WO2017078125A1

WO2017078125A1 - Electrolytic copper foil, and lithium ion secondary battery using same

Info

Publication number: WO2017078125A1
Application number: PCT/JP2016/082761
Authority: WO
Inventors: 篠崎　淳; 政登胡木; 季実子藤澤
Original assignee: 古河電気工業株式会社
Priority date: 2015-11-04
Filing date: 2016-11-04
Publication date: 2017-05-11
Also published as: CN107075707B; KR101851515B1; JP6067910B1; TWI648415B; CN107075707A; TW201728764A; JP2017088997A

Abstract

The purpose of the present invention is to provide an electrolytic copper foil suitable as a copper foil for lithium ion secondary batteries, which has high durability against stress associated with charging and discharging as in the case with lithium ion secondary batteries and which is required to have good battery characteristics. The electrolytic copper foil is characterized by having a tensile strength (Ts(50)) of 450 MPa or more at a rate of elongation of 50 mm/min under normal conditions, and a tensile strength (Ts(0.1)) of 400 MPa or more at a rate of elongation of 0.1 mm/min under normal conditions, and in that the ratio of the two tensile strengths, Ts(0.1)/Ts(50), is 0.70 or more.

Description

Electrolytic copper foil and lithium ion secondary battery using the electrolytic copper foil

The present invention relates to an electrolytic copper foil for a negative electrode current collector of a lithium (Li) ion secondary battery and a lithium ion secondary battery using the electrolytic copper foil, and more particularly, at the time of manufacturing a lithium ion secondary battery. The present invention relates to an electrolytic copper foil and a lithium ion secondary battery suitable for a lithium ion secondary battery having improved handling properties and durability against expansion and contraction stress during battery charging / discharging.

A lithium ion secondary battery is composed of, for example, a positive electrode, a negative electrode having a negative electrode active material layer formed on the surface of a negative electrode current collector, and a non-aqueous electrolyte. In recent years, it has been increasingly used for automobile applications.
A copper foil is generally used for the negative electrode current collector of a lithium ion secondary battery. In many cases, an electrolytic copper foil is used as the copper foil. This is because the electrolytic copper foil has advantages such as the ability to reduce the thickness of the copper foil at a low cost and the easy compatibility between the electrical conductivity and the strength.
An electrode is manufactured by applying carbon particles or the like as a negative electrode active material layer on the surface of the copper foil, drying, and further pressing. At this time, depending on the application conditions of the active material layer and the pressing conditions, the copper foil may be broken, such as wrinkles and cracks, and the cycle characteristics of the battery may deteriorate. In addition, the copper foil may be broken, which may cause a problem in battery manufacture. Thus, it has been reported that the tensile strength of the copper foil is set to a predetermined value or higher, or the physical properties are improved by setting the elongation of the copper foil to a predetermined value or higher.

In addition, when the lithium ion secondary battery is charged and discharged, the active material layer expands and contracts, and the stress is applied to a member typified by copper foil. As a result, peeling of the active material layer from the copper foil, destruction of the copper foil such as wrinkles and breakage, many problems such as short circuit and ignition due to deterioration of the cycle characteristics of the battery and destruction of other members such as the separator May cause. Against this, it has been reported that the physical properties are improved by setting the tensile strength of the copper foil to a predetermined value or more, or by setting the elongation of the copper foil to a predetermined value or more (see, for example, Patent Documents 1 to 5).

JP 2005-135856 A Japanese Patent No. 5588607 Japanese Patent No. 5074611 Japanese Patent No. 4465084 Japanese Patent Laid-Open No. 04-088185

However, the stress applied to the copper foil during battery manufacture is rapidly applied at a high speed, while the stress applied to the copper foil during battery charge / discharge is gradually applied at a very low speed. The correlation between copper foil characteristics such as general tensile strength and elongation, battery manufacturability and battery cycle characteristics alone is insufficient for evaluation, and even if these are controlled, battery manufacturability and In some cases, the cycle characteristics of the battery could not be improved.
In addition, with the increase in capacity and weight of batteries in recent years, the stress applied to the copper foil during battery manufacturing and battery charging / discharging becomes larger, while the copper foil has a thinner foil thickness and mechanical characteristics. It is required to satisfy.
The present invention has been made in view of the above circumstances, and is durable against both a rapidly applied stress such as during battery manufacture and a gradually applied stress such as when charging and discharging a battery. The object is to provide an excellent electrolytic copper foil.

In order to solve the above problems, the present inventor made an electrolytic copper foil by setting the current density distribution during the production of the electrolytic copper foil to an appropriate range, and compared with the conventional copper foil, the tensile rate of the tensile test. A foil having a high strength when the tension was high and a relatively high strength even when the tensile speed was slow was obtained. It has been found that by using a copper foil exhibiting such characteristics, battery manufacturability and cycle characteristics are improved.

The present invention obtained on the basis of this finding has a normal tensile strength (Ts (50)) of 450 MPa or more under the condition of a tensile speed of 50 mm / min and a normal condition under the condition of a tensile speed of 0.1 mm / min. The electrolytic copper foil is characterized in that the tensile strength (Ts (0.1)) is 400 MPa or more and the ratio Ts (0.1) / Ts (50) of both is 0.70 or more.
The normal state in this specification means a state in which the copper foil is placed at room temperature (= about 25 ° C.) without receiving a thermal history such as heat treatment.

In one embodiment, the electrolytic copper foil according to the present invention has a tensile strength (Ts_HT (0.1)) of 350 MPa or more at a tensile rate of 0.1 mm / min measured at room temperature after heating at 180 ° C. for 1 hour. It is characterized by being.

In still another embodiment, the electrolytic copper foil according to the present invention is characterized in that a normal elongation (El (0.1)) under a condition of a tensile speed of 0.1 mm / min is 4.0% or more. To do.

In still another embodiment, the electrolytic copper foil according to the present invention is characterized in that the thickness is 4 to 12 μm.

In one aspect, the electrolytic copper foil according to the present invention is a copper foil for a lithium ion secondary battery negative electrode current collector.

In one aspect, the present invention is a lithium ion secondary battery using the electrolytic copper foil according to the present invention.

According to the present invention, it is possible to provide an electrolytic copper foil that is excellent in durability against both a stress that is rapidly applied during battery manufacture and a stress that is gradually applied during charge and discharge of the battery. Moreover, the negative electrode for lithium ion secondary batteries excellent in charging / discharging cycling characteristics can be provided by using this electrolytic copper foil. Furthermore, by using this negative electrode, a lithium ion secondary battery excellent in charge / discharge cycle characteristics can be provided.

FIG. 1 is a graph plotting Ts (0.1) / Ts (50) of the electrolytic copper foil according to the example and Ts (0.1) / Ts (50) of the electrolytic copper foil according to the comparative example. FIG. 2 shows a schematic diagram of an apparatus for producing the electrolytic copper foil according to the present embodiment. FIG. 3 shows the current density distribution of a conventional electrolytic copper foil. FIG. 4 is a graph showing two types of current density distributions (A and B). FIG. 5 is a diagram showing stress-strain curves of Example 1-1 and Comparative Example 1-1 when the tensile speed is 50 mm / min or 0.1 mm / min.

Hereinafter, embodiments of the present invention will be described in detail.

The copper foil of this embodiment is greatly characterized in the values of tensile strength and the ratio thereof at two different tensile speeds.
In general, when performing a tensile test on a copper foil, the tensile speed is about 10 to 50 mm / min. Also in IPC-TM-650, the tensile test speed of the copper foil at room temperature is defined as 2 inches / min (50.8 mm / min). Stress is applied at such a relatively fast rate and the strength is measured. Since the stress applied at the time of manufacturing the battery is applied at a high speed, it was found that the durability at the time of manufacturing the battery can be evaluated by conducting a tensile test at a high tensile speed of 50 mm / min. When Ts (50) is lower than 450 MPa, the stress during battery production cannot be withstood, and wrinkles and fractures occur.
On the other hand, considering that stress accompanying charging / discharging in the lithium ion secondary battery is loaded on the copper foil, the speed is relatively slow. At this time, it discovered that the durability at the time of charging / discharging of a battery could be evaluated by performing a tensile test with a slow tensile speed of 0.1 mm / min. When Ts (0.1) is lower than 400 MPa, the wrinkles of the copper foil and the peeling of the active material layer occur due to the stress accompanying the charging / discharging of the battery.
Furthermore, it has been found that the ratio of the tensile strength at two different tensile speeds expressed by Ts (0.1) / Ts (50) correlates well with the cycle characteristics of the battery. That is, a copper foil having a small Ts (0.1) / Ts (50) is high in strength when stress is applied at a high speed but is low in strength when stress is applied at a low speed. There are wrinkles and breakage of the copper foil due to the stress associated with charging / discharging of the battery, and the cycle characteristics deteriorate.

Generally, pure copper and copper alloys have Ts (0.1) / Ts (50) smaller than 1. However, among the conventional electrolytic copper foils, this Ts (0.1) / Ts (50) is a relatively small value. Specifically, as shown in the comparative example of FIG. 1, specifically, about 0.50 to 0.67. Indicates. Therefore, when stress is applied at a slow speed, it breaks at a very low strength.

On the other hand, according to the electrolytic copper foil according to the embodiment of the present invention, unlike the conventional electrolytic copper foil, Ts (0.1) / Ts (50) is 0 as shown in the example of FIG. .70 or more. This demonstrates high durability against stress during charging / discharging of the battery, and Ts (50) is 450 MPa or more and Ts (0.1) is 400 MPa or more. It shows high durability in both stress and stress during charging / discharging of the battery, and greatly improves battery manufacturability and cycle characteristics.
In addition, since the distance between chuck | zippers of the tension test in the Example of this specification is 70 mm, if 50 mm / min of tensile speeds are recalculated to a strain speed, it will be 71.43% / min. Similarly, a tensile speed of 0.1 mm / min becomes a strain speed of 0.1429% / min. Accordingly, the tensile speed of 50 mm / min can be replaced with a strain speed of 71.43% / min (distance between chucks: 70 mm), and the tensile speed of 0.1 mm / min can be replaced with a strain speed of 0.1429% / min (between chucks). (Distance: 70 mm).

The electrolytic copper foil according to the embodiment of the present invention has a tensile strength (Ts_HT (0.1)) of 350 MPa or more at a tensile rate of 0.1 mm / min measured at room temperature after heating at 180 ° C. for 1 hour. It is. 180 ° C. and 1 hour are typical heating conditions for battery mounting. Thereby, the strength fall of the copper foil in the heating at the time of battery manufacture reduces, and a battery characteristic improves.

The electrolytic copper foil according to the embodiment of the present invention preferably has a normal elongation (El (0.1)) of 4.0% or more under the condition of a tensile speed of 0.1 mm / min. Thereby, durability with respect to the stress at the time of charging / discharging of a battery further improves.

The electrolytic copper foil according to the embodiment of the present invention preferably has a foil thickness of 4 to 12 μm. Thereby, the occurrence of pinholes in the copper foil is prevented, and the battery surface characteristics are improved because the surface area per weight of the copper foil is large.

When the electrolytic copper foil according to the embodiment of the present invention is manufactured, for example, as shown in FIG. 2, a sulfuric acid-copper sulfate aqueous solution is used as an electrolytic solution, and it is made of titanium coated with a platinum group element or its oxide element. The electrolyte drum is supplied between an insoluble anode and a titanium cathode drum provided opposite to the anode, and a DC current is passed between both electrodes while rotating the cathode drum at a constant speed, thereby providing a cathode drum surface. Copper is deposited on the surface, and the deposited copper is peeled off from the surface of the cathode drum and continuously wound. In addition, the example of this apparatus is an example.

At this time, in the conventional electrolytic copper foil, the current density distribution is caused by various factors in the rotating direction of the cathode drum. For example, in the electrolyte supply section, since there is a gap in the anode facing the cathode drum, the current density is reduced in the vicinity thereof. The closer to the surface of the electrolytic solution, the more bubbles are generated by electrolysis, so that the liquid resistance increases, and the current density changes because the state of stirring of the liquid is also different. Particularly in the vicinity of the liquid level, there is no anode facing the cathode drum because there is an outlet for the electrolyte, and the current density is reduced. In order to adjust the foil thickness in the width direction, an insulating plate is inserted between the electrodes from the liquid surface side to shield the current, thereby intentionally creating a low current density portion. Unevenness in the state and thickness of the oxide covering the anode surface results in a different local surface resistance, which may change the current density. Unless otherwise specified, in this specification, the expression of the current density distribution refers to the current density distribution in the rotating direction, not the width direction of the cathode drum.

The current density distribution thus generated can be investigated as follows, for example. First, an electrolytic solution is supplied between the anode and the cathode drum without energization. Next, energization is performed for a certain period of time in a stationary state without rotating the cathode drum. By doing so, copper is electrodeposited at a position indicated by a dotted line between AB in FIG. 2 with a thickness corresponding to the current density at that position. Thereafter, the energization is stopped, the cathode drum is quickly rotated, and the electrodeposited static electrodeposited copper foil is peeled off. By measuring the foil thickness distribution in the drum rotation direction of the static electrodeposited copper foil thus obtained, the current density distribution can be indirectly investigated. The foil thickness is measured with a soft X-ray thickness gauge at a pitch of 1 mm. When calculating the current density from the foil thickness, the current efficiency is assumed to be 100%, and only the reaction of Cu2 + + 2e- → Cu is considered. Specifically, the following formula (1) is used. (Hereinafter, this method is referred to as a static electrodeposition method).

i: current density (A / dm2), d: foil thickness (dm), ρ: copper density (g / dm3), F: Faraday constant (C / mol), m: atomic weight of copper (g / mol), t: Static electrodeposition time (s)
FIG. 3 shows the current density distribution of a conventional electrolytic copper foil. Focusing on the maximum current density (imax) and the minimum current density (imin) with respect to the average current density (iave), imax / iave is 1.10 and imin / iave is 0.80. It can be seen that the current density is distributed in the range of 0.80 to 1.10 times. It can also be seen that the relative standard deviation of the current density is distributed at about 3.84%.
Since the electrolytic copper foil manufactured by the equipment of this example is manufactured by increasing the thickness with the rotation of the drum, the current density distribution in the rotation direction of the cathode drum is that of the thickness direction of the copper foil. Current density distribution. That is, the electrolysis was performed under conditions where the current density was different in the thickness direction of the copper foil. It is known that current density has a great influence on the deposition behavior of copper foil. Specifically, it has a great influence on the copper foil characteristics such as tensile strength and elongation, crystal orientation, and residual stress. Therefore, in a strict sense, it can be said that there are layers having different characteristics such as tensile strength and elongation and crystal structure in the thickness direction of the copper foil. However, measuring the properties of only layers with different tensile strength, elongation, and crystal structure is often due to the fact that the layers are very thin and are changing continuously rather than stepwise. It is very difficult.

When such a conventional electrolytic copper foil tensile test is performed, an average mechanical property is measured in the thickness direction of the copper foil at a normal tensile test speed of 50 mm / min. In other words, it is not easily affected by differences in characteristics and structure in the thickness direction of the copper foil due to local current density.
On the other hand, at a slow tensile test speed of 0.1 mm / min, minute cracks enter from a layer having low strength or elongation in the thickness direction of the copper foil. Moreover, since crystal orientation and residual stress differ with respect to the thickness direction of copper foil, it becomes easy to enter the micro crack at the time of a tensile test by the difference.
And once a crack enters, since the crack progresses and it leads to the fracture | rupture of copper foil, the intensity | strength and elongation to be measured fall significantly. That is, the value of Ts (0.1) / Ts (50) is small and is about 0.50 to 0.67.
The electrolytic copper foils described in

Patent Documents

2 and 3 and the electrolytic copper foils produced by the methods described in

Patent Documents

4 and 5 exhibit such characteristics and have low durability against stress associated with battery charging / discharging. It can be said that this is a copper foil corresponding to Comparative Example 1-1.

On the other hand, when the copper foil of the present invention is subjected to a tensile test at a slow tensile test speed of 0.1 mm / min, the measured strength is unlikely to decrease. That is, the value of Ts (0.1) / Ts (50) is 0.70 or more, which is larger than that of the conventional electrolytic copper foil. This feature is attributed to exhibiting uniform characteristics in the thickness direction of the copper foil.

The present invention is greatly characterized in that foil production is performed with a uniform current density distribution in the thickness direction of the copper foil. For example, the following can be considered as a specific method. In order to prevent a decrease in current density in the vicinity of the liquid supply unit, the gap between the anodes is reduced. Further, a mesh-like anode is disposed in the liquid supply unit. In order to prevent the influence of bubbles, the distance between the electrodes is narrowed as the liquid level is closer. In order to adjust the foil thickness in the width direction, the insulating plate inserted between the electrodes is made into a mesh structure to prevent a sudden drop in current density. The oxide state and thickness on the anode surface are kept uniform.
Further, in the equipment other than this example, unlike the equipment of FIG. 2, for example, since one continuous anode is used, there is no liquid supply portion directly under the cathode drum, and electrolysis is performed from one liquid surface. It is conceivable to have a structure in which the liquid is supplied by a pump and the liquid flows out from the other liquid surface. However, even in such equipment, the current density distribution occurs due to the influence of air bubbles and the state of the anode surface. In this case as well, the distance between the electrodes is changed, the anode surface state is managed, and the structure of the liquid supply section and liquid outlet Make the current density distribution uniform by changing it.
In the present invention, any method may be used, but the current density distribution in the thickness direction of the copper foil is made uniform. Specifically, in the current density distribution measured at a 1 mm pitch in the drum rotation direction, imax / iave is less than 1.05 and imin / iave is greater than 0.90. In addition, the relative standard deviation of the current density should be less than 2.0%. Manufacturing with such a uniform current density distribution is suitable for obtaining an electrolytic copper foil having the characteristics of the present invention.

In the electrolyte, 1 to 30 mg / L of chloride ions is added using a sulfuric acid-copper sulfate aqueous solution having a copper concentration of 50 to 100 g / L and a sulfuric acid concentration of 40 to 120 g / L as an electrolytic solution.

In order to increase the strength of the copper foil, at least one organic or inorganic additive is added to the electrolytic solution. Examples of organic additives include thiourea (CH ₄ N ₂ S) or water-soluble thiourea derivatives, polymers such as glue, gelatin, polyethylene glycol, starch, and cellulose-based water-soluble polymers (such as carboxyl methyl cellulose and hydroxyethyl cellulose). Water-soluble polymer compounds such as polysaccharides, polyethyleneimine, and polyacrylamide are used. As an inorganic additive, NaCl, HCl, and a very small amount of metal elements are used as a source of chloride ions.

Electrolyte temperature is adjusted to 40-60 ° C and the average current density on the cathode electrode surface is adjusted to 45-60A / dm2 to produce copper foil.

It is desirable to perform surface treatment on at least one surface of the electrolytic copper foil of the present embodiment.
As a copper foil surface treatment, for example, chromate treatment, or Ni or Ni alloy plating, Co or Co alloy plating, Zn or Zn alloy plating, Sn or Sn alloy plating, or a further chromate treatment on the above various plating layers Inorganic rust prevention treatment such as benzotriazole or organic rust prevention treatment such as benzotriazole may be applied. Furthermore, a silane coupling agent treatment or the like may be performed. In addition to rust prevention, these surface treatments increase the adhesion strength with the active material, and play a role of preventing a decrease in charge / discharge cycle efficiency of the battery.

It is also possible to perform a roughening treatment on the surface of the copper foil as necessary before the surface treatment is applied to the copper foil. As the roughening treatment, for example, a plating method or an etching method can be suitably employed.

As the roughening by the plating method, an electrolytic plating method and an electroless plating method can be employed. Roughened particles are formed by one type of plating or two or more types of alloy plating among Cu, Co and Ni.

As the roughening by the etching method, for example, a method by physical etching or chemical etching is preferable. For example, physical etching includes a method of etching by sandblasting or the like. For example, chemical etching includes a method of etching with a processing solution or the like. Many treatment liquids containing inorganic or organic acids, oxidizing agents, and additives have been proposed.

Examples of the present invention will be shown below, but the present invention is not intended to be limited to the following examples, and various forms can be taken without departing from the spirit of the present invention.

As shown in FIG. 2, an electrolyte is supplied between an insoluble anode made of titanium coated with a platinum group element or an oxide element thereof and a titanium cathode drum provided to face the anode, While rotating at a constant speed, a direct current was passed between both electrodes to deposit copper on the surface of the cathode drum, whereby the copper foils of each Example and each Comparative Example were produced with a thickness of 10 μm.
About the Example, it adjusted so that electric current density distribution might become uniform before manufacturing copper foil. In order to prevent a decrease in current density in the vicinity of the liquid supply unit, the gap between the anodes was made as small as possible without impeding liquid supply. In order to prevent the influence of bubbles, the distance between the anode and the cathode was continuously changed to be 13 mm near the liquid supply part and 10 mm near the liquid surface part. In order to prevent a decrease in current density in the vicinity of the liquid surface, a mesh anode was disposed at the outlet of the electrolyte. The result of investigating the current density distribution at this time by the static electrodeposition method is the current density distribution A in FIG. imax / iave was 1.04, imin / iave was 0.90, and it was confirmed that the current standard had a uniform current density distribution with a relative standard deviation of 1.97%. The copper foil was manufactured in this current density distribution.
Comparative Examples 1-1 and 1-2 were manufactured in the conventional equipment state without paying particular attention to the current density distribution. The result of investigating the current density distribution at this time by the static electrodeposition method is the current density distribution B in FIG. Copper foil was manufactured in a current density distribution in which imax / iave was 1.10, imin / iave was 0.80, and the relative standard deviation of current density was 3.84%.
For Comparative Example 2-1, copper foil was produced in the current density distribution A.

For the examples and comparative examples, a sulfuric acid-copper sulfate electrolytic solution prepared with a copper concentration of 80 g / L, a sulfuric acid concentration of 80 g / L, and a chloride ion concentration of 10 mg / L was used. The electrolyte was manufactured under the conditions of a temperature of 50 ° C., an average current density of 40 A / dm 2, and a liquid flow rate of 1.0 m / s. Tables 1 and 2 show the types and concentrations of additives added to the electrolyte.

The copper foils of the examples and comparative examples were all subjected to chromate treatment immediately after the foil production. The copper foil was dipped in a 7 g / L chromic anhydride aqueous solution at 45 ° C. for 5 seconds, then drained and air-dried.

The copper foil of each example and each comparative example was evaluated in the following items.

(1) Tensile test A tensile tester type 1122 manufactured by Instron was used. The sample was cut into a size of 0.5 inch × 6 inch, and the distance between chucks was 70 mm. The tensile strength and elongation in the normal state were measured under two conditions of a tensile speed of 50 mm / min or 0.1 mm / min, respectively. For other conditions, measurements were made based on IPC-TM-650. In addition, elongation rate shows the elongation rate when a test piece fractures in a tensile test. In addition, under the condition of 0.1 mm / min, the tensile strength and elongation after the copper foil was heat-treated at 180 ° C. for 1 hour were measured. All measurements were performed at room temperature. The results are shown in Tables 1 and 2.

(4) Evaluation of battery characteristics (4-1) Production of positive electrode 90% by mass of LiCoO ₂ powder, 7% by mass of graphite powder, 3% by mass of polyvinylidene fluoride powder, N-methylpyrrolidone and ethanol are added as solvents and kneaded. The positive electrode paste was prepared. This positive electrode agent paste was uniformly applied to an aluminum foil having a thickness of 15 μm. The coated aluminum foil was dried in a nitrogen atmosphere to volatilize the solvent, and then roll-rolled to produce a sheet having a total thickness of 150 μm. The sheet was cut into a width of 43 mm and a length of 290 mm, and then an aluminum foil lead terminal was attached to one end thereof by ultrasonic welding to form a positive electrode.

(4-2) Manufacture of negative electrode and evaluation of manufacturability The copper foil used in the negative electrode was the copper foil of Examples and Comparative Examples in a normal state.
A natural graphite powder (average particle size 10 μm) 90 mass% and a polyvinylidene fluoride powder 10 mass% were mixed, N-methylpyrrolidone and ethanol were added as solvents and kneaded to prepare a negative electrode agent paste. Subsequently, this negative electrode agent paste was applied to both sides of the copper foil. The coated copper foil was dried in a nitrogen atmosphere to volatilize the solvent, and then roll-rolled to form a sheet having an overall thickness of 150 μm. The sheet was cut to a width of 43 mm and a length of 285 mm, and then a nickel foil lead terminal was attached to one end thereof by ultrasonic welding to form a negative electrode.
At this point, whether or not abnormalities such as wrinkles and fractures were observed in the copper foil or the active material layer was visually confirmed and evaluated as battery manufacturability. The case where no wrinkles or breakage occurred was evaluated as “good”, and the case where wrinkles or breakage occurred was evaluated as “impossible”. A copper foil with an evaluation of “good” indicates that the copper foil is suitable for this application, and a copper foil with an evaluation of “impossible” indicates that the copper foil is not suitable.

(4-3) Production of Battery (4-1) Production of Positive Electrode and (4-2) Production of Negative Electrode A polypropylene separator having a thickness of 25 μm is sandwiched between the positive electrode and the negative electrode produced as a whole. This was wound, accommodated in a nickel-plated battery can, and the negative lead terminal was spot welded to the bottom of the can. Next, place the top cover of the insulating material, insert the gasket, and connect the lead terminal of the positive electrode and the aluminum safety valve by ultrasonic welding to connect the non-aqueous electrolyte consisting of propylene carbonate, diethyl carbonate and ethylene carbonate in the battery can. Then, a lid was attached to the safety valve, and a sealed structure type lithium ion secondary battery having an outer shape of 14 mm and a height of 50 mm was assembled.

(4-4) Measurement of battery characteristics The battery of (4-3) is charged to 4.3 V at a charging current of 100 mA and discharged to 2.5 V at a charging current of 100 mA. A discharge cycle test was conducted. The number of cycles when the discharge capacity of the battery at that time fell below 800 mAh was defined as an item for evaluating the superiority or inferiority of the battery characteristics. The results are shown in Tables 1 and 2.
The cycle life was evaluated as “excellent” for 500 times or more, “good” for 400 times or more and less than 500 times, and “impossible” for less than 400 times. A copper foil whose evaluation is “impossible” indicates that the copper foil is not suitable for this application. “Good” indicates that the copper foil is suitable, and among them, “Excellent” indicates that the copper foil has better battery characteristics.

Example 1-1 and Comparative Example 1-1 produced copper foils under the same electrolytic solution composition but different current density distribution. As the additive, Nika, a polymer compound, was used. Ts (50) is similar in both cases, but Ts (0.1) / Ts (50) is as large as 0.82 in Example 1-1 and as small as 0.62 in Comparative Example 1-1. This is because the current density distribution A of Example 1-1 is relatively uniform and shows uniform characteristics in the thickness direction, while the current density distribution B of Comparative Example 1-1 is relatively non-uniform. For this reason, there are layers having different tensile properties in the thickness direction of the copper foil. Also, as a result, the battery using the copper foil of Example 1-1 has good cycle characteristics, and the battery using the copper foil of Comparative Example 1-1 has poor cycle characteristics. FIG. 5 shows the stress-strain curves of Example 1-1 and Comparative Example 1-1 when the tensile speed is 50 mm / min or 0.1 mm / min. It can be seen that Example 1-1 has a small decrease in strength when the tensile speed is slow, but Comparative Example 1-1 has a large decrease in strength.

In Example 1-2 and Comparative Example 1-2, the same electrolytic solution composition was used, and copper foils were produced under different conditions only in the current density distribution. As the additive, thiourea, which is a monomolecular compound, was used. Here, Ts (50) is similar in both cases, but Ts (0.1) / Ts (50) is as large as 0.87 in Example 1-2 and as small as 0.63 in Comparative Example 1-2. . Also, as a result, the battery using the copper foil of Example 1-2 has good cycle characteristics, and the battery using the copper foil of Comparative Example 1-2 has poor cycle characteristics. In this way, even if the type of additive is greatly changed, if the current density distribution is not uniform, Ts (0.1) / Ts (50) becomes small, and the battery using the copper foil has poor cycle characteristics. The trend is unchanged.

In Examples 2-1 to 2-4, copper foils were produced by changing the additive concentration in order to change Ts (50). In any case, since the current density distribution is uniform at A, Ts (0.1) / Ts (50) is relatively large and the cycle characteristics are good.
In Comparative Example 2-1, the current density distribution is uniform with A and Ts (0.1) / Ts (50) is relatively large, but the increase in strength is insufficient and Ts (50) is 380 MPa and Ts (0.1) is as small as 311 MPa. For this reason, some wrinkles are generated during the manufacture of the battery, and the cycle characteristics are poor.

Claims

The tensile strength (Ts (50)) in the normal state under the condition of the tensile speed of 50 mm / min is 450 MPa or more, and the tensile strength (Ts (0.1)) in the normal state under the condition of the tensile speed of 0.1 mm / min is 400 MPa. An electrolytic copper foil characterized in that the ratio Ts (0.1) / Ts (50) is 0.70 or more.
The tensile strength (Ts_HT (0.1)) at a tensile rate of 0.1 mm / min measured at room temperature after heating at 180 ° C. for 1 hour is 350 MPa or more. The electrolytic copper foil of description.
The electrolytic copper foil according to claim 1 or 2, wherein an elongation (El (0.1)) in a normal state under a condition of a tensile speed of 0.1 mm / min is 4.0% or more.
4. The electrolytic copper foil according to claim 1, wherein the thickness of the electrolytic copper foil is 4 to 12 μm.
The electrolytic copper foil according to any one of claims 1 to 4, wherein the copper foil is a copper foil for a negative electrode current collector of a lithium ion secondary battery.
A negative electrode for a lithium ion secondary battery using the electrolytic copper foil according to claim 5.
A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to claim 6.