Ferritic stainless steel foil, electrode, and battery

Abstract

To provide a ferritic stainless steel foil in which an increase in interface resistance when an oxide film is made thick is sufficiently suppressed.SOLUTION: The ferritic stainless steel foil according to the present disclosure has a chemical composition containing, in mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.030% or less, N: 0.050% or less, Mo: 1.00% or less, Cr: 14.00% to 18.00%, Ni: 0.60% or less, Ti: [Timin] to 1.00%, Nb: 0 to 1.00%, and Zr: 0 to 0.80%, with the balance being Fe and impurities, satisfying 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).SELECTED DRAWING: None

Description

[Technical Field]

[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. [Background technology]

[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] Traditionally, electrolyte solutions have been used as electrolytes in secondary batteries. However, because electrolyte solutions contain flammable organic solvents, the temperature range in which they can be used is narrow. For this reason, in recent years, development of all-solid-state batteries that use solid electrolytes instead of electrolyte solutions has been progressing. Because all-solid-state batteries do not contain organic solvents, stable battery performance can be achieved 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 results in a decrease in battery performance. In other words, the current collector must have high corrosion resistance. For this reason, the use of stainless steel as a current collector material is being considered. Stainless steel has an oxide film (also known as 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] Technology relating to ferritic stainless steel applicable to current collector applications is proposed in International Publication No. 2021 / 006089 (Patent Document 1).

[0007] Patent Document 1 discloses a ferritic stainless steel sheet for use as a current collector in a sulfide-based solid battery, which 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 being Fe and unavoidable impurities. Patent Document 1 also describes that this ferritic stainless steel sheet can achieve excellent sulfidation resistance by increasing the Cr content to 18.00% or more. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2021 / 006089 Summary of the Invention [Problem to be solved by the invention]

[0009] In the ferritic stainless steel sheet disclosed in Patent Document 1, 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 this decrease in workability, it is necessary to increase 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. [Means for solving the problem]

[0012] The ferritic stainless steel foil of the present disclosure is The chemical composition is, in mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.030% or less, N: 0.050% or less, Mo: 1.00% or less Cr: 14.00~18.00%, Ni: 0.60% or less, Ti:[Ti min]~1.00%, Nb: 0 to 1.00%, and Zr: 0 to 0.80%, the balance being Fe and impurities, Satisfies equation (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 formula (1) and formula (2).

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

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

[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. [Brief explanation of the drawings]

[0016] [Figure 1]FIG. 1 is a diagram showing an equivalent circuit used for fitting a Cole-Cole plot in an interface resistance increase rate evaluation test. [Figure 2] FIG. 2 is a schematic diagram showing an example of a Cole-Cole plot after fitting in an interfacial resistance increase rate evaluation test. DETAILED DESCRIPTION OF THE INVENTION

[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 may be one possible factor: Cr and Mo contained in ferritic stainless steel foils enhance the repair ability of the oxide film. Therefore, reducing the Cr and Mo contents increases the time required to repair fine 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 present inventors have developed a steel sheet having a chemical composition, in mass %, of C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.030% or less, N: 0.050% or less, Mo: 1.00% or less, Cr: 14.00 to 18.00%, Ni: 0.60% or less, Ti: [Ti min It was thought that a ferritic stainless steel foil containing Zn: 0-1.00%, Nb: 0-1.00%, and Zr: 0-0.80%, with the remainder being Fe and impurities, would be able to sufficiently suppress the increase in interfacial resistance when the oxide film is made thick. 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) Each element symbol in formula (2) is substituted with the content of the corresponding element in mass %.

[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 present inventors further investigated the chemical composition of the ferritic stainless steel foil. As a result, it was found that by having the above-described chemical composition and further satisfying formula (1), an increase in the interfacial resistance with the electrode mixture layer when the oxide film is thickened can be sufficiently suppressed. (Mo+Cr) / Ti≦80 (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1).

[0020] Although it is unclear why satisfying formula (1) can adequately suppress the increase in interfacial resistance when the oxide layer is thickened, the following is thought to be one possible factor: Ti may concentrate in the oxide layer and reduce the electrical resistance of the oxide layer. Therefore, as mentioned 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 layer is thickened, it is possible to adequately suppress the increase in interfacial resistance when the oxide layer 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 is The chemical composition is, in mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.030% or less, N: 0.050% or less, Mo: 1.00% or less Cr: 14.00~18.00%, Ni: 0.60% or less, Ti:[Ti min ]~1.00%, Nb: 0 to 1.00%, and Zr: 0 to 0.80%, the balance being Fe and impurities, Satisfies equation (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 formula (1) and formula (2).

[0024] The electrodes of the first configuration are: a ferritic stainless steel foil of a 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 in a first configuration; and an electrolyte.

[0026] The battery in the second configuration is 1. A battery of a first configuration, comprising: 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 "%" for 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: 0.050% or less Carbon (C) increases the strength of the ferritic stainless steel foil, and even if only a small amount of C is contained, the above effect can be obtained to some extent. 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, which in turn suppresses the embrittlement of the oxide film and improves the corrosion resistance of the ferritic stainless steel foil. Therefore, the C content is 0.050% or less. The lower limit of the C content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, and still more preferably 0.005%. The upper limit of the C content is preferably 0.040%, more preferably 0.030%, and even more preferably 0.025%.

[0031] Si: 1.00% or less Silicon (Si) deoxidizes steel during the steelmaking process, and even if the steel contains even a small amount of Si, the above effect can be obtained to some extent. 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 1.00% or less. The lower limit of the Si content is preferably more than 0%, more preferably 0.01%, even more preferably 0.03%, and still more preferably 0.05%. The upper limit of the Si content is preferably 0.90%, more preferably 0.80%, and even more preferably 0.50%.

[0032] Mn: 1.00% or less Manganese (Mn) increases the strength of ferritic stainless steel foil, and even if only a small amount of Mn is contained, this effect can be obtained to some extent. 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, and as a result, the corrosion resistance of the ferritic stainless steel foil is improved. Therefore, the Mn content is 1.00% or less. The lower limit of the Mn content is preferably more than 0%, more preferably 0.01%, even more preferably 0.03%, and still more preferably 0.05%. The upper limit of the Mn content is preferably 0.90%, more preferably 0.80%, and even more preferably 0.50%.

[0033] P:0.050% or less Phosphorus (P) is an impurity. If the P content exceeds 0.050%, P segregates at grain boundaries. As a result, the workability and corrosion resistance of the ferritic stainless steel foil deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.050% or less. The lower the P content, the better. However, excessively reducing the P content increases the production cost. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.010%. The upper limit of the P content is preferably 0.045%, and more preferably 0.040%.

[0034] S: 0.030% or less Sulfur (S) is an impurity. 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 0.030% or less. The S content is preferably as low as possible. However, excessively reducing the S content increases the production cost. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.010%. The upper limit of the S content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0035] N: 0.050% or less Nitrogen (N) is an impurity. If the N content exceeds 0.050%, Cr carbonitrides are formed, causing a Cr-depleted zone. As a result, the workability and corrosion resistance of the ferritic stainless steel foil are reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.050% or less. The N content is preferably as low as possible. However, if the N content is reduced too much, the production cost increases. Therefore, in consideration of 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: 1.00% or less Molybdenum (Mo) increases the strength and corrosion resistance of ferritic stainless steel foil, and even if even a small amount of Mo is contained, these effects can be obtained to some extent. On the other hand, if the Mo content is 1.00% or less, the workability of the ferritic stainless steel foil is improved, and further, an increase in interface resistance when the oxide film is made thick can be suppressed. Therefore, the Mo content is 1.00% or less. The lower limit of the Mo content is preferably more than 0%, more preferably 0.01%, even more preferably 0.03%, and still more preferably 0.05%. The upper limit of the Mo content is preferably 0.90%, more preferably 0.80%, and even more preferably 0.50%.

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

[0038] Ni: 0.60% or less Nickel (Ni) improves the corrosion resistance of ferritic stainless steel foil, and even if the Ni content is small, this effect can be obtained to some extent. On the other hand, if the Ni content is 0.60% or less, the workability of the ferritic stainless steel foil is improved. Therefore, the Ni content is 0.60% or less. The lower limit of the Ni content is preferably more than 0%, more preferably 0.01%, even more preferably 0.03%, and still more preferably 0.05%. The upper limit of the Ni content is preferably 0.50%, more preferably 0.40%, and even more preferably 0.35%.

[0039] Ti:[Ti min ]~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. This suppresses the formation of a Cr-deficient layer. 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 required to fix 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, if 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, resulting in improved corrosion resistance of the ferritic stainless steel foil. Therefore, the Ti content is [Ti min ]~1.00%. When X is 0.10 or more, the lower limit of the Ti content is preferably X+0.05(%), and more preferably X+0.10(%). When X is less than 0.10, the lower limit of the Ti content is preferably 0.15%, and more preferably 0.20%. The upper limit of the Ti content is preferably 0.90%, 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 more than 0%, Nb fixes C and N and suppresses the formation of Cr carbonitrides. Therefore, the formation of a Cr-depleted 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 Nb content is 1.00% or less, the occurrence of scuff marks caused by inclusions and the excessive concentration of Nb in the oxide film can be suppressed, thereby improving the corrosion resistance of the ferritic stainless steel foil. Therefore, the Nb content is 0 to 1.00%. The lower limit of the Nb content is preferably more than 0%, more preferably 0.01%, and even more preferably 0.03%. The upper limit of the Nb content is preferably 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 more than 0%, Zr fixes C and N and suppresses the formation of Cr carbonitrides. This suppresses the formation of a Cr-depleted layer. 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, resulting in improved corrosion resistance of the ferritic stainless steel foil. Therefore, the preferred Zr content is 0 to 0.80%. The lower limit of the Zr content is preferably more than 0%, more preferably 0.01%, and even more preferably 0.03%. The upper limit of the Zr content is preferably 0.70%, and 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 this embodiment further satisfies formula (1). (Mo+Cr) / Ti≦80 (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1).

[0044] Define F1 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. When 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 more preferably 70.

[0046] Plate 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 film] The thickness of the oxide film provided on the ferritic stainless steel foil of this embodiment is not particularly limited. The thickness of the oxide film is, for example, 0.5 to 25.0 nm. The thickness of the oxide film 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, so the lower limit of the oxide film thickness is preferably 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 upper limit of the oxide film thickness is preferably 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 JEOL Ltd. product name: JAMP-9500F. The FE-AES analysis measures the component profile in the depth direction from the measurement surface. 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. SiO2 is used as a standard sample when converting the sputtering rate and depth position.

[0050] [Applications of ferritic stainless steel foil] When the ferritic stainless steel foil of this embodiment is used as a current collector, the increase in 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 require 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 of manufacturing ferritic stainless steel foil] An example of a method for producing the ferritic stainless steel foil of this embodiment will be described. The method for producing the ferritic stainless steel foil described below is one example for producing the ferritic stainless steel foil of this embodiment. Therefore, the ferritic stainless steel foil having the above-mentioned configuration may be produced by a production method other than the production method described below. However, the production method described below is a preferred example of a method for producing 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 process] In the material preparation step, a ferritic stainless steel sheet having a thickness of several hundred μm to several mm is prepared as a 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 process for preparing the material is not particularly limited.

[0054] When manufacturing a raw material, for example, it is manufactured by the following method. Molten steel having the above-mentioned chemical composition is manufactured. A slab is manufactured using the molten steel by continuous casting. The manufactured slab is hot worked and cold rolled to manufacture a steel plate having 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 process] In the intermediate cold rolling process, 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 process, 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 reversing rolling mill. There is no particular limitation on the cumulative reduction ratio in the intermediate cold rolling process.

[0056] [Intermediate annealing process] In the intermediate annealing step, bright annealing is performed on the intermediate steel sheet after the intermediate cold rolling step. Bright annealing is an annealing treatment performed in an extremely low oxygen atmosphere. The surface of the intermediate steel sheet that has been subjected to bright annealing is hardly oxidized, and the surface gloss can be maintained. The extremely low oxygen atmosphere in bright annealing is preferably a mixed gas atmosphere of H2 gas and N2 gas. The N2 fraction in the atmospheric gas is, for example, 35 to 65% by volume. 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, followed by a second intermediate cold rolling step and a second intermediate annealing step.

[0058] [Final cold rolling process] 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 to 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. 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 well-known active materials can be used. When the electrode is a positive electrode, the positive electrode active material may be, for example, LiCoO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 The negative electrode active material may be, for example, a carbon-based material such as graphite, an alloy material such as CuSn alloy and NiTiSi alloy, a Si-based material such as Si and SiO, or Li4Ti5O 12 It may also be an oxide-based material 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, for example, a rubbery polymer such as styrene-butadiene rubber or isoprene rubber, a synthetic resin such as polyethylene, polyimide, polyacrylic acid, or polyamide, a styrene-butadiene-styrene block copolymer or its hydrogenated product, a thermoplastic elastomer such as styrene-ethylene-butadiene, styrene copolymer, styrene-isoprene, or styrene block copolymer or their hydrogenated products, a soft resinous polymer such as syndiotactic 1,2-polybutadiene, ethylene-vinyl acetate copolymer, or copolymer of ethylene with an α-olefin having 3 to 12 carbon atoms, or a fluorinated polymer such as polytetrafluoroethylene, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride, polypentafluoropropylene, or 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 can be used. The conductive additive may be, for example, acetylene black, carbon black, or ketjen black.

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

[0066] [Electrode slurry preparation process] 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. In other words, the electrode slurry preparation step may be performed by a known method.

[0067] [Electrode mixture layer formation process] 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. The battery of this embodiment may have any known configuration other than the electrode of this embodiment, and is 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 interfacial 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. Examples of sulfide-based solid electrolytes include Li3PS4 and Li7P3S 11 It may be an LPS system represented by Li6PS5Cl x Br (1-x) (0≦x≦1) may be an argyrodite type, 10 GeP2S 12 and Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 The thiolicon represented by the following may also be used.

[0072] [Battery manufacturing method] The method for manufacturing the battery of the present embodiment is not particularly limited. For example, the battery of the present embodiment is manufactured 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. [Example]

[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] [Table 1]

[0076] The material of each test number was subjected to an intermediate cold rolling process once to produce an intermediate steel sheet. The intermediate steel sheet after the intermediate cold rolling process was subjected to an intermediate annealing process once. Specifically, the intermediate steel sheet of each test number was subjected to bright annealing in an atmosphere of a mixed gas of 35 to 65 volume % N2 gas and the remainder H2 gas. In the bright annealing, the intermediate steel sheet was heated and held at 900 to 1200°C, and then cooled to room temperature. The intermediate steel sheet after the intermediate annealing process was subjected to a final cold rolling process. In the final cold rolling process, the intermediate steel sheet of each test number was cold rolled using a reverse rolling mill to produce a ferritic stainless steel foil with a thickness of 10 μm.

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

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

[0079] [Interface resistance increase rate evaluation test] Three test pieces were taken from the ferritic stainless steel foil of each test number. The size of the test pieces was not particularly limited, but was, for example, 100 mm × 100 mm. The three test pieces were designated as test piece A, test piece B, and test piece C. Test piece 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 piece 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 piece A, test piece B, and test piece C of each test number was measured based on the method described above in [Method for measuring oxide film thickness].

[0080] Evaluation cells were prepared 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 prepared from test piece B, and two circular current collector samples C with a diameter of 11 mm were prepared from test piece C. In addition, 0.12 g of Li3PS4 was weighed out as a sulfide-based solid electrolyte and placed in a circular mold with an inner diameter of 11 mm. Li3PS4 pellets were prepared by uniaxial pressing at a pressure of 315 MPa. One Li3PS4 pellet was sandwiched between two current collector samples A and sealed in a measurement cell with an inner diameter of 11 mm. In this manner, evaluation cell A was prepared. Similarly, evaluation cell B was prepared by sandwiching one Li3PS4 pellet between two current collector samples B, and evaluation cell C was prepared by sandwiching one Li3PS4 pellet between two current collector samples C. The process from preparing the Li3PS4 pellets to sealing them in the measurement cell was carried out in a glove box with an Ar atmosphere.

[0081] Evaluation cell A, evaluation cell B, and evaluation cell C of each test number were kept in a constant temperature bath at 60°C for 8 hours. After keeping at high temperature, AC impedance measurements were performed on each evaluation cell using a frequency response analyzer. The measurement frequency was 1Hz to 1MHz, and the amplitude was 10mV. 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 20kHz to 1MHz 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 interfacial resistance R e (Ω). The difference between the real component value at the right end of the arc and the real component value at the left end of the arc is the resistance value R due to the sulfide-based solid electrolyte. SE In this way, the interface resistance R of each evaluation cell can be calculated from the Cole-Cole plot after fitting. e The interface resistance R (Ω) of 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 on the test piece used in each evaluation cell. The plot obtained 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 for that test number.

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

[0084] [Table 2]

[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 interface 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 thicker 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: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.030% or less, N: 0.050% or less, Mo: 1.00% or less, Cr: 14.00-18.00%, Ni: 0.60% or less, Ti:[Ti ｍｉｎ ]~1.00%、 Nb: 0 to 1.00%, and Zr: 0 to 0.80%; the balance being Fe and impurities; Satisfying formula (1), Ferritic stainless steel foil. (Mo+Cr) / Ti≦80 (1) Here, [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 (1) and formula (2).

2. The ferritic stainless steel foil according to claim 1, an electrode mixture layer formed on the surface of the ferritic stainless steel foil, electrode.

3. The electrode according to claim 2; an electrolyte; battery.

4. 4. The battery of claim 3, The electrolyte is a sulfide-based solid electrolyte. battery.

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