Ferritic stainless steel foil, electrode, and battery

Abstract

Provided is a ferritic stainless steel foil in which an increase in interface resistance when an oxide film is made thick is sufficiently suppressed. The ferritic stainless steel foil according to the present disclosure has a chemical composition containing, in mass%, C: over 0% to 0.050%, Si: over 0% to 1.00%, Mn: over 0 % to 1.00%, P: over 0% to 0.050%, S: over 0% to 0.030%, N: over 0% to 0.050%, Mo: 0% to 1.00%, Cr: 14.00% to 18.00%, Ni: 0% to 0.60%, Ti: [Timin] to 1.00%, Nb: 0% to 1.00%, and Zr: 0% to 0.80%, with the balance being Fe and impurities. The chemical composition satisfies formula (1).　Formula (1): (Mo+Cr) / Ti≤80. [Timin] is defined as follows.　When X defined by formula (2) is 0.10 or more: [Timin] = X. When X defined by formula (2) is less than 0.10: [Timin] = 0.10. Formula (2): X = 16 × (C + N)

Description

Ferritic stainless steel foil, electrode, and battery

[0001] The present disclosure relates to a ferritic stainless steel foil, an electrode using the ferritic stainless steel foil as a current collector, and a battery using the electrode.

[0002] Batteries such as primary batteries and secondary batteries have been used as power sources for various electronic devices. In recent years, secondary batteries, typified by lithium-ion batteries, have become increasingly popular due to the widespread use of small electronic devices such as home video cameras, laptops, and smartphones.

[0003] A secondary battery includes electrodes having a positive electrode and a negative electrode, and an electrolyte. Both the positive electrode and the negative electrode have an electrode mixture layer formed on a current collector. The electrode mixture layer is a layer containing an active material. The current collector has the function of supplying current to the active material and the function of serving as a substrate for supporting the electrode mixture layer.

[0004] Conventionally, electrolyte solutions have been used as electrolytes in secondary batteries. However, because electrolyte solutions contain flammable organic solvents, their usable temperature range is narrow. For this reason, in recent years, development of all-solid-state batteries using solid electrolytes instead of electrolyte solutions has been progressing. Because all-solid-state batteries do not contain organic solvents, stable battery performance can be obtained over a wide temperature range. Among the solid electrolytes used in all-solid-state batteries, sulfide-based solid electrolytes, including LPS (lithium phosphorus sulfide), have particularly high ionic conductivity. Therefore, the use of sulfide-based solid electrolytes can achieve high output in all-solid-state batteries.

[0005] On the other hand, when a sulfide-based solid electrolyte is used, the current collector may be corroded by sulfides. Corrosion of the current collector reduces battery performance. In other words, high corrosion resistance is required for the current collector. For this reason, the use of stainless steel as a current collector material is being considered. Stainless steel has an oxide film (also called a passive film) on its surface. Because the oxide film functions as a protective film against corrosion, stainless steel has high corrosion resistance. Among stainless steels, ferritic stainless steel in particular has lower electrical resistance and higher conductivity than austenitic stainless steel. For this reason, ferritic stainless steel is suitable as a current collector material.

[0006] A technology relating to ferritic stainless steel applicable to current collector applications is proposed in International Publication No. 2021 / 006089 (Patent Document 1).

[0007] The ferritic stainless steel sheet for use as a current collector in a sulfide-based solid battery disclosed in Patent Document 1 contains 0.001 to 0.050% C, 0.01 to 2.00% Si, 0.01 to 1.00% Mn, 0.050% or less P, 0.010% or less S, 18.00 to 32.00% Cr, 0.01 to 4.00% Ni, 0.001 to 0.150% Al, and 0.050% or less N, with the balance consisting of Fe and unavoidable impurities. Patent Document 1 states that this ferritic stainless steel sheet can achieve excellent sulfidation resistance by increasing the Cr content to 18.00% or more.

[0008] International Publication No. 2021 / 006089

[0009] In the ferritic stainless steel sheet disclosed in Patent Document 1, the corrosion resistance is improved by increasing the Cr content. However, as the Cr content of ferritic stainless steel increases, the workability of the ferritic stainless steel decreases. Therefore, in order to ensure sufficient corrosion resistance while suppressing such a decrease in workability, it is necessary to increase the corrosion resistance by other means without increasing the Cr content. Here, an effective means of increasing the corrosion resistance of ferritic stainless steel without increasing the Cr content is to thicken the surface oxide film.

[0010] However, in ferritic stainless steel foils used as current collectors, the thicker the oxide film on the surface, the greater the interfacial resistance with the electrode mixture layer formed on the ferritic stainless steel foil. High interfacial resistance between the current collector and the electrode mixture layer is undesirable because it increases the internal resistance of the battery and reduces output. Therefore, there is a need for the development of ferritic stainless steel foils that sufficiently suppress the increase in interfacial resistance even when the oxide film is thickened to improve corrosion resistance.

[0011] The object of the present disclosure is to provide a ferritic stainless steel foil in which the increase in interfacial resistance when the oxide film is made thick is sufficiently suppressed, an electrode using the ferritic stainless steel foil as a current collector, and a battery using the electrode.

[0012] The ferritic stainless steel foil of the present disclosure has a chemical composition, in mass%, of C: more than 0 to 0.050%, Si: more than 0 to 1.00%, Mn: more than 0 to 1.00%, P: more than 0 to 0.050%, S: more than 0 to 0.030%, N: more than 0 to 0.050%, Mo: 0 to 1.00%, Cr: 14.00 to 18.00%, Ni: 0 to 0.60%, Ti: [Ti ｍｉｎ ] to 1.00%, Nb: 0 to 1.00%, and Zr: 0 to 0.80%, with the balance being Fe and impurities, and satisfying formula (1). (Mo+Cr) / Ti≦80 (1) where [Ti ｍｉｎ ] is defined as follows: When X defined in formula (2) is 0.10 or more: [Ti ｍｉｎ ]=X When X defined in formula (2) is less than 0.10: [Ti ｍｉｎ ]=0.10 X=16×(C+N) (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) and (2).

[0013] The electrode of the present disclosure comprises the ferritic stainless steel foil of the present disclosure and an electrode mixture layer formed on the surface of the ferritic stainless steel foil.

[0014] The battery of the present disclosure comprises: an electrode of the present disclosure; and an electrolyte.

[0015] In the ferritic stainless steel foil of the present disclosure, an increase in interfacial resistance when the oxide film is thickened is sufficiently suppressed. In the electrode of the present disclosure, an increase in interfacial resistance with the electrode mixture layer is sufficiently suppressed when the oxide film of the ferritic stainless steel foil used as a current collector is thickened. In the battery of the present disclosure, an increase in interfacial resistance with the electrode mixture layer is sufficiently suppressed when the oxide film of the ferritic stainless steel foil used as a current collector is thickened.

[0016] Fig. 1 is a diagram showing an equivalent circuit used for fitting a Cole-Cole plot in an evaluation test of an interfacial resistance increase rate, and Fig. 2 is a schematic diagram showing an example of a Cole-Cole plot after fitting in an evaluation test of an interfacial resistance increase rate.

[0017] The present inventors have investigated, from the standpoint of chemical composition, ferritic stainless steel foils that can sufficiently suppress the increase in interfacial resistance when the oxide film is thickened. As a result, they found that the degree of increase in interfacial resistance when the oxide film is thickened is reduced by limiting the Cr content to 18.00% or less and the Mo content to 1.00% or less. While the reason for this is unclear, the following is thought to be one of the factors: Cr and Mo contained in ferritic stainless steel foil enhance the repair ability of the oxide film. Therefore, reducing the Cr and Mo contents increases the time required to repair microscopic defects that occur in the oxide film. As a result, the number of defects per unit thickness of the oxide film increases, which is thought to suppress the increase in interfacial resistance.

[0018] Based on the above findings, the inventors have discovered a steel sheet having a chemical composition, in mass %, of C: more than 0 to 0.050%, Si: more than 0 to 1.00%, Mn: more than 0 to 1.00%, P: more than 0 to 0.050%, S: more than 0 to 0.030%, N: more than 0 to 0.050%, Mo: 0 to 1.00%, Cr: 14.00 to 18.00%, Ni: 0 to 0.60%, Ti: [Ti ｍｉｎIt was thought that a ferritic stainless steel foil containing [Ti] to 1.00%, Nb: 0 to 1.00%, and Zr: 0 to 0.80%, with the balance being Fe and impurities, would be able to sufficiently suppress an increase in interface resistance when the oxide film is made thick. ｍｉｎ ] is defined as follows: When X defined in formula (2) is 0.10 or more: [Ti ｍｉｎ ]=X When X defined in formula (2) is less than 0.10: [Ti ｍｉｎ ]=0.10 X=16×(C+N) (2) In the formula (2), the content of the corresponding element in mass % is substituted for each element symbol.

[0019] However, even when a ferritic stainless steel foil having the above-described chemical composition is used as a current collector, there are cases where an increase in the interfacial resistance with the electrode mixture layer cannot be sufficiently suppressed when the oxide film is thickened. Therefore, the inventors further investigated the chemical composition of ferritic stainless steel foil. As a result, it was found that an increase in the interfacial resistance with the electrode mixture layer when the oxide film is thickened can be sufficiently suppressed by having the above-described chemical composition and further satisfying formula (1). (Mo+Cr) / Ti≦80 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %.

[0020] Although it is unclear why satisfying formula (1) makes it possible to sufficiently suppress an increase in interfacial resistance when the oxide film is thickened, the following is thought to be one of the factors: Ti may concentrate in the oxide film and reduce the electrical resistance of the oxide film. Therefore, as described above, it is thought that by appropriately adjusting the Ti content in accordance with the combined content of Cr and Mo, which increases the degree of increase in interfacial resistance when the oxide film is thickened, it is possible to sufficiently suppress an increase in interfacial resistance when the oxide film is thickened.

[0021] The above mechanism is merely a guess. Therefore, it is possible that the ferritic stainless steel foil of this embodiment suppresses the increase in interfacial resistance when the oxide film is thickened through a mechanism different from that described above. However, the examples described below demonstrate that the ferritic stainless steel foil can sufficiently suppress the increase in interfacial resistance when the oxide film is thickened by satisfying the above-mentioned chemical composition and formula (1).

[0022] The ferritic stainless steel foil of this embodiment, the electrode of this embodiment, and the battery of this embodiment have been completed based on the above technical concept, and have the following configurations.

[0023] The ferritic stainless steel foil of the first configuration has a chemical composition, in mass %, of C: more than 0.050%, Si: more than 0.00%, Mn: more than 0.00%, P: more than 0.050%, S: more than 0.030%, N: more than 0.050%, Mo: 0 to 1.00%, Cr: 14.00 to 18.00%, Ni: 0 to 0.60%, Ti: [Ti ｍｉｎ ] to 1.00%, Nb: 0 to 1.00%, and Zr: 0 to 0.80%, with the balance being Fe and impurities, and satisfying formula (1). (Mo+Cr) / Ti≦80 (1) where [Ti ｍｉｎ ] is defined as follows: When X defined in formula (2) is 0.10 or more: [Ti ｍｉｎ ]=X When X defined in formula (2) is less than 0.10: [Ti ｍｉｎ ]=0.10 X=16×(C+N) (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) and (2).

[0024] The electrode of the first configuration comprises: a ferritic stainless steel foil of the first configuration; and an electrode mixture layer formed on the surface of the ferritic stainless steel foil.

[0025] The battery of the first configuration comprises: electrodes of the first configuration; and an electrolyte.

[0026] The battery of the second configuration is the battery of the first configuration, wherein the electrolyte is a sulfide-based solid electrolyte.

[0027] The ferritic stainless steel foil of this embodiment, the electrode of this embodiment, and the battery of this embodiment will be described in detail below. Note that "%" regarding elements means mass % unless otherwise specified.

[0028] [Ferritic Stainless Steel Foil] The ferritic stainless steel foil of this embodiment comprises a foil body made of ferritic stainless steel and an oxide film formed on the surface of the foil body. Here, ferritic stainless steel refers to a steel with a Cr content of 10.5% or more and a microstructure mainly composed of ferrite. In this specification, a microstructure mainly composed of ferrite means that the volume fraction of ferrite in the microstructure is 95% or more. Furthermore, in this specification, the chemical composition of the ferritic stainless steel foil refers to the chemical composition of the foil body. The oxide film is mainly composed of Cr oxide.

[0029] [Chemical Composition] The chemical composition of the ferritic stainless steel foil of this embodiment contains the following elements.

[0030] C: More than 0 to 0.050% Carbon (C) increases the strength of ferritic stainless steel foil. Even if even a small amount of C is contained, the above effect can be obtained to some extent. Therefore, the C content is more than 0%. On the other hand, if the C content is 0.050% or less, the formation of a Cr-deficient layer due to the generation of Cr carbides can be suppressed. As a result, embrittlement of the oxide film is suppressed, and the corrosion resistance of the ferritic stainless steel foil is improved. Therefore, the C content is more than 0 to 0.050%. The preferred lower limit of the C content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the C content is 0.040%, more preferably 0.030%, and even more preferably 0.025%.

[0031] Si: More than 0 to 1.00% Silicon (Si) deoxidizes steel during the steelmaking process. Even if even a small amount of Si is contained, the above effect can be obtained to some extent. Therefore, the Si content is more than 0%. On the other hand, if the Si content is 1.00% or less, the workability of the ferritic stainless steel foil is improved. Therefore, the Si content is more than 0 to 1.00%. The preferred lower limit of the Si content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Si content is 0.90%, more preferably 0.80%, and even more preferably 0.50%.

[0032] Mn: More than 0 to 1.00% Manganese (Mn) increases the strength of ferritic stainless steel foil. Even if even a small amount of Mn is contained, the above effect can be obtained to some extent. Therefore, the Mn content is more than 0%. On the other hand, if the Mn content is 1.00% or less, the formation of MnS, which is the starting point for rust, is suppressed. As a result, the corrosion resistance of the ferritic stainless steel foil is improved. Therefore, the Mn content is more than 0 to 1.00%. The preferred lower limit of the Mn content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Mn content is 0.90%, more preferably 0.80%, and even more preferably 0.50%.

[0033] P: More than 0 to 0.050% Phosphorus (P) is an impurity. Therefore, the P content is more than 0%. If the P content exceeds 0.050%, P segregates at grain boundaries. As a result, even if the contents of other elements are within the ranges of this embodiment, the workability and corrosion resistance of the ferritic stainless steel foil will be reduced. Therefore, the P content is more than 0 to 0.050%. The P content is preferably as low as possible. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is 0.001%, and more preferably 0.010%. The preferred upper limit of the P content is 0.045%, and more preferably 0.040%.

[0034] S: More than 0 to 0.030% Sulfur (S) is an impurity. Therefore, the S content is more than 0%. If the S content exceeds 0.030%, S segregates at grain boundaries. As a result, even if the contents of other elements are within the ranges of this embodiment, the workability and corrosion resistance of the ferritic stainless steel foil are reduced. Therefore, the S content is more than 0 to 0.030%. The S content is preferably as low as possible. However, excessive reduction of the S content increases production costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is 0.001%, and more preferably 0.010%. The preferred upper limit of the S content is 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0035] N: More than 0 to 0.050% Nitrogen (N) is an impurity. Therefore, the N content is more than 0%. If the N content exceeds 0.050%, Cr carbonitrides are formed, causing a Cr-depleted layer. As a result, even if the contents of other elements are within the ranges of this embodiment, the workability and corrosion resistance of the ferritic stainless steel foil are reduced. Therefore, the N content is more than 0 to 0.050%. The N content is preferably as low as possible. However, excessive reduction of the N content increases production costs. Therefore, considering normal industrial production, the lower limit of the N content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.010%. The upper limit of the N content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.030%.

[0036] Mo: 0 to 1.00% Molybdenum (Mo) does not necessarily have to be contained. That is, the Mo content may be 0%. When Mo is contained, that is, when the Mo content is greater than 0%, Mo enhances the strength and corrosion resistance of the ferritic stainless steel foil. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. On the other hand, when the Mo content is 1.00% or less, the workability of the ferritic stainless steel foil is improved. Furthermore, an increase in interfacial resistance when the oxide film is thickened can be suppressed. Therefore, the Mo content is 0 to 1.00%. The preferred lower limit of the Mo content is greater than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Mo content is 0.90%, even more preferably 0.80%, and even more preferably 0.50%.

[0037] Cr: 14.00 to 18.00% Chromium (Cr) forms an oxide film and improves the corrosion resistance of ferritic stainless steel foil. When the Cr content is 14.00% or more, the above effect is sufficiently obtained. On the other hand, when the Cr content is 18.00% or less, the workability of the ferritic stainless steel foil is improved. Furthermore, an increase in interfacial resistance when the oxide film is thickened can be suppressed. Therefore, the Cr content is 14.00 to 18.00%. The preferred lower limit of the Cr content is 14.50%, and more preferably 15.00%. The preferred upper limit of the Cr content is 17.50%, and more preferably 17.00%.

[0038] Ni: 0 to 0.60% Nickel (Ni) does not necessarily need to be contained. In other words, the Ni content may be 0%. When Ni is contained, that is, when the Ni content is greater than 0%, Ni enhances the corrosion resistance of the ferritic stainless steel foil. Even if even a small amount of Ni is contained, the above effect can be obtained to some extent. On the other hand, when the Ni content is 0.60% or less, the workability of the ferritic stainless steel foil is improved. Therefore, the Ni content is 0 to 0.60%. The preferred lower limit of the Ni content is greater than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Ni content is 0.50%, even more preferably 0.40%, and even more preferably 0.35%.

[0039] Ti: [Ti ｍｉｎ ] to 1.00% where [Ti ｍｉｎ ] is defined as follows: When X defined in formula (2) is 0.10 or more: [Ti ｍｉｎ ]=X When X defined in formula (2) is less than 0.10: [Ti ｍｉｎ ]=0.10 X=16×(C+N) (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2). Titanium (Ti) fixes C and N and suppresses the formation of Cr carbonitrides. Therefore, the formation of a Cr-deficient layer is suppressed. As a result, embrittlement of the oxide film is suppressed and the corrosion resistance of the ferritic stainless steel foil is improved. X is an index of the Ti content necessary to fix the C and N contained in the ferritic stainless steel foil. When X is 0.10 or more, the above effect can be sufficiently obtained as long as the Ti content is X (%) or more. When X is less than 0.10, the above effect can be sufficiently obtained as long as the Ti content is 0.10% or more. On the other hand, when the Ti content is 1.00% or less, the occurrence of scuff marks caused by inclusions and excessive concentration of Ti in the oxide film can be suppressed. As a result, the corrosion resistance of the ferritic stainless steel foil is improved. Therefore, the Ti content is [Ti ｍｉｎ] to 1.00%. When X is 0.10 or more, the preferred lower limit of the Ti content is X + 0.05(%), and more preferably X + 0.10(%). When X is less than 0.10, the preferred lower limit of the Ti content is 0.15%, and more preferably 0.20%. The preferred upper limit of the Ti content is 0.90%, and more preferably 0.80%, and even more preferably 0.60%.

[0040] Nb: 0 to 1.00% Niobium (Nb) may not be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content is greater than 0%, Nb fixes C and N and suppresses the formation of Cr carbonitrides. Therefore, the formation of a Cr-deficient layer is suppressed. As a result, embrittlement of the oxide film is suppressed, and the corrosion resistance of the ferritic stainless steel foil is improved. On the other hand, when the Nb content is 1.00% or less, the occurrence of scuff marks caused by inclusions and excessive concentration of Nb in the oxide film can be suppressed. As a result, the corrosion resistance of the ferritic stainless steel foil is improved. Therefore, the Nb content is 0 to 1.00%. A preferable lower limit of the Nb content is greater than 0%, more preferably 0.01%, and even more preferably 0.03%. A preferable upper limit of the Nb content is 0.90%, more preferably 0.80%, and even more preferably 0.60%.

[0041] Zr: 0 to 0.80% Zirconium (Zr) may not be contained. That is, the Zr content may be 0%. When Zr is contained, that is, when the Zr content is greater than 0%, Zr fixes C and N and suppresses the formation of Cr carbonitrides. Therefore, the formation of a Cr-deficient layer is suppressed. As a result, embrittlement of the oxide film is suppressed, and the corrosion resistance of the ferritic stainless steel foil is improved. On the other hand, if the Zr content is 0.80% or less, the occurrence of scuff marks caused by inclusions and excessive concentration of Zr in the oxide film can be suppressed. As a result, the corrosion resistance of the ferritic stainless steel foil is improved. Therefore, the Zr content is preferably 0 to 0.80%. The lower limit of the Zr content is preferably greater than 0%, more preferably 0.01%, and even more preferably 0.03%. The upper limit of the Zr content is preferably 0.70%, and even more preferably 0.60%.

[0042] The balance of the chemical composition of the ferritic stainless steel foil of this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials or the manufacturing environment during industrial production of the ferritic stainless steel foil, and are acceptable within a range that does not adversely affect the ferritic stainless steel foil of this embodiment.

[0043] [Regarding Formula (1)] The chemical composition of the ferritic stainless steel foil of the present embodiment further satisfies formula (1): (Mo+Cr) / Ti≦80 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass%.

[0044] F1 is defined as follows: F1 = (Mo + Cr) / Ti Mo and Cr promote an increase in interfacial resistance when the oxide film is thickened. On the other hand, increasing the Ti content in accordance with the total content of Mo and Cr reduces the degree of increase in interfacial resistance when the oxide film is thickened. If F1 is 80 or less, the Ti content is sufficiently high relative to the total content of Mo and Cr. In this case, provided that the content of each element in the chemical composition of the ferritic stainless steel foil satisfies the above-mentioned range, the increase in interfacial resistance when the oxide film is thickened can be sufficiently suppressed.

[0045] The lower limit of F1 is not particularly limited, but is, for example, 18. In consideration of the corrosion resistance of the ferritic stainless steel foil, the lower limit of F1 is preferably 20, more preferably 30, and even more preferably 40. The upper limit of F1 is preferably 75, and even more preferably 70.

[0046] [Thickness] The thickness of the ferritic stainless steel foil of this embodiment is not particularly limited, and is, for example, 5 to 20 μm.

[0047] [Oxide Coating] The thickness of the oxide coating provided on the ferritic stainless steel foil of this embodiment is not particularly limited. The thickness of the oxide coating is, for example, 0.5 to 25.0 nm. The thickness of the oxide coating can be appropriately adjusted by a heat treatment step or the like in the manufacturing process of the ferritic stainless steel foil.

[0048] The thicker the oxide film, the higher the corrosion resistance of the ferritic stainless steel foil. Therefore, the preferred lower limit of the oxide film thickness is 0.8 nm, and more preferably 1.0 nm. The thinner the oxide film, the lower the interfacial resistance with the electrode mixture layer when the ferritic stainless steel foil is used as a current collector. Therefore, the preferred upper limit of the oxide film thickness is 20.0 nm, and more preferably 15.0 nm.

[0049] [Method for Measuring Oxide Film Thickness] The thickness of the oxide film is determined by the following method. A test piece is taken from the ferritic stainless steel foil of this embodiment. The size of the test piece is not particularly limited. One surface of the test piece is used as the measurement surface. Field emission Auger electron spectroscopy (FE-AES) analysis is performed on the measurement surface. The FE-AES used is, for example, a JAMP-9500F manufactured by JEOL Ltd. The component profile in the depth direction from the measurement surface is measured by the FE-AES analysis. The depth up to which the detected oxygen intensity is 50% of the maximum value is defined as the thickness (nm) of the oxide film. Note that a standard sample used for converting the sputtering rate and depth position is SiO ２ is used.

[0050] [Uses of Ferritic Stainless Steel Foil] When the ferritic stainless steel foil of this embodiment is used as a current collector, an increase in the interfacial resistance with the electrode mixture layer when the oxide film is thickened is sufficiently suppressed. Therefore, it is suitable for use as a current collector in all-solid-state batteries using a sulfide-based solid electrolyte, which requires a thick oxide film to improve corrosion resistance. Note that the ferritic stainless steel foil of this embodiment can also be used for applications other than as a current collector for secondary batteries.

[0051] [Method for manufacturing ferritic stainless steel foil] An example of a method for manufacturing the ferritic stainless steel foil of this embodiment will be described. The method for manufacturing the ferritic stainless steel foil described below is one example for manufacturing the ferritic stainless steel foil of this embodiment. Therefore, the ferritic stainless steel foil having the above-mentioned configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the ferritic stainless steel foil of this embodiment.

[0052] An example of a method for producing the ferritic stainless steel foil of this embodiment includes a material preparation step, an intermediate cold rolling step, an intermediate annealing step, and a final cold rolling step.

[0053] [Material Preparation Step] In the material preparation step, a ferritic stainless steel sheet having a thickness of several hundred μm to several mm is prepared as the material for producing the ferritic stainless steel foil of this embodiment. The material is, for example, a cold-rolled coil obtained by cold-rolling a hot-rolled coil. The material may be prepared by manufacturing or by purchasing from a third party. In other words, the material preparation step is not particularly limited.

[0054] When manufacturing a raw material, for example, it is manufactured by the following method: Produce molten steel having the above-mentioned chemical composition. Produce a slab using the molten steel by continuous casting. Produce the manufactured slab by hot working and cold rolling to produce a steel plate with a thickness of several hundred μm to several mm. Through the above steps, the raw material for the ferritic stainless steel foil of this embodiment is prepared.

[0055] [Intermediate Cold Rolling Step] In the intermediate cold rolling step, cold rolling is performed on the prepared material to produce an intermediate steel plate having a thickness of several tens of μm to several hundreds of μm. In the intermediate cold rolling step, for example, cold rolling may be performed using a continuous rolling mill equipped with multiple rolling stands, or cold rolling may be performed using a reverse rolling mill. The cumulative reduction ratio in the intermediate cold rolling step is not particularly limited.

[0056] [Intermediate Annealing Step] In the intermediate annealing step, the intermediate steel sheet after the intermediate cold rolling step is subjected to bright annealing. Bright annealing is an annealing treatment carried out in an extremely low oxygen atmosphere. The intermediate steel sheet that has been subjected to bright annealing has its surface hardly oxidized and can maintain its surface gloss. The extremely low oxygen atmosphere in bright annealing is preferably an H ２ Gas and N ２ The atmosphere is a mixed gas of N and N gas. ２ The volume fraction is, for example, 35 to 65%. The heating temperature for bright annealing is, for example, 800 to 1200°C.

[0057] The intermediate cold rolling step and the intermediate annealing step may be alternately repeated multiple times. For example, when the intermediate cold rolling step and the intermediate annealing step are alternately repeated twice, a first intermediate cold rolling step and a first intermediate annealing step are performed, and then a second intermediate cold rolling step and a second intermediate annealing step are performed.

[0058] [Final Cold Rolling Step] In the final cold rolling step, the intermediate steel sheet after the intermediate annealing step is cold rolled again to obtain a ferritic stainless steel foil of a predetermined thickness. In the final cold rolling step, for example, cold rolling may be performed using a continuous rolling mill equipped with multiple rolling stands, or cold rolling may be performed using a reverse rolling mill. The cumulative reduction rate in the final cold rolling step is not particularly limited.

[0059] The ferritic stainless steel foil of this embodiment is manufactured by the above manufacturing method. The manufacturing method of the ferritic stainless steel foil of this embodiment may include steps other than those described above. For example, a heat treatment step may be performed on the ferritic stainless steel foil after the final cold rolling step. In the heat treatment step, the ferritic stainless steel foil after the final cold rolling step is heated and held at a predetermined temperature. This makes it possible to remove residual stress accumulated in the ferritic stainless steel foil and impart ductility. It also makes it possible to adjust the thickness of the oxide film provided on the ferritic stainless steel foil. The heat treatment temperature is, for example, 300 to 1000°C.

[0060] [Electrode] The electrode of this embodiment comprises the ferritic stainless steel foil of this embodiment and an electrode mixture layer formed on the surface of the ferritic stainless steel foil. That is, in the electrode of this embodiment, the ferritic stainless steel foil of this embodiment is used as a current collector. The electrode of this embodiment may be a positive electrode or a negative electrode. That is, the configuration of the electrode of this embodiment is not particularly limited as long as it comprises the ferritic stainless steel foil of this embodiment and an electrode mixture layer.

[0061] [Electrode Mixture Layer] In the electrode of this embodiment, the electrode mixture layer is not particularly limited as long as it has a known configuration. The electrode mixture layer contains an active material. Note that the electrode mixture layer may contain materials other than the active material. The electrode mixture layer may contain, for example, a binder and a conductive additive. Furthermore, the electrode mixture layer used in an all-solid-state battery may contain, for example, a solid electrolyte.

[0062] [Active Material] In the electrode of this embodiment, the active material contained in the electrode mixture layer is not particularly limited, and a well-known active material can be used. When the electrode is a positive electrode, the positive electrode active material is, for example, LiCoO ２ And LiNi １／３ Co １／３ Mn １／３ O ２ and ternary systems such as LiFePO ４ Olivine system represented by S, Fe ２ S, Mo ３ S ４When the electrode is a negative electrode, the negative electrode active material may be, for example, a carbon-based material represented by graphite, an alloy material represented by CuSn alloy and NiTiSi alloy, an Si-based material represented by Si and SiO, or Li ４ Ti ５ O １２ and oxide-based materials represented by the following.

[0063] [Binder] In the electrode of this embodiment, the binder contained in the electrode mixture layer is not particularly limited, and well-known binders can be used. The binder may be any one or more selected from the group consisting of rubbery polymers represented by styrene-butadiene rubber and isoprene rubber, synthetic resins represented by polyethylene, polyimide, polyacrylic acid, and polyamide, styrene-butadiene-styrene block copolymers or hydrogenated products thereof, thermoplastic elastomers such as styrene-ethylene-butadiene, styrene copolymers, styrene-isoprene, or styrene block copolymers or hydrogenated products thereof, soft resinous polymers represented by syndiotactic-1,2-polybutadiene, ethylene-vinyl acetate copolymers, and copolymers of ethylene and an α-olefin having 3 to 12 carbon atoms, and fluorinated polymers represented by polytetrafluoroethylene, tetrafluoroethylene-ethylene copolymers, polyvinylidene fluoride, polypentafluoropropylene, and polyhexafluoropropylene.

[0064] [Conductive additive] In the electrode of this embodiment, the conductive additive contained in the electrode mixture layer is not particularly limited, and any known conductive additive may be used. The conductive additive may be, for example, one or more selected from the group consisting of graphite material, acetylene black, carbon black, ketjen black, carbon fiber, carbon nanotube, and graphene.

[0065] [Electrode Manufacturing Method] The electrode manufacturing method of this embodiment is not particularly limited. The electrode of this embodiment is manufactured by a well-known method using the ferritic stainless steel foil of this embodiment as a current collector. The electrode manufacturing method of this embodiment includes, for example, an electrode slurry preparation step and an electrode mixture layer formation step.

[0066] [Electrode Slurry Preparation Step] In the electrode slurry preparation step, a composition (electrode slurry) for forming an electrode mixture layer is prepared. The electrode slurry may be prepared according to the electrode mixture layer to be obtained. For example, the electrode slurry may be prepared by kneading an active material, a binder, and a solvent. For example, the electrode slurry may be prepared by further kneading an active material, a conductive additive, a binder, and a solvent. Furthermore, when preparing an electrode slurry to be used in an all-solid-state battery, the electrode slurry may be prepared by kneading an active material, a solvent, a solid electrolyte, and a binder. The kneading method is adjusted appropriately depending on the active material, the conductive additive, the binder, the solid electrolyte, and the solvent. That is, the electrode slurry preparation step may be performed by a known method.

[0067] [Electrode Mixture Layer Forming Step] In the electrode mixture layer forming step, an electrode mixture layer is formed on the surface of the ferritic stainless steel foil of this embodiment. Specifically, the kneaded electrode slurry is applied to the ferritic stainless steel foil of this embodiment. The application method is not particularly limited and may be a well-known method. For example, application may be performed using an applicator with a gap. Furthermore, application may be performed by spraying using a sprayer.

[0068] The electrode of this embodiment can be manufactured by the above steps.

[0069] [Battery] The battery of this embodiment includes the electrode of this embodiment and an electrolyte. As long as the battery of this embodiment includes the electrode of this embodiment, other configurations may be well-known and are not particularly limited. The battery of this embodiment may further include, for example, a separator. The shape of the battery of this embodiment is not particularly limited, and may be cylindrical, prismatic, coin-shaped, or sheet-shaped. Furthermore, the battery of this embodiment may be a secondary battery or a primary battery. When the battery according to this embodiment is a secondary battery, it may be, for example, a non-aqueous electrolyte secondary battery, an aqueous electrolyte secondary battery, or an all-solid-state secondary battery.

[0070] [Electrolyte] The electrolyte conducts ions between the positive electrode and the negative electrode. In the battery of this embodiment, the electrolyte is not particularly limited, and a well-known electrolyte can be used. The electrolyte may be a liquid electrolytic solution or a solid electrolyte.

[0071] The electrodes provided in the battery of this embodiment use the ferritic stainless steel foil of this embodiment as a current collector. Therefore, even if the oxide film is made thicker to improve the corrosion resistance of the current collector, an increase in the interface resistance with the electrode mixture layer is sufficiently suppressed. Therefore, a sulfide-based solid electrolyte, which is highly corrosive to the current collector, can be used as the electrolyte of the battery of this embodiment. The sulfide-based solid electrolyte can be, for example, Li ３ P.S. ４ and Li ７ P ３ S １１ LPS systems, such as Li ６ P.S. ５ Cl ｘ Br （１－ｘ） Argyrodite type represented by (0≦x≦1) and Li １０ GeP ２ S １２ and Li ９．５４ Si １．７４ P １．４４ S １１．７ Cl ０．３ and thiolisicones represented by the following:

[0072] [Method for Manufacturing Battery] The method for manufacturing the battery of the present embodiment is not particularly limited. The battery of the present embodiment is manufactured, for example, by a well-known method, by placing a laminate of the electrode of the present embodiment, an electrolyte, and a counter electrode in a battery case.

[0073] The effects of the ferritic stainless steel foil of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the ferritic stainless steel foil of this embodiment. Therefore, the ferritic stainless steel foil of this embodiment is not limited to this one example of conditions.

[0074] As a material for producing ferritic stainless steel foil, a steel plate having the chemical composition shown in Table 1 was prepared. The thickness of the steel plate was 300 μm.

[0075]

[0076] The material of each test number was subjected to an intermediate cold rolling process once to produce an intermediate steel plate. The intermediate steel plate after the intermediate cold rolling process was subjected to an intermediate annealing process once. Specifically, the intermediate steel plate of each test number was subjected to an intermediate annealing process once to produce an intermediate steel plate containing 35 to 65% by volume of N. ２ gas and the remainder is H ２ Bright annealing was carried out in a mixed gas atmosphere with a gas. In bright annealing, the intermediate steel sheets were heated and held at 900 to 1200°C, and then cooled to room temperature. A final cold rolling process was carried out on the intermediate steel sheets after the intermediate annealing process. In the final cold rolling process, cold rolling was carried out on the intermediate steel sheets of each test number using a reverse rolling mill, and ferritic stainless steel foils with a sheet thickness of 10 μm were produced.

[0077] By the above manufacturing process, ferritic stainless steel foils with each test number were manufactured.

[0078] [Evaluation Test] The ferritic stainless steel foils produced with each test number were subjected to an evaluation test of the rate of increase in interfacial resistance by the following method.

[0079] [Test for evaluating the rate of increase in interfacial resistance] Three test specimens were taken from the ferritic stainless steel foil of each test number. The size of the test specimens was not particularly limited, but was, for example, 100 mm × 100 mm. The three test specimens were designated as test specimen A, test specimen B, and test specimen C. Test specimen B was subjected to a heat treatment at 400°C for 5 minutes in a nitrogen atmosphere with a dew point controlled to -20°C. Test specimen C was subjected to a heat treatment at 400°C for 5 minutes in an air atmosphere with a dew point controlled to 14°C. Thereafter, the thickness (nm) of the oxide film provided on test specimen A, test specimen B, and test specimen C of each test number was measured based on the method described in the above-mentioned [Method for measuring oxide film thickness].

[0080] Evaluation cells were fabricated using test piece A, test piece B, and test piece C of each test number. Specifically, two circular current collector samples A with a diameter of 11 mm were taken from test piece A. Similarly, two circular current collector samples B with a diameter of 11 mm were taken from test piece B, and two circular current collector samples C with a diameter of 11 mm were taken from test piece C. In addition, Li was used as the sulfide-based solid electrolyte. ３ P.S. ４ 0.12 g of the above was weighed out and set in a circular mold having an inner diameter of 11 mm. A uniaxial press was carried out at a pressure of 315 MPa to obtain Li ３ P.S. ４ A pellet was prepared. ３ P.S. ４ The pellet was sandwiched between two current collector samples A to prepare a laminate. The prepared laminate was placed in a measurement cell with an inner diameter of 11 mm, and a pressure of 5 MPa was applied by tightening bolts. The measurement cell was then sealed. In this way, evaluation cell A was prepared. ３ P.S. ４ The pellet was sandwiched between two current collector samples B to form an evaluation cell B, and one Li ３ P.S. ４ The pellet was sandwiched between two current collector samples C to prepare an evaluation cell C. ３ P.S. ４ The steps from the preparation of the pellets to the sealing of the measurement cell were carried out in a glove box under an Ar atmosphere.

[0081] Evaluation cell A, evaluation cell B, and evaluation cell C of each test number were held in a constant temperature bath at 60°C for 8 hours. After holding at high temperature, AC impedance measurements were performed on each evaluation cell using a frequency response analyzer. The measurement frequency was 1 Hz to 1 MHz, and the amplitude was 10 mV. Based on the measurement results, a Cole-Cole plot was created with the horizontal axis representing the real component (Z') and the vertical axis representing the imaginary component (Z''). Furthermore, fitting was performed in the range of 20 kHz to 1 MHz using the equivalent circuit shown in Figure 1. In Figure 1, R ｅ represents the interface resistance between the current collector sample and the sulfide-based solid electrolyte, and R ＳＥ represents the resistance due to the sulfide-based solid electrolyte, and CPE represents the constant phase element in the equivalent circuit.

[0082] A schematic diagram showing an example of a Cole-Cole plot after fitting is shown in Figure 2. As shown in Figure 2, the value of the real component at the left end of the arc is the interface resistance R ｅ (Ω). The difference between the value of the real component at the right end of the arc and the value of the real component at the left end of the arc is the resistance value R ＳＥ In this way, the interface resistance R of each evaluation cell was calculated from the Cole-Cole plot after fitting. ｅ The interface resistance R (Ω) of the evaluation cells A to C was calculated for each test number. ｅ The interfacial resistance (Ω) was plotted against the thickness (nm) of the oxide film provided on the test piece used in each evaluation cell. The obtained plot was linearly approximated, with the horizontal axis representing the oxide film thickness (nm) and the vertical axis representing the interfacial resistance R ｅ The slope of the graph obtained for each test number was taken as the interfacial resistance increase rate (Ω / nm) of the ferritic stainless steel foil of that test number.

[0083] The interface resistance increase rate (Ω / nm) for each test number is shown in Table 2. When the interface resistance increase rate was 2.50 Ω / nm or less, it was determined that the increase in interface resistance when the oxide film was thickened was sufficiently suppressed. When the interface resistance increase rate exceeded 2.50 Ω / nm, it was determined that the increase in interface resistance when the oxide film was thickened was not sufficiently suppressed.

[0084]

[0085] [Evaluation Results] Referring to Tables 1 and 2, in the ferritic stainless steel foils of test numbers 1 to 4, the content of each element in the chemical composition satisfied the above-mentioned range, and F1 satisfied formula (1). Therefore, an increase in interfacial resistance when the oxide film was thickened was sufficiently suppressed.

[0086] On the other hand, the Mo content was too high in Test No. 5. As a result, the increase in interface resistance when the oxide film was made thick was not sufficiently suppressed.

[0087] In test number 6, the Cr content was too high, and as a result, the increase in interface resistance when the oxide film was made thick was not sufficiently suppressed.

[0088] In test number 7, F1 did not satisfy formula (1), and as a result, the increase in interface resistance when the oxide film was made thick was not sufficiently suppressed.

[0089] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. The chemical composition, in mass%, is: C: over 0 to 0.050%, Si: over 0 to 1.00%, Mn: over 0 to 1.00%, P: over 0 to 0.050%, S: over 0 to 0.030%, N: over 0 to 0.050%, Mo: 0 to 1.00%, Cr: 14.00 to 18.00%, Ni: 0 to 0.60%, Ti: [Ti ｍｉｎ ] to 1.00%, Nb: 0 to 1.00%, and Zr: 0 to 0.80%, with the balance consisting of Fe and impurities, and satisfying formula (1): (Mo+Cr) / Ti≦80 (1) where [Ti ｍｉｎ ] is defined as follows: When X defined in formula (2) is 0.10 or more: [Ti ｍｉｎ ]=X When X defined in formula (2) is less than 0.10: [Ti ｍｉｎ ]=0.10 X=16×(C+N) (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) and (2).

2. An electrode comprising the ferritic stainless steel foil according to claim 1 and an electrode mixture layer formed on the surface of the ferritic stainless steel foil.

3. A battery comprising the electrode according to claim 2 and an electrolyte.

4. The battery according to claim 3, wherein the electrolyte is a sulfide-based solid electrolyte.

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