JP5490761B2 - Rolled copper foil for secondary battery negative electrode current collector, negative electrode material for lithium ion secondary battery and lithium ion secondary battery using the same - Google Patents

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-scale equipment such as electric vehicles and distributed power sources for general households, and are lighter and have higher energy density than other secondary batteries. Widely used in equipment that requires 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, a current collector is obtained by concentrating stress in the vicinity of the inner peripheral portion of the wound structure in which the radius of curvature becomes small due to expansion and contraction of the active material accompanying charging and discharging. Cracks are easily generated or broken, causing deterioration of 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 copper foil as a current collector is subjected to repeated stress concentration, thereby forming a transition cell due to fatigue. It turns out that by rotating with the progress of fatigue due to stress loading, a minute angle difference is produced in the crystal orientation in the crystal grain, and eventually it breaks. Here, not only when using recrystallized copper foil, but also when manufacturing a negative electrode using non-recrystallized copper foil, slurry is applied to the copper foil that is the current collector and dried. In the heat treatment step, the copper foil is recrystallized, and the recrystallized copper crystal grains have almost uniform and almost no small angle difference. After incorporating the negative electrode material thus fabricated into the battery, the expansion and contraction due to charging and discharging repeatedly causes an angle difference in the crystal orientation due to the stress applied to the copper foil, and cracks and the like are generated due to the above stress concentration. This is a cause of a decrease in discharge performance.

  Accordingly, the present invention provides a rolled copper foil for a secondary battery negative electrode current collector that, when used as a current collector for a lithium ion secondary battery, can effectively suppress the occurrence of cracks and breaks even when charging and discharging are repeated. An object is to provide a negative electrode material for a lithium ion secondary battery and a lithium ion secondary battery using the same.

  As a result of intensive studies, the present inventors have determined that rolled copper that has been controlled so that the difference in crystal orientation in the crystal after recrystallization annealing satisfies a predetermined condition as a copper foil for a current collector of a lithium ion secondary battery. It has been found that by using the foil, the occurrence of cracks and breakage of the current collector can be well controlled even when charging and discharging are repeated.

The present invention completed on the basis of the above knowledge, in one aspect, the crystal orientation obtained by irradiating the measurement point a of the metal structure of the crystal with an electron beam when heat-treated at 200 ° C. for 30 minutes, and the measurement point Centering on the measurement point a where the average value of the azimuth angle difference with the crystal orientation obtained by irradiating the electron beam to a plurality of adjacent measurement points located 200 nm apart from a is less than 0.4 ° The area of the regular hexagon whose distance between the measurement point a and each side is 100 nm is area A, and the total of the areas A is area AT.
The crystal orientation obtained by irradiating an electron beam to the measurement point b of the metallographic structure of the crystal and the electron beam irradiated to a plurality of adjacent measurement points located 200 nm apart around the measurement point b. An area of a regular hexagon centered on the measurement point b where the average value of the orientation angle difference with the crystal orientation is not less than 0.4 ° and less than 2.0 °, and the distance between the measurement point b and each side is 100 nm. Is area B, and the total of area B is area BT,
Area BT / Area AT x 100 (%) <20 (%)
It is the rolled copper foil for secondary battery negative electrode collectors which satisfy | fills.

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

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

  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 includes 10 to 100 mass ppm of Sn, and is formed using oxygen free copper standardized to JIS-C1020. .

  In yet another embodiment of the rolled copper foil for a secondary battery negative electrode current collector according to the present invention, 1 is further selected from the group consisting of Fe, In, Mg, Zn, Si, Ni, Pb, Cr and Zr. It contains 0 to 0.01% by mass in total of seeds or two or more kinds.

  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.

It is a schematic diagram showing the measurement aspect of the crystal orientation of rolled copper foil.

(Composition of rolled copper foil)
As a material of the rolled copper foil for the secondary battery negative electrode current collector, tough pitch copper standardized to JIS-C1100 or oxygen-free copper standardized to JIS-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.
Furthermore, 500 mass ppm or less of Ag may be included, and 100 mass ppm or less of Sn may be included. 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 It becomes difficult to recrystallize the copper foil. 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.
Further, in the same manner as when Ag or Sn is added excessively, the total amount of these elements is 0.01 mass for Fe, In, Mg, Zn, Si, Ni, Pb, Cr and Zr which are impurities in copper. If it exceeds 50%, the electrical conductivity decreases and the recrystallization temperature increases. Therefore, the total amount of these elements is preferably 0.01% by mass or less.
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, if the oxygen concentration exceeds 500 ppm by mass, the copper oxide particles increase, and adverse effects such as starting of cracking of the copper foil in the battery charge / discharge cycle test can be considered. Is preferred.
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.

(Rolled copper foil thickness)
As thickness of the rolled copper foil which can be used for this invention, 5-20 micrometers is preferable. When the thickness of the copper foil is less than 5 μm, the handling of the copper foil is deteriorated, and therefore, preferably 5 μm or more. When the foil is thick, the energy density per unit weight of the battery is lowered, and the cost of the material is also increased. Therefore, the thickness is preferably 20 μm or less.

(Crystal orientation control)
The rolled copper foil of the present invention is in a state where a copper foil as a current collector for a negative electrode material for a lithium ion secondary battery is recrystallized, for example, heat treatment at 200 ° C. for 30 minutes, which is a condition for sufficiently recrystallizing the copper foil. In this case, the orientation angle difference between crystal orientations at different measurement points is controlled so as to satisfy the following predetermined conditions. The conditions regarding the azimuth angle difference will be specifically described.
First, the measurement point a of the metal structure of the crystal is determined. This measurement point a includes a crystal orientation obtained by irradiating an electron beam, and a crystal orientation obtained by irradiating an electron beam to six adjacent measurement points located 200 nm apart around the measurement point a. The average value of the azimuth angle differences is less than 0.4 °. Further, the measurement point b of the metal structure of the crystal is determined. The measurement point b includes a crystal orientation obtained by irradiating an electron beam, and a crystal orientation obtained by irradiating an electron beam to six adjacent measurement points located 200 nm apart around the measurement point b. The average value of the azimuth angle differences is 0.4 ° or more and less than 2.0 °. In addition, it is determined that the adjacent measurement point where the azimuth angle difference between the measurement point and the crystal orientation of the adjacent measurement point is 2.0 ° or more is a crystal grain boundary, and is taken into consideration in calculating the average value of the above azimuth angle difference. There wasn't. Therefore, for example, if two of the six adjacent measurement points are determined to be crystal grain boundaries, the crystal orientation obtained by irradiating the measurement points with an electron beam and the remaining four points other than the crystal grain boundaries The average value of the azimuth angle difference from the crystal orientation of the adjacent measurement point is the orientation between the crystal orientation obtained by irradiating the measurement point with the electron beam and the crystal orientation obtained by irradiating the adjacent measurement point with the electron beam. The average value of the angle difference.
When the rolled copper foil of the present invention is heat-treated at 200 ° C. for 30 minutes, the area A is a regular hexagon whose center is the measurement point a and the distance between the measurement point a and each side is 100 nm. The total area is AT, the area of the regular hexagon having the measurement point b as the center and the distance between the measurement point b and each side being 100 nm is defined as area B, and the total of the areas B is defined as area BT / area. AT x 100 (%) <20 (%)
Meet.

  In FIG. 1, the schematic diagram showing the measurement aspect of the crystal orientation of the rolled copper foil of this invention is shown. First, the measurement point is determined. In FIG. 1 (hereinafter referred to as measurement point 1). Further, a regular hexagon having the measurement point 1 as the center and the distance between the measurement point 1 and each side being 100 nm is determined. Adjacent measurement points (measurement points 2 to 7) are located around the measurement point 1 and spaced apart by 200 nm. And the crystal orientation obtained by irradiating an electron beam about the measurement points 1-7 is measured, and the azimuth | direction angle difference of the measurement point 1 and the measurement points 2-7 is calculated | required, respectively. When the average value of the azimuth angle differences thus obtained is less than 0.4 °, the measurement point 1 is defined as the measurement point a, and the regular hexagonal area centered on the measurement point a is defined as the area A. Further, when the average value of the azimuth angle difference is 0.4 ° or more and less than 2.0 °, the measurement point 1 is defined as a measurement point b, and the area of a regular hexagon centering on the measurement point b is defined as an area B.

  Further, for these adjacent measurement points (measurement points 2 to 7), similarly to the measurement point 1, a regular hexagon whose distance from each side is 100 nm is determined. When the regular hexagons are sequentially determined in this way, the metal structure of the copper foil is filled with a plurality of regular hexagons in contact with each other as shown in FIG. Then, the measurement points a or b are determined for each measurement point in the same manner as described above, and the area A or B is obtained. Thus, when the total area A at each measurement point obtained by measuring the entire measurement surface of the copper foil is defined as area AT and the total area B at each measurement point is defined as area BT, area BT / area AT × 100 (%) <20 (%) is satisfied, that is, the area BT with respect to the area AT is less than 20%. In the rolled copper foil before fatigue, the area of the region where the average value of the crystallographic orientation angle difference is not less than 0.4 ° and less than 2.0 ° is small if it is smaller than the area of the region less than 0.4 °. The inventor has found that the fatigue resistance is improved. In this respect, the rolled copper foil of the present invention has an area BT of the region that is 0.4 ° or more and less than 2.0 °, and is less than 20% with respect to the area AT of the region that is less than 0.4 °. Since it is small, it has good fatigue resistance. The area BT is more preferably less than 15% with respect to the area AT.

  The measurement of the crystal orientation described above is based on a so-called KAM (Kernel Average Misorientation) value of EBSP (Electron Back Scattering Pattern) in 200 nm steps. Locations with an azimuth angle difference of 2 ° or more are omitted because they are grain boundaries. The KAM value varies greatly depending on the step interval for measuring EBSP, but as the step interval is shortened, the KAM value does not change gradually. In the rolled copper foil of the present invention, the KAM value is almost constant as long as it is 200 nm or less. For this reason, the KAM value measured at 200 nm steps can be used.

(Anode material for lithium ion secondary battery and lithium ion secondary battery)
The negative electrode material for a lithium ion secondary battery of the present invention includes a negative electrode current collector made of the above-described rolled copper foil of the present invention, and a negative electrode active material provided on the surface of the current collector. As the negative electrode active material, for example, a material mainly composed of carbon or the like can be used. As a method of providing the negative electrode active material on the surface of the negative electrode current collector, a known method can be used. For example, in addition to a method of applying and drying a slurry made of a mixture of a general active material, a binder and a solvent (adding a thickener in the case of an aqueous binder) on a current collector, sputtering, vapor deposition, CVD, etc. Various methods such as vacuum film formation techniques, chemical film formation techniques such as plating and sol-gel methods, and coating techniques such as ink jet, gravure printing, screen printing, and spin coating can be employed.
The lithium ion secondary battery of the present invention has an electrode group composed of a positive electrode, a negative electrode material of the present invention, and a separator formed by a porous polyethylene film or the like interposed therebetween, The group is impregnated with an electrolyte having lithium ion conductivity. The lithium ion secondary battery of the present invention can also be produced by a known method using the negative electrode material of the present invention.

  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.

(Examples 1-20, Comparative Examples 1-6)
An ingot produced by adding the elements shown in Table 1 to tough pitch copper or oxygen-free copper is processed into a 7 mm thick plate by hot rolling, and after removing the oxide by surface grinding, cold rolling, annealing, Pickling was repeated to a thickness of 1.0 mm. Thereafter, cold rolling to the thickness shown in Table 1 was performed by performing cold rolling under the conditions shown in Table 1 so that the average degree of processing in each pass was 10% or less. Moreover, when the workability of 1 pass exceeded 10% by cold rolling of 0.1 mm or less, it moved to the next pass after annealing at 80 to 120 ° C. after that pass. Here, “pass” refers to an operation in which a material is passed between a pair of rolls and finished to a desired thickness. For example, the “first pass” indicates the first operation in the operation of finishing to a desired thickness by the above operation.
Note that the average degree of processing of each pass was a value obtained by dividing the total degree of processing of each pass by the number of passes, as in the following equation.
Average machining degree of each pass (%) = (First pass machining degree (%) + Second pass machining degree (%) + ... + Final pass machining degree (%)) / (Pass number)
After finish rolling, the degreased copper foil was used as a negative electrode current collector. In Examples 15 to 18, the copper foil was annealed at 180 ° C. for 60 minutes in a non-oxidizing atmosphere and recrystallized in advance, and then used as a current collector.

[Area BT / Area AT]
For the obtained copper foil, after the preparation of the following negative electrode material, the portion where the active material was not applied was cut out, and the EBSP was taken using the analysis software manufactured by TSL using the electron microscope JEOL FE-SEM, and the KAM value was obtained. Was calculated. Thus, a range of 500 μm × 500 μm on the surface of the copper foil was measured, the above-described area AT and area BT on the measurement surface were obtained, and the ratio of the area BT to the area AT was calculated using them.

[Charge / discharge cycle life]
Using the obtained copper foil as a negative electrode current collector, 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 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. This was repeated twice per side of the copper foil to form negative electrode active material layers on both sides of the copper foil. Furthermore, an electrode having an active material density of 1.4 g / cm ³ and an active material thickness of 80 μm was formed by a pressure press. Further, the negative electrode material was dried at 200 ° C. for 30 minutes in a vacuum for the purpose of sufficiently evaporating moisture.
A slurry was prepared by dispersing lithium cobaltate (LiCoO2) 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. This was repeated twice per side of the aluminum foil to form positive electrode active material layers 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 connecting the electrode lead of the positive electrode to the lid of the battery case, a non-aqueous electrolyte solution in which ethylene carbonate and diethyl carbonate as a solvent was dissolved in a volume ratio of 2: 3 and 1 mol / L LiPF6 as an electrolyte was poured into the battery case. Then, 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 evaluation of the cycle life 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” indicate oxygen-free copper (JIS-C1020) and tough pitch copper (JIS-C1100), respectively. It shows what added ppm.

(Evaluation)
In each of Examples 1 to 14, 19, and 20, the area BT / area AT was less than 20%, and the cycle life of the battery was good.
Examples 15 to 18 use a copper foil that has been pre-recrystallized and annealed before the preparation of the negative electrode current collector, and in all cases, the area BT / area AT after the production of the negative electrode material is less than 20%, and the battery The cycle life was good.
In Comparative Examples 1 and 2, the average workability during rolling with a thickness of 0.1 mm or less exceeds 10%, and further, annealing is performed when the workability of one pass exceeds 10% during rolling. Therefore, the area BT / area AT exceeded 20%, and the cycle life of the battery was poor.
In Comparative Example 3, the average workability during rolling with a thickness of 0.1 mm or less is 10% or less, but annealing is not performed when the workability of one pass exceeds 10% during rolling. The area BT / area AT exceeded 20%, and the cycle life of the battery was poor.
In Comparative Example 4, annealing is performed when the degree of processing in one pass exceeds 10% during rolling, but the average degree of processing during rolling with a thickness of 0.1 mm or less exceeds 10%. The area BT / area AT exceeded 20%, and the cycle life of the battery was poor.
In Comparative Example 5, the Sn concentration exceeded 100 ppm by mass, and recrystallization was not sufficiently performed in the negative electrode preparation step. Therefore, the area BT / AT exceeded 20%, and the cycle life of the battery was poor.
In Comparative Example 6, since the Ag concentration exceeded 500 ppm by mass and the crystal was not sufficiently recrystallized in the negative electrode preparation process, the area BT / AT exceeded 20%, and the cycle life of the battery was poor.

Claims (8)

When heat-treated at 200 ° C. for 30 minutes, the crystal orientation obtained by irradiating the measurement point a of the crystalline structure of the crystal with an electron beam and a plurality of adjacent measurement points located 200 nm apart around the measurement point a Centering on the measurement point a where the average value of the azimuth angle difference with the crystal orientation obtained by irradiating the electron beam with less than 0.4 ° is the center, and the distance between the measurement point a and each side is 100 nm. The area of a regular hexagon is defined as area A, and the total of the areas A is defined as area AT.
The crystal orientation obtained by irradiating an electron beam to the measurement point b of the metallographic structure of the crystal and the electron beam irradiated to a plurality of adjacent measurement points located 200 nm apart around the measurement point b. An area of a regular hexagon centered on the measurement point b where the average value of the orientation angle difference with the crystal orientation is not less than 0.4 ° and less than 2.0 °, and the distance between the measurement point b and each side is 100 nm. Is area B, and the total of area B is area BT,
Area BT / Area AT x 100 (%) <20 (%)
The rolled copper foil for secondary battery negative electrode collectors which satisfy | fills.   The rolled copper foil for secondary battery negative electrode collectors of Claim 1 formed using the tough pitch copper specified to JIS-C1100, or the oxygen-free copper specified to JIS-C1020.   The rolled copper foil for secondary battery negative electrode collectors of Claim 2 containing 10-500 mass ppm of Ag.   Furthermore, the rolled copper foil for secondary battery negative electrode collectors of Claim 1 or 3 formed using the oxygen-free copper which contains Sn in 10-100 mass ppm and is standardized to JIS-C1020.   Furthermore, 0-0.01 mass% in total including 1 type, or 2 or more types selected from the group which consists of Fe, In, Mg, Zn, Si, Ni, Pb, Cr, and Zr is included. The rolled copper foil for secondary battery negative electrode collectors of description.   The rolled copper foil for a secondary battery negative electrode current collector according to any one of claims 1 to 5, having 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 1-6.   A lithium ion secondary battery comprising the negative electrode material according to claim 7.