Steel foil for current collector

Abstract

To provide a steel foil for a current collector which is less likely to be buckled or bent by heat treatment at high temperature enough to crystallize active material after an active material layer is formed.SOLUTION: A steel foil for a current collector according to the present disclosure comprises a ferritic stainless steel foil. The ferritic stainless steel foil has a half-value width Fw of a peak of a {110} plane of 0.40 to 0.52° in an X-ray diffraction profile by a CoK α ray.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a current collector steel foil used for an electrode of a battery, and more particularly to a current collector steel foil based on a ferrite stainless steel foil as a base material.

Background Art

[0002] Heretofore, batteries such as primary batteries and secondary batteries have been used as power sources for various electronic devices. Specifically, in recent years, due to the spread of small electronic devices such as home video cameras, notebook personal computers, and smartphones, secondary batteries represented by lithium ion batteries have been rapidly spreading.

[0003] A secondary battery includes an electrode having a positive electrode and a negative electrode, and an electrolyte. In both the positive electrode and the negative electrode, an active material layer is formed on a current collector. That is, the current collector is a base material of the electrode. The active material layer is a layer containing an active material. The current collector has a function of supplying current to the active material and a function of holding the active material.

[0004] Heretofore, a metal foil has been used as a base material of a current collector of a secondary battery. Specifically, for example, in a lithium ion battery at present, a copper foil is used as a negative electrode current collector, and an aluminum foil is used as a positive electrode current collector. On the other hand, a secondary battery may be assumed to be used in a more severe environment than before. Therefore, the base material of the current collector has been required to have higher strength and excellent heat resistance than a copper foil or an aluminum foil. Therefore, a stainless steel foil having higher strength and heat resistance than a copper foil and an aluminum foil has been attracting attention as a base material of the current collector.

[0005] Japanese Unexamined Patent Application Publication No. 2003-178753 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2016-186881 (Patent Document 2) propose technologies for applying a stainless steel foil to a current collector.

[0006] Patent Document 1 discloses a lithium battery, and the current collector of its positive electrode is made of a ferrite-based stainless steel foil with a thickness of 12 μm or less. Using a ferrite-based stainless steel foil as the current collector is because ferrite-based stainless steel is more suitable for producing thin foils with good workability compared to austenitic stainless steels such as SUS304 and SUS316, and can improve productivity, as disclosed in Patent Document 1.

[0007] Patent Document 2 discloses a negative electrode of a lithium-ion battery, and the current collector of its negative electrode is a stainless steel foil with a thickness of 15 μm or less or a Ni-plated steel sheet. Using a stainless steel foil and a Ni-plated steel sheet as the current collector is because they have high strength and do not cause a decrease in strength even during heat treatment at about 300 °C, and can maintain the initial strength even after imidization and imination treatments at high temperatures, as disclosed in Patent Document 2.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] As proposed in the above Patent Documents 1 and 2, by using a stainless steel foil as the base material of the current collector, a current collector with high strength can be obtained with high productivity. By the way, in recent years, for the purpose of achieving both higher energy density and safety, the development of all-solid-state secondary batteries has been promoted. An all-solid-state secondary battery is a secondary battery in which a solid electrolyte is used instead of the electrolytic solution used in conventional secondary batteries, and all components such as the current collector and the active material layer, and the electrolyte are composed of solids.

[0010] Here, in all-solid-state secondary batteries, after forming an active material layer on the surface of a current collector, the active material may be crystallized in order to obtain good battery characteristics. In this case, heat treatment is performed on the current collector with the active material layer formed on its surface at a high temperature of about 700°C, for example. On the other hand, when the active material is crystallized by such heat treatment at a high temperature, the active material layer shrinks. As a result, due to the dimensional change caused by the thermal shrinkage of the active material layer and the current collector, the current collector may buckle or bend.

[0011] That is, in the steel foil for current collectors assumed to be applied to all-solid-state secondary batteries, it is preferable that buckling or bending is less likely to occur even when the active material layer shrinks due to heat treatment at a high temperature of about 700°C, for example. However, in Patent Documents 1 and 2 above, when crystallizing the active material as in all-solid-state secondary batteries, the possibility of buckling or bending has not been examined.

[0012] An object of the present disclosure is to provide a steel foil for a current collector that is less likely to buckle or bend by heat treatment at a high temperature to the extent that the active material is crystallized after forming the active material layer.

Means for Solving the Problems

[0013] The steel foil for a current collector according to the present disclosure comprises a ferritic stainless steel foil, in the X-ray diffraction profile by CoKα rays, the half-value width Fw of the peak of the {110} plane of the ferritic stainless steel foil is 0.40 to 0.52°.

Effects of the Invention

[0014] The steel foil for a current collector according to the present disclosure is less likely to buckle or bend by heat treatment at a high temperature to the extent that the active material is crystallized after forming the active material layer.

Modes for Carrying Out the Invention

[0015] First, the inventors considered using ferritic stainless steel foil as the base material for the current collector foil. Ferritic stainless steel foil has higher stability at high temperatures compared to non-ferrous metal foils such as aluminum foil. Furthermore, ferritic stainless steel foil has a lower electrical resistance and a higher conductivity compared to austenitic stainless steel foil. From these points, the inventors decided to use ferritic stainless steel foil as the base material for the current collector foil.

[0016] As described above, in the current collector foil assumed to be applied to an all-solid-state secondary battery, it is preferable that buckling or bending is less likely to occur even when the active material layer shrinks due to heat treatment at a high temperature of about 700°C, for example. Therefore, the inventors considered that if the current collector foil could also be thermally shrunk to some extent during the thermal shrinkage of the active material layer accompanying the crystallization of the active material, the occurrence of buckling or bending of the current collector foil could be suppressed.

[0017] Specifically, the inventors focused on the strain of the ferritic stainless steel foil that is the base material of the current collector foil. If the amount of strain of the ferritic stainless steel foil can be controlled, there is a possibility of controlling the thermal shrinkage of the current collector foil during heat treatment at about 700°C after the active material layer is formed. As a result, when the current collector foil with the active material layer formed is heat-treated at about 700°C, there is a possibility of suppressing the occurrence of buckling or bending.

[0018] As a result of the inventors' detailed examination based on the above findings, it was clarified that if the half-value width Fw of the peak of the {110} plane in the X-ray diffraction profile by CoKα rays of the ferritic stainless steel foil is 0.40 to 0.52°, the occurrence of buckling or bending can be suppressed when the current collector foil with the active material layer formed is heat-treated at about 700°C.

[0019] If the half-value width Fw of the peak of the {110} plane in the X-ray diffraction profile of the ferritic stainless steel foil by CoKα rays (hereinafter, also simply referred to as "the half-value width Fw of the {110} plane") is too small, the amount of strain of the ferritic stainless steel foil is too small. In this case, no strain release occurs during heat treatment at about 700 °C, and no thermal contraction occurs during heat treatment. As a result, it is pulled by the thermal contraction of the active material layer, and bending occurs in the steel foil for the current collector. On the other hand, if the half-value width Fw of the {110} plane of the ferritic stainless steel foil is too large, the amount of strain of the ferritic stainless steel foil becomes too large. In this case, too much strain is released during heat treatment at about 700 °C, and excessive thermal contraction occurs during heat treatment. As a result, since it thermally contracts beyond the thermal contraction of the active material layer, buckling occurs in the steel foil for the current collector.

[0020] Therefore, the steel foil for the current collector according to the present embodiment has the half-value width Fw of the peak of the {110} plane of the ferritic stainless steel foil set to 0.40 to 0.52°. As a result, even after forming an active material layer on the surface of the steel foil for the current collector according to the present embodiment and performing heat treatment at about 700 °C, buckling and bending are less likely to occur.

[0021] The gist of the steel foil for the current collector according to the present embodiment completed based on the above findings is as follows.

[0022] [1] A steel foil for a current collector, comprising a ferritic stainless steel foil, wherein the ferritic stainless steel foil has a half-value width Fw of the peak of the {110} plane in the X-ray diffraction profile by CoKα rays of 0.40 to 0.52°. Steel foil for a current collector.

[0023] [2] The steel foil for a current collector according to [1], wherein the thickness of the ferritic stainless steel foil is 5 to 60 μm. Steel foil for a current collector.

[0024] [3] The steel foil for a current collector according to [1] or [2], wherein a resin film is formed on the surface of the ferritic stainless steel foil, Steel foil for a current collector.

[0025] Hereinafter, the steel foil for a current collector according to this embodiment will be described.

[0026] [Steel foil for a current collector] The steel foil for a current collector according to this embodiment includes a ferritic stainless steel foil as a base material. Note that the steel foil for a current collector according to this embodiment may include components other than the ferritic stainless steel foil.

[0027] [Ferritic stainless steel foil] The base material of the steel foil for a current collector according to this embodiment is a ferritic stainless steel foil. Specifically, the ferritic stainless steel foil means a steel foil having a Cr content of 10.5% or more and a microstructure mainly composed of ferrite. In this specification, that the microstructure is mainly composed of ferrite means that the volume ratio of ferrite in the microstructure is 95% or more.

[0028] In this embodiment, as the ferritic stainless steel foil, a metal foil made of a well-known ferritic stainless steel can be used. The ferritic stainless steel foil according to this embodiment may be, for example, SUS405 defined in JIS G 4305 (2012), SUS410L, SUS429, SUS430, SUS430LX, SUS430J1L, SUS434, SUS436L, SUS436J1L, SUS443J1, SUS444, SUS445J1, SUS445J2, SUS447J1, or SUSXM27.

[0029] The ferritic stainless steel foil according to the present embodiment may be, for example, 403 defined in ASTM A 280 (2006), 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, or 448.

[0030] Preferably, the thickness of the ferritic stainless steel foil according to the present embodiment is 5 to 60 μm. The thinner the ferritic stainless steel foil, the thinner the current collector steel foil. As a result, the energy density of a battery using an electrode manufactured using the current collector steel foil increases. However, if the ferritic stainless steel foil is too thin, it becomes difficult to manufacture the current collector steel foil. Therefore, in the present embodiment, it is preferable that the thickness of the ferritic stainless steel is 5 to 60 μm.

[0031] The ferritic stainless steel foil according to the present embodiment has a half-value width Fw of the peak of the {110} plane in the X-ray diffraction profile by CoKα rays of 0.40 to 0.52°. As a result, when the current collector steel foil based on the ferritic stainless steel foil according to the present embodiment is heat-treated at about 700°C after the active material layer is formed, buckling or bending hardly occurs.

[0032] If the half-value width Fw of the {110} plane of the ferritic stainless steel foil is too small, the amount of strain in the ferritic stainless steel foil is too small. In this case, no strain release occurs during heat treatment at about 700°C, and no thermal contraction occurs during heat treatment. As a result, the ferritic stainless steel foil for the current collector bends due to being pulled by the thermal contraction of the active material layer. On the other hand, if the half-value width Fw of the {110} plane of the ferritic stainless steel foil is too large, the amount of strain in the ferritic stainless steel foil becomes too large. In this case, too much strain is released during heat treatment at about 700°C, and excessive thermal contraction occurs during heat treatment. As a result, since thermal contraction occurs beyond the thermal contraction of the active material layer, buckling occurs in the ferritic stainless steel foil for the current collector.

[0033] The preferable lower limit of the half-value width Fw of the {110} plane of the ferritic stainless steel foil according to the present embodiment is 0.41, more preferably 0.43, and even more preferably 0.45. If the half-value width Fw of the {110} plane is 0.45 or more, after the active material layer is formed, buckling during heat treatment at about 700°C can be further suppressed. The preferable upper limit of the half-value width Fw of the {110} plane of the ferritic stainless steel foil according to the present embodiment is 0.51, more preferably 0.50, and even more preferably 0.48.

[0034] In this embodiment, the half-value width Fw of the {110} plane of the ferritic stainless steel foil can be measured by the following method. A test piece is prepared from the steel foil for a current collector according to this embodiment. The size of the test piece is not particularly limited, and the thickness of the test piece is the same as that of the steel foil. Measurement is carried out on the observation surface of the test piece by the grazing incident X-ray diffraction method (GIXD). Specifically, the radiation source is CoKα ray, the tube voltage is 40 kV, and the tube current is 135 mA. The X-ray beam is collimated by a mirror. The solar slit has an incident side of 5° and a diffraction side of 2.5°. The incident angle is set to 12 conditions in the range of 1 to 25° so that the measurement depth from the observation surface of the test piece is 12 points in the range of 0.19 to 4.60 μm. At each measurement point, the peak of the {110} plane is identified and the half-value width is obtained. The arithmetic mean value of the 12 obtained half-value widths is defined as the half-value width Fw of the {110} plane. The measurement depth is obtained by finding the X-ray penetration depth t that satisfies μt = 1 with the linear absorption coefficient being μ, and is used after converting it to the depth from the observation surface of the test piece. Also, the mass absorption coefficient of 19.11Cr - 1.77Mo - 78.38Fe (mass%) is used. Further, the density of the ferritic stainless steel foil is 7.75 g / cm 3 is used.

[0035] [Resin coating] The steel foil for a current collector according to this embodiment may be provided with a resin coating formed on the surface of the ferritic stainless steel foil. By forming a resin coating on the surface of the ferritic stainless steel foil, the insulation, heat resistance, and flatness of the steel foil for a current collector can be enhanced. In this embodiment, the resin coating may be formed on both sides of the ferritic stainless steel foil, on one side, or not formed at all. The resin coating may further be formed on a part or all of one side of the ferritic stainless steel foil.

[0036] The resin coating is not particularly limited, and for example, an inorganic-organic hybrid resin coating may be used. The inorganic-organic hybrid resin coating means a resin coating formed by the combination of an inorganic component and an organic component. Specifically, the inorganic-organic hybrid resin coating may be a siloxane coating having an inorganic skeleton with a siloxane bond developed in a three-dimensional network structure as a main skeleton, and at least one of the cross-linking oxygens in the skeleton is substituted with an organic group and / or a hydrogen atom. That is, in the present embodiment, as the resin coating, a siloxane coating in which the oxygen concentration [O] (mol / L) and the silicon concentration [Si] (mol / L) in the coating satisfy 1 < [O] / [Si] < 2 can be used.

[0037] As described above, the thinner the steel foil for a current collector, the higher the energy density of the battery using the electrode manufactured using the steel foil for a current collector. Therefore, in the present embodiment, when the resin coating is formed, it is preferable that the thickness of the resin coating is thinner. On the other hand, if the thickness of the resin coating is too thin, the above-described insulation, heat resistance, and flatness may not be sufficiently obtained. Therefore, in the present embodiment, when the resin coating is formed, the thickness of the resin coating is preferably 0.3 to 5.0 μm.

[0038] [Other configurations] The steel foil for a current collector according to the present embodiment may include configurations other than the ferritic stainless steel foil and the resin coating. For example, a layer other than the resin coating may be provided on the surface of the steel foil for a current collector. In this case, it is preferable that the layer other than the resin coating has conductivity.

[0039] [Electrode] The steel foil for a current collector according to this embodiment can be used as an electrode of a battery by forming an active material layer on its surface. Note that the steel foil for a current collector according to this embodiment can be used for the positive electrode of a battery or the negative electrode of a battery. When the steel foil for a current collector according to this embodiment is used as an electrode, the active material layer is not particularly limited as long as it has a well-known configuration. Note that the active material layer may contain a composition other than the active material. For example, the active material layer may contain a conductive assistant. Further, the electrode according to this embodiment may have a conductive layer between the steel foil for a current collector and the active material layer.

[0040] As described above, in an all-solid-state secondary battery, after forming an active material layer on the surface of the current collector, the active material may be crystallized. In this case, after forming the active material layer on the current collector surface, for example, heat treatment is performed at a high temperature of about 700°C. The steel foil for a current collector according to this embodiment is less likely to buckle or bend even when heat treatment at about 700°C is performed after forming the active material layer.

[0041] Here, the active material is not particularly limited, and well-known active materials can be used. When the electrode is a negative electrode, the negative electrode active material may be, for example, a carbon-based material typified by graphite, an alloy material typified by a CuSn alloy or a NiTiSi alloy, or a Si-based material typified by SiO. When the electrode is a positive electrode, the positive electrode active material may be, for example, lithium cobalt oxide, a ternary system material, lithium manganate, lithium iron phosphate, or high-nickel.

[0042] More specifically, when the steel foil for a current collector according to this embodiment is used in an all-solid-state lithium-ion secondary battery, as the active material, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMn2O4), solid solution oxide (Li2MnO3-LiMO2 (M = Co, Ni, etc.)), lithium-manganese-nickel oxide (LiNi 1 / 3 Mn 1 / 3 Co 1 / 3Composite oxides such as olivine-type lithium phosphate (LiFePO4) can be used.

[0043] In this embodiment, the method for forming the active material layer using the active materials exemplified above is not particularly limited. As the method for forming the active material layer, for example, vacuum deposition methods typified by sputtering and electron beam evaporation methods, and coating methods can be used.

[0044] [Battery] As described above, the steel foil for a current collector according to this embodiment can be used as an electrode of a battery by forming an active material layer on its surface. Note that the battery used in this embodiment is not limited. The steel foil for a current collector according to this embodiment can also be used as an electrode of an all-solid-state secondary battery, a non-aqueous electrolyte secondary battery, an aqueous electrolyte secondary battery, or a primary battery.

[0045] Preferably, the steel foil for a current collector according to this embodiment is used as an electrode of an all-solid-state secondary battery. In an all-solid-state secondary battery, the electrolyte is solid. In this case, the electrolyte is not particularly limited, and well-known electrolytes can be used.

[0046] [Buckling] The steel foil for a current collector according to this embodiment is less likely to buckle or bend by heat treatment at a high temperature to the extent that the active material is crystallized after forming the active material layer. In this embodiment, the fact that buckling is less likely to occur can be evaluated by the following method.

[0047] On the surface of the steel foil for a current collector according to this embodiment, an active material layer is formed using lithium cobalt oxide (LiCoO2) as the active material. Note that the size of the steel foil for a current collector is, for example, 300 to 600 mm in the width direction of the steel foil. The steel foil for a current collector coated with the active material is heat-treated at 700°C for 20 minutes while applying a tension using a transport roll with a diameter of 70 mm or more. The applied tension is, for example, 5 to 50 N / mm in the rolling direction of the steel foil. 2Let it be so. Also, the conveyance speed during heat treatment shall be 0.1 to 30.0 m / min. For the current collector steel foil after heat treatment, visually check for the presence or absence of buckling. In this specification, buckling means unevenness formed on the surface of the current collector steel foil. As a result of visual confirmation, if no buckling is confirmed in the current collector steel foil, it is determined that buckling is unlikely to occur.

[0048] [Buckling] The current collector steel foil according to this embodiment is less likely to have buckling or bending during heat treatment at a high temperature sufficient to crystallize the active material after forming the active material layer. In this embodiment, the fact that bending is unlikely to occur can be evaluated by the following method.

[0049] A test piece is prepared from the current collector steel foil according to this embodiment. The size of the test piece shall be 30 mm in the rolling direction and 150 mm in the width direction of the ferrite stainless steel foil. An active material layer is formed on the surface of the test piece using lithium cobalt oxide (LiCoO2) as the active material, and heat treatment is performed at 700 °C for 20 minutes. One end in the longitudinal direction of the test piece after heat treatment is fixed to the upper end of a surface plate at a right angle, and the test piece is arranged so as to be in contact with the surface plate at a right angle. The distance between the other end (lower end) in the longitudinal direction of the test piece and the surface plate at a right angle is measured with a ruler and defined as the "amount of warp" of the test piece. In this embodiment, when the amount of warp is 5 mm or less, it is determined that bending is unlikely to occur. In this embodiment, furthermore, when the amount of warp is 3 mm or less, it is determined that bending is very unlikely to occur.

[0050] [Manufacturing method of current collector steel foil] An example of the manufacturing method of the current collector steel foil according to this embodiment will be described. The manufacturing method described below is an example for manufacturing the current collector steel foil according to this embodiment, and the manufacturing method of the current collector steel foil according to this embodiment may be a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the manufacturing method of the current collector steel foil according to this embodiment. The manufacturing method of the current collector steel foil according to this embodiment includes an intermediate steel material preparation step, a final cold rolling step, and a heat treatment step.

[0051] [Intermediate steel material preparation step] In the intermediate steel material preparation process, an intermediate steel material having the same chemical composition as the ferrite stainless steel foil, which is the base material of the current collector steel foil, is prepared. That is, the intermediate steel material is an intermediate product for manufacturing the ferrite stainless steel foil according to the present embodiment, and means a steel sheet with a thickness of several tens to several hundreds of μm. The intermediate steel material is, for example, a cold-rolled coil obtained by performing cold rolling on a hot-rolled coil. The intermediate steel material may be manufactured and prepared, or may be prepared by purchasing from a third party. That is, the process of preparing the intermediate steel material is not particularly limited.

[0052] When manufacturing the intermediate steel material, for example, it is manufactured by the following method. Molten steel having a desired chemical composition is manufactured. Using the molten steel, a slab (slab, bloom, or billet) is manufactured by the continuous casting method. A steel ingot (ingot) may be manufactured by the ingot method using the molten steel. If necessary, the slab, bloom or ingot may be block-rolled to manufacture a billet.

[0053] Hot working and cold rolling are performed on the manufactured slab or steel ingot (slab, bloom, billet, or ingot) to manufacture a steel sheet with a thickness of several tens to several hundreds of μm. The method of hot working is not particularly limited, and a well-known method may be used. Hot working is, for example, hot rolling. When manufacturing the intermediate steel material by hot rolling, for example, it can be manufactured by the following method.

[0054] After heating the manufactured slab or steel ingot, rough rolling and finish rolling are performed. At this time, the conditions of hot rolling are not particularly limited, and well-known conditions may be set as appropriate. For the hot-rolled intermediate steel material, cold rolling and annealing treatment may be repeatedly performed as necessary. For the hot-rolled intermediate steel material, skin pass rolling may be further performed as necessary. Annealing treatment is further performed on the hot-rolled and / or cold-rolled intermediate steel material. Through the above processes, the intermediate steel material according to the present embodiment is prepared.

[0055] [Final Cold Rolling Process] In the final cold rolling process, cold rolling is performed on the intermediate steel material prepared in the intermediate steel material preparation process. In this embodiment, the cold rolling can be carried out using a well-known device and is not particularly limited. For example, a continuous rolling mill equipped with a plurality of cold rolling stands may be used.

[0056] In the final cold rolling process according to this embodiment, the preferable reduction ratio R is 30 to 98%. Here, the reduction ratio R in the final cold rolling process means the reduction rate (%) of the thickness of the intermediate steel material from before the final cold rolling process to after the final cold rolling process. That is, the reduction ratio R in the final cold rolling process is defined by the following formula (A). R(%) = [{(thickness of the intermediate steel material before the final cold rolling process) - (thickness of the intermediate steel material after the final cold rolling process)}] / (thickness of the intermediate steel material before the final cold rolling process) × 100 (A)

[0057] If the reduction ratio R in the final cold rolling process is too low, the amount of strain energy introduced into the intermediate steel material by cold rolling decreases. In this case, in the heat treatment process described later, the heat treatment temperature required to release the strain becomes higher. That is, if the reduction ratio R in the final cold rolling process is too low and the heat treatment temperature in the heat treatment process described later is too low, the half-value width Fw of the {110} plane in the ferritic stainless steel foil of the produced current collector steel foil may become too high. As a result, buckling is likely to occur in the current collector steel foil by heat treatment at a high temperature sufficient to crystallize the active material.

[0058] On the other hand, if the reduction ratio R in the final cold rolling process is too high, the amount of strain introduced into the intermediate steel material by cold rolling becomes too high. In this case, there is a concern that breakage may occur at the end of the intermediate steel material. Therefore, in the final cold rolling process according to this embodiment, the preferable reduction ratio R is 35 to 98%. A more preferable lower limit of the reduction ratio R in the final cold rolling process is 40%, and more preferably 45%. A more preferable upper limit of the reduction ratio R in the final cold rolling process is 96%, and more preferably 95%.

[0059] In the final cold rolling process according to this embodiment, processes other than cold rolling may also be performed. For example, slitting may be performed on the intermediate steel material to process the intermediate steel material into an arbitrary size. In this case, in the final cold rolling process, slitting may be performed after cold rolling, slitting may be performed after slitting, or slitting may be performed between cold rollings.

[0060] [Heat treatment process] In the heat treatment process, heat treatment is performed on the intermediate steel material after the final cold rolling process. In this embodiment, the heat treatment can be performed by passing the well-known heat treatment furnace through the plate. In this embodiment, the atmosphere of the heat treatment process is preferably an inert gas atmosphere such as nitrogen or a reducing gas atmosphere such as hydrogen.

[0061] The preferable heat treatment temperature in the heat treatment process is 350 to 700 °C. If the heat treatment temperature is too low, the strain cannot be sufficiently released from the intermediate steel material. As a result, in the ferrite stainless steel foil of the current collector steel foil produced, the half-value width Fw of the {110} plane may become too high. In this case, buckling is likely to occur in the current collector steel foil by heat treatment at a high temperature enough to crystallize the active material. On the other hand, if the heat treatment temperature is too high, the strain is released too much from the intermediate steel material. As a result, in the ferrite stainless steel foil of the current collector steel foil produced, the half-value width Fw of the {110} plane may become too low. In this case, bending is likely to occur in the current collector steel foil by heat treatment at a high temperature enough to crystallize the active material.

[0062] Therefore, in this embodiment, the heat treatment temperature in the heat treatment process is preferably 350 to 700 °C. A more preferable lower limit of the heat treatment temperature is 400 °C. A more preferable upper limit of the heat treatment temperature is 650 °C. A more preferable heat treatment temperature is 350 to 550 °C. In this case, in the ferrite stainless steel foil of the current collector steel foil produced, the half-value width Fw of the {110} plane is stably 0.45 to 0.52 °, and bending of the current collector steel foil is more likely to be suppressed by heat treatment at a high temperature enough to crystallize the active material.

[0063] In the heat treatment step, the preferable heat treatment time is 1 to 400 seconds. If the heat treatment time is too short, the strain cannot be sufficiently released from the intermediate steel material. As a result, in the ferrite stainless steel foil of the current collector steel foil produced, the half-value width Fw of the {110} plane may become too high. In this case, by heat treatment at a high temperature enough to crystallize the active material, buckling is likely to occur in the current collector steel foil. On the other hand, if the heat treatment time is too long, the strain may be released too much from the intermediate steel material. As a result, in the ferrite stainless steel foil of the current collector steel foil produced, the half-value width Fw of the {110} plane may become too low. In this case, by heat treatment at a high temperature enough to crystallize the active material, bending is likely to occur in the current collector steel foil.

[0064] Therefore, in the present embodiment, it is preferable that the heat treatment time in the heat treatment step is 1 to 400 seconds. A more preferable lower limit of the heat treatment time is 2 seconds, and more preferably 3 seconds. A more preferable upper limit of the heat treatment time is 300 seconds, and more preferably 250 seconds. In this specification, the heat treatment temperature in the heat treatment step means the temperature (°C) of the heat treatment furnace for performing the heat treatment. In this specification, the heat treatment time in the heat treatment step means the time (seconds) taken for the intermediate steel material to pass through the heat treatment furnace for performing the annealing treatment.

[0065] Through the above manufacturing process, the current collector steel foil according to the present embodiment having the above configuration can be manufactured. The above manufacturing process is an example of the manufacturing method of the current collector steel foil according to the present embodiment, and the manufacturing method of the current collector steel foil according to the present embodiment is not limited to the above manufacturing method. Further, the manufacturing method of the current collector steel foil according to the present embodiment may further perform any of the steps described below. The steps described below are optional and do not have to be performed.

[0066] [Tension Annealing Step] The method for manufacturing a steel foil for a current collector according to the present embodiment may further include a tension annealing step after the final cold rolling step and before the heat treatment step. Tension annealing means performing an annealing treatment while applying tension. The intermediate steel material subjected to tension annealing can maintain its flatness due to the tension.

[0067] When the tension annealing step is carried out, the preferable annealing temperature is 350 to 450°C. If the annealing temperature is too low, the effect of tension annealing may not be obtained sufficiently. On the other hand, if the annealing temperature is too high, in the microstructure of the intermediate steel material, the introduced dislocation density decreases too much. As a result, in the manufactured ferritic stainless steel foil, the half-value width Fw of the peak of the {110} plane in the X-ray diffraction profile by CoKα rays may not be increased sufficiently. Therefore, in the present embodiment, when carrying out the tension annealing step, the annealing temperature is preferably 350 to 450°C.

[0068] When the tension annealing step is carried out, the annealing time is not particularly limited. The annealing time is, for example, 1 to 10 seconds. In this specification, the "annealing temperature" of tension annealing means the temperature (°C) of the heat treatment furnace for performing the annealing treatment. In this specification, the "annealing time" of tension annealing means the time (seconds) required for the intermediate steel material to pass through the heat treatment furnace for performing the annealing treatment. Also, when carrying out the tension annealing step, the tension applied to the intermediate steel material is not particularly limited.

[0069] [Resin coating formation step] The method for manufacturing a steel foil for a current collector according to the present embodiment may further include a resin coating formation step after the final cold rolling step and before the heat treatment step. As described above, by forming a resin coating, the insulation, heat resistance, and flatness of the steel foil for a current collector can be enhanced.

[0070] When performing the resin film forming step, a resin film is formed on the surface of the intermediate steel material cold-rolled by the final cold rolling step. As described above, in the present embodiment, the resin film is not particularly limited, but for example, it is an inorganic-organic hybrid resin film. The method for forming the resin film is not particularly limited and may be a well-known method. For example, a resin film may be formed by applying a composition containing the components of the resin film and drying it.

[0071] Also, if necessary, a heat treatment for curing the resin film may be performed. In this case, in the heat treatment step described above, it is preferable to cure the resin film. When curing the resin film by the heat treatment in the heat treatment step, the distortion of the ferritic stainless steel foil can be appropriately controlled while curing the resin film at the same time by the heat treatment step. In this case, productivity can be improved.

[0072] Hereinafter, the steel foil for current collector according to the present embodiment will be described more specifically by way of examples. Note that the examples described below are an example for confirming the effects of the steel foil for current collector according to the present embodiment and do not limit the present invention.

Examples

[0073] Intermediate steel materials of each test number having the thickness (μm) shown in Table 1 were prepared. Note that the intermediate steel materials of each test number were all intermediate steel materials made of ferritic stainless steel corresponding to SUS444 defined in JIS G 4305 (2012). Also, all the intermediate steel materials were those subjected to an annealing treatment at 840 to 950°C for 3 to 30 seconds.

[0074]

Table 1

[0075] For the intermediate steel materials of each test number, cold rolling was carried out at the reduction ratio (%) described in Table 1. In this way, a base material (ferritic stainless steel foil) with the base material thickness (μm) described in Table 1 was obtained. Among the obtained base materials, annealing described in the "Annealing Treatment" column was carried out on the base materials of some test numbers. Tension annealing was carried out on the base materials of Test Numbers 5 and 6 (denoted as "TA (Tension Annealing)" in Table 1). In this example, as the tension annealing, a heat treatment was carried out at 400 °C for 4 seconds. Also, bright annealing was carried out on the base materials of Test Numbers 14 and 15 (denoted as "BA (Bright Annealing)" in Table 1). Bright annealing means annealing carried out under conditions where the surface of the ferritic stainless steel foil is not nitrided. In this example, as the bright annealing, the nitrogen concentration in the atmosphere was set to 0.1% or less, and a heat treatment was carried out at 950 °C for 4 seconds.

[0076] A composition for forming a siloxane-based resin film was applied to one side of the obtained base material of each test number. The base material with the composition applied to one side was heat-treated at the heat treatment temperature described in Table 1 for 20 to 300 seconds. Note that "-" in the "Heat Treatment Temperature (°C)" column of Table 1 means that no heat treatment was carried out. Through the above steps, the steel foil for a current collector according to this embodiment was manufactured.

[0077] [Evaluation Test] For the steel foil for a current collector of each test number, a half-value width Fw measurement test, a buckling evaluation test, and a bending evaluation test were carried out.

[0078] [Half-Value Width Fw Measurement Test] For the steel foils for current collectors with each test number, a half-value width Fw measurement test was carried out to obtain the half-value width Fw of the {110} plane. Specifically, test pieces were prepared from the steel foils for current collectors with each test number, and X-ray diffraction profiles were obtained by the grazing incidence X-ray diffraction method (GIXD). Among the test pieces, the surface on which the resin coating was not formed was specified and used as the observation surface. In the grazing incidence X-ray diffraction method (GIXD), the radiation source was CoKα ray, the tube voltage was 40 kV, and the tube current was 135 mA. Also, the X-ray beam was collimated with a mirror. The soller slit had an incident side of 5° and a diffracted side of 2.5°. Twelve conditions were set for the incident angle in the range of 1 to 25° so that there were 12 points in the range of the measurement depth from the observation surface of the test piece of 0.19 to 4.60 μm.

[0079] At each measurement point, the peak of the {110} plane was identified from the X-ray profile, and the half-value width was obtained. The arithmetic mean value of the 12 obtained half-value widths was defined as the half-value width Fw of the {110} plane. Note that the measurement depth was obtained by finding the X-ray penetration depth t that satisfies μt = 1 with the linear absorption coefficient as μ and converted to the depth from the observation surface of the test piece. Also, the mass absorption coefficient of 19.11Cr-1.77Mo-78.38Fe (mass%) was used. Furthermore, the density of the ferritic stainless steel foil was 7.75 g / cm 3 was used. For the steel foils for current collectors with each test number, the obtained half-value width Fw (°) of the {110} plane is shown in Table 1.

[0080] [Buckling evaluation test] For the steel foils for current collectors with each test number, a buckling evaluation test was carried out to evaluate the occurrence of buckling. Specifically, on the surface of the steel foils for current collectors with each test number where the siloxane-based resin coating was not formed, an active material layer was formed by vapor deposition. In this example, lithium cobalt oxide (LiCoO2) was used as the active material. Note that for each test number, the size of the steel foil for the current collector was 300 to 600 mm in the width direction of the steel foil. The steel foil for the current collector coated with the active material was heat-treated at 700 °C for 20 minutes while applying a tension using a transport roll with a diameter of 70 mm. The applied tension was 5 to 50 N / mm in the rolling direction of the steel foil 2 and the transport speed during heat treatment was 0.1 to 30.0 m / min.

[0081] For the current collector steel foils of each test number after heat treatment, the presence or absence of buckling was visually confirmed. More specifically, for the current collector steel foils of each test number, when no streak-like unevenness extending in the rolling direction of the steel foil was confirmed, it was judged that buckling was unlikely to occur (denoted as "E (Excellent)" in Table 1). On the other hand, for the current collector steel foils of each test number, when streak-like unevenness extending in the rolling direction of the steel foil was formed at intervals of several millimeters to several tens of millimeters, it was judged that buckling was likely to occur (denoted as "NA (Not Acceptable)" in Table 1).

[0082] [Buckling Evaluation Test] For the current collector steel foils of each test number, a buckling evaluation test was carried out to evaluate the occurrence of buckling. In addition, for the current collector steel foils of the test numbers judged to be unlikely to buckle in the buckling evaluation test, a buckling evaluation test was carried out. Specifically, test pieces for buckling evaluation were prepared from the current collector steel foils of each test number. The test pieces were 30 mm in the rolling direction of the steel foil and 150 mm in the width direction. An active material layer was formed on the surface of the test pieces by vapor deposition. In this example, lithium cobalt oxide (LiCoO2) was used as the active material. The test pieces with the active material layer formed were heat-treated at 700 °C for 20 minutes. One end of the longitudinal direction of the heat-treated test piece was fixed to the upper end of a surface plate at a right angle, and the test piece was arranged to be in contact with the surface plate at a right angle.

[0083] The distance between the other end (lower end) of the longitudinal direction of the test piece and the surface plate at a right angle was measured with a ruler and defined as the "amount of warping" of the test piece. For the current collector steel foils of each test number, when the amount of warping was 5 mm or less, it was judged that buckling was unlikely to occur (denoted as "G (Good)" in Table 1). For the current collector steel foils of each test number, when the amount of warping was further 3 mm or less, it was judged that buckling was very unlikely to occur (denoted as "E (Excellent)" in Table 1). On the other hand, for the current collector steel foils of each test number, when the amount of warping exceeded 5 mm, it was judged that buckling was likely to occur (denoted as "NA (Not Acceptable)" in Table 1).

[0084] [Evaluation Results] For the steel foils for current collectors with test numbers 1 to 8, the half-value width Fw of the {110} plane of the ferritic stainless steel foil satisfied 0.40 to 0.52°. As a result, in the buckling evaluation test, it was judged that buckling was unlikely to occur. Furthermore, as a result, in the bending evaluation test, the amount of warpage was 5 mm or less, and it was judged that bending was unlikely to occur. That is, after forming the active material layer, these steel foils for current collectors were less likely to buckle or bend due to heat treatment at a high temperature sufficient to crystallize the active material.

[0085] For the steel foils for current collectors with test numbers 1 to 6, the half-value width Fw of the {110} plane of the ferritic stainless steel foil further satisfied 0.45 to 0.52°. As a result, in the bending evaluation test, the amount of warpage was 3 mm or less, and it was judged that bending was extremely unlikely to occur.

[0086] On the other hand, for the steel foils for current collectors with test numbers 9 and 10, the heat treatment process was not carried out. As a result, for these steel foils for current collectors, the half-value width Fw of the {110} plane of the ferritic stainless steel foil exceeded 0.52°. As a result, in the buckling evaluation test, it was judged that buckling was likely to occur.

[0087] For the steel foil for current collector with test number 11, the heat treatment temperature in the heat treatment process was too low. As a result, for this steel foil for current collector, the half-value width Fw of the {110} plane of the ferritic stainless steel foil exceeded 0.52°. As a result, in the buckling evaluation test, it was judged that buckling was likely to occur.

[0088] For the steel foil for current collector with test number 12, the reduction ratio in the final cold rolling process was too low. As a result, for this steel foil for current collector, the half-value width Fw of the {110} plane of the ferritic stainless steel foil exceeded 0.52°. As a result, in the buckling evaluation test, it was judged that buckling was likely to occur.

[0089] For the steel foil for current collector with test number 13, the heat treatment temperature in the heat treatment process was too high. As a result, for this steel foil for current collector, the half-value width Fw of the {110} plane of the ferritic stainless steel foil was less than 0.40°. As a result, in the bending evaluation test, the amount of warpage exceeded 5 mm, and it was judged that bending was likely to occur.

[0090] For the steel foil for current collector of Test No. 14, bright annealing was carried out before the heat treatment process. Furthermore, the heat treatment process was not carried out. As a result, for this steel foil for current collector, the half-value width Fw of the {110} plane of the ferritic stainless steel foil was less than 0.40°. As a result, in the bending evaluation test, it was judged that the warpage amount exceeded 5 mm and bending was likely to occur.

[0091] For the steel foil for current collector of Test No. 15, bright annealing was carried out before the heat treatment process. As a result, for this steel foil for current collector, the half-value width Fw of the {110} plane of the ferritic stainless steel foil was less than 0.40°. As a result, in the bending evaluation test, it was judged that the warpage amount exceeded 5 mm and bending was likely to occur.

[0092] 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 the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. A steel foil for a current collector, comprising a ferritic stainless steel foil, wherein in the X-ray diffraction profile by CoKα rays, the half-value width Fw of the peak of the {110} plane of the ferritic stainless steel foil is 0.40 to 0.52°, the steel foil for a current collector.

2. The steel foil for a current collector according to Claim 1, wherein the thickness of the ferritic stainless steel foil is 5 to 60 μm, the steel foil for a current collector.

3. The steel foil for a current collector according to Claim 1 or Claim 2, wherein a resin coating is formed on the surface of the ferritic stainless steel foil, the steel foil for a current collector.

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