JP7805250B2 - Rolled copper foil for secondary battery, negative electrode for secondary battery using the same, and method for manufacturing secondary battery - Google Patents

Description

The present invention relates to rolled copper foil for secondary batteries, as well as a secondary battery negative electrode and a method for manufacturing a secondary battery using the same.

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, and mobile phones. Lithium-ion secondary batteries are also beginning to be used as power sources for larger devices such as electric vehicles and distributed power sources in ordinary homes. Because they are lighter and have a higher energy density than other secondary batteries, they are widely used in devices that require a variety of power sources.

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 comprises 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 comprises 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.

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 at 350°C for 30 minutes, and an electrical conductivity of 65% IACS or more after heat treatment at 350°C for 30 minutes. It is disclosed that this rolled copper foil has excellent strength and elongation at break after heat treatment.

Furthermore, Patent Document 2 (JP 2017-179490 A) discloses a copper alloy for electronic and electrical devices, which contains 0.15 mass% or more and less than 0.35 mass% Mg, the balance being Cu and unavoidable impurities, and is characterized by having an electrical conductivity greater than 75% IACS and a low-angle grain boundary and subgrain boundary length ratio L _LB /( _LLB + _LHB )>20%. According to this invention, it is possible to provide a copper alloy for electronic and electrical devices, a copper alloy plastically worked material for electronic and electrical devices, a component for electronic and electrical devices, a terminal, and a bus bar, which are excellent in electrical conductivity, strength, bending workability, and stress relaxation resistance.

JP 2013-001982 A JP 2017-179490 A

The current collector is coated with an electrode active material, which expands and contracts during charging and discharging as ions migrate from the active material, subjecting the current collector to repeated stress with each charge and discharge. Therefore, partial breakage 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 is being considered. However, because Si-based active materials have a large volume change rate during charge and discharge, there are concerns that the active material may peel off from the current collector after repeated cycles.

The invention described in Patent Document 1 produced a rolled copper foil with increased strength and breaking elongation by adding 0.10 to 0.30 wt% 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.

On the other hand, the heat treatment temperature when applying active material to current collector foil has decreased as technology has improved, and the heat treatment temperature is approximately 150-200°C, especially when using a water-based binder to bind the active material to the 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 prioritize strength at room temperature rather than strength after heat treatment, and to develop copper foil for batteries that is even stronger than conventional ones.

In addition, when lithium-ion batteries are charged, Joule heat is generated due to internal resistance, causing 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, to reduce the amount of heat generated, a current collector copper foil with low electrical resistance (high conductivity) is required. 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.

The present invention was completed in consideration of the above-mentioned problems, and in one embodiment, it is an object of the present invention to provide a rolled copper foil for secondary batteries that combines high strength and high conductivity. In another embodiment, it is an object of the present invention to provide a method for manufacturing a secondary battery negative electrode and a secondary battery using such rolled copper foil for secondary batteries.

As a result of extensive research, the inventors have discovered that by devising a manufacturing process for rolled copper foil for secondary batteries, it is possible to obtain rolled copper foil for secondary batteries with higher strength than conventional techniques, even with a similar Mg concentration. In other words, it is possible to increase the strength of rolled copper foil for secondary batteries without reducing electrical conductivity. The present invention was completed based on the above findings, and is exemplified below.

[1]
A rolled copper foil for a secondary battery, comprising 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 of 50% IACS or more, and a thickness of 50 μm or less.
[2]
A rolled copper foil for a secondary battery, comprising 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 of 640 MPa or more in a direction parallel to the rolling direction, an electrical conductivity EC of 50% IACS or more, and a thickness of 50 μm or less.
[3]
The rolled copper foil for a secondary battery according to [1] or [2], having a conductivity EC of 55% IACS or more.
[4]
The rolled copper foil for secondary batteries 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 a direction parallel to the rolling direction.
[5]
The rolled copper foil for a secondary battery according to any one of [1] to [4], containing 0.0001 to 0.005 wt % of P.
[6]
[1] A method for producing a secondary battery negative electrode using the rolled copper foil for secondary batteries 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 secondary batteries according to any one of [1] to [5] as a raw material for a current collector.

The present invention provides a rolled copper foil for secondary batteries that combines high strength and high conductivity, as well as a method for manufacturing a secondary battery negative electrode and a secondary battery using such rolled copper foil for secondary batteries.

1 is a graph showing the relationship between Mg concentration and tensile strength TS in the direction parallel to rolling in examples and comparative examples of the present invention.

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

(Composition of rolled copper foil for secondary batteries)
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.

If the Mg content exceeds 1.0 wt%, the electrical conductivity will decrease significantly. From this perspective, 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.

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 Simultix 14 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 molten metal is cast into a shape of approximately 30 to 40 mmφ and a thickness of approximately 50 to 80 mm, and then cut to a thickness of approximately 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 may 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, a 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.

The material for the rolled copper foil for secondary batteries of the present invention, i.e., the base copper containing Mg, 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 pure copper, the conductivity of the copper foil does not decrease, making them suitable for current collectors. The oxygen concentration contained 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.

The rolled copper foil for secondary batteries according to the present invention is made from industrially used copper and contains unavoidable impurities. Even trace amounts of these unavoidable impurities, Fe, Zr, S, Ge, and Ti, are undesirable because they can easily rotate the crystal orientation and create shear bands when the copper foil is bent, making the current collector more susceptible to cracking and breakage when repeatedly bent. For this reason, it is preferable that the copper foil according to the present invention contain one or more unavoidable impurities selected from the group consisting of Fe, Zr, S, Ge, and Ti, in total, controlled to 0.002 wt % or less.

The rolled copper foil for secondary batteries of this embodiment may contain 0.0001 to 0.005 wt. % P. If oxygen is present in copper, it will react with hydrogen during high-temperature heat treatment, making it more susceptible to hydrogen embrittlement. By adding P, the P will react preferentially with oxygen, removing the 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.

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.

(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 order to achieve both high strength that can withstand expansion of the active material and high electrical conductivity that suppresses heat generation in lithium ion batteries (especially lithium ion batteries using Si-based active materials), 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 TS (MPa) to be 500×Mg (wt %)+654, for example.

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. A tensile strength TS of 640 MPa or more in the direction parallel to the rolling ensures high strength that can withstand the expansion of the active material. Preferably, the tensile strength TS of the rolled copper foil for secondary batteries in the direction parallel to the rolling is 700 MPa or more. This is expected to further expand the applications of rolled copper foil for secondary batteries.

The tensile strength (TS) in the direction parallel to the rolling direction refers to the value measured at room temperature (23°C) when a tensile strength test is conducted in the direction parallel to the rolling direction based on JIS-Z2241 (2011) or IPC-TM-650 Test Method 2.4.18 (2012).

(Conductivity of rolled copper foil for secondary batteries)
The electrical conductivity EC of the rolled copper foil for secondary batteries of this embodiment is 50% IACS (International Annealed Copper Standard) 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 preferably 55% IACS or more, and more preferably 60% IACS or more. The electrical conductivity can be measured in accordance with JIS-H0505 (2018).

(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 setting the thickness to 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 setting it to, for example, 5 μm or more, the handleability can be improved.

(Method for 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.

Increasing the processing rate in the final cold rolling step is advantageous for increasing the strength of the rolled copper foil for secondary batteries. Furthermore, increasing the processing rate in the final cold rolling step can enhance the effect of improving electrical conductivity. In one embodiment of the present invention, the processing rate in the final cold rolling step is preferably 99.0% or more. The processing rate is expressed by the following formula:
Machining rate = (T ₀ - T ₁ )/T ₀ ×100%
In the formula, T ₀ is the thickness of the material after the last heat treatment step (hot rolling or intermediate annealing), and T ₁ is the thickness of the material at the end of the final cold rolling step.

(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.

The present invention will be explained in detail below using examples, but the explanations here are for illustrative purposes only and are not intended to be limiting.

(Example)
Copper foils were produced using copper ingots (the remainder being copper and unavoidable impurities) having the Mg contents shown in Table 1. The reduction ratios in the final cold rolling step were the final cold rolling reduction ratios shown in Table 1. The Mg contents were measured by the ICP atomic emission spectroscopy described above.

The following characteristics were evaluated for each test piece obtained in this manner. The results are shown in Table 1.

<Tensile strength>
For Examples 1 to 3, Type 13B test pieces (gauge length 50 mm, width direction 12.5 mm) were prepared based on JIS-Z2241 (2011), and tensile tests were performed parallel to the rolling direction using a tensile tester (AutoCom C-type universal testing machine AC-100kN-C, manufactured by TSE Co., Ltd.) to measure the tensile strength. Note that for copper foils with a thickness of 35 μm or less, it is desirable to perform a tensile test based on IPC-TM-650 Test Method 2.4.18 (2012).

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

(Comparative Example)
Comparative Examples 1 to 4 shown in Table 1 are quoted 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 working ratio, recrystallization annealed at 450°C, and then a final cold-rolling step was carried out according to the working ratio shown in Table 1. 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.

(Consideration)
As can be seen from Table 1, by performing the final cold rolling process at a reduction ratio of 99.0% or more, the Examples had higher tensile strength than the Comparative Examples at the same Mg concentration level (FIG. 1). It was also found that the electrical conductivity remained high.

It is presumed that in Comparative Examples 1 to 4, the reduction rate in the final cold rolling process was insufficient, resulting in insufficient tensile strength.

Claims (7)

A rolled copper foil for a secondary battery, comprising 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 of 50% IACS or more, and a thickness of 50 μm or less. A rolled copper foil for a secondary battery, comprising 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 of 640 MPa or more in a direction parallel to the rolling direction, an electrical conductivity EC of 50% IACS or more, and a thickness of 50 μm or less. The rolled copper foil for secondary batteries according to claim 1 or 2, having an electrical conductivity EC of 55% IACS or more. The rolled copper foil for secondary batteries according to claim 1 or 2, containing 0.4 to 0.6 wt. % Mg and having a tensile strength TS of 700 MPa or more in the direction parallel to the rolling direction. The rolled copper foil for secondary batteries according to claim 1 or 2 contains 0.0001 to 0.005 wt. % P. A method for manufacturing a secondary battery negative electrode using the rolled copper foil for secondary batteries described in claim 1 or 2 as a raw material for a current collector. A method for manufacturing a secondary battery using the rolled copper foil for secondary batteries described in claim 1 or 2 as a raw material for a current collector.