TWI608109B - Rolled copper foil and lithium ion secondary battery and lithium ion capacitor using the same - Google Patents

Description

Calendered copper foil and lithium ion secondary battery and lithium ion capacitor using same

The present invention relates to a rolled copper foil for a secondary battery suitable for use in a current collector or an anode active material carrier of an electrode including a secondary battery of a lithium ion battery, and a lithium ion secondary battery and a lithium ion capacitor using the same.

Lithium-ion batteries are being used in many fields due to their light weight and high energy density. Further, as a current collector of an electrode (negative electrode) of a lithium ion battery, a rolled copper foil or an electrolytic copper foil called 99.9% of a copper component has been conventionally used.

However, the current collector is coated with the electrode active material, but the active material expands and contracts during charge and discharge with the movement of lithium ions, and the copper foil as the current collector receives a repeated load every time the charge and discharge are performed. Further, if the copper foil is plastically deformed by expansion, the wrinkles are concentrated on the copper foil at the time of the next shrinkage. On the other hand, if the copper foil is plastically deformed at the time of shrinkage, there is a possibility that the copper foil is broken at the time of the next expansion. In order to avoid such an abnormality, the copper foil is required to have high strength.

Further, in the manufacturing step of the negative electrode, heat of about 350 ° C is applied to the copper foil, so that the copper foil is required to have high heat resistance after the heat treatment.

In this case, a rolled copper foil for a current collector to which 0.1% by mass or more of Zr or Ti is added is disclosed (Patent Document 1). Further, it is disclosed that 0.01% by mass of Ti or 0.05 is added. Rolled copper foil for a flexible printed circuit board of mass % Zr (Patent Document 2). Further, a rolled copper foil to which Zr is added in an amount of 0.01 to 0.20% by mass is disclosed (Patent Document 3).

[Patent Document 1] Japanese Patent No.5654911

[Patent Document 2] Japanese Patent No. 5055088 (Embodiment 5, 7)

[Patent Document 3] Japanese Patent No. 4254488

However, in the case of the rolled copper foil described in Patent Document 1, the conductivity is lowered. This is considered to be because the addition amount of Zr or Ti is large, and the temperature of annealing before final cold rolling after hot rolling is low temperature (200 ° C or lower).

Further, the electrode active material is applied to the rolled copper foil for a current collector and then dried, but in the drying step, the heat history of the copper foil is increased. Therefore, if the strength of the copper foil is lowered by heat, wrinkles may occur in the copper foil during the drying step. However, the rolled copper foil described in the above Patent Documents 1 to 3 has a problem that the strength is largely lowered due to heat.

Further, if a large number of coarse inclusions of Zr or Ti (including particles existing during the melting and casting) are present on the surface of the copper foil, the pinhole may be formed, or the battery may be assembled after the electrode active material is applied. When the object falls off, the action of the battery causes an abnormality.

In other words, the present invention has been made to solve the above problems, and an object of the invention is to provide a rolled copper foil for a secondary battery which is excellent in strength, heat resistance and electrical conductivity, and a lithium ion secondary battery and a lithium ion capacitor using the same.

As a result of investigations by the inventors of the present invention, it has been found that by reducing the rate of change of the tensile strength before and after the specific heat treatment and reducing the inclusions of Zr or Ti on the surface of the copper foil, it is possible to suppress the decrease in strength due to heat.

In other words, the rolled copper foil of the present invention contains one or more selected from the group consisting of Ti and Zr in a total amount of 100 to 500 ppm by weight, and has an oxygen concentration of 50 ppm by weight or less, and is heat-treated at 350 ° C for 1 hour, according to JIS-Z2241. The tensile strength parallel to the rolling direction is 350 MPa or more, and the electrical conductivity after the heat treatment is 90% IACS or more. After the above heat treatment, the rate of change of the tensile strength is 10% or less, and 1000 μm ² of the surface of the copper foil. In the range, the number of inclusions of Zr or Ti having a long axis of 1 μm to 5 μm is 10 or less.

The pinhole having a long axis of 10 μm or more and 50 μm or less is preferably 50/m ² or less.

A preferred thickness is 20 μm or less.

It is preferable to repeat annealing and cold rolling one time or more after hot rolling, and to perform final cold rolling with a working degree of 80% or more and 95% or less, and to perform at least 700 ° C to 1000 ° C or less before the final cold rolling after the hot rolling. 1 time high temperature annealing.

The lithium ion secondary battery of the present invention uses the above-described rolled copper foil for a secondary battery.

The lithium ion capacitor of the present invention uses the above-described rolled copper foil for a secondary battery.

According to the present invention, a rolled copper foil for a secondary battery excellent in strength, heat resistance, and conductivity can be obtained, and a lithium ion secondary battery and a lithium ion capacitor using the same can be used.

1 is a secondary electron of a scanning electron microscope on the surface of a copper foil of Example 1. image.

Fig. 2 is a reflection electron image of a scanning electron microscope of the surface of the copper foil of Comparative Example 4.

Hereinafter, a rolled copper foil for a secondary battery according to an embodiment of the present invention will be described.

<Component composition>

As the rolled copper foil, oxygen-free copper (OFC) which is a pure copper-based composition of JIS-H3100 (C1100)-compliant refined copper (TPC) or JIS-H3100 (C1020) can be preferably used. In addition, pure copper containing no added element is completely recrystallized by heat treatment, and therefore contains one or more groups selected from the group consisting of Ti and Zr in a total amount of 100 to 500 ppm by weight, and the oxygen concentration is 50 ppm by weight or less.

Ti and Zr can improve heat resistance, and even if subjected to heat treatment, coarsening of crystal grains caused by recrystallization can be suppressed. When the total content of Ti and Zr is less than 100 ppm by weight, the heat resistance is not improved, and when it exceeds 500 ppm by weight, the electrical conductivity is lowered.

Further, when the oxygen concentration exceeds 50 ppm by weight, the added Zr or Ti is oxidized, the effect by the added element is lowered, or the inclusions are increased.

<tensile strength>

The rolled copper foil of the present invention has a tensile strength of 350 MPa or more after heat treatment at 350 ° C for 1 hour. When the tensile strength is 350 MPa or more, even if the copper foil as the current collector or the negative electrode active material carrier receives a repeated load, the wrinkles are prevented from being concentrated on the copper foil or the copper foil.

Furthermore, the tensile strength is measured and calendered according to JIS-Z2241 by a tensile tester. The tensile strength (breaking strength) in the parallel direction was determined.

<Electrical conductivity>

The electrical conductivity of the rolled copper foil of the present invention after heat treatment at 350 ° C for 1 hour is 90% IACS or more. If the above conductivity is less than 90% IACS, it is not suitable as a current collector of a secondary battery. The conductivity was measured by a 4-terminal method in accordance with JIS-H0505.

The thickness of the rolled copper foil of the present invention is preferably 20 μm or less, more preferably 5 μm to 18 μm, still more preferably 7 μm to 15 μm, most preferably 10 μm to 15 μm.

<Change rate of tensile strength caused by heat treatment>

The rolling strength of the rolled copper foil of the present invention after the heat treatment at 350 ° C for 1 hour is 10% or less. After the electrode active material is applied to the rolled copper foil for a current collector, the heat history of the copper foil is increased in the drying step. Therefore, when the above-described rate of change of the tensile strength of the copper foil exceeds 10%, the strength reduction due to heat becomes large, and the copper foil wrinkles in the drying step. Further, the one-hour heat treatment at 350 ° C becomes an accelerated test which is more severe than the actual drying conditions, and if the rate of change of the tensile strength after the heat treatment is 10% or less, wrinkles in the actual drying step can be suppressed.

<Inclusions of Zr or Ti>

The rolled copper foil of the present invention has a range of 1000 μm ^{2 on} the surface of the copper foil, and 10 or less inclusions of Zr or Ti having a long axis of 1 μm to 5 μm. The inclusion of Zr or Ti is usually an oxide. When the number of the inclusions exceeds 10, there is a cause of pinholes in the copper foil, or when the battery is assembled by applying the electrode active material, the inclusions are detached, and the operation of the battery is abnormal.

The measurement method of the long axis of inclusions will be described below. In addition, since the number of inclusions having a long axis of 1 μm to 5 μm does not change after heat treatment at 350 ° C for 1 hour, it can be heated. Measured in any case before and after treatment.

<number of pinholes>

In the rolled copper foil of the present invention, the pinhole having a long axis of 10 μm or more and 50 μm or less is preferably 50/m ² or less. When the pinhole exceeds 50 pieces/m ² , when the slurry of the electrode active material is applied to the copper foil, the slurry may bleed out to the back surface of the copper foil, and it is difficult to keep the coating thickness constant. When the long axis of the pinhole is less than 10 μm, since the slurry does not easily bleed out to the back surface of the copper foil, it does not become a problem, and almost no long axis of the pinhole exceeds 50 μm.

Furthermore, the measurement method of the long axis of the pinhole will be described below. Further, since the number of pinholes having a long axis of 10 μm to 50 μm does not change after heat treatment at 350 ° C for 1 hour, it can be measured in any of the cases after the heat treatment.

The rolled copper foil of the present invention can be preferably used for a current collector or a negative electrode active material carrier of an electrode (negative electrode) such as a lithium ion secondary battery or a lithium ion capacitor, but the use is not limited. In particular, when the thickness of the copper foil is 20 μm or less, the strength reduction due to the heat treatment becomes remarkable, and thus the present invention can be effectively applied.

<Manufacture of rolled copper foil>

The rolled copper foil of the present invention can be subjected to hot rolling after casting the ingot of the above composition, and then repeatedly subjected to annealing and cold rolling one time or more, and finally cold rolled. In the case of melt-casting an ingot which is a raw material of the rolled copper foil of the present invention, it is preferred to use such a master alloy for the addition of Zr or Ti. The reason for this is that Zr and Ti have a high melting point, and when added as a metal, it is difficult to dissolve in Cu as a base material. Further, the mother alloy is preferably processed into a sheet shape or the like to increase the contact area with the molten metal of Cu.

However, since the mother alloy having a large surface area is oxidized during storage, the molten metal is increased. Since the oxygen concentration is high, it is necessary to use a mother alloy having a small oxygen content. Specifically, the master alloy may be stored in an inert gas, pulverized into a sheet form immediately before being added to the molten metal, and heated in a reducing gas before use. In order to reduce the oxygen content of the master alloy, other than the above may be used. method.

The degree of processing of the final cold rolling is set to be 80% or more and 95% or less. If the degree of processing of the final cold rolling is set to less than 80%, there is a case where the required strength cannot be obtained as a current collector. If the degree of processing of the final cold rolling exceeds 95%, the strength after rolling is increased by work hardening. However, when the copper foil is heat-treated, the processing strain is lowered and the strength is greatly lowered, so that the film is stretched before the heat treatment. The rate of change in strength exceeds 10%.

Further, at least 700 times of high temperature annealing is performed at 700 ° C or more and 1000 ° C or less before final cold rolling after hot rolling.

When the temperature of the high-temperature annealing is less than 700 ° C, Ti and Zr as additive elements are not sufficiently diffused into the copper, and most of the added elements are in a precipitation state. In this case, the electrical conductivity is 90% IACS or more, but the precipitated particles of Ti or Zr cause pinholes or etching failure. On the other hand, if the temperature of the high-temperature annealing exceeds 1000 ° C, the copper material is partially melted and the composition becomes uneven, so that the material is easily broken during the subsequent processing.

Further, the high temperature annealing may be performed in any of a batch furnace and a continuous annealing furnace.

Further, after the high-temperature annealing described above, the low-temperature annealing may be performed at 300 ° C to 500 ° C for 0.5 to 4 hours. By this low-temperature annealing, Zr or Ti which is dissolved in the Cu mother phase during high-temperature annealing is precipitated, and the conductivity can be improved.

[Examples]

First, the elements described in Table 1 were added to the oxygen-free copper of JIS-H3100 (C1020), and the copper ingot of the composition shown in Table 1 was produced (the residual part was copper and unavoidable impurities). After hot rolling to a thickness of 10 mm, the annealing and cold rolling were repeated one time or more, and final cold rolling was performed to obtain copper foils having the thicknesses shown in Table 2 (various examples and comparative examples). Further, it was annealed under the conditions shown in Table 1 before final cold rolling after hot rolling.

Further, in Comparative Example 4, instead of oxygen-free copper, elements described in Table 1 were added to JIS-H3100 (C1100) refined copper (TPC) to produce a copper ingot.

<evaluation>

The tensile strength of the copper foil sample obtained by final rolling at 350 ° C for 1 hour before and after heat treatment and the electrical conductivity after the above heat treatment were measured.

The test piece for measurement of tensile strength and elongation at break was set to have a width of 12.7 mm and a length of 110 mm, and the distance between the chucks of the tensile tester (stretching length) was set to 50 mm, according to JIS-Z2241. It was stretched in parallel with the rolling direction as described above and measured.

Further, the electrical conductivity was measured by a 4-terminal method in accordance with JIS-H0505.

<Number of inclusions of Zr or Ti>

The surface of the copper foil sample before heat treatment at 350 ° C for 1 hour was subjected to appropriate electrolytic polishing or pickling to remove the deposit, and then observed by a scanning electron microscope (XL30SFEG manufactured by FEI Corporation) at a magnification of 1,000 times. The field of view was observed at 1000 μm ² , and the image having different color tone and copper foil substrate was subjected to image analysis and taken out, and the maximum value was set to the long axis in the interval between the two straight lines which are adjacent to each other at the outer periphery of each of the extracted portions. The long axis is measured for each particle in the above observation field, and the number of the long axis is 1 to 5 μm. The observation can be performed by either of the secondary electron image and the reflected electron image, but it is preferable to observe the reflected electron image of the inclusion easily.

<number of pinholes>

The long axis and the number of pinholes were measured by an optical inspection method. The optical inspection method was performed by irradiating light from the back surface of the copper foil sample before heat treatment at 350 ° C for 1 hour to detect the presence or absence of transmitted light from the pinhole. The detection of the pinholes was carried out in the following manner. First, the copper foil was placed on a light table, and the position of each pinhole was confirmed by the transmitted light from the pinhole, and an image obtained by magnifying the vicinity of the position with a microscope was obtained. Then, the image was subjected to image analysis, and the bright portion corresponding to the pinhole was taken out, and the maximum value was set to the long axis in the interval between the two straight lines parallel to the outer circumference of each portion to be taken out. The long axis is measured for all the pinholes in a specific observation area, and the number of the long axis of 10 μm or more and 50 μm or less is counted.

The results obtained are shown in Tables 1 and 2. Further, the oxygen concentration in Table 1 is the oxygen concentration contained in the ingot.

As is clear from Tables 1 and 2, in the case of each of the examples after high-temperature annealing at 700 ° C to 1000 ° C before final cold rolling after hot rolling, the tensile strength after heat treatment at 350 ° C for 1 hour was 350 MPa. As described above, the electric conductivity after the heat treatment is 90% IACS or more, the rate of change in tensile strength before and after the heat treatment is 10% or less, and the inclusions of Zr or Ti having a long axis of 1 μm to 5 μm are 10 or less.

On the other hand, in the case of Comparative Example 1 in which the total content of Ti and Zr was less than 100 ppm by weight, the tensile strength after heat treatment at 350 ° C for 1 hour was lowered to less than 350 MPa, and tensile strength before and after heat treatment. The rate of change is over 10%.

In the case of Comparative Example 2, when the annealing temperature before the final cold rolling was less than 700 ° C after hot rolling, the inclusions of Zr or Ti having a long axis of 1 μm to 5 μm exceeded 10, and heat treatment was performed at 350 ° C for 1 hour. The tensile strength thereafter was lowered to less than 350 MPa, and the rate of change in tensile strength before and after the heat treatment exceeded 10%.

In the case of Comparative Example 3 in which the annealing temperature before the final cold rolling after hot rolling exceeded 1000 ° C, the material was broken at the final cold rolling, and the copper foil could not be produced.

In the case of Comparative Example 4 in which the oxygen concentration exceeded 50 ppm by weight, the rate of change in tensile strength before and after the heat treatment exceeded 10%, and the inclusions of Zr or Ti having a long axis of 1 μm to 5 μm exceeded 10.

In the case of Comparative Example 7 in which the degree of processing of the final cold rolling was more than 95%, the rate of change in tensile strength before and after the heat treatment exceeded 10%, and the inclusions of Zr or Ti having a long axis of 1 μm to 5 μm exceeded 10.

In the case of Comparative Examples 5, 6, and 8 in which the total content of Ti and Zr exceeded 500 ppm by weight, the electrical conductivity after heat treatment at 350 ° C for 1 hour was lowered to less than 90% IACS.

In the case of Comparative Examples 9 to 10 in which neither Ti nor Zr was added, the tensile strength after heat treatment at 350 ° C for 1 hour was lowered to less than 350 MPa.

In the case of Comparative Examples 11 and 12 in which a large amount of Sn (more than 1000 ppm by weight) was added without adding any of Ti and Zr, the rate of change in tensile strength after the heat treatment was more than 10%.

Moreover, embodiment 1 is a scanning electron microscope the surface of the copper foil in Example 1 (observation magnification 1000 times, the observation field 1000μm ²⁾ of the secondary electron image. 2 is a reflection electron image of the surface of the copper foil of Comparative Example 4.

Claims (6)

A rolled copper foil containing one or more selected from the group consisting of Ti and Zr in a total amount of 100 to 500 ppm by weight, an oxygen concentration of 50 ppm by weight or less, and heat treatment at 350 ° C for 1 hour, according to JIS-Z2241 and rolling direction. The parallel tensile strength is 350 MPa or more, and the electrical conductivity after the heat treatment is 90% IACS or more. After the heat treatment, the tensile strength changes by 10% or less, in the range of 1000 μm ^{2 on} the surface of the copper foil. The inclusions of Zr or Ti having a long axis of 1 μm to 5 μm are 10 or less. The rolled copper foil according to the first aspect of the invention, wherein the pinhole having a long axis of 10 μm or more and 50 μm or less is 50/m ² or less. A rolled copper foil according to claim 1 or 2, wherein the thickness is 20 μm or less. A method for producing a rolled copper foil according to any one of the first to third aspects of the present invention, which, after hot rolling, repeats annealing and cold rolling one time or more, and performs final cold rolling at a working degree of 80% or more and 95% or less. At least one high temperature annealing is performed at 700 ° C or more and 1000 ° C or less before the final cold rolling after the hot rolling. A lithium ion secondary battery using the rolled copper foil according to any one of claims 1 to 3. A lithium ion capacitor using the rolled copper foil according to any one of claims 1 to 3.