Ferritic stainless-steel foil, electrode, and battery

Abstract

To provide a ferritic stainless-steel foil in which the irreversible consumption of Li is sufficiently suppressed when it is used as a current collector of a lithium-ion battery.SOLUTION: A ferritic stainless steel foil comprises: a foil body made of ferritic stainless steel; and an oxide film formed on the surface of the foil body. [Cr]OF, which is an average Cr concentration as atom% in an oxide film, and [O]OF, which is an average O concentration, satisfy the expression (1). [O]OF / ([Cr]OF+[O]OF)≤0.895 (1)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 comprises 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 electrode mixture layer of a lithium-ion battery is composed of, for example, an active material capable of absorbing and releasing Li ions during charging and discharging, a conductive additive that helps improve electronic conductivity, and a binder that binds the active materials together or between the active material and the current collector. The current collector has the function of supplying current to the active material and the function of serving as a substrate that supports the electrode mixture layer.

[0004] Metal foils are used as current collector materials for secondary batteries. For example, in lithium-ion batteries, copper foil is used as the negative electrode current collector and aluminum foil is used as the positive electrode current collector. However, secondary batteries are sometimes expected to be used in harsher environments than before. Therefore, current collector materials are required to have higher strength and excellent heat resistance and corrosion resistance than copper foil or aluminum foil. As a result, stainless steel foil, which has superior strength, heat resistance, and corrosion resistance to copper foil and aluminum foil, has been attracting attention as a current collector material. Among stainless steel foils, ferritic stainless steel foil in particular has lower electrical resistance and higher conductivity than austenitic stainless steel foil. Therefore, ferritic stainless steel foil is suitable as a current collector material.

[0005] Ferritic stainless steel applicable to current collector applications in secondary batteries is proposed, for example, in International Publication No. 2021 / 006089 (Patent Document 1).

[0006] The ferritic stainless steel sheet disclosed in Patent Document 1 contains 0.001-0.050% C, 0.01-2.00% Si, 0.01-1.00% Mn, 0.050% or less P, 0.010% or less S, 18.00-32.00% Cr, 0.01-4.00% Ni, 0.001-0.150% Al, and 0.050% or less N, with the balance being Fe and unavoidable impurities. This ferritic stainless steel sheet inhibits corrosion of the steel sheet by sulfide-based solid electrolytes. Therefore, it is applicable as a current collector for secondary batteries that use sulfide-based solid electrolytes. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2021 / 006089 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, ferritic stainless steel is suitable as a material for current collectors in secondary batteries, such as lithium-ion batteries. Therefore, the present inventors conducted cyclic voltammetry (CV) measurements to further evaluate the electrochemical properties of ferritic stainless steel foils used as current collectors in lithium-ion batteries. Specifically, an evaluation cell was fabricated by stacking ferritic stainless steel foils, solid electrolyte pellets, and metallic Li foils. Measurements were performed using the ferritic stainless steel foil as the working electrode and the metallic Li foil as the counter and reference electrodes. As a result of the measurements, a reduction current peak was detected at a specific potential when the potential was swept in a less noble direction. However, no corresponding oxidation current peak was detected when the potential was subsequently swept in a more noble direction. This indicates that the reaction consuming the metallic Li foil is proceeding irreversibly. Therefore, it was found that when ferritic stainless steel foils are used as current collectors in lithium-ion batteries, the Li contained in the electrode mixture layer is irreversibly consumed, resulting in a decrease in capacity.

[0009] That is, it is preferable that the ferritic stainless steel foil used as a current collector for a lithium ion battery can suppress the irreversible consumption of Li as described above. Naturally, Patent Document 1 does not consider the suppression of the irreversible reaction.

[0010] An object of the present disclosure is to provide a ferritic stainless steel foil that, when used as a current collector in a lithium-ion battery, sufficiently suppresses irreversible consumption of Li; an electrode that uses the ferritic stainless steel foil as a current collector; and a battery that uses the electrode. [Means for solving the problem]

[0011] The ferritic stainless steel foil of the present disclosure is a foil body made of ferritic stainless steel; an oxide coating formed on the surface of the foil body; [Cr] is the average Cr concentration in atomic percent in the oxide film. OFand the mean O concentration is [O] OF and satisfy the formula (1). [O] OF / ([Cr] OF +[O] OF )≦0.895 (1)

[0012] 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.

[0013] The battery of the present disclosure comprises: The electrode; and an electrolyte. [Effects of the Invention]

[0014] When the ferritic stainless steel foil of the present disclosure is used as a current collector in a lithium-ion battery, irreversible consumption of Li is sufficiently suppressed. The electrode of the present disclosure includes the ferritic stainless steel foil of the present disclosure. The battery of the present disclosure includes the electrode of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have investigated ferritic stainless steel foils that can sufficiently suppress irreversible Li consumption when used as current collectors in lithium-ion batteries, and as a result, have made the following findings.

[0016] The present inventors first investigated the cause of the irreversible reaction that consumes Li when ferritic stainless steel foil is used as a current collector. Here, the present inventors focused on the oxide film (also called a passivation film) formed on the surface of the ferritic stainless steel foil itself. The oxide film is primarily composed of Cr oxide and functions as a protective film against corrosion reactions. Therefore, ferritic stainless steel foil has high corrosion resistance. However, the present inventors considered that when ferritic stainless steel foil is used as a current collector for a lithium-ion battery, an irreversible reaction between components in the oxide film and Li may occur, resulting in Li consumption.

[0017] Therefore, the inventors investigated the relationship between the concentration of each element in the oxide film and the amount of Li that is irreversibly consumed. As a result, the average Cr concentration in atomic percent in the oxide film, [Cr] OF and the mean O concentration is [O] OF However, it was found that satisfying equation (1) sufficiently suppresses irreversible consumption of Li. [O] OF / ([Cr] OF +[O] OF )≦0.895 (1)

[0018] Although it is unclear why satisfying formula (1) sufficiently suppresses irreversible consumption of Li, the following is thought to be one possible factor. Li may be irreversibly consumed by reacting with oxygen in the oxide film and being oxidized. Satisfying formula (1) sufficiently reduces the amount of oxygen in the oxide film that can react with Li. As a result, it is thought that irreversible consumption of Li is sufficiently suppressed.

[0019] The above mechanism is merely a guess. Therefore, in the ferritic stainless steel foil of this embodiment, irreversible Li consumption may be sufficiently suppressed by a mechanism different from that described above. However, the fact that irreversible Li consumption is sufficiently suppressed when the ferritic stainless steel foil satisfies formula (1) is demonstrated in the examples described below.

[0020] 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.

[0021] The ferritic stainless steel foil of the first configuration is a foil body made of ferritic stainless steel; an oxide coating formed on the surface of the foil body; [Cr] is the average Cr concentration in atomic percent in the oxide film. OF and the mean O concentration is [O] OF and satisfy the formula (1). [O] OF / ([Cr] OF +[O] OF )≦0.895 (1)

[0022] The ferritic stainless steel foil of the second configuration is A ferritic stainless steel foil of a first configuration, The chemical composition of the foil body is, in mass%, C: 0.001 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.050% or less, S: 0.030% or less, Ni: 0.01 to 0.50% Cr: 12.00~20.00%, Mo: 0.01 to 2.50% N: 0~0.100%, Ti: 0 to 0.80% Nb: 0 to 0.80%, and Zr: 0 to 0.80%; The balance is Fe and impurities.

[0023] The electrodes of the first configuration are: A ferritic stainless steel foil having a first or second configuration; and an electrode mixture layer formed on the surface of the ferritic stainless steel foil.

[0024] The battery of the first configuration comprises: a first configuration of electrodes; and an electrolyte.

[0025] The battery in the second configuration is 1. A battery of a first configuration, comprising: The electrolyte is a sulfide-based solid electrolyte.

[0026] 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.

[0027] [Ferritic stainless steel foil] The ferritic stainless steel foil of this embodiment comprises a foil body and an oxide coating. The foil body is made of ferritic stainless steel. 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.

[0028] [Oxide film] The oxide film is formed on the surface of the foil body. The oxide film is mainly composed of Cr oxide. The thickness of the oxide film is not particularly limited. Considering normal industrial production, the thickness of the oxide film is, for example, 0.5 to 25.0 nm.

[0029] [Method for measuring oxide film thickness] The thickness of the oxide film is determined by the following method. Three test pieces are 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 FE-AES analysis obtains a profile of O concentration in the depth direction from the measurement surface. The depth up to which the detected oxygen intensity is 50% of the maximum is defined as the thickness (nm) of the oxide film on the test piece. SiO2 is used as a standard sample for converting the sputtering rate and depth position. The arithmetic mean of the oxide film thicknesses on the three test pieces is defined as the thickness (nm) of the oxide film provided on the ferritic stainless steel foil. The thickness (nm) of the oxide film provided on the ferritic stainless steel foil is a value obtained by rounding off to one decimal place.

[0030] [Formula (1)] In the ferritic stainless steel foil of this embodiment, the average Cr concentration in atomic % in the oxide film is [Cr] OF and the mean O concentration is [O] OF and satisfy the formula (1). [O] OF / ([Cr] OF +[O] OF )≦0.895 (1)

[0031] Define F1 as follows: F1=[O] OF / ([Cr] OF +[O] OF ) As mentioned above, Li may be irreversibly consumed by reacting with oxygen in the oxide film and being oxidized. The smaller F1 is, the less oxygen in the oxide film is available to react with Li. If F1 is 0.895 or less, the amount of oxygen in the oxide film available to react with Li is sufficiently reduced. As a result, the irreversible consumption of Li is sufficiently suppressed.

[0032] The lower limit of F1 is not particularly limited. Considering normal industrial production, the lower limit of F1 is, for example, 0.700. The upper limit of F1 is preferably 0.892, more preferably 0.891, and even more preferably 0.890.

[0033] [Method for measuring the concentration of each element in the oxide film] Average Cr concentration in atomic percent in the oxide film [Cr] OF , and mean O concentration [O] OF is calculated in the following way: 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. FE-AES analysis is performed on the measurement surface to obtain profiles of O concentration and Cr concentration in the depth direction from the measurement surface. The thickness (nm) of the oxide film on the test piece is determined using a method similar to the above-mentioned [Method for measuring oxide film thickness]. Then, the arithmetic mean value of the Cr concentration in atomic % at each measurement depth position up to the oxide film thickness is determined. The obtained value is used as the average Cr concentration [Cr] in atomic % in the oxide film. OF Furthermore, the arithmetic mean value of the O concentration in atomic % at each measurement depth position up to the thickness of the oxide film is calculated. The obtained value is the average O concentration in atomic % in the oxide film [O]. OF Let's say.

[0034] The obtained [Cr] OF and [O] OF F1 is calculated based on the above. Note that F1 is a value rounded off to three decimal places.

[0035] [Chemical composition] In the ferritic stainless steel foil of this embodiment, the chemical composition of the foil body may be that of a known ferritic stainless steel.

[0036] The chemical composition of the foil body of the ferritic stainless steel foil of this embodiment may satisfy, for example, any one selected from the group consisting of SUS405, SUS410L, SUS429, SUS430, SUS430LX, SUS430J1L, SUS434, SUS436L, SUS436J1L, SUS443J1, SUS444, SUS445J1, SUS445J2, SUS447J1, and SUSXM27 as specified in JIS G 4305 (2015).

[0037] The chemical composition of the foil body of the ferritic stainless steel foil of this embodiment may further satisfy, for example, any one selected from the group consisting of 403, 405, 409L, 410, 410L, 410S, 415, 420J1, 420J2, 420, 429, 429J1, 430, 430J1L, 430LX, 430Ti, 434, 436, 436J1L, 439, 441, 444, 445, 445J1, 445J2, 446, 447, and 448 as specified in ASTM A 240 (2006).

[0038] The chemical composition of the foil body of the ferritic stainless steel foil of this embodiment may contain, for example, the following elements.

[0039] C: 0.001 to 0.030% Carbon (C) increases the strength of the ferritic stainless steel foil. If the C content is 0.001% or more, this effect can be sufficiently obtained. On the other hand, if the C content is 0.030% 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 preferred C content is 0.001 to 0.030%. A more preferable lower limit of the C content is 0.003%, and even more preferably 0.005%. The upper limit of the C content is more preferably 0.025%, and even more preferably 0.020%.

[0040] Si: 0.01 to 1.00% Silicon (Si) deoxidizes steel during the steelmaking process, and if the Si content is 0.01% or more, this effect can be sufficiently obtained. 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 preferred Si content is 0.01 to 1.00%. The lower limit of the Si content is more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Si content is more preferably 0.90%, and even more preferably 0.80%.

[0041] Mn: 0.01 to 1.00% Manganese (Mn) increases the strength of the ferritic stainless steel foil, and if the Mn content is 0.01% or more, this effect can be sufficiently obtained. On the other hand, if the Mn content is 1.00% or less, the corrosion resistance of the ferritic stainless steel foil is improved. Therefore, the preferred Mn content is 0.01 to 1.00%. A more preferable lower limit of the Mn content is 0.03%, and even more preferably 0.05%. A more preferable upper limit of the Mn content is 0.90%, and even more preferably 0.80%.

[0042] P:0.050% or less Phosphorus (P) is an impurity. If the P content is 0.050% or less, the workability and corrosion resistance of the ferritic stainless steel foil are improved. Therefore, the preferred 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%. A more preferable upper limit of the P content is 0.045%, and even more preferably 0.040%.

[0043] S: 0.030% or less Sulfur (S) is an impurity. If the S content is 0.030% or less, the workability and corrosion resistance of the ferritic stainless steel foil are improved. Therefore, the preferred 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 more preferably 0.025%, and even more preferably 0.020%.

[0044] Ni: 0.01 to 0.50% Nickel (Ni) improves the corrosion resistance of the ferritic stainless steel foil, and if the Ni content is 0.01% or more, this effect can be sufficiently obtained. On the other hand, if the Ni content is 0.50% or less, the workability of the ferritic stainless steel foil is improved. Therefore, the Ni content is preferably 0.01 to 0.50%. A more preferable lower limit of the Ni content is 0.03%, and even more preferably 0.05%. A more preferable upper limit of the Ni content is 0.40%, and even more preferably 0.30%.

[0045] Cr: 12.00~20.00% Chromium (Cr) forms an oxide film and improves the corrosion resistance of the ferritic stainless steel foil. If the Cr content is 12.00% or more, this effect can be sufficiently obtained. On the other hand, if the Cr content is 20.00% or less, the workability of the ferritic stainless steel foil is improved. Therefore, the preferred Cr content is 12.00 to 20.00%. The lower limit of the Cr content is more preferably 12.50%, and even more preferably 13.00%. The upper limit of the Cr content is more preferably 19.00%, and even more preferably 18.00%.

[0046] Mo: 0.01 to 2.50% Molybdenum (Mo) enhances the strength and corrosion resistance of ferritic stainless steel foil, and if the Mo content is 0.01% or more, these effects can be sufficiently obtained. On the other hand, if the Mo content is 2.50% or less, the workability of the ferritic stainless steel foil is improved. Therefore, the preferred Mo content is 0.01 to 2.50%. A more preferable lower limit of the Mo content is 0.03%, and even more preferably 0.05%. A more preferable upper limit of the Mo content is 2.40%, and even more preferably 2.30%.

[0047] N: 0 to 0.100% Nitrogen (N) may not be contained. On the other hand, if the N content is 0.100% or less, the strength of the ferritic stainless steel foil increases. Therefore, the preferred N content is 0 to 0.100%. A more preferable lower limit of the N content is more than 0%, even more preferably 0.001%, and even more preferably 0.003%. The upper limit of the N content is more preferably 0.090%, and even more preferably 0.080%.

[0048] Ti: 0 to 0.80% Titanium (Ti) may not be contained. On the other hand, if the Ti content is 0.80% or less, it fixes C and suppresses the formation of Cr carbides, thereby suppressing 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. Therefore, the preferred Ti content is 0 to 0.80%. A more preferable lower limit of the Ti content is more than 0%, even more preferably 0.01%, and even more preferably 0.03%. A more preferable upper limit of the Ti content is 0.70%, and even more preferably 0.60%.

[0049] Nb: 0 to 0.80% Niobium (Nb) may not be contained. On the other hand, if the Nb content is 0.80% or less, the corrosion resistance of the ferritic stainless steel foil is improved. Therefore, the Nb content is preferably 0 to 0.80%. A more preferable lower limit of the Nb content is more than 0%, even more preferably 0.01%, and even more preferably 0.03%. The upper limit of the Nb content is more preferably 0.70%, and even more preferably 0.60%.

[0050] Zr: 0 to 0.80% Zirconium (Zr) may not be contained. On the other hand, if the Zr content is 0.80% or less, the corrosion resistance of the ferritic stainless steel foil is improved. Therefore, the preferred Zr content is 0 to 0.80%. A more preferable lower limit of the Zr content is more than 0%, even more preferably 0.01%, and even more preferably 0.03%. A more preferable upper limit of the Zr content is 0.70%, and even more preferably 0.60%.

[0051] The balance of the preferred chemical composition of the foil body 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.

[0052] 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.

[0053] [Applications of ferritic stainless steel foil] When the ferritic stainless steel foil of this embodiment is used as a current collector for a lithium ion battery, irreversible consumption of Li is sufficiently suppressed. Therefore, it is suitable for use as a current collector for lithium ion batteries. The ferritic stainless steel foil of this embodiment can also be widely used in applications other than current collectors for secondary batteries, such as lithium ion batteries.

[0054] [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.

[0055] An example of a method for producing the ferritic stainless steel foil of this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Final cold rolling process (Step 3) Degreasing process (Step 4) Final heat treatment step Each step will be described below.

[0056] [(Process 1) Material preparation process] In the material preparation step, a ferritic stainless steel sheet having a thickness of several tens to several hundreds of μm 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.

[0057] When manufacturing a material, it is manufactured, for example, by the following method. A slab is manufactured by continuous casting using molten steel. The manufactured slab is hot worked and, if necessary, cold rolled to manufacture a steel plate having a thickness of several tens to several hundreds of μm. The method of hot working is not particularly limited, and any well-known method may be used. An example of the hot working is hot rolling. When manufacturing a material by hot rolling, it can be manufactured, for example, by the following method.

[0058] After heating the produced slab, rough rolling and finish rolling are carried out. At this time, the conditions of the hot rolling are not particularly limited, and well-known conditions may be set appropriately. The hot-rolled material may be repeatedly subjected to cold rolling and annealing treatment as needed. The hot-rolled material may further be subjected to skin-pass rolling as needed. The hot-rolled and / or cold-rolled material is further subjected to annealing treatment. Through the above steps, the material for the ferritic stainless steel foil of this embodiment is prepared.

[0059] [(Process 2) Final cold rolling process] In the final cold rolling step, the material prepared in the material preparation step is cold rolled to obtain a ferritic stainless steel foil of a predetermined thickness. In this embodiment, cold rolling can be performed using a known device, and is not particularly limited. For example, cold rolling may be performed using a continuous rolling mill equipped with multiple rolling stands, or may be performed using a reverse rolling mill. The cumulative reduction ratio in the final cold rolling step is not particularly limited.

[0060] [(Step 3) Degreasing process] In the degreasing process, the ferritic stainless steel foil obtained in the final cold rolling process is subjected to a degreasing process to remove lubricating oil and the like remaining on the surface. Specifically, alkaline electrolytic degreasing, which has a high degreasing power, is performed as the degreasing process. The alkaline electrolytic degreasing may be cathodic electrolytic degreasing, anodic electrolytic degreasing, or PR electrolytic degreasing. The type of alkaline cleaning solution used in alkaline electrolytic degreasing is not particularly limited. After the degreasing process, the ferritic stainless steel foil is rinsed with pure water. The ferritic stainless steel foil is then thoroughly dried.

[0061] The degreasing process satisfies the following conditions: (Condition 1) Current density d in alkaline electrolytic degreasing is 8.0 A / dm 2 The following applies. Condition 1 will be explained below.

[0062] [Current density d] In alkaline electrolytic degreasing performed in the degreasing treatment step, the higher the current density d, the higher the ratio of O concentration to Cr concentration in the oxide film provided on the ferritic stainless steel foil. As a result, the F1 of the manufactured ferritic stainless steel foil increases. On the other hand, when the current density d is 8.0 A / dm 2 If F1 satisfies the formula (1), assuming that the final heat treatment process satisfies the condition 2 described below, then the current density d in alkaline electrolytic degreasing is 8.0 A / dm 2 The following applies. The lower limit of the current density d is not particularly limited. Considering normal industrial production, the lower limit of the current density d is, for example, 1.0 A / dm 2 is.

[0063] [(Step 4) Final heat treatment process] In the final heat treatment step, the ferritic stainless steel foil after the degreasing treatment step is heated and maintained at a predetermined temperature. As a result, the residual stress accumulated in the ferritic stainless steel foil is reduced. Furthermore, the ductility of the ferritic stainless steel foil is increased. The heat treatment temperature is, for example, 300 to 1000°C.

[0064] The final heat treatment step satisfies the following conditions: (Condition 2) The heat treatment atmosphere is a non-oxidizing atmosphere, and the dew point is -40°C or less. Condition 2 will be explained below.

[0065] If the heat treatment atmosphere in the final heat treatment step is an oxidizing atmosphere such as air, the O concentration relative to the Cr concentration in the oxide film provided on the ferritic stainless steel foil will be high. Furthermore, even if the heat treatment atmosphere is a non-oxidizing atmosphere, a high dew point will increase the O concentration relative to the Cr concentration in the oxide film. As a result, the F1 of the manufactured ferritic stainless steel foil will be high. On the other hand, if the heat treatment atmosphere is a non-oxidizing atmosphere and the dew point is −40°C or lower, F1 will satisfy formula (1), provided that the degreasing treatment step satisfies condition 1. Therefore, the heat treatment atmosphere is a non-oxidizing atmosphere with a dew point of −40°C or lower. A non-oxidizing atmosphere refers to an inert gas atmosphere such as nitrogen or argon, a reducing gas atmosphere such as hydrogen, or a mixture of these gases. The lower limit of the dew point of the heat treatment atmosphere is not particularly limited, and in consideration of normal industrial production, the lower limit of the dew point is, for example, −90° C.

[0066] The ferritic stainless steel foil of this embodiment is manufactured by the above-described manufacturing method.

[0067] [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.

[0068] [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.

[0069] [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 can be, for example, LiCoO2 and LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 The negative electrode active material may be any one or more selected from the group consisting of ternary materials represented by O2, olivine materials represented by LiFePO4, and sulfur materials represented by S, Fe2S, Mo3S4, and sulfur-modified polyacrylonitrile. When 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, and Li4Ti5O 12 and oxide-based materials represented by the following.

[0070] [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 at least one selected from the group consisting of rubbery polymers such as styrene-butadiene rubber and isoprene rubber, synthetic resins such as polyethylene, polyimide, polyacrylic acid, and polyamide, thermoplastic elastomers such as styrene-butadiene-styrene block copolymers, hydrogenated styrene-butadiene-styrene block copolymers, styrene-ethylene-butadiene, styrene copolymers, styrene-isoprene, and styrene block copolymers, or hydrogenated versions thereof, soft resinous polymers such as syndiotactic 1,2-polybutadiene, ethylene-vinyl acetate copolymers, and copolymers of ethylene and α-olefins having 3 to 12 carbon atoms, and fluorinated polymers such as polytetrafluoroethylene, tetrafluoroethylene-ethylene copolymers, polyvinylidene fluoride, polypentafluoropropylene, and polyhexafluoropropylene.

[0071] [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. For example, the conductive additive may be any one or more selected from the group consisting of acetylene black, carbon black, and ketjen black.

[0072] [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.

[0073] [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.

[0074] [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 any well-known method. For example, application may be performed using an applicator with a gap. Application may also be performed by spraying using a sprayer.

[0075] The electrode of this embodiment can be manufactured by the above steps.

[0076] [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.

[0077] [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. The solid electrolyte may be, for example, a sulfide-based solid electrolyte. Examples of sulfide-based solid electrolytes include Li3PS4 and Li7P3S 11 LPS system represented by Li6PS5Cl x Br (1-x) Argyrodite type represented by (0≦x≦1) and Li 10 GeP2S 12 and Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 and thiolisicones represented by the following.

[0078] [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]

[0079] 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.

[0080] Steel sheets having the chemical compositions shown in Table 1 were prepared as materials for manufacturing ferritic stainless steel foil. The thickness of the steel sheets was 25 μm. All of the steel sheets prepared as materials had been annealed at 800 to 1200°C for 3 to 30 seconds.

[0081] [Table 1]

[0082] The material of each test number was subjected to a final cold rolling process. In the final cold rolling process, the intermediate steel plate of each test number was subjected to cold rolling using a reverse rolling mill. In this way, a ferritic stainless steel foil with a thickness of 10 μm was produced.

[0083] The ferritic stainless steel foil after the final cold rolling process was subjected to a degreasing treatment process. In the degreasing treatment process, alkaline electrolytic degreasing was carried out using the ferritic stainless steel foil of each test number as the anode. A 70°C aqueous sodium hydroxide solution was used as the alkaline cleaning solution. The current density d (A / dm 2 ) are shown in Table 2. After alkaline electrolytic degreasing, the ferritic stainless steel foil was rinsed with pure water. Thereafter, the ferritic stainless steel foil was thoroughly dried.

[0084] [Table 2]

[0085] The ferritic stainless steel foils after the degreasing treatment step were subjected to a final heat treatment step. In the final heat treatment step, the ferritic stainless steel foils of each test number were held for 5 minutes in a heat treatment furnace heated to 400°C. For test numbers 1 to 7 and 9, the atmosphere in the heat treatment furnace was a nitrogen atmosphere (shown as "non-oxidizing atmosphere" in the "heat treatment atmosphere" column in Table 2). The dew point (°C) of the heat treatment atmosphere for each test number is shown in Table 2. For test number 8, the atmosphere in the heat treatment furnace was an air atmosphere (shown as "oxidizing atmosphere" in the "heat treatment atmosphere" column in Table 2). Note that for test number 8, the dew point (°C) of the heat treatment atmosphere was not measured.

[0086] By the above manufacturing process, ferritic stainless steel foils with each test number were manufactured.

[0087] [About the evaluation test] The following evaluation tests were carried out on the produced ferritic stainless steel foils with each test number. (Test 1) Measurement of the concentration of each element in the oxide film (Test 2) Reduction current measurement test Each test will be explained below.

[0088] [(Test 1) Measurement of the concentration of each element in the oxide film] Based on the method described in the above [Method for measuring the concentration of each element in an oxide film], the average Cr concentration in atomic % [Cr] in the oxide film provided on the ferritic stainless steel foil of each test number was measured. OF , and mean O concentration [O] OF The size of the test piece used for the measurement was 5 mm x 5 mm. The obtained [Cr] OF and [O] OF F1 was calculated based on the above. F1 for each test number is shown in Table 2.

[0089] [(Test 2) Reduction current measurement test] When the ferritic stainless steel foil of each test number was used as a current collector in a lithium ion battery, the amount of Li irreversibly consumed was evaluated by the following method.

[0090] Circular test pieces with a diameter of 11 mm were cut from ferritic stainless steel foil. Test pieces representing each test number were used to prepare evaluation cells simulating lithium-ion batteries. Specifically, 0.12 g of solid electrolyte was placed in a mold with an inner diameter of 11 mm and then pressurized at 300 MPa to form a cylindrical solid electrolyte pellet with a thickness of 700 μm and a bottom diameter of 11 mm. β-Li3PS4 was used as the solid electrolyte. The pellet was placed on top of the test piece. Furthermore, a cylindrical Li foil with a thickness of 1.2 mm and a bottom diameter of 11 mm was placed on top of the solid electrolyte pellet. These were placed in a measurement cell with an inner diameter of 11 mm, which could be sealed from the outside air, and the cell was sealed with bolts at a pressure of 5 MPa. The process from preparing the solid electrolyte pellet to sealing the evaluation cell was carried out in a glove box with an Ar atmosphere. The evaluation cells for each test number were prepared using the above process.

[0091] Cyclic voltammetry (CV) measurements were carried out using the evaluation cells of each test number. In the cyclic voltammetry measurements, the test piece was used as the working electrode, and metal Li foil was used as the counter electrode and reference electrode. Specifically, the voltage was measured for the evaluation cell held at 60°C for 24 hours, and the open-circuit voltage V OC The voltage was swept from 0.005V to 3.0V, and then swept to 0.005V. The current density (μA / cm 2 The open-circuit voltage V OC was 2.0 to 2.5 V for the evaluation cells of all test numbers.

[0092] From the measurement results, the maximum value of the reduction current (μA / cm ) that flowed in the range of 0.5 to 1.0 V during the sweep process from 3.0 V to 0.005 V was 2 The obtained value was used as the reduction current value I (μA / cm 2 ) and used as an index to represent the amount of Li that is irreversibly consumed.

[0093] The reduction current value I (μA / cm 2 ) are shown in Table 2. The reduction current value I is 100 μA / cm 2 When the reduction current value I was 100 μA / cm or less, it was determined that the irreversible consumption of Li was sufficiently suppressed. 2 When the temperature exceeded this range, it was determined that the irreversible consumption of Li was not sufficiently suppressed.

[0094] [Evaluation results] Referring to Tables 1 and 2, in the ferritic stainless steel foils of test numbers 1 to 6, F1 satisfied formula (1). Therefore, when used as a current collector for a lithium ion battery, irreversible consumption of Li was sufficiently suppressed.

[0095] On the other hand, in test number 7, the current density d (A / dm 2 ) was too large. Therefore, F1 did not satisfy formula (1). As a result, when used as a current collector in a lithium-ion battery, the irreversible consumption of Li was not sufficiently suppressed.

[0096] In test number 8, the heat treatment atmosphere in the final heat treatment step was an oxidizing atmosphere. Therefore, F1 did not satisfy formula (1). As a result, when used as a current collector for a lithium-ion battery, irreversible consumption of Li was not sufficiently suppressed.

[0097] In test number 9, the dew point of the heat treatment atmosphere in the final heat treatment step exceeded -40°C. Therefore, F1 did not satisfy formula (1). As a result, when used as a current collector in a lithium-ion battery, irreversible consumption of Li was not sufficiently suppressed.

[0098] 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. a foil body made of ferritic stainless steel; an oxide coating formed on the surface of the foil body; [Cr] is the average Cr concentration in atomic % in the oxide film. ＯＦ and the average O concentration [O] ＯＦ and satisfy the formula (1). Ferritic stainless steel foil. [O] ＯＦ / ([Cr] ＯＦ +[O] ＯＦ )≦0.895 (1)

2. The ferritic stainless steel foil according to claim 1, The chemical composition of the foil body is, in mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, P: 0.050% or less, S: 0.030% or less, Ni: 0.01-0.50%, Cr: 12.00-20.00%, Mo: 0.01-2.50%, N: 0-0.100%, Ti: 0 to 0.80%, Nb: 0 to 0.80%, and Zr: 0 to 0.80%; The balance consists of Fe and impurities. Ferritic stainless steel foil.

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

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

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

Images