Ni-plated steel foil for negative electrode collector of nickel-hydrogen secondary battery, negative electrode collector of nickel-hydrogen secondary battery, and nickel-hydrogen secondary battery

Abstract

Provided is a high-strength steel foil for positive and negative electrode current collectors of nickel-hydrogen secondary batteries, which is lightweight and economical, thin, and excellent in strength, rust resistance, and resistance to metal elution. Also provided is a high-strength steel foil for positive and negative electrode current collectors of nickel-hydrogen secondary batteries, which is excellent in stretchability. A nickel-plated steel foil for hydrogen secondary battery current collectors contains, 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 %, one or both of Ti and Nb: 0.800 % or less each, and the remainder Fe and impurities, and has a nickel-plated layer on both surfaces. The thickness of the nickel-plated layer on both surfaces of the nickel-plated steel foil is 0.15 μm or more each, the thickness of the nickel-plated steel foil is 5 μm to 50 μm, the tensile strength is higher than 400 MPa and 1200 MPa or lower, and the surface defect area ratio is 5.00 % or less on both surfaces of the nickel-plated steel foil.

Description

TECHNICAL FIELD

[0001] The present application relates to a Ni-plated steel foil for a nickel-hydrogen secondary battery current collector, a nickel-hydrogen secondary battery current collector, and a nickel-hydrogen secondary battery. BACKGROUND

[0002] In recent years, with the rapid spread of electronic devices such as computers or cellular phones, batteries capable of large charge and discharge, such as nickel-hydrogen secondary batteries, lithium ion secondary batteries, and lithium polymer secondary batteries, are used. In particular, since nickel-hydrogen secondary batteries have a high energy density, they are used as power sources for mobile body communication and portable information terminals. In addition, although the energy density or output characteristics of nickel-hydrogen secondary batteries are mediocre, they have advantages in terms of reliability, safety, or cost, and in recent years, they have gradually been applied to vehicle-mounted applications, and their market has rapidly expanded. Along with this, further miniaturization and weight reduction have been pursued, and thus further performance improvements for miniaturization and weight reduction have been pursued for batteries that occupy a large volume among devices.

[0003] The basic structure of such a secondary battery is composed of an electrode of a foil-shaped metal current collector coated with a substance that causes a reversible electrochemical reaction, i.e., a so-called active material, a separator that separates the positive electrode and the negative electrode, an electrolyte, and a battery case. In nickel-hydrogen secondary batteries, nickel foam is generally used as a current collector or a core, but nickel foam is formed into a porous body through a complex manufacturing process, and thus is expensive. In addition, the current collector or the core itself does not directly contribute to the battery capacity. Therefore, in order to cope with the recent requirement for high capacity, research has been started on the use of inexpensive and thin metal foils for current collectors.

[0004] As the above-mentioned foil-shaped metal current collector, a scheme using an iron-based foil has been proposed in the past. Although iron has a larger electrical resistance than copper, with the recent research on the structure of batteries and the diversification of the purpose and characteristics of batteries, the electrical resistance does not necessarily become a problem.

[0005] As a scheme for using an iron foil in a negative electrode current collector, Patent Literature 1 proposes a scheme in which an electrolytic iron foil having a thickness of 35 micrometers or less is used for a current collector of a negative electrode of a lithium secondary battery. In addition, from the viewpoint of rust prevention, a scheme using an electrolytic iron foil after Ni plating has also been proposed.

[0006] Patent Literature 2 proposes a scheme in which a metal foil in which ferric oxide is formed on the surface of an iron foil or an iron foil subjected to Ni plating is used as a negative electrode current collector of a nonaqueous electrolyte secondary battery such as a lithium secondary battery. However, this iron-based metal foil cannot avoid Fe elution at the time of overdischarge, and is likely to cause a side reaction at a negative electrode potential, and as a result, is likely to impair the efficiency or the life of the battery.

[0007] Patent Documents 3 and 4 propose a steel foil that can be used as a negative current collector for a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery. These steel foils are thin and have strength, are lightweight and economical, and have rust resistance, resistance to metal ion elution at overdischarge, and stability at a negative electrode potential.

[0008] As described in the above patent documents, the steel foils are thin and have strength, are lightweight and economical, have rust resistance, resistance to metal ion elution at overdischarge, and stability at a negative electrode potential, and are excellent as a negative current collector for a lithium ion secondary battery.

[0009] However, the iron foils and the steel foils proposed in these patent documents are used as a current collector for a lithium ion secondary battery, and are not used as a current collector for a nickel-hydrogen secondary battery.

[0010] The steel foils described in Patent Documents 3 and 4 are thin and have strength, rust resistance, resistance to metal ion elution at overdischarge, and stability at a negative electrode potential, and thus it is intended to use these steel foils as a current collector for a nickel-hydrogen secondary battery.

[0011] However, it is known that if the steel foils described in Patent Documents 3 and 4 are used as a current collector for a nickel-hydrogen secondary battery, only a capacity far lower than the theoretical capacity (Ah / kg) of a nickel-hydrogen secondary battery can be obtained. The reason is considered to be that the electrolyte of a lithium ion secondary battery is different from the electrolyte of a nickel-hydrogen secondary battery. The electrolyte of a lithium ion secondary battery is a nonaqueous electrolyte used on the basis of the characteristics of a lithium battery. On the other hand, in a nickel-hydrogen secondary battery, an alkaline aqueous solution is generally used. Thus, elution of metal ions from the current collector in an alkaline aqueous solution, which is not a problem in the steel foils for a lithium ion secondary battery, is considered to be related to the decrease in the capacity of the battery in a nickel-hydrogen secondary battery.

[0012] Prior Art Documents

[0013] Patent Documents

[0014] Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 06-310126

[0015] Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 06-310147

[0016] Patent Document 3: Japanese Patent Application Laid-Open (JP-A) No. 2013-222696

[0017] Patent Document 4: Japanese Patent No. 6124801 SUMMARY

[0018] The present application has an object to provide a high-strength steel foil for a nickel-hydrogen secondary battery current collector, which is light and economical, thin and has strength, excellent rust resistance and metal ion elution resistance, realizes a high capacity of a nickel-hydrogen secondary battery, a nickel-hydrogen secondary battery current collector provided with the plated Ni steel foil, and a nickel-hydrogen secondary battery provided with the plated Ni steel foil.

[0019] Technical means for solving the technical problem

[0020] The present inventors have found that the reason for the low capacity of the above nickel-hydrogen secondary battery is that the metal components of the steel foil used as the current collector, particularly the Fe component, is eluted in the electrolyte, oxidized on the positive electrode, and by using a steel foil provided with a plated Ni layer that suppresses elution of the Fe component as the nickel-hydrogen secondary battery positive and negative electrode current collector, the present application has been completed.

[0021] Furthermore, according to the above insight, a high-strength steel foil for a nickel-hydrogen secondary battery current collector is obtained, for a steel foil provided with a plated Ni layer that suppresses elution of the Fe component, by performing heat treatment after cold rolling under specific conditions, thereby maintaining the performance of preventing elution of Fe and having excellent strength and elongation at break.

[0022] The object of the present application to achieve the above is as follows.

[0023] (1) A plated Ni steel foil for a nickel-hydrogen secondary battery current collector, characterized by containing, in mass%,

[0024] C: 0.0001 to 0.0200%;

[0025] Si: 0.0001 to 0.0200%;

[0026] Mn: 0.005 to 0.300%;

[0027] P: 0.001 to 0.020%;

[0028] S: 0.0001 to 0.0100%;

[0029] Al: 0.0005 to 0.1000%;

[0030] N: 0.0001 to 0.0040%;

[0031] one or both of Ti and Nb: 0.800% or less, respectively,

[0032] and the remainder is composed of Fe and impurities, and has plated Ni layers on both surfaces,

[0033] the thickness of the plated Ni layer of the first surface and the second surface of the plated Ni steel foil is 0.15 μm or more, respectively,

[0034] The Ni-plated steel foil has a thickness of 5 μm to 50 μm,

[0035] The tensile strength is higher than 400 MPa and 1200 MPa or lower,

[0036] The area ratio of surface defects is 5.00% or lower on both the first and second faces of the Ni-plated steel foil.

[0037] (2) The Ni-plated steel foil for a nickel-hydrogen secondary battery current collector according to the above (1), characterized by further having an elongation at break of 3% or more.

[0038] (3) The Ni-plated steel foil for a nickel-hydrogen secondary battery current collector according to the above (1) or (2), characterized in that the Ni-plated steel foil has a thickness of 10 μm to 30 μm.

[0039] (4) The Ni-plated steel foil for a nickel-hydrogen secondary battery current collector according to any one of the above (1) to (3), characterized in that the thickness of the Ni-plated layer on the first and second faces of the Ni-plated steel foil is 0.20 μm or more and 1.50 μm or less, respectively.

[0040] (5) A nickel-hydrogen secondary battery current collector composed of the Ni-plated steel foil for a nickel-hydrogen secondary battery current collector according to any one of the above (1) to (4).

[0041] (6) 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 sequentially stacked on a positive electrode current collector, at least one of the positive electrode current collector and the negative electrode current collector being the nickel-hydrogen secondary battery current collector according to the above (5).

[0042] Effects of the Invention

[0043] According to the present application, a thin, lightweight and economical nickel-hydrogen secondary battery current collector and a nickel-hydrogen secondary battery having strength, less elution of Fe component which is a cause of low battery capacity, and excellent stretchability can be provided. The Ni-plated steel foil for a nickel-hydrogen secondary battery current collector of the present application can be preferably used in either of a positive electrode current collector and a negative electrode current collector of a nickel-hydrogen secondary battery. Furthermore, a Ni-plated steel foil for a nickel-hydrogen secondary battery current collector having further excellent stretchability can be obtained. DETAILED DESCRIPTION

[0044] The Ni-plated steel foil for a nickel-hydrogen secondary battery collector according to the present application (hereinafter, sometimes referred to as "Ni-plated steel foil") is characterized in that it is composed of the following steel components (% by mass), the thickness of the Ni-plated layer on both surfaces (first surface and 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 50 μm, the tensile strength is higher than 400 MPa and 1200 MPa or less, and the surface defect area ratio on both the first surface and the second surface of the Ni-plated steel foil is 5.00% or less.

[0045] The Ni-plated steel foil according to the present application is characterized in that, in particular, the surface defect area ratio on both the first surface and the second surface of the Ni-plated steel foil is 5.00% or less. As described above, if the steel foil for a lithium-ion secondary battery collector is used as the collector of a nickel-hydrogen secondary battery, the problem is that the elution of metal ions of the collector component, which is not a problem except at the time of over-discharge, causes a decrease in the battery capacity.

[0046] The present inventors have found that the reason for the decrease in the battery capacity is that, due to the surface defects of the Ni-plated steel foil, the metal components of the steel foil, in particular, the Fe component, are eluted in the alkaline aqueous solution.

[0047] As the surface defects of the Ni-plated steel foil, there are, for example, the contact with the rolling roll at the time of rolling the steel sheet (thin plate) after Ni plating to make the Ni-plated steel foil, the cracks or flaws of the Ni-plated layer introduced by the deformation of the rolled material, peeling, and the like. It is known that, from the defective portions of the Ni-plated layer, the Fe component of the metal components of the steel foil is eluted in the alkaline aqueous solution of the electrolyte, and the battery capacity of the nickel-hydrogen secondary battery rapidly decreases.

[0048] The present inventors have found that the reason for the decrease in the battery capacity is that, due to the surface defects of the Ni-plated steel foil, the metal components of the steel foil, in particular, the Fe component, are eluted in the alkaline aqueous solution.

[0049] The surface defects of the Ni-plated steel foil are generally evaluated by the iron reagent test. The surface defect area ratio of the Ni-plated steel foil according to the present application is calculated from the photographs of the surface defects of the first surface and the second surface of the test piece obtained according to the following test method.

[0050] As a specific operation, first, a ferrous reagent test solution in which sodium ferrocyanide (calcium hexacyanoferrate(II) trihydrate) 10 g / L, potassium ferricyanide (calcium hexacyanoferrate(III) trihydrate) 10 g / L, and sodium chloride 5 g / L are dissolved in pure water is prepared. The test solution is immersed in a 50 mm square Ni-plated steel foil test piece having a plated Ni layer on the first and second surfaces for 3 minutes. The test piece is taken out of the test solution, washed with water, and dried at 65°C for 5 minutes. The first and second surfaces of the test piece on which blue spots appear are photographed, introduced into a computer, and subjected to binary processing using image analysis software, and the defect area ratio of the first and second surfaces is numerically evaluated. As one example, a method of recognizing the aforementioned blue spots using a binary processing function based on two thresholds of ImageJ (image analysis software) is described. First, the photograph introduced into the computer is subjected to grayscale processing at 8 bits. Further, in the grayscale image saved at 8 bits, black is indicated by a luminance of 0, and white is indicated by a maximum value of 255. It is ascertained that when 0 and 215 are set as thresholds of luminance, the blue spots can be recognized with good accuracy. Therefore, the image is processed so that the luminance is changed in the range of 0 to 215, and the blue spots are recognized. Thereafter, the area ratio of the blue spot portion is calculated using an analysis function. Further, the binary processing can also use image analysis software other than ImageJ.

[0051] The composition of the Ni-plated steel foil of the present application includes: 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 each, and the remainder being Fe and impurities.

[0052] First, the reason for limiting the composition is described. Further, in the present specification, a numerical range indicated using “~” indicates a range in which the values described before and after “~” are included as lower limit values and upper limit values. Values indicated as “less than” or “higher than” are not included in the numerical range. The % of the components indicates mass %.

[0053] (C: 0.0001 to 0.0200%)

[0054] C is an element that increases the strength of the steel and easily causes work hardening as the C content increases. As the deformation resistance during cold rolling increases as the C content increases, high pressurization in the rolling rolls is required, and thus defects in the Ni plating layer increase when the steel sheet (thin sheet) after Ni plating is rolled to become a Ni-plated steel foil. Also, if C is excessively contained, there is a case where the electrical resistance of the steel deteriorates, and thus the upper limit of the C content is set to 0.0200%. The lower limit of the C content is not particularly specified, but the limit in the current refining technology is about 0.0001%, and thus this is set as the lower limit. The C content is more preferably 0.0010% to 0.0100%.

[0055] (Si: 0.0001 to 0.0200%)

[0056] Si is an element that increases the strength of the steel, but if it is excessively contained, there is a case where the electrical resistance of the steel deteriorates, and thus the upper limit of the Si content is set to 0.0200%. If the Si content is set to less than 0.0001%, the refining cost becomes extremely large, and thus the lower limit of the Si content is set to 0.0001%. The Si content is more preferably 0.0010% to 0.0080%.

[0057] (Mn: 0.005 to 0.300%)

[0058] Mn is an element that increases the strength of the steel, but if it is excessively contained, there is a case where the electrical resistance of the steel deteriorates, and thus the upper limit of the Mn content is set to 0.300%. If the Mn content is set to less than 0.005%, the refining cost can become extremely large, and there is a case where the steel is excessively softened and the rollability is reduced, and thus the lower limit of the Mn content is set to 0.005%. The Mn content is more preferably 0.050% to 0.200%.

[0059] (P: 0.001 to 0.020%)

[0060] P is an element that increases the strength of the steel, but if it is excessively contained, there is a case where the electrical resistance of the steel deteriorates, and thus the upper limit of the P content is set to 0.020%. If the P content is less than 0.001%, the refining cost can become extremely large, and thus the lower limit of the P content is set to 0.001%. The P content is more preferably 0.001% to 0.010%.

[0061] (S: 0.0001 to 0.0100%)

[0062] S is an element that deteriorates the hot workability and corrosion resistance of the steel, and thus is preferably as little as possible. Also, in the case of a thin steel foil such as the steel foil of the present embodiment, if S is present in a large amount, there is a case where the presence of S causes inclusions that deteriorate the electrical resistance or the strength of the steel, and thus the upper limit of the S content is set to 0.0100%. If the S content is set to less than 0.0001%, there is a case where the refining cost becomes extremely large, and thus the lower limit of the S content is set to 0.0001%. The S content is more preferably 0.0010% to 0.0080%.

[0063] (Al: 0.0005 to 0.1000%)

[0064] Al is contained as a deoxidizing element of the steel in an amount of 0.0005% or more. If Al is contained in excess, the electrical resistance deteriorates, and in addition, there is a case where the manufacturing cost increases, and thus the upper limit of the Al content is set to 0.1000%. The Al content is more preferably 0.0100% to 0.0500%.

[0065] (N: 0.0001 to 0.0040%)

[0066] N is an element that deteriorates the hot workability and workability of the steel, and thus is preferably as little as possible, and the upper limit of the N content is set to 0.0040%. If the N content is set to less than 0.0001%, there is a case where the cost becomes extremely large, and thus the lower limit of the N content is set to 0.0001%. The N content is more preferably 0.0010% to 0.0030%.

[0067] (Ti and / or Nb: one or both of 0.800% or less)

[0068] The steel foil of the plated Ni steel foil of the present embodiment also contains Ti and / or Nb in an amount of 0.800% or less. Ti and / or Nb fix C and N in the steel as carbides and nitrides, and thus the workability of the steel can be improved. However, if Ti and / or Nb are added in excess, there is a case where the manufacturing cost increases and the electrical resistance deteriorates. The preferable content range is Ti: 0.010 to 0.800% and Nb: 0.005 to 0.050%. Also, the more preferable content range is Ti: 0.010 to 0.100% and Nb: 0.005 to 0.040%.

[0069] (impurities)

[0070] The term "impurities" used in the present specification indicates impurity elements derived from raw materials, elements mixed in during the process of manufacturing the plated Ni steel foil, and elements intentionally added, and is an element within a range that does not interfere with the characteristics of the present application. In the steel foil of the present embodiment, the mixing in of impurities is allowed within a range that does not interfere with the characteristics of the present application.

[0071] The steel foil of the present application can further contain, in addition to Fe, elements such as B, Cu, Ni, Sn, Cr, etc. in a range not impairing the characteristics of the steel foil of the present embodiment.

[0072] The components of the Ni-plated steel foil described above are measured by a general analysis method. The measurement position of the components is set to the central portion of the steel foil. Here, the so-called central portion is an arbitrary position except for a portion 1 cm from the end of the Ni-plated steel foil. The components are measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, C and S are measured using a combustion-infrared absorption method, and N is measured using an inert gas fusion-thermal conductivity method. The analysis of the chemical components is performed after removing the Ni-plated layer on the surface by mechanical polishing.

[0073] The Ni-plated steel foil of the present application has a Ni-plated layer on the first surface and the second surface of the steel foil. Here, the so-called first surface means one surface of the Ni-plated steel foil, and the so-called second surface means the other surface of the Ni-plated steel foil.

[0074] The thickness of the Ni-plated layer attached to the first surface and the second surface of the steel foil is 0.15 μm or more, respectively. The thicker the thickness of the Ni-plated layer, the higher the rust resistance and the improvement in the metal exudation from the steel foil, but the cost increases. Even if the thickness of the Ni-plated layer on each surface is higher than 2.00 μm, the performance cannot be confirmed to be significantly improved, and therefore, from the viewpoint of cost versus effect, the substantial upper limit of the thickness of the Ni-plated layer is 2.00 μm. The more preferable thickness of the Ni-plated layer is 0.20 μm or more and 1.50 μm or less.

[0075] Ni plating is performed on the first surface and the second surface of the steel sheet before the foil rolling (pre-rolling plating), and after the Ni-plated steel sheet is annealed to form an Fe-Ni diffusion layer (also referred to as an Fe-Ni alloy layer), the foil rolling is performed. Careful attention is required for the foil rolling of the steel sheet having the Ni-plated layer. For example, in the case where the extension of the Ni-plated layer at the time of the foil rolling is smaller than the extension of the steel sheet, defects such as cracks can sometimes occur in the Ni-plated layer, which can cause a decrease in the strength of the foil.

[0076] As the Ni-plated layer which does not cause a decrease in the strength of the foil, a soft Ni-plated layer is particularly preferable. Specifically, a pure Ni-plated layer attached to the steel sheet, which does not contain impurities, is subjected to heat treatment at 300°C or higher, whereby a Ni-plated layer in which the strain of the plated layer is eliminated is set as the soft Ni-plated layer in the Ni-plated steel foil of the present application. In addition, the Ni-plated layer can be additionally performed after the foil rolling for the purpose of repairing defects of the Ni-plated layer introduced in the foil rolling.

[0077] Further, the plating thickness of the Ni-plated steel sheet is measured according to the test method prescribed in JIS H 8501-1999. That is, the first surface and the second surface of the steel sheet before being subjected to foil rolling are directly subjected to Ni plating (pre-rolling plating) by controlling the plating current value in the production, and the amount of Ni plating (g / m 2 ) is measured using chemical analysis such as ICP before the Ni-plated steel sheet is annealed. A calibration curve is prepared in advance for the amount of Ni plating per unit area, and the plating thickness of the Ni plating layer is calculated from the fluorescent X-ray intensity of Ni. However, the detection intensity of Ni diffused into the inside is reduced after the Ni-plated steel sheet is annealed, and thus the fluorescent X-ray intensity is detected at a lower level even if the amount per unit area is the same. Therefore, it is necessary to prepare a calibration curve again after the Ni-plated steel sheet is annealed.

[0078] Further, the plating thickness of the Ni-plated steel foil is measured by glow discharge spectroscopy (GDS). Specifically, in the distribution of the amount of Ni atoms in the depth direction measured by GDS, the depth at which the content ratio of Ni atoms is 1 / 2 of the maximum value is set as the Ni plating thickness. The region in which the content of Fe atoms measured by GDS is 90% by mass or more is set as the steel foil. Further, the region present between the Ni plating layer and the steel foil is set as the Fe-Ni alloy layer. Here, the reference of the depth uses the thickness obtained by converting the product of the sputtering time and the sputtering speed into a silicon single crystal. Further, the maximum value of the content of Ni in the Ni plating layer of the Ni-plated steel foil is 90% by mass or more. The maximum content of Ni is obtained by measuring the content of each element by GDS. In the present application, the measurement is performed by glow discharge spectroscopy.

[0079] The thickness of the Ni-plated steel foil of the present application includes the Ni plating layer of 5 μm to 50 μm. This is because, on the basis of the use of the Ni-plated steel foil having a mechanical strength sufficiently high like the present application for the miniaturization and the weight reduction of the battery, a thinner current collector, that is, a thinner steel foil is expected. From the viewpoint of the miniaturization and the weight reduction, the thinner the steel foil is, the more preferable it is, but the lower limit is not particularly limited. However, 5 μm or more is sufficient in consideration of the cost or the uniformity of the thickness. The thickness of the Ni-plated steel foil is preferably 5 μm to 40 μm, more preferably 10 μm to 30 μm.

[0080] The tensile strength of the Ni-plated steel foil of the present application is higher than 400 MPa and 1200 MPa or lower. Further, the tensile strength is a measured value at normal temperature. When the tensile strength is 400 MPa or lower, there is a possibility that the occurrence of a wrinkle or a bend due to a process at the time of coating an active material layer on a current collector, or expansion and shrinkage of the active material accompanying charge and discharge, causes deformation of the steel foil or peeling of the active material. Further, the tensile strength of the steel foil is measured according to a test method based on the metal material tensile test method prescribed in JIS Z 2241. The shape of a test piece is No. 13B, and the tensile direction is the rolling direction. The test speed is set to 1 mm / min. The lower limit of the preferable tensile strength is 600 MPa.

[0081] From the viewpoint of preventing a wrinkle, a bend, deformation of the steel foil, and peeling of the active material, the upper limit of the tensile strength need not be particularly set. However, when the strength is considered in terms of the ease of processing and the stability of work hardening due to rolling in industry, 1200 MPa is a substantial upper limit of the tensile strength of the steel foil. The upper limit of the preferable tensile strength is 1000 MPa.

[0082] For the Ni-plated steel foil of the present application, stretch can be imparted by performing a heat treatment after cold rolling. The stretch of the Ni-plated steel foil after the heat treatment is preferably 3% or higher. More preferably, it is 4% or higher, and still more preferably, it is 5% or higher. If the elongation at break is 3% or higher, breakage at the time of winding the Ni-plated steel foil can be sufficiently prevented. Here, the stretch means the elongation at break. The stretch is measured according to a test method based on the metal material tensile test method prescribed in JIS Z 2241-2011. The shape of a test piece is set to No. 13B, and the tensile direction is set to the rolling direction. The test speed is set to 1 mm / min.

[0083] The manufacturing method of the Ni-plated steel foil of the present application is as follows. First, a thin plate (steel sheet) having the prescribed composition of the aforementioned components is manufactured in accordance with the usual method of manufacturing a thin plate. Thereafter, Ni plating is performed on the first and second surfaces of the steel plate before foil rolling. After annealing the Ni-plated steel plate to form an Fe-Ni diffusion layer (also referred to as an Fe-Ni alloy layer), a Ni-plated steel foil having a thickness of 5 μm to 40 μm is manufactured by cold rolling (foil rolling). By work hardening due to the cold rolling, a high-strength Ni-plated steel foil having a strength higher than 600 MPa and 1200 MPa or lower is manufactured.

[0084] The cumulative reduction ratio at the time of foil rolling is set to 70% or more. Here, the cumulative reduction ratio is the percentage of the cumulative reduction amount (the difference between the entry sheet thickness before the first pass and the exit sheet thickness after the final pass) with respect to the entry sheet thickness of the first rolling stand. When the cumulative reduction ratio is less than 70%, sufficient foil strength is not found. The cumulative reduction ratio at the time of foil rolling is preferably 80% or more. The upper limit of the cumulative reduction ratio is not particularly limited. However, under the usual rolling capacity, it is possible to achieve a cumulative reduction ratio of about 98%. In addition, in order to reduce plating defects caused by rolling at each pass, the reduction ratio at each pass of rolling is preferably set to a range of 5 to 40%.

[0085] In addition, the unit rolling load (kN / mm) at each pass is controlled in an appropriate range. The unit rolling load is obtained by dividing the load applied to the workpiece from the rolling roll by the sheet width of the workpiece. The preferred unit rolling load is 0.5 to 1.2 kN / mm. If it is less than 0.5 kN / mm, the processing heat accompanying rolling is small, the softness of the Ni-plated layer decreases, and thus cracks occur in the Ni-plated layer, and the surface defect area ratio increases. In addition, even if it is higher than 1.2 kN / mm, the processing heat becomes too much, and thus the Ni-plated layer is picked up by the rolling roll (Ni adheres to the rolling roll), and thus the surface defect area ratio increases.

[0086] In addition, heat treatment can be performed on the Ni-plated steel foil after cold rolling manufactured as described above. The heat treatment is performed at a temperature range of 700°C to 850°C, and for a period of 3 seconds to 30 seconds. By performing the heat treatment, the elongation can be set to 3% or more. In addition, here, the elongation means the breaking elongation. The elongation is measured according to a test method based on the metal material tensile test method prescribed in JIS Z 2241-2011. The shape of the test piece is set to No. 13B, and the tensile direction is set to the rolling direction. The test speed is set to 1 mm / min.

[0087] When the temperature of the heat treatment is less than 700°C, the recrystallization of the Ni-plated steel foil does not sufficiently proceed, and the elongation does not reach 3% or more. Therefore, the temperature of the heat treatment is set to 700°C or more. When the temperature of the heat treatment is higher than 850°C, the Ni of the Ni-plated layer diffuses in the steel foil, and thus the surface defect ratio is higher than 5%. Therefore, the temperature of the heat treatment is set to 850°C or less.

[0088] When the treatment time of the heat treatment is less than 3 seconds, the recrystallization of the Ni-plated steel foil does not sufficiently proceed, and the elongation does not reach 3% or more. Therefore, the treatment time of the heat treatment is set to 3 seconds or more. When the treatment time of the heat treatment is higher than 30 seconds, the Ni of the Ni-plated layer diffuses in the steel foil, and thus the surface defect ratio is higher than 5%. Therefore, the treatment time of the heat treatment is set to 30 seconds or less.

[0089] By using the Ni-plated steel foil of the present application as the positive electrode current collector or the negative electrode current collector of a nickel-hydrogen secondary battery, a nickel-hydrogen battery with a long battery life, i.e., a battery with a less likely decrease in battery capacity, can be obtained. Specifically, a conventional nickel-hydrogen secondary battery has a positive electrode active material layer, a separator, a negative electrode active material layer, and a negative electrode current collector sequentially stacked on the positive electrode current collector, but the nickel-hydrogen secondary battery current collector composed of the Ni-plated steel foil of the embodiment of the present application can be used for at least one of the above-described positive electrode current collector and the above-described negative electrode current collector. The nickel-hydrogen secondary battery current collector of the embodiment of the present application can directly use the Ni-plated steel foil, or can be subjected to surface processing in order to improve the contact area with the active material layer.

[0090] The Ni-plated steel foil is preferably used for either of the positive electrode current collector and the negative electrode current collector, but is particularly preferably used as the positive electrode current collector from the viewpoint of excellent resistance to elution of metal ions.

[0091] In the nickel-hydrogen battery of the embodiment of the present application, each of the components other than the Ni-plated steel foil of the embodiment of the present application can use a publicly known component.

[0092] As the positive electrode current collector and the negative electrode current collector other than the Ni-plated steel foil, for example, a nickel foil is given.

[0093] As the active material used for the positive electrode active material layer, for example, nickel hydroxide is given.

[0094] As the active material used for the negative electrode active material layer, for example, a hydrogen storage alloy is given.

[0095] As the separator, for example, a polyolefin nonwoven fabric, a polyamide nonwoven fabric is given.

[0096] In addition to these components, a publicly known exterior container, a current collecting lead, an electrolyte, a conductive aid, a binder can be used as the components.

[0097] [Example]

[0098] Next, the embodiment of the present application will be described, but the conditions of the example are one example of conditions adopted in order to confirm the practicability and effects of the present application, and the present application is not limited to this one example. The present application can adopt various conditions within the scope of the object of the present application, i.e., within the scope of achieving the object of the present application.

[0099] [Preparation of the Ni-plated steel foil for test]

[0100] Steels B and A having the following compositions were melted. The remaining portion was iron and impurities, and the unit was mass %.

[0101] Table 1 Table 1

[0102] billet C Si MnP S AlN Ti Nb A 0.0020 0.0100 0.0900.005 0.0060 0.03300.0024 0.037 - B 0.0043 0.0130 0.1700.012 0.0050 0.03400.0023 - 0.023

[0103] (unit: mass %)

[0104] (Example 1)

[0105] Steels A, B consisting of the components shown in Table 1 were subjected to hot rolling, cold rolling by a usual sheet manufacturing method to obtain a sheet having a thickness of 0.15 mm.

[0106] [Operation of Ni plating]

[0107] In the Ni plating, a plating bath containing nickel sulfate: 320 g / L, nickel chloride: 70 g / L, boric acid: 40 g / L was used under conditions of bath temperature: 65°C, electrolytic current: 20 A / dm 2 to form a Ni plating layer of 3.9 to 8.5 μm on both surfaces (first surface and second surface) of the steel sheet due to a difference in the passage speed. Subsequently, a continuous annealing treatment was performed under an atmosphere of 5% H2 (remainder N2) at a temperature of 820°C for 40 sec.

[0108] [Operation of foil rolling]

[0109] The foil rolling operation was performed by setting the minimum / maximum (kN / mm) of the rolling load per pass as shown in Table 2. By the above operation, steel foils No. 2 to 10 were obtained.

[0110] [Measurement of thickness of Ni-plated steel foil]

[0111] The thickness of the obtained Ni-plated steel foil was measured by an electric micrometer.

[0112] [Measurement of plating layer thickness]

[0113] The plating layer thickness of the obtained steel foil was measured by the above-described glow discharge emission spectrometry. The results are shown in Table 2.

[0114] [Measurement of tensile strength]

[0115] The tensile strength of the obtained Ni-plated steel foil was measured according to a test method based on the metal material tensile test method prescribed in JIS Z 2241-2011.

[0116] [Measurement of surface defect area ratio]

[0117] The surface defect area ratio of the plated steel foils No. 2 to 10 was measured according to the test method using the above-described potassium ferricyanide. The surfaces of the first surface and the second surface of the obtained test pieces were photographed and the surface defect area ratio was calculated.

[0118] The photos were binarized using the image analysis software ImageJ, and the area ratios of the defects on the first and second surfaces were numerically evaluated. Thereafter, the area ratio of the cyan spot portion was calculated using the analysis function. The results are shown in Table 2.

[0119] Constant potential test under alkaline conditions

[0120] To evaluate the metal ion elution resistance, constant potential tests were performed on the steel foils of Nos. 2 to 10, and the constant potential current values (μA / cm 2 ) after 24 hours under alkaline conditions were measured.

[0121] The test sample, which was immersed in a product No. 647, 0.05 mm thick, ring-shaped tape produced by Tenryo Manufacturing Co., Ltd., was spot-welded at one end with a Ni wire, and the connection portion was protected in a Teflon (registered trademark) container filled with 6N (specified) KOH test solution. The test temperature was set to 65°C, and the potential was applied under the conditions of +0.4 V vs. SHE, counter electrode: Pt, reference electrode: alkaline mercury electrode (BAS manufactured RE-61AP). The device used was a constant potential meter HA-151B manufactured by Hokuto Electric Co., Ltd., and the current change after 24 hours of applied voltage was measured. The constant potential current value after 24 hours was 4 μA / cm 2 The following cases were set as acceptable, and those other than the above were set as unacceptable. The results are shown in Table 2.

[0122] As a reference for evaluation of the constant potential current value after 24 hours under alkaline conditions, a pure Ni foil (foil thickness 200 μm) was prepared as steel foil No. 1 (Comparative Example 1). As shown in Table 2, in the steel foils of Nos. 3 to 5 and 7 to 9 of the present application examples, by controlling the unit rolling load of each pass of rolling within an appropriate range, the surface defect area ratio of the plated Ni layer was 5.00% or less on both the first and second surfaces. In the steel foil of No. 8 of the application example, the constant potential current value after 24 hours under alkaline conditions was improved to the same level as the pure Ni foil (steel foil No. 1, Comparative Example 1). On the other hand, in the steel foils of Nos. 2, 6, and 10 of the comparative examples, since the unit rolling load of each pass of rolling was out of the appropriate range, although the thickness of the plated Ni layer was within the range of the present application, the surface defect area ratio of the plated Ni layer increased, and thus the constant potential current value after 24 hours under alkaline conditions greatly deteriorated.

[0123] (Example 2)

[0124] In Example 2, it was confirmed that by performing a heat treatment on the plated Ni steel foil after cold rolling, an elongation of 3% or more was obtained.

[0125] Preparation of test plated Ni steel foils

[0126] From the steel billets A, B melted in Example 1, hot-rolled, cold-rolled by the same sheet manufacturing method as in Example 1, a sheet having a thickness of 0.15 mm was obtained.

[0127] [Operation of plating Ni]

[0128] In the plating of Ni, using the same plating bath as in Example 1, under the same conditions, the sheet passing speed was changed, and a plated layer having a thickness of 3.9 to 8.5 μm was formed on both sides of the steel sheet. Subsequently, continuous annealing treatment was performed at 820°C for 40 seconds in an atmosphere of 5% H2 (remainder N2).

[0129] [Operation of foil rolling]

[0130] As shown in Table 2, the minimum / maximum (kN / mm) of the rolling load per pass was set, and the operation of foil rolling was performed. According to the above operation, steel foils No. 11 to 20 were obtained.

[0131] [Operation of heat treatment]

[0132] After the operation of foil rolling, heat treatment was performed under the heat treatment conditions shown in Table 2. Further, " - " in the column of heat treatment temperature and heat treatment time in Table 2 indicates that heat treatment was not performed.

[0133] [Measurement of thickness of plated Ni steel foil]

[0134] The thickness of the obtained plated Ni steel foil was measured by the electric micrometer as in Example 1. The results are shown in Table 2.

[0135] [Measurement of plated layer thickness]

[0136] The plated layer thickness of the obtained steel foil was measured by the glow discharge emission spectrometry as in Example 1. The thickness of the plated Ni layer of 0.15 μm or more was set as pass, and others were set as fail. The results are shown in Table 2.

[0137] [Measurement of tensile strength and elongation]

[0138] The tensile strength and the elongation at break of the obtained plated Ni steel foil were measured according to the test method prescribed in JIS Z 2241-2011, Metal Materials Tensile Test Method, as the standard test method as in Example 1. The elongation at break of 3% or more was set as pass, and others were set as fail. The results are shown in Table 2.

[0139] [Measurement of surface defect area rate]

[0140] The surface defect area ratio of the Ni-plated steel foil of the steel foils No. 11 to 20 was measured in the same manner as in Example 1 according to the test method using potassium ferricyanide. The photographs of the surface defects of the first and second surfaces of the obtained test pieces were taken, and the surface defect area ratio of the first and second surfaces was numerically evaluated using the image analysis software ImageJ in the same manner as in Example 1. Thereafter, the area ratio of the cyan spot portion was calculated using the analysis function. The results are shown in Table 2.

[0141] Constant potential test under alkaline conditions

[0142] To evaluate the resistance to metal ion elution, the constant potential test was performed on the steel foils No. 11 to 20 in the same manner as in Example 1, and the constant potential current value (μA / cm 2 ) after 24 hours under alkaline conditions was measured.

[0143] The case where the constant potential current value after 24 hours was 4 μA / cm 2 or less was regarded as acceptable, and the other cases were regarded as unacceptable. The results are shown in Table 2.

[0144] As shown in Table 2, in the steel foils No. 13 to 15, 17 and 18, by appropriately controlling the unit rolling load of each pass of the rolling and the heat treatment conditions, the surface defect area ratio of the Ni-plated layer was 5.00% or less on both the first and second surfaces, and the elongation was 3% or more. In the steel foils No. 11, 12 and 19, there was no heat treatment step, or the conditions of the heat treatment were out of the appropriate range, and thus the elongation was low, but the thickness of the Ni-plated layer and the surface defect area ratio were within the range of the present application. In addition, in the steel foil No. 16, the heat treatment temperature was too high, and thus the surface defect area ratio of the Ni-plated layer increased, and the constant potential current value after 24 hours under alkaline conditions greatly deteriorated. In the steel foil No. 20, the rolling was too heavy, and thus the surface defect area ratio of the Ni-plated layer increased, and the constant potential current value after 24 hours under alkaline conditions greatly deteriorated.

[0145] Table 2

[0146]

Claims

1. A plated Ni steel foil for a nickel-hydrogen secondary battery current collector, characterized by comprising, in mass %, 0.0001 to 0.0200% of Si; C：0.0001～0.0200％； 0.005 to 0.300% of Mn; 0.800% or less of one or both of Ti and Nb, P：0.001～0.020％； S：0.0001～0.0100％； Al：0.0005～0.1000％； N：0.0001～0.0040％； the remainder being Fe and impurities, and having a plated Ni layer on both surfaces, the thickness of the plated Ni layer on the first and second surfaces of the plated Ni steel foil being 0.15 μm or more, respectively, the thickness of the plated Ni steel foil being 5 μm to 50 μm, the tensile strength being higher than 400 MPa and 1200 MPa or lower, the surface defect area ratio being 5.00% or less on both the first and second surfaces of the plated Ni steel foil.

2. The plated Ni steel foil for a nickel-hydrogen secondary battery current collector according to claim 1, characterized by further having an elongation at break of 3% or more.

3. The plated Ni steel foil for a nickel-hydrogen secondary battery current collector according to claim 1 or 2, the thickness of the plated Ni steel foil being 10 μm to 30 μm.

4. The plated Ni steel foil for a nickel-hydrogen secondary battery current collector according to any one of claims 1 to 3, the thickness of the plated Ni layer on the first and second surfaces of the plated Ni steel foil being 0.20 μm or more and 1.50 μm or less, respectively.

5. A nickel-hydrogen secondary battery current collector, comprising the plated Ni steel foil for a nickel-hydrogen secondary battery current collector according to any one of claims 1 to 4.

6. A nickel-hydrogen secondary battery, which is a nickel-hydrogen secondary battery configured by sequentially stacking 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, at least one of the positive electrode current collector and the negative electrode current collector being the nickel-hydrogen secondary battery current collector according to claim 5. ​ ​

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