TWI686003B - Rolled copper foil for lithium ion battery collector and lithium ion battery - Google Patents

Abstract

An object of the present invention is to provide a rolled copper foil for a lithium ion battery current collector that has good adhesion to a negative electrode active material and generates little metal powder during ultrasonic welding. The solution of the present invention: a rolled copper foil for current collector of lithium ion battery, which satisfies the wetting tension [mN/m] + arithmetic average roughness Ra [μm]×60≥41, and 0.01≤ arithmetic average roughness Ra [μm] ≤ 0.25, and wetting tension [mN/m] ≥ 35.

Description

Rolled copper foil for lithium ion battery collector and lithium ion battery

The invention relates to rolled copper foil for lithium ion battery collectors and lithium ion batteries.

Lithium-ion batteries have the characteristics of high energy density and relatively high voltages, and are mostly used for small electronic devices such as notebook computers, video cameras, digital cameras, and mobile phones. In the future, it is also expected to be used as a power source for large-scale devices such as distributed power sources for electric vehicles and general households.

Fig. 1 is a schematic diagram of a stack structure of a lithium ion battery. The electrode body of a lithium ion battery generally has a stack structure in which the positive electrode 11, the separator 12, and the negative electrode 13 are wound or stacked several dozen times. Typically, the positive electrode is composed of a positive electrode active material made of a positive electrode current collector that can be made of aluminum foil and a lithium composite oxide such as LiCoO ₂ , LiNiO _2, and LiMn ₂ O ₄ provided on the surface, The negative electrode is composed of a negative electrode active material made of a negative electrode current collector that can be made of aluminum foil and carbon or the like provided on the surface thereof. Between the positive electrode and the negative electrode are welded by the respective lead plates (14, 15). In addition, the positive electrode and the negative electrode are connected to lead terminals made of aluminum or nickel, but this is also performed by welding. Welding is usually carried out by ultrasonic welding.

Examples of the characteristics required for the copper foil used as the current collector of the negative electrode include adhesion to the negative electrode active material and low metal powder generated during ultrasonic welding.

As a general method for improving the adhesion to the active material layer, a surface treatment for forming irregularities on the surface of the copper foil, which is called a pre-roughening treatment, may be mentioned. As a method of roughening treatment, there are known methods such as sand blasting, rolling using a rough roll, mechanical polishing, electrolytic polishing, chemical polishing, and plating of electrodeposited particles. Of these, especially those using electrodeposited particles are mostly used. Plating. Perform the following technique: use copper sulfate acid plating bath, electrodeposit a large amount of copper on the surface of the copper foil in the form of dendrites or small balls to form fine irregularities, and improve the adhesion by the projection effect to prevent volume When the active material changes greatly, the stress concentrates in the concave portion of the active material layer to form a crack, and the stress concentrates on the current collector interface to cause peeling (for example, Japanese Patent No. 3733067).

In addition, for a copper foil used as a current collector of a lithium-ion battery, an active material of Li is applied to the surface of the copper foil. In this case, the active material may be thickly coated to increase the capacity of the battery. However, if the active material is thickly coated, there may be a problem related to the adhesion between the copper foil and the active material, such as peeling of the active material. In addition, as another method for increasing the capacity of batteries, the use of Si-based active materials has been studied. However, the expansion and contraction ratio of Si-based active materials is higher than that of existing active materials, so there is a concern that adhesion may cause problems.

In addition, the copper foil used as a current collector of a lithium-ion battery may peel off into a powder and generate metal powder when ultrasonic welding is performed. Such a large amount of metal powder is generated, and if left in the electrode body, it may cause an internal short circuit and the like, which may degrade the performance of the lithium ion battery. As a method of suppressing the generation of metal powder, for example, Japanese Patent Laid-Open No. 2007-305322 describes the following method: annealing removes the internal strain of the negative electrode current collector and softens it, so that during ultrasonic welding, It suppresses the peeling of part of the current collector into powder, and reduces the residual of metal powder of 50 μm or more.

In addition, as a factor that determines the battery life of the lithium ion secondary battery, the adhesion in the interface between the current collector and the active material layer may be cited. Most of the commercially available lithium ion batteries use a negative electrode produced by applying a slurry mixed with an active material, a binder, and an organic solvent to a copper foil that becomes a current collector, and then drying. If the slurry cannot be uniformly wetted and spread on the surface of the current collector, it may cause peeling of the active material or the like. Since this is undesirable, the wettability (wetting tension) of the electrode surface Also important. For example, in Japanese Patent Laid-Open No. 10-212562, a method of not overlapping copper foils laminated on a wound product (coil) obtained by winding a copper foil obtained by cold rolling is described. A copper foil wound product in which copper foil is wound after cleaning the surface of the copper foil before winding, removing fine powder of copper and the like adhering to the surface, and setting residual oil content such as rolling oil remaining on the surface to a predetermined value or less The final annealing method.

Existing technical literature Patent Literature Patent Document 1: Japanese Patent No. 3733067 Patent Document 2: Japanese Patent Laid-Open No. 2007-305322 Patent Document 3: Japanese Patent Laid-Open No. 10-212562

Problems to be solved by the invention

Although the technology development for improving the characteristics of the copper foil used as the collector of the lithium ion battery has been carried out in this way, the technology for simultaneously improving the adhesion of the active material and suppressing the generation of metal powder during ultrasonic welding is still There is room for development.

Therefore, an object of the present invention is to provide a rolled copper foil for a lithium ion battery current collector and a lithium ion battery that have good adhesion to a negative electrode active material and generate little metal powder during ultrasonic welding. Solutions for solving problems

In order to solve the above-mentioned problems, the inventors have repeatedly studied and found that by controlling the relationship between the wetting tension of the rolled copper foil and the wetting tension of the rolled copper foil and the arithmetic average roughness Ra, the arithmetic average roughness Ra is further controlled The range of the numerical value can provide a rolled copper foil for a lithium ion battery current collector that can improve the adhesion with the negative electrode active material and generate less metal powder during ultrasonic welding.

For the present invention completed on the basis of the above findings, on the one hand, it is a rolled copper foil for a lithium ion battery current collector, which satisfies the wetting tension [mN/m] + arithmetic mean roughness Ra [μm ]×60≥41; 0.01≤arithmetic mean roughness Ra [μm]≤0.25; and wetting tension [mN/m]≥35.

In one embodiment, the rolled copper foil for a lithium ion battery current collector of the present invention satisfies the wetting tension [mN/m] + arithmetic mean roughness Ra [μm]×60≥44 and the wetting tension [mN/m ] ≥37.

In another embodiment, the rolled copper foil for a lithium ion battery current collector of the present invention satisfies the arithmetic mean roughness Ra [μm] ≥ 0.03 and the wetting tension [mN/m] ≥ 37.

Another aspect of the present invention is a lithium ion battery using the rolled copper foil for a current collector of a lithium ion battery according to an embodiment of the present invention as a current collector. Invention effect

According to the present invention, it is possible to provide a rolled copper foil for a lithium ion battery current collector and a lithium ion battery that have good adhesion to a negative electrode active material and generate less metal powder during ultrasonic welding.

(Rolled copper foil for lithium ion battery collector)

Rolled copper foil is used as the copper foil base material of the rolled copper foil for lithium ion battery current collectors of an embodiment of the present invention. The rolled copper foil also includes rolled copper alloy foil. There is no particular restriction on the material of rolled copper foil, and it may be appropriately selected according to the application and required characteristics. For example, it is not limited, but in addition to high-purity copper (oxygen-free copper, tough copper, etc.), Sn-containing copper, Ag-containing copper, Cu-Ni-Si-based copper alloys added with Ni, Si, etc., added Copper alloys such as Cu-Cr-Zr-based copper alloys such as Cr and Zr.

The thickness of the rolled copper foil is not particularly limited, and can be appropriately selected according to the required characteristics. Generally, it is 1 to 100 μm, but when it is used as a current collector for a negative electrode of a lithium secondary battery, a battery with a higher capacity can be obtained when the rolled copper foil is thinned. From such a viewpoint, it is typically 2 to 50 μm, and more typically about 5 to 20 μm.

The rolled copper foil for a lithium ion battery current collector according to an embodiment of the present invention satisfies the wetting tension [mN/m] + arithmetic average roughness Ra [μm]×60≧41. By controlling the relationship between the wetting tension of the rolled copper foil and the arithmetic average roughness Ra in this way, it is possible to obtain a lithium ion battery collector that has good adhesion to the active material and generates little metal powder during ultrasonic welding Rolled copper foil for body. The rolled copper foil for lithium ion battery current collector preferably satisfies the wetting tension [mN/m] + arithmetic mean roughness Ra [μm]×60≥44, more preferably satisfies the wetting tension [mN/m] + arithmetic mean The roughness Ra [μm] × 60 ≥ 45, and further preferably satisfies the wetting tension [mN/m] + arithmetic mean roughness Ra [μm] × 60 ≥ 50.

The rolled copper foil for a lithium ion battery current collector according to an embodiment of the present invention also satisfies 0.01≦arithmetic mean roughness Ra[μm]≦0.25. When the arithmetic mean roughness Ra is less than 0.01 μm, the anchoring effect may be reduced, and the adhesion with the negative electrode active material may deteriorate. In addition, when the arithmetic average roughness Ra is greater than 0.25 μm, there are many oil pits on the surface of the copper foil, and rolling oil intrudes into the surface, so it is difficult to remove the rolling oil, and the amount of metal powder generated during ultrasonic welding is significantly increased. When there is much residual oil on the surface of the copper foil, the wetting tension tends to deteriorate. The rolled copper foil for a lithium ion battery current collector according to an embodiment of the present invention satisfies 0.01 ≤ arithmetic mean roughness Ra [μm] ≤ 0.2 in one embodiment, and 0.03 ≤ arithmetic mean roughness Ra in another embodiment [Μm] ≤ 0.15, and in another embodiment, 0.05 ≤ arithmetic mean roughness Ra [μm] ≤ 0.1 is satisfied.

The rolled copper foil for a lithium ion battery current collector according to an embodiment of the present invention also satisfies the wetting tension [mN/m] ≥ 35. When the wetting tension is less than 35mN/m, a large amount of rolling oil may be present on the surface of the copper foil, and the slurry may not be uniformly wetted and spread on the surface of the copper foil, which causes the deterioration of the adhesion of the active material, which is not preferable . The rolled copper foil for a lithium ion battery current collector according to an embodiment of the present invention preferably satisfies the wetting tension [mN/m] ≥ 37, and more preferably satisfies the wetting tension [mN/m] ≥ 39. The upper limit of the wetting tension is not particularly limited, but in order to obtain a wetting property exceeding 70 mN/m, a large amount of degreasing time may be required, so the productivity deteriorates.

For the rolling of the lithium ion battery current collector according to the embodiment of the present invention, the relationship between the wetting tension of the rolled copper foil and the arithmetic average roughness Ra and the wetting tension and the arithmetic average roughness Ra are controlled. The copper foil can be constructed by controlling the unevenness of the surface caused by oil pits without roughening such as polishing or plating of electrodeposited particles. The oil pit refers to a fine depression that is partially formed on the surface of the material to be rolled by the rolling roller and the material to be rolled in which the rolling oil is enclosed in the roll gap. Since the roughening process is omitted, there is an advantage of improved economy and productivity.

The shape of the oil pits of rolled copper foil, that is, the surface properties can be adjusted by adjusting the surface roughness of the rolling roller, the rolling speed, the viscosity of the rolling oil, and the average reduction rate per pass (especially the final pass pressure Rate) etc. to control. For example, if a rolling roll with a large surface roughness is used, the surface roughness of the obtained rolled copper foil also becomes larger. Conversely, if a rolling roll with a small surface roughness is used, the surface of the obtained rolled copper foil The roughness also tends to become smaller. In addition, by accelerating the rolling speed, increasing the viscosity of the rolling oil, or reducing the average reduction rate per pass, the amount of oil pits also tends to increase. Conversely, by slowing the rolling speed, reducing the viscosity of the rolling oil, or increasing the average reduction rate per pass, the amount of oil pits produced is likely to decrease. (Lithium Ion Battery)

A negative electrode composed of a current collector using the rolled copper foil of the present invention as a material and an active material layer formed thereon can be used to produce a lithium ion battery by a common method. Lithium-ion batteries include lithium-ion primary batteries and lithium-ion secondary batteries in which lithium ions in an electrolyte serve as a conductor. The negative electrode active material is not limited, but examples thereof include tin oxide and indium in which carbon, silicon, tin, germanium, lead, antimony, aluminum, indium, lithium, tin oxide, lithium titanate, lithium nitride, and indium are dissolved. Tin alloy, lithium-aluminum alloy, lithium-indium alloy, etc. (Manufacturing method)

The rolled copper foil for a lithium ion battery current collector according to an embodiment of the present invention can be produced by the following production method, for example. First, an ingot as a raw material is manufactured and rolled by hot rolling. Next, repeated annealing and cold rolling, in the final cold rolling, the work roll diameter is set to 50~100mm, the work roll surface roughness Ra is set to 0.03~0.1μm, and the final pass rolling speed is set to 300~ 500m/min, finishing to a thickness of 1~100μm. The viscosity of the rolling oil can be set to 3.0 to 5.0 cSt (25°C). The copper foil after the final cold rolling has oils such as rolling oil used in the final cold rolling adhered to it. Therefore, the copper foil is washed with a solution containing a petroleum-based solvent and an anionic surfactant to remove the copper The powder and rolling oil are removed, and then air drying is performed.

In addition, as a method of removing rolling oil and the like from the surface of the copper foil, a conventionally known degreasing treatment or cleaning treatment can be used. Examples of organic solvents (degreasing solvents) to be further used include, for example, n-alkanes and isopropyl Alcohols such as alcohol, acetone, dimethylacetamide, tetrahydrofuran, ethylene glycol.

As degreasing treatment or cleaning treatment, to satisfy the relationship between arithmetic average roughness Ra of copper foil surface and wetting tension (wetting tension [mN/m] + arithmetic average roughness Ra [μm]×60≥41) Take control. For example, the degreasing treatment is performed so that the copper foil having an arithmetic average roughness Ra of 0.068 μm has a wetting tension after degreasing of 37 mN/m or more. The immersion time in the degreasing liquid is preferably adjusted as shown in FIG. 2 according to the roughness of the copper foil surface.

In the copper foil manufacturing process, an oxide film is formed on the surface of the copper foil. If there is an oxide film on the surface of the copper foil, the wetting tension of the copper foil decreases. Therefore, it is desirable to remove the oxide film on the surface of the copper foil.

In the degreasing treatment, cleaning treatment, and oxide film removal treatment, the immersion time of the copper foil in the degreasing solvent can be set to 2.5 s or more. On the other hand, if the immersion time is too long, there may be poor productivity and discoloration caused by alkali ablation may occur on the surface of the copper foil. For copper foils with large Ra, that is, with many or deep oil pits, in order to remove the rolling oil entering the oil pits and the oxide film formed on the surface of the copper foil, the longer the immersion time, the better. The immersion time of the copper foil in the degreasing solvent can be set to 2.5 to 12 s, more preferably 2.5 to 8.5 s. [Example]

Examples of the present invention are shown below, but they are provided for a better understanding of the present invention and are not intended to limit the present invention. (Examples 1-9, Comparative Examples 1-6) [Manufacture of rolled copper foil]

An ingot of 600 mm wide tough copper is produced and rolled by hot rolling. Next, repeated annealing and cold rolling, and finally in cold rolling, the work roll diameter is set to 60 mm, the work roll surface roughness Ra is set to 0.03 μm, and the final pass is finished to a thickness of 0.01 with a rolling speed of 400 m/min. mm. The viscosity of the rolling oil is 4.0 cSt (25°C). In this state, oil components such as rolling oil used in the final cold rolling are attached to the copper foil. The copper foil was washed with a solution containing a petroleum-based solvent and an anionic surfactant, the copper fine powder, rolling oil, etc. adhering to the surface of the copper foil were removed, and then air-dried.

The rolling oil on the surface of the copper foil uses n-paraffin as an organic solvent (degreasing solvent), and is removed by degreasing treatment. Table 1 shows the immersion time of the copper foil implemented in this degreasing process in an organic solvent (degreasing solvent). It should be noted that in Examples 1 to 9, the relationship between the arithmetic average roughness Ra of the copper foil surface and the wetting tension (wetting tension [mN/m] + arithmetic average roughness Ra[ μm]×60≥41). [Arithmetic average roughness Ra]

The arithmetic average roughness Ra is a value obtained by measuring in accordance with JIS B0601 2001, using a confocal microscope (LASERTEC Corporation, model: HD100D), and measuring the surface of the sample at a length of 175 μm in the rolling parallel direction. [Adhesion with active material]

The adhesion to the active material was evaluated in the following order. (1) Artificial graphite with an average diameter of 9 μm and polyvinylidene fluoride are mixed in a weight ratio of 1:9, and dispersed in the solvent N-methyl-2-pyrrolidone. (2) The above-mentioned active material is coated on the surface of the copper foil. (3) The copper foil coated with the active material is heated at 90° C.×30 minutes by a dryer. (4) After drying, cut out a 20 mm square and apply a load of 1.5 tons/mm ² × 20 seconds. (5) Cut the above samples in a checkerboard pattern with a cutter, stick a commercially available adhesive tape (Sellotape (registered trademark)), place a roller with a weight of 2 kg, and reciprocate once to make the adhesive tape crimped. (6) Peel off the adhesive tape, and for the active material remaining on the copper foil, import the image of the surface into the PC, and binarize the metallic shiny part of the copper surface from the black part of the active material to calculate the residual of the active material rate. The residual rate was set as the average value of three times for each sample. In the determination of the adhesiveness of the active material, the residual rate is less than 50% and is set to “×”, and 50% or more is set to “○”. [Wetting Tension]

The wetting tension used the wetting tension test mixture (made by Wako Pure Chemical Industries, Ltd.) and was measured according to JIS K6768. [Number of metal powder produced in ultrasonic weldability]

The ultrasonic weldability was evaluated in the following order. (1) Cut the copper foil to a size of 100 mm × 30 mm and overlap 30 sheets. (2) Install a welding head (pitch 0.8mm, height 0.4mm) on the actuator (model: Ultraweld L20E) manufactured by Branson. Use 0.2mm pitch for the anvil. (3) Use the adhesive surface of 20mm wide tape as the surface and install it on both sides of the anvil. The size of the bonding surface is 20mm×60mm. (4) Welding conditions are pressure 40 psi, amplitude 60 μm, vibration frequency 20 kHz, and welding time is set to 0.1 second. (5) Under the above conditions, the welding position was changed, and after welding 30 times, the number of metal powders adhering to the adhesive surfaces of the adhesive tapes installed on both sides of the anvil were counted.

Table 1 shows the evaluation conditions and evaluation results. [Table 1]

In Examples 1 to 9, the wetting tension [mN/m] + arithmetic mean roughness Ra [μm] × 60 ≥ 41, and 0.01 ≤ arithmetic mean roughness Ra ≤ 0.25, and wetting tension [mN/m ] ≥35. Therefore, the adhesion of the active material is good, and the number of metal powders generated is small.

In Comparative Example 1, the wetting tension [mN/m] + arithmetic average roughness Ra [μm]×60 is less than 41, and the arithmetic average roughness Ra exceeds 0.25 μm. Therefore, the adhesion of the active material is poor and the resulting metal The number of powders is obviously more than the copper foil satisfying 0.01 ≤ arithmetic mean roughness Ra [μm] ≤ 0.25.

In Comparative Examples 2 and 3, the wetting tension [mN/m] + arithmetic mean roughness Ra [μm]×60 is less than 41, and therefore, the active material has poor adhesion. More specifically, in Comparative Examples 2 and 3, the immersion time in the degreasing liquid is shorter than the arithmetic average roughness Ra, and therefore, the residual oil content increases, and as a result, the wetting tension becomes small. Copper foil that satisfies the active material adhesion and wetting tension [mN/m] + arithmetic mean roughness Ra [μm] × 60 deteriorates.

In Comparative Examples 4 to 6, since the arithmetic average roughness Ra is greater than 0.25 μm, the number of metal powders produced is significantly more than the copper foil satisfying 0.01≦arithmetic average roughness Ra [μm]≦0.25.

FIG. 3 shows a graph showing the relationship between the wetting tension of Examples 1 to 9 and the arithmetic mean roughness Ra. Within the range enclosed by the dotted line, the relationship that satisfies the wetting tension [mN/m] + arithmetic mean roughness Ra [μm] × 60 ≥ 41, and 0.01 ≤ arithmetic mean roughness Ra [μm] ≤ 0.25 The ultrasonic weldability of Examples 1 to 9 is good, and the number of metal powders produced is small.

11‧‧‧Positive 12‧‧‧ Diaphragm 13‧‧‧Negative 14, 15‧‧‧lead plate

FIG. 1 is a schematic diagram of a stack structure of a lithium ion battery according to an embodiment of the present invention. FIG. 2 is a graph showing the relationship between the surface roughness Ra of the copper foil of the example and the immersion time in the degreasing liquid. FIG. 3 is a graph showing the relationship between the wetting tension and the arithmetic average roughness Ra of the examples and the comparative examples.

Claims (4)

Rolled copper foil for current collector of lithium ion battery, wherein, The following conditions: Wetting tension [mN/m] + arithmetic mean roughness Ra [μm] × 60 ≥ 41; 0.01≤arithmetic mean roughness Ra[μm]≤0.25; and Wetting tension [mN/m] ≥35. The rolled copper foil for lithium ion battery current collector as described in item 1 of the patent scope, wherein, The following conditions: Wetting tension [mN/m] + arithmetic mean roughness Ra [μm] × 60 ≥ 44; and Wetting tension [mN/m] ≥ 37. The rolled copper foil for a lithium ion battery current collector as described in item 1 or item 2 of the patent application scope, wherein, The following conditions: The arithmetic mean roughness Ra [μm] ≥ 0.03; and Wetting tension [mN/m] ≥ 37. A lithium ion battery using the rolled copper foil for a current collector of a lithium ion battery as described in any one of claims 1 to 3 as a current collector.

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