JP2013001982A - Rolled copper foil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolled copper foil which is excellent in strength and breaking elongation after heat treatment.SOLUTION: The rolled copper foil includes: 0.10-0.30 wt.% Mg and the balance being unavoidable impurities and copper. The rolled copper foil has a tensile strength TSA after heat treatment at 350°C for 30 minutes of 400 MPa or more and a conductivity after heat treatment at 350°C for 30 minutes of 65% IACS or more.

Description

  The present invention relates to a rolled copper foil suitable for a current collector used for an electrode of a secondary battery including a lithium ion battery.

Lithium ion batteries are being adopted in many fields because of their light weight and high energy density. As a current collector for an electrode (negative electrode) of a lithium ion battery, conventionally, a rolled copper foil called tough pitch copper having a copper content of 99.9% or an electrolytic copper foil has been used.
By the way, although the electrode active material is coated on the current collector, the active material expands and contracts during charge and discharge as ions move from the active material, and the current collector repeatedly loads each time it is charged and discharged. Will receive. Therefore, if the copper foil as the current collector is partially broken or peeled off, the battery life is shortened. In particular, in order to increase the capacity of the battery, it is required to make the current collector thinner. However, as the thickness of the copper foil becomes thinner, the resistance against external force decreases. Furthermore, after apply | coating an active material on a collector, a drying process (for example, 200-400 degreeC) is performed, and there exists a problem that a copper foil softens and the intensity | strength falls further in this case.

  Therefore, as a copper foil that is difficult to soften even by drying treatment, it contains one or more of Ag, Bi, Cd, Cr, Sn, Sb, Zn, and has a tensile strength after heating at 200 ° C. for 30 minutes. A rolled copper foil for a battery having a capacity of 400 N / mm @ 2 or more is disclosed (Patent Document 1). Moreover, the rolled copper foil for batteries which contains Zr and a strength does not fall even if it heat-processes by giving heat processing is disclosed (patent document 2). On the other hand, it is not a copper foil for batteries, but as rolled copper foil with excellent bending properties, Ag, Sn, Zr, Fe, Co, Ni, Mg, Zn, Ti, Si, B, Bi, Sb, Mn and Cr The rolled copper foil for flexible printed wiring boards containing 1 or more types is disclosed (patent document 3).

JP 2000-303128 A JP 2003-217595 A JP 2010-150598 A

However, when the present inventor examined a copper foil that can withstand repeated loads caused by charging and discharging, it was found that not only the strength after heat treatment was high, but also the elongation at break after heat treatment was required to be high to some extent. And when this inventor measured the strength and breaking elongation after heat processing about the copper foil of the composition described in patent documents 1-3, the copper foil of patent document 2 has small elongation at break, patent documents 1, It was found that the copper foil described in 3 has a low strength.
That is, the present invention has been made to solve the above-described problems, and an object of the present invention is to provide a rolled copper foil excellent in both strength and elongation at break after heat treatment.

As a result of various studies, the present inventors have found that the inclusion of Mg: 0.10-0.30 wt% improves both the strength and elongation at break of the rolled copper foil after heat treatment.
That is, the rolled copper foil of the present invention contains Mg: 0.10-0.30 wt%, the balance consists of inevitable impurities and copper, and the tensile strength TSA after heat treatment at 350 ° C. for 30 minutes is 400 MPa or more and at 350 ° C. The conductivity after heat treatment for 30 minutes is 65% IACS or more.

The strength reduction rate represented by {(TSB-TSA) / TSB} × 100 (%) is preferably 30% or less with respect to the tensile strength TSB before heat treatment at 350 ° C. for 30 minutes.
With respect to the breaking elongation EL ₁ before heat treatment at 350 ° C. for 30 minutes and the breaking elongation EL ₂ after heat treatment at 350 ° C. for 30 minutes, the breaking elongation increase ratio represented by {(EL ₂ -EL ₁ ) / EL ₁ } is It is preferable that it is 0.5-9.
Further, it is preferable to contain 0.05 to 0.25 wt% of Cr and / or Mn in total.
It is preferable that the oxygen content is 60 wtppm or less

  According to the present invention, it is possible to obtain a rolled copper foil excellent in both strength and elongation at break after heat treatment.

6 is a diagram showing an SEM image of the surface of a sample of Example 6. FIG. 10 is a diagram showing an SEM image of the surface of a sample of Example 7. FIG. And 350 ° C. for 30 minutes elongation at break EL ₂ copper foil after heating is a diagram showing the relationship between the cycle life of the battery.

  Hereinafter, the rolled copper foil which concerns on embodiment of this invention is demonstrated.

<Ingredient composition>
The rolled copper foil contains Mg: 0.10-0.30 wt%, and the balance consists of inevitable impurities and copper. Mg dissolves in copper and suppresses recrystallization by heat treatment to prevent softening (heat resistance is improved). Further, when Mg is added to copper, twins are formed in the recrystallized grains, and this twinning is considered to contribute to heat resistance.
Generally, when a rolled copper foil is heat-treated, there is a tendency that processing strain during rolling is reduced, strength is lowered, and elongation is improved. Therefore, when Mg is added to copper, the strength after heat treatment is increased by twinning, so that the strength reduction due to heat treatment is counteracted, and both the strength and elongation at break after heat treatment are excellent.
On the other hand, twinning is observed even when Zr is added to copper as in Patent Document 2, but the elongation at break after heat treatment is not improved. This is thought to be because Zr is an active metal, and a Zr oxide or a compound of copper and Zr is likely to be formed, and coarse particles are formed in copper to reduce elongation. Moreover, since Zr solid-dissolved in copper extremely suppresses recrystallization, strain is hardly reduced by heat treatment at 200 to 400 ° C., and ductility after heat treatment is difficult to increase.

When the Mg content in the rolled copper foil is less than 0.10 wt%, the tensile strength TSA after heat treatment at 350 ° C. for 30 minutes decreases to less than 400 MPa. When the Mg content exceeds 0.30 wt%, the elongation at break decreases. This is presumably because large Mg oxide particles are generated and become the starting point of fracture.
Further, Cr and / or Mn may be added as additive elements in a total amount of 0.05 to 0.25 wt%. In particular, when Cr is added, the heat resistance is further improved. If the total amount of Cr and / or Mn is less than 0.05 wt%, there will be no effect of improving heat resistance.If the total amount exceeds 0.25 wt%, the elongation at break will decrease after heat treatment at 350 ° C for 30 minutes and the conductivity will be 65%. Less than IACS.

  Furthermore, it is preferable that the oxygen content is 60 wtppm or less, because breakage due to oxides in the copper foil is suppressed, and elongation at break after heat treatment is further improved.

<Tensile strength>
The rolled copper foil of the present invention has a tensile strength TSA of 400 MPa or more after heat treatment at 350 ° C. for 30 minutes. When the TSA is less than 400 MPa, when the rolled copper foil is used as a battery current collector, if the current collector is repeatedly loaded with expansion and contraction of the active material during charge and discharge, the current collector It becomes easy to break.
The strength reduction rate represented by {(TSB-TSA) / TSB} × 100 (%) is preferably 30% or less with respect to the tensile strength TSB before heat treatment at 350 ° C. for 30 minutes. When the strength reduction rate is 30% or less, not only the strength after the heat treatment is high, but also the degree of softening after the heat treatment relative to the strength before the heat treatment is small, and the strength of the copper foil when the active material on the current collector is dried It is not lowered, and handling properties at the time of producing the current collector are improved.
The tensile strengths TSA and TSB are obtained by measuring the tensile strength (breaking strength; TS) in a direction parallel to the rolling direction according to JIS-Z2241 using a tensile tester.

<Elongation at break>
When a rolled copper foil is used as a current collector, not only the strength after heat treatment of copper foil is high, but also the elongation at break after heat treatment is preferably high to some extent in order to withstand repeated loads due to charge and discharge as described above. . However, when the plate thickness of the rolled copper foil is changed, the preferable range of the elongation at break after the heat treatment is also changed. This is because the coating amount of the active material on the rolled copper foil, the strain amount of the copper foil during charging / discharging, and the like differ depending on the plate thickness.
Therefore, in the present invention, a preferable range of the elongation at break EL ₂ after heat treatment at 350 ° C. for 30 minutes of the rolled copper foil was experimentally obtained and formulated. FIG. 3 shows the relationship between the elongation at break EL2 after heat treatment at 350 ° C. for 30 minutes of the rolled copper foil and the cycle life of the battery when this rolled copper foil is used as a negative electrode current collector. The cycle life evaluation method will be described later. FIG. 3 is a plot of a sample having a TSA of 400 MPa or more and a conductivity of 65% IACS or more after heat treatment at 350 ° C. for 30 minutes.

As shown in FIG. 3, a straight line C representing EL ₂ at the boundary between a sample having a good cycle life (◯) and a sample having a poor cycle life (×) is experimentally measured with respect to the plate thickness t (mm). Is represented by the formula 1:
C = 181.4 × t−0.51 (1)
It became.
The reason for using the straight line C is that when a current collector using copper foil is incorporated into a cylindrical battery, the copper foil is wound. At this time, the bending strain generated in the current collector (copper foil) This is because the thickness of the foil becomes smaller in proportion to the thickness. In other words, the thinner the copper foil, the smaller the bending strain applied during the expansion and contraction of the current collector due to battery charging / discharging, and it is difficult to break even if the elongation at break is small. Become.
In FIG. 3, for each plate thickness, the data of the sample with the smallest EL ₂ value among the samples with excellent cycle life and the sample with the largest value of EL ₂ among the samples with inferior cycle life. Data was extracted (data of a total of 10 points), and C was determined by the least square method so that the sum of squares of errors between these data and the straight line C was minimized.

The elongation at break is determined by the tensile tester in accordance with JIS-Z2241 in the direction parallel to the rolling direction, the length L between the gauge points when the test piece breaks, and the gauge distance L0 (50 mm) before the test. It is the elongation at break when the difference is obtained in%. Elongation at break (%) = (L−L ₀ ) / L ₀ × 100.
Further, the value of elongation at break varies depending on the dimensions of the test piece. Accordingly, in the present invention, the dimensions of the test piece used for measuring the tensile strength and elongation at break are 12.7 mm in width and 110 mm in length, and the distance between the gauges (tensile length) of the tensile tester is as described above. 50mm.

Accordingly, if the elongation at break EL ₂ (%) ≧ 181.4 × t−0.51 after heat treatment at 350 ° C. for 30 minutes, the elongation of the copper foil current collector increases after the active material is dried (heat treatment). The shrinkage of the copper foil current collector accompanying the charging / discharging of the battery is absorbed with high elongation, and it is difficult to break, which is preferable. On the other hand, if EL ₂ (%) <181.4 × t−0.51, a current collector that can withstand repeated loads due to charge and discharge may not be obtained.

Incidentally, using the above formula 1, indicating a preferred elongation at break EL ₂ after heat treatment typical 30 minutes at 350 ° C. in the sheet thickness t, the thickness t = 18μm, 15μm, 12μm, 10μm, with 7 [mu] m, respectively EL ₂ = 2.8% or more, 2.2% or more, 1.7% or more, 1.3% or more, 0.8% or more.

Also, elongation at break EL ₁ before the heat treatment for 30 minutes at 350 ° C. of rolled copper foil of the present invention is preferably 15% or less. When breaking elongation EL ₁ exceeds 15%, the control of the tension of a copper foil current collector of the production line for coating and drying an active material on a current collector is difficult.
Further, it is preferable that elongation at break EL ₂ after the heat treatment for 30 minutes at 350 ° C. of rolled copper foil of the present invention is also 15% or less. When breaking elongation EL ₂ exceeds 15%, the rolled copper foil is used as the current collector buckles during charging and discharging.
Further, the elongation at break EL ₁ before heat treatment at 350 ° C. for 30 minutes of the rolled copper foil of the present invention is preferably 1.6% or more when the thickness is 18 μm, and 1.3% when the thickness is 15 μm. It is preferably 1.1% or more when the thickness is 12 μm, preferably 0.8% or more when the thickness is 10 μm, and 0.3% when the thickness is 7 μm. % Or more is preferable. This is because in the production line in which the active material is applied to and dried on the copper foil current collector, the uneven tension on the copper foil before coating and drying is eliminated by elongation, and breakage in the production line is prevented. The reason why the preferable range varies depending on the thickness of the rolled copper foil is that the conveyance tension of the production line varies depending on the thickness of the rolled copper foil, and the degree of non-uniformity of the conveyance tension in the width direction also changes.

<Increase ratio of elongation at break>
In addition to defining the above-described breaking elongation EL ₂ , it is preferable to reduce the breaking elongation increase ratio M = {(EL ₂ −EL ₁ ) / EL ₁ } before and after heating at 350 ° C. for 30 minutes to 9 or less. When the increase ratio M is larger than 9, the copper foil current collector is plastically deformed with the expansion of the active material at the time of charge / discharge of the battery, and the deformed copper foil current collector does not return to the original state at the next contraction. Sometimes. In this case, when the copper foil current collector contracts, it buckles and breaks inside the battery, thereby reducing the cycle life.
However, when the increase ratio M is less than 0.5, EL ₂ or EL ₁ becomes smaller without releasing the processing strain accumulated even by the heat treatment, and the copper foil is accompanied by the expansion of the active material during charging / discharging of the battery. The current collector is not preferable because it may break without extending and the cycle life may be reduced. Moreover, even if the degree of work after recrystallization annealing is lowered, the change in work strain before and after heat treatment is small, so the increase ratio M is less than 0.5. In this case, the strength of the copper foil is originally It becomes low and handling property falls.
In addition, as a method of controlling the increase ratio M to 0.5 to 9, the composition of the copper foil (addition amount of Mg and, if necessary, addition amount of Cr and / or Mn), or processing of the final cold rolling Control the degree.
The increase ratio M is preferably 0.5 to 5, more preferably 0.8 to 3.0, and most preferably 0.8 to 2.5.

The rolled copper foil of the present invention preferably has a conductivity of 65% IACS or more after heat treatment at 350 ° C. for 30 minutes. If the conductivity is less than 65% IACS, it is not suitable as a battery current collector. The conductivity is measured by a four-terminal method according to 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, more preferably 7 μm to 15 μm, and most preferably 10 μm to 15 μm.

The rolled copper foil of the present invention preferably has a crystal grain size of 1 to 15 μm after heat treatment at 450 ° C., which is a temperature necessary for recrystallization annealing, for 30 minutes. In the case of an annealing condition in which the crystal grain size is 1 μm or less, there is a high possibility that an unrecrystallized structure remains. On the other hand, if the crystal grain size exceeds 15 μm, sufficient strain cannot be applied in the final rolling, and sufficient strength may not be obtained. Moreover, as described above, the rolled copper foil of the present invention is recrystallized after the final cold rolling (for example, heated at 450 ° C. for 30 minutes), and the rolled copper foil may contain twins.
The crystal grain size is measured according to the cutting method of JIS-H0501, and is performed on the rolled surface.

  Although the rolled copper foil of this invention can be used conveniently for the electrical power collector of electrodes (negative electrode), such as a lithium ion secondary battery, an application 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, so that the present invention can be effectively applied.

<Manufacture of rolled copper foil>
The rolled copper foil of the present invention can be produced by hot rolling an ingot having the above composition and then cold rolling. Further, as cold rolling, rolling before annealing, recrystallization annealing, and final rolling may be performed.
In the case of performing recrystallization annealing, it is preferable to set the workability of the final rolling to 80% or more because tensile strength (strength) is improved.

  First, copper ingots having the compositions shown in Tables 1 and 2 (the balance was copper and unavoidable impurities) were manufactured and hot-rolled to a thickness of 10 mm. Then, after chamfering, rolling was performed at a predetermined workability before annealing, and recrystallization annealing was performed at 450 ° C. Furthermore, the final cold rolling was carried out at the working degree shown in Tables 1 and 2, and copper foils (each example and comparative example) having thicknesses shown in Tables 1 and 2 were obtained.

<Evaluation>
The copper foil samples obtained by final rolling were measured for tensile strength, elongation at break, and electrical conductivity before and after heat treatment at 350 ° C. for 30 minutes. Note that some samples were evaluated by changing the heat treatment conditions at 400 ° C. for 30 minutes.
The dimensions of the test piece used for measurement of tensile strength and elongation at break were 12.7 mm in width and 110 mm in length, and the distance between chucks (tensile length) of the tensile tester was 50 mm. The conductivity was measured by a 4-terminal method in accordance with JIS-H0505.

<Cycle life>
About the obtained copper foil, the cylindrical lithium ion secondary battery was produced in the following procedures, and the cycle life was measured.
(1) Thickener aqueous solution 23 dissolved in 99 parts by weight of water with respect to 50 parts by weight of flaky graphite powder as a negative electrode active material, 5 parts by weight of styrene butadiene rubber as a binder, and 1 part by weight of carboxymethylcellulose as a thickener. A weight part was kneaded and dispersed to obtain a negative electrode paste. This negative electrode paste was applied to both surfaces of a rolled copper foil (sample) by a doctor blade method to a predetermined thickness, heated at 350 ° C. for 30 minutes and dried. Further, the thickness was adjusted by applying pressure to 80% of the original thickness, and then molded by shearing to obtain a negative electrode plate.
(2) 50 parts by weight of LiCoO ₂ powder as the positive electrode active material, 1.5 parts by weight of acetylene black as the conductive agent, 7 parts by weight of PTFE 50% aqueous dispersion as the binder, 41.5% aqueous solution of carboxymethyl cellulose as the thickener A weight part was kneaded and dispersed to obtain a positive electrode paste. The positive electrode paste was applied to both surfaces of a current collector made of an aluminum foil having a thickness three times the thickness of the copper foil by a doctor blade method, heated at 200 ° C. for 1 hour, and dried. Further, the pressure was adjusted to 80% of the original thickness to adjust the thickness, and then molded by shearing to obtain a positive electrode plate.

(3) The positive electrode plate and the negative electrode plate were spirally wound in a state of being insulated via a separator made of a polypropylene resin microporous film having a thickness of 20 μm, and this electrode group was accommodated in a battery case.
(4) The negative electrode lead connected from the negative electrode plate was electrically connected to the battery case via the lower insulating plate. Similarly, the positive electrode lead connected from the positive electrode plate was electrically connected to the internal terminal of the sealing plate via the upper insulating plate. Thereafter, a non-aqueous electrolyte is injected, the sealing plate and the battery case are caulked and sealed through an insulating gasket, and a cylindrical lithium ion secondary battery having a diameter of 17 mm and a height of 50 mm and a battery capacity of 780 mAh is obtained. Produced.
(5) The electrolytic solution was obtained by dissolving 1.0 mol of lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte in a mixed solvent of 30% by volume of ethylene carbonate, 50% by volume of ethyl methyl carbonate, and 20% by volume of methyl propionate. An electrolyte solution was prepared. A predetermined amount of this electrolytic solution was poured into the battery case and impregnated in the positive electrode active material layer and the negative electrode active material layer.

The charge / discharge cycle characteristics were evaluated using the produced battery. Charging / discharging was performed in an environment of 20 ° C., the discharge capacity at the third cycle was taken as the initial capacity, the number of cycles was counted until the discharge capacity was reduced to 80% of the initial capacity, and this was taken as the cycle life.
The charging condition is a constant current-constant voltage charge at 4.2V for 2 hours, and after the constant current charge of 550mA (0.7CmA) until the battery voltage reaches 4.2V, the current value further attenuates. The battery was charged to 40 mA (0.05 CmA).
The discharge conditions were a discharge at a constant current of 780 mA (1 CmA) to a discharge end voltage of 3.0 V. It was determined that good cycle characteristics were obtained when the cycle life reached 400 times or more.

  The obtained results are shown in Tables 1 and 2. In the cycle life column, “◯” indicates that the cycle life is 400 times or more, and “×” indicates that the cycle life is less than 400 times.

As is apparent from Table 1, in the case of each example containing Mg: 0.10-0.30 wt%, the balance being inevitable impurities and copper, the tensile strength TSA after heat treatment at 350 ° C. for 30 minutes is 400 MPa or more, The conductivity after heat treatment at 350 ° C for 30 minutes was 65% IACS or more. Further, in each example, the elongation at break EL ₂ was not less than the value calculated from Equation 1, and the cycle life was improved to 400 times or more.
Furthermore, in Examples 7 and 8 containing 0.05 to 0.25 wt% of Cr, the tensile strength TSA after the heat treatment was not changed even when the heat treatment conditions were higher than other examples (350 ° C.) (400 ° C. for 30 minutes). The conductivity after heat treatment at 350 ° C for 30 minutes was 65% IACS or more at 400MPa or more.

On the other hand, in Comparative Examples 1 to 3 in which the amount of Mg added was less than 0.10 wt%, the tensile strength TSA after heat treatment at 350 ° C. for 30 minutes decreased to less than 400 MPa, and the heat resistance was poor. Furthermore, in the case of Comparative Examples 1 to 3, elongation at break EL ₂ is less than the value calculated from Equation 1, the cycle life was reduced to less than 400 times.
In the case of Comparative Example 4 in which the added amount of Mg exceeded 0.30 wt%, the elongation at break EL ₂ was less than the value calculated from Equation 1 due to the breakage due to the oxide, and the cycle life was reduced to less than 400 times.
In Comparative Examples 5 and 6 in which Cr / or Mn was added in excess of 0.25 wt%, the elongation at break EL ₂ was less than the value calculated from Equation 1, and the cycle life was reduced to less than 400 times. Also, the conductivity decreased to less than 65% IACS.

In the case of Comparative Example 8 using the electrolytic copper foil, the tensile strength TSA after heat treatment at 350 ° C. for 30 minutes was reduced to less than 400 MPa, and the heat resistance was poor. Further, the elongation at break EL ₂ was less than the value calculated from Equation 1, and the cycle life was reduced to less than 400 times.
In Comparative Examples 9, 10, and 11 in which Mg was not added and Zr was added, the elongation at break EL ₂ was less than the value calculated from Equation 1, and the cycle life was reduced to less than 400 times.

In Comparative Example 2 described above, the amount of Mg added is less than 0.10 wt% and Ag is added. Even if additives other than Mg are added, the strength and elongation at break after heat treatment are balanced. It cannot be improved, and it is understood that it is important that the amount of Mg added is 0.10 wt% or more.
In Comparative Example 12 in which the oxygen content exceeds 60Wtppm, elongation at break EL ₂ by fracture due to oxide is less than the value calculated from Equation 1, the cycle life was reduced to less than 400 times.

Table 2 shows the change in elongation at break after heat treatment depending on the thickness of the sample, and the change in tensile strength after heat treatment depending on the workability of the final cold rolling. In each example, the tensile strength TSA after heat treatment at 350 ° C. for 30 minutes was 400 MPa or more, and the conductivity after heat treatment at 350 ° C. for 30 minutes was 65% IACS or more. Further, in each example, the elongation at break EL ₂ was not less than the value calculated from Equation 1, and the cycle life was improved to 400 times or more.

  When the sets of Examples 1, 13, 18, 19 and Comparative Example 21, and the sets of Example 5 and Comparative Example 22 having the same thickness and composition were compared, the degree of workability of the final cold rolling was 40 in all sets. As the percentage decreases, the tensile strength after heat treatment decreases. And in any group, when the workability becomes 40%, the tensile strength after heat treatment decreases to less than 400 MPa, and the workability of the final cold rolling needs to exceed 40%.

  1 and 2 are SEM images of the surfaces of the samples of Examples 6 and 7, respectively, and twins were observed as indicated by arrows.

Claims (5)

Mg: Containing 0.10-0.30wt%, the balance is inevitable impurities and copper, tensile strength TSA after heat treatment at 350 ° C for 30 minutes is 400MPa or more, and conductivity after heat treatment at 350 ° C for 30 minutes is 65 Rolled copper foil that is at least% IACS. 2. The rolled copper according to claim 1, wherein the strength reduction rate represented by {(TSB-TSA) / TSB} × 100 (%) is 30% or less with respect to the tensile strength TSB before heat treatment at 350 ° C. for 30 minutes. Foil. With respect to the breaking elongation EL ₁ before heat treatment at 350 ° C. for 30 minutes and the breaking elongation EL ₂ after heat treatment at 350 ° C. for 30 minutes, the breaking elongation increase ratio represented by {(EL ₂ -EL ₁ ) / EL ₁ } is It is 0.5-9, The rolled copper foil of Claim 1 or 2. Furthermore, the rolled copper foil in any one of Claims 1-3 which contains 0.05-0.25 wt% of Cr and / or Mn in a total amount. The rolled copper foil according to any one of claims 1 to 4, wherein the oxygen content is 60 wtppm or less.