Electrolytic copper foil, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

The present invention provides an electrolytic copper foil which is not susceptible to breaking. With respect to this electrolytic copper foil, if t (unit: μm) is the foil thickness, Gs (unit: %) is the gloss of the electrolytic deposition end surface as determined by irradiating the electrolytic deposition end surface with light at an angle of incidence of 60° in the length direction, and E (unit: %) is the elongation as determined by pulling the foil in the length direction, the foil thickness t is from 10 to 20, Gs / t that is obtained by dividing the gloss Gs by the foil thickness t is from 10 to 40, and E / t that is obtained by dividing the elongation E by the foil thickness t is from 0.9 to 1.8.

Description

[Technical Field]

[0001] The present invention relates to an electrolytic copper foil, a negative electrode for a lithium ion secondary battery using the electrolytic copper foil, and a lithium ion secondary battery including the negative electrode for a lithium ion secondary battery. [Background technology]

[0002] Copper foil is sometimes used as the negative electrode current collector in lithium-ion secondary batteries, but the expansion and contraction of the negative electrode material during charging and discharging of the lithium-ion secondary battery can cause the copper foil to break.In addition, the copper foil and negative electrode material, which are in close contact with each other, can locally peel off during charging and discharging, and the stress caused by the expansion and contraction can concentrate in the peeled area, causing the copper foil to break. Patent Documents 1 and 2 disclose electrolytic copper foils that can be used as negative electrode current collectors for lithium ion secondary batteries. However, the electrolytic copper foils disclosed in Patent Documents 1 and 2 may have insufficient mechanical properties and adhesion to the negative electrode material, and may therefore break during charge and discharge of the lithium ion secondary battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 217787, 2007 [Patent Document 2] Japanese Patent Publication No. 204747, 2016 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide an electrolytic copper foil that is resistant to fracture. Another object of the present invention is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery in which the negative electrode current collector is resistant to fracture during charge and discharge. [Means for solving the problem]

[0005] The electrodeposited copper foil according to one embodiment of the present invention has a foil thickness t (unit: μm), a gloss Gs (unit: %) of the surface after electrolytic deposition measured by irradiating the surface at which electrolytic deposition was completed with light at an incident angle of 60° along the length direction, and a gloss Gs (unit: %) of the surface after electrolytic deposition measured by pulling the foil along the length direction. Break When the elongation is E (unit: %), the foil thickness t is 10 to 20, and the gloss Gs divided by the foil thickness t is Gs / t of 10 or more. 37.73 is as follows: Break E / t, which is the elongation E divided by the foil thickness t, is 1.08 The gist is that it is 1.8 or less.

[0006] A negative electrode for a lithium ion secondary battery according to another aspect of the present invention includes the electrolytic copper foil according to the above aspect. Furthermore, a lithium ion secondary battery according to another aspect of the present invention includes the negative electrode for a lithium ion secondary battery according to the above-described other aspect. [Effects of the Invention]

[0007] The electrolytic copper foil of the present invention is less likely to break, and the negative electrode for a lithium ion secondary battery and the lithium ion secondary battery of the present invention are less likely to break in the negative electrode current collector during charge and discharge. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating a method for producing an electrolytic copper foil using an electrolytic deposition apparatus, and is an explanatory diagram of a step of performing anodic oxidation. [Figure 2] FIG. 1 is a diagram illustrating a method for producing an electrolytic copper foil using an electrolytic deposition apparatus, and is an explanatory diagram of a copper plating step. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of the present invention will be described. Note that the embodiment described below is merely an example of the present invention. Furthermore, various modifications and improvements can be made to this embodiment, and such modifications and improvements can also be included in the present invention. The electrodeposited copper foil according to one embodiment of the present invention has a foil thickness of t (unit: μm), a gloss of the surface after electrolytic deposition measured by irradiating the surface with light at an incident angle of 60° along the length direction, and a gloss of the surface after electrolytic deposition measured by pulling the foil along the length direction. Break When the elongation is E (unit: %), the foil thickness t is 10 to 20, and the gloss Gs divided by the foil thickness t is Gs / t of 10 or more. 37.73 is as follows: Break E / t, which is the elongation E divided by the foil thickness t, is 1.08 Above 1.8 or below. Due to this configuration, the electrolytic copper foil of this embodiment is less likely to break.

[0010] The electrodeposited copper foil of this embodiment can be used as a negative electrode current collector for a lithium ion secondary battery (mainly a cylindrical lithium ion secondary battery). That is, the negative electrode for a lithium ion secondary battery of this embodiment includes the electrodeposited copper foil of this embodiment. Furthermore, the lithium ion secondary battery of this embodiment includes the negative electrode for a lithium ion secondary battery of this embodiment. Since the electrolytic copper foil of this embodiment is less likely to break, the negative electrode for a lithium ion secondary battery and the lithium ion secondary battery of this embodiment are less likely to break in the negative electrode current collector during charge and discharge.

[0011] The electrolytic copper foil of this embodiment will be described in more detail below. As a result of extensive investigations, the present inventors have found that an electrodeposited copper foil having both high extensibility and high gloss at the surface after electrolytic deposition is less likely to break even when the negative electrode material expands and contracts during charging and discharging of a lithium ion secondary battery.

[0012] If the electrolytic copper foil has high extensibility, the electrolytic copper foil can follow the expansion and contraction of the negative electrode material, and therefore fracture is unlikely to occur. Furthermore, if the surface is high in gloss and flat, the adhesion between the closely attached electrolytic copper foil and the negative electrode material is uniform over the entire contact surface, and therefore local peeling between the closely attached electrolytic copper foil and the negative electrode material during charge and discharge is suppressed. If local peeling occurs between the electrolytic copper foil and the negative electrode material, stress during expansion and contraction is concentrated in the peeled portion, making the electrolytic copper foil prone to fracture. However, the electrolytic copper foil of this embodiment is less prone to local peeling, and therefore less prone to fracture during charge and discharge.

[0013] There is a correlation between gloss Gs and elongation E. The higher the gloss, the finer the copper crystal grains and the stronger the driving force for recrystallization, so heat softening treatment increases the crystal grain size, making the electrolytic copper foil highly extensible. It is known that electrodeposited copper foil undergoes a cold softening phenomenon in which recrystallization of crystal grains proceeds at about room temperature, and it is believed that the finer the crystal grains immediately after deposition, the greater the driving force for recrystallization, and the larger the crystal grains become after grain growth stops after cold softening. Therefore, it is believed that if finer crystal grains are deposited by controlling the thickness of the oxide film described below, the crystal grains inside the electrodeposited copper foil after cold softening will become larger, and elongation will be improved.

[0014] In general, gloss increases with increasing foil thickness. Therefore, the increase in gloss is influenced by two factors: the contribution of denser crystal grains inside the electrodeposited copper foil and the contribution of increasing foil thickness. Therefore, when estimating the degree of crystal grain fineness from gloss for electrodeposited copper foils of different foil thicknesses, it is considered necessary to normalize gloss by foil thickness and eliminate the contribution of foil thickness to gloss before making a comparison. Therefore, in the present invention, the parameter Gs / t, which normalizes gloss Gs by foil thickness t, is defined. Furthermore, two factors are thought to contribute to elongation: foil thickness and crystal grain size. When estimating the contribution of crystal grain refinement immediately after foil production to elongation, it is necessary to define E / t, which is elongation E normalized by foil thickness t. It was found that by specifying both the parameter Gs / t, which is gloss Gs normalized by foil thickness t, and the parameter E / t, which is elongation E normalized by foil thickness t, an electrodeposited copper foil that is less prone to breakage can be obtained.

[0015] [Gs / t] The parameter Gs / t must be 10 or more and 40 or less, and is preferably 25 or more and 40 or less. If the parameter Gs / t is within the above range, the surface of the electrodeposited copper foil is flat, and therefore the adhesion between the electrodeposited copper foil and the negative electrode material is likely to be uniform over the entire adhesion surface. Therefore, localized peeling between the closely adhered electrodeposited copper foil and the negative electrode material during charge and discharge is suppressed, and breakage is unlikely to occur during charge and discharge. On the other hand, if the parameter Gs / t is within the above range, the adhesion between the electrodeposited copper foil and the negative electrode material is sufficiently exhibited, and therefore breakage is unlikely to occur during charge and discharge.

[0016] The gloss Gs is measured by irradiating the surface after electrolytic deposition with light at an incident angle of 60° along the longitudinal direction. In the present invention, the "longitudinal direction" of the electrodeposited copper foil means the MD (Machine Direction), and for example, in the case where a copper foil is formed on the surface of a rotating electrode by plating using a rotating electrode during production of the electrodeposited copper foil, it means the rotation direction of the rotating electrode.

[0017] [E / t] The parameter E / t must be 0.9 or more and 1.8 or less, preferably 1.2 or more and 1.7 or less, and more preferably 1.3 or more and 1.6 or less. If the parameter E / t is within the above range, the electrodeposited copper foil has high extensibility and is therefore less likely to break during charge and discharge.

[0018] [Root mean square height Sq] The root mean square height Sq of the electrodeposited copper foil of the present embodiment at the surface after electrolytic deposition, measured using a white light interference microscope, is preferably 0.1 μm or more and 0.4 μm or less, and more preferably 0.1 μm or more and 0.25 μm or less.

[0019] If the root-mean-square height Sq of the surface at the end of electrolytic deposition is within the above range, the anchor effect tends to increase the adhesion between the electrolytic copper foil and the negative electrode material. Furthermore, if the root-mean-square height Sq of the surface at the end of electrolytic deposition is within the above range, the surface at the end of electrolytic deposition is sufficiently flat, so that the adhesion between the adhered electrolytic copper foil and the negative electrode material becomes uniform over the entire adhesion surface. Therefore, localized peeling between the adhered electrolytic copper foil and the negative electrode material during charge and discharge is suppressed, making breakage less likely to occur during charge and discharge.

[0020] [Tensile strength] The electrodeposited copper foil of this embodiment preferably has a tensile strength of 300 MPa or more and 380 MPa or less, measured by pulling it along the length direction. If the tensile strength is within the above range, the electrodeposited copper foil is less likely to break and has better followability to the expansion and contraction of the negative electrode material. The definition of the "length direction" of the electrodeposited copper foil is the same as that of the gloss Gs.

[0021] The electrolytic copper foil of this embodiment can be used not only as a negative electrode current collector for a lithium ion secondary battery but also for other applications. For example, the electrolytic copper foil of this embodiment can be suitably used for circuit applications. Since the adhesion between the electrolytic copper foil and the resin tends to be uniform over the entire adhesion surface, the occurrence of wrinkles in the electrolytic copper foil at high temperatures is suppressed, and the occurrence of defects such as blistering due to local unevenness in the adhesion is also suppressed.

[0022] [Method for producing electrolytic copper foil] An example of the method for producing the electrolytic copper foil of this embodiment will be described below. The electrodeposited copper foil can be produced, for example, by using an electrolytic deposition apparatus as shown in Figures 1 and 2. The electrolytic deposition apparatus in Figures 1 and 2 includes an insoluble electrode 12 made of titanium coated with a platinum group element or an oxide thereof, and a rotating electrode 11 made of titanium and disposed opposite the insoluble electrode 12.

[0023] The electrolytic copper foil of this embodiment can be manufactured by performing copper plating using an electrolytic deposition apparatus to deposit copper on the surface (cylindrical surface) of the cylindrical rotating electrode 11 to form copper foil, and then peeling the copper foil from the surface of the rotating electrode 11. However, before copper plating, the surface of the rotating electrode 11 may be oxidized (hereinafter, sometimes referred to as "anodic oxidation") to form an oxide film that is thicker and more uniform than a natural oxide film.

[0024] A native oxide film of several nanometers thick is usually formed on the surface of the rotating electrode 11 at room temperature. However, if the surface on which the native oxide film is formed is further anodized to form an anodic oxide film, and an oxide film consisting of the native oxide film and the anodic oxide film is formed, an oxide film of a thicker and more uniform thickness than the native oxide film is formed. If there is a variation in the thickness of the oxide film, the resistance during copper plating increases in the thicker parts, resulting in a reduced plating amount. As a result, there is a risk of the foil thickness of the electrolytic copper foil becoming uneven or pinholes being formed in the electrolytic copper foil.

[0025] Forming an oxide film thicker and more uniformly than the native oxide film by anodic oxidation can suppress foil thickness variations and pinholes. Furthermore, the presence of an oxide film thicker than the native oxide film allows copper plating to be performed at a higher overvoltage, resulting in finer crystal grains in the initial plating deposit. As a result, the elongation of the electrolytic copper foil after room-temperature softening is improved, and the gloss of the surface after electrolytic deposition is improved and the surface roughness is reduced. Therefore, it is possible to produce an electrolytic copper foil that is less likely to break during expansion and contraction when used as a negative electrode current collector.

[0026] The thickness of the anodic oxide film formed by anodic oxidation depends on the amount of electricity (unit: C / dm 2 Specifically, it can be controlled by the amount of electricity per unit area on the surface of the rotating electrode 11. By controlling the thickness of the anodic oxide film, it is possible to control the thickness of the oxide film consisting of the native oxide film and the anodic oxide film.

[0027] The amount of electricity applied to the rotating electrode 11 is 1000 C / dm 2 More than 5000C / dm2 Within the above-mentioned range, it is easy to control the parameter Gs / t to be 10 or more and 40 or less. Furthermore, within the above-mentioned range, it is possible to prevent the oxide film from becoming too thick, thereby suppressing the occurrence of abnormal deposition in copper plating or the like, which would otherwise increase the surface roughness of the electrolytic copper foil and cause uneven adhesion to the negative electrode material.

[0028] A method for producing electrolytic copper foil by performing anodization followed by plating will be described below. First, anodization will be described with reference to FIG. 1. When performing anodization, a current is applied to a rotating electrode 11 as the anode and an insoluble electrode 12 as the cathode. For example, a DSE (Dimensionally Stable Electrode) electrode (registered trademark) can be used as the insoluble electrode 12. Furthermore, for example, a 20% phosphoric acid aqueous solution can be used as the electrolyte 13.

[0029] An electrolyte 13 is supplied between the rotating electrode 11 and the insoluble electrode 12 from an electrolyte supply unit (not shown) (see the outline arrow), and while the rotating electrode 11 is rotated at a constant speed in the direction indicated by the dotted arrow, a direct current is applied between the rotating electrode 11 and the insoluble electrode 12. Then, an anodic oxide film is formed on the native oxide film on the surface of the rotating electrode 11, and an oxide film that is thicker and more uniform than the native oxide film is formed.

[0030] Next, copper plating will be described with reference to Fig. 2. When copper plating is performed, a current is applied between the rotating electrode 11 on which an oxide film is formed as the cathode and the insoluble electrode 12 as the anode. Furthermore, an aqueous solution containing sulfuric acid and copper sulfate can be used as the electrolytic solution 13. The copper concentration in the electrolytic solution 13 can be, for example, 50 to 150 g / L, and the sulfuric acid concentration can be, for example, 20 to 200 g / L.

[0031] When an electrolyte 13 is supplied between the rotating electrode 11 and the insoluble electrode 12 from an electrolyte supply unit (not shown) (see the outline arrow), and a direct current is applied between the rotating electrode 11 and the insoluble electrode 12 while the rotating electrode 11 is rotated at a constant speed in the direction indicated by the dotted arrow, copper is deposited on the surface of the rotating electrode 11. The deposited copper is peeled off from the surface of the rotating electrode 11, pulled up as indicated by the solid arrow in Fig. 2, and continuously wound up to obtain an electrolytic copper foil 14.

[0032] The electrolytic solution 13 used for copper plating may contain additives such as organic additives and inorganic additives from the viewpoint of smoothing the electrolytic copper foil and controlling its mechanical properties. By adding additives, the strength, elongation, surface roughness, and gloss in the normal state can be improved. One type of additive may be used alone, or two or more types may be used in combination.

[0033] Examples of organic additives include ethylenethiourea, polyethylene glycol, and Janus Green. As the inorganic additive, for example, metal chlorides such as sodium chloride (NaCl) or hydrogen chloride (HCl) can be used as a source of chloride ions.

[0034] The electrolytic solution 13 used for copper plating preferably contains 10 to 50 mass ppm of chloride ions (chlorine) as an inorganic additive, and preferably contains 3 to 30 mass ppm in total of at least one of ethylene thiourea, polyethylene glycol, and Janus Green as an organic additive. The electrolysis conditions for copper plating can be, for example, as follows: the temperature of the electrolytic solution 13 is 18 to 67°C, and the current density is 3 to 67 A / dm 2 is.

[0035] The surface of the electrodeposited copper foil produced as described above may be subjected to a surface treatment as desired. The surface treatment will be described below. The surface of the electrolytic copper foil may be subjected to a rust-proofing treatment. Examples of the rust-proofing treatment include inorganic rust-proofing treatment and organic rust-proofing treatment. Examples of the inorganic rust-proofing treatment include chromate treatment and plating treatment, and the chromate treatment may be applied to the plating layer formed by the plating treatment. Examples of the plating treatment include nickel plating, nickel alloy plating, cobalt plating, cobalt alloy plating, zinc plating, zinc alloy plating, tin plating, and tin alloy plating. Examples of the organic rust-proofing treatment include surface treatment using benzotriazole.

[0036] The surface that has been subjected to the rust prevention treatment may be further subjected to a surface treatment using a silane coupling agent (silane treatment). The surface treatment using a silane coupling agent provides the surface of the electrolytic copper foil (the surface that is bonded to the negative electrode material or resin) with functional groups that have a strong affinity for adhesives, thereby further improving the adhesion between the electrolytic copper foil and the negative electrode material or resin, and further improving the anti-rust properties and moisture absorption heat resistance of the electrolytic copper foil. Therefore, such an electrolytic copper foil is suitable as an electrolytic copper foil for a negative electrode current collector of a lithium ion secondary battery. The rust prevention treatment and silane coupling agent treatment increase the adhesive strength between the active material of the lithium ion secondary battery and the electrolytic copper foil, and serve to prevent a decrease in the charge / discharge cycle characteristics of the lithium ion secondary battery.

[0037] Furthermore, before the above-mentioned rust prevention treatment is performed, the surface of the electrodeposited copper foil may be subjected to a roughening treatment. For example, a plating method, an etching method, or the like can be suitably employed as the roughening treatment. The plating method is a method for roughening the surface of an untreated electrodeposited copper foil by forming a thin film layer having irregularities on the surface. Examples of the plating method include electrolytic plating and electroless plating.

[0038] A preferred roughening treatment by plating is, for example, a method in which a copper-based plating film, such as copper or a copper alloy, is formed on the surface of an untreated electrodeposited copper foil. A preferred roughening treatment by etching is, for example, a method using physical etching or chemical etching. An example of physical etching is an etching method using sandblasting, and an example of chemical etching is etching using a treatment solution containing an inorganic or organic acid, an oxidizing agent, and an additive.

[0039] [Example] The present invention will be explained in more detail below with reference to examples and comparative examples. Electrodeposited copper foils of Examples 1 to 27 and Comparative Examples 1 to 13 were produced, and negative electrode current collectors were produced using these electrolytic copper foils. Lithium ion secondary batteries were produced using these negative electrode current collectors. Various properties of the electrolytic copper foils and lithium ion secondary batteries were evaluated. The methods for producing the electrolytic copper foils and lithium ion secondary batteries and the methods for evaluating the various properties will be explained.

[0040] (A) Anodic oxidation Anodization was carried out using the same apparatus as in Figure 1 and the same operation as described above to form an oxide film on the surface of the rotating electrode. A 20% aqueous solution of phosphoric acid was used as the electrolyte. The thickness of the oxide film was controlled by the amount of electricity applied to the rotating electrode. The amount of electricity applied to the rotating electrode is shown in Table 1. Comparative Examples 8 to 10 are examples in which no anodic oxidation was carried out.

[0041] [Table 1]

[0042] (B) Copper plating Following the anodic oxidation in (A) above, copper plating was carried out in the same manner as described above, and copper was deposited on the surface of the rotating electrode on which the oxide film had been formed. The deposited copper was then peeled off from the surface of the rotating electrode and continuously wound up, thereby producing electrodeposited copper foils of Examples and Comparative Examples (see FIG. 2). The temperature of the electrolyte and the current density during copper plating were as shown in Table 1.

[0043] The electrolyte used was an aqueous solution containing sulfuric acid, copper sulfate pentahydrate, and additives. The additives used were ethylene thiourea, polyethylene glycol, and Janus Green. The concentrations of sulfuric acid, copper sulfate pentahydrate, and each additive are shown in Table 1. The concentration of copper sulfate pentahydrate is the concentration as copper. The chlorine concentration in the electrolyte is also shown in Table 1.

[0044] (C) Chromate treatment The surface of each of the electrolytic copper foils produced in the above section (B) was subjected to a chromate treatment to form an anti-rust treatment layer, thereby preparing a negative electrode current collector. The chromate treatment conditions were as follows: The plating solution used for the chromate treatment contained potassium dichromate, and the chromium concentration was preferably in the range of 6 to 12 g / L, and the treatment time for the chromate treatment was preferably in the range of 8 to 12 seconds. In this example and comparative example, the chromium concentration was 10 g / L, and the treatment time for the chromate treatment was 10 seconds.

[0045] (D) Fabrication of the positive electrode A cathode paste was prepared by mixing 90% by mass of lithium cobalt oxide (LiCoO2) powder, 7% by mass of graphite powder, and 3% by mass of polyvinylidene fluoride powder, adding N-methyl-2-pyrrolidone and ethanol as solvents, and kneading the mixture. This cathode paste was uniformly applied to an aluminum foil to a thickness of 15 μm. The aluminum foil coated with the cathode paste was dried in a nitrogen atmosphere to volatilize the solvent, and then rolled to produce a sheet with an overall thickness of 150 μm. This sheet was then cut into strips 43 mm wide and 285 mm long, and an aluminum foil lead terminal was attached to one end of the strip by ultrasonic welding to form a cathode.

[0046] (E) Manufacturing of the negative electrode A negative electrode material paste was prepared by adding N-methyl-2-pyrrolidone and ethanol as solvents to a mixture of 90 mass % of natural graphite powder having an average particle size of 10 μm and 10 mass % of polyvinylidene fluoride powder and kneading the mixture.

[0047] Each negative electrode current collector manufactured in the above section (C) was cut into a strip with a width of 720 mm. At this time, the width direction of the electrolytic copper foil was aligned with the width direction of the strip obtained by cutting. Next, the negative electrode material paste was applied to both sides of the strip in a double stripe pattern. The width of the linear negative electrode material paste coating was 300 mm, and the direction in which the linear negative electrode material paste coating extended was parallel to the longitudinal direction of the strip.

[0048] The strip coated with the negative electrode paste was dried in a nitrogen atmosphere to volatilize the solvent, and then rolled to produce a sheet with an overall thickness of 150 μm. This sheet was cut into a rectangular shape with a width of 43 mm and a length of 280 mm, and a nickel foil lead terminal was attached to one end of the rectangular shape by ultrasonic welding to form a negative electrode.

[0049] (F) Fabrication of lithium-ion secondary batteries A 25 μm thick polypropylene separator was sandwiched between the positive and negative electrodes prepared as described above, and the whole was wound to obtain a wound body. This wound body was placed in a cylindrical battery can, and the lead terminal of the negative electrode was spot-welded to the bottom of the battery can. The battery can was made of mild steel with a nickel-plated surface.

[0050] Next, an insulating top cover was placed on the battery can, a gasket was inserted, and the positive electrode lead terminal and an aluminum safety valve were connected by ultrasonic welding. A non-aqueous electrolyte consisting of propylene carbonate, diethyl carbonate, and ethylene carbonate was then poured into the battery can, and the top cover was attached to the safety valve to assemble a cylindrical, sealed lithium-ion secondary battery with an outer diameter of 14 mm and a height of 50 mm.

[0051] Next, various properties of each of the electrodeposited copper foils produced in the above section (B) and each of the lithium ion secondary batteries produced in the above section (F) were evaluated. The evaluation methods are explained below. The foil thickness of each of the electrodeposited copper foils produced in the above section (B) is as shown in Table 2.

[0052] [Table 2]

[0053] [Glossiness Gs of electrolytic copper foil] The gloss of the electrodeposited copper foil was measured on the surface after electrolytic deposition using a gloss meter VG7000 manufactured by Nippon Denshoku Industries Co., Ltd., according to the method specified in JIS Z8741-1997. The gloss was measured by irradiating the surface after electrolytic deposition with light at an incident angle of 60° along the machine direction (MD) of the electrodeposited copper foil. The gloss was measured five times, and the average value was taken as the gloss (Gs). The results are shown in Table 2. The gloss measurement was carried out on an electrodeposited copper foil in its normal state. In the present invention, "normal state" means a state in which the electrodeposited copper foil is kept at normal temperature and normal humidity (for example, a temperature of 23±2°C and a humidity of 50±5% RH).

[0054] [Elongation E and tensile strength of electrolytic copper foil] Unheat-treated electrodeposited copper foil was cut into a rectangular shape with a width of 12.7 mm and a length of 130 mm to serve as a measurement sample. A tensile test was then conducted on the measurement sample using an Instron tensile tester, Model 1122, in accordance with the method specified in IPC-TM-650 to measure the elongation at break and the tensile strength. In this tensile test, the chuck distance was 70 mm and the tensile speed was 50 mm / min. The tensile test was conducted on five measurement samples, and the average values ​​were used as the elongation E and the tensile strength. The results are shown in Table 2.

[0055] [Root mean square height Sq of electrolytic copper foil] With reference to ISO 25178, the surface shape of the electrodeposited copper foil in its normal state was measured using a white light interference optical microscope (Wyko Contour GT-K) manufactured by BRUKER, and shape analysis was performed to determine the root mean square height (Sq). The surface shape was measured at five arbitrary locations on the electrodeposited copper foil, and shape analysis was performed at each of the five locations to determine the root mean square height (Sq) for each of the five locations. The average of the results obtained at the five locations was taken as the root mean square height (Sq) of the electrodeposited copper foil in its normal state.

[0056] Shape analysis was performed using a high-resolution CCD camera with the VSI measurement method (vertical scanning interferometry). The conditions were: white light source, 1280 x 980 pixel resolution, 10x magnification, 477 μm x 357.8 μm measurement range, and 3% threshold. Furthermore, the entire 477 μm x 357.8 μm measurement range was subjected to filtering using Terms Removal (Cylinder and Tilt) and Data Restore (Method: legacy, iterations 5), followed by Fourier filtering.

[0057] For Fourier filter processing, High Freq Pass is used as Fourier filtering, Gaussian is used as Fourier filter window, and the high cutoff is set to 12.5 mm. -1 It was decided. Furthermore, statistical filtering (Filter Size: 3, Filter Type: Median) was performed. The root mean square height (Sq) was calculated by the S parameters-height analysis with Remove Tilt set to True. The results are shown in Table 2.

[0058] [Evaluation of charge / discharge cycle characteristics of lithium-ion secondary batteries] A charge-discharge cycle test was conducted on the lithium-ion secondary battery, in which one cycle consisted of charging at a charge current of 100 mA to 4.2 V, and then discharging at a discharge current of 100 mA to 2.4 V. After repeating this cycle, the lithium-ion secondary battery was disassembled and the presence or absence of breakage in the electrolytic copper foil was examined. The results are shown in Table 2.

[0059] In Table 2, if no fracture was observed even after 500 cycles or more, it is indicated as "A", if fracture occurred after 300 to 500 cycles, it is indicated as "B", and if fracture occurred after less than 300 cycles, it is indicated as "C". It can be said that an electrodeposited copper foil that breaks in less than 300 cycles is not suitable for use as a negative electrode current collector. It can be said that an electrodeposited copper foil that breaks in 300 cycles or more but less than 500 cycles is suitable for use as a negative electrode current collector. An electrodeposited copper foil that does not break even after 500 cycles or more is particularly suitable for use as a negative electrode current collector, and can improve the charge / discharge cycle characteristics of a lithium ion secondary battery.

[0060] As can be seen from Table 2, in the lithium ion secondary batteries using the electrodeposited copper foils of Examples 1 to 27 as negative electrode current collectors, the foil thickness t of the electrodeposited copper foil was 10 or more and 20 or less, Gs / t was 10 or more and 40 or less, and E / t was 0.9 or more and 1.8 or less. Therefore, even after repeated charge and discharge, the electrodeposited copper foil was less likely to break, and the charge and discharge cycle characteristics of the lithium ion secondary batteries were excellent. [Explanation of symbols]

[0061] 11. Rotating electrode 12. Insoluble electrode 13...electrolyte 14... Electrolytic copper foil

Claims

1. 1. An electrolytic copper foil having a foil thickness of t (unit: μm), a gloss of a surface at the end of electrolytic deposition measured by irradiating the surface at the end of electrolytic deposition with light at an incident angle of 60° along the length direction, Gs (unit: %), and a breaking elongation measured by pulling the surface along the length direction, Gs / t, which is obtained by dividing the gloss Gs by the foil thickness t, is 10 or more and 20 or less, and E / t, which is obtained by dividing the breaking elongation E by the foil thickness t, is 1.08 or more and 1.8 or less.

2. 2. The electrodeposited copper foil according to claim 1, wherein the foil thickness t is 12 or more and 20 or less, Gs / t, which is obtained by dividing the gloss Gs by the foil thickness t, is 25 or more and 37.73 or less, and E / t, which is obtained by dividing the breaking elongation E by the foil thickness t, is 1.2 or more and 1.7 or less.

3. An electrolytic copper foil according to claim 1 or claim 2, wherein E / t, obtained by dividing the breaking elongation E by the foil thickness t, is 1.3 or more and 1.6 or less.

4. The electrodeposited copper foil according to any one of claims 1 to 3, wherein the root mean square height Sq of the electrolytic deposition finished surface measured using a white light interference microscope is 0.1 µm or more and 0.4 µm or less.

5. The electrodeposited copper foil according to any one of claims 1 to 3, wherein the root mean square height Sq of the surface at the end of the electrolytic deposition measured using a white light interference microscope is 0.1 µm or more and 0.25 µm or less.

6. The electrodeposited copper foil according to any one of claims 1 to 5, wherein the tensile strength measured by pulling along the longitudinal direction is 300 MPa or more and 380 MPa or less.

7. The electrodeposited copper foil according to any one of claims 1 to 6, which is used for a negative electrode current collector of a lithium ion secondary battery.

8. A negative electrode for a lithium ion secondary battery comprising the electrolytic copper foil according to any one of claims 1 to 7.

9. A lithium ion secondary battery comprising the negative electrode for lithium ion secondary batteries according to claim 8.

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