. The test piece for measuring the discharge capacity was set as a working electrode, and a metal lithium foil was used for each of the reference electrode and the counter electrode. A polypropylene microporous membrane (thickness: 25 μm) was used as a separator for partitioning between the working electrode and the reference electrode and between the counter electrode and the reference electrode. As the electrolytic solution, a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / L in a solvent in which ethylene carbonate (EC) and diethylene carbonate (DEC) were mixed at a volume ratio of 1: 1 was used.

For each test cell, the theoretical capacity of the active material was calculated. Next, after fully charging using the current value indicated by [theoretical capacity (mAh)] / 5 (h), discharging was performed at the same current value. The discharge capacity at this time was defined as [battery capacity (mAh)] of each test cell. Subsequently, after fully charging at a constant charge rate of 0.5 CmA, a cycle of discharging at a constant discharge rate of 1.0 CmA is repeated 10 times, and the discharge capacity Q ₁₀ per unit volume of the active material layer in the 10th cycle is measured. did. The test temperature is 25 ° C. Here, the charge rate and the discharge rate are expressed by the following formulas (2) and (3).
[Charging rate (CmA)] = [Battery capacity (mAh)] / [Charging time (h)] (2)
[Discharge rate (CmA)] = [Battery capacity (mAh)] / [Discharge time (h)] (3)
The evaluation of the discharge capacity is based on the discharge capacity ratio defined by the following formula (4) using a sample (No. 12 in Table 2) using a metal sheet that has been subjected to rust prevention treatment as a metal sheet. went.
[Discharge capacity ratio] = [Evaluation of Q ₁₀ of the target sample] / [the Q ₁₀ of the standard sample] ... (4)
The results are shown in Table 2. “Metal sheet thickness t” in Table 2 is an average thickness (corresponding to t in FIG. 3) of the metal sheet portion not including the rust preventive agent layer. The example symbol of “manufacturing method” corresponds to the above examples a to g adopted when the copper-coated steel sheet is prepared.

  Comparative Example No. 12 in Table 2 is intended to increase the density of the active material layer by using a material having a rust preventive agent layer formed on the surface of a copper sheet and performing a strong roll press that causes intermediate elongation. Therefore, the density of the active material layer is improved as compared with the conventional negative electrode of a lithium ion secondary battery. In Comparative Example No. 13, the same copper sheet as in No. 12 was used and an even stronger roll press was attempted. However, the copper sheet had a low strength, and thus the material was broken in the roll press process.

The present invention example in Table 2 is a comparative example No. 12 (standard sample) using a material in which a rust preventive layer is formed on the surface of a copper-coated steel sheet having a copper coating layer having a thickness of 0.02 μm or more on the surface. The active material layer is densified by performing a strong roll press equivalent to or better than (1). All of these obtained good shapes as electrodes. Among them, in the case of performing strong roll press that causes breakage in the copper sheet (Nos. 1 to 5, 8 to 11, and 14 to 18), the density of the active material layer is further improved, and the discharge capacity is accordingly increased. Increased significantly.
On the other hand, of the copper-coated steel sheets, Comparative Example No. 6 was inferior in discharge capacity because the thickness t _Cu of the copper coating layer was too small.
In this experiment, the rust preventive layer was not thinned after the rust prevention treatment, but even when thinned, the experiment on the effect of increasing the discharge capacity by using a high strength material for the metal sheet. The evaluation result in the example is reflected.

Example 2
Samples with various thicknesses of the rust preventive agent layer were prepared, and surface resistance and active material layer adhesion were evaluated.
[Production of rust-proof metal sheet]
A plurality of cold-rolled steel strips (annealing materials) having a sheet thickness of 0.3 mm and a sheet width of 200 mm having the following chemical composition were prepared.
Chemical composition:% by mass, C: 0.003%, Al: 0.038%, Si: 0.003%, Mn: 0.12%, P: 0.012%, S: 0.122%, Ni : 0.02%, Cr: 0.02%, Cu: 0.01%, Ti: 0.073%, N: 0.0027%, balance Fe and inevitable impurities

Using this steel strip, a rust-proof metal sheet was prepared in accordance with Step C or D. The degree of thinning of the rust inhibitor layer was changed by adjusting the rolling rate after the rust prevention treatment. The thicknesses of the copper coating layers on both sides and the conditions for forming the rust preventive agent layer were uniform. The rust prevention treatment was performed in the same manner as in Example 1. In step C, copper strike plating was performed prior to electrolytic copper plating. These plating conditions were the same as in Example a above, and the amount of copper plating adhesion was adjusted so that the final copper coating layer thickness t _Cu would be a predetermined value. In Step D, the clad rolling rate was adjusted so that the rolling rate after the rust prevention treatment was 70%. In addition, as a comparative material, a metal sheet made of a copper sheet (copper foil) having a thickness of 10 μm and not subjected to rust prevention treatment, a metal sheet subjected to the rust prevention treatment on the copper sheet surface, and the surface of the cold rolled steel strip Then, after forming a copper coating layer having a thickness of 3 μm per side by an electrolytic copper plating method, a metal sheet subjected to the above rust prevention treatment was prepared. The following investigation was conducted for each metal sheet.

[Analysis of rust prevention material layer]
Using a TOF-SIMS analyzer (manufactured by Physical Electronics; TRIFT II type), the primary ion ⁶⁹ Ga ⁺ (15 kV), the analysis area 10000 μm ² , and the ionic current 0.6 nA are used for the metal sheet subjected to rust prevention treatment. went. Various fragment ions from the rust preventive agent layer are observed. Here, the ionic strength of Cu (I) -BTA derived from the reaction of Cu and BTA under the rust preventive agent layer is used as an evaluation index. The sample (No. 32 in Table 3) with a rust inhibitor layer formed on the copper sheet surface is used as a standard sample, and the ratio of the ionic strength of each sample to the ionic strength of the standard sample (hereinafter referred to as “ionic strength ratio”) Attempts were made to evaluate the film thickness (degree of thinning) of the rust agent layer.

[Changes in surface resistance due to atmospheric exposure]
Measure the surface resistance of each metal sheet immediately after rust prevention treatment (immediately after final rolling for a copper sheet that has not been rust prevention treatment), and samples exposed to air for 2 weeks in an indoor atmosphere, and then expose to the atmosphere. The change of surface resistance due to was investigated. Using an electrical contact simulator (manufactured by Yamazaki Seiki Co., Ltd .; CSR-1), the measurement load is changed from 0 to 100 gf, the measurement current is set to 10 mA, the resistance value is measured with the number of tests n = 5, and the measurement load is 30 gf. The average value of the resistance values was adopted as the surface resistance value (mΩ) of the sample.

[Adhesion of active material layer]
“Example 1” is applied to one side of the sample immediately after the rust prevention treatment of each metal sheet (immediately after the final rolling for the copper sheet not subjected to the rust prevention treatment) and the sample at the time when the air exposure was performed for two weeks in an indoor atmosphere. An active material layer was formed in the same manner as in “Preparation of negative electrode sample”. However, here, the linear pressure of the roll press was 980 kN / m. A test piece having a side of 5 cm is cut out from a metal sheet on which an active material layer is formed, and this is taken out after being immersed in an electrolytic solution (1 mol / L LiPF ₆ in EC / DEC (1 vol%: 1 vol%)) for 1 week. , Dried. The test piece after this immersion test was observed for the presence or absence of swelling and peeling of the active material layer, and the adhesion was evaluated according to the following criteria.
○: The area where the active material layer is swollen or peeled is 50% or less of the test piece area (25 cm ² ) (adhesion; good)
X: Other than the above (adhesion; poor)
The above results are shown in Table 3. “Rolling ratio after rust prevention treatment” in Table 3 is a rolling ratio in rolling accompanied by thinning of the rust inhibitor layer, and this rolling ratio is due to the elongation of the metal sheet portion not including the rust inhibitor layer. is there.

  As can be seen from Table 3, the ionic strength ratio by TOF-SIMS is reduced with an increase in the rolling rate after the rust prevention treatment, so that the rust inhibitor layer is thinned by the rolling after the rust prevention treatment. That is supported.

  In the samples (Nos. 32 and 33) in which the rust preventive agent layer was not thinned, the surface resistance before exposure to the atmosphere was significantly increased as compared to the sample (No. 31) in which the rust preventive treatment was not performed. This is because the electric resistance of the rust preventive agent layer itself is large. However, after the exposure to the atmosphere, the surface resistance of the sample not subjected to the rust prevention treatment was remarkably increased, and the sample without the thinned rust preventive layer exhibited a better surface resistance. Therefore, it can be seen that the effect of suppressing the increase in surface resistance due to the formation of the oxide film is exhibited without thinning the rust preventive agent layer.

  In the sample (Nos. 34 to 37) in which the rust preventive agent layer was thinned, the surface resistance before exposure to the air decreased as the degree of thinning of the rust preventive agent layer increased by rolling after the rust preventive treatment. Even after exposure to the atmosphere, the increase in surface resistance of these samples was remarkably suppressed, and it was confirmed that the surface resistance was lower than that of the sample without thinning the rust preventive layer by the thin rust preventive layer. It was.

  It was also confirmed that the adhesion of the active material layer was improved by forming a rust preventive layer.

DESCRIPTION OF SYMBOLS 1 Foil-like metal sheet 2 Coating film 3 Roll 4 Active material layer 5 Unapplied part 6 Steel sheet 7 Copper coating layer 8 Rust preventive agent layer 10 Copper coating steel sheet 40 Densified negative electrode active material layer

Claims (14)

A rust-proof metal sheet having a copper coating layer on both surfaces of a steel sheet and a rust preventive layer on the surface of the copper coating layer, the total thickness t of the steel sheet and the copper coating layers on both surfaces being 3 Each of the copper coating layers on both sides has a mean film thickness t _Cu of 0.02 to 5.0 μm and a t _Cu / t of not more than 0.3. Rust-proof metal sheet.   2. The lithium ion according to claim 1, wherein the rust preventive layer is a thin layer formed by rolling the rust preventive agent layer attached to the surface of the copper-coated steel sheet as an intermediate product together with the copper-coated steel sheet. A rust-proof metal sheet for supporting a negative electrode active material of a secondary battery.   The rust preventive metal sheet for supporting a negative electrode active material of a lithium ion secondary battery according to claim 1 or 2, wherein the rust preventive agent contains benzotriazole.   The rust-proof metal sheet for carrying | supporting the negative electrode active material of the lithium ion secondary battery in any one of Claims 1-3 in which a copper coating layer is formed through electrolytic copper plating.   The rust-proof metal sheet for supporting a negative electrode active material of a lithium ion secondary battery according to any one of claims 1 to 3, wherein the copper coating layer is formed through clad rolling of a steel sheet and a copper sheet.   The steel sheet is made of a cold-rolled steel sheet (including a steel strip) defined in JIS G3141: 2009, for supporting a negative electrode active material of a lithium ion secondary battery according to any one of claims 1 to 5. Antirust metal sheet.   6. The rust preventive metal for supporting a negative electrode active material of a lithium ion secondary battery according to claim 1, wherein the steel sheet has an austenitic or ferrite chemical composition defined in JIS G4305: 2005. Sheet.   The steel sheet is in mass%, C: 0.001 to 0.15%, Si: 0.001 to 0.1%, Mn: 0.005 to 0.6%, P: 0.001 to 0.05. %, S: 0.001 to 0.5%, Al: 0.001 to 0.5%, Ni: 0.001 to 1.0%, Cr: 0.001 to 1.0%, Cu: 0 to A composition comprising 0.1%, Ti: 0 to 0.5%, Nb: 0 to 0.5%, N: 0 to 0.05%, the balance Fe and unavoidable impurities. 6. A rust-proof metal sheet for supporting a negative electrode active material of a lithium ion secondary battery according to any one of 5 above.   The steel sheet is, in mass%, C: 0.0001 to 0.15%, Si: 0.001 to 4.0%, Mn: 0.001 to 2.5%, P: 0.001 to 0.045. %, S: 0.0005 to 0.03%, Ni: 6.0 to 28.0%, Cr: 15.0 to 26.0%, Mo: 0 to 7.0%, Cu: 0 to 3. 5%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 0.1%, N: 0 to 0.3%, B: 0 to 0.01%, V: 0 to 0.5%, W: 0 to 0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0 to 0.1%, balance Fe and inevitable impurities The rustproof metal sheet for negative electrode active material carrying | support of the lithium ion secondary battery in any one of Claims 1-5.   The steel sheet is in mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.2%, Mn: 0.001 to 1.2%, P: 0.001 to 0.04. %, S: 0.0005 to 0.03%, Ni: 0 to 0.6%, Cr: 11.5 to 32.0%, Mo: 0 to 2.5%, Cu: 0 to 1.0% Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 0.2%, N: 0 to 0.025%, B: 0 to 0.01%, V: 0 0.5%, W: 0 to 0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0 to 0.1%, balance Fe and inevitable impurities Item 6. A rust-proof metal sheet for supporting a negative electrode active material of a lithium ion secondary battery according to any one of Items 1 to 5.   The rustproof metal sheet for supporting a negative electrode active material of a lithium ion secondary battery according to any one of claims 1 to 10, wherein the tensile strength is 450 to 900 MPa.   The negative electrode of the lithium ion secondary battery which formed the active material layer for lithium ion secondary battery negative electrodes on the at least one surface of the antirust metal sheet in any one of Claims 1-11. The process of forming the coating film containing the active material for lithium ion secondary battery negative electrodes on the surface of the at least one copper coating layer of the rust prevention metal sheet in any one of Claims 1-10,
A step of densifying the coating film by reducing the coating thickness by 30 to 70% by a roll press after the coating film is dried;
The manufacturing method of the negative electrode of the lithium ion secondary battery which has NO.   A lithium ion secondary battery using the negative electrode according to claim 12.

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