JPH11273683A - Copper foil for negative electrode current collector of nonaqueous solvent secondary battery and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide copper foil for the negative electrode current collector of a nonaqueous solvent secondary battery having a large battery capacity when discharged for the first time and being excellent in charge/discharge cycle life, particularly an Li ion secondary battery, and a method for manufacturing the same. SOLUTION: This copper foil has the inverse number (1/C) of an electric double layer capacity on at least one side of the copper foil being 0.1 to 0.3 cm<2> /μF, and is manufactured by dipping either degreased rolled copper coil or electrolytic copper foil, which is water washed and dried after electrolytic foiling, either in a solution obtained by dissolving at least triazoles in a solvent or in an aqueous solution obtained by dissolving in water at least one kind selected from the group consisting of chromium trioxide, chromates, and bichromates.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copper foil for a negative electrode current collector of a non-aqueous solvent secondary battery and a method for producing the same. And a copper foil capable of improving the charge / discharge characteristics of the battery and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, secondary batteries using an aqueous electrolyte typified by nickel-cadmium secondary batteries have been widely used as driving power sources for various electric and electronic devices. However, in a secondary battery using an aqueous electrolyte, when the operating voltage exceeds a certain value, the electrolyte is decomposed, so that the electromotive force is generally low, and the energy density is also low. There is a problem as a power source in a field that maintains a high output for a long time, such as a power source for an expected electric vehicle. Furthermore, in the case of a nickel-cadmium secondary battery, for example, there is a concern about environmental pollution due to cadmium.

[0003] Under such circumstances, research and development of a secondary battery using a non-aqueous electrolyte has recently been promoted. A secondary battery using this non-aqueous electrolyte (hereinafter referred to as a non-aqueous solvent secondary battery) is generally characterized by a high electromotive force and a high energy density.
i-secondary batteries have been developed and are being put to practical use. Among these Li secondary batteries, those using Li foil for the negative electrode have a short charge-discharge cycle life, decrease the battery capacity in a short time, and remain uneasy in terms of safety. However, a Li-ion secondary battery has been put to practical use.

In general, this Li-ion secondary battery has a battery can which also serves as a negative electrode terminal and an electrode group formed by interposing a separator having electrical insulation and liquid retaining properties between a positive electrode described later and a negative electrode described later. And a predetermined non-aqueous electrolyte solution, and the opening of the battery can is sealed with a sealing plate provided with a positive electrode terminal through an insulating gasket. Also called type structure.

Here, the positive electrode is manufactured as follows. First, for example, LiC that functions as a positive electrode active material
A positive electrode mixture paste having a predetermined composition obtained by kneading oO ₂ powder, a conductive material such as carbon black or graphite powder, and a binder such as PVDF with a non-aqueous solvent such as N-methylpyrrolidone Is prepared. Next, a predetermined amount of the paste is applied to both surfaces of, for example, an aluminum foil functioning as a positive electrode current collector, dried, and then press-formed to produce a positive electrode sheet having a predetermined thickness. Then, a lead is attached to a predetermined portion of the positive electrode sheet to form a positive electrode.

On the other hand, the negative electrode is manufactured as follows. First, for example, graphite powder or amorphous carbon powder that functions as a field for insertion (during discharge) and desorption (during charging) of Li ions and a binder such as PVDF are mixed with N-methylpyrrolidone or the like. A negative electrode mixture paste is prepared by kneading with a non-aqueous solvent.

Next, a predetermined amount of the paste is applied to both surfaces of, for example, a copper foil functioning as a negative electrode current collector, dried, and then pressed to form a negative electrode sheet having a predetermined thickness. Then, a lead is attached to the predetermined location to form a negative electrode. Here, copper foil is generally adopted as the current collector of the negative electrode for the following reason. In other words, the copper foil has a high conductivity because of its excellent electrical conductivity, and its mechanical strength is also high, so that it is easy to handle at the time of manufacturing an electrode group, and an alloy is formed between Li ions coming and going in the process of charging and discharging. It is not generated. Furthermore, copper foil can be made into a thin film at low cost, and even if it is a wide current collector required for the production of a large capacity battery, it can be easily produced as an electrolytic copper foil. This is a major reason copper foil is used.

[0008]

By the way, a non-aqueous solvent secondary battery represented by a Li-ion secondary battery is also considered.
Recently, there has been an increasing demand for higher energy density and a longer charge / discharge cycle life, and research is being conducted to meet the demand. Under such a trend, the following has recently become a problem regarding battery characteristics when a copper foil is used as a negative electrode current collector.

The first problem is that if the composition of the above-mentioned negative electrode mixture paste is different, the characteristics of the assembled battery will be different even though copper foil of the same specification is used as the current collector. That is the fact. The second problem is the fact that when the composition of the negative electrode mixture paste is the same, the use of copper foils having different specifications other than the thickness results in different battery characteristics.

In particular, the charge-discharge cycle life characteristics and the battery capacity at the initial stage of charging, which are the most important characteristics for a battery, may be reduced depending on the specifications of the copper foil as the negative electrode current collector, and such a problem is caused. The provision of no copper foil is required. The present invention solves the above-described problems of the copper foil when the copper foil is used as the negative electrode current collector of the non-aqueous solvent secondary battery, even when the composition of the negative electrode mixture paste changes, excellent. It is an object of the present invention to provide an electrode for a negative electrode current collector of a non-aqueous solvent secondary battery capable of maintaining charge / discharge cycle life characteristics and high battery capacity, and a method of manufacturing the same.

[0011]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in order to achieve the above-mentioned object.
A study on the copper foil making process was added. First, the copper foil, the raw material melting-casting of the ingot-rolling of the ingot-rolled copper foil obtained through the process of annealing and homogenization of the obtained rolled foil, and wet electrolysis in an aqueous solution containing copper ions. There are two types of electrolytic copper foil to make foil. Then, the copper foil after the foil making is stored after being subjected to a rust-preventive treatment for forming a rust-preventive film by thinly applying a rust-preventive oil, for example, to the surface thereof, and placed in a standby state for shipment.

Therefore, when a negative electrode of a battery is manufactured, the copper foil in the above-described state is supplied to a manufacturing line as a negative electrode current collector, and the negative electrode mixture paste is applied to the surface thereof. By the way, the above-mentioned rust prevention film is usually electrically insulating. And, in the case of the manufactured negative electrode, the current obtained by the battery reaction through the conductive material in the negative electrode mixture supported on the surface of the copper foil is collected from the copper foil surface, If the problem of rust prevention properties is neglected, the presence of this rust prevention film becomes a factor inhibiting battery characteristics and should be removed.

However, even if this rust-preventive film is removed, there are the following problems. That is, the surface of the produced copper foil is active at all times, and the surface thereof is easily formed with an electrically insulating oxide film having a thickness of about 10 to 50 ° in the air or water. is there. Therefore, when a copper foil is used as a negative electrode current collector, a dielectric layer having a certain thickness is inevitably formed on the surface of the copper foil. It is considered that the current collecting ability of the copper foil, and eventually the battery characteristics are affected by the thickness of the dielectric layer.

Therefore, the present inventors have searched for means for confirming the thickness of the dielectric layer on the surface of the copper foil, and as a result,
The electric double layer capacity (C: μF) is measured, and the following formula is generally known: A / C = A · d + B (1) (d is the thickness of the electric double layer formed on the copper foil surface,
(A and B are constants) based on the idea that the thickness of the dielectric layer can be grasped.

The present inventors have examined the relationship between the thickness (d) of the dielectric layer and the battery characteristics, and have found the thickness (d) suitable for improving the initial charge capacity and the charge / discharge cycle life. Further, the present inventors have developed the present invention by grasping that the surface roughness of the copper foil is also an important factor for improving the battery characteristics. That is, the reciprocal (1 / C) of the electric double layer capacity on at least one side of the copper foil for a negative electrode current collector of the nonaqueous solvent secondary battery of the present invention has a value of 0.1 to 0.3 cm ² / μF.
Characterized in that the surface roughness of both surfaces is JIS B0
2.5 μm in 10-point average roughness (Rz) specified in 601
It is preferable that the following is true.

In the present invention, the degreased rolled copper foil or the electrolytic copper foil obtained by subjecting the electrolytic foil to rinsing and drying treatment is then used to form a solution having a pH of 5 to 8.5 obtained by dissolving at least a triazole in a solvent. A method for producing a copper foil for a negative electrode current collector of a non-aqueous solvent secondary battery characterized by being immersed in a solution, or washing with rolled copper foil or electrolytic foil after degreased, or subjected to water washing and drying treatment. Negative electrode for a non-aqueous solvent secondary battery, characterized in that the electrolytic copper foil is immersed in an aqueous solution obtained by dissolving at least one selected from the group consisting of chromium trioxide, chromate and dichromate in water. A method for producing a copper foil for a current collector is provided.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the copper foil of the present invention will be described. The material for producing this copper foil may be any of a rolled copper foil and an electrolytic copper foil. In case of electrolytic copper foil,
Since a wide product can be made at low cost, it can be used as a negative electrode current collector of a battery for an electric vehicle, for example, which requires a large electrode area.

The thickness of the dielectric layer formed on the surface of the copper foil was determined by measuring the electric double layer capacitance on the surface of the copper foil using a commercially available direct-reading electric double layer capacitance measuring instrument and expressed by the following equation (1). Thus, it is calculated as its reciprocal value (1 / C). In the copper foil of the present invention, the 1 / C value is 0.1 to 0.3 cm ² / μF.
Is set within the range. In the case of a copper foil on which a dielectric layer is formed whose 1 / C value is measured as a value smaller than 0.1 cm ² / μF, the surface is oxidized during storage or transport before the actual use as a negative electrode current collector. And discoloration, spot rust, etc. occur, making it unusable. Further, the copper foil having a 1 / C value larger than 0.3 cm ² / μF reduces the current collecting ability as a negative electrode current collector, and inserts / desorbs Li ions progressing in the positive electrode / negative electrode mixture through a separator. The separation reaction is also reduced, and eventually, the charge / discharge cycle life characteristics of the battery and the battery capacity at the time of the first charge are reduced.

The dielectric layer has an extremely thin oxide film inevitably formed even on a normal copper foil surface, and an organic film formed on the oxide film at the time of rust prevention treatment described later. It has a laminated structure with a dielectric film or an inorganic dielectric film. In that case, in the case of only the oxide film,
The 1 / C value is measured as a value of about 0.05 cm ² / μF.

Further, the surface roughness of the copper foil of the present invention is determined according to JIS.
2.5-point average roughness (Rz) specified in B0601
It is preferable that it is not more than μm. Such copper foil is
The dielectric film described above can be formed with a thin and uniform thickness at the time of rust prevention treatment described below, and the negative electrode mixture carried thereon can be uniform, and the current collecting ability of the entire negative electrode can be increased, This is because the charge-discharge cycle life characteristics of the battery and the battery capacity at the time of the first charge are improved.

The dielectric layer is generally formed on both sides of the copper foil, but may be formed on only one side in consideration of the battery manufacturing process, the form of the negative electrode, and the purpose of use. However,
In the case of a positive electrode and a negative electrode of a jelly roll type secondary battery, since both have a structure in which an active material is supported on both surfaces of each current collector, in the case of this copper foil which is a negative electrode current collector It is preferable that the above-mentioned dielectric layer is formed on both surfaces thereof.

When the target battery has a jelly-roll type structure, the thinner the copper foil used, the larger the electrode area as a whole, and the above-mentioned surface roughness can be reduced. This is advantageous for high energy density and charge / discharge cycle life characteristics. For example, a battery for a portable electric / electronic device has a copper foil thickness of 20 mm.
μm or less, and preferably 50 μm or less even in the case of large batteries for electric vehicles and the like.

However, if the thickness of the copper foil is too small, the mechanical strength of the copper foil is reduced and the number of pinholes is increased. Therefore, the lower limit of the thickness of the copper foil that can withstand actual use is usually 7 μm. About. Next, the method for producing a copper foil of the present invention will be described. First, a first production method is a method of dissolving a rolled copper foil or an electrolytic copper foil after degreased, followed by washing and drying, followed by dissolving at least a triazole in a solvent at a pH of 5 to 8.5. This is a method in which an organic dielectric film is further formed on an already formed oxide film by immersion in the film.

Preferred examples of the triazole include benzotriazole (BTA) and tolyltriazole (TTA). Various organic rust preventives having a rust-preventing effect on copper, thiazoles, imidazoles, mercaptan , Toluethanolamine and the like can also be used. As the solvent to be used, in addition to water, alcohols or other organic solvents may be used, but the uniformity of the formed organic dielectric film and the thickness control during mass production are easy to perform, and it is also simple. Considering the effect on the environment, it is preferable that the water is mainly deionized water.

The dissolution concentration of the triazole is appropriately determined depending on the thickness of the organic dielectric film to be formed and the processable time, but is usually about 0.005 to 1% by weight. Further, the temperature of the solution may be room temperature, but it may be heated if necessary. The pH value of the solution to be used is set at 5 to 8.5. If the pH value is lower than 5, the oxide film that is inevitably generated is easily dissolved and removed, which results in preventing the formation of a stable organic dielectric film. If the pH value is higher than 8.5, copper oxide may be generated. This increases the thickness of the oxide film, thereby preventing formation of a stable organic dielectric film.

The immersion time of the copper foil in the solution is appropriately determined depending on the dissolution concentration of the triazoles and the thickness of the organic dielectric film to be formed, but is usually about 0.5 to 30 seconds. . In the second production method, a rolled copper foil after degreasing or electrolytic copper foil is washed with water or subjected to a water washing and drying treatment to select an electrolytic copper foil from the group of chromium trioxide, chromate, and dichromate. In this method, at least one of the chromium hydrated oxides is further formed on the already formed oxide film by immersing at least one of the oxidized films in an aqueous solution of water.

The chromate is preferably, for example, potassium chromate or sodium chromate, and the dichromate is, for example, potassium dichromate or sodium dichromate. And the dissolution concentration is
Usually, it is set to 0.1 to 10% by weight, and the liquid temperature may be room temperature to about 60 ° C. The pH value of the aqueous solution is not particularly limited from the acidic region to the alkaline region, but is usually set to 1 to 12.

The immersion time of the copper foil is appropriately selected depending on the thickness of the inorganic dielectric film to be formed. Thus, in the first manufacturing method, the organic dielectric film is formed on the oxide film already formed on the copper foil surface, and in the second manufacturing method, the chromate layer or its hydrate is formed. A layer-based inorganic dielectric film is formed.

Since the organic dielectric film formed by the first manufacturing method is bonded to the copper foil surface via the above-mentioned oxide film, it is directly bonded to copper to form an organic chelate film. The electric double layer capacity is more stable than in the case. And also in the first manufacturing method, the second
In the production method of (1), the oxide film (in the case of the first production method) or the hydrated oxide layer (in the case of the second production method) located immediately below the film located on the uppermost layer is flexible in a copper foil environment. It is considered that a good moisturizing effect is exhibited, and a passivation film is formed on the negative electrode active material together with the uppermost film, thereby improving the charge-discharge cycle life characteristics.

[0030]

EXAMPLES Examples 1 to 14 and Comparative Examples 1 to 4 (1) Production of copper foil Intermediate annealing was repeatedly applied to a pure copper plate after hot rolling, and during the process, solvent degreasing and pickling with a sulfuric acid aqueous solution were performed. After being polished and further sufficiently washed with water, final rolling was performed to obtain a rolled copper foil having a thickness shown in Table 1.

On the other hand, sulfuric acid 100 g / dm ³ , copper 100 g / dm ³ ,
An aqueous solution consisting of 30 ppm of chloride ions is added to hydroxycellulose ether 0 to 1 in accordance with the required surface roughness.
0 ppm, or 0.5 to 1.5 ppm of sodium 3-mercapto 1-propanesulfonate, and 0 to 6 ppm of glue are added, and electrolysis is performed at a bath temperature of 58 ° C. and a current density of 50 A / dm ² .
Copper was electrodeposited on the surface of the rotating drum made of Ti, and continuously rolled up to make an electrolytic copper foil. At this time, the plating time was changed to the thickness shown in Table 1.

Next, both surfaces of the rolled copper foil and the electrolytic copper foil were coated with a surface roughness meter (Surfcoder S) manufactured by Kosaka Laboratory.
Rz value was measured using E-30H). The results are shown in Table 1. The rolled copper foil and the electrolytic copper foil were subjected to a rustproofing treatment as follows.

Prior to the rust preventive treatment, in the case of rolled copper foil, the surface is subjected to a degreasing treatment in which the surface is washed with a solvent containing toluene or the like. After being dried, it was subjected to a rust-proof treatment continuously before winding. In the case of forming an organic dielectric film: an aqueous solution having a concentration indicated by triazoles shown in Table 1 was prepared, and the pH value was adjusted to about 5 to 5.
The copper foil was immersed in the aqueous solution at room temperature.

In the case of forming an inorganic dielectric film: A CrO ₃ aqueous solution having a concentration shown in Table 1 was prepared, and each copper foil was immersed in the aqueous solution at room temperature. On both sides of each copper foil subjected to rust prevention treatment, a direct electric double layer capacity measuring device (continuous 10-point automatic measuring type) manufactured by Hokuden Co., Ltd., using 0.1N potassium nitrate as an electrolytic solution. The electric double layer capacity was measured under the conditions of a step current of 50 μA / cm ² and the reciprocal (1 / C) was calculated. The results are shown in Table 1.

[0035]

[Table 1]

(2) Production of Battery 90% by weight of LiCoO ₂ powder, 7% by weight of graphite powder and 3% by weight of polyvinylidene fluoride powder are mixed, and a solution obtained by dissolving N-methylpyrrolidone in ethanol is added to the mixture. And kneaded to prepare a positive electrode mixture paste. This paste was uniformly applied to a 15-μm-thick aluminum foil, dried in an N ₂ atmosphere to volatilize ethanol, and then roll-rolled to a total thickness of 100 μm.
A sheet having a size of μm was formed. Then, the sheet was cut into a width of 43 mm and a length of 290 mm, and a lead terminal of an aluminum foil was attached to one end of the sheet by ultrasonic welding to obtain a positive electrode.

On the other hand, a negative electrode was manufactured as follows. First, prior to the production of the negative electrode, each copper foil shown in Table 1 was heated at a temperature of 40 ° C.
It was left standing in an air atmosphere at a temperature of 80 ° C. and a relative humidity of 80% for 2 days to perform an appearance inspection, and one having no abnormality was selected as a negative electrode current collector. The results of the appearance inspection at this time are shown in Table 1. Next, the following carbon materials were prepared as conductive materials.

Carbon material 1: Graphite powder having an average particle size of 10 μm and a non-oriented fine structure. Carbon material 2: Mesophase graphitized powder (average particle size 1
0 μm). Carbon material 3: Non-graphitizable carbon powder having a large number of micropores (average particle size: 10 μm). Carbon material 4: natural graphite powder (average particle size: 10 μm).

Carbon material 5: Carbon fiber having an average minor diameter of 5 μm. 90% by weight of each carbon material and 10% by weight of polyvinylidene fluoride powder were mixed, and a solution obtained by dissolving N-methylpyrrolidone in ethanol was added to the mixture and kneaded to prepare a negative electrode mixture paste. Then, this paste was uniformly applied to both surfaces of each copper foil shown in Table 1. The relationship between the type of carbon material in the paste and the copper foil at this time is as shown in Table 2.

The copper foil coated with each paste was dried in an N ₂ atmosphere to volatilize ethanol, and then roll-rolled to form a sheet having a total thickness of 100 μm. Then, this sheet was cut into a width of 43 mm and a length of 285 mm, and a lead terminal of nickel foil was attached to one end of the sheet by ultrasonic welding to form a negative electrode.

A separator made of polypropylene having a thickness of 25 μm is sandwiched between the positive electrode and the negative electrode manufactured as described above, and the whole is wound. The lead terminal of the negative electrode was spot-welded to the bottom of the can. Then, an insulating plate is placed on the wound body, and after inserting a gasket, the lead terminal of the positive electrode and the aluminum safety valve are connected by ultrasonic welding to form a non-aqueous solution composed of propylene carbonate, diethyl carbonate, and ethylene carbonate. After the electrolyte was injected into the battery can, a lid was attached to the safety valve, and a sealed Li-ion secondary battery having an outer diameter of 14 mm and a height of 50 mm was assembled.

(3) Measurement of Battery Characteristics With the above-mentioned battery, charging at a charging current of 50 mA until it reaches 4.2 V and discharging at 50 mA until it reaches 2.5 V are as follows.
A charge / discharge cycle test as a cycle was performed. Table 2 shows the battery capacity and the cycle life at the time of the first charge. The cycle life in Table 2 is the number of cycles when the discharge capacity of the battery falls below 300 mAh.

[0043]

[Table 2]

The following is clear from Tables 1 and 2. 1. First, as is clear from Table 1, both Comparative Examples 1 and 2 generate rust and the like before the actual use, and thus cannot be used as a negative electrode current collector. .

2. Each of the batteries using the copper foil of the example as the negative electrode current collector had a battery capacity of 400 mA at the time of the first charge.
h, and the cycle life also exceeds 400 cycles, resulting in a battery with high capacity and long life. The batteries using the negative electrode current collectors of Comparative Examples 3 and 4 had a battery capacity of 400 mAh at the time of the first charge, and the cycle life did not reach 400 cycles. It is very inferior to a battery using.

3. As is clear from the comparison between the charge / discharge cycle characteristics of the batteries 1 to 20 in Table 2 and the surface roughness of the copper foils of Examples 1 to 14 in Table 1, the smaller the surface roughness of the copper foil, the better the battery characteristics. doing. In particular, the surface roughness (R
A battery using a copper foil having z) of 2.5 μm or less as a negative electrode current collector shows good charge / discharge cycle characteristics.

4. Batteries using a thick copper foil as a negative electrode current collector (for example, battery 9 and battery 10) have slightly reduced charge / discharge cycle characteristics, but this is an evaluation result of the characteristics of small batteries having the same shape. This is because, as the thickness increases, the electrode area in the wound body decreases. However, a thick copper foil is often used for a large battery, but the practical battery characteristics at that time are further improved.

[0048]

As is clear from the above description, when the copper foil of the present invention is used as a negative electrode current collector, even if the composition of the negative electrode mixture carried thereon changes, It is possible to manufacture a non-aqueous solvent secondary battery having a jelly roll structure, which has a large battery capacity during charging and an excellent charge / discharge cycle life. This is an effect brought about by regulating the thickness of the dielectric layer on the copper foil surface to 0.1 to 0.3 cm ² / μF in 1 / C value.

Claims (4)

[Claims] 1. A negative electrode current collector for a non-aqueous solvent secondary battery, wherein the reciprocal (1 / C) of the electric double layer capacity on at least one side is 0.1 to 0.3 cm ² / μF. Copper foil. 2. The copper foil for a negative electrode current collector of a non-aqueous solvent secondary battery according to claim 1, wherein the surface roughness of both surfaces is not more than 2.5 μm in terms of a 10-point average roughness (Rz) specified in JIS B0601. 3. A rolled copper foil after degreasing, or an electrolytic copper foil formed by electrolytic washing, followed by washing with water, or washing and drying, is prepared by dissolving at least a triazole in a solvent.
A method for producing a copper foil for a negative electrode current collector of a non-aqueous solvent secondary battery, wherein the method is immersed in a solution of H5 to 8.5. 4. An electrolytic copper foil, which is obtained by subjecting a degreased rolled copper foil or an electrolytic foil to a washing and drying treatment after the degreased copper foil,
A method for producing a copper foil for a negative electrode current collector of a non-aqueous solvent secondary battery, characterized by immersing at least one selected from the group consisting of chromate and dichromate in water.