JP2013247017A - Rolled copper foil for secondary battery negative electrode collector, negative electrode material for lithium ion secondary battery including the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a rolled copper foil for a secondary battery negative electrode collector, successfully suppressing generation of cracks and fractures even after repeated charging and discharging, when used as a collector of a lithium ion secondary battery; a negative electrode material for a lithium ion secondary battery including the rolled copper foil; and a lithium ion secondary battery.SOLUTION: When thermal treatment at 180°C is performed for 30 minutes, a rolled copper foil for a secondary battery negative electrode collector has a cross section in which the fraction of an area A, where the crystal orientation is within the range of 10° from the (001) orientation as the center, is 10% or more.

Description

  The present invention relates to a rolled copper foil for a secondary battery negative electrode current collector, a negative electrode material for a lithium ion secondary battery using the rolled copper foil, and a lithium ion secondary battery.

  Lithium ion secondary batteries have a high energy density and can obtain a relatively high voltage, and are widely used in small electronic devices such as notebook computers, video cameras, digital cameras, and mobile phones. Lithium ion secondary batteries have also begun to be used as power sources for large equipment such as electric vehicles and general household distributed power sources, and are lighter and more energy efficient than other secondary batteries such as nickel metal hydride batteries. Due to its high density, it is widely used in devices that require various power sources.

An electrode body of a lithium ion secondary battery generally has a winding structure or a stack structure in which electrodes are stacked. The positive electrode of the lithium ion secondary battery is composed of a current collector made of aluminum foil and a positive electrode active material made of a lithium composite oxide such as LiCoO ₂ , LiNiO ₂ and LiMn ₂ O ₄ provided on the surface thereof, The negative electrode is generally composed of a negative electrode active material made of a copper foil current collector and carbon or the like provided on the surface thereof.

  By the way, in a battery having a structure in which an electrode body is wound, the active material expands and contracts due to charge / discharge, and particularly near the inner peripheral portion of the wound structure in which the radius of curvature becomes small, or with an uncoated portion of the active material. When stress is concentrated near the boundary of the work part, the current collector is easily cracked or ruptured, which deteriorates the cycle characteristics of the battery. As a method of avoiding such a problem, a method of preventing breakage by adjusting the elongation of a copper foil as a negative electrode current collector to 2 to 15% is disclosed (Patent Document 1).

JP 2000-208149 A

However, when the present inventors examined, even if it used the copper foil with large elongation for a negative electrode electrical power collector, it turned out that a crack and a fracture | rupture may occur in copper foil by charging / discharging. As a result of detailed investigation, the active material expands and contracts due to charge and discharge, and the current collector copper foil undergoes repeated stress concentration, causing the current collector to partially bend and deform. The bending deformation is repeated by.
Bending deformation is accompanied by expansion and contraction of the active material, and bending and unbending are repeated alternately. Under such harsh conditions, cracks and breaks may occur in the copper foil as a current collector, resulting in an increase in energization resistance and deterioration of the cycle characteristics of the battery.

  Therefore, when the present invention is used as a current collector of a lithium ion secondary battery, a secondary battery excellent in cycle characteristics can be obtained by suppressing the occurrence of cracks and breaks even when charging and discharging are repeated. It is an object of the present invention to provide a rolled copper foil for a negative electrode current collector, a negative electrode material for a lithium ion secondary battery using the rolled copper foil, and a lithium ion secondary battery.

  The present invention relates to a copper foil used as a current collector of a lithium ion secondary battery, and is susceptible to cracking and breakage due to repeated bending caused by charging and discharging, and the crystal orientation is centered on the 001 direction in the cross section of the copper foil. Has been found to be related to the proportion of the area in the range of 10 °. And based on such knowledge, by controlling the ratio of the area in which the crystal orientation is in the range of 10 ° centered on the 001 direction in the copper foil cross section, the rolled copper for secondary battery negative electrode current collector excellent in cycle characteristics It has been found that a foil can be provided.

  In one aspect of the present invention completed based on the above knowledge, when a heat treatment is performed at 180 ° C. for 30 minutes, the ratio of the area A in which the crystal orientation is in the range of 10 ° centering on the 001 orientation is 10% or more. It is the rolled copper foil for secondary battery negative electrode collectors which has the cross section which is.

  In one Embodiment of the rolled copper foil for secondary battery negative electrode collectors which concerns on this invention, the ratio of the said area A in the said cross section is 60% or more.

  In another embodiment of the rolled copper foil for a secondary battery negative electrode current collector according to the present invention, tough pitch copper standardized to JIS-H3100-C1100 or oxygen-free copper standardized to JIS-H3100-C1020 is used. Is formed.

  In still another embodiment of the rolled copper foil for a secondary battery negative electrode current collector according to the present invention, 10 to 500 ppm by mass of Ag is contained.

  In yet another embodiment of the rolled copper foil for a secondary battery negative electrode current collector according to the present invention, the rolled copper foil further comprises 10 to 100 mass ppm of Sn, and is formed using oxygen-free copper standardized to JIS-H3100-C1020. ing.

  In yet another embodiment of the rolled copper foil for a secondary battery negative electrode current collector according to the present invention, 1 selected from the group consisting of P, Fe, Zr, Mg, S, Ge and Ti as inevitable impurities A seed or 2 or more types is 20 mass ppm or less in total.

  In still another embodiment of the rolled copper foil for a secondary battery negative electrode current collector according to the present invention, the thickness is 5 to 20 μm.

  Another aspect of the present invention is a negative electrode material for a lithium ion secondary battery including the rolled copper foil according to the present invention.

  In still another aspect, the present invention is a lithium ion secondary battery including the negative electrode material of the present invention.

  According to the present invention, when used as a current collector of a lithium ion secondary battery, the rolled copper foil for a secondary battery negative electrode current collector is satisfactorily suppressed from generating cracks and breaks even after repeated charge and discharge. A negative electrode material for a lithium ion secondary battery and a lithium ion secondary battery using the same can be provided.

(Composition of rolled copper foil)
As a material of the rolled copper foil for secondary battery negative electrode current collector of the present invention, tough pitch copper standardized to JIS-H3100-C1100 or oxygen-free copper standardized to JIS-H3100-C1020 is preferable. Since these compositions are close to pure copper, the conductivity of the copper foil does not decrease and is suitable for a current collector. The oxygen concentration contained in the copper foil is 0.01 to 0.02% by mass in the case of tough pitch copper, and 0.001% by mass or less in the case of oxygen-free copper.
The copper foil according to the present invention is made of industrially used copper and contains inevitable impurities. Even if a small amount of P, Fe, Zr, Mg, S, Ge and Ti as inevitable impurities is present, the crystal orientation is likely to rotate due to bending deformation of the copper foil, and a shear band is likely to enter. It is not preferable because cracks and breaks are likely to occur when the electric body repeatedly undergoes bending deformation. For this reason, the copper foil which concerns on this invention makes 1 mass or 2 types or more selected from the group which consists of P, Fe, Zr, Mg, S, Ge, and Ti as an unavoidable impurity to 20 mass ppm or less in total It is preferable to control.
Further, Ag may be contained in an amount of 500 ppm by mass or less, and Sn may be contained in an amount of 100 ppm by mass or less in order to improve material properties. When Ag or Sn is added to the copper foil, the material strength after finish rolling is increased, and the handleability of the material is improved, but the addition amount of Ag exceeds 500 mass ppm and the addition amount of Sn exceeds 100 mass ppm. And the conductivity decreases and the recrystallization temperature rises, it is difficult to recrystallize and suppress the surface oxidation of the copper alloy, or the current collector is dried after the active material coating in the negative electrode material manufacturing process When the copper foil is difficult to recrystallize, the characteristics of the present invention cannot be expressed. Therefore, the addition amount of Ag is preferably 500 ppm by mass or less, and the addition amount of Sn is preferably 100 ppm by mass or less. In addition, although the minimum of the addition amount of Ag and Sn is not specifically limited, Usually, it is 10 ppm or more in total. Moreover, you may add Ag and Sn simultaneously.
Here, since Ag is harder to oxidize than Cu, it can be added in either a tough pitch copper or oxygen-free copper melt. However, when the oxygen concentration exceeds 500 mass ppm, the copper oxide particles increase, and adverse effects such as the starting point of cracking of the copper foil in the battery charge / discharge cycle test can be considered. It is preferable to do.
In addition, since Sn is easier to oxidize than Cu, adverse effects such as the formation of an oxide in the copper foil and starting cracks in the charge / discharge cycle test of the battery can be considered. It is common to add.
In addition, when the term “copper foil” is used alone in this specification, the copper alloy foil is also included. When “tough pitch copper and oxygen-free copper” is used alone, copper alloy based on tough pitch copper and oxygen-free copper is used. Includes foil.

(Manufacturing method of copper foil)
The copper foil which concerns on embodiment of this invention can be manufactured as follows, for example. After hot rolling an ingot cast with a specified composition, the oxide is removed by surface grinding, and cold rolling, annealing, and pickling are repeated to process to a predetermined thickness to produce a copper foil. In order to control the ratio of the range of 10 ° centered on the 001 orientation which is the crystal orientation to 10% or more with respect to the cross section in the thickness direction, in the final rolling process, the minimum degree of processing in one pass of rolling is 15% or more. The kinematic viscosity of the oil is 5 mm ² / s or less, and the rolling strain rate is 30 to 800 / s. Here, the final rolling process refers to a process of rolling to product thickness after recrystallization annealing. In rolling, the material is repeatedly passed between a pair of rolls to finish the thickness. At this time, passing the material once between the rolls is called one pass.

(Rolled copper foil thickness)
As thickness of the rolled copper foil which can be used for this invention, 5-20 micrometers is preferable. There is no particular lower limit to the thickness of the copper foil, but if it is less than 5 μm, the handling of the copper foil becomes worse, and therefore it is preferably 5 μm or more. The upper limit of the thickness of the foil is not particularly limited, but the thickness is preferably 20 μm or less because the energy density per unit weight of the battery decreases as the thickness increases and the cost of the material also increases.

  The copper foil according to the present invention has a cross section in which the ratio of the area A in the range of 10 ° centered on the 001 orientation is 10% or more after recrystallization by heat treatment at 180 ° C. for 30 minutes, for example. The crystal orientation of the copper foil rotates when the bending is repeated several times. This rotation of crystal orientation causes cracking. If the 001 orientation of the crystal is in the cross section in the thickness direction of the copper foil and the direction parallel to the cross section is the bending direction, the crystal orientation becomes difficult to rotate, and the repeated bendability is improved. Moreover, it becomes difficult to enter a shear band which is one of the causes of cracks, and the bendability is improved repeatedly. The copper foil according to the present invention has good bending workability because the ratio of the area A in the range of 10 ° centered on the 001 orientation is 10% or more, and cracks and breaks due to repeated bending caused by charging and discharging are good. Hard to occur. The ratio of the area A in the cross section is more preferably 60% or more. The crystal orientation can be measured by an EBSD (Electron Back Scattering Diffraction) method.

  EXAMPLES Examples of the present invention will be described below, but these are provided for better understanding of the present invention and are not intended to limit the present invention.

As Examples 1 to 18, ingots were prepared by adding the elements shown in Table 1 to high-purity electrolytic copper (Cu concentration: 99.99% or more). At this time, P, Fe, and Zr as inevitable impurities are prevented by not mixing impurities from the crucible, the mold, and the refractory and further preventing P, Zr, and Mg from being mixed in the deoxidation treatment. , Mg, S, Ge and Ti were controlled so that one or more selected from the group consisting of Mg, S, Ge and Ti would be 20 ppm by mass or less in total.
Subsequently, this ingot is processed into a 7 mm thick plate by hot rolling, oxide is removed by surface grinding, and then cold rolling, annealing, and pickling are repeated, and in the final rolling process, the minimum per pass The degree of processing, strain rate, and kinematic viscosity of the rolling oil were processed under the conditions shown in Table 1.

On the other hand, as Comparative Examples 1 to 9, ingots were prepared by adding the elements shown in Table 1 to high-purity electrolytic copper. Further, at this time, inevitable impurities are not suppressed for Comparative Examples 1 to 4, and one or more selected from the group consisting of P, Fe, Zr, Mg, S, Ge, and Ti are combined. It became more than 20 mass ppm. In Comparative Examples 5 to 9, inevitable impurities were suppressed in the same manner as in Examples.
The processing conditions of the ingots of Comparative Examples 1 to 9 were the same as those of the example, and the final rolling process was performed under the conditions shown in Table 1.

  The specimens of Examples 1 to 18 and Comparative Examples 1 to 9 thus produced were heat-treated at 180 ° C. for 30 minutes in an Ar atmosphere, and then subjected to the rolling parallel direction using the CP method (cross session polisher method). Cut in the thickness direction to obtain a copper foil cross section. Immediately thereafter, the crystal orientation was measured by the EBSD method using the electron microscope FE-SEM manufactured by JEOL in an area of “copper foil thickness described in Table 1” × “300 μm width”, and analysis software manufactured by TSL was used. The KAM value was calculated by using this, and the ratio of the area A in which the crystal orientation was in the range of 10 ° around the 001 orientation was calculated.

Subsequently, the copper foils obtained in the examples and comparative examples were used for the negative electrode current collector, and a 18650 size cylindrical battery type lithium ion secondary battery having a rated capacity of 1 Ah was prepared by the following procedure, and the charge / discharge cycle life was obtained. Was measured.
A slurry was prepared by dispersing natural graphite having an average particle diameter of 15 μm as a negative electrode active material and PVDF as a binder in NMP (N-methyl-2-pyrrolidone) at a weight ratio of 92: 8. The slurry was applied on a copper foil, dried at 90 ° C. for 30 minutes, and further dried at 120 ° C. for 10 minutes. By carrying out this for each side of the copper foil, negative electrode active material layers were formed on both sides of the copper foil. Furthermore, after adjusting the electrode density by a pressure press, the negative electrode material was dried at 180 ° C. for 30 minutes in a vacuum for the purpose of evaporating water.
A slurry was prepared by dispersing lithium cobaltate (LiCoO ₂ ) as a positive electrode active material, PVDF as a binder, and acetylene black as a conductive additive in NMP at a weight ratio of 92: 4: 4. The slurry was applied on an aluminum foil having a thickness of 20 μm and then dried at 120 ° C. for 30 minutes. By carrying out this on each side of the aluminum foil, positive electrode active material layers were formed on both sides of the aluminum foil. Further, an electrode having an active material density of 3.2 g / cm ³ and an active material thickness of 75 μm was produced by a pressure press.
The battery was wound with a separator made of a porous polyethylene film having a thickness of 20 μm interposed between the positive electrode and the negative electrode produced as described above, and stored in a battery case.
After the positive electrode lead is connected to the lid of the battery case, a non-aqueous electrolyte solution in which ethylene carbonate and diethyl carbonate as a solvent are dissolved in a volume ratio of 2: 3 and 1 mol / L LiPF ₆ as an electrolyte is dissolved in the battery case. The solution was poured, and the lid of the battery can was crimped and sealed to produce a cylindrical lithium ion secondary battery.

The produced 18650 size cylindrical battery was repeatedly charged and discharged in an environment of 25 ° C., and the capacity retention rate was examined. With the second charge / discharge as the initial capacity, the number of charge / discharge cycles until the discharge capacity is reduced to 80% or less of the initial capacity is less than 100 “x”, 100 to 300 times “Δ”, 300 The cycle characteristics were evaluated with “○” as the number exceeding the number of times. If the cycle life evaluation is ○ or Δ, there is no practical problem.
The charging / discharging conditions were a constant current of 4.2 A and a charging time of 2 hours after charging to 4.2 V with a constant current of 1 A, and a discharging of 3.0 V with a constant current of 1 A.

The obtained results are shown in Table 1. In the composition of Table 1, “OFC” and “TPC” represent oxygen-free copper (JIS-H3100-C1020) and tough pitch copper (JIS-H3100-C1100), respectively. This shows the addition of 100 ppm by weight of Ag.

(Evaluation)
In Examples 1 to 18, one or more selected from the group consisting of P, Fe, Zr, Mg, S, Ge and Ti as inevitable impurities are 20 ppm by mass or less in total. The ratio of area A was 10% or more, and the cycle life of the lithium ion secondary battery was good.
In Comparative Examples 1 to 4, one or two or more selected from the group consisting of P, Fe, Zr, Mg, S, Ge and Ti as inevitable impurities are in total more than 20 mass ppm, The ratio of area A was less than 10%, and the cycle life of the lithium ion secondary battery was inferior.
In Comparative Example 5, the Sn addition concentration exceeds 100 ppm by mass, recrystallization does not proceed sufficiently in the heat treatment at 180 ° C. for 30 minutes, and the ratio of area A is less than 10%. The cycle life of the secondary battery was inferior.
In Comparative Example 6, the minimum value of the degree of one-pass processing in the final rolling is less than 15%, and the ratio of the area A after recrystallization is less than 10%. Therefore, the cycle life of the lithium ion secondary battery is inferior. It was.
In Comparative Example 7, since the kinematic viscosity of the rolling oil in the final rolling exceeds 5.0 mm ² / s and the ratio of the area A after recrystallization is less than 10%, the cycle life of the lithium ion secondary battery is inferior.
In Comparative Example 8, the strain rate of the final pass in the final rolling exceeded 800 / s, and the ratio of the area A after recrystallization was less than 10%. Therefore, the cycle life of the lithium ion secondary battery was inferior.
In Comparative Example 9, the minimum value of the degree of one-pass processing in the final rolling and the kinematic viscosity of the rolling oil are out of the specified values, and the ratio of the area A after recrystallization is less than 10%. The cycle life of was poor.

Claims (9)

  Rolled copper for secondary battery negative electrode current collector having a cross section in which the ratio of area A in the range of 10 ° centered on 001 orientation is 10% or more when heat treatment is performed at 180 ° C. for 30 minutes Foil.   The rolled copper foil for a secondary battery negative electrode current collector according to claim 1, wherein a ratio of the area A in the cross section is 60% or more.   The rolled copper foil for secondary battery negative electrode collectors of Claim 1 or 2 formed using the tough pitch copper specified to JIS-H3100-C1100, or the oxygen-free copper specified to JIS-H3100-C1020.   The rolled copper foil for secondary battery negative electrode collectors of Claim 3 containing 10-500 mass ppm of Ag.   Furthermore, the rolled copper foil for secondary battery negative electrode collectors of Claim 1, 2 or 4 formed using the oxygen free copper which contains Sn in 10-100 mass ppm and standardizes to JIS-H3100-C1020.   The total amount of one or more selected from the group consisting of P, Fe, Zr, Mg, S, Ge, and Ti as inevitable impurities is 20 mass ppm or less. Rolled copper foil for secondary battery negative electrode current collector.   The rolled copper foil for a secondary battery negative electrode current collector according to any one of claims 1 to 6, which has a thickness of 5 to 20 µm.   The negative electrode material for lithium ion secondary batteries provided with the rolled copper foil in any one of Claims 1-7.   A lithium ion secondary battery comprising the negative electrode material according to claim 8.