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

Abstract

Provided is an electrolytic copper foil that is less likely to break. When the foil thickness of the electrolytic copper foil is t (unit: μm), the glossiness of the electrolytic deposition end surface, which is determined by irradiating light at an incident angle of 60° along the length direction with respect to the electrolytic deposition end surface, is Gs (unit: %), and the elongation, which is determined by stretching along the length direction, is E (unit: %), the foil thickness t is 10 or more and 20 or less, the glossiness Gs divided by the foil thickness t (Gs / t) is 10 or more and 40 or less, and the elongation E divided by the foil thickness t (E / t) is 0.9 or more and 1.8 or less.

Description

TECHNICAL FIELD

[0001] The present application 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 provided with the negative electrode for a lithium-ion secondary battery. BACKGROUND

[0002] A copper foil is sometimes used as a negative current collector of a lithium-ion secondary battery, but the copper foil is sometimes broken due to expansion and contraction of a negative electrode material at the time of charge and discharge of the lithium-ion secondary battery. In addition, since the adhered copper foil and the negative electrode material are partially peeled at the time of charge and discharge, stress at the time of expansion and contraction is concentrated in the peeled portion, and the copper foil is sometimes broken.

[0003] An electrolytic copper foil that can be used as a negative current collector of a lithium-ion secondary battery is disclosed in Patent Literatures 1 and 2. However, the electrolytic copper foil disclosed in Patent Literatures 1 and 2 is sometimes insufficient in mechanical properties or adhesion to a negative electrode material, and thus a lithium-ion secondary battery can be broken at the time of charge and discharge.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Publication No. 2007-217787

[0007] Patent Literature 2: Japanese Patent Publication No. 2016-204747 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] The present application 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 provided with the negative electrode for a lithium-ion secondary battery. The technical problem of the present application is to provide an electrolytic copper foil that is less likely to be broken. In addition, the technical problem of the present application is to provide a negative electrode for a lithium-ion secondary battery and a lithium-ion secondary battery in which a negative current collector is less likely to be broken at the time of charge and discharge.

[0010] TECHNICAL SOLUTION TO THE PROBLEM

[0011] An electrolytic copper foil according to one embodiment of the present application is characterized in that, when a foil thickness is set to t (unit: pm), a glossiness of an electrolytic deposition end surface determined by irradiating light at an incident angle of 60° along a length direction with respect to the electrolytic deposition end surface is set to Gs (unit: %), and an elongation determined by stretching along the length direction is set to E (unit: %), the foil thickness t is 10 or more and 20 or less, Gs / t, which is the glossiness Gs divided by the foil thickness t, is 10 or more and 40 or less, and E / t, which is the elongation E divided by the foil thickness t, is 0.9 or more and 1.8 or less.

[0012] Further, another aspect of the present application relates to a negative electrode for a lithium-ion secondary battery, which is characterized by including the electrolytic copper foil according to the above aspect.

[0013] Further, another aspect of the present application relates to a lithium-ion secondary battery, which is characterized by including the negative electrode for a lithium-ion secondary battery according to the above aspect.

[0014] Effects of the Invention

[0015] The electrolytic copper foil according to the present application is less likely to be broken. Further, the negative electrode for a lithium-ion secondary battery and the lithium-ion secondary battery according to the present application are less likely to be broken at the negative electrode current collector at the time of charge and discharge. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a view for explaining a method of manufacturing an electrolytic copper foil using an electrolytic deposition apparatus, and is a view for explaining a process of anodizing.

[0017] Figure 2 is a view for explaining a method of manufacturing an electrolytic copper foil using an electrolytic deposition apparatus, and is a view for explaining a process of copper plating. DETAILED DESCRIPTION

[0018] An embodiment of the present application will be described. Note that the embodiment described below shows one example of the present application. In addition, various changes and modifications can be made to the present embodiment, and such changes and modifications are also included in the present application.

[0019] The electrolytic copper foil according to one embodiment of the present application has a foil thickness t (unit: μm), a glossiness Gs (unit: %) of an electrolytic deposition end surface measured by irradiating light at an incident angle of 60° in the length direction with respect to the electrolytic deposition end surface, and an elongation rate E (unit: %) measured by stretching in the length direction, in which the foil thickness t is greater than or equal to 10 μm and less than or equal to 20 μm, the glossiness Gs divided by the foil thickness t (Gs / t) is greater than or equal to 10 and less than or equal to 40, and the elongation rate E divided by the foil thickness t (E / t) is greater than or equal to 0.9 and less than or equal to 1.8.

[0020] With such a structure, the electrolytic copper foil according to the present embodiment is less likely to be broken.

[0021] The electrolytic copper foil according to the present embodiment can be used as a negative electrode current collector of a lithium-ion secondary battery (mainly, a cylindrical lithium-ion secondary battery). That is, the negative electrode for a lithium-ion secondary battery according to the present embodiment includes the electrolytic copper foil according to the present embodiment. Further, the lithium-ion secondary battery according to the present embodiment includes the negative electrode for a lithium-ion secondary battery according to the present embodiment.

[0022] Since the electrolytic copper foil of the present embodiment is less likely to be broken, the negative electrode for a lithium-ion secondary battery and the lithium-ion secondary battery of the present embodiment are less likely to have the negative electrode current collector broken during charge and discharge.

[0023] Hereinafter, the electrolytic copper foil of the present embodiment will be described in more detail.

[0024] The present inventors have conducted intensive studies, and as a result, have found that the electrolytic copper foil having both high stretchability and high glossiness of the electrolytic deposition end surface is less likely to be broken even if the negative electrode material of the lithium-ion secondary battery expands and shrinks during charge and discharge.

[0025] If the electrolytic copper foil has high stretchability, since the electrolytic copper foil can follow the expansion and shrinkage of the negative electrode material, the electrolytic copper foil is less likely to be broken. In addition, if the glossiness is high and the surface is flat, since the adhesion force of the adhered electrolytic copper foil to the negative electrode material is uniform over the entire adhered surface, local peeling between the adhered electrolytic copper foil and the negative electrode material during charge and discharge can be suppressed. If local peeling occurs between the electrolytic copper foil and the negative electrode material, stress at the time of expansion and shrinkage is concentrated on the peeled portion, and thus the electrolytic copper foil is easily broken, but since the electrolytic copper foil of the present embodiment is less likely to be peeled, the electrolytic copper foil is less likely to be broken during charge and discharge.

[0026] Note that the glossiness Gs and the elongation rate E have a correlation. The higher the glossiness, the finer the crystal grains of copper, and the higher the driving force for recrystallization, and thus the crystal grain diameter increases by the heating softening treatment, and the electrolytic copper foil becomes highly stretchable.

[0027] It is known that the electrolytic copper foil undergoes recrystallization of the crystal grains at a temperature around room temperature, and a normal temperature softening phenomenon occurs, but it can be considered that the finer the crystal grains immediately after deposition, the greater the driving force for recrystallization, and the greater the crystal grains after the crystal growth is stopped after the normal temperature softening. Therefore, it can be considered that if finer crystal grains are deposited by the control of the thickness of the oxide film described later, the crystal grains in the inside of the electrolytic copper foil after the normal temperature softening become larger, and the elongation rate increases.

[0028] In a general two-side gloss copper foil, the glossiness increases as the foil thickness increases. Therefore, the increase in the glossiness is affected by two factors, the densification of the crystal grains in the inside of the electrolytic copper foil and the increase in the foil thickness. Therefore, regarding electrolytic copper foils of different foil thicknesses, when the fineness of the crystal grains is inferred from the glossiness, it can be considered that the foil thickness needs to be used to normalize the glossiness, and the contribution of the foil thickness to the glossiness needs to be eliminated before comparison. Therefore, in the present application, a parameter Gs / t after the normalization of the glossiness Gs with the foil thickness t is defined.

[0029] In addition, regarding the elongation, the contribution of both the foil thickness and the crystal grain is considered, and when the contribution of the grain refinement after the foil is produced to the elongation is analogized, it can be considered that the elongation E normalized by the foil thickness t, E / t, needs to be regulated.

[0030] It is known that by regulating both the parameter Gs / t in which the glossiness Gs is normalized by the foil thickness t and the parameter E / t in which the elongation E is normalized by the foil thickness t, the electrolytic copper foil in which the breakage is less likely to occur can be obtained.

[0031] (Gs / t)

[0032] The parameter Gs / t needs to be 10 or more and 40 or less, but it is preferably 25 or more and 40 or less. If the parameter Gs / t is within the above range, since the surface of the electrolytic copper foil is flat, the adhesion of the electrolytic copper foil to the negative electrode material is easily made uniform over the entire adhesion surface. Therefore, since the partial peeling between the adhered electrolytic copper foil and the negative electrode material at the time of charge and discharge can be suppressed, the breakage at the time of charge and discharge is less likely to occur. On the other hand, if the parameter Gs / t is within the above range, the adhesion of the electrolytic copper foil to the negative electrode material is sufficiently exhibited, so the breakage at the time of charge and discharge is less likely to occur.

[0033] Note that the glossiness Gs is measured by irradiating light at an incident angle of 60° in the length direction of the electrolytic deposition end surface, and the "length direction" of the electrolytic copper foil in the present application means MD (Machine Direction). For example, if the copper foil is formed by plating on the surface of a rotating electrode when the electrolytic copper foil is produced using the rotating electrode, it means the rotation direction of the rotating electrode.

[0034] (E / t)

[0035] The parameter E / t needs to be 0.9 or more and 1.8 or less, is preferably 1.2 or more and 1.7 or less, and is more preferably 1.3 or more and 1.6 or less. If the parameter E / t is within the above range, since the electrolytic copper foil has high stretchability, the breakage at the time of charge and discharge is less likely to occur.

[0036] (Sq)

[0037] The root mean square height Sq of the electrolytic deposition end surface of the electrolytic copper foil of the present embodiment measured using a white light interference microscope is preferably 0.1 μm or more and 0.4 μm or less, and is more preferably 0.1 μm or more and 0.25 μm or less.

[0038] If the root mean square height Sq of the electrolytic termination surface is within the aforementioned range, the adhesion between the electrolytic copper foil and the negative electrode material tends to be higher due to the anchoring effect. Furthermore, if the root mean square height Sq of the electrolytic termination surface is within the aforementioned range, the adhesion between the tightly bonded electrolytic copper foil and the negative electrode material becomes uniform across the entire bonding surface because the electrolytic termination surface is sufficiently flat. Therefore, localized peeling between the tightly bonded electrolytic copper foil and the negative electrode material during charging and discharging can be suppressed, making breakage less likely during charging and discharging.

[0039] [Tensile Strength]

[0040] The electrolytic copper foil of this embodiment preferably has a tensile strength of 300 MPa or more and 380 MPa or less, measured along its length. If the tensile strength is within this range, the electrolytic copper foil is less prone to breakage and exhibits better conformity to the expansion and contraction of the negative electrode material. The definition of "length direction" for the electrolytic copper foil is the same as that for gloss Gs.

[0041] It should be noted that the electrolytic copper foil of this embodiment can be used not only as the negative electrode current collector of lithium-ion secondary batteries, but also for other applications. For example, the electrolytic copper foil of this embodiment can also be used for circuit applications. Since the adhesion between the electrolytic copper foil and the resin can easily become uniform across the entire bonding surface, wrinkles on the electrolytic copper foil at high temperatures can be suppressed, and adverse conditions such as expansion caused by local unevenness in the adhesion can be suppressed.

[0042] [Method for manufacturing electrolytic copper foil]

[0043] The following describes an example of the method for manufacturing electrolytic copper foil according to this embodiment.

[0044] Electrolytic copper foil, for example, can be used Figure 1 , 2 The electrolytic extraction device shown is used for manufacturing. Figure 1 , 2 The electrolytic extraction apparatus includes: an insoluble electrode 12 made of titanium coated with platinum group elements or their oxides, and a rotating titanium electrode 11 disposed opposite to the insoluble electrode 12.

[0045] Copper plating is performed using an electrolytic deposition apparatus, where copper is deposited on the surface (cylindrical surface) of a cylindrical rotating electrode 11 to form a copper foil. The copper foil is then peeled off from the surface of the rotating electrode 11, thereby manufacturing the electrolytic copper foil of this embodiment. However, it is also possible to oxidize the surface of the rotating electrode 11 (hereinafter, sometimes referred to as "anodic oxidation") before copper plating to form an oxide film that is thicker and more uniform in thickness than a natural oxide film.

[0046] A natural oxide film of several nm or so in thickness is usually formed on the surface of the rotating electrode 11 at room temperature, but if an anodized film is formed by further anodizing the surface on which the natural oxide film is formed, an oxide film composed of the natural oxide film and the anodized film is formed, and an oxide film thicker than the natural oxide film and having a uniform thickness is formed. If the thickness of the oxide film has a distribution, the resistance during copper plating in the thicker portion becomes large, and the plating amount decreases. As a result, it is possible that a distribution occurs in the foil thickness of the electrolytic copper foil, or pinholes are generated in the electrolytic copper foil.

[0047] If an oxide film thicker than the natural oxide film and having a uniform thickness is formed by anodization, the distribution of the foil thickness and pinholes can be suppressed. In addition, since an oxide film thicker than the natural oxide film is present, copper plating can be performed at a higher overvoltage, and therefore the crystal grains of the precipitated layer at the initial stage of plating become fine. As a result, the elongation of the electrolytic copper foil after softening at room temperature is improved, and the glossiness of the electrolytic precipitated end surface is improved, and the surface roughness becomes small. Therefore, in the case of use as a negative current collector, it is possible to produce an electrolytic copper foil which is less likely to break during expansion and contraction.

[0048] The thickness of the anodized film formed by anodization can be controlled by the amount of electricity (unit: C / dm 2 ) applied to the rotating electrode 11, and in detail, can be controlled by the amount of electricity per unit area of the surface of the rotating electrode 11. By controlling the thickness of the anodized film, the thickness of the oxide film composed of the natural oxide film and the anodized film can be controlled.

[0049] The amount of electricity applied to the rotating electrode 11 is preferably 1000 C / dm 2 or more and 5000 C / dm 2 or less. If it is in the above numerical range, it is easy to control the parameter Gs / t to be 10 or more and 40 or less. In addition, if it is in the above numerical range, it is possible to avoid the thickness of the oxide film from being too thick, and therefore it is possible to suppress the surface roughness of the electrolytic copper foil from becoming large and the adhesion to the negative material from becoming uneven due to abnormal deposition during copper plating, and the like.

[0050] The method of producing an electrolytic copper foil by performing plating after anodization will be described below. First, the method of producing an electrolytic copper foil by performing plating after anodization will be described with reference to FIG. 2. Figure 1 Anodization will be described. In the case of anodization, a current is applied with the rotating electrode 11 as an anode and the insoluble electrode 12 as a cathode. As the insoluble electrode 12, for example, a DSE (Dimensionally Stable Electrode) electrode (registered trademark) can be used. In addition, as the electrolyte 13, for example, a 20% concentrated aqueous phosphoric acid solution can be used.

[0051] The electrolyte 13 is supplied from an electrolyte supply unit not shown between the rotating electrode 11 and the insoluble electrode 12 (refer to the hollow 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 certain speed in the direction indicated by the dotted arrow. As a result, an anodic oxide film is formed on the natural oxide film on the surface of the rotating electrode 11, and an oxide film thicker than the natural oxide film and having a uniform thickness is formed.

[0052] Next, the description will be made with reference to Figure 2 The copper plating will be described. In the case of copper plating, a current is applied with the rotating electrode 11 on which the oxide film is formed as a cathode and the insoluble electrode 12 as an anode. In addition, as the electrolyte 13, for example, an aqueous solution containing sulfuric acid and copper sulfate can be used. The copper concentration of the electrolyte 13 can be, for example, 50 to 150 g / L, and the sulfuric acid concentration can be, for example, 20 to 200 g / L.

[0053] If the electrolyte 13 is supplied from an electrolyte supply unit not shown between the rotating electrode 11 and the insoluble electrode 12 (refer to the hollow 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 certain speed in the direction indicated by the dotted arrow, copper is deposited on the surface of the rotating electrode 11. If the deposited copper is peeled from the surface of the rotating electrode 11, pulled up and continuously wound as indicated by the solid arrow in FIG. 14, an electrolytic copper foil 14 can be obtained. Figure 2

[0054] In the electrolyte 13 for copper plating, an additive such as an organic additive, an inorganic additive or the like can be added from the viewpoint of control of the smoothness or mechanical properties of the electrolytic copper foil. By adding the additive, the strength, elongation, surface roughness and gloss in the normal state can be improved. The additive can be used singly or in combination of two or more.

[0055] As the organic additive, for example, ethylene thiourea, polyethylene glycol and genamine can be listed.

[0056] As the inorganic additive, for example, a metal chloride such as sodium chloride (NaCl) or hydrogen chloride (HC1) can be used as a source of supply of chloride ions.

[0057] In the electrolyte 13 for copper plating, as the inorganic additive, 10 to 50 mass ppm of chloride ions (chlorine) is preferably added, and as the organic additive, 3 to 30 mass ppm in total of at least one of ethylene thiourea, polyethylene glycol and genamine is preferably added.

[0058] The electrolysis conditions in the copper plating can be, for example, as follows. That is, the liquid temperature of the electrolyte 13 is 18 to 67°C, the current density is 3 to 67 A / dm 2 ​.

[0059] On the surface of the electrolytic copper foil manufactured as described above, surface treatment can also be performed as needed. The surface treatment is described below.

[0060] Rust-proof treatment can also be performed on the surface of the electrolytic copper foil. As the rust-proof treatment, inorganic rust-proof treatment and organic rust-proof treatment can be listed. As the inorganic rust-proof treatment, for example, chromate treatment, plating treatment can be listed, and the chromate treatment can also be performed on the plating layer of the plating treatment. As the plating treatment, for example, plating of nickel, plating of nickel alloy, plating of cobalt, plating of cobalt alloy, plating of zinc, plating of zinc alloy, plating of tin, plating of tin alloy can be listed. As the organic rust-proof treatment, for example, surface treatment using benzotriazole can be listed.

[0061] Further surface treatment using a silane coupling agent (silane treatment) can also be performed on the surface on which the rust-proof treatment is performed. By the surface treatment using a silane coupling agent, a functional group having strong affinity with an adhesive is imparted to the surface of the electrolytic copper foil (the surface of the side to be joined with the negative electrode material or the resin), so the adhesion of the electrolytic copper foil to the negative electrode material or the resin is further improved, and the rust-proof property or the moisture absorption and heat resistance of the electrolytic copper foil are also further improved. 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.

[0062] The rust-proof treatment or the silane coupling agent treatment functions to improve the adhesion strength of the active material of the lithium ion secondary battery to the electrolytic copper foil, and prevent the decrease in the charge and discharge cycle characteristics of the lithium ion secondary battery.

[0063] In addition, before performing the above-described rust-proof treatment, roughening treatment can also be performed on the surface of the electrolytic copper foil. As the roughening treatment, for example, plating method, etching method and the like can be appropriately used. The plating method is a method of roughening the surface by forming a thin film layer having unevenness on the surface of the untreated electrolytic copper foil. As the plating method, electrolytic plating method, non-electrolytic plating method can be listed.

[0064] As the roughening treatment using the plating method, for example, a method of forming a plating film of copper or copper alloy or the like having copper as the main component on the surface of the untreated electrolytic copper foil is preferred. As the roughening treatment using the etching method, for example, a method using physical etching or chemical etching is preferred. As the physical etching, a method of etching using sandblasting or the like can be listed, and as the chemical etching, etching using a treatment liquid containing inorganic acid or organic acid, oxidizing agent and additive can be listed.

[0065] [Example]

[0066] The following examples and comparative examples illustrate the present invention in more detail. Electrolytic copper foils of Examples 1-27 and Comparative Examples 1-13 were manufactured, and negative electrode current collectors were manufactured using these electrolytic copper foils, and lithium-ion secondary batteries were manufactured using these negative electrode current collectors. Then, various characteristics of the electrolytic copper foils and lithium-ion secondary batteries were evaluated. The manufacturing methods of the electrolytic copper foils and lithium-ion secondary batteries, and the methods for evaluating various characteristics, are described.

[0067] (A) Anodizing

[0068] Use and Figure 1 Using the same apparatus, anodizing was performed through the same procedures described above, forming 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 amounts of electricity applied to the rotating electrode are shown in Table 1. Comparative Examples 8-10 are examples where anodizing was not performed.

[0069] [Table 1]

[0070]

[0071] (B) Copper plating

[0072] Following the anodizing process described in section (A) above, copper plating is performed using the same operation as described above, depositing copper on the surface of the rotating electrode where an oxide film has been formed. Then, the deposited copper is peeled off from the surface of the rotating electrode and continuously wound, thereby manufacturing the electrolytic copper foil of the embodiments and comparative examples (see [reference]). Figure 2 The temperature and current density of the electrolyte during copper plating are shown in Table 1.

[0073] The electrolyte used was an aqueous solution containing sulfuric acid, copper sulfate pentahydrate, and additives. Ethylene thiourea, polyethylene glycol, and Janus Green were used as additives. The concentrations of sulfuric acid, copper sulfate pentahydrate, and each additive are shown in Table 1. The concentration of copper sulfate pentahydrate is used as the concentration of copper. Additionally, the chlorine concentration in the electrolyte is shown in Table 1.

[0074] (C) Chromate treatment

[0075] The surfaces of each electrolytic copper foil manufactured in section (B) above are subjected to chromate treatment to form an anti-rust layer, which serves as the negative electrode current collector. The conditions for chromate treatment are as follows: the plating solution used in the chromate treatment contains potassium dichromate, the chromium concentration is preferably in the range of 6 to 12 g / L, and the treatment time is preferably in the range of 8 to 12 seconds. In this embodiment and the comparative example, the chromium concentration is 10 g / L, and the treatment time is 10 seconds.

[0076] (D) Manufacturing of the positive electrode

[0077] An electrode material paste was prepared by adding N-methyl-2-pyrrolidone and ethanol as solvents to a mixture of 90 mass% of lithium cobaltate (LiCoO2) powder, 7 mass% of graphite powder, and 3 mass% of polyvinylidene fluoride powder, and kneading the mixture. The electrode material paste was uniformly coated on an aluminum foil at a thickness of 15 μm. The aluminum foil coated with the electrode material paste was dried in a nitrogen atmosphere to volatilize the solvents, and then roll-pressed to produce a sheet having a total thickness of 150 μm. The sheet was cut into a strip having a width of 43 mm and a length of 285 mm, and a lead terminal of an aluminum foil was attached to one end of the strip by ultrasonic welding to serve as a positive electrode.

[0078] (E) Production of a negative electrode

[0079] An 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 diameter of 10 μm and 10 mass% of polyvinylidene fluoride powder, and kneading the mixture.

[0080] Each of the negative electrode current collectors produced in the above (C) was cut into a strip having a width of 720 mm. At this time, the width direction of the electrolytic copper foil was made to coincide with the width direction of the strip. Then, the electrode material paste was coated in a double-stripe pattern on both surfaces of the strip. The width of the linear electrode material paste coating film was 300 mm, and the direction in which the linear electrode material paste coating film was elongated was made to coincide with the long side direction of the strip.

[0081] The strip coated with the electrode material paste was dried in a nitrogen atmosphere to volatilize the solvents, and then roll-pressed to produce a sheet having a total thickness of 150 μm. The sheet was cut into a rectangular shape having a width of 43 mm and a length of 280 mm, and a lead terminal of a nickel foil was attached to one end of the sheet by ultrasonic welding to serve as a negative electrode.

[0082] (F) Production of a lithium ion secondary battery

[0083] A separator made of polypropylene having a thickness of 25 μm was interposed between the positive electrode and the negative electrode produced as described above, and the entire roll was wound to obtain a roll body. The roll body was housed in a cylindrical battery can, and the lead terminal of the negative electrode was spot-welded to the bottom of the battery can. Note that the battery can was formed of mild steel on the surface of which nickel plating was performed.

[0084] Next, an upper cover made of an insulating material was placed on the battery can, and a gasket was inserted, and the lead terminal of the positive electrode and a safety valve made of aluminum were ultrasonically welded to be connected. Then, a nonaqueous electrolyte solution composed of propylene carbonate, diethyl carbonate, and ethylene carbonate was injected into the battery can, and the upper cover was attached to the safety valve to assemble a lithium ion secondary battery of a sealed structure type having a cylindrical shape with an outer diameter of 14 mm and a height of 50 mm.

[0085] Next, various properties of each of the electrolytic copper foils produced in the above item (B) and each of the lithium-ion secondary batteries produced in the above item (F) were evaluated. The evaluation methods are described below. Note that the foil thickness of each of the electrolytic copper foils produced in the above item (B) is shown in Table 2.

[0086] [Table 2]

[0087]

[0088] 〔Glossiness Gs of electrolytic copper foil〕

[0089] The glossiness of the electrolytic deposition end surface of the electrolytic copper foil was measured using a glossiness meter VG7000 manufactured by Nippon Denshoku Industries Co., Ltd., based on the method prescribed in JIS Z8741-1997. In the measurement of the glossiness, the electrolytic deposition end surface was irradiated with light at an incident angle of 60° along the length direction (MD) of the electrolytic copper foil, and the glossiness was measured. The measurement of the glossiness was performed five times, and the average value thereof was taken as the glossiness Gs. The results are shown in Table 2.

[0090] Note that the measurement of the glossiness described above was performed on the electrolytic copper foil in the normal state. In the present application, the "normal state" means a state in which the electrolytic copper foil is left at normal temperature and humidity (for example, temperature 23 ± 2°C, humidity 50 ± 5% RH).

[0091] 〔Elongation E and tensile strength of electrolytic copper foil〕

[0092] The normal electrolytic copper foil which had not been subjected to heat treatment was cut into a rectangular shape having a width of 12.7 mm and a length of 130 mm, and used as a measurement sample. Then, a tensile test of the measurement sample was performed using a tensile testing machine Model 1122 manufactured by INSTRON Corporation, based on the method prescribed in IPC-TM-650, and the breaking elongation and the tensile strength were measured. In the tensile test, the distance between the chucks was 70 mm, and the tensile speed was 50 mm / min. The tensile test was performed on five measurement samples, and the average value thereof was taken as the elongation E and the tensile strength. The results are shown in Table 2.

[0093] 〔Root mean square height Sq of electrolytic copper foil〕

[0094] With ISO 25178 as a reference, the surface shape of the electrolytic deposition end surface of the electrolytic copper foil was measured using a white light interferometric optical microscope, Wyko Contour GT-K, manufactured by BRUKER, Inc., as a measurement object, and shape analysis was performed to obtain the root mean square height Sq. The measurement of the surface shape was performed at five arbitrary points on the electrolytic deposition end surface, and shape analysis was performed for each of the five points to obtain the root mean square height Sq of each of the five points. Then, the average of the results of the five points was taken as the root mean square height Sq of the electrolytic deposition end surface of the electrolytic copper foil.

[0095] The shape analysis was performed using a high-resolution CCD camera in a VSI measurement mode (vertical scanning interferometry). The conditions were as follows: the light source was white light, the resolution was 1280 x 980 pixels, the measurement magnification was 10 times, the measurement range was 477 μm x 357.8 μm, and the threshold was 3%. In addition, Fourier Filter processing was performed after filter processing of Terms Removal (Cylinder and Tilt) and Data Restore (Method: legacy, iterations 5) was performed for the entire measurement range of 477 μm x 357.8 μm.

[0096] In the Fourier Filter processing, High Freq Pass was used as the fourier filtering, Gaussian was used in the Fourier Filter Window, and High Cut off was set to 12.5 mm as the Frequency Cutoff. -1 .

[0097] In addition, Statistic Filter (Filter Size: 3, Filter Type: Median) processing was performed.

[0098] The root mean square height Sq was calculated by S parameters-height analysis with Remove Tilt as True. The results are shown in Table 2.

[0099] 〔Evaluation of charge-discharge cycle characteristics of lithium-ion secondary batteries〕

[0100] A charge-discharge cycle test was performed on the lithium-ion secondary batteries as follows: a cycle in which the lithium-ion secondary battery was charged to 4.2 V at a charge current of 100 mA and then discharged to 2.4 V at a discharge current of 100 mA was taken as one cycle. After repeating the cycle, the lithium-ion secondary battery was disassembled, and the presence or absence of a crack in the electrolytic copper foil was investigated. The results are shown in Table 2.

[0101] In Table 2, "A" indicates that no breakage was found even at 500 cycles or more, "B" indicates that breakage occurred at 300 cycles or more and less than 500 cycles, and "C" indicates that breakage occurred at less than 300 cycles.

[0102] It can be said that the electrolytic copper foil that broke at less than 300 cycles is not suitable for use as a negative current collector. It can be said that the electrolytic copper foil that broke at 300 cycles or more and less than 500 cycles is suitable for use as a negative current collector. The electrolytic copper foil that did not break at 500 cycles or more is particularly suitable for use as a negative current collector, and enables good charge-discharge cycle characteristics of a lithium ion secondary battery.

[0103] As is apparent from Table 2, the lithium ion secondary battery using the electrolytic copper foils of Examples 1 to 27 as a negative current collector has a foil thickness t of 10 or more and 20 or less, Gs / t of 10 or more and 40 or less, and E / t of 0.9 or more and 1.8 or less, and thus the electrolytic copper foil is not likely to break even when charge and discharge are repeated, and the lithium ion secondary battery has excellent charge-discharge cycle characteristics.

[0104] BRIEF DESCRIPTION OF DRAWINGS

[0105] 11: rotating electrode; 12: insoluble electrode; 13: electrolytic solution; 14: electrolytic copper foil.

Claims

1. An electrolytic copper foil, When the foil thickness is set to t, the gloss of the electrolytically degraded end surface measured by irradiating it with light at an incident angle of 60° along the length direction relative to the electrolytically degraded end surface is set to Gs, and the elongation measured by stretching along the length direction is set to E, the foil thickness t is 12 or more and 20 or less, the gloss Gs divided by the foil thickness t (Gs / t) is 10 or more and 37.73 or less, and the elongation E divided by the foil thickness t (E / t) is 1.08 or more and 1.8 or less. in, The unit for foil thickness t is μm, the unit for gloss Gs is %, and the unit for elongation E is %. The determination of gloss and elongation is carried out using electrolytic copper foil under normal conditions, where normal conditions refer to the state in which the electrolytic copper foil is placed at a temperature of 23±2℃ and a humidity of 50±5%RH.

2. The electrolytic copper foil according to claim 1, wherein, The gloss Gs divided by the foil thickness t, Gs / t, is 25 or higher and 37.73 or lower, and the elongation E divided by the foil thickness t, E / t, is 1.2 or higher and 1.7 or lower.

3. The electrolytic copper foil according to claim 1 or 2, wherein, The elongation E divided by the foil thickness t, E / t, is above 1.3 and below 1.

6.

4. The electrolytic copper foil according to claim 1 or 2, wherein, The root mean square height Sq of the electrolytic termination surface, as measured using a white interference microscope, is greater than 0.1 μm and less than 0.4 μm. The root mean square height was measured using normal electrolytic copper foil.

5. The electrolytic copper foil according to claim 1 or 2, wherein, The root mean square height Sq of the electrolytic termination surface, as measured using a white interference microscope, is greater than 0.1 μm and less than 0.25 μm.

6. The electrolytic copper foil according to claim 1 or 2, wherein, The tensile strength measured along the length direction is above 300 MPa and below 380 MPa. The tensile strength was measured using normal electrolytic copper foil.

7. The electrolytic copper foil according to claim 1 or 2, wherein, The electrolytic copper foil is used as the negative electrode current collector in lithium-ion secondary batteries.

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 a lithium-ion secondary battery as described in claim 8.

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