JP5496139B2 - Copper foil and secondary battery using the same - Google Patents

Description

  The present invention relates to a copper foil suitable as a current collector of a secondary battery such as a lithium ion secondary battery and a secondary battery using the copper foil.

  Lithium ion secondary batteries have a high energy density and can obtain a relatively high voltage, and are widely used for 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, especially in a battery having a structure in which an electrode body is wound, the current collector is easily cracked or easily broken due to expansion and contraction of the electrode accompanying charging and discharging. For this reason, the method of preventing a fracture | rupture is disclosed by adjusting elongation of the copper foil which is a negative electrode collector to 2 to 15% (patent document 1).

JP 2000-208149 A

However, when the present inventors examined, even if it used copper foil with large elongation for the negative electrode collector of a battery, it became clear that a collector and a crack may generate | occur | produce by charging / discharging.
That is, an object of the present invention is to provide a copper foil that prevents the occurrence of cracks and breaks due to charge and discharge when used as a current collector of a secondary battery, and a secondary battery using the copper foil.

The present inventor has found that the work hardening index (n value) of the copper foil affects the charge / discharge cycle life of the secondary battery.
That is, the present invention
(1) a thickness of 5 to 30 [mu] m, work hardening index at 0.3 to 0.45, and I (220) / I (200) is Ri der 0.11, the tough pitch copper or JIS-C1020 to standards JIS-C1100 Rudohaku such from oxygen-free copper to standard,
(2) and a thickness 5 to 30 [mu] m, after 0.5 hours annealing at 350 ° C., work hardening coefficient becomes 0.3 to 0.45, and I (220) / I (200) Ri is Do 0.11 or less, standards JIS-C1100 Rudohaku such oxygen-free copper to standards tough pitch copper or JIS-C1020 to,
(3) The copper foil according to (1) or (2) , wherein the semi-softening temperature is 150 ° C. or lower,
(4) the A g 500 ppm by mass or less, and / or contain Sn than 100 mass ppm, and copper according to any one of the total amount of these is less 500 ppm by weight (1) ~ (3) Foil,
(5) Any one of (1) to ( 4 ), wherein the total degree of work at the time of final cold rolling is 85% or more, and the oil film equivalent in the final three passes in the final cold rolling is rolled under the following conditions: Crab copper foil,
Oil film equivalent before the final pass; 25000 or less, oil film equivalent before the final pass; 30000 or less, oil film equivalent of the final pass; 35000 or less
(6) A secondary battery using the copper foil according to any one of (1) to ( 5 ) as a current collector,
It is.

  ADVANTAGE OF THE INVENTION According to this invention, when it uses for the collector of a secondary battery, the copper foil which prevented generation | occurrence | production of the crack by a charging / discharging and a fracture | rupture is obtained.

  Hereinafter, the copper foil which concerns on embodiment of this invention is demonstrated. Unless otherwise specified,% represents mass%.

(Work hardening index (n value))
The copper foil of the present invention has a work hardening index of 0.3 or more and 0.45 or less.
When copper foil is used as a current collector for a secondary battery, the work hardening index (n value) is not the elongation of the copper foil but the charge / discharge cycle life of the secondary battery (the current collector breaks or cracks due to charge / discharge) ) Is considered as follows.
First, particularly in a battery (such as a cylindrical battery) having a structure in which an electrode body is wound, a copper foil, which is a negative electrode current collector constituting the electrode body, is wound and bent and held in the battery. When the active material layer on the current collector is repeatedly expanded and contracted by charging and discharging, the copper foil as the current collector is also deformed. Is likely to occur.
Here, the work hardening index is one of the values indicating the work hardening behavior of the material, and the larger the value, the easier the material is to work harden. When the material is subjected to tensile deformation, the material is locally constricted and fractured. However, in a material having a large work hardening coefficient, the constricted portion is work hardened and the constricted portion is hardly deformed. Therefore, instead of the constricted portion that is difficult to deform, other portions begin to deform. By repeating this, the entire material is uniformly deformed. On the other hand, elongation is an index that is captured macroscopically without taking such circumstances into consideration. Therefore, even if the elongation is large, the work hardening index is not always large, and whether the bent portion of the current collector is uniformly deformed. It is not an indicator of whether or not.

Then, the present inventors succeeded in improving the charge / discharge cycle life of the secondary battery by defining the work hardening index (n value) of the copper foil.
The work hardening index (n value) is represented by an index n when the relationship between stress and strain in the plastic deformation region above the yield point is approximated by the following formula 1 (Hollomon formula).
[True stress] = [material constant] × [true strain] n (1)
As the work hardening index is larger, local deformation is less likely to occur and is less likely to break when deformed.

  When the copper foil has a work hardening index of 0.3 or more, when the copper foil is used as a current collector and the charge / discharge cycle is repeated, local deformation of the copper foil hardly occurs, and the entire bent portion is not deformed. Because it bears, copper foil is hard to break. However, a material having a work hardening index exceeding 0.45 is not preferable because the strength after annealing is low and the handleability deteriorates.

(Thickness)
Conventionally, there is an example in which a work hardening index, which is an index of the ease of uniform deformation of the entire material, is used in drawing a thick material such as a structural member, but it is as thin as a copper foil. Since the material is not subjected to processing such as drawing, the work hardening index was not used as an index.
For this reason, the thickness of the copper foil of the present invention is specified to be 5 to 30 μm.

  Also, when the copper foil thickness is relatively thin, 5-30 μm, the ratio of crystal grains facing the material surface increases, so dislocations introduced by deformation do not accumulate at the crystal grain boundaries and are released from the material surface. The ratio becomes higher. For this reason, the work hardening index of copper foil becomes low compared with a comparatively thick material. On the other hand, the work hardening index is determined by the amount of dislocations introduced into the material by deformation and the ease of movement of dislocations. That is, the work hardening index increases if there are precipitates that can cause dislocation loops, solid solution elements that hinder the movement of dislocations, and grain boundaries. However, the addition of a solid solution element that greatly hinders the movement of dislocations or an alloy element that can generate a precipitate causes a decrease in electrical conductivity, which is not preferable as a battery current collector.

(composition)
Therefore, the copper foil of the present invention preferably has a composition of tough pitch copper standardized to JIS-C1100 or oxygen-free copper standardized to JIS-C1020. If it is set as the composition close | similar to the above-mentioned pure copper, the electrical conductivity of copper foil will not fall and it is suitable for a battery electrical power collector. The oxygen concentration contained in the copper foil is 0.01 to 0.05% by mass in the case of tough pitch copper, and 0.001% by mass or less in the case of oxygen-free copper.
Furthermore, it is good also as Ag containing 500 mass ppm or less and / or 100 mass ppm or less of Sn, and making these total addition amount into 500 mass ppm or less. 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. If the total amount of Ag and Sn added to the copper foil exceeds 500 ppm by mass, the conductivity will decrease and the recrystallization temperature will rise, making it difficult to recrystallize while suppressing the surface oxidation of the copper foil in the final annealing. It may become. In addition, although the minimum in particular of the total addition amount of Ag and Sn is not prescribed | regulated, it is usually 10 mass ppm or more in total.

As a method of adjusting the work hardening index of the copper foil of the present invention to the above range, the total degree of work at the time of final cold rolling is 85% or more, and the oil film equivalent in the final three passes in the final cold rolling is a predetermined value. An example of the condition is rolling.
As described above, it is not preferable to increase the work hardening index using a large amount of additive elements in terms of lowering the conductivity. Therefore, by controlling the angle of the sliding surface with respect to the deformation direction regardless of the additive elements. The present inventors have found that the work hardening index can be increased.
Here, the material undergoes slip deformation due to the stress applied by the deformation, and at this time, work hardening increases due to the action of a plurality of equivalent slip surfaces. By utilizing this fact, the work hardening index can be increased without impairing electrical conductivity.
That is, specifically, the oil film equivalent in the final three passes in the final cold rolling of the copper foil is rolled under the following conditions.
Oil film equivalent before the final pass; 25000 or less, oil film equivalent before the final pass; 30000 or less, oil film equivalent of the final pass; 35000 or less

Since the oil film equivalent tends to increase as the material thickness decreases, the oil film equivalent value in the final three passes gradually increases. Therefore, it is necessary to set the oil film equivalent in the above range for the last three passes having different thicknesses.
For example, if the rolling oil viscosity and the material yield stress are equal in all passes in the final cold rolling, the oil film equivalent is proportional to (rolling speed) / (engagement angle). And, since the biting angle becomes smaller as the material thickness becomes thinner, the oil film equivalent tends to increase as it approaches the final pass. In order to maintain productivity, it is necessary to increase the rolling speed as the material passes closer to the final pass, and the oil film equivalent tends to increase as the material approaches the final pass. In this case, if the oil film equivalent in the second pass and the previous pass in the final cold rolling is greater than 35000, sufficient effects can be obtained even if only the oil film equivalent is kept low in the final pass. Can not. For this reason, the oil film equivalent in the final three passes in the final cold rolling is controlled within the above range.
Furthermore, in order to reduce the oil film equivalent, the rolling degree of the final pass in the final cold rolling is preferably 25% or more.

The oil film equivalent is represented by the following formula.
(Oil film equivalent) = {(rolling oil viscosity, kinematic viscosity at 40 ° C .; cSt) × (rolling speed; m / min)} / {(yield stress of material; kg / mm ² ) × (roll biting angle; rad )}
The rolling oil viscosity is about 4.0 to 8.0 cSt, the rolling speed is 200 to 600 m / min, and the biting angle of the roll is, for example, 0.0005 to 0.005 rad, preferably 0.001 to 0.04 rad.

(I (220) / I (200))
In the copper foil according to the embodiment of the present invention, when the X-ray diffraction of the rolled surface was performed, the ratio I (220) / I (200) of the integral value (I) of the intensity of the (220) plane and (200) plane, respectively. ) Is 0.11 or less.
The recrystallized texture of pure copper type copper is characterized in that the (200) plane faces in the surface direction, and at this time, the (200) plane also faces in the rolling direction and the direction perpendicular to the rolling. In general, when bending copper foil, the bending axis is taken in the rolling direction (RD) or the perpendicular direction to rolling (TD). At this time, the deformation on the copper foil causes the {111} surface, which is the main sliding surface of copper, to undergo multiple sliding. Resulting in a high work hardening index. On the other hand, when the degree of orientation of the (220) plane, which is a crystal orientation other than the (200) plane, increases, sufficient multiple slip does not occur and a high work hardening index cannot be obtained. From the above, the degree of orientation of the (200) plane is high and the degree of orientation of the (220) plane is low, that is, the ratio I (220) / I (200) is preferably low, and the ratio is 0.11 or less. Preferably there is.
Here, since the recrystallized texture of pure copper type copper has the {001} direction in each of the ND direction (rolling surface normal direction), the RD direction, and the TD direction, Instead of the direction, the (200) plane diffraction intensity in the ND direction, which is easy to measure, is used as an index.

Moreover, in order to adjust the work hardening index to a specified range, it is necessary to obtain a recrystallized structure sufficiently by annealing. When recrystallization is insufficient, it is difficult to adjust the work hardening index within the above range, and therefore, the semi-softening temperature of the copper foil is preferably 150 ° C. or lower. In order to sufficiently obtain a recrystallized structure, it is necessary to adjust the composition and processing degree of the copper foil and appropriately control the recrystallization temperature, but if the copper foil composition and the processing degree are defined in the above range, The recrystallization temperature is about 130 to 200 ° C., and the semi-softening temperature can be 150 ° C. or less.
Here, the non-recrystallized structure has a work strain remaining, and since it has already been work hardened, it is difficult to work harden by bending deformation, the work hardening index becomes small, and the bendability deteriorates. In order to increase the work hardening index, it is necessary that the state of drying and heating after applying the active material with the copper foil as the current collector is not work hardened, that is, the state where the work strain is removed. In other words, the copper foil undergoes a heat treatment during the application and drying process of the active material, but the strain is removed by the heat treatment and recrystallization is required. And if the semi-softening temperature of copper foil is 150 degrees C or less, recrystallization of copper foil can be anticipated also by the heat processing received to the extent of application | coating of an active material and a drying process, and a work hardening index | exponent can be enlarged.

In addition, as the copper foil of the present invention, the work hardening index and I (220) / I (200) that have already been heat-treated and the like are in the above range, and the work hardening index and I after annealing at 350 ° C. for 0.5 hour. Also included is an unrecrystallized copper foil in which (220) / I (200) is in the above range. Here, annealing at 350 ° C. for 0.5 hours imitates the heating in the drying process after the application of the active material to the copper foil. In particular, an alloy material that has a large expansion and contraction due to charge and discharge as the active material is used. This is because polyimide is generally used as a binder when used, and the heating conditions after applying an active material to the copper foil current collector at this time are simulated. The annealing is preferably performed in a non-oxidizing atmosphere.
Further, the work hardening index after annealing at 250 ° C. for 0.5 hour is preferably 0.3 or more and 0.45 or less. This is because when a graphite-based active material is used as the active material, the heating temperature after applying the active material to the copper foil current collector is generally about 250 ° C. or less.
Here, when the copper foil is recrystallized by heating, the work hardening index increases. Therefore, if the work hardening index is 0.3 or more at 250 ° C. lower than 350 ° C., the work hardening index is 0.3 or more even at 350 ° C.

The copper foil of the present invention is preferably used for a current collector (particularly a negative electrode current collector) of a secondary battery.
Although it does not specifically limit as a secondary battery to which the copper foil of this invention is applied, Preferably a lithium ion secondary battery can be used. The lithium ion secondary battery includes a battery that uses metallic lithium for the negative electrode, and a battery that does not contain metallic lithium in the battery and that is responsible for electrical conduction by lithium ions in the electrolyte. The negative electrode active material of the lithium ion secondary battery is not limited, but carbon, silicon, tin, germanium, lead, antimony, aluminum, indium, lithium, silicon oxide, tin oxide, lithium titanate, lithium nitride, indium Or a combination of two or more of them.
Examples of the secondary battery include a structure in which an electrode body (a structure in which a negative electrode and a positive electrode are sandwiched between separators) is wound, or a stack structure in which each electrode body is stacked.

  The copper foil of the present invention is obtained by, for example, adding an alloying element to electrolytic copper as necessary, casting an ingot (usually 100 to 300 mm in thickness), and hot rolling the ingot (usually after hot rolling) 5 to 20 mm), and cold rolling and annealing are repeated, and the final cold rolling is finished to a predetermined thickness.

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.
Example 1
[Manufacture of copper foil]
Oxygen-free copper (JIS-C1020: indicated as “OFC” in Table 1) or tough pitch copper (JIS-C1100: indicated as “TPC” in Table 1) is dissolved, and Ag or Sn is indicated in Table 1 as necessary. An ingot having a thickness of 20 mm and a width of 60 mm was cast by casting the amount, and rolled to 10 mm by hot rolling.
Next, annealing and cold rolling were repeated and rolled to the thickness shown in Table 1 to obtain a copper foil. In order to adjust the softening temperature, the total workability during final cold rolling should be 85% or more, and in order to reduce the surface roughness, a roll with a smooth surface (Ra ≦ 0.1μm in the roll axis direction) should be used. Used for final cold rolling. The final cold rolling oil viscosity is about 4.0 to 8.0 cSt, the rolling speed is adjusted to 200 to 600 m / min, and the roll bite angle is 0.003 to 0.03 rad, and the oil film in the final three passes in the final cold rolling. The equivalent oil film equivalent was 25000 or less before the final pass; 25000 or less, the oil film equivalent immediately before the final pass; 30000 or less, the oil film equivalent of the final pass; 35000 or less.

<Work hardening index>
The obtained copper foil was annealed in a non-oxidizing atmosphere at 250 ° C. for 0.5 hour and 350 ° C. for 0.5 hour, respectively, and then subjected to a tensile test (based on JIS-Z2241) to obtain a work hardening index. Note that the work hardening index needs to be obtained using the uniform elongation and stress after the material yields, so the value from the elongation of 2% to the maximum stress point was used. Then, a logarithmic graph of true strain and true stress obtained from the measured elongation and stress was approximated by the method of least squares, and a work hardening index was obtained from the slope of the graph. True strain and true stress were determined by the following formulas.
[True strain] ＝ ln (1+ [Strain])
[True stress] = (1+ [true strain]) x [stress]

<Semi-softening temperature>
The obtained copper foil was annealed in a non-oxidizing atmosphere at 100 to 400 ° C. for 0.5 hour, respectively, and then a tensile test was performed to determine the strength (tensile strength) against the heat treatment conditions. The annealing temperature at which the strength after annealing was an intermediate value between the strength after rolling (before annealing) and the strength after being completely softened (annealed at 400 ° C. for 30 minutes) was defined as the semi-softening temperature.

<I (220) / I (200)>
The obtained copper foil was annealed in a non-oxidizing atmosphere at 350 ° C. for 0.5 hours and 250 ° C. for 0.5 hours, and then subjected to X-ray diffraction on the rolled surface, and the (220) plane and (200) plane, respectively. The integral value (I) of the diffraction peak intensity was determined.

[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.
(1) 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.
(2) 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. This was repeated twice per side of the aluminum foil to form positive electrode active material layers on both sides of the aluminum foil. Furthermore, 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.
(3) It wound with the separator which consists of a 20-micrometer-thick porous polyethylene film interposed between the positive electrode produced as mentioned above, and the negative electrode, and accommodated in the battery case.
(4) 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 are dissolved in a volume ratio of 2: 3 as a solvent and 1 mol / L LiPF6 is dissolved as an electrolyte is used in the battery case. The solution was poured into the inside, and the lid of the battery can was crimped and sealed to produce a cylindrical lithium ion secondary battery.

With respect to the produced 18650 size cylindrical battery, the cycle of charging and discharging was repeated 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 charge / discharge conditions were a constant current of 1A and a constant current of 4.2V until the charging time was 2 hours, and a constant current of 1A and a discharge of 3.0V.

  The obtained results are shown in Table 1. In the composition of Table 1, OFC and TPC indicate oxygen-free copper and tough pitch copper (JIS H3100), respectively, and Ag100 ppm TPC indicates that 100 mass ppm of Ag is added to tough pitch copper.

  As is clear from Table 1, in the case of Examples 1 to 14 where the work hardening index after annealing at 350 ° C. for 0.5 hour is 0.3 or more and I (220) / I (200) is 0.11 or less, the charge / discharge cycle life It was excellent. In Examples 15 to 17, before the negative electrode slurry was applied to the current collector, it was previously annealed at 250 ° C. for 30 minutes and recrystallized, and the work hardening index was 0.3 or more, and I (220 ) / I (200) is 0.11 or less, the charge / discharge cycle life is excellent.

On the other hand, in Comparative Examples 2, 3, and 4 in which the total degree of work during the final cold rolling was less than 85%, the work hardening index after annealing at 350 ° C. for 0.5 hour was less than 0.3, and the charge / discharge cycle life was deteriorated. .
Further, in the case of Comparative Example 1 in which the amount of Sn added in the copper foil exceeded 100 mass ppm, the work hardening index after annealing at 350 ° C. for 0.5 hour was less than 0.3, and the charge / discharge cycle life was deteriorated. This is presumably because the semi-softening temperature exceeded 150 ° C.
As the oil film equivalent in the final three passes in the final cold rolling, in the case of Comparative Example 5 in which the oil film equivalent of the previous one of the final pass exceeds 30000 and the oil film equivalent of the final pass exceeds 35000, the work hardening index is 0.45. When used for the negative electrode current collector, wrinkles were generated in the copper foil, and the active material could not be applied, and the charge / discharge cycle life test could not be performed.

Claims (6)

And a thickness of 5 to 30 [mu] m, work hardening index at 0.3 to 0.45, and I (220) / I (200 ) is Ri der 0.11 or less,
Rudohaku such oxygen-free copper to standards tough pitch copper or JIS-C1020 standards to JIS-C1100. And a thickness of 5 to 30 [mu] m, after 0.5 hours annealing at 350 ° C., work hardening coefficient becomes 0.3 to 0.45, and I (220) / I (200 ) Ri is Do 0.11 or less,
Rudohaku such oxygen-free copper to standards tough pitch copper or JIS-C1020 standards to JIS-C1100. The copper foil according to claim 1 or 2, wherein the semi-softening temperature is 150 ° C or lower. Furthermore, the copper foil in any one of Claims 1-3 which contains Ag 500 mass ppm or less and / or Sn 100 mass ppm or less, and makes these total addition amounts 500 mass ppm or less .   The copper according to any one of claims 1 to 4, wherein the total degree of work at the time of final cold rolling is 85% or more, and the oil film equivalent in the final three passes in the final cold rolling is rolled under the following conditions. Foil.
    Oil film equivalent before the final pass; 25000 or less, oil film equivalent before the final pass; 30000 or less, oil film equivalent of the final pass; 35000 or less   A secondary battery using the copper foil according to claim 1 as a current collector.