JP7474096B2 - Ni-plated steel foil for nickel-hydrogen secondary battery current collector, nickel-hydrogen secondary battery current collector, and nickel-hydrogen secondary battery - Google Patents

Description

The present invention relates to Ni-plated steel foil for nickel-hydrogen secondary battery current collectors, nickel-hydrogen secondary battery current collectors, and nickel-hydrogen secondary batteries.

In recent years, with the rapid spread of electronic devices such as personal computers and mobile phones, a large number of rechargeable and dischargeable batteries, such as nickel-metal hydride batteries, lithium-ion secondary batteries, and even lithium polymer secondary batteries, are being used. Nickel-metal hydride secondary batteries in particular have a high energy density and are therefore used as power sources for mobile communications and portable information terminals. Nickel-metal hydride secondary batteries have moderate energy density and output characteristics, but are advantageous in terms of reliability, safety, and cost, and have recently been put to practical use in vehicles, with the market growing rapidly. Accordingly, in order to further pursue miniaturization and weight reduction, there is a demand for performance improvements to further reduce the size and weight of batteries, which occupy a large volume in devices.

The basic structure of such a secondary battery consists of an electrode coated with a material that undergoes a reversible electrochemical reaction (so-called active material) on a foil-shaped metal current collector, a separator that separates the positive and negative electrodes, an electrolyte, and a battery case. Nickel-hydrogen secondary batteries generally use nickel foam as the current collector or core, but nickel foam is expensive because it is formed into a porous body through a complex manufacturing process. In addition, the current collector or core itself does not directly contribute to the battery capacity. Therefore, in order to meet the recent demand for higher capacity, the use of inexpensive and thin metal foil as the current collector has begun to be considered.

Iron-based foils have been proposed as the foil-shaped metal current collectors mentioned above. Iron has a higher electrical resistance than copper, but recent improvements in battery structure and the diversification of battery applications and required characteristics mean that electrical resistance is no longer necessarily a problem.

As an example of using iron foil as a negative electrode current collector, Patent Document 1 proposes using electrolytic iron foil with a thickness of 35 microns or less as a current collector for the negative electrode of a lithium secondary battery. It also proposes using Ni-plated electrolytic iron foil from the viewpoint of rust prevention.

Patent Document 2 proposes using a metal foil made by forming ferric oxide on the surface of iron foil or nickel-plated iron foil as a negative electrode current collector for non-aqueous electrolyte secondary batteries such as lithium secondary batteries. However, this iron-based metal foil inevitably causes iron elution during overdischarge, and is prone to side reactions at the negative electrode potential, which can result in impaired battery efficiency and lifespan.

Patent Documents 3 and 4 propose steel foils that can be used as negative electrode current collectors for non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries. These steel foils are described as being thin and strong, lightweight, and economical, and are high-strength steel foils for negative electrode current collectors that are excellent in rust prevention, resistance to metal ion elution during overdischarge, and stability at the negative electrode potential.

As described in the above patent documents, steel foil is thin and strong, lightweight and economical, and has rust resistance, resistance to metal ion elution during overdischarge, and stability at negative electrode potential, and is known to be excellent for use as a negative electrode current collector in lithium ion secondary batteries.
However, the iron foils and steel foils proposed in these patent documents are all intended for use in lithium ion secondary batteries, and are not intended for use as current collectors in nickel-hydrogen secondary batteries.

The steel foils described in Patent Documents 3 and 4 are thin and strong, and have excellent rust prevention properties, resistance to metal elution during overdischarge, and stability at negative electrode potential, so it is intended to use these steel foils as current collectors for nickel-metal hydride secondary batteries.
However, it has been found that when the steel foils described in Patent Documents 3 and 4 are used as current collectors for nickel-hydrogen secondary batteries, only a capacity far below the theoretical capacity (Ah/kg) of the nickel-hydrogen secondary battery can be obtained. This is believed to be because the electrolytes used are different. Due to the characteristics of lithium batteries, a non-aqueous electrolyte is used as the electrolyte for lithium-ion secondary batteries. On the other hand, an alkaline aqueous solution is usually used for nickel-hydrogen secondary batteries. Therefore, it is believed that the elution of metal ions from the current collector in an alkaline aqueous solution environment, which was not a problem with steel foils for lithium-ion secondary battery current collectors, is related to the decrease in battery capacity in nickel-hydrogen secondary batteries.

Japanese Patent Application Laid-Open No. 06-310126 Japanese Patent Application Laid-Open No. 06-310147 JP 2013-222696 A Patent No. 6124801

The present invention aims to provide a high-strength steel foil for nickel-hydrogen secondary battery current collectors that uses lightweight, economical steel foil to realize nickel-hydrogen secondary batteries that are thin, strong, and have excellent rust resistance and resistance to metal ion elution, as well as a nickel-hydrogen secondary battery current collector that includes the Ni-plated steel foil, and a nickel-hydrogen secondary battery that includes the Ni-plated steel foil.

The inventors of the present application discovered that the cause of the capacity decrease in the above-mentioned nickel-hydrogen secondary battery is that the metal components of the steel foil used as the current collector, especially the Fe component, dissolve into the electrolyte and are oxidized on the positive electrode. They then completed the present invention by using Ni-plated steel foil, in which the dissolution of the Fe component is suppressed, for the positive and negative electrode current collectors of nickel-hydrogen secondary batteries.

The gist of the present invention to achieve the above objective is as follows:

(1) In mass%,
C: 0.0001 to 0.0200%,
Si: 0.0001 to 0.0200%,
Mn: 0.005 to 0.300%,
P: 0.001 to 0.020%,
S: 0.0001 to 0.0100%,
Al: 0.0005 to 0.1000%,
N: 0.0001 to 0.0040%,
A Ni-plated steel foil containing one or both of Ti and Nb: 0.800% or less each, with the balance being Fe and unavoidable impurities, and having Ni-plated layers on both sides,
The thickness of the front and back Ni plating layers of the Ni-plated steel foil is 0.15 μm or more,
The thickness of the Ni-plated steel foil is 5 μm to 40 μm,
The tensile strength is more than 600 MPa and not more than 1200 MPa,
A Ni-plated steel foil for a current collector of a nickel-hydrogen secondary battery, characterized in that the surface defect area rate of both the front and back surfaces of the Ni-plated steel foil is 5.00% or less.
(2) The Ni-plated steel foil according to (1), wherein the thickness of the Ni-plated steel foil is 10 μm to 30 μm.
(3) The Ni-plated steel foil according to (1) or (2), wherein the thicknesses of the Ni-plated layers on the front and back surfaces of the Ni-plated steel foil are 0.20 μm or more and 1.50 μm or less, respectively.
(4) A nickel-hydrogen secondary battery current collector comprising the Ni-plated steel foil for nickel-hydrogen secondary battery current collector according to any one of (1) to (3) above.
(5) A nickel-hydrogen secondary battery comprising a positive electrode current collector, a positive electrode active material layer, a separator, a negative electrode active material layer, and a negative electrode current collector laminated in that order on a positive electrode current collector, wherein at least one of the positive electrode current collector and the negative electrode current collector is the nickel-hydrogen secondary battery current collector described in (4) above.

The present invention provides a nickel-hydrogen secondary battery current collector and nickel-hydrogen secondary battery that are thin, strong, and have little elution of Fe components that cause a decrease in battery capacity, and that are lightweight and economical. The Ni-plated steel foil for nickel-hydrogen secondary battery current collectors of the present invention can be suitably used for both the positive and negative electrode current collectors of nickel-hydrogen secondary batteries.

The Ni-plated steel foil for nickel-hydrogen secondary battery current collector of the present invention (hereinafter sometimes referred to as "Ni-plated steel foil of the present invention") has the following steel composition (% is mass %), and is characterized in that the thickness of the Ni plating layer on both sides (front and back) of the Ni-plated steel foil is 0.15 μm or more, the thickness of the Ni-plated steel foil is 5 μm to 40 μm, the tensile strength is more than 600 MPa and 1200 MPa or less, and the surface defect area ratio is 5.00% or less on both the front and back sides of the Ni-plated steel foil.

The Ni-plated steel foil of the present invention is particularly characterized in that the surface defect area ratio is 5.00% or less on both the front and back sides of the Ni-plated steel foil. As described above, when a steel foil for a lithium-ion secondary battery current collector is used as a current collector for a nickel-metal hydride secondary battery, a decrease in battery capacity due to the elution of metal ions from the current collector components, which has not been a particular problem except during overdischarge, becomes a problem.
As a result of research, the present inventors have found that this decrease in battery capacity is caused by surface defects in the Ni-plated steel foil causing metal components, particularly Fe components, to dissolve in an alkaline aqueous solution.

Surface defects in Ni-plated steel foil include defects such as cracks, scratches, and peeling in the Ni-plated layer that are introduced by contact with the rolling rolls or deformation of the rolled material when Ni-plated steel sheet (thin sheet) is rolled to make Ni-plated steel foil. It has been found that Fe, a metal component of the steel foil, dissolves from the defective parts of the Ni-plated layer into the alkaline aqueous solution of the electrolyte, causing a rapid decrease in the battery capacity of nickel-metal hydride secondary batteries.

The inventors of the present application have discovered that defects in the Ni plating layer can be dramatically reduced by controlling the foil rolling process when rolling a Ni-plated steel sheet (thin sheet) into Ni-plated steel foil. The surface defect area ratio of the Ni-plated steel foil of the present invention is 5.00% or less on both the front and back sides. If the surface defect area ratio exceeds 5.00%, the amount of eluted Fe ions becomes large, and the battery capacity obtained is only half or less of the theoretical capacity.

Surface defects in nickel-plated steel sheets are generally evaluated by the ferroxyl test. The surface defect area ratio of the Ni-plated steel foil specified in this invention is calculated from photographs of surface defects on the front and back surfaces of a test piece obtained based on the following test method.

As a specific procedure, first, prepare a ferroxyl test solution by dissolving 10 g/L of sodium ferrocyanide (potassium hexacyanoferrate (II) trihydrate), 10 g/L of potassium ferricyanide (potassium hexacyanoferrate (III) trihydrate), and 5 g/L of sodium chloride in pure water. Immerse a 50 mm square Ni-plated steel foil test piece having Ni plating layers on the front and back sides in this test solution for 3 minutes. Remove the test piece from the test solution, rinse with water, and dry at 65°C for 5 minutes. Photograph the front and back sides of the test piece on which blue spots have appeared, import it into a computer, and perform binarization processing using image analysis software to quantify the defect area rate of the front and back sides. As an example, a method of identifying the aforementioned blue spots using the binarization processing function using two thresholds of ImageJ (image analysis software) is described. First, convert the photo imported into the computer into grayscale at 8 bits. In grayscale images saved in 8-bit, a luminosity of 0 represents black, and a maximum value of 255 represents white. It has been found that blue spots can be accurately identified by setting the luminosity thresholds to 0 and 215. Therefore, the image is processed so that the luminosity range from 0 to 215 changes color to identify blue spots. After that, the area ratio of the blue spots is calculated using an analysis function. Note that image analysis software other than ImageJ may be used for the binarization process.

The components of the Ni-plated steel foil of the present invention are:
C: 0.0001 to 0.0200%,
Si: 0.0001 to 0.0200%,
Mn: 0.005 to 0.300%,
P: 0.001 to 0.020%,
S: 0.0001 to 0.0100%,
Al: 0.0005 to 0.1000%,
N: 0.0001 to 0.0040%,
One or both of Ti and Nb: 0.800% or less of each;
The balance is Fe and unavoidable impurities.

First, we will explain the reasons for limiting the component composition. In the following, % means mass %.

(C: 0.0001 to 0.0200%)
C is an element that increases the strength of steel, and work hardening is more likely to occur as the C content increases. If the deformation resistance during cold rolling increases with an increase in the C content, high pressure is required in the rolling rolls, and defects in the Ni-plated layer introduced when rolling a Ni-plated steel sheet (thin sheet) to form a Ni-plated steel foil increase. Furthermore, since excessive C content may deteriorate the electrical resistance of the steel, the upper limit of the C content is set to 0.0200%. The lower limit of the C content is not particularly specified, but since the limit in current refining technology is about 0.0001%, this is set as the lower limit. The C content is more preferably 0.0010% to 0.0100%.

(Si: 0.0001 to 0.0200%)
Although Si is an element that increases the strength of steel, excessive inclusion of Si may deteriorate the electrical resistance of steel, so the upper limit of the Si content is set to 0.0200%. If the Si content is less than 0.0001%, the refining cost becomes very high, so the lower limit of the Si content is set to 0.0001%. The Si content is more preferably 0.0010% to 0.0080%.

(Mn: 0.005 to 0.300%)
Mn is an element that increases the strength of steel, but if contained in excess, the electrical resistance of the steel may deteriorate, so the upper limit of the Mn content is set to 0.300%. If the Mn content is less than 0.005%, the refining cost becomes high and the steel may become too soft, resulting in reduced rollability, so the lower limit of the Mn content is set to 0.005%. The Mn content is more preferably 0.050% to 0.200%.

(P: 0.001 to 0.020%)
P is an element that increases the strength of steel, but if contained in excess, the electrical resistance of the steel may deteriorate, so the upper limit of the P content is set to 0.020%. If the P content is less than 0.001%, the refining cost may become very high, so the lower limit of the P content is set to 0.001%. The P content is more preferably 0.001% to 0.010%.

(S: 0.0001 to 0.0100%)
Since S is an element that reduces the hot workability and corrosion resistance of steel, the less the content, the better. Furthermore, in the case of a thin steel foil such as the steel foil according to the present embodiment, if there is a large amount of S, the electrical resistance may deteriorate due to inclusions caused by the presence of S, and the strength of the steel may decrease, so the upper limit of the S content is set to 0.0100%. If the S content is less than 0.0001%, the refining cost may become very high, so the lower limit of the S content is set to 0.0001%. The S content is more preferably 0.0010% to 0.0080%.

(Al: 0.0005 to 0.1000%)
Al is contained in an amount of 0.0005% or more as a deoxidizing element for the steel. Excessive content of Al may deteriorate the electrical resistance and may also lead to an increase in manufacturing costs, so the upper limit of the Al content is set to 0.1000%. The Al content is more preferably 0.0100% to 0.0500%.

(N: 0.0001 to 0.0040%)
Since N is an element that reduces the hot workability and workability of steel, the less N it contains, the better, and the upper limit of the N content is set to 0.0040%. If the N content is less than 0.0001%, the cost may become very high, so the lower limit of the N content is set to 0.0001%. The N content is more preferably 0.0010% to 0.0030%.

(Ti and Nb: 0.800% or less each)
The Ni-plated steel foil of the present invention further contains Ti and/or Nb in an amount of 0.100% or less. Ti and/or Nb can fix C and N in the steel as carbides and nitrides, improving the workability of the steel. However, excessive addition of these elements may lead to an increase in manufacturing costs and a deterioration in electrical resistance. The preferred content ranges are Ti: 0.010-0.800%, Nb: 0.005-0.050%. More preferred content ranges are Ti: 0.010-0.100%, Nb: 0.005-0.040%.

(impurities)
The term "impurities" used in this specification means impurity elements derived from raw materials, elements mixed during the production of Ni-plated steel sheets, and elements intentionally added within a range that does not impair the characteristics of the present invention. In the steel foil according to the embodiment of the present invention, the inclusion of impurities is permitted within a range that does not impair the characteristics of the present invention.

The steel foil according to the present invention may further contain elements such as B, Cu, Ni, Sn, and Cr in place of a portion of the Fe, to the extent that the properties of the steel foil according to the present embodiment are not impaired.

The components of the Ni-plated steel foil described above may be measured by a general analytical method. The components are measured at the center. Here, the center refers to any location excluding a portion 1 cm from the end of the Ni-plated steel foil. For example, the components may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method. The Ni-plated layer on the surface may be removed by mechanical grinding before analyzing the chemical composition.

The Ni-plated steel foil of the present invention has a Ni-plated layer on a first side and a second side of the steel foil, where the first side refers to one side of the Ni-plated steel foil and the second side refers to the other side of the Ni-plated steel foil.
The thickness of the Ni plating layer attached to the first and second sides of the steel foil is 0.15 μm or more. The thicker the Ni plating layer, the more improved the metal elution from the steel foil, but the cost increases. Even if the thickness of the Ni plating layer on each side exceeds 2.00 μm, no significant performance improvement is observed, so from the viewpoint of cost-effectiveness, the substantial upper limit of the thickness of the Ni plating layer is 2.00 μm. A more preferable thickness of the Ni plating layer is 0.20 μm or more and 1.50 μm or less.

Ni plating is applied to the first and second sides of the steel sheet before foil rolling (pre-rolling plating), and the Ni-plated steel sheet is annealed to form an Fe-Ni diffusion layer (also called an Fe-Ni alloy layer), and then the sheet is foil rolled. Foil rolling of steel sheet with a Ni plating layer requires careful attention. For example, if the elongation of the Ni plating during foil rolling is small compared to the elongation of the steel sheet, defects such as cracks may occur in the Ni plating layer, and these defects may cause a decrease in the foil strength.

As a Ni plating that does not reduce the foil strength, soft Ni plating is particularly suitable. Specifically, pure Ni plating that does not contain anything other than impurities and is applied to a steel sheet is heat-treated at 300°C or higher, and the Ni plating that releases the strain in the plating layer is the soft Ni plating in the Ni-plated steel foil of the present invention. In addition, Ni plating may be additionally performed after foil rolling in order to repair defects in the Ni plating layer that were introduced during foil rolling.

The plating thickness of the Ni-plated steel sheet is measured based on the test method specified in JIS H8501-1999. That is, in manufacturing, it is controlled by the plating current value, but directly, Ni plating is applied to the first and second sides of the steel sheet before foil rolling (pre-rolling plating), and before annealing the Ni-plated steel sheet, the Ni plating adhesion weight (g/m ² ) is measured using chemical analysis such as ICP. A calibration curve is prepared in advance for each Ni plating weight, and the Ni plating thickness is calculated from the fluorescent X-ray intensity of Ni. However, after annealing the Ni-plated steel sheet, the detection intensity of Ni diffused inside is lowered, so that the fluorescent X-ray intensity is detected as lower even for the same weight. Therefore, it is necessary to prepare a new calibration curve after annealing the Ni-plated steel sheet.

Furthermore, the plating thickness of the Ni-plated steel foil is measured by glow discharge spectroscopy (GDS). Specifically, the Ni plating thickness is the depth at which the Ni atom content ratio is 1/2 of the maximum value in the Ni atom depth profile measured by GDS. The area of the Ni-plated steel foil other than the Ni plating layer is steel foil. Here, the depth standard is the thickness obtained by converting the thickness into the product of the sputtering time and the sputtering rate of the silicon single crystal. The maximum Ni content of the Ni plating layer of the Ni-plated steel foil is 90 mass% or more. This maximum Ni content is obtained by measuring the content of each element by GDS.

The thickness of the Ni-plated steel foil of the present invention is 5 μm to 40 μm, including the Ni-plated layer. This is because a thin current collector foil, i.e., a thin steel foil, is desired when making batteries smaller and lighter using Ni-plated steel foil with sufficiently high mechanical strength as in the present invention. From the standpoint of making batteries smaller and lighter, a thinner steel foil is preferable, and there is no need to set a lower limit. However, when considering cost or thickness uniformity, a thickness of 5 μm or more is preferable. The thickness of the Ni-plated steel foil is preferably 10 μm to 30 μm.

The tensile strength of the Ni-plated steel foil of the present invention is more than 600 MPa and not more than 1200 MPa. The tensile strength is measured at room temperature. If the tensile strength is 600 MPa or less, problems may occur such as wrinkles or folds caused by handling when applying the active material layer to the current collector, or the steel foil may be deformed or the active material may peel off due to expansion and contraction of the active material caused by charging and discharging. The tensile strength of the steel foil is measured based on a test method that conforms to the metallic material tensile test method specified in JIS Z2241. The shape of the test piece is No. 13B, and the tensile direction is the rolling direction. The test speed is 1 mm/min.

From the viewpoint of preventing wrinkling, folding, and deformation of the steel foil and peeling of the active material, there is no particular need to limit the upper limit of the tensile strength. However, taking into consideration ease of handling and the stability when obtaining strength by work hardening through industrial rolling, 1200 MPa is the practical upper limit of the tensile strength of the steel foil.

The method for manufacturing the Ni-plated steel foil of the present invention is as follows. First, a thin sheet (steel sheet) with the above-mentioned predetermined composition is manufactured according to a normal thin sheet manufacturing method. Then, Ni plating is applied to the front and back surfaces of the steel sheet before foil rolling. This Ni-plated steel sheet is annealed to form an Fe-Ni diffusion layer (also called an Fe-Ni alloy layer), and then cold-rolled (foil rolling) to produce Ni-plated steel foil with a thickness of 5 μm to 40 μm. By utilizing the work hardening caused by cold rolling, a high strength of more than 600 MPa and not more than 1200 MPa is achieved.

The cumulative reduction during foil rolling is 70% or more. Here, the cumulative reduction is the percentage of the cumulative reduction amount (the difference between the entrance thickness before the first pass and the exit thickness after the final pass) relative to the entrance thickness of the first rolling stand. If the cumulative reduction is less than 70%, sufficient foil strength is not achieved. The cumulative reduction during foil rolling is preferably 80% or more. There is no particular upper limit to the cumulative reduction. However, with normal rolling capacity, the limit of the cumulative reduction that can be achieved is about 98%. In addition, in order to reduce plating defects due to rolling in each pass, it is preferable that the reduction in each rolling pass is in the range of 5 to 40%.

In addition, the unit rolling load (kN/mm) for each pass is controlled to an appropriate range. The unit rolling load is the load applied to the workpiece from the rolling roll divided by the width of the workpiece. A preferred unit rolling load is 0.5 to 1.2 kN/mm. If it is less than 0.5 kN/mm, the processing heat generated during rolling is small and the flexibility of the Ni plating layer is reduced, causing cracks in the Ni plating layer and increasing the number of surface defects. If it exceeds 1.2 kN/mm, the processing heat becomes too high, causing the Ni plating layer to be picked up by the rolling roll (Ni adheres to the rolling roll), resulting in a high surface defect area ratio.

In addition, by using the Ni-plated steel foil according to the embodiment of the present invention as a positive electrode collector or a negative electrode collector for a nickel-hydrogen secondary battery, a nickel-hydrogen battery in which the battery capacity is not easily reduced, that is, a long battery life can be obtained. Specifically, a typical nickel-hydrogen secondary battery is formed by sequentially laminating a positive electrode active material layer, a separator, a negative electrode active material layer, and a negative electrode current collector on a positive electrode current collector, but a nickel-hydrogen secondary battery current collector made of the Ni-plated steel foil according to the embodiment of the present invention is used for at least one of the above-mentioned positive electrode current collector and the above-mentioned negative electrode current collector. For the nickel-hydrogen secondary battery current collector according to the embodiment of the present invention, the Ni-plated steel foil may be used as it is, or may be surface-treated to improve the contact area with the active material layer.
The Ni-plated steel foil can be suitably used for both the positive electrode current collector and the negative electrode current collector, but is particularly suitable for use as the positive electrode current collector from the viewpoint of excellent resistance to metal ion elution.

In the nickel-metal hydride battery according to the embodiment of the present invention, known components can be used for each component other than the Ni-plated steel foil according to the embodiment of the present invention.
An example of the positive electrode current collector and the negative electrode current collector other than the Ni-plated steel foil is nickel foil.
An example of the active material used in the positive electrode active material layer is nickel hydroxide.
The active material used in the negative electrode active material layer may be, for example, a hydrogen storage alloy.
The separator may be, for example, a polyolefin nonwoven fabric or a polyamide nonwoven fabric.
In addition to these components, known outer containers, current collecting leads, electrolytes, conductive assistants, and binders can be used as components.

Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[Preparation of Ni-plated steel foil for testing]
A slab A having the following composition was produced by melting. The remainder is iron and impurities, and the units are mass %.

Slab A, whose composition is shown in Table 1, was hot-rolled and cold-rolled using conventional sheet metal manufacturing methods to obtain a sheet metal with a thickness of 0.15 mm.

[Ni plating operation]
For nickel plating, a plating bath containing 320 g/L nickel sulfate, 70 g/L nickel chloride, and 40 g/L boric acid was used, and nickel plating layers of 3.9 to 8.5 μm were formed on ^both sides of the steel sheet by varying the sheet passing speed under conditions of a bath temperature of 65° C. and an electrolysis current of 20 A/dm 2. Next, continuous annealing treatment was performed in an atmosphere of 5% H ₂ (balance N ₂ ) at a holding temperature of 820° C. for a holding time of 40 sec.

[Foil rolling operation]
The foil rolling operation was carried out by setting the minimum/maximum rolling load (kN/mm) for each pass as shown in Table 2. Through the above operation, steel foils with steel foil numbers 2 to 9 were obtained.

[Measurement of thickness of Ni-plated steel foil]
The thickness of the resulting Ni-plated steel foil was measured with an electric micrometer.

[Measurement of plating thickness]
The plating thickness of the resulting steel foil was measured by the glow discharge luminescence spectroscopy described above, and the results are shown in Table 2.

[Measurement of tensile strength]
The tensile strength of the obtained Ni-plated steel foil was measured based on a test method conforming to the metallic material tensile test method specified in JIS Z2241-2011.

[Measurement of surface defect area ratio]
The surface defect area ratios of the plated steel foils of steel foil numbers 2 to 9 were measured based on the test method using potassium ferricyanide described above. Photographs of the surface defects on the front and back surfaces of the obtained test pieces were taken and binarized using image analysis software ImageJ to quantify the defect area ratios on the front and back surfaces. The area ratios of blue spots were then calculated using an analysis function. The results are shown in Table 2.

[Constant potential test in alkali]
To evaluate resistance to metal ion elution, a constant potential test was carried out on the steel foils of steel foil numbers 2 to 9, and the constant potential current value (μA/cm ² ) after 24 hours in an alkali was measured.
Immersion size: A Ni wire was spot welded to one end of a sample of about 50 mm square, and the connection was protected with a circuit tape of 0.05 mm thickness (Teraoka Seisakusho, product number No. 647). The sample was then immersed in a Teflon container with a lid filled with 6N (standard) KOH test solution. The test temperature was 65°C, and a potential was applied under the conditions of +0.4V vs. SHE, counter electrode: Pt, reference electrode: alkaline mercury electrode (RE-61AP manufactured by BAS). Equipment used: A potentiostat HA-151B manufactured by Hokuto Denko was used to measure the current change up to 24 hours after voltage application. If the constant potential current value after 24 hours was 4 μA/cm ² or less, it was considered to have passed, and if not, it was considered to have failed. The results are shown in Table 2.

As a standard for the evaluation of the constant potential current value after 24 hours in alkali, pure Ni foil (foil thickness 200 μm) was prepared as Comparative Example 1. As shown in Table 2, in the steel foils Nos. 3, 4, and 6 to 8 of the present invention, the surface defect area ratio of the Ni plating layer was 5.00% or less on both the front and back sides by controlling the unit rolling load of each rolling pass to an appropriate range. In the steel foil No. 7 of the present invention, the constant potential current value after 24 hours in alkali was improved to a level equivalent to that of the pure Ni foil (Comparative Example 1). On the other hand, in the steel foils Nos. 2, 5, and 9 of the comparative examples, the unit rolling load of each rolling pass was outside the appropriate range, so that although the thickness of the Ni plating layer was within the range of the present invention, the surface defect area ratio of the Ni plating layer was large, and the constant potential current value after 24 hours in alkali was significantly deteriorated.

Claims (5)

In mass percent,
C: 0.0001 to 0.0200%,
Si: 0.0001 to 0.0200%,
Mn: 0.005 to 0.300%,
P: 0.001 to 0.020%,
S: 0.0001 to 0.0100%,
Al: 0.0005 to 0.1000%,
N: 0.0001 to 0.0040%,
A Ni-plated steel foil containing one or both of Ti and Nb: 0.800% or less each, with the balance being Fe and impurities, and having a Ni-plated layer on both sides,
The thickness of the Ni-plated layer on the first surface and the second surface of the Ni-plated steel foil is 0.15 μm or more,
The thickness of the Ni-plated steel foil is 5 μm to 40 μm,
The tensile strength is more than 600 MPa and not more than 1200 MPa,
1. A Ni-plated steel foil for a current collector of a nickel-hydrogen secondary battery, wherein the surface defect area rate of both the first surface and the second surface of the Ni-plated steel foil is 5.00% or less. The Ni-plated steel foil according to claim 1, wherein the thickness of the Ni-plated steel foil is 10 μm to 30 μm. The Ni-plated steel foil according to claim 1 or 2, wherein the thickness of the Ni-plated layer on the front and back surfaces of the Ni-plated steel foil is 0.20 μm or more and 1.50 μm or less, respectively. A nickel-hydrogen secondary battery current collector made of the Ni-plated steel foil for nickel-hydrogen secondary battery current collector according to any one of claims 1 to 3. A nickel-hydrogen secondary battery in which a positive electrode active material layer, a separator, a negative electrode active material layer, and a negative electrode current collector are laminated in this order on a positive electrode current collector, and at least one of the positive electrode current collector and the negative electrode current collector is the nickel-hydrogen secondary battery current collector described in claim 4.