Rolled copper foil for secondary battery, negative electrode for secondary battery using the same, and method for manufacturing secondary battery

Abstract

To provide a rolled copper foil for secondary batteries that achieves both of high strength and high conductivity.SOLUTION: A rolled copper foil for secondary batteries contains Mg of 0.25-1.0 wt.%, with the balance being Cu and inevitable impurities. The tensile strength TS in the rolling parallel direction satisfies the formula of TS(MPa)≥250×Mg(wt.%)+554. The conductivity EC satisfies the formula of EC(%IACS)≥-0.156×TS(MPa)+174. The thickness is 50 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a rolled copper foil for a secondary battery, a negative electrode for a secondary battery using the same, and a method for producing a secondary battery. [Background technology]

[0002] Secondary batteries, especially lithium-ion secondary batteries, are characterized by their high energy density and ability to produce relatively high voltages, and are widely used in small electronic devices such as laptops, video cameras, digital cameras, mobile phones, etc. Lithium-ion secondary batteries are also beginning to be used as power sources for large devices such as electric vehicles and distributed power sources in ordinary homes, and because they are lighter in weight and have a higher energy density than other secondary batteries, they are widely used in devices requiring various types of power sources.

[0003] The electrode assembly of a lithium-ion secondary battery generally has a wound structure or a stack structure in which each electrode is laminated. The positive electrode of a lithium-ion secondary battery generally consists of an aluminum foil current collector and a positive electrode active material made of a lithium composite oxide such as LiCoO2, LiNiO2, or LiMn2O4 provided on its surface, while the negative electrode generally consists of a copper foil current collector and a negative electrode active material made of carbon or the like provided on its surface. Conventionally, rolled copper foil called tough pitch copper with a copper content of 99.9% or electrolytic copper foil has been used as the current collector for the electrode (negative electrode) of a lithium-ion battery.

[0004] For example, Patent Document 1 (JP 2013-001982 A) discloses a rolled copper foil containing 0.10 to 0.30 wt% Mg, with the remainder consisting of unavoidable impurities and copper, which has a tensile strength TSA of 400 MPa or more after heat treatment for 30 minutes at 350° C. and an electrical conductivity of 65% IACS or more after heat treatment for 30 minutes at 350° C. It is disclosed that this rolled copper foil is excellent in both strength and elongation at break after heat treatment.

[0005] Furthermore, Patent Document 2 (JP 2017-179490 A) describes a material containing Mg in a range of 0.15 mass% or more and less than 0.35 mass%, with the remainder being Cu and unavoidable impurities, and having a conductivity exceeding 75% IACS and a low-angle grain boundary and subgrain boundary length ratio L LB / (L LB +L HB ) > 20% is satisfied. This invention makes it possible to provide copper alloys for electronic and electric devices, copper alloy plastically worked materials for electronic and electric devices, parts for electronic and electric devices, terminals, and bus bars that are excellent in electrical conductivity, strength, bending workability, and stress relaxation resistance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-001982 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-179490 Summary of the Invention [Problem to be solved by the invention]

[0007] The current collector is coated with an electrode active material, which expands and contracts during charge and discharge due to the migration of ions from the active material. This causes the current collector to be subjected to repeated stress with each charge and discharge. Therefore, partial fracture and peeling of the copper foil current collector leads to a shortened battery life. Meanwhile, in recent years, there has been a demand for higher capacity lithium-ion batteries, and the replacement of existing C-based active materials with Si-based active materials has been considered. Because Si-based active materials have a large volume change rate during charge and discharge, there is concern that the active material may peel off from the current collector after repeated cycles.

[0008] The invention described in Patent Document 1 achieved a rolled copper foil with increased strength and breaking elongation by adding 0.10 to 0.30 wt% of Mg, but if the concentration of Si-based active material is increased to achieve even higher capacity, the existing tensile strength and breaking elongation may become insufficient. A current collector copper foil with even higher strength that can accommodate high-concentration Si-based active material is needed.

[0009] On the other hand, the heat treatment temperature when applying active material to current collector foil has decreased with technological improvements, and the heat treatment temperature is approximately 150-200°C, especially when using a water-based binder to bind the active material and current collector foil (for Cu-Mg systems, the strength loss rate at heat treatment temperatures below 200°C is 5% or less). Therefore, it is necessary to focus on strength at room temperature rather than strength after heat treatment, and to develop copper foil for batteries that is even stronger than conventional ones.

[0010] Furthermore, when lithium-ion batteries are charged, Joule heat is generated due to internal resistance, which causes heat generation. If the amount of heat generated is large, it can deteriorate the battery's characteristics and, in some cases, cause serious accidents such as fire. Therefore, a current collector copper foil with low electrical resistance (high conductivity) is required to suppress the amount of heat generated. However, simply increasing the Mg concentration to increase strength will result in a decrease in conductivity. Therefore, it is necessary to increase strength without increasing the Mg concentration too much.

[0011] The present invention has been completed in view of the above problems, and an object of the present invention is to provide a rolled copper foil for a secondary battery that has both high strength and high electrical conductivity in one embodiment. An object of the present invention is to provide a method for manufacturing a secondary battery negative electrode and a secondary battery using such a rolled copper foil for a secondary battery in another embodiment. [Means for solving the problem]

[0012] As a result of extensive research, the present inventors have found that by devising a manufacturing process for rolled copper foil for secondary batteries, it is possible to obtain rolled copper foil for secondary batteries having higher strength than that of conventional techniques, even with the same Mg concentration. That is, it has been possible to increase the strength of the rolled copper foil for secondary batteries without reducing the electrical conductivity. The present invention has been completed based on the above findings, and is exemplified below.

[0013] [1] A rolled copper foil for a secondary battery containing 0.25 to 1.0 wt % of Mg, with the remainder being Cu and unavoidable impurities, A rolled copper foil for secondary batteries having a thickness of 50 μm or less, wherein the tensile strength TS in the direction parallel to the rolling direction satisfies the formula TS (MPa) ≥ 250 × Mg (wt%) + 554, the electrical conductivity EC satisfies the formula EC (%IACS) ≥ -0.156 × TS (MPa) + 174. [2] The rolled copper foil for secondary batteries according to [1], which has a tensile strength TS in a direction parallel to the rolling of 640 MPa or more. [3] The rolled copper foil for a secondary battery according to [1] or [2], which has an electrical conductivity EC of 50% IACS or more. [4] The rolled copper foil for a secondary battery according to any one of [1] to [3], which contains 0.4 to 0.6 wt % of Mg and has a tensile strength TS of 700 MPa or more in the direction parallel to the rolling. [5] The rolled copper foil for a secondary battery according to any one of [1] to [4], containing 0.0001 to 0.005% by weight of P. [6] A method for producing a secondary battery negative electrode using the rolled copper foil for a secondary battery according to any one of [1] to [5] as a raw material for a current collector. [7] A method for producing a secondary battery using the rolled copper foil for a secondary battery according to any one of [1] to [5] as a raw material for a current collector. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a rolled copper foil for a secondary battery that has both high strength and high conductivity, and a method for producing a secondary battery negative electrode and a secondary battery using such a rolled copper foil for a secondary battery. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a graph showing the relationship between Mg concentration and tensile strength TS in the direction parallel to rolling in examples of the present invention and comparative examples. [Figure 2] 1 is a graph showing the relationship between electrical conductivity EC and tensile strength TS in the direction parallel to rolling in examples of the present invention and comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[0016] Next, embodiments of the present invention will be described in detail with reference to the drawings. It should be understood that the present invention is not limited to the following embodiments, and that appropriate design changes and improvements may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0017] (Composition of rolled copper foil for secondary batteries) The material for the rolled copper foil for secondary batteries of the present invention is preferably tough pitch copper as specified in JIS-H3100-C1100 (2018) or oxygen-free copper as specified in JIS-H3100-C1020 (2018). Because their compositions are close to those of pure copper, the conductivity of the copper foil is not reduced, making them suitable for current collectors. The oxygen concentration in the copper foil is 0.05% by weight (i.e., 500 ppm by weight) or less for tough pitch copper and 0.001% by weight (i.e., 10 ppm by weight) or less for oxygen-free copper.

[0018] The rolled copper foil for secondary batteries of this embodiment contains 0.25 to 1.0 wt% Mg. If the Mg content is less than 0.25 wt%, the tensile strength will decrease significantly. From this viewpoint, the Mg content is more preferably 0.3 wt% or more, and even more preferably 0.4 wt% or more.

[0019] If the Mg content exceeds 1.0 wt%, the electrical conductivity decreases significantly. From this viewpoint, the Mg content is preferably 0.9 wt% or less, more preferably 0.8 wt% or less, even more preferably 0.7 wt% or less, and even more preferably 0.6 wt% or less.

[0020] The composition of the rolled copper foil for secondary batteries of this embodiment can be measured by X-ray fluorescence analysis. Specifically, X-ray fluorescence analysis is performed using a Simultix14 manufactured by Rigaku Corporation. The analysis surface may be cut or mechanically polished so that the maximum surface roughness Rz (JIS-B0601 (2013)) is 6.3 μm or less. When an analysis sample is collected from the molten metal during melting and casting, the foil is cast into a shape of about 30 to 40 mm in diameter and about 50 to 80 mm in thickness, and then cut into a thickness of about 10 to 20 mm, and the cut surface is used as the analysis surface. The analysis surface is repeatedly cut or mechanically polished until the maximum surface roughness Rz (JIS-B0601 (2013)) is 6.3 μm or less. The composition of the rolled copper foil for secondary batteries can be measured by ICP optical emission spectroscopy as a wet analysis in addition to X-ray fluorescence analysis. Specifically, the measurement can be performed using an ICP optical emission spectroscopy analyzer (ICP-OES) SPS3100 manufactured by Hitachi High-Tech Science Corporation. In the case of ICP optical emission spectroscopy, the sample is dissolved in a mixed acid of hydrochloric acid and nitric acid (2 parts hydrochloric acid, 1 part nitric acid, 2 parts water) and then diluted.

[0021] The rolled copper foil for secondary batteries according to the present invention is made of industrially used copper and contains unavoidable impurities. Even trace amounts of these unavoidable impurities, such as Fe, Zr, S, Ge, and Ti, are undesirable because they tend to rotate the crystal orientation and create shear bands when the copper foil is bent, making the current collector prone to cracking and breakage when repeatedly bent. Therefore, it is preferable that the copper foil according to the present invention contains one or more unavoidable impurities selected from the group consisting of Fe, Zr, S, Ge, and Ti in a total content of 0.002 wt % or less.

[0022] The rolled copper foil for a secondary battery of this embodiment may contain 0.0001 to 0.005 wt % of P. If oxygen is contained in copper, it will react with hydrogen during heat treatment at high temperatures, making it more likely to cause hydrogen embrittlement. By adding P, P will react preferentially with oxygen and can remove oxygen from the copper. If the P content exceeds 0.005 wt %, it may cause a decrease in electrical conductivity, so the content is preferably 0.005 wt % or less.

[0023] In this specification, when the term "copper foil" is used alone, it also includes copper alloy foil, and when the term "tough pitch copper and oxygen-free copper" is used alone, it also includes copper alloy foil based on tough pitch copper and oxygen-free copper.

[0024] (Tensile strength of rolled copper foil for secondary batteries) In one embodiment, the rolled copper foil for secondary batteries of the present invention has a tensile strength (TS) in the direction parallel to the rolling direction that satisfies the formula: TS (MPa) ≥ 250 × Mg (wt%) + 554. If the tensile strength (TS) in the direction parallel to the rolling direction is too low, when the rolled copper foil is used as a current collector of a battery, the current collector is prone to fracture when repeatedly subjected to loads due to expansion and contraction of the active material during charge and discharge. Increasing the Mg concentration increases the tensile strength (TS) in the direction parallel to the rolling direction and decreases the electrical conductivity. However, in lithium-ion batteries (especially lithium-ion batteries using Si-based active materials), in order to achieve both high strength that can withstand the expansion of the active material and high electrical conductivity that suppresses heat generation, the practical relationship between the tensile strength (TS) in the direction parallel to the rolling direction and the Mg concentration must be TS (MPa) ≥ 250 × Mg (wt%) + 554. It is not necessary to set an upper limit for TS (MPa), but it is common for the relationship to be, for example, TS (MPa) ≤ 500 × Mg (wt%) + 654.

[0025] In another embodiment, the rolled copper foil for secondary batteries of the present invention has a tensile strength TS of 640 MPa or more in the direction parallel to the rolling. If the tensile strength TS of 640 MPa or more in the direction parallel to the rolling is high enough to withstand the expansion of the active material, the tensile strength TS of the rolled copper foil for secondary batteries is preferably 700 MPa or more in the direction parallel to the rolling. This is expected to further expand the applications of the rolled copper foil for secondary batteries.

[0026] The tensile strength TS in the direction parallel to the rolling direction means the value measured at room temperature (23°C) in a tensile strength test conducted in the direction parallel to the rolling direction according to JIS-Z2241 (2011) or IPC-TM-650 Test Method 2.4.18 (2012).

[0027] (Conductivity of rolled copper foil for secondary batteries) The electrical conductivity EC (%IACS (International Annealed Copper Standard)) of the rolled copper foil for secondary batteries of this embodiment satisfies the formula EC (%IACS) ≧ −0.156 × TS (MPa) + 174. The addition of Mg improves tensile strength, but generally, the higher the tensile strength, the lower the electrical conductivity. The rolled copper foil for secondary batteries of this embodiment can achieve both high strength and high electrical conductivity at a high level by performing a predetermined recrystallization annealing as described below. It is not necessary to set an upper limit for the electrical conductivity EC, but it is usually EC (%IACS) ≦ −0.05 × TS (MPa) + 125, for example.

[0028] Furthermore, the electrical conductivity EC of the rolled copper foil for secondary batteries of this embodiment is preferably 50% IACS or more. This allows the rolled copper foil for secondary batteries to be effectively used as an electronic material. The electrical conductivity EC of the rolled copper foil for secondary batteries is more preferably 55% IACS or more, and even more preferably 60% IACS or more. The electrical conductivity can be measured in accordance with JIS-H0505 (2018).

[0029] (Thickness of rolled copper foil for secondary batteries) The rolled copper foil for secondary batteries of this embodiment has a thickness of 50 μm or less. By making the thickness 50 μm or less, the energy density per unit weight of the battery can be increased. From this viewpoint, the thickness of the rolled copper foil for secondary batteries is preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. There is no particular lower limit to the thickness of the rolled copper foil for secondary batteries, but by making it 5 μm or more, for example, the handleability can be improved.

[0030] (Method of manufacturing rolled copper foil for secondary batteries) Although the method for producing the rolled copper foil for secondary batteries of this embodiment is not particularly limited, the rolled copper foil is generally produced by casting an ingot, hot rolling, appropriately repeating annealing and cold rolling, and finally cold rolling. Pickling may be performed between or during each step as appropriate.

[0031] Adjusting the average crystal grain size to 5.0 μm or less before the final cold rolling step is advantageous for achieving both high strength and high electrical conductivity in the rolled copper foil for secondary batteries. This is because grain refinement strengthening increases strength while maintaining electrical conductivity. More specifically, by performing recrystallization annealing at 450 to 550°C for 10 minutes to 1 hour before the final cold rolling step, it is possible to adjust the average crystal grain size to 5.0 μm or less before the final cold rolling step.

[0032] (Secondary battery negative electrode and secondary battery) The rolled copper foil for secondary batteries of this embodiment can be suitably used as a current collector in a secondary battery negative electrode. Therefore, in another aspect, the present invention is a secondary battery negative electrode or a secondary battery including the rolled copper foil for secondary batteries of the present invention. Furthermore, in another aspect, the present invention is a method for producing a secondary battery negative electrode or a secondary battery using the rolled copper foil for secondary batteries of the present invention as a raw material for a current collector. [Example]

[0033] The present invention will be specifically described below with reference to examples, but the description here is for the purpose of illustration only and is not intended to be limiting.

[0034] (Examples 1-2, Comparative Examples 1-3) Copper ingots with the Mg content listed in Table 1 (the remainder being copper and unavoidable impurities) were melted and hot-rolled to a thickness of 9 mm to obtain sheets. Subsequently, cold rolling, recrystallization annealing, and final cold rolling were performed to produce copper foils. The Mg content was measured by the above-mentioned ICP atomic emission spectroscopy. The manufacturing process was the same for each example and comparative example, except for the recrystallization annealing conditions. The average grain size before the final cold rolling was measured using EBSD (JSM-IT500HR) under conditions of 300 × 300 μm, step size of 0.2 μm, and calculated using OIM analysis. The average grain size was calculated using the line method (intercept line orientation: horizontal, number of lines: 96) under conditions of grain tolerance: 0.5 after cleanup of the measurement data under conditions of grain tolerance: 0.5, minimum grain size: 10.

[0035] The test pieces thus obtained were subjected to the following characteristic evaluations, the results of which are shown in Table 1.

[0036] <Tensile strength> Based on JIS-Z2241 (2011), a No. 13B test piece (gauge length 50 mm, width direction 12.5 mm) was prepared and a tensile test was performed parallel to the rolling direction using a tensile testing machine (AutoCom C-type universal testing machine AC-100kN-C, manufactured by TSE) to measure the tensile strength. For copper foil with a thickness of 35 μm or less, it is recommended to perform a tensile test based on IPC-TM-650 Test Method 2.4.18 (2012).

[0037] <Conductivity> Test specimens were taken so that the longitudinal direction of the test specimen was parallel to the rolling direction, and electrical conductivity (EC: %IACS) was measured using the four-terminal method in accordance with JIS-H0505 (2018).

[0038] (Comparative Examples 4 to 7) Comparative Examples 4 to 7 shown in Table 1 are cited from JP 2013-001982 A. JP 2013-001982 A describes that a copper ingot having the Mg content shown in Table 1 (the remainder being copper, 12 to 18 wtppm of oxygen, and unavoidable impurities) was manufactured, hot-rolled to a thickness of 10 mm, then faced, pre-annealed at a predetermined reduction ratio, recrystallization annealed at 450°C, and then a final cold-rolling step was carried out. It also describes that for each test example, a copper foil having the thickness shown in Table 1 was obtained, and that the tensile strength and electrical conductivity shown in Table 1 were obtained. Since the average crystal grain size before the final cold-rolling step was unknown, it is indicated in Table 1 as " / ".

[0039] [Table 1]

[0040] (Consideration) As can be seen from Table 1, by setting the average grain size before final cold rolling to 5.0 μm or less, the Examples were found to have higher tensile strength than the Comparative Examples at the same Mg concentration level (Fig. 1). It was also found that the tensile strength improved while maintaining high electrical conductivity (Fig. 2).

[0041] It is presumed that in Comparative Examples 1 to 3, the average crystal grain size before the final cold rolling exceeded 5.0 μm, and therefore sufficient tensile strength was not obtained.

Claims

1. A rolled copper foil for a secondary battery containing 0.25 to 1.0 wt % of Mg, with the remainder being Cu and unavoidable impurities, A rolled copper foil for a secondary battery, having a tensile strength TS in a direction parallel to the rolling satisfying the formula TS (MPa) ≥ 250 × Mg (wt%) + 554, an electrical conductivity EC satisfying the formula EC (% IACS) ≥ -0.156 × TS (MPa) + 174, and a thickness of 50 μm or less.

2. The rolled copper foil for a secondary battery according to claim 1, wherein the tensile strength TS in the direction parallel to the rolling is 640 MPa or more.

3. The rolled copper foil for a secondary battery according to claim 1 or 2, having an electrical conductivity EC of 50% IACS or more.

4. 3. The rolled copper foil for secondary batteries according to claim 1, which contains 0.4 to 0.6 wt % of Mg and has a tensile strength TS of 700 MPa or more in a direction parallel to the rolling direction.

5. The rolled copper foil for a secondary battery according to claim 1 or 2, containing 0.0001 to 0.005% by weight of P.

6. A method for producing a secondary battery negative electrode using the rolled copper foil for secondary batteries according to claim 1 or 2 as a raw material for a current collector.

7. A method for producing a secondary battery using the rolled copper foil for secondary batteries according to claim 1 or 2 as a raw material for a current collector.

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